Failure properties, gels

The relationship between the structure of the disordered heterogeneous material (e.g., composite and porous media) and the effective physical properties (e.g., elastic moduli, thermal expansion coefficient, and failure characteristics) can also be addressed by the concept of the reconstructed porous/multiphase media (Torquato, 2000). For example, it is of great practical interest to understand how spatial variability in the microstructure of composites affects the failure characteristics of heterogeneous materials. The determination of the deformation under the stress of the porous material is important in porous packing of beds, mechanical properties of membranes (where the pressure applied in membrane separations is often large), mechanical properties of foams and gels, etc. Let us restrict our discussion to equilibrium mechanical properties in static deformations, e.g., effective Young s modulus and Poisson s ratio. The calculation of the impact resistance and other dynamic mechanical properties can be addressed by discrete element models (Thornton et al., 1999, 2004). 

In addition, other measurement techniques in the linear viscoelastic range, such as stress relaxation, as well as static tests that determine the modulus are also useful to characterize gels. For food applications, tests that deal with failure, such as the dynamic stress/strain sweep to detect the critical properties at structure failure, the torsional gelometer, and the vane yield stress test that encompasses both small and large strains are very useful. 

The addition of small amounts of alpha-omega dienes to the reactor, such as 1,9-decadiene, greatly increase LCB levels in polymers made with most catalysts, including Phillips catalysts. Even Ziegler catalysts, which are not known for producing LCB, produce large amounts when certain dienes are incorporated. Such polymers often exhibit some unusual properties, such as failure to dissolve in the normal solvents, probably because of the cross-linked gel-type matrix. Rheological measurements indicate extreme elasticity and memory. 

The authors noted "...This enantioselective catalysis should extend the seope of our strategy for tailored specific catalysts by the molecular imprinting method...". A review of these works exists Indeed, these results are of great interest, but it is necessary to note that Kaiser and Anderson were unable to repeat the results with the benzamide imprinted silicas. Their failure probably was the result of a short lifetime for the specific properties of such Al-doped silica gels. 

When isobornyl acrylate was used in place of MCEA, cured films with good pressure-sensitive adhesive properties were again obtained although somewhat longer irradiation times were required. For example, blends of poly (vinyl ethyl ether ) in IBA displayed properties ranging from 2 pli with adhesive failure and over 90 hours of shear to nearly 9 pli (cohesive failure) and 1 hour of shear (Table 7). The latter product was examined closely for complete cure and found to have 1.6 percent unreacted monomer at 6 sec. radiation and 0.23 percent at 9 sec. Again, a large gel fraction was present in the cured adhesive as noted earlier. 

Although the gel content indicated the presence of substantial cross-linking, the pressure-sensitive adhesive properties could be improved further by incorporating a small amount (1 percent) of pentaerythritol triacrylate (PETA). This system developed a peel value of 5.6 pli with adhesive failure and 22 hours of shear with only 6 seconds of cure time in air (Table 7). 

Finally, we are left with the conclusion that there does not exist a single answer, for all random coil polymers, to the question we have posed. This has been attributed (ref. 73) to the rather unusual properties of PEG in aqueous solution, and it may be that the conclusions drawn from the dextran data will prove to be more typical of random coil polymers in solution. This conclusion also seems to supply an explanation for the discrepancies observed by Kuga (ref. 75) in using dextran and PEG as probes to determine pore size distributions of gel substances used in SEC, as well as the failure of data from these two polymers to fall on the same universal calibration curve (ref. 76). The discrepancies in the two sets of data (Figs I and 2 of reference 56) are quite dramatic. Thus we have three independent studies of these two polymers by three different methods of analysis and they all clearly show that the differences in hydrodynamic behavior of these two polymers are real, and not artifacts due to the method of analysis. 

Gel texture properties for both heat-treated and pressure-treated gels were determined by torsion. The gels were cut into 2.8 cm lengths and formed into an hourglass shape with a 1 cm die on a lathe-type apparatus (Gel Consultants Inc., Raleigh, NC). The samples were subjected to torsional strain in a modified Brookfield viscometer (Gel Consultants Inc., Raleigh NC). Shear stress and shear strain, at failure, were calculated using equations developed by Hamann (1983). 

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