Investigation of nonlinear rheological behavior of linear and 3-arm star 1,4-cis-polyisoprene (PI) under medium amplitude oscillatory shear (MAOS) flow via FT-rheology

Enhancing melt strength and crystallization kinetics in polylactide: Influence of chain topology

The generation of long-chain branches (LCB) in biobased and biodegradable polylactide (PLA) by adding different amounts of a chain extender is studied. The rheological and calorimetric behavior have been used to determine the effect of LCB presence and their topology on PLA melt strength and crystallization behavior. Rheological modeling of linear and non-linear viscoelastic shear and extensional properties identified several possible branched structures. Moreover, remarkable differences were observed for the different topologies regarding the intrinsic non-linear parameters and the intra-cycle elastic and viscous non-linearities. Differential scanning calorimetry and polarized light optical microscopy measurements revealed a significant increase in the nucleation density and rate of PLA with increasing the amount of LCB, albeit they provoke a decrease in the growth rate due to a reduction in chain diffusion. Nevertheless, overall crystallization rate values revealed a predominant effect of nucleation over crystal growth. The introduction of LCB within the chains is highly beneficial as they increase nucleation, crystallinity, and elongational viscosity, thus improving the properties of biodegradable PLA.

Distinguishing between Linear and Star Polystyrenes with Unentangled Arms by Dynamic Oscillatory Shear Tests

Linear and nonlinear viscoelastic properties of star polystyrene (PS) melts with unentangled arms were measured by using small-amplitude and medium-amplitude oscillatory shear (SAOS and MAOS) tests. For comparison purposes, such tests were also conducted on entangled linear and star PS melts. Interestingly, the linear viscoelastic properties of unentangled star PS were quantitatively described using the Lihktman-McLeish model for entangled linear chains, indicating that unentangled stars were indistinguishable from linear chains by using relaxation spectra. By contrast, the relative intrinsic nonlinearity (Q0), one of the MAOS material functions, exhibited a difference between unentangled star and linear PS. When the maximum Q0 value (Q0,max) was plotted against the entanglement number of span molecules (Zs), unentangled star PS exhibited larger Q0,max values than linear PS, which was quantitatively predicted via the multimode K-BKZ model. Therefore, in the unentangled regime, star PS was concluded to be characterized by intrinsically higher relative nonlinearity than linear PS.

Large amplitude oscillatory shear study of a colloidal gel near the critical state

A system undergoing sol-gel transition passes through a unique point, known as the critical gel state, where it forms the weakest space spanning percolated network. We investigate the nonlinear viscoelastic behavior of a colloidal dispersion at the critical gel state using large amplitude oscillatory shear rheology. The colloidal gel at the critical point is subjected to oscillatory shear flow with increasing strain amplitude at different frequencies. We observe that the first harmonic of the elastic and viscous moduli exhibits a monotonic decrease as the material undergoes a linear to nonlinear transition. We analyze the stress waveform across this transition and obtain the nonlinear moduli and viscosity as a function of frequency and strain amplitude. The analysis of the nonlinear moduli and viscosities suggests intracycle strain stiffening and intracycle shear thinning in the colloidal dispersion. Based on the insights obtained from the nonlinear analysis, we propose a potential scenario of the microstructural changes occurring in the nonlinear region. We also develop an integral model using the time-strain separable Kaye-Bernstein-Kearsley-Zapas constitutive equation with a power-law relaxation modulus and damping function obtained from experiments. The proposed model with a slight adjustment of the damping function inferred using a spectral method, compares well with experimental data at all frequencies.

Linear and nonlinear oscillatory rheology of chemically pretreated and non-pretreated cellulose nanofiber suspensions

Linear and nonlinear rheological properties of cellulose nanofiber (CNF) suspensions were measured under small and large amplitude oscillatory shear (SAOS and LAOS) flow. Four different CNFs were produced, two by only mechanical disintegration and two with chemical pretreatments. Linear viscoelastic properties distinguished chemically treated CNFs from two untreated fibers via a different scaling exponent of the elastic modulus. However, different mechanical fibrillation degree was not characterized via linear viscoelastic properties. In contrast, nonlinear viscoelastic properties reflected both effects of chemical pretreatments and mechanical fibrillation. More fibrillated CNFs exhibited nonlinear rheological phenomena at larger deformations. In addition, chemically treated CNFs exhibited greater network stiffness and higher network recovery rates due to the presence of charged functional groups on the fiber surfaces. A material-property co-plot showed that network stiffness and recovery rate were in a trade-off relationship.

A Simple Approach to Control the Physical and Chemical Features of Custom-Synthesized N-Doped Carbon Nanotubes and the Extent of Their Network Formation in Polymers: The Importance of Catalyst to Substrate Ratio

This study intends to reveal the significance of the catalyst to substrate ratio (C/S) on the structural and electrical features of the carbon nanotubes and their polymeric nanocomposites. Here, nitrogen-doped carbon nanotube (N-MWNT) was synthesized via a chemical vapor deposition (CVD) method using three ratios (by weight) of iron (Fe) catalyst to aluminum oxide (Al₂O₃) substrate, i.e.,1/9, 1/4, and 2/3, by changing the Fe concentration, i.e., 10, 20, and 40 wt.% Fe. Therefore, the synthesized N-MWNT are labelled as (N-MWNTs)₁₀, (N-MWNTs)₂₀, and (N-MWNTs)₄₀. TEM, XPS, Raman spectroscopy, and TGA characterizations revealed that C/S ratio has a significant impact on the physical and chemical properties of the nanotubes. For instance, by increasing the Fe catalyst from 10 to 40 wt.%, carbon purity increased from 60 to 90 wt.% and the length of the nanotubes increased from 1.2 to 2.6 µm. Interestingly, regarding nanotube morphology, at the highest C/S ratio, the N-MWNTs displayed an open-channel structure, while at the lowest catalyst concentration the nanotubes featured a bamboo-like structure. Afterwards, the network characteristics of the N-MWNTs in a polyvinylidene fluoride (PVDF) matrix were studied using imaging techniques, AC electrical conductivity, and linear and nonlinear rheological measurements. The nanocomposites were prepared via a melt-mixing method at various loadings of the synthesized N-MWNTs. The rheological results confirmed that (N-MWNTs)₁₀, at 0.5–2.0 wt.%, did not form any substantial network through the PVDF matrix, thereby exhibiting an electrically insulative behavior, even at a higher concentration of 3.0 wt.%. Although the optical microscopy, TEM, and rheological results confirmed that both (N-MWNTs)₂₀ and (N-MWNTs)₄₀ established a continuous 3D network within the PVDF matrix, (N-MWNTs)₄₀/PVDF nanocomposites exhibited approximately one order of magnitude higher electrical conductivity. The higher electrical conductivity of (N-MWNTs)₄₀/PVDF nanocomposites is attributed to the intrinsic chemical features of (N-MWNTs)₄₀, such as nitrogen content and nitrogen bonding types.

Small and Medium Amplitude Oscillatory Shear Rheology of Model Branched Polystyrene (PS) Melts

：Linear and nonlinear rheological properties of model comb polystyrenes (PS) with loosely to densely grafted architectures were measured under small and medium amplitude oscillatory shear (SAOS and MAOS) flow. This comb PS set had the same length of backbone and branches but varied in the number of branches from 3 to 120 branches. Linear viscoelastic properties of the comb PS were compared with the hierarchical model predictions. The model underpredicted zero-shear viscosity and backbone plateau modulus of densely branched comb with 60 or 120 branches because the model does not include the effect of side chain crowding. First- and third-harmonic nonlinearities reflected the hierarchy in the relaxation motion of comb structures. Notably, the low-frequency plateau values of first-harmonic MAOS moduli scaled with Mw-2 (total molecular weight), reflecting dynamic tube dilution (DTD) by relaxed branches. Relative intrinsic nonlinearity Q0 exhibited the difference between comb and bottlebrush via no low-frequency Q0 peak of bottlebrush corresponding to backbone relaxation, which is probably related to the stretched backbone conformation in bottlebrush.