Interplay of Segmental and Normal Mode Dynamics in Polymer Networks Undergoing Chemical Cross-Linking. Epoxy/Amine-Terminated Linear and Star PPO Formulations

Interplay between structure and relaxation in polyurea networks: the point of view from a novel method of cooperativity analysis of dielectric response

The influence of structural constraints on the relaxation dynamics of three polyurea networks with a varying degree of crosslinking, has been studied by means of a thorough analysis of broadband dielectric spectroscopy measurements. Two different relaxation processes are observed, namely, a fast process involving the soft poly(propylene oxide) chains, and a slower and much broader process associated with the immediate surroundings of the hard crosslinkers. Microphase separation in soft and hard domains characterizes the systems in the presence of hydrogen bonding. In this case, different confinement conditions are explored by varying the soft chain length; overall, so called "adsorption" effects dominate. With respect to both cooperativity and the rearrangement energy threshold in fast relaxation, it is found that the enhancement of configurational constraints is similar to cooling, but only on qualitative grounds. An upper bound of the hard domains' interface thickness, in which the slow relaxation is believed to take place, is estimated from the analysis of the fast relaxation in the system characterized by the highest degree of confinement, taking into account the results of the structural analysis. Dropping the hydrogen bonding mechanism, phase separation does not occur anymore and the configurational constraints at the ends of the soft chains are reduced, leaving just those imposed by the rigid crosslinkers. This leads to a significant increase in cooperativity on approaching the glass transition, and to a complex behavior that is thoroughly discussed in comparison with those observed in the micro-segregated systems.

Study of the infrared spectral features of an epoxy curing mechanism

In this work, the isothermal curing process of diglycidyl ether of bisphenol A (DGEBA) cured with 4,4'-diaminodiphenylmethane (DDM) was monitored in situ by mid-infrared (MIR) and near-infrared (NIR) spectroscopy. With the help of generalized two-dimensional (2D) correlation analysis, the results obtained showed that, during curing, the change of amine and epoxy groups was simultaneous, taking place prior to the change of hydroxyl groups, followed by the change of CH(2)/CH groups, resulting from the ring-opening reaction of epoxy groups. In addition, 2D MIRxNIR hetero-spectral correlation analysis and second-derivative analysis were also employed, by means of which direct evidence of the curing mechanism could be obtained and obscure NIR band assignments in the overlapped CH combination region could be made.

Interconversion of mechanical and dielectrical relaxation measurements for dicyclohexylmethyl-2-methyl succinate

A comparison between results of dielectrical relaxation and dynamic mechanical spectroscopies is carried out for the alpha-relaxation of the ester dicyclohexyl methyl-2-methyl succinate (DCMMS). The results for the dielectric permittivity and the shear modulus measurements are presented according to the empirical Havriliak-Negami (HN) equation. By using the time-temperature principle a master curve in each case was obtained for several temperatures. The comparative analysis presented here is based on the assumption of a relationship between rotational and shear viscosities. The former one is associated to the dielectrical relaxation, whereas the latter is associated to mechanical relaxation. Both viscosities are not necessarily equal in general, and we assume that the difference between them is an important factor to appropriately compare the dielectrical and mechanical results.