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Solid-state variable-temperature NMR study of the phase separation of polybutadiene polyurethane zwitterionomers
Journal of
MOLECULAR STRUCTURE ELSEVIER
Journal of Molecular Structure 323 (1994) 209-214
Solid-state variable-temperature NMR study of the phase separation of polybutadiene polyurethane zwitterionomers G. Yang”, Q. Chena>*, Y. Wanga, C. Yangb, X. Wu” aAnalytical Center, East China Normal University, Shanghai 200062, China bDepartment of Chemistry, Nanjing University, Nanjing 210008, China
(Received 3 February 1994)
Abstract
Polybutadiene polyurethane (PBDPU) zwitterionomers based on 4,4’-diphenylmethane diisocyanate (MDI), methyldiethanolamine (MDEA), and hydroxy terminated polybutadiene are studied with variable-temperature (VT) wide-line ‘H NMR. Spin-spin relaxation times (T2) and spin-lattice relaxation times (T,) are measured. It is found that phase separation of PBDPU does not change significantly upon ionization. The initial incorporation of ionization groups destroys the crystallinity of the hard segment while further ionization enhances physical crosslinks in the hard phase. The results are compared with a previous VT NMR study on polyether polyurethane zwitterionomers based on MDI, MDEA and 1000 Da molecular weight polytetramethylene oxide.
1. Introduction Polyurethane elastomers are a kind of widely used thermoplastic with a two-phase microstructure. As with other thermoplastics, the mechanical properties of the material are greatly influenced by the phase separation between the soft and the hard segments. In turn, the phase structure of the system is influenced by many factors, including chemical structure, molecular weight, the relative contents of both hard and soft segments, introduction of ionization groups into the hard segments, and so on [ 11. Through changing the above factors, there exists a great potential to tailor the mechanical properties of the material. For this reason, the influence of the factors mentioned above on the morphology of the material has been extensively studied by *Corresponding
author.
various methods including electron microscopy, small angle X-ray scattering (SAXS), infrared dichroism, dynamic mechanical analysis, differential scanning calorimetry (DSC), stressstrain testing, etc. [1,7] Unfortunately, there often lacks a conformity among the results from different methods. Both solid-state and solution NMR spectroscopy have been used to study polyurethane elastomers and have proved to be power tools in probing the phase structure of the material. Wideline ‘H NMR has often been used to study the microphase structures of polyurethanes [2-41, and the two components of the free-induction decay (FID) signal are often directly related to the two phases at room temperature. Spin diffusion experiments are also often employed to study the interface or domain sizes of the two phases [3]. Solid-state high-resolution 13C NMR has proved
0022-2860/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI 0022-2860(94)07994-L
G. Yang et al.lJ. Mol. Strut.
210
to be useful in studying the microstructure of polyurethanes [3, 51. However, its application is often restricted due to the broad linewidth of the hard segment. High-resolution NMR of concentrated solutions is also used to study the phase structure and the interactions between the hard and soft segments [lo] on the assumption that some strong interactions between the hard and soft segments remain the same as they are in the solid-state. In this study, we employ variable temperature (VT) wide-line ‘H NMR to trace the variation of the segmental motions with temperature and probe the phase structure of the system indirectly. Compared with the above-mentioned NMR approaches, VT NMR has the following advantages. (1) Since a very broad distribution of correlation time in a single domain can also result in two-component FID [6], it is unreliable to use the two FID components to represent the hard and the soft phase directly, although it is a typical practice in wide-line NMR. (2) The high sensitivity and the quantitative character of the wide-line NMR has made a semi-quantitative study of the hard segment possible. (3) The interaction between the hard and soft segment can be studied without the influence of the solvent.
2. Experimental 2.1. Material Polybutadiene polyurethane (PBDPU) zwitterionomers are based on hydroxy terminated polybutadiene (HTPBD) with a molecular weight of 1350 Da and 4,4’-diphenylmethane diisocyanate
323 (1994) 209-214
+I
D7
) ACQ
Fig. 1. Pulse sequence for windowless solid echo. Dl is a 90” pulse; D6 = 8 ps. The echo begins at D7 = D6-1/2Dl.
extended with methyldiethanolamine (MDEA). The ratio of the contents of the three components is MDI:MDEA:HTPBD = 3:2:1. Ionization groups are incorporated into MDEA with the addition of a controlled amount of y-propane sultone [g]. Four samples, as listed in Table 1, are used in NMR measurements. Samples for NMR studies are prepared by solution casting from dimethylformamide solution with no further disposal. NW
2.2. NMR experiments ‘H NMR experiments are performed on a Bruker MSL-300 spectrometer with resonance frequency at 300.13 MHz. FID signals are acquired with a windowless solid echo pulse sequence as shown in Fig. 1. Spin-lattice relaxation time Ti is measured using the inverserecovery method. The ‘H 90” pulse width is 2~s while number of scans is typically 4-8. All samples are studied at temperatures ranging from 173 to 353 K, with FID signals acquired and Ti measured. Temperature is controlled with a Bruker VT- 1000 temperature control unit. Deviation of temperature is f 1 K.
3. Results and discussion 3.1. VT wide-line NA4R method
Table 1 Samples used in NMR measurement Sample
Degree of ionization
Proton content of HTBPD’
PBD-0 PBD-35 PBD-65 PBD-95
0 0.35 0.65 0.95
0.721 0.707 0.695 0.684
a The proton content of HTPBD is calculated from the formula of the polymer.
A typical FID of polyurethane at room temperature is plotted in Fig. 2. Obviously it can be split into two components, one gaussian fast decay and one lorentzian slow decay. The variation of magnetization with time can be represented by the following expression
M:
=ifexp(-&)
+M,exp(--$)
211
G. Yang et al./J. Mol. Struct. 323 (1994) 209-214
O~fMltfAl -C%kXlkit6d ~~.HardComponent .-CGoflcomponerlt
I
.I
-O=PBD35 J PBD-65 r-PBD-95
0
50
loo
150
260
280
300
320
340
360
380
Temperature (K)
Time@) Fig. 2. Scheme of FID decomposition of polyurethane zwitterionomers.
Fig. 3. Variation of contents of the slow decay component with temperature.
where Mf and M, represent the initial values of ma~etization of fast and slow decay respectively and T2f, TZs are the spin-spin relaxation times of corresponding components. By applying nonlinear least squares regression to the FID signals acquired, we can get Tzs and relative proton contents of the fast and slow decay components. Traditionally, FID signals are acquired at room temperature and decomposed. The fast and the slow decay components resulting from FID decomposition are directly related to the hard and soft phases respectively. The relative contents of the two phases can also be determined from the relative content of the two FID components. In this study, FID signals are acquired and decomposed over a wide temperature range. Only one gaussian component can be found in the FID signals acquired below 293 K. The variations of Tzs and the contents of the lorentzian components of four samples at temperatures higher than 293 K are plotted against temperature in Figs. 3 and 4 respectively. Tz of the gaussian component increases slightly with the temperature from about 9ps at 173 K to about 11 ps at 363 K. Since the experiment temperature is well below the Tgof the hard segment and the measurement of a sample is completed in several hours, the phase structure of the samples can be regarded as
invariable in the VT experiments. However, from Figs. 3 and 4, we can see clearly that as the temperature changes, both the T2 and content of the slow decay component change dramatically. Thus it can be concluded that the traditional method of relating two FID signal components to hard and soft phases directly is not reliable, at least in the systems studied here. 140 t 120 loo
ol,..>. 160
ePB5-0 4PBD-35 -cPB5-65 + PBD-95
.‘.
2ocl
240
I
280
320
360
Temperature (K) Fig.4. Variation of T2 of the slow decay component temperature.
with
212
G. Yang et 01./J. Mol. Struct. 323 (1994) 209-214
Since the variation of the FID signals with temperature is dominated by the change in the segmental motion in the samples, which in turn depends largely on the phase structure of the samples, we can use the VT experiments to study the molecular motion and probe the phase structure of the system indirectly. 3.2. Phase structure of samples At temperatures lower than 273 K, both the hard and soft segments are in a rigid state (in T2 sense), so only one gaussian fast decay is presented in the FID signals of the samples. As the temperature increases, there emerges an exponential slow decay component, whose T2 value and content increase with the temperature. It can be seen clearly from Figs. 3 and 4 that the Tzs values and contents of four samples show no notable difference below 313 K. The initial increase in Tzs and contents of the slow decay component is obviously caused by the onset of the segmental motions of the soft segment (HTPBD). Similarity between the initial increase of T2 and the content of slow decay component indicates a similarity in the mobility of the soft segment. Since the mobility of the soft segment is not influenced by the incorporation of ionization groups into the hard segment, it can be concluded that there exists a clean phase separation between the hard and the soft segments in all samples. A further increase of temperature enhances the segmental motion of the hard segment. At temperatures higher than 3 13 K, MD1 and MDEA begin to contribute to the slow decay component and the contents of the slow decay components soon surpass the proton contents of the HTPBD segment in the samples, which are about 70%, as listed in Table 1. As shown in Figs, 3 and 4, divergence between T2 and the contents of the slow decay components emerges among the four samples when the hard segments begin to contribute to the slow decay components. This divergence in T2 and the content of the slow decay components indicates a difference of the mobility of the hard segment, which further implies that introducing ionization groups into hard segments changes the structure of the hard phase. It can be seen from Figs. 3 and 4 that the slow
decay component in polybutadiene (PBD-0) bears the lowest contents and the longest T2 of the four samples. This means the slow decay component in PBD-0 contains the least amount of hard segments, which are less mobile and make T2 shorter. It can be further concluded that the onset of the segmental motion of the hard segment in PBD-0 is the most difficult of the four samples. For the same reason, since PBD-35 possesses the highest content and shortest T2 of the slow decay component, the onset of the segmental motion of its hard segment is the easiest. According to the above discussions, the following conclusions can be drawn. (1) The initial introduction of ionization groups into the hard segment destroys the regularity of the hard phase and renders the hard segments more mobile. The hard segment in PBD-0 is the least mobile in all samples due to its high level of regularity. (2) A further increase in the degree of ionization increases the polarity of the hard segment and enhances physical crosslinks in the hard segment, thus restricting the segmental motions of the hard segment. So the hard segments of both PBD-65 and PBD-95 are less mobile than that of PBD-35. (3) Since the FID signals of PBD-65 and PBD-95 show no notable difference in Figs. 3 and 4, it can be shown that an increase of the degree of ionization above 65% has a limited influence on the phase structure of PBDPU. It can also be seen from Fig. 3 that the content of the soft decay component of PBD-0 remains almost constant at temperatures higher than 343 K. This indicates that the segmental motion of a portion of the hard segments is very difficult, which also suggests that a high order of regularity exists in the hard phase in PBD-0. ‘H spin-lattice relaxation of all samples can be presented in a single-component exponential. Results of simulations are shown in Fig. 5, in which the relaxation time Tl is plotted against temperature. The variation of T, with temperature can be clearly divided into two sections. At temperatures lower than 293K, when both hard and soft segments are in a relatively rigid state, the spin relaxation time is determined by the mobility of both segments. Since the soft phases of four samples are almost the same, PBD-0, whose hard segment is the most rigid, exhibits the longest T, while PBD-35, whose hard segment is
213
C. Yang et al./J. Mal. Sfruct. 323 (1994) 209-214
90
ePBD-0
33PBD-35 aPBD.65 +PBD95
I 160
200
240
280
Temper&ure
320
I
Go
(K)
Fig. 5. Variation of T, with temperature.
the most mobile, exhibits the shortest Tt. However, as the temperature increases, with the rapid increase of the mobility of the soft segments, the relaxation of the soft phase soon becomes so efficient that the soft phase becomes the relaxation center and the ma~etization of the hard segment is mainly relaxed through the mechanism of spin diffusion. Since the mobility of the soft segments is very similar in all samples due to the clean phase separation, the convergence of Ti at temperatures higher than 313 K is quite understandable. So the results of Ti measurement entirely support the conclusions obtained from FID decomposition.
It is very interesting and beneficial to compare this study with a previous study on polyether polyurethane zwitterionomers based on MDI, MDEA, and polytetramethylene oxide glycol (PTMO) [9]. The structures of the samples are similar to that of PBDPU zwitterionomers except that the soft segment is 1000 Da molecular weight PTMO. The samples with ionization degrees 0, 0.35, 0.65 and 0.95 are named PUA, PUB, PUC and PUD respectively. Phase separation in polyether polyurethane zwitterionomers is not as clean as it is in PBDPU zwitterionomers perhaps due to a smaller difference
IO 0 Zxl
250
300
J 350
Temperature(K) Fig. 6. Variation of the contents of the soft segments of polyether polyurethane zwitterionomers.
in the polarity of the hard and soft segments. As in PBDPU, the initial incorporation of ionization groups into the hard segment destroys the regularity of the hard segment and decreases the degree of phase separation of the system. A further increase in the degree of ionization increases the difference of polarity between the hard and soft segments and enhances the physical crosslinks in the hard segment, thus helping to regain some degree of phase separation. As shown in Fig. 6, variation of the slow decay component of polyether polyurethane zwitterionomers with temperature is quite different from that of PBDPU. At lower temperatures, when only PTMO contributes to the slow decay components, PUA, which possesses the highest degree of phase separation, bears the highest content of slow decay components of the four samples. At higher temperatures when MDEA and MD1 begin to contribute to the slow decay components, PUB, whose hard segments are the most mobile due to the influence of the soft se~ents, possesses the highest content of the hard segment. As in PBDPU, the content of the slow decay component of PUA remains almost constant at higher temperatures, which implies a certain order of regularity in the hard phase of PUA. From the above comparison, it can be seen clearly that while FID decomposition at one
214
G. Yang et al./J. Mol. Strut.
temperature gives no meaningful information on the phase structure of the polymer, VT NMR is a very powerful tool for indirectly probing the phase structure of polymers.
323 (1994) 209-214
5. Acknowledgment This work is supported by National Natural Science Foundation of China, Foundation of the Nation’s Education Committee of China.
4. Conclusions 6. References
1. The phase separation of PBDPU is very clean, perhaps due to the extremely large difference of polarity between the hard and soft segments. Introduction of ionization into the hard segment of PBDPU has limited influence on the phase separation of PBDPU but affects the structure of the hard phase. 2. It can be seen from VT experiments that while the phase structures are allowed no time to change, the FID signals acquired under different temperatures differ dramatically. In such cases, the fast and slow decay components cannot be directly related to hard and soft phases, although spin diffusion experiments have proved that they are spatially separated. As an alternative, VT NMR provides a sensitive although not quantitative, method of probing the phase structure of the polymer systems indirectly.
[l] J.A. Miller, K.K.S. Hwangand S.L. Cooper, J. Macromol. Sci., Phys., 22 (1984) 153. [2] R.A. Assink, Macromolecules, 11 (1978) 1233, and references therein. [3] X. Lu and Y. Wang, Polym. Commun., 32 (1991) 426. [4] Y. Wang and M. Guo, Chinese J. Polym. Sci., 4 (1986) 72. [5] Q. Chen, Y. Wang, X. Yu and X. Wu, Chinese J. Polym. Sci., 10 (1992) 287. [6] H.A. Resing, Adv. Mol. Relaxation Processes, 3 (1972) 199. [7] T.A. Speckhard, K.K.S. Hwang, C.Z. Yang, W.R. Laupan and S.L. Cooper, J. Macromol. Sci., Phys., 23 (1984) 175. [8] C.Z. Yang, K.K.S. Hwang, T.A. Speckhard and S.L. Cooper, Macromol. Chem., 183 (1983) 651. [9] G. Yang, Q. Chen, Y. Wang, X. Wu and C. Yang, Chinese J. Polym. Sci., submitted for publication. [lo] X. Lu, Y. Wang and X. Wu, Polymer, 33 (1992) 958.
MOLECULAR STRUCTURE ELSEVIER
Journal of Molecular Structure 323 (1994) 209-214
Solid-state variable-temperature NMR study of the phase separation of polybutadiene polyurethane zwitterionomers G. Yang”, Q. Chena>*, Y. Wanga, C. Yangb, X. Wu” aAnalytical Center, East China Normal University, Shanghai 200062, China bDepartment of Chemistry, Nanjing University, Nanjing 210008, China
(Received 3 February 1994)
Abstract
Polybutadiene polyurethane (PBDPU) zwitterionomers based on 4,4’-diphenylmethane diisocyanate (MDI), methyldiethanolamine (MDEA), and hydroxy terminated polybutadiene are studied with variable-temperature (VT) wide-line ‘H NMR. Spin-spin relaxation times (T2) and spin-lattice relaxation times (T,) are measured. It is found that phase separation of PBDPU does not change significantly upon ionization. The initial incorporation of ionization groups destroys the crystallinity of the hard segment while further ionization enhances physical crosslinks in the hard phase. The results are compared with a previous VT NMR study on polyether polyurethane zwitterionomers based on MDI, MDEA and 1000 Da molecular weight polytetramethylene oxide.
1. Introduction Polyurethane elastomers are a kind of widely used thermoplastic with a two-phase microstructure. As with other thermoplastics, the mechanical properties of the material are greatly influenced by the phase separation between the soft and the hard segments. In turn, the phase structure of the system is influenced by many factors, including chemical structure, molecular weight, the relative contents of both hard and soft segments, introduction of ionization groups into the hard segments, and so on [ 11. Through changing the above factors, there exists a great potential to tailor the mechanical properties of the material. For this reason, the influence of the factors mentioned above on the morphology of the material has been extensively studied by *Corresponding
author.
various methods including electron microscopy, small angle X-ray scattering (SAXS), infrared dichroism, dynamic mechanical analysis, differential scanning calorimetry (DSC), stressstrain testing, etc. [1,7] Unfortunately, there often lacks a conformity among the results from different methods. Both solid-state and solution NMR spectroscopy have been used to study polyurethane elastomers and have proved to be power tools in probing the phase structure of the material. Wideline ‘H NMR has often been used to study the microphase structures of polyurethanes [2-41, and the two components of the free-induction decay (FID) signal are often directly related to the two phases at room temperature. Spin diffusion experiments are also often employed to study the interface or domain sizes of the two phases [3]. Solid-state high-resolution 13C NMR has proved
0022-2860/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI 0022-2860(94)07994-L
G. Yang et al.lJ. Mol. Strut.
210
to be useful in studying the microstructure of polyurethanes [3, 51. However, its application is often restricted due to the broad linewidth of the hard segment. High-resolution NMR of concentrated solutions is also used to study the phase structure and the interactions between the hard and soft segments [lo] on the assumption that some strong interactions between the hard and soft segments remain the same as they are in the solid-state. In this study, we employ variable temperature (VT) wide-line ‘H NMR to trace the variation of the segmental motions with temperature and probe the phase structure of the system indirectly. Compared with the above-mentioned NMR approaches, VT NMR has the following advantages. (1) Since a very broad distribution of correlation time in a single domain can also result in two-component FID [6], it is unreliable to use the two FID components to represent the hard and the soft phase directly, although it is a typical practice in wide-line NMR. (2) The high sensitivity and the quantitative character of the wide-line NMR has made a semi-quantitative study of the hard segment possible. (3) The interaction between the hard and soft segment can be studied without the influence of the solvent.
2. Experimental 2.1. Material Polybutadiene polyurethane (PBDPU) zwitterionomers are based on hydroxy terminated polybutadiene (HTPBD) with a molecular weight of 1350 Da and 4,4’-diphenylmethane diisocyanate
323 (1994) 209-214
+I
D7
) ACQ
Fig. 1. Pulse sequence for windowless solid echo. Dl is a 90” pulse; D6 = 8 ps. The echo begins at D7 = D6-1/2Dl.
extended with methyldiethanolamine (MDEA). The ratio of the contents of the three components is MDI:MDEA:HTPBD = 3:2:1. Ionization groups are incorporated into MDEA with the addition of a controlled amount of y-propane sultone [g]. Four samples, as listed in Table 1, are used in NMR measurements. Samples for NMR studies are prepared by solution casting from dimethylformamide solution with no further disposal. NW
2.2. NMR experiments ‘H NMR experiments are performed on a Bruker MSL-300 spectrometer with resonance frequency at 300.13 MHz. FID signals are acquired with a windowless solid echo pulse sequence as shown in Fig. 1. Spin-lattice relaxation time Ti is measured using the inverserecovery method. The ‘H 90” pulse width is 2~s while number of scans is typically 4-8. All samples are studied at temperatures ranging from 173 to 353 K, with FID signals acquired and Ti measured. Temperature is controlled with a Bruker VT- 1000 temperature control unit. Deviation of temperature is f 1 K.
3. Results and discussion 3.1. VT wide-line NA4R method
Table 1 Samples used in NMR measurement Sample
Degree of ionization
Proton content of HTBPD’
PBD-0 PBD-35 PBD-65 PBD-95
0 0.35 0.65 0.95
0.721 0.707 0.695 0.684
a The proton content of HTPBD is calculated from the formula of the polymer.
A typical FID of polyurethane at room temperature is plotted in Fig. 2. Obviously it can be split into two components, one gaussian fast decay and one lorentzian slow decay. The variation of magnetization with time can be represented by the following expression
M:
=ifexp(-&)
+M,exp(--$)
211
G. Yang et al./J. Mol. Struct. 323 (1994) 209-214
O~fMltfAl -C%kXlkit6d ~~.HardComponent .-CGoflcomponerlt
I
.I
-O=PBD35 J PBD-65 r-PBD-95
0
50
loo
150
260
280
300
320
340
360
380
Temperature (K)
Time@) Fig. 2. Scheme of FID decomposition of polyurethane zwitterionomers.
Fig. 3. Variation of contents of the slow decay component with temperature.
where Mf and M, represent the initial values of ma~etization of fast and slow decay respectively and T2f, TZs are the spin-spin relaxation times of corresponding components. By applying nonlinear least squares regression to the FID signals acquired, we can get Tzs and relative proton contents of the fast and slow decay components. Traditionally, FID signals are acquired at room temperature and decomposed. The fast and the slow decay components resulting from FID decomposition are directly related to the hard and soft phases respectively. The relative contents of the two phases can also be determined from the relative content of the two FID components. In this study, FID signals are acquired and decomposed over a wide temperature range. Only one gaussian component can be found in the FID signals acquired below 293 K. The variations of Tzs and the contents of the lorentzian components of four samples at temperatures higher than 293 K are plotted against temperature in Figs. 3 and 4 respectively. Tz of the gaussian component increases slightly with the temperature from about 9ps at 173 K to about 11 ps at 363 K. Since the experiment temperature is well below the Tgof the hard segment and the measurement of a sample is completed in several hours, the phase structure of the samples can be regarded as
invariable in the VT experiments. However, from Figs. 3 and 4, we can see clearly that as the temperature changes, both the T2 and content of the slow decay component change dramatically. Thus it can be concluded that the traditional method of relating two FID signal components to hard and soft phases directly is not reliable, at least in the systems studied here. 140 t 120 loo
ol,..>. 160
ePB5-0 4PBD-35 -cPB5-65 + PBD-95
.‘.
2ocl
240
I
280
320
360
Temperature (K) Fig.4. Variation of T2 of the slow decay component temperature.
with
212
G. Yang et 01./J. Mol. Struct. 323 (1994) 209-214
Since the variation of the FID signals with temperature is dominated by the change in the segmental motion in the samples, which in turn depends largely on the phase structure of the samples, we can use the VT experiments to study the molecular motion and probe the phase structure of the system indirectly. 3.2. Phase structure of samples At temperatures lower than 273 K, both the hard and soft segments are in a rigid state (in T2 sense), so only one gaussian fast decay is presented in the FID signals of the samples. As the temperature increases, there emerges an exponential slow decay component, whose T2 value and content increase with the temperature. It can be seen clearly from Figs. 3 and 4 that the Tzs values and contents of four samples show no notable difference below 313 K. The initial increase in Tzs and contents of the slow decay component is obviously caused by the onset of the segmental motions of the soft segment (HTPBD). Similarity between the initial increase of T2 and the content of slow decay component indicates a similarity in the mobility of the soft segment. Since the mobility of the soft segment is not influenced by the incorporation of ionization groups into the hard segment, it can be concluded that there exists a clean phase separation between the hard and the soft segments in all samples. A further increase of temperature enhances the segmental motion of the hard segment. At temperatures higher than 3 13 K, MD1 and MDEA begin to contribute to the slow decay component and the contents of the slow decay components soon surpass the proton contents of the HTPBD segment in the samples, which are about 70%, as listed in Table 1. As shown in Figs, 3 and 4, divergence between T2 and the contents of the slow decay components emerges among the four samples when the hard segments begin to contribute to the slow decay components. This divergence in T2 and the content of the slow decay components indicates a difference of the mobility of the hard segment, which further implies that introducing ionization groups into hard segments changes the structure of the hard phase. It can be seen from Figs. 3 and 4 that the slow
decay component in polybutadiene (PBD-0) bears the lowest contents and the longest T2 of the four samples. This means the slow decay component in PBD-0 contains the least amount of hard segments, which are less mobile and make T2 shorter. It can be further concluded that the onset of the segmental motion of the hard segment in PBD-0 is the most difficult of the four samples. For the same reason, since PBD-35 possesses the highest content and shortest T2 of the slow decay component, the onset of the segmental motion of its hard segment is the easiest. According to the above discussions, the following conclusions can be drawn. (1) The initial introduction of ionization groups into the hard segment destroys the regularity of the hard phase and renders the hard segments more mobile. The hard segment in PBD-0 is the least mobile in all samples due to its high level of regularity. (2) A further increase in the degree of ionization increases the polarity of the hard segment and enhances physical crosslinks in the hard segment, thus restricting the segmental motions of the hard segment. So the hard segments of both PBD-65 and PBD-95 are less mobile than that of PBD-35. (3) Since the FID signals of PBD-65 and PBD-95 show no notable difference in Figs. 3 and 4, it can be shown that an increase of the degree of ionization above 65% has a limited influence on the phase structure of PBDPU. It can also be seen from Fig. 3 that the content of the soft decay component of PBD-0 remains almost constant at temperatures higher than 343 K. This indicates that the segmental motion of a portion of the hard segments is very difficult, which also suggests that a high order of regularity exists in the hard phase in PBD-0. ‘H spin-lattice relaxation of all samples can be presented in a single-component exponential. Results of simulations are shown in Fig. 5, in which the relaxation time Tl is plotted against temperature. The variation of T, with temperature can be clearly divided into two sections. At temperatures lower than 293K, when both hard and soft segments are in a relatively rigid state, the spin relaxation time is determined by the mobility of both segments. Since the soft phases of four samples are almost the same, PBD-0, whose hard segment is the most rigid, exhibits the longest T, while PBD-35, whose hard segment is
213
C. Yang et al./J. Mal. Sfruct. 323 (1994) 209-214
90
ePBD-0
33PBD-35 aPBD.65 +PBD95
I 160
200
240
280
Temper&ure
320
I
Go
(K)
Fig. 5. Variation of T, with temperature.
the most mobile, exhibits the shortest Tt. However, as the temperature increases, with the rapid increase of the mobility of the soft segments, the relaxation of the soft phase soon becomes so efficient that the soft phase becomes the relaxation center and the ma~etization of the hard segment is mainly relaxed through the mechanism of spin diffusion. Since the mobility of the soft segments is very similar in all samples due to the clean phase separation, the convergence of Ti at temperatures higher than 313 K is quite understandable. So the results of Ti measurement entirely support the conclusions obtained from FID decomposition.
It is very interesting and beneficial to compare this study with a previous study on polyether polyurethane zwitterionomers based on MDI, MDEA, and polytetramethylene oxide glycol (PTMO) [9]. The structures of the samples are similar to that of PBDPU zwitterionomers except that the soft segment is 1000 Da molecular weight PTMO. The samples with ionization degrees 0, 0.35, 0.65 and 0.95 are named PUA, PUB, PUC and PUD respectively. Phase separation in polyether polyurethane zwitterionomers is not as clean as it is in PBDPU zwitterionomers perhaps due to a smaller difference
IO 0 Zxl
250
300
J 350
Temperature(K) Fig. 6. Variation of the contents of the soft segments of polyether polyurethane zwitterionomers.
in the polarity of the hard and soft segments. As in PBDPU, the initial incorporation of ionization groups into the hard segment destroys the regularity of the hard segment and decreases the degree of phase separation of the system. A further increase in the degree of ionization increases the difference of polarity between the hard and soft segments and enhances the physical crosslinks in the hard segment, thus helping to regain some degree of phase separation. As shown in Fig. 6, variation of the slow decay component of polyether polyurethane zwitterionomers with temperature is quite different from that of PBDPU. At lower temperatures, when only PTMO contributes to the slow decay components, PUA, which possesses the highest degree of phase separation, bears the highest content of slow decay components of the four samples. At higher temperatures when MDEA and MD1 begin to contribute to the slow decay components, PUB, whose hard segments are the most mobile due to the influence of the soft se~ents, possesses the highest content of the hard segment. As in PBDPU, the content of the slow decay component of PUA remains almost constant at higher temperatures, which implies a certain order of regularity in the hard phase of PUA. From the above comparison, it can be seen clearly that while FID decomposition at one
214
G. Yang et al./J. Mol. Strut.
temperature gives no meaningful information on the phase structure of the polymer, VT NMR is a very powerful tool for indirectly probing the phase structure of polymers.
323 (1994) 209-214
5. Acknowledgment This work is supported by National Natural Science Foundation of China, Foundation of the Nation’s Education Committee of China.
4. Conclusions 6. References
1. The phase separation of PBDPU is very clean, perhaps due to the extremely large difference of polarity between the hard and soft segments. Introduction of ionization into the hard segment of PBDPU has limited influence on the phase separation of PBDPU but affects the structure of the hard phase. 2. It can be seen from VT experiments that while the phase structures are allowed no time to change, the FID signals acquired under different temperatures differ dramatically. In such cases, the fast and slow decay components cannot be directly related to hard and soft phases, although spin diffusion experiments have proved that they are spatially separated. As an alternative, VT NMR provides a sensitive although not quantitative, method of probing the phase structure of the polymer systems indirectly.
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