Shear induced molecular alignments of a side-chain liquid crystalline polyacetylene containing biphenyl mesogens

Mesomorphic properties of a side chain liquid crystalline polyacetylene, poly(11-{[(4'-heptyloxy-4-biphenylyl)carbonyl]oxy}-1-undecyne) (PA9EO7), are investigated using polarized optical microscope, X-ray diffraction, and transmission electron microscope. Polymer PA9EO7 forms enantiotropic smectic A and smectic B phases. It also exhibits an additional high order smectic phase, a sandwich structure consisting of different molecular packing of biphenyl mesogenic moieties from that of alkyl spacers and terminals, when it is prepared from its toluene solution. Shearing the polymer film at its smectic A phase generates banded texture with the alignment of the backbones parallel to the direction of shear force. While at its high order smectic phase, the mesogen pendants of the polymer are arranged parallel to the direction of shear. The different mesomorphic behaviors arise from different molecular alignments influenced by the fluidity.

Synthesis and characterization of chiral smectic side-chain liquid crystalline polysiloxanes and ionomers containing sulfonic acid groups

The synthesis of new chiral smectic A (S-A) side-chain liquid crystalline polysiloxanes (LCPs) and ionomers (LCIs) containing 4-allyloxy-benzoyl-4-(S-2-ethylhexanoyl) p-benzenediol his ate (ABB) as mesogenic units and 4-[[4-(2-propenyloxy)phenyl] azo]benzensulfonic acid (AABS) as nonmesogenic units is presented. The chemical structures of the monomers and polymers are confirmed by FTIR spectroscopy or H-1-NMR. Differential scanning calorimetry (DSC), optical polarizing microscopy, and X-ray diffraction measurements reveal that all the polymers P-I-P-IV and ionomers P-V-P-VI exhibit S-A texture. The results seem to demonstrate that the tendency toward the S-A-phase region increases with increasing sulfonic acid concentration, and the thermal stability of the S-A phase is determined by the flexibility of the polymer backbones and the interactions of sulfonic acid groups. (C) 2001 John Wiley & Sons, Inc.

Synthesis and characterization of novel crosslinkable side-chain liquid crystalline polyesters

Crosslinkable side-chain liquid crystalline polyesters PCn from N-[n-(4-(4-nitrophenylazo)phenyloxy)alkyl]diethanolamine (Cn, n = 3, 5, 6, 10) as mesogenic monomers and maleic anhydride were synthesized and characterized. The thermal properties of PCn's were studied by means of DSC, polarized optical microscopy (POM) and wide angle X-ray diffraction (WAXD), and the results showed that all the polymers studied exhibit enantiotropic liquid crystallinity. In the molar mass independent region, the relatively high content of cis -CH=CH- groups in the polymer backbone of PC3 causes an increase of the melting temperature (T-m) and a decrease of T-g and isotropisation temperature (T-i). The crosslinking of PCn in the radical polymerization with styrene was confirmed by FTIR spectroscopy. The absorption band at 1300 cm(-1) attributed to the in-plane C-H-bending vibration of trans -CH=CH- in the polymer backbone disappeared after crosslinking, indicating that the trans -CH=CH- functions are consumed in the crosslinking polymerization of styrene.

Synthesis and Characterization of Side-Chain Liquid Crystalline (M6MPP/M6NPAP) Copolymers with NLO Properties

A series of novel thermotropic side-chain liquid crystalline polymer based on polymethacrylate backbone containing electron-accepting 4-(4'-nitrophenylazo)phenoxy as nonlinear optical active group and electron-donating 4(4'-methoxyphenyl) phenoxy group as mesogen attached covalently to the backbone through the flexible spacer was prepared and characterized, respectively. The results from differential scanning calorimetry showed that these series of copolymers were enantiotropic liquid crystal with single mesophase. The melting points and the relative enthalpy change of the copolymers depressed with increasing the molar percent of 4'-nitroazobenzene monomer units over 0 similar to 50mol%, but the enthalpies change of the transition from mesophase to isotropic state increased for the copolymers containing 0 similar to 50mol% 4'-nitroambenzene units. The texture observed under polarized optical microscope identified that the copolymers containing 24molar% or more than 24mol% 4-nitroambenzene monomer units could form smectic mesophase with the focal-conic texture. The results detected by WAXD were in good agreement with the results observed by POM.

Studies on the side chain liquid crystalline copolymer containing nonlinear optical groups

A new side chain liquid crystalline poly[4-(methacryloxy)hexanoloxy-4'-benzyloxy biphenyl] was synthesized. The phase behavior of the monomer and homopolymer was studied. The monomer shows a smectic B phase, while the homopolymer shows a nematic phase. The nonmesogenic nonlinear optical group was introduced to synthesize a series of side chain liquid crystalline copolymers which also show a nematic phase. Owing to the liquid crystallinity of the copolymer has been the orientational stability improved, down the relax of the orientation slowed and the nonlinear optical properties enhanced.

Synthesis and characterization of the side chain liquid crystalline poly(monoester {6-[4-(p-nitrophenyl)azo]phenoxy-1-hexyloxy} of maleic anhydride)

The poly(monoester (6-[4-(p-nitrophenyl) azo]phenoxy-1-hexyloxy) of maleic anhydride) shows a smectic phase with a focal conic fan texture. With the decrease of the monoestering degree the phase transition temperature decreases and the mesomorphic temperature range becomes narrow. The hydrogen bonding between two carboxylic acid groups was found to play a very important role in forming the smectic phase structure. The smectic bilayer structure has been built through self-assembly via. intermolecular hydrogen bonding.

Synthesis and thermal transitional properties of side-chain liquid crystalline polyacrylates containing para-nitroazobenzene as mesogenic groups

The synthesis and characterization of side-chain liquid crystalline (LC) polyacrylates containing para-nitroazobenzene (Pn) as mesogenic groups were described. Homopolymers with 3 and 4 carbon atoms in the spacers were non-LC polymers; for homopolymers with 6 carbon atoms in the spacer, nematic LC behavior was observed. Copolymers with acrylic acid as one component exhibited an S-Ad phase according to the WAXD results which showed the d/l of 1.4-1.54 for the copolymers with 3, 4, and 6 carbon atoms in the spacers. Considering the molecular structure as well as the WAXD results of the copolymers, the possible molecular arrangement in the smectic Sad phase was proposed, in which the smectic layers were composed of the antiparallel mesogens and the antiparallel arrangement was considered to be enhanced due to the H bond between - COOH and - NO2. The stress-induced orientational phenomena of Pn in the LC states was also discussed. (C) 1996 John Wiley & Sons, Inc.

Electro-optical properties of liquid crystal copolymers and their relationship to structural order

A range of side chain liquid crystal copolymers have been prepared using mesogenic and non-mesogenic units. It is found that high levels of the non-mesogenic moieties may be introduced without completely disrupting the organization of the liquid crystal phase. Incorporation of this comonomer causes a marked reduction in the glass transition temperature (Tg), presumably as a result of enhanced backbone mobility and a corresponding lowering of the nematic transition temperature, thereby restricting the temperature range for stability of the liquid crystal phase. The effect of the interactions between the various components of these side-chain polymers on their electro-optic responses is described. Infrared (i.r.) dichroism measurements have been made to determine the order parameters of the liquid crystalline side-chain polymers. By identifying a certain band (CN stretching) in the i.r. absorption spectrum, the order parameter of the mesogenic groups can be obtained. The temperature and composition dependence of the observed order parameter are related to the liquid crystal phase transitions and to the electro-optic response. It is found that the introduction of the non-mesogenic units into the polymer chain lowers the threshold voltage of the electro-optic response over and above that due to the reduction in the order parameter. The dynamic electro-optic responses are dominated by the temperature-dependent viscosity and evidence is presented for relaxation processes involving the polymer backbone which are on a time scale greater than that for the mesogenic side-chain units.

Liquid crystal polymers: macromolecular design for enhanced performances

During this work, done mainly in the laboratories of the department of Industrial Chemistry and Materials of the University of Bologna but also in the laboratories of the Carnegie Mellon University in collaboration with prof. K. Matyjaszewski and at the university of Zaragoza in collaboration with prof. J. Barberá, was focused mainly on the synthesis and characterization of new functional polymeric materials. In the past years our group gained a deep knowledge about the photomodulation of azobenzene containing polymers. The aim of this thesis is to push forward the performances of these materials by the synthesis of well defined materials, in which, by a precise control over the macromolecular structures, better or even new functionality can be delivered to the synthesized material. For this purpose, besides the rich photochemistry of azoaromatic polymers that brings to the application, the control offered from the recent techniques of controlled radical polymerization, ATRP over all, gives an enormous range of opportunity for the developing of a new generation of functional materials whose properties are determinate not only by the chemical nature of the functional center (e.g. azoaromatic chromophore) but are tuned and even amplified by a synergy with the whole macromolecular structure. Old materials in new structures. In this contest the work of this thesis was focused mainly on the synthesis and characterization of well defined azoaromatic polymers in order to establish, for the first time, precise structure-properties correlation. In fact a series of well defined different azopolymers, chiral and achiral, with different molecular weight and highly monodisperse were synthesized and their properties were studied, in terms of photoexpansion and photomodulation of chirality. We were then able to study the influence of the macromolecular structure in terms of molecular weight and ramification on the studied properties. The huge amount of possibility offered by the tailoring of the macromolecular structure were exploited for the synthesis of new cholesteric photochromic polymers that can be used as a smart label for the certification of the thermal history of any thermosensitive product. Finally the ATRP synthesis allowed us to synthesize a total new class of material, named molecular brushes: a flat surface covered with an ultra thin layer of polymeric chain covalently bond onto the surface from one end. This new class of materials is of extreme interest as they offer the possibility to tune and manage the interaction of the surface with the environment. In this contest we synthesized both azoaromatic surfaces, growing directly the polymer from the surface, and mixed brushes: surfaces covered with incompatible macromolecules. Both type of surfaces acts as “smart” surfaces: the first it is able to move the orientation of a LC cell by simply photomodulation and, thanks to the robustness of the covalent bond, can be used as a command surface overcoming all the limitation due to the dewetting of the active layer. The second type of surface, functionalized by a grafting-to method, can self assemble the topmost layer responding to changed environmental conditions, exposing different functionality according to different environment.

Synthesis and characterization of photo-crosslinkable main-chain liquid-crystalline polymers containing bis(benzylidene)cycloalkanone units

A series of new photo-crosslinkable main-chain liquid-crystalline polymers containing bis(benzylidene)cycloalkanone units have been studied. These units in the polymers function as mesogens as well as photoactive centres. Polyesters with three different bis(4-hydroxybenzylidene)cycloalkanones corresponding to three cycloalkanones, namely cyclopentanone, cyclohexanone and cycloheptanone, have been prepared. Three dicarboxylic acids with ether linkages, which were derived from oligoethylene oxides, namely triethylene glycol, tetraethylene glycol and pentaethylene glycol, have been used as spacers in these polymers. Polymerization was carried out by both solution and interfacial polycondensation; the latter method gave high-molecular-weight polymers. Structural characterizations were done by ultra-violet, infra-red and H-1 nuclear magnetic resonance spectroscopy. Liquid-crystalline properties were studied by differential scanning calorimetry and polarized-light optical microscopy. These polymers show a nematic mesophase. Liquid-crystalline transition temperatures were correlated with polymer structure. The decrease in transition temperature with increase in cycloalkanone ring size was explained in terms of the change in geometrical anisotropy of bis(benzylidene)cycloalkanone units. MNDO (modified neglect of differential overlap) calculations were performed on the model compounds, bis(4-acetyloxybenzylidene)cycloalkanone to elucidate the geometrical variation of the mesogenic units with cycloalkanone ring size. Studies of photolysis reveal the two kinds of photoreactions that proceed in these polymer systems, namely photoisomerization and photo-crosslinking. The former reaction disrupts the parallel stacking of the chromophores and is reflected as an increase in the ultra-violet spectral intensity. The favourability of these two reactions depends on the mobility of the polymer chains. When the photolysis was done below T-g, photo-crosslinking dominates over photoisomerization. Above T-g, photoisomerization is followed by photo-crosslinking. The photosensitivity of the polymers decreases with increase in size of the cycloalkanone ring.

Ordering on multiple lengthscales in a series of side group liquid crystal block copolymers containing a cholesteryl-based mesogen

Hierarchical ordering in a side group liquid crystal block copolymer is investigated by differential scanning calorimetry, polarized optical microscopy, small-angle X-ray and neutron scattering (SAXS and SANS) and transmission electron microscopy (TEM). A series of block copolymers with a range of compositions was prepared by atom transfer radical polymerization, comprising a polystyrene block and a poly(methyl methacrylate) block bearing chiral cholesteryl mesogens. Smectic ordering is observed as well as microphase separation of the block copolymer. Lamellar structures were observed for far larger volume fractions than for coil-coil copolymers (up to a volume fraction of liquid crystal block, f(LC) = 0.8). A sample with f(LC) = 0.86 exhibited a hexagonal-packed cylinder morphology, as confirmed by SAXS and TEM. The matrix comprised the liquid crystal block, with the mesogens forming smectic layers. For the liquid crystal homopolymer and samples with high f(LC), a smectic-smectic phase transition was observed below the clearing point. At low temperature, the smectic phase comprises coexisting domains with monolayer S-A,S-1 coexisting with interdigitated S-A,S-d domains. At high temperature a SA,1 phase is observed. This is the only structure observed for samples with lower f(LC). These unprecedented results point to the influence of block copolymer microphase separation on the smectic ordering.

Coupling and memory in liquid crystal elastomers

The polymer backbone of a side-chain liquid crystal polymer exhibits an anisotropic shape due to the coupling of the liquid crystal orientational order of the mesogenic side-chains to the backbone. The magnitude and sign of this coupling may be controlled by chemical design. The introduction of chemical cross-links in to such a system provides both a memory of the anisotropic organisation and a mechanism by which the microscopic anisotropy can be realised at a macroscopic level. We show how this anisotropic network structure yields new phenomena when electric or mechanical fields are applied.

Side-chain liquid crystalline elastomers: the relationship between the orientational ordering of the polymer backbone and the length of the coupling chain

The influence of cross-linking on the phase behaviour of a series of side-chain liquid crystalline elastomers has been studied. For samples cross-linked in the temperature range corresponding to the nematic phase, the phase transition was shifted compared to that observed when an identical sample was cross-linked in the isotropic phase. This shift represented a stabilisation of the nematic phase in the former case, in line with theoretical expectations. By utilising a novel, slow cross-linking method, which allows the polymer backbone to take up an equilibrium conformation prior to network formation, it proved possible to monitor the shifts in phase transition temperature as a function of the length of the methylene chain coupling the mesogenic units to the polymer backbone. The results obtained are related to the backbone anisotropy and indicate that the level of orientational order of the polymer in the nematic phase backbone increases with a reduction in the length of the coupling chain.