Spiros H. Anastasiadis

The morphology of symmetric diblock copolymers as revealed by neutron reflectivity

The specular reflectivity of neutrons has been used to characterize quantitatively the microphase separated morphology of symmetric, diblock copolymers of polystyrene (PS), and polymethylmethacrylate (PMMA), as a function of the total molecular weight of the copolymer where either block is perdeuterated. It is shown that the hyperbolic tangent function, as opposed to a linear or cosine‐squared function, most closely describes the concentration gradient at the interface between the lamellar copolymer microdomains. The effective width of the interface is found to be independent of the molecular weight of the copolymer blocks and has a value of 50±3 A. This interface is also found to be identical to that between PS and PMMA, homopolymers. However, using measured values of the Flory–Huggins interaction parameter for PS and PMMA, current theoretical treatments cannot describe the observed widths of the interface.

Concentration fluctuation induced dynamic heterogeneities in polymer blends

The presence of two distinctly different local segmental mobilities found in the case of several phase mixed polymer blends by two‐dimensional 2H‐NMR, dielectric spectroscopy and depolarized dynamic light scattering is rationalized through a simple concentration fluctuation model. Our primary hypothesis is that, although the probability of the occurrence of concentration fluctuations is symmetric about the mean value in a given volume, the ‘‘cooperative volume’’ over which a fluctuation must occur for it to be detected by a dynamic probe is not a constant, but rather depends on the composition of the cooperative volume. Consequently, we suggest that the cooperative volume associated with a concentration fluctuation be determined by the local composition in a self‐consistent manner. In the case of systems with weak interactions and large Tg contrast, these ideas are shown to create a bimodal probability density function for dynamic concentration fluctuations, which has a local maximum corresponding to smal...

The determination of interfacial tension by video image processing of pendant fluid drops

Abstract A novel technique is presented for the determination of interfacial tension by analysis of axisymmetric fluid drop profiles. The technique couples recent developments in digital image acquisition and processing with modern methods for robust shape comparison. The entire algorithm for drop profile acquisition and analysis is executable on a personal computer and includes a facility for rotationally resistant smoothing of the drop profile. The general performance of the robust shape comparison algorithm and improvement in accuracy resulting from smoothing is illustrated by analysis of simulated pendant drop profiles. The overall performance of the image acquisition hardware and profile analysis software is assessed by evaluating the surface tension of glycerin. The resultant value of 62.6 ± 0.3 dyn/cm (at 24.2 ± 0.2°C) agrees well with the literature data for glycerin.

Random laser action in organic–inorganic nanocomposites

Random laser action is demonstrated in organic–inorganic, disordered hybrid materials consisting of ZnO semiconductor nanoparticles dispersed in an optically inert polymer matrix. The ZnO particles provide both the gain and the strong scattering power that leads to light trapping due to multiple elastic scattering, whereas the polymer matrix offers ease of material fabrication and processability in view of potential applications. Excitation of the nanohybrids by a laser pulse with duration shorter than the ZnO photoluminescence lifetime leads to a dramatic increase in the emitted light intensity accompanied by a significant spectral and temporal narrowing above a certain threshold of the excitation energy density. Critical laser and material parameters that influence the observed laser-like emission behavior are investigated in a series of nanocomposites.

The work of adhesion of polymer/wall interfaces and its association with the onset of wall slip

The interfacial characteristics of a variety of polymer/wall interfaces were measured by using the sessile drop method in order to calculate the work of adhesion. Polymers included linear low-density as well high-density polyethylenes, while wall substrates included clean stainless steel and modified stainless steel by applying two different fluoropolymers in order to alter its surface energy. A linear correlation is found between the critical shear stress for the onset of slip and the work of adhesion of the corresponding polymer/wall interface, in agreement with earlier publications of Hill et al. (1991) and Hatzikiriakos et al. (1993). In the present work, the experimental results are interpreted in terms of parameters defined by these two theories. It is suggested that small deviations from the no-slip boundary condition in the case of polymer melt flow are due to a stress-induced chain detachment/desorption of polymer chains from the wall.

Probing Collective Motions of Terminally Anchored Polymers

Polymer chains attached by one end to an impenetrable surface at high coverage exemplify a tethered layer of mesoscopic dimensions. At equilibrium, thermal fluctuations of the segment density profile of the brushlike layer reflect the tethered chain dynamics; the probing of these fluctuations by evanescent-wave dynamic light scattering is reported. By utilizing a set of terminally attached layers with thicknesses (L0) from 45 to 130 nanometers, it was found that there is a preferred wavelength of order L0 of these fluctuations with a concurrent slowing down of their thermal decay rate. This technique could open the route for the investigation of the largely unexplored area of polymer surface dynamics.

Synthesis of distillation sequences with several multicomponent feed and product streams

Abstract In this paper the synthesis problem of distillation-based separation sequences that separate a number of multicomponent feed streams into a number of multicomponent products is addressed. A synthesis strategy is proposed that consists of: (a) the development of a distillation-based superstructure that has embedded many separation configurations as well as options for splitting, mixing and by-passing; (b) simulation runs performed for each of the distinct distillation columns involved in the superstructure; and (c) the mathematical formulation of the distillation-based superstructure as a mixed-integer linear programming problem which has as unknowns the stream interconnections. The solution of this mathematical formulation results in the optimal configuration of distillation columns that can perform the desired separation task, as well as in the optimal allocation of by-passes. The synthesis strategy is illustrated with the first two example problems while the third example demonstrates the use of the proposed approach as an initialization procedure for the general separation problem.

Rheology and phase separation in a model upper critical solution temperature polymer blend

The viscoelastic properties of a model binary polymer blend exhibiting an upper critical solution temperature phase diagram were investigated by utilizing small amplitude oscillatory and steady shear measurements. A mixture of unentangled monodisperse polystyrene and poly(phenyl methyl siloxane), exhibiting Newtonian shear viscosity, was used, and its phase diagram was established by turbidity and dynamic light scattering measurements. In the miscible region, the concentration dependence of the viscosity was adequately described by a mixing rule accounting for the surface fractions instead of volume fractions. Near the phase separation temperature and far from the glass transition, critical concentration fluctuations dominated the linear viscoelastic response and were responsible for the observed thermorheological complexity. An appropriate quantitative account of these fluctuations resulted in the accurate rheological determination of both the binodal and spinodal temperatures, extending thus the applica...

Development of Functional Polymer Surfaces with Controlled Wettability

There is a demand for surfaces with new functional properties in almost all industrial branches. During the next few years, research input will be required for the development of coatings exhibiting an easy-to-clean or self-cleaning ability, switchability so that they can act as sensors/actuators, and defined tribological/mechanical properties and long-term stability. To achieve such behavior, the development of new advanced functional coatings that exhibit the proper chemistry and surface structure is necessary. In this Feature Article, we provide a review of the research activities in our laboratory on the development of functional and, especially, reversibly switchable polymer surfaces where the emphasis is on controlling their wettability. We will first discuss the fabrication of superhydrophobic surfaces by hierarchically micro- and nanostructuring a substrate surface with an ultrafast laser followed by appropriate hydrophobization. Then, we will summarize the development of surfaces that can alter their wetting behavior in response to changes in external stimuli such as humidity and light illumination. Finally, we will present our investigations on utilizing responsive (organic) coatings on hierarchically roughened substrates for the development of surfaces, which would be able to switch reversibly from superhydrophilic to superhydrophobic and water-repellent in response to an external stimulus (in this case, pH).

Micellization in pH-sensitive amphiphilic block copolymers in aqueous media and the formation of metal nanoparticles.

Dynamic light scattering, potentiometric titration, transmission electron microscopy and atomic force microscopy have been used to investigate the micellar behaviour and metal-nanoparticle formation in poly(ethylene oxide)-block-poly(2-vinylpyridine), PEO-b-P2VP, poly(hexa(ethylene glycol) methacrylate)-block-poly(2-(diethylamino)ethyl methacrylate), PHEGMA-b-PDEAEMA, and PEO-b-PDEAEMA amphiphilic diblock copolymers in water. The hydrophobic block of these copolymers (P2VP or PDEAEMA) is pH-sensitive: at low pH it can be protonated and becomes partially or completely hydrophilic leading to molecular solubility whereas at higher pH micelles are formed. These micelles consist of a P2VP or PDEAEMA core and a PEO or PHEGMA corona, respectively, where the core forming amine units can incorporate metal compounds due to coordination. The metal compounds (e.g., H2PtCl6, K2PtCl6) can either be introduced in a micellar solution, where they are incorporated within the micelle core via coordination with functional groups, or can be added to a unimer solution at low pH, where they lead to a metal-induced micellization. In these micellar nanoreactors, metal nanoparticles nucleate and grow upon reduction with sizes in the range of a few nanometers as observed by TEM. The effect of the metal incorporation method on the characteristics of the micelles and of the synthesized nanoparticles is investigated.