An automated spring balance for kinetic gravimetric sorption of gases and vapors in polymers

A method for automation of a McBain-type spring balance using a charge coupled device camera, a computer equipped with a frame-grabber card, and National Institutes of Health Image software is presented. This balance is used to study the sorption and transport of small molecules in polymeric materials. Kinetic gravimetric sorption data of acetone uptake in a random copolymer of 50 wt % poly(ethylene terephthalate) and 50 wt % poly(ethylene 2,6-naphthalate) at 35 degrees C are provided to illustrate the utility of the method. The diffusion coefficients determined from the cathetometer and camera experiments are 1.1+/-0.2x10(-12) and 1.0+/-0.2x10(-12) cm(2)/s, respectively. At an acetone partial pressure of 5.4 cm Hg, the equilibrium acetone uptake was 1.47+/-0.15 g acetone/100 g polymer using the cathetometer to determine spring extension and 1.52+/-0.15 g acetone/100 g polymer when the camera was used to determine mass uptake of acetone by the polymer. The camera-based balance was determined to be sensitive to weight changes as small as +/-1 mu g. (C) 1998 American Institute of Physics. [S0034-6748(98)02406-X]