Increase In Tg Values Research Articles

Thiol‐ene Hybrid Organic/Inorganic Nanostructured Coatings Based on Thiol‐Functionalized Zirconium Oxoclusters

AbstractThe thiol‐functionalized zirconium oxocluster Zr12(µ3‐O)8(µ3‐OH)8(MP)24 · 4MPA was used as inorganic nanosized building block in the thiol‐ene photopolymerization of APE and TH in a 1:1 molar mixture. Transparent and crack‐free coatings were obtained, and TEM analysis showed that the inorganic particles are well dispersed within the polymeric network with no significant macroscopic agglomeration. An increase of Tg values, storage modulus in the rubbery region, and thermal stability were evidenced by increasing the zirconium oxocluster content in the photocurable formulations. XPS analysis and SIMS depth profile were carried out on UV cured films and showed the presence of a homogeneously distributed zirconium oxocluster.magnified image

Composite Polyether Electrolytes with Lewis Acid Type Additives

The effects of AlBr3, AlCl3, and α-Al2O3 on the conductivity and ultrastructure of electrolytes based on LiClO4 and polyethers have been studied. The results obtained are analyzed in terms of Lewis acid−base interactions occurring between various chemical moieties within these composite systems. It is shown that aluminum halides form complexes with ClO4- anion acting as plasticizing agents for polyether matrixes. However, aluminum halides interact also with polyethers leading to the formation of polyether−aluminum halide complexes, thus stiffening polymeric electrolytes. It is shown that for low additive concentrations the addition of Lewis acid results in a decrease in the degree of crystallinity of poly(ethylene oxide)-based electrolytes; this is followed by an increase in the conductivity. A significant increase in conductivity was obtained at 0 and 25 °C for samples containing up to 25 mass % of aluminum halides or α-Al2O3. For samples containing more than 30 mass % of the additive, the effect of the stiffening of polymer hosts dominates (and is confirmed by the increase in Tg values observed from DSC experiments) and the conductivity decreases.

Synthesis and characterization of halogen‐containing polyisophthalamides

AbstractTwo series of aromatic polyamides were prepared from m‐phenylenediamine (MPD) and 4,4′‐diaminodiphenyl sulfone (DDS) in combination with a set of new halogenated diacyl chlorides, namely, 5‐fluoroisophthaloyl chloride, 5‐chloroisophthaloyl chloride, 5‐bromoisophthaloyl chloride and 5‐iodoisophthaloyl chloride. The polymers were synthesized in high yields and in high molecular weight (as determined by gel permeation chromatography) by means of the low temperature condensation method. Noticeable changes in the spectral characteristics of the halogen‐modified polyisophthalamides were observed compared to those of the corresponding unmodified polyisophthalamides. Slight improvements were observed for the glass transition temperatures, which increased 5–30°C relative to those of the conventional polyisophthalamides. Moreover, a correlation was found between the size of the halogen substituents and the increase in Tg values. Thermogravimetric analysis showed that the new polymers are almost as thermally resistant as the unmodified ones, while solubility in organic solvents was some‐what poorer.

Modification of DGEBA with diimide‐diacids derived from bicyclo[2.2.2]oct‐7‐ene‐2,3,5,6‐tetracarboxylic dianhydride and crosslinking study

Epoxy resins derived from diimide–diacids obtained from bicyclo[2.2.2.]oct-7-en-2,3,5,6-tetracarboxylic dianhydride and diglycidyl ether of bisphenol A were synthesized and characterized by spectroscopic techniques and thermal analyses. A further crosslinking using hydroxylic groups using dianhydride or diisocyanate as a hardener was also carried out. These processes led to crosslinked materials, but a DSC study does not show a straightforward relationship between the amount of hardener used and reaction enthalpiies, which suggests a complicated reaction mechanism. On the other hand only a slight increase in Tg values was observed during the curing process. No substantial differences between the thermal characteristics of linear and crosslinked polymers wer observed. © 1996 John Wiley & Sons, Inc.

Modification of poly(vinyl alcohol) with acid chlorides and crosslinking with difunctional hardeners

The reaction of acid chlorides with poly(vinyl alcohol) (PVA) leads to vinyl alcohol-vinyl ester copolymers. The esterification by Schotten-Baumann enables degrees of modification to be reached which depend on the chemical structure of the pendant unit introduced. High degrees of modification were obtained in the reaction with water-stable acid chlorides such as benzoyl or cinnamoyl chloride. The copolymers obtained were characterized by spectroscopic techniques, elemental analysis, thermal methods, and viscosity determinations. Since in all cases they were only partially modified, the remaining hydroxyl groups were reacted with difunctional hardeners, Epiclon B-4400, hexamethylene diisocyanate, and tolylene 2,4-diisocyanate, to obtain tridimensional networks. Vinyl alcohol-vinyl benzoate copolymers with different degrees of modification were crosslinked to study how the percentage of remaining hydroxylic groups affects the crosslinking process. The exotherm maximum of the curing process was lower when diisocyanates were used as hardeners and the degree of crosslinking was higher when hexamethylene diisocyanate was used according to the observed increase in Tg values. Whereas the TGA curves of linear polymers only showed one degradation, in crosslinked polymers two degradation steps were detected, suggesting several degradation mechanisms. © 1996 John Wiley & Sons, Inc.

A comparative study of the kinetic theories of the glass transition phenomenon in high polymers

AbstractThe effect of thermal history on the glass transition temperature (Tg) of poly(vinyl chloride) was studied. The parameters like the hole energy (Eh), the activation energy for the disappearance of holes (Ej), the activation enthalpy for the structural relaxation (Δh1* and Δh2*) and the activation energy for the glass transition process (E) were calculated. The increase of Eh value with the increase in Tg value shows that there exists a holesize distribution. The Ej (26.9 ± 0.5 kcal/mol) value calculated according to Wunderlich's treatment, the Δh1* (27.8 ± 0.6 kcal/mol) value obtained by Moynihan's procedure and the E (35.3 kcal/mol) value from Barton‐Critchley's method agree well with one another in the poly(vinyl chloride) system. The Δh2* quantity, obtained through Moynihan's formulations, increase as the rate of heating was increased, a result similar to the variation of Eh value with the rate of heating.