arXiv subjects

Angel J. Moreno

Publications and source records attributed to Angel J. Moreno.

At least 19 recordsLinked to original sources

arXiv2023

On the structure-viscoelasticity relationship of a dually crosslinked reversible polymer network

We perform equilibrium Langevin dynamics simulations to understand the structure-viscoelasticity relationship of a dually crosslinked reversible polymer network. The cross-linking is achieved by introducing orthogonal crosslinkers (A and B) in to the polymer backbone, where only intra-species bonds (A-A or B-B) are allowed to be formed. We study the systems at infinite dilution and above the overlap concentration of the chains. We find the dually cross-linked chains to be more compact compared to the singly cross-linked chains at infinite dilution. At the finite concentration, we also explored the role of weak (A-A) vs. strong (B-B) bonds in tuning the stress relaxation behavior of the networks, by systematically varying the relative composition of A monomers (x) while keeping the total fraction of crosslinks as constant. Interestingly, we find a non-monotonic trend in the diffusivity and hence in the stress auto-correlation function of the system w.r.t. x. Furthermore, we find that in the dynamics in the plateau regime of the stress relaxation function is dictated by the strength of the weak bonds, whereas the terminal relaxation behavior of the stress auto-correlation function depends on the strength of the strong bonds. We also study the influence of the distribution of the A and B on the stress relaxation by carrying out additional simulations for a symmetric distribution of the chains. Interestingly, we find the inter-molecular bonds to be significantly lower in the symmetric case which makes it to have a faster stress relaxation compared to its random counterpart. Our study highlight the influence of the composition, distribution, and the bond energy disparity of the crosslinking monomers in tuning the viscoelasticity of the dual networks.

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arXiv2021

On the Validity of Effective Potentials in Crowded Solutions of Linear and Ring Polymers with Reversible Bonds

We perform simulations to compute the effective potential between the centers-of-mass of two polymers with reversible bonds. We investigate the influence of the topology of the unbonded precursor (linear or ring) and the specific sequence of the reactive groups (from purely random to periodic), establishing that both have a strong impact on the effective potential. When the reactive sites of both polymers are chemically orthogonal so that only intramolecular bonds are possible, the interactions become more repulsive the closer to periodic the sequence is. The opposite effect is found if both polymers have the same type of reactive sites and intermolecular bonds can be formed. We test the validity of the effective potentials in solution, from high dilution to far above the overlap concentration, by comparing simulations of the effective fluid with those of the real all-monomer system. Very good agreement is found for the reversible linear polymers, indicating that unlike in their non-bonding counterparts many-body effects are minor even far above the overlap concentration. The agreement for the reversible rings is less satisfactory, and at high concentration the real system does not show the clustering behavior predicted by the effective potential. Results similar to the former ones are found for the partial self-correlations in ring/linear mixtures. Finally, we investigate the possibility of creating a gel of two interpenetrated reversible networks. For this purpose we simulate a 50/50 two-component mixture of reversible polymers with orthogonal chemistry for the reactive sites, so that intermolecular bonds are only formed between polymers of the same component. As predicted by both the theoretical phase diagram and the simulations of the effective fluid, the two networks in the all-monomer mixture do not interpenetrate and phase separation (demixing) is observed instead.

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arXiv2020

Gel Formation in Reversibly Cross-Linking Polymers

By means of Langevin dynamics simulations, we investigate the gel formation of randomly functionalized polymers in solution, with the ability to form both intra- and intermolecular reversible bonds. Under highly dilute conditions, these polymers form soft nano-objects (so-called single-chain nanoparticles, SCNPs), resulting from the purely intramolecular cross-linking of the reactive functional groups. Here we show that the competition between intra- and intermolecular bonds at finite concentration is governed by a delicate balance of various entropic contributions and leads to a density dependent effective valence. System-spanning networks are formed at relatively low monomer densities and their stability is mediated by just a small number of intermolecular connections per chain. The formation of intermolecular bonds furthermore can induce a non-monotonic dependence of the polymer size on the density for long bond lifetimes. Concomitantly, the polymers in the percolating cluster adopt an intramolecular structure characteristic for self-avoiding chains, which constitutes a strong contrast to the fractal globular behavior of irreversible SCNPs in crowded solutions with purely topological interactions (no intermolecular bonds). Finally, we study the dynamics of the system, which displays signatures expected for reversible gel-forming systems. An interesting behavior emerges in the reorganization dynamics of the percolating cluster. The relaxation is mostly mediated by the diffusion over long distances, through breaking and formation of bonds, of chains that do not leave the percolating cluster. Regarding the few chains that are transiently free, the time they spend until they reattach to the cluster is solely governed by the bond strength.

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arXiv2019

Coarsening Kinetics of Complex Macromolecular Architectures in Bad Solvent

This study reports a general scenario for the out-of-equilibrium features of collapsing polymeric architectures. We use molecular dynamics simulations to characterize the coarsening kinetics, in bad solvent, for several macromolecular systems with an increasing degree of structural complexity. In particular, we focus on: flexible and semiflexible polymer chains, star polymers with 3 and 12 arms, and microgels with both ordered and disordered networks. Starting from a powerful analogy with critical phenomena, we construct a density field representation that removes fast fluctuations and provides a consistent characterization of the domain growth. Our results indicate that the coarsening kinetics presents a scaling behaviour that is independent of the solvent quality parameter, in analogy to time-temperature superposition principle. Interestingly, the domain growth in time follows a power-law behaviour that is approximately independent of the architecture for all the flexible systems; while it is steeper for the semiflexible chains. Nevertheless, the fractal nature of the dense regions emerging during the collapse exhibits the same scaling behaviour for all the macromolecules. This, suggests that the faster growing length scale in the semiflexible chains originates just from a faster mass diffusion along the chain contour, induced by the local stiffness. The decay of the dynamic correlations displays scaling behavior with the growing length scale of the system, which is a characteristic signature in coarsening phenomena.

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arXiv2019

Deformability and Solvent Penetration in Soft Nanoparticles at Liquid-Liquid Interfaces

Soft nanoparticles hold promise as smart emulsifiers due to their high degree of deformability, permeability and stimuli responsive properties. By means of large-scale simulations we investigate the structural properties of nanogels at liquid-liquid (A-B) interfaces and the miscibility of the liquids inside the nanogels, covering the whole range of interfacial strength from the limit of single-liquid to the case of stiff interfaces. To study the role of the internal architecture and deformability of the nanogel we simulate a realistic disordered and an ideal regular network, for a broad range of cross-linking degrees. Unlike in previous investigations on liquid miscibility, excluded volume interactions are considered for both the monomers and the explicit solvent particles. The nanogel permeability is analysed by using an unbiased grid representation that accounts for the surface fluctuations and adds to the density profiles the exact number of liquid particles inside the nanogel. The better packing efficiency of the regular network leads to higher values of the total liquid uptake and the invasive capacity (A-particles in B-side and viceversa than in the disordered network, though differences vanish in the limit of rigid interfaces. Uptake and invasion are optimized at a cross-linking degree that depends on the interfacial strength, tending to 15 - 20% for moderate and stiff interfaces. As the interfacial strength increases, the miscibility inside the nanogel is enhanced by a factor of up to 5 with respect to the bare interface, with the disordered networks providing a better mixing than their ideal counterparts. The emerging scenario reported here provides general guidelines for tuning the shape, uptake, invasive, and mixing capacities of nanogels adsorbed at liquid-liquid interfaces.

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arXiv2019

Crowded Solutions of Single-Chain Nanoparticles under Shear Flow

Single-chain nanoparticles (SCNPs) are ultrasoft objects obtained through purely intramolecular cross-linking of single polymer chains. By means of computer simulations with implemented hydrodynamic interactions, we investigate for the first time the effect of the shear flow on the structural and dynamic properties of SCNPs in semidilute solutions. We characterize the dependence of several conformational and dynamic observables on the shear rate and the concentration, obtaining a set of power-law scaling laws. The concentration has a very different effect on the shear rate dependence of the former observables in SCNPs than in simple linear chains. Whereas for the latter the scaling behavior is marginally dependent on the concentration, two clearly different scaling regimes are found for the SCNPs below and above the overlap concentration. At fixed shear rate SCNPs and linear chains also respond very differently to crowding. Whereas, at moderate and high Weissenberg numbers the linear chains swell, the SCNPs exhibit a complex non-monotonic behavior. These findings are inherently related to the topological interactions preventing concatenation of the SCNP loops, leading to less interpenetration than for linear chains.

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arXiv2018

Single-Chain Nanoparticles under Homogeneous Shear Flow

Single-chain nanoparticles (SCNPs) are a new class of macromolecular objects, synthesized through purely intramolecular cross-linking of single polymer chains. We use a multi-scale hydrodynamics simulation approach to study, for the first time, SCNPs under shear flow. We investigate the case of irreversible SCNPs (permanent cross-links) in dilute solution. SCNPs emerge as a novel class of macromolecular objects with response to shear distinct from other systems such as linear chains, star polymers, rings or dendrimers. This is evidenced by the observed set of scaling exponents for the shear rate dependence of the SCNP static and dynamic properties. Surprisingly these exponents are, at most, marginally dependent on the specific topology of the SCNP (globular or sparse), suggesting that they are inherently related to the network-like character of the molecular architecture and not to its specific connectivity. At high Weissenberg numbers the dynamics of the sparse SCNPs is dominated by tumbling motion, whereas tank-treading predominates for the most globular SCNPs.

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arXiv2018

Computational Investigation of Microgels: Synthesis and Effect of the Microstructure on the Deswelling Behavior

We present computer simulations of a realistic model of microgels. Unlike the regular network frameworks usually assumed in the simulation literature, we model and simulate a realistic and efficient synthesis route, mimicking cross-linking of functionalized chains inside a cavity. This model is inspired, e.g., by microfluidic fabrication of microgels from macromolecular precursors and is different from standard polymerization routes. The assembly of the chains is mediated by a low fraction of interchain crosslinks. The microgels are polydisperse in size and shape but globally spherical objects. In order to deeply understand the microgel structure and eventually improve the synthesis protocol we characterize their conformational properties and deswelling kinetics, and compare them with the results found for microgels obtained via underlying regular (diamond-like) structures. The specific microstructure of the microgel has no significant effect on the locus of the volume phase transition (VPT). However, it strongly affects the deswelling kinetics, as revealed by a consistent analysis of the domain growth during the microgel collapse. Though both the disordered and the regular networks exhibit a similar early growth of the domains, an acceleration is observed in the regular network at the late stage of the collapse. Similar trends are found for the dynamic correlations coupled to the domain growth. As a consequence, the fast late processes for the domain growth and the dynamic correlations in the regular network are compensated, and the dynamic correlations follow a power-law dependence on the growing length scale that is independent of the microgel microstructure.

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arXiv2017

Effect of Chain Stiffness on the Structure of Single-Chain Polymer Nanoparticles

Polymeric single-chain nanoparticles (SCNPs) are soft nano-objects synthesized by purely intramolecular cross-linking of single polymer chains. By means of computer simulations, we investigate the conformational properties of SCNPs as a function of the bending stiffness of their linear polymer precursors. We investigate a broad range of characteristic ratios from the fully flexible case to those typical of bulky synthetic polymers. Increasing stiffness hinders bonding of groups separated by short contour distances and increases looping over longer distances, leading to more compact nanoparticles with a structure of highly interconnected loops. This feature is reflected in a crossover in the scaling behaviour of several structural observables. The scaling exponents change from those characteristic for Gaussian chains or rings in $θ$-solvents in the fully flexible limit, to values resembling fractal or `crumpled' globular behaviour for very stiff SCNPs. We characterize domains in the SCNPs. These are weakly deformable regions that can be seen as disordered analogues of domains in disordered proteins. Increasing stiffness leads to bigger and less deformable domains. Surprisingly, the scaling behaviour of the domains is in all cases similar to that of Gaussian chains or rings, irrespective of the stiffness and degree of cross-linking. It is the spatial arrangement of the domains which determines the global structure of the SCNP (sparse Gaussian-like object or crumpled globule). Since intramolecular stiffness can be varied through the specific chemistry of the precursor or by introducing bulky side groups in its backbone, our results propose a new strategy to tune the global structure of SCNPs.

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arXiv2017

Effects of Precursor Topology and Synthesis under Crowding Conditions on the Structure of Single-Chain Polymer Nanoparticles

By means of molecular dynamics simulations we investigate the formation of single-chain nanoparticles through intramolecular cross-linking of polymer chains, in the presence of their precursors acting as purely steric crowders in concentrated solution. In the case of the linear precursors, the structure of the resulting SCNPs is weakly affected by the density at which the synthesis is performed. Crowding has significant effects if ring precursors are used: higher concentrations lead to the formation of SCNPs with more compact and spherical morphologies. Such SCNPs retain in the swollen state (high dilution) the crumpled globular conformations adopted by the ring precursors in the crowded solutions. Increasing the concentration of both the linear and ring precursors up to $30 \%$ leads to faster formation of the respective SCNPs.

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arXiv2016

Tunable Slow Dynamics in a New Class of Soft Colloids

By means of extensive simulations, we investigate concentrated solutions of globular single-chain nanoparticles (SCNPs), an emergent class of synthetic soft nano-objects. By increasing the concentration, the SCNPs show a reentrant behaviour in their structural and dynamical correlations, as well as a soft caging regime and weak dynamic heterogeneity. The latter is confirmed by validation of the Stokes-Einstein relation up to concentrations far beyond the overlap density. Therefore SCNPs arise as a new class of soft colloids, exhibiting slow dynamics and actualizing in a real system structural and dynamical anomalies proposed by models of ultrasoft particles. Quantitative differences in the dynamical behaviour depend on the SCNP deformability, which can be tuned through the degree of internal cross-linking.

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arXiv2015

An Anisotropic Effective Model for the Simulation of Semiflexible Ring Polymers

We derive and introduce anisotropic effective pair potentials to coarse-grain solutions of semiflexible rings polymers of various lengths. The system has been recently investigated by means of full monomer-resolved computer simulations, revealing a host of unusual features and structure formation, which, however, cannot be captured by a rotationally-averaged effective pair potential between the rings' centers of mass [M. Bernabei et al., Soft Matter 9, 1287 (2013)]. Our new coarse-graining strategy is to picture each ring as a soft, penetrable disc. We demonstrate that for the short- and intermediate-length rings the new model is quite capable of capturing the Physics in a quantitative fashion, whereas for the largest rings, which resemble flexible ones, it fails at high densities. Our work opens the way for the physical justification of general, anisotropic penetrable interaction potentials.

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arXiv2013

Cluster Glasses of Semiflexible Ring Polymers

We present computer simulations of concentrated solutions of unknotted nonconcatenated semiflexible ring polymers. Unlike in their flexible counterparts, shrinking involves a strong energetic penalty, favoring interpenetration and clustering of the rings. We investigate the slow dynamics of the centers-of-mass of the rings in the amorphous cluster phase, consisting of disordered columns of oblate rings penetrated by bundles of prolate ones. Scattering functions reveal a striking decoupling of self- and collective motions. Correlations between centers-of-mass exhibit slow relaxation, as expected for an incipient glass transition, indicating the dynamic arrest of the cluster positions. However, self-correlations decay at much shorter time scales. This feature is a manifestation of the fast, continuous exchange and diffusion of the individual rings over the matrix of clusters. Our results reveal a novel scenario of glass-formation in a simple monodisperse system, characterized by self-collective decoupling, soft caging and mild dynamic heterogeneity.

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arXiv2012

Cluster glasses of ultrasoft particles

We present molecular dynamics (MD) simulations results for dense fluids of ultrasoft, fully-penetrable particles. These are a binary mixture and a polydisperse system of particles interacting via the generalized exponential model, which is known to yield cluster crystal phases for the corresponding monodisperse systems. Because of the dispersity in the particle size, the systems investigated in this work do not crystallize and form disordered cluster phases. The clustering transition appears as a smooth crossover to a regime in which particles are mostly located in clusters, isolated particles being infrequent. The analysis of the internal cluster structure reveals microsegregation of the big and small particles, with a strong homo-coordination in the binary mixture. Upon further lowering the temperature below the clustering transition, the motion of the clusters' centers-of-mass slows down dramatically, giving way to a cluster glass transition. In the cluster glass, the diffusivities remain finite and display an activated temperature dependence, indicating that relaxation in the cluster glass occurs via particle hopping in a nearly arrested matrix of clusters. Finally we discuss the influence of the microscopic dynamics on the transport properties by comparing the MD results with Monte Carlo simulations.

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arXiv2012

Anomalous Molecular Weight Dependence of Chain Dynamics in Unentangled Polymer Blends with Strong Dynamic Asymmetry

We address the general question of how the molecular weight dependence of chain dynamics in unentangled polymers is modified by blending. By dielectric spectroscopy we measure the normal mode relaxation of polyisoprene in blends with a slow matrix of poly(ter-butylstyrene). Unentangled polyisoprene in the blend exhibits strong deviations from Rouse scaling, approaching 'entangled-like' behavior at low temperatures in concomitance with the increase of the dynamic asymmetry in the blend. The obtained results are discussed in the framework of the generalized Langevin equation formalism. On this basis, a non trivial relationship between the molecular weight dependence of the longest chain relaxation time and the nonexponentiality of the corresponding Rouse correlator is found. This result is confirmed by molecular dynamics simulations.

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arXiv2010

Static and dynamic contributions to anomalous chain dynamics in polymer blends

By means of computer simulations, we investigate the relaxation of the Rouse modes in a simple bead-spring model for non-entangled polymer blends. Two different models are used for the fast component, namely fully-flexible and semiflexible chains. The latter are semiflexible in the meaning that static intrachain correlations are strongly non-gaussian at all length scales. The dynamic asymmetry in the blend is strongly enhanced by decreasing temperature, inducing confinement effects on the fast component. The dynamics of the Rouse modes show very different trends for the two models of the fast component. For the fully-flexible case, the relaxation times exhibit a progressive deviation from Rouse scaling on increasing the dynamic asymmetry. This anomalous effect has a dynamic origin. It is not related to particular static features of the Rouse modes, which indeed are identical to those of the fully-flexible homopolymer, and are not modified by the dynamic asymmetry in the blend. On the contrary, in the semiflexible case the relaxation times exhibit approximately the same scaling behaviour as the amplitudes of the modes. This suggests that the origin of the anomalous dynamic scaling for semiflexible chains confined in the blend is esentially of static nature. We discuss implications of these observations for the applicability of theoretical approaches to chain dynamics in polymer blends.

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arXiv2010

From caging to Rouse dynamics in polymer melts with intramolecular barriers: a critical test of the Mode Coupling Theory

By means of computer simulations and solution of the equations of the Mode Coupling Theory (MCT), we investigate the role of the intramolecular barriers on several dynamic aspects of non-entangled polymers. The investigated dynamic range extends from the caging regime characteristic of glass-formers to the relaxation of the chain Rouse modes. We review our recent work on this question, provide new results and critically discuss the limitations of the theory. Solutions of the MCT for the structural relaxation reproduce qualitative trends of simulations for weak and moderate barriers. However a progressive discrepancy is revealed as the limit of stiff chains is approached. This disagreement does not seem related with dynamic heterogeneities, which indeed are not enhanced by increasing barrier strength. It is not connected either with the breakdown of the convolution approximation for three-point static correlations, which retains its validity for stiff chains. These findings suggest the need of an improvement of the MCT equations for polymer melts. Concerning the relaxation of the chain degrees of freedom, MCT provides a microscopic basis for time scales from chain reorientation down to the caging regime. It rationalizes, from first principles, the observed devations from the Rouse model on increasing the barrier strength. These include anomalous scaling of relaxation times, long-time plateaux, and non-monotonous wavelength dependence of the mode correlators.

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arXiv2010

Increasing the density melts ultrasoft colloidal glasses

We use theory and simulations to investigate the existence of amorphous glassy states in ultrasoft colloids. We combine the hyper-netted chain approximation with mode-coupling theory to study the dynamic phase diagram of soft repulsive spheres interacting with a Hertzian potential, focusing on low temperatures and large densities. At constant temperature, we find that an amorphous glassy state is entered upon compression, as in colloidal hard spheres, but the glass unexpectedly melts when density increases further. We attribute this re-entrant fluid-glass transition to particle softness, and correlate this behaviour to previously reported anomalies in soft systems, thus emphasizing its generality. The predicted fluid-glass-fluid sequence is confirmed numerically.

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