The effect of molecular properties on the rheological and fatigue behaviors of solid polystyrene (PS)/polyisoprene (PI) block copolymers was investigated. Linear model systems of PS–PI (SI), PS–PI–PS ...(SIS), and PI–PS–PI (ISI) block copolymers were synthesized via anionic polymerization with well-defined molecular structure variation such as block order, PI content, molecular weight, microstructure, and polydispersity. The different block sequences (SI vs SIS vs ISI) result, for similar PI contents, in different microdomain sizes, which correlate with the phase-separated morphologies as quantified via small-angle X-ray scattering (SAXS). The samples were mechanically characterized in the solid state via strain sweep tests to obtain their storage G′(γ0) and loss G″(γ0) moduli at room temperature as well as their nonlinear properties determined via the Fourier transform (FT) of the stress response. The fatigue behavior was determined via strain-controlled oscillatory torsion tests. First, the effect of strain amplitude on the number of cycles to failure was analyzed via Wöhler curves, specifically strain amplitude vs fatigue lifetime. A significant effect of the block sequence order, the microdomain size, and the chain dynamics on fatigue resistance was found. The fatigue resistance of SI diblock with 30 mol % PI outperforms the SIS or ISI triblock copolymer with a similar composition and, compared to neat PS, increases by a factor of 10 and even 4500, respectively. Second, the time-dependent stress response was analyzed via Fourier transform rheology to better quantify the time-dependent behavior of the nonlinear mechanical parameters and to determine quantitative parameters related to failure onset. Since the fatigue tests were performed under large amplitude oscillatory shear (LAOS), higher harmonics were detected and the time evolution was quantified in the FT spectra. Linear parameters such as the storage (G′) and loss (G″) moduli, as well as the third (I 3) harmonic over the fundamental one (I 1), were analyzed, leading to clear indications related to both brittle or ductile failure mechanisms.
Cannabidiol (CBD), a phytocannabinoid compound of
, shows limited oral bioavailability due to its lipophilicity and extensive first-pass metabolism. CBD is also known for its high intra- and ...inter-subject absorption variability in humans. To overcome these limitations a novel self-emulsifying drug delivery system (SEDDS) based on VESIsorb
formulation technology incorporating CBD, as Hemp-Extract, was developed (SEDDS-CBD). The study objective was to evaluate the pharmacokinetic profile of SEDDS-CBD in a randomized, double-blind, cross-over design in 16 healthy volunteers under fasted conditions. As reference formulation, the same Hemp-Extract diluted with medium-chain triglycerides (MCT-CBD) was used. CBD dose was standardized to 25 mg. Pharmacokinetic parameters were analyzed from individual concentration-time curves. Single oral administration of SEDDS-CBD led to a 4.4-fold higher C
and a 2.85-/1.70-fold higher AUC
/AUC
compared to the reference formulation. T
was substantially shorter for SEDDS-CBD (1.0 h) compared to MCT-CBD (3.0 h). Subgroup analysis demonstrated a higher bioavailability in women compared to men. This difference was seen for MCT-CBD while SEDDS-CBD mitigated this gender effect. Overall, SEDDS-CBD showed a significant improvement for all determined pharmacokinetic parameters: increased CBD plasma values (C
), favorably enhanced bioavailability (AUC) and fast absorption (T
). No safety concerns were noted following either administration.
► We review large amplitude oscillatory shear (LAOS) rheological measurements. ► LAOS tests can investigate nonlinear viscoelastic response of complex fluids ► We introduce nonlinear response of ...complex fluids under LAOS flow. ► We compare several quantitative methods to analyze LAOS tests. ► LAOS characterization is rigorous test for rheological models and quality control.
Dynamic oscillatory shear tests are common in rheology and have been used to investigate a wide range of soft matter and complex fluids including polymer melts and solutions, block copolymers, biological macromolecules, polyelectrolytes, surfactants, suspensions, emulsions and beyond. More specifically, small amplitude oscillatory shear (SAOS) tests have become the canonical method for probing the linear viscoelastic properties of these complex fluids because of the firm theoretical background
1–4 and the ease of implementing suitable test protocols. However, in most processing operations the deformations can be large and rapid: it is therefore the nonlinear material properties that control the system response. A full sample characterization thus requires well-defined nonlinear test protocols. Consequently there has been a recent renewal of interest in exploiting large amplitude oscillatory shear (LAOS) tests to investigate and quantify the nonlinear viscoelastic behavior of complex fluids. In terms of the experimental input, both LAOS and SAOS require the user to select appropriate ranges of strain amplitude (
γ
0) and frequency (
ω). However, there is a distinct difference in the analysis of experimental output, i.e. the material response. At sufficiently large strain amplitude, the material response will become nonlinear in LAOS tests and the familiar material functions used to quantify the linear behavior in SAOS tests are no longer sufficient. For example, the definitions of the linear viscoelastic moduli
G′(
ω) and
G″(
ω) are based inherently on the assumption that the stress response is purely sinusoidal (linear). However, a nonlinear stress response is not a perfect sinusoid and therefore the viscoelastic moduli are not uniquely defined; other methods are needed for quantifying the nonlinear material response under LAOS deformation. In the present review article, we first summarize the typical nonlinear responses observed with complex fluids under LAOS deformations. We then introduce and critically compare several methods that quantify the nonlinear oscillatory stress response. We illustrate the utility and sensitivity of these protocols by investigating the nonlinear response of various complex fluids over a wide range of frequency and amplitude of deformation, and show that LAOS characterization is a rigorous test for rheological models and advanced quality control.
We present a model-driven predictive scheme for the uniaxial extensional viscosity and strain hardening of branched polymer melts, specifically for the pom-pom architecture, using the small amplitude ...oscillatory shear mastercurve and the polymer architecture. A pom-pom shaped polymer is the simplest architecture with at least two branching points, needed to induce strain hardening. It consists of two stars, each with
arms of the molecular weight
, connected by a backbone of
. Despite the pom-pom constitutive model, experimental data of systematic investigations lack due to synthetic complexity. With an optimized approach, we synthesized polystyrene pom-pom model systems with systematically varied
and
. Experimentally, we identify four characteristic strain rate dependent regimes of the extensional viscosity, which can be predicted from the rheological mastercurve. Furthermore, we find that the industrially important maximum strain hardening factor depends only on the arm number by
. This framework offers a model-based design of branched polymers with predictable melt flow behavior.
Shear flow is a versatile strategy to align microphase-separated morphologies of diblock copolymers over macroscopic scales. Details of the local mechanism of reorientation toward the steady, ...nonequilibrium state, however, are only incompletely understood. Using large scale molecular simulation as well as experiments, we study the shear-alignment mechanism using lamella-forming, symmetric, unentangled diblock copolymers in steady and oscillatory shear flow. First we study homogeneously oriented systems and investigate the stability of different orientations with respect to the shear flow by the Rayleighian. Second, we investigate the process of reorienting an unstable grain with parallel orientation embedded in a matrix of stable, perpendicularly oriented lamellae. We observe two different reorientation mechanisms as a function of the shear rate: a fast transition, which is comparable to experimental conditions in large-amplitude oscillatory shear (LAOS) tests, and a slower transition occurring at lower shear rates. We show that for high shear rates the long-range orientational order of the lamellae inside the unstable grain disintegrates, while the grain remains spatially structured with the same characteristic length scale (similar to a microemulsion). At lower shear rates, however, we observe a shrinking of the unstable grain, i.e., a directed movement of the grain boundaries. Additionally, we compare the results of steady shear with oscillatory shear.
This work investigates the phenomena of self-heating, also called intrinsic heating, and thermoelastic coupling during non-linear dynamic mechanical fatigue testing via surface temperature ...measurement coupled with the mechanical behavior of polymers. Static tensile tests and dynamic strain controlled fatigue tests under tension/tension were performed at a frequency of ω1/2π = 5 Hz, as well as in the low cycle fatigue regime at ω1/2π = 0.2 Hz, on six polymers: high density polyethylene (HDPE), low density polyethylene (LDPE), ultra high molecular weight polyethylene (UHMWPE), polyamide 6 (PA6), and two grades of polypropylene (PP).
In dynamic testing, the surface temperature rises to a plateau value (ΔT) when an equilibrium between the viscous/plastic dissipated energy and heat convection is reached. Power-law correlations were found between the strain amplitude (ε0) and ΔT, as well as between ε0 and the calculated dissipated energy density (Wdiss,p) obtained from the mechanical stress response, with similar exponents for both correlations. Thermoelastic coupling is firstly investigated in uniaxial tension, revealing a linear relation between the strain rate and the rate of temperature decrease, which is more distinct with decreasing polymer chain mobility. In dynamic fatigue testing, the surface temperature was found to oscillate with an amplitude T1, which was analyzed via Fourier transform. A direct relation between T1 and ε0 at small deformations was observed. At large strain amplitudes, T1 (ε0) follows a similar trend as the complex modulus E*(ε0). At low frequencies and large strain amplitudes, additional higher harmonics at two (T2) and three (T3) times the fundamental frequency were also detected as fingerprints of plastic deformation, resulting in additional heat dissipated during the loading half cycle. From the results obtained, the advantages of the calculated dissipated energy density over the surface temperature analysis was analyzed to predict the fatigue behavior. This analysis is believed to be valid for all materials due to the mathematical/physical principles involved. The results are thus expected to hold for other materials such as composites, rubbers, ceramics and metals.
Display omitted
•Intrinsic heating follows mechanical nonlinearity quantified by the loss modulus.•Onset of mechanical nonlinearity correlates with 106 cycles lifetime for polypropylene.•Thermoelastic coupling is proportional to the nonlinear behavior of the complex modulus.•Plastic deformation results in higher harmonics in thermoelastic coupling at low frequency.•Higher harmonics in thermoelastic coupling can be quantified via Fourier transform.
Quantification of diffusion coefficient distribution (DCD) and correlation with molecular weight distribution (MWD) of polymers is still an issue in pulsed field‐gradient nuclear magnetic resonance ...(PFG‐NMR). The conventional scaling law utilized so far to relate diffusion coefficient and molecular weight only holds true for the determination of MWD at sufficiently low concentrations. To extend measurement limits and to get a good signal‐to‐noise ratio, an exponential correlation is introduced to describe the effect of polymer concentration on diffusion in PFG‐NMR. Two model polymers (polystyrene and poly(methyl methacrylate)) dissolved in deuterated chloroform are studied at different concentrations in the range of 0.16–8 wt%. The DCDs are determined by modeling the measured signal attenuation with three methods (gamma distribution, log normal distribution, and tailored norm regularization). It is shown that the proposed method applies to the PFG‐NMR measurements on polymer solutions over a wide concentration range, providing almost the same MWDs as those obtained at low concentrations. The MWDs retrieved from NMR experiments agree well with those by size exclusion chromatography.
Diffusion coefficient distribution and molecular weight distribution (MWD) of polymers are determined by pulsed field‐gradient nuclear magnetic resonance (PFG‐NMR). The retrieved MWDs agree well with those obtained by size exclusion chromatography. The measurements and data interpretation are extended to higher polymer concentrations, improving the signal‐to‐noise ratio, i.e., the accuracy and the applicability of the PFG‐NMR in the study of polymer solutions.
FT-Rheology as a novel procedure to determine emulsion properties like droplet size or interfacial tension based on the simulation of a universal curve for emulsions via ellipsoidal models. Display ...omitted
► FT-Rheology characterizes emulsions under nonlinear oscillatory shear. ► Simulation of dilute monodisperse emulsions with variable properties under LAOS. ► Maffettone Minale model with Batchelor theory calculate interfacial shear stress. ► Development of a universal emulsion curve E for characterizing emulsion properties. ► Determined droplet size via E comparable to result obtained via Palierne model.
A new protocol to gain interfacial tension and droplet size of dilute monodisperse emulsions from Fourier Transform Rheology (FTR), is proposed. Specifically, a universal dimensionless quantity E was found at small strain amplitudes to correlate with the droplet size of the emulsion where E is inversely related to the square of the capillary number Ca and directly proportional to the relative intensities of the fifth and third harmonics, I5/I3. The limiting value E0 at small strain deformations can be used as a universal parameter to calculate different emulsion properties. Different morphological constitutive models for emulsions were used to establish the universality of the parameter E0. Preliminary analysis on experimental data confirms the validity of this approach for the characterization of emulsion properties, including the estimation of interfacial tension and droplet radius.
PDF
