The kinetics of the sphere-to-rod transition was studied in aqueous micelle solutions of triblock copolymer poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) pluronic P103 (PEO(17)PPO(60)PEO(17)). This transition was triggered by a temperature jump from the sphere phase to the rod phase and monitored with dynamic light scattering. The combination of the scattering intensity and the hydrodynamic radius were used to show that the micelles grow steadily as rods throughout the growth process. The transition was found to exhibit a single exponential behavior even in the case of large deviations from equilibrium. The linear increase in the decay rate with increasing copolymer concentration shows that the transition is dominated by a mechanism involving fusion and fragmentation of proper micelles. The decays of the sphere-to-rod transition were simulated for two pathways: random fusion fragmentation and successive addition of spherical micelles to rods. We show that micelle growth most likely occurs via random fusion-fragmentation. The second order rate constant for fusion and the fragmentation rate are calculated for the case of random fusion-fragmentation.
DNA dynamics and
flow properties are of great importance for understanding
its functions. DNA is a semiflexible polymer chain characterized by
having a large persistence length of around 50 nm and high charge
density; DNA chains are interacting efficiently at high concentrations,
in dependence of the ionic concentration. In relation with DNA molecular
characteristics, it is also known that DNA solutions are able to form
liquid crystalline phases over a critical polymer concentration. In
this work, the supramolecular organization in calf-thymus DNA solution,
with low degree of entanglement, appearing under flow was studied
in a wide DNA concentration range from 2 to 10 mg/mL, at a pH of 7.3
and 20 °C. The rheological behavior of the system was studied
using steady state flow and oscillatory measurements. Transient regimes
were also tested by imposing controlled shear rates on a short time
up to steady state. Furthermore, a combination of visual observations
and flow birefringence measurements was proposed to reach a better
understanding of the obtained rheological behavior. The presence of
a shear-induced texture is revealed under flow for the calf-thymus
DNA solutions at C
DNA> 5 mg/mL and
attributed
to organized domains of DNA molecules, named in the text as crystalline
parts, which are progressively oriented under shear. Finally, at high
shear rates (over 100 s–1), it is shown that for
the DNA solutions the orientation of these organized DNA domains and
connecting chains under flow goes to an anisotropic monodomain.
Due to their slow kinetics, the dynamical pathways of block copolymer micelles are as important as the copolymer architecture, particularly when making self-assembled nanostructures.Two types of dynamical pathways could govern these dynamics: insertion/expulsion of copolymer chains and fusion/fission of micelles. However, quantifying and understanding the fusion and fission processes remains very limited in copolymer micelles, especially at equilibrium. In this article, it was demonstrated the use of a fluorescent technique, using randomization of a highly hydrophobic pyrene derivative between micelles, as a tool to quantify the fusion and fission at equilibrium in a series of triblock copolymers micelles Poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) in aqueous solution. The temperature dependence of fusion and fission is investigated for copolymers with various core block lengths (NPPO) in the spherical regime. Fusion and fission rates were found to strongly decrease with increasing the core block length. The dependence of the fission rate on NPPO is analyzed in terms of thin corona and starlike micelle models which suggest that fission is mainly dominated by the core interfacial tension as predicted by Halperin et al. The comparison between fission and expulsion kinetics and their dependence on PPO block is also reported. Finally, fusion is found to follow the same temperature and core length dependencies as fission, which is an indication that the interfacial tension plays a relevant role in the fusion kinetic.
Nanostructured films with electrical conductivity in the semiconductor region were prepared in a polymeric matrix of poly(vinyl alcohol) (PVA) with nanostructures of chitosan-gold nanoparticles (AuNPs)/single-wall carbon nanotubes carboxylic acid functionalized (SWCNT-COOH) (chitosan-AuNPs/SWCNT-COOH) self-assembled. Dispersion light scattering (DLS) was used to determine the average particle sizes of chitosan-AuNPs, z-average particle size (Dz) and number average particle size (Dn), and the formation of crystalline domains of AuNPs was demonstrated by X-ray diffraction (XRD) patterns and observed by means of transmission electron microscopy (TEM). The electrostatic interaction was verified by Fourier transform infrared spectroscopy (FTIR). The electrical conductivity of PVA/chitosan-AuNPs/SWCNT-COOH was determined by the four-point technique and photocurrent. The calculated Dn values of the chitosan-AuNPs decreased as the concentration of gold (III) chloride trihydrate (HAuCl4·3H2O) increased: the concentrations of 0.4 and 1.3 mM were 209 and 90 nm, respectively. Average crystal size (L) and number average size (D) of the AuNPs were calculated in the range of 13 to 24 nm. Electrical conductivity of PVA/chitosan-AuNPs/SWCNT-COOH films was 3.7 × 10−5 σ/cm determined by the four-point technique and 6.5 × 10−4 σ/cm by photocurrent for the SWCNT-COOH concentration of 0.5 wt.% and HAuCl4·3H2O concentration of 0.4 mM. In this investigation, the protonation of the amine group of chitosan is fundamental to prepare PVA films with nanostructures of self-assembled chitosan-AuNPs/SWCNT-COOH.
The
interactions of proteins and other molecules and their adsorption
onto substrates is a fascinating topic that has been applied to surface
technologies, biosensors, corrosion studies, biotechnologies, and
other fields. The success of these applications requires a previous
characterization using some analytical techniques that, ordinarily,
are not electrochemical. This work proposes analyzing the variation
of the double-layer capacitance obtained through impedance electrochemical
spectroscopy as an alternative strategy to show evidence of the interactions
between proteins and triblock copolymers. The proposal is supported
through the study of the interaction and adsorption of bovine serum
albumin (BSA) and a commercial triblock copolymer (P103) in phosphate
buffer on a gold electrode. The double-layer capacitance and the apparent
interface thickness vs polarization potential curves as well as the
potential of zero charge for pure P103 (0.6 wt %, corresponding to
6 g L
–1
), pure BSA (3 mg mL
–1
),
and P103-BSA solutions (0.6 wt % and 3 mg mL
–1
,
respectively) are sensitive enough to show not only the interaction
and the adsorption of the species but also the polarization potential
where these interactions are taking place. A qualitative and quantitative
analysis concerning the double-layer capacitance behavior is given.
The significance and impact of this work is also presented.
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