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1. Polyimide/ZIF-7 mixed-matrix membranes: understanding the in situ confined formation of the ZIF-7 phases inside a polymer and their effects on gas separations

   https://doi.org/10.1039/d0ta02761h  

   Park, Sunghwan; Cho, Kie Yong; Jeong, Hae-Kwon (June 2020, Journal of Materials Chemistry A)

   null (Ed.)

   Polymer-modification-enabled in situ metal–organic framework formation (PMMOF) is potentially a paradigm-shifting preparation method for polymer/MOF mixed-matrix membranes (MMMs). However, the actual reaction conditions of the in situ formation of MOFs in a confined polymer free volume are expected to be quite different from that in a bulk solution. ZIF-7 is an interesting filler material not only due to its use in selective light gas separations but also for its three different crystal phases. Herein, we carried out systematic investigations on the in situ confined formation of ZIF-7 phases inside a polymer (6FDA-DAM) by PMMOF. The reaction conditions of ZIF-7 in the polymer free volume were deduced based on a bulk-phase ZIF-7 phase diagram constructed by varying the ZIF-7 precursor concentrations and ratios. Based on the understanding of the reaction conditions, the ZIF-7 crystal phases formed inside the polymer during the PMMOF process were controlled, yielding 6FDA-DAM/ZIF-7 MMMs with three different crystal phases. The ZIF-7 phases had significant effects on the gas transport of MMMs with layered ZIF-7-III fillers exhibiting the highest performance enhancement for H 2 /CO 2 separation ( i.e. , H 2 permeability of ∼1630 Barrer and H 2 /CO 2 selectivity of ∼3.8) among other phases. Furthermore, the MMMs obtained by the PMMOF process showed enhanced H 2 /CO 2 separation performance, surpassing the upper bound. 

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2. Spectral attributes of sub-amorphous thermal conductivity in cross-linked organic–inorganic hybrids

   https://doi.org/10.1039/D0NR02657C  

   Morshedifard, Ali; Moshiri, Amir; Krakowiak, Konrad J.; Abdolhosseini Qomi, Mohammad Javad (July 2020, Nanoscale)

   null (Ed.)

   Organic–inorganic hybrids have found increasing applications for thermal management across various disciplines. Such materials can achieve thermal conductivities below the so-called “amorphous limit” of their constituents’ thermal conductivity. Despite their technological significance, a complete understanding of the origins of this thermal conductivity reduction remains elusive in these materials. In this paper, we develop a prototypical cross-linked organic–inorganic layered system, to investigate the spectral origins of its sub-amorphous thermal conductivity. Initially, we study the atomic structure of the model and find that besides polymer chain length, the relative drift of the layers governs the reduction in computed basal spacing, in agreement with experimental measurements. We, subsequently, find that organic cross-linking results in up to 40% reduction in thermal conductivity compared to inorganic samples. An in-depth investigation of vibrational modes reveals that this reduction is the result of reduced mode diffusivities, which in turn is a consequence of a vibrational mismatch between the organic and inorganic constituents. We also show that the contribution of propagating modes to the total thermal conductivity is not affected by organic cross-linking. Our approach paves the path toward a physics-informed analysis and design of a wide range of multifunctional hybrid nanomaterials for thermal management applications among others. 

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3. Conformation of Network Strands in Polymer Gels

   https://doi.org/10.1021/acsmacrolett.3c00006  

   Beech, Haley K.; Johnson, Jeremiah A.; Olsen, Bradley D. (February 2023, ACS Macro Letters)

   Small angle neutron scattering was used to measure single chain radii of gyration of end-linked polymer gels before and after cross-linking to calculate the prestrain, which is the ratio of the average chain size in a cross-linked network to that of a free chain in solution. The prestrain increased from 1.06 ± 0.01 to 1.16 ± 0.02 as gel synthesis concentration decreased near the overlap concentration, indicating that the chains are slightly more stretched in the network than in solution. Dilute gels with higher loop fractions were found to be spatially homogeneous. Form factor and volumetric scaling analyses independently confirmed that elastic strands stretch by 2–23% from Gaussian conformations to create a space-spanning network, with increased stretching as network synthesis concentration decreases. Prestrain measurements reported here serve as a point of reference for network theories that rely on this parameter for the calculation of mechanical properties. 

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4. Adaptable Eu-containing polymeric films with dynamic control of mechanical properties in response to moisture

   https://doi.org/10.1039/c9sm02440a  

   Wei, Zichao; Thanneeru, Srinivas; Margaret Rodriguez, Elena; Weng, Gengsheng; He, Jie (March 2020, Soft Matter)

   Self-healing polymers often have a trade-off between healing efficiency and mechanical stiffness. Stiff polymers that sacrifice their chain mobility are slow to repair upon mechanical failure. We herein report adaptable polymer films with dynamically moisture-controlled mechanical and optical properties, therefore having tunable self-healing efficiency. The design of the polymer film is based on the coordination of europium (Eu) with dipicolylamine (DPA)-containing random copolymers of poly( n -butyl acrylate- co -2-hydroxy-3-dipicolylamino methacrylate) (P( n BA- co -GMADPA)). The Eu–DPA complexation results in the formation of mechanically robust polymer films. The coordination of Eu–DPA has proven to be moisture-switchable given the preferential coordination of lanthanide metals to O over N, using nuclear magnetic resonance and fluorescence spectroscopy. Water competing with DPA to bind Eu 3+ ions can weaken the cross-linking networks formed by Eu–DPA coordination, leading to the increase of chain mobility. The in situ dynamic mechanical analysis and ex situ rheological studies confirm that the viscofluid and the elastic solid states of Eu-polymers are switchable by moisture. Water speeds up the self-healing of the polymer film by roughly 100 times; while it can be removed after healing to recover the original mechanical stiffness of polymers. 

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5. Structure and morphology of calcium-silicate-hydrates cross-linked with dipodal organosilanes

   https://doi.org/https://doi.org/10.1016/j.cemconres.2020.106076  

   Moshiri, A. (July 2020, Cement and concrete research)

   Coupling of organic and inorganic chemistry presents a new degree of freedom in nano-engineering of thermo-mechanical properties of cement-based materials. Despite these vast technological benefits, molecular scale cross-linking of calcium-silicate-hydrate (C-S-H) gel with organic molecules still presents a significant challenge. Herein, we report experimental results on sol-gel synthesis, structure and morphology of nanocrystalline C-S-H cross-linked with dipodal organosilanes. These novel organic-inorganic gels have layered turbostratic molecular structure with similarities to C-S-H precipitating in hydrating cement paste. The organic molecules' chain length controls the interlayer spacing, which shows little to no shrinkage upon dehydration up to 105 °C. However, the structure gets distorted in the basal crystallite plane, in which dimer and trimer Si-polyhedra structures condense on a 2D hexagonal Ca-polyhedra layer. Cross-linked C-S-H gels display plate-like morphology with tendency toward stacking into agglomerates at the larger scale. If successfully realized in cement environment, e.g. high concentration seed, such novel organic-inorganic C-S-H gels could potentially provide cement-based matrices with unique properties unmatched by classical inorganic systems. 

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