Synthesis and Characterization of Zwitterion-Containing Acrylic (Block) Copolymers for Emerging Electroactive and Biomedical Applications
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Zwitterionomer / ionic liquid binary compositions of poly(nBA-co-SBMA) and 1-ethyl-3-methylimidazolium ethylsulfate (EMIm ES) were prepared using both the â swellingâ and the â cast withâ methods. Dynamic mechanical analysis revealed that the â swollenâ membranes maintained their thermomechanical performance with up to 18 wt% EMIm ES incorporation, while that of the â cast withâ membranes decreased gradually as the ionic liquid concentration in the composite membranes increased. Small-angle X-ray scattering results indicated that the â swollenâ and the â cast withâ membranes have different morphologies, with the ionic liquid distributed more evenly inside the â cast withâ membranes. Impedance spectroscopy results showed that the â cast withâ membranes had better ionic conductivity than the â swollenâ membrane at high ionic liquid concentration, in agreement with our proposed model. The results indicated that the different processing methods had a significant impact on thermomechanical properties, ionic conductivities, as well as morphologies of the zwitterionomer / ionic liquid binary compositions.
Reversible addition-fragmentation chain transfer polymerization (RAFT) strategy afforded the synthesis of well-defined poly(sty-b-nBA-b-sty). 2-(Dimethylamino)ethyl acrylate (DMAEA), a tertiary amine-containing acrylic monomer, exhibited radical chain transfer tendency toward itself, which is undesirable in controlled radical polymerization processes. We employed a higher [RAFT] : [Initiator] ratio of 20 : 1 to minimize the impact of the chain transfer reactions and yielded high molecular weight poly[sty-b-(nBA-co-DMAEA)-b-sty] with relatively narrow PDIs. The presence of the tertiary amine functionality, as well as their quaternized derivatives, in the central blocks of the triblock copolymers afforded them tunable polarity toward polar guest molecules, such as ionic liquids. Gravimetric measurements determined the swelling capacity of the triblock copolymers for EMIm TfO, an ionic liquid. DSC and DMA results revealed the impact of the ionic liquid on the thermal and thermomechanical properties of the triblock copolymers, respectively. Composite membranes of DMAEA-derived triblock copolymers and EMIm TfO exhibited desirable plateau moduli of ~ 100 MPa, and were hence fabricated into electromechanical transducers.
RAFT synthesized poly(sty-b-nBA-b-sty) triblock copolymer phase separates into long-range ordered morphologies in the solid state due to the incompatibility between the poly(nBA) phases and the poly(sty) phases. The incorporation of DMAEA into the central acrylic blocks enabled subsequent quaternization of the tertiary amines into sulfobetaine functionalities. Both DSC and DMA results suggested that the electrostatic interactions in the low Tg central blocks of poly(sty-b-nBA-b-sty) enhanced block copolymer phase separation. SAXS results indicated that the presence of the sulfobetaine functionalities in acrylate phases increased electron density differences between the phases, and led to better defined scattering profiles. TEM results confirmed that the block copolymers of designed molecular weights exhibited lamellar morphologies, and the lamellar spacing increased with the amount of electrostatic interactions for the zwitterionic triblock copolymers.
Acrylic radicals are more susceptible to radical chain transfer than their styrenic and methacrylic counterparts. Controlled radical polymerization processes (e.g. RAFT, ATRP and NMP) mediate the reactivity of the acrylic radical and enable the synthesis of well-defined linear poly(alkyl acrylate)s. However, functional groups such as tertiary amine and imidazole on acrylic monomers interfere with the controlled radical polymerization of functional acrylates. Model CFR and RAFT polymerization of nBA in the presence of triethylamine and N-methyl imidazole revealed the interference of the functional group on the polymerization of acrylate. Various RAFT agents, RAFT agent to initiator ratios, degree of polymerization and monomer feed concentrations were screened with an imidazole-containing acrylate for optimized RAFT polymerization conditions. The results suggest that the controlled radical polymerization of functional acrylates, such as 2-(dimethylamino)ethyl acrylate and 4-((3-(1H-imidazole-1-yl)propanoyl)oxy)-butyl acrylate (ImPBA), remained challenging.
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