Glass Transition and Moisture Content of PA6 by IsoStep DSC

Sample

Polyamide film, PA6. The aim of the experiment was to investigate the influence of moisture on the glass transition. To do this, one part of the film was vacuum-dried and stored over silica gel (sampled named "dry"); the other part was immersed in water (sample named "saturated"). To obtain a sample of intermediate moisture content, the saturated sample was kept in air at approx. 60% relative humidity for 4 hours (sample named "moist").

Conditions

Measuring Cell: DSC822^e with IntraCooler

Pan: Aluminum 40 μl with sealed and pierced (1-mm hole) lids (for conventional DSC); aluminum 20 μl with lid for Iso-Step measurements

Sample preparation: Samples were cut into a few small pieces. Sample mass was about 4 mg. The excess water of the saturated sample was completely removed before preparing the specimen. 

DSC measurement:

• Conventional program: Heating from -60 °C to at 130 °C at a heating rate of 10 K/min
• IsoStep program: Stepwise heating from -60 °C to 100 °C with isothermal period of 60 S and heating segments of 2 K/min, with temperature increments of 2 C. The blank sapphire reference samples were measured under the same conditions for calibration purposes. IsoStep is a special DSC measurement technique. It allows a change in heat capacity to be seperated from an overlapping kinetic effect such as a reaction or evaporation (also called a non-reversing effect). This is done by means of a temperature program consisting of a large number of dynamic segments that each begin and end with isothermal segment. The dedicated IsoStep evaluation yields the specific heat capacity (c_p) curve free from the overlapping evaporation process. The non-reversing curve is the heat flow of evaporation

Atmosphere: Nitrogen, 50 ml/min

Interpretation

The conventionally measured DSC curves of PA6 films with different moisture contents (dry, moist and saturated) display one of two thermal effects, namely the step at the glass transition and the endothermic evaporation peak. With increasing moisture content, the glass transition temperature of the PA6 film decreases. The dry film exhibits a glass transition temperature (T_g) at 57 °C, and the saturated film at -22 °C. The water therefore acts as a plasticizer. Depending on its concentration, the film can either be stiff or soft at room temperature

The moisture is held in the sample if the pan is hermetically sealed; no evaporation is detected up to 100 C. At higher temperatures, the internal pressure becomes large and might rupture the pan. To withstand the increased pressure, measurements would have to be performed with pressure pans. These, however, result in poorer sensitivity. 

Conclusion

Moisture acts as a plasticizer in the polyamide samples and shifts the glass transition temperature from 57 °C under dry conditions to 22 °C for the water-saturated PA6 samples. The T_g of those samples with extreme moisture contents can be easily determined by conventional DSC experiments. 

Difficulties arise if the moisture content is sufficiently large to cause a significant cant rise in pressure in the sealed pan. This can then deform or rupture it. If pans with pierced lids are used, the evaporation overlaps the glass transition. In such cases, the IsoStep method is able to determine the heat capacity changes and the heat flow of the evaporation separately. This allows the T_g of moist samples to be identified even if the evaporation masks the transition because the thermal effects overlap.

^{Glass Transition and Moisture Content of PA6 by IsoStep DSC | Thermal Analysis Handbook No.HB234 | Application published in METTLER TOLEDO TA Application Handbook Thermoplastics}