Isothermal Crystallization of Poly(l-lactide) Induced by Graphene Nanosheets and Carbon Nanotubes: A Comparative Study

Mussel-inspired polydopamine modified mica with enhanced mechanical strength and thermal performance of poly(lactic acid) coating

Polylactic acid (PLA), as a green functional polymer, has been useful in various coating applications. However, due to the low mechanical strength and thermal stability of PLA, it needs to be improved in order to expand its application areas. In this work, a series of polylactic acid (PLA) nanocomposite films were prepared through introducing polydopamine-modified mica (PDA@MICA) as a self-assemble nanofiller to enhance its mechanical and thermal properties. The results demonstrated that PLA/PDA@MICA shows excellent mechanical properties. Tensile tests showed that PLA/PDA@MICA exhibits a 58.3 % increase in tensile strength and a 16.8 % increase in Young's modulus compared to pure PLA. Meanwhile, thermal performance testing shown the introduction of PDA@MICA led to an increase in crystallinities (Xc = 24.78 %). And the thermal decomposition temperature of PLA/PDA@MICA film (374 °C) was slightly higher than that of PLA film (367 °C). The simultaneous improvement of the mechanical and thermal properties was attributed to the formation of hydrogen bonds between PLA and PDA@MICA. In addition, the parallel arrangement of PDA@MICA and PLA macromolecular chains forms a unique "brick and mortar" structure in the coating, which enhances the mechanical properties of PLA/PDA@MICA composite coatings. This study reports a successful approach to simultaneously address the drawbacks of PLA, specifically its low thermal stability and mechanical strength, thereby promoting its widespread application in the coatings industry.

Poly(ethylene succinate-co-lactic acid) as a Multifunctional Additive for Modulating the Miscibility, Crystallization, and Mechanical Properties of Poly(lactic acid)

Polymer blending offers an effective and economical approach to overcome the performance limitations of poly(lactic acid) (PLA). In this study, a series of copolymers poly(ethylene succinate-co-lactic acid) (PESL) were synthesized, featuring lactic acid (LA) contents that ranged from 20 to 86 wt %. This synthesis involved a one-pot industrial melt polycondensation process using succinic acid (SA), ethylene glycol (EG), and LA, catalyzed by titanium tetraisopropoxide (TTP). The goal was to produce a fully biobased copolymer expected to exhibit partial miscibility with pure poly(lactic acid) (PLA). To assess the capability of PESL copolymers in toughening PLA, we conducted tensile testing on PLA/PESL blends containing 15 wt % PESL. As a result, an elongation at break for the blends with 15 wt % loading of the copolymer PESL72 was directly enhanced to 250% with an ultimate strength of 35 MPa, compared to brittle PLA with less 10% tensile length. The morphological features of interfacial adhesion before and after tensile failure were measured by scanning electron microscopy (SEM). A significant enhancement in the chain mobility of the PLA/PESL blends was further evidenced by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). These findings hold promise for the development of functional packaging materials based on PLA. The proposed copolymer design, which boasts strong industrial feasibility, can serve as a valuable guide for enhancing the toughness of PLA.

Crystallization of Polylactic Acid with Organic Nucleating Agents under Quiescent Conditions

Polylactic acid (PLA) is a versatile and sustainable polymer used in various applications. This research explores the use of orotic acid (OA) and ethylene bis-stearamide (EBS) as nucleating agents to enhance the quiescent crystallization of PLA within the temperature range of 80 °C to 140 °C. Different blends were produced via melt processing before analyzing via DSC, XRD, and SEM. Our results show that both nucleating agents significantly accelerated the crystallization process and reduced the incubation time and the crystallization half-time. The most promising results were obtained with 1% EBS at 110 °C, achieving the fastest crystallization. The XRD analysis showed that at 80 °C, the disordered α'phase predominated, while more stable α phases formed at 110 °C and 140 °C. Combining the 1% nucleating agent and 110 °C promotes densely packed crystalline lamellae. The nucleated PLA exhibited a well-organized spherulitic morphology in agreement with the Avrami modeling of DSC data. Higher nucleating agent concentrations yielded smaller, more evenly distributed crystalline domains. Utilizing OA or EBS in PLA processing could offer enhanced properties, improved processability, and cost-efficiency, making PLA more competitive in various applications.

Crystallisation Kinetics and Associated Electrical Conductivity Dynamics of Poly(Ethylene Vinyl Acetate) Nanocomposites in the Melt State

Nanocomposite systems comprised of a poly(ethylene vinyl acetate) (EVA) matrix and carbon black (CB) or graphene nanoplatelets (GNPs) were used to investigate conductivity and crystallisation dynamics using a commercially relevant melt-state mixing process. Crystallisation kinetics and morphology, as investigated by DSC and SEM, turn out to depend on the interplay of (i) the interphase interactions between matrix and filler, and (ii) the degree of filler agglomeration. For the GNP-based systems, an almost constant conductivity value was observed for all compositions upon cooling, something not observed for the CB-based compositions. These conductivity changes reflect structural and morphological changes that can be associated with positive and negative thermal expansion coefficients. GNP-based systems were observed to exhibit a percolation threshold of approximately 2.2 vol%, lower than the 4.4 vol% observed for the CB-based systems.

Molecular dynamics simulations of the effects of carbon nanotube content on stretch-induced crystallization of polyethylene/carbon nanotube nanocomposites

In the present work, we used molecular dynamics simulations to study the effects of carbon nanotube (CNT) content on stretch-induced crystallization behaviors in CNT filled polyethylene systems. During high-temperature stretching,...

In-Situ Isothermal Crystallization of Poly(l-lactide)

The isothermal crystallization of poly(l-lactide) (PLLA) has been investigated by in-situ wide angle X-ray diffraction (WAXD) and polarized optical microscopes (POM) equipped with a hot-stage accessory. Results showed that the spherulites of PLLA were formed at high temperature, whereas irregular morphology was observed under a low temperature. This can be attributed to the varying rates of crystallization of PLLA at different temperatures. At low temperatures, the nucleation rate is fast and hence the chains diffuse very slow, resulting in the formation of imperfect crystals. On the other hand, at high temperatures, the nucleation rate is slow and the chains diffuse fast, leading to the formation of perfect crystals. The change in the value of the Avrami exponent with temperature further verifies the varying trend in the morphological feature of the crystals.

Effect of supports on plasma catalytic decomposition of toluene using in situ plasma DRIFTS

Plasma catalysis technology has been demonstrated to be effective for the decomposition of volatile organic compounds (VOCs). It is highly desired to explore the effect of supports on VOCs oxidation processes during plasma catalysis. In this work, four supports of SiO₂, ZSM-5-300, ZSM-5-38 and γ-Al₂O₃ loading with transition metal oxides were used to decompose toluene at room temperature. It was found that toluene decomposition with 1 wt%Mn/γ-Al₂O₃ was highest, which was strongly proportional to the ozone decomposition ability of the catalyst. The plasma catalytic decomposition of toluene over 1 wt% MnO₂ on different supports were characterized using in situ plasma diffuse reflectance infrared Fourier transform spectrometer. The results showed that 1 wt%Mn/γ-Al₂O₃ could further catalyze toluene to carbonate and bicarbonate via the breakage of C-C bonds from benzoic acid, while that was difficult for 1 wt% Mn/SiO₂, 1 wt%Mn/ZSM-5-300 and 1 wt%Mn/ZSM-5-38. The reaction mechanism of toluene decomposition on different catalysts were proposed.

Improved Weathering Performance of Poly(Lactic Acid) through Carbon Nanotubes Addition: Thermal, Microstructural, and Nanomechanical Analyses

To understand the interrelationship between the microstructure and degradation behavior of poly(lactic acid) (PLA), single-walled carbon nanotubes (CNTs) were introduced into PLA as nucleating agents. The degradation behavior of PLA-CNT nanocomposites was examined under accelerated weathering conditions with exposure to UV light, heat, and moisture. The degradation mechanism proceeded via the Norrish type II mechanism of carbonyl polyester. Differential scanning calorimetry (DSC) studies showed an increase in glass transition temperature, melting temperature, and crystallinity as a result of the degradation. However, pure PLA showed higher degradation as evidenced by increased crystallinity, lower onset decomposition temperature, embrittlement, and a higher number of micro-voids which became broader and deeper during degradation. In the PLA-CNT nanocomposites, CNTs created a tortuous pathway which inhibits the penetration of water molecules deeper into the polymer matrix, making PLA thermally stable by increasing the initial temperature of mass loss. CNTs appear to retard PLA degradation by impeding mass transfer. Our study will facilitate designing environmentally friendly packaging materials that display greater resistance to degradation in the presence of moisture and UV light.

Mechanical Behavior of Melt-Mixed 3D Hierarchical Graphene/Polypropylene Nanocomposites

The mechanical properties of novel low percolation melt-mixed 3D hierarchical graphene/polypropylene nanocomposites are analyzed in this study. The analysis spans a broad range of techniques and time scales, from impact to tensile, dynamic mechanical behavior, and creep. The applicability of the time-temperature superposition principle and its limitations in the construction of the master curve for the isotactic polypropylene (iPP)-based graphene nanocomposites has been verified and presented. The Williams-Landel-Ferry method has been used to evaluate the dynamics and also Cole-Cole curves were presented to verify the thermorheological character of the nanocomposites. Short term (quasi-static) tensile tests, creep, and impact strength measurements were used to evaluate the load transfer efficiency. A significant increase of Young's modulus with increasing filler content indicates reasonably good dispersion and adhesion between the iPP and the filler. The Young's modulus results were compared with predicted modulus values using Halpin-Tsai model. An increase in brittleness resulting in lower impact strength values has also been recorded.

Correlation between molecular weight and confined crystallization behavior of polymers grafted onto a zero-dimensional filler

In this work, polymer chains of different molecular weights are grafted onto a zero-dimensional filler.

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).

Exploration of hybrid nanocarbon composite with polylactic acid for packaging applications

Polylactic acid (PLA) nanocomposite films were fabricated with graphene oxide (GO) and single-walled carbon nanotubes (CNT) as a hybrid-co-filler with GOCNT fraction varying from 0.05 to 0.4% by weight. The effect of the GOCNT on the physical, thermal, morphological, gas permeation, and optical properties was investigated. The X-ray diffraction test reveals no restacking and coagulation of GOCNT in the composite films. Differential Scanning Calorimetry analysis shows an insignificant shift of glass transition and melting temperature but enhanced crystallization resulting from the existence of GOCNT as a nucleating agent. Scanning Electron Microscope scans indicate GOCNT embedded homogeneously without considerable aggregates in the PLA. Transmission of ultraviolet-visible radiation decreases to 30% with increasing fraction of GOCNT while Oxygen Transmission Rate diminishes to 67% in the film. These are attributed to the tortuous pathways provided by the well-dispersed hybrid GOCNT in the PLA. Compared to the pristine PLA film, the composite film shows an increase of 75% and 130% in the tensile strength and Young's modulus, respectively. Taken together, all of these improvements observed in the hybrid GOCNT-PLA composites should provide useful guidelines in customizing designs for applications across a range of fields including packaging, life sciences, cosmetics, and conventional synthetic plastics.

Modulating the properties of polylactic acid for packaging applications using biobased plasticizers and naturally obtained fillers

The properties of PLA films intended for packaging applications have been modulated by using bio-based platicizers and naturally obtained fillers. Triethyl citrate (TEC) and glycerol triacetate (GTA) have been used as platicizers and halloysite nanotubes (HNT) and chitosan have been used as fillers. The addition of 10 wt% TEC, 10 wt% GTA and 3 wt% HNT improves the ductility of PLA films, however, reduces the tensile modulus and tensile strength. Addition of chitosan (1 wt%), on the other hand, acts as a good reinforcing filler and improves the tensile strength and tensile modulus. PLA-HNT-chitosan film show comparable tensile strength, tensile modulus and ~12 times higher elongation at break compared to pure PLA. Besides, PLA-HNT-chitosan film demonstrates very good barrier properties against moisture and ultraviolet (UV) rays. Additionally, its antibacterial efficacy against E. coli and S. aureus are found to be around 80% and 70%, respectively. The study demonstrates the complementary effects of HNT and chitosan to modulate the properties of PLA film and indicates that the PLA-HNT-chitosan film can emerge as a very potent material for packaging applications.

Poly(l-lactic acid)/Boron Nitride Nanocomposites: Influence of Boron Nitride Functionalization on the Properties of Poly(l-lactic acid)

Differently functionalized boron nitride nanosheets (BNNSs) [hydroxyl (OH_BNNSs), amine (NH₂_BNNSs), and poly(ethylene glycol) (PEG) (PEG_BNNSs)] were synthesized, and their effects on the structure and thermal properties of poly(l-lactic acid) (PLLA) along with those of the pristine BNNSs were studied. Highly dispersed nanocomposites were prepared using PLLA and 0.5 wt % of pristine/functionalized BNNSs via a solvent blending method. Homogeneous dispersion of BNNSs in the polymer matrix was confirmed using X-ray diffraction and scanning electron microscopy. Pristine BNNSs and OH_BNNSs accelerated the crystallization of PLLA as effective nucleating agents and favored the formation of the α form in melt-crystallized samples. On the other hand, NH₂_BNNSs and PEG_BNNSs incorporated samples result in the moderate crystallization rate of PLLA and lead to the formation of a mixture of α and α' forms similar to the PLLA. It is also found that thermal stability and thermal conductivity of PLLA nanocomposites significantly depend on the type of functionalization of BNNSs. At 0.5 wt % loading, the thermal conductivity enhancement is maximum for PEG_BNNSs incorporated PLLA (∼62%), and that is only 9% for pristine BNNSs incorporated PLLA. The thermal stability of PLLA nanocomposites was significantly improved by 32-41 °C depending on the type of functionalized BNNSs compared to PLLA. It is proposed that the strong interaction between functionalized BNNSs and PLLA matrix is responsible for the improved thermal management properties.

Interconnected Microdomain Structure of a Cross-Linked Cellulose Nanocomposite Revealed by Micro-Raman Imaging and Its Influence on Water Permeability of a Film

Substantial adsorption of water vapor triggered by hydrogen-bonding interactions between water molecules and cellulose chains (or nanoplates) is hard to avoid in nanocomposite films, although the addition of nanoplates can improve the oxygen (or carbon dioxide) barrier property. In the present work, an effective strategy is raised to decline adsorption by weakening hydrogen-bonding interactions via chemical cross-linking by epichlorohydrin (ECH) without sacrificing the homogeneous dispersion of nanoplates. The generated microdomain structure of the chemical cross-linking reaction via ECH is explicitly revealed by micro-Raman imaging. Unambiguously, Raman maps of scanning elucidate the distribution and morphology of physical and chemical cross-linking domains quantitatively. The chemical cross-linking domains are nearly uniformly located in the matrix at a low degree of cross-linking, while the interconnected and assembled networks are formed at a high degree of cross-linking. ECH boosts the formation of chemical cross-linking microdomains, bringing out the terrific water vapor barrier property and alleviating the interfacial interactions in penetration, consequently magnifying the water contact angle and holding back the water vapor permeability. Our methodology confers an effective and convenient strategy to obtain remarkable water vapor-resistant cellulose-based films that meet the practical application in the packaging fields.

Millisecond Response of Shape Memory Polymer Nanocomposite Aerogel Powered by Stretchable Graphene Framework

Shape memory polymers (SMPs) change shapes as-designed through altering the chain segment movement by external stimuli, promising wide uses in actuators, sensors, drug delivery, and deployable devices. However, the recovery speed of SMPs is still far slower than the benchmark shape memory alloys (SMAs), originating from their intrinsic poor heat transport and retarded viscoelasticity of polymer chains. In this work, monolithic nanocomposite aerogels composed of bicontinuous graphene and SMP networks are designed to promote the recovery time of SMP composites to a record value of 50 ms, comparable to the SMA case. The integration of a stretchable graphene framework as a fast energy transformation grid with ultrathin polycaprolactone nanofilms (tunable at 2.5-60 nm) enables the rapid phase transition of SMPs under electrical stimulation. The graphene-SMP nanocomposite aerogels, with a density of ∼10 mg cm^-3, exhibit a fast response (175 ± 40 mm s^-1), large deformation (∼100%), and a wide response bandwidth (0.1-20 Hz). The ultrafast response of SMP nanocomposite aerogels confers extensive uses in sensitive fuses, micro-oscillators, artificial muscles, actuators, and soft robotics. The design of bicontinuous ultralight aerogels can be extended to fabricate multifunctional and multiresponsive hybrid materials and devices.

Stereocomplexation of Poly(Lactic Acid)s on Graphite Nanoplatelets: From Functionalized Nanoparticles to Self-assembled Nanostructures

The control of nanostructuration of graphene and graphene related materials (GRM) into self-assembled structures is strictly related to the nanoflakes chemical functionalization, which may be obtained via covalent grafting of non-covalent interactions, mostly exploiting π-stacking. As the non-covalent functionalization does not affect the sp2 carbon structure, this is often exploited to preserve the thermal and electrical properties of the GRM and it is a well-known route to tailor the interaction between GRM and organic media. In this work, non-covalent functionalization of graphite nanoplatelets (GnP) was carried out with ad-hoc synthesized pyrene-terminated oligomers of polylactic acid (PLA), aiming at the modification of GnP nanopapers thermal properties. PLA was selected based on the possibility to self-assemble in crystalline domains via stereocomplexation of complementary poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) enantiomers. Pyrene-initiated PLLA and PDLA were indeed demonstrated to anchor to the GnP surface. Calorimetric and X-ray diffraction investigations highlighted the enantiomeric PLAs adsorbed on the surface of the nanoplatelets self-organize to produce highly crystalline stereocomplex domains. Most importantly, PLLA/PDLA stereocomplexation delivered a significantly higher efficiency in nanopapers heat transfer, in particular through the thickness of the nanopaper. This is explained by a thermal bridging effect of crystalline domains between overlapped GnP, promoting heat transfer across the nanoparticles contacts. This work demonstrates the possibility to enhance the physical properties of contacts within a percolating network of GRM via the self-assembly of macromolecules and opens a new way for the engineering of GRM-based nanostructures.

Functionalized Boron Nitride Nanosheets/Poly(l-lactide) Nanocomposites and Their Crystallization Behavior

In this work, hydroxyl-functionalized boron nitride nanosheet (OH-BNNS) was prepared and was blended with poly(l-lactide) (PLLA) to yield PLLA/OH-BNNS nanocomposites with excellent dispersion of OH-BNNS via the interaction of carbonyl in PLLA and hydroxyl in OH-BNNS. The effects of OH-BNNS on the crystallization and melting behaviors, isothermal crystallization kinetics, macroscopic crystal morphology and crystal structure of PLLA were studied by means of various techniques. The addition of OH-BNNS nanofillers can effectively accelerate the crystallization of PLLA and enhance the nucleation density, leading to a smaller spherulite size, increased crystallinity, a more obvious crystallization peak upon cooling but weakened cold crystallization behavior upon heating. Low OH-BNNS loading can increase the relative content of α-crystal, but the relative content of less perfect α′-crystal is increased at high OH-BNNS loading due to the strong interaction between PLLA and OH-BNNS.

Effects of Graphene Nanoplatelets and Multiwall Carbon Nanotubes on the Structure and Mechanical Properties of Poly(lactic acid) Composites: A Comparative Study

Poly(lactic acid)/graphene and poly(lactic acid)/carbon nanotube nanocomposites were prepared by an easy and low-cost method of melt blending of preliminary grinded poly(lactic acid) (PLA) with nanosized carbon fillers used as powder. Morphological, structural and mechanical properties were investigated to reveal the influence of carbon nanofiller on the PLA–based composite. The dependence of tensile strength on nanocomposite loading was defined by a series of experiments over extruded filaments using a universal mechanical testing instrument. The applying the XRD technique disclosed that compounds crystallinity significantly changed upon addition of multi walled carbon nanotubes. We demonstrated that Raman spectroscopy can be used as a quick and unambiguous method to determine the homogeneity of the nanocomposites in terms of carbon filler dispersion in a polymer matrix.

Stereocomplex formation in mixed polymers filled with two-dimensional nanofillers

Nanosheets promote the formation of stereocomplex crystallites due to the heterogeneous nucleation of mixed polymer chains on filler surfaces.

Effect of Reinforcement at Length Scale for Polyurethane Cellular Scaffolds by Supramolecular Assemblies

This study is aimed to represent the role of carbonaceous nanofillers to reinforce the commercially available polyurethane porous structure. The effect of dimensionality of fillers to anchor the construction of stable three-dimensional (3D) cellular architectures has been highlighted. The cellular frameworks of commercially available thermoplastic polyurethane (TPU) have been fabricated through the thermoreversible supramolecular self-assembly route. It was established that the minimum shrinkage of TPU lattice structures occurred when the solid-state network is strengthened by the topologically engineered 3D hierarchical nanofillers, where the amount of reinforcement was found to play a critical role. It has been established by series of structure-property correlations that reinforcing the cellular structure to endure the capillary stress is equally effective as supercritical drying for producing low-density porous morphologies. The removal of liquid phase from gel is as important as the presence of 3D fillers in the matrix for reinforcing the cellular structures when replacing the solvent phase with air to generate a two-phase solid-gas engineered morphology. The insight into the polyurethane network structure revealed that the dimensionality, amount, and distribution of fillers in the matrix are critical for reinforcing the cellular scaffolds in solid gel without any cross-linking.

The Preparation, Characterization, Mechanical and Antibacterial Properties of GO-ZnO Nanocomposites with a Poly(l-lactide)-Modified Surface

Graphene oxide (GO) was employed for the preparation of GO-zinc oxide (ZnO). The hydroxyl group on the surface was exploited to trigger the l-lactide ring-opening polymerization. A composite material with poly(l-lactide) (PLLA) chains grafted to the GO-ZnO surface, GO-ZnO-PLLA, was prepared. The results demonstrated that the employed method allowed one-step, rapid grafting of PLLA to the GO-ZnO surface. The chemical structure of the GO surface was altered by improved dispersion of GO-ZnO in organic solvents, thus enhancing the GO-ZnO dispersion in the PLLA matrix and the interface bonding with PLLA. Subsequently, composite films, GO-ZnO-PLLA and GO-ZnO-PLLA/PLLA, were prepared. The changes in interface properties and mechanical properties were studied. Furthermore, the antibacterial performance of nano-ZnO was investigated.

Functionalization of Graphene Oxide with Low Molecular Weight Poly (Lactic Acid)

In this paper, the hydroxyl groups on the surface of graphene oxide (GO) were used to initiate the ring-opening polymerization of a lactic acid O-carboxyanhydride. GO grafted with poly (l-lactic acid) molecular chains (GO-g-PLLA) was prepared. Lactic acid O-carboxyanhydride has a higher polymerization activity under mild polymerization conditions. Thus, the functionalization of the polymer chains and obtaining poly (lactic acid) (PLLA) was easily achieved by ring-opening polymerization with 4-dimethylaminopyridine (DMAP) as the catalyst. The results showed that with this method, PLLA can be rapidly grafted to the surface of GO in one step. As a result, the chemical structure of the GO surface was altered, improving its dispersion in organic solvents and in a PLLA matrix, as well as its bonding strength with the PLLA interface. We then prepared GO/PLLA and PLLA/GO-g-PLLA composite materials and investigated the differences in their interfacial properties and mechanical properties. GO-g-PLLA exhibited excellent dispersion in the PLLA matrix and formed excellent interfacial bonds with PLLA through mechanical interlocking, demonstrating a significant enhancement effect compared to PLLA. The water vapor and oxygen permeabilities of the GO-g-PLLA/PLLA composite decreased by 19% and 29%, respectively.