Eugene M. Terentjev

An analytical solution to the kinetics of breakable filament assembly.

Dissecting Amyloid Formation Amyloid fibrils are associated with clinical disorders ranging from Alzheimers disease to type II diabetes. Their self-assembly can be described by a master equation that takes into account nucleation-dependent polymerization and fragmentation. Knowles et al. (p. 1533) now present an analytical solution to the master equation, which shows that amyloid growth kinetics is often limited by the fragmentation rate rather than by the rate of primary nucleation. In addition, the results reveal relationships between system properties (scaling laws) that provide mechanistic insight not only into amyloid growth, but also into related self-assembly processes. The growth kinetics of amyloid fibrils and related self-assembly phenomena are revealed by analytical theory. We present an analytical treatment of a set of coupled kinetic equations that governs the self-assembly of filamentous molecular structures. Application to the case of protein aggregation demonstrates that the kinetics of amyloid growth can often be dominated by secondary rather than by primary nucleation events. Our results further reveal a range of general features of the growth kinetics of fragmenting filamentous structures, including the existence of generic scaling laws that provide mechanistic information in contexts ranging from in vitro amyloid growth to the in vivo development of mammalian prion diseases.

uv manipulation of order and macroscopic shape in nematic elastomers

A range of monodomain nematic liquid-crystal elastomers containing differing proportions of photoisomerizable mesogenic moieties, which turn from a rodlike to a kinked shape upon ultraviolet (uv) irradiation, was studied. Depending on the proportion and positional role of the photosensitive groups in the crosslinked polymer network, different types and magnitudes of response were found. The principle consequence of such photoisomerization is the destabilization of the nematic phase, whose order parameter depends on temperature in a near-critical fashion. Accordingly, the effect of uv irradiation is dramatically enhanced near the critical temperature, with the associated reduction in the nematic order parameter manifesting as a change in the macroscopic shape of the elastomer samples, producing a large uniaxial contraction. Theoretical analysis of this phenomenon gives a good quantitative agreement with experiment.

Mouldable liquid-crystalline elastomer actuators with exchangeable covalent bonds

Liquid-crystal elastomers (LCEs) are a class of actively moving polymers with remarkable practical potential for converting external stimuli into mechanical actuation. However, real-world applications of LCEs are lacking because macroscopic orientation of liquid-crystal order, which is required for reversible actuations, is hard to achieve in practice. Here we show that the processing bottleneck of LCEs can be overcome by introducing exchangeable links in place of permanent network crosslinks, a concept previously demonstrated for vitrimers. Liquid-crystal elastomers with exchangeable links (xLCEs) are mouldable, allow for easy processing and alignment, and can be subsequently altered through remoulding with different stress patterns, thus opening the way to practical xLCE actuators and artificial muscles. Surprisingly, instead of external-stress relaxation through the creep of non-liquid-crystal transient networks with exchangeable links, xLCEs develop strong liquid-crystal alignment as an alternative mechanism of mechanical relaxation.

Nucleated polymerization with secondary pathways. I. Time evolution of the principal moments

Self-assembly processes resulting in linear structures are often observed in molecular biology, and include the formation of functional filaments such as actin and tubulin, as well as generally dysfunctional ones such as amyloid aggregates. Although the basic kinetic equations describing these phenomena are well-established, it has proved to be challenging, due to their non-linear nature, to derive solutions to these equations except for special cases. The availability of general analytical solutions provides a route for determining the rates of molecular level processes from the analysis of macroscopic experimental measurements of the growth kinetics, in addition to the phenomenological parameters, such as lag times and maximal growth rates that are already obtainable from standard fitting procedures. We describe here an analytical approach based on fixed-point analysis, which provides self-consistent solutions for the growth of filamentous structures that can, in addition to elongation, undergo internal fracturing and monomer-dependent nucleation as mechanisms for generating new free ends acting as growth sites. Our results generalise the analytical expression for sigmoidal growth kinetics from the Oosawa theory for nucleated polymerisation to the case of fragmenting filaments. We determine the corresponding growth laws in closed form and derive from first principles a number of relationships which have been empirically established for the kinetics of the self-assembly of amyloid fibrils.

Spontaneous thermal expansion of nematic elastomers

Abstract:We study the monodomain (single-crystal) nematic elastomer materials, all side-chain siloxane polymers with the same mesogenic groups and crosslinking density, but differing in the type of crosslinking. Increasing the proportion of long di-functional segments of main-chain nematic polymer, acting as network crosslinking, results in dramatic changes in the uniaxial equilibrium thermal expansion on cooling from the isotropic phase. At higher concentration of main chains their behaviour dominates the elastomer properties. At low concentration of main-chain material, we detect two distinct transitions at different temperatures, one attributed to the main-chain, the other to the side-chain component. The effective uniaxial anisotropy of nematic rubber, r(T) = / proportional to the effective nematic order parameter Q(T), is given by an average of the two components and thus reflects the two-transition nature of thermal expansion. The experimental data is compared with the theoretical model of ideal nematic elastomers; applications in high-amplitude thermal actuators are discussed in the end.

Nematic elastomers with aligned carbon nanotubes: New electromechanical actuators

We demonstrate, for the first time, the large electromechanical response in nematic liquid-crystalline elastomers filled with a very low ( ~ 0.01%) concentration of carbon nanotubes, aligned along the nematic director at preparation. The nanotubes create a very large effective dielectric anisotropy of the composite. Their local field-induced torque is transmitted to the rubber elastic network and is registered as the exerted uniaxial stress of order ~ 1 kPa in response to a constant field of order ~ 1 MV/m. We investigate the dependence of the effect on field strength, nanotube concentration and reproducibility under multiple field-on and -off cycles. The results indicate the potential of the nanotube-nematic elastomer composites as electrically driven actuators.

Nematic elastomers : A new state of matter ?

Recent experimental and theoretical work shows that nematic elastomers have a highly mobile director, despite being nominally solid liquid crystals. We review this work and show that such a system represents an essentially new state of matter with non-symmetric elasticity, new nematic transitions, discontinuous stress-strain relations and an ability to behave like liquids for some special class of deformations. These phenomena have been seen in large monodomain samples made in nematic fields or by multistage crosslinking under stresses. Modelling of monodomain elastomers is straightforward and successfully explains the unusual mechanical behaviour observed. The theory is a direct extension of the classical theory of conventional rubbers; we call it the neo-classical theory of elastomers. Being a molecular theory, it describes the very large extensions at which the new transitions occur. We outline problems for the future, principally those of polydomains.

UV isomerisation in nematic elastomers as a route to photo-mechanical transducer

Abstract:The macroscopic shape of liquid-crystalline elastomers strongly depends on the order parameter of the mesogenic groups. This order can be manipulated if photo-isomerisable groups, e.g. containing N=N bonds, are introduced into the material. We have explored the large photo-mechanical response of such an azobenzene-containing nematic elastomer at different temperatures, using force and optical birefringence measurements, and focusing on fundamental aspects of population dynamics and the related speed and repeatability of the response. The characteristic time of “on” and “off” regimes strongly depends on temperature, but is generally found to be very long. We were able to verify that the macroscopic relaxation of the elastomer is determined by the nematic order dynamics and not, for instance, by the polymer network relaxation.

Dispersion and Alignment of Carbon Nanotubes in Liquid Crystalline Polymers and Elastomers

Despite extraordinarily high expectations envisaged for carbon nanotubes (CNTs) across a wide range of disciplines, several inherent defi ciencies have impeded their progress towards mainstream applications. [ 1 ] Poor solubility is one of the most fundamental and diffi cult problems. [ 2 ] In the past few years, a number of surface-active molecules have been developed to help debundle pristine CNTs into aqueous solutions, [ 3 ] and organic solvents. [ 4 ] Thermotropic liquid crystalline polymers (LCPs) are seldom investigated for this purpose: only a few poly-oxazolines have been reported to disperse CNTs to a very low concentration, e.g., below 0.4wt% in organic solutions. [ 5 ]

Micro-raman spectroscopy of algae: composition analysis and fluorescence background behavior.

Preliminary feasibility studies were performed using Stokes Raman scattering for compositional analysis of algae. Two algal species, Chlorella sorokiniana (UTEX #1230) and Neochloris oleoabundans (UTEX #1185), were chosen for this study. Both species were considered to be candidates for biofuel production. Raman signals due to storage lipids (specifically triglycerides) were clearly identified in the nitrogen‐starved C. sorokiniana and N. oleoabundans, but not in their healthy counterparts. On the other hand, signals resulting from the carotenoids were found to be present in all of the samples. Composition mapping was conducted in which Raman spectra were acquired from a dense sequence of locations over a small region of interest. The spectra obtained for the mapping images were filtered for the wavelengths of characteristic peaks that correspond to components of interest (i.e., triglyceride or carotenoid). The locations of the components of interest could be identified by the high intensity areas in the composition maps. Finally, the time evolution of fluorescence background was observed while acquiring Raman signals from the algae. The time dependence of fluorescence background is characterized by a general power law decay interrupted by sudden high intensity fluorescence events. The decreasing trend is likely a result of photo‐bleaching of cell pigments due to prolonged intense laser exposure, while the sudden high intensity fluorescence events are not understood. Biotechnol. Bioeng. 2010;105: 889–898.