Evaluating the Time-Dependent Melting Behavior of Semicrystalline Polymers Through Strobl's 3-Phase Model

The melting behavior of polymers can provide information on their crystallization mechanism. However, the origin of the time-dependent low endotherm, or annealing peak, and the extent of melting-recrystallization-remelting during heating are still debated. The crystallization and subsequent melting behavior of isotactic polystyrene are explored in the context of Stroblâ "s 3-Phase model using differential scanning calorimetry (DSC), small angle X-ray scattering (SAXS), and wide angle X-ray diffraction. DSC experiments confirm the existence of a crystallization time-dependent low endotherm, and melting-recrystallization-remelting processes during heating. SAXS analysis using the correlation function confirms that the lamellar thickness increases with crystallization temperature and is independent of time. The spread between equilibrium melting and crystallization temperatures determined in this work (Tfâ"" = 533K, Tcâ"" = 544K) is much smaller than reported by Strobl et al. (Tfâ"" = 562K, Tcâ"" = 598K). These differences are partially attributed to overestimation in lamellar thicknesses calculated through the interface distribution function. Analysis of diffraction broadening shows that the apparent crystal size decreases with crystallization time, suggesting the formation of smaller/less perfect crystals during secondary crystallization. These results are consistent with observations that the glass transition temperature increases with crystallization time and supports the idea that secondary crystallization leads to increased amorphous conformational constraints. These results also suggest that the upward shift of the annealing peak during secondary crystallization is associated with increased amorphous constraints rather than increased crystal dimensions. The lack of distinction between Tfâ"" and Tcâ"" and the evolution of crystal size during crystallization stand in direct contrast with Stroblâ "s model.

Name thesis:degree_name

Master of Science

Level thesis:degree_level

masters

Discipline thesis:degree_discipline

Materials Science and Engineering

Department dc:contributor.department

Materials Science and Engineering

Grantor dc:publisher

Virginia Tech

Year dc:date.issued

2013

Author dc:creator

• Hoang, Jonathan Dan

Chair dc:contributor.committeechair

• Marand, Hervé

Committee members dc:contributor.committeemember

• Corcoran, Sean G.
• Whittington, Abby R.

• polymer crystallization
• melting
• Lauritzen-Hoffman secondary nucleation theory
• Strobl 3-Phase model
• isotactic polystyrene

Statement dc:rights

• In Copyright

Licence dc:rights.uri

• http://rightsstatements.org/vocab/InC/1.0/

Dc Identifier Other

vt_gsexam:387

OAI identifier oai:identifier

oai:vtechworks.lib.vt.edu:10919/19303