Thermal conductivity enhancement of liquid crystalline epoxy/MgO composites by formation of highly ordered network structure

To develop a high thermal conductive composite, an MgO filler was incorporated into a liquid crystalline (LC) epoxy containing a mesogenic moiety. The thermal conductivity of the obtained composite was 1.41 W/(m∙K) at 33 vol% content, which was remarkably higher than the value predicted using Bruggeman's model. To investigate the reason for this significant enhancement of the thermal conductivity in the LC epoxy composites, the LC phase structure of the composite was analyzed by a polarized optical microscope, an X-ray diffractometry (XRD) and a polarized IR mapping measurement. An XRD analysis indicated the local formation of a highly ordered smectic phase structure, even in the high-loading composite. This result indicated the promotion of the self-assembly of the mesogenic network polymer chains by the MgO filler loading. We considered that this highly ordered structural formation can lead to an increase in the matrix resin's thermal conductivity, which can result in the effective enhancement of the thermal conductivity in the LC epoxy/MgO composite.     

环氧树脂/环氧液晶/蒙脱土纳米复合材料研究

采用液晶环氧预聚物(PHQEP)与有机蒙脱土(OMMT)共混改性环氧树脂制备三元共混体系的环氧基复合材料。用X射线衍射法(XRD)测试了有机化蒙脱土在被插层前后片层间距的变化,通过DSC、TGA及SEM等对PHQEP/OMMT增韧改性环氧树脂固化体系的力学性能,热性能及微观相态结构进行了研究。结果表明:当PHQEP质量分数为5%,添加1.5%的有机蒙脱土可以使环氧树脂的冲击强度达到最大值23.43 kJ/m2,比纯环氧树脂提高2倍左右,玻璃化转变温度及5%热分解温度比纯环氧树脂分别高出15℃和27℃。PHQEP与OMMT的加入使纳米复合材料的力学性能和热性能得到明显提高。     

Mechanical properties of glass fiber composites with an epoxy resin modified by a liquid crystalline epoxy

The effect of liquid crystalline networks on epoxy + glass fiber composites is investigated. The matrix obtained from in‐situ curing of liquid crystalline (LC) diglycidyl ether of 4,4‐dihydroxybiphenol (DGE‐DHBP) with diglycidyl ether of bisphenol F (DGEBP‐F) by anhydride curing agent was used as the matrix for polymer composites. Impact, tensile, and flexural testing results are compared between the unmodified and modified systems. The interlaminar fracture toughness of commposites in the shear mode was determined by end notch flexure (ENF) tests. Scanning electron microscopy is used to study the fracture surfaces to understand the mechanism of fracture and interphase formation between the fiber and the matrix.     

改性环氧树脂对碳纤维表面上浆性能的影响

以双酚F型EP(环氧树脂)为基体、PMCB(含磷三聚氰胺席夫碱)为改性剂,合成了碳纤维表面上浆用新型含磷阻燃型EP上浆剂。研究结果表明:该上浆剂具有一定的阻燃性和热稳定性(其热分解温度为283.6℃),能有效提高碳纤维的综合性能;将该上浆剂应用于PAN(聚丙烯腈)基碳纤维表面,则上浆后的碳纤维复丝表面光滑、黏附性和集束性明显提高,并且其极限氧指数(29.4)提升了31.8%、拉伸强度(6 300 MPa)明显增大。     

Fracture toughness of highly ordered liquid crystalline epoxy thermosets achieved by optimized composition of curing agents

Liquid crystalline (LC) epoxy resin was prepared by using different compositions of aromatic amine as curing agents, in order to control curing rates and chemical compositions. The progress of the curing reaction was investigated based on the gel fraction and epoxy groups of conversion determined by Fourier-transform infrared spectroscopy. The ordered networked polymer structure was analyzed by polarized optical microscopy and X-ray diffraction. Highly oriented network chains in the obtained epoxy thermosets were promoted by the incorporation of flexible chains in the network and the provision of sufficient time for vitrification. Furthermore, it was clarified that a curing temperature higher than T_g is required to promote the transition to the smectic LC phase in order to prepare highly ordered epoxy thermosets. The increase in the formed smectic LC phase in the network chains resulted in significant higher fracture toughness and achieved up to 2.7 times higher value.     

Toughening epoxy resins by liquid crystalline polymers

Toughening epoxy resins can be achieved by blending a low percentage of Liquid Crystalline Polymer with the thermoset. The processing technique consists of the spinning of blends of a thermoplastic (Ardel) with the LCP (PET/PHB60). The bundled filaments are subsequently dissolved in the uncured epoxy resin (Epon 825). After curing, the LCP phase separates from the matrix in the form of microfibers with a very high aspect ratio, which act as crack stoppers and improve fracture toughness of the material.     

Enhancement in the thermal and dynamic mechanical properties of high performance liquid crystalline epoxy composites through uniaxial orientation of mesogenic on carbon fiber

In this work, a high performance liquid crystalline epoxy composite was prepared and the effect of the alignment of LCE with long lateral substituent on the carbon fiber surface curing at low temperature on fracture toughness, dynamic mechanical, and thermal properties of liquid crystalline epoxy with lateral substituent (LCE6) was investigated by polarized optical microscopy (POM), wide angle X‐ray diffraction measurements (WAXS), dynamic mechanical analysis (DMA), thermogravimetric (TGA), and scanning electron microscopy (SEM). Curing degree of the composite was observed by FTIR. The experimental results indicate that the fracture toughness, glass transition temperature (T_g), thermal stability, degradation kinetics are associated with the alignment of LCE6 along long axis of carbon fiber. The alignment of LCE6 on carbon fiber surface can increase mesogen network density, which leads to higher fracture toughness, higher thermal stability, increase of the activation energies and higher T_g of the composite. The dynamic mechanical analysis shows that the compoaite possesses extremely higher dynamic storage moduli, which indicates that this LCE6/DDM/CF composite can be a high performance composite. Thus, the compoaite can be a potential candidate for advanced composites. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40363.     

联苯型环氧液晶改性环氧塑封材料的性能研究

合成了一种端环氧基的联苯型液晶(LCEP),采用红外光谱(FT-IR)、示差扫描量热法(DSC)和偏光显微镜(POM)对其结构和热性能进行了分析,并以LCEP、环氧树脂、无机粉体(Al2O3和AlN)制备了环氧塑封料,通过对其力学性能、导热系数、线膨胀系数、电性能及热稳定性的测试研究了LCEP及无机粉体的加入对塑封料性能的影响。结果表明:加入LCEP后,塑封材料的冲击性能、弯曲性能均有所提高。同时,塑封料的导热系数、介电常数、热分解温度随着Al2O3、AlN含量的增加而增大,LCEP对于提高材料的导热性和热稳定性也有一定贡献。而线膨胀系数、介电损耗随着Al2O3和AlN含量的增加而降低,加入LCEP后塑封料的线膨胀系数略有降低。     

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.     

Laser ablation surface preparation for adhesive bonding of carbon fiber reinforced epoxy composites

Adhesive bonding of carbon fiber reinforced plastic (CFRP) epoxy composites provides many advantages over mechanical fastening for assembling aerospace structures including weight savings, reduced manufacturing flow, and added structural efficiency. To ensure the reliability of bonded joints in primary airframe structures, the surface preparation method and execution are critical. Surface preparation is widely recognized as a key step in the bonding process and is one element of a bonding method that must be controlled to produce robust and predictable bonds in a precise and repeatable manner. Laser ablation of composite surface resin can provide an efficient, precise, and reproducible means of preparing composite surfaces for adhesive bonding. Advantages include elimination of physical waste (i.e., grit media and sacrificial peel ply layers that ultimately require disposal), reduction in process variability due to increased precision (e.g. monitoring laser parameters), and automation of surface preparation. This paper describes a surface preparation technique using a nanosecond, frequency-tripled Nd:YAG laser source. Lap shear specimens were laser treated and tested and apparent shear strength and failure modes of lap shear specimens were used to assess mechanical performance over a three-year accelerated aging study by exposing bonded specimens to 71 °C (160 °F) and 85% relative humidity.     

液体成型用环氧树脂体系与碳纤维表面浸润性能研究

采用1,4-丁二醇二缩水甘油醚(BDDGE)、聚乙二醇二缩水甘油醚(PEGDGE)、双酚A聚氧乙烯醚06(BPE-06)3种活性环氧树脂稀释剂,分别制备了低黏度适合复合材料液体成型工艺(LCM)的环氧树脂体系,研究了体系与国产碳纤维(HF10)的表面浸润性。首先,研究了稀释剂结构、用量对环氧树脂体系与碳纤维湿润性的影响;其次,研究了稀释剂/树脂/固化剂体系的湿润温度、反应程度对树脂与碳纤维表面的浸润性影响。采用DCAT21表面/界面张力仪分析了树脂与碳纤维界面的前进接触角;采用Young-Dupre法,计算了树脂与碳纤维的热力学粘附功。结果表明,采用稀释剂降低黏度,可以有效改善树脂体系与碳纤维的浸润性;相同黏度时,不同结构稀释剂提高浸润性效果顺序为:PEGDGEBPE-06BDDGE;升高温度可以提高环氧树脂与碳纤维的浸润性;随着反应程度的提高,树脂体系与碳纤维的湿润性变好。     

Synthesis,morphology and physical properties of multi-walled carbon nanotube/biphenyl liquid crystalline epoxy composites

We have developed multi-walled carbon nanotube/liquid crystalline epoxy composites and studied the effects of incorporation carbon nanotubes (CNTs) on the morphology, thermal and mechanical properties of the composites. The CNTs are functionalized by liquid crystalline (LC) 4,4′-bis(2,3-epoxypropoxy) biphenyl (BP) epoxy resin for the ease of dispersion and the formation of long range ordered structure. The epoxy functionalized CNT (ef-CNT) were dispersed in the LC BP epoxy resin that can be thermal cured with an equivalent of 4,4′-diamino-diphenylsulfone to form composite. The curing process was monitored by polarized optical microscopy. The results indicate the LC resin was aligned along the CNTs to form fiber with dendritic structure initially then further on to obtain micro-sized spherical crystalline along with fibrous crystalline. With homogeneous dispersion and strong interaction between nanotubes and matrix, the composite containing 2.00 wt.% ef-CNT exhibits excellent thermal and mechanical properties. When the amount of ef-CNT exceeds 2.00 wt.%, vitrification stage of curing is fast reached, which lowers the degree of conversion. As compared with the neat resin, the composite containing 2.00 wt.% ef-CNT increases the glass transition temperature by 70.0 °C, the decomposition temperature by 13.8 °C, the storage modulus by 40.9%, and the microhardness by 63.3%.     

Curing behavior of liquid crystalline epoxy/DGEBA blend

The effect of liquid crystalline epoxy (LCE) resin on the curing behavior and thermomechanical properties of diglycidylether of bisphenol A (DGEBA) was investigated. LCE was blended with DGEBA and curing behavior of the blend was studied according to LCE content in the blend. Curing of DGEBA was accelerated and thermomechanical properties of DGEBA were considerably improved by the addition of LCE, which acted as a molecular reinforcement. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Water sorption in a novel liquid crystalline epoxy resin

Water sorption tests have been carried out for a novel epoxy resin having liquid crystalline character. The sorption isotherm. Relative to the expoxy resin cured in the nematic phase, does not indicate any substantial difference from the resin cured in the isotropic state. This is an indication that, as far as this class of thermosets is considered, the liquid crystalline order does not affect mechanism and equilibrium values of water uptake.     

环氧/碳纤维复合材料性能研究

本文主要分析了碳纤维帘子布复合材料的性能,并与T300单向碳纤维复合材料性能进行对比。结果表明,碳纤维帘子布复合材料性能低于T300单向碳纤维复合材料性能。     

Annealing of drawn monofilaments of liquid crystalline polymer vectra/vapor grown carbon fiber nanocomposites

The annealing treatment of drawn LCP nanocomposite monofilaments is discussed here. Upon annealing, an unstable structure is generated which presents a strong dependence on the annealing time sequence and temperature as observed by DSC. Moreover, it seems that increasing the draw down of the fiber restricts the formation of this new (unstable) structure. Based on dynamic mechanical analysis and on solid state ¹³C NMR, the nature of the newly-formed structure is elucidated, and based on sequence ordering within the non-oriented amorphous phase in the copolymer, a 10% change in the monomer proportion in the ordered phase is observed after annealing.     

Synthesis and curing of liquid crystalline epoxy resin based on asymmetric mesogen

A novel liquid crystalline epoxy resin (LCER) derived from asymmetric mesogen unit was synthesized. Its structure and liquid crystalline behavior were characterized by hydrogen nuclear magnetic resonance (H‐NMR), differential scanning calorimetry (DSC), polarized optical microscopy (POM). The results indicated that the LCER converted to a nematic phase at 85°C during heating and finally became isotropic at 145°C. The curing behavior and phase behavior of the LCER with 4,4′‐diaminodiphenyl methane and methyl hexahydrophthalic anhydride were also studied by DSC and POM, respectively. Their apparent activation energy (E_a) was evaluated according to the Ozawa's isoconversional method. The results suggested that autocatalytic reaction had occurred in these two systems. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Circulating flow reactor for recycling of carbon fiber from carbon fiber reinforced epoxy composite

For the purpose of development of a chemical recycling process for carbon fiber from carbon fiber reinforced epoxy composite, a new chemical recycling system using nitric acid aqueous solution has been proposed. The recycling system is composed of hexahedral circulating flow reactor made of quartz, Teflon supporter, acid resistance pump and auxiliaries. Epoxy matrix in the composite was effectively decomposed by nitric acid aqueous solution in the circulating flow reactor and carbon fiber could be recycled without any tangle or disturbance. Optimum conditions for the recycling process have been experimentally established. Tensile strength loss of recycled carbon fiber and composition of liquid phase decomposition products were analyzed.     

Dielectric relaxation and mechanical properties of liquid crystalline epoxy thermosets

This article describes the dielectric relaxation behavior and mechanical properties of novel liquid crystalline (LC) epoxy thermosets. Thermal simulated current experiments show that there is an additional relaxation caused by the local orientation of mesogens. From the mechanical test, it is found that the LC thermoset exhibits higher tensile strength and even little more deformation. It shows that the cured networks can be strengthened by LC domain orientation. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1568–1573, 2000     

Liquid crystalline epoxy resins in polymer dispersed liquid crystal composites

Liquid crystalline epoxy resins can be used as a matrix to encapsulate low molecular weight liquid crystalline droplets to produce PDLC devices. The presence of the low mass liquid crystal decreases the rate of reaction of the resin, that also hardens in a mesomorphic structure for high concentrations of liquid crystal. The optical characterization of the PDLC indicates an improvement of the angular transmission of visible light. ©1997 SCI