Macromolecular mobility

Aukland, K. (1991) Distribution volumes and macromolecular mobility in rat tail tendon interstitium. Am. J. Physiol.,... 

It is well established that between Tg and about Tg + 50 K, the relaxation kinetics obeys the WLF law (Williams et al., 1955). If Pr is a property depending on the macromolecular mobility (relaxation modulus, complex modulus, viscosity, diffusion rate, etc.), the time-temperature equivalence principle may be formulated as... 

Variables Influencing Tg. Since Tg is a function of macromolecular mobility, any changes to the macromolecular structure that increases or decreases this mobility will have an effect upon Tg. The following discussion highlights several variables that can impact the observed glass transition temperature. 

For example, enhanced macromolecular mobility was observed at the surface of polystyrene films [33]. In 200 nm thick films of monodisperse PS on silicon, measurements of the lateral force (under a constant applied load of 10 nN) vs. 

The presence of stresses in the adhesive layer alters the polymer s physical and chemical properties. The glass-transition temperature depends on the internal stresses that appear in the polymer. For example, the glass-transition temperature is known to decrease with increase in internal stresses in films based on polystyrene [169] and other pol5rmers. Note that restriction of the macromolecular mobility... 

Miscibility is one of the key factors for the successful increase of tensile properties and decrease of Tg. Most literature studies used a maximum plasticizer concentration of 20 wt%, which allowed for the fabrication of a priori fully miscible blends. Correlation between yield stress and plasticizer concentration is a sign of miscible blends, as yield stress can serve as a measurement of macromolecular mobility. Miscibility limits in PLA observed by some authors were 5 wt% DOA measured by DSC, 10 wt% DINCH (by DSC and FESEM), 20 wt% for DBS (DSC) and CMS 15 wt% (DSC). Furthermore, below 10 wt% DOA, Muriaru et al. observed a decrease in the impact strength, explained by the antiplasticization effect. 

MACROMOLECULAR MOBILITY AND INTERNAL VISCOSITY. THE ROLE OF STEREOREGULARITY... 

T RT (well below Tg) High entanglement density and high macromolecular mobility (yield stress lower than disentanglement stress) result in homogeneous yielding (shear bands and deformation zones). 

On the other hand, the mechanical properties also depend on the materials molecular composition and structure, i.e., intrinsic parameters. Intrinsic parameters are, for instance, chemical composition or constitution, configuration, conformation, chain cross section, entanglement molecular weight, free volume, chain stiffness, macromolecular mobility, crystallinity, and others [4, 16, 17]. Chain length and chain length distribution (or molecular weight M ) have a basic influence on mechanical properties, which is illustrated in Fig. 1.17. Three regions can be identified ... 

The elastic nature of fiber and shear stress concentration at the fiber ends affected the tan 6 value when incorporated in a composite material, which is related to the additional viscoelastic energy dissipation in the matrix material [15], Figure 13.8 shows a selection of tan 6 curves in which it can be seen that the tan 6 peak shifted to the higher temperatures, broadened, and decreased as the kenaf whiskers content increased. The decrease of tan 6 value indicated the reduction of macromolecular mobility of the fiber surface environment, whereas better interaction between kenaf whiskers and matrix can be concluded [16]. 

The diffusion of solvating media results in addition of diffused-in media molecules to polymer macromolecules due to intramolecular forces (in polar polymers such as PVC due to dipole-dipole interaction). The macromolecules are then virtually encased by media molecules. This increases the spacing between molecules, thus leading to reduced bond forces and increased macromolecular mobility. In turn, this allows even more molecules to penetrate into the intermediate spaces, resulting in a reduction in strength and an increase in elasticity the plastic becomes soft and under the effect of solvents decomposes completely [243]. 

Dynamic mechanical analysis (DMA) has been used to study the flow behavior of hot-melt adhesives.Drummer and co-workers used DMA to study the viscoelastic behavior of adhesives. They found that dynamic mechanical measurements in adhesives provided insight in the macromolecular mobility of the polymer or rubber system studied. The viscoelastic behavior at various temperatures can be correlated with standard measurements such as adhesive force, shear strength, and tack. The authors concluded that three-dimensional DMA plots from frequency-temperature sweeps provide a complete overview of the frequency and temperature dependence ofthe adhesive. Foster, etal., characterized the hot-tack differences in hot-melt adhesives using DMTA. 

Braga, J. Desterro, J. M. et al. (2004). "Intracellular macromolecular mobility measured by fluorescence recovery after photobleaching with confocal laser scanning microscopes." Mol Biol Cell, 15(10), 4749-60. 

Bose and co-authors have shown that CNTs cause a macromolecular mobility and an onset separation phase temperature reduction. This behavior was attributed to free energy changing and probably an earlier second-phase nucleation promoted by CNT. The anthors have also observed that nanotubes migrate to a preferential phase forming a network-like structure, hindering the phase separation. Figure 1.11a and b show a poly(a-methyl styrene)-co-acrylonitrile/poly(methyl methacrylate) blend (with different proportions) reinforced by MWCNT produced by Bose et al. [34], where the MWCNT are dispersed in one polymer phase only. 

---