Search

Mechanoresponsive dynamic covalent networks were designed and synthesized by embedding radical mechanophores into the polymer backbone as crosslinkers. The mechanical properties and mechanoresponsiveness of the polymers were reinforced by hydrogen bonds.

A novel polymer network with NIR induced shape memory behaviour and plasticity.

A mechanically active spiropyran (SP) mechanophore is incorporated into the backbone of prepolymer which is further end-capped with ureidopyrimidinone (UPy) or urethane. Strong mechanochromic reaction of SP arises in the bulk films of UPy containing materials whereas much weaker activation occurs in urethane containing counterparts, coincident with their stress–strain responses. The difference in the magnitudes of supramolecular interactions leads to different degrees of chain orientation and strain induced crystallization (SIC) in the bulk and consequently distinct capabilities to transfer the load to the mechanophores. This study may aid the design of novel mechanoresponsive materials whose mechanoresponsiveness can be tailored by tuning supramolecular interactions.

Spirothiopyran (STP) is particularly attractive when used as a mechanophore to endow polymers with both damage-signaling and self-reinforcing capacity. It is, however, not clear the actual force required to induce the cycloreversion of STP into ring-opened thiomerocyanine (TMC), which reacts spontaneously with activated C-C bonds. Here, we used atomic force microscopy (AFM)-based single molecule force spectroscopy (SMFS) to study the mechanochemistry of STP mechanophore. It is found that the ring-opening of STP at room temperature requires forces of ∼ 200–400 pN, depending on the pulling speed. In addition, the reversibility of STP to TMC isomerization is demonstrated. Finally, mechanochemically induced intermolecular Click addition is achieved in single molecule level by pulling STP in the presence of maleimide.

Incorporating hidden length into polymer chains can improve their mechanical properties, because release of the hidden length under mechanical loads enables localized strain relief without chain fracture. To date, the design of hidden length has focused primarily on the choice of the sacrificial bonds holding the hidden length together. Here we demonstrate the advantages of adding mechanochemical reactivity to hidden length itself, using a new mechanophore that integrates (Z)-2,3-diphenylcyclobutene-1,4-dicarboxylate, with hitherto unknown mechanochemistry, into macrocyclic cinnamate dimers. Stretching a polymer of this mechanophore more than doubles the chain contour length without fracture. DFT calculations indicate that the sequential dissociation of the dimer, followed by cyclobutene isomerization at higher forces yields a chain fracture energy 11 times that of a simple polyester of the same initial contour length and preserves high energy-dissipating capacity up to ∼3 nN. In sonicated solutions cyclobutene isomerizes to two distinct products by competing reaction paths, validating the computed mechanochemical mechanism and suggesting an experimental approach to quantifying the distribution of single-chain forces under diverse loading scenarios.

It is appealing to develop dynamic polymer systems with multifunctionl properties. Herein, we report a polyurethane elastomer with a dynamic covalent polymer network containing a radically exchangeable 2-arylindane-l,3-dione dimer as thermally sensitive and reversible cross-links. In addition, the carbolong complex, an excellent photothermal agent, is incorporated into the dynamic network backbone. With the irradiation of NIR light, the carbolong complex rapidly generates thermal energy, which subsequently triggers the cleavage of the dynamic covalent bond to generate radicals and activate the polyurethane network. In proof-of-concept experiments, we demonstrate that the utility of a combination of radically exchangeable covalent bond and carbolong moiety brings multiple functional characteristics to the polymer network with a capability of spatiotemporal control, including thermochromism, photochromism, rewritability, malleability, and self-healing. This study holds potentials for exploring more tunable dynamics and improved material properties.

Covalently linked π-stacked dimers represent the most significant platform for elucidating the relationship between molecular alignments and their properties. Here, we present the one-pot synthesis of two intramolecularly π-stacked dimers and disclose how intramolecular stacking modes dictate photoswitching properties. The dimer, which features cofacially stacked chromophores and geometrically favours intramolecular photochemical [2 + 2] cycloadditions, displays a nearly irreversible photoswitching behaviour. By contrast, the dimer, bearing crosswise stacked chromophores, is geometrically unfavourable for the cycloaddition and exhibits a highly reversible photoswitching process, in which the homolysis and reformation of carbon−carbon single bonds are involved. Moreover, the chiral carbon centres of both dimers endow these photoswitches with chirality and the separated enantiomers exhibit tuneable chiroptical properties by photoswitching. This work reveals that intramolecular stacking modes significantly influence the photochemical properties of π-stacked dimers and offers a design strategy toward chiral photoswitchable materials., Covalently bridged π-stacked dimers are excellent molecular platforms for understanding the relationship between stacking orientation and properties. Here, the authors synthesize a pair of π-stacked dimers that are aligned either cofacially or crosswise, allowing them to compare how the intramolecular stacking mode affects each dimer’s photoswitching properties.

Smart molecules that undergo orthogonal chemical transformations in response to force and other stimuli are highly attractive in materials science. Such moieties, when incorporated into polymer chains, allow fine tuning of material responses including mechanical properties to mechanical loads. Here we show that the unique mechanochemical and photochemical properties of the methanone tethered cinnamate dimer (MTCD) make it a promising candidate to achieve multimodal stress-responsive and mechanically adaptable polymeric materials. Stretching a polymer of the MTCD results in a force induced retro [2 + 2] cycloaddition and produces methanone-biscinnamate (MBC) moieties. This mechanochemical reaction simultaneously increases the fluorescence and releases a stored length of >6 A per MTCD, suggesting its potential in damage indication and material toughening. Moreover, on-demand strengthening/healing of the pristine and/or the mechanically degraded polymers can be achieved by the photochemical reaction of the MTCD and/or [2 + 2] cycloaddition of the mechanically generated MBC and this feature is orthogonal to the mechanochemical reaction of the MTCD.

Metallopolymers combine the functionality of metal centres with the processabilities of polymers, leading to rapidly expanding interest in the design and development of novel functional materials and devices. Carbolong complexes are a brand-new family of Mobius metalla-aromatics among which specific structures exhibit excellent photothermal properties under near infrared (NIR) light. Here, NIR photothermally healable carbolong polyurethanes (CLPUs) carrying covalently embedded 12C-carbolong motifs in the backbone are reported for the first time. CLPUs with a very low content of 12C-carbolong (down to 0.10 wt%) exhibit fast ( 5 cycles) healing whose efficiency can be tuned by the content of the carbolong complex and NIR power. The healing mechanism is ascribed to the accelerated dissociation of physical interactions, melting of polymer crystallites and chain diffusion induced by the NIR photothermal effect of carbolong embedded in the CLPUs. The fruitful chemistry and the unique photothermal properties of carbolong complexes make them versatile for the design of light-controlled and light-powered self-healing materials.

International audience; Multi-network elastomers are both stiff and tough by virtue of containing a pre-stretched stiff network that can rupture and dissipate energy under load. However, the rupture of this sacrificial network in all described covalent multi-network elastomers is irreversible. Herein, we describe the first example of multi-network elastomers with a reformable sacrificial network containing mechanochemically sensitive anthracene-dimer cross-links. These cross-links also make our elastomers mechanochromic, with coloration that is both persistent and reversible, because the fluorogenic moiety (anthracene dimer) is regenerated upon irradiation of the material. In proof-of-concept experiments we demonstrate the utility of incorporating anthracene dimers in the backbone of the sacrificial network for monitoring mechanochemical remodeling of multi-network elastomers under cycling mechanical load. Stretching or compressing these elastomers makes them fluorescent and irradiating them eliminates the fluorescence by regenerating anthracene dimers. Reformable mechanochromic cross-links, exemplified by anthracene dimers, hold potential for enabling detailed studies of the molecular origin of the unique mechanical properties of multi-network elastomers.

We demonstrate an intermolecular reaction cascade to control the force which triggers crosslinking of a mechanochromic polymer of spirothiopyran (STP). Mechanochromism arises from rapid reversible force‐sensitive isomerization of STP to a merocyanine, which reacts rapidly with activated C=C bonds. The concentration of such bonds, and hence the crosslinking rate, is controlled by force‐dependent dissociation of a Diels–Alder adduct of anthracene and maleimide. Because the adduct requires ca. 1 nN higher force to dissociate at the same rate as that of STP isomerization, the cascade limits crosslinking to overstressed regions of the material, which are at the highest rate of material damage. Using comb polymers decreased the minimum concentration of mechanophores required to crosslinking by about 100‐fold compared to previous examples of load‐strengthening materials. The approach described has potential for controlling a broad range of reaction sequences triggered by mechanical load., A mechanochemical reaction cascade for controlling load‐strengthening of a mechanochromic polymer is realized by using two comb polymers, each containing either spirothiopyran or a Diels–Alder adduct in the proximity of the branch points. The threshold force to trigger crosslinking is increased by ca. 1 nN and the minimum concentration of mechanophores required for crosslinking is decreased by 100‐fold compared to previous examples.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Two chiral tertiary carbon centers bearing one mesityl group at each center are introduced into the molecular backbone of imide-fused corannulene derivatives to produce four stereoisomers (i.e., (S, S), (R, R), (R, S), or (S, R) configurations on two chiral carbons) in one pot, which are separated into two portions through column chromatography over silica gel. Portion 1, containing a pair of enantiomers ((S, S) and (R, R)), adopts layered packing in the crystal. Portion 2, consisting of a pair of mesomers ((R, S) and (S, R)), exhibits columnar packing in their cocrystal. Theoretical calculations are performed on these two packing motifs, revealing that Portion 1 displays hole-dominated transport, whereas Portion 2 shows electron-dominated transport.

Mechanochemistry offers exciting opportunities for molecular-level engineering of stress-responsive properties of polymers. Reactive sites, sometimes called mechanophores, have been reported to increase the material toughness, to make the material mechanochromic or optically healable. Here we show that macrocyclic cinnamate dimers combine these productive stress-responsive modes. The highly thermally stable dimers dissociate on the sub-second timescale when subject to a stretching force of 1–2 nN (depending on isomer). Stretching a polymer of the dimers above this force more than doubles its contour length and increases the strain energy that the chain absorbs before fragmenting by at least 600 kcal per mole of monomer. The dissociation produces a chromophore and dimers are reformed upon irradiation, thus allowing optical healing of mechanically degraded parts of the material. The mechanochemical kinetics, single-chain extensibility, toughness and potentially optical properties of the dissociation products are tunable by synthetic modifications., Mechanochemistry offers exciting opportunities for molecular engineering of stress-responsive properties of polymers. Here the authors show that macrocyclic cinnamate dimers in a polymer chain can undergo dissociation on the sub-second timescale under 1–2 nN stretching to yield a chromophore that then can be optically healed.

The compositional- and time-dependent mechanical properties, recovery, and fracture toughness of hybrid gels having chemical and transient cross-links are systematically studied. The hybrid gels are prepared via micellar copolymerization of acrylamide (AAm) with n -Butyl acrylate (nBA) and the hydrophobic associations of nBA form the transient cross-links. The hybrid gels exhibited excellent mechanical properties (ultimate strength ∼ 400 kPa), fast recovery, and high tolerance to cracks (fracture toughness up to 378.0 J m − 2 ). Swelling, tensile, and pure shear measurements all revealed that the reinforcement is more pronounced when the mole fraction of nBA is above 0.05 and/or the stretch rate is above 0.1 s −1 . The compositional dependence is related to the onset of the formation of transient network while the time-dependent mechanical responses are ascribed to the dynamics of the hydrophobic associations. We also confirmed the separability of strain and time in hydrophobically reinforced hybrid gels for the first time. Our results emphasize the role of the transient network and its responses under external loading in the design of next-generation tough gels.

A pair of interconvertible stereoisomers of imide-fused corannulene derivatives was mixed with C

The last decade has witnessed a growing interest in the field of polymer mechanochemistry, where exogenous forces are utilized to trigger the chemical transformation of covalent and non-covalent bonds embedded in polymer chains. This chapter summarizes the effects of non-covalent interactions on the mechanochemical reactivity of covalent bonds, including the degradation of polymer chains, the unfolding of biomacromolecules, and the activation of mechanophores (mechanical sensitive groups). After a brief overview of contemporary polymer mechanochemistry, we will discuss in detail the effects of non-covalent interactions (i.e. hydrogen bonding, van der Waals and metal–ligand interactions) on polymer mechanochemistry, specifically the physical aspects of these interactions at different length scales, followed by discussions of stress-responsive materials. It is shown how the mechanochemical reactivity of covalent bonds is tuned by the incorporation of supramolecular motifs in both isolated polymer chains and bulk materials, and how the non-covalent interactions of oligomers – and hence the microscopic structures of polymers – are altered by mechanical force. We expect that this chapter will aid in the future development of polymer mechanochemistry, especially the design of advanced mechanophores and stress-responsive materials that utilize non-covalent interactions.

Mechanochemistry refers to the study of the evolution of the formation and disruption of chemical bonds upon application of an external force. In this chapter, the roles of mechanical forces in different biological systems are highlighted along with mechanisms and mechanotransduction pathways showing how complex biological systems can provide inspiration for materials design. Examples of how mechano-based systems have been mimicked by other scientists are also discussed including self-healing systems.

In this study, a simple and versatile approach to self-healing polymers and electrically conductive composites is reported. A series of self-healing polymers are readily synthesized by radical copolymerization of two acrylate monomers bearing a hydrogen-bonding motif. Subsequent blending with nanofillers leads to self-healing electrically conductive composites. Their glass transition temperature, the mechanical and electrical properties, and the self-healing capability can be readily tuned by the composition of the polymer as well as the filler fraction. The composite exhibits mechanical force sensing capabilities and high efficiency of both mechanical and electrical self-healing properties. Our design starts from simple chemistry and inexpensive materials, and may offer a new route towards economic self-healing electronic/electrical devices.

An azobenzene derivative (F-azo-COOH) was synthesized and could be E/Z isomerized by visible light. The host–guest interaction between F-azo-COOH and cyclodextrins (CDs) in alkaline aqueous solution was studied by NMR spectroscopy for the first time. The results revealed that F-azo-COOH did not form a stable host–guest complex with α-CD. However, both trans- and cis-F-azo-COOH could form stable 1 : 1 complexes with β-CD. Most interestingly, cis-F-azo-COOH could fit the cavity of β-CD more tightly (3.0 ± 0.3 × 103 M−1) than its trans form (2.1 ± 0.2 × 103 M−1), which was completely opposite to the conventional azobenzene/β-CD system.

For the first time, the corannulene unit was incorporated directly into the backbone of conjugated polymers. A new donor–acceptor (D–A) copolymer PICBT using imide-fused corannulene as acceptor was synthesized and its performance in organic field-effect transistors (OFETs) was tested. PICBT exhibited ambipolar transporting property with a hole mobility of 0.025 cm2 V−1 s−1 and electron mobility of 7.45 × 10−5 cm2 V−1 s−1 when the substrates were treated with octyltrimethoxysilane (OTS). If the substrates were not modified with OTS, PICBT showed lower device performances with a hole mobility of 4.62 × 10−3 cm2 V−1 s−1 and electron mobility of 1.54 × 10−4 cm2 V−1 s−1. The device performances are competitive among the amorphous materials. This work paved the way for incorporating the corannulene unit into conjugated materials.

We study the mechanical activation of spiropyran (SP) in a doubly cross-linked polyurethane elastomer. Besides chemical cross-linking, the elastomer comprises polytetrahydrofuran as soft segments and hydrogen-bonding 2-ureido-4-pyrimidone (UPy) as hard segments. The material shows two color changes because of the ring-opening reaction of SP to merocyanine (MC) at strained state and the isomerization about the methane bridge of MC at relaxed state. Increasing tensile strain rate leads to stiffer and stronger elastomer as well as earlier activation of SP. The activation point of SP to MC always coincides well with strain hardening of the stress–elongation curves. We further use the two-color transitions of SP to study the fracture of the elastomer during crack propagation.

In this study, we develop a series of new materials that can simultaneously and reversibly self-heal without external stimuli based on metallo-supramolecular interactions. Multiple tridentate 2,6-bis(1,2,3-trizaol-4-yl)pyridine (BTP) ligand units synthesized via a copper-catalyzed azide–alkyne cycloaddition (CuAAC) “click” reaction are incorporated into the polymer backbone of a ligand macromolecule through a thiol–ene “click” reaction. 3D transient supramolecular networks are formed from the ligand macromolecule upon coordination with transition and/or lanthanide metal ions. As compared to the ligand macromolecule, the resultant supramolecular films exhibit improved mechanical properties, such as Young's modulus, strength and toughness, which can be readily tuned by the stoichiometric ratio of Zn2+ to Eu3+ to Tb3+. The supramolecular films exhibit characteristics of weakly crosslinked networks where the storage modulus G′ and loss modulus G′′ scaled with normalized frequency ωaT by the same slope of 0.5. Both the supramolecular bulk films and gels are found to exhibit fast and effective self-healing properties by virtue of the kinetically labile nature of the metal–ligand interactions.

National Natural Science Foundation of China [21074103]; Fundamental Research Funds for the Central Universities [2010121018]; Scientific Research Foundation for Returned Scholars

Natural Science Foundation of China [21074103, 21174115]; Fundamental Research Funds for the Central Universities [2010121018]; Scientific Research Foundation for Returned Scholars

With the aim of designing novel polymeric materials that exhibit superb mechanical performance and at the same time can sense stress, the hydrogen bonding ureidopyrimidinone (UPy) and covalent mech...

National Natural Science Foundation of China [21074103, U1205113]; Fundamental Research Funds for the Central Universities [2010121018]; Special Program for Key Research of Chinese National Basic Research Program [2011CB612303]; Scientific Research Foundation for Returned Scholars

National Natural Science Foundation of China [21074103]; Natural Science Foundation of Fujian Province [2010J05033]; Fundamental Research Funds for the Central Universities [2010121018]

Incorporation of small reactive moieties, the reactivity of which depends on externally imposed load (so-called mechanophores) into polymer chains offers access to a broad range of stress-responsive materials. Here, we report that polymers incorporating spirothiopyran (STP) manifest both green mechanochromism and load-induced addition reactions in solution and solid. Stretching a macromolecule containing colorless STP converts it into green thiomerocyanine (TMC), the mechanically activated thiolate moiety of which undergoes rapid thiol-ene click reactions with certain reactive C=C bonds to form a graft or a cross-link. The unique dual mechanochemical response of STP makes it of potentially great utility both for the design of new stress-responsive materials and for fundamental studies in polymer physics, for example, the dynamics of physical and mechanochemical remodeling of loaded materials.

An elastomeric, healable, supramolecular polymer blend comprising a chain-folding polyimide and a telechelic polyurethane with pyrenyl end groups is compatibilized by aromatic pi-pi stacking between the pi-electron-deficient diimide groups and the pi-electron-rich pyrenyl units. This interpolymer interaction is the key to forming a tough, healable, elastomeric material. Variable-temperature FTIR analysis of the bulk material also conclusively demonstrates the presence of hydrogen bonding, which complements the pi-pi stacking interactions. Variable-temperature SAXS analysis shows that the healable polymeric blend has a nanophase-separated morphology and that the X-ray contrast between the two types of domain increases with increasing temperature, a feature that is repeatable over several heating and cooling cycles. A fractured sample of this material reproducibly regains more than 95% of the tensile modulus, 91% of the elongation to break, and 77% of the modulus of toughness of the pristine material.

Through the utilization of a mixed solvent system, consisting of a good solvent (DMSO) with either a nonsolvent (water) or a poor solvent (ethylene glycol), the morphology and properties of a class of stimuli-responsive metallo-supramolecular gels can be systematically tailored. These gels are formed via self-assembly of a ditopic ligand, consisting of a 2,6-bis(1′-methylbenzimidazolyl)-4-oxypyridine moiety attached to either end of a penta(ethylene glycol) core, in the presence of a transition metal ion (ZnII). In each solvent system, a composition window is located in which gels, varying in turbidity from highly opaque (water- or ethylene glycol-rich) to highly transparent (DMSO-rich), are formed. Morphological observations suggest gelation occurs by a common mechanism throughout the composition range, i.e., via the flocculation of semicrystalline colloidal particles. Increase of DMSO content leads to a reduction in the particle size, accompanied by an increase in sol concentration. WAXD and SAXS analys...

Complexation, between a ditopic ligand, consisting of a 2,6-bis-(1′-methylbenzimidazolyl)-4-oxypyridine moiety (O-Mebip) attached to either end of a penta(ethylene glycol) core, with transition metal and lanthanide ions, results in the formation of metallosupramolecular polymers, soluble in acetonitrile at high temperatures. Cooling the hot sol to room temperature causes phase separation and crystallization, and produces mechanically-strong gels, which exhibit a highly thixotropic behavior. Optical microscopy indicates that the gel morphology consists of spherulitic particles, which are easily broken by mechanical shear. Reproducible gel properties are produced when the gel is formed by cooling in a sonication bath, which produces a finely-divided globular morphology, and increases the modulus of the gels. Wide angle X-ray diffraction study shows that the crystalline structures of the gels are strongly dependent on the thermal history of gel formation and the nature of the metal ion. The gel properties are a result of the interactions between the colloidal particles produced by the phase separation and crystallization process. These interactions, which may reflect electrostatic forces and possibly metal–ligand binding, in addition to the usual van der Waals interactions, give rise to the formation of a network structure. The disruption of this network by mechanical shear, and its facile reformation when shear is removed, are the origin of the pronounced thixotropic behavior of the gels.

Although existing since the concept of macromolecules, polymer mechanochemistry is a burgeoning field which attracts great scientific interest in its ability to bias conventional reaction pathways and its potential to fabricate mechanoresponsive materials. We review here the effect of topology on the mechanical degradation of polymer chains and the activation of mechanophores in polymer backbones. The chapter focuses on both experimental and theoretical work carried out in the past 70 years. After a general introduction (Sect. 1), where the concept, the history, and the application of polymer mechanochemistry are briefly described, flow fields to study polymer mechanochemistry are discussed (Sect. 2), results of mechanochemistry study are presented for linear polymers (Sect. 3), cyclic polymers (Sect. 4), graft polymers (Sect. 5), star-shaped polymers (Sect. 6), hyperbranched polymers and dendrimers (Sect. 7), and systems with dynamic topology (Sect. 8). Here we focus on (1) experimental results involving the topological effect on the coil-to-stretch transition and the fracture of the polymer chains, (2) the underlying mechanisms and the key factor that determines the mechanical stability of the macromolecules, (3) theoretical models that relate to the experimental observations, and (4) rational design of mechanophores in complex topology to achieve multiple activations according to the existing results observed in chain degradation.

A three-dimensional conductive nanocomposite with an ordered conductive network and low percolation threshold has been successfully prepared by blending graphite nanosheets (GNs) with polyethylene on a two-roll mill. The conductive nanosheets orient intensively in the composite, leading to highly anisotropic properties. The nanocomposite with the fraction of conductive nanosheets closest to the percolation threshold possesses a sharp positive pressure coefficient of resistivity, in which the abrupt transition can be attributed to compressive-stress-induced deformation of the conductive network. Such piezoresistive effects depend strongly on filler morphology, filler spatial arrangement, and filler concentration.

In this work, we analyze the conductivity data of the nylon 6/FG nanocomposites using the normalized percolation equations and the general effective equation. From the interpretations of the derived results, we demonstrate that the microstructure of the nanocomposites can be readily deduced. Taking several factors into account, it turns out that the tunneling mechanism should be responsible for the observed non-universality of the critical exponents. Experimental evidences show that the existence of the tunneling conduction should be attributed to the particular structure of the prepared materials.

In this study, the nylon 6/foliated graphite (FG) electrically conducting nanocomposites with a low percolation threshold of less than 0.75 vol % have been prepared via an in situ polymerization approach in the presence of FG nanosheet filler. Based on laser counting, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction characterization techniques, the structures and morphologies of the nanoscale filling particles and the resulting nanocomposites were examined. Using percolation theory, the conductivity behavior of the nanocomposite samples were modeled and analyzed. Through the use of mean-field and excluded volume approaches, it was demonstrated that the experimentally observed percolation threshold values could be approximately estimated, and a correlation between the percolation threshold and the aspect ratio of FG particles could be quasi-quantitatively established. Also, preliminary studies on the effects of FG nanosheets on the thermal properties of the host nylon 6 were performed. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2844–2856, 2004

In this study, HDPE/expanded graphite electrically conducting composites were prepared in the presence of expanded graphite (EG) via a melt-blending process. The microstructure, electrical, mechanical and thermal properties of the resulting HDPE/EG composites were investigated. The experimental results revealed that EG could be dispersed in the form of nanosheets in the HDPE matrix. The composite exhibited a low percolation threshold ( c ) of about 10.0 wt%, due to the high aspect ratio (width-to-thickness) of graphite nanosheets. With increase of EG content, the conductivity increases, while the tensile strength, elongation at break and impact strength decrease. When EG content reaches 30.0 wt%, the conductivity is about 3.5 × 10-2 S/cm, the tensile strength, elongation at break and impact strength remain at 23.1 MPa, 76% and 5.7 KJ/m2, respectively.

Natural flake graphite was exfoliated into exfoliated graphite via an acid intercalation procedure. The resulting exfoliated graphite was a worm-like particle composed of graphite sheets with thickness in the nanometer scale. Subjecting it to ultrasonic irradiation, the exfoliated graphite was effectively further foliated into isolated graphite nanosheets. SEM, TEM, SAD, laser counting, and BET measurements revealed that the graphite nanosheets prepared with 10 h irradiation were about 52 nm in thickness and 13 μm in diameter. FTIR examination showed that there were oxygen-containing groups presented on the surface of the exfoliated graphite. This result substantiated the statement reported in the literature that acid treatment could result in oxidization of carbon bonds on graphite surface.

This article studies the crystallization kinetics and melting behaviors of nylon 6/foliated graphite electrically conducting nanocomposites. The crystallization kinetics under isothermal conditions is described by the Avrami equation. The values of the exponent found for the neat nylon 6 and the nanocomposite samples are about 3.0 and 1.2, respectively. For non-isothermal studies, the Avrami equation modified by Jeziorny, the Ozawa theory, the modified Ozawa equation and an equation combining the Avrami and Ozawa equation are employed. It is found that the Ozawa analysis fails to provide an adequate description of the non-isothermal crystallization process in both the neat nylon 6 and the nanocomposite samples. While the combination of Avrami and Ozawa equations exhibit great advantages in treating the non-isothermal crystallization kinetics. The activation energies are also determined by the Arrhenius relation and the Kissinger method for isothermal and non-isothermal crystallization, respectively. In both cases, the activation energy for the nanocomposite sample is greater than that for the neat nylon 6. In addition, subsequent DSC scans have also been performed to investigate the melting behaviors of the isothermally and non-isothermally crystallized samples. Results reveal that the isothermally crystallized samples exhibit two melting endotherms, while only the neat nylon 6 sample shows two melting endotherms after non-isothermal crystallization. The equilibrium melting temperature obtained is lower for the nanocomposite sample than for the neat nylon 6.

Graphite nanosheets (NanoG) were prepared by treating the expanded graphite with sonication in aqueous alcohol solution. Nanocomposites of poly(methyl methacrylate) (PMMA) with NanoG were prepared via an in situ polymerization of MMA in the presence of NanoG with the aid of sonication. The nanocomposites were then dispersed with chloroform (CHCl3) and casted on glass slides to form conducting films. The percolation threshold of PMMA/NanoG conducting films at room temperature was as low as 0.31 vol%, much lower than that of the composites filled with conventional graphite particles. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area diffraction (SAD) and etc. were used to characterize the structure of the graphite nanosheets and the nanocomposites. Results showed that the high-aspect-ratio structure of graphite nanosheets played an important role in forming conducting network in PMMA matrix. The conducting behavior of the composite was interpreted by percolation theory.

Exfoliation of graphite was performed in ammonia medium via an electrochemically oxidizing method. Based on dynamic light scattering (DLS), transmission electron microscopy (TEM), selected area diffraction (SAD), X-ray diffraction (XRD), high resolution transmission electron microscopy (HREM), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) measurements and elemental analysis, it was found that during electrolysis process graphite was exfoliated into particles of about 40 nm in diameter. These nanoparticles consist of multilayer and monolayer structures. The carbon sheets in these particles contain oxygen-containing groups derived from oxidization of graphite. Due to the presence of these polar groups, the nanoparticles can be readily dispersed in water and form a stable colloid.

A new process for the dispersion of graphite in the form of nanosheets in a polymer matrix was developed via in situ polymerization of monomer at the presence of sonicated expanded graphite during sonication. Graphite nanosheets prepared via powdering the expanded graphite had a thickness ranging 30–80 nm and a diameter ranging 0.5–20 μm and was an excellent nanofiller for the fabrication of polymer/graphite conducting nanocomposite. The process fabricated electrically conducting polystyrene/graphite nanosheet nanocomposite films with much lower percolation threshold and much higher conductivities than those of composites made by conventional methods.

The nonlinear conduction behavior of composite materials of foliated graphite nanosheets and nylon-6 subjected to a variable direct-current electric field has been studied. Corresponding to the onset of the nonlinearity, there is a crossover current density/electric field (or current–voltage) couple. The current density or current decreases as the foliated graphite concentration decreases. Through discussions of the nonlinearities within the frameworks of the two theoretical models, the nonlinear random resistor network and the dynamic random resistor network, it is shown that neither of these models can explain fully the results obtained for this system. On the basis of the microscopic structures and conduction processes of the nanocomposites, it is found that a combination of the models can generally account for the nonlinear characteristics. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 155–167, 2004

Although existing since the concept of macromolecules, polymer mechanochemistry is a burgeoning field which attracts great scientific interest in its ability to bias conventional reaction pathways and its potential to fabricate mechanoresponsive materials. We review here the effect of topology on the mechanical degradation of polymer chains and the activation of mechanophores in polymer backbones. The chapter focuses on both experimental and theoretical work carried out in the past 70 years. After a general introduction (Sect. 1), where the concept, the history, and the application of polymer mechanochemistry are briefly described, flow fields to study polymer mechanochemistry are discussed (Sect. 2), results of mechanochemistry study are presented for linear polymers (Sect. 3), cyclic polymers (Sect. 4), graft polymers (Sect. 5), star-shaped polymers (Sect. 6), hyperbranched polymers and dendrimers (Sect. 7), and systems with dynamic topology (Sect. 8). Here we focus on (1) experimental results involving the topological effect on the coil-to-stretch transition and the fracture of the polymer chains, (2) the underlying mechanisms and the key factor that determines the mechanical stability of the macromolecules, (3) theoretical models that relate to the experimental observations, and (4) rational design of mechanophores in complex topology to achieve multiple activations according to the existing results observed in chain degradation.

Poly(methyl methacrylate)(PMMA)/expanded graphite composite has been made via an in situ polymerization of methyl methacrylate(MMA) in the presence of expanded graphite obtained by rapid heating of the graphite intercalation compound (GIC). The composite was then blended with poly(vinyl chloride) (PVC) to form an electrically conducting composite. SEM, TEM and XRD showed that the graphite had been dispersed throughout the polymer matrix in the form of nanosheets with thicknesses of about 20 nm. The resulting composite showed excellent electrical conductivity despite a low concentration of graphite. The transition from an electrical insulator to an electrical semiconductor for the composite occurred when the graphite content was 3.5 wt%, much lower than that of conventional conducting polymer composites. Conductivity reached a maximum of 10 -4 s/cm at a graphite concentration of 5.0 wt%. This improvement of conductivity could be attributed to the high aspect ratio (width-to-thickness) of the graphite nanosheets dispersed in the polymer matrix.

An in situ polymerization was conducted in the presence of expanded graphite obtained by rapid heating of the graphite intercalation compound (GIC) to form a polymer/expanded graphite conducting composite. Study showed that the graphite was dispersed in the form of nanosheets in the polymer matrix. The transition from an electrical insulator to an electrical semiconductor for the composite occurred when the expanded graphite content was 1.8 wt %, which was much lower than that of conventional conducting polymer composite. The composite exhibited high electrical conductivity of 10−2 S/cm when the graphite content was 3.0 wt %. This great improvement of conductivity could be attributed to the high aspect ratio (width-to-thickness) of the graphite nanosheets. Study suggested that extensive rolling of the blend greatly affected the conductivity of the composite. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2506–2513, 2001