Liquid Crystalline Thermosets Research Articles

Synthesis and improved mechanical properties of twin‐mesogenic epoxy thermosets using siloxane spacers with different lengths

ABSTRACTA series of twin‐mesogenic epoxy resins with siloxane (di‐, tri‐, and tetra‐siloxane) chain spacers of different lengths were successfully synthesized. Results showed that the melting point of the twin‐mesogenic epoxy monomers decrease by more than 50 °C due to the change of siloxane spacer length. In particular, the tri‐ and tetra‐incorporated siloxane twin‐mesogenic epoxy monomers showed a liquid crystalline (LC) phase below 100 °C during the cooling process. Epoxy thermosets with isotropic and smectic (Sm) LC phases were then prepared using an aromatic amine and the dependence on curing temperature was investigated. Furthermore, the relationship between the ordered network chain structure and mechanical properties was investigated. Results were compared with isotropic and Sm LC phases. The tensile strain and fracture energy in the Sm type LC thermosets increased without a decrease in the tensile modulus, and this occurred in all systems irrespective of siloxane spacer length. The maximum value of the fracture energy (283 kJ m−2) reached by LC thermosets with increased spacer lengths was twice as large as that using the alkyl chain spacer (144 kJ m−2). © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47891.

Molecular-level dispersion of rigid-rod sulfonated aromatic polyamides in epoxy resin for extraordinary improvement in both strength and toughness

Molecular composites using rigid-rod macromolecules as the reinforcement have attracted tremendous research interest. However, dispersion of rigid-rod macromolecules at molecular level in the matrix remains a great challenge. Herein, we report new epoxy (EP)-based molecular composites with sulfonated aromatic polyamides, poly(p-sulfophenylene terephthalamide) (sPPTA) and poly(benzidine-2,2′-disulfonic acid terephthamide) (PBDT), as the reinforcement, which disperse uniformly in epoxy. Covalent bonds formed by the reaction between the sulfonic acid groups of the polyamides and epoxy prevent reaction-induced phase separation during high temperature curing. The novel molecular composites exhibit extraordinarily high strength and toughness, with maximum tensile strength of sPPTA/EP and PBDT/EP (100.2 ± 4.8 MPa and 121.9 ± 12.5 MPa, respectively) increased by ∼81% and ∼120%, respectively. To the best of our knowledge, the improvement of mechanical properties surpasses those of molecular composites, nanofiller reinforced thermosets and liquid crystalline thermosets ever reported.

Liquid crystalline thermosets based on anisotropic phases of cellulose nanocrystals

A new class of liquid crystalline thermosets (LCTs) was successfully produced containing lyotropic cellulose nanocrystals (CNCs) as the primary mesogenic component (up to 72 wt%) by the addition of non-mesogenic epoxy monomers. Cellulose-based LCTs were produced by totally aqueous processing methods and ultimately cured at elevated temperatures to produce ordered networks of ‘frozen’ liquid crystalline (LC) phases. Various degrees of birefringence were obtained via self-assembly of CNCs into oriented phases as observed by polarized optical microscopy and transmission electron microscopy. X-ray diffraction measurements highlighted the effects of texture of CNCs within LCT films compared to lyophilized CNCs. Cellulose-based LCT films uniquely exhibited thermo-mechanical properties of both traditional LCTs and LC elastomers, such as high elastic modulus (~1 GPa) under ambient conditions and low glass transition temperature (~−25 °C), respectively. The development of LCTs based on CNCs and aqueous processing methods provides a renewable pathway for designing high performance composites with ordered network structures and unique optical properties.

Moisture Permeation in Liquid Crystalline Epoxy Thermosets

AbstractA liquid crystalline thermoset (LCT), 4,4′‐diglycidyloxy‐α‐methylstilbene/sulfanilamide, is studied by dynamic mechanical and vapor sorption measurements. Sub‐Tg transitions are assigned to the specific segmental motions through molecular modeling and used to define the environmental temperatures for the study of water transport properties in LCT. The variation of moisture transport with amine‐epoxy ratio reveals that the crosslink density and local concentration of polar groups play the key role in determining both the diffusivity and solubility. The LCT exhibits much lower water diffusion than analogous isotropic networks while the solubility is essentially the same. Segmental flexibility and an ordered smectic layered structure are responsible for the lower permeability.

전방향족 폴리에스터 열경화성 액정의 합성과 특성

용융법을 이용해서 말단에 가교 반응이 가능한 메틸 말레이미드(methyl maleimide)기를 가진 전방향족 에스터 결합의 액정(liquid crystal, LC)을 합성하였고, 합성된 LC를 이용해서 적당한 열처리 과정을 통해 열경화성 액정(liquid crystalline thermoset, LCT) 필름을 제조하였다. 합성된 LC 및 LCT 필름은 FTIR(Fourier transform infrared) 분광기, WAXD(wide angle X-ray diffraction), DSC(differential scanning calorimetry), TGA(thermogravimetric analysis), TMA(thermomechanical analysis), 그리고 가열판이 장착된 편광 현미경으로 특성 분석을 하였다. 유리전이온도(<TEX>$T_g$</TEX>)와 열팽창 계수는 주사슬 구조의 메소겐에 의해 강한 영향을 받는 것으로 확인되었고, <TEX>$para$</TEX>-로 치환된 비페닐구조를 가진 LCT 필름이 가장 좋은 열적 성질을 보여 주었다. We prepared a series of aromatic liquid crystals (LCs) based on wholly aromatic ester units with the reactive end group methyl maleimide by means of melt condensation method, and the resulting LCs were thermally crosslinked to produce liquid crystalline thermoset (LCT) films. The synthesized LCs and LCTs were characterized with Fourier transform infrared (FTIR) spectroscopy, wide angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), thermomechanical analysis (TMA), and polarizing optical microscopy (POM) with a hot stage. The glass transition temperature (<TEX>$T_g$</TEX>) and coefficient of thermal expansion are strongly affected by the mesogen units in their main chain structures. The <TEX>$p$</TEX>-substituted biphenyl LC was found to have the highest thermal property value.

Probing electric field response of LC thermosets via time-resolved X-ray and dielectric spectroscopy

Curing of LCTs in an AC electric field was studied to understand the interplay between crosslink reaction and formation of a liquid crystalline (LC) phase and its effect on mechanical thermal properties on resulting thermosetting films. We show that dicyanate thermosetting monomers with ester functionality exhibit the ability to realign even after long curing times and allow poling of the LC director in resulting thermosetting films. To probe the details of the underlying process that leads to reorientation of the LC director model compounds were synthesized and their frequency and temperature dependent behavior under electric fields was studied. Curing reactions under electric fields show that the dual-frequency characteristics of the dicyanate mesogens behave very differently than epoxy thermosets. Size exclusion experiments reveal a prolonged gelation point in this system responsible for orientational switching even after long curing times. Finally, the ability to change orientation of the LC director allows tuning of modulus and thermal coefficient of expansion, making these thermoset films potential candidates for underfills, encapsulation materials and protective coatings for LC displays and active matrix substrates which require match in refractive index and mechanical/thermal properties.

High-performance composites based on all-aromatic liquid crystal thermosets

In this study, a new high-performance liquid crystal ester-based thermoset for composite applications was investigated. All-aromatic liquid crystalline thermosets (LCTs) are a promising class of polymers that offer a unique combination of properties such as solvent resistivity, high modulus, high strength, low coefficient of thermal expansion and high after cure glass-transition temperatures ( T g ⩾ 150 °C). Fully cured LCTs offer superior thermo-mechanical properties over high-performance thermoplastic polymers such as PPS, PEEK and PEI. For this study we used a 9000 g mol −1 ester-based LCT based on cheap and readily available monomers, i.e. 4-hydroxybenzoic acid (H), isophthalic acid (I) and hydroquinone (Q), abbreviated by us as HIQ-9. Composite panels prepared from T300 carbon fiber (5-harness satin weave) showed in-plane shear strength of 154 MPa and an in-plane shear modulus of 3.7 GPa. The tensile strength and modulus were measured to be 696 MPa and 57 GPa, respectively. A post-mortem inspection showed that the interfacial strength was excellent and no delamination was observed in the test specimen. Preliminary results show that LCT-based composites exhibit a better combination of (thermo) mechanical properties over PPS and PEI-based composites.

New liquid crystals and liquid crystalline thermosets based on wholly aromatic rigid-rod mesogens

A series of new liquid crystals (LCs) and liquid crystalline thermosets (LCTs) were prepared from wholly aromatic mesogens containing 2,6-naphthalene diol (2,6-ND), 2,7-naphthalene diol (2,7-ND), hydroquinone (HQ), and biphenol (BP). Cross-linked LCTs were obtained by crosslinking their citraconimide reactive end groups under a range of heat treatment conditions. The LCs and cross-linked LCTs were characterized by Fourier transform infrared (FTIR) spectroscopy, 1H nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), polarizing optical microscopy (POM) with a hot stage, and wide angle X-ray diffraction (WAXD). All LCs were thermotropic and formed a Schlieren nematic texture. The LCTs did not exhibit a liquid crystalline mesophase, which indicates that they are non-mesogenic phases.

Thermal properties and cure kinetics of a liquid crystalline epoxy resin with biphenyl-aromatic ester mesogen

A liquid crystalline epoxy resin diglycidyl ether of 3,3′,5,5′-tetramethylbiphenyl-4,4′-diyl bis(4-hydroxybenzoate) (DGE-TMBPBHB) was synthesized and its thermal properties were studied by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). Thermal history markedly affected liquid crystalline (LC) phase behaviors. The peak values of the isotropized temperature ( T i), crystallized temperature ( T c) and enthalpy change (Δ H i) caused by the transition from the LC phase to the isotropic state of DGE-TMBPBHB decreased slightly during repetitive heat–cool cycles. Two liquid crystalline thermosets were prepared based on the epoxy monomer with two different curing agents, 4,4′-diaminodiphenyl methane (DDM) and 4,4′-diaminodiphenyl sulfone (DDS), respectively. Cure kinetics of mixtures of DGE-TMBPBHB/DDM and DGE-TMBPBHB/DDS were investigated by DSC under nonisothermal processes.

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.

All-aromatic liquid crystalline thermosets as high temperatures adhesives

In this paper we will describe the synthesis and characterization of a novel series all-aromatic liquid crystal thermosets (LCTs) for high temperature (>100 °C) adhesive applications. Our ester-based oligomers, end-capped with reactive phenylethynyl functionalities, were synthesized using a standard one-pot melt condensation technique. The obtained reactive oligomers exhibit low melting temperatures and low melt viscosities, which resulted in improved flow and surface wetting. The adhesive properties of our LCTs based on 4-hydroxybenzoic acid, hydroquinone and isophthalic acid were investigated using standard (ASTM) lap-shear experiments conducted at room temperature (25 °C) and at elevated temperatures, i.e. 150, 200 and 250 °C. Our LCTs were used on Al2024, MS and Ti6Al4V substrates and we found lap-shear values of 11, 12 and 16 MPa for Al, MS and Ti, respectively. For the Al2024 substrate the lap-shear strength remained constant up to 150 °C and dropped to 4 MPa at 200 °C. A post-mortem inspection of the fracture surface, using high-resolution scanning electron microscopy (HRSEM), showed that there was no adhesive failure at the metal substrate at room temperature or at elevated temperatures. The lap-shear values of our polymers are significantly higher as reported for other commercial available LCPs such as Vectra™.

Synthesis and characterization of liquid crystals and their thermoset films

We prepared a series of aromatic liquid crystals (LCs) based on aromatic ester units with the reactive end groups methyl-maleimide and nadimide, and the resulting LCs were thermally cross-linked to produce liquid crystalline thermoset (LCT) films by means of solution-casting and heat treatment. The synthesized LCs and LCTs were characterized with Fourier transform infrared (FT-IR) spectroscopy, 1H nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC), thermomechanical analysis (TMA), and polarizing optical microscopy with a hot stage. We found that all these LCs form nematic phases. The coefficients of thermal expansion (CTEs) of the LCT films are strongly affected by the reactive end group and the mesogen units in their main-chain structures. The methyl-maleimide-terminated biphenyl LCT was found to have the lowest CTE.

1,3,4-Oxadiazole epoxy resin-based liquid crystalline thermosets and their cure kinetics

Liquid crystalline thermosets were synthesized based on a difunctional liquid–crystal (LC) epoxy resin monomer, namely 2,5-bis(4-glycidyloxyphenyl)-1,3,4-oxadiazole with various tetrafunctional crosslinkers such as diaminodiphenyl methane, diaminodiphenyl sulfone, and diaminodiphenyl ether. The epoxy monomer was characterized by infrared and nuclear magnetic resonance spectroscopy. Cure kinetics of a stoichiometric mixture of epoxy monomer and diaminodiphenyl methane was investigated by differential scanning calorimetry (DSC) for specimens cured under various cure conditions. The nematic LC texture for the cured specimen was identified by polarized microscopy and confirmed by X-ray diffractometry. Phase diagram of cure time versus transition temperature was constructed based on the DSC data for epoxy/DDM system. The diagram displayed the changes of melting transition, isotropic transition, and glass transition temperatures as curing proceeds.

DIGLYCIDYL BIPHENYLPHOSPHATE BASED FLAME RETARDANT LIQUID CRYSTALLINE THERMOSETS

Flame retardant liquid crystalline thermosets (FR‐LCTs) were synthesized from diglycidyl biphenylphosphate (DGBP), with various aromatic diamines namely, 2,5‐bis(p‐aminophenyl)‐1,3,4‐oxadiazole, 4,4′‐diaminodiphenyl sulphone, 4,4′‐ diaminodiphenyl methane, and biphenyldiamine. Cure kinetics and mesomorphic behavior of DGBP/diamine mixtures were performed on differential scanning calorimetry and hot stage optical microscope. Mechanical and cross‐linking properties of cured LCTs were characterized by dynamic mechanical analysis. The DGBP/diphenyldiamine combination exhibited high modulus and Tg values attributed to presence of more local order. The thermogravimetric analysis revealed that the LCTs were stable in the range 278–297°C with >43 Limiting Oxygen Index (LOI) values.

All‐aromatic liquid crystalline thermosets with high glass transition temperatures

AbstractWe have synthesized and characterized a new family of low melting all‐aromatic ester‐based liquid crystal oligomers end‐capped with reactive phenylethynyl end groups. In a consecutive, high‐temperature step, the reactive end groups were thermally activated and polymerization was initiated. This reactive oligomer approach allows us to synthesize liquid crystal thermosets with outstanding mechanical and thermal properties, which are superior to well‐known high‐performance polymers such as PPS and PEEK. We have modified an intractable LC formulation based on hydroquinone and terephthalic acid, with Mn = 1000, 5000, and 9000 g mol−1, and varied the backbone composition using isophthalic acid, resorcinol, 4‐hydroxy‐benzoic acid, 6‐hydroxy‐2‐naphthoic acid, and chlorohydroquinone. All fully cured polymers showed glass transition temperatures in the range of 164–275 °C, and high storage moduli at room temperature (∼ 5 GPa) and elevated temperature (∼ 2 GPa at 200 °C). All oligomers display nematic mesophases and in most cases, the nematic order is maintained after cure. Rheology experiments showed that the phenylethynyl end group undergoes predominantly chain extension below 340 °C and crosslinking above this temperature. Highly aligned fibers could be spun from the nematic melt, and we found that the order parameter 〈P2〉 was not affected by the chain extension and crosslink chemistry. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1368–1380, 2009

Diglycidylphenylphosphate based fire retardant liquid crystalline thermosets

Organophosphorus liquid crystalline thermosets (LCTs) were developed using diglycidylphenylphosphate (DGPP) and various aromatic diamines. DGPP was prepared by the esterification of phenylphosphorodichloridate with glycidol. 2,5-Bis( p-aminophenyl)-1,3,4-oxadiazole (BPOD) was synthesized and three different commercial diamines, 4,4′-diaminodiphenyl sulphone (DDS), 4,4′-diaminodiphenyl methane (DDM) and biphenyl diamine (BPD) were used as curing agents. The mesomorphic behaviour of DGPP/diamine mixtures and their curing kinetics were monitored by differential scanning calorimetry and hot stage optical polarized microscopy. The thermogravimetric analysis data showed that the LCTs are stable in the range 261–292 °C and afford 36–48% char yield. The limiting oxygen index values are between 35 and 47 proving fire retardance. The mechanical properties of the cured LCTs were characterized by dynamic mechanical analysis.

Synthesis and Properties of a Liquid Crystalline Thermoset Epoxy Resin Containing 1,3,4-Oxadiazole Groups

A new liquid crystalline thermoset (LCT) based on 1,3,4-oxadiazole was prepared from liquid crystalline epoxy resin containing oxadiazole and diaminodiphenyl methane. The LCT was characterized and compared with the conventional thermoset. The Time—Temperature Transformation diagram was constructed to determine an appropriate set of cure cycles. The goal of this paper was centered on the evaluation of the mechanical performances of this new class of thermoset. In the case of LC thermoset, a higher Tg and good fracture resistance were noticed than that of the conventional thermoset. The LC thermoset exhibited higher thermal stability when compared with the conventional non-LC thermosets. The results tempted to consider the LCT materials as potential candidates for advanced composites.

Structure‐Property Relationships in Liquid Crystalline Thermosets

Liquid crystalline thermosets are low molar mass liquid crystals to which crosslinking endgroups have been attached. Thus, these materials may combine the behavior of typical liquid crystalline polymers and thermosets, resulting in properties not attainable in other materials. Here we review work in our laboratory conducted over the last several years at investigating how the molecular structure and liquid crystalline phases impact the properties, with emphasis on those properties that are relevant to applications as structural materials. Topics discussed include the mutual influence of the cure reaction and the liquid crystalline phase, moisture diffusion, and fracture toughness. We also discuss areas of research that are still needed to demonstrate their use for practical application.

Transition diagrams for a liquid crystalline thermoset containing a rigid‐rod epoxy

AbstractThe liquid crystalline thermoset monomers 4,4′‐diglycidyloxy‐α‐methylstilbene (DOMS) and D2A1 that resulted from the reaction between DOMS and aniline were synthesized. DOMS was cured with sulfanilamide and D2A1 was cured with catalytic curing agent 1‐methyl imidazole. Transition diagrams have been constructed and the percent conversion data were determined from dynamic differential scanning calorimetry scan to examine the applicability of Flory's gelation theory. The results reveal that even though the amount of reaction that occurs in the liquid crystalline phase is different at different cure temperatures, the isoconversion theory of gelation fits quite well. POLYM. ENG. SCI. 46:623–629, 2006. © 2006 Society of Plastics Engineers