Self-assembled poly(2-ethyl-2-oxazoline)/malonic acid hollow fibers in aqueous solutions

Abstract

Well-defined poly(2-ethyl-2-oxazoline) (PEOX)/Malonic Acid (MA) fibers having hollow tubular morphology were shown to form in aqueous solutions at 25 °C by complexation induced self-assembly between PEOX and MA. The fibers had diameter of ~1–3 μm and a wall thickness of ~40 nm. Different interactions between PEOX and MA were identified for complexation as a function of pH. At pH2, when both ends of MA were protonated, H-bonded complexation was the driving interaction in the fiber formation. IR data showed both PEOX -CO band and MA -COOH band in dried fibers formed at pH2. The downshift in the CO stretching of PEOX by as much as 15 cm−1 confirmed the H-bonded complexation. The interaction enthalpy of PEOX and MA was determined by isothermal titration calorimetry (ITC) as −49.39 kJ/mol which is consistent with H-bonding. Thermogravimetric analysis (TGA) of the fibers showed two distinct decomposition temperatures one between 100 and 150 °C corresponding to MA and the other one at 350–450 °C corresponding to PEOX which also indicated the presence of both components in the fibers. At pH4, when one end of MA was protonated and the other end was ionized, electrostatic complexation between carboxylate (–COO−) group of MA and the amide group of PEOX was the driving interaction in the fiber formation. At pH7, when both ends of MA were ionized, fiber formation was significantly hindered. The results are important in understanding the role of different interactions in the hollow fiber formation mechanism as a function of pH. pH-responsive hollow fibers have great potential to be used in biomedical applications for drug delivery and release purposes.