Photoswitchable Smectic Liquid‐Crystalline Elastomers

Summary: We succeeded in the synthesis of azo side chain containing polysiloxanes with broad smectic C* and A phases. In these polymers the phase transition temperatures can be shifted reversibly by up to 17 °C by irradiation with UV (cis) or VIS (trans) light. Thin films of these polymers in the smectic phase (both on substrates and as free‐standing films) orient perfectly in a homeotropic manner. As a consequence, the azo chromophores do no longer absorb during a perpendicular illumination with light (dichroism). It is thus possible to crosslink these films photochemically to prepare “photoswitchable smectic LC elastomers”.

The trans‐cis isomerization in homeotropically oriented LC elastomers.     

Phase Biaxility in Smectic‐A Side‐Chain Liquid Crystalline Elastomers

²H NMR investigations on the biaxial phase behavior of smectic‐A liquid crystalline side‐chain elastomers are presented. Biaxiality parameters were determined by measuring the quadrupolar splitting of two spin probes, namely benzene‐d₆ and hexamethylbenzene‐d₁₈, at various angles between the principal director and the external magnetic field: while for a uniaxial sample the angular dependence can be described by the second Legendre polynomial, an additional asymmetric term needs to be included to fit the data of the two investigated biaxial systems. Two elastomers synthesized from mesogens that differ in the molecular geometry in order to study the molecular origin of biaxiality were compared. Biaxiality is observed for both elastomers when approaching the glass transition, suggesting that the network dynamics dominate the formation of the biaxial phase.

     

Synthesis of a Photoresponsive Liquid‐Crystalline Polymer Containing Azobenzene

The synthesis of an oriented liquid‐crystalline photoresponsive polymer, prepared by polymerization of mono‐ and di‐acrylates, both of which contain azobenzene chromophores, is reported. The prepared free‐standing polymer film shows strong reversible photoinduced deformation upon exposure to unpolarized UV light at 366 nm, as a result of an optically induced isomeric change of the azobenzene moieties in the polymer network. The synthesis process is relatively simple and more efficient compared to conventional ones, and can be used to synthesize other liquid‐crystalline photoresponsive polymers. The use of this photoresponsive polymer film as an optical high‐pass/low‐pass switch under UV or natural light irradiation for a laser beam is demonstrated. This photoresponsive polymer may have applications in robotic systems, artificial muscles, and actuators in microelectromechanical systems (MEMS) and labs on chips.

     

Nanosized Shape‐Changing Colloids from Liquid Crystalline Elastomers

A method to prepare shape‐changing nanospheres from liquid crystalline elastomers is reported. The nanosized colloids are prepared by a miniemulsion process. During this process, colloids are prepared from a liquid crystalline (LC) main‐chain polyester and subsequently crosslinked into a nanometer‐sized LC elastomer. The ability of these LC elastomers to change their shape at the phase transition temperature from the smectic A to the isotropic phase was detected by temperature‐dependent transmission electron microscopy. The phase transition‐induced shape change leads to strongly shape anisotropic nanosized elastomer particles.

     

Phase Separation and Organisation of Colloidal Spheres Suspended in Sheared Lyotropic Liquid‐Crystalline Polymers

Summary: Organisation behaviours of spherical particles suspended in sheared, lyotropic, liquid‐crystalline polymer solutions have been investigated using polarizing optical microscopy. We find that in a nematic phase the particles phase separate and adopt anisotropic chain‐like structures along the director. An earring defect is observed around a single particle whereas a cross or strings defect between neighbouring particles is found to serve as a repulsive barrier to prevent the particles from contacting each other. A theoretical analysis is presented to explain this new phenomenon.

An optical micrograph of 0.01 wt.‐% glass spheres suspended in a nematic solution of 40 wt.‐% ethyl cellulose in chloroform under an external shear force.     

New Architecture of Liquid‐Crystalline Polymers from Fluorinated Vinylcyclopropanes

A new class of fluorinated polymers was prepared by radical ring‐opening homopolymerization of vinylcyclopropane monomers with a perfluorinated (CF₂)_nF chain (n = 6, 8, or 10). The polymers were in fact copolymers composed of 1,5‐linear and cyclobutane isomer units, the relative content of which depended on n. Surprisingly, they formed liquid‐crystalline mesophases (SmB_d and/or SmA_d), which was attributed to phase separation of the incompatible fluorocarbon and hydrocarbon components of the repeat unit.     

The Estimation of Statistical Parameters Determining the Relaxation Times of Nematic Elastomers: A Three‐Chain Network Model

A simplified “three‐chain” network model formed from freely jointed polymer chains consisting of Gaussian elements with fixed mean‐square lengths is proposed for describing local dynamic properties of nematic elastomers. The boundaries of a polymer network are supposed to be fixed when sample volume and shape do not change with ordering. Relaxation times characterising intrachain motions in both isotropic and ordered states are determined by two factors. The first (“dynamic”) factor is related to the friction of chain elements and the second one (“statistical” factor) is determined by statistical mean–square fluctuations of segment projections on the three axes of rectangular frame of reference. The “statistical” factor of relaxation times is calculated here as a function of the order parameter and the parameter characterising the degree of network crosslinking. Statistical factor obtained in the framework of a network model consisting of Gaussian subchains is compared with that calculated here by using freely‐jointed‐rods chain model. Good agreement is shown between statistical factors obtained in the framework of the two chain models considered. This result confirms the validity of describing the dynamics of real rod‐like mesogenic groups in nematic elastomers in terms of a simplified chain model consisting of Gaussian segments with fixed average lengths which do not change with ordering. The influence of “dynamic” factor on the relaxation spectrum of a nematic elastomer is discussed qualitatively.     

Enthalpy Relaxation of Polymers: On the Possible Role of Liquid‐Crystalline Order

Isothermal physical ageing experiments were performed by differential scanning calorimetry to probe the enthalpy relaxation in a methacrylate copolymer carrying azobenzene mesogenic side groups. Further evidence of the ability of the configurational entropy model developed by Gomez Ribelles in describing the structural relaxation mechanism of polymers is provided. The trend of the equilibrium structural relaxation time was also determined as a function of the reduced temperature T_g/T. The comparison of the aging dynamics of the copolymer with those of previous analogous copolymers containing different amounts of azobenzene counits allowed us to highlight effects of the liquid‐crystalline nematic order on the properties of structural relaxation.

     

Precise Control of Photoinduced Birefringence in Azobenzene‐Containing Liquid‐Crystalline Polymers by Post Functionalization

A series of functionalized liquid‐crystalline polymer materials with different degrees of functionality was synthesized by a post Sonogashira cross‐coupling reaction of a polymer precursor. The post‐functionalization was carried out under mild conditions and showed a high yield. Although a highly birefringent azotolane group was introduced into the polymer precursor, the photoresponse of the functionalized liquid‐crystalline materials was not obviously changed. By adjusting the content of azotolane groups, precise control of the photoinduced birefringence was successfully obtained after thermal enhancement upon annealing. The present method to gain precise control of photoinduced birefringence might enable one to finely photocontrol the optical performances of materials, and may have a potential application as an advanced process for photonic materials.

     

Opto‐Mechanical Effect in Photoactive Nematic Side‐Chain Liquid‐Crystalline Elastomers

The mechanical behaviour of monodomain nematic side‐chain liquid‐crystalline elastomers containing azoderivatives as pendant groups or crosslinkers has been studied under UV irradiation and in the darkness at different temperatures. From the evaluation of the opto‐mechanical experiments, the mechanical efficiency, kinetic rates, activation energies and the isomerization mechanism of the azocompounds in the liquid‐crystalline matrix could be determined, as well as the effect of the chemical constitution of the azobenzene derivatives and their role in the elastomeric network.

     

Optical Mechanotransduction with Carbazole‐Based Luminescent Liquid Single‐Crystal Elastomers

Carbazole‐based liquid single‐crystal elastomers (LSCEs) are valuable fluorescent flexible materials to perform optical mechanotransduction under ambient conditions. Indeed, the covalent incorporation of carbazole derivatives into nematic LSCEs allows to tune their luminescence on demand under mechanical control in a quick and reversible fashion. Specifically, the fluorescence intensity for these materials can be switched back and forth in less than a second. Moreover, such a process can be performed several times without detecting any sign of fatigue in the system. In addition, these materials show excellent resistance to aging; 2 years after their preparation they exhibit the very same mechanofluorescent behavior as when freshly prepared. In fact, the here reported fluorescent systems are highly sensitive; the application of a force of 70 mN decreases the fluorescence in the elastomeric material by 7%. Thus, mechanical forces are attractive external stimuli to modulate the fluorescence of nematic elastomers rapidly and reversibly enabling thereby mechanotransduction.

     

Normal‐stress differences and the detection of disclinations in nematic elastomers

We used a continuum model to investigate the isochoric radial expansion of a right circular cylindrical specimen composed of a nematic elastomer that is crosslinked in a uniaxial state and then annealed. Numerical solutions show that, above a certain radial expansion, the material has a definitive energetic preference for a state involving a disclination of strength +1 along the cylinder axis. Surrounding such a disclination is a core with radial dimension on the order of 10^?2 μm, which coincides with observations in conventional liquid‐crystal melts. Examination of the normal‐stress differences indicates that the first of these differences depends nonmonotically on the extent of radial expansion and possesses a local minimum at the point where a disclination becomes energetically preferred. This suggests a practical experimental method for testing the predictions of our model. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2098–2106, 2002     

Thermotropic Liquid‐Crystalline Polymer Derived from Natural Cinnamoyl Biomonomers

Summary: The compound 4‐hydroxycinnamic acid (4HCA), a natural biomonomer, is polymerized by melt polycondensation to yield a liquid‐crystalline biopolymer (P4HCA) with UV reactivity. L929 cells were successfully incubated on P4HCA films at 37 °C.

Structure of poly(4‐hydroxycinnamic acid) (P4HCA) and its crossed‐polarizing optical micrograph in the nematic state. Inset image: optical micrograph of L929 mouse fibroblasts adhered on P4HCA film after 24 h incubation at 37 °C.     

Helical Mechanoclinic Deformation of Main‐Chain Chiral Smectic Elastomers

The helical mechanoclinic deformation of a main‐chain chiral smectic elastomer, which is prepared by a crosslinking reaction under twist deformation, is investigated. The twist deformation induces a layer tilt angle that depends on the handedness of twist. The layer tilt angle in the right‐handedly twisted elastomer, of which the handedness is consistent with that of the helix in the SmC* phase of the non‐crosslinked backbone polymer, is estimated to be up to 16° at room temperature, although that in the left‐handedly twisted elastomer is less than several degrees. The experiments provide evidence of chiral coupling between tilt and twist for helical mechanoclinic deformation in the chiral smectic system.

     

Lightly crosslinked,mesomorphic networks obtained through the reaction of dimeric,liquid‐crystalline epoxy–imine monomers and heptanedioic acid

We reacted various dimeric, liquid‐crystalline epoxy–imine monomers, differing in the length of the central aliphatic spacer or the dipolar moments, with heptanedioic acid. The resulting systems showed a liquid‐crystalline phase in some cases, depending on the dimer and on the reaction conditions. The systems were characterized with respect to their mesomorphic properties and then were submitted to dynamic mechanical thermal analysis in both fixed‐frequency and frequency‐sweep modes in the shear sandwich configuration. The arrangement in the liquid‐crystalline phase seemed to be mainly affected both by the polarization of the mesogen and by the reaction temperature, which favored the liquid‐crystalline arrangement when it was lying in the range of stability of the dimer mesophase. In agreement with other recent literature data, dynamic mechanical thermal analysis results suggested that the presence of the mesogen directly incorporated into the main chain increased the lifetimes of the elastic modes both in the isotropic phase and in the liquid‐crystalline phase with respect to side‐chain liquid‐crystalline elastomers and that the time–temperature superposition principle did not hold through the liquid‐crystalline‐to‐isotropic transition. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44:6270–6286, 2006     

Studies on Lyotropic Liquid‐Crystalline N‐Alkyl Chitosans in Formic Acid

A series of derivatives of chitosan – N‐alkyl(methyl, ethyl, propyl and butyl) chitosans – were synthesized from completely deacetylated chitosan. The degree of substitution (from 0.15 to 0.81) of the N‐ethyl chitosan were obtained by controlling the molar ratio of the reactants. All the products showed lyotropic liquid‐crystalline properties regardless of the length of the side chains and the degree of substitution. The critical concentration (C*) of the samples were measured by both microscopy and refractometry. C* seemed not to vary with the degree of substitution (ds) in the case of a given subsitituent chain, but rose dramatically depending on the length of the substituent group as this was varied from methyl to butyl. The results were explained according to Flory's classical theory as well as experimental of X‐ray diffraction measurements.     

A New Chemo‐Enzymatic Route to Side‐Chain Liquid‐Crystalline Polymers: The Synthesis and Polymerization of 6‐(4‐Methoxybiphenyl‐4′‐oxy)hexyl Vinyl Hexanedioate

A novel synthetic method combining chemo and enzymatic synthesis strategies was employed to prepare a vinyl acetate type monomer, 6‐(4‐methoxybiphenyl‐4′‐oxy)hexyl vinyl hexanedioate (VA‐LC). Homo‐ and copolymers of VA‐LC with maleic anhydride (MAn) were prepared by conventional free radical polymerization using 2,2′‐azobisisobutyronitrile (AIBN) and 1,1′‐azobis (cyclohexane carbonitrile) (AHCN) as an initiator at 95 and 60 °C, respectively. The thermal properties of the generated polymeric material were investigated by differential scanning calorimetry (DSC), and the optical texture was inspected by polarizing optical microscopy (POM). While the monomer VA‐LC does not exhibit liquid‐crystalline properties, poly(VA‐LC), and the alternating copolymer of VA‐LC with maleic anhydride both displayed such properties.

     

A Main‐Chain Liquid‐Crystalline Oligomer Prepared by in situ Photopolymerization of an LC Monomer Having Cinnamate Moieties

Summary: A liquid‐crystalline (LC) compound, having a cinnamate moiety on each end of the molecule, was synthesized and irradiated with UV light in its LC phase in the presence of a triplet sensitizer. Various measurements of the irradiated sample revealed that the linearly structured LC oligomers were formed by [2+2] cycloaddition of the cinnamate moieties, and that the resultant cyclobutane units dominantly assumed an anti head‐to‐head configuration.

Schematic structures of the LC oligomer obtained by photopolymerization.     

Side‐Chain‐Functionalized Polyacetylenes, 1. Liquid Crystalline and Stereomutational Properties

Stereospecific polymerizations of functionalized acetylenes were successfully performed using a Rh complex as the catalyst and triethylamine as a base yielding poly(3,4,5‐tridodecyloxyphenylacetylene) ( Poly5 ) and poly[3,4,5‐tris((S)‐3,7‐dimethyloctyloxy)phenylacetylene] ( Poly6 ). Poly5 and Poly6 are liquid crystalline with presumably a bilayer‐type architecture in which the mesogens are arranged in an antiparallel overlapping interdigitated manner. Poly6 showed unusual thermal stereomutation in solution in a very narrow temperature range.     

Liquid‐crystalline thermosets by the curing of dimeric liquid‐crystalline epoxyimine monomers with 2,4‐toluene diisocyanate

We studied the curing processes of several series of dimeric liquid‐crystalline epoxyimine monomers with 2,4‐toluene diisocyanate (TDI) alone or with added catalytic proportions of 4‐(N,N‐dimethylamino)pyridine. We obtained isotropic materials or liquid‐crystalline thermosets with different degrees of order, which depended on the structures of the monomers. To fix ordered networks, we had to do the curing in two steps when TDI was used alone as the curing agent. However, when a tertiary amine was added in catalytic proportions, the ordered networks were fixed in just one step. In this way, we were able to fix both nematic and smectic mesophases. The significance of the polarization of the mesogen for obtaining liquid‐crystalline thermosets was demonstrated. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2521–2530, 2003