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Structure, deformation behavior and properties in polyundecanamide (nylon 11) and high-density polyethylene (HDPE) subjected to planar (equibiaxial) deformation by forging
Abstract
Cross-laminates composed of unidirectionally-reinforced composites are usually used to prepare polymeric materials with enhanced properties in a plane. In the present investigation, solid-state forging is considered as an alternate method to increase planar properties in semicrystalline flexible chain polymers by imparting equibiaxial orientation and extension of molecules in the deformation plane. Polyundecanamide (nylon 11) and High Density Polyethylene (HDPE) are chosen in this study to illustrate several aspects of forging on Polyamides and Polyolefins. They represent two families of polymers with already a large impact in the fiber technology. First, novel aspects of the nature and origin of polymorphism in melt crystallized nylon 11 observed both prior and after forging are presented. Two distinct crystalline species (semidisordered smectic $\delta\sp\prime$-form and 3-dimensional crystal $\alpha$-form) have been identified. Their respective content is highly dependent upon thermal history. The smectic $\delta\sp\prime$-form is kinetically favored but does progressively transform into the thermodynamically preferred crystal form on heat treatment. Thermal analysis provides data on the thermodynamics and kinetics of melting and crystallization for each form. Also, new enthalpic measurements were obtained which correspond to the change of symmetry in the crystalline phase from the low-temperature $\alpha$-form to the high-temperature $\delta$-form. It exhibits a characteristic broad transition typical of crystal-condis crystal transition (introduced by Wunderlich). Differences in the mode of hydrogen-bonding are given as an explanation of polymorphism. Both nylon 11 and HDPE exhibit alternate behaviors when forged to their maximum compression ratios over temperature ranges above and below about 100$\sp\circ$C. Modifying the forging rate slightly shifts this temperature. For nylon 11, low temperature deformation favors the formation and/or stability of the smectic which is found to significantly inhibit large deformation without mechanical failure. Optimal forging conditions however are found over the high temperature range where the crystal forms ($\alpha$- and $\delta$-forms) are stable during the process. Accordingly, the in-plane modulus increases by an amount comparable to the one predicted by the composites theory. Inversely, for HDPE, optimal forging conditions are found at low temperature where the stress-induced orthorhombic to monoclinic transformation is observed. Results are analysed in terms of differences in slip (or shear) mechanisms at the molecular scale with forging conditions.
Subject Area
Polymer chemistry|Plastics
Recommended Citation
Autran, Jean-Philippe M, "Structure, deformation behavior and properties in polyundecanamide (nylon 11) and high-density polyethylene (HDPE) subjected to planar (equibiaxial) deformation by forging" (1990). Doctoral Dissertations Available from Proquest. AAI9035376.
https://scholarworks.umass.edu/dissertations/AAI9035376