Thermoplasticity in Polyurethane-Imide Elastomers

Polyurethane-imide elastomers (PUIEs) are formed from isocyanates, polyols, diamines, and acid anhydrides through liquid polymerization. However, thermoplastic PUIEs have rarely been reported because the synthesis of thermoplastic PUIEs remains a challenge in polymer chemistry. In this study, PUIEs were prepared from 4,4'-diphenylmethane diisocyanate, polyols (polytetramethylene glycol (Mw = 1000), polycaprolactone diol (Mw = 1000), and polycarbonate diol (Mw = 1000)), 4,4'-oxydianiline, and acid anhydrides (pyromellitic dianhydride and 4,4'-oxydiphtaric anhydride). The thermoplasticities of the resultant PUIEs were investigated, and only PUIEs synthesized using 4,4'-oxydiphtaric anhydride expressed thermoplasticity.


Introduction
Polyurethane (PU) has been widely used in a variety of applications in many different fields. In general, PU suffers from heat resistance. However, by improving this fundamental property, PU may gain some industrial advantages. Therefore, chemists have actively pursued PU heat resistance. Polyurethane-imide (PUI), which has stability in high-temperature environments, excellent electrical and mechanical properties, and good chemical resistance, can be prepared via an organic-organic hybrid between urethanes and imides [1]- [18]. PUI possesses some distinct properties such as heat resistance and solvent resistance. Therefore, PUI has found special applications in selected industries and continues to gain importance in a variety of applications. PUI is used in a surprising number of commercial applications. For convenience, PUI is classi-Open Journal of Polymer Chemistry fied into three major types of products: foams [19] [20] [21] [22], elastomers [23]- [30], and resins [5] [31] [32] [33]. However, a particularly interesting material is polyurethane-imide elastomer (PUIE). We focused on PUIE because it is a novel heat-resistant elastomer that is widely used for commercial products, and we were successful in improving the synthetic method for creating PUIEs [4] [9] [15]. Moreover, we focused on thermoplastic PUIEs (TPUIEs) that could be utilized as industrial materials. Some of the most important TPUIE attributes include their ability to be reprocessed, the wide range of colors obtainable, and their worldwide availability. Although TPUIEs are industrially utilized, they are often reported on in the field of PUIEs. From both the theoretical and practical standpoints, it is important to synthesize TPUIEs that maintain their thermoplasticity. This becomes a key challenge in the field of polymer chemistry, and it is important that an expression of PUIE thermoplasticity is clarified.

Synthesis
Scheme 1 shows our synthetic method for preparing the PUIEs. Table 1 summarizes the composition of the PUIE composites. All the PUIEs were prepared by liquid polymerization. PUIEs containing 35% polyimide were synthesized. MDI and polyols (PTMG1000, PCL1000, and PCD1000) were placed in a 100 mL four-necked separable reaction flask equipped with a mechanical stirrer, a gas inlet tube, and a reflux condenser, and the mixture was stirred at 80˚C for 30 min. To this reaction mixture was added PMDA and NMP (15 mL), and the mixed solution was stirred at 150˚C for 15 min. ODA in NMP (10 mL) was added, and the mixture was then stirred at 150˚C for another 2 h. The reaction was carried out under an Ar atmosphere. A test film was next prepared from the synthesized PUIEs by casting half of the obtained polymer solution using a centrifugal casting machine at 150˚C overnight, after which the film was treated at

Mechanical Properties
Tensile tests were performed on an OrientecRTC-1225A (Tokyo, Japan) with a model-U-4300 using a JIS 3-dumbbell as the standard sample and a crosshead speed of 100 mm/min.

Molecular Calculations (Gaussian)
Molecular calculations were carried out using the Gaussian09W program at the B3LYP/6-31G(d) level. The resulting molecular models were visualized using Gauss View 5.0.

Expression of Thermoplasticity
The definition of a thermoplastic elastomer in JIS appears to be the following: "A polymer, or blended polymer, showing a vulcanized rubber-like nature at the operating temperature that can be processed and reprocessed to recover the polymer, or blended polymer, at high temperature." Currently, there is no defined test method for evaluating thermoplasticity. Therefore, we defined thermoplasticity as follows: "An elastomer showing elasticity at the operating temperature that can be reprocessed at higher temperatures. Moreover, the nature of the reformed elastomer must demonstrate the same properties as the original material." Synthesis of the PUIEs was carried out using various raw materials and the PUIE films fused during the process of imidization. A heating test was carried out on the PUIE films ( Figure 1). The PUIE films that contained ODPA became fused, whereas the PMDA-containing PUIEs burned and carbonized. This suggests that thermoplasticity only appears in the PUIEs that contained ODPA as the acid anhydride. To confirm the thermoplasticity of PUIEs synthesized with ODPA, a press molding was carried out. Then, tests for the chemical and physical properties (swelling test, tensile test, DSC, DMA, and TGA) of the PUIEs before and after the molding were carried out.
The swelling rate and T g values of PUIEs both before and after the molding were nearly identical ( Table 2). Figures 2-4 show the stress-strain curves, tanδ, and T 5 and 50 , respectively. The stress-strain curves, tanδ, and T 5 and 50 of the PUIEs both before and after the molding were also identical. Table 3 shows that the PUIEs expressed thermoplasticity. It has been suggested that the structure of ODPA in the PUIEs is related to the thermoplasticity. Therefore, to clarify correlation between the thermoplasticity and the structure of the acid anhydride raw material, the molecular orbitals of the PUIEs were calculated using the Gaussian program. ODPA, PMDA, ODPA-containing PUIE, and PMDA-containing PUIE were used as the model chemical compounds. Figure 5 and Figure 6 show the calculated results. The flexibility of the PMDA molecule, which has an aromatic ring and an imide group in the same plane, is relatively low and the molecule is fairly restrained. In contrast, the flexibility of the ODPA molecule, which has an oxygen atom between the two T. Ueda, S.-I. Inoue Open Journal of Polymer Chemistry   aromatic rings, is relatively high and the molecule is not restrained ( Figure 5).
Moreover, the flexibility of the PUIE is restrained by the π-π interactions between the diamine (ODA) and the aromatic ring. The expression of thermoplasticity for the ODPA-containing PUIE is thought to be related to the flexibility of the PUIE molecule, and this flexibility results from the molecular structure of ODPA ( Figure 5). This concept of thermoplasticity is supported by the swelling tests and the DMA results as well.
PMDA ODPA Open Journal of Polymer Chemistry

Conclusion
The expression of PUIE thermoplasticity for the various PUIEs synthesized by solution-phase polymerization was investigated. The thermoplasticity of ODPA-containing PUIEs was identified by the heating tests, swelling tests, tensile tests, DSC, DMA, and TGA of the PUIEs both before and after a molding treatment. We found that using ODPA as the acid anhydride greatly increased the thermoplasticity of the synthesized PUIE. The knowledge obtained in this study will be useful for designing TPUIEs for various applications.