Preparation Of Different Aspect Ratio Nanocellulose And Its Effect On The PLLA/PDLA Composite Films

Polylactic acid is a high-molecular material whose raw materials are derived from biomass and has excellent biodegradability.It is extensive used in biomedical,textile,packaging and other fields.Because polylactic acid has demerits such as slow crystallization speed,weak mechanical strength and poor heat resistance,it cannot satisfy the needs of packaging materials.Its applications get restricted.Poly-L-lactic acid?PLLA?and poly-D-lactic acid?PDLA?are blended in a 1:1 ratio to prepare stereocomplex polylactic acid,which has a melting point that is about 50 ^oC higher than that of its homopolymer,which enhances the heat resistance of polylactic acid.In order to improve the performance of polylactic acid,adding nanoparticles is also an effective method.Nanocellulose has large quantity of benefits,such as large specific surface area,high modulus,excellent mechanical properties,low cost and non-toxic and harmless.Chemical modification of nanocellulose is more conducive to exert its advantages and expand its application range.In this study,waste cotton fibers dyed by actively,direct and indigo dye were used to prepare the different aspect ratio nanocellulose.The different aspect ratio nanocellulose included cellulose nanospheres?CNS?,cellulose nanocrystals?CNC?and cellulose nanofibers?CNF?.The processing of prepared nanocellulose was dispersed three steps roughly:Na OH solution treatment,HNO₃/HCl/H₂O treatment and H₂SO₄ hydrolytic treatment.From TEM testing,the different aspect ratio nanocellulose were prepared successfully.The diameter of CNS was15-30 nm.The CNC's diameter and length were 10-15 nm,200-500 nm,respectively.The diameter and length of CNF were 15-20 nm,1000-3000 nm.The XRD testing showed that the degree of crystallinity CNS?CNC and CNF were 87.7%,85.9%and 79.4%,respectively.What is more,the crystal form was not changed and kept the crystal structure of nanocellulose I.The FTIR spectrograph of CNC and ACNC?acetylated cellulose nanocrystals?proved the acetylation treatment was successfully.The degree of substitution of acetylation was 1.24calculated by FTIR result.The contact angle of ACNS,ACNC and ACNF also displayed the hydrophilia of CNS,CNC and CNF became poor.And the contact angle of ACNS,ACNC and ACNF was increased from 52.5°to 64.3°,65.1°and 66.8°,respectively.The PLLA/PDLA?LD?composite films were prepared by adding the 0 wt%,1 wt%,3 wt%,5wt%,10 wt%of ACNS,ACNC and ACNF,respectively.According to the result of DSC,the ACNS,ACNC and ACNF had positive effect on the formation of stereocomplex PLA?sc-PLA?.The degree of crystallinity of ACNS/LD composite films was higher than other two kinds of composite films.ACNS had the most effective heterogeneous nucleation effect.Above all the LD composite films,ACNS-5 has the highest degree of crystallinity,that was44.9%.In addition,ACNS could restrain the production of homocrystallite PLA?hc-PLA?.At the same time,the result of POM was accordance with DSC.Moreover,the size of crystal nucleus of ACNS-5 was smallest and the density was largest.The TGA result indicated ACNS-5 had most outstanding mechanical property.About the mechanical property testing,the Young's modulus?E?and tensile strength???of ACNF/LD composite films were higher than other two kinds of composite films.ACNF-3 had the highest E and?.Otherwise,the elongation at break of all the LD composite films had decreased.From the result of oxygen barrier property of LD composite films,ACNC/LD composite films proved the excellent barrier property.When the content of ACNC was 5 wt%,the oxygen permeability coefficient was smallest value,3.5×10^-10 cm³·cm/m²·s·cm Hg.Test results of optical properties showed the transmittance of the LD composite film did not change significantly,but the haze value slightly increased.