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Bottlebrush polymers are complex macromolecules with tunable physical properties dependent on the chemistry and architecture of both the side chains and the backbone. Prior work has demonstrated that bottlebrush polymer additives can be used to control the interfacial properties of blends with linear polymers but has not specifically addressed the effects of bottlebrush side chain microstructures. Here, using a combination of experiments and self-consistent field theory (SCFT) simulations, we investigated the effects of side chain microstructures by comparing the segregation of bottlebrush additives having random copolymer side chains with bottlebrush additives having a mixture of two different homopolymer side chain chemistries. Specifically, we synthesized bottlebrush polymers with either poly(styrene- ran -methyl methacrylate) side chains or with a mixture of polystyrene (PS) and poly(methyl methacrylate) (PMMA) side chains. The bottlebrush additives were matched in terms of PS and PMMA compositions, and they were blended with linear PS or PMMA chains that ranged in length from shorter to longer than the bottlebrush side chains. Experiments revealed similar behaviors of the two types of bottlebrushes, with a slight preference for mixed side-chain bottlebrushes at the film surface. SCFT simulations were qualitatively consistent with experimental observations, predicting only slight differences in the segregation of bottlebrush additives driven by side chain microstructures. Specifically, these slight differences were driven by the chemistries of the bottlebrush polymer joints and side chain end-groups, which were entropically repelled and attracted to interfaces, respectively. Using SCFT, we also demonstrated that the interfacial behaviors were dominated by entropic effects with high molecular weight linear polymers, leading to enrichment of bottlebrush near interfaces. Surprisingly, the SCFT simulations showed that the chemistry of the joints connecting the bottlebrush backbones and side chains played a more significant role compared with the side chain end groups in affecting differences in surface excess of bottlebrushes with random and mixed side chains. This work provides new insights into the effects of side chain microstructure on segregation of bottlebrush polymer additives.
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Coarse-grained molecular dynamics integrated with convolutional neural network for comparing shapes of temperature sensitive bottlebrushes
Abstract Quantification of shape changes in nature-inspired soft material architectures of stimuli-sensitive polymers is critical for controlling their properties but is challenging due to their softness and flexibility. Here, we have computationally designed uniquely shaped bottlebrushes of a thermosensitive polymer, poly(N-isopropylacrylamide) (PNIPAM), by controlling the length of side chains along the backbone. Coarse-grained molecular dynamics simulations of solvated bottlebrushes were performed below and above the lower critical solution temperature of PNIPAM. Conventional analyses (free volume, asphericity, etc.) show that lengths of side chains and their immediate environments dictate the compactness and bending in these architectures. We further developed 100 unique convolutional neural network models that captured molecular-level features and generated a statistically significant quantification of the similarity between different shapes. Thus, our study provides insights into the shapes of complex architectures as well as a general method to analyze them. The shapes presented here may inspire the synthesis of new bottlebrushes.
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- Award ID(s):
- 1933525
- PAR ID:
- 10364050
- Publisher / Repository:
- Nature Publishing Group
- Date Published:
- Journal Name:
- npj Computational Materials
- Volume:
- 8
- Issue:
- 1
- ISSN:
- 2057-3960
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract By combining the unique characteristics of molecular bottlebrushes (MBBs) and the properties of stimuli‐responsive polymers, we show that MBBs with randomly grafted poly(n‐butyl acrylate) and pH‐responsive poly(2‐(N,N‐diethylamino)ethyl methacrylate) (PDEAEMA) side chains are efficient and robust pH‐responsive emulsifiers. Water‐in‐toluene emulsions were formed at pH 4.0 and disrupted by increasing the pH to 10.0. The emulsion generation and disruption was reversible over the ten cycles investigated, and the bottlebrushes remained intact. The exceptional emulsion stability stemmed from the high interfacial binding energy of MBBs, imparted by their large molecular size and Janus architecture at the interface, as evidenced by the interfacial jamming and wrinkling of the assemblies upon reducing the interfacial area. At pH 10.0, PDEAEMA became water‐insoluble, and the MBBs desorbed from the interface, causing de‐emulsification. Consequently, we have shown that the judicious design of MBBs can generate properties of particle emulsifiers from their large size, while the responsiveness of the MBBs enables more potential applications.more » « less
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Although the behavior of single chains is integral to the foundation of polymer science, a clear and convincing image of single chains in the solid state has still not been captured. For bottlebrush polymers, understanding their conformation in bulk materials is especially important because their extended backbones may explain their self-assembly and mechanical properties that have been attractive for many applications. Here, single-bottlebrush chains are visualized using single-molecule localization microscopy to study their conformations in a polymer melt composed of linear polymers. By observing bottlebrush polymers with different side chain lengths and grafting densities, we observe the relationship between molecular architecture and conformation. We show that bottlebrushes are significantly more rigid in the solid state than previously measured in solution, and the scaling relationships between persistence length and side chain length deviate from those predicted by theory and simulation. We discuss these discrepancies using mechanisms inspired by polymer-grafted nanoparticles, a conceptually similar system. Our work provides a platform for visualizing single-polymer chains in an environment made up entirely of other polymers, which could answer a number of open questions in polymer science.more » « less
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Using dissipative particle dynamics, we characterize dynamics of aggregation of molecular bottlebrushes in solvents of various qualities by tracking the number of clusters, the size of the largest cluster, and an average aggregation number. We focus on a low volume fraction of bottlebrushes in a range of solvents and probe three different cutoff criteria to identify bottlebrushes belonging to the same cluster. We demonstrate that the cutoff criteria which depend on both the coordination number and the length of the side chain allows one to correlate the agglomeration status with the structural characteristics of bottlebrushes in solvents of various qualities. We characterize conformational changes of the bottlebrush within the agglomerates with respect to those of an isolated bottlebrush in the same solvents. The characterization of bottlebrush conformations within the agglomerates is an important step in understanding the relationship between the bottlebrush architecture and material properties. An analysis of three distinct cutoff criteria to identify bottlebrushes belonging to the same cluster introduces a framework to identify both short-lived transient and long-lived agglomerates; the same approach could be further extended to characterize agglomerates of various macromolecules with complex architectures beyond the specific bottlebrush architecture considered herein.more » « less
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null (Ed.)This article reports on the conformational behavior of binary heterografted three-arm star molecular bottlebrushes composed of poly(ethylene oxide) (PEO) and either poly(2-( N , N -dimethylamino)ethyl methacrylate) (PDMAEMA, the brushes denoted as SMB-1) or poly(2-( N , N -diethylamino)ethyl methacrylate) (PDEAEMA, the brushes denoted as SMB-2) side chains in aqueous solutions in response to pH changes and addition of salts containing chaotropic anions (CAs). PEO was introduced into the brushes as a stabilizer when the tertiary amine-containing side chains collapsed. While a small size decrease of SMB-1 was observed with increasing pH from acidic to basic, SMB-2 exhibited a large and abrupt size transition caused by the pH-induced solubility change of PDEAEMA. Atomic force microscopy imaging revealed a star-to-globule shape transition of SMB-2 upon increasing pH across the p K a ; in contrast, SMB-1 stayed in the starlike state at both low and high pH values. Intriguingly, both SMB-1 and -2 displayed star-to-globule shape transitions in acidic solutions upon addition of salts containing sufficiently strong CAs such as ClO 4 − , with SMB-2 showing a greater sensitivity to moderate CAs than SMB-1. Moreover, superchaotropic anions ( e.g. , Fe(CN) 6 3− and S 2 O 8 2− ) were significantly more efficient in inducing shape changing than common CAs. The CA-induced shape transitions resulted from the ion pairing of CAs and protonated tertiary amine groups and the high propensity of CAs to associate with hydrophobic moieties in the brushes, which decreased the solubility of the tertiary amine-containing side chains and caused the brushes to collapse. The findings reported here may enable potential applications of molecular bottlebrushes in, e.g. , encapsulation and release of ionic substances.more » « less
