Synthesis and properties of polyester imides based on 2,2-bis(4-hydroxyphenyl)propanedibenzoate-3,3ʹ,4,4ʹ-tetracarboxylic acid dianhydride

Abstract

Evidence for the Higgs boson, for which the Nobel Prize in physics was awarded in 2013, was discovered by the European Organization for Nuclear Research (CERN) in 2012, using the Large Hadron Collider (LHC). PIXEO BP-S exhibited superior resistance to both radiation and cryogenics and this polymer played a crucial role in the operation of the LHC, thus contributing to the Higgs boson discovery. Herein, we report the synthesis and characterization of various polyester imides for a cryogenic and radiation-resistant polyimide adhesive tape. These polyester imides (3) were synthesized from 2,2-bis(4-hydroxyphenyl)propanedibenzoate-3,3ʹ,4,4ʹ-tetracarboxylic acid anhydride (ESDA) and various diamines (1a–1l). All polymers showed high solubility in various solvents, especially 3a, which was synthesized from ESDA and 4,4ʹ-bis(3-aminophenoxy)diphenyl sulfone and dissolved even in ether solvents such as dioxane and dioxolane. Dynamic mechanical analysis and X-ray diffraction studies indicated that the excluded volume of the polyester imides had a length of 150 Å, and that this volume affected the thermoplastic properties and solubilities of the polymers. The degree of imidization of 3a dissolved in dimethyl sulfoxide was examined by ¹H-Nuclear magnetic resonance(NMR) imaging utilizing a ¹⁵N-enriched amine model compound in conjunction with ¹H–¹⁵N heteronuclear single quantum coherence 2-dimensional NMR and was found to be 99.5%.

Introduction

In 2013, the Nobel Prize in Physics was awarded to Dr François Englert and Dr Peter W Higgs for their work on the Higgs boson.¹ Evidence for the existence of this particle had been obtained the previous year by the European Organization for Nuclear Research (CERN), using the Large Hadron Collider (LHC).²

In the late 1990s, CERN announced that the Large Electron-Positron Collider, located 100 m below ground at a site approximately 27 km from Geneva, would be replaced by the LHC as part of their efforts to discover proof of the existence of the Higgs boson shown in Figure 1. At the time, CERN had a requirement for a polyimide film and a polyimide adhesive tape with good resistance to radiation and cryogenics. In response to this requirement, 24.5 MT of a polyimide adhesive tape and 66.5 MT of a polyimide film itself were provided to CERN between 2000 and 2005.

Figure 1

(a) The Large Hadron Collider at CERN (Home Page of High Energy Accelerator Research Organization (KEK), http://semrl.t.u-tokyo.ac.jp/supercom/85/85_1.html). (b) The cross section of LHC Dipole, and (c) polyimide tape lapping.

The use of a polyimide tape that can be adhered at low temperatures and pressures on the superconducting wires was important so as to avoid quench cracking phenomenon.^{3, 4} Towards this purpose, we successfully designed a series of solvent soluble thermoplastic polyester imides that could be coated on a base polyimide film. Upon wrapping of this coated film on to the cables, it exhibited good adherence when subjected to low pressure–temperature conditions. The polyimide tape was lapped on to the superconducting strands by a 50% overlap of each winding for thorough coverage and insulation. The robust property of this material allowed it to withstand radiation (alpha particles) and exposure to cryogenic temperatures (liquid helium flow) from the collider, thus meeting all requirements set by CERN.

As a result of the contribution of the polyimide adhesive tape to the discovery of the Higgs Boson, this product has been presented with the Award of the Society of Polymer Science, Japan (2014). Herein, we report the synthesis of polyester imides for a polyimide adhesive tape, as well as the characterization of these materials.

Polyester imides

Importance and novelty of polyester imides

Loncrini⁵ first reported the synthesis of polyester imides from 2,2-bis(4-hydroxyphenyl propanedibenzoate-3,3ʹ,4,4ʹ-tetracarboxylic acid anhydride (ESDA) via trimellitate acetate, bisphenol-A and various diamines in the late 1960s. Although Kurita⁶ has reviewed the subsequent progress of research into polyester imides, there have been only a few literature reports concerning the use of ESDA to date.

In the present study, the HOMO and LUMO energy levels of ESDA were calculated and compared with those of other dianhydrides. As can be seen from Table 1, it was found that the LUMO energy level of ESDA was lower than those of Da and Db (see Table 1 for the full names of all compounds), nearly equal to that of TMHQ, and higher than those of ODPA, BTDA, BPDA, and PMDA. It has been proposed that LUMO energy levels⁷ have a significant effect on the reactivities of dianhydrides during electrophilic attack (an SN₂-type reaction) by diamines. Therefore, it follows that a lower LUMO energy level should result in higher reactivity. In keeping with this theory, ESDA was found to have moderate reactivity among the series of dianhydrides listed in Table 1. ESDA was thus considered as one of several potential dianhydrides for polyimide syntheses.

Table 1 MOPAC3 Calculation results for HOMO and LUMO orbital levels of dianhydrides

Syntheses of polyester imides

A series of polyester imides (3) were synthesized from the reaction of ESDA with various diamines (1a–1l) by a newly developed rapid thermal imidization method using polyamic acid intermediates (2). The chemical structures of the resulting polyester imides are presented in Figure 2. Each of these polymers showed high solubility in various solvents, especially 3a, which was synthesized from 4,4ʹ-bis(3-aminophenoxy) diphenyl sulfone and ESDA, and was soluble even in the ether solvents dioxane and dioxolane.

Figure 2

Chemical structures of polyester imides (3) derived from ESDA and various diamines (1).

The imidization ratios of the polyester imides were determined by a newly developed method based on a ¹H–¹⁵N heteronuclear single quantum coherence (HSQC) 2-dimensional (2D) nuclear magnetic resonance (NMR) imaging technique utilizing a ¹⁵N-enriched amine model compound.

Characteristics of polyester imides

Table 2 summarizes the solubilities of the polyester imides in various common solvents. It was found that 3a, 3c, 3d, 3e and 3f, all of which had ether linkages, ester linkages or were meta-substituted, showed solubility in THF and some other ether solvents. Compounds 3a and 3c were particularly soluble in dioxane and dioxolane, while 3l was also soluble in CHCl₃.

Table 2 Solubilities of polyester imides (3)

Casting of 3a from a dioxolane solution generated a transparent, light yellow film with the properties summarized in Table 3.

Table 3 Film properties of polyester imide 3a

Dynamic mechanical analysis (DMA) of the resulting polymers was also performed, and a typical DMA plot is presented in Figure 3. This plot shows that above its glass transition temperature (Tg) of 185 °C, the storage elastic modulus of the material (Eʹ) drops rapidly to a value of approximately 10⁵. In the temperature range above its Tg, 3a thus exhibited excellent flow properties.

Figure 3

DMA curve of polyester imide 3a.

X-ray diffraction (XRD) crystalline structure analysis of ESDA was performed after obtaining a pure crystal from DMF solution. An ORTEP drawing of the ESDA repeat unit is shown in Figure 4. The high solubility of this polyester imide can be attributed to the bent structure of its repeat units. The 3D structure of 3i, composed of ESDA and 1i, was calculated with Gaussian 03 HF/STO-3G using the ORTEP data for ESDA, and is shown in Figure 5.

Figure 4

ORTEP drawing of an ESDA repeat unit.

Figure 5

3D structure of the polyester imide from ESDA data.

The DMA and XRD data indicated that the excluded volume of the polyester imide was 150 Å in length, and that this volume was associated with the unique thermoplastic properties and solubility of the polymer.^{8, 9} As a result, it can be said that the properties of polyester imide 3i result from its amorphous nature.^{10, 11}

The determination of imidization ratios by infrared spectroscopy measurements has been reported, although not in a quantitative manner.¹² Imidization ratios of polyimides in organic solvents have also been assessed using ¹H-NMR spectroscopy, but there are associated difficulties in confirming the N–H peak positions.¹³ Therefore, it was necessary to develop a new method to assess the degree of imidization of these polyester imides. For this purpose, 3a was dissolved in DMSO and studied using a ¹⁵N-enriched amine model compound and ¹H–¹⁵N HSQC 2D NMR.^{14, 15} In addition, a ¹⁹F-NMR method capable of elucidating the trifluoroamide-NH structures of the derivatized polymer was employed.

The chemical structure of the model compound A, composed of ESDA and aniline-¹⁵N, is shown in Figure 6, and the ¹H-NMR spectra of A in its original and its imidized forms are presented in Figures 7a and b, respectively. These spectra confirmed that the N–H peak is found at 10.5 p.p.m. and the COO–H peak at 13.5 p.p.m. The ¹H–¹⁵N HSQC spectrum of compound A is given in Figure 7c. From this, the N–H peak of compound A was found to appear at 10.4–10.6 p.p.m.

Figure 6

Chemical structure of model compound A composed of aniline-¹⁵N and ESDA.

Figure 7

¹H-NMR spectra of (a) the imidized form of model compound A, (b) the model compound A, and (c) the ¹H−¹⁵N HSQC spectrum of A in DMSO-d₆.

The ¹H-NMR spectrum of 3a is shown in Figure 8.

Figure 8

¹H-NMR spectrum of 3a in DMSO-d₆.

In Figure 8, an N–H peak is observed at 10.5 p.p.m. and the protons of the aromatic group appear from 7 to 9 p.p.m.

From these data, the imidization ratio of 3a was calculated using Equation (1).

Here A_am and A_ar represent the integration values of the N–H and aromatic protons. Based on Figure 8 and Equation (1), the imidization ratio of 3a was determined to be 99.5%.

Conclusion

Evidence for the existence of the Higgs boson, for which the Nobel Prize in Physics was awarded in 2013, was discovered by CERN in 2012, using the LHC. During this work, a polyester imide having both high radiation resistance and superior cryogenic properties, was adopted as the superconducting magnet insulation adhesive tape within the LHC. In this study, we synthesized various polyester imides, including 3a, and examined the properties of these polymers. Recently, in 2013, it was further confirmed during a maintenance and repair process of LHC that the PIXEO BP-S polyimide adhesive tape still retained its radiation and cryogenic temperature resistance as same as when it was received.¹⁶