Surface replicas of drawn polyethylene. II. Annealing of cold-drawn samples with different draw ratio

Abstract

Quenched linear polyethylene was drawn at 60[ddot]C to draw ratios between 9 and 22. The samples were annealed at 120, 122, and 130[ddot]C for between 90 and 120 min and subsequently etched in fuming nitric acid at 80[ddot]C for different amounts of time. The surface replicas obtained by a shadow-transfer technique show a strong dependence of annealing effects on draw ratio. The annealed drawn material is fibrillar, each fibril consisting of a stack of lamellae initially oriented perpendicular to the draw direction. The higher the draw ratio the more the sample resists lamella rotation during annealing. Variation of draw ratio in adjacent fibrillar regions of the same sample produces an inhomogeneous surface structure. Unchanged areas alternate with regions exhibiting a high degree of lamella rotation about an axis situated in the surface plane and perpendicular to the draw direction. With further annealing the fibrils originally parallel to the draw direction may locally markedly change their orientation and even become perpendicular to draw. At the same time the rotated lamellae grow beyond the fibril boundaries so that adjacent fibrils coalesce to larger entities. Rotation of the lamellae also produces a rather irregular superstructure with a “period” fluctuating irregularly between 1000 and 4000 Å. Lamellar rotation during annealing is most likely the consequence of tie molecules which in every single fibril must be so arranged that in spite of a high degree of randomness a nonvanishing torque results which becomes operational at sufficiently high temperature and chain mobility. During such a rotation the adjacent lamellae slide over each other. High draw ratio produces a very rough surface with partially interpenetrating blocks of folded chains so that surface gliding is hampered and hence annealing with crystal rotation is shifted to higher temperatures. The heating and cooling rate significantly affects the annealing. At a slow rate the amorphous material resulting from melting the less stable crystals has ample time for recrystallization, so that the sample starts annealing with relatively high crystallinity with very little material left over for crystallization during final cooling to room temperature. A high rate, however, produces a larger amount of melted material which crystallizes only during the final cooling, yielding crystals with a small thickness. In the first paper of this series [1], electron micrographs of surface replicas of highly drawn PE films as drawn and after annealing, with and without fuming nitric acid treatment, were reported. In this paper we intend to report annealing effects on films with different draw ratio. The high-density PE used was a nonfractionated Fortiflex A60/500 sample (trademark of Celanese Corporation). The molecules are very nearly unbranched, with M_n = 6000 and M_w = 85,000. The received pellets were compression-molded at 150[ddot]C into a 0·5-mm-thick film and quenched in ice water. Strips of 5 mm width were cut and drawn at 60[ddot]C in a constant-temperature water bath. The draw rate was 0·5 cm/mm. The draw ratio varied between 9 and 22. At 60[ddot]C annealing effects do not yet matter, so that we have to do with cold drawing [2]. The drawn samples were annealed with free ends so that they could shrink during annealing. For annealing they were heated in a vacuum oven at a rate of 10[ddot]C/hr to the annealing temperature of 120 or 130[ddot]C, left at this temperature between 90 mm and 2 hr, and again cooled to room temperature at a rate of 10[ddot]C/hr. The samples annealed at 122[ddot]C were rapidly heated to this temperature and after annealing rapidly cooled to room temperature. The annealed samples were treated with fuming nitric acid (FNA) at 80[ddot]C for different amounts of time and washed with water and acetone. The dry sample was shadowed by Pt carbon at about 30[ddot] and covered with a carbon film (evaporating at 90[ddot]) in high vacuum. Finally the PE was dissolved by exposure to vapor of boiling xylene.