It is well known that polymerization-induced self-assembly (PISA) can be used to prepare thermoresponsive diblock copolymer nano-objects. Herein, we have employed poly(tert-octyl acrylamide) (POAA) as a steric stabilizer for the PISA syntheses of spherical nanoparticles in n-heptane via reversible addition-fragmentation chain transfer (RAFT) dispersion polymerization of N,N-dimethylacrylamide (DMAC) at 70 °C. Interestingly, the colloidal stability of such dispersions depends on the type of n-alkane. Spherical nanoparticles were produced in n-heptane, n-octane and n-decane that remained colloidally stable on cooling to 20 °C. However, in n-dodecane and n-hexadecane, the colloidally stable POAA-PDMAC nanoparticles synthesized at 70 °C became flocculated on cooling to 20 °C. This is because the POAA stabilizer chains exhibit upper critical solution temperature (UCST)-type behavior in higher n-alkanes. Nanoparticle aggregation was characterized by variable temperature turbidimetry and dynamic light scattering experiments. Owing to the strongly amphiphilic nature of these nanoparticles (POAA is extremely hydrophobic while PDMAC is highly water-soluble), they were evaluated as putative Pickering emulsifiers. Thus POAA85-PDMAC150 nanoparticles with a z-average diameter of 45 nm were prepared in n-heptane. Such dispersions were homogenized with an equal volume fraction of water at various nanoparticle concentrations to produce a series of Pickering emulsions, which were sized by optical microscopy and laser diffraction. Unexpectedly, oil-in-water emulsions were invariably produced when using these highly hydrophobic nanoparticles, as confirmed by conductivity measurements. To investigate whether nanoparticle inversion occurred during high shear homogenization, core-crosslinked nanoparticles were prepared by chain-extending linear POAA85-PDMAC195 diblock copolymer nanoparticles using ethylene glycol diacrylate (EGDA). However, emulsification using these crosslinked nanoparticles also produced oil-in-water Pickering emulsions. Since nanoparticle inversion cannot occur in this case, we conclude that the hydrophobic nanoparticles must remain within the oil droplets and hence be adsorbed at their inner surface.

RAFT dispersion polymerization of N,N-dimethylacrylamide in a series of n-alkanes using a poly(tert-octyl acrylamide) steric stabilizer

ACS Spring Meeting 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.