Glass-forming polymers

Structural relaxation in glass-forming polymers has been studied for many years using chemically realistic simulations. Most of the early work that examined incoherent, as well as coherent scattering functions, is more of a qualitative nature because of the unsatisfactory quality of the force fields employed and the severe limitations on the length of the MD simulations performed. Roe studied the slowdown of structural relaxation in a PE-like model140,141 as well as for polystyrene.142 More recently Okada et al.143,144... 

Investigating the Influence of Different Thermodynamic Paths on the Structural Relaxation in a Glass-Forming Polymer Melt. 

Phys. Condens. Matter, 15, S1127 (2003). Self-Motion and the a-Relaxation in Glass-Forming Polymers. Molecular Dynamic Simulation and Quasielastic Neutron Scattering Results in Polyisoprene. 

Spatially Correlated Dynamics in a Simulated Glass-Forming Polymer Melt Analysis of Clustering Phenomena. 

To date, incoherent quasi-elastic neutron scattering experiments on the a-relaxation regime of glass-forming polymers have revealed the following main features for the self-motion of hydrogens ... 

There is a fundamental question concerning the nature of the self-motion of protons in glass-forming polymers. In Sect. 4.1 we have shown that the existing neutron scattering results on the self-correlation function at times close to the structural relaxation time r (Q-region 0.2t) with a KWW-like functional form and stretching exponents close to jSsO.5. 

Fig. 11-1. Voluine-temperaluie relations for a glass-forming polymer and a material that crystallizes completely on cooling. is a melting point, and and T are glass transition temperatures of an uncrystallized tnaterial that is cooled quickly and slowly, respectively.

Alba-Simionesco, C., Cailliaux, A., Alegria A., and Tarjus, G. (2004) Scaling out the density dependence of the a relaxation in glass-forming polymers, Europhys. Lett. 68, 58... 

The KWW function is used widely to describe relaxation behavior in glass-forming polymers and other condensed systems. The distribution of relaxation times that is associated with the KWW function may also be used to shed some light on the molecular processes that occur during physical aging. To illustrate this, some results for a series of neat polymers and blend systems are presented and discussed in the following sections. 

Gomez-Ribelles, J. L., and Monleon-Pradas, M., Structural relaxation of glass-forming polymers based on an equation for configurational entropy 1. DSC experiments on polycarbonate. Macromolecules, 28, 5867-5877 (1995). 

One of the characteristics of the relaxation processes in glass forming polymers is the heterogeneity. A monotonic relaxation process can in general be expressed as... 

Typical PVT data for a glass-forming polymer are shown in Figure 7.12. It is apparent that as the pressure is increased so the specific volume is reduced and there is a corresponding increase in the value of Jg. This type of behaviour is generally found for most polymers. As we will see later, the exact behaviour is dictated by the cooling regime that is adopted. 

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