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Conformations of a Long Polymer in a Melt of Shorter Chains: Generalizations of the Flory Theorem
Large-scale simulations of the swelling of a long N-mer in a melt of chemically identical P-mers are used to investigate a discrepancy between theory and experiments. Classical theory predicts an increase of probe chain size R ∼ P–0.18 with decreasing degree of polymerization P of melt chains in the range of 1 < P < N1/2. However, both experiment and simulation data are more consistent with an apparently slower swelling R ∼ P–0.1 over a wider range of melt degrees of polymerization. This anomaly is explained by taking into account the recently discovered long-range bond correlations in polymer melts and corrections to excluded volume. We generalize the Flory theorem and demonstrate that it is in excellent agreement with experiments and simulations.
Crystallization of Polymers under the Influence of an External Force Field
We simulated the crystallization and melting behavior of entangled polymer melts using molecular dynamics where each chain is subject to a force dipole acting on its ends. This mimics the deformation of chains in a flow field but represents a well-defined equilibrium system in the melt state. Under weak extension within the linear response of the chains, the mechanical work done on the system is about two orders of magnitude smaller as compared with the heat of fusion. As a consequence, thermodynamic and simple arguments following the secondary nucleation model predict only small changes of the crystalline phase. By contrast, an increase of the stem length up to a factor of two is observed in our simulations. On the other hand, the lamellar thickening induced by the external force is proportional to the increase of the entanglement length in the melt prior to crystallization as measured by the primitive path method. While the mechanical work done on the system is only a small perturbation for thermodynamics of polymer crystallization, the change of the primitive path is large. This suggests that a strong increase in the lamellar thickness induced, by external deformation, a topological rather than a thermodynamic origin.
Pseudo-chemotaxis of active Brownian particles competing for food
Active Brownian particles (ABPs) are physical models for motility in simple life forms and easily studied in simulations. An open question is to what extent an increase of activity by a gradient of fuel, or food in living systems, results in an evolutionary advantage of actively moving systems such as ABPs over non-motile systems, which rely on thermal diffusion only. It is an established fact that within confined systems in a stationary state, the activity of ABPs generates density profiles that are enhanced in regions of low activity, which is thus referred to as ‘anti-chemotaxis’. This would suggest that a rather complex sensoric subsystem and information processing is a precondition to recognize and navigate towards a food source. We demonstrate in this work that in non-stationary setups, for instance as a result of short bursts of fuel/food, ABPs do in fact exhibit chemotactic behavior. In direct competition with inactive, but otherwise identical Brownian particles (BPs), the ABPs are shown to fetch a larger amount of food. We discuss this result based on simple physical arguments. From the biological perspective, the ability of primitive entities to move in direct response to the available amount of external energy would, even in absence of any sensoric devices, encompass an evolutionary advantage.
Reversible thermosensitive biodegradable polymeric actuators based on confined crystallization
We discovered a new and unexpected effect of reversible actuation of ultrathin semicrystalline polymer films. The principle was demonstrated on the example of thin polycaprolactone-gelatin bilayer films. These films are unfolded at room temperature, fold at temperature above polycaprolactone melting point, and unfold again at room temperature. The actuation is based on reversible switching of the structure of the hydrophobic polymer (polycaprolactone) upon melting and crystallization. We hypothesize that the origin of this unexpected behavior is the orientation of polycaprolactone chains parallel to the surface of the film, which is retained even after melting and crystallization of the polymer or the “crystallization memory effect”. In this way, the crystallization generates a directed force, which causes bending of the film. We used this effect for the design of new generation of fully biodegradable thermoresponsive polymeric actuators, which are highly desirable for bionano-technological applications such as reversible encapsulation of cells and design of swimmers.
Discontinuous switching of position of two coexisting phases
Liquid–liquid phase separation leads to the formation of condensed phases that coexist with a fluid. Here we investigate how the positions of a condensed phase can be controlled by using concentration gradients of a regulator that influences phase separation. We consider a mean field model of a ternary mixture where a concentration gradient of a regulator is imposed by an external potential. A novel first order phase transition occurs at which the position of the condensed phase switches in a discontinuous manner. This mechanism could have implications for the spatial organisation of biological cells and provides a control mechanism for droplets in microfluidic systems.
Cononsolvency Transition of Polymer Brushes: A Combined Experimental and Theoretical Study
In this study, the cononsolvency transition of poly(N-isopropylacrylamide) (PNiPAAm) brushes in aqueous ethanol mixtures was studied by using Vis-spectroscopic ellipsometry (SE) discussed in conjunction with the adsorption-attraction model. We proved that the cononsolvency transition of PNiPAAm brushes showed features of a volume phase transition, such as a sharp collapse, reaching a maximum decrease in thickness for a very narrow ethanol volume composition range of 15% to 17%. These observations are in agreement with the recently published preferential adsorption model of the cononsolvency effect.
Correlations in multithermostat Brownian systems with Lorentz force
We study the motion of a Brownian particle subjected to Lorentz force due to an external magnetic field. Each spatial degree of freedom of the particle is coupled to a different thermostat. We show that the magnetic field results in correlation between different velocity components in the stationary state. Integrating the velocity autocorrelation matrix, we obtain the diffusion matrix that enters the Fokker-Planck equation for the probability density. The eigenvectors of the diffusion matrix do not align with the temperature axes. As a consequence the Brownian particle performs spatially correlated diffusion. We further show that in the presence of an isotropic confining potential, an unusual, flux-free steady state emerges which is characterized by a non-Boltzmann density distribution, which can be rotated by reversing the magnetic field. The nontrivial steady state properties of our system result from the Lorentz force induced coupling of the spatial degrees of freedom which cease to exist in equilibrium corresponding to a single-temperature system. © 2020 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft.