scispace - formally typeset
Search or ask a question
Journal ArticleDOI

Structure and dynamics of atactic Na+-poly(acrylic) acid (PAA) polyelectrolyte in aqueous solution in dilute, semi-dilute and concentrated regimes

14 May 2019-Molecular Simulation (Taylor & Francis)-Vol. 45, Iss: 11, pp 876-895
TL;DR: In this paper, structural and dynamic properties of aqueous solution of atactic poly(acrylic) acid in dilute, semi-dilute and concentrated regimes were studied by fully atomistic molecular dynamics simulati...
Abstract: Structural and dynamic properties of aqueous solution of atactic poly(acrylic) acid (PAA) in dilute, semi-dilute and concentrated regimes were studied by fully atomistic molecular dynamics simulati...
Citations
More filters
Journal ArticleDOI
TL;DR: In this paper, the authors investigate theoretically charge regulation of weakly dissociating polyacids by potentiometric titration of their aqueous solutions, by treating deprotonation and cation binding to the polyacid as reversible reactions in their model, the ionization constant of acid groups along the polyacid chain is adjusted from its intrinsic value by electrostatic correlations.

20 citations

Journal ArticleDOI
TL;DR: It is observed that at low polymer concentrations PDMAEMA chains adopt a stiffer and slightly extended conformation due to excluded-volume effects and electrostatic repulsions within the polymer chains, as the polymer concentration increases above 20 wt %, and adopt more flexible conformations.
Abstract: A combined experimental and molecular dynamics (MD) study is performed to investigate the effect of polymer concentration on the zero shear rate viscosity η0 of a salt-free aqueous solution of poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA), a flexible thermoresponsive weak polyelectrolyte with a bulky 3-methyl-1,1-diphenylpentyl unit as the terminal group. The study is carried out at room temperature (T = 298 K) with relatively short PDMAEMA chains (each containing N = 20 monomers or repeat units) at a fixed degree of ionization (α+ = 100%). For the MD simulations, a thorough validation of several molecular mechanics force fields is first undertaken for assessing their capability to accurately reproduce the experimental observations and established theoretical laws. The generalized Amber force field in combination with the restrained electrostatic potential charge fitting method is eventually adopted. Three characteristic concentration regimes are considered: the dilute (from 5 to 10 wt %), the semidilute (from 10 to 20 wt %), and the concentrated (from 20 to 29 wt %); the latter two are characterized by polymer concentrations cp higher than the characteristic overlap concentration cp*. The structural behavior of the PDMAEMA chains in the solution is assessed by calculating the square root of their mean-square radius of gyration «Rg 2»0.5, the square root of the average square chain end-to-end distance «Ree 2»0.5, the ratio «Ree 2»/«Rg 2», and the persistence length Lp. It is observed that at low polymer concentrations, PDMAEMA chains adopt a stiffer and slightly extended conformation because of excluded-volume effects (a good solvent is considered in this study) and electrostatic repulsions within the polymer chains. As the polymer concentration increases above 20 wt %, the PDMAEMA chains adopt more flexible conformations, as the excluded-volume effects seize and the charge repulsion within the polymer chains subsides. The effect of total polymer concentration on PDMAEMA chain dynamics in the solution is assessed by calculating the orientational relaxation time τc of the chain, the center-of-mass diffusion coefficient D, and the zero shear rate viscosity η0; the latter is also measured experimentally here and found to be in excellent agreement with the MD predictions.

13 citations

Posted Content
TL;DR: In this paper, the authors reported self-diffusion and viscosity data for sodium polystyrene sulfonate (NaPSS) in semidilute salt-free aqueous solutions measured by pulsed field gradient NMR and rotational rheometry respectively.
Abstract: We report chain self-diffusion and viscosity data for sodium polystyrene sulfonate (NaPSS) in semidilute salt-free aqueous solutions measured by pulsed field gradient NMR and rotational rheometry respectively. The observed concentration dependence of $\eta$ and $D$ are consistent with the Rouse-Zimm scaling model with a concentration dependent monomeric friction coefficient. The concentration dependence of the monomeric friction coefficient exceeds that expected from free-volume models of diffusion, and its origin remains unclear. Correlation blobs and dilute chains with equivalent end-to-end distances exhibit nearly equal friction coefficients, in agreement with scaling. The viscosity and diffusion data are combined using the Rouse model to calculate the chain dimensions of NaPSS in salt-free solution, these agree quantitatively with SANS measurements.

11 citations

Journal ArticleDOI
TL;DR: Molecular dynamics simulations are used to study structure and dynamics of poly(vinyl alcohol) and water in aqueous solution as a function ofconcentration at different temperatures in the range 278.
Abstract: Molecular dynamics simulations are used to study structure and dynamics of poly(vinyl alcohol) and water in aqueous solution as a function ofconcentration at different temperatures in the range 278...

5 citations

References
More filters
Journal ArticleDOI
TL;DR: In this paper, all-atom molecular dynamics simulations of aqueous solutions of model oligomers of HPMC and HPMCARAS excipients interacting with a representative poorly soluble active pharmaceutical ingredient (API), phenytoin, were performed.
Abstract: We have performed all-atom molecular dynamics simulations of aqueous solutions of model oligomers of hydroxypropyl methylcellulose (HPMC) and hydroxypropyl methylcellulose acetate succinate (HPMCAS) excipients interacting with a representative poorly soluble active pharmaceutical ingredient (API), phenytoin. Simulations reveal formation of excipient-API complexes for some of the oligomers, which results in a reduction of API aggregation. API aggregation and diffusivity decreased with an increase in excipient content. Excipients form a "gel-like" phase spanning the simulation box beyond ∼10 wt %; API diffusivity within this gel phase is much smaller than API diffusivity without excipient, and decreases exponentially, by 5 orders of magnitude, with increased polymer concentration. Substantial differences are observed with variations in methyl, hydroxypropyl, acetate, and succinate substitution levels in the model oligomers and with the deprotonation state of succinate groups, with strongest interactions with hydrophobic phenytoin observed in the case of acetate substitution. These are used to develop quantitative measures of excipient-API interactions and excipient efficiency in the inhibition of API aggregation. We also find that for model oligomers based on Methocel E (manufactured by Dow Pharma & Food Solutions) chemistry, oligomers of length 10 monomers and simulation boxes of size 7 nm give results similar to those for longer oligomers and bigger boxes. The quantitative measures developed in this study are expected to prove useful as computational screening tools in excipient design.

53 citations

Journal ArticleDOI

51 citations


"Structure and dynamics of atactic N..." refers result in this paper

  • ...Our results are also in agreement with the experimental calorimetric measurements of atactic PAA in dilute aqueous solution which showed that the enthalpy of dissociation becomes more negative with increase in degree-of-neutralisation of PAA [91]....

    [...]

Journal ArticleDOI
TL;DR: This work finds that the molecular topology influences the fraction of counter-ions transiently associating with the polyelectrolyte on a scale of the order of the chain segments, forming a "condensed" counter-ion interfacial layer.
Abstract: We explore the monovalent counter-ion distribution around flexible highly-charged polyelectrolytes with different molecular architectures (linear chains, stars, and unknotted and trefoil rings) using molecular dynamics simulations that include an explicit solvent that interacts with the polyelectrolyte. In particular, we find that the molecular topology influences the fraction of counter-ions transiently associating with the polyelectrolyte on a scale of the order of the chain segments, forming a "condensed" counter-ion interfacial layer. As with the hydrogen bonding of water to proteins and other polymers, the persistence time of these interfacial "bound" counter-ions is relatively short, O(1 ps), and we characterize the fluctuations in the number of the counter-ions populating the interfacial layer. We also find that the counter-ions are distributed in a non-uniform fashion on the polyelectrolyte backbone, forming dynamical clusters whose form and average size is sensitive to molecular architecture. In addition, we find that the residual bound counter-ions, not located in either the interfacial layer or the bulk solution, form a diffuse ionic cloud around the polyelectrolyte due to the uncompensated polyelectrolyte charge along the backbone. Generally charge valence strongly influences the extent of the diffuse counter-ion cloud, but in the case of monovalent counter-ions, we find that the size of the diffuse counter-ion cloud nearly coincides with the polyelectrolyte radius of gyration, independent of molecular topology.

50 citations


"Structure and dynamics of atactic N..." refers background in this paper

  • ...For this chain length of PAA periodic box size 8 nm chosen for our simulations meets the necessary condition L > 5Rg for the counter-ions to not interact with their images [66]....

    [...]

Journal ArticleDOI
TL;DR: In this article, the static and dynamic properties of dilute and semidilute salt-free polyelectrolyte solutions are studied using Brownian dynamics simulations, where the polymer molecules and counterions are modeled as charged bead-spring chains and charged spheres, respectively.
Abstract: The static and dynamic properties of dilute and semidilute salt-free polyelectrolyte solutions are studied using Brownian dynamics simulations. The polymer molecules and counterions are modeled as charged bead–spring chains and charged spheres, respectively. Results are presented for the static properties, polymer rotational dynamics, self and collective diffusion coefficients, intermediate scattering functions, and dynamic structure factors of polyelectrolytes and counterions. The simulations reveal a strong dynamical coupling between counterions and polyions at low concentrations; this coupling becomes weaker as the concentration is increased. The polyion self-diffusion coefficient shows a nonmonotonic concentration dependence, but the counterion self diffusion coefficient decreases monotonically as the concentration is increased. In semidilute solutions, the collective density relaxation dynamics are slowed down at wave vectors where there is a peak in the corresponding static structure factor. This ca...

49 citations


"Structure and dynamics of atactic N..." refers background or methods or result in this paper

  • ...The observed trend of the decrease in R with fp at f = 1, is in qualitative agreement with results of Brownian Dynamics simulations available in the literature [6]....

    [...]

  • ...The shift of peak position qmax to higher values of the wave vector, with increase in polymer concentration, has been shown by coarsegrained Brownian dynamics simulations [6]....

    [...]

  • ...When the crossover concentration from semidilute to concentrated regime approaches, the diffusion mechanism of PAA is accompanied by short-ranged intermolecular interaction due to close packing of PAA and Na counter-ions, and thus self-diffusion coefficient of PAA decreases [6]....

    [...]

  • ...ent with the observation from earlier work in the literature [6] using Brownian dynamic simulation of model polyelectrolytes...

    [...]

  • ...Our results are also in qualitative agreement with earlier work on model polyelectrolytes in solution in the semi-dilute regime [6]....

    [...]

Journal ArticleDOI
TL;DR: In this paper, a coarse-grained (CG) model for methylcellulose polymers, including random copolymers with compositions representative of modeling commercial METHOCEL A polymer, using one CG bead per monomer.
Abstract: We develop a systematic coarse-grained (CG) model for methylcellulose polymers, including random copolymers with compositions representative of modeling commercial METHOCEL A polymer, using one CG bead per monomer. We parametrize our CG model using the RDFs from atomistic simulations of short methylcellulose oligomers, extrapolating the results to long chains. Using a LJ 9–6 potential, the CG model captures the effect of monomer substitution type and temperature observed in detailed atomistic simulations. We use dissociation free energy to validate our CG model against the atomistic model. We then use this CG model to simulate single chains up to 1000 monomers long, and we calculate persistence lengths for a selection of homogeneous and heterogeneous methylcellulose chains, which show good agreement with experimental results. Interestingly, simulations of 600-mer heterogeneous chains show a collapse transition at 50 °C and form a stable ring structure with outer diameter around 14 nm. This structure appea...

48 citations