Multiple shape memory behavior of highly oriented long‐chain‐branched poly(lactic acid) and its recovery mechanism

YesThe shape memory effect of highly oriented long‐chain‐branched poly(lactic acid) (LCB‐PLA) prepared through solid‐phase die drawing technology was studied by comparison with PLA. When the recovery temperature increased from 60°C to 120°C, for PLA, only one‐step recovery at about 80°C can be observed and the recovery ratio was below 21.5%, while, for LCB‐PLA, multiple recovery behavior with high recovery ratio of 78.8% can be achieved. For oriented PLA, the recovery curve of the final sample showed the same trend with that of sample suffering just free drawing; while for oriented LCB‐PLA, the recovery curve of the final sample showed the same trend with that of sample suffering just die drawing. After shape recovery, the mechanical properties of LCB‐PLA showed a linear downward trend with the recovery temperature. Together with amorphous phase, the oriented mesomorphic phase, which formed during solid die drawing, can act as switching domains. And thus, upon heating, the chain segment of amorphous phase relaxed at first and triggered the first macroscopical shape recovery, leading to the decrease of long period (Lac) and the thickness of the amorphous layer (La). Then, with further increasing temperature, the oriented mesomorphic phase gradually relaxed resulting subsequently multi‐shape recovery, and the Lac and the La further decreased. Therefore, by regulating the recovery temperature of oriented LCB‐PLA, the shape recovery ratio and mechanical strength can be controlled effectively, and thus the self‐reinforced and self‐fastening effect can be achieved simultaneously for PLA as bone fixation material

Structure and blood compatibility of highly oriented PLA/MWNTs composites produced by solid hot drawing

YesHighly oriented poly(lactic acid) (PLA)/multi-walled carbon nanotubes (MWNTs) composites were fabricated through solid hot drawing technology in an effort to improve the mechanical properties and blood biocompatibility of PLA as blood-contacting medical devices. It was found that proper MWNTs content and drawing orientation can improve the tensile strength and modulus of PLA dramatically. With the increase in draw ratio, the cold crystallization peak became smaller, and the glass transition and the melting peak of PLA moved to high temperature, while the crystallinity increased, and the grain size decreased, indicating the stress-induced crystallization of PLA during drawing. MWNTs showed a nucleation effect on PLA, leading to the rise in the melting temperature, increase in crystallinity and reduction of spherulite size for the composites. Moreover, the intensity of (002) diffraction of MWNTs increased with draw ratio, indicating that MWNTs were preferentially aligned and oriented during drawing. Microstructure observation demonstrated that PLA matrix had an ordered fibrillar bundle structure, and MWNTs in the composite tended to align parallel to the drawing direction. In addition, the dispersion of MWNTs in PLA was also improved by orientation. Introduction of MWNTs and drawing orientation could significantly enhance the blood compatibility of PLA by prolonging kinetic clotting time, reducing hemolysis ratio and platelet activation

Structure and blood compatibility of highly oriented poly(l-lactic acid) chain extended by ethylene glycol diglycidyl ether

YesHighly-oriented poly(l-lactic acid) (PLLA) with fibrillar structure and micro-grooves was fabricated through solid hot drawing technology for further improving the mechanical properties and blood biocompatibility of PLLA as blood-contacting medical devices. In order to enhance the melt strength and thus obtain high orientation degree, PLLA was first chain extended with ethylene glycol diglycidyl ether (EGDE). The extending degree as high as 25.79 mol% can be obtained at 0.7 wt% EGDE content. The complex viscosity, storage and viscous modulus for chain extended PLLA were improved resulting from the enhancement of molecular entanglement, and consequently higher draw ratio can be achieved during the subsequent hot stretching. The tensile strength and modulus of PLLA were improved dramatically by stretching. The stress-induced crystallization of PLLA occurred during drawing. The interfacial tension (γs·blood) between PLLA surface and blood decreased by chain extension and molecular orientation, indicating the weakened interaction between bioactive substance in the blood and the surface of PLLA. Modification and orientation could significantly enhance the blood compatibility of PLLA by prolonging clotting time and decreasing hemolysis ratio, protein adsorption and platelet activation. The bionic character of oriented PLLA and its anti-coagulation mechanism were tried to be explored.This research was supported by National Natural Science Foundation of China (Grant No. 51303109

Enhancing poly(lactic acid) microcellular foams by formation of distinctive crystalline structures

YesBy controlling the crystallization behavior of poly(lactic acid) (PLA) in the presence of a hydrazide nucleating agent (HNA), PLA-HNA foams with enhanced microcellular structures were prepared via supercritical CO2 foaming. It was found that HNA can self-assemble into fibrillar networks, inducing the crystallization of PLA on their surface, and "shish-kebab"crystalline structures with high crystallinity formed, which can be maintained during the whole foaming process. Incorporation of HNA promoted the formation of gt conformers, improved the amount of dissolved CO2, hindered the escape of CO2, and increased the viscoelasticity of PLA. Compared with neat PLA foam, for PLA-HNA foam, the average cell diameter decreased obviously, from 64.39 to 6.59 μm, while the cell density increased up to nearly three orders of magnitudes, from 6.82 × 106 to 4.44 × 109 cells/cm3. Moreover, lots of fibrillar structures appeared and entangled with each other on the cell wall of the foam. By forming such dense micropores and enhanced fibrillar structures, PLA foam was highly reinforced with significantly improved compressive strength.This research was financially supported by National Natural Science Foundation of China (grant no. 51773122) and State Key Laboratory of Polymer Materials Engineering (grant no. sklpme2019-2-21)

Structure evolution and orientation mechanism of long-chain-branched poly (lactic acid) in the process of solid die drawing

YesHighly oriented long-chain-branched poly (lactic acid) (LCB-PLA) was prepared and the structure evolution was studied in the process of solid die drawing by compared with poly (lactic acid) (PLA). During drawing, samples underwent not only die drawing process but also free drawing process. Drawing speed presented a prominent effect on the free drawing process, while die thickness showed a more obvious influence on the die drawing process. For PLA, free drawing process mainly contributed to its final orientation degree and crystallinity, and thus the mechanical properties of PLA were greatly influenced by drawing speed. However, for LCB-PLA, die drawing process made a greater contribution to the final orientation degree and crystallinity, and its mechanical properties were mainly affected by die thickness. Under the same drawing condition, the tensile strength and modulus of LCB-PLA were always higher than those of PLA, and reached up to 228 MPa and 7.2 GPa, respectively, which basically met the requirement for born fixation materials. Samples which only underwent die drawing showed obvious “sandwich” structure, and the thickness of the oriented skin layer for LCB-PLA was thicker than that for PLA, suggesting that shear-induced orientation can be easily retained due to the enhanced entanglement between long branched chains. After drawing, LCB-PLA samples showed smaller lamellae size (Llateral) but larger long period (Lac) compared with PLA, suggesting that the low chain mobility restricted the motion of chain slipping of LCB-PLA and thus resulted in the fragmentation of neighboring crystal lamella by chain stretched-out

Long-chain branched poly(lactic acid)- b-poly(lactide- co-caprolactone): Structure, viscoelastic behavior, and triple-shape memory effect as smart bone fixation material

YesA novel fully biosbased poly(lactic acid)-b-poly(lactide-co-caprolactone) (PLA-b-PLCL) with a two-phase structure and long-chain branches was specifically designed and prepared through reactive melt processing. The results showed that PLCL segments were introduced onto PLA chains successfully. With the increase of PLCL content, the blockier distribution of LA/CL chain sequences of the sample was exhibited. PLA-b-PLCL showed two distinct thermal transitions, corresponding to the glass transition of PLA and PLCL domains, respectively, whereas the phase morphology changed from a sea-island to a co-continuous structure with increasing PLCL content. Because of the long-chain branched structure, PLA-b-PLCL samples showed a much higher viscoelasticity, strong molecular entanglement, and obvious strain-hardening behavior, resulting in a high draw ratio of the sample during orientation process, whereas the tensile strength and the modulus of the oriented sample reached up to 173 MPa and 5.4 GPa, respectively, which basically met the requirements of bone screws. Moreover, PLA-b-PLCL showed a triple-shape memory effect at 55 and 120 °C, respectively. For PLA-b-30 wt % PLCL, the recovery ratio can reach up to 98.1% under 55 °C, while high mechanical properties can be maintained, realizing self-reinforcement and self-fastening effect simultaneously as a smart bone fixation material

Orientation direction dependency of cavitation in pre-oriented isotactic polypropylene at large strains

YesOrientation direction dependency of whitening activated at large strains was studied using four pre-oriented isotactic polypropylene (iPP) samples with different molecular weights stretched along different directions with respect to the pre-orientation (0°, 45°, and 90°) by means of in situ wide-, small-, and ultra-small-angle X-ray scattering techniques. A macroscopic fracture of iPP materials was also observed following the stress whitening at large strains. These two associated processes in pre-oriented iPP samples at elevated temperatures were found to be governed by not only the molecular weight of iPP but also the pre-orientation direction. For a certain pre-orientation direction of iPP, both the critical stress of cavitation induced-whitening and failure stress increased with increasing molecular weight. For one given molecular weight, the pre-oriented iPP showed the smallest critical stress for whitening and failure stress along the pre-orientation direction (0°) while the samples displayed larger values for the same behaviors when stretched at 45° or 90° with respect to the pre-orientation direction. Such behavior suggested that oriented amorphous networks, with different mechanical strengths, can be generated during the second deformation processes in these pre-oriented iPP samples. The evolution of inter-fibrillar tie chains in highly oriented amorphous networks was considered as the main factor controlling the response of the inner network to the external stress since the cavitation-induced whitening activated at large strains was caused by the failure of load bearing inter-fibrillar tie chains in the oriented amorphous network

Nanoindentation analysis of oriented polypropylene: Influence of elastic properties in tension and compression

YesPolypropylene has been oriented by solid-phase deformation processing to draw ratios up to ∼16, increasing tensile stiffness along the draw direction by factors up to 12. Nanoindentation of these materials showed that moduli obtained for indenter tip motion along the drawing direction (3) into to 1–2 plane (axial indentation) were up to 60% higher than for indenter tip motion along the 2 direction into the 1–3 plane (transverse indentation). In static tests, tensile and compressive determinations of elastic modulus gave results differing by factors up to ∼5 for strain along the draw direction. A material model incorporating both orthotropic elasticity and tension/compression asymmetry was developed for use with Finite Element simulations. Elastic constants for the oriented polypropylene were obtained by combining static testing and published ultrasonic data, and used as input for nanoindentation simulations that were quantitatively successful. The significance of the tension/compression asymmetry was demonstrated by comparing these predictions with those obtained using tensile data only, which gave predictions of indentation modulus higher by up to 70%

Die geometry induced heterogeneous morphology of polypropylene inside the die during die-drawing process

YesThe morphology distribution of isotactic-polypropylene (iPP) shaped through a die during hot stretching process was investigated via wide-angle X-ray diffraction technique. The evolution of micro-structures in the outer layer (layer closer to the die wall) and the inner layer (layer in the center of die) of die-drawn iPP were both recorded. It turned out that the difference of morphology distribution between outer and inner layers changes with the distance from the die entrance to exit. In general, a larger difference between outer and inner layers could be found at the intermediate deformation region inside the die while such difference disappeared at both of the entrance and exit regions of die. These behaviors could be interpreted as a result of the existence of a heterogeneous distribution of force field inside the die, which was caused by the die geometry and inclination of the drawing force. This work showed that the heterogeneous force field inside the die could be revealed through analyzing the morphology of a die-drawn sample

Suppressed cavitation in die-drawn isotactic polypropylene

YesCavitation is an important phenomenon in solid-phase deformation of polymers, which either has potential adverse effects on physical properties or creates potential opportunities for new properties. In either case, it needs to be better understood to help achieve better control of cavitation and its effects. Cavitation associated with solid-phase deformation in a β-nucleated isotactic polypropylene was found to depend on the solid-phase deformation route employed. Compared with samples obtained by free tensile stretching, cavitation was suppressed in samples deformed via die-drawing, although an almost identical β-to α-phase transition was observed for both deformation routes. Even when die-drawn samples were subsequently deformed to large strains by free stretching, there was still no comparable cavitation compared with the single free tensile-stretching route. The die-drawing process appears to suppress cavitation by fundamentally diminishing the number of growable nuclei of cavities, rather than merely hindering the growth of cavities. A relationship between cavitation intensity and the fractions of lamellae along specific directions has been established. During subsequent free stretching of die-drawn samples, newly created cavities were suggested to be initiated within the crystalline layers. The reduction of the cavity nuclei in the die-drawing process originated from the stabilization of the connections between the crystalline blocks within the lamellae.This work is supported by the National Natural Science Foundation of China (21704102 and 51525305), Newton Advanced Fellowship of the Royal Society, United Kingdom (NA 150222) and ExxonMobil