Polydimethylsiloxane Curing Research Articles

Crosslinking behaviors and mechanical properties of curable PDMS and PEG films with various contents of glycidyl methacrylate

ABSTRACTThe real‐time curing behaviors and surface mechanical properties of two curable systems, polydimethylsiloxane (PDMS) and poly(ethylene glycol) (PEG) films containing different portions of glycidyl methacrylate (GMA), were investigated via various measurements for potential application as anti‐fouling coatings. In these mixtures, methacryloxypropyl‐terminated PDMS and poly(ethylene glycol)dimethacrylate were used as the difunctional crosslinkers, and GMA was employed as the epoxy component. During the coating process, UV irradiation generated network structures for both mixtures (PDMS‐GMA and PEG‐GMA), while they exhibited the different evolution of the elastic modulus with respect to the GMA content. It was found that a small portion of the crosslinkers formed at early stage has a dominant contribution to the overall film properties, leading to a small swelling ratio and large gel fraction. The indentation and scratch mechanical properties of the cured films were qualitatively well linked with the real‐time rheological data for the curable mixtures acquired during the curing process. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47088.

Patternable Poly(chloro-p-xylylene) Film with Tunable Surface Wettability Prepared by Temperature and Humidity Treatment on a Polydimethylsiloxane/Silica Coating.

Poly(chloro-p-xylylene) (PPXC) film has a water contact angle (WCA) of only about 84°. It is necessary to improve its hydrophobicity to prevent liquid water droplets from corroding or electrically shorting metallic circuits of semiconductor devices, sensors, microelectronics, and so on. Herein, we reported a facile approach to improve its surface hydrophobicity by varying surface pattern structures under different temperature and relative humidity (RH) conditions on a thermal curable polydimethylsiloxane (PDMS) and hydrophobic silica (SiO2) nanoparticle coating. Three distinct large-scale surface patterns were obtained mainly depending on the contents of SiO2 nanoparticles. The regularity of patterns was mainly controlled by the temperature and RH conditions. By changing the pattern structures, the surface wettability of PPXC film could be improved and its WCA was increased from 84° to 168°, displaying a superhydrophobic state. Meanwhile, it could be observed that water droplets on PPXC film with superhydrophobicity were transited from a “Wenzel” state to a “Cassie” state. The PPXC film with different surface patterns of 200 μm × 200 μm and the improved surface hydrophobicity showed wide application potentials in self-cleaning, electronic engineering, micro-contact printing, cell biology, and tissue engineering.

Hybrid 2D patterning using UV laser direct writing and aerosol jet printing of UV curable polydimethylsiloxane

The hybrid technique of aerosol jet printing and ultraviolet (UV) laser direct writing was developed for 2D patterning of thin film UV curable polydimethylsiloxane (PDMS). A dual atomizer module in an aerosol jet printing system generated aerosol jet streams from material components of the UV curable PDMS individually and enables the mixing in a controlled ratio. Precise control of the aerosol jet printing achieved the layer thickness of UV curable PDMS as thin as 1.6 μm. This aerosol jet printing system is advantageous because of its ability to print uniform thin-film coatings of UV curable PDMS on planar surfaces as well as free-form surfaces without the use of solvents. In addition, the hybrid 2D patterning using the combination of UV laser direct writing and aerosol jet printing achieved selective photo-initiated polymerization of the UV curable PDMS layer with an X-Y resolution of 17.5 μm.

Introduction of a Chemical-Free Metal PDMS Thermal Bonding for Fabrication of Flexible Electrode by Metal Transfer onto PDMS.

Polydimethylsiloxane (PDMS) is a flexible and biocompatible material widely used in the fabrication of microfluidic devices, and is often studied for the fabrication of flexible electrodes. The most popular method of fabricating a flexible electrode using PDMS is done by transferring a metal electrode onto said PDMS. However, the transfer process is difficult and the transferred metal layer is easily damaged due to inherently weak adhesion forces between the metal and PDMS, thus requiring a chemical treatment or sacrificial layer between the two. The fabrication process using a chemical treatment or sacrificial layer is complicated and expensive, which is the major limitation of using PDMS in the fabrication of flexible electrodes. This paper discusses the findings of a possible solution to create strong bonding between PDMS and various metals (copper, nickel and silver) using a chemical-free metal to PDMS thermal bonding technique. This method is the same as the PDMS curing process, but with a variation in the curing condition. The condition required to create strong bonding was studied by observing copper transferred by various PDMS curing conditions, including the standard condition. The condition creating the strong bonding was baking PDMS (5:1 = base polymer: curing agent) at 150 °C for 20 min. Experimentation showed that the optimum thickness of the transferred metal shows that the optimum thickness is approximately 500 nm, which allows for a higher resistance to stresses. The successful transfer of copper, nickel and silver layers onto PDMS with a stronger adhesion force opens up many new applications dealing with the fabrication of flexible electrodes, sensors, and flexible soft magnets.

UV laser direct writing of 2D/3D structures using photo-curable polydimethylsiloxane (PDMS)

Additive manufacturing with UV curable polydimethylsiloxane (PDMS) was achieved using UV laser direct writing. In these experiments, UV curable PDMS was locally polymerized to fabricate 1D and 2D single layer structures, as well as 3D multilayer structures. Line arrays with line widths between 18 and 47 µm were produced, and it was observed that good stability and repeatability of the photo-polymerization in the UV curable PDMS was possible. The 3D structures demonstrated the absorption depth of the UV curable PDMS, which was deeper than 3 mm, and enabled the fabrication of 3.1 mm tall structures with an aspect ratio of 2 in only a single layer. The 3D structures were sufficiently strong to show elastic properties. All surfaces were smooth and transparent. In addition, UV laser direct writing of UV curable PDMS realized patterning with uniform resolutions at each layer.

Fabrication of large all-PDMS micropatterned waveguides for lab on chip integration using a rapid prototyping technique

A simple method for manufacturing centimeter-long all-PDMS embedded and rib microwaveguides is presented. It allows for the fabrication of centimeter-long micromolds by direct laser ablation of a desired waveguide pattern inside an acrylic sheet to create a pattern which is then transferred to a poly-dimethylsiloxane (PDMS) layer using soft-lithography. A refractive index difference between the core and cladding of 1.3x10−3 was achieved by controlling the PDMS curing and linear attenuation of 1.27 dB/cm for embedded and 2.36 dB/cm for a rib waveguide, similar to other techniques. Finally, a beamsplitter was fabricated to demonstrate that our low-cost process is suitable to integrate waveguide devices on lab on chip platforms.

Flow lithography in ultraviolet-curable polydimethylsiloxane microfluidic chips.

Flow Lithography (FL) is the technique used for the synthesis of hydrogel microparticles with various complex shapes and distinct chemical compositions by combining microfluidics with photolithography. Although polydimethylsiloxane (PDMS) has been used most widely as almost the sole material for FL, PDMS microfluidic chips have limitations: (1) undesired shrinkage due to the thermal expansion of masters used for replica molding and (2) interfacial delamination between two thermally cured PDMS layers. Here, we propose the utilization of ultraviolet (UV)-curable PDMS (X-34-4184) for FL as an excellent alternative material of the conventional PDMS. Our proposed utilization of the UV-curable PDMS offers three key advantages, observed in our study: (1) UV-curable PDMS exhibited almost the same oxygen permeability as the conventional PDMS. (2) The almost complete absence of shrinkage facilitated the fabrication of more precise reverse duplication of microstructures. (3) UV-cured PDMS microfluidic chips were capable of much stronger interfacial bonding so that the burst pressure increased to ∼0.9 MPa. Owing to these benefits, we demonstrated a substantial improvement of productivity in synthesizing polyethylene glycol diacrylate microparticles via stop flow lithography, by applying a flow time (40 ms) an order of magnitude shorter. Our results suggest that UV-cured PDMS chips can be used as a general platform for various types of flow lithography and also be employed readily in other applications where very precise replication of structures on micro- or sub-micrometer scales and/or strong interfacial bonding are desirable.

On-demand curing of polydimethylsiloxane (PDMS) using the photothermal effect of gold nanoparticles.

The photothermal effect of gold nanoparticles (AuNPs) produces extremely localized heat that can be harnessed to drive large scale chemical reactions by simultaneously generating many individual reactions on the nanoscale. We use the photothermal effect to enhance the curing rate of polydimethylsiloxane (PDMS) by a factor of 4.9 × 109. Photothermal curing occurs via crosslinking reactions between vinyl and Si-H groups of the pre-polymer, and the course of the reaction was followed by monitoring the disappearance of infrared bands associated with these functional groups. Using mass spectroscopy, we verify that the major polymer m/z peaks are identical for both traditionally and photothermally cured polymers, indicating that the photothermal effect of AuNPs is an effective way in which to supply on-demand curing of PDMS.

High fidelity 3D thermal nanoimprint with UV curable polydimethyl siloxane stamps

A two-step replication process chain is developed for a microlens array structure with deep three dimensional (3D) reliefs and sharp features enabling the transfer of a photocured acrylic resist patterns into thermoplastic poly-methyl methacrylate (PMMA) with the same structural polarity via an intermediate stamp. By using ultraviolet (UV)-curable polydimethyl siloxane (PDMS), high fidelity negatives were cast from the original microstructures made by two-photon-polymerization and subsequently replicated into PMMA using thermal imprint. The mechanical properties of the new UV-PDMS (X-34-4184, Shin-Etsu Chemical Company, Ltd.), along with its nearly zero process shrinkage, proved to be highly suitable to replicate both 50 μm high concave features and sharp tips with an apex diameter of 500 nm. The results prove that silicone rubber, despite its elasticity, has specific advantages in thermal imprint in structures where both tall microstructures and submicron surface structures have to be replicated. This way, high fidelity PMMA structures with low defects could be prepared by the optimized processing found in this work to have a replication of 3D masters for further upscaling.

Fabrication of pneumatic valves with spherical dome-shape fluid chambers

This paper describes a simple fabrication method for pneumatic valves with spherical dome-shape fluidic chambers and channels. The proposed method is based on replicating a deformed polydimethylsiloxane (PDMS) diaphragm using a variable PDMS mold during the liquid PDMS curing process. A PDMS mold structure with a sealed cavity is prepared through a typical replica molding process and structural bonding with a glass substrate. The PDMS diaphragm inflates due to thermal expansion of air in the sealed cavity when it is placed on a hotplate during the thermal curing process, and this shape is transferred to liquid PDMS poured on the active PDMS structure. The proposed pneumatic valve can then be fabricated by bonding the valve chamber to the diaphragm. The effectiveness of the proposed valve was verified through optical observations of the valve pattern and measurements of the temporal response of the flow rate using pressurized liquid flow rectified by the fabricated valve. A finite element method was used to provide a structural analysis of the valve diaphragm to examine the characteristics of the rectified flow rate of the working fluid due to structural deformation of the valve diaphragm. The proposed fabricated valve effectively overcame the drawbacks of micro-fluid channels with rectangular cross sections and was also suitable for other micro-devices with spherical dome-shape cross sections.

Highly elastic and transparent multiwalled carbon nanotube/polydimethylsiloxane bilayer films as electric heating materials

Abstract Highly elastic and transparent bilayer films composed of MWCNT and polydimethylsiloxane (PDMS) layers were fabricated by spin-coating of MWCNT aqueous solution on glass plates and following curing of PDMS applied on the MWCNT layer. Morphological feature, optical transparency, tensile property, electrical property, and electric heating behavior of the bilayer films with different MWCNT layer thicknesses of 65–185 nm were investigated. SEM images confirmed that pristine MWCNTs were uniformly deposited on glass substrates and the PDMS layer was combined well with the MWCNT layer, resulting in high structural stability of the bilayer films to high elongational or twisting deformations. With the increase of the thickness of the MWCNT layer, the sheet resistance of the bilayer films decreased substantially from ~ 105 Ω/sq to ~ 103 Ω/sq, in addition to the change of the optical transmittance from ~ 75% to ~ 40% at a 550 nm wavelength. The electric heating behavior of MWCNT/PDMS bilayer films was strongly dependent on the thickness of the MWCNT layer as well as the applied voltage. Even under high twisting by 540° or continuous stepwise voltage changes for long periods of time, the MWCNT/PDMS bilayer films retained stable electrical heating performance in aspects of temperature responsiveness, steady-state maximum temperature, and electric power efficiency.

Flexible Pressure Sensor Based on Photo Paper-Molded Micro-Structural AgNWs/PDMS Electrode

The novel flexible pressure sensor with skin-like stretchability and sensibility has attracted tremendous attention in academic and industrial world in recent years. And it also has demonstrated great potential in the applications of electronic skin and wearable devices. It is significant and challenging to develop a highly sensitive flexible pressure sensor with a simple, low energy consuming and low cost method. In this paper, the silver nanowires (AgNWs) as electrode material were synthesized by polyol process. The polydimethylsiloxane (PDMS) was chosen as a flexible substrate and polyimide (PI) film as dielectric layer. The AgNWs based electrode was prepared in two methods. One is coating the AgNWs on photographic paper followed by in situ PDMS curing. Another one is suction filtration of the AgNWs suspension followed by glass slide transfer and PDMS curing. Then the capacitive pressure sensor was packaged in a sandwich structure with two face to face electrodes and a PI film in the middle. The sensitivity of the sensor as well as the micro-structure of the electrodes was compared and studied. The results indicate that the roughness of the electrode based on AgNWs/PDMS micro-structure plays an important role in the sensitivity of sensor. The as-prepared flexible pressure sensor demonstrates high sensitivity of 0.65kPa-1. In addition, the fabrication method is simple, low energy consuming and low cost, which has great potential in the detection of pulse, heart rate, sound vibration and other tiny pressure.

Enhanced adhesion of bioinspired nanopatterned elastomers via colloidal surface assembly.

We describe a scalable method to fabricate nanopatterned bioinspired dry adhesives using colloidal lithography. Close-packed monolayers of polystyrene particles were formed at the air/water interface, on which polydimethylsiloxane (PDMS) was applied. The order of the colloidal monolayer and the immersion depth of the particles were tuned by altering the pH and ionic strength of the water. Initially, PDMS completely wetted the air/water interface outside the monolayer, thereby compressing the monolayer as in a Langmuir trough; further application of PDMS subsequently covered the colloidal monolayers. PDMS curing and particle extraction resulted in elastomers patterned with nanodimples. Adhesion and friction of these nanopatterned surfaces with varying dimple depth were studied using a spherical probe as a counter-surface. Compared with smooth surfaces, adhesion of nanopatterned surfaces was enhanced, which is attributed to an energy-dissipating mechanism during pull-off. All nanopatterned surfaces showed a significant decrease in friction compared with smooth surfaces.

Fabrication of silver nanorods embedded in PDMS film and its application for strain sensing

Highly reflective and surface conductive strain gauges have been prepared by embedding the silver nanorods (AgNRs) into polydimethylsiloxane (PDMS). Thermal curing of PDMS on AgNRs grown Si wafer leads to a flexible, reflective and conductive silver surface. The reflectance of the as prepared films were observed to be 60% with a low value of sheet resistance. The reflectance of the film was able to be tuned from 60% to 15% in the visible region. The fabrication of a parallel plate capacitor strain sensor from AgNRs embedded PDMS, and tuning of the capacitance with respect to the applied strain, leads to a gauge factor of ~1. These mechanically tunable AgNRs/PDMS films demonstrate potential application as a strain sensor.

Polydimethylsiloxane Microstencils Molded on 3-D-Printed Templates

Microstencils have been utilized in biomedical engineering to pattern cell cultures, engineer tissues, and pattern conductive materials in microelectronics for the measurement of bioelectric activity. However, fabricating these microstencils can be considerably time consuming, expensive, and cleanroom processing or laser micromachining intensive. We present microfabrication strategies for producing stencils rapidly and cost effectively, with minimal use of cleanroom facilities, ideal for prototyping or applications where microfabrication costs need to be conserved. The process utilizes 3-D-printed templates as master structures from, which polydimethylsiloxane (PDMS) microstencils are molded. The entire process, from concept to completed stencil, requires approximately one day; however, the majority of this time is budgeted for PDMS curing cycles, so it requires only ~1-2 person hours to complete. These microstencils were used to pattern metal traces ~160-1000-μm wide, on three commonly used BioMEMS substrate materials- to pattern rat cortical neuronal cell cultures with radii between ~300 and 1000 μm-and to pattern organic materials. With the advancement of 3-D-printing technologies, we anticipate that our presented processes will improve in resolution and gain a greater cost advantage over traditional microstencilling methods.

Fabrication and characterization of non-linear parabolic microporous membranes

Large scale fabrication of non-linear microporous membranes is of technological importance in many applications ranging from separation to microfluidics. However, their fabrication using traditional techniques is limited in scope. We report on fabrication and characterization of non-linear parabolic micropores (PMS) in polymer by utilizing flow properties of fluids. The shape of the fabricated PMS corroborated well with simplified Navier–Stokes equation describing parabolic relationship of the form L–t1/2. Here, L is a measure of the diameter of the fabricated micropores during flow time (t). The surface of PMS is smooth due to fluid surface tension at fluid–air interface. We demonstrate fabrication of PMS using curable polydimethylsiloxane (PDMS). The parabolic shape of micropores was a result of interplay between horizontal and vertical fluid movements due to capillary, viscoelastic, and gravitational forces. We also demonstrate fabrication of asymmetric “off-centered PMS” and an array of PMS membranes using this simple fabrication technique. PMS containing membranes with nanoscale dimensions are also possible by controlling the experimental conditions. The present method provides a simple, easy to adopt, and energy efficient way for fabricating non-linear parabolic shape pores at microscale. The prepared parabolic membranes may find applications in many areas including separation, parabolic optics, micro-nozzles/-valves/-pumps, and microfluidic and microelectronic delivery systems.

Development of a rapid cure polydimethylsiloxane replication process with near-zero shrinkage

Replicated polydimethylsiloxane (PDMS) micro/nanostructures are widely used in various research fields due to their inexpensiveness, flexibility, low surface energy, good optical properties, biocompatibility, chemical inertness, high durability, and easy fabrication process. However, the application of PDMS micro/nanostructures is limited when an accurate pattern shape or position is required because of the shrinkage that occurs during the PDMS curing process. In this study, we analyzed the effects of processing parameters in the PDMS replication process on the shrinkage of the final structure. Although the shrinkage can be decreased by decreasing the curing temperature, this reduction also increases the unnecessary curing time. To minimize the inherent shrinkage in the PDMS replica without an accompanying curing time increase, we propose a PDMS replication process on a high modulus substrate (glass and polymer films) with compression pressure, in which the adhesion force between the substrate and the PDMS, and the compression pressure prevent shrinkage during the curing process. Using the proposed method, a PDMS replica with less than 0.1% in-plane and vertical shrinkage was obtained at a curing temperature of 150°C and a curing time of 10 min.

Self-Enclosed Nanofiber-Reinforced PDMS Channels Using Ultrafast Laser Irradiation

We report on an innovative method to fabricate self-enclosed reinforced Polydimethylsiloxane (PDMS) microchannels using femtosecond laser pulses of megahertz pulse frequency. It involves nanofibrous structure generation and site-specific curing of PDMS. The rapid curing is due to accumulated heating at repetitive irradiation and can be explained by a volume-constrained temperature rise. The proposed technique permits both reinforcement and rapid curing and is ideal for fabricating reinforced structures in microscale. No additives are used in fabricating the nanofiber-reinforced enclosed channels. Since our method does not require a mask, it is relatively cheaper and faster compared with other conventional lithography techniques.

Polydimethylsiloxane (PDMS) Coating onto Magnetic Nanoparticles Induced by Attractive Electrostatic Interaction

In this article, we present an efficient synthesis pathway of polydimethylsiloxane (PDMS) coated magnetic nanoparticles from hydrophilic polyacrylate coated ferrofluids (NPPAA). A block copolymer based on polydimethylsiloxane is selected for its propensity to interact with the carboxylate functions on the NPPAA. The interaction is due to negative charges on NPPAA and positive ones on the amphiphilic copolymer. The synthesis is achieved by interfacial interaction, simplifying the purification of the PDMS-coated nanoparticles (NPPDMS) from subproducts such as ions and water. NPPDMS are well dispersed in hydrophobic solvents (toluene, diethyl ether) and can then be embedded into a curable PDMS polymer.

Fabrication of novel lightweight composites by a hydrogel templating technique

A non-conventional way of preparation of lightweight porous materials by templating hydrogels with a range of hydrophilic and hydrophobic scaffolding materials was explored. Sub-millimetre hydrogel slurries of polyacrylamide and gellan gum were templated with aqueous slurries of cement, gypsum and clay–cement mixtures or alternatively, dispersed in curable polydimethylsiloxane (PDMS). After the solidification of the scaffolding material, the evaporation of structured hydrogel produced porous composite material whose pores mimic the hydrogel meso-structure. We studied the density, volume contraction and the compression strength of the formed porous materials as function of the hydrogel initial volume fraction. This versatile hydrogel templating method can be applied very inexpensively to a range of scaffolding materials to yield lightweight porous materials with a great potential for use in the building industry in heat and sound insulation panels, an alternative to aerated concretes, lightweight building blocks, porous rubber substitutes and foam shock absorbers.