Abstract
In the case of cationic‐type photopolymerized epoxy networks, a new type of chain transfer agent based on hydroxyl functional acrylic cross‐linked polymer microparticles (CPM, also called microgels) has been tested. The CPM functionality was obtained through hydroxyethyl acrylate (HEA) monomer, used as comonomer with butyl acrylate (BA) and hexane diol diacrylate (HDDA). Stabilizing monomers were also required for the synthesis of CPM. In order to compare their particular effects, 4 sets of CPM were synthesized with 4 different stabilizing agents, either hydroxyl functional or not. Consequently, two types of OH groups were present in the particles: primary groups coming from HEA and preferentially located in the particles, and optional hydroxyl groups due to the hydroxyl functional stabilizing agents which were mainly placed onto the particles' surface. The viscoelastic properties of the photopolymerized films have been used to obtain information about the chain transfer reaction and the network microstructure. In all cases, the rubber modulus, $E'_{\rm r}$ was improved because of the decrease of the number of dangling epoxy chains in the epoxy network. When hydroxyl groups were only present in the particles, the mobility of the linkages was assumed to be low, and the mechanical relaxation temperature, T~α~, strongly increased. However, CPM aggregation occurred at high CPM concentrations, lowering $E'_{\rm r}$ and T~α~. On the contrary, when OH groups were located both on the surface as well as inside the particles, no large CPM aggregation took place, even with [CPM] = 40 wt.‐%. Nevertheless, the presumably higher mobility of the linkages on the particles' surface prevented any T~α~ increase. The water absorption of all systems based on CPM was very low, around 2% whatever the concentration of chain transfer agent.
Schematic description of the transfer reaction between a propagating cationic‐type epoxy chain and a hydroxyl functional CPM.
magnified imageSchematic description of the transfer reaction between a propagating cationic‐type epoxy chain and a hydroxyl functional CPM.