Synthesis and characterization of liquid crystalline epoxy resins

Fiber reinforced polymer matrix composites (FRPs) have been developed for many decades and used in a wide variety of applications. However, the residual stresses caused by the mismatch in the coefficient of thermal expansion (CTE) between the polymer matrices and the fiber reinforcements during the processing of FRPs is a crucial factor affecting the performance of the composites, which can lead to a reduction of mechanical properties and loss of dimensional stability, thereby limiting the use of FRPs in high performance applications. Additionally, the relatively poor matrix properties is another factor affecting overall performance of the composites, including chemical resistance, moisture absorption, and long term durability of FRPs. A potential strategy to solve the problems mentioned above involves the development of novel polymer matrices with improved physical, thermal, and mechanical properties with low thermal expansion to ensure minimal mismatch in CTE with the fiber reinforcements, which can reduce the magnitude of residual stresses, facilitating the development of FRPs for advanced applications. Liquid crystalline epoxy resins (LCERs) are a unique class of thermosetting materials formed upon curing of low molecular weight, rigid rod epoxy monomers, resulting in the retention of a liquid crystalline (LC) phase by the three dimensional networks. LCERs exhibit a polydomain structure, thereby combining the outstanding properties of liquid crystals and thermosets. The rigid and ordered structure of LC domains is expected to reduce the CTE of the resins as well as improve the thermal and mechanical properties of the resins. In addition, liquid crystals possess properties that can be controlled by external fields, greatly improving the design flexibility. These attractive features make LCERs good candidates for polymer matrices in high performance composites. The goal of this research is to synthesize a LCER based on biphenyl mesogen, characterize the thermal, physical, and mechanical properties of the resin, and evaluate the potential use of LCERs as polymer matrices in high performance composites

Functional liquid crystalline epoxy networks and composites: from materials design to applications

Liquid crystalline epoxy networks (LCENs) are a class of materials that combine the useful benefits of both liquid crystals and epoxy networks exhibiting fascinating thermal, mechanical, and stimuli-responsive properties. They have emerged as a new platform for developing functional materials suitable for various applications, such as sensors, actuators, smart coatings and adhesives, tunable optical systems, and soft robotics. This article provides an overview of LCENs and their composites as functional materials, including their synthesis and characterisation, focusing on structure-processing–property relationships. We provide objective analyses on how materials engineers can use these relationships to develop LCENs with desired functionalities for targeted applications. Emerging areas, including advanced manufacturing and multi-functional design of LCENs are covered to show the overall progress in this field. We also survey the forward-looking status of LCEN research in designing novel materials for future technologies

High performance thermosets with tailored properties derived from methacrylated eugenol and epoxy-based vinyl ester

A renewable chemical, eugenol, is methacrylated to produce methacrylated eugenol (ME) employing the Steglich esterification reaction without any solvent. The resulting ME is used as a low viscosity comonomer to replace styrene in a commercial epoxy-based vinyl ester resin (VE). The volatility and viscosity of ME and styrene are compared. The effect of ME loadings and temperatures on viscosity of the VE-ME resin is investigated. Moreover, the thermo-mechanical properties, curing extent, and thermal stability of the fully cured VE-ME thermosets are systematically examined. The results indicate that ME is a monomer with low volatility and low viscosity, and therefore the incorporation of ME monomer in VE resins allows significant reduction of viscosity. Moreover, viscosity of the VE-ME resin can be tailored by adjusting ME loadings and processing temperature to meet commercial liquid molding technology requirements. The glass transition temperatures of VE-ME thermosets range from 139 to 199 °C. In addition, more than 95% of the monomer is incorporated and fixed in the crosslinked network structure of VE-ME thermosets. Overall, the developed ME monomer exhibits promising potential to replace styrene as an effective low viscosity comonomer. The VE-ME resins show great advantages for use in polymer matrices for high performance fiber-reinforced composites. This work showed great significance to the vinyl ester industry by providing detailed experimental support

A Fault Diagnosis Scheme for Gearbox Based on Improved Entropy and Optimized Regularized Extreme Learning Machine

The performance of a gearbox is sensitive to failures, especially in the long-term high speed and heavy load field. However, the multi-fault diagnosis in gearboxes is a challenging problem because of the complex and non-stationary measured signal. To obtain fault information more fully and improve the accuracy of gearbox fault diagnosis, this paper proposes a feature extraction method, hierarchical refined composite multiscale fluctuation dispersion entropy (HRCMFDE) to extract the fault features of rolling bearing and the gear vibration signals at different layers and scales. On this basis, a novel fault diagnosis scheme for the gearbox based on HRCMFDE, ReliefF and grey wolf optimizer regularized extreme learning machine is proposed. Firstly, HRCMFDE is employed to extract the original features, the multi-frequency time information can be evaluated simultaneously, and the fault feature information can be extracted more fully. After that, ReliefF is used to screen the sensitive features from the high-dimensional fault features. Finally, the sensitive features are inputted into the optimized regularized extreme learning machine to identify the fault states of the gearbox. Through three different types of gearbox experiments, the experimental results confirm that the proposed method has better diagnostic performance and generalization, which can effectively and accurately identify the different fault categories of the gearbox and outperforms other contrastive methods.</p

Synthesis and characterization of liquid crystalline epoxy resins

Fiber reinforced polymer matrix composites (FRPs) have been developed for many decades and used in a wide variety of applications. However, the residual stresses caused by the mismatch in the coefficient of thermal expansion (CTE) between the polymer matrices and the fiber reinforcements during the processing of FRPs is a crucial factor affecting the performance of the composites, which can lead to a reduction of mechanical properties and loss of dimensional stability, thereby limiting the use of FRPs in high performance applications. Additionally, the relatively poor matrix properties is another factor affecting overall performance of the composites, including chemical resistance, moisture absorption, and long term durability of FRPs. A potential strategy to solve the problems mentioned above involves the development of novel polymer matrices with improved physical, thermal, and mechanical properties with low thermal expansion to ensure minimal mismatch in CTE with the fiber reinforcements, which can reduce the magnitude of residual stresses, facilitating the development of FRPs for advanced applications. Liquid crystalline epoxy resins (LCERs) are a unique class of thermosetting materials formed upon curing of low molecular weight, rigid rod epoxy monomers, resulting in the retention of a liquid crystalline (LC) phase by the three dimensional networks. LCERs exhibit a polydomain structure, thereby combining the outstanding properties of liquid crystals and thermosets. The rigid and ordered structure of LC domains is expected to reduce the CTE of the resins as well as improve the thermal and mechanical properties of the resins. In addition, liquid crystals possess properties that can be controlled by external fields, greatly improving the design flexibility. These attractive features make LCERs good candidates for polymer matrices in high performance composites. The goal of this research is to synthesize a LCER based on biphenyl mesogen, characterize the thermal, physical, and mechanical properties of the resin, and evaluate the potential use of LCERs as polymer matrices in high performance composites.</p

Liquid crystalline epoxy resin based on biphenyl mesogen: Effect of magnetic field orientation during cure

A biphenyl based epoxy monomer, 4,4′-diglycidyloxybiphenyl (BP), was synthesized and cured with a tetra-functional amine, sulfanilamide (SAA), to obtain a liquid crystalline epoxy network. The curing behavior of BP with SAA was studied using differential scanning calorimetry, polarized optical microscopy, and parallel plate rheology. Macroscopic orientation of the liquid crystalline epoxy resins (LCERs) was achieved by curing in a high strength magnetic field, and quantified by an orientation parameter determined with wide angle X-ray diffraction. The effects of orientation on the glass transition temperature, coefficient of thermal expansion, and dynamic mechanical properties of the LCERs were investigated. The results reveal that the formation of the liquid crystalline phase has a dramatic influence on the curing reaction, leading to a decrease in viscosity of the reacting system. Oriented LCERs exhibit anisotropic thermal expansion behavior and significant improvements of thermomechanical properties

Creep-resistant behavior of self-reinforcing liquid crystalline epoxy resins

The creep behavior of a liquid crystalline epoxy resin (LCER) was investigated and compared with that of a non-LCER prepared from the same epoxy monomer. The experimental data was evaluated using Burgers' model to explain the reinforcing effect of the liquid crystalline (LC) phase. The long-term performance of the material was predicted using the time–temperature superposition principle. The results revealed that the introduction of an LC phase into the resin network can reduce creep strain and creep strain rate of the material, especially at elevated temperatures. Parameters extracted from the simulation indicated that instantaneous elasticity, retardant elasticity, and permanent flow resistance of the resins were enhanced by the presence of the LC phase. A rigid filler effect and a crosslinking effect are proposed to explain the reinforcing mechanisms

Cure kinetics of liquid crystalline epoxy resins based on biphenyl mesogen

The cure kinetics of a biphenyl-based liquid crystalline (LC) epoxy resin (LCER) was studied using differential scanning calorimetry (DSC) and polarized optical microscopy. The effects of LC phase formation on the cure kinetics were investigated. Both a model-free isoconversional method and a model-fitting method were used to analyze the DSC data. Results from the isoconversional analysis were applied to develop tentative multi-step kinetic models describing the curing reaction. Kinetic analysis showed that compared to the resins cured in amorphous phase, LCERs exhibited higher values of reaction enthalpy and a complex dependence of activation energy on the degree of cure. The formation of the LC phase resulted in a decrease in activation energy, leading to higher degree of reaction

Liquid crystalline epoxy resin based on biphenyl mesogen: Thermal characterization

An epoxy monomer of 4,4′-diglycidyloxybiphenyl (BP) was synthesized and cured with a tetra-functional amine, sulfanilamide (SAA), to produce novel liquid crystalline epoxy resins (LCERs). The thermal properties, liquid crystalline morphologies, and cure behavior of the monomer were studied using differential scanning calorimetry, wide angle X-ray diffraction, and polarized optical microscopy. The effects of curing condition on the glass transition temperature, coefficient of thermal expansion, and dynamic mechanical properties of the resins were determined through thermomechanical analysis and dynamic mechanical analysis, respectively. The effects of cure condition on the formation of the liquid crystalline phase were also examined. The results show that BP is not a liquid crystalline epoxy monomer and an irreversible crystal transition exists in the temperature range of 120 °C–140 °C. The use of SAA results in the formation of a smectic liquid crystalline phase. Compared to the resins cured into an amorphous network, the LCERs exhibited a polydomain structure with individual liquid crystalline domain distributed in the resin matrix, which results in better thermomechanical properties