The key interactions between elastomer, filler, and coupler in uncured and cured compounds were studied by two complementary analytical techniques, low field 1H solid state nuclear magnetic resonance (LF NMR) and pyrolysis gas chromatography / mass spectrometry (PY GC/MS). LF NMR was utilized to characterize the macrostructure, and more specifically, the mobility of the elastomer chains in the polymeric matrix, while PY GC/MS provided information on the microstructure of the compounds analyzed.
The networks of the compounds studied were found to have three components: a tightly bound region (Tr), a loosely bound region (Tm1), and an extractable unbound rubber region (Tm2). Based upon the mobility of these three regions and the peak intensity ratios determined from PY GC/MS, conclusions were drawn about the nature of the filler-elastomer interaction in samples with different polymers (PI and SBR), fillers (CB and silica), couplers (NXT and Si-75) and cure states (uncured and cured).
It was found that the network of the SBR compounds was more tightly bound and significantly different from that of PI compounds regards of cure state, filler type, or coupler addition. This was attributed to the SBR network having restricted motion due to the phenyl ring on the backbone of the elastomer chain causing steric effects.
In regards to the PI compounds, the nature of the interaction between elastomer chains and the different fillers was determined. Strong physical adsorption interactions between elastomer and filler dominated when CB was used as the filler. The addition of coupler to this system resulted in increased mobility of elastomer chains and higher concentrations of dimer species. This suggested that the coupler served as a molecular lubricant between the CB and PI chains and thus reduced physical adsorption effects.
A strong physical adsorption interaction was proposed between silica and PI in the uncured state. The addition of coupler in the uncured state resulted in an increased dispersion of the silica aggregates and less polarization effects of the silica on the PI chains. Upon curing, the nature of the interaction between coupler, filler, and elastomer was confirmed to be chemisorption. Furthermore, a more developed silica-coupler network occurred with coupler loading based on the increasing mobility of the elastomer chains in the loosely bound region and higher dimer concentrations. The silica-coupler network was found to be the most developed when NXT was used as the coupler based on the PY GC/MS.