HIGH-PRESSURE LIGHT-SCATTERING APPARATUS TO STUDY PRESSURE-INDUCED PHASE-SEPARATION IN POLYMER-SOLUTIONS

A new high-pressure time-and angle-resolved light scattering apparatus has been developed to study the kinetics of phase separation in polym er solutions and other fluid mixtures under pressure at near-and super critical conditions. The system consists of a high-pressure polymer lo ading chamber, a solvent charge line, a variable-volume scattering cel l (with a built-in movable piston connected to a pressure generator, a nd an expansion rod driven by an air-actuated diaphragm), and a recirc ulation pump which are all housed in a temperature-controlled oven. Th e system is operable at pressures up to 70 MPa, and temperatures up to 473 K. The scattering cell is a short path-length cell made of two fl at sapphire windows that are separated by 250 mu m. It is designed to permit measurements of transmitted and scattered light intensities ove r an angle range from 0 degrees to 30 degrees. A linear image sensor w ith 256 elements is used to monitor the time evolution of the scattere d light intensities at different angles. With this sensor, the angle r ange from 2 degrees to 13 degrees is scanned at a sampling rate of 3.2 ms/scan. The pressure quenches are achieved by movement of the air-ac tuated movable expansion rod, or by the movement of the piston with th e aid of the pressure generator to bring about either rapid (at rates approaching 2000 MPa/s) or slow pressure changes in the system. Quench depth is also adjustable, and very deep (70 MPa) or very shallow (as low as 0.1 MPa) pressure quenches are readily achievable. The temperat ure and the pressure of the solution in the scattering cell, and the t ransmitted and scattered light intensities at different angles are rec orded in real time through a computerized data acquisition system befo re and during phase separation. The experimental system is especially suited to follow the kinetics of phase separation in polymer solutions and to assess the metastable and unstable regions where phase separat ion proceeds by the nucleation and growth, and the spinodal decomposit ion mechanisms, respectively. The capability of the instrument is demo nstrated for polystyrene/methylcyclohexane solutions. (C) 1998 America n Institute of Physics.