Cooperative modes of molecular mobility in liquid crystalline comb‐like oligoacrylates and oligomethacrylates with 4‐cyanoazobenzene side groups

Cooperative modes of molecular mobility in comb‐like liquid crystalline oligoacrylates and oligomethacrylates containing mesogenic 4‐cyanoazobenzene side groups and spacers of 3, 5, 7, 9, 10, and 11 CH₂ groups were studied by means of dielectric spectroscopy. Near T_g the existence of two cooperative relaxation processes denoted as α and δ was revealed. The δ‐relaxation is connected to the motion of mesogenic groups with respect to the short mesogenic axis and the α‐relaxation is connected to the motion of mesogenic groups with respect to the long axis and with the segmental mobility of the main chains. The temperatures of α‐ and δ‐transitions decrease with increasing spacer length. The liquid‐crystalline structure formation increases main chain rigidity and temperatures of the α‐ and δ‐transitions. For these oligomers aligned by an external electric field or by mechanical shear, the planar or homeotropic orientation of mesogenic groups is established. The choice of orientation conditions makes it possible to obtain the predetermined order parameters.     

Stress-induced orientation in lightly crosslinked liquid-crystalline side-group polymers

X-ray measurements of stretched crosslinked liquid-crystalline side group polymers show that two types of orientation of the mesogenic groups relative to the polymer chains (parallel or perpendicular) are possible, depending on the structure of the polymer chain. For two liquid-crystalline polymethacrylates with long spacer groups (6 methylene units) the mesogenic groups orient preferably perpendicularly to the polymer chains (axis of strain). For three liquid-crystalline polyacrylates with long or short spacer groups (6 or 2 methylene units), the mesogenic groups orient preferably parallel to the polymer chains (axis of strain). The reason for this difference is not yet clear. These results are compared with earlier results of melt-drawn fibres of uncrosslinked polymers.     

Optical anisotropy of comb-like macromolecules of lateral and semilateral structure

Maxwell effect and Kerr effect techniques were used to study the intramoleculare axial ordering of anisotropic side groups of an isolated macromolecule in solution. Three comb-like polymers with a methacrylic main chain were investigated. The side chains contain aliphatic spacers separating the mesogenic fragments R from the backbone. R contains two or three phenylene rings with ester linking groups and cyano end groups. Fragments R are attached to the side chains, by the end or middle-point, through the para- or ortho-phenylene ring. The main-chain conformation in solution was identified to assess the hydrodynamic data using the Gaussian coil approximation. The optical anisotropy of the macromolecule segment was determined and related to the monomer unit structure. The specific Kerr constant obtained in diluted benzene solution of polymer was compared with those of low molecular weight analogues of the polymer mesogenic groups. The mobility and intramolecular ordering of the mesogenic groups in the isolated macromolecules are discussed. These properties are compared with those of comb-like polymers with another type of attachment of mesogenic groups, and with another (siloxane) chemical structure of the backbone chain. The differences in restrictions of side-chain mobility due to the backbone linkage in different types of comb-like polymers are also discussed.     

Nematic Elastomers: The Dependence of Phase Transformation and Orientation Processes on Crosslinking Topology

The orientation process in response to a mechanical field and the phase transformation of nematic elastomers are investigated for different types of crosslinkers and their concentration. For the corresponding linear polymer melt a mechanical field causes a director orientation perpendicular to the external field axis. For networks with rod‐like crosslinkers, however, an orientation with the director parallel to the external field is observed. Only a trifunctional, disk‐like crosslinker at low concentrations causes the perpendicular orientation as obtained for the linear polymer. At higher concentrations the director again becomes ordered parallel to the field. The orientation behaviour is analysed by stress‐optical measurements and the thermal expansion behaviour of the network, where the thermal expansion directly reflects the change of the network conformation at the isotropic to nematic phase transformation. The different orientation behaviour of the networks can be simply explained. Because of the contour length of the crosslinking molecules the crosslinker can no longer be considered as “point‐like” crosslink, but rather as a part of the network chains. Due to interactions with the mesophase it orients the mesogenic groups in an antagonistic manner compared to the orientation of the pure polymer melt.     

Alignment of Photo‐Crosslinkable Copolymer Liquid Crystals Induced by Linearly Polarized Ultraviolet Light Irradiation and Annealing: Effect of Heating Rate

We describe thermally enhanced molecular reorientation of photo‐crosslinkable methacrylate copolymer liquid crystal (CPLC) films with a photoreactive 6‐[4‐(2‐cinnamoyloxyethoxy)biphenyloxy]hexyl side group and a photoinactive n‐(4‐cyanobiphenyloxy)alkyl (nCB, n = 5 or 6) side group. The alignment of the mesogenic groups was induced by irradiation with a linearly polarized ultraviolet (LPUV) light and subsequent annealing at different heating rates. The orientational behavior was investigated by polarized UV and FT‐IR spectroscopy. For a film of copolymer P1 (n = 6), both mesogenic groups aligned homogeneously in a direction parallel to the electric vector ( E ) of the incident LPUV light regardless of the annealing conditions. On the other hand, the heating rate highly influenced the orientational behavior of a film of P2 (n = 5). The out‐of‐plane alignment of mesogenic side groups was observed when the heating rate after the LPUV light exposure was 90°C/min, while in‐plane alignment was more prevalent in the mesogenic groups for the slow heating rate. Tilt angle control of the alignment direction was feasible by the use of oblique LPUV light irradiation. In addition, the inclination angle of the mesogenic groups for the P2 film could be regulated at the same irradiation angle by changing the heating rate.     

Liquid crystalline side chain polymers and their behaviour in the electric field

The electro-optical properties of liquid crystalline polymers with polar mesogenic units in the side chain were investigated. It could be shown, that in an electric field, they show all effects known from low molecular weight liquid crystals, e.g. the Freedericksz-transition, the DAP-effect, the formation of William's domain, and the dynamic scattering. The optical response times are slightly longer than for low molecular weight liquid crystals, but in some cases at higher temperatures as short as 200 ms. By comparison of different polymers and some corresponding liquid crystalline monomers, it is found that: (1) the difference between the measuring temperature and the glass transition temperature mainly determines the optical response times, (2) the fixation of mesogenic groups to a polymer main chain via spacers influences the threshold voltage. Low values are found for long spacers. These results allow a discussion of the influence of polymer fixation of the mesogenic groups on their liquid crystalline properties, pointing to a partial decoupling of the motions of main chains and side chains.     

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.

     

Synthesis and characterization of frequency doubling nonlinear optical liquid crystalline polyepichlorohydrins and polyoxetanes functionalized with alkoxycyano-azobenzene and alkoxynitrostilbene dyes. Second harmonic generation in corona poled thin films

New copolyethers were prepared by chemical modification of polyepichlorohydrin and poly(3,3-bis(chloromethyl)oxetane) with the sodium salt of either 4-hydroxy-4′-nitrostilbene or 4-hydroxy-4′-cyanoazobenzene, employing the concept of having the nitrostilbene or cyanoazobenzene species serve concomitantly as both the nonlinear optical (NLO) chromophore and the mesogenic moiety in the polymer. Their thermal behavior was established by means of differential scanning calorimetry (DSC) and optical microscopy. The NLO properties of two nematic copolyethers were studied. The NLO mesogenic groups were oriented by the standard corona poling technique and the degree of axial ordering was determined as a function of poling conditions. The second harmonic coefficients d₃₁ and d₃₃ were measured. The results showed that liquid crystallinity enhances field-induced polar ordering. The ratio d₃₃/d₃₁ was found to be much larger than 3, in agreement with the theoretical models for electric field poling of liquid crystals. We were able to obtain d₃₃ values up to 130 pm/V at λ = 1,064 μm, thus exceeding many values previously reported for second harmonic generation measurements on isotropic polymers containing similar side chain chromophores.     

Photocontrol of Birefringence and In‐Plane Molecular Orientation in Copolymer Liquid Crystal Films with 4‐Methoxyazobenzene and Photo‐Cross‐Linkable Side Groups

Control of the in‐plane orientation direction in a polymethacrylate copolymer liquid crystal film substituted with 4‐methoxyazobenzene (4MAz) and 4‐methoxycinnamoyloxybiphenyl (4MCB) side groups, and control of the reversible birefringence of the resultant oriented film are described. High‐order in‐plane molecular orientation was generated by irradiating with linearly polarized (LP) 365 nm light and subsequent annealing. By adjusting the exposure dose, the reorientation direction was controlled parallel and perpendicular to the electric vector ( E ) of the LP light. Cooperative reversion of the orientation direction for both the 4MAz and 4MCB groups was observed, and the orientational order, S, varied from ?0.79 to +0.75, without the side groups aggregating. The degree of the photoreaction and the distribution of the photoproducts from the 4MAz and 4MCB side groups through the axis‐selective photoreaction explain the reversion of the orientation direction. Optical recording, erasing, and birefringent control of the resultant oriented films were demonstrated by employing the photoisomerization of the 4MAz side groups using nonpolarised (NP) 405 nm light.

Order parameters of a P1 film after irradiating with LP 365 nm light (circles) and that following subsequent annealing (squares), as a function of exposure energy.     

Mechanischer Scherabbau von Polymeren in Lösung mittels turbulenter Strömung, 1. Kinetik,Molekulargewichtsabhängigkeit und Frage des Bruchmechanismus

The mechanical shear degradation of poly(decyl methacrylate) with weight average molecular weights 1,0·10⁶ ≤ M?_w ≤ 1,7·10⁶, and molecular polydispersity ratio M?_w/M?_n = 5 in dilute solutions is studied in turbulent flow as a function of molecular weight using a special apparatus consisting of two vessels connected by a capillary. Shear stress and shear rate remained constant during degradation. The rate of degradation was followed up by molecular weight distribution curves using gel permeation chromatography and described by ?dc_i/dt = k_i·cⁿ, i being a high molecular weight species of the distribution. The reaction was found to be of the first order (n = 1) independent of solvent and of capillary length. Rate constants k_i in the molecular weight range from 3,2·10⁶ to 13,5·10⁶ proved to be proportional to the hydrodynamic volume of the polymer molecules expressed in terms of the product of intrinsic viscosity and molecular weight [η]_i·M_i. This corresponds to a linear relationship between k_i and M_i^1,75. Additional experiments show that this type of dependence on molecular weight holds only for turbulent flow; in laminar flow the result of the literature could be confirmed that there is a linear relation between k_i and M_i¹. Both results are independent of capillary length. As to the mechanism of breakage in turbulent flow it seems that in one step each macromolecule is broken simultaneously into several smaller parts.     

Polymers by 1,3-dipolar cycloaddition reactions: the “criss-cross” cycloaddition

1,3-Dipolar cycloaddition reactions have been used to prepare a large number of different heterocyclic and heteroaromatic polymers. Most of this work has been done in the 60's and 70's. Since then, interest in this field has vanished almost completely. While the number of polymer structures accessible by 1,3-dipolar cycloaddition is very large, the high reactivity and tendency of the 1,3-dipoles to undergo side reactions limits the successful applications in step growth polymer synthesis considerably. The “criss-cross” cycloaddition reaction also proceeds via two consecutive 1,3-dipolar cycloaddition steps. Its starting compounds, azines and diolefins or diisocyanates, are stable, can be purified easily, and do not undergo side reactions if the reaction conditions are chosen properly. Initially, the reaction between hexafluoroacetone azine and various diolefins was studied. This work was aimed at the preparation and structural characterization of soluble polymers. Later on, the “criss-cross” cycloaddition between aromatic aldazines and diisocyanates was included in this research. Different azines and diisocyanates were studied to find the best compromise between reactivity of the monomers and solubility and glass transition temperature of the products. Due to the special reaction mechanism, the polymers have exclusively isocyanate end groups, independent of stoichiometry of the monomers and the conversion. Thus, this reaction is ideally suited for the preparation of telechelics. Prepolymers obtained by “criss-cross” cycloaddition between 4-methoxybenzaldazine and 4,4′-diisocyanatodiphenyl ether were used as hard segments in segmented block copolymers. Those block copolymers with a hard segment content below 36% showed elastomeric behavior with ultimate elongations above 700% and tensile strengths between 2 and 6 MPa.     

Liquid crystals from side-chain mesogenic poly(vinyl ether-alt-maleic anhydride)

Alternating copolymers of maleic anhydride with vinyl ethers, bearing a 4′-methoxy-4-biphenylyl mesogenic group via an oligo(ethylene oxide) spacer, were synthesized and structurally characterized. From X-ray diffraction observations, a smectic E phase was observed. Smectic layers are single layers of ribbon-like polymer chains, with all pendant groups arranged in a single row and pending on the same side of the backbone. Mesogenic groups are oriented up and down at random, and polymer chains are aligned along the [110] direction of the rectangular lattice describing the packing of the pendant groups.     

Synthesis and characterization of biaxial nematic side chain polymers with laterally attached mesogenic groups

The synthesis and characterization of liquid crystalline side chain polymers with mesogenic groups laterally attached to the polymer main chain are described. By variation of the terminal group, mesogenic moiety or spacer length, a change in the glass transition temperature, the nematic to isotropic transformation temperature and the stability of the nematic phase can be achieved. With increasing length of terminal groups, a distinct odd-even effect of the nematic to isotropic phase transformation temperature and also a strong decrease of the glass transition temperature is observed. Macroscopic homeotropic alignment can be achieved in an electric or magnetic field. Conoscopic investigations show optical biaxial behavior of the nematic phase. A chiral nematic copolymer is the first example of a thermotropic biaxial cholesteric phase.     

Ordering of liquid crystalline solutions of rigid-rod aramids using mechanical shearing and electric-field poling

2′,5′-Diamino-4-(dimethylamino)-4′-nitrostilbene was polymerized with terephthaloyl dichloride and 2,6-difluoroterephthaloyl dichloride in a low-temperature solution polycondensation to give two novel, fully aromatic rigid-rod polyamides. Nematic solutions of these polymers were processed into fibers and films that were characterized by wide-angle X-ray diffraction measurements. A post-spin annealing process was employed to enhance the chain orientation in the fibers. The dominating crystal structure was found to be similar to “modification II” of the fibers from poly(p-phenyleneterephthalamide), but the fibers do not suffer a comparable structural transformation upon heat treatment. A corona-discharge poling process gave rise to a remarkable gain in average chain orientation in the films. Again the crystal structure was found to be similar to “modification II”. The aramids investigated in this work represent a new approach to the design of liquid-crystalline rigid-rod polymers, where different mechanisms of orientation can be combined. In the nematic phase, the rigid-rod molecules form highly oriented domains that can be oriented using mechanical processes such as shearing. In addition the stilbene units that are fixed in the polymer backbone with their dipole moments oriented transverse to the main chain can be oriented in an electric field. The combination of both orientation mechanisms seems to cause a synergistic effect, probably since each affects different levels of the polymer microstructure in the solid.     

Synthesis and characterization of polyoxetanes containing 4-cyanobiphenyl-based side groups for nonlinear optical applications, 1. Chemical modification of poly[3,3-bis(chloromethyl)oxetane]with sodium 4-cyano-4′-biphenyl oxide

Two copolyethers were prepared by chemical modification of poly[3,3-bis(chloromethyl)oxetane] with sodium 4-cyano-4′-biphenyl oxide, employing the concept of having the cyanobiphenyl species serve concomitantly as both the nonlinear optical chromophore and the mesogenic moiety in the polymer. Their thermal behavior was established by means of DSC and optical microscopy. Some preliminary Corona poling experiments were performed, and the second harmonic generation coefficients d₃₁ and d₃₃ were measured by Maker fringe analysis. The results showed that liquid crystallinity enhances field-induced polar ordering. The ratio d₃₃/d₃₁ was found to be much larger than 3, in agreement with the theoretical models for electric field poling of liquid crystalline polymers where hyperpolarizable chromophores are attached as side groups to a polymer backbone.     

Dielectric properties of a liquid crystalline siloxane copolymer

An investigation on the molecular dynamics of a liquid crystalline side chain polymer using the dielectric relaxation method on oriented samples in the frequency range of 0,1 to 10 000 kHz is presented. The compound under investigation is a polysiloxane copolymer with two different mesogenic side chains and a phase sequence glassy-smectic A-nematic-isotropic. Three main relaxation processes are found and assigned to the rotation of the side chains around the main chain, the glass transition process coupled with side chain motions, and a local motion in the glassy state, respectively. Evidence is found for a layered arrangement of the polymer main chain in the smectic phase and for a parallel correlation between the transverse components of the dipole moments.     

Structures of liquid crystalline side group polymers oriented by drawing

The fiber patterns of a series of liquid crystalline polymers were analysed. For polymers with smectic phases and for polymers with long spacers and nematic phases, the mesogenic groups orient perpendicularly to the polymer main chains which are parallel to the fiber axis. The fiber pattern indicate a short range order of the mesogenic groups analogous to that in low-molecular weight liquid crystals. For polymers with short spacer groups and nematic phases qualitatively different fiber patterns are found. These fiber patterns are indicative for a special nematic phase formed by ribbon-like molecules.     

Spectroscopic investigation on poly[bis(carboxylatophenoxy)]-phosphazene polyelectrolyte interactions with cationic dyes in dilute aqueous solution

The results of spectroscopic investigations on poly[bis(carboxylatophenoxy)]phosphazene, (NaPCPP), interactions with a cationic dye (methylene blue, MB) and with a cationic fluorescent probe ((1-dimethylaminonaphthalene-5-sulfonamidoethyl)trimethylammonium perchlorate, DA) in dilute aqueous solution are reported. The metachromatic behaviour and the DA fluorescence spectral perturbations induced by NaPCPP was measured as a function of polymer concentration and univalent added salt concentration and compared with that found using different, highly charged polyelectrolytes. NaPCPP is shown to strongly bind both dye molecules, with hydrophobic dye/polymer side chain interactions playing a significant role in the aggregation (at low polyelectrolyte/dye molar ratios, P/D) and redistribution (at high P/D values) of the dye molecules along the chains. The findings of this work are discussed also in the light of the results obtained in previous studies on other features of NaPCPP polyelectrolytic behaviour in dilute aqueous solution.     

Frustrated Behavior of a Side‐Chain Liquid Crystalline Polyacrylate

Polyacrylate with phenyl benzoate mesogenic side groups was synthesized and fractionated into narrow fractions. Structure and phase behavior of the narrow‐molecular‐weight samples have been studied by X‐ray diffraction. Depending on the degree of polymerization the polymer shows nematic, smectic A_d, re‐entrant nematic and new phase with two‐dimensional (2D) periodicity. Such behaviour is implicit to so‐called frustrated systems. However, the mesogenic side chains are nonpolar and thus the origin of frustration is different from that in conventional polar liquid crystals. We found that this phenomenon is steric (entropic) in nature and depends on the length of the polymer backbone. The phase transformations of mesogen side chain polyacrylates are discussed in terms of coupling between the smectic ordering of the side groups and the polymer backbone conformations.     

Aggregation states and bistable light switching effects in a side chain ferroelectric liquid crystalline polysiloxane

Aggregation states and transient light scattering in the chiral smectic C phase of a ferroelectric liquid crystalline polymer (FLCP) were studied. The smectic phase shows an interlayer spacing of 38–39.5 Å, which is about 1.41 times the length of the side chain mesogenic group. The transient scattering mode obtained by repeated voltage polarity reversal. A reversible transparent-opaque (light scattering) change was observed in the chiral smectic C state upon application of electric fields with low and high frequencies, respectiely. Both transparent and opaque states were maintained stably even when the electric field was turned off (bistable light swiching). A novel type of electro-optical effect based on light scattering was obtained for the FLCP in the chiral smectic C state.