Abstract
Polymer-grafted nanomaterials based on carbon allotropes and their derivatives (graphene oxide (GO), etc.) are typically prepared by successive reaction stages that depend upon the initial functionalities in the nanostructure and the polymerization type needed for grafting. However, due to the multiple variables involved in the functionalization steps, it is commonly difficult to predict the properties in the final product and to correlate the material history with its final performance. In this work, we explored the steps needed to graft the carboxylic acid moieties in GO (COOH@GO) with a pH-sensitive polymer, poly[2-(diethylamino)ethyl methacrylate] (poly[DEAEMA]), varying the reactant ratios at each stage prior to polymerization. We studied the combinatorial relationship between these variables and the behavior of the novel grafted material GO-g-poly[DEAEMA], in terms of swelling ratio vs. pH (%Q) in solid specimens and potentiometric response vs. Log[H+] in a solid-state sensor format. We first introduced N-hydroxysuccinimide (NHS)-ester moieties at the –COOH groups (GO-g-NHS) by a classical activation with N-ethyl-N′-(3-dimethylaminopropyl)carbodiimide (EDC). Then, we substituted the NHS-ester groups by polymerizable amide-linked acrylic moieties using 2-aminoethyl methacrylate (AEMA) at different ratios to finally introduce the polymer chains via radical polymerization in an excess of DEAEMA monomer. We found correlated trends in swelling pH range, interval of maximum and minimum swelling values, response in potentiometry and potentiometric linear range vs. Log[H+] and could establish their relationship with the combinatorial stoichiometries in synthetic stages.
Highlights
Nanostructured carbon allotropes such as carbon nanotubes [1] and graphene [2] are among the most versatile nanomaterials due to their useful chemical, mechanical and electrical properties: e.g., size-dependent interfacial reactivity/solubility, high Young’s modulus, excellent electrical conductivity, high double-layer capacitance, etc. [1,2,3,4,5,6,7]
All reagents and solvents were used as received, unless otherwise stated. 2-(N-morpholino) ethanesulfonic acid (MES; 99%), N-(3-dimethylaminopropyl)-N -ethylcarbodiimide hydrochloride (EDC; 98%), N-hydroxysuccinimide (NHS; 98%), Na2HPO4·12H2O (99%), NaH2PO4·2H2O (99%), 2-aminoethyl methacrylate hydrochloride (AEMA; 90%), azobisisobutyronitrile (AIBN; 12 wt % in acetone), 2-(diethylamino)ethyl methacrylate (DEAEMA; 99 %), tetrahydrofuran (THF; 99%) and carboxylated graphene oxide (GO; 4.09 mmol COOH/g and 75.3%C content according to provider specifications) used in synthesis Stages I, II and III were provided by Sigma-Aldrich Co. (Saint-Louis, MI, USA)
As will be shown ahead, such a minor difference in the amount of functionalization would represent an important modulator of performance in the materials once being grafted with poly[DEAEMA], exhibiting that the hybrid material behavior strongly depends on the material history
Summary
Nanostructured carbon allotropes such as carbon nanotubes [1] and graphene [2] are among the most versatile nanomaterials due to their useful chemical, mechanical and electrical properties: e.g., size-dependent interfacial reactivity/solubility, high Young’s modulus, excellent electrical conductivity, high double-layer capacitance, etc. [1,2,3,4,5,6,7]. In all cases, it is necessary to first introduce superficial reactive sites at the starting carbonaceous material via suitable chemical reactions such as carboxylation, hydroxylation, amidation, esterification or silanization [25,28,29], commonly through intermediate activation steps (e.g., the classical use of carbodiimides prior to the formation of labile esters with N-hydroxysuccinimide for further amidation of carboxylated nanocarbons [30,31,32]) In this way, the range of useful reactive groups that can be introduced to the carbon nanomaterial surface, for example, for further polymer grafting, is as large as the polymerization chemistries available in the literature, which rely on three approaches [33]: (i) grafting through, that consists of polymer chains incorporated into the superficial reactive groups through the propagation reaction; (ii) grafting from, i.e., propagation of the polymer chains from surface-attached initiators; and (iii) grafting to, or attachment of, previously synthesized polymer chains to the reactive groups. Vinylic or acrylic moieties can be superficially incorporated to the nanocarbons for further grafting through radical polymerization [16,34,35], alkyl halides or alkyl trithiocarbonates can be, respectively, used in grafting from for successive atom transfer radical polymerization (ATRP) [36] or reversible addition–fragmentation chain-transfer (RAFT) [37] polymerization and azido-terminated polymers can be otherwise exploited in grafting to using click chemistry [38,39]
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