Improvement of the wet tensile properties of nanostructured hydroxyapatite and chitosan biocomposite films through hydrophobic modification

Abstract

Hydrophobic modification of chitosan/hydroxyapatite composites and chitosan films was performed using four n-alkyl acids with varying alkyl chain lengths (6 to 16 carbons), and these films were characterized for their hydrophobicity and tensile properties under wet conditions. Grafting was performed by a heterogeneous reaction in a 2.0 mol L⁻¹DMF solution of the acid at 80 °C for either one or five days. Characterisation by Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) verified the introduction of amide functionalities and alkyl chains. The degree of substitution (DS) ranged from 2–5% for the composite films (based on XPS N 1s narrow scans) reflecting the heterogeneous nature of the reaction. Water contact angle measurements and the tensile testing under wet conditions showed, in general, that the grafting time and alkyl chain length had little effect on the surface hydrophobicity and the tensile properties of films, respectively. However, these modifications had a pronounced effect on decreasing the water content of the films, suggesting that the grafting had occurred in the bulk of the film for longer grafting times. In contrast, the tensile properties of the grafted composite films were significantly improved relative to non-grafted films and a modulus of 393 ± 68 MPa and an UTS of 18.7 ± 1.5 MPa were reached for composite films grafted for one day. This study provides a simple method to improve the wet tensile properties of chitosan/hydroxyapatite composite films making them more suitable for biomedical applications.