Viscoelastic behavior block copolymers

In 97- it was also shown on the basis of dilatometric data that the free-volume of PMMA in the mixture with polyvinylacetate increases with the increase in FVA concentration. In 98) a large difference was reported in the viscoelastic behavior of block copolymer from that predicted by WLF theory. This theory is believed to be useful only near the Te of each component, not in the broad temperature interval including the transition from glassy to rubberlike state. This anomaly is thought to be connected with certain motions in the interphase regions, which should be looked upon as independent components of the mixture. 

In an earlier section, we have shown that the viscoelastic behavior of homogeneous block copolymers can be treated by the modified Rouse-Bueche-Zimm model. In addition, the Time-Temperature Superposition Principle has also been found to be valid for these systems. However, if the block copolymer shows microphase separation, these conclusions no longer apply. The basic tenet of the Time-Temperature Superposition Principle is valid only if all of the relaxation mechanisms are affected by temperature in the same manner. Materials obeying this Principle are said to be thermorheologically simple. In other words, relaxation times at one temperature are related to the corresponding relaxation times at a reference temperature by a constant ratio (the shift factor). For... 

Few examples of the homogeneous diblock-incompatible homo-polymer behavior have been reported. One that has received considerable attention is the system polystyrene-poly-a-methylstyrene (2). Block copolymers of styrene and a-methylstyrene exhibit a single loss peak in dynamic experiments (2,3) and have been shown to be thermorheologi-cally simple (4) hence they are considered to be homogeneous. Mechanical properties data on these copolymers also has been used to validate interesting extensions of the molecular theories of polymer viscoelasticity (2,3,4). 

Morphology and Dynamic Viscoelastic Behavior of Blends of Styrene-Butadiene Block Copolymers... 

On a global scale, the linear viscoelastic behavior of the polymer chains in the nanocomposites, as detected by conventional rheometry, is dramatically altered when the chains are tethered to the surface of the silicate or are in close proximity to the silicate layers as in intercalated nanocomposites. Some of these systems show close analogies to other intrinsically anisotropic materials such as block copolymers and smectic liquid crystalline polymers and provide model systems to understand the dynamics of polymer brushes. Finally, the polymer melt-brushes exhibit intriguing non-linear viscoelastic behavior, which shows strainhardening with a characteric critical strain amplitude that is only a function of the interlayer distance. These results provide complementary information to that obtained for solution brushes using the SFA, and are attributed to chain stretching associated with the space-filling requirements of a melt brush. 

In the ordered state, lamellar block copolymers frequently show departures from linear viscoelastic behavior at low strain amplitudes of around 1% (Rosedale and Bates 1990 Winey et al. 1993a). Homogeneous polymers, on the other hand, typically show departures... 

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. 

Theoretical calculations for llquld/llquid systems predict that the viscosity goes through a maximum at the spinodal. Depending on the type of system and its regularity, the increase may be quite large for example, Larson and Frederickson ( ) predicted that for block copolymers. These authors concluded that in the spinodal region a three-dimensional network is formed and that the system exhibits non-linear viscoelastic behavior. Experimentally, sharp Increases of n near the phase separation have been reported for low molar mass solutions as well as for oligomeric and polymeric mixtures (21). 

Several studies have been published where different aspects of the solid state behavior of nonlinear block copolymers (like the adhesive properties, viscoelastic properties after aging and solvent treatment, thermal properties, etc.) vary with varying architectures [367-373]. Much work has still to be done in order to understand the structure-property relationship of these complex macromolecules. 

Generally PSAs are well known for their very viscoelastic behavior, which is necessary for them to function properly. It was therefore important to characterize first the effect of the presence of diblocks on the linear viscoelastic behavior. Since a comprehensive study on the effect of the triblock/diblock ratio on the linear viscoelastic properties of block copolymer blends has recently been reported [46], we characterized the linear viscoelastic properties of our PSA only at room temperature and down to frequencies of about 0.01 Hz. Within this frequency range all adhesives have a very similar behavior in terms of elasticity, as can be seen in Fig. 22.10. The differences appear at low frequency, a regime where the free iso-prene end of the diblock chain is able to relax. This relaxation process is analogous to the relaxation of an arm of a star-like polymer [47], and causes G to drop to a lower plateau modulus, the level of which is only controlled by the density of triblock chains actually bridging two styrene domains [46]. 

Kraus, G., and Rollman, K.W., Dynamic viscoelastic behavior of ABA block copolymers and the nature of the domain boundary. 7. Polym. Sci. Polym. Phys. Ed., 14, 1133-1148 (1976). 

G. Krause, F.B. Jones, O.L. Marrs, and K.W. Rollmann, "Morphology and Viscoelastic Behavior of Styrene-Diene Block Copolymers in Pressure Sensitive Adhesives", J. Adhesion, 8, pp. 235-258 (1977). 

C.D. Han, D.M. Baek, J. Kim, K. Kimishima, T. Hashimoto, Viscoelastic behavior, phase-equilibria, and microdomain morphology in mixtures of a block copolymer and a homopolymer, Macromolecules 25 (1992) 3052-3067. 

As previously mentioned, the growth of the block copolymer thermoplastic elastomer industry has now reached a high level of commercial importance. However, it is still difficult to study the rheological behaviors of thermoplastic elastomers because they exhibit complex behavior that combines the elastomeric final product properties with the processing characteristics of thermoplastics. Recently, the rheological behavior of the styrene block copolymers (SBS, SIS, and SEES) was studied by Kim and Han [31]. A series of systematic investigations for the viscoelastic behavior of triblock copolymer thermoplastic elastomers also were conducted by Mathew et al. [22,23]. Kim and Hyun [20] reported the viscoelastic properties of SEES. 

A structural model has been developed to describe the viscoelastic/thermal behavior of phase-separated block copolymers. Success is achieved in fitting the G (T) and G (T) data of five samples (involving three SBS copolymers and three solvents) from -120 C to +100 C, a range which... 

Block Ionomers. The block ionomers to be discussed are of AB or ABA type, in which one of the blocks is ionic (eg, sodium methacrylate) and the other consists of nonionic units (eg, polystyrene). While ionic block copolymers in a micelle form in both aqueous and nonaqueous solutions have been studied extensively (99-101,130,131), the viscoelastic properties of block ionomers in bulk have not received much attention (132-137). If the short ionic blocks formed micelle-like aggregates, which were surrounded by nonionic blocks, the viscoelastic properties of the diblock ionomers would be very similar to those of stars or polymers of low molecular weight (136). Thus, above the Tg of the nonionic blocks, as the temperature increased the modulus dropped rapidly with a very short rubbery plateau. For example, in a dynamic mechanical study, it was found that a homopolymer containing 490 styrene units showed a Tg at ca 115°C, and started to flow at ca 150°C. However, in the case of a diblock ionomer containing 490 styrene units and 40 sodium methacrylate ionic units showed a Tg at ca 116°C, and flow behavior was observed above ca 165°C, which was only 15°C higher than that of nonionic polystyrene (135). 

Rheology is a part of continuum mechanics that assumes continuity, homogeneity and isotropy. In multiphase systems, there is a discontinuity of material properties across the interface, a concentration gradient, and inter-dependence between the flow field and morphology. The flow behavior of blends is complex, caused by viscoelasticity of the phases, the viscosity ratio, A (that varies over a wide range), as well as diverse and variable morphology. To understand the flow behavior of polymer blends, it is beneficial to refer to simpler models — for miscible blends to solutions and mixtures of fractions, while for immiscible systems to emulsions, block copolymers, and suspensions [1,24]. 

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