Polyoxymethylene hexagonal crystals

The many commercially attractive properties of acetal resins are due in large part to the inherent high crystallinity of the base polymers. Values reported for percentage crystallinity (x ray, density) range from 60 to 77%. The lower values are typical of copolymer. Polyoxymethylene most commonly crystallizes in a hexagonal unit cell (9) with the polymer chains in a 9/5 helix (10,11). An orthorhombic unit cell has also been reported (9). The oxyethylene units in copolymers of tiioxane and ethylene oxide can be incorporated in the crystal lattice (12). The nominal value of the melting point of homopolymer is 17S°C, that of the copolymer is 165°C. Other thermal properties, which depend substantially on the crystallization or melting of the polymer, are listed in Table 1. See also reference 13. 

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). 

Finally the oligomers form a crystalline nucleus and the reaction becomes now exothermic due to the added heat of crystallization during simultaneous or successive polymerization and crystallization. The crystals of polyoxymethylene observed are hexagonal with the polymer chain peuallel to the c-axis (2,82). They increase with time in lateral dimensions as well as in thickness, taking a p5mmidal shape 82). The addition of new oligomers on the lateral surface would explain an autocatalytic acceleration of the reaction with time as well as the pyramidal shape 82). 

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