Polymers from immiscible monomers

An excellent review of composite RO and nanofiltration (NF) membranes is available (8). These thin-film, composite membranes consist of a thin polymer barrier layer formed on one or more porous support layers, which is almost always a different polymer from the surface layer. The surface layer determines the flux and separation characteristics of the membrane. The porous backing serves only as a support for the barrier layer and so has almost no effect on membrane transport properties. The barrier layer is extremely thin, thus allowing high water fluxes. The most important thin-film composite membranes are made by interfacial polymerization, a process in which a highly porous membrane, usually polysulfone, is coated with an aqueous solution of a polymer or monomer and then reacts with a cross-linking agent in a water-immiscible solvent. 

The blends of polysulfone with the a-methyl styrene polymers are immiscible, as evidenced by the double glass-transition temperatures in Table II. To improve the miscibility characteristics, polysulfone was modified in two ways. First, 25% of the bisphenol A was replaced by monomer I which contains a pendant ester group and, when no improvement resulted, the whole 50% of the bisphenol A was replaced. Again, the blends remain immiscible as evidenced from Figures 4 and 5 and from Table II. Further, the presence of the pendant ester group in polymer C does not improve the miscibility picture even though one would expect a favorable contribution from the carbonyl group on account of the miscibility of polycarbonate with the a-methyl styrene polymers. 

Texier-Picard et al. analyzed a polymerization front in which the molten polymer was immiscible with the monomer and predicted that the front could exhibit the Marangoni instability even though comparable unreactive fluids would not exhibit the instability [84]. However, no liquid/liquid frontal system with an immiscible product has been identified. Even if such a system could be found, the experiment would have to be performed in weightlessness to prevent buoyancy-induced convection from interfering. 

The polymers formed from the immiscible monomers seem to hold promise for the development of new materials that cannot be synthesized on the ground except, perhaps in the presence of a solvent. We are seeing a mixing that is driven, in large part, by surface tension differences. The benefit of mechanical stirring needs to be investigated further. 

In the search for new polymer materials people have polymerized new monomers, or made new random, block or graft copolymers from existing monomers. A third alternative has been to blend existing polymers to produce materials with new properties. An obvious advantage of this approach is that it usually requires little or no capital expenditure relative to the production of new polymers. It is also possible to produce a range of materials with properties completely different from those of the blend constituents. These materials may be one phase or two phase. It is convenient to define miscibility as the ability to be mixed at a molecular level to produce one homogeneous phase. The term compatibility is often used interchangeably but is also used practically to mean able to be mixed to produce useful materials and as such is often used to describe immiscible materials. 

According to the solubility of the core-forming monomer in the reaction media, two different methods—emulsion polymerization and dispersion polymerization— have been exploited to obtain self-assembled nanoparticles by PISA [37, 38]. The dispersion polymerization can be carried out either in water or in organic solvents. The emulsion polymerization starts from a monomer-in-water emulsion, where a water-soluble polymer precursor is chain-extended by polymerizing a water-immiscible monomer, resulting in self-assembled block copolymers. In contrast to the emulsion polymerization, dispersion polymerization is conceptually much simpler and the initial reaction solution is homogeneous. 

Polymer networks can be formed by chemical reactions between polymer chains (cross-linking) or by using trifunctional comonomers during the polymerisation. If such a network is dissolved in a second monomer and this second monomer is again polymerized into a second network, one obtains a structure in which both polymers are intertwined. These polymer chains only have very local mobility. In cases where both polymers are partially or completely immiscible the L1/L2 phase-separation is reduced to a very small scale. The properties of such an IPN are completely different from the uncross-linked polymer blend [15]. 

Variables Affecting Conversion to Polystyrene. The conversion of styrene in dioxan (30% w/w) reaches a maximum at approximately 0.1M H2SO4 (Table 1). Beyond this acidity, the yield of hompolymer is not significantly increased under the radiation conditions used. At acid concentrations in excess of 0.7M there is a fall off in yield of polystyrene which may be attributed to the lower solubility of polystyrene at higher acid levels so that continued polymerisation is hampered by increasing immiscibility. The data in the same table show that the effect of water on the homopolymerisation is different from that of acid. The polymer conversion is also favored at the lower monomer concentrations (Table II) and is enhanced by the presence of acid at each monomer concentration studied. The conversion is also directly proportional to the total radiation dose (Table III) and inversely proportional to the dose rate (Table IV). The addition of acid in each case enhanced the resulting yields of homopolymer. ... 

If one were to choose more reactive monomers, it would be possible to carry out polycondensations at considerably lower temperatures in solution. For example, consider the reaction of a diamine and a diacid to make a polyamide (nylon), a polymerization that requires relatively high temperatures (see Equation 9). A much faster reaction would occur between the diamine and a corresponding diacid chloride (see Equation 10). Both reactions would produce the same polymer, although the reaction conditions would be much different, and the byproduct HC1 from the acid chloride reaction would have to be carefully trapped. One technique for performing a polymerization such as that in Equation 10 is to dissolve the monomers in different, immiscible solvents, forcing the polymerization to occur only at the interface of the two solvents, a process called interfacial polymerization. Because of the high reactivity of an acid chloride, these reactions can be carried out at very low temperatures. This polymerization can be carried out rather dramatically in a beaker and is known as the nylon rope trick (see Section 4). 

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