Poly(lactic acid)-Based in Situ Microfibrillar Composites with Enhanced Crystallization Kinetics, Mechanical Properties, Rheological Behavior, and Foaming Ability

PLA/PA Bio-Blends: Induced Morphology by Extrusion

The effect of processing conditions on the final morphology of Poly(Lactic Acid) (PLA) with bio-based Polyamide 10.10 (PA) 70/30 blends is analyzed in this paper. Two types of PLA were used: Commercial (neat PLA) and a rheologically modified PLA (PLAREx), with higher melt elasticity produced by reactive extrusion. To evaluate the ability of in situ micro-fibrillation (f) of PA phase during blend compounding by twin-screw extrusion, two processing parameters were varied: i) Screw speed rotation (rpm); and ii) take-up velocity, to induce a hot stretching with different Draw Ratios (DR). The potential ability of PA-f in both bio-blends was evaluated by the viscosity (p) and elasticity (k') ratios determined from the rheological tests of pristine polymers. When PLAREx was used, the requirements for PA-f was fulfilled in the shear rate range observed at the extrusion die. Scanning electron microscopy (SEM) observations revealed that, unlike neat PLA, PLAREx promoted PA-f without hot stretching and the aspect ratio increased as DR increased. For neat PLA-based blends, PA-f was promoted during the hot stretching stage. DMTA analysis revealed that the use of PLAREx PLAREx resulted in a better mechanical performance in the rubbery region (T > Tg PLA-phase) due to the PA-f morphology obtained.

Improving the Continuous Microcellular Extrusion Foaming Ability with Supercritical CO2 of Thermoplastic Polyether Ester Elastomer through In-Situ Fibrillation of Polytetrafluoroethylene

In-situ fibrillated polytetrafluoroethylene (PTFE) enhanced nanocomposites were successfully prepared by mixing thermoplastic polyether ester elastomer (TPEE) and PTFE using a twin-screw extruder. Well-dispersed, long aspect ratio PTFE nanofibrils with a diameter of less than 200 nm were generated and interwoven into networks. Differential scanning calorimetry and in-situ polarized optical microscopy showed that the PTFE nanofibrils can greatly accelerate and promote crystallization of the TPEE matrix and the crystallization temperature can be increased by 6 °C. Both shearing and elongational rheometry results confirmed that the introduction of PTFE nanofibrils can significantly improve the rheological properties. The remarkable changes in the strain-hardening effect and the melt viscoelastic response, as well as the promoted crystallization, led to substantially improved foaming behavior in the continuous extrusion process using supercritical CO2 as the blowing agent. The existing PTFE nanofibrils dramatically decreased the cell diameter and increased cell density, together with a higher expansion ratio and more uniform cell structure. The sample with 5% PTFE fibrils showed the best foaming ability, with an average diameter of 10.4-14.7 μm, an expansion ratio of 9.5-12.3 and a cell density of 6.6 × 107-8.6 × 107 cells/cm3.

Fabrication of bimodal open-porous poly (butylene succinate)/cellulose nanocrystals composite scaffolds for tissue engineering application

The design of porous tissue engineering scaffold with multiscale open-pore architecture (i.e., bimodal structure) promotes cell attachment and growth, which facilitates nutrient and oxygen diffusion. In this study, a porous poly (butylene succinate) (PBS)/cellulose nanocrystals (CNCs) composite scaffold with a well-defined controllable bimodal open-pore interconnected structure was successfully fabricated. The bimodal open-porous scaffold architecture was designed by synergistic control of temperature variation and a two-step depressurization in a supercritical carbon dioxide (Sc-CO₂) foaming process. The microstructure and properties of the bimodal open-porous PBS/CNCs scaffold, such as morphology, open porosity, hydrophilic and degradation performance, and mechanical compression properties, were analyzed. In the experiments, the scaffold with unimodal pore structure was used for comparison. The results showed that the bimodal open-porous PBS5 scaffold displayed a well-defined bimodal open-pore structure composed of large pore (~68.9 μm in diameter) and small pore (~11.0 μm in diameter), with a high open porosity (~95.2%). In addition, the scaffolds exhibited good mechanical compressive properties (compressive strength of 2.76 MPa at 50% strain), hydrophilicity (water contact angle of 71.7 °C) and in vitro degradation rate. Moreover, in vitro biocompatibility was determined with NIH-3T3 fibroblast cells using MTT assay and live/dead cell viability assay. Results indicated that the obtained bimodal open-porous scaffolds had a good biocompatibility and the viability of cells grown on the scaffolds reached up to 98% after 7th day of culture. Therefore, our work provides new insights into the use of biodegradable polymeric composite scaffolds with bimodal open-pore structure and balanced properties in tissue engineering.

Nanofibrillar Green Composites of Polylactide/Polyhydroxyalkanoate Produced in Situ Due to Shear Induced Crystallization

This study addresses the new concept of in situ inducing fibrillar morphology (micro or nanofibrils) of a minority component based on the simultaneous occurrence of orientation and shear induced crystallization of polymer fibers directly at the stage of extrusion in a single step. This possibility is demonstrated by using two entirely bio-sourced polymers: polylactide (PLA) and polyhydroxyalkanoate (PHA) as components. The shear induced crystallization allowed crystallization of PHA nanofibers immediately after applying high shear rate and elongation strain, avoiding subsequent cooling to initiate crystallization. Shearing of PHA increased non-isothermal crystallization temperature by 50 °C and decreased the temperature range in which the transition from a molten state to a crystallized one occurs by 17 °C. SEM observations demonstrate the successful transformation of the dispersed PHA phase into nanofibrils with diameters of nearly 200 nm. The transition from the droplets of PHA to fibers causes the brittle-to-ductile transition of the PLA matrix at a low concentration of PHA and contributes to the simultaneous increase of its rigidity and strength.

Nanofibrillar Green Composites of Polylactide/Polyhydroxyalkanoate Produced in Situ Due to Shear Induced Crystallization

This study addresses the new concept of in situ inducing fibrillar morphology (micro or nanofibrils) of a minority component based on the simultaneous occurrence of orientation and shear induced crystallization of polymer fibers directly at the stage of extrusion in a single step. This possibility is demonstrated by using two entirely bio-sourced polymers: polylactide (PLA) and polyhydroxyalkanoate (PHA) as components. The shear induced crystallization allowed crystallization of PHA nanofibers immediately after applying high shear rate and elongation strain, avoiding subsequent cooling to initiate crystallization. Shearing of PHA increased non-isothermal crystallization temperature by 50 °C and decreased the temperature range in which the transition from a molten state to a crystallized one occurs by 17 °C. SEM observations demonstrate the successful transformation of the dispersed PHA phase into nanofibrils with diameters of nearly 200 nm. The transition from the droplets of PHA to fibers causes the brittle-to-ductile transition of the PLA matrix at a low concentration of PHA and contributes to the simultaneous increase of its rigidity and strength.

Manipulation of Chain Entanglement and Crystal Networks of Biodegradable Poly(butylene adipate-co-butylene terephthalate) During Film Blowing through the Addition of a Chain Extender: An In Situ Synchrotron Radiation X-ray Scattering Study

One prerequisite for the large-scale application of biodegradable polymers is the manipulation of macroscopic performances of commercially available biopolymers during processing according to different real service requirements. Herein, the microstructural evolution of poly(butylene adipate-co-butylene terephthalate) (PBAT) modified by chain extender during film blowing was investigated by in situ synchrotron radiation X-ray scattering to unveil the origin of different performances. The chain dynamics difference induced by the chain extender was first characterized by the rheological measurement and ¹H Multiple Quantum (MQ) NMR. It shows that the terminal relaxation is significantly slowed down, while the locally segmental dynamics is not apparently changed. With the assistance of the custom-built film blowing apparatus, the microstructure right above the die exit (D = 13-165 mm) was in situ, simultaneously captured by small- and wide-angle scattering (SAXS/WAXS), where four distinct regimes can be defined. Only the PBAT melt signals are found in regime I, whereas the formation of the mesomorphic domains as shown by the SAXS streaks appearing in regime II. The crystal shows up in regime III, where the WAXS signal appears. A dramatic increment of the crystallinity is found in regime III, which contributes to the continuous increasing bubble modulus with the formation of the crystal-based network. Such a crystal-based network is filled with crystals in regime IV, where the diameter of the PBAT bubble remains constant. The addition of the chain extender is found to significantly influence the structural evolution within different regimes. These dynamics and structure information could supply general guidance for bubble stability improvement and modification of macroscopic performances of biodegradable polymer products.

The Strengthening and Toughening of Biodegradable Poly (Lactic Acid) Using the SiO2-PBA Core-Shell Nanoparticle

The balance of strengthening and toughening of poly (lactic acid) (PLA) has been an intractable challenge of PLA nanocomposite development for many years. In this paper, core-shell nanoparticles consisting of a silica rigid core and poly (butyl acrylate) (PBA) flexible shell were incorporated to achieve the simultaneous enhancement of the strength and toughness of PLA. The effect of core-shell nanoparticles on the tensile, flexural and Charpy impact properties of PLA nanocomposite were experimentally investigated. Scanning electron microscopy (SEM) and small-angle X-ray scattering (SAXS) measurements were performed to investigate the toughening mechanisms of nanocomposites. The experimental results showed that the addition of core-shell nanoparticles can improve the stiffness and strength of PLA. Meanwhile, its elongation at break, tensile toughness and impact resistance were enhanced simultaneously. These observations can be attributed to the cavitation of the flexible shell in core-shell nanoparticles and the resultant shear yielding of the matrix. In addition, a three-dimensional finite element model was also proposed to illustrate the damage processes of core-shell nanoparticle-reinforced polymer composites. It was found that, compared with the experimental performance, the proposed micromechanical model is favorable to illustrate the mechanical behavior of nanocomposites.

Investigation of the influence of pressurized CO2 on the crystal growth of poly(l-lactic acid) by using an in situ high-pressure optical system

Since CO2 is a kind of nontoxic, non-flammable and biocompatible fluid, introducing CO2 in the PLLA formation process has been regarded as a green way to the manufacture of biological products or medical supplies. However, it is still a challenge to understand the influence of CO2 on the crystal growth behavior of PLLA. Here, we developed an in situ high-pressure observation system, composed of optics, polarization optics and a small angle laser scattering system, to record the growth process of PLLA crystals in a pressurized CO2 environment. It is found that, at a low temperature (near Tg), low pressure CO2 (0.5 MPa in this work) can still induce the formation of numerous micron-sized spherulites of PLLA. Therefore, the introduction of CO2 can significantly enhance the crystallization ability of PLLA and decrease the crystallization temperature, which is helpful in improving the mechanical properties of PLLA products. We also found that a snowflake-shaped crystal was assembled by rhombic lamellae under pressurized CO2. There is a melt accumulation zone surrounding the growth front of the snowflake-shaped crystal, indicating that the growth front nucleation is limited by the pressurized CO2. This melt accumulation zone is quite different from the melt depletion zone existing ahead of the reported dendritic crystal front. Interestingly, in a high-pressure CO2 environment, a kind of bamboo-like branch is formed in a rhythmic growth mode. The repeating unit of the bamboo-like branch is constructed by an asymmetric terrace crystal originated from screw dislocation in the melt accumulation zone. These results demonstrated that CO2 has a remarkable tunability on the polymer crystal morphology.

Preparation of Thermoplastic Polyurethane (TPU) Perforated Membrane via CO2 Foaming and Its Particle Separation Performance

The way in which a perforated structure is formed has attracted much interest in the porous membrane research community. This novel structure gives materials an excellent antifouling property as well as a low operating pressure and other benefits. Unfortunately, the current membrane fabrication methods usually involve multi-step processes and the use of organic solvents or additives. Our study is the first to offer a way to prepare perforated membrane by using a physical foaming technique with CO2 as the blowing agent. We selected thermoplastic polyurethane (TPU) as the base material because it is a biocompatible elastomer with excellent tensility, high abrasion resistance, and good elastic resilience. Various processing parameters, which included the saturation pressure, the foaming temperature, and the membrane thickness, were applied to adjust the TPU membrane's perforated morphology. We proposed a possible formation mechanism of the perforated membrane. The as-prepared TPU membrane had good mechanical properties with a tensile strength of about 5 MPa and an elongation at break above 100%. Such mechanical properties make this novel membrane usable as a self-standing filter device. In addition, its straight-through channel structure can separate particles and meet different separation requirements.

Morphology control of extruded polystyrene foams with benzene-trisamide-based nucleating agents

Polystyrene is a low-priced, amorphous polymer, showing excellent foaming behavior. Polystyrene foams are widely used in a variety of applications including insulation panels for building and construction. In this context, foam morphology plays a significant role to tune the macroscopic properties of the foams and research focusses on the fabrication of foams with homogenous morphology and an average cell size distinctly below 100 µm at densities lower than 100 kg/m³. Here, we demonstrate how 1,3,5-benzene-trisamides can be used as supramolecular foam nucleating agents to control the morphology of extruded amorphous polystyrene foams. Depending on the concentration and the processing temperature, benzene-trisamides can be homogeneously dissolved in the polystyrene melt. Upon cooling, the benzene-trisamides self-assemble into finely dispersed, solid supramolecular nano-objects, which subsequently act as nucleating sites for foam cell formation. Various concentrations of the benzene-trisamide-based additive were selected to systematically study the influence of the morphology of the extruded polystyrene foams. In the same way, neat polystyrene foams were produced as a reference. We found that for extruded polystyrene foams with 0.2 wt% of additive, the cell sizes were significantly reduced by a factor of 35 from 632 to 18 µm compared to those of a neat extruded polystyrene reference foam.

The preparation of carbon nanofillers and their role on the performance of variable polymer nanocomposites

New synergic behavior is always inspiring scientists toward the formation of nanocomposites aiming at getting advanced materials with superior performance and/or novel properties. Carbon nanotubes (CNT), graphene, fullerene, and graphite as carbon-based are great fillers for polymeric materials. The presence of these materials in the polymeric matrix would render it several characteristics, such as electrical and thermal conductivity, magnetic, mechanical, and as sensor materials for pressure and other environmental changes. This review presents the most recent works in the use of CNT, graphene, fullerene, and graphite as filler in different polymeric matrixes. The primary emphasis of this review is on CNT preparation and its composites formation, while others carbon-based nano-fillers are also introduced. The methods of making polymer nanocomposites using these fillers and their impact on the properties obtained are also presented and discussed.

Strong and thermal-resistance glass fiber-reinforced polylactic acid (PLA) composites enabled by heat treatment

Polylactic acid (PLA) is a biodegradable polymer derived from renewable resources, showing potentials in replacing traditional petroleum-based polymers, yet its brittleness and low thermal-resistance limits its applications. Thus, glass fibers (GF) combined with heat treatment were used to prepare high-performance PLA. Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) were employed to analyze crystallization behavior of PLA/GF composite. Tensile, flexural and impact tests were conducted to investigate mechanical properties, and heat deflection temperature was measured to evaluate thermal resistance. GF can coincidently enhance strength, rigidity, and toughness of PLA. Isothermal heat treatment can further improve the mechanical properties regardless of GF content. Compared with neat PLA, the tensile strength, flexural modulus, and impact strength can be increased by 162.5%, 266.4%, 232.5%, respectively, in the presence of 20 wt% GF after isothermal heat treatment, and meanwhile heat deflection temperature can be increased from 50.6 °C to 148.8 °C. Both DSC and XRD analysis results indicated that GF can significantly enhance crystallization of PLA. Thus, not only GF but also enhanced crystallization led to the outstanding mechanical performance of PLA/GF composites. While GF shows little effect on thermal resistance, heat treatment can remarkably improve thermal stability, in particular for PLA/GF composites.

Improved Impact Properties in Poly(lactic acid) (PLA) Blends Containing Cellulose Acetate (CA) Prepared by Reactive Extrusion

Poly(lactic acid)/triacetine plasticized cellulose acetate (PLA/pCA) blends were prepared by extrusion at two different temperatures and tetrabutylammonium tetraphenyl borate (TBATPB) was added as a transesterification catalyst to reactively promote the formation of PLA-CA copolymer during the reactive extrusion. The occurrence of chain scission in the PLA phase and branching/crosslinking in the CA phase in the presence of TBATPB, resulting also in a darkening of the material, were demonstrated by studying torque measurements and by performing proper thermogravimetric tests on CA with the different additives. Tensile and impact tests onto the blends prepared at the lower temperature showed better properties than the ones obtained at a higher temperature. Then, the mechanical properties of PLA/plasticized cellulose acetate (pCA) blends prepared at the lower temperature were investigated as a function of the content of plasticized CA in the blend. A range of compositions was observed where blends exhibited improved impact properties with respect to pure PLA without a significant decrease in their elastic modulus. The study of the phase morphology of the blends revealed that the occurrence of reactive compatibilization did not significantly affect the phase distribution. In general, fibrillar CA particles were formed in the PLA matrix during extrusion, thus allowing the preparation of CA fibre reinforced composites. The trend of morphology as a function of the composition and processing conditions was then discussed by considering the evolution of phase morphology in immiscible polymer blends.

In situ generation of sustainable PLA-based nanocomposites by shear induced crystallization of nanofibrillar inclusions

In situ formation of polymer nanofibrils during compounding with a second polymer followed by their immediate solidification due to shear induced crystallization for two pairs of polymers is described.

Morphology, Thermal, Mechanical Properties and Rheological Behavior of Biodegradable Poly(butylene succinate)/poly(lactic acid) In-Situ Submicrofibrillar Composites

In this study, biodegradable poly(butylene succinate)/poly(lactic acid) (PBS/PLA) in-situ submicrofibrillar composites with various PLA content were successfully produced by a triple-screw extruder followed by a hot stretching−cold drawing−compression molding process. This study aimed to investigate the effects of dispersed PLA submicro-fibrils on the thermal, mechanical and rheological properties of PBS/PLA composites. Morphological observations demonstrated that the PLA phases are fibrillated to submicro-fibrils in the PBS/PLA composites, and all the PLA submicro-fibrils produced seem to have a uniform diameter of about 200nm. As rheological measurements revealed, at low frequencies, the storage modulus (G’) of PBS/PLA composites has been increased by more than four orders of magnitude with the inclusion of high concentrations (15 wt % and 20 wt %) of PLA submicro-fibrils, which indicates a significant improvement in the elastic responses of PBS melt. Dynamic Mechanical Analysis (DMA) results showed that the glass transition temperature (T_g) of PBS phase slightly shifted to the higher temperature after the inclusion of PLA. DSC experiments proved that fiber morphology of PLA has obvious heterogeneous nucleation effect on the crystallization of PBS. The tensile properties of the PBS/PLA in-situ submicrofibrillar composites are also improved compared to neat PBS.

Preparation of open microcellular polylactic acid foams with a microfibrillar additive using coreback foam injection molding processes

Open microcellular polylactic acid foams with a fibrous polytetrafluoroethylene additive were prepared by a coreback foam injection molding technique. The effects of this fibrous additive on the foam cell structure were investigated. Fibrous polytetrafluoroethylene forms a network structure in polylactic acid in metering and mixing processes. The fibrous polytetrafluoroethylene network increased the viscoelasticity of polylactic acid and provided polylactic acid with a strain-hardening property. The network also provided heterogeneous bubble nucleation sites for physical foaming. However, because of the slow crystallization rate of polylactic acid, the fibrous polytetrafluoroethylene additive did not promote the nucleation of polylactic acid crystals under fast cooling conditions. During fast cooling, such as injection molding cooling conditions, the crystals induced by the fibrous polytetrafluoroethylene network could not behave as bubble nucleation sites. Thus, changes in rheological properties and the increased number of heterogeneous sites contributed to the decrease in cell size, the increase in the number density of cells and the increase in the open cell content. As the number density of cells increased, the cell walls with the fibrous polytetrafluoroethylene fibrous additive became so thin that they could be easily fibrillated by a stretching operation during the coreback operation, while their strain-hardening property prevented the walls from complete breakage. Synergistically conducting cell reduction and stretching (coreback) operations, high expansion ratio foams with high open cell content were prepared. When we adjusted the foaming temperature and holding time, five-fold expansion (i.e. 80% void ratio) foams with cell diameters less than 25 µm and open cell contents (OCC) higher than 80% were produced.

Ultralow-Threshold and Lightweight Biodegradable Porous PLA/MWCNT with Segregated Conductive Networks for High-Performance Thermal Insulation and Electromagnetic Interference Shielding Applications

Lightweight, biodegradable, thermally insulating, and electrically conductive materials play a vital role in achieving the sustainable development of our society. The fabrication of such multifunctional materials is currently very challenging. Here, we report a general, facile, and eco-friendly way for the large-scale fabrication of ultralow-threshold and biodegradable porous polylactic acid (PLA)/multiwalled carbon nanotube (MWCNT) for high-performance thermal insulation and electromagnetic interference (EMI) shielding applications. Thanks to the unique structure of the microporous PLA matrix embedded by conductive 3D MWCNT networks, the lightweight porous PLA/MWCNT with a density of 0.045 g/cm³ possesses a percolation threshold of 0.00094 vol %, which, to our knowledge, is the minimum value reported so far. Furthermore, the material exhibits excellent thermal insulation performance with a thermal conductivity of 27.5 mW·m^-1·K^-1, which is much lower than the best value of common thermal insulation materials. Moreover, it also shows outstanding EMI shielding performance characterized by its high shielding effectiveness (SE) values and absorption-dominated shielding feature. More importantly, its specific EMI SE is as high as 1010 dB·cm³·g^-1, which is superior to those of other shielding materials reported so far. Thus, this novel multifunctional material and its general fabrication methodology provide a promising way to meet the growing demand for high-performance multifunctional materials in sustainable development.

Multiple actions of poly(ethylene octene) grafted with glycidyl methacrylate on the performance of poly(lactic acid)

Poly(ethylene octene) grafted with glycidyl methacrylate (POE-g-GMA) was employed to improve the rheological and thermal properties, toughness, and foaming behaviors of poly(lactic acid) (PLA) through a chain extension effect.

Improved viscoelastic, thermal, and mechanical properties of in situ microfibrillar polypropylene/polyamide 6,6 composites via direct extrusion using a triple-screw extruder

Improved viscoelastic, thermal, and mechanical properties ofin situmicrofibrillar polypropylene/polyamide 6,6 compositesviadirect extrusion using a triple-screw extruder.

Self-nanofibrillation strategy to an unusual combination of strength and toughness for poly(lactic acid)

The self-nanofibrillation strategy paves a new way to an unprecedented combination of strength and toughness for pure PLA.

Biomimetic Nanofibrillation in Two-Component Biopolymer Blends with Structural Analogs to Spider Silk

Despite the enormous potential in bioinspired fabrication of high-strength structure by mimicking the spinning process of spider silk, currently accessible routes (e.g., microfluidic and electrospinning approaches) still have substantial function gaps in providing precision control over the nanofibrillar superstructure, crystalline morphology or molecular orientation. Here the concept of biomimetic nanofibrillation, by copying the spiders’ spinning principles, was conceived to build silk-mimicking hierarchies in two-phase biodegradable blends, strategically involving the stepwise integration of elongational shear and high-pressure shear. Phase separation confined on nanoscale, together with deformation of discrete phases and pre-alignment of polymer chains, was triggered in the elongational shear, conferring the readiness for direct nanofibrillation in the latter shearing stage. The orderly aligned nanofibrils, featuring an ultralow diameter of around 100 nm and the “rigid−soft” system crosslinked by nanocrystal domains like silk protein dopes, were secreted by fine nanochannels. The incorporation of multiscale silk-mimicking structures afforded exceptional combination of strength, ductility and toughness for the nanofibrillar polymer composites. The proposed spider spinning-mimicking strategy, offering the biomimetic function integration unattainable with current approaches, may prompt materials scientists to pursue biopolymer mimics of silk with high performance yet light weight.

From micro/nano structured isotactic polypropylene to a multifunctional low-density nanoporous medium

A homogeneous low-density nano-porous medium of isotactic polypropylene (iPP) with a low thermal conductivity was fabricated using supercritical carbon dioxide (scCO₂).

Effect of nanoparticles on fibril formation and mechanical performance of olefinic block copolymer (OBC)/polypropylene (PP) microfibrillar composites

Adding nanoparticles provides an alternative strategy to tune the morphological evolution and mechanical performances of OBC/PP microfibrillar composites.