Computational studies on the effect of composition and interactions on structure, dynamics and gas adsorption properties of polymeric materials 

Abstract

Properties of functional polymeric materials can be tuned by either varying the composition of a multi-component system, or by chemical modification of the constituents. Thus it is extremely crucial to develop a molecular level understanding of these interactions and the role of composition that manifests into desired functional properties. The present thesis involves computer simulation studies of various polymeric systems investigating the effect of composition and interactions on the polymer properties at a molecular level. The thesis has been classified into three major parts based on the selected polymer or property of interest as follows. In Part A and B, interactions in polymeric systems are modified by changing the compositions, by incorporating additives in part A and altering chemical structure of polymer in part B. Part C investigates the gas adsorption characteristics of a polymeric system and the role of structural and dynamical heterogeneity therein. In Chapter 1, the thesis introduces the systems under consideration and presents a brief review of the literature. Subsequently, a description of applied computational methods and concepts is given in Chapter 2. These chapters are common to all parts A, B and C. Separate introductions and computational details relevant to each part are further discussed in individual parts. Part A It includes chapter 3, 4 and 5. The systems under study in part A are rubber and rubberplasticizer mixtures, focusing majorly on deciphering the molecular mechanisms related to glass transition and its impact on structural and dynamic properties of rubbers. Chapter 3: A brief overview of properties of rubber, rubber composites and rubber blends is presented. Emphasizing the fact that a systematically validated force-field and proper equilibration are the major pre-requisites for any polymer simulation. In the first working chapter of this thesis, we have validated quantum-chemically derived force-fields of rubber by calculating glass transition temperature (Tg), density and local chain characteristics like end-to-end distance and radius of gyration (Rg). All calculated properties have been compared to corresponding experimental results and force fields are also tuned in cases where calculated properties do not match with experiments. A potential energy based equilibration protocol has been proposed and tested for rubbers under study: cis and trans- Polybutadiene and Polyisoprene. Chapter 4: Additives are incorporated in rubber-matrix to enhance the mechanical and physico-chemical properties of rubbers for their optimum use in tire and other rubber industries. Plasticizers are the additives which increases the flexibility and processibility of rubbers. This chapter is focused on deducing the molecular mechanisms of plasticizer action in rubbers. Effect of plasticizers on structural and dynamic properties of rubber has been analyzed. Various polymer properties like free volume, end-to-end distance, Rg, autocorrelation functions, mean square diffusion, structural and dynamic heterogeneity have been explored. Chapter 5: The most common method employed for calculating Tg of polymers from MD simulations involves deducing temperature dependence of properties like density and specific volume. The slope of density-temperature plot gives Tg. However this protocol is computationally expensive involving polymer equilibration at temperatures below Tg. In this chapter, we have proposed a method of calculating Tg from segmental (α) relaxation times at temperatures higher than Tg. Incoherent intermediate scattering function Fs(q,t) are used to calculate relaxation times and then Tg is calculated using Vogel-Fulcher-Tamman equations. Part B It includes chapter 6. The system under study is ionomer melt of star telechelic D, L-polylactide and the property under investigation is viscosity. Chapter 6: Star D,L-Poly-lactic acid (PDLLA) shows the typical exponential decrease in viscosity with temperature. However ionomer formed by replacing acid groups of chain ends with sodium carboxylate ions, shows a significant increase in elasticity and non-monotonic temperature dependence of viscosity at temperatures above Tg, which is unusual. The molecular mechanism responsible for the non-monotonic temperature dependence of viscosity has been investigated in this chapter. Part C It includes chapter 7. The system under study is Polyethyleneimine (PEI) and its carbon dioxide capture properties have been investigated. Chapter 7: A brief introduction of existing carbon dioxide capture technologies focusing on the merits and demerits of each method is provided. Characteristics of CO2 capture through polymeric membranes, especially nitrogen containing polymers is discussed. The mechanism of CO2 adsorption in PEI is investigated using Grand Canonical Monte Carlo (GCMC) and Molecular dynamics (MD) simulation studies. A detailed analysis of intermolecular interactions between PEI and CO2 at the interface, bulk and local structural regions of PEI melt has been performed to assess the adsorption effectiveness. Effect of structural and dynamic heterogeneities on adsorption process has been analyzed. Chapter 8: A brief summary of the research work and derived conclusions is provided. Scope for the future work has also been discussed.