Chain extensibility, maximum

The formation of a phenolic resin is often formally separated into two steps, though it probably should be three. If we use a three-step model, the first step is activation of the phenol or aldehyde. The second step is methylolation, and the third is condensation or chain extension. In addition to the clarity provided by the formalism, these steps are also generally separated in practice to provide maximum control of exothermic behavior, with the strategy being to separate the exotherm from each step from that of the others as much as possible. As there are significant differences in the activation step and in the details of the methylolation and condensations steps of novolacs and resoles, we will treat the two types separately. 

A second important characteristic is the value otj. of the elongation at which rupture occurs. The corresponding values of r/rm show that rupture generally occurred at approximately 80-90% of maximum chain extensibility (12). These quantitative results on chain dimensions are very important but may not apply directly to other networks, in which the chains could have very different configurational characteristics and in which the chain length distribution would presumably be quite different from the very unusual bimodal distribution intentionally produced in the present networks. 

Figure 9. The end-to-end distance per skeletal bond n for regular conformations of polydimethylsiloxane and polyethylene network chains (12). Maximum extensibility rm of this chain molecule occurs at rm/n = 1.34 A.

By neglecting the chain extension it is assumed that Wb=Wbs. Thus the observation that Wb is approximately constant for fibres of the same polymer with different degrees of orientation means that not only a constant critical shear stress, rb, but also a maximum shear strain, (0-0b), is a useful criterion of... 

Equations (28) and (29) are derived from the statistical theory based on the Gaussian statistics which describes the network behaviour if the network is not deformed beyond the limit of the applicability of the Gaussian approximation33). For long chains, this limit is close to 30 % of the maximum chain extension. For values of r, which are comparable with rmax, the force-strain dependence is usually expressed using the inverse Langevin function 33,34)... 

Using Fourier Transform Infra Red (FTIR) spectroscopy, the reaction can be monitored. The progress of the reaction also can be followed by studying the reduction of the primary hydroxyl peaks in the mid-infrared spectra. The urethane band with the approximate wave number 1739 will be forming. The NCO band at 2273 will decrease from a maximum and then stabilize as the chain extension proceeds. 

The temperature of the transition maximum, for solution processed films, decreased significantly with increasing BF MEA concentration. This indicates a lower degree of crosslinking as catalyst is added to the resin. The decrease in crosslinking can be attributed to the increased importance of the catalyzed homopolymerization reaction, which is primarily a chain extension reaction due to the high diffusive mobility of molecules in solution. 

It is possible to characterize this non-Gaussian limited extensibility more quantitatively in a number of ways. The first involves the interpretation of limited chain extensibility in terms of the configurational characteristics of the PDMS chains making up the network structure. The upturn in modulus generally begins at approximately 60-70% of maximum chain extensibility. " This value is approximately twice that estimated previously- from stress-strain isotherms of elastomers that may have been undergoing strain-induced crystallization. 

A second important characteristic of an elastomeric network is the the elongation at rupture. Results on PDMS indicate that rupture occurs at approximately 80-90% of maximum chain extensibility. ... 

It is also useful, at this juncture, to consider Fig. 9.7 which is based on PPDI and observe that the maximum temperature for thermal stability appears to be 140°C when the chain extension system BDO + 1,4-CHDM is used this corresponds to the analogous system used with CHDI. 

If the chain extension approaches its maximum value, it is possible to use a more precise expression for the distribution of end-to-end distances as derived by Kuhn et... 

The experiments show that the increase of the elasticity modulus starts at a lower degree of swelling for suffer and shorter subchains, because thqr approach their maximum extensibility at smaller content of solvent. Thus, in swollen gels the mechanical behavior is largely influenced by chain extension leading to non-Gaussian effects. 

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