Morphological characteristics of modified freeze-dried poly(<Emphasis Type="Italic">N</Emphasis>-isopropylacrylamide) microspheres studied by optical microscopy,SEM, and DLS

Influence of the initiator and additional hydrophobic copolymer on the morphology of thermosensitive poly(N-isopropylacrylamide) (pNIPAM) microspheres, and their presumed application for the stabilization of biologically active molecules were evaluated in this study. Three different types of pNIPAM were synthesized, applying various components: PN1 is a polymer with terminal anionic groups resulting from potassium persulfate initiator; PN2 was synthesized with a 2,2′-azobis(2-methylpropionamidine) dihydrochloride initiator introducing cationic amidine terminal groups; in the PN3 polymer, anionic terminals were implemented, however, increased hydrophobicity was maintained using N-tert-butyl functional groups. Turbidity measurements of the obtained dispersions confirmed specific thermosensitivity of synthesized microspheres in the range of 32–33°C. The polymerization course was proved by infrared spectroscopy and ¹H NMR assessments, whereas the size of the synthesized microspheres, expressed as planar area, was evaluated by dynamic light scattering (DLS), scanning electron microscopy (SEM) and optical microscopy (OM). The respective surface patterns of the freeze-dried microspheres were evaluated by SEM. Planar area of the synthesized macromolecules was in the range between 0.41–3.22 μm, depending on the substrates composition and the method applied for the measurements. The assessments performed in the dry stage gave higher values of the diameter and planar area of the observed microspheres. The measured diameter and planar area increased in the following order for the PN3 microspheres: DLS, OM, SEM. In the case of PN1 and PN2, the observed diameters were positioned as: DLS, SEM, OM. These differences were assigned both to varied intramolecular hydrophobic-hydrophilic interactions of the polymer chains and to the environment, i.e. low pressure in the SEM conditions and aqueous solvent in the DLS measurements. The observed gaps in the freeze-dried PN2 polymer resulted in an attempt to evaluate the application of this polymer for mechanical stabilization of certain macromolecules or nanocrystals in the size range between 10 nm and 20 nm.