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N Hadjichristidis

Publications and source records attributed to N Hadjichristidis.

12 recordsLinked to original sources
PubMed2005

Tailoring the flow of soft glasses by soft additives.

We examine the vitrification and melting of asymmetric star polymer mixtures by combining rheological measurements with mode coupling theory. We identify two types of glassy states, a single glass, in which the small component is fluid in the glassy matrix of the big one, and a double glass, in which both components are vitrified. Addition of small-star polymers leads to melting of both glasses, and the melting curve has a nonmonotonic dependence on the star-star size ratio. The phenomenon opens new ways for externally steering the rheological behavior of soft matter systems.

Computer Simulation↗
PubMed2005

"Glass transition" in peptides: temperature and pressure effects.

We report on the origin of the liquid-to-glass transition in a series of oligopeptides of gamma-benzyl-L-glutamate up to the polymer (PBLG), and in Poly-Z-L-lysine (PZLL) and Polyglycine (PGly) using dielectric spectroscopy as a function of temperature and pressure. We show that temperature is the dominant control variable of the dynamics associated with the peptidic "glass transition." This is an intrinsic feature of the peptide dynamics, irrespective of the type of amino acid and of the peptide secondary structure. The influence of the type of secondary structure (alpha helix vs beta sheet) on the liquid-to-glass dynamics is discussed.

Journal Article↗
PubMed2004

Micelles of poly(isoprene-b-2-vinylpyridine-b-ethylene oxide) terpolymers in aqueous media and their interaction with surfactants.

Well-defined poly(isoprene-b-2-vinylpyridine-b-ethylene oxide) (PI-P2VP-PEO) triblock terpolymers were synthesized by anionic polymerization high-vacuum techniques. The terpolymers formed spherical three-layer (onion-type) micelles in neutral and acidic pH aqueous media as evidenced by static and dynamic light scattering. In pure water, kinetically frozen micelles with a core composed of a soft PI inner part and a hard P2VP outer shell and protected by a neutral PEO corona were formed. In acidic media the core was formed by the soft PI hydrophobic segment, whereas the corona consisted of an inner cationic polyelectrolyte P2VPH+ part and an outer PEO shell. The aggregation numbers were found to be high in all cases, due to the high hydrophobicity of the core-forming blocks. In the latter case an increase in size was observed due to the electrostatic repulsions between the P2VPH+ chains in the inner part of the corona, which is also responsible for the lower aggregation numbers observed in the acidic solutions. The interaction of these onion-type micelles with cationic (DTMAB) and anionic (SDS) surfactants led to the formation of mixed polymer/surfactant aggregates. Their structural characteristics could be varied by combining changes in surfactant type and concentration, solution pH and type of electrostatic interaction, leading to interesting, block-copolymer-based, environmentally responsive colloidal systems.

Journal Article↗
PubMed2004

Effect of end-group sticking energy on the properties of polymer brushes: comparing experiment and theory.

Using surface force balance measurements we have established that polystyrene chains bearing three zwitterionic groups have a higher end-group sticking energy than equivalent chains bearing a single zwitterionic group. In a good solvent, polystyrene chains end-functionalized with three zwitterionic groups form brushes of a higher surface coverage than those bearing a single zwitterion. The increase in surface coverage is slow compared with the initial formation of the brush. Measurements of the refractive index allow us to directly quantify the variation of surface coverage, permitting comparison with models for the kinetics of brush formation based on scaling theory and an analytical self-consistent field. We find qualitative support for associating the kinetic barrier with the energy required for an incoming chain to stretch as it penetrates the existing brush.

Journal Article↗
PubMed2004

High frequency acoustic excitations in ordered diblock copolymer studied by inelastic x-ray scattering.

The phonon propagation in lamellar nanostructures formed via self-assembling of short styrene-b-isoprene (SI) as well as of its more incompatible styrene-b-(ethylene-alt-propylene) (SEP) counterpart was studied by inelastic x-ray scattering. Irrespective of the physical state of the block copolymers, a single acoustic phonon was observed in SI (ordered and disordered) and SEP (ordered). At GHz frequencies, inelastic light scattering from the same samples revealed very small dispersion in the sound phase velocity but a short phonon lifetime.

Journal Article↗
PubMed2002

Pattern formation in homogeneous polymer solutions induced by a continuous-wave visible laser.

We report an unexpected nonphotothermal material organization induced by continuous-wave visible laser light at low power levels. This effect is observed along the laser beam propagation direction in fully transparent entangled solutions of common homopolymers featuring sufficiently high molecular mass and optical anisotropy along the chain backbone. The resulting formation of long-lived stringlike or dotlike patterns on the micrometer scale, probed by dark-field coherent imaging, depends on the molecular mass, architecture, solvent nature, and polymer concentration. Electrostrictive and alignment forces as well as chain cooperativity are responsible for the osmotic compression of the polymer solute. Subsequent waveguiding effects induce autoamplification and "pattern writing" upon prolonged illumination. This wave-medium coupling could potentially lead to photorefractive, microoptics, and nanotechnology applications.

Journal Article↗
PubMed1999

Ordered bicontinuous nanoporous and nanorelief ceramic films from self assembling polymer precursors

Three-dimensional ceramic nanostructured films were produced from silicon-containing triblock copolymer films exhibiting the double gyroid and inverse double gyroid morphologies (space group Ia3d). A one-step room-temperature oxidation process that used ozonolysis and ultraviolet irradiation effected both the selective removal of the hydrocarbon block and the conversion of the silicon-containing block to a silicon oxycarbide ceramic stable to 400 degrees C. Depending on the relative volume fraction of the hydrocarbon block to the silicon- containing block, either nanoporous or nanorelief structures were fabricated with calculated interfacial areas of approximately 40 square meters per gram and pore or strut sizes of approximately 20 nanometers.

Journal Article↗
PubMedDate not supplied

Nanodomain-induced chain folding in poly(gamma-benzyl-L-glutamate)-b-polyglycine diblock copolymers.

We report on the self-assembly mechanism and dynamics in a series of poly(gamma-benzyl-l-glutamate)-b-poly(glycine) (PBLG-b-PGly) diblock copolymers within the composition range 0.67 < or = f(PBLG) < or = 0.97 and the temperature (T) range 303 < T < 433 K. Small- and wide-angle X-ray scattering, (13)C NMR, and differential scanning calorimetry are used for the structure investigation coupled with dielectric spectroscopy for both the peptide secondary structure and the associated dynamics. These techniques provide not only the nanophase morphology but also the type and persistence of peptide secondary structures. The thermodynamic confinement of the blocks within the nanodomains and the disparity in their packing efficiency results in multiple chain folding of the PGly secondary structure that effectively stabilize a lamellar morphology for high f(PBLG). Nanoscale confinement proves to be important in controlling the persistence length of secondary peptide motifs.

Magnetic Resonance Spectroscopy↗