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| 研究生： |
林冠宇 Lin, Guan-Yu |
|---|---|
| 論文名稱： |
在側鏈不對稱中心影響下之聚(S)-4-甲基-1-己烯分子相變化及結晶成長特性 Influence of Lateral Chiral Center on Phase Transformation and Crystal Growth Behavior of Poly((S)-4-methyl-1-hexene) |
| 指導教授： |
阮至正
Ruan, Jr-Jeng |
| 學位類別： |
碩士 Master |
| 系所名稱： |
工學院 - 材料科學及工程學系 Department of Materials Science and Engineering |
| 論文出版年： | 2009 |
| 畢業學年度： | 97 |
| 語文別： | 中文 |
| 論文頁數： | 68 |
| 中文關鍵詞： | 單晶 、螺旋構形 、結晶成長 、中間相 、六方柱狀堆積結構 、介穩態 、相變化 、不對稱中心 、聚(S)-4-甲基-1-己烯 |
| 外文關鍵詞： | hexagonal columnar packing, helical conformation, mesophase, metastable state, phase transformation, chiral center, crystal growth, single crystal, P(S)4MH, poly((S)-4-methyl-1-hexene) |
| 相關次數： | 點閱：85 下載：2 |
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聚(S)-4-甲基-1-己烯(poly((S)-4-methyl-1-hexene)，P(S)4MH) 為一側鏈具有不對稱中心的聚烯烴分子。這個不對稱中心的存在，限制了主鏈螺旋構形(helical conformation)的轉換。這個螺旋構形轉換的限制(conformational rigidity)，將使得此分子於熔融態(melt state)中，採取特定的左手螺旋構形，並讓系統具有光學活性(optically active polyolefin)。熱分析實驗指出，P(S)4MH分子可在結晶相與熔融態之間，形成一穩定的中間相，表現出雙變型相變化行為(enantiotropic behavior)。所獲得的電子與X-ray繞射進一步證實，此中間相為分子鏈以六方柱狀堆積所形成(hexagonal columnar phase)，而低溫的結晶相則具有正交晶系的結晶結構(tetragonal structure)。
但結晶度的增加，將使得系統的光學活性降低。因此推論結晶相的成長，將牽涉到分子螺旋構形的轉變。對於結晶相內分子排列結構的分析亦指出，分子將分別以採左手螺旋與右手螺旋構形，來進行規則排列。以偏光顯微鏡觀察雙折射現象(birefringence)於降溫過程的變化，可知中間相轉變至結晶相，將經歷一類似熔融-再結晶(melt-recrystallization process)的過程。然而在較緩慢的降溫過程中，所獲得的電子繞射圖譜則指出，結晶成長所牽涉的分子排列方式轉變，會依循類似固-固相轉變的機制(solid-solid transition)進行。基於這樣的實驗觀察，可以推論P(S)4MH分子的相變化或排列方式的轉變，會隨著轉變速率的不同，而以不同的機制進行。
在經歷多次結晶成長與局部溶解(partial melting)的過程後，可於薄膜中得到P(S)4MH分子之方形正立單晶(flat-on single crystal)。一般高分子以鏈摺疊方式進行二維層晶成長，會於單晶中發展出具有不同分子鏈摺疊方向的晶區(growth sectors)。然而P(S)4MH分子所形成的方形單晶，並沒有呈現不同的晶區。所以P(S)4MH分子是否以鏈摺疊(chain folding)的方式進行結晶成長，成為一個值得進一步瞭解的議題。
電子顯微鏡的觀察指出，緩慢升溫過程中的單晶局部溶解，傾向自晶區中心處開始，沿著{100}晶面發生。而分子於局部溶解後的重新排列，亦依循著特定的方向。然而，隨著局部熔融的速率不同，P(S)4MH分子的層晶重組行為(lamellar reorganization)，受到中間相的影響也不同。實驗觀察發現一開始較不穩定的結晶區，可在較快的升溫熔融過程，完全轉變至中間相。由於在尚未抵達結晶熔融溫度前，中間相為一介穩態，結晶相的晶核可以於中間相的區域中再度形成，並發展成較穩定的結晶相。
Poly((S)-4-methyl-1-hexene)(Ps4MH) is one of the Polyolefins that bear a chiral side chain (typically an isobutyl group) on repeat unit. This kind of polymers experience a so-called macromolecular amplification of chirality: the chiral side-chain induces a slight preference for either tg or tg- main chain conformation. This slight conformational bias is amplified cooperatively along the chain, and results in preferred chirality of the main chain helical conformations. This conformational preference enhances the rigidity of main chain, as a result, this polymer displays a mesophase and optical activity both in solution and, in the melt. For the crystalline phase, however, it develops an anti-chiral type of molecular packing, which means both right-handed and left-handed helices are included. In this case, the preferred left-handed helical conformation in the melt needs to be reversed for crystal growth of Ps4MH, which should come with a free-energy penalty. But for the racemic blend of Ps4MH and Pr4MH, both left-handed and right-handed helices are present in the melt.
Therefore depends on whether the reversal of helical conformation is needed or not for the crystal growth, molecular packing within mesophase can be a stable state after crystal melting, or simply a metastable transient phase on the way to crystallization. Not only the existence of mesophase, the nucleation barrier and stability of crystallization will be influenced also if the conformational reversal is needed. Through in-situ small-angle and wide-angle X-ray scattering as well as electron diffraction pattern, the mesophase can be identified as a hexagonal columnar phase. According to the literature, the crystal phase has tetragonal structure.
The transition behavior between mesophase to crystal phase is found to severely depend on the transition rate. During slow cooling process, the structural consequences can be recognized by obtained electron diffraction pattern, that is usually only found for solid-solid transition. But with slightly faster transition rate, according to the observation by polarized microscope, the transition takes place in a different way. Further revealed by the observation under transmission electron microscope, changing the degree of partial melting of grown single crystal will result in different reorganization processes and final morphologies.
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校內：2011-07-24公開校外：2011-07-24公開