| Chitosan, exhibiting a good film forming property, is a potential biomass environmental friendly packaging material resource. In this paper, deacetylation degree (DD) 85% and 95% chitosan are selected as material, and sorbitol was acted as plasticizer, and acetic acid was played a role for chitosan solution. pH, conductivity and rheological properties of different chitosan solution were firstly analyzed. Based on these investigations, and then different kinds of chitosan films were prepared by film casting method, and the structure of chitosan films were characterized. At last, physicochemical, mechanical and thermal properties of different chitosan films were determined. The main results were showed as follow:1. The results of chitosan film forming solution properties analyzed showed:(1) chitosan presented little influence on pH and conductivity in the film forming solution system. With the concentration change of sorbitol, acetic acid and time, the pH and conductivity varied in a way. Based on the changing regularities of the chitosan-sorbitol-acetic acid blend solution system, sorbitol and acetic acid might be cross-linking with chitosan molecules, and then the stable, homogeneous and sticky solution were formed. (2) Different kinds of film-forming solution all presented a pseudoplastic fluid, and the addition of sorbitol did not modify the rheological behavior of chitosan solution. With the increase of shear rate, apparent viscosity decreased first and then gradually kept constant. The shear stress increased first, and then gradually remained nearly constant. The apparent viscosity and shear stress of deacetylation degree (DD) 85% chitosan solution were higher than those of corresponding deacetylation degree (DD) 95% chitosan solution. For two kinds of deacetylation degree (DD) chitosan solution, apparent viscosity and shear stress of solution with sorbitol were higher than those of solution without sorbitol.2. The structural investigation of chitosan powder and their films presented:(1) the surface and cross-section of chitosan films were amplified at different multiples to observe the microstrucrure. We found the forming films were homogenous and continuous. It suggested that sorbitol was a good crosslinking agent, and could generate the desired miscibility with both types of chitosan. (2) XRD analysis revealed that compared with original chitosan powder, an increase in crystallinity for films without sorbitol and a decrease for films spiked with sorbitol. (3) FT-IR showed that compared with original powder, beside the intensity change of characteristic absorption peak, films with and without sorbitol did not present unknown chemical bonds in films. (4) CP/MAS 13C-NMR results indicated that the acetic acid solution partially destroyed chitosan chemical bond in chitosan film forming system firstly, and emerged some carbocation ion group. Thus, sorbitol could be inserted in the polymer matrix, and then formed more hydrogen bonds causing homogenous and stable chitosan films.3. The physicochemical, mechanical and thermal properties of films presented some differences between two kind deacetylation degrees chitosan crosslinking sorbitol:(1) thickness, water content (MC), water solubility (WS) and water vapor transmission (WVP) of four kind chitosan films after analyzed showed sorbitol addition significantly reduced (p<0.05) moisture content and water vapor permeability (p<0.05), and significantly increased (p<0.05) the water solubility of both film types. For the films spiked and unspiked sorbitol, deacetylation degree (DD) 95% films had lower MC and WVP, and higher WS than those of deacetylation degree (DD) 85% films. (2) Static thermomechanical analysis (TMA) tests showed that for two kind deacetylation degrees chitosan films, compared with films unspiked sorbitol, sorbitol decreased stress while increased strain of chitosan film. For chitosan films sipked and unspiked sorbitol, strain of deacetylation degree (DD) 95% films were higher than that of the corresponding deacetylation degree (DD) 85% films, while stress of deacetylation degree (DD) 95% were lower than those of deacetylation degree (DD) 85% films. Dynamic mechanical analysis (DMA) tests indicated that compared with films unspiked sorbitol, sorbitol decreased storage modulus and loss modulus of two kind deacetylation degree chitosan films, showing that sorbitol increased elongation at break while decreased tensile strength of chitosan films. In addition, chitosan films with and without sorbitol, storage and loss modulus of deacetylation degree (DD) 95% films were higher than those of deacetylation degree (DD) 85% films. Curves of loss factor (Tan delta) analysis demonstrated that sorbitol increased (3 and a phase transition temperature which led to the rigidity decrease and elasticity increase of films, meanwhile, the phase transition temperature of deacetylation degree (DD) 95% films were higher than that of deacetylation degree (DD) 85% films.(3) Thermogravimetrics analysis indicated that for two kind deacetylation degrees (DD) chitosan, compared with chitosan power, thermostability improved to some extent after forming films. Compared with films unspiked sorbitol, thermostability of films spiked sorbitol were lower. For films spiked and unspiked sorbitol, thermostability of deacetylation degree (DD) 95% films was higher than that of deacetylation degree (DD) 85% films. Differential scanning calorimetry (DSC) presented that the thermodynamic properties changed of two kind deacetylation degree (DD) chitosan fore-and-aft films spiked and unspiked sorbitol was consistent with the results of thermogravimetrics analysis. Moreover, during two thermal degradation stages, the onset temperature, peak temperature and enthalpy of films with sorbitol were lower than those of films without sorbitol, indicating that sorbitol decreased the thermostability of chitosan films. The film thermostability was affected by crystal form transition during film forming process. |
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