Influence of Cross-Link Density on the Thermal Properties of Thin Polymer Network Films

The properties of thin polymer films are known to be different than the properties of bulk polymers. Both dewetting studies1,2 and temperature-dependent thickness measurements3 showed that thin polystyrene films can exhibit polymer mobility even at temperatures well below that of the bulk-polymer glass transition (Tg). Subsequent work by a wide range of groups has shown that the thermal properties in thin polymer films can be different than those of the bulk and will depend on the polymer/substrate interactions.4-20 With favorable polymer/substrate interactions, Tg can increase and the coefficients of thermal expansion (CTE) can decrease.4-11 For weak polymer/ substrate interactions, Tg can decrease and CTE can increase.4,5,8,10,12,13 For free-standing films, Tg decreases dramatically.14 Most of the work to date probing the thermal properties of ultrathin polymer films has focused on low-polydispersity thermoplastic polymers [predominantly polystyrene-based polymers and poly(methyl methacrylate)]. However, one class of polymer films that has not been as thoroughly studied is cross-linked polymers. The interfacial properties of cross-linked polymers are important in technical applications such as fiber-reinforced composites, nanocomposites, artificial tissue scaffolds, electronics packaging, antireflective coatings, and general adhesive applications. In this work, we use X-ray reflectivity to probe the thermal properties of thin cross-linked polymer films adhered to silicon wafers. Networks with two different cross-link densities were used. The network was composed of an epoxy monomer cross-linked by a diamine hardener (see Figure 1). The cross-link density of the network was varied by adjusting the length, n, of the propylene glycol spacer in the diamine hardener.