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Probing of the Reaction Progress at a PMMA/PS Interface by Using Anthracene-Labeled Reactive PS Chains Z. Yin, C. Koulic, C. Pagnoulle, and R. Je´roˆme* Center for Education and Research on Macromolecules (CERM), University of Lie` ge, B6a, Sart-Tilman, 4000 Lie` ge, Belgium Received July 8, 2002. In Final Form: October 16, 2002 The progress of the interfacial reaction of polystyrene chains end-capped by a primary amine (PS-NH2) and PMMA chains end-capped by an anhydride (PMMA-anh) has been monitored by SEC-UV, by using anthracene-labeled polystyrene chains (anth-PS-NH2) as a probe. Assemblies of an anth-PS-NH2 layer and a PMMA-anh layer were annealed at 200 °C for various periods of time. The interfacial reaction rate depends on the molecular weight (MW) of the reactive precursors in relation to the χN value of the chains. For chains of low χN (χN ) 6), the reaction is faster because the interface becomes more diffuse with time, as observed by TEM and AFM, consistent with compatibilization of the weakly immiscible polymers by the copolymer formed in-situ. For chains of higher molecular weight and χN (10, instead of 6), the interface is much sharper and the residence time at the interface of the symmetric diblock copolymer of higher molecular weight is also increased, which dramatically restricts the progress of the interfacial reaction under the annealing conditions used in this work.

Introduction Formation of block or graft copolymer at the interface of immiscible polymers is a key issue to promote compatibilization and to improve interfacial adhesion and interfacial fracture toughness.1-7 Kinetics of interfacial reactions in the melt (reactive blending) mainly depends on mutual reactivity and molecular weight of the reactive precursors.8 Because only a low amount of copolymer is required to modify substantially the phase morphology and the physicochemical properties of polyblends,5,9,10 it is quite a problem to monitor the progress of the interfacial reaction in reactive blending. The flat interface in layered sandwiches was recently studied as a model, such as the interface between PS and either styrene/maleic anhydride copolymer11 or PMMA.12 Reactive PS chains were diluted in PS and deuterated for being detected by forward recoil spectrometry (FRES), which is a technique well-suited to the depth profiling of the interfacial region. Jiao et al.11 and Schulze et al.12 accordingly reported that the interfacial reaction was controlled by the reaction kinetics rather than by the diffusion of the reactive chains at the * To whom correspondence should be addressed. (1) Koning, C.; Van Duin, M.; Pagnoulle, C.; Je´roˆme, R. Prog. Polym. Sci. 1998, 23, 707-757. (2) Orr, C. A.; Adedeji, A.; Hirao, A.; Bates, F. S.; Macosko, C. W. Macromolecules 1997, 30, 1243-1246. (3) Ide, F.; Hasegawa, A. J. Appl. Polym. Sci. 1974, 18, 963-974. (4) Sundararaj, U.; Macosko, C. W. Macromolecules 1995, 28, 26472657. (5) Beck-Tan, N. C.; Tai, S. K.; Briber, R. M. Polymer 1996, 37, 35093519. (6) Boucher, E.; Folker, T. P.; Hervet, H.; Lodger, L.; Creton, C. Macromolecules 1996, 29, 774-782. (7) Norton, L.; Smigolova, V.; Pralle, M.; Unbenko, A.; Dai, K.; Kramer, E. J.; Hahn, S.; Bergland, C.; Peckoven, B. Macromolecules 1995, 28, 1999-2008. (8) Yin, Z.; Koulic, C.; Pagnoulle, C.; Jerome, R. Macromolecules 2001, 34, 5132-5139. (9) Gue´gan, P.; Macosko, C. W.; Ishizone, T.; Hirao, A.; Nakahama, S. Macromolecules 1994, 27, 4993-4997. (10) Nakayama, A.; Gue´gan, P.; Hirao, A.; Inoue, T.; Macoscko, C. W. ACS Polym. Prepr. 1993, 34 (2), 840-841. (11) Jiao, J.; Kramer, E. J.; Vos, S.; Moller, M.; Koning, C. Macromolecules 1999, 32, 6261-6269. (12) Schulze, J. S.; Cernohous, J.; Hirao, A.; Loger, T. P.; Macosko, C. W. Macromolecules 2000, 33, 1191-1198.

interface. Size exclusion chromatography (SEC) is a traditional technique that was used to measure the content of copolymer formed in reactive blending.9,13 In the case of experiments conducted under static conditions, the sensitivity is however too low for the technique to be useful.14 Recently, Macosko et al.15 reported that anthracene-labeled reactive PMMA chains could be used to monitor the progress of the interfacial reaction at a model flat interface by SEC with a fluorescence detector. This paper aims at investigating the influence of the molecular weight of the reactive precursors on the reaction occurring at a model flat interface. Anthracene-labeled PS chains end-capped by a primary amine (anth-PS-NH2) have been used to quantify the reaction progress at the interface of PS/PMMA sandwich assemblies, that is, bilayers of anth-PS-NH2 and PMMA end-capped by an anhydride (PMMA-anh). After annealing at 200°C for different periods of time, the copolymer formed has been analyzed by SEC-UV and the topology of the interface has been observed by TEM and AFM. Experimental Section Polymer Synthesis and Characterization. The characteristics of the polymers used in this study are listed in Table 1. Anhydride-terminated PMMA (PMMA-anh) was synthesized by atom transfer radical polymerization (ATRP),8 and the end functionality was determined by 1H NMR. Polystyrene labeled by anthracene and end-capped by a primary amine group (anthPS-NH2) was also synthesized by ATRP as reported elsewhere.16 Styrene was actually copolymerized with 10 wt % 3-isopropenylR,R-dimethylbenzyl isocyanate (m-TMI) in the bulk, with 1phenylethylene bromide as an initiator. The pendant isocyanate groups of the copolymer were reacted with 9-methyl(aminomethyl)anthracene with formation of anthracene multilabeled chains. The ω-bromide end-group was derivatized into a primary amine by reaction with sodium azide and further reduction with (13) Moon, B.; Hoye, T. R.; Macosko, C. W. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 2177-2185. (14) Lyu, S. P.; Cernhous, J. J.; Bates, F.; Macosko, C.W. Macromolecules 1999, 32, 106-110. (15) Schulze, J. S.; Cernohous, J.; Moon, M.; Loger, T. P.; Macosko, C. W. Macromolecules 2001, 34, 200-205. (16) Yin, Z.; Koulic, C.; Pagnoulle, C.; Je´roˆme, R. Macromol. Chem. Phys. 2002, 203, 2021-2028.

10.1021/la020614c CCC: $25.00 © 2003 American Chemical Society Published on Web 12/17/2002

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Table 1. Molecular Characteristics of the Polymers Used in This Study polymer

Mn (kg/mol)

Mw/Mn

PMMA-anh PMMA-anh anth-PS-NH2 anth-PS-NH2 PMMA PS

15 28 17 27 18 17

1.1 1.2 1.2 1.25 1.1 1.1

anth functa

amine functa

anh functa 0.94b 0.82c

3.8d 4.6d

0.91c 0.87c

a Functionality in moles per chain. b Based on NMR. c Based on SEC-UV. d Based on UV.

LiAlH4 in THF. The amino functionality of the anthracene-labeled PS-NH2 was estimated by SEC-UV (366 nm) analysis of the chains before and after coupling with an anhydride-terminated PMMA in THF. The molecular weight and molecular weight distribution of the functional polymers were analyzed by size exclusion chromatography (SEC) in THF at 40°C, using a Hewlett-Packard 1090 liquid chromatograph equipped with a dual HewlettPackard 1037A refractive index and UV detector. PMMA and PS standards were used for calibration. Melt Coupling Reaction at the Interface of Thin Films. A 5.0 wt % solution of PMMA-anh in toluene was spin-coated on a silicon wafer (1.2 × 1.4 cm2) as a ∼250 nm thick film. A 5 wt % solution of anth-PS-NH2 in a 87/13 (v/v) cylohexane/toluene mixture was spin-coated on top of the PMMA-anh layer, with the thickness being estimated at 670 nm by AFM. The sandwich assemblies were annealed at 200 °C in a vacuum oven for 20, 40, 60, and 120 min, respectively. One sample was however kept unannealed as a reference for the SEC-UV measurements. After annealing, the sandwiched films were dissolved in phenyl isocyanate-containing THF, and a solution of 80.0 µg/mL was analyzed by SEC (UV detector at 255 nm) in order to measure the conversion of anth-PS-NH2 into a PS-b-PMMA diblock. Atomic Force Microscopy (AFM). The PS top layer of the sandwiched films annealed for 60 min at 200 °C was dissolved by immersion in a 87/13 (v/v) cyclohexane/toluene mixture overnight. The topology of the released PMMA surface was examined by AFM (Digital Instruments Nanocope 3 atom force microscope) in the tapping mode with a silicon probe. Scan sizes were 1 µm × 1 µm, the scan angle was 90°, and the scan rate was 2 Hz. Transmission Electron Microscopy (TEM). Approximately 100 µm thick films of PS-NH2 and PMMA-anh were solvent cast from toluene solutions (5 wt %). They were deposited one on top of the other, heated at 120 °C for 5 min, and finally annealed at 200 °C in a vacuum oven for 60 min. After annealing, the samples were embedded in an epoxy resin, and ultrathin slides (ca. 50 nm) were ultramicrotomed perpendicularly to the film surface (Reichert-Jung Ultracut FC4 microtome). Samples were stained by RuO4 vapor for 30 min, and the phase morphology was observed with a Philips CM 100 transmission electron microscope. The PS phase was observed as the darker phase.

Results Whenever a reaction occurs at the interface of thin films of two mutually reactive polymers PA and PB, it is quite a problem to quantify the very small amount of the PA/PB copolymer formed. Moreover, the total amount of PA and PB is also small (e.g. less than 0.15 mg for a 1 µm thick film with a 1.5 cm2 surface area), which requires sensitive analytical techniques. Although (neutron or X-ray) scattering techniques can probe the atomic structure of the respective interfaces with high accuracy, they cannot discriminate the PA(PB) chains that have actually reacted from those ones left unreacted at the interface. In contrast, chromatographic methods have the capacity to tackle this analytical problem, provided that the detector is very responsive to the chains,13 which is the case for chains tagged with UV absorbing groups and analyzed by SEC with a UV detector (SEC-UV). Anthracene is known for

Figure 1. SEC traces of the reactive product of anth-PS-NH2 (Mn ) 17 kg/mol) with PMMA-anh (Mn ) 15 kg/mol) at 200 °C for different annealing times of the sandwich assembly.

Figure 2. Conversion of low and high molecular weight anthPS-NH2 into PS-b-PMMA copolymer.

sensitivity to UV and fluorescence detection.13,17 A previous study has shown that the extent of the copolymer formation at the interface of thin films of PS-NH2 (670 nm thick) and PMMA-anh (250 nm thick) coated on a 1.2 × 1.4 cm2 silicon wafer could be quantified as a result of the anthracene labeling of the PS-NH2 chains.16 The same technique was used in this work. Typical SEC traces of annealed bilayer samples of low molecular weight PMMAanh (15 kg/mol) and anth-PS-NH2 (17 kg/mol) are shown in Figure 1. Expectedly, the major part of anth-PS-NH2 remains unreacted under the static conditions used. The SEC traces have been deconvoluted by Gaussian fitting. The fraction of anth-PS-NH2 coupled with PMMA-anh observed at low elution volume has been quantified from the relative area of the two deconvoluted peaks. Figure 2 compares the conversion of anth-PS-NH2 of different molecular weights into a diblock as a function of time. The experimental data for five independent experiments are reproducible within the limits of the experimental bars shown in Figure 2. The low molecular weight chains [PMMA-anh (15 kg/mol)/PS-NH2 (17 kg/mol)] react more extensively than the longer chains [PMMA-anh (28 kg/ mol)/PS-NH2 (27 kg/mol)] for the same period of time. (17) Porouchani, R.; Graramszegi, L.; Nguyen, T. Q.; Hilborn, J. Macromol. Rapid Commun. 2000, 21, 837-840.

Reaction Progress at a PMMA/PS Interface

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Figure 3. AFM images of the interface of (a) nonreactive PMMA/PS, (b) low molecular weight PMMA-anh (15 kg/mol)/anthPS-NH2 (17 kg/mol), and (c) high molecular weight PMMA-anh (28 kg/mol)/anth-PS-NH2 (27 kg/mol). The top layer of PS has been washed away by a selective solvent (cyclohexane/toluene 87/13 v/v) after reaction at 200 °C for 60 min. The scale bar in height is 100 nm.

Figure 4. TEM observation of the PMMA/PS interface after 60 min at 200 °C: (a) nonreactive PMMA/PS interface; (b) PMMA-anh (15 kg/mol)/anth-PS-NH2 (17 kg/mol) interface; (c) PMMA-anh (28 kg/mol)/anth-PS-NH2 (27 kg/mol) interface. Scale bars are 500 nm. The dark PS phase has been stained by RuO4.

Figure 3 shows the topology of the nonreactive (Figure 3a) and reactive (Figure 3b and c) PMMA/PS interfaces observed by AFM. The interface remains flat in the nonreactive sample (Figure 3a), in contrast to the roughness of ∼100 nm in the low MW reactive sample (Figure 3b). Although the interface is also rough in the high molecular weight reactive sample, this characteristic feature is much less pronounced (Figure 3c). Although qualitative, these observations were confirmed for at least three independent samples. Figure 4 shows TEM micrographs of the nonreactive and reactive PMMA/PS interfaces. This interface, which is sharp and flat in the nonreactive sample (Figure 4a), becomes corrugated as result of the interfacial reaction. A polymer emulsion is formed in the interfacial region when the polymers are of a low MW (Figure 4b); this rough interface is ∼200 nm thick and thus larger than that observed by AFM (Figure 3b). The explanation might be found in the preparation method of the interface to be observed by AFM. Indeed, part of the diblock copolymer might have been dissolved together with the PS layer by the cyclohexane/toluene (87/13 v/v) mixture. The corrugated topology of the interface of the high MW reactive sample (Figure 4c) is intermediate between the ones of the nonreactive and the low MW reactive samples. Discussion Figure 2 shows that conversion of low molecular weight polystyrene after 20 min of reaction is higher compared to that of the high molecular weight chains. On the

assumption that the in-situ formed copolymer remains at the PS/PMMA interface, the coverage of the planar interface by the copolymer Σ (chain/nm2) can be estimated by eq 1:18

Σ ) hCFNAv/Mn

(1)

where h is the initial thickness of the anth-PS-NH2 layer determined by AFM, C is the conversion of anth-PS-NH2 into copolymer, F is the PS density (1.05 g/cm3),19 Mn is the number average molecular weight of anth-PS-NH2 and NAv is Avogadro’s number. Figure 5 shows the reaction time dependence of the interfacial coverage Σ for both the high and low molecular weight copolymers. Although the low MW copolymer is continuously formed at the polymer/ polymer interface at least for 2 h, conversion of the high MW precursors into diblocks levels off after ∼20 min. It must be noted that the interfacial coverage estimated by eq 1 (Figure 5) is higher than the maximum value (Σ0), which can be extracted from the lamellar spacing of the symmetric PMMA-b-PS diblock copolymer.18 Σ0 ) 0.174 and 0.145 chain/nm2 for Mn ) 32 and 55 kg/mol, respectively. The same observation was reported by Schulze et al.15 for the interfacial reaction of PMMA-anh (31 kg/mol) and PS-NH2 (22 kg/mol) diluted in the parent homopolymers. Moreover, the interfacial coverage (Σ ) 0.46 chains/ (18) Macosko, C. W.; Gue´gan, P.; Khandpur, A. K.; Nakayama, A.; Mare´chal, P.; Inoue, T. Macromolecules 1996, 29, 5590-5598. (19) Brandrup, J.; Immergut, E. H. Polymer Handbook, 3rd; John Wiley and Sons, Inc.: Canada, 1989; p V77.

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Figure 5. Coverage (Σ) of the PMMA/PS interface in the case of low and high molecular weight reactive precursors.

nm2) after annealing for 4 h at 180 °C was close to the value noted in this work after 2 h at 200 °C (Σ ) 0.41 chains/nm2). Because Σ is higher than Σ0 (e.g. by an order of magnitude for the low MW diblock after 2 h), the copolymer formed at the interface is not merely accumulated as an interfacial monolayer. At least part of the copolymer leaves the interface and/or modifies it into an interphase, which is beneficial to the progress of the coupling reaction. This interfacial modification is illustrated by Figures 3 and 4, particularly when the reactive precursors are of low molecular weight. SEC data (Figure 2) show that the instantaneous rate of the interfacial reaction is higher from the very beginning when the reactive chains are shorter. Although the diffusion coefficient depends on the chain length, it does not control the kinetics of the interfacial reaction. Indeed, Figure 4b shows that the interfacial roughness extends over ∼0.2 µm after 1 h of reaction. Russell et al.20 measured the diffusion coefficient D* of PMMA and PS in symmetric PS-b-PMMA copolymers. D*, which is smaller for PMMA, was found to be 10-12 cm2/s for PMMA with Mn ) 29 000 into a 42 000/42 000 diblock at 185 °C. One can estimate that the low molecular weight PMMA (Mn ) 15 000) used in this study covers more than 0.6 µm within 60 min even below 200 °C. Therefore, at least for the first hour, the polymer diffusion to the interface is not the rate determining step of the interfacial reaction. The efficient collision of the mutually reactive groups at the interface would actually control the kinetics of the interfacial reaction. The χN product (where χ is the PS/PMMA segmental interaction parameter and N the degree of polymerization) directly depends on molecular weight, being ∼6 and 10, for the two series of reactive chains analyzed in this study (χ is 0.036 at 200 °C according to eq 2),21 and the critical value of χN is ∼2.22

χ ) 0.028 + 3.9/T

(2)

Broseta et al.22 proposed an analytical expression for the interfacial width (a) between two immiscible polymers of finite molecular weight (eq 3)

[

a ) (a)H 1 + 1n2

(

)]

1 1 + χNA χNB

(3)

where Ni is the degree of polymerization of component i (20) Green, P. T.; Russell, T. P.; Je´roˆme, R.; Granville, M. Macromolecules 1988, 21, 3266-3273. (21) Russell, T. P.; Hjelm, P. R.; Seeger, P. A. Macromolecules 1990, 23, 890-893. (22) Broseta, D.; Friddricken, G.; Helfand, E.; Leibler, L. Macromolecules 1990, 23, 132-139.

and (a)H is the width of the interface in the strong segregation limit. This equation shows that the initial interfacial thickness is as high as the molecular weight of the reactive precursors is low. The interfacial thickness in the low molecular weight sandwich is 6.8 times larger than that of the high molecular weight counterpart at 200 °C. Because the interface is the locus of reaction, the reaction progress is more limited at the sharp interface of the high molecular weight bilayer than at the broader interface formed by the shorter reactive chains at the same reaction time. Moreover, the concentration of the reactive groups in the low molecular weight sample is 2 times higher compared to the high molecular weight one. The copolymer formed at the interface is expected to decrease the interfacial tension between the two homopolymers. Whenever this tension becomes very small, instability and roughening of the interface is observed.11,14 Self-consistent mean field theory11,23 predicts that the interfacial tension is zero at a critical chemical potential or interfacial excess of diblock copolymer and that this critical value dramatically decreases with the molecular weight of the copolymer. Therefore, the low molecular weight copolymer has a greater intrinsic ability to reduce the interfacial tension at a given chemical potential. Moreover, SEC analysis shows that more copolymer is formed at the interface at the same reaction time when the reactive chains are short, which accounts for a faster drop in interfacial tension and the very pronounced instability of the interface (Figures 3 and 4), which is favorable to the progress of the interfacial reaction. After ∼40 min of reaction, conversion of high molecular weight PS-NH2 into diblock copolymer levels off, which is not the case of the short PS-NH2 chains. This observation has to be related to the behavior of the copolymer at the interface,24,25 which depends on molecular weight. Indeed, the interfacial coverage by the copolymer increases with reaction time and an energy barrier to the diffusion of the reactive chains to the interface, as well. This energy barrier, which results from the entropy loss associated with the localization of the chain ends at the interface and the chain stretching, increases with molecular weight. High molecular weight diblock at the interface forms a high energy barrier, consistent with the conversion, which rapidly levels off. It must also be noted that Mn of the short reactive chains (Mn PMMA-anh ) 15 000, and Mn anth-PS-NH2 ) 17 000) is close to the critical molecular weight for entanglements (Me), that is, 10 000 for PMMA and 13 000 for PS.26 The excess of low molecular weight copolymer accumulated at the interface can thus leave the interface easily, which allows for the progress of the interfacial reaction (Figure 2). The interfacial region is deeply modified in the low molecular weight samples, which look like a PS/PMMA/PS-b-PMMA emulsion (Figure 4b). The clear dependence of the interfacial roughness on the molecular weight of the reactive precursors is illustrated by Figures 3 and 4. Lyu et al.14 also observed roughness at the interface formed by PMMA-anh (Mn ) 29 000) and shorter PS-NH2 (Mn ) 17 000). The roughness (500 nm) was however an order of magnitude higher than that in this work for the low molecular weight sample, (23) Shull, K. R.; Kramer, E. J. Macromolecules 1990, 23, 47694779. (24) O’Shaughnessy, B.; Sawhney, U. Macromolecules 1996, 29, 7230-7239. (25) Fredrickson, G. H.; Milner, S. Macromolecules 1996, 29, 73867390. (26) Fetters, L. J.; Lohse, D. L.; Richter, D.; Witten, T. A.; Zirkel, A. Macromolecules 1994, 27, 4649-4647.

Reaction Progress at a PMMA/PS Interface

more likely because an asymmetric diblock copolymer was formed, which is more prone to leave the interface and to migrate to the PMMA phase compared to the symmetric diblock.27 In contrast, the symmetric diblock formed by PMMA-anh (Mn ) 28 000) and anth-PS-NH2 (Mn ) 27 000) remains localized at the interface, it prevents rapidly the interfacial reaction from progressing, and it accounts for the sharpness of the interface. Even under conditions of mechanical blending, this copolymer remains at the interface and rapidly blocks the interfacial reaction.8 So, the fate of the copolymer formed at the interface depends not only on the absolute molecular weight of the individual blocks but also on their ratio (copolymer structure). Conclusions Polystyrene chains end-capped by a primary amine and labeled by anthracene have been successfully used to measure the progress of the reaction at the interface with PMMA chains end-capped by an anhydride. Bilayer samples of anth-PS-NH2 and PMMA-anh have been annealed at 200 °C, and conversion of anth-PS-NH2 into diblock copolymer has been monitored by SEC with a UV detector. The interfacial reaction is faster when the reactive chains are shorter (17 000 vs 27 000). In the case of the higher molecular weight chains, their conversion into diblocks levels off after ∼40 min. This dependence of the reaction rate on the molecular weight of the reactive precursors may be related to the interaction parameter, χN, which is low (χN ) 6) for the shorter chains and (27) Leibler, L. Makromol. Chem. Macromol. Symp. 1998, 16, 1-17.

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consistent with a large interfacial thickness, favorable to the reaction progress. The larger amount of the low molecular weight copolymer accordingly formed results in a rapid decrease of the interfacial tension, which results in the roughening of the interface and an increase of the interfacial area. When the longer chains are concerned, χN ) 10, the interface is sharp, which accounts for a slower conversion into diblocks. Moreover, the symmetric diblock copolymer of higher molecular weight formed at the interface does not leave the interface, consistent with an interfacial reaction rate that tends to zero. This situation is not observed for the low molecular weight chains that form diblocks unable to block the interface. Rather, a kind of polymer emulsion (TEM observations) is formed, which contributes to the roughening of the interface. This interfacial modification is slower than the diffusion of the reactive chains, which indicates that the interfacial reaction is not controlled by diffusion but rather by the kinetics of reaction of the anhydride and amine end-groups. Acknowledgment. The authors are grateful to the “Services Fe´de´raux des Affaires Scientifiques Techniques et Culturelles” for general support to CERM and for a fellowship to Z.Y. in the frame of the “PAI-5/03: Supramolecular Chemistry and Supramolecular Catalysis”. C.K. is “Aspirant” by the “Fonds National de la Recherche Scientifique” (F.N.R.S.). The authors gratefully acknowledge N. Willet and G. Chapelle for assistance in the AFM observations. LA020614C