The coil‐globule transition of short hydrophobic‐polar (HP) chains, composed of 24 hydrophilic monomers and 24 polar monomers, in solution and on hydrophobic surface and the adsorption of the HP chain on hydrophobic surface are simulated. The coil‐globule transition point of the HP chain is dependent on sequence of chain but is roughly independent of the surface adsorption strength. Whereas the critical adsorption point of the HP chain is roughly independent of sequence. In addition, the lowest energy states can be obtained for the HP chain in solution or on surface by Monte Carlo simulated annealing method. Results show that the statistical conformation is strongly dependent on the intrachain H‐H attraction strength and the surface adsorption strength.
The distribution function P(S) of the radius of gyration S, the corresponding elastic free energy A(S) and the mean force were computed from simulations based on the wormlike chain (WLC) model. The relation of the S‐conjugated elastic functions to the analogous functions based on the chain vector R and their connection to the statistical‐mechanics ensembles was elucidated. Simulation data revealed that available analytical functions for P(S) fail to predict the behavior of semiflexible chains. When the power‐law function P(S) was used instead, the exponents sizeably raised with stiffness at chain expansion. The exponents deduced from elastic compression of a chain agreed fairly with the scaling exponents for chain confinement into a sphere.
Quantification of the stimuli‐responsive phase transition in polymers is topical and important for the understanding and development of novel stimuli‐responsive materials. The temperature‐induced phase transition of poly(N‐isopropylacrylamide) (PNIPAm) with one thiol end group depends on the confinement—free polymer or polymer brush—on the molecular weight and on the nature of the second end. This paper describes the synthesis of heterotelechelic PNIPAm of different molecular weights with a thiol end group—that specifically binds to gold nanorods and a hydrophilic NIPAm end group by reversible addition‐fragmentation chain‐transfer polymerization. Proton high‐resolution magic angle sample spinning NMR spectra are used as an indicator of the polymer chain conformations. The characteristics of phase transition given by the transition temperature, entropy, and width of transition are obtained by a two‐state model. The dependence of thermodynamic parameters on molecular weight is compared for hydrophilic and hydrophobic end functional‐free polymers and brushes. 相似文献
The transition of a solvated flexible macromolecular chain from random coil behavior in the θ‐state to a globular compact form in the collapsed state has been the subject of extensive theoretical and experimental studies. Most of the coil‐globule transition studies of macromolecules have concentrated on the prototypical polystyrene‐cyclohexane system. However, chain contractions reported in this system have been around 75% of those in the unperturbed θ‐state. This relatively small decrease in size does not satisfy the criterion for a densely packed, collapsed globule. Experimentally, the collapse from a coil to a true compact globular state has now been established for two flexible macromolecules: poly(N‐isopropylacrylamide) in water and poly(methyl methacrylate) in various solvents. In this contribution, we review recent theoretical studies covering phenomenological and Langevin models as well as computer simulations. In addition, we outline recent experimental studies of the coil‐globule transition of various flexible polymers, copolymers, and polyelectrolytes.
Expansion factor, αequation/tex2gif-stack-1.gif, versus temperature for NaPSS in 4.17 M aqueous NaCl solution. (•): NaPSS‐1, (○): NaPSS‐2. 相似文献
Summary: The addition of spermidine (SPD) into turbulent flow as a condensing agent showed the abrupt change of turbulent drag reducing (DR) efficiency of λ‐DNA in turbulent flow for the first time. The resultant asymptote DR efficiency explains the origin of those changes, which can be conclusively verified via the electrophoresis experiment. Despite the different fluid conditions, with and without condensing agent, all λ‐DNA molecules possessed the same half‐cut dimension, implying that the discrete change of DNA conformation can dramatically alter the flow characteristics.
Coil‐globule transition of DNA by spermidine. 相似文献
Molecular dynamics simulations have been extensively carried out to study glass transition of polymers for over 20 years, in which a central but still open question is how to quickly and reliably determine the glass transition temperature (Tg). To address this issue, the two “moderate big” bulk model systems comprised of the mono‐disperse isotactic or syndiotactic poly(methyl methacrylate) (iPMMA, sPMMA) chains have been separately simulated with the chemically specific coarse‐grained potentials. The model systems are first equilibrated at an enough high temperature and then cooled to an enough low temperature with three different rates. It is demonstrated that the densities and non‐bonded interaction energies qualitatively reveal glass transition whereas the reorientation functions of bond vectors quantitatively locate the Tg. Despite all, consistent result is yielded that sPMMA exhibits a higher Tg than iPMMA, which is in good agreement with the experimental observation. More importantly, the tacticity effects can be well dictated by the activation energy for the fast relaxation mode identified in the rubbery domain, which paves the way for quick assessment of polymer Tg. 相似文献
All conjugated polymers examined to date exhibit significant cumulative lattice disorder, although the origin of this disorder remains unclear. Using atomistic molecular dynamics (MD) simulations, the detailed structures for single crystals of a commonly studied conjugated polymer, poly(3‐hexylthiophene‐2,5‐diyl) (P3HT) are obtained. It is shown that thermal fluctuations of thiophene rings lead to cumulative disorder of the lattice with an effective paracrystallinity of about 0.05 in the π–π stacking direction. The thermal‐fluctuation‐induced lattice disorder can in turn limit the apparent coherence length that can be observed in diffraction experiments. Calculating mobilities from simulated crystal structures demonstrates that thermal‐fluctuation‐induced lattice disorder even enhances charge transport in P3HT. The mean inter‐chain charge transfer integral is enhanced with increasing cumulative lattice disorder, which in turn leads to pathways for fast charge transport through crystals. 相似文献
Thermophysical and mechanical properties of two conjugated polymers, poly(p‐phenylene vinylene) (PPV) and polyacetylene (PA), are predicted using molecular dynamics simulations and compared with results obtained from differential scanning calorimetry, nanoindentation, and dynamic mechanical analysis experiments. Glass transition temperature (Tg) is calculated from the changes in the slopes of the specific volume versus temperature and cohesive energy density versus temperature plots, obtained from constant pressure and constant temperature simulations (NPT ensemble). The effects of temperature on the torsion angle distributions and characteristic ratio are analyzed. PPV is found to have a Tg of 416 ± 8 K. PA does not exhibit a glass transition in the temperature range of 120 to 500 K. Using the static deformation method, the values of Young's modulus are calculated to be 1.81 ± 0.34 GPa for PA and 9.20 ± 0.57 GPa for PPV at 298 K. These values are in good agreement with the experimental measurements, validating the suitability of these techniques in the prediction of the polymer properties.
Styrene‐based thermoset polymers using vinyl ester crosslinkers offer the potential ability to tune material properties via control of network topology, measured primarily by the distribution of poly(styrene) molecular weight between vinyl esters. Clearly, demonstrating a relationship between topology and properties in the glassy state has so far proven difficult using experimental approaches. Here, an approach to control network topology in molecular simulations of randomly crosslinking systems via relative reactivity is presented. Several models of nearly fully cured (vinyl ester)/styrene (VE/ST) thermosets at fixed monomer composition but with a large range of distributions of molecular weight between crosslinks, Mc, are generated. It is found that fully cured VE/ST thermosets have glass transition temperatures and glassy Young's moduli that are insensitive to Mc distributions at fixed monomer composition. Using extreme cases of relative reactivity, it is also established that glassy‐state thermomechanical properties are sensitive to the overall density of crosslinks. 相似文献
Using a continuous space rod‐bead model and an off‐lattice Monte Carlo technique we investigate interfacial properties between two incompatible polymers of different stiffnesses. The interfacial tension is determined by using virial theorem and analyzing the spectrum of capillary waves. Detailed interfacial profiles for segment and chain densities and orientations are obtained. The simulation results agree with mean field approaches for not too large stiffness disparities and show a marked tendency towards a plateau at higher stiffness disparities where the differences to mean field approaches increase. 相似文献
A predictive CG model based on a conventional freely rotating chain was developed to describe semiflexible polymers on a relatively large length/time scale. Parameterization of the model requires only two material properties such as, the Kuhn length and coil density. The diameter of spherical “beads” employed in the model is used as an effective parameter that needs to be determined from preliminary data. Once determined for a particular solvent system, this parameter can then be used to model general solvent systems on a parameter‐free basis. Comparison with SANS data on dilute conjugated polymer solutions reveals that the CG polymer model can well describe material properties ranging from local rodlike segments to bulk interchain aggregates.
