Improvement on the Mechanical Performance and Resistance Towards Hydrolysis of Poly(glycolic acid)via Solid-state Drawing

Poly(glycolic acid)is a biocompatible as well as biocomposable polymer with superior mechanical and barrier properties and,consequently,has found important applications in both medical and packaging fields.However,the high hydrolysis rate in a high humidity environment restricts its application.In this work,a solid-state drawing process after melt extrusion is applied in order to produce fibrous PGA with enhanced mechanical properties and a much better resistance towards hydrolysis.The crystal structure of PGA gradually transformed from spherulites into oriented fibrous crystals in the stretching direction upon solid-state drawing.Meanwhile,both the length of microfibril and the size of lamellae increased initially with the drawing ratio(DR),while the chain-folded lamellae transformed into extended-chain fibrils at high(er)DR.The oriented structures lead to an overall improvement of the mechanical properties of PGA,e.g.,the tensile strength increased from 62.0±1.4 MPa to 910±54 MPa and the elongation at break increased from around 7%to 50%.Meanwhile,the heat capacity of totally mobile amorphous PGA(∆C_(p)^(0)=0.64 J·g^(−1)·℃^(−1))was reported for the first time,which was used to analyze the content of mobile amorphous fraction(XMAF)and rigid amorphous fraction(XRAF).Both the oriented chain-folded lamellae crystals and the tightly arranged RAF are beneficial to prevent water molecules from penetrating the matrix,thus improving the resistance towards hydrolysis.As a consequence,the fibrous PGA with a DR of 5 showed a tensile strength retention rate of 17.3%higher in comparison with the undrawn sample after 7-days accelerated hydrolysis.Therefore,this work provides a feasible method to improve the mechanical and resistance towards hydrolysis performance of PGA,which may broaden its application and prolong the shelf-life of PGA products.

Robust Poly(glycolic acid) Films with Crystal Orientation and Reinforcement of Chain Entanglement Network

The inherent brittleness of biodegradable poly(glycolic acid) (PGA) restricts its utilization in the packaging field. Traditional toughening methods usually require additional components accompanied by a sacrifice of strength. In the present work, PGA films with simultaneously enhanced strength and ductility are achieved via “casting-stretching-annealing” technology. The reinforced chain entanglement network of PGA induced by the intense extensional stress and the highly oriented crystals grown and refined during the stretching and annealing process endowed the improved ductility and strength of the PGA films, respectively. The relationships among the stretching process, microstructure and mechanical properties of the PGA films have been systematically investigated. As a result, the PGA film (SA-2) with low stretch ratios exhibits excellent ductility with an increase in elongation at break from 22% to 220% and tensile strength from 56 MPa to 130 MPa. Meanwhile, the PGA film (SA-4) with large stretch ratios features much higher tensile strength (335 MPa) while maintaining good ductility (elongation at break of 66%). In addition, highly oriented crystals result in obvious enhancement of heat resistance and dimensional stability of the PGA films. Therefore, this work provides an effective route to fabricate PGA films with both high strength and ductility which may promote the application of PGA materials.

Stereocompfexed Poiy(iactide) Composites toward Engineering Plastics with Superior Toughness, Heat Resistance and Anti-hydrolysis

Polyl(lactide),PLA,suffers from bitleness and low heat deflection temperature(HDT),which limits its application as an engineering plastic.In this work,poly(L-lactide)/poly(D-lactide)/ethylene-vinyl acetate glycidyl methacrylate random copolymer(PLLA/PDL A/EVM-GMA=1/1/x)composites were prepared by melt blending,and the in situ formed EVM-g PLA copolymers improved the compatibility between PLA and EVM-GMA.Subsequently,the blends were subjected to a two-step annealing process during compression molding,i.e.first annealing at 120℃ to rapidly form a certain amount of stereocomplex(sc)crystallites as nucleation sites,and then annealing at 200℃ to guide the formation of new sccrystallites.Both differential scanning calorimetry(DSC)and wide angle X-ray dffrction(WAXD)measurements confirmed the formation of highly stereocomplexed PLA products.Mechanical results showed that the PLLA/PDLA blend with 20 wt%of EVM-GMA had a notched impact strength up to 65 kJ/m2 and an elongation at break of 48%,while maintaining a tensile strength of 40 MPa.Meanwhile,dynamic mechanical analysis(DMA)and heat deflection tests showed that the PLA composite had an HDT up to 142℃ which is 90℃ higher than that of normal PLA products.Scanning electron microscopy(SEM)confirmed the fine dispersion of EVM-GMA particles,which facilitated to understand the toughening mechanism.Furthermore,the highly stereocomplexed PLA composites simultaneously exhibited excellent chemical and hydrolysis resistance.Therefore,these fascinating properties may extend the application range of sc PLA material as an engineering bioplastic.

Tailoring the Crystallization Behavior and Mechanical Property of Poly(glycolic acid) by Self-nucleation

Biocompostable poly(glycolic acid)(PGA)crystallizes slowly under fast cooling condition,leading to poor mechanical performance of the final products.In this work,a self-nucleation(SN)route was carried out to promote the crystallization of PGA by regulating only the thermal procedure without any extra nucleating agents.When self-nucleation temperature(Ts)decreased from 250℃ to 227℃,the nuclei density was increased,and the non-isothermal crystallization temperature(Tc)of PGA was increased from 156℃ to 197℃ and the half-life time(t0.5)of isothermal crystallization at 207℃ was decreased by 89%.Consequently,the tensile strength and the elongation at break of the PGA were increased by 12%and 189%,respectively.According to the change of Tc as a function of Ts,a three-stage temperature domain map(Domain I,II and III)was protracted and the viscoelastic behavior of the self-nucleation melt and the homogeneous melt was studied.The results indicated that interaction among PGA chains was remained in Domain IIb,which can act as pre-ordered structure to accelerate the overall crystallization rate.This work utilizes a simple and effective SN method to regulate the crystallization behavior and the mechanical properties of PGA,which may broaden the application range of resulting materials.

Enhancing the Crystallization Performance of Poly(L-lactide) by Intramolecular Hybridizing with Tunable Self-assembly-type Oxalamide Segments

In this work, hydroxyl-terminated oxalamide compounds N^(1),N^(2)-bis(2-hydroxyethyl)oxalamide(OXA1) and N^(1),N^(1)′-(ethane-1,2-diyl)bis(N^(2)-(2-hydroxyethyl)oxalamide(OXA2) were synthesized to initiate the ring-opening polymerization of L-lactide for preparation of oxalamide-hybridized poly(L-lactide)(PLA_(OXA)), i.e., PLA_(OXA1) and PLA_(OXA2). The crystallization properties of PLA were improved by the self-assembly of the oxalamide segments in PLA_(OXA) which served as the initial heterogeneous nuclei. The crystal growth kinetics was studied by HoffmanLauritzen theory and it revealed that the nucleation energy barrier of PLA_(OXA1) and PLA_(OXA2) was lower than that of PLA. Consequently, PLA_(OXA) could crystallize much faster than PLA, accompanied with a decrease in spherulite size and half-life crystallization time by 74.8% and 86.5%(T=125 ℃), respectively. In addition, the final crystallinity of PLA_(OXA1) and PLA_(OXA2) was 6 and 8 times higher, respectively, in comparison with that of neat PLA under a controlled cooling rate of 10 ℃/min. The results demonstrate that the hybridization of oxalamide segments in PLA backbone will serve as the self-heteronucleation for promoting the crystallization rate. The higher the content of oxalamide segments(PLA_(OXA2) compared with PLA_(OXA1)) is, the stronger the promotion effect will be. Therefore, this study may provide a universal approach by hybridizing macromolecular structure to facilitate the crystallization of semi-crystalline polymer materials.