Flexible Polymeric Materials Research Articles

Highly Flexible, Tough, and Self-Healing Supramolecular Polymeric Materials Using Host-Guest Interaction.

Flexible, tough, and self-healable polymeric materials are promising to be a solution to the energy problem by substituting for conventional heavy materials. A fusion of supramolecular chemistry and polymer chemistry is a powerful method to create such intelligent materials. Here, a supramolecular polymeric material using multipoint molecular recognition between cyclodextrin (CD) and hydrophobic guest molecules at polymer side chain is reported. A transparent, flexible, and tough hydrogel (host-guest gel) is formed by a simple preparation procedure. The host-guest gel shows self-healing property in both wet state and dry state due to reversible nature of host-guest interaction. The practical utility of the host-guest gel as a scratch curable coating is demonstrated.

Dielectric properties of castor oil derived polymer filled with sol‐gel derived strontium barium niobate (Sr0.3Ba0.7Nb2O6) ferroelectric powder

Sol‐gel derived strontium barium niobate (Sr0.3Ba0.7Nb2O6) (SBN) nanocrystalline powder was homogeneously dispersed in castor oil derived resinous polymer to develop a polymer nanocomposite. The main objective of this work was to develop a large surface area flexible polymeric material having desired mechanical, dielectric, and pyroelectric properties for use in pyroelectric detectors. Sol‐gel synthesized nanocrystalline SBN powder was characterized using X‐ray diffraction, FTIR, and Raman spectroscopy to study their structure. These powders were used to fabricate a ceramic as well as a polymer nanocomposite, which were then characterized for their morphological, thermal, and dielectric behavior using SEM, DSC‐TGA, and Agilent LCR meter, respectively. Dielectric constant of unfilled polymer at room temperature and at all frequencies was found to be around 16–18, which steadily increased with increasing temperature. However, when filled with SBN powder, the resulting polymer nanocomposite showed an increase in the dielectric constant and attained a value of 20–30 at room temperature, which further increased with increasing temperature. For SBN ceramic, the value obtained for ε′max (dielectric constant at Tc = 250°C) was around 600 (at 1 kHz) and ∼2000 (at 100 Hz). POLYM. COMPOS., 38:2119–2127, 2017. © 2015 Society of Plastics Engineers

Patterning of Polymer Arrays with Enhanced Aspect-Ratio Using Hybrid Substrate Conformal Imprint Lithography

Ultraviolet Substrate Conformal Imprint Lithography (SCIL) is an economic nanolithography technique of imprinting high-fidelity patterns over large areas. It is an improvement to conventional UV-Nanoimprint lithography (NIL) with high resolution, flexibility and conformal imprinting. However the aspect-ratio of SCIL is limited due to its patterning by capillary forces and the PDMS material has very low Young’s modulus (<2Mpa) hence, resulting in deformation of the feature sizes during the imprint process. In general the residual layer between the imprinted structures is etched away to obtain high aspect ratio. But in industrial applications an array of different 3D patterns are imprinted in a single step, thus, the residual layer of each structure depends on its volume and viscosity of the imprint material, thereby making the etching process impossible to increase the feature size and aspect-ratio of each microstructure individually. Hence in this paper a new SCIL stamp technique is demonstrated to implement an enhancement of aspect ratio. In this process, a flexible polymer material, Polydimethylsiloxane (PDMS) is used as the stamp material. The heights of the structure on the stamp are supported by a metal layer (higher Young’s modulus), which provides higher stiffness and rigidity to the stamp and thereby avoiding distortion of the patterns on the stamp. Also the metal layer acts as an UV blocking mask: hence while patterning on an UV curable imprint material, the residual edges of the microstructures, which are subjected to decrease the feature size, are removed in the development process of the uncured polymer areas. Thus, irrespective of the number and shape of the microstructures, an enhancement of aspect ratio is attained in a single step. Conventional imprinting of circular polymer arrays with dimensions 90μm / 70nm on their master template resulted in increased feature size up to 150 μm, while the height varied between 90-66nm and thereby decreasing the aspect ratio. Imprinting with the proposed SCIL technique resulted in 96 μm / 74nm dimension, thus around 60 μm of feature size improvement is achieved

Synthesis of flexible polymeric shielding materials for soft gamma rays: Physicomechanical and attenuation characteristics of radiation crosslinked polydimethylsiloxane/Bi2O3 composites

Flexible lead‐free high energy radiation shielding material was developed through internal compounding. Polymer‐filler interaction, crosslinking density, specific gravity, physicomechanical characteristics, percentage attenuation, and thermal stability of the crosslinked composites were estimated. It was found that even at very high filler loading composites can be crosslinked; however, the crosslinking density was composition dependent and was highest in 10–50 wt% loading range at 100 kGy and 200 kGy. The Nielsen model was applied to understand the micromechanics of the system. Attenuation of gamma radiation from Am241 was not affected by the crosslinking density. Thermal stability of the composites was found to be significantly affected with bismuth oxide loading. POLYM. COMPOS., 37:756–762, 2016. © 2014 Society of Plastics Engineers

High sensitivity gas sensor based on high-Q suspended polymer photonic crystal nanocavity

We present high-sensitivity, multi-use optical gas sensors based on a one-dimensional photonic crystal cavity. These devices are implemented in versatile, flexible polymer materials which swell when in contact with a target gas, causing a measurable cavity length change. This change causes a shift in the cavity resonance, allowing precision measurements of gas concentration. We demonstrate suspended polymer nanocavity sensors and the recovery of sensors after the removal of stimulant gas from the system. With a measured quality factor exceeding 104, we show measurements of gas concentration as low as 600 parts per million (ppm) and an experimental sensitivity of 10 ppm; furthermore, we predict detection levels in the parts-per-billion range for a variety of gases.

Photonic Sintering of Inkjet Printed Copper Oxide Layer

Recently, printing technologies have been adapted for the manufacturing of flexible electronic devices such as RFID antennas, capacitors, rectifiers, organic thin film transistors, photovoltaics, etc. In contrast to traditional production of electronics, printing technologies can enable low cost, high throughput, and large-area processing on flexible polymer materials. Photolithographical processes can be substituted by direct printing, e.g. of metal nanoparticles (NPs) or organic inks on flexible polymer foils. After the deposition of the materials by printing, curing, drying and/or sintering is required to remove solvents and additives, and to develop a functional layer. The operating temperatures of these post-printing processes are usually higher than the tolerable temperature of the polymer foils, which results in plastic thermal deformations of the foils. Photonic sintering is considered as one of the promising technology, especially for R2R processing, to prevent the thermal deformation of the substrate. It allows processing in ambient conditions without damaging the polymer foils due to energy exposure in microseconds-scale.In this paper, the effect of photonic sintering conditions on inkjet printed copper oxide (CuO) layers was investigated. We found that the conductivity is proportional to the exposure energy. However, excessive energy will lead to “over-sintering” and thus destroys the copper layer. Optimized photonic sintering parameters were proposed to obtain inkjet-printed copper layers with high conductivity and no layer ablation.

Increased electroaction through a molecular flexibility tuning process in TiO2–polydimethylsilicone nanocomposites

Flexible polymer materials with obvious electrostriction characteristics display a significant potential for application as novel potential actuators in the future. We report advanced TiO2–polydimethylsilicone (TiO2–PDMS) nanocomposites with electroaction that is effectively increased through a molecular flexibility tuning process. The increase in the electromechanical sensitivity (by 550%) and actuation strain (by 230%) under a low electric field in low elastic modulus TiO2–PDMS composites originates from the flexibility tuning process by the introduction of dimethylsilicone oil (DMSO). The DMSO is miscible with PDMS resulting in a uniform composition at the molecular level, which can significantly decrease the elastic modulus of the dielectric elastomer composites from 820 kPa to 95 kPa. The experimental results are interpreted using the swelling elastomers theory. It suggests that reducing the elastic modulus could be a good strategy to improve the actuation performance with a low electric field.

Implanted, inductively-coupled, radiofrequency coils fabricated on flexible polymeric material: Application to in vivo rat brain MRI at 7 T

Combined with high-field MRI scanners, small implanted coils allow for high resolution imaging with locally improved SNR, as compared to external coils. Small flexible implantable coils dedicated to in vivo MRI of the rat brain at 7T were developed. Based on the Multi-turn Transmission Line Resonator design, they were fabricated with a Teflon substrate using copper micromolding process and a specific metal–polymer adhesion treatment. The implanted coils were made biocompatible by PolyDimethylSiloxane (PDMS) encapsulation. The use of low loss tangent material achieves low dielectric losses within the substrate and the use of the PDMS layer reduces the parasitic coupling with the surrounding media. An implanted coil was implemented in a 7T MRI system using inductive coupling and a dedicated external pick-up coil for signal transmission. In vivo images of the rat brain acquired with in plane resolution of (150μm)2 thanks to the implanted coil revealed high SNR near the coil, allowing for the visualization of fine cerebral structures.

Temperature-Dependent Electrical Properties of Graphene Inkjet-Printed on Flexible Materials

Graphene electrode was fabricated by inkjet printing, as a new means of directly writing and micropatterning the electrode onto flexible polymeric materials. Graphene oxide sheets were dispersed in water and subsequently reduced using an infrared heat lamp at a temperature of ~200 °C in 10 min. Spacing between adjacent ink droplets and the number of printing layers were used to tailor the electrode's electrical sheet resistance as low as 0.3 MΩ/□ and optical transparency as high as 86%. The graphene electrode was found to be stable under mechanical flexing and behave as a negative temperature coefficient (NTC) material, exhibiting rapid electrical resistance decrease with temperature increase. Temperature sensitivity of the graphene electrode was similar to that of conventional NTC materials, but with faster response time by an order of magnitude. This finding suggests the potential use of the inkjet-printed graphene electrode as a writable, very thin, mechanically flexible, and transparent temperature sensor.

Prediction of compressive creep behaviour in flexible polyurethane foam over long time scales and at elevated temperatures

Compressive creep gradually affects the structural performance of flexible polymeric foam material over extended time periods. When designing components, it is often difficult to account for long-term creep, as accurate creep data over long time periods or at high temperatures is often unavailable. This is mainly due to the lengthy testing times and/or inadequate high temperature testing facilities. This issue can be resolved by conducting a range of short-term creep tests and applying accurate prediction methods to the results. Short-term creep testing was conducted on viscoelastic polyurethane foam, a material commonly used in seating and bedding systems. Tests were conducted over a range of temperatures, providing the necessary results to allow for the generation of predictions of long-term creep behaviour using time-temperature superposition. Additional predictions were generated, using the William Landel Ferry time-temperature empirical relations, for material performance at temperatures above and below the reference temperature range. Further tests validated the results generated from these theoretical predictions.

Development of a multilayered polymeric DNA biosensor using radio frequency technology with gold and magnetic nanoparticles

This study utilized the radio frequency (RF) technology to develop a multilayered polymeric DNA sensor with the help of gold and magnetic nanoparticles. The flexible polymeric materials, poly (p-xylylene) (Parylene) and polyethylene naphtholate (PEN), were used as substrates to replace the conventional rigid substrates such as glass and silicon wafers. The multilayered polymeric RF biosensor, including the two polymer layers and two copper transmission structure layers, was developed to reduce the total sensor size and further enhance the sensitivity of the biochip in the RF DNA detection. Thioglycolic acid (TGA) was used on the surface of the proposed biochip to form a thiolate-modified sensing surface for DNA hybridization. Gold nanoparticles (AuNPs) and magnetic nanoparticles (MNPs) were used to immobilize on the surface of the biosensor to enhance overall detection sensitivity. In addition to gold nanoparticles, the magnetic nanoparticles has been demonstrated the applicability for RF DNA detection. The performance of the proposed biosensor was evaluated by the shift of the center frequency of the RF biosensor because the electromagnetic characteristic of the biosensors can be altered by the immobilized multilayer nanoparticles on the biosensor. The experimental results show that the detection limit of the DNA concentration can reach as low as 10pM, and the largest shift of the center frequency with triple-layer AuNPs and MNPs can approach 0.9 and 0.7GHz, respectively. Such the achievement implies that the developed biosensor can offer an alternative inexpensive, disposable, and highly sensitive option for application in biomedicine diagnostic systems because the price and size of each biochip can be effectively reduced by using fully polymeric materials and multilayer-detecting structures.

High-Q, low index-contrast polymeric photonic crystal nanobeam cavities

We present the design, fabrication and characterization of high-Q (Q=36,000) polymeric photonic crystal nanobeam cavities made of two polymers that have an ultra-low index contrast (ratio=1.15) and observed thermo-optical bistability at hundred microwatt power level. Due to the extended evanescent field and small mode volumes, polymeric nanobeam cavities are ideal platform for ultra-sensitive biochemical sensing. We demonstrate that these sensors have figures of merit (FOM=9190) two orders of magnitude greater than surface plasmon resonance based sensors, and outperform the commercial Biacore(TM) sensors. The demonstration of high-Q cavity in low-index-contrast polymers can open up versatile applications using a broad range of functional and flexible polymeric materials.

Improvement of Soft Tooling Process Through Particle Reinforcement with Polyurethane Mould

Use of conventional flexible polymeric mould materials yields to longer solidification time of (wax/plastic) patterns in soft tooling process, thereby reducing the rapidity of the process to a great extent, which is not desirable in present competitive market. In this work, approach of particle-reinforcement with mould materials is introduced to reduce the cycle time of Soft tooling process and the resulting cooling time is experimentally investigated in considering a case of manufacturing of a typical wax pattern with aluminium particle filled polyurethane. It is observed that cooling time is significantly reduced particularly with higher loading condition of aluminium filler. This happens due to the increase of effective thermal conductivity of mould material. However, it is also found that the stiffness of mould becomes simultaneously high due to increase of effective modulus of elasticity of mould material. Realizing these facts, an extensive study is carried out to find the effect on equivalent thermal properties and modulus of elasticity of polyurethane composite mould materials with the reinforcement of aluminium and graphite particles independently through rigorous experimentation and correlation of experimental findings with the models cited in literatures.

Experimental investigation on equivalent properties of particle reinforced silicone rubber: Improvement of soft tooling process

The main disadvantage in soft tooling process using conventional flexible reactive polymeric mould materials, namely silicone rubber, is the longer solidification time of (wax/plastic) patterns that yields to reduce the rapidity of the process to a great extent, which is not desirable in the present competitive market. The approach of particle reinforcement with mould materials is introduced here to reduce the cooling time of soft tooling process and the resulting cooling time is experimentally investigated in considering a case of manufacturing of a typical wax pattern with aluminium particle–reinforced silicone rubber mould material. It is observed that cooling time is significantly condensed, particularly with higher loading condition of aluminium filler. This is due to the increase of equivalent thermal conductivity of mould material. However, it is also noticed that at the same time, the stiffness of mould becomes high due to the increase of equivalent modulus of elasticity of mould material. Therefore, in order to effectively utilize the particle-reinforced silicone rubber for preparing mould, it is important to have a perceptive on the equivalent thermal conductivity and modulus of elasticity of composite mould materials with different filler loadings. Based on this realization, an extensive study is carried out to find the effect on equivalent thermal properties and modulus of elasticity of silicone rubber composite mould materials with the reinforcement of various metallic and non-metallic filler particles independently through rigorous experimentation and correlation of experimental findings with the models cited in literatures.

Studies on effective thermal conductivity of particle-reinforced polymeric flexible mould material composites

Due to poor thermal conductivity of conventional flexible polymeric mould materials, the solidification of (wax/plastic) patterns in soft tooling (ST) process takes a longer time. This problem can be solved by increasing the effective thermal conductivity of mould materials through (high thermal conductive) particle reinforcement. Therefore in this study, the equivalent thermal conductivities (ETCs) of particle-reinforced polymeric mould materials, namely silicone rubber and polyurethane are experimentally observed using hot disc technique. Findings show that not only the amount of filler content and type of filler material, but also particle size has significant influence on the effective thermal conductivity of polymer and it starts increasing drastically at 20–30 per cent volume fraction of filler content. To predict the cooling time in ST process, it is important to have an appropriate model of ETC. In this study, a new method is proposed based on a genetic algorithm fuzzy (GA-fuzzy) approach to model the effective thermal conductivity of a two-phase particle-reinforced polymer composites (PCs). The effectiveness of the model is extensively tested in comparison with various empirical expressions reported in literature based on the experimental measurements. It has been found that the model based on GA-fuzzy approach not only outperforms the existing models, but also possesses a generic one applicable to a wide range of two-phase particle-reinforced PCs.

Investigating the Role of Nonmetallic Fillers in Particulate-Reinforced Mold Composites using EAs

In the soft tooling (ST) process, flexible polymeric materials (namely, silicone rubber, polyurethane, etc.) are used for making mold for producing wax pattern. Due to low thermal conductivity of mold materials, the ST process takes longer time for cooling. Hence, to reduce the cooling time, thermal conductive fillers are included in mold materials. But addition of fillers affects various properties of ST process (such as stiffness of the mold box) and the influences may vary according to the types of materials used. Therefore, in the present work, multiobjective optimizations of equivalent thermal conductivity and effective modulus of elasticity of composite mold materials are conducted using evolutionary algorithms in order to investigate the role of various nonmetallic fillers in particulate reinforced mold material composites. We have adopted NSGA-II to optimize the conflicting objectives—maximization of thermal conductivity and minimization of modulus of elasticity. A recently proposed innovization procedure is used to unveil salient properties associated with the trade-off solutions. The obtained Pareto fronts are used successfully to study the role of various parameters influencing the equivalent thermal conductivity and modulus of elasticity of the composite mold material. The optimal selection of materials is suggested in consideration with the cost implication factor based on the findings through investigations.

Cytotoxicity of Plasticizers and Ionic Liquids Using Drosophila melanogaster S2 Cell Culture

AbstractOne successfully demonstrated application of ionic liquids (ILs) is to produce flexible polymeric materials, i.e., as plasticizers. However, the use of ILs as plasticizers, especially in medical plastics and children's toys, requires a thorough toxicological investigation. This is particularly important considering the safety concerns associated with leaching of many traditional plasticizers. This paper encompasses cellular biology and engineering aspects of the application of ILs as novel plasticizers by analyzing the cytotoxic effects of a variety of ILs on Drosophila melanogaster (fruit fly) S2 cell culture. While the effects observed have been correlated with previous observations, the cytotoxicity of ILs was found to depend on the selection of specific cations, anions, exposure time, concentration and solubility of the ILs in the culture media, and the toxicity of the parent chemicals used to synthesize the ILs.

Effect on cooling time in soft tooling process using particle reinforcement with polyurethane mould material

Use of conventional flexible polymeric mould materials yields to longer solidification time of (wax/plastic) patterns in soft tooling (ST) process. Thereby, reducing the rapidity of the process to a great extent which is not desirable in present competitive market. In this work, approach of particle reinforcement with mould materials is introduced to increase the productivity of ST process and the resulting cooling time is experimentally investigated in considering a case of manufacturing of a typical wax pattern with aluminium filled polyurethane (PU). It is observed that cooling time is significantly reduced particularly with higher loading condition of aluminium filler due to the increase of equivalent thermal conductivity (ETC) of mould material. But simultaneously, it is also found that the stiffness of mould becomes high due to increase of equivalent modulus of elasticity of mould material. Realising these facts, an extensive experimental study is carried out to find the effect on equivalent thermal properties and modulus of elasticity of PU composites by reinforcing aluminium and graphite particles independently with different loading conditions. The existing models of ETC and modulus of elasticity of composites are also tried to validate based on the experimental findings.

Cyanoacrylate adhesives curable to flexible polymeric materials by poly( l-lactide- co-ε-caprolactone)

To improve the mechanical properties such as flexibility and bond strength of cyanoacrylates, poly( l-lactide- co-ε-caprolactone) (PLCL) copolymers were dissolved into 2 kinds of cyanoacrylates, ethyl 2-cyanoacrylate (EC) and allyl 2-cyanoacrylate (AC). Mechanical properties such as bond strength, bending-stress recovery, and crystallization intensity were measured. The optimal concentration for high bond strength was determined to be 8% PLCL (molar ratio 70:30). EC/8% PLCL (70:30) exhibited three times higher bond strength as compared with that of EC. Further, AC/8% PLCL (50:50) exhibited better bending recovery than other materials. These results indicate that cyanoacrylate/PLCL materials can be widely used as adhesives in various fields.

Selective binding and removal of organic molecules in a flexible polymeric material with stretchable metallosalen chains

A dynamic porous coordination network with a hydrophobic channel was assembled from stretchable 1D metallosalen polymer, which has the ability to recognize and, particularly, separate aromatic hydrocarbons from aliphatic mixtures with high selectivity.