Silica-Based Sol-Gel 

Organic-Inorganic 

Nanocomposite Materials: 

A Review of Different Material 

Technologies 

by Sandra K. Young 

ARL-TR-2732 May 2002 

Approved for public release; distribution is unhnited. 
The findings in this report are not to be construed as an 

official Department of the Army position unless so 

designated by other authorized documents. 

Citation of manufacturer’s or trade names does not 

constitute an official endorsement or approval of the use 

thereof. 

Destroy this report when it is no longer needed. Do not 

return it to the originator: 
Army Research Laboratory 

Aberdeen Proving Ground, MD 21005-5069 

ARL-TR-2732 Mav 2002 

Silica-Based Sol-Gel 

Organic-Inorganic 

Nanocomposite Materials: 

A Review of Different Material 

Technologies 

Sandra K. Young 

Weapons and Materials Research Directorate, ARL 

Approved for public release; distribution is unlimited. 
Abstract 

Organic-inorganic nanocomposite materials continue to interest researchers. The 

idea of combining the properties of organic systems with the properties of 

inorganic systems in a synergistic fashion is not only desirable but also widely 

sought in materials. It is the difficulties with which these materials are often met 

which keep many researchers from entering this field. However, the area of 

silica-based organic-inorganic nanocomposites is not new and is quickly 

becoming one of the emerging technologies for new materials. This work 

represents an overview of the different types of silica-based organic-inorganic 

nanomaterials. 

ii 
Acknowledgments 

The author would like to thank Dr. Nora Beck Tan and Dr. Steven M&night, 

whose continuous encouragement for independent scientific pursuits in the 

polymers field constantly lead to new emerging research. In addition, the 

support of the Polymers Research Branch in continuing research in the area of 

organic-inorganic silica-based sol-gel nanocomposites is acknowledged. 

. . . 

ill 
INTENTIONALLYLEFTBLANK. 
Contents 

Acknowledgments 

List of Figures 

. . . 

111 

vii 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

Introduction 1 

Inorganic-Organic Monomers 1 

2.1 ORMOSIL Monomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

. . . . . . . . . . . . . . 1 

2.2 Silsesquioxane Monomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

2.2.1 Bridging Silsesquioxanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

2.2.2 Polyhedral Oligomeric Silsesquioxanes (POSS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 

Organic-Inorganic Associations 9 

Organic-Inorganic Interpenetrating Networks 11 

4.1 Noncovalent IPN Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 

4.2 Covalent IPN Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 

Organic-Inorganic Telechelic Polymers 13 

Summary 14 

References 15 

Distribution List 21 

Report Documentation Page 23 

, 

V 

List of Figures 

Figure 1. TEOS, a tetrafunctional alkoxysilane. ......................................................... .2 

Figure 2. The ORMOSIL functionality spectrum.. ..................................................... .2 

Figure 3. Incorporating porosity in sol-gel networks ................................................. 3 

Figure 4. ORMOSIL connectivity through sol-gel chemistry.. ................................. .3 

Figure 5. Two basic structures of silsesquioxanes. .................................................... .4 

Figure 6. Bridging silsesquioxane monomers. ........................................................... .4 

Figure 7. Formation of precise porosity through usage of bridged aryl 

silsesquioxanes. ......................................................................................................... .6 

Figure 8. Common bulky substituents. ....................................................................... .6 

Figure 9. Creating an acrylic-POSS material. ............................................................. .7 

Figure 10. Creating an epoxy-POSS material . ............................................................. 7 

Figure 11. Several polymers that interact with an inorganic phase. ..................... .lO 

Figure 12. Inorganic association/coupling equilibrium with polymers. ............. .lO 

Figure 13. End group in-situ association materials. ............................................... .ll 

Figure 14. IPN materials. ............................................................................................. .12 

Figure 15. Some monomers utilized in covalent IPN materials. ........................... .13 

Figure 16. Covalent IPN material synthesis. ............................................................ .13 

Figure 17. Telechelic organic-inorganic polymers .................................................... 14 

Vii 
INTENTIONALLYLEFTBLANK. 

. . . 

Vlll 
1. Introduction 

Since the initial interest in sol-gel science and technology in the 197Os, a wide 

variety of research has been performed, including fundamental kinetic 

evaluations [l, 21, mechanism studies [3,4], and structural variations of the metal 

alkoxide from traditional silica to other metals such as aluminum [5], zirconium 

[6, 71, and titanium [8, 91. In the mid-1980s, the emphasis began to shift from 

pure inorganic systems to organic-inorganic composite materials. This report 

details silica-based sol-gel chemistries and resulting materials. This area of 

research is becoming so significant that entire books [l, lo] and journal issues 

[ll] are being dedicated to advances in the area. The study of silica-based 

organic-inorganic composites may be divided into four distinct areas of research: 

monomers, associations, interpenetrating networks (IPNs), and telechelic 

polymers. 

2. Inorganic-Organic Monomers 

In the context of sol-gel chemistry, inorganic-organic monomers are molecules 

containing a central metal atom, such as silicon, bonded to reactive alkoxy 

groups and/or organic groups. These monomers, when assembled together, 

form organic-inorganic siloxane materials. There are two distinctive types of 

inorganic-organic monomers: organically modified silicate (ORMOSIL) 

monomers and silsesquioxane monomers. Of the silsesquioxane monomers, 

there are two distinct types -bridging silsesquioxanes and polyhedral oligomeric 

silsesquioxane. All of these monomers will be addressed separately. 

2.1 ORMOSIL Monomers 

In the mid-1980s, considerable research was conducted in the field of ORMOSIL 

chemistry [12-211. ORMOSILs are derived from tetrafunctional silicon alkoxides 

such as tetraethylorthosilicate (TEOS), as shown in Figure 1. Since ethyl groups 

are commonly used in sol-gel chemistry and many of the silanes available 

contain ethoxy groups, most of the structures in this report will show ethoxy 

groups. However, it might be useful to note that there are also methoxy, 

propoxy, butoxy, and other long-chain hydrocarbon alkoxy groups. 

1 
CH$H20- tji- OCHzCH3 

OCH2CH3 

Figure 1. TEOS, a tetrafunctional alkoxysilane. 

Organic modification may take place by substitution of an alkyl, aryl, or any 

moiety, replacing one of the reactive alkoxide groups, as shown in Figure 2. 

Here, n is representative of the number of organic moieties connected to the 

silicon atom, and f is representative of the number of reactive alkoxy groups 

connected to the silicon. For a monofunctional silicon alkoxide (f = 1, n = 3), the 

monomer “terminates” the polymer chain since there is only one reaction site. A 

difunctional silicon alkoxide (f = 2, n = 2) behaves as a “bridging” agent, 

connecting molecules in a linear fashion. A trifunctional silicon alkoxide (f = 3, 

n = 1) behaves as a “crosslinker,” allowing for branching in the network. A 

tetrafunctional silicon alkoxide (f = 4, n = 0) behaves as a “networking” agent, 

allowing for complete connectivity between all functional arms of the molecule. 

Terminating Bridging Endcapping 

R’ R’ R 

R’- s i-R’ R’- s i-OCH2CH3 CH$H20- s i-OCH2CH3 

A CH2CH3 A CH2CH3 A CH2CH3 

f= 1, n=3 f = 2, n=2 f = 3, n=l 

Network Forming 

OCH2CHj 

CH3CH20- s i-OCH2CH3 

b CH2CH3 

f = 4, n=O 

Figure 2. The ORMOSIL functionality spectrum. 

Traditional inorganic glasses require temperatures in excess of 1000 “C in order 

to have densification of the siloxane network [22]. These temperatures prohibit 

the organic modification of traditional glasses due to the high-temperature 

instability of the Si-C and C-H bonds. Sol-gel derived glasses, however, do not 

require high temperatures for formation, making organic modification possible 

(see Figure 3). 

2 
l Bulk glasses 

l Sol-gel glasses 

= pores 

ORMOSILs: 

R’,Si(OQ-, 

e 

heat 

glass 

e limited 

densification 

Figure 3. Incorporating porosity in sol-gel networks. 

Upon sol-gel polymerization of tetrafunctional silicon alkoxides, network 

formation and crosslinking is extensive. Careful drying of this highly connected 

system removes excess water and alcohol, allowing for complete vitrification of 

the network. With organic modification of the silicon, sol-gel polymerization 

limits both the crosslinking (by inclusion of monomers of lower functionality) 

and material den&cation (by creation of specific free volume or porosity in the 

sol-gel network). Represented by R, the functional group on an organically 

modified silicate can be any organic moiety including methyl, vinyl, or benzyl. 

Figure 4 depicts ORMOSIL connectivity achieved through sol-gel polymerization. 

It is reasonable to expect that variation of the number and type of organic 

moieties included on the silicon monomer results in a variety of pore sizes that 

can be created in the sol-gel network. The pore size corresponds to the physical 

size of the organic moiety and the excluded volume of the moiety. Due to their 

organic nature, these pores can attract organic or divert polar gas molecules. In 

essence, ORMOSILs form precisely defined porous networks, each possessing 

unique physical and chemical properties. 

I 

0 

OCH2CH3 OCH2CH3 -O-ii-oCH#2H2O-&-OCH2CH3 + CH3--Ai-CH3 -+ 

H+ d 

&H2CH3 bCH2CHJ H20 H&--!&-CH3 

Figure 4. ORMOSIL connectivity through sol-gel chemistry. 

3 
2.2 Silsesquioxane Monomers 

Similar to ORMOSIL monomers, silsesquioxane monomers are useful for 

creating sol-gel glasses with controlled porosities. Silsesquioxanes are silica 

atoms that are covalently bound to organic molecules. These molecules can have 

two very different structures, as shown in Figure 5. 

OR 

Si&OR 

OR 

Figure 5. Two basic structures of silsesquioxanes. 

Bridging silsesquioxanes contain an organic bridging agent that behaves as a 

spacer unit and resides between two reactive di- or trialkoxysilane end groups 

[23-251. Polyhedral oligomeric silsesquioxanes are cube siloxane structures with 

eight potentially reactive sites. These materials wiIl be discussed separately in 

more detail. 

2.2.1 Bridging Silsesquioxanes 

The general structure of silsesquioxane-type monomers is shown in Figure 6. 

These bridging units have traditionally been hydrocarbon, containing 

consecutive methyl or benzyl groups, but can be synthesized to have any kind of 

organic spacer functionalities. The organic bridging agent can be synthetically 

developed to resemble a polymer backbone for use as an interface compatibilizer 

or for use as a material of a specific size that can be burned out to create a specific 

porosity. Upon initial sol-gel network formation, porosity is limited due to the 

presence of the bridged organic groups. Limiting porosity through usage of a 

spacer unit, with its specific organic character, can be beneficial for specific gas or 

liquid permeation. 

CHjCH20 OCH2CH3 CH3CH20 OCH2CH3 

R’JSi-R-Si’&R 

CH3CH2d 

CH3CH2O-&i- R-SikOCH2CH3 , 

OCH2CH3 CH3CH20/ OCH2CH3 

Figure 6. Bridging silsesquioxane monomers. 
Lacan et al. [23] synthesized functional bridging-silsesquioxane materials in 

order to investigate the mechanical properties and gelation process in proceeding 

from a viscous liquid to a viscoelastic solid. Two silsesquioxane materials were 

made-an amine-acid chloride organic spacer unit and a methacrylic spacer unit. 

The results obtained for these two materials were compared to those of a pure 

TEOSderived system in order to establish a comparison between mixed organicinorganic and purely inorganic materials. In the gel state, the amine-acid 

chloride silsesquioxane material with the short organic chain spacer unit was 

observed to possess the network with the most strength and greatest elongation. 

This behavior was expected for the pure inorganic network where the hydrolysis 

was very fast, resulting in a short gelation time, and the resultant network was 

ordinarily more viscous. The amine-acid chloride spacer unit between the two 

silicon alkoxides did not restrict the three-dimensional (3-D) network formation, 

resulting in increased viscous character through organized network formation. 

Silsesquioxanes are often used to create inorganic materials with specific 

controlled porosities. Shea [26] has investigated the use of alkyl and aryl 

silsesquioxanes in hybrid organic-inorganic materials in order to control the 

morphology of the material on a molecular level. The use of these organic spacer 

units (mono-, di-, and tri- methyl or phenyl, anthracene, and ethynyl groups) 

allowed for the “engineering” of material pore size by varying the size of the 

spacer unit. The incorporation of these materials into sol-gel processed films was 

confirmed by solid-state r3C and 2% nuclear magnetic resonance (NMR) 

spectroscopy, which also indicated that the ethynyl linkage was the most 

hydrolytically unstable during the sol-gel process, resulting in cleavage of the 

Si-C bond. Scanning electron microscopy (SEM), transmission electron 

microscopy (TEM), and atomic force microscopy (AFM) were used to study the 

silica morphology. Gas sorption porosimetry was used to investigate the pore 

structure in greater detail. The results from this analysis indicated that the 

materials showed substantial microporosity with additional mesopores and that 

the micropore structure was more dependent on the macromolecular structure 

than on the length of the organic spacer unit. Thermal analysis via differential 

scanning calorimeter (DSC) and thermal gravimetric analyzer (TGA) showed the 

relatively high thermal stability of these materials. These limited porosity 

systems can be modified to form well-defined porous systems through removal 

of the spacer units by ozonolysis (see Figure 7) [24,26]. By removing the spacer 

units after the network formation, well-defined pores corresponding to the 

organic spacer were left in the sol-gel network. 

Along these same lines, Brinker [27] used organic templates in an attempt to 

separate the pore size and pore volume. The organic templates (mono-, bi-, and 

triphenyl groups) were embedded in a dense silica matrix and subsequently 

removed by thermal oxidation or ozonolysis. The resultant pores mimicked the 
EtO, OEt PCH2CH3 H+ EtW,Si 

Et0 

Si$OEt + CH3CH20-F-OCH2CH3 B 

-9 

OEt Hz0 

-@,Si 

OCH2CH3 -0 

&IA 

-9 

-*,Si 0 S&L O3 * 

-0. 

-9 r”--\ ./O- 

-0 

--0-,Si-OH HDSyO- 

-0 

Figure 7. Formation of precise porosity through usage of bridged aryl silsesquioxanes. 

size and shape of the organic ligand with no collapse of the surrounding matrix. 

Adsorption and ellipsometry results indicated that the removal of the biphenyl 

groups (1) created porosity in the films with a volume fraction approximately 

equal to that of the template, (2) caused the formation of a continuous network of 

pores by removal of the template with no creation of mesoporosity, and 

(3) resulted in the presence of very small pores consistent with the removal of the 

organic template. 

2.2.2 Polyhedral Oligomeric Silsesquioxanes (POSS) 

The usage of bulky substituents in polymer chemistry, such as norbomene, 

adamantane, or fullerene (see Figure 8), to increase thermal and mechanical 

properties by restriction of chain mobility is becoming commonplace. As an 

added substituent on the main chain, adamantane, for example, can increase the 

glass transition by as much as 100 “C [28]. However, with very low 

incorporations of the bulky substituents, the polymers tend to become very 

brittle. Some of these substituents can be used as the main-chain polymer. 

However, as a polymer, norbornene only has a glass transition temperature of 

55 “C [29, 301. In both cases of substituent and polymer, the solubility and 

pliability of the end polymeric material has decreased significantly. Therefore, 

the challenge to find a bulky substituent that can not only increase the thermal 

and mechanical properties but also maintain some of the pliability and solubility 

that the organic polymer possesses has remained. 

Norbornene Adamantane 

Figure 8. Common bulky substituents 

6 
POSS materials have been studied for the last 15 years as alternative bulky, 

inorganic, ring-structured substituents to the characteristic bulky organics. The 

main researchers in the field include the following: Dr. Rich Laine’s research 

group at the University of Michigan at Ann Arbor [28-351, a research group 

headed by Dr. Joseph D. Lichtenhan at the U.S. Air Force Research Laboratory - 

Propulsion Directorate (Wright-Patterson Air Force Base, Dayton, OH) [36-421, 

and Dr. Frank Feher’s research group at the University of California at Irvine 

[43-451. In addition, Dr. Lichtenhan and Sean Phillips [46] have been involved 

with the start of the Hybrid Plastics Company, which promotes the usage of 

POSS molecules in nanocomposite materials. On this website, 56 variations of 

the POSS molecule are available for utilization in composite materials. 

POSS materials can be easily chemically altered to include acrylate arms [35], as 

shown in Figure 9, or epoxy arms [33,34], as shown in Figure 10. These chemical 

modifications can supposedly be controlled. By using specific equivalents of 

modifier per number of arms, in Figure 9, for example, only four arms are 

chemically modified. This, while theoretically correct, is unlikely to produce just 

that four-armed molecule, but also a variety of other multireactive armed 

species. 

H 

Pt catalyst 

(CH2C12), 80°C, 10 h 

Figure 9. Creating an acrylic-POSS material. 

mCPBA (10 equiv.) * 

Figure 10. Creating an epoxy-POSS material. 

7 
Therefore, in the case where there are eight equally reactive sites on the 

molecule, it might be more practical to have a complete chemical modification, as 

is shown in Figure 10 of the epoxy system. However, a POSS material with lessreactive arms, such as the H-Si (silane), as opposed to H-Si-Mez-0-, may be able 

to exhibit more control over the reaction because of the slower reaction rate. 

POSS cubes are characterized via FTIR, iH, 1X, 29Si NMR, and mass (chemical 

ionization [CI], electronic ionization [EI], or fast atom bombardment [FAB]) 

spectroscopies and size exclusion chromatography (SEC). 

POSS cubes can then be made into polymeric nanocomposite materials through 

curing of the POSS molecule. The specific polymer composite structure 

developed depends on the number of reactive arms involved in the 

polymerization. Sellinger and Laine [34] reported polymerizing two-arm and 

eight-arm epoxy POSS cubes. These materials were polymerized using ZnCh, 

BF30Etz initiators or ethylene diamine as a cross-linking reagent at room 

temperature in CHKL. Confirmation was made that the polymerizations 

involved only the epoxy arms, not the vinyl arms, by subjecting the original 

eight-armed vinyl POSS molecules to the same reaction conditions. All POSS 

polymers were easily made and had increased stability compared to their 

acrylate counterparts, but resulted in insoluble polymers. 

A more recent study by Lee and Lichtenhan [36] discussed the incorporation of 

epoxy-POSS molecules as reinforcing agents in epoxy systems. The study varied 

the level of POSS molecule loading (0%, 5%, and 9%) into diglycidyl ether of 

bisphenol A (622.76 g/mol). The epoxy-POSS molecules used were 

monofunctional (1442.06 g/mol- minimum mass), terminating the polymer 

chain. The glass transition temperature was shown to increase with increasing 

POSS content, but only by about 7 “C. Isothermal aging and stress relaxation 

experiments were performed and indicated that POSSreinforcements slowed 

down the motion of the polymer chains and, therefore, the time required to reach 

structural equilibrium was significantly increased with respect to nonreinforced 

polymers. However, by observing no change in the endothermic transition, it 

can be suggested that the POSS cages are completely incorporated within the 

network without the formation of macroscopic phase-separated domains. 

Feher and coworkers [43-451 have reported linking silsesquioxane molecules 

together using silane coupling agents to form a silsesquioxane nanocomposite. 

By changing the moieties on the silane-coupling agent, the thermal properties of 

the polymer could be modified. For example, the researchers reported a polymer 

utilizing a silane-coupling agent with -MeO(SiMe20)$Xvle2 arms had a T, = 23 “C, 

while a polymer utilizing a silane-coupling agent with -MeO(SiMezO),,SiMe2 

arms (n - 3.4) had a T, = 58 “C. While these glass transitions overall were low, it 

was important that the study showed control of the structure-property 

relationships. 
While this was not an all-inclusive study of the POSS-based nanocomposite 

materials, it was significant to note that for the epoxy-based systems, the research 

published was very preliminary work. If considering the potential of the POSS 

materials with respect to the bulky, organic substituents, these materials have 

been shown in preliminary studies to increase the thermal and mechanical 

properties of the polymer. Silanes and siloxanes materials typically have one 

dimension in the range of l-100 run, forming transparent nanocomposite 

materials. In addition, silane and siloxane materials can be chemically tailored, 

resulting in a spectrum of chemical and mechanical environments. 

3. Organic-Inorganic Associations 

Organic-inorganic association systems are those in which there is no covalent 

bonding between the organic polymer and the inorganic network but exhibit 

characteristics similar to those which are covalently bonded. This synergistic 

relationship can occur through two types of associative behavior-hydrogen 

bonding and association/coupling equilibrium. 

A wide variety of polymers contains moieties, which would ideally have the 

capability for hydrogen bonding. Various polyesters, polycarbonates, 

polyacrylates, and polyamides possess moieties such as carbonyls, ethers, and 

amine groups, which have free electrons available for hydrogen bonding. Yet 

most polymers, when mixed noncovalently with inorganic sol-gel systems, are 

completely phase separated. 

Some polymers do exist (see Figure 11) which possess moieties capable of 

hydrogen bonding with an inorganic sol-gel network [47-491. Hydrogen 

bonding of the sol-gel component with these polymers retards severe phase 

separation during network densification. Researchers in organic-inorganic 

associations have utilized the specific polymers that have hydrogen-bonding 

capabilities in copolymers. The other polymer in the copolymer typically does 

not have the capability of interacting with an inorganic phase. In these 

copolymers, the hydrogen-bonding polymer is used as a compatibilizer with the 

other polymer system. Ideally, the compatibilizing comonomer, which is only a 

small fraction of the copolymer, will diffuse to the interface of the two immiscible 

polymer systems and reduce the interfacial tension through favorable 

interactions. This reduction of interfacial tension creates a stable organicinorganic composite interface by reducing the dispersed-phase particle size and 

increasing the adhesion between the two phases. 
PMMA PVAc PHSty PVP PeOx 

[Poly(Methyl Methacrylate)] [Poly(Vinyl Acetate)] [Poly(HydroxyStpne)] [Poly(Vinyl Pyrrolidonc)] [Poly(ethylOxazolinc) 

Figure 11. Several polymers that interact with an inorganic phase. 

An association/coupling equilibrium occurs between polymer systems which 

hydrogen bond [50]. Here, there is a dynamic equilibrium between hydrogenbonded groups and a covalently bound inorganic network producing -C-O-Sibonds (see Figure 12). The reason for the equilibrium is the hydrolytic instability 

of -C-O-Si- bonds. Generally, the equilibrium is shifted towards the hydrogenbonded moieties. Polymers containing carboxylic acid end groups can also 

undergo this type of association/coupling equilibrium (see Figure 13). The 

carboxylic acid end groups are acidic enough to catalyze the reaction. The end 

groups are then involved in the dynamic equilibrium between hydrogen-bonded 

chain ends and covalently bound chain ends. This in situ behavior also occurs in 

template systems, where an ionomeric membrane is utilized as a morphological 

template for in situ sol-gel reactions. 

$lEt 

+ Em- Si-OEt 

OEt 

PHSty TEOS 

H+ 

yo 

Figure 12. Inorganic association/coupling equilibrium with polymers. 

10 
P Et 

HO ‘G(F’o~ymer)n&)H 

+ Etv-oEt 

OEt 

Figure 13. End group in-situ association materials. 

4. Organic-Inorganic Interpenetrating Networks 

An interpenetrating polymer network (IPN) is any material containing two 

polymers, each in network form. The three conditions for eligibility as an IPN 

are as follows: (1) the two polymers are synthesized and/or crosslinked in the 

presence of the other, (2) the two monomers have similar reaction kinetics, and 

(3) the resulting two polymers are not dramatically phase separated [lo, 51-531. 

Of course, these are loosely held guidelines. IPNs that have only one polymer 

crosslinked (where the polymers are synthesized separately) or where the 

polymers have vastly different reaction kinetics are still considered to be IPNs. 

IPNs are distinguishable from blends, block copolymers, and graft copolymers in 

two ways: (1) an IPN swells but does not dissolve in solvents, and (2) mechanical 

creep and flow are suppressed [lo]. 

Several kinds of IPN architectures exist (see Figure 14). These systems differ 

mainly because of the number and types of crosslinks that exist in the system. A 

noncovalent semi-IPN is one in which only one of the polymer systems is 

crosslinked. A noncovalent full IPN is one in which the two separate polymers 

are independently crosslinked. A covalent semi-IPN contains two separate 

polymer systems that are crosslinked to form a single polymer network. This 

covalent semi-IPN is similar to a noncovalent IPN because one of the polymer 

systems can be crosslinked without networking with the second linear system. 

However, the two systems tend to be networked for better synergistic property 

development. Examples of covalent semi-IPNs are found almost exclusively in 

the realm of organic-inorganic composite materials. 

4.1 Noncovalent IPN Materials 

A wide variety of organic-inorganic noncovalent IPN materials has been 

formulated in an effort to improve material properties. Inorganic incorporation 

into polymers ranging from polyacrylates and polyesters to polyimides and 

nylons has been attempted [54-571. The problem with the noncovalent systems, 

which can also be a problem with the covalent systems, is the lack of a 

11 
Semi-IPN Full IPN Semi-IPN 

Non-Covalent Covalent 

Figure 14. IPN materials. 

compatible interface. This problem could stem from several factors, including 

surface energy phenomena and lack of molecular interactions between phases. 

Figure 11 shows several polymers that can interact with the inorganic phase. 

These polymers are proposed to hydrogen bond with the inorganic phase, 

creating an interface between the two materials. However, the key to having 

noncovalent organic-inorganic materials is not only utilizing a polymer that can 

have hydrogen bonding between the two phases but also to have low loading of 

the inorganic phase. Low loading of the inorganic phase will result in an 

increase in the overall material properties without sacrificing the interfacial 

bonding. 

4.2 Covalent IPN Materials 

A variety of polymers and copolymers has been synthesized incorporating 

reactive silicon alkoxides along the backbone of the polymer (see Figure 15) 

[57-631. Silicon alkoxide incorporation into the polymer backbone can be 

accomplished with many monomers and through various synthetic means (see 

Figure 16). For example, silicon alkoxides can be incorporated into a polymer 

backbone via free-radical polymerization through a vinyl moiety, via a 

condensation reaction with an organic moiety on the silicon alkoxide monomer, 

or via a postreaction (such as a hydrosilylation reaction). Through covalent 

attachment of the reactive silicon alkoxides, polymer-polymer interfacial 

problems were reduced. However, covalent IPN materials can have similar 

problems with the interface as the noncovalent materials. 

Again, similar to the noncovalent systems, a general lack of cohesiveness 

between the two phases can exist at molecular weight loadings higher than 10%. 

This problem with the gross phase separation at the interface is under 

investigation by researchers. Utilization of a variety of intermolecular bonding 

forces seems to improve upon the overall separation problems of the material. 

For example, by utilizing a polymer with a covalently bound silicate material 

that can also hydrogen bond with the organic polymer backbone creates more 

opportunities for better interfacial interactions. 

12 
d OCH2CH3 

a CHz=CH 

b CH3CH20,Ai,0CH2CH3 

Q 

7H3 C 

0 

CH2=7 0CRG-b AH2 

C=O 

? 

CHQM,~i,0CH2CH3 AH2 e H 

CHjCH20’SixOCH2CHj CH2 

AH2 

OCH2CH3 ;z; 

kH2 

AH2 CH$H$I-ki-OCH2CH2 

#-I2 

b 

kH2 

bCH2CH3 

CH$H20’Si\OCH$Hj NH2 

OCH2CHj &Hz 

Hydrosilylation 

Vinyl Polymerization Monomers (a,b) Condensation Monomers (c,d) Monomer (e) 

Figure 15. Some monomers utilized in covalent IPN materials. 

-(CHz 

,CH3 ,CH3 

-C),,--(CHz-C\m- + CH3CH20 

‘p=o PO 

6CH3 i) 

g!Il 

CH2 ! 

CH,CI-I~~s~OCHzCH3 

OCHzCH3 

,W3 ,CH3 

-(CH2-C)n-(CHTC m- 

‘C=O 

>\ 

c=o 

OCHzCH3 

Figure 16. Covalent IPN material synthesis. 

5. Organic-Inorganic Telechelic Polymers 

The formation of telechelic materials is an alternate method to have covalent 

bonding between the organic and inorganic phases of the composite material. A 

telechelic polymer is one containing one or more functional end groups (see 

Figure 17) that have the capacity for selective reaction to form bonds with 

another molecule [64]. Telechelic polymers typically contain hydroxyl or 

carboxylic acid end groups. Composite materials or block copolymers can then 

be made through these end groups by postreactions. A polymerization could be 

started at the -OH or -COOH end groups or by changing the end group 

functionality. Modifying the -OH or -COOH end group through an endcapping 

reaction with an acid chloride yields a vinyl end group where a free-radical 

reaction could occur. 

13 
CH3CH20 ,OCH2CH3 

CH$H20--$i-(POLYMER)n-Si--CCl+CI+ 

CHjCH20 OCHZCHJ 

Figure 17. Telechelic organic-inorganic polymers. 

Organic-inorganic telechelic polymers can be made by many methods, two of 

which are condensation reactions or condensation/hydrosilylation reactions. In 

the first method, a condensation reaction can be performed utilizing an 

ORMOSIL monomer that contains such functionalities as an amine, epoxy, or 

cyano. Typically, little or no catalyst is needed for these condensation reactions 

to occur. In the second two-step method, the -OH or -COOH end groups are first 

endcapped with vinyl groups via a condensation reaction with, for example, 

acrylic acid for a hydroxyl end group. Then, in a second step, a hydrosilylation 

reaction is done utilizing the telechelic vinyl-endcapped polymer and a silane 

monomer. Here, a catalyst is needed. However, the catalyst used is dependent 

upon the polymer backbone. Certain catalysts used can add at a carbonyl 

instead of at a vinyl; therefore, catalyst choice is critical. 

Tian et al. [65, 661 studied the properties of similar poly(s-caprolactone) (PCL) 

ceramers for use as a coating material for bone implants and prosthetic devices 

and as support for enzyme immobilization. TEM analysis showed some degree 

of co-continuity between the two phases when the composition was 50/50 

(PCL/SiO$. Al so studied was the effect of varying the ethanol content, as well 

as varying the polymer-to-alkoxysilane ratio on the final appearance of the films. 

The gelation time was observed to be independent of the PCL content, but the 

final appearance of the films was related to the amount of PCL and ethanol in the 

system. An increase in the amount of ethanol and a decrease in the amount of 

PCL lead to an increase in the optical clarity of the films. 

6. Summary 

A diverse body of research is ongoing in the very important area of organicinorganic silica-based nanocomposites. No limitations appear to exist for 

variations of chemical structure and composition of these materials. New 

materials and possible technology applications are constantly being explored. 

The Polymers Branch of the U.S. Army Research Laboratory has a few ongoing 

projects in the Emerging Materials (EMAT) Program and to the epoxycomposites program related to the type of work described in this technical 

report. These programs involve simple sol-gel reactions based on ORMOSIL, 

silsesquioxane, and telechelic polymer technologies. It is likely that technical 

reports will be submitted regarding these important works in the near future. 

14 
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1. TITLE AND SUBTITLE 5. FUNDING NUMBERS 

Silica-Based Sol-Gel Organic-Inorganic Nanocomposite Materials: A Review of EMATOlA 

Xfferent Material Technologies 

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Sandra K. Young 

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13. ABSTRACT(Maximum 200 words) 

3rganic-inorganic nanocomposite materials continue to interest researchers. The idea of combining the properties 01 

organic systems with the properties of inorganic systems in a synergistic fashion is not only desirable but also widely 

;ought in materials. It is the difficulties with which these materials are often met which keep many researchers fron 

zntering this field. However, the area of silica-based organic-inorganic nanocomposites is not new and is quickly 

>ecoming one of the emerging technologies for new materials. This work represents an overview of the different types 

If silica-based organic-inorganic nanomaterials. 

14. SUBJECT TERMS 15. NUMBER OF PAGES 

sol-gel chemistry, nanocomposite materials, inorganic-organic monomers, ORMOSILs, 27 

silsesquioxanes, IPNs, hyperbranch 16. PRICE CODE 

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