Effect of Gamma Irradiation on the Mechanical Properties and Thermal Properties of Poly(3-Hydroxybutyrate-Co-3-hydroxyhexanoate)(PHBHHx) Based Films

Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) is a naturally sourced polymer with good biocompatibility and potential applications in biomedical field after sterilization. Radiation sterilization technology has been extensively applied on medical products. However, for polymer-based biomedical materials, gamma irradiation can cause random chain scission on the polymer backbone. In this paper, we studied the effect of gamma irradiation on the mechanical and thermal properties of PHBHHx. The tensile strength and fracture strain of PHBHHx decreased after irradiation, while the brittleness increased after irradiation. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) results showed that the melting temperatures of irradiated PHBHHx are lower than the non-irradiated films, but the irradiated specimen shows a higher decomposition temperature than non-irradiated PHBHHx.


Introduction
Polyhydoxyalkanoates (PHAs) are polyesters produced by microorganisms under unbalanced growth conditions. The polymers are generally biodegradable and have good biocompatibility, which makes them attractive for biomedical applications. Several PHAs have been applied in biomedical field as implant materials, including poly 3-hydroxybutyrate (PHB), poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV), poly 4-hydroxybutyrate (P4HB), poly(3-hydroxyoctanoate) (PHO), and PHBHHx. PHBHHx was reported to have better performance over all other PHAs as mentioned above.
Biological materials must be sterilized before use. The traditional sterilization method, such as gas sterilization with ethylene oxide and steam sterilization, are not suitable for PHBHHx-based films. Gamma irradiation is the standard method in the medical device industry as it is cost-effective, and can be applied at room temperature.
However, gamma irradiation can cause random chain scission or chain crosslinking on the polymer backbone. The aim of this study is to investigate the effect of gamma irradiation on the properties of PHBHHx by thermal and mechanical analysis.

Materials
PHBHHx containing 12% hydroxyhexanoate was kindly donated by department of biological science and biotechnology of Tsinghua University (produced by Shantou Lianyi Biotech Co., Guangdong, China). PHBHHx was purified before use: PHBHHx was dissolved in chloroform under vigorous agitation for 2 h at 65 o C, then precipitated in methanol, and finally dried under vaccum. Purified PHBHHx is a white powder. All other solvents were of analytical grade.

Film preparation and irradiation
Sample films were prepared by solution casting method. All samples were irradiated in sealed packaging using a 60 Co source at the Department of Applied Chemistry of Peking University at room temperature, and the dose was 25, 45, and 70 kGy.

Methods
The spectra were obtained with an accumulation of 100 scans and with a resolution of 4 cm -1 . The tensile properties of PHBHHx films were measured by a SANS universal testing machine with a loadcell of 100N with a strain rate of 20 mm/min. The differential scanning   .Tensile testing is very useful to determine how gamma irradiation affects the mechanical properties of PHBHHx films. Fig. 2 shows the typical stress-strain plots of control and irradiated PHBHHx films. Mechanical properties are generally characterized by two parameters, the tensile strength and the elongation at break. Table 1 summarizes the two parameters of the control and irradiated specimens. As shown in Fig. 2   Three melting peaks were observed in the DSC heating cruves for control and irradiated specimens as reported (Fig. 3), the occurrence of peak I was a result of the melting of crystals formed upon long-time annealing, the other two main melting edothermic peaks II and III are caused by the model of melting, recrystallization and remelting of PHB crystals during the heating process. With the increase of irradiated dose, the melting temperature of peak I shifted towards a lower temperaturĊ. A similar shift was also observed in peak II, which may due to the decrease of PHBHHx crystal size. Thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTG) curves are shown in Fig. 4. The TGA curves of irradiated specimens shifted towards a higher temperature compared to control specimen (Fig. 4a), the initial decomposition temperature of PHBHHx irradiated with 70 kGy located at 287.16 o C, which is 6 o C higher than that of control specimen (281.28 o C ). The derivation of the thermal decomposition curve (Fig. 4b) shows the difference of peak temperatures between the irradiated and non-irradiated samples. The temperature of total weight loss of the control PHBHHx sample was 308.45 o C, and the value of irradiated PHBHHx with 25 kGy, 45 kGy, and 70 kGy are 314.70 o C, 314.05 o C, and 313.93 o C, respectively. We also notice that the irradiated specimen shows a slightly higher decomposition temperature than the control PHBHHx, indicating that the scissions that occur due to gamma irradiation do not affect the thermal decomposition temperature of the sample.

Conclusion
We have studied the effect of gamma irradiation on the structure and properties of PHBHHx films. The mechanical studies showed that tensile strength and fracture strain of PHBHHx decreased after irradiation with the increase of brittleness. Thermogravimetric analysis and differential scanning calorimetry results showed that the melting temperature (T m ) of PHBHHx films decreased with the increase of irradiation dose,