Modification Of Cellulose Via Aminosilane-Assisted Ball Milling And The Properties Of Poly (Lactic Acid) Composites

Nanocellulose has excellent properties such as renewable,biodegradable,high specific strength and easy-modified surface,making it an ideal choice for enhancing biodegradable polymers such as poly(lactic acid)(PLA).However,the application of nanocellulose in hydrophobic polymers has been limited by its strong hydrophilicity and poor thermal stability.In order to improve the interface compatibility between nanocellulose and PLA matrix,it is necessary to modify the surface of nanocellulose.However,most studies adopt a two-step method,namely preparation of nanocellulose first and then modification,which has the problems of long cycle and complicated steps.In this paper,the simultaneous fibrillation and modification of microfibrillated cellulose(MFC)were carried out by mechanochemical method,using 3-aminopropyltriethoxysilane(APTES)as modifier,combined with the physical fibrillation effect of ball milling.Then,the modified MFC(m MFC)was compounded with PLA to explore the effects of m MFC on the crystallization properties,heat resistance and mechanical properties of the composites.The main results are as follows:(1)MFC was fibrillated and modified via a one-step APTES-assisted ball milling,and then the cellulose films with excellent hydrophobicity were prepared.The results show that there is synergism between ball milling and APTES-modification on the fibrillation and surface-modification of MFC.Scanning electron microscopy(SEM)results show that the fibrillation degree of cellulose can be improved with increasing the amount of APTES or prolonging the milling time when ball milling was assisted by APTES.In addition,the sizes of the cavities in the cellulose films are reduced,the bonds between fibrillated cellulose are improved and therefore the cellulose films become more and more compact.The results of ultraviolet-visible-near infrared spectrophotometer show that APTES-assisted ball milling is an effective method to increase the transparency of the cellulose films.Fourier transform infrared spectroscopy and wide angle X-ray diffraction results show that the MFC is successfully modified by the APTES during ball milling and the crystal form of cellulose is preserved.The results of water contact angle(WCA)show that compared with ball milling or APTES-modification,the mechanochemical modification could significantly improve the hydrophobicity of the cellulose films.The WCAs of the cellulose films increase gradually with increasing the amount of APTES or prolonging the milling time,and the maximum is 133.2 ± 3.4°.Thermogravimetric analysis(TGA)results show that the thermal stability of the cellulose films is improved effectively under the synergism between ball milling and APTES-modification.The tensile test results show that the mechanical properties of the cellulose films in terms of strength and toughness are simultaneously improved with increasing the amount of APTES or prolonging the milling time.Some fractured and pulled-out nanofibers can be easily detected by SEM in the fracture surfaces,indicating that nanofibers act as reinforcement in the cellulose films.(2)The influence of mMFC on the properties of PLA composites was investigated in detail.PLA/m MFC composites were prepared by solution flocculation method,and then PLA/m MFC composite specimens were prepared by vacuum molding process.The crystallization behavior and morphology of PLA matrix in the composites were studied by differential scanning calorimetry and polarized optical microscopy.The results show that the addition of m MFC promotes the crystallization of PLA.TGA results show that the thermal degradation temperatures of PLA composites are slightly decreased with the addition of m MFC compared with that of pure PLA.However,compared with the m MFC modified by APTES directly,the m MFC obtained by APTES-assisted ball milling has much less influence on the thermal stability of composites.The tensile test results show that compared with pure PLA,m MFC can effectively improve the modulus of composites.And for the m MFC obtained by mechanochemical modification,the tensile strength of the composites is better than that of PLA composites in the control groups.Dynamic mechanical analysis results show that the storage modulus of the composites at 90 ℃ is significantly higher than that of pure PLA,and it gradually increases with increasing the content of cellulose,indicating that mMFC can effectively improve the heat resistance of PLA composites.