A novel difunctional alkoxyamine initiator, DEPN2, was synthesized and utilized as an efficient
initiator in nitroxide-mediated controlled radical polymerization of triblock copolymers. Complementary hydrogen-bonding triblock copolymers containing adenine (A) and thymine (T) nucleobase-functionalized outer blocks
were synthesized. These thermoplastic elastomeric block copolymers contained short nucleobase-functionalized
outer blocks (M
n ∼ 1K−4K) and n-butyl acrylate rubber blocks of variable length (M
n ∼ 14K−70K). Hydrogen-bonding interactions were observed for blends of the complementary nucleobase-functionalized block copolymers
in terms of increased specific viscosity as well as higher scaling exponents for specific viscosity as a function of
solution concentration. In the solid state, the blends exhibited evidence of a complementary A−T hard phase,
which formed upon annealing, and dynamic mechanical analysis (DMA) revealed higher softening temperatures.
Morphological development of the block copolymers was studied using SAXS and AFM, which revealed
intermediate interdomain spacings and surface textures for the blends compared to the individual precursors.
Hydrogen-bonding interactions enabled the compatibilization of complementary hydrogen-bonding guest molecules
such as 9-octyladenine.
Phosphonium ion-containing acrylate triblock (ABA) copolymers were synthesized using nitroxide mediated radical polymerization. The polymerization of styrenic phosphonium-containing ionic liquid monomers using a difunctional alkoxyamine initiator, DEPN2, afforded an ABA triblock copolymer with an n-butyl acrylate soft center block (DP ∼ 400) and symmetric phosphonium-containing external reinforcing blocks (DP < 30). Two phosphonium monomers with different alkyl substituent lengths enabled an investigation of the effects of ionic aggregation of phosphonium cations on the physical properties of ABA block copolymer ionomers. Subsequently, the thermomechanical properties and morphologies of these materials were compared to a noncharged triblock copolymer analogue with neutral polystyrene external blocks. Shortening the alkyl substituents on the phosphonium cation enhanced the hydrophilicity of tributyl-4-vinylbenzyl phosphonium chloride (BPCl) relative to trioctyl-4-vinylbenzyl phosphonium chloride (OPCl). In both cases, phosphonium cations promoted microphase-separation and thermoplastic elastomer performance for the OPCl- and BPCl-containing triblock copolymers compared to a less well-defined, microphase segregated morphology for the styrene analogue. Dynamic mechanical analysis (DMA) of phosphonium-containing triblock copolymers exhibited well-defined rubbery plateau regions, whereas the plateau was shortened for the nonionic analogue. The solid state morphologies of the block copolymers were studied using small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM), and both techniques revealed phase separation at the nanoscale. DMA studies indicated that phosphonium aggregation governed flow activation energies.
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