Nanoparticle Surface Cross-Linking: A Universal Strategy to Enhance the Mechanical Properties of Latex Films

Keto-hydrazide cross-linking chemistry between diacetone acrylamide (DAAM) and adipic acid dihydrazide (ADH) has been widely applied in conventional emulsion polymerization to render materials with specific chemical and physical properties. However, significant drawbacks are usually associated with the conventional process, such as the migration of low-molar-mass surfactants, and the low cross-linking efficiency due to the random distribution of the cross-linkers throughout the nanoparticles. We demonstrate here the concept of surface cross-linking of latexes in a RAFT-mediated surfactant-free procedure that overcomes the migration of surfactants while improving the cross-linking efficiency of latexes. Specifically, the "surface cross-linking" here refers to the cross-linking of colloidal particles' surfaces during the film formation process. First, an amphiphilic RAFT agent with the incorporation of a cross-linking unit (DAAM) was designed and synthesized; the following controlled emulsion polymerizations provide a series of surfactant-free latexes with excellent colloidal stabilities, high solid contents (>40.0 wt %), and a size range of 150–250 nm. To access RAFT latexes with different cross-linking degrees and mechanisms, amphiphilic RAFT agents with different units of cross-linking component (DAAM) were designed and implemented for the controlled emulsion polymerization of styrene and n-butyl acrylate. Next, the surface cross-linkable and randomly cross-linkable RAFT latexes and corresponding films were prepared. It was found that the surface cross-linked films exhibited not only improved Young's modulus and storage modulus but also better solvent resistance compared to the control group with DAAM units randomly distributed throughout the particle systems. Remarkably, it was noticed that the surface cross-linking process can simultaneously increase the strain at break and ultimate stress, and this is the first observation of such a phenomenon in DAAM/ADH cross-linked latex films. Overall, this study represents the first attempt that combines surfactant-free RAFT-mediated emulsion polymerization with the surface cross-linking chemistry to produce cohesive films with enhanced and tunable mechanical properties. This universal synthetic strategy is applicable to a broad range of commercial monomers and can be used as a platform technology for advanced industrial applications.