Abstract
Polyamide 6 (PA6) was studied using positron annihilation lifetime spectroscopy (PALS). From the orthoβpositronium lifetime Ο~3~ the size of local free volumes (holes) was estimated. In dry PA6 the mean hole volume v varied between 70 Γ
3 and 128 Γ
^3^ when the temperature increased from 25 to 220Β°C. From the comparison of the coefficient of thermal expansion of macroscopic volume with that of hole volume a number density of holes N~h~ of 0.8 nm^β3^ was estimated. The mean hole volume of PA6 exposed to atmospheres of different relative humidity (RH) was compared with the v(T) behavior of dry PA6. We propose a phenomenological model in which T~g~ and v~g~, the glass transition temperature and the mean hole volume at T~g~, are allowed to vary with RH. Assuming T~g~(RH) as given in the literature, the v~g~(RH) were estimated from our experiments. In the range 0 < RH < 90%, the v~g~ of the humid PA6 was always smaller than in the dry polymer, while near RH = 100% both values became approximately identical. Two ranges of sorption behavior were observed. For small and medium RH, v~g~ decreases by up to 19 Γ
^3^ for RH = 45%, which corresponds to more than half of the volume of a water molecule. This behavior of v~g~ was interpreted as antiplasticization (loss of free volume) of the glassy polyamide. For larger RH, v~g~ increases again toward the value of dry PA6, indicating a plasticization (gain of free volume) behavior that compensates and finally nullifies the former antiplasticization. The behavior of the local free volume in moist PA6 corresponds well to the known variation of the specific volume. Our results deliver experimental evidence for the decrease in the unrelaxed free volume of the (glassy) waterβPA6 mixture with respect to that of the dry glassy polymer. The results are also discussed in terms of a simple holeβfilling model and of the hypothesis of firmly and loosely bound water molecules in polyamides. Β© 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 244β255, 2002; DOI 10.1002/app.10319