Xuecong Li

Polymers and functional surfaces are often described by bulk-average characterization methods, but these do not fully capture how organization and properties arise across hierarchical length scales. This thesis addresses this limitation through nanoscale chemical and structural characterization of polymers and functional surfaces. By combining atomic force microscopy (AFM) with acoustic-mechanical suppressed photothermal infrared nanospectroscopy (AMS-AFM-IR), this thesis resolves local heterogeneity, hierarchical structural organization, and intermolecular interactions beyond bulk-average readouts.

Chapter 2 establishes an AFM-based methodology for single-feature-resolved molecular-weight estimation of surface-supported polymer features. Individual polymer features are analyzed through their apparent area and height, followed by density-based conversion to molecular weight. The results show that surface supported polymer systems are composed of discrete features that differ in morphology, molecular weight, and aggregation state, and therefore cannot be treated as equivalent units within one averaged population. AFM-based analysis preserves feature-level molecular-weight distributions beyond bulk-averaged readouts and resolves clustered populations consistent with different aggregate states. Chapter 2 therefore shows that nanoscale variation is an intrinsic feature of the polymer sample rather than a deviation from its average description.

Chapter 3 shows that chirality in chiral and racemic polymer systems emerges across molecular, single-chain, and supramolecular length scales. Bulk Chiral-HPLC, circular dichroism (CD), infrared (IR), and UV-Vis spectroscopy provide average information, but AFM imaging combined with AMS-AFM-IR resolves the nanoscale chemical-structural heterogeneity of these systems directly. Chirality is not expressed as a single uniform average state, but as the sum of locally distinct conformations, assembly states, and nanoscale chemical signatures. Central chirality is present at the molecular level, while polymerization gives rise to backbone chirality in the chiral polymer and supramolecular chirality in the racemic polymer. Chapter 3 therefore demonstrates that chirality can be resolved hierarchically across different levels of organization.

Chapter 4 investigates self-assembled monolayers formed by monomers and polymers on highly ordered pyrolytic graphite (HOPG). AFM and AFM-IR show that the monomer system forms a coverage-invariant monolayer, whereas the polymer system forms a cooperative monolayer with coverage-dependent structural changes, lateral association, and an emergent carbonyl band at approximately 1725 cm⁻¹. Together with the increase in water contact angle, these results provide direct evidence that the polymer monolayer is governed by adsorbate-adsorbate intermolecular interactions, whereas the monomer monolayer remains dominated by substrate-adsorbate interactions. Chapter 4 therefore shows that local monolayer organization and intermolecular interactions govern the formation of these functional surfaces.

Taken together, this thesis shows that polymers and functional surfaces are best understood beyond bulk-average characterization methods and at the level of local organization. Across single polymer features, supramolecular assemblies, and self-assembled monolayers, local heterogeneity, hierarchical structural organization, and intermolecular interactions are shown to govern how organization and properties arise. Overall, this thesis establishes nanoscale chemical-structural characterization as a direct route to understanding polymers and functional surfaces at the level where their relevant organization and properties are formed.