Dielectric Properties of Nanocomposites Based on Polystyrene and Polyhedral Oligomeric Phenethyl-Silsesquioxanes

Novel Reactive Polyhedral Oligomeric Silsesquioxane-Reinforced and Toughened Epoxy Resins for Advanced Composites

Most toughening methods for epoxy resins are usually used at the expense of other properties. Some polyhedral oligomeric silsesquioxanes (POSSs) with both a rigid Si-O-Si structure and flexible organic chain segments could be expected to be effective toughening agents. In this study, three reactive polyhedral oligomeric silsesquioxanes with a thiol group (OMPPS), a carboxyl group (OCOPS), and an epoxy group (OGCPS) were synthesized and characterized. They were utilized as modifiers to toughen 3-(oxiran-2-ylmethoxy)-N,N-bis(oxiran-2-ylmethyl)aniline (AFG-90MH)/4,4'-methylenebis(2-ethylaniline) (MOEA) (epoxy resin) with different molar ratios to obtain hybrid resins named OMPPS-EP-i, OCOPS-EP-j, and OGCPS-EP-k. The effects of the amount of modifier added and the length of the organic chain on the cage structure on various properties of the hybrid resins were investigated. The results show that all three modifiers show good compatibility with the epoxy resin. The hybrid resins have a low viscosity at 45~85 °C and can be cured at a low temperature (110 °C). The cured hybrid resins display improved toughness. Typically, the critical stress intensity factor (KIC) and impact strength of OGCPS-EP-0.6-C are 2.54 MPa∙m-1/2 and 19.33 kJ∙m-2, respectively, which increased by 58.75% and 22.48% compared with the pristine epoxy resin, respectively. In addition, the glass transition temperature and flexural strength of the hybrid resins are basically unchanged.

Exploring a unified description of the super-Arrhenius region above and below the glass transition temperature

A new approach is presented in order to check whether the hypothesis of an Arrhenius component surviving in the α-relaxation region is consistent with experimental data. The temperature dependence of the dynamics in the whole glassy regime is described by an equation which assumes an Arrhenius component in the cooperative diffusion. Based on thermodynamic arguments, the dynamic heterogeneities close to the glass transition region are related to structural heterogeneities in a manner consistent with the idea of a sigmoidal shape in the cohesion energy. By doing so, a characteristic temperature which can be identified as the glass transition temperature (T_g) emerges, while an additional parameter for the extension of the super-Arrhenius region is introduced. In the analysis of experimental data, the activation energy parameter, determined from the temperature dependence of the β-relaxation, is fixed, and the relation between the experimental and the predicted glass transition temperature is examined. The results of this comparison support the idea that dynamics above and below T_g can be described in a unified manner. The proposed model is tested against experimental data of glass-forming liquids, polymers and polymer composites. In the latter systems, it is shown that the Arrhenius-like behavior characterizing the dynamics of the polymeric bound-layer can be accounted for by such an extension of the super-Arrhenius region.

Opposite Effects of SiO2 Nanoparticles on the Local α and Larger-Scale α' Segmental Relaxation Dynamics of PMMA Nanocomposites

The segmental relaxation dynamics of poly(methyl methacrylate)/silica (PMMA/SiO₂) nanocomposites with different compositions (ϕSiO2) near and above the glass transition temperature were investigated by mechanical spectroscopy. At ϕSiO2 ≤ 0.5%, the α peak temperature hardly changes with ϕSiO2, but that of α’ relaxation composed of Rouse and sub-Rouse modes decreases by 15 °C due to the increase of free volume. At ϕSiO2 ≥ 0.7%, both α and α’ relaxations shift to high temperatures because of the steric hindrance introduced by nanoparticle agglomeration. On the other hand, with increasing ϕSiO2, the peak height for α relaxation increases at ϕSiO2 ≤ 0.5% and then decreases at ϕSiO2 ≥ 0.7%, but that for α’ relaxation shows an opposite behavior. This is because at low ϕSiO2, the short-chain segments related to α relaxation can easily bypass the particles, but the longer-chain segments related to α’ relaxation cannot. At high ϕSiO2, the polymer chains were bound to the nanoparticles due to the physical adsorption effect, leading to the decrease of relaxation unit concentration involved in α relaxation. However, the dissociation of those bonds with heating and the concentration heterogeneity of polymer chains result in the increase of peak height for α’ relaxation.

Thermal Stability and Flame Retardancy of Polypropylene Composites Containing Siloxane-Silsesquioxane Resins

A novel group of silsesquioxane derivatives, which are siloxane-silsesquioxane resins (S4SQ), was for the first time examined as possible flame retardants in polypropylene (PP) materials. Thermal stability of the PP/S4SQ composites compared to the S4SQ resins and neat PP was estimated using thermogravimetric (TG) analysis under nitrogen and in air atmosphere. The effects of the non-functionalized and n-alkyl-functionalized siloxane-silsesquioxane resins on thermostability and flame retardancy of PP materials were also evaluated by thermogravimetry-Fourier transform infrared spectrometry (TG-FTIR) and by cone calorimeter tests. The results revealed that the functionalized S4SQ resins may form a continuous ceramic layer on the material surface during its combustion, which improves both thermal stability and flame retardancy of the PP materials. This beneficial effect was observed especially when small amounts of the S4SQ fillers were applied. The performed analyses allowed us to propose a possible mechanism for the degradation of the siloxane-silsesquioxane resins, as well as to explain their possible role during the combustion of the PP/S4SQ composites.

Unveiling the hybrid interface in polymer nanocomposites enclosing silsesquioxanes with tunable molecular structure: Spectroscopic, thermal and mechanical properties

Organic-inorganic nanobuilding blocks (NBBs) based on silsesquioxanes (SSQs) have potential applications as nanofillers, thermal stabilizers, and rheological modifiers, which can improve thermomechanical properties of polymer hosts. The possibility to tune both siloxane structure and pendant groups can promote compatibilization and peculiar interactions with a plethora of polymers. However, the control on SSQs molecular architecture and functionalities is usually delicate and requires careful synthetic details. Moreover, investigating the influence of NBBs loading and structure on the hybrid interface and, in turn, on the polymer chains mobility and mechanical properties, may be challenging, especially for low-loaded materials. Herein, we describe the preparation and characterization of polybutadiene (PB) nanocomposites using as innovative fillers thiol-functionalized SSQs nanobuilding blocks (SH-NBBs), with both tailorable functionality and structure. Swelling experiments and, more clearly, solid-state NMR, enlightened a remarkable effect of SH-NBBs on the molecular structure and mobility of the polymeric chains, envisaging the occurrence of chemical interactions at the hybrid interface. Finally, thermal and DMTA analyses revealed that nanocomposites, even containing very low filler loadings (i.e. 1, 3 wt%), exhibited enhanced thermomechanical properties, which seem to be connected not only to the loading, but also to the peculiar cage or ladder-like architecture of SH-NBBs.

Temperature fluctuations and the thermodynamic determination of the cooperativity length in glass forming liquids

The aim of this paper is to decide which of the two possible thermodynamic expressions for the cooperativity length in glass forming liquids is the correct one. In the derivation of these two expressions, the occurrence of temperature fluctuations in the considered nanoscale subsystems is either included or neglected. Consequently, our analysis gives also an answer to the widely discussed problem whether temperature fluctuations have to be generally accounted for in thermodynamics or not. To this end, the characteristic length-scales at equal times and temperatures for propylene glycol were determined independently from AC calorimetry in both the above specified ways and from quasielastic neutron scattering (QENS), and compared. The result shows that the cooperative length determined from QENS coincides most consistently with the cooperativity length determined from AC calorimetry measurements for the case that the effect of temperature fluctuations is incorporated in the description. This conclusion indicates that-accounting for temperature fluctuations-the characteristic length can be derived by thermodynamic considerations from the specific parameters of the liquid at glass transition and that temperature does fluctuate in small systems.

Temperature Assisted in-Situ Small Angle X-ray Scattering Analysis of Ph-POSS/PC Polymer Nanocomposite

Inorganic/organic nanofillers have been extensively exploited to impart thermal stability to polymer nanocomposite via various strategies that can endure structural changes when exposed a wide range of thermal environment during their application. In this abstraction, we have utilized temperature assisted in-situ small angle X-ray scattering (SAXS) to examine the structural orientation distribution of inorganic/organic nanofiller octa phenyl substituted polyhedral oligomeric silsesquioxane (Ph-POSS) in Polycarbonate (PC) matrix from ambient temperature to 180 °C. A constant interval of 30 °C with the heating rate of 3 °C/min was utilized to guise the temperature below and above the glass transition temperature of PC followed by thermal gravimetric, HRTEM, FESEM and hydrophobic analysis at ambient temperature. The HRTEM images of Ph-POSS nano unit demonstrated hyperrectangular structure, while FESEM image of the developed nano composite rendered separated phase containing flocculated and overlapped stacking of POSS units in the PC matrix. The phase separation in polymer nanocomposite was further substantiated by thermodynamic interaction parameter (χ) and mixing energy (E_mix) gleaned via Accelrys Materials studio. The SAXS spectra has demonstrated duplex peak at higher scattering vector region, postulated as a primary and secondary segregated POSS domain and followed by abundance of secondary peak with temperature augmentation.

Hybrid nanoporous polystyrene derived from cubic octavinylsilsesquioxane and commercial polystyrene via the Friedel–Crafts reaction

A series of POSS-based nanoporous materials were prepared from commercial polystyrene and octavinylsilsesquioxane via the Friede–Crafts reaction.

N-Phenylaminomethyl hybrid silica, a better alternative to achieve reinforced PU nanocomposites

A novel NPAM-silica was synthesized by an economic and environmentally friendly method. It was chemically cooperated into polyurethane with enhanced mechanical properties. Compared with N-phenylaminomethyl POSS/PU composites, the crosslinking junction states and the mechanical properties are different. NPAM-silica provided a better alternative for polymer/nanocomposites.

Segmental dynamics and physical aging of polystyrene/silver nanocomposites

We report the complicated variation trend of calorimetric T_g and physical aging in PS/Ag nanocomposites, despite the invariant segmental dynamics with increasing silver nanoparticle loading.

Polyhedral Oligomeric Silsesquioxane (POSS)-Containing Polymer Nanocomposites

Hybrid materials with superior structural and functional properties can be obtained by incorporating nanofillers into polymer matrices. Polyhedral oligomeric silsesquioxane (POSS) nanoparticles have attracted much attention recently due to their nanometer size, the ease of which these particles can be incorporated into polymeric materials and the unique capability to reinforce polymers. We review here the state of POSS-containing polymer nanocomposites. We discuss the influence of the incorporation of POSS into polymer matrices via chemical cross-linking or physical blending on the structure of nanocomposites, as affected by surface functional groups, and the POSS concentration.

Hepta(3,3,3-trifluoropropyl) polyhedral oligomeric silsesquioxane-capped poly(N-isopropylacrylamide) telechelics: synthesis and behavior of physical hydrogels

Hepta(3,3,3-trifluoropropyl) polyhedral oligomeric silsesquioxane (POSS)-capped poly(N-isopropylacrylamide) (PNIPAAm) telechelics with variable lengths of PNIPAAm midblocks were synthesized by the combination of reversible addition-fragmentation chain transfer polymerization (RAFT) and the copper-catalyzed Huisgen 1,3-cycloaddition (i.e., click chemistry). The POSS-capped trithiocarbonate was synthesized and used as the chain transfer agent for the RAFT polymerization of N-isopropylacrylamide. The organic-inorganic amphiphilic telechelics were characterized by means of nuclear magnetic resonance spectroscopy (NMR) and gel permeation chromatography (GPC). Atomic force microscopy (AFM) shows that all the POSS-capped PNIPAAm telechelics exhibited microphase-separated morphologies, in which the POSS terminal groups were self-assembled into the microdomains and dispersed into the continuous PNIPAAm matrices. The POSS nanodomains could behave as the physical cross-linking sites and as a result the physical hydrogels were formed while these POSS-capped PNIPAAm telechelics were subjected to the solubility tests with water. These physical hydrogels possessed well-defined volume phase transition phenomena and displayed rapid reswelling and deswelling thermoresponsive behavior compared to control PNIPAAm hydrogel.