Search Results
Abstract
The long-term environmental ageing of conductive composite films containing ethylene-co-vinyl-acetate (EVA) copolymer and a complex of polyaniline (PANI) and dodecylbenzenesulfonic acid (DBSA) was studied by using differential scanning calorimetry (DSC). We assume that both phase separation and crosslinking of PANI main chains occur in the systems. On the other hand, the competition between PANI–DBSA complex self-organization and crystallization of EVA matrix result in structural changes and formation of continuous conductive network, responsible for significantly increased (ca five orders of magnitude) electrical conductivity of the aged films.
Abstract
An extensive study has been carried out on aqueous ‘extremely diluted solutions’ (EDS). The employed experimental methodologies were well established physico-chemical techniques: flux calorimetry, conductometry, pH-metry, e.m.f. of suitable galvanic cell. The obtained results show that the preparation procedure significantly alters the physico-chemical behaviour of such solutions. Moreover, the analysis of the experimental data vs. the ‘arrow of time’ turned out to be astonishingly important. In fact some measured physico-chemical parameters evolve with time. Some experimentally measurable physico-chemical properties of the solvent water were largely affected by both time and the ‘life path’ of the samples. In particular, we evidenced two new experimental phenomena characterizing the EDS: the presence of a series of maximums in the measured electrical conductivity vs. the sample age; the dependence of said maximums on the volume of the EDS during its ageing. All of these new experimental results clearly suggest the presence of an extended and ‘ordered’ dynamics involving the whole of the water molecules in the liquid. A temporal evolution, featuring three maximums in the course of four years of ageing and the dependence on the ageing volumes do not fit the framework of classical thermodynamics. It therefore seems appropriate to interpret these phenomena on the basis of the thermodynamics of dissipative structures, which are far from equilibrium systems.
Abstract
It is well known that SiO2 -based inorganic-organic hybrid materials present significant differences due to the organic moieties bound to the inorganic network and to the preparation conditions. In the present work the ageing effect on the thermal stability of the SiO2 -based inorganic-organic hybrid materials prepared using tetraethoxysilan (TEOS), triethoxymethylsilan (MTEOS), triethoxyvinylsilan (VTEOS), tetramethoxysilan (TMOS), trimethoxymethylsilan (MTMOS) and trimethoxyvinylsilan (VTMOS) was studied. TG/DTA-MS was used to study the decomposition process of the materials. The structural modifications of the materials during ageing were studied using spectral methods. The gels obtained starting with more reactive alkoxides, of methoxy-type, present more complex structures and are less stable during ageing.
Introduction Cracking in the early age concrete often results from restrained volume changes due to temperature change during the hydration. Cracking may reduce the concrete durability against aggressive substance as well as
Summary
The hydration products in two aged cement pastes (DTA/DTG/TG) were compared with those in fresh ones (static heating, SH) and were also studied by mass spectrometry (MS), IR and thermo XRD-analysis. The products considered here were: the sorbed water, the CSH gel including hydrates, portlandite, calcite, aragonite and vaterite. Except carbonates their content was higher in the stronger paste C-43, than in C-33, but lowered with ageing (only the CSH gel water remained approximately unchanged). The sorbed water content became with time lower and similar in both pastes (it evaporated up to 155-185C in TG); the escape of the rest moved to higher temperatures (500-700C). The three DTG peaks at 200-400C indicated jennite-like phase in the CSH gel; the mass loss (155-460C) was higher on ageing due to development of organic matter, especially in C-43 (DTA, TG, IR). Portlandite content changed little and carbonate content increased considerably. They decomposed in air at 470 and 720-740C, in argon at 450 and 680-710C and in vacuum at 400 and 630C, respectively (DTG peak, XRD). Between 500 and 700C the simultaneous evolution of H2O and CO2was observed by MS, which is attributed to dehydroxylation of jennite-like phase and/or to decomposition of some carbonate hydrate and/or hydrocarbonate (three peaks on CO2evolution curve, MS). The d(001) peak of portlandite exceeded the nominal value and will be analyzed separately.
Vitrification phenomena and further structural relaxation processes or physical ageing occurring in the isothermal curing reaction of an epoxy resin are studied by Differential Scanning Calorimetry (DSC). The vitrification time,t v , the limiting conversion degree and the limiting glass transition temperature (T g) are evaluated at curing temperatures (T c) between 30 and 100‡C. The dependence of limitingT g withT c permits the determination of the maximumT g of the resin (109‡C). The physical ageing, which appears as the the last step of curing reaction for curing times above to vitrification, is analyzed through the endothermic peak superposed to the glass transition temperature. The results obtained in partially reticulated resin show the kinetics of the physical ageing to slow down asT c increases, as a consequence that the segmental mobility is reduced.
Abstract
Simulation of waterlogged archaeological woods was carried out by immersion of fir and chestnut wood samples into sea water at different temperatures (room temperature and 40°C). The effects of metals in contact with woods were simulated by inserting in some specimens of the two types of wood copper or iron nails, the most important metals from the archaeological point of view. The effects of this ageing simulation on woods were studied by different characterization methods. At first we have performed gravimetric analyses, controlling the mass increase of immersed wood in function of the time of immersion and the temperature of the bath. Then, thermogravimetry, differential thermal analysis, differential scanning calorimetry in oxygen flux were used. The alteration of wood was observed by means of the peak temperatures of DTA, DTG and DSC variation and by the mass losses observed during heating, evaluated on the basis of the measured thermal data. The samples were woods powder obtained by milling. Complementary characterization of the woods was performed by evaluating the crystallinity of cellulose by means of X-ray powder diffraction. The change in colour of woods during ageing was checked by means of spectrophotometric measurements in the visible region. X-ray fluorescence was used to investigate the penetration of metals into wood samples. An artificial ageing treatment with NaOH and O3 was also performed. Finally, a comparison between the effects of artificial alteration realised in our specimens and natural degradation observed in archaeological woods, was performed.
Abstract
To reveal the fire injuring of parchment, the changes in the thermal behaviour of some goat parchments, obtained from skins originating from different animals, as a result of thermal aging were determined by thermal analysis methods (DSC; simultaneous TG/DTG, DSC; micro hot table (MHT)). Thermal aging of parchments was revealed to bring about the decrease in shrinkage temperature, absolute value of enthalpy of denaturation in water and some changes in non-isothermal parameters characteristic for dehydration process in static air atmosphere. The results obtained by DSC analysis performed in N2 and O2 flows as well as those obtained by simultaneous TG/DTG, DSC analyses have shown that both softening (melting) process parameters and parameters of thermo-oxidative processes have not been changed by thermal ageing. The results obtained by thermal analysis methods were correlated with those obtained by microscopic investigation of parchment samples immersed in water and scanning electron microscopy (SEM). The application of these microscopic techniques has revealed the morphology changes in the investigated parchments as a result of thermal degradation.
Abstract
Thermal investigation has allowed us to show the changes undergone by a sort of nitrile-butadiene rubber (NBR) as a consequence ofγ-radiation-induced ageing. The parameters of the processes, which occur at progressive heating of the investigated samples, were determined. It was shown that for γ-irradiated samples the activation parameters corresponding to the thermo-oxidative process leading to solid products are correlated through the relation of compensation effect. Also, it was shown that, by γ-irradiation, NBR undergoes a relatively rapid change of its thermal behaviour which can be due to structural changes.
Abstract
Physical aging of polyetherimide (PEI) was studied using a bellows dilatometer based on Zoller's design. A linear variable differential transformer (LVDT) is used to measure the displacement of the bellows. The voltage output of the LVDT is interfaced to a computer for automated data collection. Isothermal aging experiments were carried out at temperatures near the glass temperature (206–209 ‡C) using a constant temperature oil bath maintained at the desired aging temperature. The time required to reach equilibrium and the reduced curve produced by aging time-temperature superposition are given. The results compare well with data obtained by capillary dilatometry for the same material.
