The effect of halloysite nanotubes and N,N′-ethylenebis (stearamide) on morphology and properties of polylactide nanocomposites with crystalline matrix

Poly(lactide)/halloysite nanotubes (PLA/HNT) nanocomposites with crystalline matrix were obtained by cold crystallization and examined. Neat HNT and HNT treated with N,N′- ethylenebis(stearamide) (EBS) were used as nanofillers. Reference materials, PLA and PLA/EBS blend, prepared in the same way, were also considered. The influence of HNT and/or EBS content on the crystallinity and morphology of PLA matrix, as well as on the dynamic mechanical and optical properties of the materials, was determined.The nanocomposites contained well-distributed HNT, with only occasional agglomerates. HNT, EBS-treated HNT and EBS influenced the morphology of the crystalline PLA matrix and the amounts of the disorder α’ (termed also δ) and order α crystallographic forms of PLA. Crystallinity increased stiffness of the materials compared to their counterparts with the amorphous matrix. Owing to the crystallinity and the presence of the nanofillers, the storage modulus at 20 °C and 60 °C increased by up to 30 and 60%, respectively, compared to neat amorphous PLA. Interestingly, at lower nanofiller content the crystalline nanocomposites with EBS were more transparent than neat crystalline PLA.     

Polylactide/exfoliated graphite nanocomposites with enhanced thermal stability,mechanical modulus,and electrical conductivity

We have prepared a series of polylactide/exfoliated graphite (PLA/EG) nanocomposites by melt‐compounding and investigated their morphology, structures, thermal stability, mechanical, and electrical properties. For PLA/EG nanocomposites, EG was prepared by the acid treatment and following rapid thermal expansion of micron‐sized crystalline natural graphite (NG), and it was characterized to be composed of disordered graphite nanoplatelets. It was revealed that graphite nanoplatelets of PLA/EG nanocomposites were dispersed homogeneously in the PLA matrix without forming the crystalline aggregates, unlike PLA/NG composites. Thermal degradation temperatures of PLA/EG nanocomposites increased substantially with the increment of EG content up to ～3 wt %, whereas those of PLA/NG composites remained constant regardless of the NG content. For instance, thermal degradation temperature of PLA/EG nanocomposite with only 0.5 wt % EG was improved by ～10 K over PLA homopolymer. Young's moduli of PLA/EG nanocomposites increased noticeably with the increment of EG content up to ～3 wt %, compared with PLA/NG composites. The percolation threshold for electrical conduction of PLA/EG nanocomposites was found to be at 3–5 wt % EG, which is far lower graphite content than that (10–15 wt % NG) of PLA/NG composites. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 850–858, 2010     

Plasticized polylactide/clay nanocomposites. I. The role of filler content and its surface organo‐modification on the physico‐chemical properties

Polylactide (PLA)‐layered silicate nanocomposites plasticized with 20 wt % of poly(ethylene glycol) 1000 were prepared by melt blending. Three kinds of organo‐modified montmorillonites—Cloisite® 20A, Cloisite® 25A, and Cloisite® 30B—were used as fillers at a concentration level varying from 1–10 wt %. Neat PLA and plasticized PLA with the same thermomechanical history were considered for comparison. Nanocomposites based on amorphous PLA were obtained via melt‐quenching. The influence of both plasticization and nanoparticle filling on the physicochemical properties of the nanocomposites were investigated. Characterization of the systems was achieved by size exclusion chromatography (SEC), thermogravimetric analysis (TGA), thermally modulated differential scanning calorimetry (TMDSC), X‐ray diffraction (XRD), and dynamic mechanical analysis (DMTA). SEC revealed a decrease of the molecular weight of the PLA matrix with the filler content. Thermal behavior on heating showed one cold crystallization process in the reference neat PLA sample, while two cold crystallization processes in plasticized PLA and plasticized nanocomposites. The thermal windows of these processes tend to increase with the filler content. The crystalline form of PLA developed upon heating was affected neither by the plasticization nor by the type and content of Cloisite used. It was found that the series of organo‐modified montmorillonites with decreasing affinity to PLA is Cloisite® 30B, Cloisite® 20A, and Cloisite® 25A, respectively. The dynamic mechanical properties were sensitive to the sample composition. Generally, the storage modulus increased with the filler content. Glassy PEG, well dispersed within unfilled PLA matrix, exhibited also a reinforcing effect, since the storage modulus of this sample was higher than for unplasticized reference at temperature region below the glass transition of PEG. Moreover, loss modulus of all plasticized samples revealed an additional maximum ascribed to the glass transition of PEG–rich dispersed phase, indicating partial miscibility of organic components of the systems investigated. The magnitude of this mechanical loss was correlated with the filler content, and to some extent, also with the nanofiller ability to be intercalated by polymer components. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 299–311, 2006     

Mechanical and thermal properties of green polylactide composites with natural fillers

Green composites of PLA with micropowders derived from agricultural by-products such as oat husks, cocoa shells, and apple solids that remain after pressing have been prepared by melt mixing. The thermal and mechanical properties of the composites, including the effect of matrix crystallization and plasticization with poly(propylene glycol), have been studied. All fillers nucleated PLA crystallization and decreased the cold-crystallization temperature. They also affected the mechanical properties of the compositions, increasing the modulus of elasticity but decreasing the elongation at break and tensile impact strength although with few exceptions. Plasticization of the PLA matrix improved the ductility of the composites.     

Adding Value in Production of Multifunctional Polylactide (PLA)–ZnO Nanocomposite Films through Alternative Manufacturing Methods

Due to the added value conferred by zinc oxide (ZnO) nanofiller, e.g., UV protection, antibacterial action, gas-barrier properties, poly(lactic acid) (PLA)–ZnO nanocomposites show increased interest for utilization as films, textile fibers, and injection molding items. The study highlights the beneficial effects of premixing ZnO in PLA under given conditions and its use as masterbatch (MB), a very promising alternative manufacturing technique. This approach allows reducing the residence time at high processing temperature of the thermo-sensitive PLA matrix in contact of ZnO nanoparticles known for their aptitude to promote degradation effects onto the polyester chains. Various PLA–ZnO MBs containing high contents of silane-treated ZnO nanoparticles (up to 40 wt.% nanofiller specifically treated with triethoxycaprylylsilane) were produced by melt-compounding using twin-screw extruders. Subsequently, the selected MBs were melt blended with pristine PLA to produce nanocomposite films containing 1–3 wt.% ZnO. By comparison to the more traditional multi-step process, the MB approach allowed the production of nanocomposites (films) having improved processing and enhanced properties: PLA chains displaying higher molecular weights, improved thermal stability, fine nanofiller distribution, and thermo-mechanical characteristic features, while the UV protection was confirmed by UV-vis spectroscopy measurements. The MB alternative is viewed as a promising flexible technique able to open new perspectives to produce more competitive multifunctional PLA–ZnO nanocomposites.     

The influence of matrix crystallinity,filler grain size and modification on properties of PLA/calcium carbonate composites

Thermal and mechanical properties of polylactide (PLA) composites with different grades of calcium carbonate, 40 nm and 90 nm nanoparticles, and also with submicron particles, unmodified and modified with calcium stearate or stearic acid, obtained by melt mixing, were compared. Films with amorphous and crystalline matrices were prepared and examined.T_g of PLA in the composites remained unaffected whereas its cold crystallization was enhanced by the fillers and predominantly depended on filler content. Filling decreased thermal stability of the materials but their 5% weight loss temperatures well exceeded 250 °C, evidencing stability in the temperature range of PLA processing. The amorphous nanocomposites with modified nanoparticles exhibited improved drawability and toughness without a significant decrease of tensile strength; nearly two-fold increase of the elongation at break and tensile toughness was achieved at 5 wt% content of the modified nanofiller. Lack of surface modification of the filler, larger grain size with an average of 0.9 μm, and matrix crystallinity had a detrimental effect on the drawability. However, the presence of nanofillers and crystallinity improved tensile modulus of the materials by up to 15% compared to neat amorphous PLA.     

PLA-ZnO nanocomposite films: Water vapor barrier properties and specific end-use characteristics

PLA nanocomposite films with multifunctional characteristics such as mechanical, anti-UV, antibacterial, electrical, gas barrier properties are potentially of high interest as packaging biomaterials. Occasionally, desired and beneficial effects obtained by addition nanofillers come along with some drawbacks, leading to the sharp drop in the molecular weights of the polyester chains, and consequently an important loss of mechanical and thermal properties. Novel PLA-ZnO nanocomposite films were produced by melt-compounding PLA with 0.5–3% ZnO rod-like nanoparticles. The surface treatment of nanofiller by silanization (with triethoxy caprylylsilane) was necessary to obtain a better dispersion and to limit the decrease of molecular mass of PLA. The morphology, molecular, thermo-mechanical and transport properties to water vapor of PLA-ZnO films were analyzed with respect to the neat PLA. According to DSC and to XRD, the produced films were essentially amorphous. The changes in PLA permeation properties were strongly dependent on temperature and nanofiller loading. The well dispersed ZnO nanoparticles within the polyester matrix were effective in increasing the tortuosity of the diffusive path of the penetrant molecules. The activation energy remained similar for PLA and PLA-1% ZnO, but was found greater at higher loading of ZnO (3%), confirming the increased difficulty of travelling molecules to diffuse through PLA. In comparison to the neat PLA (presenting no antimicrobial efficacy), the nanocomposites were active against both Gram-positive and Gram-negative bacteria, stronger antibacterial activity being evidenced after 7 days elapsed time. By considering the multifunctional properties of PLA-ZnO nanocomposites, the films produced by extrusion can be considered a promising alternative as environmental-friendly packaging materials.     

Ageing of polylactide and polylactide nanocomposite filaments

An experimental study was carried out in order to check the influence of various parameters on the ageing of PLA filaments performed in a climatic room. Initially, two grades of PLA with different d-isomer contents were analysed. Even after several weeks of ageing, properties of PLA filaments with less than 0.5% d-isomer content did not vary extensively. On the other hand, the mechanical and thermal properties of PLA filaments with 4% d-isomer content underwent strong modifications. Subsequently, a selected organomodified bentonite (Bentone^®104 - noted as B104) was blended with PLA and then melt-spun to study the influence of a nanofiller on the ageing of PLA filaments. Faster degradation of PLA nanocomposite filaments was then observed.     

Polylactide compositions. The influence of ageing on the structure, thermal and viscoelastic properties of PLA/calcium sulfate composites

High-performance composites prepared by melt-blending polylactide (PLA, l/d isomer ratio of 96/4) with various amounts of β-anhydrite II (AII), the dehydrated form of calcium sulfate hemihydrate obtained by a specific thermal treatment at 500 °C, have been aged to study the evolution of their physical and mechanical properties with time. The effect of 1-year ageing under ambient conditions (below T_g of PLA) for selected composites, i.e., filled with 20 and 40 wt% AII, was determined and compared to unfilled PLA with the same processing and ageing history. Samples with an initial amorphous PLA matrix, obtained by fast quenching from the melt, were characterized before and during ageing. The changes in physical parameters have been studied using dynamic mechanical thermal analysis (DMTA), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and density measurements. Surprisingly, for all the samples, an increase of the storage modulus (E′) was recorded, as a result of ageing. This improvement was ascribed to the reorganization of the PLA structure induced by ageing. The structural reorganization was also reflected by a slight increase of PLA density and changes in thermal behaviour. The X-ray investigations showed unchanged crystallographic structure of AII both during blending with molten PLA and in the composite systems after ageing. The surprising stability of the thermo-mechanical properties of PLA and PLA/AII composites is in agreement with the results of size exclusion chromatography analysis (SEC) which did not show significant changes of PLA molecular weights brought out by ageing.     

The effect of halloysite nanotubes and N,N'- ethylenebis (stearamide) on the properties of polylactide nanocomposites with amorphous matrix

Amorphous polylactide/halloysite nanotube (PLA/HNT) nanocomposites were prepared and examined. Neat HNT and HNT treated with N,N'- ethylenebis(stearamide) (EBS) were used as nanofillers. The role of HNT and/or EBS content on the cold crystallization of amorphous PLA matrix, HNT dispersion, as well as on the dynamic mechanical and optical properties of the materials was determined.The PLA/HNT-based nanocomposites contained well-distributed nanotubes and occasionally micron-sized aggregates, especially at high loading. HNT, EBS treated HNT and EBS influenced the cold crystallization of PLA, therefore the formation of the disorder α′ and the order α crystallographic forms of PLA.The nanocomposites exhibited increased stiffness and decreased transparency compared to the neat PLA. Due to the reinforcing effect and additional specific features of HNT, the addition of the nanofiller allows tuning of the properties of the nanocomposites with amorphous PLA matrix.     

Graphitic nanofillers in PMMA nanocomposites—An investigation of particle size and dispersion and their influence on nanocomposite properties

Mechanical, thermal, and electrical properties of graphite/PMMA composites have been evaluated as functions of particle size and dispersion of the graphitic nanofiller components via the use of three different graphitic nanofillers: “as received graphite” (ARG), “expanded graphite,” (EG) and “graphite nanoplatelets” (GNPs) EG, a graphitic materials with much lower density than ARG, was prepared from ARG flakes via an acid intercalation and thermal expansion. Subsequent sonication of EG in a liquid yielded GNPs as thin stacks of graphitic platelets with thicknesses of ～10 nm. Solution‐based processing was used to prepare PMMA composites with these three fillers. Dynamic mechanical analysis, thermal analysis, and electrical impedance measurements were carried out on the resulting composites, demonstrating that reduced particle size, high surface area, and increased surface roughness can significantly alter the graphite/polymer interface and enhance the mechanical, thermal, and electrical properties of the polymer matrix. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2097–2112, 2007     

Development of new approach based on Raman spectroscopy to study the dispersion of expanded graphite in poly(lactide)

A new approach has been developed to study the dispersion/delamination of expanded graphite (EG) in poly(lactide) (PLA) by using Raman spectroscopy. This technique is more sensitive and therefore fully complementary to more standard dispersion characterization techniques like SEM, TEM and X-ray diffraction. The incorporation of EG into PLA was carried out by a twin-screw micro-extruder. The effects of the dispersion and delamination of EG on the thermal and thermo-mechanical properties of polylactide-EG nanocomposites were investigated. In contrast to the standard techniques, Raman spectroscopy was able to show a partial exfoliation, which could therefore explain the slight improvement of the PLA-EG thermal and thermo-mechanical properties.     

Selective localization of graphene oxide in electrospun polylactic acid/poly(ε-caprolactone) blended nanofibers

Despite their immiscibility, blending polylactic acid (PLA) with poly(ε-caprolactone) (PCL) provides an efficient strategy for obtaining a biopolymer blend with tailored properties due to their complementary physical properties. In this study, graphene oxide (GO) was employed as a 2-D nanofiller and nucleating agent to improve the properties of the immiscible PLA/PCL blends at 70/30, 50/50, and 30/70 wt ratios. Nanofibers of PLA/PCL blends and PLA/PCL/GO composites were investigated. It was interesting to find that the GO selectively localized in the minor phase resulting from the phase separation. The selective localization of the GO as the nucleating agent had an influence on the degree of crystallinity and crystalline morphology in the blended composites. This study also demonstrated that the molecular chains in the PLA phase oriented along the fiber axes, while in the PCL phase, the partial crystallites changed their orientation direction to be perpendicular to the fiber axes with the addition of GO.     

Graphene oxide crosslinker for the enhancement of mechanical properties of polylactic acid

Polylactic acid (PLA) biopolymer appears to provide environmental advantages over the petroleum-derived polymers but often ends up with limited applications owing to their poor mechanical performance and brittleness. Herein, we present a PLA polymer compatible graphene oxide (GO) based crosslinker with the intention of improving the mechanical properties. Lactic acid (LA) functionalized GO (GO-LA) crosslinker was prepared and had been crosslinked with the PLA chains through a one-step polycondensation reaction. The mechanical properties of the as-synthesized GO crosslinked PLA (GO-C-PLA) were investigated by compression tests and compared with neat PLA, and GO reinforced PLA (GO-PLA) with no crosslinking. With 0.3% of GO-LA crosslinker in GO-C-PLA, the compressive modulus increased by nine times compared to that of the neat PLA. The compressive strength also increased to 46 MPa, which was four times higher than the neat PLA. This strategy for improving the mechanical properties by introducing GO-based crosslinker can be used potentially for many polycondensation polymers and thus be useful for many high-performance applications.     

醋酸淀粉/聚乳酸的制备及性能的影响研究

采用自设计的双螺杆结构挤出制备聚乳酸(PLA)/醋酸淀粉(AS)的全生物降解材料,考察材料的AS的含量和取代度对复合材料动态流变性能、机械性能的影响。研究结果表明,AS含量明显影响复合材料的力学性能、复合黏度和储能模量:当AS含量从45%增加到70%,材料的拉伸强度下降,复数黏度和储能模量则提高。随着AS取代度由1.0上升为3.0,复合材料的复数黏度和储能模量下降,拉伸强度由12.0MPa上升为15.5MPa。对复合材料进行电镜扫描分析发现,AS以海岛结构形式分散在PLA的连续相中,取代度2.0的AS与PLA相容性最好,当其质量含量达到70%,材料的拉伸强度仍然不低于10.0MPa,具有较好的机械强度。     

Rheological and thermal behavior of PLA modified by chemical crosslinking in the presence of ethoxylated bisphenol A dimethacrylates

Crosslinking structures can be partly introduced into PLA by melt mixing in a twin screw extruder with dicumyl peroxide (DCP) and ethoxylated bisphenol A dimethacrylates (Bis‐EMAs) as a crosslinking coagent. The study of DCP and Bis‐EMA contents on the melt rheology, thermal properties, dynamic mechanical properties and morphology of the reactive extruded pellets is presented. The results show that PLA with a DCP content higher than 3 phr exhibits increases in both the melt modulus and complex viscosity as compared with PLA. The introduction of DCP into PLA improved the thermal stability of the PLA. PLAs with various Bis‐EMA contents showed the optimum storage modulus and complex viscosity to occur at 5 phr Bis‐EMAs. Moreover, the glass transition, cold crystallization and melting temperature of PLAs decreased with increasing Bis‐EMA content. The crystallinity of the partly crosslinked PLAs was lower than that of PLA. Similar to the rheological results, the thermo‐mechanical properties showed that the storage modulus and loss modulus of the partly crosslinked PLAs increased with increasing Bis‐EMA contents up to 5 phr. In addition, these partly crosslinked PLAs showed rough surface or sea island‐like structure. Copyright © 2016 John Wiley & Sons, Ltd.     

Development and thermal behaviour of ternary PLA matrix composites

Biodegradable PLA composites were prepared using microcrystalline cellulose (MCC) and silver (Ag) nanoparticles. The main objective of the present study is to develop new biopolymer composites with good mechanical properties, thermal stability, maintaining the optical transparency and also providing antimicrobial properties through silver nanoparticle introduction. Composites were prepared with 1%wt of Ag nanoparticles and 5%wt of MCC using a twin-screw microextruder; film parameters were optimized in order to obtain a thickness range between 20 and 60 μm.PLA composites maintained optical transparency properties of the matrix, while MCC was able to reduce polymer permeability. Thermal analysis revealed that MCC increased PLA crystallinity and the mechanical properties of the composites demonstrated that tensile modulus was improved by microcrystalline cellulose.     

Enhanced Nucleation and Crystallization in PLA/CNT Composites via Disperse Orange 3 with Corresponding Improvement in Nanomechanical Properties

The potential for the material property improvement through the addition of carbon nanotubes (CNTs) in composite materials is often limited due to CNT agglomeration. In this work, Disperse Orange 3 (DO3) was investigated to determine its effectiveness in dispersing CNTs in a poly (lactic acid) (PLA) matrix. First, adsorption studies of DO3 onto CNTs were performed to determine the appropriate amount of DO3 to add so that the CNT surface will be nearly saturated with DO3 while limiting the excess DO3 dissolved in the polymer. The resultant improvements in the mechanical properties were determined via nanoindentation. Highly stable dispersion of CNTs in tetrahydrofuran with DO3 was observed 72 hours after sonication. Scanning electron microscopy confirmed that DO3‐functionalized CNTs were able to separate and disperse well inside of the PLA matrix. Addition of DO3 to the nanocomposite resulted in an increase in the glass transition temperature and crystallinity of the composite due to the more effective dispersion of the nanofiller which serves as a nucleation agent. The CNTs treated with DO3 also increased the elastic modulus and hardness of the composite compared to neat PLA and untreated PLA‐CNT composites. From this study, DO3 was demonstrated to be an effective dispersing agent in the solvent and the PLA matrix which allowed for enhanced crystallization and improved nanomechanical properties in the resultant composite.     

Impact of coated calcium carbonate nanofillers and annealing treatments on the microstructure and gas barrier properties of poly(lactide) based nanocomposite films

Amorphous poly(lactide) (PLA) and nanocomposite films were prepared from melt‐blending with precipitated calcium carbonate nanofillers (PCC). Nanocomposites based on uncoated PCC (PCC‐UT), stearic acid coated PCC (PCC‐S), and poly(ε‐caprolactone) coated PCC (PCC‐P) were investigated for an inorganic content fixed to 8 wt %. Using coated nanofillers allowed preserving both PLA average molar mass and thermal stability while enhancing the nanofiller dispersion state. Poly(ε‐caprolactone) was identified as the best coating for optimized morphology and thermal properties. Maxwell law accurately described the increase in oxygen barrier properties observed for the nanocomposites based on PCC‐S. A modified Maxwell law was proposed to take account of the additional increase in barrier properties evidenced for the PLA/PCC‐P nanocomposites and assigned to the particularly strong compatibility between PCL and PLA. Different annealing conditions were investigated to respectively study the impact of physical ageing and PLA crystallization on gas permeability. Different extents of physical ageing did not significantly modify the oxygen transport properties. However, a high permeability decrease was observed for the semicrystalline nanocomposites with respect to the amorphous reference PLA film. Finally, the gain in barrier properties was shown to result from both contribution of the nanofillers and the crystalline phase. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 649–658     

Effect of thermo-mechanical cycles on the physico-chemical properties of poly(lactic acid)

Polylactic acid (PLA) has now become an economically viable commodity plastic in many industries. This raises the question of recyclability of industrial production waste and some packaging wastes as well. The evolution of rheological and mechanical properties of polymer with the number of recycling cycles up to seven was investigated. For PLA, only the tensile modulus remains constant with the thermo-mechanical cycles. In contrast, stress and strain at break, rheological factors and the modulus and hardness probed by nanoindentation decrease for PLA. This dramatic effect is ascribed to a large decrease in the molecular weight due to several different complex degradation processes which are discussed. The effect of two stabilizers is also assessed.