A poly(n-butyl acrylate)-based star polymer, polyEGDA-(polyBA) n , was synthesized by atom transfer radical polymerization using a modified core-first method. Further polymerization of a disulfide (SS) cross-linking agent bis(2-methacryloyloxyethyl) disulfide from the arm end produced a SS cross-linked star polymer. The disulfide functionality could be cleaved via reducing reactions, generating individual stars containing SH groups at the chain ends. The transformation between SS cross-linked star and SH-star was reversible via repetitive reduction and oxidation. Dynamic light scattering measurements showed that the average diameter of the cleaved star polymer was around 20 nm. The dynamic mechanical properties of these star copolymers were characterized through examination of the temperature dependencies of their shear moduli. The results showed that SS-functionalized star polymers respond to reduction−oxidation conditions, indicating that the disulfide bonds do cleave and re-form. These stimuli responsive star polymers have potential utility as intelligent polymeric materials such as self-healing materials.

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A series of 10- and 20-arm starlike block copolymers containing inner soft poly(n-butyl acrylate) (PBA) block and outer hard poly(methyl methacrylate) (PMMA) block were synthesized by atom transfer radical polymerization (ATRP). Short macroinitiators for preparation of starlike copolymers, poly(2-bromoisobutyryloxyethyl acrylate) (PBiBEA) with degree of polymerization DP = 10 and 20, was prepared by ATRP of trimethylsilyloxyethyl acrylate (HEATMS) and subsequently esterified. Partial star coupling during the star extension with PMMA blocks was observed, and the coupling increased with increasing number of arms and arm length. Phase-separated morphologies of cylindrical hard PMMA block domains arranged in the soft PBA matrix were observed by atomic force microscopy and small-angle X-ray scattering. The mechanical and thermal properties of the copolymers were also thoroughly characterized, and their thermoplastic elastomer behavior was studied. Tensile strength of the starlike copolymers was considerably higher compared to linear and three-arm stars with similar compositions.

Fluorescence correlation spectroscopy (FCS) was employed to study the diffusion of molecular tracers in different polymer melts (polydimethysiloxane (PDMS), 1,4-cis-polyisoprene (PI), poly-(vinylethylene) (PVE), and a symmetric PI/PVE blend) as a function of molecular weight (M w ) and temperature (T). The single molecule sensitivity of the FCS technique precludes any modification of the matrix polymer properties. In all studied systems, the small tracer diffusion coefficient D(M w ,T) senses local segmental dynamics depending on the glass transition temperature T g (M w ) of the polymer matrix and not its macroscopic viscosity. From the good representation of the D(T) data by the common non-Arrhenius (VFT) function, we found that the activation energy (B D ) increases with tracer size (R) and for a given tracer the value of B D in PI is almost 2 times bigger than in PDMS. The possibility to establish a direct relation between D(T) and the segmental relaxation time τ(T) of the polymer matrix was critically addressed based on experimental data in dynamically homogeneous (homopolymers) and heterogeneous (miscible blend) systems and discussed in view of recent computer simulations of polymer/penetrant mixtures.

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