Synchrotron X‐Ray Study of Liquid Crystalline Polyoxetanes Containing Two Mesogenic Groups Connected by a Flexible Spacer in the Side Chain

The study of the mesophase behaviour of a family of new side chain liquid crystalline polymers by means of synchrotron X‐ray diffraction is reported. These systems contain two bibenzoate rigid units in the side chain that are interconnected through a flexible spacer. A polymerisable oxetane unit is linked to one end of the dimer structure, and a flexible terminal chain is linked to the other end. All the polymers form phases with variable degree of order (from low ordered smectic to crystalline) depending on the chemical constitution of the different segments. The influence of the length, parity, and lateral substitution of the spacers on the transitional parameters and the symmetry of the mesophases that are formed is analysed. Close similarities were found between the phase behaviour of the monomer precursors and the derived polyoxetanes. The arrangement of the mesogens, the flexible groups and the polymeric backbone in the layered structure is discussed with respect to the L/d ratio. Different values were obtained depending on the parity of the central spacer and on the degree of order of the mesophase. Interpenetrated structures, where the flexible groups of different lengths are mixed, seem to be compatible with low ordered smectic phases, but sterically disfavoured when building up highly ordered mesophases.

     

Synthesis of New Side‐Chain Liquid‐Crystalline Polyoxetanes with Two Mesogenic Groups Connected by a Flexible Spacer in the Side Chain

The synthesis of a different geometry of side‐chain liquid‐crystalline polymers is reported where two mesogenic units connected by a flexible spacer are attached as pendent groups of a polymeric chain. The resulting structure is the following, where R₁, R₂ and R₃ are methylenic or oxymethylenic segments: The monomers were obtained by linking the dimer structure to an oxetane ring. These substituted oxetanes were polymerised by a boron trifluoride‐initiated cationic ring‐opening reaction to obtain the corresponding polymers. The polymerisation proceeds with high yields, in spite of the steric hindrance derived from the bulkiness of the side chain, and the presence of some functional groups in the molecule that compete against the cyclic ether for the nucleophilic attack at the propagating centre. A fraction of cyclic oligomer is obtained together with high molecular‐weight polyoxetane, as a result of back‐biting reactions during the polymerisation. Several systems with spacers of different length and parity were synthesised in order to analyse the influence of the nature of the spacer on the phase behaviour. The formation of mesophases was proved by means of differential scanning calorimetry, X‐ray diffraction and microscopic analysis. Most of the monomers form smectic structures at subambient temperatures. The polymers display a similar phase behaviour, although shifted to higher temperatures.     

Dielectric Relaxation of Side‐Chain Liquid Crystalline Ionomers Containing Alkaline Metal Ions

Summary: Two sets of side‐chain, liquid‐crystalline ionomers based on copolymers of butyl acrylate and 4‐{[6‐(acryloyloxy)hexyl]oxy}benzoic acid containing 2.5–20 mol‐% of lithium or rubidium ions were studied. As the content of ions of alkali metals increases, segmental mobility markedly decreases. As compared with rubidium‐containing ionomers, ionomers containing lithium ions are characterized by a far more efficient decrease in their dielectric relaxation times because ionic radius of lithium ions is lower than that of rubidium ions.

     

Tailoring Linear and Nonlinear Optical Properties of a Side‐Chain Liquid Crystalline Azo‐Polymethacrylate

The molecular orientation in a side‐chain liquid crystalline azopolymer with push–pull chromophores ( Pol‐PZ‐CN ) is studied by UV‐vis spectroscopy, refractive index and second harmonic generation measurements. Fresh films show homeotropic arrangement, but irradiation with polarized light induces “in‐plane” birefringence, which is enhanced by subsequent thermal annealing up to values as high as 0.34. The nonlinear response of Corona poled films with “in‐plane” anisotropy is governed by three independent NLO d_ij coefficients, evidencing symmetry different from the usual (C_∞v). For irradiated films, stable values d₃₁ = 1.4, d₃₂ = 0.9 and d₃₃ = 11 pm·V^?1 are measured at 1.9 µm. A noticeable higher d₃₁/d₃₂ ratio is obtained for preirradiated and annealed films. The proposed two‐step method consisting of light irradiation and heating is an effective strategy to tailor the polar molecular orientation of nonlinear liquid crystalline azopolymers.

     

Two‐Step Smectic CA Phase Formation from Isotropic Liquid upon Supercooling in Main‐Chain Liquid‐Crystalline BB‐5(1‐Me) Polyester

The mesophase formation process of a main‐chain BB‐5(1‐Me) polyester in the supercooled isotropic‐liquid state has been studied using DSC, WAXD, and optical microscopy. The supercooled isotropic liquid follows two different SmCA formation processes. Above 85 °C, it forms the expected SmCA phase. Below, it yields a metastable SmA phase that displays a layer spacing almost twice the one of the SmCA phase. The SmA phase develops from a homogeneous nucleation followed by 3D growth and thereafter transforms to the SmCA phase. This two‐step SmCA formation involves a change in polymer chain configuration: first, the chains are folded in each two units to form the SmA phase and then they stretch to achieve the characteristic zigzag arrangement of the mesogens in the SmCA phase.

     

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.     

Orientation of the Main Chain in Liquid‐Crystalline Side‐Chain Polyesters with Variation of Main‐Chain Spacer Length

Previous X‐ray investigations on liquid‐crystalline side chain polyesters with variable spacer length in the side chain as well as in the main chain led to the assumption that the structure will change qualitatively if the main chain spacer length exceeds a certain value. We obtained indications supporting this assumption by applying two‐dimensional ¹³C NMR. It was shown that for a sample with main chain spacer shorter than the critical length, the main chain segments align perpendicular to the side chains (〈P₂〉 = ?0.46), and for spacer length larger than critical length, the main chain segments preferentially orient themselves parallel to the side chain (〈P₂〉 = 0.28).

Visualization of the main‐chain order as revealed by the values of Table 2 . The opening angle of the cones corresponds to the average angle of deviation as defined in the text.     

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).     

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.     

Preparation and Characterisation of Hydrophobically Modified Hydroxypropylcellulose: Side‐Chain Crystallisation

Summary: Hydroxypropylcellulose has been hydrophobically modified by reaction with different amounts of palmitoyl chloride. When the degree of substitution is sufficiently high, the side chains are able to produce lateral crystallisation, which has been analysed by DSC and diffraction experiments, using both conventional and synchrotron radiation. The melting temperatures and enthalpies depend on the degree of substitution, in such a way that it is relatively easy to get melting temperatures close to that of the human body, although a rather wide melting region is obtained.

X‐ray diffractograms of the original HPC sample and of the modified specimens.     

Synthesis and Properties of Side‐Chain Liquid Crystalline Polypeptides Bearing Various Alkyl Spacers and Oligo‐Ethylene‐Glycol Tails

A series of polypeptides bearing various alkyl (i.e., propyl, hexyl, or octyl) spacers, biphenyl mesogens, and oligo‐ethylene‐glycol (OEG) tails (i.e., PPLG₂₆‐BPOEG _m, PHLG₃₄‐BPOEG_m, and POLG₃₂‐BPOEG_m [m = 3 or 7]) has been synthesized via 1,3‐dipolar cycloaddition reactions with quantitative grafting densities. Fourier transform infrared results reveal that the polypeptides adopted an α‐helical conformation in the solid‐state. Differential scanning calorimetry (DSC) analysis reveals that the glass transition temperatures (T_gs) of the polypeptides with OEG₃ tails decrease with the increasing length of the alkyl spacers. For the samples containing alkyl spacers of the same length, T_g significantly decreases with the increasing lengths of the OEG tails. Polarized optical microscope (POM) results reveal that all polypeptides exhibit strong birefringence at room temperature. Polypeptides with long alkyl spacers (i.e., hexyl or octyl) or OEG₇ tails show isotropic transitions, while PPLG₂₆‐BPOEG₃ show no clear point temperature before thermal decomposition. DSC, POM, and wide‐angle X‐ray scattering results collectively reveal that polypeptides with long alkyl spacers (i.e., hexyl or octyl) and OEG₃ tails undergo a reversible smectic E phase to isotropic phase transition, while polypeptides with OEG₇ tails undergo a reversible liquid crystalline phase to isotropic phase transition. The transition temperatures increase with the increasing length of alkyl spacers.

     

New Optically Active Methacrylic Polymers Bearing Side‐Chain Bisazoaromatic Moieties

The synthesis of three novel optically active monomers containing a side‐chain chiral moiety linked to a photochromic bisazoaromatic chromophore, such as (S)‐3‐methacryloyloxy‐1‐[4′‐phenylazo‐(4‐azobenzene)]‐pyrrolidine [(S)‐ MPAAP ], (S)‐3‐methacryloyloxy‐1‐[4′‐cyanophenylazo‐(4‐azobenzene)]‐pyrrolidine [(S)‐ MPAAP‐C ] and (S)‐3‐methacryloyloxy‐1‐[4′‐nitrophenylazo‐(4‐azobenzene)]‐pyrrolidine [(S)‐ MPAAP‐N ], is described. Each monomer has been radically homopolymerized to afford the corresponding optically active polymeric derivative, which have been characterized, in solution and in the solid state, and their properties compared to those of the monomers and of analogous homopolymers bearing only one azoaromatic chromophore in the side chain. The optical activity displayed by the polymers is discussed in terms of extent of chiral conformation assumed by the macromolecules as a consequence of their stiffness and the dipole‐dipole interactions between the bisazoaromatic chromophores.

     

A Triple‐Shape Memory Material via Thermal Responsive Behavior of Liquid Crystalline Network Incorporating Main‐Chain/Side‐Chain LC Units

In the last several years, multiple‐shape memory liquid crystalline networks (LCNs) have received more and more attention due to the basic theoretical research on them and their wide potential applications. In this article, a novel main‐chain/side‐chain liquid crystalline monomer and its corresponding polymer networks based on the thiol‐ene click reaction are reported. Properties of the synthesized liquid crystalline monomer are well studied with nuclear magnetic resonance (NMR), Fourier transform infrared (FTIR) spectra, differential scanning calorimetry (DSC), and polarized optical microscopy (POM). The as‐prepared free‐standing LCN films are investigated well by FTIR, DSC, POM, and X‐ray diffraction (XRD), which show them having good liquid crystalline properties. Tensile test and dynamical mechanical analysis (DMA) results indicate the LCN films have excellent thermal mechanical properties. By adjusting the crosslinking densities, LCN films exhibit two thermal transition temperatures (T_g and T_NI) that can be utilized to trigger the triple‐shape memory behaviors. The cyclic thermal mechanical analysis conducted by DMA reveals that LCN films exhibit good triple‐shape memory properties with high‐shape fixity ratio (R_f (S1→S2) is 99.2% and R_f (S2→S3) is 99.3%) and shape recovery ratio (R_r (S3→S2) is 92.4% and R_r (S2→S1) is 98.5%).     

Synthesis of Laser‐Alignable Liquid Crystalline Conducting Polymers

Summary: The synthetic routes to three series of liquid crystal conducting polymers (based on pyrrole, thiophene, and aniline monomers) are reported and the optimum conditions for polymer preparation are described. These polymers show increased conductivity when laser‐aligned, the greatest effect being shown by the N‐substituted pyrrole‐based system. Information on their liquid crystal and spectroscopic properties and other characteristics are also included.

Structure of the monomers M1, M2, and M3.     

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.     

Atom Transfer Radical Polymerization and Third‐Order Nonlinear Optical Properties of New Azobenzene‐Containing Side‐Chain Polymers

The atom transfer radical polymerization (ATRP) technique has been successfully applied to synthesize a series of nonlinear optically (NLO) active homopolymers, 4‐(4‐nitrophenyl‐diazenyl) phenyl acrylate ( P ‐ NPAPA ) and 4‐(4‐methoxyphenyl‐diazenyl) phenyl acrylate ( P ‐ MPAPA ), containing azobenzene groups on the side chain. The third‐order NLO properties of the polymer films were measured by the degenerated four‐wave mixing (DFWM) technique. A dependence of the χ⁽³⁾ values and response times of polymers on their number‐average molecular weight and the electronic effect of the substituent (nitro‐ or methoxy‐) on the azobenzene group have been evidenced. The increasing χ⁽³⁾ value of the polymer films at the magnitude of about 10^?10 was displayed with increasing molecular weight and the presence of the push‐pull electronic system contributes much in enhancing the third‐order NLO susceptibility of polymers.

     

Structure and Properties of Phosphorous‐Containing Thermotropic Liquid‐Crystalline Aliphatic–Aromatic Copolyesters

A series of thermotropic liquid crystalline aliphatic–aromatic copolyesters derived from various ratios of dodecane‐1,12‐diol ( 1 ), terephthaloyl bis‐(4‐oxybenzoyl‐chloride) ( 2 ), and 2‐(6‐oxido‐6H‐dibenz〈c,e〉〈1,2〉 oxaphosphorin‐6‐yl)‐1,4‐naphthalene diol ( 3 ), has been synthesized. The chemical structures of the monomers and polymers have been confirmed by elemental analyses, and FT‐IR and ¹H NMR spectroscopy. The mesomorphic properties of polymers have been investigated by differential scanning calorimetry, polarizing optical microscopy, and X‐ray diffraction measurements. The influence of the content of the aliphatic unit on the phase behavior of the polymers has been examined. The polymers that contain >30 mol‐% aliphatic diol showed smectic phases while the polymers that contained <30 mol‐% aliphatic diol displayed nematic phases. The polymers revealed a reversible mesomorphic phase transition, wide mesophase temperature ranges, and high thermal stability. The degree of crystallinity increased upon increasing the content of aliphatic moieties. The char yield at high temperature increased by increasing the content of phosphorous‐containing bisphenol.

     

Self‐Assembly of a Hydrogen‐Bonded Association Chain Liquid Crystalline Polymer (LCP)

A hydrogen‐bonded association chain liquid crystalline polymer (LCP) based on tetraethyleneglycoxy‐bis(2,6‐dimethyl‐4‐benzoic acid) and 4,4′‐(p‐phenylenedi‐1,2‐ethenediyl)bipyridyl in a 1 : 1 stoichiometric ratio was synthesized using melting mixing method. X‐ray scattering from fiber drawn from its nematic phase exhibits preferred orientation, indicating that hydrogen bonding association chain polymer can also be strong enough to withstand the shear or tensile force and could be macroscopically oriented under shear or tensile force. Two complimentary components are likely to form a co‐crystallized structure to form a monoclinic crystal.     

Tuning Structures of Mesogen‐Jacketed Liquid Crystalline Polymers and Their Rod–Coil Diblock Copolymers by Varying Chain Rigidity

Two mesogen‐jacketed liquid crystalline (LC) polymers with different rigid side‐chain cores, poly[2,5‐bis(tert‐butoxylcarbonyl)styrene] (PM1) and poly[2,5‐bis(4‐tert‐butoxylcarbonyl phenyl)styrene] (PM3), as well as their corresponding block copolymers (BCPs), poly(dimethylsiloxane)‐b‐PM1 (PDMS‐b‐PM1) and PDMS‐b‐PM3, are synthesized and characterized. The LC phase structures of the homopolymers and the microphase‐separated structures of the BCPs can be controlled by changing the rigidity of the polymer chains with the different rigid side‐chain cores used. The LC phase structure of PM1 homopolymer is dependent on its molecular weight (MW). On the other hand, PM3 is always amorphous below thermal decomposition temperature regardless of its MW, owing to much higher rigidity of PM3 compared with that of PM1. Although both BCPs can self‐assemble into lamellae (LAM) and hexagonally packed cylinders (HEX) with varying compositions, PDMS‐b‐PM3 can form the HEX structure at a smaller weight fraction of the PM3 block because PDMS‐b‐PM3 possesses a larger geometric asymmetric factor compared with PDMS‐b‐PM1. For PDMS‐b‐PM1 with a large enough MW, it can form hierarchically ordered nanostructures, including the LAM or HEX nanostructure of the BCP and the columnar nematic phase of the PM1 block.     

Thermotropic Phase Behavior of a Liquid‐Crystalline Poly(ether ester) Derived from Hydroxydibenzoic Acid, 2‐Methyl‐1,3‐propanediol and R‐1,3‐Butanediol

The thermotropic phase behavior of a liquid‐crystalline poly(ether ester) derived from 4′‐hydroxybiphenyl‐4‐carboxylic acid and two different spacer diols (2‐methyl‐1,3‐propanediol and R‐1,3‐butanediol) has been analyzed by differential scanning calorimetry, real time synchrotron X‐ray diffraction and solid‐state ¹³C NMR. It has been found that the polymer develops a smectic mesophase that presents order inside the layers. This mesophase has a slow rate of formation, in such a way that it is possible to obtain either an amorphous glass or a liquid‐crystalline glass. These two phases exhibit different and independent glass transition temperatures: 95 °C for the amorphous one and 85 °C for the mesophase. The diffractogram of this mesophase shows two clear diffractions in the wide‐angle region and a long spacing (of low intensity) in the small‐angle region. All those features are characteristic of a mesophase of intermediate order, presumably of the type SmI, in contrast to the low‐ordered SmC_alt mesophase reported for the racemic polymer. Thus, the chirality of the spacer, which provides structural regularity, makes easier the packing of the chains, so that a more ordered mesophase is obtained. In addition, both the solid‐state ¹³C NMR line shapes and the value of are practically the same for the quenched and the annealed samples, indicating that the “ordered” phase obtained by annealing is not a three‐dimensional crystal, thus confirming its assignment as mesophase, with a considerable degree of conformational disorder.