Confined Side Chain Crystallization in Microphase-Separated Poly(styrene-block-octadecylmethacrylate) Copolymers

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1 Confined Side Chain Crystallization in Microphase-Separated Poly(styrene-block-octadecylmethacrylate) Copolymers Mario Beiner 1, Elke Hempel 1, Hendrik Budde 2, Siegfried Höring 2 1 FB Physik, Martin-Luther-Universität Halle-Wittenberg, D-699 Halle (Saale), Germany 2 FB Chemie, Martin-Luther-Universität Halle-Wittenberg, D-699 Halle (Saale), Germany The crystallization behavior of microphase-separated poly(styrene-b-octadecylmethacrylate) block copolymers with lamellar and cylindrical morphology is studied by DSC and scattering techniques. In these block copolymers confined side chain crystallization occurs in PODMA domains of size of 1-2nm surrounded by glassy polystyrene. The calorimetric crystallization behavior of PODMA lamellae is similar to the situation in the homopolymer and practically unaffected by the confinement. Strong confinement effects are observed in cylindrical PODMA domains with a diameter of about 1nm: The degree of crystallinity is 5% reduced and the crystallization kinetics slows down. The Avrami coefficients changes from n~3 for homopolymers and PODMA lamellae to n~1 for PODMA cylinders with a diameter of 1nm. These findings are discussed in the context of a change from heterogeneous to homogeneous nucleation and/or from three- to one-dimensional crystal growth. A speculative picture for the internal structure of the PODMA domains explaining qualitatively differences between the crystallization in lamellae and cylinders in a glassy environment is discussed. 1. INTRODUCTION Confined crystallization in block copolymers has been investigated since the 198s by scattering techniques [1], calorimetry [2,3], atomic force microscopy [3] and other methods. Significant changes in the crystallization kinetics have been observed depending on domain size and morphology due to changes in crystal growth as well as nucleation behavior. Characteristic for the confined crystallization case is that the microphase morphology is not significantly affected by the crystallization process [4]. So far the confined crystallization of polymers like polyethylene, polyethyleneoxide or poly(ε-caprolactone) has been studied in detail. Chain folding is a relevant effect in all these polymers. Less is know about the side chain crystallization under confinement. In general, detailed studies to the side chain crystallization process seem to be rare. Thus, we have focused our work on the crystallization of poly(n-octadecylmethacrylate), PODMA, as crystallizable component of microphase-separated block copolymers. In this side chain polymer long alkyl groups belonging to different monomeric units and PODMA chains are able to crystallize. Poly(styrene-b-octadecylmethacrylate) copolymers are studied. In these systems side chain crystallization occurs in a glassy environment because the glass temperature of the polystyrene block (T g 85 C) is significantly higher than the crystallization temperature of the PODMA block (T c =1 3 C) [5]. A special feature of such block copolymers is a hierarchy of length scales in the nanometer range: One length scale in the range 1-3nm due to the microphase separation of the two incompatible blocks and a second scale due to nanophase separation of methacrylate main chains and long alkyl groups in the PODMA domains [6]. In this paper we present calorimetric data as obtained from DSC experiments during the isothermal crystallization of P(S-b-ODMA) block copolymers. The influence of the confinement on the crystallization behavior is studied and compared with the findings for PODMA homopolymers. 1

2 2. RESULTS Scattering data in Fig.1 show the different length scales in P(S-b-ODMA) block copolymers and their changes during crystallization. At low q-values the typical features of a lamellar block copolymer are observed. The thickness of the POMDA lamellae is about 17nm for this example. The position of the first order peak at q.15nm -1 is obviously not shifting during the crystallization process, i.e., the morphology is basically unaffected and side crystallization occurs in a strong confinement [4]. The disappearance of the odd order peaks is probably a consequence of a change in the average density of PODMA during crystallization. The peak at near q 2nm -1 indicates nanophase separation of main and side chain parts in the PODMA domains. The peak is observed already in the amorphous state but sharpens during the crystallization process indicating an increase of the perfection of the relevant structure. A slight shift to lower q-values during crystallization indicates an increase of the average main chain to main chain distance although the average density of PODMA should increase. Higher orders belonging to the nanophase separation peak obtained for crystalline PODMA homopolymers indicate that regular packing of thin lamellar crystals is typical for the final structure of crystalline PODMA. In the WAXS range near q 15nm -1 a sharp peak develops on top of a amorphous halo reflecting the development of the crystalline fraction in the PODMA domains. The maximum of the peak occurs at larger q values than the maximum of the amorphous halo for PODMA consistent with a decrease of the average distance between non-bonded alkyl carbons and an increase of the PODMA density during side chain crystallization. The absence of additional peaks in the WAXS range q>15nm -1 indicates hexagonal packing of crystalline alkyl groups. Densification of PODMA during side chain crystallization is obviously anisotropic. The packing density of the alkyl groups in the direction parallel to the main chains increases while the main chain to main chain distance increases slightly. Further details of the structure of semi-crystalline PODMA will be discussed elsewhere [7]. d nps d loc log(i / a.u.) d mps (a) (b) (c) q / nm -1 Figure 1. Scattering data for a lamellar P(S-b-ODMA) block copolymer containing Φ=44vol% ODMA (N PS =257, N ODMA =52, d ODMA 17nm) measured during isothermal crystallization (bold line: t c =1min - amorphous; thin line: t c =2min semi-crystalline) at T c =27 C measured on beamline BM26B at the ESRF in Grenoble. Different structures are indicated in different q ranges: (a) microphase separation of PS and PODMA blocks (d mps 4nm), (b) nanophase separation of main chain and alkyl groups in PODMA domains (d nps 3.1nm), (c) local structure of the alkyl groups (d loc.41nm). 2

3 The isothermal crystallization process has been studied using a Perkin-Elmer DSC7. We have performed isothermal crystallization experiments at different temperatures. The sample was quenched (dt/dt=-4k/min) from the molten state (T m +2K) to the crystallization temperature T c and was then isothermally annealed for a certain crystallization time t c. Afterwards a heating curve (dt/dt=+1k/min) was measured and the heat of melting q m was determined based on the area of the melting peak [5]. For a given crystallization temperature T c the crystallization time t c was varied in the range from about five to a few thousand minutes. From these data isotherms as shown in Fig.2a have been constructed. Based on the assumption that the heat of melting per CH 2 unit is identical to those for alkanes we have used the equation D c = q m / q m,od with D c being the degree of crystallinity and q m,od =61.4kJ/mol being the heat of melting for octadecene as obtained from analogous DSC measurements. This q m,od value corresponds to values for the heat of melting per CH 2 unit q m,ch2 3.4kJ/mol reported in the literature [8]. The time dependence of heat of melting q m and degree of crystallinity D c for a lamellar block copolymer measured under isothermal conditions at different temperatures T c is shown in Fig.2a. As expected the characteristic crystallization time of the sigmoidal step in D c (t c ) increase with increasing crystallization temperature. In general, primary crystallization occurs in a narrow time interval for all T c values. Note, that primary crystallization is followed by a strong secondary crystallization in PODMA which leads to a linear increase of D c on logarithmic time scales. This is a common feature of the side chain crystallization in PODMA under different conditions, i.e., it occurs for homopolymers as well as block copolymers containing PODMA. Apart from the shift of the characteristic times for the primary crystallization process the shape of the D c (t c ) curves is quite similar. Thus, master curves can be constructed shifting the individual D c isotherms horizontally until they optimally overlap (Fig.2b). The method to determine the half time τ c for the reference temperature from the master curve using a tangent construction is indicated. The shift factors were used to calculate half times τ c for all other crystallization temperatures T c. 6 T c / C q m / Jg (a) t c / min 2 1 D c / mol% q m / Jg T (b) ref = 29 C t c / min D c / mol% Figure 2. Crystallization curves for a lamellar P(S-b-ODMA) block copolymer (d ODMA 17nm): (a) Heat of melting q m and degree of crystallinity D c vs. isothermal crystallization time t c measured at different temperatures T c. (b) Master curve for a reference temperature T ref =29 C as obtained from the individual isotherms in part (a). The tangent construction used to determine the half time τ c (T ref ) is indicated. 3

4 The effects of confinement size and block copolymer morphology on the side chain crystallization in the PODMA domains are shown in Fig.3. Master curves for a PODMA homopolymer as well as PODMA in lamellar and cylindrical domains are compared (Fig.3a). The q m data indicate a reduction of the achieved degree of crystallinity D c in cylindrical PODMA domains compared to the situation in homopolymers and PODMA lamella. Moreover, the crystallization kinetics is significantly different. In homopolymers and PODMA lamellae crystallization occurs in a narrow time interval corresponding to Avrami exponents n~3 usually observed for three- or two-dimensional growth and heterogeneous nucleation. The influence of the lamella thickness seems to be negligible. In case of small PODMA cylinders with a diameter of about 1nm, however, the crystallization behavior is different. Broad transformation intervals and Avrami exponents n < 1 are observed. This indicates strong confinement effects due to a change from two- or three-dimensional to one-dimensional crystal growth and/or a change from heterogeneous to homogeneous nucleation. Note, that the crystallization temperature in small PODMA cylinders is 15K smaller than those for homopolymers and PODMA lamellae. The melting temperatures T m as obtained from heating curves (dt/dt = +1K/min) measured after cooling the sample with the same rate (dt/dt=-1k/min) show a similar trend [5]. q m / Jg (a) Homo Lam 9 Cyl a T *t c / min D c / mol% half time τ c / s Homo 11nm (b) T c / C Figure 3. Master curves (a) and temperature-dependent half times (b) for lamellar (triangles) and cylindrical (circles) P(S-b-ODMA) block copolymers and a PODMA homopolymer (squares). The labels correspond to the thickness of the PODMA lamellae (9,17) or the diameter of the PODMA cylinders (11,16,24) in nanometer. The reference temperatures for the master curves in part (a) are 31, 27 and 12 C for homopolymer, lamellar and cylindrical block copolymers, respectively. Open symbols in part (b) correspond to crystallization temperatures as obtained from heat flow rate measurements during isothermal crystallization. The values are taken from Ref.[5]. The temperature-dependent half times τ c for different block copolymers are compared with those for a PODMA homopolymer in Fig.3b. While the crystallization temperatures for homopolymer and lamellar samples are quite similar, the T c values for PODMA cylinders seem to decrease systematically with decreasing domain size. The slopes dlogτ c /dt c are similar for the different PODMA containing systems and vary in the range from.36 dec/k for the ho- 4

5 mopolymer to.77 dec/k for the block copolymer containing 11nm PODMA cylinders. For a given half time τ c the slope seems to increase with decreasing T c of the system. However, the slopes observed for the side chain polymer PODMA are generally much larger than the dlogτ c /dt c values reported for other polymers like poly(ε-caprolactone) (.14 dec/k), polyethyleneoxide (.16 dec/k) or polyethylene (.26 dec/k) [5]. 3. DISCUSSION AND CONCLUSIONS The experimental results for P(S-b-ODMA) block copolymers indicate clearly strong confinement effects for PODMA in cylindrical domains. If the diameter is smaller than 2nm degree of crystallinity D c, crystallization temperature T c and Avrami coefficient n decrease systematically. In contrast, the crystallization behavior in PODMA lamellae with comparable thickness is nearly unaffected and similar to that of PODMA homopolymers. A speculative picture for the internal structure of lamellar and cylindrical PODMA domains explaining qualitatively differences in the degree of crystallinity D c is given in Fig.4. In this context the observed changes in D c are related to the fact that the structure of semi-crystalline PODMA is lamellar and fits to the morphology of lamellar block copolymers. In case of PODMA cylinders the PS-POMDA interfaces are significantly curved and disturb the side chain crystallization within the PODMA domains. This effect is more pronounced for smaller cylinders and leads finally to a reduction of the D c values. A preliminary evaluation of scattering data for oriented lamellar P(S-b-ODMA) block copolymers and the dependence of the spacing d mps on the PODMA block length in case of samples with cylindrical morphology seem to support this picture. glassy polystyrene lamella glassy polystyrene matrix Figure 4. Speculative picture for the internal structure of the PODMA domains in case of P(S-b-ODMA) block copolymers with lamellar (l.h.s.) and cylindrical morphology (r.h.s.). The small beads represent the carboxyl groups close to the backbone. The shift of the crystallization temperature T c with decreasing diameter of the PODMA cylinders might be a consequence of a transition from heterogeneous to homogenous nucleation. Although the cylinders are not a priory isolated from each other a trend in this direction is observed for several block copolymers with cylindrical morphology [9]. Consistent with this interpretation is the broadening of the transformation interval in the D c (t c ) curves and the corresponding decrease in the Avrami coefficients from n 3 to n < 1. However, the T c reduction is relatively small of the order of 15K. This is much smaller than the undercooling values re- 5

6 ported for other systems ( 5K) where long main chains crystallize in small spherical domains [3]. Otherwise, 1-2K is a typical order of magnitude for undercooling effects in homogenously nucleated alkanes [1]. In general the side chain crystallization in PODMA has some similarities to the crystallization of alkanes although the alkyl groups in these side chain polymers are bonded to a less mobile main chain reducing the crystallization tendency. Only if the alkyl groups are long and mobile enough side chain crystallization occurs. If the side chain are too short and the main chain is immobile the alkyl groups are frustrated. The alkyl groups aggregate also under these conditions indicated by the nanophase separation peak in scattering data (Fig.1) but they do not crystallize [6]. Another approach to explain changes based on the reduced melting temperature T m in small cylinders is to relate this observation to the smaller degree of crystallinity. According to the Gibbs-Thomson relation thinner crystallites are connected with smaller melting temperatures. This was shown for alkanes with different length by calorimetry [8]. However, the assumption would be in this case that the reduction of D c in PODMA cylinders is basically due to a smaller thickness of the individual crystallites. This idea is not really consistent with the picture for the internal structure in small PODMA cylinders presented in Fig.4. Moreover, it should be mentioned that the Gibbs-Thomson equation is based on equilibrium thermodynamics while semi-crystalline polymers are kinetically hindered systems in a non-equilibrium state which are able to reorganize. The changes in the shape of the D c (t c ) curves could be also interpreted as a consequence of a transition from three-dimensional to one-dimensional crystal growth. In case of small PODMA cylinders crystal growth is more or less restricted to one direction while in case of PODMA lamellae it should be basically undisturbed in two directions. This would also lead to a change in the Avrami coefficients n and broad transformation intervals in isothermal crystallization curves as obtained for small PODMA cylinders (Fig.2a). In order to distinguish between different effects and interpretations additional information about the structure of semi-crystalline PODMAs, especially about the thickness of the thin crystalline lamellae in the alkyl nanodomains, is required. Work along this line is in progress. ACKNOWLEDGMENT The authors thank the German Science Foundation (SFB418) and the European Synchrotron Radiation Facility (ESRF) for financial support. REFERENCES 1. Hamley, I.W., The Physics of Block Copolymers, Oxford Universty Press, Oxford, Xu, J.T. et al., Polymer 44 (23) Röttele, A., Thurn-Albrecht, T., Sommer, J.U. and Reiter, G., Macromolecules 36 (23) Loo, Y.L., Register, R.A., Ryan, G.T., Macromolecules 35 (22) Hempel, E., Budde, H., Höring, S. and Beiner, M., Thermochim. Acta 432/2 (25) Beiner, M. and Huth, H., Nature Materials 2 (23) Hempel, E., Darko C. et al., to be published. 8. Höhne, G.W.H., Polymer 43 (22) Loo, Y.L., Register, R.A., Ryan, A.J. and Dee, G.T., Macromolecules, 34 (21) Kraack, H., Sirota, E.B. and Deutsch, M., J. Chem. Phys. 112 (2)