Polymer-Induced Inverse-Temperature Crystallization of Nanoparticles on a Substrate

Abstract

Using molecular dynamics simulations, we study the properties of liquid state polymer-nanoparticle composites confined between two parallel substrates, with an attractive polymer-substrate interaction. Polymers are in the semidilute regime at concentrations far above the overlap point, and nanoparticles are in good solvent and without enthalpic attraction to the substrates. An increase of temperature then triggers the crystallization of nanoparticles on one of the two substrate surfaces-a surprising phenomenon, which is explained in terms of scaling theory, such as through competing effects of adsorption-and correlation blobs. Moreover, we show that the first, closely packed layer of nanoparticles on the substrate increases the depletion attraction of additional nanoparticles from the bulk, thereby enhancing and stabilizing the formation of a crystalline phase on the substrate. Within the time frame accessible to our numerical simulations, the crystallization of nanoparticles was irreversible; that is, their crystalline phase, once created, remained undamaged after a decrease of the temperature. Our study leads to a class of thermoreactive nanomaterials, in which the transition between a homogeneous state with dissolved nanoparticles and a surface-crystallized state is triggered by a temperature jump.

Adding Nonconnected Monomers to Manage the Layering Crystallization of Polymers on Athermal Substrate

“…Finally, 2D thin layer of polymer chains, with thickness as large as about one monomer diameter, develops at the substrate surface when system is in the concentrated regime of polymers . However, polymer chains beyond the first layer keep their 3D structure, and the corresponding monomer distribution is thereby amorphous without order appearing in the dimension perpendicular to the substrate surface . In this communication, after a large scale of molecular dynamics (MD) simulations and theory analysis, we confirm and clarify that layering crystallization of polymers beyond the first layer can be intrigued after blending polymers with nonconnected monomers, and the thickness of polymer layering crystal is targetedly controllable.…”

Thin polymer-layer decorated, structure adjustable crystals of nanoparticles

“…The presence of the substrates can significantly alter the equilibrium phases that have been developed in bulk polymer-NP mixtures. [16][17][18][19][20] Our MD simulations show that controlling the competition from different kinds of entropies is a possible way to achieve the desired phase behaviors of NP mixing with athermal polymer chains confined between two hard substrates. As shown in the part (a) of Fig.…”

“…For samples of polymer-NP mixtures on a substrate or confined between two parallel walls, previous theory and experimental studies have mostly shown that NPs prefer to aggregate on the surface due to the polymer induced depletion of attraction between NPs and substrate and among NPs. [15][16][17][18][19][20] NPs and the substrate surface give a minimum restriction on the local conformation of polymer segments after phase separation between polymer chains in the central region and NPs at the substrate surfaces. In this case, the order pattern of crystallized NPs is solely decided by the geometric structure of the substrate surface.…”

“…From a previous study on pure polymer chains under confinement, we already know that the range of the depletion region between the substrate surfaces and the most approaching mass center of polymer chains, contracts with increasing the polymer concentration. 20,35 It completely disappears when the bulk polymer concentration is larger than a certain value, which is variable depending on the polymeric degree of chains. In such cases, those polymer chains touching the substrates are flattened against the substrate surfaces.…”

Entropic Effects in Polymer Nanocomposites

“…The effective attraction between colloids induced by polymer depletion force was reduced, because the polymer radius of gyration decreased as the θ -temperature was approached, which raised the effective temperature, leading to ‘melting’ of colloidal gels. Similarly, Cao et al [ 88 ] simulated the mixture of nanoparticles and polymer confined between sandwiched substrates, demonstrating that nanoparticles aggregated in the center at a low temperature, but were absorbed by substrate and created a ‘crystalline’ layer at a higher temperature. The inverse temperature crystallization of nanoparticles was also originated from the entropic depletion attraction between nanoparticle and the substrate, which leads to a gain of free energy.…”