Molecular liquids formed by nanoparticles

Highlights

• Polymethylsilsesquioxanes are nanoobjects which form a viscoelastic liquid.

• Molecular weight dependence is described by a power law with unusual high exponent.

• Nanoparticles occupy an intermediate position between macromolecules and colloids.

• In decreasing temperature, these nanofluids loss fluidity and form a physical gel.

Abstract

The rheology of a rather special case of molecular nanoliquids formed by polymethylsilsesquioxane nanoparticles which occupy an intermediate position between colloidal particles and macromolecules has been studied. These objects are liquids, homogeneous up to submicron size in a wide temperature range. They demonstrate viscoelastic behavior with very strong dependence of viscosity on molecular weight of these nanoobjects. A new scaling model based on the concept of friction in viscoelastic outer layers describing this kind of behavior has been proposed. Relaxation properties of smaller nanoparticles can be described by a single-mode Maxwell model, while relaxation for larger particles takes place in a wide frequency rage covering three orders. Interpretation of the temperature dependence of viscosity within the framework of the standard WLF equation allowed us to find the “glass” (or gel)transition point. Transition from fluid to global gel-like state was clearly observed at this temperature. As part of finding on the nature of this transition, it was shown that no structural effects are related to this transition and it should be treated as a relaxation phenomenon.

Introduction

The emergence of new polymeric forms, such as hyperbranched systems and their regular counterparts – dendrimers, multiarm stars, dense macromolecular brushes, and molecular nanogels, that is, everything that we call nanoobjects [1], requires a comprehensive study of their properties and interpretation of the results obtained in the coordinates of the classical polymer science.

Pioneering research of dendrimer rheology immediately demonstrated their unusual solution and bulk behavior [[2], [3], [4]], and lately unique results were obtained [5,6] for the behavior of carbosilane dendrimers in the bulk.

The subject of our research was polymethylsilsesquioxane nanosized densely cross-linked polycyclic formations, the transition of which to cross-linked systems of infinite size is artificially limited by blocking agents [7]. The number of similar nanoobjects is gradually increasing, and most importantly, many of the previously known objects that have many practical applications can be unambiguously attributed to this type (MQ resins, many organoelement systems), and, therefore, the study of their rheological properties is highly relevant.

Interest in these objects also arises from the fact that they lie in the borderline between the polymers and colloidal particles [8,9]. The thermodynamics and the phase state of such systems were an object of intensive investigations [[10], [11], [12]] as well as their behavior in solutions [13], rheological properties of MQ-systems [14], and correlation between their phase state and the rheological properties [15]. These materials are of wide practical interest including oil recovery [16], improving compatibility of polymer blends [17], platform for targeted drug-delivery [18] and others. Different aspects of silica-soles processing and various fields of their applications were discussed in [19].

Earlier we dealt with particles of a rigid silica core and trimethylsiloxane shell – molecular silica-sols [[20], [21], [22]], in this work molecular nanoparticles with polymethylsilsesquioxane core and trimethylsiloxane shell were investigated. That is, the outer shell of the new objects remained the same, but the rigidity of the core became substantially less with the replacement of the silica structure by the methylsilsesquioxane one, as the functionality of monomer units changed from 4 to 3. This led to the fact that in the whole range of the core-shell ratios the synthesized systems remained liquids, which allowed us to investigate these objects in a wide range of molecular weights and core-shell ratios. At the same time, they remained close analogues of particles with silica cores, retained a compact form, core-shell morphology, and very low intrinsic viscosity [13]. These objects could be classified as nanoparticles, but are completely different from solid nanoparticle, such as metal oxides, carbon allotropic derivatives (nanotubes, fullerenes, graphenes), some clays.

Thus, the subject of this study was quite new nanoobjects, namely fluids formed by polymethylsilsequioxane nanoparticles, which stay homogeneous up to submicron size level and can flow in a wide temperature range. We expected that the mode of flow of these molecular liquids is different from the mechanism of flow of usual long-chain linear polymers and therefore it was necessary to understand the nature of their viscosity-MW dependence.