ABA Block Copolymer

Chapter 17

Gel Network Development in AB, ABA, and AB/ABA Block Copolymer Solutions in a Selective Solvent 1

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Elizabeth A. Wilder , Scott A. White , Steven D. Smith , and Richard J. Spontak Downloaded by GRIFFITH UNIV on September 9, 2017 | http://pubs.acs.org Publication Date: October 15, 2002 | doi: 10.1021/bk-2002-0833.ch017

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1

Department of Chemical Engineering,

North

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State U n i v e r s i t y ,

R a l e i g h , NC 2 7 6 9 5 - 7 9 0 5 2

Medical

Device Technologies Department, Becton Dickinson Technologies, Research Triangle

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R e s e a r c h D i v i s i o n , The P r o c t e r a n d

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Freiburg,

D-79104 Freiburg I. Br., Freiburg, Germany 5

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corresponding address: Department of C h e m i c a l Engineering,

North

Carolina State University, Raleigh, NC 27695-7905

Abstract In the presence of a selective solvent, ordered block copolymers form micelles that, at sufficiently high copolymer concentrations, serve to stabilize a three -dimensional network and promote physical gelation. This study examines the steady and dynamic rheological properties of micellar solutions composed of AB diblock, ABA triblock and bidisperse mixtures of AB and ABA copolymer molecules. Of particular interest is the unexpected improvement in network development upon addition of an AB copolymer to an ABA copolymer at con -stant solution composition. This behavior is observed for ABA/solvent systems above and below the critical gelation concentration, and is interpreted in terms of the volume exclusion that occurs in bidisperse mixture of grafted chains.

Introduction At sufficiently high thermodynamic incompatibility, AB diblock and ABA triblock copolymers order into several periodic nanostructures, the curvature of which is governed by a combination of interfacial area minimization and packing considerations (1,2). Comparable nanostructures are likewise generated in concentrated solutions of block copolymers in the presence of either a neutral (3) or at least one selective (4-6) solvent. At high solvent concentrations, block copolymer molecules behave as surfactants and micellize in a selective solvent (see Fig. 1). If the copolymer is an ABA triblock copolymer and the solvent is B-selective, the B blocks of the copolymer are capable of adopting looped,

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© 2003 American Chemical Society Bohidar et al.; Polymer Gels ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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Downloaded by GRIFFITH UNIV on September 9, 2017 | http://pubs.acs.org Publication Date: October 15, 2002 | doi: 10.1021/bk-2002-0833.ch017

bridged and dangling-end conformations (7,8). These conformations affect the rheological properties of the solution and promote the formation of physical gels above the critical gelation concentration (cgc). We consider a physical gel as a bi- or multicomponent system possessing a liquid matrix and exhibiting two characteristic rheological properties (9): (i) the dynamic storage modulus (G') is independent of oscillatoryfrequency(co); and (ii) G' exceeds the dynamic loss modulus (G"). Recent rheological studies of block copolymer micellar solutions have focused on elucidating the effects of composition (10,11), temperature (10,12,13) and shear (14,15) on gel characteristics. In this work, we use steady and dynamic rheology to examine micellar solutions of an AB copolymer, an ABA copolymer and a bidisperse mixture of the two to establish the roles of molecular conformation and coronal packing in gel network development. Experimental Two copolymers, a poly(styrene-fc-isoprene-Z)-styrene) (SIS) triblock (60 wt% S; M =100,000, M /M =1.04) and a poly(styrene-Msoprene) (SI) diblock (70 wt% S; M =50,000, M /M =1.05), were synthesized by anionic polymeriza tion. The selective solvent used here was an aliphatic white mineral oil (MO) produced by Witco (380PO). Specific masses of each copolymer and MO were dissolved in cyclohexane and cast into molds. Upon solvent evaporation, the resultant films were vacuum-dried for up to 7 h at 120°C. Steady-shear tests were performed on a Rheometrics dynamic stress rheometer (DSR) as a function of shear stress (x) to measure the solution viscosity (r\), while dynamic tests were performed here to discern G' and G" as functions of T, to and temperature. n

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Figure 1. Transmission electron micrograph of a micellar SIS triblock copoly mer gel containing MO. The stained I blocks comprising the coronae are dark.

Bohidar et al.; Polymer Gels ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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x (dyn/cm ) Figure 2. Steady-shear viscosity (rj) evaluated at ambient temperature and presented as a function of shear stress (x) for SI diblock copolymer/MO micellar solutions differing in (|) i (in wt%): 15 (O), 13 (•), 11 (A), 9 (A) and 5 (O). S

Results and Discussion A. Pure Copolymer Solutions In this section, we first address micellar solutions composed of either the SI or SIS copolymer, but not both. The concentration (expressed here in wt%) of copolymer in these solutions is designated by (fa (i=SI or SIS). Shown in Fig. 2 is the dependence of steady-shear r) on x for SI copolymer solutions differing in Si- These solutions consist of glassy micellar cores surrounded by a swollen brush of I tails. Intermicellar interactions may occur only through entanglement of the tails comprising the coronae of adjacent micelles. As seen in Fig. 2, an increase in (|>si promotes a monotonic increase in T|, which is weakly dependent on x for solutions with si up to 13 wt%. As (|>si is increased further to 15 wt%, however, T| is found to increase substantially and become more dependent on x. This abrupt change in viscosity behavior suggests that a more highly correlated micellar morphology develops within the solution. Values of T| evaluated at two arbitrary x are presented for comparison as a function of (|>si in Fig. 3. The regressed solid lines included in this figure reveal that rj increases exponentially with increasing copolymer concentration.


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1 Downloaded by GRIFFITH UNIV on September 9, 2017 | http://pubs.acs.org Publication Date: October 15, 2002 | doi: 10.1021/bk-2002-0833.ch017

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Dynamic rheological measurements of the SI copolymer solutions described in Fig. 2 indicate that these solutions are not physical gels, according to the criteria specified earlier. In marked contrast, the SIS copolymer solutions behave as physical gels, in which the S-rich micelles serve as physical crosslink sites for highly swollen I midblocks. Frequency measurements (not shown here for brevity) confirm that G is both independent of © (from 10 to 10 rad/s) and always greater than G" in these solutions. Figure 4 displays the dependence of G on x for four SIS solutions differing in (|>sis- hi all these solutions, two trends are apparent: (i) G' increases with increasing (|>sis> and (ii) G' exceeds G" by at least an order of magnitude. Solutions in which sis * l 9 wt% do not exhibit gel behavior, implying that the cgc in this series lies between 7 and 9 wt% at ambient temperature. According to the data in Fig. 4, G' is invariant with respect to x over a sisdependent range, which identifies the linear viscoelastic (LVE) regime. Within this regime, the dynamic moduli are independent of x, and the properties of the gel network existing in each solution can be probed without irreversibly damaging the nanostructure. The LVE regime, denoted by X L V E d presented as a function of sis Fig- 5, increases with increasing (|>sis. Such dependence !

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T (dyn/cm ) Figure 4. Dynamic storage moduli (G\ open; G", filled) displayed as functions of x for SIS diblock copolymer/MO micellar solutions differing in siS wt%): 15 (circles), 13 (triangles), 11 (diamonds) and 9 (squares). In these and later dynamic stress measurements, co is maintained constant at 1 rad/s. is representative of more highly connected networks in gels with high copoly mer content. Values of G' measured in the L V E regime are also displayed in terms of sis in Fig. 6. These data, acquired from both co and x measurements, are accurately fitted by a power-law expression (represented by the solid line in Fig. 6), indicating that G'~sis with n^2.3. This relationship, with n>l, is consistent with theoretical predictions (16) and experimental evidence (6,10,11) for flowered block copolymer micelles. In this case, both the looped and bridged midblocks of triblock copolymer micelles contribute to the measured modulus. As ^ S I S is reduced, the intermicellar distance increases, eventually inhibiting entanglements (due to looped midblocks) between neighboring coronae. In this limit, G' only depends on the network formed by bridged midblocks. Another composition-dependent feature of the moduli in Fig. 4 is the dynamic yield stress (x ), the stress at which the gel network is disrupted. Values of x identified either as the stress at which a catastrophic reduction in G' occurs or from the point at which two tangent lines (reflecting the data) intersect are included in Fig. 5. In similar fashion as X L V E > y increases with increasing sis due to reduced intermicellar distance and improved network development. n

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254 B. Bidisperse Copolymer Solutions In the previous section, each micellar solution consisted of one copolymer (either the SI diblock or SIS triblock) at a specified composition. Here, we turn our attention to micellar solutions composed of bidisperse mixtures of both the SI and SIS copolymers. Three solution series with fixed copolymer composi tions (BC) of 15, 11 and 7 wt% are examined. The concentration of the SI copolymer in each of these blends, expressed relative to the solution, is designated by si- The maximum value attainable by 4>si I 0BC* which occurs in solutions containing only the SI copolymer. To facilitate comparison between the three series, we also introduce w$i as the concentration (in wt%) of SI copolymer in each SI/SIS blend used to prepare a bidisperse copolymer solution. This blend composition is related to si 9 wt%, this occurs gradually over a broad x range. This is clearly not the case for the three remaining solutions shown in Fig. 7a. In these SIS-rich solutions, the transition from high- to low-T| appears relatively sharp, and the value of x responsible for the onset of the transition is sensitive to siFigure 7b corresponds to the solution series with BC7 wt% and reveals two very interesting features. The first is that both the pure SI (si7 wt%) and the pure SIS (si0 wt%) copolymer solutions exhibit comparable magnitudes of T|, on the order of about 3 P. In both cases, T| is virtually independent of x, as was evident for only the SI solution in Fig. 2. This behavior does not change substantially if si is reduced to 6 wt% or increased to 1 wt%. Within these bounds, however, r| is seen to increase dramatically, and its shear dependence becomes sensitive to solution composition. Consider the solution with 3 wt% SI copolymer. Its steady-shear rj increases by up to ~5 orders of magnitude at low x, and then decreases by ~2 orders of magnitude over the range of x shown in Fig. 7b. Comparison of Figs. 7a and 7b reveals that, between 3 and 5 wt% SI, the r| curves displayed in Fig. 7b resemble the curves for SIS-containing solutions in Fig. 7a. Such similarity suggests that solutions (BC7 wt%) lying within this composition window may possess copolymer nanostructures that are comparable to those found in the solutions with (^sr^ wt%. To compare


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induced increase of about 30% in G' beyond that of the pure SIS solution at constant copolymer concentration. The reproducibility in G' of the micellar solutions examined here is estimated to be about 10%, which means that the increase in G' evident in Fig. 9a is beyond experimental uncertainty. Similar, but less pronounced, behavior is observed for die solutions with fec^H in Fig. 9b. Values of G' do not differ substantially with solution composition up to si=5 wt%. In the series with 7 wt% block copolymer (Fig. 9c), only solutions possessing intermediate blend compositions are found to exhibit a measurable viscoelastic response. This observation indicates that the solutions composed of either the pure SI or pure SIS copolymer are below the cgc and do not form physical gels in MO under the present conditions, a finding that agrees with the results presented in the first section of this study. Values of G extract ed from Fig. 9, as well as from co measurements (not shown), are provided as a function of w$i in Fig. 10. The maxima discussed above with regard to the three bidisperse copolymer solution series are more clearly evident in this figure. The dashed lines identify values of G' corresponding to the pure SIS solutions, whereas the dotted lines denote linear fits to the data beyond G ' . In the series with copolymer concentrations of 15 and 11 wt%, these fits correctly extrapolate to vanishing G (for the pure SI copolymer solutions). f

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Figure 10. Variation of G' with blend composition for bidisperse SI/SIS block copolymer micellar solutions with fee of 15 (O), 11 (A) and 7 (•) wt%. The solid lines connect the data, and the horizontal dashed lines identify G' for the solutions composed of only the SIS copolymer. Dotted lines are linear fits to the data beyond G ' . All data have been acquired at ambient temperature. m a x

According to the data presented in Fig. 10, incorporation of SI copolymer molecules within S-rich micelles composed of SIS molecules either improves or promotes physical gelation at constant copolymer concentration, implying that bidisperse copolymer mixtures synergistically enhance network development. Results from mixtures of polymer chains differing in length and grafted at one end to an impenetrable surface (17) reveal that monolayer stratification induces extension of the longer chains. In the present study, the looped midblocks of the SIS copolymer are further extended relative to the corresponding tails of the SI copolymer (the molecular weight of the I block in the SI copolymer is 15,000, while the half molecular weight of the I block in the SIS copolymer is 20,000). Such extension is expected to inhibit the formation of midblock loops due to coronal volume exclusion of the SIS molecules, thereby favoring the formation of intermicellar bridges and, hence, improving network connectivity (18). Note that at constant copolymer concentration, two SI molecules replace each SIS molecule upon substitution, which is consistent with coronal volume exclusion in bidisperse block copolymer micellar solutions. We recognize, however, that other explanations for the rheological behavior observed here may exist (19).


260 1

ABA Block Copolymer

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