Solid-state and time domain NMR to elucidate degradation behavior of thermally aged poly (urea-urethane)

https://doi.org/10.1016/j.polymdegradstab.2019.109052Get rights and content

Highlights

  • Two types of poly (urea-urethane) were thermally aged at 100 °C for up to 10000 h.

  • Their degradation proceeded in urea bonds and urethane bonds, respectively.

  • Spin-spin relaxation time, T2, comprised three components of T2S, T2I and T2L.

  • Molecular chain scission in urea bonds changed the fractions of T2S and T2L, whereas all the T2 fractions were stable during that in urethane bonds.

Abstract

We have demonstrated that solid-state and time-domain Nuclear Magnetic Resonance spectroscopy (NMR) techniques are effective in monitoring the degradation site of poly (urea-urethane). We prepared two types of commercial poly (urea-urethane), Urethane-A and Urethane-B, and aged them at 100 °C for up to 10000 h. We performed tensile strength tests and solid-state NMR experiments, in which 13C and 15N NMR spectra and T2 were collected. The tensile strength of Urethane-A decreased, increased and decreased again, whereas that of Urethane-B decreased at a lower rate with aging. Solid-state NMR experiments clearly showed that the degradation of Urethane-A was caused by molecular chain scission and cross-linking in its urea bonds, and that of Urethane-B occurred owing to molecular chain scission in its urethane bonds. T2 had three components, short T2 (T2S), intermediate T2 (T2I) and long T2 (T2L), whose fractions were xS, xI and xL, respectively. We have demonstrated that molecular chain scission in urea bonds decreases xS and increases xL, and that in urethane bonds increases T2I and T2L without changing all the T2 fractions.

Introduction

To use polymeric materials effectively, it is important to evaluate their degradation behavior and lifetime. Structure analysis techniques, such as Fourier Transform Infrared spectroscopy (FT-IR) and Nuclear Magnetic Resonance spectroscopy (NMR), are widely used to investigate the degradation behavior. However, depending on the types of polymeric materials, it would be challenging to reveal the degradation behavior.

Poly (urea-urethane), which has urethane and urea bonds, is one of those types of polymeric materials. Urea and urethane bonds are synthesized through a polyaddition reaction of polyamine and polyisocyanate and that of polyisocyanate and polyol, such as polyether and polyester, respectively. There are many types of polyisocyanate, polyol and polyamine, and various combinations of them. Degradation behaviors of poly (urea-urethane) vary depending on their compositions and surroundings. Therefore, although the degradation behavior of polyurethane has already been reported by many authors [[1], [2], [3], [4], [5]], it should be evaluated for each composition and surroundings. To reveal the change in chemical structure with degradation, FT-IR and NMR spectra are usually used [[6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]]. However, some peaks are not easily assigned in these spectra. For instance, in FT-IR spectra, the peaks of >Cdouble bondO in urea and urethane bonds are observed between 1600 and 1800 cm−1, and hydrogen bonding makes them shift. In addition, the carboxyl group (-C(=O)-OH), which is generated owing to degradation, also appears in the same range. When a plasticizer having carbonyl carbons is used, their peaks can be overlapped. In 13C NMR spectra, the peaks of >Cdouble bondO of urea and urethane bonds generally overlap each other. Although 15N NMR spectra are useful in elucidating the degradation behavior of urea and urethane bonds, peak assignments are often challenging, and an experiment of 15N NMR takes a much longer time than that of 13C NMR. Moreover, both cross-linking and molecular chain scission can occur in poly (urea-urethane) degradation, and they are not easily distinguished on FT-IR and NMR spectra. As a result, it is challenging to reveal the degradation of poly (urea-urethane), especially when it proceeds in urethane or urea bonds.

Because polyisocyanate, amine and urethane/urea bonds are incompatible with polyol, poly (urea-urethane) has a microphase-separated structure composed of hard segment, soft segment and interfacial domains [19]. The hard segment comprises polyisocyanate, polyamine and urea bonds, while the soft segment consists of polyol. Because urethane bonds connect these segments, they could contribute to the interfacial domain. 1H spin-spin relaxation time (T2), which is easy to measure by using low field NMR techniques, is useful for investigating the microphase-separated structure of poly (urea-urethane), that is, the molecular mobilities of the hard segment, interfacial and soft segment domains and their fractions [[20], [21], [22], [23], [24], [25]]. Therefore, T2 could be an effective parameter to follow the degradation site of poly (urea-urethane) when the degradation proceeds in urea or urethane bonds. We investigated the usefulness of the combination of the solid-state and time-domain NMR techniques for elucidating degradation behavior of poly (urea-urethane). We prepared two types of thermally aged poly (urea-urethane), measured tensile strength and T2, and recorded 13C NMR and 15N NMR spectra. We have shown the degradation behavior of them and the usefulness of the combination of the solid-state and time-domain NMR techniques.

Section snippets

Materials

Two types of commercial poly (urea-urethane), Urethane-A and Urethane-B, were used for this evaluation. Fig. 1 shows their structures. Urethane-A consists of a plasticizer (diisononyl phthalate) and two types of poly (urea-urethane). One was made of 4,4′-methylene diphenyl diisocyanate (MDI), poly (tetra methylene glycol), PTMG, and 4,4′-bis-(sec-butylamino)-diphenylmethane (Urethen-A-1). The other (Urethane-A-2) was composed of MDI, PTMG and diethyl toluene diamine (DETDA). These components

Results

Fig. 2(a) and (b) show the tensile strength and elongation at break of aged Urethane-A and Urethane-B against the aging time, respectively. Those of virgin Urethane-A and Urethane-B are shown in Table 1. The tensile strength of Urethane-A decreased until 800 h (the first degradation stage), increased until 5000 h (the second degradation stage) and decreased again until 10000 h (the final degradation stage). The elongation at break of Urethane-A decreased until 800 h, increased until 4000 h and

Degradation behavior of poly (urea-urethane)

The degradation behaviors of Urethane-A and Urethane-B are obviously different from each other. The tensile strength of Urethane-A decreased in the first degradation stage, increased in the second stage, and decreased again in the final stage. 13C DP/MAS and 15N CP/MAS NMR spectra clearly show that Urethane-bond-1 and PTMG units were unchanged in all the degradation stages. The T2 measurement results show that the fraction of the hard segment domain, which comprises polyisocyanate, polyamine

Conclusions

We have demonstrated the usefulness of the combination of the solid-state and time-domain NMR techniques for revealing degradation behavior of poly (urea-urethane). We prepared two types of commercial poly (urea-urethane), Urethane-A and Urethane-B, and aged them at 100 °C for up to 10000 h. We performed tensile strength tests and solid-state NMR experiments. The tensile strength of Urethane-A decreased, increased and decreased again, while that of Urethane-B decreased at a lower rate. The 13C

CRediT author statement

Kaori Numata:Conceptualization, Methodology, Writing- Original draft preparation, Investigation.

Atsushi Asano:Reviewing and Editing, Supervision.

Yasumoto Nakazawa:Reviewing and Editing, Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

All the NMR spectra were obtained on a JEOL ECA II spectrometer owned by the Institute of Physical and Chemical Research (RIKEN). Authors are grateful to Dr. Fumiaki Hayashi of RIKEN for the valuable opinions and Mr. Hitoki Matsuno of Tokyo Gas Co.,ltd for the EGA and GC/MS analysis.

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