Polymers characteristic temperature

In many of the above polymers characteristic temperature drifts of the calorimeter were observed above and below Tg depending upon the previous thermal history of the sample. In the case of the synthetic rubber, GR-S, Rands,... 

Approximate values of Tg are included in Table II for the purpose of indicating the temperature region in which the polymer characteristically changes from a hard, more or less brittle glass to a rubbery or viscous polymer within which motions of portions of the chains, usually called segments, are comparatively unhampered by the interactions... 

Our theory of polymer melt glasses distinguishes four characteristic temperatures of glass formation that are evaluated for a given pressure from the configurational entropy s T) or the specific volume v(T). Specifically, these four... 

Figures 8a and 8b display the polymer mass dependence of Ta, Ti, Tg, and Tq for the F-F and F-S classes of polymers, respectively. All four characteristic temperatures exhibit the same physical trend, that is, growing with increasing M and saturating to constants in the high molar mass limit M oo. Comparison of Figs. 8a and 8b reveals that the variation of these characteristic temperatures with M is generally stronger for the F-S polymers. For instance, the slope of Tg versus 1/M is nearly a factor of 3 larger for the F-S class than the F-F class. All four characteristic temperatures T (a = 0, g, I, A) for the F-S polymers exceed their counterparts for the F-F polymers.

Figure 13. Ratios T/ /To and Ti/Tg of the characteristic temperatures from Figs. 8a and 8b as a function of the inverse number l/M oi united atom groups in individual chains for constant pressure (P = 1 atm 0.101325 MPa)F-F (open symbols) and F-S (filled symbols) polymer fluids. The single data point denoted by refers to high molar mass F-S polymer fluid at a pressure of P = 240 atm (24.3 MPa).

Figure 22. The configurational entropy Sc per lattice site as calculated from the LCT for a constant pressure, high molar mass (M = 40001) F-S polymer melt as a function of the reduced temperature ST = (T — To)/Tq, defined relative to the ideal glass transition temperature To at which Sc extrapolates to zero. The specific entropy is normalized by its maximum value i = Sc T = Ta), as in Fig. 6. Solid and dashed curves refer to pressures of F = 1 atm (0.101325 MPa) and P = 240 atm (24.3 MPa), respectively. The characteristic temperatures of glass formation, the ideal glass transition temperature To, the glass transition temperature Tg, the crossover temperature Tj, and the Arrhenius temperature Ta are indicated in the figure. The inset presents the LCT estimates for the size z = 1/of the CRR in the same system as a function of the reduced temperature 5Ta = T — TaI/Ta. Solid and dashed curves in the inset correspond to pressures of P = 1 atm (0.101325 MPa) and F = 240 atm (24.3 MPa), respectively. (Used with permission from J. Dudowicz, K. F. Freed, and J. F. Douglas, Journal of Physical Chemistry B 109, 21350 (2005). Copyright 2005, American Chemical Society.)...

The Tg of elastomers must be below the use temperature. The high degree of cold flow which is characteristic of polymers at temperatures above the Tg is reduced by the incorporation of a few crosslinks to produce a network polymer with a low crosslink density. 

The following polymer characteristics were obtained on the variously irradiated test specimens melt index (ASTM 1238-62T), Vicat softening temperature (modified ASTM 1525-58T) heat distortion (ASTM D 1220-63T) physical strength yield strength ultimate tensile strength percent elongation at break (ASTM D 412-64T) chemical resistance to boiling toluene. 

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