Conversion topological limit

The reaction can be truly adiabatic for a wide range of experimental conditions. It initiates in the kinetic region and reaches a limit of 94-96 % conversion, which corresponds to the topological limit for these systems 5>14, l617-5169). The reaction kinetics is well described by the adopted mechanism 51,69 and the resulting experimental kinetic and thermodynamic reaction parameters are similar to those obtained in isothermal experiments. 

In conclusion, it can be said that the theory can well describe the development of the gel structure. The correlation between the equilibrium modulus and sol fraction is very good so that the sol fraction can alternatively be used for determination of the concentration of EANC s if an accurate and precise determination of conversion meets with difficulties. It is to be recalled here that the Gaussian rubber elasticity theory does not apply to highly crosslinked networks of usual stoidiiometric systems. When a good theory is available, the calculated value of taking possibly into account the topological limit of the reaction will be ne ed. 

Figure 3.11 Conversion versus temperature transformation diagram showing conditions under which gelation, vitrification and chemical degradation take place, as well as restriction arising from topological limitations for novolac epoxy resin cured with DDS. Reprinted with permission from RA. Oyanguren and R.J.J. Williams,/owmia/ of Applied Polymer Science, 1993, 47,1361 1993,...

The concept of an ultimate conversion lower than 100% is reasonable in view of steric restrictions in a highly crosslinked network. Oleinik has reported an ultimate conversion of 92% in a diepoxide-diamine stem. This ultimate conversion value is attributed to topological limitations and is consistent with computer simulations Hale reports an experimental ultimate conversion of less than 85% for a 1 1 stoichiometry ECN-phenolic cresol novolac (PCN) system which is similar to the one in this study. The lower ultimate conversion seen in the ECN-PCN stem is in agreement with greater steric hindrances arising from the cresol pendant group. The topological constraints on the ultimate conversion will increase as functionahty of the reactive molecules also increases. 

The rate-limiting step in the kinetic pathway of nucleotide incorporation is the conversion of the E p/t dNTP complex to the activated complex, E p/t dNTP (Step 3 in Fig. 1). This step is crucial in many respects. First, it is essential for the phosphoryl transfer reaction to occur. During the E p/t dNTP to E p/t dNTP transition, all the components of the active site are assembled and organized in a topological and geometrical arrangement that allows the enzyme to proceed with the chemical step (Step 4). Second, Step 3 plays a major role in the mechanism of discrimination between correct versus incorrect nucleotides. Interpretation of the kinetic measurements has led to the hypothesis that the E p/t dNTP... 

This assumption leads to a very important conclusion about the existence of the topological conversion limit a °p in the completely cured system. 

Following the work of Mishra and co-workers (53), Dean and co-workers (54,55) investigated the SIN syntheses of dimethacrylate- and epoxy-based materials by near-infrared and other methods. The rates and order of the polymerizations were systematically altered. The final conversion of the dimethacrylate was limited by the vitrification or topological restraint of the SIN. Whether one or two glass transitions are observed depends on the relative rates and order of the polymerizations. 

For some highly crosslinked thermosetting polymers with rigid backbones, topological restrictions limit the maximum attainable conversion. For the reaction between epoxidized novolacs and cresol-based novolacs, a maximum conversion, Xmax= 0.80 was reported (Hale et al., 1991). A similar value of x ,ax was reported for the cure of an epoxidized novolac with 4,4 -diaminodiphenylsulfone (DDS) (Oyanguren and WilKams, 1993a). In these cases, partially reacted networks may be modeled as a random solution of reacted functionalities with a concentration equal to x/x ax... 

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