Crystallinity, macromolecules, thermal analysis

Hu, X., Kaplan, D. and Cebe, P. (2006) Determining beta-sheet crystallinity in fibrous proteins by thermal analysis and infrared spectroscopy. Macromolecules, 39, 6161-6170. 

In the last two chapters of the book on Thermal Analysis of Polymeric Materials the link between microscopic and macroscopic descriptions of macromolecules will be discussed with a number of examples based on the thermal analysis techniques which are described in the prior chapters. Chapter 6 deals with single-component systems, Chap. 7 with multiple-component systems. It is shown in Sect. 6.2, as suggested throughout the book, that practically aU partially crystalline polymers represent nonequilibrium systems, and that thermodynamics can establish the equilibrium limits for the description. It was found, however, more recently, that equilibrium thermodynamics may be applied to local areas, often small enough to be called nanophases [1]. These local subsystems are arrested and cannot establish global equilibrium. 

M. E. Nichols and R. E. Robertson, The origin of multiple melting endotherms in the thermal analysis of polymers, J. Polym. Sci., Polym. Phys. Ed.,30 305-307,1992. M. Yokouchi, Y. Sakakibara, Y. Chatani, T. Tadokoro, T. Tanaka and K. Yoda, Structures of two crystalline forms of polyfbutylene terephthalate) and reversible transition between them by mechanical deformation. Macromolecules, 9 266, 1976. 

Crystallization, one of the two first-order transitions encountered in the thermal analysis of polymers, is a process in which a material from the amorphous state is transformed into the crystalline state from either solution or the melt. Crystallization of macromolecules is different from the crystallization of low-molecular-mass materials. First, similar to the melting process, it takes place at conditions far from equilibrium. When compared to low-molecular-mass substances, the crystallization process of polymers is much slower because of the lower mobility of the polymer chain segments therefore in nonisother-mal conditions this process takes place over much wider temperature ranges. Crystallization of low-molecular-mass materials is mentioned here very briefly, and only for the purpose of comparison with macromolecules. 

The bottom curve in Fig. 5.26 illustrates the ultimate result of thermal analysis of two crystalline linear macromolecules. Two isomers of 1,4-polybutadiene (PB) are analyzed. All data were extrapolated to 100% crystallinity and equilibrium. The entropy was then computed using Eq. (2) of Fig. 5.11, and adding the transition entropies at the equilibrium temperatures. Clearly, the trans isomer melts in two steps, while the cis isomer melts in a... 

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