Nano- and Microgels of Poly(Vinyl Methyl Ether) Obtained by Radiation Techniques

Janusz M. Rosiak

Institute of Applied Radiation Chemistry, Technical University of Lodz,

Wroblewskiego St. 15, 93-590 Lodz, Poland

Abstract

Hydroxyl radicals were generated radiolytically in N2O-saturated aqueous solutions in the presence of poly(vinyl methyl ether) (PVME, 6104 Da, 10-3-10-2 moldm-3 in monomer units). As measured by pulse radiolysis, they react (k = 2.2108 dm3mol-1s-1) with PVME by giving mainly rise to a-alkoxyalkyl radicals (~72%) that reduce (k 2109 dm3mol-1s-1) Fe(CN)63-, IrCl62- or tetranitromethane. Based on the formaldehyde yield in the presence of the latter two oxidants (~40% of OH), it is concluded that OH radicals undergo H-abstraction at ROCH2H, R3CH and R2HCH with probabilities of ~40%, ~32% and ~28%, respectively. The momentary rate constant of the decay of the PVME radicals depends on the number of radicals per polymer chain and drops as they decay. The yield of intermolecular crosslinks, as measured by an increase of the molecular weight, strongly increases with decreasing dose rate, and it is concluded that the majority of crosslinks occur intramolecularly, even at the lowest dose rate used (0.0015 Gys-1, G(intermolecular crosslinks) = 0.6210-7 molJ-1).

At low temperatures and low polymer concentrations the intramolecular crosslinking reaction is favored. A large number of radicals are formed in a short period of time during each pulse of accelerated electrons. The combination of the formed polymeric radicals is in the intramolecular way and nanogels formation occurs. Without changes in the molecular weight the dimension (radius of gyration and hydrodynamic radius), as well as the intrinsic viscosity of the nanogels decreases with increasing radiation dose. At temperatures above the LCST PVME molecules collapse to globular particles. Electron beam irradiation of these stable phase-separated structures leads to the formation of temperature-sensitive microgel particles. The additive-free method of crosslinking of polymers in aqueous solutions by high-energy radiation offers the application of these microgels in the field of medicine because of no remaining toxic substances (monomers, initiators, crosslinkers, etc.). The variation of the crosslinking density and the particle diameter can be performed by varying the polymer concentration and the radiation dose. Applying the closed-loop system reduces the amount of un-crosslinked molecules (sol content).