Synthesis and Characterization of Ureidopyrimidone Telechelics by CuAAC “Click” Reaction: Effect of <i>T</i>_g and Polarity

Telechelic oligomers functionalized with 2-ureido-4­[1H]-pyrimidone (UPy), a quadruple hydrogen bonding group, have been synthesized using a combination of atom-transfer radical polymerization and click reaction. Ureidopyrimidone (UPy) synthons with propargyl and azide functionality were used for clicking with azido and propargyl telechelic oligomers, respectively. The effect of triazole linker and types of oligomers differing in Tg and polarity, such as poly­(n-butyl acrylate) (PnBA), polystyrene (PS), and polybutadiene (PBd) on UPy hydrogen bonding have been examined. High solution viscosity and deviation from the normal terminal relaxation in melt state were observed, suggesting the presence of UPy aggregates that are in equilibrium between linear and network polymers. Differential scanning calorimetry studies confirm dissociation of UPy aggregates as an endothermic peak for PBd system, whereas the high Tg and polar polymers (PS and PnBA) had no such peaks associated with Tm indicating the significance of the polymer chain dynamics in supramolecular hydrogen bonding. The triazole linker interferes with the UPy association and reduces the sizes of hydrogen-bonded UPy aggregates and thereby improves the physical property of supramolecular polymers

A Novel Reactive Processing Technique: Using Telechelic Polymers To Reactively Compatibilize Polymer Blends

Difunctional reactive polymers, telechelics, were used to reactively form multiblock copolymers in situ when melt-blended with a blend of polystyrene and polyisoprene. To quantify the ability of the copolymer to compatibilize the blends, the time evolution of the domain size upon annealing was analyzed by SEM. It was found that the most effective parameter to quantify the ability of the copolymer to inhibit droplet coalescence is Kreltstable, the relative coarsening constant multiplied by the stabilization time. These results indicate that intermediate-molecular-weight telechelic pairs of both highly reactive Anhydride-PS-Anhydride/NH2-PI-NH2 and slower reacting Epoxy-PS-Epoxy/COOH-PI-COOH both effectively suppress coalescence, with the optimal molecular weight being slightly above the critical molecular weight of the homopolymer, Mc. The effects of telechelic loading were also investigated, where the optimal loading concentration for this system was 0.5 wt %, as higher concentrations exhibited a plasticizing effect due to the presence of unreacted low-molecular-weight telechelics present in the blend. A determination of the interfacial coverage of the copolymer shows that a conversion of ∼1.5−3.0% was required for 20% surface coverage at 5.0 wt % telechelic loading, indicating a large excess of telechelics in this system. At the optimal loading level of 0.5 wt %, a conversion of 15% was required for 20% surface coverage. The results of these experiments provide a clear understanding of the role of telechelic loading and molecular weight on its ability to reactively form interfacial modifiers in phase-separated polymer blends and provide guidelines for the development of similar reactive processing schemes that can use telechelic polymers to reactively compatibilize a broad range of polymer blends

Living Anionic Surface Initiated Polymerization (SIP) of Styrene from Clay Surfaces

Living Anionic Surface Initiated Polymerization (SIP) of Styrene from Clay Surface

Atomistic and Coarse-Grained Molecular Dynamics Simulation of a Cross-Linked Sulfonated Poly(1,3-cyclohexadiene)-Based Proton Exchange Membrane

Atomistic and coarse-grained (CG) molecular dynamics (MD) simulations were conducted for a cross-linked and sulfonated poly­(1,3-cyclohexadiene) (xsPCHD) hydrated membrane with λ­(H₂O/HSO₃) = 10 and 20. From the atomistic level simulation results, nonbonded pair correlation functions (PCFs) of water–water, water–H₃O⁺ ion, H₃O⁺–H₃O⁺, polymer–water, and polymer–H₃O⁺ ion pairs were obtained and studied. The water self-diffusivity and H₃O⁺ vehicular self-diffusivity were also obtained. Membrane structure was further studied at CG level using a multiscale modeling procedure. Nonbonded PCFs of polymer–polymer pairs were obtained from atomistic simulation of hydrated membrane with λ = 10 and 20. Two sets of CG nonbonded potentials were then parametrized to the PCFs using the iterative Boltzmann inversion (IBI) method. The CGMD simulations of xsPCHD chains using potentials from above method satisfactorily reproduced the polymer–polymer PCFs from atomistic MD simulation of hydrated membrane system at each hydration level. The transferability of above two set of CG potentials was further tested through CGMD simulation of hydrated membrane at an intermediate hydration level (λ = 15). Limited transferability was observed, presumably due to the use of an implicit solvent. Using an analytical theory, proton conductivity was calculated and compared with that from experimental measurement under similar conditions. Good agreement was obtained using inputs from both atomistic and CG simulation. This study provides a molecular level understanding of relationship between membrane structure and water and H₃O⁺ ion transport in the xsPCHD membrane

Model Branched Polymers: Synthesis and Characterization of Asymmetric H-Shaped Polybutadienes

A new type of model branched polymer, asymmetric H-shaped polybutadienes, consisting of central crossbars having various combinations of short and long arms attached to the ends of the crossbars, was synthesized using living anionic polymerization and chlorosilane linking chemistry. The linking agent 4-(dichloromethylsilyl)­diphenylethylene provides selective reactivity to attach short or long arms on one side or both sides as desired. The samples were characterized thoroughly by size exclusion chromatography with light scattering detection (SEC-LS) and found to exhibit controlled molecular weights, as well as narrow polydispersity indices (PDIs of 1.01–1.06). Temperature gradient interaction chromatography, a method with far superior resolution as compared to SEC, also shows that these materials are well-defined, with minimal and identifiable impurities

Preparation of Aggregation Stable Gold Nanoparticles Using Star-Block Copolymers

Nanoparticles of improved stability against long-term aggregation were prepared using poly(styrene)-b-poly(2-vinylpyridine) (PS-b-P2VP) star-block copolymer architectures. The star-block copolymers, physically resembling diblock copolymer micelles, were synthesized by anionic polymerization and coupling with ethylene glycol dimethacrylate. They contain P2VP core segments that facilitated conversion of HAuCl4 to single gold nanoparticles. The size distribution and long-term stability against aggregation of the gold nanoparticles were investigated both as solution and as films. UV−vis spectroscopy and transmission electron microscope images reveal long-term stability against aggregation up to 1 month

All-Acrylic Multigraft Copolymers: Effect of Side Chain Molecular Weight and Volume Fraction on Mechanical Behavior

We present the synthesis of poly­(<i>n</i>-butyl acrylate)-<i>g</i>-poly­(methyl methacrylate) (P<i>n</i>BA-<i>g</i>-PMMA) multigraft copolymers via a grafting-through (macromonomer) approach. The synthesis was performed using two controlled polymerization techniques. The PMMA macromonomer was obtained by high-vacuum anionic polymerization followed by the copolymerization of <i>n</i>-butyl acrylate and PMMA macromonomer using reversible addition–fragmentation chain transfer (RAFT) polymerization to yield the desired all-acrylic multigraft structures. The P<i>n</i>BA-<i>g</i>-PMMA multigraft structures exhibit randomly spaced branch points with various PMMA contents, ranging from 15 to 40 vol %, allowing an investigation into how physical properties vary with differences in the number of branch points and molecular weight of grafted side chains. The determination of molecular weight and polydispersity indices of both the PMMA macromonomer and the graft copolymers was carried out using size exclusion chromatography with triple detection, and the structural characteristics of both the macromonomer and P<i>n</i>BA-<i>g</i>-PMMA graft materials were characterized by ¹H and ¹³C NMR. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry was employed for monitoring the macromonomer synthesis. Thermal characteristics of the materials were analyzed using differential scanning calorimetry and thermogravimetric analysis. The mechanical performance of the graft materials was characterized by rheology and dynamic mechanical analysis, revealing that samples with PMMA content of 25–40 vol % exhibit superior elastomeric properties as compared to materials containing short PMMA side chains or <25 vol % PMMA. Lastly, atomic force microscopy showed a varying degree of microphase separation between the glassy and rubbery components that is strongly dependent on PMMA side chain molecular weight

Nature of Steady-State Fast Flow in Entangled Polymer Melts: Chain Stretching, Shear Thinning, and Viscosity Scaling

Understanding the nonequilibrium dynamics of topologically entangled polymers under strong external deformation has been a grand challenge in polymer science for more than half a century. Important deformation-induced single-polymer structural changes have been identified, such as chain orientation and stretching. But how these changes impact the physical entanglement network and bulk viscoelasticity remains largely elusive in the fast flow regime that involves highly oriented and stretched polymer chains. Here, through new experimental and theoretical developments, we establish a unified understanding of the steady-state shear viscosity, η, of entangled polymer melts at high Rouse Weissenberg numbers, WiR > 1. New capillary rheometry measurements in the absence of flow instabilities reveal a dramatic change in shear-thinning scaling from η ∼ γ̇–0.7 ± 0.1 at WiR N/γ̇)0.50 at WiR > 1, where N is the degree of polymerization and γ̇ is the shear rate. Moreover, the viscosity scaling exponent with polymer molecular weight decreases with applied shear stress, and a remarkable unentangled melt scaling η ∼ N emerges under ultrahigh constant stress conditions σ/Ge ≥ 2, where Ge is the equilibrium entanglement elastic modulus. These new observations are not consistent with existing molecular theories. We construct a dynamic scaling model based on tension blob concepts as extended to entangled polymers, resulting in a (near) universal expression for the shear-thinning behavior controlled by purely dissipative considerations associated with orientational stress. This physical picture is in sharp contrast to the predictions of various state-of-the-art tube-based models based on the widely adopted factorization approximation of the total stress into stretching and orientational contributions, and also qualitatively differs from predictions of non-tube-based slip-link models based on a transient network perspective

Synthesis and Characterization of Comb and Centipede Multigraft Copolymers P<i>n</i>BA‑<i>g</i>‑PS with High Molecular Weight Using Miniemulsion Polymerization

Comb and centipede multigraft copolymers, poly­(<i>n</i>-butyl acrylate)-<i>g</i>-polystyrene (P<i>n</i>BA-<i>g</i>-PS) with P<i>n</i>BA backbones and PS side chains, were synthesized via high-vacuum anionic polymerization and miniemulsion polymerization. Single-tailed and double-tailed PS macromonomers were synthesized by anionic polymerization and Steglich esterification. Subsequently, the copolymerization of each macromonomer and <i>n</i>BA was carried out in miniemulsion, and multigraft copolymers were obtained. The latex particles of multigraft copolymers were characterized using dynamic light scattering. The molecular weights of macromonomers and multigraft copolymers were analyzed by size exclusion chromatography. Moreover, the molecular weights and structures of macromonomers were investigated by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and ¹H nuclear magnetic resonance spectroscopy. The weight contents of PS in comb and centipede multigraft copolymers were calculated by ¹H nuclear magnetic resonance spectroscopy. The thermal properties of multigraft copolymers were characterized by thermogravimetric analysis and differential scanning calorimetry. The microphase separation of multigraft copolymers was observed by atomic force microscopy and transmission electronic microscopy. Rheological measurements showed that comb and centipede multigraft copolymers have elastic properties when the weight content of PS side chains is 26–32 wt %. Centipede multigraft copolymers possess better elastic properties than comb multigraft copolymers with the similar weight content of PS. These findings are similar to previous results on poly­(isoprene-<i>g</i>-polystyrene) comb and centipede copolymers made by anionic polymerization

Synthesis and Characterization of Graft Copolymers Poly(isoprene‑<i>g</i>‑styrene) of High Molecular Weight by a Combination of Anionic Polymerization and Emulsion Polymerization

In this work, high molecular weight “comb-shaped” graft copolymers, poly­(isoprene-<i>g</i>-styrene), with polyisoprene as the backbone and polystyrene as side chains, were synthesized via free radical emulsion polymerization by copolymerization of isoprene with a polystyrene macromonomer synthesized using anionic polymerization. A small amount of toluene was used in order to successfully disperse the macromonomer. Both a redox and thermal initiation system were used in the emulsion polymerization, and the latex particle size and distribution were investigated by dynamic light scattering. The structural characteristics of the macromonomer and comb graft copolymers were investigated through use of size exclusion chromatography, spectroscopy, microscopy, thermal analysis, and rheology. While the macromonomer was successfully copolymerized to obtain the desired multigraft copolymers, small amounts of unreacted macromonomer remained in the products, reflecting its reduced reactivity due to steric effects. Nevertheless, the multigraft copolymers obtained were very high in molecular weight (5–12 × 10⁵ g/mol) and up to 10 branches per chain, on average, could be incorporated. A material incorporating 29 wt % polystyrene exhibits a disordered microphase separated morphology and elastomeric properties. These materials show promise as new, highly tunable, and potentially low cost thermoplastic elastomers