Influence of Curing Accelerators on the Imidization of Polyamic Acids and Properties of Polyimide Films

In order to lower the imidization temperature of polyamic acids（PAA）, the catalytic activities of the curing agents p-hydroxybenzoic acid（PHA）, quinoline（QL）, benzimidazole（BI）, benzotriazole（BTA）, triethylamine（Et_3N） and 1, 8-diazabicyclo [5.4.0]undec-7-ene（DBU） were investigated in the process of thermal imidization of PAA. In addition, the effect of these various curing agents on the thermal stabilities and mechanical properties of the resultant polyimide（PI） films was determined. Quinoline was found to be an effective curing accelerator in the use of two-step method for synthesizing PI. Due to its moderate base strength, low steric crowding effect and moderate boiling point, quinoline could not only accelerate PAA to achieve imidization completely at 180 ℃, but also maintain the mechanical properties and thermal stability of the ordinary PI film. Any residual quinoline could be removed from PI films by heating at 250 ℃ for 4 h.

Influence of Curing Duration on Thaumasite Formation of Portland-limestone Cement Pastes

Extensive researches have been carried out on the conventional sulfate attack, while it has been found that the thaumasite form of sulfate attack（TSA）, sulfate attack at low temperature, has just been discovered and its mechanism is not well understood so far. In this study, the sulfate attack of cement paste incorporating 30% mass of limestone powder was investigated. After 20 ℃ water cured for 7, 14, and 28 days,respectively, 20 mm cube specimens were exposed in a 5% magnesium sulfate solution at（6 ±1） ℃ for periods up to 240 days. Their appearance change, compressive strength development were examined at different storage time, and selected paste samples were examined by X-ray diffraction（XRD）, Fourier transform infrared spectroscopy（FTIR）, scanning electron microscopy（SEM） and energy dispersive spectroscopy（EDS）. The results indicate that all Portland-limstone cement pastes suffer from appearance deterioration to some extent. The compressive strength of cement paste initially increases and after 120 days decreases with increasing exposed period. In addition, the cement paste with short curing time is more susceptible to sulfate attack, which directly leads to the formation of non-binder thaumasite crystal accompanied by the formation of ettringite, gypsum and brucite, and becomes a white, mushy, and incohesive matrix. Additionally, the extent of sulfate attack is greater and the formation of thaumasite is observed earlier for shorter curing time.

Low-Energy Driven Ring-Opening Behavior of Benzocyclobutene Derivatives

It's urgent to develop benzocyclobutene(BCB)-based polymers with low curing temperature for temperature-sensitive applications such as liquid crystal displays(LCDs)and flexible electronics.Herein,the effect of substituents on the ring-opening behavior of BCB derivatives was investigated.The ring-opening activation energy barriers(ΔGA)of BCB derivatives with one or two substituents on the four-membered alkyl ring were systematically calculated using the B3LYP function.Both mono-and di-substituted BCBs adopted the conrotatory ring-opening process,obeying the Woodward-Hoffmann's Rules upon heating.The mono-/di-substituted BCBs exhibited 8.2%—69%lowerΔGA compared with BCB,attributed to the electronic effects of the substituents.Disubstituted BCBs with both electron-donating and electron-withdrawing groups,e.g.,1-NH_(2)-8-NO_(2)-BCB,demonstrated the lowestΔGA.In addition,BCB derivatives with amide/ester/acyloxy group modified on C1 position were synthesized as model molecules,and their ring-opening temperature can be decreased by 20℃ compared to the unsubstituted one,also consistent with our calculation results.This work combined theoretical calculation methods with experimental results to provide valuable insights into the design and synthesis of BCB derivatives and next-generation BCB functional packaging materials with low ring-opening temperature.