Tensile Properties and Morphology of the

Dynamically Cured EPDM and PP/HDPE

Ternary Blends
CHANG SIK HA, Department of Polymer Science and Engineering,
Pusan National University, Pusan 609-735,Korea and SUNG CHUL
KIM, Department of Chemical Engineering, Korea Advanced
Institute of Science and Technology, Seoul 136-791,Korea

Synopsis
The tensile properties and morphology of the polyolefin ternary blends of ethylene-
propylene-diene terpolymer (EPDM), polypropylene and high density polyethylene were studied.
Blends were prepared in a laboratory internal mixer where EPDM was cured in the presence of
PP and HDPE under shear with dicumyl peroxide (DCP). For comparison, blends were also
prepared from EPDM which was dynamically cured alone and blended with PP and HDPE later
(cure-blend). The effect of DCP concentration, intensity of the shear mixing, and rubber/plastics
composition was studied. The tensile strength and modulus increased with increasing DCP
concentration in the blends of EPDM-rich compositions but decreased with increasing DCP
concentration in blends of PP-rich compositions. In the morphological analysis by scanning
electron microscopy (SEM), the small amount of EPDM acted as a compatibilizer to HDPE and
PP. It was also revealed that the dynamic curing process could reduce the domain size of the
crosslinked EPDM phase. When the EPDM forms the matrix, the phase separation effect
becomes dominant between the EPDM matrix and PP or HDPE domain due to the crosslinking
in the matrix.

INTRODUCTION
Recently the rheological properties and crystalline structure of the dynami-
cally cured ethylene-propylene-diene terpolymer (EPDM) and PP/HDPE
ternary blends was reported. The blends of crosslinked EPDM and PP/HDPE
were prepared in a laboratory internal mixer by the dynamic curing method
where EPDM was cured under shear with peroxide.
Several works have been published on the rheological and mechanical
properties of the dynamically cured EPDM and polyolefin blends,2-6 but
virtually no morphological studies have been reported as yet on the dynami-
cally cured blends.
The objective of this study is to clarify the effect of DCP concentration, the
intensity of the shear mixing, and the rubber/plastics composition on the
morphology and mechanical properties of the dynamically cured EPDM and
PP/HDPE ternary blends.
EXPERIMENTAL
The polymers used in this study are listed in Table I. The polypropylene
(PP), high density polyethylene (HDPE), and ethylene-propylene-diene
terpolymer (EPDM) with ethylidene-2-norbornene (ENB) as a termonomer

Journal of Applied Polymer Science, Vol. 37, 317-334 (1989)

0 1989 John Wiley & Sons,Inc. CCC 0021-8995/89/020317-18$O4.~
318 HA AND KIM

TABLE I
Materials

Materials Characteristics Source

PP MFI 6.0 PP4017

Mu 2.201 x lo5 (Korea Petrochem.)
Mll 2.829 X lo4
HDPE MFI 0.8 E308
Mlu 1.002 x lo5 (Korea Petrochem.)
Mll 1.310 X lo4
EPDM" ?inh (dL/g) 1.22 Roy. 521
IVb 15.2 (Uniroyal)
PE/PP (by mol W)' 52.0/48.0
DCP Granule type Mitsui Chem.

"0.5 g/dL xylene solution at 70C.

bby ICl titration method.
by I R analysis.

were all commercially available grades. The polymers were cured with various
amounts of dicumyl peroxide (DCP). Blends were prepared by two different
methods, blend-cure process and cure-blend process. The preparation of the
blends is described elsewhere.' The sample notation is listed and described in
Table 11.

TABLE I1
Sample Description

Composition (wt. W)
Sample Notation EPDM PP + HDPE PP/(PP + HDPE) HDPE/(PP + HDPE)
PP75PE25 0 100 75 25
PP50PE50 0 100 50 50
PP25PE75 0 100 25 75
RL25P75" 25 75 75 25
RL25S75 25 75 50 50
RL25E75 25 75 25 75
RB(C)25P75b 25 75 75 25
RB(C)25S75 25 75 50 50
RB(C)25E75 25 75 25 75
RL50P50 50 50 75 25
RWOS50 50 50 50 50
RL50E50 50 50 25 75
RB(C)50P50 50 50 75 25
RB(C)50S50 50 50 50 50
RB(C)50E50 50 50 25 75
RL75P25 75 25 75 25
RL75S25 75 25 50 50
RL75E25 75 25 25 75
RB(C)75P25 75 25 75 25
RB(C)75S25 75 25 50 50
RB(C)75E25 75 25 25 75

'P denotes PP-rich composition; S denotes same wt % of PP and HDPE compositions;

E denotes HDPE-rich composition in the ternary blends.
bRL means linear E P D M RB(C) means blend-cure (RB) and cure-blend (RC).
 EPDM AND PP / HDPE TERNARY BLENDS 319

TABLE 111
Tensile Properties of the EPDM/PP/HDPE Homopolymers and Linear Ternary Blends

Tensile strength Elongation Youngs modulus

Sample notation (kg/cm2 at break (9) (kg/cm2 1

PPlOO 285 780 3850

PElOO 243 890 3060
EPDMlOO 4 60 20
RL25P75 168 51.5 2750
RL25S75 140 73.7 2220
RL25E75 97 370.7 1360
RL50P50 57 159.7 1190
RL50S50 57 237.0 660
RIAOE50 38 237.7 560
RL75P25 20 121.3 70
RL75S25 19 130.9 75
RL75E25 6 168.9 30

The tensile tests were carried out using an Instron tensile tester (Model
1127) on the samples prepared according to ASTM D638-84. The crosshead
speed was 100 mm/min, and the gauge length was 20 mm.
Scanning electron micrographs (SEM) were obtained by JEOL JSM35-CF
SEM. Samples were cryogenically fractured in liquid nitrogen and metallized
by gold coating prior to the installation in the SEM chamber.

RESULTS AND DISCUSSION

Tensile Properties
Tensile properties of the linear ternary blends are given in Table I11 and
Figure 1. The tensile strength and modulus are reduced as the EPDM content
increases. The reduction in tensile properties should be expected as the result
of a rubbery nature of EPDM.
Table IV and Figure 2 show the effect of DCP concentration on the tensile
properties of the dynamically cured EPDM and PP/HDPE ternary blends. I t
is generally observed that in the PP-rich blend the tensile strength and
modulus slightly decrease with increasing DCP concentration due to the
peroxide-induced mechanochemical degradation of PP whereas, for the
EPDM-rich blends, the tensile strength and modulus increase with increasing
DCP concentration due to the chemical crosslinking of EPDM by DCP.
I t is also noted, as shown in Table IV and Figure 3, that the elongation a t
break in the dynamically cured PP-rich blends (for instance, RB25P75 blends)
decrease with increasing DCP content and that the values are much lower
when compared to those of any other blends. This behavior may also be
related to the mechanochemical degradation of PP.
It is also interesting to note that all the EPDM-rich (RB75 designated
compositions) blends show maximum elongation a t break a t intermediate
DCP concentration. This may be caused by a number of factors acting
independently and simultaneously, including the decreasing crystallinity of
PP and HDPE by blending with EPDM, the mechanochemical degradation of
320 HA AND KIM
PPlOO KL25P75 RL50P50 RL15P25 EPDfllOO

350

300

250

200

150

100

50

0
EPDNlOO RL75EZ5 RL50E5G RL25E75 PElOO

BLEND C O M P O S I T I O N
Fig. 1. Tensile strength with blend composition for the linear EPDM/PP/HDPE ternary
blends.

PP, and the chemical crosslinking of the EPDM, etc. The results obtained
may be a combination of all the factors mentioned above.
Table V shows that the tensile strength and modulus of the dynamically
cured EPDM and PP/HDPE ternary blends are not significantly affected by
the intensity of the shear mixing or preparative methods over all the composi-
tion ranges examined. It must be pointed out, however, that the elongations
a t break are generally greater when the blends are prepared at 60 rpm than
when prepared at 20 rpm for the blend-cure materials except for the RB25-
designated samples of 25 w t %5 of EPDM. The increase in elongation at break
with increasing shear intensity is likely to be ascribed to the morphological
changes in the blends, even though the accurate interpretation is at the
moment not easy.

Morphology
The morphology of the EPDM/HDPE and PP/HDPE binary blends as
well as the linear EPDM/PP/HDPE ternary blends and the dynamically
cured EPDM/PP/HDPE blends by scanning electron microscopy (SEM) is
analyzed.

EPDM/HDPE and PP/HDPE Binary Blends

For PP/HDPE binary blends, as shown in Figure 4, the PP and HDPE are
incompatible and their phases are grossly separated. The SEM micrographs
show that small HDPE particles of the order of 1-3 pm are dispersed in the
continuous matrix of PP for the blend of PP75PE25 composition, but large PP
 EPDM AND PP / HDPE TERNARY BLENDS 321

TABLE IV
Tensile Properties of the Dynamically Cured EPDM/PP/HDPE Ternary Blends;
Effect of DCP Concentration (Mixer Speed 60 rpm)

Sample DCP conc Tensile Elongation Young's

notation (PW strength (kg/cm') at break (%) modulus (kg/cm2)

RB25P75 0 168 51.5 2750

0.33 141 15.3 2660
0.67 139 14.7 2280
1.00 131 13.8 1590
1.33 134 8.4 2140
RB25S75 0 140 73.7 2220
0.33 125 52.1 1960
0.67 130 14.4 2110
1.00 129 13.8 2240
1.33 128 10.2 2590
RB25E75 0 97 370.7 1360
0.33 96 255.9 1380
0.67 100 274.8 1730
1.00 112 253.3 1980
1.33 95 279.2 2370
RB50P50 0 57 159.7 1190
0.33 52 75.5 820
0.67 63 115.7 750
1.00 75 233.7 870
1.33 83 224.7 820
RB50S50 0 57 237.0 660
0.33 72 240.0 660
0.67 58 258.1 660
1.00 66 243.3 840
1.33 91 315.2 600
RB50E50 0 38 237.7 560
0.33 50 424.4 570
0.67 60 380.0 570
1.00 78 365.5 870
1.33 70 317.7 460
RB75P25 0 20 121.3 70
0.33 30 177.1 100
0.67 47 265.2 150
1.00 59 169.9 140
1.33 66 169.9 155
RB75S25 0 19 130.9 75
0.33 31 188.2 85
0.67 32 304.4 155
1.00 73 180.7 170
1.33 78 184.9 190
RB75E25 0 6 168.9 30
0.33 19 289.1 65
0.67 30 250.2 105
1.00 68 184.0 110
1.33 63 167.8 70
322 HA AND KIM

quuu
200
t 1
150 5uuo

1uu 2uou

50 lUUU

U -0
t
0 0.33 0,67 1.00 1 . 3 3

UCr' CONCENTRATION, PHR

Fig. 2. The effect of DCP concentration on the tensile strength (A) and the modulus (3) for
the dynamically cured EPDM and PP/HDPE ternary blends.

350

300

250
v
x
6
2 21.0
n
I-
a
150
-
0

I-
5 lUU
z
0
J

50

n
0.00 0.33 0.67 1.00 1,33
DCP CONCENTRATION,PHR
Fig. 3. Elongations at break with DCP concentration for the dynamically cured EPDM and
PP/HDPE ternary blends.

domains of 6-10 pm in size are dispersed in the continuous HDPE matrix for
the PP25PE75 blend.
It is interesting to note that the blends show two-phase structure over the
composition ranges examined and the phase separation between two compo-
nents is more evident for the HDPE-rich blend than the PP-rich blend.
Careful inspection of the micrograph for the PP50PE50 blend reveals that
both HDPE and PP phases are interconnected. Lovinger and Williams: who
 EPDM AND PP / HDPE TERNARY BLENDS 323

TABLE V
Tensile Properties of the Dynamically Cured EPDM/PP/HDPE Blends;
Effect of Shear Intensity and Preparative Methods (DCP 0.67 phr)

Sample Mixer rotation Tensile Elongation Young's

notation (wm) strength (kg/cm2) at break (%) modulus (kg/cm2)

RB25P75 20 120 21.3 2420

60 139 14.7 2280
RC25W5 60 139 19.7 2140
RB25S75 20 135 21.7 1660
60 130 14.4 2110
RC25S75 60 132 18.8 2820
RB25E75 20 100 359.2 1665
60 100 274.8 1730
RC25E75 60 85 220.2 1890
RB50P50 20 64 68.6 820
60 63 115.7 750
RC50P50 60 63 133.1 410
RB50S50 20 65 151.6 840
60 58 258.1 660
RC50S50 60 58 258.1 290
RB50E50 20 66 229.5 380
60 60 380.0 570
RC50E50 60 55 376.6 280
RB75P25 20 57 194.7 135
60 47 265.2 150
RC75P25 60 47 265.2 150
RB75S25 20 31 211.8 150
60 32 304.4 155
RC75S25 60 17 224.4 140
RB75E25 20 21 330.3 140
60 30 250.2 105
RC75E25 60 30 250.2 105

studied the relationship of tensile properties and morphology of the blend,

concluded that the lower elongation at break of the blend might be attributed
to the two-phase structure consisting of interconnecting networks of domains
of HDPE in a PP matrix.
For the EPDM/HDPE binary blend, it is shown that the EPDM particles
of 0.4-0.6 pm in diameter are dispersed in HDPE matrix for PE75EPDM25
blend and HDPE particles of similar size are dispersed in EPDM matrix for
PE25EPDM75 blend (Fig. 5).
The SEM micrograph of the PE75EPDM25 blend with higher magnifica-
tion (Fig. 6) shows that the volume fraction of EPDM in the blend is
somewhat larger than that of the initially added amount of EPDM. This
result may be due to the semicompatibility of the EPDM and HDPE. Careful
inspection of Figure 6 reveals rough fractured surfaces, which indicate good
adhesion of EPDM and HDPE.
The different compatibilities or segmental solubilities of the EPDM with
PP or HDPE depend on the molecular parameters such as ethylene content,
molecular weight, termonomer type and content, stereoblock length of the
ethylene and/or propylene segments, etc.8-14
324 HA AND KIM

Fig. 4. SEM micrographs of PP/HDPE binary blends: (a) PP75PE25; (b) PP50PE50;
(c) PP25PE75.

In this system, the segmental mobilities of EPDM and HDPE seem to be

higher than those of EPDM and PP because of slightly higher ethylene units
in the EPDM chain.

EPDM/PP/HDPE Ternary Blends with 25%EPDM

SEM micrographs of the fractured surfaces of the linear blend of
EPDM/PP/HDPE with 25% EPDM (RL25-designated blend) are shown in
Figure 7. The addition of 25 w t %
' of EPDM to the binary HDPE/PP blends
drastically changes the overall morphology.'6,'6 The blends of RL25P75 show
that the EPDM and HDPE are combined to form composite particles dis-
persed in PP matrix [Fig. 7(a)]. Stehling et al.17 reported that EPDM inter-
layer between the HDPE domain and the PP matrix was the possible
 EPDM AND PP / HDPE TERNARY BLENDS 325

Fig. 5. SEM micrographs of EPDM/HDPE binary blends: (a) PE75EPDM25;

(b) PEWEPDMW; (c) PE25EPDM75.

Fig. 6. SEM micrographs of PE75EPDM25 blend; higher magnification ( X 15,000).

326 HA AND KIM

Fig. 7. SEM micrographs of EPDM/PP/HDPE ' EPDM:

linear ternary blends of 25 wt %
(a) RL25P75; (b) RL25S75; (c) RL25E75.

morphology from the interfacial energy considerations.", l9 The RL25E75

blend [Fig. 7(c)] shows very different morphology compared to those of
RL25P75 and RL25S75 blends. It. is apparent, as indicated by the arrows, that
the PP domain of 2-4 pm and EPDM domains of 0.4-0.6 pm in size are
distributed separately in the HDPE matrix. Comparison of Figures 4(c) and
7(c) reveals that the domain size of PP phase ranges from 6 to 10 pm in
PP25PE75 binary blend and from 2 to 4 pm in RL25E75 ternary blend, which
indicates the compatibilizing effect of the EPDM phase.
The dynamically cured PP-rich blends (RB25P75) also show the composite
particles of EPDM and HDPE dispersed in the PP matrix (Fig. 8). The
composite particles are 3-7 pm in diameter. It is noteworthy that the shape of
HDPE particles encapped in EPDM is irregularly deformed, and the sur-
rounding EPDM layer is also irregular in shape. The magnified composite
 EPDM AND PP / HDPE TERNARY BLENDS 327

Fig. 8. SEM micrographs of the dynamically cured RB25W5 ternary blends with DCP
concentration of 0.67 phr, the effect of the shear intensity. Mixer speed: (a) 20 rpm ( X 2600);
(b) 20 rpm ( X 15,000); (c) 60 rprn ( x 2600); (d) 60 rpm ( X 15,000).

particle micrograph [Fig. qb)] shows intermixed HDPE and EPDM phases
within the composite particles. The effect of the shear intensity during the
blend-cure process is shown by comparing Figures 8(a) and 8(c). Under high
shear, the composite particle size becomes smaller, and the surrounding
EPDM layer becomes more continuous. This feature may be due to the poorer
compatibility of crosslinked EPDM with HDPE and PP when prepared a t
lower shear intensity of mixing.
The dynamically cured HDPE-rich blends (RB25E75) show that the PP
and EPDM domains are distributed separately in the HDPE matrix, which is
similar to the RL25E75 linear ternary blend. [Compare Figs. 7(c) and 9(b).]
Figure 9 shows the effect of the shear intensity on the morphology of the
HDPE-rich ternary blends. It is clearly shown that very fine dispersions of PP
domain are observed for the RB25E75 blend prepared at 60 rpm [Fig. 9(b)]
whereas the distinct segregated PP domains of 2-3 pm in size are observed in
HDPE matrix for RB25E75 prepared at 20 rpm [Fig. 9(a)].
The effect of the DCP concentration for the RB25P75 and RB25E75
compositions is shown in Figures 10 and 11. For the RB25P75 blends, when
DCP content is 1.0 phr, the EPDM domain surrounding the HDPE particle is
3-5 pm in diameter and is dispersed in PP matrix, whereas at DCP content of
0.33 phr the EPDM domain is much larger (about 10 pm in diameter) (Fig.
10). This may be attributed to the fact that the rubber particle size is reduced
with crosslinking at high DCP content. Dao20.21reported that the rubber
particle growth is inhibited and stabilized at high crosslinking density.
328 HA AND KIM

Fig. 9. SEM micrographs of the dynamically cured RB25E75 ternary blends with DCP
concentration of 0.67 phr; the effect of the shear intensity. Mixer speed: (a) 20 rpm ( X2600);
(b) 60 rpm ( x 2600).

Fig. 10. SEM micrographs of the dynamically cured RB25P75 ternary blends prepared at 60
rpm; the effect of DCP concentration: (a) 0.33 phr ( X 2600); (b) 0.33 phr ( X 15,000); (c) 1.00 phr
( X 2600); (d) 1.00 phr ( X 15,000).
 EPDM AND PP / HDPE TERNARY BLENDS 329

Fig. 11. SEM micrographs of the dynamically cured RB25E75 ternary blends prepared at 60
rpm; the effect of DCP concentration: (a) 0.33 phr (X2600); (b) 0.33 phr (X 15,000); (c) 1.00 phr
( x 2600); (d) 1.00 phr (X 15,000).

The stabilizing effect by crosslinking is more evidently observed for HDPE-

rich RB25E75 blends, as shown in Figure 11. At lower DCP content the
EPDM domains are interconnected, but at higher DCP content the EPDM
particle is spherical in shape. I t is apparent that the domain of EPDM might
be stabilized a t 2-4 pm in size during the dynamic curing process when DCP
content is higher.

EPDM/PP/HDPE Ternary Blends of 50 and 75 wt % EPDM

The morphology of the ternary linear blends of 50 w t % EPDM (RL50-des-
ignated samples) is shown in Figure 12. For these samples, the irregularly
shaped HDPE and PP domains are dispersed in EPDM matrix.
The morphology of the RL50 designated blends is somewhat different from
that of the RL25 designated blends and the phase-separated structure is more
evident in RL50 designated blends. Comparison of Figures 7 and 12 reveals
that the size of the dispersed domain of PP and HDPE is larger in the RL50
designated samples.
Figure 13 shows the comparison between the linear ternary blend RL50P50
and the dynamically cured RB50P50 under two different shear intensities.
The linear blend has the finer dispersion while the dynamically cured blend
under low shear (20 rpm) shows distinct and large domains of PP and HDPE.
Figure 14 shows the effect of DCP concentration on the morphology of the
RB50 designated samples. I t is seen that the phase separation is more clearly
330 HA AND KIM

Fig. 12. SEM micrographs of EPDM/PP/HDPE linear ternary blends of 50 w t B EPDM:

(a)RL50P50; (b) RL50S50; (c) RL50E50.

observed at higher DCP concentration due to their poor compatibility be-

tween the components for both RB50P50 and RB50E50 blends.
The effect of DCP concentration on the morphology of RB50 designated
samples is different from the RB25 designated samples. When EPDM is
dispersed in PP or HDPE matrix, as in RB25 designated samples, the
stabilizing effect by crosslinking is dominant, but when EPDM forms the
matrix phase, as in the RB50 designated samples, the phase separation effect
becomes dominant between the EPDM matrix and PP or HDPE domain due
to the crosslinking in the matrix phase.
The SEM micrographs of the EPDM-rich linear blends (RL75 designated
samples) are shown in Figure 15. Small particles of HDPE and PP domains
are dispersed in EPDM matrix. However, in the dynamically cured blends
of RB75P25 and RB75E25, the identification of PP and HDPE domains
 EPDM AND PP / HDPE TERNARY BLENDS 331

Fig. 13. SEM micrographs of EPDM/PP/HDPE ternary blends of 50 wt % ' EPDM;

(a) RL50P50; (b) RB50PW (mixer speed 20 rpm; DCP 0.67 phr); (c) RB50P50 (mixer speed 60
rpm; DCP 0.67 phr).

is difficult, and the blends look homogeneous even at high magnification

(Fig. 16). The cracks shown in the micrographs are due to the volume
contraction in the EPDM matrix during quenching in liquid nitrogen.

SUMMARY
This study reveals the influences of the blend composition, peroxide con-
tent, and intensity of the shear mixing on the tensile properties and the
morphology of the dynamically cured EPDM and PP/HDPE ternary blends.
It was found that the tensile strength and the modulus decrease with
increasing DCP concentration in the PP-rich blends of 25 wt 5% EPDM. The
decrease in the tensile properties for the PP-rich blends was ascribed to the
332 HA AND KIM

Fig. 14. SEM micrographs of the dynamically cured RB50P50 and RB50E50 ternary blends
prepared at 60 rpm; the effect of DCP concentration: (a) RB50P50, DCP 0.33 phq (b) RB50P50,
DCP 1.00 phq (c) RB50E50, DCP 0.33 phq (d) RB50E50, DCP 1.00 phr.

mechanochemical degradation of PP by shear and peroxide during the dy-

namic curing process.
It was also revealed from the morphological analysis by scanning electron
microscopy (SEM) that dynamic curing process could reduce and stabilize the
domain size of the crosslinked EPDM when the composition of EPDM is
25 w t %. The effect is prominent when DCP concentration is higher or the
intensity of the shear mixing is higher. The phase separation effect becomes
dominant between the EPDM matrix and PP/HDPE domain due to the
crosslinking in the matrix when the EPDM forms the matrix in the blends of
the EPDM composition of more than 50 wt S.
Even though it would be required to deduce the accurate mechanism for the
tensile properties as a function of the morphology, we did not attempt such an
undertaking in the present study but summarized some of the important
factors to be related to the mechanism. Those factors affecting independently
and simultaneously the tensile properties and the morphology of the blends
studied include the decreasing crystallinity of PP or HDPE with blending
with EPDM, the mechanochemical degradation of PP by peroxide and shear,
the chemical crosslinking of EPDM by peroxide, and even the morphological
changes by the various process conditions of the dynamic curing themselves,
etc.
Further studies will be made in more detail and reported elsewhere.

The authors wish to thank Mr. Nam Gyun Yun for his help in SEM experiments.
 EPDM AND PP / HDPE TERNARY BLENDS 333

Fig. 15. SEM micrographs of EPDM/PP/HDPE linear ternary blends of 75 w t % EPDM:

(a) RL75P25; (b) RL75S25; (c) RL75E25.

Fig. 16. SEM micrographs of the dynamically cured RB75P25 and RB75E25 ternary blends
prepared at 60 rpm with DCP concentration of 0.67 p k (a) RB75P25; @) RB75E25.
334 HA AND KIM

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Received November 24,1987

Accepted December 15, 1987