Multifunctional liquid metal polymer composites

Liquid metal polymer composites are an emerging class of functional materials with potentially transformative impacts in wearable electronics, soft robotics, and human-computer interactions. By employing different processing methods, room temperature liquid metal inclusions can be embedded in insulating polymers like elastomers to incorporate functional properties of metals while the matrix remains soft and stretchable. These solid–liquid composites offer an interesting, yet complex multifunctional material system. In this review, we present an exclusive overview of the synthesis methods, structural and functional properties, and applications of gallium-based liquid metal polymer composites. Common methods to control the size of liquid metal inclusions and their interaction in polymers are discussed. Moreover, the effect of liquid metal microstructures on the overall properties of the composites is summarized. We also highlight the new trends in terms of material composition, printing process, and novel applications of liquid metal polymer composites in intelligent systems.     

The formation of a liquid crystalline main chain polymer by means of photopolymerization

A nematic liquid crystal material containing a mercapto group and an olefinic group was synthesized. Photopolymerization of this compound in the liquid crystalline state resulted in a liquid crystalline main chain polymer which showed a relatively high birefringence. Due to scattering, the material had a low transmission at ambient temperatures.     

Kinetics of formation of polymer dispersed liquid crystals in a liquid-crystalline polymer matrix

With the use of optical polarization microscopy, the kinetics of phase separation during cooling of molten mixtures of a nematic low-molecular-mass liquid crystal and a liquid crystalline polymer is studied to produce polymer dispersed liquid crystals. The statistical drop-size distribution of a low-molecular-mass liquid crystal is described in the terms of equilibrium thermodynamics of irreversible processes. For a nematic polymer component of a mixture, the analysis of time dependences of the average diameter of drops of a low-molecular-mass liquid crystal makes it possible to reveal two stages in the kinetics of their growth and to describe this process according to the universal law of cluster growth. For a smectic polymer component, the Avrami equation is used to quantitatively describe the kinetics of growth of low-molecular-mass liquid-crystal drops.     

Flow-induced structure in a thermotropic liquid crystalline polymer as studied by SANS

Small-angle neutron scattering is utilized to determine the flow induced alignment of a model thermotropic liquid crystalline polymer (LCP) as a function of shear rate and temperature. The results demonstrate that the flow-induced structures in thermotropic liquid crystalline polymers have similarities and differences to those in lyotropic liquid crystalline polymer solutions. The shear rate dependence of the alignment shows that the flow-induced alignment correlates very well to the viscosity behavior of the LCP in the shear thinning regime, while temperature variation results in a change in the extent of alignment within the nematic phase. Relaxation results also demonstrate that the flow-induced alignment remains essentially unchanged for up to an hour after the shear field has been removed. Last, there exists a regime at low shear rate and low temperature where alignment of the LCP molecule perpendicular to the applied shear flow is stable. These results provide important experimental evidence of the molecular level changes that occur in a thermotropic liquid crystalline polymer during flow, which can be utilized to develop theoretical models and more efficiently process thermotropic polymers. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 3017–3023, 1998     

Computer simulation of the liquid crystal formation in a semi-flexible polymer system

Molecular dynamic simulations are reported for system of semi-flexible linear rod-like molecules. The molecules are composed of N_c tangent soft spheres, connected by elastic springs. Rigidity is introduced by additional springs between all pairs of spheres along the molecule. The formation of only a nematic LC phase is shown for all systems with N_c = 8 and different flexibility. The effect of flexibility on the order parameter and the volume fraction at the LC phase transition is compared with theoretical predictions by Khokhlov-Semenov and with available simulation data. The dependence of the anisotropy of diffusion on chain flexibility in LC phase was studied. The polymer brushes consisting of flexible and semi-flexible (composed of linear rod-like segments) chains were simulated at different grafting densities. Height of brush, order parameter, distribution of density and chain ends in brush were obtained in both cases and compared with theoretical predictions.     

Estimation of flocculation structure in filled polymer composites by dynamic rheological measurements

 The frequency and concentration dependences of the storage modulus (G ^′) for carbon black and short-carbon-fiber-filled polymer composites were investigated by means of dynamic rheological measurements. It was found that G ^′ at low frequencies and amplitudes could be used as a sensitive experimental parameter for detecting the flocculation structure of the ultra-fine-particle-filled polymer composites. Correlation of electrical resistivity of the composites to the relative storage modulus, G ^′ _r(=G ^′ _c/ G ^′ _p), revealed that the three-dimensional interparticle networks start to construct through the matrix when G ^′ _r increases to 7 regardless of the composite systems. Quantitative calculations in order to determine the flocculation structure were carried out by means of the modified Kerner equation. A plot of the calculated value, defined as the floc index A, dependence of electrical resistivity for various systems was found to be a universal curve. Accordingly, we suggest that A might universally correspond to the flocculation structure of the filler, which is independent of the nature of the filler, the molecular weight, the chemical composition of the polymer and the temperature at which the measurement is made. This method is particularly effective for estimating the flocculation structure of ultra-fine-particle-filled polymer composites no matter whether the filler is conductive or not. Received: 26 May 1999/Accepted in revised form: 28 September 1999     

Sandwich structure containing liquid crystal polymer grafted on polymer support

Graft post-polymerization of mesogenic monomers onto fluorine-containing polymer support was initiated by the simultaneous action of vacuum ultraviolet radiation and atomic oxygen. The resultant two-layer structure possesses the combined physical–mechanical properties of the polymer-supporting film and the optical characteristics of the anisotropic liquid crystal layer.     

Features of the intramolecular structure of branched polymer systems in solution

The review is devoted to the synthesis and properties of branched polymer structures of a controlled molecular architecture. The work considers the main methods to obtain branched macromolecules. The results of both theoretical calculations and experimental studies of the structure and features of the behavior of the mentioned systems in solution are analyzed and generalized. The promising use of branched polymer systems as an alternative to linear systems is noted.     

Polymerization of polymer/ferroelectric liquid crystal composites formed with branched liquid crystalline bismethacrylates

Polymerization and phase behaviour of a branched liquid crystalline bismethacrylate in a ferroelectric liquid crystal (FLC) were characterized. Addition of the monomer increases the temperature range of the smectic A phase, and, at relatively low concentrations of monomer, the temperature range increases to more than 10 times that observed in the neat FLC. Other phases such as the smectic C* phase are no longer exhibited as the monomer interferes with the inherent tilt of the FLC molecules. After polymerization, the polymer network phase separates and the phase transition temperatures return to values close to those of the FLC. The monomer also shows a high degree of orientational order before polymerization, which is retained to a large extent after polymerization. The order in the polymer network results in considerable birefringence at temperatures well above the clearing point of the pure FLC. This behaviour is induced by the order of the polymer network and interactions of the FLC molecules with the network which prevent a fraction of the FLC molecules from exhibiting typical phase behaviour. In the formation of the branched liquid crystalline bismethacrylates/FLC composites, a steady increase is observed in the polymerization rate as the polymerization temperature increases and the order of the FLC phase decreases, a behaviour significantly different from that observed in other monomer/FLC mixtures for which the polymerization rate increases as the order of the FLC phase increases. Additionally, at appropriate polymerization temperatures around the clearing point, polymerization driven endothermic phase transitions may be observed.     

Rheological effects of structure formation in polymer melts

The viscosity of a number of molten polystyrenes and polypropylenes obtained in various ways from solutions is found to be very strongly dependent on the history of dissolution of the polymer (the concentration of the solution and the “goodness” of the solvent), and on the presence of certain organic oligomeric compounds in very small amounts. The viscosity of the melt may be orders of magnitude less than that of the original samples. The present discussion of these effects is based on the concept of structure formation in amorphous polymers. The structure formed on the solution appears to be very stable with respect to heating and prolonged standing at elevated temperature, although it (and hence the viscosity) strongly depends on the initial concentration of the solution and the nature of the solvent. The role played by additives, present in amounts of small fractions of a percent, is accounted for by the fact that the melt retains the microheterogeneity of the medium. The material moves without its micromosaic structure being disturbed, and the role of the additives is reduced to surface (with respect to structural elements) phenomena.     

Structure and electrical properties of Y,Fe-based perovskite mixed conducting composites fabricated by a modified polymer precursor method

In this work, samples of Y_0.07Sr_0.93Ti_1-xFe_xO_3-δ with 20, 40, 60 and 80 mol% of iron amount were prepared by a low-temperature polymer precursor method. The SEM-EDS analysis proved that analyzed Y_0.07Sr_0.93Ti_1-xFe_xO_3-δ samples were composites of two Ti- and Fe-rich perovskite samples. This kind of composite consists of two phases in which one has a good ionic and the other electronic conductivity, which makes such a composite a potential mixed ionic and electronic conductors (MIECs) material. The total electrical conductivities of analyzed samples were measured in air atmosphere (cathode conditions in Solid Oxide Fuel Cell). The values changed from ∼10⁻³ to 10⁻¹ S cm⁻¹ and depended on the ratio between two observed perovskite phases. The 0.12 S cm⁻¹ conductivity value at 800 °C for sample with the highest amount of Fe-rich perovskite in the structure makes this composite material a candidate for air electrode in electrochemical devices.     

Photorefractive effect in composites of ferroelectric liquid crystal and photoconductive polymer

The photorefractive effect in composites of a ferroelectric liquid crystal (FLC) and several photoconductive polymers was investigated. The photorefractivity of mixtures of photoconductive polymers and an FLC (polymer/FLC), as well as that of photoconductive‐polymer‐stabilized ferroelectric liquid crystals (PPS‐FLCs) was examined. The polymer/FLC samples exhibited two‐beam coupling gain coefficients of about 6～12 cm^?1 in a 5 µm gap cell. The photopolymerization of a methacrylate monomer in the FLC medium established a polymer‐stabilized state in which the alignment of FLC molecules was mechanically stabilized. The noise in a two‐beam coupling signal was reduced significantly in the PPS‐FLC samples.     

The effect of temperature on the curing of unsaturated polyester resins modified with a liquid polymer

The effect of temperature on the cure kinetics of two different unsaturated polyester resins, Estratil 1.112 and G102, modified with a liquid polymer, has been studied by Fourier transform infrared (FTIR) spectroscopy in the range of 60–80°C and 70–90°C, respectively. The experimental results have been related to the rheological changes during cure.     

Preparation and properties of self-reinforced polypropylene/liquid crystalline polymer composites

A fast and potentially economic method for creating a composite material of polypropylene (PP) with liquid crystalline polymers (LCPs) was explored. The LCPs were dispersed in the PP matrix in a conventional extrusion process and subsequently drawn in order to obtain a highly uniaxially molecular orientation of the LCP fibres. The strongest mechanical properties were found after rapid cooling of the blend, which was optimally exploited in thin filaments where cooling was fast enough to prevent fibre break-up and/or orientation relaxation. To enable the production of extrudates which are thick enough to be cut into self-reinforced pellets, a fused multi-filament technology was developed. In this technology several thin filaments were extruded/drawn and rapidly cooled to a temperature between the crystallisation temperatures of the LCP and the PP. After solidification of the LCP, the single filaments were fused to a multifilament strand and further cooled to room temperature. The thick multifilament LCP reinforced PP strands were cut into pellets and used in injection-and compression-moulding processes. This processing took place between the melting temperature of the PP and the LCP, with retention of the aspect ratio and the molecular orientation of the LCP fibres.     

Photorefractive effect in composites of ferroelectric liquid crystal and photoconductive polymer

The photorefractive effect in composites of a ferroelectric liquid crystal (FLC) and several photoconductive polymers was investigated. The photorefractivity of mixtures of photoconductive polymers and an FLC (polymer/FLC), as well as that of photoconductive-polymer-stabilized ferroelectric liquid crystals (PPS-FLCs) was examined. The polymer/FLC samples exhibited two-beam coupling gain coefficients of about 6∼12 cm^-1 in a 5 µm gap cell. The photopolymerization of a methacrylate monomer in the FLC medium established a polymer-stabilized state in which the alignment of FLC molecules was mechanically stabilized. The noise in a two-beam coupling signal was reduced significantly in the PPS-FLC samples.     

Striped pattern formation in polymer dispersed liquid crystal films

Composite films of polymer and liquid crystal (LC) have been prepared by a simple solution casting technique. The films obtained exhibit a polymer dispersed liquid crystal structure where LC droplets are dispersed in the polymer matrix. Casting the mixture of polymer solution and LC on a tilted substrate results in a specific texture in which arrays of LC droplets align parallel to each other, i.e. a striped pattern can be formed. The size of the droplets and the spacing between the lines are dependent on the substrate tilt angle and the preparation temperature. By using a dip-coating technique, a similar striped texture appears at a fast dipping velocity. From in situ observation of the stripe development, it is seen that the translation of the phase-separating region, accompanying the flow, plays a more important role in the formation of the striped patterns than the flow of the solution itself.     

Surfactant boundary lubricant film modified by an amphiphilic diblock copolymer

The effect of the uptake of a low-molecular-weight amphiphilic diblock copolymer on the morphology of didodecyldimethylammonium bromide (DDAB) adsorbed layers on mica, the interactions between two coated surfaces, and the frictional properties of the boundary film have been studied using an atomic force microscope and a dynamic surface forces apparatus nanotribometer. When DDAB-coated surfaces in aqueous solution were compressed, hemifusion or removal of the adsorbed surfactant bilayers could not be induced, and no frictional force could be measured between the surfaces, which display superior lateral cohesion and lubricant properties. Coadsorbing octadecyl end modified poly(ethylene oxide) chains at low density facilitates hemifusion, generating significant shear stress and leading to stick-slip instabilities. The mixed films regain their lateral cohesion at higher adsorbed copolymer densities, but an extra short-range attraction brings the adsorbed layers into adhesive contact without causing bilayer hemifusion. Here, noticeable frictional forces are also measured.     

Striped pattern formation in polymer dispersed liquid crystal films

Composite films of polymer and liquid crystal (LC) have been prepared by a simple solution casting technique. The films obtained exhibit a polymer dispersed liquid crystal structure where LC droplets are dispersed in the polymer matrix. Casting the mixture of polymer solution and LC on a tilted substrate results in a specific texture in which arrays of LC droplets align parallel to each other, i.e. a striped pattern can be formed. The size of the droplets and the spacing between the lines are dependent on the substrate tilt angle and the preparation temperature. By using a dip-coating technique, a similar striped texture appears at a fast dipping velocity. From in situ observation of the stripe development, it is seen that the translation of the phase-separating region, accompanying the flow, plays a more important role in the formation of the striped patterns than the flow of the solution itself.     

Characterisation of polymer wall formation in nematic liquid crystal devices

A phase-sensitive measurement technique is proposed to characterise polymer wall formation in a nematic liquid crystal cell. The impact of photopolymerisable monomer concentration and curing time on the electro-optical properties of polymer-wall LC cells was studied. The experimental results indicate that optimum curing conditions for constructing the polymer wall in an LC cell can be easily achieved by means of the proposed method. The structure of the polymer walls was also investigated using scanning electron microscopy and a polarising optical microscope.