📚 Synthetic Polyamides | 合成聚酰胺
Synthetic polyamides are condensation polymers that contain the characteristic amide linkage, -CONH-, repeated along the polymer chain. They are manufactured industrially from petrochemical monomers and include familiar materials such as nylon-6,6, nylon-6 and Kevlar. This article explores how their monomer structures, hydrogen bonding and crystallinity control their physical properties and real-world applications.
合成聚酰胺是一类缩聚聚合物，其分子链中重复出现特征性的酰胺键（-CONH-）。它们通常以石化产品为单体进行工业生产，包括人们熟知的尼龙-6,6、尼龙-6 和凯夫拉等材料。本文将介绍单体结构、氢键和结晶度如何决定它们的物理性质与实际用途。
1. What Are Synthetic Polyamides? | 什么是合成聚酰胺？
A polyamide is any polymer whose repeat units are joined by amide groups, -CONH-. Synthetic polyamides are made by chemical polymerisation rather than by living organisms. Naturally occurring polyamides include proteins, silk and wool, but synthetic nylons and Kevlar are designed with simpler, repeating chains to give high strength, toughness and thermal stability.
聚酰胺是指重复单元通过酰胺基团（-CONH-）连接起来的聚合物。合成聚酰胺是通过化学聚合而非生物体合成的。天然聚酰胺包括蛋白质、蚕丝和羊毛，而合成尼龙和凯夫拉则具有更简单、重复的链结构，从而获得高强度、高韧性和热稳定性。
The amide group is polar and planar. The carbon-nitrogen bond in an amide has partial double-bond character, which restricts rotation and stiffens the polymer chain. This is a key reason why polyamides are strong engineering materials.
酰胺基团具有极性和平面结构。酰胺中的碳氮键具有部分双键性质，这限制了旋转并使聚合物链更加刚性。这是聚酰胺能成为高强度工程材料的关键原因之一。
2. Key Monomers and Repeat Units | 关键单体与重复单元
Synthetic polyamides are usually made from two monomer types: a diamine and a dicarboxylic acid, or from a single monomer containing both an amine and a carboxylic acid group. The repeat unit must contain one amide linkage formed between monomers.
合成聚酰胺通常由两类单体制备：二胺与二羧酸，或由同时含有氨基和羧基的单一单体制备。重复单元中必须含有单体之间形成的酰胺键。
For nylon-6,6, the monomers are hexane-1,6-diamine, H₂N(CH₂)₆NH₂, and hexanedioic acid, HOOC(CH₂)₄COOH. The repeat unit is -[NH(CH₂)₆NHCO(CH₂)₄CO]-. The name ‘6,6’ comes from the six carbon atoms in the diamine and the six carbon atoms in the dicarboxylic acid.
以尼龙-6,6 为例，其单体是 1,6-己二胺 H₂N(CH₂)₆NH₂ 和己二酸 HOOC(CH₂)₄COOH，重复单元为 -[NH(CH₂)₆NHCO(CH₂)₄CO]-。”6,6″ 这个名称来源于二胺中有 6 个碳原子、二羧酸中也有 6 个碳原子。
Nylon-6 is made from caprolactam, a cyclic amide containing six carbon atoms. Its repeat unit is -[NH(CH₂)₅CO]-. Kevlar is made from aromatic monomers, giving the repeat unit -[NH-C₆H₄-NHCO-C₆H₄-CO]-.
尼龙-6 由含有 6 个碳原子的环状酰胺——己内酰胺制备，其重复单元为 -[NH(CH₂)₅CO]-。凯夫拉由芳香族单体制备，重复单元为 -[NH-C₆H₄-NHCO-C₆H₄-CO]-。
When drawing repeat units, exam answers should show the amide group clearly and use brackets with the suffix n to indicate repetition.
在绘制重复单元时，考试答案应清楚地标出酰胺基，并用方括号和下标 n 表示重复。
3. Formation by Condensation Polymerisation | 缩聚反应形成
Most synthetic polyamides are formed by condensation polymerisation, in which each new amide link releases a small molecule. When a diamine reacts with a dicarboxylic acid, the small molecule eliminated is water.
大多数合成聚酰胺通过缩聚反应形成，每形成一个酰胺键都会释放一个小分子。当二胺与二羧酸反应时，释放的小分子是水。
The formation of nylon-6,6 can be represented as follows:
尼龙-6,6 的生成可用下式表示：
n H₂N(CH₂)₆NH₂ + n HOOC(CH₂)₄COOH → H—[NH(CH₂)₆NHCO(CH₂)₄CO]ₙ—OH + (2n−1)H₂O
Each amide link forms when the -NH₂ group of a diamine reacts with the -COOH group of a dicarboxylic acid. One water molecule is lost for each amide bond created, so the polymer chain grows stepwise rather than by addition of many monomers at once.
每个酰胺键的形成都是二胺的 -NH₂ 基团与二羧酸的 -COOH 基团发生反应，同时失去一分子水。聚合物链逐步增长，而不是一次性加成许多单体。
When an acyl chloride is used instead of a carboxylic acid, as in Kevlar production, hydrogen chloride is eliminated instead of water. This reaction is faster and is often used in the laboratory or in interfacial polymerisation demonstrations.
当使用酰氯代替羧酸时，例如生产凯夫拉时，消除的是氯化氢而不是水。该反应更快，常用于实验室或界面聚合演示中。
n H₂N-C₆H₄-NH₂ + n ClOC-C₆H₄-COCl → [NH-C₆H₄-NHCO-C₆H₄-CO]ₙ + (2n−1)HCl
4. Nylon-6,6 Production and Structure | 尼龙-6,6 的生产与结构
Nylon-6,6 is produced industrially by first mixing hexane-1,6-diamine and hexanedioic acid in a 1:1 ratio to form a salt known as nylon salt. This salt is then heated under pressure to temperatures around 250-300 °C, driving off water and forming long polymer chains.
尼龙-6,6 的工业生产首先将 1,6-己二胺和己二酸按 1:1 比例混合，生成一种称为尼龙盐的盐。然后将尼龙盐在加压条件下加热至约 250-300 °C，除去水分并形成长链聚合物。
The resulting polymer is a thermoplastic. When molten, it can be extruded through spinnerets to make fibres, or injection-moulded into solid objects. The long aliphatic carbon chains give flexibility, while the regular amide groups allow strong hydrogen bonding between chains.
所得聚合物是一种热塑性塑料。熔融状态下，它可以通过喷丝头挤出制成纤维，也可以通过注塑制成固体部件。较长的脂肪族碳链赋予其柔韧性，而规则的酰胺基团使链间能够形成强氢键。
In the solid fibre, nylon-6,6 chains pack closely to form semi-crystalline regions. The crystalline areas provide strength and rigidity, while the amorphous areas allow some flexibility and impact resistance.
在固态纤维中，尼龙-6,6 的分子链紧密排列形成半结晶区域。结晶区域提供强度和刚性，而无定形区域则赋予一定的柔韧性和抗冲击性。
5. Nylon-6 via Ring-Opening Polymerisation | 尼龙-6 的开环聚合
Nylon-6 is synthesised from caprolactam, a seven-membered cyclic amide with the formula C₆H₁₁NO. Under the influence of heat, water and a catalyst, the ring opens and the monomers join to form a linear polyamide.
尼龙-6 由己内酰胺合成。己内酰胺是一种七元环状酰胺，分子式为 C₆H₁₁NO。在加热、水和催化剂的作用下，环打开，单体相互连接形成线性聚酰胺。
n C₆H₁₁NO → [NH(CH₂)₅CO]ₙ
Unlike nylon-6,6, this reaction is not a simple condensation polymerisation because no small molecule is eliminated. The amide group is already present inside the lactam ring, so ring-opening polymerisation can be classified as a form of addition polymerisation that produces a polyamide.
与尼龙-6,6 不同，该反应不是简单的缩聚反应，因为没有小分子被消除。酰胺基团已经存在于内酰胺环中，因此开环聚合可以看作一种生成聚酰胺的加聚反应。
Nylon-6 and nylon-6,6 have similar repeat unit masses and very similar properties, but nylon-6 has a slightly lower melting point because its repeat unit is less symmetrical than that of nylon-6,6.
尼龙-6 和尼龙-6,6 的重复单元质量相似，性质也非常接近，但尼龙-6 的熔点略低，因为其重复单元的对称性不如尼龙-6,6。
6. Kevlar: An Aromatic Polyamide | 凯夫拉：芳香族聚酰胺
Kevlar is an aromatic polyamide, or aramid, made from benzene-1,4-diamine and terephthaloyl dichloride. The presence of benzene rings in the polymer backbone makes the chains very rigid and rod-like.
凯夫拉是一种芳香族聚酰胺（芳纶），由对苯二胺和对苯二甲酰氯制备。聚合物主链中的苯环使分子链非常刚硬并呈棒状。
The aromatic rings reduce chain flexibility and allow the polymer chains to align parallel to one another. Combined with strong hydrogen bonding between the amide groups, this gives Kevlar exceptionally high tensile strength and heat resistance.
芳香环降低了链的柔韧性，使聚合物链能够彼此平行排列。再加上酰胺基团之间的强氢键，凯夫拉具有极高的抗拉强度和耐热性。
Kevlar is widely used in body armour, bulletproof vests, aerospace composites, high-performance ropes and protective clothing. Its strength-to-weight ratio is much higher than that of steel.
凯夫拉广泛用于防弹衣、防弹背心、航空航天复合材料、高性能绳索和防护服。其强度与重量之比远高于钢。
7. Intermolecular Forces and Hydrogen Bonding | 分子间作用力与氢键
Polyamide chains contain both N-H groups and C=O groups. These polar groups allow hydrogen bonds to form between the N-H hydrogen of one chain and the C=O oxygen of a neighbouring chain.
聚酰胺分子链中同时含有 N-H 基团和 C=O 基团。这些极性基团使得一条链上的 N-H 氢与相邻链上的 C=O 氧之间形成氢键。
Hydrogen bonding is the dominant intermolecular force in polyamides. It is much stronger than the van der Waals forces that hold simple polyalkenes such as poly(ethene) together. This explains why nylon has a much higher melting point and greater tensile strength than poly(ethene).
氢键是聚酰胺中最主要的分子间作用力。它比简单聚烯烃（如聚乙烯）中的范德华力强得多。这就解释了为什么尼龙的熔点和抗拉强度远高于聚乙烯。
The long hydrocarbon sections in nylon also experience van der Waals forces. These forces increase with chain length and contribute to the overall strength, especially when the chains pack closely in crystalline regions.
尼龙中较长的碳氢链段之间也存在范德华力。这些力随链长增加而增强，并且在分子链紧密排列的结晶区域中对整体强度作出贡献。
The number of hydrogen bonds per unit length depends on the density of amide groups. Aromatic polyamides such as Kevlar have a high density of amide groups and rigid rings, producing far stronger hydrogen-bonded sheets.
单位长度上的氢键数量取决于酰胺基团的密度。凯夫拉等芳香族聚酰胺具有较高的酰胺基密度和刚性的苯环，因而能形成强得多的氢键网络。
8. Crystallinity, Strength and Thermal Properties | 结晶度、强度与热性能
Synthetic polyamides are semi-crystalline polymers. When fibres are drawn, the polymer chains become aligned along the fibre axis, increasing crystallinity. This alignment allows more hydrogen bonds to form and greatly increases tensile strength and stiffness.
合成聚酰胺是半结晶聚合物。当纤维被拉伸时，聚合物链沿纤维轴方向排列，结晶度增加。这种排列使更多氢键得以形成，从而大幅提高抗拉强度和刚性。
Nylon-6,6 has a melting point of about 264 °C, while nylon-6 melts at about 220 °C. Kevlar does not melt under normal conditions but begins to decompose at temperatures above 500 °C. These differences arise from chain symmetry, hydrogen bonding density and chain rigidity.
尼龙-6,6 的熔点约为 264 °C，尼龙-6 的熔点约为 220 °C。凯夫拉在通常条件下不会熔化，但在 500 °C 以上开始分解。这些差异来源于链对称性、氢键密度和链刚性的不同。
The table below compares some key physical properties arising from structure.
下表比较了由结构决定的一些关键物理性质。
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