Biomacromolecules 2001, 2, 80-84
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Polyurethaneurea/Vinyl Polymer Hybrid Aqueous Dispersions Based on Renewable Material Yong Sheng Hu, Yong Tao, and Chun Pu Hu* Institute of Material Science and Engineering, East China University of Science and Technology, Shanghai 200237, People’s Republic of China Received July 12, 2000; Revised Manuscript Received November 8, 2000
A series of polyurethaneurea-vinyl polymer (PUA) hybrid aqueous dispersions were synthesized from castor oil and/or difunctional poly(oxypropylene) polyol (GE-210, Mn ) 1000), butyl acrylate, and styrene. The effect of hybrid between the polyurethaneurea (PUU) and vinyl polymer (PA) on the morphologies and the mechanical properties for these PUA films was examined and studied. The experimental results showed that there was appreciable miscibility between the PUU and PA phases for the PUA prepared with GE-210, giving rise to less ordered hard segments in PUU and resulting in lowering of some mechanical properties compared with those of pure PUU synthesized with GE-210. For PUA based on castor oil, the compatibility between the PUU and PA phases was also observed, although there was a network structure in the system. Moreover, the particle-accumulation morphology during the film formation process for this PUA system disappeared, but this behavior was found for a similar PUU. In this case, a great improvement of mechanical properties for such a PUA specimen was observed. The other PUA specimens with high content of castor oil exhibit excellent comprehensive mechanical properties, resulting from the reinforcement of cross-linked PUU phase existing in the systems. 1. Introduction
2. Exprimental Section
Recently for protection of the environment, waterborne polyurethane (WBPU) is receiving more attention and is widely used to prepare some coatings and adhesives that are applied in different ways, since WBPU has the advantages of low volatile organic compounds (VOC) and excellent mechanical properties. Polyether and polyester polyols are generally used to prepare WBPU, and the relationships between the structures and properties for such systems have been studied.1-3 The benefits of the hybrid of WBPU with some vinyl polymers include the lowering of cost and the improvements of some properties as well as some conditions of the preparation process, but the systematic experimental data on the structure-property relationships for such hybrid systems are sparsely reported.4 Environmental protection can be better realized when the polyol is replaced with renewable sources, such as some vegetable oils, to synthesize the WBPU hybrid, although this kind of research work has not been developed at present. Some polyurethaneurea-vinyl polymer (PUA) hybrid aqueous dispersions based on renewable vegetable oil as polyols were prepared in our laboratory successfully.5 In the present paper, a series of PUA hybrid aqueous dispersions have been synthesized from castor oil and/or polyether polyol, and the effect of a hybrid between the polyurethaneurea (PUU) and vinyl polymer (PA) on the morphologies and mechanical properties for these PUA films is reported.
2.1. Materials. Castor oil (hydroxyl number, 163 mg KOH/g) provided by the Kunshan Oils & Fats Plant and difuctional poly(oxypropylene) polyol (GE-210, hydroxyl number, 99 mg KOH/g, Mn ) 1000) supplied by the Gaoqiao No. 3 Chemical Plant were dried under vacuum at 110 °C for 2 h. Dimethylpropionic acid (DMPA, Perstop Co.) was dried at 60 °C for 24 h under vacuum. Isophorone diisocyanate (IPDI, Hu¨ls Co.) and all other standard laboratory reagents obtained from various manufacturers were used as received. 2.2. Synthesis of PUA Aqueous Dispersions. Castor oil and/or GE-210, DMPA, and IPDI were charged into a 250mL four-necked flask equipped with mechanical stirrer, nitrogen inlet, condenser, and thermometer, and the mixture was heated to 90 °C for 3-4 h until the theoretical value of -NCO groups was reached. The obtained NCO-terminated prepolymer was neutralized by adding tertiary amine at 60 °C for 20 min, then styrene and butyl acrylate (weight ratio 1/1) were added into the system at 50 °C. The prepolymer/ monomer mixture was then dispersed into deionized water under vigorous stirring, and ethylenediamine and azobisisobutyronitrile were added into the dispersion subsequently. PUA aqueous dispersion was prepared after the chain extension of prepolymer and the copolymerization of vinyl monomers at 70 °C. Two kinds of polyurethaneurea (PUU) aqueous dispersions were also synthesized from castor or GE-210 according to the similar procedure without adding any vinyl monomers. PUA and PUU films were prepared by casting the aqueous dispersions into a poly(tetrafluoroethylene) mold at room
* To whom correspondence should be addressed. E-mail address:
[email protected].
10.1021/bm0000687 CCC: $20.00 © 2001 American Chemical Society Published on Web 12/28/2000
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Hybrid Aqueous Dispersions
Table 1. DSC Scan Results for Castor Oil, Polyether Polyol, and Different PUUs and PUAs samples castor oil GE-210 CP-10 CP-01 PUA-10 PUA-11 PUA-01
Figure 1. The DSC scan plots for castor oil, polyether polyol, different PUUs, and PUAs: (A) CP-01; (B) CP-10; (C) PUA-01; (D) PUA-11; (E) PUA-10; (F) GE-210; (G) castor oil.
temperature for 2 days and then at 80 °C for 3 h. For simplicity, the PUA films synthesized with different weight ratios of castor oil and GE-210, i.e., 1/0, 3/1, 1/1, 1/3, and 0/1, were called PUA-10, PUA-31, PUA-11, PUA-13, and PUA-01, respectively. The weight ratio of styrene (St) and butyl acylate (BA) copolymer and PUU was at 37.5/62.5, while the hard segment content of PUU was 51.7 wt % for all PUA samples. The pure PUU films prepared from castor oil or GE-210 were simply called CP-10 or CP-01, in which the contents of hard segments are also at 51.7 wt %. The solid content was 33 wt % for all PUA and PUU aqueous dispersions. 2.3. Morphology Characterization and Mechanical Property Measurement. Differential scanning calorimetry (DSC) was measured with a TA Instruments modulated DSC 2910 analyzer at a heating rate of 20 °C/min under nitrogen atmosphere. The morphology of brittle rupture surface at liquid nitrogen temperature for PUA specimens was observed by means of scanning electron microscopy (SEM, Stereoscan 250 MK3, Cambridge), and the surface was coated with gold vapor before examining. FTIR spectra of PUA samples were recorded by using a Nicolet 5DXC FTIR spectrometer at 25 °C. Wide-angle X-ray diffraction (WAXD) measurement was performed on a D/MAX-γ B X-ray diffractometer employing nickel-filtered Cu KR radiation (1.5418 Å) at an operational voltage of 40 kV. The tensile strength and elastic modulus measurements for all specimens were conducted on an Instron 4465 testing machine under 50 mm/min crosshead rate, and the specimens were made in accordance with GB1040-79. The hardness of PUA and PUU samples was measured according to GB/G1703-93. 3. Results and Discussion 3.1. Morphologies of PUA Films. Figure 1 shows the DSC scan curves for CP-01, CP-10, and different PUA films,
Tg1 (°C)
Tg2 (°C)
Tg3 (°C)
Ta1 (°C)
∆Ha1 (J/g)
Ta2 (°C)
∆Ha2 (J/g)
61 57 51 55 64
130 129 123 120 123
7.5 11.4 2.9 3.8 4.7
232 237 236 225 226
4.6 17.8 3.3 5.3 6.7
-44 -68 -34 26 21
and the scan results are given in Table 1. All PUA samples exhibit two endothermic peaks at 120-123 °C and 225236 °C, which may be attributed to the disruption of domains composed of hard segments with limited short-range order and long-range order, respectively.6,7 The values of enthalpy for these two endothermic peaks decreased with the increase of castor oil content in PUA systems, resulting from the suppressive effect of cross-linking introduced by trifunctional castor oil on the formation of ordered hard segments in PUU. These ordered structures of hard segments in PUU should be related to the hydrogen bonding in the systems and were determined by using a FTIR spectrometer, as shown in Figures 2 and 3. The iteration procedure of damping least squares was used to separate the absorption peaks in the carbonyl region corresponding to different hydrogen bondings8,9 (see Table 2), and the curve-fitting results are listed in Table 3. The hydrogen bonding between the urea linkages was chosen for investigation, as the absorption peaks for urethane groups would be affected by the carbonyl absorption peaks of castor oil and BA. However, it should be pointed out that the carbonyl absorption peak of DMPA units existing in the PUU macromolecular chains is located at 1550 cm-1, which would not influence the study of hydrogen bonding for urea groups.10 The degree of hydrogen bonding for urea groups (Xb,UA) and the percentage of ordered urea-urea hydrogen bonds (Xo,UA) in Table 3 were defined as follows: Xb,UA )
Xo,UA )
∑area(bonded urea carbonyl) area(1690 cm-1) + ∑area(bonded urea carbonyl) area(1640 - 1632 cm-1) area(1690 cm-1) +
∑area(bonded urea carbonyl)
As mentioned above, the PA component in PUA was a copolymer of St(M1) and BA(M2) (the initial monomer feed composition for M1 and M2 was at 50/50), and the glass transition temperatures of poly(butyl acrylate) and polystyrene were reported at -56 and 100 °C, respectively.11 If the copolymeriztion conversion of these two monomers was assumed to be complete and this copolymer was considered as a “random copolymer” (r1 ) 0.48 ( 0.04, r2 ) 0.15 ( 0.04)12, the Tg of PA could be estimated as 1.4 °C according to the Fox equation: 1/Tg ) w1/Tg1 + w2/Tg2 Figure 1 shows that PUA-01 synthesized with GE-210 only
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Figure 2. FTIR spectra of different PUUs and PUAs: (A) CP-01; (B) CP-10; (C) PUA-01; (D) PUA-11; (E) PUA-10.
Figure 3. FTIR spectra of different PUUs and PUAs in the carbonyl region: (A) CP-10: (B) CP-01: (C) PUA-10: (D) PUA-11: (E) PUA-01.
exhibits a wide second transition region without showing the Tg of PA or polyether soft segment of PUU, while the value of enthalpy for the disruption of ordered hard segment structure reduced remarkably compared with the pure PUU (CP-01). These experimental results indicated that there was appreciable compatibility between the PUU and the PA phases in the system, and part of the ordered hard segment structure should be destroyed resulting from the hybrid of the two components and giving rise to better microphase
miscibility between the hard segments and polyether soft segments in PUU. Table 3 shows that both values of Xb,UA and Xo,UA for PUA-01 were lower than those for CP-01, which implied that the hydrogen bonding in PUA-01 system was depressed by the hybrid of PUU and PA as consistent with the DSC scan results. Furthermore, it also exhibited that the lowering of Xo,UA for PUA-01 was less than that of Xb,UA compared with those of CP-01. In this case, for such a hybrid PUA-01 the disordered hydrogen bonding is mainly
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Hybrid Aqueous Dispersions Table 2. Assignment of Absorption Band in Carbonyl Region of FTIR Spectra for PUA wavenumber (cm-1) 1747-1728 1717
1700-1708 1690 1678-1651 1640-1632
assignment free carbonyl stretching of urethane linkages, castor oil based soft segment and butyl acrylate disordered hydrogen-bonded carbonyl of urethane linkages, castor oil based soft segment and butyl acrylate ordered hydrogen-bonded urethane carbonyl free urea carbonyl disordered hydrogen-bonded urea carbonyl ordered hydrogen-bonded urea carbonyl
Table 3. Least-Squares Curve Fitting FTIR Spectra in the Carbonyl Region for Different PUAs
Figure 4. WAXD spectra of (A) PUA-01 and (B) CP-01.
peak area (%) samples
1690 cm-1
1666 cm-1
1651 cm-1
1636 cm-1
Xb,UA (%)
Xo,UA (%)
CP-10 CP-01 PUA-10 PUA-11 PUA-01
32.4 0 64.9 40.9 25.9
31.0 22.0 17.0 25.2 42.3
18.5 47.9 8.2 19.4 11.9
18.1 28.3 9.9 14.5 19.9
67.6 100 35.1 59.1 74.1
18.1 28.3 9.9 14.5 19.9
Table 4. Curve-Fitting Results of WAXD Spetra for PUU and PUA peak areas (%) samples
0.926 nm
0.519 nm
0.413 nm
amorphous
Xc (%)
CP-01 PUA-01
3.9 3.5
20.6 6.7
21.9 18.2
53.8 71.6
46.2 28.4
affected. These phenomena could be observed further from the WAXD measurement. Figure 4 represents the WAXD patterns for PUA-01 (A) and CP-01 (B), in which two sharp reflections were observed at 9.4° and 21.6° (2θ), and the lattice spacings calculated from the Bragg formula were 0.926 and 0.413 nm, respectively. The reflection at 0.413 nm can belong to the planar reflection caused by the hydrogen bonding between the urea groups in hard segments,13-15 corresponding to the urea carbonyl absorption peak at 1640-1632 cm-1 in the FTIR spectrum, while the peak at 0.926 nm may be attributed to some ordered structures of soft segments in PUU.16 Figure 4 reveals that there was an obscure reflection at 0.541 nm, which may be related to some imperfect crystals of hard segments.17 The curve-fitting results of WAXD patterns are listed in Table 4, and the crystallinity (Xc) of the PUU phase was definded as follows: Xc ) area(9.26) + area(5.19) + area(4.13) area(9.26) + area(5.19) + area(4.13) + area(amorphous) It was obvious that the value of Xc for PUA-01 decreased considerably with adding PA phase in the system. Figure 1 also indicated that, for PUA-10 prepared from castor oil, the Tg1 corresponding to soft segments composed of castor oil did not appear while the Tg2 of the PA phase was observed, but it shifted toward a higher temperature compared with that estimated according to the Fox equation. The Tg3, which might be related to the compatible region
between the hard and soft segments in PUU, shifted to a lower temperature compared to that of pure PUU (CP-01). In addition, both the values of enthalpy for the disruption of ordered hard segment structure reduced dramatically. These experimental results revealed that there should have been some compatibility between the PUU and PA phases in this hybrid system, although the network structure existed in the system, as the castor oil is a trifunctional polyol. Table 3 shows that the values of Xb,UA and Xo,UA resulting from the hydrogen bonding of urea carbonyl groups in PUA-10 were seriously suppressed by introducing the castor oil soft segment in PUU, indicating that it destroys both the ordered and disordered hydrogen bonding of urea carbonyl groups in PUU. The experimental data of the DSC scan results and the FTIR measurements for PUA-11 were just between those for PUA-10 and PUA-01. These experimental behaviors should be expected. To examine the morphologies for these systems directly, the rupture surfaces of different PUU and PUA specimens were observed by means of SEM, as shown in Figure 5. The photomicrographs of PUA-01 (D) exhibited the fiberlike crystals composed of hard segments in PUU clearly, which grew up from the crystal nucleus, although they were depressed somewhat by the PA phase, as compared with those of CP-01 (A). Figure 5 also shows the ordered hard segments in PUU for PUA-11 (E), but their interfaces were not so clear. For PUA-10 (F), such ordered hard segments were more obscure compared with those of PUA-01 (D) and PUA-11 (E). Moreover, a particle-accumulation morphology was observed for CP-10 (C), which was formed during the film-formation process. To confirm this experimental phenomenon, a solution-cast film (called as SCP-10) was prepared with the same composition of CP-10 and was observed by using SEM (see Figure 5B). It was found that such particle-accumulation morphology disappeared, but the clear ordered hard segment structure was observed for SCP10. 3.2. Mechanical Properties. The mechanical properties for different PUAs are listed in Table 5. For PUA-01, as there was appreciable compatibility between the PUU and PA phases, the lowering of some mechanical properties was observed compared with those of CP-01. For PUA-10 based on castor oil, with some compatibility between the two phases and with network structure in the system, the hybrid of PUU and PA components changed the particle-accumula-
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Figure 5. SEM photomicrographs of different PUUs and PUAs: (A) CP-01; (B) SCP-10; (C) CP-10; (D) PUA-01; (E) PUA-11; (F) PUA-10. Table 5. Mechanical Properties of Different PUUs and PUAs castor oil/ tensile elastic GE-210 strength elongation at module samples (wt/wt) (MPa) break (%) (MPa) hardnessa CP-10 CP-01 PUA-10 PUA-31 PUA-11 PUA-13 PUA-01 a
100/0 0/100 100/0 75/25 50/50 25/75 0/100
11.5 34.2 16.0 18.6 23.0 26.0 27.7
51.9 452.7 113.8 185.8 265.6 356.4 448.3
217.4 110.1 251.6 253.8 225.2 193.0 171.9
0.67 0.52 0.64 0.59 0.54 0.44 0.37
Relative hardness based on the value of hardness for glass as 1.00.
tion morphology existing in the CP-10 specimen, giving rise to great improvement of mechanical properties for PUA-10 compared to those of CP-10. Other PUA specimens (PUA11 and PUA-31) exhibited excellent comprehensive mechanical properties, resulting from the reinforcement of the crosslinked PUU phase in the systems. In conclusion, the appreciable miscibility between the PUU and PA phases was observed for PUA-01 prepared with GE210, resulting in lowering of some mechanical properties compared with those of pure PUU synthesized with GE210 (CP-01). For the PUA-10 based on castor oil, the hybrid of PUU and PA components changed the particle-accumulation morphology, which was found in another pure PUU (CP10) made from castor oil. In this case, a great improvement of mechanical properties for PUA-10 was observed. The other PUA specimens with high content of castor oil exhibited excellent comprehensive mechanical properties, which could be attributed to the reinforcement of the crosslinked PUU phase existing in the systems.
Acknowledgment. The authors sincerely thank Professor Zhang Z. P. for discussing the SEM photomicrographs and the WAXD patterns. References and Notes (1) Xiao, H.; Xiao, H. X.; Frisch, K. C.; Malwitz, N. J. Macromol. Sci.s Pure Appl. Chem. 1995, A32 (2), 169-177. (2) Kim, B. K.; Lee, J. C. Polymer 1996, 37 (3), 469-475. (3) Bao, H.; Zhang, Z. P.; Ying, S. K. Polymer 1996, 37 (13), 27512754. (4) Xu, Q.; Jin, H.; Hu, C. P.; Ying, S. K. J. Gongneng Gaofenzi Xuebao (Funct. Polym.) (in Chinese) 1999, 12 (4), 405-408. (5) Hu, C. P.; Hu, Y. S.; Tao, Y. Chinese Invention Patent Application No. 991113871.6. (6) Seymour, R. W.; Cooper, S. L. Macromolecules 1973, 6, 48-53. (7) Chen, N. P.; Chen, Y, L.; Wang, D. N.; Hu, C. P.; Ying, S. K. J. Appl. Polym. Sci. 1992, 46, 2075-2080. (8) Luo, N.; Wang, D. N.; Ying, S. K. Macromolecules 1997, 30, 44054409. (9) Wang, G. Y.; Zhu, M. Q.; Hu, C. P. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 136-144. (10) Xue, Q. The Application of Spectum Methods in the Studies of Polymer Structures (in Chinese); Higher Education Press: Beijing, 1995; pp 49-54. (11) He, M. J., Cheng, W. X., Dong, X. X., Eds. Macromolecular Physics (in Chinese); Revised Edition; Fudan University Press: Shanghai, 1990; pp 248-252. (12) Ying, S. K., Yu, F. N., Eds. Principle of Copolymerization (in Chinese); Chemical Industry Press: Beijing, 1984; p 520. (13) Luo, N.; Wang, D. N.; Ying, S. K. Polymer, 1996, 37 (16), 35773583. (14) Ishihara, H.; Kimura, I.; Yoshihara, N. J. Macromol. Sci., Phys. 1983-84, B22 (5&6), 713-733. (15) Born, L. Hespe, H. Colloid Polym. Sci. 1985, 263, 335. (16) Wang, G. Y.; Hu, C. P. Acta Polym. Sin. 1999, (1), 24-30. (17) Wang, G. C.; Fang, B.; Zhang, Z. P. Polymer 1994, 35 (15), 3178-3183.
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