Poly crystal unit parameters

Properties of PET Molding Resins. The fliU crystal stmcture of poly(ethylene terephthalate) has been estabhshed by x-ray diffraction (134—137). It forms triclinic crystals with one polymer chain per unit cell. The original cell parameters were estabhshed in 1954 (134) and numerous groups have re-examined it over the years. Cell parameters are a = 0.444 nm, b = 0.591 nm, and c = 1.067 nm a = 100°, (3 = 117°, and 7 = 112° and density = 1.52 g/cm. One difficulty is determining when crystallinity is fliUy developed. PET has been aimealed at up to 290°C for 2 years (137). 

The crystal structure of PHB is a orthorhombic form with unit cell parameters fl = 0.576 nm, = 1.320 nm, and c(fiber axis)=0.596 nm, and space group P2,2,2, (Alper et al. 1963 Okamura and Marchessault 1967). The conformational analysis by intermolecular energy calculation has indicated that the PHB molecule has a left-handed 2j helix conformation (Comibert and Marchessault 1972 Yokouchi et al. 1973 Bruckner et al. 1988). The crystal structure of random copolymers of 3HB and 3HV has been investigated extensively (Bloembergen et al. 1986 Kamiya et al. 1991 VanderHart et al. 1995). A structural characteristic of poly(3HB-co-3HV) is isodimorphism, i.e., cocrystallization, of the two monomer units. In contrast, the cocrystallization of 3HB with 3HH or (7 )-6-hydroxyhexanoate (6HH) does not occur. 

The crystal in Fig. 5.34 illustrates the concept of isomorphism, the possibility that two different motifs fit into the same crystal. For polymers isomorphism of repeating units is most important. Isomorphism of complete chains is seldom possible. Polyfvinyl fluoride) that fits into a crystal of poly(vinylidene fluoride) provides a rare exception. The repeating-unit isomorphism can be separated into three types Type 1 occurs when both homopolymers have die same crystal structure, and a smooth change of the lattice parameters occurs on changing the concentration. An example of type 1 repeating-unit isomorphism is poly(vinylidene fluoride-co-vinyl fluoride). Type 2 is also called isodimorphism and occurs if the homopolymers have different crystal structures. A change in structure occurs at an intermediate concentration. Type 3 occurs if one homopolymer does not crystallize by itself, but participates in the crystal of the other. 

The WAXD patterns of the prepared series of polymers, from poly(propylene succinate) (X=2) to poly(propylene sebacate) (X=8), are quite different as one can see in Figure 4. In general the parameters of the unit cell characterizing the crystals of the polymers have not... 

---