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Register a new userSelective molecular transport in thermo-responsive polymer membranes: role of nanoscale hydration and fluctuations
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For a wide range of modern soft functional materials the selective transport of sub-nanometer-sized molecules (`penetrants) through a stimuli-responsive polymeric membrane is key to the desired function. In this study, we investigate the diffusion properties of penetrants ranging from non-polar to polar molecules and ions in a matrix of collapsed Poly(N-isopropylacrylamide) (PNIPAM) polymers in water by means of extensive molecular dynamics simulations. We find that the water distributes heterogeneously in fractal-like cluster structures embedded in the nanometer-sized voids of the polymer matrix. The nano-clustered water acts as an important player in the penetrant diffusion, which proceeds via a hopping mechanism through `wet transition states: the penetrants hop from one void to another via transient water channels opened by rare but decisive polymer fluctuations. The diffusivities of the studied penetrants extend over almost five orders of magnitude and thus enable a formulation of an analytical scaling relation with a clear non-Stokesian, exponential dependence of the diffusion coefficient on the penetrants radius for the uncharged penetrants. Charged penetrants (ions) behave differently as they get captured in large isolated water clusters. Finally, we find large energetic activation barriers for hopping, which significantly depend on the hydration state and thereby challenge available transport theories.
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In the popular solution-diffusion picture, the membrane permeability is defined as the product of the partition ratio and the diffusivity of penetrating solutes inside the membrane in the linear response regime, i.e., in equilibrium. However, of practical importance is the penetrants flux across the membrane driven by external forces. Here, we study nonequilibrium membrane permeation orchestrated by a uniform external driving field using molecular computer simulations and continuum (Smoluchowski) theory in the stationary state. In the simulations, we explicitly resolve the penetrants transport across a finite monomer-resolved polymer network, addressing one-component penetrant systems and mixtures. We introduce and discuss possible definitions of nonequilibrium, force-dependent permeability, representing `system and `membrane permeability. In particular, we present for the first time a definition of the differential permeability response to the force. We demonstrate that the latter turns out to be significantly nonlinear for low-permeable systems, leading to a high amount of selectiveness in permeability, called `permselectivity, and is tunable by the driving force. Our continuum-level analytical solutions exhibit remarkable qualitative agreement with the penetrant- and polymer-resolved simulations, thereby allowing us to characterize the underlying mechanism of permeabilities and steady-state transport beyond the linear response level.
Improving lithium-ion batteries (LIBs) safety remains in a challenging task when compared with the tremendous progress made in their performance in recent years. Embedding thermo-responsive polymer switching materials (TRPS) into LIB cells has been proved to be a promising strategy to provide consistent thermal abuse protections at coin-cell level. However, it is unrealistic to achieve large-scale applications without further demonstration in high-capacity pouch cells. Here, we employed tungsten carbide (WC) as a novel conductive filler, and successfully overcame the intrinsic processing difficulty of polyethylene (PE) matrix in a scalable solvent-based method to obtain ultra-thin, uniform, highly conductive TRPS. Moreover, by integrating TRPS directly into LIB electrodes, no extra fabrication facilities or processes are required for making the cells. As a result, multi-layer pouch cells with consistent electrochemical performance and thermal abuse protection function were fabricated using industry relevant manufacturing techniques, which brings TRPS one step further to the real application scenarios.
Motivated by a freely suspended graphene and polymerized membranes in soft and biological matter we present a detailed study of a tensionless elastic sheet in the presence of thermal fluctuations and quenched disorder. The manuscript is based on an extensive draft dating back to 1993, that was circulated privately. It presents the general theoretical framework and calculational details of numerous results, partial forms of which have been published in brief Letters (Le Doussal and Radzihovsky 1992). The experimental realization of atom-thin graphene sheets has driven a resurgence in this fascinating subject, making our dated predictions and their detailed derivations timely. To this end we analyze the statistical mechanics of a generalized D-dimensional elastic membrane embedded in d dimensions using a self-consistent screening approximation (SCSA), that has proved to be unprecedentedly accurate in this system, exact in three complementary limits: d --> infinity, D --> 4, and D=d. Focusing on the critical flat phase, for a homogeneous two-dimensional membrane embedded in three dimensions, we predict its universal length-scale dependent roughness, elastic moduli exponents, and a universal negative Poisson ratio of -1/3. We also extend these results to short- and long-range correlated random heterogeneity, predicting a variety of glassy wrinkled membrane states. Finally, we also predict and analyze a continuous crumpling transition in a phantom elastic sheet. We hope that this detailed presentation of the SCSA theory will be useful for further theoretical developments and corresponding experimental investigations on freely suspended graphene.
The uptake and sorption of charged molecules by responsive polymer membranes and hydrogels in aqueous solutions is of key importance for the development of soft functional materials. Here we investigate the partitioning of simple monoatomic (Na$^+$, K$^+$, Cs$^+$, Cl$^-$, I$^-$) and one molecular ion (4-nitrophenolate; NP$^-$) within a dense, electroneutral poly($N$-isopropylacrylamide) membrane using explicit-water molecular dynamics simulations. Inside the predominantly hydrophobic environment water distributes in a network of polydisperse water nanoclusters. The average cluster size determines the mean electrostatic self-energy of the simple ions, which preferably reside deeply inside them; we therefore find substantially larger partition ratios $Ksimeq>$10$^{-1}$ than expected from a simple Born picture using a uniform dielectric constant. Despite their irregular shapes we observe that the water clusters possess a universal negative electrostatic potential with respect to their surrounding, as is known for aqueous liquid-vapor interfaces. This potential, which we find concealed in cases of symmetric monoatomic salts, can dramatically impact the transfer free energies of larger charged molecules because of their weak hydration and increased affinity to interfaces. Consequently, and in stark contrast to the simple ions, the molecular ion NP$^-$ can have a partition ratio much larger than unity, $Ksimeq>$10-30 (depending on the cation type) or even $10^3$ in excess of monovalent salt, which explains recent observations of enhanced reaction kinetics of NP$^-$ reduction catalyzed within dense polymer networks. These results also suggest that ionizing a molecule can even enhance the partitioning in a collapsed, rather hydrophobic gel, which strongly challenges the traditional simplistic reasoning.
We investigate by means of molecular dynamics simulation a coarse-grained polymer glass model focusing on (quasi-static and dynamical) shear-stress fluctuations as a function of temperature T and sampling time $Delta t$. The linear response is characterized using (ensemble-averaged) expectation values of the contributions (time-averaged for each shear plane) to the stress-fluctuation relation $mu_{sf}$ for the shear modulus and the shear-stress relaxation modulus $G(t)$. Using 100 independent configurations we pay attention to the respective standard deviations. While the ensemble-averaged modulus $mu_{sf}(T)$ decreases continuously with increasing T for all $Delta t$ sampled, its standard deviation $delta mu_{sf}(T)$ is non-monotonous with a striking peak at the glass transition. The question of whether the shear modulus is continuous or has a jump-singularity at the glass transition is thus ill-posed. Confirming the effective time-translational invariance of our systems, the $Delta t$-dependence of $mu_{sf}$ and related quantities can be understood using a weighted integral over $G(t)$. This implies that the shear viscosity $eta(T)$ may be readily obtained from the $1/Delta t$-decay of $mu_{sf}$ above the glass transition.
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