Research On Crystallization Behavior And Mechanical Properties Of Poly(Lactic Acid) And Its Degradable Composites

Biodegradable polymers have become a research hotspot in the field of materials due to their eco-friendly.Poly(lactic acid)(PLA)is an important component of polymer in ecological environment.PLA is widely used in biomedical,fiber spinning,food packaging and other fields due to its easy processing,good biocompatibility and excellent mechanical properties.However,there are some disadvantages of PLA such as high brittleness,poor thermal resistance,and high cost,which limits its engineering applications.The process methods and parameters affect the dimensional accuracy,microstructure and morphology,which further influences the properties of products.The PLA materials undergo complex thermal mechanical coupling fields during processing.As for semicrystalline polymers processing,the external thermal and flow fields even coupled with phase transition(crystallization)and enthalpy,which leads to a more complex issue.Crystallization is an essential factor for the physical properties of PLA.Investigating the influence of flow field and thermal field on crystallization behavior and the contribution of crystallization to thermal and mechanical properties is helpful to study the process-structure-property relationship of degradable polymers.In this work,the aim is to accelerate the crystallization rate and improve the thermal and mechanical properties of PLA without sacrificing biodegradability or increasing cost.The effect of flow field and temperature field on crystallization behavior(crystal form,morphology and crystallinity),and the effect of crystallization on the thermal and mechanical properties of PLA and its composites are comprehensively studied.The results indicate that excellent macro properties can be obtained by tailoring microstructures.This work contributes to the engineering applications of PLA.The main contents and conclusions are as follows.(1)Effect of shear on the crystallization behavior of neat PLA.The relationships between the shear-affected range and the morphology of interfacial cylindrite were built under the strong shear field.The effect of shear rate(fiber-pulling speed 500-900μm/s)and shear time(fiber-pulling time 5-11 s)on the structure and morphology of PLA interfacial cylindrite was investigated by pulling a single glass fiber in PLA melt.The results suggested that with increasing fiber-pulling speed and fiber-pulling time,there were hierarchical cylindrites with two layers formation.The inner layer of interfacial cylindrite is composed of high-density crystal nuclei,and the outer layer is a common cylindrite.The thickness of the inner layer increases first and then reaches a maximum of 18.5 μm with the increase of fiber-pulling speed.The relative degree of crystallinity and orientation outer layer have the same trend with the inner layer thickness.(2)Effect of annealing on the crystallization and properties of injection molded neat PLA.PLA injection molded samples with different crystallization behavior were obtained by tailoring annealing temperature(80-120℃)and annealing time(0.5-2 h).The effect of crystallization behavior(crystal form,crystal size and crystallinity)on the thermal and mechanical properties of PLA specimens was investigated in detail.Finally,the corroletions between microstructure and macro properties were built.The results are as follows.1)The crystal form of PLA samples annealing at 80/100/120℃ is α’form/α’ and α-form/α-form.2)The fine crystals obtained at 80℃ were beneficial to toughness,and the elongation at break for samples annealed for 2 h was 16.9%,which was 3.5 times of original samples.3)The thermal stability is directly controlled by the degree of crystallinity(Xc).The quantied relationship between HDT and Xc was built and found to be such that ΔHDT has a linear relationship with ΔXc,regardless of crystal form(α’ or α).(3)Crystallization behavior and mechanical properties of natural fiber reinforced PLA composites.The ramie fiber reinforced PLA composites were fabricated by melt blending and injection molding.The effect of fiber content,aspect ratio and interfacial property on the crystallization behavior,thermal and mechanical properties was investigated.Based on the results,the rule of ramie fiber as reinforcing filler or nuclear agent depends on the physical state of PLA.At glass state,ramie fiber acts as a reinforcing filler,while at rubber state,it acts as a nuclear agent inducing PLA crystallization.The results are as follows.1)The aspect ratio of ramie fiber mainly distributed in the range of 3-12,belonging to short fiber.2)Ramie fiber cound accelerate the crystallization rate,reduce half crystallization time and increase degree of crystallinity of PLA.3)The addition of ramie fiber improved the mechanical strength of PLA.(4)Effect of fiber surface treatment on the properties of PLA/ramie fiber(PLA/RF)composites.The effect of alkali concentrations(1/3/5 wt%)and treatment periods(3/6/9 h)on interfacial adhesion,crystallization behavior and mechanical and thermal properties for PLA/ramie fiber composites was carefully investigated.It suggests that the improvement in mechanical strength and toughness of the alkali treated composites is mainly due to good interfacial property rather than crystallization.The results are as follows.1)After fiber surface treatment,the effective contact area between RF and PLA was increased,and the interfacial adhension was improved.2)The PLA/RF-3-6 composites with the lowest crystallinity had the best tensile properties after 6 h treatment with 3 wt%alkali concentration,the tensile strength and elongation at break of which increased by 19.6%and 23.9%,respectively.(5)Huge toughened PLA blends were obtained by multi-component melt blending.Carbon nanotubes(CNTs)were used to tune the microstructure/morphology and toughness of PLA/PBAT blends.The mechanisam of carbon nanotubes toughened PLA/PBAT blends is the formation of CNTs networks.The results are as follows.1)The crystallization behavior of PLA matrix in blends has no obvious change due to CNTs selective located in PBAT phase.2)The volume-filling effect of the captured CNTs resulted in the enlargement of PBAT particles size.The phase morphology of PLA/PBAT blends changes from "sea-islead" to "quasi-cocontiunous".3)The addition of CNTs sharply improved the toughness of PLA.The elongation at break of PLA/PBAT/CNTs blends with 1 wt%CNTs is 296.5%,which is almost 50 times more than neat PLA.