A new technique for measuring retrograde vitrification in polymer–gas systems and for making ultramicrocellular foams from the retrograde phase
✍ Scribed by Y. Paul Handa; Zhiyi Zhang
- Publisher
- John Wiley and Sons
- Year
- 2000
- Tongue
- English
- Weight
- 241 KB
- Volume
- 38
- Category
- Article
- ISSN
- 0887-6266
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✦ Synopsis
A stepwise temperature-and pressure-scanning thermal analysis method was developed to measure glass-transition temperature T g in the two-phase polymergas systems as a function of gas pressure p, and was used to confirm recent theoretical predictions that certain polymer-gas systems exhibit retrograde vitrification, that is, they undergo rubber-to-glass transition on heating. A complete T g -p profile delineating the glass-rubber phase envelope was established for the PMMA-CO 2 system. The retrograde vitrification behavior observed, where at certain gas pressures the polymer exists in the rubbery state at low and high temperatures and in the glassy state at intermediate temperatures, was similar to that reported previously based on the creep-compliance measurements. The existence of the rubbery state at low temperatures was used to generate foams by saturating the polymer with CO 2 at 34 atm and at temperatures in the range Ϫ0.2 to 24 °C followed by foaming at temperatures in the range 24 to 90 °C. Foams with very fine cell structure never reported before could be prepared by this technique. For example, PMMA foams with average cell size of 0.35 m and cell density of 4.4 ϫ 10 13 cells/g were prepared by processing the low temper- ature rubbery phase.