Phase Behavior of Poly(2-vinylpyridine)-block-Poly(4-vinylpyridine

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Phase Behavior of Poly(2-vinylpyridine)-block-Poly(4-vinylpyridine) Copolymers Containing Gold Nanoparticles Jaeyong Lee, Jongheon Kwak, Chungryong Choi, Sung Hyun Han, and Jin Kon Kim* National Creative Research Initiative Center for Smart Block Copolymers, Department of Chemical Engineering, Pohang University of Science and Technology, Pohang, Kyungbuk 790-784, Republic of Korea S Supporting Information *

ABSTRACT: We studied the phase behavior of the poly(2-vinylpyridine)-blockpoly(4-vinylpyridine) copolymer (P24VP) containing gold nanoparticles, by rheometry, small-angle X-ray scattering, transmission electron microscopy (TEM), and differential scanning calorimetry (DSC). Although both blocks of P24VP exhibited attractive interaction to gold precursors, unusual phase behavior was observed depending on the amount of gold nanoparticles. As the amount of gold nanoparticles increased, the order-to-disorder transition temperature (TODT) of P24VP with gold nanoparticles decreased first, then increased, and finally decreased again. To explain this phenomenon, we prepared two block copolymers: polystyrene-block-poly(2vinylpyridine) copolymer (PS2VP) and polystyrene-block-poly(4-vinylpyridine) copolymer (PS4VP) containing gold nanoparticles. With increasing the amount of gold particles, the TODT of PS2VP increased continuously, whereas that of PS4VP gradually decreased. For PS4VP containing gold nanoparticles, because P4VP chains can interact with the gold nanoparticle surface, density fluctuations exist near the gold nanoparticle surfaces, which causes the TODT to decrease. On the other hand, although the pyridine ring in P2VP could be associated with the gold surface, P2VP chains become stretched due to steric hindrance arising from the ortho position of nitrogen in P2VP. The chain stretching increases the TODT. Thus, the decrease of TODT for P24VP originates from P4VP microdomains containing gold nanoparticles, while the increase of TODT is attributed to the P2VP microdomains containing gold nanoparticles. With increasing the amounts of gold nanoparticles, the contribution of P4VP microdomains containing gold nanoparticles on the TODT becomes dominant, causing the TODT to redecrease. To verify the gold nanoparticle position in both P2VP and P4VP microdomains, we performed TEM and scanning transmission electron microscopy (STEM) experiments. At lower amounts of gold nanoparticles, they are mainly located inside P4VP microdomains. With increasing the amount of the gold nanoparticles, they are distributed in both P2VP and P4VP microdomains, though the amount of gold nanoparticles in P4VP microdomains is larger than that in P2VP microdomains.

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INTRODUCTION Block copolymers have been extensively studied due to their self-assembled nature into various periodic nanoscale structures.1−10 Among many block copolymers, poly(2-vinylpyridine) (P2VP) or poly(4-vinylpyridine) (P4VP) containing block copolymers have received great attention11−14 due to their easy inclusion of nanoparticles,15 nanofibers,16 photonic band gap,17 and plasmonic materials.18 When nanoparticles are incorporated into a block copolymer, the microdomain structures change dramatically. Fahmi and co-workers19 demonstrated that polystyrene (PS)-b-P4VP copolymer (PS4VP) containing gold nanoparticles significantly affects the glass transition temperature (Tg) of P4VP block and the order−disorder transition temperature (TODT). Balazs and coworkers20,21 and Russell and co-workers22,23 investigated TODT and morphological change of block copolymers with nanoparticles. We previously reported that P2VP-b-P4VP (P24VP) showed well-ordered self-assembled nanostructures, though the only difference between P2VP and P4VP is the position of nitrogen in the pyridine ring (ortho vs para position). The Flory’s © XXXX American Chemical Society

interaction parameter (χ) of P2VP/P4VP was between PS/ P2VP and PS/P4VP.24 Chang and co-workers25 showed via Fourier transform infrared spectroscopy that the degree of the hydrogen bonding power between P4VP and phenolic resin is higher than that between P2VP and phenolic resin. We also reported that the degree of the hydrogen bonding power between P4VP and poly(4-hydroxy styrene) (PHS) is greater than that between P2VP and PHS.26 This is because the hydrogen bonding depends on the direction of two moieties (− N in the pyridine ring and −OH in the phenolic resin or PHS). Nitrogen in the ortho position in the pyridine ring of P2VP is difficult to associate with the para position of OH in PHS due to the steric hindrance. Because most metal (for instance, gold) nanoparticles have been easily incorporated into both P2VP and P4VP microdomains by the coordination between metal precursors (for example, HAuCl4 for gold) and nitrogen atom in pyridine Received: July 25, 2017 Revised: October 10, 2017

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DOI: 10.1021/acs.macromol.7b01590 Macromolecules XXXX, XXX, XXX−XXX

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group followed by the reduction,27−31 it is difficult to directly determine the coordination power difference between P2VP and P4VP toward metal precursors. However, based on previous reports on the protonation and counterion binding phenomena for poly(vinylpyridine),32−34 one can assume that P4VP shows stronger coordination power toward metal precursors than P2VP due to stronger counterion binding. Nevertheless, there has been no research on investigating metal precursor coordination when P2VP and P4VP simultaneously exist. Thus, the use of P24VP, where both P2VP and P4VP microdomains exist, would allow one to understand fully the coordination power difference between P2VP and P4VP toward metal precursors. Moreover, one can easily fabricate multimetal nanopatterns,35−37 once the amount of metal precursors in P2VP microdomains would be different from that in P4VP microdomains (namely, asymmetric sequestering of metal nanoparticles into two microdomains). Previously, Kim and co-workers38,39 found that when cadmium chloride (CdCl2) was coordinated with pyridine ring of P2VP and P4VP, the lamellar domain spacing (D) of the PS-b-P2VP copolymer (PS2VP) increased with increasing amount of CdCl2 arising from the intramolecular coordination, while D of PS4VP first slightly increased and then decreased due to intermolecular coordination. On the other hand, TODT of both PS2VP and PS4VP increases with increasing amount of CdCl2. This is due to the increased χ by the addition of metal salts (or precursors).40−43 Therefore, the change of D and TODT of P24VP with the amount of a metal precursor would not allow one to determine the coordination power difference between P2VP and P4VP toward the metal precursor. Especially, gold precursor (HAuCl4) coordinated with P4VP chains became fully reduced to gold nanoparticles at high temperatures (>170 °C),19 suggesting that the TODT of P24VP in the gold precursor state is not experimentally possible because the TODT should be higher than Tg (∼150 °C) of the P4VP block. Thus, we assume that the coordination power between P2VP and P4VP chains of P24VP toward a metal precursor can be determined by probing into distribution (or location) of metal nanoparticle in each microdomain, via transmission electron micrograph (TEM) measurements, after it is chemically or physically reduced. In this study, we investigated the phase behavior of P24VP containing gold nanoparticles by rheometry, small-angle X-ray scattering (SAXS), differential scanning calorimetry (DSC), and transmission electron micrograph (TEM). With increasing the amount of gold nanoparticles, the TODT of P24VP with gold nanoparticles decreased first, then increased, and finally decreased. The unusual behavior is strongly related to the different coordination power between P2VP and P4VP toward gold precursors. To explain this phenomenon, we prepared two block copolymers: a polystyrene-block-poly(2-vinylpyridine) copolymer (PS2VP) and a polystyrene-block-poly(4-vinylpyridine) copolymer (PS4VP) containing gold nanoparticles. With increasing the amount of gold nanoparticles, the TODT of PS2VP increased continuously, whereas that of PS4VP decreased. Therefore, for P24VP, gold precursors (and thus gold nanoparticles after the reduction) are first incorporated into P4VP microdomains, which decreases TODT. Once gold nanoparticles are sequestered into P2VP microdomains, the TODT increases. At large amounts of gold nanoparticles, the contribution of P4VP microdomains containing gold nanoparticles on the TODT becomes dominant, causing the TODT to redecrease.

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EXPERIMENTAL SECTION

Materials and Molecular Characterization. P24VPs with two different molecular weights and weight fractions of P4VP block (wP4VP) were synthesized by sequential anionic polymerization, described in detail in our previous paper.24 The number and weightaverage molecular weights, Mn and Mw, respectively, were measured by size exclusion chromatography (SEC) using P2VP standards in dimethylformamide (DMF). wP4VP was determined by 1H nuclear magnetic resonance spectroscopy (NMR). The gold precursor (HAuCl4) and all solvents were purchased from Sigma-Aldrich. PS2VP and PS4VP were purchased from Polymer Source, Inc., Canada. Molecular characteristics for all neat P24VPs, PS2VP, and PS4VP employed in this study are given in Table 1.

Table 1. Molecular Characteristics of All Block Copolymers Employed in This Study sample code

Mna (g/mol) a

wP4VPb

Mw/Mna

P24VP-C

19 400

0.3

1.12

P24VP-L PS4VP PS2VP

14 100a 13 200 18 000

0.53 0.5 0.5c

1.13 1.20 1.09

microdomains P4VP cylinders (TODT > 300 °C) lamellae (TODT > 300 °C) lamellae (TODT > 300 °C) lamellae (TODT = 159 °C)

a

Measured by SEC using P2VP standards in DMF. bMeasured by 1H NMR. cWeight fraction of P2VP block measured by 1H NMR

Preparation of the Sample. P24VP with various amounts of gold nanoparticles were prepared by adding predetermined gold precursor (HAuCl4) in THF. After THF was completely removed under vacuum, the samples were annealed at 210 °C for 12 h. During this thermal annealing, almost all of the gold precursors were reduced to gold nanoparticles, which was confirmed by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and combustion ion chromatography (CIC) (Figure S1 in the Supporting Information). This result is consistent with a previous report.19 Rheometry. Temperature dependence of the shear modulus (G′) was measured by using an Advanced Rheometrics Expansion System (ARES) with 8 mm parallel plate geometry and 1 mm gap. Samples were first annealed at 170 °C for 1 h to remove thermal history and then the temperature was increased at a heating rate of 1 °C/min. A strain amplitude of 0.05 and angular frequency (ω) of 0.1 rad/s were used. Small Angle X-ray Scattering (SAXS). SAXS measurements were performed on beamline 4C at the Pohang Light Source (Korea), where the X-ray wavelength was 0.1115 nm. The scattered X-rays were collected onto a CCD detector (Rayonix SX165, U.S.A.). The sample to detector distance was 3 m, and the sample thickness and the exposure time were 1.0 mm and 10 s, respectively. Differential Scanning Calorimeter (DSC). The glass transition temperature (Tg) of samples was measured during the second heating run with DSC (PerkinElmer DSC-7) at a rate of 10 °C/min from 60 and 180 °C. Electron Microscopy. The samples were ultrasectioned with a Leica Ultracut Microtome (EM UC6 Leica Ltd.) at room temperature with a thickness of ∼40 nm, and stained by exposure to I2 vapor for 40 min at room temperature. The micrographs were obtained at room temperature with bright field TEM (S-7600 Hitachi Ltd.) at 80 kV. To find the nanoparticle distribution in P2VP and P4VP microdomains, high-angle annular dark-field (HAADF)-STEM and bright field (BF)-STEM were also performed by JEOL JEM-2100F (with Cs Corrector on STEM) at 200 kV. Gold nanoparticles appeared bright in the HADDF-STEM image, because the annular dark field (Z-contrast) image is sensitive to the atomic number of the species.44,45 B

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Figure 1. Temperature dependence of G′ at ω = 0.1 rad/s and SAXS profile (inset) at 210 °C of P24VP-C (a) and P24VP-L (d). SAXS profiles at various temperatures of P24VP-C (b) and P24VP-L (e). Bright field TEM images stained by I2 vapor for 40 min for P24VP-C (c) and P24VP-L (f). The inset of panel c was obtained when the sample was cut perpendicular to the cylinder axis.

Figure 2. Temperature dependence of G′ (upper panel) and change of TODT with various molar ratios of gold precursor to 4VP monomer (rgold = [HAuCl4]/[4VP]) for P24VP-C (a) and P24VP-L (b).

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with θ and λ being the scattering angle and the wavelength of the incident beam, respectively; Figure 1b,e). However, as shown in the insets of Figure 1a,d, both samples do not exhibit higher order peaks except the first order peak because of similar electron densities of 2VP and 4VP. P24VP-C and P24VP-L should exhibit P4VP cylindrical and lamellar microdomains,

RESULTS AND DISCUSSION

We investigated the morphology of neat P24VP-C and P24VPL. Both samples are fully ordered up to 300 °C based on rheological properties (Figure 1a,d), and SAXS profiles (I(q) vs scattering vector q, which is defined by q = (4π/λ) sin(θ/2) C

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Figure 3. SAXS profile at 210 °C for (a) P24VP-L[0.02] and (d)P24VP-L[0.03]. SAXS profiles at various temperatures for (b) P24VP-L[0.02] and (e) P24VP-L[0.03]. Plots of 1/Im(q*) vs 1/T (●) and fwhm vs 1/T (O) for (c) P24VP-L[0.02] and (f) P24VP-L[0.03].

Figure 4. Changes of TODT and Tg of each block with rgold for (a) PS4VP, (b) PS2VP, and (c) P24VP-L. Here, rgold = [HAuCl4]/[4VP] for PS4VP and P24VP-L, while it is [HAuCl4]/[2VP] for PS2VP. (d) Schematic of the location of gold nanoparticles for P24VP-L[0.02] and P24VP-L[0.04].

Figure 2 gives the temperature dependence of G′ for P24VPC and P24VP-L with various mole ratios of gold precursor to 4VP monomer (rgold = [HAuCl4]/[4VP]). TODT was defined as the temperature where G′ drops precipitously,47,48 marked by an arrow in Figure 2a,b. For neat block copolymers, the TODT is higher than 300 °C. However, after adding a certain amount of gold nanoparticles (rgold = 0.01 for P24VP-C and rgold = 0.005 for P24VP-L), TODT is clearly observed below 300 °C. Both samples show unusual change of TODT with rgold. With increasing rgold, TODT of first decreased for rgold less than ∼0.02, then increased for 0.02 < rgold < 0.03, and finally, it decreased again with further increase of rgold. This observation is interesting, because previous reports49,50 showed the decreased TODT with increasing amounts of nanoparticles. It is noted that the measured TODTs represent equilibrium phenomena, not kinetic trapping (Figure S2 in the Supporting Information).

respectively, judging from their volume fraction. To verify this argument, we preformed TEM experiments by short staining time (40 min) of I2 vapor. It is well-known that P2VP and P4VP microdomains are well stained by I2 vapor.11,46 Very interestingly, we clearly see cylindrical and lamellar microdomains for P24VP-C and P24VP-L, respectively, in TEM images (Figure 1c,f). Since the weight fraction of P4VP in P24VP-C is 0.3, the dark region in the TEM images should correspond to the P4VP microdomains. This indicates that P4VP microdomains are heavily stained by I2 vapor compared with P2VP microdomains. Although the exact explanation is not clear, the staining of I2 of 4-position nitrogen groups would be easier than that of 2-position nitrogen groups. Once the staining time was longer than ∼1 h, we could not distinguish P2VP and P4VP microdomains because both microdomains looked dark. D

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nanoparticles. Regarding Tg of PS2VP, even if it is impossible to distinguish the Tg between PS and P2VP blocks, Tg for PS2VP increases slightly when gold nanoparticles are incorporated (Figure 4b). Thus, the increase of Tg, though small, is attributed to the favorable interaction between gold nanoparticle and P2VP block chains. The different degree of association (or interaction) between PS2VP and PS4VP to gold precursors (nanoparticles) can explain the change of Tg and TODT of P24VP containing gold nanoparticles. At smaller rgold (up to 0.02), TODT decreases, while Tg of P4VP increases. This indicates that gold nanoparticles are predominantly located in the P4VP microdomains. Moreover, Tg of P2VP does not change, similar to that of the PS block in PS4VP. The increased TODT from the increase of rgold from 0.02 to 0.03 suggests that, in addition to gold nanoparticles located in the P4VP microdomains, they are also located in the P2VP microdomains. Interestingly, the increase of Tg of P2VP is very small, because the amount of gold nanoparticles in the P2VP microdomain is not enough to much increase Tg. For rgold larger than 0.03, TODT decreases again because the gold nanoparticles in the P4VP microdomains could contribute more to TODT than those in P2VP microdomains. This suggests that the total amount of gold nanoparticle in P4VP microdomains is larger than P2VP microdomains. Since Tg of P2VP block increases and TODT redecreases, the amount of gold nanoparticles in both microdomain increases. Finally, we obtained TEM images for P24VP-L with two values of rgold. The gold nanoparticles look like dark spots in TEM images and the size of gold nanoparticles is 6 (±1) nm estimated from TEM images. At rgold = 0.02, gold nanoparticles marked by the blue box are located only inside the P4VP microdomain (Figure 5a). At rgold = 0.04, the gold nanoparticles

To determine whether the microdomain changes (or not) with the addition of gold nanoparticles, we performed SAXS experiments. The SAXS profile at 210 °C of P24VP-L[0.02] and P24VP-L[0.03] is given in Figure 3a,d, respectively. Here, the value inside the bracket represents rgold. From Figure 3a,d, we clearly observed a higher order peak at 3q*, which does not appear in neat P24VP-L. We also see a higher order peak at √7q* corresponding to hexagonally packed P4VP cylinders for P24VP-C[0.02] and P24VP-C[0.03]. This indicates that the gold nanoparticles are not equally distributed in both P2VP and P4VP microdomains.(see Figure S3 in the Supporting Information). A sharp first-order SAXS peak of P24VPL[0.02] was observed up to 220 °C and then became broad at temperatures higher than 230 °C; thus, the TODT of P24VPL[0.02] is 225 ± 5 °C. This is also determined by 1/I(q*) as well as full width at the half-maximum (fwhm) versus 1/T, as shown in Figure 3c. For P24VP-L[0.03], a sharp SAXS peak appeared up to 230 °C, and became broad at temperatures higher than 240 °C. Therefore, the TODT of P24VP-L[0.03], is defined to be 235 ± 5 °C. The TODT determined by SAXS profiles are well consistent with that measured by rheometry. Thus, the unusual change of TODT with increasing rgold as shown in Figure 2 is not related to the change of microdomains. This is because of smaller values of rgold used in this study (rgold < 0.04). Previously, Fahmi and co-workers19 showed that the cylindrical P4VP microdomains in PS4VP were transformed into lamellar microdomains at larger rgold (∼1). To explain the unusual behavior of TODT for P24VP containing gold nanoparticles, we prepared PS4VP and PS2VP containing gold nanoparticles. Figure 4 shows changes of TODT and Tg of each block with rgold for PS4VP, PS2VP, and P24VP. The determination of TODT by rheometry for PS4VP and PS2VP with various values of rgold is given in Figure S4 in the Supporting Information, and DSC thermograms of PS4VP, PS2VP, and P24VP-L with various values of rgold are shown in Figure S5. Interestingly, TODT of PS4VP gradually decreased with increasing rgold, while that of PS2VP gradually increased with increasing rgold. The difference might originate from the different position of nitrogen in the pyridine ring of P2VP and P4VP. Paulus and co-workers reported that there exist interactions between nitrogen of pyridine ring and gold surface.51 When the P4VP chains in PS4VP were surrounded by gold nanoparticles, the density fluctuation in the P4VP microdomain was expected.50 Thus, the number of unfavorable contacts of P4VP and PS chains was decreased, causing the TODT to decrease. Meanwhile, the Tg of P4VP in the presence of gold nanoparticles increased (finally leveled off) due to the restriction of P4VP chain mobility, while Tg of PS block did not change because gold nanoparticles were located only inside P4VP microdomains (Figure 4a). On the other hand, for PS2VP, even though P2VP chains have pyridine moiety, these have to be stretched to make the interaction between the ortho position nitrogen and gold precursors (finally gold nanoparticle). This chain stretching causes the increase of TODT.52 To confirm the stretching effect, we investigated the change of lamellar domain spacing (D) for PS4VP and PS2VP with the various values of rgold. (Figure S6 in the Supporting Information). D of PS4VP slightly increases with increasing rgold, while D of PS2VP increases much. For instance, at a given rgold = 0.04, the increase of D for PS4VP is 5.7% (15.6 to 16.9 nm), but for PS2VP it is 20% (14.3 to 17.0 nm). Thus, a large increase in D for PS2VP could be mainly attributed to the P2VP chain stretching in the presence of gold

Figure 5. TEM images and scheme of location of the gold nanoparticles in P24VP-L[0.02] (a,b) and P24VP-L[0.04] (c,d). The samples are stained by I2 vapor under 40 min. Gold nanoparticles located in P4VP and P2VP microdomains are drawn as blue and green boxes, respectively. The inset of TEM image is an enlarged image of the dotted box.

are located at both P4VP (marked by the blue box) and P2VP microdomains (marked by the green box) as shown in Figure 5c. However, because the amount of gold nanoparticles in P4VP microdomains is larger than that in P2VP microdomains, the TODT decreases. To confirm whether the very dark region indeed represents gold nanoparticles, we obtained HAADFSTEM and BF-STEM images (Figure S7 in the Supporting Information), where gold nanoparticles show bright spots in the HAADF-STEM image. We also found the exact correlation of the black spots in the BF-STEM image and the bright spots E

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in the HAADF-STEM image. From both TEM and STEM studies, at low amounts of gold nanoparticles, they are predominantly located inside the P4VP microdomains. However, at large amounts of gold nanoparticles, they are positioned in both P4VP and P2VP microdomains, though the amount of gold nanoparticles in P4VP microdomains is larger than that in P2VP microdomains.

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CONCLUSION

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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.7b01590. Additional experimental details and results as discussed in the text, including figures and references. (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: (+) 82-54-279-8298. ORCID

Jin Kon Kim: 0000-0002-3872-2004 Notes

The authors declare no competing financial interest.

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REFERENCES

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In this study, we investigated the coordination power difference between P2VP and P4VP with gold precursors by investigating the phase behavior of P24VP with various amounts of gold nanoparticles. With increasing the amount of gold nanoparticles, the TODT decreased, then increased, and finally decreased again. To elucidate this unusual phase behavior, we also studied the phase behavior of PS4VP and PS2VP containing gold nanoparticles. Very interestingly, with increasing amount of gold nanoparticles, TODT of PS4VP containing gold nanoparticles decreased, while TODT of PS2VP containing gold nanoparticles increased. The different behavior is due to the different degree of the interaction between the nitrogen in the pyridine ring and the gold surface. For P4VP chains, nitrogen in the pyridine ring could easily bind the gold surface and induce density fluctuation near the surface. Then, the density fluctuation prevented unfavorable interaction between PS and P4VP chains, resulting in reduced χ (and TODT). However, for P2VP chains, the association of gold nanoparticles to P2VP chains induced chain stretching due to steric hindrance from the ortho position nitrogen in pyridine. The stretched P2VP chains causes χ to increase (and TODT). We successfully found, by TEM and HAADF-STEM images, the location of gold nanoparticles in both P4VP and P2VP microdomains. At low amounts of gold nanoparticles, they are predominantly located inside the P4VP microdomains. However, with increasing the amount of gold nanoparticles, they are positioned in both P4VP and P2VP microdomains.

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ACKNOWLEDGMENTS

This work was supported by the National Creative Research Initiative Program supported by the National Research Foundation of Korea (2013R1A3A2042196; NRF). Smallangle X-ray scattering was performed at PLS beamline 4C supported by POSCO and KOSEF. F

DOI: 10.1021/acs.macromol.7b01590 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.7b01590 Macromolecules XXXX, XXX, XXX−XXX