Polymers in crowded environment under stretching force: Globule-coil transitions

Pulling on grafted flexible polymers can cause twisted bundles

Bundles of semiflexible polymers can twist at low temperatures to balance energy gain from attraction and energy cost from bending. This raises the question whether twisting can be also observed for bundles of rather flexible grafted polymers stretched out by pulling force. Here, we address this question using Monte Carlo computer simulations of small bundles. Our data show that for weak forces F < Fl, intertwined globular conformations are favored, whereas for strong forces F > Fu, rod-like bundles emerge. In the intermediate force window Fl < F < Fu, bundles with a helical twist can be clearly identified. Applying a field to all monomers yields qualitatively the same effect. This suggests the conclusion that rather flexible polymers under pulling force or field behave effectively like semiflexible polymers without external pull.

Non-monotonic dependence of polymer chain dynamics on active crowder size

Configuration dynamics of flexible polymer chains is of ubiquitous importance in many biological processes. Here, we investigate a polymer chain immersed in a bath of size-changed active particles in two dimensional space using Langevin dynamics simulations. Particular attention is paid to how the radius of gyration R_g of the polymer chain depends on the size σ_c of active crowders. We find that R_g shows nontrivial non-monotonic dependence on σ_c: The chain first swells upon increasing σ_c, reaching a fully expanded state with maximum R_g, and then, R_g decreases until the chain collapses to a compact coil state if the crowder is large enough. Interestingly, the chain may oscillate between a collapse state and a stretched state at moderate crowder size. Analysis shows that it is the competition between two effects of active particles, one stretching the chain from inside due to persistence motion and the other compressing the chain from outside, that leads to the non-monotonic dependence. Besides, the diffusion of the polymer chain also shows nontrivial non-monotonic dependence on σ_c. Our results demonstrate the important interplay between particle activity and size associated with polymer configurations in active crowding environments.

Asymptotic scaling behavior of self-avoiding walks on critical percolation clusters

We study self-avoiding walks on three-dimensional critical percolation clusters using a new exact enumeration method. It overcomes the exponential increase in computation time by exploiting the clusters' fractal nature. We enumerate walks of over 10^{4} steps, far more than has ever been possible. The scaling exponent ν for the end-to-end distance turns out to be smaller than previously thought and appears to be the same on the backbones as on full clusters. We find strong evidence against the widely assumed scaling law for the number of conformations and propose an alternative, which perfectly fits our data.

Role of solvation dynamics and local ordering of water in inducing conformational transitions in poly(N-isopropylacrylamide) oligomers through the LCST

Conformational transitions in thermo-sensitive polymers are critical in determining their functional properties. The atomistic origin of polymer collapse at the lower critical solution temperature (LCST) remains a fundamental and challenging problem in polymer science. Here, molecular dynamics simulations are used to establish the role of solvation dynamics and local ordering of water in inducing conformational transitions in isotactic-rich poly(N-isopropylacrylamide) (PNIPAM) oligomers when the temperature is changed through the LCST. Simulated atomic trajectories are used to identify stable conformations of the water-molecule network in the vicinity of polymer segments, as a function of the polymer chain length. The dynamics of the conformational evolution of the polymer chain within its surrounding water molecules is evaluated using various structural and dynamical correlation functions. Around the polymer, water forms cage-like structures with hydrogen bonds. Such structures form at temperatures both below and above the LCST. The structures formed at temperatures above LCST, however, are significantly different from those formed below LCST. Short oligomers consisting of 3, 5, and 10 monomer units (3-, 5-, and 10-mer), are characterized by significantly higher hydration level (water per monomer ~ 16). Increasing the temperature from 278 to 310 K does not perturb the structure of water around the short oligomers. In the case of 3-, 5-, and 10-mer, a distinct coil-to-globule transition was not observed when the temperature was raised from 278 to 310 K. For a PNIPAM polymer chain consisting of 30 monomeric units (30-mer), however, there exist significantly different conformations corresponding to two distinct temperature regimes. Below LCST, the water molecules in the first hydration layer (~12) around hydrophilic groups arrange themselves in a specific ordered manner by forming a hydrogen-bonded network with the polymer, resulting in a solvated polymer acting as hydrophilic. Above LCST, this arrangement of water is no longer stable, and the hydrophobic interactions become dominant, which contributes to the collapse of the polymer. Thus, this study provides atomic-scale insights into the role of solvation dynamics in inducing coil-to-globule phase transitions through the LCST for thermo-sensitive polymers like PNIPAM.