Interplay between Chain Collapse and Microphase Separation in Bottle-Brush Polymers with Two Types of Side Chains

Abstract

Conformations of a bottle-brush polymer with two types (A,B) of grafted side chains are studied by molecular dynamics simulations, using a coarse-grained bead−spring model with side chains of up to N = 50 effective monomers. Varying the solvent quality and the grafting density, the crossover from the “pearl-necklace” structure to dense cylinders is studied. Whereas for small grafting density, A- and B-chains form separate collapsed chains, at intermediate grafting density, larger “pearls” containing several chains are observed, exhibiting microphase separation between A and B in “dumbbell”-type configurations. At still larger grafting density, short-range order of “Janus dumbbell”-type is observed. It is argued that because of the quasi-1D character of bottle-brush polymers with stiff backbones, all phase changes occur gradually, and no sharp-phase transitions like in bulk polymer mixtures or block copolymer melts can be observed.

Effect of the solvent quality on the structural rearrangement of spherical brushes: coarse-grained models

“…For small grafting densities, where under good solvent conditions the grafted chains can be described as ''polymer mushrooms'' 32 interacting only weakly with each other, the grafted chains collapse individually into dense globules. [34][35][36][37] This behavior is similar to the collapse of free chains in dilute solution under poor solvent conditions. 38 For high enough grafting density, the thickness of the grafted polymer layer shrinks uniformly, until the melt density is reached, [28][29][30][31][32][33][34][35][36][37]39 in the bulk of the film coating the planar or cylindrical surface.…”

“…The details of the microphase separation of the chain monomers depend on several parameters such as the grafting density, the number of monomer per chain, the temperature and the geometry of the substrate. [28][29][30][31][32][33][34][35][36][37] Here we want to focus on a spherical grafted surface.…”

“…Indeed several chains cluster together in ''dimples'' for planar brushes, 28,29 or form pearl-necklace structures when the chains are grafted to thin cylinders. [34][35][36][37] The detailed conformational scenario strongly depends on the geometry of the grafted surface. In this paper we analyze the phase behavior of spherical polymer brushes under poor solvent conditions, by means of coarsegrained models.…”

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

“…Of course, our study does not address any effectsdue to a particular chemistry relat ing to the synthesis of thesebottle-brush polymers, but as usually done [40,44,45], we address universal features of the conformat ional properties of thesemacromo lecules. There is one important distinction relating to our previouswork [10][11][12][13][14] on bottle-brush polymers with rig id backbones: following Grest and Murat [40], there the backbone was taken as aninfinitely straight line in continuous space, thus allowingarbit rary values of the distances between neighbouring graftingsites, and hence the grafting density σ could be continuouslyvaried. For the present model, where we disregard anypossible quenched disorder resulting fro m the graft ing process, ofcourse, the grafting density σ is quantized: we denote here byσ=1 the case that every backbone monomer carries a sidechain, σ=0.5 means that every second backbone monomer hasa side chain, etc.…”

“…Such macro mo lecules 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 macro molecules.The response of the large scale structure of bottle-brush polymers tosolvent conditions is an intriguing Biopolymers with a related arch itecture are also abundant in nature;fo r examp le proteoglycans [8] o r the agg recan mo lecules kept responsible for the very good lubricating properties in human joints [9]. In thiscontext the change in the solvent conditions is an important parameter, and the influence of these parameters on bottle-brush polymers has beenstudiedfor the case of single [10][11][12] and two-component [12][13][14] bottle brushes with a rig id backbone,suggesting in agreement with theoretical predictions structures rangingfrom individual collapsed chains at low g rafting densities to the so-called "pearl-necklace" structures for intermed iate densities andto homogenous cylinders and Janus-like structures at even higher densities.…”