ガラスの移行時間スケールとポリマーの構成エントロピー:実験的な分子視野
J E Wolak1, X Jia, Jeffery L White
1Department of Chemistry, North Carolina State University, Campus Box 8204, Raleigh, North Carolina 27695-8204, USA.
Journal of the American Chemical Society
|November 6, 2003
まとめ
研究者は,ガラスの移行温度 (Tg) で,ポリマーチェーンダイナミクスを直接観察しました. 先進的なNMRを用いて,ポリマーの相変化と構成エントロピーの理解に不可欠な分子運動を明らかにした.
科学分野:
- ポリマーサイエンスの科学
- 物理化学 物理化学
- マテリアルサイエンス 材料科学
背景:
- ガラスの移行温度 (Tg) は,ポリマー科学における重要なパラメータであり,ガラスの状態からゴム状の状態への移行を定義します.
- Tgで発生する分子ダイナミクスを理解することは,ポリマーの行動と性質を予測するために不可欠です.
- Tgを研究する伝統的な方法は,局所的な形状動態を観察するための解像度が欠けていることが多い.
研究 の 目的:
- カロリメトリックガラス変換温度 (Tg) で,ポリマー鎖の局所形状動態を直接観察する.
- 先進的なNMR技術を使用して構成エントロピーの変化を定量的に評価するための一般的な戦略を実証します.
- ポリマー相変異の分子レベルの理解を図る.
主な方法:
- 変温二次元 (2D) 固体交換型核磁共振 (NMR) スペクトロスコーピーは,自然に豊富に存在する.
- グラス移行温度範囲を決定するディフェンショナル・スキャニング・カロメトリー (DSC).
- 混合可能なポリマー混合物を用いて,定量評価戦略をテストする.
主要な成果:
- Tg (203-208 K) で数秒の時間スケールでポリイソブチレン (PIB) のセグメンタルダイナミクスの直接実験観察.
- カロリメトリックガラス移行温度における分子レベルのコンフォマー交換 (トランス-トランス/トランス-左/左-左) の最初の直接観測.
- 2D交換NMRとアダムズ・ギブス理論を組み合わせて構成エントロピーの変化を評価するための定量的な方法を示した.
結論:
- この研究は,Tgでのポリマー動力学に関する直接的な分子レベルの洞察を提供し,散発技術を補完します.
- 開発された戦略は,ポリマー物理学にとって不可欠な構成エントロピーを定量的に評価するための強力なツールを提供します.
- 結果は,Tg.で地域を協調的に再編成する長さのスケールを理解するために重要です.
関連する概念動画
01:34Fluid Mosaic Model
The fluid mosaic model was first proposed as a visual representation of research observations. The model comprises the composition and dynamics of membranes and serves as a foundation for future membrane-related studies. The model depicts the structure of the plasma membrane with a variety of components, which include phospholipids, proteins, and carbohydrates. These integral molecules are loosely bound, defining the cell’s border and providing fluidity for optimal function.LipidsThe most...
02:00Molecular Models
Physical models representing molecular architectures of chemical compounds play essential roles in understanding chemistry. The use of molecular models makes it easier to visualize the structures and shapes of atoms and molecules.
02:18Network Covalent Solids
Network covalent solids contain a three-dimensional network of covalently bonded atoms as found in the crystal structures of nonmetals like diamond, graphite, silicon, and some covalent compounds, such as silicon dioxide (sand) and silicon carbide (carborundum, the abrasive on sandpaper). Many minerals have networks of covalent bonds.
To break or to melt a covalent network solid, covalent bonds must be broken. Because covalent bonds are relatively strong, covalent network solids are typically...
To break or to melt a covalent network solid, covalent bonds must be broken. Because covalent bonds are relatively strong, covalent network solids are typically...
02:46Crystal Field Theory - Tetrahedral and Square Planar Complexes
Tetrahedral Complexes
Crystal field theory (CFT) is applicable to molecules in geometries other than octahedral. In octahedral complexes, the lobes of the dx2−y2 and dz2 orbitals point directly at the ligands. For tetrahedral complexes, the d orbitals remain in place, but with only four ligands located between the axes. None of the orbitals points directly at the tetrahedral ligands. However, the dx2−y2 and dz2 orbitals (along the Cartesian axes) overlap with the ligands less than the dxy,...
Crystal field theory (CFT) is applicable to molecules in geometries other than octahedral. In octahedral complexes, the lobes of the dx2−y2 and dz2 orbitals point directly at the ligands. For tetrahedral complexes, the d orbitals remain in place, but with only four ligands located between the axes. None of the orbitals points directly at the tetrahedral ligands. However, the dx2−y2 and dz2 orbitals (along the Cartesian axes) overlap with the ligands less than the dxy,...
01:21Polymer Classification: Crystallinity
Unlike ionic or small covalent molecules, polymers do not form crystalline solids due to the diffusion limitations of their long-chain structures. However, polymers contain microscopic crystalline domains separated by amorphous domains.
Crystalline domains are the regions where polymer chains are aligned in an orderly manner and held together in proximity by intermolecular forces. For example, chains in the crystalline domains of polyethylene and nylon are bound together by van der Waals...
Crystalline domains are the regions where polymer chains are aligned in an orderly manner and held together in proximity by intermolecular forces. For example, chains in the crystalline domains of polyethylene and nylon are bound together by van der Waals...
01:19Crystal Density
The crystal lattice structure of a material allows us to determine how many molecules exist in its unit cell. With this information, alongside the unit-cell parameters - three distance parameters (a, b, c) and three angular parameters (α, β, γ).Density (ρ) = (Z × M) / (a × b × c × NA)where:Z is the number of formula units per unit cellM is the molar mass of the substancea, b, and c are the edge lengths of the unit cellNA is Avogadro’s numberFor a simple cubic lattice, atoms are located only at...