Liquid Crystalline Epoxy Research Articles

Molecular Orientation and Thermal Conductivity in Liquid Crystalline Epoxy Resins by Anionic Ring-Opening Polymerization

Molecular Orientation and Thermal Conductivity in Liquid Crystalline Epoxy Resins by Anionic Ring-Opening Polymerization

Biobased Imine-Containing Liquid Crystalline Epoxy Resins with Versatile Recyclability and Enhanced Intrinsic Thermal Conductivity

Biobased Imine-Containing Liquid Crystalline Epoxy Resins with Versatile Recyclability and Enhanced Intrinsic Thermal Conductivity

Effect of Flexible Chains on Thermal Conductivity of Liquid Crystalline Epoxy Resins

Effect of Flexible Chains on Thermal Conductivity of Liquid Crystalline Epoxy Resins

3D Printing of Thermally Responsive Shape Memory Liquid Crystalline Epoxy Networks.

A two-component liquid crystalline epoxy network (LCEN) with shape memory behavior was developed and evaluated as a candidate material for 3D printing. The cure kinetics of the uncured material and the shape memory properties of the cured LCEN were investigated by using parallel plate rheology and dynamic mechanical analysis, respectively. A commercially available fumed silica additive was introduced to the neat, uncured material to improve the rheological properties for 3D printing. The addition of fumed silica was found to increase the yield stress, shear-thinning behavior, and toughness of the uncured epoxy ink. Polarized light microscopy, differential scanning calorimetry, and wide-angle X-ray scattering measurements between the neat and additive-modified LCEN suggested a reduction in liquid crystalline alignment in the modified LCEN, owing to interactions between crystalline domains and fumed silica, which in turn influenced the mechanical behavior. Overall, the additive was found to be successful in preserving the shape memory properties of LCEN while improving its printability.

Investigating Cross-Linking Parameters and Molecular Arrangement in Liquid Crystalline Epoxy Monomer with Aromatic Diamine: DSC-TOPEM® and WAXS Analysis.

This study presents the characterization of cross-linking parameters of a liquid crystalline epoxy monomer with an aromatic diamine as a curing agent. The mixture tested consisted of bis [4-(10,11-epoxyundecanoyloxy)benzoate] p-phenylene (LCEM) and 1,3-phenylenediamine (1,3-PDA). This paper focuses on the structural characterization of such systems using X-ray analysis. To investigate this, a comprehensive analysis was conducted using Differential Scanning Calorimetry (DSC) and Wide-Angle X-ray Scattering (WAXS). Through DSC analysis, the curing behavior and transition temperature of the liquid crystal epoxy system were established. To fully characterize the cross-linking of the system, a novel technique called DSC-TOPEM® was employed. The use of this technique enabled real-time monitoring of the curing process and provided precise information on the cross-linking energy, which resulted in the finding that the mixture was cross-linking faster than expected. WAXS analysis was performed to assess the structural changes formed during the cross-linking. The results of this analysis confirm that lower cross-linking temperatures of the mixture cause better ordering of mesogens than higher ones.

The Effect of Nonterminal Liquid Crystalline Epoxy Resin Structure and Curing Agents on the Glass Transition of Polymer Networks.

Modern science and technology demand a low glass transition temperature, yet one tailored to specific thermoset needs and specific to individual hardener applications. Two novel, nonterminal liquid crystalline epoxy resins (LCER) were synthesised, with their structures characterized via nuclear magnetic resonance (NMR), mass spectrometry (MS), and elemental analysis. Their liquid crystalline nature and thermal properties were determined using polarized optical microscopy (POM), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). A set of seven aromatic amines serving as curing agents was used to perform curing in fourteen different systems in order to assess the glass transition temperature (Tg) of the obtained polymer networks using DSC. The liquid crystalline elastomers were obtained with vitrification occurring in a low temperature range (-10-40 °C), with a more predictable outcome for amines with two aromatic rings in the structure than with one. Moreover, the resin with a core consisting of four aromatic rings produces networks with higher Tg than the three-aromatic resin. The use of nonterminal LCER allowed the lowering of the glass transition temperature of the polymers to more than 70 °C compared to a terminal analogue. This brings new possibilities of designing highly elastic yet cured polymers with potential for use in smart applications due to the LC nature of the resin.

Enhancement of thermal conducting properties in epoxy thermoset systems using an aligned liquid-crystalline mesophase

Liquid crystalline epoxy resins (LCERs) with high thermal conductivity (T/C) are an attractive solution that meet the thermal management requirements of electronic devices.

Nonterminal liquid crystalline epoxy resins as structurally ordered low Tg thermosets with potential as smart polymers

Nonterminal liquid crystalline epoxy resins as structurally ordered low Tg thermosets with potential as smart polymers

Thermal Conductivity and Orientation Structure of Liquid Crystalline Epoxy Thermosets Prepared by Latent Curing Catalyst

Improvements in the performance of electronic devices necessitate the development of polymer materials with heat dissipation properties. Liquid crystalline (LC) epoxies have attracted attention because of the orientation of their polymer network chains and their resultant high thermal conductivity. In this study, a diglycidyl ether of 1-methyl-3-(4-phenylcyclohex-1-enyl)benzene was successfully synthesized as an LC epoxy and the LC temperature range was evaluated via differential scanning calorimeter (DSC). The synthesized LC epoxy was cured with m-phenylenediamine (m-PDA) as an amine-type curing agent and 1-(2-cyanoethyl)-2-undecylimidazole (CEUI) as a latent curing catalyst, respectively. The LC phase structure and domain size of the resultant epoxy thermosets were analyzed through X-ray diffraction (XRD) and polarized optical microscopy (POM). High thermal conductivity was observed in the m-PDA system (0.31 W/(m·K)) compared to the CEUI system (0.27 W/(m·K)). On the other hand, in composites loaded with 55 vol% Al2O3 particles as a thermal conductive filler, the CEUI composites showed a higher thermal conductivity value of 2.47 W/(m·K) than the m-PDA composites (1.70 W/(m·K)). This difference was attributed to the LC orientation of the epoxy matrix, induced by the hydroxyl groups on the alumina surface and the latent curing reaction.

Effect of the Structure of Epoxy Monomers and Curing Agents: Toward Making Intrinsically Highly Thermally Conductive and Low-Dielectric Epoxy Resins.

The low intrinsic thermal conduction and high dielectric properties of epoxy resins have significantly limited their applications in electrical and electronic devices with high integration, high frequency, high power, and miniaturization. Herein, a liquid crystalline epoxy (LCE) monomer with a biphenyl mesogenic unit was first synthesized through an efficient one-step reaction. Subsequently, bisphenol AF (BPAF) containing low-polarizable -CF3 groups and 4,4'-diaminodiphenylmethane (DDM) were applied to cure the LCE and commercial diglycidyl ether of bisphenol A-type epoxy (E-51), respectively, to afford four kinds of epoxy resins with various intrinsic thermal conductivity and dielectricity values. Owing to the dual effect of microscopically stacking of mesogens and the contribution of fluorine to the formation of liquid crystallinity, ordered microstructures of the nematic liquid crystal phase were formed within the cross-linking network of LCE as confirmed by polarized optical microscopy and X-ray diffraction. Consequently, phonon scattering was suppressed, and the intrinsic thermal conductivity was improved considerably to 0.38 W/(m·K), nearly twice as high as that of E-51 cured with DDM (0.20 W/(m·K)). Additionally, the ordered microstructure and ultralow polar -CF3 groups within LCE cured with BPAF enabled the epoxy resin to exhibit a remarkably lower and stable dielectric constant (ε) and dielectric loss tangent (tan δ) over both low and high frequencies compared to E-51 cured with DDM. The ε decreased from 3.40 to 2.72 while the tan δ decreased from 0.044 to 0.038 at 10 GHz. This work presents a scalable and facile strategy for breaking the bottleneck of making epoxy resins simultaneously with high inherent thermal conduction and low dielectric performance.

Forming a Homeotropic SmA Structure of Liquid Crystalline Epoxy Resin on an Amine-Modified Surface.

The molecular orientation of a liquid crystalline (LC) epoxy resin (LCER) on silane coupling surfaces of amorphous soda-lime-silica glass substrates was investigated. The LC epoxy monomer on the silane coupling surfaces of the substrates was revealed to form a smectic A (SmA) phase with planar alignments because of the relatively low surface free energy. An LCER with a curing agent, however, formed a homeotropically aligned SmA structure by curing on a substrate surface modified using a silane coupling agent with amino groups. This formation of homeotropic alignment was considered due to the attribution of the reaction between the amino group on the surface of the substrate and the epoxy group of the LCER. The homeotropic alignment had a relatively high orientation parameter of 0.95. Therefore, it is expected to possess high thermal conductivity and be applied as high-thermal-conductivity adhesives or packaging materials for electrical and electronic devices.

Programmable Patterned Dynamic Scattering of Liquid Crystals/Epoxy‐based Polymer Composite Film Sandwiched between Two Fully Covered Electrodes

AbstractDynamic scattering has been a prominent technique in the display industry for several years. However, conventional approaches have required the use of tailored conductive layers to enable modular control in display panels. This has resulted in an increase in process complexity and device cost. To overcome these limitations, the present work proposes a novel method for fabricating a patterned dynamic scattering film using masked photopolymerization of liquid crystalline epoxy in negative liquid crystals. Specifically, the film is sandwiched between two fully covered electrodes, and an electric field is applied causing dynamic scattering only in the area that had been exposed to ultraviolet light during the preparation process. The remaining area remained transparent. The underlying mechanism is attributed to the formation of polymer solely in the UV‐exposed region via masked photopolymerization. This polymer is capable of absorbing ions, resulting in the formation of a concentration gradient of ions between the UV‐exposed and unexposed areas. The low concentration of ions in the unexposed area prevents the induction of dynamic scattering under an electric field, leading to the patterned dynamic scattering phenomenon. Building on this mechanism, the researchers are able to achieve several multilevel patterned dynamic scattering films sandwiched between two fully covered electrodes.

Phase-controllable topochemical polymerization of liquid crystalline epoxy according to spacer length

A new liquid crystal epoxies (LCEs) system based on the twin mesogenic core and imidazole curing agent is investigated. The LCEs records high thermal conductivity (0.49 W m−1 K−1) which are highly affected by microscopic orientation.

Colorfully Patterned Epoxy Resin Films with a Cholesteric Structure Prepared through a Photopolymerization Approach

Structurally colored epoxy resin films were prepared using a liquid crystalline (LC) epoxy monomer of E11M and a chiral additive of CA-iso through a photopolymerization approach. With the addition of the co-photoinitiator isopropyl thioxanthone and comonomer 3′,4′-epoxycyclohexylmethyl 3,4-epoxycyclohexylcarboxylate, the polymerization rate was increased. Since the epoxy resin films can be obtained within several seconds, they are feasible for large-area preparation on the coating line. The reflective wavelength was tunable by changing the concentration of CA-iso in the LC mixture. Diffuse-reflectance circular dichroism spectra indicated a right-handed helical supramolecular structure. Field-emission scanning electron microscopy images indicated a multilayer structure. Since the LC mixtures were thermochromic, colorfully patterned epoxy resin films were prepared by controlling the polymerization temperature. Moreover, LC gratings were also prepared, which were expected to be applied for anticounterfeiting.

The application of liquid crystalline epoxy resin for forming hybrid powder coatings

The application of liquid crystalline epoxy resin for forming hybrid powder coatings

Increasing strengths of liquid crystalline polymers while minimizing anisotropy via topological rearrangement assisted bi-directional stretching of reversibly interlocked macromolecular networks

To prepare oriented crosslinked liquid crystalline polymers (LCPs) while preventing the unwanted anisotropy, the authors fabricated interlocked networks from liquid crystalline epoxy and amorphous polyurethane, which are crosslinked by reversible Diels-Alder (DA) bonds and Schiff base, respectively. By making use of dynamicity of the reversible bonds as well as relative mobility of the parent networks of the interlocked networks, the material is subjected to stepwise solid-state bi-directional drawing. Owing to the directional alignment of the mesogens and molecular chains and the increased crosslinking density during stretching caused by the networks reshuffling, the strengths of the material in orthogonal directions are simultaneously improved and gradually converged to the values comparable to the reported highest level of unidirectionally orientated crosslinked LCPs. Furthermore, the biaxially oriented interlocked networks have inherited the intrinsic self-healing capability from the included reversible bonds. The proposed method is easy to use and expected to contribute to the practical applications of crosslinked LCPs, which is becoming more and more important in modern microelectronics and communication technology.

The application of liquid crystalline epoxy resin for forming hybrid powder coatings

The application of liquid crystalline epoxy resin for forming hybrid powder coatings

The application of liquid crystalline epoxy resin for forming hybrid powder coatings

The purpose of the study was to check whether the addition of a liquid crystalline epoxy resin containing rigid mesogenic groups would have a positive effect on the properties of epoxy/polyester hybrid powder coating. The research included the preparation of three hybrid compositions of epoxy/polyester powder coating consisting of polyester resin GS 7371 with the addition of commercial bisphenol A-based epoxy resin E 011 (Composition 1), mixture of epoxy resin E 011 and liquid crystalline diepoxy resin MU22 (Composition 2) and only liquid crystalline diepoxy resin MU22 (Composition 3). By using the DSC method, the course of the cross-linking process of the prepared compositions was analyzed, and the curing conditions were selected. Based on the DSC analysis, it was also found that the addition of diepoxide MU22 reduces the glass transition temperature of the obtained coatings. For coatings cured on a steel substrate, a number of tests were carried out on the basis of which the surface structure was correlated with the chemical structure of the coatings and macroscopic surface behavior, i.e.: color, thickness, gloss, roughness, abrasion and scratch resistance, hardness, elasticity, contact angle, surface free energy, as well as adhesion to steel surface. It has been found that the modification of polyester/epoxy hybrid powder paint with liquid crystalline epoxy resin had a positive effect on its properties.

High toughness and thermal conductivity of thermosets from liquid crystalline epoxy with low melting point

AbstractA diphenylcyclohexene‐type liquid crystalline (LC) epoxy was successfully synthesized and characterized by proton nuclear magnetic resonance (1H‐NMR) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and gel permeation chromatography (GPC). Even though the synthesized epoxy monomer showed a low melting temperature (99°C), the LC (smectic and nematic) phase was maintained over a wide temperature range of 99–184°C during the heating process. Moreover, the synthesized LC epoxy was cured with 4,4′‐diaminodiphenylmethane under four different temperature conditions. The LC phase structure, domain size, and LC fraction of the resulting epoxy thermosets were analyzed by X‐ray diffraction (XRD) and polarized optical microscopy (POM). The fracture toughness and thermal conductivity increased with an increase in the size and volume fraction of the LC domains. It was also found that the enhancement of the fracture toughness was more sensitive than that of the thermal conductivity.

Liquid Crystalline Composite Stabilized by Epoxy Polymer with Boscage‐Like Morphology for Energy‐Efficient Smart Windows with High Stability

AbstractSmart windows, which can provide comfortable indoor conditions for cars or buildings and protect the privacy of people, have attracted much attention. Traditional acrylate‐based smart windows have the disadvantages of high energy consumption and poor stability. Herein, an energy‐efficient smart window based on a liquid crystalline composite stabilized by an epoxy polymer with boscage‐like morphology is reported. This epoxy polymer is prepared by cationic polymerization of the liquid crystalline epoxy monomer E6M in vertically oriented negative nematic liquid crystals. The orientation of the liquid crystals is controlled by the epoxy polymer, which makes the composite film transparent and switchable to scattering by an electric field. Therefore, the composite film can be used in an energy‐efficient reverse‐mode smart window. The reverse‐mode smart window has better cycling stability than does acrylate, owing to the better mechanical properties endowed by the polymer matrix with a unique morphology and high stress tolerance.