Structure and properties of highly oriented polyoxymethylene produced by hot stretching

Structure and Morphology of Polyoxymethylene

“…Th e POM polymorphs show diff erent physical properties and, amongst other methods, they can be distinguished also by means of infrared and 13 C NMR spectra because of their diff erent vibrational frequencies and chemical shift s [15]. Th e hexagonal, that is the prevalent form, has unit cell dimensions a = b= 4.45 Å and c= 17.3 Å. Th e a and b axes are in the same plane perpendicular to the c axis, which is parallel to the molecular chain [1,10]. 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 one-dimensional, wide-angle X-ray diff raction curve of isotropic (non-oriented) hexagonal POM shows four peaks at increasing 2θ (θ is the Bragg diff raction angle) corresponding to lattice planes having Miller indexes (100) 115) and (205) [1,4]. While the position of the diff raction peaks of POM samples depends on the crystalline structure, the intensity of the peaks depends on crystallization order and orientation.…”

“…While the position of the diff raction peaks of POM samples depends on the crystalline structure, the intensity of the peaks depends on crystallization order and orientation. In highly oriented POM obtained through hot stretching from the solid phase, the intensity of the peaks changes with the drawing conditions [1]. In particular, being the (100) plane parallel to the c axis of the crystal, which is also the direction of chain axes, the (100) refl ection gives information on the molecular chain orientation [4].…”

“…Polyoxymethylene (POM) is a linear polymer with a high degree of crystallinity (up to 70-80% in commercial, more than 90% in highly oriented [1] and even 100% in laboratory synthesized samples [2]), so that, diff erently from the majority of thermoplastics, it is considered an engineering material. However, the high degree of crystallinity makes POM diffi cult to process and, moreover, under rapid conditions signifi cant shrinkage and void formation arise due to the diff erent densities and thermal expansion coeffi cients between the amorphous and crystalline phases [3].…”

Polyoxymethylene Processing

“…Th e same authors also obtained highly oriented self-reinforced POM using solid phase hot stretching technology. Th ey observed that the tensile strength and modulus increased with draw ratio reaching 900 MPa and 12 GPa, respectively, without remarkable drop of the elongation at break [143].…”