Investigation of Deformation Behavior of Thiourethane Elastomers

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Investigation of Deformation Behavior of Thiourethane Elastomers Using In Situ X‑ray Scattering, Diffraction, and Absorption Methods Rahmawati Rahmawati,† Shiori Masuda,† Chao-Hung Cheng,† Chigusa Nagano,† Shuhei Nozaki,† Kazutaka Kamitani,‡ Ken Kojio,*,†,‡,§ Atsushi Takahara,†,‡,§ Naoki Shinohara,∥ Kazuki Mita,⊥ Kiminori Uchida,⊥ and Satoshi Yamasaki#

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Graduate School of Engineering, ‡Institute for Materials Chemistry and Engineering, and §WPI-I2CNER, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan ∥ Synthetic Chemicals Laboratory and ⊥Process Technology Laboratory, R&D Center, Mitsui Chemicals, Inc., 580-32, Nagaura, Sodegaura, Chiba 299-0265, Japan # Coatings & Engineering Materials Division, Food & Package Business Sector, Mitsui Chemicals, Incorporation, Shiodome City Center, 1-5-2 Higashi-shimbashi, Minato-ku, Tokyo 105-7122, Japan S Supporting Information *

ABSTRACT: The deformation behavior of polythiourethane (PTU) elastomers was investigated using in situ small-angle X-ray scattering (SAXS), wide-angle X-ray diffraction (WAXD), and X-ray absorption fine structure (XAFS) methods. Two PTUs were prepared from poly(oxytetramethylene) glycol, 1,4-bis(isocyanatomethyl) cyclohexane, and 1,4-butanedithiol (PTU-B) or 1,5-pentanedithiol (PTU-P). The effect of methylene length of the chain extender on molecular aggregation structure of PTU during the elongation process was evaluated. SAXS measurement revealed that the spacing of hard segment domains of PTUs increased and decreased in the directions parallel and perpendicular to the elongation direction and showed a constant value of strain above 2. The strain calculated from the spacing of the hard segment domains for PTU-B was larger than that for PTU-P, suggesting that well-developed hard segment domains were formed for PTU-B. WAXD measurement showed that strain-induced crystallization of the soft segment occurred at around the strain of 2. XAFS measurement showed that at the strain of 2 or 3, atoms in the vicinity of sulfur became more ordered, which is confirmed by the decrement of the extended XAFS Debye−Waller factor. It seems reasonable from these SAXS, WAXD, and XAFS results that the hard segment domains orientation occurred for both PTUs during the deformation process, followed by strain-induced crystallization of the soft segment. In addition, PTU-B exhibits more ordered hard segment domains that maintain their aggregation structure upon uniaxial deformation in comparison with PTU-P.

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Furthermore, Cooper and co-workers11,12 studied the orientation of hard and soft segments in polyether and polyester urethane under uniaxial deformation using infrared dichroism. They found that the orientation of hard segments is affected mainly by the hard segment length, while soft segment orientation is less affected by polymer composition. Moreover, the hard segment orientation is also influenced by the type of ordering inside the hard segment domains. In addition, the hard segment domains have a tendency to retain a partially oriented conformation after the stress is removed, whereas the soft segments return to an unoriented state. Furthermore, Lee et al.13 reported the uniaxial deformation of PU elastomers using Fourier-transform infrared (FT-IR) and synchrotron SAXS methods and found that the deformation behavior of PU elastomers depends on the orientation of the hard segment

INTRODUCTION Polyurethane (PU) is one of the most well-known polymers which has been extensively explored and used in a wide range of applications, including, among others, household appliances, electronic devices, medical implants, and automotive parts. The vast majority of the existing literature on PUs has mainly focused on studying the structure−property relationship in order to obtain more superior materials.1−10 Such studies involve a comprehensive analysis of structural change of hard and soft segments of PU during deformation by relying on small-angle X-ray scattering (SAXS), wide-angle X-ray diffraction (WAXD), and infrared dichroism method. For instance, Bonart et al.1,2 investigated the deformation behavior in PUs with polyether- and polyester-based soft segment using SAXS and WAXD. They found that the deformation behavior of the hard segment domains in PUs is highly influenced by the nature of the hard segment, exhibiting orientation of hard segments along the elongation direction, which is likely due to the local torque force from the oriented soft segment. © XXXX American Chemical Society

Received: May 12, 2019 Revised: July 16, 2019

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Figure 1. Chemical structures of PTU-B and PTU-P.

using the sulfur K-edge EXAFS. In addition, the effect of the methylene length of chain extender on the deformation behavior of PTUs will be evaluated.

domains in the initial state. The hard segment domains with a parallel orientation to the elongation direction underwent increasing spacing of hard segment domains at low strain, whereas the ones with perpendicular orientation showed decreasing propensity. The abovementioned literature on deformation behavior of PUs which relies mainly on SAXS, WAXD, and FT-IR focuses on identifying the changes in the orientation and spacing of hard segment domains, strain-induced crystallization of the soft segment, and orientation of functional groups. However, such studies provide no structural information of the short-range order in the vicinity of the absorbing atom. Therefore, the extended X-ray absorption fine structure (EXAFS) method, which enables us to obtain a set of structural parameters in the range below 0.5 nm, is required as a complement for SAXS, WAXD, and FT-IR analyses. EXAFS is a method which has been extensively utilized to investigate the molecular structure of the material, such as determining the vicinity of the cation in ionomers14−16 and molecular structure behavior under uniaxial deformation.4 The ability of EXAFS spectra to observe the changes in the short-range order make it suitable for investigating the degree of order at this level, which is essential for studying the deformation behavior of polymers. Despite its potential contribution, only a few studies have utilized EXAFS for deformation behavior analysis. For instance, Grady and Cooper17 investigated the uniaxial deformation in sulfonated polystyrene using EXAFS and found different patterns of bond vectors alignment and degree of order during uniaxial deformation. However, for the case of PUs, although it has not been used for studying the deformation behavior, the EXAFS method has been used for studying the changes in the degree of order due to variations in neutralization level in PU ionomers.18 Polythiourethanes (PTUs) are sulfur analogues of PUs, which are produced from the reaction between polyol, diisocyanate, and dithiol chain extender. Like PUs, PTUs comprise soft and hard segments, where the hard segments act as physical crosslinks between the soft segments. Because of their thermodynamic incompatibility, the hard and soft segments tend to form a microphase-separated structure in the nanometer scale which strongly affects the mechanical properties of PTUs.19−22 Despite their potential elastomeric properties, the deformation-induced structural changes in the PTUs have not been comprehensively investigated. This article aims to investigate the deformation behavior of PTUs related to the microphase-separated structure. The microphaseseparated structure of PTU elastomers during uniaxial elongation will be studied using in situ SAXS and WAXD measurements, whereas the degree of order will be investigated

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EXPERIMENTAL SECTION

Materials. The chemical structures of PTU elastomers are shown in Figure 1. The polymer glycol and diisocyanate used in this study were poly(oxytetramethylene)glycol (PTMG: Mn = 1800, Asahi Kasei Chemicals Co., Ltd., Japan) and 1,4-bis(isocyanatomethyl)cyclohexane (FORTIMO 1,4-H6XDI, Mitsui Chemicals Inc.), respectively. Whereas, the chain extenders used were 1,4-butanedithiol (BDT) and 1,5-pentanedithiol (PDT) (FUJIFILM Wako Pure Chemicals Co., Ltd., Japan) with a weight ratio of hard segment approximately 16%. The diisocyanate was used as received, whereas chain extenders were purified by distillation. The number average molecular weights (Mn), weight average molecular weight (Mw), and polydispersity index (Mw/Mn) for PTU-B (PTU with BDT) are 53 000, 171 000 g mol−1, and 3.2, respectively, whereas for PTU-P (PTU with PDT), Mn, Mw, and Mw/Mn are 56 000, 210 000 g mol−1, and 3.8, respectively. The gel permeation chromatography curves for both PTUs are given in the Supporting Information (Figure S1). Sample Preparation. The segmented PTUs were prepared by a two-step method in bulk, as described in the previous work.22 PTMG was dried under reduced pressure with dry nitrogen in a four-neck flask and then 1,4-H6XDI was added to the flask with the ratio of K = [NCO]iso/[OH]PTMO = 1.40 and reacted under N2 atmosphere for 3 h at 80 °C. Dibutyltin dilaurate (FUJIFILM Wako Pure Chemical Co., Ltd. Japan) was used as a catalyst. The prepolymer reaction was monitored by using an amine equivalent method and stopped when the NCO group reaction ratio exceeds 90%. Moreover, the prepolymer was degassed in a vacuum, followed by the addition of the chain extender. The relative amount of the chain extender to NCO groups in the prepolymer was [NCO]pre/[SH] = 1.02. After agitating, the product was molded and heated at 110 °C for 24 h to perform polymerization. Characterization. In situ SAXS/WAXD measurements were carried out at the SPring-8 facility, frontier softmaterial beamline (BL03XU), Japan.23,24 The size and wavelength of the X-ray beam were 150 μm × 150 μm and 0.1 nm, respectively. The samples were elongated at a certain rate and scattering patterns were measured with an exposure time of 200−500 ms. The scattering in the small-angle region was obtained using a PILATUS 1M detector (DECTRIS, Ltd., the pixel size of 172 μm × 172 μm) with approximately 4 m camera length. Whereas a flat panel detector was operated for the wide-angle region, at 74 mm camera length. Data processing was performed using the FIT-2D (Andy Hammersley/ESRF, France). The stress−strain curves for PTUs were obtained by a tensile testing machine at room temperature. The dimension of the samples was 100 mm × 5 mm × 2 mm. The initial length and elongation rate were 25 mm and 0.82 mm s−1, respectively. Engineering strain and stress were used in this study. The engineering strain and stress were calculated by elongation divided by initial length and force divided by initial cross-section. True strain and stress are also plotted in the Supporting Information (Figure S2). True strain and stress were B

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mean free path of photoelectrons. σj2 corresponds to the mean-square relative displacement, which, also known as the EXAFS Debye− Waller factor, is the variation in the distance of atoms from the absorbing atom

calculated force divided by cross-sectional area at a certain time by elongation divided by length at a certain time. Figure 2 shows the schematic setup for X-ray absorption fine structure (XAFS) measurements during the elongation process of

σ 2 = ⟨(r − r ̅ )2 ⟩

Figure 2. Schematic setup for XAFS measurements during the elongation process of PTU.

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PTU. XAFS measurements were carried out at Saga Synchrotron Light Research Center, Kyushu University Beamline (BL06), Japan with the storage ring operating at the energy of 1.4 GeV. The energy range of this light source (bending magnet) is 2.1−23 keV. A silicon (111) double-crystal monochromator was used to obtain the incident X-ray beam. The typical photon flux is 1010 photons/s. The intensities of the incident beam and the transmitted beam were monitored using ionization chamber detector filled with He gas. The XAFS spectra of the PTUs was measured with a transmission mode at room temperature using sulfur K-edge, and the absorption peak was calibrated using the sodium thiosulfate standard, with the lowest energy of 2469 eV and the maximum of the first pre-edge at 2472 eV. It took ca. 5 min to sweep energy and measurements were conducted at a certain strain. The dimension of the sample was 30 mm × 5 mm × 0.1 mm. In this study, the EXAFS χ(k) spectrum is the superposition of sinusoidal wave functions (eq 1) of the sulfur-neighboring atoms in the PTU hard segments. This spectrum is Fourier-transformed from wavenumber (nm−1) to radial distance (nm) in order to filter the frequency.25,26 The FT magnitude, |χ(R)|, is the most common way to present the data, hiding the oscillation in the complex χ(R), as the FT makes χ(R) a complex function, with real and imaginary parts. The FT magnitude is related to the radial distribution function, which accounts the contribution of multiple-scattering paths, k-weighting, disorder, dependence of scattering amplitude on k, an infinite mean free path, and so on. The positions of the peaks are related to the distance between the absorber and the neighboring atoms, whereas the magnitude or size of the peaks is related to the numbers and types of the neighboring atoms. χ (k) = =

RESULTS AND DISCUSSION The stress−strain curves for PTU elastomers during uniaxial elongation is provided in Figure 3 and the true stress-true

Figure 3. Stress−strain curves for PTU elastomers during uniaxial elongation measured at 25 °C.

strain curves for PTUs during uniaxial elongation are provided in Figure S2. The Young’s modulus and tensile strength for PTU chain extended with butanedithiol (PTU-B) were greater than those for PTU chain extended with pentanedithiol (PTUP) due to the stronger degree of phase separation and higher hard segment chains ordering. Whereas the elongation at break for PTU-P was larger than that for PTU-B, indicating that the packing of the soft segment chains in PTU-P was more difficult.22 The hard segment domains of PTU-B are spherical in shape with the spacing between hard segment domains and the degree of phase separation is 19.1 nm and 0.35, respectively. The degree of phase separation for PTU-B is higher than that for PTU-P, which possesses the overall degree of phase separation of 0.26. In addition, the spacing between hard segment domains for PTU-P is 24.5 nm. This spacing between hard segment domains was determined from threedimensional correlation function (γ3(r)) analysis.30 Figure 4a shows the 2D-SAXS patterns for PTU-B during uniaxial elongation. Figure 4b exhibits the 1D meridional and equatorial SAXS profiles, and Figure 4c depicts the azimuthal intensity profiles of 2D-SAXS patterns at q = 0.26−0.33 nm−1 at various strains for PTU-B. The elongation direction corresponds to the meridional direction in the 2D-SAXS patterns. At the initial state, one can see the ring-shaped 2D-

μ(E) − μ0 (E) μ0 (E)

∑ S02(k)Nj j

Fj(k) krj 2

2 2

e−2σj k e−2rj / λj sin[2krj + δj(k)] (1)

μ(E) is the measured absorption coefficient and μ0(E) is the background corresponding to absorption of an isolated atom. k is the wave vector, which obtained from k=

2m (E − E0) ℏ2

(3)

An individual path will have contributions from scattering pairs that are closer or farther than the average due to the static and/or thermal disorder. The EXAFS Debye−Waller factor is, therefore, a measure of both kinds of disorders.26−28 The EXAFS curves were analyzed to obtain the structural parameters, that is, the distance to neighboring atoms, the number of neighboring atoms, EXAFS Debye−Waller factor, edge energy, and amplitude reduction factor. All parameters left free for the initial state with the coordination number followed the crystal structure which is obtained from powder WAXD. Data processing, including background removal, edge calibration, data normalization, Fouriertransforms, and statistical analysis, was performed using ATHENA and ARTEMIS.29 The model structures used in the calculations were obtained from the crystal structure solutions of powder WAXD pattern of the hard segment models.22

(2)

where E0 is the edge energy, m is the electron mass, and ℏ is the Planck’s constant divided by 2π. Nj is the number of neighboring atoms of type j in the jth shell, S02(k) is the amplitude reduction factor due to excitations of electrons other than 1s, Fj(k) is the backscattering amplitude, and δj(k) is the phase shift of the photoelectron from the Nj atom. rj is the root mean square distance between the absorbing atom (sulfur) and neighboring atom, λj is the C

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Figure 4. (a) 2D-SAXS patterns, (b) 1D meridional and equatorial SAXS profiles, and (c) azimuthal profiles of 2D-SAXS pattern at q = 0.26−0.33 nm−1 for PTU-B at various strains.

Figure 5. (a) 2D-SAXS patterns, (b) 1D meridional and equatorial SAXS profiles, and (c) azimuthal profiles of 2D-SAXS pattern at q = 0.26−0.33 nm−1 for PTU-P at various strains.

hard segment domains fragmentation (see Figure 4b).13 The strain dependence of the orientation function for PTU elastomers is provided in the Supporting Information, Figure S3. The hard segment domains and crystals of PTU-B show random orientation at the initial state, which changes to an orientation parallel to the elongation direction with increasing strain. Figure 5a shows the 2D-SAXS patterns for PTU-P during uniaxial elongation. Figure 5b exhibits the 1D meridional and equatorial SAXS profiles, and Figure 5c exhibits the azimuthal intensity profiles of 2D-SAXS patterns at q = 0.26−0.33 nm−1 at various strains for PTU-P. At the initial state, one can see the ring-shaped 2D-SAXS pattern, suggesting the random orientation of the hard segment domains. This pattern becomes elliptical with increasing strain, as presented in Figure 5a. At strain 1.0, a four-point pattern was observed at approximately 50° from the elongation direction (Figure 5b,c) and continued with increasing strain, indicating the tilted hard segment domains and crystals from the elongation direction. As strain increased, the four-point pattern tilted to 60° from the elongation direction, as shown in Figure 5c. When the strain increased further, the equatorial streak occurred at strain 2.0, with its intensity increased with increasing strain. Similar to PTU-B, it is also seen that the scattering intensity decreased with increasing strain (Figure 5b), suggesting the fragmenta-

SAXS pattern, suggesting the random orientation of the hard segment domains, with long-range correlation as a result of the microphase separation. This pattern becomes elliptical at the initial stage of elongation, indicating that the spacing between hard segment domains increased and decreased in the directions parallel and perpendicular to the elongation direction, respectively. At strain 0.3, a four-point scattering was observed at approximately 45° from the elongation direction (Figure 4b,c), which is attributable to the tilted hard segment domains and crystals along the elongation direction. This four-point pattern caused by shearing mechanisms due to the local stress concentration was induced by soft segment chains alignment. As strain increased, the fourpoint pattern tilted to 60° from the elongation direction, as shown in Figure 4c. At this stage, the hard segment domains and crystals inclination progressed and the periodic correlation of the hard segment domains was shifted.31,32 The hard segment domains and crystals inclination continued until the soft segment was extremely elongated. Furthermore, the equatorial streak occurred at strain 2.5, which is observed as a sharp peak at 90° and 270° in Figure 4c, suggesting a random correlation of scattering objects aligned in the direction of elongation, which is an indication of nanofibril formation.33,34 The decrease in the scattering intensity with increasing strain suggests decrease in population of scattering objects due to the D

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respectively. Whereas Δd is the change of spacing of hard segment domains, equal to ds − d. Both PTU elastomers show increasing and decreasing trend in the Δd/d with strain along meridional and equatorial directions, respectively. PTU-B exhibits an increment of the Δd/d along the meridional direction, which remains constant afterward. It is possible that the hard segment domains maintained their aggregation structure due to the well-developed hard segment crystallites in PTU-B. On the other hand, PTU-P exhibits a decreasing trend in the Δd/d along the meridional direction after reaching its peak at strain 1.5, suggesting significant fragmentation of hard segment domains due to the less ordered hard segment crystalline.22 In addition, the equatorial Δd/d decreased gradually with increasing strain for both PTU elastomers. This is because the hard segment domains tend to come closer toward each other, as a result of the hard segment domain destruction.36 In the WAXD patterns of PTU-B, the diffraction of hard segment domains was observed at the initial state, namely at q = 12.5, 14.2, and 16.4 nm−1. They can be assigned to (10−2), (020), and (11−1) in the triclinic cell, respectively. Moreover, the strain-induced crystallization of soft segment chains occurred at around elongation 1.5. Two peaks assigned to crystallized PTMO chains were observed at q = 14.2 and 16.8 nm−1, which correspond with (020) and (110) in the monoclinic cell, respectively.37 During uniaxial elongation, the diffraction peak of the (10−2) plane of hard segment domains in the PTU-B at q = 12.5 nm−1 became weaker at strain above 3.0. In contrast, the (020) and (11−1) lattice diffractions of PTU-B hard segment domains at q = 14.2 and 16.4 nm−1, respectively, were overlapped with the crystallized PTMO chains, thus they could not be observed. The change in the (10−2) interplanar spacing suggests the fragmentation of

tion of the hard segment domains. The shift of the scattering peak to higher q region after a gradual shift to lower q region can be explained as follows. Spacing of hard segment domains are observed, with the most frequent type possessing longer spacing with increasing strain up to strain 1.5. The peak shifted to a higher q region with increasing strain, indicating that the hard segment domains with longer spacing undergo disruption.35 Furthermore, PTU-P shows a gradual increase of the orientation of hard segment domains in the direction parallel to the elongation direction, with a slightly lower magnitude compared to that in PTU-B (Figure S3). Figure 6 shows the relation between film strain and strain obtained from spacing of hard segment domains of PTUs, Δd/

Figure 6. Strain obtained from spacing of hard segment domains, Δd/ d for PTU elastomers obtained from 1D-SAXS profiles at various strains, calculated using three-dimensional correlations functions.

d during uniaxial elongation. d and ds is the spacing of hard segment domains at the initial state and at a certain strain,

Figure 7. 2D-WAXD patterns and 1D-WAXD profiles for (a) PTU-B and (b) PTU-P at various strains. E

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Figure 8. Sulfur K-edge (a) XANES and (b) EXAFS spectra (open symbol) and best fit model (dotted line) for PTU-B at various strains, measured at 25 °C.

shell, respectively.41,43 As the strain increased to 3.0, a slight shift to higher R region and the gradual increase in the first peak magnitude were observed. This is because the uniaxial elongation induces rearrangement in the vicinity of sulfur, thus decreasing the disorder within the first-coordination shell. In other words, the uniaxial elongation increases the degree of ordered structure in the vicinity of sulfur. As it is difficult to distinguish the modification of the S−C bond length, the distribution of bond length is preferred to express as a staticdisorder contribution to the EXAFS Debye−Waller factor. The EXAFS fitting results show a decrease in the EXAFS Debye− Waller factor with elongation until strain 3.0. Furthermore, at strain 4.0, the EXAFS Debye−Waller factor increased, indicating that the sulfur-neighboring atoms can no longer maintain their ordered structure. For the case of the secondcoordination shell of sulfur, there are three single scatterings involved, namely sulfur to oxygen carbonyl (S−O, r2), sulfur to nitrogen (S−N, r3), and sulfur to second-nearest carbon of methylene (S−C2, r4). The uniaxial elongation not only affects the peak position and magnitude but also the shape of the second-coordination shell. Although the number of scatterers that contributes to this peak is the same, the peak area of this second curve increased with increasing strain. The change in the peak position, magnitude, and shape are due to the change in the distance and angle between sulfur and scattering atoms, for which the distribution is not exactly in phase. Besides, the multiple scattering might give an additional contribution. It is to be mentioned that the FT is not the same with a radial distribution function because the number of scatterers would not be directly reflected by the area under the curve due to the presence of the scattering factor, Fj(k), exponential damping 2 2 factor (e−2σj k and e−2rj/λj), and phase shift [δj(k)] of about −0.05 nm.27,44,45 The increase in the magnitude of the secondcoordination shell with increasing strain is likely to occur due to the formation of a more ordered structure. However, the peak becomes poorly defined because of the magnitude attenuation. The EXAFS Debye−Waller factor tends to decrease until strain 3.0. It is, therefore, reasonable to conclude that uniaxial elongation increases the degree of ordered structure in the vicinity of the sulfur atom until strain 3.0. The third-coordination shell of sulfur is contributed from sulfur to sulfur (S−S, r5) single scattering, other single scattering, and multiple scattering with lower contributions. However, the damping function at high r attenuates the

hard segment domains. This result corresponds well with the EXAFS data mentioned in the later section. The S−N1 (r3) distance shows remarkable change at strain 3.0 and 4.0. On the other hand, at the initial state, PTU-P showed an amorphous halo and shoulder at q = 14.2 and 13.1 nm−1, respectively, as shown in Figure 7b. This indicates the less ordered crystalline hard segment chains compared to PTU-B. The strain-induced crystallization of the soft segment for PTUP occurred at strain 2.0, with a lower intensity of diffraction peak than that for PTU-B. This is due possibly to the lower alignment capability of the soft segment chains due to weak aggregation force of hard segment chains in PTU-P.24,38,39 After elongation, SAXS patterns returned to the initial state without intensity and WAXD patterns totally recover to the initial state. Therefore, it is likely that the hard segment domains had a small change in structure and the soft segment returned to the unoriented state after elongation. To probe the atomic arrangement in the hard segment of PTUs, the EXAFS measurements were conducted at various strains. Figure 8a shows the normalized sulfur K-edge X-ray absorption near-edge structure (XANES) spectra of PTU-B. The intense absorption peaks, called white lines, are shown in the sulfur K-edge XANES spectra of PTU-B at 2472 eV. These peaks correspond to the transition from S 1s to σ*(S−C) with contribution from π* character of the thiourethane carbonyl group.40 An increase in the intensity of the white line and a slight shift of the absorption threshold (edge region) to lower energy with increasing strain were observed until strain 3.0. This can be associated with a rearrangement in the vicinity of sulfur and change in the S−C bond length, respectively,41,42 with the most ordered structural arrangement of sulfur neighborhood achieved at strain 3.0. Figure 8b shows the FT magnitude of experimental sulfur Kedge EXAFS spectra and the best fit model for PTU-B at various strains. The sulfur K-edge EXAFS spectra and individual scattering contribution for PTU-B can be seen in Figure S5a−e. To estimate the type of atoms that contribute to the EXAFS spectra and determine the reasonable values of the structural parameters, the hard segment crystal structure of 1,4H6XDI−1,4-BDT (Figure S6a) was utilized. The obtained structural parameters for PTU-B are shown in Table S1. The main peak of the PTU-B EXAFS spectra reflects the sulfur firstcoordination shell with the position and magnitude related to the S−C bond length and disorder within this coordination F

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Figure 9. Sulfur K-edge (a) XANES and (b) EXAFS spectra (open symbol) and best fit model (dotted line) for PTU-P at various strains, measured at 25 °C.

This signifies that PTU-P possesses a less ordered structure in the sulfur nearest neighbor in comparison with PTU-B. Moreover, the second-coordination shell of sulfur is contributed from single scattering of sulfur to oxygen carbonyl (S−O, r2), sulfur to nitrogen (S−N, r3), and sulfur to secondnearest carbon of methylene (S−C2, r4). The most obvious feature is the increase in peak magnitude and peak splitting at strain 3.0. This is because the rearrangement of the sulfurneighboring atoms leads to the more ordered structure of sulfur second-coordination shell, which is confirmed by the decrease in the Debye−Waller factor and increase in the high R distribution of sulfur and scattering atoms. Furthermore, at strain 4.0, the sulfur-neighboring atoms become less ordered, which is confirmed by the increase in the Debye−Waller factor. This is possibly due to the fragmentation of the hard segment domains. The third-coordination shell of sulfur is contributed from sulfur to sulfur (S−S, r5) single scattering, other single scattering, and multiple scattering with lower contributions. The schematic illustration as a proposed model for the molecular aggregation structure of PTUs during uniaxial elongation is provided in Figure 11. PTU-B exhibited random orientation of the hard segment domains at the initial state, with a rather shorter spacing of hard segment domains compared to PTU-P. As the strain increased to 2.0, the hard segment domains of PTU-B showed parallel orientation to the elongation direction. The spacing between the hard segment domains in the direction parallel to the elongation direction increased and remains constant after strain 2.0, whereas the spacing between the hard segment domains in the direction perpendicular to the elongation direction gradually decreased. On the other hand, PTU-P showed random orientation of the hard segment domains at the initial state, which changes slightly with increasing strain up to 1.5. The spacing between the hard segment domains in the direction parallel to the elongation direction slightly increased and then decreased after reaching its peak at strain 1.5, whereas the spacing between the hard segment domains in the direction perpendicular to the elongation direction gradually decreased. At strain above 2.0, the orientation of the hard segment domains of PTUs in the direction parallel to the elongation direction increased, with higher magnitude observed in PTU-B. The strain-induced crystallization of soft segment chains occurred at elongation of approximately 2.0. And finally, fragmentation of the hard

amplitude exponentially, resulting in a very low magnitude of the FT. Figure 9a shows the normalized sulfur K-edge XANES spectra of PTU-P. The white lines are observed at 2472 eV, indicating the transition from S 1s to σ*(S−C) with contribution from π* character of the thiourethane carbonyl group. An increment in the intensity of the white line and shift of the absorption threshold were greater than that for PTU-B. This can be explained by the lower degree of the ordered structure in the sulfur neighborhood of PTU-P becoming more ordered with increasing strain. The FT magnitude of sulfur K-edge EXAFS spectra and the best fit model of PTU-P at various strains are shown in Figure 9b. The sulfur K-edge EXAFS spectra and individual scattering contribution for PTU-P can be seen in Figure S5f−j. The hard segment crystal structure of 1,4-H6XDI−1,5-PDT and the obtained structural parameters for PTU-P are provided in Figure S6b and Table S2, respectively. The main peak of the EXAFS spectra of PTU-P represents the sulfur firstcoordination shell. As the strain increased, a gradual increase in the first peak magnitude and a slight shift to lower R were observed, with more noticeable changes detected at strain 3.0 and 4.0. This indicates an increase in the degree of ordered structure and change in the S−C bond length within the sulfur first-coordination shell. PTU-P shows relatively higher values of EXAFS Debye−Waller factor for the first-coordination shell of sulfur compared to PTU-B, which can be seen in Figure 10.

Figure 10. Strain dependence of EXAFS Debye−Waller factor for the first-coordination shell of sulfur for PTU elastomers. G

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Figure 11. Schematic illustration of (a) PTU-B and (b) PTU-P during uniaxial elongation.

segment domains is likely to occur as an indication of permanent deformation.

CONCLUSIONS The deformation behavior of PTU elastomers was investigated using in situ synchrotron SAXS, WAXD, and XAFS spectroscopy. PTU elastomers (PTU-B or PTU-P) were prepared from PTMG, 1,4-H6XDI, and BDT or PDT. SAXS measurement revealed that the spacing of hard segment domains of PTUs increased in the parallel directions to the elongation direction and showed the constant value of strain above 2. The strain obtained from the spacing of the hard segment domains for PTU-B was larger than that for PTU-P, and Young’s modulus and tensile strength of PTU-B were larger than those for PTUP, suggesting that well-developed hard segment domains were formed for PTU-B. The crystalline peaks were observed along the direction perpendicular to the elongation direction in WAXD patterns and Debye−Waller factor decreased at around strain of 2−3. Therefore, it seems reasonable to consider that ordering of hard segment chains occurred and then straininduced crystallization of the soft segments subsequently occurred during the elongation process.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00982. GPC curves for PTU elastomers; true stress-true strain curves for PTUs during uniaxial elongation; Herman’s orientation function for PTU elastomers; DSC thermograms for PTU elastomers; sulfur K-edge EXAFS spectra and individual scattering contribution for PTU-B and PTU-P; structure and structural changes; and best-fit values determined for the S K-edge data for PTU-B and PTU-P obtained from EXAFS fitting (PDF)

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ACKNOWLEDGMENTS

This work was supported by the Impulsing Paradigm Change through Disruptive Technology (ImPACT) Program, the Photon and Quantum Basic Research Coordinated Development Program from the Ministry of Education, Culture, Sports, Science and Technology, Japan. In situ simultaneous SAXS/ WAXD measurements were conducted at the BL03XU SPring8 facility with the approval of the Japan Synchrotron Radiation Research Institute (JASRI; proposal nos. 2012B1506, 2013B1186, 2014B1198, 2014B7266, 2015A1514, 2015A7216, 2015B7267, 2016A7217, 2016B7266, 2017A7215, 2017B7267, 2018A7217, and 2018B7267). We gratefully acknowledge Dr. Hiroyasu Masunaga and Dr. Taizo Kabe (JASRI), for their assistance on the SAXS and WAXD measurements. R.R. was supported by Research and Innovation in Science and Technology Project (RISETPRO), Ministry of Research, Technology, and Higher Education of Indonesia [loan number 8245-ID] and Center for Isotope and Radiation Application, National Nuclear Energy Agency of Indonesia (BATAN).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ken Kojio: 0000-0002-6917-7029 Atsushi Takahara: 0000-0002-0584-1525 Kazuki Mita: 0000-0001-9508-1621 Notes

The authors declare no competing financial interest. H

DOI: 10.1021/acs.macromol.9b00982 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.9b00982 Macromolecules XXXX, XXX, XXX−XXX