Design of covalently dynamic films and coatings: Analysis of biocatalyzed kinetic behavior in soft condensed matter

Pirro Cipi


Reversibly adjusting surface and bulk properties of materials is advantageous to many applications and will vastly expand current polymer capabilities. A novel strategy was developed to render materials capable of "smart" surface and bulk properties with the aid of embedded biocatalyst and latent functional groups that are selectively activated in the presence of biocatalyst to undergo chemical transformation. The research presented herein focused on systematically characterizing the activity of lipase catalysts embedded in poly (alkyl methacrylate)s simultaneously with functionalized components, either as diffusing within the polymeric matrix or tethered to the matrix. Lipase activity, ester forming (synthetic) or ester breaking (hydrolytic), provided the pathway for the polymer matrix to self-tune its properties by catalytic transformation of untethered or tethered functionalized component. Therefore, the primary research goal was to understand and quantify embedded biocatalyst mediated transformation rates of functional species, diffusing or covalently tethered in films and coatings, as a function of polymer physical parameters and processing conditions. In vivo, biocatalysts mediate complex biochemical reactions and pathways in macromolecular crowded environments. The excluded volume established by macromolecular complexes largely controls cellular enzyme turnover rates by establishing concentration gradients and diffusion regimes within cells. Initially, three poly (alkyl methacrylate) free-standing films were embedded with Porcine Pancreas Lipase and -OH and -COOH functional small molecules capable of undergoing lipase catalyzed acyl transfer reactions. Design of films with embedded functionality along with embedded biocatalyst was inspired by the confined nature of cellular microenvironments. A systematic study was conducted to understand how initial confining matrix properties influence transformation rates of embedded functionality mediated by embedded enzymes in amorphous matter. Biocatalytic activity in films embedded with enzyme and functionality was diffusion-limited relative to both free enzyme and enzyme-only embedded films with surface exposure to small molecule functionality. The diffusion-limited biocatalytic activity exhibited linear dependency on the fraction of polymer free volume in glassy and rubbery films at < 8% by weight (8.8 - 9.7% by volume) loadings of stoichiometric amounts of functionality. The biocatalytic activity in the glassy state increased linearly with film thickness, which suggested the fugitive solvent left behind more free volume sites within the confining matrix at higher film thicknesses facilitating the transport of the functionality within the matrix. Temperature studies confirmed that biocatalytic activity in the glassy state was dependent on small molecule migration in kinetically trapped excess free volume within interconnected OA and 1-NN rich regions. Solvents in the glassy systems behaved as transient templates during film formation as determined by the correlation of biocatalytic activity with the difference between the solubility parameters of the film and the solvent. Poly (alkyl methacrylate) films with embedded biocatalyst alongside functionality manifested reversible conversion of octanoic and 1-nonanol to nonyl octanoate based on the equilibrium water content. These formulations exhibit potential as hybrid materials with switchable bulk properties via reaction equilibrium control of the functionality by altering environmental conditions. We also investigated an approach to engineer materials capable of reversible crosslinking by immobilizing enzymes in matrices containing multifunctional comb-like -COOH and -OH copolymers for adhesives, elastomers, composites, and laminates applications. Coatings comprised of the polyfunctional polymer blend and lipase at 1-4 wt% on solids were cast on glass and aluminum substrates. Acid equivalents, final molecular weight, and T g , progression over time were used to quantitatively assay the enzyme-catalyzed reaction between the two functionalized components up to 250 hours. In rubbery systems, the consumption of acid functionality corresponded to an increase in molecular weight with similar reactivity rates. Finally, immobilized lipase B from Candida Antarctica was used to synthesize novel poly (ester amide)s with pendant and backbone amides. Polymerization kinetics, molecular weight, and structure of the resulting poly (ester amide)s were determined via gel permeation chromatography and nuclear magnetic resonance spectroscopy. Rate constants for lipase from Candida Antarctica -catalyzed polyesterifications were examined as a function of monomer structure. Polymerization kinetics were characterized by initial linear molecular weight growth and a plateau in the later stages of polymerization. Thermal analysis of poly (ester amide)s suggested that the pendant amide improved thermal stability. In addition, the polymerization of γ-acetamido-[varepsilon]- caprolactone was carried out by a two-step, one-pot process, despite lower reactivity relative to the unsubstituted analog, -caprolactone. Initially, base catalyzed ring opening was conducted in methanol at room temperature followed by bulk polymerization in the presence of Candida Antarctica lipase acrylic resin at 60°C for 150 h. The resulting polymer was determined to be amorphous in nature due to the presence of pendant groups along the polyester backbone.