Keywords

1 Introduction

Silsesquioxanes, with a general formula (RSiO1.5)n, are one kind of molecular-level organic/inorganic hybrid silica-based materials. They combine many unique physical properties (thermal, mechanical, and structural stability) from inorganic part and chemical properties (possibility for functionalization and high flexibility) from organic part that traditional composite materials do not exhibit. Silsesquioxanes conventionally are synthesized from the hydrolysis and condensation of trialkoxysilane [RSi(OR’)3] or trichlorosilane (RSiCl3) monomers with active or inactive organic groups. By utilizing various silane precursors, various functional groups can be introduced to control the structures including ladderlike polysilsesquioxane (LPSQ) and cage-like polyhedral oligomeric silsesquioxane (POSS) in which a cubic core is connected with eight armlike organic substituents [1, 2].

The construction of hierarchical structured SSQs with different length scales usually is necessary to transfer the properties of molecule or atom to macroscopic materials. There are some reports on SSQ materials, focusing on the structure-composition-performance relationship of this kind of materials with well-defined morphology and nanostructure [3, 4]. Recently, design, fabrication, and applications of hierarchically structured porous SSQ materials have become a rapidly developing field due to their promising applications in separation, sensor, catalysis, and drug delivery [5]. A large series of synthesis methods have been developed involving templating method, conventional technique, sol-gel method, and self-formation method [6].

In recent decades, considerable research effort has been focused on the preparation of inorganic silicon-based materials modified with organic functional groups [7]. The integration or hybrid of different building block with silsesquioxanes has drawn much research attention, as this approach offers novel functionalities to the hybrid composite. In fact, the synthesis of hybrid silsesquioxane nanocomposites for potential application has becoming one of the attractive fields. SSQ-containing hybrid composites, mostly hybrid with polymers, have been designed due to their excellent thermomechanical properties [8]. SSQs could offer inorganic materials with biocompatible or physicochemical property which make them potential in biomedical applications. In fact, SSQs also could encapsulate and protect guest molecules from destruction or degradation.

In this chapter, we review some significant progresses made recently in the field silsesquioxane-based materials with well-controlled hierarchical structures. The fabrication process, structure-forming mechanism, the enhanced properties, and the final applications have been emphasized. We also review the main synthetic processes and applications of silsesquioxane hybrid materials. We will focus on the interaction between the SSQs and the hybrid materials, in order to reveal the hybrid mechanism.

2 Hierarchical Silsesquioxanes

Considerable attention has been paid to the design and fabrication of materials containing hierarchical silsesquioxane with improved physical and chemical properties. Hierarchical silsesquioxanes refer to the structure with morphology on different length scales. In detail, the SSQs involved hierarchical structures including core/shell, hollow, bowl-shaped, golf ball-like, etc.

2.1 SSQs with Core/Shell Structure

Core/shell structured materials based on silsesquioxanes have been extensively explored through the process such as sol-gel process, emulsion polymerization, template methods, electrospinning process, etc. The silsesquioxanes could not only offer the final materials with enhanced physical or chemical properties but also could protect the molecules in the core from leaching or degradation. Conventionally, the core/shell structured SSQs could be prepared from emulsion polymerization process. For example, the polysilsesquioxane/polyacrylate/polydimethylsiloxane core/shell particles have been successfully prepared via seeded emulsion polymerization of acrylate monomers and octamethylcyclotetrasiloxane with emulsifier as seeds [9]. Similarly, polystyrene/poly(γ-methacryloxypropyltrimethoxysilane) core/shell latex particles also were obtained through emulsion polymerization by adding methacryloxypropylene functionalized SSQ sol into the emulsion system of styrene monomer [10].

As Ha et al. reported, core/shell structured microspheres with raspberrylike to flowerlike morphology were fabricated with polysilsesquioxane (PSQ) shell grown stepwise on polystyrene template (Fig. 6.1). Time-dependent study on the forming of the core/shell structures demonstrates that the diameter and the topography of the microspheres could be well tailored by adjusting the silane precursor content. The core/shell structure and the low surface energy from PSQs enable the assembled particulate film to exhibit superhydrophobic property [11]. By adding the mercaptopropyl trimethoxysilane precursor into PS aqueous suspension, Deng et al. also prepared PS@mercaptopropyl-PSQs core/shell structured materials [12].

Fig. 6.1
figure 1

The core/shell structured polysilsesquioxane/polystyrene microspheres. SEM (a, c) and TEM (b) images of the hierarchical microspheres with (d) the corresponding size distribution measured by DLS technique. (Reprinted with permission from Ref. [11]. Copyright 2017 Royal Society of Chemistry)

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In addition, core/shell structured hybrids with cross-linked octa-methacrylate-POSS (MA-POSS) as shell and multi-walled carbon nanotubes (MWCNTs) as center have been fabricated through an in situ free radical polymerization process (Fig. 6.2). Due to the coating of PSQs, the obtained materials could disperse well in organic solvent and show a controlled electrical performance [13, 14].

Fig. 6.2
figure 2

Cross-linked silsesquioxane shell coated on carbon nanotube. (Reprinted with permission from Ref. [13]. Copyright 2014, American Chemical Society)

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In the core/shell structured materials, silsesquioxanes could be used as a layer to protect the guest molecules in the core. For example, Tolbert et al. encapsulated hydrophobic sunscreens in the polysilsesquioxane-silica shell to reduce the leaking and photodegradation of the materials [15]. 3-Glycidoxypropyl-silsesquioxane materials were also coated on ferrite nanoparticles to help the particles’ dispersion well in the epoxy and prevented sedimentation of the nanoparticles [16]. Core/shell structured polymer/POSS composites also have been fabricated through electrospinning process and been utilized to protect bioactive molecules in the core from thermal and chemical destruction [17, 18].

2.2 SSQs with Hollow Structures

As a specific member of SSQ-based materials, hollow SSQs have attracted much attention due to their unique micro- or nano-sized free volume in the structure, which can act as large reservoirs for guest molecule including catalysts or drugs [19]. For example, hybrid hollow mesoporous silsesquioxane nanoparticles have shown improved therapeutic performance and enhanced biomedical property [20].

Hollow-structured silsesquioxanes can be fabricated through the procedures like chemical etching, emulsion templating, hard templating, etc. Usually, hard template method for hollow polysilsesquioxane involves the formation of PSQ shell onto a template core which could be removed to leave behind a hollow shell. Ha et al. have offered a green method to fabricate a series of shape uniform and monodisperse hollow spheres based on organoalkoxysilanes and polystyrene template [21, 22]. With silica as a template, Koike et al. also have fabricated one kind of hollow silsesquioxane nanoparticles by stirring a biphasic mixture of organoalkoxysilane precursors and a silica aqueous suspension (Fig. 6.3). The tunable particle size, shell thickness, high surface areas, and large pore volumes of the hollow particles make them potential for practical application in many fields [23]. Similarly, with silica nanospheres as hard template, Zou et al. prepared organic groups, modified periodic mesoporous organosilica (PMO) hollow spheres, which are potential in the fields of drug delivery, bio-imaging, sensing, and heterogeneous catalysis [24].

Fig. 6.3
figure 3

(a) Schematic of the formation of hollow organosilica nanoparticles using silica nanoparticles as templates. (b) Dynamic light scattering curves of the dispersions of (a) silica particles sized ca. 40 nm, (b) silica core-organosilica shell nanoparticles, and (c) hollow organosilica nanoparticles. (c) Photograph of the dispersion of hollow organosilica nanoparticles (ca. 1.2 wt.%). (Reprinted with permission from Ref. [23]. Copyright 2013, Royal Society of Chemistry)

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Through an in situ polymerization method, Xing et al. prepared one kind of well-defined silsesquioxane hollow nanospheres based on the methacryloxypropyl silsesquioxanes and styrene monomer. The obtained hollow spheres show excellent methylene blue adsorption performance [25]. Via a sol-gel process, Fatieiev et al. prepared o-nitrophenylene-ammonium bridged silsesquioxane hollow spheres with photoresponsive properties (Fig. 6.4) [26]. About 50% organic content homogeneously distributed in the hybrid composition of silsesquioxanes, which make it potential for on-demand delivery of plasmid DNA in HeLa cancer cells via light actuation.

Fig. 6.4
figure 4

Design of bridged silsesquioxane (BS) and hollow BS NPs via the sol-gel reactions of the photoresponsive bridged alkoxysilane (PBA) precursor, as shown by TEM micrographs. (Reprinted with permission from Ref. [26]. Copyright 2015, American Chemical Society)

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Via reversible addition-fragmentation chain transfer (RAFT) polymerization, Zhang et al. prepared hollow polymeric capsules by self-assembly of an amphiphilic POSS-based block copolymer (Fig. 6.5). The obtained hollow polymeric capsules are responsive to pH and redox potential, so the capsules could be further utilized in the responsive drug release and photodynamic therapy [27]. In addition, Jiang et al. fabricated hollow mono adamantane-functionalized POSS/β-cyclodextrin spheres by the assistance of interface of H2O/toluene, and the obtained materials show excellent behavior of adhesion and proliferation of cells [28].

Fig. 6.5
figure 5

Illustration of polymeric capsule preparation and the process of loading and release of drugs. (Reprinted with permission from Ref. [27]. Copyright 2016, American Chemical Society)

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2.3 SSQs with Other Hierarchical Structures

Besides core/shell and hollow-structured silsesquioxane-based materials, there are still some other specially structured materials such as bowl-shaped, golf-like, grapelike materials, nanofibrillar micelles, etc. [29]. Bowl-shaped polysilsesquioxane particles have been also fabricated from two kinds of organoalkoxysilane precursors through a facile and controllable sol-gel polymerization procedure. The key factors for the formation of a bowl-shaped structure are the flexibility of the shell, osmotic pressure, as well as the centrifugal force. The concentrations of the two silane precursors, methyltriethoxylsilane (MTES) and phenyltriethoxysilane (PTES) monomers, as well as the ammonia concentration show a significant effect on the morphology of final particles [30].

Golf ball-like polymethylsilsesquioxane microspheres were fabricated through the hydrolysis and co-condensation of methyltrimethoxysilane (MTMS) and tetraethoxysilane (TEOS) (Fig. 6.6). The golf ball-like wrinkled surface was proved to be obtained from the self-assembly of polymethylsilsesquioxane and silica under Ostwald ripening process [31].

Fig. 6.6
figure 6

(a) SEM image of the polysilsesquioxane/silica spheres and (b) the formation mechanism of the golf-ball like spheres. (Reprinted with permission from Ref. [31]. Copyright 2011, Royal Society of Chemistry)

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With polystyrene colloidal crystals as templates, grapelike silica-based hierarchical porous interlocked aminopropyl polysilsesquioxanes have been fabricated, whose structure is the integration of open-mouthed structure, hierarchical porous nanostructure, and interlocked architecture (Fig. 6.7) [32].

Fig. 6.7
figure 7

(a) SEM image of polystyrenes and average size distribution of the microspheres (inset); (b) SEM image of polystyrene@silica composites; (c) SEM (1.00 kV) image and (d) STEM (5.00 kV) image of grapelike microspheres; (e) schematic illustration of the preparation process of the grapelike microspheres; (f) cross-sectional SEM image of grapelike microspheres. All the scale bars are 1 μm. (Reprinted with permission from Ref. [32]. Copyright 2015, American Chemical Society)

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Yolk-shell structured mesoporous inorganic-organic hybrid spheres with well-controlled size have been fabricated by Teng et al. with TEOS and 1,2-bis (triethoxysilyl) ethane (BTSE) as precursors. The monodisperse hybrid spheres with tunable diameter, shell thickness, and core size show excellent hemocompatibility and have a great promise for various applications [33].

Rodlike polysilsesquioxane also has been successfully prepared by oxidation and hydrolytic polycondensation of 3-mercaptopropyltrimethoxysilane (MPTMS) in a mixed aqueous solution of NaOH and H2O2. The obtained rodlike sulfo-group containing PSQs have a hexagonally stacked structure and show high proton conductivity [34].

Besides one-dimensional shape, two dimension layers also have been developed. Kataoka et al. fabricated microporous layered perovskites from metal halides and cage-like silsesquioxane. In the forming process, POSS could form micropores between the metal halide perovskite layers, which may offer new property for perovskites [35]. Similarly, hierarchical porous structured films on PET matrix were also achieved with cross-linked POSS as the silica source and a copolymer surfactant as the porogen. The final obtained films, with high surface areas and well-controlled pore sizes, could be used in many fields such as supercapacitors, sensors, filtration, and catalysis [36]. Much attention also has been drawn to three-dimensional porous scaffold. Through sol-gel polymerization from bridged silane precursors, microscale aerogels containing nanoparticle-constructed networks have been fabricated by vacuum drying process [37]. Through a facile cross-linking and solution extraction process, ionic gels and scaffolds with interconnected mesopores derived from POSS have been fabricated. The obtained POSS-based porous materials show excellent catalytic performance and superior lithium ion battery performance [38].

3 Silsesquioxane Hybrid Composites

3.1 SSQ Hybrid with Polymer

Recently, much attention has been attracted by POSS-based molecules and polymers, especially the design, preparation, and applications of the POSS hybrid polymers [39,40,41,42,43,44,45]. Due to the organic/inorganic hybrid structure in the molecular level, silsesquioxane has become an ideal building for polymer/SSQ nanocomposite through sol-gel or melt mixing process [46,47,48,49,50,51]. POSS consists of a stable silica core and eight active/inactive functional arms. The active organic functional groups on the POSS could work as a nucleus for covalent bonding to create multi-armed polymers with enhanced mechanical and biological properties [52]. In fact, polymer incorporated with suitable organic functionalized POSS derivatives as side chains could be obtained for targeted applications [53]. For example, the star-shaped POSS-polycaprolactone- polyurethane (POSS-PCL-PU) film with high porosity has been fabricated. The nanocomposites show unique surface nanotopography and excellent biocompatibility which make it a great candidate as a tissue engineering scaffold biomaterial [54]. Through atom transfer radical polymerization (ATRP), Qiang et al. synthesized two different eight-arm star-shaped POSS fluorinated acrylates, which show a great potential in filtration, cell culture, tissue engineering, and marine antifouling applications [55]. Zhang et al. prepared the star-shaped organic/inorganic hybrid poly (L-lactide) (PLLA) based on octa(3-hydroxypropyl) polyhedral oligomeric silsesquioxane via ring-opening polymerization (ROP) of L-lactide (LLA) for biological and medical applications [56]. In electrochromic applications, PANI is the most widely used due to their good environmental stability and electrical property, which could be improved by hybrid with POSS. Jia et al. prepared polyaniline (PANI)-tethered cubic POSS via oxidative copolymerization of octa(aminophenyl) silsesquioxane and aniline in the presence of HCl. The as-prepared POSS-PANI copolymer in emeraldine salt (ES) was filtered, washed, and treated with triethylamine to achieve POSS-PANI copolymer in emeraldine base (EB) form. The films fabricated via layer-by-layer (LBL) assembly show enhanced electrical conductivity (Fig. 6.8) [57]. Lin et al. also fabricated multi-armed polyaniline-octa-aminophenylsilsesquioxane conjugates (PANI-SSQ) with hierarchical porous structure, exhibiting excellent specific capacitance and stability, which make it potential for electrode material applications [58].

Fig. 6.8
figure 8

Oxidative copolymerization of multi-armed polyaniline (PANI)-tethered cubic POSS. POSS-PANI-ES was formed from POSS and aniline, followed by conversion to POSS-PANI-EB. (Reprinted with permission from Ref. [57]. Copyright 2009, American Chemical Society)

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Lin et al. fabricated one kind of carbon nanofiber-silsesquioxane-polyaniline nanohybrids with hierarchical structure and used as flexible supercapacitor electrodes [59]. Firstly, through amide linkage, octa-aminophenylsilsesquioxane was first attached onto the carboxylated nanocarbon (CNF-COOH). And then phenylamino-modified CNF surface was copolymerized with aniline via chemical oxidative polymerization to create a unique CNF-conjugated polymer hybrid (CNFS-PANI). The obtained flexible CNFS-PANI nanohybrid shows excellent electric conductivity and specific capacitance, suggesting its potential as electrochemical electrode material (Fig. 6.9).

Fig. 6.9
figure 9

Schematic design for the preparation of carbon nanofiber-silsesquioxane-polyaniline (CNFS-PANI) nanohybrids by functionalizing CNF with octa-aminophenylsilsesquioxane (OASQ) through amidation followed by a reaction with aniline to form a hyperbranched PANI-grafted CNF. (Reprinted with permission from Ref. [59]. Copyright 2015, Royal Society of Chemistry)

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Based on poly(styrene-b-butadiene-b-styrene) (SBS) cross-linked by POSS, Bai et al. have fabricated nanostructured thermoplastic elastomeric composites [60]. Through ATRP and the copper-catalyzed azide-alkyne “click” reaction process, Zhang et al. have prepared dumbbell-shaped POSS/poly(tert-butyl acrylate)(PtBA) from alkyne-functionalized POSS and azido-terminated PtBA [61]. Through electrospinning process, Pisuchpen et al. have fabricated polystyrene polyhedral oligomeric silsesquioxane-derived methacrylate (PS-co-PMAPOSS) copolymer fibers. The obtained fibers with highly porous structure are potential for further novel applications [62]. This polymerization process is a combination of reversible addition-fragmentation chain transfer (RAFT) and activator regenerated by electron transfer for ATRP (ARGET ATRP).

3.2 SSQ Hybrid with Silica

Silsesquioxane mesoporous frameworks are a kind of synergistic combination of inorganic silica, mesopores, and organics with some physicochemical and biocompatible properties which could be used as bio-imaging agent and drug delivery system [63]. Via the hydrolysis and condensation polymerization from the mixture of TEOS and MTMS, Hayashi et al. have prepared mesoporous and hydrophobic silicate silsesquioxane hybrid copolymers under a strong basic condition [64]. Azo-functionalized silsesquioxanes have been used as gatekeeper for drug-loaded mesoporous silica, and the obtained drug release system shows enzyme-responsive drug release behavior [65]. Dopierala et al. have reported hydrophobic POSS materials containing silica nanoparticles which could be used as self-cleaning coating. In this system, a hybrid of silica nanoparticles make the POSS monolayer more condensed and rigid; meanwhile, POSS molecules prevent silica nanoparticles from aggregations (Fig. 6.10) [66].

Fig. 6.10
figure 10

The chemical structures of the silsesquioxane cage with substituents (on the top) and 3-aminopropyl- (3-oxobutanoic acid) functionalized silica (on the bottom). (Reprinted with permission from Ref. [66]. Copyright 2016, Royal Society of Chemistry)

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3.3 SSQ Hybrid with Fe3O4

Magnetic nanoparticles could offer SSQ materials with interesting properties for applications such as medical diagnostic, wastewater treatment, catalyst carrier, and drug delivery [67,68,69]. Via a one-pot coprecipitation and surface grafting approach, Ha et al. have fabricated magnetite-polysilsesquioxane hybrid nanoparticles with ferrous, ferric chloride and various silane monomers as raw materials (Fig. 6.11) [70]. The magnetic PSSQ hybrid nanoparticle composites have excellent adsorption and selectivity behavior for iron (Fe3+). The magnetic and functional magnetic nanoparticles can be separated easily from the adsorbed solution by a bar magnet and reused for the repeated cycles of adsorption.

Fig. 6.11
figure 11

Schematic diagram of the formation of magnetite-polysilsesquioxane hybrid nanoparticles. (Reprinted with permission from Ref. [70]. Copyright 2017, Royal Society of Chemistry)

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In another report, octavinyl POSS was first constructed on the Fe3O4 nanoparticles by surface polymerization process. After modification with dithiol via thiol-ene addition reaction on the as-prepared Fe3O4@POSS hybrid composite, the ultimate material (Fe3O4@POSS-SH) could also be used to remove heavy metal ions and organic dyes from wastewater (Fig. 6.12) [71].

Fig. 6.12
figure 12

Schematic representation of the route for synthesis of Fe3O4@POSS-SH. (Reprinted with permission from Ref. [71]. Copyright 2013, American Chemical Society)

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3.4 SSQ Hybrid with Noble Metals

Hierarchically porous SSQ with well-defined macropores or mesopores could be embedded with noble metal nanoparticles, which are promising as heterogeneous catalysts or antimicrobial materials [72].

Silver nanoparticles could be hybrid with POSS and used for antimicrobial applications [73]. To avoid the aggregation of silver nanoparticles, hydrophobic POSS have been used to protect the particles [74]. Schneid et al. synthesized spherical silver nanoparticles hybrid with charged silsesquioxane containing a quaternary ammonium group and applied as an antibacterial agent. The cytotoxicity assay showed that the system also is safe for mammalian cells at the studied concentrations [75].

The SSQ hybrid silver particles could also be utilized as a smart colorimetric probe for rapid and accurate detection of hydrogen sulfide. With a poly-POSS formaldehyde polymer (PPF) cage as a ligand and reductant, Zhang et al. developed a novel strategy for the fabrication of a positively charged silver nanoparticle probe. POSS cage works as a capping ligand and reducing agent for the fabrication of well-dispersed silver nanoparticles; the final system has excellent performance on the rapid and accurate detection of hydrogen sulfide (Fig. 6.13) [76].

Fig. 6.13
figure 13

Schematic illustration of the preparation of poly-POSS formaldehyde polymer-AgNPs and its colorimetric detection of H2S. (Reprinted with permission from Ref. [76]. Copyright 2017, American Chemical Society)

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Functional polysilsesquioxanes containing different organic groups could be utilized as carriers to load and protect gold nanoparticles for applications such as catalyst, sensors, DNA assays, etc. [77, 78]. Ha’s group has loaded gold nanoparticles on the shell of cyanopropyl polysilsesquioxane (CPSQ) hollow spheres which initially combined cyanopropyl groups and pores on the shell (Fig. 6.14). The pores make it possible for guest molecules to diffuse through the shell, and the cyanopropyl groups make the noble metals stable due to high affinity between cyanopropyl group and gold. PSQ hollow spheres with well-dispersed gold nanoparticles demonstrate excellent catalytic performances [79]. Scholder et al. have fabricated gold nanoparticles on hydrophilic dithiol-modified POSS scaffolds, showing highly efficient catalytic performance [80].

Fig. 6.14
figure 14

Fabrication of CPSQ hollow spheres and Au/CPSQ hybrid nanocomposites loaded with highly dispersed Au nanoparticles. (Reprinted with permission from Ref. [79]. Copyright 2012, Royal Society of Chemistry)

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Silva et al. have designed nanocomposites by water-soluble silsesquioxane polymer hybrid with gold nanoparticles for detection of pollutant (nitrophenol isomers) [81]. Zapp et al. loaded gold nanoparticles on silsesquioxane based on liquid crystal and applied them as nanostructured immunosensor to detect a protein troponin T [82]. Brigo et al. deposited an aryl-bridged polysilsesquioxane film on Au nanoparticle to form an aryl-bridged polysilsesquioxane system for xylene gas optical sensors [83].

Silsesquioxane hybrid with palladium (Pd) was utilized to protect the nanoparticles [84,85,86]. Pd nanoparticles capped with SSQ possessing stable reactivity of Pd-catalyzed reactions have been quickly and easily synthesized [87]. Tanabe et al. have produced a palladium complex with an O,O-chelating silsesquioxanate ligand from an incompletely condensed silsesquioxane which reacts with a palladium precursor [88]. Through on-site reduction-based methodology, Moitra et al. have loaded Pd nanoparticles onto the hierarchically porous hydrogen silsesquioxane (HSQ) monolithic (Fig. 6.15). The obtained Pd@HSQ catalyst shows high catalytic activity, reusability, and easy handling [89].

Fig. 6.15
figure 15

(a) Schematic representation of on-site reduction of PdCl2 on a hydrogen silsesquioxane (HSQ) monolith; simultaneous transformation of Pd2+ to Pd(0) nanoparticles and Si-H to Si-OB. (b, c) Digital camera images of the HSQ monolith right after immersing in PdCl2 solution and after reacting for 3 h, respectively. (Reprinted with permission from Ref. [89]. Copyright 2014, Royal Society of Chemistry)

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3.5 SSQ Hybrid with Carbon Nanomaterials

Silsesquioxane nanocomposites hybrid with carbon through covalent or uncovalent linkage will improve the physical or chemical property of the carbon materials [69]. Octaglycidyldimethylsilyl POSS worked as coupling agents to graft carbon nanotubes onto carbon fibers (Fig. 6.16) [90]. Due to the introduction of POSS, the properties of the resulting composites such as toughness, service temperature, oxidation, and chemical resistance have been enhanced.

Fig. 6.16
figure 16

The schematic of binary grafting procedure including grafting carbon nanotubes onto carbon fibers using polyhedral oligomeric silsesquioxanes as coupling agents. (Reprinted with permission from Ref. [90]. Copyright 2011, Royal Society of Chemistry)

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Chemical functionalization of carbon nanotube with POSS, as a kind of innovative 0D–1D nanohybrid structure, is an important strategy to improve the thermal or electrical property of the composite. Zhang et al. prepared a core/shell structured composite with a MWCNT at the center and cross-linked octa-acrylate POSS as shell, which could improve the dielectric permittivity and low dielectric loss of polymers [91]. Through the formation of amide bonds, Sabet et al. fabricated POSS covalent-bonded MWCNT, and the obtained material has the high thermal stability which could effectively reinforce polymer materials [92]. Through Friedel-Crafts acylation and amidation chemistry, cube-like octa-organsilsesquioxane hybrid with one-dimensional single-walled carbon nanotubes (SWNTs) has been reported by Xu et al. [93].

Polyoligomeric silsesquioxane (POSS)-decorated graphene or graphene oxide (GO) nanoplatelets have been obtained by various methods [94, 95]. Typically, POSS-GO could be successfully synthesized via amide formation between the octa(aminopropyl) POSS (OAPOSS) and GO [96]. Utilizing similar amide formation process, POSS-functionalized graphene nanosheets have been fabricated from amine-functionalized POSS and oxygen-containing groups, graphene oxide (GO). The obtained nanocomposites are highly soluble in various organic solvents and show potential in multifunctional applications (Fig. 6.17) [97, 98]. Through a one-step hydrothermal process, Bai et al. have prepared POSS/reduced graphene oxide nanocomposite (POSS/rGO) as a hydrophobic electrochemical sensor for nitrite detection [99].

Fig. 6.17
figure 17

Proposed model of the reinforcing (a) GO and (b) octa(aminopropyl) POSS-GO in polyimide (PI) polymeric matrix. (Reprinted with permission from Ref. [97]. Copyright 2014, American Chemical Society)

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Cheng’s group designed and fabricated a sphere-cubic-shaped amphiphile based on 3-hydroxypropylheptaisobutyl-POSS and modified fullerene (Fig. 6.18). To connect the two building blocks and prevent macroscopic phase separation, a short covalent linkage was thus selected. The hybrid nanocomposite of layered structure with an alternating conductive fullerene and insulating POSS layer structure is of great interest for various potential applications such as nano-capacitors [100]. In the same group, POSS-[60]fullerene (POSS-C60) dyad was designed and used as a novel electron acceptor for polymer solar cells with an inverted device configuration [101].

Fig. 6.18
figure 18

Chemical structures of octaisobutyl-POSS (1), C60 (2), and POSS-C60 (3) and the synthetic route to POSS-C60 (3). (Reprinted with permission from Ref. [100]. Copyright 2011, Royal Society of Chemistry)

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3.6 Hybrid with Quantum Dot

Utilizing organic substituted POSS as stabilizer, hybrid CdSe quantum dots (QDs) could be fabricated [102, 103]. By embedding water-soluble N, S-co-doped carbon dots into a POSS matrix, highly efficient solid-state luminophores with strong deep blue emission could be obtained [104]. Due to the bulkiness of the siloxane core, mercapto-substituted POSS(SH-POSS) was utilized as a ligand and a steric stabilizer to produce wurtzite phase CdSe QDs. The obtained SH-POSS capped QDs show excellent optical properties including photoluminescence quantum efficiencies and fluorescence lifetimes (Fig. 6.19) [105].

Fig. 6.19
figure 19

Schematic drawing of a CdSe QD capped with mercaptopropylisobutyl-POSS and a photograph showing the photoluminescence color change of the POSS-CdSe QDs under a UV lamp with different sizes of CdSe QDs. (Reprinted with permission from Ref. [105]. Copyright 2013, American Chemical Society)

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4 Conclusions

In summary, recent advances and applications in various aspects of the silsesquioxane-based hierarchical and hybrid materials have been highlighted. From the recent trend, it is clear that more and more hierarchical silsesquioxanes have been designed and fabricated for unique chemical and physical properties. At the same time, a wide category of SSQ hybrid materials has been developed to meet the requirement for a versatile application. It is believed that, in the near future, more sophisticated silsesquioxane composite with various morphologies will be created for more and better high performance.