The effects of drawing polyethylene in the solid state on crystallographic, morphological and mechanical properties

The major purpose of this experimental research was to investigate the effects of drawing condition on the microstructure of high density polyethylene (HDPE) and its influence on the mechanical properties. A number of experiments on the mechanical properties of low density polyethylene (LDPE) were also carried out.

The microstructures of the samples were studied by a range of experimental techniques, including wide angle and small angle x-ray diffraction, scanning electron microscopy, thermal analysis and dynamic mechanical measurements. The macroscopic mechanical properties were also investigated.

The Warren-Averback Fourier analysis and the Guinier-Warren-Hoseman paracrystal analysis were used to determine crystal sizes and their perfection in a series of drawn filaments.

The polyethylene filaments were drawn both by a one-stage draw (100 percent/min) and by a two-stage method which utilizes 100 percent/min draw rate in the first stage and a 1 percent/min draw rate in the second stage (superdraw). The structural differences resulting from drawing by both methods and their relationship to the mechanical properties were studied.

The superdrawn HDPE samples showed a great increase of modulus with draw ratio (10 GPa for natural drawn, 25 GPa for superdrawn). The density, crystallinity and melting temperature measurements also increased with draw ratio. These changes are interpreted as an increase of order in the structure of HDPE filaments with increasing deformation. An increase in drawing temperature results in a decrease of moduli at high draw ratios above the optimum temperature which corresponds to that of the α-transition.

A comparison of HDPE samples drawn by the one-stage and two-stage methods showed a rate effect in the one-stage method above 22X beyond which the increase of macroproperties of samples with draw ratio were lessened (i..e., modulus, crystallinity melting temperature).

The viscoelastic properties of HDPE samples showed a remarkably high storage modulus at high draw ratios (87 GPa). The distinction between the effects of one-stage and two-stage drawing methods on the properties of the samples is apparent in the dynamic mechanical measurements, although not readily discernible in the Young's modulus.

The SEM studies on strained superdrawn samples showed lateral cracks for which the number average size decreased with draw ratio.

X-ray integrated breadth studies of HDPE filaments showed that drawing results in an increase of weight average crystal sizes in the radial and also in the fiber direction. The Fourier analysis of these x-ray data also showed an increase in the number average crystal size in the radial direction with draw ratio. In addition, the root mean square strain in the samples was reduced; correspondingly, the a,b unit cell dimensions were decreased with draw ratio.

The application of paracrystallinity theory to the WAXS patterns showed that the number average size of microfibrils was not affected by deformation. On the other hand, the number average size of fibril clusters increased. While the microfibril perfection increased, the interfibrillar and intracluster paracrystalline domains originating from disordered amorphous regions showed a decrease of perfection due to the increase of sizes of paracrystalline domains.

The above findings indicate that the optimum drawing temperature for LDPE and HDPE is directly related to the chain mobility within the crystallites (α-transition). The HDPE samples showed that at the α-transition temperature, the "friction" between and within the microfibrils resulted in unfolding and orientation of chains in the draw direction. However, this friction decreased as the drawing temperature increased and was greatly reduced at ll0°C. At this temperature, the refolding process dominates in the structure, resulting in a lowering of modulus values. Higher draw rates above the draw ratio of 22 resulted in greater crystal imperfections.

The HDPE samples drawn by the two-stage method show an increase of perfection and size with draw ratio. The crystallites comprise the main backbones of the drawn polymer, thus an increase in size and perfection results in improved mechanical properties. The results of the Fourier analysis and the Guinier-Warren-Hosemann analysis are found to be complementary in supporting these interpretations.