Perturbing effects on the structure and properties of physically and chemically crosslinked phenolic resins

Crosslinked polymers may consist of physical crosslinks such as hydrogen bonds, or covalent bonds or both. The network of crosslinked phenolic resins involves the both. The formation of each type of crosslinking has not been studied in detail. Their contributions to the physical properties are also unclear. Utilizing a combination of infrared spectroscopy, thermal analysis, low field NMR and a cantilever deflection technique, the relative contributions of two types of crosslinks have been characterized as a function of temperature and moisture. The results show that the increased chemical crosslinks reduced the formation of hydrogen bonds. As expected, the covalent bonds dominate at elevated temperatures. However, the contribution of hydrogen bonds is unexpectedly important at low temperatures. As expected, the hydrogen bonding contribution of crosslinked structures is significantly decreased by the moisture absorbed. However, segmental relaxation behavior measured under different humidity levels depends on not only the two types of crosslinks achieved but also the morphological features affecting the moisture absorption process.