Reduced crystallinity

The introduction of a perfluoromethyl side chain (Teflon FEP) greatly reduces the crystallinity of PTFE. Crystallinity is reduced even further by replacing the short side chain with a long side chain, such as perfluoropropyl ether. In contrast to Teflon EEP, only a small amount of vinyl ether is requited to reduce crystallinity and develop adequate toughness. 

A further class of ethylene-vinyl acetate copolymer exists where the vinyl acetate content is of the order of 3 mole %. These materials are best considered as a modification of low-density polyethylene, where the low-cost comonomer introduces additional irregularity into the structure, reducing crystallinity and increasing flexibility, softness and, in the case of film, surface gloss. They have extensive clearance as non-toxic materials. 

Table 1. Some imide ions and carbanions used in salts to enhance polymer electrolyte conductivity and reduce crystallinity...

Several soluble precursor approaches were reported to produce high-molecular-weight soluble precursors, which could be later reacted to remove the solubilizing groups. The introduction of bulky substitutions can reduce crystallinities or even suppress crystallization of a polymer, thereby improving its solubility substantially. Thus, the incorporation of removable substituents provides a strategy for... 

Regiodefects are less readily incorporated into crystallites than defect-free chain sequences. In semicrystalline polymers, increasing levels of misinsertion result in reduced crystallinity. This can affect numerous physical properties, resulting in reduced modulus, lower heat distortion temperature, and decreased tensile strength. 

Stereodefects in syndiotactic polymers have similar effects to those in their isotactic counterparts, reducing crystallinity levels and changing the associated physical characteristics. 

These representative aliphatic polyesters are often used in copolymerized form in various combinations, for example, poly(lactide-co-glycolide) (PLGA) [66-68] and poly(lactide-co-caprolactone) [69-73], to improve degradation rates, mechanical properties, processability, and solubility by reducing crystallinity. Other monomers such as 1,4-dioxepan-5-one (DXO) [74—76], 1,4-dioxane-2-one [77], and trimethylene carbonate (TMC) [28] (Fig. 2) have also been used as comonomers to improve the hydrophobicity of the aliphatic polyesters as well as their degradability and mechanical properties. 

When considering reduction of particle size, one should be aware of potential instability enhancement. It is well known that grinding can reduce crystallinity [65] the resulting noncrystalline material may be more hygroscopic... 

Random copolymerization of one or more additional monomers into the backbone of PET is a traditional approach to reducing crystallinity slightly (to increase dye uptake in textile fibers) or even to render the copolymer completely amorphous under normal processing and use conditions (to compete with polycarbonate, cellulose propionate and acrylics in clear, injection molded or extruded objects). 

Plasticizers weaken the intermolecular forces in the PVC reducing crystallinity. A relatively stable suspension, called a plastisol, of finely divided PVC in a liquid plasticizer, can be poured into a mod and heated to about 175°C producing a solid flexible plastic as a result of fusion of the plasticizer in the PVC. 

One method of reducing crystallinity in PEO-based systems is to synthesize polymers in which the lengths of the oxyethylene sequences are relatively short, such as through copolymerization. The most notable hnear copolymer of this type is oxymethylene-linked poly(oxyethylene), commonly called amorphous PEO, or aPEO for short. Other notable polymer electrolytes are based upon polysiloxanes and polyphosphazenes. Polymer blends have also been used for these applications, such as PEO and poly (methyl methacrylate), PMMA. The general performance characteristics of the polymer electrolytes are to have ionic conductivities in the range of cm) or (S/cm). 

Copolymers offer the twin advantages of reduced crystalline melting temperature and introduction of crosslinking sites. One of the best for. both of these purposes that has been reported so far is the thiocarbonyl fluoride-allyl chloro-formate copolymer (39). Products containing as little as 2-3 mole % of allyl chloroformate melt below 0°C and are readily crosslinked by incorporation of zinc oxide followed by heating. 

Copolymerization. Vinyl chloride can be copolymerized with a variety of monomers. Vinyl acetate, the most important commercial comonomer, is used to reduce crystallinity, winch aids fusion and allows lower processing temperatures. Copolymers are used in flooring and coatings. This copolymer sometimes contains maleic add or vinyl alcohol (hydrolyzed from the poly(vinyl acetate ) to improve the coating s adhesion to other materials, including metals, Copolymers with vinylidene chloride are used as barrier films and coatings. Copolymers of vinyl chlonde with acrylic esters in latex from are used as film formers in paint, nonwoven fabric binders, adhesives, and coatings. Copolymers with olefins improve thermal stability and melt flow, but at some loss of heat-deflection temperature,... 

Ball milling is the most commonly used pretreatment. It reduces crystallinity and particle sizes, while it increases surface area, bulk density, and the water soluble fraction. The major drawbacks of the milling are cost and the fact that noncellulosic substances are not removed. 

The results of the calculations and a comparison with experimental data are shown in Fig. 2.36. For convenience in comparison, the degree of crystallinity is normalized to its equilibrium value, so that the calculated function of the reduced crystallinity a changes from 0 to 1. Fig. 2.36 shows that a solution with the appropriate values of the constants fits the experimental data with high degree of accuracy. Both calculations and experiment show a pronounced inhomogeneity in temperature T and degree of transformation a, even for a cylinder with a relatively small diameter the difference in temperature can reach some tens of K. 

PFA is a copolymer of TFE and perfluoro(propylvinyl ether) (PPVE) in a mole ratio approximately 100 1. Even such a small amount of comonomer is sufficient to produce a copolymer with a greatly reduced crystallinity. The relatively long side chains also markedly reduce the cold flow. MFA, a copolymer of TFE and perfluoro(methylvinyl ether) (PMVE) has similar properties with a somewhat lower melting point. 

---