Molecular liquids formed by nanoparticles
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.
Section snippets
Objects and methods
The object of this study were polymethylsilsesquioxane nanoparticles, synthesized as described earlier [7,13] by condensation and blocking of hyperbranched polymethylethoxysiloxane. Principle stages of synthesis and structure are shown in Fig. 1. Samples of different molecular weight were obtained by varying condensation time.
Main characteristics of the samples under study are summarized in Table 1 where the objects of the study are denoted as S-samples and the subscript designates approximate
Viscosity at room temperature
The results of measuring viscous properties are shown in Fig. 3 for room temperature. All samples are Newtonian liquids and the viscosity strongly increases with the growth of MW as shown in Fig. 4.
The dependence of the viscosity on MW as presented in Fig. 4 has a typical power law type, but the exponent is very high, close to 8.3, that is rather unusual for the η(M) dependence.
At present, there is more or less general agreement about rheological properties of linear polymers [[25], [26], [27]
Conclusion
- 1.
The study of polymethylsilsesquioxane nanoparticles showed that these objects are viscoelastic liquids in the temperatures above 0 °C in opposite to usual nanoparticles which are always solids.
- 2.
The viscosity of these nanoliquids strongly depends on the size of particles and in flow they demonstrate their dual nature (like colloidal particles and like liquids). Unusual strong dependence of the viscosity on MW is explained by a new molecular model based on scaling arguments.
- 3.
According to the
Acknowledgements
The authors are grateful for the financial support of Russian Science Foundation (grant 17-79-30108).
Synthesis of polymethylsilsesquioxane nanoparticles was supported by the Ministry of Science and Higher Education of the Russian Federation (Grant of the Government of the Russian Federation No. 14.W03.31.0018).
Polymethylsilsesquioxane nanoparticles were examined within the State Program of TIPS RAS.
The authors are grateful for measuring particle size distribution to Mr. M. Kuzin from TIPS RAS (
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