Controlled and scalable torsional actuation of twisted nylon 6 fiber

Large‐scale torsional actuation occurs in twisted fibers and yarns as a result of volume change induced electrochemically, thermally, photonically, and other means. A quantitative relationship between torsional actuation (stroke and torque) and volume change is here introduced. The analysis is based on experimental investigation of the effects of fiber diameter and inserted twist on the torsional stroke and torque measured when heating and cooling nylon 6 fibers over the temperature range of 26–62 °C. The results show that the torsional stroke depends only on the amount of twist inserted into the fiber and is independent of fiber diameter. The torque generated is larger in fibers with more inserted twist and with larger diameters. These results are successfully modeled using a single‐helix approximation of the twisted fiber structure. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1278–1286     

First principle studies of charge transport in PPV polymer under conformational deformation

We consider the inclusion of torsional deformations in the structure of an infinite chain of poly‐p‐phenylene vinylene and study the consequences on charge transport along the polymer length. Calculations of the electronic transport are performed with density functional theory combined with Keldysh nonequilibrium Green's function method. Deformations are modeled either as a sharp rotation of the polymer backbone about a single chemical bond or as a continuous twist extending along various monomer units. We study current‐voltage (I‐V) characteristics in a two probe configuration as a function of angle and degree of torsional sharpness and demonstrate that when the backbone torsion is abrupt a barrier to electron transport builds up that becomes maximum at an angle of 100°. The outcome of our calculations is that the abrupt twist of the polymer backbone creates two virtually disconnected segments, which validates models that treat a real polymer as distribution of chains of different sizes and conjugation lengths. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 578–586     

Closed-form solution of the finite torsion problem by use of the λ form of the strain-energy function

A closed-form solution for the stress distribution in a cylindrical elastic rod under finite torsion is given involving use of the logarithmic λ form of the strain-energy function proposed by Valanis and Landel. Experimental torque data on natural rubber which give support to the present solution over a large range of angles of twist are shown. It is also shown how torsion data may be used to determine the λ form for deformations that lie outside the range of applicability of the logarithmic function if certain conditions apply. A solution, also in closed form, is given by using another λ form proposed by Dickie and Smith on the basis of experiments on styrene—butadiene rubber.     

A Minimal Sequence for Left‐Handed G‐Quadruplex Formation

Recently, we observed the first example of a left‐handed G‐quadruplex structure formed by natural DNA, named Z‐G4. We analysed the Z‐G4 structure and inspected its primary 28‐nt sequence in order to identify motifs that convey the unique left‐handed twist. Using circular dichroism spectroscopy, NMR spectroscopy, and X‐ray crystallography, we revealed a minimal sequence motif of 12 nt (GTGGTGGTGGTG) for formation of the left‐handed DNA G‐quadruplex, which is found to be highly abundant in the human genome. A systematic analysis of thymine loop mutations revealed a moderate sequence tolerance, which would further broaden the space of sequences prone to left‐handed G‐quadruplex formation.     

Linking number analysis of a pentadecanuclear metallamacrocycle: a Möbius-Craig system revealed

An analysis of the pi-electronic topology of a recently reported pentadecanuclear manganese metallacycle in terms of linking number, twist, and writhe confirms its previous assignment as a M?bius system of order six. The total degree of local torsion is reduced by a significant contribution from writhe, thus correcting the previously inferred local torsion at each metal center from 120 degrees to approximately 90 degrees . This latter value suggests a model for the cyclic pi conjugation around the metallacycle, which involves 3d pi-2p pi interactions at the metal center, of a type first suggested by Craig 50 years ago in the context of main group cyclic aromatic systems.     

Characterisation of torsional actuation in highly twisted yarns and fibres

Highly twisted oriented polymer fibres and carbon nanotube yarns show large scale torsional actuation from volume expansion that can be induced, for example, thermally or by electrochemical charging. When formed into spring-like coils, the torsional actuation within the fibre or yarn generates powerful tensile actuation per muscle weight. For further development of these coil actuators and for the practical application of torsional actuators, it is important to standardise methods for characterising both the torsional stroke (rotation) and torque generated. By analogy with tensile actuators, we here introduce a method to measure both the free stroke and blocked torque in a one-end-tethered fibre. In addition, the torsional actuation can be measured when operating against an externally applied torque (isotonic) and actuation against a return spring fibre (variable torque). A theoretical treatment of torsional actuation was formulated using torsion mechanics and evaluated using a commercially available highly-oriented polyamide fibre. Good agreement between experimental measurements and calculated values was obtained. The analysis allows the prediction of torsional stroke under any external loading condition based on the fundamental characteristics of the actuator: free stroke and stiffness.     

Binding of proteins to the minor groove of DNA: What are the structural and energetic determinants for kinking a basepair step?

The structural and energetic determinants for kinking a basepair step by minor groove-insertion of the protein side chains of PurR, LacI, LEF-1, IHF, Sac7d, and Sso7d, have been calculated by molecular dynamics/potential of mean force simulations. The structural determinants of the kinked structures are: two contiguous furanose rings achieve different conformations, in the region of C3'endo (A-DNA) and C2'endo (B-DNA); the chi torsion angle always takes values characteristic of the C2'endo conformation of B-DNA, independently of sugar puckering; and protein side chain insertion increases slide (from negative to positive values), rise, and roll, and decreases twist. The energetic determinants of DNA kinking are: the conformational transition of the sugar-phosphate backbone is not energetically demanding; the relative importance of the interbase parameters in the free energy penalty is slide, followed by twist and rise, and concluding with shift and roll; and the characteristic increase of roll and decrease of twist, upon side chain insertion, tends to stabilize the process of DNA kinking.     

A Helicate‐Based Three‐State Molecular Switch

The control of structural transformations triggered by external signals is important for the development of novel functional devices. In the present study, it is demonstrated that helicates can be designed to structurally respond to the presence of different counterions and to adopt either a compressed or an expanded structure. Reversible switching is not only possible between those two states, furthermore, the twist of the aggregate also can be controlled. Thus, three out of four possible states of a helicate (expanded/left‐handed, expanded/right‐handed, compressed/left‐handed) based on an enantiomerically pure ester bridged dicatecholate ligand are specifically addressed by introduction, exchange, or removal of countercations. This approach is used to reversibly switch between the different states or to successively address them.     

Helical DNA Origami Tubular Structures with Various Sizes and Arrangements

We developed a novel method to design various helical tubular structures using the DNA origami method. The size‐controlled tubular structures which have 192, 256, and 320 base pairs for one turn of the tube were designed and prepared. We observed the formation of the expected short tubes and unexpected long ones. Detailed analyses of the surface patterns of the tubes showed that the short tubes had mainly a left‐handed helical structure. The long tubes mainly formed a right‐handed helical structure and extended to the directions of the double helical axes as structural isomers of the short tubes. The folding pathways of the tubes were estimated by analyzing the proportions of short and long tubes obtained at different annealing conditions. Depending on the number of base pairs involved in one turn of the tube, the population of left‐/right‐handed and short/long tubes changed. The bending stress caused by the stiffness of the bundled double helices and the non‐natural helical pitch determine the structural variety of the tubes.     

A tensor model of liquid crystalline polymers: application to basic disclination processes

One of the features of liquid crystalline polymers (LCPs) is their strong elastic anisotropy, which means they have unequal elastic constants. Elastic anisotropy plays a crucial role in the microstructure and macroscopic properties of LCPs. In this paper, the effect of unequal elastic constants on microstructure is investigated without an external field by using a deterministic tensorial approach. In this model, the evolution of the director field can be viewed as a process driven towards the state of zero elastic torque. A tensor expression of the elastic torque is used so that the nematic symmetry is automatically conserved. In simulations of bulk samples, disclination lines of strength half and escaped integer disclinations are observed. The distortion fields around the disclinations are found to depend on elastic anisotropy. If the twist constant is the lowest, as is the case for main chain liquid crystalline polymers, the disclination lines are predominantly of the twist type.     

A tensor model of liquid crystalline polymers: application to basic disclination processes

One of the features of liquid crystalline polymers (LCPs) is their strong elastic anisotropy, which means they have unequal elastic constants. Elastic anisotropy plays a crucial role in the microstructure and macroscopic properties of LCPs. In this paper, the effect of unequal elastic constants on microstructure is investigated without an external field by using a deterministic tensorial approach. In this model, the evolution of the director field can be viewed as a process driven towards the state of zero elastic torque. A tensor expression of the elastic torque is used so that the nematic symmetry is automatically conserved. In simulations of bulk samples, disclination lines of strength half and escaped integer disclinations are observed. The distortion fields around the disclinations are found to depend on elastic anisotropy. If the twist constant is the lowest, as is the case for main chain liquid crystalline polymers, the disclination lines are predominantly of the twist type.     

Light scattering from a nematic monodomain in an electric field Twist elastic constant and viscosity coefficient of nematic polymer-solvent mixtures

The light scattering technique was used to investigate the viscoelastic parameters characterizing director twist distortions in miscible nematic mixtures of 5CB (pentacyanobiphenyl) with two side chain liquid crystal polymers and a main chain liquid crystal polymer. By applying an AC electric field to homeotropically-aligned nematic monodomains of the mixtures, the field-dependent scattering intensities and director orientation fluctuation relaxation rates yield, respectively, the twist elastic constant K₂₂ and viscosity coefficient γ₁. The results directly demonstrate that the addition of liquid crystal polymers causes substantial decreases of the relaxation rates for dynamic light scattering from the twist mode and these changes are due to small decreases in K₂₂ coupled with large increases in γ₁. The decrements in K₂₂ are comparable for both side chain and main chain liquid crystal polymers. The relative increase in the twist viscosity for the side chain liquid crystal polymers is much smaller than those of main chain polymers. A theoretical model is used to qualitatively interpret the difference between the viscous behaviour of the twist mode for both side chain and main chain liquid crystal polymers in a nematic solvent.     

Measurement of twist angle and surface torsional anchoring strength in a nematic liquid crystal cell

In this paper a novel method is developed for measuring the twist angle and surface torsional anchoring strength in nematic liquid crystal (NLC) cells. This method is based on a technique developed from the Jones optical propagation matrix. From the measurement, the actual twist angle in NLC cell and the deviation of the LC director on the boundary of the cell from the rubbing direction of the substrate are obtained. A theoretical discussion shows that the surface torsional anchoring strength has a great influence on the value of the deviation, and hence so it can be calculated.     

Light scattering from a nematic monodomain in an electric field Twist elastic constant and viscosity coefficient of nematic polymer–solvent mixtures

Abstract

The light scattering technique was used to investigate the viscoelastic parameters characterizing director twist distortions in miscible nematic mixtures of 5CB (pentacyanobiphenyl) with two side chain liquid crystal polymers and a main chain liquid crystal polymer. By applying an AC electric field to homeotropically-aligned nematic monodomains of the mixtures, the field-dependent scattering intensities and director orientation fluctuation relaxation rates yield, respectively, the twist elastic constant K ₂₂ and viscosity coefficient γ₁. The results directly demonstrate that the addition of liquid crystal polymers causes substantial decreases of the relaxation rates for dynamic light scattering from the twist mode and these changes are due to small decreases in K ₂₂ coupled with large increases in γ₁. The decrements in K ₂₂ are comparable for both side chain and main chain liquid crystal polymers. The relative increase in the twist viscosity for the side chain liquid crystal polymers is much smaller than those of main chain polymers. A theoretical model is used to qualitatively interpret the difference between the viscous behaviour of the twist mode for both side chain and main chain liquid crystal polymers in a nematic solvent.     

Tricks of Light on Helices: Transformation of Helical Polymers by Photoirradiation

The following polymer structural transitions were achieved using light: preferred‐handed helix formation for poly(9,9‐di‐n‐octylfluoren‐2,7‐diyl), helix racemization (helix–helix transition) for poly(2,7‐bis(4‐t‐butylphenyl)fluoren‐9‐yl acrylate) and poly(2,5‐bis[4‐((S)‐2‐methylbutyloxy)phenyl]styrene), and helix decomposition for poly(2,7‐bis(4‐t‐butylphenyl)‐9‐methylfluoren‐9‐yl acrylate) and poly(2,7‐bis(4‐t‐butylphenyl)fluoren‐9‐ylmethyl methacrylate). Although these types of transitions and chemical transformations have been studied mainly using heat or chemicals as stimuli, light can also cause these structural alterations. In the helix construction and the helix–helix transition, a key transition is a twist‐coplanar conformational change of a biphenyl or an aryl–aryl unit in the side chain or the main chain of the polymer. Furthermore, the helix–helix transition was caused only by light and not by heat. The examples discussed in this review are expected to trigger off a new direction in synthesis and reaction of chiral polymers.     

Dynamic Elasticity of Cubic Diamond

Previously, the structure of the carbon allotrope glitter has been disclosed, and a theory accompanying the structural report as to its bulk modulus at pressure predicted it would be among the hardest materials possible. The dynamic elasticity theory developed in that paper, involving the forces generated in elastic chemical bond deformations resulting from applied mechanical forces, is here applied to the cubic diamond lattice. Stresses, both lateral and axial, contribute to the bulk modulus of cubic diamond at pressure. The ultimate strength of the cubic diamond lattice, in the approximations of the dynamic elasticity theory presented in this paper, is estimated to be in excess of 1 TPa, at modest bond length deformations of about 0.1 ?, and when including the zero pressure bulk modulus B ₀ in the computation. In particular, the dynamic elasticity model predicts the hardest direction of cubic diamond will be for an isotropic mechanical force applied along 〈111〉 directions of the structural unit cell.