Abstract: | The variation of the indentation hardness of a high molecular weight poly(methyl methacrylate) (PMMA) subjected to CO2 and Ar at high pressure was measured in situ. The samples were subjected to gas exposure for 3 h at 40 °C before a conical indenter of an included angle at 105 °, with a fixed load of 0.237 kg, was applied for a loading time of 60 s. The data show that both CO2 and Ar reduce the hardness of PMMA to a comparable extent at low pressures. The hardness of PMMA subjected to Ar indicates a minimum at about 4 MPa and then increases. CO2 produced a monotone decreasing trend in hardness in the pressure range studied, and the glass‐transition temperature (Tg) was achieved at about 6.0 MPa. The change in hardness is attributed to plasticization of the polymer matrix that is more extensive for CO2. The relationship between the change in hardness for this PMMA subjected to high‐pressure CO2, the corresponding change in the Tg, and the associated swelling of the polymer is discussed. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 3020–3028, 2001 |