PACS numbers: 61.41. + e, 47.50. + d, 64.70.Md, 83.50.Ax
The effect of shear constraint on the behavior of liq- uid crystals or on the behavior of “ordinary” polymers has been studied for more than two decades both experi- mentally and theoretically . In contrast, the investiga- tion of liquid crystal polymer behavior under shear flow has only recently begun . Very interesting behavior can be expected for liquid crystal polymers owing to the competition between the internal strain produced by the side-chain mesogens on the main chain and the external strain brought by the shear process. Recently, we have studied the evolution of the main-chain conformation in the shear plane of a liquid crystal polymethacrylate as a function of the shear rate . It is shown that macro- scopic shear is transmitted at a microscopic level by the smectic layers, ensuring an efficient shear of the poly- mer main chains. The outcome is a macroscopic orienta- tion of the smectic monodomains whose smectic planes are established parallel to the shear plane. The poly- mer main chains already confined by the mesogenic lay- ers become more elongated along the velocity direction with increasing shear (Fig. 1). These results were ob- tained from measurements carried out in situ in the shear plane, whereas other planes were observed on a quenched sample after shearing. In this article we consider mea- surements made in situ in the plane of the velocity gradient (vorticity plane). This plane is particularly in- teresting since it allows the simultaneous observation of the formation of the smectic phase and of the conforma- tion of the polymer main chain versus shear rate. Such a direct study has never been carried out in bulk, even for nonliquid crystallinepolymers. This last experiment  confirmed the theory that bulk polymer conformation un- der shear flow is a tilted elongated shape resulting from a combination of rotational and translational motions. We obtained the same result using our cell with a polysty- rene melt.
FIG. 1. Section of the shear cell ring (placed in a vertical position for the three-axis geometry) defining the three axes 共 y, = ? y, z兲. The scattering vector is defined by q 苷 k i 2 k f ,
where k i is the incident beam and k f is the diffracted one. intensity [5(a),6(a)] and of the layer thickness [Fig. 2(a)] as the temperature decreases. The layer thickness increases about 0.5% for a temperature decrease of 50 ± . This vari- ation is usually explained by an increase of the mesogen length due, in particular, to a mobility loss and thus an ordering of the alkyl part of the mesogen. This layer thick- ness corresponds to the most extended side-chain configu- ration forming thus a monolayer S A1 phase; it is a “rigid”
In the case of side-chainliquid-crystallinepolymers, the smectic phase is a result of the packing of the side-chain land so the primary question in such systems is to know where and how the main chains are arranged. The picture usually accepted is that the main chains and the side chains remain, on average, perpendicular to each other, thus repelling the main chains from the mesogenic zone and concentrating them at the interfaces. This intuitive picture was adopted as a basic assumption for the theoretical predictions concerning the main-chain anisotropy in the smectic phase . The first observations of the main-chain conformation (carried out by Small-Angle Scattering ) shows that the main-chain extension is favoured in the direction perpendicular to the director which is in good agreement with the predictions.
Abstract: Very few studies concern the isotropic phase of Side-ChainLiquid-CrystallinePolymers (SCLCPs). However, the interest for the isotropic phase appears particularly obvious in flow experiments. Unforeseen shear-induced nematic phases are revealed away from the N-I transition temperature. The non-equilibrium nematic phase in the isotropic phase of SCLCP melts challenges the conventional timescales described in theoretical approaches and reveal very long timescales, neglected until now. This spectacular behavior is the starter of the present survey that reveals long range solid-like interactions up to the sub-millimetre scale. We address the question of the origin of this solid-like property by probing more particularly the non-equilibrium behavior of a polyacrylate substituted by a nitrobiphenyl group (PANO2). The comparison with a polybutylacrylate chain of the same degree of polymerization evidences that the solid-like response is exacerbated in SCLCPs. We conclude that the liquid crystal moieties interplay as efficient elastic connectors. Finally, we show that the “solid” character can be evidenced away from the glass transition temperature in glass formers and for the first time, in purely alkane chains above their crystallization temperature. We thus have probed collective elastic effects contained not
have demonstrated that the chain is deformed at times scales longer than the viscoelastic the terminal time. These observations imply that extra long length and time scales exist in the melt which do not originate from mesomorphic properties. Finally, our present study supports and extends at a much larger macroscopic scale, the first observations of elasticity carried out with a piezorheometer . Indeed, from the comparison of the results obtained with two different techniques, using different substrates, with different polymers, at different temperatures away from transition temperatures, similar solid-state properties have been evidenced. These similarities demonstrate the fundamental elastic character of LC- polymers which are wrongly considered as flowing fluids above the glass transition temperature. It is also interesting to note that previous studies , regarding the low stress creep behaviour of a nematic main- chainliquidcrystalline polymer, had also reported on a solid-like behaviour at T<T NI . It was interpreted as
ethyl-oxy]coumarin} (PAzoMACMA). The polymer’s side groups comprise photo- isomerizable azobenzene in majority and photo-dimerizable coumarin in minority, with the former as mesogens and the latter for intra-chain photo-crosslinking. Despite the sub-15 nm size, confinement and crosslinking, the liquidcrystalline (LC) phases of bulk PAzoMACMA persist in SCLCPs. Such LC-SCNPs exhibit a number of interesting and peculiar properties. While their dispersion in THF is non-fluorescent, when dispersed in chloroform, the nanoparticles appear to agglomerate to certain degree and display significant fluorescence that is different for SCNPs rich in the trans or cis isomer of azobenzene. The azobenzene LC-SCNPs also undergo photo-induced deformation, similar to azobenzene micro- or colloidal particles. However, the elongational deformation of the nanoparticles is dependent upon the linearly polarized excitation wavelength. While under polarized 365 nm UV irradiation the SCNP stretching direction is perpendicular to the light polarization, under polarized 400-500 nm visible light irradiation, the stretching takes place along the light polarization direction. Finally, an all-polymer nanocomposite was prepared by dispersing the LC-SCNPs in poly(methyl methacrylate) (PMMA), and mechanically stretching-induced orientation of azobenzene mesogens developed along the strain direction. The interesting properties of LC-SCNPs unveiled in this study suggest new possibilities for applications including bio-imaging and LC materials.
chain mobility in this region is instrumental in mediating action of positive and negative allosteric modulators and their respective stabilization of the active or inhibited receptor state ( Hackos et al., 2016 ; Yi et al., 2016 ). A straightforward explanation of the partial nature of UV inhibition of GluN1- P532PSAA receptors is the presence of mixed subunits harboring either a PSAA or a natural amino acid, the later contributing to light-insensitiveness. This possibility could be excluded, however, based on our control experiments performed in the absence of the UAA showing negligible unspe- cific background (at the GluN1-P532 site and all other positions tested; Figure 2—figure supple- ment 3 and Figure 6—figure supplement 2 ). Since the absorption spectra of the azobenzene trans- and cis-states overlap substantially ( Beharry and Woolley, 2011 ; Hoppmann et al., 2014 , 2011 ), UV irradiation produces a photostationary state with <100% of the cis-isoform. This limited cis-isomer induction is also unlikely to account for the incomplete nature of UV inhibition of maxi- mally-activated GluN1-P532PSAA receptors. Indeed, nearly 95% of photoinhibition was achieved when the glycine concentration was reduced to 1 mM ( Figure 4d ). This indicates that under our con- ditions where light duration and intensity are not limiting ( Figure 3—figure supplement 2 ), the vast majority of the azobenzene side chains had switched to the cis-state following UV exposure, in good agreement with previous estimations with PSAA introduced into small model peptides ( Hoppmann et al., 2011 ). Hence, the incomplete UV inhibition of GluN1-P532 receptors unlikely stems from an inefficient PSAA photochemistry. Rather, it likely finds its origin in the biological mechanism underlying the UV photoinhibition.
It may seem odd to devote so much eﬀort to achieving good docking performance on data sets with backbone conforma- tions in the bound state since these will rarely be encountered in real-world scenarios. An even stronger case might be made against the relevance of docking performance with side chains in the BB state. However, the ability to dock under these more favorable conditions can be seen as an advantage and a minimum requirement in order to address the more diﬃcult unbound docking problems where backbone sampling will need to be introduced. Of course, this advantage can only be fully realized if and when a robust backbone sampling method that visits the bound state is achieved, which remains a very challenging task. For many current algorithms, the impaired ability to dock these idealized cases by softening their scoring functions is presented as an acceptable trade-oﬀ for achieving some success in the true unbound cases. However, in our opinion such a compromise greatly limits their ability to take advantage of any future improvement in backbone sampling as demonstrated by their modest docking success when presented
(18) Carlton, R. J.; Hunter, J. T.; Miller, D. S.; Abbasi, R.; Mushenheim, P. C.; Tan, L. N.; Abbott, N. L. Chemical and biological sensing using liquid crystals. Liq. Cryst. Rev. 2013, 1, 29−51.
(19) (a) Woltman, S. J.; Jay, G. D.; Crawford, G. P. Liquid-crystal materials find a new order in biomedical applications. Nat. Mater. 2007, 6, 929 −938. (b) Bera, T.; Freeman, E. J.; McDonough, J. A.; Clements, R. J.; Aladlaan, A.; Miller, D. W.; Malcuit, C.; Hegmann, T.; Hegmann, E. Liquid Crystal Elastomer Microspheres as Three- Dimensional Cell Scaffolds Supporting the Attachment and Proliferation of Myoblasts. ACS Appl. Mater. Interfaces 2015, 7, 14528 −14535. (c) Worthington, K. S.; Green, B. J.; Rethwisch, M.; Wiley, L. A.; Tucker, B. A.; Guymon, C. A.; Salem, A. K. Neuronal Differentiation of Induced Pluripotent Stem Cells on Surfactant Templated Chitosan Hydrogels. Biomacromolecules 2016, 17, 1684− 1695. (d) Martella, D.; Paoli, P.; Pioner, J. M.; Sacconi, L.; Coppini, R.; Santini, L.; Lulli, M.; Cerbai, E.; Wiersma, D. S.; Poggesi, C.; Ferrantini, C.; Parmeggiani, C. Tissue Engineering: LiquidCrystalline Networks toward Regenerative Medicine and Tissue Repair. Small 2017, 13, 1702677−1702677. (e) Hirst, L. S.; Charras, G. Liquid crystals in living tissue. Nature 2017, 544, 164−165. (f) Prévôt, M. E.; Andro, H.; Alexander, S. L. M; Ustunel, S.; Zhu, C.; Nikolov, Z.; Rafferty, S. T.; Brannum, M. T.; Kinsel, B.; Korley, L. T. J; Freeman, E. J.; McDonough, J. A.; Clements, R. J.; Hegmann, E. Liquid crystal elastomer foams with elastic properties specifically engineered as biodegradable brain tissue scaffolds. Soft Matter 2018, 14, 354−360. (20) (a) Donnio, B.; Guillon, D.; Bruce, D. W.; Deschenaux, R. Comprehensive Coordination Chemistry II: From Biology to Nano- technology; McCleverty, J. A.; Meyer, T. J., Eds.; Elsevier: Oxford, U.K., 2003; Vol. 7, pp 357 −627. (b) Pucci, D.; Donnio, B. Metal- containing liquid crystals. In Handbook of Liquid Crystals. Goodby, J. W., Collings, P. J., Kato, T., Tschierske, C., Gleeson, H., Raynes, P., Eds.; Wiley−VCH:Weinheim, Germany, 2014; Vol. 5.
the rapid diversity-oriented synthesis of hybrid macrocyclic peptide libraries with varied chemical and structural complexities.
Cyclic polypeptides represent a unique class of naturally occurring biomolecules often with enhanced biological properties compared to their linear counterparts. 1 These bioactive macromolecules can be found in a number of bacteria, plants, and mammals. 2 Chemists have aimed to synthesize hybrid macrocyclic polypeptides that mimic the structure and function of naturally produced bioactives. 3 Side-chain to side-chain macrocyclization has emerged as one method to access such species. This approach entails the cross-linking between two or more side-chain functional groups with natural or non-natural amino acid residues. Several methods exist, including nucleophilic substitution with benzyl/allyl halide electrophiles, 4 a myriad of carbon-carbon 5 and carbon-nitrogen 6 bond-forming processes, cycloadditions, 7 disulfide formation, 8 metal-based coordination 9 and non-covalent routes. 10 Each approach may have drawbacks associated with chemoselectivity, tunability, and synthetic practicality. Most importantly, regions within a polypeptide are often found to undergo inefficient macrocyclization with certain chemical approaches. These limitations reduce the ability to synthesize analogues for potential therapeutic applications. 11
ID2 high brilliance beamline at the European Synchrotron Radia- tion Facility (ESRF) 16 . Experiments performed on very dilute isotropic suspensions showed an X-ray scattering intensity, I, that monotonously decreases with scattering vector modulus q (q 4p sin v=l, where 2v is the scattering angle) as I ~ q 22 in all the range of concentrations studied (0:033% , f , 0:18%). This scattering law is typical of two-dimensional planar objects, and its q-range shows that sheets are of considerable lateral spatial exten- sion (at least 300 nm). These experiments prove that the layers remain ¯at in suspension and do not crumple or fold. At higher volume fractions, typical diffraction patterns of unoriented samples of birefringent suspensions (Fig. 4a) display many sharp peaks (up to 12) that can be indexed to the 001 re¯ections of a lamellar phase. The small width of these re¯ections proves that long-range posi- tional ordering does take place, as expected for a lyotropic lamellar phase. The numerous higher-order re¯ections observed indicate that there is very little positional ¯uctuation 17,18 and that the lamellar positional order is very high. Even at maximum swelling, quite a large number (up to 7) of lamellar re¯ections are still detected. This contrasts with more usual swollen lamellar phases comprised of liquid-like layers, and suggests that this lamellar phase comprised of solid-like sheets has very different elastic constants.
The chiral cholesteric-liquid-crystalline structure, which concerns the organization of chromatin, collagen, chitin, or cellulose, is omnipresent in living matter . In technology, it is found in temperature and pressure sensors, supertwisted nematic liquid crystal displays, optical filters, reflective devices, or cosmetics . The cholesteric liquid crystal (CLC) phase exhibits a helical structure with a twist axis perpendicular to the local director (Fig. 1.a), which comes in whole or in part from the molecular chirality. The cholesteric phase is characterized by two structural parameters: the helical pitch p and the twist sense. The helical pitch p gives the distance along the helical axis that corresponds to a rotation of 360° in the orientation of the rod-like molecules (Fig. 1.b).
FIGURE 5. IR-thermography taken at steady-state temperature (a) and the back surface temperature of PE plate obtained via
numerical and experimental analyses (b)
The model presented in this study proposes a numerical approach to consider the optical scattering and the change in its behavior under heating condition indirectly, without modeling how the light scatters inside of polymer medium. In other words, the effect of optical scattering on the optical characteristics of semi-crystalline PE was taken into account without modeling the spatial distribution of the scattered light intensity which offers computationally cost-effective numerical solutions. The change in the directional-hemispherical optical characteristics of PE under heating conditions were experimentally analyzed and, it was demonstrated especially in the temperature ranges close to the melting range of PE and eventually used as numerical input. In this study, the experimental and numerical analyses showed that the model predicts the temperature closely in the middle zone of the PE plate, whereas there is a remarkable difference at regions close to the edge of the plate considering both the computed and measured surface temperature profiles defined on the normal and alongside to the IR-lamp. This may be attributed to ignorance of potential heat losses due to conduction between the barrier and PE plate as the barrier around the plate was not modeled in this study. In addition, the convergence studies for the number of modeled rays were not performed at this step which may cause to change the final temperature field predictions.
a b s t r a c t
Porous catalyst layers (CLs) containing short-side-chain (SSC) perfluorosulfonic acid (PFSA) ionomers of different ion exchange capacity (IEC: 1.3, 1.4 and 1.5 meq g −1 ) were deposited onto Nafion 211 to
form catalyst-coated membranes. The porosity of SSC-PFSA-based CLs is larger than Nafion-CL analogues. CLs incorporating SSC ionomer extend the current density of fuel cell polarization curves at elevated temperature and lower relative humidity compared to those based on long-sidechain PFSA (e.g., Nafion)- based CLs. Fuel cell polarization performance was greatly improved at 110 ◦ C and 30% relative humidity
data. 72 The liquid crystal cells are made up by two glass plates covered by a 100 Ω / square Indium Tin Oxide transparent electrode.
A thin layer of polyimide wass deposited by spin-coating on each substrate and cured at 180°C. Both plates were rubbed in order to give a homogeneous alignment direction to the liquid crystal. A gasket of glue was deposited on the edge of one plate by a liquid dispenser and then, they were assembled and pressed. Spacers of 5µm were dispersed in the glue to force a gap of 5 µm between the two plates. After UV curing of the gasket, the cells were filled by capillarity with liquid crystal mixture. Lifetime measurements were performed using APD110A Thorlabs Si avalanche photodiode and MPL-F-355 CNI 355 Q-switched laser with ~7ns - 1µJ pulses and 6kHz repetition rate.Traces were recorded using Lecroy 12 bit HDO4022 200MHz oscilloscope with 1000 averaging. Electroswitchable device integrated luminescence was measured using PDA36A-EC Thorlabs Si avalanche photodiode and 405nm 50mW Oxxius laser diode circularly polarized impinging at ~15° incidence. For latter both measurements, luminescence was collected at normal incidence with numerical aperture of ~0.16 using parabolic mirrors, spectrally filtered with a high pass 500nm filter and focused on the detector using another parabolic mirror. The device temperature was controlled using a MR-hei-Tec- Heidolph hotplate. Temperature was controlled with a 2°C uncertainty. The luminescence quantum yields in the solid state were measured with a C9920-03 Hamamatsu system equipped with a 150 W xenon lamp, a monochromator, a Spectralon integrating sphere and a PMA-12 photomultiplier.
3 Results and Discussion
3.1 Synthesis of macrocyclic monomers
In the synthesis of macrocyclic bile acids via ring-closure metathesis (RCM), cyclic monomers 3a-d (38-membered rings) and 3e (35-membered rings) were prepared (Figure 3.1). There are three main differences between these cyclic monomers. First, different bile acid cores are used (cholic acid core for cyclic monomers 3a and 3e, lithocholic acid core for cyclic monomers 3b-d). Cholic acid has two extra hydroxyl groups at positions 7 and 12 compared to lithocholic acid. Consequently, cyclic monomers with a cholic acid core are more hydrophilic than those with a lithocholic acid core. Second, the types of linkages between bile acid core and long alkane chain are different. Some of the linkages are ester bonds and some are amide bonds. Generally, amide groups are more stable and more hydrophilic than ester groups. Third, the number of linkages is different. Cyclic monomers 3a-d possess three linkages and cyclic monomer 3e has only two. These differences make polymers prepared from cyclic monomers 3a-e different and interesting.
controlled hydrodynamic conditions. It consists of a double wall glass vessel, 0.065 m in internal diameter and tightly closed. The vessel is filled with a 0.035 m height of liquid (H L ).
A magnetic agitator enables bulk agitation of liquid without appreciable wave motion. The free surface remains flat in the whole range of rotation speeds used in the experiments (N = 50 - 120 rpm). The rotation speed is kept very small so as to maintain a constant surface of the gas-liquid interface offered to the mass transfer whatever the experiments. The temperature’s control is ensured by a liquid circulation through the vessel’s jacket associated to a thermo-regulated system. The temperature in the cell is measured by means of a thermometer. The experiments are carried out batch wise with respect to the liquid- and continuous to the gas-phase. Gas is fed above the liquid surface (connection through the cell’s cap) and is controlled by a gas flow meter. A gas flow rate of 2.85.10 -6 m 3 .s -1 is fixed whatever the experiments: this low value hinders any surface deformation and enables a constant interfacial shear stress to be imposed. A three-way valve is used to inject either air or nitrogen (atmosphere flushing).