# Radiation-Assisted Synthesis of Polymer-Based Nanomaterials
## 4 Polymer-Based Nanomaterials
### Nanogels
Irradiation of these complex structures by pulsed electrons in dilute, deoxygenated solutions induced intercomplex crosslinking evidenced by decrease in viscosity and radius of gyration, leading to the formation of permanent PVP/PAA nanogels. A recipe for preparing multifunctional nanogels with double amphiphilic properties using PVP and PAA was developed. Interpolymer complexes of these two polymers with coil size range of 40-300 nm were prepared under different experimental conditions of polymer molecular weight, concentration, mixing ratios, pH and temperature and irradiated with gamma rays. At relatively low absorbed doses of 5 and 10 kGy, nanogels with size range of 30-250 nm, zeta potential of \(-12\) to \(-28\) mV and polydispersities lower than 0.17 were obtained. The PVP/PAA complex nanogels obtained were found to show high colloidal stability [56]. Multifunctional poly(ethylene oxide)-poly(acrylic acid), PEO/PAA nanogels were also obtained from their interpolymer complexes by radiation-induced intramolecular crosslinking. PEO is a water-soluble nonionic polymer showing very high biocompatibility. Its complex with another biocompatible polymer, PAA, makes their nanogels very promising nanobiomaterials for biomedical applications. Prior to irradiation, the size of interpolymer complexes were reduced by using acetone, which is a non-solvent for both polymers. The effect of acetone on the contraction of coils was carefully investigated by dynamic light scattering technique before the formation of corresponding nanogels by gamma irradiation. A wide range of surface charge values of the nanogels prepared with equimolar stoichiometric ratio of polymer repeating units was achieved between \(-5.31\) mV (pH = 2) and \(-45.00\) mV (pH = 11). The size of the nanogels in the swollen state was between 100 and 240 nm. PEO/PAA nanogels synthesized in 25% acetone solution exhibited size stability over a period of 2 months. It was concluded that this simple method of preparing nanogels from polymer pairs avoids the synthesis of corresponding block copolymers as precursors of functional nanogels [57]. PAA at pH 3.5 was complexed with poly(acrylamide) in their aqueous solutions and irradiated with gamma rays up to 7 kGy dose. Biocompatibility of submicron gels were assessed by MIT assay [58]. Nanosilver embedded antimicrobial poly(acrylamide) bioactive nanogels were prepared by solution polymerization of monomer/AgNO\({}_{3}\) in nanoemulsion using gamma radiation. PAAm/Ag nanogel matrix with the size range of 10-50 nm was achieved. Bactericidal action of nanogels was found to be suitable for developing antimicrobial surfaces in surgical devices and synthetic implants [59]. More sophisticated nanogels were prepared by e-beam irradiation of micelle forming graft copolymers of thermosensitive poly(\(N\)-isopropylacrylamide) backbone and poly(2-ethyl 2-oxazoline) hydrophilic graft chains for specific compositions above the transition temperature of the backbone \(\sim\)32 \({}^{\circ}\)C. A low number of side chains (3 mol% graft sites) and a degree of polymerization at least 100 for the polyoxazoline chains were found to be necessary to keep the thermoresponsive behavior of the backbone and to stabilize the micelles with the hydrophilic side chains to avoid formation of large aggregates. These graft copolymers were used to form core/shell type nanogel with thermo-reversible swelling and deswelling by electron beam crosslinking in the micellar state at 38 \({}^{\circ}\)C. The crosslinking occurred mainly in the collapsed hydrophobic core, allowing further stabilization by the polyoxazoline chains. Thus, stable thermoresponsive nanogels of about 30 nm in the collapsed state were achieved. The reversibility of the volume change of the nanogels were verified eight times by dynamic light scattering [60]. Multifunctional nanogels with carefully controlled chemistry, size, surface charge and other properties can carry drugs and bioactive substances, providing a safer and effective therapy than conventional chemotherapy.