The kinetics of the crystallization of thermoresponsive poly(2-isopropyl-2-oxazoline) in water and the time-dependent evolution of the morphology were examined using wide-angle X-ray scattering and conventional and cryogenic scanning electron microscopy. Results indicate that a temperature-induced phase separation produces a bicontinuous polymer network-like structure, which with the onset of crystallization collapses into individual particles (1-2 mm in diameter) composed of a porous fiber mesh. Nanofibers then preferentially form at the particle surface, thus wrapping the microspheres like a ball of wool. The particle morphology is severely affected by changes in temperature and less by the initial polymer concentration.
A single-step process to polymer nanofiber meshes that possess biofunctional peptide segments on their surfaces is described here, which requires a standard electrospinning setup only. Spinning a homogeneous mixture composed of a valuable polymer-peptide conjugate (poly(lactic acid)-block-CGGRGDS) and a biocompatible commodity poly(lactic-co-glycolic acid) (PLGA) leads to nonwovens where the bioactive peptide part is enriched up to 11 times on their fiber surface. This is determined by X-ray photoelectron spectroscopy (XPS). The surface accessibility of the peptide is proved on the macroscale by contact angle measurements comparing pure PLGA fibers with GRGDS-functionalized fiber meshes as well as on the nanoscale by probing electrostatic interaction between CGGRGDS surface functionalities and a colloidal silica probe via atomic force microscopy (AFM). Ultimately, bioavailability and bioactivity of the peptides on the fiber surfaces are demonstrated, showing that the meshes promote adhesion and migration of fibroblasts in comparison to pure PLGA meshes. The one-step production of hydrophilic PLGA-based fibers could be exploited to electrospin into living cell culture without indication of toxic adverse effects on cell proliferation. This might be useful for directly production of cell-loaded scaffolds or biohybrid materials.
The static and dynamic properties of a range of molecular weights (2 × 10 4 to 1.6 × 10 5 g/mol) of poly(benzyl methacrylate) have been assessed in four different imidazolium-and pyrrolidinium-based ionic liquids over a wide temperature range (27−155 °C), primarily using light scattering techniques. All four systems exhibit lower critical solution temperature phase behavior. The relevant structural, dynamic, and thermodynamic parameters were examined as a function of concentration, temperature, and molecular weight. Some interesting observations were revealed. The phase boundaries suggest a shift of the critical composition toward the polymer-rich region, in contrast to the low critical concentrations for polymers commonly observed in polymer solutions. Surprisingly, the second virial coefficient (A 2 ) remains positive, even at temperatures close to phase separation, where A 2 < 0 is anticipated. Furthermore, A 2 also shows stronger dependence on molecular weight than commonly observed for polymers in good solvents. On the dynamic side, the diffusion virial coefficients (k d ) remained positive over the given temperature range, further corroborating the apparent good solvent behavior of A 2 . The excluded volume exponents (ν ≈ 0.53−0.54) obtained from the dependence of hydrodynamic radii on molecular weight also indicate good solvent characteristics.
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