Synthesis and surface modification of sunflower oil-based non-isocyanate polyurethane coatings containing chitosan: Physicochemical and biological characterization

Abstract

ABSTRACT : Polyurethanes are a very versatile family of polymers widely used in many applications. Traditionally, they are synthesized from the reaction between di-isocyanates and diols, with the drawback that both monomers are petroleum derivatives, very toxic and polluting. Thus, the aim of this research was the synthesis of environmentally friendly non isocyanate polyurethanes (NIPU), obtaining both monomers from sunflower oil (SFO), which is a renewable source, and using a solvent and catalyst-free route. The surfaces of the NIPU films obtained were modified with chitosan to give them antibacterial properties so that they could be used as biomaterials for medical applications. For the synthesis of NIPU, the double bonds of unsaturated fatty acids from sunflower oil, were epoxidized with formic acid and excess of hydrogen peroxide. The obtained epoxidized sunflower oil (ESFO) (epoxy content of 4.07 mmol g-1) was used to synthesize both monomers. First, ESFO was catalytic carbonatized with CO2 under 120 °C and 50 bar to obtain carbonated sunflower oil (CSFO), yielding 90% conversion to cyclic carbonate. Then, polyamine polyol (PAPO) was synthesized by ring-opening polymerization from ESFO and ethylenediamine (EDA), which leads to a non-crosslinked elastomer with a low glass transition temperature (Tg) of −42.97 °C and an amine value of 478.84 mg KOH/g. Crosslinking reaction at 90°C between CSFO and 50% excess of PAPO generated the NIPU films. Urethane group formation occurred between 18 to 24h of curing. The obtained NIPU was a heterogeneous crosslinked network, which was easily shaped like a film. Then, a two step-treatment was employed to perform the surface modification of NIPU films. For this purpose, ultraviolet radiation was used to graft acrylamide (AM), which was then bonded to chitosan (CS) through glutaraldehyde. Chemical, morphological, thermal, mechanical, and biological characterization of the materials was carried out. Proton Nuclear Magnetic Resonance (1H NRM) and Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR) were used to identify the structures and to confirm the formation of urethane groups. FTIR-ATR was also used to confirm the graft polymerization as the presence of AM and CS groups on the NIPU, after each step of the surface modification treatment. The changes in glass transition temperatures were studied by differential scanning calorimetry (DSC). NIPUs Tg lied between 5-10°C, associated with the crosslinking degree of 1857.32-14.065 mol/m3. After the surface modification, Tg decreases to -10.41°C, which may be related to a plasticizing effect, due to the presence of AM and CS related groups. 6 Unmodified NIPU films showed promising mechanical properties, for future coating purposes, displaying high elongation at break (313.47%). However, CS-modified NIPU films were not suitable for the mechanical test. The thermal stability of NIPU precursors, CSFO, and PAPO as well as of the unmodified and modified NIPU was investigated by thermogravimetric analysis (TGA). After the reaction between CSFO and PAPO, a polymer with enhanced thermal stability was obtained. However, the thermal stability of CS-modified NIPU decreased. Surface modification of NIPU with AM and CS was corroborated throughout each step by measurement of contact angle and scanning electron microscopy (SEM). The contact angle of unmodified NIPU revealed that it had a hydrophobic surface. After the treatment, an improvement of CS modified NIPU hydrophilicity was observed as a decrease in contact angle from 114.2° to 84.2° and the morphological analysis showed topographic changes in the SEM micrographs. CS-modified NIPU films did not show antibacterial performance inhibiting the growth of S. aureus and E. coli. Additionally, the percentage of cell viability on Detroit ATCC 551 Fibroblasts of the treated and untreated NIPU films was about 75%. Therefore, NIPU films showed potential cytotoxicity. These results showed a facile and green method to obtain high elastic isocyanate-free polyurethanes, starting from SFO, a renewable resource. However, after the first surface modification treatment, the expected performance of NIPU films as antibacterial biomaterials was not obtained. Therefore, it was presented a promising alternative method to perform the surface modification that involves the use of biomolecules such as heparin, alginate, and chitosan as well as the implementation of a common process as the layer by layer (LbL) self-assembly, which may have a significant role in improving the surface biocompatibility properties of the obtained NIPU films1. Besides, NIPU film’s flexibility and easy handling might make them useful as coating materials.