Synthesis, curing behavior and properties of siloxane
and imide-containing tetrafunctional epoxy
Ming-Wei Wang&Ho-ying Wu&Mu-Shih Lin
Received: 18 April 2007 / Accepted: 11 July 2007 / Published online: 20 September 2007
# Springer Science + Business Media B.V. 2007
Abstract A novel tetrafunctional epoxy resin containing siloxane and imide groups, i.e., N,N,N′,N′-tetraglycidyl-bis (4-aminophenyl)-5,5′ (1,1,3,3-tetramethyl-1,1,3,3-disiloxane-bisnorbornane-2,3-dicaroximide, was synthesized and char-acterized. The curing behavior of this resin and the properties of its cocured material with commercial tetraglycidyl meta-xylene (GA-240) was studied. Functional group changes during cocuring reactions were investigated with FTIR. Kinetic parameters were analyzed with dynamic DSC. Thermal properties were measured with TGA, TMA and DMA. Curing kinetics revealed that this novel tetrafuc-tinal epoxy indicated a lower activation energy and lower curing temperature than GA-240. The cocured materials, due to the presence of siloxane and imide groups in the polymer matrix, show higher glass transition temperature, better dimensional stability and toughness, and also enhanced properties than pure GA-240.
Keywords Siloxane and imide-containing tetrafunctinal epoxy . Co-cured materials . Toughness . Enhanced properties
Introduction
Epoxy resin, because of its easy processing and excellent adhesion, is one of the most widely used thermosets, especially in the field of package and PC board in electronics industry. Demand for the epoxy resin in the
new era of electronic application lies in improved thermal properties, better toughness, lower dielectric constant, lower curing temperature and easy processing, etc. Further-more, the vertrification of the bulk state curing would result in internal residual stress in the polymer matrix, and subsequently leads to device failure. Siloxane, due to its flexibility, when incorporated in the epoxy chain, would therefore improve the toughness and lower the internal residual stress. In addition, since siloxane is an excellent water repelling material, an epoxy resin containing such a siloxane group would somehow show a lower dielectric constant. Similar ideal of molecular design was also applied to imide group for its good mechanical property and long term reliability. Tetrafunctional epoxide was deliberately designed in this new resin to enhance the cross-linking and further improve its glass transition temperature and me-chanical properties. Synthesis and characterization of other tetrafunctioinal epoxies can also be referred from literature [1,2]. Since the commercially available tetraglycidyl meta-xylediamine ( electronic grade, GA-240) is relatively hard and brittle, When GA-240 is blended with this siloxane and imide-containing tetrafunctional epoxy, the cocured materi-al apparently show enhanced thermmateri-al properties and toughness, but lower residual stress. In this work, we would like to report such a study.
Experimental
Chemicals
5-norbornane-2,3-dicarboxylic anhydride (i.e., nadic anhydride) was purchased from Merck Co. Tetramethyldi-siloxane was bought from Lancaster Co., catalyst. Platinum– cyclovinyl–methylsiloxane complex (as catalyst for
DOI 10.1007/s10965-007-9137-3
M.-W. Wang
:
H.-y. Wu:
M.-S. Lin (*) Department of Applied Chemistry, National Chiao Tung University, Hsinchu 300, Taiwanhydrosilation) was provided from UCT, and 4,4′-methylene diamine, from CVC, epichlorohydrin from Tedia Co., and 4-nitroaniline from Janssen Co. Solvents toluene and N,N-dimethyl formamide were purified by adding calcium chloride and were distilled. All other chemicals were used directly without further purification.
Instruments
All 1HNMR spectra were obtained with Varian Unity 300 MHz NMR, using CDCl3as solvent. IR spectra were
obtained from Avatar 360 FT-IR (Nicolet Co.) with a
resolution of 4 cm−1. Dynamic differential scanning calorimeter (DSC) thermograms were performed with Seiko Instrument DSC 200 under 100 ml/min of nitrogen flow with heating rates of 3, 5, 10 and 15 °C/min. Thermogravi-metric analyzer (TGA) and thermomechanical analyzer (TMA) thermograms were performed with Du Pont TA 2950 and 2940 with a heating rate of 10 °C/min. Samples with dimension of 4×1×0.2 cm were tested with dynamic mechanical analyzer (DMA) at 1 Hz with a heating rate of 3 °C/min., temperature ranging from 30 to 300 °C. Dielectric constants of cured samples were measured with a DEA Du Pont 2970 dielectric analyzer at 1 MHz.
O O O Si O Si CH3 CH3 CH3 CH3 H H O O O Si CH3 CH3 O Si CH3 CH3 O O O
+
2 Pt complexI
O Si Si CH3 CH3 CH3 CH3 O O O O O O NO2 H2N + 2 DMF/BenzeneH
2 Pd / C -H2O O Si Si CH3 CH3 CH3 CH3 N N O O O O O2N NO2 O Si Si CH3 CH3 CH3 CH3 N N O O O O H2N NH2 Si O Si CH3 CH3 CH3 CH3 N N O O O O NH2 N H2+
O Cl 4 NaOH (aq) 110°C O O O O Si O Si CH3 CH3 CH3 CH3 N N O O O O N NIII
II
b
II
a
toluene,80°C Scheme 1 Synthesis ofPreparation of materials (Scheme1)
Synthesis of 5,5′-(1,1,3,3-tetramethyl-1,1,3,3-disiloaxane-dialyl) bisnorbornane-2,3-dicarboxylic anhydride (I) Nadic anhydride (131.33 g, 0.8 mole) in 500 ml of toluene, to which tetramethyldisiloxane (53.6 g, 0.4 mole) and catalyst platinum– cyclovinyl methyl–siloxane complex were added and stirred at 80 °C for 46 h. The reaction mixture was concentrated to precipitate the product. The product was recrystallized from ether, giving a white crystal with mp. of 135–7 °C (70% yield), which shows1H-NMR (CDCl3, ppm):δ:0.03∼
0.05 (m, 12H), 0.63∼0.68 (m, 2H), 1.56∼1.68 (m, 8H), 2.75∼ 2.78 (m, 2H), 2.85 (m, 2H), 3.39∼3.43 (m, 4H).
IR: anhydride νc=o 1,856, 1,780 cm−1; siloxane, νsi–o–si
1,082 cm−1,νc–si1,225 cm−1.
Synthesis of N,N’-bis(4-aminophenyl)-5,5′-(1,1,3,3- tetramethyl-1,1,3,3-disiloxanedialyl)-bisnorbornane-2,3-dicarboximide (IIb) Into a three necked flask, I (11.58 g, 0.025 mole) in 20 ml DMF and 4-nitroaniline (7.60 g, 0.055 mole) in 20 ml DMF were charged. Reaction pro-ceeded at ambient temperature for 6 h, then benzene was
added, imidization proceeded in a Dean-Stark apparatus. After removal of water, a yellow solid (IIa) was obtained. IIa (7.07 g, 0.01 mole) was dissolved in 300 ml ethanol and Pd/C (0.4 g) was added under Hydrogen atmosphere to reduce the nitro group into NH2at room temperature for 24 h. A red
product of IIb was obtained (5.67 g, 88% yield). 1H-NMR (CDCl3, ppm) shows:δ:0.00∼0.01 (m, 12H), 0.61 (m, 2H),
1.54∼1.61 (m, 8H), 2.72 (m, 2H), 2.78 (m, 2H), 3.15 (m, 4H), 3.66 (s, 4H), 6.59∼6.63 (d, 4H), 6.87∼6.90 (d, 4H).
IR: imide, νc=o 1,768, 1,705 cm−1; siloxane, νsi–o–si
1,082 cm−1,νc–si1,225 cm−1.
Synthesis of N,N,N′,N′-tetraglycidyl-bis(4-aminophenyl)-5,5′-(1,1,3,3- tetramethyl-1,1,3,3-disiloxanedialyl)-bisnorboenane-2,3-dicarboximide (III) IIb (20.00 g, 0.031 mole), epichlorohydrin (96.79 ml, 1.24 mole), NaOH (7.44 g, 0.186 mole) in 15 ml H2O were charged into a
flask and was refluxed at 110 °C for 8 h. Product III was extracted with dichloromethane, washed with water and dried with MgSO4. A greenish solid (20.6 g, 76.7% of
yield) with mp of 127–31 °C was isolated.1
H-NMR and IR spectra of compound III are given in Figs.1 and 2, where
1
H-NMR (CDCl3, ppm) shows:δ:0.04∼0.06 (m, 12H), 0.67
(m, 2H), 1.60 (m, 8H), 1.82∼1.87 (m, 4H), 2.57 (m, 4H), 2.78∼2.85 (m, 8H), 3.16∼3.23 (m, 8H), 6.83∼6.85 (d, 4H), 7.04 (d, 4H).
Fig. 1 1H NMR spectrum of compound I
Fig. 2 FT-IR spectrum of compound I
Table 1 Curing conditions of samples
Sample Equivalent ratio Curing conditions [temp (°C)/time (h)] GA-240/III/MDA A(1/0/1) 150/2+180/4+200/2 B(0.95/0.05/1) 140/2+170/4+200/2 C(0.9/0.1/1) 140/2+170/4+200/2 D(0.85/0.15/1) 130/2+160/4+200/2 E(0.8/0.2/1) 130/2+160/4+200/2
IR shows: epoxide,ν 910 cm−1.
This tetrafunctional epoxy (III) was titrated by the pyridinium chloride method [3] to find an epoxide equivalent weight (EEW) of 432.
Preparation of cocured materials
Blends of GA-240/III/MDA in equivalent ratios of (A) 1/0/1, (B) 0.95/0.05/1, (C) 0.9/0.1/1, (D) 0.85/0.15/1, (E) 0.8/0.2/1, and (F) 0/1/1 were mixed thoroughly and was each filled into DSC aluminum pan and scanned from 30 to 300 °C under 100 ml/min of nitrogen flow at a heating rate of 10 °C/min. Maximum curing exothermic peak for each sample was found at (A) 129.9 °C, (B) 173.1 °C, (C) 170.3 °C, (D) 166.6 °C, (E) 165.2 °C and (F) 152.2 °C. Test samples were then cured at the three stages shown in Table1.
Results and discussions
The synthesis of siloxane and imide containing tetrafunc-tional epoxy (III) is given in Scheme1.
Structures determination
Compound I was prepared from the hydrosylation of nadic anhydride and tetramethyldisiloxane in the molar ratio of 2:1 under the catalysis of active Pd complex. Trace of the reaction was monitored with 1H-NMR (Fig. 1). In the beginning, the two vinylic H’s in norbornene occurs at δ0.63 ppm, and the two Si–H’s in tetramethyldisiloxane occurs atδ4.7 ppm. After 24 h of reaction, both absorption disappeared and a new absorption at δ0.6 ppm was observed, indicating 2 H’s at the C–Si, while δ0.03 to 0.05 ppm indicating 12 H’s of the four methyl groups on Si, and those at δ0.63 to 3.43 ppm indicating the 4 H’s on anhydride, an evidence of producing I. FT-IR spectrum of I (Fig. 2) shows an absorption of anhydride at 1,782, 1,852 cm−1 anhydride, C=O stretching) and 1,220 cm−1 (anhydride, C–O–C stretching). The disappearance of absorption at 1,680 cm−1 (C=C stretching) of norbornene, the new absorptions at 1,083 cm−1 (Si–O–Si stretching,)
Fig. 4 1H NMR spectrum of compound II b
Fig. 5 FT-IR spectrum of compound III
Fig. 6 1H NMR spectrum of compound III
Fig. 7 Dynamic DSC of samples (GA-240/III/MDA), A= 1/0/1; B=0.95/0.05/1; C=0.9/0.1/1; D=0.85/0.15/1; E=0.8/0.2/1; F=0/1/1
and 1,258 cm−1 (Si–CH3 stretching) of the product
confirms the structure of I.
For the structure determination of IIb, the IR absorption at 1,768, 1,705 cm−1 (imide, C=O stretching) strongly supports the existence of imide group (Fig.3). The coupled doublet of asymmetric N–H stretching occuring at 3,460 and 3,369 cm−1 proves the reduction of nitro group into primary amine.1H-NMR spectrum (Fig.4) shows 12 H’s of tetramethyl groups in the siloxane at δ0.00∼0.01 ppm, 4 H’s at δ3.66 ppm for the two primary amine, and δ6.59∼6.90 ppm for the 8 H’s on benzene rings. All these evidence conforms the structure of IIb.
IR absorption (Fig. 5) at 910 cm−1 (epoxy ring stretching) and broad absorption at 3,200–3,500 cm−1,
(νO–H) indicates that III is an oligomer. The 1H-NMR
spectrum (Fig.6) shows Hgatδ0.04∼0.06 ppm (12H), Hf
at δ0.67 ppm (2H), Hd.e at δ1.60 ppm (8H), Hm:
atδ1.82∼1.87 ppm (4H), Hn atδ 2.57 ppm (4H), Hb.c.l at
δ2.78∼2.85 ppm (8H), Ha.j atδ3.16∼3.23 ppm (8H), Hiat
δ6.83∼6.85 ppm (4H), Hh atδ7.04 ppm (4H). Where Hh
represents one H on the benzene ring , Hmrepresents one H
on the epoxide, and from the structure of III, one molecule of III contains 4 Hh’s, and 4 Hm’s. If the product III
contains tetraglycidyl group, the ratio of integrated areas of Hh to Hm=1: 1. It is observed that 1H-NMR spectrum
(Fig.6) shows this is the case, thus conforms the structure of III. A reasonable structure of III is giving in the following:
III was titrated by the HCl–pyridine method [3], and an epoxide equivalent weight (EEW) of 432 was found, which means III has a molecular weight of 1728. The calculated repeating unit, n, is 0.97, very close to 1, which reveals that the tetrafunctional epoxy thus prepared is a dimmer.
n = 0.095
Dynamic DSC
Maximum exothermic peaks for various blends of GA-240/ III/ MDA are listed in Fig. 7 and are summarized in Table2.
It is obvious that Sample A, GA-240 cured with equivalent MDA, shows the highest curing temperature at 179.9 °C. As more III incorporated into GA 240, The curing reaction shifts to lower temperature. Sample F, only III cured with equivalent MDA, shows a maximum exothermic temperature at 152.2 °C. As III contains a tertiary amine, (imide group), which played a catalytic effect on the epoxy cure [4]. This explain the reason why blends containing more quantity of III would show lower exothermic temperature.
Curing kinetics
The activation energy of curing reaction can be studied with dynamic DSC [5–9]. Figure 8 gives the typical dynamic DSC thermograms for sample E at heating rates of 3; 5; 10°, and 15 °C/min. According to the method developed by O Si Si CH3 CH3 CH3 CH3 N N O O O O N N O O O O a b c d e f g h i j k l m n N O Si Si CH3 CH3 CH3 CH3 N N O O O O N O O Si Si CH3 CH3 CH3 CH3 N N O O O O N N CH CH2 O O O CH2 CH OH O CH CH2 OH O CH2 CH OH n OH
Table 2 The maximum exothermic temp. of samples (MDA) Sample Equivalent ratio Max. exothermic temp (°C) GA-240/III/MDA A(1/0/1) 179.9 B(0.95/0.05/1) 173.1 C(0.9/0.1/1) 170.3 D(0.85/0.15/1) 166.6 E(0.8/0.2/1) 165.2 F(0/1/1) 152.2
Kissinger [5, 6, 10], the activation energy, Ea, can be
calculated by:
2:303 d log ΦTm2d 1=Tð mÞ ¼ Ea=R ð1Þ
where Tmis the Maximum peak temperature in K,Φ is the
heating rate in °C/min, R is Gas constant (8.314 JK−1mol−1), and Ea is the activation energy (kJ/mol). A plot of
ln ΦT2
mÞ versus 1/Tm is given in Fig. 9. The calculated
Eavalues are listed in Table3.
Result shows that Sample A has the highest activation energy of 55.51 kJ/ mol. As more III blended into GA-240, lower activation energies were found. This result is in consistent with the finding in Maximum exothermic peak shift (Fig. 7 and Table 2), because imide group played a catalytic role during curing reaction. Furthermore, the tetrafunctional epoxy III contains a flexible siloxane in the main chain which may, in part, show higher chain mobility and thus account for its easy processing.
Functional group changes during curing reaction
Typical functional group changes during curing reactions for Sample E is given in Fig.10, and the difference spectra using the absorbance of Si–O–Si at 1,050 to 1,060 cm−1as
internal reference, is giving in Fig. 11. The absorption of epoxide at 910 cm−1 decreases with time. The absorptions of NH2 (from the curing agent, MDA) at 3,460 and
3,369 cm−1are clearly decreased, and eventually broadened because of overlapping with the absorption of newly produced OH group.
Gel fractions
The fully cured samples were continuously extracted with acetone in Soxhlet extractor for 24 h to extract the uncured portions. Gel fractions of samples are listed in Table4. It is found that all the gel fractions of the five samples are high enough over 97%, meaning that these samples are potentially easy to process and contains high cross-linking.
Fig. 8 Typical dynamic DSC of sample (GA-240/III/MDA=0.8/0.2/1) with different heating rate
Fig. 9 Plot of ln (Φ/Tm2) vs 1/Tm of sample (GA-240/III/MDA= 0.8/0.2/1)
Table 3 Kinetic data of samples Sample Equivalent
ratio
Heating rate (°C/min) Ea, kJ /mol 3 5 10 15 GA-240/ III/MDA A(1/0/1) 151 163.1 179.9 193.6 55.51 B(0.95/ 0.05/1) 142.8 156 173.1 184.9 53.80 C(0.9/ 0.1/1) 139.6 151.7 170.3 181 52.97 D(0.85/ 0.15/1) 137.1 148.3 166.6 179.1 51.81 E(0.8/ 0.2/1) 135.2 147.6 165.2 178.1 50.75
Fig. 10 FT-IR spectrum of sample (GA-240/III/MDA=0.8/0.2/1), A=initial; B=130 °C/10 min; C=160 °C/10 min; D=160 °C/20 min; E=200 °C/10 min
Coefficients of thermal expansion (CTE)
Thermal expansion coefficient of each sample was mea-sured from the smooth slope of TMA thermogram before Tg (Fig.12) and the values are listed in Table5. Although each TMA was tested in a temperature range of 30 to 280 C, the TMA thermogram after Tg appears not very regular, and the CTE after Tg is difficult to be found. Since all these samples are highly cross-linked with high gel fractions as indicated in Table 4, they are, therefore, thermally stable and show relatively low coefficient of thermal expansion. Compared the pure GA-240 (sample A in which GA-240/ III/MDA=1/0/1) with other co-cured samples, It is found that as III is added into GA-240, thermal expansion coefficient can be effectively reduced. That means these co-cured materials have improved dimensional stability and are good for device fabrication. The low thermal expansion coefficient and good dimensional stability of these samples are due to high cross-linking and, also are, probably in part, due to the presence of hard and strong molecular interaction of imide group in III.
Thermal stability
The on-set temperature of 5% weight loss and %residual weight at 450 °C are found from TGA thermograms (Fig.13) and are listed in Table6.
TGA thermogram of Sample A indicates one stage of pyrolysis. While other samples pyrolyze in two stages but
show higher residual weight, which is probably due to the presence of thermally stable imide group [11].
Dynamic mechanical analyzer
Damping peaks (tan δ) and Storage Modulus (G′) of samples are giving in Figs. 14 and 15 and data are listed in Table7.
GA-240 has a Tg of 217.1 °C, as III incorporated into GA-240, these co-cured samples still have relatively high Tg’s. Sample E shows two Tg’s at 228.5 °C and a shoulder at 185.5 °C (probably a transition of siloxane), indicating possible phase separation. Therefore, 20% of III is the maximum amount that can be incorporated into GA-240, or otherwise, more III might cause drawback in the mechan-ical property because of phase separation [12]. Sample A has a storage modulus of 1,775 MPa, other samples indicate storage modulus decreases as III incorporated. This is because the existence of soft siloxane in the samples. However, the rubber siloxane play a toughness role in the samples [12]. In general, high cross-inking materials are hard and brittle. The siloxane portions in III, when incorporated into a high cross-linking material, clearly improve the toughness of these samples, as can be seen from the increased damping peak height and the area of Tan δ in Table8. This phenomenon has been discussed in literature [13, 14]. Furthermore, it appears that the G’
Fig. 11 Different spectra (At−Ao, t in min), showing the increase and decrease of absorbances during cocuring reaction at 160°C
Table 4 Gel fraction results of samples
Sample Equivalent ratio Gel fraction (%)
GA-240/III/MDA A (1/0/1) 98.3
B (0.95/0.05/1) 97.6 C (0.9/0.1/1) 97.4 D (0.85/0.15/1) 98.0 E (0.8/0.2/1) 97.1
Table 5 Thermal expansion coefficients of samples
Sample Equivalent ratio CTEα (μm/m°C)a
GA-240/III/MDA A (1/0/1) 79.6
B (0.95/0.05/1) 56.55 C (0.9/0.1/1) 55.95 D (0.85/0.15/1) 53.68 E (0.8/0.2/1) 55.33
aThe value of CTE was measured from the smooth slope of TMA thermogram before Tg.
Table 6 Thermogravimetric analysis of samples
Sample Equivalent ratio 5% Weight loss temp(°C) Residue weight (%) at 450°C GA-240/III/MDA A(1/0/1) 302.2 43.3 B(0.95/ 0.05/1) 278.3 47.8 C(0.9/0.1/1) 283.6 53.4 D(0.85/ 0.15/1) 257.4 54.8 E(0.8/0.2/1) 251.4 60.7
profiles of samples A, C and D near Tg’s are not regular (Figs. 14, 15). This phenomenon happens presumably because of incomplete cure. As incompletely cured samples are reheated during DMA test, probably post-cure would occur to some extent, leading to some fluctuations as observed from the G’ profiles.
Dielectric constants
Dielectric constants and dissipation factor of cured samples were measured with a DEA Du Pont 2970 dielectric analyzer at 1 MHz of frequency and data are listed in Table 9. In general, dielectric constant and dissipation factor would be reduced as water repellent material is incorporated in a polymer blend [8]. Unfortunately, this is
Fig. 14 Tanδ of samples (GA-240/III/MDA), A=1/0/1; B=0.95/0.05/ 1; C=0.9/0.1/1; D=0.85/0.15/1; E=0.8/0.2/1
Fig. 13 TGA thermograms of samples (GA-240/III/MDA), A=1/0/1; B=0.95/0.05/1; C=0.9/0.1/1; D=0.85/0.15/1; E=0.8/0.2/1
Table 7 Storage modulus and glass transition temperatures of samples Sample Equivalent ratio Storage modulus (MPa) Tg (°C) GA-240/III/ MDA A(1/0/1) 1,775 217.1 B(0.95/0.05/1) 1,210 213.8 C(0.9/0.1/1) 1,099 226.3 D(0.85/0.15/1) 636 227.1 E(0.8/0.2/1) 443 185.5, 228.5
Table 8 DMA results (peak height and area of Tanδ) of samples Sample Equivalent ratio Peak height Area of Tanδ GA-240/III/ MDA A(1/0/1) 0.24 13.8 B(0.95/0.05/1) 0.37 20.6 C(0.9/0.1/1) 0.40 25.4 D(0.85/0.15/1) 0.45 29.0 E(0.8/0.2/1) 0.48 34.4
Fig. 15 Storage modulus of samples (GA-240/III /MDA), A=1/0/1; B =0.95/0.05/1; C=0.9/0.1/1; D=0.85/0.15/1; E=0.8/0.2/1
Fig. 12 TMA thermograms of samples (GA-240/III/MDA), A=1/0/1; B=0.95/0.05/1; C=0.9/0.1/1; D=0.85/0.15/1; E=0.8/0.2/1
not the case expected as observed from data. As it is well known that OH group produces when epoxy cures [4]. The high polarity of OH group in III would no doubt play a drawback in dielectric constant and dissipation factor. Furthermore, since GA-240 and III each contains four epoxides which must produce lots of OH groups after they are cured. This probably explain why relative high dielectric constants and dissipation factors are observed for these samples. Use of less MDA as curing agent could be one way to lower the dielectric constant and dissipation factor. Because less diamine used would produce less OH’s but in the meantime cause less cross-linking. It seems apparently, dielectric constant and mechanical properties must compromise in some way.
Conclusion
Since commercial tetrafunctional GA-240 is a relatively hard and brittle material. Blending siloxane and imide-containing tetrafunctional epoxy III into GA-240 to improve the physical properties appears permissible. The imide group in III contains tertiary amine and played a catalytic role to the curing reaction of epoxy and lower the
activation energy and curing temperature. The soft rubber section of siloxane played a toughness role. The co-cured materials apparently showed improved dimensional stabil-ity, lower thermal expansion coefficient, and enhanced toughness. However, increased dielectric constant of sam-ples is the drawback because more OH’s were produced in the cured material. This drawback could be possibly improved by using less MDA as curing agent.
Acknowledgement The authors would like to express their appre-ciation to National Science Council for financial support under grant contract NSC 94-2216-E-009-005.
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Sample Equivalent ratio Dielectric constant Dissipation factor GA-240/III/ MDA A(1/0/1) 5.71 0.036 B(0.95/0.05/1) 5.16 0.036 C(0.9/0.1/1) 5.43 0.038 D(0.85/0.15/1) 7.21 0.081 E(0.8/0.2/1) 6.02 0.069
