Glass Transition - Glass Transition

When metals, inorganic substances, organic substances, polymers, and other substances are cooled rapidly from a molten state, there is no time for crystallization, and each substance freezes in an amorphous state similar to the melt, becoming a glassy amorphous solid. The volume change from melt to amorphous solid is that when a melt is cooled below its melting point T _m , it does not crystallize, but remains a supercooled melt as it is cooled, its viscosity increases, and its volume decreases, but at a certain temperature (glass transition temperature T _g ), the contraction of its volume (expansion coefficient) slows down, it loses almost all fluidity, and it changes to a hard glassy state (viscosity of approximately 10 ¹³ P (poise) or more). This change from a melt to an amorphous solid is called the glass transition. This glass transition is not a phase transition as a thermodynamic thermal equilibrium, but a kinetic change in which the microscopic motion of atoms or molecules suddenly slows down, resulting in a frozen, metastable, non-equilibrium state. Near the glass transition temperature Tg , the increase in volume bends, _and the expansion coefficient, specific heat, etc. (second derivative of the chemical potential) change discontinuously at the glass transition temperature; therefore, this was formerly called a thermodynamic second-order phase transition, but is now understood to be a kinetic transition caused by the freezing of cooperative molecular motion, rather than a thermodynamic transition. On the other hand, when a melt is cooled slowly, crystallization Tc occurs at temperatures below _the melting point Tm . In general, the volume of a crystal is smaller than the volume of glass (it has a higher density), _and the difference in volume is called the free volume. In other words, the free volume is almost constant below the glass transition temperature, but increases rapidly above the glass transition temperature. This glass transition phenomenon has been explained by considering it to be a state of equal free volume, equal coordination entropy, and equal viscosity, but this has not yet been fully explained.

Source: Morikita Publishing "Chemical Dictionary (2nd Edition)" Information about the Chemical Dictionary 2nd Edition

金属，無機，有機，高分子などの物質を溶融状態から急激に冷却すると，結晶化する余裕がなく，各物質は融体と同様な無定形状態のまま凍結され，ガラス状の非晶質固体となる．この融体から非晶質固体になる体積変化は，融体は融点 T_m 以下に冷却されても，結晶化せず，過冷却融体のまま冷却され粘度は増加し，体積は減少するが，ある温度(ガラス転移温度 T_g)でその体積の収縮(膨張係数)が緩やかになり，流動性はほとんどなくなり，ガラス状の硬い状態(粘度約 10¹³ P(ポアズ)以上)に変化する．このように融体から非晶質固体に変化することをガラス転移とよぶ．このガラス転移は，熱力学的な熱平衡としての相転移ではなく，原子または分子のミクロな運動が急激に緩慢になり，凍結された準安定な非平衡状態であり速度論的変化である．ガラス転移点 T_g の付近では，体積の増加は折れ曲がり，膨張係数，比熱など(化学ポテンシャルの二次導関数)はガラス転移温度で不連続に変化することから，古くは熱力学的二次相転移とよばれたが，現在では熱力学転移ではなく，共同的な分子運動の凍結による速度論的転移として理解されている．一方，融体をゆっくり冷却すると，融点 T_m 以下の温度で結晶化 T_c が起こる．一般に結晶の体積はガラスの体積より小さく(密度は高い)，その体積の差を自由体積とよぶ．いいかえると，自由体積はガラス転移温度以下ではほぼ一定であるが，ガラス転移温度以上では急激に増加する．このようなガラス転移現象を等自由体積状態，等配位エントロピー状態，等粘性状態としてとらえて説明されているが，まだ十分に説明はできていない．

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