Configurational studies of asymmetric star polymers

Abstract

A series of miktoarm star polybutadienes were investigated in the melt and dilute solution state by neutron and light scattering and viscometry techniques. The stars had 8-arms, with a single asymmetric arm of varying molecular weight. Living anionic polymerisation synthesised the arms to be coupled by chlorosilane core molecules, with the molecular weight of the hydrogenous arms being 3 x 10(^4) g mol(^-1) and increasing for the deuterated arm from 3 x 10(^4) g mol(^-1) to 3 x 10(^5) g mol(^-1). Global properties of Mw, RG, A(_2), D(_0) and [η] were ascertained for these stars in the good solvent of cyclohexane, and over a temperature range in the 6-solvent of 1,4-dioxane. Branching ratios calculated from the cyclohexane data indicated that increasing the asymmetric arm length yielded values similar to a linear polymer of equivalent molecular weight, with a reduction of segment density near the star core. This was seen more clearly by calculating the size ratios of Rt/Rg, RhIRg and RylRc from the equivalent sphere radii. Aggregation of the stars was found with the dioxane solutions, as the Mw of these stars increased with decreasing temperature to values higher than obtained in cyclohexane, where they remained constant and equal to the good solvent data for the linear polymers. Small-angle neutron scattering was used to determine the Rg and interaction parameters for the asymmetric arm in the melt and solution states. The increase of Rg in the order of unperturbed linear equivalent<melt<cyclohexane was due to the presence of the branch point and the excluded volume effect. Higher Rg values obtained in the 9-solvent suggested aggregation. An increase in temperature in the melt state was found to promote inter-star mixing of the asymmetric arms and this was greater for shorter arms. In cyclohexane, the intra-star interaction parameters were found to decrease with increasing asymmetric arm length, and lower values were found in 1,4-dioxane as the solvent became thermodynamically more favourable.