Eloisa B. Mano and Leni Akcelrud DUGO Universidade Federal do Rio de Janeiro Rio de Janeiro, Brazil
I
I
Review of Laboratory Methods for the Preparation of Polymer
The difficulty of establishing a sharp distinction hetween a film and a membrane has been pointed out in the literature. Frequently the terms are used interchangeably. Carnell and Cassidy ( I ) define "membrane" as a solid, porous sheet which, when placed between two phases, may allow the passage of small particles but hinders or prevents the passage of large particles. The authors consider films to be relatively non-porous, and membranes to be porous enough for the study of the movement of small ions in solution. The higher degree of porosity in membranes mav he the result of s ~ e c i aformulative l and manipulative devices to enlarge -the intermolecular spaci@s beyond the extent normally inherent in the film form of the bulk polymer ( 2 ) . General reviews on films or membranes have been published (1, 3, 4 ) . Specific cases such as cellulosic membranes ( 2 ) and styrene-isoprene block copolymers ( 5 ) have also been considered. Polymer films and membranes are useful for many purposes in laboratory work, either in scientific or in technological research. In most cases it is important to prepare highly homogeneous, flaw-free, uniform thickness samples. Films are necessary for nuclear studies such as windows of Geiger-Muller counter tubes and source mounts for the purposes of &spectrometry (6-17). Electron microscopy studies use films as specimen supports (18). Polymers may he crystallized by casting films from their dilute solution on an appropriate substratum. Single crystals of polyethylene, polypropylene, poly-1-hutylene, polyacrylonitrile, and cellulose triacetate were obtained by this method (19). The bulk cwstallinitv of Nylon-6 (20) and were the cryst&ine morpholog; of synthetic studied using films of these materials (21). Thermal T22, 23) and mechanical (24-26) properties of polymers can he evaluated if they are film-forming materials. Membranes have extremely important applications. According to the mode of transport through them they can he classified as molecular diffusion, ultra-filter and microporous membranes (27). Permeability and diffusion of gases, vapors and liquids through polymeric membranes have been discussed by many authors (28-36). Osmotic membranes have been studied as well (37, 38). Membranes for desalination of water have been developed since Loeb and Sourirajan's work, in 1960 (39-46).Ion-exchange membranes are also of great interest (47-59). Biological research is another important field of membrane application (2, 60-67). Attempts have been made to establish possible relations hetween the method of sample preparation and tbe film properties (68). This paper intends, without exhausting the subject, to This work was sponsored jointly by Banco Nacional do Desenvolvimento Econamico (BNDE), Conselho Nacional de Pesquisas (CNPq) and Coordenaqh de Aperfei~oarnentodo Pessoal de Nivel Superior (CAPES). 228
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present literature information on laboratory methods for ore~arationof ~olvmericfilms. As "film" it will he understood a relativ&non-porous sheeting having a nominal thickness not ereater than 0.010 in. (2, 69), although someauthors consider 0.003 in. as being the limit (70). The industrial aspects of film preparation are more than adequately considered in the literature (71) and will not be discussed in the present work Preparation on Solid Surfaces
To prepare films on solid surfaces the polymeric material should be dissolved or dispersed in a solvent or mixture of solvents or molten. The choice of solvent is of major importance. It has been recognized for some time that mechanical properties of films depend on the solvent used for casting. For instance, for ethyl cellulose thermodynamically poorer solvents lead to films of higher hirefringence, higher density, lower brittle point temperatures, and in general greater toughness, although this effect is ohsewed only when the casting is on solid surfaces (72).It has been found that the purity of the solvent influences the film properties (46).After filtration of the polymer solution under vacuum or pressure techniques, entrapped air bubbles should he removed either by letting the solution stand or by vacuum application. Planar Surfaces Any solid planar surface can he used when the technique developed hy Blodgett (73) for fatty acids is employed. This technique is based on Langmuir's work on oil monomolecular film and consists on dypping the solid surface heneath a water surface covered with the film and withdrawing the solid slowly. The molecules in the film retain the orientation they had on water (73).Mechanical devices were develo~edto remove the film without damaging it (74-77). Glass Plates. The use of rectaneular shaned . elass " plates is very common. Circular plates (78) or Petri dishes (50) or a polished flat bottom of a flask (46) may also he used. Glass surfaces should be completely clean and highly polished. In some cases, the conditioning of the plates betore use (53, 79, 80) is reported. The preparation of films can he achieved by spreading, casting, or dipping. In the first two cases, the levelling of the surface is of major imnortance to obtain homoeeneous films. A technique for setting the glass plates in the horuontal position consiats of floatine (hem i n a Petri dish fullof mercurv. .(in,.. Uniform thickness is accomplished by spreading with special devices. For cellulose acetate hutyrate melts the use of a coating knife (81) has been reported. A Stainless Steel spreader bar has been employed for SBR latexes (25). A special spreader bar cut along one edge to give a uniform depression of 0.04 to 0.05 in. in depth has also been used successfully for SBR emulsions (26). For viscous solutions, a Gardner knife (45, 68, 82) or a doctor blade (22, 33, 64, 72, 83, 84) is usually employed. For instance, spreading of a 5% benzene or toluene solution of
polystyrene on glass plates with a doctor hlade results in films as thin as 0.0009 in. (22). A blade opening of 0.5 mm gave a film of about 25p for solutions of about 12% solids (64). Casting of polymeric solutions on glass plates can be ohtained by limiting the film boundary with brass rings (22) or other suitable devices (58). This technique is adequate for thicker films. As example, 15% benzene or toluene solutions of polystyrene produced films up to 0.010in. thick (22). Dipping a glass plate into polymer solutions and removing it vertically is another laboratory technique for film preparation (I, 9, 13, 79, 80, 85). The rate of withdrawal of the plate influences the thickness of the resulting film. As an example, cellulose acetate dipped films 0.6 to 4 . 5 ~ thick were ohtained using a 30 cm/min rate (68). On the other hand, a 10 pg/cm2 film of vinyl acetate-vinyl chloride copolymer resulted from a 1 cm/min rate (7). A string may be used to remove the dipped plate (19). A modified technique is reported for films of low-density polyethylene: a glass plate is half-dipped in 75 ml of a 0.2 g/100 ml xylene solution in a wide neck bottle immersed in a water bath at 100°C and heated until the solution is clear: the nlate is then lifted and held verticallv. iust ahovk the hot surface for a few seconds and removed &om the bottle (18). A more sovhisticated techniaue is reported in which a film of polyknyl formal (3 pg/cm2) with a film thickness distribution within 3% is produced: a glass funnel fitted with a tap is filled with the chloroform-polymer solution, a glass plate is immersed in it and the solution is drained off through the tap at a given rate (14). Another device is described for vinyl acetate-vinyl chloride copolymer dipped films, in which the glass plate is fixed inside a cvlindrical funnel having a glass rod adavted in such a way to obstruct the stem of the f i n nel. A chosen drainina rate can thus be obtained (11). There are several ways by which the solvent h a y be removed whenever a film is prepared from a polymer solution. Spontaneous drying, at room temperature requires a shield to prevent dust and retard solvent evaporation, so that a more uniform surface is obtained. The evaporation can also he retarded by maintaining a solvent atmosphere during drying (45, 72). When volatile solvents such as trifluoroacetic acid and trifluoroethanol are employed, watch glasses inverted over the glass plates are a practical procedure to prevent a fast evaporation (21). Some authors report other means such as oven drying, with or without air circulation, for instance, for solutions of polyvinyl formal in tetrachloroethane (75-80°C) (23), polyvinyl butyral in dimethyl formamide (75°C) (331, alcohol-soluble polyamides in a mixture of ethanol 50-glycerol 15 (60°C) (36) and amylose in a mixture of water 200-n-butanol 30 (21°C) (83). For cellulose nitrate films from acetone solution (34) as well as for cellulose acetate solutions (86) a stream of dry air over the film surface was used. A nitrogen stream was employed for cellulose acetate in acetone as well as vacuumdr),ing (461. An infrared lamp was also utilized for polyvin?lidene fluoride in propylene oxide (8. 8;). For rubber latexes, drying at room (26) or higher temperatures (24) is described. To facilitate the release of the polymer film from the glass plate it is common to score or cut the edges with the help of a hlade (9) and immerse the film adhered to the plate in distilled water. For special purposes a device is described for the removal of the film (9). The release is easier when a demolding agent is previously applied to the glass plate. For example, methyl cellulose (I), polyvinyl hutyral (25), paraffin dissolved in carbon tetrachloride (88) and commercial products like silicone fluids (83) and Teepol' (89) are common releasing agents.
an
The polymer film may also be prepared between glass plates. This technique is specially useful when an oven cure is required. For instance, an ion-exchange membrane can be prepared pouring the hot polymer solution into a pre-heated glass former made of two glass plates separated by about 0.4 mm and sealed at the edges. This assembly is placed upright in an air-oven kept at a chosen temperature (53). A 5p thick film of styrene-divinyl benzene copolymer is produced when the monomer mixture is placed in a mold composed of a frame of aluminum foil interposed between two glass plates, covered with rubber sheets and finally with metal plates. The whole sandwich is kept tightly assembled by means of a screw device and heated in an oven at the proper polymerization temperature (55). Immersing a glass plate in an 1 N aqueous alkaline solution, wiping the plate with a special paper, and finally with a piece of chamois, results in a very thin layer of sodium hydroxide which will act as a parting agent (9). Another technique describes the use of a few drops of hydrofluoric acid to the water in which the film is dipped (90). When the film tends to he brittle it is convenient to remove it from the glass plate while still plasticized with solvent (82). Inorganic Crystal Plates. In some circumstances, it is advantageous to use salt plates as a support to form polymer films. For instance, weight loss analysis was carried out on polyvinyl formal films obtained from 8-10% tetrachloroethane solutions on salt plates (23). Infrared quantitative analysis of vulcanized natural and synthetic rubber were p!rformed using rock salt plate cast films (91). Refluxing in o-dichlorohenzene the acetone 32 chloroform 68 extracted vulcanized sample, uniform reproducible films were prepared on sodium chloride or potassium bromide plates by spreading the concentrated mucillage between parallel metal spacers with the aid of two razor blades (92). Infrared studies on preferential chain orientation of poly(ethy1ene oxide) cast films were carried out by depositing the chloroform polymer solution on rolled silver chloride plates and allowing the solvent to evaporate. The resultant film was then sandwiched between silver chloride plates by cold pressing to provide a sample which could be melted and thermally treated without change in its physical dimensions (93). In electron microscopy studies, carbon coated mica was successfully employed to prepare films from fluorinated solvent solutions which tend to cling tenaciously to glass surfaces, as in the case of synthetic polypeptides (21). Despite being reported only for inorganic films like antimony trisulfide, lead chloride, and zirconium oxide, a method based on depositing the film on a substratum which can he subsequently removed by volatilization might he convenient for thermostable film preparation. Films were deposited by vacuum evaporation on pressed disks of ammonium chloride which were then mounted, film-face down, on gold plates having a central slot. On gentle heating in uacuo, the ammonium salt sublimes readily leaving the unbacked film. Coherent, transparent, pinhole-free, very thin films were obtained (12). Metallic Plates. Whenever heat transmission is an important factor in the preparation of a polymer film, a metallic plate is adequate as a support. Polymers that need cure by heat treatment, such as some polyurethanes, may he cast on iron plates (94). A terpolymer of vinyl chloride, vinyl acetate and vinyl alcohol compounded with an epoxy 'Trademark for a detergent of the type RIRzCHOSO*ONa; R1 and Rz are alkyl groups. Volume 50, Number 3. March 1973 1 229
resin may be cast on aluminum panels and baked at 300°F to give a film 0.006 in. thick (95). For chloro-trifluoro ethylene polymers, which need heat treatment at 250'C to melt or sinter, a film may he prepared from dispersions containing about 20% solids by spraying it in a criss-cross manner on a chromium or stainless steel plate (96). An imnortant characteristic for coatine materials is the minimum filming temperature of a polymer emulsion: this temnerature was reoroduciblv determined for a 55/45 methyl methacrylate/ethyl acrylate emulsion, spread on a metal bar along which a temperature gradient was maintained (97). Plastic Plates. Teflon surfaces are used for preparation of films that demand curing at a reasonably high temperature because of its inertness and heat and solvent resistance. For instance, a compound of acrylonitrile grafted onto polydimethyl siloxane was vulcanized as a film using a Teflon support at 250°C (98). If polymer aqueous solutions can he prepared, polymethyl metbacrylate polished plates may he used. Polyvinyl alcohol films can he ohtained this way (99). Metallic Sheets. One of the advantages of metallic sheets as support for plastic films is the possibility of suhsequent removal of the metal by dissolving it. This is specially important when the polymeric film is highly adhesive to the substratum. Copper strips have been used for polyvinyl formal films (23). Polyvinyl chloride and polymethyl methacrylate solutions were spread with a doctor blade on tin foil, which was then removed by mercury (100, 101). Aluminum foil as a support can he destroyed by alkali solutions (102). Stainless steel sheets have also been employed either for casting (103) or pressing the resin between two sheets (104). Cellulose Sheets. I t has been pointed out (105) that cellophane, when used as a support for film preparation, allows the full amount of shrinkage to take place during the evaporation of the solvent. In contrast, when films are formed directly on glass, the adhesion to the glass reduces the normal shrinkage of the film. Cellophane may he used as a lining on a Petri dish (105) or held taut between closely fitting steel hoops, the uniformity being obtained by using dilute solutions and numerous coats. The film is released by covering the uncoated surface of the cellophane with water. Thin films of rubbers, cellulose derivatives and oleoresinous varnishes were prepared in this manner (lC6). Natural and SBR ruhher films have been prepared by spreading the latex or the solution on tightly stretched cellophane sheets (24). A device is descrihed in the literature (107) to apply polymer dispersions on cellophane. The cellulose sheet is placed under a spreading bar supported on two shims (which will determine the thickness of the film) fixed on a plane metal base. An excess of the film-forming solution is poured on the cellophane sheet across its width and the sheet is drawn with a uniform motion.
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Non-Planar Surfaces A plaster of Paris cylindrical mold is reported for preparation of latex films in which drying takes place simultaneously a t both surfaces, through the plaster mold and at the side exposed to the atmosphere. A cylindrical shape for the mold was chosen since it gives an even distribution of stresses that develop during drying and prevents the development of an impervious surface skin, which would cause entrapment of water in subsurface layers. The technique consists of pouring a slurry of 2100 g of plaster of Paris into 700 ml water in a 1-1 beaker and keeping inserted in it a 250-ml, high-form heaker which had been immersed in an alcoholic soap solution and dried. On 230
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standing, the plaster hardens and expands slightly, breaking the outer glass heaker. The inner one is removed easily. The plaster mold is then dried for 24 hr in a circulating air oven at 60°C. To prepare films, the dry plaster mold is filled with filtered latex usually containing 50% solids. After 30-120 min, which is the time required for deposition of a ruhher layer on the plaster mold walls, the excess latex is poured out and the mold is returned to the oven for 3-12 hr until dry. The film is then stripped from the mold, cut along one side to form a sheet and given any further vulcanized treatment (108). Preparation on Liquid Surfaces Among the advantages of a liquid surface as a support for film formation are its suitability for very adherent films and the lack of defects and imperfections due to irregular solid surfaces as well as absence of levelling problems. It is specially recommended for very thin films. Water Surface Depending on the density and the chemical nature of the water insoluble polymer solvent system, a water surface may he employed for film preparation. A method is descrihed for cases in which the solvent is denser than water, as reported for poly(viny1 formal) dissolved in 1,2-dichloroethane to produce films about 500 A thick. A motor driven hydrophobic plate (the spreader) is moved horizontally at a uniform rate on guide rails so that its lower edge dips slightly into the water surface and creates a trough. Before the spreader is lowered into position, drops of polymer solution are laid on the upper surface of the spreader near the bottom edge. As the spreader is moved across the water surface, a film is formed (16). To remove films from the water surface it is common to use flat rings provided with suitable handles which are slipped under the water surface and raised so as to lift off the film. For larger films, the use of a brass (109) or aluminum (10) frame has been reported. For very thin films, this technique can lead to the film breaking; a more convenient method is to press an upside down glass upon the film covering the water, and submerging the two of them. The film will stick to the rim and outer wall of the glass which is then removed from the water and the adherent film allowed to dry (14). A method has been descrihed in the literature which is applicable to polymer solutions which do not spread satisfactorily as in the case of copolymers of vinyl chloride and vinyl acetate in cyclohexanone. A trough or sink is filled with water and a floating wooden harrier placed in contact with one end. One or two mililiters of resin solution are pipeted between the harrier and the trough so as to wet both the side of the trough and the harrier. The latter is released and the resin solution allowed to expand into a hand about 2-3 cm wide, the outer edge (near the harrier) of which immediately begins to solic]ify. The harrier is lifted from the water, lowered lightly onto the solidified film and then moved along the water surface a t a speed of about 30-40 cmjsec. During this process a film of resin is observed to feed out of the solution band, and continues to do so covering the water surface until either the harrier reaches the far end of the trough, or the hand of solution is exhausted. The lifting may he accomplished by the use of wire frames which allow larger areas of film to he ohtained, e.g., up to 40 x 20 cm of 10-20 pg/cmZ film (6). To free the film from wrinkles, stretching it on frames after removal from water has been reported (110). Mercury Surface One of the advantages of preparing films on mercury is that the liquid surface allows the polymer film to undergo
unrestrained shrinkage during its formation. For ethyl cellulose it is reported that mercury surface cast films are essentially isotropic, except for a small area around the borders. Since other metallic surfaces, such as polished chromium, lead to highly hirefringent films similar to those ohtained on glass plates, it must he the mobility of the mercury surface which allows planar shrinkage (72). Very thin films of cellulose nitrate plasticized with camphor have been prepared from 0.2% alcohol-ether solutions when drops of this solution are allowed to evaporate in an iron ring 2-cm in diameter floating on perfectly clean mercury (109). Cellulose nitrate osmometer membranes were prepared by slow evaporation of solution poured inside a chromium-plated steel ring which has floated on mercury. After 60-70 min evaporation, the ring carrying the partially dried film was lifted by means of two pins set vertically in the ring (37). Polymers from diisocyauates and polyols were cast on mercury which covered the bottom of a flat metallic pan. The metal was chosen so as to form an amalgam thus preventing the formation of a meniscus a t the edges, resulting in a smoother surface. Before casting the 50% solution of the adduct in benzene, acetone was poured over the clean mercury surface and allowed to evaporate, so that most of the air was displaced by acetone vapor. The polymer solution was then added to the layer of acetone (111). Other Liquid Surfaces Besides water and mercury surfaces, other liquids can he selected for the preparation of films. Carbon tetrachloride, for example, has been used as a liquid support for gelatin films from aqueous solutions (112). Other Methods of Preparation Ultrathin sections of polymers may be obtained with the use of an ultramicrotome. An instrument has been recently developed which freezes the polymer sample and by means of a glass or diamond knife, cuts it to a size suitable for electron microscopy studies. Films of polyurethane elastomer, polypropylene, polytetrafluoroethylene and polyvinyl chloride have been ohtained in this way. It has been found that materials will section best if the sample is maintained well below the glass transition temperature (113). Spin or centrifugal casting is a convenient technique for the preparation of thin films. A cylindrical mold has been used for elastomer solutions or latexes, for thermoplastic melts, and for thermoset resins, in which case casting and curing are achieved simultaneously. The equipment includes an aluminum cylinder with one open face provided with lips. A 3-in. cylinder spun a t 3,600 rpm distributes the polymer with a force up to 550 g and provides outstanding thickness uniformity and freedom of interior defects. The cylinder may he rotated in a vacuum oven (114). A rotating cylinder on which a 5% solution of polyvinyl butyral in 95% ethanol is cast has been reported. Three coats are applied in 5-min intervals and, 5 min after the last cast, when the film reaches a gel stage, i.e., a uonflowing consistency, the whole cylinder is immersed in water (97). A method has been patented for heat resistant polymer films. The polymer solution is introduced into a vessel the bottom of which is made of a membrane of a suitable material. i.e.. silicone rubber s u o ~ o r t e dhv a screen. The hottom musthe permeable to tce'solvent b u t impermeable to the ~olvmer.Because of the reduced Dressure maintained in che'chamber under the screen, fhe solvent diffuses through the membrane to give a useful film which does not adhere to the membrane (115). Literature Cited (11 Carnel1.P.H.. and Cassidy. H . . J Polym. S r i . 55. 233 119611. 121 Jameron, T . A.. and Meltzcr. T . H.. Appl Pojvm. Symp.. 13.267 1~9701.
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1371 Fuas. R.M.. andMead. D. J..J. Phys. Chem., 47.5911943). 138) Hmkwsy, HT..sndTovnsend, J Chem Sor. P3.3190i1952l. 1391 Chrm. Enp. Neus38.64. April 11,1960. 1401 Agrswal, J. P.. and Sourirajan. S..Ind. & Eng. Chem.. 61.61 119691. 141) Kunst.B..andSourirajan.S..%Appl. PolvmSci.. 14.723119701. 1421 . . Chandoriks~.M. V.. Kano. A. S.. and Motha.. Ohirsilal. Indian C h m . J.. 4. 27 (19691;c h b m 73, i h o a ~ i v o ) . 1431 Brock. J. L.. Fshey, P. M.. Miller. C. S.. and Spatz. 0. 0.. US. Office Saline WaUr,Res. Develop. Pmgr.Rep., n.4111969I: Chsm. Abs.. 71.71901v(19691. I441 Mat*. R.. Tulin. M. P.. Gollan. A,. Preiner. H. S.. and Alcalav. H.. U.S. Offlce Saline w a ~ c Pes. , Develop. Pmgr. Pep., n.542 119701; ~ h Abs.. k 73. 77955e (1970). I451 Knspp. C. H.. and Ward, W. J., Ind. & E n g Chem., Rod. Res. D~uelop,7. 169
. .
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1461 Dick, R., andNicalar. L..Anpem. Makromol. Chem.. 7.39119691. 1471 Wyllie, M. R. J., andKanasn. S.L.. J. Phys. Chem.. 58.73(19541. (48)Yoshikawa, S.. Uchida, Y.. and Kuwsta. T.. Dmhamo Monograph. 47, 515,805 119621; Chem Abr. 59,8296l1963l. 1491 Frilstts. V. J., J. P h s . Chem., 60. 435 119561. I501 Grsydon. W. F.. and Stewart. R. J.. J. Phyr. Chem., 59.8611955l. , J Chem Soc ~ o p o nlnd. ; Cham. Seer.. 56. (ELI~ o s a k a Y., . Sato, A . and ~ a j i m a8.. 279(19531; Chzm Abs, 48.9759(19541. 1521 Nasini, A,, andostacoli. G . J . CollaidSci.. 11.63711956). (531 Hills, G. J., Kitchoner, J. A , and Ouender. P. J.. Tmnr. Faroday Sac., 51. 719 ,>az-a>
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161) Hills, B. A , Tmns. Am.,. Sac. Anif. Intern. Oaonr. 14.111968). I621 Wyllie. M.R., andPatnodo, H. W.. J . Phys. Chem.. 54.M4lL9501. 163) Neihof.R..and Soliner K . J . Phys. C h m . 54,157 (195Ul. 164) Gregor. H.P.,and Wetstone, 0. M.,Disr. ForodavSoc.. 21, 162119561. 165) Bloek, M.. Chem. lnc. ilondonl, 50, 3099119671; Chem. Abs. 68,30496n (1968). 1661 Mizufani. Y., Ysmane, R.. Ihara. H., and Moromura, H.. Bull. Chem Soc. Japan, 36.361 (19631. 1671 Jacobson, H.. Polytechnic Inst. of Bmklyn. Univ. Micmfilms, L. C. Card n.~Mic 59-3629,Diss~rfolionAbs..20. 1191 (1959); Chem.Abs..54.4WbI19M)l. 1681 Kokfa. B..Luner.P.. andSuen. R..Aool. Palvm. Svmo.. 13. 169119691. . 1691 ~ o d e r n~ ~ a s t i c s ~ n i ~ c i o p ep d3i7a i, i ~ o p 1 1 . 1701. Modern Packaging Encyclopedia. p. 129 (19701. 1711 Wolinrki. L. E., "Films and Sheeting," in Encyclopedia of Polymer Science and Technolw. Editors: Mark. H. F.. Gavlord. N. G.. and Bikales. N. M.. Wilev~nterscie& New Yark. vol. 6. 1967. p. 764. 1721 Haas. H. C.. Farnay. L.,snd Velle, C.. Jr.. J Colloid. Sci. 7.58411952l 1131 Rlodgetl, K. B..J.Amer Chem. Sac.. 56.495119541. 1741 ~ i o d p e t tK. , B . , J . A ~c ~h p~m s n c , 57.1007 (isas). (751 Asthury, W. T.. Bell. F. 0.. Gortor. E.. and Van Ormondf. J.. Naturr. 142. 33 (19381.
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