2,937 research outputs found

    Tuning Interparticle Hydrogen Bonding in Shear-Jamming Suspensions: Kinetic Effects and Consequences for Tribology and Rheology

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    The shear-jamming of dense suspensions can be strongly affected by molecular-scale interactions between particles, e.g. by chemically controlling their propensity for hydrogen bonding. However, hydrogen bonding not only enhances interparticle friction, a critical parameter for shear jamming, but also introduces (reversible) adhesion, whose interplay with friction in shear-jamming systems has so far remained unclear. Here, we present atomic force microscopy studies to assess interparticle adhesion, its relationship to friction, and how these attributes are influenced by urea, a molecule that interferes with hydrogen bonding. We characterize the kinetics of this process with nuclear magnetic resonance, relating it to the time dependence of the macroscopic flow behavior with rheological measurements. We find that time-dependent urea sorption reduces friction and adhesion, causing a shift in the shear-jamming onset. These results extend our mechanistic understanding of chemical effects on the nature of shear jamming, promising new avenues for fundamental studies and applications alike

    A Critical Approach to Polymer Dynamics in Supramolecular Polymers

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    Over the past few years, the concurrent (1) development of polymer synthesis and (2) introduction of new mathematical models for polymer dynamics have evolved the classical framework for polymer dynamics once established by Doi-Edwards/de Gennes. Although the analysis of supramolecular polymer dynamics based on linear rheology has improved a lot recently, there are a large number of insecurities behind the conclusions, which originate from the complexity of these novel systems. The interdependent effect of supramolecular entities (stickers) and chain dynamics can be overwhelming depending on the type and location of stickers as well as the architecture and chemistry of polymers. This Perspective illustrates these parameters and strives to determine what is still missing and has to be improved in the future works

    Rheology of Supramolecular Polymers

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    Dynamics and viscoelastic properties of Hydrogen-bonding telechelic associating polymers

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    Supramolecular polymers (also termed as associating polymers), which are connected by non-covalent interactions between polymer chains, have become an increasingly important class of polymers and gained tremendous interest in the last few decades. The co-existence of reversible secondary interactions and covalent bonding makes supramolecular polymers promising candidates for functional materials. Immense effort has been put on the development of chemical structure design, while the understanding of their physical properties is rather limited, especially in the melt state. In this dissertation, we studied the dynamics and viscoelastic properties of H-bonded telechelic associating polymers by tuning the association strength, main chain length, flexibility and polarity. A systematical analysis was conducted by employing a combination of experimental techniques: dielectric spectroscopy, differential scanning calorimetry, rheology and small angle X-ray spectroscopy. We demonstrated that hydrogen-bonding has a strong influence on both segmental and slower dynamics in the polydimethylsiloxane (PDMS) and poly(propylene glycol) (PPG) systems with low molecular weights. The supramolecular association of hydroxyl-terminated PDMS chains leads to the emergence in dielectric and mechanical relaxation spectra of the so-called Debye process traditionally observed in monohydroxy alcohols. Then we investigated telechelic associating PMDS with different hydrogen bonding end groups, e.g. NH2, NHCO-COOH (amide-acid groups). Remarkably, a single species of end group forms two qualitatively different types of associates in PDMS-NHCO-COOH: transient bonds which allow stress release by a bond-partner exchange mechanism, and effectively permanent bonds formed by a phase segregated fraction of end groups which are stable on the timescale of the transient mechanism. In the following work, we studied telechelic PDMS and PPG with three types of H-bonding end-groups possessing different interaction strengths and a non-H-bonding end-group as reference were compared. Unraveling the mechanisms of many molecular processes and structure-dynamics-property relationship in supramolecular polymers is of great importance for both fundamental studies and industrial applications. Findings in this work suggested that the backbone length, flexibility and polarity, the strength and lifetime of the associating groups, and the ratio of characteristic time scales between backbone and chain ends should be considered in the design of associating polymers to achieve the desired properties

    Supramolecular polymers

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    Enhancing the Macroscopic Properties of Parts Printed via Fused Filament Fabrication by Incorporating Nanoscopic Additives

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    Additive Manufacturing (AM), or 3D printing, provides an alternative route to generate end-stage products by coupling advanced manufacturing techniques with computer modeling. However, parts fabricated by AM are known to have inferior mechanical properties compared to parts prepared by traditional methods, such as injection molding. This principal drawback is attributed to the presence of voids and inefficient adhesion between adjacent filaments, or beads, due to limited diffusion of polymer chains across interbead interfaces. Together these shortcomings also lead to anisotropic mechanical properties in printed parts. While optimizing print conditions or applying post-printing procedures decreases the anisotropy, these methods are incapable of obtaining significant enhancements in material properties. To address this, my dissertation work examines how incorporating nanoscopic additives, including bare (unfunctionalized) nanoparticles, poly(methyl methacrylate)-grafted-nanoparticles (PMMA-g-NPs), and macromolecules containing self-complementary, multiple hydrogen bonding motifs that trigger supramolecular assembly, into PMMA filaments affects structure formation at the nanoscale and impacts the resultant macroscopic properties of PMMA parts manufactured by Fused Filament Fabrication (FFF). Results indicate that incorporating bare nanoparticles, which arrange as well-dispersed mass fractals throughout the matrix, into PMMA filaments leads to a slight increase the thermomechanical properties. Adding PMMA-g-NPs significantly improves material properties relative to samples printed with bare nanoparticles. These enhancements are attributed to increased interactions across grafted nanoparticle/matrix interfaces because there is a direct correlation between loading level and changes in thermomechanical properties. In addition to using inorganic additives, my research efforts demonstrate that copolymeric additives capable of forming thermoreversible physical crosslinks are advantageous. They increase part performance at use temperatures, but the dissociation of physical crosslinks at high temperatures (used for polymer melt processing) alleviates any deleterious effect on the viscosity, rendering them highly processable. These results demonstrate that molecular engineering can be used to effectively manage interactions on the nanoscale, leading to substantial increases in the performance of FFF-printed parts. These studies, which highlight the importance and potential of non-bonded interactions, provide a compelling and useful pathway for addressing challenges associated with the inferior performance of 3D printed polymeric materials

    Benzene Tetraamide:A Covalent Supramolecular Dual Motif in Dynamic Covalent Polymer Networks

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    In dynamic polyamide networks, 1,2,4,5-benzene tetraamide (B4A) units act simultaneously as a dynamic covalent cross-linker and as supramolecular stacking motif. This results in materials with a rubbery plateau modulus that is about 20 times higher than that of a corresponding reference network in which the supramolecular interaction is suppressed. In branched polyamides with the same B4A dynamic motif, hydrogen bonding and stacking lead to strong and reversible supramolecular networks, whereas a branched polyamide with the nonstacking reference linker is a viscous liquid under the same conditions. Wide-angle X-ray scattering and variable-temperature infrared experiments confirm that covalent cross-linking and stacking cooperatively contribute to the dynamics of the network. Stress relaxation in the reference network is dominated by a single mode related to the dynamic covalent chemistry, whereas relaxation in the B4A network has additional modes assigned to the stacking dynamics.</p
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