Mechanical & Materials Engineering, Department of

 

Date of this Version

8-20-2013

Comments

A thesis Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of the Requirements For the Degree of Master of Science, Major: Mechanical Engineering and Applied Mechanics, Under the Supervision of Professor Lucia Fernandez-Ballester. Lincoln, Nebraska: August, 2013.

Copyright 2013 Frederic Aubin

Abstract

Flow-induced crystallization (FIC) has been investigated for decades but the effect of molecular characteristics that hinder crystallization—such as the presence of random comonomers— on FIC remains scarcely explored. Flow greatly enhances crystallization kinetics and can induce the formation of highly oriented morphologies that affect final properties (i.e. stiffness, permeability, thermal conductivity, elastic modulus, etc.). Therefore, understanding FIC remains particularly important for developing predictive models to allow optimization of processing techniques involving flows.

Polypropylene represents ~ 40% of the global polyolefin production. The addition of ethylene comonomer allows their use as engineering plastics because of their particular toughness and flexibility. This study explores the influence of ethylene content on FIC by using isotactic polypropylene and random ethylene copolymers and imposing high shear stress in a pressure driven flow. The types of morphologies formed are compared for all three grades, as well as the required critical shear stress σcrit for formation of oriented crystallites. A depth sectioning method has been used to explore the oriented nucleation densities within the highly oriented skin in all three grades. A saturation behavior has been detected for iPP and 3% ethylene—but not for 7% content—which correlates with an increase in viscosity.

Adviser: Lucia Fernandez-Ballester