Dielectric probe of intermolecular interactions in poly(methyl methacrylate) (PMMA) and PMMA+SiO2 matrixes doped with luminescent organics

Modifications in the molecular dynamics of the side-chain relaxation in pol y(methyl methacrylate) (PMMA) have been recorded by monitoring the thermall y stimulated depolarization currents (TSDC) beta -relaxation band of PMMA i n three systems: bulk PMMA, PMMA polymerized in situ in porous SiO2, and la sing matrixes with rhodamine 6G/Cl- (R6G) and a perylene derivative (PG). T o study the different types of inter- and intramolecular interactions, a ra nge of dye concentrations and different polymerization initiators have been tested. In PMMA + SiO2, the low-temperature (LT) shift of the beta -band a nd the increase of the energy barriers (W) associated with the side-chain ( re)orientation are attributed to various counterbalancing effects. A scheme of extensive hydrogen-bond interactions (surface or chemical effect) betwe en the ester carbonyls of PMMA and the silicic acid pore surface is used to explain the overall increase in the distributed energy barriers. In contra st, the reduction of the polymer's chain entanglements and the increase in the free volume (structural or physical effect), due to the pore-directed p olymerization, are considered to loosen up several steric hindrances on the rotational motion. The modification of the relaxation times spectrum promp ts the shift of the beta -relaxation in PMMA + SiO2. In R6G + PMMA, the dra stic LT shift of the beta -band indicates the partial coupling between the chromophores and the side-group rotations. The TSDC spectrum does not prese nt a rotational relaxation of the polar rhodamine. dye. In R6G + PMMA + SiO 2, the presence of the chromophores on the pore surfaces and the correspond ing decrease of the "effective" average pore diameters available for MMA di ffusion and PMMA growth balance the physical and chemical effects, as depic ted in the similarity of the energy distributions. The speedup of the beta -relaxation with increasing dye content is ascribed to the reduced exposure of the side groups to chemical effects. The positive energy shift in PG PMMA + SiO2 can be explained by considering the mixing of PG and PMMA, whic h permits strong hydrogen-bonding interactions and simultaneously reduces P MMA's free volume. TSDC signals around room temperature (RT) are tentativel y discussed in terms of a Maxwell-Wagner-Sillars (MWS) polarization mode at the PMMA-SiO2 interface.