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La escala de tiempo de la transición de vidrio y la entropía de configuración en polímeros: una visión molecular
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
Resumen
Los investigadores observaron directamente la dinámica de la cadena de polímeros a la temperatura de transición de vidrio (Tg). Usando RMN avanzada, revelaron el movimiento molecular crucial para comprender las transiciones de fase de polímeros y la entropía configuracional.
Área de la Ciencia:
- La ciencia de los polímeros es la ciencia de los polímeros.
- Química Física es la química física.
- Ciencia de los materiales Ciencia de los materiales.
Sus antecedentes:
- La temperatura de transición vítrea (Tg) es un parámetro crítico en la ciencia de los polímeros, que define la transición de un estado vítreo a un estado caucho.
- Comprender la dinámica molecular que ocurre en Tg es esencial para predecir el comportamiento y las propiedades del polímero.
- Los métodos tradicionales para estudiar Tg a menudo carecen de la resolución para observar la dinámica conformacional local.
Objetivo del estudio:
- Para observar directamente la dinámica conformacional local en las cadenas de polímeros a la temperatura calorimétrica de transición de vidrio (Tg).
- Demostrar una estrategia general para evaluar cuantitativamente los cambios de entropía de configuración utilizando técnicas avanzadas de RMN.
- Proporcionar una comprensión a nivel molecular de las transiciones de fase de los polímeros.
Principales métodos:
- Espectroscopia de intercambio de estado sólido bidimensional (2D) de temperatura variable de resonancia magnética nuclear (RMN) en abundancia natural.
- Calorimetría de barrido diferencial (DSC) para determinar el rango de temperatura de la transición de vidrio.
- Utilizando una mezcla de polímeros miscibles para probar la estrategia de evaluación cuantitativa.
Principales resultados:
- Observación experimental directa de la dinámica segmentaria en poliisobutileno (PIB) en una escala de tiempo de varios segundos a Tg (203-208 K).
- Primera observación directa del intercambio del conformador a nivel molecular (trans-trans/trans-izquierda/izquierda-izquierda) a la temperatura calorimétrica de transición de vidrio.
- Demostró un método cuantitativo para evaluar los cambios de entropía configuracional mediante la combinación de RMN de intercambio 2D y la teoría de Adams-Gibbs.
Conclusiones:
- El estudio proporciona información directa, a nivel molecular, sobre la dinámica de los polímeros en Tg, complementando las técnicas a granel.
- La estrategia desarrollada ofrece una poderosa herramienta para evaluar cuantitativamente la entropía configuracional, vital para la física de los polímeros.
- Los resultados son relevantes para comprender la escala de longitud de las regiones que reorganizan cooperativamente en Tg.
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