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Journal of Sol-Gel Science and Technology

, Volume 46, Issue 3, pp 335–347 | Cite as

Perfect and nearly perfect silsesquioxane (SQs) nanoconstruction sites and Janus SQs

  • R. M. Laine
  • M. Roll
  • M. Asuncion
  • S. Sulaiman
  • V. Popova
  • D. Bartz
  • D. J. Krug
  • P. H. Mutin
Original Paper
First Online:
Received:
Accepted:

Abstract

Cubic silsesquioxanes (SQs) offer perfect and nearly perfect 3-D symmetry with eight functional groups with each functional group occupying a different octant in Cartesian space. This paper is concerned with the synthesis of both highly symmetrical octa-monofunctional SQs and statistically bifunctional SQs wherein because of similar properties each functionality separates to one side of the cubically symmetric SQ giving Janus-like behavior. In the latter case we find these materials to offer novel properties of use in making both coatings with controlled contact angles ranging from 50–120 °C, with hardnesses (pencil) as high as 6H and the opportunity to make layer-by-layer coatings.

Keywords

Silsesquioxanes Perfect 3-D nanobuilding blocks  3-D star molecules Janus silsesquioxanes Layer-by-layer coatings Spray coating 

1 Introduction

The synthesis and assembly of 2- and 3-D structures from molecular components (nanobuilding blocks) is of great current interest because of the potential to realize novel properties in nanostructured materials. The use of space filling 2- and 3-D structures requires components that fulfill the mathematical relationships found for Bravais lattices. Cubic structures are expected to offer the highest symmetry and therefore would require the least energy to assemble [1].

Numerous symmetrical 2-D molecules are described in the literature. Sets of molecules with highly symmetrical 3-D functionality, e.g., tetrahedranes, adamantanes, cubanes, dodecahedral boranes, etc., are also known [1, 2, 3, 4, 5, 6, 7, 8, 9]. However, few molecules offer perfect cubic symmetry and octafunctionality with one functional group in each octant in Cartesian space. Such compounds include the difficult to prepare cubanes [1, 2, 3], and easily prepared cubic silsesquioxanes (SQs), Q8 (RMe2SiOSiO1.5)8 and T8 (RSiO1.5)8 of Fig. 1 [10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54]. SQs offer potential access to macroscopic structures assembled nm × nm in 2- or 3-D. They also offer unique properties in their own right.
Fig. 1

SQs. (a) Q8 (Q = SiO2/2) R = H, vinyl, epoxy, methacrylate, etc. (b) RPh8 (T = R-SiO3/2) R = alkyl, alkene, alkyne, R’X (X = halogen, amine, epoxy, etc. R’ = R same or mixed)

Silsesquioxanes contain rigid silica cores with eight vertices (body diagonal ≈ 0.5 nm) with an organic functional group in each octant in Cartesian space giving spherical, nanobuilding blocks 1–2 nm in dia [10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54]. The positioning of the functional groups, the variety possible, and their size provide unique opportunities to build nanocomposites in 1-, 2-, or 3-D with tailorability at nanometer length scales [10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54]. In addition, the core adds the rigidity and heat capacity of silica making these compounds quite robust, e.g. [PhSiO1.5]8 (OPS) [31] is air stable to >500 °C. As such, cubes (e.g., Fig. 1) are subjects of diverse studies [10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54].

In the past decade, we have published more than 30 papers on octafunctional SQs and nanocomposites from them [32, 33, 34, 35, 36, 37, 38, 39, 40, 41]. We have developed routes to numerous octafunctional SQs (Fig. 1a) including mixed-substituent systems. Other researchers have made star polymers [25a], octaphenols [29], octaisocyanates [48], oligoethylene oxides [49], and liquid crystalline materials [50]. We also developed routes to nitro, amino, halo, alkyl, and aryl, OPS (RxOPS) (Fig. 1b) and the T12 SQs (RxPhSiO1.5)12 = RxDPS [39, 44, 45, 46, 51]. A brief overview of our work on structure-property relationships of nanocomposites and on the synthesis of star compounds is given in Sect. 2.

Silanizing surfaces is practiced commercially in applications ranging from sizing on particulates and fibers (carbon, glass, polymer) used as reinforcing media for processing composites, to coatings applied to optical fibers for communication applications to modifying surface properties to be hydrophobic or hydrophilic to making bactericidal surfaces. In general, one silanizes surfaces by reacting organo chloro- or alkoxysilanes, R’MexSiCl3–x or R’MexSi(OEt)3–x, with surface hydroxyl groups, SUR-OH, as suggested by Scheme 1 to form R’MexSi(–O–SUR)3–x bonds.
Scheme 1

Modification of hydroxylated surfaces by silanization

Researchers at Mayaterials recently developed (see Sect. 4) low-cost SQs prepared directly from an agricultural waste product, rice hull ash, that are bifunctionalized, with about four of each type of functional group. These bifunctional SQs offer the potential to: (1) serve as superior silanizing agents, (2) build layer-by-layer structures at nanometer length scales, (3) build graded property thin films and also (4) porous, high strength interlayer dielectrics; as a direct result of the Janus properties of these materials.

2 Background

2.1 Structure–processing–property relationships

Silsesquioxanes provide access to epoxy nanocomposites with nearly complete definition (nm length scales) of all organic and inorganic components [37, 42, 43, 44, 47]. In turn, these nanocomposites provide structure–processing–property relationships that delineate the effects of changes in the organic component (tether) architectures on global mechanical properties [34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 53]. For example, tailoring provides control of coefficients of thermal expansion of epoxy resin composites from 25 to 250 ppm/ °C [52]. Likewise, we were able to tailor O2 barrier properties to equal commercial systems (<1 cc · 20 μm/m2 · day · atm) and superior to polysiloxanes at 19,000 cc · 20 μm/m2 · day · atm) [53]. Moreover optimal values obtain from epoxy:amine ratios of 1:1 rather than the 1:2 stoichiometry used traditionally, pointing to the novelty of these materials [43, 44, 45, 51, 52].

Other novel properties include [(NO2)2PhSiO1.5]8, which is stable to ≈400 °C, but decomposes explosively thereafter [51]. In contrast, i-butyl and i-hexyl alkylated OPS behave as “self-lubricating nanoball bearings,” forming liquids air stable to >400 °C because the alkyl chains interdigitate in the melt [54].

2.2 Star compounds from octavinylsilsesquioxane

We recently identified several new and exciting areas of research based on the synthesis of high density functionalized, robust and easily purified cubes with chemistries and properties that complement dendrimers and hyperbranched materials [55, 56, 57]. For example, we have synthesized styrenyl and vinylstilbene stars from octavinylsilsesquioxane, Scheme 2, extending work of Feher [11b, 58], Marciniec [59], and Sellinger [60, 61, 62].
Scheme 2

Star cubes with conjugated functionality produced via Grubbs and Heck reactions [62]

These stars are typical of conjugated organics with normal π–π* absorptions, emissions, and 0–0 band overlaps (Fig. 2). Quantum emission yields (THF) are as expected ≈1% and ≤30% for the styrenyl and vinylstilbene systems, respectively. Basically, the (SiO1.5)8 core appears to have no effect on the photonic properties as might be expected for a good insulator.
Fig. 2

UV–Vis, Emission (THF) [62]

2.3 Star compounds from octaiodophenylsilsesquioxane

We have also synthesized I8 phenyl functionalized SQs (Fig. 3). Octaiodophenylsilsesquioxane (I8OPS) [64] is >95% p-substituted as confirmed by a single crystal structure. These compounds are easily functionalized via Heck, Suzuki, Songashira, etc. [63, 64], coupling chemistries accessing compounds with high densities of functional groups/unit volume, some greater than any dendrimer cores ever made [54, 55, 56, 57] (see Scheme 3 and Table 1) [64, Asuncion and Laine, Octaalkynylsilsesquioxanes, Sea Urchin Molecular connectors for 3-D-Nanostructures, "unpublished data"]. We have also made [RxPhSiO1.5]12 RxDPS compounds and find similar properties [65]. I8OPS and I12DPS (not shown) provide access (as illustrated in Scheme 3) to a wide variety of other highly symmetrical compounds with novel UV–Vis and emission behaviors that differ from those shown in Fig. 2. These results are discussed elsewhere in detail [64].
Fig. 3

(a) I8OPS: (b) MALDI TOF of 99% I8OPS, >90% p, 3% o-I, 4% m-I [64]

Scheme 3

I8OPS derivatives (i) Cu2O/carbazole/150°C/1d. (ii) (Ph3P)4Pd/Zn(CN)2/120 °C/1d. (iii) RC2H/CuI/(Ph3P)4Pd/TEA/dioxane/60 °C/1d. (iv) BipyB(OH)3/(Ph3P)4Pd/PR3/120 °C/1d. v. (v) w/2-vinypy [45, 46, 64, Asuncion and Laine, Octaalkynylsilsesquioxanes, Sea Urchin Molecular connectors for 3-D-Nanostructures, "unpublished data"]

Table 1

Characteristic absorptions of various functional groups of Janus cubes

Functional group

Wavenumber (cm−1)

Vibration type

Si–H

2,200

νs

Si–C

1,250

νs

Si–O–Si

1,030–1,110

νs

C–H, aliphatic

2,840–3,000

νs, νas

Epoxy

1,250

νs

Epoxy

810–950

νas

3 Results and discussion

The concept of Janus cubes or double sticky sided SQs arises from a striking single crystal X-ray structure of (octylSiO1.5)8, one of several reported by Bassindale et al. [66] that points to a feature common to long chain flexible groups, they align uniformly to either side of the cage or silica core rather than in a typical 3-D SQs structure as seen in Fig. 3, with each R group lying in a different octant in Cartesian space. This has very important implications.

Access to bifunctional SQs with an average very close to four of each type of functional group via reactions akin to (1)–(3) [OHS = Octadimethylsiloxyoctasilsesquioxane] has been described in the literature [38, 41]. It must always be an average because grafting on oligomers will always give statistical numbers of additions. However, because the SQs offer cubic symmetry, there are six faces and therefore even if the groups add in a statistical manner to the cage vertices, there will be one face where three and possibly four of the same group can be found. Furthermore, it is possible to imagine a second type of Janus cube with a C3V configuration (Fig. 4).
Fig. 4

Two Janus configurations possible

On a statistical basis alone, and assuming that it is possible to find SQs with at least three groups on a face or vertex, it is possible to calculate that nearly 90% of all bifunctional SQs will be Janus-like!

Indeed it may be that the catalytic addition of a second group is driven to occur more frequently on the same face as the first group of that same type by its chemical affinity to the first group. In either case, one face is very likely to have more than two groups whose affinity for each other should drive lateral alignment such as seen by Bassindale et al. [66].

If one functionality is slightly more polar or hydrophobic than the other, then it is possible that this set of functionalities lies to one side and the opposing set is then “forced” to (or prefers) the opposite side. Thus statistically, it seems reasonable to argue that solely based on polarity or “phobicity,” bifunctional, or Janus structures will form. This creates the potential for making novel surface coatings, and films with structuration controlled at nanometer length scales vertically away from the surface.
Consider what happens if the product of reaction (3) is hydrolyzed, reaction (4). If we assume one face is likely to have more –Si(OH)3 groups than another face, we can suggest that hydrogen bonding followed by condensation
will lead to structures suggested but not meant to be limiting as shown in reactions (5)–(6). The product(s) of reaction (6) is (are) equivalent to the structures seen by Ro et al. [67] and offer potential to make super hard, tough, thin films like theirs from low-cost starting materials.
Recognizing that one molecular face likely consists of 3–4 Si(OH)3 groups, there is potential for exceptional degrees of silanization, which in turn should lead to multiple adhesive bonds to surfaces much superior in terms of resistance to decohesion, hydrolysis, and/or chemical oxidation to typical silanization agents that depend on single silanes to form surface bonds, e.g. R–MexSi(Cl)3–x or R–MexSi(OR)3–x.

At any point in the (3)-(6) reaction sequence, the Si–H bonds of these intermediates can be modified by hydrosilylation to introduce a second type of functional group. As we demonstrate below, adding bifunctionality provides access to an enormous number of new coating systems and to novel methods of surface modification.

Baseline studies were run as part of a Phase II AFRL program (FA8650-05-C-5046) at Mayaterials targeting corrosion resistant coatings for aircraft fuselages using ambient temperature hydrolysis, spray coating, and curing of TRTSEs (tetra-R-tetratriethoxysilylethylsilsesquioxanes, Fig. 5) on Al 2024 T3. The surface modification expected is that shown in Scheme 4. These and related coatings offer considerable promise as discussed below and provided the impetus to develop double-sided sticky tape or Janus SQs as discussed above. Thus, we have now made a whole series of TRTSEs per Fig. 5 and coatings there from using a very simple standard solution spray method of application, as follows.
Scheme 4

OTSE binding to an Al surface

Fig. 5

Bifunctional SQs synthesized to date see experimental for standard procedures and FTIR and GPC data

Figure 6 demonstrates the change in wetting of a freshly cleaned Al surface compared with a TCTSE coating. TCTSE also provides one of the hardest coatings prepared. The upper surface of all of these coatings are amenable to further modification. One can envision building on the hydrolyzed TGTSE of reaction (8) to create a strongly Al adherent coating and then an adhesive interlayer to a polymer overcoat per reaction (9). Octanilino (OAPS) of reaction (9) plays an important role in all the resin properties noted in the structure properties section above.
Fig. 6

Three water droplets placed on uncoated and coated Al 7075 surface. (a) Bare aluminum substrate. (b) Aluminum substrate coated with TCTSE

Reaction (10) below adds a second layer suggesting that the potential to make not only adhesive coatings on surfaces but also to create flexible layers by reversing the process. Thus it is possible to put epoxy layers on top of the first adhesive coating and then a coating of TGTSE on the amine layer and thereafter some quantity of hydrolyzed OTSE to create hard layers akin to those produced by Ro et al. [67] but without heating. These represent the potential commercial utility of these materials.

Note that in reactions (8)–(10) and (12) only two arms are shown attaching to the surface. This is done in part because it is difficult to draw high quality 3-D structures showing all four arms to the surface and in part to also signify that these arms could also cross-link with other arms of other Janus cubes per the work of Ro et al. [67].
We finish with one more set of examples based on TCPTSE synthesized as follows.
TCPTSE coatings were processed using the standard method (see Sect. 4) to give films with excellent hardnesses, 4–6 H. The propylchloride reacts easily with simple nucleophiles, e.g., OAPS per reaction (12) by brief exposure to acetone or EtOH solutions leading to the changes in contact angles seen in Fig. 7. We find that OTSE derived coatings are typically about 0.1–0.4 μm thick, we believe the TCPTSE coatings offer similar thicknesses with some surface irregularities. After reaction with amines, reactions (13) and (14), done to produce coatings predisposed to form strong bonds to epoxy and/or isocyanate overlayers, we expected that the contact angles would fall and indeed they drop. As seen in Fig. 8 for diethanolamine (DEA), the contact angles are 50–70°; however, contrary to our expectations with N-methylethanolamine (MEA), the resulting coatings are highly hydrophobic for reasons that cannot at present be explained.
Fig. 7

Comparison of contact angles for TCPTSE coated Al and after reaction with OAPS

Fig. 8

Contact angles for TCPTSE coated Al after reaction with (a) DEA and (b) MEA

In still a third set of experiments, the OAPS, DEA, and MEA coatings were then exposed to the octachloropropyl SQs: [ClCH2CH2CH2SiMe2OSiO1.5]8 or OCP by immersion in an acetone solution for 0.5 h. At this point, the contact angles all returned to 90°, the same as found for TCPTSE but treatment of the MEA coatings with DEA now gives contact angles of 60–75°. The history of this layering is shown in Fig. 9, below.
Fig. 9

Contact angles for a series of coatings on Al made sequentially with TCPTSE, MAE, OCP, and DEA showing the changes in contact angles with each application. Only the first coating is sprayed on

Basically, we appear to have developed a very simple coating method for forming multilayer systems. Figure 10 illustrates potential uses for our methods and possible TCPTSE coating derivatives that might be made, all of which are potential commercially important.
Fig. 10

Possible modifications to TCPTSE coatings as a route to new coatings with tailored properties

4 Experimental

4.1 Chemical compounds

All solvents, allyl chloride, glycidoxyallylether, N-methylethanolamine, and diethanolamine, were purchased from Aldrich Inc. The solvents were distilled from drying agents under nitrogen prior to use. Vinyl triethoxysilane, was purchased from Gelest. Octahydridodimethylsilox octasilsesquioxane is produced commercially by Mayaterials and was recrystallized from hexane prior to use. Octaaminooctaphenyloctasilsesquioxane is a product of Mayaterials and was used as produced (>95% purity).

4.2 Spray coating methods

For a typical coating, 6 g of TRTSE is added to 25 mL of 70% MeOH 30% acetone mixture and stirred for 30 min. Thereafter, 1 mL of an HCl/water solution (1 mL of 37% HCl to 99 mL water) is added and the solution stirred for ∼10 min. Then the solution is applied by dipcoating, spin casting, or spraying. For example, it can be sprayed using a Binks M1-G HVLP spray gun across the substrate to a thickness of 2.5–15 μm.

4.3 Gel permeation chromatography

All gel permeation chromatography (GPC) analyses were run on a Waters 440 system with Breeze software equipped with Waters Styragel columns (7.8 × 300, HT 0.5, 2, 3, 4) with RI detection and THF as solvent. The system was calibrated using polystyrene standards and toluene as a reference. Note that it is common for GPC analyses to show molecular weight data that are 15–20% lower than the calculated molecular weight data because SQs are spherical with very similar hydrodynamic radii rather than the hydrodynamic radii of linear molecules as used for the GPC standards.

4.4 Fourier transform infrared analyses

Diffuse reflectance infrared Fourier transform (DRIFT) spectra were obtained using a Thermo-Fisher Nicolet 6700 Fourier transform infrared (FTIR), using a Praying Mantis DRIFT accessory (Harrick Scientific Products, Inc.). Optical grade potassium bromide (International Crystal Laboratories) was used as the supporting medium. Attenuated total reflectance spectra were obtained neat using an FTIR 4100 (Jasco, Inc.) using a MIRacle diamond accessory.

4.5 Contact angle measurements

Spray-coated samples from above were placed horizontally in the specimen holder in an optical microscope and three drops of water of varying size were applied to the surface. A digital camera was used to photograph each set of droplets. The resulting images were then imported into iPhoto and the angles measured from these images. The error for each measurement is estimated to be ±4°.

General synthesis procedures followed the synthesis illustrated for TCPTSE below.

4.6 TCPTSE synthesis, reaction (11)

TTSE (25 g, 14 mmol) is dissolved in 250 mL of solvent, typically THF. Allyl chloride (6 mL, 56 mmol) and 5 wt% 0.1 g Pt/C catalyst are added. The reaction is refluxed for 24–48 h and followed by FTIR until the νSi–H peak 2,200 cm−1 disappears. The product is isolated by filtration and obtained in 85% yield. Analytical results for the majority of the compounds shown in Fig. 5 are presented below.

4.7 Octatriethoxysilylethyloctadimethylsiloxyoctasilsesquioxane (OTSE)

Selected characterization data: IR (cm−1) ν C–H: 2,980, 2,930, 2,850; ν Si–C: 1,260; ν Si–O: 1,110. GPC analysis: Mn = 2,210 (2.2 × 103), Mw = 2,377 (2.3 × 103), PDI 1.07 (FW of OTSE is 2,528).

4.8 Tetratriethoxysilylethyldimethylsiloxy(tetradimethylsiloxy)octasilsesquioxane (TTSE)

Selected characterization data: IR (cm−1) ν C–H: 2,980, 2,930, 2,850; ν Si–H: 2,200; ν Si–C: 1,260; ν Si–O: 1,110. GPC analysis: Mn = 1,542 (1.5 × 103), Mw = 1,742 (1.7 × 103), PDI 1.1 (FW of TTSE is 1,772).

4.9 Tetratriethoxysilylethyldimethylsiloxy(tetraglycidoxypropyldimethylsiloxy)octasilsesquioxane (TGTSE)

Selected characterization data: IR (cm−1) ν C–H: 2,980, 2,930, 2,850; ν Si–C: 1,260; ν epoxy ring: 1,250, 900; ν Si–O: 1,110. GPC analysis: Mn = 1,891 (1.8 × 103), Mw = 2,369 (2.3 × 103), PDI 1.2 (FW of TGTSE is 2,225).

4.10 (tetracyclohexenylethyldimethylsiloxy)tetratriethoxysilylethyldimethylsiloxyoctasilsesquioxane (TCTSE)

Selected characterization data: IR (cm−1) ν C–H: 2,980, 2,930, 2,850; ν Si–C: 1,260; ν epoxy ring: 1,250, 900; ν Si–O: 1,110. GPC analysis: Mn = 1,955 (1.9 × 103), Mw = 2,392 (2.3 × 103), PDI 1.2 (FW of TCTSE is 2,201).

4.11 (tetra-3-chloropropyldimethylsiloxy)Tetratriethoxysilylethyldimethylsiloxyoctasilsesquioxane (TCPTSE)

Selected characterization data: IR (cm−1) ν C–H: 2,980, 2,930, 2,850; ν Si–C: 1,260; ν Si–O: 1,110. GPC analysis: Mn = 1,416 (1.4 × 103), Mw = 1,814 (1.8 × 103), PDI 1.2 (FW of TCPTSE is 2,074).

Notes

Acknowledgments

The authors would like to thank AFRL (Wright Patterson AFB) through contract FA8650-05-C-5046 for support of the coating work. The NSF, Canon Ltd, Kuraray, Delphi Inc., Nippon Shokubai Ltd, and Matsushita Electric Ltd. provided partial support as well.

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Copyright information

© Springer Science+Business Media, LLC 2008

Authors and Affiliations

  • R. M. Laine
    • 1
  • M. Roll
    • 1
  • M. Asuncion
    • 1
  • S. Sulaiman
    • 1
  • V. Popova
    • 2
  • D. Bartz
    • 2
  • D. J. Krug
    • 2
  • P. H. Mutin
    • 3
  1. 1.Departments of Materials Science and Engineering, and Macromolecular Science and EngineeringUniversity of MichiganAnn ArborUSA
  2. 2.Mayaterials IncAnn ArborUSA
  3. 3.Chimie Moléculaire et Organisation du SolideUniversité de Montpellier IIMontpellierFrance

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