Siloxane Spacer Research Articles

Synthesis and mesomorphic properties of new organosiloxane chiral calamitic liquid crystals

A new chiral calamitic compound consisting of a phenyl core that is connected through ester linkers to a benzoate carrying a chiral (S)-2-methylbutoxy group at one side and a biphenyl carboxylate with an undecenyloxy chain at other side has been synthesized. Additionally, the siloxane substituted derivatives and a dimer with these two identical chiral calamitics linked by a disiloxane spacer have been obtained via hydrosilylation reaction to study the effect of combining the chiral moiety and siloxane segments within the same structure on liquid crystalline properties. The chiral calamitic compound and derivatives with a siloxane end-group or a siloxane spacer have been characterized using classical spectroscopic methods (1H NMR, 13C NMR, 29Si NMR and MS) and elemental analysis (EA). The liquid crystalline properties of all new compounds were investigated by differential scanning calorimetry, optical polarizing microscopy, X-ray scattering and electro-optic methods. With the introduction of a siloxane end-group to the vinyl-terminated calamitic compound exhibiting enantiotropic chiral nematic phase (N*) and chiral tilted smectic phase (SmC*), the occurrence of stable smectic phase was strongly promoted. In a similar way, the dimer with two identical calamitic mesogens connected by a disiloxane spacer exhibited a wider smectic mesophase interval in addition to the presence of N* mesophase. The new mesogens exhibit a ferroelectric switching with PS of around 200 nC cm−2 in their chiral tilted smectic (SmC*) mesophase range. The transition temperatures of organosiloxanes are considerably lower than that of olefinic precursor.

Mitigating the Shielding Effect of Ether Oxygen in Poly(ethylene glycol) on Boron Atoms in Boron‐Doped Poly(ethylene glycol) Hybrid Polymer Electrolyte by Introducing Siloxane Spacers

AbstractDoping boron (B) atoms into poly(ethylene glycol) (PEG)‐based solid polymer electrolytes (B‐PEG) is believed to be an attractive method to increase the lithium‐ion transference number ( ) because of the interaction between the electropositive B atoms and the anions of lithium salts. However, this effect can be largely impeded by the interaction between B and the electronegative ether oxygen atoms in the attached PEG molecular chains, which is referred to as a shielding effect. Enlarging the distance between the center B atoms and the electronegative ether oxygens is believed to be an effective way to reduce such shielding effect. In this work, different number of siloxane spacing groups (Six) are introduced in between the B atom and ether oxygen in the B‐PEG macromolecules (B‐Six‐PEGs x=0,1,3,5). As the siloxane oxygen has less electronegativity to ether oxygen, the shielding effects should be weakened and the ability of B atoms to trap the anions of lithium salt will increase. As the introduction of siloxane groups into the B‐Six‐PEGs polymer electrolytes will also reduce the neat contents of both B and PEG, which has the opposite effect on the whole ionic conductivities and values, both the ionic conductivities and values of B‐Six‐PEGs electrolytes first increase then decrease along with x increases from 0 to 5 when the [Li+]:[EO] mole ratio is fixed at 1 : 10. The peak value exists at x=1. Meanwhile, both the mobilities of Li+ ions (confirmed via 7Li nuclear magnetic resonance (NMR) results) and bis(trifluoromethane) sulfonimide (TFSI−) anions (confirmed from Raman spectra analysis) get their maximum values when x=1. However, if both the concentrations of TFSI− and B atoms in B‐Six‐PEGs electrolytes are taken into consideration, the normalized diffusion coefficient of TFSI− anions obtained by molecular dynamic (MD) simulation decrease monotonically when x increases from 0 to 5. As the diffusion coefficient of TFSI− anions is determined by the interaction probability and strength of TFSI− and B atoms; these results confirm that the interaction between the B atoms and TFSI− anions increase with x. Namely, the deshielding effects of siloxane spacing groups on B atoms increase along with the increase of x in the B‐Six‐PEGs polymer electrolytes, while it levels off when x exceeds 3.

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.

Hydrophobic, amorphous metal–organic network readily prepared by complexing the aluminum ion with a siloxane spaced dicarboxylic acid in aqueous medium

ABSTRACTA less common dicarboxylic acid with siloxane spacer, 1,3‐bis(carboxypropyl)tetramethyldisiloxane, whose structure is for the first time determined crystallographically, is used to coordinate aluminum through an environment‐friendly preparation process, resulting in a polymeric structure of amorphous metal–organic framework (aMOF) type. The high flexibility of the spacer induced by the siloxane bond gives the polymeric product a glass transition slightly below room temperature, while the long length of the former creates the premises for collapse of the network and consequently reduced porosity. At the same time, the high hydrophobicity of the tetramethyldisiloxane fragment and its low surface energy, which causes it to migrate to the air interface, gives the network low moisture sorption capacity guaranteeing the stability of its properties in a wet environment. The particulate complex, of the order of 1–200 nm, as generated by the synthesis, proves to be suitable as filler for silicone with outstanding reinforcement effect, without significantly affecting their transparency. The presence of the dimethylsiloxane units in the structure of both the matrix and the filler ensures good incorporation of the latter without the need for special compatibility treatments. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47144.

Three tetrasiloxane-tailed cationic gemini surfactants: The effect of different spacer rigidity on surface properties and aggregation behaviors

Gemini cationic surfactant with tetrasiloxane-tailed has potential to improve performance and biocompatibility in a range of applications. With different rigidity spacers, structure effects on their self-assembling behavior and interface properties are obvious. Hence, it should be possible to tune system surface properties and aggregation morphology in aqueous solution by changing rigidity of spacers. A series of novel cationic tetrasiloxane Gemini surfactants MR8M, MDM, MDOM were designed and synthesized by a simple process. Their surface property, hydrolysis resistance ability, wetting ability, aggregation behavior as well as antibacterial activity in aqueous solutions were studied. These three surfactants exhibited high hydrolysis resistance in both acidic and neutral aqueous solutions. On one hand, surface tension of MR8M (γCMC = 19 mN/m) and MDOM (γCMC = 23 mN/m), significantly lower than C8-PSi-C8 (γCMC = 37 mN/m) with siloxane spacer and pure hydrocarbon equivalents. On the other hand, MR8M can wet the highly hydrophobic PTEF plate completely at low concentration (0.4 mM). In addition, different aggregations, MDM with only vesicle, MDOM with spherical micelle and vesicle micelle and MR8M spherical micelle, wormlike micelle and vesicle were observed to assemble. Moreover, the antibacterial activity of these surfactants increased as the rigidity of spacer increased and MDM exhibited best antibacterial activity.

Structure, Dynamics, and Host–Guest Interactions in POSS Functionalized Cross-Linked Nanoporous Hybrid Organic–Inorganic Polymers

We report the synthesis and structural studies of new hybrid organic–inorganic porous materials with flexible siloxane spacers with different lengths bonded to polyhedral oligomeric silsesquioxane (POSS) cages. Positron annihilation lifetime spectroscopy (PALS) was used to measure the pore sizes of two polymers. For sample 1, which has a shorter linker, the free volume diameters were found to be 5.32 ± 0.06 A, 8.28 ± 0.05 A, and 10.42 ± 0.05 A at 25 °C. The first value represents the POSS cage, while the others describe the space between linkers. For polymer 2, the appropriate diameters are 4.96 ± 0.04 A and 7.96 ± 0.06 A for the POSS cage and linker spacing, respectively. This sample is extremely well organized and homogeneous. The evolution of the free volume sizes as a function of temperature in the range from −20 °C to +60 °C was investigated. Solid-state NMR spectroscopy (SS NMR) was employed to study the structure and molecular dynamics of the polymers. The signal assignment was performed using 1H M...

Dielectric Study of Liquid Crystals with Large Electroclinic Effect

Here we report results of the study of tilt angle and birefringence of some “high-electroclinic” materials having similar molecular structure with two and three siloxane spacers. The structure of these LC materials were studied by dielectric spectroscopy, birefringence and tilt angle measurements. Three of the four studied materials with a siloxane fragment are the deVries materials, but the siloxane-free sample is conventional SmA phase and is thus different. Possible models for the molecular structures of deVries SmA* phases are discussed.

Effect of siloxane spacer length on organosilicon bi-quaternary ammonium amphiphiles

A series of organosilicon Bola-form bi-quaternary ammonium amphiphiles, [OH5C3(C2H5)2N+-(CH2)3-Si(CH2)3-O-(Si(CH3)2O)n-Si(CH2)3⋯(CH2)3-N+(C2H5)2C3H5O]Cl2− (SinN2Cl2, n=0, 4, 6, 8), with the same headgroups and different length of hydrophobic linkage has been synthesized. The critical micelle concentration (CMC) of each amphiphiles was determined by equilibrium surface tension. With the increased length of hydrophobic siloxane spacer, the CMC values follow the order of Si8N2Cl2<Si6N2Cl2<Si4N2Cl2<Si0N2Cl2. At the same time, thermodynamic parameters (ΔGm0, ΔHm0, ΔSm0) of micellization were calculated by Gibbs equation under the mass action model. It indicates that the micellization of the amphiphiles in aqueous solution is entropy-driven. Especially, the surface area (Amin) of Si4N2Cl2 is the largest attributing to the interaction of free-rotated siloxane spacer and its internal oxygen atom, which make the molecular stretch free. The antimicrobial property of Si4N2Cl2 is more effective than others below the concentration of CMC, while the antimicrobial property of Si8N2Cl2 is more effective above the concentration of CMC, which indicated that both the adsorbability in formation of micellar and the hydrophobicity arising from the different length of siloxane spacer are important to inhibit microbe. Moreover, their wetting ability has been characterized by contact angles on various material surfaces. It shows that the higher weight of lipophilic siloxane spacer leads to lower contact angels.

Synthesis and characterization of novel fluorinated siloxane star-like copolymer with short perfluoroalkyl chain and used for modification the epoxy resin

Two series of fluorinated siloxane star-like copolymers, with different molecular weight of siloxane segments and large fluorinated segments at the tail, were synthesized from dicarboxyl terminated poly(2,2,3,4,4,4-hexafluorobutyl acrylate) (CTHFA) and dihydroxypropyl-terminated poly(dimethyl siloxane) (PDMS). The chemical structures of the star-like copolymer (CTHFA-PDMS) were measured by FT-IR, 1H NMR, 13C NMR and GPC. Long siloxane spacers were employed to increase the migration efficiency of the large fluorinated segments. The long siloxane spacers also played a part in toughening the epoxy resin. Then, the copolymers reacted with 4′4-diphenylmethane dissocyanate and were used as a surface modifier of cured commercial diglycidyl ether of bisphenol A (DGEBA) at different concentration (0.1–0.5wt% with respect to DGEBA). For both kinds of the CTHFA-PDMS modified DGEBA, high water contact angles (122° and 110°) were obtained at very low concentration (0.2wt%). The surface chemical composition was measured by X-ray photoelectron spectroscopy (XPS). It was found that the fluorine and silicon atoms selectively migrated toward the outermost surface of modified DGEBA resin. The surface enrichment factors of fluorine and silicon atoms (SmF and SmSi) of modified resin were up to 2.18×105 and 2.93×104 times than the theoretical value expected for the bulk formulations at very low concentration (0.2wt%). Moreover, the surface of modified epoxy resins showed good chemical stability by immersing in acidic and salt solutions. Scanning electron microscopy (SEM) images observed many ridges and rough crack structures of the fracture surface of modified DGEBA resin, indicating the CTHFA-PDMS toughened the DGEBA network.

The design and investigation of porphyrins with liquid crystal properties at room temperature

The design, synthesis and investigation of a series of porphyrins displaying smectic liquid crystal phase behaviour at room temperature are described. The molecules are constructed in a modular approach, consisting of porphyrin units linked via hydrocarbon and siloxane spacers to calamitic cyanobiphenyl units. The investigation of the phase behaviour, based on optical polarizing microscopy, differential scanning calorimetry and XRD studies, is discussed in detail.

Characteristic smectic structures of main-chain liquid-crystalline polyimides driven by a microphase separation between aromatic imide mesogen and a siloxane spacer

We investigated the phase transition behavior and liquid-crystalline structure for main-chain liquid-crystalline polyimides with aromatic imide cores and alkyl and dimethyl siloxane spacers, BPDA/3SiO4-3F. They formed crystal, smectic C (SC), smectic A (SA), and isotropic phases in order of increasing temperature. The characteristic structures of the SA and SC phases were extracted with wide-angle X-ray diffraction (WAXD) and Fourier transform infrared (FTIR) spectroscopy measurements. In the SC phase, the mesogenic groups were tilted by approximately 40° to the smectic layer and the unusual quasi two-dimensional structural order exists irrespective of the liquid-like packing of aromatic cores within a layer. On heating, the SC phase transformed into the SA phase with a nature of first-order transition. The resulting SA structure was also unusual. The orientational order parameter was 0.43, which was lower than 0.71 in the SC phase. This indicates that a “de Vries”-type of smectic A is formed in which the mesogenic groups are tilted away from the optic axis, but in azimuthally random directions. The average tilt angle was estimated to be 42°. These characteristics were produced because of the strong segregation effect of the dimethyl siloxane spacer groups with a relatively larger molecular area compared to alkyl chain and aromatic imide cores.

Phase Transition Behavior of Main Chain Nematic Liquid-Crystalline Polymers Based on 2-methyl-1,4-bis[4-(4-pentenyloxy)benzoyl]hydroquinone and 2-tert-butyl-1,4-bis[4-(4-pentenyloxy)benzoyl]hydroquinone

ABSTRACTThe control of phase transition behavior in liquid crystalline polymers could enable potential application in the field of actuators and sensors by enabling a higher actuator performance of liquid crystalline elastomers (LCE). In this context the phase transition behavior of siloxane based liquid crystalline copolymers synthesized from 1,1,3,3-tetramethyldisiloxane, 2-methyl-1,4-bis[4-(4-pentenyloxy)benzoyl]hydroquinone (M-MeHq), and 2-tert-butyl-1,4-bis[4-(4-pentenyloxy)benzoyl]hydroquinone (M-tBHq) was explored. The selected monomers provided different thermal stabilities of the nematic phase, while the non-flexible siloxane spacer suppressed a smectic phase. The mesogenic properties were studied by means of differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and wide angle X-ray scattering (WAXS). With increasing fraction of M-MeHq the nematic phase of the copolymer was stabilized and a tailoring of relatively low TNI was achieved.

Unsymmetric main-chain liquid crystal elastomers with tunable phase behaviour: elastic response

In this paper are reported the results of an investigation of the mechanical and thermoelastic properties of a new series of polysiloxane-based MC-LCEs containing different unsymmetric mesogenic units, siloxane spacers and cross-linkers. The structure of the networks has been varied systematically in order to study their properties. The mechanical properties of polydomain and monodomain elastomers were characterised by stress and strain measurements. The mechanical behaviour of the elastomer networks can be divided into two groups according to the phases they presented. Elastomers that contained flexible cross-linker, C1, and showed a SmC phase, demonstrated a non-linear elastic response when stress was applied in the axis of the domain director, in contrast to earlier experiments. In the axis normal to the director, at low strains, the response was linear and the Young's modulus was ten times lower compared to that in the axis of the director, demonstrating the marked anisotropic nature of these systems. The elastomers containing cross-linker C2 were nematic and showed lower Young's moduli compared to the smectic systems. The thermoelastic response of these systems showed spontaneous elongations up to 88% of strain when the samples were cooled from the isotropic to the liquid crystal phase, supporting stresses of 120 kPa. The properties are discussed in terms of the chemical structure of the networks in order to establish a correlation.

Study of the biaxiality in the nematic phase of liquid crystals in terms of orientational order parameters by infrared spectroscopy

The observation and quantification of the biaxiality in the nematic phase of thermotropic liquid crystals is one of the topical problems in the science of liquid crystals. The biaxiality in the refractive index and/or relative permittivity, both constituting tensors of the second rank, are expressed in terms of the four scalar order parameters S, D, P and C and the relevant molecular quantities. In this paper we review the theory of determining these order parameters using polarised infrared spectroscopy and present the method that we have developed in obtaining results for the order parameters of tetrapodes (with symmetrical and asymmetrical mesogens) and tripodes with symmetrical mesogens. In the tetrapodes, four mesogens are linked through the siloxane chains to the central Si atom. In the tripode, three mesogens are linked to a benzene ring via the oxygen atom through the siloxane spacers. In the first case a platelet-like structure is formed where the major director corresponds to the highest refractive index. A disc-like structure is formed for a tetrapode with symmetrical mesogens and for a tripode. The order parameters are calculated in the framework of the quasi-flat platelets. For the discotic-like structure, the major director corresponds to the lowest of the three refractive indices. For the three cases, the compounds are shown to exhibit a biaxial nematic phase. Biaxiality in the refractive indices is expressed in terms of the biaxial order parameters. A comparison of the results on the I–NU and NU–NB transitions for tetrapodes compare favourably with the predictions from the mean field models of de Gennes, Virga and his co-workers.

Thermotropic Liquid Crystalline Polyimides with Siloxane Linkages: Synthesis, Characterization, and Liquid Crystalline Behavior

Thermotropic liquid crystalline (LC) semialiphatic polyimides have been developed from diamines containing siloxane spacer units with pyromellitic dianhydride (PMDA) or 3,3′,4,4′-tetracarboxybiphenyl dianhydride (BPDA). Monomer diamines with siloxane units were prepared by three steps, which include Williamson’s synthesis, hydrosilylation, and reduction. The polymerization was conducted by the conventional method, solution polymerization, followed by thermal imidization. A highly viscose poly(amic acid) solution was obtained and its inherent viscosities were 0.65−1.73 dL/g. These novel polyimides exhibited high thermal stability and a lower crystal-LC transition temperature compared to reported polyimides containing alkylene or oxyethylene spacer units. X-ray studies were performed for investigating the oriented smectic mesophases of each polyimide. In the polyimides derived from PMDA, a smectic A (SmA) phase was formed on gradual cooling from the isotropic liquid phase. On the other hand, the polyimides ...

Synthesis of Thermotropic Liquid Crystalline Polyimides with Siloxane Linkages

Abstract Thermotropic liquid crystalline (LC) semi-aliphatic polyimides have been developed from 3,3′,4,4′-tetracarboxybiphenyl dianhydride (BPDA) and diamines containing siloxane spacer units. These polyimides exhibited high thermal stability and lower transition temperature compared to the corresponding polyimides containing alkyl or oxyethylene spacer units.

Columnar phase structures of an organic-inorganic hybrid functionalized with eight calamitic mesogens.

A liquid-crystalline octapode, formed by laterally connecting calamitic mesogens to an inorganic silsesquioxane cube through flexible siloxane spacers, is studied using polarized light microscopy, differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The studies are extended to mixtures of the octapode with the respective monomer mesogens. The monomer and the octapode show a nematic phase. At lower temperatures, the octapode exhibits additionally a columnar hexagonal phase (6 lattice), which, on further cooling, undergoes a transition to a columnar rectangular phase (2 lattice). A similar phase-transition sequence is observed for mixtures of the octapode with moderate concentrations of the monomer. The columnar-columnar transition is discussed combining XRD and DSC results, and a possible model of the molecular self-organization is presented.

High performance holographic polymer dispersed liquid crystal systems using multi-functional acrylates and siloxane-containing epoxides as matrix components

High performance transmission holographic polymer dispersed liquid crystals were fabricated using a mixture of radically photopolymerizable multifunctional acrylates and ring-opening photopolymerizable epoxides as monomers for matrix components with Nd:YAG laser (λ=532 nm). Functionality of multifunctional acrylates and concentration of LC were varied to optimize the performance of holographic gratings. Gratings with much higher diffraction efficiency (83% or 67%, respectively) were obtained from recording solution composed of dipentaerythritol penta/hexaacrylate,1vinyl2pyrrolidone, commercial liquid crystal E7, and 1,3-bis(3-glycidoxypropyl)-1,1,3,3-tetramethyldisiloxane with glycidyl function linked with disiloxane, or 1,3-bis[2-(1,2-epoxycyclohex-4-yl)ethyl]-1,1,3,3-tetramethyldisiloxane with cyclohexene oxide function and the same siloxane spacer (45:9:10:36), compared with the ordinary recording solution composed of dipentaerythritol penta/hexaacrylate, 1vinyl2pyrrolidone and E7 (<2%). The recording solution composed of 1,5-bis(3-glycidoxypropyl)-1,1,3,3,5,5-hexamethyltrisiloxane and 1,5-bis[2-(1,2-epoxycyclohex-4-yl)ethyl]-1,1,3,3,5,5-hexamethyltrisiloxane with trisiloxane spacer gave further improved diffraction efficiency (97% and 75%, respectively). Recording solutions contain trisiloxane derivatives gave gratings with considerably or moderately reduced angular deviation (0.83, 0.66 degree for trisiloxane derivatives from 1.2, 0.7 degree for disiloxane derivatives, respectively, for signal beam, and 0.76, 0.70 degree from 1.1, 1.0 degree, respectively, for the reference beam at 32 degree of external incident angle) from Bragg profile, namely 5.2 and 4.5% volume shrinkage for trisiloxane derivatives, and 7.5, 5.6% volume shrinkage for disiloxane derivatives, respectively. High diffraction efficiency over 95% with angular selectivity of 4.0 degree was obtained for the gratings formed from 1,5-bis[2-(1,2-epoxycyclohex-4-yl)ethyl]-1,1,3,3,5,5-hexamethyltrisiloxane when the concentration of E7 was reduced to 5%. Clearly phase-separated liquid crystal domains were observed by scanning electron microscopy. Grating spacing (about 900 nm) was close to the calculated value (965 nm) by Bragg equation.

Dipole relaxation, intramolecular mobility, and molecular self-organization in solutions of alternating polymers with phenylazomethine and siloxane fragments

Dielectric relaxation in solutions of polyesters containing flexible siloxane and rigid phenylazomethine fragments of various structures is studied. For these polymers in solution, five relaxation processes that reflect the mobility of certain portions of the macrochain are observed. The kinetic rigidity of the macrochain increases with increasing length of the siloxane spacer. This trend is associated with rearrangement of the electronic structure of silicon atoms and phenyl rings and with ordering of rigid phenylazomethine fragments.

Cholesteric Silatranes

ABSTRACT Most liquid crystals with a C3 symmetry have so far been discotic systems, e.g., triphenylenes. In the context of calamitic systems research has concentrated on triptycenes and to a lesser extend on silatranes. In this contribution we report our investigations of cholesteric silatrane systems, which have been obtained by connecting a tribenzylsilatrane core to cholesteric groups via alkyl and siloxane spacer groups.