Porous synthetic resin film
BACKGROUND OF THE INVENTION 
     1. FIELD OF THE INVENTION 
     The present invention relates to a porous synthetic resin film having many fine communicating holes which is useful as a filter. 
     2. DESCRIPTION OF THE PRIOR ART 
     Heretofore, porous films used for an air permeable diaphragms, filters and the like include paper, glass fiber sheets, synthetic fiber sheets, cellulose derivative filters and the like. It is desired to use membranes having narrow hole size distribution for filters. However, commercial filters have broad hole size distribution. For example, filters having an average hole diameter of 5 μm will pass particles having a diameter of 10 μm. Typical of filter problems is where the communicating holes in the filter have various portions, each having different diameters. Accordingly, a particle may pass into one communicating hole, but may be caught at a part of the hole which has a smaller diameter than the hole opening which causes clogging to occur. The clogging cannot be prevented by stirring the solution on the surface of film and the life of film is remarkably decreased. 
     Accordingly, there exists a need for porous films having uniform, fine cylindrical communicating holes. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a porous synthetic resin film having uniform, fine, cylindrical communicating holes. 
     This and other objects of the present invention have been attained by providing a porous, synthetic resin film formed by removing a fine cylindrical component from a film of the AB-type or the ABA-type block copolymer which has a fine cylindrical A component or B component. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     It is known that when the miscibility between component A and component B is low in the AB-type or ABA-type block copolymer which has narrow distribution of components and molecular weight, in a specific ratio of component A to component B, uniform regular cylindrical components having uniform diameter in the range of 70 A to 3,000 A are formed in the film because of microphase separation. The present inventors have succeeded in preparing a porous, synthetic resin film in which each of the fine communicating holes has uniform diameter throughout by removing this cylindrical component from the film. That is, the present invention provides a porous, synthetic resin film which is formed by removing the fine cylindrical component from a film of AB-type or ABA-type block copolymer which has a fine cylindrical component of component A or component B. 
     The AB-type block copolymers used in the present invention are the block copolymers having one molecular chain connecting polymer chain A and polymer chain B at the terminals thereof. The ABA-type block copolymers used in the present invention are block copolymers having one molecular chain connecting polymer chain A, polymer chain B and polymer chain A in that order. The invention will be further illustrated in detail. 
     One component of the two components of the block copolymer, which forms the continuous phase without forming the fine cylindrical phase, should be the polymer of the porous synthetic film of the invention, and accordingly, the component should have sufficient mechanical strength so as to be used as the film, it is preferable that it have a softening point higher than 60° C. Suitable components include vinyl type polymers such as polyacrylonitrile, polymethacrylonitrile, polymethylmethacrylate, polystyrene, poly-t-butyl crotonate, polybutylisocyanate, polyamylisocyanate, polyhexylisocyanate, polyoctylisocyanate etc.; and polycondesnation-type polymers such as polycarbonates, polyesters etc. Suitable combinations of the component for the continious phase and the component for the fine cylindrical phase are as follows. The following is the list of components. 
     Pst = Polystyrene 
     Pmma = polymethylmetacrylate 
     Ptbcr = Polytert-Butyl crotonate 
     Ptbc = polytert-butyl cinnamate 
     Pman = polymethacrylonitrile 
     Pan = polyacrylonitrile 
     Pbic = poly n-butylisocyanate 
     Paic = poly n-amylisocyanate 
     Pbd = Polybutadiene 
     Pip = Polyisoprene 
     Pcp = Polychloroprene 
     Pmvk = polymethylvinylketone 
     Ppvk = polyphenylvinylketone 
     Phic = poly n-hexylisocyanate 
     The following is a list of the combinations. 
     
         ______________________________________                                    
Continuous                                                                
        Fine cylindrical                                                  
                     Continuous                                           
                               Fine cylindrical                           
phase   phase        phase     phase                                      
______________________________________                                    
PSt     PBd          PMAN      PIp                                        
&#34;       PIp          PMAN      PMVK                                       
&#34;       PCp          PAN       PBd                                        
&#34;       PMVK         PAN       PIp                                        
PMMA    PBd          PAN       PMVK                                       
&#34;       PIp          PAN       PPVK                                       
&#34;       PCp          PBIC      PIp                                        
&#34;       PMVK         PBIC      PBd                                        
PTBCr   PBd          PBIC      PMVK                                       
&#34;       PIp          PAIC      PIp                                        
PTBCr   PMVK         PAIC      PBd                                        
PTBC    PBd          PAIC      PMVK                                       
PTBC    PIp          PHIC      PIp                                        
PTBC    PMVK         PHIC      PBd                                        
PTBC    PPVK         PTBCr     PBIC                                       
PMAN    PBd          PTBCr     PAIC                                       
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     The other component which forms the fine cylindrical phase should be easily removable from the film of the AB-type or ABA-type block copolymer by decomposition etc. The components having the desirable properties for the fine cylindrical component include polybutadiene, polyisoprene and polychloroprene which are easily oxidized by ozone. The oxidized polymers can be easily decomposed by treatment with an acid or an alkali. The photodisintegratable polymethylvinyl ketone, polyphenylvinyl ketone etc. can be easily decomposed by irradiating with ultraviolet light, so as to remove it from the film. Accordingly, these are also suitable as the fine cylindrical component of said block copolymer. Polyesters, polycarbonates and polyamides which can be easily hydrolyzed with an acid or an alkali can also be used as the fine cylindrical component. 
     When the bond between component A and component B is an ester bond or an amide bond which is easily hydrolyzed, the fine cylindrical component can be dissolved by a solvent after decomposing the bonds with an acid or an alkali to obtain the porous synthetic resin film. The thickness of the film is preferably less than 20 μm. When it is more than 20 μm, the decomposition and separation of the fine cylindrical component are not smoothly conducted. The minimum thickness is about 0.5 μm. When the thickness is less than 0.5 μm, the handling of the film is quite difficult. Control of the diameter of the fine cylindrical component, that is the control of the diameter of the fine cylindrical holes, can be attained by controlling the molecular weight of the block copolymer. The diameter of the cylindrical holes is typically in the range of from 70 A to 3000 A. The diameter of the fine cylindrical holes can also be varied depending upon the types of components and the types of solvents used for preparing the block copolymer film. 
     Having generally described the invention, a more complete understanding can be obtained by reference to certain specific examples, which are provided herein for purposes of illustration only and are not intended to be limiting unless otherwise specified. 
    
    
     EXAMPLE 1 
     A film of AB-type block copolymer of t-butyl crotonate and butadiene having a thickness of 4 μm (the fine cylindrical component of polybutadiene having a diameter of 500 A was observed by the electron microscopic observation) was kept in a flow of air containing 6% of ozone for 36 hours and then the film was immersed in a methanol solution of nitric acid (30%) for 24 hours. According to electron microscopic observation, it was confirmed to form a porous poly-t-butyl crotonate film having fine cylindrical holes 500 A in diameter. The porous film had water permeability of 0.7 ml/min./cm 2  at 25° C under the pressure difference of 700 mm Hg. The porous film was used to concentrate a polystyrene latex having solid content of 5% and an average particle diameter of 900 A. When the latex on the surface of the porous film was stirred, the flow rate was 0.5 ml/min./cm 2 , under a pressure difference of 700 mm Hg at a 20% solid content. 
     EXAMPLE 2 
     A film of a block copolymer of methacrylonitrile and methylvinyl ketone at a ratio of 2 : 1 by weight, having a thickness of 2 μm was immersed in a methanol solution of 2,6-di-tert-butyl-p-cresol (1%) and the film was irradiated with ultraviolet rays for 80 hours. Devitrification of the film occurred. According to electron microscopic observation, it was confirmed to form a porous polymethacrylonitrile film having fine cylindrical holes 150 A in diameter. The porous film had an air permeability of 0.25 l/cm 2  /min. under a pressure difference of 700 mm Hg. Asbestos dust having a minimum dimeter of 300 A could be completely removed from air by passing the air through the porous film. The air permeability of the porous film was not lower than 0.18 l/cm 2  /min. under a pressure difference of 700 mm Hg. 
     EXAMPLE 3 
     The polymerization of styrene by the living anion method, was stopped by feeding carbon dioxide gas when the average molecular weight attained 80,000. The terminals of the polystyrne were acylated with thionyl chloride, and reacted with polyethyleneoxide having an average molecular weight of 30,000 in toluene in the presence of pyridine. The reaction mixture was separated by a large size column separation to obtain the AB-type block copolymer. The bonds between the polystyrene chain and the polyethyleneoxide chain the block copolymer were ester bonds. A film of the block copolymer, having a thickness of 18 μm, was immersed in a 1N aqueous solution of sodium hydroxide at 60° C for 2 days. According to electron microscopic observation, it was confirmed to form a porous polystyrene film having fine cylindrical holes 80 A in diameter. The porous film had methanol permeability of 0.2 ml/cm 2  /min. under a pressure difference of 700 mm Hg. A 2% methanol solution of polyvinylpyrrolidone having an average molecular weight of 60,000 was concentrated by using the porous film to obtain a  60% methanol solution of polyvinylpyrrolidone. No permeation of polyvinylpyrrolidone was found to occur. 
     EXAMPLE 4 
     A film of the block copolymer of acrylonitrile and butadiene, having a thickness of 5 μm (the fine cylindrical component of polybutadiene having a diameter of 110 A) was treated in accordance with the process of Example 1, whereby a porous polyacrylonitrile film having fine cylindrical holes, 110 A in diameter, was obtained. A 2% methanol solution of polyethyleneoxide (average molecular weight of 20,000) was passed through the porous film to methanol for 3 days for dialysis. The methanol phase was concentrated and tested. According to high speed liquid chromatography, the elution curve of polyethyleneoxide which permeated to the methanol phase through the porous film was compared with the elution curve of polyethyleneoxide in the original methanol solution. The results are shown in FIG. 1. The polyethylenoxide permeated had sharp molecular weight distribution. 
     EXAMPLE 5 
     A film of the AB-type block copolymer of t-butyl crotonate and isoprene (the fine cylindrical component of polyisoprene having a diameter of 1500 A) was treated in accordance with the process of Example 1, whereby a porous poly-t-butyl crotonate film having fine cylindrical holes 1500 A in diameter was obtained. A polystyrene latex having a solid content of 10%, an average particle diameter of 900 A was passed through the porous film. The permeability was 13 ml/cm 2  /min. under a pressure difference of 700 mm Hg which was not changed during the operation. When the same polystyrene latex was passed through a cellulose ester film having fine holes 0.22 μm in diameter (Miripore Co. Ltd.) the permeability was 19 ml/cm 2  /min. at the initial stage but was decreased to 14 ml/cm 2  /min. after 5 minutes. The results show the fact that the porous poly-t-butyl crotonate film had narrow diameter distribution of the fine cylindrical holes. 
     EXAMPLE 6 
     A film of n-butylisocyanate-isoprene AB block copolymer (0.8 μm thickness), having a polyisoprene cylindrical component 100 A in diameter, was kept in air containing 5% of ozone for 20 hours. Then, the film was immersed in a 10% hydroxyperoxide aq. for 5 hours and then into methanol for a day. The porous film so obtained showed a methanol permeation of 0.08 ml/cm 2  /min. under a pressure difference of 700 mm Hg. 
     Having now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit or scope of the invention as set forth herein.