Study on the Melting Behavior and Crystal Structures of Poly(butylene-2,6-naphthalate)
| Autor: | Ming-Yih Ju, 朱明毅 |
|---|---|
| Rok vydání: | 2001 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 89 In this study, the multiple melting behaviors, crystallization characteristics and crystal structures of poly(butylene-2,6-naphthalate) (PBN) were investigated systematically and in detail. Multiple melting behaviors of PBN prepared by various crystallization conditions were studied by differential scanning calorimetry (DSC) at the heating rate of 10°C/min. It is found that the crystallization rate of PBN is so rapid that the glass transition temperature cannot be detected by simply heating the quenched sample. Two melting peaks are generally perceptible in a heating scan for isothermally crystallized samples that can be properly explained by the model proposed by Zhou and Clough. The low temperature-melting peak can be ascribed to the melting of primary and secondary crystals generated during crystallization or annealing process, while the high temperature-melting peak comes from the significant contribution of the melting of recrystallized species formed during heating. The step-crystallization was performed to clarify the relationship between the melting behavior and crystal modifications of PBN. It is found that the melting behavior of PBN is independent of the crystal forms and dominated by the crystal lamellar thickness and the perfection of crystals, which are dependent upon the thermal history. Furthermore, according to the Hoffmann-Weeks plot, an equilibrium melting temperature of 276°C is obtained for the neat PBN. Previous investigations have generally believed that the complete crystalline phase transformation of PBN from the α form to the β form is impossible without applying external stress. In this work, the exclusive β crystalline form of PBN is obtained through unique thermal treatment by crystallizing the bulk PBN sample nonisothermally at a rate of 0.1°C/min from the melt under a static state. The crystalline forms that exist in these thermal-treated samples were identified by wide-angle X-ray diffraction (WAXD) measurements. It was found that annealing in the solid state or crystallizing at a lower temperature from the static melt (TC < 205°C) only generated the α form, while using a higher TC obtained both the α and β forms. In the solid state, the exclusive β form prepared by thermal treatment is quite stable, and the crystalline form transition never occurs. Optical microscopic observations indicate that the α form crystal exhibits a typical spherulite morphology, while the β form crystal displays a dendritic spherulite morphology due to the extremely slow spherulite growth rate. The crystal structures of the α and β crystalline forms of PBN prepared by different thermal treatments were studied by infrared and solid-state 13C CP/MAS NMR spectroscopy. The infrared spectra of these two forms are discussed in detail. It is proposed that the main difference in the crystal structures between these two forms of PBN lies in the packing efficiency of the crystal chains, not in the conformation change of the glycol residue. The crystal chains are packed more tightly in the β form than in the α form, and the glycol residue adopts a similar conformation in both two forms. The conformational arrangements of the naphthalene ring differ in both two forms of PBN, and the planarity of the naphthalate residue in the β form exceeds that in the α form. Consequently, the greater planarity provides a better environment for closing packing of the crystal chains. Through the high-resolution solid-state 13C CP/MAS NMR results, the crystal structures of the α and β forms of PBN were described in great detail. Precise chemical shift assignments for each carbon of PBN are achieved through the application of the short contact time pulse sequence and the interrupted decoupling pulse sequence. A comparison of the chemical shift values of the interior methylene carbons leads to the conclusion that the glycol residues of PBN adopt a similar conformation in both the α and β forms. The identified conformation would be non-gauche non-trans, which is close to a gauche-trans-gauche sequence. The planarity of the naphthalate residues in the β form is greater than that in the α form, thus resulting in the higher packing efficiency of the β crystals. Finally, based on the chemical shift observations, the proposed conformation for the naphthalate residues in the crystalline phases of PBN is that the carbonyl groups are trans to the naphthalene ring with the oxygens heading toward the C3,7 carbons. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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