China 
Africa 
Explore chapters and articles related to this topic
Poly(3-alkylthiophenes): Controlled Manipulation of Microstructure and its Impact on Charge Transport
Published in John R. Reynolds, Barry C. Thompson, Terje A. Skotheim, Conjugated Polymers, 2019
Michael McBride, Guoyan Zhang, Martha Grover, Elsa Reichmanis
One of the key metrics describing an individual polymer chain in solution is its persistence length. The persistence length provides an indication of the degree of stiffness or flexibility of a chain. A persistence length on the order of magnitude of the polymer contour length indicates a semiflexible chain. Figure 11.3 provides a visualization of the persistence length and its relation to entanglements. Notably, this parameter is related to the solubility of a polymer in a particular solvent, so chain planarity is governed by relative polymer–solvent interactions [31–33]. As the solvent quality increases, polymer–solvent interactions increase relative to polymer-polymer interactions, leading to planarization.
Nonionic Block Copolymer Wormlike Micelles
Published in Raoul Zana, Eric W. Kaler, Giant Micelles, 2007
The stiffness characteristics of long chain-like objects (such as polymers and wormlike micelles) in solution are typically quantified in terms of the persistence length lp, defined as the orientational correlation length between two unit tangent vectors along the contour of the chain.130 Using this parameter, one can write the root-mean-square end-to-end distance (R) of a semiflexible (Kratky-Porod) chain as
*
Published in Chad A. Mirkin, Spherical Nucleic Acids, 2020
Christine R. Laramy, Matthew N. O’Brien, Chad A. Mirkin
Despite these advances driven by DNA flexibility, the presence of the double-stranded regions remains an important design element for building blocks with nanoparticle cores. These double-stranded regions help to pre-orientate sticky ends for hybridization and to facilitate collective interactions or rolling [29, 66, 67]. When the DNA becomes too flexible relative to the particle core size, order can decrease, and, eventually, crystallization behavior may deviate from expectations [64, 68–71]. This behavior can be attributed to the random coiling behavior of highly flexible ligands, analogous to flexible polymer systems. To illustrate the effect of DNA ligand flexibility in these systems, it is useful to consider a quantitative measure of flexibility, namely, persistence length. The persistence length is defined as the length over which a polymer acts as a rigid rod [72]. In the case of DNA, hybridization to its complementary strand changes the characteristic persistence length: ~1–3 nm for single-stranded DNA (depending on the ionic strength) [73] and ~40–50 nm for double-stranded DNA [74, 75]. Although single-stranded DNA anchored to the surface of a densely functionalized particle would be more orientated than the persistence length implies (as a result of the polymer brush effect [76]), its flexibility would still allow for random coiling. Furthermore, the effects of this random coiling become more pronounced further from the surface towards the sticky end, which can limit sticky-end access for hybridization events [39, 77, 78]. Random coiling behavior also reduces the energetic favorability of sticky-end hybridization through the introduction of a configurational entropy cost [29, 79]. Together, these sources of DNA variability can lead to a broader distribution of particle positions within a crystal (more specifically, decreased order) and can lead to a loss of directional interactions templated by the shape of the underlying particle core. In the subsequent discussion on microparticle cores, we will revisit the role of flexibility and the conclusions drawn from that community (DNA design 2).
Elastocaloric effect in liquid crystal elastomers from molecular simulations
Published in Liquid Crystals, 2018
We consider main-chain (rather than side-chain) LCE because of their extreme strain-alignment coupling [6] leading to large values of and, presumably, a large elastocaloric . The simulations are based on the soft-core Gay–Berne (GB) potential [14,15], favouring parallel alignment of elongated particles. Uniaxial soft-core GB ellipsoids are used here to assemble LCE networks, representing bonded mesogenic molecules, as well as the non-bonded mesogenic swelling monomers. For compatibility with previous work [16,17], the ratio of potential well depths for a pair of aligned GB particles in the end-to-end and side-to-side relative positions is fixed at 0.2, and the length-to-width ratio of all GB ellipsoids is taken to be equal to 3. Bonding sites are assumed at the GB particle ends (head/tail) as well as on the equator wherever needed to provide cross-links creating an LCE network. The interparticle bonds are modelled by means of the finitely extensible nonlinear elastic (FENE) [18] potential applied to both bond stretching and bending. The maximum bond length is set to molecular widths. The polymer chains are assumed to be rather flexible, with a persistence length approaching molecular lengths for isolated chains. The easy bond orientation is taken parallel to the long molecular axis for the head/tail sites and along the short molecular axis for the equatorial site. Further details regarding the potential parameters are given in Ref. [16] and in references therein.