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Lamellar microphase-separated structure of ABA triblock copolymers
PflYSICA ELSEVIER
Physica B 213&214 (1995) 697 699
,
Lamellar microphase-separated structure of ABA triblock copolymers Y. M a t s u s h i t a a'*, M. N o m u r a a' 1, I. N o d a a, M. Imai b aDepartment oj Applied Chemistry, Naqoya Unieersity, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan b Institute for Solid State Physics, Neutron Scatterin 9 Laboratory, University oJ'Tokyo. Tokai-mura, Naka-oun, lbaraki-ken 319-11, Japan
Abstract Alternating lamellar structures of 2-vinylpyridine-styrene-2-vinylpyridinetriblock copolymers of the ABA type with two different compositions were studied by small-angle neutron scattering (SANS). The volume fractions of polystyrene (S), ~bs, of the copolymers are 0.32 and 0.5. It was revealed that the middle block polymers (S) shrink in the direction parallel to the lamellar interface, but they are elongated normal to the interface, so that the volume occupied by the middle block polymers are the same as that in the unperturbed state irrespective of ~bs. However, the degree of deformation for the sample with ~bs of 0.32 is higher than that for the sample with ~bs of 0.5.
1. Introduction Microdomain structures of block copolymers have been extensively studied in the most basic and the simplest molecules, i.e., diblock copolymers of the AB type [1]. The alternating lamellar structure is the best studied morphology among several well-known structures. Chain conformations of block polymers in lamellar microdomains have been measured for several polymers by SANS [2-5], and it was found that block chains shrank in the direction parallel to the domain interface so as to compensate the elongation in the normal direction. The most striking feature of the microdomain structure of ABA triblock copolymers consists of having middle block polymers (B) both of whose ends must be anchored either on the same domain boundary or on the
* Corresponding author; present address: Institute for Solid State Physics, Neutron Scattering Laboratory, University of Tokyo, Tokai-mura, Naka-gun, Ibaraki-ken, 319-11, Japan. ~Present address: Toyota Motor Corporation, 1, Toyotacho, Toyota, Aichi, 471 Japan.
adjacent boundary, while the two A block polymers have their free-ends in microdomains. The former chain conformation of a center block polymer can be called the "loop-type" conformation and the latter can be called "bridge-type". The purpose of the present work is to clarify the features of the microdomain structures of the ABA triblock copolymers in comparison with those of diblock copolymers. Poly(2-vinylpyridine-b-styrene-b-2-vinylpyridine) (PSP) and their deuterium-labelled counterparts were prepared and used as samples in this study.
2. Experimental Samples were prepared by an anionic block copolymerization of styrene and 2-vinylpyridine monomers with dipotassium salt of a-methylstyrene tetramer as a bifunctional initiator in tetrahydrofuran (THF) at 78C. Styrene-d8 monomer was also used to prepare labelled block copolymers. Details of sample preparation and characterization will be described elsewhere [6].
0921-4526/95/$09.50 ¢ 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 5 ) 0 0 2 5 3 - 7
698
Y. Matsushita et al./ Physica B 213&214 (1995) 697-699
Table 1 Molecular characteristics of samples Blend code
Table 2 Chain dimensions of polystyrene block
Sample Mn × 10 4 code 2-VP S
2-VP
1
PDP- 1 PSP-8
3.00 3.58
2.96 3.10
3.00 3.58
1.02 1.03
0.34 0.32
II
PDP-5 PSP- 11
2.30 2.45
4.25 4.49
2.30 2.45
1.03 1.04
0.50 0.50
Mw/Mn
~s
Blend code
1 11
Chain dimension (nm) Rg,~
R,,~,
Re, =
2.48 3.12
4.11 3.85
2.48 3.25
R,.,~o
R,,x/R,,~,o
2.80 3.35
0.89 0.93
v
~
_~=0°
Rg,y
¢..
to _J
/iew l , , l i , I L I L L ~ i '
0,00
0.02
. . . .
I
. . . .
0.04
, . . . .
I
. . . .
0.06
, . . . .
I . . . .
0.08
, . . . .
O,10
q2 (nm-2) Fig. 2. Guinier plot of the circularly-averaged through-view data. Sample; blend I. Fig. 1. Geometrical relationships of neutron beam, sample and the detector for two views,
Table 1 summarizes the molecular characteristics of labelled and unlabelled samples. Blend films were cast from dilute solutions of T H F and annealed at 150°C under vacuum for four days. Morphological observations were carried out by T E M and SAXS separately [6] and two blends were confirmed to have alternating lamellar structures. Measurements were performed with the SANS-U spectrometer of the Institute for Solid State Physics (ISSP) of the University of Tokyo in JRR-3M at Tokai [7]. The wavelength ). used was 0.7 n m and its distribution A2/2 was 0.1. The sampleto-detector distance adopted was 6 m. SANS intensities were measured in two geometries, i.e., edge- and throughviews, in which the neutron beams were incident in the directions parallel and perpendicular to the film surface as shown in Fig. 1. The composition-matching was examined to eliminate the domain scattering from the total coherent scattering intensity in the edge-view. Since the calculated ratio of poly(styrene-ds) to poly(styrene-hs) to match the scattering length of the polystyrene blend to that of
poly(2-VP) is 0.107/0.893 by volume [8] several blend samples were prepared around this ratio for each of the P D P / P S P pairs in Table 1.
3. Results and discussion Since neither diffraction nor anisotropy was observed in the SANS through-view contour map, through-view data were circularly-averaged. Fig. 2 shows the Guinier plot of the coherent scattering intensities of the circularly averaged data for PDP-1/PSP-8 in the through-view as an example. From the initial slope of this plot, we evaluated the radius of gyration of polystyrenes in the direction parallel to lamellar interface [5] assuming that the orientation of lamellae is perfect. Though the coordinates x and z are equivalent in Fig. 1, we define the component of the radius of gyration obtained from through-view data as R,., for simplicity [5]. Table 2 lists R,., thus obtained for two P D P / P S P blends. This table also lists the unperturbed radii of gyration R,,~,o, of the center block, polystyrene, and also the ratios ax = R,,x/Rg.xo'S. Rg,xo'S were evaluated by using an empirical equation Rg, kO = 0.0165M 1/2 (nm) [9], where k is x or y or z. It is apparent from this table that the a / s are smaller than
Y. Matsushita et al. /Physica B 213&214 (1995) 697 699
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E. p.
© ©
._1
,
,
,
,
0.00
I
,
,
,
0.02
,
I
,
A
,
0.04
,
l
. . . .
0.06
I
. . . .
0.08
0.10
q2 (nm2) Fig. 3. Guinier plots of the sector-averaged edge-view data. Sample; blend II.
2
E ............-'"'"' ..,~.
x c7~ rr
3
1
- "
................
....-"'
.
.
.
.
•
i
i
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................
i
699
segments in the polystyrene microdomains, x, was around 0.1. The details are however not described here. Fig. 3 compares the Guinier plots of the sums of the sector-averaged edge-view intensities at q~ = 0 ° + 5 ° and 180 ° + 5 ° (filled symbols) and at 90 ° + 5 ° and 270 ° + 5 ° (open symbols) for blend I with x of 0.103, where the minimum contrast was attained. The radii of gyration along the y- and z-axes, Rg, y and R,,= defined in Fig. 1 were estimated from the initial slopes of the data. The radii of gyration thus obtained were also listed in Table 2. Fig. 4 shows double logarithmic plots of R,,x and Rg. r against the molecular weight of polystyrene, together with the data o f a diblock copolymer [5]. One can see from Table 2 and Fig. 4 that the degree of deformation of polystyrene block of blend I, with ~b~ of 0.32 and with Mn(s) of 31 K, is higher than that of blend II, with ~b~, of 0.50 and with Mn(s) of 43 K, even though the molecular weight of the center block, polystyrene, of the former is smaller than that of the latter. Apparently, the chain dimension along the y-axis, which is represented by Rg.r in the present study, for bridge-type conformation may be larger than that of the loop-type one, while chain dimension along x-axis for bridge may be smaller than the loop. Thus, we speculate that the fraction of bridgetype conformations of the center block of a PSP triblock copolymer with lower polystyrene content is higher than that for a block chain with higher polystyrene content if both have alternating lamellar structures. This work was done under the approval of the Neutron Scattering P r o g r a m Advisory Committee (Proposal numbers 91241, 23727, 3677).
i 10 5
Mo
Fig. 4. The radii of gyration of polystyrene blocks in x and y directions. The filled symbols are for triblock copolymers obtained in this work, whereas the open symbols are for SP diblock copolymers [5]. The upper data points are for Rg.~,, while the lower data points are for Rg.x.
unity and the value for blend I is significantly smaller than that for blend II. Composition matching was successfully achieved for two blends when the volume fraction of labelled
References [_1] B.R.M. Gallot, Adv. Polym. Sci. 29 (1978) 85. [2] G. Hadziioannou et al., Macromolecules 15 (1982) 263. [3] H. Hasegawa et al., Macromolecules 18 (1985) 67. [4] X. Quan et al., J. Polym. Sci. B 25 (1987) 641. [5] Y. Matsushita et al., Macromolecules 23 (1990) 4317. [6] Y. Matsushita, M. Nomura, I. Noda, in preparation. [7] Y. ltoh, M. Imai and S. Takahashi, unpublished. [8] Y. Matsushita et al., Macromolecules 25 (1988) 641. [9] J.P. Cotton et al., Macromolecules 6 (1974) 863.
Physica B 213&214 (1995) 697 699
,
Lamellar microphase-separated structure of ABA triblock copolymers Y. M a t s u s h i t a a'*, M. N o m u r a a' 1, I. N o d a a, M. Imai b aDepartment oj Applied Chemistry, Naqoya Unieersity, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan b Institute for Solid State Physics, Neutron Scatterin 9 Laboratory, University oJ'Tokyo. Tokai-mura, Naka-oun, lbaraki-ken 319-11, Japan
Abstract Alternating lamellar structures of 2-vinylpyridine-styrene-2-vinylpyridinetriblock copolymers of the ABA type with two different compositions were studied by small-angle neutron scattering (SANS). The volume fractions of polystyrene (S), ~bs, of the copolymers are 0.32 and 0.5. It was revealed that the middle block polymers (S) shrink in the direction parallel to the lamellar interface, but they are elongated normal to the interface, so that the volume occupied by the middle block polymers are the same as that in the unperturbed state irrespective of ~bs. However, the degree of deformation for the sample with ~bs of 0.32 is higher than that for the sample with ~bs of 0.5.
1. Introduction Microdomain structures of block copolymers have been extensively studied in the most basic and the simplest molecules, i.e., diblock copolymers of the AB type [1]. The alternating lamellar structure is the best studied morphology among several well-known structures. Chain conformations of block polymers in lamellar microdomains have been measured for several polymers by SANS [2-5], and it was found that block chains shrank in the direction parallel to the domain interface so as to compensate the elongation in the normal direction. The most striking feature of the microdomain structure of ABA triblock copolymers consists of having middle block polymers (B) both of whose ends must be anchored either on the same domain boundary or on the
* Corresponding author; present address: Institute for Solid State Physics, Neutron Scattering Laboratory, University of Tokyo, Tokai-mura, Naka-gun, Ibaraki-ken, 319-11, Japan. ~Present address: Toyota Motor Corporation, 1, Toyotacho, Toyota, Aichi, 471 Japan.
adjacent boundary, while the two A block polymers have their free-ends in microdomains. The former chain conformation of a center block polymer can be called the "loop-type" conformation and the latter can be called "bridge-type". The purpose of the present work is to clarify the features of the microdomain structures of the ABA triblock copolymers in comparison with those of diblock copolymers. Poly(2-vinylpyridine-b-styrene-b-2-vinylpyridine) (PSP) and their deuterium-labelled counterparts were prepared and used as samples in this study.
2. Experimental Samples were prepared by an anionic block copolymerization of styrene and 2-vinylpyridine monomers with dipotassium salt of a-methylstyrene tetramer as a bifunctional initiator in tetrahydrofuran (THF) at 78C. Styrene-d8 monomer was also used to prepare labelled block copolymers. Details of sample preparation and characterization will be described elsewhere [6].
0921-4526/95/$09.50 ¢ 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 5 ) 0 0 2 5 3 - 7
698
Y. Matsushita et al./ Physica B 213&214 (1995) 697-699
Table 1 Molecular characteristics of samples Blend code
Table 2 Chain dimensions of polystyrene block
Sample Mn × 10 4 code 2-VP S
2-VP
1
PDP- 1 PSP-8
3.00 3.58
2.96 3.10
3.00 3.58
1.02 1.03
0.34 0.32
II
PDP-5 PSP- 11
2.30 2.45
4.25 4.49
2.30 2.45
1.03 1.04
0.50 0.50
Mw/Mn
~s
Blend code
1 11
Chain dimension (nm) Rg,~
R,,~,
Re, =
2.48 3.12
4.11 3.85
2.48 3.25
R,.,~o
R,,x/R,,~,o
2.80 3.35
0.89 0.93
v
~
_~=0°
Rg,y
¢..
to _J
/iew l , , l i , I L I L L ~ i '
0,00
0.02
. . . .
I
. . . .
0.04
, . . . .
I
. . . .
0.06
, . . . .
I . . . .
0.08
, . . . .
O,10
q2 (nm-2) Fig. 2. Guinier plot of the circularly-averaged through-view data. Sample; blend I. Fig. 1. Geometrical relationships of neutron beam, sample and the detector for two views,
Table 1 summarizes the molecular characteristics of labelled and unlabelled samples. Blend films were cast from dilute solutions of T H F and annealed at 150°C under vacuum for four days. Morphological observations were carried out by T E M and SAXS separately [6] and two blends were confirmed to have alternating lamellar structures. Measurements were performed with the SANS-U spectrometer of the Institute for Solid State Physics (ISSP) of the University of Tokyo in JRR-3M at Tokai [7]. The wavelength ). used was 0.7 n m and its distribution A2/2 was 0.1. The sampleto-detector distance adopted was 6 m. SANS intensities were measured in two geometries, i.e., edge- and throughviews, in which the neutron beams were incident in the directions parallel and perpendicular to the film surface as shown in Fig. 1. The composition-matching was examined to eliminate the domain scattering from the total coherent scattering intensity in the edge-view. Since the calculated ratio of poly(styrene-ds) to poly(styrene-hs) to match the scattering length of the polystyrene blend to that of
poly(2-VP) is 0.107/0.893 by volume [8] several blend samples were prepared around this ratio for each of the P D P / P S P pairs in Table 1.
3. Results and discussion Since neither diffraction nor anisotropy was observed in the SANS through-view contour map, through-view data were circularly-averaged. Fig. 2 shows the Guinier plot of the coherent scattering intensities of the circularly averaged data for PDP-1/PSP-8 in the through-view as an example. From the initial slope of this plot, we evaluated the radius of gyration of polystyrenes in the direction parallel to lamellar interface [5] assuming that the orientation of lamellae is perfect. Though the coordinates x and z are equivalent in Fig. 1, we define the component of the radius of gyration obtained from through-view data as R,., for simplicity [5]. Table 2 lists R,., thus obtained for two P D P / P S P blends. This table also lists the unperturbed radii of gyration R,,~,o, of the center block, polystyrene, and also the ratios ax = R,,x/Rg.xo'S. Rg,xo'S were evaluated by using an empirical equation Rg, kO = 0.0165M 1/2 (nm) [9], where k is x or y or z. It is apparent from this table that the a / s are smaller than
Y. Matsushita et al. /Physica B 213&214 (1995) 697 699
"-I v
"E) r-
E. p.
© ©
._1
,
,
,
,
0.00
I
,
,
,
0.02
,
I
,
A
,
0.04
,
l
. . . .
0.06
I
. . . .
0.08
0.10
q2 (nm2) Fig. 3. Guinier plots of the sector-averaged edge-view data. Sample; blend II.
2
E ............-'"'"' ..,~.
x c7~ rr
3
1
- "
................
....-"'
.
.
.
.
•
i
i
10 4
................
i
699
segments in the polystyrene microdomains, x, was around 0.1. The details are however not described here. Fig. 3 compares the Guinier plots of the sums of the sector-averaged edge-view intensities at q~ = 0 ° + 5 ° and 180 ° + 5 ° (filled symbols) and at 90 ° + 5 ° and 270 ° + 5 ° (open symbols) for blend I with x of 0.103, where the minimum contrast was attained. The radii of gyration along the y- and z-axes, Rg, y and R,,= defined in Fig. 1 were estimated from the initial slopes of the data. The radii of gyration thus obtained were also listed in Table 2. Fig. 4 shows double logarithmic plots of R,,x and Rg. r against the molecular weight of polystyrene, together with the data o f a diblock copolymer [5]. One can see from Table 2 and Fig. 4 that the degree of deformation of polystyrene block of blend I, with ~b~ of 0.32 and with Mn(s) of 31 K, is higher than that of blend II, with ~b~, of 0.50 and with Mn(s) of 43 K, even though the molecular weight of the center block, polystyrene, of the former is smaller than that of the latter. Apparently, the chain dimension along the y-axis, which is represented by Rg.r in the present study, for bridge-type conformation may be larger than that of the loop-type one, while chain dimension along x-axis for bridge may be smaller than the loop. Thus, we speculate that the fraction of bridgetype conformations of the center block of a PSP triblock copolymer with lower polystyrene content is higher than that for a block chain with higher polystyrene content if both have alternating lamellar structures. This work was done under the approval of the Neutron Scattering P r o g r a m Advisory Committee (Proposal numbers 91241, 23727, 3677).
i 10 5
Mo
Fig. 4. The radii of gyration of polystyrene blocks in x and y directions. The filled symbols are for triblock copolymers obtained in this work, whereas the open symbols are for SP diblock copolymers [5]. The upper data points are for Rg.~,, while the lower data points are for Rg.x.
unity and the value for blend I is significantly smaller than that for blend II. Composition matching was successfully achieved for two blends when the volume fraction of labelled
References [_1] B.R.M. Gallot, Adv. Polym. Sci. 29 (1978) 85. [2] G. Hadziioannou et al., Macromolecules 15 (1982) 263. [3] H. Hasegawa et al., Macromolecules 18 (1985) 67. [4] X. Quan et al., J. Polym. Sci. B 25 (1987) 641. [5] Y. Matsushita et al., Macromolecules 23 (1990) 4317. [6] Y. Matsushita, M. Nomura, I. Noda, in preparation. [7] Y. ltoh, M. Imai and S. Takahashi, unpublished. [8] Y. Matsushita et al., Macromolecules 25 (1988) 641. [9] J.P. Cotton et al., Macromolecules 6 (1974) 863.