Mahdi Abbasi

Investigation of the rheological behavior of industrial tubular and autoclave LDPEs under SAOS, LAOS, transient shear, and elongational flows compared with predictions from the MSF theory

The molecular structure of two different types of industrial low-density polyethylenes (LDPE), i.e., tubular and autoclave, was investigated by gel permeation chromatography and different rheological methods and then compared with predictions from theoretical modeling. Linear rheological data generated from small amplitude oscillatory shear (SAOS) are presented using the framework of a Van Gurp-Palmen plot while nonlinear rheological data obtained from either transient shear, transient extension, or medium amplitude oscillatory shear (MAOS) are compared with simulations using a generalized form of the molecular stress function theory with only three nonlinear material parameters: a 2, β, and f max which represent the dissipation in simple shear flow, the ratio of molar mass of the branched polymer to the molar mass of the backbone, and the maximum stretching of the polymer chain before chains slip past one another without further stretch, respectively. The effect of these parameters on the predictions is investigated for the aforementioned nonlinear deformations. As a result of these comparisons, a 2 was found to have a constant value within experimental error (a 2 = 0.07 ± 0.03) when simulating the shear stress growth coefficient. Next, β was defined by fitting the extensional data with this model resulting in a constant value of 1.8 for tubular LDPEs and a value between 1.6 and 2.1 for autoclave LDPEs depending on the molecular structure. The universality of these β values was found by simulating the intrinsic nonlinearity Q 0 ( ω ) ≡ lim γ 0 → 0 I 3 / 1 / γ 0 2 (defined by Fourier transform rheology method) in the MAOS region. Finally, f max was evaluated by fitting the model to tensile stress growth coefficient data. It was found that f max is independent of the extensional rate for tubular LDPEs but exhibits a power law behavior with the extensional rate for autoclave LDPEs. Our investigations demonstrated that nonlinear deformations instead of SAOS deformations are the preferred method for characterizing the molecular structure of these polymers due to its sensitivity to the complex structure of LCB present in these materials along with their broad molecular weight distribution.

Linear and Nonlinear Rheology Combined with Dielectric Spectroscopy of Hybrid Polymer Nanocomposites for Semiconductive Applications

The linear and nonlinear oscillatory shear, extensional and combined rheology-dielectric spectroscopy of hybrid polymer nanocomposites for semiconductive applications were investigated in this study. The main focus was the influence of processing conditions on percolated poly(ethylene-butyl acrylate) (EBA) nanocomposite hybrids containing graphite nanoplatelets (GnP) and carbon black (CB). The rheological response of the samples was interpreted in terms of dispersion properties, filler distortion from processing, filler percolation, as well as the filler orientation and distribution dynamics inside the matrix. Evidence of the influence of dispersion properties was found in linear viscoelastic dynamic frequency sweeps, while the percolation of the nanocomposites was detected in nonlinearities developed in dynamic strain sweeps. Using extensional rheology, hybrid samples with better dispersion properties lead to a more pronounced strain hardening behavior, while samples with a higher volume percentage of fillers caused a drastic reduction in strain hardening. The rheo-dielectric time-dependent response showed that in the case of nanocomposites containing only GnP, the orientation dynamics leads to non-conductive samples. However, in the case of hybrids, the orientation of the GnP could be offset by the dispersing of the CB to bridge the nanoplatelets. The results were interpreted in the framework of a dual PE-BA model, where the fillers would be concentrated mainly in the BA regions. Furthermore, better dispersed hybrids obtained using mixing screws at the expense of filler distortion via extrusion processing history were emphasized through the rheo-dielectric tests.

Influence of molecular structure on the foamability of polypropylene: Linear and extensional rheological fingerprint

The foaming structure and rheological properties of four different isotactic homo-polypropylenes with various molecular weights and an isotactic long chain branched polypropylene were investigated to find a suitable rheological fingerprint for PP foams. The molecular weight distribution and thermal properties were measured using GPC-MALLS and differential scanning calorimetry, respectively. Small amplitude oscillatory shear data and uniaxial extensional experiments were analyzed using the frameworks of van Gurp-Palmen plot (δ vs. |G*|) and the molecular stress function model, respectively. These analyses were used to find a correlation between the molecular structure, rheological properties and foaming structures of linear and long chain branching polypropylenes. Two linear viscoelastic characteristics, |G*| at δ = 60° and |η*| at ω = 5 rad/s were used as criteria for foamability of these polymers, where decreasing of both parameters by increasing the long chain branching content results in smaller cell size and higher cell density. The molecular stress function model was able to quantify the strain hardening properties of long chain branching blends using small amplitude oscillatory shear data and two nonlinear material parameters, 1 ≤ β ≤ 2.2 and 1 ≤  f max 2  ≤ 600, where the minimum and maximum values of these parameters belong to the linear and long chain branched polypropylene, respectively. Increasing the long chain branched polypropylene content of the PP blends increased strain hardening, and therefore improved the foaming characteristics significantly by suppressing the coalescence of cells. Dilution of linear PP with only 10 wt% of long chain branched polypropylene enhanced the cell density from 5.7 × 106 to 2.7 × 107 cell/cm3 and reduced the average cell diameter from 58 to 26 µm, respectively, while their volume expansion ratio remained in the same range of 2–3. Increasing of long chain branching to 50 and 100 wt% enhanced the V.E.R. to 6.2 and 7.8, respectively.

Stability of Diels–Alder photoadducts in macromolecules

We explore the stability of o-quinodimethane-based ligation points generated by a popular photo-induced Diels–Alder reaction, assessing their ability to withstand challenging environments. Our molecular assessment includes size-exclusion chromatography as well as high resolution electrospray ionization mass spectrometry (SEC-HR ESI MS), finding that their stability is dependent on the macromolecular chain length.

The intrinsic mechanical nonlinearity 3Q0(ω) of linear homopolymer melts

Medium amplitude oscillatory shear (MAOS) in combination with Fourier Transformation of the mechanical stress signal (FT rheology) was utilized to investigate the influence of molecular weight, molecular weight distribution and the monomer on the intrinsic nonlinearity 3Q0(ω). Nonlinear master curves of 3Q0(ω) have been created, applying the time-temperature superposition (TTS) principle. These master curves showed a characteristic shape with an increasing slope at small frequencies, a maximum 3Q0,max and a decreasing slope at high frequencies. 3Q0(De) master curves of monodisperse polymers were evaluated and quantified with the help of a semi-empiric equation, derived from predictions from the pom-pom and molecular stress function (MSF) models. This resulted in a monomer independent description of the nonlinear mechanical behavior of linear, monodisperse homopolymer melts, where 3Q0(ω,Z) is only a function of the frequency ω and the number of entanglements Z. For polydisperse samples, 3Q0(ω) showed a hig...