Can Random Copolymers Serve as Effective Polymeric ...

Can Random Copolymers Serve as Effective PolymericCompatibilizers?M. S. LEE,* ,1 T. P. LODGE, 2 C. W. MACOSKO 11 Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Ave.,Minneapolis, Minnesota 554552 Department of Chemistry, University of Minnesota, 207 Pleasant St., S.E. Minneapolis, Minnesota 55455Received 17 April 1997; accepted 11 August 1997ABSTRACT: We investigate the compatibilizing performance of a random copolymer inthe melt state, using transmission electron microscopy. Blends of polystyrene (PS) andpoly(methyl methacrylate) (PMMA) are chosen as a model system, and a randomcopolymer of styrene and methyl methacrylate (SMMA) with 70 wt % styrene is used asa compatibilizer. From TEM photographs it is clear that SMMA moves to the interfacebetween PS and PMMA domains during melt mixing, and forms encapsulating layers.However, the characteristic size of the dispersed phase increases gradually with annealingtime for all blend systems studied. This demonstrates that the encapsulating layerof SMMA does not provide stability against static coalescence, which calls into questionthe effectiveness of random copolymers as practical compatibilizers. We interpret theencapsulation by random copolymers in terms of a simple model for ternary polymerblends. 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2835–2842, 1997Keywords: polymer blends; random copolymer; compatibilizer; encapsulation; coalescenceINTRODUCTIONmethod for compatibilization of immiscible polymerblends. 8 There are many experimental andBlends of immiscible polymers often require the theoretical results that support this possibility.addition of a compatibilizer to improve the disperterfacialFirst, random copolymers can enhance the in-sion and adhesion of phases and to stabilize theadhesion between immiscible phases, 8–13morphology. Block or graft copolymers having the although the mechanism whereby a random co-same monomeric units as the blend components polymer layer strengthens the interface is stillhave been successfully used for this purpose. 1–4 controversial. 10,14 Second, random copolymers lo-Such compatibilizers can be introduced by adding cated at the interface can reduce the interfaciala premade copolymer, or by in situ reactive forma- tension between immiscible phases. For example,tion during melt processing. 5–7 However, these Balazs and co-workers recently examined the efcompatibilizersare typically expensive, and can fect of molecular architecture of potential compatcausea substantial viscosity increase during promers)on their compatibilizing performance usingibilizers (random, alternating, and diblock copoly-cessing.Random copolymers may offer an alternative the self-consistent mean field approach. 15 Theycalculated the extent of the interfacial tension reductionas a measure of the efficiency of the com-* On leave from Chonnam National University, Kwangju, patibilizers, and predicted that long random co-Koreapolymers are more effective than short diblocksCorrespondence to: T. P. Lodgewhen they compare random copolymers withJournal of Polymer Science: Part B: Polymer Physics, Vol. 35, 2835–2842 (1997) 1997 John Wiley & Sons, Inc. CCC 0887-6266/97/172835-08 diblocks of different molecular weight. However,28358Q46 / 8q46$$3012 10-20-97 05:06:51 polpa W: Poly Physics 9713012

2836 LEE, LODGE, AND MACOSKOat fixed molecular weight, diblock copolymers are were quenched in ice water. The sample code 70/more effective in reducing interfacial tension. 20/10 denotes 70% PS, 20% PMMA, and 10%Third, the spreading of random copolymers be- SMMA by weight. To investigate the melt stability,tween immiscible phases is thermodynamicallyas-blended samples were annealed in an ovenfavored. Winey and co-workers demonstrated that at 200C for a specified period of time and thenencapsulation by random copolymers can occur quenched in ice water. Specimens were wrappedduring drying of a solvated blend. 16in aluminum foil to prevent flow during annealing.It is not yet clear that random copolymers preferentiallyThe blend morphology of as-blended and aninglocate at the interfaces under melt mix- nealed samples was characterized using a JEOLconditions, or whether they effectively stabilize1210 transmission electron microscope (TEM),the resultant morphology during annealing, operated at 120 kV. Sections of 70 nm thicknessas is necessary for commercial application. Therefore,were microtomed at room temperature with amore data on the compatibilizing perfor- Reichert Ultracut S ultramicrotome. The sectionsmance of random copolymers in the melt state were stained for 15 min with the vapor of 0.5%are needed, because polymer blends are generally RuO 4 in water solution. The number (D n ) and volume-to-surfaceprepared by melt mixing. Furthermore, it is noteworthy(D VS ) average Waddel diameterthat the efficiency of compatibilizers can (the diameter for an equivalent sphere) of the disperseddepend on the blend preparation method. For example,phase were calculated from TEM photodependin melt compatibilization with premade graphs, averaging over 200 to 400 particles. Fordiblock copolymers, there appears to be an optimumthe case that SMMA forms encapsulating layersmolecular weight, 7 whereas when blends between PS and PMMA domains, the layers wereare prepared by solution blending, compatibilizersincluded to calculate D n or D VS .with higher molecular weight are more effec-tive in reducing the dispersed phase size. 17In this study we investigate the compatibilizing RESULTS AND DISCUSSIONperformance of a random copolymer in the meltstate. Polystyrene (PS) and poly(methyl methac- As styrene is preferentially stained under the conditionsrylate) (PMMA) are chosen as model blend components,used in this study, 18 PS appears dark,and a random copolymer of styrene and SMMA gray, and PMMA white in bright-fieldmethyl methacrylate (SMMA) is used as the compatibilizer.TEM. This contrast makes it possible to differentiwhetherThrough this study we will determine ate the SMMA phase in the blends. Figure 1random copolymers move to the interface shows TEM photographs of three as-blended sam-between matrix and dispersed phase during melt ples, where PS comprises the majority componentmixing and, if so, whether encapsulating layers at a constant concentration of 70 wt %. As seen inof random copolymers can stabilize blend mor- Figure 1(b) and (c), SMMA forms encapsulatingphology during melt annealing or not. Experimentallayers between the PS matrix and the PMMA par-answers to these two questions will be helpful ticles during melt mixing. The encapsulatingto determine whether random copolymers can be SMMA layer for the 70/20/10 blend is clearly seencommercially effective as compatibilizers.in the magnified TEM photograph inserted in Figure1(b). The ratio of PMMA to SMMA has nodiscernible effect on the observed encapsulationEXPERIMENTALby SMMA, although for the 70/10/20 blend [Fig.1(c)] some pure SMMA domains exist. AlthoughThe source, molecular weight, and melt viscosity the blends were prepared by melt mixing ratherof the polymers used in this study are listed in than solution casting, the morphologies seen inTable I. Note that at the blending temperature Figure 1 are equivalent to those observed bythe ranking of viscosities is h SMMA ú h PS ú h PMMA . Winey and co-workers. 16 This suggests that encapsulationAll polymers have broad molecular weight distributions,by the random copolymer is indepen-and were provided in pellet form; they dent of the blend preparation method. Moreover,were used as received. Blend samples were preparedbecause SMMA has the highest melt viscosity un-using a Haake Rheomix 600 batch mixer der the mixing conditions employed, we canconcludethat the driving force for the encapsulation,that is, the interfacial tension reduction by randomcopolymers, is strong enough for random co-operating at 210C for 20 min. The rotor speedwas 50 rpm, which corresponds to a maximumshear rate of 65 s 01 . After mixing, the samples8Q46 / 8q46$$3012 10-20-97 05:06:51 polpa W: Poly Physics 9713012

RANDOM COPOLYMERS AS EFFECTIVE POLYMERIC COMPATIBILIZERS 2837Table I.Polymers Used for This StudyPolymers Source M w (g/mol) PI a h b (Pa.s)PS Dow 200,000 1.8 894PMMA Polysciences, Inc. 100,000 1.9 660SMMA c Polysciences, Inc. 270,000 1.9 1383a Polydispersity index, measured using gel permeation chromatography.b Measured using a Rheometrics DSR rheometer at a shear rate of 50 s 01 and 210C.c 70 wt % styrene.polymers to move to the interfaces during melt the plot of (D n ) 3 vs. annealing time is almost themixing. A second observation from Figure 1 is the same within the experimental error.change in the size of dispersed phase with the Figure 4 shows the morphology change beforeaddition of SMMA. The dispersed phase size of and after melt annealing for a 20/70/10 blendthe 70/20/10 blend is significantly reduced com- with a PMMA matrix. As with blends with a PSpared to the 70/30/0 blend. As predicted by Balazsmatrix, SMMA encapsulates the minor phase andand co-workers, 15 this results from the in- dispersed phase coarsening is observed after an-terfacial tension reduction by SMMA layers locatednealing. Thus, the matrix type has no significantat the interfaces. However, as seen in the effect on the encapsulation by random copolymers70/10/20 blend, SMMA used in excess does not and coarsening of encapsulated particles.contribute to further size reduction; rather, the Recently Macosko et al. attributed the role ofthickness of the SMMA shell increases.block copolymers in preventing static coalescenceIn accord with other experimental results re- to the steric stabilization mechanism of colloidalferred to in the Introduction, Figure 1 clearly particles, 7 i.e., the copolymer forms a brush atdemonstrates that random copolymers are poly- the interface. They proposed an elastic repulsionmeric compatibilizers: SMMA moves to the interfacebetween approaching particles, caused by the retheduring melt mixing and contributes to reduce duction in the compatibilizer chain conformation.dispersed phase size. However, as noted in From the balance between van der Waals at-the Introduction, to be actually effective as a com- traction between approaching particles and thepatibilizer the layer located at the interface must entropic repulsion, they postulated that Ç 20%provide melt stability during other processing surface coverage by block copolymers is necessaryconditions, such as annealing. Figure 2, TEM photographsto impart static stability. In contrast, the confor-of annealed blend samples, may provide mation of SMMA at the interface is very differentan answer. In all cases, samples show that after from that of a block copolymer brush; in effect,60 min at 200C, the domain size has increased the SMMA chains may be considered as a separatesignificantly compared to that of the as-blendedphase. Consequently, there is no repulsionsamples. This is similar to the coarsening process between encapsulated particles.observed for uncompatibilized polymer blends. 19,20 Figure 5 illustrates how the particles with en-Although the encapsulating layer of SMMA is well capsulating layers grow during annealing. Thedeveloped for 70/20/10 and 70/10/20 blends after photographs were obtained from a 70/10/20 blendannealing, it is clear that the layer does not pre- annealed for 60 min, and reorganized according tovent static coalescence. Considering that the the coalescence mechanism of immiscible polymerphysical properties of polymer blends depend crit- blends proposed by Chesters and others. 21–23 Asically on the size of dispersed phase as well as on seen in Figure 5, coalescence of the encapsulatedthe adhesion between the matrix and the dis- particles is two-step process: coalescence of encapsulatingpersed phase, the coarsening of encapsulated particlesparticles in the PMMA matrix, followedmay be a crucial disadvantage in commer- by coalescence of dispersed domains within thecial applications.encapsulating SMMA layer. The latter processThe change in the domain size with annealing might be slower than the first because the melttime is summarized in Figure 3(a). Upon annealing,viscosity of SMMA is higher than that of PS. It isthe particle size increases significantly, irre- clear that the encapsulating layer acts as a dis-spective of the blend composition. As shown in crete phase and does not provide stability againstFigure 3(b), the coarsening rate calculated from static coalescence. The force associated with the8Q46 / 8q46$$3012 10-20-97 05:06:51 polpa W: Poly Physics 9713012

2838 LEE, LODGE, AND MACOSKOparticle movement is sufficient to squeeze out theencapsulating SMMA layer when the encapsulatedPMMA particles collide. After a bridge betweentwo inner particles is formed by rupture ofFigure 2. TEM photographs of samples annealed at200C for 60 min: (a) 70/30/0 (D n Å 1.15; D VS Å 1.75);(b) 70/20/10 (D n Å 0.85; D VS Å 1.19); (c) 70/10/20 (D nÅ 0.87; D VS Å 1.18). The unit of D n and D VS is mm.Figure 1. TEM photographs of as-blended samples:(a) 70/30/0 (D n Å 0.80; D VS Å 1.03); (b) 70/20/10(D n Å 0.28; D VS Å 0.36); (c) 70/10/20 (D n Å 0.47; D VSÅ 0.56). The unit of D n and D VS is mm.the SMMA layer, a coalesced particle, which hasa well-developed encapsulating layer, is formed.The encapsulation by random copolymers canbe interpreted with a model proposed by Hobbsand co-workers for ternary polymer blends. 24 They8Q46 / 8q46$$3012 10-20-97 05:06:51 polpa W: Poly Physics 9713012

RANDOM COPOLYMERS AS EFFECTIVE POLYMERIC COMPATIBILIZERS 2839When both l 31 and l 13 are negative, we can expecttwo separate dispersed phases of 1 and 3. In otherwords, 3 moves to the interface and encapsulatesminor phase 1 or 2, when the sum of the interfacialtensions associated with 3 is smaller thanthe interfacial tension between the original blendcomponents. In mean-field theory g ij is proportionalto the square root of the Flory–Hugginsinteraction parameter x 25 ij as√g ij Å kT xijb 2 6(2)where b is the effective length per monomericunit, k is the Boltzmann constant, and T is temperature.Since for homopolymer/copolymer blendsx ij can be estimated from a binary interactionmodel, 26 we can estimate l ij for the PS/PMMA/Figure 3. (a) variation of the number-average particlediameter with annealing time; (b) plots of (D n ) 3 vs.annealing time. Slopes are proportional to coalescencerates.considered the spreading coefficient l ij , which determineswhether the third component should encapsulatethe minor phase or not. For the casethat a polymer, 3, is added to an immiscible polymerblend of components 1 and 2, l 31 is definedasl 31 Å g 12 0 [g 31 / g 32 ] (1)where g ij is the interfacial tension between i andj. Encapsulation of 3 onto 1 will occur if l 31 ú 0.Figure 4. TEM photographs of PS/PMMA/SMMA20/70/10 blends: (a) as-blended; (b) annealed at 200Cfor 60 min.8Q46 / 8q46$$3012 10-20-97 05:06:51 polpa W: Poly Physics 9713012

2840 LEE, LODGE, AND MACOSKOFigure 5. Coalescence of encapsulated particles: (a) approach and overlap of encapsulatinglayer; (b) deformation of dispersed particles and squeezing out the encapsulatinglayer; (c) rupture of the encapsulating layer; (d) mass flow and coalescence. Note thatthese pictures do not represent a time series of two particular drops.SMMA system. We designate PS, PMMA, and The values of b PS and b PMMA obtained from neu-SMMA as 1, 2, and 3. tron scattering measurements are 0.68 and 0.69For blends of PS, PMMA, and SMMA, x 13 and nm, respectively. 27 From eqs. (3), (4), and (5),x 23 are obtained from the binary interaction we can calculate l ij for a given matrix. The l ijmodel as follows:values calculated, and the corresponding morphologies,x 13 Å x 12 (1 0 f ) 2 (3) are given in Figure 6. The calculationpredicts that, except for the case of a random cox23 Å x 12 f 2 (4)polymer matrix, encapsulation by random copoly-mers is thermodynamically favored, regardless ofthe content and type of comonomer and matrix.where f is the volume fraction of styrene in This prediction is consistent with the TEM resultsSMMA. Assuming that the b values of PS, PMMA, of Figures 1 and 4. For the SMMA matrix, Wineyand SMMA are the same, we can approximate eq. et al. reported that two separate PS and PMMA(1) as domains are formed in the SMMA matrix and nol 31 É √ x 12 0 [ √ x 13 / √ x 23 ]encapsulation between PS and SMMA occurs. 16(5) Their observation is also in agreement with the8Q46 / 8q46$$3012 10-20-97 05:06:51 polpa W: Poly Physics 9713012

RANDOM COPOLYMERS AS EFFECTIVE POLYMERIC COMPATIBILIZERS 2841Figure 6. Spreading coefficients, l ij , for PS/PMMA/SMMA and possible morphologies for a given matrix.nealed at high temperature, the size of the encapsulatedparticles increases with annealing time.This means that the encapsulating layer of randomcopolymers does not provide stability againststatic coalescence. The amount of SMMA and thematrix type, PMMA or PS, have no effect on theencapsulation by SMMA and the coarsening processof encapsulating particles. It is not yet clearwhether the coarsening behavior observed in thisstudy is fatal for the physical properties of polymerblends compatibilized by random copolymersor not. To answer this question further experimentswill be necessary.The morphologies observed after annealing areequivalent to those seen after long drying of asolvated blend. 16 Moreover, the same morphologywas also seen in as-blended samples. It is clearthat encapsulation by SMMA is thermodynami-cally favored and a driving force effectively acts tolocate SMMA at a preferred position, the interfacebetween PS and PMMA phases. The driving forcefor encapsulation is apparently the reduction ofinterfacial tension caused by the addition of ran-dom copolymers. From spreading coefficients cal-culated from Helfand and Tagami theory and abinary interaction model we can predict that theencapsulation by random copolymers is thermo-dynamically favored. Also, the encapsulation byrandom copolymers is independent of copolymertype and comonomer composition. However, to establishwhether this result is a general rule willrequire further work.prediction of Figure 6. Further evidence for theencapsulation by random copolymers may befound for polypropylene (PP)/polyethylene (PE)/ethylene–propylene rubber (EPR) blends, whereEPR encapsulate the minor PE phase. 28The encapsulation by random copolymers maybe extended to other blend systems. For misciblepolymers, the interfacial tension is effectivelyzero. Therefore, it is possible to prepare polymerblends with encapsulation by random copolymerwhen we replace one or both of blend componentswith corresponding miscible components. Becausepolycarbonate (PC) is partially miscible withPMMA, and poly(phenylene oxide) (PPO) is completelymiscible with PS, 3,29 PS/PC/SMMA, PPO/PMMA/SMMA, and PPO/PC/SMMA blends arecandidates for encapsulation. From TEM photographswe have found that in all cases, SMMAmoves to the interface during melt mixing and This work was supported in part by the Center for Informsan encapsulating layer.terfacial Engineering, an NSF-sponsored EngineeringResearch Center at the University of Minnesota. M.S.L.thanks the Korea Science and Engineering FoundationSUMMARY(KOSEF) and Chonnam National University (CNU)for fellowship support.We investigate whether random copolymers canact as effective compatibilizers for blends preparedby melt mixing. Similar to previous experimentaland theoretical studies, the possibility ofREFERENCES AND NOTESrandom copolymers acting as polymeric compatiandS. Newman, Eds., Academic Press, New York,1. D. R. Paul, in Polymer Blends, Vol. 2, D. R. Paulbilizers can be seen from TEM photographs of asblendedsamples. SMMA moves to the interfaces 1978.between PS and PMMA domains during melt mix-2. L. A. Utracki, Polymer Alloys and Blends, HanserPublishers, Munich, 1989.ing and forms encapsulating layers. As a result,3. L. A. Utracki, Ed., Encyclopaedic Dictionary ofthe size of the dispersed phase is significantly re-Commercial Polymer Blends, ChemTec Publishing,duced. When SMMA is used in excess, separate Toronto, 1994.domains of pure SMMA are formed. This is analo- 4. S. Datta and D. Lohse, Polymeric Compatibilizers,gous to micelle formation of block copolymers in Hanser Publishers, Munich, 1996.the case of compatibilization by premade block 5. C. D. Park, W. H. Jo, and M. S. Lee, Polymer, 37,copolymers. However, when the blends are an- 3055 (1996).8Q46 / 8q46$$3012 10-20-97 05:06:51 polpa W: Poly Physics 9713012

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