Temperature dependent variations in the lamellar structure of poly(l-lactide)

Super-Tough PLA-Based Blends with Excellent Stiffness and Greatly Improved Thermal Resistance via Interphase Engineering

Super-tough poly(lactic acid)/polycarbonate (PLA/PC) (50/50) blends with an excellent balance of stiffness, toughness, and thermal stability were systematically designed and characterized. Poly(methyl methacrylate) (PMMA) was utilized as a novel, highly effective nonreactive interphase to promote PLA-PC phase compatibility. Partial miscibility of PMMA with both PLA and PC produced strong molecular entanglements across the PLA-PC phase boundary followed by an excellent phase adhesion. This was predicted from interfacial energy measurements and supported by dynamic mechanical thermal analysis, morphological observations, and mechanical tests. Ternary PLA/PC/PMMA blends exhibited an exceptional set of stiffness, tensile and flexural strength, tensile and flexural ductility, and thermal stability together with improved impact strength compared with neat PLA and uncompatibilized PLA/PC blends. Addition of nonreactive polybutadiene-g-styrene-co-acrylonitrile (PB-g-SAN) impact modifier to the compatibilized blend resulted in further dramatic improvements in the dispersion state of PC and PMMA phase domains followed by the development of an interconnected structure of PC, PMMA, and PB-g-SAN domains in the PLA matrix. Such a network-like morphology, with rubbery particles percolated at the interface between the dispersed structures and surrounding PLA matrix, produced a tremendous increase in impact resistance (≈700 J/m) and tensile ductility (≈200% strain) while maintaining excellent stiffness (≥2.1 GPa). The combined effects of interfacial localization of impact modifier particles, network-like morphology (extended over the entire volume of the blend), and strong phase interactions between the components (due to mutual miscibility) are described to be responsible for super-tough behavior. The role of PMMA as an efficient interphase adhesion promoter in the toughened quaternary blends is also clarified. Impact fractography revealed multiple void formations, plastic growth of microvoids, and the formation of void-fibrillar structures around as well as inside the dispersed structures as the main micromechanical deformation processes responsible for massive shear yielding and plastic deformation of blends. Blends designed in this work offer remarkable improvements in tensile and flexural ductility, impact resistance, and heat deflection temperature compared with neat PLA resin. The overall characteristics of these blend systems are comparable and/or superior to those of several commercial thermoplastic resins.

Micro-structure and tensile properties of microfluidic spinning konjac glucomannan and sodium alginate composite bio-fibers regulated by shear and elongational flow: experiment and multi-scale simulation

Microfluidic spinning has been widely used to produce bio-fibers with excellent tensile performances by regulating the conformation of biological macromolecules. However, the effect of channel shapes on fiber tensile performances is unclear. In this study, bio-fibers were prepared using konjac glucomannan and sodium alginate by five channels. The micro-morphology and tensile performance of fibers were characterized and measured. Then, the dynamical behaviours of macromolecule clusters in flow fields were simulated by multi-scale numerical methods. The results show that the elongational flow with increasing extension rates produced fibers with a tensile strength of 32.34 MPa and a tensile strain of 18.72 %, which were 1.37 and 1.55 times that for a shear flow, respectively. The difference in tensile performances was attributed to the micro-morphology regulated by flow fields. The continuously increasing extension rate of flow was more effective than the shear rate or the maximum extension rate for the stretching of macromolecule clusters. We conclude that the channel shapes significantly influence flow fields, dynamical behaviours of molecule clusters, the morphology of fibers, and tensile performances. This study provides a novel numerical method and understanding of microfluidic spinning, which will promote the optimization and applications of bio-fibers.

Correlation between Processing Parameters, Morphology, and Properties of Injection-Molded Polylactid Acid (PLA) Specimens at Different Length Scales

Polylactic acid (PLA) is one of the most promising bioplastic representatives that finds application in many different areas, e.g., as single-use products in the packaging industry, in the form of mulch film for agriculture, or in medical devices. For the development of new areas, especially in terms of long-term applications and the production of recyclable products, the material properties controlled by processing must be known. The state of the art is investigations at the global scale (integral values) without consideration of local structure inhomogeneities and their influence on the material properties. In this work, morphological, thermal, and mechanical properties of injection-molded PLA tensile bars are investigated at different length scales (global and local) as a function of processing parameters. In addition to the processing parameters, such as melt temperature, mold temperature, and cooling time in the mold, the influence of the D-isomer content on the crystallization behavior and the resulting material properties are investigated. The material was found to form crystalline structures only when cooled in a mold tempered above T_g. In addition, PLA with a lower content of D-isomer was found to have a higher degree of crystallinity. Since the mechanical properties obtained by tensile tests could not be correlated with the degree of crystallinity, detailed analysis were performed showing a characteristic inhomogeneous morphology within the tensile bars. By means of micromechanical investigations on samples with different microstructure ranges, the relationship between local morphology and failure behavior could be explained.

Young's modulus of the different crystalline phases of poly (l-lactic acid)

Young's modulus of α'- and α-crystals of poly (l-lactic acid) (PLLA), more precisely, of aggregates of isotropically arranged lamellae, has been estimated based on dynamic-mechanical analysis of sets of isotropic film samples containing largely different though well-defined amounts of crystals. Evaluation of the modulus of elasticity of these film samples yielded the dependence of Young's modulus as a function of the enthalpy-based crystallinity, increasing with the crystal fraction in the assessed range, from zero to about 75% crystallinity. Extrapolation towards 100% crystallinity suggests values of Young's modulus of around 3.7 and 4.6 GPa for isotropic aggregates of α'- and α-crystals, respectively, being only slightly higher than the modulus of the unaged glassy amorphous phase of 3.0 GPa. Noting the inherent anisotropy of the crystal modulus, suggested in the literature, the average modulus determined in this work seems to be controlled by weaker interchain secondary bonding but not the modulus in chain direction. Great effort has been undertaken to minimize errors by keeping the lamellar thickness in samples of different crystallinity constant, and by providing evidence for independence of the moduli on the spherulitic superstructure.

Temperature-Dependent Polymorphism and Phase Transformation of Friction Transferred PLLA Thin Films

Poly(L-lactic acid) (PLLA) thin films with a highly oriented structure, successfully prepared by a fast friction transfer technique, were investigated mainly on the basis of synchrotron radiation wide-angle X-ray diffraction (WAXD) and Fourier transform infrared spectroscopy (FTIR). The crystalline structure of the highly oriented PLLA film was remarkably affected by friction transfer temperatures, which exhibited various crystal forms in different friction temperature regions. Interestingly, metastable β-form was generated at all friction transfer temperatures (70-140 °C) between T_g and T_m, indicating that fast friction transfer rate was propitious to the formation of β-form. Furthermore, the relative content among β-, α'-, and α-forms at different friction temperatures was estimated by WAXD as well as FTIR spectroscopy. In situ temperature-dependent WAXD was applied to reveal the complicated phase transition behavior of PLLA at a friction transfer temperature of 100 °C. The results illustrated that the contents of β- and α'-forms decreased in turn, whereas the α-form increased in content due to partially melt-recrystallization or crystal perfection. Moreover, by immersing into a solvent of acetone, β-, α'-form were transformed into stable α-crystalline form directly as a consequence. The highly oriented structure was maintained with the chain perfectly parallel to friction transfer direction after acetone treatment, evidenced by polarized FTIR and polarized optical microscopy (POM) measurements.

Crystallization of polylactides examined by vibrational circular dichroism of intra- and inter-chain chiral interactions

Herein, vibrational circular dichroism (VCD) measurements were carried out to study the kinetics of cold-crystallized poly(D-lactide) (PDLA) at the molecular level via qualitative analysis. The amplification of the VCD signals from intra- and inter-chain chiral interactions suggests the formation of partially ordered PDLA, followed by heterogeneous nucleation for crystallization. These results were further supported by differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and Fourier transform infrared (FTIR) spectroscopy analyses.

Bulk enthalpy of melting of poly (l-lactic acid) (PLLA) determined by fast scanning chip calorimetry

The bulk enthalpy of melting of α-crystals of poly (l-lactic acid) (PLLA) has been evaluated by fast scanning chip calorimetry (FSC), by analysis of the correlation between the measured enthalpy of melting of sets of samples of different crystallinity and the corresponding heat capacity at 90°C, that is at a temperature higher than the glass transition temperature of the bulk amorphous phase and lower than the melting temperature. Extrapolation of this relationship for crystals formed at 140°C towards the heat capacity of fully solid PLLA yields a value of about 104.5±6 J/g when melting occurs at 180-200°C. The analysis is based on the presence of a two-phase structure, that is, absence of a vitrified rigid amorphous fraction (RAF) at the temperature of analysis of the solid fraction of the material (90°C). Formation and vitrification of an RAF was suppressed by avoiding both continuation of primary crystallization and secondary crystallization during cooling the system from the crystallization temperature of 140°C to 90°C, making use of the high cooling capacity of FSC. Small-angle X-ray scattering (SAXS) confirmed distinct thickening of initially grown lamellae which only is possible if these lamellae are not surrounded by a glassy RAF. Linear crystallinity values obtained by SAXS and calorimetrically determined enthalpy-based crystallinities agree close to each other. This article is protected by copyright. All rights reserved.

Simultaneously enhancing the crystallization rate and fire retardancy of poly(lactic acid) by using a novel bifunctional additive trimethylamine phenylphosphonate

Simultaneously regulating the crystallizing and combustion behaviors of poly(lactic acid) (PLA) will be conducive to its further development in the fields of electronic appliances, automotive and rail transit materials. To achieve this goal, a novel bifunctional additive triethylamine phenylphosphonate (TEAP) was synthesized through acid-base neutralization reaction between trimethylamine and phenylphosphonic acid. When TEAP was added into PLA, the crystallization behaviors of PLA/TEAP assessed by differential scanning calorimetry (DSC) and polarized optical microscopy (POM) suggested that TEAP acted as a nucleating agent and plasticizer for PLA, which effectively increased the crystallization rate of PLA. However, PLA with 3 wt% TEAP showed a slower crystallization rate than that of PLA with 1 wt% TEAP due to the filler aggregation of TEAP. Thus, the crystallization rate increased first and then slightly decreased with increasing content of TEAP. Compared with the variation of the crystallization rate, the long period (L) and amorphous layer thickness (L _a) resulting from SAXS showed opposite trends, while the average crystal thickness (L _c) changed slightly; the reason may relate to the variation of the number of lamellae with increasing the content of TEAP. Meanwhile, the results of WAXD and Raman spectra showed the crystal structure of PLA was not affected by the addition of TEAP. The combustion behaviors of PLA and PLA/TEAP were evaluated by the limiting oxygen index (LOI), UL-94 test, cone calorimetry test (CCT) and thermal gravimetric analyses coupled to Fourier transform infrared spectroscopy (TGA-FTIR). According to the results, TEAP mainly promotes the removal of melt dripping, hence brings away heat and delays the combustion. Besides, the production of phosphorus-containing free radicals can quench hydrogen or oxygen free radicals in the fire. Thus, the fire safety of PLA is significantly improved by adding a very low content of TEAP (1-3 wt%). Only 1 wt% loading of TEAP can increase the LOI value of PLA from 19.5 vol% to 28.6 vol%, pass the UL-94 V-0 rating and have a low peak heat release rate of 404 kW m^-2.

Composites Based on Poly(Lactic Acid) (PLA) and SBA-15: Effect of Mesoporous Silica on Thermal Stability and on Isothermal Crystallization from Either Glass or Molten State

Several composites based on an L-rich poly(lactic acid) (PLA) with different contents of mesoporous Santa Barbara Amorphous (SBA-15) silica were prepared in order to evaluate the effect of the mesoporous silica on the resultant PLA materials by examining morphological aspects, changes in PLA phases and their transitions, and, primarily, the influence on some final properties. Melt extrusion was chosen for the obtainment of the composites, followed by quenching from the melt to prepare films. Completely amorphous samples were then attained, as deduced from X-ray diffraction and differential scanning calorimetry (DSC) analyses. Thermogravimetric analysis (TGA) results demonstrated that the presence of SBA-15 particles in the PLA matrix did not exert any significant influence on the thermal decomposition of these composites. An important nucleation effect of the silica was found in PLA, especially under isothermal crystallization either from the melt or from its glassy state. As expected, isothermal crystallization from the glass was considerably faster than from the molten state, and these high differences were also responsible for a more considerable nucleating role of SBA-15 when crystallizing from the melt. It is remarkable that the PLA under analysis showed very close temperatures for cold crystallization and its subsequent melting. Moreover, the type of developed polymorphs did not accomplish the common rules previously described in the literature. Thus, all the isothermal experiments led to exclusive formation of the α modification, and the observation of the α’ crystals required the annealing for long times at temperatures below 80 °C, as ascertained by both DSC and X-ray diffraction experiments. Finally, microhardness (MH) measurements indicated a competition between the PLA physical aging and the silica reinforcement effect in the as-processed amorphous films. Physical aging in the neat PLA was much more important than in the PLA matrix that constituted the composites. Accordingly, the MH trend with SBA-15 content was strongly dependent on aging times.

Synergic Effect of Nanolignin and Metal Oxide Nanoparticles into Poly(l-lactide) Bionanocomposites: Material Properties, Antioxidant Activity, and Antibacterial Performance

Binary and ternary poly(l-lactide) (PLLA)-based nanocomposites, containing nanolignin (1 wt %) and different metal oxide nanoparticles (0.5 wt %, Ag₂O, TiO₂, WO₃, Fe₂O₃, and ZnFe₂O₄), were realized by solvent casting, and their morphological, thermal, surface, optical, antioxidant, and antimicrobial characterizations were performed. The presence of metal oxide nanoparticles at the selected weight concentration affects the surface microstructure of the PLLA polymer, and this outcome is particle-type dependent, according to the shape, morphology, and chemical properties of the selected nanoparticles (NPs). Analogously, wettability of PLLA-based nanocomposites was slightly modified by the presence of hydrophobic lignin nanoparticles and different shaped metal oxides. Results of differential scanning calorimetry (DSC) and X-ray powder diffraction (XRD) tests confirmed that nanoparticle addition confined the mobility of the amorphous phase, increasing at the same time the formation of more numerous but less perfect PLLA crystals. Interestingly, antioxidant activity was also obtained in ternary-based nanocomposites, where a synergic effect of lignin and metal oxide nanoparticles was obtained. Antibacterial tests showed manifest activity of TiO₂ and Ag₂O nanoparticles containing PLLA films, and the time dependence was more evident for Staphylococcus aureus than for Escherichia coli. Lignin nanoparticles are able to provide protection against UV light while still allowing visible light to pass and even surpass the UV-protection capacity provided by many inorganic nanoparticles. This makes them an attractive renewable additive for the realization of PLLA/metal oxide nanocomposites in the fields of food, drug packaging, and biomedical industry, where antibacterial and antioxidant properties are required.

Plasticization of Polylactide after Solidification: An Effectiveness and Utilization for Correct Interpretation of Thermal Properties

Polylactide/triethyl citrate (PLA/TEC) systems were prepared in two ways by introducing TEC to solidified polymer matrix (SS) and by blending in a molten state (MS) to investigate the effectiveness of the plasticization process after solidification of polylactide. The plasticization processes, independently of way of introducing the TEC into PLA matrix, leads to systems characterized by similar stability, morphology and properties. Some differences in mechanical properties between MS and SS systems result primarily from the difference in the degree of crystallinity/crystal thickness of the PLA matrix itself. Based on the presented results, it was concluded that the plasticization process after solidification of polylactide is an alternative to the conventional method of modification-blending in a molten state. Then, this new approach to plasticization process was utilized for interpretation of thermal properties of PLA and PLA/TEC systems. It turned out that double melting peak observed at DSC thermograms does not result from the melting of a double population of crystals with different lamellar thickness, or the melting of both the α' and α crystalline phase (commonly used explanations in literature), but is associated with the improvement of perfection of crystalline structure of PLA during heating process.

High Tm poly(L-lactide)s via REP or ROPPOC of l-lactide

A new kind of high melting (HT_m) pol(l-lactide) was discovered when cyclic poly(l-lactide)s were prepared by ring-expansion polymerization with cyclic tin catalysts at 130–160 °C in bulk.

Pressure induced crystallization and in situ simultaneous SAXS/WAXS investigations on structure transitions

A phase diagram of PLLA crystal structures as a function of crystallization temperature (T_c) and pressure (P_c).

Effect of Porosity and Crystallinity on 3D Printed PLA Properties

Additive manufacturing (AM) is a promising technology for the rapid tooling and fabrication of complex geometry components. Among all AM techniques, fused filament fabrication (FFF) is the most widely used technique for polymers. However, the consistency and properties control of the FFF product remains a challenging issue. This study aims to investigate physical changes during the 3D printing of polylactic acid (PLA). The correlations between the porosity, crystallinity and mechanical properties of the printed parts were studied. Moreover, the effects of the build-platform temperature were investigated. The experimental results confirmed the anisotropy of printed objects due to the occurrence of orientation phenomena during the filament deposition and the formation both of ordered and disordered crystalline forms (α and δ, respectively). A heat treatment post-3D printing was proposed as an effective method to improve mechanical properties by optimizing the crystallinity (transforming the δ form into the α one) and overcoming the anisotropy of the 3D printed object.

Rapid crystallization and mesophase formation of poly(L-lactic acid) during precipitation from a solution

Very rapid crystallization behaviors of poly(L-lactic acid) (PLLA) are observed at room temperature when it is precipitated from a chloroform solution into a large amount of alcohols (non-solvents). The resulting crystalline phase contains both a highly ordered (α) and less ordered (α′) modifications, and the fraction of these phases depends on the alcohols used as the non-solvents: methanol tends to produce the highly ordered phase. The degree of crystallinity tends to be high for lower alcohols. When the precipitation occurs in n-hexane, almost no crystalline phase is formed, but a mesomorphic phase is formed as a precursor to the crystalline phase. The results suggest that the hydroxyl group of alcohols tends to promote the crystallization of PLLA. However, it is found that the precipitation in methanol at lower temperatures, such as 0°C, does not yield any crystalline phase. It is suggested that the present rapid crystallization during precipitation originates from the enhanced mobility of PLLA molecules in a metastable (non-equilibrium) liquid state.