Effect of thermal history on the deformation and failure of polyimides

Abstract

Thermal-processing structure-property relationships for polyetherimide (PEI), poly(4,4Ј-oxydiphenylene pyromellitimide) (POPPI), and phenylethynyl-terminated imide (PETI-5) composite matrices are reported from a fundamental perspective. For thermoplastic PEI, deformation and failure depend primarily on free volume as evidenced by moisture-absorption, mechanical-property, and mass-density changes as a function of annealing. The deformation of POPPI can be divided into the following three regimes as a function of annealing temperature: (1) physical aging-induced glassy state free-volume decreases, (2) thermally activated microvoid collapse, and (3) chemical degradation. In the case of PETI-5, macroscopic defects, free volume, and polymer morphology control deformation. The effects of residual crystallinity on deformation are reported, and it is shown that mechanical toughness can be significantly decreased upon annealing below the glass-transition temperature.

Proton Mobility in Sulfonated PolyEtherEtherKetone (SPEEK): Influence of Thermal Crosslinking and Annealing

“…Another important strategy for the improvement of polymers is annealing [34][35][36][37]: in that case the polymer is treated at a distinctly lower temperature, below the one needed for building covalent bonds between macromolecules and creating cross-links, but sufficiently high to induce irreversible changes of conformations or remove entanglements of macromolecular chains. In general, annealing leads to a reduction of free volume in the polymer [38].…”

Analysis of Lightweight Materials for the AM2 System

“…Literature showed that PETI-5, with only a minimal increase in density (1234 kg/m 3 to 1335 kg/m 3 ), displayed a 35% increase in its elastic modulus, an 80.4% increase in its ultimate strength and was seen to be much more ductile at equal strain rates compared to 3900 (Lincoln et al 2001). Researchers also noted that PETI-5 was able to remain fairly ductile at much higher strain rates compared to the 3900.…”

“…This fact is very important in that solid control of the cooling process established a stable set of mechanical properties. Cooling rates of 1.1°C/m (Lincoln et al 2001) were shown to produce a more ductile polymer matrix with the highest recorded glass transition temperature and, therefore, the best thermal stability of all of the samples.…”

“…However, the most significant issues were caused by debonding between the ceramic reinforcement particles or fibers and the metal matrix material produced by differences in material thermal expansions and lower material yield strengths at these elevated thermal conditions (Lincoln et al 2001). This issue is even more pronounced since the F-35 will be one of the heaviest joint strike fighters ever deployed, weighing almost twice as much as a conventional F-16.…”

“…When the material temperature passes over this transition temperature, being a thermoset, the molecular bonding will not reset if it plastically deforms. Therefore, even though the material will maintain its shape, the material's tensile strength will decrease and a weak segment in the structure will develop (Lincoln et al 2001). …”