Polymer melts local structure

K. S. Schweizer and J. G. Curro, Macromolecules, 21, 3070 (1988). Integral Equation Theory of Polymer Melts Intramolecular Structure, Local Order, and the Correlation Hole. 

Chapter 4 deals with the local dynamics of polymer melts and the glass transition. NSE results on the self- and the pair correlation function relating to the primary and secondary relaxation will be discussed. We will show that the macroscopic flow manifests itself on the nearest neighbour scale and relate the secondary relaxations to intrachain dynamics. The question of the spatial heterogeneity of the a-process will be another important issue. NSE observations demonstrate a subhnear diffusion regime underlying the atomic motions during the structural a-relaxation. 

Polymers of different structures do not form homogeneous blends. Mixture of polymers that differ in structure and polarity are compatible with one another only to a very limited extent. They separate in the melt or at the latest after cooling in the solid state, as a result of weak interfaces between the components and local stress concentrations. Therefore, they cannot be... 

Crystallites grow from nuclei. In homogeneous nucleation, nuclei are created by random statistical thermodynamic fluctuations of the local structure of the polymer. In heterogeneous nucleation, nuclei are created on surfaces of randomly distributed microscopic insoluble particles present in the melt. Since thermodynamic fluctuations are very small in magnitude while impurities are almost always present in significant amounts in a polymer, heterogeneous nucleation normally dominates. However, homogeneous nucleation can also become important at low values of Tx [186,187]. 

When a polymer is melted, the molecular conformations become disordered. As a result, the vibrational spectrum is significantly different from the spectrum of the crystalline state. Because of the presence of new conformations in the melt, many new vibrational bands appear in the spectrum, or splitting of the bands observed in crystalline phase disappears. The vibrational modes are broad because of the variety of structures in the melt. Because the structures are disordered, no specific selection rules can be applied. The Raman spectrum of molten POE is presented in Figure 2 (dashed line). The band assignments for the molten state become difficult, not only because of the broadening, but also because of the large number of local structures with small vibrational energy differences that may be present. 

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