Activated Transport in Polymer Grafted Nanoparticle Melts

Jhalaria, Mayank; Huang, Yucheng; Ruzicka, Eric; Tyagi, Madhusudan; Zorn, Reiner; Zamponi, Michaela; Sakai, Victoria García; Benicewicz, Brian C.; Kumar, Sanat K.

doi:10.1021/acs.macromol.1c00601

Cited by 13 publications

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References 43 publications

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“…First, the gas permeability of the grafted layers goes through a maximum in the vicinity of this critical MW g . Similarly, the segmental diffusivity and segmental dynamics activation energy as measured by quasielastic neutron scattering go through a minimum. , Finally, rheology also demonstrates a transition from a jammed glass-like state (“colloid”-dominated) to a liquid-like state (polymer-dominated) with increasing MW g . , …”

Section: Resultsmentioning

confidence: 94%

“…18 Similarly, the segmental diffusivity and segmental dynamics activation energy as measured by quasielastic neutron scattering go through a minimum. 34,38 Finally, rheology also demonstrates a transition from a jammed glass-like state ("colloid"-dominated) to a liquid-like state (polymer-dominated) with increasing MW g . 18,39 In gas transport applications, for example, it has been found that increasing molecular weight for short chains increasingly enhances gas permeability in GNPs relative to the neat polymers, but beyond a critical MW g , the normalized gas permeability enhancements decrease, which is dependent on grafting density, graft chain chemistry, and core size.…”

Section: Resultsmentioning

confidence: 94%

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Local Structure of Polymer-Grafted Nanoparticle Melts

et al. 2022

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Polymer-grafted nanoparticle (GNP) membranes show unexpected gas transport enhancements relative to the neat polymer, with a maximum as a function of graft molecular weight (MWg ≈ 100 kDa) for sufficiently high grafting densities. The structural origins of this behavior are unclear. Simulations suggest that polymer segments are stretched near the nanoparticle (NP) surface and form a dry layer, while more distal chain fragments are in their undeformed Gaussian states and interpenetrate with segments from neighboring NPs. This theoretical basis is derived by considering the behavior of two adjacent NPs; how this behavior is modified by multi-NP effects relevant to gas separation membranes is unexplored. Here, we measure and interpret SAXS data for poly(methyl acrylate)-grafted silica NPs and find that for very low MWgs, contact between GNPs obeys the two-NP theorynamely that the NPs act like hard spheres, with radii that are linear combinations of the NP core sizes and the dry zone dimensions; thus, the interpenetration zones relax into the interstitial spaces. For chains with MWg > 100 kDa, the interpenetration zones are in the contact regions between two NPs. These results suggest that for MWgs below the transition, gas primarily moves through a series of dry zones with favorable transport, with the interpenetration zone with less favorable transport properties in parallel. For higher MWgs, the dry and interpenetration zones are in series, resulting in a decrease in transport enhancement. The MWg at the transport maximum then corresponds to the chain length with the largest, unfavorable stretching free energy.

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Section: Resultsmentioning

confidence: 94%

Section: Resultsmentioning

confidence: 94%

Local Structure of Polymer-Grafted Nanoparticle Melts

et al. 2022

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“…Under these conditions, quasielastic neutron scattering (QENS) experiments indicate that the apparent segmental diffusion of the grafted chains goes through a maximum. 37,38 This brush height, h max =√3R c , is predicted by the theory to be independent of chain chemistry and σ. Thus, the maximum permeability must occur at an apparently universal value of the NP loading, ϕ NP , ϕ NP,max = [R c /(R c + h max )] 3 ≈ 0.049.…”

Section: ■ Introductionmentioning

confidence: 99%

Understanding Gas Transport in Polymer-Grafted Nanoparticle Assemblies

et al. 2022

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We rationalize the unusual gas transport behavior of polymer-grafted nanoparticle (GNP) membranes. While gas permeabilities depend specifically on the chemistry of the polymers considered, we focus here on permeabilities relative to the corresponding pure polymer, which show interesting, “universal” behavior. For a given NP radius, R c, and for large enough areal grafting densities, σ, to be in the dense brush regime, we find that gas permeability enhancements display a maximum as a function of the graft chain molecular weight, M n. Based on a recently proposed theory for the structure of a spherical brush in a melt of GNPs, we conjecture that this peak permeability occurs when the densely grafted polymer brush has the highest, packing-induced extension free energy per chain. The corresponding brush thickness is predicted to be h max = √3R c, independent of chain chemistry and σ, i.e., at an apparently universal value of the NP volume fraction (or loading), ϕNP, ϕNP,max = [R c/(R c + h max)]3 ≈ 0.049. Motivated by this conclusion, we measured CO2 and CH4 permeability enhancements across a variety of R c, M n, and σ and find that they behave in a similar manner when considered as a function of ϕNP, with a peak in the near vicinity of the predicted ϕNP,max. Thus, the chain length-dependent extension free energy appears to be the critical variable in determining the gas permeability for these hybrid materials. The emerging picture is that these curved polymer brushes, at high enough σ, behave akin to a two-layer transport mediumthe region in the near vicinity of the NP surface is comprised of extended polymer chains that speed up gas transport relative to the unperturbed melt. The chain extension free energy increases with increasing chain length, up to a maximum, and apparently leads to an increasing gas permeability. For long enough grafts, there is an outer region of chain segments that is akin to an unperturbed melt with slow gas transport. The permeability maximum and decreasing permeability with increasing chain length then follow naturally.

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“…Apart from the data at 80 °C, which is very close to the empirical temperature lower bound of 1.2 T g , below which dynamical heterogeneities of the polymer matrix will affect the polymer relaxation, the characteristic times can be described by an Arrhenius-like activated relaxation process with an apparent activation energy of 45 kJ/mol [Figure C]. This number is lower than the apparent activation energy extracted from William–Landell–Ferry (WLF) fits to the shift factors obtained from rheology for the PMA homopolymer at high temperatures (70 kJ/mol); it is also lower than the apparent activation energy for segmental dynamics in PMA (∼120 kJ/mol) at comparable temperatures . Clearly, neither of these two processes directly drives the NP reorganization process.…”

Section: Resultsmentioning

confidence: 57%

“…This number is lower than the apparent activation energy extracted from William−Landell−Ferry (WLF) fits to the shift factors obtained from rheology for the PMA homopolymer at high temperatures (70 kJ/mol); 74 it is also lower than the apparent activation energy for segmental dynamics in PMA (∼120 kJ/ mol) at comparable temperatures. 75 Clearly, neither of these two processes directly drives the NP reorganization process. The scaling of the curves with their 1/e relaxation times (t 1/e ) only results in an approximate collapse of the data onto a single curve (Figure S6), implying that principles associated with time−temperature superposition are not exactly valid in describing this process.…”

Section: Resultsmentioning

confidence: 99%

Long-Term Aging in Miscible Polymer Nanocomposites

et al. 2022

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We find that the initial, solvent-cast state of nanoparticles (NPs) in a polymer matrix temporally evolves during thermal annealing such that, at steady state, NPs maximize their distance from each other subject to mass balance constraints. The observed timescales for this unexpected structural reorganization, as probed by small-angle X-ray scattering, are temperature-dependent and can be prohibitively large, especially at temperatures around and below 1.2T g. X-ray photon correlation spectroscopy measurements during reorganization reveal that the collective NP dynamics slow down with annealing at constant temperature; this is accompanied by changes in the low-frequency regime in macroscopic viscoelastic measurements in equilibrated materials. By ruling out other potential sources for these effects (i.e., electrostatic interactions, adsorbed layers), we attribute these results to a long-ranged repulsive force between the NPs caused by fluctuations in the polymer phase, i.e., the “anti-Casimir” effect proposed by Obhukhov and Semenov [Long-range interactions in polymer melts: The anti-Casimir effect038305Phys Rev Lett200595 Thus, our results highlight the important role of long-term, slow NP reorganization on the structure and, subsequently, the properties of polymer nanocomposites (PNCs), even in the case of nominally miscible polymer nanoparticle hybrids.

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Activated Transport in Polymer Grafted Nanoparticle Melts

Cited by 13 publications

References 43 publications

Local Structure of Polymer-Grafted Nanoparticle Melts

Local Structure of Polymer-Grafted Nanoparticle Melts

Understanding Gas Transport in Polymer-Grafted Nanoparticle Assemblies

Long-Term Aging in Miscible Polymer Nanocomposites

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