Semiflexible Behavior Research Articles

Effect of lamina fiber orientation interfaced with semi-flexible adhesive layer on strength and failure mode of composite single-lap joints

Various geometric and material parameters such as adhesive thickness, adhesive overlap, adherend thickness, and composite layup may affect the strength of the joint under quasi-static loading and failure mode of bonded composite-to-composite single-lap joints (SLJs) and are investigated by previous studies. The study herein broadens these findings by looking into the effect of lamina fiber angle adjacent to the adhesive layer on the damage initiation and evolution in detail. In this regard, a composite-to-composite adhesively bonded SLJ with adherends made of E-glass/epoxy composites and [04//θ/03] (where//shows the adhesive location) layups are manufactured and tested under quasi-static tensile loading. The adhesive type is semi-flexible Araldite 2015. Experimental results show that by increasing the fiber angle from 0° to 90°, the shear stress in the adhesive layer is decreased while the peel stress is increased. In examining typical fracture interfaces for each layup configuration, a full description of failure mode assessment is obtained. In particular, the SLJ is modeled in Abaqus using cohesive elements with bilinear traction-separation law. Numerical results indicate that the bilinear cohesive law cannot model the exact load-displacement curve due to semi-flexible behavior of the epoxy adhesive, but it can predict maximum strength precisely. The failure of composite joints is significantly influenced by shear stress.

Advancing the computational methodology of rigid rod and semiflexible polymer systems: A new solution to the wormlike chain model with rod‐coil copolymer calculations

ABSTRACTA wormlike chain model for rod type blocks in a rod‐coil diblock copolymer is implemented in the self‐consistent field theory (SCFT) formalism. A pseudo‐spectral method is used to solve for the single‐chain partition function of this copolymer system. Orientation degrees of freedom are discretized using Lebedev sphere rules such that orientation integrations are carried out through a Lebedev quadrature, an approach not used previously in tandem with the pseudo‐spectral method. Phase behavior in the rigid‐rod limit as a function of rod segment volume fraction, Flory–Huggins interaction parameter χ, degree of polymerization N, and rod contour length ratio β are examined in detail in one and two dimensions. Examples extending to three dimensions are included. Semiflexible behavior via the rod bending rigidity κ is explored. An approximation is used for rigid‐rods that do not need spherical harmonics leading to increased speed in finding equilibrium morphologies. The results show that standing vertical structures may be more easily produced with rigid‐rod blocks compared to coil‐coil lamellae, an important feature in nanolithographic applications. Suggestions are made for using the model in future molecular orientation studies where the model can be used with inverse search methods to measure the values of the model parameters for the real systems. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 29–39

Microrheology and Microstructure of Fmoc-Derivative Hydrogels

The viscoelasticity of hydrogel networks formed from the low-molecular-weight hydrogelator Fmoc-tyrosine (Fmoc-Y) is probed using particle-tracking microrheology. Gelation is initiated by adding glucono-δ-lactone (GdL), which gradually lowers the pH with time, allowing the dynamic properties of gelation to be examined. Consecutive plots of probe particle mean square displacement (MSD) versus lag time τ are shown to be superimposable, demonstrating the formation of a self-similar hydrogel network through a percolation transition. The analysis of this superposition yields a gel time t(gel) = 43.4 ± 0.05 min and a critical relaxation exponent n(c) = 0.782 ± 0.007, which is close to the predicted value of 3/4 for semiflexible polymer networks. The generalized Stokes-Einstein relation is applied to the master curves to find the viscoelastic moduli of the critical gel over a wide frequency range, showing that the critical gel is structurally and rheologically fragile. The scaling of G'/G″ as ω(0.795±0.099) ≈ ω(3/4) at high frequencies provides further evidence for semiflexible behavior. Cryogenic scanning electron micrographs depict a loosely connected network close to the gel point with a fibrillar persistence length that is longer than the network mesh size, further indications of semiflexible behavior. The system reported here is one of a number of synthetic systems shown to exhibit semiflexible behavior and indicates the opportunity for further rheological study of other Fmoc derivatives.

Semiflexibility Highlights the Polymers' Topology: Monte Carlo Studies

AbstractSummary: In this article we analyze in how far semiflexible behavior enhances the differences in the properties of classes of polymers whose topological structure varies. We focus on three pairs of macromolecular classes: stars vs. chains, unknotted rings vs. rings with one knot (trefoils), and stars vs. unknotted rings. For this we determine the mean‐square radii of gyration and the bond‐bond correlation functions through Monte Carlo simulations which use the bond fluctuation model. We show that introducing semiflexibility magnifies the differences between experimentally measurable quantities and may even lead to qualitative changes. Our simulation results are supported by theoretical studies which make use of the maximum entropy principle.

High‐ and low‐frequency mechanical properties of living starfish oocytes

We studied the mechanical properties of living starfish oocytes belonging to two species, Astropecten Auranciacus and Asterina pectinifera, over a wide range of timescales. We monitored the Brownian motion of microspheres injected in the cytoplasm using laser particle-tracking (LPT) and video multiple-particle-tracking (MPT) techniques, to explore high- and low-frequency response ranges, respectively. The analysis of the mean-square-displacements (MSD) allowed us to characterize the samples on different timescales. The MSD behavior is explained by three power-law exponents: for short times (τ < 1 ms) it reflects the semiflexible behavior of the actin network; for intermediate timescales (1 ms < τ < 1 s) it is similar to that of a soft-glass material; finally for long times (τ > 1 s) it behaves mainly like a viscous medium. We computed and compared the viscoelastic moduli using a recently proposed model describing the frequency response of the cell material. The large fluctuations found in the MSD over hundreds of trajectories indicate and confirm the significant cytoplasm heterogeneity.

Liquid Crystalline Phase Behavior of Protein Fibers in Water: Experiments versus Theory

We have developed a new method allowing the study of the thermodynamic phase behavior of mesoscopic colloidal systems consisting of amyloid protein fibers in water, obtained by heat denaturation and aggregation of beta-lactoglobulin, a dairy protein. The fibers have a cross section of about 5.2 nm and two groups of polydisperse contour lengths: (i) long fibers of 1-20 microm, showing semiflexible behavior, and (ii) short rods of 100-200 nm long, obtained by cutting the long fibers via high-pressure homogenization. At pH 2 without salt, these fibers are highly charged and stable in water. We have studied the isotropic-nematic phase transition for both systems and compared our results with the theoretical values predicted by Onsager's theory. The experimentally measured isotropic-nematic phase transition was found to occur at 0.4% and at 3% for the long and short fibers, respectively. For both systems, this phase transition occurs at concentrations more than 1 order of magnitude lower than what is expected based on Onsager's theory. Moreover, at low enough pH, no intermediate biphasic region was observed between the isotropic phase and the nematic phase. The phase diagrams of both systems (pH vs concentration) showed similar, yet complex and rich, phase behavior. We discuss the possible physical fundamentals ruling the phase diagram as well as the discrepancy we observe for the isotropic-nematic phase transition between our experimental results and the predicted theoretical results. Our work highlights that systems formed by water-amyloid protein fibers are way too complex to be understood based solely on Onsager's theories. Experimental results are revisited in terms of the Flory's theory (1956) for suspensions of rods, which allows accounting for rod-solvent hydrophobic interactions. This theoretical approach allows explaining, on a semiquantitative basis, most of the discrepancies observed between the experimental results and Onsager's predictions. The sources of protein fibers complex colloidal behavior are analyzed and discussed at length.

The Self-Assembly, Elasticity, and Dynamics of Cardiac Thin Filaments

Solutions of intact cardiac thin filaments were examined with transmission electron microscopy, dynamic light scattering (DLS), and particle-tracking microrheology. The filaments self-assembled in solution with a bell-shaped distribution of contour lengths that contained a population of filaments of much greater length than the in vivo sarcomere size (∼1 μm) due to a one-dimensional annealing process. Dynamic semiflexible modes were found in DLS measurements at fast timescales (12.5 ns–0.0001 s). The bending modulus of the fibers is found to be in the range 4.5–16 × 10 −27 Jm and is weakly dependent on calcium concentration (with Ca 2+ ≥ without Ca 2+). Good quantitative agreement was found for the values of the fiber diameter calculated from transmission electron microscopy and from the initial decay of DLS correlation functions: 9.9 nm and 9.7 nm with and without Ca 2+, respectively. In contrast, at slower timescales and high polymer concentrations, microrheology indicates that the cardiac filaments act as short rods in solution according to the predictions of the Doi-Edwards chopsticks model (viscosity, η ∼ c 3, where c is the polymer concentration). This differs from the semiflexible behavior of long synthetic actin filaments at comparable polymer concentrations and timescales (elastic shear modulus, G′ ∼ c 1.4, tightly entangled) and is due to the relative ratio of the contour lengths (∼30). The scaling dependence of the elastic shear modulus on the frequency ( ω) for cardiac thin filaments is G′ ∼ ω 3/4 ± 0.03 , which is thought to arise from flexural modes of the filaments.