Multi-functionalized Polymer Nanomaterials for Environmental Applications

Demand for food, energy and other critical resources is increasing with the growth of the global population. Existing materials and industrial practices cannot meet these demands sustainably. Polymer nanomaterials with densified functional groups and engineered specific physicochemical properties, including size, charge, shape, hydrophobicity and surface to volume ratio can be used to create highly efficient functional nanomaterials for environmental applications,

including heavy metal removal from environmental media, resource recovery from waste streams and targeted agrochemical delivery. This thesis develops and tests two kinds of polymer nanomaterials: (1) polymer nanogel sorbents with a high density of chelating groups for efficient

and selective heavy metal removal and rare earth element (REE) recovery, and (2) environmentally responsive polymer nanocarriers for controlled and efficient agrochemical delivery. The first objective of this work was to develop and test polymer nanogels with abundant, densely packed thiol chelating groups for mercury removal from both water and hydrocarbons, and for REE recovery from large scale industrial aqueous waste streams such as coal fly ash. Under the first objective, a thiol abundant polymer nanogel was synthesized and the hydrophilic thiol groups and hydrophobic benzene groups in the nanogel enabled its dispersability in both liquid hydrocarbons and water. This unique amphiphilicity and abundance of thiol groups allowed the nanogel to efficiently remove relevant mercury species from both produced water and gas condensate. A phosphate abundant polymer nanogel (PPN) was developed for rare earth elements (REE) recovery from waste streams. The PPN had one of the highest REE sorption capacities (qm)

and distribution coefficients (Kd) reported and could concentrate REEs in coal fly ash leachate twenty-one-folds in one sorption-desorption cycle. The second objective of this thesis was to develop polymer nanocarriers with temperature and pH responsive functional groups that enable temperature programmed agent delivery to

combat plant heat stress. A temperature of 40 °C was used to trigger agent release from poly(acrylic acid)-block-poly(N-isopropyl acrylamide) (PAA-b-PNIPAm) star polymers and high aspect ratio P[BiBEM-g-(PAA-b-PNIPAm)] polymer bottlebrushes in vivo in tomato plants. Both star polymers and bottlebrushes had significant (~20-40%) phloem loading, indicating that a high aspect ratio does not inhibit nanocarrier translocation in plants. Both nanocarriers had near complete uptake into leaf mesophyll and were mainly transported through apoplasts to the vasculature. Near the vascular bundle, nanocarrier was found inside mesophyll cells, suggesting symplastic phloem loading. Overall, these materials can provide nearly 100% efficient uptake of

agrochemicals into leaves and can deliver those agents in response to an environmental stress (temperature), making them highly promising agents for increasing the sustainability of agriculture. The third objective was to design and synthesize a reactive oxygen species (ROS)

responsive nanocarrier that alleviates plant stress by scavenging ROS in the leaf while releasing plant nutrients. ROS responsive poly(acrylic acid)-block-poly((2-(methylsulfinyl)ethyl acrylate)- co-(2-(methylthio) ethyl acrylate)) (PAA-b-P(MSEA-co-MTEA)) star polymer (RSP) was synthesized and the ROS scavenging functionality was confirmed both in-vitro and in-vivo. Apart from scavenging ROS, RSP can be loaded with plant nutrients such as Mg2+. After foliar application, RSP penetrated through the leaf epidermis and distributed around the chloroplasts. The foliar applied RSP increased the plant carbon assimilation rate, quantum yield of CO2 assimilation, Rubisco carboxylation rate and photosystem II quantum yield compared to untreated plants under heat and light stress. Mg loaded RSP enhanced carbon assimilation of Mg deficient plants by promoting Rubisco activity. The fourth objective was to determine how size, charge content and hydrophobicity could be tuned for efficient polymer nanocarrier foliar uptake and translocation to specific plant tissues. PAA-b-PMSEA and PAA-b-P(MSEA-co-MTEA) star polymers with well controlled size, negative charge content and hydrophobicity were synthesized, and their leaf uptake and distribution in tomato plants were quantified. Despite the differences in star polymer properties, ~30% of applied star polymers translocated to other plant organs, much higher than traditional agrochemicals (<3%). The smaller star polymer exhibited more transport to younger leaves, while the larger star polymers had enhanced transport to roots. For the same sized star polymers, lower charge content promoted translocation to younger leaves and roots, whereas larger negative charge content had lower overall mobility in plants. Star polymer hydrophobicity affected their leaf uptake

pathway, but not translocation in plants. This fundamental insight into the property-transport relationships of polymer nanocarriers in plants can be leveraged to greatly improve agrochemical delivery over current approaches. Overall, this thesis invented several new polymer nanomaterials for environmental applications: polymer nanogels for metal sequestering and environmentally responsive polymer

nanocarriers for efficient agrochemical delivery. It determined the nanocarrier properties affecting plant uptake and translocation, providing design parameters for polymer nanomaterials that can be used to reduce environmental impacts during resource acquisition, including removing hazardous metals from oil and gas, recovering REEs from waste streams, delivering agrochemicals and alleviating plant stress according to plants’ demand.