Visualizing Spatial Distribution of Poly( n -Alkyl Acrylates) Aggregates in Pressure Sensitive Adhesive Films

Poly( n -alkyl acrylates) exhibit tunable melting temperatures, which can be precisely adjusted by varying the length and composition of their alkyl side chains. These polymers serve as promising thermoresponsive additives in pressure-sensitive adhesive (PSA) systems, particularly for medical tapes designed for individuals with sensitive skin. However, the mechanism behind their heat-triggered debonding remains unclear due to the chemical similarity between the additive and the hybrid rubber-acrylic PSA matrix, which hinders contrast in conventional analytical techniques. In this study, we present a noncovalent fluorescent labeling strategy that leverages the cocrystallization of long-alkyl-chain dyes with semicrystalline additives to selectively visualize additive distribution within the matrix without chemical modification. Two cocrystallizing dyes were employed to validate labeling reproducibility, while a non-cocrystallizing dye served as a negative control to confirm the spatial location of aggregates through complementary contrast. Confocal fluorescence microscopy revealed a stratified distribution of additive aggregates. Numerous small poly( n -alkyl acrylates) aggregates (approximately 5 μm 3 ) were densely distributed near the film surfaces. In contrast, larger aggregates (up to ∼100 μm 3 ) were fewer in number and with a distribution tending toward the midplane of the film. Upon heating, the fluorescence intensity of the polymer aggregates diminished, corresponding to crystalline melting (onset melting temperature is 40 °C according to differential scanning calorimetry) that correlates with adhesive performance changes. Heating the adhesive film transforms the additive aggregates from rigid, load-bearing reinforcements into soft, non-load-bearing inclusions, weakening the cohesive zone at the peel front and thus reducing the peeling force. This work represents the first use of confocal microscopy to directly visualize thermoresponsive additive behavior within a PSA film and provides spatially resolved insights into the mechanism of temperature-triggered adhesive debonding.