- Email: [email protected]
Hydrogen bonding interactions in polyureas as studied by variable-temperature high-resolution 1HNMR spectroscopy
Journal of Molecular Structure 471 (1998) 183±188
Hydrogen bonding interactions in polyureas as studied by variable-temperature high-resolution 1H NMR spectroscopy Q. Chen a,*, G. Yang a, Y. Wang a, X. Wu a, H. Kurosu b, I. Ando c a
Analytical Center, East China Normal University Shanghai 200062, People's Republic of China b Department of Apalel Science, Nara Woman's University, Nara-shi, Nara, Japan c Department of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo, Japan Received 13 March 1998; accepted 6 April 1998
Abstract Variable-temperature high-resolution 1 H NMR experiments on four polyurea samples with different kinds of hard segments and soft segments were carried out in dimethylformamide solution. From these experimental results, it was found that the NH groups of the hard segments of the polymers form different styles of hydrogen-bonds between the amide group of dimethylformamide and the oxygen atom of the soft segment, and that the formation of hydrogen-bonds between the NH groups and oxygen atoms greatly depends on the molecular weight of the soft segment of the polymers. q 1998 Elsevier Science B.V. All rights reserved Keywords: Hydrogen bonding; Intermolecular interaction; Polyureas; Structure; Variable-temperature high-resolution H NMR spectroscopy
1
1. Introduction Polyurethane and polyurea are classes of thermalplastic polymers with signi®cant industrial importance [1]. It is now well known that the hydrogenbonding interactions play an important role in determining physical properties of polyurethanes and polyureas. For this, it is very important to elucidate hydrogen-bonded structure of the polymers. Infrared spectroscopy (IR) [2±5] has been proved to be a powerful technique for studying the hydrogen bonds formed between the NH group and the carbonyl groups. However, the low resolution nature of IR often hinders the deep-going or quantitative study of the hydrogen-bonding interactions. Recently, Lu et al. * Corresponding author. E-mail: [email protected]
[6, 7] have shown from high-resolution NMR studies that in concentrated solution of certain kinds of polyurethane zwitterionomers and polyureas, the hard segments can form micelles and the hydrogenbonding plays an important role for the formation of micelles. They have shown that the 1 H spectrum of polyurethanes and polyureas often has the number of NH peaks more than that expected from chemical structure of the polymers and the extra NH peaks come from hydrogen bondings with different styles. They also showed that the high-resolution solutionstate NMR can be employed to study the hydrogenbonding interactions of bulk polymer materials like polyurethanes and polyureas. On the other hand, Natansohn et al. have shown from 1 H NMR study on polyurethanes based on poly(caprolactone) diol and polymers based poly(tetramethylene oxide) and
0022-2860/98/$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII S0 022-2860(98)004 02-5
184
Q. Chen et al. / Journal of Molecular Structure 471 (1998) 183±188
Scheme 1. (a) MDI-650 and MDI-1900; (b) HDI-650 and HDI-1000.
diphenylmethane 4,4-diisocynate that the extra NH peaks come from the by-produced structures of NH groups formed in the synthesis process. As mentioned before, there are two interpretations for the appearance of the extra NH peaks. From such situation, in this paper we aim to clarify the origin of the extra NH peaks of polyereas. Furthermore, the hydrogen-bonding interaction in the polymer systems will be studied in details, on the basis of the variable-temperature (VT) high-resolution 1 H NMR spectra and the 1 H spin-lattice relaxation times of a series of polyureas with different kinds of hard segments and soft segments with different lengths. 2. Experimental 2.1. Materials Polyurea samples were kindly provided by Professor X. Yu of Nanjing University and the synthesis procedure of the samples was described elsewhere [8]. These samples are polymers based on poly(tetramethylene oxide), di-p-aminobenzonate (PA) with different molecular weight and diphenylmethane 4,4 0 -diisocynate(MDI) (Polyscience) or hexamethylene diisocynate (HDI). The samples are, therefore, named as MDI-650, MDI-1900, HDI-650 and HDI-1000 (Scheme 1), respectively, and the numbers here represent the average molecular weight of PA. 2.2. Nmr measurements 1
H high resolution spectra were recorded on a JEOL-GSX500 NMR spectrometer and a Bruker
MSL-300 NMR spectrometer at 500 and 300 MHz, respectively, with a single pulse under the quadrature detection mode. The scan number is 16 or 32. All samples are dissolved in deuterated N, N-dimethylformamide (DMF) at more than 10%(w/v) concentration. The chemical shifts were determined relative to the lower ®eld signal (8.0 ppm) of DMF. The 1 H spinlattice relaxation time (T1) was measured by a standard inverse-recovery pulse sequence on the MSL300 spectrometer. 3. Results and discussion The 500 MHz 1 H NMR spectra of MDI-650 and MDI-1900 samples at room temperature were shown in the range from 8.5 to 10 ppm in Fig. 1, since we are concerned with NH peaks. In the 1 H spectrum of MDI-1900, ®ve NH peaks appear at 9.8 ppm (A), 9.4 ppm (B), 9.1 ppm (C), 8.8 ppm (D) and 8.6 ppm (E). However, only two NH peaks in the 1 H spectrum of MDI-650 corresponding to Peaks C and D exhibit intense intensity. From the molecular structure of these two polymers (Scheme 1) under consideration, these experimental results lead to the following two questions immediately: (1) why do ®ve NH peaks appear in the 1 H spectrum of MDI-1900 which has only two structurally different sites of NH according to its structural formula?; (2) why does the number of the 1 H signals for NH group of two samples with almost same in the hard segments differ signi®cantly from each other? There are two possible answers to the ®rst question. The ®rst one is that some secondary reactions occurred in the synthesis process of MDI-1900,
Q. Chen et al. / Journal of Molecular Structure 471 (1998) 183±188
Fig. 1. 500 MHz 1 H NMR spectra of MDI-650 and MDI-1900 in 10%(w/v) DMF solution at room temperature in the 8±10 ppm range.
which lead to more than two structural different sites of NH in the hard segment. The second one is that there exist several styles of hydrogen-bonds formed between the NH and other groups such as the oxygen in soft segment, the carbonyl group in the hard segment and in solvent. In order to clarify which explanation is suitable for our results, variable temperature (VT) 1 H NMR measurements on both of MDI-1900 and MDI-650 in the temperature range from room temperature (RT) to 1208C were carried out. The temperature dependence of NH peaks of MDI-1900 are shown in Fig. 2, and the chemical shifts of NH peaks of both of the samples are plotted against temperature in Figs. 3 and 4. As the temperature is increased, all of the NH peaks shift to up®eld, while other signals except for water signal (3.5 ppm at RT) are independent of temperature. This shows that all of the NH protons which appear in the region of 8±10 ppm form hydrogen bonds and the exchange between hydrogen-bonded and non-hydrogen-bonded states is fast enough to give only one peak for each style of hydrogen bonding in the NMR timescale. As the temperature is increased, hydrogen bonding becomes weaker, and so these peaks shift to up®eld.
185
Fig. 2. Variable-temperature high-resolution 500 MHz 1 H NMR spectra of MDI-1900 in 10%(w/v) solution at room temperature in the 8±10 ppm.
Fig. 3. The dependence of the 1 H chemical shifts of ®ve NH peaks of MDI-1900 and peak of DMF solvent on temperature. Peak A, B, C, D and E, and peak of DMF solvent are corresponding to (W), (X), (S), (V), (K) and (1), respectively.
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Scheme 2. Structure of MDI-1900 by-produced.
Fig. 4. The dependence of the 1 H chemical shifts of the NH peaks of MDI-650 on temperature.
If we look at these observed spectra carefully, the following results can be obtained: 1. Chemical shifts of two NH peaks of MDI-650 and their dependences on temperature are the same as those of peaks C and D of MDI-1900. These two peaks have nearly the same intensity. From the structure of the polymers, we can see that there are two chemically-different NH sites and their content ratio is 1:1. Therefore, it is reasonable to assign peaks C and D to these two sites, of which is each forming a certain style of hydrogen bonding. 2. The contents ratio of peaks A and B is also about 1:1. The difference between these two peaks is close to that between peaks C and D. As the temperature is increased, these two peaks move to up®eld and its dependence is larger than that of peaks C and D. These experimental results lead to following conclusive assignments: peaks A and B come from the same NH sites as that of peaks C and D, but the NH groups are forming different style of hydrogen bonds. If considering the strength of hydrogen bondings, it is reasonable to assign peaks A and B to the NH groups hydrogen-bonded with the oxygen atom in the soft segment, and to assign peaks C and D to the NH groups hydrogen-bonded with the carbonyl group of the solvent.
3. As the temperature is raised to 1208C, peaks A and B suddenly disappear. This suggests that the hydrogen-bonds between the NH and the oxygen of soft segment are decomposed above 1208C; This also supports that peaks A and B come from different style of hydrogen bonds and does not come from the by-produced structure formed in polymerization process. 4. By comparising with model compounds reported previously [9], peak E of MDI-1900 can be assigned to structure as shown in Scheme 2. The peak moves up®eld with an increase in temperature like the case of peaks C and D. This suggests that the corresponding NH groups are hydrogenbonded with the carbonyl group of the solvent. 5. The change in conformation and the increase in mobility of the polymer chain allows chemical exchanges between different hydrogen-bonds. In order to verify this, 1 H high-resolution NMR spectra of both the samples were measured at different ®eld strengths (500, 300.13 and 90 MHz). The spectra were independent of the ®eld strength. Therefore, it can be said that the above-mentioned chemical exchange process may not occur or may be rather slow. This suggests that hard segments may form micelles in the solution [6, 7, 10], which will circumvent such an exchange. In order to verify above assignments, 1 H spinlattice relaxation time (T1) of the NH groups of both samples were measured at room temperature. The obtained results are listed in Table 1. From this table, it can be found that two NH peaks of MDI650 exhibit the same value of T1 as peaks C and D of MDI-1900. This obviously supports the above assignment. Five NH peaks of MDI-1900 can be divided into two groups according to T1. The T1 values
Q. Chen et al. / Journal of Molecular Structure 471 (1998) 183±188 Table 1 1 H spin-lattice relaxation times of the NH groups of MDI-650 and MDI-1900 ppm 9.8 9.4 9.1 8.8 8.6
MDI-650 T1(s)
0.40 0.40
MDI-1900 T1(s) 0.73 0.92 0.41 0.40 0.48
of peaks A and B are longer than those of other three peaks. Since the difference in T1 value re¯ects the difference in molecular motion in the MHz region, the NH groups of peaks C and D are in different chemical environments from the NH groups of peaks A, B and E. This also supports our above interpretation. The remaining question is why the spectrum of MDI-650 is so different from that of MDI-1900. One reason which leads to the difference in these
187
two spectra may be different the length of soft segments in both of the samples. The soft segments of MDI-1900 are longer than those of MDI-650. In other words, the ratio of the number of oxygen atoms in the soft segment to the number of NH groups in the hard segment of MDI-1900 is about three times larger than that of MDI-650. Therefore, the possibility of forming hydrogen-bonds between the NH groups and oxygen atoms in the soft segment in MDI-1900 is higher than that for MDI-650. However, Fig. 1 shows that the intensity of two down®eld NH peaks to the two strong NH peaks of MDI-1900 is about 10 times higher than that of MDI-650. This suggests that the different length of the soft segments leads to different conformation of polymers in solution. In the case of MDI-1900, the oxygen atom can access the NH groups in the hard segment more easily compared with the case of MDI-650. Another question we must answer is that why the most up®eld NH peak in the spectrum of MDI-1900 is not observed in the spectrum of MDI-650. This can easily be
Fig. 5. 500 MHz 1 H NMR spectra of HDI-650 and HDI-1000 in 10%(w/v) DMF solution at room temperature in the 6±10 ppm. The asterisks indicate the signals of NH groups.
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Q. Chen et al. / Journal of Molecular Structure 471 (1998) 183±188
visualized in the following way: the higher the molecular weight of the soft segment means the higher the viscosity of the reactant system in the synthesis process and, therefore, higher possibility of by-producing reactions. Although the above results are obtained from solution NMR studies, it is believed that to some extent these discussions are also suitable to elucidate hydrogen-bonding interactions of these polymers in the bulk state, at least for solution-casted samples. In our previous work [8], it was found by solid-state high-resolution and 1 H wide-line NMR spectroscopy that the regularities of the hard-segment of MDI-650 is higher than that of MDI-1900. At that stage, no clear explanation about this phenomenon could be given, but now the explanation can be easily visualized, in the way that the formation of hydrogen-bonds between the NH group in the hard segment and the oxygen atom in the soft segment will enhance the compatibility of both of the segments, and so destroy the regularity of the hard-segment domain. In addition to the NMR experiments on MDI based polyureas, similar experiments are also carried out on HDI based samples. Both of HDI-650 and HDI-1000 gave the spectrum with two NH signals with almost same intensity (Fig. 5). It can be said that in both of the HDI based samples, a small amount of the NH groups in the hard segments are hydrogen-bonded with the oxygen atoms in the soft segment like the case of MDI-650. Therefore, it appears that the length of the soft segment is an important factor for the formation of hydrogen-bonds between the soft and hard segments. 4. Conclusions From the above results, it can be concluded that in
DMF solution of polyureas, the NH groups of the hard segment can form different kind of hydrogen-bonds with the polar groups of the polymer and the solvent. The number of the hydrogen-bonds formed between the NH groups and the oxygen atoms of the soft segment depends on the molecular weight of the soft segments. It appears that the soft segment with long length can access the hard segment more easily compared with the soft segment with short length. Such a result gives a good explanation about our previous observations obtained by solid-state 13 C CP/MAS and 1 H wideline NMR experiments. Acknowledgements This work is partly supported by the KUASHIJI RENCAI foundation of Nation's Education Committee. References [1] S.L. Cooper, A.V. Tobolsky, J. Appl. Polym. Sci. 10 (1966) 1837. [2] Yu.M. Boyaychuk, L. Ya, Rappoport, V.N. Nikitin and N, P. Apukhtina, Polym. Sci. USSR (Engl. Transl.) 7 (1965) 859. [3] G.A. Senich, W.J. MacKnight, Macromolecules 13 (1980) 106. [4] K.K.S. Hwang, T.A. Speckhard, S.L. Cooper, J. Macromol. Sci. B23 (2) (1984) 153. [5] D.J. Skrovanek, S.E. Howe, P.C. Painter, M.M. Coleman, Macromolecules 18 (1985) 1676. [6] X. Lu, Y. Wang, X. Wu, Polymer 33 (1992) 958. [7] X. Lu, Y. Wang, X. Wu, Polymer 35 (1994) 2315. [8] Q. Chen, Y. Wang, X. Yu, X. Wu, Chin. J. Polym. Sci. 10 (1992) 287. [9] A. Natansohn, M. Rutkowska, A. Eisenberg, Polymer 28 (1987) 885. [10] G. Yang, Q. Chen, Y. Wang, X. Wu, Polym. J. 29 (1997) 108.
Hydrogen bonding interactions in polyureas as studied by variable-temperature high-resolution 1H NMR spectroscopy Q. Chen a,*, G. Yang a, Y. Wang a, X. Wu a, H. Kurosu b, I. Ando c a
Analytical Center, East China Normal University Shanghai 200062, People's Republic of China b Department of Apalel Science, Nara Woman's University, Nara-shi, Nara, Japan c Department of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo, Japan Received 13 March 1998; accepted 6 April 1998
Abstract Variable-temperature high-resolution 1 H NMR experiments on four polyurea samples with different kinds of hard segments and soft segments were carried out in dimethylformamide solution. From these experimental results, it was found that the NH groups of the hard segments of the polymers form different styles of hydrogen-bonds between the amide group of dimethylformamide and the oxygen atom of the soft segment, and that the formation of hydrogen-bonds between the NH groups and oxygen atoms greatly depends on the molecular weight of the soft segment of the polymers. q 1998 Elsevier Science B.V. All rights reserved Keywords: Hydrogen bonding; Intermolecular interaction; Polyureas; Structure; Variable-temperature high-resolution H NMR spectroscopy
1
1. Introduction Polyurethane and polyurea are classes of thermalplastic polymers with signi®cant industrial importance [1]. It is now well known that the hydrogenbonding interactions play an important role in determining physical properties of polyurethanes and polyureas. For this, it is very important to elucidate hydrogen-bonded structure of the polymers. Infrared spectroscopy (IR) [2±5] has been proved to be a powerful technique for studying the hydrogen bonds formed between the NH group and the carbonyl groups. However, the low resolution nature of IR often hinders the deep-going or quantitative study of the hydrogen-bonding interactions. Recently, Lu et al. * Corresponding author. E-mail: [email protected]
[6, 7] have shown from high-resolution NMR studies that in concentrated solution of certain kinds of polyurethane zwitterionomers and polyureas, the hard segments can form micelles and the hydrogenbonding plays an important role for the formation of micelles. They have shown that the 1 H spectrum of polyurethanes and polyureas often has the number of NH peaks more than that expected from chemical structure of the polymers and the extra NH peaks come from hydrogen bondings with different styles. They also showed that the high-resolution solutionstate NMR can be employed to study the hydrogenbonding interactions of bulk polymer materials like polyurethanes and polyureas. On the other hand, Natansohn et al. have shown from 1 H NMR study on polyurethanes based on poly(caprolactone) diol and polymers based poly(tetramethylene oxide) and
0022-2860/98/$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII S0 022-2860(98)004 02-5
184
Q. Chen et al. / Journal of Molecular Structure 471 (1998) 183±188
Scheme 1. (a) MDI-650 and MDI-1900; (b) HDI-650 and HDI-1000.
diphenylmethane 4,4-diisocynate that the extra NH peaks come from the by-produced structures of NH groups formed in the synthesis process. As mentioned before, there are two interpretations for the appearance of the extra NH peaks. From such situation, in this paper we aim to clarify the origin of the extra NH peaks of polyereas. Furthermore, the hydrogen-bonding interaction in the polymer systems will be studied in details, on the basis of the variable-temperature (VT) high-resolution 1 H NMR spectra and the 1 H spin-lattice relaxation times of a series of polyureas with different kinds of hard segments and soft segments with different lengths. 2. Experimental 2.1. Materials Polyurea samples were kindly provided by Professor X. Yu of Nanjing University and the synthesis procedure of the samples was described elsewhere [8]. These samples are polymers based on poly(tetramethylene oxide), di-p-aminobenzonate (PA) with different molecular weight and diphenylmethane 4,4 0 -diisocynate(MDI) (Polyscience) or hexamethylene diisocynate (HDI). The samples are, therefore, named as MDI-650, MDI-1900, HDI-650 and HDI-1000 (Scheme 1), respectively, and the numbers here represent the average molecular weight of PA. 2.2. Nmr measurements 1
H high resolution spectra were recorded on a JEOL-GSX500 NMR spectrometer and a Bruker
MSL-300 NMR spectrometer at 500 and 300 MHz, respectively, with a single pulse under the quadrature detection mode. The scan number is 16 or 32. All samples are dissolved in deuterated N, N-dimethylformamide (DMF) at more than 10%(w/v) concentration. The chemical shifts were determined relative to the lower ®eld signal (8.0 ppm) of DMF. The 1 H spinlattice relaxation time (T1) was measured by a standard inverse-recovery pulse sequence on the MSL300 spectrometer. 3. Results and discussion The 500 MHz 1 H NMR spectra of MDI-650 and MDI-1900 samples at room temperature were shown in the range from 8.5 to 10 ppm in Fig. 1, since we are concerned with NH peaks. In the 1 H spectrum of MDI-1900, ®ve NH peaks appear at 9.8 ppm (A), 9.4 ppm (B), 9.1 ppm (C), 8.8 ppm (D) and 8.6 ppm (E). However, only two NH peaks in the 1 H spectrum of MDI-650 corresponding to Peaks C and D exhibit intense intensity. From the molecular structure of these two polymers (Scheme 1) under consideration, these experimental results lead to the following two questions immediately: (1) why do ®ve NH peaks appear in the 1 H spectrum of MDI-1900 which has only two structurally different sites of NH according to its structural formula?; (2) why does the number of the 1 H signals for NH group of two samples with almost same in the hard segments differ signi®cantly from each other? There are two possible answers to the ®rst question. The ®rst one is that some secondary reactions occurred in the synthesis process of MDI-1900,
Q. Chen et al. / Journal of Molecular Structure 471 (1998) 183±188
Fig. 1. 500 MHz 1 H NMR spectra of MDI-650 and MDI-1900 in 10%(w/v) DMF solution at room temperature in the 8±10 ppm range.
which lead to more than two structural different sites of NH in the hard segment. The second one is that there exist several styles of hydrogen-bonds formed between the NH and other groups such as the oxygen in soft segment, the carbonyl group in the hard segment and in solvent. In order to clarify which explanation is suitable for our results, variable temperature (VT) 1 H NMR measurements on both of MDI-1900 and MDI-650 in the temperature range from room temperature (RT) to 1208C were carried out. The temperature dependence of NH peaks of MDI-1900 are shown in Fig. 2, and the chemical shifts of NH peaks of both of the samples are plotted against temperature in Figs. 3 and 4. As the temperature is increased, all of the NH peaks shift to up®eld, while other signals except for water signal (3.5 ppm at RT) are independent of temperature. This shows that all of the NH protons which appear in the region of 8±10 ppm form hydrogen bonds and the exchange between hydrogen-bonded and non-hydrogen-bonded states is fast enough to give only one peak for each style of hydrogen bonding in the NMR timescale. As the temperature is increased, hydrogen bonding becomes weaker, and so these peaks shift to up®eld.
185
Fig. 2. Variable-temperature high-resolution 500 MHz 1 H NMR spectra of MDI-1900 in 10%(w/v) solution at room temperature in the 8±10 ppm.
Fig. 3. The dependence of the 1 H chemical shifts of ®ve NH peaks of MDI-1900 and peak of DMF solvent on temperature. Peak A, B, C, D and E, and peak of DMF solvent are corresponding to (W), (X), (S), (V), (K) and (1), respectively.
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Q. Chen et al. / Journal of Molecular Structure 471 (1998) 183±188
Scheme 2. Structure of MDI-1900 by-produced.
Fig. 4. The dependence of the 1 H chemical shifts of the NH peaks of MDI-650 on temperature.
If we look at these observed spectra carefully, the following results can be obtained: 1. Chemical shifts of two NH peaks of MDI-650 and their dependences on temperature are the same as those of peaks C and D of MDI-1900. These two peaks have nearly the same intensity. From the structure of the polymers, we can see that there are two chemically-different NH sites and their content ratio is 1:1. Therefore, it is reasonable to assign peaks C and D to these two sites, of which is each forming a certain style of hydrogen bonding. 2. The contents ratio of peaks A and B is also about 1:1. The difference between these two peaks is close to that between peaks C and D. As the temperature is increased, these two peaks move to up®eld and its dependence is larger than that of peaks C and D. These experimental results lead to following conclusive assignments: peaks A and B come from the same NH sites as that of peaks C and D, but the NH groups are forming different style of hydrogen bonds. If considering the strength of hydrogen bondings, it is reasonable to assign peaks A and B to the NH groups hydrogen-bonded with the oxygen atom in the soft segment, and to assign peaks C and D to the NH groups hydrogen-bonded with the carbonyl group of the solvent.
3. As the temperature is raised to 1208C, peaks A and B suddenly disappear. This suggests that the hydrogen-bonds between the NH and the oxygen of soft segment are decomposed above 1208C; This also supports that peaks A and B come from different style of hydrogen bonds and does not come from the by-produced structure formed in polymerization process. 4. By comparising with model compounds reported previously [9], peak E of MDI-1900 can be assigned to structure as shown in Scheme 2. The peak moves up®eld with an increase in temperature like the case of peaks C and D. This suggests that the corresponding NH groups are hydrogenbonded with the carbonyl group of the solvent. 5. The change in conformation and the increase in mobility of the polymer chain allows chemical exchanges between different hydrogen-bonds. In order to verify this, 1 H high-resolution NMR spectra of both the samples were measured at different ®eld strengths (500, 300.13 and 90 MHz). The spectra were independent of the ®eld strength. Therefore, it can be said that the above-mentioned chemical exchange process may not occur or may be rather slow. This suggests that hard segments may form micelles in the solution [6, 7, 10], which will circumvent such an exchange. In order to verify above assignments, 1 H spinlattice relaxation time (T1) of the NH groups of both samples were measured at room temperature. The obtained results are listed in Table 1. From this table, it can be found that two NH peaks of MDI650 exhibit the same value of T1 as peaks C and D of MDI-1900. This obviously supports the above assignment. Five NH peaks of MDI-1900 can be divided into two groups according to T1. The T1 values
Q. Chen et al. / Journal of Molecular Structure 471 (1998) 183±188 Table 1 1 H spin-lattice relaxation times of the NH groups of MDI-650 and MDI-1900 ppm 9.8 9.4 9.1 8.8 8.6
MDI-650 T1(s)
0.40 0.40
MDI-1900 T1(s) 0.73 0.92 0.41 0.40 0.48
of peaks A and B are longer than those of other three peaks. Since the difference in T1 value re¯ects the difference in molecular motion in the MHz region, the NH groups of peaks C and D are in different chemical environments from the NH groups of peaks A, B and E. This also supports our above interpretation. The remaining question is why the spectrum of MDI-650 is so different from that of MDI-1900. One reason which leads to the difference in these
187
two spectra may be different the length of soft segments in both of the samples. The soft segments of MDI-1900 are longer than those of MDI-650. In other words, the ratio of the number of oxygen atoms in the soft segment to the number of NH groups in the hard segment of MDI-1900 is about three times larger than that of MDI-650. Therefore, the possibility of forming hydrogen-bonds between the NH groups and oxygen atoms in the soft segment in MDI-1900 is higher than that for MDI-650. However, Fig. 1 shows that the intensity of two down®eld NH peaks to the two strong NH peaks of MDI-1900 is about 10 times higher than that of MDI-650. This suggests that the different length of the soft segments leads to different conformation of polymers in solution. In the case of MDI-1900, the oxygen atom can access the NH groups in the hard segment more easily compared with the case of MDI-650. Another question we must answer is that why the most up®eld NH peak in the spectrum of MDI-1900 is not observed in the spectrum of MDI-650. This can easily be
Fig. 5. 500 MHz 1 H NMR spectra of HDI-650 and HDI-1000 in 10%(w/v) DMF solution at room temperature in the 6±10 ppm. The asterisks indicate the signals of NH groups.
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Q. Chen et al. / Journal of Molecular Structure 471 (1998) 183±188
visualized in the following way: the higher the molecular weight of the soft segment means the higher the viscosity of the reactant system in the synthesis process and, therefore, higher possibility of by-producing reactions. Although the above results are obtained from solution NMR studies, it is believed that to some extent these discussions are also suitable to elucidate hydrogen-bonding interactions of these polymers in the bulk state, at least for solution-casted samples. In our previous work [8], it was found by solid-state high-resolution and 1 H wide-line NMR spectroscopy that the regularities of the hard-segment of MDI-650 is higher than that of MDI-1900. At that stage, no clear explanation about this phenomenon could be given, but now the explanation can be easily visualized, in the way that the formation of hydrogen-bonds between the NH group in the hard segment and the oxygen atom in the soft segment will enhance the compatibility of both of the segments, and so destroy the regularity of the hard-segment domain. In addition to the NMR experiments on MDI based polyureas, similar experiments are also carried out on HDI based samples. Both of HDI-650 and HDI-1000 gave the spectrum with two NH signals with almost same intensity (Fig. 5). It can be said that in both of the HDI based samples, a small amount of the NH groups in the hard segments are hydrogen-bonded with the oxygen atoms in the soft segment like the case of MDI-650. Therefore, it appears that the length of the soft segment is an important factor for the formation of hydrogen-bonds between the soft and hard segments. 4. Conclusions From the above results, it can be concluded that in
DMF solution of polyureas, the NH groups of the hard segment can form different kind of hydrogen-bonds with the polar groups of the polymer and the solvent. The number of the hydrogen-bonds formed between the NH groups and the oxygen atoms of the soft segment depends on the molecular weight of the soft segments. It appears that the soft segment with long length can access the hard segment more easily compared with the soft segment with short length. Such a result gives a good explanation about our previous observations obtained by solid-state 13 C CP/MAS and 1 H wideline NMR experiments. Acknowledgements This work is partly supported by the KUASHIJI RENCAI foundation of Nation's Education Committee. References [1] S.L. Cooper, A.V. Tobolsky, J. Appl. Polym. Sci. 10 (1966) 1837. [2] Yu.M. Boyaychuk, L. Ya, Rappoport, V.N. Nikitin and N, P. Apukhtina, Polym. Sci. USSR (Engl. Transl.) 7 (1965) 859. [3] G.A. Senich, W.J. MacKnight, Macromolecules 13 (1980) 106. [4] K.K.S. Hwang, T.A. Speckhard, S.L. Cooper, J. Macromol. Sci. B23 (2) (1984) 153. [5] D.J. Skrovanek, S.E. Howe, P.C. Painter, M.M. Coleman, Macromolecules 18 (1985) 1676. [6] X. Lu, Y. Wang, X. Wu, Polymer 33 (1992) 958. [7] X. Lu, Y. Wang, X. Wu, Polymer 35 (1994) 2315. [8] Q. Chen, Y. Wang, X. Yu, X. Wu, Chin. J. Polym. Sci. 10 (1992) 287. [9] A. Natansohn, M. Rutkowska, A. Eisenberg, Polymer 28 (1987) 885. [10] G. Yang, Q. Chen, Y. Wang, X. Wu, Polym. J. 29 (1997) 108.