morphology of nylon-6 blends with styrenic polymers

Morphology of Nylon-6 Blends with Styrenic Polymers

Y. TAKEDA* and D. R. PAUL

Department of Chemical Engineering and Center for Polymer Research, The University of Texas at Austin, Austin, Texas 78712

SY NOPSlS

The morphology of blends of styrenic polymers in a matrix of 75% Nylon-6 prepared in a Brabender Plasti-Corder was examined by scanning electron microscopy. Styrene/acrylonitrile copolymers (SAN) form smaller particles as the AN level increases owing to the corresponding decrease in the SAN-polyamide interfacial tension. Various styrenic polymers containing functional groups, maleic anhydride or oxazoline type, that can react with Nylon- 6 during melt processing were added to the SAN phase which also led to a decrease in the particle size owing to the graft copolymer formed in situ. The effects of functional group type, amount of functional groups per chain, amount of functional polymer added, and the miscibility of the styrene/maleic anhydride (SMA) and SAN copolymers on the morphology of the styrenic phase in the Nylon-6 matrix are described. 0 1992 John Wiley & Sons, Inc. Keywords: Nylon-6 blends with styrenic polymers, morphology of blends of Nylon-6 with styrenic polymers, morphology of morphology of immiscible blends of styrenic polymers in Nylon-6 matrix

INTRODUCTION

The performance of multiphase blends is controlled in large measure by their phase morphology and the nature of the interface between the phases.'-5 Be- cause of how morphology is generated, factors that affect the interface also influence phase morphol- ~ g y . ' - ~ This report explores some of these issues and was motivated by the current interest in compatibilized blends of Nylon-6 and ABS The study is restricted to the particle size and shape of the styrenic phase in Nylon-6-rich blends. We first examine how the particle size is affected by the interfacial tension between Nylon-6 and styrene /acrylonitrile copolymers ( SAN ) . Next, we explore the effect of adding to the SAN phase another styrenic copolymer having chemical functionality that reacts with Nylon-6 during melt processing. The primary focus is on styrene /maleic anhydride copolymers (SMA) ; however, a styrenic polymer containing a small amount of oxazoline functionality ( RPS) has

* Permanent address: Mitsubishi Gas Chemical Co., 22 Wadai, Tsukuba-shi, 300-42, Japan Journal of Polymer Science: Part B: Polymer Physics, Vol. 30,1273-1284 (1992) 0 1992 John Wiley & Sons, Inc. CCC 0sS7-6266/92/01101273-12504.00

also been used. The effects of level of functionality (%MA), interfacial tension (%AN), and the miscibility of the two styrenic materials are considered (i.e., the relationship between %MA and %AN).

EXPERIMENTAL

Table I summarizes pertinent information about the various polymers used in this work. Blends were prepared from them by melt mixing in a Brabender Plasti-Corder for ten minutes at 240°C using a rotor speed of 60 rpm. The content of Nylon-6 was held fixed at 75% by weight to limit the number of variables in the examination of morphology but to en- sure that the polyamide formed the continuous phase. When the blend contained two styrenic polymers, these components were premixed in the Bra- bender for 10 min at 24OoC using a rotor speed of 60 rpm and mechanically granulated prior to the final mixing with Nylon-6. Torque readings were recorded after 10 minutes of mixing. The molten mixtures were removed from the Brabender mixing bowl and allowed to solidify.

Blend samples were microtomed at room temperature using a glass knife to create a flat surface.

1273

1274 TAKEDA AND PAUL

Table I. Polymers Used in This Study

MW Melt Polymer Designation Information Viscosity" Source

Poly(t-caprolactam)b Polystyrene Poly (styrene-co-acrylonitrile)

6.3% AN

14.7% AN

25% AN

34% AN

40% AN

58% AN 68% AN

2% MA Poly(styrene-co-maleic anhydride)

6% MA 13% MA

1% ox Poly(styrene-co-vinyl oxazoline)

Nylon-6 PS

SANG

SAN15

SAN25

SAN34

SAN40

SAN58 SAN68

SMA2

SMA6 SMA13

RPS

Mn = 25,000 Mu = 350,000

Mw = 343,000

Mw = 182,000 Mn = 83,000 Mu = 152,000 Mn = 77,000 Mu = 145,000 M,, = 73,000

M,, = 61,000 nd' nd

Mu = 346,000 M,, = 140,000

TJ = 3.92 cPd nd

Mn = 121,000

Mu = 122,000

Mu = 200,000

1.00 0.84

0.71

0.81

1.00

1.12

1.32

1.37 1.40

1.42

1.31 1.16

0.47

Allied Signal, Inc. American Petrofina Chemical Co.

Dow Chemical Co.

Asahi Chemical Co.

Dow Chemical Co.

Asahi Chemical Co.

Asahi Chemical Co.

Monsanto Co. Monsanto Co.

Arco Chemical Co.

Dow Chemical Co. Arco Chemical Co.

Dow Chemical Co.

* Relative melt viscosity, Brabender torque at 240°C and 60 rpm after 10 min divided by that of Nylon-6.

' nd = not determined. End groups, 40 pEq/g NH2 and 40 pEq/g COOH.

Viscosity of 10 wt '36 solution in methyl ethyl ketone at 25°C.

Cryofracture was also used to prepare surfaces for some samples with very small particles to avoid de- formation by the glass knife. The sample surfaces were exposed for 3 minutes to methylene chloride in an ultrasonic bath at room temperature to dissolve away the dispersed styrenic phase except in the case of SAN58 and SAN68 where N,N-dimethyl form- amide was used. The etched samples were washed with methylene chloride, dried in a vacuum oven for several hours at room temperature, and then the surface was gold-palladium coated with a Pelco sputter coater prior to viewing with a JEOL 35C scanning electron microscope operated at 25 kV. An attached feature analyzer was used to determine the apparent diameter (i.e., the diameter of a circle having equal area) for each particle in the field of view (ca. 150-250 particles). The weight-average particle diameter, d,, was computed from this information. No attempt was made to correct for the fact that the microtome does not cut each particle at its equator.

MORPHOLOGY OF BLENDS CONTAINING NONREACTIVE SAN COPOLYMER

Blends of Nylon-6 containing 25% by weight of each of the SAN copolymers listed in Table I were prepared in the Brabender as described above. Figure 1 shows representative $EM photographs. In every case, the particles appear to be nearly circular in shape; but their average size depends significantly on the acrylonitrile content of the copolymer as shown in Figure 2. The trend of smaller size with increased AN content is intuitively consistent with the accompanying increase in polarity; however, this fact can be analyzed more quantitatively as shown next.

Morphology generation during mixing of miscible polymer components involves a balance between the competing processes of fluid drop break-up and co- alescence. The final product has the morphology that is captured when the mixture solidifies. Drop break- up can be envisioned, at least qualitatively, in terms

MORPHOLOGY OF NYLON-6 BLENDS 1275

75/25 Nylon 6/PS

75/25 Nylon 6lSAN25

- 10 pm

- 10 pm

75/25 - Nylon 6/SAN68 10 pm

Figure 1. SAN blends (microtomed) .

SEM photomicrographs of 75/25 Nylon-61

of a model for Newtonian fluids that dates to the pioneering work of Taylor.' A stress field will cause a drop of radius R to break-up when the ratio of the surface force ( - y / R , where y = interfacial tension)

n

E 3. Y

75/25 Nylon 6/SAN

Brabender 240°C/1 0 min/60 rpm

3 -

2 -

1 -

0 " " " I ' " ' I ' " "

0 1 0 2 0 30 4 0 50 6 0 70 8 0

wt% AN

Figure 2. Nylon-6/SAN blends versus AN content of SAN.

Weight-average particle diameter of 75 /25

to the shearing force ( - qmG, where qm = matrix viscosity and G = local shear rate) is less than some critical ratio. The latter is a function of viscosity of the dispersed phase, v d , relative to that of the matrix phase, v m . Without explicitly considering the co- alescence process, successful correlations of q,GR / y versus q d / q , have been developed for specific pro-

- E 0 Q, C >

. z. C 0 fn c Q I-

.-

Nylon 6/SAN

8 0 100 0 ' . " I " ' " '

0 2 0 40 60

wt?" AN

Figure 3. Calculated interfacial tension a t 24OoC between Nylon-6 and SAN as a function of AN content in SAN.


h

E

3 I0

% Y

4 75/25 Nylon WSAN

24O0C/1O min/bO rpm

3 I Brabender / I 2 -

1 -

0 2 4 6 8

Interfacial Tens ion (d y ne/cm)

Figure 4. Weight-average diameter of SAN particles in 75/25 Nylon-6/SAN blends as a function of interfacial tension.

cessing condition^.^^-^^ For the fixed Nylon-6 matrix and the Brabender mixing process used here, the only variables are R , y, and ‘qd. The Brabender torque for each component can be used as a relative measure of viscosity. As shown in Table I, the torque for the SAN phase varies some relative to that of Nylon-6; however, based on the experiments of

Miscibility of SANlSMA blends

z tn C

a

0 1 0 20

wt%MA in SMA

SMA2 SMA6 SMAl3

Figure 5. indicate SMA/SAN mixtures used in this study.

Miscibility of SMA/SAN mixtures. Circles

5000

4000

1 Nylon G/Styrenic Copolymer

SMA , 3 Brabender 24OoC/10 min/60 rpm

0 2 0 4 0 6 0 80 1 0 0

Nylon 6 Styrenic Copolymer

Figure 6. Brabender torque of various Nylon-6/styrenic copolymer blends after fluxing for 10 min at 240°C and 60 rpm.

wt%

Wu,” torque ratios in this narrow range about unity should not affect particle size significantly relative to errors in measurement. Within this approximation, we expect the particle size to be directly pro- portional to the interfacial tension.

3000

200a

? E v

0 3 P 0 1ooc F

C

75wl% Nylon 6 Brabender 240°C/1 0 min/6O rprn

2 0 40 6 0 80 100 Fs

wt% W S SMA2

Figure 7. Brabender torque of 75% Nylon-6 blends with miscible mixtures of polystyrene and reactive styrenic copolymers as a function of composition of the styrenic phase after 10 min at 240°C and 60 rpm.


7516.2511 8.75 Nylon 6lRPSlPS

7511 2.511 2.5 Nylon 6IRPSlPS

- 7511 8.7516.25 - 75/25 Nylon 6IRPSIPS 5 Pm Nylon 6/RPS 5 Pm

Figure 8. SEM photomicrographs of 75% Nylon-6 blends with 25% of RPS/PS mixtures (microtomed) .

The interfacial tension between Nylon-6 and the various SAN copolymers was estimated using the data and techniques described by Wu16 and Hobbs et al.17 We have assumed that the surface tension characteristics of Nylon-6 are the same as Nylon- 6,6.16 Interfacial tension characteristics of SAN copolymers were calculated by an additivity model using data for polystyrene and polyacrylonitrile.16 The interfacial tensions for Nylon-6-SAN pairs estimated from the harmonic mean e q ~ a t i o n ' ~ , ' ~ are shown in Figure 3 as a function of the AN content of the SAN copolymer.

Interestingly, there appears to be a minimum in the interfacial tension at about 70%AN. Figure 4 shows a plot of the experimentally determined SAN particle size versus the calculated interfacial tension. The proportionality suggested earlier is realized to a remarkably good approximation.

MORPHOLOGY CONTROL USING REACTIVE COPOLYMERS

When physical factors fail to give the desired blend morphology, in situ chemical reaction of functionalized components may offer an attractive route to additional control reactive functional groups as in the present case of the series of SAN materials already discussed, but they can be introduced by mixing into this phase materials with functional groups. Here, we consider a reactive polystyrene containing approximately 1% oxazoline units (RPS) that pre- sumably would react with the carboxyl chain ends of the polyamide 6~18,24 and styrene copolymers with maleic anhydride units (SMA) whose predominate reaction is with the amine end groups of the polyamide.5,18-24


7516.2511 8.75 Nylon 6lSMA2lPS

- 7511 2.511 2.5 5 Pm Nylon 6/SMA2/PS

75125 75125 - Nylon 6lSMA2 2 Pm Nylon 6lSMA6

Figure 9. tures (microtomed) , SMAZ, and SMA6 (cryofractured) .

SEM photomicrographs of 75% Nylon-6 blends with 25% of SMAZ/PS mix-

Graft copolymer formed a t the interface affects morphology by lowering the interfacial tension 25,26

and by providing stabilization against c o a l e ~ c e n c e . ~ ~ ~ ~ ~ In the solid state, the chemical bonds undoubtedly strengthen the usually weak Important variables that must be considered include: the inherent reactivity of the functional groups, the amount of functional polymer added, the order in which the functionalized polymer is added to the blend, the number of functional groups per chain (i.e., the molecular architecture of the graft chain formed), etc. I t has been implicitly assumed that the functionalized molecule (e.g., RPS or SMA) should be miscible with the nonreactive phase (i.e., the SAN materials in the current case); however, this is an issue that needs to be explicitly addressed by experiment. In the current case, we expect RPS and SMAZ to be miscible with polystyrene, PS.32-34

In general, SMA copolymers are miscible with SAN copolymers when their MA and AN contents do not differ by more than known limits32 as shown in Figure 5.

Rheological factors also play some role in morphology generation in reactive sy~ te rns .~* '~ .*~ In addition, rheological measurements provide a useful tool for monitoring the reaction As an ex- ample of the latter, Figure 6 shows the Brabender torque measured for blends of Nylon-6 with various reactive and nonreactive styrenic polymers. For polystyrene, the torque relation is nearly an additive function of composition; although careful examination reveals a slight minimum. The relation for the oxazoline-containing polystyrene, RPS, is com- pletely linear suggesting a low extent of reaction. On the other hand, when the various SMA copolymers are mixed with Nylon-6, a strong maximum is


75 wt% Nylon 6 Brabender 24O"CilO rnin/60 rpm

n

E

I'c1

2 Y

3

U 0 20 4 0 60 8 0 100

Ps RPS wt% SMA2

Figure 10. Weight-average particle diameters for 75% Nylon-6 blends with 25% of RPS/PS mixtures and SMA2/PS mixtures as a function of composition of the styrenic phase.

observed whose peak shifts to lower SMA content as the MA content of the copolymer increases. In- terestingly, the peak seems to occur within a relatively narrow range of the ratio of maleic anhydride units per amine end groups ( MA/NH2 N 2.7-3.8). Figure 7 shows Brabender torque ratios for blends containing 75% Nylon-6 and 25% of a phase based on polystyrene to which varying amounts of either SMA2 or RPS was added. For the RPS case, the torque remains constant regardless of RPS content; whereas, there is a strong increase as SMA2 is added. While the content of functional groups in SMA2 is about two times that of an equivalent mass of RPS, this does not seem enough to explain the large dif- ference in responses. Evidently the oxazoline units are less reactive with the polyamide than maleic anhydride units.

Figures 8 and 9 show how the morphology of 75 Nylon-6/25 styrenic polymer blends changes as a functionalized polymer miscible with polystyrene is added. Average particle diameters obtained from analysis of the SEM photomicrographs are shown in Figure 10. For unfunctionalized polystyrene, the average particle diameter is 3.5 pm. Pure RPS yields smaller particles with an average diameter of 0.9 pm despite the rheological evidence in Figures 6 and 7 that suggests the extent of reaction with Nylon-6 is very limited. Mixtures of PS and RPS show a

monotonic change in particle size between these limits. Blends of pure SMA2 or SMA6 with Nylon- 6 show no evidence of a dispersed styrenic phase by this SEM technique as may be seen in Figure 9. A few holes are seen on the surface of Nylon-6/ SMAB blends; however, their extent is far less than expected from the amount of SMA2 present. Either the SMA copolymers are not extracted by this pro- cedure or their particles are too small to be seen by SEM (< -0.1 pm). The results shown in Figure 9 indicate that the particle size drops rapidly as SMA2 is added to PS and when the styrenic phase is more than half SMA2, they become too small for viewing by SEM. Further investigation by transmission electron microscopy would be of interest. The greater reactivity of SMAS with Nylon-6 compared with RPS (see Fig. 6 ) is dramatically reflected in the morphology of these blends (Fig. 10). The behavior of mixtures of PS and SMAB is quite analogous to results reported recently35 for mixtures of functionalized and nonreactive block copolymer blended with Nylon-6.

We examine now the issue of miscibility of the functional SMA copolymer with the nonreactive styrenic phase. Figure 11 shows the average particle diameter of a SAN phase in blends containing 75% Nylon-6 as a function of the amount of SMAS added and the AN content of the SAN copolymer. On the

Nylon 6/SAN/SMA2 blends 75 wt% Nylon 6

', wt% SMA2

3 I'CJ

" 0 20 40 6 0 8 0

Wt% AN

Figure 11. Weight-average particle diameters of 75% Nylon-6 blends with 25% of SMA2/SAN mixtures as a function of AN content of SAN. Dotted line is drawn from Figure 2.


75/25 Nylon 6/PS

75/2.5/22.5 Nylon 6/SMA2/PS

75/2.5/22.5 - 75/2.5/22.5 - Nylon 6/SMA6/PS 5 Pm Nylon 6/SMA13/PS 10 pm

Figure 12. crotomed) as a function of MA content of SMA.

SEM photomicrographs of 75 /2 .5 /22 .5 Nylon-G/SMA/PS mixtures (mi-

basis of previous we expect SANG to be very near but just outside the limits of miscibility with SMA2, and certainly all higher AN copolymers are not miscible with SMAB (see Fig. 5). As shown earlier, addition of SMA2 to PS reduces the particle size; however, a t a fixed SMA2 content, the particles become larger with increasing AN content. This trend may reflect, a t least in part, the lack of com- plete miscibility of SMA2 with the dispersed phase. It is possible that some of the SMAB migrates from this phase and disperses separately in the Nylon- 6,17s20s37s38 thereby raising its viscosity. Even if all the SMA2 does so, the total amount is relatively small (20% SMA2 in the SAN phase amounts to 6% in polyamide phase). The change in viscosity expected on the basis of Figure 6 would not seem enough to explain on purely rheological grounds the particle size increase observed. The limited attach-

ment to the SAN phase of the SMAB chains that graft to Nylon-6 may be a significant factor. At higher AN levels, the particle size decreases again for fixed SMAB content. This is caused by the decrease in Nylon-6-SAN interfacial tension as AN level increases since the curves for 10 and 20% SMA2 appear to asymptotically approach the dotted line taken from Figure 2 for 0% SMA2.

An alternate way to examine the issue of miscibility is to fix the AN content of the SAN copolymer and vary MA content of the SMA copolymer. Figure 5 shows the pattern of experiments in relation to the experimental miscibility window for these copolymers. Figures 12 to 14 show selected photomicrographs for 0, 6, and 15% AN in the SAN copolymer. Figure 15 shows the quantitative response of effective particle size. In Figures 12 to 14, the reactive SMA copolymer comprises 10% by weight of


75125 Nylon 6lSAN6

- 7512.5122.5 Nylon 6lSMA2/SA N 6 5 Pm

7512.5122.5 - 7512.5122.5 - Nylon 6/SMA6lSAN6 5 Pm Nylon GISMAI 3lSAN6 5 Pm

Figure 13. (microtomed) as a function of MA content of SMA.

SEM photomicrographs of 75/2.5/22.5 Nylon-6/SMA/SANG mixtures

the styrenic component while Figure 15 also shows results for other SMA contents.

For all these SAN copolymers, addition of SMA2 and SMAG causes significant reductions in particle size with SMAG being most efficient in each case. The addition of SMA13 causes the particles to become complex in shape and larger in size. In addition to the issue of miscibility, these results reflect changes in the number of functional groups per chain as the MA content of the SMA copolymer varies and in the interfacial tension as the AN content of the SAN copolymer varies. Considering all of these factors, we tentatively conclude the follow- ing. Up to a certain limit, increasing the MA content improves the efficiency of the SMA copolymer for emulsifying the styrenic phase (i.e., reducing its particle size) in Nylon-6. Levels beyond this point (e.g., SMA13) lead to complex morphology and particle enlargement. It appears that the styrenic phase

which is the minor component is attempting to become co-continuous with the Nylon-6 phase. This might be explained on rheological grounds if SMA migrates into the Nylon-6 and raises its viscosity by reaction such that the styrenic phase is now the least viscous p h a ~ e . ~ ~ ' ~ ~ ~ ~ The comb-like structure of graft copolymers formed from such highly functional ad- ditives may also play a role. Close examination of these results does reveal something about the role of miscibility. The pairs SMA2/SAN13 and SMA13/PS are the furthest from the region of miscibility (see Fig. 5) and their blends with Nylon-6 have relatively large particles. The pairs SMA2/PS and SMA6/SAN6 fall more or less in the center of the miscibility region, and their blends with Nylon- 6 have relatively small particles. The pair SMA13 / S A N E also falls in the miscibility region, and their blends with Nylon-6 have small particles relative to blends based on SMA13 but not to those based on


75125 Nylon 6lSAN 15

- 7512.5122.5 Nylon 6lSMA2/SAN15 5 Pm

- 75/2.5/22.5 - 7512.5122.5 Nylon 6/SMA6/SAN15 5 Pm Nylon 6lSMA13/SAN15 5 Pm

Figure 14. (microtomed) as a function of MA content of SMA.

SEM photomicrographs of 75/2.5/ 22.5 Nylon-G/SMA/SAN15 mixtures

SMA2 or SMA6. The issues associated with the high level of functionality of SMA13 have been discussed already. The SMAG/SANG system seems to have the most optimum balance of all the factors men- tioned since at 10% SMA6 in SANG, the particles of this phase in nylon 6 blends are below the reso- lution of SEM (see Figs. 13 and 15).

SUMMARY

The experiments reported here were designed to gain insight into morphology generation in immiscible blends where in situ grafting reactions are possible. This is an important issue in the optimal design of reactively compatibilized polymer alloys, and the specific systems considered here most closely relate to Nylon-6/ABS materials which are of current

commercial interest. Since this was a model study, the rubber phase found in ABS materials was omit- ted. It was found that, in the absence of any reaction, the size of SAN particles in Nylon-6-rich blends decreases dramatically as the AN content of the copolymer increases. This is quantitatively related to the decrease in SAN-polyamide interfacial tension as the styrenic phase becomes more polar like the polyamide. Effects of this interfacial tension still exist when chemical reactions occur.

It appears that maleic anhydride is more reactive with Nylon-6 than oxazoline units. Certain SMA copolymers when added to the SAN phase cause im- pressive reduction in the size of the particles of the styrenic polymer phase. The experiments with SAN and SMA copolymers of different compositions tentatively suggest that particle size reduction is great- est when the two materials are fully miscible with


h

E 3. Y

3 I-

4 SANISMA2 blends J

2 b 4 75 Wh Nylon 6

SANlS

1 "

0 5 10 15 20 25

wPhSMA2 in SANISMAP

SANlSMA6 blends 75 wi% Nylon 6

0 5 10 15 20 25

wt%SMA6 in SANISMA6

SANlSMAl3 blends 75 wt% Nylon 6

A c

0 5 10 15 20 25

wt%SMAl3 in SANEMA1 3

Figure 15. Weight-average particle diameters for 75% Nylon-6 blends with 25% of SMA/SAN mixtures as a function of MA content of SMA, AN content of SAN, and SMA/SAN proportion.

each other. There seems to be an optimal content of maleic anhydride in the SMA copolymer for reduction of particle size. At high MA contents the

particles become enlarged and complex in shape, and it appears as though the styrenic phase is tending to become co-continuous with the Nylon-6 phase. Several factors may be operative, and further studies are needed to better understand the contribution of each.

The authors express their appreciation to Mitsubishi Gas Chemical Co., The U.S. Army Research Office, and The Texas Advanced Technology Program for their support of various aspects of this research.

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Received July 23, 1991 Accepted February 18, 1992

morphology of nylon-6 blends with styrenic polymers

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