fruitful discussions with S. M. Lindsay. References and Notesproperties of PDMS in the temperature range 223-323 K.' As shown in ref 11, this is barely into the temperature interval where any deviation from a simple Arrhenius behavior is detected and surely cannot be used to estimateTo. More recent attempts13 to determine To from viscosities produced even more outrageous results (To = 67 K).The very rapid change in relaxation frequency with temperature near 150 K demands that To be well above 81 K.The empirical parameters in eq 10 can also be interpreted in terms of the free volume of the liquid according to the WLF12 equation. PDMS was thought to have an unusually high fractional free volume at the glass transition.12 The WLF parameters calculated from our results are clg = 14.6 and c2g = 20 K. This leads to a calculated fractional free volume at the glass transition of f g / B = 0.030. This is higher than some polymers, but well within the normal range. The value of Tg listed in ref 12 is 150 K. It now appears that this value was obtained with semicrystalline PDMS. The value reported by Yim and St. Pierre' was obtained with shock-quenched amorphous PDMS. The difference can be rationalized by the results of Adachi et obtained by dielectric relaxation on both amorphous and crystalline PDMS. The dielectric loss maximum shifts by approximately 2 orders of magnitude when the sample crystallizes. This corresponds to approximately 4 K in the region of Tr The elevation of the glass transition temperature of the amorphous part in a semicrystalline polymer is a Yim, A.; St. Pierre, L. E.

The rheological properties of star-shaped polyisoprenes having a wide range of arm numbers and arm molecular weights are reported. In contrast to linear polymers, stars have a broad relaxation spectrum and a viscosity that increases exponentially with arm molecular weight. A comparison of eight pairs of samples having 3 and 4 arms and identical arm molecular weights showed that the viscosity of 3-arm stars is approximately 20% lower. For higher degrees of functionality, 4 < f < 33, the effect of functionality saturates and the viscosity is determined by arm molecular weight only. The nonlinear properties of one sample were studied using step-strain tests and found to be essentially identical to those of linear polymers. The predictions of a molecular theory for star polymers based on an extension of the reptation model are reviewed and shown to be in good agreement with the experimental data.

No abstract

No abstract

No abstract