Controlled In Situ Nanocavitation in Polymeric Materials

and biomedical applications, [ 5 ] have attracted increasing attention in recent years. Cross-linking reactions generally involve the polymerization of low-viscosity monomers or oligomers, thereby eliminating the need for organic compounds as diluents. [ 6 ] However, one undesirable feature for many crosslinking reactions is volume shrinkage, which can be greater than 20% depending on the chemical composition. [ 6 ] In polymer coatings, combined effects of shrinkage and constraint due to adhesion to a rigid substrate result in the accumulation of a signifi cant amount of in-plane stress, which can compromise the interface integrity, leading to cracks and delamination. [ 6 ] Various methods have been applied to reduce polymerization shrinkage and the associated stress development, including incorporating a plasticizer or increasing the reaction temperature to promote stress relaxation. [ 6 ] When used as matrix resins for structural (such as bridges or aircraft panels) or biomedical applications, polymerization shrinkage causes signifi cant internal stress buildup at the resin-fi ller interface. In addition, interfacial stress can be overwhelming if the composite is bonded to a substrate, as in the case of dental restorative composites. Both internal and interfacial stress can lead to premature composite failure. Common strategies for reducing polymerization shrinkage and the corresponding stress include modifying the resin chemistry, [ 7 ] increasing the fi lling content, [ 8 ] or optimizing the matrix-fi ller interface, [ 9 ] none of which have eliminated polymerization shrinkage. Thus, a challenge remains in identifying a convenient and practical strategy to minimize or effectively eliminate polymerization volume shrinkage for thermal and photo-cross-linking polymers. Here, we demonstrate a new concept to reduce the volume shrinkage by introducing a small amount of soluble cavitation agent (up to 2.0% mass fraction), which decomposes simultaneously with cross-linking to produce gaseous molecules. The volatile components are trapped in the cross-linked networks, resulting in nanovoids in the polymerized system, and thereby counteract polymerization shrinkage ( Scheme 1 a ). A key requirement for nanovoids is that they do not adversely affect the composite performance, such as mechanical properties. As a demonstration of the concept, acetone dicarboxylic