Longitudinal growth of polymer crystals from flowing solutions. VI. Melting behavior of continuous fibrillar polyethylene crystals

Abstract

The melting behavior of continuous fibrillar crystals of high‐molecular‐weight polyethylene has been investigated. The macrofibers were grown from dilute solutions in xylene subjected to Couette flow in the temperature range between 103 and 118.5°C. The thermograms, as determined by differential scanning calorimetry, exhibit three melting endotherms with peak temperatures at 141, 150.5, and 159.5°C after extrapolation to zero scan speed. All peaks were found to be strongly superheatable. Reduction of fiber length, in particular by etching with fuming nitric acid, led to the disappearance of the melting peaks at 150.5 and 159.5°C. The remaining peak at 136°C appeared not to be superheatable. The heat of fusion of the fragmented fibers was 69.8 cal/g. Wide‐angle x‐ray diffractograms taken on a macrofiber while gradually heated at a rate of 0.35°C/min at constant length showed that the triclinic phase present in the fiber disappeared at 130°C and that the orthorhombic cell transformed into the hexagonal modification at 150°C. This hexagonal phase was still observable at 180°C. The retractive force developed on heating at constant length displays first a slight decrease followed by a maximum at 150°C. Beyond the latter temperature the stress decays abruptly corresponding to the temperature at which fracture of the fiber could be observed visually. From all these observations it is inferred that the first melting endotherm in the differential scanning calorimeter (DSC) thermograms arises from the melting of unconstrained fibrillar crystal regions which are able to shrink during fusion. Moreover, the melting of lamellar overgrowths on the elementary fibrils on shish‐kebab type may contribute to this endotherm. The second melting endotherm at about 150°C is associated with the transformation of the orthorhombic into the hexagonal lattice in constrained parts of the sample. This latter “rotator” phase allows slippage of the polymer chains past each other, giving rise to stress relaxation. The third endotherm arises from melting of this hexagonal phase and the heat take‐up connected with the formation of higher energy gauche states upon randomization of the chains in the melt. Almost smooth, fully constrained fibrillar crystals grown at high temperature absorb more than 15.5 cal/g during this process, indicating that the polymer chains in such fibers must be highly extended.