Structural, mechanical and thermal properties of gamma irradiated SF_HPMC blend films

Abstract

Introduction: The properties of the silk fibroin can be improved by blending silk with some other biopolymers. The improvement in properties of the blend depends on the degree of compatibility or miscibility of the polymers at the molecular level (1). Depending upon the degree of molecular mixing, blends can be compatible, semi-compatible or incompatible. Gamma irradiation is one of the techniques used to promote the compatibility between the individual polymer molecules of the polymer blends. Here gamma irradiation is performed in order to investigate the effect of gamma radiation on the silk fibroin blends, which may be useful for development of silk fibroin blends for biomedical applications in the future. The goal of this study was to prepare and characterize SF_HPMC blend films, aiming at an insoluble film, with a more stable and crystalline structure. Later the blend film was exposed to gamma radiation of different doses. Methods: The Silk Fibroin solution was prepared from Bombyx mori CSR4 bivoltine cocoons. Prepared SF solution was added with Hydroxypropyl Methylcellulose solution of same weight percentage (5 wt%), with constant stirring at room temperature. The final solution mixture is tuned into free standing flexible film of SF_HPMC by solution casting method. Gamma irradiation of SF_HPMC blend films was done at CARRT centre, Mangalore university, using Cobalt-60 source, with a dose range of 0-250 kGy (50 kGy interval) at a dose rate of 7.065 kGy/h. The average energy of the gamma photons was 1.25 MeV. The irradiation was carried at normal temperature and pressure. Structural property of the samples was studied by using Rigaku Miniflex-II, X-ray Diffractometer. Mechanical studies of the sample were done by universal testing machine (Zwick Roell Z020, Germany) according to ASTM D882. Thermal properties of the samples were studied using TA instruments, SDT Q600 analyzer. Results & Discussions: The XRD results suggested that both α-helical (silk I) and β sheet (silk II) components were present in the SF_HPMC irradiated with the high doses of gamma radiation. It also seems that up to 100 kGy of gamma energy the composition of silk fibroin and its crystal conformation did not change significantly. The increase in doses of gamma radiation increases the crystallite sizes and decreases the lattice strain slightly. Mechanical strength analysis shows the breaking strength and elongation at break of SF_HPMC films showed almost a linear downward trend due to free radical formation, chain scissions, and disordering of silk fibroin chains due to the breakage of hydrogen bonds and peptide bonds in amorphous region, which results loss of the average molecular orientation in the polymer assembly. The thermal stability of SF_HPMC blend film decreases with gamma irradiation, which is due to the weakening of molecular interactions between SF and HPMC components and decrease in the percentage composition of silk II (β- sheet components). Conclusions: It was found from results that the increase in the energy of incident gamma radiation the thermal stability and mechanical strength of the SF_HPMC blend films decreased. Also, the structural and chemical properties of the blend films were slightly altered with increase in the dose of gamma radiation. The various characterization techniques used here to study the properties of the gamma radiated Silk fibroin-HPMC blend revealed that when silk fibroin materials were sterilized by gamma irradiation smaller radiation doses should be used, otherwise irreversible damages (breakdown of secondary bonds and covalent bonds) on the materials would be caused. Keywords: Silk fibroin, Gamma irradiation, structural properties, chemical, mechanical, thermal properties References Shetty GR, Rao BL, Asha S, Wang Y, Sangappa Y, Preparation and Characterization of Silk Fibroin/Hydroxypropyl Methyl Cellulose (HPMC) Blend Films. Fiber Polym, 2015;16(8):1734-1741.