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Letter Cite This: ACS Macro Lett. 2018, 7, 822−827

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Polymerization-Induced Nanostructural Transitions Driven by In Situ Polymer Grafting Everett S. Zofchak,†,‡ Jacob A. LaNasa,† Wenwen Mei,† and Robert J. Hickey*,†,§ †

Department of Materials Science and Engineering, ‡Department of Chemical Engineering, and §Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States

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S Supporting Information *

ABSTRACT: Polymerization-induced structural transitions have gained attention recently due to the ease of creating and modifying nanostructured materials with controlled morphologies and length scales. Here, we show that order−order and disorder−order nanostructural transitions are possible using in situ polymer grafting from the diblock polymer, poly(styrene)-block-poly(butadiene). In our approach, we are able to control the resulting nanostructure (lamellar, hexagonally packed cylinders, and disordered spheres) by changing the initial block polymer/monomer ratio. The nanostructural transition occurs by a grafting from mechanism in which poly(styrene) chains are initiated from the poly(butadiene) block via the creation of an allylic radical, which increases the overall molecular weight and the poly(styrene) volume fraction. The work presented here highlights how the chemical process of converting standard linear diblock copolymers to grafted block polymers drives interesting and controllable polymerization-induced morphology transitions.

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conditions.8,11 Although there has been significant progress in the area of polymerization-induced structural transitions, there are still limitations, especially for controlling order−order nanostructural transitions and the chain architecture. Molecular architecture is an important parameter in controlling the resulting nanoscale morphologies of block polymers.2,24 Nonlinear block polymers with architectures such as graft,25,26 bottlebrush,27,28 and miktoarm star29,30 exhibit fascinating phase behavior that is dependent on the location of grafts and degree of grafting, in addition to the degree of polymerization (N), Flory−Huggins interaction parameter (χ), and polymer block volume fraction (f).1,2 In most studies of nonlinear block polymer phase behavior, polymerization methods resulting in well-defined polymers have been the focus. Less established is taking advantage of in situ polymer grafting to control the final morphology of the material. The most common example of in situ polymer grafting is in the production of high-impact poly(styrene) (HIPS).31−33 The morphology of HIPS consists of complex poly(butadiene) (PBD) droplets grafted with poly(styrene) (PS) embedded within a PS matrix. The process of making HIPS involves the polymerization of styrene in the presence of PBD, which results in PS chains grafted from PBD during the styrene polymerization.31−33 HIPS is a perfect example how polymer grafting during polymerization leads to intricate phase behavior.

lock polymer materials exhibit well-defined morphologies with tunable length scales that are controlled by the polymer architecture and the choice of monomer chemistry.1,2 The ability to precisely control the necessary parameters using polymer chemistry for creating the desired nanostructures for specific technologies has led to their uses as separation membranes,3 polymer electrolytes,4,5 and photonic crystals.6 The final morphology dictates the material performance for specific applications, and is directly related to the processing conditions.7 For the most part, the correlation between linear block polymer architecture and the self-assembled morphology is well-established and has led to high-performance materials.7 On the other hand, in situ methods that simultaneously initiate polymerization and drive nanoscale organization to fabricate desired morphologies for end uses are only starting to be fully exploited.8−10 Colloidal and bulk nanostructured materials fabricated via polymerization-induced structural transitions take advantage of the microphase separation thermodynamics of block polymers while using controlled polymerization methods to tailor the polymer chemistry and molecular structure. In the two most common methods, polymerization-induced self-assembly (PISA)9−19 and polymerization-induced microphase separation (PIMS),8,20−23 the starting reagents, macro chain-transfer agent (macro-CTA), monomer, and solvent, are initially well solubilized, creating a homogeneous mixture. During the macro-CTA chain-extension process, the growing polymer becomes incompatible with the initial polymer segment and/or the solvent, leading to the formation of nanoscale domains. Depending on the application, structures ranging from polymersomes to disordered continuous nanochannels are obtainable by tailoring monomer chemistry and polymerization © XXXX American Chemical Society

Received: May 14, 2018 Accepted: June 20, 2018

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DOI: 10.1021/acsmacrolett.8b00378 ACS Macro Lett. 2018, 7, 822−827

Letter

ACS Macro Letters Here, we present a method using nitroxide-mediated radical polymerization (NMP) to create polymeric nanostructures using polymerization-induced nanostructural transitions in which polymer chains are grafted from one polymer block of a diblock copolymer during polymerization (Scheme 1). Scheme 1. Nanostructural Transition from Lamellar-toHexagonally-Packed Cylinders During In Situ Polymer Grafting

Differing from previously published PISA and PIMS examples, in our approach, styrene is blended with a lamellar-forming diblock copolymer, poly(styrene)-block-poly(butadiene) (PSPBD). After polymerization of the styrene, the PS-PBD converts from a standard linear diblock copolymer to a grafted block polymer consisting of poly(styrene)-block-[poly(butadiene)-graf t-poly(styrene)] (PS-PBD-g-PS). The nitroxide, 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl (4-hydroxy-TEMPO), used in this work prevents the free-radical polymerization of styrene, therefore, allowing one to control the propagation of the PS chains. The resulting morphology of the materials after polymerization of PS is controlled by changing the initial diblock copolymer/monomer ratio. The work presented here is the first report in which a controlled polymerization method is used to convert standard linear diblock copolymers to grafted block polymers, driving interesting nanostructural transitions. In a typical experiment, PS-PBD, styrene, 4-hydroxyTEMPO, and benzoyl peroxide (BPO), are added together and stirred to create a homogeneous mixture. The PS-PBD diblock copolymer was synthesized by living anionic polymerization.34 The number average molecular weight (Mn), volume fraction of PS (f PS), dispersity (Đ), and 1,4 versus 1,2PBD microstructural content for the PS-PBD were 27.5 kg/mol, 0.58, 1.03, and 0.94 1,2PBD, respectively (Supporting Information). The reaction mixture was then added to a quartz capillary tube and sealed with epoxy to prevent styrene evaporation. The room temperature small-angle X-ray scattering (SAXS) pattern for a PS-PBD volume fraction of 60% (ϕPS‑PBD = 60%) in styrene sample before polymerization, which forms a lamellar morphology (q/q* = 1, 3, where q* is the primary scattering peak), is shown on the left-hand panel of Figure 1a. To induce styrene polymerization, the capillary filled with the PS-PBD/styrene reaction mixture was thermally initiated using microwave heating (100 W, 125 °C, for 3 h). The conditions used in the polymerization of PS were modified from a previously published microwave-assisted PS synthesis using NMP.35 Microwave heating was chosen to

Figure 1. Room temperature SAXS patterns for the polymerizationinduced morphology transitions in PS-PBD/styrene blends. (a) Lamellar-to-hexagonal (ϕPS‑PBD = 60%) and (b) disordered-tohexagonal (ϕPS‑PBD = 40%) transitions occur on polymerization of styrene. The red arrow indicates that the transition after polymerization of styrene.

induce the polymerization of styrene because microwave heating will uniformly heat the sample, which is hypothesized to prevent disruption of the nanostructured morphology due to thermal gradients generated in conventional heating. After 3 h, the microwave heating was stopped, and the reaction mixture was cooled to room temperature. As seen in on the right-hand panel of Figure 1a, the room temperature SAXS pattern drastically changes from the initial lamellar morphology to a pattern that is indexed as hexagonal (q/q* = √1, √3, √4, √7, √12). When the PS-PBD volume fraction is decreased, ϕPS‑PBD = 40%, the blend morphology is disordered before polymerization, indicating that all of the reagents are homogeneously mixed. (There are likely concentration fluctuations, which is expected in binary diblock copolymer and neutral solvent blends36; Figure 1b.) After polymerization, the room temperature SAXS pattern indicates that the mixture transformed into a hexagonal morphology (q/q* = √1, √3, √7, right-hand side of Figure 1b). The disorder-to-hexagonal nanostructural transition suggests that a more complicated mechanism is at play, as simply segregating the homopolymer PS that potentially forms during the polymerization from the PBD domains would not induce order. For example, if the same PSPBD diblock copolymer is blended with a PS homopolymer (Mn = 6.1 kg/mol and Đ = 1.03) of similar molecular characteristics of the PS synthesized using microwave heating 823

DOI: 10.1021/acsmacrolett.8b00378 ACS Macro Lett. 2018, 7, 822−827

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ACS Macro Letters

ally, two different thermally activated radical generators were used in the polymerization, BPO and 2,2′-azobis(2-methylpropionitrile) (AIBN), as both are known to exhibit different grafting mechanisms.37−39 BPO will abstract an allylic hydrogen from both 1,4PBD and 1,2PBD units, yet the reactivity of the radials are drastically different. The radical formed via hydrogen abstraction is located on a secondary or a tertiary carbon for the 1,4PBD or the 1,2PBD units, respectively.38,39 The allylic radical on the 1,4PBD unit will initiate the polymerization of reactive monomers like styrene, but the allylic radical on the 1,2PBD unit will not.38,39 It is unlikely for AIBN to abstract a hydrogen due to resonance stability.40 The most likely grafting mechanism for AIBN is the addition of a propagating PS chain grafting to the PBD vinyl group.37−39 Figure 2b shows the SEC traces for the polymerization of styrene in the presence of 1,2PBD using a 40% 1,2PBD volume fraction in styrene (ϕ1,2PBD). The 1,2PBD homopolymer used in the polymerization was synthesized using living anionic polymerization (Mn = 20.3 kg/mol and Đ = 1.04) using conditions that favored the 1,2 microstructure (90%) to mirror the 1,2PBD content in the PS-PBD diblock copolymer (Supporting Information).34 As seen in Figure 2b, when the 1,2PBD, styrene, 4-hydroxy-TEMPO, and BPO mixture is reacted using microwave heating (100 W, 125 °C, for 3 h), a large molecular weight species elutes at earlier times, which is similar to Figure 2a (there is some 1,2PBD and PS homopolymers present as well). Interestingly, when AIBN is used as the radical generator, the SEC trace (purple) is significantly different as compared to the BPO radical generator. In the case of AIBN, only a minor high molecular weight shoulder is seen in addition to a larger fraction of homopolymer PS, as compared to the BPO sample. The SEC trace of the AIBN sample suggests that there is a slight increase in the 1,2PBD homopolymer molecular weight, indicating a small amount of 1,2PBD-g-PS graft copolymer is formed. The SEC traces in Figure 2a,b suggest that during the styrene polymerization using BPO, in either the presence of 1,2PBD and PS-PBD polymers, that the formation of PS graft on the PBD backbone is more efficient than AIBN. From the work shown here, and previously published work related to graft copolymerization of PBD polymers using BPO, we predict that the majority of the PS chains graft from the PBD backbone via the creation of an allylic radial.37−39 The proposed PS grafting mechanism from the PS-PBD diblock copolymer, which leads to the polymerization-induced nanostructural transition, is described in Scheme 2. When the temperature of the reaction mixture reaches 125 °C, the BPO will split into two radicals that are able to abstract an allylic hydrogen from the 1,4PBD units, forming an allylic radical. The allylic radial will then initiate the polymerization of PS from the backbone, increasing the overall molecular weight of the PS-PBD-g-PS block polymer and the f PS. We propose that the allylic radical is generated as a result of an allylic hydrogen abstraction from the 1,4PBD unit, and not the 1,2PBD unit because the abstraction of a hydrogen from the 1,2PBD tertiary carbon leads to a stable radical.39 Although a grafting from mechanism is unexpected for the 1,2PBD unit, two additional grafting mechanisms are possible: (1) addition of a propagating PS chain to a PBD vinyl group and (2) attack of primary radicals to the PBD vinyl group.39,41 More in-depth mechanistic studies are needed to fully determine which

at the same volume fraction (ϕPS‑PBD = 40%), a microemulsion structure forms, as confirmed using SAXS (Supporting Information). To better understand the polymerization-induced nanostructural transitions seen in Figure 1, size-exclusion chromatography (SEC) was used to analyze the products after the microwave heating of the ϕPS‑PBD = 60% sample (Figure 2a). As seen in Figure 2a, the SEC trace of the ϕPS‑PBD = 60% after polymerization (blue trace) contains a large molecular weight species that elutes at an earlier time as compared to the PS homopolymer synthesized using identical microwave heating conditions (green trace) and the neat PSPBD diblock copolymer (red trace). In addition to the large molecular weight species in the ϕPS‑PBD = 60% after polymerization, there is a slight shift to earlier elution times for the PS-PBD diblock along with a small amount to PS homopolymer as indicated from the slight increase in intensity where the PS homopolymer would be expected to elute. We hypothesize that the high molecular weight species in the SEC trace in Figure 2a of the ϕPS‑PBD = 60% after polymerization is the formation of a PS-PBD-g-PS grafted block polymer. To test the hypothesis that PS-PBD-g-PS grafted block polymer forms during the polymerization, and to gain further insight into the possible formation of the PS-PBD-g-PS polymer, styrene was polymerized in the presence of a 1,2PBD homopolymer using similar microwave-assisted polymerization conditions for PS-PBD (Figure 2b). Addition-

Figure 2. SEC traces of the homopolymers, diblock copolymer, and grafted block polymers. (a) Comparison of PS homopolymer (green trace), PS-PBD diblock copolymer (red trace), and PS-PBD/styrene mixture (ϕPS‑PBD = 60%) after polymerization (blue trace). (b) Analysis of 1,2PBD homopolymer (red trace) after polymerization of styrene using BPO (blue trace) and AIBN (purple trace). 824

DOI: 10.1021/acsmacrolett.8b00378 ACS Macro Lett. 2018, 7, 822−827

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ACS Macro Letters Scheme 2. Hypothesized In Situ Polymer Grafting From Mechanism

grafting process is more favorable under different reaction conditions. The nanostructural transitions shown in Figure 1 are a result of PS grafts from the PBD backbone. In both the ϕPS‑PBD = 60% and 40% samples, there is an increase in the molecular weight, and as a result, f PS, which is driving the phase transition (Figure 1). Exactly how the graft density influences the resulting morphology is an open question because the number of PS grafts from the 1,4PBD and 1,2PBD units are not easily discernible. Additionally, the entire sample is expected to be highly swollen with unreacted styrene (styrene acts as a neutral solvent for both PS and PBD domains) as the polymerization goes to ∼70% completion. Therefore, to fully understand the phase behavior of the PS-PBD-g-PS block polymer, we removed all of the unreacted styrene and annealed the samples at 125 °C under vacuum for 15 h, and then cooled to room temperature (Figure 3). By changing the initial PS-PBD/styrene ratio, we are able to access lamellar, hexagonally packed cylinders, and disordered spheres when all of the unreacted styrene is removed, as confirmed by transmission electron microscopy (TEM) and SAXS (Figure 3). As seen in Figure 3, as the PS-PBD/styrene ratio is decreased, asymmetric morphologies form. We predict that as the PBD/styrene ratio is decreased, the PS graft length, and as a result f PS, will increase, driving the morphology transition. Furthermore, we can deduce that the minority phase in the ϕPS‑PBD = 40% and 20% samples is indeed the PBD block as it is the darker phase in the TEM images (OsO4 will only stain the vinyl group of the PBD block, and it will not react with the PS block42). Synthetic25,26 and computational43−47 work related to graft copolymers has shown that polymer grafting plays an important role in the nanoscale phase behavior of these materials. In previous synthetic work, the graft copolymers had well-defined graft parameters, which is drastically different

Figure 3. TEM images and corresponding SAXS patterns for the products produced using polymerization-induced nanostructural transitions for (a, b) ϕPS‑PBD = 60%, (c, d) ϕPS‑PBD = 40%, and (e, f) ϕPS‑PBD = 20%. All samples were annealed at 125 °C under vacuum for 15 h and cooled to room temperature. The TEM samples were microtomed and stained with OsO4. The scale bars of the inset TEM micrographs are 100 nm.

from the work presented here because the exact number, location, and molecular weight of the PS grafts from the PBD backbone is difficult to determine experimentally. One parameter that we can explicitly determine is the change in the volume fraction of PS by measuring the increase in sample mass after the polymerization and the removal of unreacted styrene. If we assume that the majority of the reacted styrene results only in PS grafts from the PBD block after styrene removal, we would expect an increase of the PS content in PSPBD-g-PS to be f PS ≈ 0.67, 0.76, and 0.87 for the ϕPS‑PBD = 60%, 40%, and 20% samples, respectively. Further studies are needed to determine exactly how many grafts there are per PBD block, and if the 1,4 versus 1,2PBD microstructural content will lead to changes in the number of grafts, and as a result changes in the morphology transitions of the system. We have developed a polymerization-induced nanoscale structural transition method using microwave heating to graft polymer chains from linear diblock copolymers. The grafting leads to interesting order−order and disorder−order morphology transitions. In the process, a linear diblock copolymer is blended with monomer that acts as a neutral solvent, and swells both domains equally. During the polymerization of the monomer, the PBD block in the PS-PBD diblock copolymer are grafted with PS chains. The polymer grafting on the PBD backbone is hypothesized to be due to the initiation of styrene monomer from an allylic radial that forms on the 1,4PBD units of the PS-PBD diblock copolymer. The grafting process leads 825

DOI: 10.1021/acsmacrolett.8b00378 ACS Macro Lett. 2018, 7, 822−827

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ACS Macro Letters

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to an increase in the PS volume fraction of the grafted copolymer, which drives the morphology transitions. As the styrene content is increased, we are able to access hexagonally packed cylinders and disordered spheres where PBD is the minority phase.

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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/acsmacrolett.8b00378.

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Experimental details and supporting figures (PDF).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Robert J. Hickey: 0000-0001-6808-7411 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the PPG/MRI Undergraduate Fellowship. All of the SAXS and TEM measurements were taken at the Materials Characterization Lab (MCL) in the Materials Research Institute (MRI) at Penn State University. We are grateful to Missy Hazen for help with microtoming and staining the polymer samples for TEM, and Dr. Chao Lang for taking the TEM images. The authors thank Profs. Scott Milner and Mike Hickner for insightful discussions.

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DOI: 10.1021/acsmacrolett.8b00378 ACS Macro Lett. 2018, 7, 822−827

Letter

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DOI: 10.1021/acsmacrolett.8b00378 ACS Macro Lett. 2018, 7, 822−827