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Advances in Materials Research
  Volume 1, Number 4, December 2012 , pages 245-268

Structure-property relations for polymer melts: comparison of linear low-density polyethylene and isotactic polypropylene
A.D. Drozdov, A. Al-Mulla and R.K. Gupta

    Results of isothermal torsional oscillation tests are reported on melts of linear low density polyethylene and isotactic polypropylene. Prior to rheological tests, specimens were annealed at various temperatures ranging from Ta = 180 to 310oC for various amounts of time (from 30 to 120 min). Thermal treatment induced degradation of the melts and caused pronounced decreases in their molecular weights. With reference to the concept of transient networks, constitutive equations are developed for the viscoelastic response of polymer melts. A melt is treated as an equivalent network of strands bridged by junctions (entanglements and physical cross-links). The time-dependent response of the network is modelled as separation of active strands from and merging of dangling strands with temporary nodes. The stress-strain relations involve three adjustable parameters (the instantaneous shear modulus, the average activation energy for detachment of active strands, and the standard deviation of activation energies) that are determined by matching the dependencies of storage and loss moduli on frequency of oscillations. Good agreement is demonstrated between the experimental data and the results of numerical simulation. The study focuses on the effect of molecular weight of polymer melts on the material constants in the constitutive equations.
Key Words
    thermal properties; isotactic polypropylene; linear low-density polyethylene; molecular weight; viscoelasticity; thermal degradation
A.D. Drozdov: Department of Chemical Engineering, West Virginia University, WV, USA; A. Al-Mulla: Department of Chemical Engineering, Kuwait University, Safat, Kuwait; R.K. Gupta: Department of Chemical Engineering, West Virginia University, WV, USA

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