Abstract
Ultra-high molecular weight (UHMW, M n > 1000 kDa) polymeric drift control adjuvants (DCAs) for agricultural spraying are prone to mechanical degradation and rapidly lose performance. To overcome this, we have designed linear coordination polymers (LCPs) composed of 400 kDa telechelic bis-terpyridine end-functionalised polyacrylamide units, which 'self-heal' upon shearing through reformation of coordination bonds. After addition of Fe(ii) to dilute aqueous solutions of the terpyridine telechelics, UHMW LCPs were obtained as demonstrated by UV-vis spectroscopy, MALS GPC and intrinsic viscosity measurements. Importantly, these UHMW LCPs were shown to function as effective DCAs, reducing the formation of fine 'driftable' droplets during spray testing at concentrations as low as 100 ppm. Following mechanically-induced coordination bond-scission, the UHMW LCPs were found to recover up to 90% of their performance compared to un-sheared samples, at a rate dependent on the transition metal ion used to form the complex.
Highlights
Ultra-high molecular weight (UHMW, Mn > 1000 kDa) linear polymers are able to increase solution extensional viscosity at concentrations as low as 100 ppm, with application in mist control,[1] drag reduction[2,3] and reducing agrichemical spray dri .4,5 Extensional stresses along the elongated chain can cause mechanical degradation of the polymer, with longer chains being more susceptible due to a greater accumulation of stress.[6]
To overcome this loss of performance for drift control adjuvants (DCAs), we investigated if linear coordination polymers (LCPs) could associate into UHMW species in dilute aqueous solution, and if so, reversibly break and reassociate a er shearing
Macro-chain transfer agent (CTA) were rst synthesised with either AM or N,N-dimethylacrylamide (DMA) monomer in DMSO, before chain-extending with AM to approximately 400 kDa (Scheme 2). Both macro-CTA and chain-extension polymerisations were initiated by photolysis of the reversible addition– fragmentation chain transfer (RAFT) agent under visible light irradiation (DP-RAFT),[44] which we have found is well-suited to high molecular weight (Mn > 1000 kDa) AM polymerisations.[41]
Summary
Ultra-high molecular weight (UHMW, Mn > 1000 kDa) linear polymers are able to increase solution extensional viscosity at concentrations as low as 100 ppm, with application in mist control,[1] drag reduction[2,3] and reducing agrichemical spray dri .4,5 Extensional stresses along the elongated chain can cause mechanical degradation of the polymer (covalent bond scission), with longer chains being more susceptible due to a greater accumulation of stress.[6]. Spray dri during agrichemical spraying can cause damage to neighbouring crops and the environment; and is more likely to occur for sprays with ne, ‘dri able droplets’ (generally < 150 mm in diameter).[7,8] UHMW polyacrylamide and polyethyleneoxide dri control adjuvants (DCAs) have been shown to initially reduce formation of ne droplets, a er repeated circulations through a centrifugal pump (causing mechanical degradation) they were found to have similar performance to water alone.[9] To overcome this loss of performance for DCAs, we investigated if linear coordination polymers (LCPs) could associate into UHMW species in dilute aqueous solution, and if so, reversibly break and reassociate a er shearing. The DP can be crudely described by a function of the concentration (c) and association constant (Ka) (eqn (1)),[14,15] many other factors not accounted for in this equation (such as end-group delity) can have an effect.[16]
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