Relaxation in Polyethylene: Orientation of Lamellar Crystals

Abstract

In polymers with a lamellar morphology, crystallization in the presence of a temperature gradient can result in oriented lamellar crystals. Although the orientation is imperfect because of the helical [twisted] nature of the lamellae, shear stress applied by a torsion pendulum can be oriented with respect to the long lamellar dimension. Data of this type have been obtained to provide insight into the mechanical relaxation of polyethylene. The data assist the interpretation of experiments in which samples of the same density are obtained by different thermal histories.

Mechanical relaxation in polyethylene crystallized with various degrees of lamellar orientation

“…Concerning the two weaker second ary relaxations also observed, it can be further s tated that it appears reaso nable to associate the tail , observed both from the present work and from that of Eby and Colson [11], with the loss mechanism found by Illers on the low temperature side of the y relaxation peak. How-, ever, due to the as yet obscure nature of this process, no statement can be made with respect to possible orientational effects.…”

“…In addition, investigations by Eby and Colson [11] and by Flocke [12], using similar techniques, indicate analogous results for samples of polyethyle ne que nched from the melt and slowly cooled from the melt. However, in the latter two inves tigations no distinct secondary maxima, or shoulders, associated with the y process were observed, although the data of Eby and Colson indicate a definite tail on the low te mperature side of this peak, the tail being slightly hi gher for the que nch ed sample.…”

“…Inasmuch as th e oriented specimens show a lower Gil than the unoriented specimens in both the a and a' regions , it may be tentatively co ncluded that the relaxation is preferentially activated by shearing the (010), (010), and (100), or (010) and (001) faces of th e crystal. When our results are compar ed with those of Eby and Colso n [11] , it appears most likely that the relaxation is preferentially activated by shearing of th e (010) and (l00), or (010) and (001) faces of the crystal, but th e te nuou s nature of the co nclu sion is apparent.…”

“…In addition , Eb y a nd Colson [11] have s hown for sample s of polye th yle ne prepared with a n orie nte d surface layer a nd for the s am e sa mples with the oriented layer r e moved that the mecha ni c al r elaxation behavior in s he ar is me a s ura bly differ ent.…”