Films viscoelastic behavior

In Section 4.2.2 the central role of atomic diffusion in many aspects of materials science was underlined. This is equally true for polymers, but the nature of diffusion is quite different in these materials, because polymer chains get mutually entangled and one chain cannot cross another. An important aspect of viscoelastic behavior of polymer melts is memory such a material can be deformed by hundreds of per cent and still recover its original shape almost completely if the stress is removed after a short time (Ferry 1980). This underlies the use of shrink-fit cling-film in supermarkets. On the other hand, because of diffusion, if the original stress is maintained for a long time, the memory of the original shape fades. 

Sucrose changes the dynamic structure of water molecules, which, in turn, affects the manner of aggregation of the DPPE. Citric acid changes the degree of dissociation of the head group of the DPPE molecules. It becomes, therefore, apparent that each chemical species affects the viscoelastic behavior of the lipid thin film in a characteristic manner. 

It is also obvious that many such films will exhibit complex viscoelastic behavior, the same as found in bulk phases. The flow behavior then can be treated in terms of viscous and elastic components. 

Absorption of a solute liquid or vapor into a polymer film can profoundly affect the viscoelastic behavior of the polymer. The magnitude of this effect depends on the nature of the solute/polymer interactions and on the amount of solute absorbed. The solute/polymer interactions can range fttun simple dispersion to hydrogen-bonding and other specific interactions. The extent of absorption can be described by the partition coefficient, AT, which quantifies the thermcxlynamic distribution of the solute between two phases (K = coiKentration in polymer divided by die concentration in the liquid or vapor phase in contact with the polymer). It has long been known that acoustic wave devices can be used to probe solubility and partition coefficients (53,67). Due to the relevance of these topics to chemical sensors, more comprehensive discussions of these interaction mechanisms and the significance of the partition coefficient are included in Chapter 5. 

Another mechanism is related to polymer viscoelastic behavior. The interfacial viscosity between polymer and oil is higher than that between oil and water. The shear stress is proportional to the interfacial viscosity. Because of polymer s viscoelastic properties, there is normal stress between oil and the polymer solution, in addition to shear stress. Thus, polymer exerts a larger pull force on oil droplets or oil films. Oil therefore can be pushed and pulled out of dead-end pores. Thus, residual oil saturation is decreased. This mechanism is detailed in Chapter 6. 

If a proper emnlsifier is selected, the interfacial film can be tolerant to local mechanical compression and has some viscoelastic behavior. When the film is damaged, fhe viscoelasticity can heal the film. The film s viscoelasticity behavior therefore plays an important role in stabilizing emulsions. 

The buffering action of a coating in this situation is determined by the relaxation modulus of the coating material. The relaxation modulus may be measured on a film cast from the material by carrying out tensile-stress relaxation measurements with a suitable apparatus such as a Rheovibron dynamic viscoelastometer operated in a static mode. Figure 13 (inset) displays such measurements for the four coating materials used on the fibers measured in Figure 12. The measurements were carried out at 23 °C at small tensile strains, where the materials exhibit linear viscoelastic behavior. 

Poon [32] explained the film behavior of grease in terms of viscoelastic behavior as the lubricant passes through the conjunction zone. Kauzlarich and Greenwood [25] suggested that because of its gel structure grease heats up by shear faster than oil and loses the heat by conduction more slowly. In their estimation, a thermal rather than an isothermal treatment of the elastohydrodynamic problem is required. 

As discussed in Chapter 11, experimental results suggest that material properties of UTR films can differ in significant ways from their bulk counterparts. Of particular interest, because of its influence on the viscoelastic behavior of the spin-coated films, is the effect of film thickness on Eg (see Eig. 17.29), which decreases with film thickness for the particular substrate investigated in the study. 

Numata and Kinjo (52) have shown rubber-modified isocyanurate-oxazolidone resins may be effectively modified with carboxyl-reactive nitrile liquids. The viscoelastic behavior of models using a polyglycidyl ether of phenol-formaldehyde novolac resin and di-phenylmethane-4,4 -diisocyanate is discussed. Such resins have suggested utility in thin films as electrical varnishes. 

Tadros summarized the fundamental prineiples of emulsion rheology (61). Emulsions stabilized by surfactant films (such as resins and asphaltenes) behave like hard sphere dispersions. These dispersions display viscoelastic behavior. Water-in-oil emulsions show a transition from predominantly viscous to predominantly elastic response as the frequency of oscillation exceeds a critical value. Thus, a relaxation time can be determined for the system which increases with the volume fraction of the discontinuous phase. At the critical value, the system shows a transition from predominantly viscous to predominantly elastic response. This reflects the increasing steric interaction with increases in volume of the discontinuous phase. 

In Sect. 2.7.3.7.1, appropriate control of polymer composition of a redox polymer (PVF) was shown to lead to the introduction of viscoelastic phenomena and to thermal sensitivity. For polypyrrole, deposition from micellar surfactant media (dodecylsulfate and dodecylbenzenesul-fonate) also leads to changes in film morphology and viscoelastic behavior [139]. 

In accordance with the results of the adntittance measurements, the dependence of the change in the resonant frequency corresponding to the reduced state of the polymer on the charge injected during the electropolymerization is linear, except for very thick films (Fig. 3.15). Usually such a deviation indicates a transformation from elastic to viscoelastic behavior however, in this case it was assigned to the poor adherence of the deposited polymer, since the energy loss measured was small even for thick films [157]. 

Extruded films of PP/rubber blends [89] showed that the CO2 concentration is higher in the rubber domains than in the PP matrix. In addition, as expected, the rubber domains were the only place where the porous phase can develop according to different CO2 solubility and viscoelastic behavior of both phases. Nanostructured PP/TPS blends were foamed with a saturation stage performed at 20 MPa and 25°C during 1 h, followed by a pressure release at a rate of 1 MPa/s and a foaming stage at 120°C. In this study the 80-PP/20-HSIS blend presented the best results, with pore sizes of approximately 200-400nm but low porosities below 25% (Fig. 9.19). Another remarkable output from this study is the perfect relationship found between the sizes of the rubber domains of the precursor and the pores of the foam as well as between the shape and orientation of the nanostructure of the precursor and the porous structure of the foam. 

The details of the deposition and dissolution processes of the dendrimers in TBAH/AN solution were investigated using an EQCM technique as well as an admittance measurement of the quartz crystal resonator by Takada et al. [82]. EQCM measurements showed that upon reduction of the cobaltocene sites, electrodeposition of multilayer equivalents of the dendrimers took place. The additional material gradually desorbed upon reoxidation so that only a monolayer equivalent remained on the electrode surface. Admittance measurements of the quartz crystal resonator revealed that films of the lower dendrimer generations behave rigidly, whereas the higher generations exhibit viscoelastic behavior. 

As mentioned above, interfacial films exhibit non-Newtonian flow, which can be treated in the same manner as for dispersions and polymer solutions. The steady-state flow can be described using Bingham plastic models. The viscoelastic behavior can be treated using stress relaxation or strain relaxation (creep) models as well as dynamic (oscillatory) models. The Bingham-fluid model of interfacial rheological behavior (27) assumes the presence of a surface yield stress, cy, i.e.. 

Calvo and Etchenique summarized in their review some further in situ combinations of EQCM with non-electrochemical techniques (see [35] and references therein). For example, EQCM was also combined with ellipsometry in order to study the nucleation and growth of polyaniline films (reference 24 in [35]) or the viscoelastic behavior of poly(7-methyl-L-co y-n-octadecyl-L-glutamate) [17]. EQCM was combined with UV-visible absorption spectroscopy, in order to investigate the redox reactions of viologens. A combination of EQCM and probe beam deflection, PBD, was also reported in the literature (references 29, and 30 in [35], and [81]). PBD can discriminate between anion, cation, and solvent fluxes that might be generated on the electrode surface. 

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