Study on the Melting Behavior and Crystal Structures of Poly(butylene-2,6-naphthalate)
|Keywords:||聚-2,6-對□二甲酸丁酯;熔融行為;晶體結構;poly(butylene-2,6-naphthalate);melting behavior;crystal structure|
之前的研究一直認為除非對PBN試樣施加一外部機械力，否則其結晶態要從α晶態轉變成β晶態是不可能達到的。然而在本研究中，PBN的完全β晶態可以藉由對處於巨量（bulk）且靜態的PBN試樣施予一個特殊的熱處理方法來得到：將此試樣自熔融態以0.1°C/min的冷卻速率作非恆溫熔融結晶降溫至室溫。經過熱處理後的PBN試樣之結晶態以廣角X光繞射譜形來作辨別。研究發現在PBN的固態下作退火或在較低的溫度下（TC < 205°C）作恆溫熔融結晶只能得到α晶態，而在較高的TC下則α與β晶態可以同時生成。在固態下，使用熱處理方法得到之完全β晶態相當穩定，且二個結晶態之間不會發生轉變。在偏光顯微鏡的觀察中則看到α晶態晶體表現出一般典型的球晶相形態，而β晶態晶體則因為極緩慢的球晶成長速率而出現樹枝狀球晶的相形態。
經由熱處理所得到之PBN的α及β晶態之晶體結構利用紅外線光譜及固態13C CP/MAS NMR光譜來分析。根據紅外線光譜的分析結果，發現此二個PBN晶體結構最大的不同在於分子的堆疊程度，而非雙醇鏈段構形的改變。PBN分子在β晶態中的堆疊程度要比在α晶態中為高，而雙醇鏈段在二個晶態中的構形則十分接近。此外，在二個晶態中□環的構形也有些微的不同，而在β晶態中PBN之對□二酸鏈段的平面性要高於在α晶態中的平面性。因此，較佳的平面性提供給PBN分子有更好的環境以產生緊密堆疊。
透過高解析度固態13C CP/MAS NMR的結果，PBN的α與β晶態的晶體結構得以更詳細地描述。經由短接觸時間脈衝序列及插斷去偶合脈衝序列的使用，PBN分子結構上每一個碳核的化學位移得以作準確的標定。比較內亞甲基碳核的化學位移值得到一個結論：PBN的雙醇鏈段在α及β晶態中幾乎相同，其構形處於non-gauche、non-trans的狀態而接近gauche-trans-gauche的序列。13C NMR實驗進一步證實對□二酸鏈段在β晶態中的平面性確實較在α晶態中高，導致在β晶態中分子的堆疊較緊密。最後，依據化學位移的觀察結果，推論出PBN的對□二酸鏈段在結晶相中的構形應是羰基相對於□環處於trans的位置上，其中羰基的氧原子則朝向C3,7碳核的方向。|
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.
|Appears in Collections:||Thesis|