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Macromolecules 2011, 44, 440–443 DOI: 10.1021/ma102322w
Lower Critical Ordering Transition of Poly(ethylene oxide)-block-poly(2-vinylpyridine) Chao-Lin Yeh,† Ting Hou,† Hsin-Lung Chen,*,† Lin-Ya Yeh,‡ Fang-Chyou Chiu,‡ Alejandro J. M€ uller,§ and ^ N. Hadjichristidis † Department of Chemical Engineering, National Tsing Hua University, Hsin-Chu 30013, Taiwan, ‡Department of Chemical and Materials Engineering, Chang Gung University, Kwei-Shan Tao-Yuan 33302, Taiwan, §Grupo de Polı´meros USB, Departamento de Ciencia de los Materiales, Universidad Sim on Bolı´var, Apartado 89000, Caracas 1080-A, Venezuela, and ^ Department of Chemistry, University of Athens, 15771 Panepistimiopolis Zografou, Athens, Greece
Received October 10, 2010 Revised Manuscript Received December 8, 2010 Microphase separation of block copolymers can yield a series of long-range ordered nanostructures governed by the degree of stretching of the block chains and the interfacial energy.1,2 These factors are parametrized into the segregation strength (χN) and the volume fraction of constituent blocks for constructing the universal phase diagram3 that predicts an order-to-disorder transition (ODT) with increasing temperature due to the reduction of the Flory-Huggins interaction parameter χ. Such an “upper critical ordering transition” (UCOT) has been predominately observed among a wide variety of block copolymer systems. It is known that many blends of hompolymers exhibit an opposite “lower critical solution temperature” (LCST) phase diagram, where phase separation of an initially miscible mixture occurs upon heating. This behavior has been understood by the equation-of-state theories that consider the difference in thermal expansion coefficients of the constituents.4-6 Such an effect leads to increasing difference in the pure component volumes with increasing temperature and hence a positive volume change on demixing.7 A phase separation process then becomes entropically favorable at elevated temperature because the volume expansion increases the available configurations. The LCST type of phase behavior may also be operative in block copolymers (which is called “lower critical ordering transition” (LCOT) in this case); however, due to the significantly larger interfacial energy and the entropic loss from the stretching of block chains on microphase separation, the “disorder-to-order transition (DOT) temperature” (TDOT) on heating of a block copolymer can be much higher than the phase separation temperature of the corresponding homopolymer blend.8 As a result, LCOT is rarely observed among block copolymers. The first block copolymer exhibiting LCOT behavior was reported by Russell et al.9 It was found that a symmetric polystyrene-block-poly(n-butyl methacrylate) (PS-b-PnBMA) with a lower molecular weight (Mw = 68 000) exhibited solely LCOT, while the other sample with a higher molecular weight (Mw = 99 000) underwent a conventional ODT followed by a DOT on heating. Later, Ryu et al. also found similar LCOT behavior for polystyrene-block-poly(n-pentyl *To whom correspondence should be addressed. E-mail: hlchen@ che.nthu.edu.tw. pubs.acs.org/Macromolecules
Published on Web 01/12/2011
methacrylate) (PS-b-PnPMA).10-12 However, this copolymer underwent an additional ODT following the DOT on heating, meaning that it displayed a closed-loop phase diagram. Apart from the diblocks based on PS-block-poly(alkyl methacrylate)s, to our knowledge, no other system has been shown to display LCOT behavior because the TDOT may easily exceed the thermal degradation temperature. In a previous study by Hashimoto et al., a PS-block-poly(vinyl methyl ether) was found to display a disordered state; however, the associated small-angle neutron scattering (SANS) intensity enhanced with increasing temperature, signaling an increased amplitude of concentration fluctuations.13 It was thus suggested that this system should tend to microphase separate at higher temperature, although a true ordered state was not accessed. In this study, we disclose a new diblock copolymer exhibiting the unconventional LCOT behavior. The system is composed of a poly(ethylene oxide) (PEO) and a poly(2vinylpyridine) (P2VP) block. Here we probe the phase transition process of a symmetric and an asymmetric PEOb-P2VP by temperature-dependent SAXS and compare their TDOTs. This copolymer system is special considering that P2VP is susceptible to various functionalizations; in this case, the physical or chemical modification of P2VP block could fine-tune the interaction energy in the diblock and would thus allow for systematic investigation of the corresponding perturbation of the phase diagram. For instance, P2VP block may be selectively complexed with many metal ions and amphiphilic surfactants such as dodecylbenzenesulfonic acid.14,15 The segregation strength between PEO and P2VP block may then be controlled by the degree of binding of metal ions or surfactants to P2VP. Moreover, the complexation with surfactant would introduce an additional self-organization mechanism associated with the microphase separation between the polar moiety of the headgroupbound P2VP and the nonpolar tail of the surfactant.16 The action of such a driving force is expected to result in richer self-assembled structures and more complex phase transition behavior of the system. The PEO-b-P2VP samples studied here were synthesized by sequential anionic polymerization of 2-vinylpyridine and ethylene oxide.17,18 The number-average molecular weights (Mn) of both blocks in the symmetric diblock were ca. 40 000, which prescribed the volume fraction of PEO ( fPEO) as 0.49. The Mn of PEO and P2VP blocks in the asymmetric diblock were 20 000 and 100 000, respectively, and the corresponding fPEO was 0.16. The thermally induced phase transition of PEO-b-P2VP was detected by the temperature-dependent SAXS measurement performed at station BL23A at the National Synchrotron Radiation Research Center (NSRRC) located at Hsin-Chu, Taiwan. The energy of X-ray source and sampleto-detector distance were 8 keV and 2259 mm, respectively. The scattering signals were collected by MarCCD detector of 512 512 pixel resolution. The scattering intensity profile was output as the plot of the scattering intensity (I ) vs the scattering vector, q = (4π/λ) sin(θ/2) (θ = scattering angle), after corrections for background, sample transmission, empty cell transmission, empty cell scattering, and the detector sensitivity. For the temperature-dependent study, the r 2011 American Chemical Society
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Figure 2. Temperature-dependent SAXS profiles of the asymmetric PEO-b-P2VP (fPEO = 0.16) collected in a heating and subsequent cooling cycle.
Figure 1. (a) Temperature-dependent SAXS profiles of the symmetric PEO-b-P2VP ( fPEO = 0.49) collected in a heating and subsequent cooling cycle. (b) SAXS profiles at temperatures from 230 to 300 C obtained in the heating cycle to show the enhancement of structure order with increasing temperature.
sample was equilibrated at each prescribed temperature for 2 min followed by data collection for ca. 20 s. Figure 1a shows the temperature-dependent SAXS profiles of the symmetric PEO-b-P2VP collected in a heating and subsequent cooling cycle. The copolymer displayed a broad peak at 0.23 nm-1 at the onset of heating (i.e., 30 C). This peak was associated with the crystalline morphology formed by the crystallization of PEO blocks, as it vanished when the system was heated to 80 C (>melting point of PEO). The fact that the SAXS profile was characterized by a monotonically decayed curve at 80 C indicates that the copolymer was in the disordered melt state. With further heating, a scattering peak at 0.14 nm-1 emerged at about 120 C. The intensity of this peak grew with increasing temperature, and a second-order peak was discernible above 160 C, signaling the formation of a microphase-separated lamellar morphology. The temperature dependence of SAXS profiles clearly showed the occurrence of a DOT process in the heating cycle. The formation of lamellar structure upon microphase separation was reasonable considering the symmetric copolymer composition. The thermal reversibility of the phase transition was verified by the subsequent cooling experiment, with the corresponding SAXS profiles also being displayed in Figure 1a. It can be seen that the peak intensity reduced progressively with decreasing temperature, and the scattering peak became essentially unobservable below 120 C. In addition to the change of intensity, the position of the peak was found to shift to lower q with increasing temperature (and vice versa), signaling a larger interdomain distance at higher temperature. This was consistent with the stronger “effective
Figure 3. Im-1 vs T-1 plots of the symmetric and asymmetric PEO-bP2VP for determinations of TDOT.
repulsion” between the two blocks at higher temperature that caused a greater stretching of the block chains to reduce the contacts or mixing of PEO and P2VP monomer units at the microdomain interface. Moreover, the SAXS profiles in Figure 1b, showing clearer higher-order lamellar peaks at higher temperature, demonstrate that the ordering of the nanostructure was enhanced by increasing temperature in spite of the intensification of thermal fluctuations. These results are in clear contrast to the system exhibiting the conventional UCOT behavior. Figure 2 displays the temperature-dependent SAXS profiles of the asymmetric PEO-b-P2VP. It was obvious that this diblock also exhibited LCOT phase behavior, as a scattering peak emerged at ca. 150 C and its intensity grew with increasing temperature. The higher-order or form factor peaks became discernible at 190 C; however, the number of peaks was not enough to unambiguously assign the morphology formed by microphase separation. Figure 3 plots the inverse of primary peak intensity (Im-1) against the inverse of absolute temperature (T-1) for determination of TDOT. The plots showed opposite trend to that associated with the UCOT behavior. The TDOTs determined
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from the intersection of the two straight lines at the higher and lower temperature side were 148.7 and 166.0 C for the symmetric and asymmetric PEO-b-P2VP, respectively. The higher TDOT of the asymmetric diblock is consistent with the prediction by the LCOT phase diagram. The LCOT behavior of PEO-b-P2VP was verified by calculating the phase diagram of PEO/P2VP mixture using a theoretical model developed by Ruzette et al.4,5 This simple free energy model took compressibility into account for weakly interacting polymer blends. In this case, the free energy of mixing per unit volume, Δgmix, at atmospheric pressure is given by
Δgmix
~ ~ φ F φ F ¼ kT A A ln φA þ B B ln φB N A vA NB vB
Table 1. Properties of PEO and P2VP Used To Calculate the Phase Diagrams of Their Blends polymer
G1* (g/cm3)
R (10-4 K-1)
δi(298) (J1/2/cm3/2)
δi,0 (J1/2/cm3/2)
PEOa 1.38 7.09 21.30 23.67 1.334 4.97 21.36 23 P2VPb a Obtained from ref 4. b Determined from the group contribution calculations.20
~A F ~B ðδA, 0 - δB, 0 Þ2 þ φA φB ð~ FA - ~ FB ÞðδA 2 - δB 2 Þ þ φA φB F ð1Þ ~i is the reduced density or, equivalently, one minus where F the “fractional free volume” in the compressible fluid, δi2 is the temperature- and pressure-dependent cohesive energy density, and δi,02 is the hard-core cohesive energy density evaluated at 0 K and zero pressure. The reduced density ~ Fi is ~i = Fi/Fi*, with Fi and Fi* being the temperaturegiven by F and pressure-dependent mass density and the hard core density, respectively. Equation 1 contains three contributions, namely, the combinatorial entropy of mixing, the exchange interaction energy considered in the classical regular solution model, and the free energy change from compressibility. The third contribution is expressed as a function of pure component properties, which are obtainable from experimental P-V-T data19 or group contribution calculations.20 Thus, this model allows one to predict the thermodynamic phase diagram of a given polymer mixture based on pure component properties of mass density, cohesive energy density (or solubility parameter), and thermal expansion coefficient. It is however not able to predict the phase diagram of block copolymers quantitatively because additional effects including the excess free energy from block chain stretching and interfacial free energy are not considered. Table 1 lists the properties of PEO and P2VP homopolymers used to compute the phase diagrams of their blend. The values of δi(298) were calculated using group contribution methods15 and were then extrapolated to absolute zero temperature to obtain the hard core cohesive energy density via the following manner: δi, 0 2 δi 2 ð0Þ ¼ δi 2 ð298Þ
~i ð0Þ F δi 2 ð298Þ ¼ ~i ð298Þ ~i ð298Þ F F
ð2Þ
The phase diagram was then constructed by connecting the spinodal envelope obtained by4
gφφ
D2 Δgmix ¼ DφA 2 ¼ kT
T, P
D2 Δgmix DφA 2
T , P, Ff
~A ~ FB F ~B ðδA, 0 - δB, 0 Þ2 þ 2~ FA F þ φ A N A vA φ B N B vB ~B ÞðδA 2 - δB 2 Þ ¼ 0 - 2ð~ FA - F
ð3Þ
The solid curves in Figure 4 represent the calculated phase diagrams of PEO/P2VP blends with the molecular weights
Figure 4. Calculated phase diagrams of the PEO/P2VP blends with (a) symmetric molecular weights and (b) asymmetric molecular weights of the constituents.
being identical to those of the corresponding block chains in the copolymers. It can be seen that both UCST and LCST envelops were predicted by the theory. As expected, the calculated spinodal temperatures at the compositions of fPEO = 0.49 and 0.16 were significantly lower than the observed TDOTs. According to the theoretical analysis of Yeung et al.,8 the TDOT of a diblock exhibiting LCOT behavior is approximately equivalent to the phase separation temperature of the corresponding blend with ca. 1/5 molecular weight for the homopolymer constituents. Therefore, we have also calculated the phase diagrams of PEO/P2VP blend with molecular weights of 8000/8000 and 4000/20 000, as shown by the dashed curves in Figure 4. The calculated phase separation temperatures were ca. 93 and 99 C at fPEO = 0.49 and 0.16, respectively, which were much closer to the observed TDOTs of the corresponding diblocks. It is noted that in all cases the critical temperatures of the UCST envelops were found to
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situate far below the Tg of the miscible mixture. As a result, the UCOT behavior was never identified in the diblocks under study. Below the melting point of PEO, the crystallization of PEO blocks from the disordered melt may dominate the morphological formation, and the resultant crystalline lamellar morphology gave rise to the broad scattering peak for the symmetric diblock (Figure 1a). However, crystallization of PEO in the asymmetric PEOb-P2VP appeared to be completely hindered, presumably because the Tg of the miscible mixture of PEO and P2VP (=65 C as estimated from the Fox equation 21 ) was higher than the PEO melting point. In summary, the LCOT behavior, which is uncommon among block copolymers, has been revealed for PEO-bP2VP. The symmetric and asymmetric diblocks studied here displayed the disorder-to-order transition temperature at ca. 149 and 166 C, respectively, on heating, as determined from Im-1 vs T-1 plots. In contrast to the system showing the conventional UCOT behavior, the interdomain distance of PEO-b-P2VP was found to increase with increasing temperature due to the greater stretching of the block chains to reduce the contacts or mixing of the PEO and P2VP monomer units in the microdomain interface. The LCOT behavior was qualitatively consistent with the theoretical calculation of the phase diagram of PEO/P2VP blends using a thermodynamic theory considering the disparity in solubility parameter and thermal expansion coefficient. The calculated phase diagram also predicted a UCST envelop; however, the corresponding critical temperature situated well below the Tg of the miscible mixture, making the UCOT behavior unobservable for the diblocks studied. Acknowledgment. We acknowledge the financial support of the National Science Council of the Republic of China under Grant No. NSC 97-2221-E-007-034.
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References and Notes (1) Hamley, I. W. The Physics of Block Copolymers; Oxford University Press: New York, 1998. (2) Ruzzete, A. V.; Leibler, L. Nature Mater. 2005, 4, 19. (3) Matsen, M. W.; Bates, F. S. Macromolecules 1996, 29, 1091. (4) Ruzette, A.-V. G.; Mayes, A. M. Macromolecules 2001, 34, 1894. (5) Ruzette, A.-V. G.; Mayes, A. M.; Russell, T. P. J. Chem. Phys. 2001, 114, 8205. (6) Cowie, J. M. G.; Arrighi, V. Polymers: Chemistry and Physics of Modern Materials, 3rd ed.; CRC Press: New York, 2008. (7) Pollard, M.; Russell, T. P.; Ruzette, A.-V. G.; Mayes, A. M.; Gallot, Y. Macromolecules 1998, 31, 6493. (8) Yeung, C.; Desai, R. C.; Shi, A.-C.; Noolandi, J. Phys. Rev. Lett. 1994, 72, 1834. (9) Russell, T. P.; Karis, T. E.; Gallot, Y.; Mayes, A. M. Nature 1994, 368, 729. (10) Ryu, D. Y.; Jeong, U.; Kim, J. K.; Russell, T. P. Nature Mater. 2002, 1, 114. (11) Ryu, D. Y.; Park, M. S.; Chae, S. H.; Jang, J.; Kim, J. K.; Russell, T. P. Macromolecules 2002, 35, 8676. (12) Ryu, D. Y.; Jeong, U.; Lee., D. H.; Kim, J.; Youn, H. S.; Kim, J. K. Macromolecules 2003, 36, 2894. (13) Hashimoto, T.; Hasegawa, H.; Hashimoto, T.; Katayama, H.; Kamigaito, M.; Sawamoto, M.; Imai, M. Macromolecules 1997, 30, 6819. (14) Nandan, B.; Lee, C.-H.; Chen, H.-L.; Chen, W.-C. Macromolecules 2005, 38, 10117. (15) Nandan, B.; Lee, C.-H.; Chen, H.-L.; Chen, W.-C. Macromolecules 2006, 39, 4460. (16) Ruokolainen, J.; M€akinen, R.; Torkkeli, M.; M€akel€a, T.; Serimaa, R.; ten Brinke, G.; Ikkala, O. Science 1998, 280, 557. (17) Fragouli, P.; Iatrou, H.; Hadjichristidis, N. Polymer 2002, 43, 7141. (18) C ordoba, M.; Lorenzo, A. T.; M€ uller, A. J.; Fragouli, P.; Iatrou, H.; Hadjichristidis, N. Macromol. Symp. 2010, 282, 101. (19) Rodgers, P. A. J. Appl. Polym. Sci. 1993, 48, 1061. (20) van Krevelen, D. W.; Hoftyzer, P. J. Properties of Polymers. Correlation with Chemical Structure; Elsevier: New York, 1972. (21) Fox, T. G. Bull. Am. Phys. Soc. 1956, 1, 123.
