The development of rheological nonlinearity of NR during aging has been investigated. • The evolution of the crosslink density and defects correlate well to rheological response. • The parameter from FT-Rheology I 3/1 exhibits higher relative sensitivity. • With the limitations of NMR and microscope, I 3/1 respond to the network change at both nano-and micrometer scales.
Production of controlled rheology polypropylenes (CRPPs) is practiced industrially by modifying existing commodity Ziegler-Natta resins through peroxide-induced β-scission reactions, resulting in materials with controlled rheological properties and accompanying narrower molecular weight distributions (MWDs). In this work, this methodology was studied using both metallocene-based polypropylenes (mPPs) and Ziegler-Natta-based polypropylenes (ZN-PPs). Numerical simulations based on a previously proposed kinetic model indicated that the nature of the starting resin has a significant effect on the control of MWD polydispersity index (PDI) and weightaverage molecular weight (M W ) of the resulting CRPP. Based on these observations, experiments were carried out to demonstrate the feasibility of producing CRPP with targeted molecular and rheological characteristics. Commercial mPP and ZN-PP resins were selected to produce CRPP with similar M W or melt flow rates (MFRs) but varying PDIs. The rheological properties and MWDs of these materials were evaluated through oscillatory shear and gel permeation chromatography (GPC) measurements and their extrusion behavior was briefly studied and assessed with respect to these properties.
The cross-linking process of natural rubber (NR) is studied for the first time by a unique combination of rheology and time-domain (TD) NMR, two methods that are commonly conducted separately. The combined setup consists of a high-end rheometer and a compact 25 MHz 1 H NMR relaxometer (rheo-NMR), which allows rheological characterization and NMR relaxometry to be operated simultaneously on the same sample. During vulcanization, the elastic modulus G′(t), representing the macroscopic elasticity, was directly correlated to the molecular dynamic parameters derived from NMR, including the populationaveraged transverse relaxation rate, 1/T 2ave , and the logarithmic mean of the residual dipolar coupling constant, D̅ res . It is found that, at the same level of macroscopic elasticity, the conformational flexibility of network chains in NR cross-linked with the so-called efficient vulcanization (EV) is more restricted than that with conventional vulcanization (CV). This can be attributed to the shorter and consequently less flexible (monosulfidic) cross-links formed by EV, whereas polysulfidic links dominate in the CV vulcanizate as confirmed by high-field 13 C NMR spectroscopy.
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