Session V34: Where Simulation, Theory, and Experiments Meet Across Length Scales II

What drives hydrophobic polymer collapse and re-entry transitions in miscible good solvents?

Herein, we study co-nonsolvency of poly(N-isopropylacrylamide) (PNiPAM) in methanol aqueous solutions. Our results show that both the coil-to-globule transition at low methanol concentrations and the globule-to-coil re-entrance at high methanol concentrations are entropy driven. At low alcohol content, methanol preferentially binds to the PNiPAM globule and drives polymer collapse. Rather than being driven by electrostatic, hydrogen bonding or bridging-type interactions with the globule, preferential methanol binding is found to result from a significant increase of the chain configurational entropy, stabilizing methanol-enriched globular structures over wet globular structures in neat water. The globule-to-coil re-entrance at high methanol concentrations is instead driven by changes in solvent-excluded volume of the coil and globular states imparted by a decrease in solvent density with increasing methanol content of the solution. The co-nonsolvency mechanism proposed in this contribution provides a new angle on how to develop Coarse Grained simulation models for responsive soft matter systems. Moreover, several of the solvation effects described in this contribution can be incorporated in theories for cosolvent-induced conformational transitions in dilute polymer solutions.   

Co-non-solvency: Depletion forces or preferential adsorption?

Co-non-solvency is a phenomenon that occurs when a polymer is added to a mixture of two (perfectly) miscible and competing good solvents. As a result, the same polymer collapses into a globule within intermediate mixing ratios. More interestingly, polymer collapses despite the fact that the solvent quality remains good or even gets increasingly better by the addition of the better cosolvent [1]. This puzzling phenomenon, where the solvent quality is completely decoupled from the polymer conformation, is driven by strong local preferential adsorption of the better cosolvent to the polymer [1,2,3]. Because a polymer collapses in good solvent, the depletion forces, that are responsible for standard poor solvent collapse, do not play any role in describing co-non-solvency [4]. [1] D. Mukherji and K. Kremer, Macromolecules (2013). [2] D. Mukherji, C. M. Marques, and K. Kremer, Nature Communications (2014). [3] D. Mukherji, C. M. Marques, T. Stuehn and K. Kremer, Journal of Chemical Physics (2015). [4] T. E. de Oliveira, P. A. Netz, D. Mukherji, and K. Kremer, Soft Matter (2015).   

Depleting depletion: Polymer swelling in poor solvent mixtures

A polymer collapses in a solvent when the solvent particles dislike monomers more than the repulsion between monomers. This leads to an effective attraction between monomers, also referred to as depletion induced attraction. This attraction is the key factor behind standard polymer collapse in poor solvents. Strikingly, even if a polymer exhibits poor solvent condition in two different solvents, it can also swell in mixtures of these two poor solvents. This collapse-swelling-collapse scenario is displayed by poly(methyl methacrylate) (PMMA) in aqueous alcohol. Using molecular dynamics simulations of a thermodynamically consistent generic model and theoretical arguments, we unveil the microscopic origin of this phenomenon. Our analysis suggests that a subtle interplay of the bulk solution properties and the local depletion forces reduces depletion effects, thus dictating polymer swelling in poor solvent mixtures.   

Effects of dipolar interactions in polymer brushes

Effects of dipolar interactions on structure of polymer brushes are studied using a combination of semi-analytical theory, numerical simulations based on the self-consistent field theory (SCFT) and experiments. In this talk, insights obtained by studying brushes in the presence and absence of various polar solvents will be discussed. Possibility of vertical phase segregation in planar polymer brushes immersed in polar solvents and interpenetration as well as forces between opposing brushes will be discussed.   

Molecular dynamics simulations of poly (ethylene oxide) hydration and conformation in solutions.

Polyethylene oxide (PEO) is one of the most actively used polymers, especially in biomedical applications due to its high hydrophilicity, biocompatibility and potency to inhibit protein adsorption. PEO solubility and conformation in water depends on its capability to form hydrogen bonds. Using atomistic molecular dynamics simulations we investigated the details of water packing around PEO chain and characterized the type and lifetime of hydrogen bonds in aqueous and mixed solvent solutions. The observed polymer chain conformation varies from an extended coil in pure water to collapsed globule in hexane and a helical-like conformation in pure isobutyric acid or isobutyric acid --water mixture in agreement with experimental observations.~~We'll discuss the implications of protic solvent arrangement and stability of hydrogen bonds on PEO chain conformation and mobility.   

A Coarse-Grained Model for Thermoresponsive Poly(\textit{N}-isopropylacrylamide)

Poly(\textit{N}-isopropylacrylamide) (PNIPAM) is a thermoresponsive polymer that undergoes a phase transition at its lower critical solution temperature (LCST). Although atomistic simulations have been effective to study PNIPAM single chains in solution, they are limited in reaching longer length- and time-scales. In this work, a coarse-grained (CG) model is developed for PNIPAM that captures its thermoresponsive behavior. Nonbonded parameters are fit to experimental thermodynamic data, with minor adjustments to provide better agreement with radial distribution functions from atomistic simulations. Bonded parameters are fit to probability distributions from atomistic simulations using multi-centered Gaussian-based potentials. The temperature-dependent potentials derived for the CG model in this work properly capture the coil-globule transition of PNIPAM single chains and yield a chain-length dependence consistent with atomistic simulations and experiment. The self-assembly of PNIPAM surfactants is also explored. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.   

A Coarse Grained Model for Methylcellulose: Spontaneous Ring Formation at Elevated Temperature

Methylcellulose (MC) is widely used as food additives and pharma applications, where its thermo-reversible gelation behavior plays an important role. To date the gelation mechanism is not well understood, and therefore attracts great research interest. In this study, we adopted coarse-grained (CG) molecular dynamics simulations to model the MC chains, including the homopolymers and random copolymers that models commercial METHOCEL A, in an implicit water environment, where each MC monomer modeled with a single bead. The simulations are carried using a LAMMPS program. We parameterized our CG model using the radial distribution functions from atomistic simulations of short MC oligomers, extrapolating the results to long chains. We used dissociation free energy to validate our CG model against the atomistic model. The CG model captured the effects of monomer substitution type and temperature from the atomistic simulations. We applied this CG model to simulate single chains up to 1000 monomers long and obtained persistence lengths that are close to those determined from experiment. We observed the chain collapse transition for random copolymer at 600 monomers long at 50C. The chain collapsed into a stable ring structure with outer diameter around 14nm, which appears to be a precursor to the fibril structure observed in the methylcellulose gel observed by Lodge \textit{et al.} in the recent studies. Our CG model can be extended to other MC derivatives for studying the interaction between these polymers and small molecules, such as hydrophobic drugs.   

Modeling helical polymer brushes using self-consistent field theory (SCFT)

We investigate structure of helical polymer brushes in terms of segment density distribution and local helical ordering using SCFT. A flexible chain model with vector potential was used to model liquid crystalline-like ordering in the brushes. The effects of surface grafting density, polymer molecular weight and the solvent quality on the brush structure were investigated. For densely grafted polymer brushes or the brushes made up of high molecular weight polymers, immersed in good quality solvent, stronger orientational ordering was found near the edge of the brushes (i.e., far from the grafting surface). Furthermore, an increase in the orientational ordering near the grafted end was found with decrease in solvent quality or decrease in molecular weight and decrease in surface grafting density.   

The effects of bonded interactions on the structural phase properties of flexible elastic homopolymer

By means of advanced parallel-tempering replica-exchange Monte Carlo methods we systematically examine the effects of an asymmetric bond potential between the bonded monomers on the structural formations of an elastic flexible polymer model. Employing microcanonical inflection-point analysis and conformational analysis based on a suitable set of structural order parameters, we identify diverse structural phases in the low-temperature region of the microcanonical hyperphase diagram. In addition to the icosahedral phase occurring if the symmetry of the bonded interaction is broken by strong bonded Lennard-Jones potential, amorphous structures with bihexagonal cores appear for small values of the asymmetry control parameter in the bond potential. Another remarkable feature is the observation of the hierarchy of freezing transitions associated with the formation of the surface layer after nucleation.   

Crystal Growth in Lennard-Jones Mixtures: A Model System to Study Generic Effects in Biomineralization Processes

In various scientific fields regulating the growth of crystalline structures and tuning their morphologies plays an important role. E.g. in pharmaceutical delivery the crystallization of a supersaturated drug solution is inhibited by the addition of stimuli-responsive polymers. While past simulation studies rather focused on a detailed understanding of the binding modes of specific additives to likewise specific crystal surfaces here we investigate the characteristics of a generic model system in which we modify the growth mechanisms and the emerging shapes of Lennard-Jones crystallites by tuning the specific interaction parameters and/or adding polymer chains - represented by linear bead-spring molecules - to the system. We performed molecular dynamics simulations on samples containing a crystalline phase embedded in a supersaturated solution applying an adaptive simulation scheme in order to keep the chemical potential difference between the solid and the surrounding liquid constant. We report on different crystal properties depending on a systematic variation of simulation parameters as the solvent content, the solubility and the density of polymer chains. We analyze our results by means of various approaches within the framework of non-equilibrium statistical physics.   

Thermodynamics of polymer nematics described with a worm-like chain model: particle-based simulations and SCF theory calculations

Polymer liquid crystals, apart from traditional applications as high strength materials, are important for new technologies, e.g. Organic Electronics. Their studies often invoke mesoscale models, parameterized to reproduce thermodynamic properties of the real material. Such top-down strategies require advanced simulation techniques, predicting accurately the thermodynamics of mesoscale models as a function of characteristic features and parameters. Here a recently developed model [1] describing nematic polymers as worm-like chains interacting with soft directional potentials is considered. We present a special thermodynamic integration scheme delivering free energies in particle-based Monte Carlo simulations of this model, avoiding thermodynamic singularities. Conformational and structural properties, as well as Helmholtz free energies are reported as a function of interaction strength. They are compared with state-of-art SCF calculations [2] invoking a continuum analog of the same model, demonstrating the role of liquid-packing and fluctuations. [1] P. Gemünden and K.Ch. Daoulas, Soft Matter 2015, 11, 532; [2] Y. Jiang and J.Z.Y. Chen, Macromolecules 2010, 43, 10668.   

Molecular Dynamics Modeling of Dielectric Polarization and Ferroelectricity in Poly(vinylidene fluoride) and Related Polymers

Molecular dynamics studies of the dielectric polarization response of a constrained bond length and bond angle, united-atom-based model of lamellar crystals of poly(vinylidene fluoride) (PVDF) are reported. Classical ferroelectricity is observed in PVDF, and when variations in the basic PVDF-like interaction parameters are allowed, a transition between classical and relaxor ferroelectricity is found to depend systematically on the polymer repeat unit dipole moment and on the united atom radius of the non-CH$_{2}$ functional group. The effects of step and ramp electric field reversal are studied. A complicated sequence of reorientation processes occurs over a wide range of time scales, including a weak, temperature-independent response of 1-2 ps duration associated with local torsional motion, followed by a slow-rising delay regime lasting 10s of ns or longer that involves trans-gauche (TG) transitions in the amorphous phase. After the delay, a large-amplitude primary reorientation occurs over a relatively short additional duration (0.1 to 2 ns), which is due to rotation of large sub-segments in the crystalline phase with few TG transitions. The overall sequence concludes with a slow terminal rise lasting several 100s of ns involving an improvement in crystalline order.