Molecular Dynamics Simulations of Bottle-Brush Polymers with a Flexible Backbone under Theta and Good Solvent Conditions

Author(s)

Panagiotis Theodorakis, Nikolaos G. Fytas

Abstract

Using molecular dynamics simulations of a standard bead-spring model for

polymer chains,bottle-brush polymerswith a flexible backbone of N_beffective units,where side chains of length N are grafted under theta and good solvent conditionsin the range^, are studied.The range of backbone and side chains' length varies correspondingly asandfor

two different grafting densities σ, namely σ=0.5 and 1.0.Even at

temperatures T close to the theta point the side chains are

significantly stretched, as it has been confirmed for bottle brushes

with a rigid backbone, their linear dimension depending on the solvent

quality only weakly. However, the distribution of monomers shows a more

pronounced dependence, which we characterize through the asphericity and

acylindricity as functions of σ, T, N_b, and N. In particular, increase of σ, T, N_b,

and N increases the normalized asphericity and acylindricity of the

macromolecule. Interestingly, we also find that the dimensions of the

side chains reveals differences in the distributions of side chain

monomers by changing the backbone length N_b as the region

between the two backbone-ends increases. A method to extract the

persistence length of bottle-brush macromolecules and its drawbacks is

also discussed given that different measures of the persistence length

are not mutually consistent with each other and depend distinctly both

on N_b and the solvent quality.Macromolecules which consist of

a backbone where side chains are graftedrandomly or regularly have

recently found much interest[1-6]. Such macromolecules are described in

terms of their structure by a multitudeof parameters, such as the

backbone length N_b and the grafting densitythat the side

chains with length N are grafted ontothe flexible backbone, while

solvent conditions may also varyby variation of the temperature T or the

pH of the solution resulting inthe structural change of these

stimuli-responsive macromolecules.The response of the large scale

structure of bottle-brush polymers tosolvent conditions is an intriguing

We recall that for linear chains, the theta temperaturefor the present

(implicit solvent) model has been roughlyestimated[46] as T_theta≈ 3.0 (note, however, that there is still some uncertainty about the precise value of T_theta,for a similar model[47] T_theta=

3.18 in this case, couldonly be established for chain lengths exceeding

N= 200).Thus, in the present work we have thoroughly studied

thetemperature range.

From previous work[48] on rather long chains in polymer brushes on flat

surfaces, using the same model[Eqs. (1) and (2)] to describe the

interactions, it is known that for T= 4.0 one finds a behaviour

characteristicfor (moderately) good solvents. Very good solvent

conditionscould be obtained from a slightly different model that

hasextensively been studied for standard polymer brushes[40,49],where

the cut-off in Eq. (1) is chosen to coincide with theminimum of the

potential, (and then also T= 1 can be chosen for this essentially a-thermal model).

 

Organisation(s)

Computational and Soft Matter Physics

External organisation(s)

University of Patras

Journal

American Journal of Condensed Matter Physics

Volume

2

Pages

101-108

No. of pages

8

ISSN

2163-1115

DOI

https://doi.org/10.5923/j.ajcmp.20120204.05

Publication date

2012

Peer reviewed

Yes

Austrian Fields of Science 2012

103002 Acoustics

Portal url

https://ucrisportal.univie.ac.at/en/publications/d1e72f5e-302f-44a0-9b45-c5ccf7012e0c