材料导论

1. Define thermoplastics, thermosetting plastics, elastomers and thermoplastic elastomers.

Thermoplastics: linear or branched polymers in which chains of molecules are not interconnected to one another.

Thermosetting plastics: polymers that are heavily cross-linked to produce a strong three dimensional network structure.

Elastomers: these are polymers (thermoplastics or lightly cross-linked thermosets) that have an elastic deformation ﹥200%.

Thermoplastic elastomers: polymers that behave as thermoplastics at high temperatures, but as elastomers at low temperatures.

热塑性聚合物：线形或支链形大分子以物理力聚集而成，加热时可熔融，并能溶于适当溶剂中的一类聚合物。

热固性聚合物：线形或支链形大分子在加热或外加交联剂作用下发生交联形成不溶不熔的空间网络结构聚合物。

2. What electrical and optical applications are polymers used for? Explain using examples.

PE, PVC are used for wire insulation, PP is used for insulation foams, lighting panels, Polychlorotrifluoroethylene, PEEK are used for electrical insulation, PET is used for photographic film. PA, PES, PAI are used for electrical components.

3. What are the advantages associated with plastics compared to ceramics, glasses and metallic materials?

Compared with most metals and ceramics, plastics have low strength, stiffness, and melting temperatures; however, they also have a low density and good chemical resistance.

与大多都数金属材料，陶瓷材料相比，尽管高分子材料的强度，硬度和熔点都不高，但是它们仍然具有质轻，弹性好，耐化学性能优良，绝缘性好，易成型加工等特点。

4. Explain why low-density polyethylene is good to make grocery bags, however, super high-molecular weight polyethylene must be used where strength and very high wear resistance is needed.

食品袋一般是采用吹塑工艺制成，因而要求拉伸性能好，透明，柔韧性好，耐应力破裂性能好等。低密度聚乙烯与高密度聚乙烯相比，支链多，密度小，分子量低，分子量分布宽，所以易拉伸，适合制造吹塑薄膜制成食品袋。而高密度聚乙烯支链少，密度大，分子量大，分子量分布窄，机械强度大，因而具有较好的冲击强度和耐磨性能，适合做高抗冲耐磨材料。

5. The molecular weight of PMMA is 250,000 g/mol. If all of the polymer chains are the same length,

(a) Calculate the degree of polymerization, and

(b) The number of chains in 1 g of the polymer.

6. Analysis of a sample of PAN shows that there are six lengths of chains, with the following number of chains of each length. Determine

(a) The weight average molecular weight and degree of polymerization, and

(b) The number average molecular weight and degree of polymerization.

7. Explain what the following terms mean: decomposition temperature, heat

distortion temperature, glass temperature, and melting temperature. Why is it that thermoplastics do not have a fixed melting or glass temperature? Decomposition temperature: the temperature above which a polymer burns, chars, or decomposes.

Heat distortion temperature: the temperature at which a polymer will deform a given amount under a standard load.

Glass temperature: the temperature range below which the amorphous polymer assumes a rigid glassy structure.

熔点：熔点是物质的物态由固态转变为液态的温度。但高聚物的熔点并不是一个固定的值，而是一个较宽的熔融范围，称为熔限。

由于结晶高聚物中含有完善程度不同的晶体的缘故，因而结晶高聚物具有一个较宽温度范围的熔限，而不是一个固定值的熔点。

8. How do the glass temperatures of PE, PP, and PMMA? Explain their differences, based on the structure of monomer.

PE﹤PP﹤PMMA；玻璃化温度与高分子链柔性有关，减少高分子链柔性或增加分子间作用力的因素，如引入刚性基团或极性基团，交联或结晶等，都会使玻璃化温度增加，从PE，PP到PMMA，重复单元上的侧基从无到有，从一个侧基到两个侧基，空间位阻不断增大，柔性不断减小，因而玻璃化温度依次增大。9. What factors influence the crystallinity of polymers? Explain the development and role of crystallinity in PET and nylon.

（a）链的对称性：高分子链的结构对称性越高，越容易结晶；（b）链的规整性，等规度越高结晶能力就越大；（c）一定的链柔顺性是结晶时链段向结晶表面扩散和排列所必必需的，所以链的柔顺性不好在一定程度上也降低了聚合物的结晶能力；（d）支化破坏链的对称性和规整性，使结晶能力降低；（e）交联降低了链的活动性，随着交联度的增加结晶能力下降；（f）分子间形成氢键则有利于结晶结构的稳定。

在外力作用或低温冷却下PET进行结晶，由于PET的大分子链上有—C —O —基使对称性下降,主链上还有苯环,使分子链的刚性增大,对链段运动起到了一定的阻碍作用,影响了分子链扩散的速度，因而其结晶速度较慢。而尼龙由于分子间氢键作用力的影响，有利于其结晶结构的稳定，结晶能力较好。

在外力或低温冷却作用下，聚合物中的链段开始运动扩散，沿一定方向进行取向或作规整的链堆积，形成有序的结晶结构。结晶有利于改善PET和尼龙的密度，耐高温性，耐化学腐蚀性和机械性能等。

10. For each of the following pairs, recommend the one that will most likely have the better impact properties at 25℃. Explain each of your choices.

（a）PE vs PS

（b）LDPE vs HDPE

（c）PMMA vs PTFE

（a）PE;与PE相比，PS上有苯环侧基，从而使得PS的空间位阻比PE大，阻碍链段的运动，在应力作用下，PS的强迫高弹形变低于PE，其冲击强度也就不如PE。

（b）LDPE;与HDPE相比，LDPE支链多且长，增大了链的柔性，链段运动较容易，易发生强迫高弹形变，冲击强度优于HDPE。

（c）PTFE；在PTFE中的F原子与H原子大小相差不大，可类似与看成聚乙烯