Effect of Polymer Concentration on the Structure and Dynamics of Short Poly(N, Ndimethylaminoethyl methacrylate) in Aqueous Solution : A Combined Experimental and Molecular Dynamics Study Mintis, Dimitris G. ; Dompé, Marco ; Kamperman, Marleen ; Mavrantzas, Vlasis G.  \ 2020
The Journal of Physical Chemistry Part B: Condensed Matter, Materials, Surfaces, Interfaces & Biophysical 124 (2020).  ISSN 15206106  p. 240  252. A combined experimental and molecular dynamics (MD) study is performed to investigate the effect of polymer concentration on the zero shear rate viscosity η_{0} of a saltfree aqueous solution of poly(N,Ndimethylaminoethyl methacrylate) (PDMAEMA), a flexible thermoresponsive weak polyelectrolyte with a bulky 3methyl1,1diphenylpentyl 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 c_{p} 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 meansquare radius of gyration «R_{g} ^{2}»^{0.5}, the square root of the average square chain endtoend distance «R_{ee} ^{2}»^{0.5}, the ratio «R_{ee} ^{2}»/«R_{g} ^{2}», and the persistence length L_{p}. It is observed that at low polymer concentrations, PDMAEMA chains adopt a stiffer and slightly extended conformation because of excludedvolume 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 excludedvolume 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 τ_{c} of the chain, the centerofmass diffusion coefficient D, and the zero shear rate viscosity η_{0}; the latter is also measured experimentally here and found to be in excellent agreement with the MD predictions. 

Continuum formulation of the ScheutjensFleer lattice statistical theory for homopolymer adsorption from solution Mavrantzas, V.G. ; Beris, A.N. ; Leermakers, F.A.M. ; Fleer, G.J.  \ 2005
Journal of Chemical Physics 123 (2005)17.  ISSN 00219606  p. 1  11. Homopolymer adsorption from a dilute solution on an interacting (attractive) surface under static equilibrium conditions is studied in the framework of a Hamiltonian model. The model makes use of the density of chain ends n1,e and utilizes the concept of the propagator G describing conformational probabilities to locally define the polymer segment density or volume fraction ¿; both n1,e and ¿ enter into the expression for the system free energy. The propagator G obeys the Edwards diffusion equation for walks in a selfconsistent potential field. The equilibrium distribution of chain ends and, consequently, of chain conformational probabilities is found by minimizing the system free energy. This results in a set of model equations that constitute the exact continuumspace analog of the ScheutjensFleer (SF) lattice statistical theory for the adsorption of interacting chains. Since for distances too close to the surface the continuum formulation breaks down, the continuum model is here employed to describe the probability of chain configurations only for distances z greater than 2l, where l denotes the segment length, from the surface; instead, for distances z¿2l, the SF lattice model is utilized. Through this novel formulation, the lattice solution at z=2l provides the boundary condition for the continuum model. The resulting hybrid (lattice for distances z¿2l, continuum for distances z>2l) model is solved numerically through an efficient implementation of the pseudospectral collocation method. Representative results obtained with the new model and a direct application of the SF lattice model are extensively compared with each other and, in all cases studied, are found to be practically identical.
