719 research outputs found
Evidence of Overcharging in the Complexation between Oppositely Charged Polymers and Surfactants
We report on the complexation between charged-neutral block copolymers and
oppositely charged surfactants studied by small-angle neutron scattering. Two
block copolymers/surfactant systems are investigated, poly(acrylic
acid)-b-poly(acrylamide) with dodecyltrimethylammonium bromide and
poly(trimethylammonium ethylacrylate methylsulfate)-b-poly(acrylamide) with
sodium dodecyl sulfate. The two systems are similar in terms of structure and
molecular weight but have different electrostatic charges. The neutron
scattering data have been interpreted in terms of a model that assumes the
formation of mixed polymer-surfactant aggregates, also called colloidal
complexes. These complexes exhibit a core-shell microstructure, where the core
is a dense coacervate microphase of micelles surrounded by neutral blocks.
Here, we are taking advantage of the fact that the complexation results in
finite-size aggregates to shed some light on the complexation mechanisms. In
order to analyze quantitatively the neutron data, we develop two different
approaches to derive the number of surfactant micelles per polymer in the mixed
aggregates and the distributions of aggregation numbers. With these results, we
show that the formation of the colloidal complex is in agreement with the
overcharging predictions. In both systems, the amount of polyelectrolytes
needed to build the core-shell colloids always exceeds the number that would be
necessary to compensate the charge of the micelles. For the two
polymer-surfactant systems investigated, the overcharging ratios are 0.66 and
0.38.Comment: 20 pages, 7 Figures, 6 Table
Time Resolved Correlation measurements of temporally heterogeneous dynamics
Time Resolved Correlation (TRC) is a recently introduced light scattering
technique that allows to detect and quantify dynamic heterogeneities. The
technique is based on the analysis of the temporal evolution of the speckle
pattern generated by the light scattered by a sample, which is quantified by
, the degree of correlation between speckle images recorded at
time and . Heterogeneous dynamics results in significant
fluctuations of with time . We describe how to optimize TRC
measurements and how to detect and avoid possible artifacts. The statistical
properties of the fluctuations of are analyzed by studying their
variance, probability distribution function, and time autocorrelation function.
We show that these quantities are affected by a noise contribution due to the
finite number of detected speckles. We propose and demonstrate a method to
correct for the noise contribution, based on a extrapolation
scheme. Examples from both homogeneous and heterogeneous dynamics are provided.
Connections with recent numerical and analytical works on heterogeneous glassy
dynamics are briefly discussed.Comment: 19 pages, 15 figures. Submitted to PR
Structure of Extremely Nanosized and Confined In-O Species in Ordered Porous Materials
Perturbed-angular correlation, x-ray absorption, and small-angle x-ray
scattering spectroscopies were suitably combined to elucidate the local
structure of highly diluted and dispersed InOx species confined in porous of
ZSM5 zeolite. These novel approach allow us to determined the structure of
extremely nanosized In-O species exchanged inside the 10-atom-ring channel of
the zeolite, and to quantify the amount of In2O3 crystallites deposited onto
the external zeolite surface.Comment: 4 pages, 5 postscript figures, REVTEX4, published in Physical Review
Letter
pi-Conjugation and conformation in a semiconducting polymer: small angle x-ray scattering study
Small angle X-ray scattering (SAXS) in
poly[2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) solution
has shown the important role of pi-electron conjugation in controlling the
chain conformation and assembly. By increasing the extent of conjugation from
30 to 100 %, the persistence length (l_p) increases from 20 to 66 Angstrom.
Moreover, a pronounced second peak in the pair distribution function has been
observed in fully conjugated chain, at larger length scales. This feature
indicates that the chain segments tend to self-assemble as the conjugation
along the chain increases. Xylene enhances the rigidity of PPV backbone to
yield extended structures, while tetrahydrofuran solvates the side groups to
form compact coils in which the l_p is much shorter.Comment: accepted (J. Phy. Cond. Mat.
Using Dynamic Covalent Chemistry To Drive Morphological Transitions: Controlled Release of Encapsulated Nanoparticles from Block Copolymer Vesicles
Dynamic covalent chemistry is exploited to drive morphological order–order transitions to achieve the controlled release of a model payload (e.g., silica nanoparticles) encapsulated within block copolymer vesicles. More specifically, poly(glycerol monomethacrylate)–poly(2-hydroxypropyl methacrylate) (PGMA–PHPMA) diblock copolymer vesicles were prepared via aqueous polymerization-induced self-assembly in either the presence or absence of silica nanoparticles. Addition of 3-aminophenylboronic acid (APBA) to such vesicles results in specific binding of this reagent to some of the pendent cis-diol groups on the hydrophilic PGMA chains to form phenylboronate ester bonds in mildly alkaline aqueous solution (pH ∼ 10). This leads to a subtle increase in the effective volume fraction of this stabilizer block, which in turn causes a reduction in the packing parameter and hence induces a vesicle-to-worm (or vesicle-to-sphere) morphological transition. The evolution in copolymer morphology (and the associated sol–gel transitions) was monitored using dynamic light scattering, transmission electron microscopy, oscillatory rheology, and small-angle X-ray scattering. In contrast to the literature, in situ release of encapsulated silica nanoparticles is achieved via vesicle dissociation at room temperature; moreover, the rate of release can be fine-tuned by varying the solution pH and/or the APBA concentration. Furthermore, this strategy also works (i) for relatively thick-walled vesicles that do not normally exhibit stimulus-responsive behavior and (ii) in the presence of added salt. This novel molecular recognition strategy to trigger morphological transitions via dynamic covalent chemistry offers considerable scope for the design of new stimulus-responsive copolymer vesicles (and hydrogels) for targeted delivery and controlled release of cargoes. In particular, the conditions used in this new approach are relevant to liquid laundry formulations, whereby enzymes require protection to prevent their deactivation by bleach
Time-Resolved SAXS Studies of the Kinetics of Thermally Triggered Release of Encapsulated Silica Nanoparticles from Block Copolymer Vesicles
Silica-loaded poly(glycerol monomethacrylate)-poly(2-hydroxypropyl methacrylate) diblock copolymer vesicles are prepared in the form of concentrated aqueous dispersions via polymerization-induced self-assembly (PISA). As the concentration of silica nanoparticles present during the PISA synthesis is increased up to 35% w/w, higher degrees of encapsulation of this component within the vesicles can be achieved. After centrifugal purification to remove excess non-encapsulated silica nanoparticles, SAXS, DCP, and TGA analysis indicates encapsulation of up to hundreds of silica nanoparticles per vesicle. In the present study, the thermally triggered release of these encapsulated silica nanoparticles is examined by cooling to 0 °C for 30 min, which causes in situ vesicle dissociation. Transmission electron microscopy studies confirm the change in diblock copolymer morphology and also enable direct visualization of the released silica nanoparticles. Time-resolved small-angle X-ray scattering is used to quantify the extent of silica release over time. For an initial silica concentration of 5% w/w, cooling induces a vesicle-to-sphere transition with subsequent nanoparticle release. For higher silica concentrations (20 or 30% w/w) cooling only leads to perforation of the vesicle membranes, but silica nanoparticles are nevertheless released through the pores. For vesicles prepared in the presence of 30% w/w silica, the purified silica-loaded vesicles were cooled to 0 °C for 30 min, and SAXS patterns were collected every 15 s. A new SAXS model has been developed to determine both the mean volume fraction of encapsulated silica within the vesicles and the scattering length density. Satisfactory data fits to the experimental SAXS patterns were obtained using this model
Recent studies of cements and concretes by synchrotron radiation crystallographic and cognate methods
The portfolio of available synchrotron radiation techniques is increasing notably for cements
and pastes. Furthermore, sometimes the terminology is confusing and an overall picture highlighting
similarities and differences of related techniques was lacking. Therefore, the main
objective of this work is to review recent advances in synchrotron techniques providing a
comprehensive overview. This work is not intended to gather all publications in cement chemistry
but to give a unified picture through selected examples. Crystallographic techniques are
used for structure determination, quantitative phase analyses and microstructure characterization.
These studies are not only carried out in standard conditions but synchrotron techniques
are especially suited to non-ambient conditions: high temperatures and pressures, hydration,
etc., and combinations. Related crystallographic techniques, like Pair Distribution Function,
are being used for the analysis of ill-crystalline phase(s). Furthermore, crystallographic tools
are also employed in imaging techniques including scanning diffraction microscopy and
tomography and coherent diffraction imaging. Other synchrotron techniques are also reviewed
including X-rays absorption spectroscopy for local structure and speciation characterizations;
small angle X-ray scattering for microstructure analysis and several imaging techniques for
microstructure quantification: full-field soft and hard X-ray nano-tomographies; scanning
infrared spectro-microscopy; scanning transmission and fluorescence X-ray tomographies.
Finally, a personal outlook is provided.I am grateful to all my coauthors, collaborators, colleagues and PhD students, for all our work together
during more than two decades. I thank the University of Malaga and ALBA Synchrotron Light Source
for the support and the stirring environments. I acknowledge the Spanish science funding agencies (they
change the name quite often) for funding my studentship, to do the PhD and the three summer research
stays at Oxford University, to the last ongoing research project. To all synchrotrons I have been allowed to
enjoy carrying out experiments: SRS, ESRF, Max-Lab, DLS, APS, SLS and ALBA. Finally, this work has
been supported by the Spanish MINECO through the BIA2014-57658-C2-1-R research grant
Solvent Mediated Assembly of Nanoparticles Confined in Mesoporous Alumina
The controlled self-assembly of thiol stabilized gold nanocrystals in a
mediating solvent and confined within mesoporous alumina was probed in situ
with small angle x-ray scattering. The evolution of the self-assembly process
was controlled reversibly via regulated changes in the amount of solvent
condensed from an undersaturated vapor. Analysis indicated that the
nanoparticles self-assembled into cylindrical monolayers within the porous
template. Nanoparticle nearest-neighbor separation within the monolayer
increased and the ordering decreased with the controlled addition of solvent.
The process was reversible with the removal of solvent. Isotropic clusters of
nanoparticles were also observed to form temporarily during desorption of the
liquid solvent and disappeared upon complete removal of liquid. Measurements of
the absorption and desorption of the solvent showed strong hysteresis upon
thermal cycling. In addition, the capillary filling transition for the solvent
in the nanoparticle-doped pores was shifted to larger chemical potential,
relative to the liquid/vapor coexistence, by a factor of 4 as compared to the
expected value for the same system without nanoparticles.Comment: 9 pages, 9 figures, appeared in Phys. Rev.
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