Crystallizable and Tough Aliphatic Thermoplastic Polyureas

Article pubs.acs.org/IECR

Crystallizable and Tough Aliphatic Thermoplastic Polyureas Synthesized through a Nonisocyanate Route Suqing Li, Zhihui Sang, 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 S Supporting Information *

ABSTRACT: A simple nonisocyanate route for synthesizing crystallizable and tough aliphatic thermoplastic polyureas (TPUreas) is described. Melt transurethane polycondensation of diethylene glycol bis(3-aminopropyl) ether with bis(hydroxyethyl) hexanediurethane and bis(hydroxyethyl) isophoronediurethane was conducted at 170 °C under a reduced pressure of 3 mmHg, and a series of TPUreas were prepared. The TPUreas were characterized by GPC, FT-IR, 1H NMR, 13C NMR, 2D 13C−1H HSQC NMR, DEPT-135 13C NMR, DSC, TGA, wide-angle X-ray scattering, atomic force microscopy, and tensile tests. The TPUreas exhibited an Mn up to 12 000 g/mol, an Mw up to 17 600 g/mol, Tg between 2.8 and 18.1 °C, Tm from 140.5 to 149.8 °C, initial decomposition temperature of over 278.4 °C, and tensile strength up to 37.81 MPa with elongation at break of 691.25%. TPUreas with high Tm, good tensile strength, and good toughness were prepared through a nonisocyanate route.

1. INTRODUCTION Polyurethanes (PUs) including polyurethane-ureas (PUUs) and polyureas (PUreas) are important commercial polymers that are widely used as foams, fibers, adhesives, coatings, and medical materials because of their outstanding mechanical properties, abrasion resistance, and good biocompatibility.1 PUUs are currently prepared from diisocyanates and polyols with diamines as chain extenders.2−7 PUreas are prepared from diisocyanates and polyamines and are usually used as twocomponent systems in the protective coating/lining industry.8−11 PUUs and PUreas normally exhibit better mechanical properties than common PUs because urea groups provide additional hydrogen bonds. Meanwhile, phosphorus-containing monomers can be used to synthesize phosphorus-containing PUreas with improved flame-retarding properties.12 Diisocyanates are currently prepared from extremely toxic phosgene. Diisocyanates also show high toxicity. They are harmful to the human body during the production and application of PUs including PUUs and PUreas. Nonisocyanate polyurethanes (NIPUs), a green and sustainable route to synthesize PUs, attract more and more attention from the chemists working in the PU field.13−15 In recent decades, many investigations have been reported on the cross-linked NIPUs prepared from cyclic carbonates and diamines or multiamines.16−23 Loontjens et al. also reported NIPUs prepared from carbonyl biscaprolactamate.24−26 Transurethane polycondensation of diurethanes has been developed to synthesize nonisocyanate thermoplastic polyurethanes (TPUs). This kind of TPU includes amorphous27,28 or crystallizable29,30 polyurethanes, alternating31−33 or segmented34 poly(amide urethane)s, segmented poly(ether amide urethane)s,35 segmented poly(ether urethane)s,36,37 and the thermoplastic elastomers.38 Nonisocyanate thermoplastic polyureas (TPUreas) were also synthesized from © XXXX American Chemical Society

solution polycondensation of the dimethyl dicarbamates containing multiurea segments, with amino-terminated poly(propylene glycol).39 Meanwhile, Tan et al. synthesized phosphorus-containing nonisocyanate TPUreas from solution polycondensation of a phosphorus-containing dicarbamate with aliphatic diamines.40 Dai et al. synthesized nonisocyanate TPUreas through solution and solution-free polycondensation of diamines with diphenyl carbonate.41−43 In our previous work, we established a simple method for synthesizing nonisocyanate thermoplastic PUUs (TPUUs) and TPUreas directly from melt polycondensation of diamines with simple diurethanediols.44 We found that most of the TPUUs and TPUreas are amorphous polymers. Merely a crystallizable TPUrea was synthesized from the copolycondensation of diethylene glycol bis(3-aminopropyl) ether (DGBAE) with bis(hydroxyethyl) piperazinediurethane. The TPUrea merely exhibited a low melting temperature (Tm) at 77.7 °C and a low tensile strength of 6.46 MPa with an elongation at break of 180.20%. This TPUrea is hardly used practically because of its low Tm and poor mechanical properties. Meanwhile, we found that when melt transurethane polycondensation of DGBAE with bis(hydroxyethyl) hexanediurethane (BHHDU) was conducted, a solid TPUrea was obtained. The TPUrea did not melt up to 220 °C and was not soluble in organic solvents including N,N′-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO), maybe because of its high Tm and high regularity of main chains. In this paper, direct melt transurethane polycondensation of DGBAE with BHHDU and bis(hydroxyethyl) isophoronediurethane (BHIDU) was conReceived: October 29, 2015 Revised: February 1, 2016 Accepted: February 4, 2016

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DOI: 10.1021/acs.iecr.5b04083 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

3. RESULTS AND DISCUSSION 3.1. Synthesis of TPUreas. TPUreas were prepared through a melt transurethane polycondensation of DGBAE

ducted, and a series of TPUreas were prepared. The copolycondensation of DGBAE with BHHDU is to introduce regular and crystallizable polyurea segments. The copolycondensation of DGBAE with BHIDU is to introduce irregular polyurea segments, which prevent the formation of too long crystallizable polyurea segments. The aim of this paper is to prepare TPUreas with proper Tm and good mechanical properties. A simple method may be established to synthesize crystallizable and tough nonisocyanate TPUreas, which might replace common isocyanate TPUs that are used widely in many fields.

Scheme 1. Synthesis Reaction of TPUreas from DGBAE, BHHDU, and BHIDU

2. EXPERIMENTAL SECTION 2.1. Materials. DGBAE was purchased from TCI Japan. Other materials, such as DMF and DMSO, were of reagent grade and used directly. BHHDU and BHIDU were prepared from the reaction of ethylene carbonate with 1,6-hexanediamine29,34 or isophorone diamine.38,45 2.2. Synthesis of TPUreas. A total of 2.34 g (0.008 mol) of BHHDU, 2.77 g (0.008 mol) of BHIDU, and 3.52 g (0.016 mol) of DGBAE were added to a 100 mL three-necked flask and mechanically stirred at 170 °C under a N2 atmosphere. The reaction was maintained at a reduced pressure of 30 mmHg for 0.5 h and at 3 mmHg for 3.3 h, until the Weissenberg effect took place.36,46 A total of 6.73 g (77.98%) TPUrea-1 was formed, with 1.90 g (22.02%) of byproduct ethylene glycol collected. Similarly, TPUrea-2, TPUrea-3, TPUrea-4, and TPUrea-5 were prepared from BHHDU, BHIDU, and DGBAE at a BHHDU/BHIDU/DGBAE molar ratio of 3:1:4, 5:1:6, 7:1:8, and 9:1:10, respectively. 2.3. Characterization. The Mn, Mw, and polydispersity index (PDI) of TPUreas were determined via gel permeation chromatography (GPC) on an Agilent-2600 system (PLgel 5 μm 1000 Å column, refractive index detector) at 25 °C, with DMF containing 10 mM LiBr as the eluent (flow rate: 1 mL/ min) and polystyrene (PS) as the standard. TPUreas were purified before FT-IR and NMR characterization.44 FT-IR spectra were acquired on a NICOLET 60SXB FTIR spectrometer. 1H NMR spectra were obtained in deuterated DMSO (d6-DMSO) on a Bruker 400 AVANCE with tetramethylsilane as the internal label. 13C NMR, 2D 13 C−1H HSQC NMR, and DEPT-135 13C NMR spectra were detected on a Bruker AV600 in d6-DMSO at 25 °C. The second heating and cooling differential scanning calorimetric (DSC) curves of TPUreas were measured similarly as described in the literature47on a TA Q200 differential scanning calorimeter in a N2 atmosphere. Thermogravimetric analysis (TGA) was conducted on a TGA Q50 analyzer at a heating rate of 10 °C/min in N2. Wide angle X-ray scattering (WAXS) measurements were taken on a Rigaku D/Max 2500 VB2+/PC diffractometer with Cu Kα radiation. Tapping mode atomic force microscopy (AFM) was performed on a Dimension FastScan Atomic Force Microscope. The sample was first dissolved in DMF (1 wt %); then, drops of the solution were spun on a silicon plate of 2 cm × 2 cm via a Spin coater WS-400B-6NPP-LITE at 2000 rpm for 130 s. The spin-coated films were annealed in a vacuum oven from their Tm to a crystallization temperature of 60 °C. Polymer bars (50 mm × 4 mm × 1 mm) used for tensile testing were prepared by a melt-press method.47 Mechanical analysis was conducted on a LLOYD LR30K tensile testing machine at a crosshead speed of 200 mm/min at 25 °C.

with BHHDU and BHIDU at 170 °C under a reduced pressure of 3 mmHg (Scheme 1), with byproduct ethylene glycol distilled out. The synthesis reaction was conducted at a temperature no higher than 170 °C due to less thermal stability of BHHDU. Temperatures higher than 170 °C lead to obvious thermal decomposition of BHHDU30,34 and impede the copolycondensation between DGBAE and BHHDU. Meanwhile, lower reaction temperatures than 160 °C often lead to longer reaction time. TPUreas were synthesized at a BHHDU/ BHIDU/DGBAE molar ratios from 1:1:2 to 9:1:10. Copolycondensation of DGBAE with BHHDU formed DGBAE hexanediurea (−DGBAE−HDU−) segments, and copolycondensation of DGBAE with BHIDU formed DGBAE isophoronediurea (−DGBAE−IPDU−) segments. The −DGBAE− HDU− segments were introduced as crystallizable polyurea segments because of their linear and regular structural units. The −DGBAE−IPDU− segments were introduced as irregular polyurea segments because BHIDU has a trimethyl-substituted cyclic and irregular structural unit. The irregular −DGBAE− IPDU− polyurea segments prevent the copolycondensation of DGBAE with BHHDU from the formation of too long regular and crystallizable −DGBAE−HDU− segments. TPUrea with a Tm below 200 °C is proper, because too high Tm and processing temperature causes severe thermal decomposition of polyureas. The properties of TPUreas are compiled in Table 1. These TPUreas are soluble in high polar solvents such as DMF and DMSO. They exhibit an Mn ranging from 8200 g/mol to 12 000 g/mol and an Mw ranging from 9400 g/mol to 17 600 g/mol. The synthesis reaction of TPUreas was conducted at a reduced pressure to 3 mmHg until the Weissenberg effect took place. As TPUreas contain many urea linkages, many hydrogen bonds are formed between different macromolecular chains. This effect makes the TPUreas show relatively high viscosity, and easy to climb onto the stirrer blades; i.e., the Weissenberg effect takes place easily. Thus, TPUreas with relatively low Mn and Mw were obtained. As polymers with low molecular weights usually show low PDIs, these TPUreas exhibit low PDI values due to their relatively low molecular weight. Meanwhile, sufficient stirring also leads to low PDIs. TPUreas with a higher content of regular −DGBAE−HDU− segments also form hydrogen bonds easier and show higher viscosity, which leads to the Weissenberg effect taking place easier. Thus, TPUreas B


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Industrial & Engineering Chemistry Research Table 1. Synthesis and Tensile Testing Data of TPUreas TPUreasa TPUrea-1 TPUrea −2 TPUrea-3 TPUrea-4 TPUrea-5

BHHDU/BHIDU/DGBAE (molar ratio)

reaction timeb (h)

Mnc (g mol−1) (GPC)

Mwc (g mol−1) (GPC)

PDIc (GPC)

tensile strength (MPa)

elongation at break (%)

1:1:2 3:1:4

3.3 4.5

12000 8800

17600 11600

1.47 1.32

37.81 21.83

691.25 275.22

5:1:6 7:1:8 9:1:10

5.0 6.0 5.0

12000 9200 8200

14400 11000 9400

1.20 1.20 1.15

30.99 18.45

474.66 462.75

Reaction conditions: 170 °C, 30 mmHg, 0.5 h. bReaction time under a reduced pressure of 3 mmHg. cGPC data detected in DMF containing 10 mM LiBr with PS as the standard.

a

Figure 1. FT-IR spectrum of TPUrea-1.

Figure 2. 1H NMR spectrum of TPUrea-1. C


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(Table 2). The residual urethane content is related to the molecular weight of TPUreas. TPUreas with lower Mn or Mw show higher residual urethane content, because they contain more HEIPU and HEU terminal groups. Figure 3 shows the 13C NMR spectrum of TPUrea-1 and the chemical shifts related to different carbon atoms. Correlation between different carbons and the hydrogens on them was clearly demonstrated on the 2D 13C−1H HSQC NMR spectrum (Figure 4). The DEPT-135 13C NMR spectrum in Figure 5 confirmed further the difference of carbons. Signals lying upward correspond to CH and CH3 groups, while signals corresponding to CH2 groups lie downward. Structures described in Figures 2 and 3 are fully verified by these NMR characterization. 3.3. WAXS Characterization. Figure 6 shows the WAXS curves of TPUreas. TPUrea-1, TPUrea-2, TPUrea-3, TPUrea-4, and TPUrea-5 show an obvious diffraction peak at 21.2°, 22.6°, 23.5°, 23.5°, or 23.3°, which is ascribed to the intermolecular spacing and is related to chain folding.36,48,49 The layer spacing related is calculated from Bragg eq 1. The calculated intermolecular spacing of TPUrea-1, TPUrea-2, TPUrea-3, TPUrea-4, and TPUrea-5 is 4.2, 3.9, 3.8, 3.8, and 3.8 Å, respectively. TPUreas from TPUrea-1 to TPUrea-5 were synthesized at a BHHDU/BHIDU/DGBAE molar ratio from 1:1:2 to 9:1:10. Chain folding becomes tight as the regular −DGBAE−HDU− composition increases and the irregular −DGBAE−IPDU− composition decreases. Thus, intermolecular spacing decreases from 4.2 Å to a steady value of 3.8 Å.

Table 2. Residual Content of the Urethane Groups in TPUreas

a

sample

A20

A24

A2

urethanea (%)

TPUrea-1 TPUrea-2 TPUrea-3 TPUrea-4 TPUrea-5

1.00 1.00 1.00 1.00 1.00

1.41 1.23 0.42 0.39 0.57

67.29 33.02 32.67 20.37 21.37

3.45 6.33 4.16 6.39 6.84

Urethane content: Urethane% =

A20 + A24 A2 + A20 + A24

with decreasing molecular weights and decreasing PDIs were prepared from TPUrea-1 to TPUrea-5. 3.2. FT-IR and NMR Characterization. Figure 1 shows the FT-IR spectrum of TPUrea-1 and the major absorption peaks. Absorption peaks corresponding to DGBAE segments and hexanediurea (HDU) or isophoronediurea (IPDU) units derived from BHHDU or BHIDU are clearly found. TPUrea1 was constructed with the DGBAE segments as well as the HDU and IPDU units. Meanwhile, some hydroxyethyl urethane (HEU) terminal groups are present in TPUrea-1. Figure 2 shows the 1H NMR spectrum of TPUrea-1 and the chemical shifts related to different hydrogen atoms. Signals are assigned based on the literature.34,36,38,44 The structure of TPUrea-1 is described in Figure 2. TPUrea-1 is mainly constructed with −DGBAE−HDU− segments and −DGBAE−IPDU− segments, which were formed from the copolycondensation of DGBAE with BHHDU and BHIDU, respectively. Some hydroxyethyl isophorone urethane (HEIPU) groups, HEU groups, and aminopropyl (AP) groups, which derived from BHIDU, BHHDU, and DGBAE, respectively, were left as terminal groups. The aminohexyl (AH) terminal groups might result from the decomposition of BHHDU due to its low thermal stability.30,36 The residual content of the urethane groups in TPUreas was detected

nλ = 2d sin θ

(1)

The crystallinity of TPUrea-1, TPUrea-2, TPUrea-3, TPUrea-4, and TPUrea-5 calculated based on the X-ray method is at 4.3%, 17.5%, 19.9%, 21.4%, and 25.8%, respectively (Table 3). TPUrea-1 is nearly an amorphous polyurea, because of its low crystallinity. TPUrea-1 was synthesized at a BHHDU/BHIDU/DGBAE molar ratio of

Figure 3. 13C NMR spectrum of TPUrea-1. D


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Figure 4. 2D 13C−1H HSQC NMR spectrum of TPUrea-1.

Figure 5. DEPT-135 13C NMR spectrum of TPUrea-1.

1:1:2. It contains nearly the same amount of regular or crystallizable −DGBAE−HDU− segments and irregular or amorphous −DGBAE−IPDU− segments. Thus, TPUrea-1 does not crystallize well and is nearly amorphous. TPUrea-2, TPUrea-3, TPUrea-4, and TPUrea-5 were synthesized at a higher BHHDU/BHIDU molar ratio. They are all crystallized

polyureas with higher crystallinity. As the BHHDU/BHIDU molar ratios increase from 3:1 to 9:1, their crystallinity increases, because longer regular and crystallizable −DGBAE−HDU− segments were formed, and the amount of irregular and amorphous −DGBAE−IPDU− segments E


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Figure 6. X-ray diffraction diagrams of TPUreas.

Table 3. Second Heating DSC and TGA Data of TPUreas (10 °C·min−1 under N2) sample TPUrea-1 TPUrea-2 TPUrea-3 TPUrea-4 TPUrea-5 TPUrea6b TPUrea7c

Tg/ °C 18.1 6.4 5.0 2.8 3.2

51.5

Tc/ °C

ΔHc/J· g−1

59.5 50.6

13.7 14.7

Tm/°C

ΔHm/J· g−1

140.5 141.5 149.5 149.8

17.6 17.4 19.2 17.1

Xca/%

Ti/°C

4.3 17.5 19.9 21.4 25.8 61.2

292.4 296.9 287.7 278.4 302.1 323.7

0.8

297.6

Figure 8. Cooling DSC scans of TPUreas (10 °C·min−1).

Table 4. DSC Data of TPUreas in the Cooling Scans (10 °C/ min)

a

The crystallinity was determined by the X-ray method. bTPUrea-6 is composed of 100% DGBAE-HDU segments.44 cTPUrea-7 is composed of 100% DGBAE-IPDU segments.44

sample

Tg (°C)

Tcca (°C)

TPUrea-1 TPUrea-2 TPUrea-3 TPUrea-4 TPUrea-5

11.8 −0.7 −0.1 −6.2 −7.8

61.5 115.1 118.2

Tcc: The cooling crystallization temperature.

a

HDU segments and 100% DGBAE−IPDU segments, respectively. They were synthesized as described in the literature.44 TPUrea-7 is an amorphous polyurea with a Tg at 51.5 °C. As TPUrea-1 was synthesized at a BHHDU/BHIDU/DGBAE molar ratio of 1:1:2, it is nearly an amorphous polyurea because TPUrea-1 contains a high amount of amorphous −DGBAE− IPDU− segments. As the BHHDU/BHIDU molar ratio increased from 3:1 to 9:1, the portion of the regular −DGBAE−HDU− segments increased, and that of the amorphous −DGBAE−IPDU− segments decreased. Semicrystallized TPUreas including TPUrea-2, TPUrea-3, TPUrea4, and TPUrea-5 were prepared. They exhibit a Tm between 140.5 and 149.8 °C. The Tm of TPUrea-6 cannot be detected, because TPUrea-6 is a solid polyurea which cannot melt up to 220 °C. TPUrea-2 and TPUrea-3 also show a Tc at 59.5 or 50.6 °C because they crystallized slowly and their crystallization did not finish after the cooling period. TPUrea-4 and TPUrea-5 do not show Tc because they crystallized well. Their crystallization had completed in the rapid cooling period. Figure 8 shows the cooling DSC scans of TPUreas. The cooling crystallization temperature (Tcc) and Tg are compiled in Table 4. TPUrea-1 is an amorphous polymer. TPUrea-2 does not show Tcc because of its low −DGBAE−HDU− amount. TPUreas from TPUrea-3 to TPUrea-5 show a Tcc which increases from 61.5 to 118.2 °C as the regular −DGBAE− HDU− amount increases. Their crystallization becomes better. Figure 9 shows the cooling (a) and the second heating (b) DSC scans of TPUrea-2 at different cooling rates in the cooling period. TPUrea-2 cannot crystallize in the cooling period at different cooling rates from 10 °C/min to 40 °C/min. But, TPUrea-2 shows a steady Tc at about 59 °C in the second

Figure 7. Second heating DSC scans of TPUreas detected in a heating (40 °C·min−1)−cooling (40 °C·min−1)−heating (10 °C·min−1) mode.

decreased. Semicrystallized TPUreas including TPUrea-2, TPUrea-3, TPUrea-4, and TPUrea-5 were prepared. 3.4. Thermal Properties. Figure 7 shows the second heating DSC curves of TPUreas detected in a heating− cooling−heating mode.47 The glass transition temperature (Tg), crystallization temperature (Tc), enthalpy of crystallization (ΔHc), Tm, and enthalpy of melting (ΔHm) of TPUreas are listed in Table 3. TPUreas exhibit a Tg from 51.5 to 2.8 °C. TPUrea-6 and TPUrea-7 are composed of 100% DGBAE− F


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Figure 9. Cooling (a) and second heating (b) DSC scans of TPUrea-2 at different cooling rates in the cooling period.

Figure 10. Cooling (a) and second heating (b) DSC scans of TPUrea-4 at different cooling rates in the cooling period.

Information), maybe because TPUrea-2 contains a lesser amount of regular −DGBAE−HDU− segments, which make TPUrea-2 crystallization difficult. Although the cooling rates are different, TPUrea-2 scarcely crystallizes in the cooling period. Crystallization merely occurs in the second heating scans. As a result, TPUrea-2 exhibits almost the same Tc, Tm, and ΔHm at different cooling rates in the cooling period. A similar phenomenon is observed for TPUrea-3, which also hardly crystallizes in the cooing period at different cooling rates from 20 °C/min to 40 °C/min and merely starts crystallization in the second heating period, with almost the same Tc, Tm, or ΔHm observed at different cooling rates. Figure 10 shows the cooling (a) and the second heating (b) DSC scans of TPUrea-4 at different cooling rates in the cooling period. Different from TPUrea-2 and TPUrea-3, TPUrea-4 merely crystallizes in the cooling period and shows a Tcc from 115.1 to 77.5 °C (Table S1 in the Supporting Information). As the cooling rate increases from 10 °C/min to 40 °C/min, the Tcc decreases. TPUrea-4 does not crystallize in the second heating period. A Tm from 157.8 to 149.5 °C is observed in the second heating period. Meanwhile, the cooling crystallization

Figure 11. TGA curves of TPUreas (10 °C·min−1; N2 atmosphere).

heating DSC scans, and its Tm and ΔHm are little influenced by different cooling rates (Table S1 in the Supporting G


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Tm. They show thermal stability high enough and are suitable to be processed via the common thermal processing method. 3.5. Morphological Behavior. The morphology of the annealed TPUreas was imaged using a tapping-mode AFM. Figure 12 shows the phase and 3D height images of TPUrea-2 and TPUrea-4, respectively. The bright regions in the phase image (a, c) correspond to hard or crystalline −DGBAE− HDU− phases. The dark regions correspond to soft or irregular −DGBAE−IPDU− phases.50 Microphase separation occurred in TPUrea-2 and TPUrea-4. TPUrea-2 exhibits a lower density of hard phases due to its lower amount of crystallizable −DGBAE−HDU− segments. TPUrea-4 exhibits a higher density of hard phases because of its higher amount of −DGBAE−HDU− segments. Increasing density of hard domains is formed when the amount of regular and crystallizable −DGBAE−HDU− segments increases. 3.6. Tensile Testing. Figure 13 shows the stress−strain curves of TPUreas, and the results are listed in Table 1. TPUrea-1 is an amorphous polyurea with a Tg of 18.1 °C. It is a strong and tough polymer with a tensile strength of 37.81 MPa and an elongation at break of 691.25%. TPUrea-2, TPUrea-3, and TPUrea-4 are semicrystallized polyureas with a Tm from 140.5 and 149.8 °C. They are all tough polymers with a tensile strength from 18.45 to 30.99 MPa and an elongation at break from 275.22% to 474.66%. TPUrea-2 and TPUrea-4 show lower tensile strength and elongation at break maybe because of their low molecular weight. As TPUrea-5 contains many bubbles, homogeneous TPUrea-5 film was not prepared. No tensile testing data of TPUrea-5 were given.

Figure 12. AFM images of TPUrea-2 (a, phase; b, 3D height) and TPUrea-4 (c, phase; d, 3D height).

4. CONCLUSION A H2N-terminated ethylene glycol oligomer, DGBAE, was copolymerized with two diurethanediols, a linear diurethanediol BHHDU and a trimethyl-substituted cyclic diurethanediol BHIDU, through melt transurethane polycondensation, and a series of thermoplastic polyureas, TPUreas, were prepared. Copolycondensation of DGBAE with BHHDU formed the linear and crystallizable polyurea segments, while copolycondensation of DGBAE with BHIDU formed the irregular and amorphous polyurea segments, which were introduced to prevent longsequence formation of the crystallizable polyurea segments. TPUreas with proper Tm and crystallization were prepared. The TPUreas exhibited Mn up to 12 000 g/mol, an Mw up to 18 000 g/mol, Tg between 2.8 and 18.1 °C, Tm ranging from 140.5 to 149.8 °C, initial decomposition temperature at over 278.4 °C, and tensile strength up to 37.81 MPa with elongation at break of 691.25%. Nonisocyanate aliphatic TPUreas with good thermal and mechanical properties were successfully prepared.

Figure 13. Stress−strain curves of TPUreas.

enthalpy (ΔHcc) in the cooling DSC scans and the ΔHm in the second heating DSC scans increase as the cooling rate increases. The reason may be that TPUrea-4 contains a higher amount of regular −DGBAE−HDU− segments, which make TPUrea-4 crystallize easier. A higher cooling rate in the cooling period may cause a bigger temperature difference and more nucleating centers to form, which lead to higher ΔHcc in the cooling scans and higher ΔHm in the second heating scans. At lower cooling rate, merely a lower portion of the easily crystallized −DGBAE−HDU− segments can crystallize. Thus, higher Tcc, higher Tm, lower ΔHcc, and lower ΔHm are observed. At a cooling rate of 40 °C/min, the largest ΔHm or crystallinity is exhibited. A similar trend is observed for TPUrea-5, because it also contains a high amount of regular −DGBAE−HDU− segments. Figure 11 shows the TGA curves of TPUreas, which exhibit initial decomposition temperatures (Ti) all above 278.4 °C (Table 3). TPUreas have a Ti nearly 130 °C higher than their

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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.iecr.5b04083. (Figure S1) ESI-MS spectrum of the concentrated filtrate in TPUrea-1 purification and (Table S1) cooling and second heating DSC data of TPUreas at different cooling rates (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: +8610-6443-4864. E-mail: [email protected]. H


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Industrial & Engineering Chemistry Research Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant Nos. 21244006 and 50873013).

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Crystallizable and Tough Aliphatic Thermoplastic Polyureas

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