Constrained-state measurement

Obviously, the constrained-state and free-to-shrink measurements may give vastly different results depending on the orientation of the fiber. The melting point of drawn fibers measured in a constrained state is always higher than in the free-to-shrink measurements. It can be intuitively understood that in a constrained measurement, the entropy increase is smaller than in the free-to-shrink measurement, since in the constrained measurement the macromolecules are stretched, and they will remain partially stretched in the melt. They will lose their orientation in the melt, but that will require some time. Then, supposing that the heat of fusion is the same in both types of measurement (which is not always true, but can be accepted as a first approximation)... 

Performing constrained-state DSC measurements on oriented films is more complex than similar measurements on drawn fibers, especially when the film has biaxial orientation it is more difficult to immobilize the total film sample during the DSC measurement, than simply tying together the ends of a single fiber after winding it up on a steel plate. Therefore, various approaches have been suggested to solve this problem. The two most important of these are as follows ... 

Clough is one of the very few authors who have performed constrained-state film measurements. Most DSC film experiments reported in the literature are prepared unconstrained (free-to-shrink) because of the technical difficulties of sample preparation just mentioned. 

Figure 2.60. The sample pan developed by Clough (1970a) for measuring melting of stretched polymeric films in constrained state (Clough 1970a). (Reprinted with permission of Taylor Francis)...

It is now necessary to attend to the second important function of the column. It has already been stated that, in order to achieve the separation of two substances during their passage through a chromatographic column, the two solute bands must be moved apart and, at the same time, must be kept sufficiently narrow so that they are eluted discretely. It follows, that the extent to which a column can constrain the peaks from spreading will give a measure of its quality. It is, therefore, desirable to be able to measure the peak width and obtain from it, some value that can describe the column performance. Because the peak will be close to Gaussian in form, the peak width at the points of inflexion of the curve (which corresponds to twice the standard deviation of the curve) will be determined. At the points of inflexion... 

The new interface model and the concept for the carbon black reinforcement proposed by the author fundamentally combine the structure of the carbon gel (bound mbber) with the mechanical behavior of the filled system, based on the stress analysis (FEM). As shown in Figure 18.6, the new model has a double-layer stmcture of bound rubber, consisting of the inner polymer layer of the glassy state (glassy hard or GH layer) and the outer polymer layer (sticky hard or SH layer). Molecular motion is strictly constrained in the GH layer and considerably constrained in the SH layer compared with unfilled rubber vulcanizate. Figure 18.7 is the more detailed representation to show molecular packing in both layers according to their molecular mobility estimated from the pulsed-NMR measurement. 

Crosslinked polymer networks formed from multifunctional acrylates are completely insoluble. Consequently, solid-state nuclear magnetic resonance (NMR) spectroscopy becomes an attractive method to determine the degree of crosslinking of such polymers (1-4). Solid-state NMR spectroscopy has been used to study the homopolymerization kinetics of various diacrylates and to distinguish between constrained and unconstrained, or unreacted double bonds in polymers (5,6). Solid-state NMR techniques can also be used to determine the domain sizes of different polymer phases and to determine the presence of microgels within a poly multiacrylate sample (7). The results of solid-state NMR experiments have also been correlated to dynamic mechanical analysis measurements of the glass transition (1,8,9) of various polydiacrylates. 

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