Structure Development in a Thermoplastic Polyimide. Cold Crystallization as Revealed by Microhardness

Abstract

ABSTRACT:The isothermal "cold crystallization" of the thermoplastic polyimide (New TPI) has been studied using the microhardness technique to examine the property-microstructure correlation. By using wide-and small-angle X-ray diffraction and differential scanning calorimetry. the influence of the experimental parameters, i.e., treatment temperature T, and time t, on the micromechanical properties of the "cold crystallized" samples has been examined. It is shown that both macroscopic hardness H and volume crystallinity x, increase with T, and 1,. For the samples prepared in the secondary crystallization range. it is demonstrated that H values strongly depend on the hardness of the crystalline units//,. In this range, long spacing L. crystal thickness /, and crystallinity (both linear XcL• and derived from density x,) slightly increase with T,. From the DSC study, it is demonstrated that the proportion of "liquid-like" and "rigid" amorphous fraction present in each sample is directly related with the crystallization conditions. Finally, from the SAXS and DSC combined study, information concerning the secondary crystallization mechanism has been obtained.KEY WORDS Thermoplastic Polyimide / Microhardness / Crystallinity / "Liquid-Like" and "Rigid" Amorphous Fraction / Aromatic polyimides have many technological applications. mainly due to their specific chemical structure. The presence of aromatic and imide rings in the monomeric units imparts rigidity to the polymeric chain. As a result, aromatic polyimides exhibit high Tg (glass transition temperature) and Tm (melting temperature) values. Besides good thermal stability, these materials show excellent mechanical and electrical properties and high radiation and solvent resistance. Aromatic polyimides are therefore widely used in specific applications: electronic packaging for printed circuits. insulation for lead wire, etc, being particularly useful in the aerospace industry.Some specific products of this group, e.g., the so called Kapton (developed by DuPont de Nemours Company) and BTDA-ODA based polyimide (obtained from benzophenone tetracarboxylic dianhydride and 4,4' -oxydianiline) have very rigid chains, being for this reason extremely difficult to melt-process and crystallize. To overcome this inconvenience, synthetic methods have been developed in order to lower the transition temperatures by different routes: incorporation of substitutes in the aromatic rings, introduction of more flexible groups ((-CH 2-)., -0-) or meta-phenyl linkages into the main chain, 1 · 2 etc. The thermoplastic polyimide has been synthesized using the last route. Its monomeric unit contains both ether groups and phenyl rings substituted in meta-position. 3 . The result is a crystallizable material with a glass transition temperature Tg of about 250°C. This value is notably lower than those exhibited by other aromatic polyimides. For this reason, the crystallization of New TPI can be easily followed from the molten 3 or from the amorphous solid state ("cold crystallization"). 4 The d...

Crystallization behavior of a thermoplastic polyimide derived from 3,3′,4,4′‐oxydiphthalic dianhydride and 4,4′‐oxydianiline

“…Therefore, polyimide materials have special applications in automobile and aircraft parts and packaging imprinted electronic circuitry, and continue to gain importance in a wide variety of applications like high‐temperature adhesives and composites, microelectronics, membranes, and as photosensitive materials 1–3. In general, introduction of crystallinity in these materials can further substantially improve the thermal stability and enhance solvent resistance,4–6 as well as increase radiation resistance and partial retention of mechanical properties above the T g 7, 8. Therefore, semicrystalline polyimides have been the focus of considerable research over the recent years 9–29.…”

Crystallization of a thermoplastic polyimide

“…The Aurum thermoplastic polyimide, derived from pyromellitic anhydride and 4,4′‐bis(3‐aminophenoxy)biphenyl, has been claimed as a crystalline polyimide with excellent heat resistance and injection moldability, and can be used to produce high‐performance parts 13, 14. Various properties and the crystalline properties of the Aurum thermoplastic polyimide have been characterized to some extent in the literature 15–24. The temperature resistance of Aurum is significantly dependent on its crystallinity.…”

Density Fluctuations as Precursors of Crystallization in a Thermoplastic Polyimide

“…The development of crystallinity in the amorphous thermoplastic polyimide (New-TPI) has been studied, in detail, using differential scanning calorimetry, wideand small-angle X-ray scattering (WAXS and SAXS) 1 · 2 and microhardness, 3 by cold crystallization from the amorphous state. Preceding SAXS studies show that during cold crystallization, New-TPI develops a periodic structure with crystals a few molecular repeat-units thick.…”