The Crystalline Melting Temperature

FIGURE 10-37 Schematic diagram depicting the overall rate of crystallization versus temperature. 

FIGURE 10-39 Melting curves of volume versus temperature for and CWH1W (plotted from the data of L. Mandelkem, Comprehensive Polymer Science, Vol 2. Pergamon Press, Oxford, 1989). 

FIGURE 10-40 Typical melting curves of specific volume versus temperature for polyechylenes [plotted from the data of R. Chiang and P. Flory, JACS, 83,2857 (1961)]. 

---

Differential Thermal Analysis (DTA). One of the characteristics of a rubber useful in tire rubber compounds is that it is amorphous at room temperature but readily undergoes strain induced crystallization. For this reason, copolymers were prepared in order to appropriately adjust the crystalline melt temperature. 

Figure 7.6 Effect of ethylene glycol concentration on the crystalline melting temperature of PCTG copolyesters...

In this chapter it will be demonstrated that the two main transition temperatures, viz. the glass-rubber transition temperature and the crystalline melting temperature can be correlated with the chemical structure by means of a method based on group contributions. 

Microscopic crystalline regions found within a solid polymer below the crystalline melting temperature, (p. 1237)... 

The glass transition temperature of amorphous polymers is a function of the chemical structure of the polymer chain. It varies widely with the types of skeletal atoms present, with the t T)es of side groups, and with the tacticity of side groups along the polymer backbone. Table 14.11 demonstrates the effects of structural variations on the crystalline melting temperature and glass transition temperature for several polymers. 

The glass transition temperature, T, the crystalline melting temperature, and the enthalpy for melting. A//, for PMMA, PETG... 

Amorphous materials usually exhibit an apparent second-order thermal transition, the glass transition, at about two-thirds of the crystalline melting temperature (measured in Kelvin)... 

For blends of LDPE with EMA-salts, G is superposable but there is a clear breakdown of the time-temperature superposition principle at high frequencies for G". Furthermore, the frequency range over which G" is superposable decreases with increasing ionomer (EMA-salt) content. In other words, the inability of EMA-salt to Increase G" at high frequencies, as a function of temperature, increases as the content of EMA-salt in the blend is Increased. These results lead to the conclusion that above the crystalline melting temperature of the two components, the breakdown of the time-temperature superposition principle in G" is due solely to the presence of ionic domialns in PE/EMA-salt blends. 

The transition temperatures were essentially the same for a similar type of polymers regardless of molecular weight range In this study. The lower transition (-86 C) In the B-CL of 70-30 wtX dlblock Is associated with Tg of the polybutadiene block and the upper transition (56 C) with the crystalline melting temperature of polycaprolactone segment No polycaprolactone Tg (about -60 C for Its amorphous part) was observed In the B-CL dlblock, evidently because of Its lowr content and proximity to the polybutadiene Tg. It can be detected at -59 C, however. In the S/B-Cl of 33/44-23 wt% dlblock terpolymer. In the latter, the Tg of the S/B block (-20 ) was essentially the same as the random S/B copolymer control which Is coded as S/B-CL 33/44-0 iftZ In Table IV. 

A new correlation will be developed in Section 6.B, to enable the prediction of reasonable values of Tg without requiring and thus being limited by the availability of group contributions. This correlation will account for the effect of the composition and the structure of a polymer, as reflected in the chain stiffness and the cohesive forces between different chains, on Tg. A new correlation will be presented for the effects of the average molecular weight on Tg in Section 6.C. The effects of plasticization on Tg will be discussed in Section 6.D. A new correlation will be presented for the effects of crosslinking on Tg in Section 6.E. The effects of tacticity on Tg will be discussed in Section 6.F. Secondary relaxations will be discussed in Section 6.G. The crystalline melting temperature will be discussed in Section 6.H. Finally, the ratio Tm/Tg... 

The stmctural dependence of the crystalline melting temperature is essentially the same as that for the glass transition temperature. The only dilTerence is the effect of structural regularity, which has a profound influence on crystallizability of a polymer. T is virtually unaffected by structural regularity. From a close examination of data for semicrystalline polymers it has been established that the ratio Tg/T , (K) ranged from 0.5 to 0.75. The ratio is formd to be closer to 0.5 in symmetrical polymers (e.g., polyethylene and polybutadiene) and closer to 0.75 in unsymmetrical polymers (e.g., polystyrene and polychloro-prene). This behavior is shown in Figure 4.9. 

Solid phase pressure forming (SPPF) combines the use of pressure forming and plug-assist to form crystalline plastics at temperatures below their normal forming range, 5 to 8% below the crystalline melting temperature, in order to produce stiffer parts with less material. 

---