# Radiation-Assisted Synthesis of Polymer-Based Nanomaterials
## 4 Polymer-Based Nanomaterials
### Nanogels
A very simple and effective method to quantitatively measure formation of double bonds on PVP was also proposed [47]. Although PVP is not a thermosensitive polymer, at relatively high temperature, the coils start to shrink due to loss of water of hydration with eventual collapse into tight coils. When thermally collapsed PVP coils are irradiated at relatively high temperatures intrachain crosslinks form leading to the formation of corresponding nanogels [48]. The results of a recent detailed parametric study give the guidelines for the radiation synthesis of poly(vinyl pyrrolone) (PVP) nanogels in the size range of 30-200 nm, which is the range most suitable for biomedical applications by using either gamma rays or accelerated electrons. Various parameters--polymer-based; molecular weight, concentration and radiation source-based; Co-60 vs. E-beam, dose rate and absorbed dose--were investigated systematically [49]. * PAA nanogels  
Poly(acrylic acid) is another water-soluble polymer approved by the FDA as biocompatible. Because of its polyelectrolyte nature, PAA has been shown to undergo predominantly chain scission over a broad pH range. Only in dilute solutions below pH = 3, where carboxylic acid groups are protonated, does crosslinking start to dominate. In other words, in order to synthesize PAA nanogels by radiation, one should work in dilute solutions at low pH under high dose rate irradiations. Hence, PAA nanogels were synthesized by pulse irradiation of deoxygenated PAA solutions with 6 MeV electrons [50,51]. In another work from the same group, PAA nanogels with hydrodynamic radii of 25-44 nm were synthesized by irradiating various concentrations of air-saturated PAA solutions with e-beam. When the same solutions were irradiated with gamma rays in the same dose range, the hydrodynamic radii of nanogels were found to be between 50-250 nm. One can control the dimensions of PAA nanogels by adjusting the concentration of polymer, pH of the solutions and absorbed dose and dose rate [52]. pH-sensitive nanogels of narrow size distribution were directly prepared by gamma irradiation of acrylic acid in aqueous solution of PVP as a template polymer. Once the radiation-formed PAA reached the proper length for complex formation with PVP, further irradiation allowed the formation of intra-particle crosslinks and hence, the development of nanogels. Hydrogen bonding between PVP and radiation polymerized PAA during the course of irradiation has facilitated crosslinking with the formation of nanogels. The size control of PAA-rich nanogels was achieved by controlling the feed concentration,  
Figure 2: The fate of polymer-bound radicals generated by OH radical attack in aqueous systems. Reprinted with permission from Ref. [47]. Copyright 2021 Elsevier. composition and absorbed dose [53]. Very similar PAA-based nanogels prepared from acrylic acid rich feed solutions at 20 kGy by the same group were tested as low-viscosity biomimetic fluid tears [54]. * Synthesis of nanogels from interpolymer complexes  
When complex formation between two different polymers in their aqueous solutions is favored, either due to hydrogen bonding or electrostatic or hydrophobic interactions, irradiation of these binary systems results with intra-coil crosslinks leading to the formation of nanogels composed of two polymers with different functionalities. This is an easy, straightforward method for the synthesis of multifunctional nanogels instead of using copolymeric precursors. Naturally, the two most frequently studied water-soluble polymers, namely PVP and PAA, form a good pair for the radiation synthesis of PVP/PAA nanogels in combined form. Detailed studies on the radiation formation of PVP/PAA nanogels from their hydrogen bonded complexes were performed by [PERSON] et al. [55]. A comprehensive analysis was made by using high molecular weight PVP and oligomeric PAA to obtain interpolymer complexes in the pH around 4.0.