The interfacial properties of polymer matrix composites are critical to the retention of the mechanical properties of the composites in a wet environment. The objective of this study is to investigate the effect of moisture on the interfacial properties of glass bead- epoxy composites by measuring mechanical properties obtained through dynamic mechanical testing. The viscoelastic material properties of glass bead-composites, including glassy and rubbery moduli and the loss tangent (tand) were measured. In order to characterize the relationship between dynamic mechanical properties and interfacial structure, composites with different interfacial structures were tested and the results were compared. The different interfacial structures are created by treating or not treating the glass beads with a silane coupling agent before the production of the composites. The plain epoxy matrix was also tested. The sorption behavior of the specimens was monitored by recording the percent weight gain of the water uptake periodically.
In general, the measured properties reflect the effect of moisture on the properties of matrix, composites, and interphase. For the plain matrix, moisture reduces Tg,. by about 15oC, and reduces tand at Tg. These are all characteristic of plasticization of the matrix. The effects of water on composites strongly depend on the interfacial structure of the composites. The interphase with silane coupling agent provides better interfacial adhesion, thus improving stress transfer between matrix and reinforcement. The silane also reduces the mobility of the epoxy network near the glass surface. This hypothesis was supported by the greater E’, higher Eaa and smaller tand at Tg for silane-treated composite compared to corresponding properties for the untreated composite.
Water seems to weaken the interfacial strength of composites since water reduces the rubbery modulus of the composites but not of the plain matrix. Water reduces the interfacial bond strength less for silane- treated composites than for untreated composites. In addition, composites with poorer interfacial bond strength showed larger “additional” tand peaks net to the a-transition. If we assume that poor interfacial bond strength leads to more interfacial cracks or voids, then the tand spectrum may be useful for characterizing the extent of cracking.