Letter Cite This: ACS Macro Lett. 2017, 6, 1232-1236
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Tricontinuous Nanostructured Polymers via Polymerization-Induced Microphase Separation Stacey A. Saba,† Bongjoon Lee,‡ and Marc A. Hillmyer*,‡ †
Department of Chemical Engineering and Materials Science and ‡Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, United States S Supporting Information *
ABSTRACT: Nanostructured tricontinuous block polymers allow for the preparation of single-component materials that combine multiple properties. We demonstrate the synthesis of a mesoporous material from the selective orthogonal etching of a microphase-separated tricontinuous block polymer precursor. Using the synthetic approach of polymerization-induced microphase separation (PIMS), divinylbenzene (DVB) is polymerized from a mixture of poly(isoprene) (PI) and poly(lactide) (PLA) macro-chain transfer agents. In the PIMS process in situ cross-linking by the DVB arrests structural coarsening, resulting in a disordered block polymer morphology that we posit exhibits three nonintersecting continuous domains. Selective etching of the PI domains by olefin cross metathesis and PLA domains by hydrolytic degradation produces a mesoporous polymer with two independent pore networks arising from the different etch mechanisms.
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compositions.15,16 Given that the incompatibility between poly(isoprene) (PI) and PLA is much greater than either polymer with poly(styrene),17 we hypothesized that blends of PLA-CTA and PI-CTA could be used to prepare tricontinuous nanostructured polymers via PIMS if macrophase separation can be avoided. More specifically, the solubility of PLA/PI in DVB, the kinetics of structure arrest via in situ cross-linking, and the thermodynamics of both phase separation of the PLA/ PI and microphase separation of the growing block polymer must be balanced. Under the right conditions, application of the PIMS process to mixtures of PLA-CTA, PI-CTA, and DVB is expected to result in a macroscopically homogeneous material with nanostructured PI and PLA domains that are independently etchable. After initial optimization experiments, we settled on the polymerization of the mixture depicted in Figure 1, which comprised PLA-CTA (25 wt %, Mn = 11 kg mol−1) and PICTA (25 wt %, Mn = 9.5 kg mol−1) dissolved in technical grade DVB (50 wt %). Polymerization of the DVB resulted in 88 wt % conversion of the liquid precursor to an insoluble product. The unreacted volatile components (comprising 12 wt %) after polymerization were identified as a mixture of unreacted DVB and ethylstyrene (present in commercial DVB) by 1H NMR spectroscopy (Figure S1). The presence of unreacted DVB is likely due to hindered diffusion of the monomer after network gelation and vitrification.16 While thermal gravimetric analysis of the monoliths postpolymerization and postdrying was not useful for determining the monolith composition (Figure S2a),
hile multiply continuous inorganic materials from templating techniques are actively being pursued,1−3 tricontinuous polymers themselves are useful in technologies requiring disparate properties in a single material. This may include multiple mechanical attributes such as high ultimate elongation, elastic modulus, and impact strength4 or a combination of mechanical rigidity, ion conductivity, and electron conductivity.5 One route to such materials relies on the self-assembly of block polymers that can adopt structures with multiple continuous independent networks.6−9 Recently, the selective etching of such materials was demonstrated and resulted in a mesoporous (pore size 2−50 nm) polymer with two distinct pore networks. 10 Alternatively, disordered tricontinuous polymers have been produced through the phase separation of polymer blends. In these cases, the morphology is comprised of one phase segregating to the interface of two interpenetrating and incompatible domains.11−13 We combine these two strategies to target a disordered and cross-linked tricontinuous block polymer with three independent nonintersecting networks, two of which can be selectively etched by orthogonal chemistries to produce a mesoporous polymer. Our group has reported the preparation of mesoporous polymers via polymerization-induced microphase separation (PIMS).14 In this route, a poly(lactide) macro-chain transfer agent (PLA-CTA) is dissolved in a multifunctional monomer mixture of styrene and divinylbenzene (DVB). The reversible addition−fragmentation chain transfer (RAFT) polymerization of the monomers induces microphase separation during incipient formation of a block polymer accompanied by in situ cross-linking that covalently fixes a disordered bicontinuous structure. Selective etching of the PLA domains results in a percolating mesopore network over a wide range of © XXXX American Chemical Society
Received: September 1, 2017 Accepted: October 17, 2017
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DOI: 10.1021/acsmacrolett.7b00677 ACS Macro Lett. 2017, 6, 1232−1236
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ACS Macro Letters
respective homopolymers, indicating relatively pure microdomains.18 At T > 100 °C the parent monolith exhibits an irreversible exotherm that we attribute to PI cross-linking (Figure S3a, top); a third Tg for the cross-linked DVB microphase was not apparent by DSC. Dynamic mechanical analysis also did not reveal a Tg for the poly(DVB) microphase due to the high degree of cross-linking (Figure S3 bottom). Further evidence of the disordered microphase-separated structure was revealed by small-angle X-ray scattering (SAXS) data that showed a broad maximum indicating overall compositional heterogeneities at the 16 nm length scale (Figure S4a). An image of the nanostructure was obtained in real space using transmission electron microscopy (TEM) (Figure 3a). Before TEM imaging, the polymer was stained with OsO4 to provide contrast. OsO4 stains the PI blocks, rendering it darkest, and leaves the PLA and PDVB unstained making these blocks the lightest in the TEM images.19 A higher magnification TEM micrograph clearly shows the disordered microphase-separated structure, and the corresponding Fourier transform of this image indicates compositional heterogeneities on the 14 nm length scale, consistent with the SAXS data (Figure S5). The tricontinuity of the polymer was established through orthogonal etching of the PI and PLA domains. Etching of PLA from disordered bicontinuous monoliths prepared via PIMS is well-established and results in a pore network that should contain carboxylic acid groups lining the pore walls based on the chain transfer agent used (Figure 1).11 The selective etching of PI from other nanostructured block polymers to produce mesoporous materials has been accomplished by ozonolyis20−23 and metathesis;24,25 however, these proesses have generally been applied to thin films. A few examples of PI etching from thicker films have been demonstrated but typically require stronger oxidation conditions including UV irradiation7 or a combination of UV and ozone.26 A metathesis etching procedure was demonstrated for the removal of polybutadiene (PB) from microphase-separated monolithic PB-b-PLA, but the trisubstituted olefins in PI render the metathesis degradation more challenging.22,27 Thus, we developed a cross-metathesis etching procedure to remove PI from bulk polymers using Grubbs’ second-generation catalyst in cyclohexane with trans-4octene as a cross metathesis reactant. PI is soluble in cyclohexane, while the PLA and PDVB blocks are insoluble, facilitating the selective removal of PI fragments without compromising the mechanical integrity of the matrix. Using this approach 82 wt % of the PI block was removed based on gravimetric analysis, possibly due to the inaccessibility of alkene units in close proximity to the pore walls, producing a pore network that should contain alkene groups lining the pore walls. FTIR spectra are consistent with the selective etching of the PLA and PI under the aforementioned conditions (Figure S6). Furthermore, selective etching of the PI block results in the disappearance of only the low temperature Tg in DSC thermograms (Figure 2b), and similarly, etching of the PLA block results in the disappearance of only the high temperature Tg (Figure 2c). Aspects of this disordered mesoporous structure, after removal of either the PI or PLA block, are visible by scanning electron microscopy (SEM) (Figure S7), and the broad peak in the SAXS profiles is consistent with coincidental length scales in the nanostructure pre- and postetching (Figure S4b−e). After etching of both the PLA and PI blocks, an irreversible exotherm is still observed in DSC
Figure 1. Preparation of tricontinuous polymers via PIMS and orthogonal degradation of the PI and PLA blocks which produces a mesoporous polymer.
differential scanning calorimetry (DSC) analysis of the assynthesized monolith showed Tg’s at −56 and 47 °C, corresponding to the PI and PLA blocks, respectively, corroborating a microphase-separated structure containing PLA and PI (Figure 2a). The Tg values are close to their
Figure 2. DSC thermograms of tricontinuous nanostructured polymers (a) before and (b−d) after etching, as indicated. The vertical dashed lines at −64 and 48 °C correspond to the Tg values of PI-CTA and PLA-CTA homopolymers, respectively. 1233
DOI: 10.1021/acsmacrolett.7b00677 ACS Macro Lett. 2017, 6, 1232−1236
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Figure 4. Nitrogen sorption isotherms of etched polymers after (a) PI only etching, (b) PLA only etching, and (c) PI then PLA etching. The corresponding BET specific surfaces areas and pore volumes are indicated. Filled circles indicate nitrogen adsorption, and empty circles indicate desorption.
Figure 3. (a) TEM micrograph of the tricontinuous cross-linked polymer after staining with OsO4. The larger length scale lighter and darker regions in the image are attributed to thickness variation across the sample. (b) SEM micrograph of the mesoporous polymer after etching both the PI and PLA blocks and coating with approximately 1 nm of Ir.
complete PI etching followed by PLA etching was consistent with independent domains as gravimetric analysis reflected the removal of the individual components. However, orthogonal etching resulted in a significant increase in porosity as determined by nitrogen sorption measurements. The BET specific surface area and pore volume are greater than the sum of the individual contributions, implying that there is some porosity arising from the PDVB matrix that is only accessible after removal of both the PI and PLA.30 Indeed, mesopore size distributions show a second population of smaller mesopores that are apparent only after etching of both PI and PLA domains (Figure S9).31 These submesostructural pores have been observed where inefficient chain packing results in additional porosity in the cross-linked PDVB matrix
thermograms, and in this case, we attribute it to partial pore collapse (Figure S3b−d).28 SEM of the mesoporous polymer post etching of both PLA and PI domains shows a high degree of porosity (Figure 3b), and after heating in the DSC the sample shows a textured surface with some retained porosity (Figure S8). This polymer has improved thermal stability compared to non-cross-linked mesoporous poly(styrene), as the putative partial pore collapse exotherm is shifted by more than 30 °C to 125 °C.28 After each etch step, nitrogen sorption analysis gave Type IVa isotherms with H2a hysteresis, which is indicative of a disordered networked pore structure (Figure 4).29 Near 1234
DOI: 10.1021/acsmacrolett.7b00677 ACS Macro Lett. 2017, 6, 1232−1236
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block.15,32 Our first attempts at etching PLA followed by PI were unsuccessful as the presence of residual base from the PLA etch likely caused premature catalyst decomposition. Instead, etching PLA with trifluoroacetic acid, which was readily removed under reduced pressure, permitted the subsequent successful metathesis etching of PI.33 Using this route, the mesoporous polymer had comparable porosity to the etching in the reverse order (Figures S9 and S10). Incredibly, due to the high degree of cross-linking in the PDVB matrix, which lacks a Tg up to 280 °C (Figure S3), approximately half of the porosity as determined by nitrogen sorption measurements (i.e., surface area and pore volume, Figure S11) is retained after heating at 280 °C for 30 min, and the mesostructure is still observed in SAXS and SEM (Figure S12). The ability to form a tricontinuous structure is a direct consequence of the high level of incompatibility between PLA and PI which promotes segregation of the PI into relatively pure percolating domains.14 This was confirmed when we attempted to prepare bicontinuous polymer monoliths of PI-bPDVB, similar to our well-established approach with PLA-bPDVB.11−13,29 Although DSC and SAXS analysis suggests that this polymer is microphase-separated (Figures S13 and S14) with a disordered morphology (Figures S15 and S16), the PI Tg was still observed after cross metathesis etching (Figure S13), and 66 wt % of the PI was removed. In terms of the PIMS process, this implies that the kinetic trapping of the PI-b-PDVB structure is occurring prior to the formation of well-segregated continuous domains of PI, likely because of the relatively small interaction parameter between PI and PS.14 However, in the presence of PLA-CTA, the PI microphase separates into continuous domains facilitating the preparation of tricontinuous samples. In summary, we have demonstrated the synthesis of a disordered tricontinuous block polymer representing a unique combination of the disordered structure of polymer blends and the nanometer-scale features of ordered self-assembled nanostructures from block polymers. Importantly, orthogonal etching of two of the blocks renders the material mesoporous, which should have two distinct pore wall functionalities, and good control over pore size is attained. DSC and SAXS provide strong evidence of the microphase-separated structure, and nitrogen sorption measurements show the increase in porosity with orthogonal etching. SEM and TEM provide further confirmation of the disordered network structure.
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ACKNOWLEDGMENTS The authors thank Dr. Yanzhao Wang, Dr. Michael Larsen, and Dr. Thomas Vidil for helpful discussions, Sujay Chopade for DMTA assistance, Dr. Hanseung Lee and Chris Frethem for SEM assistance, and Altasorb for the generous donation of lactide. This work was supported by the National Science Foundation (DMR-1609459). S.A.S. thanks the University of Minnesota (UMN) for the Louise T. Dosdall Fellowship. The Hitachi SU8320 SEM was provided by NSF MRI DMR1229263. Parts of this work were carried out in the Characterization Facility, UMN, which receives partial support from NSF through the MRSEC program. Portions of this work were performed at the DuPont-Northwestern-Dow Collaborative Access Team (DND-CAT) located at Sector 5 of the Advanced Photon Source (APS). DND-CAT is supported by Northwestern University, E.I. DuPont de Nemours & Co., and The Dow Chemical Company. This research used resources of the APS, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC0206CH11357. Data were collected using an instrument funded by the NSF under Award Number 0960140.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b00677. Experimental details and detailed characterization data (PDF)
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Letter
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Stacey A. Saba: 0000-0002-2606-2520 Marc A. Hillmyer: 0000-0001-8255-3853 Notes
The authors declare no competing financial interest. 1235
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