Amrita Sarkar

Sustainable thermoplastic elastomers derived from plant oil and their “click-coupling” via TAD chemistry

We report the preparation of plant oil based triblock copolymers based on soybean oil monomers. The monomers were polymerized via atom transfer radical polymerization with subsequent chain extension, resulting in poly(styrene-b-soybean oil acrylate-b-styrene) (PS-b-PSBA-b-PS) and poly(styrene-b-soybean oil methacrylate-b-styrene) (PS-b-PSBMA-b-PS) triblock copolymers. These polymers, ranging from thermoplastics to thermoplastic elastomers (TPEs), were obtained by tuning molecular structures. We employed a “click coupling” strategy using triazolinedione (TAD) chemistry to create chemical junctions between the soft middle blocks of the triblock copolymers, which behave similar to physical chain entanglements. This method helps to overcome the drawbacks of plant oil based polymers, allowing for increase of tensile strength without sacrificing elongation. Cyclic tensile tests show that the “click coupled” triblock copolymers exhibit excellent elastic recovery characteristics.

2D matrix engineering for homogeneous quantum dot coupling in photovoltaic solids

Colloidal quantum dots (CQDs) are promising photovoltaic (PV) materials because of their widely tunable absorption spectrum controlled by nanocrystal size1,2. Their bandgap tunability allows not only the optimization of single-junction cells, but also the fabrication of multijunction cells that complement perovskites and silicon3. Advances in surface passivation2,4–7, combined with advances in device structures8, have contributed to certified power conversion efficiencies (PCEs) that rose to 11% in 20169. Further gains in performance are available if the thickness of the devices can be increased to maximize the light harvesting at a high fill factor (FF). However, at present the active layer thickness is limited to ~300 nm by the concomitant photocarrier diffusion length. To date, CQD devices thicker than this typically exhibit decreases in short-circuit current (JSC) and open-circuit voltage (VOC), as seen in previous reports3,9–11. Here, we report a matrix engineering strategy for CQD solids that significantly enhances the photocarrier diffusion length. We find that a hybrid inorganic–amine coordinating complex enables us to generate a high-quality two-dimensionally (2D) confined inorganic matrix that programmes internanoparticle spacing at the atomic scale. This strategy enables the reduction of structural and energetic disorder in the solid and concurrent improvements in the CQD packing density and uniformity. Consequently, planar devices with a nearly doubled active layer thicknesses (~600 nm) and record values of JSC (32 mA cm−2) are fabricated. The VOC improved as the current was increased. We demonstrate CQD solar cells with a certified record efficiency of 12%.A new matrix engineering strategy enables improvements of CQD solar cell efficiency via considerable enhancement of the photocarrier diffusion length.

Hydrogen-Bonding-Directed Ordered Assembly of Carboxylated Poly(3-Alkylthiophene)s

Hydrogen-bonding-induced ordered assembly of poly(3-alkylthiophene)s derivatives bearing carboxylic acid groups has been investigated from diluted and concentrated solutions to solid films using ultraviolet–visible absorption spectroscopy, polarized optical microscopy, and four-point probe conductivity measurements. In dilute solutions, the polymer undergoes a spontaneous structural transition from disordered coil-like to ordered rodlike conformations, which is evidenced by time-dependent chromism. Many factors such as alkyl-chain length, types of solvents, and temperature are studied to understand the assembly behavior. Transition kinetics of the assembly process reveals a universal second-order rate law, indicating an intermolecular origin due to hydrogen bonding. When more concentrated, hydrogen bonding drives nematic liquid-crystalline gelation above a critical concentration and the gels are thermally reversible. Under an appropriate balance of mechanical and thermal stresses, uniform liquid-crystalline monodomains are obtained through the application of a mechanical shear force. The dried films made from the sheared solutions display both optical and electrical anisotropies, with a more than 200% increase in charge transport parallel to the direction of shear as opposed to that in the perpendicular one.

Robust porous polymers enabled by a fast trifluoroacetic acid etch with improved selectivity for polylactide

Polylactide is a widely used sacrificial block for the preparation of porous polymers from ordered block copolymers. Although numerous etching strategies were developed in the past decade, demonstrations to date are limited by slow etch rates that require as long as a week for the etching of few-mm thick films. Recent studies have also shown that NaOH etching of thin-films can degrade the morphology, highlighting the need for more selective processes. Here we report an aqueous trifluoroacetic acid etchant that results in an enhanced etch rate of 14 nm s−1 with greatly improved selectivity for poly(styrene-block-lactide). The high etch rate enables the complete removal of polylactide from 2 mm thick block copolymer films in 19 h. Furthermore, the improved etch selectivity enables the macroscopic preservation of morphologies as confirmed by both SAXS and SEM and yields pristine porous PS as confirmed by NMR and GPC.

How to make persistent micelle templates in 24 hours and know it using X-ray scattering

The controlled fabrication of nanoscale materials can enable new behaviors and properties as well as improved performance. For example, many electrochemical devices are made from porous materials where the architecture of both the porosity and the material each affect distinct processes. Recently, persistent micelle templating (PMT) emerged as a unique nanofabrication technique that enables decoupled control over the porosity and wall material dimensions via self-assembly. PMT control relies upon kinetic entrapment to preserve the micelle diameter while adding material. However, the development of PMT is currently cumbersome where time-intensive polymerizations and solution parameter searches are both required. Here we report simple SAXS based geometric models that significantly expedite the identification of the PMT window with a one-pot titration-approach. The models also quantitatively predict the nominal template diameter and wall-thickness within the PMT window. This approach yielded the first PMT criteria for a low molar mass block copolymer with ∼13 nm mesopores and continuously tunable wall-thickness with 2 A increments. Furthermore, we demonstrate an accelerated synthesis that includes custom polymer fabrication and micelle templating within 24 h. The polymer synthesis was demonstrated without high-vacuum equipment and only used low-cost, commercially available reagents. These advances will ease and accelerate the use of PMT for a wide gamut of nanomaterials investigations.

Expanded Kinetic Control for Persistent Micelle Templates with Solvent Selection.

The precision control of nanoscale materials remains a challenge for the study of nanostructure-performance relationships. Persistent micelle templates (PMT) are a kinetic-controlled self-assembly approach that decouples pore and wall control. Here, block copolymer surfactants form persistent micelles that maintain constant template size as material precursors are added, despite the shifting equilibrium dimensions. Earlier PMT demonstrations were based upon solvent mixtures where kinetic rates were adjusted with the amount of water cosolvent. This approach is however limited because ever-higher water contents can lead to secondary porosity within the material walls. Herein, we report an improved method to regulate the PMT kinetics via the majority solvent. This enables a new avenue for the expansion of the PMT window to realize templated materials with a greater extent of tunability. In addition, we report a new small-angle X-ray scattering (SAXS)-based log-log analysis method to independently test the micelle-templated series for consistency with the expected lattice expansion with an increasing material:template ratio. The PMT window identified by the log-log analysis of the SAXS data agreed well with independent scanning electron microscopy measurements. The combination of improved micelle control with solvent selection along with SAXS validation will accelerate the development of a myriad of nanomaterial applications.

Deciphering magnesium stearate thermotropic behavior

ABSTRACT Magnesium stearate (MgSt) is the most commonly used excipient for oral solid dosage forms, yet there is significant commercial physicochemical variability that can lead to variable performance of critical product attributes. Differential scanning calorimetry (DSC) is often used as a quality control tool to characterize MgSt, but little data is available regarding the physicochemical relevance for the DSC thermograms. The main aim of this study was to decipher MgSts complex thermotropic behavior using DSC, thermogravimetric analysis, capillary melting point, polarized hot‐stage microscopy, and temperature dependent small‐angle X‐ray scattering (SAXS) and assign physicochemical relevance to the DSC thermograms. Several DSC thermal transitions are irreversible after the first heating cycle of a heat‐cool‐heat‐cool‐heat cycle. Interestingly, after the first heat cycle, the complex cool‐heat‐cool‐heat DSC thermograms were highly reproducible and exhibited 6 reversible exothermic‐endothermic conjugate pairs. SAXS identified 5 distinct mesophases at different temperatures with Phase C′ persisting to 250°C. MgSt maintained molecular ordering beyond 276°C and did not undergo a simple melting phenomena reported elsewhere. This research serves as a starting point to design heat‐treatment strategies to create more uniform MgSt starting material.