Mechanical Properties of Polyimides

The incorporation of 40% glass fiber into polyimide increased the tensile strength from 72 to 79 MPa (Table 3.1) and the flexural modulus from 2.46 to 13.2 GPa with hardly any effect on impact strength (Table 3.2). The elongation of break falls from 

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Additional drawbacks to the use of polyimide insulators for the fabrication of multilevel structures include self- or auto-adhesion. It has been demonstrated that the interfacial strength of polyimide layers sequentially cast and cured depends on the interdiffusion between layers, which in turn depends on the cure time and temperature for both the first layer (Tj) and the combined first and second layers (T2) [3]. In this work, it was shown that unusually high diffusion distances ( 200 nm) were required to achieve bulk strength [3]. For T2 > Tj, the adhesion decreased with increasing T. However, for T2 < Tj and Tj 400 °C, the adhesion between the layers was poor irrespective of T2. Consequently, it is of interest to combine the desirable characteristics of polyimide with other materials in such a way as to produce a low stress, low dielectric constant, self-adhering material with the desirable processabiHty and mechanical properties of polyimide. 

Another strategy employed to enhance the mechanical strength, lies in the integration of aromatic imide bonds into the backbone of the polyanhydrides. This particular choice aims at combining the good mechanical properties of polyimides with the degradative properties of polyanhydrides. 

The most extensively investigated polyanhydride is poly [(carboxyphenoxy) propane-sebacic acid] p(CPP-SA). Ghadel is based on p(CPP-SA), which is used as a dehvery matrix for controlled dehvery of the chemotherapeutic carmustine to treat brain cancer. Septacin, a copolymer based on 1 1 S A and EAD, is used for the delivery of gentamicin in the treatment of osteomyehtis. Poly(anhydrides-co-imides) have been developed to combine the mechanical properties of polyimides with surface-eroding properties of polyanhydrides, and they are being explored as scaffolds for tissue engineering. 

Sager, K., Schroth, A., Nakaldal, A. Gerlach, G. [1996]. Humidity-dependent mechanical properties of polyimide films and their use for ic-compatible humidity sensors. Sensors and Actuators A Physical Vol. 53(No. 1-3) 330-334. 

Xiaowen J, Yuezhen B, Masam M (2006) Electrical and mechanical properties of polyimide-carbon nanotubes composites fabricated by in situ polymerization. Polymer 46(18) 7418-7424 Xie F, Liang H, Ren XJ, Yifa C (2013) Isothermal crystallization of PET/PTT-CNTs composites. 

V. E. Yudin, J. U. Otaigbe, V. M. Svetlichnyi, E. N. Korytkova, O. V. Almjasheva, V. V. Gusarov, Effects of nanofiller morphology and aspect ratio on the rheo-mechanical properties of polyimide nanocomposites, eXPRESS Polymer Letters, 2 (2008), 485-93. 

Likewise, the mechanical properties of the copolymers were nearly identical or even somewhat enhanced towards the polyimide homopolymer in terms of the modulus and tensile strength values [44,47]. For most of the block copolymers, the elongations to break were substantially higher than that of PMDA/ODA polyimide (Table 4). The shape of the polyimide stress-strain curve is similar to that of a work-hardened metal with no distinguishable yield point... 

In light of the higher imidization temperatures associated with poly(amic alkyl esters), this approach could be extended to yield improved coating formulations [82]. Since it was now possible to balance the relative reactivity of the chain-extender with the imidization temperature of an oligofamic alkyl ester), here R = methyl, ethyl, etc., less reactive chain-extenders could be utilized. The lower reactivity of the chain-extender would be reflected in improved solution stability and shelf-life of the formulation without sacrificing the mechanical properties of the final polyimide, see Table 10 where EGX, TFE,... 

Table 10. Mechanical properties of PMDA/ODA based polyimides derived from low molecular weight, chain-extendable precursors...

Table 14.4. Mechanical Properties of Perfluorinated (10FEDA/4FMPD), Partially Fluorinated (6FDA/TFDB), and Unfluorinated (PMDA/ODA) Polyimide Films ...

Syntactic materials based on polybenzimidazole and glass or phenolic microspheres (y = 40-500 kg/m3) have been described in the literature109 nl). They have been used as ablation materials that consist of two layers, one of which is a monolithic carbon plastic. Carbon fiber was added to the polyimide binder to improve the mechanical properties of the material. 

Svetlichnii VM, Denisov VM, Kudryavtsev VV Polotskaya GA, Kuznetsov YP (1991) Soluble ether imide-oligodimethylsiloxane copolymers kinetic mechanical and gas separation properties of polyimides and other high-temperature polymers. Proc Eur Tech Symp 2nd Abadie MJ, Sillion B (eds). Elsevier, Amsterdam,pp 525-535... 

Earlier investigations on the dynamic mechanical properties of PPMI over a wide range of temperatures indicated the existence of two distinct relaxations at around 250 and 400 K [15,16]. The former relaxation was assigned to adsorbed water molecules in the polyimide chain and the latter was due to local relaxation modes of the backbone. Recently Ahlbom reported the mechanical relaxation of various polymers at low temperatures [17]. In the film sample of PPMI, the small relaxation at 93 K. attributed to the motion of phenyl rings was observed, although the dominant damping peak at 198 K is not yet explained. 

A similar kind of association may be responsible for the mechanical properties of the polyimide discussed at the beginning of this article. Thus, the 10-second modulus shows only a gradual decrease over the temperature interval 30° to 450°C. rather than an abrupt decrease over a small temperature span, as is usually observed for linear amorphous polymers at temperatures near the glass transition (10). It was suggested that a small amount of crystallinity was responsible for this behavior, which is similar to the explanation advanced for the properties of BBB. Despite this indication of limited molecular mobility, the polyimide is reported to be ductile in elongation and thus again similar to BBB. 

The dielectric constants of polymers are strongly influenced by their chemical structure. Generally, molecules with more polar groups have higher dielectric constants. Polymers with very low dielectric constants (<3) such as polyethylene and polytetrafluoro-ethylene, do not always meet the combined thermal and mechanical requirements for application in electronic devices, nor do they have desirable adhesive characteristics. Polyimides with much better thermal and mechanical properties have dielectric constants between 3 and 4. Efforts to combine the desirable features of the fluoro polymers with those of the polyimides have been successful in that the resulting compositions have dielectric constants equal to or smaller than 3 without sacrificing too much of the thermal and mechanical properties of the polyimides (23). 

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