NMR and Macromolecules - American Chemical Society

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Solid State H NMR Studies of Molecular Motion

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Poly(butylene terephthalate) and Poly(butylene terephthalate)Containing Segmented Copolymers L. W. JELINSKI and J. J. DUMAIS Bell Laboratories, Murray Hill, NJ 07974 A. K. ENGEL Ε. I. du Pont de Nemours and Company, Wilmington, DE 19898

Solid state deuterium NMR spectroscopy is used to provide information concerning the motional heterogeneity and homogeneity of segmented co-polyesters containing poly(butylene terephthalate) as the hard segment. The results presented here provide the first clear evidence that there are two distinct motional environments for the hard segments in the co-polyester. One of the environments is identical to that observed in the poly(butylene terephthalate) homopolymer, in which Helfand-type motions about three bonds (Helfand, E. J. Chem. Phys. 1971, 54, 4651) occur with a correlation time of 7 x 10 s at 20°C. The other motional environment is more-nearly isotropic. The residues in the mobile environment are attributed to short blocks of hard segments residing in the soft segment matrix, or to hard segments forming the loop regions in the poly(butylene terephthalate) lamellae. Approximately 10% of the hard segments reside in the mobile environment in the segmented copolymer with 0.87 mole fraction of poly(butylene terephthalate) hard segments. -6

Poly (butylène terephthalate) has been used as a model for observing motions about three bonds as an isolated motional mode in polymers. Early carbon N M R studies involving carbon N M R relaxation data (1,2) and carbon chemical shift anisotropy considerations (3) showed that the terephthalate residues can be considered static in comparison to the motions exhibited by the alkyl residues. Furthermore, the alkyl portion of poly (butylène terephthalate) contains the shortest sequence that is able to undergo motions about three bonds (4), with the terephthalate groups acting as "molecular anchors" to prevent the longer range motional modes. Poly (butylène terephthalate) is thus ideally set up to undergo three-bond types of motions. Solid state deuterium N M R spectroscopy was used to study these motions in detail (5), using the selectively labeled polymer: 0097 6156/ 84/ 0247 0055506.00/0 © 1984 American Chemical Society Randall; NMR and Macromolecules ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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C ~0-CH CD CD CH -Of2

2

2

2

X

The temperature-dependent spectra were interpreted in terms of a two-site hop model, in which the deuterons undergo jumps through a dihedral angle of 1 0 3 ° . This type of motion is consistent with gauche-trans conformational transitions. At -88 °C these motions appear static on the time scale of the deuterium N M R experiment, and at +85 ° C the motions are in the fast exchange limit. The rate constants for these motions were obtained from the calculated spectra. A n Arrhenius plot of these data show that the apparent activation energy is 5.8 kcal/mol. (Dynamic mechanical data (20 Hz) fall on the Arrhenius plot.) The transitions have an intermediate rate on the deuterium N M R time scale at 20 °C, with the correlation time for the motion being 7 x 1 0 s at this temperature. -6

The solid state deuterium N M R results for the selectively labeled poly (butylène terephthalate) homopolymer have been interpreted in terms of various models for polymer motion (5). The data are consistent with the models proposed by Helfand (6), in which counter rotation occurs about second neighbor parallel bonds. Termed gauche migration and pair gauche production, these motions are illustrated in Figure la and b, respectively. Gauche migration consists of a transition from g tt to ttg , and is part of the pathway for pair gauche production. In this latter motional mode, a ttt sequence goes to g tg . These motional models allow gauche-trans conformational transitions to occur without concomitant large-scale reorientation of the ends of the polymer chain. In addition, they require slightly more than one C - C bond rotational barrier height, and are thus consistent with the apparent activation energy of 5.8 kcal/mol. ±

±

±

T

Having established the rates and types of motion that occur in the poly (butylène terephthalate) homopolymer (5), it is of interest to extend these studies to the investigation of the types of motions that occur in segmented copolymers that contain poly (butylène terephthalate) as the hard segment. The structure of such a polymer is shown below, where m and η refer to the "hard" and "soft" segments, respectively. (This structure also illustrates the sites of the deuterium labels.)

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Randall; NMR and Macromolecules ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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The poly (butylène terephthalate) "hard" segments and the poly (tetramethyleneoxy) terephthalate "soft" segments are not completely miscible and thus lead to phase separation. However, in contrast to the discrete domain structures found in polystyrene — polybutadiene or polystyrene — polyisoprene, for example, the hard and soft segments of this copolyester are more intimately dispersed. This leads to a hard segment morphology that has been described as "continuous and interpenetrating lamellae." Such a morphology is shown in schematic form in Figure 2. This copolymer thus presents a unique opportunity to study a system in which there is a large interfacial area. The specific questions to be addressed by solid state deuterium N M R studies of this polymer are the following: (1) Are the motions of the C D groups in the segmented copolymer identical to the motions observed in the poly (butylène terephthalate) homopolymer? (2) What effect do variations in the hard:soft content ratio have on the motions of the C D groups? And (3), is there evidence for hard segments which reside in the soft segment matrix? If so, what per cent of them are in non-poly (butylène terephthalate) -like environments? 2

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EXPERIMENTAL The selectively labeled poly (butylène terephthalate) -containing segmented copolymers were prepared according to literature methods (7), using 2,2,3,3-d butylene glycol (Merck) as the starting diol. The polymers used in this study correspond to m:n ratios (see the structure, above, for the meaning of m and n) of 24:1 and 7:1. The mole fractions of hard segments are 0.96 and 0.87, respectively, corresponding to 81 and 57 weight per cent of hard segments. The polymers were characterized by thermal measurements and by solution state deuterium and carbon N M R spectroscopy (8). No end groups were observed by carbon spectroscopy, and the deuterium N M R spectra in solution attested to the integrity of the labeling pattern. 4

The samples for N M R spectroscopy were melted into glass tubes and allowed to cool from the melt. The observed deuterium N M R spectra are reproducible with temperature cycling, thus providing evidence that the thermal history induced by acquiring temperature-dependent spectra of the samples does not greatly affect the properties that we are measuring. The solid state deuterium N M R spectra were recorded on a home-built spectrometer operating at 55.26 M H z for deuterium (360 M H z for protons). The spectrometer has been described previously (5). Routine spectra were obtained in quadrature using the quadrupolar echo pulse sequence (Figure 3) ( 9 0 - t - 9 0 - t ) (9-11), 4K data points, a digitization rate of 200 ns/point (5 M H z ) , and a 4.3 #s 90 degree pulse width. Unless otherwise noted, the length of t was generally set at 30 μ&. The value of t was set several microseconds shorter than the time needed to start digitization at the top of the quadrupolar echo maximum. After data accumulation, the FID was left-shifted by the correct number of points, so that for each spectrum, the part of the FID which was transformed began at the exact top of the echo. The quadrupolar echo pulse sequence is used to circumvent problems with receiver recovery times. The solid ± x

1

y

2

x

2


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Figure 1. Helfand-type motions about three bonds (6). The segments represented here correspond to the —OCH2CD2CD2CH2O— portion of the poly (butylène terephthalate) hard segments. These motions are consistent with the solid state deuterium N M R data for poly (butylène terephthalate) (5).

Figure 2. Schematic representation of continuous and interpenetrating hard segment lamellae. The hard segment blocks that are too short to crystallize are shown in the soft segment matrix.


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state deuterium N M R spectra are very broad (ca. 250 kHz), and thus the free induction decay signal dies away very rapidly. The quadrupolar echo pulse sequence causes the magnetization to refocus while the radio frequency circuits have time to recover from the transmitter pulses. Inversion-recovery deuterium N M R spectra were obtained by performing a 180°-r-90° pulse sequence, followed by the quadrupolar echo sequence (12). Spin lattice relaxation times were estimated from the null points in the inversionrecovery spectra. RESULTS AND DISCUSSION In Figure 4, the solid state deuterium N M R spectrum for poly(butylene terephthalate) at 20 °C is compared to the spectra for the segmented copolymers, also obtained at 20 °C. The spectrum of poly (butylène terephthalate) (Figure 4a) can be simulated by assuming the C - D bond jumps between two sites separated by a dihedral angle of 1 0 3 ° , with a rate constant of 1.4 x 10 s" . A quadrupolar splitting (A*>) of 124 kHz is used for this calculation (5). (The calculated deuterium spectrum is obtained by also taking into account pulse power fall-off as a function of frequency (13) and the distortions that arise when motions occur during the quadrupolar echo delay time (14,15).) Figure 4b shows the deuterium N M R spectrum of the segmented copolymer with 0.96 mole fraction hard segments, and below it in Figure 4c is the spectrum of the segmented copolymer with 0.87 mole fraction hard segments. The spectra of the segmented copolymers are clearly different from the spectrum of poly (butylène terephthalate), and suggest, particularly in the case of the softest segmented copolymer (Figure 4c), that there are at least two motional environments for the hard segments in the copolymers. 5

1

q

Inversion-recovery solid state deuterium N M R spectra can be used to show that the spectrum of the copolymer with 0.87 mole fraction of hard segments is composed of at least two components. Figure 5 shows such spectra. At an inversion-recovery delay time of 50 ms, the broad poly (butylène terephthalate)-like part of the line is almost nulled, yet the sharp component is positive (Figure 5b). At shorter inversion-recovery delay times, the sharp component goes through its null, and the broad component is negative. Inversion-recovery spectra such as these indicate that the solid state deuterium N M R spectra for the segmented copolymers shown in Figures 4b and 4c are composed of two components with different T values. The Ί of the sharp component in the copolymer with 0.87 mole fraction hard segments is estimated to be 10 ms, and that of the broad component is approximately 60 ms. The difference in these deuterium T values is a factor of six, and indicates that the deuterons that give rise to the broad component are in markedly different motional environments from those that give the sharp line. x

γ

{

It is noteworthy that the sharp component gives a signal with the quadrupolar echo pulse sequence. The observation of a quadrupolar echo can be interpreted to indicate that the constraints on the motion of the deuterons persist for a long time (16). Although the deuterium line for the T = 10 ms component is sharp, the observation of a quadrupolar echo proves that the line is not isotropically mobile on the deuterium N M R timescale. The sharp component also gives a spectrum with a r


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Figure 3. The quadrupole echo pulse sequence. The time t is usually set at 30 /us, and t is approximately 25 MS. {

2

(a)

(b)

(c)

-100

0

100

kHz Figure 4. Solid state deuterium N M R spectra of (a) poly (butylène terephthalate) ; (b) segmented co-polyester containing 0.96 mole fraction of poly (butylène terephthalate) hard segments; and (c) segmented copolymer containing 0.87 mole fraction hard segments. (See text for specific deuterium labeling patterns.) A l l spectra were obtained with the quadrupole echo pulse sequence at 2 0 ° C and 55.26 M H z , using 30 MS at the t quadrupole echo delay time. {


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standard 90° — τ pulse sequence using a 30 MS receiver dead time (spectrum not shown). The width of this line at half-height is approximately 5 kHz. The spectra shown in Figure 5 clearly indicate that there are two distinct motional environments in the segmented copolymers. The broad component of the spectrum arises from the majority of the hard segments which show motional characteristics similar to that of poly (butylène terephthalate). The sharp component is attributed to hard segments which reside in the soft segment matrix, either by virtue of being very short blocks, or because they form loops on the surfaces of the hard segment lamellae. It is of interest to estimate the amount of hard segment which gives the sharp line, and further, to determine if the poly (butylène terephthalate) -like broad line represents deuterons which undergo motions which are identical to those observed in the poly (butylène terephthalate) homopolymer. These points are addressed for the copolymer containing 0.87 mole fraction hard segments, by the solid state deuterium N M R spectra shown in Figure 6. In Figure 6a is shown the quadrupole echo deuterium N M R spectrum of the segmented copolymer with 0.87 mole fraction of hard segments. Adjacent to it in Figure 6b, is the corresponding calculated spectrum. The dashed line represents the sum of the sharp and broad components. The sharp line is Lorentzian, and the broad line is Gaussian. The broad line is calculated assuming a quadrupolar splitting (Δί/ ) of 124 kHz, a two-site dihedral hop angle of 1 0 3 ° , and a rate constant for the hop of 1.4 x 10 s . Corrections have been made for pulse power fall-off (13) and for distortions that arise when motions occur during the quadrupolar echo pulse sequence (14,15). This calculation indicates that the sharp and broad components are present in an approximately 1:10 ratio. ς

5

_1

The spectrum shown in Figure 6c illustrates that the motions of the C D groups of poly (butylène terephthalate) in the segmented copolymer are identical to the motions in the homopolymer. This spectrum is a difference spectrum, obtained by subtracting the spectrum of the poly (butylène terephthalate) homopolymer (Figure 4a) from the spectrum of the segmented copolymer (Figure 4c). The difference spectrum (Figure 6c) shows an excellent null for the broad part of the deuterium N M R pattern. This result indicates that the motions of approximately 90% of the hard segment C D deuterons in the segmented copolymer are wellrepresented by the motions observed for the poly (butylène terephthalate) homopolymer. 2

2

It is emphasized that the semi-quantitative treatment above provides only an estimate of the amounts of hard segments in the two motional environments. A more-quantitative answer is probably unlikely, due to the quadrupolar echo pulse sequence necessary to obtain the data. The quadrupolar echo pulse sequence preserves the inhomogeneously broadened part of the solid state deuterium N M R pattern. Homogeneous T effects will cause some signal loss during the quadrupolar echo delay times. In comparing sharp and broad components such as those shown in Figure 6b, it is also necessary to know that both components have approximately the same homogeneous T . 2

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62

^

(a) (b)

A ,

1 1

1 1 1

I

-200

'~T~

^

'

1 1

-100

MACROMOLECULES

τ-, . I I . ι I !" 100

0

200

kHz Figure 5. Inversion-recovery solid state deuterium N M R spectra of the segmented copolymer containing 0.87 mole fraction hard segments. The spectra were obtained with a 1 8 0 ° — 1 - 9 0 ° — t j — 9 0 ° — 1 pulse sequence. The spectrum in (b) was obtained with a t of 50 ms. 3

±x

y

2

3

kHz Figure 6. Experimental (a), calculated (b), and difference (c) solid state deuterium N M R spectra for the segmented copolymer with 0.87 mole fraction of hard segments. The spectrum in (a) was obtained at 55.26 M H z and 20 °C, using the quadrupole echo pulse sequence. The dashed line in (b) represents the sum calculated for the broad and narrow components in a respective 10:1 ratio. (See text for details of the calculation.) The spectrum in (c) is the difference spectrum obtained by subtracting the spectrum of poly (butylène terephthalate) (Figure 4a) from the segmented copolymer spectrum (Figure 6a).


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Homogeneous T effects can be studied by obtaining quadrupolar echo N M R spectra as a function of the quadrupolar echo delay times, t and t . Such experiments have been performed for all samples. The results indicate that the observed lineshapes are not markedly dependent upon the quadrupolar echo delay times, although signal intensity clearly is lost as the delay times are increased. Figure 7 shows a plot of the relative signal intensity versus quadrupolar echo delay time for the solid state deuterium N M R spectra of the segmented copolymer containing 0.87 mole fraction hard segments. This figure shows that the relative intensity is a linear function of the quadrupolar echo delay times. When extrapolated back to zero time, Figure 7 illustrates that approximately 2 5 % of the signal intensity is lost at the usual quadrupolar echo delay time of 30 MS. However, the lack of distortion in these spectra indicates that representative patterns are obtained at a quadrupolar echo delay time of 30 MS. 2

x

2

SUMMARY The results presented here show that there are two distinct motional environments for the "central" deuterons of poly (butylène terephthalate) in the segmented copolyesters containing poly (butylène terephthalate) as the hard segment. One of the motional environments is identical to that observed in the poly (butylène terephthalate) homopolymer, in which Helfand-type motions (6) about three bonds occur with a correlation time of 7 x 10~ s at 20 °C. The Ί for these deuterons is ca. 60 ms at 20 °C. Approximately 90% of the hard segments reside in these organized lamellar environments in the segmented copolymer with 0.87 mole fraction hard segments. 6

ι

The other 10% of the hard segments in this copolymer undergo motions that are more-nearly isotropic. The T for the "central" deuterons in the mobile regions is approximately 10 ms at 2 0 ° C . The mobile residues are attributed to short blocks of hard segments residing in the soft segment matrix, or to hard segments forming the loop regions of the poly (butylène terephthalate) lamellae. As the mole fraction of hard segments is increased from 0.87 to 0.96, a lower per cent of the hard segments are found to reside in mobile environments. t

Taken together, these solid state deuterium N M R experiments provide otherwise unobtainable answers to questions concerning motional homogeneity and heterogeneity in these segmented copolymer systems.


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20

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QUADRUPOLAR ECHO DELAY TIME (ps) Figure 7. A plot of the relative intensity of the quadrupole echo deuterium N M R signal versus the echo delay time, tj, for the segmented copolymer containing 0.87 mole fraction hard segments. The line is extrapolated back to zero time.


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ACKNOWLEDGEMENT We are grateful to Dr. F. A . Bovey for his interest and encouragement in this project.

LITERATURE CITED 1. Jelinski, L. W.; Dumais, J. J. Polym. Prepr., Am. Chem. Soc. Div. Polym. Chem. 1981, 22 (2), 273. 2.

Jelinski, L. W.; Dumais, J. J.; Watnick, P. I.; Engel, A. K.; Sefcik. M. D. Macromolecules 1983, 16, 409.

3.

Jelinski, L. W. Macromolecules 1981, 14, 1341.

4.

Schatzki, T. F. Polym. Prepr., Am. Chem. Soc. Div. Polym. Chem. 1965, 6, 646.

5.

Jelinski, L. W.; Dumais, J. J.; Engel, A. K. Macromolecules 1983, 16, 492.

6.

Helfand, E. J. Chem. Phys. 1971, 54, 4651.

7.

Wolfe, J. R., Jr. Polym. Prepr., Am. Chem. Soc. Div. Polym. Chem. 1978, 19 (1), 5.

8.

Jelinski, L. W.; Schilling F. C.; Bovey, F. A. Macromolecules 1981, 14, 581.

9.

Davis, J. H.; Jeffrey, K. R.; Bloom, M.; Valic, M. I.; Higgs, T. P. Chem. Phys. Lett. 1976, 42, 390.

10.

Blinc, R.; Rutar, V.; Seliger, J.; Slak, J.; Smolej, V. Chem. Phys. Lett. 1977, 48, 576.

11.

Hentschel, R.; Spiess, H. W. J. Magn. Resonance 1979, 35, 157.

12.

Batchelder, L. S.; Niu, C. H., Torchia, D. A. J. Am. Chem. Soc. 1983, 0000.

13.

Bloom, M.; Davis, J. H.; Valic, M. I. Can. J. Phys. 1980, 58, 1510.

14. 15.

Spiess, H. W.; Sillescu, H. J. Magn. Resonance 1981, 42, 381. Mehring, M. In "NMR-Basic Principles and Progress", Diehl, P.; Fluck, E.; Kosfeld, R.; Eds.; Springer-Verlag: New York, 11, 1976.

16.

Hentschel, D.; Sillescu, H.; Spiess, H. W. Macromolecules 1981, 14, 1605.

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