TRANSPORT-COEFFICIENTS OF ISOTACTIC OLIGO(METHYL METHACRYLATE)S AND POLY(METHYL METHACRYLATE)S IN DILUTE-SOLUTION

The intrinsic viscosity [eta] was determined for 25 samples of isotact ic oligo- and poly(methyl methacrylate)s (iPMMA), each with the fracti on of racemic diads f(r) similar or equal to 0.01, in the range of wei ght-average molecular weight M(w) from 3.58 x 10(2) (trimer) to 1.71 x 10(6) in acetonitrile at 28.0 degrees C (Theta). The translational di ffusion coefficient D was also determined from dynamic light scatterin g 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 [eta] is proportional to M(w)(1/2) for M(w) greater than or simi lar to 5 x 10(4) and its deviation from this asymptotic behavior is sm all even for smaller M(w), while D is inversely proportional to M(w)(1 /2) except for M(w) less than or similar to 2 x 10(3). Such apparent G aussian behavior of [eta] and D over a wide range of M(w) is the resul t expected from that previously obtained for the mean-square radius of gyration [S-2]. From an analysis of these transport coefficients on t he basis of the helical wormlike (HW) chain model, it is shown that th e above M(w) dependences of [eta] and D may be well explained by the H W theories with the values of the model parameters consistent with tho se previously determined from [S-2]. A comparison is also made of the present results for [r] and D for i-PMMA with the previous ones for at actic (a-) PMMA with f(r) = 0.79. This leads to the confirmation of th e previous conclusion derived from [S-2] concerning the f(r) dependenc e of the chain stiffness and local chain conformation of PMMA. That is , the iPMMA chain is of weaker helical nature than the a-PMMA chain th at retains rather large and clearly distinguishable helical portions i n dilute solution.