Mesomorphic Phase Transitions in Semicrystalline Polymers
Open Access
- Author:
- Chen, Qin
- Graduate Program:
- Chemical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 05, 2017
- Committee Members:
- Scott Thomas Milner, Dissertation Advisor/Co-Advisor
Scott Thomas Milner, Committee Chair/Co-Chair
Enrique Daniel Gomez, Committee Member
Janna Kay Maranas, Committee Member
Robert John Hickey III, Outside Member - Keywords:
- Crystallization
polyethylene
polypropylene
strobl
rotator phase
nucleation
mesomorphic phase
molecular dynamics - Abstract:
- Polyethylene (PE) and isotactic polypropylene (iPP) are the two most widely used commercial polymers in the plastics industry. These polymers are semicrystalline, that is, they contain both amorphous and crystallizable domains. While there exists a large empirical knowledge base of processing techniques that allows one to control crystallization, fundamental understanding of the underlying mechanisms is still lacking. In particular, there is yet an unified molecular theory that satisfactorily describes the process of polymer nucleation. Recently, new evidences have emerged from studies of PE and alkanes that contradicts conventional wisdom. Rather, they support Strobl's theory which suggests that polymers crystallize via metastable rotator phases. However, it is unknown whether the theory is universal to all polymers. In this dissertation, we use molecular dynamics (MD) simulation to investigate polymer nucleation, under the premise of Strobl's theory. We investigate rotator phase formation in PE and iPP by developing simulation techniques that allow one to measure thermodynamic and spectroscopic properties of bulk phases. First, we examine the melting transitions of polymer crystal and rotator phases by presenting a simple method to accurately simulate polymer melting points. Next, we demonstrate an approach to predict the Raman spectra of bulk structures from MD simulation, that can be used to identify the spectral signatures of rotator phases. Lastly, we develop a novel ``plunger" method to measure the polymer crystal-melt interfacial tension, which is a key parameter that governs the nucleation barrier. Using the above methods, we provide support for mesomorphic phase transitions in the nucleation pathway of iPP, which at present is not well-characterized experimentally.