Microstructure probe diffusion

A rheological measurement is a useful tool for probing the microstructural properties of a sample. If we are able to perform experiments at low stresses or strains the spatial arrangement of the particles and molecules that make up the system are only slightly perturbed by the measurement. We can assume that the response is characteristic of the microstructure in quiescent conditions. Here our convective motion due to the applied deformation is less than that of Brownian diffusion. The ratio of these terms is the Peclet number and is much less than unity. In Equation (5.1) we have written the Peclet number in terms of stresses ... 

The orientation of crystalline stems with respect to the interface of the microstructure in block copolymers depends on both morphology and the speed of chain diffusion, which is controlled by block copolymer molecular weight and the crystallization protocol (i.e. cooling rate). In contrast to homopolymers, where folding of chains occurs such that stems are always perpendicular to the lamellar interface, a parallel orientation was observed for block copolymers crystallized from a lamellar melt phase perpendicular folding was observed in a cylindrical microstructure. Both orientations are shown in Fig. 8. Chain orientation can be probed via combined SAXS and WAXS on specimens oriented by shear or compression. In PE, for example, the orientation of (110) and (200) WAXS reflections with respect to Bragg peaks from the microstructure in the SAXS pattern enables the unit cell orientation to be deduced. Since PE stems are known to be oriented along the c axis, the chain orientation with respect to the microstructure can be determined. 

Structure. This may in turn have resulted in an encapsulation of the active center by the dendrimer, and influenced the diffusion of substrates into the catalytically active core of the dendrimer. Hence, the asymmetric hydrogenation reaction may also be used for probing the dendrimer microstructure and overall shape [45]. 

When multiple scattering is discarded from the measured signal, DLS can be used to study the dynamics of concentrated suspensions, in which the Brownian motion of individual particles (self-diffusion) differs from the diffusive mass transport (gradient or collective diffusion), which causes local density fluctuations, and where the diffusion on very short time-scales (r < c lD) deviates from those on large time scales (r c D lones and Pusey 1991 Banchio et al. 2000). These different diffusion coefficients depend on the microstructure of the suspension, i.e. on the particle concentration and on the interparticle forces. For an unknown suspension it is not possible to state a priori which of them is probed by a DLS experiment. For this reason, a further concentration limit must be obeyed when DLS is used for basic characterisation tasks such as particle sizing. As a rule of thumb, such concentration effects vanish below concentrations of 0.01-0.1 vol%, but certainty can only be gained by experiment. 

Some of these parameters are directly measured with self-diffusion NMR spectroscopy (79, 80, 89) or indirectly by fluorescence probes (88). In addition, the relaxation of the microstructure after a sudden jump in a field variable such as temperature or pressure yields information on the dynamics of the structures (86). Depending upon composition of the mixtures, the time-scales of the dynamic processes range from microseconds to milliseconds (14, 88). 

The physical and mechanical properties of polymeric systems are connected with their solid state morphology. NMR spectroscopy of the nuclear spins attached to a polymeric system is a very applicable means to gain insight into the microstructure as well as into the dynamics of the system. An alternative way is to make use of a probe, such as a xenon atom, which diffuses over the environment and gives information on the microscopic heterogeneity. 

I. M. Wong, M. L. Gardel, D. R. Reichman, et al. Anomalous diffusion probes microstructure dynamics of entangled F-actin networks. Phys. Rev. Lett., 92 (2004),... 

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