Quantifying macromolecular growth and hierarchical structuring on interfaces

Abstract

For the development of the next generation of soft matter based functional surfaces it is critical to obtain an in-depth molecular understanding of the processes that govern the functionalization. There exists a multidisciplinary need for advanced surface performance ranging from wear/chemical resistance to biomedical applications. Especially the interplay between theoretical and experimental assessments is of substantial value for generating an encompassing picture of the macromolecular functionalisation process on the molecular level. In the present thesis, the current methods for determining quantitative grafting densities are critically assessed and a user's guide is provided to estimate maximum chain coverage. More importantly, the most frequently employed approaches for determining grafting densities, i.e., dry thickness measurements, gravimetric assessment, and swelling experiments are examined. An estimation of the reliability of these determination methods is provided via carefully evaluating the underpinning assumptions, their simplicity as well as the stability of the deriving equations. The assessment is concluded with a perspective on the development of advanced approaches for determination of grafting density. By functionalisation of a quartz crystal microbalance sensor via the 'grafting-to' approach of poly(methyl methacrylate) (PMMA), one of the proposed possibilities of precise grafting density determination was experimentally assessed. By virtue of this experimental approach, it was demonstrated that grafting a distribution of polymer chains onto a surface critically affects the shape of the distribution, with shorter chains being preferentially attached. The preferred surface attachment of shorter chains was unambiguously underpinned by single-molecule force spectroscopy measurements, establishing a preferential grafting factor. The preferential grafting factor allows to predict the molar mass distribution of polymers on the surfaces compared to the initial distribution in solution. These findings not only have serious consequences for functional polymer interface design, yet also for the commonly employed methods of grafting density estimation. Furthermore, the reaction conditions were found to influence the resulting grafting density and molar mass distribution when grafting polymers onto surfaces. Theoretically and experimentally the application of poor solvents is proven to be beneficial for the 'grafting-to' approach. The effect is demonstrated by grafting PMMA chains on silica nanoparticles in different solvents and comparison of the molar mass distributions via size exclusion chromatography. The shorter polymer chains are preferentially grafted onto the surface, leading to a distortion effect between the molar mass distribution in solution and on surfaces. The molecular weight distortion effect is significantly higher for better solvent quality than for poor solvents. In summary, the current thesis is exploring theoretical and experimental aspects of functionalising surfaces with macromolecules, focusing on the precise determination of grafting densities. Especially for the 'grafting-to' approach novel procedures of surface characterisations were introduced, enabling an advanced understanding of the grafting procedure on the molecular level. The gained information result in the introduction of a preferential grafting factor, which can be used in any further investigation of surface grafting by the 'grafting-to' method.