Chain Architecture and Hydrogen Bonding Induced Co-Ordering and

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Chain Architecture and Hydrogen Bonding Induced Co-Ordering and Segregation of Block Copolymer/Graft Copolymer Blends Chia-Chen Wang, Kuang-Hsin Wu, and Chieh-Tsung Lo* Department of Chemical Engineering, National Cheng Kung University, No. 1, University Road, Tainan City 701, Taiwan

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S Supporting Information *

ABSTRACT: We investigated the effects of chain architecture and hydrogen-bonding interaction on the phase behavior of binary mixtures containing nearly symmetric polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) block copolymer and highly asymmetric polystyrenegraf t-poly(acrylic acid) (PS-g-PAA) graft copolymer. When PS-g-PAA was added to PS-b-P2VP, hydrogen bonds between PAA and P2VP chains improved the miscibility of the copolymers and facilitated localization of PS-g-PAA at the PS-b-P2VP interface, which reduced the interfacial free energy of the blends. However, positioning PS-g-PAA with one PS main chain and two PAA grafted chains at the PS-b-P2VP interface increased the stretching free energy of PS-b-P2VP. Consequently, the interfacial coverage of PS-g-PAA reached saturation. Residual PS-g-PAA was segregated into the PS microdomains formed by PS-b-P2VP to regain translational entropy and reduce the stretching free energy. When the molecular weight ratio of PS-b-P2VP to PS-g-PAA (R) was smaller than 8, PS-g-PAA could not swell the PS microdomains formed by PS-b-P2VP. Therefore, the morphology of PS-b-P2VP/PS-g-PAA blends remained lamellar. By contrast, when R > 8, PS-g-PAA effectively swelled the PS microdomains formed by PS-b-P2VP. This behavior amplified the asymmetry effect caused by the branched-chain architecture of PS-g-PAA on altering the interfacial curvature of PS-b-P2VP. Consequently, the morphology of the blends transformed into a cylindrical structure.

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units but different molecular weights.13−26 When both block copolymers were symmetric (meaning that the chain lengths of chemically different sequences in any block copolymer are nearly identical), the total molecular weight ratio of the two block copolymers (r) determined the miscibility criterion of the blends. When r was smaller than 5, two block copolymers in any blend composition microphase-separated at the molecular level with their chemical junctions located at the common interface and the distinct blocks microphaseseparated into respective microdomains.13,17,21 This phenomenon is called a “cosurfactant effect”.15,20,21,23,24 By contrast, when r was larger than 10, macrophase separation occurred, which resulted in coexisting microdomain morphologies.15 In binary block copolymer blends containing at least one asymmetric block copolymer, the critical r value at the regime between microphase and macrophase separation was strongly correlated with the composition of the asymmetric block copolymer.25,26 Furthermore, when two block copolymers microphase-separated following cosurfactant behavior, the interfacial curvature exhibited a function of the total chemical composition (f), in which f = (1 − ϕ)f1 and ϕf 2, where f1 and f 2 are the volume fractions of the chemically identical block in each block copolymer and 1 − ϕ and ϕ are the volume fractions of each block copolymer in the blend.16 This is similar to the phase behavior of neat block copolymers. By

INTRODUCTION Block copolymers exhibit two or more chemically distinct sequences joined by covalent bonds. Because of the repulsive interaction between their repeating units, block copolymers tend to microphase-separate, forming well-organized morphologies on a length scale of tens of nanometers. The resulting equilibrium morphology and domain size depend on the balance between the enthalpic contribution associated with short-range segmental interaction and entropic contribution associated with chain packing and distribution within microdomains to maintain a constant segment density in entire microdomains. Well-defined morphologies formed by block copolymers include alternating lamellae, gyroids, hexagonally packed cylinders, and body-centered-cubic spheres.1,2 The volume fraction of components, degree of polymerization, and segmental interaction parameter are used for manipulating the phase behavior of block copolymers. Another strategy to tailor the phase behavior of block copolymers involves blending a block copolymer with another polymer, such as a homopolymer3−12 and another block copolymer.13−31 This second component adds an additional degree of freedom into the original neat block copolymer, leading to more complex and richer phase behavior of the system when compared with that of a neat block copolymer; this enhanced system behavior includes microphase separation, macrophase separation, order−order transition, and order− disorder transition. For example, several key findings have been established for systems composed of binary block copolymer blends in which two block copolymers have the same repeating © XXXX American Chemical Society

Received: January 24, 2019 Revised: April 3, 2019

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DOI: 10.1021/acs.macromol.9b00184 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

In this study, we extended our previous work and investigated the role of hydrogen-bonding interaction on the phase behavior of blends composed of a block copolymer and a graft copolymer. We used polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) block copolymer and polystyrene-graf tpoly(acrylic acid) (PS-g-PAA) graft copolymer. In these molecules, P2VP forms hydrogen bonds with PAA, which allows co-organization of PS-g-PAA with PS-b-P2VP in microdomain morphologies. We modulated the phase behavior of the blends by changing the molecular weight of PS-b-P2VP but left the chain architecture and molecular weight of PS-gPAA unchanged. Our results suggested the importance of the chain architecture in PS-g-PAA for affecting its dispersion in the ordered microdomains formed by PS-b-P2VP, which did not entirely agree with the phase behavior of conventional binary block copolymer blends. The unique self-assembly behavior in block copolymer/graft copolymer blends resulted from the interplay between the changes in the stretching free energy and interfacial free energy to localize PS-g-PAA at the PS-b-P2VP interface. No study has provided such experimental data and theoretical calculations of the phase behavior of block copolymer/graft copolymer blends. Our results provide a novel perspective and contribute experimental data to the scarce literature on the phase behavior of block copolymer/graft copolymer blends.

contrast, when binary block copolymer blends contained a highly asymmetric block copolymer (i.e., either f1 or f 2 near 1), the highly asymmetric block copolymer tended to segregate into the preferred domain of the second block copolymer. In this case, the interfacial curvature could not be simply determined using f. These properties suggest that the increasing molecular parameters caused by the blending of two block copolymers result in increasingly complex phase behavior of the blends, thereby enhancing the possibility of creating new phases of the blends. In addition to the aforementioned methods, manipulation of the morphology of blends containing a block copolymer can be achieved through noncovalent bonding, such as hydrogenbonding interaction.9−12,27−33 Hydrogen bonding increases the compatibility of two chemically different polymers, thereby allowing them to co-organize in microdomain morphologies without causing macrophase separation. Additionally, the phase behavior of these blends exhibits a strong function of hydrogen-bonding strength. Through manipulation of the hydrogen-bonding strength, hierarchical microphase-separated structures dissimilar to those in miscible blends without hydrogen bonds can be obtained. Furthermore, hydrogenbonded polymer blends can respond to external stimuli, such as temperature and solvents. Through external stimulation, hydrogen bonds can be broken and miscible blends can be transformed into parent polymers. These supramolecular selfassemblies and tunable microstructures provide an alternative approach for creating complex morphologies with a high level of hierarchy. Compared with block copolymer/homopolymer blends and binary block copolymer blends, experimental results and theoretical predictions of the phase behavior of blends containing a block copolymer and a graft copolymer are scarce. Graft copolymers exhibit a branched molecular structure, which consists of a backbone and multiple side chains. This unique chain architecture enables modification of molecular properties of graft copolymers by varying the relative chain lengths between the backbone and side chains, areal density of side chains, and average distance between side chains. Thus, blends composed of a graft copolymer provide highly complex molecular parameters for manipulation of the phase behavior of the blends. We have previously investigated the phase behavior of blends composed of an A-b-B block copolymer and an A-g-C graft copolymer.34,35 In these blends, the hydrogen-bonding interaction between the B and C blocks improved the miscibility of the two copolymers, which resulted in their co-ordering in a microphase-separated morphology with their chemical junctions located at the common interface. Additionally, the A blocks in both copolymers coexisted in an ordered microdomain, whereas the B and C blocks coorganized in the other ordered microdomain. However, when A-g-C was localized at the A-b-B interface, the branched-chain architecture of A-g-C induced a large discrepancy between the radii of gyration of the A and C chains parallel to the microdomain interface. This discrepancy further amplified a mismatch of the volume fraction of the two microdomains, which resulted in a substantial change in the interfacial curvature. Consequently, an order−order transition occurred.35 Although we demonstrated the chain architecture effect on the phase behavior of block copolymer/graft copolymer blends, detailed studies are required to understand fully the crucial parameters affecting the phase behavior of the blends.

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EXPERIMENTAL SECTION

Sample Preparation. All of the polymers used in this study were purchased from Polymer Source and used as received. Table 1

Table 1. Molecular Characteristics of Polymers polymer PS385-b-P2VP419a PS721-b-P2VP633 PS981-b-P2VP924 PS-g-PAAb PS-b-PAA PAA

molecular weight (g/mol)

polydispersity index (PDI)

f PSd

Mnc of PS = 40 000 Mn of P2VP = 44 000 Mn of PS = 75 000 Mn of P2VP = 66 500 Mn of PS = 102 000 Mn of P2VP = 97 000 Mn of PS = 20 000 Mn of PAA = 2300 Mn of PS = 15 000 Mn of PAA = 1600 3000

1.10

0.50

1.18

0.55

1.12

0.53

1.20

0.92

1.10

0.93

1.15

a

PSm-b-P2VPn: m and n are the degree of polymerization of PS and P2VP, respectively. bPS-g-PAA graft copolymer contains two PAA chains grafted onto one PS main chain. cMn: number-averaged molecular weight. df PS: volume fraction of PS in copolymers, which were calculated using the densities of PS, P2VP, and PAA of 1.05, 1.14, and 1.41 g/cm3, respectively. provides the detailed molecular properties of these polymers. The three PS-b-P2VP block copolymers were nearly symmetric in composition but differed in molecular weights. The PS-g-PAA and PS-b-PAA copolymers were highly asymmetric with the ratio of PS composition in copolymers of approximately 0.90. The PS-b-P2VP/PS-g-PAA blends and PS-b-P2VP/PS-b-PAA blends were prepared by dissolving the copolymers with various compositions in tetrahydrofuran (THF, J. T. Baker). These mixtures were placed in a sample holder, and the solvent was removed completely at room temperature. The bulk samples were then annealed at 170 °C for 7 days under vacuum. Characterization. The hydrogen bonds between the P2VP and PAA molecules were examined using Fourier transform infrared B


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Macromolecules (FTIR) spectroscopy, operated on a Nicolet 6700 FTIR spectrometer with a resolution of 2 cm−1 at room temperature. Small-angle X-ray scattering (SAXS) measurements were conducted both at Sector 23A1 at the National Synchrotron Radiation Research Center in Taiwan and using a Bruker diffractometer (NanoSTAR U System, Bruker AXS GmbH, Karlsruhe, Germany). Samples used for the measurements were sealed in an aluminum sample pan, and the measurements were conducted at room temperature. The raw scattering data were processed following routine procedures, which included corrections for incident beam flux, absorption, detector sensitivity variation, and dark current. Scattering intensity was converted to one-dimensional intensity (I) data using azimuthal average for structural analysis. The scattering intensity was recorded as a function of the scattering vector (q), and q = (4π sin θ)/λ, where θ is the half-scattering angle and λ is the wavelength of the incident beam. The real-space observation of the morphology of the blends was conducted through transmission electron microscopy (TEM) on a Hitachi H7500 transmission electron microscope with an accelerating voltage of 80 keV. Samples embedded in epoxy resins were microtomed to a thickness of 55 nm using an Ultracut R Microtome (Reichert, Leica, MI). The thin sections were subsequently exposed to iodine vapor for 1 day to selectively stain the P2VP molecules. SAXS Analysis. Block copolymers and their blends exhibiting a lamellar morphology can be characterized using a one-dimensional correlation function, γ(x), in which x is the coordinate along the electron density distribution, to obtain the size of each lamellar microdomain. The one-dimensional correlation function is the Fourier transform of the Lorentz-corrected SAXS profile, which is calculated as follows36 γ(x) =

1 Q

∫0

∞

q2I(q) cos(qx) dq

Figure 1. FTIR spectra of PS385-b-P2VP419/PS-g-PAA (w/w %) blends.

P2VP419/PS-g-PAA (w/w %) blends with various compositions at wavenumbers between 970 and 1020 cm−1. For neat PS385b-P2VP419, absorbance at a wavenumber of approximately 993 cm−1 was observed, which corresponded to the free pyridine group in P2VP.40 When PS-g-PAA was added to PS385-bP2VP419, absorbance at a wavenumber of approximately 1003 cm−1 considerably increased, indicating the formation of intermolecular hydrogen bonds between the pyridine group in P2VP and the carboxyl group in PAA. The absorbance associated with the hydrogen-bonding interaction increased monotonically with the addition of PS-g-PAA. The fraction of hydrogen-bonded pyridine in PS-b-P2VP/ PS-g-PAA blends (f b) was estimated using the following equation40

(1)

Here, Q is the scattering invariant, which is expressed as follows

Q=

∫0

∞

q2I(q) dq

(2)

To operate the Fourier transform, I(q) must be available over a sufficiently extended angular range. This criterion is not practical because the measured I(q) is only available to a limited q range. I(q) for q values through infinity can be approximated using the Porod− Ruland model37 I(q) =

K p exp(− σ 2q2) q4

+ Ib

fb =

I(0) (1 + q2ξ 2)2

Ab ar

+ Af

(5)

where Ab and Af are the areas of absorbance corresponding to the hydrogen-bonded and free pyridine rings, respectively, and ar is the absorbance ratio, which is assumed as 1.40 Table 2 summarizes the fractions of hydrogen-bonded pyridine in various PS-b-P2VP/PS-g-PAA blends. The fractions of hydrogen-bonded pyridine in PS-b-P2VP/PS-b-PAA blends and PSb-P2VP/PAA blends are also included in Table 2 for comparison. In the PS-b-P2VP/PS-g-PAA blends containing different-molecular-weight PS-b-P2VP, adding PS-g-PAA increased the fraction of hydrogen-bonded pyridine. When the PS-g-PAA content was high, the fraction of hydrogen-bonded pyridine approached nearly constant values of 0.20 ± 0.03, depending on the molecular weight of PS-b-P2VP. The maximum fraction of hydrogen-bonded pyridine slightly increased with increasing molecular weight of PS-b-P2VP. Compared with the fractions of hydrogen-bonded pyridine in the blends containing different architectures of PAA, incorporation of PS-b-PAA in PS385-b-P2VP419 considerably increased the fraction of hydrogen-bonded pyridine and no apparent maximum fraction of hydrogen-bonded pyridine was observed. Moreover, the fraction of hydrogen-bonded pyridine in PS385-b-P2VP419/PAA blends was much higher than those in PS385-b-P2VP419/PS-g-PAA and PS385-b-P2VP419/PS-b-PAA blends. Nearly 80% of the pyridine formed hydrogen bonds with PAA in the PS385-b-P2VP419/PAA blend containing 40%

(3)

where Kp is the Porod constant, σ is the transition thickness between the two microdomain phases, and Ib is the background scattering intensity arising from thermal density fluctuations. By contrast, the extrapolation of I(q) to the zero-q region can be accomplished through the Debye−Bueche approximation38,39 I(q) =

Ab ar

(4)

where ξ is the correlation length. In the plot of γ(x) versus x, according to eq 1, the average long period (L) is obtained from the first maximum of the correlation curve. The extrapolation of the linear fragment near the position at x = 0 to the intersection with the level of the minimum of the correlation function determines the size of the thinner microdomain. The difference between the long period and thinner microdomain size results in the size of the thicker microdomain.

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RESULTS AND DISCUSSION When P2VP and PAA molecules are blended, nitrogen in the pyridine group can form hydrogen bonds with carboxylic acid in PAA. Hydrogen bonds can be detected through FTIR spectroscopy. Figure 1 displays the FTIR spectra of PS385-bC


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Macromolecules Table 2. Fractions of Hydrogen-Bonded Pyridine in Various Blendsa blend PS385-b-P2VP419/PS-g-PAA 90/10 80/20 70/30 60/40 PS721-b-P2VP633/PS-g-PAA 90/10 80/20 70/30 60/40 PS981-b-P2VP924/PS-g-PAA 90/10 80/20 70/30 60/40 PS385-b-P2VP419/PS-b-PAA 90/10 80/20 70/30 60/40 PS385-b-P2VP419/PAA 90/10 80/20 70/30 60/40

fb 0.10 0.14 0.17 0.18 0.08 0.13 0.18 0.20 0.08 0.11 0.20 0.23

Figure 2. SAXS patterns of PS385-b-P2VP419/PS-g-PAA (w/w %) blends. Scattering patterns are shifted to increase clarity, and numbers in the plot indicate the ratio of peak positions to the value of firstorder peak.

0.20 0.26 0.32 0.45 0.20 0.32 0.47 0.77

a

The blend compositions are designated in wt %.

PAA. Notably, the fraction of hydrogen-bonded pyridine in the various blends strongly depended on the dispersion of the PAA homopolymer or PAA-containing copolymers in PS-b-P2VP. The PAA homopolymer was completely solubilized in the P2VP microdomains formed by PS-b-P2VP, thereby allowing all of the PAA chains to form hydrogen bonds with P2VP chains. This differed from the PAA-containing copolymers, in which the covalent bonds in PS-PAA (i.e., PS-b-PAA and PS-gPAA) restricted their uniform dispersion in the P2VP microdomains. The dispersion states of PAA-containing copolymers in PS-b-P2VP are discussed in a later section. Figure 2 displays the SAXS patterns of neat PS385-b-P2VP419 and its blends with PS-g-PAA. The SAXS pattern of the neat PS385-b-P2VP419 exhibited multiple diffraction peaks at q = qmi, where qm is the q value at the maximum peak intensity and i is the order of diffraction, with the qmi/qm1 ratio equal to 1:2:3:4:5:6:7; this indicated the formation of a lamellar microdomain morphology composed of PS and P2VP lamellae. The relatively weak intensity of even-order peaks resulted from the comparable thicknesses of the alternating lamellae, causing the form factor minimum from the lamellae to occur at the even-order peak positions of the lamellar structure. The TEM image in Figure 3a confirms the morphology of PS385-bP2VP419; the dark areas are composed of selectively strained P2VP lamellar microdomains, whereas the bright regions consist of PS lamellar microdomains. When PS-g-PAA was added to PS385-b-P2VP419, the SAXS patterns of the blends still exhibited multiple peaks with integer qmi/qm1 ratios, which suggested that the blends retained a lamellar structure. The first-order peak position shifted slightly to the lower q with an increase in the PS-g-PAA content. According to Bragg’s law, the first-order peak position

Figure 3. TEM micrographs of (a) PS385-b-P2VP419 and (b) 60:40 PS385-b-P2VP419/PS-g-PAA blends. The blend compositions are designated in wt %.

is associated with the domain size (D) of the lamellar phases according to the expression D = 2π/qm1. The calculated domain size increased monotonically from 46.5 nm for the neat PS385-b-P2VP419 to 50.7, 52.4, 53.7, and 55.6 nm for the blends containing 10, 20, 30, and 40% PS-g-PAA, respectively. Figure 3b presents the TEM image of the 60:40 PS385-bP2VP419/PS-g-PAA blend. Addition of a high amount of PS-gPAA in PS385-b-P2VP419 caused distortion of lamellae and created defects in the morphology of the blend. These defects were presumably caused by the packing constraint of the branched-chain architecture of PS-g-PAA that disturbed the ordered phases of the blend. In Figure 3b, the area of the bright regions is marginally larger than that of the dark regions, which was in contrast to the nearly equal areas of the two microdomains for the neat PS385-b-P2VP419. This suggests that the addition of PS-g-PAA provided greater contribution to the volume fraction of the PS microdomains than that of the P2VP microdomains. The SAXS patterns of neat PS721-b-P2VP633 and its blends with PS-g-PAA are shown in Figure 4. The SAXS profiles of neat PS721-b-P2VP633 and the 90:10 PS721-b-P2VP633/PS-gPAA blend exhibited several distinct peaks with integer qmi/qm1 ratios, and the higher-order peaks were easily identified, indicating that a highly ordered lamellar structure was formed. However, when the PS-g-PAA content increased to 20% or D


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higher, only three broad peaks with considerably low intensities were observed, indicating that the ordering of the blends became poor. Based on the relative peak positions, the morphologies of the blends containing high concentrations of PS-g-PAA were considered lamellar. Structural features of these blends were characterized through TEM, as shown in Figure 5. The neat PS721-b-P2VP633 formed well-organized alternating lamellae with nearly equal thicknesses because of the nearly equal volume fractions of the PS and P2VP microdomains. When 10% PS-g-PAA was incorporated into PS721-b-P2VP633, the blend exhibited a lamellar structure with some dislocation of microdomains. The area of the PS microdomains appeared to be slightly larger than that of the P2VP microdomains. Further increasing the PS-g-PAA content to 30% resulted in short lamellae composed of P2VP microdomains surrounded by long and interconnected PS microdomains. These findings suggest that the addition of PS-g-PAA disturbed well-organized PS and P2VP chains in the respective microdomains and PS-gPAA increased the size of the PS microdomains more than that of the P2VP microdomains. This behavior is similar to that in the PS385-b-P2VP419/PS-g-PAA blends. Figure 6 displays the SAXS patterns of neat PS981-b-P2VP924 and its blends with PS-g-PAA. Similar to the PS385-b-P2VP419/ PS-g-PAA and PS721-b-P2VP633/PS-g-PAA blends, the nearly symmetric PS981-b-P2VP924 microphase-separated into a lamellar structure, as displayed in the TEM image (Figure

7a). Addition of 10 or 20% PS-g-PAA did not alter the microphase-separated morphology but caused a discrepancy in the individual domain thicknesses where the PS microdomain was marginally thicker than the P2VP microdomain (Figure 7b). When the PS-g-PAA concentration increased to 30%, the SAXS patterns exhibited peaks with the qmi/qm1 ratio of 1:31/2:121/2, indicating that the morphology of the blends transformed into a hexagonally packed cylindrical structure. As shown in Figure 7c, the cylindrical structure was composed of P2VP cylinders, whereas the PS microdomains formed a continuous phase. The order−order transition suggested that the addition of PS-g-PAA caused a huge mismatch between the volumes of PS and P2VP microdomains and PS-g-PAA increased the volume of PS microdomains more effectively than it did the volume of P2VP microdomains. Figure 8 shows the microdomain size of various PS-b-P2VP/ PS-g-PAA blends analyzed using the one-dimensional correlation function. We used TEM to determine whether the thinner domain size would be assigned to the PS or P2VP microdomains. When PS-g-PAA was added to PS-b-P2VP, the P2VP microdomain size was nearly unchanged, whereas the PS microdomain size increased with an increase in the PS-g-PAA content regardless of the molecular weight of PS-b-P2VP. By comparison, the incorporation of PS-b-PAA in PS-b-P2VP caused a reduction of both the PS and P2VP microdomain sizes (Figure S1). The changes in the PS and P2VP

Figure 5. TEM micrographs of (a) PS721-b-P2VP633, (b) 90:10 PS721-b-P2VP633/PS-g-PAA, and (c) 70:30 PS721-b-P2VP633/PS-g-PAA blends. The blend compositions are designated in wt %. E


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Figure 7. TEM micrographs of (a) PS981-b-P2VP924, (b) 90:10 PS981-b-P2VP924/PS-g-PAA, and (c) 60:40 PS981-b-P2VP924/PS-g-PAA blends. The blend compositions are designated in wt %.

Figure 8. Changes in microdomain sizes in various blends with respect to the PS-g-PAA content obtained by a one-dimensional correlation function: (a) PS385-b-P2VP419/PS-g-PAA, (b) PS721-b-P2VP633/PS-g-PAA, and (c) PS981-b-P2VP924/PS-g-PAA.

into PS microdomains (model 2), PS-g-PAA segregated into P2VP microdomains (model 3), and PS-g-PAA macrophaseseparated from the PS-b-P2VP microdomains (model 4). We can easily exclude model 4 because neither SAXS nor TEM demonstrated features supporting macrophase separation. In model 3, the intermolecular hydrogen-bonding interaction between P2VP and PAA chains could act as a driving force to induce the segregation of PS-g-PAA into the P2VP microdomains. However, sequestering PS-g-PAA in the P2VP microdomains could cause substantial expansion of the P2VP microdomains, resulting in a larger P2VP microdomain than the PS microdomain. This contradicts the TEM results. Moreover, the segregation of PS-g-PAA into the P2VP

microdomain sizes with the addition of PS-b-PAA were expected because the expansion of the average distance between chemical junction points caused by the cosurfactant behavior led to the compression of microdomain sizes. However, the explanation of changes in the microdomain sizes in PS-b-P2VP/PS-b-PAA blends could not be applied to those in PS-b-P2VP/PS-g-PAA blends. The most crucial parameter influencing the phase behavior of the PS-b-P2VP/PS-g-PAA blends is the location of minority PS-g-PAA in the microphases formed by majority PS-b-P2VP. The dispersion state of PS-g-PAA can be categorized into four possible scenarios (Figure 9): PS-g-PAA localized at the PS-bP2VP microdomain interface (model 1), PS-g-PAA segregated F


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Figure 9. Schematics depicting possible dispersions of PS-g-PAA in lamellar microdomain morphology formed by PS-b-P2VP: (a) model 1, PS-gPAA localized at the PS-b-P2VP interface; (b) model 2, PS-g-PAA segregated into PS microdomains formed by PS-b-P2VP; (c) model 3, PS-g-PAA segregated into P2VP microdomains formed by PS-b-P2VP; (d) model 4, PS-g-PAA macrophase-separated from lamellar microdomain morphology.

systems, PS-b-P2VP were highly immiscible with degrees of segregation (NχPS/P2VP) of 169, 284, and 400 for PS385-bP2VP419, PS721-b-P2VP633, and PS981-b-P2VP924, respectively, where N is the degree of polymerization of block copolymers and χPS/P2VP is the segmental interaction parameter of PS and P2VP, assumed to be 0.21.47 By contrast, the degree of segregation of PS-g-PAA was 49 with χPS/PAA of 0.22, calculated using the solubility parameter expression.48 The smaller segregation power of PS-g-PAA than PS-b-P2VP indicates that the localization of PS-g-PAA at the PS-b-P2VP interface is energetically favorable. The positioning of PS-g-PAA at the PSb-P2VP interface resembled the self-assembly of two block copolymers with the same molecular components but different molecular weights into ordered microdomains, which is consistent with the cosurfactant effect.15,20,21,23,24 Satisfying the cosurfactant criterion, the two PS blocks in PS-g-PAA and PS-b-P2VP co-organized into common PS microdomains. By contrast, the PAA and P2VP blocks microphase-separated into other common microdomains through hydrogen-bonding interaction. The chemical junction points of PS-g-PAA and PS-b-P2VP were located at the common interface between the PS and P2VP/PAA microdomains. The model 1 assumptions allow for hydrogen bonding between the P2VP and PAA chains, which was observed from the FTIR spectra. Moreover, the addition of the branched-chain architecture of PS-g-PAA causes a large discrepancy between the radii of gyration of the PS chains and the PAA chains parallel to the microdomain interface. This discrepancy can alter the interfacial curvature between the PS and P2VP microdomains from a flat twodimensional interfacial plane to concave curvature toward the P2VP/PAA microdomains. This in turn induces an order−

microdomains indicates that the fraction of hydrogen-bonded pyridine should increase monotonically with the addition of PS-g-PAA. This disagrees with the FTIR results, in which a nearly constant fraction of hydrogen-bonded pyridine was obtained when the PS-g-PAA content was high. Therefore, model 3 cannot be used to explain the dispersion of PS-g-PAA in PS-b-P2VP. The nonlocalization of PS-g-PAA in the P2VP microdomains was presumably caused by the repulsive interaction between the long PS main chain in PS-g-PAA and P2VP chains in PS-b-P2VP. The intermolecular hydrogenbonding interaction between the short, grafted PAA chains and P2VP chains was unable to compensate for the huge repulsive interaction. In model 2, PS-g-PAA is assumed to be completely segregated into the PS microdomains formed by PS-b-P2VP. We can expect a substantial expansion of the PS microdomains. This is consistent with the TEM results and analysis of the one-dimensional correlation function, including the larger PS microdomain than the P2VP microdomain with the addition of PS-g-PAA and occurrence of an order−order transition, which formed P2VP cylinders surrounded by continuous PS phases. However, the partition of PS-g-PAA in the PS microdomains indicated that no hydrogen bonds were formed between the PAA and P2VP chains. This did not concur with the nearly 20% hydrogen-bonded pyridine observed through FTIR spectroscopy. The only possible model is the localization of PS-g-PAA at the PS-b-P2VP interface (model 1). As suggested in the literature, graft copolymers act as surfactants, which migrate to the interface between two immiscible homopolymers, resulting in a substantial reduction of the interfacial tension.41−46 In our G


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Macromolecules

This is consistent with a finding that an A-g-B graft copolymer with the A chain as the backbone tended to disperse into the homopolymer A phase domains in immiscible homopolymer A/homopolymer B blends when the A-g-B graft copolymer at the A/B interface was saturated.43 Similar behavior was observed when an A-g-B graft copolymer was incorporated into immiscible homopolymer A/homopolymer C blends, in which the grafted B chains in A-g-B had attractive interactions with the homopolymer C.43 The dispersion of PS-g-PAA in the PS microdomains cannot contribute to hydrogen-bonding formation and the fraction of hydrogen-bonded pyridine nearly levels off. This concurs with the FTIR spectroscopy results. The segregation of PS-g-PAA in the PS microdomains enables the system both to gain translational entropy and to reduce the elastic free energy of stretching in contrast to the localization of PS-g-PAA at the PS-b-P2VP interface. These contributions surpass the enthalpic penalty, which results from the repulsive interaction between the PS chains in PS-b-P2VP and PAA chains in PS-g-PAA when PS-g-PAA is segregated into the PS microdomains formed by PS-b-P2VP. Furthermore, regarding the changes in microdomain sizes, the cosurfactant behavior assumed in model 1 tends to reduce the size of both PS and P2VP microdomains through the expansion of the chemical junction points of PS-b-P2VP along the interface. However, PS-g-PAA segregation in the PS microdomains offsets the decrease in the PS microdomain size and further translates into a substantial increase in PS microdomain size. The change in the microdomain size of PS-b-P2VP with the addition of PS-gPAA is in accordance with a combination of model 1 and model 2 assumptions. The coexistence of graft copolymers at the interface and one of preferred phases was also observed in highly immiscible homopolymer A/homopolymer B blends mixed with a graft copolymer.41−44 The maximum interfacial coverage of block copolymers (∑*) as a compatibilizer at the interface of immiscible polymer blends can be estimated by the following equation49

order transition from lamellae to cylinders, which is consistent with the morphological transition upon addition of PS-g-PAA. The model 1 assumption seems to agree with our experimental data. However, if low-molecular-weight PS-gPAA was located at the high-molecular-weight PS-b-P2VP interface, PS-g-PAA would enlarge the average distance between the chemical junctions of PS-b-P2VP along the interface, resulting in the expansion of the lamellar microdomains parallel to the interface. Simultaneously, the lamellar microdomains normal to the interface must be compressed to satisfy the incompressibility of the microdomains. In our SAXS profiles of all of the blends, regardless of the molecular weight of PS-b-P2VP, the total domain size of the blends increased with the addition of PS-g-PAA. Additionally, the PS microdomain size increased, whereas the P2VP microdomain size was nearly unchanged on addition of PS-g-PAA. These changes in the domain size violated the principles of cosurfactant behavior. Thus, model 1 could not fully express the scenarios that occurred in the PS-b-P2VP/PS-g-PAA blends. None of the aforementioned single models could be used to describe the phase behavior of the PS-b-P2VP/PS-g-PAA blends. This suggests that the dispersion of PS-g-PAA is not unified and a combination of at least two models may be required to explain the phase behavior of the PS-b-P2VP/PS-gPAA blends. We propose that PS-g-PAA is localized at both the PS-b-P2VP interface (model 1) and PS microdomains formed by PS-b-P2VP (model 2). When PS-g-PAA is added to PS-bP2VP, PS-g-PAA is localized at the PS-b-P2VP interface to screen the repulsive interaction between the PS and P2VP microdomains. The localization of PS-g-PAA at the PS-b-P2VP interface allows the hydrogen-bonding formation between PAA and P2VP. Therefore, the fraction of hydrogen-bonded pyridine increased. However, the organization of PS-g-PAA with a branched-chain architecture at the PS-b-P2VP interface causes chain stretching of PS-b-P2VP, especially of the PS chains (Figure 10a). Therefore, the amount of PS-g-PAA that can localize at the PS-b-P2VP interface is limited. When the PS-g-PAA at the PS-b-P2VP interface is saturated, the additional PS-g-PAA is segregated into the PS microdomains.

∑ * = 2.06N −0.39

(6)

The maximum interfacial coverage of graft copolymers as a compatibilizer is reported to be approximately 1.7 times smaller than that of block copolymers with the same copolymer composition and molecular weight.50,51 According to these expressions, the maximum interfacial coverage values of PS-g-PAA and PS-b-PAA were 0.15 and 0.28 chain/nm2, respectively. On the other hand, the interfacial coverage of copolymers (∑) at the interface of PS-b-P2VP in the strong segregation regime is approximated to the interfacial area per junction (σ) as follows22 1 = σ = [2{[(1 − nSV )NSA,PS + nSV NSV,PS]vPS ∑ + (1 − nSV )NSA,PAAvPAA + nSV NSV,P2VPvP2VP}] /D

(7)

Here, NSA,PS and NSA,PAA are the degrees of polymerization of the PS and PAA chains in PS-PAA copolymers, respectively, and NSV,PS and NSV,P2VP are the degrees of polymerization of the PS and P2VP chains in PS-b-P2VP, respectively. Finally, vPS, vPAA, and vP2VP are the volumes of a styrene, an acrylic acid, and a 2-vinylpyridine monomer unit, respectively, which are calculated as follows

Figure 10. Comparison of packing of PS-g-PAA with different chain architectures at the PS-b-P2VP interface: (a) PS-g-PAA composed of one PS main chain and two PAA grafted chains and (b) PS-g-PAA composed of one PS main chain and one PAA grafted chain. H


Article

Macromolecules vi =

Mi ρi NA

A previous study demonstrated that when highly asymmetric PS-g-PAA (Mn of PS and PAA of 11 500 and 2300 g/mol, respectively) was incorporated into symmetric PS-b-P2VP (Mn of PS and P2VP of 40 500 and 40 000 g/mol, respectively), the intermolecular hydrogen-bonding interaction between PAA and P2VP chains induced the co-ordering of PS-b-P2VP and PS-g-PAA in a lamellar structure with chemical junctions of the two copolymers located at a common interface.34 Consequently, the domain size of lamellar microdomains decreased monotonically with an increase in the PS-g-PAA content. Notably, the PS-g-PAA graft copolymer used in the literature was composed of a PS main chain grafted with only one short PAA chain; this differs from the two grafted PAA chains in PSg-PAA used in this study. The packing of PS-g-PAA with more grafted chains at the PS-b-P2VP interface caused higher stretching free energy than that from packing with fewer grafted chains (Figure 10). Therefore, the critical amount of PS-g-PAA with two grafted PAA chains that could be localized at the PS-b-P2VP interface was less than that of PS-g-PAA with one grafted PAA chain; this is similar to immiscible polymer blends mixed with a graft copolymer.41,42 As a result, in the blends containing PS-g-PAA with one grafted PAA chain, almost all PS-g-PAA chains were localized at the PS-b-P2VP interface. By contrast, in the blends containing PS-g-PAA with two grafted PAA chains, the PS-g-PAA chains were partly localized at the PS-b-P2VP interface and partly segregated into the PS microdomains. The different dispersion states of PS-gPAA resulted in an opposite change in the domain size when PS-g-PAA was added to PS-b-P2VP. Another study suggested that when highly asymmetric PS-gPAA (Mn of PS and PAA of 20 000 and 2300 g/mol, respectively) was incorporated into symmetric polystyreneblock-poly(ethylene oxide) (PS-b-PEO, Mn of PS and PEO of 21 500 and 20 000 g/mol, respectively), in which the molecular weights of the PS chains in the two copolymers were nearly identical, PS-g-PAA was preferentially located at the PS-b-PEO interface.35 The partition of PS-g-PAA in the PS microdomains formed by PS-b-PEO was restricted possibly because the molecular weight of the PS chains in PS-b-PEO was close to that of the PS chains in PS-g-PAA. In this case, the incorporation of PS-g-PAA in the PS microdomains caused the PS chains in PS-b-PEO to stretch considerably; therefore, the model 2 assumption might not be feasible. This behavior is similar to the phase behavior of blends composed of a block copolymer (A-b-B) and a homopolymer (A) in which macrophase separation of blends occurs when the molecular weight of the homopolymer is higher than that of the chemically identical block in the block copolymer.3,4 These results suggest that the number of grafted chains plays a key role in the dispersion of graft copolymers and the resulting microphase-separated morphologies of the A-b-B/A-g-C blends. Similarly, Ye et al. developed the random phase approximation method and real-space self-consistent field theory to study the phase behavior of blends comprising an A-g-B graft copolymer and homopolymer A.52 At a given blend composition, the microphase separation regions decreased with an increase in the number of grafted chains. However, the spinodal of macrophase separation was independent of the number of grafted chains. Based on the results of the self-assembly of the PS-b-P2VP/ PS-g-PAA blends, we summarized the resulting morphologies in a phase diagram (Figure 11). This phase diagram was expressed as functions of the molecular weight ratio of PS-b-

(8)

where vi is the volume of monomer i, ρi is the density of polymer i, Mi is the molar mass of monomer i, and NA is Avogadro’s number. By assuming ρPS, ρPAA, and ρP2VP to be 1.05, 1.41, and 1.14 g/cm3, respectively, we obtained vPS = 0.165 nm3, vPAA = 0.085 nm3, and vP2VP = 0.153 nm3. The number fraction of PS-b-P2VP (nSV) is expressed as22 nSV =

wSV M n,SV wSV M n,SV

+

1 − wSV M n,SA

(9)

where wSV and wSA are the weight fractions of the PS-b-P2VP and PS-PAA copolymers, respectively, and Mn,SV and Mn,SA are their respective number-averaged molecular weights. For the PS385-b-P2VP419/PS-g-PAA blends as an example, the ∑ values in the blends containing 10, 20, 30, and 40% PS-g-PAA were calculated to be 0.25, 0.32, 0.38, and 0.46 chain/nm2, respectively. All of these values were larger than the interfacial coverage of PS-g-PAA at saturation (0.15 chain/nm2), which suggested that only some PS-g-PAA could localize at the PS385b-P2VP419 interface. By contrast, the ∑ values in the blends containing 10, 20, 30, and 40% PS-b-PAA were calculated to be 0.24, 0.28, 0.32, and 0.34 chain/nm2, respectively. These values were close to the interfacial coverage of PS-b-PAA at saturation (0.28 chain/nm2), which indicated that nearly all PS-b-PAA chains were positioned at the PS385-b-P2VP419 interface. Therefore, the fraction of hydrogen-bonded pyridine did not exhibit a maximum in the PS-b-P2VP/PS-b-PAA blends, as shown in Table 2. Figure S2 compares the SAXS patterns of the PS385-bP2VP419/PS-g-PAA blend and PS981-b-P2VP924/PS-g-PAA blend with 20% PS-g-PAA before and after thermal annealing. For both blends, thermal annealing resulted in neither shifting of the first-order peak position nor development of a new morphology but increased the intensity of diffraction peaks. Therefore, we can conclude that the microphase-separated morphology of the PS-b-P2VP/PS-g-PAA blends was developed through the following steps. PS-b-P2VP and PS-g-PAA were dissolved in THF, which is a good solvent for both copolymers. THF molecules screened the intermolecular hydrogen-bonding interaction between P2VP and PAA chains. Thus, PS-b-P2VP and PS-g-PAA exhibited an unperturbed random-coiled conformation in the dilute solution. The solution was then cast on a substrate, and the solvent was evaporated. During solvent evaporation, the segregation power of both copolymers increased, which caused PS-b-P2VP to form a microphase-separated morphology. PS-g-PAA, which had a small degree of segregation, remained disordered and, subsequently, was segregated into the PS-b-P2VP microdomain interface with the assistance of hydrogen-bonding interaction to reduce the interfacial tension. When the interfacial coverage of PS-g-PAA at the interface was at saturation, the residual PS-g-PAA was segregated into the PS microdomains formed by PS-b-P2VP. The PS-g-PAA graft copolymers were then trapped at the interface and the preferred microdomains. Further thermal treatment removed grain boundaries and defects, resulting in highly ordered arrays of microphases with the same PAA chemical potential in all microdomains and interfaces. I


Article

Macromolecules

dispersities are needed to self-assemble into highly ordered morphologies. However, the literature has shown that wellordered morphologies can still be achieved in copolymers with wide molecular weight distributions.53−56 In polydisperse block copolymers, shorter chains preferred to localize at the interface between two microdomains.53 Simultaneously, the tails of longer chains were squeezed to maintain the segment density in the microdomains at a constant value, leading to an increase in the domain size. Furthermore, high polydispersities resulted in the shifting of order−order54−56 and order− disorder55,56 transition temperatures. Similar to the phase behavior of polydisperse block copolymers, in the polydisperse PS-b-P2VP/PS-g-PAA blends, we presume that shorter chains in PS-b-P2VP and PS-g-PAA would localize at the microdomain interface because shorter chains exhibit higher conformational entropy at the interface than longer chains. However, the polydispersity effect on the phase behavior of PS-b-P2VP/PS-g-PAA blends would be augmented because of the branched molecular structure of graft copolymers, leading to more complex phase behavior.

Figure 11. Phase behavior of PS-b-P2VP/PS-g-PAA blends as functions of copolymer molecular weight ratio and PS-g-PAA concentration.

P2VP to PS-g-PAA (R) and the amount of PS-g-PAA in the blends. In all of the blend systems, PS-g-PAA coexisted at the interface and PS microdomains of PS-b-P2VP, as discussed in the preceding section. When R < 8, all of the blends microphase-separated into a lamellar structure. Some PS-gPAA chains, which were located at the microdomain interface, provided a large asymmetry in radii of gyrations parallel to the interface (Rg∥) between the PS main chain, (Rg∥,PS)PS‑g‑PAA, and the PAA side chains, (Rg∥,PAA)PS‑g‑PAA, which caused a change in the interfacial curvature of PS-b-P2VP from zero mean curvature to concave curvature toward the P2VP/PAA microdomain (Figure 10a). The remaining PS-g-PAA chains, which were sequestered in the PS microdomains formed by PS-b-P2VP, presumably self-assembled into micelles with PS in the corona and PAA in the core to screen the repulsive interaction between the PAA chains in PS-g-PAA and PS chains in PS-b-P2VP. Because the size of micelles and radii of gyration of PS in PS-b-P2VP were comparable, micelles would be expelled to the PS microdomain center, creating a scenario of “dry brush”. The dry brush behavior expanded the PS microdomain size normal to the interface but could not effectively alter the interfacial curvature of PS-b-P2VP. Consequently, the domain size of the blends increased with the addition of PS-g-PAA and the morphology of the blends remained lamellar. This behavior was similar to the phase behavior of A-b-B/A blends where the molecular weight of homopolymer A was comparable to that of the A block in the A-b-B block copolymer.3,4 By contrast, when R > 8, similar to the phase behavior in the blends with R < 8, some PS-g-PAA was localized at the PS-b-P2VP interface and the remaining PSg-PAA was solubilized in the PS microdomains formed by PSb-P2VP. Because the radius of gyration of the PS chains in PSb-P2VP was much larger than the size of micelles composed of PS-g-PAA, PS-g-PAA was able to uniformly swell the PS microdomains, exhibiting a “wet brush” behavior. The wet brush behavior effectively increased the interfacial curvature of PS-b-P2VP, which amplified the asymmetry effect caused by the mismatch between (Rg∥,PS)PS‑g‑PAA and (Rg∥,PAA)PS‑g‑PAA on the interfacial curvature of PS-b-P2VP. Consequently, the morphology of PS-b-P2VP/PS-g-PAA blends transformed into a cylindrical structure when the content of PS-g-PAA reached 30%. One concern of the PS-b-P2VP/PS-g-PAA blends is how the polydispersity affects their phase behavior. Intuitively, copolymers with well-defined molecular weights and small poly-

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CONCLUSIONS We demonstrated the phase behavior of nearly symmetric PSb-P2VP and highly asymmetric PS-g-PAA blends with hydrogen bonds between the P2VP and PAA chains. In these blends, the relatively small-molecular-weight PS-g-PAA acted as a compatibilizer to reduce the interfacial free energy of PS-b-P2VP. The localization of PS-g-PAA at the PS-b-P2VP interface induced the co-ordering of PS-g-PAA and PS-b-P2VP with the two PS chains microphase-separated into the PS microdomains, whereas the hydrogen-bonded PAA and P2VP microphase-separated into the other microdomains. The chemical junction points of PS-g-PAA and PS-b-P2VP shared a common interface in a manner similar to binary block copolymer blends. Although the cosurfactant behavior reduced the interfacial free energy of the blends, the unique chain architecture of PS-g-PAA caused a substantial increase in the stretching free energy of PS-b-P2VP. Consequently, interfacial coverage of PS-g-PAA reached saturation. Residual PS-g-PAA was segregated into the PS microdomains formed by PS-bP2VP to regain the translational entropy and decrease the stretching free energy. Furthermore, the resulting morphology of the blends was a result of the asymmetry effect caused by the chain architecture of PS-g-PAA at the PS-b-P2VP interface and PS-g-PAA dispersion in the PS microdomains. When the molecular weight ratio of PS-b-P2VP to PS-g-PAA was small, PS-g-PAA could not swell the PS microdomains and the dry brush behavior did not effectively alter the interfacial curvature. By contrast, when the molecular ratio between PS-b-P2VP and PS-g-PAA was large, PS-g-PAA dispersed uniformly in the PS microdomains, creating the a wet brush scenario. The wet brush behavior amplified the asymmetry effect caused by PS-gPAA on the interfacial curvature, which resulted in the morphological transition from lamellae to cylinders.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00184. Changes in microdomain sizes of PS385-b-P2VP419/PS-bPAA blends with respect to PS-b-PAA content; SAXS J


Article

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patterns of PS-b-P2VP/PS-g-PAA blends before and after thermal annealing (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +886-6-2757575 ext. 62647. Fax: +886-6-2344496. ORCID

Chieh-Tsung Lo: 0000-0002-5031-1945 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Ministry of Science and Technology in Taiwan under Grant Nos. 101-2628-E-006001 and 105-2628-E-006-009-MY3.

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Chain Architecture and Hydrogen Bonding Induced Co-Ordering and

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