DYNAMIC DEPOLARIZED LIGHT-SCATTERING AND NUCLEAR MAGNETIC-RELAXATION STUDIES OF ISOTACTIC OLIGO(METHYL METHACRYLATE)S AND POLY(METHYL METHACRYLATE)S IN DILUTE-SOLUTION

The (excess) power spectrum J(Gamma) of the depolarized component of s cattered light intensity was measured for eight samples of isotactic o ligo- and poly(methyl methacrylate)s (i-PMMA), each with the fraction of racemic diads f(r) similar or equal to 0.01, in the range of weight -average degree of polymerization x(w) from 4 to 70.1 in acetonitrile at 28.0 degrees C (Theta). The spin-lattice relaxation time T-1 was al so determined for the three samples with x(w) = 4, 5, and 70.1, and th e nuclear Overhauser enhancement NOE, for the two samples with x(w) = 4 and 70.1, all in acetonitrile at 35 degrees C. As in the cases of at actic (a-) polystyrene (a-PS) and a-PMMA previously studied, it is fou nd that J(Gamma) may be well represented in terms of a single Lorentzi an independently of x(w) and that the relaxation time sr defined from J(Gamma) at infinite dilution increases with increasing x(w) and level s off to its asymptotic value in the limit of x(w) --> infinity, being consistent with the recent theoretical prediction on the basis of the helical wormlike (HW) chain model. A comparison is made of the presen t data for tau(Gamma), T-1, and NOE with the HW theory, and it is show n that the theory may explain satisfactorily the data in the range of x(w) greater than or similar to 10, although semiquantitatively for ta u(Gamma). For x(w) less than or similar to 10, the rigid sphere model having the radius equal to the apparent root-mean-square radius of gyr ation of the HW chain may give a good explanation of tau(Gamma) but no t of T-1, indicating that the dynamic depolarized light scattering and nuclear magnetic relaxation cannot be described in terms of a common single relaxation time. However, there is shown to be an effective (me an) magnetic relaxation time tau(M) approximately equal to 0.6 tau(Gam ma). From a comparison of the present results for tau(Gamma) for i-PMM A with the previous ones for a-PS and a-PMMA, it is shown that there i s good correlation between the static and dynamic chain stiffness, the latter being defined as the ratio of the value of tau(Gamma) in the l imit of x(w) --> infinity to that of the corresponding isolated repeat unit (monomer), as predicted by the HW theory.