Poly reversing heat capacities

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%. 

Figure 2.102. Use of the derivative reversing heat capacity to study interfacial effects this figure shows the application of this method to poly(methylmethacrylate)-poly(vmyl acetate) (PMMA-PVAc), (50 50) latex film [from Song et al. (1999) reprinted with permission from Springer-Verlag],...

By introducing TMDSC [10-12,16] it was possible to examine directly the kinetics and thermodynamics of the reversibility and irreversibility of the latent heat of melting [12-18]. In contrast to small molecules, well-crystallized macromolecules melt irreversibly. The irreversible melting process of low molar mass POE [43] is illustrated in Figure 9.21. Results show lack of reversing heat capacity during melting of a well-crystallized poly-... 

Figure 9.34 (a) The comparison of the reversing heat capacities of poly(lactic acid) (PLA) resulting from different the time domains in the melting region by quasi-isothermal method of temperature-modulated DSC in the frame of vibrational and liquid Cp [55]. (See text for full caption.)... 

For some polymers such as isotactic polystyrene (iPS), polycarbonate, and PHB, reversing heat capacity from TMDSC equals baseline heat capacity for temperatures near glass transition. For other polymers also in isothermal TMDSC experiments latent heats may contribute to the measured reversing heat capacity at the generally low frequencies available by TMDSC. Figure 11 shows the development of measirred reversing heat capacity during isothermal crystallization of polyamide 12. Similar observations were made for PE, poly(c-caprolactone) (PCL), and polyetheretherketone (PEEK) to name a few. 

Pyda M, Wunderlich B (2000) Reversible and Irreversible Heat Capacity of Poly(tri-methylene Terephthalate) Analyzed by Temperature-modulated Differential Scanning Calorimetry. J Polymer Sci, Part B Polymer Phys 38 622-631. 

Poly(ethylene oxide) of high molar mass behaves similarly to the PET and PTT and other polymers analysed, although special effects are seen for many analysed polymers [78,82]. Figure 4.80 represents an example of PEO of a molar mass of 35,000 Da. As before, at low temperature, standard DSC and quasi-isothermal MTDSC give the same result. Most of the melting is irreversible and shows only in the total apparent heat capacity. A small amount, however, is reversing. The irreversible melting occurs at a temperature expected for 4 folds per molecule [52]. 

Fig. 12. Modulated DSC curve for Makroblend UT-400, an impact-modified polycarbon-ate/poly(ethylene terephthalate) blend. Curve A shows the conventional DSC curve curve B shows the heat capacity extracted from the reversing component of the signal curve C shows the modulus of the same material measured by DMA (Hale and Bair, in Ref 5).

Figure 9.35 Comparison of the reversing (by TMDSC) and apparent heat capacity (by DSC) of semicrystalline poly(oxy-2,6-dimethyl-l,4-phenylene) (PPO) [57].

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