Ind. Eng. Chem. Res. 1991,30, 211-217
211
Penetrant Transport in Epoxy Resins Jennifer J. Sahlin and Nikolaos A. Peppas* School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907
Cured epoxy resins were prepared by reacting N,N,N’,N’-tetraglycidyl-4,4’-diamina iphenylmethane (TGDDM) with the cross-linking agent 4,4’-diaminodiphenyl sulfone (DDS). The components were cured at 80 “C for 2 h, 100 OC for 1h, 150 OC for 4 h, and 200 “C for 7 h. Epoxy resin samples were prepared with compositions of 25, 30, and 35 wt %’ DDS. Thin slabs were exposed to a number of penetrants, which acted as molecular probes of the resins’ structure. The transport data were analyzed by using Fickian diffusion with a constant diffusion coefficient or a coupled relaxation/ diffusion model containing a characteristic relaxation constant in addition to the diffusion coefficient. Equilibrium water uptake varied from 5.6 to 7.1 g/100 g of resin and methanol uptake from 16.4 to 19.1 g/100 g of resin. The dynamic uptake behavior of methanol was highly anomalous. The equilibrium uptake values of ethanol, acetone, and methyl ethyl ketone (MEK) were all lower than 2.5 g/100 g of resin. There was a slight increase in the equilibrium uptake with increasing DDS content in the resin composition range examined.
Introduction General. Epoxy resins can exhibit excellent toughness, adhesive strength, chemical, electrical, and thermal resistance, and mechanical properties varying from good flexibility to exceptional strength and hardness (May, 1988). These characteristics promote their extensive use in an array of applications. A major limitation of epoxy resins is their sensitivity to environmental conditions such as humidity and temperature fluctuations. Variations in temperature can cause a substantial degradation of the resin’s mechanical properties; these deleterious effects are enhanced when the resin is also exposed to moisture. Since exposure to both fluctuating temperatures and moisture is inevitable in both aerospace and naval applications, explaining the mechanisms that promote the degradation of the epoxy resin’s physical properties and the structural strength and stability of the composite system could lead to improvements in both the system’s chemical formulation and the mechanical designs employed in their use. One of the widely studied resin systems used in these endeavors is N,N,N’,N1-tetraglycidyl-4,4’-diaminodiphenylmethane (TGDDM) cured with 4,4’-diaminodiphenyl sulfone (DDS) (Figure 1). The effect of moisture on the resin produced from the reaction of these monomers has been the focal point of previous work because of its importance in aeronautical applications. TGDDM has four epoxy functional groups that can react with the two amine functional groups in DDS; the epoxy functionals can also interact with each other to form ether linkages. Additional reactions can occur between the hydroxyl groups that form from the primary amine-epoxy reaction and other epoxy groups (Figure 1). The multiplicity of reaction pathways leads to a molecularly heterogeneous, highly cross-linked network structure. Which reaction process predominates depends on both the temperature program used during the cure and the initial composition of the reactant mixture. Different distributions of free functional groups are present as the curing process passes beyond the gel point. Furthermore, as the gel point is approached, the mobility of the chain segments in the resin is restricted. Thus, an increasing fraction of the molecular species present become trapped in an unreacted state. These factors lead to different network structures with unique physical properties (Mikols et al., 1982). It is important to note that two major reaction pathways lead to the formation of hydroxy end groups. These hydroxy functional groups may not react completely due to steric and configurational limitations in the reacting 0888-5885/91/2630-0211$02.50/0
system. They have an affinity for penetrant molecules such as water and can have a significant effect on the system’s properties. Moisture Sorption in Glassy Epoxy Resins. Moy and Karasz (1980) concluded that non-Fickian diffusion observed in TGDDM-DDS resins could be due either to diffusion coupled to relaxation processes or to irreversible chemical reactions. Since the experimental isotherms examined were measured at temperatures significantly lower than the glass transition temperatures of the plasticized resin, the time for diffusion should be much shorter than the relaxation times. Thus, it was proposed that the water was interacting with the polymer by hydrogen bonding as verified by differential scanning calorimetry (DSC). This conclusion was supported by the presence of residual water, which could only be removed from the resin by exposing it to a dry atmosphere heated above 100 “C. Wong and Broutman (1985) pointed out that the nonFickian process observed during a first sorption cycle may be due to insufficient cross-linking. Additional crosslinking could potentially occur during this sorption process. Fickian behavior was observed for subsequent sorption cycles. Similarly, the concentration-dependent diffusion coefficient observed by Mikols et al. (1982) was eliminated by removing the unattached, low molecular weight species from the matrix by sorption and desorption cycles. Mijovic and Weistein (1985) found that the diffusion process in this system was anomalous; the transport behavior became increasingly Fickian with increasing temperature. It was concluded that at lower temperatures moisture was contained within the regions of the matrix with lower crosslinking densities. As the temperature increased, the thermal energy of the water and the segmental motions of the polymer chains increased. This allowed moisture to penetrate into the higher density regions of the resin. Tsou and Peppas (1988) studied the diffusion of water in TGDDM-DDS resins with compositions of 5,15,25,35, and 45 wt % DDS. Swelling studies were done at temperatures that were 80-140 “C below the observed TBof the “water-swollen” resin. They observed that all of the samples with DDS concentrations greater than 5 wt % DDS vitrified during cure. This vitrification prevented the polymer chains from relaxing to their quasi-equilibrium conformation in the glassy state. Thus, additional free volume was trapped within the material. Since the 5 w t % DDS sample could rearrange to its quasi-equilibrium state, this case could be readily fit with Fick’s law. A t 30
0 1991 American
Chemical Society
212 Ind. Eng. Chem. Res., Vol. 30, No. 1, 1991
Table I. Equilibrium Water Uptake of TGDDM-DDS SamDles
temD. OC 25 ~
-CH2 -CH
/O\
CHz
* RNH2
-
60
-CHz -CHOH -CH2
I
90
RNH +CH, -CH -CHzOH
-CH2 -CH
-
CH2 ROH
-
I
RNH
"2H2 -CHOH -CHz -0 -R
-CHZ -CH -CHzOH
1
OR
CH2 + 9, R2 NH --CH2
-CHOHCHz NR, Rz ETC
0
I
-CH2 -CH -CHz -0
-
Figure 1. Chemical structures of TGDDM and DDS and the major curing reactions.
"C, the absorption data showed a slight inflection at low sorption levels. For relaxation times of the same order of magnitude as the diffusion times of the system, the vitrified resin would relax to its quasi-equilibrium state. The diffusion process was faster than the relaxation processes when absorption occurred at 60 "C. The quasi-equilibrium state was reached for all of the samples that were postcured. These samples could be fit with Fick's law by using a constant diffusion coefficient. A substantial amount of work has been done by focusing on the transport of water into the TGDDM-DDS resin system. An effort has been made to describe this transport behavior as Fickian or non-Fickian. However, the use of other low molecular weight penetrants to study the processes involved has not been examined. By using other penetrants, the differences in the penetrants' molecular structure, their affinity for the resin, and their diffusion behavior could potentially be correlated. This could be beneficial in trying to improve this type of high glass transition epoxy resin by reducing its sensitivity to moisture. This, in turn, would minimize the deleterious effects of moisture uptake. Experimental Section Sample Preparation. Epoxy resin samples were prepared by continuously heating TGDDM (MY720, CibaGeigy, Palo Alto, CA) at approximately 120 OC while a predetermined amount of DDS (Eporal976, Ciba-Geigy, Palo Alto, CA) was thoroughly mixed in. The mixture was then poured into aluminum weighing pans and degassed for 20 min in a vacuum oven at 20 inHg and 130 "C.
DDS in resin. wt % 25 30 35 25 30 35 25 30 35
water uptake, g/100 g of resin as-cast post-cured (90% CI) (90% CI) 6.32 f 0.80 6.30 f 0.45 6.76 f 0.31 6.31 f 0.40 7.13 f 0.49 6.97 f 1.65 6.02 f 0.27 5.66 f 0.27 5.56 f 0.67 6.45 f 1.38 6.57 0.27 6.07 h 0.27 6.37 f 0.94 5.91 f 0.13 6.84 f 0.22 6.48 f 0.04 6.94 0.36 7.05 f 0.36
The samples were stored at 4 "C prior to curing. The samples were cured following the manufacturer's specifications: 2 h at 80 "C, 1 h at 100 "C, 4 h at 150 "C, and 7 h at 200 "C. This staged curing process was also used by Tsou and Peppas (1988). The samples were allowed to cool to room temperature and were then stored in a desiccator. Thin films were cut from the epoxy samples with a diamond-edged saw. The aspect ratio of each film (length and width to thickness) exceeded 50:l to ascertain one-dimensional transport as suggested by Ritger and Peppas (1987). Some samples underwent a post-curing process; the epoxy films were heated to 200 "C for 4 h, cooled to room temperature, and stored with the as-cast samples in a desiccator. Dynamic Penetrant Transport. The uptake of various penetrants by epoxy resin samples was measured as a function of time. After the dimensions and initial mass of each sample were noted, the thin slab was submerged in the penetrant. Each sample was supported by a small mesh so that it would not settle to the bottom of the vial and prevent diffusion through the bottom surface. Each slab was removed at various time intervals, patted dry with tissue, weighed to the nearest 0.1 mg, and returned to the diluent. This process continued until equilibrium was attained. In each case, near-replicate samples were used. These samples were cut from the same epoxy sample so each pair had identical compositions and cure histories; they may have varied slightly in their actual size. The absorption of water was studied at 25, 30, and 35 w t % DDS for both as-cast and post-cured samples. Data were obtained for each case at room temperature (25 "C), 50 "C, and 90 "C. The absorption of acetone, ethanol, methanol, and methyl ethyl ketone (MEK)was studied at room temperature for each of the aforementioned compositions and cures. Results and Discussion Water Transport into Epoxy Resins. Equilibrium moisture sorption results for all the samples prepared are given in Table I. The 90% confidence intervals for the average value of the two near replicates at every experimental condition were determined with a t-test analysis. These data show that the samples which underwent a post-curing process consistently absorbed a higher quantity of moisture. Furthermore, while the moisture sorption trend for the as-cast samples was not as consistent, the post-cured samples showed a trend toward higher equilibrium moisture content with increasing DDS content. Typical sorption data are shown in Figures 2 and 3 for as-cast and post-cured samples containing 30 wt % DDS, which were exposed to water at 25 OC. The sorption curves exhibited a slight inflection at lower times, which could be due to a relaxation process that increased the diffusion rate. This would appear as a lag time (induction period)
Ind. Eng. Chem. Res., Vol. 30,No. 1, 1991 213 n
d
** m
8.0
0 0
6.0
e, k
r(
2
v
0)
4.0
3 (d 3
5
2.0
2k
3 al
9
lf
ti/*//
(hr'n/mm)
the square root of the sorption time normalized with respect to the sample's thickness, t1/2/l.Circles: 1 = 0.55 mm. Solid line: eq 1. Dashed line: eq 2.
D)
d
8.0
k
M
w 0 0
6.0
W
4
4.0
3
4
5
2.0
d
d
*,
4
8 4'0
2.0
2
2
e,
9)
pl:
0
W
3
!
6.0
a
a +,
8.0
d
rl
3
(hr'/*/mm)
mm.
e, k
9)
20.0
15.0
Figure 4. Water uptake in as-cast samples of TGDDM-DDS epoxy resin a t 60 OC, prepared with 35 wt % DDS as a function of the square root of the sorption time normalized with respect to the sample's thickness, t1/2/1.Circles: 1 = 0.54 mm. Squares: 1 = 0.59
9)
0 0
I
I
10.0
n
n 4
I
5.0 ti'*//
Figure 2. Water uptake in an as-cast sample of TGDDM-DDS epoxy resin a t 25 OC, prepared with 30 wt % DDS as a function of
d
c
0.0 0.0
0.0 0.0
B
10.0
20.0
30.0
40.0
(hr*D/mm) Figure 3. Water uptake in a post-cured sample of TGDDM-DDS epoxy resin a t 25 OC, prepared with 30 wt % DDS as a function of the square root of the sorption time normalized with respect to the sample's thickness, t 1 / 2 / l . Circles: I = 0.49 mm. Solid line: eq 1. Dashed line: eq 2. ti/*//
in the inception of moisture uptake. No inflection was apparent for moisture sorption at 60 O C , while at 90 "C the data available were insufficient to characterize the initial stage of water sorption. The dynamic sorption behavior at 60 and 90 "C is shown in Figures 4 and 5, respectively. In the compositions examined, increasing the DDS content increased the amount of moisture retained in these samples. The rate of moisture uptake increased significantly as the sorption temperature was raised, as can be concluded from comparison of the data of Figures 2 and 5. The moisture sorption data were analyzed with two models for the diffusion process. The first model was one
p:
0.0 0.0
I
I
10.0 2 0 . 0
I
I
30.0 40.0
50.0
tal*// (hr'/*/mm) Figure 5. Water uptake in as-cast samples of TGDDM-DDS epoxy
resin at 90 OC, prepared with 30 wt % DDS as a function of the square root of the sorption time normalized with respect to the sample's thickness, t l / * / l .Circles: 1 = 0.38 mm. Squares: 1 = 0.40 mm.
of unidirectional diffusion into a thin slab described by Fick's law with a constant diffusion coefficient and lead to the following expression for the fractional penetrant uptake (Crank, 1959): g -MMt-, - 1 - n-n-o(2n z + I)**'
ex.[
-D(2n
+ 1)'a't 12
]
(1)
Here, Mt is the amount of penetrant absorbed at time t ,
M, is the equilibrium penetrant uptake, the D is the apparent diffusion coefficient. The second model was proposed by Berens and Hopfenberg (1978,1979) to describe the anomalous transport
214 Ind. Eng. Chem. Res., Vol. 30, No. 1,1991
25 O C for an as-cast sample containing 30 w t % DDS. Although the sum-squared error analysis implies that the model proposed by Berens and Hopfenberg (1978,1979) fit the experimental results more adequately than the Fickian solution, the difference was very small. The post-cured samples, which were exposed to identical experimental conditions, on the other hand, were more adequately fit by the model that accounted for relaxational effects (Figure 3). In several instances, the regression analysis converged on a minimum in which the fraction of Fickian behavior was relatively small. The high percentage of anomalous behavior described by this model could not be attributed to changes in the degree of swelling since the dimensional changes were within the error of their measurement. Thus, this heuristic model did not specify the physical processes that caused the observed anomalous behavior. Instead, it simply facilitated the determination of non-Fickian diffusion behavior. This analysis indicates that the relaxation times are of the same magnitude as the diffusion times at 30 “C.This is observed macroscopically as an inflection in the moisture uptake curve. Relaxation phenomena predominate when the relaxation time is an order of magnitude or larger than the diffusion time. Such anomalous behavior is unusual in glassy polymers that are exposed to penetrants at temperatures significantly below their glass transition temperature. As the temperature increases, the rate of diffusion increases and the two processes are no longer coupled. Transport of Organic Penetrants. The dynamic and equilibrium sorption behavior of different penetrants was examined in order to determine how the penetrant’s diffusion behavior was affected by its molecular weight and affinity for the epoxy resin. This information, combined with spectroscopic observations, could help identify the major interactions between the epoxy resin and water. The interactions which had a major effect on the resin’s sorption behavior could then be minimized by changing the chemistry of the system. The diffusion of low molecular weight organic penetrants in TGDDM-DDS resins gave mixed results. The dynamic uptake data for acetone were monitored for 1 month in post-cured resin samples; the uptake behavior is shown in Figure 6. Less than 0.015 g of acetone/g of resin was absorbed during this time, and the samples maintained their equilibrium sorption level for the last 3 weeks. The dynamic uptake of methyl ethyl ketone and ethanol was monitored for over 3 months. Again, equilibrium readings (f0.1mg) were obtained for several weeks. The equilibrium penetrant uptake is less than 0.020 g of MEK/g of resin and less than 0.025 g of ethanol/g of resin. The low
Table 11. Apparent Water Diffusion Coefficient at 25 “C, Equation 1
DDS in resin, w t 70 sample 25 1 2 30 1 2 35 1 2 DDS in resin, wt 70 sample 25 1 2 30 1 2 35 1 2
as-cast
D, cmz/s 1.16 X 9.68 X 1.00 x 10-9 9.91 X 8.48 X 9.75 X D, cmz/s 1.05 X lo4 9.02 X 9.51 X 9.46 X 8.32 X 9.05 X
SSE 0.0154 0.0237 0.0202 0.0138 0.0101 0.0161 post-cured
SSE 0.0263 0.0800 0.0260 0.0357 0.0809 0.0220
data pta fit 33 35 36 37 38 37 data pts 35 36 35 36 36 35
observed in glassy polymer films, notably polystyrene. An identical result was also derived by Joshi and Astarita (1979)by introducing the degree of swelling as an internal state variable for the initially glassy polymer. This model assumes that while the penetrant fugacity is constant at the interface, the penetrant concentration need not be. Instead, the concentration can be function of the fugacity and an activity parameter that is a function of time: -Mt- -
M,
dR(1 - e - 9 (2) The parameters dF and dR are the relative fractions of Fickian and relaxational behavior and k is a relaxation constant. This heuristic model incorporates both Fickian diffusion and a first-order relaxation process in a directly additive fashion. The relaxation term can be extended to consider a spectrum of relaxation times. Since the propagated error in the fractional uptake increases as equilibrium is approached, the data analysis was based on minimizing the sum-squared error for the first 80% of the data by using nonlinear regression. The results are given with the calculated sum-squared error and the number of data points used in the analysis. Table I1 contains the regression results for Fick’s law, while representative results for the three-parameter model are given in Table 111. The estimated uptake curves for these two models are shown in Figure 2 with the experimental data obtained at Table 111. Eatimated Model Parameters at 25 O C from Eauation 1
__ ____
as-cast
DDS in resin, wt 7% 25 30 35
DDS in resin, wt 25
%
sample 1 2 1 2 1 2
D, cm2/s 1-07x 10-9 9.03 X 1.11 x 10-9 1.07 X 1.63x 10-9 1.00 x 10-9
sample 1 2
D, cmz/s 2.12 x 10-8 1.21 x 10-8 1.11 x 104 1.63 X 2.92 X 2.267 X loT8
30
1
35
2 1 2
@F
SSE
0.817 0.901 0.899 0.831 0.542 0.823
0.0119 0.0172 0.0151 0.00923 0.0110 0.00993
k, s-l 8.47 x 10” 8.57 X 10“ 2.07 X 10” 3.24 X 10” 1.97 X 10” 4.04 X 10“ Dost-cured @F k, s-l 0.107 5.49 x 10” 0.0735 3.88 X 10” 0.642 5.49 x 10” 0.478 2.99 X 10” 0.122 3.15 X 10” 0.110 3.86 X 10“
SSE 0.00524 0.00825 0.00524 0.00811 0.0742 0.00679
analyzed data pts 35 36 35 36 36 35 analyzed data pts 35 36 35 36 36 35
Ind. Eng. Chem. Res., Vol. 30, No. 1, 1991 215 n
h
d m 2.0 Q)
k
M 0 0
I
M 0 0
1.5
rl
rl
\
\
M
t
15.0
W
-!
sa 3
l.O 5.0
0.5 0
0 0
0 . o hd
I
I
I
0.0
I
0.0 10.0 20.0 30.0 40.0 50.0 6 0 . 0 (hrl/./mm)
tl/./L
Figure 6. Acetone uptake in post-cured samples of TGDDM-DDS epoxy resin at 25 "C, prepared with 30 wt % DDS as a function of the square root of the sorption time normalized with respect to the sample's thickness, t1/2/1.Circles: 1 = 0.55 mm. Squares: 1 = 0.55 mm. n
d 0 L
15.0
10.0
*,
0 0
10.0
k
p,
0
0
6.0
d
2
4 0
0.0 0
20
40
60
80
100
120
t-// (hrl-/mm) Figure 7. Methanol uptake in as-cast samples of TGDDM-DDS epoxy resin at 25 O C , prepared with 30 wt 70DDS as a function of
the square root of the sorption time normalized with respect to the sample's thickness, t1/2/1.Circles: 1 = 0.38 mm. Squares: 1 = 0.45 mm.
level of penetrant uptake results in a substantially larger percentage of error in the sorption data. The sorption behavior of methanol in the TGDDMDDS epoxy resin was significantlydifferent. Figures 7 and 8 illustrate the excellent reproducibility and the anomalous behavior observed for these samples. After 3 months of exposure to the methanol, the equilibrium sorption capacity of the resin samples ranged from 0.16 to 0.18 g of methanol/g of resin. The equilibrium sorption level increases with increasing initial DDS content. Initially, the sorption process proceeded very slowly. Then, it passed beyond this lag phase and began to exhibit a very rapid increase in the methanol uptake rate. The onset of the time when this dramatic transition occurred also increased
0
20
40
60
80
100
120
ti/.// (hr'/./mm) Figure 8. Methanol uptake in pt-cured samples of TGDDM-DDS
epoxy resin at 25 OC, prepared with 35 wt % DDS as a function of the square root of the sorption time normalized with respect to the sample's thickness, t1/2/1. Circles: 1 = 0.51 mm. Squares: 1 = 0.52 mm.
as the amount of DDS increases. Furthermore, while the equilibrium uptake was the same for the as-cast and post-cured samples, the inception of rapid penetrant uptake occurred more quickly in the post-cured samples. Unlike the samples exposed to other organic penetrants, the volume of methanol-exposed samples increased by 25-35% during the sorption process. The high resin uptake could be attributed to several factors. As the methanol diffused into the network, its hydroxy groups could interact with the resin's free hydroxy groups through hydrogen bonding. This would promote the disruption of intramolecular hydrogen bonding in the resin matrix itself and could allow the resin's network to expand. When the methanol-exposed samples were dried under vacuum, they regained their original dimensions; this implied that there were no irreversible relaxation processes occurring. More significantly,they weighed approximately 1 wt % less than the original samples; this meant that, during the swelling process, some low molecular weight impurities that were initially trapped in the network diffused out. Characterization of Samples. Figure 9 shows the FTIR difference spectra for as-cast epoxy resins of each composition studied, and Figure 10 illustrates the effect of drying a specimen that had been previously exposed to methanol; the sample used was prepared with 25 w t % DDS and was post-cured. The peaks in these spectra were identified by comparing them with literature data for epoxy resins (Levy et al., 1979; Antoon et al., 1981; Netravali et ai., 1984, 1985). Finally, Figure 10 shows that the chemical structure of the epoxy is changed after being exposed to methanol. This can be seen in the spectral range from 1500 to 1400 em-'. Several groups have characteristic absorption bands in this region including secondary amines, the 6(NH) bending mode, and primary alcohols, and these groups may be involved with the observed spectral changes. Furthermore, the peak that was originally at 1280 cm-' in the water-exposed samples shifted to 1290 cm-' in the methanol-exposed samples. Upon drying, this peak shifted to lower frequencies (1270 cm-') and had a stronger absor-
216 Ind. Eng. Chem. Res., Vol. 30, No. 1, 1991 1.woo
..M
4 AW
A
1510
I
1.00
CI I
I
I
1600
1400
1
m
'.\
1
1
I
lo00
a00
e 1 1 0 0
Figure 9. FTIR spectra of as-cast epoxy resins exposed to water at 90 "C with the corresponding dry sample's spectra subtracted. Solid lines: 25 wt % DDS. Dashed line: 30 w t % DDS. Dotted-dashed line: 35 wt % DDS.
..om
I
I
bance. This indicated that the methanol interacted with the SOagroup in DDS. After drying the methanol-exposed samples, there was still a small band apparent at 1017 cm-*. This peak was due to an increased number of hydroxy groups present in the network.
Conclusions In this work, the dynamic and equilibrium water uptake behavior of TGDDM-DDS resins was examined in more detail. The compositions and curing conditions selected
I
I
I
i
were chosen to verify and expand upon previously observed phenomena (Tsou and Peppas, 1988). Anomalous transport was observed in resins that were exposed to water at 25 OC. This is indicative of relaxational phenomena that occur on time scales of the same magnitude as diffusion. The inflections observed in the water uptake curves at low sorption times for samples exposed to water at 25 "C disappeared as the sorption temperature increased. The equilibrium moisture uptake increased in post-cured samples as a function of composition over the range studied.
217
Ind. Eng. Chem. Res. 1991,30, 217-221
The uptake behavior of several low molecular weight penetrants was also examined to determine the effect of molecular size and the penetrant’s affinity for the resin network on the uptake behavior. While the equilibrium uptake of ethanol, methyl ethyl ketone, and acetone was less than 2.5 g of penetrant/100 g of resin, the resin samples absorbed a significant amount of methanol (approximately 17.0 g/100 g of resin). The dynamic uptake behavior observed for methanol-exposed samples was anomalous. The inflection observed in these data was significantly more dramatic than observed in the water uptake behavior. The desorption of methanol from methanol-exposed samples indicated that the resin’s volume collapses back to its original unswollen dimensions. Furthermore, the loss of mass signifies that impurities, such as low molecular weight oligomers, have been removed from the original network. The methanol uptake behavior could be correlated with the dynamic changes in the sample’s dimensions. This would indicate whether the rapid change in uptake behavior corresponds with the dramatic inception of volumetric expansion.
Acknowledgment This work was supported in part by a grant from the National Science Foundation (CBT-86-17719). Registry No. (TGDDM)(DDS) (copolymer), 63804-34-2;water, 7732-18-5;acetone, 67-64-1;ethanol, 64-17-5; methanol, 67-56-1; methyl ethyl ketone, 78-93-3.
Literature Cited Antoon, M. K.; Koenig, J. L.; Serafini, T. Fourier-Transform Infrared Study of the Reversible Interaction of Water and A Crosslinked Epoxy Matrix. J . Polym. Sci., Polym. Phys. Ed. 1981, 19, 1567-1575.
Berens, A. R.; Hopfenberg, H. B. Diffusion and Relaxation in Glassy Polymer Powders: 2. Separation of Diffusion and Relaxation Parameters. Polymer 1978,19,489-496. Berens, A. R.; Hopfenberg, H. B. Induction and Measurement of Glassy-State Relaxations by Vapor Sorption Techniques. J . Polym. Sci., Polym. Phys. 1979,17, 1757-1770. Crank, J. The Mathematics of. Diffusion: Oxford University Press: .. London, 1959. Joshi, S.; Astarita, G. Diffusion-Relaxation Coupling in Polymers which Show Two-Stage - Sorption Phenomena. Polymer 1979.20, . . 455-458. Levy, R. L.; Fanter, D. L.; Summers, C. J. Spectroscopic Evidence for Mechanochemical Effects of Moisture in Epoxy Resins. J. Appl. Polym. Sci. 1979,24,1643-1664. May, C. A. Introduction to Epoxy Resins. Epoxy Resins: Chemistry and Technology, 2nd ed.; May, C. A., Ed.; Dekker: New York, 1988. Mijovic, J.; Weinstein, S. A. Moisture Diffusion into a GraphiteEpoxy Composite. Polym. Commun. 1985,26,237-239. Mikols, W. J.; Seferis, J. C.; Apicella, A.; Nicolais, L. Evaluation of Structural Changes in Epoxy Systems by Moisture Sorption-Desorption and Dynamic Mechanical Studies. Polym. Comp. 1982, 3, 118-124. Moy, P.; Karasz, F. E. Epoxy-Water Interactions. Polym. Eng. Sci. 1980,20,315-319. Netravali, A. N.; Fornes, R. E.; Gilbert, R. D.; Memory, J. D. Investigations of Water and High Energy Radiation Interactions in an Epoxy. J . Appl. Polym. Sci. 1984,29,311-318. Netravali, A. N.; Fomes, R. E.; Gilbert, R. D.; Memory, J. D. Effects of Water Sorption at Different Temperatures on Permanent Changes in an Epoxy. J. Appl. Polym. Sci. 1985,30,1573-1578. Ritger, P. L.;Peppas, N. A. Transport of Penetrants in the Macromolecular structure of Coals, 4. Models for Analysis of Dynamic Penetrant Transport. Fuel 1987,66,815-826. Tsou, A.; Peppas, N. A. Transport Properties of Water in Epoxy Resins and Composites. Polym. Mater. Sci. Eng. Proc. 1988,58, 952-956. Wong, T. C.; Broutman, L. J. Water in Epoxy Resins Part 11. Diffusion Mechanism. Polym. Eng. Sci. 1985,25,529-534.
Received for review December 5, 1989 Revised manuscript received July 17,1990 Accepted August 1, 1990
Decomposition of Ozone on a Silver Catalyst Seiichiro Imamura* and Masaaki Ikebata Department of Chemistry, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606,Japan
Tomoyasu Ito Department of Chemistry, Faculty of Science, Tokyo Metropolitan University, Setagaya-ku, Tokyo 158, Japan
Takashi Ogita Matsui Kagaku Co. Ltd., Jodori-cho, Fushimi-ku, Kyoto 612,Japan
Decomposition of ozone was carried out on metal oxide catalysts. The activity of the metal oxide catalysts increased roughly in the order of the increase in their surface area and in the amount of surface oxygen on them. Conductance change of these metal oxides on an introduction of ozone suggested that negatively charged oxygen species were formed on their surface. The Ag catalyst showed the highest activity, and the reactivity of the oxygen species produced on the surface of Ag catalyst toward carbon monoxide was much higher than that of the oxygen species on Co, Ni, Fe, and Mn oxides. Formation of superoxide ions and their precursors on the Ag catalyst was suggested by ESR analysis and by an activity measurement of these oxygen species. The latter, probably oxygen ion (0-),seemed to be an active species for low-temperature oxidation of CO. Ozone, a powerful oxidizing agent, is useful for sterilization (Tsuruta, 1988),deodorization (Stevens and Brown, 198% and destruction of pollutants in wastewaters (Gould and Weber, 1976; Teramoto et al., 1981). However, as ozone itself is toxic to organisms, its release into the en-
vironment must be avoided, The most commonly used technique of ozone detoxification is adsorption and decomposition on activated charcoal (Emel’yanova and Atyaksheva, 1979). Although this method is simple and convenient, activated charcoal loses its activity after pro-
0888-5885/91/2630-0217$02.50/0 Q 1991 American Chemical Society
