Pesticide-treated water is used frequently in agricultural applications; nonetheless, because small droplets drift off-target, 50 to 80% of sprayed pesticide does not interact with the targeted plants. Covalent polymers suppress the formation of small droplets; however, high molecular weight polymers, highly effective candidates, are also highly prone to degradation by chain scission. Cameron and coworkers have demonstrated that terpyridine-ended poly(acrylamide) (PAM) can reduce small droplet formation from spray. The associative nature of the terpyridine-ended polymers maintains the improvement after mechanically induced scission. However, simply increasing the population of large droplets may result in increased droplet bouncing and breakup, rendering the crop untreated and pesticide released into the environment. We study the ultimate droplet fate and its relationship to the fluid properties of the polymer solution. In the target application, solutions undergo both elongational and shear flow. Thus, studying the polymer solution behavior under elongational flow, in addition to shear, is essential to understanding of the behavior. We have synthesized covalent and associatively bonded PAM molecules and measured the elongational viscosity and relaxation time of solutions using dripping-onto-substrate extensional rheometry. In the solutions of terpyridine-ended polymers, we observe a three-fold increase in both elongational properties in the presence of iron ions, indicating most active polymers have dimerized. Aqueous solutions with long polymer chains have high elongational viscosity and demonstrate dampened droplet bounce behavior upon impact with a hydrophobic surface, resulting in improved droplet retention. Overall, the effects are more pronounced at higher polymer molecular weights and concentrations. The associative solutions of PAM result in improved droplet adhesion, recovery from mechanically induced scission, and the reduced fine “driftable” droplets demonstrated in literature. This combination yields an aqueous solution that can reduce use of pesticide and fertilizer without reducing efficacy by ensuring more treated water ultimately stays on target surfaces.

Long, associative, telechelic poly(acrylamide) under shear and elongational flow

ACS Spring 2022

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