Aspect Ratio Effect of Nanorod Surfactants on the Shape and Internal Morphology of Block Copolymer Particles This manuscript is dedicated to Prof. Jean Frechet on the occasion of his 70th birthday and to his extraordinary contributions to polymer science. Kang Hee Ku, Hyunseung Yang, Jae Man Shin, Bumjoon J. Kim Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305 701, Republic of Korea Correspondence to: B. J. Kim (E- mail: bumjoonkim@kaist.ac.kr) Received 14 June 2014; accepted 22 July 2014; published online 6 August 2014 DOI: 10.1002/pola.27333 KEYWORDS: aspect ratio; block copolymers; emulsion particle; nanocomposites; nanoparticle surfactant; nanoparticles; nanorod INTRODUCTION Controlled assembly of nanoparticles (NPs) within a polymer matrix can create novel nanostructured materials with enhanced properties. 1 4 This is of particular interest in the case of anisotropically shaped nanorods (NRs), because the collective electrical and optical properties of their organizations depend strongly on both their aspect ratio (AR) and directional assembly. 5 11 Self-assembly of block copolymers (BCPs) can direct the position of the NRs, their orientation, and three-dimensional assembly. 12 16 Despite their interesting and attractive properties, much fewer studies on the morphological behavior of BCP/NR assembly have been conducted when compared with BCP/NP system. 6,17 22 The influence of the AR values of the NRs on their positioning in the BCP domain and on their morphological behavior has been theoretically simulated; 23 25 however, it is still an open question experimentally. Although the assembly of NPs in BCP domains can be relatively well understood by the interplay of entropy loss of the chains to accommodate the NPs and enthalpic interactions between the NPs and BCPs, 26,27 other factors including the anisotropic geometry, rotational freedom, and NR NR interactions should be considered for the NR positioning within the BCP domains. The self-assembly of BCPs confined in a three-dimensional emulsion particle can produce novel structured materials that are not available in bulk. 28,29 Their morphological behavior is strongly dependent on the interfacial interactions between the BCP emulsions and the surrounding media. 30 32 Therefore, the effect of NR location on the morphological transition of BCP domains can be systematically investigated if the NRs are used as surfactants, and thus involved in tuning the interfacial properties of the BCP particles. In particular, the role of the NP surfactants should be emphasized in affecting the morphology of the BCP particles because the interfacial interactions are greatly amplified by the high surface area of the particles. Recently, controlling the position of NPs at the interface between the BCP particle and the surrounding media led to a dramatic change in the internal morphology and overall shape of the BCP particles by tuning their interfacial properties. 33 35 For example, we achieved precise positioning of size-controlled Au NPs in the BCP particles and controlled the interfacial properties at selective locations on the particle surface, generating the interesting morphological transitions of the BCP particles. 33 Herein, we exploited the particles of polystyrene-b-poly(4- vinylpyridine) (PS-b-P4VP) BCPs to investigate the AR effect of NRs on their location in the BCP domains and on the internal morphology and the overall shape of the BCP particles. The lengths (l) of the CuPt NRs were tuned from 2.6 to 40 nm with a fixed width (w) of 2.6 nm, thus producing five different AR values of 1, 3, 6, 10, and 15. To generate strong favorable interaction between the NRs and the BCP chains, 3-n-pentadecylphenol (PDP) molecules were added into the BCP solution as the linkers between the P4VP blocks and the alkyl ligands of the NRs. 36 38 The molar ratio of PDP to 4VP unit was fixed at 0.5 to produce PS 27k -b- P4VP 7k (PDP) 0.5, and the size of P4VP(PDP) domain (L) was estimated to be 18.5 nm. The molecular weight and PS fraction of PS 27k -b-p4vp 7k were 34 kg/mol and 0.79, respectively. Figure 1 illustrates the fabrication of BCP particles from the mixture of PS 27k -b-p4vp 7k (PDP) 0.5 and ARcontrolled CuPt NRs using the emulsion encapsulation and evaporation process. 31 33,39 Depending on the AR value of the CuPt NRs, the ratio of the length of the NRs over the size of the NR-hosting P4VP(PDP) domain (l/l) was varied from 0.14 to 2.16. It was found that the l/l value was a key parameter in determining the positioning of the NRs in the Additional Supporting Information may be found in the online version of this article. VC 2014 Wiley Periodicals, Inc. 188, PART A: POLYMER CHEMISTRY 2015, 53, 188 192
FIGURE 1 Schematic illustration of the fabrication of PS 27k -b-p4vp 7k (PDP) 0.5 BCP particles including AR-controlled CuPt NRs through the emulsion encapsulation and evaporation process : the locations of the NRs in the BCP particles depended strongly on their AR values, thereby determining the overall shapes of the BCP particles. BCP particles. As a result, we observed a dramatic morphological transition of the BCP particles from spherical to the convex lens-shaped particles and then back to spherical particles as the AR and l/l values increased. First, we synthesized a series of oleic acid/oleylamine-capped CuPt NRs with various ARs ranging from 1 to 15 by the thermal degradation method. 40,41 Figure 2 shows the CuPt NRs with different AR values of 1.0 (NR-1), 3.0 (NR-3), 5.9 (NR-6), 10.1 (NR-10), and 15.3 (NR-15). All of the CuPt NRs had high monodispersity in their AR value. Although the width of all of the NRs (w) was fixed at 2.6 nm, the length of the NRs (l) varied over a wide range, that is, from 2.6 to 40 nm. To characterize the weight and volume fractions of the organic ligands and the CuPt core, five different NRs were characterized with thermogravimetric analysis (TGA) and transmission electron microscopy (TEM). The detail characteristics are summarized in Supporting Information Figure S1. Figure 3 shows the SEM and TEM images of PS-b- P4VP(PDP) 0.5 particles with different AR values of NRs. After adding each of the different NRs into the PS-b-P4VP(PDP) 0.5 solution in chloroform, they were emulsified using cetyl trimethyl ammonium bromide (CTAB) as the surfactant, and the solvent was evaporated to produce BCP particles. The volume factions (/ NR ) of NRs in the BCP/NR blends were fixed at 0.037 for all NRs. It should be noted that the BCP particles exhibited both different outer shape and internal morphology, depending on the AR values. When NR-1 was added, the spherical BCP particles with cylindrical internal morphology consisting of PS coronae and P4VP(PDP) cores were formed with preferential wetting of the PS layer to the particle surface [Fig. 3(a,b)]. 37,39 In addition, for NR-1, most of the NRs were dispersed within the P4VP(PDP) domain inside the BCP particles due to the strong favorable interaction between the aliphatic chain of PDP and the alkyl ligands on the surface of the NRs. In contrast, the addition of NR-6 rods resulted in a dramatic change in the overall shape of the BCP particles from spherical to a convex lens shape with regularly ordered pores on the surfaces [Fig. 3(c,d)]. All of the particles had hexagonally arranged, dark P4VP(PDP) domains, which indicated that the standing-up cylinders were developed inside the particles. The mean diameter of the P4VP(PDP) cylinders and the center-to-center distance between the cylinders were measured as 21.3 6 1.8 nm and 42.2 6 3.2 nm, respectively. It is notable that the cylinders with pores were well oriented and highly ordered within the BCP particles. These findings agreed well with the observations in our previous works using size-controlled Au NPs. 33 However, these convex lens-shaped BCP particles could not be obtained when the longer NRs (i.e., NR-10 and NR-15) FIGURE 2 TEM images of CuPt NRs with different AR values: (a) AR 5 1.0 (NR-1); (b) AR 5 3.0 6 0.1 (NR-3); (c) AR 5 5.9 6 0.2 (NR-6); (d) AR 5 10.1 6 0.3 (NR-10); and (e) AR 5 15.3 6 0.9 (NR- 15). The scale bars are 50 nm. WWW.MATERIALSVIEWS.COM, PART A: POLYMER CHEMISTRY 2015, 53, 188 192 189
FIGURE 3 SEM and TEM images of PS 27k -b-p4vp 7k (PDP) 0.5 particles including (a and b) NR-1, (c and d) NR-6, and (e and f) NR-15; NR-1 and NR-15 generated spherical particles, whereas NR-6 produced convex lens-shaped particles. were incorporated. Instead, the BCP particles remained almost spherical even with the addition of longest NR-15 rods [Fig. 3(e,f)]. To gain a deeper insight into the effect of the ARs of the NRs on the morphology of the BCP particles, the morphologies of the BCP particles and the location of the NRs were characterized by cross-sectional TEM measurements. As shown in Figure 4(a,b), NR-1 NRs were well dispersed within the P4VP(PDP) domain of the inner BCP particles. In contrast, the NR-6 NRs were strongly segregated at the surface of the BCP particles, which was also evidenced by the absence of NRs inside the particles [Fig. 4(c,d)]. It is worth noting that the NRs were in contact with P4VP domain at the BCP particle surface. When long NRs were incorporated into the P4VP(PDP) 0.5 domain, the entropic penalty associated with the stretching of BCP chains to host the long NRs became the dominant factor in determining their location when compared with the enthalpic interaction between the NRs and P4VP(PDP) chains, thus driving them to be excluded from the P4VP(PDP) 0.5 domain. In addition, when the NRs were segregated at the surfaces of the BCP particles, significant gains of enthalpic energy were expected by reducing the unfavorable interfacial interactions between the BCP emulsion/water. 27,33 In this case, both CuPt NRs and CTAB surfactants rearranged themselves at the surface of the BCP emulsion by interacting with two different domains of the PS-b-P4VP(PDP), respectively, driven by favorable interactions between the NRs and the P4VP(PDP) domain and between the CTAB and the PS domain. 35,36 This could generate a balanced interaction between the PS/P4VP domains of the BCP particles and the surrounding water during emulsion, leading to the formation of the convex-shaped BCP particles. Interestingly, for the longer NR-15 rods, the BCP particles were not changed to the convex shape; rather, their overall shape remained spherical, although all of the NR-15 rods were clearly segregated at the surface of the BCP particles, similar to the case of the NR-6 rods [Fig. 4(e,f)]. This feature can be understood by the large l/l value (5 2.16) for NR- 15. When NR-15 rods orient parallel to the tangential plane at the surface of the BCP particle, the entire portion of the NR cannot be confined within the P4VP(PDP) domain at the surface. Instead, some part of the NR should stick out to the PS domain. To confine the long NR within the P4VP(PDP) cylinder, the NR should be rotated in a direction normal to the surfaces of the BCP particle and penetrated into the P4VP domain. However, in such a case, the large energy penalty will be accompanied by (1) an increase in the stretching penalty of P4VP chains by penetration of the long NRs and (2) an increase in the interfacial energy between the BCP emulsion and water because the area at the BCP emulsion/ water occupied by NRs will be reduced. 5,24,42,43 In particular, because the interfacial tension between the BCP emulsion and the surrounding water is very large, the second contribution will be very important. Thus, NR surfactants were oriented preferentially to be parallel to the interfacial plane rather than perpendicular to it in order to maximize the area of the BCP particle/water interface occupied by the NRs. As a result, as shown in Figure 4(f), the NRs were segregated at the BCP surface and oriented parallel to the surface. However, in this case, the NR-15 rods were too long to be confined within the P4VP(PDP) domain. Thus, they were unable to induce the selective nucleation of the P4VP(PDP) cylinders at the interface of BCP emulsion/water, which is one of the critical steps for producing the convex-shaped BCP particles with highly ordered cylinders. 33 In addition, highly anisotropic NRs often tend to aggregate side-by-side due to the rod rod interactions, which could reduce the efficiency of NRs as surfactants to generate the morphological transition of the BCP particles. 6,7 190, PART A: POLYMER CHEMISTRY 2015, 53, 188 192
FIGURE 4 Normal and cross-sectional TEM images of BCP particles including (a and b) NR-1, (c and d) NR-6, (e and f) NR-15; NR- 6 and NR-15 rods were segregated at the surfaces of the BCP particles, whereas the NR-1 were dispersed within the P4VP domain of the inner BCP particles. We observed a dramatic morphological transition of the BCP particles from spherical to the convex lens-shaped particles; however, as the AR values of the NRs further increased, they became spherical again. To examine the quantitative effect of l/l on the external and internal morphology of the PS-b- P4VP particle, we summarized the morphological behavior of the BCP particles containing five different NRs, that is, NR-1, NR-3, NR-6, NR-10, and NR-15, which produced l/l ratios from 0.14 to 2.16 (Fig. 5). Only NR-3 (l/l 5 0.42) and NR-6 (l/l 5 0.83) generated anisotropic, convex lens-shaped particle driven by the positioning of the NRs on the selective location (i.e., the P4VP domain) at the surfaces of the BCP particles. The NR-1 NRs that had a very small l/l value (5 0.14) were well dispersed within the P4VP domain inside the BCP particles and did not have any effect on the shape of the BCP particles. Finally, for l/l values larger than 1 (NR- 10, l/l 5 1.42, and NR-15, l/l 5 2.16), spherical BCP particles were produced with decoration of NRs on the surface. Therefore, the key to the successful control of the shape of the BCP particles was the use of AR-controlled NRs, in which the l/l ratio was critical for precise positioning to determine their ability to function as surfactants in emulsions. In summary, we demonstrated AR-dependent positioning of NRs in the PS 27k -b-p4vp 7k (PDP) 0.5 BCP particles and investigated their effects on the external and internal morphologies of the BCP particles. At a small value of l/l (0.14), the NRs were well dispersed in the P4VP domain inside the BCP particles, and the spherical BCP particles were produced. The CuPt NRs were localized preferentially and confined at the P4VP domain on the surface of the particles only in the specific region of AR values (0.4 l/l 1.0). Consequently, a balanced interfacial interaction between two different PS/ P4VP domains of the BCP particles and water produced convex lens-shaped BCP particles. In contrast, when the length of the NRs was larger than the domain size of the P4VP cylinder (l/l 1.42), the NRs were segregated at the surface of the BCP particle but could not induce the morphological transition of the BCP particle surface. Based on our study, FIGURE 5 Summary of the morphological transition of BCP particles depending on AR values and l/l ratios. The scale bars for the TEM images are 300 nm. WWW.MATERIALSVIEWS.COM, PART A: POLYMER CHEMISTRY 2015, 53, 188 192 191
when the NRs have anisotropic geometry with two different dimensions of w and l, the greater dimension (l) is a key parameter to determine the positioning of the NRs within the BCP domains and their efficiency as surfactants. EXPERIMENTAL A full experimental procedure and the TGA data of alkyl ligand-capped CuPt NRs are given in the Supporting Information. ACKNOWLEDGMENTS This research was supported by the Korea Research Foundation Grant (2012R1A1A2A10041283, 2012M1A2A2671746) funded by the Korean Government. Authors acknowledge the KAIST- KUSTAR Research Project for the financial support. This work was also supported by Samsung Research Funding Center of Samsung Electronics (Project Number SRFC-MA1301-07). REFERENCES AND NOTES 1 M. R. Bockstaller, R. A. Mickiewicz, E. L. Thomas, Adv. Mater. 2005, 17, 1331 1349. 2 R. A. Vaia, J. F. Maguire, Chem. Mater. 2007, 19, 2736 2751. 3 M. J. A. Hore, R. J. Composto, Macromolecules 2014, 47, 875 887. 4 A. Haryono, W. H. 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