Photoinitiated free-radical polymerization of nonionized acrylic acid in aqueous solutions: Kinetics study using rheology

In this paper, a shear rheometer with a bottom quartz plate and a top reflective plate was modified to emit UV light [with wavelength ∼350–410 nm and absorbable light intensity ∼14.3 (W/m2)] on samples containing water, acrylic acid (0.15–0.30 gg−1), and photoinitiator (PI, 3.77–7.74 mM). The viscosity rise over time was used to in situ probe photoinduced free-radical polymerization (Photo-FRP). UV–vis measurements showed a small extinction coefficient [ε ∼29 1/(cm M)] at 357 nm with absorption coefficient ≤23 m−1, in line with the fact that sample thickness (0.55–2.2 mm) did not affect measurements, consistent with Beer–Lambert law predictions. Experimental steps involved oxygen removal, photon emission, inhibition, polymerization onset, and monitoring monomer conversion (X∼0%−99%). The inhibition period correlated with oxygen transport calculations. Size exclusion chromatography revealed Mw ∼2 000 000 g/mol and polydispersity ∼3.4. Frequency sweeps showed the progressive development of an elastic response during early polymerization (at ∼50 s), indicating polymer entanglement at Ce ≥ 0.005 gg−1 (∼0.33% monomer conversion). Viscosity–concentration relations (μ = 5.4*c0.99 for C < Ce and μ = 16.28*c4.28 for C > Ce) were used to estimate monomer conversion over time. A kinetic model assuming steady-state radical concentration and including inhibition, PI decomposition, propagation, and termination captured species evolution. Fractional free volume accounted for rate variations with monomer conversion, fitting experimental data well and yielding the photoinduced decomposition constant for the PI kd ∼ 1.75 × 10−5 1/s/(W/m2). The results of this work highlight the effectiveness of in situ rheology for studying liquid-state Photo-FRP and linking kinetic parameters to polymer molecular weight and process design.