A RIS treatment of the mean-square dipole moment of PMMA chains in consideration of the pendant ester group orientations

According to the six-state scheme proposed by Vacatello and Flory, the mean-square dipole moments 〈M²〉/x (x: number of monomeric units) of poly(methyl methacrylate) (PMMA) chains and their temperature dependence were calculated by the matrix algebra method of rotational isomeric state theory. To take account of the orientation effect of polar ester groups, the statistical weight matrices, U′ and U″, were recast into an expanded form. A Bernoullian probability λ introduced to measure the preference between the two alternative orientations (χ=0 and χ=π) of the side ester groups was found to give rise to a large effect on the mean-square dipole moment of polymers. When the skeletal bonds of a meso dyad are both in trans, an exceptionally strong dipole-dipole interaction may take place. If the two adjacent ester groups adopt the same orientation, the dipoles must be situated nearly parallel to each other in a close proximity. Such repulsive interactions may be alleviated by taking a staggered conformation, i.e. (χ,χ′)=(0,π) or (π,0). Such neighbor-dependent correlation inherent to the meso dyad is considered by assigning a parameter λ′ to the proper element of the U_m″ matrix. The values of 〈M²〉/x observed at 30 °C were satisfactorily reproduced by adopting λ=0.270 and λ′=0.865. The thermal coefficients were found to be reconciled with the available experimental data by allowing the orientation parameters λ and λ′ to be slightly temperature dependent. The stereochemical characteristics of the chain terminals were examined in order to extend the treatment to include the dipole moment of PMMA oligomers.