M. Lanza

Luminescent Mononuclear and Dinuclear Iridium(III) Cyclometalated Complexes Immobilized in a Polymeric Matrix as Solid-State Oxygen Sensors.

Oxygen quenching of the luminescence of mononuclear and dinuclear Ir(III) cyclometalated complexes immobilized in the pPEGMA matrixes has been studied. Linear Stern-Volmer plots, even when experiments at different emission wavelengths have been performed, were evidenced. Despite the different luminescence lifetimes of the chromophores in the absence of quencher, similar Stern-Volmer slopes have been calculated. This behavior was tentatively interpreted by taking into account the size and charge of the chromophores. Increased sizes and lower charges seem to enhance the sensitivity of the systems. Such findings could be of interest for the design of new solid-state luminescent oxygen sensors with improved performance.

DSC and DMTA study of annealed cold-drawn PET: a three phase model interpretation

Cold-drawn poly(ethylene terephthalate) (PET) samples annealed at different undercoolings are studied by means of differential scanning calorimetry and dynamic mechanical thermal analysis. When heating from room temperature, the onset of the glass transition region in cold-drawn, un-annealed samples is found to be significantly lower than in the case of un-oriented PET. On the contrary, the presence of crystalline lamellae in oriented PET cause a shift (and spread out) of the glass transition region towards higher temperatures. The crystal thickening process caused by heating above the annealing temperature, is suggested to take place after a rigid amorphous phase linked to the basal surface of the lamellae has softened. It is found that the low-temperature (between 100 and 140 °C) annealed samples have a glass dispersion region ranging significantly above the annealing temperature itself. This circumstance leads to envisage vitrification as a possible mechanism able to limit lamellar thickening during the annealing process at these low temperatures.

Solid state electrochromic device: behaviour of different salts on its performance

Optical and electrical properties of transmissive solid state electrochromic devices based on a sandwich structure of conductive K-glass®1/WO3/polymeric electrolyte/conductive K-glass®, have been investigated in correlation with the type of salt added to the polymer in order to increase its ionic conductivity. Lithium, sodium and ammonium triflate salts have been used with poly (ethylene glycol) ethylethermethacrylate (pPEGMA) to obtain transparent and conductive semi-solid solutions. The physical properties of the three different types of electrolytes and the operational performances exhibited by devices in terms of colour contrast, response time and stability are discussed. The colouring and bleaching mechanism and the electrochromic behaviour appear to be very similar in the three cases, but a rapid optical degradation is present for devices made up with the ammonium triflate salt when they are operated under cyclic biasing condition.

Influence of Poly(Ethylene Glycol)Methacrylate on the Morphology and Conductivity of Poly(Ethylene Oxide)-Sodium Thiocyanate Complexes

The effect of the introduction of poly(ethylene glycol)methacrylate (PEGMA) on structural, thermal and electrical properties of solid electrolytes based on poly(ethylene oxide) (PEO) and sodium thiocyanate, is investigated by means of calorimetric, mechanical, X-ray diffraction and conductivity measurements. PEO-PEGMA systems show in general a lower value of the glass transition temperature as compared with pure PEO. The crystallinity of the latter has been found to reach a minimum value at about 17% weight of PEGMA. In this situation it is possible to dissolve salt to a concentration χ = 0.051 at least, without formation of the crystalline complex. The ionic conductivity has been found to be almost two orders of magnitude larger than that of PEO-NaSCN systems.

Molecular mobility in semi-IPNs of linear polyurethane and heterocyclic polymer networks

Abstract A study of the thermal and mechanical properties of new semi-interpenetrating polymer networks (IPNs) based on linear polyurethane (PU) and crosslinked trimerized dicyanate (TDC) reveals the existence of structures characterized by the absence of chemical interactions. Two distinct glass transitions are observed in the thermograms, as an indication of the fact that the two polymeric components preserve their molecular structure. The interpenetration affects markedly the glass transition temperatures revealed in the pure components in consequence of modifications in the local environments of the relaxing molecular units in the two phases. The primary and secondary relaxations of these systems show features which can be explained by accounting for the free-volume decrease due to the inclusion of PU in the network of TDC. Below the glass transition two molecular relaxations have been observed which have been ascribed to the secondary relaxation motions characterizing each polymeric component. Both ...

Locally heterogeneous dynamics in miscible blends of poly(methyl methacrylate) and poly(vinylidene fluoride)

Comparative measurements of specific heat capacities (temperature interval between 2 and 500 K), and of low frequency mechanical spectroscopy (temperature interval between 120 and 400 K) in poly(methyl methacrylate) (PMMA) and poly(vinylidene fluoride) (PVDF) amorphous blends show the existence of single calorimetric and mechanical glass transition temperatures, as a clear indication of the existence of homogeneous single-state structures. Below T g , the experimental data reveal distinct local relaxation processes within the backbone of the individual components, while the heat capacities below 15 K can be explained in terms of a two-phase model (i.e., a simple linear overlap of the contributions from wholly amorphous PMMA and PVDF, weighted by their proportions). These findings are associated with locally heterogeneous relaxation and vibrational motions, and are regarded as experimental evidence for the existence of a nanoscopic length scale where the dynamics of a blend exhibits a heterogeneous regime.

DYNAMICAL MECHANICAL MEASUREMENTS IN DRY PHEMA AND ITS HYDROGELS

SummaryThe elastic modulus and tg δ of poly(2-hydroxyethyl methacrylate) (PHEMA) and its hydrogels have been measured over, the frequency range 0.3–30Hz and the temperature range 120–450 K. The mechanical spectra reveal the presence of a low-temperature γ-relaxation plus a shoulder (β-relaxation) that merges into the α-relaxation. The absorbed water lowers the temperature and rises the hight of the lowest temperature peak (the γ-peak for dry PHEMA) while lowering both temperature and height of the α-peak. This behaviour is interpreted in terms of hydrogen-bonding effects between water and polymer which weaken the intra- and/or interchain interactions. Furthermore, the calorimetric analysis in dry polymer is consistent with the mechanical behaviour in the higher-temeprature region.

Structural relaxation processes in poly(ethylene glycol) methacrylate macromonomers

The investigation of the structural relaxation properties of poly(ethylene glycol) methacrylates are of practical interest, due to their ability to work as the inert backbone to which poly(ethylene oxide) oligomers can be attached to obtain highly amorphous polymeric matrices. Two poly(ethylene glycol) methacrylate, PEGMA, macromonomers, with different side chain lengths, are investigated by the dielectric technique, in the frequency range 0.3-300 kHz, and by Brillouin scattering. The analysis of the Brillouin spectra gives evidence for the existence of a relaxation process in both the systems. Furthermore, a comparison of the normalized absorption data with their corresponding classical values (deduced from the shear viscosity data) suggests that what the Brillouin scattering experiment detects is a dissipative relaxational process in which the shear viscosity plays the main role. The comparison of the Brillouin scattering results with the dielectric data shows that what we are observing, in both the systems, is a single relaxation process. The temperature dependences of the relaxation times, observed on more than 10 decades, fail to follow simple Arrhenian behaviours. Both the systems can be interpreted as intermediate between strong and fragile liquids, following the Angell classification, and appear characterized by the existence of a wide variety of local structural environments, triggered by some relaxation process. Such a situation is more clearly evidenced by the macromonomer with longer chain. From the whole body of experimental data, it can be deduced that shorter-side-chain PEGMA macromonomers are better candidates for the formation of highly amorphous comb-branched polymers.

Thermodynamic analysis of two‐dimensional crystal growth in nucleated isotactic polypropylene

A thermodynamic model is used to describe the two‐dimensional crystallization kinetics (as observed by means of a calorimeter) of polypropylene nucleated with small amounts of indigo. The customary expression of the free enthalpy associated to secondary nucleation is initially used to analyze the experimental data. A significant dependence of the apparent surface tension (either basal or lateral) of the nuclei on the concentration of indigo is pointed out. To account for this effect, a phenomenological correction to the free enthalpy of nucleation is considered afterwards, which is connected with an entropy contribution arising from the formation of the crystal–melt interface. A more detailed data analysis is then carried out in light of this model.

Mechanical behavior of polycyanurate-polyurethane sequential full-interpenetrating polymer networks

Differential scanning calorimetry and dynamic mechanical spectroscopy studies have been performed between 120 and 600 K in a new series of sequential full-interpenetrating polymer networks (full-IPNs) of crosslinked polyurethane and polycyanurate network (PCN), based on dicyanate ester of 1,1 0 -bis-4,cyanatophenyl-ethane. The measurements have revealed the existence of two distinct glass transitions in the thermograms and of relaxation losses given by the overlap of two aa-relaxations in the mechanical spectra whose temperature locations change with changing weight ratio of polymeric component. These observations indicate that the morphology of this new class of full-IPNs is based on double-phase structures characterized by weak interchain interactions. The interpenetration affects markedly the glass transition temperatures revealed in the pure components as a consequence of modifications in the local environments of the relaxing molecular units in the two phases. Below the glass transition, two secondary relaxation losses have been observed which have been ascribed to local molecular transitions within each polymeric component. The analysis of their concentration behaviors leads to a conclusion that the interpenetration process affects markedly the local motion of the PCN component. 2002 Elsevier Science B.V. All rights reserved.