Viscoelastic Properties of Alkoxy Silane-Epoxy 

Interpenetrating Networks 

by Robert E. Jensen, Steven H. McKnight, and Giuseppe R. Palmese 

ARL-TR-3109 November 2003 

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Army Research Laboratory 

Aberdeen Proving Ground, MD 21005-5069 

ARL-TR-3109 November 2003 

Viscoelastic Properties of Alkoxy Silane-Epoxy 

Interpenetrating Networks 

Robert E. Jensen and Steven H. McKnight 

Weapons and Materials Research Directorate, ARL 

Giuseppe R. Palmese 

Center for Composite Materials, University of Delaware 

Approved for public release; distribution is unlimited. 
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4. TITLE AND SUBTITLE 

Viscoelastic Properties of Alkoxy Silane-Epoxy Interpenetrating Networks 

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AH42 

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6. AUTHOR(S) 

Robert E. Jensen, Steven H. McKnight, and Giuseppe R. Palmese* 

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U.S. Army Research Laboratory 

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*Center for Composite Materials, University of Delaware, Newark, DE 19716 

14. ABSTRACT 

A model epoxy-silane interpenetrating network (IPN) was synthesized to simulate the molecular structure found at the fibermatrix interphase. The Young’s modulus (E) of the epoxy-silane IPN was determined through micromechanical analysis both 

quasi-statically and in the frequency domain. The epoxy-silane IPN was synthesized by diffusion of uncured diglycidyl ether of

bisphenol A (DGEBA) epoxy resin and bis (p-aminocyclohexyl) methane (PACM) curing agent into spherical particles of 

condensed and crosslinked 3-glycidoxypropyltrimethoxysilane (GPS). This IPN composition was chosen to simulate the typical 

properties of a silane modified interphase found in glass reinforced composites. Differential scanning calorimetry showed that

the glass transition temperature (Tg) of the initial crosslinked siloxane network increased upon cure of the DGEBA and PACM, 

but was still significantly lower than that of the neat epoxy matrix. Additionally, dynamic mechanical analysis was used in 

conjunction with the micromechanical C-Combining Rule to show that the Young’s modulus of the epoxy-silane IPN spherical 

inclusions (Ei) was decreased in comparison to the Young’s modulus of the matrix epoxy (Em) at all temperatures. The timetemperature superposition (tTsp) principle was successfully applied to the epoxy-silane IPN to determine viscoelastic properties 

at high frequencies. The viscoelastic properties of the epoxy-silane IPN may have implications with respect to the ballistic 

impact resistance of composite structures used for U.S. Army applications. 

15. SUBJECT TERMS

interphase, silane, IPN, modulus, viscoelasticity, epoxy, amine, composite, adhesion 

16. SECURITY CLASSIFICATION OF: 

19a. NAME OF RESPONSIBLE PERSON

Robert E. Jensen 

a. REPORT 

UNCLASSIFIED 

b. ABSTRACT 

UNCLASSIFIED

c. THIS PAGE 

UNCLASSIFIED

17. LIMITATION 

OF ABSTRACT 

UL 

18. NUMBER 

OF PAGES 

 40 19b. TELEPHONE NUMBER (Include area code) 

410-306-1910 

Standard Form 298 (Rev. 8/98) 

Prescribed by ANSI Std. Z39.18
iii

Contents 

List of Figures iv

List of Tables v

Acknowledgments vi

1. Introduction 1

2. Experimental and Analysis 3

2.1 Materials..........................................................................................................................3 

2.2 Bulk Silane Powder Synthesis and Blend Processing.....................................................4 

2.3 Micromechanical Analysis..............................................................................................7 

3. Discussion and Results 9

3.1 Diffusion Time vs. Reaction Time..................................................................................9 

3.2 Epoxy-Silane IPN Structure ..........................................................................................10 

3.3 Quasi-Static Micromechanical Analysis .......................................................................13 

3.4 Dynamic Modulus Analysis ..........................................................................................15 

3.5 Conversion of Modulus Ratio to Modulus Value .........................................................19 

4. Conclusions 21

5. References 24

Appendix. Explicit Inclusion Modulus Determination 27

Distribution List 30 
iv

List of Figures 

Figure 1. Schematic of polymer-matrix interphase.........................................................................1 

Figure 2. Schematic outline of approach used to characterize interphase properties. ....................3 

Figure 3. Matrix epoxy, curing agent, and silane coupling agent...................................................4 

Figure 4. Particle size distributions of bulk condensed and crosslinked silane particles prior 

to incorporation into the epoxy matrix. Particle size varied with dispersion solvent 

(water vs. acetone shown)..........................................................................................................5 

Figure 5. SEM cross section of epoxy-silane IPN inclusion filled composite showing 

spherical IPN particles embedded in the cured and glassy epoxy matrix..................................6 

Figure 6. DSC results for nonreacted bulk silane powder (top curve), epoxy-silane IPN 

inclusion composite (Vi = 0.49, center curve), and neat PEO-PPO-PEO triblock 

copolymer surfactant (bottom curve).......................................................................................11 

Figure 7. β-relaxation for neat epoxy matrix as measured by DMA as a function of 

temperature at constant frequency (□ = 0.1 Hz, ○ = 0.3 Hz, Î = 1 Hz, « = 3 Hz, 

‘ = 10 Hz, + = 30 Hz)..............................................................................................................13 

Figure 8. β-relaxation for epoxy-silane powder inclusion composite (Vi = 0.49) as measured 

by DMA as a function of temperature at constant frequency (□ = 0.1 Hz, ○ = 0.3 Hz, 

Î = 1 Hz, « = 3 Hz, ‘ = 10 Hz, + = 30 Hz)..............................................................................14 

Figure 9. Arrhenius activation energies (equation 18) for the neat epoxy β-relaxation (■), 

nonreacted bulk silane powder α-transition (▲), and epoxy-silane IPN α-transition (●). 

Transition temperatures are taken from E'' spectra in figure 8. ...............................................15 

Figure 10. C-Combining Rule predictions of epoxy-silane IPN static modulus. Modulus 

values measured via experimental methodology outlined in ASTM D 790-96a. 

Experimental error was negligible, but is listed in table 1. (Test temperatures: 

■ = −50 °C, ● = 25 °C, and ▲ = 100 °C.)..............................................................................16 

Figure 11. DMA E' plots comparing the neat epoxy matrix (●) and epoxy-silane IPN 

inclusion composite (Vi = 0.49, ■)...........................................................................................17 

Figure 12. Tan δ signals measured from DMA. (□ = neat epoxy matrix, ○ = 0.12 volume % 

epoxy-silane IPN filler, Î = 0.29 volume % epoxy-silane IPN filler, « = 0.49 volume % 

epoxy-silane IPN filler)............................................................................................................18 

Figure 13. Experimental master curve for epoxy-silane IPN inclusions. .....................................19 

Figure 14. Shift factor plot (data points) for the epoxy-silane IPN inclusions with fit to WLF 

equation (solid line). ................................................................................................................20 

Figure 15. Comparison of modulus plots between experimental DMA (1-Hz) data and those 

predicted from the tube-junction model. Fitting parameters were K = 2.5 GPa, 

ε = 0.0006, μ = 63.7 kJ/mol, σ = 12 kJ/mol, Z = 60 kJ/mol, and Tg = 370 K. All other 

fitting parameters needed for the tube-junction model were kept identical, as cited by 

Simon and Ploehn (40). (« = experimental data, and Î = tube-junction model.) ....................22 
v

Figure 16. Corrected master curves for matrix epoxy (▲) and epoxy-silane IPN inclusions 

(▼)...........................................................................................................................................23 

Figure A-1. Arbitrary data set fit to the Voigt upper (solid line) and Reuss lower 

(dashed line) bound predictions using a least squares regression............................................28 

Figure A-2. Rigorous master curve obtained from the C-Combining Rule using equation 22. 

Isotherms were shifted using identical aT values as used in figure 14. ...................................29 

List of Tables 

Table 1. Corresponding E values and error illustrated in figure 10..............................................16 
vi

Acknowledgments 

This research was supported in part by an appointment to the Research Participation Program at 

the U.S. Army Research Laboratory (ARL) administered by the Oak Ridge Institute for Science 

and Education through an interagency agreement between the U.S. Department of Energy and 

ARL. The authors also wish to thank the ARL Composite Materials Research Materials Center 

of Excellence contract DAAL01-96-2-0048 to the University of Delaware Center for Composite 

Materials (UD-CCM) for partial funding of this project. The authors would also like to thank 

Owens Corning Fiberglass for supply of the epoxy film former emulsion used in the model silane 

formulation. The authors also wish to acknowledge the technical assistance provided by John 

Mullin, Gene Napadensky, and Phil Patterson at ARL. The authors are also very grateful for the 

assistance of Professor Roy L. McCullough at UD-CCM. 
1

1. Introduction 

The role of silane coupling agents on adhesion promotion between polymers and inorganic 

surfaces has been widely documented. Much attention has been placed on the specific 

interaction of alkoxy and chloro silane species with adherend surfaces (1–5). Another large body 

of research has focused on the interaction and reaction of the organic functional group of the 

silane coupling agent with the matrix polymer (6–8). It is also widely recognized that, in 

application, the adsorbed silane usually exists as a surface-tethered crosslinked network of varied 

thickness. The three-dimensional interphase (figure 1) is created by interdiffusion and chemical 

reactions between the polymer matrix and silane layer, subsequently forming an interpenetrating 

network (IPN) type of molecular architecture in the fiber-matrix interphase region (9, 10). While 

many have noted the role of subsequent diffusion and/or chemical reaction of the polymer resin 

within the multilayer silane network, very little research has been performed to characterize the 

nature of the structure and properties of this interphase material (11, 12). 

Figure 1. Schematic of polymer-matrix interphase. 

Many resulting adhesive bonding properties may be directly affected by the nature of this 

interphase network structure. For example, it is well known that the fiber-matrix interphase 

plays the most critical role with respect to the micromechanical failure mechanisms and hygrothermal resistance that govern the overall durability of glass fiber reinforced composites (13–16). 

The adhesive bond strength of the interphase also influences the ballistic properties of 

Substrate

Polymer

Interphase
2

composites used in lightweight armor applications (17). The time- and temperature-dependent 

mechanical properties (modulus, Poisson’s ratio, etc.) of the interphase network may also 

strongly influence these aspects of composite performance. Indeed, recent research has shown 

that a rate dependence on the response of the fiber-matrix interphase can be observed during 

fiber push-out tests (18). This research also revealed that the rate dependence of the initial fibermatrix loading and failure was related to the type of silane-based sizing package applied to the 

glass fiber. However, the role of the complicated polymer network structure that is formed near 

the solid fiber surface with respect to rate-dependent composite properties is largely unknown. 

Previous attempts to quantify the mechanical properties of the fiber-matrix interphase in 

composite materials using techniques such as atomic force microscopy and dynamic mechanical 

analysis (DMA) have not been successful due to the small volume fraction (Vf) of material 

located in the interphase region as well as possible fiber artifacts governing the interphase 

mechanical response (19–21). An alternative approach to elucidate the properties of interphase 

materials by studying model epoxy-silane networks purposely formed in the absence of a fiber 

surface has been recently reported (22). Model interphase materials were synthesized to simulate 

commercial-based silane glass fiber sizings, which were subsequently confirmed to match their 

commercial counterparts in chemical composition via nuclear magnetic resonance (NMR) 

spectroscopy. The primary advantage of synthesizing a model interphase material is that large 

volume fractions (Vf ~0.50) can then be incorporated into a matrix resin as micro-sized spherical 

inclusions to form particulate filled composites. During processing, the unreacted matrix resin 

monomers diffuse into the inclusions and react with each other and the organo-functional groups 

of the siloxane network to form an IPN morphology upon cure. The material properties of the 

model interphase IPN can then be determined via the mechanical characterization of the model 

particulate filled composites and suitable application of traditional micromechanics. 

The present study is focused on determining the quasi-static and viscoelastic mechanical 

properties of a model epoxy-silane IPN that is representative of the interphase region formed 

between a silane surface bound to a glass fiber and a thermosetting resin (e.g., glass-fiber epoxy 

composite materials). The experimental technique developed by Tanoglu et al. (22), which is 

outlined schematically in figure 2, is further expanded to investigate the frequency-dependent 

viscoelastic properties of the model epoxy-silane IPN using DMA. The model IPN was chosen 

to simulate the polymeric fiber-matrix interphase microstructure found in typical glass-reinforced 

composites. Micromechanical models were used to analyze the results of DMA experiments on 

a series of model particulate filled composite specimens (epoxy-silane IPN inclusions in an 

epoxy matrix) and used to quantify the viscoelastic properties of the embedded epoxy-silane 

IPN. These results point to the role of the IPN on the properties of epoxy-silane interphase 

structures typical of those found in composite materials and other silane-modified bonds. 
3

Figure 2. Schematic outline of approach used to characterize interphase properties. 

2. Experimental and Analysis 

2.1 Materials 

The IPN constituent materials used in this study were composed of a matrix resin and a model 

silane-based sizing compound. The matrix resin consisted of diglycidyl ether of bisphenol A 

(DGEBA EPON 828) obtained from the Shell Chemical Company. The DGEBA resin was 

cured with stoichiometric amounts of bis (p-aminocyclohexyl) methane (PACM), which was 

acquired from Air Products and Chemical, Incorporated. The model sizing material used was 

intended to mimic typical glass fiber sizing formulations used commercially, and contained 

3-glycidoxypropyltrimethoxysilane (GPS) (Dow Corning Corporation Z-6040). The initial 

synthesis step of the model-sizing material preparation also included the incorporation of a 

water-based dispersion of high molecular weight DGEBA film former, which was emulsified 

using a nonionic polyethylene oxide-polypropylene oxide-polyethylene oxide (PEO-PPO-PEO) 

Matrix

Fiber

 Interphase

chemical

characterization bulk synthesis

composition

 Spherical inclusion

filled composite

μm- powder properties
4

triblock copolymer surfactant. The Owens Corning Company generously provided the film 

former emulsion. A detailed chemical and solids analysis is provided elsewhere (22). Chemical 

structures of the matrix epoxy, amine curing agent, and silane coupling agent are provided in 

figure 3. 

Figure 3. Matrix epoxy, curing agent, and silane coupling agent. 

2.2 Bulk Silane Powder Synthesis and Blend Processing 

The objective of this research is to determine the properties of a model interphase of typical 

commercially treated glass fiber composites, thus the incorporation of film former and surfactant 

into the formulation. Processing of the sizing formulation proceeded in a manner similar to 

commercial treatment of glass fibers except that the sizing was allowed to condense and 

crosslink in the absence of any glass surface. A 10 weight-percent solution of GPS in distilled 

water (pH adjusted to 4.5 with acetic acid to promote hydrolysis of the silane) is added to the 

film former emulsion in a ratio to yield 15 weight-percent solids upon evaporation of the 

aqueous phase. This evaporation was carried out in a forced air convection oven at a 

temperature of 65 °C over a period of time of several days to allow for the condensation and 

crosslinking of the silane coupling agent. The soluble portions of the dried solids are removed 

using a Soxhlet extraction procedure in acetone, which has a similar solubility parameter (δ) to 

the DGEBA epoxy resin, δacetone = 20.3 (J/cm3

)

1/2 vs. δDGEBA = 21.1 (J/cm3

)

1/2 (23). Upon 

completion of solubles extraction, a gelled insoluble silane powder remains. The insoluble silane 

powder has a composition of 73% crosslinked GPS, 19% surfactant, and 8% film former by 

DGEBA

GPS

PACM
5

weight, which is very comparable to what is found on the surface of commercially treated fibers 

after similar extraction experiments (22, 24). 

Particle size distributions of the crosslinked silane powder were measured using a Horiba 

Instruments LA 900 light-scattering particle-size analyzer. The particle-size analysis, illustrated 

in figure 4, displayed bimodal distributions, with the larger particle sizes attributed to 

agglomeration. The average particle size measured using water as the dispersing medium was 

1.9 μm. When acetone was substituted as the dispersing medium significant swelling was 

observed and the average particle size increased to 7.2 μm, including a substantial increase in the 

larger particle diameter portion of the distribution plots. 

Figure 4. Particle size distributions of bulk condensed and crosslinked silane particles prior to 

incorporation into the epoxy matrix. Particle size varied with dispersion solvent (water vs. 

acetone shown). 

The bulk silane powder was then dispersed into the epoxy-amine matrix at high-volume fractions 

using a high-speed/high-shear mixer. The unreacted epoxy-bulk silane mixtures were degassed 

under vacuum, cast into rectangular high-temperature silicone molds, and thermally cured at 

80 °C for 2 hr and post-cured at 160 °C for 1 hr. Under these curing conditions, the time for 

diffusion (tdiffusion) of resin/curing agent into the inclusion is much less than the time for reaction 

0 20 40 60 80 100

0

3

6

9

 Dispersion Medium = Acetone

 Dispersion Medium = Water

Particle Size (μm)

Frequency (%)
6

(treaction). The diffusion of the resin/curing into the silane inclusions to form dispersed particles of 

epoxy-silane IPN material was verified by studying the matrix epoxy Tg while varying the matrix 

amine:epoxy stoichiometry ratio, which is discussed further in the experimental results section. 

Upon completion of the post-cure step, the oven was allowed to slow-cool overnight to minimize 

residual cure stresses. A scanning electron microscopy (SEM) image of a cross section of the 

epoxy-silane IPN inclusions embedded in the cured epoxy matrix is illustrated in figure 5. 

Figure 5. SEM cross section of epoxy-silane IPN inclusion filled 

composite showing spherical IPN particles embedded in 

the cured and glassy epoxy matrix. 

The static tensile Young’s moduli of the neat epoxy matrix (Em) and epoxy-silane IPN particulate 

filled composites (Ec) were measured in a 3-point bending mode configuration using an Instron 

tensile testing instrument. Rectangular samples were prepared with dimensions of width, 

thickness, and length provided by the recommended guidelines listed in ASTM D 790-96a (25) 

to ensure that the proper support span-to-depth ratio was maintained. A bending mode was used 

to measure Em and Ec statically because a similar oscillatory configuration could be used in 

DMA. Dynamic mechanical analysis was carried out using a TA Instruments 2980 DMA with a 

20-mm dual cantilever clamp frame. The samples were tested at an oscillatory displacement 

amplitude of 7.5 μm. The displacement amplitude was verified to ensure linear viscoelastic 

response by deriving corresponding stress-strain (σ-ε) curves. Constant heating rate experiments 

were carried out at 2.0 °C/min at a frequency of 1 Hz from −135 to 225 °C. Master curves were 

constructed from data obtained from multiple frequency sweeps, which were measured over 

three decades of frequency (0.1, 0.3, 1, 3, 10, and 30 Hz) in 3 °C isothermal steps. The 
7

α and β transition temperatures were taken as the peak maximum of the loss modulus (E'') 

curves measured at 1 Hz. The glass transition temperatures (Tg) of the cured samples were also 

measure using a TA Instruments 2980 differential scanning calorimetry (DSC) at a heating rate 

of 15 °C/min. 

2.3 Micromechanical Analysis 

Micromechanical combining rules can be used to describe the elastic and viscoelastic behavior of 

a polymer composite using constituent properties and compositional information. With some 

effort these rules can be used to determine constituent properties from composite measurements. 

Thus, by measuring the properties of the model epoxy-silane IPN particulate filled composite 

and the matrix epoxy, the unknown properties of the epoxy-silane IPN spherical inclusions can 

be determined. 

In this work, we selected the C-Combining Rule (equation 1) as our model. The C-Combining 

Rule can predict properties of composites (Pc) filled with spherically shaped particulate 

inclusions based upon knowledge of the volume fractions (V), Poisson’s ratios (ν), properties (P) 

of the matrix (m), and inclusion (i) phases (26). 

( ) Pc =Vm Pu +ViPl + λ pVi

Vm Pu − Pl , (1) 

where 

( )

( ) 



















 





− −

− = −

m m m i Pm

i m i

u m P V P P

V P P

P P

η

1 , (2) 

( )

( ) 



















 





− −

− = +

i i m i Pi

m m i

l i P V P P

V P P

P P

η

1 , (3) 

and

[ ] ( ) c c c Pm

c

p φ φ φ η

φ λ − −− = 1 2 12 1 . (4) 

The C-Combining Rule uses the Hashin and Shtrikman (27) improved upper (Pu) and lower 

bounds (Pl) and incorporates a phase contiguity parameter (λp). The term φc represents a critical 

packing volume fraction, which for the case of simple spherical packing φc is equal to 0.74. 

Equations 5 and 6 define the term ηPj, where j = i or m. 

( )j

j

j j

j

Kj K G

K

ν

ν

η −

+ = + = 3 1

1

3 4

3

; (5) 

( )

( )

( )

( )j

j

j j

j j

Gj K G

K G

ν

ν

η −

− = ++ = 15 1

2 4 5

5 3 4

6 2

. (6) 
8

The C-Combining Rule, as is the case for most predictive modulus relationships, is only 

rigorously correct for either bulk modulus (K) or shear modulus (G). The Young’s modulus, 

which is more dependent upon Poisson’s ratio, is derived using equations 7 and 8. 

K G

KG E

+ = 3

9 ; (7) 

( ) K G

K G

+

− = 2 3

3 2

ν . (8) 

Thus, by measuring the global properties of the composites that include epoxy-silane IPN 

inclusions as well as the properties of the cured matrix resin, the C-Combining Rule can be used 

to “back-out” the actual epoxy-silane IPN inclusion properties. While simple in theory, the 

application of the C-Combining Rule, due to the numerous embedded equations (equations 2–8), 

can be very time consuming when multiple data sets must be evaluated (e.g., DMA runs with 

several tens of curves at several different frequencies). One may wish to employ a more 

convenient approximation of equation 1 for analysis of modulus data. 

One can show from equations 7 and 8 that the C-Combining Rule can be expressed as the ratio 

Ec/Em in the following exponential series expansion form: 

( ) ... 2

1 1 exp 21

3 0 2 2 +













 ′  +



 



 − 













 ′ = = − μ

μ

μ

μ Z

m

c H H Z

E

E ; (9) 

( ) * Z H0 0 Vi −Vi = μ′ ; (10) 

( ) ( ) 





























 − + − 













 − = − +

m

m i

m

m

m i

m K

K K

G

G G

μ 2 1 ν 1 2ν . (11) 

The series expansion shown equation 9 can be used to solve for Ei by defining Ec in terms of Ei

and Em through any micromechanical model. The expansion is then solved about an arbitrary 

reference volume fraction ( * Vi ) with 0 μ 0 H = ′ at * Vi =Vi . Determination of Ec in terms of Ei

and Em through the C-Combining Rule and subsequent substitution into equation 9 yields a 

6th-order polynomial solution set for Ei. Although a physical interpretation of equation 9 

through more complicated micromechanical models is difficult, it can be seen that a simple 

exponential fitting expression may be used to approximate the rigorous exponential series 

expansion derived in equation 9. Hence it may be convenient to approximate the C-Combining 

Rule as follows: 

[ ]i

m

c bV

E

E = exp − , (12) 

where b captures the terms in equations 10 and 11. This approximation greatly simplifies the 

following viscoelastic analysis of the composite specimens. 
9

Dickie (28) and Schapery (29) have shown that the imaginary term of the complex modulus (E*

) 

must be carried through combining rule analysis of polymer blends in the case of a viscoelastic 

inclusion surrounded by a viscoelastic matrix. 

E = E ′ + iE ′′ * . (13) 

If the imaginary term of equation 13 is carried through the combining rule analysis of Ei, then the 

difficulty of the equation manipulation increases substantially. Fortunately, it can be proved that 

the imaginary component of E*

 can be disregarded from the modulus calculations through the 

following relationships: 

( )( )

2 '' 2 '

' '' ' ''

' ''

' ''

*

*

m m

i i m m

m m

i i

m

i

E E

E iE E iE

E iE

E iE

E

E

+

+ − = ++ = ; (14) 

'

''

'

''

tan , tan

m

m

m

i

i

i E

E

E

E δ = δ = ; (15) 

( )

( ) ( ) ( ) ( ) ( ) i m

m

i m

m

i

m

i i

E

E

E

E δ δ

δ

δ δ

+ −

+

+ = 1 tan

1 tan

1 tan tan

2 '

'

*

*

. (16) 

Therefore, based on the derivation of equation 16, the imaginary component of E*

 can be 

dropped from the ratio of the complex modulus of the inclusion to matrix if the difference in 

magnitude between the loss tangent (tan δ) signals is small. Additionally, if the tan δ values of 

the matrix and inclusion are each small then the remaining tan δ coefficients of equation 16 

reduce to unity, which leaves the complex modulus ratio approximately equal to the storage 

modulus ratio. Therefore E' from DMA can be directly substituted for E in equation 12 to give 

the following expression: 

[ ]i

m

c bV

E

E = − ′

′ exp , (17) 

where Ei Ec ′ = ′ at an extrapolated volume fraction of 1 Vi = . 

3. Discussion and Results 

3.1 Diffusion Time vs. Reaction Time 

A primary assumption of this approach is that the chemistry of the model epoxy-silane IPN 

closely resembles the interphase in actual fiber-reinforced composites. The average particle 

diameter of the epoxy-silane IPN spherical inclusions is much greater than the length scale of the 

interphase actually found in a fiber-reinforced composite. It is also known that tdiffusion for the 

matrix resin and curing agent into the fiber sizing with subsequent equilibration and reaction can 
10

occur rapidly (30). This raises the question of whether or not the DGEBA and PACM can 

adequately diffuse into the larger condensed and crosslinked bulk silane spherical inclusions to 

form a true epoxy-silane IPN structure prior to reaching the gel point of the matrix epoxy phase. 

Fortunately, a simple verification of the assumption tdiffusion << treaction can be made by measuring 

the glass transition temperature (Tg) of the matrix epoxy phase in highly loaded particulate filled 

composites as a function of amine to epoxy stoichiometry ratios. The Tg of cured 

DGEBA-PACM epoxy resin is a strong function of reaction stoichiometry and peaks near 

160 °C at an amine to epoxy ratio of 1:1 (31). Additional PACM was added during the 

processing of the epoxy-silane IPN particulate filled composites to account for the excess 

epoxide functional groups of the GPS and film former found in the condensed and crosslinked 

bulk silane powder. For example, if the particulate filled composite is comprised of 45 weightpercent bulk silane powder then the amine to epoxy ratio of the matrix epoxy phase must be 

adjusted to 1.3:1 prior to blending to compensate for the additional reactivity of the bulk silane 

powder toward the PACM curing agent. This formulation is based on a measured epoxy 

equivalent weight of 500 g/eq (determined via titration and NMR spectroscopy) for the bulk 

silane powder (22). Epoxy-silane IPN particulate filled composite test specimens were prepared 

with global amine to epoxy ratios of 0.9:1, 1:1, and 1.1:1 (these ratios include the epoxide groups 

present in the bulk silane powder and matrix epoxy), which yielded matrix epoxy Tg’s of 139.5, 

155.1, and 152.3 °C, respectively, as measured by DSC. The Tg of the matrix epoxy in the 

composite samples peaked for the global 1:1 amine to epoxy stoichiometry. If the DGEBA and 

PACM were not diffusing, swelling, and chemically reacting with the bulk silane filler then a 

surplus of PACM would remain in the matrix phase, leading to a depressed Tg for the matrix 

phase. It was determined by DSC that if none of the extra compensatory PACM added to the 

matrix epoxy resin diffused into and reacted with the bulk silane powder filler to form an 

epoxy-silane IPN that the matrix epoxy Tg would be depressed to 130 °C. Therefore, based upon 

the study of Tg vs. amine-to-epoxy stoichiometry measurements, the assumption that the average 

bulk silane powder diameter was small enough to allow for adequate penetration of a high 

proportion of the DGEBA and PACM prior to gelation of the matrix phase seems reasonable. 

3.2 Epoxy-Silane IPN Structure 

The results obtained in this research demonstrate the dramatic changes to network properties as 

the matrix resin and curing agent diffuse into and cure with the crosslinked silane powder to 

form an epoxy-silane IPN. Figure 6 shows the DSC results for the bulk silane powder before and 

also after incorporation into the epoxy matrix to form the particulate filled composite consisting 

of epoxy-silane IPN inclusions embedded in the epoxy matrix. The virgin bulk silane powder 

(top curve of figure 6) is compliant at room temperature and has a relatively broad 

glass-to-rubber transition with a Tg of ~ −50 °C. Unfortunately, the cooling capability of the 

DSC is realized near −60 °C; therefore, the glass-to-rubber transition of the bulk silane powder 
11

Figure 6. DSC results for nonreacted bulk silane powder (top curve), epoxy-silane IPN inclusion 

composite (Vi = 0.49, center curve), and neat PEO-PPO-PEO triblock copolymer surfactant 

(bottom curve). 

begins as soon as the temperature of the instrument is ramped. The Tg of the epoxy-silane IPN is 

increased to ~0 °C (indicated in the center trace of figure 6) after diffusion and reaction of the 

epoxy-amine matrix into the bulk silane powder filler. The additional network coupling 

reactions of the epoxy organofunctional groups of the silane with the PACM curing agent forms 

epoxy-silane IPN inclusions with increased crosslink density (ρc) in comparison to the bulk 

silane powder, containing only siloxane crosslinks, which correspondingly increase the Tg of the 

epoxy-silane IPN in comparison to the initial bulk silane powder. The Tg of the matrix-epoxy 

resin is observed near 155 °C. The magnitudes of the endothermic glass-to-rubber transitions are 

decreased for the particulate filled composite due to decreased volume fractions of constituent 

material present in the sample. 

The DSC results also showed a small endothermic peak with an onset near 35 °C, attributed to 

the melt transition of the block copolymer surfactant, which was transported into the particulate 

filled composite from within the initial loading of bulk silane powder. For comparison, figure 6 

also shows the melt transition of a neat PEO-PPO-PEO triblock copolymer surfactant (BASF 

-50 0 50 100 150 200

Heat Flow

Temperature (°C)

endotherm

Bulk silane powder Tg

epoxy-silane IPN Tg

surfactant melting

endotherm matrix epoxy Tg

surfactant melting

endotherm

(control)

-50 0 50 100 150 200

Heat Flow

Temperature (°C)

endotherm

Bulk silane powder Tg

epoxy-silane IPN Tg

surfactant melting

endotherm matrix epoxy Tg

surfactant melting

endotherm

(control)
12

Pluronic F-108), which is a typical representative surfactant used to prepare a commercial-grade 

aqueous film former dispersion. A rough estimate of the amount of the phase-separated 

surfactant can be calculated if the melt energy is assumed to be equal to the BASF surfactant 

(141.7 J/g). The melt energy for the endothermic peak recorded in the epoxy-silane powder 

inclusion composite sample can be normalized based upon the weight fractions and 

compositional properties of the bulk silane powder and surfactant added to the matrix epoxy to 

yield a value of 17.7 J/g, which implies that ~12.5% of the surfactant present in the initial bulk 

silane powder has phase-separated upon formation of the epoxy-silane IPN. It was not 

determined if the small amount of phase separation of the surfactant is occurring in the matrix 

phase or within the epoxy-silane IPN inclusions. 

The network structures of the blends were also probed via DMA. Figures 7 and 8 show the E''

spectra in the temperature range of −100–50 °C for the neat matrix epoxy and particulate filled 

composite (Vi = 0.49), respectively. The broad low-temperature β-relaxation in the neat epoxy 

matrix (figure 7) is caused by the localized chair-to-boat conformational changes experienced in 

the cyclohexyl rings of the PACM curing agent (32). The α-relaxation of the epoxy-silane IPN, 

seen as the shoulders near −10° in the E'' spectra of figure 8, is partially obscured by the βrelaxation of the matrix epoxy, which complicates the analysis of this data. The peaks observed 

near −60 °C in the E'' spectra of the particulate filled composite in figure 8 are presumable due to 

a small volume fraction of larger particles or agglomerations of particles of bulk silane powder, 

where diffusion of the matrix resin and curing agent was not completed and the formation of a 

homogeneous epoxy-silane IPN did not occur. Multiple frequency sweeps were used to resolve 

the peak shoulders embedded in the β relaxation E'' spectra of the matrix epoxy to characterize 

each contributing molecular relaxation mechanism. The α-relaxation (segmental motion) of the 

epoxy-silane IPN and the β-relaxation (short-range motion) of the glassy matrix epoxy will shift 

along the temperature (T) axis with varying extents as a function of frequency (f) due to differing 

Arrhenius activation energies (Ea) as defined by equation 18, 





 



−  = R TE f a 1

2.303 log . (18) 

Figure 7 illustrates the E'' spectra for the β-relaxation of the neat matrix epoxy as a function of 

temperature over three decades of frequency. The shape of the β-relaxation for the matrix epoxy 

is fairly symmetric, ranging from ~ −100 °C to nearly 50 °C, and does not change in shape as f is 

swept. The Ea value for the β-relaxation of the matrix epoxy was calculated to be equal to 

63.7 kJ/mol, which is a typical value lying below 100 kJ/mol for secondary glassy motion (33). 

Figure 8 portrays the multiple low-temperature E'' relaxations present in the epoxy-silane IPN 

inclusion filled composite system. The overall shape of the E'' curves remains the same except 

for the presence of the low-temperature peak near −60 °C and a slight shoulder at −10 °C. The 

Arrhenius analysis yielded Ea values for these transitions of 226.7 and 366.2 kJ/mol for the lowand high-temperature transitions, respectively. Both transitions have Ea values greater than 
13

Figure 7. β-relaxation for neat epoxy matrix as measured by DMA as a function of temperature at 

constant frequency (□ = 0.1 Hz, ○ = 0.3 Hz, Î = 1 Hz, « = 3 Hz, ‘ = 10 Hz, + = 30 Hz). 

100 kJ/mol, which is indicative of an α-transition. The Arrhenius plots are shown in figure 9. 

The α-transition seen as the shoulder at −60 °C could be the segmental relaxation of a small 

amount of unreacted silane powder within the matrix epoxy. Despite a generally good dispersion 

of particles some agglomeration was observed on the fracture surface SEM images. These 

agglomerates were too large to allow for complete diffusion of the epoxy resin and curing agent 

into the largest bulk silane particle prior to reaching the gel point of the matrix (tdiffusion > treaction). 

The α-transition shoulder at −10 °C corresponds to the fully reacted epoxy-silane IPN structure, 

as indicated by the increased value of Tg and Ea. Both the low- and high-temperature shoulder 

locations in the E'' spectra for the epoxy-silane IPN particulate filled composite sample observed 

via DMA are consistent with the transition temperatures trends obtained from DSC. 

3.3 Quasi-Static Micromechanical Analysis 

Figure 10 illustrates the quasi-static Ec vs. Vi plots for the particulate filled composites and the 

least squares fits to the rigorous C-Combining Rule (equation 1) as a function of temperature. 

-150 -100 -50 0 50 100

7.0

7.2

7.4

7.6

7.8

8.0

8.2

8.4

8.6

Temperature (°C)

Log E’’

-100 -50 0 50 100 150 200

5.5

6.0

6.5

7.0

7.5

8.0

8.5

-150 -100 -50 0 50 100

7.0

7.2

7.4

7.6

7.8

8.0

8.2

8.4

8.6

Temperature (°C)

Log E’’

-100 -50 0 50 100 150 200

5.5

6.0

6.5

7.0

7.5

8.0

8.5
14

Figure 8. β-relaxation for epoxy-silane powder inclusion composite (Vi = 0.49) as measured by DMA as a 

function of temperature at constant frequency (□ = 0.1 Hz, ○ = 0.3 Hz, Î = 1 Hz, « = 3 Hz, 

‘ = 10 Hz, + = 30 Hz). 

The experimental data with error is listed in table 1. The extrapolated values of Ei were 

predicted using the E, G, and K conversion factors (equations 7 and 8), based on the assumption 

that νi = 1/3 at T = −50 °C (glassy) and νi ∼1/2 at T = 25 and 100 °C (rubbery). For the matrix 

epoxy, νm was assumed to equal 1/3 for the entire temperature range because at 100 °C the 

matrix epoxy is well below Tg and is still in the glassy state. Two observations are readily 

apparent from the data plotted in figure 10. The first prominent observation is that Ei is always 

less than Em. The second observation is that the decrease in modulus for the epoxy-silane IPN 

inclusions upon transitioning from a glassy to a rubbery polymer at temperatures T > Tg is small. 

Qualitatively, this relatively high rubbery modulus is indicative of a high value of ρc for the 

epoxy-silane IPN material. The DMA E' plots in the temperature range from −100 to 225 °C 

for the neat epoxy matrix and epoxy-silane IPN inclusion filled composite are illustrated in 

figure 11. 

-100 -50 0 50 100

7.4

7.6

7.8

8.0

8.2

8.4

8.6

Temperature (°C)

Log E’’

-100 -50 0 50 100

7.4

7.6

7.8

8.0

8.2

8.4

8.6

Temperature (°C)

Log E’’
15

Figure 9. Arrhenius activation energies (equation 18) for the neat epoxy β-relaxation (■), nonreacted bulk 

silane powder α-transition (▲), and epoxy-silane IPN α-transition (●). Transition temperatures 

are taken from E'' spectra in figure 8. 

The simplified exponential numerical approach (equation 12) for extrapolating to Ei at Vi = 1 was 

also tested using the quasi-static Ec data. These results are also summarized in table 1. The 

values of Ei estimated via equation 12 are in good agreement with those calculated rigorously 

using equation 1, differing by the greatest extent for the 100 °C data set. The exponential fit 

always predicts a slightly elevated value of Ei in comparison to the rigorous C-Combining Rule, 

due to the lack of a phase contiguity parameter. The exponential function was judged to provide 

an adequate approximation of the C-Combining Rule and was subsequently used to fit the 

dynamic modulus data. 

3.4 Dynamic Modulus Analysis 

The rationalization outlined in equations 14–16 is justified by the plots of the tan δ signals for 

the neat epoxy matrix and epoxy-silane IPN inclusion-filled composites illustrated in figure 12. 

In this plot, the region of interest is at subambient temperatures where the secondary β-relaxation 

of the matrix epoxy overlaps with the α-transition of the epoxy-silane IPN inclusions. The 

0.0036 0.0038 0.0040 0.0042 0.0044 0.0046 0.0048 0.0050

-1.0

-0.5

0.0

0.5

1.0

1.5

log f

1/T (K -1)

366.2 kJ/mol

226.7

kJ/mol

63.7 kJ/mol

0.0036 0.0038 0.0040 0.0042 0.0044 0.0046 0.0048 0.0050

-1.0

-0.5

0.0

0.5

1.0

1.5

log f

1/T (K -1)

366.2 kJ/mol

226.7

kJ/mol

63.7 kJ/mol
16

Figure 10. C-Combining Rule predictions of epoxy-silane IPN static modulus. Modulus values measured 

via experimental methodology outlined in ASTM D 790-96a. Experimental error was 

negligible, but is listed in table 1. (Test temperatures: ■ = −50 °C, ● = 25 °C, and 

▲ = 100 °C.) 

Table 1. Corresponding E values and error illustrated in figure 10. 

Vi 

E (GPa) 

T = −50 °C 

E (GPa) 

T = 25 °C 

E (GPa) 

T = 100 °C 

0 2.93 (±0.05) 2.25 (±0.04) 1.68 (±0.07) 

0.12 2.74 (±0.10) 1.92 (±0.04) 1.26 (±0.05) 

0.30 2.70 (±0.06) 1.80 (±0.05) 1.05 (±0.02) 

0.49 2.40 (±0.03) 1.46 (±0.03) 0.784 (±0.004) 

1a 1.89 0.89 0.24 

1b 1.99 0.93 0.33 a

Predicted values obtained using the C-Combining Rule (equation 1). 

b

Predicted values using the exponential approximation function (equation 12). 

α-transition of the matrix epoxy is present near 150 °C and is shown for reference. From these 

plots, it can be seen that both of the requirements for the low and balanced tan δ signal intensity 

are fulfilled, which warrants the ignoring of the imaginary component of the modulus equation 

12. The mathematical rational for dropping the imaginary component of E*

 can also be 

envisioned from a molecular structure point of view. Dickie (28) and Schapery (29) expressed 

E

(GPa)

Vi

0.0 0.2 0.4 0.6 0.8 1.0

0

1

2

3

E

(GPa)

Vi

0.0 0.2 0.4 0.6 0.8 1.0

0

1

2

3
17

Figure 11. DMA E' plots comparing the neat epoxy matrix (●) and epoxy-silane IPN inclusion 

composite (Vi = 0.49, ■). 

concern regarding the imaginary component of the dynamic modulus for situations where the αtransition of the inclusion occurs near the temperature range of the α-transition for the matrix. 

The physical and chemical interactions between the components of a polymer blend can lead to 

changes in the cooperative relaxation time scale of the α-transition for the individual constituents 

of the blend (34, 35). Furthermore, the alteration of the segmental relaxation of polymer blend 

components can also be a function of the blend composition (36). These variations in the time 

scale of the α-transitions of the blend components with volume fraction would result in 

inaccurate bounding values used to predict composite properties if the imaginary term of E*

 is 

neglected. For the circumstance specifically presented in this research, the α-transition of the 

inclusion is superimposed upon the β-transition of the matrix. Fortunately, the molecular 

dynamics of the β-transition in polymer blends are localized and noncooperative, therefore 

remaining unaffected by compositional changes in the blend (37–39). Thus, the viscoelastic 

properties of the matrix epoxy were assumed to remain constant as a function of the epoxy-silane 

IPN inclusion concentration. 

epoxy-silane IPN

particulate filled epoxy

neat epoxy matrix

Temperature (°C)

Log E’

-100 -50 0 50 100 150 200 250

6

7

8

9

epoxy-silane IPN

particulate filled epoxy

neat epoxy matrix

Temperature (°C)

Log E’

-100 -50 0 50 100 150 200 250

6

7

8

9
18

Figure 12. Tan δ signals measured from DMA. (□ = neat epoxy matrix, ○ = 0.12 volume % epoxy-silane 

IPN filler, Î = 0.29 volume % epoxy-silane IPN filler, « = 0.49 volume % epoxy-silane IPN 

filler). 

The master curve obtained from the exponential determination (equation 12) of the epoxy-silane 

IPN inclusion modulus isotherms is portrayed in figure 13. The corresponding horizontal shift 

factor (aT) plot for the master curve is shown in figure 14. The shift factor plot was also fit to the 

Williams-Landel-Ferry (WLF) equation, which is used to describe the temperature dependence 

of the distribution of relaxation times (33), 

( )

( ) 2 0

1 0 log C T T

C T T

aT

+ −

− − = . (19) 

As can be seen in figure 14, the curvature of the experimental shift factor plot for the 

epoxy-silane IPN inclusions is well described by the WLF equation. The reference temperature 

(T0) was set equal to Tg and a nonlinear least squares analysis yielded experimental C1 and C2

constants of 18.4 and 60.5/K, respectively. The experimental values determined for C1 and C2

are in good agreement with the universal values of 17.4 and 51.6/K, respectively. The shift 

factor plot also provided the opportunity to reevaluate the Arrhenius activation energy of the 

-150 -100 -50 0 50 100 150 200 250

0.0

0.2

0.4

0.6

0.8

1.0

Temperature (°C)

Tan 

δ

-150 -100 -50 0 50 100 150 200 250

0.0

0.2

0.4

0.6

0.8

1.0

Temperature (°C)

Tan 

δ
19

Figure 13. Experimental master curve for epoxy-silane IPN inclusions. 

glass-to-rubber transition region for the epoxy-silane IPN inclusions through the following 

alternative expression for Ea: 

0

(1/ )

(log ) 2.303

T T

T

a d T

d a E R

=















 = . (20) 

Equation 20 generates a value of Ea equal to 371.2 kJ/mol in comparison to the value of 

366.2 kJ/mol, which was obtained from the E'' plots shown in figure 8 and applying equation 18. 

These values are in excellent accord and place confidence in the accuracy of the master curve 

portrayed in figure 13. 

3.5 Conversion of Modulus Ratio to Modulus Value 

The preceding derivations were used to validate dropping the imaginary component of the 

dynamic modulus, the exponential fit (equation 12), and the vertical normalization scheme Ei/Em

used to construct the master curve illustrated in figure 13. The modulus ratios ensured that the 

empirical master curve is probably very accurate, but the values provided by Ei/Em are not very 

useful for any future fiber-matrix interphase mechanical models. These modulus ratios need to 

be multiplied by Em as to determine the actual magnitude of Ei. To facilitate this conversion of 

the modulus ratios another master curve of E' vs. frequency at T0 = 0 °C for the neat matrix 

epoxy is required. The difficulty with constructing a master curve for the neat epoxy at T0 = 0 °C 

-6 -4 -2 0 2 4 6 8 10

0.40

0.45

0.50

0.55

0.60

0.65

ω a T log

'

'

m

i

E

E

To = 0°C

-6 -4 -2 0 2 4 6 8 10

0.40

0.45

0.50

0.55

0.60

0.65

ω a T log

'

'

m

i

E

E

To = 0°C
20

Figure 14. Shift factor plot (data points) for the epoxy-silane IPN inclusions with fit to WLF equation 

(solid line). 

is that this reference temperature lies 155 °C below Tg, as seen in figure 11. Traditional timetemperature superposition fails in the glassy region of polymers due to the breakdown of the 

theory of rubber elasticity (33). The isothermal values of E' vs. frequency can be shifted in the 

β-relaxation region of a glassy polymer, but the vertical shift factor correction becomes 

extremely sensitive and the value of Ea determined via equation 20 will not match the 

experimental value determined from the multiple frequency sweep plots of E'' taken across the 

same temperature range (equation 18), which invalidates the master curve. 

Simon and Ploehn (40–42) have recognized this issue and developed the tube-junction molecular 

model to successfully predict the isothermal modulus vs. frequency curves that will shift 

correctly in the glassy region of polymers. The tube-junction model balances the linear 

momentum of an axial (a) and transverse (t) polymer chain by considering the cohesive (Fc

), 

entropic (Fs

), and frictional (Fν) forces brought about by a dynamically induced stress, 

+ + + = 0 ν F F F F t

s

s

a

c . (21) 

T – T0

Log a

T

-30 -20 -10 0 10 20 30

-6

-4

-2

0

2

4

6

8

10

12

14

T – T0

Log a

T

-30 -20 -10 0 10 20 30

-6

-4

-2

0

2

4

6

8

10

12

14
21

An in-depth discussion of the molecular dynamics and mathematical derivations governing the 

tube-junction model is beyond the scope of this report, but the premise behind the fitting of 

experimental data to the model is relatively straightforward. As discussed previously, 

confidence can be placed in the accuracy of a master curve if Ea obtained from the frequency 

sweep measurements matches the value calculated from the corresponding shift factor data taken 

after the construction of the master curve. Among the experimental parameters inputted into the 

tube-junction model are Ea and the breadth of the distribution for the β-relaxation. Basically, Ea

for the β-relaxation of the neat epoxy matrix measured via the frequency dependence of the loss 

modulus is equal to 63.7 kJ/mol. Therefore, to ensure correlation, the frequency-dependent 

modulus predicted by the tube-junction model is assigned a value of Ea equal to 63.7 kJ/mol. 

The storage and loss modulus vs. temperature curves (f = 1 Hz) as predicted by the tube-junction 

model are compared directly with the experimental DMA curves for the neat matrix epoxy in 

figure 15. The agreement between the experimental data and tube-junction model is excellent in 

the β-relaxation region. The agreement in the α-transition region is not as good, but this region 

is already adequately described by the WLF equation. Once a reasonable fit to the experimental 

data is found, the tube-junction model is used to predict multiple frequency sweep DMA data, 

and a master curve in the glassy region can be generated in normal fashion. The master curve for 

the neat epoxy at T0 = 0 °C was then used to factor Em from the Ei/Em ratio used in figure 13. 

The E' master curves for the neat epoxy and the epoxy-silane IPN inclusions plotted on an 

absolute modulus scale are both portrayed in figure 16. The storage modulus for the epoxysilane IPN ranges from ~3 GPa to 750 MPa in the glassy and rubbery response regions, 

respectively. The modulus of the epoxy-silane IPN is always lower in comparison to the neat 

epoxy matrix, which ranges from ~4.75 GPa to 1.75 GPa over the same frequency range. 

4. Conclusions 

To elucidate the typical properties of a silane-modified interphase, the structure of an 

organofunctional silane/thermoset crosslinked IPN was studied. A model epoxy-silane IPN was 

synthesized to simulate the molecular structure found at a representative silane-epoxy-glass 

fiber-matrix interphase. Bulk silane was hydrolyzed and condensed in the presence of a 

surfactant and epoxy film former to mimic commercial glass fiber sizings in larger and more 

easily characterized samples. The resulting material consisted of a powder with a Tg of −50 °C. 

The model bulk silane powder was next mixed at varied volume fractions in an epoxy matrix to 

form filled composites in which spherical particulates of epoxy-silane IPN material formed the 

inclusion phase. Several important results were obtained. First, the diffusion and reaction of the 

epoxy-amine matrix resin into the bulk silane powder resulted in a significant increase in the Tg

of the particulate upon formation of the epoxy-silane IPN. This was due to the additional 
22

Figure 15. Comparison of modulus plots between experimental DMA (1-Hz) data and those predicted from 

the tube-junction model. Fitting parameters were K = 2.5 GPa, ε = 0.0006, μ = 63.7 kJ/mol, σ = 

12 kJ/mol, Z = 60 kJ/mol, and Tg = 370 K. All other fitting parameters needed for the tubejunction model were kept identical, as cited by Simon and Ploehn (40). (« = experimental data, 

and Î = tube-junction model.) 

crosslinking of the matrix resin’s curing agent with the epoxy functional groups in the bulk 

silane powder, as well as possible infusion and crosslinking with additional matrix epoxy resin. 

This Tg of the epoxy-silane IPN was still significantly lower than that of the neat epoxy matrix 

(0 vs. 155 °C), suggesting that a similar interphase in a composite would also have a lower Tg. 

Hence the mechanical response of the interphase would be significantly different than the bulk as 

was expected. 

Next, to quantify the mechanical properties of the epoxy-silane IPN inclusions embedded in the 

epoxy matrix, a micromechanical analysis was devised to determine the properties of the 

inclusion through measurements obtained on particulate filled composite specimens. 

Specifically, the C-Combining Rule was applied to data obtained under quasi-static conditions to 

predict the epoxy-silane IPN modulus based upon the overall composite and constituent matrix 

-150 -100 -50 0 50 100 150 200 250

Temperature (°C)

Modulus
23

Figure 16. Corrected master curves for matrix epoxy (▲) and epoxy-silane IPN inclusions (▼). 

properties. These results show that the modulus of the epoxy-silane IPN was always lower than 

the modulus of the epoxy matrix and, not surprisingly, this difference was greatest when the test 

temperature was much greater than the epoxy-silane IPN Tg (T >>0 °C). The rate-dependent 

storage modulus of the model epoxy-silane IPN was determined using appropriate viscoelastic 

micromechanical descriptions developed for this study and shown to differ greatly from the 

matrix response at all temperatures and rates. 

This study has attempted to quantify the properties of a specific epoxy-silane IPN that are typical 

of those found at silane-modified interphases of glass-reinforced composites. Although possible 

effects to the model epoxy-silane IPN due to the fiber surface (e.g., restriction to the molecular 

mobility, preferential adsorption of curing agent or resin, variations in ρc, entropic effects, etc.) 

are conveniently absent, these results indicate the importance of considering the properties of the 

epoxy-silane IPN interphase. The findings are expected to have relevance to efforts that seek to 

model the interphase-dependent mechanical response of composites including impact, fatigue 

and durability. Furthermore, the methods described in this work may be used to help design 

interphase materials with desired thermal and mechanical properties. 

-4 -2 0 2 4 6 8 10 12

1

2

3

4

5

( )

'

GPa

E

ω a T log

To = 0°C

-4 -2 0 2 4 6 8 10 12

1

2

3

4

5

( )

'

GPa

E

ω a T log

To = 0°C
24

5. References 

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13. Madhukar, M. S.; Drzal, L. T. J. Comp. Mat. 1991, 25 (8), 932. 

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27

Appendix. Explicit Inclusion Modulus Determination 

The use of micromechanical bounding models to “back-out” an unknown modulus of a 

particulate inclusion by extrapolating to Vi = 1 is actually incorrect. For example, a least squares 

fit of an arbitrary set of composite property (Pc) vs. Vi data to the simple Reuss lower bound 

( Pc Vm Pm Vi Pi 1/ = / + / ) and Voigt upper bound ( Pc =Vm Pm +Vi Pi ) models readily 

exemplifies the error with this analysis route. As can be seen in figure A-1, a least squares fit is 

forcing the data to assume the curvature of the model of choice and the lower bound model will 

wrongly predict a higher value of inclusion modulus than the upper bound model. To avoid this 

dilemma in analysis, the inclusion modulus of a particulate filled composite must be determined 

explicitly using the combining rules at each value of Vi. It can also be shown, after much 

derivation, that any combining rule can be used to express the ratio Ec/Em in the following 

algebraic equation: 

0

3 3 − = 















+ 















G G k G

m

c

k k

m

c n F F F

E

E

n F

E

E , (A-1) 

where 

 ) , , , ( , , , , k G G k Ei i m Vi Vm n n F F = f ν ν . (A-2) 

The input parameters for equation A-1 can all be determined experimentally with the exception 

of νi, νm, and Ei. If the values of Poisson’s ratio for the matrix and inclusion are assumed then 

the terms of equation A-1 can be rearranged to solve for Ei. The solution to the C-Combining 

Rule using equation A-1 yields a 3rd order polynomial with 2 negative roots and 1 positive root. 

The isothermal Ec/Em data at Vi = 0.49 was used to reevaluate the epoxy-silane interpenetrating 

network (IPN) inclusion master curve illustrated in figure 13, using the rigorous format of 

equation A-1. For the calculations of the new isothermal data, νm was assumed to equal a 

constant value of 1/3 for all values of T, and νi was set equal to 1/3 at T < Tg and 1/2 at T > Tg. 

The new rigorous modulus isotherms were then shifted horizontally along the frequency axis 

using the aT values plotted in figure 14 to form a new master curve, which is illustrated in figure 

A-2. Several key results can be observed from the rigorous master curve. The assumptions 

made pertaining to νi are valid only in the glassy and rubbery plateau regions where the modulus 

isotherms were shifted successfully using the aT values transposed from figure 14. Poisson’s 

ratio is the key-driving factor in equation A-1, and the lack of reasonable approximations of νi in 

the glass transition region causes the abrupt discontinuity in the rigorous master curve. The 

simplified exponential fit (equation 12) is unbiased by Poisson’s ratio in the glass transition 

region, which is why the empirical master curve shown in figure 13 is smooth and continuous. 

The magnitudes of the predicted modulus ratios are also in good agreement between the 
28

Figure A-1. Arbitrary data set fit to the Voigt upper (solid line) and Reuss lower (dashed line) bound 

predictions using a least squares regression. 

empirical and rigorous master curves. However, the rigorous master curve does expectedly 

predict slightly lower values of Ei/Em in the rubbery plateau region due to phase contiguity 

parameter of the C-Combining Rule. It is also interesting to note that if the difference in 

magnitude between the rubbery and glassy modulus of the epoxy-silane IPN inclusions were 

greater then the error due to the simplified exponential fit and extrapolation procedure, used to 

derive the master curve in figure 13, would have potentially become much greater. 

Pc

Vi

0.00 0.25 0.50 0.75 1.00

0.00

0.25

0.50

0.75

1.00

Pc

Vi

0.00 0.25 0.50 0.75 1.00

0.00

0.25

0.50

0.75

1.00
29

Figure A-2. Rigorous master curve obtained from the C-Combining Rule using equation 22. Isotherms were 

shifted using identical aT values as used in figure 14. 

-6 -4 -2 0 2 4 6 8 10

0.3

0.4

0.5

0.6

0.7

To = 0°C

'

'

m

i

E

E

ω a T log

-6 -4 -2 0 2 4 6 8 10

0.3

0.4

0.5

0.6

0.7

To = 0°C

'

'

m

i

E

E

ω a T log
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