Nanomechanical properties of plasma treated polylactic acid

In this work, a radio frequency discharged plasma generated in air atmosphere by pressure has been used to modify polylactic acid (PLA) surface. The results were evaluated through nanoindentation testing. Contact angle measurements revealed a gradual transition to a more hydrophilic state with increasing polarity after plasma treatment, while partial recovery to their untreated state during 10?day storage in air was evidenced. The results were evaluated through nanoindentation testing. All PLA samples exhibited an almost hard-like surface area where hardness and elastic modulus are enhanced. The activity of the plasma creates a higher cross-linking density within the material in the surface region. For higher displacements, both H and E tend to reach pristine PLA's values. Hardness values reveal surface hardening due to plasma treatment except for 180?s etching time, where hardness is slightly decreased possibly due to surface deformation. The change of H/E slope reveals the strengthening of oxygen plasma etched PLA with 180?s of etching time with increasing displacement.     

Production of porous polylactic acid monoliths via nonsolvent induced phase separation

Polylactic acid (PLA) is one of the most promising polymers for use as the matrix of a bone scaffold. In this work, porous PLA monoliths are fabricated via nonsolvent induced phase separation using dichloromethane as a solvent and hexane as a nonsolvent. The PLA-dichloromethane-hexane compositions which undergo liquid–liquid phase separation followed by gelation are shown to allow for the production of high quality foams. Solvent exchange with methanol after aging the gel is found to substantially reduce shrinkage during drying. Using this simple, versatile and template-free method we produced PLA foams with porosities as high as ∼90.8%, specific surface area up to 54.14 m²/g, crystallinity up to 62.6% and compressive modulus ranging from 1.8 to 57 MPa. Depending on ternary mixture concentration and standing temperature a range of mesoporous and combined meso/macroporous morphologies suitable for use as a bone scaffold are produced.     

基于静电纺丝技术的PLGA载药纳米纤维膜的制备工艺

同轴静电纺丝法制备的聚乳酸-乙醇酸(PLGA)纳米纤维具有良好的生物相容性和生物可降解性, 加之其高孔隙率和高透氧率, 使其能成为优良的药物载体。本文初步摸索了PLGA的同轴静电纺丝的工艺条件, 并通过同轴静电纺丝法制备了PLGA载氟比洛芬酯(FA)的纳米纤维膜, 应用扫描电子显微镜、红外光谱分析观察纤维的表观形貌并确定其微观结构。重点探究了不同溶剂配比的混合溶剂对载药纤维膜药物释放性能影响。研究结果表明在U⁺为+15.00kV, U^-为-2.50kV, 接受距离为15cm, 壳层推进速度为0.4mm/min, 芯层推进速度为0.1mm/min进行静电纺丝时, 所制备的PLGA(壳)/PVP+FA(核)复合载药纤维膜壳核结构良好, 且成功载了约0.5%的FA。当改变壳层混合溶剂(DCM和DMF)和芯层混合溶剂(无水乙醇和DMF)体积比时, 纤维直径会随着DMF的减少而增大。     

Influence of biaxial stretching mode on the crystalline texture in polylactic acid films

Structural evolution during simultaneous (SB) and sequential rubbery state biaxial stretching (SEQ) of polylactic acid (PLA) films from cast amorphous precursors was investigated. Simultaneous biaxial stretching always leads to films with in-plane isotropy and poor crystalline order. In the first stage of sequential biaxial stretching, oriented crystallization gradually develops while transverse isotropy is maintained. Application of transverse stretching to these films possessing semicrystalline structure gradually destroys the crystalline structure oriented in MD during this realignment while establishing a second population of oriented but poorly ordered crystallites in TD. This destruction is caused primarily by splaying action under transverse stretching as evidenced by the decrease of crystallite sizes in MD.     

Electrospinning of polymer melts: Phenomenological observations

Melt electrospinning is an alternative to solution electrospinning, however, melt electrospinning has typically resulted in fibers with diameters of tens of microns. In this paper we demonstrate that polypropylene fibers can be reduced from 35 ± 8 μm in diameter, to 840 ± 190 nm with a viscosity-reducing additive. Melt electrospun blends of poly(ethylene glycol)-block-poly(?-caprolactone) (PEG₄₇-b-PCL₉₅) and poly(?-caprolactone) (PCL) produced fibers with micron-scale diameters (2.0 ± 0.3 μm); this was lowered to 270 ± 100 nm by using the gap method of alignment for collection. The collected melt electrospun fibers often fused together where they touched, allowing the stabilization of relatively thick non-woven felts. The melt electrospun collection also included coiled circles and looped patterns of fibers approximately 150-250 μm in diameter. The polymer jet was visible between the collector and spinneret for particularly significant lengths, and underwent coiling and buckling instabilities close to the collector. The focused deposition of melt electrospun fibers was maintained when multiple jets were observed, with the collections from multiple jets separated by 3.8 ± 0.5 mm for a 5 cm collector gap. The frequent fusion points between melt electrospun fibers, and a reduction in diameter for the gap method of alignment, indicated that the melt electrospun fibers are still slightly molten at collection.     

Orthogonal design study on factors affecting the degradation of polylactic acid fibers of melt electrospinning

Melt electrospinning is a much simpler and safer method in the production of ultra-fine fibers compared with solution electrospinning. However, high-spinning temperature usually leads to serious degradation of polymer materials. In determining factors that affect the relative molecular mass of polylactic acid (PLA) fibers during melt electrospinning, an orthogonal design method was used to examine the influence of melting temperature, spinning distance, and species and content of antioxidants. Results showed that antioxidant content at the present three levels (i.e., 0.1%, 0.3%, and 0.5%) has the most considerable effect on the relative molecular mass of PLA fibers. Error analysis showed that changes in temperature, distance, and antioxidant content influence the experiment's results significantly. All interaction effects were larger than those of the single factor in the experimental results. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Modeling of melt electrospinning for semi-crystalline polymers

A comprehensive model for the stable jet region in electrospinning of crystallizing polymer melts has been presented. First, the conventional flow-induced crystallization (FIC) model by Ziabicki was coupled with the non-isothermal melt electrospinning model. The modeled initial jet profiles were compared to digitized experimental images of the stable Nylon-6 melt jet near the spinneret. The final jet diameters were also compared to the average thickness of collected fibers. The results were in good agreement with the flow visualization experiments for various melt temperatures and flow rates. The modeled crystallinity predictions were also in agreement with experimental data from collected fiber mats. Then, a new FIC model that can provide microstructure information, such as crystallite number density and average size, has been proposed and validated under isothermal and non-isothermal conditions in the bulk as well as in the confined geometry of the polymer melt jet in electrospinning. Nylon-6,6 was used as the model polymer in this crystallization study, and the results are in good agreement with the widely-used Ziabicki FIC model.     

Functionalized graphenes with polymer toughener as novel interface modifier for property-tailored polylactic acid/graphene nanocomposites

In this work, an effective strategy for engineering the interfacial compatibility between graphene and polylactic acid (PLA) was developed by manipulating the functionalization of graphene and introducing an epoxy-containing elastomer modifier. Curing between the functional groups of the modified graphene and the epoxy groups of the elastomer modifier resulted in controlled dispersion and distribution of graphene in the composite system and hence improved the interfacial adhesion between PLA and graphene. Effects of different graphene functionalization with polymer toughener on morphology, viscoelasticity, and thermal properties of the resulting PLA nanocomposites were thoroughly examined. The resulting percolated structures were the origin of the improved properties of PLA/graphene nanocomposites. The mechanism on property tailoring from interface engineering through dual modifiers are also proposed. Overall, the insight into the interface engineering between the functionalized graphene and the matrix through an elastomer modifier offers a novel way for the future design of graphene polymer nanocomposites.     

Stretching-induced crystallinity and orientation of polylactic acid nanofibers with improved mechanical properties using an electrically charged rotating viscoelastic jet

An electrospinning technique with an additional centrifugal field was employed to prepare polylactic acid (PLA) nanofibers. The results indicated that combining the strong stretching force of the additional centrifugal field and an electrostatic field can align the PLA polymer chains parallel to the nanofiber axis, producing PLA nanofibers with superior crystalline features, molecular orientation and conformation as well as good mechanical properties. The optimal stretching force for an electrically charged rotating viscoelastic jet was determined by high-speed videography and the analysis of dimensionless groups (i.e., the Re, We, and Oh numbers). The elastic modulus of PLA nanofibers with a crystallinity degree of 37% and a dichroic ratio (D) of 1.89 was measured at 2.64 GPa and 3.30 GPa by atomic-force microscopy (AFM) and nanoindenter experiments, respectively. Furthermore, the optimal PLA nanofibers produced by the proposed method can potentially be applied for the reinforcement of composites.     

Cistanche polysaccharide (CDPS)/polylactic acid (PLA) scaffolds based coaxial electrospinning for vascular tissue engineering

High rate of platelet adhesion and limited biocompatibility were regarded as the chief weaknesses of vascular tissue engineering scaffolds. The objective of this research was to overcome these drawbacks by ultilizing coaxial electrospinning to combine CDPS with PLA. CDPS and PLA were located at the inner and external layer respectively so that a “core-sheath” structure was formed. The properties of scaffolds were tested by methods such as mechanical testing, MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide] assay and subcutaneous transplantation. Compared to natural tissues, CDPS/PLA coaxial scaffolds showed excellent biomechanic properties and hemocompatibility so that CDPS owned good potentiality in vascular tissue engineering.     

Flash co-pyrolysis of biomass with polylactic acid. Part 1: Influence on bio-oil yield and heating value

High amounts of water present in bio-oil are one of the major drawbacks for its utilisation as a fuel. One technology that shows the potential to satisfy the demand for bio-oil with a reduced water content is the flash co-pyrolysis of biomass with polylactic acid, PLA. The influence of PLA on the pyrolysis of willow is investigated with a semi-continuous home-built pyrolysis reactor. Flash co-pyrolysis of willow/PLA blends (10:1, 3:1, 1:1 and 1:2) show synergetic interaction. A higher bio-oil yield and a lower water content as a function of the willow/PLA ratios are obtained. Among the tested blends, the 1:2 willow/PLA blend shows the most pronounced synergy: a reduction in the production of pyrolytic water of almost 28%, accompanied by an increase of more than 37% in the production of water-free bio-oil. Additionally, PLA shows to have a positive influence on the energetic value of the bio-oil produced and on the resulting energy recuperation.     

聚乳酸/环氧化聚丁二烯抗冲树脂结晶性能的研究

采用差示扫描量热仪(DSC)对聚乳酸/环氧化聚丁二烯抗冲聚乳酸树脂(PLA/PB(EPB))的结晶性能进行了考察,并与纯聚乳酸(PLA)的结晶行为进行了对比,研究了环氧化聚丁二烯(EPB)对PLA等温/非等温条件下结晶行为的影响规律。结果表明:聚丁二烯橡胶(PB)对PLA的结晶行为影响较小,而EPB对PLA的晶体完善程度影响较为明显。PLA和PLA/PB更倾向于在低温条件下结晶,而PLA/EPB20.9%和PLA/EPB46.5%更倾向于在高温条件下结晶。抗冲聚乳酸树脂的结晶速率高于纯PLA,并且随着EPB环氧化度的增大,抗冲聚乳酸树脂的结晶速率呈逐渐增大的趋势。     

Association behaviors between carboxymethyl cellulose and polylactic acid revealed by resonance light scattering spectra

Resonance light scattering (RLS) spectra were used to study the formation of complexes of carboxymethyl cellulose (CMC) and polylactic acid (PLA) in a mixed solvent of 10% DMSO/90% H₂O(V/V). The RLS results showed that the CMC and PLA could form a steady homogeneous complex due to the interaction of hydrogen bonding. With the increasing of CMC mass fraction in the complex, the observed durative enhancement RLS signal with two inflexion points indicated the forming of complexes and aggregation of complexes. The aggregation equilibrium and thermo stability of the complexes were also investigated based on RLS values.     

Thermo‐rheological and interfacial properties of polylactic acid/polyethylene glycol blends toward the melt electrospinning ability

The electrospinning ability of PLA/PEG system at the melt state was investigated through the viscoelastic parameters obtained from dynamic shear and extensional rheometers. PLA and PEG were melt‐blended at various composition ratios. Effect of PEG concentration on the PLA thermal behavior was studied by the differential scanning calorimetry (DSC). According to DSC and wide‐angle X‐ray diffraction, the PLA crystallinity increased and the crystalline structure became more completed (α‐crystal form) in the presence of PEG. Viscoelastic parameters such as zero‐shear viscosity and relaxation time as an indication of elasticity were obtained. The results revealed enhanced polymer chain mobility and disentanglement ought to plasticizing effect of PEG. The critical content of PEG about 20–30 wt % at which the solid–liquid phase separation occurred was in good agreement with the viscoelastic properties. Hence, more than 20% PEG the elasticity diminished and the melt strength became zero. The interfacial tension of the PLA and PEG estimated through the rheological and morphological parameters evidenced the good miscibility of PLA/PEG system at the melt electrospinnig temperature. While the high viscose samples (η₀ > 1800 Pa/s) PLA and PLA/PEG (95/5) were not spinnable at the spinning temperature of 180 °C, blends containing 10–30% PEG were easily spun. The finest and continuous fiber mats were obtained by electrospinning of PLA/PEG (80/20) blend (d_f = 4.8 ± 0.8 μm). More than the critical concentration of PEG (Φ > 30%), lacking the elasticity suppressed the melt electro‐spinnability of PLA/PEG. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44120.     

Isothermal ternary phase diagram of the polylactic acid-dichloromethane-hexane system

The ternary phase diagram of polylactic acid, dichloromethane (solvent) and hexane (nonsolvent) is experimentally developed at room conditions based on naked eye observations over 14-day periods. The experimental procedure is explained in detail, allowing it to be applied to similar ternary systems. Three regions are distinguishable in this diagram: single phase, liquid–liquid phase separated and solid–liquid phase separated. This diagram is then assessed based on the lever rule concept in ternary phase diagrams. Phase separation kinetics are also evaluated using turbidity studies to quantitatively monitor the cloudiness of the samples over time. The results show that, a small increase in polylactic acid concentration on the order of a few weight percent can drastically increase the phase separation rate. The application of polylactic acid in various fields has been increasing, and this work provides fundamental information essential for solvent-based processing (e.g., nonsolvent induced phase separation and immersion-precipitation) of this polymer in the system composed of dichloromethane and hexane.     

壳聚糖/聚乳酸复合膜的制备及其吸附性能

为了减小水处理废弃膜对环境的危害,以静电纺聚乳酸(PLA)纤维膜为基底层,采用浸涂的方式将壳聚糖(CS)涂覆层与PLA基底层复合形成了CS/PLA复合膜。采用SEM、XRD对其进行了表征。考察了CS与PLA体积比对膜性能的影响。结果发现,当CS和PLA体积比为7∶5时,CS涂覆层表面致密且平整,其厚度为7μm,CS/PLA复合膜的拉伸强度为2.55 MPa,断裂伸长率为24.96%,纯水通量为115.32 L/(m²·h),对酸性染料的渗透通量为99.43 L/(m²·h),吸附率达96%;对牛血清蛋白和卵清蛋白的吸附率分别为86%和84%;对Cu²⁺、Pb²⁺和Cd²⁺的平衡吸附量分别为165.00、248.54和307.83 mg/g。     

静电纺丝法纺制聚乳酸纳米纤维无纺毡

采用静电纺丝法制备了生物降解聚乳酸(PLLA)纳米纤维无纺毡。分析了纺丝液浓度、电压、接收距离、挤出速度等因素对纤维形态的影响。结果表明:纺丝液的浓度和挤出速度对纤维直径的影响较为明显,溶液挤出速度增大,所得纤维微孔含量及尺寸也增大;适当的电压和接收距离有利于收集无液滴纤维;随着纤维直径的减小,无纺毡的孔径呈减小趋势。在PLLA质量分数为5．7％、挤出速度0．8 mL／h、接受距离 15．5 cm、电压8 kV的静电纺丝条件下,可制备纤维直径为200-400 nm的PLLA纳米纤维无纺毡。     

共溶剂对聚乳酸相对分子质量及热性能的影响

以超临界二氧化碳(SC-CO2)为载体,分别以丙酮、无水乙醚、六氟异丙醇为共溶剂,合成聚乳酸(PLLA),并采用FT-IR、GPC和DSC等对所获得的聚合物进行了相对分子质量测定和结构性能表征,探讨了3种共溶剂对用SC-CO2为溶剂合成的PLLA相对分子质量及热性能的影响。结果表明,共溶剂对PLLA的相对分子质量、相对分子质量分布指数以及热性能影响较大;以丙酮为共溶剂时,可以获得最高相对分子质量为47 690且热性能较好的PLLA。     

聚乳酸合成工艺研究

以乳酸单体为原料,采用直接缩聚法合成了具有较高粘均分子质量的聚乳酸。研究了催化剂种类、用量、反应时间、真空度因素对聚乳酸粘均分子质量的影响。结果表明,聚乳酸合成的最佳工艺条件为:选取辛酸亚锡为催化剂,催化剂用量为乳酸量0.5%,氮气气氛,反应温度170℃,真空度30 Pa,反应时间10 h。     

Improvement of PAN separator properties using PVA/malonic acid by electrospinning in lithium ion-batteries

The prominent objective of this study is to improve the thermal shrinkage and wettability of lithium-ion battery membrane separators based on Polyacrylonitrile (PAN) formed using an electrospinning technique. To achieve this goal, a PAN blended with highly hydrophilic Polyvinylalcohol (PVA) is formed, using malonic acid (MA) as a cross-linker. Due to the excellent hydrophilic properties of PVA and the network formation inside the separators (because of the MA crosslinker presence), the results show significant improvement in the separator properties. For this reason, at the optimum concentrations of 5 wt.% PVA and 5 wt.% MA (sample F4), an increase of wettability (contact angle from 85° for pure PAN to 42° for the F4 separator) is able to be seen. The electrolyte uptake was significantly increased, as for the F4 sample is increased to 1,150%, which is 2.67 times higher than the PAN with 430%. The improved separators showed higher porosity, better tensile strength, lower thermal shrinkage, and better electrochemical performance than the pure PAN separator. It showed an ion conductivity of 3.03 mS/cm, a wide electrochemical stability window of 5.2 V and an initial discharge capacity of 156.4 mAh/g.