Dispersions cast films

One activity at DuPont s Fuel Cells Business Center is the development of a thinner membrane with sufficient mechanical stabdity. Thinner membranes translate into higher current density, which in turn means a higher electrical efficiency. The tradeoff is a less mechanically robust membrane. Nafion membranes are nonreinforced films based on Nafion resin, a PFSA/PTFE copolymer in the acid (H+) form. DuPont is especially marketing Nafion PFSA NR-111 and NR-112 membranes as nonreinforced dispersion-cast films for that purpose. These membranes are dehvered as a composite with the membrane positioned between a backing film and a coversheet. This composite is wound on a 6 in. i.d plastic core, with the backing film facing out, as shown in Figure 27.21. 

Film generated by depositing a layer of liquid plastic onto a surface and stabilizing by evaporating the solvent, by fusing after deposition or by cooling. Cast films are generated from solutions or dispersions. 

Cobalt(II) chloride was dissolved in poly(amide acid)/ N,N-dimethylacetamide solutions. Solvent cast films were prepared and subsequently dried and cured in static air, forced air or inert gas ovens with controlled humidity. The resulting structures contain a near surface gradient of cobalt oxide and also residual cobalt(II) chloride dispersed throughout the bul)c of the film. Two properties of these films, surface resistivity and bullc thermal stability, are substantially reduced compared with the nonmodified condensation polyimide films. In an attempt to recover the high thermal stability characteristic of polyimide films but retain the decreased surface resistivity solvent extraction of the thermally imidized films has been pursued. 

The self-assembling character of bilayer membranes is demonstrated by the formation of free-standing cast films from aqueous dispersions of synthetic bilayer membranes. The tendencies for association are sufficiently strong to allow the addition of guest molecules (nanoparticles, proteins, and various small molecules) to these films where the connective forces are secondary in nature and not primary. Synthetic polymer chemists have made use of these self-assembling tendencies to synthesize monolayer films. In particular, a monomer that contains both reactive groups and hydrophobic and hydrophilic areas is cast onto an appropriate template that self-assembles the monomer, holding it for subsequent polymerization. Thus, a bilayer structure is formed by... 

Although not strictly LB films, there are other types of self-assembled films containing Q-state MCs that resemble LB films. One example involves the self-assembly of the amphiphile DTG into an organized film by slow evaporation of solvent from a dispersion of the amphiphile (40). The structure of the cast film has the head-... 

A cast film was obtained in which an island/sea structure exists and the islands were cross-linked. Then they were dispersed in a good solvent. 

As manufactured, PTFE is of two principal types dispersion polymer, made by suspension polymerization followed by coagulation, and granular PTFE, polymerized and generally comminuted to a desirable particle size. Some details are given by Sperati. We have observed cast films of an aqueous colloidal dispersion and see that it consists of peanut-shaped particles, approximately 0.25 pm in size, which are composed of even finer particles. Electron micrographs of as-polymerized granular particles show three structures bands arranged in parallel, striated humps, and fibrils, some of which have the shish-kebab structure."... 

Fig. 6.29 TEM image of a 30 70 mixture of a diblock with /PS < 0.5 (M = 94 kg mol fK = 0.83) and a longer one with fK > 0.5 (Mn = 176kgmol 1,/PS — 0.81) (Koizumi et al. 1994c). The solvent-cast film was annealed at 150 °C. Domains rich in the diblock with /ps < 0.5 (consisting of PS cylinders) are dispersed in the matrix of the diblock with /ps > 0.5, which forms a microstructure of PI spheres. TOKI4 and B2 are notations of the authors for the diblocks.

Perhaps the simplest solution-precipitation membrane preparation technique is thermal gelation, in which a film is cast from a hot, one-phase polymer/solvent solution. As the cast him cools, the polymer precipitates, and the solution separates into a polymer matrix phase containing dispersed pores filled with solvent. Because cooling is usually uniform throughout the cast film, the resulting membranes are relatively isotropic microporous structures with pores that can be controlled within 0.1-10 i m. 

The major utility of PTFE dispersions is that they allow processing of PTFE resin, which cannot be processed as ordinary polymeric melt, because of its extraordinarily high melt viscosity, or as solution, because it is insoluble. Thus, PTFE dispersions can be used to coat fabrics and yams, impregnate fibers, nonwoven fabrics, and other porous structures to produce antistick and low-friction coatings on metals and other substrates and to produce cast films. 

For a successful production of a cast film, the dispersion has to wet the supporting surface and spread uniformly. In drying, the thickness of the deposited layer is very... 

Films of starch-EAA blends were prepared by either casting aqueous dispersions or by dry fluxing both the polymers.133 Cast films were relatively transparent, flexible and stable on immersion in water. In contrast, the dry blended materials were opaque at starch levels greater than 30% and had poorer tensile properties at high starch contents. No plasticizers were needed to prepare flexible films. Starch-EAA films have also been prepared by extrusion.134... 

Polymer solutions and dispersions find many practical applications. We can cast films from some polymers, and we use many others to form coatings. Coatings can be produced from either solutions or dispersions such as polymer emulsions (latexes). Examples include paint, varnish, textile finishes, adhesives, and floor waxes. Other useful latex products are latex gloves, condoms, and balloons. 

Dispersion characteristics of CNTs in PMMA could vary with the type of solvent, e.g. nitromethane, the most polar amongst the other solvents has been found to be most effective for dispersion of SWCNT in PMMA cast films (53). 

Park et al. (60) studied dispersion characteristics of MWCNT-PMMA composites synthesized by in-situ bulk polymerization using AIBN as free radical initiator. In their method, CNTs in varying amounts such as 0.001, 0.01 and 0.1 wt% with respect to MMA were first dispersed in MMA monomer by ultrasonication before polymerization. Experimental evidence such as molecular weight of free PMMA prepared via in-situ polymerization with and without CNTs, diameter of pristine MWCNT and diameter of MWCNT in composite, FTIR and SEM studies confirmed the role of AIBN and MWCNT in polymerization. The induced radicals on MWCNT by AIBN were found to trigger grafting of PMMA on to CNT surface. Solvent cast film of the composite was transparent and showed a better nanoscopic dispersion without aggregates compared to the cast film prepared from direct mixing of MWCNT and PMMA. 

Demonstier-Champagne et al. used atomic force microscopy (AFM) to observe microphase separation within cast films of PS-PMPS-PS/ PS-PMPS block copolymer mixtnre [43] that were nsed to compatibilize a blend of PMPS and PS. The fractnre snrface of blend films with the block copolymer incorporated show a far finer dispersion of particle sizes than those without. Matyjaszewski et al. studied PMPS-PS thin films by SFM (scanning force microscopy) and TEM (transmission electron microscopy) and Fig. 8 shows a TEM picture of a thin section of a film which was prepared by slow evaporation from THE, which is slightly selective for the polystyrene block [73]. 

Multicomponent polymers systems such as polyblends, and block copolymers often exhibit phase separation in the solid state which results in one polymer component dispersed in a continuous phase of a second component. The morphological properties of these systems depend upon a number of factors such as the molar ratios of the components, the molecular weights, the thermal history of the system and, for solvent cast films, the solvent and drying conditions. 

T. Kunitake, Aqueous BUayer Dispersions, Cast Multilayer Films, and Langmuir-Blodgett Films of Azobenzene-Containing Amphiphiles , Colloid Surf., 19,225 (1986)... 

Kunitake, T. Aqueous Bilayer Dispersions, Cast Multilayer Films and Langmuir-Blo%ett Films of Azobenzene Containing Amphiphile. Coll. Surf. 19, 225 (1986). 

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