Low value animal fats, notably brown grease, represent one of the most underutilized by-products from agricultural animal processing. It is estimated that more than 1.7 million tons of low value animal fats are produced in the US annually. Chemically, these low value animal fats are primarily comprised of triglycerides and fatty acids. Herein, we report the synthesis of high sulfur content materials using the atom economical inverse vulcanization process. A blend of canola or sunflower oil with brown grease was used as the organic crosslinker and yield high strength composites CanBGx and SunBGx (x = wt% sulfur, varied from 85–90 %). The thermal stability and transitions of the composites were assessed using Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). These composites were also characterized by infrared spectroscopy, dynamic mechanical analysis (DMA), mechanical test stand analysis, elemental analysis, and powder X-ray diffraction. The high sulfur content materials exhibited impressive compressive (28.7–35.9 MPa) and flexural strengths (between 6.5–8.5 MPa). The mechanical properties of these composites significantly exceed that of previously reported fatty acid-sulfur composites and traditional building materials such as Portland cement. These first investigations using low value animal fats such as brown grease to yield high sulfur content materials with improved mechanical properties represent a promising alternative to better utilize animal coproducts for value-added high-strength sustainable structural materials.
Liquid crystalline elastomers (LCEs) are functional materials whose stimuli-response is strongly influenced by mechanical properties dependent on the development of the polymer network. These networks are commonly fabricated by subjecting acrylate monomers to polymerization reactions with thiols. This work explores the network structure and properties as a result of LCE polymer networks formed via radical-mediated photopolymerization reactions using diacrylate liquid crystalline monomers with dithiols. Control experiments contrasting these reactions to conventional thiol-acrylate photopolymerizations in non-liquid crystalline media indicate that the liquid crystalline structural character strongly hinders mobility of reactive species during the formation of the LCE polymer network. Detailed analysis using monoacrylates indicates that these reactions produce LCEs composed of singly-reacted dithiols and numerous dangling ends in the structure. The unreacted thiol end of the dithiols incorporated in the LCE polymer network is utilized to facilitate post-functionalization of the LCE with reactive additives for additional degrees of control of functional material properties.
Organocatalyzed atom transfer radical polymerization (O-ATRP) is a controlled radical polymerization method employing organic photoredox catalysis to produce polymers with targeted molecular weights, narrow molecular weight distributions, and complex architectures. Previously, much of the work in this field has focused on the design of new and improved catalysts to access polymerizations that can be performed using visible light irradiation while maintaining a high degree of control over the polymerization of various acrylate and methacrylate monomers. The result of this work has been the introduction of several new organic photoredox catalysts (PCs), as well as an improved understanding of how catalyst structures impact activation in O-ATRP. However, deactivation – the process that enables control over these radical polymerizations – has historically been understudied, especially with regard to the PC radical cations thought to be responsible for this process. In this presentation, recent work investigating PC radical cations to better understand deactivation in O-ATRP is discussed. As a result of this work, new side reactions are identified that can limit polymerization control in O-ATRP, the role of PC radical cations in deactivation is probed, and factors influencing the deactivation process are uncovered.
The incorporation of renewable feedstocks into polymer backbones is of great importance in modern polymer science. We report the synthesis of 1,3-diyne polymers derived from the bispropargyl ethers of isosorbide, isomannide, and isoidide. The dialkyne monomers can be polymerized through an adaptation of the Glaser–Hay coupling using a nickel(II) cocatalyst. These well-defined diyne polymers bear an iodoalkyne end group, afforded through an unanticipated reductive elimination pathway, and display glass transition temperatures (Tg) from 55 to 64 °C. Fully saturated, analogous polyethers can be prepared from the hydrogenation of the diyne polymers, and these show Tg values between −10 and −2 °C. Both the 1,3-diyne polymers and the saturated analogues display similar trends in their Tg values vis-à-vis the stereochemical features of the isohexide unit within the backbone. This polymerization provided access to two series of isohexide-based polyethers, the thermal properties of which are influenced by the nature of the 2,4-hexadiynyl and hexamethylene linkers as well as the relative configuration of the bicyclic subunit in the backbone. The reported method represents an important step toward accessing well-defined polyethers from renewable feedstocks using readily available catalysts and convenient ambient conditions.
Click chemistries have transformed polymer chemistry, with their potential to add functionality with remarkable easy to precisely defined polymer structures. Our group has contributed to that via the first polymer formation using the SuFEx reaction based on SOF4 as building block and high-end characterization down to the individual polymer chain level (Nature Chemistry2021, 13, 858), and via the application of intrinsically chiral click reactions (ACIE2020, 59, 7494) to - most recently - the formation of polymers with configurational backbone chirality. The presentation outlines this development, will discuss both synthetic features and polymer characterization in detail and will sketch directions where to go from here.
This paper focuses on the synthesis and characterization of new bivalent folate-targeted PEGylated doxorubicin (FA2-dPEG-DOX2) made by modular chemo-enzymatic processes using Candida antarctica lipase B (CALB) as biocatalyst. Unique features are the use of monodisperse PEG (dPEG) and the synthesis of FA-SH yielding exclusive γ-conjugation of folic acid (FA). In comparison, conjugates in the literature use the activated ester method to attach FA to polymers that gives a mixture of products that need to be purified to separate the biologically active γ-conjugate. The synthetic strategies are shown in Figure 1. DOX fluoresces in the red so it has dual properties as both a diagnostic and therapeutic agent. It does not interfere with live tissue fluorescence. The modular approach with enzyme catalysis leads to selectivity, full conversion and high yield, and no transition metal catalyst residues. Flow cytometry analysis showed that at 10 µM concentration, both free DOX and FA2-dPEG-DOX2 would be taken up by 99.9% of triple-negative breast cancer cells in 2 h. Fluorescence was detected for 5 days after injecting compound IV into mice. Preliminary results showed that intra-tumoral injection seemed to delay tumor growth more than intravenous delivery.
Figure 1. Synthesis of FA2-dPEG-DOX2 (a) and FA-SH (b)
Photo-controlled Atom Transfer Radical Polymerization (Photo-ATRP) mediated by UV light (λ = 365 nm) was utilized, for the first time, to synthesize star polymers by arm-first method. Star diblock copolymers were prepared by a two-step process starting with a one pot amphiphilic diblock copolymer synthesis containing poly(oligo(ethylene glycol) methyl ether acrylate) (POEGA) blocks and poly(t-butyl acrylate) (PtBA) blocks. Star diblock copolymers were successfully prepared using various rigid and flexible crosslinkers using CuIIBr2/ Tris[2- (dimethylamino)ethyl]amine (Me6TREN) catalyst systems in trifluoroethanol (TFE) and CuIIBr2/tris(pyridin-2-ylmethyl)amine (TPMA) catalyst systems in water. Amphiphilic block copolymers POEGMA-b-PtBA form micelles in water by ultrasonication followed by crosslinking using photo-ATRP. Star-star coupling side reactions were minimized due to the preassembling of micelles. Very high molecular weight with narrow dispersity star diblock copolymers were obtained. After the deprotection of tert-butyl moieties, the star diblock copolymer will be utilized as nanoreactors in the future for fabricating functional metal nanoparticle (MNP) gels and superlattices.
When studying the chemistry of materials and their structure-property relationships, it can be valuable to consider combinations of molecular motifs with dissimilar characteristics to probe the impact of each motif on the resulting properties. Rubbery materials hold a unique place in human society and the chemistry of materials due to their viscoelastic properties, where the enigmatic glass transition plays a central role. Polyisoprene is perhaps the canonical rubber, and represents an astonishingly hydrophobic biopolymer, with a glass transition ca. -60 to -70 °C. The cis- form is amorphous and liquid at low molar masses, while the harvested biopolymer is solid, but with a hardly processable mega-dalton molar mass. The low Tg and the ability to cross-link the unsaturated chains has allowed for diene polymers to serve as a core material in human society, from tires to o-rings. In stark contrast, diamondoids are highly crystalline molecules possessing a bond geometry that may be superimposed on the diamond lattice, the smallest of which is adamantane. These highly crystalline solids have been isolated from petroleum reserves identifiable as multiple, fused adamantane units. Adamantane is a unique compound showing a surprisingly high melting point for a C10 compound, > 200 °C, though significant sublimation can be found even at room temperature. In comparison, the melting point of decane is -30 °C. This contrast strongly suggested utilizing adamantane as a pendant group on 1,3-butadiene to explore the resulting polymers, suspecting a variety of unique properties would emerge. To this end, 2-adamantyl-1,3-butadiene was synthesized and the homopolymer and copolymers were produced by emulsion and anionic polymerization. Copolymers with isoprene possess glass transition temperatures ranging from -63 to 172 °C, and give robust rubbery materials at intermediate co-monomer ratios. Further results of this research effort have included thermomechanical and rheological characterization, with the soluble fraction of the 40 wt% copolymer showing a striking resistance to viscous flow at temperatures up to 220°C. These results suggest that the adamantane cages are strongly affecting chain interactions, and it is hypothesized that the adamantyl cages are acting as “snags” that prevent chain slippage.
Stimuli-responsive polymeric nanoparticles with dynamic covalent bonds, exhibiting simultaneous multi-responsivity, open the door for the construction of a highly synchronized system in nanotechnology and biology-driven applications. Utilizing one-pot, photo-controlled atom transfer radical polymerization induced self-assembly (PhotoATR-PISA) mediated by UV light (λ = 365 nm) using parts per million (ppm) levels (ca. <20 ppm) of a copper catalyst and without any work-up between stages, varied polymeric nanostructures morphologies, from nanospheres to worm-like micelles were obtained at ambient temperature. A series of well-defined, core cross-linked polymeric nanoparticles were prepared from the solvophilic poly(oligo(ethylene oxide) methyl ether methacrylate) (POEGMA) stabilizer via one-pot chain extension with glycidyl methacrylate (GMA) as core-forming, hydrophobic block, and N, N-cystamine bismethacrylamide (CBMA) and coumarin-methacrylate as cross-linking agents. The system demonstrated UV and redox stimuli-responsiveness via the coumarin [2+2] photocycloaddition/scission and DL-dithiothreitol (DTT) reduction of disulfide bonds, respectively. Fluorescence spectroscopy was utilized to verify the Nile red encapsulated NPs’ stimuli-responsiveness. It was displayed that each mentioned stimuli were able to trigger the Nile red release to some extent and show synergic effect for their simultaneous usage. In addition, the kinetics of release could be tailored by the pH of the surrounding medium. Thus, the system also showed pH-responsive controlled release behavior. These stimuli-responsive polymeric nanoparticles are being investigated for potential applications in biomedicines, targeted in vivo drug delivery, and water purification, to name a few.
Polymeric Cross-linking and Decross-linking process
The development of alternative transportation modes such as electric or hybrid vehicles, has become a key need for a sustainable long term development. Among the different technologies, lithium-metal polymer batteries (LMPB) are the most attractive. Indeed, lithium-metal as an anode shows a specific capacity of more than ten times that of the LiC6anodes which are currently used in the widespread lithium-ion battery. Unfortunately, the use of lithium metal associated with liquid electrolyte, is the source of safety problems due to a possible irregular metallic lithium electrodeposits during the recharge. In some cases, this phenomenon could result in dendrite formation responsible for dramatic explosion hazards. In order to avoid this issue, solid polymer electrolytes (SPE) were developed. However, the development of SPE has been hampered by two hurdles i/ the inability to design a SPE that exhibits both a high ionic conductivity and good mechanical properties and ii/ during battery operation, the motions of lithium ions carry only a small fraction of the overall ionic current which leads to the formation of strong concentration gradient resulting in undesired effects like favored dendritic growth and limited energy density especially when power increases. To overcome these drawbacks, in this communication we will present our latest results on the developement of SPE composites made from the dispersion of silica nanoparticles grafted with polyethylene oxide (PEO) chains in a PEO matrix. The synthesis of the PEO-grafted nanoparticles as well as an in-depth characterization of the SPE using techniques including SAXS, SANS, DSC, TGA, rheology but also electrochemical characterization will be discussed.
Scheme 1. Synthesis of PEO-grafted silica nanoparticles
Degradable polyethylene glycol (PEG) hydrogels provide platforms for drug delivery and tissue engineering. End group modification of poly(ethylene glycol) (Mn = 2,000 g/mol) yielded polyether precursors with pH-sensitive and photocurable end groups. UV-initiated binary thiol-acrylate crosslinking with varied amount of thiol-functionalized three-arm PEG (THIOCURE® ETTMP 1300) developed pH-degradable networksSmall angle X-ray scattering (SAXS) and dynamic mechanical analysis (DMA) confirmed that the stoichiometric offset of thiols and acrylates controlled crosslink density. Spectroscopic monitoring of a released dye established controlled degradation studies, which quantified the hydrogels’ degradation rate. Hydrogels displayed bulk degradation in acidic solution. Low crosslink density gels degraded fully in aqueous solutions of pH (3.4) within 72 h while the highly crosslinked gels fully degraded over 3 wks. All hydrogels displayed long term stability in phosphate buffered saline of pH (7.4) beyond 3 mo making them ideal hydrogels for selective degradation and release in low pH environments.
Dynamic functions of biological organisms often rely on arrays of actively deformable microstructures undergoing a nearly unlimited repertoire of predetermined and self-regulated reconfigurations and motions, most of which are difficult or not yet possible to achieve in synthetic systems. Here, we introduce stimuli-responsive microstructures based on liquid-crystalline elastomers (LCEs) that display a broad range of hierarchical, even mechanically unfavored deformation behaviors. By polymerizing molded prepolymer in patterned magnetic fields, we encode any desired uniform mesogen orientation into the resulting LCE microstructures, which is then read out upon heating above the nematic-isotropic transition temperature (TN-l as a specific prescribed deformation, such as twisting, in- and out-of-plane tilting, stretching, or contraction. By further introducing light-responsive moieties, we demonstrate unique multifunctionality of the LCEs capable of three actuation modes: self-regulated bending toward the light source at T < TN-I, magnetic-field-encoded predetermined deformation at T > TN-I, and direction-dependent self-regulated motion toward the light at T > TN-I. We develop approaches to create patterned arrays of microstructures with encoded multiple area-specific deformation modes and show their functions in responsive release of cargo, image concealment, and light-controlled reflectivity. We foresee that this platform can be widely applied in switchable adhesion, information encryption, autonomous antennae, energy harvesting, soft robotics, and smart buildings.
The production of polymer products relies largely on age-old molding techniques. Casting arose roughly 7,000 years ago. Polymer materials were first used in injection molding 150 years ago. Since then, the basic approach to manufacturing polymer products at scale has not fundamentally changed. A major reason for this is that additive methods have not delivered meaningful alternatives to traditional processes. In this talk, I will describe Continuous Liquid Interface Production (CLIP) technology, which embodies a convergence of advances in software, hardware, and materials to bring the digital revolution to polymer additive manufacturing. CLIP uses software-controlled chemistry to produce commercial quality parts rapidly and at scale by capitalizing on the principle of oxygen-inhibited photopolymerization to generate a continual liquid interface of uncured resin between a forming part and a printer’s exposure window. This allows layerless parts to ‘grow’ continuously from a pool of resin, formed by light. Compatible with a wide range of polymers, CLIP opens major opportunities for innovative products across diverse industries. Previously unmakeable products are already manufactured at scale with CLIP, given its combination of performance, speed, and material choice. Additionally, at Stanford we are currently developing new high-resolution, high-throughput printing capabilities using CLIP, including for microneedle-based vaccine platform applications using unmoldable designs.
Antibiotic overuse in animals fuels the dissemination of antibiotic resistant bacterial strains. An exciting possibility is to replace antibiotic use with alternative strategies based on immunity. Animals weight gain can be attenuated by immune responses. Our goal was to develop vaccines that could increase the yield of effective antibodies against key proteins involved in immunity to promote weight gain. An incomplete understanding of how to combine vaccine elements that promote B and T cell communication and yield enhanced antibody production is a major barrier to developing effective responses. To address this deficiency, we employed the ring opening metathesis polymerization (ROMP) to generate chemically defined multivalent antigens bearing B and T cell epitopes to robustly generate antibodies In chickens. Our findings that yielded effective vaccines that can mitigate the effects of immune system-dependent weight loss. We are applying our polymeric designs to generate subunit vaccines that yield increased antibody titer to replace the use of antibiotics.
Oxidative coupling of parent free-base porphyrin or metalloporphyrins on the surface of an aqueous subphase in a Langmuir-Blodgett trough yields single or multiple sheets of porphene (C20N4H2)∞, a regular fully conjugated two-dimensional polymer similar to graphene, but composed of fused porphyrin instead of benzene rings. Porphene structure was established by several spectroscopic and imaging techniques both in situ and after transfer to solid surfaces or grids. Density functional calculations with periodic boundary conditions suggest that the most stable tautomer contains a checkerboard arrangement of NH bonds, whose directions alternate as one proceeds from a macrocycle to its four nearest neighbors. Porphene properties can be tuned without taking any π centers out of conjugation, because under redox control the two protons centered in each macrocyclic ring can be reversibly exchanged for metal ions, and these can carry ligands.
This lecture will describe our ongoing effort to engineer the physical properties of thin bacterial films by display of adhesive proteins on the cell surface. Studies of film fabrication and structure, in situ mineralization, and response to mechanical stress will be discussed.
Preparation of thin bacterial films as soft materials
We investigate the comparative self-assembly, crystalline structure and morphology, and crystallization kinetics of three sets of bio–sourced, polyethylene–like polymers, including polyesters (PEs), polycarbonates (PCs), and polyacetals (PAs) with aliphatic sequences between precisely placed functional groups (FGs) ranging in length from 18 to 48 methylene (CH2) groups. We quantify the polarity of each FG in terms of the solubility parameter and demonstrate that melting and crystallization temperatures increase with increasing FG interaction strength (PE > PC > PA) and decrease with increasing FG content. All form lamellar crystals with FGs incorporated in planar layers, evidenced by the presence of layer reflections in the X–ray patterns. PEs and PCs have an isomorphic, polyethylene–like unit cell. In contrast, PAs develop multiple crystalline polymorphic forms depending on the crystallization temperature. Polyethylene–like packing in PEs and PCs is enabled by dipole–dipole interactions between FGs of adjacent chains, which serve to compensate for the enthalpic penalty introduced by the presence of FGs in the crystals. The relatively weaker interactions as well as the drive for gauche bonding in acetal groups lead to a different unit cell in PAs. For the same undercooling and FG content, PEs tend to crystallize faster than PCs, indicating that stronger FG interactions enhance crystallization kinetics. PAs crystallize at lower temperatures than either PCs or PEs, suggesting that the weaker FG interactions in PAs slow crystallization. In general, crystallization rates are normalized when compared at the same undercooling for the same FG type, indicating that FG polarity, rather than content, is the dominant factor controlling crystallization kinetics. Unusual inversions in the crystallization rate at low undercooling with crystallization temperature observed in PEs and PAs will be discussed.
Ionenes are ion-containing polymers that have charges in their backbone rather than on pendant sites. Segmented ionenes behave like an elastomer, similar to elastomeric polyurethane, by having alternating hard and soft segments along the backbone. This work described the influence of structural difference of hard segments, linear and heterocyclic aliphatic amines, as well as the weight fraction of soft segments on the thermomechanical properties, elasticity, and microphase separation of poly(ethylene glycol) (PEG)-based ionenes. Step-growth polymerization via the Menshutkin reaction of aliphatic and aromatic ditertiary amines with dihalides afforded the synthesis of PEG-based ionene random block copolymers. 1H NMR spectroscopy confirmed the successful synthesis of PEG-2k dibromide oligomers and segmented ionenes. DSC and XRD revealed that DABCO-based ionenes have higher purity of PEG crystallites compared to aliphatic ionenes, and 75 wt% of soft segment triggered soft-hard mixing in both aliphatic and DABCO hard segments. TGA showed that both aliphatic and DABCO-based ionenes were thermally stable up to 250 °C and the possible crosslinking of DABCO upon exposure to elevated temperatures for DABCO-based ionenes. DMA revealed that DABCO-based ionenes possess superior elastomeric behavior compared to aliphatic ionenes at all weight fractions, possibly due to the better ionic aggregation and microphase separation. AFM confirmed the presence of microphase separated morphology with both aliphatic and DABCO-based ionenes having 25 wt% of the soft segment, which showed a better degree of phase separation among different weight fractions. Uniaxial tensile analysis showed that the elongation of PEG-based ionenes is highly dependent on the melting temperature of the PEG crystallites. Thus, these findings improve our understanding of microphase separated segmented ionene elastomers for a diverse set of applications.
Figure 1. Structures of poly(ethylene glycol)-based segmented ionenes with aliphatic and DABCO hard segments. XX refers to 25, 50, and 75 which are corresponding to the overall weight fractions of the soft segment.
Infiltration of polymers into nanoparticle packings has emerged as a powerful strategy to prepare highly loaded composites with superb mechanical and transport properties. The high surface energy of the nanoparticles as well as the high curvature of the pores give rise to complex interaction of nanoparticle packings with polymer and water. Leaching-enabled capillary rise infiltration (LeCaRI) is a novel technique of making polymer composites by inducing the motion of mobile polymers from polymer-filled gels into nanoparticle packing. This presentation will describe dynamics and thermodynamics of polymer infiltrated into nanoparticle packings via LeCaRI under varying humidity conditions. A hydrophobic low glass transition temperature polymer, polydimethylsiloxane (PDMS), is loaded into a PDMS elastomer. The PDMS-filled elastomer is contacted with a packing of SiO2 nanoparticles to induce infiltration of PDMS into the pores of the SiO2 nanoparticle packing by capillary forces. Atmospheric humidity controls the amount of polymer infiltrated in the pores due to capillary condensation of water. When polymer infiltration is localized by using patterned PDMS stamps, polymer can then spread laterally to unfilled regions. This room temperature, spontaneous lateral motion of polymer can be tracked to understand interfacial polymer diffusion under confinement. Results on polymer diffusivity inside silica particle packings shows that higher humidity, unexpectedly, leads to faster spreading of the polymers within the packings possibly due to reduced particle-polymer friction with water coverage on particle surface. Finally, when the polymer-filled nanoparticle packings are exposed to humid atmosphere, heterogeneity in film surface composition along with unique nanostructures at the surface leads to spontaneous drop-wise condensation of waterwithout any subcooling.
The controlled alignment of stacked MXene (Ti3C2Tx) with nano to microscale patterning on complex 3D architectures is essential for tuning the functional properties of the composite structures. In this work, the rapid and scalable method to fabricate well-aligned thin film of MXene by combining μCLIP 3D printing and capillary-driven self-assembly technique has been studied. For deposition, single/multi-layered MXene flakes solution was confined into microchannels via capillary action leading to the formation of highly ordered and aligned MXene film. During the evaporation-drying process, the solid-liquid-air contact line is molded by the shape of the gratings, and the nanoparticle experiences various long-range and short-range microfluidic forces which promote the layer-by-layer deposition of nanoparticles. The stacked film displayed strong plane to plane and out-of-plane adherence, with anisotropic electronic properties. The 3D printed template enables a large area deposition of MXene on complex architecture while protecting film structure from mechanical deformations. The device demonstrated high sensitivity, wide sensing range, and excellent durability, suitable for the piezoresistive sensor. This synergistic approach shows potential for nanomaterial assembly and broad applications, such as structural composites, sensors, actuators, human−machine interfaces, cryptosecurity, and soft robotics.
Schematic of the hybrid 3D printing combining (a) the surface patterning via mCLIP (CCD: Charge-coupled device, BS: Birefringence system, UV: Ultraviolet) with the (b) surface topography of micro features like microchannels and reservoir on the substrate and (c) DIW of MXene/ethanol ink for directed MXene assembly with anisotropic deposition and preferential alignment. (d) Schematic for the alignment mechanism with micro force balances between the shear from the ink flow (Fc1), gravity (Fg), drag force (Fd), capillarity (Fc2), and Van der Waals (Fvdw) between adjacent layers (Ln).
Recently, various block copolymer blending strategies have demonstrated that Frank-Kasper (FK) phases represent equilibrium states in particle forming block copolymers (BCP). Close examinations of the symmetry selection between this diverse set of energetically near-degenerate particle packings provides an unrivaled look into the nuances of BCP self-assembly. Building on our recent report of the ability of core-homopolymer/diblock blends to generate FK phases, we sought to investigate the impact of homopolymer molecular weight on the symmetry of the resultant particle packings. We blended a FK-phase forming poly (ethylene oxide-block-2-ethyl hexyl acrylate) (PEO-P(2-EHA)) diblock with poly (ethylene oxide) (PEO) homopolymers across a range of molecular weights. Temperature dependent synchrotron small angle X-ray scattering indicated a strong dependence of particle packing symmetry on homopolymer molecular weight. Namely, FK phases with lower volume asymmetry between constituent particles such as sigma was favored for blends with lower molecular weight homopolymers whereas high volume asymmetry phases such as the Laves C14 and C15 lattices were favored for blends with higher molecular weight homopolymers. These results may be rationalized in terms of the homopolymer distribution within each of these particle packings, demonstrating that homopolymers are a tool in the formation of FK phases and offer control over which particle packing arises.
Thermoresponsive polymers are used in numerous technological applications, including biomedicine, insulator materials, and tissue engineering. Despite their wide use, we lack well-established, direct techniques for elucidating their elevated temperature, solution-phase, nanoscale morphologies and dynamics. Presently, the accepted workflow for analyzing these materials at elevated temperatures consists of scattering techniques with static imaging via electron microscopy. However, scattering techniques require raw data to be fit to models, often creating challenges in assigning nanostructure morphologies. Alternatively, direct imaging by traditional transmission electron microscopy (TEM) methods at temperature is typically not feasible for nanomaterials that can undergo thermally-reversible transitions. To address this unmet need in the fields of nano and polymer science, we examined thermoresponsive polymeric materials by variable temperature liquid-cell TEM (VT-LCTEM), a nascent technique for imaging solvated nanomaterials and their dynamics with heating capabilities. We studied phase transitions of thermoresponsive poly(diethylene glycol methyl ether methacrylate) (PDEGMA)-based polymers, specifically a homopolymer, diblock, and triblock. We mitigated sample damage by screening imaging and solvent conditions during LCTEM and evaluated polymer survival via matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI-IMS). Additionally, we employed variable temperature small angle X-ray scattering (VT-SAXS) to correlate LCTEM data. Our multimodal approach, utilizing VT-LCTEM with MS validation and VT-SAXS, is generalizable across polymeric systems and can be used to study solvated nanomaterials and thermally-induced transitions. Moreover, utilizing SAXS with LCTEM provided direct insight into transient nanoscale intermediates formed during the thermally-triggered transformation of a PDEGMA-based triblock. Notably, we observed the temperature-triggered formation and slow relaxation of core-shell particles with complex microphase separation by both techniques.
Multimodal study of thermoresponsive polymeric nano-assemblies.
Polymers of intrinsic microporosity (PIMs) have shown excellent pure-gas separation performance due to their rigid backbones, inefficient packing, and high free volume. Their out-of-equilibrium packing structures, however, make PIMs susceptible to aging and plasticization. We recently reported mixed-gas and high-pressure transport properties of six functionalized PIMs. Low-pressure mixed-gas tests indicated a relationship between CO2 sorption affinity and increased CO2/CH4 mixed-gas selectivity compared to pure-gas calculations for PIMs considered. The best results were found for amine-functionalized PIM-1 (PIM-NH2), which shows a 2.4- and 3.5-fold increase in mixed-gas CO2/CH4 and CO2/N2 selectivity, respectively. PIM-NH2 also retained high mixed-gas selectivity up to a total mixed-gas pressure of 26 bar. Our results demonstrated the promise of amine functionalization for developing sorption-selective and plasticization-resistant membranes for gas separations. Here, we probe the generalizability of our findings by synthesizing an amine-functionalized microporous polymer based on a poly(aryl ether) (PAE) backbone (Fig. 1). PAE-NH2 shows increases in mixed-gas selectivities (compared to pure-gas) similar to those observed for PIM-NH2. Moreover, pure-gas CO2 sorption for PAE-NH2 was significantly higher than that of PAE-CN, which suggests that increases in mixed-gas performance were driven by sorption. The strength of the CO2-polymer interactions were quantified through calculation of isosteric heats of sorption for CO2 in PIM-1, PIM-NH2, PAE-CN, and PAE-NH2. Amine-functionalized samples showed significantly more exothermic interactions compared to those of nitrile-functionalized analogues, indicating stronger interactions for the amine with CO2. Additionally, PAE-NH2 showed exceptional plasticization resistance up to a total mixed-gas pressure of 26 bar. Finally, the H2S sorption capacity of all four samples was tested. Interestingly, compared to nitrile-functionalized analogues, amine-functionalized samples did not show as strong of an affinity increase with H2S as was observed with CO2.
Supramolecular chemistry is intrinsically a dynamic chemistry in view of the lability of the interactions connecting the molecular components of a supramolecular species and the resulting ability to exchange components. The same holds for molecular chemistry when the molecular entity contains covalent bonds that may form and break reversibly. These features allow for a continuous change in constitution by reorganization and exchange of building blocks and define a Constitutional Dynamic Chemistry (CDC) covering both the molecular and supramolecular levels. CDC introduces a paradigm shift with respect to constitutionally static chemistry. It takes advantage of dynamic diversity to allow variation and selection and operates on dynamic constitutional diversity in response to either internal or external factors to achieve adaptation. CDC generates networks of dynamically interconverting constituents, constitutional dynamic networks, presenting agonistic and antagonistic relationships between their constituents that may respond to perturbations by physical stimuli or to chemical effectors. In materials science, it leads in particular to the generation of dynamic polymers, dynamers, and biopolymers. The implementation of these concepts points to the emergence of adaptive and evolutive chemistry, towards systems of increasing complexity.
In this lecture I will describe an improbable tale of intersecting paths of two intellectual giants, Ralph Hirschmann and Leslie Orgel, within the context of studies on the prebiotic formation of peptides which were carried out in our laboratories by Professor Luke Leman. Almost all discussions of prebiotic chemistry assume that amino acids, nucleotides, and possibly other monomers were first formed on the Earth or brought to it in comets and meteorites, and then condensed nonenzymatically to form oligomeric products. However, attempts to demonstrate plausibly prebiotic polymerization reactions have met with limited success. We show that carbonyl sulfide (COS), a simple volcanic gas, brings about the formation of peptides from amino acids under mild conditions in aqueous solution. Depending on the reaction conditions and additives used, exposure of a-amino acids to COS generates peptides in yields of up to 80% in minutes to hours at room temperature.
This talk will provide an overview of our work in the design of self-assembled, functionalised peptide and protein nanoparticles, hydrogels and bio-interfaces for applications in healthcare. These hybrid materials are of growing importance with potential applications including drug delivery and tissue engineering. We are developing high-throughput synthesis techniques to diversify peptide libraries and quickly survey properties of interest such as fluorescence and supramolecular behaviour . We are exploiting the sensing capabilities of functionalised nanoparticles to engineer nanoprobes for in vivo disease diagnostics that produce a colorimetric response ideal naked eye read-out . Using molecular dynamics simulation, we can rationalise the different self-assembly behaviours across length scales and to understand the interplay of membrane-curvature and nanotopography at the biointerfaces to aid in the development of cell-active substrates . With the design of peptide-polymer hybrid hydrogels, we are incorporating tailorable mechanical properties to self-healing injectable scaffolds with exciting applications in regenerative medicine . This talk will also provide an overview of our recent advances in Raman spectroscopy-based characterisation techniques for tracking the functionalisation in single nanoparticles . Recent developments in this and other contexts will be discussed.
For the most part, enzymes contain one active site wherein they catalyze in a serial manner chemical reactions between substrates both efficiently and rapidly. Imagine if a situation could be created within a chiral porous crystal containing trillions of active sites where substrates can reside in vast numbers before being converted in parallel into products. Here, we report how it is possible to incorporate 1-anthracenecarboxylate as a substrate into a γ-cyclodextrin-containing metal-organic framework (CD-MOF-1), where the metals are K+ cations, prior to carrying out [4+4] photodimerizations between pairs of substrate molecules, affording selectively one of four possible regioisomers. One of the high-yielding regioisomers exhibits optical activity as a result of the presence of an 8:1 ratio of the two enantiomers following separation by HPLC. The solid-state superstructure of 1-anthracenecarboxylate potassium salt, which is co-crystallized with γ-CD, reveals that pairs of substrate molecules are, not only packed inside tunnels between spherical cavities present in CD-MOF-1, but are also stabilized—in addition to hydrogen-bonding to the C-2 and C-3 hydroxyl groups on the D-glucopyranosyl residues present in the γ-CD tori—by combinations of hydrophobic and electrostatic interactions between the carboxyl groups in 1-anthracenecarboxylate and four K+ cations on the waistline between the two γ-CD tori in the tunnels. These noncovalent bonding interactions result in preferred co-conformations that account for the highly regio- and enantioselective [4+4] cycloaddition during photoirradiation. Theoretical calculations, in conjunction with crystallography, support the regio- and stereochemical outcome of the photodimerization.
The Stupp Lab developed “magic” peptide molecules, named as peptide amphiphiles (PAs), that have fascinated not only those who have worked in the group but also the international scientific community. Their self-assembly into defined nanostructures is well established and their modular structure enables systematic variations to probe the chemical space and reveal structure/property relationships. This talk will describe our expedition around PAs [1,2], but also what we have learned in this journey to design peptides for creating materials with novel properties and biomedical interest .
Our lab is extensively involved in the minimalistic, reductionist and non-biased quest towards the most fundamental molecular recognition and self-assembling modules in nature that possess unique physical properties including mechanical, optical, electronic and piezoelectric. For many years, our identification of b-sheet-like arrangement of ultrashort dipeptides, most notably diphenylalanine (Phe-Phe) and its derivatives, had prompted the basic study and technological application of such short peptides. These peptides offer the combination of the materials properties of natural and synthetic polyamide with the ease and facile synthesis of dipeptide. We recently extended our studies in different directions. The first is the identification and utilization of minimalistic peptides that form helical structure. We realized that the Pro-Phe-Phe could form helical assemblies with notable mechanical properties. Inspired by nature, we replaced the proline with hydroxyproline to achieve Young's modulus comparable to titanium. One key direction is the use of co-assembly instead of self-assembly to achieve novel architectures and desired mechanical properties. Finally, in recent years, we become more and more interested in metabolites, both as the basis for disease but also as building blocks for materials with mechanical, optical and electronic properties. Many of the properties that are found in short peptides could be observed also in metabolite assemblies. Intriguingly, natural systems also use metabolite to form optically-active assemblies such as tapetum lucidum retro-reflectors.
Collagen is the most abundant protein in the human body and plays major roles in wound healing, tissue regeneration, cancer metastasis. The defining structural feature of collagen is the triple helix (figure a & b). Depending on the specific type of collagen, these triple helices pack into fibrillar bundles which continue to pack in a hierarchical assembly process, eventually forming macroscopic collagen fibers. This PP2 triple helix is also found in a variety of proteins outside the traditional collagen family in bacteria, viruses and the defense collagens of the innate immune system. Despite this ubiquity the structure-stability-function relationship of the collagen triple helix is poorly understood. CMPs differ in critical ways from other well studied proteins in that they lack a hydrophobic core and require a glycine every third amino acid creating a characteristic (Xaa-Yaa-Gly)n repeat (figure a). Design of synthetic triple helices which can mimic collagen are extremely challenging for many reasons, but one of the most apparent is that a system of three peptides A, B and C can self-assemble into at least 27 different compositions and registers. Proper design therefore must stabilize one of these while destabilizing the other 26. I will describe the design, synthesis and characterization of several AAB and ABC heterotrimers and computational methods which allow us to both predict their stability and allow rapid and accurate design of new triple helical systems. An additional challenge in the application of collagen triple helices is their extremely slow rate of folding and relatively poor equilibrium between monomer and trimer. To overcome these limitations we have developed a method of covalent capture (figure c & d) that perfectly preserves its three dimensional architecture while improving thermal stability. With these advances we are now working towards biomaterial applications that allow selective targeting of integrins.
Over the past two decades our laboratory has investigated supramolecular polymers based on biomolecular monomers known as peptide amphiphiles. We discovered that these amphiphiles, consisting of lipidated peptides that may also be glycosylated or conjugated to nucleic acids and synthetic segments, can self-assemble into filamentous supramolecular polymers. The materials obtained mimic the extracellular matrix of mammalian cells, and when biological signals are incorporated in their monomeric structures they display extraordinary bioactivity for the regeneration of tissues such as cartilage, bone, and the central nervous system. In a very recent discovery we found that intensifying supramolecular motion within these ensembles of thousands of molecules can have truly remarkable impact on their ability to signal cells. Strikingly, these supramolecular polymers “in motion” enhanced recovery from severe spinal cord injury. Slowing such motions greatly diminished their ability to reverse paralysis. This phenomenon could be broadly applicable to other biomedical therapies and offers an exciting frontier for supramolecular polymers.