Photoresponsive liquid crystalline and amorphous polymers

Isomerization processes of azobenzene dyes dissolved in a glassy polymeric matrix or attached in glassy amorphous or liquid crystalline polymers to the backbone as side groups are induced by light. The isomerization process, in turn, causes the dye to reorient provided that polarized light is used: the long axis of the dye is oriented perpendicular to the polarization direction in the stationary case. Such a reorientation gives rise to strong modifications of the optical properties. This contribution is concerned with the analysis of the correlation between the nature of the azobenzene dyes, the isomerization, reorientation and modulations discussed above and with possible applications in the optical holographic storage. Considered are, in particular, dye/matrix combinations giving rise to nonlinear holographic responses, two photon holography, transient holographic modes applicable for holographic displays and the optical switching of other than optical properties.     

Novel photo-crosslinkable liquid crystalline polymers containing vanillylidene cycloalkanones and azobenzene units

Two series of novel liquid crystalline photo-crosslinkable bis(vanillylidene-azobenzene) cycloalkanone containing polymers, namely poly(vanillylidene alkyloxy-4,4'-azobenzenedicarboxylic ester)s, have been synthesised from bis[m-hydroxyalkyloxy(vanillylidene)cycloalkanone] (m = 6, 8, 10) with azobenzene dicarbonylchloride by solution polycondensation method at ambient temperature. Polymers with varying spacer lengths have been synthesised and characterised by spectroscopic techniques. These variations have been correlated with the thermal properties and transition temperatures. Thermal transitions were analysed by differential scanning calorimetry (DSC) and the mesophases were identified by hot stage optical polarised microscopy (HOPM). All of the polymers were found to exhibit liquid crystalline properties. Transition temperatures were observed to decrease with increasing spacer length. The thermogravimetric analysis reveals that all of the polymers were stable up to 280°C undergo two-stage decomposition. Using the UV-visible photolysis studies we investigated the simultaneous behaviour of reactivity rates of crosslinking in the vanillylidene unit and isomerisation caused by the azobenzene unit in the photo-crosslinkable main chain liquid crystalline polymers. The photolysis of liquid crystalline bis(vanillylidene)cycloalkanone-based polymers reveals that there are two kinds of photoreactions in these systems: the EZ photoisomerisation of azobenzene unit and 2p+2p addition by vanillylidene units. The EZ photoisomerisation in the liquid crystal phase disrupts the parallel stacking of the mesogens, resulting in the transition from the liquid crystal phase to isotropic phase. The photoreaction involving 2p+2p addition of the bis(vanillylidene)cycloalkanone units in the polymers results in the conjoining of the chains. The cyclopentanone polymers exhibited a faster rate of photolysis than the cyclohexanone polymers.     

液晶聚合物的单层与Langmuir-Blodgett膜

系统研究了手性液晶聚硅氧烷和光致变色液晶聚硅氧烷两个毓的侧链液晶聚合物在空气/水界面的单层行为和Langmuir-Blodgett(LB)膜沉积特性,对LB膜结构与存在的聚集现象进行了系统的表征,并初步探讨了LB膜中液晶聚合物表现的功能性。     

Photoinduced chirality in achiral liquid crystalline azobenzene polymers containing different polar side groups

Circular dichroism (CD) was induced in the films of two achiral liquid crystalline polymers, poly[(4′-(6-(methacryloyloxy)hexyl)oxy)-(4-X)azobenzene] (PM6X, X = C and N for cyano and nitro, respectively), by irradiation with one handed circularly polarized light (CPL) at 442 nm. The CD and UV spectra of the polymeric films suggest that left-CPL induces a left helical structure and right-CPL reverses the resulting structure into a right helix. For PM6C, the photoinduced CD values of the nematic film are much higher than in the casting and isotropic film. The photoinduced chirality of PM6C films is higher than PM6N due to a higher polarity of nitro groups in PM6N. The PM6N’s CD intensity around 450 nm is increased with the irradiation time of linearly polarized light (LPL) followed by left-CPL irradiation. The correlation between azobenzene chromophores and circularly polarized light will be discussed in the paper.     

Photoinduced optical anisotropy in films of photochromic liquid crystalline polymers

The light-induced modification of the optical properties of photochromic liquid crystalline (side-group) polymers (LCPs) containing azobenzene moieties was studied. Films of such polymers were irradiated with unpolarized and linearly polarized light. Unpolarized irradiation results in a modification of the order parameter, whereas the director orientation remains constant. The light-induced disturbance of the supramolecular order is strongly dependent on the structure of the polymer. A correlation with the enthalpic stability of the liquid crystalline phases is given. Linearly polarized irradiation causes a modification of the order parameter and a reorientation of the side-groups towards a direction perpendicular to the electric vector of the actinic light. This reorientation process is caused by an angular-dependent photoselection within the steady state of the photoisomerization of the azobenzene units. The amount and kinetic of the reorientation differ considerably as a function of certain structural features of the polymers, such as the content of photochromic moieties and the lengths of the spacer chains between the polymeric backbone and the rod-like moieties. It is shown that under certain circumstances the angular-dependent photoselection process may cause a reorientation of the non-photochromic moieties by a cooperative process. The result is a light-induced rotation of the optical axis of the LCP and thus an efficient modification of the birefringent and dichroic properties. This effect can be used in optical data storage.     

Phase ordering in blend films of semi‐crystalline and amorphous polymers

A combination of optical and atomic force microscopy (AFM) is used for probing changes in the morphology of polymer blend films that accompany phase ordering processes (phase separation and crystallization). The phase separation morphology of a “model” semi‐crystalline (polyethyleneoxide or PEO) and amorphous (polymethylmethacrylate or PMMA) polymer blend film is compared to previous observations on binary amorphous polymer blend films of polystyrene (PS) and polyvinylmethylether (PVME). The phase separation patterns are found to be similar except that crystallization of the film at high PEO concentrations obscures the observation of phase separation. The influence of film defects (e.g., scratches) and clay filler particles on the structure of the semi‐crystalline and amorphous polymer films is also investigated.     

Synthesis,characterization and recent developments of liquid crystalline polymers

Recent developments of polymer liquid crystals (PLCs) are reviewed. The virial expansion method of Onsager and the lattice model used by Flory to appreciate the most relevant parameters in establishing mesomorphic behavior in polymeric systems are presented. These and other theoretical predictions are confirmed by numerous experiments. Both lyotropic (polymer solutions) and thermotropic (polymer melts) types of PLCs are considered with emphasis placed on the latter. The general properties of mesophases formed by such polymers are surveyed and some chemical structures capable of producing mesophases are classified in relation to their ability to form lyotropic and thermotropic systems. The synthetic routes, the effects of polymer structure on physical properties, and applications of two major classes of lyotropic systems (polypeptides, polyamides) and of a range of potentially important thermotropic polymers are discussed.     

Photo and electrically switchable behavior of azobenzene containing pendant bent‐core liquid crystalline polymers

New class of photo and electrically switchable azobenzene containing pendant bent‐core liquid crystalline monomers ( AZBM 1, 2 , and 3 ) and their polymers ( AZBP 1, 2 , and 3 ) are reported. The synthesized precursors, monomers, and polymers were characterized by FT‐IR, ¹H, and ¹³C NMR spectroscopy. Thermal stability of polymers was examined by thermogravimetric analysis and revealed stable up to 260 °C. The mesophase transition of monomers and polymers are observed through polarized optical microscopy (POM) and further confirmed by differential scanning calorimetry (DSC). The electrically switching property of monomers and their polymers were studied by electro‐optical method. Among the three monomers AZBM 1, 2 , and 3 , AZBM 1 and 2 exhibit antiferroelectric (AF) switching and AZBM 3 exhibits ferroelectric (F) switching behavior. On the other hand, low molecular weight polymers ( AZMP 1, 2 , and 3 ) show weak AF and F switching behavior. The photo‐switching properties of bent‐core azo polymers are investigated using UV‐vis spectroscopy, trans to cis isomerization occurs around 25 s for AZBP‐1 and 30 s for AZBP‐2 and 3 in chloroform, whereas reverse processes take place around 80 and 90 s. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Tailoring the liquid crystalline property via controlling the generation of dendronized polymers containing azobenzene mesogen

The first‐ and second‐generation dendronized polymers containing azobenzene mesogen were designed and successfully synthesized via free radical polymerization. The chemical structures of the monomers were confirmed by elemental analysis, ¹H NMR, and ¹³C NMR. The molecular characterizations of the polymers were performed with ¹H NMR and gel permeation chromatography. The phase structures and transition behaviors were studied using differential scanning calorimetry, polarized light microscopy, and small‐angle X‐ray scatter experiments. The experiment results revealed that the first‐generation dendronized polymer exhibited liquid crystalline behavior of the conventional side‐chain liquid crystalline polymer with azobenzene mesogen, that is, the polymer exhibited smectic phase structure at lower temperature and nematic phase structure at higher temperature. However, the second‐generation dendronized polymers exhibited more versatile intriguing liquid crystalline structures, namely smectic phase structure at lower temperature and columnar nematic phase structure at higher temperature, and moreover, the phase structure still remained before the decomposition temperature. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1149–1159, 2010     

Synthesis and characterization of new thermotropic liquid crystalline polymers with paired mesogens containing azobenzene unit: Poly(malonic ester)s

New thermotropic liquid crystalline poly(malonic ester)s with two symmetrical mesogenic units (paired mesogens) were prepared by the polycondensation of 1,6-dibromohexane and novel malonic ester compounds with different flexible ethyleneoxy spacer lengths linked to azobenzene group. All the prepared malonic esters and the resulting polymers exhibited monotropic liquid crystallinity with a nematic texture. The length of the spacer had influence on the temperature of phase transition of them. A possibility of application of the prepared polymers to reversible optical information storage media was also identified through a trans–cis isomerization of azobenzene by UV irradiation.     

Synthesis and characterization of halogen-containing ferroelectric liquid crystals and side chain liquid crystalline polymers

A new series of ferroelectric liquid crystals and side chain liquid crystalline polymers based on halogen-containing chiral centres has been synthesized. Chemical structures were analysed by NMR. Liquid crystal phases were characterized by differential scanning calorimetry, optical polarizing microscopy, and X-ray diffractometry. The behaviour of the liquid crystalline phases was investigated as a function of spacer units and differing terminal asymmetric moieties. It was found that phase transition temperatures decreased with increasing length of the oligooxyethylene spacer unit. Differing terminal asymmetric moieties led to differing mesophase phenomena. Furthermore, a wide temperature range (including room temperature) of a chiral smectic C phase was achieved.     

Synthesis and characterization of halogen-containing ferroelectric liquid crystals and side chain liquid crystalline polymers

A new series of ferroelectric liquid crystals and side chain liquid crystalline polymers based on halogen-containing chiral centres has been synthesized. Chemical structures were analysed by NMR. Liquid crystal phases were characterized by differential scanning calorimetry, optical polarizing microscopy, and X-ray diffractometry. The behaviour of the liquid crystalline phases was investigated as a function of spacer units and differing terminal asymmetric moieties. It was found that phase transition temperatures decreased with increasing length of the oligooxyethylene spacer unit. Differing terminal asymmetric moieties led to differing mesophase phenomena. Furthermore, a wide temperature range (including room temperature) of a chiral smectic C phase was achieved.     

Supramolecular side chain liquid crystalline polymers assembled via hydrogen bonding between carboxylic acid-containing polysiloxane and azobenzene derivatives

Supramolecular side chain liquid crystalline polymers were prepared from poly(3-carboxypropylmethylsiloxane) (PSI100) and azobenzene derivatives through intermolecular hydrogen bonding (H-bonding) between the carboxylic acid groups in the PSI100 and the imidazole rings in the azobenzene derivatives. The existence of H-bonding has been confirmed using FTIR spectroscopy. The polymeric complexes behave as liquid crystalline (LC) polymers and exhibit stable mesophases. The LC behaviour of these H-bonded polymeric complexes was investigated by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction. The complexes exhibit nematic LC phases identified on the basis of Schlieren optical textures. On increasing spacer length or the concentration of the H-bonded mesogenic unit in the complex, the clearing temperature and the temperature range of the LC phase of the polymeric complex increase. The terminal group plays a critical role in determining the LC properties of the polymeric complexes. A terminal methoxy group is more efficient than a nitro group in increasing the clearing temperature. The electron donor-acceptor interactions between the H-bonded mesogenic units containing methoxy and nitro terminal groups in supramolecular 'copolymeric' complexes lead to an increase in the clearing temperature and a wider temperature range for the LC phase.     

Thin-film polymerization and characterization of Sumitomo's Sumikasuper®-type liquid crystalline polymers

Using the thin film polymerization approach, we have studied the texture evolution when synthesizing a novel liquid crystalline polymer (LCP) system based on Sumikasuper^® LCPs. The main monomers used were p-acetoxybenzoic acid (ABA), 4,4'-biphenol (BP), isophthalic acid (IA), phthalic acid (PA) and terephthalic acid (TA). Polarizing optical microscopy (POM), FTIR, X-ray diffraction (XRD) and atomic force microscopy (AFM) were employed to study the thin film polymerization process and characterize the products. The generation and evolution of liquid crystal phases were monitored; the results revealed that there exists a composition range for the monomers to react and form liquid crystal materials. The critical temperature for LC formation in ABA/BP/IA system decreased with increasing ABA content. FTIR results confirmed the formation of polymers. AFM investigation suggested a similar process of morphological change to that observed using POM. An increasing surface roughness of the thin films with the progress of polymerization was also obtained from AFM analysis. A nematic LC texture of the polymer system was suggested by XRD examination. Results obtained by replacing BP with acetylated BP, and by conducting polymerization using two-monomer systems, suggest that BP units are included in polymers obtained by the thin film polymerization method.     

Synthesis and characterization of novel photoisomerizable liquid crystalline polymers containing cinnamoyl groups

A series of novel liquid crystalline monomers and polymers incorporating phenylbenzoate or phenylcinnamate segments as mesogenic cores have been synthesized to investigate the sensitivity of the photochromic cinnamoyl derivatives and to overcome the defects of the thermal instability of azobenzene. Their liquid crystalline, thermal, and photoinduced properties of all monomers and polymers were characterized. The polymers showed excellent solubility in common organic solvents such as CHCl₃, toluene, and DMF and exhibited good thermal stability with decomposition temperatures (T_d) at 5% weight loss greater than 340 °C and about 50% weight loss occurred beyond 430 °C under nitrogen atmosphere. The pitch length (about 574 nm) of the synthesized cholesteric polymeric film ( CP2 ) was estimated using scanning electron microscopy. These photochromic polymers exhibited strong UV–vis absorption maxima at about 264 or 320 nm. Moreover, photo induced configurational E/Z isomerization further changed the π‐electron conjugation systems leading to a decrease at the π‐π* transition and an increase in the range of 300 nm to 400 nm for photochromic copolymers. The thermal stability of the Z‐structural segment was confirmed by heating the polymer at 50 °C for over 5 h. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1289–1304, 2008     

Supramolecular side chain liquid crystalline polymers assembled via hydrogen bonding between carboxylic acid-containing polysiloxane and azobenzene derivatives

Supramolecular side chain liquid crystalline polymers were prepared from poly(3-carboxypropylmethylsiloxane) (PSI100) and azobenzene derivatives through intermolecular hydrogen bonding (H-bonding) between the carboxylic acid groups in the PSI100 and the imidazole rings in the azobenzene derivatives. The existence of H-bonding has been confirmed using FTIR spectroscopy. The polymeric complexes behave as liquid crystalline (LC) polymers and exhibit stable mesophases. The LC behaviour of these H-bonded polymeric complexes was investigated by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction. The complexes exhibit nematic LC phases identified on the basis of Schlieren optical textures. On increasing spacer length or the concentration of the H-bonded mesogenic unit in the complex, the clearing temperature and the temperature range of the LC phase of the polymeric complex increase. The terminal group plays a critical role in determining the LC properties of the polymeric complexes. A terminal methoxy group is more efficient than a nitro group in increasing the clearing temperature. The electron donor-acceptor interactions between the H-bonded mesogenic units containing methoxy and nitro terminal groups in supramolecular 'copolymeric' complexes lead to an increase in the clearing temperature and a wider temperature range for the LC phase.     

Preparation and characterization of side-chain liquid crystalline polymers containing chenodeoxycholic acid residue

A series of new side-chain liquid crystalline polymers containing chenodiol residue derived from 24-[4′-hydroxybiphenyl-4-yl-4-(allyloxy)benzoyloxy]-3α,7α-di{n-[4′-(4-ethoxybenzoyloxy)biphenyl-4-yloxy]-n-oxoalkanoyloxy}-5β-cholane was designed and prepared. The chemical structures of the monomer and polymer were confirmed by Fourier transform infrared and ¹H NMR spectra. The mesomorphic properties of monomer and polymer were investigated by differential scanning calorimetry, thermogravimetric analysis, polarizing optical microscopy, and X-ray diffraction. The side-chain liquid crystalline polymers revealed wide mesophase temperature range and high thermal stability, and they showed nematic liquid crystalline phase. The influence of flexible space group length on thermal properties and specific rotation was examined.     

Synthesis and nonlinear optical properties of side chain liquid crystalline polymers containing azobenzene push–pull chromophores

Azobenzene monomeric precursors bearing piperazine as donor moiety with different withdrawing groups and derived side chain polymethacrylates have been prepared and characterized. Monomers having terminal cyano or nitro groups, and the corresponding polymers, exhibited smectic A phases. Linear and nonlinear optical properties of every monomer and thin films of the cyano polymer ( pol‐PZ‐CN ) have been also studied. UV‐vis spectroscopy revealed out‐of‐plane orientation in the as prepared films, as confirmed by waveguide refractive index measurements. Moreover, absorption spectra indicated the presence of azo aggregates in these films. The initial molecular arrangement has been modified by applying thermal annealing within the mesophase range and UV‐blue irradiation. Although thermal annealing resulted in a significant amplification of the out‐of‐plane optical anisotropy due to thermotropic self‐organization of side chain azo moieties, irradiation with 440 nm light induced some disruption of aggregates. The nonlinear optical response of Corona poled films has been studied by second harmonic generation measurements, and the influence of the molecular arrangement on the nonlinear d_ij coefficients has been analyzed. The more efficient poling corresponded to preirradiated films. In any case, a noticeable degree of polar order (70% of the initial d₃₃ value) remained for several months after the poling in films kept at RT. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 232–242, 2010