The effect of temperature (T) is studied on the swelling of model poly(vinyl acetate) (PVAc) gels... more The effect of temperature (T) is studied on the swelling of model poly(vinyl acetate) (PVAc) gels in isopropyl alcohol. The theta temperature Θ of these gels, at which the second osmotic virial coefficient A 2 vanishes, is close to that of the corresponding high molecular mass polymer solution without cross-links so that solvent quality may be defined in the same way as the corresponding precursor polymer solution. We quantified the swelling and deswelling of PVAc gels relative to their size at the theta temperature, and also determined the effect of T on the shear modulus of these gels. It was found that both swelling and deswelling data could be reduced to a universal scaling equation of the same general form as derived from renormalization group (RG) theory for flexible linear polymer chains in solutions.
The self-assembly of dispersed polymer-coated ferromagnetic nanoparticles into micron-sized one-d... more The self-assembly of dispersed polymer-coated ferromagnetic nanoparticles into micron-sized one-dimensional mesostructures at a liquid-liquid interface was reported. When polystyrene-coated Co nanoparticles (19 nm) are driven to an oil/water interface under zero-field conditions, long (% 5 lm) chain-like assemblies spontaneously form because of dipolar associations between the ferromagnetic nanoparticles. Direct imaging of the magnetic assembly process was achieved using a recently developed platform consisting of a biphasic oil/water system in which the oil phase was flash-cured within 1 s upon ultraviolet light exposure. The nanoparticle assemblies embedded in the crosslinked phase were then imaged using atomic force microscopy. The effects of time, temperature, and colloid concentration on the self-assembly process of dipolar nanoparticles were then investigated. Variation of either assembly time t or temperature T was found to be an interchangeable effect in the 1D organization process. Because of the dependence of chain length on the assembly conditions, we observed striking similarities between 1D nanoparticle self-assembly and polymerization of small molecule monomers. This is the first in-depth study of the parameters affecting the self-assembly of dispersed, dipolar nanoparticles into extended mesostructures in the absence of a magnetic field. V
Bulletin of the American Physical Society, Mar 13, 2017
The formation of a glass is one of the most vital features of amorphous polymers. While this subj... more The formation of a glass is one of the most vital features of amorphous polymers. While this subject has been exhaustively studied for linear chain polymers, comparatively little is known about the glass formation of star polymers, one of the most important classes of branched polymers. Using molecular dynamics simulation methods, we study the dynamical heterogeneity of star-polymers. We characterize the cooperative nature of the dynamic properties melts via the non-Gaussian nature of displacements, fourpoint correlations, clusters of highly mobile monomers, and subsets of string-like monomer motion. We contrast the behavior to that of ordinary linear chains, considering the role of both number of arms and molecular weight. In doing so, we quantify the degree to which the topology of star polymer plays a role in dynamical heterogeneity.
Bulletin of the American Physical Society, Mar 15, 2017
University-It is widely appreciated that the glass transition of thin polymer films can be strong... more University-It is widely appreciated that the glass transition of thin polymer films can be strongly altered compared to bulk polymers. Furthermore, it is generally agreed that attractive substrate interactions slow relaxation and increase the glasstransition temperature T g. However, there is evidence that the magnitude of T g changes for strong substrate interactions are much smaller are expected, a phenomenon that also occurs in polymer-nanoparticle composites. Here, we use molecular simulations to reproduce this effect in supported polymer thin films; we explain the lack of sensitivity of T g to strong substrate interactions as a result of the emergence of a layer of "bound" polymer at the substrate. This bound polymer effectively cloaks the remainder of the film from the strong interfacial interactions. This bound layer manifests itself by an additional relaxation process in self-intermediate scattering function. We further characterize the temperature and substrate interaction strength dependence of the bound layer relaxation time from the overall relaxation, as well as the relaxation from polymer chains near the film center.
In living systems, it is frequently stated that form follows function by virtue of evolutionary p... more In living systems, it is frequently stated that form follows function by virtue of evolutionary pressures on organism development, but in the study of how functions emerge at the cellular level, function often follows form. We study this chicken versus egg problem of emergent structureproperty relationships in living systems in the context of primary human bone marrow stromal cells cultured in a variety of microenvironments that have been shown to cause distinct patterns of cell function and differentiation. Through analysis of a publicly available catalogue of threedimensional (3D) cell shape data, we introduce a family of metrics to characterize the "form" of the cell populations that emerge from a variety of diverse microenvironments. In particular, measures of form are considered that are expected to have direct significance for cell function, signaling and metabolic activity: dimensionality, polarizability and capacitance. Dimensionality was assessed by an intrinsic measure of cell shape obtained from the polarizability tensor. This tensor defines ellipsoids for arbitrary cell shapes and the thinnest dimension of these ellipsoids, P 1 , defines a reference minimal scale for cells cultured in a 3D microenvironment. Polarizability governs the electric field generated by a cell, and determines the cell's ability to detect electric fields. Capacitance controls the shape dependence of the rate at which diffusing molecules contact the surface of the cell, and this has great significance for inter-cellular signaling. These results invite new approaches for designing scaffolds which explicitly direct cell dimensionality, polarizability and capacitance to guide the emergence of new cell functions derived from the acquired form.
We perform molecular dynamics simulations on a coarse-grained polymer melt to study the dynamics ... more We perform molecular dynamics simulations on a coarse-grained polymer melt to study the dynamics of glass-formation in ring polymer melts of variable knot complexity. After generating melts of non-concatenated polymeric rings having a range of minimum crossing number values, mc, we compute the coherent intermediate scattering function, the segmental α-relaxation time, fragility, and the glass transition temperature as a function of mc. Variation of knot complexity is found to have a pronounced effect on the dynamics of polymer melts since both molecular rigidity and packing are altered, primary physical factors governing glass-formation in polymeric materials.
We present a novel platform, dubbed fossilized liquid assembly, for the creation of 2-D assemblie... more We present a novel platform, dubbed fossilized liquid assembly, for the creation of 2-D assemblies from nanoscale building blocks. The system consists of an oil/water interface in which the oil phase can be flash-cured upon UV exposure. The photopolymerizable material, 1,12-dodecanediol dimethacrylate, solidifies in as little as 1 s when exposed to UV light. The rapid cross-linking allows one to obtain a "snapshot" of the assembly process for particles that segregate to the oil/water interface. Among the particles investigated were nonpolar 0.39 µm poly(methyl methacrylate) latex spheres, nonpolar 10 µm polystyrene latex spheres, highly polarizable 5 nm Au nanocrystals, dipolar 10 nm CdTe quantum dots, and magnetic 25 nm magnetite nanoparticles. The aggregates formed by this process were typically either globular or fractal in appearance. By comparing with simulation, we can perform quantitative image analysis on the resulting micrographs to define a rigorous set of standards for distinguishing among the main classes of aggregation: flocculation, equilibrium phase separation, and true self-assembly.
The mean-square atomic displacement <u 2 > of thin (1015 Å to 75 Å) polycarbonate films supported... more The mean-square atomic displacement <u 2 > of thin (1015 Å to 75 Å) polycarbonate films supported on Si wafers is measured using incoherent elastic neutron scattering. The value of <u 2 > is determined by fitting the scattering intensity data to the Debye-Waller factor and we find that <u 2 > is diminished with decreasing film thickness. When the film thickness is comparable to the unperturbed dimensions of the macromolecule two characteristic temperatures start to emerge in the <u 2 > data, one above and the other below the bulk Tg. Furthermore, preliminary results with a different polymer suggests that <u 2 > does not depend on the probing direction, i.e. the in-plane and the out-of-plane <u 2 > were found to be identical within the experiment uncertainty.
A number of methods for estimating the translational friction coefficient f and the intrinsic vis... more A number of methods for estimating the translational friction coefficient f and the intrinsic viscosity [η] of polymers and nanoparticles have been proposed. These methods range from first-principles "boundaryelement" or "bead-model" solutions of the Stokes equation employing a precise description of particle shape, to coarse-grained descriptions of polymer structures and approximate computational methods at an intermediate level of description, and finally to phenomenological estimates that relate f to the surface area of the particle. Analytic treatments normally involve slender-body and various "preaveraging" approximations, etc., that render the calculation analytically tractable, but numerically uncertain. Powerful numerical path-integral methods have become available in recent years that allow the assessment of the accuracy of the various approximate methods. We compare several methods of computing f and [η] to determine their applicability to various classes of particle shapes.
Bulletin of the American Physical Society, Mar 13, 2017
The canonical model of patchy particles, a hard sphere decorated with attractive patches, has bee... more The canonical model of patchy particles, a hard sphere decorated with attractive patches, has been used to describe solutions of small globular proteins, as well as micron-size particles with attractive patches. Previously, we extended the canonical model by introducing an isotropic, attractive interaction. Using Monte Carlo simulations and an analytic, Wertheim based mean-field theory, we found that although the location of the self-assembly transition lines were dependent on the isotropic interaction strength, the nature of the self-assembly was unaffected. Specifically, we developed a formalism to describe a master curve for the average molecular mass by combining Flory-Stockmayer theory with an analysis of the thermodynamics of association. We also found that the self-assembled clusters have a fractal dimension of two; this value is consistent with randomly branched polymers swollen by repulsive self-excluded volume interactions. Extending this work, we consider the role of patch number and find that the formalism still holds but becomes dependent on the number of patches. We explore the experimental implications of this finding and investigate the role of patch number on cluster shape.
Articular cartilage is a composite hydrogel found in animal and human joints, which exhibits uniq... more Articular cartilage is a composite hydrogel found in animal and human joints, which exhibits unique load-bearing properties that have been challenging to reproduce in synthetic materials and model in molecular dynamics (MD) simulations. We computationally investigate a composite hydrogel that mimics key functional properties of articular cartilage as a potential biomimetic model to investigate its unique load-bearing properties. Specifically, we find that the emergence of prestress in composite gels derives primarily from the stiffness of the polymer matrix and the asymmetry in the enthalpic interactions of the embedded particles and polymer matrix. Our MD simulations of the development of prestress agree qualitatively with osmotic pressure measurements observed in our model composite hydrogel material.
The fragility of polymeric glass-forming liquids is calculated as a function of molecular structu... more The fragility of polymeric glass-forming liquids is calculated as a function of molecular structural parameters from a generalized entropy theory of polymer glass-formation that combines the Adam-Gibbs (AG) model for the rate of structural relaxation with the lattice cluster theory (LCT) for polymer melt thermodynamics. Our generalized entropy theory predicts the existence of distinct high and low temperature regimes of glassformation that are separated by a thermodynamically well-defined crossover temperature T I at which the product of the configurational entropy and the temperature has an inflection point. Since the predicted temperature dependence of the configurational entropy and structural relaxation time are quite different in these temperature regimes, we introduce separate definitions of fragility for each regime. Experimentally established trends in the fragility of polymer melts with respect to variations in polymer microstructure and pressure are interpreted within our theory in terms of the accompanying changes in the chain packing efficiency.
The effect of temperature (T) is studied on the swelling of model poly(vinyl acetate) (PVAc) gels... more The effect of temperature (T) is studied on the swelling of model poly(vinyl acetate) (PVAc) gels in isopropyl alcohol. The theta temperature Θ of these gels, at which the second osmotic virial coefficient A 2 vanishes, is close to that of the corresponding high molecular mass polymer solution without cross-links so that solvent quality may be defined in the same way as the corresponding precursor polymer solution. We quantified the swelling and deswelling of PVAc gels relative to their size at the theta temperature, and also determined the effect of T on the shear modulus of these gels. It was found that both swelling and deswelling data could be reduced to a universal scaling equation of the same general form as derived from renormalization group (RG) theory for flexible linear polymer chains in solutions.
The self-assembly of dispersed polymer-coated ferromagnetic nanoparticles into micron-sized one-d... more The self-assembly of dispersed polymer-coated ferromagnetic nanoparticles into micron-sized one-dimensional mesostructures at a liquid-liquid interface was reported. When polystyrene-coated Co nanoparticles (19 nm) are driven to an oil/water interface under zero-field conditions, long (% 5 lm) chain-like assemblies spontaneously form because of dipolar associations between the ferromagnetic nanoparticles. Direct imaging of the magnetic assembly process was achieved using a recently developed platform consisting of a biphasic oil/water system in which the oil phase was flash-cured within 1 s upon ultraviolet light exposure. The nanoparticle assemblies embedded in the crosslinked phase were then imaged using atomic force microscopy. The effects of time, temperature, and colloid concentration on the self-assembly process of dipolar nanoparticles were then investigated. Variation of either assembly time t or temperature T was found to be an interchangeable effect in the 1D organization process. Because of the dependence of chain length on the assembly conditions, we observed striking similarities between 1D nanoparticle self-assembly and polymerization of small molecule monomers. This is the first in-depth study of the parameters affecting the self-assembly of dispersed, dipolar nanoparticles into extended mesostructures in the absence of a magnetic field. V
Bulletin of the American Physical Society, Mar 13, 2017
The formation of a glass is one of the most vital features of amorphous polymers. While this subj... more The formation of a glass is one of the most vital features of amorphous polymers. While this subject has been exhaustively studied for linear chain polymers, comparatively little is known about the glass formation of star polymers, one of the most important classes of branched polymers. Using molecular dynamics simulation methods, we study the dynamical heterogeneity of star-polymers. We characterize the cooperative nature of the dynamic properties melts via the non-Gaussian nature of displacements, fourpoint correlations, clusters of highly mobile monomers, and subsets of string-like monomer motion. We contrast the behavior to that of ordinary linear chains, considering the role of both number of arms and molecular weight. In doing so, we quantify the degree to which the topology of star polymer plays a role in dynamical heterogeneity.
Bulletin of the American Physical Society, Mar 15, 2017
University-It is widely appreciated that the glass transition of thin polymer films can be strong... more University-It is widely appreciated that the glass transition of thin polymer films can be strongly altered compared to bulk polymers. Furthermore, it is generally agreed that attractive substrate interactions slow relaxation and increase the glasstransition temperature T g. However, there is evidence that the magnitude of T g changes for strong substrate interactions are much smaller are expected, a phenomenon that also occurs in polymer-nanoparticle composites. Here, we use molecular simulations to reproduce this effect in supported polymer thin films; we explain the lack of sensitivity of T g to strong substrate interactions as a result of the emergence of a layer of "bound" polymer at the substrate. This bound polymer effectively cloaks the remainder of the film from the strong interfacial interactions. This bound layer manifests itself by an additional relaxation process in self-intermediate scattering function. We further characterize the temperature and substrate interaction strength dependence of the bound layer relaxation time from the overall relaxation, as well as the relaxation from polymer chains near the film center.
In living systems, it is frequently stated that form follows function by virtue of evolutionary p... more In living systems, it is frequently stated that form follows function by virtue of evolutionary pressures on organism development, but in the study of how functions emerge at the cellular level, function often follows form. We study this chicken versus egg problem of emergent structureproperty relationships in living systems in the context of primary human bone marrow stromal cells cultured in a variety of microenvironments that have been shown to cause distinct patterns of cell function and differentiation. Through analysis of a publicly available catalogue of threedimensional (3D) cell shape data, we introduce a family of metrics to characterize the "form" of the cell populations that emerge from a variety of diverse microenvironments. In particular, measures of form are considered that are expected to have direct significance for cell function, signaling and metabolic activity: dimensionality, polarizability and capacitance. Dimensionality was assessed by an intrinsic measure of cell shape obtained from the polarizability tensor. This tensor defines ellipsoids for arbitrary cell shapes and the thinnest dimension of these ellipsoids, P 1 , defines a reference minimal scale for cells cultured in a 3D microenvironment. Polarizability governs the electric field generated by a cell, and determines the cell's ability to detect electric fields. Capacitance controls the shape dependence of the rate at which diffusing molecules contact the surface of the cell, and this has great significance for inter-cellular signaling. These results invite new approaches for designing scaffolds which explicitly direct cell dimensionality, polarizability and capacitance to guide the emergence of new cell functions derived from the acquired form.
We perform molecular dynamics simulations on a coarse-grained polymer melt to study the dynamics ... more We perform molecular dynamics simulations on a coarse-grained polymer melt to study the dynamics of glass-formation in ring polymer melts of variable knot complexity. After generating melts of non-concatenated polymeric rings having a range of minimum crossing number values, mc, we compute the coherent intermediate scattering function, the segmental α-relaxation time, fragility, and the glass transition temperature as a function of mc. Variation of knot complexity is found to have a pronounced effect on the dynamics of polymer melts since both molecular rigidity and packing are altered, primary physical factors governing glass-formation in polymeric materials.
We present a novel platform, dubbed fossilized liquid assembly, for the creation of 2-D assemblie... more We present a novel platform, dubbed fossilized liquid assembly, for the creation of 2-D assemblies from nanoscale building blocks. The system consists of an oil/water interface in which the oil phase can be flash-cured upon UV exposure. The photopolymerizable material, 1,12-dodecanediol dimethacrylate, solidifies in as little as 1 s when exposed to UV light. The rapid cross-linking allows one to obtain a "snapshot" of the assembly process for particles that segregate to the oil/water interface. Among the particles investigated were nonpolar 0.39 µm poly(methyl methacrylate) latex spheres, nonpolar 10 µm polystyrene latex spheres, highly polarizable 5 nm Au nanocrystals, dipolar 10 nm CdTe quantum dots, and magnetic 25 nm magnetite nanoparticles. The aggregates formed by this process were typically either globular or fractal in appearance. By comparing with simulation, we can perform quantitative image analysis on the resulting micrographs to define a rigorous set of standards for distinguishing among the main classes of aggregation: flocculation, equilibrium phase separation, and true self-assembly.
The mean-square atomic displacement <u 2 > of thin (1015 Å to 75 Å) polycarbonate films supported... more The mean-square atomic displacement <u 2 > of thin (1015 Å to 75 Å) polycarbonate films supported on Si wafers is measured using incoherent elastic neutron scattering. The value of <u 2 > is determined by fitting the scattering intensity data to the Debye-Waller factor and we find that <u 2 > is diminished with decreasing film thickness. When the film thickness is comparable to the unperturbed dimensions of the macromolecule two characteristic temperatures start to emerge in the <u 2 > data, one above and the other below the bulk Tg. Furthermore, preliminary results with a different polymer suggests that <u 2 > does not depend on the probing direction, i.e. the in-plane and the out-of-plane <u 2 > were found to be identical within the experiment uncertainty.
A number of methods for estimating the translational friction coefficient f and the intrinsic vis... more A number of methods for estimating the translational friction coefficient f and the intrinsic viscosity [η] of polymers and nanoparticles have been proposed. These methods range from first-principles "boundaryelement" or "bead-model" solutions of the Stokes equation employing a precise description of particle shape, to coarse-grained descriptions of polymer structures and approximate computational methods at an intermediate level of description, and finally to phenomenological estimates that relate f to the surface area of the particle. Analytic treatments normally involve slender-body and various "preaveraging" approximations, etc., that render the calculation analytically tractable, but numerically uncertain. Powerful numerical path-integral methods have become available in recent years that allow the assessment of the accuracy of the various approximate methods. We compare several methods of computing f and [η] to determine their applicability to various classes of particle shapes.
Bulletin of the American Physical Society, Mar 13, 2017
The canonical model of patchy particles, a hard sphere decorated with attractive patches, has bee... more The canonical model of patchy particles, a hard sphere decorated with attractive patches, has been used to describe solutions of small globular proteins, as well as micron-size particles with attractive patches. Previously, we extended the canonical model by introducing an isotropic, attractive interaction. Using Monte Carlo simulations and an analytic, Wertheim based mean-field theory, we found that although the location of the self-assembly transition lines were dependent on the isotropic interaction strength, the nature of the self-assembly was unaffected. Specifically, we developed a formalism to describe a master curve for the average molecular mass by combining Flory-Stockmayer theory with an analysis of the thermodynamics of association. We also found that the self-assembled clusters have a fractal dimension of two; this value is consistent with randomly branched polymers swollen by repulsive self-excluded volume interactions. Extending this work, we consider the role of patch number and find that the formalism still holds but becomes dependent on the number of patches. We explore the experimental implications of this finding and investigate the role of patch number on cluster shape.
Articular cartilage is a composite hydrogel found in animal and human joints, which exhibits uniq... more Articular cartilage is a composite hydrogel found in animal and human joints, which exhibits unique load-bearing properties that have been challenging to reproduce in synthetic materials and model in molecular dynamics (MD) simulations. We computationally investigate a composite hydrogel that mimics key functional properties of articular cartilage as a potential biomimetic model to investigate its unique load-bearing properties. Specifically, we find that the emergence of prestress in composite gels derives primarily from the stiffness of the polymer matrix and the asymmetry in the enthalpic interactions of the embedded particles and polymer matrix. Our MD simulations of the development of prestress agree qualitatively with osmotic pressure measurements observed in our model composite hydrogel material.
The fragility of polymeric glass-forming liquids is calculated as a function of molecular structu... more The fragility of polymeric glass-forming liquids is calculated as a function of molecular structural parameters from a generalized entropy theory of polymer glass-formation that combines the Adam-Gibbs (AG) model for the rate of structural relaxation with the lattice cluster theory (LCT) for polymer melt thermodynamics. Our generalized entropy theory predicts the existence of distinct high and low temperature regimes of glassformation that are separated by a thermodynamically well-defined crossover temperature T I at which the product of the configurational entropy and the temperature has an inflection point. Since the predicted temperature dependence of the configurational entropy and structural relaxation time are quite different in these temperature regimes, we introduce separate definitions of fragility for each regime. Experimentally established trends in the fragility of polymer melts with respect to variations in polymer microstructure and pressure are interpreted within our theory in terms of the accompanying changes in the chain packing efficiency.
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Papers by Jack Douglas