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Viscoelastic Functions: Effect of Various Parameters
Published in B. R. Gupta, Rheology Applied in Polymer Processing, 2023
Branching in the polymers increases the inter-chain distance and therefore increases the free volume and also lowers the degree of crystallization and thus is expected to reduce the viscoelastic modulus. However the different lengths of branches are expected to give different effects as the small and medium branch lengths will show the above effects but the large branch lengths tend to entangle and thus expected to give the higher moduli. The variation of the viscoelastic parameters with the degree of branching as well as with the branch lengths has been reported to be rather erratic due to the complex interaction between the entanglements and the branching. It is expected that the viscoelastic functions of high concentration solutions and melts will be the functions of structural parameter, g as well as exact topology of the branches. The parameter g is the ratio of the mean square radii of the branched and linear polymer.
Blown Film Technology
Published in Nicholas P. Cheremisinoff, Elastomer Technology Handbook, 2020
The first process developed for making polyethylene was the autoclave process. This is basically a large stirred pot reactor operating at very high temperatures and high pressures (15,000–40,000 psi and 300–500°F), using either oxygen or a peroxide to initiate the reaction. The resulting polymer is highly branched. Controlling the branching is accomplished by varying the pressure and temperature and the location in the reaction vessel for the introduction of ethylene. These variables also affect the molecular weight and molecular weight distribution (MWD). Therefore, one is not totally free to vary the polymer properties independently. Short chain branching is the prime factor in determining the polymer density, and long chain branching affects the viscoelasticity of the polymer. The elastic properties of the polymer affect its processability.
Industrial Polymers
Published in Manas Chanda, Plastics Technology Handbook, 2017
The keys to the process are active catalysts. These are special organochromium compounds on particular supports. The catalysts yield up to about 106 kg of polymer per kilogram of metallic chromium. Branching is controlled by the use of comonomers like propylene or 1-butene, and hydrogen is used as a chain transfer agent. The catalyst is so efficient that its concentration in the final product is negligible. The absence of a solvent and a catalyst removal step makes the process less expensive. The products marketed as linear LLDPE can be considered as linear polyethylenes having a significant number of branches (pendant alkyl groups). The linearity imparts strength, the branches impart toughness.
Mechanical, thermal, and morphological properties of low-density polyethylene nanocomposites reinforced with montmorillonite: Fabrication and characterizations
Published in Cogent Engineering, 2023
Safaa Kh. Al-Jumaili, Wasan A. Alkaron, Maithem Y. Atshan
The experimental results of the tensile test for pure LDPE and LDPE with different amounts of MMT as a filler are given in Figure 6. The results showed a general comparison between the tested samples. The tensile test was done to measure the tensile strength of LDPE which can be defined as a material’s resistance to breaking under tension. Theoretically, LDPE has a low tensile strength due to its highly branched polymer chains. The branching, however, prevents the chains from stacking correctly beside one another, thus reducing the intermolecular forces of attraction between them, which reduces the resistance ability. From Figure 6, it can be seen that the stress–strain curve rises with an increase in the elongation percentage and the different amounts of MMT to an optimal point where the tensile strength levels. Continued addition of the elongation percentage beyond the threshold to LDPE decreases the tensile strength.
Multi-objective optimization through a novel Bayesian approach for industrial manufacturing of Polyvinyl Acetate
Published in Materials and Manufacturing Processes, 2023
Arjun Manoj, Srinivas Soumitri Miriyala, Kishalay Mitra
In the polymerization industry, high amounts of branching in the polymer and large molecular weight serve as the key strength and quality indicators, respectively. Thus, the objectives set for the optimization problem in PVAc polymerization is namely, maximizing the molecular weight Mw, maximizing the branching index Bn, and minimizing the polymerization time tpoly. While ensuring the above, it is also important to prevent the formation of gel during the polymerization process.[31] This is enabled by implementing phenomenological constraints on Temperature, PDI, and Mw. The resulting optimization problem formulation is presented in Table 1, where the HFM is represented as , and % Conversion is measured as a time-inexpensive function of T, thus eliminating the need for a surrogate model for its computation.
Imperative role of SBS molecular structure on the performance properties of modified binders and asphalt mixes
Published in International Journal of Pavement Engineering, 2023
Sk Sohel Islam, Sumit K. Singh, G. D. R. N. Ransinchung, Sham S. Ravindranath
In addition to chemical and molecular characterisation, the melt viscosity of the four polymers was also assessed using a dynamic shear rheometer (DSR) in an oscillatory mode. Temperature sweep measurement was carried out from 120°C to 180°C (Strain: 10%, Frequency = 0.1 rad/s), and the results are produced in Figure 3(c). Amongst the four SBS polymers, it can be observed that branched and di-block polymers respectively demonstrate the highest and lowest melt viscosity. The melt viscosity of the studied SBS polymers strongly depends upon the Mw and long-chain branching. Polymers with a higher degree of long-chain branching tend to move slower, resulting in higher viscosity.