Control of Head/Tail Isomeric Structure in Polyimide and Isomerism-Derived Difference in Molecular Packing and Properties

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1 Communication Isomeric Polyimides Control of Head/Tail Isomeric Structure in Polyimide and Isomerism-Derived Difference in Molecular Packing and Properties Longbo Luo, Yu Dai, Yihao Yuan, Xu Wang, and Xiangyang Liu* Two sequence isomeric poly(amic acid)s (PAAs) are successfully synthesized from 3,3,4,4 -biphenyltetracarboxylic dianhydride and unsymmetrical 5(6)-amino-2-(4-aminobenzene) benzimidazole (PABZ). The syntheses are based on the site-selective reactivity of head/tail amino groups of PABZ and solubility differences of PABZ in good solvent (dimethyl sulfoxide, DMSO) and poor solvent (N-methyl-2-pyrrolidone, NMP). The proton nuclear magnetic resonance ( 1 H-NMR) results reveal that the content of head tail head tail (HT HT) bonding units in PAA-DMSO (PAA synthesized in DMSO) is 37%, while this content increases to 54% in PAA-NMP (PAA synthesized in NMP). The wide-angle X-ray diffraction (WAXD) results indicate polyimide (PI)-NMP film with high HT HT content exhibits a semicrystalline structure, while PI-DMSO film is amorphous. Moreover, PI-NMP also shows higher inplane orientation than PI-DMSO. The ordered molecular packing and higher in-plane orientation of PI-NMP lead to an increase in mechanical properties and a decrease in in-plane thermal expansion coefficient. 1. Introduction Aromatic polyimides (PIs) have excellent mechanical properties and thermal stability, along with good chemical resistance and electrical properties. [1 3] Because of these outstanding properties, PIs have been widely used in many applications such as in the aerospace, gas separation, microelectronics industries, and so on. [4 8] However, the physical properties of PIs are still needed to be further enhanced in order to meet the requirements for the application in the severe environment. [9] In our previous papers, [10,11] the diamine, 5(6)-amino-2-(4-aminobenzene)benzimidazole (PABZ), containing both proton donor ( NH ) and proton acceptor ( N ), was incorporated into the main chain of PI. The mechanical properties and thermal properties of those PIs containing benzimidazole units were significantly improved due to the formation of H-bonding interactions. [12,13] Moreover, this type of PI with outstanding mechanical properties has been paid more and more attention by other researchers. [14 18] The diamine, PABZ, is an unsymmetrical heterocyclic diamine, and the charge density values of head and tail amino groups were calculated by Material Studio 4.0 in our previous paper, [19] as shown in Figure 1 The charge density of amino group adjacent to benzimidazole unit (head amino group) is larger than that of amino group adjacent to benzene ring (tail amino group). [19] It means that head amino group is more reactive than tail amino group. Thus, when PABZ was reacted with dianhydride, the sequence isomerism of PAA would arise because of the regularity of head head (H H), head tail (H T), and tail tail (T T) bonding sequence. Moreover, During the study of solubility of PABZ in different polar aprotic solvents, we found that N-methyl- 2-pyrrolidone (NMP) and dimethylacetamide (DMAC) are poor solvents for PABZ, while dimethyl sulfoxide (DMSO) is a good solvent for PABZ. In this paper, based on this unique site-selective reactivity of PABZ and the solubility of PABZ in different solvent, sequence isomers of PAAs and corresponding PIs with different head/tail bonding sequence were first synthetized and the considerable differences between them in crystalline ability and physical properties were studied. 2. Experimental Section Materials, methods, analysis, and additional experimental data are supplied in the Supporting Information. 3. Results and Discussion Dr. L. Luo, Y. Dai, Y. Yuan, Dr. X. Wang, Prof. X. Liu State Key Laboratory of Polymer Material and Engineering College of Polymer Science and Engineering Sichuan University Chengdu , P. R. China lxy@scu.edu.cn The ORCID identification number(s) for the author(s) of this article can be found under DOI: /marc Model Study on the Site-selective Reactivity of PABZ It is found that DMSO is good solvent for PABZ while NMP is poor solvent for PABZ, as shown in Figure S1 (Supporting Information). Two model reactions were designed to study the site-selective reactivity of PABZ. The phthalic anhydride and PABZ were added into different solvents and the ratio of anhydride and amino groups was 1:2. In model reaction 1, PABZ was completely dissolved in DMSO and then equimolar phthalic (1 of 5)

2 67% H-AA were synthesized in NMP and the proportion of regioselectivity for the head amino groups is 75.6%. The difference in two model reactions is that phthalic anhydride was always in an excess state in NMP, leading to an increase of reaction probability of tail amino groups of PABZ. Thus, compared with the reaction in DMSO, the proportion of regioselectivity for the head amino groups decreased and the content of HT-AA showed a significant increase as the reaction carried out in NMP. Figure 1. The chemical structure of PABZ and charge density of amino groups calculated by Material Studio 4.0. anhydride was added. This reaction was carried out at 15 C for 10 h and reddish brown amide acid solution was obtained, as shown in Figure S2 (Supporting Information). In model reaction 2, PABZ was added into NMP and the solubility of PABZ in NMP is lower than 0.1%wt. The mixtures including undissolved PABZ were milk white and cloudy, as shown in Figure S1a (Supporting Information). When the equimolar phthalic anhydride was added, phthalic anhydride was first dissolved in NMP and then reacted with a tiny bit of dissolved PABZ. The undissolved PABZ gradually disappeared while reacting with phthalic anhydride. This reaction was also carried out at 15 C for 10 h and the milk white and cloudy solution was obtained, as shown in Figure S3a (Supporting Information). It is implied that two phthalic anhydrides were simultaneously reacted with head and tail amino groups of one PABZ in model 2 reaction. Thus, some unreacted diamines were residual. The obtained amide acids in model reaction 1 and 2 were characterized by 1 H-NMR spectra, as shown in Figure 2. The peaks (δ = 7 9 ppm) are attributed to aromatic protons. Three peaks attributable to the amide group are observed at ppm. The high field line at ppm (peak a) and ppm (peak b) would be assigned to head (H) amide groups of H-AA and HT-AA units, as shown in Scheme 1a,b, respectively. These shifts to high field are due to electron donating effect of amino group and benzimidazole unit. The low field line at ppm (peak c) can be reasonably assigned to tail (T) amide group. In Figure 2a, the integral area ratio of peak a, b, and c is about 11:0.9:1. It is indicated that the main compound synthesized in DMSO is H-AA and the proportion of regioselectivity for the head amino groups has been roughly estimated as 92%. In Figure 2b, the integral area ratio of peak a, b, and c is about 2.1:1:1, which indicates that about 33% HT-AA and 3.2. Preparation of Sequence Isomeric PAAs As DMSO was used for solvent, PABZ was first dissolved in DMSO, and then equimolar 3,3,4,4 -biphenyl tetracarboxylic dianhydride (BPDA) was averagely added by 10 times to guarantee the excess molar of PABZ in the early reaction. Thus, the higher reactivity of head amino groups would be preferentially reacted with BPDA, which results in the formation of H H bonding sequence. The obtained PAA is named PAA-DMSO. As the solvent was substituted for NMP, the major of PABZ does not dissolve in NMP, and then equimolar BPDA was added at one time in order to guarantee the excess molar of BPDA in this reaction. Thus, the head and tail amino groups might be reacted with BPDA simultaneously, and the H T sequence isomer would be formed. The 1 H-NMR spectra were used to characterize the chemical microstructures of PAAs, as shown in Figure S5 (Supporting Information). Aromatic protons are observed at 7 9 ppm. The difference between 8.0 and 8.4 ppm in the 1 H NMR spectrum of PAA-NMP and PAA-DMSO can be reasonably attributed to the different sequential structure. [20,21] The peaks between 10.3 and 10.8 ppm arise from protons of amide groups. The protons in benzimidazole units and carboxyl groups were observed at about 12.8 and 13.2 ppm, respectively. Those results of 1 H-NMR indicate PAAs containing benzimidazole units were successfully synthesized. In order to confirm the sequential structure of two PAAs, the 1 H-NMR spectra in the range of ppm are carefully investigated. The high field resonances at ppm and the low field resonances at ppm would be attributed to head and tail amide groups, respectively, as noted above. In theory, four sequence isomeric units would be obtained from the reaction of BPDA and PABZ, and they are HT HT HT, TH HT HT, HT HT TH, and TH HT TH bonding Figure 2. a) 1 H-NMR spectra of amide acid obtained in DMSO and b) amide acid obtained in NMP (2 of 5)

3 Scheme 1. The chemical structure of two possible amide acids a) H-AA and b) HT-AA. sequence, respectively, as shown in Figures S6 and S7 (Supporting Information). For protons of H amide group, there would be two kinds of bonding sequences, which are TH HT and HT HT bonding sequences. Similarly, there are also two kinds of bonding sequences (HT HT and HT TH) for protons in T amide group. The curve-resolved 1 H-NMR spectra were carried out, as shown in Figure 3. Peak I, peak II, peak III, and peak IV can be reasonably attributed to H amide group in TH HT sequence unit, H amide group in HT HT sequence unit, T amide group in HT HT sequence unit, and T amide group in HT TH sequence unit, respectively. On the basis of this assignment, the percentage of the HT HT sequence units (ω) can be roughly calculated by the equation AII + AIII ω (%) = 100% (1) A + A + A + A I II III IV where A I, A II, A III, and A IV are the integral area of peak I, peak II, peak III, and peak IV in 1 H-NMR spectra, respectively. The values of ω of PAA-DMSO and PAA-NMP are 37% and 54%, respectively, which indicates that PAAs synthesized in NMP show higher content of HT HT sequence units. This attributes to the lower proportion of regioselectivity for the head amino groups and less head head units in PAA-NMP The Effect of Sequence Isomers on Macromolecular Packing of PIs The PAAs were thermal imidization at 400 C for 1 h and PI-NMP and PI-DMSO films were obtained. Formation of polyimide films was confirmed by attnuated total reflection- Fourier transform infrared (ATR-FTIR) spectra, as shown in Figure S8 (Supporting Information). The characteristic absorption bands of the imide ring are observed at about 1770, 1708, and 1360 cm 1, respectively. [22,23] The characteristic absorptions of benzimidazole at 1308 cm 1 are shown in PI films [19] and the absorption band at about 840 cm 1 is assigned to C H out of plane bending of phenyl ring. [23,24] Those results indicate that PI-NMP and PI-DMSO films were successfully prepared. [25,26] Moreover, compared to PI-DMSO, the PI-NMP film shows spectral shifts from 841 to 839 cm 1 for C H out of plane bending of phenyl ring and from 1360 to 1364 cm 1 for the stretching of C N C, respectively. It indicates that PI-NMP exhibits more ordered macro molecular packing than PI-DMSO. [13,23,24] The WAXD analyses were used to further investigate the aggregation structure of two PI films, as shown in Figure 4a. Obviously, PI-DMSO film shows an amorphous structure. On the contrary, PI-NMP film exhibits two discernible crystalline peaks at 14.5 o and 23.4 o, respectively, and they are assigned to side to side packing of heterocyclic rings and π π stacking, respectively. [19] Those results imply that high content of HT HT sequence unit is in favour of the formation of crystalline structure. The macromolecular orientation was characterized by polarized ATR-FTIR spectra, as shown in Figure S9 (Supporting Information). Figure 4b shows polar diagrams of absorbance of peaks at about 1490 cm 1 (ν C C, C C stretching vibration of 1,4-C 6 H 4 in backbone) calculated from the results of Figure S9 (Supporting Information) as a function of the angle of polarization of incident polarized light. For the PI molecular backbone, the averaged direction of ν C C transition moments is parallel Figure 3. Curve-resolved 1 H-NMR spectra in the range of ppm for PAA-DMSO and PAA-NMP, respectively. Peak I belongs to the head amide group in the TH HT sequence unit; peak II belongs to the head amide group in the HT HT sequence unit; peak III belongs to the tail amide group in the HT HT sequence unit, and peak IV belongs to the tail amide group in the HT TH sequence unit (3 of 5)

4 Figure 4. a) The WAXD curves of PI-DMSO and PI-NMP films; b) Polar diagrams of absorbance of peaks at different positions as a function of the angle of polarization of incident polarized light, obtained using linearly polarized IR spectroscopy at 1490 cm 1. to the axis of molecular backbone and the polar diagrams of 1490 cm 1 of two PI films show a maximum intensity of absorbance peak at the direction 0 o and 180 o, which indicates those two PIs show a remarkable in-plane orientation. It is found that the band at 890 cm 1 was nondichroic, as shown in Figure S9 (Supporting Information). Thus, the 890 cm 1 band can be used to normalize dichroic specular reflectance data, just as 1410 cm 1 was chose to normalize dichroic data in PET film. [27] The dichroic ratio (R) can be calculated by the equation R = ( A + A )/ A (2) where A0, A90, A180 are the normalized absorbance at the angle of polarization 0, 90, and 180, respectively. The values of R of ν C C are 1.7 and 2.4 for PI-DMSO and PI-NMP, respectively. It can be confirmed that PI-NMP exhibits higher in-plane orientation than PI-DMSO. The complete HT HT bonding sequence PI and TH HT bonding sequence PI with 10 repeat units were investigated by molecular simulation. The results are shown in Figure S10 (Supporting Information). The end-to-end distance of HT HT bonding sequence PI is Å, which is 20.7% higher than that of TH HT bonding sequence PI (171.5 Å). Thus, the HT HT sequence isomer exhibits higher rigidity and linearity, which might lead to the improvement in crystalline ability and in-plane orientation The Effect of Sequence Isomers on Physical Properties of PIs Mechanical properties of PI-DMSO and PI-NMP films are summarized in Table 1. The tensile strength and elasticity modulus of PI-NMP film are 15.9% and 12.6% higher than those of PI-DMSO film, Table 1. Mechanical properties and CTE value of the polyimide films. Polyimides Tensile strength [MPa] Elasticity modulus [MPa] Elongation at break [%] CTE [ppm C 1 ] PI-DMSO 195 ± ± ± PI-NMP 226 ± ± ± respectively. The in-plane thermal expansion coefficient (CTE) of two PI samples were directly measured by the thermomechanical analysis (TMA) instrument, and the values of CTE were averaged calculated in the range of C, as shown in Figure S11 (Supporting Information) and Table 1. The values of in-plane CTE of PI-DMSO and PI-NMP films are 11.0 and 8.2 ppm C 1. The higher mechanical properties and lower CTE of PI-NMP film are due to the crystalline structure and high inplane orientation, [28 30] deriving from high content of HT HT sequence unit. 4. Conclusions Based on the site-selective reactivity of unsymmetrical diamine (PABZ), sequence isomeric PAAs have been successfully synthesized in DMSO (good solvent for PABZ) and NMP (poor solvent for PABZ). Quantitative statistical descriptions of sequence structure have been deduced from the 1 H-NMR measurements. Due to the high regioselectivity for the head amino groups, the content of HT HT sequence unit of PAA-DMSO is only 37%, while PAA-NMP reaches 54% HT HT. The results of molecular simulation further prove that HT HT sequence PI shows higher rigidity and linearity than TH HT sequence PI. Thus, PI-NMP with higher HT HT sequence unit exhibits the more ordered macromolecular packing and higher in-plane orientation, which leads to an increase in mechanical properties and a decrease in CTE. The method presented in this paper opens up a wide range of structure/properties relationship studies on high performance PIs containing sequence isomerism. Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant Nos and ) and State Key Laboratory of Polymer Materials Engineering (Grant (4 of 5)

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