Dynamics of PDMS- g-PDMS Bottlebrush Polymers by Broadband Dielectric Spectroscopy

Copyright © 2020 American Chemical Society. Poly(dimethylsiloxane) (PDMS)-based bottlebrush polymers, PDMS-g-PDMS, have been synthesized by anionic polymerization in combination with a condensation-based grafting reaction. Bottlebrush polymers show intriguing features, e.g., extremely low viscosities. Hereby, studies of their dynamics are rare. Therefore, we focus on the segmental relaxation by broadband dielectric spectroscopy. An increasing cross-sectional radius proportional to the increasing side chain length has been observed by small-angle neutron scattering over three samples. A comparison of the segmental relaxation times of the bottlebrushes with the respective linear chains reveals slower dynamics in the former. For longer chains, this effect vanishes

Dynamics of bottlebrush polymers

Bottlebrushes are an interesting class of polymers which shows intriguing material properties often associated with dynamics. While dynamical phenomena in linear polymers are well understood and existing theories can describe them in a good way, bottlebrush dynamics have only rarely been investigated. Therefore, we performed dielectric spectroscopy and quasi-elastic neutron scattering to study the dynamics of polydimethylsiloxane-based bottlebrush polymers, PDMS-g-PDMS focusing mostly on the segmental dynamics of the side chains. Comparing the relaxation times of the α – relaxation, tracked with dielectric spectroscopy, of bottlebrush polymers with those of their respective linear side chains show a slowing down once the side chains are attached to the backbone. This effect diminishes and finally vanishes with increasing side chain length. The time and length scale, offered by quasi-elastic neutron scattering, fits for the segmental dynamics together with faster processes. The Q-dependence of the segmental relaxation times allows to classify bottlebrush polymers as heterogenous including a non-Gaussian character. For such a dynamical system, the mean square displacement needs to be separated into single processes before an overall mean square displacement can be generated by applying the time temperature superposition principle

Dynamical Comparison of Different Polymer Architectures - Bottlebrush vs Linear Polymer

© 2021 American Chemical Society. 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

Structural Analysis of Ultrasoft PDMS- g-PDMS Shell-Only Particles

Copyright © 2019 American Chemical Society. We have used anionic polymerization to synthesize polymers of linear and bottlebrush architecture each with a polydimethylsiloxane backbone. The blending of polymer architectures has the effect of changing material properties, e.g., the viscoelasticity, which are connected to the chain conformation. Thus, we explore the conformation of bottlebrush polymers in a linear host melt both as a function of the concentration and for various molecular weights of the linear host matrices. Our bottlebrush polymers are seen as shell-only particles with a negligible core size. We find a substantial influence of the molecular weight of the linear matrices on the structure of the bottlebrushes and their interactions. In samples with a low molecular weight matrix that have the same degree of polymerization as the side chains, the bottlebrush behavior is consistent with an effective theta solvent condition for all concentrations. With increasing molecular weight of the host matrix, this condition is only reached at the highest concentration of the bottlebrush polymers. The increase of the molecular weight of the host matrix leads to a shrinkage of the bottlebrushes and subsequently to a formation of clusters at higher volume fractions. None of the scattering patterns show a pronounced correlation peak; however, decreased forward scattering associated with a structure factor effect is observed

Side Chain Dynamics of poly(norbornene)-g-Poly(propylene oxide) Bottlebrush Polymers

The segmental dynamics of the side chains of poly(norbornene)-g-poly(propylene oxide) (PNB-g-PPO) bottlebrush polymer in comparison to PPO is studied by quasi-elastic neutron scattering. Having experimental time and length scale information simultaneously allows to extract spatial information in addition to relaxation time. Tethering one end of the PPO side chain onto a stiff PNB backbone slows down the segmental relaxation, over the length and time scales investigated. The power law dependence of the relaxation time on the momentum transfer, Q, indicates a more heterogeneous relaxation pattern for the bottlebrush polymer, whereas the linear PPO has less deviations from a homogenous relaxation. Similar conclusions can be drawn from the time dependent mean square displacement, 〈r (t)〉, and the non-Gaussian parameter, α (t). Here the bottlebrush polymer shows a more restricted dynamics, whereas the linear PPO reaches 〈r (t)〉∝t at the highest temperature. The deviations from Gaussian behavior are evident at the α (t). Both samples show a decaying α (t). The non-Gaussian parameter supports the results from the power law dependence of the relaxation times, with lower α (t) values for PPO compared to those for PNB-g-PPO, pointing to less deviations from Gaussian behavior. This article is protected by copyright. All rights reserved

Short-Time Dynamics of PDMS- g-PDMS Bottlebrush Polymer Melts Investigated by Quasi-Elastic Neutron Scattering

© We have studied the short-time dynamical behavior of polydimethylsiloxane (PDMS) bottlebrush polymers, PDMS-g-PDMS. The samples have similar backbone lengths but different side-chain lengths, resulting in a shape transition. Quasi-elastic neutron scattering was used to observe the dynamical changes inherent to these structural changes. The combination of data from three spectrometers enabled to follow the dynamics over broad frequency and temperature ranges, which included segmental relaxations and more localized motions. The latter, identified as the methyl group rotation, is described by a threefold jump model and shows higher activation energies compared to linear PDMS. The segmental relaxation times, τs, decrease with increasing molecular weight of the side chains but increase with momentum transfer, Q, following a power law, which suggests a non-Gaussian behavior for bottlebrush polymers

Universality of TimeTemperature Scaling Observed by Neutron Spectroscopy on Bottlebrush Polymers

[Image: see text] The understanding of materials requires access to the dynamics over many orders of magnitude in time; however, single analytical techniques are restricted in their respective time ranges. Assuming a functional relationship between time and temperature is one viable tool to overcome these limits. Despite its frequent usage, a breakdown of this assertion at the glass-transition temperature is common. Here, we take advantage of time- and length-scale information in neutron spectroscopy to show that the separation of different processes is the minimum requirement toward a more universal picture at, and even below, the glass transition for our systems. This is illustrated by constructing the full proton mean-square displacement for three bottlebrush polymers from femto- to nanoseconds, with simultaneous information on the partial contributions from segmental relaxation, methyl group rotation, and vibrations. The information can be used for a better analysis of results from numerous techniques and samples, improving the overall understanding of materials properties