Study On Electrospun Polylactic Acid Nanofibers Reinforced With Cellulose Nanocrystals

Cellulose is one of the most abundant natural macromolecules and the main source of environmentally friendly materials in nature.Cellulose has been widely used as reinforcing material because of its excellent properties,such as high strength,high crystallinity,biodegradable and renewable characteristics.Nanocomposites can be prepared by self-assembly,phase separation,electrospinning and so on.Among these methods,electrospinning is widely used to prepare nanocomposites because of its simple device,low cost,ease of control.In recent years,researchers have focused to study the effects of different polymers and electrospinning process parameters on the structure and properties of composite fibers.Nevertheless there are limited studies on the morphology and properties of electrospun aligned nanocomposite fibers.In this study,polylactic acid(PLA)was selected as the polymer matrix,and cellulose nanocrystals(CNCs)were implanted into PLA matrix.Electrospinning technology was used to prepare nanocomposite fibers.The effects of CNCs loading on the morphology and properties of PLA/CNC nanocomposite films were discussed.The mechanism of CNCs reinforcement was further clarified.In addition,the differences between the morphology and the performances of the fibers with aligned structure and disordered structure were also compared.It is of great significance to provide more application possibility for the biomaterial-strengthed polymer via electrospinning.The specific contents are as below:(1)The effects of acid hydrolysis time and temperature on CNCs were investigated.The prepared CNCs were characterized by TEM,WXRD and FTIR.With the increase of hydrolysis time,the average length of CNCs decreased from 254±31nm to 180±30nm.On the contrary,its aspect ratio,crystallinity and agglomeration degree were increased.With the increase of hydrolysis temperature,the average length of CNCs decreased from 235±23nm to 161±22nm.Its aspect ratio,crystallinity and agglomeration degree were also increased.FTIR analysis showed that microcrystalline cellulose and sulfuric acid esterification reaction occurred.It was found that the degree of esterification was higher with the increase of acid hydrolysis time and acid hydrolysis temperature.Stable CNCs suspension in water was formed due to the presence of the ester sites on the CNCs.(2)The effects of different CNCs loading on the properties of electrospun PLA/CNCs nanocomposite films were investigated.An electrospinning process was successfully utilized to fabricate PLA/CNCs nanofibrous films.The morphological,crystallinity,thermal and mechanical properties of the PLA/CNCs nanofibrous composite films were investigated.At the same time,the specific surface area and pore size of the electrospinning PLA/CNCs composite film were investigated.The SEM results showed that the bead-on-string nanofibers gradually decreased with increasing the CNCs content,while the fiber diameter increased.The crystallinity index of the nanofibrous film with 11wt%CNCs content was up to 87.9%,compared with degree of crystallinity for the pure PLA nanofibrous film.The maximum thermal decomposition temperature was increased from 369.4~oC to 380.0 ~oC as CNCs content increased from 0%to 11%.Tensile test indicated that the tensile strength of the PLA/CNCs nanofibrous films was significantly increased with increasing CNCs content.The nanofibrous film with 11wt%CNCs showed the maximum tensile stress(3.76N)and the highest tensile strength(4.58MPa),which improved 289%and 159%respectively compared with the pure PLA film.The results provide a theoretical basis and a wider application field for the further study of electrospun nanocomposite fiber films based on PLA.(3)The nanocomposite films with disordered and aligned structures were prepared by means of electrospinning,using low and high speed drum collecting device respectively.The morphology,thermal stability and mechanical strength of the nanocomposite fibers of electrospun PLA/CNCs were analyzed by systematically comparing the disordered and aligned structures.The average diameters of electrospun PLA/CNCs nanocomposite fibers with disordered and aligned structures decreased first and then increased with the increase of CNCs loading.Compared with the pure electrospun PLA nanofiber films,the maximum pyrolysis temperature of the nanocomposite fiber membranes was increased from 374.9~oC to 385.7~oC.When the CNCs content was 10wt%,the electrospun PLA/CNCs nanocomposite fiber membrane had a rich mesoporous structure,and the maximum adsorption capacity of nitrogen reached to 48.07cm~3/g.Tensile test analysis showed that the tensile properties of nanocomposite fiber films increased significantly with the increase of CNCs.When the additions of CNCs was 10wt%,the maximum tensile strengths of disordered and aligned PLA/CNCs nanocomposite films were 4.74MPa and 10.39MPa respectively.The study provides a theoretical basis for the application of aligned electrospun porous fibers in the adsorption field.The above results demonstrated that,with CNCs incorporation,PLA/CNCs composite fibers prepared by electrospinning technology were improved in both thermal and mechanical properties.The properties of electrospun aligned fibersare significantly better than those of disordered fibers.It is of a certain guiding significance to the application of electrospun aligned composite films.This study provides a theoretical basis for the application of electrospun PLA/CNCs composite film materials in filtration and adsorption fields.