Nanoporous Thermosets with Percolating Pores from Block Polymers Chemically Fixed above the Order-Disorder Transition.

A lamellar diblock polymer combining a cross-linkable segment with a chemically etchable segment was cross-linked above its order-disorder temperature (TODT) to kinetically trap the morphology associated with the fluctuating disordered state. After removal of the etchable block, evaluation of the resulting porous thermoset allows for an unprecedented experimental characterization of the trapped disordered phase. Through a combination of small-angle X-ray scattering, nitrogen sorption, scanning electron microscopy, and electron tomography experiments we demonstrate that the nanoporous structure exhibits a narrow pore size distribution and a high surface to volume ratio and is bicontinuous over a large sample area. Together with the processability of the polymeric starting material, the proposed system combines attractive attributes for many advanced applications. In particular, it was used to design new composite membranes for the ultrafiltration of water.

Polymeric materials with nanoscopic interpenetrating and percolating domains that exhibit high surface to volume ratios are very useful for separation,[1] catalysis,[2] and energy technologies.[3] A rich variety of synthetic strategies to these materials have been developed with the aim of combining two discrete phases that form stable and interconnected domains.[4,5] Block polymers have been extensively used for this purpose[6,7] as they provide self-assembled nanostructures with well-defined geometries and domain sizes that can be readily controlled by tuning the molar mass and the chemical composition.[5,8] However, commonly observed equilibrium morphologies typically either require additional postsynthetic processing to align the domains (cylinders and lamellae)[9−11] or exist over a narrow range of composition (double gyroid).[12] As a result, numerous reports have outlined strategies to kinetically trap nonequilibrium, bicontinuous morphologies.[13−15] In particular, microphase separation in cross-linkable systems involving polymeric precursors (e.g., polymerization induced microphase separation, PIMS,[13] or randomly end-linked copolymer networks, RECNs[15]) has been successfully employed for the straightforward preparation of large-area samples with cocontinuous nanodomains. A fine balance between the kinetics of the phase separation process and the cross-linking reaction ensures trapping of a nonequilibrium morphology comprised of interconnected domains. The bicontinuity of these morphologies has been correlated to their global disorder.[16]

We now report a new strategy for the preparation of robust bicontinuous nanostructured materials by exploiting the well-known order–disorder transition (ODT) of block polymers.[17] Previous studies indicate that this transition is characterized by strong, thermally induced local composition fluctuations that disrupt the long-range order of the system.[18−21] As the order–disorder transition temperature (TODT) is approached from the disordered state, the fluctuations become large in amplitude and spatially correlated over short distances, resulting in the persistence of local microphase segregation despite the absence of global long-range order. Some view the ODT as a “pattern transition” resulting in only small changes in chain stretching and interfacial area, despite a loss in long-range order.[22,23] Early works have also postulated that the resulting structure is bicontinuous, with a morphology that is topologically similar to other disordered, microphase separated systems.[17,19,23] In two important experimental studies, the persistence of microphase separation above TODT was demonstrated by the direct imaging of the morphology of reactive block polymers cross-linked in the disordered state.[24,25] However, there have been very few attempts to characterize the three-dimensional connectivity of these domains or exploit this interesting state in nanotechnology with the important exception of Balsara et al., who demonstrated that the conductivity of a lamellar diblock polymer loaded with a lithium salt increases in the vicinity of TODT, most likely as a consequence of the interconnectivity of the conductive domains induced by composition fluctuations.[26] Consequently, there is a strong motivation to further investigate the morphological development in the vicinity of the TODT so that the ODT could be leveraged for the design, discovery, and development of new materials.

Our aim is to utilize the order–disorder transition of reactive block polymers to prepare mesoporous materials with well-defined and continuous three-dimensional network structures (Figure ). This approach involves heating a reactive diblock polymer above its TODT in the presence of a thermally latent initiator that can trigger the cross-linking reaction for curing temperatures Tcuring ∼ TODT, thus kinetically trapping the disordered state. The concurrent use of a chemically etchable block allows for the subsequent removal of one of the nanodomains, affording a nanoporous structure that can be easily characterized under ambient conditions. This method represents a new and versatile strategy for the design of mesoporous materials that retains all of the processability advantages of polymer-based systems. To demonstrate a potential application of this work, we prepared composite membranes for the ultrafiltration of water.

Figure 1

Strategy employed to obtain a bicontinuous nanoporous structure by chemical cross-linking of a diblock polymer in the disordered state. The chemical structures of the components are represented at the top. (a) Lamellar diblock copolymer with an etchable block (blue) and a cross-linkable block (green) containing a thermally latent cross-linking initiator. (b) This mixture is heated above its order–disorder transition temperature (TODT) to adopt a disordered bicontinuous morphology. (c) The thermolatent initiator triggers the cross-linking reaction above TODT. (d) Once the cross-linkable phase is cured (darker green), the etchable block (blue) is removed to obtain well-defined percolating nanopores.

We synthesized a block polymer containing a chemically etchable polylactide (PLA) and a cross-linkable block consisting of a statistical copolymer of styrene (S) and glycidyl methacrylate (GMA) [PLA-b-P(S-s-GMA)] (Figure , top). The synthetic route starts with a functionalized PLA precursor obtained by ring opening polymerization of ±-lactide initiated by an hydroxyl-functionalized chain transfer agent (CTA) for reversible addition–fragmentation chain transfer (RAFT) polymerization (see the Supporting Information for synthetic details and supporting characterization data). A series of near symmetric PLA-b-P(S-s-GMA) samples with controlled molar masses and mole fractions of GMA in the cross-linkable block as well as low values of dispersity (Đ ∼ 1.1) were obtained (Figures S4–S6 and Table S2). The diblocks are labeled as PLA-α-P(S-s-GMA)-β-XGMA-γ where α and β are the molar masses (in kg mol–1) of the PLA and P(S-s-GMA) blocks, respectively, and γ is the molar percentage of GMA in the P(S-s-GMA) block (XGMA). Two glass transition temperatures, Tg, were observed by differential scanning calorimetry (Tg(PLA) ∼ 55 °C and Tg(P(S-s-GMA) ∼ 70–80 °C, Figure S10, Table S3), implicating microphase separation.

Figure a shows the temperature dependence of the low-frequency dynamic elastic shear modulus (G′) for PLA-17-P(S-s-GMA)-11-XGMA-29. The TODT, 174 ± 5 °C, was identified as the onset of the discontinuous drop in G′ resulting in a liquid-like response (see the Supporting Information for details, Figure S11). The linear viscoelastic responses of the melt at low frequency (0.01 ≤ ω ≤ 100 rad s–1) for two temperatures above and below TODT are also shown in Figure a (insets). For T = 160 °C (T < TODT), the moduli obey a G′ ∼ G″ ∼ ω1/2 power law typically observed for lamellar ordering.[18] Above TODT at 190 °C the moduli scale as G′ ∼ ω1.5 and G″ ∼ ω1. The frequency dependence exponent of G′ (1.5) is indicative of composition fluctuations in disordered phases close to the ordering transition[20,27] where the longest relaxation time is shifted to lower frequencies due to the collective motion of the segregated domains in the frequency range of the experiment (G′ ∼ ω2.0 would be expected at even lower frequencies).

Figure 2

(a) Temperature dependence of the low-frequency dynamic storage modulus (G′) for PLA-17-P(S-s-GMA)-11-XGMA-29. Ramp rate = 1 °C min–1 (strain, ε = 1%, frequency, ω = 1 rad s–1). Insets: isothermal frequency sweeps (frequency, 0.01 ≤ ω ≤ 100 rad s–1) acquired at 160 and 190 °C. (b) SAXS patterns obtained for PLA-17-P(S-s-GMA)-11-XGMA-29. The sample was annealed 2 min at each temperature. The TODT was determined as 174 ± 1 °C through more precise SAXS measurements (Figure S12). For T < TODT, higher ordering peaks are marked by inverse triangles and are consistent with a lamellar morphology.

Figure b illustrates representative small-angle scattering (SAXS) data for PLA-17-P(S-s-GMA)-11-XGMA-29 upon heating from 25 to 220 °C. The SAXS pattern acquired at 25 °C exhibits a principal scattering peak q* = 0.23 nm–1 and higher order peaks 2q* and 3q* that persist up to 160 °C and are consistent with a lamellar ordering and a domain spacing d = 27 nm. Above 160 °C, the principal scattering peak broadens and a single broad reflection is evident at 180 °C. More precise SAXS measurements give TODT = 174 ± 1 °C (Figure S12), in agreement with TODT determined by rheological measurements. In a series of related block polymers we demonstrated that TODT is a decreasing function of the molar fraction of GMA in the cross-linkable block, XGMA. This trend is consistent with the expectation that inclusion of GMA in the PS block increases its solubility parameter, making it more compatible with PLA (Table S4). We also showed that the tunable nature of TODT in this new block polymer system allows for independent tuning of the cross-linking temperature and nanodomain size (Figures S13 and S14).

Benzyl triphenylphosphonium hexafluoroantimonate (BTPH, Figure top, the synthetic procedure is available in the Supporting Information, Figures S15–S17) was used to initiate the cationic polymerization of the pendant epoxide moieties.[28] BTPH was selected as the catalyst as its reaction kinetics are appropriate for the temperature window around the TODT of PLA-17-P(S-s-GMA)-11-XGMA-29 and its high efficiency allows for low catalyst loadings. A film was prepared by casting a THF solution of PLA-17-P(S-s-GMA)-11-XGMA-29 (24.7 wt %) and BTPH (0.3 wt %) (dried for 1 d at room temperature and 1 d at 60 °C). The film was annealed at 110 °C for 1 h, and complete removal of THF was confirmed gravimetrically. SEC and SAXS analysis of the initiator-loaded system indicated that the molar mass, dispersity, and morphology of the block polymer were not impacted by these low-temperature drying procedures (Figures S18 and S19), and independent 1H NMR analysis suggested that BTPH was located in both the P(S-s-GMA) and PLA blocks prior to cross-linking. Moreover, variable-temperature SAXS experiments show that the TODT of PLA-17-P(S-s-GMA)-11-XGMA-29+BTPH(0.3 wt %) (TODT = 178 °C) is only 4 °C higher than the TODT of the sample without initiator (TODT = 174 °C) (Figures S20 and S21, Table S5). The TODT of the initiator-loaded polymer (178 °C) is used as the reference TODT.

Initiator-loaded samples were then cured at various temperatures in the proximity of TODT of PLA-17-P(S-s-GMA)-11-XGMA-29+BTPH(0.3 wt %) (see detailed procedure in the Supporting Information). Samples were cured below TODT, Tcuring = 160 or 170 °C, and above TODT, Tcuring = 180, 190, 200, 210, or 220 °C. Curing times (tcuring) were chosen on the basis of estimations of the gel times (tgel) through independent rheological measurements (Figures S22 and S23, Table S6; typically we utilized tcuring > 3tgel). Thermogravimetric analysis was used to demonstrate that the polymer did not lose significant mass during curing (Figure S24), and in all cases the gel fraction of the cured samples was >90% (Table S7).

The cured samples were first characterized by SAXS experiments at room temperature. For Tcuring = 160 °C i.e., 18 °C below TODT (ΔT = Tcuring – TODT = −18 °C), the room temperature SAXS pattern exhibits a single sharp scattering peak (q*) and two higher order peaks (2q* and 3q*), indicating that a lamellar microstructure is kinetically trapped during the curing step (Figure a, solid line). At Tcuring = 190 °C i.e., 12 °C above TODT (ΔT = +12 °C), a single broad reflection peak is consistent with a microphase-separated, but disorganized, structure (Figure b, solid line). These room temperature SAXS data are consistent with the SAXS data for the initiator-free system acquired at temperatures corresponding to the curing conditions (see Figure b).

Figure 3

Characterization of the structure of the materials obtained by curing PLA-17-P(S-s-GMA)-11-XGMA-29+BTPH(0.3 wt %) (TODT = 178 °C) at two different temperatures: Tcuring = 160 °C (tcuring = 4 h) (a and b) and Tcuring = 190 °C (tcuring = 1 h) (c and d). (a) SAXS pattern acquired at room temperature for the sample cured at 160 °C before (solid line) and after (dashed line) etching in a basic solution. (b) Representative SEM image of a cryofractured surface coated with Pt (∼2 nm) for the sample cured at 160 °C after etching in a basic solution. (c, d) These panels represent the corresponding SAXS patterns and SEM image for the sample cross-linked at Tcuring = 190 °C.

For the sample cross-linked in the ordered state at 160 °C, variable temperature SAXS experiments show that the microstructure remains ordered at temperatures as high as 250 °C, i.e., higher than the TODT of the original non-cross-linked system (Figure S25). The sample cross-linked in the disordered state is unable to recover lamellar ordering post-cross-linking at temperatures below the TODT of the unreacted system. Even after thermal annealing at 120 °C (i.e., well below the original TODT and above the highest Tg of the pristine diblock) for 15 h (Figure S26) the sample did not recover the lamellar microstructure. Together, these observations confirm that the efficiency of the cross-linking reaction is sufficient to kinetically and irreversibly trap the microstructure adopted at the curing temperature. In both cases the domain spacing is only slightly affected by the curing reaction (within 8% of spacing prior to curing, Figure b and Table S8). For most other curing temperatures the SAXS results correlate well with the observations made for Tcuring = 160 °C when ΔT < 0 and for Tcuring = 190 °C when ΔT > 0 (see Figure S27 and Table S8). A notable exception is the sample cured at 180 °C, where the SAXS pattern exhibits a sharp principal scattering peak despite a curing temperature above the TODT of the non-cross-linked system. This unexpected behavior is attributed to its close proximity to the TODT (ΔT = +2 °C), and further discussion of this case is in the Supporting Information (Figure S27B).

The PLA was subsequently removed in the cured samples by immersing the monoliths in a 0.5 M methanol (40% by volume)/water solution of NaOH. Complete removal of PLA was confirmed by gravimetric analysis with a mass loss in close agreement with the weight fraction of PLA in the block polymer (Table S9), the absence of the PLA C=O vibration by IR spectroscopy (Figure S28), and the absence of a glass transition corresponding to PLA in the DSC thermogram of the etched materials (Figure S29). The removal of PLA was accompanied by a dramatic change of the SAXS pattern of the samples cross-linked below TODT (ΔT < 0) with a total loss of the scattering characteristic of the lamellar ordering, likely due to the collapsing of the nanoporous structure (Tcuring = 160 °C, ΔT = −18 °C, Figure a, dashed line, and Tcuring = 170 °C, ΔT = −8 °C, Figure S27a), consistent with previous reports of partial collapsing in nanoporous lamellar microstructure.[29,30] Conversely, for all the samples cross-linked above 180 °C, the SAXS patterns retain a broad reflection post etching consistent with a disordered structure (Tcuring = 190 °C, ΔT = +12 °C, Figure c, dashed line, and Tcuring = 200 °C, 210 °C, and 220 °C, ΔT = +22 °C, +32 °C, and +42 °C, Figure S27c–e) and the domain spacing is within 9–14% of the spacing prior to etching (Table S10). The dramatic difference in stability for the porous microstructures obtained from samples cross-linked above and below TODT is likely due to a difference in terms of the continuity of their porous network. Indeed, while the etched lamellar structures result in large 2-dimensional sheets that are only separated by voided domains and eventually collapse,[29] bicontinuous network structures (e.g., gyroid[31] or samples from PIMS,[13] RCENs,[16] and bicontinuous microemulsions[32]) are known for providing three-dimensional structures that percolate the entire sample and are stable provided that their thermal and mechanical properties are appropriate. Thus, the SAXS results for the etched material shown in Figure c are consistent with a cured sample containing bicontinuous domains: one consisting of mesopores and one of the cross-linked P(S-s-GMA) polymer.

Scanning electron microscopy (SEM) observations of cryofractured and coated (Pt, ∼2 nm) surfaces of the samples are consistent with the results obtained from SAXS experiments. In agreement with the lamellar ordering indicated by the SAXS patterns in Figure a, sheet-like and evidently collapsed layered objects are observed in Figure b for the sample cured at Tcuring = 160 °C (ΔT = −18 °C). The SEM image is also consistent with the lack of strong scattering in Figure a for the etched sample. Similar images were obtained at Tcuring = 170 °C (see Figure S30a). For the sample cured at Tcuring= 190 °C (ΔT = +15 °C) the SEM micrograph clearly reveals a nanoporous structure with interconnected pores homogeneously distributed throughout the material. Similar images were obtained for other curing temperatures above the ODT (see Figure S30c–e for Tcuring = 200, 210, and 220 °C). This structure closely resembles the morphologies of other polymeric materials with percolating nanoporous network obtained from PIMS[13] and RECNs[16] or using polymeric bicontinuous microemulsions.[32] This supports well the idea that the morphology of the disordered state of a symmetric diblock polymer is truly bicontinuous.

Transmission electron microscopy (TEM) of a microsection of porous sample cured at 190 °C is also consistent with a homogeneous disordered network of pores (Figure S32). The bicontinuity of the nanostructure was further assessed using TEM tomography.[33] A tilt series of TEM micrographs of the same sample was collected for a layer-by-layer reconstruction of the 3D structure (Figure S33 and Movie S1). The tomogram reveals a network of highly branched channels that traverse the thickness of the sample and supports the idea of a bicontinuous network of pores. A computational reconstruction of the volume also supports highly interconnected porous domains (Figure S34).

Nitrogen sorption was further used to quantitatively analyze the porosity. As expected from their collapsed lamellar structures, the samples cross-linked below TODT (ΔT < 0 °C) exhibit essentially featureless nitrogen adsorption isotherms (Figure S35a,b), indicating that the materials are not porous. For all the curing temperatures above TODT (ΔT > 0 °C), the materials produced type IV isotherms with H2 hysteresis (Figure S35c–g). This again supports the idea that the porous structures associated with composition fluctuations in the disordered state are bicontinuous. For Tcuring ranging from 190 to 220 °C (12 °C ≤ ΔT ≤ 42 °C), the pore size distributions, modeled using the adsorption branch of the isotherms and a quenched solid density functional theory kernel (QSDFT),[34] are monomodal with a sharp peak centered on 10 nm (Figure S36). This correlates well with the values of the domain spacing determined by SAXS d ∼ 22 nm and the overall volume fraction of the sacrificial PLA domains of 54% (Table S4). Nanoporous samples prepared using a block polymer with a smaller PLA block, PLA-10- P(S-s-GMA)-10-XGMA-14+BTPH(0.3 wt %) (Figures S37–S42), gave pore diameters of about 7–8 nm, indicating that the pore diameter can be controlled by the molar mass of the PLA block.

Figure represents the pore volume determined at P/P0 = 0.95 as well as the surface area estimated by a Brunauer–Emmett–Teller (BET) analysis as a function of ΔT = Tcuring – TODT, i.e., the relative distance between the curing conditions and the ODT in °C.[35] Maxima in both sets of data are reached for Tcuring = 190 °C with a total pore volume of 0.47 cm3 g–1 and an estimated surface area of 190 m2 g–1. These values are essentially the same at higher curing temperature. The material cured at 180 °C exhibits a low total pore volume of 0.22 cm3 g–1 and a surface area of 90 m2 g–1. Again, this is attributed to the close proximity to the TODT (ΔT = +2 °C, see Figures S35c and S36c).

Figure 4

Plot of the pore volume (filled squares) and the surface area (empty squares) as a function of ΔT = Tcuring – TODT, for PLA-17-P(S-s-GMA)-11-XGMA-29+BTPH(0.3 wt %) cross-linked at seven different temperatures (Tcuring = 160, 170, 180, 190, 200, 210, and 220 °C). The cured samples are subsequently etched in a basic solution and characterized by nitrogen sorption experiments. The pore volumes are determined at P/P0 = 0.95, and the surface areas are estimated on the basis of a Brunauer–Emmett–Teller (BET) analysis. Error bars represent the typical range of the data obtained from three independent measurements for the sample cross-linked at 190 °C.

The system described in this report provides nanoporous material with narrow pore size distribution while retaining all of the processability advantages associated with polymeric materials. In particular, it can be solution processed to fabricate nanoporous thin film membranes that have the potential to combine both high permeability and high size selectivity, two appealing attributes for ultrafiltration applications.[36] To demonstrate this, an 8 wt % solution of PLA-17-P(S-s-GMA)-11-XGMA-29+BTPH(0.3 wt %) in chlorobenzene was coated onto a water-filled poly(ether sulfone) (PES) membrane using a wire-wound rod.[37] The resulting film was dried and cured above TODT (Tcuring = 190 °C, Figure top, see the Supporting Information for experimental details). After selective etching of PLA, SEM imaging of the composite membrane demonstrates that the cross-linked polymer forms a ∼500 nm homogeneous layer (Figure S43) that effectively covers the pores of the PES support (Figures a and 5b). In Figure b the nanopores from the block polymer coating can clearly be seen within the larger support pores.

Figure 5

Characterization of the composite membrane obtained by coating an 8 wt % solution of PLA-17-P(S-s-GMA)-11-XGMA-29+BTPH(0.3 wt %) (TODT = 178 °C) in chlorobenzene onto a PES support membrane followed by curing at Tcuring = 190 °C and etching with a basic solution. The strategy employed for the design of the membrane and the rejection experiments is represented at the top. (a) SEM micrograph of the bare PES support membrane and (b) SEM micrograph of the nanoporous selective layer obtained after coating the PES support with PLA-17-P(S-s-GMA)-11-XGMA-29+BTPH(0.3 wt %) followed by curing at Tcuring = 190 °C and PLA etching. (c) UV–vis absorbance results for the feed solution (solid line) of fluorescent TRITC-dextran (0.5 mg mL–1, M = 150 kg mol–1, Rh ∼ 7 nm) and the filtrate (dashed line) obtained after passing the feed solution through the composite membrane. Rejection was calculated as 98% based on the ratio of the absorbance of the feed solution to the absorption of the filtrate at 521 nm. Photographs of the feed solution (1) and the filtrate (2) are included in the inset.

Pure water permeability of the composite membrane was determined to be 7 L m–2 h–1 bar–1 as compared to 3700 L m–2 h–1 bar–1 for the bare PES support (∼100 nm nominal pore size) based on a linear fit of flux vs ΔP (Figures S44 and S45b). The possibility to use the membrane for water ultrafiltration was first investigated by measuring the rejection of a solute with a hydrodynamic radius, Rh, greater than the dimensions expected for the pores of the selective layer. On the basis of the average pore radius measured by N2 sorption for porous monoliths obtained from the corresponding polymer (∼5 nm), we used a 0.5 mg mL–1 solution of TRITC-dextran (Mw = 155 kDa, Rh ∼ 7 nm), a polysaccharide labeled with a fluorescent dye.[38] UV–vis analysis of the filtrate passed through the membrane demonstrates that 98% of the solute was rejected (Figure c). The size selectivity of the membrane was further investigated by using a mixed feed of dextran standards with molar masses ranging from 5 kg mol–1 to 410 kg mol–1.[39] Aqueous SEC of the filtrate indicates that the membrane displayed a sharp molecular weight cutoff around 50 kg mol–1, i.e., Rh ∼5 nm (Figures S45 and S46). Again, this is consistent with the average pore diameter expected for the selective layer. The same polymer was also spin coated onto the PES membrane to provide a thinner selective layer (150 nm, Figure S47), resulting in a significantly higher permeability (196 L m–2 h–1 bar–1, Figure S48) and only a slight decrease of the TRITC-dextran rejection (96%, Figure S49).

In this work we demonstrate that the ODT of a diblock polymer comprised of a cross-linkable block and an etchable block can be used to design new mesoporous materials with narrow pore size distributions and high surface to volume ratios. The characterization of the porous structure provides an unprecedented set of data confirming that the morphology of the disordered state is microphase separated and bicontinuous over a large sample area. The solution processability of the proposed system was demonstrated by directly coating the cross-linkable diblock on top of a commercial PES membrane. After curing and etching, the resulting composite membrane exhibits high permeability and sharp molecular weight cutoff that are suited for the ultrafiltration of water. Together with the chemical versatility and tunability of block polymers (e.g., using triblocks and other architectures) and the variety of cross-linking reactions available in thermoset technology, this new strategy may have utility for a large spectrum of advanced applications.