VISCOELASTIC PROPERTIES OF AMORPHOUS POLYMERS .5. A COUPLING MODEL ANALYSIS OF THE THERMORHEOLOGICAL COMPLEXITY OF POLYISOBUTYLENE IN THE CLASS-RUBBER SOFTENING DISPERSION

Isothermal data of high molecular weight polyisobutylene obtained by m echanical measurements with a spectral range over eight decades and ad ditional photon correlation measurements have found that there are thr ee distinct viscoelastic mechanisms in the glass-rubber transition zon e. Theoretical considerations have helped to identify these three mech anisms to originate separately from local segmental (alpha) modes, sub -Rouse (sR) modes, and Rouse (R) modes. The temperature dependences of the shift factors of these mechanisms, a(T,alpha), a(T,sR) and a(T,R) , determined over a common temperature range are found to be all diffe rent. The differences in temperature dependences are explained quantit atively by the coupling model. The local segmental motion contributes to compliances ranging from the glassy compliance, J(g), up to 10(-8.5 ) Pa-1. The sub-Rouse modes contribute in the compliance range, 10(-8. 5) less than or equal to J(t) less than or equal to 10(-7) Pa-1. The R ouse modes account for the compliances in the range of 10(-7) Pa-1 les s than or equal to J(t) less than or equal to J(plateau), where J(plat eau) is the plateau compliance. The magnitudes of the bounds given her e are only rough estimates. Shift factors, a(T), obtained by time-temp erature superpositioning of viscoelastic data taken in the softening t ransition over a limited experimental window are shown to be a combina tion of the three individual shifts factors, a(T,alpha), a(T,sR), and a(T,R). Consequently, care must be exercised in interpreting or using the WLF equation that fits the shift factors of the entire softening d ispersion, because the latter do not describe the temperature dependen ce of any one of the three viscoelastic mechanisms. (C) 1997 John Wile y & Sons, Inc.