Polyoxymethylene, crystal structure

The common crystal form of polyoxymethylene is the hexagonal form. Mortillaro et al. [56—58] found that another crystalline form of polyoxymethylene was produced when the polymerization of aqueous formaldehyde was carried out at 20° to 35° C at pH > 10 in high salt concentrations (>20%). It is shown in Table 12 that high molecular weight polymer is only achieved below 35°C. The rate of polymerization is slow it takes about 10 days to obtain maximum molecular weight. The formation of the proper seed crystal is important almost any crystalline polyoxymethylene regardless of the crystal structure can be used as seed. The type of salt used as catalyst is critical to obtain orthorhombic polyoxymethylene of reasonable molecular weight (Table 13). 

These crystal modifications differ in their molecular and crystal structures as well as in their physical properties. Many types of crystalline modifications are reported, including a stable orthorhombic phase and metastable monoclinic phase for PE a, and y forms for isotactic polypropylene (/-PP) trigonal and orthorhombic phases for polyoxymethylene a and y forms for Nylon 6 and others. Poly(vinylidene fluoride) (PVF), for example, appears in at least four types of crystalline modification (Lovinger, 1985 Dunn Carr, 1989). 

Kobayashi, M., Matsumoto, Y, Ishida, A., Ute, K. and Hatada, K. (1994). Polymorphic structures and molecular vibrations of linear oligomers of polyoxymethylene studied by polarized infrared and Raman spectra measured on single crystals. Spectrochim. Acta A, 50, 1605-17. [128]... 

A number of studies have been carried out on the helical chain structure. Two crystals i.e. polyoxymethylene (POM) and poly(4-methylpentene 1) (P4MP1) have been extensively studied by Choy and Nakafuku [64] and White et al [42], respectively. Choy and Nakafuku... 

Figure 6.14 Surface replica of polyoxymethylene fractured at liquid nitrogen temperatures. Lamellae at lower left are oriented at an angle to the fracture surface. Lamellae elsewhere are nearly parallel to the fracture surface, being stacked up like cards or dishes in the bulk state. These structures closely resemble stacks of single crystals, and they have led to ideas about...

Figure 5.1 (a) The solution-grown polyoxymethylene single crystal, (b) spherulites of melt-cooled polyethylene sample and the hierarchical structure, (c) extended-chain crystals of polyoxymethylene (whisker), and (d) the shish-kebab structure. (See color insert.)... 

S.2.3 Generation of Tie Chains in Isothermal Crystallization of Polyoxymethylene The infrared spectra of POM is highly sensitive to the aggregation structure of helical chains [77]. Assuming that the crystalline region of POM is a cylindrical shape, the infrared... 

Figure 5.20 Structural evolution in the isothermal crystallization of polyoxymethylene at (a) 130°C and (b) 150°C. The difference between these two processes is seen in the generation of tie chains passing through the neighboring lamellae (ECC) in Figure 5.19 [49,65].

---