Sotiria Kripotou

Polymer Dynamics in Polyurethane/clay Nanocomposites Studied by Dielectric and Thermal Techniques

Differential scanning calorimetry (DSC), broadband dielectric relaxation spectroscopy (DRS), and thermally stimulated depolarization current (TSDC) techniques were employed to investigate glass transition and polymer dynamics in nanocomposites of polyurethane (PU) and organically modified montmorillonite (MMT) (weight fraction 0%–15%) prepared by solution casting. The PU matrix was obtained from oligo(oxytetramethylene glycol) of molar mass 1000 g/mol, 4,4′-diphenylmethane diisocyanate and 1,1-dimethylhydrazine as chain extender. Wide-angle X-ray scattering confirmed the formation of partly exfoliated structures at low MMT content. DSC, DRS, and TSDC show, in agreement with each other, that a fraction of polymer makes no contribution to the glass transition and to the corresponding α relaxation, whereas the rest exhibits similar glass transition dynamics as the pure matrix. This fraction of immobilized polymer reaches a maximum at about 5 wt% MMT. Effects of MMT on the microphase-separated structure of PU are negligible, as indicated by the study of glass transition and interfacial dielectric polarization/relaxation. No effects of MMT on the local, secondary γ and β relaxations were observed. Mechanical properties show a maximum improvement at about 5 wt% MMT, in good correlation with morphology and dynamics.

Molecular Mobility in Polycyanurate/Clay Nanocomposites Studied by Dielectric Techniques

Nanocomposites, having neat polycyanurate networks (PCN) and modified by polytetramethylene glycol polycyanurate networks (PCN/PTMG) as matrices and montmorillonite (MMT) as filler, were prepared using thermal polymerization, and studied with respect to polymerization kinetics, morphology and in particular, molecular mobility. In order to investigate the influence of the clay modifier and the way of preparation two types of commercial MMT were used, while the mixing was obtained mechanically and by using ultrasound. IR-spectroscopy, Wide Angle X- ray Scattering (WAXS) and dielectric techniques including Dielectric Relaxation Spectroscopy (DRS) and Thermally Stimulated Depolarization Current (TSDC) techniques were used to provide information on polymerization kinetics, morphology and molecular mobility, respectively.

Polycyanurate -Organically Modified Montmorillonite Nanocomposites: Structure -Dynamics- Properties Relationships

Polycyanurate‐modified montmorrilonite (PCN‐MMT) nanocomposites were synthesized by polymerization of dicyanate ester of bisphenol A in the presence of MMT dispersed by ultrasound. Techniques of IR spectroscopy, WAXD, and TEM were applied to study polymerization kinetics and structure of the nanocomposites prepared, whereas their dynamics and thermal/mechanical properties over the −30 to 420°C range were studied by using DSC, laser‐interferometric creep rate spectroscopy (CRS), and dielectric relaxation spectroscopy (DRS) techniques. It was shown that a small amount of MMT additive acts as a catalyst of polymerization and results in the formation of complicated intercalated/exfoliated structures, as well as strongly modifies the dynamics in the PCN network. Pronounced dynamic heterogeneity was observed for PCN/MMT nanocomposites. Along with the main PCN glass transition, two new glass transitions, at much higher and much lower temperatures, were revealed as a consequence of constrained dynamics in matrix interfacial nanolayers and due to incomplete local cross‐linking in the PCN matrix, respectively. In addition, increased sub‐T g mobility was observed in these nanocomposites. A two‐fold rise of modulus of elasticity as well as increasing thermal stability and arising microplasticity at low temperatures, promoting, obviously, improved crack resistance in a brittle PCN network, were found for the PCN‐MMT nanocomposites.

Thermal Transitions of Polypropylene in Blends and Composites with Polypyrrole and Polypyrrole/Montmorillonite

Thermal transitions of the semi-crystalline isotactic polypropylene, in polypropylene/polypyrrole blends and polypropylene/polypyrrole/montmorillonite composites, processed by two different ways, were investigated by differential scanning calorimetry. Glass transition temperature of polypropylene was found to remain unchanged at 269 K, whereas the crystallization rate was found to be higher in the blends and composites, which is explained in terms of increased concentration of extrinsic crystallization nuclei. The effect is larger in the materials processed by mixing and subsequent compression molding as compared to the materials prepared directly by compression molding. The degree of crystallinity of polypropylene did not show any systematic variation with composition, whereas it is slightly higher for the samples prepared by direct compression molding, being in the range of 50–59%. The polypropylene in the blends and the composites crystallizes in the stable α form, whereas metastable crystallites of the β form were observed as a minor component, depending on the thermal history of the samples. The results are discussed on the basis of the picture emerging from morphological studies.

Porous Polylactic Acid–Silica Hybrids: Preparation, Characterization, and Study of Mesenchymal Stem Cell Osteogenic Differentiation

A novel approach to reinforce polymer porous membranes is presented. In the prepared hybrid materials, the inorganic phase of silica is synthesized in-situ and inside the pores of aminolyzed polylactic acid (PLA) membranes by sol-gel reactions using tetraethylorthosilicate (TEOS) and glycidoxypropyltrimethoxysilane (GPTMS) as precursors. The hybrid materials present a porous structure with a silica layer covering the walls of the pores while GPTMS serves also as coupling agent between the organic and inorganic phase. The adjustment of silica precursors ratio allows the modulation of the thermomechanical properties. Culture of mesenchymal stem cells on these supports in osteogenic medium shows the expression of characteristic osteoblastic markers and the mineralization of the extracellular matrix.

Structure and dynamic/compositional heterogeneity in polycyanurate : Poly (tetramethylene glycol) hybrid networks

Structure and dynamics over the range from −150 to 300°C were studied in a series of polycyanurate‐poly(tetramethylene glycol) (PCN‐PTMG) hybrid networks. Wide‐angle x‐ray diffraction (WAXD), small‐angle x‐ray scattering (SAXS), laser‐interferometric creep rate spectroscopy (CRS), differential scanning calorimetry (DSC), and thermally stimulated depolarization currents (TSDC) techniques were applied. The networks were synthesized from the dicyanate ester of bisphenol A (DCEBA) and hydroxyl‐terminated PTMG with M n =1000, 2000 or 5000 g/mol and weight fraction of 10, 20, 30, and 40%. The noncrystalline structure and the pronounced structural nanoheterogeneity, depending on PTMG chain length and PCN‐PTMG ratio in the hybrid systems, were evidenced. Combined CRS/DSC analysis revealed a complex dynamics in these networks, in particular a dispersion of glass transitions in a wide temperature range. On this basis, the presence of nanodomains with different degrees of rigid cross‐linking (X CN→PCN), i.e., compositional nanoheterogeneity in these hybrids was shown. The results obtained turned out to be of applied interest because of a substantial increase in tensile strength because of arising microplasticity at low temperatures, and retaining some creep resistance, at low stresses, up to temperatures much higher the basic T g . In Commemoration of the Contributions of Professor Valery P. Privalko to Polymer Science.

Phase Transitions and Molecular Mobility in 5CB and CE8 Studied by Dielectric Techniques

Two complementary dielectric techniques, broadband dielectric relaxation spectroscopy (DRS) and thermally stimulated depolarization currents (TSDC), covering together broad ranges of frequency and temperature, were employed to study molecular mobility in the molecular liquid crystal 5CB. Three relaxations were observed and analyzed with respect to time scale, strength and shape, a sub-glass β relaxation and the α and δ relaxations in the supercooled nematic phase. In addition, TSDC and isochronal DRS measurements were used to follow phase transitions in 5CB and CE8 in comparison with DSC. The results demonstrate the power of dielectric techniques, in particular TSDC, for monitoring phase transitions in liquid crystals.

Dielectric and hydration properties of segmental polyurethanes

Abstract Novel polyurethanes (PUs), based on oligo(oxytetramethylene glycol), 4,4’-diphenylmethanediisocyanate and 1,1-dimethylhydrazine as chain extender, were prepared in a two-step process. The ratio prepolymer (PP) to chain extender (CE) was systematically varied in extreme ranges, from 1:1 to 10:1. Dielectric relaxation spectroscopy and thermally stimulated depolarization currents (TSDC) techniques were employed to investigate molecular dynamics and to conclude on microphase separation (MS). In that respect TSDC was proven to be very powerful, in particular as far as the investigation of the interfacial Maxwell-Wagner-Sillars polarization is concerned. Additional information on micromorphology is obtained from water sorption/diffusion measurements. A part of the results suggest that MS improves with increasing the PP:CE ratio. The whole body of results can be explained if, at the same time, it is assumed that a branched structure is developed for samples out of stoichiometry and branching increases with increasing the PP:CE ratio. Preliminary experiments with solutions of the PUs in organic solvents provide support for that assumption.

Study of Isothermal Crystallization Kinetics of 5CB with Differential Scanning Calorimetry and Broadband Dielectric Spectroscopy

The isothermal crystallization kinetics of 4-n-pentyl-4–cyanobiphenyl (5CB) has been studied with Differential Scanning Calorimetry (DSC) and Dielectric Relaxation Spectroscopy (DRS). 5CB is a well characterized material which makes it ideal for a dielectric and thermal comparative study. The effect that isothermal crystallization exerts on the behavior of relaxation processes α and δ by cooling the isotropic liquid below the nematic phase or through the process of cold crystallization by heating it from the supercooled state is explored by comparing the DRS and DSC. Furthermore, by employing the Avrami equation, we compare the ability to probe the crystallization with each technique.

Dielectric relaxation in polymer–clay nanocomposites

Abstract: This chapter deals with the investigation of polymer dynamics in polymer–clay nanocomposites by using dielectric techniques. These have been established as a powerful tool for investigation of chain dynamics in polymers and composites. The chapter first reviews briefly dielectric techniques, in particular broadband dielectric relaxation spectroscopy and thermally stimulated depolarization currents. It then presents and discusses results on polymer dynamics obtained first with a model system, poly(propylene oxide) amines intercalated in clay, and, in the following, with three families of polymer/clay nanocomposites with different polymer matrices, polyurethane, epoxy resin, and polycyanurate network.