The Polymer Mixing Problem

The biconstituent and bicomponent polymer pairs, discussed in Section 9.2. 

The interesting features of a true polymer/polymer eutectic might include a highly controllable fine structure, especially in fiber formation. In this respect, the materials might be made to behave like the unidirection-ally solidified metallic eutectics of Kraft (Sheffler et al., 1969). 

Starting with recent studies by Meier (1969, 1970) on block copolymers (see Section 4.6.3), we have begun to learn about the phase relationships and degree of mixing at the phase boundaries in polymer blends. In fact. 

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Another portion of this chapter will treat some polymer mixing problems. Brief consideration will be given to possible polymer/polymer eutectic systems, which have never been made. The status of our knowledge of polymer/polymer mixing at phase boundaries will be reviewed, with an attempt to emphasize unknown or poorly understood factors. 

In this mixing process, contaminants such as solvent and/or diluents as well as their removal problems can be avoided. Degradation of the polymers is avoided by proper maintenance of the viscosity and shearing rates. 

The solution to this problem has been to isolate the lactide and to polymerize this directly using a tin(ii) 2-(ethyl)hexanoate catalyst at temperatures between 140 and 160 °C. By controlling the amounts of water and lactic acid in the polymerization reactor the molecular weight of the polymer can be controlled. Since lactic acid exists as d and L-optical isomers, three lactides are produced, d, l and meso (Scheme 6.11). The properties of the final polymer do not depend simply on the molecular weight but vary significantly with the optical ratios of the lactides used. In order to get specific polymers for medical use the crude lactide mix is extensively recrystallized, to remove the meso isomer leaving the required D, L mix. This recrystallization process results in considerable waste, with only a small fraction of the lactide produced being used in the final polymerization step. Hence PLA has been too costly to use as a commodity polymer. 

The symbol x is used here and in following pages in a somewhat different sense than in earlier portions of the book, where it represents the number of structural units. The segment employed in mixing problems often is conveniently defined as that portion of a polymer molecule requiring the same space as a molecule of solvent it is unrelated to the size of the structural unit, which is of no interest here. The present x, like the previous one, defines the size of a polymer species, however. 

Other water-soluble polymers which do not tend to suffer from problems of retrogradation are sometimes used when different properties are required. Soluble cellulose derivatives, particularly car-boxymethyl cellulose, which is prepared by reaction of high purity cellulose with chloroacetic acid in the presence of alkali (equation 8.1), is popular for surface sizing base papers which are subsequently to be coated, because it assists in water removal when the coating mix is applied. 

For applications where only mechanical properties are relevant, it is often sufficient to use resins for the filling and we end up with carbon-reinforced polymer structures. Such materials [23] can be soft, like the family of poly-butadiene materials leading to rubber or tires. The transport properties of the carbon fibers lead to some limited improvement of the transport properties of the polymer. If carbon nanotubes with their extensive propensity of percolation are used [24], then a compromise between mechanical reinforcement and improvement of electrical and thermal stability is possible provided one solves the severe challenge of homogeneous mixing of binder and filler phases. For the macroscopic carbon fibers this is less of a problem, in particular when advanced techniques of vacuum infiltration of the fluid resin precursor and suitable chemical functionalization of the carbon fiber are applied. 

Solvent-polymer compatibility problems are often encountered in industry, such as in the selection of gaskets or hoses for the transportation of solvents. A rough guide exists to aid the selection of solvents for a polymer, or to assess the extent of polymer-liquid interactions. A semi empirical approach has been developed by Hildebrand based on the principle of like dissolves like. The treatment involves relating the enthalpy of mixing to a solubility parameter, S, and its related quantity, 8, called the cohesive energy density. 

A different approach to the problem has been adopted by Candau and co-workers (85), They make a useful distinction between the temperature 0 that occurs in their equations as a parameter characterizing the polymer-solvent interactions which may be assumed to be independent of MW or branching,- and the temperatures 0Al and 0a at which A2 =0 and os = I respectively, which do depend on both factors. They assume that the free energy of mixing AGm of nl molecules of solvent with n2 molecules of polymer is given by ... 

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