Liquid Crystalline Polyurethanes Research Articles

Shape-Programmable Liquid-Crystalline Polyurethane-Based Multimode Actuators Triggered by Light-Driven Molecular Motors.

In recent years, light-driven soft actuators have been rapidly developed as enablers in the fabrication of artificial robots and biomimetic devices. However, it remains challenging to amplify molecular isomerization to multiple modes of macroscopic actuation with large amplitude and complex motions. Here, a strategy is reported to build a light-responsive liquid-crystalline polyurethane elastomer by phototriggered overcrowded alkene-based molecular motors. A trifunctional molecular motor modified with an ethylene glycol spacer on the rotor and stator functions as a crosslinker and unidirectional stirrer that amplifies molecular motion into macroscopic movement. The shape-programmable polymeric film presents superior mechanical properties and characteristic shape-memory effect. Furthermore, diverse modes of motions including bending, unwinding, and contracting with tunable actuation speed over a wide range are achieved. Such research is hoped to pave a new way for the design of advanced light-responsive soft actuators and robots.

Molecular mobility of liquid crystalline polyurethanes modified by polyhedral oligomeric silsesquioxanes

Molecular mobility and liquid crystalline order formation of liquid crystalline polyurethanes (LCPU) modified by trisilanolphenyl-POSS was investigated by using dielectric spectroscopy (DRS), differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). Hybrid materials with 2, 4 and 6 wt% of POSS load were studied. Small-angle X-ray scattering (SAXS) results suggested that the nanoscale morphology was not significantly altered by the introduction of POSS into the LCPU matrix. The presence of liquid crystal phase transitions was confirmed by both DRS and DSC measurements. Dielectric study showed changes in relaxation steps and conductivity in the LC transitions region due to a shift in phase order. Using partially cross-linked POSS particles, an enhancement of the material's rubber modulus was observed in the DMA measurement. Deterioration in charge transfer was observed for the composites, which was attributed to the crosslinking nature of the silsesquioxane used. Further, dynamic calorimetric studies in different cooling rates were performed for detailed study of liquid crystalline transitions. In summary, it was found that the addition of POSS causes a nucleation effect and does not impair LC phase formation.

Transient Current Investigations in Ion Irradiated Liquid Crystalline Polyurethane

Transient Current Investigations in Ion Irradiated Liquid Crystalline Polyurethane

Phase transitions and orientational behavior of liquid crystalline polyurethane mixtures with polar compounds

Phase transitions and orientational behavior of liquid crystalline polyurethane mixtures with polar compounds

Liquid crystalline polyurethanes modified by Trisilanolisobutyl-POSS

A series of liquid crystalline polyurethanes based on 4,4′-bis(hydroxyhexoxy)biphenyl moieties and modified by Trisilanolisobutyl-POSS (TRIS-POSS) was synthesized via a polyaddition reaction. ATR-FTIR confirmed introduction of POSS moieties into the macrochain. Formation of liquid crystalline phases along with higher melting and crystallization temperatures due to the addition of the POSS moieties has been observed by Differential Scanning Calorimetry (DSC) and Polarized Optic Microscopy (POM). The influence of the POSS on the improvement of thermal stability and char residue after degradation is also discussed. Influence of the POSS particles on glass transition was found to be relevant for high mesogenic unit load. Morphological investigations by Scanning Electron Microscopy showed different distribution of the POSS nanoparticles for each examined matrix and absence of POSS originated agglomerates which was also confirmed by X-ray diffraction technique. Reinforcement effect of the POSS on mechanical properties was also investigated. Mechanically induced anisotropy was hypothesized to be one of the factor responsible for improved tensile strength at high strain.

A facile approach for the preparation of liquid crystalline polyurethane for light-responsive actuator films with self-healing performance

A facile approach was developed to prepare liquid crystalline polyurethane by quaternization between ordinary polyurethane and liquid crystalline monomers to yield a fast light-responsive actuator film with self-healing performance.

Thermal- and light-responsive programmable shape-memory behavior of liquid crystalline polyurethanes with pendant photosensitive groups

A liquid crystalline polyurethane bearing side-chain photosensitive moieties prepared via the quaternization reaction exhibits thermal- and light-responsive programmable shape-memory behavior.

The Synthesis and Properties of Liquid Crystalline Polyurethanes, Chemically Modified by Polyhedral Oligomericsilsesquioxanes.

In this work, we report for the first time on the influence of polyhedral oligomericsilsesquioxanes (POSS) on the structure and properties of liquid crystalline polyurethane (LCPU). LCPU/POSS hybrids were synthesized via a two-step method. In the first step, 4,4′-methylenephenyl diisocyanate (MDI) and polytetramethylene ether glycol (PTMG) reacted with functionalized trisilanolphenyl POSS (TSP-POSS) bearing three hydroxyl groups. In the second step, the growing chain was extended with 4,4′-bis(hydroxyhexoxy)biphenyl (BHHBP). FTIR measurements confirmed the chemical bonding between the POSS and LCPU matrix and showed the influence of the silsesquioxane modification on the intensity of hydrogen bonds. The DSC and POM techniques confirmed the formation of liquid crystalline phases. The incorporation of silsesquixanes into the LC matrix leads to higher melting and isotropization temperatures along with the broadening phase transition effect. Scanning electron microscopy showed a good distribution of POSS moieties, both in the bulk and on the surface of the liquid crystalline PU matrix, whereby wide-angle X-ray diffraction (WAXD) patterns revealed halos from both the liquid crystalline and unmodified polyurethane matrix. The stress at the breaking points for LCPU/POSS hybrids containing 50% and 60% of elastic segments is greater than the stress at the breaking point of the reference material (LCPU), what is due to good dispersion of POSS in less elastic matrix. Thermal properties of the LCPU/POSS materials obtained, determined by TGA, revealed that the char residue increased with the amount of POSS for 40% of elastic segments materials.

Liquid crystalline polyurethane composites based on supramolecular structure with reversible bidirectional shape memory and multi-shape memory effects

In this study, a novel series of supramolecular liquid crystalline (LC) polyurethane composites, named SMPU–#HOBA (# represents the molar ratio of HOBA/BINA), were successfully prepared by incorporating hexadecyloxybenzoic acid (HOBA) into pyridine-containing polyurethane (PU).

High Mechanical and Thermal Properties of Epoxy Composites with Liquid Crystalline Polyurethane Modified Graphene.

Graphene nanosheets (GNs) often result in incompatibility with the hydrophobic polymer matrix, and the tendency to form aggregates during processing. Herein, liquid crystalline polyurethane modified GNs (GPLP) were obtained by π–π stacking interactions between GNs and perylene bisimide derivatives, and then in-situ polymerization of liquid-crystalline polyurethane. Spectroscopic studies, elemental analysis, and thermal properties confirmed the successful π–π stacking and the integrated structure of GPLP. The good dispersion state of GPLP in the epoxy matrix (EP), and the strong interactions between GPLP and EP, lead to the significant improvement of the thermal and mechanical performance of the GPLP/EP composites. The impact strength, Young’s modulus, tensile strength, and toughness of the GPLP/EP composites with 1.47 wt % GNs reached the highest values of 54.31 kJ/m2, 530.8 MPa, 112.33 MPa and 863 J/m3, which significantly increased by 210%, 57%, 143%, and 122% compared to that of neat epoxy, respectively. As well, the glass transition temperature increased by a notable 33 °C. It is hoped that this work can be used to exploit more efficient methods to overcome the poor adhesion between GNs and polymers.

Development of liquid crystalline polyurethane composites with stage-responsive shape memory effects

Novel liquid crystalline shape memory polymer composites with interesting staging-responsive shape memory properties were successfully prepared. The liquid crystalline shape memory polymer composites maintain the intrinsic reversible liquid crystalline properties of Azo11.

Fabrication of Liquid Crystalline Polyurethane Networks with a Pendant Azobenzene Group to Access Thermal/Photoresponsive Shape-Memory Effects.

Herein, we report a novel thermal/photoresponsive shape-memory polyurethane network with a pendant azobenzene group by utilizing its anisotropic-isotropic phase transitions and photoresponsive feature concurrently. To achieve this goal, the side-chain liquid crystalline polyurethane networks based on the pendant azobenzene group [SCLCPU(AZO)-Ns] were developed in a well-defined architecture. The smectic C nature of an LC phase in the polyurethane networks was confirmed by differential scanning calorimetry, polarized optical microscopy, and one-dimensional and two-dimensional wide-angle X-ray diffraction. The well-defined architecture-made SCLCPU(AZO)-N displays two distinct transition temperatures (Ttrans) (Tg and Tcl), with a difference of about 40 °C. Consequently, the excellent triple-shape-memory effect in this network was demonstrated by cyclic thermomechanical analysis. By making full use of the trans-cis photoisomerization of azobenzene, the reversible bending and unbending behaviors were realized under the light irradiation with wavelengths of 450 and 550 nm, respectively.

The Effect of 4-Octyldecyloxybenzoic Acid on Liquid-Crystalline Polyurethane Composites with Triple-Shape Memory and Self-Healing Properties.

To better understand shape memory materials and self-healing materials, a new series of liquid-crystalline shape memory polyurethane (LC-SMPU) composites, named SMPU-OOBAm, were successfully prepared by incorporating 4-octyldecyloxybenzoic acid (OOBA) into the PEG-based SMPU. The effect of OOBA on the structure, morphology, and properties of the material has been carefully investigated. The results demonstrate that SMPU-OOBAm has liquid crystalline properties, triple-shape memory properties, and self-healing properties. The incorporated OOBA promotes the crystallizability of both soft and hard segments of SMPU, and the crystallization rate of the hard segment of SMPU decreases when the OOBA-content increases. Additionally, the SMPU-OOBAm forms a two-phase separated structure (SMPU phase and OOBA phase), and it shows two-step modulus changes upon heating. Therefore, the SMPU-OOBAm exhibits triple-shape memory behavior, and the shape recovery ratio decreases with an increase in the OOBA content. Finally, SMPU-OOBAm exhibits self-healing properties. The new mechanism can be ascribed to the heating-induced “bleeding” of OOBA in the liquid crystalline state and the subsequent re-crystallization upon cooling. This successful combination of liquid crystalline properties, triple-shape memory properties, and self-healing properties make the SMPU-OOBAm composites ideal for many promising applications in smart optical devices, smart electronic devices, and smart sensors.

Enhancement in Mechanical and Shape Memory Properties for Liquid Crystalline Polyurethane Strengthened by Graphene Oxide.

Conventional shape memory polymers suffer the drawbacks of low thermal stability, low strength, and low shape recovery speed. In this study, main-chain liquid crystalline polyurethane (LCPU) that contains polar groups was synthesized. Graphene oxide (GO)/LCPU composite was fabricated using the solution casting method. The tensile strength of GO/LCPU was 1.78 times that of neat LCPU. In addition, shape recovery speed was extensively improved. The average recovery rate of sample with 20 wt % GO loading was 9.2°/s, much faster than that of LCPU of 2.6°/s. The enhancement in mechanical property and shape memory behavior could be attributed to the structure of LCPU and GO, which enhanced the interfacial interactions between GO and LCPU.

Synthesis and Shape Memory Property of a MDI Based Liquid Crystalline Polyurethane

Liquid crystalline polyurethanes (LCPU) were prepared from 4,4’-methylenediphenyl diisocyanate (MDI), 1,6-hexanediol (HDO), 2,2-dimethylol propionic acid (DMPA) and polytetramethylene ether glycol (PTMG). The experiments synthesized three liquid crystalline polyurethane films with different soft/hard segment ratio. Chemical and structural characterization of the polyurethanes were investigated by Fourier transform infrared, X-ray diffraction, thermogravimetric analysis, differential scanning calorimeter and polarized microscopy respectively. Swelling rate and shape memory property were tested. The results indicated that the polyurethane with 62% soft segment and large group of carboxyl displayed excellent swelling and shape memory properties, and the shape recovery rate reached 100%. It was found that the crystallinity, thermal stability decreased and the temperature flexibility, water absorption and shape recovery rate increased with the increase of polytetramethylene ether glycol.

Elaborate fabrication of well-defined side-chain liquid crystalline polyurethane networks with triple-shape memory capacity

A series of side-chain liquid crystalline polyurethane networks (SCLCPU-Ns) with well-defined architecture are prepared via an elaborate strategy. The SCLCPU-Ns display dual thermal transition temperatures (Tg and Tcl), which can be utilized as Ttrans to trigger the triple shape memory effect.

Low-Temperature TSDC Studies of Molecular Dynamics in Pristine and O7+ Ion–Modified Liquid Crystalline Polyurethane

The molecular dynamics of pristine and 100 MeV O7+ ion–modified liquid crystalline polyurethane (LCPU) was carried out using the thermally stimulated depolarization currents (TSDC) method. The TSDC spectra of pristine LCPU samples consist of current peaks appearing around 110,160, 210, and 260 K and are called the γ, β, α, and δ peaks respectively. The γ-peak is attributed to the crankshaft motion of the spacer group (but-2-ene group). The β-peak is due to the localized rotational fluctuations of C˭O groups. The α-peak is associated with the dipolar orientation of imines group present in the urethane linkage, and the δ-peak has its origin in the space charge trapping mechanism. In the TSDC spectra of O7+ ion–irradiated LCPU samples the γ-peak almost disappears, which shows that the ion induced modification in the methylene linkage. A relaxation process owing to new sub-polar groups formed due to dehydrogenation of CH2 groups and occurring in the vicinity of β-relaxation is discussed. The radiation-induced chain scissioning at NH groups in LCPU is also confirmed from the fluence effect on the α-peak. The irradiation results in the production of a large number of defect sites and an increase in the same results in the enhancement of δ-relaxation.

Dielectric Spectroscopy of Pristine and O7+ Ion Irradiated NCO‐Terminated Polybutadiene‐Based Liquid Crystalline Polyurethane

AbstractThe dielectric constant (ε′) and loss factor (ε′′) have been measured in NCO‐terminated polybutadiene‐based pristine and 100 MeV O7+ ion irradiated liquid crystalline polyurethane in the temperature region of 80–300 K at frequencies ranging from 1 KHz to 1 MHz. The variation in ε′with temperature in different temperature regions has been analyzed in relation to the dipole structure of the molecules. The ε′′–T plots of pristine liquid crystalline polyurethane (LCPU) reveal the presence of multiple relaxation processes associated with the crankshaft motion of but‐2‐ene groups, dipolar nature of carbonyl groups, and glass transition temperature, respectively. An additional dielectric loss peak is observed in irradiated LCPU samples associated with the Maxwell–Wagner–Sillars (MWS) polarization. There is an overall increase in ε′ in irradiated samples due to the dominance of MWS polarization arising from the large number of radiation‐induced defect sites and free radicals. The loss in γ relaxation due to chain scissions of but‐2‐ene groups and the formation of new subpolar groups providing a new dipolar relaxation process is observed in irradiated LCPU samples. The variation in ε′ with fluence shows the occurrence of a secondary radiation induced crystallization in irradiated LCPU samples. The X‐ray diffraction analysis also shows the effect of fluence on the crystalline nature of LCPU samples.

Effects of 100 MeV O7+ ion beam irradiation on the optical, chemical and structural properties of NCO-terminated polybutadiene based liquid crystalline polyurethane

NCO-terminated polybutadiene based liquid crystalline polyurethane films were irradiated with O7+ ion beam at various fluences ranging 3 × 1010 to 1 × 1012 ions/cm2. Significant modifications in the structural, optical and chemical properties have been observed after irradiation. UV-visible spectra show a shift toward the higher wavelength regime after irradiation owing to the decrease in the band gap. FTIR spectra show the formation of NH2 group and CC bonds. The XRD spectra show the semi-crystalline nature of LCPU and with increasing fluence of irradiation a decrease in crystallinity is observed. However an increase in crystallinity at certain fluence followed by a decrease at the higher fluence of irradiation reveals the presence of secondary radiation induced crystallinity in LCPU.

Development of supramolecular liquid-crystalline polyurethane complexes exhibiting triple-shape functionality using a one-step programming process

Supramolecular shape memory liquid crystalline polyurethanes showing smectic C properties and exhibiting triple-shape functionality using a one-step programming process are prepared.