Temperature of polymers, melting

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(r = 32) system (occupation density is 0.9375 in a 32-sized cubic box) with a template substrate (Hu and Frenkel, unpublished results)...

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 The segments are drawn as a guide for the eye (Hu and Frenkel, unpublished results)...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 ... 

Table 2. Morphologies, Glass Transition Temperatures, and Melting Temperatures of Polymers Treated with Carbon Dioxide...

As occurs with other crystalline substances, the melting temperature of polymers decreases in the presence of solvents (melting point depression). Thermodynamic arguments lead to expressions that relate the depression of the melting point to the solution concentration. The equation that gives the melting temperature of a polymer solution as a function of the concentration (17) can be written as... 

An identical crosslink density does not necessarily lead to the same effect of cross-linking on properti. This is particularly evident in a case when crosslinks are formed between segments of polymer chains which are in the amorphous ase of semicrystalline polymer below the melting temperature of polymer crystallites. Ifcre, the effect of crosslinking on crystallinity is Icwer. 

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... 

TABLE 1.5 Glass Transition Temperatures (Tg) and Crystalline Melting Temperatures of Polymers... 

Injection molding is probably the most suitable fabrication process for polymers in high-volume production, due to its fast process time and high replication accuracy. As shown in Figure 20.12, the polymer is fed into a heated screw and melted (heating can reach 200 C-350 C depending on the melting temperature of polymer). It is then injected into the mold cavity, usually made in steel, and cooled to form a replica. 

Once these empirical rules have been established, it is possible to link the melting temperatures of polymers to chain flexibility, interactions (cohesive energy densities), internal heats of fusion, and the possible presence of mesophases (see Fig. 2.103). Such analyses are of importance for the design of new polymers and of changes in existing polymers when there is a need to alter thermal stabiUty. 

Does copolymerization lower or raise the melting temperature of polymers Explain... 

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)... 

Differential thermal analysis is widely used for determining exotherm and endotherm of reactive substances. It can also be used for determining transition temperatures, such as the glass transition temperature and the melt temperature of polymers (ASTM D3418). ASTM D472 provides directions for reporting thermoanalytical data. 

On the other hand, to lower the melting temperature of polymers based on HQ/TPA, two approaches had been employed [11] ... 

Thermodynamic Equilibrium Melting Temperature of Polymer Crystals... 

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