The polymerization of crystalline trioxane. III. experiments with irradiated trioxane single crystals

Abstract

Irradiation with x-rays imparts to trioxane crystals a perennial capacity for later polymerization to high conversion. This capacity remains latent at room temperature, but becomes manifest above a threshold temperature varying with the history of the crystal (40–55°c). Yet, even at room temperature, intensive reorganization of the crystals takes place within two hours of the irradiation, the radiolysis products being segregated into cavities and cracks in the lattice, and in part exuded to the outer surface (Stage 1). Polymer crystal nucleation, leading to the formation of a white, reflecting haze, also occurs at room temperature, though at a slower rate, or somewhat later (Stage 2). In view of the spontaneous polymerization observed to occur at dry trioxane surfaces (van der Heijde et al. 1965), it is concluded that this is what the irradiation does to promote later polymerization: creating dry surface within the monomer crystal. The topokinetics of the polymerization above the threshold temperature lead to the conclusion that this reaction proceeds in at least two more stages (3 and 4). Stage 3 involves the growth of a tenuous structure of primary ribbons sprouting from the nuclei-carrying defect surfaces into the monomer crystal. Stage 4 consists of the multiplication of these primary ribbons by fast collateral development of secondary ribbons, and leads to a densening of the pseudomorph. This conclusion is supported by the micro- and macromorphology of the product in its various stages of development. The twins, which Carazzolo et al. (1963) proved to exist in the polymer by x-ray diffraction—‘Z’-crystals ∥ the monomer c-axis, and ‘W’-crystals in three equivalent orientations at 76° to the c-axis—cannot be traced in the morphology of the product, as the ribbons are invariably extended along the c-axis. The crystal optics of incipient ribbon aggregates do reveal the existence of the W -orientations, but only in the region where growth of these aggregates started. A tentative explanation for this phenomenon is offered.