COUPLED RELAXATIONS IN A BLEND OF PMMA AND A POLYCARBONATE

Proton lineshape and proton spin-lattice relaxation, T-1 rho, measurem ents were made on blends of bisphenol chloral polycarbonate and perdeu tero poly(methyl methacrylate) (PMMA). The solution-cast blends are op tically clear and display a single thermal glass transition temperatur e, T-g, as reported. The proton lineshape and proton relaxation data a re used to follow the dynamics of the polycarbonate in the blend. The glass transition process is monitored by lineshape collapse as a funct ion of temperature and the sub-glass transition process associated wit h pi flips of the phenylene group is monitored by the proton T-1 rho v alues as a function of temperature. The glass transition as viewed fro m the polycarbonate remains similar in character to the behaviour obse rved in pure polycarbonate. Below T-g, the lineshape consists of a bro adened Fake doublet and above T-g a narrow Lorentzian grows in on top of the Fake pattern. The bimodal behaviour and the growth of the narro w line with temperature can be described by the Vogel-Tamman-Fulcher e quation with T-0 values which decrease with PMMA concentration. The pr oton T-1rho data show antiplasticization in the form of suppression of the pi flip process as PMMA is added. In addition, a new minimum asso ciated with the motion of the polycarbonate is observed at the tempera ture and time scale of the ester group rotation of the PMMA. This appa rent coupling of the sub-T-g relaxation processes is related to the be haviour of polycarbonate upon addition of low-molecular-weight ester d iluents. The level of antiplasticization and the amount of coupling is underestimated by a lattice model and an assumption of random mixing. On the very local scale of the pi flip motion and at the higher conce ntrations of PMMA, there are apparently fewer PMMA-polycarbonate conta cts than a random mixing assumption indicates.