A Study of the Thermal Transition of Poly(2-vinylpyridine) in Solution by Carbon-13 NMR

Abstract

ABSTRACT:The variation, as a function of temperatute, of the carbon-13 spinlattice relaxation time of individual carbons of poly(2-vinylpyridine) in solution was measured and the molecular motion was analysed quantitatively. The motion of the polymer can be characterized by three contributions: namely, a segmental motion and two kinds of internal rotation of the pyridine ring. The temperature dependence of the rate of the internal rotation revealed a transitional change at about 25°C, while that of the segmental motion changed linearly. It is suggested that the internal rotation of the pyridine ring is interrupted by steric hindrance, which is probably due to a repulsive interaction between the hydrogen atoms in the pyridine ring and the methine group and by a formation of hydrogen bonding at lower temperatures. Above the transition temperature, the thermally activated motion overcomes such hindrances and the pyridine ring rotates more freely.KEY WORDS Poly(2-vinylpyridine) / 13 C-NMR / Spin-Lattice Relaxation / Molecular Motion / Internal Rotation / Transition / Recently much interest has been devoted by several investigators to transitions observed in the temperature dependence of the conformation of a macromolecular chain. It has been generally accepted that solutions of polystyrene show an anomalous temperature dependence in solution properties at about 50 and 80°c. 1 -3 A transition was observed at 80°C for both atactic and isotactic polystyrenes in several solvents in the temperature dependence of the second virial coefficient, in the mean radius of gyration, and so on. Reiss 4 ' 5 has proposed that this arises from the transition between a 31 helix and a random coil in the isotactic sequences of both polymers. On the other hand, a transition at about 50°C for isotactic and atactic polystyrenes both in solution and bulk has been reported. 1 In a previous paper, 6 this phenomenon was investigated by using carbon-13 nuclear-magneticresonance spectroscopy (NMR) and it was concluded that this transition is due to the sudden modal change in the mobility of the phenyl ring.Several properties, such as viscosity7 and optical density,8 of poly(2-vinylpyridine) in solution have also been investigated, and a transition was observed at about 25°C in several solvents in the temperature dependence of these properties. Chachaty, et al.,9 have investigated the temperature dependence of the carbon-13 spin-lattice relaxation time of backbone methine carbon of isotactic poly(2-vinylpyridine); but they found no anomaly because the temperature range of their measurement (30-80°C) was above the transition temperature (25°C), and they observed only linearity in the temperature dependence of the correlation time. The process of the transition is different according to the kind of solvents. A break point has been observed in the temperature dependence of the intrinsic viscosity of poly(2-vinylpyridine) in benzene and tetrahydrofuran solution, but no break point has been observed in chloroform. 8 In this paper we will present the r...