Hexamethylene, and 4,4’-dicyclohexyl diisocyanates and Polytetramethylene glycol were supplied by Tosoh Industry and were purified by distillation or dehydrated before use. Pyromellitic dianhydride, 4,4’-Diphenylmethane diamine, and dibutyltindilaurate were purchased from Tokyo Chemical Industry and were used without further purification. 1,4-Diazabicyclo[2.2.2]octane and N-Methyl-2- pyrrolidone (NMP) were purchased from Nacalai Tesque, Inc. and NMP was purified by distillation.

The PUIEs were synthesized from the isocyanates (HDI and H₁₂MDI), PTMG1000, PMDA, and MDA via liquid polymerization (Scheme 1). Table 1 shows the recipe and imide content for each PUIE. The syntheses were performed as follows: 1) In a 100 mL four-necked separable reaction flask equipped with a mechanical stirrer, a gas inlet tube, and a reflux condenser were added PTMG1000 and DABCO, and the DABCO was dissolved completely at 100˚C. HDI and DBTL were added to the separable flask. The prepolymers were prepared by stirring at 100˚C for 30 min. Then, PMDA and NMP (10 mL) were added to the separable flask and stirred at 150˚C for 15 min. Finally, MDA and NMP (30 mL) were added to the separable flask and were stirred at 150˚C for 4 h; 2) H₁₂MDI, PTMG1000, and DBTL were added to the separable flask. The prepolymers were prepared by stirring at 80˚C for 30 min. Then, PMDA and NMP (10 mL) were added to the separable flask and were stirred at 150˚C for 15 min

Scheme 1. Synthesis of polyurethane-imide elastomers (PUIEs) by the solution method.

^aHDI: Mw = 168.19, H₁₂MDI: Mw = 262.35; ^bPTMG1000: Mw = 1000; ^cMDA: Mw = 200.24; ^dPMDA: Mw = 218.12; ^eDBTL: Mw = 631.56; ^fDABCO: Mw = 112.17; ^eimide content is the theoretical value.

besides MDA and NMP (30 mL) were also added to the separable flask and were stirred at 150˚C for 2 h. The degree of imide content was calculated using the following formula:

Imidecontent ( wt % ) = ( imide a / ( isocyanate + PTMG + MDA + PMDA ) ) × 1 00 (1)

a: weight of isocyanate^b + MDA + PMDA;

b: (isocyanate − PTMG) mol × isocyanate (Mw).

The resultant PUIE solutions were treated at 150˚C for 24 h under the atmosphere using a centrifugal casting machine. The resultant films were heated at 200˚C for 4 h under reduced pressure (267 - 400 Pa) to complete the imide reaction.

To examined the existence of the imide content in the PUIEs, FTIR spectral measurements were carried out. Figure 1 shows Carbonyl C=O group absorptions. Three peaks were observed at around1700, 1720, and 1780 cm⁻¹, which correspond to the hydrogen-bonded carbonyl stretching ν (C=O_bonded), the free carbonyl ν (C=O_free), and the imide carbonyl ν (C=O_imide), respectively. The peak-strength of the imide carbonyl group increased as the imide content increased.

Figure 2 illustrates the XRD patterns of the PUIEs composed of the isocyanates. Broad diffraction peaks that were mainly attributed to the amorphous nature of these PUIEs are observed at around 2θ = 6, 17.5 and 20.

Generally speaking, molecular angles are smaller for smooth surfaces than for the rough surfaces. In order to understand the distinction between the PUIEs with the isocyanates, it was necessary to observe and investigate the surface wettability of the films. The contact angles to water were measured at room temperature and atmospheric pressure on the surfaces of films. Table 2 and Figure 3 show the contact angles of the PUIE films with the isocyanates, HDI and H₁₂MDI. The values of the contact angles for the PUIE films were nearly identical, showing similar hydrophobicities among the films. However, although the values for the contact angles of the PUIE-HDI films were nearly identical as the imide content increased, the values for the contact angles of the PUIE-H₁₂MDI films decreased as the imide content increased, showing that the hydrophobicity of these PUIE films gradually decreases.

The surface topographies of the PUIEs were examined by AFM (Figure 4), and phase-segregated morphologies of the PUIEs with various hard segment

structures were observed. The topographical heterogeneity was studied from the images. In these images, dark and bright regions are appeared as the urethane and imide contents and other content, respectively. Changes were observed in the surface morphology as the imide content increased. Dark-colored spots which are urethane and imide contents were dispersed all over the matrix parts. Inclusions of the urethane and imide contents could be seen in some limited areas. Comparing the two kinds of films, the AFM images of PUIE-HDI and PUIE-H₁₂MDI revealed a rough smooth surface.

The solvent resistances of the PUIEs (PUIE-HDI_15-55 and PUIE-H₁₂MDI_15-55) were tested by immersing each PUIE films in various solvents, including benzene, hexane, acetone, THF, NMP, methanol, DMF, and DMSO. The results are presented in Table 3. All of the PUIEs were resistant to hexane at room temperature. The PUIE-HDIs were resistant to DMSO at room temperature. However, no PUIE-H₁₂MDIs were resistant to NMP at room temperature. Generally speaking, the PUIE films show good solvent resistance. The reason for this is considered to be the uniformity caused by the formation of networked structures between the imide and the urethane.

Swelling tests were then performed using the PUIEs. As evident from Table 4, each of the swelling rates of the PUIE-HDIs decreased as the imide content

○: completely dissolved, △: slightly dissolved, ×: undissolved. ^aMeasurement conditions: benzene, hexane, acetone, THF, NMP, methanol, DMF or DMSO as the solvent at room temperature (23˚C ± 2˚C).

^aMeasurement conditions: shore A type, total thickness = 6 mm, room temperature (23˚C ± 2˚C). ^bMeasurement condition: benzene solvent at room temperature (23˚C ± 2˚C) for 24 h. ^cDifferential scanning calorimetry was performed at heating rate of 10˚C/min from −100˚C to 300˚C under an Ar atmosphere. ^dThermogravimetric analysis was performed at a heating rate of 10˚C/min from 30˚C to 500˚C under an Ar atmosphere.

increased, demonstrating the increase in crosslinking density. However, the swelling rates of the PUIE-H₁₂MDIs were unsteady. Additionally, the swelling rate increased in the following order based on the type of isocyanate: PUIE-H₁₂MDI > PUIE-MDI. This difference is considered to be due to the structure of the isocyanate.

Table 4 shows the hardness. The hardness of the PUIE-HDIs and PUE-H₁₂MDIs are almost same and increased as imide content increased. This is due to the molecular chains of the PUIEs with the isocyanates (HDI and H₁₂MDI), which are similar to the mobility of the molecular chains.

Figure 5 shows stress-strain curves of the PUIEs. Table 5 shows the tensile strength and elongation at the breaking point. Comparing the tensile strength and elongation at the breaking point of the PUIEs, it can be seen that each of the tensile strength values are almost the same and that the values for the elongation at the breaking point follow this order for the isocyanates: PUIE-HDI > PUIE- H₁₂MDI. Also, the tensile strength increased with increasing imide content, and the elongation at the breaking point decreased with increasing imide content. This is caused mainly by the ratio of imide content, and it suggests both that the network chain density in the composite increases with increasing imide content and that cross-linking occurs between the imide contents. It is important to mention that the PUIEs almost lose their elasticity, especially, with high imide content. Therefore, the ratio of urethane content and imide content need to be controlled in synthesis of PUIEs.

^aTensile properties measured at room temperature (23˚C ± 2˚C) with strain speed of 100 mm/min. ^bTensile strength at breaking point. ^cElongation at breaking point.

Table 4 reports the Tg values of the PUIEs. The Tgs of the PUIE-HDIs were nearly identical as the imide content increased. However, PUIE-H₁₂MDIs fell as the imide content increased. The chemical structure and crosslinking density of PUIE influence to the Tgs of PUIEs. The Tgs of PUIEs fell as the imide content or the number of crosslink increased. Moreover, when the steric hindrance caused by the formation of physical and chemical crosslinks between molecular chains, probably the Tgs of PUIEs fell.

Figure 6 shows the viscoelastic behavior of the PUIEs through the storage modulus (E’) and tan δ (tan δ = E”/E’). None of the PUIEs had a rubbery plateau. These results suggest that the PUIEs have much more crosslinking structure, but their molecular chains can still move easily above the Tg. Additionally, the thermal properties of the PUIEs increased with increasing imide content.

Table 4 shows the TGA curves of the PUIEs. All of the PUIEs were thermally stable up to around 300˚C and began to lose weight at a higher temperature. The 5% weight loss temperature (T₅) of the PUIEs changed according to the following isocyanate order: PUIE-HDI > PUIE-H₁₂MDI. The 50% weight loss temperatures (T₅₀) of the PUE-HDI and PUIE-H₁₂MDI were identical. In general, the decomposition of PUIE is believed to take place in two main steps: the urethane linkage first dissociates to the isocyanate and alcohol at 300˚C - 350˚C, followed by imide degradation at temperatures above 400˚C. From these results, it is obvious that the PUIEs with higher imide content are more thermally stable. Also, the weight loss temperatures increased according to the following isocyanate order: PUIE-H₁₂MDI > PUIE-HDI, and the T₅s and T₅₀s of the PUIEs increased as the imide content increased.