Correlating the Diffusion of Water to Performance in Model Reverse Osmosis Polyamides with Controlled Crosslink Densities.

We systematically reduce the cross-link density of a PA network based on m-phenylene diamine by substituting a fraction of the trifunctional trimesoyl chloride cross-linking agent with a difunctional isophthaloyl analog that promotes chain extension, in order to elucidate robust design cues for improving the polyamide (PA) separation layer in reverse osmosis (RO) membranes for desalination. Thin films of these model PA networks are fully integrated into a composite membrane and evaluated in terms of their water flux and salt rejection. By incorporating 15 mol % of the difunctional chain extender, we reduce the cross-link density of the network by a factor of two, which leads to an 80 % increase in the free or unreacted amine content. The resulting swelling of the PA network in liquid water increases by a factor of two accompanied by a 30 % increase in the salt passage through the membrane. Surprisingly, this leads to a 30 % decrease in the overall permeance of water through the membrane. This conundrum is resolved by quantifying the microscopic diffusion coefficient of water inside the PA network with quasi-elastic neutron scattering. In the highest and lowest cross-link density networks, water shows strong signatures of confined diffusion. At short length scales, the water exhibits a translational diffusion that is consistent with the jump-diffusion mechanism. This translational diffusion coefficient is approximately five times slower in the lowest cross-linked density network, consistent with the reduced water permeance. This is interpreted as water molecules interacting more strongly with the increased free amine content. Over longer length scales the water diffusion is confined, exhibiting mobility that is independent of length scale. The length scales of confinement from the quasi-elastic neutron scattering experiments at which this transition from confined to translational diffusion occurs is on the order of (5 to 6) Å, consistent with complementary X-ray scattering, small angle neutron scattering, and positron annihilation lifetime spectroscopy measurements. The confinement appears to come from heterogeneities in the average inter-atomic distances, suggesting that diffusion occurs by water bouncing between chains and occasionally sticking to the polar functional groups. The results obtained here are compared with similar studies of water diffusion through both rigid porous silicates and ion exchange membranes, revealing robust design cues for engineering high-performance RO membranes.

Hierarchical Nature of Nanoscale Porosity in Bone Revealed by Positron Annihilation Lifetime Spectroscopy.

Bone is a hierarchical material primarily composed of collagen, water, and mineral that is organized into discrete molecular, nano-, micro-, and macroscale structural components. In contrast to the structural knowledge of the collagen and mineral domains, the nanoscale porosity of bone is poorly understood. In this study, we introduce a well-established pore characterization technique, positron annihilation lifetime spectroscopy (PALS), to probe the nanoscale size and distribution of each component domain by analyzing pore sizes inherent to hydrated bone together with pores generated by successive removal of water and then organic matrix (including collagen and noncollagenous proteins) from samples of cortical bovine femur. Combining the PALS results with simulated pore size distribution (PSD) results from collagen molecule and microfibril structure, we identify pores with diameter of 0.6 nm that suggest porosity within the collagen molecule regardless of the presence of mineral and water. We find that water occupies three larger domain size regions with nominal mean diameters of 1.1, 1.9, and 4.0 nm-spaces that are hypothesized to associate with intercollagen molecular spaces, terminal segments (d-spacing) within collagen microfibrils, and interface spacing between collagen and mineral structure, respectively. Subsequent removal of the organic matrix determines a structural pore size of 5-6 nm for deproteinized bone-suggesting the average spacing between mineral lamella. An independent method to deduce the average mineral spacing from specific surface area (SSA) measurements of the deproteinized sample is presented and compared with the PALS results. Together, the combined PALS and SSA results set a range on the mean mineral lamella thickness of 4-8 nm.

Water sensitivity in Zn4O-based MOFs is structure and history dependent.

Moisture can cause irreversible structural collapse in metal-organic frameworks (MOFs) resulting in decreased internal surface areas and pore volumes. The details of such structural collapse with regard to pore size evolution during degradation are currently unknown due to a lack of suitable in situ probes of porosity. Here we acquire MOF porosity data under dynamic conditions by incorporating a flow-through system in tandem with positronium annihilation lifetime spectroscopy (PALS). From the decrease in porosity, we have observed an induction period for water degradation of some Zn4O-based MOFs that signals much greater stability than commonly believed to be possible. The sigmoidal trend in the degradation curve of unfunctionalized MOFs caused by water vapor has been established from the temporal component of pore size evolution as characterized by in situ PALS. IRMOF-3 is found to degrade at a lower relative humidity than MOF-5, a likely consequence of the amine groups in the structure, although, in contrast to MOF-5, residual porosity remains. The presence of an induction period, which itself depends on previous water exposure of the sample (history dependence), and sigmoidal temporal behavior of the moisture-induced degradation mechanism of MOFs was also verified using powder X-ray diffraction analysis and ex situ gas adsorption measurements. Our work provides insight into porosity evolution under application-relevant conditions as well as identifying chemical and structural characteristics influencing stability.

Interpenetration, porosity, and high-pressure gas adsorption in Zn4O(2,6-naphthalene dicarboxylate)3.

Microporous coordination polymers (MCPs) have emerged as strong contenders for adsorption-based fuel storage and delivery in large part because of their high specific surface areas. The strategy of increasing surface area by increasing organic linker length has shown only sporadic success; as demonstrated by many members of the iconic Zn4O-based IRMOF series, for example, accessible porosity is often limited by interpenetration or pore collapse upon guest removal. In this work, we focus on Zn4O(ndc)3 (IRMOF-8, ndc = 2,6-naphthalene dicarboxylate), which exhibits typical surface areas of only 1000-2000 m(2)/g even though a surface area of more than 4000 m(2)/g is expected from geometric analysis of the originally reported crystal structure. We recently showed that a high surface area could be produced with zinc and ndc by room-temperature synthesis followed by activation with flowing supercritical CO2. In this work, we investigate in detail the porosity of both the low- and high-surface-area materials. Positron annihilation lifetime spectroscopy (PALS) is used to show that the low-surface-area material suffers from near-complete interpenetration, explaining why traditional synthetic routes have failed to yield materials with the expected porosity. Furthermore, the high-pressure hydrogen and methane sorption properties of noninterpenetrated Zn4O(ndc)3 are examined, and PALS is used to show that pore filling is not operative during room-temperature CH4 sorption even at pressures approaching 100 bar. These results provide insight into how gas adsorbs in high-surface-area materials at high pressure and reinforce previous contentions that increasing surface area alone is not sufficient for the simultaneous optimization of deliverable gravimetric and volumetric gas uptake in MCPs.

Evidence of Positronium Bloch states in porous crystals of Zn4O-coordination polymers.

Positronium (Ps) is shown to exist in a delocalized state in self-assembled metalorganic crystals that have large 1.3-1.5 nm cell sizes. Belonging to a class of materials with record high accessible specific surface areas, these highly porous crystals are the first to allow direct probing with simple annihilation lifetime techniques of the transport properties of long-lived triplet Ps in what is hypothesized to be a Bloch state. Delocalized Ps has unprecedented (high) Ps mobility driven primarily by weak phonon scattering with unusual and profound consequences on how Ps probes the lattice.

Reconciling the discrepancies between crystallographic porosity and guest access as exemplified by Zn-HKUST-1.

There are several compounds for which there exists a disconnect between porosity as predicted by crystallography and porosity measured by gas sorption analysis. In this paper, the Zn-based analogue of Cu(3)(btc)(2) (HKUST-1), Zn(3)(btc)(2) (Zn-HKUST-1; btc = 1,3,5-benzenetricarboxylate) is investigated. Conventional analysis of Zn-HKUST-1 by powder X-ray diffraction and gas sorption indicates retention of crystalline structure but negligible nitrogen uptake at 77 K. By using positron annihilation lifetime spectroscopy, a densified surface layer preventing the entry of even small molecular species into the crystal framework is revealed. The material is shown to have inherent surface instability after solvent removal, rendering it impermeable to molecular guests irrespective of handling and processing methods. This previously unobserved surface instability may provide insight into the failure of other microporous coordination polymers to exhibit significant porosity despite crystal structures indicative of regular, interconnected, microporous networks.

Role of molecular architecture on the vitrification of polymer thin films.

We show that thin film star-shaped macromolecules exhibit significant differences in their average vitrification behavior, in both magnitude and thickness dependence, from their linear analogs. This behavior is dictated by a combination of their functionality and arm length. Additionally, the glass transition temperature at the free surface of a star-shaped molecule film may be higher than that of the interior, in contrast to their linear analogs where the opposite is true. These findings have implications for other properties, due largely to the origins, entropic, of this behavior.