Pulse-Induced Critical Scattering (Pics) from Polymer Solutions

Abstract

Pulse-induced critical scattering (PICS) is a new, fast technique for determining thermodynamic and kinetic parameters of polymer solutions. Relative intensities of scattered laser light are measured over periods of seconds after a fast temperature step into the region of Debye critical opalescence. In the mode of operation using thermal pulses to progressively lower temperatures, the intensity pulse shape gives sensitive information on the fluctuation equilibrium and on the kinetics of phase separation with formation of emulsions of sub-wavelength particles. Spinodal and critical loci for the system polystyrenecyclohexane (PS-CH) are measured about 10 times faster and more accurately than in recent work based respectively on conventional light scattering equipment and on turbidimetry. The classical Debye theory of scattering near a critical point, or Scholtes extension to spinodal points, gives excellent semiempirical extrapolations to spinodal temperatures Ts. Though there are signs that the classical theory needs amendment in the sense of the generalized nonlinear theory of Fisher, Chu, and co-workers, this would generally give results on Ts. in close agreement with the simpler classical theory in the experimental range accessible to PICS. The effects of heterodispersity on the phase diagram are studied, comparing data by Scholte and new data by PICS on PS-CH. The evidence suggests that spinodal loci are governed mostly by Mw(as suggested by recent theories). Marked secondary effects seem, however, to arise, not (as recently suggested) from Mn, but from Mz. Results obtained by the technique of approaching phase equilibrium from the heterogeneous side, using an emulsion carefully 'seeded’ by a thermal step, suggest that supercooling is more serious in cloud-point determinations of dilute polymer solutions than hitherto thought. The formation of sub-wavelength droplets follows kinetics bearing more than formal resemblance to that for the nucleation and growth of spherulitic crystals in bulk. © 1974, IEEE. All rights reserved.