Simultaneous differential scanning calorimetry (DSC), small-angle (SAXS) and wide-angle X-ray scattering (WAXS) measurements were adopted for more precise determination of the equilibrium melting temperatures (Tm*) of a and b phases in bulk-crystallized syndiotactic polystyrene. On the basis of Kratky-Porod approximation, a new method for determining crystalline lamellar thickness from SAXS profiles obtained at high temperatures where there are only limited number of discrete crystalline lamellae dispersed in the melt matrix was developed. This method is shown to be reliable as it gave comparable results obtained from the conventional 1D correlation function method for SAXS profiles obtained at lower temperatures where lamellae are closely stacked. Results of the subsequent Gibbs-Thomson analysis indicated that the trigonal a phase is the entropically favored high temperature phase with Tm* = 355 oC whereas the b phase is enthalpically favored at lower temperatures, with Tm* = 314 oC. Compared to previous held contention in the temperature-dependent phase stability of these two phases, the current phase stability assignment is more consistent with both the density and the symmetry of the corresponding crystal structures. It also explains various observations reported previously on the competition between the two polymorphs during crystallization and during melting.
| Identifer | oai:union.ndltd.org:NSYSU/oai:NSYSU:etd-0721107-174437 |
| Date | 21 July 2007 |
| Creators | Su, Chiu-Hun |
| Contributors | Hsin-Lung Chen, U-ser Jeng, Po-Da Hong, Rong-Ming Ho, Ming Chen, An-Chung Su |
| Publisher | NSYSU |
| Source Sets | NSYSU Electronic Thesis and Dissertation Archive |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.lib.nsysu.edu.tw/ETD-db/ETD-search/view_etd?URN=etd-0721107-174437 |
| Rights | withheld, Copyright information available at source archive |
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