Article pubs.acs.org/Macromolecules
Intrinsically Hierarchical Nanoporous Polymers via PolymerizationInduced Microphase Separation Michael B. Larsen,† J. David Van Horn,‡ Fei Wu,‡ and Marc A. Hillmyer*,† †
Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, United States Department of Chemistry, University of Missouri, Kansas City, Missouri 64110, United States
‡
S Supporting Information *
ABSTRACT: The synthesis of microporous polymers generally requires postpolymerization modification via hypercross-linking to trap the polymeric network in a state with high void volume. An alternative approach utilizes rigid, sterically demanding monomers to inhibit efficient packing, thus leading to a high degree of free volume between polymer side groups and main chains. Herein we combine polymers of intrinsic microporosity with polymerization-induced microphase separation (PIMS), a versatile methodology for the synthesis of nanostructured materials that can be rendered mesoporous. Copolymerization of various styrenic monomers with divinylbenzene in the presence of a poly(lactide) terminated with a chain-transfer agent (PLA-CTA) results in kinetic trapping of a microphaseseparated state. Subsequent etching of PLA provides a bicontinuous mesoporous network. Using equilibrium and kinetic nitrogen sorption experiments as well as positron annihilation lifetime spectroscopy (PALS), we demonstrate that variations in the steric characteristics of the styrenic monomer impart the network with microporosity, resulting in hierarchically (meso and micro) porous materials. Additionally, structure−property relationships of the styrenic monomer with total surface area and pore volume indicate that the glass transition temperature (Tg) of the corresponding styrenic homopolymers provides a reasonable measure of the steric interactions and resultant microporosity in these systems. Finally, PALS provides insight into micro- and mesoscopic void volume differences between porous monoliths containing either tert-butyl or TMS-modified styrenic monomers compared to the parent, unmodified styrene.
■
INTRODUCTION Hierarchically nanoporous materials exhibiting two or more distinct pore size distributions have found utility in diverse applications1,2 including catalysis,3,4 biomaterials,5 and energy conversion and storage.6 The defining characteristic of these materials is the marriage of small pore sizes that give high surface area and larger pores that allow for rapid mass transport. Provided there is sufficient connectivity between the two different pore size distributions, the resulting hierarchical materials combine the attributes afforded by a combination of micropores (50 nm). The majority of known hierarchically nanoporous materials are zeolites,7,8 silicates,9 and carbons.10 However, the versatile synthesis and processing techniques afforded by polymeric materials have led to increased interest in the development of hierarchically porous polymers. Although a variety of methods have been utilized to generate meso-11−14 or macropores15−17 in these materials, microporosity is generally derived from hypercross-linking of aromatic side groups via Friedel−Crafts reactions in solvent-swollen networks.18 In this method, the swollen state of the polymer network is effectively locked, thus arresting chain relaxation and increasing the free volume in the material.19 At a high extent of hyper-cross-linking, this free volume can percolate throughout the network, forming a microporous structure with a high internal surface area. © XXXX American Chemical Society
An alternative approach to microporosity in polymeric materials is to maximize inter- and intrachain free volume using highly rigid and sterically encumbered molecular structures that cannot efficiently pack.20,21 In general, most polymers possess enough conformational freedom to maximize energetically favorable cohesive interactions by assuming optimal packing; by designing structures incapable of such relaxation, percolating free volume can result in significant levels of microporosity. These polymers of intrinsic microporosity differ from hyper-cross-linked polymer networks in that high free volume is inherent to their structures, and no secondary reaction or processing step is necessary to form micropores.22 Thus, we envisioned that integrating this concept with a method for producing meso- or macroporous materials could lead to a more efficient means of synthesizing hierarchically porous polymers. Block polymers are a versatile class of materials capable of selfassembly into a number morphologies on nanometer length scales due to covalent linkage of thermodynamically incompatible segments.23,24 Through judicious choice of the chemical structure and functionality of each component, blocks can be selectively degraded and the self-assembled materials can be rendered mesoporous.25,26 To maximize accessibility and Received: April 18, 2017 Revised: May 18, 2017
A
DOI: 10.1021/acs.macromol.7b00808 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
similar approach in PIMS would result in hierarchical materials after removal of the PLA phase. In this study, we report the use of PIMS to synthesize hierarchically porous polymers exhibiting intrinsic microporosity. We demonstrate that the use of sterically demanding styrenic monomers in the synthesis of PIMS-derived monoliths results in materials that exhibit intrinsic microporosity, while the mesoscale morphology of these materials is unaffected by the identity of the styrenic component. A direct probe of free volume and porosity via positron annihilation lifetime spectroscopy (PALS) provides additional evidence of hierarchical porosity in these monoliths. Finally, an empirical correlation between BET surface area and glass transition temperature (Tg) of the styrenic homopolymers suggests that Tg is an overall appropriate measure of relative steric interactions in these differing polymer systems.
connectivity to the envisioned microporous polymer component, the mesopores should percolate throughout the entire material; thus, we required a method that produces a bicontinuous structure with percolating mesopores in a microporous matrix. Previous work has demonstrated that polymerization-induced microphase separation (PIMS) is a facile route to polymer monoliths possessing percolating mesopores upon removal of a sacrificial block.27−29 In this method, poly(lactide) terminated with a chain-transfer agent (PLA-CTA) is dissolved in a mixture of styrene and divinylbenzene (S/DVB), forming an initially homogeneous reaction solution. Upon thermal initiation, the S/ DVB copolymer grows from the PLA-CTA via reversible addition−fragmentation chain transfer polymerization (RAFT). Cross-linking due to the presence of DVB kinetically arrests the microstructure prior to the adoption of the thermodynamically preferred nanostructures. As a result, the structure becomes locked in a bicontinuous architecture consisting of interpenetrating domains of PLA and cross-linked polystyrene. Subsequent basic etching of the PLA domain results in fully percolating mesopores in a cross-linked P(S/DVB) monolith. The adaptation of PIMS methodology to hierarchically structured systems, including macro- and mesoporous materials via addition of a nonreactive macromolecular porogen29 and micro- and mesoporous powders via Friedel−Crafts hyper-crosslinking,28 has demonstrated versatility of PIMS toward additives and secondary reactions. We hypothesized that the rigid structure of the cross-linked phase combined with the use of bulky styrenic monomers could more easily lead to intrinsic microporosity, precluding the need for postpolymerization modification to obtain a hierarchically porous material (Figure 1). Previous reports by Turner and Svec utilized styrenic monomers substituted at the 4-position with sterically demanding esters to endow intrinsic microporosity in rigid poly(styrene-alt-maleimide) beads,30,31 and we reasoned that a
■
EXPERIMENTAL SECTION
Materials. Unless otherwise noted, all reagents were purchased from commercial sources and used as received. Styrene (99%), 4-tertbutylstyrene (93%), 4-methylstyrene (99%), and divinylbenzene (technical grade, 80%) were passed through basic alumina to remove inhibitor prior to use. ±-Lactide was provided by Ortec, Inc., and recrystallized twice from toluene and stored under N2 at −20 °C. AIBN was recrystallized from methanol and dried under reduced pressure. (S)1-Dodecyl-(S′)-(α,α′-dimethyl-α″-acetic acid) trithiocarbonate was synthesized as previously reported.32 Tetrahydrofuran and dichloromethane were purified on a home-built solvent purification system. Instrumentation. NMR spectra were recorded on a Bruker Avance III HD 500 spectrometer. Chemical shifts are reported in δ units, expressed in ppm downfield from tetramethylsilane using the residual protio-solvent as an internal standard (CDCl3, 1H: 7.26 ppm). Size exclusion chromatography (SEC) was performed in THF (25 °C, 1 mL/ min) on an Agilent Infinity 1260 HPLC system equipped with three Waters Styragel HR columns, a Wyatt HELEOS-II multiangle laser light scattering detector, and a Wyatt Optilab T-rEX differential refractive index detector. FTIR spectra were recorded on a Bruker Alpha Platinum ATR spectrometer. Differential scanning calorimetry (DSC) analyses were performed on a TA Instruments Discovery DSC using hermetically sealed aluminum T-zero pans. Scans were conducted under a nitrogen atmosphere at a heating rate of 10 °C/min. Small-angle X-ray scattering (SAXS) profiles were collected at the Advanced Photon Source at Argonne National Laboratories using the Sector 5-ID-D beamline, which is maintained by the DuPont−Northwestern−Dow Collaborative Access Team. Scattering experiments were performed using X-rays of wavelength 0.729 or 1.378 Å, and the scattering intensity was collected on a 2D Mar CCD detector at room temperature with a sample-todetector distance of 850 cm. Intensity as a function of the wavevector, q, where q = (4π/λ) sin(θ/2) (θ is the scattering angle and λ is the X-ray wavelength), was obtained by azimuthally integrating the 2D patterns. SEM micrographs were obtained on a Hitachi S-4700 cold FEG-SEM with a working distance of 5 mm and an accelerating voltage of 3 kV. Prior to imaging, monoliths were cryo-fractured and sputter coated with ca. 2 nm of Ir or Pt. Nitrogen sorption isotherms were obtained using a Quantachrome Autosorb iQ2-MP at 77 K; prior to measurement, samples were degassed at room temperature for 20 h. Brunauer− Emmett−Teller (BET) surface areas were obtained from the adsorption branch of the isotherm from P/P0 = 0.05−0.35, total pore volume was determined at P/P0 = 0.95, and pore size distributions were estimated using a quenched solid density functional theory kernel (QSDFT) for the adsorption branch of nitrogen on carbon using a cylindrical pore model. Micropore surface area and volume were estimated using the tplot method from P/P0 = 0.20−0.50 and the carbon black thickness equation according to ASTM D6556-16.33,34 Nitrogen sorption kinetics were obtained via a constant volume dosing method in which a fixed volume of gas was dosed into a sample of monolith at 77 K and the pressure recorded every 5 s. From this, an adsorbed volume at STP was calculated. To compare samples of disparate total surface areas, the adsorbed volume of each sample at time t (Vt) was normalized to the
Figure 1. Preparation of nanoporous polymer monoliths via PIMS and subsequent basic etching of PLA. Use of small, lower Tg styrenic monomers results in mesoporosity typical of PIMS-derived monoliths (left); in contrast, sterically bulky, high-Tg styrenic monomers endow the system with intrinsic microporosity as a result of poor chain packing in the kinetically trapped state (right). B
DOI: 10.1021/acs.macromol.7b00808 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
added, and the mixture was stirred for 1 h at room temperature. A small amount of benzoic acid was added to quench the DBU catalyst, and the polymer was isolated following precipitation twice into cold methanol (12.6 g, 84%; Mn,NMR = 34 kDa, Mn,SEC = 25 kDa, Mw,SEC = 30 kDa, Đ = 1.20). (S)-1-Dodecyl-(S′)-(α,α′-dimethyl-α″-acetic acid) trithiocarbonate (976 mg, 2.68 mmol, 9.1 equiv) was dissolved in dry dichloromethane (10 mL) under an Ar atmosphere in a two-neck round-bottom flask fitted with a stopcock. Oxalyl chloride (0.30 mL, 3.57 mmol, 12.1 equiv) and a drop of DMF were added, and the solution was stirred at room temperature for 2 h, after which bubbling ceased and the volatiles were removed under reduced pressure. A solution of hydroxy-terminated PLA (10.0 g, 0.29 mmol of hydroxy end group, 1 equiv; the polymer was previously dried via azeotropic distillation of a toluene solution under reduced pressure) and triethylamine (0.50 mL, 3.57 mmol, 12.1 equiv) in dry dichloromethane (60 mL) was added to the remaining solids, and the reaction was stirred for 16 h at room temperature. The polymer was precipitated twice into cold methanol and once into hexanes and dried under reduced pressure at 60 °C (8.4 g, 84%). Full CTA functionalization was confirmed by NMR end group analysis as well as SEC analysis of linear PLA-b-PS diblock polymers synthesized using the PLA-CTA as the macroinitiator. Synthesis of PIMS-Derived Polymer Monoliths. All monoliths examined in this study were synthesized utilizing the 34 kDa PLA-CTA described above as macroinitiator. To compare properties across differing styrenic polymers, the initial volume fraction of PLA-CTA in the reaction mixture was held constant at 0.25 in all cases; the remainder consisted of mono- and difunctional styrenic monomers at a constant molar ratio of 4:1. Polymer monoliths were synthesized via dissolution of PLA-CTA in the styrenic monomers in single dram vials; in most cases, gentle heating was required for complete dissolution of the PLACTA. In the case of 4-cyclohexylstyrene and 4-trimethylsilylstyrene, a small amount of dioxane (ca. 10 vol %) was added to aid dissolution; this amount of solvent was found to minimally affect microphase separation, as evidenced by SAXS compared to monoliths that did not require dioxane addition (Figure S3). PIMS was affected via the addition of AIBN (0.4 equiv relative to CTA end group) and heating at 120 °C for 20 h. The resulting monoliths were dried under reduced pressure at 60 °C. PLA was etched in basic solution by submerging the monolith in 0.5 M NaOH solution in 40% methanol/water and heating at 70 °C for 3 days in a polypropylene bottle. Following this, the etched monoliths were rinsed with water and methanol and dried under reduced pressure. Full PLA removal was confirmed by gravimetric analysis and FTIR spectroscopy (Figures S4−S9).
equilibrium-adsorbed value (Vads). However, likely due to swelling of the polymer matrix, nitrogen uptake very slowly increased after the relatively more rapid initial uptake due to adsorption. For consistency, the Vads of each sample was set as the last collected data point, well after rapid adsorption had ceased (Figure S1). PALS measurements were performed by using a conventional fast−fast coincidence method with coincidence events compiled using a multichannel analyzer.35,36 The time resolution of the spectrometer was
