Miscibility in Random Copolymer Blends - Advances in Chemistry

4 Miscibility in Random Copolymer Blends F . E . KARASZ and W. J. M A C K N I G H T

Downloaded by PURDUE UNIVERSITY on September 14, 2013 | http://pubs.acs.org Publication Date: December 9, 1985 | doi: 10.1021/ba-1986-0211.ch004

Polymer Science and Engineering Department, University of Massachusetts, Amherst, M A 01003

Compatibility in homopolymer-copolymer and copolymer-copolymer mixtures may be enhanced by the repulsion of dissimilar segments in the random copolymer chain. A mean field approach for this phenomenon is presented that gives a quantitative description of this phenomenon and that accounts for many of the phenomena observed. It can also be used to derive segmental interaction parameters from miscibility measurements. The treatment is generalized to cover other situations: (a) homopolymers with copolymers containing a common segment, (b) copolymer-copolymer blends in which the blend constituents contain the same two segments but in differing ratios, and (c) copolymer-copolymer blends containing one common segment. Graphical representatives of model calculations for several of these systems are shown.

THEGENERALIMMISCIBILITYOFPOLYMERShas been established experimentally and accounted for in thermodynamic terms. Exceptions have often been explained by invoking "specific interactions," which are associated with either an extremely small positive or, indeed, a negative noneonfigurational free energy of mixing. This thermodynamic requirement is necessitated by the negligible configurational entropy associated with binary high molecular weight polymer mixtures. In random copolymers blended either with a homopolymer or a second random copolymer, an alternative mechanism can lead to miscibility. Miscibility may occur if the mutual repulsion between the dissimilar segments i n the copolymer is sufficient to, in effect, overcome the repulsion between these segments and those in the second component of the mixture. In thermodynamic terms, this effect can lead to the negative net interaction energy necessary to induce miscibilization. Such effects have been recognized on an empirical basis. A n example is the well-known miscibility of polyvinyl chloride with random copolymers of ethylene and vinyl acetate for certain composition ranges of the latter system (I). These and similar findings recently were explained in a quantitative manner in a mean field theory for miscibility in copolymer blends 0065-2393/86/0211/0067$06.00/0 © 1986 American Chemical Society

In Multicomponent Polymer Materials; Paul, D., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1985.

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MULT C IOMPONENT POLYMER MATERA ILS

(2). This theory has permitted substantial insights into the effect of chemi

cal structure and composition, of temperature, and of molecular weight on phase behavior in such systems. T h e theory parallels the well-established mean field approach for concentrated polymer solutions in that it divides the free energy of mixing, AGm9

into purely configurational and noncon-

figurational terms. T h e nonconfigurational terms are described in terms of a dimensionless interaction parameter, Xbiend- Thus AGm


R

= ( Φ ι / η ι ) 1 η Φ ι + ( Φ 2 / η 2 ) In Φ 2 + ΦιΦ 2 Χι>ι β η 0, and xBC is suffi ciently positive to render Xbiend < 0 for certain ranges of x. For copolymers containing styrene homopolymer, a single boundary m the T-c plot is observed; this observation is consistent with the fact that P P O and PS are miscible. More recently, mixtures of PS with copolymers of o- and pchlorostyrenes have been studied (5). These data have permitted us to re fine values of χ ί ; · not only for this system but also to calculate a self-consistent set of values for a wider range of χψ A l l the systems studied to date have displayed lower critical solution temperature ( L C S T ) behavior; how ever, upper critical solution temperature ( U C S T ) behavior may also be found. T h e predicted T - c behavior for this system is shown in Figure 2. T h e extremum in this plot corresponds to the instance in which the two consolute points just merge. Systems containing PS and copolymers of o- and p-styrene exhibit ex treme sensitivity to the degree of polymerization of either or both compo nents. This effect is partially a result of the very low positive value of the interaction parameter representing styrene and o-chlorostyrene interac tions. A recent extension of our investigations to lower molecular weight systems has shown that the temperature maxima of the miscibility windows increase substantially as the degree of polymerization is lowered and can readily be selected to cover the entire accessible experimental temperature range. T h e resulting data have enabled us to calculate the temperature de pendence of the respective χ ί ? values with some accuracy (8). (AxBi-x)nl(AyBi-y)n\ Blends of random copolymers containing identical segments but of different overall compositions are predicted to be


4.

KARASZ AND MACKNIGHi

Miscibility

in Random Copolymer Blends

71


two phases

Β

COPOLYMER COMPOSITION

C

Figure 2. Predicted miscibility boundary for A n / ( B x C ] _ J n ' blend (XAB < 0 and the two other χ values are positive) displaying both upper and lower critical solution temperatures. The boundary is the locus of UCSTs and LCSTsfor positive and negative slope temperature regimes, respectively.

incompatible when the respective polymers are of infinite molecular weight. T h e limiting miscibility condition \x - y\ - 0 is relaxed, how ever, for η , η ' < oo. This behavior has been verified (9) for chlorinated polyethylenes (which may conveniently be treated as random copolymers of C H 2 and C H C 1 segments). In our work on this system, an extensive series of U C S T values has been observed for chlorinated polyethylene blends whose glass transition temperature (T g ) values are sufficiently low (JO).

(AxBi-x)J(CyDi-y)n'. N o systematic blend study of miscibility in two random copolymers has yet been reported. As already mentioned, be havior in this system is a function of six independent χ*;· values. A conve nient representation of the predictions for this system is in the form of iso thermal composition-composition plots ("c-c plots") displaying the miscibility-immiscibility boundary as a function of χ and y (0 0 (and η , η ' oo)? domains of miscibility can still be found (Figure 3) in the c-c representation. T h e ellipse shown cannot extend to the corners of the diagram because these areas correspond to immiscible blends of homopolymers of A and C and A and D , for example. However, for certain χ ί ? · the ellipse will intersect one or more of the boundaries of the diagram (e.g., the line χ = 0, 0 < t/ < 1 in Figure 4 corresponding to the instance of homo polymer-copolymer miscibility that displays the "window of miscibility") shown in Figure 1. A different type of miscibility domain is predicted if one or more χ$ are negative. See Figure 5. The special blend of copolymers containing a common segment [e.g., (AxBi-x) J\AyC\-y)„'] may also be conveniently represented in this man ner. In this system, again only three χ*7· values are required to describe the system, and the regimes of miscibility are bound by straight lines intersect ing at the origin, x,y = 0. See Figure 6.

Conclusions The theory provides explanations for a range of phenomena involving ran dom copolymer containing blends. T h e interaction parameters that may be derived are sufficiently quantitative to be of predictive value in at least one system. Experimental determination of miscibility boundaries for copoly-

D ir

C 0» 0

1

L

A

1

~*

1

«

I

Β

Figure 3. Calculated miscibility domain (shaded region) in copolymer blends. The degree of polymerization of the two copolymers is infinite. The interaction parameters χ , \ , χ , χ Β ο XBD, and x C D are 0.3, 0.1, 0.2, 9.2, 0.1, and 0.4, respectively. ΑΒ

AC

ΑΌ



4.

KARASz AND MACKNiGHT

Miscibility

in Random Copolymer Blends

Figure 4. Calculated miscibility domain for a copolymer blend (infinite mo lecular weights) with values of 0.3, 0.2, 0.1, 0.1, 0.1, and 0.5 (same order as in Figure 3).

Figure 5. Calculated miscibility domains for a copolymer blend (infinite mo lecular weights) with χ y values of 0.1, -0.1, 0.17, 0.37, -0.1, and 0.2 (same order as in Figure 3).


73


74

MULT C IOMPONENT POLYMER MATERA ILS

Figure 6. Calculated miscibility domain for a copolymer blend (infinite mo lecular weights) containing the common segment A. The interaction param eters XAB? XAC> and XBC are 0.7, 0.1, and 0.2,

respectively.

mer-copolymer systems provides the means for further verification of the treatment. A n important aspect is the possibility for a rational design of copoly mers to yield miscible systems. For example, miscibility that occurs in the AB-AC system may be viewed as a miscibilization of homopolymers of Β and C by the respective copolymerization with the common segment A . Special requirements, such as conditions for minimizing the total consump tion of A , for example, may be readily evaluated. Extensions of the theory to take into account composition and concen tration dependencies of the interaction parameters are formally possible. In addition the effect of chain microstructure (i.e., a relaxation of the re quirement of random monomer placement in the copolymer) is feasible and is being incorporated in extensions of this work.

Acknowledgment W e acknowledge with thanks support from A F O S R Grant 84-0100 for this research.

Nomenclature AGmIRT Φ*

Reduced free energy of mixing per monomer mole Volume fraction of component i


4.

KARASZ A N D MACKNiGHT

Miscibility

in Random Copolymer

Blends

75

Hi

Degree of polymerization of component i

Xij

Thermodynamic interaction parameter (dimensionless) be-

Xbiend

Net thermodynamic interaction parameter for blend (dimen-

tween segment i and ; sionless)


Literature Cited 1. Hammer, C . F. Macromolecules 1971, 4, 69. 2. ten Brinke, G . ; Karasz, F. E . ; MacKnight, W . J. Macromolecules 1983, 16, 1827. See also Paul, D . R.; Barlow, J. W . ; Polymer 1984, 25, 487. 3. Koningsveld, R.; Kleintjens, L . A . J. Polym. Sci. Polym. Symp. 1977, 61, 221. 4. Alexandrovich, P.; Karasz, F. E.; MacKnight, W . J. Polymer 1977, 17, 1023. 5. ten Brinke, G.; Rubinstein, E.; Karasz, F. E.; MacKnight, W . J.; Vukovic, R. J. Appl. Phys. 1984, 56, 2440. 6. Karasz, F. E . ; MacKnight, W . J. Polym. Mater. Sci. Eng. Prepr. 1984, 51, 280. 7. Karasz, F. E . "Polymer Blends and Mixtures"; Walsh, D . J., E d . ; Nijhoff: The Hague, Neth., 1985; p. 25-36. 8. Cimmino, S.; Karasz, F. E.; MacKnight, W . J., unpublished data. 9. Chai, Z . ; Sun, R. Polymer 1983, 24, 1279. 10. Ueda, H . ; Karasz, F. E . Macromolecules, in press.

RECEV IED for review November 15, 1984.ACCEPTEDFebruary 12, 1985.


Miscibility in Random Copolymer Blends - Advances in Chemistry

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