Liquid Crystalline Epoxy Monomer Research Articles

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.

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.

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.

Rational design and synthesis of a novel LCE material and its curing behaviours

ABSTRACT Two triaromatic liquid crystalline epoxy monomers (LCEMs) containing different lengths of spacer units terminated with epoxy groups were synthesised to investigate the effect of spacer unit on the properties of liquid crystalline epoxy resins (LCERs). Their chemical structures were characterised by FT-IR and 1HNMR, and the liquid crystalline (LC) behaviour was studied by using DSC and POM. After curing with three different aromatic amines, the LC properties of the cured products were investigated by using DSC, WAXD and POM. The effects of curing agents on the formation of LC networks were also studied. The monomer cured with the selected curing agent yielded a highly ordered mesomorphic region in the network. Flexible spacers are found to significantly decrease the melting point of epoxy monomer and to promote the curing reaction occurs at the LC state.

Curing Reaction and Dielectric Properties of Rigid and Elastic Liquid Crystal Epoxy Networks Modified with Nanofillers

The aim of the paper is to compare the progress of the curing reaction and the dielectric properties of liquid crystalline epoxy networks of different elasticity and to investigate how modification with nanofillers influences their properties. The study focuses on a liquid crystalline epoxy monomer with four aromatic rings in the mesogen, cross-linked with 4,4′-diaminodiphenylmethane (DDM) and pimelic acid (PA) to produce rigid and elastic polymer networks. The obtained results are compared to networks modified with nanofillers (diphenyl aluminum phosphate nanorods). The curing process is monitored in situ with broadband dielectric spectroscopy, which is a good tool to view the progress of the reaction. Two stages with different reaction dynamics are observed in the studied cases. Dielectric properties of the products cured with two chosen agents show significant differences in the obtained parameters (activation energy, glass transition, and fragility index) as well as in the dynamics of the α and β relaxations. Although the curing agent is the main factor determining the properties of the product, the nanofiller additive also modifies the values of specific parameters that characterize both the process of the reaction and the properties of the final composites. Particularly, adding the nanofiller raises the glass transition temperature in both the cases. The obtained results are in good agreement with the earlier calorimetric study.

Highly Oriented Liquid Crystalline Epoxy Film: Robust High Thermal-Conductive Ability.

The molecular orientation effect of a liquid crystalline (LC) epoxy resin (LCER) on thermal conductivity was investigated, with the thermal conductivity depending on the surface free energy of amorphous soda–lime–silica glass substrate surfaces modified using physical surface treatments. The LC epoxy monomer was revealed to form a smectic A (SmA) phase with homeotropic alignments on the surfaces of substrates that possess high surface free energies of 71.3 and 72.7 mN m–1, but forming a planar alignment on the surface of a substrate that possesses a relatively low surface free energy of 46.3 mN m–1. The optical microscopy observations and the X-ray analyses revealed that the LC epoxy monomer also induced a homeotropically aligned SmA structure due to cross-linking with a curing agent on the high-free-energy surface. The orientational order parameter of the resulting homeotropic SmA structure was calculated from the grazing incidence small-angle X-ray scattering patterns to be 0.73–0.75. The thermal conductivity of the cross-linked LCER forming a homeotropically aligned SmA structure was also estimated to be 2.0 and 5.8 W m–1 K–1 for the average and maximum in the direction of the Sm layer normal. The value of the thermal conductivity was remarkable among the thermosetting polymers and ceramic glass, and the LCER could be applied for high-thermal-conductive adhesives and packaging materials in electrical and electronic devices.

Highly thermal conductive benzoxazine‐epoxy interpenetrating polymer networks containing liquid crystalline structures

ABSTRACTA diamine‐based benzoxazine monomer (Bz) and a liquid crystalline epoxy monomer (LCE) are synthesized, respectively. Subsequently, a benzoxazine‐epoxy interpenetrating polymer network (PBEI) containing liquid crystalline structures is obtained by sequential curing of the LCE and the Bz in the presence of imidazole. The results show that the preferential curing of LCE plays a key role in the formation mechanism of liquid crystalline phase. Due to the introduction of liquid crystalline structures, the thermal conductivity of PBEI increases with increasing content of LCE. When the content of LCE is 80 wt %, the thermal conductivity reaches 0.32 W m−1 K−1. Additionally, the heat‐resistance of PBEI is superior to liquid crystalline epoxy resin. Among them, PBEI55 containing equal weight of Bz and LCE has better comprehensive performance. Its thermal conductivity, glass transition temperature, and the 5 % weight loss temperature are 0.28 W m−1 K−1, 160 °C, and 339 °C, respectively. By introducing boron nitride (BN) fillers into PBEI55, a composite of PBEI/BN with the highest thermal conductivity of 3.00 W m−1 K−1 is obtained. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 1813–1821

Liquid‐crystalline epoxy thermosets as matrices for ordered nanocomposites—a summary of experimental studies

This work focuses on obtaining ordered liquid crystalline epoxy networks and applying them as matrices for nanofillers. The article summarizes research carried out by us on the synthesis and curing of a series of composites based on liquid crystalline epoxy monomers. The authors experimented with several factors which influence the crosslinking reaction and determined curing conditions which are the most favorable for developing liquid crystalline‐like molecular order in the final products. New results of curing selected mixtures with the addition of anisotropic fillers are also presented. The crosslinking process was followed by differential scanning calorimetry (DSC) measurements and the morphology of liquid crystalline epoxy resin (LCER) composites was investigated by wide‐angle X‐ray scattering technique (WAXS‐2D). The experiments demonstrated that nanorods only have a small impact on the thermal properties of the composite (e.g., the change of Tg is just about 2°C) as well as structural ones (diffraction images of the plain matrix and the nanocomposite only differ in a single additional reflection near 0.73 nm−1 which appears in all the studied matrices containing nanorods). In one case monodomain ordering could be achieved in both the matrix alone and that with the nanofiller, when curing was conducted in the presence of a magnetic field. POLYM. COMPOS., 38:277–286, 2017. © 2015 Society of Plastics Engineers

A novel wholly aromatic liquid crystalline epoxy monomer based on 2,7-naphthalene mesogen: Synthesis and curing with aromatic diamine

A novel all aromatic 2,7-naphthalene-based liquid crystalline epoxy (LCE) was synthesized successfully in this study. The thermal transition of the synthesized LCE was characterized by wide angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), and polarized optical microscopy (POM). The characterization showed that the LCE had a low melting temperature of 65 °C and was in a nematic state in temperature range of 65–75 °C. The curing reaction between LCE and 4,4′-diaminodiphenylsulfone (DDS) was investigated, using 2-methylimidazole (2MI) as the reaction catalyst. Upon the addition of 1 phr of 2MI, the curing temperature was dramatically decreased from 227 to 128 °C, which is 99 °C lower than that of a reaction without the use of a catalyst system.

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.

The Curing Behavior and Properties of Diglycidyl Ether of 4,4′-Bis(4-hydroxybenzoyloxy)-3,3′,5,5′-tetramethylbiphenyl and its Composites with Multi-Wall Carbon Nanotubes

The curing system of a liquid crystalline epoxy monomer, diglycidyl ether of 4,4′-bis(4-hydroxybenzoyloxy)-3,3′,5,5′-tetramethylbiphenyl (DGE-BHBTMBP), and curing agent diaminodiphenylsulfon (DDS) was investigated by means of differential scanning calorimetry (DSC). Physical properties of the cured polymer and its composites with carbon nanotubes were evaluated with dynamic mechanical thermal analysis (DMA). Test of transmission electron microscopy (TEM) showed that oxidated multi-wall carbon nanotubes (OX-MWNT) performed a better dispersion in the cured polymer than untreated MWNT did. The storage modulus of the cured polymer containing OX-MWNT increased, while that containing untreated MWNT decreased. The results also showed that the Tg decreased when the MWNT was filled.

Dimeric liquid crystalline thermosets from azo-containing diglycidyl ether cured by anhydride

A series of dimeric liquid crystalline (LC) epoxy monomers containing azobenzene group with central spacers of different lengths were synthesized and structurally characterized by 1H NMR, 13C NMR, and elemental analysis. The mesogenic behavior of these monomers was measured by differential scanning calorimetry (DSC), polarized optical microscopy (POM), and X-ray diffraction (XRD). Like other dimeric LC epoxy monomers, the melting points, clear points, and mesophase of these compounds are influenced by the carbon numbers of central spacers. Anhydride was employed to cure the monomers and LCTs with nematic phase were obtained. Thermal properties, dynamic mechanical properties and UV–Vis spectra of the LCTs were studied. The decomposition of the LCTs occurred near 220 °C and followed three stages. The LCTs also had the natural absorbance bonds of azo compounds.

Dimeric Liquid Crystalline Epoxy Resins Containing Azo Group: Mesogenic Behavior and Curing with Anhydride

A series of dimeric liquid crystalline (LC) epoxy monomers containing azo groups with different length of central spacers were synthesized. The mesogenic behavior of these monomers was characterized by differential scanning calorimetry(DSC), polarized optical microscopy (POM) and wide-angle X-Ray diffraction(WAXS). Like other dimeric LC epoxy monomers, the melting points, clear points and mesophase of these compounds were influenced by the carbon numbers of the central spacers. Cure kinetics of these monomers with anhydride was studied by non-isothermal DSC at different heating rates. With the increasing of conversion, the values of activation energy show a tendency to decrease. The formation and development of LC phase during curing were also studied by POM. Finally LC thermosets with nematic phase were obtained.

Anisotropic Epoxy Networks

AbstractWe present the synthesis, characteristics and curing process of liquid crystalline epoxy monomers containing a central triaromatic mesogenic group. The epoxy monomers were cross‐linked with aromatic amines or dicarboxylic acids. In most cases, solid, thermally stable and anisotropic products were obtained. The curing reactions of the monomer were also carried out with selected fillers. The cross‐linking process of the composites was monitored by differential scanning calorimetry and dielectric relaxation spectroscopy to compare with analogous liquid crystalline systems not containing filler. Some properties of the cured resin and the composites are described and discussed.

Effects of the Heating Rate and the Amount of Organic Montmorillonite on the Thermal Properties of the Novel Liquid Crystalline Epoxy Nanocomposite

A novel nanocomposite was prepared by liquid-crystalline epoxy (LCE)/diaminodiphenylsulfone (DDS)/organic montmorillonite (MMT). A novel liquid-crystalline epoxy monomer, diglycidyl ether of 4,4′-bis(4-hydroxylbenzoyloxy)-3,3′,5,5′-tetramethylbiphenyl (DGE-BHBTMBP), was chosen as the matrix material. The curing processes and thermal properties of the novel LCETs obtained were investigated by nonisothermal DSC. The peak temperature increased with an increase in the heating rate for the different amount of organic MMT. The exothermal peaks of the samples containing organic MMT shift toward a lower temperature compared with the samples without organic MMT in the same curing condition. The glass transition temperature (T g ) of cured composites increases as the heat rates decrease.

Synthesis and characterization of a liquid crystalline epoxy containing azomethine mesogen for modification of epoxy resin

AbstractA novel liquid crystalline epoxy monomer, 1,1′‐bis [4‐(2,3‐epoxypropoxyphenyleneininomethyl)]‐2,2′‐dimethylbiphenylene (BMPE) was synthesized and characterized by infrared (IR) and Nuclear magnetic resonance (NMR) spectroscopy. The effect of BMPE content on mechanical and thermal properties of its blends with Diglycidyl Ether of Biphenol A (DGEBA) was investigated. BMPE presented a Schlieren texture in the range of 150 to 215°C as observed by differential scanning calorimeter (DSC) and polarizing optical microscope (POM). The improvement of mechanical properties of DGEBA modified with BMPE was achieved without sacrificing thermal resistance. Scanning electronic microscopy (SEM) graphs of fracture surfaces of the cured blends showed that microfiber‐like structure formed in the cured blends, which would be a result of self‐oriented alignment of azomethine mesogen component. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007

Liquid‐crystalline epoxy resins

AbstractA new liquid‐crystalline epoxy monomer (LCEM) containing central triaromatic mesogen and aliphatic spacers consisting of eight methylene units terminated with epoxy groups was synthesized and characterized by means of IR and NMR spectrometry, differential scanning calorimetry (DSC), polarized optical microscopy, and wide‐angle X‐ray scattering. The cure reaction of LCEM with primary aromatic diamines in a stoichiometric ratio and with a tertiary amine catalyst of polymerization and the properties of the cured products were investigated with a DSC technique. The effects of the curing conditions (polyaddition vs anionic polymerization) and the structures of the amines on the formation of liquid‐crystalline networks were studied. The monomer cured with selected amines yielded an anisotropic network. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 105: 224–228, 2007

Liquid Crystalline Epoxy Azomethine Thermoset

An investigation was carried out on the curing reaction of a liquid crystalline epoxy monomer, N, N'-bis[4-(2,3-epoxypropoxy)benzylydene]-1,4-phenylenediamine, with an aromatic diamine, sulfanilamide (SAA). The complexity of the curing involves the competition of the reactivities of a primary amine and a secondary amine and also a difference in reactivity of the amine and the amide functional groups from SAA. These phenomena have been studied by differential scanning calorimetry (DSC) under isothermal and non-isothermal conditions. The stability of the mesophase during cross-linking reaction was investigated by polarized optical microscopy (POM) and X-ray diffraction (WAXS).

Development of Liquid Crystalline Order During Cure of Mesogenic Epoxy Resins

AbstractIn the present paper we describe the curing process of a new liquid crystalline epoxy monomer with 4,4′‐diaminodiphenylenemethane (DDM). The characteristic of cured product is presented. Both the monomer and cured product have been characterized by IR and NMR spectroscopy, differential scanning calorimetry (DSC), polarized optical microscopy (POM), wide‐angle X‐ray scattering (WAXS) and dielectric relaxation spectroscopy (DRS).The curing process was analysed with the use of DSC and real‐time DRS.

Synteza i sieciowanie nowych cieklokrystalicznych monomerow epoksydowych

On the basis of literature data, the structures, types and properties of liquid crystalline polymers have been characterized. The subject of the own research was two-step synthesis of four new liquid crystalline epoxy monomers. They were obtained by esterification of four various diphenols, showing rigid structure, with 4-pentenoic acid and subsequent epoxidation of obtained unsaturated esters using m-chloroperbenzoic acid (Scheme A). The structures of the cured products obtained as well as the courses of the monomers' curing were investigated using DCS, WAXS and polarization microscopy methods. The results concerning the curing of p-phenylene bis[4-(4,5--epoxypentanoyloxy)benzoate] (MU2 according to Scheme A) with p-phenylene diamine (PDA), 4,4'-diaminodiphenylmethane (DDM) and 4,4'-oxyphenylenediamine (DDE) (Figs. 3-6 and 8) were described in details. It was found that as a result of the process of mesogenic diepoxy monomers' curing carried out at the temperature of mesophase existence, liquid crystalline order had been mostly maintained.