Rigid-amorphous fraction, RAF, in semicrystalline polymers

The first quantitative analysis of RAF was carried out by analysis of the heat capacity of semicrystalline poly(oxymethylene), POM, as shown in Fig. 6.16 [20]. Clearly, the 67% of crystallinity computed from the heat of fusion, which would require a 33% amorphous fraction as indicated by the dotted line in the left trace, does 

Crystallinity Dependence of the Heat Capacity of Poly(oxy-methylene) 

The amorphous phase is at this temperature only partially mobile. 

The reversing heat capacity and the total heat-flow rate of an initially amorphous poly(3-hydroxybutyrate), PHB, are illustrated in Fig. 6.18 [21]. The quasi-isothermal study of the development of the crystallinity was made at 296 K, within the cold-crystallization range. The reversing specific heat capacity gives a measure of the crystallization kinetics by showing the drop of the heat capacity from the supercooled melt to the value of the solid as a function of time, while the total heat-Uow rate is a direct measure of the evolution of the latent heat of crystallization. From the heat of fusion, one expects a crystallinity of 64%, the total amount of solid material, however, when estimated from the specific heat capacity of PHB using the ATHAS Data Bank of Appendix 1, is 88%, an indication of a rigid-amorphous fraction of 24%. 

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