Different Molecular Weight And Structure Of Polylactic Acid Polymer With Three-dimensional Copolymer Of Condensed Matter, Thermodynamics And Crystallization Kinetics Study

During the past two decades,biodegradable polymers have attracted much interest in both basic research and chemical industry.Aliphatic polyesters present excellent degradation properties,with degradation by-products harmless to the environment and animal body.Especially,polylactide(PLA) is regarded as the most promising biodegradable polyester.The raw material of PLA,lactic acid,can be obtained from renewable sources.PLA is a thermoplastic with high strength and high modulus,and can be processed by using conventional industrial equipments and techniques.Up to now,a great deal of work has been reported on biomedical applications of PLA such as tissue engineering scaffolds,resorbable medical implants, sustained drug delivery systems and wound dressings.All these applications strongly depend on the specific physical properties of PLA.For example,high molecular weigh(MW) and highly condensed structure are required for higher-modulus PLA used as osteosynthetic materials,while drug delivery systems generally require amorphous copolymers of relatively low MW.This work consists in investigating the morphology and crystallization of PLA,including the thermodynamics,crystallization kinetics,condensed structure,morphology,and the influence of MW and configurational structure.Many researchers have studied the crystallization of PLA obtained by using Sn(Oct)₂ as catalyst.However,Sn(Oct)₂ is more or less cyto-toxic,which could limit the potential medical applications of PLA.Zinc lactate was used as catalyst in this work for the sake of biocompatibility.PLA with various MWs and configurational structures were thus synthesized for studies on the thermodynamics,crystal structure, and morphology etc.The main contents are shown as follows.1.Poly(L-lactide)(PLLA) was synthesized by ring opening polymerization of L-lactide using zinc lactate as catalyst.The basic thermal behaviors and morphologies were examined by using DSC,POM and SEM.The results show that the catalyst has no significant effect on the crystallization.Isothermally crystallized PLLA exhibits double melting behavior,which could be ascribed to melt recrystallization mechanism. Limited lamellar thickening was detected at certain temperatures,withαform crystal structure.Avrami analysis showed that PLLA crystallization starts with heterogeneous nucleation,followed by three-dimensional growth.The maximal overall crystal growth rate was obtained at approximately 105℃.Concentric cracks,either circular or hexagonal,were observed during melt crystallization at 135℃and quenching in liquid nitrogen,which was assigned to rhythmic growth and thermal shrinkage.In addition,the crystal growth rate of PLLA spherulites was evaluated by using self nucleation combined with non-isothermal method.Lauritzen-Hoffman equation was used for nonlinear fitting of the obtained data,which allowed deducing the kinetic parameters of PLLA crystallization.2.The influences of MW on the thermodynamics and kinetics of PLLA crystallization were investigated.The glass transition temperature of PLLA with infinite MW was estimated to be 60.0℃,and the melting temperature 178.8℃,based on the relevant equations from Flory.In Avrami analysis,PLLA with higher MW presents lower overall crystal growth rate,higher super-cooling degree leading to longer induction period for crystallization.And there exists a scale law between the maximal crystal growth rate and MW with power of-0.5.Linear and nonlinear Hoffman-Weeks expressions were applied to calculate equilibrium melting temperature of isothermally melt crystallized PLLA.Linear extrapolation was found not applicable to PLLA with low MW.The equilibrium melting temperature of PLLA was determined to be 207.6℃.Double endothermic peaks were observed during PLLA glass transition.The two peaks result from enthalpy recovery of both confined and free amorphous fractions in crystallized PLLA.SEM shows that lathlike lamellae align along radial direction and are well organized in chemically etched PLLA spherulites.The transition temperature from Regimeâ…¡to Regimeâ…¢was found to be around 115-120℃for PLLA with different MWs according to Lauritzen-Hoffman theory.3.PLA samples with various configurational structures were synthesized from different L-lactide and DL-lactide feeds.DSC shows that the PLA samples present the same crystallization mechanism.ESEM observations confirmed that proteinase K could degrade both free and confined amorphous fractions of PLA through L-lactic acid units.During enzymatic degradation,lamellae inside the spherulites preserve their structural organization without collapsing.Two kinds of PLA spherulitic morphology were observed,which probably result from different nucleation locations, i.e.inside or at the surface.Furthermore,both crystal and amorphous characteristic peaks were detected on FTIR spectra.Splitting of the crystal peak at 921 cm^-1 seems to indicate that degradation of confined amorphous fraction influences the vibration state of chains in lamellae,while a shoulder peak appearing near the amorphous peak at 955 cm^-1 is considered as an evidence of physical aging of PLLA amorphous fraction.4.The crystallization behaviors of PLLA/PDLA stereocomplex were investigated by using DSC and ESEM.PLLA and PDLA with similar MWs were mixed at 1:1 ratio.Three different types of PLLA/PDLA blends were utilized to illustrate the influence of MW and configurational structure on PLA stereocomplexation.In particular,sterecomplexation was found to weaken or even disappear when PLLA/PDLA was melted at higher temperature or for prolonged period.This interesting phenomenon was assigned to the initial melt state,i.e.homogeneous or heterogeneous.