Deuteration Impact on Micellization Pressure and Cloud Pressure of

Oct 27, 2009 - The deuterated homopolymers and their corresponding polystyrene-block-polybutadiene and polystyrene-block-polyisoprene copolymers requi...
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Deuteration Impact on Micellization Pressure and Cloud Pressure of Polystyrene-block-polybutadiene and Polystyrene-block-polyisoprene in Compressible Propane Winoto Winoto, Youqing Shen, and Maciej Radosz* Soft Materials Laboratory, Department of Chemical & Petroleum Engineering, UniVersity of Wyoming, Laramie, Wyoming 82071-3295

Kunlun Hong and Jimmy W. Mays Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6494 ReceiVed: May 26, 2009; ReVised Manuscript ReceiVed: October 1, 2009

The deuterated homopolymers and their corresponding polystyrene-block-polybutadiene and polystyreneblock-polyisoprene copolymers require lower cloud pressures than their hydrogenous analogues to dissolve in a compressible alkane solvent, such as propane. For symmetric diblocks, deuteration reduces the micellization pressure. By contrast, for asymmetric diblocks with a long diene block relative to the styrene block, deuteration can increase the micellization pressure. All in all, however, the deuteration effects, while measurable, do not qualitatively change the principal diblock properties in compressible propane solutions, such as pressureinduced micelle decomposition, micelle formation and micelle size, and their temperature dependence. Therefore, isotope labeling should be a useful approach to neutron-scattering characterization for styrenediene block copolymers in compressible alkane systems. Introduction In our recent work, we focused on well-defined and uniform polystyrene-block-polydiene copolymers in small near-critical solvents, such as propane or propylene, to understand their bulk and micellar phase behavior in a compressible environment. For example, symmetric polystyrene-block-polyisoprene was demonstrated to exhibit robust micellization and micelle decomposition in subcritical and supercritical propane induced either by changing pressure or temperature or both.1 At a constant copolymer concentration, both micellization temperatures and pressures were found to fall around a monotonically decreasing micellar phase boundary curve in pressure-temperature coordinates, which lies above the copolymer cloud-pressure curve and below the polystyrene cloud-pressure curve. The copolymer cloud-pressure curve consists of two distinct branches, a conventional cloud-pressure (CP) branch above the micellar end point (MEP, a pressure-generalized critical micelle temperature) and a micellar cloud-pressure (MCP) branch below MEP.2 At CP, we observe the onset of copolymer precipitation upon decompression of a random nonmicellar solution. At MCP, we observe the onset of copolymer precipitation upon decompression of a micellar solution. The previous work demonstrated that MCP is consistently lower than CP estimated for a hypothetical random solution2,3 and that it is not very sensitive to the diblock molecular weight, composition (block ratio), or concentration. This is in contrast to the micellization pressure (MP), which depends on the diblock molecular weight, composition (block ratio), and concentration.3 However, there are no data available for compressible block copolymer systems that can help understand the effects of replacing hydrogen by deuterium, in one or both blocks, while * Corresponding author. E-mail [email protected]. Tel.: +1 307 766 4926. Fax: +1 307 766 6777.

keeping the carbon structure exactly the same. Such data are available for incompressible liquid solutions, for example, for styrene-diene micellar solutions in liquid organic solvents,4–6 for small compounds7 and other polymers,8–10 and for hydrogenbonding systems, where deuteration of water was found to enhance hydrogen bonding with poly(ethylene oxide).11 The question is to what extent, if any, CP, MCP, and MP in a compressible solvent will change upon full or partial diblock deuteration, even in the absence strong specific interactions. This is an important question if one wants to exploit isotope labeling for micelle structure characterization using neutron scattering, which is our longer term objective. The neutron scattering approaches take advantage of the scattering contrast between regular and isotope-labeled copolymer analogues. Ideally, the isotope and hydrogenous analogues should behave identically, but we know that this is a matter of approximation, not an ideal match, and hence need to understand the extent of such an approximation for a full (both blocks) or partial (one block only) deuteration, as opposed to a limit of very low deuteration,12 which is less fruitful. Therefore, the purpose of this work is to characterize fully deuterated, partially deuterated, and nondeuterated polystyreneblock-polybutadiene and polystyrene-block-polyisoprene analogues in propane to understand deuteration effects on micellizationpressuresandcloudpressures,whichreflectsolvent-polymer interactions. Such analogues will be matched by the number of repeating units rather than by molecular size. The micellization pressures and cloud pressures are needed for thermodynamic analysis using equation-of-state-type models that account for compressibility effects and for structural analysis, both of which are beyond the scope of this work.

10.1021/jp904917w CCC: $40.75  2009 American Chemical Society Published on Web 10/27/2009

Polystyrene-block-polybutadiene and -polyisoprene

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TABLE 1: Polymers Used in This Work polymer polystyrene deuterated polystyrene polybutadiene deuterated polybutadiene polystyrene-block-polybutadiene hydrogenous-hydrogenous deuterated-deuterated deuterated-hydrogenous hydrogenous-deuterated polystyrene-block-polyisoprene hydrogenous-hydrogenous deuterated-hydrogenous

short name

Mna

PDIb

refc

S(37) dS(42) B(38) dB(38)

36.8 41.7 38.2 38.3

1.02 1.03 1.05 1.04

3 this work 3 this work

S-B(37-13) S-B(37-36) S-B(38-113) dS-dB(40-12) dS-dB(42-41) dS-dB(40-117) dS-B(42-41) S-dB(37-37)

36.8-12.9 36.8-35.9 37.8-113.4 39.7-12.4 41.7-40.9 39.7-117.0 41.7-40.8 37.2-36.7

1.03 1.01 1.01 1.02 1.02 1.01 1.02 1.03

3 3 3 this this this this this

S-I(37-37) dS-I(42-42)

36.8-37.4 41.7-42.2

1.02 1.02

3 this work

a Mn ) number-average molecular weight (kg/mol). source of tabulated data.

b

work work work work work

Polydispersity index ) Mw/Mn; Mw ) weight-average molecular weight.

c

Original

Figure 1. Chemical structure of samples used in this work.

Experimental Section Materials. The homopolymers and diblock copolymers used in this work are synthesized via living anionic polymerization using well-established vacuum line techniques, as documented in recent reviews.13,14 Briefly, the diblocks are prepared by sequential monomer addition, with sampling of the first block (polystyrene or deuterated polystyrene). The microstructure of the polybutadiene and polyisoprene is controlled through the choice of solvent and use of polar additions (tertiary amines and ethers).15,16 Molecular weights and polydispersity indices are characterized using size exclusion chromatography (SEC) with online light scattering detection. The microstructure of the polydienes and composition of the block copolymers are determined using proton nuclear magnetic resonance (1H NMR). The polybutadiene and polyisoprene samples are at least 90% of the 1,4-addition type for all homopolymer and diblock copolymers, hydrogenous and deuterated. The molecular weights and polydispersity indices of all samples are provided in Table 1, including their short names used throughout this paper. The deuterated homopolymer and copolymer analogues are designed to match the number of repeating units. A deuterated analogue with exactly the same number of repeating unit as the hydrogenous analogue will have a slightly higher molecular weight because deuterium has a higher molecular mass than hydrogen. Table 1 lists the actual molecular weights, and Figure 1 illustrates the chemical structure of the samples. Propane, 99.0% grade from Matheson Trigas, Inc., is used without further purification. Experimental Method. The onset of bulk phase separation in a polymer solution is usually observed as the onset of its

turbidity and hence is referred to as the “cloud point”, that is, the cloud-pressure (CP) and the cloud-temperature (CT) at which the homogeneous solution turns cloudy. When the bulk phase transition occurs from a micellar solution, as opposed to a molecularly homogeneous solution, such a transition is referred to as the micellar cloud point, that is, the micellar cloudpressure (MCP) and the micellar cloud-temperature (MCT). The micelle formation or decomposition in a block copolymer solution is measured from the change of the scattered-light intensity upon changing pressure at constant temperature, which results in the micellization pressure (MP), or changing temperature at constant pressure, which results in the micellization temperature (MT). The micelle-containing solution is referred to as the micellar solution, in contrast to the molecular solution observed following micelle decomposition upon compression or heating or both. In this work, CP, MCP, and MP are measured in a small high-pressure variable-volume cell coupled with transmittedand scattered-light intensity probes and equipped with a borescope for visual observation of the phase transitions and with pressure and temperature probes accurate to within (2 bar and (0.1 °C, respectively. In a typical experiment, a known amount of polymer and solvent is loaded into the cell, which is then brought to and maintained at a desired temperature and pressure, high enough for the polymer and the solvent to become completely miscible. A simplified schematic of the experimental setup is shown in Figure 2. A detailed description of the apparatus and experimental procedure can be found in Winoto et al.1 In this work,

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Figure 2. Simplified schematic of the apparatus.

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Figure 3. Deuterated polystyrene (dS) and deuterated polybutadiene (dB) are more soluble in propane than their hydrogenous analogues, (S) and (B); 0.5 wt % homopolymer in propane.

attempts to reproduce CP, MCP, and MP for different cell loads suggest that the measured data are reproducible to within 30 bar. The hydrodynamic micelle diameters in propane are derived from the light-scattering data taken in this work. As reported before,1 the Stokes-Einstein equation requires viscosity and refractive index of the solution, but dilute-solution (say, 0.5 wt %) properties are assumed to be identical to those of pure propane. The density and viscosity data for pure propane are obtained from the National Institute of Standards and Technology.17 The propane density at high pressure is approximated by extrapolation of the available data using an exponential decay equation. The viscosity at high pressure is also approximated by extrapolation utilizing the extrapolated density data and the residual viscosity correlation proposed by Brebach and Thodos.18 The refractive index is calculated using the Lorentz-Lorenz correlation19 of the refractive index and density. Results and Discussion All new experimental results obtained in this work are presented in Tables S1-S4 in the Supporting Information. These and other available data are analyzed to understand the micellization and cloud transitions of deuterated homopolymers, corresponding diblock copolymers, and their hydrogenous analogues in near critical propane. Deuterated Homopolymers. Figure 3 illustrates that deuterated homopolymers, dS(42) and dB(38), despite having the same (dB) or slightly higher (dS) molecular weight, exhibit lower cloud pressure (CP) and hence are easier to dissolve in propane than their hydrogenous analogues, S(37) and B(38). This is somewhat counterintuitive, especially for the styrene pair, because increasing molecular weight within the same homologous series normally increases the cloud pressure; it makes a large solute harder to dissolve in a small solvent. For example, dS(42) exhibits roughly the same CP as a 30 kg/mol hydrogenous polystyrene, S(30), and dB(38) exhibits roughly the same CP as a 18 kg/mol hydrogenous polybutadiene, B(18). In other words, replacing hydrogen by deuterium makes the deuterated analogue easier to dissolve in propane. Fully Deuterated Symmetric Polystyrene-block-polybutadiene. As a result, one would expect that a fully deuterated polystyrene-block-polybutadiene should also be easier to dissolve in propane (exhibit a lower cloud-pressure and micellar cloud-pressure) than its hydrogenous analogue. While no plain CP data for a nonmicellar solution are available for a matched

Figure 4. Symmetrical (deuterated polystyrene)-block-(deuterated polybutadiene) copolymer, dS-dB(42-41) (filled triangle), exhibiting lower micellization pressure (MP) and micellar cloud-pressure (MCP) than its hydrogenous analogue, S-B(37-36) (open triangles); 0.5 wt % polymer in propane.

pair of deuterated and hydrogenous diblocks, an example shown in Figure 4 confirms that dS-dB(42-41) has a lower micellar cloud-pressure (MCP) than S-B(37-36). The same turns out to be true for the micellization pressure (MP). The differences are not large, but they are measurable and consistent. The corresponding homopolymer curves are not shown in Figure 4, which is busy enough, but a comparison with Figure 3 suggests that the MCP curves are reminiscent of the butadiene CP curves in Figure 3 and that the MP curves are reminiscent of the polystyrene CP curves in Figure 3. This is consistent with our earlier findings3 that the corona-forming block (butadiene) dominates the micelle aggregation and precipitation behavior, as captured by MCP, while the core-forming block (styrene) dominates the micellization behavior, as captured by MP. Since deuterated polystyrene exhibits lower CP than its hydrogenous analogue, as shown in Figure 3, the deuterated diblock, dS-dB(42-41), has as a lower MP than its hydrogenous analogue, S-B(37-36), as shown in Figure 4. One of our previous observations3 for hydrogenous diblocks was that MCP is not very sensitive to polymer molecular weight, block ratio, or concentration in the same solvent. Deuteration,

Polystyrene-block-polybutadiene and -polyisoprene

Figure 5. Micellization pressure (MP) and micellar cloud-pressure (MCP) as a function of temperature for 0.5 wt % solutions of high S/B ratio (deuterated polystyrene)-block-(deuterated polybutadiene) copolymer, dS-dB(40-12) (filled squares), and its hydrogenous analogue, S-B(37-13) (open squares), both in propane.

Figure 6. Micellization pressure (MP) and micellar cloud-pressure (MCP) as a function of temperature for 0.5 wt % solutions of low S/B ratio (deuterated polystyrene)-block-(deuterated polybutadiene) copolymer, dS-dB(40-117) (filled circles), and its hydrogenous analogue, S-B(38-113) (open circles), both in propane.

however, can have a more significant impact on MCP as shown in Figure 4. For example, below 100 °C, MCP of dS-dB(42-41) is reduced by 60-150 bar relative to its hydrogenous analogue, S-B(37-36). For comparison, MCP of S-B(15-13)3 is only 7-75 bar lower than that of S-B(37-36) over the same temperature range. In other words, the diblock deuteration has a stronger effect on MCP than reducing its molecular weight by a factor of 2. Fully Deuterated Asymmetric Polystyrene-block-polybutadiene. To probe for the effect of block ratios, we consider two fully deuterated asymmetric (deuterated polystyrene)-block(deuterated polybutadiene) samples having nearly the same styrene block, about 40 kg/mol, similar to that of the symmetric sample used in the previous section. The results are illustrated in Figure 5 for the diblock with a short butadiene block and hence a high styrene/butadiene (S/B) ratio, dS-dB(40-12), and in Figure 6 for the diblock with a long butadiene block and hence a low S/B ratio, dS-dB(40-117). Figure 5 suggests that for the short butadiene block deuteration has almost no impact on MP and it only slightly decreases MCP.

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Figure 7. Monotonic S/B ratio effect on the MP of 0.5 wt % (deuterated polystyrene)-block-(deuterated polybutadiene) solutions in propane.

Deuteration of the long butadiene block also slightly decreases the diblock MCP, as shown in Figure 6 for dS-dB(40-117). However, the small but reproducible deuteration effects on the MP are more convoluted. For example, deuteration slightly reduces MP for the medium butadiene block in the symmetric diblock shown in Figure 4 (styrene deuteration effect dominant); it makes essentially no difference for the short butadiene block shown in Figure 5 (butadiene and styrene deuteration effects in balance); and it increases MP for the long butadiene block shown in Figure 6 (butadiene deuteration effect dominant). This trend is qualitatively consistent with the data obtained for asymmetric hydrogenous diblocks, if one observes that deuteration produces results that are analogous to reducing the chain length of the hydrogenous analogue. Let us recall3 that for hydrogenous diblocks with a fixed diene block decreasing styrene block length decreases the diblock MP. For hydrogenous diblocks with a fixed styrene block, however, decreasing butadiene block length increases the diblock MP. In other words, the lower the S/B ratio, the easier it is to decompose (randomize) the micelles in propane. This is because propane “likes” butadiene more than it does styrene. As it turns out, propane “likes” deuterated butadiene even more (it also “likes” deuterated styrene more than it does styrene). Therefore, styrene deuteration reduces MP while butadiene deuteration increases MP, and the net impact on MP depends on the block ratio. A summary of the MP curves is shown in Figure 7 for all three deuterated diblocks, dS-dB(40-12) with squares, dS-dB(42-41) with triangles, and dS-dB(40-117) with circles; the longer the deuterated butadiene, the lower the micellization pressure (the easier it is to decompose the micelles). Figure 8 illustrates a nonmonotonic S/B ratio effect on MCP. Namely, increasing deuterated butadiene block decreases MCP on going from dS-dB(40-12) to dS-dB(42-41) (from squares to triangles), where the butadiene ratio is dominant. However, itincreasesMCPongoingfromdS-dB(42-41)todS-dB(40-117) (from triangles to circles), where the total molecular weight effect becomes dominant. This is consistent with the block ratio effects on MCP recently reported for hydrogenous copolymers.3 For the record, hydrodynamic micelle diameters measured for the deuterated and hydrogenous copolymers discussed in this section fall within a 60-70 nm range for the symmetric diblocks, S-B(37-36) and dS-dB(42-41), and within a 110-130 nm range for the asymmetric diblocks with a long

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Figure 8. Nonmonotonic S/B ratio effect on the MCP of 0.5 wt % (deuterated polystyrene)-block-(deuterated polybutadiene) solutions in propane.

butadiene block, S-B(38-113) and dS-dB(40-117). Sample micelle diameter data are reported in Table S4 in Supporting Information, but more data are needed to analyze the deuteration effect on micelle diameters, if any. Partially Deuterated Diblocks: (Deuterated Styrene)Diene. Figure 9 illustrates the effect of partial (styrene only) deuteration of polystyrene-block-polybutadiene and polystyreneblock-polyisoprene. Such partial deuteration does not affect MCP much, as illustrated for dS-B(42-41) in Figure 9(a) and for dS-I(42-42) in Figure 9(b). This is because MCP is primarily dependent on the corona block3 not on the styrene block. By contrast, both dS-B(42-41) and dS-I(42-42) copolymers exhibit slightly but consistently lower MP than their hydrogenous analogues, as also illustrated in Figure 9(a) and (b). This is because it is the styrene block that dominates MP.3 Partially Deuterated Diblocks: Styrene-(Deuterated Diene). Figure 10 illustrates micellization pressures and micellar cloud-pressures measured for a polystyrene-block-(deuterated polybutadiene), S-dB(37-37), solution in propane. The effect of partial (butadiene only) deuteration of polystyrene-blockpolybutadiene shifts the MP and MCP to higher pressures. The structure and properties of this sample have been confirmed,

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Figure 10. Unexpectedly high MP (half-filled squares) and MCP (halffilled circles) measured for 0.5 wt % solutions of symmetrical polystyrene-block-(deuterated polybutadiene), S-dB(37-37), in propane. Dotted curve, MP; and solid curve, MCP of the hydrogenous analogue, S-B(37-36) in the same solvent.

and the results have been replicated; however, the results presented in Figure 10 are surprising and hence call for more work, which is in progress. Conclusion The deuterated homopolymers and their corresponding polystyrene-block-polybutadiene and polystyrene-block-polyisoprene copolymers require slightly lower cloud-pressures than their hydrogenous analogues to dissolve in propane. For symmetric diblocks, deuteration reduces the micellization pressure. By contrast, for asymmetric diblocks with a long diene block relative to the styrene blocks, deuteration can increase the micellization pressure. All in all, however, the deuteration effects, while measurable and consistent, do not qualitatively change the principal diblock properties in compressible solutions, such as pressure-induced micelle decomposition, micelle formation and micelle size, and their temperature dependence. Therefore, isotope labeling should be a useful approach to neutron scattering characterization for styrene-diene block copolymers in compressible alkane systems.

Figure 9. Deuteration on styrene block does not affect the MCP much but shifts the MP of styrene-diene copolymer to lower pressure as shown for: (a) symmetrical (deuterated polystyrene)-block-polybutadiene, dS-B(42-41) (half-filled triangles), and its hydrogenous analogue, S-B(37-36) (open triangles), and (b) symmetrical (deuterated polystyrene)-block-polyisoprene, dS-I(42-42) (half filled circles), and its hydrogenous analogue, S-I(37-37) (open circles); 0.5 wt % polymer solutions in propane.

Polystyrene-block-polybutadiene and -polyisoprene Acknowledgment. This work is funded by a National Science Foundation Grant (CTS-0625338) at the University of Wyoming. Part of this research was done at Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences, which was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy, through User Project CNMS2006-114. A preliminary account of the paper, presented at AIChE Annual Meeting 2007, was entitled: Pressure-Induced Micellization of Polystyrene-block-polybutadiene, Polystyrene-block-polyisoprene and Their Deuterated Analogs in Near Critical Propane. Supporting Information Available: Tables of data. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Winoto, W.; Adidharma, H.; Shen, Y.; Radosz, M. Macromolecules 2006, 39, 8140. (2) Winoto, W.; Tan, S. P.; Shen, Y.; Radosz, M.; Hong, K.; Mays, J. W. Macromolecules 2009, 42, 3823. (3) Winoto, W.; Tan, S. P.; Shen, Y.; Radosz, M.; Hong, K.; Mays, J. W. Macromolecules 2009, 42, 7155.

J. Phys. Chem. B, Vol. 113, No. 46, 2009 15161 (4) Pedersen, J. S.; Svaneborg, C.; Almdal, K.; Hamley, I. W.; Young, R. N. Macromolecules 2003, 36, 416. (5) Bang, J.; Viswanathan, K.; Lodge, T. P. J. Chem. Phys. 2004, 121, 11489. (6) Stepanek, P.; Tuzar, Z.; Nallet, F.; Noirez, L. Macromolecules 2005, 38, 3426. (7) Fenby, D. V.; Kooner, Z. S.; Khurma, J. R. Fluid Phase Equilib. 1981, 7, 327. (8) Strazielle, C.; Benoit, H. Macromolecules 1975, 8, 203. (9) Kayillo, S.; Shalliker, R. A.; Dennis, G. R. Macromol. Chem. Phys. 2005, 206, 2013. (10) Harton, S. E.; Stevie, F. A.; Ade, H. Macromolecules 2006, 39, 1639. (11) Hammouda, B.; Ho, D.; Kline, S. Macromolecules 2002, 35, 8578. (12) Wignall, G. D.; Melnichenko, Y. B. Rep. Prog. Phys. 2005, 68, 1761. (13) Hadjichristidis, N.; Pitsikalis, M.; Pispas, S.; Iatrou, H. Chem. ReV. 2001, 101, 3747. (14) Uhrig, D.; Mays, J. W. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 6179. (15) Hattam, P.; Gauntlett, S.; Mays, J. W.; Hadjichristidis, N.; Young, R. N.; Fetters, L. J. Macromolecules 1991, 24, 6199. (16) Mays, J. W.; Hadjichristidis, N.; Fetters, L. J. Macromolecules 1984, 17, 2723. (17) National Institute of Standards and Technology Chemistry WebBook: http://webbook.nist.gov/chemistry/fluid/. (18) Brebach, W. J.; Thodos, G. Ind. End. Chem. 1958, 50, 1095. (19) Ha¨drich, J. Appl. Phys. 1975, 7, 209.

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