Segmental dynamics in poly(methylphenylsiloxane) networks by dielectric relaxation spectroscopy

The segmental dynamics of PMPS chains are determined in the bulk liquid and crosslinked networks of varying cross-link density by broad-band dielectri c relaxation spectroscopy. A large fragility index [tau(T-g*/T) dependence] , F-1/2 = 0.77, is found, independent of cross-link density, molecular weig ht, and T-g. Also independent of the above variables is the relaxation shap e (characterized by a KWW beta parameter of 0.45). Since these parameters q uantifying the alpha process in PMPS networks are insensitive to cross-link ing, it is concluded that the length scale of cooperatively rearranging dom ains in PMPS networks is smaller than the distance between cross-links, i.e ., less than 5 nm, in agreement with the current consensus. An argument bas ed on the temperature dependence of the dielectric relaxation strength, dip ole moment, and ensemble average chain configuration was advanced, suggesti ng that the high fragility of PMPS has an intramolecular origin.