Session F38: Padden Award Symposium

Dispersion-Aggregation and Wetting-Dewetting Phase Transitions in Mixtures of Polymer Grafted Nanoparticles and a Chemically Dissimilar Polymer Matrix

Significant efforts have been focused towards controlling morphology of the nanoscale fillers and matrix polymer in polymer nanocomposites as the composite morphology is directly related to the macroscopic properties of that material. For nanocomposites with chemically identical graft and matrix polymers, it is well understood that the polymer grafted particle dispersion to aggregation transition is directly linked to and synonymous with wetting/dewetting of the graft and matrix polymer. Our recent work has focused on exploring composites with chemically different graft and matrix polymers, specifically those with attractive graft-matrix interactions that lead to a dispersed filler state at low temperature and aggregated filler state at high temperatures. We show, using coarse-grained molecular simulations, that the sharp phase transition from dispersed to aggregated states is distinct from the continuous wetting-dewetting transition. The onset of wetting to dewetting occurs at temperatures lower than the dispersion to aggregation transition, and dewetting continues at temperatures above the transition temperature in the aggregated state. Furthermore, the graft and matrix chain composition can be varied to tune the dispersion-aggregation transition temperature and the degree of wetting of the grafted layer. Experiments using SANS and SAXS of deuterated poly(styrene) grafted silica particles in a poly(vinyl methyl ether) matrix show remarkable agreement with our simulations.   

Dynamics of associating networks

Associating polymers offer important technological solutions to renewable and self-healing materials, conducting electrolytes for energy storage and transport, and vehicles for cell and protein deliveries. The interplay between polymer topologies and association chemistries warrants new interesting physics from associating networks, yet poses significant challenges to study these systems over a wide range of time and length scales. In a series of studies, we explored self-diffusion mechanisms of associating polymers above the percolation threshold, by combining experimental measurements using forced Rayleigh scattering and analytical insights from a two-state model. Despite the differences in molecular structures, a universal super-diffusion phenomenon is observed when diffusion of molecular species is hindered by dissociation kinetics. The molecular dissociation rate can be used to renormalize shear rheology data, which yields an unprecedented time-temperature-concentration superposition. The obtained shear rheology master curves provide experimental evidence of the relaxation hierarchy in associating networks.   

Tuning the Assembly of Spherical Nanoparticles in Semicrystalline Polymers

We propose a simple, novel strategy to controlling nanoparticle (NPs) dispersion states in a semi-crystalline polymer matrix exploiting the kinetics of polymer crystallization. The system consists of poly(methyl methacrylate) grafted spherical silica NPs and poly(ethylene oxide) matrices, which are thermodynamically miscible in the melt. We first show that no remarkable change was observed in the spatial dispersion of NPs upon fast crystallization. However, for slow crystallization, both TEM and X-ray/neutron scattering reveal that the system starts to be organized in a ``layer-by-layer'' architecture, where the NPs are aligned in the amorphous phases intercalated by the crystalline lamellar phases. More importantly, we have found that the resulting ``sheet-like'' NP morphology gives rise to a 2-fold increase in the storage modulus but without compromising the fracture toughness of the neat polymer. These results open pathways for creating in-situ biomimetic hierarchical structures with improved mechanical properties through a simple, single-step crystallization processing, which could lead to new applications for this largest class of commercially relevant polymeric materials.   

Direct measurement of the critical pore size in a polymer membrane

The formation of pores is an important process in cellular membranes. Here we use freestanding polymer films as model membranes to study the stability of nucleated pores. Polymer membranes with pores of varying size are patterned using a lithographic technique. The membranes are heated above their glass transition temperature to allow viscous flow to occur. Pores with a radius larger than a critical value grow, while pores smaller than the critical radius are observed to shrink and eventually close. Remarkably, holes that are close enough to the critical radius neither grow nor shrink, even though the film is in the melt state. A simple model which takes into account the energy cost of having additional surface area at the edge of a pore describes the experiments with no free parameters. Biological membranes have an additional energetic cost of forming a pore, which we mimic using a lamellar-forming diblock copolymer. Indeed, we find that the critical pore radius is increased when pore formation is frustrated by molecular architecture.   

Pinpointing the onset of mechanical rejuvenation in a polymer glass by monitoring segmental dynamics before and after a constant strain rate pulse

Over time, dynamics in polymer glasses become slower through physical aging; it is thought that post-yield mechanical deformation may reverse the effects of physical aging in what has been termed ``rejuvenation''. We have monitored segmental dynamics in a poly(methyl methacrylate) glass before and after reversing constant strain rate pulses of varying magnitude to explore the onset of mechanical rejuvenation. We find that the segmental dynamics in the glass is unperturbed after pulses to $\varepsilon $/$\varepsilon _{\mathrm{yield}} \quad =$ 0.6 or less. For pre-yield pulses of higher magnitude, we find evidence of rejuvenation, which is indicated by faster dynamics after the pulse. We find that full rejuvenation only occurs at a strain of $\varepsilon $/$\varepsilon_{\mathrm{yield}} \quad =$ 3 or higher. This work is qualitatively consistent with recent simulations of Smessaert and Rottler and additionally shows quantitative agreement with predictions from the theory of Chen and Schweizer. However, in spite of observed enhanced dynamics on a molecular level, we find that large pre-yield pulses do not alter the mechanical response of the polymer during subsequent deformation. We explore the apparent contradiction between the macroscopic mechanical and molecular-level dynamical response of the glass to deformation.   

High resolution imaging of the dynamics of nanoparticles in/on liquids

Electron microscopy for the study of nanoscale structure and dynamics in solvated soft materials has only recently been proposed, and since this technique requires high vacuum, significant challenges must be confronted. Specimens can be encapsulated in vacuum-sealed devices for TEM but this approach is not without difficulties, including beam damage, cumbersome specimen handling, and propensity for wall artifacts. Here, we report an alternative SEM approach, obviating need for a liquid cell by exploiting the nonvolatility of ionic liquids, which is illustrated by visualizations of nanoscale dynamics for two solvated systems, dispersed nanospheres and nanorods in/on thin, free-standing IL films. The translational and rotational Brownian of these nanoparticles were quantitatively tracked. In ultra-thin films, a striking and unanticipated dynamical pairing of the nanospheres was observed, manifesting a balance of capillary and hydrodynamic interactions. Concentrated nanorods were seen to assemble into finite stacks that could be tracked over their entire lifetimes. Broadly applicable to solvated soft nanoscopic materials, the new imaging protocol offers a breakthrough in the study of their structure and dynamics.   

Predicting Themomechanical Responses of Polymer Thin Films and Nanocomposites via an Innovative Coarse-grained Approach

Understanding and predicting the thermomechanical responses of nanoscale polymer systems are very challenging as their responses are greatly influenced by many factors, such as interfacial energy, filler volume fraction and molecule weight, giving rise to the presence of nanoscale interface and free surface. To overcome these issues, here we employ a novel atomistically informed coarse-grained computational technique, called thermomechanically consistent coarse graining (TCCG), to investigate how the nanoscale interface and free surface influence the elastic modulus (E) and glass transition temperature (Tg) of polymer films and nanocomposites. By performing tensile tests and nanoindentation simulations, we are able to predict the size dependent elastic properties of polymer films and quantify the length scale of the local mechanical interphase. Finally, taking cellulose nanocrystal (CNC) and poly(methyl-methacrylate) (PMMA) nanocomposites as a relevant model system, we present a multi-scale framework built upon our CG approach to allow the prediction of Tg of nanocomposite as a function of interfacial energy and filler volume fractions by drawing the analogy between thin film and nanocomposites. Our established multi-scale framework is validated by recent experiments and breaks new ground in predicting, without any empirical parameters, key structure-property relationships for polymer nanomaterials.   

Nanoparticle Order through Entropic Confinement

As has been addressed in colloidal science, visual order transitions can be achieved with entropy contributions alone. Herein, entropy-driven ordering of nanoparticle (NP) structures is generated where entropy increase and visual order are achieved simultaneously. We study an ``athermal'' NP-polymer blends where NPs are densely grafted with polymer brush of the same chemical composition as the polymer matrix. Visual order of the NPs is induced by geometrically confining the thin film blends with meso-scale topographic patterns. When the residual layer thickness of the patterned blend films approaches the nanoparticle dimension, exclusive segregation of NPs to less confining imprinted mesa region occurs. This preferential segregation of NPs, defined by partition coefficient $K=$0, is attributed to purely entropic penalty, where $K$ denotes the particle density ratio at highly confined residual layer to that at mesa region. We further demonstrate $K$ is fully tunable and even invertible with increasing matrix chain dimension. The associated entropic free energy change ($\Delta F=-$ln$K)$ is calculated to explain NP segregation preference. Accordingly, variation of residual layer thickness and polymer matrix molecule size can both affect NP distribution among patterned thick and thin regions.   

Design of Bicontinuous Donor/Acceptor Morphologies for Use as Organic Solar Cell Active Layers

Two of the primary challenges limiting the marketability of organic solar cells are i) the smaller device efficiency of the organic solar cell relative to the conventional silicon-based solar cell and ii) the long term thermal instability of the device active layer. The achievement \textit{of equilibrium} donor/acceptor morphologies with the characteristics believed to yield high device performance characteristics could address each of these two challenges. In this work, we present the results of a combined simulations and experiments-based approach to investigate if a conjugated BCP additive can be used to control the self-assembled morphologies taken on by conjugated polymer/PCBM mixtures. First, we use single chain in mean field Monte Carlo simulations to identify regions within the conjugated polymer/PCBM composition space in which addition of copolymers can lead to bicontinuous equilibrium morphologies with high interfacial areas and nanoscale dimensions. Second, we conduct experiments as directed by the simulations to achieve such morphologies in the PTB7 $+$ PTB7-$b$-PNDI $+$ PCBM model blend. We characterize the results of our experiments via a combination of transmission electron microscopy and X-ray scattering techniques and demonstrate that the morphologies from experiments agree with those predicted in simulations. Accordingly, these results indicate that the approach utilized represents a promising approach to intelligently design the morphologies taken on by organic solar cell active layers.   

Predicting the Phase Behavior of Polymer Nanocomposites using Field-Based Simulations

Polymer nanocomposites (PNCs) have been shown to improve mechanical, electric, and optical properties, which are not achievable with polymers or nanoparticles alone. The parameter space associated with PNCs is vast and having efficient tools to study and characterize PNCs is critical to understand parameter-structure-property relationships. In recent years, we have extended the powerful polymer field theory framework to capture particle correlations and allow for efficient characterization of PNCs. We have made numerous strides in extending the class of PNC systems that are able to be studied with polymer field theory; namely, nanospheres, nanorods, complex grafted particles, and liquid crystals. In this talk, I provide details in developing this framework and illustrate its potential by demonstrating its applicability to bulk polymer nanocomposite systems where we can relax the mean-field approximation, study systems with several nanoparticles, and systems that can macro- or microphase separate (e.g. polymer blends or block copolymers). I will also discuss recent advances we have made in incorporating dynamics into our framework which has exciting implications in understanding the phase behavior of PNCs.