1,4 Hydroxyl-Terminated Polybutadiene-Based Polyurethanes with

Feb 4, 2016 - Since their discovery,1 polyurethanes (PUs) have been used in ... ical and mechanical properties, the HTPB-based PUs are also used in so...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/IECR

High cis-1,4 Hydroxyl-Terminated Polybutadiene-Based Polyurethanes with Extremely Low Glass Transition Temperature and Excellent Mechanical Properties Zhe Cao, Qinzhuo Zhou, Suyun Jie,* and Bo-Geng Li State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China ABSTRACT: A series of polyurethanes have been prepared from hydroxyl-terminated polybutadiene (HTPB) with high cis-1,4 content and commercial free-radical-polymerized HTPB (FHTPB). The mechanical properties of cured polyurethane (PU) samples have been studied by tensile testing and dynamic mechanical analysis (DMA), and their thermal properties have been tested by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The NCO/OH ratio, chain extension coefficient (r), and microstructure of HTPB and FHTPB have a large influence on the mechanical properties of PUs. Meanwhile, the cyclic tensile testing indicated that the HTPB-based PUs had an outstanding elasticity. Both DSC and DMA analyses showed that the HTPB-based PUs had a glass transition temperature much lower than that of the FHTPB-based PUs. The hydrophobicity of the surface of PUs was verified by measuring their water contact angles.

1. INTRODUCTION Since their discovery,1 polyurethanes (PUs) have been used in wide applications, such as vehicles, clothes, shipbuilding, construction industries, and so on.2,3 Polyurethanes are mainly based on the reaction between diisocyanates and telechelic oligomers with hydroxyl groups (polyols). Among the widely used polyols (mainly polyether and polyester backbones), hydroxyl-terminated polybutadiene (HTPB)-based polyurethanes are usually used as the main components of solid propellants.4−9 Apart from the application as a polymeric binder in solid propellants, because of their good physicochemical and mechanical properties, the HTPB-based PUs are also used in some other essential and important areas in both industrial and civilian research,10−12 such as adhesives, coatings, membranes, sealants, liner materials, etc.13−16 The HTPB-based PUs, including the relationship between structure and properties, mechanical properties, thermal stability, aging performance, susceptibility toward oxygen, and low-temperature performance, have been studied in the literature.17−24 The influence of NCO/OH ratio on the mechanical properties of PUs prepared from HTPB and isophorone diisocyanate has been also investigated.25 Different types of diisocyanates, such as toluene diisocyanate (TDI), 4,4′di(isocyanatocyclohexyl) methane (H12MDI), and isophorone diisocyanate (IPDI), have been found to affect the tensile strength, equilibrium relaxation modulus, and the elongation of HTPB-based PUs, but the dynamic mechanical studies showed that the amount of diisocyanates did not change the glass transition temperature of PUs.26 Different types and amounts of chain extender have been reported to affect the mechanical properties by changing the value of chemical cross-links and sol fraction.27 In addition, the terminal-functionalized HTPBs with dinitrobenzene groups (HTPB-DNB) or triazine groups, namely, cyanuric chloride (HTPB-CYC), 2-chloro-4,6-bis(dimethylamino)-1,3,5-triazine (HTPB-CBDT), and 1-chloro3,5-diazido-2,4,6-triazine (HTPB-CDT), being attached to the © 2016 American Chemical Society

terminal carbon atoms, have been also used to react with various diisocyanates to improve the tensile strength and understand the structure−property relationship of HTPB-based PUs.28−30 Although considerable efforts have been made in the above literature, the HTPB-based PUs did not have good mechanical properties because of the flaw of cross-links caused by the nonuniform distribution of hydroxyl groups in the free-radical polymerized HTPB.31 Very recently, we have reported a new type of HTPB with high cis-1,4 content and extremely low glass transition temperature prepared via oxidolysis of butadiene rubber.32,33 Furthermore, the hydroxyl functionality of these HTPBs was nearly 2.0, ensuring that each chain end of HTPB was terminated with only one hydroxyl group. In the current study, the HTPBs with high cis-1,4 content were used to react with 4,4′-diphenylmethane diisocyanate (MDI) to prepare the HTPB-based PUs. The influences of different conditions, such as NCO/OH ratio and chain extension coefficient, on the properties of PUs were investigated. The relationship between the microstructure of HTPB and properties of PUs has been also studied by comparing the PUs prepared from HTPB with high cis-1,4 content and commercial free-radical polymerized HTPB (FHTPB).

2. EXPERIMENTAL SECTION 2.1. Materials. Hydroxyl-terminated polybutadiene (HTPB) with high cis-1,4 content was prepared via the oxidolysis of commercial butadiene rubber (BR9000) according to the procedure that we reported previously.32,33 The HTPB via free-radical polymerization (FHTPB) was purchased from Zibo Qilong Chemical Industrial Group. 4,4′-Diphenylmethane Received: Revised: Accepted: Published: 1582

December 24, 2015 January 22, 2016 January 25, 2016 February 4, 2016 DOI: 10.1021/acs.iecr.5b04921 Ind. Eng. Chem. Res. 2016, 55, 1582−1589

Article

Industrial & Engineering Chemistry Research Table 1. Characteristics of HTPB and FHTPB Used for the Synthesis of HTPB-PUs

microstructuree (mol %)

Mn (g/mol) a

samples

NMR

HTPB1 HTPB2 HTPB3 FHTPB1 FHTPB2

2600 4100 5150 3090 4620

GPC (PDI)

b

c

d

OH value (mmol/g)

f OH

0.78 0.50 0.38 0.76 0.50

2.02 2.05 1.96 2.35 2.31

6260 (2.24) 9620 (2.67) 12270 (2.12) 5630 (2.16) 8680 (2.02)

cis-1,4/trans-1,4/1,2-vinyl 96/1/3 96/1/3 96/1/3 22/60/18 22/60/18

a Determined by 1HNMR spectroscopy. bDetermined by GPC against polystyrene standards and reported uncorrected. cDetermined by chemical titration method. dDegree of functioinality: f OH = Mn,NMR × OH value/1000. eDetermined by 1H and 13C NMR spectroscopy.

Scheme 1. Synthesis of Cured HTPB-Based Polyurethanes

diisocyanate (MDI) and dibutyltin dilaurate (DBTDL) were received from J&K Scientific company and used without further purification. 1,4-Butanediol (BDO) and tetrahydrofuran (THF) were received from Sinopharm Chemistry Company and dried with 4 Å molecular sieves. 2.2. Preparation of Polyurethane Prepolymer and Chain-Extended Polyurethanes. The polyurethane prepolymers were synthesized under nitrogen atmosphere. The materials, HTPB (FHTPB) (4.00 g) and MDI, whose amount was adjusted with different NCO/OH molar ratio of 1.2, 1.4, 1.6, 1.8, and 2.0, were first dissolved in 30 mL of THF. Then a catalytic amount of DBTDL (0.5 wt %) as a catalyst was added, and the reaction was continued at 30 °C for 50 min with careful and controlled magnetic stirring. In the subsequent chain-extension step for the preparation of polyurethanes, a calculated amount of BDO dissolved in 30 mL of THF was added slowly to the above solution. In the different cases, the chain extension coefficient (r), representing the molar ratio of −OH in BDO and −NCO in the prepolyur-

ethane prepolymer, was maintained at 0.2, 0.4, 0.6, 0.8, and 0.9, respectively. The reaction was stirred at 30 °C for another 40 min. The solvent THF in the solution was partially removed under reduced pressure. A polyurethane sheet was cast from the viscous polymer solution on a clean flat-base Teflon dish and cured under vacuum at 80 °C for 5 h. Finally, the PU films were cut to the standard samples for testing after being stored at room temperature for 3 d. 2.3. Characterization. 2.3.1. Structural Characterization. Gel permeation chromatography analyses (GPC) were measured on a Waters 2414 series system in THF at 25 °C calibrated with polystyrene standards. 1H NMR and 13C NMR spectra of HTPB were recorded on a Bruker DMX-400 instrument in CDCl3 with tetramethylsilane (TMS) as the internal standard at room temperature. Fourier transform infrared (FT-IR) spectra were recorded on a Pekin-Elmer FTIR 2000 spectrometer using solid films over the range of 4000− 400 cm−1. 1583

DOI: 10.1021/acs.iecr.5b04921 Ind. Eng. Chem. Res. 2016, 55, 1582−1589

Article

Industrial & Engineering Chemistry Research Table 2. Composition and Mechanical Properties of Cured HTPB-Based PUs samples

HTPB

NCO/ OHa

rb

hard segment contentc (wt %)

tensile strength (MPa)

PU1 PU2 PU3 PU4 PU5 PU6 PU7 PU8 PU9 PU10 PU11 FPU10 FPU11

HTPB3 HTPB3 HTPB3 HTPB3 HTPB3 HTPB3 HTPB3 HTPB3 HTPB3 HTPB1 HTPB2 FHTPB1 FHTPB2

1.2 1.4 1.6 1.8 2.0 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6

0.6 0.6 0.6 0.6 0.6 0.2 0.4 0.8 0.9 0.8 0.8 0.8 0.8

5.6 6.6 7.6 8.5 9.5 7.2 7.4 7.8 7.9 14.6 10.0 14.4 10.0

0.767 2.32 3.23 3.38 3.67 3.25 4.34 1.42 0.74 3.03 4.57 0.92 0.86

elongation at break (%)

Young’s modulus (MPa)

effective cross-linking ved (mol/m3)

± ± ± ± ± ± ± ± ± ± ± ± ±

1.23 2.36 3.47 4.62 4.82 2.80 3.56 3.38 4.06 6.97 3.55 7.06 2.12

165.5 317.5 466.8 621.6 648.5 376.7 478.9 454.7 546.2 937.7 477.6 949.8 285.2

464 560 625 506 519 524 649 559 273 302 481 65 159

46 36 58 46 13 47 46 8 25 18 8 6 9

NCO/OH = nMDI/nHTPB. bChain extension coefficient (r) = nBDO/(nMDI − nHTPB). cHard segment content (wt %) = (WMDI + WBDO)/(WHTPB + WMDI + WBDO). dEffective cross-linking density ve = E/(3RT), where the E is the Young’s modulus, R the gas constant (8.314 J·mol−1·K−1), and T the absolute temperature in Kelvin. a

2.3.2. Thermal Properties. Thermogravimetric analysis (TGA) was carried out in nitrogen atmosphere on a TA Q500 TGA analyzer. The samples in a platinum crucible were first heated to 100 °C to remove the residual solvent and moisture and then cooled back to 50 °C, and the data were recorded from 50 to 600 °C with a scanning rate of 10 °C/min. Differential scanning calorimetry analyses (DSC) were recorded on a TA Q200 thermal analyzer under nitrogen atmosphere. The samples were first cooled from ambient temperature to −150 °C at 50 °C/min, and the data were recorded from −130 to 30 °C at 10 °C/min. Glass transition temperature (Tg) was identified from the slope change in the thermograms. 2.3.3. Mechanical Properties. The thermo-mechanical properties of PU films were measured using a dynamic mechanical analysis (DMA) instrument (TA Instrument, model Q800). The films were clamped on the film tension clamp of the precalibrated instrument. The samples were annealed at −120 °C for 5 min and then scanned from −120 to 50 °C at a heating rate of 3 °C/min. The storage modulus (E′), loss modulus (E″), and tan δ values were measured at 10 Hz frequency with preload force of 0.01 N, and the strain was constantly kept at 20%. Tensile properties of PU samples (standard dull-bell specimens) were measured using a Zwick/ Roell Z020 tensile machine at room temperature with a cross head speed of 20 mm/min. For each sample, at least three specimens were tested in the machine to check the reproducibility. The cyclic tensile testing (10 cycles) of PU samples were tested at room temperature with the cyclic speed of 50 mm/min. Three different deformations (50%, 100%, and 200%) were performed at least three times to ensure the reproducibility. 2.3.4. Contact Angle Measurement. Contact angles were measured with a contact angle measuring device (OCA 20, Dataphysics, Germany) using the sessile drop method to evacuate the hydrophobic nature of the films. Thin films were used for this measurement and each sample was dried for a whole day to remove the moisture. At least three different locations of each film were tested to ensure the reproducibility.

3. RESULTS AND DISCUSSION 3.1. Synthesis of HTPB-Based PUs. The hydroxylterminated polybutadienes (HTPB) with high cis-1,4 content were prepared via the oxidolysis of commercial butadiene rubber according to the reported procedure.32,33 The physical characteristics of HTPB and FHTPB used for the synthesis of HTPB-PUs are listed in Table 1. It is well-known that the properties of HTPB are greatly influenced by its chain microstructures.7 The microstructure of HTPB prepared via oxidolysis and FHTPB prepared by radical polymerization had large difference. The ratio of cis-1,4/trans-1,4/1,2-vinyl in HTPB was 96/1/3; in contrast, the cis-1,4 content in FHTPB was only 22% along with 60% of trans-1,4 and 18% of 1,2-vinyl. In addition, the degree of functionality ( f OH) of HTPB with high cis-1,4 content was maintained around 2.0, whereas the f OH of commercial FHTPB was around 2.3. All the HTPB and FHTPB samples had similar molecular weight distribution. The synthesis of cured HTPB-based polyurethanes was carried out in three steps (Scheme 1). First, the polyurethane prepolymers were prepared by reacting HTPB with MDI under nitrogen. The polyurethane prepolymers were end-capped by −NCO groups and the content of −NCO groups was determined by chemical titration. Second, the certain amount of BDO was then added as a chain extender to the solution of polyurethane prepolymers. Finally, the cured yellow PU samples were achieved. According to the different ratio of NCO/HTPB, chain extension coefficient (r), and microstructure of HTPB, the cured polyurethane samples (PU1− PU11, FPU10, and FPU11) were obtained (Table 2). 3.2. Characterization of HTPB-Based PUs. The typical functional groups of MDI, HTPB, and PUs were characterized by FT-IR spectra (Figure 1). In the FT-IR spectra of MDI, an intense peak appeared at 2280 cm−1, which was assigned to the stretching vibration band of free −NCO groups. The characteristic peak of −OH groups in HTPB showed up as a broad absorption peak between 3200 and 3600 cm−1. In the FT-IR spectra of both HTPB and PU, the strong peak at 735 cm−1 representing cis-1,4 isomers manifested their high cis-1,4 content, and the weak peaks at 965 and 910 cm−1 were assigned to the traces of trans-1,4 and 1,2-vinyl isomers, respectively. The absorption peaks of −NH− and CO groups in the PU samples appeared at 3300 and 1720 cm−1, respectively. The 1584

DOI: 10.1021/acs.iecr.5b04921 Ind. Eng. Chem. Res. 2016, 55, 1582−1589

Article

Industrial & Engineering Chemistry Research

from the slope of the linear portion of stress−strain plot. The effective cross-linking density (ve) was estimated according to the equation ve = E/(3RT), where E is the Young’s modulus, R the gas constant (8.314 J·mol−1·K−1), and T the absolute temperature in Kelvin.20 3.3.1. Effect of NCO/OH Ratio. As shown in Table 2 and Figure 2a, the mechanical properties of PUs were greatly affected by the NCO/OH ratios. As the NCO/OH ratio increased, the elongation at break of PUs first increased and then decreased. When the 1.6 NCO/OH ratio was used, the polyurethane sample was found to be the most elastomeric (ca. 625% elongation before rupture for PU3). On the other hand, the Young’s modulus increased gradually along with the increase in NCO/OH ratio, which was ascribed to the significant correlation between the Young’s modulus and the content of hard segments. The higher NCO/OH ratio obviously led to the increase in the content of hard segments in the PU samples. When the NCO/OH ratio increase from 1.2 to 1.6, the tensile strength increased remarkably, whereas the increase in tensile strength became relatively slow when the NCO/OH ratio ranged from 1.6 to 2.0. The maximum of tensile strength (3.67 MPa) was obtained at the NCO/OH ratio of 2.0. 3.3.2. Effect of Chain Extension Coefficient (r). It was evident from Figure 2b that the chain extension coefficient (r) also had a large influence on the mechanical properties. When the r values ranged from 0.4 to 0.8, the tensile strength, elongation at break, and Young’s modulus of PUs decreased as the r value increased. However, the r value less than 0.4 or more

Figure 1. FT-IR spectra of MDI, HTPB, and cured PU samples.

characteristic peaks of free −NCO and −OH groups disappeared after the curing reaction was completed. 3.3. Mechanical Properties of HTPB-Based PUs. To investigate the influence of the different ratio of NCO/OH, chain extension coefficient (r), and different types of HTPB on the mechanical properties of polyurethanes, the different PU samples (PU1−PU11, FPU10, and FPU11) were prepared by varying the NCO/OH ratio from 1.2 to 2, chain extension coefficient (r) from 0.2 to 0.9, and five different HTPBs (HTPB1, HTPB2, HTPB3, FHTPB1, and FHTPB2) (Table 2 and Figure 2). In addition, the Young’s modulus was calculated

Figure 2. Stress−strain curves of HTPB-based PUs prepared under various conditions: different NCO/OH ratio (a), different r value (b), and different HTPB type (c). 1585

DOI: 10.1021/acs.iecr.5b04921 Ind. Eng. Chem. Res. 2016, 55, 1582−1589

Article

Industrial & Engineering Chemistry Research

Figure 3. Cyclic tensile response curves (a) and terminal stress (b) for 10 cycles of PU10 at 50%, 100%, and 200% of deformation.

3.4. Elasticity Recovery of the HTPB-Based PUs. To understand the elasticity of the HTPB-based PUs, the cyclic tensile loading with a maximum strain at 50%, 100%, and 200% of deformation was performed with PU10. As shown in Figure 3a, the sample exhibited an apparent hysteresis loop in the first cycle, and the curves were very similar from the second cycle to the 10th cycle. Comparing the terminal stress of each cycle for the three different deformations (Figure 3b), it was obvious that the terminal stress slowly declined with the increase of the cycle number. What’s more, the terminal stress at high deformation (200%) descended faster than that at low deformation (50% and 100%). As a result, the terminal stress in the 10th cycle decreased to 98.8%, 88.1%, and 83.0% of that in the first cycle at 50%, 100%, and 200%, respectively. The results demonstrated that the HTPB-based PUs prepared from HTPB with high cis-1,4 content had an outstanding elasticity. 3.5. Thermal Properties and Modulus of PUs. The important DMA data for the PU samples (PU10, PU11, FPU10, and FPU11) are summarized in Table 3, and the

than 0.8 led to poor mechanical properties of PUs. The reason was probably that the r value controlled the chain length of PUs and either too long or too short chain length was detrimental to the mechanical properties. The best mechanical properties were achieved with PU7, possessing 4.34 MPa of tensile strength, 3.56 MPa of Young’s modulus, and 649 ± 46% of elongation at break. 3.3.3. Effect of Various HTPBs. When the NCO/OH ratio was 1.6 and the chain extension coefficient (r) equaled 0.8, the various HTPBs (HTPB1, HTPB2, HTPB3, FHTPB1, and FHTPB2) with different molecular weight, OH value, and microstructure were used to prepare the HTPB-based PUs, and their influence was investigated (Figure 2c). Comparing PU8, PU10 and PU11 samples, whose precursor corresponded to HTPB3, HTPB1, and HTPB2 with different molecular weight and OH value, their Young’s modulus increased as the hard segment increased (PU10 > PU11 > PU8), whereas the elongation at break had an inverse order (PU8 > PU11 > PU10) because the different molecular weight of HTPBs caused the different content of hard segments in the corresponding PUs. The microstructure of HTPB had great influence on the properties of PUs. As a result, the tensile strength and the elongation at break of PU10 (from HTPB1) and PU11 (from HTPB2) were much larger than those of FPU10 (from FHTPB1) and FPU11 (from FHTPB2) although they were prepared with the same NCO/OH ratio, chain extension coefficient, and the same content of hard segments. On one hand, HTPB and FHTPB precursors had very different chain microstructure due to the different synthetic methods. HTPB1 and HTPB2, prepared by the oxidolysis of butadiene rubber,32,33 had high cis-1,4 content up to 96%, which resulted in the good elasticity of the corresponding PUs. However, FHTPB prepared by free-radical polymerization of butadiene had much lower cis-1,4 content (22%). On the other hand, the distribution of functional groups in FHTPB was uneven and the degree of functionality ( f OH) was more than 2.0,31 which led to the existence of defects in the network of PUs and poor mechanical properties. However, the functional hydroxyl groups in HTPB1 and HTPB2 located in the chain ends of polybutadiene made the cross-linked network of PUs uniform and resulted in better mechanical properties. In addition, the content of hard segments resulted in similar Young’s modulus for PU10 and FPU10 (PU11 and FPU11).

Table 3. Modulus and Glass Transition Temperatures of PUs Determined by DMA storage modulus (MPa)

a

loss modulus (MPa)

samples

−120 °C

30 °C

−120 °C

30 °C

Tga (°C)

PU10 PU11 FPU10 FPU11

3360 2260 2690 2240

7.58 2.77 6.16 1.98

169 134 114 91

0.34 0.01 1.03 0.14

−81 −82 −54 −58

Obtained from tan δ versus temperature plot of DMA studies.

storage modulus (E′), tan δ, and loss modulus (E″) plots as a function of temperature are shown in Figure 4. The results clearly showed that PU10 and PU11 had much lower glass transition temperature than FPU10 and FPU11, although all four samples were elastic at room temperature. This was caused by the different microstructure of soft segments, in which the cis-1,4 content of HTPB1 and HTPB2 precursors reached up to 96% and FHTPB1 and FHTPB2 precursors contained 22% of cis-1,4 content. It is well-known that the higher the cis-1,4 content, the lower the glass transition temperature. It was also clearly shown that the storage modulus of PU10 and PU11 was slightly higher than that of FPU10 and FPU11 at 30 °C, whereas the loss modulus of PU10 and PU11 was smaller than 1586

DOI: 10.1021/acs.iecr.5b04921 Ind. Eng. Chem. Res. 2016, 55, 1582−1589

Article

Industrial & Engineering Chemistry Research

Figure 4. DMA plots of PUs: storage modulus (a), tan δ (b), and loss modulus (c).

that of FPU10 and FPU11. This observation indicated that the PU10 and PU11 samples were more resilient and had better mechanical properties than FPU10 and FPU11. At −120 °C, all four PU samples were in glassy state and the storage modulus and loss modulus of PU10 and PU11 were larger than those of FPU10 and FPU11. The DMA studies proved that the PU samples prepared from HTPB had much lower Tg and better elasticity than the corresponding PU samples from FHTPB, as well as the PU samples prepared from hydroxytelechelic cis-1,4polyisoprene (HTPI),34,35 and this will further expand the applied range of HTPB-based PUs. To reconfirm the Tg value and rubbery nature of HTPBbased PUs, differential scanning calorimetry experiments were carried out (Figure 5). The DSC curves showed that the Tg of PU10 and PU11 was around −104 °C and the Tg of FPU10 and FPU11 was −79 °C, which also suggested the elastic rubbery nature of the PU samples at room temperature. Comparing the data determined by DSC, we also noted that the Tg of PU10 and PU11 was about 25 °C lower than that of FPU10 and FPU11. According to our previous research,32 the Tg values of HTPB and FHTPB were −104 °C and −81 °C, respectively. From the DSC results, it was concluded that the Tg of PUs mainly depended on the HTPB precursors. The difference of Tg values between DSC and DMA results was ascribed to the difference of measurement techniques. TGA of PU10 and FPU10 were conducted in nitrogen atmosphere from 50 to 600 °C with a heating rate of 10 °C/ min (Figure 6). To diminish the inaccuracy, the samples were first heated to 100 °C to remove the residual solvent and moisture and then cooled to 50 °C. Thermal stability of PUs primarily depended upon the polyols and isocyanate structures.

Figure 5. DSC curves of PUs.

In comparison with HTPB precursors, the PUs generally showed less thermal stability because of the presence of labile urethane and urea linkages in PUs.36,37 Both PU10 and FPU10 showed thermal degradation in three stages, which were similar to those of the HTPI-based polyurethane.34,35 In the first stage, PUs started to decompose at around 220 °C and reached a short plateau at 305 °C with about 8% of weight loss. The first stage was mainly caused by the decomposition of urethane/ urea bonds in polyurethane materials. The following two stages were similar to those of HTPB.32,33 In the second stage, the thermal degradation of PUs began at 305 °C and ended at 380 °C in a relatively slow rate with 10% of weight loss. In the third stage, the decomposing rate was much faster than those in the 1587

DOI: 10.1021/acs.iecr.5b04921 Ind. Eng. Chem. Res. 2016, 55, 1582−1589

Article

Industrial & Engineering Chemistry Research

4. CONCLUSION A series of polyurethane samples with excellent mechanical properties and low glass transition temperature have been successfully prepared from high cis-1,4 HTPB as soft segments, MDI as hard segments, and BDO as a chain extender. The different mechanical properties could be achieved by adjusting the NCO/OH ratio and chain extension coefficient (r) and controlling the content of hard segments and the chain length of PUs. In comparison with the FHTPB-based PUs, the HTPBbased PUs had the better mechanical properties due to the uniform hydroxyl functional distribution of HTPB and the ordered cross-linked networks. Meanwhile, the cyclic tensile testing demonstrated that the HTPB-based PU had an outstanding elasticity. Based on the data of both DSC and DMA, the HTPB-based PUs had a glass transition temperature much lower than that of the FHTPB-based PUs because of high cis-1,4 content in soft segments. The water contact angles above 100° indicated that the HTPB-based PUs had a hydrophobic surface. The HTPB-based PUs will be expected to be applied in specialized fields because of their excellent mechanical properties and low glass transition temperature.

Figure 6. TGA curves of PU10 and FPU10.

first two stages, and the extreme degradation rate occurred at 475 °C. Comparing the curves of PU10 and FPU10, it was found that the difference in the thermal stability was not very notable, which might indicate that the categories and the numbers of bonds in these samples had nearly no difference. 3.6. Water Contact Angle Analysis of PUs. Water contact angle responds to the property of the outermost monomolecular layer and is usually used to evaluate the degree of hydrophobicity and wettability of the material surface, which is governed by both surface energy and the geometrical arrangement or microstructure of the surface. When the water contact angle is above 90°, the material displays hydrophobicity; otherwise, it is hydrophilic.38−40 As shown in Figure 7, the water contact angles over 100° indicated that the HTPBbased PUs were hydrophobic. The PU samples from HTPB had similar water contact angle around 104°, which was slightly lower than that of PUs from FHTPB. This result might be caused by the different distribution of −OH groups in HTPB and FHTPB; thus, the urethane/urea groups were distributed diversely on the surface of PUs. In general, the HTPB-based PUs had a hydrophobic surface, which makes them have potential applications in biological fields, such as use as biological pipes, due to the good biocompatibility of polyurethanes.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The work was supported by the Zhejiang Provincial Natural Science Foundation (LY16B040001) and the National Natural Science Foundation of China (21536011).

■

REFERENCES

(1) Bayer, O. Das Di-Isocyanat-Polyadditio-nsverfahren (Polyurethane). Angew. Chem. 1947, 59, 257−272. (2) Calle, M.; Lligadas, G.; Ronda, J. C.; Galià, M.; Cádiz, V. An Efficient Nonisocyanate Route to Polyurethanes via Thiol-Ene SelfAddition. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 3017−3025. (3) Yin, J.; Wildeman, J.; Loontjens, T. Lysine-Based Functional Blocked Isocyanates for the Preparation of Polyurethanes Provided with Pendant Side Groups. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 2036−2049.

Figure 7. Images of water contact angles of PUs. 1588

DOI: 10.1021/acs.iecr.5b04921 Ind. Eng. Chem. Res. 2016, 55, 1582−1589

Article

Industrial & Engineering Chemistry Research (4) Whitmore, S. A.; Peterson, Z. W.; Eilers, S. D. Deep Throttle of a Nitrous Oxide and Hydroxyl-Terminated Polybutadiene Hybrid Rocket Motor. J. Propul. Power 2014, 30, 78−86. (5) Pant, C. S.; Santosh, M. S. S. N.; Banerjee, S.; Khanna, P. K. Single Step Synthesis of Nitro-Functionalized Hydroxyl-Terminated Polybutadiene. Propellants, Explos., Pyrotech. 2013, 38, 748−753. (6) Abdullah, M.; Gholamian, F.; Zareiee, M. R. Investigation of Composite Propellant Based on Hydroxyl Terminated Polybutadiene−1-Chloro-2,4-Dinitrobenzene. J. Propul. Power 2013, 29, 1343− 1345. (7) Sankar, R. M.; Saha, S.; Meera, K. S.; Jana, T. Functionalization of hydroxyl terminated polybutadiene with biologically active fluorescent molecule. Bull. Mater. Sci. 2009, 32, 507−514. (8) Sankar, R. M.; Roy, T. K.; Jana, T. Functionalization of terminal carbon atoms of hydroxyl terminated polybutadiene by polyazido nitrogen rich molecules. Bull. Mater. Sci. 2011, 34, 745−754. (9) Shankar, R. M.; Roy, T. K.; Jana, T. Terminal Functionalized Hydroxyl-Terminated Polybutadiene: An Energetic Binder for Propellant. J. Appl. Polym. Sci. 2009, 114, 732−741. (10) Kothandaraman, H.; Nasar, A. S. The Kinetics of the Polymerization Reaction of Toluene Diisocyanate with HTPB Prepolymer. J. Appl. Polym. Sci. 1993, 50, 1611−1617. (11) Manjari, R.; Somasundaran, U. I.; Joseph, V. C.; Sriram, T. Structure-Property Relationship of HTPB-Based Propellants. II. Formulation Tailoring for Better Mechanical Properties. J. Appl. Polym. Sci. 1993, 48, 279−289. (12) Harris, D. J.; Assink, R. A.; Celina, M. NMR Analysis of Oxidatively Aged HTPB/IPDI Polyurethane Rubber: Degradation Products, Dynamics, and Heterogeneity. Macromolecules 2001, 34, 6695−6700. (13) Yang, J. M.; Lin, H. T. Wettability and protein adsorption on HTPB-based polyurethane films. J. Membr. Sci. 2001, 187, 159−169. (14) Chen, S. H.; Yu, K. C.; Houng, S. L.; Lai, J. Y. Gas transport properties of HTPB based polyurethane/cosalen membrane. J. Membr. Sci. 2000, 173, 99−106. (15) Yang, J. M.; Lin, H. T.; Lai, W. C. Properties of modified hydroxyl-terminated polybutadiene based polyurethane membrane. J. Membr. Sci. 2002, 208, 105−117. (16) Sarkar, S.; Adhikari, B. Thermal stability of lignin−hydroxyterminated polybutadiene copolyurethanes. Polym. Degrad. Stab. 2001, 73, 169−175. (17) Huang, S. L.; Chang, P. H.; Tsai, M. H.; Chang, H. C. Properties and pervaporation performances of crosslinked HTPB-based polyurethane membranes. Sep. Purif. Technol. 2007, 56, 63−70. (18) Tien, Y. I.; Wei, K. H. Hydrogen bonding and mechanical properties in segmented montmorillonite/polyurethane nanocomposites of different hard segment ratios. Polymer 2001, 42, 3213−3221. (19) Huang, S. L.; Lai, J. Y. Structure-tensile properties of polyurethanes. Eur. Polym. J. 1997, 33, 1563−1567. (20) Sekkar, V.; Bhagawan, S. S.; Prabhakaran, N.; Rao, M. R.; Ninan, K. N. Polyurethanes based on hydroxyl terminated polybutadiene: modelling of network parameters and correlation with mechanical properties. Polymer 2000, 41, 6773−6786. (21) Alibeik, S.; Rizkalla, A. S.; Mequanint, K. The effect of thiolation on the mechanical and protein adsorption properties of polyurethanes. Eur. Polym. J. 2007, 43, 1415−1427. (22) Reshmi, S.; Arunan, E.; Nair, C. P. R. Azide and Alkyne Terminated Polybutadiene Binders: Synthesis, Cross-linking, and Propellant Studies. Ind. Eng. Chem. Res. 2014, 53, 16612−16620. (23) Mowery, D. M.; Assink, R. A.; Celina, M. Sensitivity of proton NMR relaxation times in a HTPB based polyurethane elastomer to thermo-oxidative aging. Polymer 2005, 46 (24), 10919−10924. (24) Nagle, D. J.; Celina, M.; Rintoul, L.; Fredericks, P. M. Infrared microspectroscopic study of the thermo-oxidative degradation of hydroxy-terminated polybutadiene/isophorone diisocyanate polyurethane rubber. Polym. Degrad. Stab. 2007, 92, 1446−1454. (25) Haska, S. B.; Bayramli, E.; Pekel, F.; Ö zkar, S. Mechanical properties of HTPB-IPDI-based elastomers. J. Appl. Polym. Sci. 1997, 64, 2347−2354.

(26) Sekkar, V.; Gopalakrishnan, S.; Devi, K. A. Studies on allophanate-urethane networks based on hydroxyl terminated polybutadiene: effect of isocyanate type on the network characteristics. Eur. Polym. J. 2003, 39, 1281−1290. (27) Gupta, T.; Adhikari, B. Chain-extended, hydroxyterminatedpolybutadiene-based polyurethaneureas: Synthesis, reaction kinetics, and properties. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 2978− 2992. (28) Malkappa, K.; Jana, T. Simultaneous Improvement of Tensile Strength and Elongation: An Unprecedented Observation in the Case of Hydroxyl Terminated Polybutadiene Polyurethanes. Ind. Eng. Chem. Res. 2013, 52, 12887−12896. (29) Malkappa, K.; Jana, T. Hydrophobic, Water-Dispersible Polyurethane: Role of Polybutadiene Diol Structure. Ind. Eng. Chem. Res. 2015, 54, 7423−7435. (30) Rao, B. N.; Yadav, P. J. P.; Malkappa, K.; Jana, T.; Sastry, P. U. Triazine functionalized hydroxyl terminated polybutadiene polyurethane: Influence of triazine structure. Polymer 2015, 77, 323−333. (31) Ninan, K. N.; Balagangadharan, V. P.; Catherine, K. B. Studies on the functionality distribution of hydroxyl-terminated polybutadiene and correlation with mechanical properties. Polymer 1991, 32, 628− 635. (32) Zhou, Q.; Jie, S.; Li, B.-G. Preparation of Hydroxyl-terminated Polybutadiene with High cis-1,4 Content. Ind. Eng. Chem. Res. 2014, 53, 17884−17893. (33) Zhou, Q.; Jie, S.; Li, B.-G. Facile synthesis of novel HTPBs and EHTPBs with high cis-1,4 content and extremely low glass transition temperature. Polymer 2015, 67, 208−215. (34) Kébir, N.; Campistron, I.; Laguerre, A.; Pilard, J.-F.; Bunel, C.; Couvercelle, J.-P.; Gondard, C. Use of hydroxytelechelic cis-1,4polyisoprene (HTPI) in the synthesis of polyurethanes (PUs). Part 1. Influence of molecular weight and chemical modification of HTPI on the mechanical and thermal properties of PUs. Polymer 2005, 46, 6869−6877. (35) Kébir, N.; Campistron, I.; Laguerre, A.; Pilard, J.-F.; Bunel, C.; Couvercelle, J.-P. Use of new hydroxytelechelic cis-1,4-polyisoprene (HTPI) in the synthesis of polyurethanes (PUs): Influence of isocyanate and chain extender nature and their equivalent ratios on the mechanical and thermal properties of PUs. e-Polym. 2006, 6, 619− 632. (36) Lligadas, G.; Ronda, J. C.; Galià, M.; Cádiz, V. Poly(ether urethane) Networks from Renewable Resources as Candidate Biomaterials: Synthesis and Characterization. Biomacromolecules 2007, 8, 686−692. (37) Coutinho, F. M. B.; Delpech, M. C. Degradation profile of films cast from aqueous polyurethane dispersions. Polym. Degrad. Stab. 2000, 70, 49−57. (38) Xu, S.; Xie, L.; Yu, X.; Xiong, Y.; Tang, H. Synthesis and characterization of phenyl polysiloxane modified polyurea/polyurethanes. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 1794−1805. (39) Wu, W.; Zhu, Q.; Qing, F.; Han, C. C. Water Repellency on a Fluorine-Containing Polyurethane Surface: Toward Understanding the Surface Self-Cleaning Effect. Langmuir 2009, 25, 17−20. (40) Dikić, T.; Ming, W.; Thune, P. C.; van Benthem, R. A. T. M.; de With, G. Well-defined polycaprolactone precursors for low surfaceenergy polyurethane films. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 218−227.

1589

DOI: 10.1021/acs.iecr.5b04921 Ind. Eng. Chem. Res. 2016, 55, 1582−1589