2,937 research outputs found
Tuning Interparticle Hydrogen Bonding in Shear-Jamming Suspensions: Kinetic Effects and Consequences for Tribology and Rheology
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
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
Dynamics and viscoelastic properties of Hydrogen-bonding telechelic associating polymers
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
Recommended from our members
Rheological Investigations of Self-Assembled Block Copolymer Nanocomposites with Complex Architectures
The self-assembly of block copolymers (BCP) into microphase separated structures is an attractive route to template and assemble functional nanoparticles (NP) into highly ordered nanocomposites and is central to the “bottom up” fabrication of future materials with tunable electronic, optical, magnetic, and mechanical properties. The optimization of the co-assembly requires an understanding of the fundamentals of phase behavior, intermolecular interactions and dynamics of the polymeric structure. Rheology is a novel characterization tool to investigate these processes in such systems that are not accessible by other means. With the combination of X-ray scattering techniques, structure-property relationships are determined as a function of NP loading in self-assembled hybrid composites.
This thesis examines two classes of BCP templates used for nanocomposite assembly. First, low molecular weight, disordered (low χN) BCP surfactants are considered. The addition of NPs with enthalpically favored interactions between the NP and one of the BCP domains boosts the phase segregation strength and drives self-assembly, resulting in highly filled nanocomposites (φNP ~ 30 vol.%) with small domain spacing (d0 ~ 10 nm) due to the low N. The effect of NPs on the self-assembly dynamics, material properties, and temperature dependent phase transitions are considered in the high loading regime. Oscillatory shear rheology reveals a transition from liquid-like to solid-like behavior with increasing NP content. The addition of stiff NPs to a soft polymer matrix, along with favorable intermolecular interactions, produces a x103 increase in the magnitude of G*. Phase transitions are investigated by correlating time-resolved rheology and time-resolved SAXS. Structure development and viscoelasticity scale with the NP content, and a general master curve describing behavior across all NP loadings is constructed. The access to new material properties and transitional phenomenon provides further insight into the complex structure-property relationships of this class of nanocomposites.
The second BCP template is microphase separated bottlebrush block copolymers (BBCP), macromolecules with discrete blocks of densely grafted side chains tethered to a molecular backbone. Highly extended backbone conformations and significant repulsion between grafted side chains are believed to suppress chain entanglements, resulting in rapid self-assembly (order of minutes) into large nanostructures (d0 \u3e 100 nm) advantageous for optically active materials. A systematic study of model poly(styrene)-block-poly(ethylene oxide) (PS-b-PEO) BBCPs with short side chains below entanglement molecular weight is conducted. We measure dynamic moduli G’(ω) and G”(ω) over a wide range of timescales. The scaling relationships in dynamic data show distinct power law behavior analogous to critical gels. The relaxation mechanisms are a consequence of the reduced entanglements and mobile microstructure. This interplay of high molecular mobility and rapid self-assembly contrasts the viscoelasticity of linear BCP materials with comparable microstructure.
The role that applied shear plays on the directed alignment of microphase separated lamellae in bulk BBCP samples is considered. The periodic structures are found to align at exceptionally low strain amplitudes and mild processing temperatures as confirmed by SAXS. Alignment over several mm3 is realized by high throughput synchrotron experiments and we hypothesize that this method can be applied as a means of fabricating and processing BCP-based hybrid materials with exceptional long-range order.
Building on the understanding from the highly loaded NP/BCP composites, similar considerations are taken towards the investigation of phase behavior, morphology, and rheological response in NP/BBCP hybrids. The goal is to understand how NPs and intermolecular interactions impacts the unique relaxation processes inherent to BBCP melts. From oscillatory shear rheology measurements, systematic transitions in the long-time relaxations towards solid-like behavior is observed with increasing NP loading, suggesting the NP inhibits the highly mobile microstructure and rapid side chain relaxations. The structure-property relationships realized by both rheology and SAXS lay the groundwork as we explore future manipulation and processing of these diverse structures for both well-established and emergent applications
Enhancing the Macroscopic Properties of Parts Printed via Fused Filament Fabrication by Incorporating Nanoscopic Additives
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
Recommended from our members
Amorphous-Crystalline Brush Block Copolymers: Phase Behavior, Rheology and Composite Design
Bottlebrush block copolymers are polymers with chemically distinct polymer side chains grafted onto a common backbone. The unique architecture induced properties make these materials attractive for applications such as photonic materials, stimuli responsive actuators and drug delivery vehicles to name a few. This dissertation primarily investigates the phase transitions and rheological behavior of amorphous-crystalline bottlebrush brush block copolymers and their composites. The temperature induced phase behavior is investigated using time resolved synchrotron X-ray source. Irrespective of volume fraction and backbone length, the samples display strong segregation even at high temperatures (200 °C) and there is no accessible order-disorder transition in the temperature range of 25-200 °C. The isothermal crystallization kinetics reveal a two-stage crystallization process, different than that observed in linear block copolymers. A considerable part of this dissertation is focused on the study of the effect of hydrogen bonding additives on the relaxations of brush block copolymer. The additives greatly enhance the mechanical and physical properties of the pristine polymer. The small molecules also impart a gel-like characteristic due to the formation of physically crosslinked network. Finally, the understanding of the brush block copolymer systems from the previous studies and literature is used to study the self-assembly of nanorods using brush block copolymers as templates
Benzene Tetraamide:A Covalent Supramolecular Dual Motif in Dynamic Covalent Polymer Networks
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
New polymers based on the quadruple hydrogen bonding motif : from molecules towards materials
- …