A rapid and efficient method to obtain self-healing epoxy resins is discussed. This method is based on the use of a thiol-disulfide oligomer obtained by partial oxidation of a multifunctional thiol using a hypervalent iodine (III) compound as oxidant. The oligomer was characterized by Fourier transform infrared spectroscopy (FTIR), Raman and nuclear magnetic resonance spectroscopies, and gel permeation chromatography (GPC). The oligomer was a joint component of the thiol-ene system along with a tetra-allyl-functionalized curing agent. The kinetics of the photopolymerization of diglycidylether of bisphenol A (DGEBA) revealed that conversions of the epoxy groups as high as 80% were achieved in only 15 minutes by increasing the concentration of the thiol-ene system in the formulation. The disulfide bonds introduced in the copolymer using the thiol-disulfide oligomer allowed the repairing of the test specimens in as little as 10 minutes when the specimens were heated at 80°C or for 500 minutes at room temperature. The analysis of the mechanical properties using dynamic mechanical analysis (DMA) showed that the specimens displayed a healing efficiency up to 111% compared with the unhealed specimens, depending on the amount of polythioethers present in the copolymer.
The dynamic mechanical behavior of linear and slightly cross-linked poly(vinyl chloride) was studied in the region of the ß relaxation. The relaxation spectrum has been described by an alternative relaxation time distribution based on a log normal function. By this method we have obtained distributions of the activation energy and entropy. We have found a very large distribution for the activation energy but not for the activation entropy. Moreover, cross-linking removes the high-energy components of the ß relaxation process.
I. IntroductionGenerally, amorphous polymers show at least two relaxations: the main one, named the a relaxation, is related to the glass transition. The relaxations occurring at lower temperature are called secondary (or sub-Tg) relaxations. The secondary relaxations are labeled, ß, , and 6 in order of decreasing temperature.Secondary relaxations manifest neither a very noticeable change in the state of the material (such a change from the "glassy" to the "rubbery" state) nor a high jump in the heat capacity (Cp). Nevertheless, significant changes are observed in the physical properties of amorphous polymers going through secondary relaxations (for instance, physical aging).1 The existence of ß relaxation in a glass is actually regarded as a result of thermally activated molecular motions at local sites which are randomly dispersed in the disordered structure. In this structure the fluctuations of density remain frozen in all along measurements.2Most mechanical and dielectric studies of poly(vinyl chloride) (PVC) have been made on the conventional polymer. These studies have shown that at least two relaxation processes, a and ß, occur.Many works have been published about ß relaxation in PVC. It is generally accepted that this relaxation is due to a movement of small segments of the main chain since the chlorine substituents in PVC are attached rigidly to the main polymer chain.3-5 Moreover, Havriliak et al.6 have found that the intermolecular relaxation time is about 1/1000 that of the intramolecular value. These results suggest that the restrictions to segmental orientation come from chain stiffness and not from chain-to-chain interactions. The activation energy of the ß relaxation is of about 50-65 kJ/mol.4It is well-known that the structure of PVC is complicated and sensitive to processing conditions. However, the main features of the ß relaxation are not strongly affected by crystallinity, microstructure, tacticity, molecular weight, and the thermal history of the samples.7•8 Several authors have reported the existence of two ß relaxations in PVC. Kakutani et al.3 had found two relaxations (ß and $2) for PVC polymerized at -30 °C.
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