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The synthesis of a supramolecular polymer network based on ureidopyrimidinone (UPy) is demonstrated starting from a low-molecular weight polybutadiene (PB) polymer, which was modified with UPy quadruple hydrogen-bonding motifs through UV-initiated thiol−ene chemistry under ambient conditions. Here, the UPy units contain a branched alkyl side chain rather than a methyl group, as is the case in most literature examples. The sterically demanding aliphatic 2-ethylpentyl side chain was introduced to prevent stacking of the UPy dimers and to enhance the compatibility of the UPy unit with the apolar PB polymer matrix. The UPy units were grafted onto PB with different functionalization degrees yielding materials with a relatively broad range of glass transition temperatures (Tg) and material properties, as evaluated by thermogravimetric analysis, differential scanning calorimetry (DSC), and dynamic mechanical thermoanalysis. Importantly, only the system with 13 mol % UPy functionalization showed some macroscopic phase separation as indicated by its partial opaque appearance, even though this was not detected by DSC. Previous reports on PB end functionalized with methyl-functional UPy revealed phase separation through DSC, indicating that the branched side chains indeed suppress the phase separation. The network dynamics were assessed by rheological measurements at different temperatures, which were subsequently fitted to the sticky Rouse model and creep experiments. We found that the sticky Rouse model satisfactorily fits the linear response of the systems in the terminal regime. However, significant discrepancy from theory still persists in the rubbery plateau regime. This deviation is attributed to the presence of structural defects in the systems. Altogether, this work demonstrates the importance of designing the supramolecular physical cross-linker unit with a branched aliphatic side chain to enhance the compatibility with the polymer matrix for the formation of supramo

Thermoplastic elastomers (TPEs) are an extremely versatile class of engineering plastics that are suitable for many applications thanks to their high extensibility, reversible elasticity, and tunable hardness. They are widely used as fibers, cable insulators, medical devices, shoe soles, and many other applications. Additionally, like many other thermoplastic polymers, they are recyclable and can be easily processed. However, regardless of their exact chemical structure, many TPEs show a decline in their mechanical properties, such as toughness and ductility, at high temperatures. This behavior is in sharp contrast with standard thermoplastics and limits the range of applications where TPEs can be used as a replacement for thermoset rubbers. Understanding the different dynamics behind this behavior and finding out the key parameters that influence the high-temperature mechanical properties is critical to designing new TPEs and targeting new applications where traditional rubbers can be replaced. In this thesis, we explore the high-temperature and high-deformation mechanical properties of a model series of industrially relevant soft TPEs. Through a series of mechanical and morphological tests, we investigate the fundamental relationships between microstructure and mechanical properties. The ultimate aim is to draw a unifying picture that can describe which dynamics influence the deformation behavior and how they interplay with each other., (FSA - Sciences de l'ingénieur) -- UCL, 2022

Semi-crystalline polymers exhibit performances that are highly dependent on their micro-structure as induced by the thermo-mechanical processes they are subjected to. In this thesis we developed a multiscale modeling and simulation framework able to predict the thermo-mechanical response of semi-crystalline polymers including crystallization and porosity evolution, with an application to additive manufacturing by selective laser sintering. Firstly, an enhanced phase field model was developed for the numerical simulation of crystallization in semi-crystalline polymers. The model is based on coupling the heat equation with the Allen-Cahn equation, which is derived from the Gibbs-Thomson solid-liquid interface equation. Starting from the nucleation of spherulites, existing phase field models can simulate their evolution in a surrounding liquid and separate the amorphous and crystalline phases. However, the predictions of the morphological characteristics of the spherulites remain qualitative only. Moreover, the predicted spherulite evolution as a function of crystallization temperature is not consistent with experimental results. In our work, existing phase field models were enhanced in order to obtain experimentally consistent results. We used spherulite growth, crystal morphology, and crystallinity degree in spherulite, as measures to determine the model accuracy. The model is numerically implemented using the finite difference method so that 2D and 3D simulation results are presented and compared to experimental data, illustrating the quantitative adequacy of the predictions with experimental evidence. Secondly, full-field micromechanical simulations were conducted on the micro-structures generated by the enhanced phase field model in order to predict the effective mechanical properties. Care is taken to obtain Representative Volume Elements (RVEs) by computing the number of subcells in each spherulite and the number of spherulite nucleations in each RVE. An FFT sol, (FSA - Sciences de l'ingénieur) -- UCL, 2022

Dual-cross-linked networks (DCNs), interpenetrating polymer networks (IPNs), and IPN-derived double networks (DNs) are increasingly utilized to fabricate hydrogels with unique mechanical properties. However, the relationship between the topology of these networks and the resulting dynamics is rarely compared and little understood. To tackle this shortcoming, this work presents a systematic investigation of the viscoelastic properties of DCN, IPN, and DN hydrogels as well as their corresponding single networks by oscillatory shear rheology using both frequency and strain sweeps. All the hydrogels are based on the same orthogonal combination of a supramolecular interaction: zinc(II)-terpyridine bis-complexes, and of a reversible covalent bond: oxime, as cross-links. To understand the contribution of each sub-network to the properties of the DCN, IPN, and DN hydrogels, the corresponding single networks, i.e., cross-linked by only one type of bond, are first studied in detail. All double dynamics hydrogels have a plateau modulus much higher than the sum of the plateau modulus of the single networks, evidencing a synergetic effect between the sub-networks. However, the origin of this modulus increase varies according to the network topology. We also show that the relaxation behaviors of the DCN, IPN, and DN hydrogels are influenced by the dynamics of the corresponding single dynamic networks. Finally, the strain sweeps reveal that, for all network topologies, the amplitude of deformation at which the linear viscoelastic region of the double dynamics networks stops is governed by the oxime network, while the metallo-supramolecular network governs the amplitude of deformation up to which the sample can resist before starting to break.

We investigate the viscoelastic properties of double dynamic networks (DDNs) based on side-functionalized PnBA chains. One of these networks is highly crosslinked by metal-ligand junctions characterized by a fast association/dissociation dynamics, while the other network is sparsely crosslinked with slow dynamic covalent networks (DCNs). We first show that modulating the dynamics of the metallosupramolecular networks, by playing with the temperature, the density of reversible junctions, or the stress applied, has direct consequences on the local equilibration of the DCN. The latter takes place by a constraint release Rouse process at the rhythm of the association/dissociation of the metal-ligand junctions. Then, based on creep-recovery experiments, we investigate the ability of the DDNs to recover their initial shape after a creep test and show again the important role played by the metallosupramolecular network. In particular, the sample recovery strongly depends on the network connectivity, which is enhanced if a denser metallosupramolecular network is used as it reduces the possible creep of the double dynamic network and increases its elastic memory. The sample recovery also depends on the association-dissociation dynamics of the metallosupramolecular bonds as it fixes how fast the stretched DCN can come back to its equilibrium conformation and can recover its initial shape after a large deformation has been applied. Adjusting the dynamics of the weak network is thus a key process to govern the viscoelastic response of the slow network.

We study the linear viscoelastic properties of polymeric networks formed by poly(n-butyl acrylate) telechelic stars end-capped with 2,2:6,2″-terpyridine (Star-PnBA-tpy4) and two types of metal-ligand cross-links with different lifetimes. The influence of interactions, mediated by temperature, nature of metal ions, and ion content, on the linear viscoelastic behavior of both single and double dynamics transient networks is systematically investigated by small amplitude oscillatory shear and creep rheometry. The experimental results reveal that the dynamics of networks with two different metal-ligand cross-links is much faster than expected, characterized by the average sticker lifetime rather than a discrete contribution of each metal-ligand complex. We model the dynamics with the help of our modified tube-based time marching algorithm by accounting for both association/dissociation dynamics of metal-ligand coordination and the entanglement dynamics. Two parameters are defined in the model, namely, the proportion of dangling ends and the average time during which a sticker is free. This allows us to quantify the transient dynamics of the network and, in particular, to determine how the sticker dynamics depend on temperature and ion content.

We present a systematic experimental study of the shear rheology of metallosupramolecular assemblies based on entangled telechelic star polymers comprising one (single dynamic network) or two (double dynamics network) types of physical bonds with the aim to unravel the role of concentration and strength of these bonds on the nonlinear response. Model dynamic networks functionalized with terpyridine ligands were formed by adding different metal ions with increasing bonding strength, zinc, copper, and cobalt. The dynamics are driven by entanglement/disentanglement processes and a ligand exchange mechanism. Steady-state viscosities of single and double dynamics networks collapse onto a universal curve over a wide range of Weissenberg numbers based on terminal time (up to about 300 for single and 1000 for double), exhibiting stronger shear thinning (with an exponent of −0.76) compared to entangled neutral star polymers. Double dynamics networks consisting of two different metal ions (with different lifetimes) exhibit stronger mechanical coherence (rate-dependent fractional viscosity overshoot) and accumulate larger strain at steady-state flow compared to single-ion counterparts. The shear stress growth function signals exhibit weak, albeit unambiguous shear strain hardening, which becomes more pronounced for stronger associations. They also exhibit double overshoot, which reflects the interplay of association strength and chain deformation. Increasing the strength of associations leads to the failure of the Cox–Merz rule, which is more severe for single dynamic networks. The markedly different behavior of double dynamics networks is attributed to the fact that at sufficiently high ion content, the weaker bond acts as a sacrificial component, which provides local energy dissipation and enhances the overall deformability. This bears analogies with their linear viscoelastic response, which has revealed that the arm disentanglement (delayed due to the reversible bonds) effe

This work focuses on the temperature-dependent structural and rheological characterization of polystyrene-b-poly(n-butyl acrylate)-b-polystyrene triblock copolymers (PS-b-PnBA-b-PS) in the melt and, in particular, on their ability to show a lower disorder-to-order temperature (LDOT). To this aim, copolymers of varying block lengths, but keeping the PnBA block as a major component, were synthesized. Small-angle x-ray scattering revealed that the copolymers with short PS blocks (∼10 kg/mol) approach an LDOT but do not cross it. At room temperature, these copolymers exhibit higher moduli compared to a PnBA homopolymer due to the reinforcing effect of the PS but are flowing at temperatures above the glass transition of the PS. Increasing the PS and PnBA block length, to keep the same PS fraction, induces more profound changes in the structural and viscoelastic behaviors. Such a copolymer crosses the LDOT, leading to a microphase-separated and ordered state at high temperature. Contrary to the copolymers with short PS blocks, the flow regime was not reached, even at temperatures well above the glass transition of the PS. Instead, a low-frequency plateau was observed in rheology, showing the increased lifetime of the microphase-separated PS domains. ABA triblock copolymers exhibiting an LDOT behavior could, thus, be of interest for the design of thermoplastic elastomers or pressure-sensitive adhesives that can resist the flow at high temperatures.

We investigate the linear viscoelastic properties of industrial pressure sensitive adhesives comprising double networks with an entangled acrylate-based polymer and two types of intermolecular associations (crosslinking), permanent (epoxide) and reversible (metal-chelate), having different compositions. A combination of shear rheometry and an appropriately modified version of the Time Marching Algorithm (TMA) allows to probe and analyze the behavior of the different double dynamic networks, in particular, the effects of the type and amount of crosslinks on their linear viscoelastic spectra. To this end, the dynamics of the double networks are compared with the respective individual responses of the polymeric component without crosslinks and the single networks (possessing only physical or only chemical crosslinks), in order to quantify their contributions to the relaxation mechanisms, particularly the interplay between disentanglement and bond association/dissociation processes. With the help of the TMA model, we also examine the respective roles of the lifetime of stickers, polydispersity, and molar mass. Triggered by the good comparison between predictions and experimental data, we propose a framework to tune material parameters in order to obtain a desired viscoelastic behavior.

Dynamic covalent networks present a unique opportunity to exert molecular-level control on macroscopic material properties, by linking their thermal behaviour to the thermodynamics and kinetics of the underlying chemistry. Yet, existing methods do not allow for the extraction and analysis of the influence of local differences in chemical reactivity caused by available reactants, catalysts, or additives. In this context, we present a rheological paradigm that allows us to correlate the composition of a reactive polymer segment to a faster or slower rate of network rearrangement. We discovered that a generalised Maxwell model could separate and quantify the dynamic behaviour of each type of reactive segment individually, which was crucial to fully comprehend the mechanics of the final material. More specifically, Eyring and Van 't Hoff analysis were used to relate possible bond catalysis and dissociation to structural changes by combining statistical modelling with rheology measurements. As a result, precise viscosity changes could be measured, allowing for accurate comparison of various dynamic covalent network materials, including vitrimers and dissociative networks. The herein reported method therefore facilitated the successful analysis of virtually any type of rate-enhancing effect and will allow for the design of functional and fast (re)processable materials, as well as improve our ability to predict and engineer their properties for future applications.

Polymer networks are created by crosslinked polymer chains. These links can be covalent (permanent) or non-covalent (reversible) with consequent effects on the material properties. Both types of networks have advantages and disadvantages: covalent networks resist flow and creep but are difficult to process and recycle, while non-covalent networks are reversible but susceptible to structural changes in the long term. To overcome these limitations of individual networks, double networks work via the combination of a sacrificial network, which breaks immediately on stress and a structural network which maintains the overall integrity. In this way tough and strong materials are created. The subnetworks of a double network can be covalent or non-covalent, imparting new functionalities to the material. The dynamic nature of non-covalently crosslinked networks makes them an ideal candidate to be studied by rheology, which measures the time and frequency dependent behaviour of materials. Our dynamic networks, studied in the melt state, are based on single and interpenetrating poly(n-butylacrylate) (PnBA) networks built from reversible crosslinks with largely different dynamics. The metallosupramolecular network, based on metal-terpyridine complexes, is highly tuneable according to the type of metal ion used. The dynamic covalent network, based on imine-aldehyde bonds, is slowly exchanging at room temperature and provides contrast to the more rapid metallosupramolecular network. Finally, the slide-ring network which facilitates highly extensible materials through the pulley effect is combined with the metallosupramolecular network. Through different combinations of networks and experimental parameters, we are able to establish a relationship between the structure of these networks and their viscoelastic behaviour measured by rheological techniques. In this thesis, our aim is to understand the dynamics of these networks individually and within the double network structure., (SC - Sciences) -- UCL, 2022

The mechanical properties of multiblock copolymer thermoplastic elastomers (TPEs) are governed by the interplay of different reversible dynamics [e.g., hard block (HB) association and chain entanglements]. Understanding how these physical processes influence the hightemperature deformation behavior is relevant as many TPEs lose toughness with increasing temperature. Increasing molecular weight (Mw) improves their temperature resistance that is attributed to an increase in network connectivity. Indeed, longer chains are characterized by more HBs per chain and by a longer lifetime of the entanglements in the amorphous phase. Both the associating HB and disentanglement dynamics are temperature and rate dependent. To further understand the interconnected role of Mw, temperature and rate dependencies on the mechanical properties, we perform Temperature Scanning Stress Relaxation (TSSR) tests. The method consists of measuring the stress relaxation of the materials as the temperature monotonically increases, allowing us to probe the stress response as the HBs progressively disassociate due to the increase in temperature. The results show that increasing Mw improves the high-temperature relaxation behavior, allowing the material to retain more stress than its low Mw counterpart as the temperature increases. This distinction does not show itself when performing standard small strain dynamic mechanical thermal analyses. Depending on the deformation experienced before the TSSR is performed, different relaxation behaviors are observed illustrating the importance of the current microstructure in determining the mechanical properties. The TSSR approach is well-suited to benchmark the high-temperature stress-bearing properties of network-based polymers whose morphology and, hence, properties are strongly deformation dependent.

The current coarse-grained picture to represent polymer chain dynamics under uniaxial extensional flow (based on the Doi–Edwards model) fails to predict some scaling dependencies of material properties on deformation rate observed experimentally, specifically the monotonic thinning behavior of polymer melts. Recently, new mechanisms based on the concept of monomeric friction reduction have been proposed to explain this peculiar behavior; however, it is difficult to include them in the framework of the standard tube model. Therefore, in this work, we propose an alternative treatment which does not rule out friction reduction but uses a different approach. It considers that the chain can stretch up to a certain level that we determine based on the Pincus blob picture, in place of determining to which extend the chain stretch is reduced compared to its finite extensibility. To this end, we revisit the extensional rheological data of polystyrene melts and see how the specificities of chains under strong elongational flow can be integrated into a tube model. This requires accounting for possible flow-induced chain orientation, stretching, and disentanglement. In particular, we extend the picture of Pincus blobs and define different levels of stretch that a chain can reach as a function of the extensional rate by invoking a rate-dependent blob picture. While this approach requires introducing an additional parameter to describe the stretch relaxation time, the results are in good agreement with the experimental observations. This alternative but sound approach should contribute to the on-going discussion on the elongation of entangled polymers.

This study addresses the linear viscoelasticity of blends involving monodisperse comb and linear polymers, the latter being the matrix of varying molar mass. This is dynamically a three-component system, comprising linear chains, branches (which are short enough to undergo essentially Rouse relaxation), and backbones. We examine systematically the relaxation of the same comb diluted in the linear matrix as a function of the molar mass of the linear chain for all possible situations (when the latter is oligomer or polymer relaxing faster or slower than the branches) as well as the case of backbone self-entanglements. We develop a simple tube model for the contributions of the three components to comb relaxation by accounting for the dynamic dilution and two delay factors, one for the matrix and another for the comb. The former represents the frictional contribution of the linear polymers to the constraint release Rouse process of the branches and backbone, and the latter represents the relevant effect of the branches on the backbone motion. The model provides an excellent quantitative description of the experimental data. The resulting retardation factors controlling the relaxation process of the combs as a function of the linear polymer’s molar mass complement and extend our earlier investigations with blends involving H-polymers in linear matrices, pointing toward a universal description of the dynamic response of architecturally complex polymers in linear matrices based on constraint release mechanisms. This finding enhances our ability to tailor the dynamics of topological homopolymer blends as per our wish. The remaining challenges associated with the possible universality of the delay factor of combs and the coupling of branch and backbone motion are discussed.

A mechanical bond is not shared between atoms, instead, it develops between molecular entities entangled in space and provides higher structural integrity. The molecules resulting from the mechanical bonds are called mechanically interlocked molecules (MIMs), and the most common MIMs are catenanes and rotaxanes. These mechanically interlocked structures have been exploited in a wide range of research fields, in particular molecular machines and polymeric materials. The slide-ring gels is one of those research areas where the mechanically interlocked molecules or mechanical bonds are incorporated in a three-dimensional polymeric network. The aim of this work is to study the ring chain interactions in slide-ring gels based on the variation of the strength between the ring and the polymer chain by using palladium-based slide-ring networks. Due to the presence of palladium-based metal-ligand interactions in the network, the ring mobility can be controlled by removing or keeping the metal-ligand interactions between the ring and the thread. Thus, by using a single system, three different types of ring mobilities; from immobile, to partially mobile, to completely free mobility of the rings can be achieved without changing any other parameter. The palladium-based slide-ring gel systems are developed by two different approaches, named as polyrotaxane approach and one-pot approach. In the polyrotaxane approach, a polyrotaxane is first synthesised and then cross-linked to form the slide-ring gel, while in the one-pot approach, a gel is directly synthesised by using a tetra functional bis-pseudorotaxane and other precursors. The precursors used in both the approaches are roughly the same and utilised polyurethane chemistry to obtain the polymeric materials. The palladium-based pseudorotaxane is the most important precursor in this work. A number of pseudorotaxanes are synthesised to explore different strategies. In polyrotaxane approach, several trials were made to develop the, (SC - Sciences) -- UCL, 2021

The Doi−Edwards tube model, coupled with relaxation mechanisms, such as reptation, contour length fluctuation, and constraint release, allows us to quantitatively predict the linear viscoelastic properties of entangled polymers. However, for nonlinear elongational flows, large discrepancies between theoretical predictions based on the tube model and experimental results still persist today. This is in particular obvious for the experimentally observed strong qualitative differences in extensional flow of entangled polystyrene (PS) melts and solutions despite having the same number of entanglements and exhibiting the same linear viscoelastic behavior. The cause of this non-universality is often attributed either to a monomeric friction reduction or to an interchain pressure effect. In this work, we investigate the changes in extensional flow behavior going from polymer solutions to the melt state. For this purpose, we measure with a filament stretching rheometer the nonlinear extensional responses of differently long PS chains, both in the melt state and diluted in short-chain matrices of the same polymer at varying concentrations. These concentrations have been chosen sufficiently high, such that the chains stay entangled. This allows us to discuss the influence of concentration and molar mass on the steady-state elongational viscosity to highlight scaling relations. The purpose of the present work is to conduct well-defined experiments to further investigate how the steady extensional viscosity of polymer solutions and blends varies with the concentration and with the molecular weight of the chains.

Dynamic polymer hydrogels crosslinked by dynamic bonds are capable of a controlled lifetime, enhanced toughness, self-healing properties or responsiveness to external stimuli. Dynamic bonds generally fall into two categories: supramolecular interactions and dynamic covalent bonds, which can break and reform spontaneously or in response to external stimuli. Supramolecular interactions (e.g., hydrogen bonds, metal-ligand coordination bonds, host-guest interactions) are labile and reversible, and usually under (fast) continuous equilibrium between associated and dissociated forms. In comparison with supramolecular interactions, dynamic covalent bonds (e.g., C=N bonds, disulfide bonds, boronic ester bonds) are much more robust but also exhibit slower exchange. The pros and cons of these two types of linkages make them a promising complementary toolbox for the design and fine-tuning of the properties of hydrogels. The focus of this dissertation is the development of dynamic polymer hydrogels by the orthogonal combination of dynamic covalent bonds and supramolecular interactions, and the investigation of the contribution of the two types of dynamic bonds to the properties of the hydrogels. Metal-terpyridine coordination bonds (zinc(II)-terpyridine and iron(II)-terpyridine bis-complexes) and reversible C=N bonds (acylhydrazone and oxime) were selected and employed as crosslinks to form hydrogels. First, we sought to design four interpenetrating polymer network (IPN) hydrogels with tunable combinations by leveraging the differences in lifetime and stability of the two types of dynamic bonds. The dynamic, oscillatory mechanical properties and pH responsiveness of the four hydrogels were systematically investigated using oscillatory rheological methods. Second, we prepared hydrogels with three topological strategies, namely, IPN, dual-crosslinked network (DCN) and double network (DN), using a combination of zinc(II)-terpyridine bis-complexes and oxime bonds. The dynamic and li, (SC - Sciences) -- UCL, 2021

This work addresses the shear response of entangled linear polymers by assessing the current state-of-the-art model and proposing alternative directions. In particular, we examine the performance of the Graham-Likhtman-McLeish-Milner (GLaMM) nonlinear tube model in fast shear flows. Predictions are compared against experimental data for well-characterized, monodisperse, entangled linear polystyrene chains. Unlike previous works using the GLaMM model, finite extensibility is accounted for. Comparison of model predictions and data not only reveals an overall reasonable performance but also highlights limitations. For example, the predictions significantly depend on whether the contribution of contour length fluctuations to the retraction rate is accounted for or not. This specific sensitivity of the model is enhanced as the Rouse-Weissenberg number increases. Possible improvement of the model by modification of the existing mechanisms/model assumptions and/or by incorporation of overlooked mechanisms such as chain tumbling and CCR-driven disentanglement (CCR-D) is discussed. As a possible way to overcome the existing limitations, we propose an alternative approach based on the time marching algorithm (TMA) and focus on the description of the steady-state regime. Compared to the GLaMM model, this approach allows us to keep track of the relaxation state of each entanglement segment in a simple way, while ensuring consistency between chain stretch and constraint release mechanisms. Moreover, it accounts for CCR-D in an indirect manner. A key ingredient is the use of the recently advanced concept of the shear blob and its dependence on the shear rate. Our simple modification of TMA reproduces the experimental steady-state viscosities accurately for all samples and for all shear rates examined.