Elsevier

Polymer

Volume 47, Issue 20, 20 September 2006, Pages 7318-7327
Polymer

Bottle-brush macromolecules in solution: Comparison between results obtained from scattering experiments and computer simulations

https://doi.org/10.1016/j.polymer.2006.06.010Get rights and content

Abstract

We addressed the structure of bottle-brush macromolecules under good solvent conditions by means of scattering techniques and computer simulations. Architectural parameters, such as backbone length, side chain length and side chain stiffness were varied systematically. A consistent description of the form factors was achieved by describing the bottle-brush polymers as flexible cylinders with internal density fluctuations. The model leads to direct conclusions about parameters, such as the brush persistence length, describing the overall shape of the bottle-brush polymers. Indirect conclusions about the side chain and backbone conformation can be drawn. Experimental results were compared to those obtained from computer simulations carried out for single bottle-brush polymers using the cooperative motion algorithm. The simulation gives direct access to the pair correlation function, allowing an independent determination of the form factor. In addition direct information about the side chain and backbone conformation can be obtained. The critical parameter for lyotropic behavior of bottle-brush polymer solutions is the ratio of brush persistence length to diameter which should be of order 10 or larger. Thus the discussion of the results is focused on the impact of the architectural parameters on the persistence of the bottle-brush polymers. Experimental results on the lyotropic behavior of a bottle-brush polymer are presented.

Introduction

Bottle-brush polymers are highly-branched macromolecules where linear side chains are covalently bonded to a linear polymeric backbone in a dense manner [1]. Overcrowding of the side chains in the bottle-brush polymer leads to rather shape-persistent, stiff, cylindrical structures solely due to intra-molecular excluded volume interactions.

Despite the large number of experimental [2], [3], [4], [5], [6], [7], [8], [9], simulation [10], [11], [12], [13], [14], [15], [16], [17], [18] and theoretical [12], [19], [20], [21], [22], [23] studies dealing with the conformational properties of bottle-brush macromolecules in solutions, a number of questions still remain open. The crucial point is that the persistence of the overall bottle-brush macromolecule is significantly increased compared to the bare backbone. The backbone adopts a rather extended conformation in the brush compared to the corresponding free linear chain. However, there is no agreement on what the effect of increasing side chain length is, on both, the persistence length of the backbone as well as of the overall brush structure. Predictions vary from a strong increase of the persistence length (backbone [12], brush [7], [20], [22], [23]) over a slight increase (backbone [4], [8], [12], [13], [14], brush [4], [8], [21]) to no impact (backbone [2], [5], [6], [9], [11], brush [2]) on the persistence length with increasing side chain length. Computer simulations reveal that the impact of the side chain length on the backbone persistence depends on side chain flexibility. Stiff side chains induce a higher backbone persistence as well as a stronger increase of the persistence length with increasing side chain length than flexible side chains [15], [16]. Another question still under discussion is, whether the conformation of the side chains is unaffected by steric congestion or if the conformation is significantly inflicted in space. Predictions vary from a side chain conformation undergoing an (almost) three-dimensional (3D) self-avoiding random walk (SAW) [2], [6], [9], [11], [12], [17], [21] to a conformation being closer to that of a two-dimensional (2D) SAW [7], [10], [13], [18], [20], [22]. In the first case, the brush radius is expected to increase with the end-to-end distance of the side chainsNs3/5 whereas in the latter extreme case a Ns3/4-dependence is expected. Ns denotes the number of segments constituting the side chains.

Concerning the possibility of lyotropic behavior of bottle-brushes in solutions, the critical parameter is the ratio of the persistence length, lp, to the diameter, d, of the bottle-brush macromolecules. The driving force for ordering phenomena in bottle-brush polymer solutions are excluded volume interactions. In contrast to flexible cylinders with hard-core interactions, the bottle-brush macromolecules start to interpenetrate when the overlap concentration Φ, which can be rather low for brushes with long side chains, is exceeded. For concentration above Φ excluded volume interactions will gradually diminish, thus lyotropic behavior is expected to disappear again at somewhat higher polymer concentrations. As an estimate deduced from predictions for semi-flexible cylinders with hard-core interaction the ratio lp/d should be of the order of 10 in order to lead to lyotropic behavior at reasonable concentrations [24].

We measured the full form factor of brushes synthesized by the “grafting from” route which consist of poly(n-butyl acrylate) (pnBA) side chains grafted from a poly(alkyl methacrylic) (pAMA) backbone. Experimental results are compared to results obtained from computer simulations. Conformational changes in the bottle-brush macromolecule are discussed under systematic variation of architectural parameters, such as backbone length, side chain length and side chain stiffness. Emphasis is placed on the impact of these structural parameters on the brush persistence and the ratio lp/d. Experimental results of the lyotropic behavior of a bottle-brush polymer in concentrated solutions are shown.

Section snippets

Samples

The architecture of the bottle-brush polymers was varied in a systematic manner. Parameters changed were the side chain length (series S) and backbone length (series B), keeping one of the parameters fixed. A schematic of the chemical structure of the investigated bottle-brush macromolecules is given in Ref. [2].

The samples consist of hydroxyethyl methacrylic main chains prepared by atom transfer radical polymerization [25], [26]. As described in earlier publications [27], [28], subsequent side

Results and discussion

The aim of the simulation was to conduct allow a systematic investigation on which impact architectural parameters such as side chain length Ns, backbone length Nb as well as side chain stiffness (Nc) have on the structural properties of the bottle-brush polymers. The scattering experiments were carefully designed in a systematic manner to enable us to investigate if the results obtained by computer simulation find experimental support. A comparison between the simulation data and the

Conclusions

We investigated the structure of bottle-brush polymers by means of scattering experiments and computer simulations. Good qualitative agreement between the measured form factors and those calculated from the simulated structures was obtained. Both the simulation as well as the experimental results indicate a side chain conformation close to that of an unperturbed 3D-SAW. The backbone adopts a stretched conformation in the brush. The extent of stretching and the increase in the persistence length

Acknowledgements

We appreciate Dr. Burkhard Dünweg for the fruitful discussions.

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