X‐Ray Diffraction Studies on the Phase‐Transformational Behavior of a Smectic Liquid‐Crystalline Elastomer Composed of Chiral Mesogens

Structural changes through successive phase transformations of a chiral smectic liquid‐crystalline elastomer are investigated by X‐ray scattering technique. In uniaxially deformed elastomers, the smectic layer seemingly tilts even in the SmA phase, in which an in‐plane chevron structure formed in the tilted smectic phase. On the basis of an analysis of the layer reflection peaks, the layer correlation length in the tilted smectic phases is shorter than that in the non‐tilted SmA phase, though smectic layers in the tilted smectic phases are better ordered than those in SmA.

Experimental arrangement for X‐ray measurements of the uniaxially deformed elastomer in the tilted smectic phase at room temperature.     

Side‐chain Cholesteric Liquid Crystalline Elastomers Derived from Nematic Bis‐olefinic Crosslinking Units

The synthesis of the nematic crosslinking agent (4′‐allyloxy‐benzoyl)‐(4″‐undec‐10‐en‐1‐oyl‐benzoyl)‐p‐benzenediolate ( M ‐ 1 ), the cholesteric monomer cholesteryl undec‐10‐en‐1‐oate ( M ‐ 2 ) and side‐chain cholesteric liquid crystalline elastomers is described. The chemical structures of the obtained monomers and elastomers were confirmed by FTIR and ¹H NMR spectroscopy. Their mesomorphic properties and phase behavior were investigated by differential scanning calorimetry, polarizing optical microscopy, and X‐ray diffraction measurements. The influence of the crosslinking M ‐ 1 units on phase behavior of different elastomers prepared by a one‐step hydrosilation reaction was discussed. The elastomers containing less than 20 mol‐% of the crosslinking M ‐ 1 units showed elasticity, reversible phase transition and cholesteric texture. The experimental results demonstrated that the glass transition temperatures and isotropization temperatures of P ‐ 2 ～ P ‐ 6 increased with increasing the concentration of crosslinking M ‐ 1 units.

Synthesis and schematic representation of elastomers.     

Liquid‐Crystalline Colloidal Particles

Summary: Here we report about the synthesis of colloidal particles of nematic and smectic liquid‐crystalline polymers. For this purpose mesogen‐containing acrylate monomers were synthesized and polymerized in a special modification of a precipitation polymerization called dispersion polymerization. By variation of the polymerization conditions colloidal particles of different size and polydispersity could be obtained including very narrowly distributed samples in optimized batches. On azobenzene‐containing colloidal particles switching experiments with polarized light were performed. It could be observed that the nematic director of the mesogens within the colloidal particles can be rotated due to the photochemical trans‐cis‐isomerization of the azobenzene chromophores.

Microscope images of a monolayer of P3‐9 .     

Structure and Phase Transitions of Poly(heptamethylene p,p′‐bibenzoate): Time‐Resolved Synchrotron WAXS and DSC Studies

Time‐resolved wide‐angle X‐ray scattering (WAXS), as well as differential scanning calorimetry (DSC) and polarisation microscopy studies, were applied to investigate the structure and phase transitions of poly(heptamethylene p,p′‐bibenzoate). Temperature dependencies of several structural parameters were determined. Complete transformation from an isotropic melt to a smectic phase was suggested whereas the transition from a smectic to crystalline phase is only partial (around 30%), although it takes place from the ordered SCA phase. Crystals are formed within the SCA domains with nearly the same coherent length. On the basis of the analysis of the position and the profile of the diffuse wide‐angle X‐ray scattering and mesogenic layer spacing, it was assumed that either crystallisation modifies the smectic structure, or mesophase losses its positional order because of the lack of mobility of the spacers at low temperatures.

WAXS scattering profiles corresponding to P7MB: a) cooling from the isotropic melt at 2 °C · min^?1; b) subsequent melting at 12 °C · min^?1.     

Synthesis,Phase Behaviour and Mechanical Properties of Poly(2‐methyl‐1,3‐propanediol‐4,4′‐bibenzoate)

The synthesis and phase behaviour of PB32 is reported. The results indicate that PB32 develops a low‐ordered smectic mesophase of the type SmC_alt. The slow formation of such a mesophase allows the quenching of the isotropic melt into an amorphous state so that both amorphous glass and smectic glass can be obtained, which exhibit different glass‐transition temperatures. Mechanical tests indicate that the macromolecular chains in the mesophase of PB32 can be oriented either parallel or perpendicularly in relation to the streching direction, depending on the deformation conditions. Solid‐state ¹³C NMR spectroscopy results show that the mobility of the chains is rather similar in both phases.

     

FTIR‐Spectroscopy on Segmental Reorientation of a Nematic Elastomer under External Mechanical Fields

Summary: Fourier transform infrared (FTIR) spectroscopy with polarized light is employed to study the segmental orientation and the order of a nematic liquid crystalline elastomers (NLCEs) with a monodomain structure in response to an external mechanical field. The reorientation and the order parameter of the different molecular moieties are analyzed in detail, revealing information about angular excursion in the rearrangement of the mesogens, the spacer molecules, and the main‐chain. In case of an elongation of the NLCE films perpendicular to the initial mesogen orientation, no reorientation or change of order is observed for an elongation ratio less than 1.3. At higher strain, a molecular reorientation process is induced on all molecular segments and the order of alignment is decreased. When the NLCE‐films are stretched parallel to the mesogens, no molecular reorientation takes place and the order parameters show no significant change.

Angular reorientation of different molecular moieties at mechanical strain perpendicular to the initial mesogen orientation.     

Phase Behaviour of New Side‐Chain Liquid Crystalline Polyoxetanes

Summary: The synthesis and thermal behaviour of two new polyoxetanes with side chains having a dimer structure mesogen‐spacer‐mesogen and a long terminal alkyl group are reported. The two polymers differ in the chemical structure of the central spacer. The polyoxetanes have been obtained by cationic ring‐opening polymerisation of an appropriately substituted oxetane ring. Both polyoxetanes studied exhibit liquid crystalline properties showing a similar sequence of phases. DSC and synchrotron experiments indicate that a highly ordered phase with a probable orthorhombic packing is observed at low temperature. On heating, this phase melts into a mesophase with hexagonal order within the smectic layers, which, on further heating, is transformed into a disordered smectic mesophase.

Synchrotron X‐ray diffraction patterns of one of the poyoxetanes in a heating experiment.     

Directing the Smectic Layer Orientation by Shear Flow in Hierarchical Lamellar‐within‐lamellar Liquid Crystalline Diblock Copolymers

A self‐assembled lamellar‐within‐lamellar structure of a side chain liquid crystalline diblock copolymer was shear aligned to induce overall alignment and to direct the smectic layer orientation within the copolymer lamellae. The copolymer consisted of a polystyrene block and a poly(methyl methacrylate) block bearing cholesteryl mesogens with only short oxycarbonyloxyethyl spacers separating the mesogens from the backbone. Upon shearing, the copolymer lamellae exhibited uniaxial alignment whereas the smectic layers of the mesogens showed coexisting perpendicular and parallel orientations with respect to the copolymer lamellae. The fraction of the parallel oriented domains could be systematically increased by tuning the oscillation frequency and strain amplitude.

     

Photopolymerization of Reactive Mesogens

Summary: The photopolymerization of two reactive mesogens with photopolymerizable acrylate endgroups, the methyl substituted 1,4‐phenylene‐bis{4‐[6‐(acryloyloxy)hexyloxy]benzoate} 1 and acrylic acid 6‐{4‐[6‐(6‐acryloyloxyhexyloxy)naphthalen‐2‐yl]‐phenoxy}hexyl ester 2 has been investigated using Photo‐DSC measurements. Photocrosslinking of 1 in the nematic phase at 100 °C leads to a final conversion of 87% of the acrylate groups. It is possible to perform photopolymerization with very small amounts of photoinitiator. Even with 0.001% (10 ppm) of photoinitiator, 47% of the acrylate groups polymerize within 15 min. The polymerization of the reactive mesogen 2 proceeds faster in the smectic A phase at 100 °C compared to the isotropic phase at 120 °C and leads to a higher conversion of 75%. This can be explained by an increased local concentration of the acrylate groups between the layers of the smectic cores.

Photopolymerization of reactive mesogens.     

Photo‐Crosslinked Side‐Chain Liquid‐Crystalline Elastomers for Microsystems

Photo‐crosslinkable side‐chain liquid‐crystalline polymers (LCPs) containing photoreactive benzophenone cores are synthesized in order to obtain their corresponding side‐chain liquid‐crystalline elastomers (LCEs). This strategic synthesis allows thin elastomeric films and their integration into microsystems for actuators and micromachines to be obtained. As an example of this principle, a gripper was developed. The position of its arms can be changed by applying voltages from 1.5 to 3.5 V at different rates. Small changes in the liquid‐crystalline elastomer film cause strains of up to 150% in the microdevice and the capacity to move up to 400 times its own mass due to the nematic‐to‐isotropic transformation.

     

Synchrotron X‐ray and DSC Studies of the Phase Behaviour of Poly(diethylene glycol p,p′‐bibenzoate)

Time‐resolved synchrotron X‐ray and DSC experiments were applied to investigate the phase behaviour of poly(diethylene glycol p,p′‐bibenzoate), PDEB. The DSC results are indicative of the formation of a smectic mesophase, previously identified as a SmCA type, which can be easily quenched down to room temperature. However, the synchrotron results show that the SmCA phase undergoes some kind of ordering or transformation at temperatures below 110 °C. Moreover, the annealing of PDEB at temperatures above T_g for sufficiently long times leads to the formation of a highly ordered structure, although very thin crystals and low crystallinities are obtained.

Scattering profiles corresponding to sample PDEB85 in a melting experiment.     

Amorphous‐Smectic Glassy Main‐Chain LCPs, 1. Poly(ether esters) Derived from Hydroxybibenzoic Acid and (R,S)‐ and (R)‐2‐Methylpropane‐1,3‐diol

The synthesis and the characterization of main‐chain liquid‐crystalline poly(ether esters), derived from hydroxybibenzoic acid and (R,S)‐ and (R)‐2‐methylpropane‐1,3‐diol, are reported. These polymers show an interesting thermal behavior. They develop mesophases with a slow rate of formation, allowing the easy quenching of the melt into: a) the glassy amorphous state, b) the glassy liquid‐crystalline state, or c) a mixture of both, depending on the thermal treatment. The extent of the transformation and the symmetry of the different phases have been determined by means of calorimetric and X‐ray diffraction methods. Dielectric spectroscopy results provide additional evidence for the detection of distinct glass transitions. The results show that the racemic polymer forms a low‐ordered SmC_alt mesophase, while a more ordered phase is obtained in the case of the enantiomerically pure polymer. The comparison of the properties of the different states evidences the special behavior and properties of the glass transition (T_g) in these polymers. Emphasis is paid to the location of the T_g of the liquid‐crystalline state in comparison to the T_g of the amorphous state. It is found that the glass transition of the SmC_alt glass in R,S‐PBO3 (the poly(ether ester) derived from hydroxybibenzoic acid and (R,S)‐2‐methylpropane‐1,3‐diol) appears at lower temperatures than the glass transition of the amorphous state. However, in R‐PBO3 (the poly(ether ester) derived from hydroxybibenzoic acid and (R)‐2‐methylpropane‐1,3‐diol), where the more ordered phase is present, the glass transition follows the classical tendency of semicrystalline polymers.

     

Photodeformable Polymers: A New Kind of Promising Smart Material for Micro‐ and Nano‐Applications

Summary: Starting in the 1960s, several kinds of photodeformable polymers have been developed, such as monolayers, polymer gels, solid films and liquid‐crystalline elastomers with different photodeformation mechanisms. This field evolved slowly until recently when significant achievements have been made. Most recently, Lendlein and co‐workers have put forward another new concept – using photo‐crosslinking to prepare deformable polymers with various pre‐determined shapes (Nature 2005 , 434, 879). This highlight gives a general introduction into photodeformable polymers and brings forth future challenges.

A polymer film doped with SCAA molecules where (a) is the permanent shape, (b) is the temporary shape and (c) is the recovered shape.     

Synthesis and Characterisation of Side‐Chain Liquid Crystalline Poly[1‐({[(4‐cyano‐4′‐biphenyl)oxy]alkyl}oxy)‐2,3‐epoxypropane]

Summary: A new series of side‐chain liquid‐crystal polymers, the poly[1‐({[(4‐cyano‐4′‐biphenyl)oxy]alkyl}oxy)‐2,3‐epoxypropane], has been synthesized in which the spacer length is varied from 2 to 8 methylene units. The synthesis used for the chemical modification of polyepichlorohydrin (PECH) involved the phase transfer catalyzed etherification of the chloromethyl groups with sodium 4‐cyano‐4′‐biphenoxide and lithium n‐(4‐cyano‐4′‐oxybiphenyl)‐alkanoates. All the resulting polymers (except polymers 7 and 8), including that without spacer, characterized by ¹H NMR, and IR, exhibit thermotropic liquid crystalline mesomophism. The thermal behavior of the polymers has been characterized using differential scanning calorimetry (DSC) and polarized light microscopy (POM) equipped with hot‐stage. A more pronounced odd‐even effect in the clearing temperatures is observed on increasing the spacer length in which the odd members display slightly higher values, which were also dependent on the substitution degree of PECH. The flexible PECH chain assists nematic LC formation compared with other more rigid backbone polymers where a smectic phase is often encountered with the same mesogen. A comparison of the thermal properties of the cyanobiphenyl based series reported here. Polyacrylate (PA) and poly(methacrylate)‐based (PMA) SCLCPs containing 4‐cyanobiphenyl as the mesogenic unit are consistent with the general rule that increasing backbone flexibility for a given mesogenic unit and spacer length enhance the clearing temperature. This was not found from the PECH‐based series, which show the lower clearing temperature than the PMA and PA series, even though they have more flexible backbone than PA and PMA series. So the clearing temperature is not solely determined by the flexibility of the backbone.

Schlieren textures of PECHOC2‐B taken at 110 °C (cooling from isotropic phase after annealing 1 h).     

Ionic‐Strength‐ and pH‐Responsive Poly[acrylamide‐co‐(maleic acid)] Hydrogel Nanofibers

A novel acrylamide/maleic acid copolymer [P(AM‐MA)] hydrogel nanofibrous membrane with a fiber diameter of ca. 120 nm is prepared by electrospinning an aqueous P(AM‐MA) solution with diethylene glycol as crosslinker, followed by a heat‐induced esterification crosslinking reaction at 145 °C. This hydrogel nanofiber can maintain a fiber form, but becomes distorted and merges to form many physical crosslinking points after immersion in water. The P(AM‐MA) hydrogel nanofibers are sensitive to external stimuli ionic strength and pH. Their water‐swelling ratio decreases with increasing solution ionic strength, and it shows a characteristic two‐step increase at pH = 2.5 and 8.5 in response to the increase of solution pH. The maximum water‐swelling ratios of the P(AM‐MA) hydrogel nanofibers are 18.1 and 22.5 g · g^?1 in a solution of 0.05 mol · dm^?3 ionic strength and in an aqueous solution of pH 11, respectively.

     

Synthesis of Ethylene/Acrylonitrile Composite Elastomers with Nanosized Polyacrylonitrile Domains Using α‐Dimine‐[N,N] Nickel Dichloride/EASC

A series of ethylene and acrylonitrile composite elastomers were prepared using (1,4‐bis(2,6diisopropylphenyl)‐acenaphtenediimine‐nickel(II))‐dichloride/ethylaluminum sesquichloride (EASC). The xylene‐soluble polymer fraction showed nitrile bands in infrared spectroscopy at 2 245 and 2 214 cm^?1 and polyacrylonitrile‐enriched structures were detected in the xylene‐insoluble fraction by¹H and ¹³C NMR. In addition, TEM detected nanosized polyacrylonitrile domains dispersed in the polyethylene matrix. Differential scanning calorimetry scans conducted from ?70 to 350 °C measured exothermic bands corresponding to the cyclization and aromatization of the nitrile groups dispersed in the polyethylene matrix.

     

Optical Proof of Mesophase Transitions between Smectic and Nematic Phase of the Side‐Group Polyacrylate PAC6

The liquid‐crystalline side‐group polyacrylate PAC6 forms two mesophases in thin layers within the temperature range between 90 and 100 °C. The structures developing by self‐organization can be made clearly visible by Schlieren‐optical methods. If a low lateral temperature gradient is maintained in the temperature range mentioned above one can simultaneously and abreast observe four phases in PAC6 layers. Both new phases occur between the smectic and the nematic phase. Layer surface deformations arise during this phase transition. They were analyzed with a microscope interferometer. The PAC6 melt shows a considerable change of surface tension in this temperature range.

Schlieren‐optical image of a PAC6 layer with two mesophases.     

“One‐pot” Synthesis of 1‐Vinyl‐2‐pyrrolidone with Protic Functional Groups in 3‐Position

A synthetic route that allows modification of 1‐vinyl‐2‐pyrrolidone with an alcohol or thiol group in its 3‐position in a one‐pot reaction without using protecting groups is described. The strategy used to achieve this goal is the reaction of the carboxamide anion of 1‐vinyl‐2‐pyrrolidone with cyclic precursors of these functionalities. It is furthermore shown that VP‐monomers functionalized with OH groups are suitable for the preparation of the corresponding copolymers with pure VP. As a second application of the new compounds, we also describe the synthesis of the first asymmetric crosslinker based on VP.

     

A Liquid‐Crystalline Co‐Polysiloxane with Asymmetric Bent Side Chains

A co‐polysiloxane has been prepared with an asymmetric bent side group based on a liquid‐crystalline monomer showing a B₂ (SmCP) phase. In order to dilute the side chain mesogens along the polymer backbone, octyl side chains were also incorporated. X‐ray studies confirmed that the resulting co‐polysiloxane exhibits a liquid‐crystalline layered phase, the siloxane polymer chains being microphase segregated from the incompatible side chain mesogens. The mesophase of polymer exhibits a homeotropic structure that can be successfully aligned by unidirectional shear. This phase does not exhibit polar switching under an applied electric field unlike the bent‐core monomer used as the side chains of this polymer. This can be caused by the homeotropic alignment of the polymer samples.