Various carboxylic acid-functionalized poly(N’N-dimethylacrylamide) (PDMAC) macromolecular chain transfer agents (macro-CTAs) were chain-extended with diacetone acrylamide (DAAM) by reversible addition-fragmentation chain transfer (RAFT) aqueous dispersion polymerization at 70 °C to produce a series of PDMAC-PDAAM diblock copolymer nano-objects at 20 % w/w solids via polymerization-induced self-assembly (PISA). TEM studies indicate that a PDMAC macro-CTA with a mean degree of polymerization (DP) of 68 or higher results in the formation of well-defined spherical nanoparticles with mean diameters ranging from 40 to 150 nm. In contrast, either highly anisotropic worms or polydisperse vesicles are formed when relatively short macro-CTAs (DP = 40-58) are used. A phase diagram was constructed to enable reproducible targeting of pure copolymer morphologies. Dynamic light scattering (DLS) and aqueous electrophoresis studies indicated that, in most cases, these PDMAC-PDAAM nano-objects are surprisingly resistant to changes in either solution pH or temperature. However, PDMAC40-PDAAM99 worms do undergo partial dissociation to form a mixture of relatively short worms and spheres on adjusting the solution pH from pH 2-3 to around pH 9 at 20 °C. Moreover, a change in copolymer morphology from worms to a mixture of short worms and vesicles was observed by DLS and TEM on heating this worm dispersion to 50 °C. Post-polymerization crosslinking of concentrated aqueous dispersions of PDMAC-PDAAM spheres, worms and vesicles was performed at ambient temperature using adipic acid dihydrazide (ADH), which reacts with the hydrophobic ketone-functionalized PDAAM chains. The formation of hydrazone groups was monitored by FT-IR spectroscopy and afforded covalently-stabilized nano-objects that remained intact on exposure to methanol, which is a good solvent for both blocks. Rheological studies indicated that cross-linked worms formed a stronger gel compared to the linear precursor worms.

Preparation and cross-linking of all-acrylamide diblock copolymer nano-objects via polymerization-induced self-assembly in aqueous solution

254th National Meeting (2017)

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