Zhiyun Li

Synthesis and self-assembly of isotactic polystyrene-block-poly(ethylene glycol)

Isotactic polystyrene-block-poly(ethylene glycol) (iPS-b-PEG) was synthesized via a thiol–ene click coupling reaction of vinyl-terminated isotactic polystyrene (iPS–) with thiol-terminated poly(ethylene glycol) (PEG-SH). iPS– was prepared by the extremely highly isospecific polymerization of styrene with 1,4-dithiabutandiyl-2,2′-bis(6-tertbutyl-4-methylphenoxy) titanium dichloride and methylaluminoxane (MAO) in the presence of 1,7-octadiene as a chain transfer agent. PEG-SH was synthesized by the direct esterification of hydroxyl-terminated PEG (PEG-OH) with 3-mercaptopropionic acid using HfCl4·2THF as a catalyst. The behavior and micelle morphology of the crystallization-driven self-assembly of iPS-b-PEG in N,N-dimethylformamide (DMF) were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), dynamic light scattering (DLS), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The results revealed that the crystallinity of the iPS blocks in the micelle cores was increased with increasing the storage time of the micelle solution at room temperature, and the increase of crystallinity led to the transition from spherical to petal-like micelles. With time these petal-like micelles could self-associate into flower-like aggregates. Moreover, when the newly prepared micelle DMF solution of iPS-b-PEG was dialyzed against deionized water, bowl-like micelles were formed due to the diffusion of DMF from the iPS cores into the aqueous phase.

Glass transition of polystyrene nanospheres under different confined environments in aqueous dispersions

Two aqueous dispersions of anionic and nonionic polystyrene (PS) latex nanospheres were prepared via activators generated by electron transfer (AGET) atom transfer radical polymerization (ATRP) and reverse ATRP in microemulsions using sodium dodecyl benzene sulfonate (SDBS) and polyoxyethylene (20) oleyl ether (Brij 98) as surfactant, respectively. Moreover, the third aqueous dispersion of surfactant-free PS nanospheres was obtained by the dialysis of a PS microemulsion against deionized water. The molecular weight distributions (Mw/Mn) of the resultant PS with controlled-molecular weight were less than 1.7. We investigated the glass transition of these PS nanospheres in different aqueous dispersions by nano-DSC. We observed an apparent size-dependent glass transition on the surfactant-free PS nanospheres and nonionic PS latex nanospheres, and that the glass transition temperature (Tg) decreases with the reduction of size for PS nanospheres. However, the anionic PS latex nanospheres show an unambiguous glass transition near to the bulk PS. These results suggest that the effects of size and interface on glass transition may become significant as the diameter of the polymer nanospheres is decreased to less than 100 nm.

Crystalline-coil diblock copolymers of syndiotactic polypropylene-b- poly(ethylene oxide): synthesis, solution self-assembly, and confined crystallization in nanosized micelle cores

In this paper, two crystalline-coil diblock copolymers consisting of blocks of syndiotactic polypropylene (sPP) with different syndiotacticities ([rrrr]sPP1 = 0.78, [rrrr]sPP2 = 0.94) and poly(ethylene oxide) (PEO) were synthesized utilizing the click reaction of azide-terminated PEO (PEO–N3) with alkynyl-terminated sPP (sPP–C≡CH), which was obtained via the syndiospecific polymerization of propylene using fluorinated bis(phenoxyimine) titanium catalysts and further end-group transformation. All products and intermediates were characterized by nuclear magnetic resonance (NMR) spectroscopy and gel permeation chromatography (GPC). We probed the temperature-induced self-assembly of sPP-b-PEO in the solvent N,N′-dimethylformamide (DMF), which is selective for PEO. The self-assembly behavior and micellar morphology of each sPP-b-PEO were investigated by dynamic light scattering (DLS), transmission electron microscopy (TEM), and atomic force microscopy (AFM). Before sPP crystallization, the crystalline-coil diblock copolymers self-assembled into spherical micelles with an amorphous sPP core and a soluble PEO corona. Platelet-like micelles with a crystallized sPP core sandwiched between two solvent-swollen PEO layers then formed upon the crystallization of the sPP block. Furthermore, the confined crystallization of sPP within nanosized micelle cores was investigated by differential scanning calorimetry (DSC). An enormous degree of supercooling was observed during crystallization due to the homogeneous nucleation.

Crystallization-driven Self-assembly of Isotactic Polystyrene in N , N -Dimethylformamide

Herein we demonstrate crystallization-driven self-assembly of isotactic polystyrene (iPS) with high isotacticity and narrow molecular weight distribution and crystallization-induced switching of the morphology of iPS aggregates in N, N-dimethylformamide (DMF). The formation and morphology switching of the self-assembled aggregates of iPS are investigated by means of dynamic light scattering (DLS), scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and wide angle X-ray diffraction (WXRD). The results reveal that cooling DMF solution of iPS promotes iPS chains to self-assemble into spherical aggregates with a gelled core cross-linked by microcrystals, which is surrounded by solvent-swollen corona. Furthermore, crystallization induces the deformation of iPS aggregates from spherical to plate-like or nest-like.