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1. Confined Dynamics in Spherical Polymer Brushes

   https://doi.org/10.1021/acsmacrolett.3c00505  

   Kotkar, Shivraj B; Howard, Michael P; Nikoubashman, Arash; Conrad, Jacinta C; Poling-Skutvik, Ryan; Palmer, Jeremy C (November 2023, ACS Macro Letters)

   We investigate the dynamics of polymers grafted to spherical nanoparticles in solution using hybrid molecular dynamics simulations with a coarse-grained solvent modeled via the multiparticle collision dynamics algorithm. The mean-square displacements of monomers near the surface of the nanoparticle exhibit a plateau on intermediate time scales, indicating confined dynamics reminiscent of those reported in neutron spin–echo experiments. The confined dynamics vanish beyond a specific radial distance from the nanoparticle surface that depends on the polymer grafting density. We show that this dynamical confinement transition follows theoretical predictions for the critical distance associated with the structural transition from confined to semidilute brush regimes. These findings suggest the existence of a hitherto unreported dynamic length scale connected with theoretically predicted static fluctuations in spherical polymer brushes and provide new insights into recent experimental observations. 

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2. Impact of Grafting Density on the Assembly and Mechanical Properties of Self-Assembled Metal–Organic Framework Monolayers

   https://doi.org/10.1021/jacs.4c17748  

   Kang, Minjung; Lin, Po-An; Bunch, Jordan A; Lipomi, Darren J; Arya, Gaurav; Cohen, Seth M (February 2025, Journal of the American Chemical Society)

   Polymer-grafted metal−organic frameworks (MOFs) can be used to form free-standing self-assembled MOF monolayers (SAMMs). Polymer chains can be introduced onto MOF surfaces through either the ligands or metal nodes using both grafting-to and grafting-from approaches. However, controlling the grafting density of polymer-grafted MOFs has not yet been achieved, because a means to control the density of grafting sites on the MOF surface has not been developed. In this study, the grafting density of polymer-grafted UiO-66 (UiO = University of Oslo) was controlled by functionalizing a portion of the Zr(IV) secondary building units (SBUs) on a UiO-66 surface with a so-called blocking agent. The remaining sites on the UiO-66 SBUs were functionalized with polymerization initiation groups, and polymers were grown from these sites to obtain particles with variable grafting densities and chain lengths that form SAMMs at an air−water interface. Even under conditions of low grafting density, these materials retain the ability to form SAMMs and their freestanding ability. Changes in particle arrangement within the monolayers were investigated using SEM imaging, and the toughness of the monolayers was evaluated using a film-on-water (FOW) method. Furthermore, coarse-grained molecular dynamics simulations were carried out to elucidate the morphology and mechanical properties of the monolayers. Findings from both experiments and simulations indicate that the toughness of SAMMs is more heavily influenced by the chain length of the grafted polymers than by the overall polymer content in the composite. 

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3. Pyroelectric Polyelectrolyte Brushes

   https://doi.org/10.1002/adma.202307038  

   Wang, Jian; Hu, Fei; Sant, Sabrina; Chu, Kanghyun; Riemer, Lukas; Damjanovic, Dragan; Kilbey, S Michael; Klok, Harm‐Anton (April 2024, Advanced Materials)

   Abstract Piezo‐ and pyroelectric materials are of interest, for example, for energy harvesting applications, for the development of tactile sensors, as well as neuromorphic computing. This study reports the observation of pyro‐ and piezoelectricity in thin surface‐attached polymer brushes containing zwitterionic and electrolytic side groups that are prepared via surface‐initiated polymerization. The pyro‐ and piezoelectric properties of the surface‐grafted polyelectrolyte brushes are found to sensitively depend on and can be tuned by variation of the counterion. The observed piezo‐ and pyroelectric properties reflect the structural complexity of polymer brushes, and are attributed to a complex interplay of the non‐uniform segment density within these films, together with a non‐uniform distribution of counterions and specific ion effects. The fabrication of thin pyroelectric films by surface‐initiated polymerization is an important addition to the existing strategies toward such materials. Surface‐initiated polymerization, in particular, allows for facile grafting of polar thin polymer films from a wide range of substrates via a straightforward two‐step protocol that obviates the need for multistep laborious synthetic procedures or thin film deposition protocols. The ability to produce polymer brushes with piezo‐ and pyroelectric properties opens up new avenues of application of these materials, for example, in energy harvesting or biosensing. 

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4. polyGraft 1.0: A program for molecular structure and topology generation of polymer‐grafted hybrid nanostructures

   https://doi.org/10.1002/jcc.27206  

   Chen, Guang (October 2023, Journal of Computational Chemistry)

   Abstract Polymer‐grafted hybrid materials have been ubiquitously employed in various engineering applications. The design of these hybrid materials with superior performances requires a molecularly detailed understanding of the structure and dynamics of the polymer brushes and their interactions with the grafting substrate. Molecular dynamics (MD) simulations are very well suited for the study of these materials which can provide molecular insights into the effects of polymer composition and length, grafting density, substrate composition and curvatures, and nanoconfinement. However, few existing tools are available to generate such systems, which would otherwise reduce the barrier of preparation for such systems to enable high throughput simulations. Here polyGraft, a general, flexible, and easy to use Python program, is introduced for automated generation of molecular structure and topology of polymer grafted hybrid materials for MD simulations purposes, ranging from polymer brushes grafted to hard substrates, to densely grafted bottlebrush polymers. polyGraft is openly accessible on GitHub (https://github.com/nanogchen/polyGraft). 

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5. Effect of Filler–Polymer Interface on Elastic Properties of Polymer Nanocomposites: A Molecular Dynamics Study

   Chi Ma, 1 Tuo (September 2017, Tire science & technology)

   A coarse-grained model has been built to study the effect of the interfacial interaction between spherical filler particles and polymer on the mechanical properties of polymer nanocomposites. The polymer is modeled as bead-spring chains, and nano-fillers grafted with coupling agent are embedded into the polymer matrix. The potential parameters for polymer and filler are optimized to maximally match styrene-butadiene rubber reinforced with silica particles. The results indicated that, to play a noticeable role in mechanical reinforcement, a critical value exists for the grafting density of the filler–polymer coupling agent. After reaching the critical value, the increase of grafting density can substantially enhance mechanical properties. It is also observed that the increase of grafting density does not necessarily increase the amount of independent polymer chains connected to fillers. Instead, a significant amount of increased grafting sites serve to further strengthen already connected polymer and filler, indicating that mechanical reinforcement can occur through the locally strengthened confinement at the filler–polymer interface. These understandings based on microstructure visualization shed light on the development of new filler polymer interfaces with better mechanical properties. 

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