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In biomedicine, polymer blends are frequently applied in wound dressing design or drug delivery. Within these applications, poly­(2-alkyl/aryl-2-oxazoline)­s (PAOx) are emerging as a popular matrix due to excellent biocompatibility and miscibility with other polymers. However, as much is known of PAOx miscibility with other biocompatible polymer systems, so little is known of the miscibility within the PAOx class. We show the remarkable phase separation of two important, structurally alike, amorphous PAOx, i.e., poly­(2-ethyl-2-oxazoline) and poly­(2-n-propyl-2-oxazoline), that occurs when the polymers’ number-average molar mass exceeds 10 kg·mol–1. The (im)­miscibility as a function of average molar mass is experimentally investigated by thermal analysis, theoretically underpinned by the Flory–Huggins lattice theory, and visualized by fluorescence microscopy in both films and nanofibers, the latter being a high-potential support material in biomedicine. These results provide important knowledge on PAOx (im)­miscibility which has a crucial impact on the behavior of the many final end products they are investigated for.

Colorimetric nanofibers provide visual, easy-to-interpret sensors for personal use as well as advanced applications. The potential of 2-n-butyl-2-oxazoline (B) and 2-ethyl-2-oxazoline (E) statistical copolymers as a universal, versatile support platform for nanofibrous halochromic sensor design is demonstrated. These polymers are electrospinnable from eco-friendly solvent systems, while wettability, moist adsorption capacity, and water-solubility of the membranes can be easily tuned by changing the B/E monomer ratio, ensuring wide applicability. The halochromic sensing functionality is introduced by incorporating the alizarin yellow R (AYR) chromophore, which is covalently modified with an ethyl ester-group or a short poly(2-n-butyl-2-oxazoline) chain, which is demonstrated to simultaneously prevent dye-leaching and allows tuning of the halochromic pH-sensing window. The colorimetric nanofibrous sensors reversibly respond toward aqueous solutions of different pH, (hydrochloric) acid and alkaline (ammonia) vapors, and several biogenic amines with detection limits as low as 5 ppb. Tunability of sensor responsivity, sensitivity, and pKa via manipulation of dye-polymer interactions, determined by support polymer structure and semi-crystallinity, as well as the chain length of the AYR-modified polymer, are discussed. Preliminary proof-of-principle studies indicate the potential of the developed sensors for sub-ppm biogenic amine vapor detection, which may serve as the basis for future applications in food packaging or breath analysis.

The development of intelligent photonic systems made of stimuli‐responsive materials, i.e., with features tunable and switchable by environmental signals, is gaining increasing attention. Here, the study reports on switchable optical gain based on complex arrays of nanofibers made of thermo‐responsive poly(2‐n‐propyl‐2‐oxazoline), incorporating a blue‐emitting chromophore. The fluorescent component endows the nanofibers with optical gain in addition to the moisture absorption capability of the polymer. Light amplification is found with temperature‐ and humidity‐dependent excitation threshold. The threshold value is halved close to the polymer cloud point temperature, enabling reversible switching of the emission intensity upon temperature change. Waveguiding analysis by back‐focal plane imaging on individual fibers allows the switching mechanisms to be rationalized, in terms of moisture sorption swelling‐induced morphological changes. These responsive light‐emitting nanofibers may find application in a novel class of lasers with dynamically‐controlled properties, environmentally‐switchable optoelectronics, and smart sensors.

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