Copolymers Rheological behavior

Block (Star) Arrangement. The known star polymers, like their linear counterparts, exhibit microphase separation. In general, they exhibit higher viscosities in the melt than their analogous linear materials. Their rheological behavior is reminiscent of network materials rather than linear block copolymers (58). Although they have been used as compatibiUzers in polymer blends, they are not as effective at property enhancements as linear diblocks... 

An indication of whether block or copolymer architecture is AB, ABA, or ( AB ) n often be seen in its rheological behavior. For example,... 

Associative copolymers of acrylamide with N-alkylacrylamides, terpoly-mers of acrylamide, N-decylacrylamide, and sodium-2-acrylamido-2-methyl-propane sulfonate (NaAMPS), sodium acrylate (NaA), or sodium-3-acrylamido-3-methylbutanoate (NaAMB) have been shown to possess the required rheologic behavior to be suitable for enhanced oil-recovery processes [1184]. 

Because the existence of domain structure in heterogeneous block copolymers persists even in the molten state, thier rheological behavior is rather unique when compared with homogeneous polymer melts. 

More recently, it has been demonstrated that many of the unusual rheological behavior of block copolymers will disappear when the measurements were carried out at temperatures higher than the separation temperature proposed by Leary and Williams (43). Figure 11 shows that for bulk SBS block copolymers with a composition of 7-43-7 (xlO3), the transition occurs around 145°C (especially clear at low frequencies) (76, 77). These data are consistent with those of Pico and Williams on plasticized block copolymers (78). 

As with cylinder- and lamellae-forming block copolymers, the rheological behavior of block copolymers that form spherical domains depends on whether or not the domains possess macrocrystalline order. If the domains are disordered, then the low-frequency moduli show terminal behavior typical of ordinary viscoelastic liquids that is, G and G" fall off steeply as the frequency becomes small (Watanabe and Kotaka 1983, 1984 Kotaka and Watanabe 1987). When the spherical domains are ordered, however, elastic behavior is observed at low frequency that is, G approaches a constant at low frequency, and a yield stress is observed in steady shearing. 

McLeish and coworkers have published results on the rheological behavior of S2I2 miktoarm star copolymers [359]. For the temperature range between 100 and 150°C it was evident that the rheology of a polymer with 20wt%. PS was independent of temperature, implying a particular molecular mechanism for stress relaxation for this architecture. For the sample having 35 wt% PS, a failure of the superposition principle was observed, a fact that was attributed to the temperature sensitive effective modulus of the polymer. 

Finally, the rheological behavior of block copolymers serves as a model for well compatibilized blends, with perfect adhesion between the phases. The copolymers provide important insight into the effects of the chemical nature of the two components, and the origin of the yield phenomena. 

Effects of addition of a compatibilizing block copolymer, poly(styrene-b-methyl methacrylate), P(S-b-MMA) on the rheological behavior of an immiscible blend of PS with SAN were studied by dynamic mechanical spectroscopy [Gleisner et al., 1994]. Upon addition of the compatibilizer, the average diameter of PS particles decreased from d = 400 to 120 nm. The data were analyzed using weighted relaxation-time spectra. A modified emulsion model, originally proposed by Choi and Schowalter [1975], made it possible to correlate the particle size and the interfacial tension coefficient with the compatibilizer concentration. It was reported that the particle size reduction and the reduction of occur at different block-copolymer concentrations. 

We will focus in this paper on the rheological properties, at room temperature, of styrene-isoprene block copolymers, particularly Triblock [SISj-Diblock [SI] copolymer blends. We will describe the effect of the molecular parameters of the copolymers on the rheological behavior, and wiU propose, on the basis of molecular dynamics models derived from the reptation concept and the analysis of the dynamic behavior of the blend [SIS-SI], a model which allows calculation of the variation of the complex shear modulus as a function of frequency. Different types of macromolecules have been designed from calculations using this molecular model in order to improve the processing and end-user properties of the full formulations (HMPSAs). 

The copolymer base of the adhesives is a blend of diblock and triblock copolymers. In Fig. 16.2 we compare the rheological behavior for two blends which have different diblock contents (54% and 85%), different molecular weights for the diblock part (86000 and 104300 g mol ), and the same styrene content (16%). The molecular weight of the triblock is 128000 gmoh. Let us describe now the relaxation domains exhibited by the two blends ... 

The linear viscoelastic behavior of the pure polymer and blends has already been described quantitatively by using models of molecular dynamics based on the reptation concept [12]. To describe the rheological behavior of the copolymers in this study, we have selected and extended the analytical approach of Be-nallal et al. [13], who describe the relaxation function G(t) of Hnear homopolymer melts as the sum of four independent relaxation processes [Eq. (1)]. Each term describes the relaxation domains extending from the lowest frequencies (Gc(t)) to the highest frequencies (Ghf( )), and is well defined for homopolymers in Ref [13]. 

We have calculated from the model the characteristics of these newly designed structures, which have been synthesized [23], and we have measured their rheological properties at room temperature. As Fig. 16.4 shows, it is possible to mimic the rheological behavior of the [SIS-SI] blend (we have applied a vertical shift on the curves for greater clarity). Also, the rheological behavior of the full formulations (Fig. 16.4) based on these newly designed copolymers yields the same properties as for [SIS-SI], which demonstrates that the tackifying resins act Hke a solvent of the elastomeric part of all the copolymers presented here. 

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