X-Ray Studies of Low-Temperature Polybutadiene and Butadiene-Styrene Copolymers

Abstract

Although it is now generally recognized that the temperature of polymerization affects profoundly the properties of emulsion elastomers, there is very little evidence available pertaining to the cause of the variations of properties. It is felt by some that the improved properties of low-temperature elastomers can be related to variations in molecular weight and molecular-weight distribution. In this laboratory, however, the opinion has prevailed that the lower emulsion polymerization temperatures appreciably alter the fine structure of the molecules with an increase in the regularity of the polymer chains. If there were actually less branching and cross-linking in low-temperature polymers, and less 1,2-addition to monomer components, the increased order should be evident from x-ray diffraction patterns. To provide information on the above questions, x-ray studies were made with four purposes in view: (1) to determine the effect of polymerization temperature on the crystallization properties of unstretched and stretched polybutadienes; (2) to determine the influence of styrene content on the crystallization of butadiene-styrene copolymers; (3) to study some effects of compounding and vulcanization on crystallizable polybutadiene; and (4) to use the preferred orientation patterns obtained from some of these polymers for structural evaluations. To accomplish these objectives, x-ray patterns were obtained at several temperatures of some unstretched and stretched polybutadiene polymers, butadiene-styrene copolymers, and a vulcanized and compounded polybutadiene. The polybutadienes were prepared by emulsion polymerizations at 55°, 40°, 30°, 20°, 5°, −10° and −20° C. Since the −20° C polybutadiene showed the most marked crystallization patterns, the effects of compounding and of styrene addition were studied, using polymers prepared at this temperature for comparison. Three butadiene-styrene copolymers containing, respectively, 10, 20, and 30 per cent styrene in the monomer charge and one vulcanized polybutadiene compounded with Wyex carbon black were studied.

Approaches to Polymer Compounding

“…These results support the belief " ASTM D 813-57T modified, flexures to 1-inch break. 6 Freezing point by temperatureretraction procedure.…”

“…In both series, 5% solutions of 50 to 50 monomer Individual copolymers exhibited essentially no tensile strength after this treatment. 6 Individual copolymers had tensile strengths of 100 to 200 p.s.i. after this treatment.…”

“…for 41 hours in forced draft oven. 6 Solvent evaporated at 20°to 25°C. for 15 hours, heated in a forced draft oven for 8 hours at 60°C., and finally in a vacuum oven for 63 hours at 60°C.…”

“…At high zinc oxide contents the effect of the cross linking with zinc oxide might overshadow the attrac- " ASTM D 813-57T modified, flexures to 1-inch break. 6 Freezing point by temperature-retraction procedure. = 20-minute cure.…”

“…Early workers (6) reported that the dehydration and decomposition of aluminum nitrate crystals could be broken down into several independent steps. Limited thermodynamic data reported by Kelly (4) indicate that the reaction proceeds to completion at temperatures above 425°K.…”