Transport Coefficients of Isotactic Oligo- and Poly(methyl methacrylate)s in Dilute Solution

Abstract

The intrinsic viscosity [η] was determined for 25 samples of isotactic oligo- and poly(methyl methacrylate)s (i-PMMA), each with the fraction of racemic diads f r ≃ 0.01, in the range of weight-average molecular weight M w from 3.58 x 10 2 (trimer) to 1.71 x 10 6 in acetonitrile at 28.0°C (Θ). The translational diffusion coefficient D was also determined from dynamic light scattering measurements for 12 of them in the range of M w from 6.58 x 10 2 (hexamer) to 9.46 x 10 5 under the same solvent condition. It is found that [η] is proportional to M w 1/2 for M w ≥ 5 x 10 4 and its deviation from this asymptotic behavior is small even for smaller M w , while D is inversely proportional to M w 1/2 except for M w ≤ 2 x 10 3 . Such apparent Gaussian behavior of [η] and D over a wide range of M w is the result expected from that previously obtained for the mean-square radius of gyration (S 2 ). From an analysis of these transport coefficients on the basis of the helical wormlike (HW) chain model, it is shown that the above M w dependences of [η] and D may be well explained by the HW theories with the values of the model parameters consistent with those previously determined from (S 2 ). A comparison is also made of the present results for [η] and D for i-PMMA with the previous ones for atactic (a-) PMMA with f r = 0.79. This leads to the confirmation of the previous conclusion derived from (S 2 ) concerning the f r dependence of the chain stiffness and local chain conformation of PMMA. That is, the i-PMMA chain is of weaker helical nature than the chain that retains rather large and clearly distinguishable helical portions in dilute solution.