Simcha Srebnik

Technion - Israel Institute of Technology, Chemical Engineering, Faculty Member

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Papers by Simcha Srebnik

Vector Imitation Model of Semiflexible Polymers: Application to Polymer Adsorbed on a Spherical Particle

Macromolecules, 2007

We introduce a new type of model that imitates polymer behavior of semiflexible polymers. In this... more We introduce a new type of model that imitates polymer behavior of semiflexible polymers. In this model, the energy and entropy contributions to the free energy are treated as forces that act on the individual monomers, and thus guide the "movement" or "growth" of the chain. The model falls at the interface between theory and simulation, allowing statistical prediction of polymer conformational behavior of long polymer chains at a small computational cost. Results are compared with theoretical predictions by Spakowitz and Wang (SW) (Phys. ReV. ) of the behavior of semiflexbile chain confined to the surface of a sphere and our own Monte Carlo simulations. The SW and Monte Carlo results are reproduced with the imitation model with surprising accuracy. The simple nature of the imitation model allows us to carry out calculations for very long chains, with which we show that the surface coverage of the sphere is a non-monotonic function of persistence length for a given fixed chain length, distinguishing between random, wrapping, and ring conformations.

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Conformational behavior of polymers adsorbed on nanotubes

Chemical Physics, 2008

The importance of hydrophobic interactions in determining polymer adsorption and wrapping of carb... more The importance of hydrophobic interactions in determining polymer adsorption and wrapping of carbon nanotubes is still under debate. In this work, we concentrate on the effect of short-ranged weakly attractive hydrophobic interactions between polymers and nanotubes (modeled as an infinitely long and smooth cylindrical surface), neglecting all other interactions apart for chain flexibility. Using coarse-grained Monte Carlo simulation of such simplified systems, we find that uniform adsorption and wrapping of the nanotube occur for all degrees of chain flexibility for tubes with sufficiently large outer radii. However, the adsorbed conformations depend on chain stiffness, ranging from randomly adsorbed conformations of the flexible chain to perfect helical or multihelical conformations (in the case of more concentrated solutions) of the rigid chains. Adsorption appears to occur in a sequential manner, wrapping the nanotube nearly one monomer at a time from the point of contact. Once adsorbed, the chains travel on the surface of the cylinder, retaining their helical conformations for the semiflexible and rigid chains. Our findings may provide additional insight to experimentally observed ordered polymer wrapping of carbon nanotubes.

$FIG. 8. (Color online) Normalized end-to-end distance vs number of persis- tence units n,=L/I,. Prediction of Kratky—Porod wormlike chain model (Ref. 31) (solid curve) and our MC simulations (circles, different shades represent chains of different contour length L), respectively. The limits of the rigid rod regime at low values of L/1, ((R?)!’? ~ NI) and flexible regime ((R?)'?~N'1) at high values of L/1, are shown by the thin solid lines. The arrows indicate the values of « corresponding to the transitions observed in the helical correlation length. This two-step transition is further corroborated by the fluctuations in adsorbed fraction and energy per segment as a function of «, shown in Fig. 9. The peak in the fluctuations of adsorbed fraction demonstrates a transition from weak to strong adsorption at low values of «. Energy fluctuations are characterized by two distinct peaks; one following the ad- sorption transition and another at around «=40. These results$

$FIG. 9. Fluctuations in energy and adsorbed fraction as a function of kappa. R,=20 and N,=1.$

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Structural effect of sugars on water

The modulation of the structure of liquid water by solutes has tremendous consequences in numerou... more The modulation of the structure of liquid water by solutes has tremendous consequences in numerous fields, particularly on the stability of proteins. However, the reasons for the differences in effects of similar solutes are still unclear. Recently, Livney and coworkers [1] found a strong relationship between the hydration layer of sugars and its effect on the phase transition of a model polymer, which may be explained by the strong interaction between water and sugars leading either to cooperative structuring of the water and thus to large hydration numbers, or disrupting water structure near the sugar, resulting in lower hydration. Using atomistic Monte Carlo simulation, we studied the compatibility of various sugars with an ideal tetrahedral water structure, as embodied in hexagonal ice. Our simulations suggest the following order of compatibility with ideal water structure: galactose > glucose > mannose. In agreement, experimental measurements of isentropic compressibility show the same order of hydration numbers and kosmotropic effect. A simple physical model of the binary system is used to shed further insight on the structuring effect of sugars on water. 1. Shpigelman, A.; Portnaya, I.; Ramon, O.; Livney, Y. D. J Polym Sci Part B: Polym Phys 2008, 46, 2307-2318.

Conformational behavior of polymers adsorbed on nanotubes

We study the interaction of a dilute solution of semiflexible polymers with a weakly attractive i... more We study the interaction of a dilute solution of semiflexible polymers with a weakly attractive infinitely long nanotube using Monte Carlo simulation. Apart for bending stiffness of the polymer chains, the only interactions considered in our model are weakly attractive short-ranged Lennard-Jones interactions between the monomers and with the surface. These nonspecific interactions are found to result in stable helical and multi-helical adsorbed conformations for semiflexible chains. Adsorption of these chains is found to occur in a sequential manner through tight wrapping of the polymer around the nanotube. Adsorption occurs quickly and is characterized by a sharp peak in the heat capacity. A second transition follows whereby opening and reorganization of the adsorbed chains into nearly perfect helices and multiple helices. Extension of the model to block and triblock copolymers reveals rich conformational behavior. These results are discussed on physical grounds and implications towards polymer-carbon nanotubes composites are offered.

$FIG. 9. Fluctuations in energy and adsorbed fraction as a function of kappa. R,=20 and N,=1.$

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Conformational behavior of semi-flexible polymers confined to a cylindrical surface

Chemical Physics Letters, 2006

Uniform wrapping of a polymers around carbon nanotubes has been recently reported as a means to s... more Uniform wrapping of a polymers around carbon nanotubes has been recently reported as a means to separate individual nanotubes from their natural bundled state. It has been suggested that polymers wrap the nanotubes in a helical manner due to various interactions, including p-stacking, hydrophobic and van der Waals. In this communication, we show that surface curvature plays an important role in stabilizing helical conformations. We use Monte Carlo simulations supported by simple thermodynamics arguments to reveal that semiflexible chains adopt ordered helical conformations at a narrow range of radii, depending only on the stiffness of the chain and cylinder radius.

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Adsorption-Freezing Transition for Random Heteropolymers near Disordered 2D Manifolds due to ``Pattern Matching

Physical Review Letters, 1996

We study the simplest model for random heteropolymers near chemically disordered surfaces, and pr... more We study the simplest model for random heteropolymers near chemically disordered surfaces, and predict a new phenomenon that may have implications for pattern recognition applications. As the strength of surface fluctuations increases a smooth adsorption transition occurs first, with a multitude of chain conformations being sampled. More interestingly, beyond a larger threshold value of the strength of surface fluctuations, a sharp second adsorption transition is observed that is accompanied by the polymer freezing into a few dominant conformations that match the pattern of the surface fluctuations. [S0031-9007(96)

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$Research paper thumbnail of Adsorption-Freezing Transition for Random Heteropolymers near Disordered 2D Manifolds due to {open_quote}{open_quote}Pattern Matching{close_quote}{close_quote$

Adsorption-Freezing Transition for Random Heteropolymers near Disordered 2D Manifolds due to {open_quote}{open_quote}Pattern Matching{close_quote}{close_quote

Physical Review Letters, 1996

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Random heteropolymer adsorption on disordered multifunctional surfaces: Effect of specific intersegment interactions

Chemical Physics, 1998

Biopolymers adsorb on cell and virus surfaces with great specificity. Recently, theoretical and c... more Biopolymers adsorb on cell and virus surfaces with great specificity. Recently, theoretical and computational studies have inquired as to whether there are any universal design strategies that nature employs in order to affect such recognition. Specifically, the efficacy of multifunctionality and quenched disorder as essential design strategies has been explored. It has been found that when random heteropolymers interact with disordered multifunctional surfaces, a sharp transition from weak to strong adsorption occurs when the statistics characterizing the sequence and surface site distributions are related in a special way. The aforementioned studies consider the intersegment interactions to be much weaker than the surface site interactions. In this work we use nondynamic ensemble growth Monte Carlo simulations to study the effect of the competition between frustrating intersegment and segment-surface interactions. We find that as the intersegment interactions become stronger, the transition from weak to strong adsorption occurs at higher surface disorder strengths. This trend is reversed beyond a threshold strength of the intersegment interactions because interactions with the surface are no longer able to ''unravel'' the dominant conformations favored by the intersegment interactions.

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Pore Size Distribution Induced by Microphase Separation: Effect of the Leaving Group during Polycondensation

Chemistry of Materials, 2001

A mean field theoretical model is developed to investigate the porosity of cross-linked copolymer... more A mean field theoretical model is developed to investigate the porosity of cross-linked copolymer networks formed by polycondensation. It is found that cross-linking induces microphase separation, and when accompanied by polycondensation, the pore size distribution is affected in an unexpected manner. Pores much larger than the size of the leaving groups are predicted when the leaving group is large. The pore distribution in such cases is found to be multimodal, exhibiting a peak for small pores, a second narrow peak for pores corresponding to the monomer size, and a third peak for very large pores. The model also predicts the extent of microphase separation of the chemically different monomers as a function of the chemical constitution, the degree of cross-linking, and the size of the leaving group.

format_quoteLarger leaving groups lead to increased density fluctuations and pore size variation in cross-linked networks, suggesting a method to control porosity.format_quote

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Theoretical Investigation of the Imprinting Efficiency of Molecularly Imprinted Polymers

Chemistry of Materials, 2004

Molecular imprinting is an emerging tool for the design of structured porous materials having a p... more Molecular imprinting is an emerging tool for the design of structured porous materials having a precise arrangement of functional groups within pores of a controlled size and shape. Such controlled specificity in principle can offer a scope of opportunities for moleculespecific recognition applications. In practice, however, molecular recognition is often not fully realized, either due to distortion during the imprinting process or due to incomplete imprinting. Using a mean-field lattice model, we study imprinting efficiency of tetrafunctional monomers using stiff imprinting agents of various sizes and for various preparation conditions. Neglecting imperfections and distortions during gelation and post-treatment, we show that high imprinting efficiencies (i.e., a large number of pores of the needed size and functionality) are hard to achieve. However, monomer-template interactions and preparation conditions can be optimized for a given template size to yield a higher population of high affinity sites.

$Figure 2. (a) Number fraction and (b) volume fraction of imprinted pores as a function of the size of the imprinting molecule. Curves correspond to (— HM —) yv = x%rF= 0; (- a-) Xv = 2, xr = 0; (— @—) xv = 2, Yr = 53 (— © -) XV = Yr = 5; (-O-) xv = 8, Yr = 53 (~A-) Xv = 2, YF = —5; (“O-) Xv 2, XF —10.$

$Figure 3. Volume fraction of fully functional pores as a function of pore size for various preparation conditions. Parameter values are identical to those given in Figure 2 (—B—-) xv =x%F = 0; (— A) Xv = 2, YF = 0; (— @—-) XV = 2, te = 5; (-— @-) xv = xr = 5; (-O-) xv = 8, xr = 55 (— A-) Xv = 2, Xr 5; (0 —-)xyv= 2, XF >= —10.$

$Figure 4. Order parameter indicating the fraction of im- printed pores that are fully functional. Parameter values are identical to those given in Figure 2 (— Hl —) yv=Y%r=0;(— a—) Xv = 2, Xr = 0; (— @—) xv = 2, YF = 53 (— O—) XV = KF = S; ((“O-) v= 8, xr = 53 (— A) xv = 2, Yr = —5;(“O-) xv = 2,%¥r = —10.$

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