Effects of high pressure on amorphous polymers: Densification of polymethyl methacrylate

The densification of polymethyl methacrylate resulting from pressure and temperature treatment in a Bridgman anvil device has been studied at temperatures to 245°C and pressures to 50 kbar. Treatment in the glass transition region at pressure produces samples 1.1%-1.5% more dense than the starting materials, depending on the length of treatment. Treatment in the glassy state produces samples up to 0.7% more dense than the starting material, depending on the temperature and the internal state of the sample immediately prior to the application of pressure. Samples held in the transition region under pressure and then vitrified by rapid application of a high pressure show densifications similar to but somewhat higher than those produced by treatment in the glassy state. The results are compared with thermodynamic theories of the glass transition and with similar data on other polymers and inorganic glasses.