Evolution of Intercrystallite “Bridges” During Multi-Step Drawing of Gel-Spun Polyethylene Fibers

Abstract

In 1978, Gibson et al. [1] observed a paradoxical x-ray scattering pattern in oriented ultra-high-molecularweight polyethylene (UHMWPE) that indicated that the crystallite thickness in the fiber exceeded significantly the long period. The researchers proposed a bridging model that assumed the presence of a large number of straightened connecting molecules (SCM) joining like bridges successive crystallites along the fiber axis in order to explain this obvious dimensional disparity. Subsequently, this model was confirmed in other x-ray investigations [2-5] and independently from results of low-frequency Raman spectroscopy [6-8]. The bridging model was used successfully to interpret several mechanical properties of highly oriented UHMWPE fibers, in particular, to explain the behavior of Young’s modulus during various sample-preparation procedures and various test conditions [9 and references therein]. The model turned out to be useful also for understanding the solid-state extrusion process [9]. Nevertheless, little reliable information about the formation of intercrystallite bridges in drawn fibers is available in terms of a general concept of the oriented state. The goal of the present work was to study the formation of SCM-bridges in gel-spun UHMWPE fibers. Hot drawing of fibers from a spinning solution is usually carried out in several stages in industry. Each stage has a different special structural organization of drawn fibers that can be characterized using instrumental methods. Herein we present results from a study of the formation of intercrystallite bridges in fibers drawn at the pilot plant (SRISF, Tver´). Fibers that were obtained at various stages of oriented drawing were studied using low-frequency Raman spectroscopy and wide-angle x-ray scattering. Samples were prepared from UHMWPE reactor powder with crystallinity degree 64% (according to differential scanning calorimetry) and molecular weight 2.7"10 6 g/mol (according to viscosity measurements). The reactor powder and drawing temperature—time regime were chosen based on previous results [10, 11]. The drawing process included the following successive stages. Reactor powder was dissolved with stirring in mineral oil at 172°C. The resulting homogeneous solution of concentration 3 mass% was forced at the same temperature by a dispensing six-channel pump through six spinnerettes with 48 openings of diameter 0.5 mm each. The ratio of capillary length to diameter was 5. The solution from the spinnerette head was fed into a cooling bath filled with mineral oil at 43.0 ± 1.5°C. The fluid lost its flowability and converted into a gel as a result of phase separation during such rapid cooling. The spun gel-threads were continuously transferred from the cooling bath to a mineral-oil bath at 70°C. Then, the threads were freely placed in a container where they were shrunk to the equilibrium state.