Abstract
The behavior of the molecular motions of nine synthetic polybutadienes, two synthetic polyisoprenes, and an ethylene—propylene copolymer were studied by proton magnetic resonance absorption. The values of the line widths and the second moments were estimated as a function of temperature from −190°C. to room temperature for all samples except for the trans‐polybutadienes, for which measurements were made from −190 to 120°C. The transition temperatures determined from the inflection points of the line width versus temperature curves were compared with the transition temperatures detected from the volume versus temperature curves of dilatometric experiments for each sample. The difference between the value of the transition point obtained from the δ__H—T__ curve and that obtained from the V—T curve for each sample was not very large. The average transition temperature of cis‐polybutadienes was about −100°C., while that for cis‐polyisoprene was about −50°C. The great difference between the transition temperatures of two polymers is explained by the fact that the potential barrier for internal rotation around the carbon‐carbon single bond of cis‐polybutadiene is expected to be lower than that of cis‐polyisoprene. The second moment at liquid nitrogen temperature is found to be about 20 gauss^2^ for polybutadiene and about 18 gauss^2^ for the polyisoprenes; the value for polyisoprenes increases with descending temperature. From this fact the existance of another transition resulting from methyl group rotation is suggested.