Role of Microstructures in the Dielectric Properties of PVDF-Based Nanocomposites Containing High-Permittivity Fillers for Energy Storage.

Polymer-based nanocomposites containing inorganic ferroelectric inclusions, typically ABO3 perovskites, have emerged as innovative dielectric materials for energy storage and electric insulation, potentially coupling the high breakdown strength (BDS) and easy processing of polymers with the enhancement of dielectric constant provided by the ferroelectric phase. In this paper, experimental data and three-dimensional finite element method (3D FEM) simulations were combined to shed some light on the effect of microstructures on the dielectric properties of poly(vinylidene fluoride) (PVDF)-BaTiO3 composites. The existence of particle aggregates or touching particles has a strong effect on the effective dielectric constant and determines an increase of the local field in the neck region of the ferroelectric phase with a detrimental effect on the BDS. The distribution of the field and the effective permittivity are very sensitive to the specific microstructure considered. The degradation of the BDS can be overcome by coating the ferroelectric particles with a thin shell of an insulating oxide with a low dielectric constant, such as SiO2 (εr = 4). The local field is highly concentrated on the shell, while the field in the ferroelectric phase is reduced almost to zero and that on the matrix is close to the applied one. The electric field in the matrix becomes less homogeneous with increasing the dielectric constant of the shell material, as happens with TiO2 (εr = 30). These results provide a solid background to explain the enhanced dielectric properties and the superior BDS of composites containing core-shell inclusions.

Biobased Cryogels from Enzymatically Oxidized Starch: Functionalized Materials as Carriers of Active Molecules.

Starch recovered from an agrifood waste, pea pods, was enzymatically modified and used to prepare cryogels applied as drug carriers. The enzymatic modification of starch was performed using the laccase/(2,2,6,6-tetramethylpiperidin-1-yl)oxyl TEMPO system, at a variable molar ratio. The characterization of the ensuing starches by solution NMR spectroscopy showed partial conversion of the primary hydroxyl groups versus aldehyde and carboxyl groups and successive creation of hemiacetal and ester bonds. Enzymatically modified starch after simple freezing and lyophilization process provided stable and compact cryogels with a morphology characterized by irregular pores, as determined by atomic force (AFM) and scanning electron microscopy (SEM). The application of cryogels as carriers of active molecules was successfully evaluated by following two different approaches of loading with drugs: a) as loaded sponge, by adsorption of drug from the liquid phase; and b) as dry-loaded cryogel, from a dehydration step added to loaded cryogel from route (a). The efficiency of the two routes was studied and compared by determining the drug release profile by proton NMR studies over time. Preliminary results demonstrated that cryogels from modified starch are good candidates to act as drug delivery systems due to their stability and prolonged residence times of loaded molecules, opening promising applications in biomedical and food packaging scenarios.

Graphene Layers Functionalized with A Janus Pyrrole-Based Compound in Natural Rubber Nanocomposites with Improved Ultimate and Fracture Properties.

The ultimate properties and resistance to fracture of nanocomposites based on poly(1,4-cis-isoprene) from Hevea Brasiliensis (natural rubber, NR) and a high surface area nanosized graphite (HSAG) were improved by using HSAG functionalized with 2-(2,5-dimethyl-1H-pyrrol-1-yl)propane-1,3-diol (serinol pyrrole) (HSAG-SP). The functionalization reaction occurred through a domino process, by simply mixing HSAG and serinol pyrrole and heating at 180 °C. The polarity of HSAG-SP allowed its dispersion in NR latex and the isolation of NR/HSAG-SP masterbatches via coagulation. Nanocomposites, based either on pristine HSAG or on HSAG-SP, were prepared through traditional melt blending and cured with a sulphur-based system. The samples containing HSAG-SP revealed ultimate dispersion of the graphitic filler with smaller aggregates and higher amounts of few layers stacks and isolated layers, as revealed by transmission electron microscopy. With HSAG-SP, better stress and elongation at break and higher fracture resistance were obtained. Indeed, in the case of HSAG-SP-based composites, fracture occurred at larger deformation and with higher values of load and, at the highest filler content (24 phr), deviation of fracture propagation was observed. These results have been obtained with a moderate functionalization of the graphene layers (about 5%) and normal lab facilities. This work reveals a simple and scalable way to prepare tougher NR-based nanocomposites and indicates that the dispersion of a graphitic material in a rubber matrix can be improved without using an extra-amount of mechanical energy, just by modifying the chemical nature of the graphitic material through a sustainable process, avoiding the traditional complex approach, which implies oxidation to graphite oxide and subsequent partial reduction.

Edge Functionalized Graphene Layers for (Ultra) High Exfoliation in Carbon Papers and Aerogels in the Presence of Chitosan.

Ultra-high exfoliation in water of a nanosized graphite (HSAG) was obtained thanks to the synergy between a graphene layer edge functionalized with hydroxy groups and a polymer such as chitosan (CS). The edge functionalization of graphene layers was performed with a serinol derivative containing a pyrrole ring, serinol pyrrole (SP). The adduct between CS and HSAG functionalized with SP was formed simply with a mortar and pestle, then preparing water dispersions stable for months in the presence of acetic acid. Simple casting of such dispersions on a glass support led to carbon papers. Aerogels were prepared through the freeze-dry procedure. Exfoliation was observed in both these families of composites and ultra-high exfoliation was documented in aerogels swollen in water. Carbon papers and aerogels were stable for months in solvents in a wide range of solubility parameter and in a pretty wide range of pH. By considering that a moderately functionalized nanographite was straightforwardly exfoliated in water in the presence of one of the most abundant biobased polymers, the obtained results pave the way for the simple and sustainable preparation of graphene-based nanocomposites. HSAG-SP/CS adducts were characterized by wide angle X-ray diffraction (WAXD), scanning and transmission electron microscopy (SEM, TEM and HRTEM), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Thermal stability of the composites was studied by thermogravimetric analysis (TGA) and their direct electrical conductivity with the four-point probe method.

The Effect of the Surface Area of Carbon Black Grades on HNBR in Harsh Environments.

Concerning the still rising demand for oil and gas products, the development of new reliable materials to guarantee the facility safety at extreme operating conditions is an utmost necessity. The present study mainly deals with the influence of different carbon black (CB) filled hydrogenated nitrile butadiene rubber (HNBR), which is a material usually used in sealing applications, on the rapid gas decompression (RGD) resistance in harsh environments. Therefore, RGD component level tests were conducted in an autoclave. The supporting mechanical and dynamic mechanical property analysis, the microscopic level investigations on the material and failure analysis were conducted and are discussed in this work. Under the tested conditions, the samples filled with smaller CB primary particles showed a slightly lower volume increase during the compression and decompression phases; however, they steered to a significantly lower resistance to RGD. Transmission electron micrographs revealed that the samples filled with smaller CB particles formed larger structures as well as densified filler networks including larger agglomerates and as a consequence a decrease effective matrix component around the CB particles. Apparently, at higher loading conditions, which already deliver a certain level of mechanical stresses and strains, the densified filler network, and especially a lower amount of effective matrix material composition, adversely affect the RGD resistance. SEM-based fracture analysis did not identify any influence of the CB grades tested on the crack initiation site; however, it revealed that the cracks initiated from existing voids, hard particles, or low strength matrix sites and propagated to the outer surface.

A Green Approach for Preparing High-Loaded Sepiolite/Polymer Biocomposites.

Global industry is showing a great interest in the field of sustainability owing to the increased attention for ecological safety and utilization of renewable materials. For the scientific community, the challenge lies in the identification of greener synthetic approaches for reducing the environmental impact. In this context, we propose the preparation of novel biocomposites consisting of natural rubber latex (NRL) and sepiolite (Sep) fibers through the latex compounding technique (LCT), an ecofriendly approach where the filler is directly mixed with a stable elastomer colloid. This strategy favors a homogeneous dispersion of hydrophilic Sep fibers in the rubber matrix, allowing the production of high-loaded sepiolite/natural rubber (Sep/NR) without the use of surfactants. The main physicochemical parameters which control Sep aggregation processes in the aqueous medium were comprehensively investigated and a flocculation mechanism was proposed. The uniform Sep distribution in the rubber matrix, characteristic of the proposed LCT, and the percolative filler network improved the mechanical performances of Sep/NR biocomposites in comparison to those of analogous materials prepared by conventional melt-mixing. These outcomes indicate the suitability of the adopted sustainable procedure for the production of high-loaded clay⁻rubber nanocomposites with remarkable mechanical features.

Carbon Papers and Aerogels Based on Graphene Layers and Chitosan: Direct Preparation from High Surface Area Graphite.

In this work, carbon papers and aerogels based on graphene layers and chitosan were prepared. They were obtained by mixing chitosan (CS) and a high surface area nanosized graphite (HSAG) in water in the presence of acetic acid. HSAG/CS water dispersions were stable for months. High resolution transmission electron microscopy revealed the presence of few graphene layers in water suspensions. Casting or lyophilization of such suspensions led to the preparation of carbon paper and aerogel, respectively. In X-ray spectra of both aerogels and carbon paper, peaks due to regular stacks of graphene layers were not detected: graphene with unaltered sp2 structure was obtained directly from graphite without the use of any chemical reaction. The composites were demonstrated to be electrically conductive thanks to the graphene. Chitosan thus makes it possible to obtain monolithic carbon aerogels and flexible and free-standing graphene papers directly from a nanosized graphite by avoiding oxidation to graphite oxide and successive reduction. Strong interaction between polycationic chitosan and the aromatic substrate appears to be at the origin of the stability of HSAG/CS adducts. Cation-π interaction is hypothesized, also on the basis of X-ray photoelectron spectroscopy findings. This work paves the way for the easy large-scale preparation of carbon papers through a method that has a low environmental impact and is based on a biosourced polymer, graphene, and water.

Aqueous-based immobilization of initiator and surface-initiated ATRP to construct hemocompatible surface of poly (styrene-b-(ethylene-co-butylene)-b-styrene) elastomer.

Surface-initiated atom transfer radical polymerization (SI-ATRP) is a versatile tool for surface functionalization in a well-controlled manner. However, surface modification of styrenic thermoplastic elastomers (STPEs) faces a great challenge because immobilization of typical ATRP initiators onto STPEs needs to be carried out in organic solvent, which dissolves and destroys the STPEs film. In this work, a simple aqueous-based route is developed to immobilize ATRP initiators, Br, onto the surface of styrene-b-(ethylene-co-butylene)-b-styrene elastomer (SEBS), chosen as a model copolymer of STPEs. In such a way, functional polymer brushes of ethylene glycol methyl ether methacrylate (OEGMA) are successfully prepared from the surface of SEBS. Kinetic investigations show an approximately linear relationship between grafting density and reaction time, indicating the growth of chains is coincident with a "controlled" process. CBr bonds directly connected to benzene rings on the SEBS-Br surfaces are demonstrated to be effective initiation sites for SI-ATRP. The even coverage of the surface by well-defined P(OEGMA) brushes enable SEBS films to exhibit excellent resistance to protein adsorption and platelet adhesion as well as low hemolysis ratio. This work not only manipulates the SEBS surface to substantially improve its biocompatibility, but paves a way to facilitate SI-ATRP on the surface of styrene-based block copolymers (SBCs).

The nanostructured morphology of linear polyurethanes observed by transmission electron microscopy.

The morphology of polyester-based polyurethanes was observed by transmission electron microscopy, which highlighted a nanostructured system made by a continuous distribution of hard domains with size equal to few nanometres in the soft matrix.

Morphology development and stability of polypropylene/organoclay nanocomposites.

A series of polypropylene (PP)/organoclay nanocomposites with varied concentrations of clay, from 1 to 7 wt%, was successfully prepared via melt intercalation using a PP functionalized with maleic anhydride as compatibilizer. The morphology/property relationships of the nanocomposites were investigated by XRD, TGA and DSC analyses. Two distinct groups of composites, from a quasi-exfoliated to an intercalated/flocculated morphology, were identified. In particular, intercalated/flocculated morphologies were obtained for those composites with an organoclay concentration beyond the threshold (3 wt%), as evidenced by XRD analysis and confirmed by the increase of the glass transition temperature. This last effect was related to the confinement of polymer chains between the silicate layers, generating a reduction of the chain mobility. The variable increase of the thermal stability of the nanocomposites was also likely related to the different degree of exfoliation/intercalation of the samples. The toluene extraction of composites was used as a powerful methodology to distinguish between polymer phases differently interacting with the inorganic surface: composites having a semi-exfoliated structure were split into two fractions having a similar morphology. For those samples having the higher organoclay concentration and intercalated morphology, a toluene-residue fraction was obtained containing almost all the clay present in the pristine composite. Furthermore, in this case the morphological analysis of the residue fraction evidenced a collapse of the inorganic structure compared to that of the unextracted composite. A careful characterization of both soluble and residue fractions is reported and the results are discussed considering the interactions at the interface between the functionalized PP chains and silicate layers and their effects on the organoclay dispersion degree and stability.

New functionalized polypropylenes as controlled architecture compatibilizers for polypropylene layered silicates nanocomposites.

Polypropylene (PP) samples functionalized with Maleic Anhydride (MAH) were used as interfacial coupling agents during the preparation of PP based layered silicate nanocomposites (PP-LSN). To prepare these functionalized PP samples, butyl 3-(2-furyl)propenoate (BFA) was used as coagent during the radical post-functionalization with MAH to avoid the polymer degradation. The obtained materials, differing from the functionalization degree (FD) and structure (MW and molecular weight distribution), were accurately characterized and firstly employed as polymer matrices for PP-LSN preparation to study the influence of their architecture on clay dispersion and thus on their intercalation capability. Successively, PP-LSNs were prepared by using PP as matrix and 5 phr of the above compatibilizers. Morphological, thermal, mechanical and thermo-mechanical analyses of the nanocomposites pointed out that the higher molecular weight PP-g-MAH samples allow to achieve simultaneously a good intercalation within the filler and a significant compatibilization with pristine PP chains, leading to high performances PP-LSNs.

High throughput synthesis of polyesters using entropically driven ring-opening polymerizations.

Copolyesters were synthesized in a high throughput (HT) manner and in high yield on ca. a 90 mg scale using entropically driven ring-opening polymerizations (ED-ROPs). This synthetic approach is a valuable addition to the HT polymer synthesis arsenal in that it allows condensation-type polymers with relatively large repeat units, such as those in poly(ethylene terephthalate) and poly(butylene terephthalate), to be obtained easily. The synthetic procedure involved taking mixtures of the appropriate macrocyclic oligoesters and heating them together under neat conditions at 250-300 degrees C for 2 h in the presence of 0.1 mol % of di- n-butyltin oxide or tetra- n-butylammonium tetrafluoroborate. In most cases Mw values were >25,000 and, as expected for ED-ROPs, the polydispersity indices were close to 2.0. Higher molecular weights could be obtained by using longer reaction times, but this might lead to product decomposition. The method worked well for esters formally derived from aliphatic or aromatic acids and alcohols, but less well for esters derived from phenols. Attempts were also made to synthesize copolymers by mixing together the two homopolymers and heating with a catalyst. These reactions were successful in a few instances, but generally, they were not. This is probably because the homopolymers did not mix well. An aluminum reaction block with 36 wells lined with Teflon cups, that fitted snugly in a cylindrical Buchi oven, was the most successful method for carrying out syntheses in an HT manner.

Oxygen and water vapor barrier properties of MMT nanocomposites from low density polyethylene or EPM with grafted succinic groups.

LDPE, EPM and their derivatives containing a moderate amount (0.08-1.8 by mol) of diethylsuccinate or succinic anhydryde groups were used as matrices in blending with different amount of organophilic montmorillonites and the resulting composite morphology and structure (by XRD, SEM, TEM microscopy, DSC analysis and selective solvent extraction) were studied with reference to the polar groups/MMT ratio. Exfoliated, intercalated and mixed morphologies were achieved. High concentrations of polar groups grafted to the polyolefin and montmorillonite loading not larger than 5% wt were favourable for obtaining high exfoliation degree. Particularly in the exfoliated MMT composite LDPE had lower crystallinity degree, while EPM showed increased glass transition temperature and reduced solubility in hot toluene. Moreover, oxygen and water vapor barrier property improvement was observed in films where MMT exhibits either exfoliated or intercalated morphologies. Strong interactions with the montmorillonite particle surface through the polar groups grafted to the polyolefin seems to be the basic effect responsible for the morphology and peculiar properties. A model based on the reduced mobility of the polymer located near the particle surface or inside the MMT gallery (confined phase) was proposed to explain the observed oxygen permeability reduction, the T(g) increase and solubility of poly(ethylene-ran-propylene)/MMT nanocomposites.

Influence of the silane modifiers on the surface thermodynamic characteristics and dispersion of the silica into elastomer compounds.

A good dispersion of silica into elastomers, typically used in tire tread production, is obtained by grafting of the silica with multifunctional organosilanes. In this study, the influence of the chemical structure of a triethoxysilane (TES), octadecyltriethoxysilane (ODTES), and ODTES/bistriethoxysilylpropyltetrasulfane (TESPT) mixture was investigated by inverse gas chromatography (IGC) at infinite dilution. Thermodynamic results indicate a higher polarity of the silica surface modified with TES as compared to that of the unmodified silica due to new OH groups deriving from the hydrolysis of ethoxy groups of the silane; the long hydrocarbon substituent of the ODTES lies on the surface of silica and reduces the dispersive component of the silica surface tension. A comparison with silica modified with TESPT is discussed. An accurate morphological investigation by transmission electron microscopy (TEM) and automated image analysis (AIA) was carried out on aggregates of silica dispersed into a SBR compound loaded with 35 phr (per hundred rubber) of untreated and TESPT-treated silica. Morphological descriptors such as the projected area/perimeter ratio (A/P) and roundness (P2/4piA) provided direct and quantitative indications about the distribution of the filler into the rubber matrix.