玻璃化转变温度Glass transition

玻璃化转变不是热力学相变，而是一种动力学现象：Tg 与升温速率和测试频率有关。通过 Tg 时比热容、热膨胀系数和模量（约下降 3 个数量级）发生突变。自由体积理论认为，当自由体积分数降到约 2.5% 时链段运动被冻结。影响 Tg 的因素有：主链刚性（苯环使 Tg 升高）、侧基大小与极性、分子间作用力、交联度、分子量和增塑剂。

The glass transition is not a thermodynamic phase change but a kinetic phenomenon: Tg depends on heating rate and test frequency. Heat capacity, expansion coefficient and modulus (which falls by about three orders of magnitude) all change abruptly through it. Free-volume theory holds that segmental motion freezes when the free-volume fraction falls to about 2.5%. Tg depends on backbone stiffness (rings raise it), the size and polarity of side groups, intermolecular forces, crosslinking, molar mass and plasticisers.

1930 年代人们注意到橡胶在低温下变脆。1950 年福克斯和弗洛里研究了分子量对 Tg 的影响（福克斯-弗洛里方程 Tg = Tg∞ − K/Mₙ）。1955 年威廉斯、兰德尔和费里提出 WLF 方程，描述 Tg 以上松弛时间随温度的变化，奠定了时温叠加原理。1986 年“挑战者号”航天飞机事故的原因之一是寒冷天气使 O 形密封圈低于其工作温度范围而失去弹性。

In the 1930s rubber was noticed to turn brittle in the cold. In 1950 Fox and Flory studied how molar mass affects Tg (the Fox–Flory equation Tg = Tg∞ − K/Mₙ). In 1955 Williams, Landel and Ferry proposed the WLF equation for how relaxation times above Tg vary with temperature, founding time–temperature superposition. One cause of the 1986 Challenger disaster was that cold weather left the rubber O-rings below their working range, so they lost elasticity.

聚二甲基硅氧烷约 −125 °C，天然橡胶约 −70 °C，聚乙烯约 −120 至 −80 °C，PVC 约 80 °C，聚苯乙烯约 100 °C，PMMA 约 105 °C，聚碳酸酯约 150 °C。Tg 低于室温的非晶高分子用作橡胶，Tg 高于使用温度的用作塑料。测定方法有差示扫描量热法（DSC）、动态力学分析（DMA）和膨胀计法。

Typical values are about −125 °C for PDMS, −70 °C for natural rubber, −120 to −80 °C for polyethylene, 80 °C for PVC, 100 °C for polystyrene, 105 °C for PMMA and 150 °C for polycarbonate. Amorphous polymers with Tg below room temperature serve as rubbers and those with Tg above their service temperature as plastics. Tg is measured by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and dilatometry.