Preparation Of Long Chain Branched Poly (Lactic Acid) Via Transesterification And Its Property Study

Poly(lactic acid)(PLA)possesses excellent biodegradability.However,poor melt strength and strain hardening effect of PLA,due to its semi-rigid linear molecular chain,severely restrict its application in large strain processing occasions such as extrusion foaming.The introduction of long chain branched structure is one of the strategies to solve this problem well.Therefore,highly efficient and environmentally friendly post reaction preparation techniques,which are suitable for continuous application of long chain branched poly(lactic acid)(LCBPLA),is the research focus in recent years.The dominance of branching reaction in the competition with chain degradation in PLA melt is the key to the preparation of LCBPLA.And the branched chain length as well as chain topological structure directly affect the melt properties of LCBPLA.Therefore,new theoretical guidance and basis for the design of high-performance LCBPLA with higher efficiency and environmental friendliness should be provided via the study of novel approach for preparing LCBPLA,the novel theory establishment of long chain branched reaction,and the clarification of LCBPLA chain structure.In this paper,the chain structure of PLA was modified through transesterification by taking advantage of the large number of ester groups in the PLA main chain that can be fractured and recombined under the action of water,heat or promoters.A kind of star shaped LCBPLA was prepared by transesterification between PLA macromolecules and multi-functional ester monomers using Nano-ZnO as a promoter.The long chain branched reaction and the structure of branched products were characterized by rheology measurements,SEC,DSC,SEM,TEM and ~1H NMR.The effects of Nano-ZnO content,size,microstructure and surface modification on the rheological properties,molecular topology,molecular weight and molecular weight distribution of LCBPLA were studied.The degradation behaviors of linear PLA promoted by Nano-ZnO in the solution was studied by statistical method,and the evolution mechanism of PLA chain structure with the prolongation of reaction time in the long chain branched reaction system was revealed.Moreover,the effects of chain topology on the crystallization behavior and extrusion foaming behavior of PLA were investigated.The paper has achieved the following main results:(1)Star-shaped LCBPLA was successfully prepared by transesterification reaction between linear PLA and trifunctional monomer TMPTA in the molten state with Nano-ZnO as a promoter.The rheological properties of PLA melt were greatly modified by the presence of the star-shaped branching structure.As a result,the storage modulus in the low-frequency area was significantly increased,and the continuous shear thinning phenomenon can be observed in the whole test frequency range.Moreover,the terminal relaxation time was longer,resulting in“multiple relaxation behavior”.More importantly,the melt strength and strain hardening effect were significantly improved.Compared with linear PLA,melt strength of LCBPLA can be increased by 12 times to30 c N with the introduction of long chain branched structure even in the case of slightly lower molecular weight.It was found that the morphology and size of Nano-ZnO had a great influence on the formation of long chain branched structure.When six-prism shaped Nano-ZnO with an average size of 50 nm was applied,LCBPLA can be effectively obtained.(2)Double hybrid branched structure LCBPLA was successfully prepared by Amino-ZnO as a promoter via double hybrid branched(DHB)mechanism.It was found that the star shaped LCBPLA was respectively centered on TMPTA and Amino-ZnO in the system.Amino-ZnO can timely collect the active end groups of PLA fragment,and inhibit the excessive degradation,thus LCBPLA can be formed in a longer processing time,which is beneficial to long-time continuous production.Owing to the introduction of double hybrid branched structure,the strain hardening effect of LCBPLA melt was further enhanced,and the melt strength of PLA was up to 37 c N.(3)Based on the linear viscoelastic behavior of PLA prepared at different reaction time,the evolution of PLA chain structure was investigated.The degradation mode of linear PLA in the solution under the promotion of Nano-ZnO was simulated by statistical method.It was found that the degradation of PLA chain in the solution was firstly carried out through the long-range bite back of a few long chain end groups,which resulting in great reduction in the molecular weight of PLA in the early stage.Further experiments showed that the LCB reaction of PLA in the solution was prior to the degradation reaction.In the molten state,the competition between PLA branching and degradation was earlier and more intense.Amino-ZnO can timely collect the active end groups of PLA fragment and play the role of chain branching and expanding.As a result,the melt strength of LCBPLA was improved and the processing time was prolonged.(4)The crystallization behavior and extrusion foaming behavior of LCBPLA prepared by transesterification reaction under the promotion of Nano-ZnO were investigated.The results showed that the crystallization of LCBPLA was affected by the regularity of molecular chain,and meanwhile controlled by the restriction effect of Nano-ZnO on PLA chains.LCBPLA prepared by Amino-ZnO possessed high crystallization ability,which manifested as the increasing spherulite number and the decreasing spherulite size.Benefiting from the introduction of star shaped long branched chains,the extrusion foaming ability of PLA was effectively improved.Under the same conditions,LCBPLA foam products possessed higher foaming ratio and smaller foam hole size than those of linear PLA.