Showing papers in "Macromolecules in 2018"

Tough, Swelling-Resistant, Self-Healing, and Adhesive Dual-Cross-Linked Hydrogels Based on Polymer–Tannic Acid Multiple Hydrogen Bonds

Abstract: We demonstrate a facile and universal strategy in the fabrication of dual-cross-linked (DC) single network hydrogels with high toughness, “nonswellability”, rapid self-healing, and versatile adhesiveness based on polymer–tannic acid (TA) multiple hydrogen bonds. Two widely used hydrogels, physically cross-linked poly(vinyl alcohol) and chemically cross-linked polyacrylamide, have been transformed to TA-based DC hydrogels by dipping the corresponding aerogels into TA solution. The second cross-link via multiple polymer–TA hydrogen bonds effectively suppresses the crack propagation, resulting in both DC gels with high mechanical strength. But these two TA-based DC hydrogels go through different deformation mechanisms during the stretching based on analyzing their stress–strain curves using the Mooney–Rivlin equation. Moreover, these DC hydrogels are swelling-resistant, with strong toughness, good self-recoverability, rapid self-healing, and versatile adhesiveness. This work provides a simple route to fabric...

Reprocessable Acid-Degradable Polycarbonate Vitrimers

Abstract: Vitrimers are cross-linked polymer networks containing linkages that undergo thermally activated, associative exchange reactions, such that the cross-link density and overall network connectivity are preserved. Polycarbonates are industrially relevant polymers that, to our knowledge, have not yet been explored as vitrimers. We developed hydroxyl-functionalized polycarbonate networks that undergo transcarbonation exchange reactions at elevated temperatures in the presence of catalytic Ti(IV) alkoxides. The rate of transcarbonation within the networks, estimated through stress relaxation experiments, was tuned by adjusting the catalyst loading or hydroxyl group concentration in the networks. The polymer networks exhibit recovery of their tensile strength and plateau storage modulus (71–133%) after reprocessing. In addition to being reprocessable, the networks were hydrolyzed and decarboxylated in aqueous acid to recover 80 wt % of the precursor to the bifunctional cyclic carbonate monomer. These observation...

Phase Behavior and Salt Partitioning in Polyelectrolyte Complex Coacervates

Abstract: Polyelectrolyte complexes are omnipresent both in nature and in the technological world, including nucleotide condensates, biological marine adhesives, food stabilizers, encapsulants, and carriers for gene therapy. However, the true phase behavior of complexes, resulting from associative phase separation of oppositely charged polyelectrolytes, remains poorly understood. Here, we rely on complementary experimental and simulation approaches to create a complete quantitative description of the phase behavior of polyelectrolyte complexes that represents a significant advance in our understanding of the underlying physics of polyelectrolyte complexation. Experiments employing multiple approaches with model polyelectrolytes—oppositely charged polypeptides poly(l-lysine) and poly(d,l-glutamic acid) of matched chain lengths—led to phase diagrams with compositions of the complex and the supernatant that were in excellent agreement with simulation results. Contrary to the widely accepted theory for complexation, we...

Robust, Fire-Safe, Monomer-Recovery, Highly Malleable Thermosets from Renewable Bioresources

Abstract: Conventional thermosets are built by nonrenewable fossil resources and are arduous to be reprocessed, recycled, and reshaped due to their permanent covalent cross-linking, and their flammability makes them unsafe during use. Here, for the first time, we synthesized a novel Schiff base precursor from abundant and renewable lignin derivative vanillin and produced malleable thermosets (Schiff base covalent adaptable networks (CANs)) combining high performance, super-rapid reprocessability, excellent monomer recovery, and arbitrary permanent shape changeability as well as outstanding fire resistance. The Schiff base CANs exhibited high glass transition temperatures of ∼178 °C, tensile strength of ∼69 MPa, tensile modulus of ∼1925 MPa, excellent flame retardancy with UL-94 V0 rating and V1 rating, and high LOI of ∼30%. Meanwhile, three Schiff base CANs showed high malleability with the activation energy of the bond exchange of 49–81 kJ mol–1 and could be reprocessed in 2–10 min at 180 °C. These Schiff base CAN...

A Catalyst-Free Epoxy Vitrimer System Based on Multifunctional Hyperbranched Polymer

Abstract: To date, all epoxy vitrimer systems reported in the literature rely on addition of significant amounts of catalysts to achieve the dynamic transesterification reaction (TER). However, the catalysts used in vitrimers are often toxic and have poor miscibility with organic compounds, and they may further comprise the application performance like corrosion resistance. Moreover, the reprocessing and recycling properties are highly dependent on the loading amount and the type of catalyst. In this study, two hyperbranched epoxy (HBE) prepolymers are synthesized and then reacted with succinic anhydride to prepare a catalyst-free epoxy vitrimer system. It is demonstrated that both the curing during the preparation of the cross-linked materials and the TER in the resulting cross-linked materials proceed properly without addition of external catalyst. We attributed this phenomenon to the abundant free hydroxyl groups in HBE which serve as both reacting moiety and catalyst in both curing and the TER processes. At ele...

A Self-Healable High Glass Transition Temperature Bioepoxy Material Based on Vitrimer Chemistry

Abstract: The design of high glass transition temperature (Tg) thermoset materials with considerable reparability is a challenge. In this study, a novel biobased triepoxy (TEP) is synthesized and cured with an anhydride monomer in the presence of zinc catalyst. The cured TEP exhibits a high Tg (187 °C) and comparable strength and modulus to the cured bisphenol A epoxy. By adopting the vitrimer chemistry, the cross-linked polymer materials are imparted significant stress relaxation and reparability via dynamic transesterification. It is noted that the reparability is closely related to the repairing temperature, external force, catalyst content, and the magnitude of rubbery modulus of the sample. The width of the crack from the cured TEP can be efficiently repaired within 10 min. This work introduces the first high-Tg biobased epoxy material with excellent reparability and provides a valuable method for the design of high-Tg self-healing materials suitable for high service temperature.

Recyclable and Malleable Epoxy Thermoset Bearing Aromatic Imine Bonds

Abstract: A method for preparing reprocessable epoxy thermoset is presented. The process is composed of synthesis of a bisphenol monomer bridged by an imine bond, glycidylation of the bisphenol, and cross-linking with a hardener to form thermoset. The resulting epoxy thermoset possesses comparable properties to conventional high-performance thermosets made from bisphenol A. However, when treated by a stimulus like acid, temperature, and/or water, the described thermoset exhibits reprocessability. Degradation and recycling involve hydrolysis and re-formation of imine bonds; reshaping and repairing of the thermoset are realized through imine exchange reactions. All the described processes require no metal catalyst, press heating, or additional monomer, which significantly widens thermoset reprocessing.

Vitrimers Designed Both To Strongly Suppress Creep and To Recover Original Cross-Link Density after Reprocessing: Quantitative Theory and Experiments

Abstract: Vitrimers form a promising class of dynamic polymer networks, but they have an Achilles’ heel: elastomeric vitrimers exhibit significant creep under conditions where permanently cross-linked, elastomeric networks exhibit little or no creep. We demonstrate that vitrimers can be designed with strongly suppressed creep and excellent reprocessability by incorporating a substantial yet subcritical fraction of permanent cross-links. This critical fraction of permanent cross-links, which has little or no detrimental effect on reprocessability, is defined by the gelation point of only permanent cross-links leading to a percolated permanent network. Via a modification of classic Flory–Stockmayer theory, we have developed a simple theory that quantitatively predicts an approximate limiting fraction. To test our theory, we designed vitrimers with controlled fractions of permanent cross-links based on thiol–epoxy click chemistry. We characterized the rubbery plateau modulus before and after reprocessing as well as st...

Multistimuli-Responsive Intrinsic Self-Healing Epoxy Resin Constructed by Host-Guest Interactions

Abstract: The self-healing abilities inside polymeric materials are desirable functions in materials science. Host–guest chemistry, combined with excellent properties of graphene, was used for the construction of multistimuli-responsive intrinsic self-healing epoxy materials. By ultraviolet (UV) curing, the unsaturated epoxy resin was connected with β-cyclodextrin/graphene complex through free radical copolymerization. The introduced complex, acting as macro-cross-linker and photothermal agent, can reconnect the damage-induced broken bonds through dynamic host–guest interaction. In this work, the epoxy composites exhibited a high healing efficiency of up to 79.2% and tensile strength of up to 20.8 MPa under heating or near-infrared stimulation, which represent relatively excellent values among all the reported intrinsic self-healing epoxy resin and host–guest self-healing systems.

Grafting Polymers from Cellulose Nanocrystals: Synthesis, Properties, and Applications

Abstract: Over the past 10 years, the grafting of polymers from the surface of cellulose nanocrystals (CNCs) has gained substantial interest in both academia and industry due to the rapidly growing number of potential applications of surface-modified CNCs, which range from building blocks in nanocomposites and responsive nanomaterials to antimicrobial agents. CNCs are rod-like nanoparticles that can be isolated from renewable biosources and which exhibit high crystallinity, tunable aspect ratio, high stiffness, and strength. Upon drying, the abundance of surface hydroxyl groups often leads to a degree of irreversible aggregation, as a result of strong hydrogen bonding. Moreover, their relatively hydrophilic character renders CNCs incompatible with hydrophobic media, e.g., nonpolar solvents and polyolefin matrices. By grafting macromolecules from their surface, CNCs can be imparted with surface characteristics and other physicochemical properties that are reminiscent of the grafted polymer. This has allowed the desi...

A Tough and Stiff Hydrogel with Tunable Water Content and Mechanical Properties Based on the Synergistic Effect of Hydrogen Bonding and Hydrophobic Interaction

Abstract: Hydrogels are usually recognized as soft and weak materials, the poor mechanical properties of which greatly limit their applications as structural elements. Designing of hydrogels with high strength and high modulus has both fundamental and practical significances. Herein we report a series of tough, stiff, and transparent hydrogels facilely prepared by copolymerization of 1-vinylimidazole and methacrylic acid in dimethyl sulfoxide followed by solvent exchange to water. The equilibrated hydrogels with water content of 50–60 wt % possessed excellent mechanical properties, with tensile breaking stress, breaking strain, Young’s modulus, and tearing fracture energy of 1.3–5.4 MPa, 40–330%, 20–170 MPa, and 600–4500 J/m2, respectively. These tough hydrogels were also stable over a wide pH range (2 ≤ pH ≤ 10), resulting from the formation of dense and robust hydrogen bonds between imidazole and carboxylic acid groups. Moreover, the water content and mechanical properties of one gel can be adjusted over a wide r...

Evolution and Future Directions of Metal-Free Atom Transfer Radical Polymerization

Abstract: The increasing impact of atom transfer radical polymerization (ATRP) in fields beyond traditional polymer science has necessitated the development of alternative strategies for controlling polymer growth. Driven by applications that are sensitive to metal ion contamination, “greener” methodologies are emerging as a powerful alternative to conventional ATRP. Organic catalysis represents a major evolution of ATRP with metal-free systems holding significant potential as user-friendly methods for utility in biological and microelectronic applications. In addition, shifting from a combination of thermal activation/metal ions/ligands to simpler organic catalysis/light activation increases compatibility with functional monomers and allows the development of novel surface patterning strategies. Herein, we highlight key discoveries and recent developments in metal-free ATRP, while providing a perspective for future opportunities in this emerging area.

Water Uptake Study of Anion Exchange Membranes

Abstract: Anion exchange membrane fuel cells (AEMFCs) have attracted extensive attention in the recent years, primarily due to the distinct advantage potentials they have over the mainstream proton exchange membrane fuel cells. The anion exchange membrane (AEM) is the key component of AEMFC systems. Because of the unique characteristics of water management in AEMFCs, understanding the water mobility through AEMs is key for this technology, as it significantly affects (and limits) overall cell performances. This work presents a study of the equilibrium state and kinetics of water uptake (WU) for AEMs exposed to vapor source H2O. We investigate different AEMs that exhibit diverse water uptake behaviors. AEMs containing different backbones (fluorinated and hydrocarbon-based backbones) and different functional groups (various cations as part of the backbone or as pendant groups) were studied. Equilibrium WU isotherms are measured and fitted by the Park model. The influence of relative humidity and temperature is also s...

Toughening Effect and Flame-Retardant Behaviors of Phosphaphenanthrene/Phenylsiloxane Bigroup Macromolecules in Epoxy Thermoset

Abstract: A series of novel phosphaphenanthrene/phenylsiloxane bigroup macromolecules (DDSi-n) were synthesized and applied to obtain high-performance epoxy thermosets. DDSi-n macromolecules simultaneously enhanced the anti-impact and flame-retardant performance of epoxy thermosets. The impact strength of the DDSi-n-containing thermoset (DDSi-n/EP) was maximally increased by nearly 140% in 8% DDSi-1/EP because of the flexible phenylsiloxane block and the polar phosphaphenanthrene group interacting with matrix in DDSi-n macromolecules. Meanwhile, the evidently elevated anti-ignition and self-extinguishing performance, the suppressed combustion heat, and the enhanced charring capability of DDSi-n/EP were all caused by the flame-retardant group synergistic effect of phosphaphenanthrene and phenylsiloxane groups of DDSi-n macromolecules in thermosets. Compared with the individual phosphaphenanthrene or phenylsiloxane group in monogroup contrasts, the phosphaphenanthrene and phenylsiloxane groups in bigroup DDSi-n macro...

Vinylogous Urea Vitrimers and Their Application in Fiber Reinforced Composites

Abstract: Vitrimers are covalently cross-linked polymeric materials that can be thermally processed in a liquid state without losing their network integrity. In this work, novel vitrimers based on the dynamic amine exchange reaction of vinylogous urea moieties are introduced. Following a systematic exploration of different vinylogous acyl compounds (urethanes, amides, and urea), vinylogous urea clearly emerges as showing the fastest intrinsic exchange kinetics. The combination of these networks with a simple acid catalyst (0.5 mol % pTsOH) resulted in materials with good mechanical properties (Tg ∼ 110 °C, E ∼ 2.2 GPa) and remarkably short relaxation times above Tg, in the order of a few seconds when heated. This attractive combination of properties made it possible to prepare vinylogous urea based composites as enduring prepregs. We demonstrate that the dynamic material properties give fully cured composites that still allow an efficient thermal fusion of multiple layers as well as thermoforming. Finally, we also ...

High Performance Near-Infrared (NIR) Photoinitiating Systems Operating under Low Light Intensity and in the Presence of Oxygen

Abstract: Photopolymerization under near-infrared (NIR) light is challenging due to the low energy of the absorbed photon but, if successful, presents significant advantages For example, this lower energy wavelength is safer than UV light that is currently the standard photocuring light source Also, NIR allows for a deeper light penetration within the material and therefore resulting in a more complete curing of thicker materials containing fillers for access to composites In this study, we report the use of three-component systems for the NIR photopolymerization of methacrylates: (1) a dye used as a photosensitizer in the NIR range, (2) an iodonium salt as a photoinitiator for the free radical polymerization of the (meth)acrylates, and (3) a phosphine to prevent polymerization inhibition due to the oxygen and to regenerate the dye upon irradiation Several NIR-absorbing dyes such as a cyanine borate and a silicon–phthalocyanine are presented and studied Systems using borate dyes resulted in methacrylate monome

Relating Chemical Structure to Toughness via Morphology Control in Fully Sustainable Sebacic Acid Cured Epoxidized Soybean Oil Toughened Polylactide Blends

Abstract: The use of soybean oil or its derivatives to toughen polylactide (PLA) usually leads to limited toughening efficiency, due to the incompatibility between toughening agents and parent PLA. Herein, we report a dynamic vulcanization method to toughen PLA using sebacic acid cured epoxidized soybean oil (VESO), a fully sustainable and biodegradable component. A series of sebacic acid cured epoxidized soybean oil precursors (SEPs) were prepared with different carboxyl/epoxy equivalent ratio (R), which consequently dictates the chemical structure and the morphology of PLA/VESO blends after the dynamic vulcanization. We demonstrated that the chemical structure of VESO plays a critical role in the compatibility, morphology, and toughness of the PLA/VESO blends. By optimizing the R-value, supertoughened PLA blends can be obtained, as evidenced by the significant improvement in the tensile toughness (up to 150.6 MJ/m3) and the impact strength (up to 542.3 J/m). The results of the toughening mechanism from the morpho...

Ion Diffusion Coefficients in Ion Exchange Membranes: Significance of Counterion Condensation

Abstract: This study presents a new framework for extracting single ion diffusion coefficients in ion exchange membranes from experimental ion sorption, salt permeability, and ionic conductivity data. The framework was used to calculate cation and anion diffusion coefficients in a series of commercial ion exchange membranes contacted by aqueous NaCl solutions. Counterion diffusion coefficients were greater than co-ion diffusion coefficients for all membranes after accounting for inherent differences due to ion size. A model for ion diffusion coefficients in ion exchange membranes, incorporating ideas from counterion condensation theory, was proposed to interpret the experimental results. The model predicted co-ion diffusion coefficients reasonably well with no adjustable parameters, while a single adjustable parameter was required to accurately describe counterion diffusion coefficients. The results suggest that for cross-linked ion exchange membranes in which counterion condensation occurs condensed counterions mi...

Advances in Atomic Force Microscopy for Probing Polymer Structure and Properties

Abstract: Over the past 30 years, atomic force microscopy (AFM) has played an important role in elucidating the structure and properties of polymer surfaces. AFM-based techniques have enabled the quantitative determination of the physicochemical properties of polymer surfaces with high spatial resolution and under a wide variety of conditions. Coupled with the improvements in spatial and temporal resolution, multiparametric and multifunctional characterization has revealed the delicate interplay between structure, dynamics, and properties at the surfaces of complex systems. Here we summarize some of the significant advances that have been made in synthetic polymeric materials, most in the past 10 years, where AFM has been crucial, and we provide our perspective on where AFM will be insightful in future and instrumental in advancing emerging areas.

Direct Synthesis of Polar-Functionalized Linear Low-Density Polyethylene (LLDPE) and Low-Density Polyethylene (LDPE)

Abstract: Late-transition-metal catalysts have great potentials to incorporate polar comonomers during olefin polymerization. The preparation of polar-functionalized polyolefins with different microstructures and topologies is a highly fascinating concept. In this contribution, we demonstrate this possibility through palladium-catalyzed ethylene copolymerizations with polar-functionalized α-olefins and their terpolymerizations with α-olefins. A phosphine–sulfonate–palladium catalyst (PO-Pd) afforded highly linear polyethylene during ethylene homopolymerization. Linear low-density polyethylene (LLDPE) was obtained from the PO-Pd-catalyzed copolymerizations of ethylene with α-olefins; in these copolymerizations, the partial or complete replacement of α-olefins with polar-functionalized α-olefins led to the formation of polar-functionalized LLDPE (P-LLDPE). A specially designed α-diimine palladium catalyst (NN-Pd) afforded polyethylenes with tunable branching densities (16–37 per 1000 carbon atoms), melting points (10...

Color-Coding Visible Light Polymerizations To Elucidate the Activation of Trithiocarbonates Using Eosin Y

Abstract: We report mechanistic investigations into aqueous visible-light reversible addition–fragmentation chain transfer (RAFT) polymerizations of acrylamides using eosin Y as a photoinduced electron-transfer (PET) catalyst. The photoinduced polymerization was found to be dependent upon the irradiation wavelength and reagents, where either reduction or oxidation of the PET catalyst leads to inherently different initiation and reversible-termination steps. Using blue light, multiple mechanisms of initiation are observed, depending on the presence or absence of a sacrificial reducing agent. Using green light, both an oxidative and a reductive PET initiation mechanism can be pursued. Investigations into the role of PET catalyst, wavelength, and reducing agent demonstrated that precise polymers with predictable molecular weights are best realized under an oxidative PET-RAFT mechanism. Therefore, this study provides fundamental insight into visible-light RAFT photopolymerizations and the role of eosin Y as a photoredo...

Atomistic Description of Ionic Diffusion in PEO–LiTFSI: Effect of Temperature, Molecular Weight, and Ionic Concentration

Abstract: Understanding the ionic diffusion mechanism in polymer electrolytes is critical to the development of advanced lithium-ion batteries. We report here molecular dynamics-based characterization of structures and diffusion in poly(ethylene oxide) (PEO) with lithium and bis(trifluoromethysulfonyl)imide (TFSI) ions imbedded into the PEO structure. We consider a range of temperatures (360–480 K), molecular weights (43, 22, 10, and 2 chains with 23, 45, 100, and 450 EO monomers, respectively), and ion concentrations (r = 0.02, 0.04, 0.06, and 0.08 Li:EO) for which there is experimental data. The found dependence of the diffusion coefficients on these variables is in good agreement with experimental measurements. We then analyze how the diffusion performance depends on details of the atomistic diffusion mechanism, the motion of the Li and TFSI along the polymer chains and hopping between them, the role of polymer motion, the temperature dependence of the intrachain and interchain diffusion contributions to the tot...

Selenium-Containing Polymers: Perspectives toward Diverse Applications in Both Adaptive and Biomedical Materials

Abstract: Selenium is a semimetallic element lying in group XVI of the periodic table with its chemical properties resembling sulfur. But owing to its relatively low electronegativity and large atomic radius compared with sulfur, selenium also shows unique properties. This feature endows selenium-containing compounds with high reactivity and sensitivity. Although organic selenium chemistry has been developing very fast, the successful introduction of selenium into polymer science is rather scarce. Fortunately, we have seen a drastic rising trend in the area of selenium-containing polymers over the past decade. In this Perspective, the synthetic routes of selenium-containing polymers are summarized, and their unique stimuli-responsive properties are elaborated on, together with their diverse applications in the field of adaptive and biomedical materials.

Effect of Linking Pattern of Dibenzothiophene-S,S-dioxide-Containing Conjugated Microporous Polymers on the Photocatalytic Performance

Abstract: Conjugated microporous polymers (CMPs) exhibit great potential for photocatalytic hydrogen generation due to their tunable electronic structure. The rational molecular design is a key point for developing an efficient CMP photocatalyst. Herein, we developed two CMPs photocatalysts via the copolymerization from pyrene and dibenzothiophene-S,S-dioxide building blocks. The effect of the linking pattern of the building block of dibenzothiophene-S,S-dioxide on the photocatalytic hydrogen evolution was explored. The polymer of PyDOBT-1 with 3,7-linking pattern shows a superior photocatalytic performance to PyDOBT-2 with 2,8-linking pattern because the 3,7-linking pattern enhances the conjugation chain length and improves the coplanarity of the polymeric backbone, which facilitate the charges migration along the polymer chain. As a result, the bare PyDOBT-1 shows an impressive visible light activity with a hydrogen evolution rate (HER) of 5697 μmol h–1 g–1. Notably, an outstanding HER of 12986 μmol h–1 g–1 was a...

A Fascinating Metallo-Supramolecular Polymer Network with Thermal/Magnetic/Light-Responsive Shape-Memory Effects Anchored by Fe3O4 Nanoparticles

Abstract: Two-way shape-memory polymers (2W-SMPs) show great potential in actuating applications such as robotics due to their reversibility; indeed, multiresponsive 2W-SMPs are more expected. Inspired by the fascinating adhesion effects of mussels, we herein describe a metallo-supramolecular poly(e-caprolactone) (PCL)-based network constructed around catechol chemistry, leading to excellent thermal/magnetic/light-responsive two-way shape-memory effects (2W-SME) as well as self-healing capacity. These hybrid networks get readily self-assembled upon metal coordination interaction between superparamagnetic iron oxide nanoparticles (Fe3O4 NPs) and catechol-telechelic PCL. The incorporation of Fe3O4 NPs may act as the strong netpoints which allow the networks with excellent thermal-responsive one-way (1W) and 2W-SME, due to the ability of the semicrystalline PCL segments to present both crystallization-induced elongation (CIE) and melting-induced contraction (MIC) under constant stress. As a multifunctional medium, mor...

Highly-Toughened Polylactide- (PLA-) Based Ternary Blends with Significantly Enhanced Glass Transition and Melt Strength: Tailoring the Interfacial Interactions, Phase Morphology, and Performance

Abstract: The inherent shortcomings of polylactide (PLA) including brittleness, low glass transition temperature, and melt strength during processing were addressed through a facile melt blending of PLA with polybutadiene-g-poly(styrene-co-acrylonitrile) (PB-g-SAN) core–shell impact modifier and poly(methyl methacrylate) (PMMA). Highly tough PLA-based ternary blends with drastically enhanced glass transition temperature (≈ 21 °C) and melt strength were successfully prepared. The effect of PMMA content (ranging from 0 to 30 wt %) on the phase miscibility, morphology, mechanical properties, thermal behavior, rheological properties, and toughening mechanisms of PLA/PB-g-SAN/PMMA blends with 30% PB-g-SAN was systematically investigated. It was found that PMMA can effectively tune the interfacial interactions, phase morphology and performance of incompatible PLA/PB-g-SAN blend owing to its partial miscibility with PLA matrix and miscibility with SAN shell of PB-g-SAN, as evidenced by DMTA analysis. Increase in PMMA cont...

Inside PEF: Chain Conformation and Dynamics in Crystalline and Amorphous Domains

Abstract: A thorough vibrational spectroscopy and molecular modeling study on poly(ethylene 2,5-furandicarboxylate) (PEF) explores its conformational preferences, in the amorphous and crystalline regions, while clarifying structure–property correlations. Despite the increasing relevance of PEF as a sustainable polymer, some of its unique characteristics are not yet fully understood and benefit from a deeper comprehension of its microstructure and intermolecular bonding. Results show that in the amorphous domains, where intermolecular interactions are weak, PEF chains favor a helical conformation. Prior to crystallization, polymeric chains undergo internal rotations extending their shape in a zigzag pattern—an energetically unfavorable geometry which is stabilized by C–H···O bonds among adjacent chain segments. The zigzag conformation is the crystalline motif present in the α and β PEF polymorphs. The energy difference among the amorphous and crystalline chains of PEF is higher than in PET poly(ethylene terephthalat...

Mechanical Reactivity of Two Different Spiropyran Mechanophores in Polydimethylsiloxane

Abstract: We investigate the effect of pulling point location on the mechanochemical activation of two isomers of spiropyran in cross-linked polymeric materials through computational calculations and in situ fluorescence measurements. The threshold stress and strain required to activate the spiropyran mechanophores under tensile load are characterized. For both spiropyran isomers, applied stress favors the activated merocyanine states; however, despite differences in mechanochemical behavior predicted by quantum chemical calculations and previous single molecule experiments, both spiropyran isomers exhibit similar mechanochemical reactivity in bulk polymeric materials. The kinetics of the spiropyran–merocyanine transition under different tensile stresses are also examined. Overall, we find that varying the pulling geometry on the spiropyran mechanophore has only a minimal effect on the mechanical activation in bulk polymeric materials due to the complex nature of the macroscopic system.

Highly Active Organic Lewis Pairs for the Copolymerization of Epoxides with Cyclic Anhydrides: Metal-Free Access to Well-Defined Aliphatic Polyesters

Abstract: Polyester synthesis from the alternating copolymerization of epoxides with cyclic anhydrides via a metal-free route remains a key challenge. This work reports the development of a highly active organocatalytic route for the copolymerization of a spectrum of epoxides and cyclic anhydrides. Fully alternating polyesters were synthesized by a variety of organic Lewis acid–base pairs including organoboranes and quaternary onium salts. The effect of the acidity, type, and size of Lewis pairs on the catalytic activity and selectivity of the copolymerization is presented. The undesirable transesterification and etherification were effectively suppressed even in the case of complete conversion of the cyclic anhydride. This could be ascribed to the formation of a unique tetracoordinate bond-carboxylate (or alkoxide) anion. The Lewis pairs are highly active, with a turnover frequency of 102 and 303 h–1 for the copolymerization of propylene oxide with maleic anhydride and phthalic anhydride, respectively, at 80 °C. B...

Organic Ring-Opening Polymerization Catalysts: Reactivity Control by Balancing Acidity

Abstract: Organocatalysts derived from thioureas and amines exhibit high functional group tolerance and extraordinary selectivities for ring-opening relative to chain transesterification. The modest activities of the thiourea/amine catalysts prompted a detailed investigation of ureas and thiourea with organic bases for the ring-opening polymerization of lactones. An array of ureas or thioureas and organic bases were evaluated to assess the effect of the acidity of the urea (thiourea) and the basicity of the base cocatalyst on the activity for ring-opening polymerization. These studies reveal that for a given urea or thiourea stronger bases lead to faster rates. For a given base, the observed catalytic activity is highest when the acidity of the (thio)urea is closely matched with that of the B–H+. For ureas and thioureas of comparable acidity, the urea/base catalyst systems are considerably more active than the corresponding thiourea/base systems. These results are consistent with two mechanisms: one mediated by dep...