Melting temperature of polymer crystals

Fig. 17 B/E-p dependence of the critical temperatures of liquid-liquid demixing (dashed line) and the equilibrium melting temperatures of polymer crystals (solid line) for 512-mers at the critical concentrations, predicted by the mean-field lattice theory of polymer solutions. The triangles denote Tcol and the circles denote T cry both are obtained from the onset of phase transitions in the simulations of the dynamic cooling processes of a single 512-mer. The segments are drawn as a guide for the eye (Hu and Frenkel, unpublished results)...

Figure 1 also shows that plasticized polyvinyl chloride begins to flow at a lower temperature. This is to be expected in view of the fact that equilibrium melting temperature of polymer crystals is depressed by monomeric diluents. A statistical thermodynamic treatment by Flory (13), showed that this effect depends on the nature of the polymer, concentration of the diluent, and the degree of polymer-diluent interaction in the following manner ... 

Determination of the equilibrium melting temperature of polymer crystals linear and nonlinear Hofiman-Weeks extrapolations. Macromolecules, 31, 8219-8229. 

F igiire 6.1. Temperature dependence of Apicr nd Api, (o), concentration dependence of AGm for solution (the single-phase state) (6), and the dependence of the melting temperature of polymer crystals on polymer concentration in solution (the liquidus curve) (c) (Schematic)... 

Thermodynamic Equilibrium Melting Temperature of Polymer Crystals... 

Lovell R, Mitchell GR, Windle A (1979) Wide-angle X-ray scattering study of structural parameters in non-crystalline polymers. Faraday Discuss Chcm Soc 68 46-57 Maraud H, Xu JN, Srinivas S (1998) Determination of the equilibrium melting temperature of polymer crystals Linear and nonlinear Hoffinan-Weeks extrapolatimis. Macromolecules... 

Fig. 1 Melting temperatures of polymers k%Tm/Ec) with variable Ev/Ec values. The line is calculated from Eq. 10 and the circles are the simulation results obtained from the onset of crystallization on the cooling curves of disorder parameters, in a short-chain (r = 32) system (occupation density is 0.9375 in a 32-sized cubic box) with a template substrate (Hu and Frenkel, unpublished results)...

The reduction of the lamdlar thickness of crystallites leads to a decrease of T. On the other hand, the crosslinks stabilize the arrangement of macromolecules which contributes to an increase of T. Actual values of T , for a given sample of crosslinked polyethylene depend on initial ciystallinity of a polyrnCT, temperature at which the crosslinking is conducted, as well as on conditions of crystallization before a measurement of melting temperature of polymer crystallites. The effect of crosslinkmg on the... 

Most of the melting temperatures in the literature do not apply to perfect crystals of infinitely long chains (Tm) or even to perfect crystals of finite chains ( J m). They are more often the melting temperatures of imperfect crystals from polymers of finite molar mass, Tm. The melting temperatures, Tm, are obtained through the following considerations ... 

The thermodynamic equilibrium melting point of polymer crystals can be measured by means of DSC and small-angle X-ray diffraction. The following procedure is recommended for the measurement of the equilibrium melting temperature of a polymer crystal,... 

Polymer crystallization and melting are typically first-order phase transitions between the amorphous phase and the crystalline phase. When these two phases are in thermodynamic equilibrium, two phase transitions are thermodynamically reversible under a certain temperature. This temperature is referred to the equilibrium melting point of polymer crystallization. The free energy changes of amorphous phase and crystalline phase under various temperatures are depicted in Fig. 4.1, illustrating the definition of the equilibrium melting point 7. ... 

Glass Transition and Crystal Melting Temperatures of Polymers (°C)... 

There are thus two specific places where the dilution directly influences AG (v2) the In V2 term and AG (V2). The latter can be approximated by AG = AH iTm — r)/ Tn,. Tm is now the equilibrium melting temperature of the crystallizing polymer in the mixture. There is also the distinct possibility, which cannot be ruled out, that either, or both, of the interfacial free energies, cTen mid cTun, depend on composition. 

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