A SPIN-LATTICE RELAXATION STUDY OF DISSOLVED CYCLOHEXYL POLYCARBONATE

A dilute-solution spin-lattice relaxation time study was performed on a bisphenol polycarbonate related to the polycarbonate of bisphenol A except the two methyl, isopropylidene unit is replaced by a cyclohexyl group. C-13 spin-lattice relaxation times were measured at three Larm or frequencies: 50.3, 75.4, 125.7 MHz. The motion of cyclohexyl ring i s seen to be isotropic on the spin-lattice relaxation time scale even though cyclohexyl rings undergo slow conformational change. Cyclohexyl ring relaxation is caused by segmental motion and was well interprete d in terms of the Hall-Helfand correlation function. The apparent acti vation energies for cooperative and individual bond transitions were 1 7 and 22 kJ mol(-1), while the corresponding Arrhenius prefactors were 15 x 10(-13) and 5 x 10(-13) s. This cyclohexyl polycarbonate differs from many other polycarbonates in that the two phenylene groups are i nequivalent, one being axial and the other equatorial relative to the cyclohexyl ring. This difference could be clearly seen in the low temp erature C-13 spectrum at a Larmor frequency of 125.7 MHz. In addition to segmental motion, it was found the equatorial phenyl ring underwent anisotropic internal rotation which could be described by the Woessne r model, while the axial phenyl ring underwent restricted rotational d iffusion which could be described by the Gronski model. Since full ani sotropic rotation was observed in bisphenol A polycarbonate and restri cted anisotropic rotational diffusion was observed in norbornyl polyca rbonate, a clear picture of the effects of substitution in the isoprop ylidene units on local dynamics is developing. Copyright (C) 1996 Else vier Science Ltd.