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Compatibilizing immiscible blends with a mutually miscible homopolymer.

Addition of a small amount of PMMA effectively compatibilized four immiscible binary blends, as demonstrated by improved mechanical properties and better dispersion of the primary components.

One of the several forces propelling the rapid introduction of new polymer blends is innovation in compatibilization technology. Because the nature and properties of the interface frequently exert a limiting effect upon the bulk properties of the material, effective compatibilization involves the manipulation of the properties of the material's internal interfaces.|1~

For many polymer blends, the compatibility of the components must be enhanced to provide an attractive property balance. To achieve this enhancement, a number of strategies have emerged. Suitable block or graft copolymers are often introduced to serve as macromolecular emulsifiers providing covalent bonds that traverse and fortify the blend interface.|2~ Alternatively, block and graft copolymers may be generated in situ via reactive blending to generate a compatibilized blend.|3~ Ionomers have served as compatibilizers in many cases, ionic or other strong physico-chemical interactions generated across the interface having effected the compatibilization.|4~ Compatibilization has even been demonstrated by the addition of a third immiscible component that exhibits a relatively low interfacial tension with each of the primary blend components.|5~

This article investigates the hypothesis that the compatibility of an immiscible binary blend may be improved by the addition of a third polymer that is miscible with both of the primary components.|6~ Although many investigations into this class of ternary blend systems have been undertaken, most have focused on finding the onset of single-phase behavior.|7~ In most instances, complete homogenization of the blend occurred only when the mutually miscible component comprised the majority of the blend. In these circumstances, it is impractical to view the third component as the compatibilizing species.

This investigation focuses on the two-phase regions of the phase diagrams for the ternary blends where only a modest amount of the third component is present, and examines the effect the third component exerts on the mechanical properties and morphology of the blends. Chosen for this investigation were ternary systems for which the phase behaviors have been well characterized, and which also provide examples of a variety of material types: glassy amorphous, semi-crystalline, and rubber-toughened thermoplastics. In each case, polymethyl methacrylate (PMMA) homopolymer was the mutually miscible compatibilizer.


The materials used are commercially available thermoplastics. The styrene-acrylonitrile copolymer (SAN), Lustran 31 from Monsanto, has a 23-wt% acrylonitrile content and a melt-flow index of 7.9 g/10 min. Dylark 232 from Arco, the styrene-maleic anhydride copolymer (SMA), has an 8-wt% maleic anhydride content and a melt-flow index of 1.7 g/10 min. The high-rubber acrylonitrile-butadiene-styrene terpolymer (ABS) is Blendex 310 from GE Plastics; the polyvinylidene fluoride (PVDF) is Kynar 201 from Elf Atochem; and the PMMA, obtained from Scientific Polymer Products, has a nominal |M.sub.w~ of 75,000.

Blends were melt compounded on a Baker-Perkins MPC-15 co-rotating twin-screw extruder. For SMA blends, the melt temperature was maintained below 200 |degrees~ C to stay below the reported cloud point for SMA/PMMA blends. A 230 |degrees~ C melt temperature was maintained for PVDF blends. Extrudates were injection molded into standard test specimens on a Fama AU-20-SP machine at the same melt temperatures used for compounding. Blend compositions and their mechanical properties are given in the Table.

Morphology studies were carried out on a Jeol 100-CX electron microscope operating in the STEM mode. Specimens were microtomed from the molded plaques and stained with ruthenium tetraoxide.

SMA/SAN Blend System

SMA/SAN blends have been reported to be miscible when their respective comonomer ratios do not differ by more than 5%.|8~ In the present case, the difference between the 8-wt% MA level in the SMA and the 23-wt% AN level in the SAN is sufficient to produce immiscible blends. PMMA has been reported to be miscible with SMA copolymers having MA levels in the 6- to 40-wt% range, and with SAN copolymers having AN levels in the 10- to 30-wt% range.|9,10~ Thus, PMMA is miscible with both copolymers used in this study.

The results given in the Table show that the addition of PMMA improved the mechanical properties at every blend composition. The graphs of tensile strength as a function of SMA/SAN blend ratio both with and without PMMA in Fig. 1 show a concave shape, indicative of poor compatibility, in the absence of PMMA, and a slightly convex shape, indicative of improved compatibility, in the presence of PMMA.

Morphological studies provide additional compelling evidence for effective compatibility. A comparison of micrographs of 25/75 SMA/SAN blends with and without 10 pph PMMA clearly shows a substantial reduction in the size of the dispersed SMA droplets upon addition of PMMA to the SAN-rich blend. A similar comparison of micrographs of 75/25 SMA/SAN blends demonstrates an analogous effect on SAN droplets in the SMA-rich blend. Thus, the addition of PMMA to the blends promoted smaller domain size--presumably accomplished through a reduction of interfacial tension.

SMA/ABS Blend System

This system combines a glassy plastic with a rubber-toughened material. The behavior of SMA/ABS blends in which the comonomer contents were such that SMA exhibited miscibility with the SAN matrix of the ABS component is described elsewhere.|11~ In this case, the comonomer contents are such that the SMA and ABS components are immiscible, though both are miscible with PMMA.

The effect of compatibilization with PMMA is most readily observed in the examination of impact strength. Notched Izod impact strength as a function of SMA/ABS ratio with and without 10 pph PMMA is plotted in Fig. 3. The PMMA, a brittle material itself, nevertheless gives rise to greatly improved impact strength in the blends. For a 50/50 ratio, addition of 10 pph PMMA doubled the impact strength of the blend (despite a slight reduction in the rubber volume fraction).

PVDF/SAN Blend System

PVDF is a semicrystalline polymer immiscible with SAN copolymers. PMMA has been reported to be miscible with PVDF in the melt and in the amorphous phase of the solid.|12~

Addition of PMMA to PVDF, a ductile plastic, decreased yield strength by 10% and impact strength by 40%. However, adding PMMA to PVDF/SAN blends increased their tensile strength, tensile elongation, and impact strength. A plot of tensile strength as a function of PVDF/SAN ratio is concave in the absence of PMMA, but linear when PMMA is present, suggesting a compatibilizing effect attributable to the PMMA.

PVDF/ABS Blend System

This system combines a semicrystalline polymer with a rubber-toughened plastic. The Table shows that addition of PMMA at any given PVDF/ABS ratio has a negligible effect on tensile yield stress, but gives rise to a remarkable increase in tensile elongation and impact strength. Typical stress/strain curves for a PVDF/ABS blend with and without PMMA demonstrate the large increase in ductility afforded by the compatibilizer.
TABLE. Blend Compositions and Mechanical Properties.

 Tensile Tensile Notched
Blend, pph(*) strength, MPa elongation, % Izod, J/m

100/0/0 55 1.2 12
100/0/10 58 6.5 12
75/25/0 50 0.9 8
75/25/10 65 6.8 16
50/50/0 59 1.0 7
50/50/10 67 1.3 11
25/75/0 59 1.0 11
25/75/10 68 6.3 20
0/100/0 67 5.7 12
0/100/10 69 6.4 17

75/25/0 57 6.1 20
75/25/10 51(y) 13.6 29
50/50/0 36(y) 15.3 52
50/50/10 39(y) 14.7 109
0/100/0 18(y) 121 313

0/100/0 44(y) 69 135
0/100/10 40(y) 76 81
25/75/0 43 4.4 15
25/75/10 51(y) 26 26
50/50/0 38 2.5 58
50/50/10 52 3.8 76

25/75/0 32(y) 29 45
25/75/10 32(y) 213 89
50/50/0 27(y) 18 222
50/50/10 27(y) 134 487

* Compositions in parts per hundred blend excluding PMMA.

y Indicates yielding occurred.

Addition of 10 pph PMMA to PVDF/ABS blends increased their notched Izod impact strength by a factor of approximately two. Since PMMA is much more brittle than either PVDF or ABS, and its addition to pure PVDF decreases PVDF impact strength, this increase is almost certainly attributable to the improved compatibility afforded by the PMMA.


The foregoing results demonstrate that when added in modest amounts to an immiscible polymer mixture, a mutually miscible polymer can improve compatibility in a variety of different blend types. The compatibilizing effects of the mutually miscible component may result from its presumed tendency to become enriched in the vicinity of the blend interface. Based on studies of the behavior of the free surface of single-phase polymer blends,|13~ one may speculate that the interfacial tension of a two-phase, three-component blend could be lowered by the creation of a local composition gradient on either side of the internal blend interface such that the interfacial zone becomes enriched with the common phase component, as illustrated in Fig. 6.

On a polymer segment level, the above scheme amounts to eliminating some of the unfavorable interactions that act across the interface at the expense of reducing the number of favorable interactions within the bulk of each phase. The result of an interfacial region enriched with this common phase component is not only lower interfacial tension, but improved molecular interpenetration through the interface. This results in better adhesion between the phases and accounts for improved bulk mechanical properties for the blend.


We are grateful to D. R. Paul and J. T. Koberstein for helpful discussions.


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A Broad View of Blend Compatibilization

Compatibilization of polymer blends continues to be a fruitful area of research in the plastics industry. This research has focused on applications of compatibilizer technology such as the recycling of post-consumer plastics without extensive sorting and the production of multilayer film and packaging materials, as well as the development of new polymer alloys.

As our understanding of blend compatibility has deepened, the definition of what materials can function as compatibilizers has broadened significantly. To the traditional paradigm of block copolymers have been added functionalized polymers, ionomers, and mutually miscible polymers. This article introduces the latter class of compatibilizers as a complement to existing compatibilization strategies. Mutually miscible polymers may find commercial use in cases where a suitable block copolymer or functional polymer is not readily available.

The binary blend systems used in this work represent model systems in that their phase behaviors are described in the literature and are well known. This technology has also been applied to a family of proprietary blends of engineering plastics. These basic principles have been further extended to the development of filled and reinforced thermoplastic compounds having improved adhesion between the reinforcing material and the matrix.
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Title Annotation:Alloys & Blends; includes related article
Author:Machado, Joseph M.; Chi Sing Lee
Publication:Plastics Engineering
Date:Oct 1, 1993
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