SYNTHESIS, CHARACTERIZATION, AND KINETIC STUDIES OF IONIZING RADIATION-INDUCED INTRA- AND INTER-CROSSLINKED POLY(VINYL PYRROLIDONE) NANOHYDROGELS
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A polymer nanohydrogel can be defined as a three-dimensional polymer network composed of hydrophilic crosslinked macromolecular chains filled with liquid and possessing a diameter of 1-10<sup>2</sup> nanometers. Nanohydrogels have drawn huge interest due to their potential applications, such as target-specific drug delivery carriers, absorbents, chemical/biological sensors, and bio-mimetic materials. However, the conventional methods of nanohydrogel synthesis require toxic chemicals (e.g., initiators, crosslinking agents) to form the gel structure. The additional steps required to remove unreacted or residual (undesired) substances cause nanohydrogel fabrication to be complicated, environmentally unfriendly, and unsuitable for biomedical use. This study aims to develop simple and efficient methods of producing nanohydrogels from polymeric, aqueous solutions using ionizing radiation. Poly(vinyl pyrrolidone) (PVP) nanohydrogels of various sizes and molecular weights were prepared by pulsed electron beam and steady-state gamma irradiation at different doses (5 and 10 kGy; 1Gy = 1 J kg<sup>-1</sup>) and temperatures (20 to 77 °C). The pervaded volume of the PVP chains becomes smaller at high temperatures (above 50 °C) due to the disruption of hydrogen bonds between water and PVP molecules which reduces the size and the molecular weight of the synthesized PVP nanohydrogels. The synthesis parameters (e.g., irradiation temperature, pulse repetition rate, dose rate, and solution concentration) were varied in order to control the size and the average molecular weight of the irradiated sample. In the absence of oxygen, the radiolytically produced free radicals of the thermally collapsed PVP molecules primarily underwent intra-crosslinking reactions, along with a minor contribution from inter-crosslinking reactions. The predominance of the intra-crosslinking mechanism was exhibited at high irradiation temperature (77 °C) in dilute solutions (c = 0.9 x 10<sup>-2</sup> mol L<sup>-1</sup>). The formation of carbon-centered free radicals along the backbone of the PVP chain at higher pulse repetition rate (300 pulses per second) was found to enhance the intra-crosslinking reaction, thereby leading to the formation of smaller nanohydrogel molecules containing an average hydrodynamic radius (R<sub>h</sub>) of 9.9 ± 0.1 nm.