PDADMAC/PSS Oligoelectrolyte Multilayers: Internal Structure and Hydration Properties at Early Growth Stages from Atomistic Simulations.

Pedro A Sánchez,Marcello Sega,Martin Vögele,Jens Smiatek,Baofu Qiao,Christian Holm

doi:10.3390/molecules25081848

Abstract

We analyze the internal structure and hydration properties of poly(diallyl dimethyl ammonium chloride)/poly(styrene sulfonate sodium salt) oligoelectrolyte multilayers at early stages of their layer-by-layer growth process. Our study is based on large-scale molecular dynamics simulations with atomistic resolution that we presented recently [Sánchez et al., Soft Matter 2019, 15, 9437], in which we produced the first four deposition cycles of a multilayer obtained by alternate exposure of a flat silica substrate to aqueous electrolyte solutions of such polymers at 0.1M of NaCl. In contrast to any previous work, here we perform a local structural analysis that allows us to determine the dependence of the multilayer properties on the distance to the substrate. We prove that the large accumulation of water and ions next to the substrate observed in previous overall measurements actually decreases the degree of intrinsic charge compensation, but this remains as the main mechanism within the interface region. We show that the range of influence of the substrate reaches approximately 3 nm, whereas the structure of the outer region is rather independent from the position. This detailed characterization is essential for the development of accurate mesoscale models able to reach length and time scales of technological interest.

Highlights

Layer-by-layer (LbL) deposition is one of the most relevant methods currently available for the synthesis of soft materials from ionic building blocks [1,2,3]
In order to decrease the computing cost to a level that makes the analysis of some mesoscopic properties feasible, we focused on PEMs formed by low molecular weight polyions, or oligoelectrolyte multilayers (OEMs)
We presented the results of an extended analysis, based on local correlation measurements, of previous atomistic simulations performed to study the formation of a four layer oligoelectrolyte system created by LbL deposition on a hydrophilic substrate

Summary

Introduction

Layer-by-layer (LbL) deposition is one of the most relevant methods currently available for the synthesis of soft materials from ionic building blocks [1,2,3]. We extended our atomsitic simulations to obtain a system of four layers of the same oligoelectrolytes, (PDADMAC/PSS) , with a degree of polymerization DP = 30, deposited from water solutions with 0.1M of NaCl on a charged silica substrate and subsequently rinsed with pure water The size of this system was large enough to obtain some mesoscopic properties—as the surface roughness, film thickness and quantity of adsorbed material—comparable to experimental measurements. The characterization of the local properties at different positions within the multilayer will allow us to determine the range of influence of the substrate The latter is essential to choose the atomistic information that may serve to the future development of accurate mesoscale models, based on bottom-up approaches, intended to overcome the limitations in length and time scales of full atomistic simulations. Results for these systems are compared to that of single oligomer chains in bulk solutions of the same ionic strength

Results and Discussion

Materials and Methods

Conclusions