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DOI: 10.1039/C4RA05880A (Paper) RSC Adv., 2014, 4, 43406-43414

Crystallizable and tough aliphatic thermoplastic poly(ether urethane)s synthesized through a non-isocyanate route

Yong Deng, Suqing Li, Jingbo Zhao*, Zhiyuan Zhang, Junying Zhang and Wantai Yang
Key Laboratory of Carbon Fiber and Functional Polymers (Beijing University of Chemical Technology), Ministry of Education, State Key Laboratory of Chemical Resource Engineering, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China. E-mail: zhaojb@mail.buct.edu.cn; Tel: +86-10-6443-4864

Received 18th June 2014 , Accepted 1st September 2014

First published on 1st September 2014

Abstract

A simple non-isocyanate route synthesizing aliphatic thermoplastic poly(ether urethane)s (PEUs) with good thermal and mechanical properties is described. Melt transurethane polycondensation of 1,6-bis(hydroxyethyloxy carbonyl amino)hexane (BHCH) with two ethylene glycol oligomers, i.e. triethylene glycol (3EG) and tetraethylene glycol(4EG), was conducted, and high molecular weight PEUs were prepared. The PEUs were characterized by gel permeation chromatography, FT-IR, 1H-NMR, differential scanning calorimetry, thermogravimetric analysis, wide angle X-ray scattering, and tensile tests. The PEUs exhibited Mn above 32[thin space (1/6-em)]000, Mw above 54[thin space (1/6-em)]300, Tg ranging from 11.2 °C to 28.2 °C, Tm from 103.7 °C to 150.6 °C, and initial decomposition temperature over 237.4 °C. A PEU prepared at a BHCH/4EG molar ratio of 5[thin space (1/6-em)]:[thin space (1/6-em)]1 exhibited the best mechanical properties with tensile strength of 32.82 MPa and strain at break of 151.82%. Some urea linkages were formed due to a back-biting side reaction in the transurethane polycondensation.

Introduction

Polyurethanes are important commercial polymers that are widely used as foams, adhesives, coatings and medical materials because of their outstanding mechanical properties, abrasion resistance, and good biocompatibility.1 However, most commercial polyurethanes are prepared from highly toxic diisocyanates, the major synthesis precursor of which is the extremely toxic phosgene. Non-isocyanate synthesis is becoming a promising alternative for the isocyanate-route, and is attracting increasing attention from academic and industrial researchers who are working on polyurethanes.2–4 Most investigations on non-isocyanate polyurethanes (NIPUs) are currently focused on cross-linked polyurethanes prepared from cyclic carbonates and diamines or multi-amines.5–12 Meanwhile, Loontjens et al. synthesized NIPUs from carbonyl biscaprolactamate.13–15

Transurethane polycondensation of diurethanes has been developed to synthesize linear or thermoplastic polyurethanes (TPUs). Tang et al. synthesized TPUs having different multi-urea segments from solution polycondensation of dimethyl dicarbamates with amino-terminated poly(propylene glycol).16 Deepa et al. synthesized a series of amorphous TPUs from transurethane polycondensation of dimethyl 1,6-hexamethylene dicarbamate and dimethyl isophorone dicarbamate with glycol oligomers, 1,4-cyclohexanedimethanol, 1,4-cyclohexanediol or isosorbide.17,18 Rokicki et al. synthesized two α,ω-bis(hydroxyethyloxy carbonyl amino)alkanes and studied their solution transurethane polycondensation with 1,6-hexanediol or 1,10-decanediol.19 Ochiai studied the self-polycondensation of 1,6-bis(hydroxyethyloxy carbonyl amino)hexane (BHCH), and used the obtained polyurethane to synthesize methacrylate macromolecular monomers.20 Meanwhile, Sharma et al. synthesized a series of α-hydroxy-ω-amino-amides through the ring-opening reaction of caprolactam with excessive aminoalcohol21 or through the ring-opening reaction of ε-caprolactone with diamines,22 and turned them into α-hydroxy-ω-O-phenyl urethanes21,22 or α-hydroxy-ω-O-hydroxyethyl urethanes23 through reaction with diphenyl carbonate or ethylene carbonate. Several alternating poly(amide urethane)s were prepared via polycondensation of the α-hydroxy-ω-O-phenyl urethanes or α-hydroxy-ω-O-hydroxyethyl urethanes.21–23 In our previous work,24 we synthesized high molecular weight homopolymer of BHCH (PBHCH) and characterized its physical properties. The PBHCH had good tensile strength but was brittle. To overcome this drawback, we synthesized two nylon-6 oligomers terminated with H2N– and HO– groups (H2N-PAn-OHs) via the ring-opening polymerization of ε-caprolactam with ethanolamine in the presence of water under the catalysis of H3PO3, and transformed the H2N-PAn-OHs into HO– terminated nylon-6 oligomers (HO-PAn-OHs). Several aliphatic segmented poly(amide urethane)s with short nylon-6 segments (s-PAUs) were prepared via transurethane polycondensation of the HO-PAn-OHs with BHCH. However, the obtained s-PAUs merely had good crystallization behavior and good mechanical properties under very narrow HO-PAn-OH[thin space (1/6-em)]:[thin space (1/6-em)]BHCH molar ratios. s-PAU synthesized at a HO-PAn-OH[thin space (1/6-em)]:[thin space (1/6-em)]BHCH molar ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]9 was the only TPU with good crystallization and good mechanical properties. s-PAUs synthesized at a HO-PAn-OH[thin space (1/6-em)]:[thin space (1/6-em)]BHCH molar ratio of 2[thin space (1/6-em)]:[thin space (1/6-em)]8 or 3[thin space (1/6-em)]:[thin space (1/6-em)]7 exhibited good toughness and high tensile strength, but did not crystallize well. s-PAUs synthesized at a HO-PAn-OH[thin space (1/6-em)]:[thin space (1/6-em)]BHCH molar ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]1 crystallized very well, but was very brittle. Meanwhile, synthesis of H2N-PAn-OHs and transformation of them into HO-PAn-OHs were time-consuming, which made the synthesis of s-PAUs too complicated.

Ethylene glycol oligomers (o-EGs), e.g. triethylene glycol (3EG) and tetraethylene glycol (4EG), are major co-products of ethylene glycol. They are mainly used as solvents or plasticizers in lacquer, dehydrating agents, agricultural formulation, and components in cosmetics. 3EG and 4EG are also used in synthesis of polyester polyols25,26 and in modification of polymers to increase their toughness.27 In this paper, we conducted melt transurethane polycondensation of BHCH with 3EG and 4EG, and prepared a series of aliphatic thermoplastic poly(ether urethane)s (PEUs) with high molecular weight. As PBHCH is a brittle TPU with good crystallization property and good tensile strength, introducing flexible ether linkages might improve the toughness of PEUs and increase their tensile strength further, with good crystallization properties remaining. The obtained PEUs were characterized by gel permeation chromatography (GPC), FT-IR and 1H NMR spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), wide angle X-ray scattering (WAXS), and tensile testing.

Experiment section

Materials

Ethylene carbonate, triethylene glycol and tetraethylene glycol were purchased from Alfa Aesar UK, and were used as received. 1,6-Hexanediamine was purchased from Shanghai Reagent Plant, China. SnCl2·2H2O was purchased from Beijing Shuanghuan Weiyi Reagent Co. Ltd., China. Other materials such as N,N′-dimethyl formamide (DMF) and dimethyl sulfoxide (DMSO) were all obtained as reagent grade and used directly. BHCH was prepared from the reaction of ethylene carbonate with 1,6-hexanediamine according to literatures.19,24 Its melting point was 94 °C.

FT-IR (KBr) of BHCH: 3328.9, 2946.4, 2860.7, 1687.6, 1535.1, 1459.6, 1340.8, 1267.8, 1220.5, 1140.3, 1085.3, 1055.5, 895.2, 778.3, 735.6, 635.6.

Synthesis of PEUs

5.84 g (0.02 mol) BHCH, 3.00 g (0.02 mol) 3EG and 17 mg SnCl2 (0.19 mol%) were added to a 100 mL 3-necked flask equipped with a distilling adapter, a condenser, a receiver, and a mechanical stirrer. The mixture was stirred at 170 °C for 2 h under a nitrogen atmosphere and was reacted under a reduced pressure to 30 mm Hg for another 4 h. Then, the pressure in the flask was gradually reduced to 2 mm Hg and maintained for different periods until no further increase of viscosity was observed (Weissenberg effect took place, i.e. the polymer climbed onto the stirrer blades). The obtained faint polymer was poured out and cooled at room temperature. It was designated as PEU-1.

Similarly, PEU-2, PEU-3 and PEU-4 were synthesized at a BHCH/3EG molar ratio of 3[thin space (1/6-em)]:[thin space (1/6-em)]1, 5[thin space (1/6-em)]:[thin space (1/6-em)]1 and 9[thin space (1/6-em)]:[thin space (1/6-em)]1, respectively. PEU-5 and PEU-6 were synthesized at a BHCH/4EG molar ratio of 5[thin space (1/6-em)]:[thin space (1/6-em)]1 and 9[thin space (1/6-em)]:[thin space (1/6-em)]1.

Synthesis of PBHCH

5.84 g BHCH and 0.26 mol% SnCl2 were added to a 100 mL 3-necked flask. Polycondensation of them was conducted in a similar way to PEU-1. The PBHCH obtained was poured out and cooled at room temperature.

Characterization

Mn, Mw and molecular weight distribution of PEUs and PBHCH were determined by gel permeation chromatography using Waters Agilent-2600 Series equipped with PLgel 5 μm 1000 Å column and a refractive index detector at 25 °C. DMF was used as the eluent with a flow rate of 1 mL min−1 and polystyrene (PS) was used as standards.

Polymer samples for IR and 1H-NMR characterization were purified thrice through dissolving-precipitation cycles using DMSO as the solvent and ether as the nonsolvent. FT-IR spectra were acquired on a NICOLET 60SXB FTIR spectrometer. 1H-NMR spectra were obtained in deuterated DMSO (DMSO-d6) on a Bruker 400 AVANCE using tetramethylsilane as the internal standard.

Wide angle X-ray scattering measurements were taken on a Rigaku D/Max 2500 VB2+/PC diffractometer with Cu Kα radiation (wavelength: 0.154 nm). Samples were continuously scanned over the 2θ range from 5° to 50°.

The second heating DSC curves of polymers were measured with a TA Q200 differential scanning calorimeter in N2 atmosphere. Samples were first heated from room temperature to 200 °C at a heating rate of 60 °C min−1 and maintained for 5 min to eliminate thermal history. The samples were then cooled to −80 °C at a rate of 40 °C min−1. In the second heating scans, the samples were heated from −80 °C to 200 °C at a rate of 10 °C min−1. Thermogravimetric analysis was performed on a TGA Q50 analyzer with a heating rate of 10 °C min−1 from 25 °C to 500 °C in N2 atmosphere.

Polymer films (50 mm × 50 mm × 1 mm) for tensile tests were prepared by using a 70911-24B powder press machine (Tianjin New Technical Instrument Co. Ltd., China). Samples were heated to temperature 20 °C higher than their melting point, maintained for 5 minutes under 15 MPa, naturally cooled to room temperature with the pressure unchanged, and then cut into dumbbell-shaped bars (50 mm × 4 mm × 1 mm). Mechanical property analysis was conducted on a LLOYD LR30K tensile testing machine at a crosshead speed of 20 mm min−1 at 25 °C. Five measurements were performed for each sample, and the results were averaged to obtain a mean value.

Results and discussion

Synthesis of PEUs

Melt transurethane polycondensation of BHCH with 3EG or 4EG was conducted at 170 °C, and a series of PEUs were prepared. High vacuum up to 1 mm Hg favored the removal of ethylene glycol formed, and led to the increase of molecular weight. PEUs from PEU-1 to PEU-4 were synthesized at a BHCH/3EG molar ratio from 1[thin space (1/6-em)]:[thin space (1/6-em)]1 to 9[thin space (1/6-em)]:[thin space (1/6-em)]1, and PEU-5 and PEU-6 were synthesized at a BHCH/4EG molar ratio of 5[thin space (1/6-em)]:[thin space (1/6-em)]1 or 9[thin space (1/6-em)]:[thin space (1/6-em)]1. Self-polycondensation of BHCH and co-polycondensation with 3EG or 4EG concurrently took place during the synthesis process. Self-polycondensation of BHCH formed short PBHCH segments or ethylene hexanediurethane (EHDU) segments. Co-polycondensation with 3EG or 4EG formed hexanediurethane-polyethylene glycol (HDU-co-PEG) segments and increased the flexibility of polyurethanes (Scheme 1). Meanwhile, PBHCH was synthesized as a control to compare the thermal and mechanical properties with PEUs. High molecular weight PBHCH was also prepared at the same conditions without addition of 3EG or 4EG. Their properties were compiled in Table 1. PBHCH and PEUs with Mn above 30[thin space (1/6-em)]900 and Mw above 54[thin space (1/6-em)]300 were prepared.
image file: c4ra05880a-s1.tif
Scheme 1 PEUs prepared from BHCH and 3EG or 4EG.
Table 1 Synthesis of PEUs and PBHCH
PEUsa o-EG BHCH[thin space (1/6-em)]:[thin space (1/6-em)]o-EG (molar ratio) SnCl2b (mol%) Reaction timec (h) GPCd
Time (h) Reduced pressure (mm Hg) Mn (g mol−1) Mw (g mol−1) Mw/Mn
a Reaction conditions for PEUs: reaction temperature: 170 °C; reaction time: 760 mm Hg, 2 h; 30 mm Hg, 4 h.b SnCl2 was notated with respect to the total mole of monomers.c Reaction time under reduced pressure of 1, 2 or 3 mm Hg.d GPC data detected in DMF with PS as the standard.
PEU-1 3EG 1[thin space (1/6-em)]:[thin space (1/6-em)]1 0.19 12 2 37[thin space (1/6-em)]100 61[thin space (1/6-em)]900 1.67
PEU-2 3[thin space (1/6-em)]:[thin space (1/6-em)]1 0.23 8.5 1 50[thin space (1/6-em)]900 93[thin space (1/6-em)]900 1.84
PEU-3 5[thin space (1/6-em)]:[thin space (1/6-em)]1 0.24 12.5 2 36[thin space (1/6-em)]800 75[thin space (1/6-em)]900 2.06
PEU-4 9[thin space (1/6-em)]:[thin space (1/6-em)]1 0.25 8 1 46[thin space (1/6-em)]400 59[thin space (1/6-em)]900 1.29
PEU-5 4EG 5[thin space (1/6-em)]:[thin space (1/6-em)]1 0.25 11 2 32[thin space (1/6-em)]000 55[thin space (1/6-em)]800 1.74
PEU-6 9[thin space (1/6-em)]:[thin space (1/6-em)]1 0.25 16 3 36[thin space (1/6-em)]300 54[thin space (1/6-em)]300 1.50
PBHCH — — 0.26 10 2 30[thin space (1/6-em)]900 62[thin space (1/6-em)]700 2.03

IR and 1H NMR characterization

The obtained PEUs and PBHCH were characterized by FT-IR and 1H NMR spectra. Fig. 1 shows the FT-IR spectrum of PBHCH. The major vibration peaks are designated on Fig. 1. Absorption peaks corresponding to the EHDU segments and the C–O linkages of the terminal –CH2OH groups are clearly found. Fig. 2 shows the FT-IR spectrum of PEU-5. Besides the absorption peaks corresponding to the hexanediurethane (HDU) units derived from BHCH and the C–O linkages of the terminal –CH2OH groups, a shoulder peak at 1105.1 cm−1 corresponding to C–O–C linkages in the PEG segments is also found in Fig. 2. PEU-5 is constructed with these short PEG segments and HDU units.
image file: c4ra05880a-f1.tif
Fig. 1 FT-IR spectrum of PBHCH.

image file: c4ra05880a-f2.tif
Fig. 2 FT-IR spectrum of PEU-5.

Fig. 3 shows the 1H NMR spectrum of PEU-5 as well as the chemical shifts of different hydrogen atoms. Signals are assigned according to the literatures.19,20,28 The figure clearly shows the signals that correspond to the EHDU units and the HDU-co-PEG segments, which were formed from the self-polycondensation of BHCH and its co-polycondensation with 4EG, respectively. The 1H NMR spectrum verifies the structure of PEU-5. PEU-5 has –HNCOOCH2CH2OH terminal groups remaining. Meanwhile, some urea units were formed during synthesis reaction. Tail-biting side reaction of the –HNCOOCH2CH2OH terminal groups is the reason which leads to the formation of urea units (Scheme 2).

image file: c4ra05880a-u1.tif


image file: c4ra05880a-f3.tif
Fig. 3 1H-NMR spectrum of PEU-5.

image file: c4ra05880a-s2.tif
Scheme 2 Formation of urea units in polycondensation of BHCH with 3EG or 4EG.

WAXS characterization

Fig. 4 shows the WAXS patterns of PEUs and PBHCH. They are all crystallized polymers. PBHCH shows a series of diffraction peaks at 2θ of 6.61°, 19.68°, 21.12°, 22.35°, 24.12° and 26.34°, respectively (Table 2). PEUs crystallized in the same manner as PBHCH. EHDU and HDU-co-PEG segments are all linear structural units. Easy chain folding and regular arrangement between polymeric chains of PEUs or PBHCH make them crystallized well in solid state. Strong intermolecular hydrogen bonding formed from mono-EHDU segments (a) and multi-EHDU or short PBHCH segments (b) are the major crystallization source (Scheme 3). PEU-1 crystallized because of the intermolecular interaction between mono-EHDU segments, and PBHCH or PEUs from PEU-2 to PEU-6 crystallized due to the intermolecular interaction between short PBHCH segments. PEU-1, PEU-2, PEU-3 and PEU-4 were synthesized at a BHCH/3EG molar ratio from 1[thin space (1/6-em)]:[thin space (1/6-em)]1 to 9[thin space (1/6-em)]:[thin space (1/6-em)]1. As the BHCH/3EG molar ratio increased, the PBHCH segments in PEUs became longer. The crystallinity of PEUs increased (Table 3), because the intermolecular interaction between short PBHCH segments became stronger. PEU-4 shows similar crystallinity to PBHCH, because PEU-4 has relative long PBHCH segments in its main chains. Although PEU-1 was synthesized at a BHCH/3EG molar ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]1, in which the PEG or 3EG segments were directly connected by HDU units formed from BHCH, PEU-1 still shows similar crystallization characteristics to those of PBHCH. PEU-5 and PEU-6 were synthesized at a BHCH/4EG molar ratio of 5[thin space (1/6-em)]:[thin space (1/6-em)]1 or 9[thin space (1/6-em)]:[thin space (1/6-em)]1. Short PBHCH segments were also formed in their main chains. PEU-6 shows obviously lower crystallinity than PEU-4 (Table 3), because the former was synthesized from a longer 4EG. Longer PEG or 4EG segments were introduced in the main chains of PEU-6, resulted in the decrease of regularity in its main chains and the lower crystallinity.
image file: c4ra05880a-f4.tif
Fig. 4 X-ray diffraction diagrams of PEUs and PBHCH.
Table 2 The layer spacing related to different 2θ
PEUs 2θ da (10−10 m)
a d: layer spacing.
PEU-1 19.53 4.54
21.08 4.21
22.48 3.95
PEU-2 7.29 12.12
19.48 4.55
21.12 4.20
23.47 3.79
PEU-3 6.98 12.65
19.40 4.57
21.32 4.16
22.41 3.96
24.39 3.64
PEU-4 6.37 13.87
19.50 4.55
21.01 4.22
22.11 4.02
24.05 3.70
26.03 3.42
PEU-5 7.46 11.84
19.34 4.59
21.05 4.21
22.31 3.98
24.43 3.61
PEU-6 7.05 12.53
19.51 4.55
21.33 4.16
22.34 3.98
24.50 3.63
PBHCH 6.61 13.36
19.68 4.51
21.12 4.20
22.35 3.98
24.12 3.69
26.34 3.38


image file: c4ra05880a-s3.tif
Scheme 3 Intermolecular hydrogen bonding formed from mono-EHDU segments (a) and from multi-EHDU or short PBHCH segments (b).
Table 3 Second heating DSC data of PBHCH and PEUs (heating rate: 10 °C min−1)
PEUs Tg (°C) Tc (°C) ΔHc (J g−1) Tm (°C) ΔHm (J g−1) Xca (%)
a The degree of crystallinity was determined by X-ray method.
PEU-1 12.2 79.8 31.29 103.7 17.41 36.50
PEU-2 22.9 85.3 25.39 119.2 23.63 68.43
PEU-3 28.2 82.3 36.88 143.9 30.48 73.57
PEU-4 27.6 81.2 35.06 150.6 41.18 78.26
PEU-5 11.2 63.4 28.68 121.4 29.46 41.02
PEU-6 25.7 80.3 38.09 148.8 38.60 66.41
PBHCH 25.6 81.8 34.46 152.3 46.56 83.60

Different layer spacing related to structural feathers was calculated from Bragg eqn (1).

 
nλ = 2d[thin space (1/6-em)]sin[thin space (1/6-em)]θ (1)

In which n is an integral number (1, 2, 3…). The λ, d, and θ represent the wavelength of X-ray, the layer spacing, and the angle of incidence, respectively. Table 2 shows the layer spacing related to different 2θ. Intermolecular spacing occurred at 2θ of about 22.4°, 24.1° and 26.3°, which related to the chain folding. As the BHCH/3EG molar ratio decreased from 9[thin space (1/6-em)]:[thin space (1/6-em)]1 to 1[thin space (1/6-em)]:[thin space (1/6-em)]1, the spacing or d at a 2θ of about 24.1° and 26.3° gradually disappeared, and the d at a 2θ of about 22.4° decreased slightly except PEU-2, maybe because peaks at 22.4° and 24.1° of PEU-2 was combined into a single peak at 23.47°. The reason may be because the crystallizable PBHCH segments became shorter from PEU-4, PEU-3, PEU-2, to PEU-1. This phenomenon means that the chain-folded thickness lowered from PBHCH or PEU-4, PEU-3, PEU-2, to PEU-1. Similar trend is also found between PEU-6 and PEU-5. The intra-molecular spacing occurred at 2θ of about 19.6° and 21.1°.29 They are less influenced with the change of BHCH/o-EG molar ratios. The spacing related to orientation occurred at a 2θ in the range from 6.61° to 7.46°. From PBHCH or PEU-4, PEU-3, PEU-2, to PEU-1, as the BHCH/3EG molar ratio decreased, the spacing decreased gradually to disappearance. Orientation became worse also because the length of the PBHCH segments decreased. Similar trend is also found from PEU-6 to PEU-5.

DSC characterization

Fig. 5 shows the second heating DSC curves of PEUs and PBHCH. Glass transition temperature (Tg), crystallization temperature (Tc), enthalpy of crystallization (ΔHc), melting temperature (Tm) and enthalpy of melting (ΔHm) of them are listed in Table 3. The PBHCH has a Tg at 25.6 °C and a Tm at 152.3 °C. Tg increased with the increase of amide content from PEU-1 to PEU-4. As amide groups in the carbamate groups of PEUs exhibit higher rigidity and easily form hydrogen bonds, increases in amide content increase the Tg. The ether linkages merely act as flexible units. PBHCH has obviously higher Tm than BHCH (94 °C). PBHCH contains many EHDU segments that connected each other. Strong intermolecular hydrogen bonding is easily formed in solid PBHCH. BHCH contains only one EHDU segment with two HO– terminal groups. Intermolecular hydrogen bonding of BHCH is obvious weak. Terminal HO– groups further discount the crystallization of BHCH. Thus, the Tm of PBHCH is obviously higher than that of BHCH. PBHCH is a semi-crystallized thermoplastic polyurethane. If a polymer crystallizes very fast, the Tc will not emerge because crystallization has already finished before the second heating. A Tc at 81.8 °C means that PBHCH did not crystallize fast enough to complete crystallization immediately after the rapid cooling period before the second heating started. Crystallization still progressed in the second heating DSC scan. PEUs show similar crystallization behavior to that of PBHCH. They show Tm ranging from 103.7 °C to 150.6 °C. PEUs have lower Tm and ΔHm or crystallinity than PBHCH, because PEG segments have different structure from PBHCH, and introduction of short PEG segments lowers the regularity and crystallinity of PEUs (Table 3). PEU-1, PEU-2, PEU-3, and PEU-4 were synthesized at a BHCH/3EG molar ratio from 1[thin space (1/6-em)]:[thin space (1/6-em)]1 to 9[thin space (1/6-em)]:[thin space (1/6-em)]1. As the BHCH/3EG molar ratio increased, the PBHCH segments in PEUs became longer, with Tm, ΔHm and crystallinity increased. Similar trend is also observed between PEU-5 and PEU-6, which were synthesized at a BHCH/4EG molar ratio of 5[thin space (1/6-em)]:[thin space (1/6-em)]1 and 9[thin space (1/6-em)]:[thin space (1/6-em)]1, respectively. PEU-4 or PEU-3 shows higher Tm and ΔHm than PEU-6 or PEU-5, because the latter contains longer PEG segments in the main chains and leads to more decrease of crystallinity. Thus, PEU-6 or PEU-5 shows lower Tm and ΔHm than the corresponding PEU-4 or PEU-3. PEUs also show a Tc, which means that they did not crystallize fast enough to complete crystallization immediately after the rapid cooling period before the second heating started. Crystallization still progressed in the second heating DSC scans.
image file: c4ra05880a-f5.tif
Fig. 5 Second heating DSC scans of PEUs and PBHCH (heating rate = 10 °C min−1).

TGA characterization

The thermal stability of PBHCH and PEUs was characterized using TGA, as shown in Fig. 6. The 5% mass decomposition temperature (T5), 50% mass decomposition temperature (T50), and end mass loss are summarized in Table 4. PBHCH exhibits a T5 of 262.8 °C, and PEUs exhibit T5 between 237.4 °C and 268.2 °C. PEUs have lower T50 than PBHCH maybe because PEG segments are also low thermal stable units. PEUs all show T5 above 237.4 °C, which is not too high because carbamate and ether linkages are only moderately thermally stable. Even so, PEUs have T5 several tens degree higher than their Tm. They are thermoplastics and are still suitable for processing with normal thermal processing machines.
image file: c4ra05880a-f6.tif
Fig. 6 TGA curves of PBHCH and PEUs (heating rate: 10 °C min−1; atmosphere: N2).
Table 4 TGA (heating rate: 10 °C min−1 under N2) and tensile testing data of PBHCH and PEUs
Sample T5 (°C) T50 (°C) End mass loss (%) Tensile strength (MPa) Strain at break (%)
PEU-1 268.6 332.7 100.0 14.08 35.76
PEU-2 256.2 338.6 99.1 24.44 92.00
PEU-3 245.8 330.0 96.2 32.44 78.04
PEU-4 240.2 322.4 97.6 13.45 5.14
PEU-5 240.6 316.4 96.8 32.82 151.82
PEU-6 237.4 315.5 99.1 15.76 5.56
PBHCH 262.8 356.4 96.7 28.29 3.40

Tensile testing

Tensile testing of PBHCH and PEUs was conducted, and the stress–strain curves are shown in Fig. 7. Their tensile strength and strain at break are listed in Table 4. PBHCH has tensile strength of 28.29 MPa and strain at break of 3.40%. It has high tensile strength but is very brittle. PEU-1 shows tensile strength of 14.08 MPa and strain at break of 35.76%. PEU-1 has lower tensile strength than PBHCH maybe because the former was synthesized at a BHCH/3EG molar ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]1 and only HDU units derived from BHCH connected 3EG segments. Lack of short PBHCH segments and lower crystallinity or ΔHm (Table 3) make PEU-1 had lower tensile strength. Introduction of 3EG segments increased the flexibility, thus PEU-1 exhibits higher strain at break. PEU-2 and PEU-3 were synthesized at a BHCH/3EG molar ratio of 3[thin space (1/6-em)]:[thin space (1/6-em)]1 and 5[thin space (1/6-em)]:[thin space (1/6-em)]1, respectively. As the length of short PBHCH segments in the main chains increases, the intermolecular interaction between the multi-HDU units increases. Thus, tensile strength of PEUs increases. PEU-2 and PEU-3 all have good toughness. PEU-3 exhibits tensile strength of 32.44 MPa with a strain at break of 78.04%. PEU-4 just shows tensile strength of 13.45 MPa and strain at break of 5.14%. As PEU-4 was synthesized at a BHCH/3EG molar ratio of 9[thin space (1/6-em)]:[thin space (1/6-em)]1, crystallizable and rigid phase was formed from short PBHCH segments in its main chains. The 3EG segments introduced only forms a minor phase, which is not too high to improve the toughness of the rigid PBHCH phase. Thus, PEU-4 shows low tensile strength and low strain at break. Appropriate flexibility is essential to polymer materials allowing them to exhibit good mechanical properties. PEU-2 and PEU-3 have appropriate length of PBHCH segments and 3EG amount, so they have good tensile strength (24.44 and 32.44 MPa, respectively). PEU-5 was synthesized at a BHCH/4EG molar ratio of 5[thin space (1/6-em)]:[thin space (1/6-em)]1. It shows tensile strength of 32.82 MPa and strain at break of 151.82%. Introducing longer and more flexible 4EG segments lowers the crystallinity of PEU-5 and increases its toughness. Thus, PEU-5 exhibits higher tensile strength and strain at break than PEU-3. Similarly, PEU-6 shows higher tensile strength and strain at break than PEU-4, because the former contains longer 4EG segments. The 4EG segments introduced also form a minor component, which leads to PEU-6 as a brittle polymer with low tensile strength. Fig. 8 shows the influence of molar percentage of HDU units (mol% HDU) in PEUs on the stress, strain at break and modulus from PEU-1 to PEU-4. In a range from 75 mol% HDU to 85 mol% HDU, the PEUs like PEU-2 and PEU-3 exhibit good tensile strength, strain at break and modulus. PEUs with good tensile strength and toughness were prepared.
image file: c4ra05880a-f7.tif
Fig. 7 Stress–strain curves of PBHCH and PEUs.

image file: c4ra05880a-f8.tif
Fig. 8 Influence of mol% HDU on the stress, strain at break and modulus of PEUs from PEU-1 to PEU-4.

Conclusion

Two ethylene glycol oligomers, i.e. 3EG and 4EG, were copolymerized with a diurethanediol, BHCH, through melt transurethane polycondensation, and a series of poly(ether urethane)s, PEUs, were prepared. The PEUs exhibited Mn above 32[thin space (1/6-em)]000, Mw above 54[thin space (1/6-em)]300, Tm ranging from 103.7 °C to 150.6 °C, and initial decomposition temperature over 237.4 °C. Their tensile strength reached 32.82 MPa with a strain at break of 151.82%. A simple non-isocyanate route was established for synthesizing high molecular weight aliphatic thermoplastic polyurethanes with good thermal and mechanical properties in a wide range of BHCH[thin space (1/6-em)]:[thin space (1/6-em)]o-EG molar ratios. These new non-isocyanate TPUs can be used as plastic films and fibers in routine life, as well as medical materials because of their good safety compared to the existing TPUs prepared through isocyanate route.

Acknowledgements

The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant no. 21244006 and 50873013).

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