Dynamical Comparison of Different Polymer Architectures—Bottlebrush vs Linear Polymer

Abstract

Different polymer architectures behave differently regarding their dynamics. We have used a combination of dielectric spectroscopy, and fast field cycling nuclear magnetic resonance (NMR) to compare the dynamical behavior of two different polymer architectures, with similar overall molecular weight. The systems of interest are a bottlebrush polymer and a linear one, both based on poly(dimethylsiloxane) (PDMS). To verify the structure of the PDMS-g-PDMS bottlebrush in the melt, small-angle neutron scattering was used, yielding a spherical shape. Information about the segmental dynamics was revealed by dielectric spectroscopy and extended to higher temperatures by fast field cycling NMR. One advantage of fast field cycling NMR is the detection of large-scale chain dynamics, which dielectric spectroscopy cannot probe for PDMS. While segmental relaxation seems to be independent of the architecture, the large-scale chain dynamics show substantial differences, as represented by the mean square displacement. Here, two regions are detected for each polymer. The linear polymer shows the Rouse regime, followed by reptation. In contrast, the bottlebrush polymer performs Rouse dynamics and diffusion in the available time window, and entanglement effects are completely missing.

Influence of Side-Chain Length and Relative Rigidities of Backbone and Side Chains on Glass Formation of Branched Polymers

“…By varying the molecular mass of the side chains and comparing the segmental relaxation dynamics of bottlebrush polymers with their respective linear polymers, the bottlebrush polymers are found to exhibit slower dynamics than the linear polymers for short side chains, and differences between the bottlebrush and linear polymers become diminished as the side-chain length is elevated . For sufficiently long side chains, the segmental relaxation dynamics of bottlebrush and linear polymers becomes essentially indistinguishable. , These experimental trends are well captured by the GET for the S–F class of polymers, as shown in Figure c. Notice that the branched polymers should be compared with the respective flexible linear polymers to make a comparison between the experiment and theory.…”

“…Schneider and co-workers , conducted dielectric spectroscopy measurements for the T dependence of τ α in a series of bottlebrush polymers, where both the backbone and side chains are composed of PDMS. By varying the molecular mass of the side chains and comparing the segmental relaxation dynamics of bottlebrush polymers with their respective linear polymers, the bottlebrush polymers are found to exhibit slower dynamics than the linear polymers for short side chains, and differences between the bottlebrush and linear polymers become diminished as the side-chain length is elevated .…”

“…26 For sufficiently long side chains, the segmental relaxation dynamics of bottlebrush and linear polymers becomes essentially indistinguishable. 26,27 These experimental trends are well captured by the GET for the S−F class of polymers, as shown in Figure 2c.…”

“…In addition to the fundamental importance in the GET, the glass formation of polymers with side chains has been under close scrutiny in experiments in the context of molecular bottlebrushes, ,− a class of polymeric materials that are characterized by densely grafted side chains and exhibit many unique properties. , As clarified in ref , comb and bottlebrush polymers can be distinguished based on the ratio of the coil dimension of the side chains to the unperturbed backbone spacing in between side chains. Based on this definition, poly­(α-olefins) with alkyl chain lengths greater than 6 would exhibit a bottlebrush architecture .…”

“…18 The same trends also hold in poly(n-alkyl methacrylates), as discussed in the works of Sokolov and coworkers 17 and Ngai and co-workers. 32 Schneider and coworkers 26,27 have recently studied the glass formation of PDMSbased bottlebrush polymers using dielectric spectroscopy and fast field cycling nuclear magnetic resonance in comparison to linear polymers, and these combined techniques allow for a detailed analysis of the relaxation time describing the time scale of the segmental dynamics over a wide T range, which we term the segmental structural relaxation time for convenience. These interesting experimental observations, in conjunction with the practical importance of bottlebrush polymers, naturally invite attempts at a theoretical explanation and require simulation to better understand the glass formation of branched polymers at the molecular scale.…”