TY - JOUR
T1 - Fatigue-Crack Propagation of High-Density Polyethylene Homopolymers
T2 - Influence of Molecular Weight Distribution and Temperature
AU - Cerpentier, Robin R.J.
AU - Van Vliet, Tim
AU - Pastukhov, Leonid V.
AU - Van Drongelen, Martin
AU - Boerakker, Mark J.
AU - Tervoort, Theo A.
AU - Govaert, Leon E.
N1 - Funding Information:
The authors thank Dr. Nic. Friederichs (SABIC) for kindly providing a unique set of polyethylene homopolymers. Dr. Enrico Troisi, Dr. Harm Caelers, and Dr. Rudy Deblieck all from SABIC are acknowledged for fruitful discussions. R.R.J.C. gratefully acknowledges funding by SABIC.
Publisher Copyright:
© 2021 The Authors. Published by American Chemical Society.
PY - 2021/12/28
Y1 - 2021/12/28
N2 - The present study focuses on the influence of the molecular weight distribution (MWD) on the crack-growth kinetics of fatigue-crack propagation in high-density polyethylene (HDPE) homopolymers. Compact-tension specimens of HDPE homopolymer grades, with polydispersities ranging from 2 to 45 and weight-averaged molar mass ranging from 49 to 450 kg/mol, are tested in cyclic loading at temperatures ranging between 23 and 92 °C. Through a variation of sample thickness, linear elastic-fracture mechanics is shown to apply for the chosen geometry (compact tension). It was found that the crack-propagation kinetics obey the Paris-Erdogan law, for which the Paris-Erdogan exponent m is (highly) similar for all grades tested (m = 3.9), implying that the Paris-Erdogan prefactor A is the governing parameter for the crack-growth kinetics. Relatively poor correlations are observed when the prefactor A is plotted as a function of both the tie-molecule fraction derived from the theoretical model by Huang and Brown, J. Mater. Sci. 1988, 23, 3648, and the average number of effective physical cross-links per chain as derived by Tervoort et al., Macromolecules 2002, 35, 8467. A far better correlation is observed between prefactor A and the weight-average molecular weight (Mw), which improved further when Mw is corrected for the width of the MWD, taking into account the z-average molecular weight Mz, through the ratio Mz/Mw. A power-law correlation of prefactor A with Mw and the width-corrected Mw reveals slopes of -3.4 and -3.3, respectively. Because a molecular slip within the fibrils would require chain transport through the crystalline blocks, the temperature dependence of the fatigue-crack-growth kinetics is investigated to identify the underlying molecular processes. This investigation reveals the existence of a high-temperature and a low-temperature deformation process, both of which can be related to chain-slip mechanisms through their respective activation energies (125 and 50 kJ/mole), as their activation energies are considerably lower than that required for chain scission (430 kJ/mol). This, combined with the power-law exponent of -3.4, would suggest a possible connection between the underlying failure mechanisms of craze fibrils and reptation-like dynamics. Furthermore, experiments at elevated temperatures on a selection of homopolymer grades suggest that the MWD has no influence on the temperature dependence of fatigue-crack propagation for HDPE homopolymers.
AB - The present study focuses on the influence of the molecular weight distribution (MWD) on the crack-growth kinetics of fatigue-crack propagation in high-density polyethylene (HDPE) homopolymers. Compact-tension specimens of HDPE homopolymer grades, with polydispersities ranging from 2 to 45 and weight-averaged molar mass ranging from 49 to 450 kg/mol, are tested in cyclic loading at temperatures ranging between 23 and 92 °C. Through a variation of sample thickness, linear elastic-fracture mechanics is shown to apply for the chosen geometry (compact tension). It was found that the crack-propagation kinetics obey the Paris-Erdogan law, for which the Paris-Erdogan exponent m is (highly) similar for all grades tested (m = 3.9), implying that the Paris-Erdogan prefactor A is the governing parameter for the crack-growth kinetics. Relatively poor correlations are observed when the prefactor A is plotted as a function of both the tie-molecule fraction derived from the theoretical model by Huang and Brown, J. Mater. Sci. 1988, 23, 3648, and the average number of effective physical cross-links per chain as derived by Tervoort et al., Macromolecules 2002, 35, 8467. A far better correlation is observed between prefactor A and the weight-average molecular weight (Mw), which improved further when Mw is corrected for the width of the MWD, taking into account the z-average molecular weight Mz, through the ratio Mz/Mw. A power-law correlation of prefactor A with Mw and the width-corrected Mw reveals slopes of -3.4 and -3.3, respectively. Because a molecular slip within the fibrils would require chain transport through the crystalline blocks, the temperature dependence of the fatigue-crack-growth kinetics is investigated to identify the underlying molecular processes. This investigation reveals the existence of a high-temperature and a low-temperature deformation process, both of which can be related to chain-slip mechanisms through their respective activation energies (125 and 50 kJ/mole), as their activation energies are considerably lower than that required for chain scission (430 kJ/mol). This, combined with the power-law exponent of -3.4, would suggest a possible connection between the underlying failure mechanisms of craze fibrils and reptation-like dynamics. Furthermore, experiments at elevated temperatures on a selection of homopolymer grades suggest that the MWD has no influence on the temperature dependence of fatigue-crack propagation for HDPE homopolymers.
UR - http://www.scopus.com/inward/record.url?scp=85121026922&partnerID=8YFLogxK
U2 - 10.1021/acs.macromol.1c01945
DO - 10.1021/acs.macromol.1c01945
M3 - Article
AN - SCOPUS:85121026922
SN - 0024-9297
VL - 54
SP - 11508
EP - 11521
JO - Macromolecules
JF - Macromolecules
IS - 24
ER -