A new synthetic strategy for the preparation of macroscopically oriented smectic main chain liquid crystal elastomers (MC‐LCE) by the photocrosslinking of laterally functionalized polyesters is presented. X‐ray measurements proved the formation of the monodomain and allowed a quantitative determination of the order parameter. The thermo‐actuation of our material at the phase transition from the liquid‐crystalline to the isotropic phase was demonstrated by temperature dependent measurements of the sample length, showing a fully reversible shape change of ≈40%. Mechanical measurements showed that the sample can be stretched by up to 60% along the smectic layer normal without any reorientation of the structure, in contrast to earlier experiments and theoretical predictions. The results are discussed in the context of limited smectic layer correlation in different types of smectic materials.magnified image

The strained sila-[1]ferrocenophane monomer Fe(η-C5H3 t Bu)2SiPh2 (5) was synthesized and polymerized via thermal ring-opening polymerization (ROP) both in the melt and in solution to give the soluble high-molecular-weight polyferrocenylsilane (PFS) [Fe(η-C5H3 t Bu2)2SiPh2] n , 7 (M n = 5.2 × 104−1.7 × 106 Da, PDI = 1.38−1.99). Both monomer 5 and polymer 7 were structurally characterized by 1H, 13C, and 29Si NMR spectroscopy and elemental analysis. The electronic structure and electrochemical properties of the monomer 5 and the polymer 7 were investigated by UV−vis spectroscopy and cyclic voltammetry. The oxidation potentials were shifted to more negative values relative to non-methylated or methylated analogues, indicating that the electron-donating effect of the t Bu group is transmitted to the iron center. Polymer 7 exhibited two reversible redox waves with a redox coupling, ΔE 1/2, of 0.33 V, which is indicative of appreciable Fe···Fe interactions along the polymer backbone. The resiliency of 7 toward the redox processes was probed by measuring the molecular weights before photooxidation or chemical oxidation (with tetracyanoethylene (TCNE) or tris(4-bromophenyl)ammoniumyl hexachloroantimonate) and after subsequent reduction of the oxidized polymers to the neutral forms by using bis(pentamethylcyclopentadienyl)iron(II). Significantly, unlike the previous studies of oxidized PFS homopolymers, soluble polymeric salts were obtained even for a high degree of oxidation, and these were characterized by UV−vis and IR spectroscopy. GPC measurements demonstrated the excellent redox resiliency of polyferrocenylsilane 7 to redox cycling as no appreciable decline in molecular weight was apparent for degrees of oxidation of up to 50%.

This paper presents the synthetic route to SmA LC main‐chain polymers, that can be (photo)crosslinked without solvent in the bulk phase. They are based on soluble polymalonates, in which higher ordered phases can be suppressed by copolymerization with a laterally brominated biphenyl. Two routes were developed to incorporate the crosslinkable groups into the polyester backbone. The first consists in the incorporation of phenols into the polyester. These phenols are not reactive enough to participate in the transesterification reaction used to build up the polymer, but they can be esterified afterwards with acrylates. Thermally or photochemically created radicals then start the crosslinking. The second route is based on the incorporation of benzophenone as side group. It allows a photochemical crosslinking. Crosslinked fibers (monodomains) show the potential of the smectic LC main‐chain elastomers as actuators.magnified image

The photoresponse of ferroelectric smectic side‐chain liquid‐crystalline (LC) polymers containing a photoisomerizable azobenzene derivative as a covalently linked photochromic side group is investigated. By static measurements in different photostationary states, the effect of trans–cis isomerization on the material's phase‐transition temperatures and its ferroelectric properties (spontaneous electric polarization PS and director tilt angle θ) are analyzed. It turns out that the Curie temperature (transition SC* to SA) can be reversibly shifted by up to 17 °C. The molecular mechanism of this “photoferroelectric effect” is studied in detail using time‐resolved measurements of the dye's optical absorbance, the director tilt angle, and the spontaneous polarization, which show a direct response of the ferroelectric parameters to the molecular isomerization. The kinetics of the thermal reisomerization of the azo dye in the LC matrix are evaluated. A comparison to the reisomerization reaction in isotropic solution (toluene) reveals a faster thermal relaxation of the dye in the LC phase.

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.magnified imageThe trans‐cis isomerization in homeotropically oriented LC elastomers.