Investigation of phase behaviour of neutral and ionic microgels using scattering, rheology and microscopy techniques

Abstract

The thesis deals with the investigation of structure and dynamics of temperature and pH sensitive microgels of Poly (N-isopropylacrylamide) at very dense concentrations using scattering, rheology and microscopy techniques. The main focus of the work is on understanding how the microgel architecture/structure affects the flow properties as well as in applications. Poly (N-isopropyl acrylamide) microgels were prepared using two different synthesis procedures giving rise to core-shell particles and homogeneous particles respectively. These particles are then used to prepare dense suspensions to study non-linear rheological behaviour in the glassy region. Modern Light scattering techniques, rheology and confocal microscopy are used to investigate the role of microstructure in the flow properties of these dense suspensions. The thesis also deals with electric field induced structure formations in ionic microgels and mixtures. We also look at the effect of microgel particle morphology in a typical application like drug delivery. Colloids are heterogeneous system where one substance is dispersed as very fine particles in another. The dispersion may be foam, emulsion or sols and are indispensable in or daily life. Study of the processing of all these colloids is important because many of the colloids we use as commercial products in our daily life is subjected to shear when use. They transform from elastic to a plastic state when yielded. The study of yielding in colloids is thus very much important to design the products. In addition to its utility as materials of daily use, colloidal suspensions are extensively used as model systems to study phase behavior of atomic or molecular systems. Phase transitions in colloidal suspensions can be studied in real time using optical techniques and are analogous to phase transitions in liquids. Experiments with colloids are always simpler. The microscopic size scale of colloids allows them to be easily characterized by microscopy and various light scattering techniques. Although lot of work has been done in understanding the yielding mechanism in hard sphere colloids and colloid-polymer mixtures, there is not much progress in understanding the yielding phenomena in soft colloids. First chapter sets the motivation and objectives of the study. This chapter also contains a literature survey and introductory remarks on colloids. Hard sphere colloids and soft sphere colloids, their properties, colloidal interactions and phase diagram are described here. A brief history on the investigation of electric field driven assembly of ionic colloids and a background on the drug release studies on model soft microgels are also provided. The second chapter deals with the synthesis and characterization of the microgels synthesized. A brief introduction to various experimental techniques used is also provided. In the third chapter, I try to address the yielding mechanism in dense thermosensitive microgels of Poly (N-Isopropylacrylamide) (PNIPAm) in the glassy region as a function of particle morphology and crosslinker content. Electric field driven assembly of negatively charged copolymer microgels of NIPAm and acrylic acid at different concentration regimes were investigated in the fourth chapter. The behaviour of a binary mixture of two different sized particles towards applied electric field is also under investigation. The intension of the work is to develop a phase diagram of binary mixture of particles under the electric field with respect to different concentrations and different ratios of particles. Fifth chapter will cover the study of the effect of particle morphology on the drug release from PNIPAm microgel particles. A water-soluble drug and a water insoluble drug were taken for the case study. Percentage drug release was measured using UV-Visible spectroscopy. Second chapter provides the details of the synthesis of homo and copolymer microgels of NIPAm. Some of the preliminary characterizations of microgels using light scattering technique are also provided in this chapter. All the experimental techniques and the methods used in this work to investigate the structure and properties of microgels are explained. The technical part of instruments like 3D- Dynamic Light Scattering, rheology, confocal microscopy, UV-Visible Spectroscopy and Differential Scattering Calorimetry are elaborately discussed in this chapter. Light Scattering techniques are used to find the radius of gyration, hydrodynamic radius, swelling ratio and volume phase transition temperature of PNIPAm microgels. Yielding mechanism in the microgels was elucidated by Large Amplitude Oscillatory Shear experiments in a stress controlled rheometer. Electric field driven assembly of colloids were observed using confocal microscopy. Drug release studies were conducted and percentage release was found by UV-Visible spectroscopy. Subzero temperature DSC was done to analyse the changes thermodynamic property of water associated with the gelling process. The results of the current work will be concluded in the sixth chapter. Ours is one of the first studies on the understanding of origin of double yielding mechanism in pure PNIPAm microgel system. Although two step yielding has been observed in attraction induced colloidal glasses, we are observing two step yielding in pure PNIPAM microgel glasses, which is otherwise repulsive. Two step yielding disappears with the change in morphology from core-brush type structure to homogeneously crosslinked structure. With increase in crosslinker content, and finally like hard-sphere colloids. Electric field driven assembly can be effectively used to tune the particles assembly in fluid regime and also at high concentrations. The works may be extended to mixture of oppositely charged particles, active particles and colloids, colloid-metal hybrid particles etc. Drug release ability of microgels can be enhanced by particle morphology changes.