Chapter 7 General Conclusions and Future Works

7.1 General Conclusions Investigations of the crystallization and melting behavior of linear polyethylene and of a series of ethylene/styrene random copolymers with styrene content ranging from 0.35 mol% to 11.7 mol% were discussed in chapters 3, 4 and 5 using the techniques of DSC and temperature modulated calorimetry. The focus of these studies was placed on the correlations between the temporal evolution of the degree of crystallinity, melting behavior and excess heat capacity during isothermal crystallization. The results suggest that: 1) The mechanism for the formation of low melting crystals varies with the crystallization temperature and comonomer content. With decreasing crystallization temperature and increasing styrene content, low melting crystals evolve from chainfolded lamellae to fringed-micelle type crystals; 2) The mechanism for the generation of excess heat capacity during quasi-isothermal crystallization also depends on the crystallization temperature and comonomer content. At high temperatures or in low styrene content copolymers, the generation of the excess heat capacity is most likely associated with the reversible segmental exchange on the fold surface. However, at low temperatures or in high comonomer content copolymers, the excess heat capacity is more likely to originate from a reversible attachment and detachment of segments on the lateral surface of crystals. Therefore a connection was established between the mechanism of secondary crystallization and that of reversible crystallization and melting. In chapter 6, the chain length dependence of the spherulitic growth rate is discussed in the case poly(ethylene oxide) fractions. A regime I/II transition is observed in fractions with molecular weight ranging from 11 to 157 kg/mol while no obvious regime transition was detected for fractions of higher molar mass. The equilibrium melting temperature was estimated by assuming that the regime I/II transition occurs a fixed undercooling. It is found that the ratio of nucleation constants in regimes I and II is, in general, not equal to 2, as predicted by the LH theory, but increases with increasing chain length. In addition, the exponential prefactor, G, inferred from the LH analysis does not follow the power law predicted by Hoffman and Miller. All of these results cast some doubts on the nucleation-based regime concept and the incorporation of a simple