Combustion and thermal properties of epoxy/phenyltrisilanol polyhedral oligomeric silsesquioxane nanocomposites

Organic–inorganic hybrid composites of epoxy and phenyltrisilanol polyhedral oligomeric silsesquioxane (Ph₇Si₇O₉(OH)₃, POSS-triol) were prepared via in situ polymerization of epoxy monomers. The nanocomposites of epoxy with POSS-triol can be prepared in the presence of metal complex latent catalyst, aluminum triacetylacetonate ([Al]) for the reaction between POSS-triol and diglycidyl ether of bisphenol A (DGEBA). The dispersion morphology of organic–inorganic hybrid was characterized by scanning electronic microscopy (SEM). The thermostability of composites was evaluated by thermal gravimetric (TG) analysis. The flammability was evaluated by cone calorimeter test. The presence of [Al] latent catalyst leads to a decrease in combustion rate with respect to epoxy and epoxy/POSS composites as well as reduction in smoke, CO and CO₂ production rate. The effect of [Al] is to reduce the size of spherical POSS particles from 3–5 μm in epoxy/POSS to 0.5 μm in epoxy/POSS[Al]. Furthermore, POSS with smaller size may form compact and continue char layer on the surface of composites more efficiently.     

Polypropylene-POSS Nanocomposites: Morphology and Crystallization Behaviour

Nanocomposites of isotactic polypropylene (PP) with polyhedral oligomeric silsesquioxanes (POSS) [RSiO_1,5]₈ having different alkyl substituents (R = methyl, isobutyl, isooctyl) were obtained by melt blending and analysed with electron microscopy, optical microscopy and DSC calorimetry. The influence of POSS structure on the morphological characteristics, the crystallization and melting behaviour of PP/POSS composites was investigated with varying the filler amount. The crystallization kinetics of the composites from the melt, examined both in isothermal and non-isothermal conditions, demonstrated that the nucleation activity of the examined POSS can be related to the length of alkyl substituents which, depending on the loading amount, affect the filler dispersion in the PP matrix and the growth of polymer crystals.     

Effect of Polyhedral Oligomeric Silsesquioxane and Sorbitol on Properties of Isotactic Polypropylene

Isotactic polypropylene(iPP) was modified by the introduction of polyhedral oligomeric silsesquio- xanes(POSS) and 1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol(DMDBS). Chemical combination of (3-mercapto)- propyl-heptaisobutyl POSS with DMDBS(POSS-DMDBS), and physical mixing of DMDBS with octaisobutyl POSS (iso-POSS/DMDBS) or trisilanolisobutyl POSS(tri-POSS/DMDBS) were applied respectively to modifying iPP, and the effects of POSS and DMDBS on crystallization, rheological and mechanical properties of iPP were systematically investigated. The results indicate that iso-POSS/DMDBS and tri-POSS/DMDBS were more effective than POSS-DMDBS on the improvement of the crystallization behavior of iPP due to the higher crystallization temperature, while the crystallinity of iPP containing POSS-DMDBS was enhanced, approximately approached to that of iPP containing tri-POSS/DMDBS. The tensile strength of iPP with POSS-DMDBS was significantly increased from 34 MPa to 40 MPa, as high as that of iPP with iso-POSS/DMDBS. The different effects caused by the specific interaction between POSS and DMDBS could possibly be applied in the modification of iPP.     

Isothermal crystallization kinetics of polypropylene/POSS composites

Polypropylene (PP)/octavinyl polyhedral oligomeric silsesquioxane (POSS) composites were prepared by two different processing methods: reactive blending and physical blending, and the crystallization behavior of PP and PP/POSS composites was studied by means of differential scanning calorimetry and polarized optical microscope. The results showed that the crystallization of PP in PP/POSS composites was strongly influenced by the different processing methods. POSS particles can act as effective nucleating agent, accelerating the crystallization of PP. The crystallization rate increased more dramatically for the reactive blending composite due to the stronger nucleating effect of PP grafted POSS. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1762–1772, 2008     

Preparation and properties of POSS grafted polypropylene by reactive blending

The octavinyl polyhedral oligomeric silsesquioxane (POSS) grafted polypropylene (PP) was first prepared by reactive blending. The structure and properties of physical blending and reactive blending composites of PP/POSS were investigated by wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA). WAXD analysis shows that the POSS in the reactive blending composites has better compatibility with PP than in the physical blending composites. The β-form crystalline hence disappears even the non-reactive POSS can act as an effective β-nucleating agents. DSC analysis shows the reactive blending composites have higher crystalline temperature while POSS in the physical blending composites have little effect on the crystalline temperature. The modulus of reactive blending composites increases in the presence of POSS, while that of the physical blending composites decreases with increasing POSS content.     

POSS‐nylon 6 nanocomposites: Influence of POSS structure on surface and bulk properties

Hybrid organic/inorganic nanocomposites based on polyhedral oligomeric silsesquioxane (POSS) nanostructured chemicals and nylon 6 were prepared via melt mixing. Two structurally and chemically different POSS molecules, a closed cage, nonpolar octaisobutyl POSS (Oib‐POSS) and an open cage, polar trisilanolphenyl POSS (Tsp‐POSS) with differing predicted solubility parameters were evaluated in the nylon matrix. Surface analysis, including quasi‐static and dynamic nanoindentation and nanotribological techniques, revealed exceptional improvements in modulus and hardness along with significant reductions in friction. Additionally, surface wetting characteristics of the nylon were reversed, with POSS incorporation yielding low surface energy, highly hydrophobic surfaces. AFM, TEM/EDAX, spectroscopic techniques and thermomechanical analysis were used to evaluate nanoscale dispersion and bulk properties of the composites. Both POSS molecules exhibit preferential surface segregation behavior in the nylon matrix. Tsp‐POSS, with its higher predicted solubility in nylon, exhibited enhanced dispersion and tribomechanical properties at both nano and bulk scale. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1088–1102, 2009     

周边硅笼取代的混杂酞菁卟啉三层铽单分子磁体

通过在铽的酞菁卟啉混杂三层的卟啉周边共价连接体积庞大的笼型倍半硅氧烷(POSS), 得到了首个包含POSS的混杂三层Tb₂(Pc)[T(OPOSS)₄PP]₂ (1)[H₂Pc=phthalocyanine;H₂T(OPOSS)₄PP=5, 10, 15, 20-tetra{[[N-[heptakis(isobutyl)propoxy]phenyl]octasiloxane]}porphyrin]。为了对比研究, 同时合成了类似的三层化合物Tb₂(Pc)(TPP)₂(2)(H₂TPP=5,10,15,20-tetraphenyporphyrin)。尤其值得注意的是, 在没有外加磁场的条件下, Tb₂(Pc)[T(OPOSS)₄PP]₂(1)和Tb₂(Pc)(TPP)₂(2)分别表现出单分子磁体和非单分子磁体的性质, 这充分说明了共价连接均匀分布的POSS基团有效地分离了磁性核心, 从而改善了酞菁卟啉混杂三层的磁性。     

A comparative study of POSS as synergists with zinc phosphinates for PET fire retardancy

This paper deals with the fire behaviour of poly (ethylene terephthalate) (PET) filled with Exolit OP950, a zinc phosphinate fire retardant, and three polyhedral oligomeric silsesquioxanes (POSS) having different chemical structures. Regardless of the POSS type, intumescence occurs during combustion, but the insulation properties of the chars produced are different. Best reductions on total heat evolved (THE) and on cumulative CO₂ with no increase in CO emissions are observed when dodecaphenyl POSS is used. This may be related to its thermal degradation pathway, releasing via this process volatile organic species contributing on intumescence and producing an effective protective layer having a foliated structure.     

Electrical and thermomechanical properties of epoxy-POSS nanocomposites

Two types of the epoxy-POSS nanocomposites were prepared and their electrical/dielectric and thermomechanical properties were determined. The mono- and octa-epoxyfunctionalized POSS (POSS,E1 and POSS,E8) were covalently incorporated in the epoxy network matrix DGEBA-3,3′-dimethyl-4,4′-diaminocyclohexylmethane (Laromin C260) as pendant units or as polyhedral junctions, respectively. While the POSS junctions are well dispersed in the hybrid network DGEBA-Laromin-POSS,E8, the pendant POSS aggregate to form large crystalline POSS domains.The properties of the nanocomposites correlate with the morphology. The nanocomposites with inhomogeneously dispersed large aggregates of pendant POSS,E1 show poor properties, including low T_g and thermal stability, and the high dielectric loss factor at higher temperatures. On the contrary, the well homogeneous nanocomposites containing POSS,E8 dispersed units display improved electrical properties in the range of 1–10 wt.% POSS. The hybrid exhibits the high resistivity (1 × 10¹⁵ Ω m) and polarization index p_i1 characterizing electrical relaxation, as well as the low dielectric loss factor tan δ, mainly at temperatures above 50 °C. Moreover, the material shows improved thermo oxidative stability and enhanced thermomechanical behavior. Consequently, this nanocomposite was proved to be a prospective insulation material particularly for a high temperature electrotechnical application. In contrast to electrical properties being the best at a low POSS content, the thermomechanical properties, such as rubbery modulus and T_g, are gradually improving with increasing POSS content due to growing crosslinking density.     

Glass transition and structural properties of glycidyloxypropyl-heptaphenyl polyhedral oligomeric silsesquioxane-epoxy nanocomposites

We have used molecular simulations to study the properties of nanocomposites formed by the chemical incorporation of polyhedral oligomeric silsesquioxane (POSS) particles in the cross-linked epoxy network. The particular POSS molecule chosen—glycidyloxypropyl-heptaphenyl POSS—can form only one bond with the cross-linker and thus was present as a dangling unit in the network. Four epoxy-POSS nanocomposites containing different fractions (up to 30 mass/%) of POSS particles were studied in this work. Well-relaxed atomistic model structures of the nanocomposites were created and then molecular dynamics simulations were used to characterize the density, glass transition temperature (T _g), and the coefficient of volume thermal expansion (CVTE) of the systems. In addition to the effect of nanoparticle loading, the effect of nanoparticle chemistry on the nanocomposite properties was also characterized by comparing these results with our previous results (Lin and Khare, Macromolecules 42:4319–4327, 2009) on neat cross-linked epoxy and a nanocomposite containing a POSS nanoparticle that formed eight bonds with the cross-linked network. Our results showed that incorporation of these monofunctional POSS particles into cross-linked epoxy does not cause a measurable change in its density, glass transition temperature, or the CVTE. Furthermore, simulation results were used to characterize the aggregation of POSS particles in the system. The nanofiller particles in systems containing 11, 20, and 30 mass/% POSS were found to form small clusters. The cluster-size distribution of nanoparticles was also characterized for these systems.     

Organically modified montmorillonite as a synergist for intumescent flame retardant against the flammable polypropylene

Low flame retardant efficiency is a key bottleneck for currently available retardants against the flammable polypropylene (PP). Herein, the organically modified montmorillonite (OMMT) was utilized as a synergist for our previously reported intumescent flame retardant (IFR) that was constructed from ammonium polyphosphate (APP) and hyperbranched charring foaming agent (HCFA) to further enhance the retardant efficiency against PP. The resultant's combustion behavior was thoroughly investigated by cone calorimetry, limiting oxygen index (LOI), vertical burning test (UL‐94), and scanning electron microscopy (SEM). The results showed that 20% addition of IFR with OMMT showed a positive effect and improved the flame retardancy of the PP systems. Especially, addition of 2 wt% OMMT obviously increased the LOI values of PP systems with 20% total loading flame retardants from 29% to 31.5% and the samples meet V‐0 rating as well as the reduction of the heat release rate (HRR), total heat release (THR), CO₂, and CO production occurred. On the other hand, the SEM images were also revealed that OMMT initiated a dense and strong char on the surface of the material, which resulted in efficient flame retardancy of PP matrix during combustion. In addition, thermal degradation behavior discussed by thermogravimetric analysis (TGA) indicated that OMMT could improve the thermal stability of PP systems under high temperature, and promoted char residues of PP/IFR systems. Copyright © 2016 John Wiley & Sons, Ltd.     

Migration and surface modification in polypropylene (PP)/polyhedral oligomeric silsequioxane (POSS) nanocomposites

This paper reports on the migration of POSS‐based nanocomposites both by annealing the melt and by heating the solid blend in the microwave oven. The migration proceeds to all surfaces of the sample as verified by ATR‐FTIR spectra of the bottom and top surfaces. Concentrations of POSS on the surface, exceeding 50%, are obtained. Polarity of the matrix increases POSS migration. During annealing at 190°C, a sublimation of POSS from the upper surface occurs. In air, sublimation is decreased by oxidizing the organic side groups of POSS and the PP to non‐volatile moieties. No sublimation occurs below 100°C. The AFM and SEM‐EDAX verified the high POSS concentration on the surface and indicated very pronounced roughness of the surface of the sample. The static contact angle measurements reveal very high hydrophobicity as well as low surface free energy (SFE) of the surface of the sample, which is close to that of Teflon and of pristine POSS. The migration of POSS is due to its lower surface tension, the entropy considerations, its lower cohesive energy with the matrix chains as compared to the cohesion energy between the chains, and the density and temperature fluctuations of the matrix chains which upon relaxation repulse and propel POSS to the surfaces. Copyright © 2008 John Wiley & Sons, Ltd.     

Surface energies of linear and cross-linked polymers based on isobornyl methacrylate and methacryl-heptaisobutyl POSS

The use of monofunctional polyhedral oligomeric silsesquioxanes (POSS) as polymer modifiers requires controlling the phase separation process producing POSS-rich and polymer-rich domains that occurs in most systems due to the thermodynamic incompatibility between both components. One significant result of this study is the finding that isobornyl methacrylate (IBoMA) is an excellent reactive solvent of a commercial methacryl-heptaisobutyl POSS (MA-POSS). Formulations containing up to 30 wt% MA-POSS in IBoMA or in IBoMA (95 parts by weight)-diethylene glycol dimethacrylate (DEGDMA, 5 parts by weight), were polymerized using benzoyl peroxide as initiator up to complete conversion of CC bonds (determined by FTIR). Transparent POSS-modified materials were obtained without any evidence of a macroscopic phase separation. POSS addition produced a decrease of the glass transition temperature and the glassy and rubbery elastic modulus. A significant decrease in surface energy for both linear and cross-linked polymers was observed. This effect was particularly important for cross-linked polymers where the addition of 30 wt% POSS decreased the surface energy from about 29 mN m⁻¹ to 16 mN m⁻¹, a very low value for hydrocarbon materials. This study opens a way to obtain hydrophobic methacrylic coatings without the use of fluorinated monomers.     

周边硅笼取代的混杂酞菁卟啉三层铽单分子磁体

通过在铽的酞菁卟啉混杂三层的卟啉周边共价连接体积庞大的笼型倍半硅氧烷(POSS),得到了首个包含POSS的混 杂三层Tb₂(Pc)[T(OPOSS)₄PP]₂(1)[H₂Pc=phthalocyanine;H₂T(OPOSS)₄PP=5,10,15,20-tetra{[[N-[heptakis(isobutyl)propoxy]phenyl]octasiloxane]}porphyrin]。为了对比研究,同时合成了类似的三层化合物Tb₂(Pc)(TPP)₂(2)(H₂TPP=5,10,15,20-tetraphenyporphyrin)。尤其值得注意的是,在没有外加磁场的条件下,Tb₂(Pc)[T(OPOSS)₄PP]₂(1)和Tb₂(Pc)(TPP)₂(2)分别表现出单分子磁体和非单分子磁体的性质,这充分说明了共价连接均匀分布的POSS基团有效地分离了磁性核心,从而改善了酞菁卟啉混杂三层的磁性。     

Flame retardancy of epoxy resin nanocomposite with a novel polymeric nanoflame retardant

A wrapped nanoflame retardant, designated as polyhedral oligomeric silsesquioxane (POSS)‐poly(4‐bromostyrene) (PBS)‐carbon nanotubes (CNTs), was synthesized via π‐π stacking interactions between the walls of multiwalled carbon nanotubes and the silicon‐bromine containing hybrid copolymer (designated as POSS‐PBS) that was copolymerized by 4‐bromostyrene and acryloyloxyisobutyl polyhedral oligomeric silsesquioxane. The POSS‐PBS‐CNTs exhibited good dispersibility in epoxy resin (EP) without obvious aggregation. Furthermore, the fire behaviors of this flame‐retardant EP (FR‐EP) nanocomposites were examined via limited oxygen index (LOI) and cone calorimeter (CONE) tests. The FR‐EP had an ideal LOI value of 35.3% and its residual char yield obtained from CONE test was significantly enhanced from 5.9% to 15.3% with the incorporation of 4 wt% POSS‐PBS‐CNTs and 1.33 wt% Sb₂O₃ into EP matrix. Additionally, the addition of 4 wt% POSS‐PBS‐CNTs or POSS‐PBS can efficiently decrease the peak heat release rate (PHRR) of EP matrix by 41.0% or 45.6%, respectively.     

Synthesis of semitelechelic POSS‐polymethacrylate hybrids by thiol‐mediated controlled radical polymerization with unusual thermal behaviors

We have developed an efficient and versatile method for the synthesis of polyhedral oligomeric silsesquioxanes (POSS)‐polymethacrylate hybrids, such as POSS‐poly(methyl methacrylate) (POSS‐PMMA), POSS‐poly(ethyl methacrylate) (POSS‐PEMA), and POSS‐poly(benzyl methacrylate) (POSS‐PBzMA) of controllable molecular weights and low polydispersities by thiol‐mediated radical polymerization at elevated temperature (100 °C). By tuning the reactant concentrations and degree of polymerization of the grafted polymethacrylate chains, POSS content in these hybrid materials could be varied. MALDI‐TOF‐MS analysis of the hybrid molecule shows that the nanoscale POSS moiety is connected to the end of polymethacrylate chain through the sulfur atom bridge. These hybrid materials were further characterized using various techniques such as FTIR, XRD, NMR, TGA, and DSC. In all synthesized hybrids, the incorporation of POSS moiety at the end of polymethacrylate chain resulted in the decrease of glass transition temperature (T_g) compared to that of neat polymethacrylates of comparable molecular weights. Surprisingly, POSS‐PMMA hybrids only with relatively high POSS content (～ 10 and 16 wt %) showed physical aging behavior as reveled by DSC study. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1111–1123, 2008     

Synthesis,Characterization, and Thermal Behavior of Carboranyl–Styrene Decorated Octasilsesquioxanes: Influence of the Carborane Clusters on Photoluminescence

Novel polyhedral oligomeric silsesquioxanes (POSS) or octasilsesquioxanes with carboranyl–styrene fragments attached to each corner are described. These compounds have been synthesized by olefin‐metathesis reactions between octavinylsilsesquioxane and carboranyl–styrene compounds that possess different substituents (Ph, Me, or H). In all cases, these reactions, which were catalyzed by the Grubbs catalyst, are highly regioselective and yield exclusively the E isomers. The existence of the carborane cage in the POSS structure induces a remarkable thermal stability in these compounds. After combustion at 1000 °C, these carboranyl–POSS compounds exhibit a mass loss lower than 10 %. The UV/Vis absorption data of these carboranyl–POSS compounds shows a slight bathochromic shift with respect to the carboranyl–styrene monomers, with an absorption maximum around 262 nm. Nevertheless, important differences in the emission spectra of the carboranyl–POSS compounds with regard to their carboranyl–styrene precursors are observed; the phenyl‐o‐carborane‐containing POSS compound exhibits the highest fluorescence intensity (Φ_F=44 %), whereas for the POSS compound bearing the methyl substituent, and for the unsubstituted o‐carborane clusters, the fluorescence intensity is much lower (Φ_F=9 and 2 %, respectively). This is precisely the reverse of what occurs with the monomers, in which the unsubstituted o‐carboranyl–styrene compound exhibits the highest Φ_F, and a quenching of the fluorescence is observed in the phenyl‐o‐carboranyl–styrene compound. In addition, a large red shift of around 100 nm is observed for the POSS compounds with respect to their precursors. These experimental results can only be accounted for by the spatial ordering induced by the POSS core that eases interactions, which otherwise would not occur. These results have been confirmed by time‐dependent density functional theory (TDDFT) calculations that exclude a photoinduced electron transfer (PET) process in the POSS compounds.     

Alumina nanoflake‐coated graphene nanohybrid as a novel flame retardant filler for polypropylene

γ‐Al₂O₃ nanoflakes are decorated on the surface of graphene nanosheets by hydrothermal method. The chemical structures, composition, and morphology of the graphene nanohybrid are characterized. Alumina nanoflake‐coated graphene nanohybrid is incorporated into polypropylene (PP) matrix by master batch‐based melt mixing method. The effects of alumina‐graphene nanohybrid on the thermal stability and combustion behavior of PP are investigated. More char residue is left for PP composite containing alumina‐graphene nanohybrid. Peak heat release rate of pure PP is greatly decreased by this nanohybrid, corresponding to 30.6% reduction. Fire safety of PP is considerably enhanced. Flame retardant mechanisms are concluded based on the combustion results and char analysis. The barrier effect of graphene and catalytic charring actions of γ‐Al₂O₃ nanoflakes make the contribution to the enhanced flame retardant properties.     

Novel hybrid systems based on poly(propylene-g-maleic anhydride) and Ti-POSS by direct reactive blending

Novel hybrid systems based on maleic anhydride-grafted polypropylene (PPgMA) and home-made Ti-containing amino polyhedral oligomeric silsesquioxanes (Ti-POSS-NH₂) have been prepared by one-step reactive blending, and their properties have been compared with those of systems based on a non-reactive POSS (POSS). The occurrence of a reaction between PPgMA and the reactive POSS molecules has been assessed by Fourier Transform Infrared Resonance (FTIR) measurements, whereas dispersion of POSS into the polymer was evaluated by Scanning Electron Microscopy (SEM), showing a nanometric dispersion only for the reactive POSS. Thermo-oxidative behaviour was studied by Thermogravimetric Analysis (TGA), showing a delayed volatilization of the PPgMA/Ti-POSS-NH₂ with respect to both PPgMA/POSS and pristine PPgMA, which is attributed to the chemical activity of Ti in Ti-POSS-NH₂. To highlight the mechanism of the hybrid system decomposition, samples which underwent a thermal treatment at 250 °C, i.e. the onset temperature for polymer matrix decomposition in thermo-oxidative conditions, have been studied by FTIR and X-Ray Photoelectron Spectroscopy (XPS) measurements.     

Metallocene supported on a polyhedral oligomeric silsesquioxane‐modified silica with high catalytic activity for ethylene polymerization

A polyhedral oligomeric silsesquioxane (POSS) consisting of a mixture of perfect and imperfect polyhedra with 8–10 Si atoms bonded to bulky and branched organic groups functionalized with (β‐hydroxy)‐tertiary amines, was adsorbed on an activated silica from a toluene solution. POSS fixation was confirmed by measuring its concentration in the solutions resulting from filtering and repeatedly washing with toluene. Diffuse reflectance infrared Fourier‐transformed (DRIFTS) spectra of the modified silica showed a decrease in the absorption of isolated SiOH groups. A POSS previously marked by reaction with an isocyanate was also used, and the presence of the carbonyl in the urethane group was recorded in the DRIFTS spectra of the modified silica, confirming the presence of adsorbed POSS. X‐ray photoelectron spectroscopy (XPS) was used to measure the amount of Si (2p) of POSS adsorbed on the SiO₂ surface as a function of the POSS concentration in the toluene solution. A linear increase of adsorbed POSS as a function of its concentration in toluene was found, even after successive washings with toluene. A metallocene, (nBuCp)₂ZrCl₂, was then adsorbed on the POSS‐modified silica from a toluene solution, and the fixed Zr amount was measured by Rutherford backscattering spectrometry (RBS). The activity of the resulting catalyst for ethylene polymerization in toluene, using methylaluminoxane (MAO) as a cocatalyst, was determined in two different devices operating in the range of 1.6–7.0 bar. Under every experimental condition, POSS‐modification of the silica support led to an increase of about 50% in the activity of the resulting catalyst when compared to the use of the unmodified support. Reasons for the observed increase in activity are discussed. The molar mass distribution and crystallinity of the resulting polyethylenes was not affected by the POSS modification of the silica support. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5465–5476, 2005