Abstract
Thin films of silk fibroin were prepared by solvent evaporation from calcium chloride/ethanol aqueous solution. The influence of alcohol treatments on thermal, mechanical and optical properties of silk-fibroin-based film is presented. To understand the conformal structure of the alcohol-treated silk fibroin film, the IR spectral decomposition method is employed. The optical properties especially the optical transparency, haze and fluorescence emission of alcohol-treated silk fibroin film is systematically investigated together with the conformal structure to understand the effect of the fibril such as the beta-sheet influencing the optical properties. Monohydric alcohol treatment increased beta-turn content in the regenerated silk fibroin structure. These affected the amount of light diffusion and scattering within silk-fibroin films. With alcohol-treatment, all the silk-fibroin films exhibit exceptional optical transparency (>90%) with different levels of optical haze (2.56–14.17%). In particular, ethanol-treated silk-fibroin films contain the highest content of beta-turns (22.8%). The ethanol-treated silk-fibroin films displayed a distinct interference of oscillating crests and troughs in the UV-Vis transmittance spectra, thereby showing the lowest optical haze of 2.56%. In contrast, the silk-fibroin films treated with methanol and propanol exhibit the highest (14.17%) and second-highest (10.29%) optical transmittance haze, respectively. The beta-turn content of the silk-fibroin films treated with methanol is the lowest (20.5%). These results show the relationship between the beta-turn content and optical haze properties. The results manifestly provide a method to manufacture exceptional optically transparent silk-fibroin films with adjustable light diffusion and scattering which can be designed to meet specific applications with the potential to provide UV-shielding protection via monohydric alcohol treatment.
Highlights
Silk broin is a biopolymer extracted from Thai silkworm cocoons by subtraction of sericin using a process called degumming
The results from FTIR con rmed that the peaks at 1618 and 1512 cmÀ1, representing b-sheet conformation of Amide I and II, show relatively higher intensities than that of the peak at 1229 cmÀ1, accredited to random coil deformation of Amide III
These results suggest that the silk- broin lm is predicted to show strong inter-chain interactions, and possesses superior strength
Summary
Silk broin is a biopolymer extracted from Thai silkworm cocoons by subtraction of sericin using a process called degumming. The study of the in uence of alcohol treatments on properties of silk- broin lms, mainly for the high optical transparent coating applications, is presented. The optical properties especially the optical transparency, haze and uorescence emission of alcohol-treated silk- broin lms is systematically investigated together with the conformal structure to understand the effect of the bril compositions such as beta-sheet, beta-turn, alpha-helix and random coil, in uencing on the optical properties. The degummed silk broin (1.00 g) was dissolved in 5.00 mL of CaCl2/EtOH/H2O solution (mole ratio of 1 : 2 : 8) at 110 C for 2 h and dialyzed against deionized water at room temperature for 48 h, using cellulose dialysis bag with molecular cut-off (MWCO) of 12 000–14 000 Da (Cellu Sep®). The gel fraction was calculated from the percentage ratio of the remaining solid weight and the initial weight of the dried lm using eqn (1):
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