Leni Akcelrud

Polyethylene characterization by FTIR

Important experimental parameters for FTIR spectra acquisition were studied: apodization, beam aperture, number of scans and spectral resolution, using polyethylene samples. The optimum conditions for the analyses were established and permitted unequivocally to identify the three most important commercially available grades: low density, high density and linear low density products (LDPE, HDPE and LLDPE respectively).

Degradation profile of polyethylene after artificial accelerated weathering

Abstract Elucidation of the chemical changes that take place during the aging of polyethylene sheared electrical cables, and the correlation of these changes with physical properties need experiments with accelerated aging assays. These were carried out using the most representative polyethylene grades found in todays market: low density (LDPE), linear low density (LLDPE) and high density (HDPE). The samples were exposed to UV- and xenon arc radiation using a varying exposure time and different temperature cycles. The changes in the material structure and properties were analyzed by differential scanning calorimetry (DSC), scanning electron microscopy (SEM), FTIR-spectroscopy, density and hardness measurements. During degradation polar groups in the polyethylene are generated, as well as chain scission and cross-linking. As net effects of these processes respective increases in crystallinity, density and hardness and surface cracking at late stages of degradation are observed. By varying the angle of incidence in attenuated total reflection (ATR) experiments using FTIR-spectroscopy it was possible to detect variations in composition of distinct layers from the surface down to a depth of 1.2 μm. Deeper layers were reached by microtoming the samples in layers down to 60 μm. The main chemical modifications were carbonyl formation of various kinds which were identified in the FTIR–ATR spectra of degraded polyethylene samples.

The effects of CdSe incorporation into bulk heterojunction solar cells

Hybrid solar cells based on CdSe nanoparticles and a PPV-type polymer containing fluorene and thiophene units (PFT) were investigated. The CdSe/PFT devices showed very low photocurrent and fill factor values, which was attributed to the poor charge transport in the TOPO-capped CdSe nanoparticle phase. Thus, ternary systems based on mixtures of PFT/CdSe and the fullerene derivative PCBM were investigated. The CdSe:PCBM ratio was varied, and nanoparticles with different sizes were also used. It was observed that for the optimized composition of 20 wt% PFT + 40 wt% CdSe + 40 wt% PCBM the devices presented higher photocurrents and efficiencies. The photophysical and electrochemical properties and microscopy images (AFM and HRTEM) of the ternary systems were systematically investigated to elucidate the mechanism of action of the inorganic nanoparticles in these ternary hybrid devices.

The effect of accelerated aging on the surface mechanical properties of polyethylene

The surface mechanical properties of polyethylene subjected to accelerated aging in Weathering Tester (QUV) and weatherometer (WOM) chambers were determined by a nanoindentation technique. It was observed that the apparent surface hardness was three times greater than that of the bulk after 1600 h of exposure in the WOM chamber, and a similar increase was also observed after 800 h of exposure in the QUV chamber. The elastic modulus at tip penetration depths of 500 nm also shows a significant increase from 400 to 2000 MPa for samples aged in the QUV chamber during 800 h. For aging times of 200 h and lower the variations in surface mechanical properties are small and restricted to a thin layer, with thickness lower than 1 μm for samples aged in both WOM or QUV chambers. The modifications in nanomechanical properties were correlated with the corresponding chemical processes due to aging, measured through the carbonyl index profile obtained from ATR–FTIR. An interpretation for the surface cracking of aged LDPE based on differential increases in elastic modulus at different depths is presented.

Thermal treatment and dynamic mechanical thermal properties of polyaniline

Thermal transitions of polyaniline in the emeraldine base form (Pani-EB) were studied by DMTA using two series of thermally treated samples. In the first series the specimens were annealed at 70 °C for 5, 15 min, 1 and 3.5 h. In the second they were submitted to annealing at 100 °C during the same periods of time plus a 24 h treatment. Two transitions were observed at sub-zero temperatures and were attributed to the motion of solvated water and solvent (NMP). The glass transition and the highest temperature relaxation, assigned to crosslinking depended on the degree of solvation resulting from the thermal treatment. A linear contraction of Pani-EB films with residual water evaporation was reported for the first time.

Light-emitting copolymers of cyano-containing PPV-based chromophores and a flexible spacer

Abstract Conjugated–non-conjugated alternating block copolymers containing cyano-modified distirylbenzene units as emitting chromophores were synthesized via Knoevenagel and Wittig reactions; the non-conjugated block was an alkylene spacer. The polymers were differentiated with respect to the location of the cyano groups in the chromophoric block (double bond or phenylene ring). When photoluminescence and electroluminescence spectra of these materials were compared with those of a similar structure without the cyano group, a strong bathochromic effect was observed. A more pronounced red shift and a higher electroluminescence efficiency was observed in the polymer with the cyano group attached to the aromatic ring.

Fluorescent aggregates in naphthalene containing poly(urethane–urea)s

Abstract A series of segmented poly(urethane-urea)s containing naphthalene in the hard block and hexamethylene spacers in the soft block was prepared. The hard to soft segment ratio was varied systematically, to afford a series of polymers with various chromophore concentrations and a constant length of the chromophoric block, using a three-step synthetic procedure. The absorption, fluorescence and fluorescence-excitation spectra of solutions and films of the block copolymers provide strong evidence for aggregation. A red-shifted fluorescence spectrum peaking at 420 nm gains in intensity as the naphthalene concentration is increased. The excitation spectrum of this new emission is well to the red of the normal naphthalene absorption spectrum, consistent with the UV spectrum. Formation of a fluorescent ground state dimer (or higher aggregate) is proposed to account for these observations.

Interchain luminescence in poly(acetoxy-p-phenylene vinylene)

The synthesis of a new substituted poly(phenylene vinylene) (PPV) derivative poly(acetoxy-p-phenylene vinylene) is reported. A new pathway, combining chemical and electrochemical steps was employed, resulting in a highly regular, polar, monosubstituted PPV. The participation of excimers and/of aggregates as emitting units in PPV and some of its derivatives has been demonstrated previously in various papers. This contribution provides evidence from fluorescence studies of the participation of those associated forms in the emission of this novel structure. A pattern of progressive fluorescent aggregate formation was observed, a result not reported to date.

Hole mobility effect in the efficiency of bilayer heterojunction polymer/C60 photovoltaic cells

We report here bilayer heterojunction solar cells fabricated by using poly[9,9′-hexyl-fluorene-alt-bithiophene] (LaPPS43) polymer as active layer. The power conversion efficiency (η) displays a sevenfold increase upon annealing at 200 °C, reaching the value of 2.8%. This result is comparable to the highest η reported so far for bulk heterojunction solar cells using the same polymer. Simulation, absorbance spectra, and current versus voltage results indicate that the π–π stacking in solid state is enhanced after annealing with a reduction in traps and thus reflecting in higher hole mobility.

HTPB‐based polyurethanes. II. SINs with PMMA

Simultaneous interpenetrating networks from poly(methyl methacrylate-co-ethyleneglycol dimethacrylate) (PA) and a hydroxyl-terminated polybutadiene-based polyurethane (PU) were prepared with various hard-segment contents (X) in the PU and different ratios (PU/PA) between the components. The level of the reinforcement, the mechanism of molecular failure, and the phase inversion depended strongly on X. Dynamic mechanical results indicated that the interpenetration occurred in the rigid blocks of the PU. The improved thermal and mechanical properties observed with higher values of X were interpreted in terms of the molecular weight and polydispersity of the hard blocks in the PU.