Nanostructured Polyamide by Reactive Blending. 1. Effect of the

Macromolecules 2004, 37, 3459-3469

3459

Nanostructured Polyamide by Reactive Blending. 1. Effect of the Reactive Diblock Composition C. Koulic† and R. Je´ roˆ me* Center for Education and Research on Macromolecules (CERM), University of Lie` ge (ULg), B6, Sart Tilman, B-4000 Lie` ge, Belgium Received November 7, 2003; Revised Manuscript Received February 13, 2004

ABSTRACT: Reactive blending of phthalic anhydride end-capped polystyrene-b-polyisoprene diblock (PSb-PIP-anh) with 80 wt % of polyamide 12 (PA12) results in the very rapid formation of a PS-b-PIP-b-PA triblock copolymer, which self-assembles with formation of characteristic nanoobjects, within the polyamide matrix. For instance, a vesicular nanostructure is formed in the particular case of a symmetric, lamellarforming diblock copolymer. This morphology actually complies with the lower curvature possible for ABC lamellae diluted in a continuous C phase under shear. In contrast, when the diblock composition is typically asymmetric (at constant molecular weight), vesicles disappear in favor of a core-shell morphology with a cucumber-like suborganization. This spontaneous nanostructuration of the PA12 matrix is quite general. Indeed substitution of an amorphous primary amine end-capped styrene/acrylonitrile random copolymer (SAN-NH2) for PA12 results in exactly the same phase morphology upon reactive blending with PS-bPIP-anh.

I. Introduction Rubber toughening of thermoplastics is one of the most important applications in the rapidly expanding field of polymer blends. The morphology of rubbermodified thermoplastics is however the key issue for a successful toughening process.1 In this respect, micellar structures, such as vesicles with a rubbery shell, have been recently proposed as efficient toughening agents.2c-e This strategy has been implemented by dissolving an amphiphilic block copolymer in a reactive liquid precursor of the matrix to be toughened. The copolymer spontaneously forms micellar nanostructures in relation to its molecular composition and structure. In an example reported by Bates et al.,2c an epoxidized polyisoprene-b-polybutadiene copolymer dissolved in diglycidyl ether bisphenol A, spontaneously self-assembles into spherical micelles or vesicles depending on its composition. The epoxy resin is then hardened upon addition of 4,4′-methylenedianiline and the micellar structure is accordingly frozen in. This strategy is general whenever one constitutive block of the diblock is miscible with the matrix and the other one is an immiscible rubbery block.2 In the specific case of vesicular micelles, a drastic increase in fracture toughness is observed at low copolymer content (ca. 2.5 wt %), with however a substantial decrease in modulus.2e More recently, Pascault et al.3 reported on the formation of transparent nanostructured thermosets by blending a poly(styrene)-b-poly(butadiene)-b-poly(methyl methacrylate) triblock copolymer with the liquid precursors of an epoxy resin followed by the resin curing. Again, the major requirement for the successful nanostructuration of the resin is an intimate intermixing with the PMMA block of the copolymer until the epoxy is cured. In the same vein, Meyer et al.4 paid attention to a comparable approach for the toughening of brittle thermoplastics, * To whom correspondence should be addressed: [email protected]. † Current address: Atofina Research S.A. Zone Industrielle C, B-7181 Feluy, Belgium.

such as poly(methyl methacrylate) (PMMA) and polystyrene (PS). They successfully preformed micelles of hydrogenated polybutadiene-b-poly(methyl acrylate) in MMA and styrene, respectively, that were frozen in upon polymerization of the continuous monomer phase. Although the absence of macrophase separation was confirmed by SAXS, no mechanical data were reported. Even though micellar nanostructures are promising toughening agents for brittle thermoplastics and thermosets, the two-step strategy, based on the polymerization (or cure) of the liquid precursor of the matrix after the nanophase morphology is established, is not straightforward enough to be viable on a large scale. Moreover, this strategy can only be conveniently implemented for matrix prepared under mild conditions. Recently, we have reported on the formation and spontaneous selfassembly of a block copolymer in the preformed matrix by reactive blending. In this example, an anhydride endfunctional symmetric PS-b-PIP diblock copolymer is melt reacted with a primary amine end-capped polyamide matrix. Within 1-2 min, vesicular objects with a three-layer envelope are formed by the “in situ” generated PS-b-PIP-b-polyamide triblock.5 This paper aims to investigate the effect of the molecular structure of the triblock on the final nanostructuration of the matrix. II. Experimental Section Materials. Styrene (Sty, Aldrich, 99+%), isoprene (IP, Janssen Chimica, 99%), methyl methacrylate (MMA, Aldrich, 99+%), and ethylene oxide (EO, Meisser) were dried over CaH2 (Aldrich), distilled, and stored under Ar at -20 °C. Cyclohexane was dried over CaH2 and distilled before use. Tetrahydrofuran (THF) was dried over Na/benzophenone and distilled before use. sec-BuLi (Aldrich, 0.22 M in hexane), n-BuLi (Aldrich, 1 M in hexane), dibutylmagnesium (Aldrich, 1 M in hexane), triethylaluminum (AlEt3, Aldrich, 1 M in toluene) were used as received, diluted if necessary, and stored under Ar at -20 °C. Diphenylethylene (DPE, Aldrich, 99%) was distilled over sec-BuLi and stored under Ar at -20 °C. Trimellitic anhydride chloride (TAC, Fluka, 99%) was stored and handled in a glovebox. Fluorenyllithium (1 M in toluene) was prepared by reacting sec-BuLi with fluorene (Aldrich) in

10.1021/ma035674j CCC: $27.50 © 2004 American Chemical Society Published on Web 04/06/2004

3460 Koulic and Je´roˆme

Macromolecules, Vol. 37, No. 9, 2004

Scheme 1. Synthetic Pathway for the ABC Triblock Copolymer

toluene under Ar. Liquids were transferred under argon (Ar, Meisser) with glass syringes or stainless steel capillaries through rubber septa. Polyamide 12 was PA12 Rilsan AECHV0 from ATOFINA (Mn ∼ 20 000 g/mol, Mw/Mn ) 1.8, [NH2] ) 25µequiv/g). Copolymers Synthesis. PS-b-PIP-anh Diblocks. The classical sequential anionic polymerization of styrene and isoprene was carried out in a previously flame-dried glass reactor, under argon. The polymerization reaction was terminated either by degassed methanol (unreactive diblocks) or by ethylene oxide followed by a HCl/methanol/2-propanol mixture (5/70/25 in volume) (hydroxyl-terminated diblocks).7 In a second step, the reactive PS-b-PIP-OH diblocks were reacted with a large excess of trimellitic anhydride chloride (> 30 equiv), in dry THF, to convert the hydroxyl end groups into phthalic anhydride ones as reported in a previous communication.8 PS-b-PIP-b-PMMA and PMMA.9. Polystyrene-b-polyisoprene-b-poly(methyl methacrylate) (PS-b-PIP-b-PMMA) triblock was synthesized by sequential living anionic polymerization of styrene, isoprene and methyl methacrylate, respectively (Scheme 1). The styrene polymerization was first initiated by an R-methylstyrene/sec-butyllithium adduct in THF at -78 °C under argon and in the presence of LiCl (10 equiv with respect to the initiator). After a few minutes, the monomer conversion was complete, an aliquot of the living polystryryl chains was picked out for characterization of the first PS block. Isoprene was then added and allowed to react at -20 °C overnight. After an aliquot of the living diblock chains was isolated for analytical purposes, the living diblock chains were end-capped by reaction with a slight excess (1.5 equiv) of freshly distilled 1,1-diphenylethylene (DPE). The endcapping reaction was carried out at -20 °C for 30 min and followed by addition of MMA at -78 °C. The polymerization was complete after 30 min, and the triblock copolymer was terminated and isolated by precipitation into methanol. The copolymer was reprecipitated three times to remove LiCl completely. SEC analysis (Figure 1A) of the triblock copolymer showed contamination by diblock chains, which were however removed by Soxhlet extraction with cyclohexane, a solvent for the PS-b-PIP diblock. SEC analysis confirmed the efficiency of this purification method (Figure 1B). Poly(methyl methacrylate) (PMMA) was synthesized by anionic polymerization under the same conditions as the PMMA block of the triblock.8 Characterization. Size exclusion chromatography (SEC) was performed in THF (flow rate of 1 mL/min) at 40 °C using a HP1090 liquid chromatograph equipped with a HP1037A dual refractive index/UV detector (columns HP PL gel 5µm, 105, 104, 103, 100 Å) calibrated with polystyrene standards (Polymer Labs). 1H NMR spectra were recorded with a Bruker AM 400 MHz spectrometer at 25 °C. Infrared spectra of polymer films deposited on a NaCl disk were recorded with a Perkin-Elmer 1720X spectrometer. Differential scanning calorimetry (DSC) was carried out with a Dupont 910 DSC Thermal analyzer calibrated with indium. The glass transition temperature was measured after cooling the sample down to -100 °C and heating it at a 10 °C/min rate. (Co)polymer Blending. Melt blending was carried out in a 5 cm3 DSM microextruder at 220 °C (in the case of PA12 containing blends) or 190 °C (for PMMA containing blends) under nitrogen at 200 rpm for 2 min. The matrix was first added followed by 20 wt % of diblock or triblock copolymer. In the case of preblending of part of the blend constituents, they

Figure 1. SEC chromatograms for the PS-b-PIP-b-PMMA triblock (SIM-1, Table 2) before (A) and after (B) purification. Scheme 2. Sketch of the Directions of Ultramicrotomy of the Samples Observed by TEM

were coprecipitated in methanol from THF followed by filtration and drying at room temperature in a vacuum oven. This two-step technique made the feeding of the miniextruder by the blend constituents easier. Transmission Electron Microscopy (TEM). The phase morphology was observed with a Philips CM100 transmission electron microscope. A Reichert-Jung ultracryomicrotome equipped with a diamond knife was used to prepare ultrathin samples at -78 °C. The PIP phase was selectively stained by OsO4. The preferred orientations for TEM observation are shown in Scheme 2.

III. Results and Discussion Previous studies10 emphasized that the kinetics of interfacial reaction of immiscible polymers in the melt has a decisive effect on the phase morphology. When all the chains are reactive and the mutually reactive groups are used in stoichiometric amounts, the reaction can go to completion in the case of chains of relatively low molecular weight and high mutual reactivity (i.e., high reaction rate). Because the coupling of PS-b-PIP diblock to PA12 must be as complete as possible, the highly reactive primary amine (PA12)/anhydride (PSb-PIP) pair was considered in this study. Synthesis of the Reactive Copolymers. Living anionic polymerization was selected to synthesize reactive diblock copolymers and model triblocks, because the molecular characteristics can be fully controlled, which is essential to trigger the formation of well-defined nanostructures in the polymer matrix by the in situ selfassembly of the block copolymers. Among other advantages, the microstructure of the PIP block can be tuned merely by the solvent polarity. In this respect, PIP with a dominantly cis-1,4 microstructure has been prepared in this study, to favor a low glass transition temperature (Tg ∼ -65 °C) which is a requirement for the rubber toughening of thermoplastics. Figure 2 confirms the expected low Tg as result of polymerization in an apolar solvent (Scheme 3). A second stringent requirement for the success of this study is the quantitative anhydride end-functionalization of the PS-b-PIP diblock for it to


Figure 2. DSC of a PS-b-PIP-anh reactive diblock copolymer (R-4, Table 1).

be converted as completely as possible in the triblock which is expected to self-assemble in the PA12 matrix. For this purpose, a two-step strategy has been considered, i.e., termination of the living lithium polyisoprenyl block by reaction with an excess of ethylene oxide (EO). Only one EO unit is added under these conditions (Li counterion), which leads to the hydroxyl-terminated diblock (PS-b-PIP-OH). The hydroxyl end group is then reacted with a large excess of trimellitic anhydride chloride (TAC) in dry THF in order to derivatize the endchain into the highly reactive phthalic anhydride.5,8 Attention has been paid to the diblock purification, because any unbound anhydride can consume part of the primary amine end groups of PA12. The diblock copolymer has therefore been precipitated from THF

Nanostructured Polyamides 3461

into methanol, followed by washing with methanol (a good solvent for TAC). This treatment has been repeated until no TAC is detected by FTIR analysis of the solid content of the methanol solution. The phthalic anhydride end group of the diblock has been confirmed by FTIR (see Supporting Information) and 400 MHz 1H NMR (Figure 3), respectively. In addition to 1H NMR analysis, the end-functionality has been quantified by a SEC-UV method reported by Macosko et al.11 For this purpose, an excess of amino end-functionalized PS tagged with a UV probe (anthracene) has been reacted with the diblock, so making the actually reactive diblock chains detectable by SEC-UV (Scheme 4). Synthesis of this anthracene labeled PS has been published elsewhere.12 Prior to reaction with anth-PS-NH2, the PS-b-PIP-anh diblock has been annealed at 210 °C for converting any diacid released during isolation and handling of the diblock, into anhydride.9 The diacid is observed at 1732 cm-1 by FTIR (see Supporting Information), at which wavelength the ester linkage is also expected to absorb. After coupling of the diblock with the labeled PS, two elution peaks are observed by SECUV (Figure 4). The excess anth-PS-NH2 is eluted at the lower volume in contrast to the PS chains that have reacted with the diblock chains as consequence of their effective end reactivity. From the relative area of the deconvoluted SEC peaks, the actual end-functionalization of the PS-b-PIP-anh diblock is systematically found to be higher than 90%. Symmetric Reactive Block Copolymer.5 The symmetric PS-b-PIP-anh diblock (R-1 in Table 1; 47 wt %

Scheme 3. Synthetic Pathway for the PS-b-PIP-anh Reactive Diblock



Figure 3. 400MHz 1H NMR of reactive PS-b-PIP-anh diblock (R-2, Table 1) in CDCl3. Scheme 4. anth-PS-NH2 (S-UV; Table 3) Used as a Coupling Agent for the Quantitative Analysis of the Anhydride End Group of Diblocks

PS) has been first melt reacted with 80% PA-12. For sake of comparison, Figure 5A shows a TEM micrograph for the nonreactive blending of PA12 with the NR-1 diblock (Table 1). As expected for nonmiscible polymer blends, a coarse and heterogeneous macrophase separation is observed. Indeed, neither PS nor PIP is miscible with PA12. It is however interesting to note that the lamellar mesophases typical of symmetric diblocks can be observed in the PS-b-PIP microdomains as result of the selective staining of PIP by osmium tetroxide. The thickness of the constitutive PS lamellae is estimated at 7 nm from the TEM micrograph. When the endreactive diblock is substituted for the nonreactive counterpart, a drastic drop in the particle size is observed which is the expected consequence for the formation of a triblock with a block identical to the matrix chains (Figure 5B). Observed at high enough magnification and in the direction perpendicular to the extrusion (Scheme 2), the nanophases formed by the in situ generated triblock exhibit a general core-shell morphology which deserves, however, a more careful analysis (Figure 6). Indeed, the shell has a three-layer substructure as revealed by staining by OsO4. Two dark

Figure 4. SEC-UV chromatogram of the reaction product of PS-b-PIP-anh (R1, Table 1) with an excess of anth-PS-NH2 (S-UV, Table 3) (1/8 molar ratio).

outer layers are clearly seen that have to be assigned to PIP. PS is thought to form the intermediate layer because of a thickness (ca. 8 nm) which is consistent with the size of the lamellar morphology of the PS-bPIP diblock itself. According to this picture, PA12 should be the core-forming component leading to vesicular-type nanostructures. Indeed, the in situ formed PS-b-PIP-bPA triblock copolymer, with constitutive blocks of comparable molecular weight (MnPA12 ∼ 20 000 g/mol), tends to self-assemble into lamellar mesophases. However, continuous lamellar mesophases cannot persist in the dilute regime under shear. Under these conditions, the more stable lamellar morphology consists of bilayered vesicles with a large diameter compared to the wall thickness, thus lamellar sheets with a low curvature.13 The vesicle wall is nothing but a double layer of PS-bPIP-b-PA triblock copolymer lamellae as illustrated by



Table 1. Reactive Diblock Copolymers and Their Nonreactive Counterparts Synthesized in This Work code

type

Mn (10-3 g/mol)

R-1 R-2 R-3 R-4 R-5 R-6 NR-1 NR-2 NR-3 NR-4 NR-5 NR-6 R-7 NR-7

PS-b-PIP-anh PS-b-PIP-anh PS-b-PIP-anh PS-b-PIP-anh PS-b-PIP-anh PS-b-PIP-anh PS-b-PIP PS-b-PIP PS-b-PIP PS-b-PIP PS-b-PIP PS-b-PIP PIP-b-PS-anh PIP-b-PS

16 (PS),a 18 (PIP)b 6 (PS),a 30 (PIP)b 22 (PS), 14 (PIP)b 26 (PS), 10 (PIP)b 29 (PS), 7 (PIP)b 31 (PS),a 7 (PIP)b 16 (PS),a 18 (PIP)b 6 (PS),a 30 (PIP)b 22 (PS), 14 (PIP)b 26 (PS), 10 (PIP)b 29 (PS), 7 (PIP)b 31 (PS),a 7 (PIP)b 15 (PS),a 16 (PIP)b 15 (PS),a 16 (PIP)b

a

PS f Mw/Mna (wt %) (%)c 1.05 1.04 1.03 1.02 1.04 1.03 1.05 1.04 1.03 1.02 1.04 1.03 1.05 1.05

47 17 61 72 80 82 47 17 61 72 80 82 48 48

93 96 92 91 93 92

94

SEC with PS standards. b 400 MHz 1H NMR. c Reference 11.

the scheme shown in Figure 7. As result of the extrusion process, the nanostructures could be preferentially oriented by the flow field parallel to the extrusion direction. This expectation has been assessed by TEM observation parallel to the extrudate (Figure 8 and Scheme 2). Anisotropic nanophases are indeed observed, which look like vesicular cyclinders. This type of transverse orientation has already been observed by Lodge et al.14 in the case of extrusion of a highly concentrated solution of lamellae-forming block copolymer. Effect of the End Group Functionnality. Changing the final composition of the blend is thought to be a tool for tuning the nanostructuration of the ABC/C blends. In this respect, a direct way would consist in restricting the progress of the interfacial reaction, such that AB diblock coexists with the ABC triblock. To validate this strategy, the reactive diblock was premixed with a known amount of the unreacted counterpart. Therefore, PA12 was melt blended with a 50/50 (w/w) premixture of reactive PS-b-PIP-anh and the nonreac-

tive precursor (R-1/NR-1, Table 1). Figure 9 shows the onionlike morphology,15 which results from the selective location and self-organization of the (nonreactive) diblock copolymer within the interior of the former PS-b-PIPb-PA vesicles in the PA12 matrix. This observation illustrates the possible tuning of the vesicular morphology of an ABC/C binary blend by the addition of a third component. Effect of the Location of the Reactive Group. According to the previous experiments, an ABC triblock with approximately the same length for the constitutive blocks forms easily vesicular nanoobjects when diluted in a C matrix under shear, because of the low curvature imposed to the lamellae. In these experiments, the reactive group of the symmetric PS-b-PIP diblock was attached at the end of the PIP block. Because the composition of the triblock does not change if the reactive group is attached as an end group to PS rather than to PIP, this modification is not expected to change the nanostructuration of the matrix. To confirm this prediction, a symmetric anh-PS-b-PIP diblock has been synthesized by polymerizing isoprene first and then styrene followed by the same reaction as shown in Scheme 1. The reactive blending of 80 wt % PA12 with this reactive symmetric diblock (R-7; Table 1) leads again to ellipsoidal nanoobjects, with PIP localized in the shell as emphasized by staining with OsO4. This observation is consistent with a shell consisting of two layers of triblock lamellae (PA-PS-PIP/PIP-PS-PA) with PIP in the central position. The transition in contrast from PIP (black) to PS (gray) to PA (white) is however less suitable for the direct observation of the internal structure of the shell than the previous transition from PA to PIP and finally to PS, thus with two easily detectable dark PIP sublayers. Nevertheless, the nanoobjects have the same size and shape whatever the triblock (ABC in Figure 6 vs BAC in Figure 10). Moreover, the lamellar morphology in the bulk (not

Figure 5. TEM micrographs for the PA12/PS-b-PIP (R1, Table 1) blend (A) and the PA12/PS-b-PIP-anh (NR-1, Table 1) blend (B).



Figure 6. TEM micrographs for the PA12/PS-b-PIP-anh (R-1, Table 1) blend at increasing magnifications.

Figure 7. Sketch for the internal structure of the vesicular shell.

shown) is also the same, and these lamellae are now the key constituent of the nanoobjects formed with the same curvature under the same conditions. Although the nanostructuration remains of the vesicular type, it may not be precluded that a change in the vesicle

substructure can affect the macroscopic properties of the blends. Modeling of the Reactive System. A mixture of a PS-b-PIP-b-PA triblock copolymer within a polyamide matrix (ABC/C), is the expected outcome of the reactive



Figure 8. TEM micrographs for the PA12/PS-b-PIP-anh (R-1, Table 1) blend observed in the direction of the extrudate (direction B in Scheme 2) at different magnifications.

Figure 9. TEM micrograph for the PA12/(PS-b-PIP-anh (R1, Table 1)/PS-b-PIP (NR-1, Table 1)) blend with an onionlike morphology.

blending implemented in this work and thus responsible for the observed nanostructuration. To validate once and for all this statement, the melt blending of a preformed ABC triblock with a C matrix has been considered in the dry-brush regime (as was the case for the reactive blending). However, the controlled synthesis of a triblock copolymer with a polyamide block is quite a problem, which explains why the polyamide block has been substituted by poly(methyl methacrylate) (PMMA), readily prepared by anionic polymerization under the conditions mentioned in Scheme 1. A PS-b-PIP-b-PMMA triblock (SIM-1, Table 2) with quite similar molecular parameters as the in situ formed PS-b-PIP-b-PA triblock has thus been synthesized and melt blended with anionically synthesized PMMA (M-1, Table 2) of the same length as the PMMA block of the triblock. Figure 11 shows TEM micrographs for a binary blend with 20 wt % of triblock. Nanovesicles (with diameter up to 100 nm) are clearly observed, which are reminiscent of the

Figure 10. TEM micrograph for the PA12/PIP-b-PS-anh (R7, Table 1) blend with an inverse vesicular morphology. Table 2. Model Triblock Copolymer and Homopolymers code SIM-1 M-1 S-1 a

type

Mn (10-3 g/mol) (PS),a

PS-b-PIP-b-PMMA

17

PMMA PS

16a

(PIP),b

18 16 (PMMA)b

17a

Mw/Mn1 1.1 1.09 1.02

SEC with PS standards. b 400 MHz 1H NMR after purification.

morphological features reported in the case of reactive blending with PA12 instead of PMMA. Compared to Figure 6, the nanostructuration observed in Figure 11A



Figure 12. TEM micrograph for the bulk morphology of the PS-b-PIP-b-PMMA triblock (SIM-1, Table 2). PIP is seen black after staining with OsO4, and PMMA is the whiter phase as result of thinning by the electron beam.

Figure 11. TEM micrographs for the PMMA (M-1, Table 2)/ PS-b-PIP-b-PMMA (SIM-1, Table 2) blend before (A) and after (B) annealing (30 min, at 190 °C).

is however less ordered and homogeneous throughout the sample. However, annealing at 190 °C for 30 min is enough to convert ill-defined lamellar micelles into wellshaped nanovesicles and to restore the supramolecular organization directly achieved by reactive blending (Figure 11B). The symmetry in volume in the block copolymer, which dictates a bulk lamellar morphology (Figure 12), is thus the main driving force toward the vesicular organization in the dilute regime under shear. It must be noted that the same nanostructuration has been observed when homopolystyrene (S-1, Table 2) is the matrix instead of PMMA for the same triblock in the dry-brush regime (Figure 13). A more detailed analysis of the morphology imparted by an ABC copolymer to a C matrix is under current investigation and will be reported elsewhere.16 Asymmetric Reactive Copolymer. The effect of the PS volume fraction of the reactive PS-b-PIP-anh diblock

Figure 13. TEM micrograph of the PS (S-1, Table 2)/PS-bPIP-b-PMMA (SIM-1, Table 2) blend.

copolymer on the PA12 nanostructuration has been studied by reactive blending of an asymmetric diblock precursor with the polyamide, while keeping the total molecular weight of the diblock constant (35 000-40 000 g/mol). A PIP-rich diblock copolymer (R-2, Table 1, 17 wt % PS) has been first blended with PA12. Upon staining with OsO4, dispersion of black nanodomains is observed, which are thus basically PIP containing more likely PS inclusions. This expected substructuration of the PIP nanophases is not detectable by TEM because of the low PS content (Figure 14). This technical problem should be alleviated by inverting the composition of the diblock (R-6, Table 1, 82 wt % PS). The internal structure of the dispersed nanophases is then clearly observed by TEM at high magnification (Figure 15). The substantial increase in the PS volume fraction prevents vesicles from being formed and confirms, if necessary, that a lamellar-forming system is the key condition for the development of a vesicular nanostruc-



Figure 14. TEM micrograph for the PA12/PS-b-PIP-anh (R2, Table 1) blend. Figure 16. TEM micrograph for the PA12/(PS-b-PIP-anh (R6, Table 1)/PS-b-PIP (NR-6, Table 1)) blend.

Figure 15. TEM micrographs for the PA12/PS-b-PIP-anh (R6, Table 1) blend.

turation. So, when there is a phase transition from lamellae to cylinders (or spheres) for the neat AB diblock

precursor, there is a transition from a vesicular to a core/ shell-like morphology. Indeed, a PIP shell is clearly observed after staining, which isolates PS from the PA matrix. Substructuration of the PS core is however observed, which is reminiscent of a slice of cucumber, that we will designate as a “cucumber-like” morphology afterward. One explanation for the dispersion of PIP containing nanophases in the PS core of the core-shell objects, might be accumulation and phase separation of some unreacted diblock copolymer. To confirm this assumption, PA12 has been reactive blended with a 50/ 50 (w/w) premixture of the reactive and nonreactive asymmetric diblock (R-6/NR-6, Figure 16). The nonreactive diblock is obviously accommodated in the PS core of core-shell nanodomains, whose the size has accordingly increased. Because of the bimodal size distribution of the dispersed domains, it is not clear yet whether the two populations are distinct particles that coexist or identical particles far from equilibrium. In the former case, it might be supposed that the small cucumberlike particles consist essentially of triblock, whereas the larger particles result from the phase separation of the unreacted diblock restricted in size by the triblock that forms a stabilizing envelope. More investigation is needed to clear up this issue. TEM observation in the transversal direction shows exactly the same cucumberlike nanostructures as in Figure 15, which are therefore isotropic. A Universal Strategy? Most of this work has been carried out with a polyamide matrix, because of intrinsic amino end functionality and a toughness/stiffness balance that could be improved by a rubber-containing additive, i.e., the triblock formed in situ. Having in mind a general toughening strategy, it is of prime importance to demonstrate that the nanostructuration of PA12 can be extended to other (reactive) thermoplastics that would deserve toughening improvement. For this purpose, a glassy matrix, i.e., a styrene and acrylonitrile copolymer (SAN), has been selected because a toughened version is available (ABS resins) that might serve



Figure 17. TEM micrographs for the SAN-1/PS-b-PIP-anh (R-6, Table 1) (80/20) blend (A) and the PA12/PS-b-PIP-anh (R-6, Table 1) blend (B) for the sake of comparison. Table 3. Amino-Terminated Polymers Synthesized by ATRP code

type

Mn (10-3 g/mol)1

Mw/Mna

SAN-1 SAN-2 SAN-3 S-UV

SAN-NH2 SAN-NH2 SAN-NH2 anth-PS-NH2

11 20 25 5d

1.1 1.1 1.1 1.1

PS (wt %)b

f (%)c

69 71 70

80 79 75 95e

a SEC with PS standards. b 400 MHz 1H NMR. c Reference 11. With an average anthracene content of ∼1group per chain.12 e Reference 12. d

as a reference. In contrast to most polycondensates, including polyamides, polymers prepared from unsaturated monomers are not spontaneously end-capped by a functional group. In the case of SAN, synthesis was carried out by controlled radical copolymerization based on the “atom transfer radical polymerization” (ATRP) mechanism. Briefly, 1-phenyl ethyl bromide was used as initiator in the presence of CuBr/1,1,4,7,10,10 hexamethyltriethylene tetramine in a 1/1 molar ratio. The SAN chains (70% styrene) were systematically endcapped by a bromide, that was derivatized into an azide followed by reduction into a primary amine [SAN-NH2: SAN-1, Table 3]. This matrix was reactive blended with 20 wt % of the asymmetric PS-b-PIP-anh copolymer used previously. Figure 17 compares the nanostructuration of PA12 and SAN by the same reactive precursor of the triblock. The typical cucumber-like core-shell is observed with the same characteristic features whatever the matrix, which confirms that the molecular architecture and composition of the ABC triblock have a decisive effect on the nanostructures which are formed in a variety of C matrices ranging from semicrystalline PA12 to glassy SAN. In this respect, reactive blending of a diblock precursor with a matrix is quite a general strategy to nanodisperse a rubbery component, with well-defined shape and size, in a fragile polymer with the purpose to have it toughened. This specific issue will be addressed in a forthcoming paper.

IV. Conclusions A new concept has been illustrated that falls at the borderline between block copolymer self-assembly and reactive blending, the purpose being the extensive control of the nanostructuration of polymer matrices. Briefly, linear ABC triblock copolymers have been prepared within reactive matrices (PA12 and SANNH2) by reactive blending of the AB precursor (used at an extent of 20 wt %) with the matrix. Keys to success are the controlled synthesis of the reactive precursors and the efficient chemistry in the melt, based on the highly reactive NH2/anhydride pair. Diluted in the C matrix, the ABC copolymer self-assembles into nanophases, which are basically controlled by the relative volume fraction of the constitutive blocks. Whenever the in situ formed triblock copolymer is symmetric, the ABC/C type of reactive blend exhibits vesicular nanophases, as is the case for a nonreactive model consisting of 20 wt % of a symmetric PS-b-PIP-b-PMMA triblock in PMMA. Transition from a vesicular to a core/shelllike morphology takes place when the composition of the reactive diblock is no longer symmetric. Higher, but still well-defined, complexity is imparted to the aforementioned nanostructures by premixing the reactive diblock with substantial amounts of the nonreactive counterpart. The vesicular morphology is for instance changed into an “onionlike” morphology. Therefore, the strategy worked out in this study is an easily implemented and versatile technique for designing polymeric nanostructures. This capacity must be helpful to tailor some macroscopic properties of the nanostructured polymers and to establish fundamental relationships between nanostructuration and bulk properties (e.g., rubber toughening of thermoplastics). This aspect will be discussed in a forthcoming paper. Acknowledgment. The authors are much indebted to the “Belgian Science Policy” for financial support in the frame of the “Interuniversity Attraction Poles Program (PAI V/03-Supramolecular Chemistry and Supramolecular Catalysis)”. They are grateful to Dr.C.


Pagnoulle and Dr Z. Yin for fruitful discussions. C.K. is an “Aspirant” fellow of the “Fonds National de la Recherche Scientifique”. Supporting Information Available: Figures showing FTIR spectra of anhydride end-capped PS-b-PIP diblock and SEC-RI and SEC-UV (366 nm) traces of PS-b-PIP-anh diblock after reaction with 9-(methylaminomethyl)anthracene. This material is available free of charge via the Internet at http:// pubs.acs.org.

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Nanostructured Polyamide by Reactive Blending. 1. Effect of the

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