Controlled Acetylation Of Cellulose Nanocrystal And Its Composite With Poly(Lactic Acid)Studied By Infrared Spectroscopy And Imaging

Cellulose nanocrystal(CNC)is a highly crystalline nanomaterial with large aspect ratio,high mechanical strength and modulus,which has a promising prospect in biodegradable polymer materials.The dispersity of CNC in the hydrophobic polymer matrix is poor due to the rich hydroxyl groups on the surface.Therefore,enhancing the compatibility between the CNC and polymer matrix is essential to improve the performance of their composite.To overcome this limit,the surface hydroxyl group of CNC can be acetylated.In this paper,the acetylation of CNC and its composite with PLLA were systematically studied by FTIR spectroscopy and imaging.Firstly,the acetylated cellulose nanocrystal(ACNC)with different degree of substitution(DS)were prepared.The linear regression equations with R2>0.99 between the absorbance ratio of the band at～1745 cm-1 to the one at 1164 cm-1 in ACNC’s FTIR spectra,which indicating the degree of acetylation and the DS from 1H NMR data was acquired.The reliability of the developed method was proved by verification experiments.The DS of ACNC can be directly determined by using the powder sample,avoiding its indissoluable problem.It is a simple,reliable and rapid method,and can be directly applied to any FTIR instrument,and can be extended to the determination of DS in any cellulose derivative.Secondly,the experimental scheme was arranged by uniform design,and ACNCs with different DS and crystallinity index(CrI)were prepared at different reaction temperature,reaction time and acetic anhydride addition volume.The mathematical models of DS and CrI versus the three factors,with the adjR2 of 0.9159 and 0.8998 at p<0.0001 were obtain by using multivariate stepwise regression,respectively.The regression models are reliable.Combining these regression models with response surface method(RSM),the controllable acetylation of CNC can be realized by directyly obtaining the relevant experimental conditions for specific DS and CrI from the RSM contour maps.The relative errors with less than 16.77%were obtained from three verification experiments.Finally,the compatibility and crystallization behavior of PLLA/ACNC nanocomposites were studied by FTIR imaging.The dispersity of ACNC in PLLA was clearly visualized using univariate analysis method,and the compatibility of the two increases with the degree of substitution of ACNC.The principal component analysis(PCA)further indicated that the compatibility became worse when the added amount of ACNC was larger than 5 parts.It was also found from the PCA results of infrared images of PLLA/ACNC nanocomposites that ACNCs with different DS had different effects on the crystallization behavior of PLLA.The crystallinity of PLLA decreases with the increase of ACNC when the degree of substitution is low(DS=0.51).However,it is still higher than pure PLLA.On the other hand,when the degree of substitution is high(DS=1.84),the crystallinity of PLLA increases with the addition of ACNC,which is consistent with the results of differential scanning calorimetry(DSC).