Abstract
The morphology of molten polymeric materials is known to be less sensitive to shear than to extensional deformations. However, it is not easy to characterise molten polymeric materials in simple extensional flows due to the large number of experimental difficulties involved. This has led to the effective absence of a structure-preserving, morphology probing technique similar to the ones commonly found in shear, i.e., the equivalent of stress relaxation and oscillatory experiments. It is the aim of the present work to demonstrate the usefulness of a recently developed experimental technique that enables stress relaxation experiments after a step strain in uniaxial extension to be performed. Results are presented for two model melts (polyisobutylene, PIB, of different molecular weights) and for a series of linear low-density polyethylenes, LLDPE, in which the molecular structure (molecular weight, MW, molecular weight distribution, MWD and degree of long chain branching, LCB) is changed systematically. It is shown that, for both types of materials, stress relaxation experiments in extension yield quantitatively correct results and that this technique is more sensitive to differences in molecular structure than oscillatory experiments in shear.
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Acknowledgements
V.C. Barroso would like to thank PRAXIS XXI for the financial support (Grant No. GGP XXI/BD/4491/96) during this work. BASF, Portugal is gratefully acknowledged for kindly supplying the PIB samples used. The authors would also like to express their sincere thanks to Prof. H. Henning Winter for providing the relaxation spectra of the materials used in this work.
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Barroso, V.C., Ribeiro, S.P. & Maia, J.M. Stress relaxation after a step strain in uniaxial extension of polyisobutylene and polyethylene. Rheol Acta 42, 345–354 (2003). https://doi.org/10.1007/s00397-002-0284-5
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DOI: https://doi.org/10.1007/s00397-002-0284-5