Marián A. Gómez-Fatou

Chemical sensors based on polymer composites with carbon nanotubes and graphene: the role of the polymer

This review provides an overview of recent research on chemical sensors based on polymer composites with carbon nanotubes (CNTs) and graphene (G) for quantitative and qualitative analysis in diverse application fields such as biosensing (DNA, enzymes, proteins, antigens and metabolites), and chemical and gas sensing using electrochemical and optical detection methods. Both CNTs and G show outstanding electrical, chemical, electrochemical and optical properties that make them ideal candidates for use in chemical sensors. The incorporation of polymers into the development of this type of sensor not only improves the CNT and G dispersion, but also enhances some of their properties like redox behaviour and biocompatibility, and provides additional properties such as photoelectric or swelling capacity. Moreover, unique synergistic effects arising from the combination of the matrix and nanofiller contributions are described by means of several examples highlighting the most important achievements in this field. Special emphasis has been placed throughout the review on analysing the role of the polymer in different sensing platforms. The combination of polymers with carbon nanomaterials for the preparation of chemical sensors opens up exciting areas of research due to their biocompatibility, and excellent sensitivity and selectivity.

Covalent functionalization of MWCNTs with poly(p-phenylene sulphide) oligomers: a route to the efficient integration through a chemical approach

We report a new strategy to obtain high performance polymer-grafted multi-walled carbon nanotubes (MWCNTs). Chlorophenyl-functionalized MWCNTs, obtained through the in situ generation and reaction of diazonium compounds, were subjected to polymerization in the presence of Na2S and 1,4-dichlorobenzene in order to yield MWCNTs covalently functionalized with poly(p-phenylene sulfide) (PPS) oligomers. The MWCNT functionalization and the PPS oligomeric chain growth can be controlled throughout accessible experimental variables, including the possibility to carry out the whole process in a one-pot reaction. This represents an efficient and facile route to develop covalently grafted MWCNTs beyond those reported so far for the same polymer. These materials are promising fillers for the production of high performance PPS-based composite materials, due to the improvement of the filler–matrix compatibility. The manufacturing and characterization of some test samples show that oligomer-grafted MWCNTs induce the suppression of the intrinsic confinement effect imposed by the nanofiller, as for the observation of an increase in the PPS crystallinity. An outstanding increase in the PPS thermal stability and mechanical properties is also observed, as compared to bare MWCNTs, while leaving the electrical properties unharmed.

New hybrid nanocomposites containing carbon nanotubes, inorganic fullerene-like WS2 nanoparticles and poly(ether ether ketone) (PEEK)

Single-walled carbon nanotubes (SWCNTs) and inorganic fullerene-like WS2 nanoparticles were successfully dispersed in poly(ether ether ketone) (PEEK) by advantageously traditional melt processing techniques. This strategy offers an attractive way to combine the merits of organic and inorganic materials into novel hybrid systems that challenge emerging polymer/CNT nanocomposites in terms of performance, cost and processability. In particular, the incorporation of IF-WS2 has shown to be efficient to produce well-dispersed PEEK/SWCNT composites influenced in turn by the processing method. The morphology, thermal stability, crystallization behaviour, thermal conductivity as well as mechanical and electrical properties of PEEK/SWCNT composites were tuneable by the introduction of small amounts of IF-WS2. The overall structural, thermal, mechanical and electrical performances confirm the potential use of SWCNTs and IF-WS2 for the preparation of advanced hybrid nanocomposites as lightweight alternatives for use in critical industrial applications of high-performance and high-temperature thermoplastics like PEEK.

Isothermal crystallization kinetics of novel isotactic polypropylene/MoS2 inorganic nanotube nanocomposites.

Inorganic nanotubes (INT) were used for the first time to prepare advanced polymer nanocomposites by means of the most simple, cost-effective and ecologically friendly way (i.e., melt-processing route). The polymer matrix was isotactic polypropylene (iPP) and the inorganic fillers were molybdenum disulfide nanotubes (MoS(2)). The effect of INT-MoS(2) concentration and the crystallization temperature on the isothermal crystallization behavior of iPP was investigated using differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXS). It has been observed that INT-MoS(2) affects the crystallization of nanocomposites remarkably, which can be attributed to the nucleating effect of INT-MoS(2) on the monoclinic α-crystal form of iPP. Other parameters such as the Avrami exponent and the fold surface free energy of crystallization of iPP chains in the nanocomposites were obtained in order to determine the effect of the INT-MoS(2) on them. The addition of INT-MoS(2) remarkably influences the kinetics of nucleation and growth of iPP with a decrease in the fold surface free energy of 11-24%.

Towards a new generation of polymer nanocomposites based on inorganic nanotubes

We report for the first time the preparation and characterization of novel polymer nanocomposites based on inorganic nanotubes (INTs) processed by an advantageous melt-processing route. This strategy offers an attractive way to combine the merits of organic and inorganic materials into novel polymer/INTs and opens new perspectives for large scale production. The polymer matrix selected here is iPP and the inorganic nanofillers are MoS2 nanotubes. The morphological and structural investigations confirm the successful dispersion of the INT-MoS2 into the iPP matrix. A significant enhancement of the thermal stability of iPP and a nucleating effect of the INTs comparable to CNTs has been observed. The excellent dynamic mechanical data demonstrates the high performance of these novel nanocomposites for industrial applications.

Epoxy composites with covalently anchored amino-functionalized SWNTs: towards the tailoring of physical properties through targeted functionalization

Functionalization of single-walled carbon nanotubes (SWNTs) with covalently grafted amine moieties provides reactive fillers with potential for covalent anchoring to an epoxy matrix. Manufacturing and characterization of a high performance epoxy system reinforced with as-grown and aminated SWNTs are presented through four different approaches. Epoxy composite materials incorporating SWNTs aminated through sidewall addition reactions present enhanced mechanical, thermal and electrical properties, beyond the effect of unfunctionalized SWNTs. The functionalization pathways studied here lead to a composite with specific improvements in some of the physical properties of the epoxy matrix, which enables the tailored design of the composites properties through functionalization. The aminationviadiazonium reaction with 4-aminobenzylamine is especially effective in enhancing the tensile and impact properties of the epoxy composites (44% improvement in impact strength at 0.1 wt% loading) and leads to the highest increase in elastic modulus reported so far for the integration of aminated nanotubes into epoxy resin. Composites incorporating aminated SWNTs throughout the 1,3-dipolar cycloaddition reaction stand out for their thermo-oxidative stability and thermomechanical properties. The incorporation of as-produced arc-discharge SWNTs into the TGAP/DDS epoxy matrix leads to composite materials with the highest electrical conductivity among all the studied samples.

Dynamic Crystallization Kinetics and Nucleation Parameters of a New Generation of Nanocomposites Based on Isotactic Polypropylene and MoS2 Inorganic Nanotubes

Differential scanning calorimetry (DSC) and time-resolved synchrotron X-ray diffraction have been used to investigate the dynamic crystallization behavior and crystalline structure of novel nanocomposites based on isotactic polypropylene (iPP) and molybdenum disulfide inorganic nanotubes (INT-MoS(2)). The influence of the INT-MoS(2) content and different cooling rates on the crystallization behavior has been studied. The crystallization exothermic peak shifted to higher temperature, and the overall crystallization time was reduced by increasing the INT-MoS(2). The dynamic crystallization kinetics was analyzed using the Ozawa-Avrami method, which was successful in describing the dynamic crystallization behavior of these new nanocomposites. On the other hand, study of the nucleation activity using the Dobreva method revealed that the INT-MoS(2) had an efficient nucleation effect on the monoclinic crystal form of iPP. Moreover, this effect was corroborated by the results of the crystallization activation energy, calculated using Kissinger and Takhor methods, which also confirmed the fact that the addition of INT-MoS(2) made the molecular chains easier to crystallize and increased the crystallization rate of iPP.

Reactive fillers based on SWCNTs functionalized with matrix-based moieties for the production of epoxy composites with superior and tunable properties

Composite materials based on epoxy matrix and single-walled carbon nanotubes (SWCNTs) are able to exhibit outstanding improvements in physical properties when using a tailored covalent functionalization with matrix-based moieties containing terminal amines or epoxide rings. The proper choice of grafted moiety and integration protocol makes it feasible to tune the composite physical properties. At 0.5 wt% SWCNT loading, these composites exhibit up to 65% improvement in storage modulus, 91% improvement in tensile strength, and 65% improvement in toughness. A 15 °C increase in the glass transition temperature relative to the parent matrix was also achieved. This suggests that a highly improved interfacial bonding between matrix and filler, coupled to improved dispersion, are achieved. The degradation temperatures show an upshift in the range of 40-60 °C, which indicates superior thermal performance. Electrical conductivity ranges from ~10(-13) to ~10(-3) S cm(-1), which also shows the possibility of tuning the insulating or conductive behaviour of the composites. The chemical affinity of the functionalization moieties with the matrix and the unchanged molecular structure at the SWCNT/matrix interface are responsible for such improvements.

The overlooked role of reduced graphene oxide in the reinforcement of hydrophilic polymers

The covalent incorporation of reduced graphene oxide into a hydrophilic polymer demonstrates that, contrary to current opinion, reinforcement can be attributed to a change in water affinity. Advanced nanoindentation techniques show that the deterioration of the mechanical properties of poly(vinyl alcohol) under high relative humidity can be avoided by incorporating a small quantity of reduced graphene oxide at molecularly controlled locations.

Influence of the covalent immobilization of graphene oxide in poly(vinyl alcohol) on human osteoblast response

The differences in the response of human Saos-2 osteoblasts to nanocomposites of poly(vinyl alcohol) (PVA) and 1.5wt.% graphene oxide (GO) prepared by covalent linking (PVA/GO-c) and simple blending (PVA/GO-m) have been evaluated through different biocompatibility parameters. The effects produced on osteoblasts by these two nanocomposites were analysed in parallel and compared with the direct action of GO and with the effect of PVA films without GO. The intracellular content of reactive oxygen species (ROS) and the levels of interleukin-6 (IL-6) were measured to evaluate oxidative stress induction and protective response, respectively. The results demonstrate that the combination of GO with PVA reduces both the proliferation delay and the internal cell complexity alterations induced by GO on human osteoblasts. Moreover, the covalent attachment of GO to the PVA chains increases both cell viability and IL-6 levels, reducing both apoptosis and intracellular ROS content when compared to simple blending of both materials. The use of this strategy to modulate the biointerface reduces the toxic effects of graphene while preserving the reinforcement characteristics for application in tissue engineering scaffolds, and has enormous interest for polymer/graphene biomaterials development.