Enhanced Photostability and Red-NIR Photosensitivity of Conjugated Polymer Charge-Transfer Complexes

A promising route to photostable and low-bandgap polymer materials for optoelectronic and photonic applications could be utilization of donor-acceptor ground-state charge-transfer complexes (CTCs) of wide-bandgap conjugated polymers. In this work, we report our results on photostability and photoelectric studies of CTC between an archetypical conjugated polymer MEH-PPV and low-molecular-weight acceptor 2,4,7-trinitrofluorenone (TNF). By using the pump-probe laser photobleaching technique, we show that the photodegradation rate of MEH-PPV:TNF blend films decreases up to four orders of magnitude as compared with pristine MEH-PPV. By using photocurrent and surface photovoltage spectroscopies, we demonstrate that the photoelectric sensitivity of the MEH-PPV:TNF CTC can be deeply extended in the polymer optical gap down to 1 µm. However, the main drawback of the CTC studied is their low charge collection efficiency, and an approach to increasing it is discussed.     

Influence of degradation on the crystallization behaviour of a biodegradable segmented copolymer constituted by glycolide and trimethylene carbonate units

The influence of degradation on non-isothermal crystallization from the melt of a segmented copolymer constituted of glycolide and trimethylene carbonate units and used as a bioabsorbable surgical suture was studied by optical microscopy, differential scanning calorimetry and time-resolved X-ray diffraction. Fibrillar positive spherulites were obtained with slightly degraded samples but new axialitic morphologies were detected when samples had a molecular weight, M_w, lower than 29,000 g/mol and the crystallization started at a high temperature.Crystal growth kinetics of samples degraded under different conditions was evaluated over a wide temperature range by a non-isothermal method. Two crystallization regimes (I and II) were determined for the more degraded samples (i.e., those able to crystallize according to axialitic and spherulitic morphologies), whereas only regime II was found for samples of higher molecular weights. Primary nucleation density decreased with the extent of degradation provided no morphological changes occurred, and so did the regularity of lamellar stacking, as shown by synchrotron measurements, although the morphological parameters remained practically constant.     

The effect of extrusion processing on Zein

Extrusion processing was carried out on zein where extrusion temperatures were varied between 100 and 300 °C. Differential scanning calorimetry and thermogravimetric analysis showed that thermal degradation begins around 220 °C. The color of the extrudate changed the most above temperatures of 160 °C. Sodium dodecyl sulfate polyacrylamide gel electrophoresis shows that the protein begins cross-linking at 120 °C and chain cleavage begins above 180 °C. Examination of the protein structure of the extrudate using near and far-UV circular dichroism shows a gradual reduction in α-helix and β-sheet content between 100 and 240 °C; above 240 °C the rate of secondary structure loss is increased. Infrared spectroscopy displayed differences in the carbonyl absorption with the carbonyl peak becoming narrower and shifting towards higher wavenumber with increased extrusion temperature. The peak at 1533 cm⁻¹ becomes slowly smaller with higher extrusion temperature until 220 °C after which its loss accelerates. Nuclear magnetic resonance spectroscopy demonstrated the formation of new carbonyl peaks and the loss of alkoxyl carbons suggesting that in addition to protein backbone cleavage the alcohol moieties of serine and threonine are oxidizing to carboxylic acids. Tensile properties begin to deteriorate when extruding above 140 °C; extruding above 220 °C yields a material that cannot be molded.     

Preparation and thermal degradation kinetics of terpolymer poly(?-caprolactone-co-1,2-butylene carbonate)

Poly(?-caprolactone-co-1,2-butylene carbonate) (PBCCL) was successfully synthesized via terpolymerization of carbon dioxide, 1,2-butylene oxide(BO) and ?-caprolactone (CL). A polymer-supported bimetallic complex (PBM) was used as catalyst. The influences of various reaction conditions such as reaction content, reaction time and reaction temperature on properties of terpolymers were investigated. When CL content increased, the viscosity-average molecular weights (M_v), glass transition temperature (T_g) and decomposition temperature (T_d) of PBCCL improved relative to those of poly(1,2-butylene carbonate) (PBC). Prolonging the reaction time resulted in increase in M_v and T_g. As reaction temperature increased, the molar fractions of CL (fCL) increased obviously. When the reaction temperature went beyond 80 °C, the resulting copolymers tended to be crystalline. The thermal properties and degradation behaviors of PBCCL were investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The apparent activation energy and thermal degradation model of PBCCL was estimated by means of Ozawa-Flynn-Wall method and Phadnis-Deshpande method, respectively. The results showed that T_g and T_d of the terpolymer PBCCL were much higher than those of PBC. The thermal degradation behavior of PBCCL was evidenced by one-step thermal degradation profile. The average apparent activation energy is 77.06 kJ/mol, the thermal degradation kinetics follows the power law thermal decomposition model.     

Influence of MWCNT morphology on dispersion and thermal properties of polyethylene nanocomposites

In this study a series of multi-walled carbon nanotube (MWCNT)/Polyethylene (PE) composites with different kinds and several concentrations of carbon nanotubes (CNTs) were investigated. The morphology and degree of dispersion of the fillers in the polymer matrix at different length scales was investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Both individual and agglomerated MWCNTs were evident but a good dispersion was observed for some of them. TGA measurements were performed on nanocomposites in order to understand if CNTs affect the stabilization mechanism during thermal and oxidative degradation. The analysis demonstrates that MWCNTs presence slightly delays thermal volatilisation (15-20 °C) without modification of thermal degradation mechanism. In contrast, thermal oxidative degradation in air is delayed up to about 100 °C dependently from MWCNTs concentration, in the range used here (0.1-2.0 wt%), and degree of dispersion. The stabilization is due to the formation of a thin protective layer of entangled MWCNTs kept together by carbon char generated on the surface of the nanocomposites as shown by SEM images taken on degradation residues.     

The effect of metal catalyst on the discoloration of poly(ethylene terephthalate) in thermo-oxidative degradation

Titanium-based catalyst and ethylene glycol antimony were used as catalysts to prepare poly(ethylene terephthalate) (PET), and the effect of the catalysts on the discoloration of PET in thermo-oxidative degradation was studied by UV-visible spectroscopy, Fourier transform infrared spectroscopy (FTIR) and fluorescence spectroscopy. Hydroxylation was found during thermo-oxidative degradation of both the titanium- and antinomy-containing PETs. However, more hydroxylated products were further changed into quinonoid species in the presence of titanium catalyst, which directly results in obviously enhanced coloration of the PET. It indicates that the PET synthesized with titanium-based catalysts is more susceptible to formation of quinonoid groups and discoloration than the PET prepared with ethylene glycol antimony.     

Effect of selected residual Ziegler-Natta and metallocene catalysts on the UV-induced degradation of unstabilized ethylene homopolymer films

Catalyst creates new grades of polyethylenes while their degradation phenomena determine the resulting application profiles. The resins and the end-products eventually contain residual catalysts. Therefore, the effects of the following two residual catalysts—one Ziegler-Natta [PQ Silica/Bu₂Mg/^tBuCl/TiCl₄], and the other metallocene [PQ Silica/MAO/(ⁿBuCp)₂ZrCl₂]—on the UV-induced degradation of the resulting unstabilized ethylene homopolymer films were studied. The MetCat HomoPE film was found to be more susceptible to UV-induced degradation degradation than the Z-N HomoPE one despite the residual Zr level was 1/15th of the Ti content. Therefore, the zirconocene residual catalyst better (i) facilitated the UV-initiated reaction, and (ii) decreased the activation energy required for the decomposition of the resulting hydroperoxide. These findings were explained from the differences in (i) electronic configuration and atomic radius, and (ii) surface chemistry and solid-state electronic environment of these catalyst systems. In both films, T_pm and T_pc did not significantly vary with the exposure time. The change of % crystallinity showed to be irregular and disturbed. As a function of carbonyl index, T_pm and T_pc,varied in a similar fashion in the MetCat HomoPE film. However, they significantly differed for the Z-N HomoPE film. Here, T_pm did not appreciably change whereas T_pc initially increased very mildly; then it did not essentially differ.     

Thermal degradation studies of polyurethane/POSS nanohybrid elastomers

Reported here is the synthesis of a series of polyurethane/POSS nanohybrid elastomers, the characterisation of their thermal stability and degradation behaviour at elevated temperatures using a combination of thermogravimetric Analysis (TGA) and thermal volatilisation analysis (TVA). A series of PU elastomer systems have been formulated incorporating varying levels of 1,2-propanediol-heptaisobutyl-POSS (PHIPOSS) as a chain extender unit, replacing butane diol. The bulk thermal stability of the nanohybrid systems has been characterised using TGA. Results indicate that covalent incorporation of POSS into the PU elastomer network increases the non-oxidative thermal stability of the systems. TVA analysis of the thermal degradation of the POSS/PU hybrid elastomers have demonstrated that the hybrid systems are indeed more thermally stable when compared to the unmodified PU matrix; evolving significantly reduced levels of volatile degradation products and exhibiting a ∼30 °C increase in onset degradation temperature. Furthermore, characterisation of the distribution of degradation products from both unmodified and hybrid systems indicate that the inclusion of POSS in the PU network is directly influencing the degradation pathways of both the soft and hard-block components of the elastomers: The POSS/PU hybrid systems show reduced levels of CO, CO₂, water and increased levels of THF as products of thermal degradation.     

Structures and gas-sorption properties of carbonaceous film prepared by radio-frequency sputtering of polysaccharides pectin

Carbonaceous films with microcolumnar layer have been prepared by radio-frequency sputtering of polysaccharides pectin. The repeated sputtering has developed the densely packed seamless microcolumns, which are separated by the narrow grooves. The residual film stress has formed the honeycomb-patterned ridges. X-ray photoelectron spectroscopy and energy dispersion spectroscopy revealed the inclusion of nitrogen in the film constituents. The film surface is hydrophilic mainly due to the polar functional groups, such as the carboxyl and amino groups. Nitrogen adsorption measurement revealed that the specific surface area of the film was no less than 109 m²/g. Impedance analysis of the film-coated quartz crystal resonator clarified that the film had the higher adsorption capacities to the polar and cohesive vapors, such as ethyl alcohol. The adsorption of organic vapors has not induced the viscoelastic changes in the film.     

Silver sulfide/poly(3-hydroxybutyrate) nanocomposites: Thermal stability and kinetic analysis of thermal degradation

The non-isothermal degradation of poly(3-hydroxybutyrate) (PHB) and silver sulfide/poly(3-hydroxybutyrate) (Ag₂S/PHB) nanocomposites was investigated using thermogravimetric (TG) analysis. In the composite materials, Ag₂S caused the degradation of PHB at a lower temperature as opposed to that of neat PHB. Moreover, an increase Ag₂S loading in the PHB decreased the onset temperature (T_onset) of thermal degradation, whereas it was raised upon augmenting the heating rate. From Kissinger plots, the observed trend of the degradation activation energy, E_d, was attributed to polymer-particle surface interactions and the agglomeration of Ag₂S. The thermal degradation rate constant, k, was linearly related to the Ag₂S loading in PHB. Thus, the Ag₂S nanoparticles effectively catalyzed the thermal degradation of PHB in the Ag₂S/PHB nanocomposites. Differential scanning calorimetry (DSC) data also supported the catalytic property of Ag₂S.