Phase Transformations in Cold Rolled Polyoxymethylene

Structure and Morphology of Polyoxymethylene

“…Diff erent unit cell parameters, namely a = 4.469 Å, b = 7.740 Å, c = 73.4 Å with an uncertain but assumed 2*38/21 helix, have also been reported by Gramlick for very small single crystals of hexagonal POM [24,25]. Th e orthorhombic modifi cation has unit cell dimensions a = 4.77 Å, b = 7.65 Å, c = 17.80 Å and, according to Pready and Wheeler, it may be formed from the hexagonal by an expansion along the a axis and a contraction along the b axis when only variations in the ab plane are considered [17]. Indeed, Pready and Wheeler observed the X-ray pole fi gure of POM obtained by a strictly controlled procedure of cold rolling and explained the strong maxima in the transverse direction and the weaker maxima in the sheet normal direction in terms of a transition from the hexagonal to the orthorhombic phase at a temperature much lower than room temperature.…”

“…Notwithstanding the limited attention given in the literature to solid-solid phase transitions of POM, it is possible to fi nd a few evidences of the occurrence of the hexagonal→orthorhombic transition. For instance, Pready and Wheeler obtained orthorhombic POM at the temperature of solid CO 2 by cold rolling of hexagonal POM [17]. According to an enantiotropic polymorphism, this fi nding demonstrates that the orthorhombic phase is the more stable phase below 60 °C but is actually obtainable only at much lower temperatures because of the inertia to transform of the kinetically favored hexagonal modifi cation.…”

“…Beside the synthesis of the orthorhombic POM as a fraction of reaction products, together with the prevalent hexagonal phase, it is possible to convert trigonal POM into the orthorhombic phase by pressure-induce transition [31], according to the higher density and cohesive energy of orthorhombic POM in comparison to the hexagonal POM [18]. Although the high pressure conditions under which it is possible to observe a conversion of the hexagonal into the orthorhombic phase apparently seem to suggest a more probable monotropic transition, it must be noted that, as discussed in the above paragraph, obtaining orthorhombic POM by cold rolling [17] is a fi nding in favor of an enantiotropic process. Linear thermoplastic polymers generally achieve a crystallinity level much less than 100% and may be considered to have a two-phase structure of crystalline and amorphous regions.…”

Polyoxymethylene Processing

“…Th ey found that POM rolling at a temperature close to the melting point produced a [0001] texture, that is, with the most densely packed planes [0001] aligned parallel to the rolling plane. Preedy and Wheeler [105] studied the rolling texture of POM by X-ray pole fi gure technique. Th e observed textures were interpreted in terms of a stress-induced martensitic transformation of the hexagonal phase to an orthorhombic phase.…”