Preparation and properties of silk sericin/cellulose cross-linking films

Silk sericin/cellulose cross-linked films were successfully prepared using glutaraldehyde as crosslinkinger. FTIR was applied to characterize the chemical structure of films. Cross-linked silk sericin film was found the peak intensity of FTIR for cross-linked film decreased markedly compared to pure silk sericin, which indicating cross-linking reaction has been occurred. The increasing value of swelling ratio also indicated the cross-linking has been happened. The cross-linking reaction increased the thermal decomposition temperature.


Introduction
Biomaterials such as silk sericin and cellulose are considered as potential substitutes for synthetic polymers. Silk sericin delivered from silkworm silk, has been explored as a versatile biomaterial for various fields. It has excellent mechanical properties with high elastic modulus, elongation to break, and toughness [1][2][3]. In recent years biomaterials using flexible electronics based on organic field-effect transistors (OFETs) have attracted considerable attention. It have been introduced as gate dielectrics with low dielectric losses and leakage currents as well as sufficiently high dielectric breakdown strength. It has been introduced as gate dielectrics. B. Singh et. al. [4] fabricated an OFET device using a deoxyrionucleiv acid-based biopolymer for gate dielectric region. J.W. Chang et. al. [5] used albumen, as a gate dielectric in pentacene-and C 60 -based organic field-effect transistors.
However, Silk sericin materials are either too weak in wet state or too brittle and stiff in the dry state to application. Blending is an effective possible way to combine advantanges of each componet [6]. Blending of silk sericin and cellulsoe have been prepare to acquire biomaterial that are able to obtain good mechanical properites [7]. Meanwhile, to maintain water-insolubility chemical crosslinking reagents were added to blending film. Glutaraldehyde is an effective synthetic crosslinking agents that can react with amino acids or proteins [8]. It has been reported biopolymers using glutaradehyde as crosslinking agents show good mechnical properites.
In this work, silk sericin and cellulose cross-linking films were prepared. The effects of diffenernt cellulose content on the chemical structure of the silk sericin and the thermal behavior were confirmed by FTIR and DSC.

Materials
Silk sericin powder was purchased from Huzhou Xintiansi Biotec Co. (China). Glutaraldehyde was supplied by LingFeng Chemical Reagent Co. Ltd.. Cellulose was purchased from Aladdin Industrial Corporation (China). All chemical were of analytical grade, and used as received without further purification.

Specimens preparation
First 8% carbamide, 8% sodium hydrate and 6.5% thiocarbamide were mixed into conical flask with continuous magnetic stirring to make mixed solution. Silk sericin and cellouse with 2% contents of glutaraldehydewere dissolved into above mixed solution for 2h to generate a homogeneous solution. Finally the homogenous solution was poured into a smooth plate and placed in an oven at 60 o C until dried silk sericin/cellouse samples were obtained.

Characterizations
Fourier-transform infra-red (FTIR) spectroscopy has been used to monitor the chemical reactions. FTIR were recorded by nicolet NEXUS-670 IR spectrometer in the wavelength range of 4000-400cm -1 .
X-ray diffraction patterns were recorded in XD-6 Xray diffractometer (China). The length and width of all test samples are 20mm and 15mm, respectively. The wavelength of the monochromated X-ray from Cu Kα radiation is 0.154056nm. Scanning 2θ range was 10-60° with a scanning rate of 2°/min. Differential scanning calorimetry was used to measure the melting temperature of samples. Sample weights ranged from 3 to 5 mg. All samples were heated MATEC Web of Conferences from room temperature to 400℃ at a rate of 20℃/min. Heating curves was recorded.
The weight of 0.1g dry sample was immersed into adequate deionizer water at room temperature for 24h to reach the swelling equilibrium. Then, the swelling ratio (g/g) of sample was calculated.

Chemical structure of silk sericin/cellouse cross-linked films
The FTIR spectra of pure silk sericin, pure cellouse and silk sericin/cellouse cross-linked films with 2% glutaraldehyde were shown in Fig.1. Silk sericin exhibited characterisitc amide absorption bands of protein, amide, and at 1635, 1415, and 1093 cm -1 , respectively, which indicating random coil structures of silk sericin [6,7]. Cellouse exhibited characterisitc bands at 1635, 1419, 1310 and 1084 cm -1 , respectively. The peak at 1635 cm -1 was mainly corresponded with the stretching vibrations of C=O and -NH group. Compared to pure silk sericin and cellouse, cross-linked film with 2% glutaraldehyde was found the peak intensity decreased markedlly, which indicating crosslinking reaction has been occurred.  Fig. 2 shows the XRD patterns of pure silk sericin, pure cellouse and 2% glutaraldehyde cross-linked silk sericin films. Diffraction peaks of 2θ=21.9º, 28.0º for pure silk sericin are assigned to the silk Ⅱcrystal planes, which is the spun silk state (β-sheet silk structure). Pure cellouse shows diffraction peaks at 2θ=22º, 34.6º. For 2% glutaraldehyde cross-linked film, it can be found that the diffraction peaks of 2θ=21.9º disappears. The results show that the silk sericin crystalline structure affected by glutaraldehyde and cellouse.  Fig. 3 shows the DSC thermograms of heating scan for pure silk sericin, pure cellouse and cross-linked film with 2% glutaraldehyde from room temperature to 400 o C. Both silk sericin and cross-linked film display a peak nearly 150 o C, which is caused by desorption of water and motivation of molecular chain in amorphous region [8,9]. Meanwhile, an weak endothermic peak at 300 o C have appeared for pure silk sericin , representing the thermal decomposition of silk sericin. And then, an interse endothermic peak at 300 o C comes out in cellouse, representing the thermal decomposition of cellouse [10]. The peak at 300 o C disappeared for cross-linked film compared to pure silk sericin, which indicating the crosslinking reaction increasing the thermal decomposition temperature.

Swelling behaviors of silk sericin/cellouse cross-linked films
The swelling capacity in deionized water has been studied to evaluate the water-holding capacity. The CMPSE2017 swelling capacities of pure silk sericin and cross-linked silk sericin film were shown in Fig. 4. The swelling ratio of cross-linked silk sericin film increased compared to pure silk sericin. Swelling behavior depends on hydrophilicity/hydrophobicity, crosslinking density and the characteristics of the external solution. The increasing value of swelling ratio indicated the crosslinking have been happened.

Conclusions
In this work, we report silk sericin/cellouse cross-linked films based on glutaraldehyde as crosslinking agent. FTIR was used to confirm the cross-linking structure of cross-linked films. The cross-linking reaction increased the thermal decomposition temperature. The increasing value of swelling ratio indicated the crosslinking have been happened.