Thickness dependence of elastic modulus of polycarbonate interphase

The oligomer of bis-phenol A (oligo-PC) with M-W = 1300 and bis-phenol A polycarbonate (PC) with M-W = 20000 were deposited onto E-glass surface using SiCl4 as the grafting and cross-linking agent. Thickness of the deposited layers was varied from 30 to 10(6) nm and the layers were investigated as prepared and after thermal annealing at 245 degrees C for 10 min in the air. Vibrational piezoelectric resonator technique and the speed of Rayleigh wave measurement were used to determine elastic moduli of the ultra thin layers deposited on flat E-glass substrate as a function of their thickness. In all cases, increase of the Young modulus of the interphase, E-i, with decreasing layer thickness, t(i), was observed. At a given thickness, the E-i of PC layer was significantly lower than that for the oligo-PC layer. Thermal annealing of the deposited PC layer resulted in a significant increase of its E-i compared to the as received layer. No significant change was observed for oligo-PC inter-phases. Increase of the shear strength of the interface, tau(a), with reducing interphase thickness, t(i), was observed. The observed increase of E-i with the decreasing t(i) was ascribed to the reduction of the molecular mobility of chains near solid surface compared to their mobility in the bulk. Most probably, the observed increase of E-i after thermal annealing of PC was caused by rearrangement of both segment density distribution in individual PC coils near the solid surface and cooperative rearrangements of multiple PC chains. Since the oligomers attached to the surface attained presumably more regular extended conformations with lower conformation entropy compared to the PC random coils, the effect of thermal annealing was negligible. In agreement with theoretical predictions, increase of E-i at the same extent of interfacial interactions resulted in the observed increase of the to measured using the single embedded fiber test.