Inhomogeneities and Chain Dynamics in Diene Rubbers Vulcanized with Different Cure Systems

Abstract

In this study, we focus on qualitative differences in the network structure and dynamics of natural as well as poly(butadiene) rubber in dependence of the cure system (sulfur/accelerator or organic peroxide) used in the vulcanization process. The spatial homogeneity of the distribution of chemical and physical cross-links in the network is assessed via the quantitative measurement of proton-proton residual dipolar couplings as measured by static multiple-quantum (MQ) NMR spectroscopy at low field. The experiment also provides information on the apparent correlation time of fast segmental fluctuations that dominate chain relaxation processes at lower temperature, for which we also find characteristic differences. Vulcanization via a radical mechanism (using organic peroxides) leads to networks with a high content of nonelastic defects (loops or dangling chains), a rather inhomogeneous distribution of cross-links, and modified (slower) local dynamics, as compared to networks obtained by sulfur vulcanization. These microstructural factors can be related with the well-known differences in the macroscopic properties of diene rubbers vulcanized with different cure systems.

Epoxy resin curing reaction studied by proton multiple-quantum NMR

“…3(d)], according to the procedure explained elsewhere. 8,23 The sum of both experimental data (I DQ 1 I ref ) comprises the full magnetization of the sample, that is, signal from dipolar coupled network segments (including DQ and higher-order coherences as well as dipolar-encoded longitudinal magnetization), and signal from dipolar uncoupled (isotropically mobile) protons. The individual fractions are characterized by rather different relaxation behavior, which allow the FULL PAPER identification and subtraction of the exponential long-time contribution of dipolar uncoupled protons to the total magnetization:…”

“…Nevertheless, due to its selectivity, double-quantum (DQ) or more generally multiple-quantum (MQ) NMR is nowadays considered as the most precise and effective experimental approach for the quantification of weak RDCs of soft polymers, 8 and thus one of the most powerful and successful tools for the quantitative characterization of dynamics in polymer melts, [17][18][19] the gel point, 20 vulcanization kinetics, 21 and the complete and quantitative characterization of polymer network structures. [22][23][24][25] In those studies, MQ experiments are based upon the lengthy but quite robust Baum-Pines (B-P) sequence 8,26,27 (shown in Fig. 1), which shows ideal performance for studying soft (weaker dipolar-coupled) polymers systems such as elastomers [23][24][25]28,29 and gels.…”

“…[22][23][24][25] In those studies, MQ experiments are based upon the lengthy but quite robust Baum-Pines (B-P) sequence 8,26,27 (shown in Fig. 1), which shows ideal performance for studying soft (weaker dipolar-coupled) polymers systems such as elastomers [23][24][25]28,29 and gels. 22,30 Nevertheless, the use of B-P sequence is limited for characterizing high dipolar coupled polymers (epoxy resins would belong to this type of samples), as it was recently demonstrated in the study of ionic elastomers.…”

ESR investigation of NR and IR rubber vulcanized with different cross-link agents

“…ionic or free radical pathways, is still ongoing [1][2][3][4]. Several works conclude that radical mechanisms are required to justify the crosslink structures formed on NR after the vulcanization with sulfur [2][3][4][5][6]: sulfur cross-links of different lengths, mainly polysulfidic bonds, pendant groups, cyclic sulfides and cis/trans isomerization of the double bonds and/or conjugated sequences in the polymer backbone [7]. In addition, the low reaction efficiency and the long time required to complete vulcanization process do not recommend the use of sulfur without accelerators for commercial purposes [6].…”

“…The NR samples usually show different and characteristic mechanical and viscoelastic properties, fatigue and ageing resistance according to the sulfur/accelerator system employed in the vulcanization process [1,7]. On the other hand, vulcanization of NR with organic peroxides provides materials with higher concentration of non-elastic network defects and rubber network structures characterized by the formation of C-C cross-links with more heterogeneous spatial distribution, as compared to the samples vulcanized with sulfur-accelerators [6]. These structural differences have a critical impact on the drop of elastic properties of these materials, being caused by the complex free radical mechanism (with different radical pathways and side reactions) that characterizes the vulcanization of NR with organic peroxides.…”