*2.5. Coatings*

#### 2.5.1. Thin Film Silicone Coatings

The inclusion of silicone macromers into the formal polymeric structures of coatings, as distinguished from their use as additives, is a growing area of technology. These applications tend to be associated with release, lubricity, and mechanical properties related to direct physical interactions with humans. Indeed, the sensory appeal of coatings has always been an important driver of consumer applications in which positive tactile interaction is critical to acceptance [87], such as synthetic leather, textile finishes, and hair care.

Older approaches to urethane materials mainly use polydisperse telechelic carbinolterminated siloxane polymers, in which the two identical functionalities on the termini serve to introduce siloxanes into urethanes as soft-blocks. AROP-derived siloxane macromers (oligomeric materials with functionality on one terminus) represent a newer approach in which the functionality on one terminus of the oligomer allows the formation of a brush polymer with siloxane segments as pendant, allowing the mechanical properties of the urethane backbone to be maintained (Scheme 9) [87]. These structures also demonstrate significantly greater wear resistance as well as lower friction and release properties compared to telechelic controls [153], in which siloxane is incorporated into the urethane backbone as shown in Scheme 10. Table 8 compares tribological and contact angle properties of urethane in which siloxane has been introduced as an unreacted fluid, a soft segment, and a pendant, respectively.

**Scheme 9.** Urethane Polymer with Pendant Siloxane.

**Scheme 10.** Urethane-Siloxane Soft Segment Block Polymer.

In similar chemistry, self-lubricating cannulae for medical applications have been fabricated from thermoplastic urethanes derived from mono dicarbinol macromers [86].


**Table 8.** Comparing Contact Angle, Abrasion, and Friction in an IPDI-based Urethane.

Control (no siloxane); (A) introduction of 50 cSt dimethylsiloxane fluid as a non-reactive additive; (B) telechelic siloxane MW 5000 (DMS-C21) added to form a soft-block copolymer; (C) macromer MW 5000 (MCR-C62) forming a brush polymer with siloxane pendant.

#### 2.5.2. Thick Film Silicone Coatings

Depending on their formulation, silicone coatings elicit a wide range of tactile responses. Coatings that contain low molecular weight, particularly volatile species are associated with a "silky" feel and exhibit slip. On the other hand, silicone coatings that have been depleted of low molecular weight species are associated with a "tacky" feel. Common to both these experiences is the extreme hydrophobicity of the polymer. When silicone coatings are free of low molecular-weight species, they exhibit high coefficients of friction and, due to their relatively poor mechanical properties, failure by abrasive and adhesive spalling during continuous tactile interaction. Well-defined silicones with a central vinyl functionality and discrete PEG2 (MCS-VX15), PEG3 (MCS-VX16—Scheme 11), or tetrahydrofurfuryl (MCS-VF14—Scheme 12) pendant end-groups can be used as comonomers in addition-cure, platinum-catalyzed 2-part silicone elastomer formulations in order to introduce hydrophilicity [61]. In such formulations, the surface tribological properties are modified by introducing a hydrodynamic lubricating layer of adsorbed water. The modified silicone elastomers retain optical clarity and mechanical performance characteristic of this class of material with up to 15 wt.% comonomer in the 2-part formulation. Contact angle measurements of deionized water on the silicone elastomer surface showed improved wettability with comonomer content: at ~3 wt.% comonomer, the elastomer surface shifts from hydrophobic (contact angle ~120◦C) to hydrophilic (contact angle < 90◦C). Coefficient of friction measurements for the modified silicone elastomers demonstrate increased surface lubricity with comonomer loadings (Figure 7) [112].

**Scheme 11.** Vinyl functional Siloxane with PEG Endgroups-Reactive in Two-component Pt-Cure Silicone RTVs.

**Scheme 12.** Vinyl functional Siloxane with Tetrahydrofurfuryloxy Endgroups-Reactive in Two-component Pt-Cure Silicone RTVs.

**Figure 7.** Modification of silicone elastomers with symmetric silicone macromers. (**a**) Effect of comonomer on contact angle and hydrophobicity of silicone elastomer; (**b**) coefficient of friction of silicone elastomers modified with symmetric silicone macromers (Determined on AR-G2 Rheometer: aqueous, 37 ◦C, normal force 1N, velocity 1.0 rad/s (1.5 mm/s) and 1.0 rad/s (15 mm/s).)

Separately, in the field of dielectric elastomer actuators, monovinyl-terminated PDMS macromers have been used to selectively adjust the network behavior of silicone films between compliant electrodes [111].

#### *2.6. Cosmetics and Hair Care*

Hair-care formulations, including shampoos and conditioners with the ability to withstand multiple washings, require good film-forming properties with strong adhesion to the hair cuticle, but must simultaneously offer lubricity in order to provide combability. Copolymers of dimethylaminoethylmethacrylate and low molecular weight methacrylate functional silicone macromers (MW 2000) [89,90] have been used directly or in combination with other polymers [154] to provide increased lubricity and combability of hair. Dispersions of macromer-derived terpolymer particles have been reported in hydrocarbon vehicles such as isododecane, yielding film-forming compositions that are useful in eyeliners and mascaras [97]. Hydride-terminated macromers have been reacted with unsaturated terpenes and cannabidiol (CBD) (Scheme 13 to form emollient compounds with solubility in the siloxane vehicles preferred for skin care [155,156].

**Scheme 13.** CBD terminated PDMS macromer.

#### *2.7. Magneto-Rheological Fluids*

Well-defined biocompatible magnetic nanoparticles are of interest as materials for biomedical applications including magnetic field-directed drug delivery, biomolecule separations, and assay devices. Superparamagnetic iron oxide (Fe3O4) nanoparticles (SPION) sterically stabilized with PDMS macromers synthesized by Living AROP produced homogeneous hydrophobic ferrofluids that are stable against precipitation [157,158]. PDMS macromers with a tricarboxylate endgroup capable of binding to the surface of magnetite nanoparticles were synthesized by first making a trivinyl-terminated PDMS via Living AROP, followed by a thiol-ene reaction between the vinyl silane groups and mercaptoacetic acid, as depicted in Figure 8 [159,160]. Molecular effects of the PDMS tail on the stability of the PDMS-magnetite complexes were studied. Magnetic separation methods were developed to narrow the particle size distribution of the magnetite nanoparticles using tricarboxylate PDMS stabilizer while controlling the PDMS surface concentration [161]. In other studies, a monocarboxydecyl-terminated PDMS macromer (MCR-B12) was used to stabilize magnetite nanoparticles; however, the resulting ferrofluids had issues with stability and sedimentation, likely due to the lower number of carboxylate binding groups per macromer chain [88]. The magnetophoretic mobility of the magnetite-PDMS fluids was then studied in different magnetic field conditions (magnetic fields and field gradients), with the results demonstrating that the shape and speed of these droplets in viscous media can be independently manipulated [160,162].

**Figure 8.** PDMS macromer-modified SPIONs (Author's work).

These magnetite-polydimethylsiloxane ferrofluids were proposed by Wilson as a material that could aid in the treatment of retinal detachment disorder [159]. The proposed treatment entailed inserting a pre-aligned magne<sup>t</sup> into the conjunctiva and then injecting the PDMS-based ferrofluid into the vitreous humor. As shown below in Figure 9, the ferrofluid would close the tear as it moved toward the permanent magnetite, allowing the surgeon to repair the tear.

**Figure 9.** Retinal repair utilizing macromer-enabled magnetorheological fluids.

#### *2.8. Bulk Macroscale Materials*

Ultra-High Elongation Elastomer

An ultra-high elongation silicone elastomer has been prepared from a heterobifunctional silicone macromer compounded with reinforcing agent, achieving elongations nearing 5000%, nearly four times greater than conventional silicone elastomers. The cure mechanism of this elastomer is a step-growth polymerization of an α-vinyl-ω-hydrideterminated silicone macromer [62,113,114] via intermolecular hydrosilylation reaction, which yields a linear polymer of exceptionally high molecular weight with no apparent covalent crosslinking (Scheme 14) [73,113].

**Scheme 14.** Step-Growth Polymerization of Heterobifunctional α-Vinyl-ω-Hydride-terminated Silicone Macromer.

Atomistic modeling (Figure 10) of the cured silicone macromer shows the probability of knotting within a 50,000 Da segmen<sup>t</sup> [115], which correlates to an experimental value of critical molecular weight (Mc) for entanglement of ~42,000 Da [163]. The stress-strain curve is remarkably different from those of crosslinked silicone elastomers, as shown in Figure 11.

**Figure 10.** Atomistic model of a 50,000 Da segmen<sup>t</sup> of ultra-high elongation elastomer suggesting 1–2 knots. (Author's work).

**Figure 11.** Mechanical properties of ultra-high elongation elastomer (ExSil 100) compared to conventional silicone elastomer.

Ultra-high elongation elastomers exhibit other behaviors not shared by conventional silicone elastomers [115]: e.g., they undergo extreme multi-axial distortions and return close to their original shape, and display remarkable tear propagation mechanisms, meaning that tear failure occurs at drastically greater elongations; recovery from minor penetration is improved below the failure limits and, finally, pseudo-self-healing behavior is also displayed, as shown in Figure 12 [116].

**Figure 12.** Pseudo-self-healing demonstration of bisected specimen of ExSil® (Author's work).

#### *2.9. Nanoscale Morphology*

2.9.1. Photoresist and Contact Printing

Templated self-assembly of a cylinder-forming poly(styrene-b-dimethylsiloxane) (PS −PDMS) diblock copolymer was first described by Saam and Fearon [51]—followed by others [164–167]—and has been investigated for nanolithography applications [44,145,168, 169]. The general structure is depicted in Scheme 15. The general structure (PS −PDMS) diblock copolymer is depicted in structure 11. These copolymers undergo microphase segregation above their *Tg*, and the large X-polymer-solvent interaction parameter of the blocks is advantageous for achieving long-range ordering as well as for minimizing defect densities. Furthermore, the high Si content in PDMS leads to a robust oxide etch mask after two-step reactive ion etching (RIE) [170], as exemplified in Figure 13.

To address the critical needs of nano-dimensional photoresists, materials that possess dual surface properties are required. When cast or hot-pressed on a high-surface-energy substrate such as silicon, glass, or aluminum, the copolymer film forms both a lowersurface-energy component (PDMS)-enriched air/polymer interface and a higher-surfaceenergy component (organic block)-dominated polymer/substrate interface [171–173].

**Figure 13.** PS-b-PDMS after a single imprint cycle and annealing provides 60 nm features (Reprinted with permission from Ref. [173] Copyright 2007 Wiley).

Interface effects complicate the use of siloxane A-B block copolymers due to the low surface energy and wetting characteristics of the siloxane block.

**Scheme 15.** Polystyrene-Polydimethylsiloxane Block Copolymer.

Silylated styrene block polymers that employ conventional radical polymerization rather than AROP have been proposed [174,175], and azidopropyl functional silicone macromers have been separately "clicked" with alkynyl-terminated ATRP-generated macromers to form PDMS-b-PMMA, producing sub-10 nm structures [174,176]. A second potential solution is star block polymers generated by AROP, which provide a mechanism through which the non-siloxane block can dominate interfacial behavior [52]. The reviewers note that similar morphologies have also been reported for polybutadienepolydiethylsiloxane block copolymers (PBD-b-PDES) [19]; these copolymers possess less differentiation in surface energy, potentially mitigating issues with the styrenic and methacrylate systems, but have not been evaluated in lithographic systems. Similarly noteworthy in this context, while the synthesis of diblock polymers typically starts with an organic macroanion and then proceeds to a siloxane polymerization, the potential of the reverse process, in which a siloxane macromer starts and proceeds to an acrylate polymerization, was demonstrated using ATRP [177]. Other organic block copolymer polymerization examples initiated by carbinol-terminated macromers include caprolactone and trimethylene carbonate blocks [83,178]. This approach clearly expands the options for generating polymers with varying self-assembly structures.

The use of polystyrene macroanions was further elaborated by Bellas to form triblock and microarm polymers [44], and by Shefelbine to form continuous core–shell gyroid morphologies [179]. This work led to the observation of periodic double gyroid (DG) behavior, as well as a series of publications displaying an appreciation of the potential for extended novel 3D structures (Figure 14) [180–182]. As suggested by the DG topological visualizations of Thomas [183], potential structural, dielectric, charge transport, and mass transport material behavior can be controlled by the direct use of DG block polymers structures or by the removal, conversion, and/or infiltration of a DG microphase.

**Figure 14.** Reconstruction of a volume containing a Double Gyroid of PS-b-PDMS (Reprinted with permission from Ref. [183] Copyright 2021 National Academy of Sciences).

#### 2.9.2. Biomimetic Polymers and Bottle-Brush (BB) Architecture

Recently, grafting-through polymerization and surface-initiated polymerization have led to bottle-brush polymers and particle-brush materials that have shown potential in the fabrication of biomimetic materials. These materials can be broadly considered filamentous structures that exhibit non-linear behavior under deformation, greater relaxation times, and the potential for complex non-covalent interactions leading to the formation of supermolecular structures. Siloxane macromers appear to be of particular interest in creating these structures due to both the palette of functionality and the intrinsic flexibility of the siloxane structures, which allows the length-scale of the filaments to extend beyond

the primary polymer backbone or nano-feature without imposing a "hard" structural domain. Siloxane macromers allow entry to classes of materials that possess an unusually low modulus while maintaining mechanical failure properties consistent with the main polymer backbone for both methacrylate [96] and norbornene [84] functional siloxane macromers.

Separately, this recognition revealed such macromers' potential for generating biomimetic gel structures [95]. The complexity of the potential range and behavioral characteristics associated with bottle-brush polymers in terms of macromer molecular weight, graft density, and final molecular weight is readily apparent. Their tunable physical properties in both crosslinked and uncrosslinked states—based on the Dp of the main chain and grafting density of methacryloxypropyl-terminated polydimethysiloxane (Dp = 10) prepared by Grafting Through Atom Transfer Radical Polymerization—have been investigated in this context [103]. The dynamics of deformation have also been modeled [104,109,110]. A striking extension of this approach can be found in the "chameleon-like" color changes demonstrated in these systems—in which vibrant color, extreme softness, and intense strain stiffening on par with that of skin tissue have been observed as a consequence of placing a heterogeneous polymeric system under varying degrees of strain (Figure 15) [106].

**Figure 15.** Color alteration from turquoise to dark blue during uniaxial stretching. (Reprinted with permission from Ref. [106] Copyright 2018 AAAS).

#### 2.9.3. Liquid Crystal Siloxanes

Most polymers that exhibit liquid crystalline behavior have anisotropic side groups. Polydiethylsiloxane, which is distinct from both lost LCPs and its simpler homolog, polydimethylsiloxane, demonstrates mesophasic liquid crystalline behavior at ambient temperatures, as first reported by Beatty [184]. This behavior extends to poly(di-n-alkylsiloxane)s with side chains no longer than seven carbons that are also able to form a columnar mesophase. Such polymers are positionally and orientationally ordered in a two-dimensional hexagonal lattice, but without positional order along the chain. While early work was conducted with polymers of high polydispersity (>2.0), the desire to optimize liquid crystal behavior by controlling the PDI has led to the use of living AROP conditions for the polymerization of polydiethylsiloxane (PDES)—as first reported by Molenberg, who utilized lithium sec-butyldiethylsilanolate as the initiator [41,43]. The phase behavior of diethylsiloxane is shown in Figure 16.

**Figure 16.** Liquid Crystal-Phase Behavior of Polydiethylsiloxane.

There are few reports of living AROP with higher dialkylsiloxanes, presumably because of either ineffective termination or these systems' sluggish kinetics.

Molenberg also reported on elastomeric block polymers of butadiene and diethylsiloxane that exhibited mesophase formation under tensile stress [19]. Later work with styrene-diethylsiloxane diblock polymers showed periodic nanoscale lamellar structures with compositions possessing greater than 20% styrene content, as shown in Figure 17 [42].

**Figure 17.** Bulk structures of PolyStyrene-Polydiethylsiloxane block copolymers. Bulk structures of (**a**) PS-*b*-PDES containing 37 wt% PS and (**b**) PS-*b*-PDES containing 43 wt% PS (Reprinted with permission from Ref. [19] Copyright 1998 Wiley).

The vast majority of liquid crystal polysiloxanes based on side chain substitution that have recently been reviewed are polydisperse in nature [185]. Hepenius, however, utilized AROP to form vinylmethylsiloxane copolymers from vinylpentamethylcyclotrisiloxane and then functionalized the positions with mesogenic groups: for example, by reacting 4-cyano-4-(ω-alkenoxy)biphenyl with an excess of tetramethyldisiloxane and then reacting with the remaining Si-H group (Scheme 16) [46].

**Scheme 16.** Derivatization of AROP Siloxanes with mesogenic pendant groups.
