Polyhedral oligomeric silsesquioxane/epoxy coatings: a review

Cite this articleSeidi F, Jouyandeh M, Taghizadeh A et al. (2021)Polyhedral oligomeric silsesquioxane/epoxy coatings: a review.Surface Innovations 9(1): 3–16,https://doi.org/10.1680/jsuin.20.00037

Invited Feature ArticlePaper 2000037Received 29/05/2020; Accepted 29/07/2020Published online 24/08/2020

ICE Publishing: All rights reserved

Keywords: anti-corrosion/coatings/mechanical properties

Surface Innovations

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Polyhedral oligomeric silsesquioxane/epoxycoatings: a review
Farzad SeidiProvincial Key Lab of Pulp and Paper Science and Technology and JointInternational Research Lab of Lignocellulosic Functional Materials, NanjingForestry University, Nanjing, China
Maryam JouyandehCenter of Excellence in Electrochemistry, School of Chemistry, College ofScience, University of Tehran, Tehran, Iran

Ali TaghizadehCenter of Excellence in Electrochemistry, School of Chemistry, College ofScience, University of Tehran, Tehran, Iran

Mohsen TaghizadehCenter of Excellence in Electrochemistry, School of Chemistry, College ofScience, University of Tehran, Tehran, Iran

Sajjad HabibzadehDepartment of Chemical Engineering, Amirkabir University of Technology(Tehran Polytechnic), Tehran, Iran

[] on [10/03/21]. Copyright © ICE Publishing, all rights reserved.

Yongcan JinProvincial Key Lab of Pulp and Paper Science and Technology and JointInternational Research Lab of Lignocellulosic Functional Materials, NanjingForestry University, Nanjing, China

Huining XiaoDepartment of Chemical Engineering, University of New Brunswick,Fredericton, NB, Canada

Payam ZarrintajSchool of Chemical Engineering, Oklahoma State University, Stillwater, OK,USA

Mohammad Reza SaebCenter of Excellence in Electrochemistry, School of Chemistry, College ofScience, University of Tehran, Tehran, Iran (corresponding author:[email protected])

Nowadays, there is a need for a paradigm shift in material selection for surface coatings so as to meet therequirements of advanced systems. Polyhedral oligomeric silsesquioxanes (POSS) are three-dimensional networkswith a silica-like core covered by an organic shell, a promising nanosized structure for organic coatings. The use ofPOSS structures in coating materials has experienced an almost erratic period over the past decade, mainly becauseof the synthesis of POSS being exclusive and, to some extent, expressive. However, there has been a big return tothe use of POSS in coatings during the last years. Epoxy is best known for its versatility of usage as an organiccoating. Hybrid organic–inorganic POSS nanostructures with unique properties are a complement to the epoxy forcoating applications. The tailorable compatibility and considerable reactivity of POSS molecules towards organicgroups have made them promising candidates for emerging multifunctional epoxy coatings. In this sense, the focusof this review is placed on POSS/epoxy nanocomposite coatings. The thermal, mechanical, anti-corrosion anddielectric properties of POSS/epoxy coatings have been comprehensively studied and discussed.

1. IntroductionEpoxy nanocomposites have attracted considerable attention overthe last decade due to the promising effect of nanoscale particlesas the building blocks of innovative systems.1,2 Incorporation ofinorganic fillers in the epoxy coating has been mainly aimedat combining the auspicious features of epoxy with fillers forhigh thermal, mechanical, and protective properties.3–5 Theimprovement of physical properties of epoxy nanocompositecoatings roots in the synergic effects of the cure moieties withnanoparticles.6,7 Recent developments in epoxy nanocompositecoatings were responsible for considerable drastic enhancementsin the physical properties of epoxy – for example, mechanicalproperties, chemical resistances, thermal stability, self-healing,corrosion protection and fire retardancy.8–11 Nevertheless, novelnanomaterials with multifunctional characteristics are still on theroad of innovation to meet engineering requirements.

Polyhedral oligomeric silsesquioxanes (POSS) are sub-members ofinorganic–organic hybrid materials mostly containing zero-dimensional to three-dimensional (3D) cage-like building blocksmade of silicon/oxygen structure (RSiO3/2)n, where R is a hydrogenor a hydrocarbon-based moiety (e.g. aryl, alkylene, alkyl andarylene).12–15 In the POSS architecture, (SiO3/2)n is located at the

centre as the core but linked to the hydrocarbon-based moietieslocated outside of the centre as the shell.16,17 Therefore, withcontrolling and manipulating the chemistry of shell molecules, thechemical activity of the monomer can be changed, and its sizediameter can be tuned between 1 and 3 nm.18 POSS are a kind ofhydrophobic nanoparticle. The closed cubic silica structure in POSShas extremely low polarity.19 Therefore, most hydrocarbon-substituted POSS derivatives are soluble in weakly polar organicsolvents, such as chloroform, tetrahydrofuran and sometimeshexane.20 However, the hydrophobic nature of POSS limits theirapplication in polar environments (i.e. aqueous or ethanolsolutions).21 Hydrophilic modification of POSS results in achievingexcellent solubility in water and some nonaqueous polar medium likeethanol.22 To be more precise, the presence of these hydrocarbon-based moieties as the shell facilitates their postmodification withdifferent functional groups such as carboxylic, alcohol and aminethen increases the solubility, stability, compatibility andhydrophilicity of the POSS for further uses.23,24 Their distinctivecharacteristics such as good mechanical and thermal properties,hydrophilicity, particle size tunability, environmental neutrality andhigh thermal/physiochemical stability coupled with an abundance ofprecursors render them excellent nanofillers for the combination andenhancement of the physicochemical features of complexes.25–28

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Surface InnovationsVolume 9 Issue 1

Polyhedral oligomeric silsesquioxane/epoxy coatings: a reviewSeidi, Jouyandeh, Taghizadeh et al.

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Figure 1(a) presents the molecular architecture of POSS and someof their inherent features. Recently, according to recent publications(Figure 1(b)), researchers have employed POSS and modified POSS

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in a pristine form or as nanofillers in the composition of variouscomplexes (e.g. polymeric networks) at an exponential rate forutilisation in diversified requests from water treatment,29 gasseparation30 and catalysis31 to storage (e.g. fuel cell and lithium ionbattery),32 cosmetics,33 electronic34 and medicine (e.g. cancerresearch).35 Figure 2 depicts a broad range of POSS applications indifferent industries.

POSS have properties similar to those of inorganic materials suchas chemical resistance, high modulus and thermal stability as wellas those of organic polymers like ductility, processability and lowtoxicity.36 Their high potential to achieve desired propertiesthrough their structure, which is tailored by various functionalgroups, makes them antecedent additives for optimising theproperties of polymer matrices. Therefore, it is a promisingopinion to utilise POSS for the enhancement of polymericnetwork properties. The aim of this review is to understand howPOSS nanocages affect the ultimate properties of epoxy coatings,particularly mechanical, thermal and anti-corrosion performances.

2. POSS in epoxy coatingsCore/shell architecture of POSS comprises inorganic and organichybrid will donate them compatible with a polymer host.Different from silica or silicone, a POSS cage is surrounded byeight organic groups bonded to the silicon vertices. A variety of

R

Unreacted R group, usually forstability, solubility and

compatibility

One or more reactive R groupsfor further modification,

grafting or polymerisation

Thermally and chemically robustorganic–inorganic framework

Well defined, 3D structure forintroduction into polymers and

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O

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O

OO

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Figure 1. (a) Schematic diagram of POSS molecular architectures and their general features. Reprinted with permission from Zhenget al.14 Copyright 2020 Elsevier. (b) The number of released published articles related to ‘POSS’ and ‘POSS and coating’ (Source: Scopus,19 April 2020)

Composite resin

Catalys

tBattery

DyeCosmeticMed

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Figure 2. Summary of applications of POSS in diversifiedindustries, designed by the authors



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organic groups from methyl and isobutyl to cyclopentyl andaniline can be positioned around the silicate (Si8O12) core at thecage vertex to interact with the epoxy matrix. Furthermore, bysubstituting one or more corner groups with appropriatefunctional groups, POSS can be architected for use in advancecoatings technology. Various functional groups including amine,alcohol, phenol, styrene, epoxy, acrylate and methacrylate givethe opportunity to design diverse POSS/epoxy coating formultifunctional applications. The modification of a POSS surfaceresults in a strong interfacial area between the epoxy resin andPOSS, because functional groups can interact with epoxide ringsand lead to a denser cross-linked network that causes ananocomposite to have higher thermal and mechanical properties.

The dimensions of POSS cages are comparable with those ofpolymer segments or coils, which provide the possibility ofbonding with a polymer and reinforce it in a molecular level.24

Moreover, POSS could be categorised into highly symmetricmolecule families such as dendrimers. The rough spherical shapeof POSS cages makes them potent to interact with the epoxymatrix in the three dimensions of the surrounding space. Inaddition, POSS molecules can be architected in a distinctgeometry through aggregation or crystallisation into lowersymmetric supermolecules. Although small-sized POSS candisperse in the polymer matrix as nonreactive components, theirmore powerful implementation arises from their copolymerisationin a polymeric system.37 Therefore, POSS in the epoxy matrixcan act either as fillers or as plasticising moieties depending ontheir dispersion state in the polymer matrix.38

Various researchers have focused on the interaction of POSS withepoxy in POSS/epoxy nanocomposites, which can be categorisedas the chemical reaction of multifunctional POSS and theirparticipation in the cross-linking of epoxy,39–41 covalent bondingbetween monofunctional POSS and epoxy42–44 and physicalmixing of inert POSS in the epoxy matrix.45 Different from that inmultifunctional POSS, the cross-linking density of epoxy is notpositively affected by incorporating monofunctional pendant POSS.However, physically bonded pendant POSS act as reinforcementand increase the glass transition temperature (Tg) of the system.46

In an ideal case, epoxy networks are almost free of structuraldefects. However, by incorporating POSS in the epoxy, thenetwork structure perturbed due to the formation of defects, whichdecrease the cross-linking density. This is because of thereplacement of a diepoxide in the network by thePOSS–monoepoxide. This adverse feature of nanocompositeslimits the reinforcing effect of POSS domains in the epoxy matrix.Using the self-assembled POSS units in the epoxy network to formthe ordered organic–inorganic hybrid structure can avoid thisproblem. Matějka et al.47 prepared a novel POSS/epoxy hybridnetwork in which the POSS units self-assemble to form crystallinelamellae. They showed that the self-assembly of POSS units in thenetwork strongly affects the thermomechanical properties of theepoxy network. They found that the epoxy containing POSS with


octyl (Oct) organic substituents decreased the rubbery modulus ofthe epoxy network, while the incorporation of POSS with phenyl(Ph) and cyclopentyl (Cp) monomers reinforced the network.POSS-Oct perturbed the epoxy network and diminished the cross-linking density of epoxy. However, POSS-Ph formed crystallinedomains and hard physical junctions in the epoxy network, whichincreased the modulus.46 POSS-Cp self-assembly within the epoxynetwork produced the ordered hybrid physical cross-linking andreinforced the network. By contrast, the flexible Oct substituents ofPOSS-Oct formed only soft amorphous aggregates, which droppedthe modulus. Ma et al.48 showed that different morphologies canbe obtained by evolving cage- or linear-structured POSS into anepoxy matrix based on their dispersion state. They used cage-structured methacrylisobutyl POSS (MA-POSS), cage-structuredaminopropyllsobutyl POSS (NH2-POSS) and linear-structureddiamino-terminated poly(dimethylsiloxane) (NH2-PDMS) forcuring polyglycidyl methacrylate (PGMA), as shown in Figure 3.It was found that in the pre-cured PGMA/POSS hybrid systems,MA-POSS, NH2-POSS and NH2-PDMS aggregated into the self-assembled spherical micelles with diameters of 250, 400–500 and300–400 nm, respectively. The surface morphologies of PGMA/MA-POSS and PGMA/NH2-POSS (samples 1 and 2 in Figure 3)exhibited a rough surface due to the high agglomeration of POSSin the film surface that led to the microphase segregation from theepoxy matrix. By contrast, linear-structured NH2-PDMS resultedin the smooth surface of the PGMA film.

On the ground of the above discussion, the unique structure ofPOSS constituting of an inorganic core and an organic shellmakes POSS promising additives to obtain a wide range ofultimate properties of the epoxy matrix. The remarkable effect ofPOSS in the epoxy matrix is devoted to the enhancementof ultimate properties such as Tg, decomposition temperature,chemical resistance, mechanical strength and anti-corrosionproperty. However, optimisation of properties necessitatesdeepening the chemistry of the POSS architecture as well as theinteraction between POSS and epoxy.

2.1 Mechanical propertiesEpoxy resins are widely used as paint and coatings in our dailylife thanks to their good mechanical properties.49–51 However, theinherently brittle nature due to the rigid aromatic and phenylgroups and highly cross-linked glassy network of epoxy results inpoor impact strength.52,53 The mechanical limitation of epoxyresins impedes them from further utilisation as high-performancecoatings. Therefore, many research studies have focused oncompensating the poor ductility and alleviating the fracturebehaviour of epoxy coatings.

POSS as novel organic–inorganic hybrid materials with unique3D cage structure and possessing flexible silicon bond candedicate the coating an excellent mechanical strength.54,55

Moreover, the octa-edge organic functional group of POSSmolecules gives them flexible performance.56,57 Therefore, theaddition of pendant units of POSS into the epoxy matrix makes it

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possible to have coatings with high fracture resistance and goodbulk modulus. The inner inorganic silicon–oxygen–silicon(Si–O–Si) core with an organic shell renders excellentcompatibility and functionality to POSS molecules that let themalter the mechanical properties of epoxy coatings. Organicfunctional groups of POSS and their weight fraction are twoimportant factors that affect the fracture toughness of epoxyresins. The effect of functionality and content of POSS on themechanical properties of epoxy resins in recent studies arerepresented in Table 1.

Incorporating POSS into the epoxy resin can impart a significanteffect on the ductility and fracture toughness behaviour of epoxycoatings. POSS comprises an inorganic rigid core and organicflexible pendant groups. The inorganic core of POSS providesrigid sites that can transfer the stress and energy through theextremely ductile epoxy matrix. The hollow-structured core ofPOSS can play the role of a stress concentrator to improve thefracture toughness of epoxy.74,75 The stiff silica cage of POSStoughens epoxy coatings through the shear-yielding, crack-bridging and crack-pinning mechanism. Moreover, the pendantgroups and functionalities of POSS molecules allow them todisperse well in the epoxy matrix and create a strong interphaseregion that promotes energy absorption. The unique cage structureof POSS results in epoxy chain flexibility by increasing the freevolume in the network.76–78 Furthermore, the organic groups ofPOSS create a well-dispersed nanocomposite that enhances themolecular-level interactions between POSS and the epoxy matrix.The functional groups of POSS can contribute to the cross-linkingreaction of epoxy, permitting the formation of a continuousnetwork. POSS/epoxy nanocomposites enable load to be


distributed on the POSS units, resulting in the enhancement of theYoung’s modulus of the system.79,80

The change in the failure mode from brittle to ductile is dependentnot only on the functionality of POSS but also the loading of POSS.The change in the flexural response can be attributed to the self-assembly of POSS molecules via POSS—POSS interaction at higherloadings. It has been shown by many researchers that POSSmolecules have the capability to form self-assembled aggregation indifferent morphologies such as spheres, cylinders and vesicles, whichis a function of processing conditions.81–83 This self-aggregatedPOSS forms a soft core such as a conventional organic filler that canact as a plasticiser. Therefore, at high loading of POSS creation ofsoft shell domains raises the flexibility of the network and results inchanging the failure mode from brittle to ductile.

2.2 Anti-corrosion propertiesThe application of epoxy as a protective coating on metal surfacesfor the reduction of metal degradation in corrosive media hasreceived enormous scientific and industrial attention because of itseffectiveness and high performance and ease of use.84–86

Nevertheless, the existence of some defects such as cracks andmicropores in epoxy coatings may cause penetration of corrosivespecimens from aggressive media into the coatings and eventuallyreduce their long-term corrosion resistance.87–89 Therefore, theenhancement of the density of epoxy coatings is an urgentproblem to be solved.

Incorporation of POSS nanocages into the epoxy coatings caneffectively improve their anti-corrosion potential due to organic/inorganic hybrid systems’ unique structure, excellent chemical

PMDETA

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Figure 3. The surface morphology of PGMA containing MA-POSS (sample1), NH2-POSS (sample 2) and NH2-PDMS (sample 3), in (a–c) apre-cure system by transmission electron microscopy and (d–f) in a casted film by scanning electron microscope. Reprinted with permissionfrom Choi et al.39 Copyright 2020 Elsevier.



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stability and high insulativity.36,90,91 The presence of POSS inepoxy coatings could strongly improve its anti-permeability andavoid the direct contact of the metal surface with aggressiveenvironments and suppress the corrosion reaction. Therefore,POSS have been welcomed by researchers as additives for


corrosion-protection epoxy coatings. Recent efforts by researchersin this field are summarised in Table 2.

The corrosive components (chloride (Cl−), dioxygen (O2) andwater (H2O)) can easily penetrate into pure epoxy coatings

Table 1. The main outcomes of research studies on the mechanical properties of POSS/epoxy coatings (continued on next page)

Number
POSS Modifying agent Main finding Reference
Composites of epoxy

1
Glycidyl POSS (GPOSS) Carboxyl-terminated poly(acrylonitrile-co-butadiene)(CTBN)
From fracture toughness measurement, itwas found that the stress intensity factor(KIC) of epoxy was improved by about62% by 5 phr CTBN, 59% by 2.5 partsper hundred (phr) POSS and only 38%by their hybrids.

Konnolaet al.58

2
GPOSS DodecaPhenyl (DPHPOSS) Multi-walled carbonnanotubes (MWCNTs)
The hybrid of 0.5% MWCNT and 5%GPOSS improved strength at break ofepoxy by about 123% and elongation atbreak by about 32%. However, thehybrid of 0.5% MWCNT and 5%DPHPOSS improved strength at break ofepoxy by about 82% and elongation atbreak by about 50%.

Barra et al.59

3
Methacryl-POSS GPOSS Trisilanolphenyl-POSS
—
The fracture toughness of epoxyreinforcement by GPOSS increased by afactor of 2.3 at 5 wt% loading, whilethat by methacryl- and trisilanol phenyl-POSS at 5 wt% loading increased byfactors of 1.6 and 1.3, respectively.
Mishra et al.60

4
Octamercaptopropyl (OMP)-POSS Poly(cyclotriphosphazene-co-4,40-sulfonyldiphenol) (PZS)nanotubes
Three weight percent of OMP-POSSgrafted onto PZS nanotubes improvedthe storage modulus at 30°C and Tg ofepoxy by 88% and 16°C, respectively.

Li et al.61

5
Mono and poly double-deckersilsesquioxanes (mono DDSQ and polyDDSQ, respectively)
—
The mono DDSQ showed a more flexiblestructure and toughened the epoxy resinbetter (at a loading of 8%, thetoughness increased by about 50% overthe neat epoxy resin), while thebranched poly DDSQ exhibited betterthermal resistance.
Cao et al.62

6
Methacrylisobutyl POSS (MA-POS)Aminopropyllsobutyl POSS(NH2-POSS)
Diamino-terminated poly(dimethylsiloxane)(NH2-PDMS)

The introduction of MA-POSS into theepoxy matrix resulted in strong adhesivestrength (1113 N) and highthermostability (Tg = 282°C), but theflexible PDMS improved the storagemodulus of epoxy (519MPa) more thandid NH2-POSS (271MPa).

Ma et al.48

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Aminopropyl iso-octyl POSS (AP) Epoxide-functionalhyperbranched polymer(HBE)
Addition of 1 wt% AP + 10wt% HBE ledto a 229% increase in fracturetoughness, and 5 wt% AP + 10wt% HBEincreased fracture toughness by about223%.

Misasi et al.63

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GPOSS TriglycidylCyclohexyl (TCPOSS)Epoxycyclohexyl POSS (ECPOSS)
MWCNT
The self-healing efficiency of the epoxysystem enhanced by about 40% byaddition of 5% GPOSS + 0.5% MWCNT,45% by adding 5% DPHPOSS + 0.5%MWCNT and 30% by incorporating 5%ECPOSS + 0.5% MWCNT.
Guadagnoet al.64

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POSS-(3-glycidyl)propoxy-heptaisobutyl(Ep-POSS) POSS-(3-mercapto)propyl-heptaisobutyl (SH-POSS)
—
The compressive strength of epoxy curedwith pentaerythritol tetrakis (3-mercaptopropionate) was decreasedfrom 40 to 17 and 10MPa by additionof Ep-POSS and SH-POSS, respectively.
Lungu et al.65

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through defects and reach the metal substrate (Figure 4a). Theshielding effect of pure epoxy coatings arises only from thethickness of coating, which leads to a poor protecting effectand results in redox reactions and exacerbates the corrosion ofthe metal surface.99 For long immersion in corrosive media, thesurface of metal coated by pure epoxy may seriously corrodedue to the moderate impermeability of the epoxy coating thateventuates from the micro-size defect in the epoxy coating.


In the case of POSS-incorporated epoxy coatings, the well-dispersed nanocage can effectively fill micro-holes and defects inthe epoxy matrix and increase the density of the coating, whichconsequently reduces the risk of corrosion reactions. In addition,octafunctional POSS can act as connection points in the epoxynetwork and enhance the network formation of epoxy throughincreasing the cross-link density and blocking segment motion inthe rigid region around the POSS unit.100 The functional groups

able 1. Continued

Number
Composites of epoxy

10
Octavinyl-POSS Graphene oxide (GO) Impact strength of epoxy containingPOSS-GO enhanced 29.29% comparedto that of epoxy containing GO at 0.8phr.
Zhang et al.66

Fibre reinforced epoxy

Number
POSS Modifying agent Fibre Main finding Reference
11
GPOSS TriSilanolIsooctyl (TSI)-POSSTriSilanolPhenyl (TSP)-POSS
—
Carbon fibre (CF) The TSI-POSS-coated fibres giveabout 82% increase in theflexural strength of acid-treated CF/epoxy, while TSP-POSS- and GPOSS-coatedfibres give about 76 and 23%increase, respectively.
Ibn Afzalet al.67

12
Octaglycidyldimethylsilyl POSS GO CF CF–GO significantly increasedthe interlaminar shear strength(ILSS) of the composites by34.77% compared to that ofthe de-sized CF composites.The ILSS of the CF–GO–POSScomposites possessed thehighest increase of 53.05%.
Zhang et al.68

13
Octa(g-chloropropyl) POSS Hyperbranchhexamethylenediamine
CF
The interfacial shear stress (IFSS)of the CF–POSS–NH2/epoxycomposites had anenhancement of about 57.7%in comparison with untreatedCF/epoxy composites.
Ma et al.69

14
Octaammonium POSS (POSS-NH2) Co-incubation inthepolydopamine(PDA)
CF
The IFSS of CF-PDA/POSS is117.1MPa (63.8%amplification to that of pristineCF).
Yang et al.70

15
Octaglycidyldimethylsilyl POSS Zinc oxide (ZnO)nanowires
Poly(p-phenylenebenzobisoxazole)(PBO) fibres

The reinforcement offered an83.4% enhancement in theIFSS by addition of PBO–zincoxide–POSS.

Chen et al.71

16
Sulfonated octaphenyl POSS(SOP-POSS)
—
CF The calculated IFSS values showthat the presence of surface-adsorbed SOP-POSS onto thesurface of oxidised CFsignificantly improves theinterfacial properties of thecorresponding composites byapproximately 15%.
Kafi et al.72

17
bis(heptaphenylaluminosilsesquioxane)(AlPOSS)
Zeolite
Basalt fibre The presence of 5 wt% zeoliteand 1 wt% AlPOSS helped toimprove the impact strength ofthe composite by about 35%.
Matykiewiczet al.73

K/S, color strength



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Table 2. The main outcomes of research work on POSS/epoxy anti-corrosion coatings

[

Number

] on [10/03

POSS

/21]. Copyright © ICE P

Modifying agent

ublishing, all rights reserve

Corrosive media

d.

Metal
Main finding Reference
1
OctaaminopropylPOSS (OapPOSS)
Diethyl maleate (MA)
3.5 wt% sodiumchloride (NaCl)solution
Carbonsteel

The impedance modulus at 0.01 Hzfor the OapPOSS–MA/epoxycoating exhibits high value(1.43 × 109W cm2) after 40 dayscompared with pure epoxy(1.72 × 107W cm2).

Chen et al.92

2
EpoxycyclohexylPOSSGlycidyl POSS(GPOSS)
Hydroxyl-terminatedmethyl phenylsilicone (PSi)

5 wt% sulfuricacid (H2SO4)solution5 wt% NaOHsolution5 wt% sodiumchloride solution

Tinplate
Thirty days of immersion indicatedthat the anti-corrosion property ofthe coatings in 5 wt% acid,alkaline and saline environmentswas all improved.
Xiong et al.93

3
POSS-NH2 Graphene oxide (GO) 3.5 wt% sodiumchloride solution
Mild steel
The impedance value at 0.01 Hz ofthe epoxy matrix reached6.95 × 107W cm2 after 150 days,the |Z|0.01 Hz value of 0.5 wt%POSS-NH2 functionalised GO/EPsample was still kept at a highlevel of 2.16 × 109W cm2.
Ye et al.94

4
Glycidylisobutyl-POSS (POSSmono)Triglycidylisobutyl-POSS (POSStri)GPOSS (POSSocta)
—
3.5 wt% sodiumchloride solution
Low alloysteel

The |Z|0.01Hz value of epoxycontaining 5 wt% of POSSmono isabout 1 × 1010W cm2 for theepoxy system containing 5 wt% ofPOSStri is about 1 × 109W cm2,while the impedance valuesdecreased to about 1 × 109W cm2

for pure and POSSocta-incorporated epoxy after18 weeks.

Longhiet al.95

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POSS-NH2 Tris(p-isocyanatophenyl)thio phosphate(DESMODUR)

Mild steel
The coating system containingPOSS-NH2 and DESMODURretained an impedance value of109W cm2 after 30 days ofimmersion, while the impedancevalues of the pure specimendecreased to 1 × 106W cm2.
Kumar andSasikumar96

6
POSS-NH2 GO 3.5 wt% sodiumchloride solution
Q235 steel
The |Z|0.01Hz values of epoxy/0.5%POSS-modified GO (PG) andepoxy/1% PG coatings were up to3.16 × 109 and 1.01 × 109W cm2,respectively, which were higherthan those of pure epoxy andepoxy/0.5% GO (2.78 × 108W cm2)coatings at initial immersion. After50 days of immersion, the |Z|0.01Hzvalues of all coatings sharplydecreased; nevertheless, the |Z|0.01Hz value of epoxy/0.5% PGcoating was still kept at a high levelof 1.12 × 108W cm2.
Ye et al.97

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AminopropyllsobutylPOSS
Tetraaniline (TA)Graphene (G)


Q235 steel
As the immersion time increasedfrom 80 to 100 days, the |Z|0.01Hzvalue of POSS-TA-G0.5%/epoxycoating increased from 2.13 × 109
to 2.76 × 109W cm2. Also, the |Z|0.01Hz value of POSS-TA-G0.5%/epoxy coating wasmaintained at 109W cm2 after 120days of immersion.

Ye et al.98

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of POSS can participate in the oxirane ring opening of the epoxyresin, which provides higher adhesion and assists the stabilisationof the system by preventing rearrangement of system moieties bysubjecting the coating to sodium chloride (NaCl) immersion.101 Inthe denser cross-linked POSS/epoxy network, nanocages blockthe penetration channel and prevent the diffusion of a corrosivemedium into the coating. Moreover, the addition of POSScomplicates the penetration path and delays the reaching ofelectrolytes to the metal surface. The super-hydrophobic nature ofcage-structured POSS reduces the transition of corrosive speciesand barricades the direct contact of corrosive solution and themetal surface by forming a protective layer over the metal.102

Ye et al.98 prepared an epoxy coating containing tetraaniline (TA)grafted POSS for corrosion protection of steel. They found that theelectrons of the metal anodic dissolution were captured by POSS-TA and changed from the intermediate oxidation state emeraldinebase (EB) to the reduced state leucoemeraldine base (LEB), whichis attributed to the electroactivity of POSS. Simultaneously, in thepresence of O2 in the systems, iron(2+) (Fe2+) and iron(3+) (Fe3+)were oxidised to iron(III) oxide (Fe2O3) and iron(II,III) oxide(Fe3O4) formed a passive film on the surface of Q235 steel. Inaddition, as shown in Figure 4, released electrons through theoxidation of reduced POSS-TA to the oxidation state (PB)promote the formation of a passivation layer.103

Xiong et al.93 measured the anti-corrosion property of tinplatesubstrates coated with the blend of hydroxyl-terminated methylphenyl silicone (PSi) rubber and silanised epoxy resins containingEpoxycyclohexyl POSS and glycidyl POSS were immersed in5 wt% sulfuric acid (H2SO4) solution, 5 wt% sodium hydroxide(NaOH) solution and 5 wt% sodium chloride solution respectivelyfor 30 days. Both the POSS-modified coatings performed goodanticorrosion properties in the saline and alkaline conditions butboth showed poor performance in the acid condition. Because thepolished surface of the tinplate substrate was prone to acidcorrosion, this led to hydrogen generation and formed air bubblesbetween the substrate and the coatings. Then the bubbles in theinterface of substrate and coating detached gradually and theadhesion loosed. Another reason is the fragility of the cage


structure of POSS in the acid condition. The POSS cagedecomposed to silicon–hydroxide (OH) compounds anddisappeared in acid corrosive media.104 However, in the alkalinecondition where there is no acceleration effect of a hydrogen ionon the substrate, much less corrosion could appear.105

2.3 Thermal propertiesGenerally, the applications of epoxy coatings are restricted inharsh environments due to their limited thermal stability.106,107

The rigid cage-like structure of POSS could effectively enhancethe thermal stability and oxidation resistance of polymers dueto the formation of an inert char layer on the surface ofcoating.108,109 Three important factors may be responsible for thethermal stability of POSS/epoxy nanocomposites including theloading of POSS, the interaction between POSS and the epoxymatrix and the cross-linking density.110,111 The main findings ofrecent studies on the thermal properties of POSS/epoxynanocomposite coatings are reported in Table 3.

The enhancement of the thermal stability of POSS/epoxynanocomposites arises from the interconnections of the POSScages with the epoxy ring of the matrix at the molecular level,which promotes the formation of highly cross-linked network.The dimensional stability of the POSS/epoxy network preventsthe degradation of the epoxy coatings at elevated temperaturesand increases their thermal stability. Well-dispersed POSS in theepoxy matrix can suppress the mass loss of segmentaldecomposition of the network through the gaseous fragments.121

POSS can greatly improve the thermal stability of epoxy coatingsby promoting the formation of char residues and taking the heataway to reduce the decomposition of coating.122 It is expectedthat nano-sized POSS with a 3D cage structure and high siliconcontent offer astounding effects on the thermal properties ofcoatings.123,124 When heat is applied on the POSS/epoxycoatings, a silicon network forms on the surface of the polymermelt and results in a hard and stable residue.125 Incorporation ofPOSS molecules containing a silica-like silicon–oxygen–siliconstructure in the cured epoxy coating leads to higher inorganicmoieties in the system, which consequently results in a higher

(a) (b)

O2

O2 O2

H2O

O2

H2O

H2O

H2O

H2O

H2O

Cl– Cl–

Cl–

Cl–

2e–

2e–

2e– 2e–

+ +

2e–

2e– FeOOH Fe2O3+Fe3O4

Fe(OH)2

Cl–

Na– Na–

Cl–

H+ H+

OH

OH

OH OH

Fe2+Fe2+

Fe Fe Fe Fe Fe

Fe

Q235 steel Q235 steelPB EB LEB

Possive film

Fe2+ Fe3+

Figure 4. Protection mechanism of (a) neat epoxy coating and (b) epoxy coating containing POSS-TA. Reprinted with permission fromGoffin et al.91 Copyright 2020 Elsevier.



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char residue.126 The silicon–oxygen–silicon linkage barricades theoxidation of the underlying epoxy matrix. Silicon in the structureof POSS could form a ceramified inert silica char layer on thesurface of the coating when decomposition takes place to keep thecoating from undergoing more thermal degradation.127–129 Thisstable and highly compact protective char layer can prevent thediffusion of heat and oxygen to the inner layers of epoxy matrixand reduce the degradation rate of epoxy coatings.

In addition, as apparent from Table 3, incorporating POSS in theepoxy matrix could increase or decrease the Tg of the system, whichis a function of dispersion state and interaction quality. The nature ofinteraction between POSS and the polymer matrix strongly dependson the loading level and the organic groups. Chemical interactionbetween the epoxy matrix and POSS molecules constrains the chain


motions of the network and enhances Tg. By contrast, in the caseof a weak POSS–epoxy interaction, POSS cages act as plasticisersthat could disrupt intermolecular bonding and hinder cross-linkingreaction, resulting in a higher free volume in the system.130,131 Thecross-link density of POSS/epoxy networks is controlled byregulating POSS–POSS and POSS–epoxy interactions. Moreover, inthe case of epoxy containing monofunctional POSS, the loading ofPOSS is another key parameter. At low loading, pendant POSS havea plasticising role in the epoxy network. However, at higher loading,the aggregated and physically cross-linked POSS can act as fillersand reinforce the system.118

2.4 Dielectric propertyEpoxy resins can be used as dielectric materials for cableterminations and rotating machines in the high-voltage insulation

Table 3. The main findings of research work on thermal properties of POSS/epoxy coatings

Number
1
POSS-NH2 — The initial decomposition temperature and thechar yield of pure epoxy were 290°C and 20%,respectively, while they significantly improved to320°C and 26% for POSS-NH2-incorporatedepoxy, respectively.
Meenaksi andSudhan112

2
Octaglycidyldimethylsilyl POSS(OGPOSS)
—
An OGPOSS/epoxy nanocomposite containing10 wt% OGPOSS exhibited a reduction of 67%in mass loss compared with neat epoxy at125°C under high vacuum for 24 h.
Choi et al.113

3
Methacrylisobutyl POSS (MA-POSS) — MA-POSS could significantly increase initialthermal decomposition from 212.7°C for pureepoxy to 288.9°C, and the char residueenhanced from 14 to 12.3%.
Ma and He114

4
Aminopropylisobutyl POSS 9, 10-Dihydroxa-10-phosphaphenanthrene-10-oxide (DOPO)
Addition of 2.5% DOPO + 2.5% POSS enhancedthe char residue of epoxy at 800°C from 16 to18%, while 5% POSS increased it to 20%.

Wu et al.115

5
Aminopropylisobutyl POSS DOPO tetrabutyl titanate(TBT)
The Tg of epoxy increased from 155 to 161°C byaddition of 5% TBT + 5% POSS-bisDOPO.Moreover, the initial decomposition temperatureand the char residue improved by 22°C and19.5%, respectively.

Zeng et al.116

6
Octa-(N,N-(bis-(9,10-dihydro-9-oxa-10-phosphaphenantheneyl) methyl)aminopropyl) silsesquioxane(ODMAS)
—
The char residue of epoxy at 800°C was increasedfrom 18 to 26% by addition of 15% ODMAS.
Liu et al.117

7
Aminopropylisobutyl POSS — The char yield at 595°C increased from 16wt%for control epoxy network to 19, 18 and26 wt% with the epoxy hybrid networkcorresponding to 7.2 POSS, 21.8 POSS and54 POSS, respectively.
Sharmaet al.118

8
Glycidyl POSS (GPOSS) — The Tg of epoxy/polyurethane copolymerincreased from 53 to 64 by adding 0.2 wt% ofPOSS and then decreased in higher loading,while the char residue increased by increasingPOSS loading and reached from 6.25 to 19.77%in the case of the 10wt% POSS-incorporatedcopolymer.
Zhu et al.119

9
OGPOSS Layered zirconiumphenylphosphate (ZrPP)
One mass percent ZrPP and 4 mass % POSS wereadded to epoxy, the T50%, Tmax and char residueincreased to 407°C, 385°C and 22.2%,respectively, compared with those of pure epoxyat 383°C and 16.4%.

Zhou et al.120

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industry.132,133 However, an epoxy resin as an organic coating isexposed to degradation by electrical discharge.134,135 Theapplication of epoxy resins as low-k dielectric materials insemiconductors and large-scale integrated circuits is limited dueto their relatively high dielectric constant and dielectric loss.POSS can impart an extra excellent property like the dielectricproperty to epoxy coatings. The effectiveness of incorporatingPOSS into epoxy resins as a reinforcing agent for the dielectricperformance of coatings has been shown in literature in terms ofdielectric breakdown strengths and corona resistance.136–139 Heidet al.140 showed that the addition of 2.5 wt% oftriglycidylisobutyl-POSS into an epoxy resin resulted in asignificant increase in dielectric breakdown strength from214.9 kV/mm for neat epoxy to 241.7 kV/mm. However, byfurther increasing the POSS content, the dielectric breakdownstrength decreased, which can be due to the formation of aconductive interfacial layer around aggregated POSS, which cancreate a conductive path. It was shown by Dhanapal et al.141 that1.5 wt% of POSS-NH2 decreased the dielectric constant of anepoxy/carbon fibre composite from 4.10 to 2.05 because thepolarisability of the system was reduced.

3. Summary and future directionThis review article attempted to summarise significant research onthe diverse properties of POSS/epoxy nanocomposites occurringduring the recent years. POSS in epoxy have developed severalimproved properties such as mechanical properties, thermalstability, high corrosion protection and dielectric properties. It waslearned from the literature that the loading level, dispersibility andaggregation state of POSS in the epoxy matrix have prominent rolein determining ultimate properties of coatings. In addition, theability of POSS to interact with the epoxy host greatly affects themicrostructure and final properties of POSS/epoxy nanocomposites.It has been found that diversification of the POSS vertex (R) groupcan control the interactions between the POSS and the epoxymatrix, which has a significant impact on the microstructure andfinal properties of POSS incorporated into epoxy coatings. ThoughPOSS/epoxy nanocomposites possess outstanding properties, thelarge-scale applications of these coatings have been limited dueto economic reasons. There is a need to deeply understand thestructure–property relationship in a POSS/epoxy system to developoptimised structures. It is anticipated that research on the field ofPOSS/epoxy nanocomposite coatings will continue to develop theirapplications that exploit the unexpected properties of cage-likePOSS.

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Polyhedral oligomeric silsesquioxane/epoxy coatings: a review