Polymers with Low Degree of Crystallinity

Above Ts. the properties of such partly crystalline systems have already been illustrated in Fig. 12-15. Here the relaxation modulus for poly(vinyl alcohol) resembles those of the polyethylenes in Fig. 16-8, although the slope is steeper probably the lack of tactidty leads to substantial imperfecticnis of crystallinity even without esteriHcation. With increasing esterification, the curves become steeper, and at 40% esterification the relaxation modulus falls by two and a half logarithmic decades in the range of reduced time investigated. 

Below Tg, loss mechanisms identiHable with secondary processes in the glassy state of the amorphous regions may be observed.  

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Comparison with Crystalline Polymers. The structure of the polymers discussed above differs from that in crystallizable polymers with low degrees of crystallinity (poorly developed structure). Here the tendency to organize comes from the parallel packing of the side groups. There is no evidence that polymer... 

The crystallinity of partially crystalline polymers during elongation can increase, decrease, or remain constant (Figure 11-17). Chain orientation, on the other hand, increases continuously during elongation. With quenched samples, i.e., those with low degrees of crystallinity, the available crystallites with their molecular axes are obviously first oriented in the stress direction. 

Anionic, cationic, coordination, or radical polymerization routes ate all applicable [42, 42], Recently, enzymatic catalysed polymerization of e-caprolactone has been reported [36]. It is a semicrystalline polymer with a degree of crystallinity around 50%. It has a rather low glass transition temperature (-60°C) and melting point (61°C). 

Figure 6.25. Low-resolution NMR signal of the H nuclei of a polymer with high degree of crystallinity. Dashed lines represent two signals resulting from the deconvolution.

PCL is a semi-crystaUine polymer with a degree of crystallinity of 50%, a low Tg (-60 °C) and a melting point of 60 °C. Injection-molded samples of PCL exhibited a modulus of 400 MPa and a yield stress of 15 MPa. Moreover, the material can be processed by injection molding, film blowing and extrusion. The rate of crystallization of PCL is slower than that of conventional polymers, while poly(ether-ester)s are more flexible due to the presence of ether linkages. Although PCL and poly(DXO) resemble each other in their chemical structure, PCL is semicrystaUine whereas poly(DXO) is an amorphous polymer with a Tg of approximately -37 °C [37]. 

In the case of a crystalline polymer the maximum service temperature will be largely dependent on the crystalline melting point. When the polymer possesses a low degree of crystallinity the glass transition temperature will remain of paramount importance. This is the case with unplasticised PVC and the polycarbonate of bis-phenol A. 

Finally, the use of low-molecular-weight species to improve flow properties called plasticizers normally reduces stiffness, hardness, and brittleness. Plasticization is usually restricted to amorphous polymers or polymers with a low degree of crystallinity because of the limited compatibility of plasticizers with highly crystalline polymers. Other additives, such as antioxidants, do not affect the mechanical properties significantly by themselves, but can substantially improve property retention over long periods of time, particularly where the polymer is subject to environmental degradation. 

The choice of a suitable polymer material is a prerequisite for the success of the enrichment process. In general, all polymers with a low degree of crystallinity and low glass temperature show good enrichment capacities (Table 6.5-2). 

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