Preparation,Crystallization Behavior And Properties Of Biodegradable Polyester/Cellulose Nanocrystals Composites

To deal with the ecological environment pollution and resource shortage,biodegradable polyesters with green derivation,superior biodegradability,and biocompatibility have been considered as a potential alternative to conventional non-degradable plastics.The deficiencies in the crystallization and mechanical properties of biodegradable polyesters can be regulated and improved by preparing polyester-based composites,which benefit for further expanding their practical application.In this dissertation,a series of biodegradable polyester/cellulose nanocrystals(CNC)composites with different loadings of CNC were prepared through a simple solution and casting method.With regulating the CNC content and polyester structure,the dispersion of CNC,the interaction between CNC and polyester matrix,crystallization behavior,nucleation and crystal growth,mechanical properties,as well as rheological property of the composites were systematically investigated.The main research and conclusions are as follows:1.Homopolyester/CNC composites:Poly(ethylene succinate)(PES)is a promising biodegradable polyester with relatively high thermal degradable temperature and comparable mechanical properties similar to those of polyethylene.However,the slow crystallization rate severely limits its actual processing.Therefore,CNC was introduced into PES matrix to improve its crystallization behavior and physical properties in this dissertation.Compared with neat PES,the composties demonstrated significantly enhanced melt crystallization behavior under different conditions.As a heterogeunous nucleating agent,CNC achieved high nucleation activity in PES.Meanwhile,CNC efficiently promoted the spherulite nucleation density and reduced the crystallization activation energy of PES.The rigid nanoparticle also increased the storage modulus and glass transition temperature of the matrix,together with improving its rheological properties.In addition,the crystallization enhancement of CNC in PES displayed a strong CNC content dependence.With CNC loading incrasing,the reduction of dispersibility resulted in a trend of first rising and then declining on the performance promotion effect of the composites.The above results confirmed that CNC may promote the crystallization and mechanical properties of PES.In order to investigate whether CNC has a similar promoting effect in other homogeneous polyesters,this work further extended to poly(ethylene adipate)(PEA)homopolyester to study the universality of CNC in polyester modification.CNC effectively improved the crystallization behavior of PEA.The nucleation enhancement in PEA/CNC composites was not as effective as that in PES/CNC composites,due to the stronger interface interaction between PES and CNC than that of between PEA and CNC.In this dissertation,we also investigated and compared the effects of CNC and carboxyl-modified multi-walled carbon nanotubes(f-MWCNT)on the performance improvement of PEA at the same content(1 wt%).f-MWCNT with higher aspect ratio showed a stronger interfacial affinity with PEA and demonstrated a better nucleation promotion effect.In addition,rigid rod-like CNC displayed a better mechanical enhancement than tubular f-MWCNT.2.PBS based copolyester/CNC composites:Polybutylene succinate(PBS)with high mechanical strength,good thermal properties and renewability,is one of the successfully commercialized biodegradable aliphatic polyesters.Although a large number of PBS-based copolyesters have been developed to expand its pratical application,the crystallization properties of these copolyesters still need to be further improved.In the modification of these copolyester composites,the chain structure has an important impact on the dispersion of the fillers and final property performance of the composites.Based on the above discussed study of homopolyester/CNC composites,in this section two kind of PBS-based copolyesters with different chain structures,i.e.,linear poly(butylene succinate-co-butylene adipate)(PBSA)and long side group branched poly(butylene succinate-co-1,2-decylene succinate)(PBDS),were chosen as the model systems to prepare a series of copolyster/CNC composites,respectively.Compare with the copolyester,the composites exhibited prominently increased crystallization temperature and accelerated overall crystallization rate.CNC,acting as the nucleation agent for the two copolyesters,increased the nucleation density and mantained the crystallization mechanism and crystal structure of the matrix.The long side groups of PBDS provided more free volume,resulting in better interfacial affinity in PBDS/CNC composites.CNC demonstrated higher nucleation activity and nucleation efficiency in the PBDS matrix;therefore,the performance improvement in PBDS/CNC composites was higher than that in PBSA/CNC composites.In addition,the composites showed significantly reinforced tensile strength and modulus,while the thermal stability was barely affected by CNC.3.Ternary fully biodegradable PLLA/PDEGA/CNC compositesAs the most widely used bio-based,biodegradable polyester,poly(L-lactic acid)(PLLA)suffers two main defenciencies of both slow crystallization and poor toughness.Based on the nucleation and growth characteristics of polymer crystallization,the combination of both nucleating agent CNC and plasticizer poly(diethylene adipate)(PDEGA)was introduced at the same time to prepare fully biodegradable ternary PLPA/PDEGA/CNC composites for the first time.Compared with neat PLLA and binary modified PLLA systems(PLLA/PDEGA blends or PLLA/CNC composites),the ternary composites displayed the lowest cold crystallization peak temperature,highest melt crystallization temperature,and fastest isothermal melt crystallization rate,indicating the significantly synergistic effect of CNC and PDEGA on the crystallization of PLLA.Specifically,CNC and PDEGA promoted nucleation and crystal growth of PLLA in the matrix,respectively,so that the ternary composites possessed the shortest crystallization half-time.Increasing the content of CNC or PDEGA showed a positive effect on the further improvement of PLLA crystallization.Tensile test results suggested that the combination of PDEGA and CNC prominently increased the elongation at break of PLLA,while the strength and modulus were not greatly reduced,benefiting for the practical application of PLLA.In brief,in this dissertation the introduction of CNC into different biodegradable polyesters was performed for the modification of these polyesters.CNC displayed reinforcement and nucleation promotion in polymer matrix,affecting the crystallization behavior,mechanical and rheological properties of the composites.The influence of CNC content,different chain structures of polymer matrix,and the interfacial interaction between CNC and matrix on the enhancement of the crystallization and physical properties of polyesters were systematically studied and discussed.The results of this dissertation may provide a useful reference for the property regulation and practical application of biodegradable polyester/CNC composites.