A Carbon-13 NMR Study of Molecular Motion of Polystyrene in Solution

Abstract

ABSTRACT:The molecular motion of polystyrene in solution has been investigated quantitatively using carbon-13 NMR. It is concluded that the motion can be characterized by three contributions, namely, segmental motion and two kinds of internal rotation of the phenyl ring, and that the phenyl ring is rotating more freely above 50°C. Free rotation of the phenyl ring is hindered by a energy barrier, which is probably due to a steric hindrance between the hydrogens in the phenyl ring and the methine hydrogen in the adjacent monomer unit. This model is consistent with the fact that a transition of polystyrene at about 50°C occurs both in solutions and in bulk. KEY WORDSPolystyrene / 13 C NMR / Molecular Motion / Internal Rotation / Transition / Spin-Lattice Relaxation / It has been generally accepted that solutions of isotactic and atactic polystyrene show an anomalous temperature dependence in solution properties at about 50 and S0°c. 1 -5 A transition was observed at about 80°C for isotactic and atactic polystyrene in several solvents in the temperature dependence of the second virial coefficient, in the mean radius of gyration, and in other properties. This transition is firstorder, 6-8 and Reiss 6 • 7 has proposed that it 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 in isotactic and atactic polystyrene both in solution and in bulk has been reported. 1 As 50°C is well below the glass-transition temperature (ca. l05°C), it was thought that the major molecular process can not be a conformational change but an intramolecular change of more local molecular motions. It was proposed that rotation of the phenyl ring may be hindered by the a and /3 main-chain hydrogens, and that the transition at about 50°C may be due to a sudden change in the mobility of the phenyl ring. 9 However, the molecular motion has not been heretofore studied quantitatively; we have used 13 C NMR to elucidate the molecular process occurring in this transition.Although physical properties of polymer solutions have been studied with proton NMR, data are often rather complex to analyze because of a narrow range of chemical shifts (ca. IO ppm) and of splitting of peaks due to spin-spin coupling. Proton-decoupled natural abundance 13 C NMR spectra are easier to analyze because of a wide range of chemical shifts (ca. 250 ppm) and the absence of complex spin-spin coupling. Partially relaxed Fourier transformed (PRFT) spectra permit measurement of the 13 C spinlattice relaxation times for all resolved peaks of complex molecules. It is of interest to measure the 13 C spin-lattice relaxation times of individual carbons in molecules, as the molecular motion of individual groups can be estimated quantitatively. Proton magnetic relaxation times .are difficult to analyze quantitatively in terms of molecular motion because of the difficulty of separating intramolecular and intermolecular contributions. The latter contribution to 13 C spin-lattice relaxatio...

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

“…At the lower temperatures the internal rotation of the pyridine rings may be restricted by steric hindrance, which is probably due to a repulsive interaction between the hydrogen atoms of the methine group in the backbone and that bonded to the C(3) carbon in the pyridine ring, 6 and by the formation of hydrogen bonding between the Natom in one pyridine ring and C(6)-H in another pyridine ring or between the N-atoms in the pyridine ring and chloroform. So the pyridine ring can not rotate freely at the lower temperatures.…”

“…The a motion of the pyridine ring which has the magnetic anisotropy is expected to influence the temperature dependence of the chemical shift. 6 However this influence does not appear as a break, but as a linearity in the temperature dependence of the chemical shift, as shown in Figure 5. This may be due to the fact that the a motion at ,lower temperatures makes the anisotropy of the pyridine ring average out to some extent and the modal change of the a motion at the transition does not influence the temperature dependence of the chemical shift in a step wise way.…”

“…p= ,(I) ( 6) As in the case of polystyrene, the internal rotation of poly(2-vinylpyridine) can be divided into two motions: the first is the rotation of the pyridine ring around the C(a)-C(2) axis (we call this the a motion) and the second is the precession of the C(a)-C(2) in a plane perpendicular to the main chain (we call this the pmotion). From this model the T1 value of C (5) (which is on the axis of the a motion) is smaller than those of C(3), C(4), and C(6), and larger than that of C(a).…”

“…In Figure 5 is shown the temperature dependence of the chemical shift of poly(2-vinylpyridine) in C6D 6 and CDC13• The values of chemical shift are expressed in ppm with respect to the C(2) carbon peak and a positive value corresponds to the upper field. These dependences are linear for both solutions.…”

“…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.…”