Surface modification of poly(L-lactide) films by grafting hydrophilic polymers and branched architectures

Poly(L-lactide) (PLA) has been used as a bioabsorbable material in the medical and pharmaceutical fields. The unmodified PLA surface is hydrophobic and generally has low cell affinity; thus, modification of PLA film surface properties is necessary to improve its use as a biomaterial. The major objective of this research was to modify the surface characteristics of PLA by grafting a combination of hydrophilic polymers to produce a continuum of hydrophilicity. The PLA film was solvent cast and the film surfaces were activated by ultraviolet (UV) irradiation. A single monomer or combination of two monomers, selected from vinyl acetate (VAc), acrylic acid (AA), and acrylamide (AAm), were then grafted to the PLA film surface using UV-induced photopolymerization. Contact angle goniometry results showed that the wettability of the PLA film surface could be tailored across a broad range, from hydrophobic (contact angle ∼ 82° for neat PLA) to hydrophilic (contact angle ∼ 12° for PLA grafted with polyacrylamide), using a single monomer or a combination of monomers. It was noted that AAm dominated the wetting characteristics of the film when copolymerized with VAc or AA, while the water contact angles for PLA films grafted with poly(vinyl acetate-co-acrylic acid) varied more gradually with feed composition.; When incubated in different pH solutions, viz., pH = 4, 7, and 10, for specified time periods at room temperature, faster PLA degradation was achieved for the PLA film grafted with poly(acrylic acid). However, control studies revealed that the major contribution to the observed degradation was due to entangled poly(acrylic acid) chains resulting from acrylic acid monomer that migrated into the film bulk and not due to the surface-grafted layers. Additionally, concern of PLA degradation due to the UV irradiation during the surface-modification process was addressed by investigating the effect of UV irradiation on the representative bulk and surface properties of PLA films. The PLA molecular weight decreased significantly during UV irradiation under atmospheric conditions; however, irradiation through a pyrex container was shown to minimize polymer degradation.; To develop another experimental protocol for the PLA film surface modification, preliminary experiments were done using an ethylene-acrylic acid (EAA) copolymer film as a 'model' substrate. In this study, amine-terminated linear and branched architectures were grafted to EAA films using the so-called grafting from approach. A significant reduction in water contact angle for the EAA film grafted with a branched architecture was observed. This grafting method was altered for PLA film surface modification and involved photografting of 4,4'-diaminobenzophenone to introduce amine groups on the PLA film surface. Subsequently, wet chemistry similar to the one used for EAA-film surface modification was employed to create branched architectures containing amine functionalities on the periphery. Contact angle goniometry results showed an initial decrease and subsequent plateau in the water contact angles for the PLA films with each successive generation of the branched architectures. X-ray photoelectron spectroscopy (XPS) data provided insight into the structure of the grafted layer and revealed an increase in the nitrogen content with each generation. F3T3M mouse fibroblast cells cultured on PLA film grafted with the amine-terminated branched architectures showed enhanced cell adhesion, proliferation, and viability compared to the unmodified PLA.