Nhol Kao

Extensional rheology of polypropylene melts from the Rheotens test

Abstract In recent years the “Rheotens” melt strength test has emerged as one of the most important tools for comparing the drawability of polymer melts. This paper examined whether a reliable set of test conditions could be obtained to determine a true and accurate melt strength of a polymer. The influence of various extrusion and drawing parameters on the extensional rheological performance of two polypropylenes are discussed. Using this knowledge an attempt was made to estimate the transient extensional viscosity from the Rheotens test. An analytical Wagner model in the linear region of the Rheotens test was used for the first time to calculate the transient extensional viscosity from the Rheotens test. These results were compared to the transient extensional viscosity obtained from a constant strain rate Rheometrics melt extensional rheometer (RME). The results agree within measurement accuracy with those measured from an RME. Consequently, the viscosity determined from the Rheotens experiment can be used to estimate transient extensional viscosity. The influence of different Rheotens test conditions on transient extensional viscosity are also discussed.

Elongation-induced crystallization of a high molecular weight isotactic polybutene-1 melt compared to shear-induced crystallization

The extensional viscosity fixture (EVF) from TA Instruments was applied for elongation-induced crystallization (EIC) experiments on a high molecular weight isotactic polybutene-1 (PB-1). The results are compared to those previously obtained for shear-induced crystallization (SIC) on the same sample. After annealing the sample at a temperature well above the melting point to erase any memory effects, the sample is cooled to a temperature below the melting point (93–101°C). An increase of the transient elongational viscosity compared to the Trouton viscosity 3η(t) is used to define a crystallization onset time. Challenges caused by the chamber temperature control of the EVF in the ARES, like the delayed attainment of a stable sample temperature as well as the significant mismatch of the latter compared to the imposed nominal temperature, are addressed. By modifying the temperature program accordingly, it was finally possible to run shear and elongation experiments at a comparable sample temperature protocol to prove that quasi-quiescent crystallization occurs at the same time for both types of flow. At Hencky strain rates above 10−3s−1, the onset time decreases rapidly with increasing strain rate. Compared to SIC, the decrease in EIC is much stronger (e.g., at 10s−1 the onset time in elongation is two powers of ten shorter than that in shear) and approaches a constant Hencky strain eC=4 regime. A temperature change by 4°C had no significant effect on eC. Following a procedure introduced for SIC, a temperature-invariant plot of the onset time versus normalized flow rate was constructed containing both flow types.The extensional viscosity fixture (EVF) from TA Instruments was applied for elongation-induced crystallization (EIC) experiments on a high molecular weight isotactic polybutene-1 (PB-1). The results are compared to those previously obtained for shear-induced crystallization (SIC) on the same sample. After annealing the sample at a temperature well above the melting point to erase any memory effects, the sample is cooled to a temperature below the melting point (93–101°C). An increase of the transient elongational viscosity compared to the Trouton viscosity 3η(t) is used to define a crystallization onset time. Challenges caused by the chamber temperature control of the EVF in the ARES, like the delayed attainment of a stable sample temperature as well as the significant mismatch of the latter compared to the imposed nominal temperature, are addressed. By modifying the temperature program accordingly, it was finally possible to run shear and elongation experiments at a comparable sample temperature protocol...

The effect of temperature on the viscoelastic properties of model and industrial dispersions

This paper discusses the effect of temperature on the dynamic rheological properties of both the model polystyrene gelatin and the industrial photographic coupler dispersions. The time temperature superposition (TTS) was used to bring experimental data at various temperatures together into single master curves. An Arrhenius-type TTS principle, rather than the Williams–Landel–Ferry equation, was used in this work to bring all dynamic moduli and dynamic viscosity curves at different temperatures into single master curves. The present investigation verified that the TTS principle, which was developed for polymeric materials, could also be used for model and industrial photographic coupler dispersions as well. Furthermore, not only was the TTS principle suitable for the dispersions in the sol state, but it could also be used for the data in the gel-like state as well. The TTS allowed the estimation of the rheological properties of the dispersions over the frequency range which is otherwise inaccessible to the range of experimental measurement. Therefore, the linear viscoelastic properties of these model and photographic coupler dispersions at very low frequency (which is useful in predicting the stability of the product), as well as properties at very high frequency (or large deformation, e.g., during coating or pumping processes), could be estimated.

Fabrication of phosphate microcrystalline rice husk based cellulose particles and their electrorheological response

As a dry-based electrorheological (ER) material, phosphate microcrystalline cellulose (MCC), which exhibits ER properties under anhydrous conditions, was fabricated by the phosphorylation of MCC particles. The MCC particles were initially synthesized by the three step preparation of an alkali treatment, bleaching, and hydrolysis of cellulose particles from rice husk. The phosphate MCC was then synthesized via the phosphoric ester reaction of urea with phosphoric acid and MCC, and its chemical characteristics were examined by energy dispersive X-ray spectroscopy and Fourier transform infrared spectroscopy. The phosphate MCC particles were dispersed in silicone oil to produce an ER fluid (10vol%), and its chain structure was observed directly by optical microscopy. The rheological behavior of the ER fluid was tested using a rotational rheometer under a range of electric fields, showing a polarization mechanism with a slope of 2.0 for the yield stress as a function of the applied electric field strengths.

Chemically imaging the interaction of acetylated nanocrystalline cellulose (NCC) with a polylactic acid (PLA) polymer matrix

The non-covalent interaction of acetylated nanocrystalline cellulose (AC-NCC) with polylactic acid (PLA) in a composite blend has been studied at the micron scale by synchrotron Fourier transform infrared (FTIR) microspectroscopy. Microtomed sections of AC-NCC in PLA showed strong, localized carbonyl stretching (νC=O) absorbance characteristic of the cellulose acetylation, and this was observed on the surface of larger aggregated AC-NCC particles. A shift in the νC=O IR absorption peak of AC-NCC in PLA, relative to unblended AC-NCC was observed, which is indicative of an intermolecular interaction between AC-NCC and PLA matrix. Acetylation can therefore potentially improve the performance of the composite by enabling linkages between carbonyl groups, helping to establish a good stress transfer between the fiber and the matrix. This could in turn lead to a material with high yield elastic modulus. This is the first reported chemical imaging of acetylated nanocrystalline cellulose-based composite materials using synchrotron FTIR microspectroscopy.

MORPHOLOGICAL CHARACTERISATION AND DYNAMIC RHEOLOGY OF NANO-STRUCTURED BLENDS OF POLYSTYRENE AND SEBS

Polystyrene homopolymer (hPS) was melt-blended with styreneethylene/ butylene-styrene (SEBS) in a counter-rotating twin-screw extruder to produce hPS/SEBS blends. The morphology of the hPS/SEBS blends was investigated using environmental scanning electron microscopy (ESEM) and transmission electron microscopy (TEM). The dynamic rheological behaviour of the blends was studied using a parallel plate rheometer. TEM and ESEM images of the hPS/SEBS blends showed three different morphologies at three different SEBS concentration ranges. When SEBS concentration was less than 30 wt%, the hPS-rich blends hPS/SEBS 90/10 and 80/20 showed a droplet/matrix morphology. When SEBS concentration was between 30 and 70 wt%, the blend morphology revealed a co-continuous nano-structure. At SEBS concentrations 80 wt% and above, TEM images of the SEBS-rich blends showed that the homopolymer hPS was absorbed within the bulk of SEBS, resulting in a modification of its original microstructure. Dynamic rheological tests showed that the blend morphology had significant effect on their rheological behaviour at low frequencies.

Rheology and physical characterization of graphene nanoplatelet/poly (butylene adipate-co-terephthalate) nanocomposites

Biodegradable poly (butylene adipate-co-terephthalate) (PBAT)/ graphene nanoplatelet (GNP) nanocomposites were prepared and characterized. It was observed that GNP incorporation enhanced the thermal stability, electrical conductivity and Young’s modulus of the matrix markedly. Melt flow behaviours of the prepared nanocomposites were investigated via dynamic oscillatory measurements. Viscoelastic properties exhibited significant enhancement with increasing GNP loading. The effect of GNPs was more pronounced in the storage modulus which exhibited weaker frequency dependency in the low frequency region. Rheological percolation threshold of GNPs in PBAT was determined to be in vicinity of 9 wt% at 140°C. Interestingly, percolation was found to be temperature sensitive, occurring at lower GNP concentrations as the temperature was increased further from the system’s melting point.

Optimization and modelling of delignification process for nanocrystalline cellulose production from rice husk biomass

Nanocrystalline cellulose (NCC) has been attributed as a noble material due to its superior properties for miscellaneous applications in medical science, pharmaceutical production and engineering fields. The major problems associated with the production of NCC are low production rate and inefficient process. Alkaline delignification plays an important role for the mass production of NCC from lignocellulosic biomass. In order to fulfil these research gaps a fractal kinetic model of alkaline delignification of rice husk biomass (RHB) was studied with process optimization. Fifteen kinetic experiments of alkaline delignification for NCC production from RHB, as designed by the Minitab® 2014b software using the Box-Behnken method, were performed in a 2L jacketed glass reactor under three process parameters (time, temperature and alkali concentration) with ranges of 2-10 hours, 40-100°C and 1-4M, respectively. The nucleic growth model of fractal kinetics, developed by Nguyen & Dang (2007), has been successfully ...