Polymer nanoparticles, concentration-dependent

Xu and Liu recently reported the syntheses of the well-defined 7-arm and 21-arm PiPAAm stars with a P-cyclodextrin core [278, 568] and presented a thorough analysis of the literature on thermoresponsive stars and polymer brushes tethered to curved surfaces, such as latex particles [279, 280, 569], gold nanoparticles [282] and microgels [570], A unique feature of these architectures is that they form a densely packed spherical core and a less-dense outer shell [159]. As a result of such a non-uniform density distribution, two temperature-induced phase transitions have been observed experimentally in several systems based on PiPAAm [279, 280, 282, 568, 569], One transition has been ascribed to the phase transition of the inner segments of PiPAAm, whereas the other transition, which is concentration dependent, was assigned to the collapse of the outer PiPAAm segments [282],... 

Figure 9.1 Example of dependence of characteristic process times ( , nucleation time A, aggregation time , growth time and total mixing time (O) on inlet water jet velocity in a confined impinging jet mixer (ClJ-dl) calculated for PCL (iW = 14,000) nanoparticles formation, polymer initial concentration in acetone = 2.5 mg/mL with water/acetone volumetric flow rate ratio = 2 (redrawn from [42,43])...

The author with co-workers [39] has investigated systems with the structures Al/Cso/PANI+CdS/ITO and Al/Cgo/PPV-l-CdInS/ITO. Most worthy of note are the dependencies of the short circuit current and the open circuit voltage on the nanoparticles concentration (Fig. 21). Since the fullerene molecule acts as a strong electron acceptor, excitons generated both in the polymer matrix and in the CdS particles are decomposed, electrons are accepted by the Ceo layer and the holes are transported to the anode through the polymer. Once the concentration of the nanoparticles exceeds the percolation threshold value, the system becomes short contacted, since the electrons can pass from the anode to the cathode through the barrier-free connected network of CdS clusters this latter fact leads to the disappearance of the photovoltaic effect, as illustrated in Fig. 21b. The increase in the photocurrent and the open circuit... 

Figure 25.14 (a) Concentration-dependent fluorescence spectra of polymer blend nanoparticles under 375 nm excitation (b) Variation of the fluorescence intensity of PFO and dopant polymers in each pair with the dopant... 

Relaxation in Nanocomposites. At concentrations above the percolation threshold polymer/nanoparticle interactions dominate the viscoelastic terminal behaviour of polymer nanocomposites. As has been reported for phenoxy based nanocomposites [8], the analysis of tan 5 relaxation at low frequencies constitutes a reliable rheological method to investigate the strength of phenoxy/nanoclay interactions. Moreover, since coordinates ((o)Max (tan 5)Max)) reflect the blocking effect of nanoparticles on polymer chains, the dependence of (o)Max with nanoparticles volume fraction can be used in the percolation equation X=Xq (volume fraction threshold [Pg.69]

In the temperature interval of —70 to 0°C and in the low-frequency range, an unexpected dielectric relaxation process for polymers is detected. This process is observed clearly in the sample PPX with metal Cu nanoparticles. In sample PPX + Zn only traces of this process can be observed, and in the PPX + PbS as well as in pure PPX matrix the process completely vanishes. The amplitude of this process essentially decreases, when the frequency increases, and the maximum of dielectric losses have almost no temperature dependence [104]. This is a typical dielectric response for percolation behavior [105]. This process may relate to electron transfer between the metal nanoparticles through the polymer matrix. Data on electrical conductivity of metal containing PPX films (see above) show that at metal concentrations higher than 5 vol.% there is an essential probability for electron transfer from one particle to another and thus such particles become involved in the percolation process. The minor appearance of this peak in PPX + Zn can be explained by oxidation of Zn nanoparticles. 

What is the dependence of the mean size and the size distribution of M/SC nanoparticles on the concentration and nature of M/SC in the process of the solid-state low-temperature synthesis of polymer composite films by PVD method ... 

Three different ways have been developed to produce nanoparticle of PE-surfs. The most simple one is the mixing of polyelectrolytes and surfactants in non-stoichiometric quantities. An example for this is the complexation of poly(ethylene imine) with dodecanoic acid (PEI-C12). It forms a solid-state complex that is water-insoluble when the number of complexable amino functions is equal to the number of carboxylic acid groups [128]. Its structure is smectic A-like. The same complex forms nanoparticles when the polymer is used in an excess of 50% [129]. The particles exhibit hydrodynamic diameters in the range of 80-150 nm, which depend on the preparation conditions, i.e., the particle formation is kinetically controlled. Each particle consists of a relatively compact core surrounded by a diffuse corona. PEI-C12 forms the core, while non-complexed PEI acts as a cationic-active dispersing agent. It was found that the nanoparticles show high zeta potentials (approximate to +40 mV) and are stable in NaCl solutions at concentrations of up to 0.3 mol l-1. The stabilization of the nanoparticles results from a combination of ionic and steric contributions. A variation of the pH value was used to activate the dissolution of the particles. 

In one study, PLG microspheres bound to DNA using a cryogenic double emulsion process and injected intramuscularly into mice were found to promote transgene expression for up to 174 days after injection, dependent upon microsphere mass. More recently nanoparticles were constructed from carboxy terminated, PEG modified PGL polymers. These particles were conjugated with an aptamer to the prostate specific membrane antigen and evaluated for in vivo biodistribution in an LNCaP (PSMA+) xenograft mouse model of prostate cancer. In this study, the aptamer caused a 3.77-fold increase in polymer concentration in the tumor tissue after retro-orbital injection (79). 

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