Polymer Bilayer Research Articles

Ripple Patterns Spontaneously Emerge through Sequential Wrinkling Interference in Polymer Bilayers.

We report the formation of "ripple" patterns by the sequential superposition of nonorthogonal surface waves excited by the spontaneous buckling of polymeric bilayers. Albeit of a different nature and micron scale compared to the familiar sedimentary ripples caused by gentle wave oscillations, we find commonalities in their topography, defects, and bifurcations. The patterns are rationalized in terms of a defect density that depends on the relative angle between generations, and a constant in-plane bending angle that depends on skin thickness. A minimal wave summation model enables the design of ripple and checkerboard surfaces by tuning material properties and fabrication process.

A vesicle microrheometer for viscosity measurements of lipids and polymer bilayers

Lipid bilayers are the main structural component of cell membranes. The nanometrically thin bilayer behave as a two-dimensional fluid and its shear viscosity controls the transport of embedded biomolecules and membrane deformation. In this study, we devise a high throughput and non-invasive method to measure shear surface viscosity of bilayer membranes based on transient deformation of giant vesicles (closed membrane sacs) in an uniform electric field. Our method is probe-free and sensitive enough to allow for robust measurement of membrane viscosity of single-and multi-component lipid and polymer membranes.

Differential Polymer Chain Scission Enables Free-Standing Microcavity Laser Arrays.

Control over material architectures is essential to the performance of photonic devices and systems. Optical isolation of the photonic materials from substrates can significantly enhance their performance but suffers from complicated fabrication processes and limited applications. Here a differential polymer chain scission strategy is proposed to fabricate free-standing photonic structures based on one-step electron-beam direct writing on polymer bilayers (EOB). The polymer molecular mass-dependent sensitivity to electron beam enables differential patterning of the two layers of polymers, leading to the direct formation of suspended optical microcavities. The EOB technique features high materials compatibility and design flexibility for the optical microcavities, which significantly expands the application scope of the suspended optical microcavities. As well as providing a versatile strategy for building high-performance photonic materials, the results provide a promising platform for innovative applications of optical microstructures.

Peano‐Hydraulically Amplified Self‐Healing Electrostatic Actuators Based on a Novel Bilayer Polymer Shell for Enhanced Strain, Load, and Rotary Motion

The hydraulically amplified self‐healing electrostatic actuator is an emerging driving component for soft robotics, which is composed of a flexible dielectric polymer shell that is partially covered by conductive electrodes and filled with a liquid dielectric. However, the low permittivity and dielectric strength of the polymer shell remain a challenge that limits the actuator performance. Herein, a Peano‐hydraulically amplified self‐healing electrostatic actuator is constructed by innovatively integrating a bilayer polymer shell for combined favorable properties of high dielectric strength, dielectric permittivity, and elastic modulus. Compared with a traditional single‐layer shell actuator, the new bilayer actuator architecture generates an increased strain (164%) at 5 kV and improves load‐bearing capability (620 mN) at 6 kV, thereby providing a significantly enhanced actuation performance. The new actuator is further applied in driving a ratchet system, which converts the reciprocating motion of the actuator into a rotating motion and a flexible output torque, in order to protect the rotating components from impact. The high strain and load characteristics of the bilayer configuration and the easy‐to‐deform characteristics of the new actuator design make it an attractive approach to fabricate complex geometries and achieve a variety of motion modes in soft systems.

A pH-Responsive Liquid Crystal Hydrogel Actuator with Calcium-Induced Reprogrammable Shape Fixing

Soft polymer actuators have myriad applications and have therefore gained considerable attention in recent years. However, it remains challenging to prepare soft actuators with predefined shapes. Here, a bilayer polymer actuator with a (re)programmable shape is prepared from a microporous anisotropic polypropylene scaffold and a thin, pH-responsive liquid crystalline network (LCN) layer. The hydrogen bonds between dimerized benzoic acid derivatives in the LCN can be disrupted by an alkaline treatment, resulting in a pH-responsive LCN hydrogel layer. The pH-responsive actuation is governed by both the anisotropic mechanical properties of the scaffold and the cross-link density of the LCN hydrogel. Ca2+ ions can be used to chemically cross-link the actuator resulting in an initial programmed shape. The shape fixing can be reversed by removing the Ca2+ ions with an ethylenediaminetetraacetic acid (EDTA) solution. The shape fixing can be performed locally, resulting in pH-responsive actuators with three-dimensional initial configurations of choice.

Cell Adhesion and Migration on Thickness Gradient Bilayer Polymer Brush Surfaces: Effects of Properties of Polymeric Materials of the Underlayer.

In the field of tissue engineering and biomaterials, controlling the surface properties and mechanical properties of scaffold materials is crucial and has attracted much attention. Here, two types of bilayer polymer brushes composed of a hydrophilic underlying layer and a cationic surface layer [made of poly(2-aminoethyl methacrylate)] with a thickness gradient were prepared by surface-initiated atom-transfer radical polymerization. To investigate the influence of the stiffness as a mechanical property of the polymer brush on cell behavior, the underlayer was prepared from either 2-methacryloyloxyethyl phosphorylcholine or oligo(ethylene glycol) methyl ether methacrylate, with the bilayers designated as gradient poly(2-methacryloyloxyethyl phosphorylcholine)-block-poly(2-aminoethyl methacrylate) [grad-pMbA] and gradient poly(oligo[ethylene glycol] methyl ether methacrylate)-block-poly(2-aminoethyl methacrylate) [grad-pEGbA], respectively. Characterization of these surfaces was performed by spectroscopic ellipsometry, X-ray reflectivity, and determination of the zeta potential, static contact angle, and force curve. These diblock copolymer brushes with a thickness gradient helped to distinguish the effects of the mechanical and surface properties of the brushes on cell behavior. The attachment and motility of L929 fibroblasts and epithelial MCF 10A cells on the fabricated brushes were then assessed. L929 cells had a round shape on the thin surface layer of grad-pMbA and spread well on thicker areas. In contrast, MCF 10A cells spread well in areas of any thickness of either grad-pMbA or grad-pEGbA. Single MCF 10A cells migrated randomly on grad-pMbA, whereas grouped cells started to climb up along the thickness gradient of grad-pMbA. In contrast, both single and grouped MCF 10A cells migrated randomly on grad-pEGbA. These thickness gradient diblock copolymer brushes are simple, reproducible, and reasonable platforms that can facilitate practical applications of biomaterials, for example, in tissue engineering and biomaterials.

Surfactant induced bilayer-micelle transition for emergence of functions in anisotropic hydrogel.

Tuning the self-assembled structures in amorphous hydrogels will enrich the functionality of hydrogels. In this study, we tuned the structure of a photonic hydrogel, which consists of polymeric lamellar bilayers entrapped inside a polyacrylamide network, simply by molecular triggering using an ionic surfactant. Owing to the binding of ionic surfactants (sodium dodecyl sulfate), the lamellar bilayers comprising non-ionic polymeric surfactants [poly(dodecyl glyceryl itaconate)] changed to micelles, whereas the unidirectional lamellar structure was preserved in the hydrogel. The bilayer-micelle structure transition caused a dramatic decrease in the swelling anisotropy and mechanical softening of the photonic gel. With the micelle structure, the softened gel shows fast (0.3 s) and reversible color change over the entire visible light range in response to a small mechanical pressure (5 kPa). This low stress-induced color-changing hydrogel could be applied as a visual tactile sensor in various fields, especially in biomedical engineering.

Selective CO2 adsorption and bathochromic shift in a phosphocholine-based lipid and conjugated polymer assembly.

We assemble a film of a phosphocholine-based lipid and a crystalline conjugated polymer using hydrophobic interactions between the alkyl tails of the lipid and alkyl side chains of the polymer, and demonstrated its selective gas adsorption properties and the polymer's improved light absorption properties. We show that a strong attractive interaction between the polar lipid heads and CO2 was responsible for 6 times more CO2 being adsorbed onto the assembly than N2, and that with repeated CO2 adsorption and vacuuming procedures, the assembly structures of the lipid-polymer assembly were irreversibly changed, as demonstrated by in situ grazing-incidence X-ray diffraction during the gas adsorption and desorption. Despite the disruption of the lipid structure caused by adsorbed polar gas molecules on polar head groups, gas adsorption could promote orderly alkyl chain packing by inducing compressive strain, resulting in enhanced electron delocalization of conjugated backbones and bathochromic light absorption. The findings suggest that merging the structures of the crystalline functional polymer and lipid bilayer is a viable option for solar energy-converting systems that use conjugated polymers as a light harvester and the polar heads as CO2-capturing sites.

Polymer electret-based organic field-effect transistor memory with a solution-processable bilayer (PαMS/ cross-linked PVP) gate dielectric

Pentacene based organic field-effect transistors (OFETs) were fabricated, with both cross-linked poly vinyl phenol (CL-PVP) and a bilayer of poly(α-methylstyrene) (PαMS)/ CL-PVP as gate dielectric. The PαMS layer decreases the surface energy of the gate dielectric and increases the hydrophobic nature, which leads to favorable growth of pentacene and the corresponding field-effect mobility, though at a higher gate voltage span, increases three times compared to that of the device with only CL-PVP as the gate dielectric. OFET with bilayer polymer gate dielectric exhibited non-volatile memory behavior with an on-off ratio 103, retention time >103 s and a large memory window of −25 V. The memory effect observed in the device was due to the charge trapping in the PαMS layer, with CL-PVP acting as a blocking dielectric. Our studies indicate that the bilayer dielectric, comprising of solution-processable PαMS/CL-PVP is a good choice for obtaining non-volatile electret memory on an OFET platform.

Nanomechanical Properties of Artificial Lipid Bilayers Composed of Fluid and Polymerizable Lipids.

Polymerization enhances the stability of a planar supported lipid bilayer (PSLB) but it also changes its chemical and mechanical properties, attenuates lipid diffusion, and may affect the activity of integral membrane proteins. Mixed bilayers composed of fluid lipids and poly(lipids) may provide an appropriate combination of polymeric stability coupled with the fluidity and elasticity needed to maintain the bioactivity of reconstituted receptors. Previously (Langmuir, 2019, 35, 12483-12491) we showed that binary mixtures of the polymerizable lipid bis-SorbPC and the fluid lipid DPhPC form phase-segregated PSLBs composed of nanoscale fluid and poly(lipid) domains. Here we used atomic force microscopy (AFM) to compare the nanoscale mechanical properties of these binary PSLBs with single-component PSLBs. The elastic (Young's) modulus, area compressibility modulus, and bending modulus of bis-SorbPC PSLBs increased upon polymerization. Before polymerization, breakthrough events at forces below 5 nN were observed, but after polymerization, the AFM tip could not penetrate the PSLB up to an applied force of 20 nN. These results are attributed to the polymeric network in poly(bis-SorbPC), which increases the bilayer stiffness and resists compression and bending. In binary DPhPC/poly(bis-SorbPC) PSLBs, the DPhPC domains are less stiff, more compressible, and are less resistant to rupture and bending compared to pure DPhPC bilayers. These differences are attributed to bis-SorbPC monomers and oligomers present in DPhPC domains that disrupt the packing of DPhPC molecules. In contrast, the poly(bis-SorbPC) domains are stiffer and less compressible relative to pure PSLBs; this difference is attributed to DPhPC filling the nm-scale pores in the polymerized domains that are created during bis-SorbPC polymerization. Thus, incomplete phase segregation increases the stability of poly(bis-SorbPC) but has the opposite, detrimental effect for DPhPC. Overall, these results provide guidance for the design of partially polymerized bilayers for technological uses.

Kinetic viscoelasticity during early polymer-polymer spinodal dewetting

The dewetting kinetics of a supported polymer bilayer were measured in situ using coherent grazing-incidence x-ray scattering. X-ray photon correlation spectroscopy provides both the two-time correlation functions and the cross-correlation function which measures the average spatial shift of the speckles produced by the coherent x rays. The stress in the ultrathin top dewetting film can be directly observed due to the exquisite sensitivity to sample curvature changes provided by the x-ray speckle correlation functions. The hole-opening events in the film are found to be associated with significant changes to the stress. These results are interpreted through an analogy between viscoelastic spinodal dewetting and early-stage bulk viscoelastic phase separation. The frequency of hole-initiation events during dewetting decreases with time as a power law, and the power-law exponent can be linked to nonlinear viscoelastic effects, showing similarity in their stress relief dynamics to aftershock decays.

Patterned Actuators via Direct Ink Writing of Liquid Crystals.

Soft actuators allowing multifunctional, multishape deformations based on single polymer films or bilayers remain challenging to produce. In this contribution, direct ink writing is used for generating patterned actuators, which are in between single- and bilayer films, with multifunctionality and a plurality of possible shape changes in a single object. The key is to use the controlled deposition of a light-responsive liquid crystal ink with direct ink writing to partially cover a foil at strategic locations. We found patterned films with 40% coverage of the passive substrate by an active material outperformed “standard” fully covered bilayers. By patterning the film as two stripes, a range of motions, including left- and right-handed twisting and bending in orthogonal directions, could be controllably induced in the same actuator. The partial coverage also left space for applying liquid crystal inks with other functionalities, exemplified by fabricating a light-responsive green reflective actuator whose reflection can be switched “on” and “off”. The results presented here serve as a toolbox for the design and fabrication of patterned actuators with dramatically expanded shape deformation and functionality capabilities.

Ferroelectric properties and breakdown strength of layer-by-layer poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) and polyurethane (PU) for energy storage application

Ferroelectric polymers are one of the next- generation pulsed capacitor materials for the potential application in capacitive energy storage. This polymer with higher saturated polarization, smaller remnant polarization, and higher electrical breakdown are the most promising candidates. In this work, the dielectric properties and energy storage capacity of the bilayer polymer films of Poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) and polyurethane (PU) were studied. These bilayer polymers were prepared by layer- by- layer method at the condition of variable layer thickness. The results show that the dielectric constants and the saturated polarization of the bilayer films increased, and bilayer films with P70/PU30 exhibit electrical high breakdown strength up to 379 V/μm. Moreover, enhanced energy storage density and the energy efficiency of the bilayer constrictors will be discussed for the capacitive energy storage polymers.

High Energy Density Solid State Lithium Metal Batteries Enabled by Sub-5 µm Solid Polymer Electrolytes.

Solid-state batteries (SSBs) are considered as the most promising next-generation high-energy-density energy storage devices due to their ability in addressing the safety concerns from organic electrolytes and enabling energy dense lithium anodes. To ensure the high energy density of SSBs, solid-state electrolytes (SSEs) are required to be thin and light-weight, and simultaneously offer a wide electrochemical window to pair with high-voltage cathodes. However, the decrease of SSE thickness and delicate structure may increase the cell safety risks, which is detrimental for the practical application of SSBs. Herein, to demonstrate a high-energy-density SSB with sufficient safety insurance, an ultrathin (4.2 µm) bilayer SSE with porous ceramic scaffold and double-layer Li+ -conducting polymer, is proposed. The fire-resistant and stiff ceramic scaffold improves the safety capability and mechanical strength of the composite SSE, and the bilayer polymer structure enhances the compatibility of Li metal anode and high-voltage cathodes. The 3D ceramic facilitates Li-ion conduction and regulates Li deposition. Thus, high energy density of 506Wh kg-1 and 1514Wh L-1 is achieved based on LiNi0.8 Co0.1 Mn0.1 O2 (NCM811) cathodes with a low N/P ratio and long lifespan over 3000 h. High-energy-density anode-free cells are further demonstrated.

Intrinsically conductive polymers hybrid bilayer films for the fluorescence molecular diagnosis of the Zika virus

In 2016, the Zika virus (ZIKV) infection became a major public health problem, after the discovery that an alarming increase in the number of Brazilian newborns with microcephaly could be associated with the occurrence of this viral disease during the pregnancy of their mothers. The urgent need for simple diagnostic methods that allow rapid screening of suspected cases has stimulated the search for low-cost devices capable of detecting specific sequences of nucleic acids. The present work describes the development of nanostructured films formed by bilayers of conjugated polymers for rapid detection of the presence of Zika virus DNA, via fluorescence methods. For this, we initially deposited alternating layers of polyaniline (PANI) and polypyrrole (PPY) on the surface of polyethylene terephthalate (PET) sheets. The films obtained were then characterized by SEM, UV-Vis, ATR-FTIR, and contact angle measurements. For their use as quenchers for the diagnosis of Zika, a single DNA strand-specific for ZIKV was labeled with a fluorophore (FAM-ssDNA). We determined the time required for the saturation of the interaction between probe FAM-ssDNA and the film (180 min) and the time for the maximal hybridization between FAM-ssDNA and target DNA to occur (60 min). The detection limits were estimated as 345 pM and 278 pM for the PET/PPY-PANI and PET/PANI-PPY hybrid films, respectively. The simplicity of the procedure, coupled with the fact that a positive/negative response can be obtained in less than 60 min, suggests that the proposal of using these polymeric bilayer films is a promising methodology for the development of rapid molecular diagnostic tests.

Tunable Phase Gratings by Wrinkling of Plasma-Oxidized PDMS: Gradient Skins and Multiaxial Patterns

Wrinkling instabilities in polymeric bilayers have been exploited as optical phase gratings with tunable performance. Here, we report strain modulated 1D and 2D phase gratings fabricated by the ubiquitous process of plasma-oxidation of polydimethylsiloxane (PDMS). While surface oxidation provides a remarkably facile glassy skin formation approach, minimizing delamination and debonding, it inherently results in a gradient conversion profile emanating from the top film interface. We examine and quantitatively model the consequences of this gradient layer on the optical properties of the resulting strain-tunable phase gratings. Diffraction efficiencies up to 48% are demonstrated. We then develop and validate a surface reconstruction methodology based on the diffraction pattern of our sinusoidal gratings and our model, which we extend to the high deformation regimes and to 2D gratings, obtained by superposition of two wrinkling generations, where both amplitudes and wavelengths can be independently tuned. Overall, this approach provides a rapid, robust and predictive framework for the design and fabrication of tunable, single, and multiaxial surface gratings.

2D reflectometry for the investigation of polymer interfaces: off-specular neutron scattering

Specular and off-specular neutron reflectometry have been used in a combined approach to study thin polymer films. Our goal in this work is to illustrate the power of the off-specular scattering technique to probe the properties of the buried interface of immiscible polymer bilayers of deuterated polystyrene and protonated poly(methyl methacrylate) (h-PMMA). The diffuse scattering stemming from these systems is discussed in relation to thermal fluctuations at the polymer/polymer interface, providing a means to extract in-plane correlation lengths from buried interfaces. In addition the onset of hole formation in the top layer is evidenced by the diffuse scattering, not easily detectable by specular reflection alone.

Triboelectric Laminates with Volumetric Electromechanical Response for Mechanical Energy Harvesting

AbstractThe main requirement for triboelectric nanogenerator devices is the necessity to have separable or movable parts to harvest mechanical energy, thus complicating their integration into the energy harvesting systems. Herein, triboelectric laminates that show a volumetric electromechanical response are introduced, similar to intrinsic ferroelectric polymers. The laminates are composed of stacked bilayer polymer films with different triboelectric surface properties separated by polymer mesh. This sophisticated design provides dipole alignment across the volume of triboelectric laminate and electromechanical response without applying any poling procedure. Triboelectric laminate generates the open‐circuit voltage (VOC) of 29 V and short‐circuit current (ISC) of 15 nA. Volumetric energy density of 21.13 µJ cm−3 and a peak average power density of 46.19 µW cm−3 are observed. Triboelectric laminate shows an effective piezoelectric constant d33 of 9.83 pC N−1, higher than reported for piezoelectric polymer like nylon‐11. The triboelectric laminate is produced from cheap, mechanically flexible materials that can be produced in large quantities.

(Invited) Graphene-Based Micro Biosensors

On the surface of graphene, energy transfer occurs when molecules are located close to the surface. The energy transfer yield depends on the degree of molecular interaction between the adsorbed molecules and the graphene surface. For example, when a fluorescent molecule such as a dye is located very close to the graphene surface, the dye does not exhibit fluorescence because of the efficient fluorescence resonance energy transfer (FRET). The degree of energy transfer varies when the molecules on the surface changes their molecular states and structures. This allows us to visualize biological/chemical reactions by converting those invisible molecular interactions into the measurable physical quantities such as light and electricity. This makes graphene a promising material for a novel biosensor.We have created a unique type of biosensor, which works on a graphene surface by modifying it with a specific DNA called an aptamer, for the detection of biologically important proteins such as cancer markers. One end of the aptamer is labeled with a fluorescent dye and the other end is connected to a pyrene linker molecule, which shows a strong affinity to the graphene. Thus, the aptamer is firmly fixed to the graphene surface. The graphene aptasensor detection mechanism is as follows. In the initial stage, the dye-conjugated aptamer is adsorbed on the graphene surface via physical adsorption (π-π interactions), and thus the dye is located close to the graphene surface. Here, the fluorescence of the dye is well quenched by graphene via FRET and is barely observable (Fig. 1(a)). If the target of the aptamer is present in the system, the aptamer forms a complex with the target and leaves the graphene surface. At the same time, the dye molecule also leaves the graphene surface and the dye recovers its fluorescence (Fig. 1(b)). We can detect the target molecule by observing the fluorescence [1]. The system allows us to perform molecular detection on a solid surface, which is a powerful tool to realize a two-dimensional (2D) on-chip sensor. By using the on-chip sensor, detection of the target protein is possible simply by adding a sample smaller than 1 μL to a sensor chip and is completed in about a minute. The system also allows us to perform real-time molecular detection, the simultaneous detection of multiple target molecules on a single chip [2], and the molecular design of a probe for enhancing the sensitivity by using single-stranded DNA spacer (Fig. 2)[3].We then extend the biosensing platform from a 2D plane to a hollow three-dimensional (3D) space by building the protein detection system on the inner surfaces of flexible layered polymer films. The different strain gradients of the polymeric bilayer are used as the driving force behind the 3D transformation [4]. Detection of human serum albumin (HAS) in a 3D aptasensor has been successfully demonstrated (Fig. 1(e-f)). Since the formation of the micro-roll avoids any cytotoxic processes, they can be used for encapsulating cells in vitro [4, 5]. Our sensor is expected to be applied to spatiotemporal detection of several secreted proteins such as growth factors in near future.

Multi-Walled Carbon Nanotubes and Carbon Black Incorporated Heterogeneous Layered Polymer Composites with Enhanced Electrical Features

The present work is centered on the study of electrical properties of three different types of composites i.e. single layer carbon black (CB) and multi-walled carbon nanotubes (MWCNTs) based PMMA composites, CB/PMMA and MWCNTs/PMMA, and bilayer polymer composites CB/PMMA-MWCNTs/PMMA, prepared by solution mixing method using chloroform as dispersion medium. It is expected that two layers in bilayer polymer composites would show increased interfacial interactions resulting in improved electrical character especially dielectric properties. Prepared composites have been characterized by UV-Vis and FTIR spectroscopy for successful synthesis. Electrical properties, conductivity, dielectric constant and tangent loss of all composites were measured at frequency range of 40Hz to 1MHz by using LCR meter. Percolation threshold value was obtained by using data of electrical conductivity as a function of volume fraction of fillers. The obtained value of percolation threshold was much lower for bilayer polymer composites i.e. 0.01 vol% as compared to the single layer polymer composites. Observed data of dielectric constant encourages the synthesis of composites with this kind of structure.