Sub-θ Hydrodynamic Behavior of Poly(α-methylstyrene) in Cyclohexane

The hydrodynamic properties of poly(α-methylstyrene) (PaMS) in cyclohexane from 16 °C below the θ temperature to 10 °C above θ have been investigated by dilute-solution viscometry and photon correlation spectroscopy. Seven near-monodisperse samples covering the range 5.9 × 10⁴ ≤ M_w ≤ 3.41 × 10⁶ were used. These data have been analyzed with regard to chain contraction below θ (the so-called “coil-globule” transition). No evidence for the existence of a globular state was found. It is found that the temperature dependencies of chain contraction parameters, based on the intrinsic viscosity [η] and the hydrodynamic radius R_h, can be expressed in terms of master curves of α³∣τM^1/2 versus ∣τIM^1/2, where α is the chain expansion or contraction factor (from [η] or R_H), τ is the reduced temperature (τ = (T - θ)/θ), and M is molecular weight. An asymptotic region (plateau), generally associated with the collapsed state, is reached with the R_h data and approached with the [η] data. Attainment of the plateau occurs at higher values of α_η³∣M^1/2 than for αh³|τ|M^1/2, where α_η, and α_H are, respectively, [η]- and R_h-based contraction factors. For polystyrene the reverse order has been observed. These results suggest a strong dependence of polymer contraction on chemical structure.