Morphology and rheology of poly(methyl methacrylate)-block-poly ...

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Morphology and rheology of poly(methyl methacrylate) ... 1253The equilibrium phase morphology of block copolymersstrongly depends on the copolymer composition, asillustrated by the styrene-diene block copolymers 11, 12) .Spherical morphology is commonly observed for polystyrene(PS) contents up to l17 wt%. When the PS contentranges from 17 to 38 wt.-%, the phase morphology iscylindrical, whereas a lamellar morphology is reportedfor 36l62 wt.-% PS. For the sake of comparison, a seriesof MIM copolymers covering a wide range of PMMAcomposition has been analyzed by AFM. Fig. 2 showssome typical phase morphologies: PMMA spheres(Fig. 2a) for sample 3 of low PMMA content (12 wt.-%),PMMA cylinders (Fig. 2b) for sample 5 of intermediatePMMA content (22 wt.-%), and PMMA short lamellae(Fig. 2c) for sample 6 of higher PMMA content (36 wt.-%). Although the copolymer composition-phase morphologyrelationship is basically comparable for MIM andstyrene-diene block copolymers, MIM with very shortPMMA blocks (M — n: 3500) shows a well-defined sphericalmorphology, in contrast to the PS-block-polybutadiene(PB)-b-PS (SBS) and SIPS analogues 13) that show nophase separation.The annealing of sample 5 has a strong effect on theorientation of the PMMA cylinders with respect to thesample surface. Before annealing, the cylinders lie flat onthe surface (Fig. 2b). After annealing at 1408C, only afew flat cylinders coexist with many bright dots whosediameter is similar to the width of the flat cylinders. Thisobservation is consistent with the fact that the bright dotsare cylinders standing upright perpendicular to the surface(Fig. 2d). This reorganization is likely to be governedby the surface energy, which is smaller for PIOA(30610 –3 N/m) than for PMMA (41610 –3 N/m). As theequilibrium is approached, the PMMA cylinders tend toreorganize themselves with their apex at the surface.Viscoelastic behavior of MIM triblock copolymersIt is well established that the domain structure of blockcopolymers, such as SBS and SIPS, persists beyond theupper (PS) glass transition temperature. However, as temperatureis raised above a critical value, the microdomainstructure disappears completely, resulting in a homogeneoussystem. This transition is referred to as the orderdisordertransition (ODT), which occurs in all knownblock copolymers 14–18) . The viscoelastic behavior of theseblock copolymers significantly changes at the ODT, theblock copolymer being easily processable beyond ODT.Dynamic temperature sweep experimentsFig. 3 compares the temperature dependence of the storagemodulus (G9) and the loss factor (tand) measured at1 Hz for the 5000–140000–5000 MIM triblock (sample2, Tab. 1) and the Kraton D1107 sample. The dynamicFig. 3. Temperature dependence of the shear storage modulus(G9) and the loss factor (tan d) (1 Hz, heating rate: 28C/m). (a)MIM triblock, sample 2; (b) Kraton D1107mechanical behavior of these samples is typical of thermoplasticelastomers, i.e. two transitions, a rubbery plateaubetween them and the terminal zone at higher temperature.Compared to Kraton D1107, the MIM sample(Fig. 3a) has quite a comparable behavior, except for theG9 plateau region which is more flat (up to 1108C) asresult of the absence of diblock copolymers 19) . The tandcurves do not exhibit any clear peak at the T g of PS (orPMMA) microdomains, which can be explained by thelow PS (or PMMA) content and the simultaneous occurrenceof the flow.Fig. 4 compares the logG9 vs. temperature curves for aseries of MIM triblock copolymers covering a large rangeof PMMA molecular weight (3500–20000). The behaviorof MIM triblocks is similar to that of the Kraton TPEwhen the PMMA molecular weight is smaller than 7000.However, when this molecular weight is higher, no terminalzone is observed up to 2008C, the highest tempera-
1254 J. D. Tong et al.Fig. 4. Temperature dependence of the shear storage modulus(G9) for a series of MIM triblocks at 1 Hz (heating rate: 28C/m).For the sake of clarity, curves have been vertically shifted withrespect to sample 2 (sample 1 downwards by 0.5 unit; samples3, 4 and 5, upwards by 0.5, 1.0 and 1.5 units, respectivelyture tested. It is known that G9 measured at low frequenciesdecreases sharply at or near the order-disorder transitiontemperature (T ODT ) of the block copolymer 20 – 23) . It isthus obvious that a phase-separated morphology persistsbeyond the glass transition of the PMMA microdomainsof samples 3 to 5, at least up to 2008C. The comparisonof MIM and SIPS of comparable molecular structure(sample 4 and Kraton D1107), also shows that G9 starts todecrease only at 1208C for MIM, which is ca. 208Chigher than for the SIPS sample, in agreement with ahigher service temperature for the MIM triblock copolymer.Relaxation at temperatures beyond T g of PMMAThe rheological properties of polymers are closely relatedto the relaxation process of the polymer chains. Recently,Berglund and McKay have thoroughly studied the relaxationbehavior of SIPS triblock copolymers 19) . The completerelaxation stress proceeds in two steps:diffusion ofthe outer blocks out of the microdomains, followed bydiffusion of the released chains through the entangledmidblock chains. Fig. 5 compares the relaxation for twoMIM triblocks of different PMMA molecular weight(samples 1 and 3) but of the same spherical morphology.The relaxation curve for the MIM containing PMMAblocks of 7000 MW clearly confirms Berglund andMcKay’s conclusion, i.e. a two-step drop of G(t) in the10 –1 l10 1 s and 10 1 l2610 2 s regions, respectively,assigned to the relaxation of the PMMA blocks out of theFig. 5. Time dependence of the stress modulus for two MIMtriblocks (samples 1 and 3, in Tab. 1)hard microdomains followed by the diffusion of the entirechains through the PIOA matrix. The relaxation of MIMcontaining two times shorter PMMA blocks is quite reminiscentof that one commonly observed for monophasepolymers, so indicating that no microdomain structurepersists at 1308C.Dynamic frequency sweep experimentsFig. 6 illustrates how the complex viscosity of sample 2depends on the angular frequency. The complex viscosityat 1208C is clearly non-Newtonian, as is the case for vulcanizedrubber. A yield behavior starts to be observed at1808C, and a Newtonian behavior at low shear rate, aswell. The observation that the Newtonian behaviorbecomes more pronounced as the temperature isincreased beyond some limit, is the signature of the completerelaxation of the triblock chains at low angular frequencywhen T ODT of the block copolymer isapproached 19) . Fig. 7 compares the plots of complex viscosityvs. angular frequency for a series of MIM triblocksat 1708C. As the PMMA molecular weight is increased,the non-Newtonian behavior is continuously more pronounced,as a result of increasingly more extended phaseseparation when the molecular weight 24) is increased.Fig. 8 compares the complex viscosity of MIM/MI binaryblend (sample 9, Tab. 1) and Kraton D1107. It mustbe noted that although the MIM/MI binary blend containsthe same diblock content as the Kraton copolymer, thecontent of the hard block (ca. 11.5 wt.-%) is smaller comparedto Kraton (ca. 15 wt.-%). Moreover, the molecularweight between chain entanglements (M e ) is 60000 forPIOA 6) much higher than the 6100 for PIP 2) . For these
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