A Dislocation Mechanism for Cryogenic Relaxations in Crystalline Polymers

Abstract

ABSTRACT:The experimental properties of the 00-relaxation observed in polyethylene, poly(oxymethylene), and poly(ethylene terephthalate) at about 50°K are reviewed. A mechanism is proposed involving the thermally activated redistribution of kinks along a discontinuous dislocation under an applied stress. The activation energy and relaxation intensity are discussed with reference to specific chain conformations in the dislocation. It is suggested that the kinked dislocations arise from external stresses transferred to the crystalline or ordered regions of the polymer possibly via tie molecules. These stresses are greatest in deformed material or material annealed under constraint and accounts for the increase in peak height observed in these specimens.KEY WORDS Cryogenics/ Dislocation/ Polyethylene/ Poly(ethylene terephthalate) / Poly(oxymethylene) / a-Relaxation / Kink / Conformation / Secondary relaxations above 77°K have been observed in many crystalline polymers and the effects of orientation and crystallinity on the relaxation spectrum have been studied by various investigators. Their results are reviewed in the excellent monograph by McCrum, Williams, and Read. 1 The possibility that linear, crystalline polymers might show relaxation processes below 77°K has received less attention. Early investigators reported no relaxation processes below 77°K in polyethylene 2 and poly-(tetrafluoroethylene).3 However, with the use of more sensitive experimental techniques, very low-temperature relaxations have been observed in this laboratory in a number of linear polymers. Moreover these relaxations were found to be very sensitive to the changes in solid-state structure caused by annealing or deformation. Speculation concerning the mechanism responsible for these relaxation processes has been hampered by the lack of experimental information, and only recently has sufficient data become available to warrant a detailed consideration of the underlying mechanism. CRYOGENIC RELAXATIONS IN POLYMERSDynamic mechanical relaxations in polymers 378 in the temperature range 4.2 to 77°K have been reported by a number of authors. Previous workers however have been primarily concerned with polymers containing side groups, and relaxations observed below 100°K were attributed to motions of the pendent group. 2 -7 This assignment has been substantiated for the a-peak in polystyrene (PS) by correlation of mechanical, NMR, and dielectric measurements of polystyrene and a variety of halogenated and

Relaxation Behavior of Collagen

“…Synthetic polypeptides made from derivatives of lysine (28) and glutamic acid (29) are known to show mechanical loss peak at 260°-300°K (0.2-1 Hz). Copolymers of L-glutamic acid and L-leucine have loss peaks at about 2700K (1 cps) (30). Poly(y-benzyl-L-glutamate) exhibits (31) a decrease in second moment to a value near zero somewhat above 300°K; poly(sodium a-L-glutamate) containing traces of water shows a narrow line at temperatures as low as 260°K.…”

“…Dehl and Hoeve (16) reported that the reorientation of water molecules in oriented rat tail tendon can take place at temperatures as low as 220°K (104 Hz), and Hiltner and coworkers (30) reported small loss maxima at about 175°K (1 Hz) due to water in synthetic polypeptides. It is possible, therefore, that at least part of the drop in second moment in water containing-collagen and -gelatin around 200°K is due to reorientation of bound water molecules.…”