Superior Properties of Polyurethane Elastomers Synthesized with

Jan 20, 2017 - Polyurethane elastomers (PUEs) containing trans-1,4-bis(isocyanatomethyl)cyclohexane (1,4-H6XDI) have been synthesized by polymerizing ...
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Superior Properties of Polyurethane Elastomers Synthesized with Aliphatic Diisocyanate Bearing a Symmetric Structure Shuhei Nozaki,† Shiori Masuda,† Kazutaka Kamitani,‡ Ken Kojio,*,†,‡,§ Atsushi Takahara,†,‡,§ Goro Kuwamura,∥ Daisuke Hasegawa,∥ Krzysztof Moorthi,∥ Kazuki Mita,∥ and Satoshi Yamasaki∥ †

Graduate School of Engineering, ‡Institute for Materials Chemistry and Engineering, and §WPI-I2CNER, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan ∥ Mitsui Chemicals, Inc., 580-32, Nagaura, Sodegaura, Chiba 299-0265, Japan S Supporting Information *

ABSTRACT: Polyurethane elastomers (PUEs) containing trans-1,4-bis(isocyanatomethyl)cyclohexane (1,4-H 6 XDI) have been synthesized by polymerizing 1,4-H6XDI with poly(oxytetramethylene) glycol and 1,4-butanediol. The molecular aggregation state and mechanical properties of these PUEs have been compared with those exhibited by PUE analogues made of MDI and diols. The hard segment chains in the 1,4-H6XDI-based PUEs are found to readily crystallize and form strong hydrogen bonds due to a high symmetry of 1,4H6XDI molecule. Consequently, the 1,4-H6XDI-based PUEs exhibit well-organized hard segment domains. This leads to their generally superior mechanical properties as compared to those of the well-known MDI-based PUEs. 1,4-H6XDI’s lack of aromatic moieties is expected to greatly enhance color stability of resulting PUEs. All the above features suggest 1,4-H6XDI could replace MDI in a range of applications.

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the formation of a quinoid structure of the aromatic ring.10 Thus, MDI-based PUEs containing aromatic rings in chains show good mechanical properties, but these PUEs easily undergo yellowing. The use of aliphatic and cycloaliphatic isocyanates prevents yellowing and thus may solve the above problem. However, diisocyanate molecules such as 2,5(2,6)bis(isocyanatomethyl)bicyclo[2.2.1]heptane (NBDI)11 and isophorone diisocyanate (IPDI)12 are asymmetric, and a symmetric 4,4′-diisocyanato dicyclohexylmethane (HMDI)13 includes isomers. This results in low cohesion between hard segments, which generally results in deterioration of mechanical properties of PUEs based on the aliphatic and cycloaliphatic isocyanates. Some studies concerning 1,3- and 1,4-bis(isocyanatomethyl)cyclohexane-based PUEs attempted to address the above problem: Lin et al. reported the hydrogenbonding state of regular polyurethane synthesized by a polyaddition reaction of equimolar of 1,3-H6XDI and 4,4′bis(ω-hydroxyalkoxy)biphenyl.14,15 Xie et al. investigated mechanical properties of a mixture of 1,3-H6XDI- and 1,4H6XDI-based polyurethane elastomers to improve mechanical properties.16 However, the improvement reported for these materials is insufficient for qualifying them as elastomers. FORTIMO trans-1,4-bis(isocyanatomethyl)cyclohexane (1,4-H6XDI) (FORTIMO 1,4-H6XDI) is expected to solve the above problems. Since this diisocyanate is highly symmetric,

INTRODUCTION One of the goals of a sustainable society of tomorrow is to employ recyclable materials. Thermoplastic elastomers (TPEs) are ideal recyclable materials, since cross-links in these materials are formed by physical interactions in appropriately engineered microdomains, crystallites, or hydrogen bonds. Polyurethane elastomers (PUEs), a subclass of TPEs, exhibit versatile mechanical properties.1 These polymers are composed of alternating hard and soft segments, which form microphaseseparated structure due to the thermodynamic immiscibility of the constituent segments. The hard segment chains form crystallites, which are often hydrogen-bonded. Another important feature of polyurethanes, which makes them interesting from the point of view of sustainability, is the ability to derive these materials from a very large number of stock chemicals. Many combinations of the raw materials have been tried to control various properties of the PUEs. Poly(oxytetramethylene) glycol (PTMG), 4,4′-diphenylmethane diisocyante (MDI), and 1,4-butanediol (BD) based PUE is one of the best PUE systems in terms of mechanical properties.2,3 This is because hard segment chains, which are composed of a sequence of alternating MDI and BD moieties, can easily crystallize and form hydrogen bonds in the hard segment domains.4 Nowadays, PUE applications expand steadily, and PUEs’ properties continue to fascinate us.5−9 However, there remain open problems as well. These are yellowing and improvement of mechanical properties. If diisocyanate, a starting material for PUE preparation, includes aromatic rings, the resulting PUEs undergo yellowing due to © XXXX American Chemical Society

Received: September 19, 2016 Revised: January 5, 2017

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DOI: 10.1021/acs.macromol.6b02044 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

number of moles of NCO groups of diisocyanate and OH groups of PTMG, respectively. 10 mg of dibutyltin dilaurate (DBTL, Wako Chemical Co., Ltd. Japan) was added to 1,4-H6XDI as a catalyst in the prepolymer reaction. The extent of the reaction was monitored by an amine equivalent method, and the end of the reaction was determined by confirming that reaction ratio of NCO groups exceeded 90%. After completing the prepolymer reaction, the prepolymer was degassed using a vacuum agitator. The curing agent BD was added to the prepolymer and agitated in vacuum at a ratio of [NCO]pre/[OH]BD = 1.03, where [NCO]pre and [OH]BD are the numbers of moles of NCO groups in the prepolymer and OH groups of BD, respectively. After mixing the viscous product for 90 s, it was poured into a mold constructed using a 1 mm thick spacer and two aluminum plates heated at 120 °C. The PUEs were cured at 120 °C for 24 h in air. The 1,4-H6XDI-based PUE with hard segment content of 10 wt % was synthesized by mixing dry PTMG and 1,4-H6XDI at 80 °C and subsequently cured at 80 °C for 24 h in the mold mentioned above. The nuclear magnetic resonance (NMR) spectrum for HX-10 is shown in Figure S2. To study the crystalline structure of the hard segment chain in the PUEs, the regular polyurethanes based on diisocyanates and BD were synthesized as a hard segment models. Samples were obtained by reacting either FORTIMO 1,4-H6XDI or MDI and BD with K = 0.8 at 80 °C. −(1,4-H6XDI-BD)n− and −(MDI-BD)n− denote the hard segment model samples of 1,4-H6XDI and MDI. Swelling Test. The swelling behavior of the PUEs was investigated using toluene and N,N-dimethylacetamide (DMAc). The PUEs were soaked in each solvent at 60 °C. The gel fraction, G, was defined as G = Wb/W, where Wb and W are the dry weight after swelling and original weight, respectively. The degree of swelling, Q, of the PUEs was determined by weighting dry and swollen PUE samples. Q is defined as Q = 1 + [(Wa − Wb)/ds/(Wb/dp)], where Wa, ds, and dp are the weight of sample swollen to equilibrium state, the density of solvent, and density of the PUEs, respectively. Hydrogen-Bonding State. The state of hydrogen bond of the hard segment in the PUEs was investigated by attenuated total reflection Fourier transform infrared spectroscopy (ATR-FT-IR). ATR-FT-IR spectra were obtained with an FTS-3000 EXCALIBUR (Digilab Japan Co., Ltd. Japan) equipped with an MCT detector using a ATR cell (MIRacle, PIKE Technologies, Inc., USA). All spectra were collected with 32 scans and at a resolution of 4 cm−1. Thermal Properties. Thermal properties were analyzed based on DSC measurements. The thermograms were obtained using a DSC (Rigaku DSC 8230, Rigaku Denki Co. Ltd. Japan) within −130 and 250 °C temperature range, with a heating rate of 10 °C min−1 under a nitrogen atmosphere. At the start of measurement, as-prepared samples were simply cooled down to around −140 °C. Small-Angle X-ray Scattering (SAXS) and Wide-Angle X-ray Diffraction (WAXD) Measurements. SAXS and WAXD data for the PUEs were acquired at the BL03XU beamline18,19 in the SPring-8 facility in Japan. The photon flux was ∼1013 photons/s, and the size of beam at the sample was 150 μm × 150 μm. The X-ray with a 0.1 nm wavelength was monochromatized from an undulator using Si doublecrystal monochromator. For SAXS measurements, a CCD detector (ORCA-R2, Hamamatsu Photonics, K. K.) with a pixel size of 126 μm/pixel and a total size of 672 × 512 pixels was used to measure the scattered radiation. The detector was placed behind a vacuum path, and the camera length was ca. 2 m. For WAXD measurements, a flat panel detector was employed, and the camera length was 63 mm. Scattering patterns were collected for 200−500 ms. Data were reduced from the 2D format to 1D by integrating with FIT2D (Ver. 12.077, Andy Hammersley). Both sets of data were expressed in terms of the wave vector, q, where q = 4π sin θ/λ. Also, supplemental measurement was performed at BL40XU and BL05SS. Absolute scattering intensity was obtained by calibration with pure water.20 Intensity for empty capillary (a lindemann glass capillary with 1 mm in diameter) and the capillary filled with pure water was recorded for the calibration. Computational for Modeling Crystal Structure of Hard Segment. Similar molecules often crystallize in a similar manner.21 In an attempt to elucidate the crystal structure of the 1,4-H6XDI/BD

hard segment chains form well-organized structures, which can be tailored using an appropriate chain extender. Moreover, 1,4H6XDI is not prone to yellowing.17 These highly favorable property balance makes 1,4-H6XDI a strong contender to MDI in the elastomer field. In this study, 1,4-H6XDI-based PUEs with varying hard segment content have been synthesized and characterized. The structure and properties of 1,4-H6XDI-based PUEs were studied using Fourier-transform infrared spectroscopy (FTIR), wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS), dynamic viscoelastic property measurement, and tensile testing.

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EXPERIMENTAL SECTION

Materials. Poly(oxytetramethylene) glycol (PTMG: Mn = 1826, Asahi Kasei Chemicals Co., Ltd., Japan), FORTIMO trans-1,4bis(isocyanatomethyl)cyclohexane (1,4-H6XDI, Mitsui Chemicals Inc.), and 1,4-butanediol (BD, Wako Chemicals Co., Ltd., Japan) were used as the polymer glycol, diisocyanate, and chain extender, respectively. BD was refluxed with calcium hydride to remove water and subsequently purified by distillation. The amount of residual water was a few hundred ppm. 4,4′-Diphenylmethane diisocyanate (MDI, Tosoh Co., Japan) was used as a control diisocyanate. FORTIMO 1,4H6XDI and MDI were used without further purification. Impurity is chlorine derivative in ppm. Synthesis of PUEs. The PUEs were prepared by a bulk polymerization method using PTMG, diisocyanate, and BD. FORTIMO 1,4-H6XDI and MDI were used as a diisocyanate. Figure 1 shows synthetic procedure of a prepolymer method, which includes

Figure 1. Synthetic scheme of PUEs by a prepolymer method. two-step reaction. The composition of the PUEs samples and chemical structures of raw materials are shown in the Supporting Information (Figure S1). The 1,4-H6XDI-based PUEs with three hard segment contents, 10, 20, and 30 wt %, were synthesized. The hard segment content for MDI-based PUEs was 34 wt %. The nomenclature denotes the type of diisocyanate and hard segment content; for example, HX30 denotes 1,4-H6XDI-based PUE with hard segment conent of 30 wt %. Prior to polymerization, PTMG was dried with dry nitrogen under reduced pressure using a separable flask. Prepolymers were synthesized from PTMG and diisocyanate in a mechanical agitator and at theformulation ratio of K = [NCO]iso/[OH]PTMG = 3.08, 1.94, and 2.99 for HX-30, HX-20, and MD-34, respectively, at 80 °C for 3 h under a nitrogen atmosphere, where [NCO]iso and [OH]PTMG are the B

DOI: 10.1021/acs.macromol.6b02044 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules system, we assume it can be derived from the structure of related aliphatic linear polyurethanes of similar chemical composition. Atomistic hard segment models with explicit hydrogen atoms have been constructed using Materials Studio crystal building tools combined with molecular mechanics/dynamics tools. We test the performance of these tools to represent the zigzag form of the poly(ethylene−hexamethylene dicarbamate) (M1), listed in the Cambridge Structural Database22 under reference HIDDAZ01. The latter structure has been minimized under periodic boundary conditions with interatomic interactions described using the COMPASSII force field.23 In the minimized structure all CO and NH groups engage in intermolecular hydrogen bonds of single type as suggested in the literature.24,25 Although the crystal density obtained via minimization alone (ca. 1.38 g cm−3) is much higher than the theoretical crystal density derived in ref 24 (1.27 g cm−3), this discrepancy greatly decreases when crystal density is determined at finite temperature (1.30 g cm−3 at 300 K) in a course of short (0.5 ns) molecular dynamics runs on an assembly of cells. This result suggests that COMPASSII represents well the interatomic interactions in this polyurethane crystal. In order to derive plausible crystal structures of the −(1,4-H6XDI-BD)n− copolymer, we modify M1 in a gradual fashion by substituting the hexamethylene moiety with a 1,4dimethylcyclohexane one and an ethylene diol moiety with a butylene diol one. Each substitution event is followed by several cycles of minimization and short NPT (isothermal−isobaric ensemble) molecular dynamics runs at 300 K in order to relax the structure. In the course of several such runs, each starting from a different atom to be reconstructed, two stable structures, M2 (shown in Figure 5) and M3 have been identified. The agreement between the crystal models and the experimental X-ray diffraction pattern is characterized by fitting the derived cell constants to the experimental powder pattern using the Pawley method26 as implemented in Materials Studio. The goodness of fit is quantified by the weighted profile residual, Rwp

R wp2 =

Table 1. Density, Gel Fraction, and Degree of Swelling of 1,4-H6XDI- and MDI-Based PUEs degree of swelling, Q

a

sample

toluene

DMAca

toluene

DMAca

HX-30 HX-20 HX-10 MD-34

1.06 1.04 1.02 1.09

2.02 ± 0.01 4.09 ± 0.05 soluble 1.95 ± 0.03

18.2 soluble soluble soluble

99.3 ± 0.2 99.2 ± 0.1 soluble 99.0 ± 0.2

23.5 ± 5.3 soluble soluble soluble

N,N-Dimethylacetamide.

gel fraction, and degree of swelling of the PUEs. Gel fraction of the PUEs swollen by toluene was greater than 99% , except HX-10. This indicates that a well-developed network structure was formed in HX-20, HX-30, and MD-34 PUEs, but not in HX-10. The degree of swelling in toluene for 1,4-H6XDI-based PUEs decreased with increasing hard segment content, resulting in the formation of well-developed hard segment domains. The degree of swelling of HX-30 was almost the same as that of MD-34. Furthermore, HX-20 and MD-34 PUEs were soluble in polar solvent, DMAc, in contrast to HX-30. The linear PUEs, which form only physical cross-links, dissolve in DMAc because hydrogen bonds in physical cross-links dissociate when solvated by molecules of polar solvent.27−31 Thus, insolubility of HX-30 in DMAc suggests that HX-30 forms quite strong physical cross-links. To evaluate the hydrogen-bonding state of the urethane carbonyl groups of hard segment chains in the PUEs, ATR-FTIR measurements were carried out. Figure 2 shows ATR-FT-IR

∑i wi[cY c i − I e i + Y bi]2 ∑i wi[I e i]2

gel fraction, G (%)

density (g/cm3)

(1)

and the weighted profile residual without background, Rwbp

R wbp2 = e

∑i wi[cY c i − I e i + Y bi]2 ∑i wi[I e i − Y bi]2

(2)

b

where I and Y are the experimental and background intensities, Yc is the calculated intensity without the background contribution, c is a scaling factor independent of the diffraction angle, wi is defined as wi = 1/Iei, and the index i denotes the 2θ angle. Dynamic Viscoelastic Properties. The dynamic viscoelastic properties were measured with a DMS 6100 (Seiko Instruments, Co., Ltd., Japan) between −150 and 250 °C with a heating rate of 2 °C min−1 under a nitrogen atmosphere. The size of samples used was 25 mm × 5 mm × 2 mm. Imposed strain and frequency were 0.2% and 10 Hz, respectively. Mechanical Properties. Tensile tests were performed with an Instron type tensile tester (Orientec, Co., Ltd, RTE-1210) at room temperature. The definition of strain is (l − l0)/l0, where l and l0 are the length at a certain time and initial length, respectively. The dimension of samples were 5 mm × 100 mm × 1 mm. An initial length and elongation rate were set to be 30 mm and 10 mm min−1 (0.33 min−1), respectively. Cyclic testing was also performed using tensile tester to evaluate elastic property of the PUEs. Tensile and retraction rates were set to be 0.33 min−1.

Figure 2. ATR-FT-IR spectra of the 1,4-H6XDI- and MDI-based PUEs with various hard segment contents: (a) NH stretching and (b) carbonyl stretching band regions.

spectra in the ranges of (a) NH and (b) CO stretching band regions for the 1,4-H6XDI- and MDI-based PUEs prepared with various hard segment contents. The complete spectra are shown in Figure S3. In all PUEs, the NH stretching band (ν(NH)) and carbonyl stretching one (ν(CO)) were clearly observed at around 3300 and 1700 cm−1, respectively. In all spectra, no NCO stretching band was observed at 2260 cm−1, which indicates that the curing was properly performed for all PUEs. The carbonyl stretching band at 1660 cm−1 which is characteristic of allophanate groups was also unobserved. The hydrogen-bonding state in the hard segment chains can be ascertained from the intensity of the ν(NH) and ν(CO) peaks.32,33 The hydrogen-bonded NH groups with ether oxygen (ν(NHether)) and carbonyl groups (ν(NHcarbonyl)) and free one (ν(NHfree)) are generally observed at around 3290−

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RESULTS AND DISCUSSION The appearance of HX-20, HX-30, and MD-34 PUEs is milky opaque, and that of HX-10 is transparent. This implies that crystallites are formed from the hard segment chains in HX-20, HX-30, and MD-34, while no crystallite existed in HX-10. All PUEs, including HX-10, are elastomers. Table 1 shows density, C

DOI: 10.1021/acs.macromol.6b02044 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules 3310, 3300−3350, and 3450 cm−1, respectively. The hydrogenbonded carbonyl stretching band (ν(COH‑bond)) and the free one (ν(COfree)) are observed at 1690−1705 and 1720−1735 cm−1, respectively. These peak positions depend somewhat also on the chemical structure of diisocyanate. The bands observed at 3341, 1694, and 1723 cm−1 for HX-20 and HX-30 and at 3325, 1704, and 1731 cm−1 for the MD-34 are likely to correspond to ν(NH), ν(COH‑bond), and ν(COfree) bands, respectively. The ratios of Iν(COH‑bond) to Iν(COfree) (Iν(COH‑bond)/Iν(COfree)) for HX-30 and HX-20 were close to each other and much larger than the Iν(COH‑bond)/ Iν(COfree) ratio for MD-34. The Iν(CO) peak shifts due to the formation of hydrogen bonds were 29, 29, and 27 cm−1 for HX-30, HX-20, and MD-34, respectively, the values for HX30 and HX-20 being slightly larger than for MD-34. Moreover, the ν(NH) band of HX-30 and HX-20 observed at 3340 cm−1 was sharper than the ν(NH) band of MD-34 observed at 3325 cm−1. Furthermore, for MD-34, a broad tail was observed for the ν(NH) band in the lower frequency region. These observations suggest that the hard segment chains of HX-30 and HX-20 are well-organized and more strongly hydrogen bonded than MD-34. In contrast to the former systems, the ATR-FT-IR spectra of HX-10 exhibited both hydrogen-bonded and free ν(NH) bands at 3330 and 3450 cm−1 and ordered and disordered hydrogen bonds and free ν(CO) bands at 1690, 1703, and 1721 cm−1, respectively. It is conceivable that urethane groups partially formed hydrogen bonds in the both ordered and disordered states of the 1,4-H6XDI residue in HX10. The thermal behavior of the PUEs was investigated by DSC. Figure 3 shows the DSC thermograms for the 1,4-H6XDI- and

due to the change in the molecular interaction between the soft and hard segments. Moreover, an increase in Tm,H generally leads to a higher degree of phase separation.1,11,29 Therefore, it seems reasonable to conclude that the degree of microphase separation of the 1,4-H6XDI-based PUEs increased with an increase in hard segment content, and the degree of microphase separation of HX-30 is higher than MD-34 because of higher Tg,S of MD-34 and ATR-FT-IR data. The Tm,Hs of the −(1,4-H6XDI-BD)n− hard segment model were ca. 10 °C higher than for MDI-based model, −(MDIBD)n−. However, the Tm,H of HX-30 was comparable with MD-34. It is expected that the increase in Tm,H for HX-30 could be obtained by appropriate annealing. To investigate the crystal structure of the hard segment chains in the PUEs, WAXD measurements were carried out. Figure 4 shows WAXD profiles for the 1,4-H6XDI- and MDI-

Figure 4. WAXD profiles of the 1,4-H6XDI- and MDI-based PUEs with various hard segment contents. WAXD profiles for hard segment model are plotted as well.

based PUEs with various hard segment contents. For comparison, the WAXD profiles of hard segment models for both isocyanates, −(1,4-H6XDI-BD)n− and −(MDI-BD)n−, were also plotted. The crystalline peaks were clearly observed at q = 12.0, 13.9, 15.2, and 17.0 nm−1 for the −(1,4-H6XDIBD)n− model. For HX-20 and HX-30, one can clearly see crystalline peaks coincide with peaks of the hard segment model, though some of them are difficult to recognize because of overlapping amorphous halo of soft segment. It is conceivable that the hard segments are packed in the same crystal lattice also in the PUEs. The intensity of crystalline peak of HX-30 was much higher than that of HX-20. This is due to not only increase in hard segment content but the formation of well-organized crystallized hard segment domains. No crystalline peak was observed for HX-10, presumably due to insufficient cohesive force of 1,4-H6XDI residue at this concentration. On the other hand, in the WAXD profile for −(MDI-BD)n−, the crystalline peaks were observed at q = 13.1, 13.7, 15.4, 16.8, and 18.0 nm−1. Weak crystalline peaks of hard segment chains were observed at the same positions for MD-34 as well. These overlapped peaks also correspond well to the hard segment peaks. It is apparent that the peak intensity from crystallized hard segment domains for HX-30 is much stronger than the peak intensity for MD-34 despite lower hard segment content in HX-30. This implies that, in addition to the formation of stronger hydrogen, the 1,4-H6XDI-based hard segment chains exhibit higher crystallization ability than MDIbased hard segment chains. The detailed discussion of the latter subject will be given in following section.

Figure 3. DSC thermograms the 1,4-H6XDI- and MDI-based PUEs and hard segment models.

MDI-based PUEs with various hard segment contents and for two hard segment models, −(1,4-H6XDI-BD)n− and −(MDIBD)n−. The glass transition temperatures of the soft segment chains (Tg,S) were observed at around −70 °C for all PUEs. Obvious endothermic peaks were observed in the temperature range of −10 to 10 °C as well. This is due to the melting of partially crystallized soft segment chains (Tm,S). The Tg,S and Tm,S increased in the following order: HX-30 → HX-20 → HX10 → MD-34. With the exception of HX-10, other endothermic peaks were observed in the temperature range of 160 and 200 °C. These peaks can be assigned to melting of crystallized hard segment domains. Melting points of the crystallized hard segment domains (Tm,Hs) of HX-20 and HX-30 increased with an increase in hard segment content. Concerning Tg,S, it is wellknown that an increase in Tg of the soft segment in PUE systems results in increasing miscibility of the two components D

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Figure 5. Model crystal structure (M2) of 1,4-H6XDI-BD-based hard segment chains. A view (a) from the point perpendicular to the butylene plane and (b) along the molecular axis.

In order to gain additional insight into the structure and conformation of the crystallized hard segment chains in the 1,4H6XDI-based PUEs, molecular modeling of −(1,4-H6XDIBD)n− has been performed. Figure 5 shows the view of M2 structure from the point perpendicular to the butylene plane in the all-trans state (a) and along the molecular axis of the model (b). The experimental and refined X-ray profiles for −(1,4H6XDI-BD)n− are shown in Figure S4. The Rwp and Rwbp residuals for the refined M2 lattice cell are 0.038 and 0.105, respectively. This result as well as the inspection of the profile in Figure S4 suggests that the structure is consistent with WAXD data shown in Figure 4. The refined unit cell was triclinic, of P1 symmetry, a = 0.755 nm, b = 0.829 nm c = 1.594 nm, α = 139.0°, β = 139.7°, and γ = 36.2°. The refined cell parameters are within 1% of the initial values obtained from molecular modeling. In this crystal lattice, the BD residue is predicted to form a zigzag plane, and one of the urethane ether oxygens lies in the plane of the alkyl zigzag. However, an opposite oxygen atom is off the plane resulting in a nearly antiparallel orientation of both NH(CO)O groups, which is a necessary condition to form intermolecular hydrogen bonds. There are two kinds of hydrogen bonds in M2 structure with slightly different distances of 0.193 and 0.212 nm. The M3 structure, in which butylene chains are fully extended and collinear with urethane moieties, is calculated to be more stable (ca. 14 kcal/mol) than the M2, and its refined unit cell is triclinic of P1̅ symmetry with a = 0.736 nm, b = 0.861 nm, c = 1.784 nm, α = 140.9°, β = 142.6°, and γ = 33.9°. The M3 structure has hydrogen bonds of only one length (0.192 nm). However, the Rwp and Rwbp residuals for the M3 structure are 0.072 and 0.18, respectively; thus, the experimental WAXD profile agrees much better with the M2 diffraction pattern despite the fact that M2 is less stable than M3 (cf. Figure S4). We hypothesize that M3 structure could be obtained by stretching and adequately annealing the sample. The detailed microdomain structure in the PUEs was investigated using the SAXS technique. Figure 6 shows SAXS profiles for the 1,4-H6XDI- and MDI-based PUEs with different hard segment content. Broad scattering peaks in the q range from 0.1 to 1.0 nm−1 were detected in the SAXS profiles for PUEs, except HX-10. It seems that there is no specific microphase separation for HX-10. Peaks observed for HX-20, HX-30, and MD-34 are associated with interdomain spacing of hard segment domains in a microphase-separated structure, which consists of the hard segment domains and a surrounding soft segment matrix. For HX-20 and HX-30, an increase in hard

Figure 6. (a) SAXS profiles and (b) three-dimensional experimental correlation functions of the 1,4-H6XDI- and MDI-based PUEs with various hard segment contents.

segment content induced a peak shift to low q region. In order to make a more quantitative evaluation, diffuse phase boundary thickness of hard and soft segments was determined by analyzing the deviation from Porod’s law, and interdomain spacings were determined by analyzing the three-dimensional correlation function. These quantities were obtained according to the calculation methods by Bonart et al.34 and Koberstein et al.35 Table 2 summarizes interdomain spacings calculated from peak positions using Bragg’s law and determined by correlation function analysis. The relative degrees of overall microphase separation for the 1,4-H6XDI-based PUEs increased with an increase in hard segment content, suggesting that the PUEs are microphase-separated systems as already discussed. The boundary diffuseness (Δη2″/Δη2′ − 1), and sigmoidal and linear boundary thicknesses (σ and E) for three samples exhibited a consistent trend; that is, the boundary diffuseness values for HX-30 and MD-34 were larger than the value determined for HX-20. The PUEs containing large amount of hard segments form larger hard segment domains, and they show larger boundary diffuseness. The boundary thickness is influenced by the length and distribution of hard segments, a cohesive force originated from primary segment structure such as symmetry, and ability to phase separate from a soft segment phase. The boundary diffuseness of HX-30 was slightly smaller than for MD-34. This is because the 1,4-H6XDI-based hard segment has higher cohesive energy as clarified by ATR-FT-IR, WAXD, and crystal structure analysis for hard segment chain. Figure 7 shows temperature dependence of the dynamic storage modulus (E′), loss modulus (E″), and loss tangent (tan δ) of the 1,4-H6XDI- and MDI-based PUEs with various hard segment contents. During the heating process starting at −150 E

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Table 2. Degree of Overall Microphase Separation and Diffuse-Microphase Boundary Thickness Parameters for 1,4-H6XDI- and MDI-Based PUEs

HX-30 HX-20 HX-10 MD-34

degree of overall microphase separation

boundary diffuseness

diffuse-microphase boundary thickness/ nm

Δη2′/Δηc2

Δη2″/Δη2′ − 1

sigmoidal, σ linear, E

0.39 0.29

1.77 0.18

0.84 0.35

0.38

1.83

1.04

d-spacing from Bragg’s law (nm)

d-spacing from 3D correlation function (nm)

2.09 0.88

17.4 16.4

18.1 16.7

2.61

21.7

23.4

Figure 8. Stress−strain curves of the 1,4-H6XDI- and MDI-based PUEs with various hard segment contents.

Table 3. Young’s Modulus, Tensile Strength, and Strain at Break for the 1,4-H6XDI- and MDI-Based PUEs Obtained from Figure 8

Figure 7. Temperature dependence of dynamic viscoelastic properties of the 1,4-H6XDI- and MDI-based PUEs measured at 10 Hz.

°C, the E′ and tan δ values of the 1,4-H6XDI-based PUEs steeply decreased and increased at around −70 °C. These changes are associated with the glass transition of the soft segments. Above the glass transition of the soft segment, a shoulder in E′ curve at around −40 °C was observed, which is indicative of the soft segment to recrystallization. In the tan δ curve, two dispersions were clearly observed at around −50 °C. They originate from the glass−rubber transition and recrystallization of the soft segment chains during the heating process as already discussed. With the exception of HX-10, the rubbery plateau region was observed within 0 and 150 °C for all the PUEs. The E′ value at the rubbery plateau region decreased in the following order: HX-30 > HX-20 > MD-34 > HX10. The E′ value of HX-10, ca. 3 MPa, was much lower than the moduli for other PUEs. Further discussion on the E′ values at rubbery plateau region will be given with stress−strain curve data. The terminal temperature decreased in the following order: HX-30 ≅ MD-34 > HX-20 > HX-10. These results are consistent with the temperatures determined via DSC and strongly suggest higher heat resistance of HX-30 in practical applications. Despite of smaller hard segment content in HX-30 as compared to MD-34, the terminal temperatures of both polymers were almost the same. Therefore, it is expected that the PUEs with higher heat resistance can be obtained by tuning thermal history and consumption of diisocyanate can be reduced by using of 1,4-H6XDI. Figure 8 shows stress−strain curves of the 1,4-H6XDI- and MDI-based PUEs with various hard segment contents. The Young’s moduli estimated from initial slope of stress−strain curves, tensile strength, and strain at break value for the PUEs are summarized in Table 3. For the 1,4-H6XDI-based PUEs, Young’s modulus, and tensile strength increased with hard segment content. The value of Young’s modulus for MD-34 is close to that of HX-20, and the tensile strength of MD-34 is

sample HX-30 HX-20 HX-10 MD-34

Young’s modulus (MPa)

strain at break

tensile strength (MPa)

± ± ± ±

8.8 >10 14.8 6.0

36.3 ± 2.5 >20 3.2 ± 0.1 42.2 ± 3.9

19.1 8.0 3.4 8.2

0.1 0.1 0.2 0.4

similar to that of HX-30. The strain-induced crystallization for MD-34 was more pronounced than in HX-30 and HX-20. According to the atomic force microscopic (AFM) observation, isolated hard segment domains were detectable for MD-34 at this hard segment content; on the other hand, HX-30 and HX20 possess well-developed cylindrical hard segment domains as shown in Figure S5. Thus, it is likely that obvious breaks of anisotropic hard segment domains might have occurred for HX-based PUEs. As the result, the slope at larger strain region for HX-based PUEs was smaller than for MD-34. The tensile strength of HX-30 was slightly higher than that of MD-34 despite the fact that the hard segment content in HX-30 is lower than in MD-34. If highly isolated hard segment domains are formed in the HX-based PUE by tuning some conditions, for example, hard segment content or temperature, stronger tensile strength would be obtained. HX-10 has only 10 wt % of hard segment content. In the case of the PUEs-based on PTMG-MDI with 10 wt % of hard segment content, plastic deformation occurred predominantly. Figure 9a shows cycle test data of the 1,4-H6XDI- and MDIbased PUEs with various hard segment contents. All samples recovered quickly without slack at this retraction rate. To investigate the elastic property, residual strain (εr) was normalized by maximum imposed strain (εm). The normalized residual strain (εr/εm) (Figure 9b) is based on the data taken from Figure 9a. As shown in Figure 9b, the εr/εm of all PUEs increased with εm. The slope decreased for HX-20, HX-30, and F

DOI: 10.1021/acs.macromol.6b02044 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Notes

The authors declare no competing financial interest.

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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, and Japan Society of the Promotion of Science (JSPS) KAKENHI (No. 26410138). Small-angle X-ray scattering measurements were done at BL03XU, BL05SS, BL40XU, and the SPring-8 facility with the approval of the Japan Synchrotron Radiation Research Institute (JASRI; Proposal No. 2012B1506, 2013B1186, 2014B1198, 2014B7266, 2015A1514, 2015A7216, 2015B7267, 2016A7217, and 2016B7266). We gratefully acknowledge Dr. Hiroyasu Masunaga (JASRI) and Dr. Hiroki Ogawa (Kyoto University) for their assistance on the SAXS and WAXD measurements.

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Figure 9. (a) Cycle test of the 1,4-H6XDI- and MDI-based PUEs with various hard segment contents. (b) Normalized residual strain− maximum imposed strain relation obtained from (a).

MD-34 at εm ≅ 4, but it was close to zero for HX-10. These results suggest that the degree of breakage of the microphaseseparated structure and hydrogen bonds does not increase at high strains. The εr/εm of HX-30 was lower in comparison with that of MD-34. This result can be ascribed to the strong cohesive force of 1,4-H6XDI-based hard segment chains.

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CONCLUSIONS The PUEs based on 1,4-H6XDI were synthesized with PTMG and BD with various hard segment contents. The hard segment chains in the 1,4-H6XDI-based PUEs crystallized easily forming strong intermolecular hydrogen bonds due to the symmetry of 1,4-H6XDI molecule. Consequently, Young’s moduli, tensile strength, and elastic properties of the 1,4-H6XDI-based PUEs were higher than the respective quantities for the MDI-based PUEs. Thus, the 1,4-H6XDI-based PUEs exhibited superior mechanical properties relative to MDI-based PUE. Tailoring of PUE properties is often made by tuning the hard segment content. Thus, smaller amount of 1,4-H6XDI could be employed in comparison with MDI. All the above features suggest 1,4-H6XDI could replace MDI in a range of applications.

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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.6b02044. Figures S1−S5 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone +81-92-802-2515, Fax +81-92-802-2518, e-mail kojio@ cstf.kyushu-u.ac.jp (K.K.). ORCID

Ken Kojio: 0000-0002-6917-7029 G

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DOI: 10.1021/acs.macromol.6b02044 Macromolecules XXXX, XXX, XXX−XXX