材料导论 (35)

Video 5: Amorphous or Crystalline State of PolymersNow, let’s talk about the solid state of polymers.Structurally, materials in solid state may be amorphous or crystalline.We all know that substances having small molecules (such as water, CO2 and methane) are normally totally crystalline as solids, or totally amorphous as liquids.In contrast, polymers are macro-molecules with high-molecular weight, and may have a wide molecular weight distribution. Therefore, the concept of crystallinity in polymers must be different from that in low-molecular-weight substances.今天讲固态聚合物形态。

从结构上讲，固态材料可呈无定形和结晶两种形态。

固态小分子物质如水、二氧化碳和甲烷通常可呈完全结晶形态，而液相则是完全无定形的。

相比而言，聚合物分子量很高，且具有一定分子量分布。

因此，聚合物的结晶与小分子物质的结晶不同。

When polymers are cooled from the molten state or concentrated from a dilute solution, their chains could be randomly twisted, coiled and entangled. This twisting, coiling prevents the strict ordering of polymer chains, resulting in a disorder arrangement of polymer chains, only a short-range order of repeating units. Consequently, we can only get an amorphous solid. This is a common situation for polymers in solid state. Possibly, individual polymer chains are folded (back and forth) on themselves and packed in a regular manner or in an orderly fashion. This will form a three-dimensional long-range ordered arrangement, that is, a crystalline structure of polymer.熔融态聚合物冷却时，或聚合物稀溶液浓缩时，分子链会随机蜷缩、缠绕。

这种卷曲、缠结会阻止分子链有序排列，使分子链呈无序排布，只有重复单元结构可做短程有序排布。

因此，通常固态聚合物呈无定形状态。

当然，单根分子量也有可能自己来回折叠，以有序方式规整堆积，形成三维、长程有序排布，即形成聚合物结晶结构。

But, in fact, since polymer chains are very long, they have high molecular weights,wide distributions of molecular weight, and complex structures,It is impossible for the chains to fit into a perfect arrangement as that observed in low-molecular-weight materials. Polymer molecules are often only partially crystalline or semi-crystalline. Almost no polymer is 100% crystalline.实际上，聚合物分子量高、分布宽，分子链很长且结构复杂。

因此，聚合物分子链不大可能像低分子物质那样形成完全规整排布。

聚合物分子常常只能是部分结晶或半结晶的，几乎没有100%结晶的。

The crystalline regions or domains in polymers are dispersed within the remaining amorphous material. We can say that the crystalline regions are connected with the disordered amorphous regions. Amorphous regions separate and connect crystalline regions. Polymer chains that run through both the ordered and the disordered segments. Usually, the crystalline regions are much smaller in size, and contain many imperfections, such as vacancy, or screw dislocation.聚合物结晶区域分散于无定形区域中，可以说，结晶区域由无序的无定形区域连接，无定形区域被结晶区域分隔和连接，聚合物链穿越有序和无序区域。

通常，结晶区域非常小，且结晶部分仍含有许多缺陷，如空穴，位错等。

We can use the degree of crystallinity, called DC, to show the fraction of total polymer in crystalline regions, or to measure the fraction of existing imperfections. Generally, the value of DC of polymer materials may vary or range from a few percentage points, almost completely amorphous, to about 90-95%, almost entirely crystalline.可用结晶度表述聚合物中结晶部分、或无定形部分的含量。

通常，结晶度可从百分之几(几乎完全无定形)变化到90-95%(几乎完全结晶)。

Examples of crystalline polymers include polyethylene, polyacrylonitrile and PET. Typical examples of amorphous polymers are Poly (methyl methacrylate), PMMA and polycarbonate.Simply to say, polymers in so lid state are semi-crystalline, have a mixed amorphous and crystalline structure. Degree of crystallinity indicates the fraction of crystalline structure or imperfections existing in polymers.典型结晶聚合物：聚乙烯、聚丙烯腈、聚酯；典型无定形聚合物：聚甲基丙烯酸甲酯和聚碳酸酯。

简言之，固态聚合物通常是半结晶的，非晶-无定形和结晶结构相互混合；可用结晶度表示聚合物中结晶或非晶结构的含量。

Then, we want to know the tendency for a polymer to crystallize, or what factors will influence the formation of crystalline structure.Usually, this tendency depends on how much the intermolecular bonding forces are, and the structural features of polymer chains.If polymer molecules have specific polar groups that can form strong intermolecular bonding, they are crystallizable.那么，如何知道聚合物结晶的趋势？什么因素会影响结晶结构的形成呢？通常，结晶结构的形成与分子间作用力以及聚合物链的结构特点有关。

聚合物分子含有极性基团，该基团能够形成较强分子间作用力，聚合物容易结晶。

For example, the groups like amide i n nylon, provide sites for hydrogen bonding. Therefore, nylon is a highly crystalline, high melting polymer.PVC or PAN have polar side groups –Cl (chloride), or –CN (cyano group), respectively, exhibiting polar bonding.They are crystalline and have higher melting points than polyethylene.例如：尼龙分子链中含有酰胺基团，可形成氢键。

因此，尼龙结晶度高，熔点高。

与聚乙烯相比，聚氯乙烯或聚丙烯腈含有极性侧基–Cl，-CN，形成较强极性相互作用，更容易结晶，且熔点较高。

The structural features of polymer chains we need to consider include structural regularity, stereo-regularity or tacticity, and chain flexibilityFor example, in comparison with HDPE, LLDPE and LDPE contains lots of short and long branches. These branches bring about irregularities in polymer chains and prevent the close packing of polymer chains. As a result, degree of crystallinity and melting point of LLDPE and LDPE are much lower than those of HDPE.聚合物分子链的结构特点，包括结构规整度、空间立构规整度或立构体、及链柔顺性等会影响聚合物的结晶。