Poly paradox

It is an interesting paradox that one of the least stable of commercially available polymers should also be, in terms of tonnage consumption at least, one of the two most important plastics materials available today. Yet this is the unusual position held by poly(vinyl chloride) (PVC), a material whose commercial success has been to a large extent due to the discovery of suitable stabilisers and other additives which has enabled useful thermoplastic compounds to be produced. 

Finally, we stress that the free volume approach is only applicable to nonpolar systems. Aqueous dispersions fall outside its scope. This is vividly illustrated by the data of Evans et al. (1975), who determined experimentally that d(UCFT)/d7 = — 1 x 10 KPa for latex particles sterically stabilized by poly(oxyethylene) in aqueous 0-43 molal magnesium sulphate solutions. Both the sign and magnitude of this quantity is different from that predicted by free volume theory for the UCFT of non aqueous dispersions. Paradoxically, it falls in line with the predictions, both in sign and magnitude, published by Croucher and Hair (1979) for the pressure dependence of the LCFT of poly(a-methylstyrene) in -butyl chloride. This may be merely coincidental, but the very small pressure dependence exhibited by the UCFT of aqueous sterically stabilized dispersions emphasizes the major differences between the origins of flocculation at the UCI T for aqueous and nonaqueous dispersions. The small pressure dependence observed for aqueous systems is scarcely surprising since the UCFT of an aqueous dispersion occurs far from the critical point of water whereas that for nonaqueous dispersions is quite close to the critical point of the dispersion medium. 

In principle, the force necessary to cause a brittle break F = E jL can be calculated from the energy E required to separate chemical and physical bonds by an interbond-partner distance L. To break extended-chain poly-(ethylene) crystals perpendicular to the chain direction (i.e., breaking covalent bonds), a force of about 20 000 MPa is necessary, whereas to cause a break parallel to the chain direction (i.e., working only against dispersion forces), only 200 MPa is required. Experimentally, however, a maximum tensile strength of 20 MPa is observed (the so-called crystal paradox). Consequently, the break must occur at inhomogeneities, since these lead to an inhomogeneous distribution of the tensile stress onto disruption points and thus lead to stress concentrations. 

Water sorption in physically crosslinked poly(vinyl alcohol) membranes an experimental investigation of Schroeder s paradox. J. Memhr. Sci., 337, 291-296. 

To achieve high conductivities, the polythiophene paradox must be overcome. The polymerization process and the conductivity of the resultant material are influenced by the concentration of monomer used during poly-merization, because, if this is too low, the overoxidation reaction predominates, at least when galvanostatic polymerization is used. Synthesis at reduced temperatures will help avoid overoxidation and can be used to increase the conductivity of the resultant material. ... 

With the above assumptions, RSC were able to compute volume-recovery curves which mimic the major features of the glassy phenomenology. In Table 4 are shown the parameters used in the calculations. Figure 38 shows the asymmetry of the calculated approach curves compared with the experimental data of Kovacs for poly(vinyl acetate). In Figure 39 is shown a plot of logteff calculated from the model compared with the data for poly(vinyl acetate). As was the case for the phenomenological models, the i-effective paradox is not resolved. 

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