Terminal and segmental relaxations in epoxidized polyisoprene

Abstract

The terminal and segmental dispersions in the dynamic mechanical spectra were measured for l,4-polyisoprene (PIP) epoxidized to various levels. A systematic increase in the plateau modulus, arising from the transient entanglement network, is observed upon epoxidation. This increase is at odds with published correlations of the effect of chemical structure on chain entanglements. The segmental relaxation dispersion in the epoxidized polymers is broader and more sensitive to temperature than in the unmodified PIP, indicating enhanced intermolecular coupling of the segmental dynamics. The greater intermolecular cooperativity may reflect steric interactions promoted by the oxirane group. Dynamic mechanical data in the terminal zone were obtained with a Bohlin VOR rheometer using a parallel-plate geometry. Sample radii of 25 and 40 mm were used, with gaps in the range of 1.5-2 mm. The dynamic shear modulus was measured from 60 Hz down to as low as 1 X lo-& Hz at temperatures between 25 and 30 C. The broad range of experimental frequencies (6.3 x 10-6 to 380 rad/@ obviated the need for time-temperature superpositioning. Mechanical spectra in the vicinity of the glass transition were measured on strips (typically 24 X 12 X 1.5 mm) in tension using an Imass Co. Dynastat Mark I1 mechanical spectrometer. The Dynastat has a frequency range of 0.01-100 Hz. For the segmental relaxation measurements, the temperature was adjusted according to the TC of each polymer. Temperature control was usually better than kO.1 OC.