Interface core-shell

More recently, the scope of using hyperbranched polymers as soluble supports in catalysis has been extended by the synthesis of amphiphilic star polymers bearing a hyperbranched core and amphiphilic diblock graft arms. This approach is based on previous work, where the authors reported the synthesis of a hyperbranched macroinitiator and its successful application in a cationic grafting-from reaction of 2-methyl-2-oxazoline to obtain water-soluble, amphiphilic star polymers [73]. Based on this approach, Nuyken et al. prepared catalyticaUy active star polymers where the transition metal catalysts are located at the core-shell interface. The synthesis is outlined in Scheme 6.10. 

Model systems have been used to demonstrate that S-sensitization is important in determining the location of the latent image centers. Moisar s experiments with core/shell emulsions showed that, when the surface of the core was sulfur-sensitized, the latent image formed by exposure of the core/shell grains was situated predominantly at or very close to the core/shell interface (124). 

A general requirement for the synthesis of CS NCs with satisfactory optical properties is epitaxial type shell growth. Therefore an appropriate band alignment is not the sole criterion for choice of materials but, in addition, the core and shell materials should crystallize in the same structure and exhibit a small lattice mismatch. In the opposite case, the growth of the shell results in strain and the formation of defect states at the core-shell interface or within the shell. These can act as trap states for photogenerated charge carriers and diminish the fluorescence QY.95 The structural parameters of selected semiconductor materials are summarized in Table 5.1. 

Important parameters that control the size of micelles are the degree of polymerization of the polymer blocks, NA and NB, and the Flory-Huggins interaction parameter %. The micellar structure is characterized by the core radius Rc, the overall radius Rm, and the distance b between adjacent blocks at the core/shell-interface as shown in Fig. 1. b is often called grafting distance for comparisons to polymer brush models, b2 is the area per chain which compares to the area per head group in case of surfactant micelles. In the case of spherical micelles, the core radius Rc and the area per chain b2 are directly related to the number of polymers per micelles, i.e., the aggregation number Z=4nR2clb2. 

For quite some time, there have been indications for a phase-separation in the shell of polyelectrolyte block copolymer micelles. Electrophoretic mobility measurements on PS-PMAc [50] indicated that a part of the shell exhibits a considerable higher ionic strength than the surrounding medium. This had been corroborated by fluorescence studies on PS-PMAc [51-53] and PS-P2VP-heteroarm star polymers [54]. According to the steady-state fluorescence and anisotropy decays of fluorophores attached to the ends of the PMAc-blocks, a certain fraction of the fluorophores (probably those on the blocks that were folded back to the core/shell interface) monitored a lower polarity of the environment. Their mobility was substantially restricted. It thus seemed as if the polyelectrolyte corona was phase separated into a dense interior part and a dilute outer part. Further experimental evidence for the existence of a dense interior corona domain has been found in an NMR/SANS-study on poly(methylmethacrylate-fr-acrylic acid) (PMMA-PAAc) micelles [55]. 

Figure 5. Schematic of constrained yielding (a) interpenetrating-interface model and (b) core-shell interface model with a rubbery shell. Le is the effective deformation length of the matrix.

The evolution of optical phonon spectra of colloidal core/shell CdSe/ZnS nanocrystals with an increase of the shell thickness from 0.5 to 3.4 monolayers have been studied by resonant Raman spectroscopy. The results show that at a thickness of about 2ML the surface of the CdSe core is mainly defect free although the structure of the shell is not established yet. The latter occurs at the thickness more than 3.4 ML where the shell is, most likely, amorphous. It is concluded that the defect-free core/shell interface is more important for producing high-luminescence QD structures than the increase of the shell thickness. 

Highly-luminescent II-IV semiconductor nanocrystals, or quantum dots (QDs) have attracted much attention because of their applications in optoelectronics, non-linear optics and biology. It is known that the photoluminescence (PL) efficiency of QDs can be improved by growing a shell of a wide-band gap semiconductor around the QD core. A good example is the CdSe/ZnS core/shell QDs thatpossess highPL quantum yield (>50%) with a narrow PL line [1]. However, the dependence of PL efficiency on the shell parameters, e.g., structure of the shell (amorphous vs. crystal) and the quality of the core/shell interface have yet to be clarified. In this paper, we present results of optical phonon Raman studies of CdSe/ZnS QDs with different thickness of the ZnS shell which allow one to investigate the above mentioned problem. 

An illustrative model aids the interpretation of the current images, assuming a spherical QD shape, with a radial core-shell potential, as shown in the inset of Figure 5.8a [82]. The energy calculated for the s state is lower than the barrier height at the core-shell interface, and has about the same values for core and core-shell QDs. In... 

First, we studied the solvent relaxation in solutions of diblock copolymer micelles. A commercially available polarity-sensitive probe, patman (Fig. 10, structure I), frequently used in phospolipid bilayer studies [123], was added to aqueous solutions of PS-PEO micelles. The probe binds strongly to micelles because its hydrophobic aliphatic chain has a strong affinity to the nonpolar PS core. The positively charged fluorescent headgroup is supposed to be located in the PEO shell close to the core-shell interface. The assumed localization has been supported by time-resolved anisotropy measurements. 

Experimental results were obtained by two experimental techniques, LS and TRFS. When the micelles are formed, all attached Np molecules (potential excitation energy donors) are localized at the core-shell interface. The Np fluorescence quenching due to NRET is expected if some energy traps (An) come relatively close to Np, i.e., to distances shorter or comparable with the Forster radius, Rq (for the Np-An pair, ca. 2.1 nm [144]). Hence, the NRET study should prove whether a fraction fraction of shell-embedded An could closely approach the core. 

Quite a number of theories were developed over the years in order to predict, mainly for non-polyelectrolyte systems, the structural parameters of a micelle (CMC, association number Z, core radius shell thickness L, hydrodynamic radius as a function of the copolymer characteristics, for example its molecular weight and composition. For A-B diblock copolymers which were mainly examined and where the B sequence is forming the micellar core, these characteristics are defined by the corresponding polymerization degrees and Ng. In all these theories and by using various models and mathematical approaches, the total Gibbs free energy of the micelle is expressed as the sum of several contributions, mainly those related to the core the shell and the core/shell interface... 

Typical polyelectrolyte behavior could be found for this type of micelles. Tuzar etal. [179] have shown for PS-PMAA the steep increase of the hydrodynamic radius nd of the electrophoretic mobility at a pH around 7, corresponding to the increases in dissociation of the carboxy groups when the pH was changed from 5 to 10 in various buffer solutions. In their experiments they could also demonstrate that the degree of dissociation decreases from the shell outer layer to the core-shell interface. 

To stabilize the micelle structure, core and shell crosslinking has been studied [41,61-63]. Recently, Shen et aL reported that amphiphilic brush copolymers composed of PEO and PCL chains, which were synthesized by macromonomer copolymerization, formed polymeric micelles in which the core-shell interface was crossUnked [64]. The diameters of the crosslinked micelles increased with increasing PCL/PEO chain ratio in the range 27.4-198 nm. The crosslinked micelles were 100 times more stable against dilution compared with micelles from corresponding amphiphilic block copolymers. 

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