Porous polymer composite with tailored porosity is applied in the myriads of areas such as energy storage, oil/water absorption, bioengineering, and advanced areas of material science. The emulsion templating technology is one of the most popular methods for synthesizing porous polymer composite. It involves solidifying a two-phase mixture of porogen and polymer, then removing porogen to create pores within the continuous emulsion phase by polymerization or curing. The surfactant plays a pivotal role in accomplishing a stable emulsion, a key factor in designing the internal porous structure. This study highlights the effect of silica filler and mixing time on pore morphology, i.e., shape, size, and distribution. on polydimethylsiloxane (PDMS) porous structure utilizing the water-in-oil emulsion templating method. Span® 80 is used as a surfactant to reduce the surface tension between water, silica, and PDMS and simultaneously create a strong foaming effect. Different weight concentrations of silica (1-10 wt%) were chosen while keeping the internal phase, i.e., water (50 wt%) constant. The designed porous structures were further characterized through scanning electron microscopy (SEM). Porous composite specimens fabricated with higher silica content and mixing time consistently exhibit smaller pore sizes than specimens fabricated with lower mixing time and silica content. A breakthrough of pore morphology is seen at silica content higher than 5wt% at 1 min mixing, however, pore morphology drastically changes when mixing time increases from 1 min to 6 min. Variation of finer mixing time beyond 1 min shows stepwise changes in pore morphology from a large single-phase porous structure to a bi-modal porous structure which eventually become a smaller single-mode porous structure. Thus, the emulsion templating technique, in combination with different filler content and mixing time, will effectively aid in designing engineered porous polymer composite with varying stiffness and pore morphology.

A high-performance polymer (HPP) (polyether ether ketone, PEEK) blended with a commercial low-temperature polymer (polyamide 12, PA-12) was fabricated using a low-temperature selective laser sintering (SLS) 3D printer. Different post-processing heat treatments were implemented after fabrication to tune the thermal and mechanical properties of printed blends. The specific mechanical modulus of the blend was increased up to 79% in comparison to printed PA-12 samples with heat treatment. Additionally, the thermal resistance of the treated blends greatly exceeded the properties observed using PA-12 alone. This research proposes a methodology to fabricate and engineer HPP using desktop SLS printers through a secondary lower melting temperature material and heat treatments.

of the BTO dipoles was achieved through corona poling method, which applies an electric charge on the surface layer of the functional material, aligning the dipoles in the desired direction and thus gaining the piezoelectricity. Different BTO mixing ratios (10–50 wt. %) were evaluated in order to obtain tunable piezoelectric properties and compare the sensitivity with respect their elastic properties. Tensile testing and piezoelectric testing were carried out to characterize mechanical and piezoelectric properties. Results showed that fabricated PDMS with 50 wt. % BTO gave the highest piezoelectric coefficient (d33) of 11.5 pC N-1 and with an output voltage of 385 mV under compression loading of >200 lbF. This demonstrates feasibility of using multi-material DIW printing to fabricate piezoelectric force sensors with integrated electrodes in one-step without compromising the flexibility of the material.

Thermoplastic materials such as PA12 and PA6 have been extensively employed in Selective Laser Sintering (SLS) 3D printing applications due to their printability, processability, and crystalline structure. However, thermoplastic-based materials lack polymer inter-chain bonding, resulting in inferior mechanical and thermal properties and relatively low fatigue behavior. Therefore, 3D printing of high-performance crosslinked thermosets using SLS technology is paramount to pursue as an alternative to thermoplastics. In this work, a thermoset resin was successfully 3D printed using SLS, and its thermal stability of printed parts after a multi-step post-curing process was investigated. Dimensionally stable and high glass transition temperature (Tg: ~300 °C) thermoset parts were fabricated using SLS. The polymer crosslinking mechanism during the printing and curing process was investigated through FTIR spectra, while the mechanical stability of the SLS 3D-printed thermoset was characterized through compression tests. It is found that 100% crosslinked thermoset can be 3D printed with 900% higher compressive strength than printed green parts.

as the hydration level at the interface was reduced. The reduced mobility of the PDMS chains in the interfacial domain reduced the overall, bulk, motional properties of the polymer, thus causing an effective ''stiffening'' of the polymer matrix. The role of the long-ranged Coulombic interactions on the structural features and chain dynamics of the polymer were also examined. Both are found to be strongly influenced by the electrostatic interactions as identified by the bond orientation time correlation function and local density distribution functions. These results have important implications for the design of nanocomposite silica-siloxane materials.

This article reports the fabrication and characterization of porous polydimethylsiloxane (PDMS) structures developed by the solvent evaporation-induced phase separation technique. Ternary systems containing water/tetrahydrofuran (THF)/PDMS with various concentrations are produced to form a stable solution. The porous PDMS structures are formed by removing the solvent (THF) and nonsolvent (water) phases during the stepping heat treatment procedure. The analytical ternary phase diagram is constructed based on the thermodynamic equilibrium state in the polymer solution to explain the stable/unstable formulations and the possible composition change path. The results show that the isolated pores with the adjustable pore size ranging from 330 to 1900 mu m are obtained by tuning the water to the THF ratio. The mechanical properties of the porous PDMS structures are determined by conducting the tensile tests on the prepared dog bone-shaped specimens. A wide range of elastic modulus ranging between 0.49 and 1.05 MPa was achieved without affecting the density of the porous sample by adjusting the solvent and non-solvent content in the solution. It is shown that the flexibility of the porous structures can be improved by reducing the ratio of water to THF and decreasing the PDMS content. The porosity measurements reveal that the PDMS concentration is the major phase controlling the porosity of the structure, while the effect of water/THF is negligible.

Porous polydimethylsiloxane (PDMS) framework with adjustable pore structures has been fabricated by polymerization of the continuous phase in the emulsion templates. Different types of surfactants, including nonionic (Triton X-100), cationic (Benzalkonium chloride), anionic (sodium dodecyl sulfate), and silicone-based block copolymer were used to stabilize the water-in-oil emulsion system. Deionized water with a wide range of internal phases varying from 10% to 60% by weight was employed to make the low internal phase emulsion and medium internal phase emulsion. The effect of surfactant type, surfactant concentration, and the internal phase volume on the stability of the emulsion, pore morphology, and pore size distribution was explored. The stability of the emulsion was investigated by comparing the pore morphology of the cured sample at different set times, such as 0, 6, and 72 h. Scanning electron microscopy was employed for the characterization of the porous structures. The image analysis was conducted, and the pore size distribution, porosity, and open-cell ratio of each sample were calculated. Interconnected pores have been seen in the porous PDMS made from emulsions with an internal phase larger than 30%. The results demonstrated that the frequency of open-cell pores and the pore size is dependent on the surfactant types.

A new technology is under development to selectively recover regulated metal ions from electroplating rinse waters. The electroplating metal ions are recovered in a concentrated form with the appropriate counter ions ready for return to the original electroplating bath. The technology is based on the use of specially designed water-soluble polymers that selectively bind with the metal ions in the rinse bath. The polymers have such a large molecular weight that they can be physically separated using available ultrafiltration technology. The advantages of this technology are high metal selectivity with no sludge formation, rapid processing, low energy, low capital costs, and small size. We have tested and demonstrated the recovery of zinc and nickel (using a new alloy electroplating bath designed to replace cadmium) from rinse waters. The metal-ion concentrate was returned to the original electroplating bath. Impurity metals such as iron and copper were removed from the zinc/nickel concentrate and were not returned to the electroplating bath. Test panels were electroplated as a baseline and compared with new test panels electroplated after the addition of the recovered zinc and nickel. No adverse effects on the bath integrity were observed. The rinse water was depleted of the electroplating metals to less than 0.1 ppm zinc and nickel, and the water was satisfactory for discharge to the sewer system.

Direct-Ink-Writing (DIW) provides unprecedented flexibility in regulating the different compositions and fabricating complex geometry when compared with traditional manufacturing methods. In this paper, polydimethylsiloxane (PDMS) polymer is modified with a thixotropic filler, such as silica and conducting additives, such as carbon nanotubes (CNT) and carbon black (CB) to investigate the viscoelastic ink for the DIW printing. Different amounts of silica in the range from 10-20% are considered. Based on the percolation threshold (1.5% for CNT and 15% for CB), hybrid conducting additives of CB in the amount 1-2wt% are considered to fine-tune the viscosity and rheology of the ink which eventually transform the ink into an electrically conductive while exhibiting shear thinning characteristics. The resulting inks are used to print different cross-sectional geometries and further optimized for further use as piezoresistive sensors. It is found that silica filler in the amount beyond 15% puts a limit in the 3D printer as it takes a significant amount of force to extrude. It is found that the addition of inhibitor greatly decouples the curing process, which indeed assists in printing more complex structures over a few hours. Mechanical and piezoresistive data of some selected inks shown potential as sensor applications in soft robotics and pressure sensing.

Here, we report all-polymer polysiloxane composites that overcome the long-standing processing problems of silica-reinforced silicone rubbers. Polystyrene fillers are dispersed with styrene/dimethylsiloxane symmetric diblock and triblock copolymers that control the filler morphology, filler-matrix interactions, and filler-filler interactions. Surprisingly, the composites not only rival the traditional silica-reinforced polysiloxane in mechanical properties of cured materials but also have better processability and stability than the silica-filled compound before curing. Large amplitude oscillatory shear experiments demonstrate that the triblock copolymer addition strongly affects the rheological properties. We hypothesize that the bridges and entangled loops that were formed by the triblock copolymer can connect different PS domains to provide additional reinforcement. The aging effect that originates from PDMS chain adsorption on the filler particle surface is also avoided because of the thermodynamic repulsion between PS and PDMS phases.

Elastomeric sensors have a wide range of applications in fields such as structural health monitoring, robotics, and biomedical industries. The additive manufacturing of these sensors, achieved using Direct Ink Writing (DIW), has provided numerous advantages, including increased sensitivity and the fabrication of complex geometry. However, bulk material sensors manufactured additively or using conventional methods, display a more significant effect of hysteresis, especially at strains higher than 10%, which limits their sensitivity. They also show a higher level of material relaxation whereby the resistance change within the material decreases in cyclic loading of the same strain and strain rate. Sensors with significant amounts of porosity have decreased hysteresis and high sensitivity at the lower strain. Porosity can be introduced in the form of varying infill densities and patterns made possible by DIW, which reduces these bulk material effects. In this paper, a lattice structure with four infill densities are investigated, and the samples are 3D printed using a grid infill pattern. The fabricated samples are characterized using a scanning electron microscope (SEM) to validate the microstructural features and layer bonding. Each sensor’s pressure-sensing capability is investigated using cyclic compression loading at various maximum strains. Sensing experiments show an increase in strain sensitivity with the introduction in porosity, compared to bulk samples of the same material and geometry. It is found that introducing porosity using DIW is a sensible strategy to improve the piezoresistive performance of nanocomposites and to allow for the tunability of sensing capacity in piezoresistive strain sensors.

This paper presents the fabrication and characterization of porous polydimethylsiloxane (PDMS) plates. The framework for obtaining porous PDMS is based on the solvent evaporation induced phase separation technique. A mixture of PDMS, water, and tetrahydrofuran (THF) with different concentrations is prepared. The three phases are stirred to reach a highly stable and viscous solution. The THF and water phases are evaporated during a curing cycle by applying a stepping heat treatment. The porous PDMS sheets with a wide range of pore sizes are fabricated by controlling the ratio of water to THF in the mixture. The confocal microscopy images are used to characterize the average pore size and the pore size distribution in the structures. Dogbone samples following the ASTM standard D412 are cut from the porous plates by utilizing a designed cutting die and mechanical press. The specimens are tested under tensile loading to evaluate the effect of the pore size on the mechanical properties of the porous structure. The results demonstrate the ability of the proposed solvent evaporation method to control the stiffness of the porous structure by changing the non-solvent to the solvent ratio in the mixture.

Here we report microphase-separated poly(styrene-block-dimethylsiloxane) (PS-b-PDMS) as a reinforcing filler in PDMS thermosets that overcomes the long-standing problem of aging in the processing of silica-reinforced silicone. Surprisingly, PS-b-PDMS reinforced composites display comparable mechanical performance to silica-modified analogs, even though the modulus of PS is much smaller than that of silica and there is no evidence of percolation with respect to the rigid PS domains. We have found that a few unique characteristics contribute to the reinforcing performance of PS-b-PDMS. The strong self-assembly behavior promotes batch-to-batch repeatability by having well-dispersed fillers. The structure and size of the fillers depend on the loading and characteristics of both filler and matrix, along with the shear effect. The reinforcing effect of PS-b-PDMS is mostly brought by the entanglements between the corona layer of the filler and the matrix, rather than the hydrodynamic reinforcement of the PS phase.

This paper presents the fabrication, optimization, and characterization of in-situ pressure sensor arrays using polydimethylsiloxane (PDMS) and carbon nanofibers (CNFs) nanocomposites. We first synthesize and characterize the nanocomposites to identify the optimal material formulation and fabrication procedure. Structural optimization algorithms and finite element method are employed to optimize the geometries of sensors. Pressure sensing units in cylinder and conical shapes are fabricated using the optimized material formulation and geometries. Two prototypes of sensors arrays are assembled and tested under different pressure load conditions. The long term sensor performance is validated using cyclic compression tests.

A polydimethylsiloxane (PDMS)/carbon nanofiber (CNF) nanocomposite with piezoresistive sensing function is presented. Excellent electrical conductivity is achieved by dispersing the CNFs into PDMS. A facile, low cost, and scalable fabrication procedure allows the sensors to be made in different shapes. The piezoresistive sensors show repeatable response up to 30% tensile strain. In addition, the characterization of sensing mechanism using an in situ mechanical testing system within a scanning electron microscope reveals the reorganization of CNF network by varying fiber alignment and interfiber distance in the nanocomposites under tensile load. To validate the wearable sensing capability, nanocomposite straps are employed to monitor the finger motions under various bending speed and holding time. Two different shapes of compressive sensors, including cylinder and truncated cone, are tested in compression strain as low as 3%, with gauge factors of 18.3 and 6.3, respectively. In addition, the sensing capability is independent of the applied strain rates and is highly repeatable in 1000 cycles under compression. Finally, the developed nanocomposites are made into sensor arrays for pressure sensing ranging from 35 to 690 kPa. The versatility and ease of fabrication of the reported nanocomposites can bridge the current challenge between performance and applicability of flexible sensors.

Pristine polymers, when extruded for 3D printing purposes, often lack the ability to hold their shape after deposition due to insufficient yield stress, spreading as they wet on the deposited surface almost immediately upon exiting the nozzle. However, the addition of fillers, such as silica, can alter an ink’s rheological properties, viscoelastic properties, and shear thinning behavior, while endowing the ink with the yield stress and thixotropic behavior to hold its shape upon printing. With the addition of conductive nanofillers, such as carbon nanotubes (CNTs), these polymer-based nanocomposites can be used as 3D printable, functional inks. In this work, we study the effects of silicon-based nanofillers, on rheological behavior and 3D printability of PDMS/CNT nanocomposites. The shear thinning properties and yield point of the nanocomposite at various filler content is explored via peak hold and amplitude sweep tests, respectively. To explore the effects of the fillers on the 3D printing of the functional ink, multi-layered geometries are printed and studied under scanning electron microscopy (SEM). These images show the shape of each stacked layer and provide insight on the controllability of the print and the bonding of each layer to its adjacent. Studying the effect of fillers on the 3D printability of polymer-based functional inks is valuable, as these inks allow for the quick and facile generation of custom multifunctional structures for use in a wide array of applications.

Low-density, high-strength, and high-temperature foams are recently finding new structural applications in the aerospace sector. In many instances, these foams are comprised of a thermosetting polymer resin incorporating dispersed microballoons, of a variety of materials, to achieve low mass density and other behaviors. Bismaleimide (BMI) is a thermosetting resin that is being used in these applications, in a variety of formulations. In this study, BMI is combined with Apocure-601 (APO). While a purified grade of the APO-BMI resin is achievable, an unpurified, production-ready grade is regularly utilized. To understand the performance and aging mechanisms of the two grades, and differences between them, they have been characterized by broadband dielectric spectroscopy over frequency range of 10−1 Hz to 106 Hz and temperature range of −140°C to approximately 300°C. Additionally, the thermogravimetric analysis (TGA) coupled with an evolved gas characterization technique (mass spectrometry) has been applied. Results show that impurities in the production grade APO-BMI lead to significant differences in the dielectric alpha-relaxation, in comparison to those measured on the purified APO-BMI resin. Additionally, kinetic models based on TGA results were developed to describe the thermo-oxidative degradation of the two materials. Using these models, life predictions based on both isothermal and cyclic exposure to temperatures up to 300°C and durations up to 25 h have shown significantly more higher mass loss for production grade material compared to the purified APO-BMI. Additional use of this model for specific conditions may provide beneficial information to the aerospace industry.