Study On The Structural Morphology And Properties Of PLA/PEG/CNF Materials

Because petroleum-based polymers are not renewable and cannot be biodegraded,resulting in increasingly serious environmental pollution.It is of great significance to research and develop bio-based biodegradable polymers with excellent comprehensive properties and broaden their application fields.As an outstanding bio-based polymer,polylactide（PLA）has excellent biodegradability,high strength,modulus and transparency,and is expected to replace traditional petroleum-based plastics in many fields.However,the poor toughness of PLA greatly limits its practical application.Therefore,in this work,to overcome the poor toughness of PLA,PLA was toughened by poly（ethylene glycol）s（PEGs）or cellulose nanofibers（CNF）,and high-toughness PLA-based materials were prepared by melt-blending method.The relationship between the structure and properties of PLA-based materials was studied in depth.The synergistic effect between the components and the mechanisms of strengthening and toughening were also discussed.The main research contents and results include the following five aspects:1.PLA was toughened by PEGs with five different molecular weights（M_W=200,600,6000,10000 and 20000 g/mol,marked as PEG-200,PEG-600,PEG-6000,PEG-10000 and PEG-20000,respectively）,and five PLA/PEG blend systems were prepared by melt blending.Firstly,the effects of PEG molecular weight on the dynamic rheological properties,crystallization behavior,mechanical properties and microstructure of PLA were compared and analyzed.For the same PEG content（10 wt%）,the addition of PEG significantly reduced the glass transition temperature（T_g）,tensile strength and modulus of PLA.Moreover,with decreasing PEG molecular weight,the T_g,tensile strength and modulus of PLA/PEG blends decreased,but the elongation at break and V-notched izod impact strength increased.Among the five PLA/PEG blend systems,the comprehensive mechanical properties of the PLA/PEG-10000 blends were optimal.Then,the effect of PEG-10000 content on the properties of PLA was systematically investigated.The introduction of PEG-10000weakened the interaction between PLA chains and increased the mobility of PLA chain,thus significantly improving the crystallization ability and flexibility of PLA.After adding 10 wt%PEG-10000,the crystallinity（X_c）of PLA/PEG-10000（90/10）blend increased from 4.0%of pure PLA to 49.0%.In addition,with increasing PEG-10000 content,the T_g,tensile strength and modulus of PLA/PEG-10000 blends decreased significantly,while the elongation at break and V-notched izod impact strength increased significantly.When PEG-10000 content was 20wt%,the elongation at break（207.7%）and V-notched izod impact strength（7.3 kJ/m²）of PLA/PEG-10000（80/20）blend reached the maximum,which increased by 6008.8%and114.7 respectively compared with the corresponding values of pure PLA.SEM observation found that the fracture mode of pure PLA and PLA/PEG-10000 blends(W_PEG-10000￡5wt%)was brittle fracture,and the toughening mechanism was mainly induced by crazes.However,with further increasing PEG-10000 content(10wt%￡W_PEG-10000￡20wt%),the internal cavitations of PEG-10000 induced a large area of intense shear yielding of PLA matrix,which was the main toughening mechanism,significantly improving the toughness of PLA and undergoing the brittle-ductile transition of PLA/PEG-10000 blends.2.PLA was reinforced and toughened by CNF,and PLA/CNF nanocomposites were prepared by melt-blending method.The effects of CNF content on the dynamic rheological properties,crystallization behavior,mechanical properties and microstructure of PLA were investigated.Based on the analysis of microstructure,the effects of the dispersion of CNF in PLA matrix and interfacial bonding between CNF and PLA on the mechanical properties of PLA/CNF nanocomposites,and the mechanism of CNF reinforced and toughened PLA were investigated.The addition of CNF improved the storage modulus（G￠）,loss modulus（G￠）and complex viscosity（η*）of PLA.Moreover,with increasing CNF content（≤3 wt%）,the G￠,G￠andη*values of PLA/CNF nanocomposites gradually increased.However,when CNF content exceeded 3 wt%,with continuously increasing CNF content,CNF agglomerated in the PLA matrix and increased the free volume,resulting in a slight decrease in the G￠,G￠andη*values of PLA/CNF（95/5）nanocomposite.Due to the heterogeneous nucleation of CNF,the introduction of CNF improved the crystallization rate and crystallization ability of PLA.As CNF content increased from 0 to 5 wt%,the crystallinity of PLA/CNF nanocomposites increased from 4.0%of pure PLA to 34.4%.In addition,the tensile strength and modulus as well as V-notched izod impact strength of PLA/CNF nanocomposites were higher than those of pure PLA,and first increased and then decreased with increasing CNF content.When CNF content was 3 wt%,the tensile strength（68.3 MPa）and modulus（4438.8MPa）as well as V-notched izod impact strength（7.1 kJ/m²）of PLA/CNF nanocomposites reached the maximum values,increasing by 9.5%,18.9%and 108.8%respectively compared with the corresponding values of pure PLA.In PLA/CNF nanocomposites,CNF as a stress concentration point induced a large amount of crazes,and with increasing CNF content（≤3wt%）,the interaction between uniformly dispersed CNF and CNF or PLA increased,and formed entangled 3D network structures.It could effectively prevent the further expansion of crazes,cracks or micropores,and produced rougher fracture surfaces,which was the main mechanism for improving the strength and impact toughness of PLA/CNF nanocomposites.3.The grafting of PLA onto CNF was successfully prepared by in situ reactive extrusion using dicumyl peroxide（DCP）as a free radical initiator to improve the interfacial adhesion between CNF and PLA.The effects of DCP content on the dynamic rheological properties,crystallization behavior,mechanical properties and morphology of PLA/CNF（95/5）nanocomposite were systematically investigated.Compared with PLA/CNF（95/5）nanocomposite,DCP-induced synthesized CNF-g-PLA graft copolymer as a compatibilizer improved interfacial adhesion and stress transfer at CNF-PLA interfaces,thus enhancing the mechanical properties of CNF-g-PLA nanocomposites;the maximum increases in tensile modulus and strength,elongation at break and V-notched izod impact strength of CNF-g-PLA nanocomposites were 31.8%,20.0%,12.0%and 27.9%,respectively.Thus,the theories of synergy effect of DCP and CNF reinforced and toughened PLA were concluded,and the toughening and reinforcing models of 3D network structures of CNF-g-PLA nanocomposites were established.DCP induced grafting improved the crystallization ability of PLA/CNF（95/5）nanocomposite.As a result,the crystallinity of CNF-g-PLA nanocomposites was higher than that of PLA/CNF（95/5）nanocomposite,and it first increased and then decreased with increasing DCP content.Hot stage polarized optical microscopy（HS-POM）observation demonstrated that DCP induced grafting significantly increased the nucleation density but reduced the spherulite size of PLA.In addition,DCP induced grafting enhanced the interaction between CNF and PLA molecules,thereby improving the melt strength of CNF-g-PLA nanocomposites.4.PLA/PEG-10000/CNF nanocomposites were prepared by melt-blending method using PEG-10000 as a compatibilizer to improve the interfacial compatibility between CNF and PLA and the dispersion of CNF in the PLA matrix.The effects of CNF content on the dynamic rheological properties,crystallization behavior,mechanical properties and microstructure of PLA/PEG-10000（80/20）blend were systematically studied.The mechanisms of synergy effect of CNF and PEG-10000 reinforced and toughened PLA were discussed by comparing the changes of microstructure and properties of PLA/CNF（95/5）nanocomposite before and after adding 20 wt%PEG-10000.The addition of CNF significantly increased the G￠,G￠andη*values of PLA/PEG-10000（80/20）blend,and increased with increasing CNF content.In the low frequency region,the G￠,G￠andη*values of PLA/PEG-10000/CNF5 nanocomposite were higher than the corresponding values of PLA/CNF（95/5）nanocomposite,indicating that the addition of 20 wt%PEG-10000improved the dispersion of CNF in the PLA matrix,which was also confirmed by SEM and TEM observations.The combination of plasticization and dispersion of PEG-10000 with nucleation of CNF significantly improved the crystallization ability of PLA.As a result,the crystallinity of PLA/PEG-10000/CNF nanocomposites was significantly higher than that of PLA/PEG-10000（80/20）blend or PLA/CNF（95/5）nanocomposite.When CNF content was0.5 phr,the crystallinity of PLA/PEG-10000/CNF nanocomposites reached the maximum value（51.7%）,and increased by 19.1%and 50.3%respectively compared with the crystallinity of PLA/PEG-10000（80/20）blend and PLA/CNF（95/5）nanocomposite.The combination of plasticization and dispersion of PEG-10000 with enhancement of CNF made PLA/PEG-10000/CNF nanocomposites possess superior mechanical properties.Their tensile strength and modulus as well as V-notched izod impact strength were higher than the corresponding values of PLA/PEG-10000（80/20）blend.Moreover,when CNF content was 1phr,the tensile strength,elongation at break and V-notched izod impact strength of PLA/PEG-10000/CNF nanocomposites reached the maximum values,and increased by27.4%,27.2%and 31.5%respectively compared with the corresponding values of PLA/PEG-10000（80/20）blend.Thus,the mechanisms of synergy effect of CNF and PEG-10000 reinforced and toughened PLA were summarized,and the toughening and reinforcing models of interfacial adhesion of PLA/PEG-10000/CNF nanocomposites were established.5.To understand the effects of PEG-10000 or CNF on the crystallization behaviors of PLA,the non-isothermal crystallization behaviors of pure PLA,PLA/PEG-10000 blends and PLA/PEG-10000/CNF nanocomposites at different cooling rates were studied by differential scanning calorimetry（DSC）.Moreover,their crystallization mechanisms and non-isothermal crystallization kinetic parameters were analyzed by Jeziorny,Ozawa,Mo and Kissinger methods.With increasing cooling rate,the crystallization rates of pure PLA,PLA/PEG-10000blends and PLA/PEG-10000/CNF nanocomposites increased,while their crystallization temperatures decreased.At the same cooling rate,the addition of PEG-10000 weakened the interaction between PLA chains and improved the segmental mobility of PLA molecules,resulting in the crystallization rate and temperature of PLA/PEG-10000 blends higher than the corresponding values of pure PLA,and increased with increasing PEG-10000 content.The addition of a small amount of CNF could improve the crystallization rate and crystallization onset temperature of PLA/PEG-10000（80/20）blend due to the heterogeneous nucleation of CNF.This was consistent with the trends of crystallization rates obtained by Jeziorny and Mo methods.The PLA/PEG-10000 blends crystallized mainly in the form of three-dimensional growth with homogeneous nucleation,while the addition of CNF promoted the crystallization of PLA/PEG-10000 blends.In addition,the crystallization activation energy（E_g）of pure PLA,PLA/PEG-10000 blends and PLA/PEG-10000/CNF nanocomposites during non-isothermal crystallization was calculated by Kissinger method.The results showed that the addition of PEG-10000 or a small amount of CNF could reduce the E_g value of PLA.