CONICET | Buscador de Institutos y Recursos Humanos

Nanomechanical properties of materials is a crucial factor in the design of adaptive devices1-3. For instance, in polymer scaffolds, cell growing only can occur under correct mechanical features of the polymer3. Moreover, topographical aspects, nano and micro aspects of polymer surface, have a noticeable impact on material surface properties and therefore on the applicability. Well-known techniques for topographical patterning are nanoimprint, nano/microlithography or nanomolding strategies using hard cylindrical porous anodic aluminum oxide (AAO) nanomolds where a polymer melt is infiltrated. The in-situ polymerization method complements the more traditional polymer infiltration method and expanded the capability of anodic aluminum oxide (AAO) templates to pattern polymer nanostructures4-8. However, polymer properties, such as mechanical properties, are not easily obtained from the single monomer chemistry. Copolymerization is used to modify the properties of polymers to meet specific needs, for example to control wetting or hydrophobia properties, to reduce crystallinity, modify glass transition temperature or to improve mechanical properties. These advantages make the polymer nanostructures to be adaptable to a wide range of devices Recently, we have reported the preparation of different polymer nanostructures by in-situ homopolymerization of different monomers using AAO nanocavities as reactors. For instance, in the free radical polymerization kinetics of a styrene, methyl methacrylate, or fluorinated acrylic monomer, a large increase in the rate of polymerization in confinement in AAO templates was observed compared to bulk4,5. In the step polymerization of urethane-alcohol monomers it was also found that the speed of reaction is significantly enhanced in nanopores with respect to bulk polymerization. Nevertheless, in-situ copolymerization reaction of two monomers with different chemical functionalities in AAO reactors has never been reported. This work presents the process to obtain copolymer nanostructures based on butyl methacrylate (MBA) and 2-hidroxyethyl acrylate (HEA) monomers and the chemical, topographical and mechanical characterization. The fabrication of porous AAO templates was carried out by a two-step electrochemical anodization process following a procedure already reported in the lietrature9,10. Diameter and length of AAO nanocavities were controlled by specific parameters of anodization process. The copolymerization of BMA and HEA monomers, 1:1 feeding ratio, and AIBN initiator (0.5%vol) was carried out inside the AAO nanopores in an oven at 70ºC during 6h. The morphology of AAO templates and nanofibers obtained after copolymerization was characterized by Scanning Electron Microscopy (SEM), Philips XL-30 ESEM. Chemical composition of copolymer and copolymerization kinetics were determined by H1-RMN and by Fourier transform infrared spectroscopy (FTIR) employing the attenuated total reflectance (ATR) mode. The molecular weight of copolymers extracted from AAO templates was determined by Gel Permeation Chromatography.Topographical study of free nanopillars was carried out by atomic force microscopy (AFM). As an example, the morphology of AAO templates synthesized in this work is shown in Figure 1. Fig 1a and b correspond to the top view and cross section of tubular nanocavities, respectively. The dimensions of pores are 200 nm for diameter and 1 µm for length. Figure 1c corresponds to a cracked AAO template after in-situ BMA and HEA copolymerization. As observed some fibers are formed, going from one to the other side of the crack. It means that the reaction of monomers has taken place in the nanocavities at 70ºC, during 6 hours, being a very efficient nanostructuration process in comparison to infiltration process. The polymer infiltration process only takes place at very high temperature, higher than 150 C. Figure 1. A) SEM images of AAO template with 200nm of diameter pore (top view). B) AAO template with 1µm of length (cross section). C) Cracked AAO template with nanofibers (lateral view). D) AFM images of free copolymer nanopillars.The copolymer composition was determined by FT-ATR. The spectra showed the absence of C=C stretching peak at 1550 cm-1 indicating a complete reaction of two monomers in confinement. The copolymerization kinetic study by H1-RMN indicates that copolymerization reaction in confinement is quicker than in bulk. The results are in agreement with those previously reported in literature on radical polymerization of vinyl monomers in AAO nanoreactors4,5. The weight average molecular weight of copolymers is lower than that obtained in bulk polymerization, also being in agreement with previous results4,5. Topography and swelling properties of free nanopillars extracted from templates were study by AFM microscopy. As an example, Figure 1d image presents the topography of a MBA-HEA copolymer nanostructure.

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