Tailoring agrichemical release kinetics through material design: Understanding the counterintuitive effect of matrix hydrophobicity

Nitrification inhibitors (NIs), such as dicyandiamide (DCD), can improve the nitrogen uptake efficiency by plants and reduce environmental losses. Unfortunately, DCD degrades rapidly through microbial action in tropical conditions, limiting its efficacy. Here, the encapsulation and controlled release of DCD as a model water-soluble, crystalline agrichemical was studied within biodegradable matrices comprising blends of poly(3-hydroxybutyrate- co -3-hydroxyvalerate) (PHBV) and poly(ε-caprolactone) (PCL). DCD was mixed at 40 wt% and extruded with PHBV:PCL blends at polymer mass ratios of 1:0, 3:1, 1:1, 1:3 and 0:1 to produce ∼3 × 3 mm cylindrical pellets. The release kinetics into water were monitored over 12 weeks at 10, 23 and 40 °C. A Fickian diffusion model fitted the data well and the diffusivities followed an Arrhenius dependence on temperature. Counterintuitively, the more hydrophobic PHBV matrix released DCD the fastest. Release kinetics slowed as the PCL content increased, except for the neat PCL matrix. The lower affinity between the hydrophobic PHBV and hydrophilic DCD led to the DCD being relatively excluded from the PHBV phase and hence more accessible to interconnected voids and channels. In contrast, the higher affinity between PCL and DCD led to PCL-coated DCD crystals, which reduced direct water access to the DCD. Therefore, as the content of PCL was increased, the layer of PCL through which water and dissolved DCD must diffuse also increased, slowing release. This work demonstrated the ability to control DCD release from a biopolymer matrix through material design, with complete release taking between a few days to several months depending on the blend ratio. • Controlled-release NIs can extend their effectiveness to reduce nitrogen losses. • Extrusion is a viable processing technique to form DCD-biopolymer matrices. • PHBV and PCL are excellent polymer candidates as both readily degrade in soil. • Counterintuitively, increasing matrix hydrophobicity increased release rates.