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Keywords:

  • atomic force microscopy (AFM);
  • block copolymers;
  • phase behavior;
  • SAXS;
  • self-assembly;
  • supramolecular structures;
  • templates

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. Acknowledgements
  8. Note added in proof:
  9. REFERENCES AND NOTES
  10. Supporting Information

The phase behavior of supramolecular assemblies (SMAs) formed by poly(4-vinylpyridine)-block-polystyrene-block-poly(4-vinylpyridine) (P4VP-b-PS-b-P4VP) triblock copolymer with 2-(4′-hydroxybenzeneazo)benzoic acid (HABA) was investigated with respect to the molar ratio (X) between HABA and 4VP monomer unit in bulk as well as in thin films. The results were compared with SMAs formed by a PS-b-P4VP diblock copolymer of similar composition as the triblock but half the molecular weight to ascertain the effect of molecular architecture on microphase separation. In bulk, both the di- and triblock SMAs showed composition-dependent morphological transitions, which could be tuned by HABA/4VP molar ratio. The domain spacing of the SMA was not significantly affected by the molecular architecture of the constituting block copolymers. In thin films also, both the di- and triblock SMAs showed more or less similar morphological transitions depending on X. Interestingly, the domain orientation of the cylindrical or lamellar microdomains in the SMAs was influenced by the molecular architecture of the block copolymer. After chloroform annealing, although the diblock SMAs showed in-plane orientation of the domains, triblock SMAs showed perpendicular domain orientation. The perpendicular orientation of the microdomains in triblock was favored because it allowed the mid-PS blocks to acquire normal distribution of loop and bridged conformations. Furthermore, the orientation of the lamellar and cylindrical microdomains of the diblock SMAs was found to switch to perpendicular orientation after annealing in 1,4-dioxane vapors. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1594–1605, 2010


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. Acknowledgements
  8. Note added in proof:
  9. REFERENCES AND NOTES
  10. Supporting Information

Self-assemblies of block copolymer-based systems have attracted considerable attention in the area of nanotechnology, which relies largely on the ability to arrange functional domains at the nanoscale.1–4 Block copolymers consisting of chemically different polymer chains form periodic nanostructures driven by the repulsive interaction between the constituent blocks. The morphology depends on the volume fraction of the blocks and χN, where χ is the Flory-Huggins segmental interaction parameter and N is the degree of polymerization. The size of the nanostructures, which typically is in the range of tens of nanometers, is mostly influenced by the length of the blocks. The typical morphological patterns observed for diblock copolymers in bulk include body-centered-cubic (bcc) packed spheres, hexagonally packed cylinders (hcp), bicontinuous cubic (gyroid), and 1D stacked alternating lamellae.5–7 The phase behavior of AB diblock copolymers in bulk is now well known.

It is well known that a change in molecular architecture at constant chain length and composition dramatically influences microphase separation, and hence physical properties, in block copolymers.8–11 Like simple AB diblock copolymers, these polymeric species undergo microphase separation to form well-ordered structures. However, the different molecular architecture results naturally in much slower phase separation kinetics and modification of the phase behavior. One of the simplest block copolymers with a different molecular architecture is an ABA (or BAB) triblock copolymer, which has been well studied in the past.12–17 It has been found that the need to locate the two block junctions of such a triblock copolymer at a domain boundary significantly reduces the conformational entropy of its ordered state relative to that of an AB diblock copolymer of the same chain length and composition. As a consequence, the disordered state of a triblock copolymer melt is more stable than that of a diblock copolymer of the same length and composition.

The microphase separation of block copolymers in thin films, however, significantly differs from that in the bulk because the domain structure depends both on the surface energies and geometrical constraints.1, 2 At interfaces, such as the air–polymer interface and the film–substrate interface, additional driving forces for structure formation exist. For instance, the block with the lower interfacial energy will adsorb to the surface or interface preferentially, which can result in either an alignment of the bulk structure at the interface or a deviation of the microdomain structure from the bulk, or both. In general, the preferential wetting of the substrate with one of the blocks drives the system to the parallel alignment of nanodomains. In addition, the lowest surface tension component occupies the free surface of polymer film, enhancing the trend toward parallel alignment. The control on the orientation of cylindrical and parallel microdomains in block copolymer thin films is crucial, and substantial efforts have been addressed toward this direction. The orientation in block copolymer thin films has been controlled by applying external fields, by adjusting surface interactions, by varying film thickness, and through the control of solvent evaporation.1, 2 Thin films of ABA triblock copolymer have also been extensively studied in the past. It has been shown that such a molecular architecture of the block copolymer does not have any significant influence on the morphology in thin films except that the position of the phase boundaries between different phases differs slightly compared with the AB diblock.18

Recently, supramolecular assemblies (SMAs) based on block copolymers have received significant attention.19, 20 These SMAs are produced by attaching small molecules to the side chains of a block via noncovalent interactions. A range of morphologies can be obtained with one block copolymer by incorporating different fractions of the small molecules. The morphological transitions occur because of the change in the volume fraction of the constituting blocks on selective incorporation of the small molecule to one of the blocks. If the low-molecular-weight additive is sufficiently long, the resulting SMAs show hierarchical structures with the larger length-scale structure formed by block copolymer self-assembly, whereas the smaller length-scale structure formed by the comb block, that is, the block holding the small molecules. Furthermore, in these SMAs, as the small molecules are not covalently bound to the block copolymer, they can be selectively removed by washing with an appropriate solvent resulting in functional nanoporous materials or nanoobjects.19, 21, 22 Ikkala et al. have extensively investigated such supramolecules with hierarchical structures in bulk.19–21 Moreover, Chen and coworkers have shown that the molecular architecture of the block copolymer can significantly influence the phase transitions and periodicity of the SMAs.23–25

Although SMAs based on block copolymers have been extensively investigated in the bulk, their behavior in thin films is still to be fully understood. Moeller and coworkers26 investigated the thin film morphologies of wedge-shaped liquid crystalline amphiphilic molecules complexed with poly (2-vinylpyridine)-block-poly(ethylene oxide) (P2VP-b-PEO) diblock copolymer via proton transfer. The small molecules in this case were liquid crystalline, and their assembly dominated the assembly of the supramolecules. Recently, the thin films of SMAs formed by polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) and pentadecylphenol (PDP) were investigated by ten Brinke and coworkers.27, 28 The PDP was bound to pyridine group of 4VP unit via hydrogen bonding. They examined the phase behavior and terrace formation in this system after annealing it under different chloroform vapor pressures. It was observed that the P4VP(PDP) was present at the SiO2 interface as well as at the air interface, implying symmetric boundary conditions, and the morphologies depended on the chloroform vapor pressure as well as on the formation of terraces. Xu and coworkers29 investigated the hierarchical structures in thin films of PS-b-P4VP(PDP) SMA. They found that the lamellar and cylindrical microdomains, with a periodicity of 40 nm, could be oriented normal to the surface, whereas the assembly of comb blocks, P4VP(PDP), with a periodicity of 4 nm was oriented parallel to the surface. Recently, our group reported the synthesis of SMA from PS-b-P4VP and 2-(4′-hydroxybenzeneazo)benzoic acid (HABA), where HABA selectively associated with the pyridine nitrogen via hydrogen bonding. Thin films of SMA demonstrated well-ordered hexagonal cylindrical morphology.30–33 The cylindrical nanodomains were formed by the P4VP(HABA) block surrounded by PS matrix. Significantly, the orientation of the cylindrical microdomains with respect to the film surface plane was found to depend on the solvent used for preparing the SMA.

However, as far as we know, the effect of molecular architecture of the block copolymer on the morphologies in thin films of SMAs has still not been investigated. In this article, we investigate this issue by studying SMAs formed by a cylinder-forming P4VP-b-PS-b-P4VP asymmetric triblock copolymer and HABA. The self-assembly of the SMAs of triblock copolymer has been compared with those of a PS-b-P4VP diblock of similar composition but half the chain length. The microphase separation of the SMAs was investigated in bulk as well as in thin films in a wide range of compositions tuned by varying the molar ratio between HABA and 4VP unit.

EXPERIMENTAL

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. Acknowledgements
  8. Note added in proof:
  9. REFERENCES AND NOTES
  10. Supporting Information

Materials

PS-b-P4VP diblock copolymer with number-averaged molecular masses (Mn): PS 19,000 g mol−1, P4VP 5200 g mol−1, Mw/Mn = 1.10, and P4VP-b-PS-b-P4VP triblock copolymer with (Mn): PS 38,000 g mol−1, P4VP 4500 g mol−1, Mw/Mn = 1.10 were purchased from Polymer Source. 2-(4′-Hydroxybenzeneazo)benzoic acid (Scheme 1) was purchased from Sigma-Aldrich. Solvents—1,4-dioxane, chloroform, tetrahydrofuran, and methanol—were purchased from Acros Organics and used as received. Highly polished single-crystal silicon wafers of {100} orientation were purchased from Semiconductor Processing and used as substrates. The silicon wafers were cleaned with dichloromethane in an ultrasonic bath for 20 min and then further in a 1:1:1 mixture of 29% ammonium hydroxide, 30% hydrogen peroxide, and water (Warning: This solution is extremely corrosive and should not be stored in tightly sealed containers) for 1.5 h at 65 °C, finally rinsed several times with water, and dried in an argon flow.

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Scheme 1. Structure of 2-(4′-hydroxybenzeneazo)benzoic acid.

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Preparation of SMAs

PS-b-P4VP(HABA)X or P4VP(HABA)X-PS-P4VP(HABA)X SMAs were prepared in 1,4-dioxane, where X denotes the molar ratio given by the average number of HABA molecules bound to one monomer unit of P4VP block. The preweighed block copolymers and HABA were dissolved separately in the solvent. Block copolymer solution was then slowly added dropwise to HABA solution while heating close to the boiling point of the solvent in an ultrasonic bath. The elevated temperature during mixing was very important for reproducible formation of SMA. The resulting 1 wt % solution was kept at least overnight to complete hydrogen-bond formation. Thin films were prepared by dip coating from the filtered (200-nm pore size PTFE filter) solutions. Detailed characteristics of the SMA investigated here are shown in Table 1. Bulk samples were prepared by slowly evaporating the solvent in a petridish over 2 weeks.

Table 1. Characteristics of Di- and Triblock SMAs at Different HABA/4VP Molar Ratios
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Solvent Vapor Treatment

The SMA thin films were further annealed in vapors of appropriate solvent for the reorientation of the microdomains and for improving the long-range order. The solvent vapor treatment of the thin films was done in a crystallographic dish with a petridish containing the solvent. The silicon substrate coated with the SMA film was kept in this crystallographic dish, which then was completely closed. The sample was removed from the dish after a predetermined time during which the solvent in the film quickly evaporated. The annealing time was recorded from the moment when samples were put into or taken out from the crystallographic dish. All the samples used in this study were solvent vapor treated in the same crystallographic dish for the comparative experiments.

Ellipsometry

The thickness of the polymer film was measured by a SE400 ellipsometer (SENTECH Instruments GmbH, Germany) with a 632.8 nm laser at a 70° incident angle. The thickness of all the samples coated on silicon substrate was kept constant at 40 ± 2 nm.

Small-Angle X-Ray Scattering

SAXS measurements were performed in transmission geometry using a three-pinhole collimation system equipped with an Osmic multilayer mirror for higher photon flux at smaller beam spot and a Rigaku rotating anode generator (Cu Kα radiation λ = 0.1542 nm, operating at 4.2 kW with microfilament). For data collection, a Marccd detector with an average pixel size of 78.7 × 78.7 μm2 was used. All scattering patterns were radially averaged to obtained the intensity I(q), where q = 4π/λ(sin θ/2). The data were corrected for background scattering, X-ray absorption, and thermal diffuse scattering.

Atomic Force Microscopy

Atomic Force Microscopy (AFM) imaging was performed using a Dimension 3100 scanning force microscope (Digital Instruments, Santa Barbara) and a CP microscope (Park Scientific Instrument) in the tapping mode. The tip characteristics were as follows: spring constant 1.5–3.7 N m−1, resonant frequency 45–65 Hz, and tip radius about 10 nm.

RESULTS AND DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. Acknowledgements
  8. Note added in proof:
  9. REFERENCES AND NOTES
  10. Supporting Information

Microphase Separation in Bulk

First, the morphology of the SMAs in bulk will be discussed. Figure 1(a,b) shows the SAXS intensity profiles of P4VP(HABA)X-b-PS-b-P4VP(HABA)X triblock and PS-b-P4VP(HABA)X diblock SMAs with different HABA/4VP molar ratio (X). The neat diblock copolymer (D0) showed an intense primary scattering peak at ∼0.26 nm−1 and a second order peak in the ratio 1:31/2, which suggested a morphology consisting of hexagonally packed P4VP cylinders in PS matrix. The diblock SMAs D025 and D050 also exhibited hexagonally packed cylindrical morphology as could be seen from the peak positions of their SAXS profiles. This was expected considering the volume fractions of the component blocks. The SAXS profiles of diblock SMAs D075 and D100, however, were strikingly different. The SAXS profiles consisted of a sharp primary scattering peak and several higher order peaks in the ratio 1:2:3:4:5:6, which revealed a well-ordered one-dimensionally packed lamellar morphology consisting of alternating layers of PS and P4VP(HABA) blocks. The SAXS profile of neat triblock copolymer (T0) showed peaks in the ratio 1:31/2:71/2, which was indicative of hexagonally packed cylindrical morphology (Supporting Information). The SMAs based on the triblock copolymer at lower molar ratios also showed hexagonally packed cylindrical morphology as could be observed from the SAXS profiles of T025 and T050. The SMAs with higher HABA content (T075 and T100), however, showed very poor order in bulk because the first order peak was weak and higher order peaks were mostly absent. Nevertheless, from the composition of the respective blocks, these SMAs were expected to have a lamellar morphology.

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Figure 1. Room temperature SAXS intensity profiles of SMA based on (a) diblock copolymer and (b) triblock copolymer.

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Figure 1 also shows that the primary scattering peak in both diblock and triblock samples shifts toward low-q region as the concentration of HABA increases in the SMA signaling an increase in the interdomain spacing. Figure 2 shows the variation of Braggs spacing (D = 2π/qm, where qm is the position of the primary scattering maximum) with molar ratio of the SMA. The figure clearly shows that the Braggs spacing increased as the molar ratio increases both in the case of diblock as well as in triblock SMAs. This could be explained considering that as the molar ratio increases the P4VP chains become more densely grafted with HABA molecules and, hence, the chains will be in highly stretched conformation compared with that of the neat P4VP chains. The stretched conformation of the P4VP chains will also provide less interfacial area for the PS chains to relax and, hence, will necessitate its stretching normal to the interface. Both of these effects are expected to make a significant influence on the interdomain spacing and were clearly visible from the experimental data. Figure 2 also shows that the Braggs spacing of the SMA is not much affected by the molecular architecture of the constituting block copolymer. The issue of domain spacing differences in di- and triblock copolymers has been investigated in detail by Mai et al.13 They found that the triblock copolymers were 10% more stretched than corresponding diblock copolymers and, hence, have a larger domain spacing. In the present system, no domain spacing difference was observed between di- and triblock copolymers. This could be attributed to the fact that the conclusions drawn by Mai et al.13 were for systems in the weak segregation region and the PS-b-P4VP copolymers belong to the strong segregation regime. In fact, Mai et al.13 suggested in their study that the domain spacing difference vanishes as the system moves toward strong segregation region. However, at a molar ratio of X = 0.25, the Braggs spacing of the triblock SMA was significantly higher than that of the diblock SMA, which is still not clear to the author.

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Figure 2. Plot depicting variation of Braggs spacing (D) with HABA/4VP molar ratio.

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Microphase Separation in Thin Film

Now the microphase separation of the SMAs in thin film will be discussed. The microphase separation in thin film was investigated by tapping-mode AFM. The images were obtained after selectively washing the HABA molecules with methanol to increase the contrast between the two phases. The dark regions in the height images corresponded to holes or empty channels created by the removal of HABA, whereas the light regions corresponded to the PS matrix.

It must be noted here that the conclusion regarding the phase morphology in this study has been drawn from the composition of the SMAs and SAXS data of their bulk samples. The perpendicular or parallel orientation of the lamellae or cylinder in thin films, hence, then accordingly has been assigned based on the AFM topographical images. Such an interpretation of the phase morphology in thin films based on AFM data, though, is not completely reliable and have to be corroborated in future by techniques such as grazing incidence small-angle X-ray scattering (GISAXS) and/or cross-sectional transmission electron microscopy.

Figure 3(a–d) shows the AFM images of the as-casted diblock SMAs. The as-casted diblock SMA at all the molar ratios showed disordered morphology. The AFM images of D025, D050, and D075 were very similar and showed mixture of dots and short cylinders. The percentage of short cylinders and their length increases as the molar ratio of the SMA increases. However, the as-casted D100 SMA showed drastically different surface morphology. The surface showed highly irregular features randomly distributed along the plane of the film. Such a morphology could be explained considering that the SMA D100 was expected to have a lamellar morphology, and in this case, the lamellae were oriented parallel to the substrate. As PS has low surface energy compared with P4VP block, it will preferentially form the top layer. During the etching process of HABA, methanol diffused into the P4VP(HABA) layer through the thin top PS layer and then the only way the solvent and HABA could diffuse out was via disturbing the top PS surface layer. This created the highly irregular structures on the surface observed by AFM. Figure 4(a–d) shows the AFM topographic images of as-casted triblock SMAs. As expected, the as-casted SMAs showed disordered morphology. The SMA T100 and T075 showed mostly mixtures of randomly arranged dots and worm-like structures on the surface, whereas SMA T050 and T025 showed only randomly arranged dots on the surface.

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Figure 3. AFM height images obtained for as-casted diblock SMA samples: (a) D100, (b) D075, (c) D050, and (d) D025.

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Figure 4. AFM height images obtained for as-casted triblock SMA samples: (a) T100, (b) T075, (c) T050, and (d) T025.

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Figure 5(a–d) shows AFM topographic images of diblock SMAs after annealing in chloroform. The AFM images of SMAs D100 and D075 showed highly irregular features similar to that observed for unannealed D100 SMA. This again could be explained considering that from the volume fraction of the constituting blocks, the D100 and D075 diblock SMAs were expected to have a lamellar morphology, and, in this case, the lamellae were oriented parallel to the silicon substrate. The diffusion of the solvent and HABA from the film during methanol washing distorts the top layer formed by PS blocks resulting in the irregular features observed by AFM. The AFM images of D050 and D025 showed alternating dark and bright stripes on the surface, which was indicative of parallel cylindrical morphology. The cylindrical morphology was expected in these SMAs considering the volume fraction of the respective blocks. Hence, the orientation of lamellar and cylindrical microdomains in diblock SMAs was parallel to the substrate.

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Figure 5. AFM height images obtained for diblock SMA samples annealed in chloroform vapors: (a) D100, (b) D075, (c) D050, and (d) D025.

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Figure 6(a–d) shows AFM topographic images of triblock SMAs after annealing in chloroform. The AFM images of triblock SMAs T100 and T075 showed alternating dark and bright stripes. From the composition of the constituting blocks, it was expected that T100 and T075 will have lamellar morphology. Hence, the lamellar orientation in T100 and T075 was perpendicular to the surface, and the dark and bright stripes in the AFM images corresponded to the P4VP(HABA) and PS layers, respectively. Similarly, the AFM images of the SMAs T050 and T025, which were expected to had cylindrical morphology, showed dark spherical structures packed in a hexagonal lattice in bright matrix. This was indicative of perpendicular orientation of P4VP(HABA) cylinders in the thin films of these SMAs. Hence, the orientation of lamellar and cylindrical microdomains in the triblock SMAs was perpendicular to the substrate, which was opposite to that observed for diblock SMAs of similar compositions.

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Figure 6. AFM height images obtained for triblock SMA samples annealed in chloroform vapors: (a) T100, (b) T075, (c) T050, and (d) T025.

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It is well known that the microdomain orientation in case of thin film casted on substrate having preferential attraction for one block is in-plane. However, factors such as film thickness, solvent evaporation rate, and the selectivity of the solvent may further influence the domain orientation.34–36 In the present system, the silicon substrate is known to be attractive for P4VP block, and hence, the substrate–film interface was expected to be enriched in P4VP chains.37, 38 Moreover, as PS has lower surface energy, it is expected to occupy the film–air interface. Such a scenario will favor in-plane orientation of P4VP(HABA) microdomains. The parallel orientation of lamellar and cylindrical microdomains, thus, observed in diblock SMAs was expected. Similar behavior was also observed in the past for cylinder-forming diblock SMA with HABA.30, 31 However, the perpendicular orientation of microdomains observed in triblock SMAs was unexpected and, hence, was very interesting. It showed that molecular architecture of the block copolymer does affect the morphology in thin films.

Now the question arises how the molecular architecture of the block copolymer affects the domain orientation as observed here. Figure 7(a–c) shows schematically the chain conformations in lamellae forming diblock and triblock SMA thin films. In the case of diblock SMA, after chloroform annealing, the lamellae were oriented parallel to the surface. Considering that PS block should occupy the air–film interface because of its lower surface energy compared with P4VP(HABA) block, the chain packing arrangement in the lamellae forming diblock SMAs was expected to be like that shown in Figure 7(a). However, chain packing arrangement in triblock SMAs was more complex. As a consequence of the chain architecture of the triblock, the chain conformation of the middle PS block can have one of two different types of conformations: a loop or a bridge conformation. In the bridge conformation, the two end blocks of the triblock, that is, the P4VP blocks belong to two different P4VP domains, whereas in the loop type the ends belong to the same P4VP domain. This issue has attracted significant attention in the past, because the presence of bridges linking separate interfaces together strongly affects the mechanical properties of the material.14–16 Matsen and Thompson14 evaluated bridging fraction as a function of the triblock copolymer for different values of χN. The bridging fraction showed negligible dependence on χN but was found to depend on morphology. The bridging fraction was found to be around 0.8 for the spherical micelles, about 0.6 for the cylindrical micelles and about 0.45 for the lamellar morphology. Some of the experimental details since then have corroborated the theoretical findings.39 Recently, Huang et al.16 and Nie at al.15 measured fraction of bridges in thin films of ABA triblock copolymer by Monte Carlo simulation, and it was concluded that because of confinement the fraction of bridges in the thin films is greater than that in the bulk.

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Figure 7. Schematic illustration explaining the chain packing arrangement in diblock and triblock SMAs: (a) diblock SMA with in-plane lamellae orientation; (b) triblock SMA with in-plane lamellae orientation; and (c) triblock SMA with perpendicular orientation of lamellar domains. A portion of the cross-sectional view of the thin film has been illustrated.

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Now let us go back to Figure 7 and applying similar argument to the triblock SMA as for a diblock and assuming an in-plane lamellar orientation for triblock SMA, the chain packing arrangement in such a morphology should be as shown in Figure 7(b). In this case, as the PS layer will constitute the film–air interface, all the PS chains in the top PS layer will have to be arranged in looped conformation, which will not be favorable considering the distribution of bridged and looped conformation in a typical ABA triblock microphase-separated structure. In such a scenario, the perpendicular orientation of the lamellar (or cylindrical) microdomains will be favored. As shown in Figure 7(c), the perpendicular orientation of the domains allows the mid-PS blocks to have normal distribution of the bridge and loop conformations.

In previous work with PS-b-P4VP(HABA) SMAs, which had cylindrical microdomains of P4VP(HABA), it was shown that the orientation of the microdomains could be switched from parallel to perpendicular by exposing it to 1,4-dioxane vapors.30, 31 Hence, an experiment was carried out to observe the effect of annealing in 1,4-dioxane for the diblock and triblock SMA systems investigated here. Figure 8(a–d) shows AFM topographic images of diblock SMAs after annealing in 1,4-dioxane vapors. It could be noted from the Figure that the D100 and D075 SMAs showed alternating bright and dark stripes, which though are not highly ordered [Fig. 8(a,b)]. This suggested a perpendicular orientation of the lamella in these SMAs and clearly demonstrated that the switching aspect of such SMAs could be extended even to other morphologies apart from the cylindrical ones. The switching aspect of cylindrical morphology in such systems in the past was explained on the basis of compositional change in the system swelled in a selective system.31 Dioxane is a selective solvent for PS block, and hence it enriches the PS domains when swelled in the solvent vapors. This increases the volume fraction of PS and the system attains spherical morphology with P4VP(HABA) blocks forming the spherical microdomains. During evaporation of the solvent, the spherical microdomains merge along the thickness of the film giving perpendicular cylinders. The same argument could be extended to the present system also. Here, on being swelled in dioxane vapors, the lamellar morphology will change to cylindrical morphology, with P4VP(HABA) blocks forming the cylinders, and during evaporation of the solvent, the cylinders will merge normal to the film giving rise to perpendicular lamellae.

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Figure 8. AFM height images obtained for diblock SMA samples annealed in 1,4-dioxane vapors: (a) D100, (b) D075, (c) D050, and (d) D025.

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However, the D050 SMA, which had in-plane cylinders after annealing in chloroform vapors, did not show the expected switching to perpendicular orientation after annealing in dioxane vapors [Fig. 8(c)]. The AFM image was similar to that for D100 and D075 SMAs. This could be attributed to the fact that the composition of the D050 SMA was such that it was situated very close to the cylinder-lamellae transition region in the phase diagram, and, hence, it is plausible that during swelling in dioxane vapors, the system does not go to the spherical morphology necessary for switching to perpendicular cylinder orientation. However, a more detailed investigation involving in situ GISAXS study will be necessary to prove such a hypothesis. The D025 SMA, as expected, does show the perpendicular cylindrical morphology [Fig. 8(d)].

The triblock SMAs, which already showed perpendicular domain orientation in chloroform for the reason explained above, were not expected to show any changes in the domain orientation in 1,4-dioxane. Figure 9(a–d) shows AFM topographic images of triblock SMAs after annealing in 1,4-dioxane vapors. The notable observation from the AFM images was that the morphology of the triblock SMAs was highly disordered both for lamellae and cylinder-forming systems. This could again be explained considering the architecture of the triblock and selectivity of the solvent. As dioxane was a selective solvent for PS block, which was tethered to the insoluble P4VP blocks at both chain ends, the diffusion of the triblock chains during swelling was highly restricted. This did not allow the system to acquire any favorable ordered morphology after being solvent in dioxane vapors. The morphologies of the di- and triblock SMAs were also investigated in THF, which is a partially selective solvent for the present system, and the domain orientation was more difficult to explain in this case for some of the compositions (Supporting Information).

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Figure 9. AFM height images obtained for triblock SMA samples annealed in 1,4-dioxane vapors: (a) T100, (b) T075, (c) T050, and (d) T025.

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CONCLUSIONS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. Acknowledgements
  8. Note added in proof:
  9. REFERENCES AND NOTES
  10. Supporting Information

The molecular architecture effect of the block copolymer on the SMAs formed with a low-molecular-weight additive, HABA, was investigated by taking a P4VP-b-PS-b-P4VP triblock copolymer and a PS-b-P4VP diblock copolymer of similar composition but half the molecular weight. The SMAs showed morphological transitions from cylinders to lamellae depending on the molar ratio between HABA and 4VP units irrespective of the architecture of the block copolymer both in bulk as well as in thin film. However, interestingly, the domain orientation of the lamellar and cylindrical microdomains in thin films was found to depend on the molecular architecture of the block copolymer. The SMA based on diblock copolymer showed parallel orientation of the microdomains, whereas triblock SMAs showed perpendicular orientation of the microdomains after annealing in a nonselective solvent. It was suggested that perpendicular orientation of the microdomains in triblock SMAs allowed the mid-PS blocks to acquire its normal distribution of loop and bridged conformation. Furthermore, the microdomain orientation in diblock SMAs was found to switch to perpendicular orientation in solvent vapors selective for PS blocks.

Acknowledgements

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. Acknowledgements
  8. Note added in proof:
  9. REFERENCES AND NOTES
  10. Supporting Information

This research was supported by the priority program of Deutsche Forschungsgemeinschaft (SPP1165, Project No. STA324/31). The author thanks Prof. Manfred Stamm for useful discussions during the course of this work.

Note added in proof:

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. Acknowledgements
  8. Note added in proof:
  9. REFERENCES AND NOTES
  10. Supporting Information

Recently, I found that a similar explanation for domain orientation in thin films of a triblock copolymer containing poly(cyclohexylethylene) and poly(ethylene) blocks was also given by Kramer and coworkers in a previous work.40 The surface energy difference between the two blocks in their case was small so that according to them the entropic gain because of the normal distribution of loop and bridged conformations in domains with perpendicular orientation compensated for the increase in energy due to the presence of block chains with high surface energy on the surface. However, the surface energy difference between PS and P4VP blocks, used in this work, is significantly large, and this raises questions if some other factors also are responsible for the particular domain orientation in the triblock complexes investigated here. I would also like to note that during the review of this article, an interesting article was published by M. W. Matsen where he proposed using SCFT theory that the normal orientation of domains in an ABA triblock copolymer is due to an entropic advantage of having the end segments of the A-rich domain next to a surface.41

REFERENCES AND NOTES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. Acknowledgements
  8. Note added in proof:
  9. REFERENCES AND NOTES
  10. Supporting Information

Supporting Information

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. Acknowledgements
  8. Note added in proof:
  9. REFERENCES AND NOTES
  10. Supporting Information

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