In recent years, sugar-derived polymer materials have been actively investigated. In research of polyurethane (PU), sugar has been used as a raw material because it has properties similar to polyol. However, the elastic property of the obtained PU is substantially lost. Hence, the introduction of a sugar unit to PU while maintaining the elastic property remains a challenge in polymer chemistry. Here, we report the synthesis of a polyurethane elastomer (PUE) with a trehalose unit using raw materials such as an aromatic diisocyanate (4,4’-diphenylmethane diisocyanate), polyols including a polyether polyol (polytetramethylene glycol), two polyester polyols (polycaprolactone and polycarbonate diol), and trehalose. Novel PUEs with trehalose units are synthesized by a one-shot method. Trehalose, which has non-reducing properties, is used as sugar. The use of trehalose, which has been scarcely applied to PUE, is essential to obtain the desired PUEs with sugar units.
Polymer materials derived from sugar have been actively investigated in recent years in the field of polyurethane (PU) [
PTMG (molecular weight = 2000) (PTMG2000) (TERATHANE 2000) was supplied by Invista Industry, Texas, USA. PCL (molecular weight = 2000) (PCL2000) (PLACCEL 2000) was supplied by Daicel Industry, Osaka, Japan. MDI (MILLIONATE MT) and PCD (molecular weight = 2000) (PCD2000) (NIPPOLLAN 980N) were supplied by Tosoh Industry, Tokyo, Japan (Tosoh). MDI was purified by distillation under reduced pressure (267 - 400 Pa) at 100˚C before use. Trehalose was purchased from NacalaiTesque, Inc., Kyoto, Japan (Nacalai) and used without further purification. Tetrahydrofuran (THF) and benzene were purchased from Nacalai and distilled over calcium hydride under an Ar atmosphere. N,N-Dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) were purchased from Nacalai and stored over 4 Å molecular sieves before use. The following compounds were purchased from commercial suppliers and used as received: DMSO (Euriso Top, Saint-Aubin, France), hexane (Nacalai), and acetone (Nacalai).
PUEs with trehalose units were prepared from MDI, one of the three polyols (PTMG2000, PCL2000, and/or PCD2000)), and trehalose by a one-shot method (Scheme 1). The recipe and trehalose content for each of the PUEs are listed in
For example, the synthesis of PUE-PTMG-T1 was performed as follows. A trehalose/DMF solution was prepared from trehalose (0.34 g, 0.10 × 10−3 mol) and DMF (10 mL) at 100˚C for 15 min under an Ar atmosphere.
Sample | MDI (mol × 10−2) | Polyola (mol × 10−2) | Trehalose (mol × 10−3) | Trehalose Content (wt%) |
---|---|---|---|---|
PUE-Polyol-T1 | 2.0 | 0.90 | 0.10 | 1.4 |
PUE-Polyol-T2 | 2.0 | 0.80 | 0.20 | 3.1 |
PUE-Polyol-T3 | 2.0 | 0.70 | 0.30 | 5.0 |
PUE-Polyol-T4 | 2.0 | 0.60 | 0.40 | 7.6 |
PUE-Polyol-T5 | 2.0 | 0.50 | 0.50 | 10 |
PUE-Polyol | 2.0 | 1.0 | - | - |
aPolyols: polyoxytetramethylene glycol (molecular weight = 2000; PTMG2000), polycaprolactone diol (molecular weight = 2000; PCL2000), and polycarbonate diol (molecular weight = 2000; PCD2000).
Scheme 1. Synthesis of polyurethane elastomers containing trehalose via the one-shot method.
MDI (5.0 g, 2.0 × 10−2 mol), PTMG2000 (18 g, 0.90 × 10−2 mol), THF (20 mL), and trehalose/DMF solution (10 mL) were added to a 100 mL four-necked separable reaction flask equipped with a mechanical stirrer, a gas inlet tube, and a reflux condenser. This mixture was stirred at 80˚C for 20 min under an Ar atmosphere. The thin polymer sheets (~0.5 mm) were obtained by casting the resulting solution (20 g) using a disposable container at room temperature (23 ± 2˚C) for 15 h. The obtained sheet was cured at 80˚C for 6 h in vacuo.
All analyses and tests were performed at room temperature (23 ± 2˚C) unless otherwise indicated.
NMR spectra were recorded on a Varian (California, USA) Unity Plus-300 (1H, 300 MHz; 13C, 75.4 MHz) NMR spectrometer. Chemical shift values for protons were referenced to the resonance of tetramethylsilane as the internal standard and values of carbon were referenced to the carbon resonance of DMSO-d6 (δ49.5).
Fourier transform infrared (FTIR) spectra were recorded on a JASCO (Tokyo, Japan) FTIR-5300 spectrometer equipped with an attenuated total reflection (ATR) system, which used an ATR500/M with an ATR prism KRS-5.
The average molecular weight and molecular distributions were investigated using a Tosoh (Tokyo, Japan) gel permeation chromatograph (GPC) equipped with SD-8022, CCPD, CO-8020, and RI-8020. The measurement conditions for GPC were as follows: sample, 0.1 wt% (DMF/DMSO = 1/1 solution); solvent, DMF; column, TSK gels α-M and TSK GUARDCOLUMNα; flow rate, 500 µL/min at 40˚C; quantum, polystyrene transformation method.
Solubility tests were performed using 15 × 15 mm test pieces. Each test piece was soaked in a solvent (benzene, hexane, acetone, THF, DMF, or DMSO; 8.0 mL) at room temperature or 100˚C (for DMF and DMSO) for 24 h.
Swelling tests were performed using 15 × 15 mm test pieces. The degree of swelling (Rs) was calculated using the formula Rs(%) = W’/W × 100, where W’ is the weight of the test piece soaked in benzene for 24 h, and W is the weight of the test piece after drying at 30˚C for 24 h in vacuo.
Hardness tests were performed using a Kobunshi Keiki (Kyoto, Japan) Asker Durometer (JIS A type) with test pieces stacked to achieve a thickness of 6 mm.
Tensile tests were performed on an Orientec (Tokyo, Japan) RTC-1225A with a model-U-4300 using a JIS 3-dumbbell as the standard sample and a crosshead speed of 100 mm/min.
Dynamic mechanical analyses (DMA) were performed on a Seiko Instruments (Chiba, Japan) DMS 6100 at 5˚C/min over −100˚C to 300˚C at 20 Hz under N2 atmosphere.
Differential scanning calorimetry (DSC) measurements were performed on a Rigaku (Tokyo, Japan) Thermo-Plus DSC-8230 at 10˚C/min from −120˚C to 200˚C under an Ar atmosphere. Approximately 9.5 mg of each PUE was weighed and sealed in an aluminum pan. The samples were rapidly cooled to −120˚C and then heated to 200˚C at 10˚C/min.
Thermogravimetric analyses (TGA) were performed on a Seiko Instruments (Chiba, Japan) TG/DTA6200 at 10˚C/min from 30˚C to 500˚C under N2 atmosphere.
Atomic force microscopy (AFM) analyses were performed on dried sheets at room temperature (23˚C ± 2˚C) in air using an Olympus NV2000. Most of the images were obtained in tapping mode (ACAFM) with a silicon nitride cantilever (OMMCL-AC 240TS-C2, Olympus optical) using a spring constant of 15 N/m and a resonating frequency of 20 KHz. The scanning rates were varied from 1 to 2 Hz. All the images presented here were reproduced from images obtained from at least three points on each sample surface.
Analyses by 1H NMR (
The representative IR spectra of PUE-PTMG-T1-5 are shown in
Infrared (ATR, cm−1) ν3307 (N-H), 2937, 2851, and 2795 (C-H), 1709 (C=O), 1597 and 1535 (N-H).
The GPC of PUEs containing trehalose units is reported in
PUEs with trehalose units yielded quantitatively in 10 - 25 min, and the actual trehalose contents in each synthesized PUE agreed with the theoretical values. Interestingly, the reaction solutions showed different inherent viscosities, and the viscosities of PUEs increased with the trehalose contents. In addition, all PUEs containing trehalose units were transparent. The solvent resistances of PUEs containing trehalose units were tested by immersing each PUE sheet in various solvents including hexane, benzene, toluene, acetone, THF, DMF, and DMSO. The results are presented in
Sample | Hardnessa (JIS A) | Swelling Rateb (%) | Tgc (˚C) | T10d (˚C) | Mwe (×104) | Mw/Mne |
---|---|---|---|---|---|---|
PUE-PTMG-T1 | 61 | 340 | −78.1 | 352 | 34 | 7.4 |
PUE-PTMG-T2 | 63 | 315 | −77.1 | 345 | 14 | 18 |
PUE-PTMG-T3 | 66 | 308 | −76.7 | 340 | 11 | 9.1 |
PUE-PTMG-T4 | 73 | 290 | −76.2 | 335 | 11 | 15 |
PUE-PTMG-T5 | 83 | 275 | −76.0 | 331 | 3.3 | 3.9 |
PUE-PTMG | 77 | 229 | −67.0 | 351 | 38 | 4.8 |
PUE-PCL-T1 | 60 | 250 | −44.4 | 347 | 10 | 18 |
PUE-PCL-T2 | 67 | 230 | −41.0 | 346 | 7.0 | 18 |
PUE-PCL-T3 | 76 | 210 | −39.1 | 338 | 2.6 | 17 |
PUE-PCL-T4 | 81 | 196 | −36.5 | 346 | 2.8 | 15 |
PUE-PCL-T5 | 92 | 173 | −34.2 | 328 | 2.2 | 11 |
PUE-PCL | 79 | 204 | −45.0 | 338 | 16 | 3.5 |
PUE-PCD-T1 | 67 | 265 | −65.5 | 335 | 13 | 9.5 |
PUE-PCD-T2 | 70 | 250 | −60.9 | 333 | 15 | 12 |
PUE-PCD-T3 | 86 | 239 | −57.6 | 330 | 12 | 14 |
PUE-PCD-T4 | 91 | 223 | −53.2 | 327 | 7.5 | 12 |
PUE-PCD-T5 | 95 | 208 | −51.6 | 325 | 2.2 | 4.9 |
PUE-PCD | 73 | 199 | −54.0 | 331 | 21 | 3.6 |
aMeasurement conditions: JIS A type, total thickness = 6 mm, room temperature (23˚C ± 2˚C). bMeasurement conditions: benzene solvent at room temperature (23˚C ± 2˚C) for 24 h. cDifferential scanning calorimetry was performed at a heating rate of 10˚C/min from −120˚C to 200˚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 N2 atmosphere. eMeasurements conditions: solvent = N,N-dimethylformamide, sample = 0.1 wt% (N,N-dimethylformamide/dimethyl sulfoxide = 1/1 solution), flow rate 500 L/min, measurement temperature = 40˚C.
Samplea | Benzeneb | Hexaneb | Acetoneb | THFb | DMFc | DMSOc | ||
---|---|---|---|---|---|---|---|---|
23˚C | 100˚C | 23˚C | 100˚C | |||||
PUE-Polyol-T1 | Δ | × | × | Δ | Δ | Δ | Δ | Δ |
PUE-Polyol-T2 | Δ | × | × | Δ | Δ | Δ | Δ | Δ |
PUE-Polyol-T3 | Δ | × | × | Δ | Δ | Δ | Δ | Δ |
PUE-Polyol-T4 | Δ | × | × | Δ | Δ | Δ | Δ | Δ |
PUE-Polyol-T5 | Δ | × | × | Δ | Δ | Δ | Δ | Δ |
PUE-Polyol | Δ | × | Δ | Δ | Δ | ○ | Δ | ○ |
○: dissolved, Δ: swelled, ×: undissolved; aMeasurement conditions: benzene, hexane, acetone, THF, DMF, or DMSO as the solvent at room temperature (23 ± 2˚C) or 100˚C (for DMF and DMSO) for 24 h. bRoom temperature (23˚C ± 2˚C); cRoom temperature (23˚C ± 2˚C) and 100˚C.
The mechanical behavior of the crosslinked PUEs is dependent on the structural differences between PUEs containing trehalose units which were caused by changing the hard segment content, crosslinking density and intermolecular interactions between their hard segments. The stress versus strain curves for the trehalose-con- taining PUE sheets with different hard segment molar ratios are illustrated in
The effect of the various polyurethane microstructures on their macroscopic behavior was reflected in their hardness.
DSC analyses of the PUEs containing trehalose units were performed from −120˚C to 200˚C under an Ar atmosphere. From
from several factors including the crosslinking density of the trehalose-based network and higher content of MDI in crosslinked polyurethane.
The thermal stabilities of the PUEs containing trehalose units were examined via TGA under an N2 atmosphere. The TG curves of the PUEs without trehalose and the PUEs with different trehalose component ratios are displayed in
DMA measurements of the PUEs with trehalose units were performed from −100˚C to 300˚C. The results are displayed (
The surface topography of the PUEs containing trehalose units was examined by AFM (
utilized to study the phase-segregated morphology ofPUEs with trehalose units. AFM investigations were conducted on the surface of the polymer, with a scanning area of 1000 × 1000 nm. Topographical heterogeneity is observed in the images of PUE containing trehalose units, which may reflect the existence of ordering tendencies in the polymer structure. In these images, by convention, the hard and soft microphases appear as bright and dark regions, respectively. By increasing the hard segment concentration, changes were observed in the surface morphology. The presence of bright and dark regions indicates the presence of microphase morphology. AFM images of the trehalose-containig PUE sheets have an extended smoother surface area compared to the AFM image observed in the polyurethane sheet without trehalose. The light-colored spots represent hard domains. These are dispersed all over the matrix which is formed by the soft domains while in the case of polyurethane without trehalose and appear as thick regions. Inclusions of hard segments can been in some limited areas. This could be explained by the possibility that those areas benefitted from a better ordering.
3D structures of the PUEs containing trehalose units were predicted using the characteristic structural and chemical features of trehalose as a reference. Specifically, the trehalose composed of two glucoses has two primary alcohols that are of the highest reactivity and six secondary alcohols which are of high reactivity [
In summary, we achieved the synthesis of PUs that maintained the elastic property using the one-shot method. The use of trehalose for additive compound was essential to obtain the desired PUEs. This study demonstrated the
synthesis of PUEs by introducing trehalose units, which acted as a crosslinker in the main chain. The properties of PUE sheets were mainly governed by the stoichiometric balance of the components in the reaction and the degree of crosslinking.
Kazunori Kizuka,Shin-Ichi Inoue, (2016) Synthesis and Properties of Polyurethane Elastomers with Trehalose Units. Open Journal of Organic Polymer Materials,06,63-75. doi: 10.4236/ojopm.2016.62007