From HEMA monomers to poly-HEMA systems simulations

There is plenty of experimental literature characterizing hydroxyethyl methacrylate (HEMA)-based physical and chemical hydrogels, due to their spread applications in numerous fields, especially in the biomedical one. Transport properties of poly-HEMA hydrogels as well as systems dynamics are of particular interest owing to delivery issues, whether in terms of solvents, drugs or further carriers. Water content (WC), cross-linking and T dependence are the main investigated variables when characterizing diffusion and related properties. Nevertheless, there is an extremely low number of papers devoted to theoretical studies on both the HEMA monomer and on poly-HEMA systems. Concerning the HEMA monomer, to the best of our knowledge, just one published paper deals with conformational research, despite experimental suggestions of more conformers, and solute-solvent descriptions are missing. For these reasons, we oriented a first part of research to: (i) study its structural and spectroscopic properties through Density Functional Theory (DFT) calculations; (ii) highlight hydrogen bond interactions of the solvated monomer by running molecular dynamics (MD) simulations with four water models; (iii) compare the calculated vibrational and electronic properties to experimental spectra. Results confirmed the coexistence of two stable HEMA conformers, interacting with solvents in the same way, mainly through the -C=O and the -C-OH groups between T 230 and 360 K. The trans to cis ratio was calculated to be 56.5 : 43.5 at 298 K from vibrational data. Regarding poly-HEMA systems, there are several guesses that still require a theoretical elucidation. The major open questions on poly-HEMA systems have to do with the plasticizer role of water and with the quantity of solvent molecules per -OH moiety in systems at various WC; moreover, investigations might confirm if water diffusion remains a hydrogen bonding driven mechanism in poly-HEMA. For answering to the cited questions, the second part of our work was devolved to poly-HEMA inter- and intramolecular interactions: (iv) representative physical and chemical systems were built and the respective force fields parametrized; (v) MD simulations of such systems at WC from 10 to 40% w/w were run at room T, analyzing them in terms of radial distribution functions (RDF) and gyration radius (Rg). Outcomes showed that chain extension from 10 to 150 monomeric units has no influence on solute-solvent interactions. There are almost 2 water molecules around each -C-OH group and just 1 near the -C=O moiety at 1.8 Å distance from WC 20 to 40%, in agreement with experimental findings. Intramolecular interactions, which result in weak H-bonding, occur mainly between the alcoholic hydrogens and the carbonyl and ester oxygens. The -C-OH moiety is less involved in intramolecular interactions rather than in intermolecular ones, although the latter remains very poor in quantity and strength. The Rg varies from chain to chain, ensuring good statistics.