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Isolating the Effects of Morphology and Chain Architecture on the Mechanical Properties of Triblock Copolymers Lei Qiao,† Cora Leibig,‡ Stephen F. Hahn,‡ and Karen I. Winey*,† Laboratory for Research on the Structure of Matter, Department of Materials Science and Engineering, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104-6272, and The Dow Chemical Company, Corporate Research and DeVelopment, Midland, Michigan 48674
This study extends the understanding of the effects of morphology and chain architecture on mechanical properties of block copolymers by isolating the effect of morphology from other factors, such as composition and molecular weight. Using a single poly(isoprene-styrene-isoprene) triblock copolymer (ISI), we have used solvent casting with solvents of varying selectivity to produce three morphologies (cylinders, gyroid, and lamellae). The elastic moduli of these as-cast morphologies were measured using microtensile tests and found that at fixed molecular weight and composition (38 wt % PS) the moduli increased with increasing glassy domain continuity, in the order of PS cylinders to lamellae to gyroid morphology. At higher PS composition this trend continued in the order of lamellae to PI cylinders. Finally, with the same total molecular weight, composition, casting solvent, and morphology, SIS triblocks exhibit higher elastic moduli than ISI because both ends of the rubbery PI blocks are tethered in SIS. Introduction Much is understood about the microphase-separated morphologies of diblock and triblock copolymer melts in which the equilibrium morphologies and phase behavior of the block copolymers depend on their composition and their degree of segregation, typically described by χN, where χ is the interaction parameter and N is the degree of polymerization. In the simplest cases of diblock copolymers or ABA-type triblock copolymers, four stable morphologies have been well-established: alternating lamellae, interpenetrating double gyroid morphologies, hexagonally packed cylinders, and spheres arranged on a cubic lattice.1 The phase behavior in these block copolymers is a result of minimizing the overall free energy due to asymmetric chain stretching and packing frustration.2,3 It is also well-established that nonequilibrium morphologies are often produced due to factors such as casting solvents,4-7 rate of solvent evaporation,8 and thermal annealing.9 In particular, researchers have studied the effect of using a selective solvent in block copolymer solutions,10-14 thin films,8 and nonequilibrium bulk morphologies15,16 in diblock copolymers. While ABA-type triblock and (AB)n multiblock copolymers have considerable similarities with the observed morphologies in diblock copolymers, a major difference is that the triblock and multiblock copolymers have doubly tethered midblocks. The superior mechanical properties of triblocks relative to diblocks are often attributed to the chain conformations of the doubletethered midblocks. However, the mechanical properties of block copolymers are also influenced by molecular weight, composition, and morphology, many of which are interdependent, particularly, composition and morphology.17,18 It is the objective of this structure-property study to isolate the effect of morphology from other factors, such as composition and molecular weight of the block copolymers. We examine nonequilibrium morphologies in ABA- and BAB-type triblock copolymers, in which A and B represent * To whom correspondence should be addressed. Phone: 215-8980593. Fax: 215-573-2128. E-mail:
[email protected]. † University of Pennsylvania. ‡ The Dow Chemical Company.
polystyrene (S) and polyisoprene (I), respectively. Various nonequilibrium morphologies are produced by casting the same block copolymers using different solvents, either neutral or preferential to one of the blocks. This leads to different degrees of swelling within the blocks that modify the effective volume fraction during solvent evaporation and morphology formation to create various bulk morphologies. We report here the formation of lamellar, cylindrical, and gyroid structures, and the relation between these microdomain morphologies and mechanical properties. Experimental Section Materials. Six ABA- and BAB-type triblock copolymers of polystyrene and polyisoprene, denoted as poly(styrene-b-isoprene-b-styrene) (SIS) and poly(isoprene-b-styrene-b-isoprene) (ISI), were synthesized via anionic polymerization at the Dow Chemical Company. The polymers were prepared in a specially constructed polymerization reactor with a screw-type agitator to ensure top to bottom mixing during the polymerization process. All polymers were prepared using the sequential monomer addition process in cyclohexane at 65 °C with secbutyllithium as the initiator. Individual blocks were sampled and the block sizes were determined by size exclusion chromatography (SEC) to determine the number averaged molecular weight to within 1000 g/mol. An Agilent Liquid Chromatograph was run with tetrahydrofuran eluent at 1.0 mL/min flow rate through two Polymer Labs mixed porosity C columns, with a 50 µL injection loop. Output was detected using an Agilent 1100 Series Differential Refractive Index Detector and was calibrated using narrow polydispersity index samples of polystyrene and polyisoprene (Polymer Laboratories). The triblock copolymers have similar molecular weights (ca. 100,000 g/mol) and polydispersity indices of 1.07, Table 1. Copolymer compositions were determined to within 1 mol % by 1H NMR spectroscopy using a Varian INOVA NMR (300 MHz for 1H) on CDCl3 solutions at 25 °C. Sample Preparation. Triblock copolymers were dissolved in ∼5 wt % solutions with cyclohexane, toluene, and 1,4dioxane, having solubility parameters of 16.8, 18.5, and 20.3
10.1021/ie0511940 CCC: $33.50 © 2006 American Chemical Society Published on Web 02/09/2006
Ind. Eng. Chem. Res., Vol. 45, No. 16, 2006 5599 Table 1. Summary of Triblock Copolymers: Molecular Weight and Composition
38-SIS 52-SIS 67-SIS 38-ISI 52-ISI 67-ISI
materiala
Mn,total (g/mol)
φsb (wt %)
SIS(18-64-18) SIS(23-46-24) SIS(29-36-27) ISI(31-35-30) ISI(26-49-24) ISI(22-59-19)
101,000 96,000 94,000 95,000 102,000 100,000
38.4 51.7 66.6 38.7 52.4 66.6
a Numbers are the nominal number averaged molecular weights of each block in SIS and ISI. b Determined from 1H NMR measurements.
MPa1/2, respectively. The solutions were cast in Teflon-coated casting pans where the solvents slowly evaporated over 3 weeks. The cast films were further dried under vacuum at room temperature and then at 80 °C (below the Tg of styrene block for all six copolymers) for 1-2 days to remove residual solvents. The film thicknesses are ∼1.5 mm. Films prepared as above were cut into two halves; one is referred to as “as-cast” films and the other half underwent further thermal treatment under vacuum at 180 °C for 1 week to facilitate the formation of near equilibrium morphologies and is referred to as “annealed”. Small-Angle X-ray Scattering (SAXS). Small-angle X-ray scattering was performed using the multiple-angle X-ray scattering (MAXS) apparatus at the University of Pennsylvania. A Nonius FR591 rotating anode generator with a 0.2 × 2 mm2 filament and operated at 40 mA × 85 kV power produced X-rays that were focused by a double-focusing mirror-monochromator system; the incident X-rays have 0.154 nm wavelength. A Bruker Hi-Star multiwire area detector was located at the end of a 126 cm evacuated flight path. Two-dimensional scattering data were integrated about the azimuthal angle and then the background parasitic scattering was subtracted to yield the scattering intensity as a function of scattering vector (q). Transmission Electron Microscopy (TEM). A Philips 420T transmission electron microscope, operated at 120 kV, was used to examine the morphologies of both the as-cast and annealed triblock copolymers samples. Thin sections of ∼60 nm were prepared using a Reichert cryo-ultramicrotome operated at -120 °C and were subsequently stained in the vapor above a 4% aqueous solution of OsO4 that selectively stained the PI microdomains. Microtensile Tests. The standard microtensile tests ASTM1708 were conducted on as-cast films at ambient temperature (72 °F) using an Instron testing machine in the materials testing center at the Dow Chemical Company. Three to five specimens with dimensions of 38 × 15 × 1.5 mm were cut from each film. Experiments were carried out at a constant crosshead speed of 0.05 in./min and the distance between grips was 0.866 in. Tensile stress and strain were measured during uniaxial extension, and the Young’s modulus was determined for each specimen. Triblock Copolymer Morphology Figures 1 and 2 show the SAXS profile and TEM images for 38-ISI cast from three solvents wherein each solvent produced a different morphology. The 38-ISI film cast from cyclohexane shows three distinct reflections with the positional ratios of the q values 1:2:71/2, indicating a hexagonally packed cylindrical structure; the 31/2 reflection in the hexagonal structure is suppressed by the minimum in the form factor for scattering from cylinders with 38 wt % PS composition. TEM images of this material show hexagonally packed PS cylinders (bright) in a PI (dark) matrix, Figure 2a. The 38-ISI film cast from 1,4dioxane exhibits 5 orders of reflections with positional ratios
Figure 1. SAXS intensities, I, as a function of the scattering vector q for 38-ISI show the morphologies formed by solvent casting from different solvents: cyclohexane, toluene, and dioxane. Three morphologies were observed: hexagonally packed cylinders of PS (C1), gyroid (G), and lamellae (L). For clarity, these SAXS profiles and those in the subsequent figures were shifted vertically by 2 orders of magnitudes.
of the q values (1:2:3:4:5), corresponding to a lamellar structure, which is consistent with the TEM images, Figure 2c. The TEM images for 38-ISI cast from toluene clearly indicate the double gyroid morphology, Figure 2b. The scattering data from this sample exhibit two closely spaced reflections with a q ratio of ∼1:1.13 that corresponds to the double gyroid structure (61/2, 81/2) and a broad peak at q ∼ 0.6 nm-1 that can be assigned to higher order double gyroid reflections (321/2, 381/2, 501/2). Note that the first reflection from the double gyroid morphology occurs at approximately the same position as the first-order reflection of both the cylindrical structure in 38-ISI cast from cyclohexane and the lamellar structure cast from dioxane, which is consistent with the previously reported epitaxial relations between the gyroid {211} planes and the lamellar {100} planes or the {10} planes of hexagonally packed cylinders.19 Table 2 summarizes the SAXS and TEM results for each triblock copolymer solvent cast from the three solvents. For example, the ISI with a majority styrene composition, 67-ISI, exhibits a lamellar morphology when cast from toluene or cyclohexane, but exhibits a cylindrical morphology when cast from dioxane. There is one case when the casting solvent did not alter the morphology, 52-SIS. Finally, note that the as-cast morphology of 52-ISI prepared from cyclohexane was confirmed by TEM to contain both the lamellae and gyroid morphologies. As shown in Table 2, the bulk morphologies of triblocks with composition close to the equilibrium phase boundaries (38 SIS/ ISI and 67 SIS/ISI) were influenced by the solvent selection. This can be attributed to preferential solvent swelling, or different effective volume fractions during solvent evaporation and microphase formation. The solubility parameters for polystyrene and polyisoprene are 19.0 and 15.1 MPa 1/2, respectively. Toluene, with a solubility parameter of 18.5 MPa1/2, is a neutral solvent for PS-PI block copolymers. Cyclohexane (δ ) 16.8 MPa1/2) is slightly preferential for the PI block, while dioxane (δ ) 20.3 MPa1/2) dissolves both PS and PI blocks but substantially prefers the PS block. A neutral solvent promotes uniform distribution of solvents during solvent evaporation and microphase separation. If the solvent is preferential to one block, say the styrene block, the solvent is preferentially distributed
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Figure 3. Tensile moduli for as-cast 38-SIS and as-cast 38-ISI samples as a function of the solubility parameter of the casting solvent. Circles, squares, and triangles indicate the as-cast morphologies as PS cylinders on a hexagonal lattice (C1), gyroid (G), and lamellae (L), respectively. The horizontal bars represent the average moduli.
Figure 2. TEM images of 38-ISI show three morphologies as a result of using different solvents during casting: (a) cyclohexane produces PS cylinders on a hexagonal lattice, (b) toluene produces the gyroid morphology, and (c) dioxane produces lamellae. The PI microdomains are selectively stained by OsO4 and therefore appear dark in this and the following TEM images. Table 2. Morphologies of SIS and ISI Triblock Copolymer: As-cast and Annealed
solvent cyclohexane toluene dioxane annealedc
SIS morphologyb ISI morphologyb solubility a parameter δ (MPa1/2) 38-SIS 52-SIS 67-SIS 38-ISI 52-ISI 67-ISI 16.8 18.5 20.3
C1 L L L
L L L L
L C2 C2 C2
C1 G L G
L+G L L L
L L C2 L
a Quoted from the Polymer Handbook, 4th ed.; Wiley: New York, 1999. Determined from SAXS and TEM experiments: C1, PS cylinders; C2, PI cylinders; L, lamellae; G, gyroid. c Upon annealing for 1 week at 180 °C, the morphology is independent of the casting solvent.
b
in the PS domains, which increases the effective volume fraction of the PS blocks relative to the equilibrium volume fraction. Upon microphase separation, this nonequilibrium swelling of domains leads to a nonequilibrium bulk structure that is subsequently trapped as the PS blocks become glassy due to solvent evaporation. For example, the equilibrium morphology of 38-ISI is a gyroid structure, as found in the film cast from toluene. Cyclohexane decreases the effective styrene composition and induces a morphology of PS cylinders in a PI matrix (C1), whereas dioxane increases the effective styrene composition and therefore induces a lamellar structure.
While solvent casting block copolymers from preferential solvents can produce nonequilibrium morphologies, annealing above the glass transition temperature can convert the materials to their equilibrium morphology.16 Independent of the casting solvent, each triblock copolymer exhibits the same morphology after annealing, Table 2. For example, after annealing at 180 °C for 1 week, the three as-cast morphologies (cylinders of PS, gyroid, and lamellae) of 38-ISI all transform into the gyroid structure and the two as-cast morphologies (lamellae and cylinders of PI) of 67-ISI both exhibit a lamellar structure. Note that the annealed morphologies correspond to the morphology produced by solvent casting from toluene, which indicates that toluene is the most neutral solvent. Interestingly, 67-SIS exhibits a cylindrical structure as the equilibrium morphology, while 67-ISI exhibits a lamellar structure as the equilibrium morphology, although both triblocks have the same composition. Furthermore, 38-SIS exhibits a lamellar structure, whereas 38-ISI reveals a gyroid structure as its equilibrium structure. The phase boundaries between microphase separated morphologies apparently shift toward higher PS compositions as the chain architecture changes from SIS to ISI. The phase boundary between lamellae and the PS-rich gyroid is between 52 and 63 wt % PS in SIS and above 67 wt % in ISI, assuming a 3 wt % PS composition range for the gyroid. Also the phase boundary between PI-rich gyroid and lamellae is below 35 wt % PS in SIS, but between 39 and 52 wt % PS in ISI. It is obvious from these data that the chain architecture of a triblock copolymer, that is, ABA versus BAB, affects the equilibrium morphology in addition to the composition and the degree of segregation (χN). Elastic Modulus and Triblock Copolymer Structure Figure 3 shows the elastic moduli for the as-cast 38-SIS and as-cast 38-ISI as a function of the solubility parameters of the casting solvents. In 38-SIS, the samples cast from toluene (δ ) 18.5 MPa1/2) and dioxane (δ ) 20.3 MPa1/2) show similar average moduli (20 ksi vs 22 ksi), while 38-SIS cast from cyclohexane (δ ) 16.8 MPa1/2) exhibits a significantly lower average modulus (10 ksi). Overall, the moduli for 38-ISI are lower than those for 38-SIS when cast from the same solvent,
Ind. Eng. Chem. Res., Vol. 45, No. 16, 2006 5601 Table 3. Average Modulus Correlated with Composition and As-Cast Morphology φs (%)
solvent
morphology
SIS (ksi)
ISI (ksi)
38 38 38 38 52 52 67 67 67
cyclohexane toluene toluene dioxane toluene dioxane toluene toluene dioxane
C1 G L L L L C2 L C2
10
5 14
20 21 23 33 103 73
8 18 22 34 42
even though both triblock copolymers have the same PS content; these data indicate the importance of chain architecture at fixed composition and molecular weight. As with 38-SIS, the 38-ISI samples cast from cyclohexane show the lowest average modulus, but 38-ISI cast from toluene forms a gyroid morphology and exhibits a higher average modulus than 38-ISI cast from dioxane that forms a lamellar morphology; these data indicate the importance of morphology at fixed composition and molecular weight. The average elastic moduli are summarized in Table 3, where the results are listed from top to bottom in the sequence of increasing solubility parameters of the casting solvent and increasing styrene composition in the copolymers. The two columns in the table distinguish the average moduli between SIS and ISI for the same block copolymer composition and casting solvent. First, the tensile modulus increases significantly with increasing styrene composition, as shown in Table 3 from 38-SIS (ISI) to 52-SIS (ISI) to 67-SIS (ISI). For example, the average moduli increases significantly from 10 to 21 ksi for 38-SIS to 23-33 ksi for 52-SIS to 73-103 ksi for 67-SIS. This is consistent with previous findings on triblock copolymers20,21 and is due to the increasing content of glassy styrene segments relative to the elastomeric isoprene segments. Second, increasing the connectivity of the PS microdomains increases the elastic modulus even at fixed copolymer composition. For example, when 38-ISI is solvent-cast from cyclohexane, dioxane, and toluene, the progression of morphologies is PS cylinders, lamellae, and gyroid and the elastic moduli increase from 5.4 to 7.8 to 14.0 ksi, respectively. This sequence of morphologies (C1, L, G) has increasing PS connectivity from one, two, and three dimensions that inhibit elastic deformation. This higher modulus for the gyroid phase is consistent with the reports by Dair et al. in triblock copolymers, where a gyroid structure was stronger than lamellar or cylindrical morphologies where they accomplished the morphology change by changing the copolymer compositions.22 As presented above, the copolymer composition also impacts the elastic modulus; therefore, we produced the morphology changes using selective solvents so as to isolate the influence of copolymer morphology. In addition to the 38-ISI results, Table 3 also shows that increases in moduli exist as a function of morphology at fixed copolymer composition and molecular weight: 38-SIS C1 to L and 67-ISI L to C2 (where the PS matrix has three-dimensional connectivity). Again, the increase in elastic moduli corresponds to an increase in connectivity of the glassy domains in the morphology. To our knowledge, ours is the first report comparing triblock copolymer morphologies without changing the total molecular weight or composition. Third, in every case in which an SIS-type and an ISI-type triblock copolymer have the same copolymer composition and morphology the average modulus is higher for SIS than for ISI. For example, the average modulus for 38-SIS cast from cyclohexane is approximately twice the modulus of 38-ISI cast
from cyclohexane, although they both have 38% styrene and exhibit a cylindrical morphology. In both architectures, SIS and ISI, the styrene domains microphase-separate to form glassy microdomains that act as physical cross-links between the rubbery PI microdomains. The PI domains in all six copolymers are well-entangled, i.e., MPI_block > 2Me,PI (Me,PI ∼ 4 kg/mol).23 In the case of SIS, the entangled PI chains are pinned by the glassy PS domains at both ends, whereas in ISI, the PI chains are only tethered at one end by the glassy domains. The existence of trapped entanglements in SIS contributes to the elastic behavior because they have much less mobility than the PI blocks in ISI. Thus, at fixed total molecular weight, composition, and morphology, a higher modulus in SIS is expected compared to ISI as we observed. Bard and Chung24 and Holden et al.25 researched the stress relaxation behavior of S-B-S triblock copolymers blended with a small amount of S-B diblock copolymers. These studies indicate that SB diblocks decrease the amount of trapped PB blocks in SBS and weaken the material. This is in agreement with our results on SIS and ISI, illustrating the influence of triblock architecture. Finally, we note that within a given morphology type at fixed copolymer composition and triblock copolymer architecture there can be considerable differences in the average elastic modulus. For example, 67-SIS prepared from toluene and dioxane both exhibit a cylindrical morphology, but the modulus decreases from 103 to 73 ksi, while 38-SIS prepared from toluene and dioxane both exhibit a lamellar morphology and have comparable moduli, 20 and 21 ksi, respectively. Perhaps within a given observed morphology the casting solvent can alter interfacial width between PS and PI microdomain and/or the orientation of the microdomains within the sample and these morphological attributes impact the elastic modulus. These observations point to areas of future study. Conclusions The mechanical behavior of block copolymers depends on the composition, microdomain morphology, chain architecture, and the nonequilibrium processing conditions, as demonstrated in this study. In particular, by using various solvents during casting while holding the molecular weight and composition fixed, we have isolated the effects of morphology and chain architecture in SIS and ISI triblock copolymers. With the six triblock copolymers studied, we observed the following correlations with regard to the average elastic modulus. The average modulus increases with increasing polystyrene composition, as expected. At fixed composition the modulus increases with increasing PS connectivity from PS cylinders to lamellae to gyroid morphologies and from lamellae to PI cylinders. And finally the elastic modulus clearly depends on the chain architecture, such that the moduli for SIS triblocks are larger than ISI triblocks when both have the same total molecular weight, composition, and morphology. This finding was discussed in terms of the trapped PI entanglements when the PI midblock is double-tethered in SIS. Acknowledgment The authors would like to thank Molly T. Reinhardt for her help on the synthesis of the block copolymers in this study. The authors would like to thank Dr. Shaofu Wu for conducting microtensile mechanical tests for us. Funding was provided by The Dow Chemical Company. Literature Cited (1) Fredrickson, G. H.; Bates, F. S. Dynamics of block copolymers: Theory and experiment. Annu. ReV. Mater. Sci. 1996, 26, 501.
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ReceiVed for reView October 26, 2005 ReVised manuscript receiVed December 21, 2005 Accepted December 22, 2005 IE0511940
