Some effects of monomer sequence on the viscoelastic behavior of random copolymers of butadiene and styrene
Authors: Kraus G. , Rollmann K. W. 
Published: April 2003

The copolymerization of butadiene and styrene by lithium alkyls can be regulated to give either random or block copolymers. The block copolymers exhibit characteristic mechanical behavior which is attributable to their two‐phase domain structure. In random copolymers free of long sequences of styrene there exists, nevertheless, the possibility of varying the sequence distribution by changing the manner in which composition varies along the polymer chain. Since copolymers of butadiene and styrene differing sufficiently in composition are likewise incompatible and will form multi‐phase systems, it is likely that microheterogeneity can exist in certain “random” copolymers.

Five copolymers of monomer ratio 70 : 30 butadiene/styrene, varying from a uniformly randomized sample, in which composition was very nearly independent of conversion, to a block polymer containing 22% block styrene chemical analysis, were prepared for the present investigation. Composition vs. conversion data indicated that all but the last polymer were free of long styrene sequences, with the composition distribution (along the chain) broadening systematically throughout the remainder of the series.

The melt viscosity of the unvulcanized copolymers was distinctly affected by sequence distribution effects. Thus, the temperature coefficient of the apparent viscosity was independent of shear stress only for the uniformly randomized copolymer. In all others temperature superposition of the non‐NEWTONian flow curves was impossible, the discrepancies becoming larger the broader the composition distribution. The results can be explained qualitatively by association effects attributable to a domain structure similar to that found in block polymers. When these copolymers were cross‐linked with dicumyl peroxide at 153 °C and the dynamic properties of the networks examined, no clear evidence of a domain structure was found except in the block polymer. Only the latter exhibited more than a single loss maximum. Temperature‐frequency reduction of the dynamic measurements was successful with all but the block polymer. Whereas the parameters C1and C2in the WILLIAMS‐LANDEL‐FERRYequation appear to change systematically with the degree of randomness, there is evidence that this is attributable to a slight systematic drift toward higher vinyl unsaturation with increasing randomization of the monomer sequence. Relaxation spectra calculated for 25 °C were very nearly the same for all four random copolymers. When the polymers were cross‐linked by gamma radiation at room temperature, the resulting networks did show properties indicative of a domain structure in the compositionally more heterogeneous copolymers. It is proposed that compatibility of chain segments of varying composition at the temperature of cross‐linking leads to a suppression of the domain structure in the peroxide‐cured rubbers, as segments of different composition are joined together. Independent evidence from stress‐optical measurements supports this interpretation.

The present investigation permits the conclusion that differences in sequence distribution of butadiene‐styrene copolymers have, at best, only very minor effects on the visco‐elastic properties of conventional vulcanizates, provided the polymers contain no long sequences of styrene units, i.e., polystyrene blocks detectable by classical methods. This is not true of the low shear melt viscosity, which senses relatively small differences in the composition and/or sequence distributions of the uncured rubbers.