Synthesis and Characterization of Polymeric Nanocomposites Obtained by Frontal Polymerization

Frontal Polymerization 1,2 (FP) is an innovative technique that converts a monomer into a polymer as a result of the propagation of a reaction front along the reactor. In this work, the above technique has been used for the first time to synthesize nanocomposites, mainly based on hydrogels. Indeed, montmorillonite (MMT)-type clays and polyhedral oligomeric silsesquioxanes (POSS) have been dispersed in several polymer matrices, such as epoxy resins, polyacrylamides, and their copolymers. Nanocomposites represent a new class of materials, in which the nanofillers, dispersed inside the polymer matrix, improve the material mechanical properties and its performances, such as flame retardance, barrier property, etc. Hydrogels are three-dimensional networks of crosslinked polymers swollen in a liquid medium. Because of their good biocompatibility and hydrophilic properties, hydrogels have been proposed for many biomedical applications. Indeed, as for these materials the swelling or deswelling can be induced by stimuli, such as variation of temperature, electric field, pH, ionic force and pressure, they can be used as drug delivery systems 3,4 , where high swelling ratio and rate are mostly desired. As previously mentioned, several systems have been considered and both frontal and classical batch polymerization have been used to compare the final features of materials obtained by the two techniques. Three different hydrogel networks based on poly(dimethylacrylamide) were synthesized, by both bulk and solution (water or dimethylsulfoxide) FP. For each series of hydrogels, the concentration of radical initiator was varied from 0.4 to 2.0 wt.-%. Moreover, another system, prepared by using N-isopropylacrylamide (NIPAAm) as monomer and N,N'-methylenebisacrylamide as crosslinker has been synthesized. In addition, a variable amount of sodium MMT (from 0 to 10 wt.-%) was added to the system. Finally, copolymers of NIPAAm and N-vinylcaprolactam, characterized by different ratios of the two monomers, were prepared. The influence of the crosslinker and N-vinylcaprolactam concentrations was studied. Indeed, the property of swelling/deswelling and the lower critical solution temperature (LCST) of the hydrogels has been found to be influenced by monomer compositions. In addition, N-vinylcaprolactam seems to be responsible for the morphological disomogeneity of the samples. The maximum temperature (T max) and the front velocity (V f) were followed for all the samples. Moreover, the prepared hydrogels were characterized in terms of swelling ratio, thermal and rheological properties. The morphology was studied by SEM analysis. Some of the prepared samples have turned out to be thermoresposive, namely they swell or deswell as a result of variations …