Graft copolymers, which feature polymeric side chains attached to a polymer backbone, offer greater structural complexity than traditional linear (co)polymers, with parameters such as backbone length, sidechain sequence,1–5 sidechain length,6 grafting density,7,8 and block lengths9 defining their unique bulk properties10–14 and self-assembled morphologies15–21 and enabling their application as optical materials,22,23 elastomers,24–28 drug delivery systems,29,30 biomimetics,27,31−34 and more.35–38 Nevertheless, despite enormous recent interest in their synthesis and properties, most studies of graft copolymers to date have involved either homopolymers or copolymers derived from two different polymer side chains arranged in either block or statistical sequences; the potential for combining graft polymer segments with small-molecule comonomers, leveraging only the latter to tune material properties, has been much less explored. Sheiko and coworkers reported A-B-A triblock copolymers with linear polymer A blocks and a graft copolymer B block; these structures formed very soft thermoplastic elastomers through microphase separation of their A blocks into spherical morphologies bridged by the B segment.33 Bates and coworkers reported statistical graft copolymers featuring polydimethylsiloxane (PDMS) and smaller, oligoethylene oxide (OEO) side chains; intramolecular phase separation of the PDMS and OEO segments yielded very soft, shear thinning materials with unbridged spherical morphologies.39 In a different vein, Grubbs and coworkers explored the impact of small molecule monomers as “diluents” in the formation of diblock bottlebrush copolymers, revealing how tapered and gradient sidechains can impact diblock bottlebrush assembly. While inspiring, these disparate studies do not explore the impact of comonomer composition on the thermomechanical properties and morphologies of the materials investigated, relying primarily on tuning dimensions of the grafted chains to modulate material characteristics. Moreover, these studies do not provide access to a wide range of thermomechanical properties.
Here, we introduce a simple diversity-oriented synthetic platform for rapidly exploring and discovering graft copolymer-based silicone materials with diverse, bespoke properties. Silicones are an important category of commercial polymers that find numerous applications as lubricants, elastomers, sealants, components of household appliances, dielectrics and more, and are produced on the ~2 M ton scale annually. Despite their broad utility, however, silicones suffer from low moduli and difficult-to-tune thermal transitions, unless they are densely crosslinked or filled, two strategies which can be prohibitive in terms of material (re)processing, flexibility, and optical properties (e.g., transparency). These drawbacks motivated our interest in designing a modular, efficient synthetic approach to broadly tune silicone properties using a simple copolymerization strategy. Through ring-opening metathesis polymerization (ROMP) of an exo-norbornene-terminated PDMS macromonomer combined with one of three comonomers featuring aliphatic sidechains (n-hexyl, cyclohexyl, or n-hexadecyl), a library of 92 graft copolymers was prepared (Fig. 1a). Bulk samples from this library displayed a remarkable range of properties. For example, statistical copolymers with very high PDMS weight fractions (up to fPDMS−MM = 0.98) behaved as shear-thinning, soft (E′ ~ 100 kPa) plastic crystals with body-centered cubic (BCC) spherical morphologies. By contrast, statistical copolymers with lower PDMS volume fractions (e.g., fPDMS−MM = 0.53) formed reprocessable TPs with lamellar morphologies, broad processing windows (over 160°C), and room temperature elastic moduli (E′) ranging from 10 to 150 MPa, which are greater than leading unfilled commercial crosslinked silicone elastomers (e.g., Sylgard 184 with E′ of 1.7 MPa).40 Moreover, these graft copolymer silicones can be easily reprocessed to provide new materials with equivalent mechanical performance, offering a recyclable alternative to crosslinked silicone thermosets. Remarkably, the pendant groups of the small molecule comonomers are shown to play dominant roles in defining the thermomechanical and morphological properties of these materials, which has generally been overlooked in related studies. Meanwhile, as expected, domain spacing relationships are dictated by grafting density and backbone sequence, providing easily variable and independent parameters for controlling material structure and properties. Finally, we find that statistical copolymers, which are generated in a single polymerization step, provided materials with improved thermomechanical properties compared to block copolymers of the same fPDMS−MM, which facilitates the scalability of these materials as demonstrated through decagram-scale syntheses.
Diversity-Oriented Synthesis of a Library of Graft Copolymer Silicones
PDMS-based macromonomer Nb-PDMS and small-molecule comonomers with aliphatic pendants cHex, nHex, and nHexDec were each prepared in a single step from the appropriate exo-norbornene electrophile and commercially available hydroxyl-terminated PDMS (5 kDa), hexylamine, cyclohexylamine, or hexadecylamine, respectively (Fig. 1a). Leveraging the robust nature of Ru-initiated ROMP,41 a library of 92 unique (co)polymer compositions was prepared from these 4 (macro)monomers in parallel on the 100–500 mg scale, with each ROMP reaction taking ~5 min to reach completion (Fig. 1a,b, Fig. S1-S11, Table S1-S4). To demonstrate scalability of specific members of this library, several samples that displayed interesting mechanical properties were produced on the decagram scale with no purification steps required following polymerization. Comonomers cHex, nHex, and nHexDec differ by the identity of their aliphatic pendant groups, which vary in size, conformational freedom, and self-interaction. For statistical copolymers, graft densities of 0, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 and 1.0 were explored, the first and last of which represent homopolymers of the small-molecule and PDMS (macro)monomers, respectively. Diblock copolymer analogues were synthesized at grafting densities of 0.05, 0.2, 0.5 and 0.8. Two backbone degrees of polymerization (Nbb), 50 and 150, for each composition were prepared. For clarity, all copolymers are referred to by a sequence-pendant-Nbb-grafting density formalism. For example, a statistical copolymer with cyclohexyl pendant, Nbb of 50, and grafting density of 0.4 is referred to as stat-cHex-50-0.4.
This diversity-oriented approach allows for parallel tuning of grafting density, sequence, pendant composition, and Nbb (Fig. 1b), all of which play key roles in determining polymer conformations, self-interactions, and ultimately self-assembly and bulk properties. Specifically, grafting density influences backbone and side-chain flexibilities42 and the volume fraction (f) of components. Statistical or block sequence influences domain spacing (d*), chain conformations, and entanglement molecular weight.12 Nbb determines number average molar mass (Mn) and backbone entanglements. Lastly, the comonomer pendant groups influence copolymer backbone flexibility and self-interaction. We note that side chain degree of polymerization (Nsc) was not varied in this study. Normalized size exclusion chromatography (SEC) traces for all statistical copolymers are shown in Fig. 1c and block samples in Fig. S12-S17. Copolymerization kinetics experiments1,43 revealed that each of the three small-molecule comonomers copolymerize with Nb-PDMS with similar efficiency, yielding copolymers with slightly gradient monomer distributions that cannot explain differences in material properties (Fig. S18-S26, Table S5).
Room-Temperature Self-Assembly of Silicone-Based Graft Copolymers
To investigate the bulk morphologies of these graft (co)polymers, bulk samples were prepared by drop-casting from toluene solution and drying under vacuum at room temperature. The resulting materials were subjected to small angle X-ray scattering (SAXS, Fig. 2a,b,e, Tables S1-S4, and Figs. S27-S48) and, in some cases, transmission electron microscopy (TEM, Figs. 2c,f and S49-S54). In many cases, highly ordered morphologies, driven by phase separation of PDMS and the polynorbornene/comonomer backbone, were observed. Progressing from grafting density of 0.9 to 0.05 (fPDMS−MM from ~0.995 to ~0.53 for cHex and nHex, or 0.42 for nHexDec copolymers), and a concurrent transition from bottlebrush, to comb, to linear architecture (Fig. 2a), provided an array of morphologies including disordered (DIS), lamellar (LAM), hexagonally close packed cylinder (HEX), body centered cubic spherical (BCC), and long-range-disordered worm-like cylinders (D-CYL) as suggested by SAXS (Fig. 2a). While LAM, HEX, and D-CYL morphologies of bottlebrush and graft copolymers are well-documented, highly ordered BCC morphologies are less common for densely grafted polymers.39 Notably, BCC morphologies were not accessible to any of the diblock copolymers, nor were they observed in the nHexDec series, suggesting that backbone collapse, which is enabled by reductions in grafting density and/or with smaller comonomer pendants, is necessary to achieve this morphology.
Copolymers containing cHex and nHex displayed BCC or HEX morphologies at grafting densities of 0.7–0.1 while LAM morphologies were observed for all samples at grafting density of 0.05. Notably, the formation of LAM in the nHexDec series occurs at fPDMS−MM as high as 0.86, which is attributed to the steric bulk of the long aliphatic comonomer chains and the concomitant rigidification of the polynorbornene backbone. Lastly, the morphologies of the statistical copolymers are insensitive to Nbb, which agrees well with similar results observed for statistical and Janus bottlebrush multiblock copolymers (Fig. 2a).3,4,38,44
Interestingly, samples with highly ordered BCC morphology based on SAXS (Fig. 2b) appeared lamellar when analyzed by TEM (Fig. 2c). This observation could be due to a change in the material morphology due to thin-film confinement, or it could be the result of imaging a highly ordered BCC lattice from the (200) plane. To address this question, a select BCC sample, stat-cHex-50-0.6 was studied with grazing-incidence small-angle X-ray scattering (GISAXS, Fig. 2d, Figs. S55-S60), which revealed significant surface anisotropy and confirmed the BCC morphology of the thin film (Fig S61-S62). Samples with highly ordered LAM morphology based on SAXS (e.g., Fig. 2e) required microtoming prior to TEM imaging to visualize the lamellae (Fig. 2f). GISAXS of one such sample block-nHexDec-150-0.05 showed well-ordered lamellae in plane to the surface of deposition (Fig. 2g, S63-S64).
To correlate the composition of each sample with morphological periodicity, domain spacing (d*, nm, calculated as 10*(2π/q*) where q* is the principal scattering peak in Å−1 measured by SAXS and 10 is a unit conversion factor) was plotted against grafting density for each sidechain sequence (statistical versus block), comonomer identity, and Nbb value (Figs. 3a,b). As expected,3,5,44,45 while Nbb has little impact on d* for the statistical copolymers, it has a large effect for the diblock copolymers (Fig. 3a–c) due to their tendency to assemble in side-to-side versus head-to-head orientations (Fig. 3d), respectively. The qualitatively different relationship between d* and grafting density for diblock copolymers of Nbb = 50 versus 150 can be attributed to the presence of entanglements in the latter system when grafting density is below ~0.6 (Note: at this grafting density, the PNB homopolymer segment can become entangled; the entanglement molecular weight of linear polynorbornene with n-hexyl pendants was reported elsewhere as Nbb ~ 45).46 Despite the considerable variations in morphology as a function of comonomer composition, the trends in d* vs grafting density were similar for each comonomer, demonstrating that this simple copolymerization approach can provide facile access to materials with diverse morphologies and independently tunable d* values from a simple set of (macro)monomer precursors.
Mechanical Properties of Graft Copolymer Silicones
Next, we sought to characterize the mechanical properties of select members of our graft copolymer library. First, we focused on samples that displayed the highly ordered BCC morphology, which behaved as transparent, soft, shear-thinning, viscoelastic solids despite their very high PDMS volume fractions (fPDMS−MM of ~0.98). Small-angle oscillatory shear rheology frequency (Fig. 4a) and temperature (Fig. 4b) sweeps of stat-cHex-150-0.6 demonstrated a broad viscoelastic regime with a plateau storage modulus of 48 ± 9 kPa, which is similar to many soft biological tissues and softer than most commercial silicones. Notably, stat-cHex-150-0.7, which has an fPDMS−MM only 0.01 greater than for stat-cHex-150-0.6, was disordered (D-CYL) with a ~4 orders-of-magnitude lower G′ (Fig. S65). To explain this large difference, we propose that the structure of our BCC samples most likely comprises the comonomer pendant groups and polynorbornene backbones closely packed in BCC cores embedded in an unbridged, rubbery PDMS matrix, forming plastic crystals where the lattice energy of the assembly provides physical crosslinking.47–50 To this end, a rapid transition from viscoelastic solid to liquid is observed at 75 ± 1°C, which is consistent with the order-to-disorder transition (TODT) observed by variable-temperature SAXS (Figs. 4c). This transition underlies a shift from BCC to D-CYL, and though we note that the D-CYL arrangement does have short-range order, it is the loss of long-range order, as reflected by a morphology change and peak broadening, that causes us to denote this as an order-to-disorder transition, and not an order-to-order transition. The TODTs of the BCC samples prepared in this study ranged over ~20°C as a function of the graft copolymer composition, offering a strategy for tuning the processing conditions of these materials (Fig. S66-S70). Overall, these results highlight the advantages of our simple, diversity-oriented synthesis approach, enabling the discovery of emergent material properties from comparably small changes in polymer composition.
While the BCC samples described above formed ultrasoft materials, we noticed that LAM samples with low grafting densities (~0.05) formed unusually robust, albeit malleable, transparent solids (Fig. 5a), warranting further investigation. 5–10 g samples of stat-150-0.05 and block-150-0.05 copolymers with each pendant group were synthesized following similar procedures used for the library synthesis and were analyzed by dynamic mechanical analysis (DMA) and tensile testing.
DMA revealed that these samples behaved as thermoplastics with a wide range of properties including room temperature elastic moduli (E′) up to ~150 MPa (Fig. 5b, c, Fig. S71–S72), which is similar to cartilage, tendons, and muscle tissues, is greater than commercial crosslinked silicones, and yet is softer than typical commodity thermoplastics. A remarkable dependence of E′ on side chain sequence was observed, with the three statistical copolymers displaying much higher values than their block copolymer counterparts. For example, stat-150-0.05-cHex has a ~15-fold greater E′ than block-150-0.05-cHex despite their identical composition (Fig. 5b). Given that these samples are not chemically crosslinked and are expected to have few entanglements (at most 1 entanglement per polynorbornene block in the case of blocky samples), their disparate mechanical properties can be explained by the much greater density of high-energy phase boundaries in the statistical copolymer, as reflected by their lower d*.
Notably, the thermal transitions of these materials were not sequence dependent, changing by only ~10% between statistical and block samples, and were not related to TODTs (Fig. S73-S89, Table S6). Instead, they were dominated by the glass transition temperatures of the small molecule norbornene homopolymers (Tg,PNB) with cHex > nHex > nHexDec (201 ± 4, 95 ± 1, 64 ± 1°C respectively). The comonomers also had a major effect on E′, decreasing from ~150 MPa to ~100 MPa to ~50 MPa from cHex to nHexDec to nHex, respectively. These findings demonstrate using comonomer composition to broadly tune the properties of these silicone thermoplastics.
Universal tensile testing was conducted on the three statistical graft copolymer samples with different comonomer pendants (Fig. 5d–i) (Note: due to their lack of entanglement and limited PDMS interdigitation, Fig. 3d, the block copolymer samples were too brittle to acquire meaningful data). The Young’s moduli (E, Fig. 5f) trended with the storage moduli obtained by DMA. Interestingly, the comonomer pendant composition had major impacts on the ultimate tensile strength (Fig. 5g), strain at break (Fig. 5h), and toughness (Fig. 5i, approximated by the area underneath the stress–strain curves). For example, strain at break ranged from a remarkable 230 ± 54% to 57 ± 5% to only 16 ± 1% for nHexDec, cHex, and nHex samples, respectively. Tensile strength ranged from 2.4 ± 0.2 (nHexDec) to 6.3 ± 0.2 (cHex) MPa, on par with commercial silicone elastomers such as Sylgard 184 (ultimate tensile strength c.a. 6.8 MPa).40 Similarly, toughness varied from ~600 J•m−3 to ~300 J•m−3 to ~50 J•m−3 across these samples. Altogether, these differences are attributed to a combination of the pendant group self-interaction strength, space-filling, and ability to undergo structural rearrangements during extension. Additionally, given that these samples are uncrosslinked, we expected that they could be recycled similarly to other thermoplastics. To demonstrate this concept, an nHexDec-based sample was dissolved, recast into a dogbone shape, and subjected to tensile testing. This process could be repeated at least 3 times with negligible impact on tensile properties (Fig. S90).
Summary and Outlook
Herein, we investigated the impact of comonomer composition on graft (co)polymer-based silicones using a novel diversity-oriented synthesis approach. Using one PDMS macromonomer and three comonomers that differed only by their aliphatic pendants, a library of 92 (co)polymers with varied sequence, backbone length, grafting density, and comonomer composition was synthesized. Characterization of this library confirmed many well-established trends in graft copolymer assembly, but also revealed unprecedented and dramatic variations in material properties that correlated with comonomer composition and sequence, offering a new and simple strategy to target bespoke material properties. Moreover, members of this library displayed properties that span the entire gamut of silicones properties, with certain features surpassing those of commercial systems; for example, a new class of transparent, reprocessable silicone thermoplastics with stiffness and toughness that surpass unfilled, crosslinked silicones as well as classical rubbers (e.g., ABS/SBS formulations) was discovered. Looking forward, we note that while the impacts of comonomers in this system are undeniable, the precise molecular mechanisms that translate from pendant composition to microphase morphology to bulk properties remains to be elucidated. In the future, this general approach should be applicable to a range of chemical systems in addition to silicones.