Dynamics and Transport Properties in Polymer Nanocomposites: Role of Interfacial Entropic and Enthalpic Effects
Polymers embedded with nanoparticles polymer nanocomposites (PNCs) have emerged as a new class of hybrid materials, which combine the unique electronic, mechanical, magnetic, catalytic, and optical properties of nanoparticles with the flexibility and processability of polymers, resulting in materials with novel and much improved properties. Experimental investigations accompanied by various theoretical and computational efforts have contributed to the fundamental understanding of the thermal, mechanical, and rheological properties of such PNCs, especially at lower NP loadings. Given the diverse properties of such materials, several potential applications have also emerged. Apart from their varied applications, the soft nanoparticle-polymer composites are a platform of rich physics involving subtle entropic and enthalpic effects which eventually determine their thermo-mechanical properties. In this thesis, we studied complex interplay of interfacial entropic-enthalpic effects and nanoparticles on temperature and time-dependent dynamical changes in PNCs. A short description of the works presented in this thesis is given below. In Chapter 1, we discuss various types of interactions present in polymer nanocomposite mixtures. The theoretical background of entropic and enthalpic interactions in a binary mixture is discussed in this chapter. A brief discussion on polymer dynamics and confinement-induced finite-size effects have been introduced. Chapter 2 deals with the materials and methods used in this thesis. In Chapter 3, we discuss a systematic study of segmental dynamics in polymer nanocomposites of polystyrene (PS) and 5 nm diameter polymer grafted nanoparticles (PGNPs) using quasi-elastic neutron scattering (QENS). It provides spatial and temporal information about small length scales (~1 nm) and fast time scales (~1 ns) and, therefore, at temperatures above the glass transition. For athermal PNCs, consisting of PGNPs embedded in chemically identical polymers, interface wettability and matrix chain penetration into the grafted chain layer (or thickness of interface layer, IL) is enhanced with increasing entropic compatibility between the graft and matrix chain. The IL properties are altered by changing the grafted to matrix polymer size ratio, f which in turn changes the extent of matrix chain penetration into the grafted layer. So, the interfacial length between matrix polymer on these length PGNPs are playing a crucial role for the dynamics and time scales polymer segmental motion in bulk PNCs. In Chapter 4, we extend the discussions into a confined PNC system. Since it is widely known that various properties of these thin films, especially their thermo-mechanical behavior, can be considerably different from the bulk depending on the thickness and interaction with surrounding media, it is imperative to study these properties directly on the films. However, quite often, it becomes difficult to perform these measurements reliably due to a dearth of techniques, especially to measure mechanical or transport properties like the viscosity of thin polymer or PNC films. Here we explore the complex interplay of two interfacial widths -film/substrate interface and graft/matrix chain interface - on the viscosity of confined PNC thin films through careful atomic force microscopy (AFM). We demonstrate a new method to study the viscosity of PNC thin films using atomic force microscopy-based force-distance spectroscopy. Using this method, we investigated viscosity and the glass transition, Tg, of PNC thin films consisting of polymer grafted nanoparticles (PGNPs) embedded in un-entangled homopolymer melt films. The PGNP–polymer interfacial entropic interaction parameter, f, operationally controlled through the ratio of grafted and matrix molecular weight, was systematically tuned while maintaining good dispersion even at very high PGNP loadings, ϕ. We observed a significant reduction (low f) and giant enhancement (high f) in the viscosity of the PNC thin films, with the effect becoming more prominent with increasing ϕ. This work thus not only demonstrates the tunability of the interfacial entropic effect to facilitate a dramatic change in the viscosity of PNC coatings, which could be of great utility in various applications of these materials but also suggests a new regime of viscosity variation in athermal PNC films indicating the possible need for a new theoretical model. In Chapter 5, we discuss a facile method to prepare PGNPs-based high-density functional polymer nanocomposites using thermal activation of a high-density PGNPs monolayer to overcome entropic or enthalpic barriers to the insertion of PGNPs into the underlying polymer films. The key challenge is to attain a high loading while maintaining reasonable dispersion to attain the maximum possible benefits from the functional nanoparticle additives. We monitor the temperature-dependent kinetics of penetration of a high-density PGNP layer and correlate the penetration time to the effective enthalpic/entropic barriers. Repeated application of the methodology to insert nanoparticles by appropriate control over temperature, time and graft-chain properties can lead to enhanced densities of loading in the PNC. This method can be engineered to produce a wide range of high-density polymer nanocomposite membranes for various possible applications, including gas separation and water desalination. In Chapter 6, we discuss the potential application of such nanostructured polymer thin film membranes for water desalination. Membranes with high water flux and large salt rejection are necessary to desalinate water at scale. While polyamide composite (PA-TFC) membranes are the benchmark, there is a continuing need to improve performance systematically. Here, we discuss a novel, transformative paradigm using thin films of pure PGNPs with fixed grafting density but varying chain lengths assembled on PA-TFC membranes through the venerable Langmuir-Blodgett method. The water permeance (A), and flux (J), show a non-monotonic dependence on graft chain molecular weight, likely driven by the modification of osmotic compressibility, which goes through a minimum at 88 kDa. In contrast to most separations, the membrane transport of components of saline solutions is driven by different physics, thus providing us with two distinctly different control handles to optimize these important phenomena opening a new direction in affordable water desalination technologies. Finally, Chapter 7 summarizes the results obtained in this thesis and expresses the future scope of this work.
- Physics (PHY) 
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