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

Nucleation Enhancement in Stereodefective Poly(l-lactide) by Free Volume Expansion Resulting from Low-Temperature Pressure CO₂ Preconditioning

Nucleation enhancement in a highly stereodefective poly(l-lactide) (PLLA) with an optical purity of 88% by low-temperature pressure (0 and 35 °C under 2 MPa) CO₂ preconditioning was investigated using differential scanning calorimetry (DSC), infrared (IR) spectroscopy, polarized optical microscopy (POM) as well as positron annihilation lifetime spectroscopy (PALS). Despite the preconditioning of the melt-quenched films for 2 h, IR results indicated that no trace of mesophase was generated and the samples remained in the glassy state. However, judging from the results of DSC, IR, and POM, when compared to the untreated sample, both the treated ones showed a significantly enhanced crystal nucleation effect, resulting in the corresponding greatly enhanced crystallization kinetics. Moreover, owing to the existence of the retrograde vitrification, the conditions of the previous low-pressure CO₂ conditioning affected the nucleation enhancement effect. When compared to the case of 35 °C, the much lower temperature of 0 °C was more effective for nucleation enhancement. The PALS results indicated that the enlarged free volume, which resulted from the CO₂ conditioning, largely accounted for the formation of locally ordered structures, providing many more potential nucleation sites for forming critical nuclei and thus the resulting enhanced crystallization kinetics in glassy PLLA. The present results have implications in understanding the nucleation enhancement effect, in particular in stereodefective PLLA systems, which possess extremely low crystallization ability and are thus probably too problematic to be evaluated by conventional methods.

Nanoconfinement effects of chemically reduced graphene oxide nanoribbons on poly(vinyl chloride)

Polymeric nanocomposites with graphene-based nanocarbons (GNCs) have been extensively studied with emphasis on the percolation of nanofillers toward electrical, rheological, and mechanical reinforcement. In this study, we report an unusual indirect reinforcing phenomenon of highly defective GNCs dispersed in the poly(vinyl chloride) (PVC) matrix via densification of the polymer packing originating from nanoscale confinement. Herein, chemically reduced graphene oxide nanoribbons (C-rGONRs) are employed as a nanofiller. The inclusion of defective and oxygen-functionalized C-rGONRs resulted in a dramatic densification of the PVC host with extremely low C-rGONR loading, largely exceeding the theoretical calculation from a rule of mixture. Along with the densification, the glass transition temperature of PVC also increased by 28.6 °C at 0.1 wt% filler loading. Remarkably, the oxygen barrier property and mechanical toughness under tension for the PVC/C-rGONR nanocomposite were the maximum when the greatest densification occurred. The structure-property relationship of the nanocomposites has been discussed with an emphasis on the nanoscale confinement phenomenon.

Effect of stereocomplex crystal and flexible segments on the crystallization and tensile behavior of poly(l-lactide)

Effect of stereocomplex crystal and PEG segments in blends on the crystallization and tensile behavior of PLLA.

Surface-Induced Frustration in Solid State Polymorphic Transition of Native Cellulose Nanocrystals

The presence of an interface generally influences crystallization of polymers from melt or from solution. Here, by contrast, we explore the effect of surface immobilization in a direct solid state polymorphic transition on individual cellulose nanocrystals (CNCs), extracted from a plant-based origin. The conversion from native cellulose I to cellulose III crystal occurred via a host-guest inclusion of ethylene diamine inside the crystal. A 60% reduction in CNC width (height) in atomic force microscopy images suggested that when immobilized on a flat modified silica surface, the stresses caused by the inclusion or the subsequent regeneration resulted in exfoliation, hypothetically, between the van der Waals bonded sheets within the crystal. Virtually no changes in dimensions were visible when the polymorphic transition was performed to nonimmobilized CNCs in bulk dispersion. With reservations and by acknowledging the obvious dissimilarities, the exfoliation of cellulose crystal sheets can be viewed as analogous to exfoliation of 2D structures like graphene from a van der Waals stacked solid. Here, the detachment is triggered by an inclusion of a guest molecule inside a host cellulose crystal and the stresses caused by the firm attachment of the CNC on a solid substrate, leading to detachment of molecular sheets or stacks of sheets.

Epitaxial crystallization of precisely bromine-substituted polyethylene induced by carbon nanotubes and graphene

Epitaxial crystallization of precisely bromine-substituted polyethylene induced by carbon nanotubes and graphene.

Molecular dynamics simulations of the structure and the morphology of graphene/polymer nanocomposites

Molecular dynamics simulations are performed to scrutinize the structure and morphology of three polymer/graphene nanocomposites.

PLA composites: From production to properties

Poly(lactic acid) or polylactide (PLA), a biodegradable polyester produced from renewable resources, is used for various applications (biomedical, packaging, textile fibers and technical items). Due to its inherent properties, PLA has a key-position in the market of biopolymers, being one of the most promising candidates for further developments. Unfortunately, PLA suffers from some shortcomings, whereas for the different applications specific end-use properties are required. Therefore, the addition of reinforcing fibers, micro- and/or nanofillers, and selected additives within PLA matrix is considered as a powerful method for obtaining specific end-use characteristics and major improvements of properties. This review highlights recent developments, current results and trends in the field of composites based on PLA. It presents the main advances in PLA properties and reports selected results in relation to the preparation and characterization of the most representative PLA composites. To illustrate the possibility to design the properties of composites, a section is devoted to the production and characterization of innovative PLA-based products filled with thermally-treated calcium sulfate, a by-product from the lactic acid production process. Moreover, are emphasized the last tendencies strongly evidenced in the case of PLA, i.e., the high interest to diversify its uses by moving from biomedical and packaging (biodegradation properties, "disposables") to technical applications ("durables").

A Novel Electrostimulated Drug Delivery System Based on PLLA Composites Exploiting the Multiple Functions of Graphite Nanoplatelets

A novel drug delivery system based on poly(l-lactide) (PLLA), graphite, and porphyrin was developed. In particular, 5,10,15,20-tetrakis(4-hydroxyphenyl)porphyrin (THPP) was chosen because, besides its potential as codispersing agent of graphite, it is a pharmacologically active molecule. Graphite nanoplatelets, homogeneously dispersed in both the neat PLLA and the PLLA/porphyrin films, which were prepared by solution casting, turned out to improve the crystallinity of the polymer. Moreover, IR measurements demonstrated that unlike PLLA/porphyrin film, where the porphyrin was prone to aggregate causing variable concentration throughout the sample, the system containing also GNP was characterized by a homogeneous dispersion of the above molecule. The effect of graphite nanoplatelets on the thermal stabilization, electrical conductivity, and improvement of mechanical properties of the polymer resulted to be increased by the addition of the porphyrin to the system, thus demonstrating the role of the molecule in ameliorating the filler dispersion in PLLA. The porphyrin release from the composite film, occurring both naturally and with the application of an electrical field, was measured using an UV-vis spectrophotometer. Indeed, voltage application turned out to improve significantly the kinetic of drug release. The biocompatibility of the polymer matrix as well as the mechanical and thermal properties of the composite together with its electrical response makes the developed material extremely promising in biological applications, particularly in the drug delivery field.

N,N′-Bis(benzoyl) adipic acid dihydrazide and talc: nucleating agents for poly(l-lactic acid)

In this paper, high-heat-resistant polymeric composite products were prepared via the traditional melt blending process by incorporating N,N′-bis(benzoyl) adipic acid dihydrazide (BAAD) into poly(l-lactic acid) (PLLA), which acted as an organic nucleating agent. The heat distortion temperature (HDT) of the PLLA/BAAD composite samples was measured by an HDT apparatus, and a high value of 96.2°C was achieved at a BAAD loading fraction of 0.5 wt.%, whereas, at the same processing conditions, the HDT of PLLA/talc specimens reached a similar value at a talc content of 20 wt.%, which was much higher than the BAAD content. Differential scanning calorimetry and X-ray diffraction analyses were applied to determine the melting and crystallization behavior of the PLLA/BAAD blends. Polarized optical microscopy was used to observe the crystalline morphologies. Thermogravimetric analysis was employed to study the effect of BAAD on the thermal stability of PLLA. Measurement of the mechanical property confirmed that the addition of BAAD was beneficial to the enhancement of the mechanical properties of the resulting blends. However, the tensile strength of the PLLA/talc composites decreased with increasing weight fraction of talc.

Graphene Nanocomposites with High Molecular Weight Poly(ε-caprolactone) Grafts: Controlled Synthesis and Accelerated Crystallization

Grafting of high molecular weight polymers to graphitic nanoplatelets is a critical step toward the development of high performance graphene nanocomposites. However, designing such a grafting route has remained a major impediment. Herein, we report a "grafting to" synthetic pathway by which high molecular weight polymer, poly(ε-caprolactone) (PCL), is tethered, at high grafting density, to highly anisotropic graphitic nanoplatelets. The efficacy of this tethering route and the resultant structural arrangements within the composite are confirmed by neutron and X-ray scattering measurements in the melt and solution phase. In the semicrystalline state, X-ray analysis indicates that chain tethering onto the graphitic nanoplatelets results in conformational changes of the polymer chains, which enhance the nucleation process and aid formation of PCL crystallites. This is corroborated by the superior thermal properties of the composite, manifested in accelerated crystallization kinetics and a significant increase in the thermal degradation temperature. In principle, this synthesis route can be extended to a variety of high molecular weight polymers, which can open new avenues to solution-based processing of graphitic nanomaterials and the fabrication of complex 3D patterned graphitic nanocomposites.

Emerging trends in eco-compliant, synergistic, and hybrid assembling of multifunctional polymeric bionanocomposites

The quest to develop eco-benign polymeric hybrid materials arose out of the need to protect the environment from the harmful effects of synthetic petroleum polymeric waste and meet the specific needs of industries such as oil and gas, aerospace, automotives, packaging, electronics biomedicals, pharmaceuticals, agricultural, and construction. This has resulted in synergistic hybrid assembling of natural fibers, polymers, biopolymers, and nanoparticles. Bionanocomposites based on inorganic nanoparticle reinforced biofiber, polymers and biopolymers, and polysaccharides such as chitosan, alginate, and cellulose derivatives, and so on, exhibiting at least a dimension at the nanometer scale, are an emerging group of nanostructured hybrid materials. These hybrid bionanocomposites exhibit structural and multifunctional properties suitable for versatile applications similar to polymer nanocomposites. Their biocompatibility and biodegradability provide opportunities for applications as eco-benign green nanocomposites. This review presents state-of-the-art progress in synergistic nanotechnological assembling of bionanocomposites relative to processing technologies, product development, and applications.

From starch to polylactide and nano-graphene oxide: fully starch derived high performance composites

Starch-derived nGO is an efficient compatibilizer and property enhancer for high performance PLA/starch biocomposites.

Graphene oxide induced isotactic polypropylene crystallization: role of structural reduction

Nucleation ability of reduced GO was enhanced by the removal of oxygenic groups and the partial recovery of sp² carbon network.

Formation of thermally conductive networks in isotactic polypropylene/hexagonal boron nitride composites via “Bridge Effect” of multi-wall carbon nanotubes and graphene nanoplatelets

Efficiently thermal conductive networks were fabricated in the iPP/h-BN composites by the “bridge effect” of MWCNTs or GNPs.