SAXS and Rheological Studies on the ... - ACS Publications

Downloaded by GEORGETOWN UNIV on June 4, 2018 | https://pubs.acs.org Publication Date: July 30, 1999 | doi: 10.1021/bk-2000-0739.ch031

Chapter 31

SAXS and Rheological Studies on the Order-Disorder and Order-Order Transitions in Mixtures of Polystyrene-b-Polyisoprene-bPolystyrene and Low Molecular Weight PS Seung-Heon Lee and Kookheon Char Department of Chemical Engineering, Seoul National University, Seoul 151-742, Korea

The order-disorder and order-order transitions (ODT and OOT) of a series of mixtures of polystyrene-block-polyisoprene-blockpoiystyrene copolymers (SIS), V4411D (f = 0.377) and V4113 (f = 0.131), and low molecular weight PS homopolymers with different molecular weights have been investigated by using both small-angle X-ray scattering (SAXS) and rheological measurements. ODT temperatures determined from the discontinuous change in 1/I and σ vs 1/T plots were found to be in reasonable proximity to the ones determined from the discontinuous change in log G' vs Τ plots. OOT was also identified for pure V4113 and a few of its mixtures with PS homopolymers, as evidenced from SAXS measurements. While the phase behavior including ODT of V4411D/PS mixtures was in good agreement with theoretical predictions, that of V4113/PS mixtures showed unusual behavior. This was attributed to the change in PS homopolymer distribution in the microdomain with the addition of low molecular weight PS homopolymer to the SIS triblock copolymer. PS

PS

2

max

q

Introduction Block copolymer/homopolymer mixtures have been of considerable interest because they can be easily modified to yield desired properties in polymeric materials such as pressure sensitive adhesives. Phase behavior of these mixtures is generally quite complicated because two different natures of transition can occur at the same time: macrophase and microphase separation. If the molecular weight of a 496

© 2000 American Chemical Society

Cebe et al.; Scattering from Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

497 homopolymer is sufficiently low, it however tends to be solubilized into one of the microdomains. " In this case, the macrophase separation is suppressed and the microphase separation (or order-disorder transition; ODT) becomes dominant. There have been many theoretical and experimental works on the ODT of pure block copolymers. " A few researchers have also investigated the ODT of block copolymer/homopolymer mixtures. Whitmore and Noolandi calculated theoretical phase diagrams of AB diblock copolymer/A homopolymer mixtures using functional integral formalism. They assumed that the ordered phase has a lamellar structure, so that the third-order term in the Landau-Ginzburg expansion of the free energy vanishes. They found that the spinodal ODT temperature decreases with the addition of the homopolymer if the degree of polymerization of A homopolymer is smaller than the half of that of A block of symmetric A B diblock copolymer, de la Cruz and Sanchez also investigated theoretically the effect of the relative molecular weight and composition of the homopolymer on the spinodal ODT by modifying the scattering theory originally proposed by Leibler to AB diblock copolymer/A homopolymer mixtures, and obtained similar results with those of Whitmore and Noolandi. Recently, some researchers extended the self-consistent mean-field calculations to block copolymer/homopolymer mixtures having other ordered phases rather than lamellae. Matsen found that the addition of homopolymer stabilizes complex ordered phases in the weak segregation regime. Janert and Schick investigated the effect of relative homopolymer length on the phase behavior in the weak to intermediate segregation regime considering lamellae, hexagonal, and body centered cubic phases. ODT behavior in previous authors' works, however, showed similar trend to that obtainedfromthe calculations just assuming lamellar phase. Experimentally, Roe and Zin investigated the phase behavior of mixtures of styrene-butadiene diblock copolymer and polystyrene or polybutadiene homopolymers with different molecular weights using small-angle X-ray scattering (SAXS) and turbidity measurements. Baek et al. investigated the ODT and phase behavior of mixtures of SIS triblock copolymer/PS homopolymer mixtures by using turbidity and rheological measurements. To the best of our knowledge, however, systematic studies on the order-disorder and order-order transitions in mixtures of block copolymer and low molecular weight homopolymers have yet to been done. Since relatively high molecular weight homopolymers were used in the previous works, the ODT temperature just increased as the homopolymer weight fraction was increased and eventually macrophase separation occurred above a certain value of composition that was quite small. In present study, we employed both SAXS and rheological measurements to investigate the order-disorder and order-order transitions in a series of SIS triblock copolymer/low molecular weight PS homopolymer mixtures, which did not show macrophase separation in the whole temperature and composition range covered in this experiment. Phase diagrams obtained from both measurements were compared with the predictions based on the Whitmore-Noolandi theory . The difference between the theory and the experiment was discussed in terms of the change in homopolymer distribution and microdomain morphology by the addition of homopolymer to block copolymer. 1

4

5

11

1


12

5

13

14

15

16

1


498

Experimental Two commercial grades of polystyrene-6/oc£-polyisoprene-6/0eA-polystyrene (PS-PI-PS) copolymers, Vector 4113 and Vector 441 ID (DEXCO Polymers) were used as received. They will be denoted, hereafter, as V4113 and V441 ID, respectively. Four polystyrenes (PS) with different molecular weights were synthesized by anionic polymerization using η-butyl lithium as an initiator and benzene containing a trace amount of tetrahydrofuran as a solvent. The characteristics of these polymers are summarized in Table I. The weight-average molecular weight, M , and the polydispersity index, M / M , were determined by gel permeation chromatography (GPC) using PS standards, and the PS weight percent of the block copolymers was determined by proton nuclear magnetic resonance (H-NMR). The microstructure of PI blocks of the block copolymers was determined to be 93% 1,4 and 7% 3,4 addition by H-NMR. w


w

n

Table I. Molecular Characteristics of the Polymers Used in This Study Sample V4113 V4411D PS2 PS3 PS6 PS11 a

3

M xW' (g/mol) 13.2-6-117.4-6-13.2 17.1-6-80.0-6-17.1 1.9 2.9 5.9 11.4 w

MJM 1.19" 1.08 1.09 1.06 1.04 1.03 n

PSwt% 15.1 41.7 100 100 100 100

18 wt% of uncoupled diblocks is present in V4113.

Mixture samples were prepared by solution casting method with toluene as a solvent. First, predetermined amounts of block copolymers and PS homopolymers were dissolved in toluene with the addition of 0.3 wt% antioxidant (Irganox 1010, Ciba-Geigy Group). The solvent was slowly evaporated at ambient condition for 3 weeks, and then in a vacuum oven at 56 °C for 3 days. Finally, all the samples were annealed in a vacuum oven at 130°C for 3 days. Two different methods were used to determine the order-disorder transition temperature (T DT) of each mixture. RMS 800 (Rheometrics Inc.) in parallel plate geometry (25 mm diameter and 1.5 mm gap) was used to measure the dynamic viscoelastic storage and loss moduli, G' and G", of the mixtures as a function of temperature. A heating rate of 1 °C/min was used and the strain amplitude was small enough to ensure linear viscoelasticity (typically smaller than 5%) for all the measurements. Synchrotron small-angle X-ray scattering (SAXS) measurements were carried out at 4C2 SAXS beamline in Pohang Light Source (PLS), which consisted of 2 GeV LINAC accelerator, storage ring, S i ( l l l ) double crystal monochromator, ion chambers, and one-dimensional position sensitive detector with 2048 pixels. The 0


499 wavelength (λ) of the synchrotron beam was 1.59 À and the energy resolution (Δλ/λ) was 5xl0" , Typical beam size was smaller than l x l mm . Sample-to-detector distance was 103.7 cm. Scattering profiles were obtained as a function of temperature and then corrected for absorption, air and imide film scattering. In order to prevent oxidative degradation of the samples, all the experiments were carried out under the nitrogen blanket. 4

2

Results and Discussion


Solubility of PS in the Microdomains of SIS Triblock Copolymer It was found from the hot stage microscopy that V4411D/PS mixtures did not show any macrophase separation up to 40 wt% of PS regardless of PS molecular weight. V4113/PS mixtures with high molecular weight PS, however, showed macrophase separation when PS weight fraction exceeded a certain value. This is due to the fact that V4113 is more asymmetric and hence has more space-restricted microdomain morphology, e.g., cylindrical morphology, than V441 ID, thus being less capable of solubilizing PS homopolymer into its microdomain. It should be noted here that in present work we focussed only on the order-disorder and order-order transition behavior of the mixtures in a composition range showing no macrophase separation.

Changes in Equilibrium Microdomain Morphology of Block Copolymers with the Addition of PS It is well known that a block copolymer shows multiple X-ray scattering peaks due to its periodic microdomain structure having a long-range order. Information on the equilibrium microdomain morphology of block copolymers can be obtained from the relative positions of these multiple peaks, since they exhibit different arrays depending on the shape of the microdomain structure, e.g., 1, 2, 3, 4, ... for lamellae, 2

1, S , Λ/4 , Λ/7 , Λ/9, ... for cylinders in hexagonal array, 1, yfl , Λ/3, Λ/4 , Λ / 5 , ... for spheres in body centered cubic array, and so on. Synchrotron SAXS measurements were carried out at room temperature in order to obtain information on the change in the equilibrium microdomain morphology of SIS/PS mixtures with the addition of PS. In Figures 1 and 2, the results for V4411D/PS2 and V4113/PS2 mixtures are presented, respectively, in a semi logarithmic plot of scattered intensity as a function of scattering wave number q. q is given by (7 = (4;r//l)sin(0/2) (1) where θ is the scattering angle. All the measurements were taken after annealing the samples at 130°C for 3 days. Each data set was vertically shifted for clarification. As shown in Figure 1, the relative positions of scattering peaks of V4411D/PS2 mixtures are 1, 2, and 3 up to 30 wt% of PS2, while 1, (4/3) , and 2 at 40 wt% of PS2. It is î/2



500

Figure 1. Synchrotron SAXS profiles ofV44JW/PS2 mixtures with different composition at room temperature after annealing the samples at 130 Vfor 3 days. Each data set was vertically shiftedfor clarification.



501

Figure 2, Synchrotron SAXS profiles ofV4113/PS2 mixtures with different composition at room temperature after annealing the samples at 130 V for 3 days. Each data set was vertically shiftedfor clarification.


502 interesting to note that higher order peaks of 60/40 (w/w) V4411D/PS2 mixture appear at the relative positions of (4/3) and 2. Since the former is the unique characteristic of the gyroid phase and the latter probably results from the lamellar phase, it can be inferred that lamellae and gyroid phases coexist in the 60/40 (w/w) V4411D/PS2 mixture. This further implies that the transition from lamellae to gyroid phase occurs around w ~ 0.40, where w 2 is the weight fraction of PS2 in the mixture. Transition from gyroid to polyisoprene cylinders in hexagonal packing 1/2

PS2

occurs at 0.40 < w

PS2

PS

< 0.50, since the relative positions of the peaks change to 1, V J ,

VÏ , and 4Ï , which is the characteristic of cylinders in hexagonal array, at 50 wt% of PS2. If we assume complete solubilization of PS2 in the polystyrene microdomains of SIS triblock copolymers, overall styrene volume fraction Φ can be calculated. The values of Φ at the transition points, Φ ~ 0.614 and 0.614 < Φ < 0.676, are in reasonable agreement with those of PS volume fraction in SI diblock copolymers reported in the literature . Therefore, it can be concluded that Φ can be a good measure of the equilibrium microdomain morphology in block copolymer/ homopolymer mixtures. For V4113/PS2 mixtures, the transition from polystyrene cylinders to lamellae was found to occur at 0.20 < w < 0.25 (0.288 < Φ < 0.33). This kind of transition from one microstructure to another by adding homopolymer to the block copolymer has also been reported by many researchers, but will not be discussed here in detail. Ρ 5


Ρ5

Ρ5

Ρ δ

9

Ρ5

PS2

Ρ5

Order-Disorder and Order-Order Transitions Determined from Rheology In order to determine the order-disorder transition temperature (T DT)> two different measurements were employed in present study. Rheological measurements were first carried out. Figures 3 and 4 show log G' vs Τ plots of V4411D/PS2 and V4113/PS2 mixtures, respectively, with different composition. Angularfrequencyω was fixed to 0.5 rad/sec for all the samples except for V4113 (0.1 rad/sec) and the heating rate was 1 °C/min. As is evident in the figures, G' shows a precipitous drop at a certain temperature for each mixture. Rosdale and Bates investigated the orderdisorder transition behavior of a series of PEP-PEE block copolymers having lamellar microdomains, and found that both G' and G" drop precipitously at a certain temperature with angular frequencies below critical angular frequencies (co ' and co " respectively). They also reported that the drop in G* is more pronounced since the critical angular frequency for G' is larger than that for G" (i.e., co ' > co ). Following their criterion, the temperature at which there is an abrupt change in G' was chosen as TODT of each mixture. While T D T of V4411D/PS2 mixtures decreases as the PS2 weight fraction was increased in the mixture (see Figure 3), T D T of V4113/PS2 mixtures first decreases up to 10 wt% and increases again (see Figure 4). This unusual behavior will be discussed later. It should be mentioned here that some researchers argued Bates' criterion for the ODT of highly asymmetric block copolymers and proposed another method. Han and coworkers used log G' vs log G" plots to determine the ODT temperatures of 0

7

c

M

c

c

0

0

11,20


c


503

Figure 3. Dynamic storage modulus (G') as a function of temperature for V4411D/PS2 mixtures with different composition.


504

1


Τ

Ε

1

140

1

1

160

1

I

1

180

I

.

·

ι

200

1

ι

,

ι

220

Γ

,

ι

240

,

3

260

Temperature (°C) Figure 4. Dynamic storage modulus (G') as a function of temperature for V4113/PS2 mixtures with different composition. Each data set was vertically shifted by multiplying JO , 10 , JO , JO , and J0°, respectively, for clarification. 4

s

2

1


505 asymmetric SI diblock and SIS triblock copolymers, and proposed that the temperature at and above which log G' vs log G" plot does not change any more should be regarded as T D T - Their criterion was not checked for our V4113/PS mixture system, however, since the values of T D T determined from log G' vs log G" plots are usually much larger than those determined from log G' vs Τ plot and exceed the degradation limit for our system (-250 °C). 0

0


Order-Disorder and Order-Order Transitions Determined from SAXS Figure 5 presents typical change in the synchrotron SAXS profiles for 70/30 (w/w) V4113/PS2 mixture with temperature. Scattering intensity is plotted in logarithmic scale in order to see more clearly the change in multiple scattering peaks. Maximum scattered intensity I shows a sharp decrease between 230°C and 235 °C. This discontinuity is more pronounced when the inverse of scattering maximum 1/I and the square of the half-width at half-maximum a were plotted against inverse temperature 1/T as shown in Figure 6a. Sakamoto and Hashimoto investigated the ODT of near symmetric SI diblock copolymers (f =0.45 and 0.54) using SAXS, and reported that there exists a clear discontinuity at ODT in 1/I vs 1/T plot. They also reported that σ shows the same trend as 1/I . As can be seen in Figure 6a, σ follows exactly the same trend as 1/I . From Figure 6a, the temperature at which the discontinuity occurs in 1/I and σ vs 1/T was chosen as the T D T of the mixture. The value of the T D T was in good agreement with the one determined from the precipitous decrease in log G' vs Τ plot shown in Figure 4. In Figure 6b, domain periodicity D (= 2n/q ) was plotted against 1/T. It is interesting to note that there exists a discontinuity in D at ODT. This is in contrary to the results of Sakomoto and Hashimoto that D is continuous at ODT. Recently, however, Floudas and coworkers examined the ODT of SI simple graft copolymer, SIB terpolymer, (SI) star-block copolymers and asymmetric SI diblock copolymer using SAXS, and reported the discontinuity in D. Ogawa et αι. also reported similar results to our study for an asymmetric SI diblock copolymer (f =0.303). It can be clearly seen in Figure 5 that the position of the second order scattering peak relative to the first order scattering peak changes from 2 to 3 between 190°C and 195°C. Local maxima were also found between these temperatures in both 1/I vs 1/T and σ vs 1/T plots shown in Figure 6a. In addition to this, a discontinuous change in D occurs in the same temperature range as can be seen in Figure 6b. Above results indicate that order-order transition from lamellae to cylinders in hexagonal array occurs between these temperatures. Similar results on OOT were also observed for asymmetric SIS triblock copolymer (w =0.183) by Sakamoto et ai. It can be concluded from our results that local maxima in both 1/I and σ against 1/T and the discontinuous change in D can be attributed to OOT. I/Imax, s and D vs 1/T plots for 70/30 (w/w) V4411D/PS2 mixture are presented in Figure 7. Similarly, the discontinuity in D vs 1/T plot as well as 1/I and σ vs 1/T plots was found at ODT. The T D T value was again in good agreement with the max

max

2

q

10

PS

max

2

2

ς

max

ς

max

2

max

ς

0

0

max

10

17,

18

2

4

19

PS

1/2

max

2

ς

20

PS

2

max

ς

2

q

2

max

0


ς


506



507

Figure 6. Scattering parameters as a function of inverse temperature for 70/30 (w/w) V4113/PS2 mixture: (a) inverse of the scattering maximum (1/I .x) and square of half-width at half maximum (σ ) plotted against 1/T, and (b) domain spacing (D = 2ïï/q ) against 1/T. A heating rate of 1 V/min was used. ma

2

max


508

T(°C)


300 280 260 240

1.9

220

200

2.0

180

2.1 1

1000/T(K" )

Figure 7. Scattering parameters as a function of inverse temperature for 70/30 (w/w) V44Î1D/PS2 mixture: (a) inverse of the scattering maximum (1/I x) and square of half-width at half maximum (a ) plotted against 1/T, and (h) domain spacing (D = 27t/q ) against 1/T. A heating rate of I V/min was used. ma

2

q

may


509 one determined from the precipitous decrease in log G' vs Τ plot shown in Figure 3. OOT was not observed in this mixture. Pure V4113, 75/25 (w/w) V4113/PS2 mixture and 60/40 (w/w) V4411D/PS2 mixture were also found to show OOT (not shown here). The details of ODT and OOT of these mixtures will be discussed in future publication.


Phase Diagrams of Mixtures of SIS Triblock Copolymers and PS Homopolymers Phase diagrams of V4411D/PS and V4113/PS mixtures with different PS molecular weight obtained from both measurements are constructed in Figures 8 and 9, respectively. PS homopolymer weight fraction of the mixture was converted to the overall styrene volume fraction by assuming complete solubilization of PS in the polystyrene microdomains of SIS triblock copolymers and shown in the upper x-axis. Theoretical phase diagrams were also constructed in the insets of Figures 8 and 9 based on Whitmore-Noolandi theory for comparison with the experimental results. The following expression was used in the calculations for the temperature-dependent Flory-Huggins interaction parameter between styrene and isoprene. j = -0.0149 + 38.54/Γ (2) It should be noted here that no adjustable parameter was used in the above calculations. As shown in Figure 8, TQDT'S of V4411D/PS mixtures were found to either decrease or increase depending on PS homopolymer molecular weight as PS homopolymer weight fraction in the mixture was increased; T D T decreases with the addition of PS2 and PS3, while it increases with the addition of PSI 1. This behavior is in good agreement with the calculated phase diagram based on the WhitmoreNoolandi theory (see inset of Figure 8). Phase diagram of V4113/PS2 mixtures plotted in Figure 9, however, shows different behavior; T D T first decreases as PS2 weight fraction is increased up to 10 wt% and then increases again. This unusual trend was not predicted from the Whitmore-Noolandi theory (see inset of Figure 9). This can be attributed to the change in homopolymer distribution in the microdomain as PS2 homopolymer is added. If a small amount of low molecular weight homopolymer is added to the block copolymer, it tends to be solubilized uniformly into the microdomains. As a large amount of homopolymer is added, however, it is localized in the center of the microdomain. This finally results in the change in microdomain morphology, e.g., from cylinders to lamellae, if homopolymer is further added, as evidenced from the change in relative position of multiple SAXS peaks with increasing PS2 weight fraction in the mixture (see Figure 2). This effect of homopolymer localization on ODT would be more pronounced in mixtures with asymmetric copolymer (V4113) than those with symmetric copolymer (V441 ID). 1

21

0

0

2

Conclusion Triblock copolymer/homopolymer mixtures showing no macrophase separation have been prepared by adding a small amount of a series of low molecular weight



510

Figure 8. Phase diagram ofV44UD/PS mixtures. Filled and open symbols are TQDT'S obtainedfrom rheological and SAXS measurements, respectively. Calculations based on the Whitmore-Noolandi theory are also shown in the inset.



511

Figure 9. Phase diagram ofV4113/PS mixtures. Filled and open symbols are T DT'S obtainedfrom rheological and SAXS measurements, respectively. Calculations based on the Whitmore-Noolandi theory are also shown in the inset. 0


512 polystyrenes (PS) to near symmetric (V4411D, f = 0.377) and highly asymmetric (V4113, fp = 0.131) polystyrene-6/oc£-polyisoprene-6/oc£-polystyrene copolymers. The order-disorder and order-order transitions (ODT and OOT) of these mixtures were investigated by using both small-angle X-ray scattering (SAXS) and rheological measurements. ODT temperatures of the mixtures were determined from the discontinuous change in 1/I and σ vs 1/T plots using SAXS. They were found to be in good agreement with the ones determined from the discontinuous change in log G' vs Τ plots using rheology. OOT was also identified for pure V4113 and a few of its mixtures with PS homopolymers, as evidenced from the change in relative position of higher order scattering peaks as well as the discontinuity in D and local maxima in 1/I and o atOOT. Phase diagrams of V4411D/PS and V4113/PS mixtures were constructed from the ODT measurements using both SAXS and rheology, and compared with theoretical predictions based on the Whitmore-Noolandi theory. ODT of V4411D/PS mixtures were found to either decrease or increase depending on the molecular weight of PS homopolymer added, which is in good agreement with the Whitmore-Noolandi theory. ODT of V4113/PS2 mixtures, however, first decreased and increased again with the addition of PS2, which was not predicted by the theory. We attributed this abnormal phase behavior to the change in the homopolymer distribution in the microdomains with the addition of PS homopolymer. PS

S

2

max

ς

2


max

q

Acknowledgment Experiments performed at Pohang Light Source (PLS) were supported in part by MOST and POSCO. We are very grateful to Y . J. Park for his assistance during SAXS experiments at PLS. K. Char is also very grateful to the LG-Yonam Foundation for its financial support during a sabbatical visit (1997 ~ 1998) to Cornell University.

References 1. 2. 3.

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513 11. Han, C. D.; Baek, D. M.; Kim, J. K.; Ogawa, T.; Sakamoto, N.; Hashimoto, T. Macromolecules 1995, 28, 5043. 12. de la Cruz, M. O.; Sanchez, I. C. Macromolecules 1987, 20, 440. 13. Matsen, M. W. Macromolecules 1995, 28, 5765. 14. Janert, P. K.; Schick, M. Macromolecules 1998, 31, 1109. 15. Roe, R.-J.; Zin, W.-C. Macromolecules 1984, 17, 189. 16. Baek, D. M.; Han, C. D.; Kim, J. K. Polymer 1992, 33, 4821. 17. Floudas, G.; Hadjichristidis, N . ; Iatrou, H.; Pakula, T.; Fischer, E. W. Macromolecules 1994, 27, 7735. 18. Floudas, G.; Pispas, S.; Hadjichristidis, N . ; Pakula, T.; Erukhimovich, I. Macromolecules 1996, 29, 4142. 19. Ogawa, T.; Sakamoto, N.; Hashimoto, T.; Han, C. D.; Baek, D. Macromolecules 1996, 29, 2113. 20. Sakamoto, N.; Hashimoto, T.; Han, C. D.; Kim, D.; Vaidya, N . Y. Macromolecules 1997, 30, 1621. 21. Hashimoto, T.; Ijichi, Y.; Fetters, L. J. J. Chem. Phys. 1989, 89, 2463.


SAXS and Rheological Studies on the ... - ACS Publications

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