高分子英语unit10

UNIT 10 Glass transition temperature

An ordinary rubber ball if cooled below -70°C becomes so hard and brittle that it will break into pieces like a glass ball falling on a hard surface! Why does a rubber ball become like glass bellow -70°C?

break into：摔碎

一个普通的橡胶球被冷却到零下70度以下，就会变得又硬又脆，掉在硬表面上就会像玻璃球一样破碎成碎片。为什么橡胶球在零下70度以下就变得像玻璃一样了？

This is because there is a “temperature boundary” for almost all amorphous polymers (and many crystalline polymers) only above which the substance remains soft, flexible and rubbery and below which it becomes hard, brittle and glassy.

这是因为几乎所有无定型聚合物(及许多结晶聚合物)都有一个“温度边界”，温度高于这个边界，材料保持软的柔韧的及橡胶状的，而高于这个边界，材料则变成硬的，脆的，玻璃状的。

This temperature, below which a polymer is hard and above which it is soft, is called the “glass transition temperature” Tg.

这个温度，低于这个温度聚合物是硬的，高于这个温度聚合物就是软

的，这个温度就叫玻璃化温度，Tg.

The hard, brittle state is known as the glassy state and the soft, flexible state as the rubbery or viscoelastic state. Be known as：叫做

硬的，脆的状态叫做玻璃态，软的，柔韧的状态叫做橡胶态或粘弹态。

On further heating, the polymer (if it is uncross-linked) becomes a highly viscous liquid and starts flowing: this state is termed viscofluid state, and the another transition takes place at its flow temperature Tf.

takes place: 发生

进一步加热，聚合物（如果它是非交联的）就变成高粘性液体而开始流动，这个状态叫做粘流态，另一个转变发生在聚合物流动温度上。

Now, let us consider a polymer, say, polyethylene. At room temperature, polyethylene is solid, exhibiting all characteristics of a low molecular weight substance. 现在让我们考虑一下一个聚合物，叫聚乙烯。在室温下，聚乙烯是固体的，展现低分子量化合物的所有特征。

At high temperatures, however, the characteristic difference between high and low molecular weight substances can be seen.

然而，在高温下高分子量和低分子量物质之间的区别是显而易见的。

Depending on the temperature, the molecules of a low molecular weight substance either move apart as a whole or do not move at all. i. e., there is a definite temperature(melting point Tm) below which the molecules do not move and above which they do move.

Depending on：根据Move apart: 分开,分离,移开As a whole 作为一个整体, 整体(上)

根据温度, 低分子量的物质要么整个分子移开，要么根本不运动。有一个特定的温度(熔点Tm)在低于这一温度时分子不运动，而在高于这一温度时分子则在运动。

On the other hand, with polymers, if the temperature increases above Tg, localized units (chain segments) within the long chain molecule are first mobilized before the whole molecule starts moving.

对聚合物而言，另一方面，如果温度增加超过玻璃化温度，在整个长分子链内的局部单元（链段）在整个分子运动之前首先动起来。

In some parts within the molecule, there is a considerable localized motion, but not in other parts of the same molecule.

在这个分子的一部分有比较大的局部运动，而相同分子的另一部分则没有运动。

Thus, within the long chain of the polymer molecules, some segments have a certain freedom of movement, whereas others do not.

因此，在聚合物分子的长链内，一些链段有一定的运动自由度，而其它的链段则没有。

The molecule as a whole does not move although some of its segments do. In the case of polymers, there is indeed an intermediate state.

尽管分子的一些链段在动，分子整体是不动的。聚合物有一个中间状态。

If the temperature ranges between Tg and Tf, the localized mobility is activated, but the overall mobility is not.

如果温度在Tg和Tf之间，局部的运动被激活，而整体的运动则没有。

The local segments, where mobility is already activated, correspond to the liquid state, while the molecule as a whole, where mobility is forbidden, is in the solid state. This state, which is really a combination of liquid and solid, is called the rubbery state.

correspond to: 相当于

局部链段的运动被激活，对应着液态，而整个分子的运动性受到限制，则仍处于固态。这个状态，实际上是液态和固态的结合，叫做橡胶态。

Under the influence of an applied stress, it exhibits properties of a viscous fluid as well as an elastic solid and undergoes what is called viscoelastic deformation. 在外加应力的作用下，聚合材料既显示出粘性液体的性质，也显示出弹性固体的性质，而经历的过程叫做粘弹形变。

The glass transition temperature Tg is an important parameter of a polymeric material. The Tg value of a polymer decides whether a polymer at the “use temperature” will behave like rubber or plastics.

玻璃化转变温度Tg是聚合物材料的重要参数。聚合物的Tg决定聚合

物在使用温度下究竟是体现橡胶的还是塑料的行为。

The Tg value along with the Tm value gives an indication of the temperature region at which a polymeric material transforms from a rigid solid to a soft viscous state。

give an indication of: 表明，表示trasforms from... to... 由?转变成

Tg值和Tm值表明了一个温度范围，在这个范围内聚合材料由刚性固体转化为柔软的粘流态。

This helps in choosing the right processing temperature, i.e., the temperature region in which the material can be converted into finished products though different processing techniques such as moulding, calendering, extrusion, etc.

be converted into: 转化为