Preparation Of Strong,Tough,and Heat Resistant Polylactic Acid Nanocomposite Films And Their Properties

Polylactic acid(PLA)is a kind of macromolecule derived from corn and other plants.It has good physical and chemical properties,unique biocompatibility and biodegradability.It is called"the most promising biodegradable material",which has aroused great research interest.The excellent mechanical properties of PLA make it can be used in many fields such as packaging industry,construction industry,and biomedicine.In addition,the degradability of PLA makes it expected to become an ideal substitute for traditional petroleum-based polymers.However,PLA has some inherent disadvantages,such as poor toughness and low heat resistance,which limits its commercial large-scale application.Researchers have adopted a variety of methods to improve the properties of PLA,such as blending modification,copolymerization modification,nanocomposite modification,and so on.Nanocomposite modification process is simple and does not affect the degradability of PLA.It is a very promising research direction.Through a certain process,the nano filler can be well dispersed in the PLA matrix,provide nucleation sites and promote the crystallization of PLA.Only a small amount of modified nano filler can significantly improve the mechanical properties and heat resistance of PLA.Cellulose nanocrystalline(CNC)is a kind of nano filler derived from cellulose.It also has the advantages of biocompatibility and biodegradability.With a wide range of raw materials and recyclable regeneration,the CNC has great application potential in the field of nanocomposites.However,there are usually a large number of hydroxyl groups on the surface of CNC,which makes it difficult to disperse evenly in PLA matrix and easy to agglomerate.Moreover,the compatibility between hydrophilic CNC and hydrophobic PLA matrix is poor and needs to be improved.In this paper,the octadecylamine(ODA)was used as the modifier of CNC to prepare the ODA modified(or grafted)cellulose nanocrystals(g-CNC),which could improve the dispersion and compatibility of CNC in L-polylactic acid(PLLA).The PLA/g-CNC nanocomposite films were then fabricated for further investigation on their structure and properties.In order to further improve the heat resistance of PLA,the effect of nano silica on the properties of PLA was also studied.The main research contents and results are as follows:(1)Preparation of PLA/g-CNC nanocomposite films and their propertiesThe g-CNC was prepared by coating CNC by oxidative self-polymerization of tannic acid(TA)and grafting octadecylamine(ODA)on the surface of CNC by Michael addition/Schiff base reaction.The as-prepared g-CNC and commercial PLA were mixed in solution,and cast into film,and then combined with post-treatment crystallization process to prepare PLA/g-CNC nanocomposite films.The properties of the nanocomposite films were characterized by differential scanning calorimetry(DSC),hot stage polarizing microscope(POM),X-ray diffraction(XRD),mechanical properties test,heat resistance test,double beam ultraviolet visible spectrophotometer(UV-vis),and so on.It was found that the modified g-CNC can be evenly and stably dispersed in chloroform and the introduction of g-CNC can effectively improve the crystallization rate and mechanical properties of the nanocomposite films.Compared with the PLA/CNC nanocomposite films,the tensile strength and elongation at break of PLA/g-CNC nanocomposite films were significantly improved.When the addition of g-CNC is 0.5%,the tensile strength of the nanocomposite films reaches 65 MPa(increased by 48%),the elongation at break is 3.1%(increased by 55%),showing the effect of strengthening and toughening.With the increase of the g-CNC loading,the heat resistance and light transmittance of crystalized PLA/g-CNC nanocomposite films showed a gradual upward trend.When the addition of g-CNC was 1.0%,the average light transmittance of the nanocomposite films was 83%,which was 34%higher than that of the pure PLA films.The POM was used to observe the changes of spherulite morphology during isothermal crystallization.It was found that,with increasing the g-CNC content,the number and size of the PLA spherulites shows an increase and decrease,respectively,which was also an important reason for the increase of light transmittance of the nanocomposite films.(2)Preparation of PLA/g-Si O₂nanocomposite films and their propertiesHydroxyl groups are formed on the surface of silica(Si O₂)after hydrolysis,which can also be used as a grafting platform to introduce polymer segments.Nano silica was modified by3-Aminopropyl triethoxysilane and grafted with stearic acid long chain to obtain modified silica(g-Si O₂).The PLA/Si O₂nanocomposite films with different ratios were prepared by the same process.The properties of the nanocomposite films were characterized by mechanical properties test,dynamic thermomechanical analysis(DMA),and double beam ultraviolet visible spectrophotometer.It was found that after crystallization treatment,the addition of Si O₂and g-Si O₂can improve the mechanical properties of the PLA nanocomposite films,and the modification effect of the g-Si O₂is more significant.When only 0.2%g-Si O₂is added,the nanocomposite films have higher mechanical properties,and the tensile strength reaches 60 MPa,which is 34%higher than that of the pure PLA films.After adding 0.5%g-Si O₂,the elongation at break of the nanocomposite films is the highest(3.2%),which is 60%higher than that of the pure PLA films.After adding 1%g-Si O₂,the nanocomposite films have the highest tensile strength(63MPa),which is 43%higher than that of the pure PLA films.The DMA test results show that the storage modulus of the nanocomposite increases as the amount of g-Si O₂increases.After adding1%g-Si O₂,the storage modulus increases from 5.7 GPa to 7.2 GPa,indicating that the heat resistance of the nanocomposite films is improved.The UV-vis test results show that as the g-Si O₂content increases,the light transmittance of the crystalline composite film firstly increases and then decreases.When the addition amount is 0.2%,the nanocomposite films have the best light transmittance with an average light transmittance of 83%.Compared with PLA/g-CNC nanocomposite films,a small amount loading of g-Si O₂can obtain the better mechanical properties and light transmittance.