Nuclear Magnetic Resonance Study of Transitions in Polymers

Abstract

Nuclear magnetic resonance absorption line widths have been studied for several high polymers. The proton line widths were studied in a field of 7000 gauss and at a frequency of about 30 mc. Line widths obtained for Hevea and GR-S were very narrow, indicating a large degree of ``quasi-free rotation'' in these materials. Vulcanization, carbon loading, co-polymerization, and crystallization produced a broadening of the lines as expected from the hindrance to internal motion introduced by these factors. Line width transitions as a function of temperature were observed for several polymers. Linear thermal expansion coefficients and specific heat vs temperature curves have breaks in the region of line transitions for the polymers studied. Swelling of polymers with benzene produced a narrowing of the absorption line throughout the line transition and lowered the transition temperature. Vulcanization broadened the transition range and shifted it to higher temperatures. Of two closely cut fractions of polystyrene, the larger molecular weight sample exhibited a line transition range at a higher temperature. The styrene monomer as well as the two fractions exhibited the same low temperature line width of about 8 gauss. The line width transitions were also studied for three butadiene styrene co-polymers at temperatures where anomalous behavior was found in dynamic measurements. The theoretical implications of these experimental results, together with a survey of other pertinent work, indicate the possibilities of the nuclear resonance method as applied to the study of high polymers in conjunction with other methods.