Diffusion Dynamics in Weak Polyelectrolyte Solutions

Abstract

Diffusion anomaly is accountable for molecular transport inside the living organism on which the transference of genetic materials, vital nutrients and ions around the cell membrane along with the movement of drug particles are dependent. This simple well-ordered dynamic process in reality is based on a complex mechanism. At a given time, numerous parameters control the diffusion environment of real-life system. The simultaneous presence of these parameters turns the system into a macromolecular crowding where the structural organization and other molecular interactions also affect the dynamical process. Apart from the biological system, the diffusion dynamical process has shown its importance in the field of many industrial applications; such as the pharmaceutical industry, food packaging and processing and water treatment or management processes. Hence, understanding the basis of diffusion behaviour and garnering knowledge on the various controllable parameters, their origin and aftereffects has become an interesting and challenging task among polymer physicists and soft material scientists. Normally, the controllable parameters affecting the diffusion process offer varieties of environmental constraints and when act together, it is difficult to distinguish the exact parameter responsible for the analogous changes in the diffusion dynamics. For this reason, the “model system” approach has been utilized as an alternative adaption to avoid the lack of understanding of the dynamics of the relevant systems. These model systems provide the freedom of choosing or rather selecting the individual parameters and tweaking them appropriately to analyze the consequences of the diffusion process. The amount of crowding, the existence of other molecules, their interactions and the structural organization should be taken care of while designing the model system. Hence the research on these model systems and their behavior will be of immense help in addressing the real-life complex interactions. The model systems comprise various categories and among these, the weak polyelectrolyte solution system is one of a kind where the polymer backbone charge can be regulated under the suitable conditions of pH and the type of the polar solvent. The unique characteristic of seizing a partial degree of dissociation at the intermediate pH range of the solvent has made the weak polyelectrolyte system more interesting and applicable to replicate the complicated real-life system interactions. The genetic materials of DNA, RNA and various proteins and polysaccharides are charged macromolecules, imprisoning vital cellular information and are regarded as natural polyelectrolytes. Synthetically prepared weak polyelectrolytes with advance properties act as an appropriate model for these natural polyelectrolytes and provide us with the advantage of controlling different parameters and allow us to carry out the sophisticated study which is otherwise impossible directly in the case of natural polyelectrolyte solution system. The proper information on the diffusion dynamics of these polyelectrolyte solutions assists the pharmaceutical industry in designing the drug loading capsules and tracing the target site at a controlled speed. The waste water treatment and other biomedical applications including cell encapsulation rely on the dynamical study of weak polyelectrolyte solutions. To achieve our research objectives, two types of weak polyelectrolyte solutions have been considered anionic and cationic weak polyelectrolyte. The two weak polyelectrolytes consist of oppositely charged polymer backbone and make use of three primary parameters as, pH of the solvent, polymer concentration and external electrolyte concentration, the systematic study on the diffusion dynamics has been carried out and presented in the current thesis. The experimental techniques of Dynamic light scattering (DLS) have been used to analyze the diffusion behaviour of polymer chains under set conditions whereas Fluorescence recovery after photobleaching (FRAP) is utilized to study the probe dynamics inside the weak polyelectrolyte solutions. The complementary techniques of rheology and microviscosity have been used in the required sections of the study. The light scattering study on anionic PAA solutions has revealed the simultaneous transition of ergodic to non-ergodicity and triple to bimodal relaxation on variation of pH of the solution and the reverse effect is achieved with the addition of electrolyte concentration to the PAA solutions in the respective pH of the solution. This transition in the dynamics of polyacrylic acid (PAA) solutions is the resultant influence of concentration, crowding and charge effect owing to pH variation. The screening of charge from the salt solutions helps to restore the ergodicity in PAA solutions which is also dependent on particular concentration and pH of the solution. The fast mode relaxation remains diffusive irrespective of pH and salt solutions whereas the non-diffusive slow mode relaxation and intermediate mode change to diffusive with the addition of salt solutions. The newly observed intermediate mode has been coined as non-ergodic mode due to the coincidence of appearance with non-ergodicity in PAA solutions. The rheology study on PAA solutions has confirmed the presence of network structure without the addition of crosslinking agent into PAA solutions and well explains the great storage property at higher pH of the solutions and the loss property dominance in the lower pH and lower concentrated solutions. The storage and loss behaviour of solutions is also dependent on the degree of ionization of the PAA at particular pH of the solvent and salt concentrations. The cationic polyallylamine hydrochloride (PAH) solutions have demonstrated a similar transition from ergodic to non-ergodic behaviour and bimodal relaxation to single relaxation at the lowest polymer concentrations in each pH value. With the salt solutions in PAH, the systematic complete restoration of ergodicity can be achieved only for lower polymer concentrations. The light scattering study on PAH solutions depicts that although the transition in dynamics is due to the combined effect of concentration, charge and crowding but the influence of concentration on crowding is dominant than the charge effect in PAH due to lack of charge variation. The fast and slow modes are diffusive and non-diffusive in nature at each pH value and at optimum salt solutions, both the modes turn out to be non-diffusive and the slow mode shows greater angle dependence with scattering wave vector. The probe dynamics in both the weak polyelectrolyte solutions are analyzed with two different sizes of FITC labeled dextran of 40kDa and 2000kDa molecules. Both the types of probe dynamics in PAA and PAH solutions experience the impact of microviscosity at all studied pH and salt solutions. This effect is more pronounced in PAA solutions than the PAH solutions. Moreover, the probe diffusion inside the weak polyelectrolyte solutions is dependent on various interactions among probe-polymer, the polymer background and the flexibility of polymer chains at particular pH and salt solution environments. We are sincerely hopeful that the research works presented in the thesis will contribute to the basic understanding of real-world systems and their functional performance in intricate surroundings. Altogether, these studies will provide opportunities for the conceptual design of matrices for appropriate applications comprising controlled release systems with separation of matrices and scaffoldings for enzyme immobilization.