Dynamic light scattering studies of poly(methyl methacrylate)polystyrene copolymers in their theta solvents

The coil-globule transitions of random (statistical) and alternating copoly mers of styrene and methyl methacrylate in their theta solvents, cyclohexan ol and 2-ethoxy ethanol were studied by using dynamic light scattering tech nique. The dimensions of the copolymeric chains show smooth and continous c ontractions below theta temperatures. The hydrodynamic size, R-h, of a rand om copolymer P(St-co-MMA) (M-W = 4.59 X 10(6) g/mol) in cyclohexanol (theta = 68.6 degreesC) decreases to 48% of that in the unperturbed state at 56.5 degreesC). This copolymer shows a similar contraction behavior in a second theta solvent 2-ethoxy ethanol (theta = 58.4 degreesC. The Rh of this copo lymer decreases again to 48% of that in the unperturbed state at 45.3 degre esC. An alternating copolymer of the same monomers P(St-alt-MMA) (M-W = 2.6 2 x 106 g/mol) displayed a comparable chain contraction. The hydrodynamic s ize of this sample in cyclohexanol was reduced to 59% of that in the unpert urbed state by decreasing temperature from 60.8 to 53.3 degreesC. For the r andom and alternating copolymers studied in this work, the temperature inte rval from the theta -state to the collapsed state is narrower compared to t hat of the PMMA homopolymer. (C) 2001 Elsevier Science Ltd. All rights rese rved.