WO2009069110A1

WO2009069110A1 - A method for producing an array of pores of cylindrical shape in a polymer film, and a polymer film produced according to the method

Info

Publication number: WO2009069110A1
Application number: PCT/IE2008/000113
Authority: WO
Inventors: Ronan Daly; John Boland; Terence Connolly
Original assignee: The Provost, Fellows And Scholars Of The College Of The Holy And Undivided Trinity Of Queen Elizabeth, Near Dublin
Priority date: 2007-11-26
Filing date: 2008-11-26
Publication date: 2009-06-04

Abstract

A method for producing elongated cylindrical micro-pores (2) in a polymer film (1) comprises drop-casting a thin film (22) of a low polymer concentration solvent solution comprising approximately 3.5% by mass polymer and 96.5% by mass solvent on a planar horizontal upwardly facing surface (20) of a substrate (18) in an elongated chamber (17). A first gas flow of nitrogen gas of relative humidity of 2% to 5% is passed through the chamber (17) over a gas/film interface surface (55) of the film solution (22) for approximately 45 seconds to initially accelerate solvent evaporation from the film solution (22) in order to establish a polymer concentration gradient which extends into the film solution (22) from the gas/film interface surface (55) with the polymer concentration highest at the gas/film interface surface (55). A second gas flow of nitrogen gas of relative humidity of approximately 85% is then passed over the gas/film interface surface (55) until all the solvent has been evaporated. Water condensing on the gas/film interface surface (55) from the second humid gas flow forms water droplets, which grow and sink into the film solution (22). The second gas flow controls the downward rate of growth of the polymer concentration gradient into the film solution (22) so that it is matched with the downward rate of growth of the water droplets. This stabilises the water droplets, thereby resulting in the formation of the cylindrical pores (2).

Description

"A method for producing an array of pores of cylindrical shape in a polymer film, and a polymer film produced according to the method"

The present invention relates to a method for producing an array of pores of cylindrical shape in a polymer film, and the invention also relates to a polymer film comprising an array of cylindrical pores formed by the method according to the invention.

Films comprising arrays of cylindrical pores of micron and sub-micron size are required for many applications, for example, in photonics where micron and sub- micron cylindrical pores are used as micro-cavities for, for example, photonic light delays and the like. Additionally, such micro-pores also have use in microfluidic applications. Currently films comprising such micron and sub-micron size cylindrical pores are formed in silicon, glass, quartz and the like using reactive ion etching, electrochemical processes or electron beam lithography. The cylindrical pores thus formed are then coated or functionalised as required. While these methods do result in the formation of micron and sub-micron size cylindrical pores, the initial capital investment and the running costs of apparatus for carrying out such processes in silicon, glass or quartz are relatively high and the processes are relatively labour intensive and time consuming.

Methods for producing a polymer film comprising arrays of micro and sub-micron pores are known. In general such micron and sub-micron pores are of spherical shape. For example, U.S. Published Patent Application Specification No.2003/0106487 of Wen-Chiang Huang discloses a method for producing a photonic crystal material which requires initially producing a porous template of polymer material. In the method of Huang a polymer, an oligomer or a non- polymeric organic substance is dissolved in a volatile solvent to form an evaporative solution. A film of the solution is deposited on a substrate, and a moisture containing gas is passed over the film solution in order to evaporate the solvent. Moisture in the gas condenses on the surface of the film solution to form water droplets in an ordered array, which in turn sink into the film solution to form spherical water filled voids. When the solvent has been fully evaporated, water in the spherical voids is then evaporated thereby producing a thin polymer with ordered arrays of spherical micron pores. Other prior art documents which disclose methods for producing micron and sub-micron pores in a polymer film are U.S. Published Patent Application Specification No. 2003/0129311, U.S. Published Patent Application Specification No. 2004/0138323, U.S. Published Patent Application Specification No.2002/0143073, PCT Published Application Specification No. WO 2007/086421, PCT Published Application Specification Application No. WO 2006/112358, Chinese Patent Abstract No. CN1676204 and Chinese Patent Abstract No. CN1511874. However, none of these documents disclose a method for forming a cylindrical pore in a polymer film of substantially constant transverse cross-section and of predefined length.

There is therefore a need for a method for producing a film comprising an array of cylindrical pores which addresses this problem.

The present invention is directed towards providing such a method, and the invention is also directed towards providing a film comprising an array of cylindrical pores.

According to the invention there is provided a method for producing an array of pores of cylindrical shape in a polymer film, the method comprising forming a film of a low polymer concentration solvent solution on a substrate, initially accelerating solvent evaporation from the film solution through a gas/film interface surface thereof to produce a polymer concentration gradient in the film solution with the polymer concentration decreasing from the gas/film interface surface into the film solution, so that the polymer concentration gradient is suitable for facilitating elongated growth of liquid droplets sinking into the film solution from the gas/film interface surface thereof, and condensing a liquid to form liquid droplets on the gas/film interface surface of the film solution to sink into the film solution to define the cylindrical pores therein.

In one embodiment of the invention the elongated growth of the droplets sinking into the film solution is in a direction substantially perpendicular to the gas/film interface surface. Preferably, the polymer concentration in the polymer concentration gradient decreases from the gas/film interface surface into the film solution at a rate suitable for elongated growth of the liquid droplets into the film solution.

In another embodiment of the invention the rate of solvent evaporation from the film solution is controlled so that the rate of progress of the polymer concentration gradient into the film solution from the gas/film interface surface thereof is proportional to the rate of elongated growth of the liquid droplets therein. Preferably, the rate of solvent evaporation from the film solution is controlled so that the rate of progress of the polymer concentration gradient into the film solution from the gas/film interface thereof is substantially similar to the rate of elongated growth of the liquid droplets therein.

In one embodiment of the invention a first gas flow is passed over the gas/film interface surface of the film solution for initially accelerating the solvent evaporation therefrom through the gas/film interface surface. Preferably, the first gas flow is passed over the gas/film interface surface of the film solution for establishing the polymer concentration gradient.

In another embodiment of the invention a second gas flow having vapour of the droplet forming liquid entrained therein is passed over the gas/film interface surface of the film solution to form the liquid droplets on the gas/film interface surface of the film solution, after the first gas flow has been passed over the film solution. Preferably, the second gas flow is passed over the gas/film interface surface of the film solution for controlling the rate of progress of the polymer concentration gradient into the film solution from the gas/film interface surface thereof.

In another embodiment of the invention the vapour of the droplet forming liquid is water vapour, and the second gas flow is a humid gas flow. Preferably, the relative humidity of the second gas flow is at least 35%. Advantageously, the relative humidity of the second gas flow is at least 50%. Preferably, the relative humidity of the second gas flow lies in the range 60% to 100%, and ideally, the relative humidity of the second gas flow is in the range of 75% to 85%. In one embodiment of the invention the relative humidity of the second gas flow is tuned to control the elongated growth of the droplets in the film solution.

In another embodiment of the invention the relative humidity of the second gas flow is adjusted during the second gas flow to control the elongated growth of the droplets in the film solution.

Preferably, the flow rate of the second gas flow is less than the flow rate of the first gas flow.

In a further embodiment of the invention the flow rate of the second gas flow lies in the range of just above zero litres per minute to 7 litres per minute. Preferably, the flow rate of the second gas flow lies in the range of 0.5 litres per minute to 5 litres per minute. Advantageously, the flow rate of the second gas flow is approximately 1 litre per minute.

Preferably, the flow rate of the second gas flow over the gas/film interface surface of the film solution is less than the flow rate of the first gas flow over the gas/film interface surface of the film solution.

In another embodiment of the invention the flow rate of the gas of the second gas flow over the gas/film interface surface of the film solution lies in the range of just above zero metres per second to 0.4 metres per second.

Preferably, the flow rate of the gas of the second gas flow over the gas/film interface surface of the film solution lies in the range of 0.03 metres per second to 0.29 metres per second.

Advantageously, the flow rate of the gas of the second gas flow over the gas/film interface surface of the film solution is approximately 0.06 metres per second. Ideally, the flow rate of the second gas flow is a substantially steady state flow rate. In one embodiment of the invention the flow rate of the second gas flow is tuned to control the elongated growth of the droplets in the film solution.

In another embodiment of the invention the flow rate of the second gas flow is adjusted during the second gas flow to control the elongated growth of the droplets.

In a further embodiment of the invention the first gas flow comprises a vapour of the droplet forming liquid entrained therein. Preferably, the vapour entrained in the first gas flow is water vapour, and the first gas flow is a humid gas flow, and the relative humidity of the first gas flow is less than the relative humidity of the second gas flow.

In another embodiment of the invention the relative humidity of the first gas flow is less than 85%. Preferably, the relative humidity of the first gas flow lies in the range of 0% to 45%. Advantageously, the relative humidity of the first gas flow is approximately 5%.

In one embodiment of the invention the relative humidity of the first gas flow is tuned to control the establishment of the polymer concentration gradient in the film solution.

In another embodiment of the invention the relative humidity of the first gas flow is adjusted during the first gas flow to control the establishment of the polymer concentration gradient in the film solution.

In a further embodiment of the invention the flow rate of the first gas flow lies in the range of 0.5 litre per minute to 20 litres per minute.

Preferably, the flow rate of the first gas flow lies in the range of 3 litres per minute to 10 litres per minute. In one embodiment of the invention the flow rate of the first gas flow is approximately 5 litres per minute.

Ideally, the flow rate of the gas of the first gas flow over the gas/film interface surface of the film solution is greater than the flow rate of the gas of the second gas flow thereover.

In one embodiment of the invention the flow rate of the gas of the first gas flow over the gas/film interface surface of the film solution lies in the range of 0.03 metres per second to 1.14 metres per second. Preferably, the flow rate of the gas of the first gas flow over the gas/film interface surface of the film solution lies in the range of 0.17 metres per second to 0.57 metres per second. In one embodiment of the invention the flow rate of the gas of the first gas flow over the gas/film interface surface of the film solution is approximately 0.29 metres per second.

In one embodiment of the invention the flow rate of the first gas flow is tuned to establish the polymer concentration gradient in the film solution.

In another embodiment of the invention the flow rate of the first gas flow is adjusted during the first gas flow to establish the polymer concentration gradient in the film solution.

Preferably, the gas of the first and second gas flows is an inert gas. Advantageously, the gas of the first and second gas flows is nitrogen.

In one embodiment of the invention the film solution is formed on a substantially horizontal surface of the substrate.

In an alternative embodiment of the invention the film solution is formed on a surface of the substrate which is inclined at an angle less than 90° to the horizontal.

In a further embodiment of the invention the surface of the substrate on which the film solution is formed is an upwardly facing surface thereof, and in an alternative embodiment of the invention the surface of the substrate on which the film solution is formed is a downwardly facing surface thereof.

In a further alternative embodiment of the invention the film solution is formed on a substantially vertical surface of the substrate. Preferably, the surface of the substrate on which the film solution is formed is a planar surface.

In one embodiment of the invention the film solution is formed on the substrate by drop-casting the polymer/solvent solution onto the substrate.

Preferably, the polymer is an amphiphilic polymer, and in one embodiment of the invention the polymer is ω-dicarboxy-terminated polystyrene.

Preferably, the polymer concentration in the polymer/solvent solution prior to forming the film solution on the substrate does not exceed 8%. Advantageously, the polymer concentration in the polymer/solvent solution prior to forming the film solution on the substrate lies in the range of 2% to 5%. In a preferred embodiment of the invention the polymer concentration in the polymer/solvent solution prior to forming the film solution on the substrate lies in the range of 3% to 4%, and ideally, the polymer concentration in the polymer/solvent solution prior to forming the film solution on the substrate is approximately 3.5%.

Preferably, the solvent is a volatile solvent. Ideally, the solvent is volatile at temperature and pressure conditions of the first and second gas flows.

Preferably, the latent heat of vaporisation of the solvent is such as to reduce the temperature of the gas/film interface surface to a temperature sufficient to induce condensation of the vapour entrained in at least the second gas flow.

In one embodiment of the invention the solvent is chloroform.

In one aspect of the invention the liquid droplets are evaporated from the film solution to form the cylindrical pores. In another aspect of the invention the cylindrical pores are of micron size. In a further aspect of the invention the cylindrical pores are of sub-micron size. The invention also provides a polymer film comprising an array of cylindrical pores formed therein using the method as claimed in any preceding claim.

In one aspect of the invention the cylindrical pores are of micron size. In another aspect of the invention the cylindrical pores are of sub-micron size.

Additionally, the invention provides apparatus for carrying out the method according to the invention for producing an array of pores of cylindrical shape in a polymer film.

The advantages of the invention are many. A particularly important advantage of the invention is that it permits a film having an array of pores of cylindrical shape to be produced at a relatively low cost at normal room temperature and pressure. The method according to the invention does not require complex and highly capital intensive machinery and processes as do currently known methods for producing thin films having arrays of cylindrical pores. In fact, the pores order themselves into a hexagonally close-packed arrangement, as a result of the formation of the water droplets on the gas/film interface surface of the film solution in a similar hexagonally close-packed array, which the water droplets adopt, in order to minimise the energies of the respective water droplets. Thus, the pores effectively self assemble in the film solution.

The invention will be more clearly understood from the following description of a preferred embodiment thereof, which is given by way of example only, with reference to the accompanying drawings, in which:

Fig. 1 is a photomicrograph of a polymer film according to the invention comprising a plurality of cylindrical pores produced by a method according to the invention,

Fig. 2 is a photomicrograph illustrating a top plan view of the polymer film of

Fig. 1.

Fig. 3 is a schematic diagram of apparatus in which the method for producing the polymer film of Fig. 1 is carried out, and

Fig. 4 is an enlarged cross-sectional side elevational view of a detail of the apparatus of Fig. 3.

Referring to the drawings and initially to Figs. 1 and 2, there is illustrated photomicrographs of a polymer film according to the invention, indicated generally by the reference numeral 1 , which is produced by a method according to the invention, and which comprises a plurality of pores 2 of cylindrical shape arranged in a hexagonally close-packed array. Each pore 2 extends downwardly into the polymer film 1 from a top surface 3 thereof. Each pore 2 is of substantially constant transverse cross-section over substantially most of its axial length, and converges to a neck 4 adjacent the surface 3 of the polymer film 1 and terminates in a dome shaped lower end 6. In this embodiment of the invention the axial length of the pores 2 ranges from 9 microns to 10 microns, and the diameter of the portion of the bores 2 of substantially constant transverse cross-section ranges from 3.5 microns to 5.5 microns. The diameter of the neck 4 of the pores 2 ranges from 2.5 microns to 3.3 microns. The pores are arranged in a hexagonally close-packed array with centre-to-centre spacings in the range of 4 microns to 5.5 microns.

However, pores of other diameters and lengths may be produced, and the diameter and the length of the pores depends on process conditions. The polymer film 1 of pores 2 is formed as will be described below from a low polymer concentration solvent solution, a thin film of which is formed on a substrate. First and second gas flows of a humid inert gas, which in this case is nitrogen gas with entrained water vapour are passed sequentially over the exposed surface of the thin film of the polymer/solvent solution, and result in evaporation of the solvent and condensing of water from the water vapour to form droplets on the exposed surface of the polymer/solvent solution film corresponding to the surface 3 of the film 1. As the solvent evaporates, the water droplets grow and sink into the polymer/solvent solution film to define the cylindrical pores 2. The first and second gas flows, as will be described below, are of respective different relative humidities, the relative humidity of the second gas flow being greater than that of the first gas flow. Although the relative humidities of the first and second gas flows could in certain cases be substantially similar, it is preferable that the relative humidity of the first gas flow should be relatively low, since the primary objective of the first gas flow is to accelerate evaporation of the solvent from the polymer/solvent solution film in order to establish a polymer concentration gradient adjacent to the exposed surface of the polymer/solvent solution film with the polymer concentration decreasing into the polymer/solvent solution film, as will be described below. Once the polymer concentration gradient has been established in the polymer/solvent solution film, the first gas flow is terminated and the second gas flow is commenced. The objective of the second gas flow is to facilitate growth of the water droplets on the exposed surface of the polymer/solvent solution film as the water droplets sink into the polymer/solvent solution film to form the cylindrical pores 2. Thus, the relative humidity of the second gas flow should be relatively high in order to propagate growth of the water droplets.

The method for producing the polymer film 1 with the cylindrical pores 2 will now be described with reference to Figs. 3 and 4, in which apparatus indicated generally by the reference numeral 5 is illustrated for carrying out the method. The low polymer concentration solvent solution in this embodiment of the invention comprises an amphiphilic polymer, which in this particular case is ω-dicarboxy-terminated polystyrene, and a solvent which is volatile at room temperature and in which the polymer is highly soluble. In this case the solvent is chloroform. The polymer constitutes 3.5% by mass of the polymer/solvent solution, and the solvent constitutes 96.5% by mass of the polymer/solvent solution. The polymer is placed in a vial and the solvent is added to the polymer and minimal mixing is required to achieve a clear solution. In this embodiment of the invention the molecular weight of the polymer is approximately 100,300 grams per mol. The percentage concentration of the polymer in the polymer/solvent solution will be dependent on many variables, including but not limited to the types of polymers and the types of solvents used in the preparation of the polymer/solvent solution, whether the polymer/solvent solution comprises one type of polymer or more than one type of polymer, the molecular weight of the polymer or polymers, as well as the processing conditions and the size of the pores required, amongst other variables. The apparatus 5 in which the polymer 1 with the cylindrical pores 2 is produced comprises a support platform 7 supported on ground engaging legs δ, and defining a planar horizontal upwardly facing top surface 9. A housing 10 supported on the top surface 9 of the support platform 7 comprises a pair of spaced apart elongated longitudinally extending side walls 12 which are joined by a top wall 16. The side walls 12 and the top wall 16 define with the top surface 9 of the support platform 7 a chamber 17 in the form of an elongated tunnel, within which the film 1 of the pores 2 is produced. The chamber 17 extends from an upstream end 14 to a downstream end 15. The upstream end 14 is closed by an upstream end wall 11 , within which an input port 24 is located for accommodating the nitrogen gas with entrained water vapour therein into the chamber 17 for producing the first and second gas flows to which a film of the polymer/solvent solution is subjected during forming of the pores 2, as will be described below. A substrate 18, which in this embodiment of the invention comprises a borosilicate glass microscope cover slip, is located in the chamber 17, and is supported on the top surface 9 of the support platform 7 towards the downstream end 15 of the chamber 17. The substrate 18 defines a smooth planar horizontal upwardly facing top surface 20 for supporting a film 22 of the polymer/solvent solution. A dosing port 28 formed in the top wall 16 of the housing 10 at a location above the substrate 18 is provided for drop-casting a predefined volume of the polymer/solvent solution onto the top surface 20 of the substrate 18 using a suitable micro-pipette.

Nitrogen gas to produce the first and second nitrogen gas flows is derived from a nitrogen gas supply 30, and is fed through a first flow control valve 31 to a supply pipe 32, and in turn to a first Y-piece connector 33. A first pipeline 34 is coupled to the supply pipe 32 by the first Y-piece connector 33, and feeds into the inlet port 24 of the housing 10 through a second Y-piece connector 39 for delivering nitrogen gas from the supply pipe 32 into the chamber 17. A second pipeline 35, which is also coupled to the supply pipe 32 through the Y-piece connector 33 delivers nitrogen gas from the supply pipe 32 into a Dreschel flask 37 wherein water vapour is entrained in the nitrogen gas supplied from the supply pipe 32. A third pipeline 38 couples the Dreschel flask 37 with the first pipeline 34 through the second Y-piece connector 39 for delivering nitrogen gas with water vapour entrained therein from the Dreschel flask 37 for varying the relative humidity of the nitrogen gas being delivered through the first pipeline 34 into the chamber 17. The Dreschel flask 37 is supported on an electrically powered heater 40 which heats water in the Dreschel flask 37, and maintains the water therein at a suitable temperature to produce a sufficient level of water vapour entrained in the nitrogen gas from the second pipeline 35, so that the relative humidity of the nitrogen gas from the Dreschel flask 21 through the third pipeline 38 is approximately 100%. In this embodiment of the invention the water in the Dreschel flask 37 is de-ionised and filtered.

First and second manually operated isolating valves 42 and 43, respectively, are located in the respective first and second pipelines 34 and 35 for selectively isolating the first and second pipelines 34 and 35 from the supply pipe 32, and also for varying the flow rate of the nitrogen gas through the first and second pipelines 34 and 35.

A second flow control valve 45 located in the first pipeline 34 between the first Y-piece connector 33 and the second Y-piece connector 39 is provided for varying and controlling the flow of nitrogen gas through the first pipeline 34 between the first and second Y-piece connectors 33 and 39 for in turn varying the relative humidity of the nitrogen gas flowing through the first pipeline 34 between the second Y-piece connector 39 and the inlet port 24 for in turn varying the relative humidity of the nitrogen gas flowing through the chamber 17. The first flow control valve 31 facilitates varying the flow rate of nitrogen gas flowing through the chamber 17 for in turn varying the rate of evaporation of the solvent from the polymer/solvent solution film 22. The second flow control valve 45 facilitates varying the ratio of nitrogen gas flowing directly from the supply pipe 32 to the chamber 17 to nitrogen gas flowing through the Dreschel flask 37 to the chamber 17, for in turn varying the relative humidity of the nitrogen gas flowing through the chamber 17, for varying the rate of growth of the water droplets on the polymer/solvent solution film 22 on the substrate 18.

A first flow rate monitor 46 is located on the first pipeline 34 between the first Y -piece connector 33 and the second Y-piece connector 39 for monitoring the flow rate of nitrogen gas through the first pipeline 34 between the first and second Y-piece connector 33 and 39. A second flow rate monitor 47 is located in the third pipeline 38 for monitoring the flow rate of nitrogen gas with entrained water vapour flowing through the third pipeline 38. A third flow rate monitor 48 is located in the first pipeline 34 between the second Y-piece Connector 39 and the inlet port 24 of the housing 10 for monitoring the flow rate of nitrogen gas with entrained water vapour being delivered into the chamber 17. Flow rates of nitrogen gas read by the first, second and third flow rate monitors 46, 47 and 48, respectively, are displayed on respective flow rate indicators 52 of the first, second and third flow rate monitors 46 to 48.

A probe 50 of a hygrometer 51 which also includes a temperature sensor (not shown) extends from the hygrometer 51 through the support platform 7 and into the chamber 17 and terminates at a location adjacent the substrate 18 for monitoring the relative humidity and temperature of the nitrogen gas with the water vapour entrained therein as it passes the substrate 18 supporting the film 22. The relative humidity and temperature of the nitrogen gas flowing through the chamber 17 is displayed on a display screen 53 of the hygrometer 51.

The first and second flow control valves 31 and 45 in this embodiment of the invention are manually operated for controlling the flow rate and the relative humidity of the nitrogen gas flowing through the chamber 17 by an operator in response to the values of the flow rates, the relative humidity and the temperature displayed by the first, second and third flow rate monitors 46 to 48, and the hygrometer 51 , respectively. However, it will be readily apparent to those skilled in the art that a microprocessor or a programmable logic controller may be provided coupled to the hygrometer 51 and the first, second and third flow rate monitors 46 to 48 which would produce appropriate signals to be read by the microprocessor or the programmable logic controller. The microprocessor or programmable logic controller would then control the operation of the first and second flow control valves 31 and 45 via suitable servomotors in response to the signals from the first, second and third flow rate monitors 46 to 48 and the hygrometer 51 for in turn controlling the flow rate and the relative humidity of the nitrogen gas flowing through the chamber 17.

With the apparatus 5 set up as described, and with the substrate 18 placed on the support platform 7 and the housing 10 also placed on the support platform 7 to form the chamber 17 with the support platform 7, so that the substrate 18 is located within the chamber 17 towards the downstream end 15 thereof and beneath the dosing port 28, the apparatus 5 is ready for use. The first flow control valve 31 is set to produce the first gas flow through the chamber 17 at a flow rate of approximately 5 litres of nitrogen gas and entrained water vapour per minute. The second flow control valve 45 in conjunction with the second isolating valve 43 are set in order to produce a relatively dry first gas flow of nitrogen of relative humidity, which is maintained between 2% and 5%, through the chamber 17.

Once the first gas flow of relative humidity in the range of 2% to 5% and at a flow rate of 5 litres per minute is established in the chamber 17, an appropriate volume, which in this embodiment of the invention is 60 micro-litres of the polymer/solvent solution is drop-cast through the dosing port 28 in the housing 10 onto the top surface 20 of the substrate 18. Needless to say, the volume of polymer/solvent solution which is drop-cast onto the substrate 18 may be of any suitable volume, and will be dependent on the desired product output. Drop-casting of the polymer/solvent solution is carried out using a suitable micro-pipette (not shown). The depth t to which the film 22 of polymer/solvent solution settles on the top surface 20 of the substrate 18 depends on the polymer and solvent of the polymer/solvent solution and the proportions thereof in the solution, as well as other parameters such as surface tension, the smoothness of the top surface 20 of the substrate 18, the temperature of the polymer/solvent solution and the temperature and pressure within the chamber 17. In this embodiment of the invention the method is carried out at room temperature and pressure, and on being drop-cast onto the top surface 20 of the substrate 18, the polymer/solvent solution is at room temperature. The temperature and pressure within the chamber 17 during the carrying out of the method is substantially room temperature and pressure. Under these conditions the depth t to which the film 22 of polymer/solvent solution settles on the top surface 20 of the substrate 18 in a central area of the film 22 where the depth t is substantially constant and is in the order of 550 microns.

The polymer/solvent solution film 22 defines an upwardly facing exposed gas/film interface surface 55 over which the first gas flow passes. The gas/film interface surface 55 corresponds with the surface 3 of the film 1. Taking account of the transverse cross-sectional area of the chamber 17, the flow rate of the first gas flow of 5 litres per minute through the chamber 17 equates to a velocity of the first gas flow of nitrogen gas and entrained water vapour over the gas/film interface surface 55 of approximately 0.29 metres per second. The polymer/solvent film solution film 22 is subjected to the first gas flow for a first time period of approximately 45 seconds in order to produce a polymer concentration gradient extending from the gas/film interface surface 55 into the film 22 with the polymer concentration decreasing from the gas/film interface surface 55 in a direction into the film 25 as will be described in more detail below.

Once the polymer/solvent solution film 22 has been subjected to the first gas flow for the first time period, the relative humidity of the gas flow is immediately increased to a value of approximately 85% by appropriately setting the second flow control valve 45. However, the manually operated isolating valves 42 and 43 may also be used for controlling the relative humidity of the first and second gas flows. The first flow control valve 31 is operated to reduce the flow rate of the second gas flow to a steady state flow rate of approximately 1 litre per minute. The flow rate of 1 litre per minute equates to a velocity of the nitrogen gas and entrained water vapour over the gas/film interface surface 55 of the polymer/solvent solution film 22 of approximately 0.06 metres per second. The second gas flow of relative humidity of 85% and at a flow rate of 1 litre per minute is maintained in a steady state until all the solvent has been evaporated from the polymer/solvent solution film 22.

During the periods while the polymer/solvent solution film 22 is being subjected to the first and second gas flows, it is important that the conditions are maintained constant over the gas/film interface surface 55 of the polymer/solvent solution film 22. The relative humidities of the first and second gas flows through the chamber 17 are monitored by the hygrometer 51 and controlled by the second flow control valve 45. However, the manually operated isolating valves 42 and 43 may also be used for controlling the relative humidity of the first and second gas valves. The flow rates of the first and second gas flows through the chamber 17 are monitored by the third flow rate monitor 48 and controlled by the first flow control valve 31.

The high flow rate and low relative humidity of the first gas flow over the gas/film interface surface 55 of the polymer/solvent solution film 22 results in an initial relatively rapid evaporation of the solvent from the polymer/solvent solution film 22 at and below the gas/film interface surface 55. This produces the polymer concentration gradient which extends into the polymer/solvent solution film 22 from the gas/film interface surface 55, with the polymer concentration highest at the gas/film interface surface 55, and decreasing into the polymer/solvent solution film 22.

The flow rate of the first gas flow through the chamber 17 is set, and the relative humidity of the first gas flow is also set so that the at the end of the first time period the polymer concentration gradient is established adjacent to the gas/film interface surface 55 and extends into the polymer/solvent solution film 22. At the end of the first time period the polymer concentration gradient should extend into the polymer/solvent solution film 22 a sufficient distance to stabilise the water droplets as they commence to form on the gas/film interface surface 55 and as they commence to sink into the polymer/solvent solution film 22 through the gas/film interface surface 55. Thus, the length of the first time period is dependent on the rate of evaporation of the solvent from the polymer/solvent solution film 22, which in turn is dependent on the flow rate of the first gas flow. Additionally, depending on the relative humidity of the first gas flow, water droplets may commence to condense on the gas/film interface surface 55 of the polymer/solvent solution film 22 before the end of the first time period. It has been found that in general where the relative humidity of the first gas flow is below 35% in general, water droplets do not form on the gas/film interface surface 55 during the first time period, but when the relative humidity of the first gas flow exceeds 35% water droplets may commence to form on the gas/film interface surface 55 before the end of the first time period. The greater the relative humidity of the first gas flow, in general the earlier will water droplets commence to form on the gas/film interface surface 55 during the first time period.

However, it is important that during the first gas flow the polymer concentration and the viscosity of the polymer/solvent solution film 22 adjacent the gas/film interface surface 55 should not be allowed to increase to such an extent that droplet growth into the film solution 22 and ordering of the droplets at the gas/film interface surface 55 would be inhibited during the second gas flow. It has been found that by maintaining the relative humidity of the first gas flow at approximately 5% relative humidity and the flow rate to produce a velocity of the nitrogen gas with the entrained water vapour over the gas/film interface surface 55 of approximately 0.29 metres per second for the first time period of 45 seconds is sufficient to prevent excessive polymer concentration and excessive viscosity of the polymer/solvent solution film 22 adjacent the gas/film interface surface 55 during the first gas flow, while at the same time producing a suitable polymer concentration gradient. However, it will be appreciated that continuous relatively minor adjustments of both the relative humidity and the flow rate of the first gas flow may be required during the period while the polymer/solvent solution film 22 is being subjected to the first gas flow in order to produce a suitable polymer concentration gradient.

Additionally, the relatively rapid evaporation of the solvent from the film solution 22 results in a rapid fall in the temperature of the polymer/solvent solution film 22 at the gas/film interface surface 55 due to the latent heat of evaporation of the solvent, which in turn facilitates condensing of the water vapour entrained in the nitrogen gas to form the water droplets on the gas/film interface surface 55 during the second gas flow, and also during the first gas flow, if the relative humidity of the first gas flow is sufficiently high to result in the formation of water droplets.

During the second period while the polymer/solvent solution film 22 is subjected to the second gas flow, due to the high relative humidity of the second gas flow the water droplets commence to grow into the polymer/solvent solution film 22. Additionally, during the second gas flow the rate at which the polymer concentration gradient progresses downwardly into and through the polymer/solvent solution film 22 in a direction from the gas/film interface surface 55 is controlled, so that the rate of progress of the polymer concentration gradient downwardly into the polymer/solvent solution film 22 in a direction from the gas/film interface surface 55 is substantially equal to the downward rate of growth of the water droplets into the polymer/solvent solution film 22. By so controlling the downward rate of progress of the polymer concentration gradient into the polymer/solvent solution film 22 to be equal to the downward growth rate of the water droplets into the polymer/solvent solution film 22 results in the elongation of the water droplets in a direction downwardly into the polymer/solvent solution film 22 from the gas/film interface surface 55 to define the cylindrical pores 2 of substantially constant cross-section.

It is believed that by matching the downward rate of progress of the polymer concentration gradient with the downward rate of growth of the water droplets into the polymer/solvent solution film 22, the water droplets are further stabilised and confined into a cylindrical configuration. It has been found that by maintaining the second gas flow at a relative humidity in the order of 85% and a flow rate of approximately 1 litre per minute, the downward rate of progress of the polymer concentration gradient into the polymer/solvent solution film 22 is matched with the downward growth rate of the water droplets therein sufficiently to produce the cylindrical pores 2. However, it will be appreciated that both the relative humidity and the flow rate of the second gas flow may require continuous relatively minor adjustment during the second gas flow in order to maintain the downward rate of progress of the polymer concentration gradient into the polymer/solvent solution film 22 matched with the downward rate of growth of the water droplets.

Once all the solvent has been evaporated from the polymer/solvent solution film 22, the second gas flow is terminated, and the water droplets are allowed to evaporate from the film 22 to form the film 1 with the cylindrical pores 2.

In this particular case the photomicrographs of Figs. 1 and 2 were produced by cross-sectioning the pores 2 in a portion of the thin film 1 and sputtering the film 1 and the sectioned pores 2 with an electrically conductive metal to facilitate imaging. The sputtered film 1 was placed in a scanning electron microscope and photographed therein.

In this embodiment of the invention the pores 2 as discussed above are arranged in a hexagonally close-packed array with centre-to-centre spacings in the range of 4 microns to 5.5 microns. The pores 2 are of length in the range 9 microns to 10 microns and of diameter in the range of 3.5 microns to 5.5 microns, reducing at the neck of each pore 2 to a diameter in the range of 2.5 microns to 3.3 microns. The example of the method according to the invention just described was repeated with a polymer/solvent solution film similar to that just described, and with a similar polymer concentration. The process parameters were identical to those just described, with the exception that the relative humidity of the second gas flow was reduced to 75%. All other processing conditions were identical to those just described. Cylindrical pores were formed in the polymer film, and in this case, the pores were of axial length in the range of 5.7 microns to 7 microns, and were of diameter in the range of 2.4 microns to 4.5 microns, with a neck diameter in the range of 1.8 microns to 3 microns. The pores were arranged in a hexagonally close-packed array with centre- to-centre spacings in the range of 3 microns to 4.3 microns.

An experiment was also carried out to ascertain if the orientation of the gas/film interface of the polymer/solvent solution film was important. A film of polymer/solvent solution was drop-cast onto a substrate, which was then inverted so that the exposed gas/film interface surface was facing downwardly in the chamber. The polymer/solvent solution film was subjected sequentially to a first gas flow of nitrogen gas of low relative humidity, and a second gas flow of nitrogen gas of high relative humidity. Cylindrical pores, with each pore being of substantially constant transverse cross-section were formed in the polymer film.

Accordingly, it is believed that a polymer film with cylindrical pores may be formed on either an upwardly facing or a downwardly facing surface of a substrate on which a polymer/solvent solution film is deposited. It is also envisaged that a polymer film with cylindrical pores may be formed on other surfaces besides horizontal surfaces. For example, it is envisaged that the cylindrical pores may be formed in a polymer/solvent solution film deposited on a vertical surface or on a surface inclined to the horizontal at any angle greater than 0° and less than 90°.

In practice, the relative humidities and the flow rates of the first and second gas flows are tuned and adjusted during the periods of the first and second gas flows, so that during the first gas flow a suitable polymer concentration gradient is developed in the polymer/solvent solution film extending into the film from the gas/film interface surface thereof, and during the second gas flow the rate of progress of the polymer concentration gradient into the polymer/solvent solution film is controlled to be substantially similar to the rate of growth of the water droplets into the film in the direction from the gas/film interface surface.

While specific values of relative humidity and flow rates have been described for the first and second gas flows, and a specific first time period has also been described for the first gas flow, it is believed that the relative humidities and the flow rates of the first and second gas flows, as well as the first time period will have to be altered for films of different polymer/solvent solutions. For example, varying the proportion of polymer to solvent in the solution from that which has been described may be sufficient to require altering the flow rates and relative humidities, and quite likely, the first time period in order to produce a suitable polymer concentration gradient extending into the polymer/solvent film solution from the gas interface thereof during the first gas flow, and also, to maintain the rate of progress of the polymer concentration gradient into the polymer/solvent solution film during the second gas flow.

Additionally, varying the type of polymer and/or the solvent of the polymer/solvent solution from which the film thereof is formed will also require varying the flow rates and the relative humidity of the first and second gas flows, and also, most likely the duration of the first time period. These values of relative humidity, flow rate and first time period duration will typically be determined by trial and error in order that the first flow rate produces a polymer concentration gradient during the first time period which extends into the polymer/solvent solution film from the gas/film interface surface thereof with the polymer concentration decreasing from the gas/film interface surface. At the end of the first time period the concentration gradient of the polymer adjacent to the gas/film interface should be such to stabilise the water droplets as they form either at the end of the first time period or on commencement of subjecting the polymer/solvent solution film to the second gas flow, and also to stabilise the water droplets as they commence to grow into the polymer/solvent solution film through the gas/film interface surface. However, the polymer concentration adjacent the gas/film interface surface should not be of viscosity which would hinder growth of the water droplets into the polymer/solvent solution film during the second gas flow, or indeed during the first gas flow if towards the end of the first gas flow the relative humidity of the first gas flow was such as to facilitate the formation of water droplets on the gas/film interface surface, and the growth thereof through the gas/film interface surface into the polymer/solvent solution film. In particular, irrespective of the type of polymer/solvent solution, the rate of progress of the polymer concentration gradient into the polymer/solvent solution film from the gas/film interface thereof should be matched to the growth rate of the water droplets into the polymer/solvent solution film.

While the method according to the invention has been described as comprising subjecting the polymer/solvent film to first and second gas flows whereby the second gas flow commences immediately on termination of the first gas flow, and the relative humidity of the nitrogen gas is step changed upwardly from the value of the relative humidity for the first gas flow to the value of the relative humidity of the second gas flow, and the flow rate is step changed downwardly from the flow rate of the first gas flow to the flow rate of the second gas flow, in certain cases, it is envisaged that the value of the relative humidity may be transitioned gradually from the relative humidity value for the first gas flow to the relative humidity value for the second gas flow, and similarly, the flow rate would likewise be transitioned gradually from the flow rate of the first gas flow to the appropriate flow rate for the second gas flow. Indeed, in certain cases, it is envisaged that during the period during which the polymer/solvent film is being subjected to the first gas flow, the flow rate of the first gas flow may be ramped downwardly from a maximum flow rate to a minimum flow rate over the duration of the first time period. The maximum flow rate would be similar to the desired flow rate of the first gas flow, and the minimum flow rate would be similar to the desired flow rate of the second gas flow. Additionally, or alternatively, during the first time period, the relative humidity of the first gas flow may be increased from a minimum relative humidity, which may be 0%, to a maximum relative humidity value, which may be the relative humidity of the second gas flow. It is also envisaged that the relative humidity may be continuously ramped upwardly from the beginning of the period during which the polymer/solvent film is subjected to the first gas flow to the end of the second period during which the polymer/solvent solution is subjected to the second gas flow. Similarly, it is envisaged that the flow rate may be ramped downwardly from the beginning of the first time period during which the polymer/solvent film is being subjected to the first gas flow to the end of the second period during which the polymer/solvent film is being subjected to the second gas flow. However, it is important that the flow rate and the relative humidity of the first and second gas flows over the polymer/solvent film should be controlled so that the polymer concentration gradient is quickly established in the polymer/solvent film adjacent the gas/film interface surface thereof, and thereafter the rate of progress of the polymer concentration gradient into the polymer/solvent film is proportional, and preferably similar, to the rate of growth of the water droplets into the polymer/solvent film.

Additionally, while the difference in the flow rates between the first and second gas flows has been described as being 4 litres per minute, namely, the difference between 5 litres per minute and 1 litre per minute, the difference between the flow rates of the first and second gas flows may be greater or less than the 4 litres per minute described, and in some cases it is envisaged that it may be advantageous to have a greater difference between the flow rates of the first and second gas flows, and this most likely would be dependent on the polymer and solvent materials and the concentration of the polymer initially in the polymer/solvent solution.

It is also envisaged that in certain cases, the polymer/solvent film may be subjected initially to nitrogen gas directly from the nitrogen supply to further accelerate evaporation of the solvent from the polymer/solvent film, and after a short time period, typically not more than a few seconds, the nitrogen gas in the first pipeline 34 would be mixed with nitrogen gas with the entrained water vapour from the third pipeline 38 by appropriately operating the second flow control valve 45, and the relative humidity of the nitrogen gas to which the film solution would be subjected would be gradually increased.

While specific values for relative humidity of the first and second gas flows have been described, it is envisaged that the first and second gas flows may be of other suitable relative humidities, however, it is envisaged that the relative humidity of the second gas flow will normally be higher than the relative humidity of the first gas flow. Similarly, it is envisaged that flow rates other than those described for the first and second gas flows may be used, and in general the flow rate of the second gas flow will be less than the flow rate of the first gas flow. The flow rates and relative humidities of the first and second gas flows will in general, be dependent on the area of the gas/film interface surface of the film solution, the thickness t of the film solution, as well as on the polymer and solvent of the polymer/solvent solution and the proportions of the polymer and solvent in the polymer/solvent solution. These values typically will be determined by trial and error. It is also envisaged that by reducing the relative humidity of the first gas flow, a higher relative humidity than that which would normally be used in the second gas flow can be tolerated in the second gas flow, and vice versa.

Further, it is believed that the relative humidity of the first gas flow may be as low as 0% to 5% during the entire duration of the first gas flow. However, the relative humidity of the first gas flow could be considerably higher than 5%. It is has been found that cylindrical pores can be formed in a polymer/solvent film similar to that described even when the relative humidity of the first gas flow is up to 80%.

Additionally, while the duration of the first time period of the first gas flow has been described as being approximately 45 seconds, it is envisaged that the film solution may be subjected to the first gas flow for periods greater than or less than 45 seconds.

Needless to say, while the polymer/solvent solution has been described as being drop-cast onto the substrate, any other suitable means of forming a film of the polymer/solvent solution may be used, for example, spin casting.

It will also be appreciated that while it is desirable, it is not essential that the substrate should have a flat surface on which the polymer/solvent film is formed, and indeed, in certain cases, the surface defined by the substrate may be uneven or a dished surface, or of any desired configuration in order to produce a thin film of a corresponding configuration with cylindrical pores.

While a specific polymer has been described, it will be readily apparent to those skilled in the art that any other polymer may be used, and additionally, while the solvent has been described as being chloroform, any other suitable solvent may be used, however, the solvent should be a volatile solvent. Indeed, it is also envisaged that the solvent may comprise a mixture of a number of different solvents, and the polymer may comprise a mixture of a number of different polymers.

In general, it has been found that the cylindrical pores are formed in hexagonally close-packed arrays when viewed in plan view looking down on the surface 3 of the polymer film 1. It is believed that this results from the fact that the water droplets as they condense on the gas/film interface surface of the polymer/solvent film are configured into a plurality of hexagonally close-packed arrangements in order to minimise the energies of the respective water droplets.

While the polymer films produced by the method according to the invention which has been described have been described as comprising cylindrical pores of specific dimensions, it is envisaged that by appropriately varying one or both of the relative humidities and the flow rates of the first and/or second gas flows, polymer films with cylindrical pores of other dimensions could be produced, and it is envisaged that by further trial and error, pores with greater or lesser aspect ratios, in other words, a greater ratio of the axial length to the diameter thereof, could be produced.

While a particular type of apparatus has been described for carrying out the method according to the invention for producing the polymer films with cylindrical pores, it will be readily apparent that the apparatus described is essentially experimental apparatus, and commercial scale production apparatus may be quite different to that described. However, the operating principle of such commercial scale production apparatus would be such as to carry out the method according to the invention.

While the gas of the first and second gas flows has been described as being nitrogen gas, any other suitable gas may be used, and it is envisaged that in many cases the gas may be air. It is also envisaged that vapours of liquids other than water may be entrained in the gas of the first and second gas flows for forming liquid droplets in the film solution to define the cylindrical pores.

It will also be appreciated that where polymers and solvents other than those described are used, the concentration of polymer in the polymer/solvent solution will be such as to suit the polymer/solvent solution, and additionally, other relative humidities and flow rates of the first and second gas flows than those described may be required.

While the method has been described as comprising subjecting the polymer/solvent solution film to a first gas flow for initially accelerating the solvent evaporation from the polymer/ solvent film through the gas/film interface surface thereof to produce the polymer concentration gradient in the polymer/solvent film with the polymer concentration decreasing from the gas/film interface surface into the Polymer/Solvent film, any other suitable means for initially accelerating solvent evaporation to produce such a polymer concentration at the gas/film interface surface may be used. Additionally, it is envisaged that any other means for condensing water vapour or other vapour to form liquid droplets onto the gas/film interface surface besides subjecting the polymer/solvent film to a second gas flow may be used, and furthermore, it is envisaged that other means besides subjecting the polymer/solvent film to a second gas flow could be used in order to cause the polymer concentration in the polymer concentration gradient to decrease from the gas/film interface surface into the polymer/solvent film at a rate matched to the growth rate of the liquid droplets into the film.

While the polymer/solvent film has been described as being drop-cast to a specific depth, it is envisaged that the polymer/solvent film may be formed on the substrate to any suitable depth, and the depth to which the polymer/solvent film will be formed will be dependent on the method for depositing the polymer/solvent solution onto the substrate, the surface of the substrate, its smoothness and surface tension, the temperature of the substrate and the polymer/solvent solution, the viscosity of the polymer/solvent solution, as well as other variables. For example, if the film were formed by spincasting the polymer/solvent solution onto the substrate, the depth to which the polymer/solvent film would be formed would be considerably less than drop-casting the polymer/solvent solution onto the substrate.

While the flow rate of the second gas flow has been described as being lower than the flow rate of the first gas flow, it is envisaged in certain cases, for example, where a relatively high growth rate of water droplets is required, the flow rate of the second gas flow may be similar to or higher than the flow rate of the first gas flow, in order to ensure the delivery of sufficient moisture to adequately propagate the water droplets.

While the pores which have been formed in the polymer film by the method according to the invention have been of micron size, it is envisaged that pores of sub-micron size may be formed by appropriately altering the process conditions.

Additionally, while the polymer has been described as being an amphiphilic polymer, it is envisaged in certain cases that non-amphiphilic polymers may be used.

Claims

1. A method for producing an array of pores of cylindrical shape in a polymer film, the method comprising forming a film of a low polymer concentration solvent solution on a substrate, initially accelerating solvent evaporation from the film solution through a gas/film interface surface thereof to produce a polymer concentration gradient in the film solution with the polymer concentration decreasing from the gas/film interface surface into the film solution, so that the polymer concentration gradient is suitable for facilitating elongated growth of liquid droplets sinking into the film solution from the gas/film interface surface thereof, and condensing a liquid to form liquid droplets on the gas/film interface surface of the film solution to sink into the film solution to define the cylindrical pores therein.

2. A method as claimed in Claim 1 in which the elongated growth of the droplets sinking into the film solution is in a direction substantially perpendicular to the gas/film interface surface.

3. A method as claimed in Claim 1 or 2 in which the polymer concentration in the polymer concentration gradient decreases from the gas/film interface surface into the film solution at a rate suitable for elongated growth of the liquid droplets into the film solution.

4. A method as claimed in any preceding claim in which the rate of solvent evaporation from the film solution is controlled so that the rate of progress of the polymer concentration gradient into the film solution from the gas/film interface surface thereof is proportional to the rate of elongated growth of the liquid droplets therein.

5. A method as claimed in any preceding claim in which the rate of solvent evaporation from the film solution is controlled so that the rate of progress of the polymer concentration gradient into the film solution from the gas/film interface thereof is substantially similar to the rate of elongated growth of the liquid droplets therein.

6. A method as claimed in any preceding claim in which a first gas flow is passed over the gas/film interface surface of the film solution for initially accelerating the solvent evaporation therefrom through the gas/film interface surface.

S 7. A method as claimed in Claim 6 in which the first gas flow is passed over the gas/film interface surface of the film solution for establishing the polymer concentration gradient.

8. A method as claimed in Claim 6 to 7 in which a second gas flow having0 vapour of the droplet forming liquid entrained therein is passed over the gas/film interface surface of the film solution to form the liquid droplets on the gas/film interface surface of the film solution after the first gas flow has been passed over the film solution. 5

9. A method as claimed in Claim 8 in which the second gas flow is passed over the gas/film interface surface of the film solution for controlling the rate of progress of the polymer concentration gradient into the film solution from the gas/film interface surface thereof. 0

10. A method as claimed in Claim 8 or 9 in which the vapour of the droplet forming liquid is water vapour, and the second gas flow is a humid gas flow.

11. A method as claimed in Claim 10 in which the relative humidity of the second gas flow is at least 35%. 5

12. A method as claimed in Claim 10 or 11 in which the relative humidity of the second gas flow is at least 50%.

13. A method as claimed in any of Claims 10 to 12 in which the relative humidity0 of the second gas flow lies in the range 60% to 100%.

14. A method as claimed in any of Claims 10 to 13 in which the relative humidity of the second gas flow is in the range of 75% to 85%.

15. A method as claimed in any of Claims 10 to 14 in which the relative humidity of the second gas flow is tuned to control the elongated growth of the droplets in the film solution.

16. A method as claimed in any of Claims 10 to 15 in which the relative humidity of the second gas flow is adjusted during the second gas flow to control the elongated growth of the droplets in the film solution.

17. A method as claimed in any of Claims 10 to 16 in which the flow rate of the second gas flow is less than the flow rate of the first gas flow.

18. A method as claimed in any of Claims 10 to 17 in which the flow rate of the second gas flow lies in the range of just above zero litres per minute to 7 litres per minute.

19. A method as claimed in any of Claims 10 to 18 in which the flow rate of the second gas flow lies in the range of 0.5 litres per minute to 5 litres per minute.

20. A method as claimed in any of Claims 10 to 19 in which the flow rate of the second gas flow is approximately 1 litre per minute.

21. A method as claimed in any of Claims 10 to 20 in which the flow rate of the second gas flow over the gas/film interface surface of the film solution is less than the flow rate of the first gas flow over the gas/film interface surface of the film solution.

22. A method as claimed in any of Claims 10 to 21 in which the flow rate of the gas of the second gas flow over the gas/film interface surface of the film solution lies in the range of just above zero metres per second to 0.4 metres per second.

23. A method as claimed in any of Claims 10 to 22 in which the flow rate of the gas of the second gas flow over the gas/film interface surface of the film solution lies in the range of 0.03 metres per second to 0.29 metres per second.

24. A method as claimed in any of Claims 10 to 23 in which the flow rate of the gas of the second gas flow over the gas/film interface surface of the film solution is approximately 0.06 metres per second.

25. A method as claimed in any of Claims 10 to 24 in which the flow rate of the second gas flow is maintained at a substantially steady state flow rate.

26. A method as claimed in any of Claims 10 to 25 in which the flow rate of the second gas flow is tuned to control the elongated growth of the droplets in the film solution.

27. A method as claimed in any of Claims 10 to 26 in which the flow rate of the second gas flow is adjusted during the second gas flow to control the elongated growth of the droplets.

28. A method as claimed in any of Claims 10 to 27 in which the first gas flow comprises a vapour of the droplet forming liquid entrained therein.

29. A method as claimed in Claim 28 in which the vapour entrained in the first gas flow is water vapour and the first gas flow is a humid gas flow, and the relative humidity of the first gas flow is less than the relative humidity of the second gas flow.

30. A method as claimed in Claim 29 in which the relative humidity of the first gas flow is less than 85%.

31. A method as claimed in Claim 29 or 30 in which the relative humidity of the first gas flow lies in the range of 0% to 45%.

32. A method as claimed in any of Claims 29 to 31 in which the relative humidity of the first gas flow is approximately 5%.

33. A method as claimed in any of Claims 22 to 32 in which the relative humidity of the first gas flow is tuned to control the establishment of the polymer concentration gradient in the film solution.

34. A method as claimed in any of Claims 29 to 33 in which the relative humidity of the first gas flow is adjusted during the first gas flow to control the establishment of the polymer concentration gradient in the film solution.

35. A method as claimed in any of Claims 8 to 34 in which the flow rate of the first gas flow lies in the range of 0.5 litres per minute to 20 litres per minute.

36. A method as claimed in any of Claims 8 to 35 in which the flow rate of the first gas flow lies in the range of 3 litres per minute to 10 litres per minute.

37. A method as claimed in any of Claims 8 to 36 in which the flow rate of the first gas flow is approximately 5 litres per minute

38. A method as claimed in any of Claims 8 to 37 in which the flow rate of the gas of the first gas flow over the gas/film interface surface of the film solution is greater than the flow rate of the gas of the second gas flow thereover.

39. A method as claimed in any of Claims 8 to 38 in which the flow rate of the gas of the first gas flow over the gas/film interface surface of the film solution lies in the range of 0.03 metres per second to 1.14 metres per second.

40. A method as claimed in any of Claims 8 to 39 in which the flow rate of the gas of the first gas flow over the gas/film interface surface of the film solution lies in the range of 0.17 metres per second to 0.57 metres per second.

41. A method as claimed in any of Claims 8 to 40 in which the flow rate of the gas of the first gas flow over the gas/film interface surface of the film solution is approximately 0.29 metres per second.

42. A method as claimed in any of Claims 8 to 41 in which the flow rate of the first gas flow is tuned to establish the polymer concentration gradient in the film solution.

43. A method as claimed in any of Claims 8 to 42 in which the flow rate of the first gas flow is adjusted during the first gas flow to establish the polymer concentration gradient in the film solution.

44. A method as claimed in any of Claims 8 to 43 in which the gas of the first and second gas flows is an inert gas.

45. A method as claimed in any of Claims 8 to 44 in which the gas of the first and second gas flows is nitrogen.

46. A method as claimed in any preceding claim in which the film solution is formed on a substantially horizontal surface of the substrate.

47. A method as claimed in any of Claims 1 to 45 in which the film solution is formed on a surface of the substrate which is inclined to the horizontal at an angle greater than zero degrees and less than 90°.

48. A method as claimed in Claim 46 or 47 in which the surface of the substrate on which the film solution is formed is a generally upwardly facing surface thereof.

49. A method as claimed in Claim 46 or 47 in which the surface of the substrate on which the film solution is formed is a generally downwardly facing surface thereof.

50. A method as claimed in any of Claims 1 to 45 in which the film solution is formed on a substantially vertical surface of the substrate.

51. A method as claimed in any of Claims 46 to 50 in which the surface of the substrate on which the film solution is formed is a planar surface.

52. A method as claimed in any preceding claim in which the film solution is formed on the substrate by drop-casting the polymer/solvent solution onto the substrate.

53. A method as claimed in any preceding claim in which the polymer is an amphiphilic polymer.

54. A method as claimed in any preceding claim in which the polymer is ω-dicarboxy-terminated polystyrene.

55. A method as claimed in any preceding claim in which the polymer concentration in the polymer/solvent solution prior to forming the film solution on the substrate does not exceed 8%.

56. A method as claimed in any preceding claim in which the polymer concentration in the polymer/solvent solution prior to forming the film solution on the substrate lies in the range of 2% to 5%.

57. A method as claimed in any preceding claim in which the polymer concentration in the polymer/solvent solution prior to forming the film solution on the substrate lies in the range of 3% to 5%.

58. A method as claimed in any preceding claim in which the polymer concentration in the polymer/solvent solution prior to forming the film solution on the substrate is approximately 3.5%.

59. A method as claimed in any preceding claim in which the solvent is a volatile solvent.

60. A method as claimed in any preceding claim in which the solvent is volatile at temperature and pressure conditions of the first and second gas flows.

61. A method as claimed in any preceding claim in which the latent heat of vaporisation of the solvent is such as to reduce the temperature of the gas/film interface surface to a temperature sufficient to induce condensation of the vapour entrained in at least the second gas flow.

62. A method as claimed in any preceding claim in which the solvent is chloroform.

63. A method as claimed in any preceding claim in which the liquid droplets are evaporated from the film solution to form the cylindrical pores.

64. A method as claimed in any preceding claim in which the cylindrical pores are of micron size.

65. A method as claimed in any preceding claim in which the cylindrical pores are of sub-micron size.

66. A polymer film comprising an array of cylindrical pores formed therein using the method as claimed in any preceding claim.

67. A polymer film as claimed in Claim 66 in which the cylindrical pores are of micron size.

68. A polymer film as claimed in Claim 66 or 67 in which the cylindrical pores are of sub-micron size.

69. Apparatus for carrying out the method as claimed in any of Claims 1 to 65 for producing an array of pores of cylindrical shape in a polymer film.