Abstract
Medium or large amplitude oscillatory shear (M/LAOS) is sensitive to polymer chain structure, yet poses unsolved challenges for 'a priori' structural characterisation. In this letter, we present a MAOS protocol applied to near-monodisperse linear polymer melts, from which chain-stretch relaxation, a key structural feature, is clearly discernible. The third harmonics of MAOS frequency sweeps are decomposed into real and imaginary components and found to obey time-temperature superposition. The GLaMM molecular tube-based model of linear entangled melt rheology and structure, which has no free parameters, closely follows the form of our experimental results for the third harmonics and contains discriminatory features which depend only on the polymer's chain stretch relaxation time. Significantly, these third harmonic features occur at low frequency and are readily accessible with standard rheometers. For materials where phase transitions restrict the use of time temperature superposition, this method greatly increases the scope of rotational rheometry for structural analysis of polymers. Although the theory shows good qualitative agreement with experimental data, we find fundamental differences in magnitude and the frequency dependence of the third harmonics which must be resolved in order to fully understand the molecular basis of the stress response.
Highlights
Oscillatory shear rheology is sensitive to the microstructure of complex soft materials
LAOS is complementary to small-amplitude measurements, and results are a complex function of molecular architecture such as linear [14], star [15], and comb architectures [16] and can be used to quantify the level of branching in industrial resins such as metallocene-catalyzed sparsely branched HDPEs [17,18] or tubular-reacted randomly branched LDPEs [18,19]
In this paper we focus on Fourier transform rheology (FTR) because this has been shown to be a sensitive enough technique to isolate small nonlinearities in the material stress response, either from shear stress [5,24,25,26,27] or from the first normal stress [28]
Summary
Oscillatory shear rheology is sensitive to the microstructure of complex soft materials (e.g., polymers [1,2,3] or immiscible blends [4]). The technique subjects a fluid sample to oscillatory shear strain at a given amplitude and frequency, γ (t ) = γ0 sin(ωt ), and analyzes the stress response. LAOS is complementary to small-amplitude measurements, and results are a complex function of molecular architecture such as linear [14], star [15], and comb architectures [16] and can be used to quantify the level of branching in industrial resins such as metallocene-catalyzed sparsely branched HDPEs [17,18] or tubular-reacted randomly branched LDPEs [18,19]. LAOS is not yet a standard analytical tool for characterizing molecular architecture
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