Influence of surface confinement and curvature on the formation of loop brushes
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Authors
Wen, Mengyao
Issue Date
2025-05
Type
Electronic thesis
Thesis
Thesis
Language
en_US
Keywords
Materials engineering
Alternative Title
Abstract
Polymer chain conformation and behavior under geometric confinement have been widely explored due to their importance in applications such as nanotechnology, biomaterials, and surface functionalization. If long flexible macromolecule is covalently anchored on a substrate surface, polymer brush is obtained. Depending on molecular weight of the polymer and the number of grafting sites (graft density), surface properties such as adhesion, wetting, and lubrication can be tuned. One acquires loop brushes either by reating both ends of the chain to the substrate or reacting the free ends of grafted chains. While extensive studies have focused on linear polymer brush behavior on various substrates as a function of curvature, formation of loops and loop chain conformation as a function of curvature remains less explored. In the current study, the static properties of linear polymer chains grafted to flat and spherical surfaces and their tendency to form loop grafts were investigated using Monte Carlo simulations. The end-to-end distance (R), radius of gyration (Rg), distance between grafting sites (S), and distance between the free ends of the grafted chains (L) were analyzed to understand how confinement by an impenetrable surface and its curvature influences grafted chain conformation, providing insights into the probability of forming loop brushes on the surface.Our simulations reveal distinct power law scaling behaviors of polymer conformations influenced by surface curvature: the power scaling exponent for R decreases from 0.48 for polymers grown in free space, to 0.45 for polymers grafted onto a flat surface, and further reduces to 0.42 for polymers grafted onto a spherical surface, reflecting increased geometric constraint on polymer conformation due to surface curvature. Additionally, surface curvature significantly impacts the orientation of grafted polymers, with average angles between chain end-to-end vectors being ap- proximately 62 − 65◦ on a flat surface and 90◦ on a spherical surface. This difference in the average angle affects the distance between graft chain ends, L, explaining why polymer chains on spherical surfaces display greater average chain end separation, ⟨L⟩, despite having smaller ⟨R⟩ values.
An unexpected observation is the appearance of bimodal distributions in L histograms of short polymer chains confined to a spherical surface. It is believed that this bimodal distribution is an artifact of the way L is calculated. While L is calculated as the shortest distance between graft chain ends, S is calculated as the arc length between grafting sites. At low chain lengths, the end-to-end distance of the grafted chain is comparable to the radius of the spherical surface or less than it and the L vector might span through the sphere itself and become artificially smaller than the corresponding value of S. This is especially true when the grafting sites are far apart from each other – at large S values.
These findings advance our understanding of how geometric confinement influences polymer conformations and provide a basis for future investigations into loop brush formation and polymer surface interactions relevant to nanotechnology and biomaterials engineering.
Description
May2025
School of Engineering
School of Engineering
Full Citation
Publisher
Rensselaer Polytechnic Institute, Troy, NY