Structure of spatial heterogeneities in model pnipam hydrogels and their implications on origins and properties

Abstract

Characterizing nanoscale variations in crosslink density in gels has remained a long standing challenge in polymer science. This primarily stems from an inability of popular techniques in polymer science to evaluate local variations in mesh sizes in the native swollen state of the material. Much of our current understanding of the nature of crosslink density variations comes from scattering studies, but local information is often obscured by the reciprocal nature of the extracted information. Direct visualization routes have attempted to fill this gap in understanding but extracting volumetric information in the swollen form with the required resolutions has remained beyond reach. Over the past decade, developments in fluorescence microscopy has pushed their resolutions into the relevant length scales for polymers and colloids. Specifically, super resolution microscopy has demonstrated the ability to circumvent the diffraction limit and probe structure in the 10-100 nm regime. In this thesis, we utilize single molecule switching microscopy techniques and STED microscopy to directly visualize spatial heterogeneities in model PNIPAM colloidal and bulk gels. We achieve this by utilizing a novel molecule that tags dye molecules to crosslink points and provide labeling specificity to visualize crosslink density variations. In the case of dye tagged PNIPAM colloidal microgels, we observe nanodomains on the length scales of 20-50nm within individual particles via 4PiWSMSN microscopy. These first time observations provide real space evidence for discrepancies in the traditional core-shell model. Moreover, these images provides novel insights into fractality and nucleation and growth mechanisms driving microgel formation. Further, we demonstrate that the spatial distribution of dye tagged crosslinks in microgels provide a cost effective means to extract consumption rate of these molecules. The kinetic rate constants from fitting a terminal co-polymerization model to this consumption data, has important implications for studies on the morphology of spatial heterogeneities in microgels and hydrogels. In bulk PNIPAM bulk gels, we utilize 3D STED microscopy to formulate a new structural model for spatial heterogeneities in these systems. Specifically, we characterize the nature of the interface between the polymer rich domains and the surrounding matrix, connectivity between the domains and the variation in local crosslink density within these domains. Finally, we utilize a combination of laser scanning confocal microscopy and a library of dye functionalized crosslinkers to evaluate the origins of heterogeneities in these systems. The library of crosslinkers provide a unique platform to not only decouple the influence of phase separation and reaction kinetics on heterogeneity formation but also visualize their direct impact on local structure. Observations of the combined role of thermodynamics and kinetics is further strengthened by capturing the different stages in structure formation in time. These structural insights and their implications on origins provide important contributions towards formulating a general framework of spatial heterogeneities in thermoresponsive gels that extends beyond the current model of sponge-like hierarchical networks. Finally, we elucidate the impact of spatial heterogeneities on the mechanical properties of these model PNIPAM hydrogels. We observe an anomalous transition in the variation in elastic moduli in these gels with increasing incidence of spatial heterogeneities and further, observe a strong correlation between the observed trends and the morphologies exhibited by the gels. We conclude our investigation on structure-property relationships by elucidating the impact of an external compressive load on local structure and provide new insights into the nature of deformation at the individual heterogeneity level.