Responsiveness Of Polymer Brushes: Coarse-Grained Molecular Dynamics Studies Based On Martini Force Field Framework

Polymer brush （PB） modification is considered as the most important approach to endowmaterial surfaces with stimulus-responsiveness. PB has great potential applications in thefields such as surface wetting, microfluidic control and its electronic devices, mechanicalmotor, chemical sensors, biocompatible materials, controlled drug delivery systems, cellculture and separation and synthesis of nanomaterials etc.Aiming at exploring the environmental stimulus-responsiveness of three typical flatlygrafted polymer brushes: homopolymer brushes, binary mixed brushes and block copolymerbrushes, coarse-grained molecular dynamics （CGMD） simulations were employed to studythree different specific flatly grafted brush systems: PNIPAm polymer brushes, PS-PEObinary mixed brushes and PS-b-PAA block copolymer brushes based on Martini force fieldframework.Firstly, how the grafting density （GD, σ） and degree of polymerization （DP, n） ofPNIPAm influence the conformation change behavior of PNIPAm brushes in water andbenzene was investigated. The simulation results indicated that the thickness of brushesdisplayed dramatic changes in different solvents when σ ranges from0.25to1.0chains nm^-2.Moreover, when σ was0.4chains nm^-2and n equaled to50⁷0, the brush height experiencedthe largest change. Secondly, we studied the influence of σ and the ratio of DP for PEO andPS nPEO-PSexerted on the structural changes of PS-PEO binary mixed brushes. It is found thatbrushes have large thickness change with the σ in0.51¹.56chains nm^-2and composition ofthe top surface of brush displays solvent-driven changes for nPEO-PSin60:50⁷0:50when putinto water and benzene respectively. Finally, two CG models of water （standard Martini watermodel and polarizable water model） were employed to study the pH-responsiveness of PAAbrushes and found that both CG water models are good enough to reproduce thepH-responsive behavior of PAA brushes. Took a move forward, we explored the pH/solventdual responsiveness of PS-b-PAA block copolymer brushes using the standard Martini watermodel. There existed phase separation along the stretching direction of copolymer brusheswith PAA above and PS below. Density profile analyses showed that PS block dominatedlittle responsiveness to different solvent conditions resulting from the influence of PAA blockor the limitation of adopted force field. Density profiles of PAA block showed great variationwhen the solvent or pH was changed. Easily, we can conclude that PS25-b-PAA25brushsystem displayed a good solvent-pH dual reponsiveness when σ was0.4chains nm^-2. Nodoubt that fraction of PAA or PS （FPAAor FPS） in PS-b-PAA is a key factor in tuning the dual-responsiveness of PS-b-PAA block copolymer brushes.