Polymer crystallization, particularly near the glass transition, exhibits strong nonlinearities and prolonged metastability that enable fabrication of devices with complex hierarchal structure from nm to mm. A fascinating example arises in the production of bioresorbable scaffolds (BRS) from poly(L-lactide) (PLLA), in which a sequence of processes (extrusion, stretch-blow molding and crimping) create diverse semicrystalline morphologies, side-by-side within a span of a hundred microns (Figure 1). To discover how these structures form, we need to examine transient structure under conditions that mimic manufacturing processes. An apparatus that enables scattering measurements during the stretch-blow molding step, called “tube expansion” imposes a nearly constant-width elongation as it converts an extruded “preform” into an “expanded tube”.

To increase the range of accessible properties of PLLA-based BRS, we use this apparatus to examine inorganic nanotubes as potential reinforcing agents that also enhance radiopacity, relevant to clinical applications. Understanding how their microstructure develops during processing is relevant to increasing strength to enable thinner devices and improving radiopacity to enable imaging during implantation. Consistent with the premise of this MS, in-situ X-ray scattering reveals unanticipated phenomena in the transient microstructure of PLLA/WS₂NTs nanocomposites during “tube expansion” (Figure 2).

Surprisingly, the WS₂NT orientation hardly changes from that produced during extrusion of the preform (z-dir., defined Fig. 1A), despite significant strain in the transverse direction (at inner diameter, 500% strain in q-dir.). Although WS₂NTs promote PLLA nucleation, the NTs do not modify the orientation of crystallization (c-axis along q, just as observed in tube expansion of neat PLLA). The striking independence of the orientations of the NT and polymer crystals stems may arise from the favorable interaction between PLLA and WS₂NTs: facile and stable dispersion of WS₂NTs in PLLA enables strong NT orientation in shear (extrusion); NT that are orthogonal to the stretching direction do not reorient; remaining orthogonal to decouples WS₂NT orientation from that of PLLA crystals. Future directions include evaluating cross-reinforcement of the mutually orthogonal NT and PLLA crystals. Based on the surprising effects we have found, further discoveries likely lie ahead in the effects of WS₂NT on morphology development during crimping.

Polymeric nanoparticles are used in many fields, e.g. for drug delivery. Poly(N-isopropylacrylamide) in aqueous solution forms nanoparticles (“mesoglobules”) above its cloud point. The coexistence line of this system in the temperature-pressure frame is an ellipse with a maximum at ~60 MPa and 35 °C [1]. We investigate the formation and growth of mesoglobules as well as their disintegration after rapid pressure jumps across the coexistence line, both at low (below 20 MPa) and high pressures (above 101 MPa). Time-resolved small-angle neutron scattering at instrument D11 (ILL Grenoble) gives structural information on a large range of length scales and in a time range from 50 ms to ~1650 s after the jump [2,3].

Mesoglobule formation is found to be vastly different in the low- and the high-pressure regime. In the low-pressure regime, we find that, initially, growth of the mesoglobules proceeds via diffusion-limited coalescence, but this process is later slowed down by the appearance of a dense and rigid shell from dehydrated polymers. The deeper the target pressure in the two-phase region, i.e. the further away from the coexistence line, the earlier the slowing-down sets in and hinders further growth. In contrast, in the high-pressure regime, the chains stay hydrated and mobile, when the coexistence line is crossed towards the two-phase region, and the diffusion-limited coalescence proceeds without hindrance during the entire measuring time.

The disintegration of mesoglobules is studied by pressure jumps from the two-phase into the one-phase region, varying the target pressure. At a target pressure close to the coexistence line, the release of single polymers from the surface of the mesoglobules is the dominating mechanism, whereas for target pressures deeper in the one-phase regime, the swelling of the mesoglobules by water prevails. The disintegration time decreases with increasing jump depth. The results point to the importance of the osmotic pressure of water.

These findings are key for the tuning of the switching process in applications of responsive polymers for transport and release purposes. The comparatively simple polymer PNIPAM serves as a model system for more complex biological macromolecules, such as cellulose or proteins.

1. B. J. Niebuur, K.-L. Claude, S. Pinzek, C. Cariker, K. N. Raftopoulos, V. Pipich, M.-S. Appavou, A. Schulte, C. M. Papadakis, ACS Macro Lett. 6, 1180 (2017).
2. B.-J. Niebuur, L. Chiappisi, X. Zhang, F. Jung, A. Schulte, C. M. Papadakis, ACS Macro Lett. 7, 1155 (2018),
3. B.-J. Niebuur, L. Chiappisi, F. Jung, X. Zhang, A. Schulte, C. M. Papadakis, Macromolecules 52, 6416 (2019).

In general, when a polymer film becomes thinner, the aggregation states and physical properties will deviate from those in the corresponding bulk due to surface and interfacial effects. This is also the case for perfluorosulfonic acid polyelectrolytes such as Nafion, Aquivion, etc. We previously reported that the anisotropic conductivity of protons was induced by thinning of a Nafion film in water.[1] This could be explained in terms of the peculiar aggregation states of Nafion close to the solid interface. In this study, the impact of the solid interface on the proton conductivity in Nafion thin films under a humidity condition was examined by alternating current (AC) impedance measurements in conjunction with neutron reflectivity (NR) measurements.

Nafion films were prepared by a spin-coating method from Nafion alcohol dispersions onto the substrates and dried under vacuum at 413 K for 3 h. The films were kept under humidity condition for 5 h to reach an equilibrium swollen state. The AC impedance measurements were performed at room temperature by a two-point probe method with a Kelvin connection using an impedance analyzer combined with a micro-prober. The density profile of the Nafion film along the direction normal to the interface was examined by NR measurement under a D₂O vapor condition (RH86%) at BL-17 in J-PARC.

Figure 1 shows the thickness (h_w) dependence of in-plane proton conductivity (s) in Nafion thin films in water and under a humidity condition. While s increased with decreasing h_w in water, s decreased with decreasing h_w under the humidity condition. Since the interface-to-volume ratio increased with decreasing h_w, it was evident that the thinning-induced s variation was due to an interfacial effect.

Panel (a) of Figure 2 represents an NR curve of the 57 nm-thick Nafion thin film prepared on a quartz substrate under the D₂O vapor condition. A solid line denotes the best-fit calculated reflectivity to the experimental one based on the model scattering length density (b/V) profile shown in Figure 2(b). The model containing an interfacial segregation layer gave a better fitting for the experimental data. The interfacial layer having a lower (b/V) value than the internal region may correspond to the initially adsorbed H₂O-contained one. This was in contrast to the interfacial structure of Nafion in D₂O, showing multi-layers with a total thickness of ca. 5 nm [1]. These results make it clear that the aggregation states of Nafion at a substrate interface were strongly affected by the wet environment. Thus, it can be concluded that the presence or absence of the interfacial multi-layers, or the two-dimensional proton-conductive pathway, enhanced and suppressed the in-plane proton conductivity.

[1] Ogata, Y. Abe, T., Yonemori, S., Yamada, N. L., Kawaguchi, D., Tanaka, K. (2018). Langmuir, 34, 15483-15489.

Metal-organic frameworks (MOFs) are of great interest for applications such as energy storage and carbon capture[1] and have outstanding chemical tunability.[2] In particular, the isostructural Zr and Hf MOFs are particularly promising for real-world applications due to their stability.[3] We recently discovered that the formation during synthesis of Hf metal clusters with different nuclearities and geometries results in a dramatic change in the structure of the subsequent MOF. Selection between the resultant MOF phases can be controlled by tuning the synthesis conditions, including temperature and solvent system.[4,5,6] This finding raises the possibility of designing syntheses to obtain previously inaccessible MOF phases with new metal clusters and therefore different reactive properties. While recent studies have demonstrated the importance of understanding the formation of MOF frameworks,[7,8] the evolution of their formation, from individual clusters and their precursors through to the ordering of the full framework, during the reaction must be fully explored and understood in order to rationally synthesise new MOFs.

In our previous work, we have shown that X-ray Pair Distribution Function (XPDF) measurements are sensitive to the identity of the cluster in Zr MOFs, and can clearly distinguish between isolated Zr atoms, Zr₆ clusters, and Zr₁₂ clusters.[5] Here we show that XPDF measurements, taken in situ during reactions of both Hf precursor solutions and the full hcp UiO-66(Hf) MOF, can be used to identify critical intermediates in the materials,[9] improving our understanding of stages of growth of Hf metal-organic frameworks [Figure 1] and hence providing routes towards the efficient design of syntheses for new and unrealised members of this important MOF family.

[1] Schoedel, A., Ji, Z. & Yaghi, O. (2016). Nature Energy 1, 16034.

[2] Stock, N. & Biswas, S. (2012). Chem. Rev. 112, 933.

[3] Cavka, J.H., Jakobsen, S., Olsbye, U., Guillou, N., Lamberti, C., Bordiga, S. & Lillerud, K.P. (2008). J. Am. Chem. Soc. 130(42), 13850–13851.

[4] Cliffe, M.J., Castillo-Martínez, E., Wu, Y., Lee, J., Forse, A.C., Firth, F.C.N., Moghadam, P.Z., Fairen-Jimenez, D., Gaultois, M.W., Hill, J.A., Magdysyuk, O.V., Slater, B., Goodwin, A.L. & Grey, C.P. (2017). J. Am. Chem. Soc. 139, 5397.

[5] Firth, F.C.N., Cliffe, M.J., Vulpe, D., Aragones-Anglada, M., Moghadam, P. Z., Fairen-Jimenez, D., Slater, B., & Grey, C.P. (2019) J. Mater. Chem. A., 7, 7459.

[6] Cliffe, M.J., Wan, W., Zou, X., Chater, P.A., Kleppe, A.K., Tucker, M.G., Wilhelm, H., Funnell, N.P., Coudert, F.-X. & Goodwin, A.L. (2014) Nat. Commun., 5, 4176.

[7] Xu, H., Sommer, S., Broge, N.L., Gao, J. & Iversen, B.B. (2019) Chem. – A Eur. J., 25, 2051.

[8] Taddei, M., van Bokhoven, J.A. & Ranocchiari, M. (2020), Inorg. Chem., 59(11), 7860-7868.

[9] Firth, F.C.N., Wu, Y., Gaultois, M.W., Stratford, J., Keeble, D.S., Grey, C.P., Cliffe., M.J. (2021), ChemRxiv, https://doi.org/10.33774/chemrxiv-2021-ssr8z.

Keywords: in-situ; metal-organic frameworks; XPDF; crystallisation; metal cluster

The picture of the phase separation constitutes a continuing source of inspiration for the development of multicomponent functional materials [1]. A key point of many applications is to precisely control the size and the connectivity of the phase-separated domain to achieve the desired structure and properties. However, the region of the forming the interconnected domains does not coincide with the spinodal curve, as often supposed, but that its region is actually much narrower, instead a densely packed droplet structure forms, i.e., the so-called "droplet spinodal decomposition (DSD)" [2]. A richness of kinetic phenomena was observed in the DSD, especially the hydrodynamic motion and collision of the droplets which are different from the pure diffusion in the NG and also deserve particular attention in the theoretical investigation. Obviously, an effective control of the DSD structure is much more complicated, because, in addition to some relevant material and thermophysical parameters, we also need to know the structural details, such as the droplet size, their size distribution, and the spatial correlation between them, at any instant of the evolution.

In this work, since the scatterers are known to be a collection of spherical droplets, our aim is to construct a suitable scattering function for the DSD structure and then to solve the droplet size distribution in real space by the indirect Fourier transformation (IFT) method [3]. Furthermore, unlike the most commonly used a priori strategies, we adopt a posteriori judgment to solve the IFT problems and yield quantitatively accurate descriptions of the transient domain structure in the bulk, especially in the droplet-size distribution, the well-defined short-range order, and the stress-optical phenomenon. Furthermore, the microscopic observation shows that at high droplet densities, the droplet collision and coalescence trigger a series of chain collisions which looks like a "ripple" propagating. The IFT results of the scattering by bulk specimen also support the observation. It is interesting that this new hydrodynamic phenomenon seems to be a nonequilibrium self-organization process and can occur only if the size of the coalescing droplets is greater than a threshold value.

Liposomes have attracted increasingly higher attention due to its wide applications in bioengineering and drug transport. To prolong the circulation time of liposomes, it is advantageous to graft poly(ethylene glycol) (PEG) at the liposomal surface for so-called PEGylated liposomes. The grafted PEG layer increases the miscibility of the drug-carrier liposomes in blood, and reduces changes of being targeted by opsonins. In this study, EGylated liposome solutions, of tens of nanometers, prepared with the different surface-modified phospholipids, are studied using synchrotron small-angle X-ray scattering (SAXS). A 5-layer model is developed for the SAXS data analysis, to resolve the nanostructures of the complex vesicles of PEGylated phospholipids. The proposed model employs five Gaussian functions to represent: one central layer of the lipid-tail zone in the liposome vesicles, which is sandwiched by two layers of phosphate head groups of the lipids, and further capped by two outermost layers of PEG of the unilamellar vesicle bilayer of the liposomes. The 5-layer model could fit decently the SAXS data, and reveal the thickness and electron-density of each sublayer of the PEG-grafted vesicle bilayer of the liposome. The structural changes observed are further correlated to the drug releasing efficiency observed, providing a structural basis for the design of controlled drug delivery.