MATERIALS CHARACTERIZATION 40:1–5 (1998)© Elsevier Science Inc., 1998 1044-5803/98/$19.00655 Avenue of the Americas, New York, NY 10010 PII S1044-5803(97)00088-0

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M

icrostructure, Optical, and Dielectric Characterization of Porous Gel Silica Impregnated with PMMA

Maria Cecília Nobrega,* L. C. F. Gomes,* G. P. LaTorre,

†

and J. K. West

†

Metallurgical and Materials Engineering Department, *Federal University of Rio de Janeiro, C.P. 68505, CEP 21945-970, Rio de Janeiro, Brazil; and Materials Science and Engineering Department,

†

University of Florida, 132 Rhines Hall P.O. Box 116450, Gainesville, Florida 32611-6595

The microstructure, optical, and dielectric permittivity of gel silica impregnated with poly-methyl methacrylate (PMMA) were evaluated. Samples with and without PMMA werecompared for pore size reduction and for their dielectric response in the 1–100kHz fre-quency range. The dielectric constant was as low as 3–5, which is much less than the valuefor the wet silica gel (150). Besides its insensitivity to the moisture content in the environ-ment, this composite material is much easier to handle and to machine. The discovery ofthese characteristics points to an opportunity for the application of silica gel–PMMA com-

posites in the optical-electronic field. © Elsevier Science Inc., 1998

INTRODUCTION

It is well known that the signal-transmis-sion rate through a dielectric substrate isproportional to the square root of the di-electric permittivity of the medium. Atpresent, popular ceramic substrates used inindustry are made of alumina or its com-posites, owing to the excellent thermal andmechanical properties that alumina pos-sesses. The dielectric permittivity of alu-mina, however, is relatively high (in therange of 10); it is therefore necessary to ex-plore other potential materials, with lowerdielectric permittivity, for use in multilayerceramic substrates.

Yarbrough et al. [1] demonstrated thatboth bulk silica gels and thin films pre-pared from colloidal silica could be promis-ing for meeting the needs of high-speed,high-performance substrates with very lowdielectric permittivity; their gels, however,had to be pressed to form a desirable shapethat could be handled.

Within the past decade, sol-gel techniqueshave been developed and refined by several

researchers in various parts of the world [2,3] in a way to prepare crack-free monolithicpieces from these techniques. By control-ling parts of the sol-gel processing, one cancontrol the volume fraction, average radius,and size distribution of the porous network.These unique properties have opened upmany different applications for the porousgel silica products, including solid-statedye laser [4], laser-densified waveguides[5], wavelength shifters, and matrices fornonlinear optical materials [6].

Porous type VI gel silica (xerogels), as inBrückner’s notation [7], were produced in theAMRC laboratories. West et al. [8] showedthat this kind of porous gel silica can be suc-cessfully used as a substrate for laser-den-sifed or solution-impregnated waveguides.

In the search for substrate materials,products from silica gels and organic mate-rials with dielectric constants close to 3have been considered. However, concernsremain about some of their properties, withmoisture absorption for the silica gels andinterfacial stress being the most seriousproblems for the polymers.

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M. C. Nobrega et al.

Because the combination of silica gel andthe hydrophobic polymethyl methacrylate(PMMA) has shown better resistance toerosion, solvent crazing, weathering, ther-mal expansion, thermal conductivity, andthermal shock resistance than has bulkPMMA [9, 10], it is important to know theproperties of these composites regardingtheir application in the substrate field.

This paper describes the preliminary pro-cessing and characterization of porous gelsilica impregnated with PMMA targetedtoward the needs of the optical and elec-tronic packaging industry.

EXPERIMENTAL PROCEDURES

Porous gel silica was prepared from a solu-tion of tetramethyloxysilane, deionized wa-ter, reagent grade nitric and hydrofluoricacids, as described elsewhere [11]. Samples

were used for the PMMA impregnation asschematically shown in Fig. 1.

Gel silica rods were filled with a PMMA(powder) and methyl methacrylate (MMA),(monomer) solution [Fig. 1(a)]. Before im-pregnation, the samples were dried in avacuum oven at 125

8

C overnight.The impregnation was done by soaking

the silica gel samples in the PMMA solu-tion (PMMA dissolves in the MMA) for 1–2days [Fig. 1(b)]. The polymerization of thePMMA impregnated into the silica gel(6nm average porous sized samples) wascompleted after 7 days under irradiation ofultraviolet lamp followed by 2 h at 100

8

C.The samples were dry to the touch andtransparent after being cleaned with ben-zene and polished [Fig. 1(c, d)].

The Quantachrome Corporation’s Au-tosorb 6 was used for the pore characteriza-tion. Mercury and helium pycnometry wereused to obtain the bulk and skeletal densityof the samples, and an HP 16451B Dielec-

FIG. 1. Schematic of Type VI silica gel monolith impregnation with PMMA.

Characterization of Gel Silica with PMMA

3

tric Test Fixture combined with an HP4192A was used to measure the dielectricconstant over a frequency range of 5Hz to100 MHz.

RESULTS AND DISCUSSION

According to the quantitative N

2

adsorp-tion-desorption isotherms (test BET), thesurface area decreased from 506m

2

/g to al-most zero after PMMA impregnation. Ta-ble 1 compares the surface area, pore vol-ume, and average pore radius before andafter impregnation. These results show thatthe impregnation filled up all the pores inthe silica gel. Figure 2 compares the micro-structure of both materials before and afterthe PMMA impregnation. Both materialsexhibited brittle fracture, but the compositesilica gel plus PMMA supports much higherimpact tension before it breaks. With thebulk and skeletal density measurements, itwas possible to evaluate the open porosityand the percentage of the second-phase im-pregnation. Following the mixing rule forsilica gel–PMMA composite materials [9],we calculated that about 70% of PMMAwas impregnated into the pores and intothe skeletal structure of the silica gel. Theopen porosity was completely filled afterthe PMMA impregnation, as verified by theBET test results showed in Table 1.

Figure 3 shows that the impregnationprocedure leads to a composite materialthat is more transparent in the visible andhas an ultraviolet cutoff shifted to smallerwavelengths when compared with pure sil-ica gel. The composite material presents al-most the same optical characteristic as doesthe pure PMMA.

As shown in Fig. 4(a, b), the dielectricconstant of the impregnated samples islower than that of the pure PMMA, and it isinsensitive to the water environment. Thewater or OH

2

content, porosity, averagepore size, pore-size distribution, and resid-ual alkali ions are factors that can affect thedielectric properties of porous silica. Thehydrophobic properties of the PMMA,however, impart hydrolytic stability to thesilica-polymer combination, as shown forthe dielectric characteristic in Fig. 4(b).

Chandrashekhar and Shafer [12] proposedthat water or hydroxyl ions alone could not

Table 1

Quantitative BET Results for Pure Silica Gel (6 nm Porous Diameter) before and afterPMMA Impregnation

Before After impregnation

Surface area 506m

2

/g

2

1.30

3

10

2

2

m

2

/gPore volume 7.0

3

10

2

1

g/m

3

8.7

3

0

2

4

g/m

3

Average pore radius 2.802nm 2

3

10

2

4

nm

FIG. 2. Sannins electron micrograph of gel-silica ma-trix before and after PMMA impregnation.

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M. C. Nobrega et al.

be responsible for the high dielectric con-stants measured on different gel-silica glassesand that the elemental carbon incorporatedinto gel-silica products was the most likelycause of enhanced dielectric constant.

Porous ceramics can be treated as mix-tures of ideal dielectrics. In this case, as-suming the dielectric constant of pores tobe unity and that of bulk silica 4.5, Banno[13] derived a theoretical equation that canbe used to calculate the dielectric constantof porous gel-silica materials [14].

By applying the Banno equation to theimpregnated and pure gel-silica samples,we calculated the dielectric constants of thesamples with and without PMMA. A di-electric constant equal to 2.08, which islower than the 2.5 experimentally measured,was calculated for the pure gel-silica mate-

rial. For the impregnated samples, the dif-ference was considerably large (2.81 as calcu-lated versus 3.5 as experimentally measured).The Banno analysis assumes a closed-poremicrostructure model. As already pointedout by Cao et al. [14], the deviation fromthe measured value suggests that the con-nectivity or the pore microstructure be-comes an important factor in determiningthe dielectric permittivity of the compositewith open porosity.

Because of the very high surface area andsurface free energy of the gels, the organicmaterials in the micropores can react withthe silicic acid structure. An energeticallyfavorable bonding may have a role in themechanism of polarization that leads to anincrease in the dielectric constant of porousgel silica to 4.5 from 2.5. Further molecular

FIG. 3. Ultraviolet–visible transmission of gel silicamatrices before and after the PMMA impregnation. FIG. 4. Variation of dielectric constant with frequency.

Characterization of Gel Silica with PMMA

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orbital calculations will be performed to ex-plain the dielectric data for the PMMA im-pregnated samples.

The increase in the high-frequency di-electric constant seen for the pure gel-silicasamples may be attributed to the presenceof more micropores or bulk water, as re-ported by Wallace et al. [15].

CONCLUSIONS

1. PMMA impregnation of all pores of thetype VI gel-silica monoliths can be ob-tained by a combination of soaking thesilica gel in a PMMA solution and ultra-violet polymerization processing. Bettermechanical, electronic, and optical prop-erties than those for pure PMMA can beachieved by a low-temperature gel-silicafabrication.

2. The low dielectric constant and its insen-sitivity to environmental humidity of thegel silica–polymer composite is a desir-able characteristic for the packaging in-dustry competing with the various ceram-ics currently used as substrate materials.

The authors are grateful to the National ScientificFoundation of Brazil (CNPq) and to the AdvancedMaterials Research Center (University of Florida)for financial and technical support for this re-search.

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Received February 1997; accepted June 1997.