Let there be light: Quantitative imaging of nanoscale dynamics in polymer materials

In light of the ever-growing societal demand for high-performance, sustainable products, it is of utmost importance to unravel the inner machinery of future's materials. This pursuit is particularly relevant for polymer materials, which are highly complex yet ubiquitous. Synthetic polymer materials come in all shapes and sizes, including plastics, paints, adhesives, elastomers and gels. They play an essential role in everyday life. Yet, the Earth's diminishing resources and growing population set increasingly stringent limits on the possibilities. Modern societies must therefore make a transition from short-lifetime, polluting products to durable, environmentally friendly alternatives. Examples in this thesis are water-based paints as substitutes for organic-solvent-based systems, and sophisticated materials such as self-healing and self-cleaning polymers. The high complexity of these materials calls for more than trial-and-error testing alone. Rather, researchers must seek to gain a deeper understanding of the polymer dynamics and mechanics on all scales involved. The legendary Nobel physicist Richard Feynman (1918‒1988) once wrote on his blackboard: “What I cannot create, I do not understand.” However, this truth does not imply that we do understand what we can create. Resolving the multitude of issues we encounter today, and are about to face in the (near) future, requires fundamental insight into the nanoscopic processes driving polymer behaviour. Remarkably, this nanoscale is generally not the primary scale of interest when developing new polymer materials. In this thesis, we cast new light on the nanoscale dynamics and mechanics inside complex polymer materials. We herein focus on polymer dispersions and networks, specifically drying water-borne coatings (Parts I and II) and dynamic elastomers (Part III). Reaching down to the very core of these materials is an ambitious task, as the smallest length scales are unreachable to conventional imaging methods that operate using visible light. Indeed, the wavelength of visible light equals hundreds of nanometres, thus precluding interaction with nanometric objects. We overcome these limitations in two ways in this thesis. In Part I, we simplify matters by creating model polymer dispersions representative of water-based paints. We unleash the full power of bright-field microscopy to elucidate the various mechanisms at work. Although this method is not truly nanometric, we complement it in three ways to still realise a comprehensive picture of the governing processes down to the nanoscale: (i) we systematically vary the dispersion parameters to allow for nanoscopic insight through deduction; (ii) we examine the nanoscale surface structure in detail using electron microscopy; and (iii) we establish multi-scale theoretical frameworks to underpin our experimental results in a quantitative manner. This three-pillar approach has proven successful for semi-transparent systems, yet it severely fails for non-transparent, turbid materials ‒ which most everyday polymers are. For those cases, a more specialized microscopy technique is optimally suited: laser speckle imaging (LSI). This technique has its roots in the biomedical field, where it was pioneered in the 1980s and gained renewed attention in the early 2000s with the advent of powerful digital cameras and computers. We introduce an advanced version of LSI, and apply it to drying polymer dispersions in Part II of this thesis. Laser speckle imaging allows us to cut through the fog of opaque samples with exceptional sensitivity. In this method, we shine a powerful laser beam on the material of interest, which diffuses through the cloudiness. Subsequently, we capture the light that returns, and use dedicated computer algorithms to acquire an abundance of quantitative information about hidden phenomena deep inside the material. Not only do we use LSI to probe the inner dynamics of coatings, but we also open up a whole new world of applications related to dynamic elastomers in Part III. We quantitatively visualize various collective ‘dances’ of nanoscale building blocks in high-tech materials for the first time. We uncover the molecular path to catastrophic fracture, where ‒ analogous to a flashmob dance ‒ a rapidly growing number of molecules participate until a climax is reached. We show how polymer damage can be completely healed, because ‒ like a moshpit ‒ molecules spontaneously move in collective patterns with net directionality. Finally, we reveal how molecular collaboration can result in unique, long-ranged shape transformations ‒ similar to synchronized ballet. First and foremost, these nanoscopic glances beneath the surface of coatings and elastomers are fundamentally interesting. Yet, we hope they will also expedite the development of sustainable polymer materials with increased lifetime.