Search

Polysiloxane networks with or without phenyl groups have been subjected to isothermal, thermal sweep and cycling thermal treatments to investigate the phenyl effect on evolution of their properties during thermal aging. Polydimethyldiphenylsiloxanes (PDMDPS) containing between 5 and 30 mol% phenyl groups were more stable than polydimethylsiloxanes (PDMS) under isothermal aging, and no effect of the phenyl content was noted. Hence, only PDMS and PDMDPS with 5 mol% phenyl groups were then retained for the monitoring of their weight loss, tensile stress, elongation at break and Young's modulus during isothermal treatments between 150 and 300°C. Weight losses as well as the cracks appearance time during cycling aging were recorded simultaneously. The activation energies under the different aging conditions were determined, and found equal to 175 kJ/mol from the weight loss (characteristic of the depolymerization reaction) and 145–165 kJ/mol from the mechanical properties (characteristic of the post‐cross‐linking reaction) for PDMDPS, and 110 kJ/mol and 125–145 kJ/mol for PDMS, respectively. These activation energies confirm that silicone containing phenyl moieties are thermally more stable than those without. Finally, all these results empirically allow connecting weight loss and changes in mechanical properties to the appearance of cracks during thermal cycle aging. [ABSTRACT FROM AUTHOR]

A series of eight poly(aminobismaleimide)s containing aromatic units have been synthesized from a one-step nucleophilic addition reaction between diamine and bismaleimide, without exogeneous solvent, to provide encapsulant withstanding high temperature suitable for power electronics. To have a homogeneous medium at room temperature, the solid aromatic diamines are first mixed with different liquid aliphatic ones, and then mixed with bismaleimide. The syntheses were then carried out at 175 8C for 15 min. Thus, according to their composition, these obtained thermosetting resins are characterized by a relaxation temperature between 98 and 190 8C and a coefficient of thermal expansion between 20 and 150 ppm/K. All material surfaces are hydrophobic and their moisture uptake is lower than 2-3 wt %. Finally, as expected, the substitution of a part of the aliphatic diamine by an aromatic one improves the thermal stability under air atmosphere of the resulting materials (Td5280-300 8C increased to Td5315-340 8C). Their use as power module encapsulant can therefore be considered. [ABSTRACT FROM AUTHOR]

The article discusses the results of a study which examined the flexibility and rigidity of proton conducting poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS) and poly(ethylene oxide) dimethacrylate (PEGDM). The methods to study the homogeneity of conetworks are described, including dynamic mechanical thermal analysis and differential scanning calorimetry. Factors that influence the swelling behavior of the materials are cited.