Polymer resins are used for numerous applications, for example as matrix materials for high performance fiber-reinforced composites. Such structures operate in humid air and liquid water, which may cause swelling and degradation of the material. Because of imperfections in the chain structure such as chain ends, glassy polymer contain molecular sized voids called “free volume”. In addition, macroscopically sized voids are commonly found in polymers. When polymers absorb water, differential scanning calorimetry (DSC) studies have shown that the absorbed water may be classified as free water and bound water. Free water state is when the water molecules occupy free volume and voids. Bound water state exists when the water molecules interact with the polymer chains and form hydrogen bond.
In order to examine the influence of voids on water uptake, and the state of water in epoxy, foaming agents were utilized to produce a controllable amount of voids (void contents from 0 to about 50%). Epoxy specimens containing voids and void-free were prepared. Water uptake experiments were conducted by immersing specimens with and without voids in distilled water at 40 °C. The moisture content was monitored by gravimetric measurements. To examine the state of absorbed water, DSC analysis of dry and water saturated epoxy specimens without and with voids was conducted over a temperature range from -80 °C to room temperature. DSC data was recorded during both the cooling and heating cycles. The state of water absorbed was determined based on analysis of the appropriate transitions recorded.
The free and bound water phase transitions were absent in the dry void-free specimen. In the water saturated void-free specimen, most the moisture absorbed was non-freezable water. In the water saturated medium and high void content specimens, peaks attributed to free and bound water were observed. The contents of free and bound water were estimated from the exothermic peaks in the DSC curves. The bound water peak was much smaller than the free water peak, and decreased with increasing the void content. The area under the free water peak increased with increasing void content.