A roll to roll continuous processing method is developed for vertical alignment (“Z” alignment) of barium titanate (BaTiO3) nanoparticle columns in polystyrene (PS)/toluene solutions. This is accomplished by applying an electric field to a two-layer solution film cast on a carrier: one is the top sacrificial layer contacting the electrode and the second is the polymer solution dispersed with BaTiO3 particles. Flexible Teflon coated mesh is utilized as the top electrode that allows the evaporation of solvent through the openings. The kinetics of particle alignment and chain buckling is studied by the custom-built instrument measuring the real time optical light transmission during electric field application and drying steps. The nanoparticles dispersed in the composite bottom layer form chains due to dipole–dipole interaction under an applied electric field. In relatively weak electric fields, the particle chain axis tilts away from electric field direction due to bending caused by the shrinkage of the film during drying. The use of strong electric fields leads to maintenance of alignment of particle chains parallel to the electric field direction overcoming the compression effect. At the end of the process, the surface features of the top porous electrodes are imprinted at the top of the top sacrificial layer. By removing this layer a smooth surface film is obtained. The nanocomposite films with “Z” direction alignment of BaTiO3 particles show substantially increased dielectric permittivity in the thickness direction for enhancing the performance of capacitors.

•An equation for mutual diffusion coefficient in polymer solutions was derived using Onsager’s variational principle.•The trend of variation of mutual diffusion coefficient was studied in different hypothetical phase diagrams.•It was shown that cessation of diffusion occurs on the spinodal line in all of the phase diagrams.•It was shown that diffusion slows down by approaching the critical point and, in general, the spinodal line.This paper aims to discuss the trend of variation of mutual diffusion coefficient in polymer solutions exhibiting different phase behavior. An equation for mutual diffusion coefficient in polymer solutions was derived in the framework of classical irreversible thermodynamics using Onsager’s variational principle. The behavior of mutual diffusion coefficient in different phase diagrams was investigated by changing the Flory-Huggins interaction parameter to include concentration and temperature dependencies using Koningsveld interaction factor. The proposed diffusion coefficient satisfies the asymptotic behavior at infinite dilution, becomes zero on the spinodal line and changes sign at the onset of entering the unstable zone of phase diagrams. It was shown that the diffusion slows down near the critical point and in general by approaching the spinodal curve. Moreover, it was shown that the derived equation for mutual diffusion coefficient can be expressed in terms of osmotic bulk modulus which is consistent with the result of a variational approach using Landau-Ginsberg-Wilson Hamiltonian.

Amphiphilic polymer co-networks provide a unique route to integrating contrasting attributes of otherwise immiscible components within a bicontinuous percolating morphology and are anticipated to be valuable for applications such as biocatalysis, sensing of metabolites, and dual dialysis membranes. These co-networks are in essence chemically forced blends and have been shown to selectively phase-separate at surfaces during film formation. Here, we demonstrate that surface demixing at the air–film interface in solidifying polymer co-networks is not a unidirectional process; instead, a combination of kinetic and thermodynamic interactions leads to dynamic molecular rearrangement during solidification. Time-resolved gravimetry, low contact angles, and negative out-of-plane birefringence provided strong experimental evidence of the transitory trapping of thermodynamically unfavorable hydrophilic moieties at the air–film interface due to fast asymmetric solvent depletion. We also find that slow-drying hydrophobic elements progressively substitute hydrophilic domains at the surface as the surface energy is minimized. These findings are broadly applicable to common-solvent bicontinuous systems and open the door for process-controlled performance improvements in diverse applications. Similar observations could potentially be coupled with controlled polymerization rates to maximize the intermingling of bicontinuous phases at surfaces, thus generating true three-dimensional, bicontinuous, and undisturbed percolation pathways throughout the material.

•Thermodynamically consistent governing equations for mass and heat diffusion in ternary polymer solutions were derived.•A 3 × 3 diffusion matrix was constructed whose entries describe the mass and heat diffusion in ternary mixtures.•Entries of diffusion matrix were derived for solvent/solvent/polymer and solvent/polymer/polymer systems.•Spinodal curves of ternary polymer solutions were derived from the governing equations.•It was proved that setting cross-diffusion coefficients to zero results in a contradiction in ternary mixtures.Governing equations for evolution of concentration and temperature in three-component systems were derived in the framework of classical irreversible thermodynamics using Onsager’s variational principle and were presented for solvent/solvent/polymer and solvent/polymer/polymer systems. The derivation was developed from the Gibbs equation of equilibrium thermodynamics using the local equilibrium hypothesis, Onsager reciprocal relations and Prigogine’s theorem for systems in mechanical equilibrium. It was shown that the details of mass and heat diffusion phenomena in a ternary system are completely expressed by a 3 × 3 matrix whose entries are mass diffusion coefficients (4 entries), thermal diffusion coefficients (2 entries) and three entries that describe the evolution of heat in the system. The entries of the diffusion matrix are related to the elements of Onsager matrix that are bounded by some constraints to satisfy the positive definiteness of entropy production in the system. All the elements of diffusion matrix were expressed in terms of derivatives of exchange chemical potentials of the components with respect to concentration and temperature. The spinodal curves of ternary polymer solutions were derived from the governing equations and their correctness was checked by the Hessian of free energy density. Moreover, it was proved that setting cross-diffusion coefficients to zero results in a contradiction, and the governing equations without cross-diffusion coefficients do not express the actual phase behavior of the system.Download high-res image (240KB)Download full-size image

•A model for diffusion of mass and heat in polymer solutions was developed in the framework of irreversible thermodynamics.•Mutual diffusion and thermal diffusion coefficients enter the governing equations in terms of entries of Onsager matrix.•The dependence of polymer thermal diffusion on molecular weight, chain rigidity and solvent viscosity was explained.•Thermal diffusion manifests itself as an increase in the solvent concentration on the warm side of a temperature gradient.This paper aims to theoretically explain experimental observations regarding the thermal diffusion of polymers in polymer solutions. 3D governing equations for mass and heat diffusion in polymer solutions were derived in the framework of classical irreversible thermodynamics using local equilibrium hypothesis, Onsager reciprocal relations and Prigogine's theory in systems in mechanical equilibrium. The mutual diffusion coefficient, thermal diffusion coefficient and thermal conductivity enter the governing equations as functions of thermodynamic variables and phenomenological coefficients that are bounded to some theoretical constraints. It was shown that the derivative of the linear combination of chemical potentials of polymer and solvent with respect to temperature plays an important role in thermal diffusion in polymer solutions. Thermal diffusion coefficient derived in the model can qualitatively explain the experimental observations in the literature regarding the dependence of thermal diffusion of polymers to molecular weight, polymer chain rigidity and viscosity of the solvent. Numerical simulation of the governing equations for a 1D drying process of polymer solutions indicates that the model is able to capture the effect of thermal diffusion. This effect manifests itself as an increase in local concentration of the solvent on the warm side of a temperature gradient during a solution casting process.

A real-time spectral birefringence technique was implemented to follow the coupled relationships among birefringence, true stress, and true strain behavior of natural rubber vulcanizates containing a series of clay concentrations. The dynamics of structural evolution was also followed by off-line wide-angle X-ray diffraction to characterize the order and orientation at different extension levels. The results show that at the same true strain level the natural rubber/organomodified clay (NR/OC) nanocomposite with higher OC content exhibits higher true stress and birefringence. While the stress optical relationship shows three different linear stages during stretching, the strain optical relationship is nonlinear and found to always exhibit a positively increasing slope with increasing strain. XRD results show higher OC-content-induced earlier appearance of strain crystallization and higher orientation factor in crystalline domains of rubber, as also evidenced by the real time birefringence results.

This paper focuses on the real-time mechano-optical behavior of hydroxyl-functionalized polypropylene (PPOH) copolymer containing 0.4 mol % comonomer of 10-hydroxy-1-undecene and its comparison with unmodified polypropylene (PP). The mechano-optical behavior of the two polymers was studied in the partially molten state during four different processing steps—heating, stretching, annealing, and cooling—with an integrated system that combined uniaxial stretching with real-time spectral birefringence and real-time ultrarapid-scan polarized FTIR measurements. While the material response was dominated by the crystalline network in both polymers, the presence of intermolecular hydrogen bonding between the hydroxyl groups was found to affect the structural evolution of the PPOH copolymer more significantly compared to the PP homopolymer. The PPOH copolymer exhibited not only lower crystallinity and smaller crystals but also a lower extent of lamella thickening at similar deformation conditions due to steric hindrance by the copolymer C9–side chains and OH groups. Uniaxial deformation of PP up to lower strains (εH ∼ 0.1) and subsequent cooling had the same effect on the crystal structure as annealing the polymer for longer times (≥30 min) at the same temperature with no deformation and subsequent cooling, which was a decrease in the population of smaller crystals and an increase in the average crystal size while maintaining similar crystallinity as the undeformed PP. Surprisingly, for PP both the amorphous and crystalline chain axes were oriented in the transverse direction (TD) at low strains before orienting in the machine direction (MD) at higher strains. However, no such observation was made for PPOH. This behavior was ascribed to the difference in breakup and unraveling of the spherulitic morphology between PP and PPOH upon deformation. In addition, four different regimes in the stress–optical curve were established for both the polymers. Stress relaxation immediately following uniaxial deformation of PP in the semimolten state resulted in a loss of MD orientation of crystalline segments but an increase in the MD orientation of amorphous segments. Unlike the result for PP, in PPOH the secondary H-bonded network supported the primary crystalline network during stress relaxation, and no change in the orientation of either crystalline or amorphous segments was observed. A schematic of the structural evolution was proposed based on the experimental results.

A strategy for the synthesis of new cross-linkable bimodal amphiphilic grafts (bAPGs) was developed. These grafts are of hydrophilic PDMAAm backbones carrying low (Mn ∼ 17 200 g/mol) and high (Mn ∼ 117 000 g/mol) molecular weight hydrophobic PDMS branches, each branch carrying a vinylsilyl end-group. The bAPGs were cross-linked by Karstedt catalyst to bimodal amphiphilic conetworks (bAPCNs) by the use of polyhydrosiloxane-co-PDMS as the cross-linker. Membranes prepared from bAPCNs exhibit mechanical properties surprisingly superior to earlier APCNs prepared with APGs with monomodal low molecular weight branches. Membrane bimodality controls surface morphology and topography by means of elastic wrinkling instability during film formation. Semipermeable bAPCN membranes with precisely controlled nanochannel dimensions were prepared so as to allow rapid insulin diffusion and prevent passage of IgG. bAPCN membranes were designed for immunoprotection of live pancreatic islets and are thus key components for a bioartificial pancreas.

•We investigate film drying followed by mechano-optical behavior of PVA films in both solid and swollen state.•Aqueous PVA solution develops out of plane optical anisotropy while maintaining in-plane isotropy during drying.•Stretching of dried films lead to three regimes of stress and strain-optical behavior.•Photoelastic and strain optical constants depend on moisture level in the films.In this paper, we investigate film drying followed by mechano-optical behavior of PVA films in solid and swollen state, respectively. During drying the real time evolution of thickness, weight, and birefringence (both in and out of plane) are captured using a highly instrumented measurement system. During drying, initially isotropic solution rapidly loses water and beyond a critical stage it develops optical anisotropy in the thickness direction as reflected in rapid development of out of plane birefringence while in-plane isotropy is maintained. Stretching the dried films in the solid state yields three regimes of non-linear stress and strain-optical behavior. The temperature shows a positive effect on the birefringence value at given true strain levels. Swelling the PVA films with water, facilitate room temperature stretchability, hence providing a route of stretching that avoids decomposition and discoloration that occurs at elevated temperatures. The effects of moisture content on mechano-optical properties of PVA films are mapped and opposite dependences of photolelastic and strain optical constants on moisture content are observed.

•Drying of polyether imide was investigated using instrumented real time measurement system.•During drying isotropic to anisotropic optical transition was observed. Initially randomly oriented polymer chains become oriented in the film plane exhibiting uniaxial optical symmetry.•Effects of remnant solvent concentration on the mechano optical behavior of the films were quantified and mapped.In order to have a fundamental understanding of the temporal evolution of physical characteristics of Polyetherimide (PEI) film and processing parameters, real time measurements are essential. Because drying and stretching processes of solvent-cast polymer film are complex, off line measurements are unable to reveal the temporally varying detailed information, especially for fast “transient events”. To solve the problem, two measurement systems, one for drying that tracks real time thickness, weight, surface temperature, in-plane and out-of-plane birefringence of solution casting film, and another for uniaxial stretching that tracks real time true stress, true strain and birefringence of stretched film are used. By using these systems, we are able to systematic investigate the development of optical anisotropy through in and out of plane birefringence monitoring drying of solution cast films and their mechano-optical behavior during stretching, and quantify the influence of solvent N-methyl-2-pyrrolidinone (NMP) on these relationships.

Hydroxyl-modified polypropylenes (PPOH) with side chains containing OH groups were synthesized by copolymerization of the propylene and undecenyloxytrimethylsilane monomers. The isothermal and non-isothermal crystallization behavior of the modified polypropylenes (PPOH) with side chains containing up to 6.8 mol% OH groups were compared with that of polypropylene (PP). The introduction of the OH-comonomer decreased the overall rate of isothermal crystallization compared with PP due to steric effects of the hydroxyl-containing side-chains that hindered packing of the PP backbone chains into a lamellar structure. However, a maximum reduction in the rate of crystallization occurred at an intermediate hydroxyl concentration as a consequence of a competition between the effects of the comonomer on the nuclei density and the thermodynamic barrier to crystallization. Steric hindrance by the comonomer side-chains also reduced the radial growth rate of the crystals in PPOH and produced a coarser crystal morphology than that for PP. PP and PPOH exhibited an identical α-monoclinic crystal structure, but the introduction of only ∼6.8 mol% comonomer reduced the fold-surface free energy of the crystals by 42%. For non-isothermal crystallization, the crystallization peak temperature (Tp) decreased for low concentrations of OH, but above a critical OH concentration, Tp increased, a result similar to the isothermal crystallization rate.

•Developed multi-layer films showed enhanced gas-barrier properties and flexibility.•Organoclay content increased out-of-plane birefringence values.•Shear viscosity & stress increased with organoclay loading at low shear rates.•Crossover behavior was observed in rheological behavior as the clay concentration increased at critical shear rate of 0.35 s−1.•Permeability values decreased over 50% in comparison with the neat PAI films.Multifunctional single and triple-layer films exhibiting flexibility, enhanced modulus and gas barrier properties were developed using a soluble polyamide-imide (PAI) in dimethylacetamide (DMAc) with ammonium-modified montmorillonite (MMT, Cloisite 30B) mineral clay. The drying behavior and associated anisotropy development were determined real-time, using a newly developed real-time measurement system. Out-of-plane birefringence development takes place earlier for thinner neat samples caused primarily by increased depletion rate of solvent. Addition of organoclay content resulted in a decrease in evaporation rate of solvent due to planar orientation of well exfoliated nanoplatelets as shown by TEM images and WAXS. This is in agreement with the out-of-plane anisotropy development observed during drying. Beyond a critical solid wt%, out-of-plane birefringence started to increase earlier with organoclay addition. In the case of multi-layer organoclay reinforced PAI films, the drying behavior of each individual layer was tracked and a complementary drying model is proposed. Planar orientation of nanoplatelets resulted in high helium-barrier properties.

•Stress optical relationships in sequential biaxial stretching of PET revealed three distinct regimes.•Regime I is linear where the material remains amorphous.•In Regime II material form of nematic-like order. In regime III chains reach finite extensibility.•Crystallinity development correlated reasonably with the work input.•The transverse stretching from ordered state lead to destruction of some crystals.The main focus of this research is to identify the structural mechanisms that are responsible for changes observed in various stages of coupled mechano-optical (stress and strain optical) relationships as they are influenced by the deformation mode, level and rate. This involves in situ real time studies of the true-stress-strain-birefringence of sequential biaxially stretched PET films using a highly instrumented biaxial stretching machine and ex situ structural studies. These ex-situ studies include wide angle X-ray diffraction, Raman spectroscopy and DSC thermal analysis to examine the orientation, conformation and crystallization behaviors as well as formation of long range connectivity through physical network. The relationship between the stress and birefringence exhibited three Regimes. Regime I, linear relationship with a stress optical constant of 5.8 GPa−1. Regime II, non-linear relationship with the establishment of chain tautness and nematic like two-dimensional order. Regime III, where the chains reach their finite extensibility.

•We investigated real-time physical and chemical changes during imidization of PMDA-ODA films.•We used a custom build uniaxial tensile stretcher coupled with IR spectrometer.•Solvent evaporations prior to imidization and chemical change observed at higher temperatures.•Shrinkage (due to solvent evaporation) translates into stress and orientation before conversion.•Bound solvent evaporation and chemical conversion occur simultaneously at higher temperatures.Multiple overlapping physical and chemical changes often take place during casting/drying and imidization from PMDA-ODA polyamic acid precursors from cast solutions. To shed light into details of these complex phenomena, we designed a unique real time measurement system that combines true stress, true strain, in-plane birefringence and temperature with polarized ultra-rapid scan FT-IR spectrometry (URS-FT-IR). At the early stages of heating (21°C–130 °C), initially isotropic solution cast film was observed to develop stress and birefringence as the solvent decomplexed and evaporated without showing any imidization as it was held in uniaxially constrained state. At a temperature around 130 °C, the onset of imidization reaction was detected while the stress went through a maximum. Beyond this stage, the evaporation of bound solvent and chemical conversion was observed to take place simultaneously and this is accompanied by a steady increase in birefringence. As the majority of the bound solvent evaporated, the stress and birefringence values started leveling off at long times.

Silica aerogels are comprised of highly porous three-dimensional networks. They typically are very fragile and brittle due to the inter-particle connections in the pearl-necklace-like fractal network. This behavior prevents their wider utility. The present study aims to reinforce the silica-based gel to improve the poor mechanical strength through crosslinking the silica particles with polyimide and incorporating Lucentite STN clay into the skeletal silica–polyimide network. 3-Aminopropyltriethoxysilane (APTES) end-capped polyamic acid oligomers were first formed followed by gelation with TMOS at a range of clay concentrations to generate a silica network. The incorporation of clay leads to slightly lower BET surface area with little effect on shrinkage, porosity and density. Microscopy revealed that the aerogel preferentially grows from the edges of well dispersed clay particles while minimal growth occurs from clay surfaces. The formation of covalent bonds and hydrogen bonding through the OH functionalized clay edges is thought to enhance the connectivity with silica network and clay, leading to a substantial reinforcement effect as evidenced by an increase in modulus.

The development of optical anisotropy gradient as a result of solvent evaporation for poly(amide–imide) (PAI) solution in dimethylacetamide (DMAc) was investigated for the first time. Experiments were carried out using real-time optical measurement with spectral birefringence technique coupled with off-line optical techniques such as the Abbe refractometer and optical compensator method. We have shown that drying process induces temporal evolution of nonuniform out-of-plane birefringence profile through the thickness direction while in-plane birefringence remained zero. The highest birefringence was observed at the substrate–solution interface at early stages of drying. Beyond a critical time, the formation of highly oriented layer was observed at the air–solution interface. This oriented layer progresses through the thickness direction as the solvent concentration is disproportionately reduced in these regions. Abbe refractometer results confirmed the anisotropy is preserved at longer drying times, the air–solution interface birefringence becoming higher compared to the substrate–solution interface. Overall, observations obtained by real-time measurement system agreed with off-line measurements.

A newly developed real-time multifunctional monitoring system was used to track in-plane and out-of-plane birefringence, weight, thickness and surface temperature of semiaromatic, transparent, and soluble polyimide cast films during drying. At a critical point during drying, out-of-plane birefringence develops rapidly when the cast film undergoes vitrification. The rapid development of out-of-plane birefringence is attributed mainly to polymer chain orientation caused primarily by in-plane confined drying induced by thickness reduction and secondarily by shrinkage stress development in the process of vitrification. Shrinkage stress and its magnitude depend on the details of freezing-in process as dictated by the coating and drying variables. Some drying solutions also found to develop a small negative in-plane birefringence with the higher refractive index in the direction transverse to the air flow direction and it is attributed primarily to the residual stress development. In addition, the influence of solvent type on this phenomenon was studied using series of polar organic solvents.

Drying and imidization of PMDA–ODA solutions in NMP were investigated by a novel custom designed measurement system that tracks real time weight, thickness, surface temperature, and in-plane and out-of-plane birefringence. At low temperature drying stage, the weight and thickness reductions occurred rapidly as a result of solvent evaporation. All the parameters started leveling off while the out of plane birefringence steadily increased and reached a plateau at longer drying times. When the temperature was increased for imidization reaction, additional weight loss accompanied by temporary reduction of birefringence was observed due to evaporation of bound solvent as solvent molecules decomplexed from the polymer chains and plasticized the film. During the latter stage, out-of-plane birefringence rose rapidly due to increased intrinsic birefringence of the polymer chains due to imidization reaction and orientation of polymer chains with their chain axes preferentially oriented in the film plane. Throughout the whole process the in-plane birefringence remained zero. For the first time, these real time measurements allowed us to quantitatively show the dynamics between chain relaxation due to evaporation of the decomplexed solvent molecules, and orientation development due to decreased chain mobility caused by imidization reaction and increasing Tg for the PMDA–ODA/NMP solutions. In addition, the dynamics of this interplay was investigated by varying the processing conditions: initial casting thickness and drying temperature.

We present an experimental study of real time true stress–strain–birefringence measurements to elucidate the sequence of structural mechanisms that occur during simultaneous biaxial stretching of PET films from amorphous precursors in rubbery state. Stress–birefringence relationship, wide angle X-ray diffraction, Raman spectroscopy and DSC thermal analysis were used to identify the stages of the mechano-optical behavior of the films during stretching, and to identify their structural origins. The measurements revealed four regimes for the relationship between the stress and birefringence. In the first regime the stress has a linear relationship with birefringence where the linear stress optical rule holds and the stress optical constant for PET is 5.8 GPa−1 (5800 Brewster). In the second regime, the relationship is also nearly linear with a steeper positive slope, and in the third regime the relationship is nonlinear. At very high stretching rates, a fourth stage could be seen, where the stress increases while the birefringence reaches a plateau. This stage is reached when the polymer chains attain their finite extensibilities. This stage was not observed if low rates of stretching employed, where high relaxation movements dominate the orientation effects. The deviation from the initial linear stress optical rule coincides with the onset of the stress-induced crystallization as revealed by the off line measurements. This transition was found to be rate dependent and increased rate delays this transition to higher stresses.

Hybrid films of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) were prepared with different molecular weights of poly(ethylene oxide) (PEO). The cross-linking reaction between PEO and PEDOT:PSS was performed at high temperature and confirmed by using differential scanning calorimeter (DSC), contact angle measurement, and solid-state 1H NMR. The effect of chemical reaction on the conductivity and morphology of these hybrid films was studied by using 4-point probe and atomic force microscope (AFM), respectively. As-spun PEO/PEDOT:PSS films have lower electric conductivity due to the addition of nonconductive PEO, and exhibits no molecular weight dependence on conductivity. After chemical cross-linking reaction at high temperature, only PEDOT:PSS films with lowest molecular weight PEO additives show enhanced conductivity with increasing reaction time. AFM result indicates that the heat-treated PEO/PEDOT:PSS hybrid films show grain-like morphology compared to ethylene glycol treated PEDOT:PSS films which shows continuous PEDOT domain. In the present work we demonstrate that the cross-linking reaction can be used to improve the wet stability of PEDOT:PSS nanofiber, showing good water resistance and excellent dimensional stability.

Structural evolution during constrained annealing of PLA films biaxially stretched in simultaneous and sequential biaxial stretching was compared. Annealing of simultaneous biaxially stretched films yields films with in-plane isotropy with (100) crystallographic planes parallel to the surface. The first stage of sequential stretching where the films are stretched in Uniaxial constrained mode was found to yield films exhibiting transverse isotropy. The transverse stretching of these films lead to formation of a distinct second population of chains primarily oriented in transverse direction, generating bimodal orientation texture. When the extent of stretching in two directions are balanced, constrained annealed samples were found to exhibit uniplanar axial (100)[001] texture with the primary chain axial direction now switched to transverse direction. Two new superstructures, along [110] and [100] respectively were discovered in annealed PLA films.

Silica aerogels are sol−gel-derived materials consisting of interconnected nanoparticle building blocks that form an open and highly porous three-dimensional silica network. Flexible aerogel films could have wide applications in various thermal insulation systems. However, aerogel thin films produced with a pure sol−gel process have inherent disadvantages, such as high fragility and moisture sensitivity, that hinder wider applications of these materials. We have developed synthesis and manufacturing methods to incorporate electrospun polyurethane nanofibers into the cast sol film prior to gelation of the silica-based gel in order to reinforce the structure and overcome disadvantages such as high fragility and poor mechanical strength. In this method, a two-stage sol−gel process was employed: (1) acid-catalyzed tetraethyl orthosilicate hydrolysis and (2) base-catalyzed gelation. By precisely controlling the sol gelation kinetics with the amount of base present in the formulation, nanofibers were electrospun into the sol before the onset of the gelation process and uniformly embedded in the silica network. Nanofiber reinforcement did not alter the thermal conductivity and rendered the final composite film bendable and flexible.Keywords: electrospinning; flexible superinsulator; hybrid film; nanofiber embedding; polyurethane; silica aerogel

Silica aerogels are comprised of highly porous three-dimensional networks. They typically are very fragile and brittle due to the inter-particle connections in the pearl-necklace-like fractal network. This behavior prevents their wider utility. The present study aims to reinforce the silica-based gel to improve the poor mechanical strength through crosslinking the silica particles with polyimide and incorporating Lucentite STN clay into the skeletal silica–polyimide network. 3-Aminopropyltriethoxysilane (APTES) end-capped polyamic acid oligomers were first formed followed by gelation with TMOS at a range of clay concentrations to generate a silica network. The incorporation of clay leads to slightly lower BET surface area with little effect on shrinkage, porosity and density. Microscopy revealed that the aerogel preferentially grows from the edges of well dispersed clay particles while minimal growth occurs from clay surfaces. The formation of covalent bonds and hydrogen bonding through the OH functionalized clay edges is thought to enhance the connectivity with silica network and clay, leading to a substantial reinforcement effect as evidenced by an increase in modulus.