We report on the local microenvironment surrounding three small fluorescent probe molecules (pyrene, [6-propionyl-2-(<i>N</i>,<i>N</i>-dimethylamino) naphthalene] (PRODAN), and [4-(dicyanomethylene)-2-methyl-6-(4dimethylaminostyryl)-4<i>H</i>-pyran] (DCM)) when they are sequestered at low concentration within thin biodegradable films formed from poly( L lactic acid) (PLLA) and Pluronic P104 blends. Despite each probe molecule being neutral, they sense and report from much different microenvironments in comparison to each other. Specifically, the non-polar pyrene probe senses a slightly more polar microenvironment as the PLLA content in the polymer blend increases. In contrast, the polar PRODAN and DCM probes sense less polar microenvironments as the PLLA content within the polymer blend increases. Time-resolved fluorescence intensity decay experiments on pure PLLA films reveal that each probe molecule encounters significant heterogeneity. Pyrene emits simultaneously from three discrete microenvironments. This is consistent with pyrene molecules reporting from crystalline, amorphous-crystalline intermediate, and amorphous regions. PRODAN and DCM appear to emit from a continuum of microenvironments. These results have ramifications on the performance of biodegradable drug delivery platforms loaded with small drug molecules.

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