Journal of Applied Polymer Science, Vol.84, No.2, 244-255, 2002
Humidity-induced plasticization and antiplasticization of polyamide 6: A positron lifetime study of the local free volume
Polyamide 6 (PA6) was studied using positron annihilation lifetime spectroscopy (PALS). From the ortho-positronium lifetime tau(3) the size of local free volumes (holes) was estimated. In dry PA6 the mean hole volume u varied between 70 Angstrom(3) and 128Angstrom(3) when the temperature increased from 25 to 220degreesC. 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(-3) was estimated. The mean hole volume of PA6 exposed to atmospheres of different relative humidity (RH) was compared with the upsilon(T) behavior of dry PA6. We propose a phenomenological model in which T-g and upsilon(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 upsilon(g)(RH) were estimated from our experiments. In the range 0 < RH <90% the upsilon(g) of the humid PA6 was always smaller than in the dry polymer, while near RH = 100% both values becarne approximately identical. Two ranges of Sorption behavior were observed. For small and medium RH, upsilon(g) decreases by up to 19 Angstrom(3) for RH = 45%, which corresponds to more than half of the volume of a water molecule. This behavior of upsilon(g) was interpreted as antiplasticization (loss of free volume) of the glassy polyamide. For larger RH, upsilon(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) water-PA6 mixture with respect to that of the dry glassy polymer. The results are also discussed in terms of a simple hole-filling model and of the hypothesis of firmly and loosely bound water molecules in polyamides.