Finding the path to better desalination: Polyamide membranes have been used in large-scale desalination for decades. However, because of the thinness of the membranes and their internal variability, it has been hard to determine which aspects of the membranes most affect their performance. Culp et al. combined electron tomography, nanoscale three-dimensional (3D) polyamide density mapping, and modeling of bulk water permeability with zero adjustable parameters to quantify the effect of 3D nanoscale variations in polymer mass on water transport within the polyamide membrane (see the Perspective by Geise). They found that variability in local density most affects the performance of the membranes. Better synthesis methods could thus improve performance without affecting selectivity. Science , this issue p. 72 ; see also p. 31

The glass transition temperature (T_g) is a key property that dictates the applicability of conjugated polymers. TheT_gdemarks the transition into a brittle glassy state, making its accurate prediction for conjugated polymers crucial for the design of soft, stretchable, or flexible electronics. Here we show that a single adjustable parameter can be used to build a relationship between theT_gand the molecular structure of 32 semiflexible (mostly conjugated) polymers that differ drastically in aromatic backbone and alkyl side chain chemistry. An effective mobility value,ζ, is calculated using an assigned atomic mobility value within each repeat unit. The only adjustable parameter in the calculation ofζis the ratio of mobility between conjugated and non-conjugated atoms. We show thatζcorrelates strongly to theT_g, and that this simple method predicts theT_gwith a root-mean-square error of 13 °C for conjugated polymers with alkyl side chains.

The plant cell wall is a dynamic network of several biopolymers and structural proteins including cellulose, pectin, hemicellulose and lignin. Cellulose is one of the main load bearing components of this complex, heterogeneous structure, and in this way, is an important regulator of cell wall growth and mechanics. Glucan chains of cellulose aggregate via hydrogen bonds and van der Waals forces to form long thread-like crystalline structures called cellulose microfibrils. The shape, size, and crystallinity of these microfibrils are important structural parameters that influence mechanical properties of the cell wall and these parameters are likely important determinants of cell wall digestibility for biofuel conversion. Cellulose–cellulose and cellulose-matrix interactions also contribute to the regulation of the mechanics and growth of the cell wall. As a consequence, much emphasis has been placed on extracting valuable structural details about cell wall components from several techniques, either individually or in combination, including diffraction/scattering, microscopy, and spectroscopy. In this review, we describe efforts to characterize the organization of cellulose in plant cell walls. X-ray scattering reveals the size and orientation of microfibrils; diffraction reveals unit lattice parameters and crystallinity. The presence of different cell wall components, their physical and chemical states, and their alignment and orientation have been identified by Infrared, Raman, Nuclear Magnetic Resonance, and Sum Frequency Generation spectroscopy. Direct visualization of cell wall components, their network-like structure, and interactions between different components has also been made possible through a host of microscopic imaging techniques including scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. This review highlights advantages and limitations of different analytical techniques for characterizing cellulose structure and its interaction with other wall polymers. We also delineate emerging opportunities for future developments of structural characterization tools and multi-modal analyses of cellulose and plant cell walls. Ultimately, elucidation of the structure of plant cell walls across multiple length scales will be imperative for establishing structure-property relationships to link cell wall structure to control of growth and mechanics.