Theta temperature Polyethylene

The data in Fig. 4 refer to linear polyethylene samples with less broad distributions since they were obtained by fractionation of a commercial Marlex 50 type polyethylene into 10 fractions. The shape of the curves indicates that some of them would intersect, if extrapolated. This points to considerable residual poly-molecularity in the fractions so that identification of minima with critical points is not permitted. Nakajima et al. [35, 36] nevertheless analyzed the data assuming the identification to be allowed and, using the Shultz-Flory method [45], derived values for the critical temperatures for solutions of polyethylene of infinite molar mass in -alkanes. This specific temperature is known as the theta temperature, . Nakajima et al. s values, listed in Table 4, agree within 5 K with those... 

According to the statistical-mechanical theory of rubber elasticity, it is possible to obtain the temperature coefficient of the unperturbed dimensions, d InsjdT, from measurements of elastic moduli as a function of temperature for lightly cross-linked amorphous networks [Volken-stein and Ptitsyn (258 ) Flory, Hoeve and Ciferri (103a)]. This possibility, which rests on the reasonable assumption that the chains in undiluted amorphous polymer have essentially their unperturbed mean dimensions [see Flory (5)j, has been realized experimentally for polyethylene, polyisobutylene, natural rubber and poly(dimethylsiloxane) [Ciferri, Hoeve and Flory (66") and Ciferri (66 )] and the results have been confirmed by observations of intrinsic viscosities in athermal (but not theta ) solvents for polyethylene and poly(dimethylsiloxane). In all these cases, the derivative d In sjdT is no greater than about 10-3 per degree, and is actually positive for natural rubber and for the siloxane polymer. 

A series of well-defined A-B block copolymers of polystyrene-block-polyethylene oxide (PS-PEO) were synthesized [11] and used for emulsion polymerization of styrene. These molecules are ideal since the polystyrene block is compatible with the polystyrene formed and thus it forms the best anchor chain. The PEG chain (the stabilizing chain) is strongly hydrated with water molecules and it extends into the aqueous phase forming the steric layer necessary for stabilization. However, the PEG chain can become dehydrated at high temperature (due to the breaking of hydrogen bonds) thus reducing the effective steric stabilization. Thus the emulsion polymerization should be carried out at temperatures well below the theta (0) temperature of PEG. 

In some cases it may be advantageous to use a temperature gradient instead of, or in addition to, a nonsolvent to obtain fractions. Theta conditions for various polyolefins are summarized in Table 5. Fractionation of polyethylene using temperature... 

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