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

The coil-globule transitions of random (statistical) and alternating copolymers of styrene and methyl methacrylate in their theta solvents, cyclohexanol and 2-ethoxy ethanol were studied by using dynamic light scattering technique. The dimensions of the copolymeric chains show smooth and continous contractions below θ temperatures. The hydrodynamic size, Rh, of a random copolymer P(St-co-MMA) (Mw=4.59×106 g/mol) in cyclohexanol (θ=68.6°C) decreases to 48% of that in the unperturbed state at 56.5°C. This copolymer shows a similar contraction behavior in a second theta solvent 2-ethoxy ethanol (θ=58.4°C). The Rh of this copolymer decreases again to 48% of that in the unperturbed state at 45.3°C. An alternating copolymer of the same monomers P(St-alt-MMA) (Mw=2.62×106 g/mol) displayed a comparable chain contraction. The hydrodynamic size of this sample in cyclohexanol was reduced to 59% of that in the unperturbed state by decreasing temperature from 60.8 to 53.3°C. For the random and alternating copolymers studied in this work, the temperature interval from the θ-state to the collapsed state is narrower compared to that of the PMMA homopolymer.