Block Crystallization in Model Triarm Star Block Copolymers with Two

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Chapter 28

Block Crystallization in Model Triarm Star Block Copolymers with Two Crystallizable Blocks: A Time-Resolved SAXS-WAXD Study 1

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2

2

3

3

G. Floudas , G. Reiter , O. Lambert , P. Dumas , F.-J. Yen , and B. Chu 1

Foundation for Research and Technology-Hellas (FORTH), Institute of Electronic Structure and Laser, P.O. Box 1527, 711 10 Heraklion Crete, Greece Institut de Chimie des Surfaces et Interfaces, 15 rue Jean Starcky, B.P. 2488, 68057 Mulhouse Cedex, France Department of Chemistry, State University of New York at Stony Brook, Stony Brook, NY 11794-3400

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The kinetics of crystallization in model triarm star block copolymers of two crystallizable blocks (poly(ethylene oxide) (PEO) and poly(εcaprolactone) (PCL)) and an amorphous block (polystyrene (PS)) have been investigated with time resolved synchrotron wide-angle Xray diffraction (WAXD) and small-angle X-ray scattering (SAXS). These model block copolymers form a homogeneous melt and the crystallization of the longer block drives the microphase separation. We have explored the block crystallization in the stars as a function of (i) the length of the two crystallizable blocks and (ii) the crystallization temperature. We found "paths" of delaying or even inhibiting individual block crystallization.

Block copolymers composed of crystallizable and amorphous blocks develop structure over different length scales, from the unit cell structure of the crystalline block to the microdomain length scale to the spherulitic superstructure. Such combination of the amorphous and crystalline phases can result in materials with enhanced mechanical properties. The ability to form structures over various length scales (1)(2), the control over the mechanical response (3)(4), and the possibility to template crystallization by fluid mesophases (5) have been the driving force for investigating such materials. We have recenlty reported (6) on the structure of model triarm star block copolymers of two crystallizable blocks (PEO and PCL) and an amorphous block (PS) using X-ray scattering, differential scanning calorimetry 448

© 2000 American Chemical Society

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

449 (DSC), optical microscopy (OM) and atomic force microscopy (AFM). Our aim is to trigger the thermodynamic interactions by combining two crystallizable blocks with an amorphous block and to develop an understanding of the effect of polymer architecture on the crystallization process. The main results from the earlier study can be summarized as follows. Both PEO and PCL blocks can crystallize in the stars provided that their length ratio is below 3. For the more asymmetric stars, the longer block suppresses the crystallization of the shorter one. The crystallinity, long period and crystalline lamellar thickness are reduced in the stars as compared to the PEO and PCL homopolymers. There is a reduction in the equilibrium melting temperature in the stars (as compared to the values of 343 and 347 Κ for PEO and PCL, respectively), which is caused primarily by the amorphous PS block. The Avrami analysis of the isothermal crystallization calorimetrie experiments indicated a disc-like growth from heterogeneous nuclei, independent of the nature of the crystallized block. However, the crystallization times were sensitive to the block nature. Lastly, different superstructures were formed (spherulites/axialites) depending on the type of the crystallizable block (PEO/PCL). The nucleation sites of the two superstructures were completely independent suggesting a heterogeneous distribution of star molecules with PEO or PCL crystals. In the present study we use the different unit cells of PEO (monoclinic) and PCL (orthorhombic) of the two crystallizable blocks as a probe of crystallization and we investigate the crystallization kinetics by making temperature jumps - from the homogeneous phase to different crystallization temperatures - as a function of composition and temperature using time-resolved synchrotron SAXS/WAXD.

Experimental The synthesis of the model triarm star block copolymers has been reported elsewhere (7). Table I gives the molecular characteristics of the three copolymers used in the present study.

Table I. Molecular Characteristics of the Triarm Star Block Copolymers Sample

M (PS) n

M (PEO) n

M„(PCL)

M( total) n

X10

X10

3

3

MJM

n

3

X10

SEL4.7/20/1.8

4.7

20

1.8

27

1.15

SEL4.7/20/45

4.7

20

45

70

1.19

SEL4.7/20/87

4.7

20

87

112

1.29


450 The PEO and PCL weight fraction in the three stars are: 0.88 and 0.03 (SEL4.7/20/1.8); 0.51 and 0.44 (SEL-4.7/20/45); 0.36 and 0.6 (SEL-4.7/20/87), respectively. Real time-resolved synchrotron SAXS/WAXD measurements have been performed at the X27C beamline of the National Synchrotron Light Source at Brookhaven National Laboratory. The wavelength λ, of the X-rays was 0.1307 nm. Two position sensitive detectors were used; one at small-angle at a distance of 1.2 m from the sample and the other at a wide-angle setting of about 45° angle to the sample. The samples used in this study were prepared directly from the melt and upon crystallization had a thickness of about 1mm. First all samples were heated to 383 Κ for 5 min and subsequently brought to another temperature unit at the preset final temperature. Temperature jumps to different final temperatures in the range 312-316 Κ have been investigated. The Lorentz correction was performed to the data by multiplying the scattered intensity I by q (q=(4jcM,)sinO, q is the scattering wavevector and θ is the scattering angle). Knowledge of the absolute intensity, of the contribution from pure density fluctuations (thermal background) together with an extrapolation to q=0 and to q=°o can result in the absolute invariant. The relative invariant, Q, was calculated here from 2

(1) 2

where I „ is the background scattering, by integrating the (I-I )q curve from q=0.15 to 1 nmwhere the above quantity becomes independent of q. b

d

bgd

Results and Discussion Typical SAXS/WAXD spectra are shown in Figure 1 for the SEL-4.7/20/1.8 copolymers for a crystallization temperature of 313 K. The SAXS spectrum taken immediately before the T-jump indicated only a minor contribution from concentration fluctuations at 383 K. This is a result of the compatibility of PS and PCL in the melt state (8) and of the star architecture with PEO which further increases the intrinsic compatibility of the copolymer (9). After about 500 s the WAXD patterns develop peaks corresponding to the monoclinic unit cell of PEO. Notice that the crystallization of the shorter block is suppressed in the triarm star. At the same time the SAXS peaks develop signifying the formation of the PEO crystalline lamellar. The result of the corresponding T-jump for the SEL-4.7/20/87 triarm star block copolymer from 383 to 313 Κ is shown in Figure 2. Notice the distinctly different WAXD Bragg peaks with positions corresponding to the (110) and (200) reflections from the orthorhombic unit cell of PCL. Now, it is the crystallization of the PEO block which is suppressed, notwithstanding the high molecular weight of the PEO block (Mw=2xl0 ). Simultaneous with the WAXD reflections, a peak in the SAXS spectra appears reflecting the characteristics of the PCL crystals. We have shown earlier (6) that the crystallinity, the long period and the crystal thickness are reduced in the stars and that the magnitude of the reduction depends on the ratio of the amorphous to the crystallized block lengths. The calculated invariant from the 4


451 SEL 4.7/20/1.8

1

(nm' )

q

Figure

L

copolymer

SAXS

(left)

Time (s)

and

WAXD

taken in increments

313 K. The SAXS

invariant

the PEO are also

(right)

Channel

from

of 60s following

Q and the WAXD

the

SEL-4.7/20/1.8

a temperature

intensity

star

jump from

block

383 Κ to

of the most intense reflection

of

compared.

SEL 4.7/20/87

0,5

1,0

10

2

1(f

1

q (nm' ) Figure block The

2. SAXS copolymer

WAXD

(200)

reflections

peak

develops

intensity

(left) and WAXD

(right ) spectra

taken at increments

pattern

after about

from

300

400

for

a T-jump from

of the most intense WAXD

unit cell of PCL PCL

reflections

crystals. are

600

the SEL-4.7/20/87

400 s shows peaks corresponding

of the

500

Channel

of 60 s following

the orthorhombic

characteristic

200

Time (s)

triarm

to the (110)

At the same time a

The SAXS

star

383 to 313

invariant

compared.


and

K. and

SAXS the

452 SAXS spectra and the intensity of the most intense WAXD reflection (110) increase simultaneously, indicating that the PCL crystallization drives the microphase separation in this system as well. The situation with the third block copolymer, SEL-4.7/20/45, is very different. Earlier conventional X-ray measurements (6) have shown that both blocks are capable of undergoing crystallization, however, not within the same molecule. The distinctly different WAXD patterns corresponding to the PEO and PCL unit cells facilitate the investigation of the crystallization kinetics for both blocks. The result of the T-jump from 383 to 312 Κ is shown in Figure 3. The WAXD spectra at long times exhibit three main reflections: a peak at low q corresponding to the Bragg reflection from the monoclinic unit cell of PEO, an intermediate peak corresponding to the (110) reflection from the orthorhombic unit cell of PCL and a third peak which is an overlapping reflection from both unit cells. The existence of mixed reflections clealry shows that in SEL-4.7/20/45, both blocks can crystallize. However the results from OM have shown that (i) there is a heterogeneous distribution of star molecules with PEO or PCL crystals and (ii) crystallization of the two blocks is not simultaneous. The spectra shown in Figure 3 support the OM conclusion. In the WAXD spectra, first the intermediate reflection appears corresponding to the (110) reflection from the orthorhombic unit cell of PCL and at some later time PEO starts to crystallize.

Figure 3. SAXS (left) and WAXD (right ) spectra of the SEL-4 J/20/45 triarm star block copolymers following a T-jump from 383 Κ to 312 K. Notice the mixed reflections at the WAXD spectra indicating that both PEO and PCL crystallize in their monoclinic and orthorhombic unit cells.


453

The SAXS invariant and the WAXD peak intensities corresponding to pure PEO, pure PCL (110) and mixed PEO+PCL reflections are compared in Figure 4. The intensity of the mixed reflection follows the increase of Q indicating again that microphase separation is driven by block crystallization. The two pure reflections have a different time-evolution. The PCL block starts to crystallize first and after the PCL crystallization is completed, the PEO block starts to crystallize.

SEL 4.7/20/45

0

500

1000

1500

2000

2500

Time (s) Figure

4. Time-evolution

of the SAXS invariant

QSAXS compared

three most intense WAXD peaks for the SEL-4.7/20/45

to the intensity of the

at T=312 K.

We have investigated the dependence of the crystallization times - which are operationally defined here as the time required for the first WAXD reflection to appear - of PEO and PCL on the crystallization temperature T , and the results are shown in Figure 5. The background intensity (I ) was evaluated by monitoring the time-evolution of an area away from the Bragg reflections and analyzed by fitting to Ib d=A+Bexp(-t/x), where A, Β and τ depend on the T . Then the QSAXS d the three peak intensities were evaluated. The PCL-crystallization is always faster and for smaller undercoolings the difference in the crystallization rates increases. For example, at 316 K, the PEO crystallization is slower by a factor of 4 whereas at 312 Κ the two times are comparable. OM results on the same star block copolymer have shown a higher nucleation probability for PCL crystals than for PEO, in agreement with the data of Fig.4. Furthermore, in OM, the growth rate of PCL crystals (with an c

bgd

a n

g

c


454 axialitic superstructure) was higher than for PEO (with a spherulitie superstructure) which is in agreement with the steeper slope of the IPCL reflection in Fig. 4. The results shown in Figures 1,2 3 and 5 demonstrate that it is possible to control the nature of the crystallizing block in the stars not only at the synthesis level (by synthesizing asymmetric block copolymers) but also by delaying the crystallization of a particular block (in this case the PEO block) in cases where both blocks can crystallize (i.e., the crystallizable blocks are of comparable length).

SEL-4.7/20/45

/ /

_ o

—5

a """"

313

314

315

316

317

T (K) c

Figure (circles)

5.

Dependence

of

on the crystallization

the

crystallization

temperature

times

in the star

of

PEO

(squares)

and

PCL

SEL-4.7/20/45.

Conclusions Star block copolymers composed of two crystallizable blocks and an amorphous block offer new possibilities to control the block crystallization at the synthesis level and in the lab by changing the crystallization conditions. We have shown that in asymmetric stars only the longer block will crystallize. In more symmetric stars both blocks can crystallize but not within the same molecule. Furthermore, we have shown here with the use of time-resolved synchrotron SAXS/WAXD, that in stars with comparable block lengths of the crystallizable blocks, it is possible to delay the crystallization of a particular block for long times with a suitable choice of temperature. These features could be important in the design of new materials where a selectivity in block crystallization is required.


455

Acknowledgment This work was supported by the NATO-CRG 970551 to G.F. and B.C., by a FrenchGreek collaborative grant (CNRS-NHRF 1998) to G.R. and G.F and by a NSF grant (DMR9612386) to B.C.

References 1.

Wunderlich, B. Macromolecular Physics 2. Crystal Nucleation, Growth, Academic Press: New York, 1978. Strobl, G. The Physics of Polymers; Springer-Verlag: Berlin, 1996; Chapter4. Floudas, G.; Tsitsilianis, C. Macromolecules 1997, 30, 4381. Alig, I. Tadjbakhsch, S.; Floudas, G.; Tsitsilianis, C. Macromolecules 1998, 31, 6917. Quiram, D.J.; Register, R.A.; Marchand, G.R.; Ryan, A.J. Macromolecules 1997, 30, 8338. Floudas, G.; Reiter. G.; Lambert, O.; Dumas, P. Macromolecules 1998, 31, 7279. Lambert, O.; Dumas, P.; Hurtez, G.; Riesss, G. Macromol. Rapid Commun. 1997, 18, 343. Li, Y.; Jungnickel, B.-J. Polymer 1993, 34, 9 Floudas, G.; N. Hadjichristiidis, Y. Tselikas, I. Erukhimovich, Macromolecules 1997, 30, 3090.

Annealing;

2. 3. 4. 5. 6. 7. 8. 9.


Block Crystallization in Model Triarm Star Block Copolymers with Two

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