Stergios Pispas

Anionic polymerization: High vacuum techniques

Anionic polymerization is a powerful tool for the synthesis of a variety of model materials with well-defined molecular characteristics. However specially designed apparatuses and appropriate high vacuum techniques are needed in order to exclude from the reaction environment all reactive contaminants with the anionic centers. This review describes the basic principles of anionic polymerization as well as detailed experimental methods for the purification of the reagents usually used for the synthesis of model polymeric materials. In addition a few examples of the synthesis of polymers with complex architecture are given.

Nonlinear Block Copolymer Architectures

The synthesis and bulk and solution properties of block copolymers having nonlinear architectures are reviewed. These materials include star-block copolymers, graft copolymers, miktoarm star copolymers, and complex architectures such as umbrella polymers and certain dendritic macromolecules. Emphasis is placed on the synthesis of well-defined, well-characterized materials. Such polymers serve as model materials for understanding the effects of architecture on block copolymer self-assembly, in bulk and in solution.

Asymmetric Star Polymers: Synthesis and Properties

The synthesis and the properties, both in bulk and in solution, of asymmetric star polymers are reviewed. Asymmetry is introduced when arms of different molecular weight, chemical nature or topology are incorporated into the same molecule. The phase separation, aggregation phenomena, dilute solution properties etc. are examined from a theoretical and experimental point of view. Recent applications of these materials show their importance in modern technologies.

Optical fiber long-period grating humidity sensor with poly(ethylene oxide)/cobalt chloride coating

A long-period fiber grating (LPFG) humidity sensor is reported utilizing poly(ethylene oxide)/cobalt chloride (PEO/CoCl2) as a hybrid hygrosensitive cladding coating. A thin overlay of the material is deposited on the LPFG and with exposure to different ambient humidity levels, its spectral properties are modified. The material parameters associated with the sensing mechanism may include those of refractive index, absorption, and morphological alterations of the overlaid material. Relative humidity variations in the range from 50% to 95% have been detected with a resolution better than 0.2%. The response time constant of the fiber sensor is of the order of a few hundred milliseconds.

Star‐branched polystyrenes by nitroxide living free‐radical polymerization

Star-branched polystyrenes, with polydispersity indices of 1.15–1.56 and 4–644 equal arms, were synthesized by the reaction of 2,2,6,6-tetramethylpiperidin-1-yloxy (TEMPO)-capped polystyrene (PS-T) with divinylbenzene (DVB). The characterization of PS-T and the final star polymers was carried out by size exclusion chromatography, low-angle laser light scattering, and viscometry. The degree of branching of the star polymers depended on the DVB/PS-T ratio and the PS-T molecular weight. An asymmetric (or miktoarm) star homopolymer of the PSnPS′n type was made by the reaction of the PSn symmetric star, which had n TEMPO molecules on its nucleus and consisted of a multifunctional initiator, with extra styrene.

Biocompatible microemulsions based on limonene: formulation, structure, and applications.

The preparation of biocompatible (w/o) microemulsions based on R-(+)-limonene, water, and a mixture of lecithin and either 1-propanol or 1,2-propanediol as emulsifiers was considered. The choice of the compositions of the microemulsions used was based on the pseudo-ternary phase diagrams of the four-component system determined at 30 degrees C for different weight ratios of the components. When 1-propanol was considered as co-surfactant, the area of the microemulsion zone was remarkably increased. Interfacial properties and the dynamic structure of the emulsifiers monolayer were studied by electron paramagnetic resonance (EPR) spectroscopy using the spin-labeling technique. The rigidity and polarity of the interface were affected by the nature of the alcohol used as co-surfactant. When 1-propanol was used, the emulsifiers interface was much more flexible, indicating a less tight packing of lecithin molecules than in the case of 1,2-propanediol. In addition, the membranes polarity was decreased when the diol was added as co-surfactant in the microemulsion system. To evaluate the size of the dispersed aqueous domains as a function of water content and other additives concentration, dynamic light scattering (DLS) measurements were carried out. Radii in the range from 60 to 180 nm were observed when 1-propanol was used as co-surfactant, and the water content varied from 0 to 12% w/w. Electrical conductivity measurements of R-(+)-limonene/lecithin/1-propanol/water microemulsions with increasing weight fractions of water indicated the appearance of a percolation threshold at water content above 4% w/w. Lipase from Rhizomucor miehei was solubilized in the aqueous domains of the biocompatible microemulsions, and the esterification of octanoic, dodecanoic, and hexadecanoic acids with the short-chained alcohols used as co-surfactants for the formulation of microemulsions was studied. The enzyme efficiency was affected by the chain length of the carboxylic acids and the nature of the alcohol. In the case of 1-propanol, a preference for the long-chain carboxylic acids was observed. On the contrary, when 1,2-propanediol was used formulation of the corresponding esters was not observed. This behavior could be possibly attributed to either the specificity of the lipase toward the alcohol employed for the esterification of the acids or the structural changes induced in the system when 1-propanol was replaced by 1,2-propanediol.

Graphene exfoliation in organic solvents and switching solubility in aqueous media with the aid of amphiphilic block copolymers

The successful exfoliation of graphite to graphene sheets in a liquid phase via tip sonication was achieved. A number of solvents were examined for several time periods and it was found that o-dichlorobenzene (o-DCB) and N-methyl-1,2-pyrolidone (NMP) are ideal solvents to exfoliate graphite and produce stable dispersions of graphene. The exfoliated graphene dispersions were characterized by complementary techniques including AFM, DLS, TGA and Raman. Furthermore, treatment of stable dispersions of exfoliated graphene sheets in NMP with poly[styrene-b-(2-vinylpyridine)] block copolymer, under acidic conditions, resulted in aqueous solubilization of graphene. Similar results were obtained, i.e. transfer of graphene from the organic to the aqueous phase, when the poly(isoprene-b-acrylic acid) block copolymer was added on exfoliated graphene in NMP.

Interdiffusion of polymers across interfaces.

Neutron Reflection (NR) and Dynamic Secondary Ion Mass Spectroscopy (DSIMS) experiments were conducted on symmetrically deuterated polystyrene triblock bilayers (HDH/DHD) which directly probed the interdiffusion dynamics of the chains during welding. The HDH chains had their centers deuterated 50%, the DHD chains had their ends deuterated (25% at each end) such that each chain contained approximately 50% D. During welding, anisotropic motion of the chains produces a time-dependent oscillation (ripple) in the H and D concentration at the interface, which bears the characteristic signature of the polymer dynamics. These oscillations were compared with those predicted by Rouse, polymer mode coupling (PMC), and reptation dynamics. The following conclusions can be made from this study. (a) During the interdiffusion of high molecular weight HDH/DHD pairs, higher mobility of the chain ends caused a concentration oscillation which increased to a maximum amplitude, and eventually vanished at times, t > τD. The amplitude, or excess enrichment found, was appreciably more than that predicted by Rouse and PMC simulations, and was only slightly less than that predicted from reptation simulations. (b) The oscillations were completely missing in the 30 and 50K HDH/DHD polymers, which are only weakly entangled. The lack of oscillations for the 30 and 50K pairs may be due to a combination of surface roughness and fluctuations of order 30 A. (c) It was found that the position of the maximum in this ripple stayed at the interface during its growth. This is also consistent with reptation and has not been explained by other theories. (d) All dynamics models for linear polymers produce ripples, many of which are qualitatively similar to that predicted for reptation. However, each ripple bears the fingerprint of the dynamics in terms of its time-dependent shape, position, and magnitude, and the models are clearly distinguishable. Our results, in summary, support reptation as a candidate mechanism of interdiffusion at polymer(SINGLEBOND) polymer interfaces and its uniqueness is being further pursued.

Spherical polymeric brushes viewed as soft colloidal particles: zero-shear viscosity

We examine the concentration dependence of the zero-shear viscosity of two chemically distinct classes of spherical brushes, multiarm star polymers and giant diblock copolymer micelles. We show that the effective volume fraction based on the hydrodynamic, rather than the static, size of these objects is the appropriate scaling parameter for obtaining a generic description of the relative viscosity for systems of varying size and softness. We demonstrate that it is possible to span the whole range from polymeric behavior to that of hard colloidal spheres by tuning the sphere softness at the molecular level (arm size and functionality).

Properties, applications and functionalisation of carbon nanohorns

Functionalisation of carbon nanohorns (CNHs), by covalent bonding or non-covalent supramolecular interacting organic moieties, overcomes insolubility problems and enhances manipulation and processibility. The present review paper summarises our recent accomplishments in the field of chemical modification of CNHs together with various critical and important aspects related to their synthesis, morphology, properties and applications. Moreover, the design and preparation of some novel hybrid materials consisting of CNHs and electron donors will be presented.