Structural recovery

The effect that compression force can have on the disintegration efficiency seems, therefore, largely dependent on the mechanism of the disintegrant action. The effectiveness of swelling or structure recovery may well be dependent on attaining a compression force that achieves a critical porosity in the matrix. On the other hand, the capillary uptake of liquid, which is a necessary precursor to these mechanisms could be compromised if the tablet matrix is compressed to a porosity too low. 

Figure H3.1.4 Structure recovery after loading a sample of a grease or fat on a cone-and-plate fixture on a rotational rheometer. G was measured over a period of 17 hr at constant oscillatory stress, frequency, and temperature.

One theory that describes the temperature dependence of relaxation time and structural recovery is the Tool-Narayanaswamy-Moynihan (TNM) model developed to describe the often nonlinear relationship between heating rate and Tg. In this model, the structural relaxation time, x, is referenced as a function of temperature (T), activation enthalpy (Ah ), universal gas constant (R), hctive temperature (7)), and nonlinearity factor (x) (Tool, 1946 Narayanaswamy, 1971 Moynihan et al., 1976) ... 

Owing to the fact that the glassy state is a nonequilibrium or metastable state, the thermodynamic properties of a glassy system (volume, enthalpy, etc.) in isothermal conditions will evolve toward thermodynamic equilibrium. The evolution of the volume is usually expressed in terms of = p — Ve)/Ve, where v and are, respectively, the specific volume at time t and at equilibrium. After a T-jump cooling experiment, the variation of 8 with time, in isothermal conditions, follows trends similar to those shown (18) in Figure 12.16. This process is known as structural recovery. 

Memory effects play an important role in structural recovery (18), as can be seen in Figure 12.21, where illustrative plots of the evolution of volume with time for different thermal histories are shown. Curve 1 presents the... 

Figure 12.21 Structural recovery of poly(vinyl acetate) at 30° C after several thermal histories. See text for details, (From Ref. 18.)...

A convenient way to analyze physical aging processes (structural recovery) is to use a relaxation function < t) defined as... 

Once shearing is stopped (the external force is no longer acting), the rate of the system structural recovery is the same as the rate of structural breakdown under steady shear. In foods, the time needed for recovery may vary significantly from one thixotropic product to another. The time dependency can be observed in food systems such as concentrated emulsions, sols, and gels. 

Narfstrom K, Katz ML, Bragadottir R, et al. Functional and structural recovery of the retina after gene therapy in the RPE65-/-null mutation dog. Invest Ophtahlmol Vis Sci 2003 44 1663-1672. 

Physical aging and structural recovery at the nanoscale Summary and perspectives... 

There are only a handful of investigations of aging of materials at the nanometer size scale. The earliest was done by the author in collaboration with workers at Eastman Kodak in a structural recovery investigation of o-TP confined in controlled pore glasses having a variety of pore sizes [McKenna et al., 1992], This work is of interest because of the errors of interpretation made in that study. The... 

Zheng, Y, and McKenna, G. B., Structural recovery in a model epoxy comparison of responses after temperature and relative humidity jumps. Macromolecules, 36, 2387-2396 (2003). 

Hutchinson, J. M., Interpretation of structural recovery of amorphous polymers from DSC data. Prog. Colloid Polym. Sci., 87, 69-73 (1992). 

H ure 2 The change in Active temperature during structural recovery at selected aging tmpoatures as a functitm of aging time, lines indicate the trends in tlK data. 

Fhenmisnidogical Descripttm of Bw Vdonie Reooveiy. We have employed a hybrid (29 of the Tool-Narayanaswamy-Mt nihan-KAHR models of the kinetics of structural recovery (16-20) to obtain relevant phenomoiological fits to peiature-jump eiqieriments. The volume departure from equilibrium, 5, is related to the thermal history and material prc rties as ... 

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