Chain structure rheological behaviour

On the other hand, a power law relaxation in rheology cannot be a sure indication for a gelation threshold. Self-similar relaxations has been associated with selfsimilar structures on the molecular and supermolecular level as well as for suspensions and emulsions (Winter and Mours 1997). Similarities between gelation and long-chain branching viscoelastic behaviour have been also discussed (Garcia-Franco et al. 2001). 

The interest in the phase behaviour of block copolymer melts stems from microphase separation of polymers that leads to nanoscale ordered morphologies. This subject has been reviewed extensively [1 ]. The identification of the structure of bicontinuous phases has only recently been confirmed, and this together with major advances in the theoretical understanding of block copolymers, means that the most up-to-date reviews should be consulted [1,3]. The dynamics of block copolymer melts, in particular rheological behaviour and studies of chain diffusion via light scattering and NMR techniques have also been the focus of several reviews [1,5,6]. 

An extensive programme of research to find new synthetic polymers for lOR application has also been carried out by McCormick, Hester and co-workers at the University of Southern Mississippi. These workers maintain that the most important single property of macromolecules for mobility control in lOR is hydrodynamic volume of the polymer molecule. They found that it is this factor which most closely controls the polymer concentration/viscosity relationship, its rheological behaviour and the extent of pore and channel space penetration into the reservoir rock. They also reported that the hydrodynamic volume was a function of the chemical structure, the polymer chain length and the polymer/solvent interactions. They examined several families of co-polymers of acrylamide, which, in some cases, showed behaviour which was, in certain respects at least, an improvement over conventional HPAM (McCormick et al, 1985). 

Incorporation of filler material in polymers and subsequently cross linking them to form a filled polymer network structure leads to a system of great complexity. Numerous theoretical attempts have been made to describe this complicated rheological behaviour of the filled polymers (Leonov 1990). There is either a particle-particle or particle-matrix network formed, depending on the strength of interparticle attractions or the interactions between particles and polymer chains (Wolff and Wang 1992). 

Vinyl monomers, such as styrene, and alkenes with a side group, such as propylene, can polymerize in several molecular forms whose crystallization behaviors are quite different from each other. If the side groups are all on one side of the backbone, the structure is called isotactic, and if they are on alternating sides, it is called syndiotactic. If they are distributed in a random fashion, the polymer is said to be atactic. The isotactic and syndiotactic forms are crystallizable, often in a helical structure, while the atactic form does not crystallize and solidifies only at its glass transition temperature. Figure 2.3 illustrates the tacticities mentioned above for the case of polypropylene. It has been found that polypropylene tacticity can also have an important effect on chain dimensions [10] and on the rheological behaviour of the melt [11]. 

High molar mass epoxy prepolymers containing rabber dispersions based on carboxyl-terminated butadiene-acrylonitrile copolymer were prepared from initially miscible solution of low molar mass epoxy prepolymers, bisphenol A and carboxyl-terminated NBR. During chain extension inside a twin screw extruder due to epoxy-phenoxy and epoxy-carboxy reactions, a phase separation process occurs. Epoxy-phenoxy and epoxy-carboxy reactions were catalysed by triphenylphosphine. The effect of reaction parameters (temperature, catalyst, reactant stoichiometry) on the reactive extrasion process were analysed. The structure of the prepolymers showed low branching reactions (2-5%). Low molar mass prepolymers had a Newtonian rheological behaviour. Cloud-point temperatures of different reactive liquid butadiene aciylonitrile random copolymer/epoxy resin blends were measured for different rubber concentrations. Rubber... 

Most properties of linear polymers are controlled by two different factors. The chemical constitution of the monomers determines the interaction strength between the chains, the interactions of the polymer with host molecules or with interfaces. The monomer structure also determines the possible local conformations of the polymer chain. This relationship between the molecular structure and any interaction with surrounding molecules is similar to that found for low-molecular-weight compounds. The second important parameter that controls polymer properties is the molecular weight. Contraiy to the situation for low-molecular-weight compounds, it plays a fundamental role in polymer behaviour. It determines the slow-mode dynamics and the viscosity of polymers in solutions and in the melt. These properties are of utmost importance in polymer rheology and condition their processability. The mechanical properties, solubility and miscibility of different polymers also depend on their molecular weights. 

Finally, the structure or network of polymer chains combined with micellar clusters occurring in solution from such associative polymers also results in a modified rheological profile with respect to the viscosity versus shear rate behaviour compared with the previously known Hnear and cross-linked types. This was of particular interest in formulations such as water-based paints and will be covered in more detail in a later section on rheological profiles of acrylic thickeners. 

For polyacrylamide there are two rheological effects which can be explained in terms of its random coil structure. Firstly, it was discussed above that polyacrylamide is much more sensitive than xanthan to solution salinity and hardness. This is explained by the fact that the salinity causes the molecular chain to collapse, which results in a much smaller molecule and hence in a lower viscosity solution. The second effect which can be explained in terms of the polyacrylamide random coil structure is the viscoelastic behaviour of this polymer. This is shown both in the dynamic oscillatory measurements and in the flow through the stepped capillaries (Chauveteau, 1981). When simple models of random chains are constructed, such as the Rouse model (Rouse, 1953 Bird et al, 1987), the internal structure of these bead and spring models gives rise to a spectrum of relaxation times, Analysis of this situation shows that these relaxation times define response times for the molecule, as indicated in the simple Maxwell model for a viscoelastic fluid discussed above. Thus, because of the internal structure of a flexible coil molecule, one would expect to observe some viscoelastic behaviour. This phenomenon is discussed in much more detail by Bird et al (1987b), in which a range of possible molecular models are discussed and the significance of these to the constitutive relationship between stress and deformation rate and deformation history is elaborated. 

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