Percolation analysis of the electrical conductive network in a polymer nanocomposite by nanorod functionalization

Chemical functionalization of nanofillers is an effective strategy to benefit the formation of the conductive network in the matrix which can enhance the electrical conductivity of polymer nanocomposites (PNCs). In this work, we adopted a coarse-grained molecular dynamics simulation to investigate the effect of the nanorod (NR) functionalization on the conductive probability of PNCs under the quiescent state or under a shear field. It is found that the direct aggregation structure of NRs is gradually broken down with increasing the NR functionalization degree λ_A, which improves their dispersion state. Moreover, a local bridging structure of NRs sandwiched via one polymer layer is formed at high λ_A. Corresponding to it, the percolation threshold of PNCs first quickly decreases, then increases and last slightly decreases again with the increase of λ_A, which exhibits an anti N-type under the quiescent state. Meanwhile, it shows a non-monotonic dependence on the interaction between polymer and the functionalized beads which reaches the lowest value at the moderate interaction. However, the percolation threshold is nearly independent of λ_A under the shear field. Compared with in the quiescent state, the decrease or the increase of the percolation threshold can be tuned by λ_A under the shear field. The significant change in the percolation threshold is attributed to the orientation and the dispersion state of NRs under the shear field, which affects the conductive network. Especially, we found that the dispersion state of NRs is different for different λ_A under the shear field. However, the percolation threshold is similar which indicates that the dispersion state of NRs is not completely correlated to the conductive network. In summary, this work presents some further understanding of how the NR functionalization affects the electrical conductivity of PNCs.

Chemical functionalization of nanofillers is an effective strategy to benefit the formation of the conductive network in the matrix which can enhance the electrical conductivity of polymer nanocomposites (PNCs).     

Modeling the effect of interfacial conductivity between polymer matrix and carbon nanotubes on the electrical conductivity of nanocomposites

This article presents the role of interfacial conductivity between the polymer matrix and nanoparticles in the electrical conductivity of polymer carbon nanotube (CNT) nanocomposites (PCNT) by simple equations. In this methodology, CNT size, CNT conductivity, CNT waviness and interfacial conductivity express the effective length and effective concentration of CNT in PCNT. Additionally, the percolation threshold and the percentages of CNT in the conductive networks are defined by the above mentioned terms. Finally, a simple model is developed to suggest the electrical conductivity of PCNT by CNT dimensions, CNT conductivity, CNT waviness, interphase thickness, interfacial conductivity and tunneling distance. The developed model is applied to show the roles of all parameters in the conductivity. Also, the experimental levels of percolation threshold and conductivity for several samples are compared to the predictions to validate the developed equations. The interfacial conductivity directly controls the electrical conductivity of nanocomposites. In addition, thick interphase, low waviness and short tunneling distance increase the conductivity. Moreover, the predictions show good agreement with the experimental measurements, providing evidence in support of the developed equations.

This article presents the role of interfacial conductivity between the polymer matrix and nanoparticles in the electrical conductivity of polymer carbon nanotube (CNT) nanocomposites (PCNT) by simple equations.     

Practical PBT/PC/GNP composites with anisotropic thermal conductivity

Most of the highly thermally conductive polymer-based composites currently face problems that must be solved before they can be directly used in industrial production. Herein, a practical polybutylene terephthalate (PBT)/polycarbonate (PC)/graphite nanoplatelet (GNP) thermally conductive composite with relative low filler content was prepared by a conventional melt-blending technique. GNPs selectively distributed and oriented in the PBT phase afford the composite a low percolation threshold and anisotropic thermal conductivity. Investigation of the influence of filler content on the final comprehensive performance showed that a prepared PBT/PC/GNP composite with 20 vol% GNPs exhibited superior performance in thermal conductivity, heat resistance, and mechanical properties. The in-plane and through-plane thermal conductivities of the composite were 5.82 W m⁻¹ K⁻¹ and 1.06 W m⁻¹ K⁻¹, respectively, which were increased by 2430% and 361% as compared to that of a neat PBT/PC blend. The Vicat softening temperature increased by 17.7 °C and reached 213.7 °C, while the mechanical properties also maintained a good application level.

The selective distribution of thermally conductive fillers in a co-continuous polymer blends provides an industrialized preparation method that takes into account both the properties and functions of thermally conductive composites.     

Effect of surface wettability on the interfacial adhesion of a thermosetting elastomer on glass

Interfacial adhesion dictates properties and performance of both composites and adhesively bonded structures. Weak adhesion at the interfaces of polymer composites leads to void formation and debonding, which adversely affect composite structural integrity and mechanical performance. This work investigated the relationship between surface wettability and interfacial fracture energy with the goal of tailoring interfacial adhesion within polymer composites. A series of model functionalized surfaces was created using silane coupling agents with different organo-functionalities to alter surface wettability. Based on the analysis of interfacial fracture energy between a thermosetting elastomeric polymer network and model surfaces, interfacial adhesion was found to be positively correlated to resin wettability. The results provide a fast and simple approach to screen different material combinations for the development of novel polymeric composites and adhesively bonded structures with tailorable adhesion.

Interfacial adhesion is postively correlated with resin wettability.     

Effect of grafted graphene nanosheets on morphology evolution and conductive behavior of poly(methyl methacrylate)/poly(styrene-co-acrylonitrile) blends during isothermal annealing

A facile method was developed for directly grafting poly(methyl methacrylate) (PMMA) to graphene oxide (GO) without surface modification, with the resultant insulating PMMA-g-GO nanosheets further reduced in situ to give conductive grafted reduced graphene oxide (RGO) nanosheets. The effect of PMMA-g-RGO nanosheets on the morphological evolution and conductive behavior of partially miscible blends of poly(methyl methacrylate)/poly (styrene-co-acrylonitrile) (PMMA/SAN) upon annealing above their phase-separation temperature was investigated using phase-contrast microscopy (PCM) with a real-time online digital picoammeter. With phase separation of the blend matrix, the well-dispersed PMMA-g-RGO nanosheets in the homogeneous matrix preferentially migrated to the SAN-rich phase and showed remarkably little aggregation. Surface grafting of PMMA-g-RGO might inhibit the aggregation of nanosheets in the blend matrix and weaken the retardation effect of nanosheets on the morphology evolution of the blend matrix. Furthermore, the percolation behavior of dynamic resistivity for ternary nanocomposites was attributed to the formation of a PMMA-g-RGO conductive network in the SAN-rich phase. The activation energy of conductive pathway formation was closer to the activation energy of flow for PMMA than that of SAN.

The activation energy of conductive pathway formation for PMMA/SAN/PMMA-g-RGO nanocomposites is close to that of flow for PMMA, indicating that DC percolation is mainly related to the mobility of grafted PMMA chains, rather than that of SAN chains.     

Boron carbide composites with highly aligned graphene nanoplatelets: light-weight and efficient electromagnetic interference shielding materials at high temperatures

B₄C-based ceramic composites containing 0–2 vol% highly aligned graphene nanoplatelets (GNPs) are fabricated. The electromagnetic interference (EMI) shielding properties of the obtained composites are investigated at X-band (8.2–12.4 GHz) frequency range from room-temperature up to 800 °C. All composites exhibit outstanding EMI shielding properties with satisfactory frequency- and thermal-stability. The shielding effectiveness (SE) of GNP/B₄C composites increases monotonically with increasing GNP loading. Superior room-temperature SE close to 40 dB is achieved with only 2 vol% GNPs and high SE around 35 dB still persists at 800 °C. Considering their relatively low density, GNP/B₄C composites possess a high specific shielding effectiveness (SSE) of 16 dB cm³ g⁻¹ which is among the highest values in reported ceramic-based shielding composites. Especially, the GNP/B₄C composite with 2 vol% GNPs exhibits the highest SSE/t (SSE divided by thickness) values at temperatures above 200 °C for all reported shielding composites, indicating that GNP/B₄C composites belong to the most promising high-temperature shielding composites. The excellent shielding properties of GNP/B₄C composites arise mainly from the high electrical conductivity, high dielectric loss and the multiple reflections by the highly aligned and large-sized GNP layers.

The incorporation of a small amount of highly aligned graphene nanoplatelets into boron carbide leads to light-weight and efficient high-temperature electromagnetic interference shielding composites.     

Estimation of the tensile modulus of polymer carbon nanotube nanocomposites containing filler networks and interphase regions by development of the Kolarik model

In this paper, the Kolarik model for the tensile modulus of co-continuous blends based on cross-orthogonal skeleton structures is simplified and developed for polymer/carbon nanotube (CNT) nanocomposites assuming continuous CNT networks in the polymer matrix and the reinforcing and percolating efficiencies of the interphase. For this purpose, the Ouali model for the modulus of nanocomposites above the percolation threshold is linked with the Kolarik model and the interphase percolation is considered with the excluded volume of the nanoparticles. In addition, the simplified Kolarik model is developed with the interphase as a new phase surrounding the nanofiller. A good agreement between the experimental data and the predictions is observed in the samples containing interphases and filler networks, while the developed model cannot estimate the modulus in the absence of interphases and network structures. The developed model demonstrates the effects of all the parameters on the modulus. The interphase parameters more significantly affect the modulus compared to the concentration and modulus of the filler, demonstrating the importance of the interphase properties.

The Kolarik model for the tensile modulus of co-continuous blends is developed for polymer/carbon nanotube (CNT) nanocomposites assuming continuous CNT networks and the reinforcing and percolating efficiencies of the interphase.     

Self-assembled functional components-doped conductive polypyrrole composite hydrogels with enhanced electrochemical performances

A conductive hydrogel is a composite conductive material formed by combining a conductive polymer with a nanogel structure of a hydrogel. Conductive hydrogels not only have potential applications in supercapacitors, sensors, and modulators, they can also be synthesized by many methods, such as copolymerization, crosslinking, and grafting. In this work, we successfully prepared three conductive composite hydrogels by in situ polymerization, namely polypyrrole sodium alginate conductive hydrogel, ferric chloride-doped polypyrrole sodium alginate hydrogel and doped polypyrrole sodium alginate hydrogel with sodium dodecylbenzene sulfonate. In addition, a series of characterizations were performed for the three conductive hydrogels described above. The results show that the polypyrrole sodium alginate hydrogel doped with ferric chloride forms a nanofiber network with a more stable structure and better electrochemical performance.

New functional components-doped conductive polypyrrole composite hydrogels are prepared via a self-assembled process, demonstrating potential applications in catalysis as well as electrochemical materials.     

Synthesis of tetraphenylethylene-based conjugated microporous polymers for detection of nitroaromatic explosive compounds

Conjugated microporous polymers (CMPs) containing tetraphenylethylene (TPE) were synthesized via the Suzuki coupling polymerization. The tetrafunctional TPE moiety in the polymer backbone was linked with the difunctional phenylene group to exhibit a porous structure with high fluorescence in the solid state because of aggregation-induced emissive TPE. The porous polymer with a fluorescent TPE group successfully detected nitroaromatic explosive compounds that exhibited fluorescence quenching, in which the polymer shows high quenching efficiency to picric acid among nitroaromatic explosive compounds. The interaction between the electron-rich TPE group and the electron-deficient nitroaromatic compounds played a decisive role in fluorescence quenching via a photoinduced electron transfer (PET). Compared with a linear polymer containing TPE, the porous, crosslinked polymer showed better sensing performance toward nitroaromatic compounds, presumably because of the more efficient interaction between TPE and nitroaromatic compounds in the pores of TPE-based CMP (TPE-CMP).

Conjugated microporous polymers (CMPs) containing tetraphenylethylene (TPE) were synthesized via the Suzuki coupling polymerization.     

The chemical functionalization of graphene nanoplatelets through solvent-free reaction

Graphene nanoplatelets (GNPs) were functionalized through 1,3-dipolar cycloaddition of azomethine ylide using a solvent-free approach and under different reaction conditions. The yield and the functionality of the carboxyl-terminated pyrrolidine ring attached on the surface of GNPs could be affected by varying the reaction temperature as well as the reactant to GNP weight ratio. The functionalized GNPs were characterized extensively using a range of spectroscopic and microscopy techniques.

Carboxyl-terminated pyrrolidine functionalized graphene nanoplatelets through a solvent-free reaction.     

Synthesis,characterization and chemical degradation of poly(ester-triazole)s derived from d-galactose

α-Azide-ω-alkynyl ester monomers were designed and synthesized in order to obtain hydrolytically degradable polymers. The monomers were prepared from d-galactose, as a renewable resource. Environmentally benign azido–alkyne cycloaddition polymerizations were conducted to afford poly(ester-triazole)s, with complete atom economy. Although polymer formation prevailed under optimized polymerization conditions, variable proportions of cyclic oligomer byproducts were detected. The Cu-catalyzed click polymerization led regioselectively to 1,4-disubstituted triazole linkages, while the thermal, metal-free polymerization produced a random distribution of 1,4- and 1,5-disubstituted triazoles in the polymer backbone. The poly(ester-triazole)s exhibited high molecular weights (M_w in the range 35–85 kDa). They were soluble in organic solvents but highly insoluble in water, thus removal of the Cu(i) catalyst was simplified. The polymers were stable up to 300 °C, and had T_g values in the range 90–100 °C. The materials were hydrolysed under either basic or strong acid conditions, and the degradation products have been characterized.

Carbohydrate-derived poly(ester-triazoles), soluble in organic solvents and degradable in aqueous media, have been synthesized by CuAAC or thermal polymerization.     

Self-assembled morphologies of polyelectrolyte-grafted nanoparticles directed by oppositely charged polymer matrices

Self-assembled structure of polymer grafted nanoparticles is an interesting and growing subject in the field of hybrid electronics and high energy density materials. In light of this, the self-assembled morphologies of polyelectrolyte (PE) sparsely grafted nanoparticles tuned by oppositely charged matrix chains are studied using molecular dynamics simulations. Our focus is to elucidate the effect of matrix chain polymerization on modulating the stretching properties of tethered PE layers, on the self-assembled structuring of nanoparticles. Through varying the matrix chain length and stiffness as well as electrostatic interaction strength, rich phase behaviors of PE coated nanoparticles are predicted, including spherical micelle-like structures being preferred with short matrix chains and percolating network morphologies favored with long matrix chains, which is more pronounced with an enhanced matrix chain rigidness. To pinpoint the mechanisms of self-assembled structure formation, the thickness of grafted layers, the gyration radius of tethered chains, and pair correlation functions between nanoparticles are analyzed carefully. Additionally, electrostatic correlations, manifested as the bridging via matrix chains, are examined by identifying three states of matrix PE chains. Our simulation results may be useful for designing smart polymer nanocomposites based on PE coated nanoparticles.

Self-assembled structure of polymer grafted nanoparticles is an interesting and growing subject in the field of hybrid electronics and high energy density materials.     

Graphene nanoplatelets-sericin surface-modified Gum alloy for improved biological response

In this study a “Gum Metal” titanium-based alloy, Ti-31.7Nb-6.21Zr-1.4Fe-0.16O, was synthesized by melting and characterized in order to evaluate its potential for biomedical applications. The results showed that the newly developed alloy presents a very high strength, high plasticity and a low Young''s modulus relative to titanium alloys currently used in medicine. For further bone implant applications, the newly synthesized alloy was surface modified with graphene nanoplatelets (GNP), sericin (SS) and graphene nanoplatelets/sericine (GNP–SS) composite films via Matrix Assisted Pulsed Laser Evaporation (MAPLE) technique. The characterization of each specimen was monitored by scanning electron microscopy (SEM), atomic force microscopy (AFM), contact angle (CA) measurements, and Fourier Transform Infrared Spectroscopy (FTIR). The materials'' surface analyses suggested the successful coating of GNP, SS and GNP–SS onto the alloy surface. Additionally, the activities of pre-osteoblasts such as cell adhesion, cytoskeleton organization, cell proliferation and differentiation potentials exhibited on these substrates were investigated. Results showed that the GNP–SS-coated substrate significantly enhanced the growth and osteogenic differentiation of MC3T3-E1 cells when compared to bare and GNP-coated alloy. Collectively, the results show that GNP–SS surface-modified Gum alloy can modulate the bioactivity of the pre-osteoblasts holding promise for improved biological response in vivo.

GNP–SS functionalized Gum alloy exhibits superior bioactivity in inducing in vitro osteogenesis.     

Enzymatic synthesis of biobased aliphatic–aromatic oligoesters using 5,5′-bis(hydroxymethyl)furoin as a building block

5,5′-Dihydroxymethyl furoin (DHMF) is a novel biobased difuranic polyol scaffold, achievable from the benzoin condensation of 5-hydroxymethylfurfural (HMF), which has recently been employed as a monomer for the preparation of cross-linked polyesters and polyurethane. Its upgrading by means of enzymatic reactions has not yet been reported. Here we demonstrated that Candida antarctica lipase B (CALB) is a suitable biocatalyst for the selective esterification of the primary hydroxyl groups of DHMF. Exploiting this enzymatic activity, DHMF has been reacted with the diethyl esters of succinic and sebacic acids obtaining fully biobased linear oligoesters with number-average molecular weight around 1000 g mol⁻¹ and free hydroxyl groups on the polymer backbone. The structures of the DHMF-diacid ethyl ester dimers and of the oligomers were elucidated by NMR and MS analyses.

Fully bio-based linear oligoesters were obtained by the unprecedented enzymatic polymerization of 5,5′-bis(hydroxymethyl)furoin with succinic and sebacic acid diethyl esters.     

Segregated poly(arylene sulfide sulfone)/graphene nanoplatelet composites for electromagnetic interference shielding prepared by the partial dissolution method

Segregated conductive polymer composites have been proved to be outstanding electromagnetic interference shielding (EMI) materials at low filler loadings. However, due to the poor interfacial adhesion between the pure conductive filler layers and segregated polymer granules, the mechanical properties of the segregated composites are usually poor, which limit their application. Herein, a simple and effective approach, the partial dissolution method, has been proposed to fabricate segregated poly(arylene sulfide sulfone) (PASS)/graphene nanoplatelet (GNP) composites with superior EMI shielding effectiveness (SE) and high tensile strength. Morphology examinations revealed that the GNPs were restricted in the dissolved outer layer by the undissolved cores, and there was a strong interaction between the PASS/GNP layer and the pure PASS core. The resultant PASS/GNP composites showed excellent electrical conductivity (60.3 S m⁻¹) and high EMI SE (41 dB) with only 5 wt% GNPs. More notably, the tensile strength of the PASS/GNPs prepared by partial dissolution reached 36.4 MPa, presenting 136% improvement compared to that of the conventional segregated composites prepared by mechanical mixing. The composites also exhibited high resistance to elevated temperatures and chemicals owing to the use of the special engineering polymer PASS as a matrix.

The segregated structure prepared by controlling the dissolution process endows the composites with both excellent EMI SE and high mechanical strength.     

Theoretical analyses on water cluster structures in polymer electrolyte membrane by using dissipative particle dynamics simulations with fragment molecular orbital based effective parameters

The mesoscopic structures of polymer electrolyte membrane (PEM) affect the performances of fuel cells. Nafion® with the Teflon® backbone has been the most widely used of all PEMs, but sulfonated poly-ether ether-ketone (SPEEK) having an aromatic backbone has drawn interest as an alternative to Nafion. In the present study, a series of dissipative particle dynamics (DPD) simulations were performed to compare Nafion and SPEEK. These PEM polymers were modeled by connected particles corresponding to the hydrophobic backbone and the hydrophilic moiety of sulfonic acid group. The water particle interacting with Nafion particles was prepared as well. The crucial interaction parameters among DPD particles were evaluated by a series of calculations based on the fragment molecular orbital (FMO) method in a non-empirical way (Okuwaki et al., J. Phys. Chem. B, 2018, 122, 338–347). Through the DPD simulations, the water and hydrophilic particles aggregated, forming cluster networks surrounded by the hydrophobic phase. The structural features of formed water clusters were investigated in detail. Furthermore, the differences in percolation behaviors between Nafion and SPEEK revealed much better connectivity among water clusters by Nafion. The present FMO-DPD simulation results were in good agreement with available experimental data.

The mesoscopic structures of polymer electrolyte membrane (PEM) affect the performances of fuel cells.     

Rational and site-selective formation of coordination polymers consisting of d10 coinage metal ions with thiolate ligands using a metal ion-doped polymer substrate

Here, we report an interfacial approach for fabricating coordination polymers (CPs) consisting of d¹⁰ coinage metal ions with thiolate ligands on a polymer substrate. It was found that CPs were selectively formed on the polymer substrate, resulting in the formation of CP-based thin films. In addition, utilizing a mixed metal ion-doped polymer substrate leads to the formation of mixed-metal CP-based films.

We report an interfacial approach for fabricating coordination polymer (CP) crystal films and patterns on a polymer surface using a metal ion-doped polymer substrate.     

Predicting the electrical conductivity in polymer carbon nanotube nanocomposites based on the volume fractions and resistances of the nanoparticle,interphase, and tunneling regions in conductive networks

Some limited models have been suggested to determine the conductivity of polymer carbon nanotube (CNT) nanocomposites (PCNTs). However, earlier models (e.g., the Kovacs model) cannot properly consider the roles of the interphase regions or tunneling properties on the percolation threshold and subsequent conductivity of PCNTs. In this paper, the Kovacs model is further developed by assuming that the CNT, interphase, and tunneling regions are separate phases. Also, some simple equations are provided to calculate the percolation threshold as well as the volume fractions and resistances of the CNT, interphase, and tunneling regions in conductive networks. The experimental conductivity results for several samples are compared with the predictions of the developed model. In addition, the calculations of the developed model at different parameter levels are explained and justified. The conductivity calculations show good agreement with the experimental data. Moreover, the developed model reasonably explains the roles of the different parameters on the conductivity. For example, long, thin, and straight CNTs efficiently improve the conductivity because they form large networks in the nanocomposites. Additionally, a thick interphase enlarges the conductive networks, resulting in a desirable conductivity. The conductivity of PCNTs only depends on the tunneling resistance; this is the case because the poor resistance/significant conductivity of the CNT and interphase regions do not influence the conductivity. The developed equations can replace conventional approaches for predicting the conductivity of nanocomposites.

Some limited models have been suggested to determine the conductivity of polymer carbon nanotube (CNT) nanocomposites (PCNTs).     

Design of melt-recyclable poly(ε-caprolactone)-based supramolecular shape-memory nanocomposites

A novel poly(epsilon-caprolactone) (PCL) supramolecular network exhibiting shape-memory behavior was successfully constructed with pendant UPy units that are highly able to dimerize. The dynamic network was obtained by a simple and versatile strategy consisting of chain-extension reaction between α,ω-dihydroxyoligoPCL and hydroxylated UPy units in the presence of hexamethylene diisocyanate as a coupling agent and further intermolecular dimerization of the UPy along the polyurethane backbone. ¹H NMR analyses confirmed the dynamic features of the system, and DMTA in tensile mode was investigated to assess the SMP properties. Recyclability was also assessed by taking advantage of these supramolecular networks. Further addition of cellulose nanocrystals into the polymer network enabled adjustment of the extent of the net-points and therefore the SMP features. As confirmed by dispersion tests in solution and SEM observations, these bio-based nanofillers were homogeneously distributed in the network via supramolecular interaction between the hydroxyl groups present on their surface and UPy moieties along the polyurethane backbone. Thus, the here developed nanomaterials might reveal applicability in areas where a combination of SMP and biocompatibility is needed.

Novel melt-recyclable poly(ε-caprolactone)/cellulose nanocrystals supramolecular nanocomposite networks with shape-memory behavior have been successfully constructed by playing with UPy chemistry.     

Biorenewable rosin derived benzocyclobutene resin: a thermosetting material with good hydrophobicity and low dielectric constant

Development of bio-based polymers has been promoted by the growing concerns about the long-term sustainability and negative environmental footprint of petroleum-based polymer materials. A new monomer containing benzocyclobutene and allyl units has been developed by using rosin as the feedstock. The structure of the monomer was characterized by elemental analysis, MS, FT-IR and NMR spectroscopy. The monomer could be converted to the polymer via thermal ring-opening polymerization which was characterized via FT-IR, thermogravimetric analysis (TGA), atom force microscopy (AFM) and so on. The polymer showed good dielectric properties and hydrophobicity with an average dielectric constant of 2.51 in a range of frequencies from 0.1 to 18 MHz and a water contact angle of 106°. In addition, the polymer with other comprehensive performances exhibited a 5% weight loss temperature of 406 °C, a surface roughness (R_a) of 0.658 nm in a 5.0 × 5.0 μm² area, hardness and Young''s modulus of 0.283 and 3.542 GPa, and storage modulus of 11.46 GPa at 30 °C. These data suggest that the polymer may have potential application in electronics and microelectronics.

The rosin structure was introduced into benzocyclobutene resin for the first time, and the thermosetting material with good hydrophobicity and low dielectric constant was prepared.