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

Facile and Low Environmental Impact Approach to Prepare Thermally Conductive Nanocomposites Based on Polylactide and Graphite Nanoplatelets

In this work, the preparation of nanocomposites based on poly(l-lactide) PLLA and graphite nanoplatelets (GNP) was assessed by applying, for the first time, the reactive extrusion (REX) polymerization approach, which is considered a low environmental impact method to prepare polymer systems and which allows an easy scalability. In particular, ad hoc synthesized molecules, constituted by a pyrene end group and a poly(d-lactide) (PDLA) chain (Pyr-d), capable of interacting with the surface of GNP layers as well as forming stereoblocks during the ring-opening polymerization (ROP) of l-lactide, were used. The nanocomposites were synthesized by adding to l-lactide the GNP/initiator system, prepared by dispersing the graphite in the acetone/Pyr-d solution, which was dried after the sonication process. DSC and X-ray diffraction measurements evidenced the stereocomplexation of the systems synthesized by using the pyrene-based initiators, whose extent turned out to depend on the PDLA chain length. All the prepared nanocomposites, including those synthesized starting from a classical initiator, that is, 1-dodecanol, retained similar electrical conductivity, whereas the thermal conductivity was found to increase in the stereocomplexed samples. Preferential localization of stereocomplexed PLA close to the interface with GNP was demonstrated by scanning probe microscopy (SPM) techniques, supporting an important role of local crystallinity in the thermal conductivity of the nanocomposites

Thermally and electrically conductive nanopapers from reduced graphene oxide: Effect of nanoflakes thermal annealing on the film structure and properties

In this study, we report a novel strategy to prepare graphene nanopapers from direct vacuum filtration. Instead of the conventional method, i.e., thermal annealing nanopapers at extremely high temperatures prepared from graphene oxide (GO) or partially reduced GO, we fabricate our graphene nanopapers directly from suspensions of fully reduced graphene oxide (RGO), obtained after RGO and thermal annealing at 1700 °C in vacuum. By using this approach, we studied the effect of thermal annealing on the physical properties of the macroscopic graphene-based papers. Indeed, we demonstrated that the enhancement of the thermal and electrical properties of graphene nanopapers prepared from annealed RGO is strongly influenced by the absence of oxygen functionalities and the morphology of the nanoflakes. Hence, our methodology can be considered as a valid alternative to the classical approach

Super nucleation and orientation of poly (butylene terephthalate) crystals in nanocomposites containing highly reduced graphene oxide

The ring opening polymerization of cyclic butylene terephthalate into poly (butylene terephthalate) (pCBT) in the presence of reduced graphene oxide (RGO) is an effective method for the preparation of polymer nanocomposites. The inclusion of RGO nanoflakes dramatically affects the crystallization of pCBT, shifting crystallization peak temperature to higher temperatures and, overall, increasing the crystallization rate. This was due to a super nucleating effect caused by RGO, which is maximized by highly reduced graphene oxide. Furthermore, combined analyses by differential scanning calorimetry (DSC) experiments and wide angle X-ray diffraction (WAXS) showed the formation of a thick {\alpha}-crystalline form pCBT lamellae with a melting point of ~250 {\deg}C, close to the equilibrium melting temperature of pCBT. WAXS also demonstrated the pair orientation of pCBT crystals with RGO nanoflakes, indicating a strong interfacial interaction between the aromatic rings of pCBT and RGO planes, especially with highly reduced graphene oxide. Such surface self-organization of the polymer onto the RGO nanoflakes may be exploited for the enhancement of interfacial properties in their polymer nanocomposites

Breaking the Nanoparticle Loading-Dispersion Dichotomy in Polymer Nanocomposites with the Art of Croissant-Making

\u3cp\u3eThe intrinsic properties of nanomaterials offer promise for technological revolutions in many fields, including transportation, soft robotics, and energy. Unfortunately, the exploitation of such properties in polymer nanocomposites is extremely challenging due to the lack of viable dispersion routes when the filler content is high. We usually face a dichotomy between the degree of nanofiller loading and the degree of dispersion (and, thus, performance) because dispersion quality decreases with loading. Here, we demonstrate a potentially scalable pressing-and-folding method (P & F), inspired by the art of croissant-making, to efficiently disperse ultrahigh loadings of nanofillers in polymer matrices. A desired nanofiller dispersion can be achieved simply by selecting a sufficient number of P & F cycles. Because of the fine microstructural control enabled by P & F, mechanical reinforcements close to the theoretical maximum and independent of nanofiller loading (up to 74 vol %) were obtained. We propose a universal model for the P & F dispersion process that is parametrized on an experimentally quantifiable D factor . The model represents a general guideline for the optimization of nanocomposites with enhanced functionalities including sensing, heat management, and energy storage.\u3c/p\u3

Thermally conductive polymer/graphene-related materials nanocomposites prepared by melt reactive processing

Polymer nanocomposites containing graphene-related materials attracted a wide research interest thanks to the combination of the processability, lightweight and corrosion resistance typical of polymers, with the outstanding properties of graphene-related materials, including mechanical properties, thermal conductivity and electrical conductivity. Nanocomposites exploiting graphene-related materials are indeed showing interesting properties and several industrial applications for such nanomaterials are currently being developed, including structural materials, as well as functional materials, electrodes and conductors in flexible electronics, waste heat management, gas-barrier materials, etc., also taking into advantage of the large European initiative for graphene research, development and application called Graphene Flagship (http://graphene-flagship.eu/). This thesis aims to the preparation of polymer nanocomposites, exploiting graphene-related materials, by the development of industrially viable preparation methods, for the application as heat management materials. These are currently of interest in several application fields, including low temperature heat recovery, heat exchange in highly corrosive environments as well as heat dissipation in electronics and flexible electronics. Beside the thermal conductivity property, this PhD thesis was aimed at the fundamental understanding of phenomena controlling nanoparticle dispersion into the polymer matrix as well as the correlations between structure and properties of the prepared materials, including electrical conductivity, rheological properties and polymer crystallization phenomena. As the availability of graphene (i.e. a single layer of sp2 carbons) nanoflakes remains extremely limited and insufficient for the exploitation in large scale applications embedding graphene in the polymer bulk, different types of graphene-related materials were selected for exploitation in this PhD thesis, namely graphite nanoplatelets (GNP) and reduced graphene oxide (rGO). In particular, different grades of GNP and rGO were selected aiming at the correlation between their quality, mainly in terms of defectiveness and aspect ratio, and the properties of their corresponding polymer nanocomposite. For these reasons, the initial part of this thesis is focused on thorough characterization of nanoflake quality, i.e. defectiveness and aspect ratio, through electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and thermogravimetric analysis. On the other hand, the second part is focused on the preparation and detailed characterization of nanocomposites prepared by ring opening polymerization of polyester oligomers (CBT) during melt mixing in presence of graphene-related materials. In particular, the effects of the exploitation of different graphene-related materials, of the polymerization during reactive mixing and of the processing parameters (processing temperature, time and shear rate) on the electrical and thermal properties of polymer nanocomposites is addressed. Thorough characterization of the effect of the exploitation of pristine and high temperature-annealed reduced graphene oxide on the nanocomposite properties is also reported, in terms of both of conductivities and modification in the crystallization of the polymer matrix. The results reported in this thesis demonstrate the viability of CBT polymerization during melt mixing with graphene-related materials to produce thermally and electrically conductive polymer nanocomposites aiming at possible industrial applications

GNP DISPERSION BY REACTIVE EXTRUSION FOR THERMALLY CONDUCTIVE POLYMER NANOCOMPOSITES

Thermally conductive polymer nanocomposites are of great interest in substituting metal components in all those applications where corrosion resistance, lightweight and processability are required. Nanoparticles with extremely high thermal conductivity (Carbon Nanotubes, Graphene and Graphene Nanoplatelets, hexagonal Boron Nitride) are expected to confer a huge improvement to polymer thermal properties. Recently, Graphene Nanoplatelets (GNP) have become an interesting option due to their geometry, which allows to obtain an higher contact between nanoparticles [1,2,3]. However, it is critical to obtain a good dispersion degree of GNP in polymer matrix, especially during melt mixing. In this work, a GNP with an expanded structure has been melt mixed with cyclic butylene terephthalate (CBT). These mixed oligomers are known to have a very low viscosity that can allow to a complete filling of the expanded structure, which is preliminar to the obtainment of a good exfoliation degree. This can lead to an higher contact between nanoparticles, with a consequent improvement in the thermal conductivity of the nanocomposite. Moreover, in a second step, during melt mixing a tin catalyst was added to the compound; this is known to open the cyclic structure, starting CBT polymerization into poly (butylene terephthalate) (pCBT). During polymerization the molecular weight, and the viscosity, of the molten system increase, and consequently raise the shear stresses that can have a key role in the optimization of the exfoliation degree. This, combined to the higher crystallinity of pCBT respect to CBT, can push to the obtainment of high thermally conductive polymer nanocomposites

Decoupled trends for electrical and thermal conductivity in phase-confined CNT co-continuous blends

In the present work, the morphology and the electrical and thermal conduction properties of co-continuous poly(vinylidene fluoride) (PVDF), maleated polypropylene (PPgMA) and multiwall carbon nanotubes (CNT) nanostructured blends are investigated. CNT preferentially locates in the PPgMA phase and clearly causes a refinement in the co-continuous structure. Electrical conductivity experiments show that nanocomposites are well above the percolation threshold and evidence for one order of magnitude enhancement in conductivity for the co-continuous nanocomposites compared to the monophasic nanocomposites with the same CNT volume fraction. On the other hand, thermal diffusivity enhancement for the co-continuous blends is found lower than that for the monophasic nanocomposites at the same CNT volume fraction. An explanation is proposed in terms of large interfacial area, causing phonon scattering at the interface between immiscible PVDF and PPgMA domains. Results described in this paper open the way to the preparation of high electrical and low thermal conductivity materials with possible application as thermoelectrics

Carbon Nanotubes migration and segregation at the interface in immiscible polymer blends

Selective localization of nanoparticles in immiscible polymer blends, during melt blending, is a well-known phenomenon[1] for different kind of particles, such as lamellar clays [2], carbon black and carbon nanotubes[3], nanographite[4], etc. Particles distribution in polymer blends prepared by melt blending depends both from thermodynamic (interfacial tensions) and kinetic (viscosity, time, temperatures) factors. In this work from has been studied the diffusion of CNT from a PP matrix to another polymer (PVC or PA6)