No abstract

A series of all-acrylic poly(lauryl acrylate)− poly(benzyl acrylate) (PLA−PBzA) diblock copolymer nanoparticles are prepared by reversible addition−fragmentation chain transfer (RAFT) dispersion polymerization of benzyl acrylate in n-heptane, n-dodecane, or isohexadecane. As the PBzA block grows from the soluble PLA block it eventually becomes insoluble, which drives in situ polymerization-induced self-assembly (PISA). High monomer conversions (>99%) can be achieved and high blocking efficiencies are observed using 1 H NMR spectroscopy and gel permeation chromatography, respectively. However, final M w /M n values range from 1.36 to 2.10, which suggests that chain transfer to polymer occurs in these all-acrylic PISA formulations. The soft, film-forming nature of these all-acrylic nanoparticles makes conventional TEM studies problematic. However, inspecting the visual appearance of these dispersions combined with DLS studies allows the construction of a phase diagram, which has been validated by cryo-TEM studies of selected copolymers. The latter technique confirms that spherical, worm-like or vesicular morphologies can be obtained depending on the copolymer concentration, mean degree of polymerization of the core and stabilizer blocks, and choice of solvent. Oscillatory rheology studies indicate that PLA− PBzA worms form free-standing worm gels at 20 °C with relatively low moduli (G′ ∼ 20 Pa). Moreover, reversible thermal transitions are observed below ∼15 °C and above ∼67 °C. Finally, worm gels that exhibit critical gelation concentrations as low as 2.5% w/w at 20 °C can be prepared at up to 40% w/w solids using a convenient one-pot protocol. In summary, this new PISA formulation represents a cost-effective, facile synthesis route to all-acrylic nano-objects in non-polar solvents.

A near-monodisperse monohydroxy-terminated polydimethylsiloxane (PDMS; mean degree of polymerization = 66) was esterified using a carboxylic acid-functionalized trithiocarbonate to yield a PDMS66 precursor with a mean degree of functionality of 92 ± 5 % as determined by 1 H NMR spectroscopy. This PDMS66 precursor was then chain-extended in turn using nine different methacrylic monomers in a low-viscosity silicone oil (decamethylcyclopentasiloxane, D5). Depending on the monomer type, such PISA syntheses proceeded via either RAFT dispersion polymerization or RAFT emulsion polymerization. In each case the target DP of the core-forming block was fixed at 200 and the copolymer concentration was 25 % w/w. Transmission electron microscopy studies indicated that kinetically-trapped spheres were obtained in almost all cases. The only exception was 2-(dimethylamino)ethyl methacrylate (DMA), which enabled access to spheres, worm or vesicles. This striking difference is attributed to the relatively low glass transition temperature for this latter block. A phase diagram was constructed for a series of PDMS66-PDMAx nano-objects by systematically increasing the PDMA target DP from 20 to 220 and varying the copolymer concentration between 10 and 30 % w/w. Higher copolymer concentrations were required to access a pure worm phase, whereas only spheres, vesicles or mixed phases were accessible at lower copolymer concentrations. Gel permeation chromatography studies indicated a linear evolution of number-average molecular weight (Mn) with PDMA DP while dispersities remained below 1.39, suggesting relatively well-controlled RAFT polymerizations. Small angle x-ray scattering (SAXS) was used to characterize selected examples of spheres, worms and vesicles. PDMS66-PDMA100-112 worms synthesized at 25-30 % w/w formed freestanding gels at 20 °C. Oscillatory rheology studies performed on a 30 % w/w PDMS-PDMA105 worm dispersion indicated a storage modulus (gel strength) of 1057 Pa and a critical gelation concentration (CGC) of 12 % w/w. Finally, PDMS66-PDMAx worms could also be prepared in n-dodecane, hexamethyldisiloxane or octamethylcyclosiloxane. Rotational rheometry studies indicate that such worms are efficient viscosity modifiers for these non-polar oils.

Do isomeric core-forming blocks afford the same thermo-responsive behavior for diblock copolymer worm gels?

Increased plasma PAI-1 activity in obese human subjects may result from PAI-1 release from an increased mass of adipose tissue, particularly in association with thrombosis and elaboration of TGF-beta from platelet alpha-granules into the circulation. The increased PAI-1 may exacerbate vascular disease by shifting the balance between thrombosis and thrombolysis toward thrombosis and consequently exposing luminal surfaces of vessels to mitogens associated with microthrombi over protracted intervals.