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    Multiscale Modeling and Characterization of Radical-Initiated Modification of Molten Polyolefins

    Zou, WeizhongTupper, AmberRebello, Nathan J.Ranasinghe, Duminda S....
    15页
    查看更多>>摘要：Multiscale approaches for peroxide-initiated grafting of vinyl alkoxy silane (VAOS) monomers to polyolefins have been investigated to develop a mechanistic model for the synthesis of functional copolymers by melt-phase processing. By investigating the reaction kinetics through both molecular modeling and model compound studies, a comprehensive mechanistic view of the complex radical-mediated reactions is developed. This mechanism indicates that grafting of single monomers via intramolecular H transfer is much more probable than single grafts in close proximity, oligomeric grafts, or chain branching under typical melt processing conditions. These results clearly show that the overall mechanism is dominated by repeated grafting of single monomers of vinyl silane to a hydrocarbon substrate as a consequence of very rapid intramolecular hydrogen transfer, and the resulting radicals are terminated primarily via combination to form polymer cross-links. With the kinetic rates evaluated from a hybrid quantum calculation procedure as well as the analyses of the NMR spectrum and GC/MS data of model compounds, a fundamental kinetic model is established to depict the general chemistry involving all the critical reactions in modification of the molten polymer and their relationships to processing conditions. Predictions of the model are confirmed by Fourier transformation infrared spectroscopy (FTIR), linear theology, and high-temperature gel permeation chromatography (HT-GPC) analysis of 30 polyolefin elastomer (POE) formulations. This allows quantitative estimation of both the grafting yield and the cross-linked polymer fraction during the synthesis of polyolefin graft copolymers in the molten state.
      原文链接:
    • NSTL
    • Amer Chemical Soc

    Subcontinuum Interpretation of Mechanical Behavior for Cross- Linked Epoxy Networks

    Kim, HongdeokChoi, Joonmyung
    10页
    查看更多>>摘要：The origin of the mechanical properties of highly cross-linked epoxy networks was theoretically investigated from a subcontinuum perspective. By use of all-atom molecular dynamics (MD) simulations, the macromolecular network of epoxy formed during the cross-linking reactions was classified into subgroups according to their bonding relationship. The deformation energy density applied to the entire system under mechanical loading is expressed by the contribution of each subgroup. The load transfer capabilities according to the chemical bonding state between resin and hardener were then quantified at the atomic level. On the basis of the results, an analytic blending model was established that can predict mechanical properties of the cross-linked epoxy according to its chemical composition and associated network topology. It was confirmed that the proposed model successfully predicts the mechanical properties of materials for the range of composition ratios that can be considered in actual synthesis as well as an indepth analysis of the individual molecular components.
      原文链接:
    • NSTL
    • Amer Chemical Soc

    Sequence Effects on the Glass Transition of a Model Copolymer System

    Drayer, William F.Simmons, David S.
    12页
    查看更多>>摘要：Achieving rational control of the glass transition temperature T-g and associated dynamics is a major fundamental and practical challenge in polymer science, with applications ranging from ion conductivity of polymer electrolytes to thermal stability and processability of engineering plastics. Here we employ molecular dynamics simulations to elucidate a new strategy for rational control of glass transition temperatures by varying copolymer sequences-an approach viable due to advances enabling synthesis of copolymers with increasingly controlled sequences. Our results point to two regimes of sequence control on T-g. For sequences sufficiently blocky to allow microphase separation, T-g alterations emerge from interfacial effects on dynamics, an extensively studied phenomenon, with sequence effects on domain size playing a central role in determining T-g. For sequences that approach an alternating copolymer, we identify a second regime in which T-g is directly tuned by segmental packing. In this regime, T-g is exquisitely sensitive to sequence- far more so than in the interface-dominated regime -so that large fractional changes in T-g can be realized over a range spanning from an alternating sequence to an alternating trimer. This regime represents a largely untapped opportunity for tuning dynamics and improving transport properties in copolymers. Finally, results suggest that sequence-specified copolymers may provide a unique platform in which to probe the physics of the glass transition by leveraging their dynamical variability at fixed composition; results from this study, for example, find that classical free volume theories of glass formation do not account for the sequence dependence of dynamics in these copolymers.
      原文链接:
    • NSTL
    • Amer Chemical Soc

    PET-RAFT Enables Efficient and Automated Multiblock Star Synthesis

    Foster, HenryStenzel, Martina H.Chapman, Robert
    8页
    查看更多>>摘要：We show that photoinitiated electron/energy transfer-reversible addition-fragmentation chain transfer (PET-RAFT) enables vastly superior control over the polymerization of multiblock star copolymers compared to conventional techniques. Monomodal distributions with dispersities <1.3 could be achieved after the 10th block despite pushing the polymerization to >95% conversion in each block extension. The improvement in control is likely due to the reabsorption of the free radical at the propagating chain end by the excited catalyst, which would otherwise lead to a termination product. Simple modeling shows the dramatic effect that this has in the case of star polymerizations. Because PET-RAFT is also tolerant to oxygen, we were able to automate the synthesis of up to heptablock stars at short block lengths, providing a useful technique for screening the effect of polymer composition on the solution structure.
      原文链接:
    • NSTL
    • Amer Chemical Soc

    Liquid Crystal Ordering in DNA Double Helices with Backbone Discontinuities

    Fontana, FrancescoBellini, TommasoTodisco, Marco
    8页
    查看更多>>摘要：ABSTRACT: Short DNA double helices in highly concentrated solutions self-assemble into long noncovalent polymers and globally order as liquid crystals. What level of helical defect is compatible with this collective ordering is still unclear. In this work, we show that a series of 20 bp-long DNA double helices with different structural defects such as nicks and gaps still retain their capability to transition into liquid crystalline solutions despite the increased flexibility of their structure. These results expand our understanding of the space of hybridization motifs leading to linear physical polymerization and liquid crystal ordering. Moreover, by studying liquid-crystalline solutions made of 20 bp-long duplexes with a central dinucleotidic gap, we find that regardless of the highly crowded state of the system, guanosine monophosphate molecules can diffuse in-between the DNA polymers and effectively fill the gap. This finding enables formulation of a prebiotic scenario where fragmented DNA double helices, held together by self-interactions and supramolecular ordering, form a matrix rich in substrates for ligation, gap-filling, and primer extension reactions.
      原文链接:
    • NSTL
    • Amer Chemical Soc

    Revisiting Conjugated Polymers with Long-Branched Alkyl Chains: High Molecular Weight, Excellent Mechanical Properties, and Low Voltage Losses

    Liu, ChunhuiXiao, ChengyiWang, JingLiu, Baiqiao...
    11页
    查看更多>>摘要：High-molecular-weight conjugated polymers exhibit excellent mechanical properties due to long-chain entanglement, but it is challenging to increase molecular weight since conjugated polymers with rigid backbones show low solubility and tend to be precipitated during polymerization. In this work, we have successfully obtained high-molecular-weight polymers through the introduction of long-branched alkyl chains and optimization of polymerization conditions. Three conjugated polymers with long 2-octyldodecyl side units were obtained with number-average molecular weights (M(n)s) of 38.8 kDa (OD-PM6_L), 81.2 kDa (OD-PM6_M), and 178.3 kDa (OD-PM6_H), while the conjugated polymer PM6 with short 2-ethylhexyl units showed an M-n of 43.8 kDa. The branched side units and molecular weight have a significant influence on the mechanical properties, in which the elastic ranges and the crack-onset strains were enhanced to >20% and >170% in OD-PM6_H thin films. In addition, due to the suppressed non-nonradiative recombination via increasing the donor-acceptor spacings, organic photovoltaic devices based on OD-PM6_H showed an efficiency of over 13% with low voltage loss. These results demonstrate that introducing longer side units into conjugated polymers provides the opportunity to realize high molecular weight, resulting in excellent mechanical properties and high efficiencies in organic solar cells toward flexible applications.
      原文链接:
    • NSTL
    • Amer Chemical Soc

    n-Type Conjugated Polymers Based on an Indandione-Terminated Quinoidal Building Block

    Du, TianLiu, YingyingWang, ChengDeng, Yunfeng...
    10页
    查看更多>>摘要：Conjugated polymers (CPs) with deep LUMO energy levels (ELUMOs) are essential for developing n-type organic electronics. However, few approaches can markedly lower the ELUMOs of CPs. Herein, this issue is addressed by the introduction of a quinoidal building block into the polymer chain. Two CPs, PIQ-A and PIQ-O, are synthesized based on an indandione-terminated quinoidal unit, with alkyl or oligo(ethylene glycol) side chains on the comonomer, respectively. The CPs show ultralow ELUMOs of ca. -4.30 eV, which is lower by 0.75 eV compared to the analogous polymer with aromatic conjugated structure. The deep LUMO energy levels endow the CPs with unipolar n-type transport behavior in organic thin-film transistors and efficient n-type doping in organic thermoelectrics, with doped PIQ-O showing an optimized electrical conductivity of 0.43 S cm(-1) and a power factor of 4.24 mu W m(-1) K-2. Importantly, the electrical conductivity of doped polymer thin films (similar to 50 nm) barely changes after a 7 day exposure to air, and the doped PIQ-O film shows an electrical conductivity of 0.39 S cm(-1) even after a 15 day exposure to air. This work provides a new approach to develop CPs with ultralow LUMO energy levels, toward high-performance and potentially air -stable n-type organic electronics.
      原文链接:
    • NSTL
    • Amer Chemical Soc

    Swelling and Residual Bond Orientations of Polymer Model Gels: The Entanglement-Free Limit

    Lang, MichaelScholz, ReinhardLoeser, LucasBunk, Carolin...
    18页
    查看更多>>摘要：We investigate the swelling of polymer model networks prepared at different polymer volume fractions and in solvents of different quality. We extend the existing theory to describe residual bond orientations (the vector and the tensor order parameters) for theta, good, and athermal solvents and put these relations in context with modulus at preparation conditions and the equilibrium degree of swelling. We find good agreement with the assumption of affine swelling for the weakly entangled networks of our study. The same scaling relations (up to numerical coefficients) are obtained for the vector order parameter, m, and the tensor order parameter, S, as a function of the preparation conditions, network structure, the equilibrium degree of swelling, Q, and the modulus at swelling equilibrium, G. We obtain m & PROP; Q(-2) and G & PROP; m(3/2) for swelling in theta solvents and m & PROP; Q(-1.08) with G & PROP; m(2.14) in the good-solvent regime, in both cases independent of preparation conditions. Modulus and residual bond orientation are related by G & PROP; phi(m)(0) and G & PROP; phi(1.23)(0)m as a function of the preparation polymer volume fraction phi 0 for theta solvents and good solvents, respectively. Computer simulations and experimental data for the good-solvent regime show good agreement with the predictions.
      原文链接:
    • NSTL
    • Amer Chemical Soc

    In Situ Dissolution and Swelling of Confined Lamellar Polymer Crystals through Exposure to Humid Air

    Pfohl, ThomasHeck, BarbaraAlshetwi, YaserKhechine, Emna...
    8页
    查看更多>>摘要：Using a hydrophobic-hydrophilic poly(styrene)-b-poly(ethylene oxide) (PS-b-PEO) diblock copolymer, we formed sandwich-like micron-sized thin platelets via crystallization in solution. These platelets consisted of a crystalline PEO lamella confined between two glassy PS layers and were deposited on a solid substrate. Using in situ optical microscopy, we followed the temporal and spatial changes in the thickness and morphology of these sandwich-like platelets induced by exposure to humid air. As water is a good solvent for PEO but a nonsolvent for PS, we observed first the dissolution of the confined crystalline PEO layers characterized by a sharp dissolution front propagating at an almost constant velocity as expected for case II diffusion into a solid. The resulting hydrated PEO brushes absorbed water further until equilibration of the respective chemical potentials, accompanied by a change from a planar to a dome-shaped morphology that could be fully reversed by exposure to dry air. Repeated swelling-deswelling cycles demonstrated stability and reproducibility of these developments taking place at distinctly different transport rates. Here, we discuss the underlying processes of water permeation into and efflux from the PEO layers.
      原文链接:
    • NSTL
    • Amer Chemical Soc

    Study of the Mullins Effect in Carbon Black-Filled Styrene-Butadiene Rubber by Atomic Force Microscopy Nanomechanics

    Liang, XiaobinNakajima, Ken
    8页
    查看更多>>摘要：For a long time, it has been difficult to reach a consensus on the physical origin of the "Mullins effect" because of the limited capabilities for visualizing microscopic structures. This work investigates the microscopic mechanism of the Mullins effect in carbon black (CB)-filled styrene-butadiene rubber (SBR) after application of cyclic uniaxial tension. We used atomic force microscopy (AFM) nanomechanics to characterize the nanoscale mechanical properties of CB/SBR vulcanizate after the application of different tensile loadings. The stress-softening behavior of CB/SBR was directly observed at the nanoscale for the first time. The rubber matrix and interfacial region exhibit different softening behaviors at different elongations, which provides insight into the microscopic mechanism of the Mullins effect. Moreover, we found that recovery from the Mullins effect starts with the recovery of the interfacial region, while the rubber matrix takes longer to recover its strength. This approach could provide new research perspectives for the mechanisms that induce the Mullins effect.
      原文链接:
    • NSTL
    • Amer Chemical Soc