Fabrication of Highly Thermally Resistant and Self-Healing Polysiloxane Elastomers by Constructing Covalent and Reversible Networks.

The fabrication of self-healing elastomers with high thermal stability for use in extreme thermal conditions such as aerospace remains a major challenge. A strategy for preparing self-healing elastomers with stable covalent bonds and dynamic metal-ligand coordination interactions as crosslinking sites in polydimethylsiloxane (PDMS) is proposed. The added Fe (III) not only serves as the dynamic crosslinking point at room temperature which is crucial for self-healing performance, but also plays a role as free radical scavenging agent at high temperatures. The results show that the PDMS elastomers possessed an initial thermal degradation temperature over 380 °C and a room temperature self-healing efficiency as high as 65.7%. Moreover, the char residue at 800 °C of PDMS elastomer reaches 7.19% in nitrogen atmosphere, and up to 14.02% in air atmosphere by doping a small amount (i.e., 0.3 wt%) of Fe (III), which is remarkable for the self-healing elastomers that contain weak and dynamic bonds with relatively poor thermal stability. This study provides an insight into designing self-healing PDMS-based materials that can be targeted for use as high-temperature thermal protection coatings.

Fabrication of Lightweight Polyimide Foams with Exceptional Mechanical and Thermal Properties.

Lightweight polyimide foams (PIFs) with exceptional thermal resistance and compressive properties are fabricated by heating polyester ammonium salts (PEASs) which are prepared by copolymerizing 4, 4'-diaminobenzanilide (DABA), 4, 4'-diaminodiphenyl methane (MDA) and 3, 3', 4, 4'-benzophenone tetracarboxylic dianhydride (BTDA). Hydrogen bonds are formed between CONH and CO in the PI chains due to the addition of DABA and the melt viscosity of PEAS precursors increase with increasing content of DABA, which is advantageous to bind the foaming gases for cell expansion. The expansion ratio of PEAS precursors is increased from 633% to 1133% when the molar ratio of MDA/DABA is changed from 10:0 to 6:4. The compressive strength and modulus of PIFM9D1 (i.e., the molar ratio of MDA/DABA is 9:1, foam density: 120.8 kg m-3 ) reach as high as 0.59 and 15.0 MPa, respectively. The PIFs possess prominent thermal performance with the initial thermal degradation temperatures (under both nitrogen and air atmosphere) and glass transition temperatures (as assessed by DSC and DMA) exceeding 511 and 292 °C, respectively. The thermal conductivity of PIFs is lower than 0.049 W m-1 K-1 , which exhibits promising applications for serving as high-temperature thermal insulation materials in the fields of aerospace, marine, and nuclear sectors among others.

Injection Molding of Polymers and Polymer Composites.

Injection molding technology has been widely adopted to fabricate multifunctional polymeric components or structural parts for applications in fields such as automotives, electronics, packaging, aerospace, and many others [...].

Improving the Ablation Properties of Liquid Silicone Rubber Composites by Incorporating Hexaphenoxycyclotriphosphonitrile.

The ablative properties of epoxy-modified vinyl silicone rubber (EMVSR) composites containing hexaphenoxycyclotriphosphonitrile (HPCTP) have been systematically studied. The strength of the ablation char layer was greatly enhanced with the addition of HPCTP, which induced the formation of a more complete, denser, and thicker char during oxyacetylene ablation tests. Moreover, the HPCTP-containing EMVSR composites demonstrated lower thermal conductivity and pyrolysis rate when compared with those without HPTCP. At the same time, the thermal insulation properties of HPCTP-filled composites were improved under low heat flow ablation scenarios. The reduction of graphitic carbon content, the formation of phosphate-like crystals as well as the increase of SiC content contributed to strengthening the char layer, which was critical for improving the ablation properties. The optimum char layer strength and thermal insulation properties were achieved when the content of HPCTP was 15 phr, whereas an optimum ablation resistance was achieved at 25 phr HPCTP. This suggests that HPCTP-modified EMVSR composites can be used for thermal protection purposes, especially in the fields of aerospace and aeronautics.

Group II/III metabotropic glutamate receptors exert endogenous activity-dependent modulation of TRPV1 receptors on peripheral nociceptors.

There is pharmacological evidence that group II and III metabotropic glutamate receptors (mGluRs) function as activity-dependent autoreceptors, inhibiting transmission in supraspinal sites. These receptors are expressed by peripheral nociceptors. We investigated whether mGluRs function as activity-dependent autoreceptors inhibiting pain transmission to the rat CNS, particularly transient receptor potential vanilloid 1 (TRPV1)-induced activity. Blocking peripheral mGluR activity by intraplantar injection of antagonists LY341495 [(2S)-2-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl) propanoic acid] (LY) (20, 100 µm, group II/III), APICA [(RS)-1-amino-5-phosphonoindan-1-carboxylic acid] (100 µm, group II), or UBP1112 (α-methyl-3-methyl-4-phosphonophenylglycine) (30 µm, group III) increased capsaicin (CAP)-induced nociceptive behaviors and nociceptor activity. In contrast, group II agonist APDC [(2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate] (0.1 µm) or group III agonist l-(+)-2-amino-4-phosphonobutyric acid (l-AP-4) (10 µm) blocked the LY-induced increase. Ca(2+) imaging in dorsal root ganglion (DRG) cells confirmed LY enhanced CAP-induced Ca(2+) mobilization, which was blocked by APDC and l-AP-4. We hypothesized that excess glutamate (GLU) released by high intensity and/or prolonged stimulation endogenously activated group II/III, dampening nociceptor activation. In support of this, intraplantar GLU + LY produced heat hyperalgesia, and exogenous GLU + LY applied to nociceptors produced enhanced nociceptor activity and thermal sensitization. Intraplantar Formalin, known to elevate extracellular GLU, enhanced pain behaviors in the presence of LY. LY alone produced no pain behaviors, no change in nociceptor discharge rate or heat-evoked responses, and no change in cytosolic Ca(2+) in DRG cells, demonstrating a lack of tonic inhibitory control. Group II/III mGluRs maintain an activity-dependent autoinhibition, capable of significantly reducing TRPV1-induced activity. They are endogenously activated after high-frequency and/or prolonged nociceptor stimulation, acting as built-in negative modulators of TRPV1 and nociceptor function, reducing pain transmission to the CNS.

Improving Thermal Conductivity of Injection Molded Polycarbonate/Boron Nitride Composites by Incorporating Spherical Alumina Particles: The Influence of Alumina Particle Size.

In this work, the influences of alumina (Al2O3) particle size and loading concentration on the properties of injection molded polycarbonate (PC)/boron nitride (BN)/Al2O3 composites were systematically studied. Results indicated that both in-plane and through-plane thermal conductivity of the ternary composites were significantly improved with the addition of spherical Al2O3 particles. In addition, the thermal conductivity of polymer composites increased significantly with increasing Al2O3 concentration and particle size, which were related to the following factors: (1) the presence of spherical Al2O3 particles altered the orientation state of flaky BN fillers that were in close proximity to Al2O3 particles (as confirmed by SEM observations and XRD analysis), which was believed crucial to improving the through-plane thermal conductivity of injection molded samples; (2) the presence of Al2O3 particles increased the filler packing density by bridging the uniformly distributed BN fillers within PC substrate, thereby leading to a significant enhancement of thermal conductivity. The in-plane and through-plane thermal conductivity of PC/50 µm-Al2O3 40 wt%/BN 20 wt% composites reached as high as 2.95 and 1.78 W/mK, which were 1183% and 710% higher than those of pure PC, respectively. The prepared polymer composites exhibited reasonable mechanical performance, and excellent electrical insulation properties and processability, which showed potential applications in advanced engineering fields that require both thermal conduction and electrical insulation properties.

Anisotropic and Lightweight Carbon/Graphene Composite Aerogels for Efficient Thermal Insulation and Electromagnetic Interference Shielding.

High-performance and lightweight carbon aerogels (CAs) have attracted considerable attention in various fields such as electrochemistry, catalysis, adsorption, energy storage, and so on. However, finding an environmentally friendly and efficient preparation method and achieving a controllable performance of CAs are still a challenge. Herein, a series of anisotropic carbon/graphene composite aerogels were synthesized by unidirectional freezing of polyamic acid ammonium salt/graphene oxide (PAS/GO) suspension followed by lyophilization, thermal imidization, and carbonization. The prepared aerogels presented a tubular pore structure oriented along the freezing direction. The GO dispersed in the polymer matrix reinforced the skeleton of aerogels, which significantly inhibited the volume shrinkage during the preparation process, thus giving low densities of 0.074-0.185 g cm-3. In addition, the oriented pore structure endowed the composite aerogels with obviously anisotropic heat insulation performance. The radial thermal conductivity was as low as 0.038 W m-1 K-1 at the density of 0.074 g cm-3. When the initial content of GO rose to 20 phr, the resultant aerogels exhibited a high electrical conductivity of about 0.77 S cm-1 in the radial direction and the electromagnetic interference shielding effectiveness (EMI SE) reached 54.6 dB at the same time. Therefore, this study provided a facile and environmentally friendly method to prepare lightweight and anisotropic carbon aerogels.

Improving the Heat and Ablation Resistance of Silicone Rubber Composites by Incorporating Hollow Microspheres.

For thermal protection materials (TPMs) which are used to protect space vehicles from extreme thermomechanical environments, the thermal conductivity of the original material and the char layer that has formed during ablation plays a significant role in determining the ablation performance. In order to investigate this, in this study, we introduced glass hollow microspheres (GHMs), phenolic hollow microspheres (PHMs), and acrylonitrile-methyl methacrylate copolymer hollow microspheres (AMHMs) into silicone rubber (SR), and the ablation performance of these composites was systematically studied. The thermogravimetric results showed that the residue yield of the SR composites was increased with the incorporation of the hollow microspheres. Compared to the SR composites without the hollow microspheres, the residue weight values under 800 °C (R800) of the SR composites with the 30 parts of fumed silica per hundred of the SR (phr) addition of GHMs, PHMs, and AMHMs were increased from 10.11% to 21.70%, 18.31%, and 20.83%, respectively. The ablation tests showed that the addition of the AMHMs enhanced the ablation performance of the SR composites because the linear ablation rates and the backplane temperature were clearly decreased when compared to the SR composites without the hollow microspheres. This work provides an effective and potential method for preparing thermal protection materials with an improved ablation performance.

Carbonization of Graphene-Doped Isocyanate-Based Polyimide Foams to Achieve Carbon Foams with Excellent Electromagnetic Interference Shielding Performance.

Lightweight carbon foams with excellent electromagnetic interference (EMI) shielding performance were prepared by carbonization process, using isocyanate-based polyimide foams as carbon precursors. The influence of carbonization temperature and graphene-doping on the morphological, electrical and EMI shielding effectiveness (SE) of corresponding carbon foams was studied in detail. Results showed that the addition of graphene was beneficial to the improvement of electrical conductivity and EMI shielding performance of carbon foams. The electrical conductivity of carbon foams increased with the carbonization temperature which was related to the increase of graphitization degree. Collapse of foam cells was observed at higher carbonization temperatures, which was detrimental to the overall EMI SE. The optimal carbonization temperature was found at 1100 °C and the carbon foams obtained from 0.5 wt% graphene-doped foams exhibited a specific EMI SE of 2886 dB/(g/cm3), which shows potential applications in fields such as aerospace, aeronautics and electronics.

Crystallization and Microstructure Evolution of Microinjection Molded Isotactic Polypropylene with the Assistance of Poly(Ethylene Terephthalate).

In this work, a series of isotactic polypropylene/poly(ethylene terephthalate) (iPP/PET) samples were prepared by microinjection molding (µIM) and mini-injection molding (IM). The properties of the samples were investigated in detail by differential scanning calorimetry (DSC), Wide-Angle X-ray Diffraction (WAXD), Polarized light microscope (PLM) and scanning electron microscopy (SEM). Results showed that the difference in thermomechanical history between both processing methods leads to the formation of different microstructures in corresponding iPP/PET moldings. For example, the dispersed spherical PET phase deforms and emerges into continuous in-situ microfibrils due to the intensive shearing flow field and temperature field in µIM. Additionally, the incorporation of PET facilitates both the laminar branching and the reservation of oriented molecular chains, thereby leading to forming a typical hybrid structure (i.e., fan-shaped ß-crystals and transcrystalline). Furthermore, more compact and higher degrees of oriented structure can be obtained via increasing the content of PET. Such hybrid structure leads to a remarkable enhancement of mechanical property in terms of µIM samples.

Effect of Hybrid Carbon Fillers on the Electrical and Morphological Properties of Polystyrene Nanocomposites in Microinjection Molding.

The effect of hybrid carbon fillers of multi-walled carbon nanotubes (CNT) and carbon black (CB) on the electrical and morphological properties of polystyrene (PS) nanocomposites were systematically investigated in microinjection molding (µIM). The polymer nanocomposites with three different filler concentrations (i.e., 3, 5 and 10 wt %) at various weight ratios of CNT/CB (100/0, 30/70, 50/50, 70/30, 0/100) were prepared by melt blending, then followed by µIM under a defined set of processing conditions. A rectangular mold insert which has three consecutive zones with decreasing thickness along the flow direction was adopted to study abrupt changes in mold geometry on the properties of resultant microparts. The distribution of carbon fillers within microparts was observed by scanning electron microscopy, which was correlated with electrical conductivity measurements. Results indicated that there is a flow-induced orientation of incorporated carbon fillers and this orientation increased with increasing shearing effect along the flow direction. High structure CB is found to be more effective than CNT in terms of enhancing the electrical conductivity, which was attributed to the good dispersion of CB in PS and their ability to form conductive networks via self-assembly. Morphology observations indicated that there is a shear-induced depletion of CB particles in the shear layer, which is due to the marked difference of shear rates between the shear and core layers of the molded microparts. Moreover, an annealing treatment is beneficial to enhance the electrical conductivity of CNT-containing microparts.

Mechanism by which peripheral galanin increases acute inflammatory pain.

Galanin (GAL) is a neuropeptide involved in pain transmission. Intraplantar GAL at low doses enhances capsaicin (CAP)-induced pain behaviors in rat, suggesting an excitatory role for GAL under acute inflammatory conditions. The mechanisms underlying this pro-nociceptive action have not yet been elucidated. Thus, the present study investigated the role of protein kinase C (PKC) in the GAL enhancement of CAP-induced inflammatory pain. Ipsilateral, but not contralateral, calphostin C, a PKC inhibitor, blocked GAL-induced potentiation of CAP-evoked inflammatory pain in a dose-dependent fashion. Peripheral activation of PKC using the phorbol ester phorbol-12-myristate-13-acetate (PMA) mimicked the pro-nociceptive effect of GAL. These results suggest that GAL enhances acute inflammatory pain through activation of PKC intracellular pathways.

Somatostatin modulates the transient receptor potential vanilloid 1 (TRPV1) ion channel.

Activation of peripheral somatostatin receptors (SSTRs) inhibits sensitization of nociceptors, thus having a short term or phasic effect [Pain 90 (2001) 233] as well as maintaining a tonic inhibitory control over nociceptors [J Neurosci 21 (2001) 4042]. The present study provides several lines of evidence that an important mechanism underlying SSTR modulation of nociceptors is regulation of the transient receptor potential vanilloid 1 ion channel (TRPV1, formerly the VR1 receptor). Double labeling of L5 dorsal root ganglion cells demonstrates that approximately 60% of SSTR2a-labeled cells are positive for TRPV1. Conversely, approximately 33% of TRPV1-labeled cells are positive for SSTR2a. In vivo behavioral studies demonstrate that intraplantar injection of 20.0 but not 2.0 microM octreotide (OCT, SSTR agonist) significantly reduces capsaicin (CAP, a ligand for TRPV1) -induced flinching and lifting/licking behaviors. This occurs through local activation of SSTRs in the injected hindpaw and is reversed following co-application of the SSTR antagonist cyclo-somatostatin (c-SOM). In vitro studies using a skin-nerve preparation demonstrate that activation of peripheral SSTRs on nociceptors with 20.0 microM OCT significantly reduces CAP-induced activity and can prevent CAP-induced desensitization. Furthermore, blockade of peripheral SSTRs with c-SOM dramatically enhances CAP-induced behaviors and nociceptor activity, demonstrating SSTR-induced tonic inhibitory modulation of TRPV1. Finally, TRPV1 does not appear to be under tonic opioid receptor control since the opioid antagonist naloxone does not change CAP-induced excitation and does not effect OCT-induced inhibition of CAP responses. These data strongly suggest that SSTRs modulate nociceptors through phasic and tonic regulation of peripheral TRPV1 receptors.

Somatostatin receptors on peripheral primary afferent terminals: inhibition of sensitized nociceptors.

Somatostatin (SST) is in primary afferent neurons and reduces vascular and nociceptive components of inflammation. SST receptor (SSTR) agonists provide analgesia following intrathecal or epidural administration in humans, but neurotoxicity in the central nervous system (CNS) has been reported in experimental animals. With the rationale that targeting peripheral SSTRs would provide effective analgesia while avoiding CNS side effects, the goals of the present study are to investigate the presence of SSTRs on peripheral primary afferent fibers and determine the behavioral and physiological effects of the SST agonist octreotide (OCT) on formalin-induced nociception and bradykinin-induced primary afferent excitation and sensitization in the rat. The results demonstrate that: (1) SSTR2as are present on 11% of peripheral primary afferent sensory fibers in rat glabrous skin; (2) intraplantar injection of OCT reduces formalin-induced nociceptive behaviors; (3) OCT reduces, in a dose-dependent fashion, responses to thermal stimulation in C-mechanoheat sensitive fibers; and (4) OCT reduces the responses of C-mechanoheat fibers to bradykinin-induced excitation and sensitization to heat. Each of these actions can be reversed following co-injection of OCT with the SSTR antagonist cyclo-somatostatin (c-SOM). Thus, activation of peripheral SSTRs reduces both inflammatory pain and the activity of sensitized nociceptors, avoids deleterious CNS side effects and may be clinically useful in the treatment of pain of peripheral origin.

Pro-nociceptive role of peripheral galanin in inflammatory pain.

We investigated the peripheral function of galanin (GAL) in capsaicin (CAP)-induced inflammatory pain. Intraplantar GAL (0.1 ng/microl) alone does not produce nociceptive behaviors. However, ipsilateral but not contralateral GAL at low doses (0.1 ng/microl) significantly increases CAP-evoked nociceptive behaviors approximately twofold. This effect is attributed to activation of peripheral GAL receptor 2 (GalR2) because a selective GalR2 agonist (AR-M1896) mimics the pro-nociceptive actions of GAL. Recording from nociceptors confirms that GAL does not modify activity of nociceptors but markedly enhances CAP-induced excitation of these fibers. CAP produces a discharge rate of 0.15+/-0.05 impulses/s which increases to 0.54+/-0.17 impulses/s following CAP+GAL. Immunohistochemical studies indicate GalR2 are highly expressed (65.8%) in L5 dorsal root ganglion (DRG) cells. Also, 44.5% GalR2-positive DRG neurons label for the capsaicin receptor (vanilloid receptor 1, VR1) while 61.7% of VR1-positive DRG neurons label for GalR2; 28.1% of total DRG neurons are double-labeled supporting the hypothesis that GAL-induced effects are mediated by GalR2 on capsaicin-sensitive primary afferents. Furthermore, 68.0% unmyelinated and 23.1% myelinated digital nerve axons label for GalR2, indicating the receptor is transported out to the periphery. Immunostaining for GAL peptide in digital nerves labels 46.4% unmyelinated and 27.1% myelinated axons, suggesting that afferents are a major source of ligand for peripheral GalR2. These results suggest that peripheral GAL has an excitatory role in inflammatory pain, likely mediated by peripheral GalR2 and that GAL can modulate VR1 function.

Pain after discontinuation of morphine treatment is associated with synaptic increase of GluA4-containing AMPAR in the dorsal horn of the spinal cord.

Withdrawal from prescribed opioids results in increased pain sensitivity, which prolongs the treatment. This pain sensitivity is attributed to neuroplastic changes that converge at the spinal cord dorsal horn. We have recently reported that repeated morphine administration triggers an insertion of GluA2-lacking (Ca(2+)-permeable) α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPAR) in the hippocampus. This finding together with the reported involvement of AMPAR in the mechanisms underlying inflammatory pain led us to hypothesize a role for spinal AMPAR in opioid-induced pain behavior. Mice treated with escalating doses of morphine showed hypersensitivity to mechanical stimulation. Intrathecal administration of a Ca(2+)-permeable AMPAR selective blocker disrupted morphine-induced mechanical sensitivity. Analysis of the expression and phosphorylation levels of AMPAR subunits (GluA1/2/3/4) in homogenates and in postsynaptic density fractions from spinal cord dorsal horns showed an increase in GluA4 expression and phosphorylation in the postsynaptic density after morphine. Co-immunoprecipitation analyses suggested an increase in GluA4 homomers (Ca(2+)-permeable AMPAR) and immunohistochemical staining localized the increase in GluA4 levels in laminae III-V. The excitatory postsynaptic currents (EPSCs) recorded in laminae III-V showed enhanced sensitivity to Ca(2+)-permeable AMPAR blockers in morphine-treated mice. Furthermore, current-voltage relationships of AMPAR-mediated EPSCs showed that rectification index (an indicator of Ca(2+)-permeable AMPAR contribution) is increased in morphine-treated but not in saline-treated mice. These effects could be reversed by infusion of GluA4 antibody through patch pipette. This is the first direct evidence for a role of GluA4-containing AMPAR in morphine-induced pain and highlights spinal GluA4-containing AMPAR as targets to prevent the morphine-induced pain sensitivity.

Group II metabotropic glutamate receptor activation on peripheral nociceptors modulates TRPV1 function.

Transient receptor potential vanilloid 1 (TRPV1) receptors are critical to nociceptive processing. Understanding how these receptors are modulated gives insight to potential therapies for pain. We demonstrate using double labeling immunohistochemistry that Group II metabotropic glutamate receptors (mGluRs) are co-expressed with TRPV1 on rat dorsal root ganglion (DRG) cells. In behavioral studies, intraplantar 0.1 microM APDC, a group II agonist, significantly attenuates capsaicin-induced nociceptive behaviors through a local effect. The APDC-induced inhibition of capsaicin responses is blocked by 1 microM LY341495, a group II antagonist. At the single fiber level, nociceptor responses to capsaicin are significantly decreased following exposure to APDC and this effect is blocked by LY341495. Finally, activation of peripheral group II mGluRs inhibits forskolin-induced thermal hyperalgesia and nociceptor heat sensitization, suggesting group II receptors are negatively coupled to the cAMP/PKA pathway. The data indicate that group II mGluRs and TRPV1 receptors are co-expressed on peripheral nociceptors and activation of mGluRs can inhibit painful sensory transmission following TRPV1 activation. The data are consistent with group II and TRPV1 receptors being linked intracellularly by the cAMP/PKA pathway. Peripheral group II mGluRs are important targets for drug discovery in controlling TRPV1-induced nociception.