The PhD meeting of this month will take place on September 5th. Farhad will give a talk about:
Study of the chain dynamics in irradiated high density polyethylene (HDPE) by proton low-field double-quantum (DQ) NMR
time: 4 pm
place: SR 1.29 Von-Seckendorff-Platz 1
Abstract:
Industrial grade samples of HDPE with a broad molecular weight distribution, cross-linked via irradiation below the melting point, were probed by low-field 1H time domain NMR to provide information about the crystallinity in the semi-crystalline state and chain motion in the molten state. As the cross-linking reactions only occur in the amorphous region, the produced samples are not uniformly cross-linked, which means in the molten state, the sample contain a mixture of linear chains with different lengths, which follow different dynamic regimes, leading to very complex dynamics. For studying the chain motion with the double-quantum (DQ) NMR experiment, two methods were applied for analyzing the measured data. The first method, used for probing rubbers and hydrogels in previous studies, extracts the averaged residual dipolar coupling (Dres) which mirrors the amount of anisotropic motions caused by entanglements and cross-links. The second novel method is based on a power-law model of the orientation autocorrelation function (OACF) C(t), which estimates the amplitude of the OACF and the time scaling exponent κ (slope in the plot of C(t)~t-κ against time in logarithmic scale). The κ values, which can vary between ~0 (a perfect network) and ~0.5 (a linear entangled polymer) in this sample, can demonstrate how the cross-linking affects the chain motions in such a complex system. Complementary rheological measurements also showed that the decaying slope of storage modulus (G´) vs. angular frequency (ω) correlates with the irradiation intensity, which is similar to what κ values reflected.