Microstructural features, diffusion and molecular relaxations in polyimide/silica hybrids

Synthesis and characterization of aramid composites reinforced with silanized graphene platelets

The synthesis and characterization of aramid composites reinforced with graphene platelets are reported. Hydroxy-functionalised graphene platelets were modified with two sol-gel binders (aminopropyl- or aminophenyl-trialkoxysilanes) and then chemically linked with aramid chains. The effect of the two sol-gel binders on the physiochemical and mechanical properties was evaluated. Chemical changes during the sol-gel reaction and subsequent amidation process in the nano-composite preparation were evaluated by the XPS and FTIR analyses. Thin films of these composites with different proportions of graphene were prepared. Morphology of the hybrids prepared was studied by the SEM technique. Properties of the composite films were studied by dynamical mechanical thermal (DMT) analysis to measure their glass transition temperature (T g) and storage modulus. These properties have been compared with previously reported values using pristine graphene (Gr) as a filler. The increase in thermal mechanical properties on addition of silanized graphene (SiGr) showed a large shift in the T g and more increase in storage modulus by chemically binding SiGr sheets on the aramid chains. Aminophenyl-trialkoxysilane was found to give better results due to the presence of phenyl groups which were more rigid than propyl groups present in aminopropyl-trialkoxysilane. The effect of chemical bonding and the possible π-π secondary bond interactions between the matrix and graphene platelets on the properties of the resulting hybrids are discussed.

Water Diffusion through a Titanium Dioxide/Poly(Carbonate Urethane) Nanocomposite for Protecting Cultural Heritage: Interactions and Viscoelastic Behavior

Water diffusion through a TiO₂/poly (carbonate urethane) nanocomposite designed for the eco-sustainable protection of outdoor cultural heritage stonework was investigated. Water is recognized as a threat to heritage, hence the aim was to gather information on the amount of water uptake, as well as of species of water molecules absorbed within the polymer matrix. Gravimetric and vibrational spectroscopy measurements demonstrated that diffusion behavior of the nanocomposite/water system is Fickian, i.e., diffusivity is independent of concentration. The addition of only 1% of TiO₂ nanoparticles strongly betters PU barrier properties and water-repellency requirement is imparted. Defensive action against penetration of water free from, and bonded through, H-bonding association arises from balance among TiO₂ hydrophilicity, tortuosity effects and quality of nanoparticle dispersion and interfacial interactions. Further beneficial to antisoiling/antigraffiti action is that water-free fraction was found to be desorbed at a constant rate. In environmental conditions, under which weathering processes are most likely to occur, nanocomposite Tg values remain suitable for heritage treatments.

Comparison of hybrid polyimide films with silica and organosilica obtained via sol–gel process

A series of polyimide/silica (PI–S) and polyimide/organosilica (PI–OS) hybrid films were prepared via a sol–gel process from mixtures of poly(amic acid) (PAA) and tetraethoxysilane or a silane coupling agent in solution. The PAA was synthesized from bis-(3-phthalyl anhydride) ether and 1,4-bis (4-aminophenoxy) benzene. The hybrid films were produced via an imidization reaction to form silica particles or a silica network in a polymer matrix through a programmed heating process. The derived films were characterized and compared by Fourier transform infrared spectroscopy, field emission scanning electron microscopy, thermogravimetric analysis, tensile testing, contact angle measurements, moisture absorption measurements, and dielectric property analysis. Both the PI–S and PI–OS hybrid matrixes showed strong regularity with the increasing silica content and greatly improved thermal stability and mechanical properties. Among them, the PI–OS hybrid films possessed superior interphase interactions and were found to have better physicochemical properties.

Structure-Properties Correlation in Polyimide/Silica Hybrids

Polyimide/silica hybrids were prepared by a sol-gel process and were evaluated in terms of curing behavior, morphology, water sorption properties and mechanical performances at ambient and at higher temperatures. The spectroscopic examination showed that the presence of the inorganic phase catalyzes the imidization reaction of the polyamic acid pre-polymer. Two types of morphology were obtained by tailoring the composition of the precursor solution mixture, i.e. phase-separated or co-continuous systems. Water sorption was found to be strongly influenced by the system morphology. The presence of the inorganic phase increased considerably the amount of absorbed water and phase-separated hybrids absorbed more than co-continuous hybrids. Also the mechanical properties were found to depend on the system morphology. The largest increase in rigidity both at ambient and attemperatures up to 250°C was achieved when the two phases were co-continuous. Conventional two-phase systems, on the other hand, exhibited improved fracture toughness.

Relaxation behavior of flax-reinforced ethylene–propylene–diene rubber

The molecular dynamics of flax-reinforced ethylene–propylene–diene monomer (EPDM) composites, with increasing quantities of flax (EPDM0–EPDM20), was investigated by dynamic mechanical analysis. Multifrequency experiments were also performed in order to distinguish between the relaxation and the kinetic events. Also the time–temperature superposition was applied to the multifrequency results for obtaining the master curves. They were used to anticipate the behavior of the composites in time and to calculate the activation energies using the Williams–Landel–Ferry equation.

An investigation of physico-chemical properties of a new polyimide–silica composites

In the present study, a novel diamine 1,4-bis[4-(hydrazonomethyl)phenoxy]butane (4-BHPB) has been successfully synthesized by a facile method.

Viscoelasticity and structure of polystyrene/fumed silica nanocomposites: filler network and hydrodynamic contributions

We investigate the relationships between structure and linear viscoelasticity of a model polymer nanocomposite system based on a mixture of fumed silica nanoparticles and polystyrene. Alterations in the viscoelastic behavior are attributed to the structuring of primary silica aggregates. Above a critical filler volume fraction, a space-filling network builds up as the result of cluster aggregation, and the complex frequency-dependence of the moduli is simplified by splitting the viscoelasticity of the composites into the independent responses of the suspending polymer melt and the filler network. Specifically, we present a refinement of a two-component model recently proposed for attractive colloidal suspensions, in which hydrodynamic effects related to the presence of the filler are properly taken into account using the concept of shear stress equivalent deformation. Our approach, validated through the building of a master curve of the elastic modulus for samples of different composition, allows the estimation of the elasticity of samples in which the filler network is too tenuous to be appreciated through a simple frequency scan. In addition, the structure of the filler network is studied using both the percolation and fractal approaches, and the reliability of the critical parameters is discussed. We expect that our analysis may be useful for understanding the behavior of a wide variety of complex fluids where the elasticity of the components may be superimposed.