材料科技英语第一章part2unit13.ppt

Unit 13 ;Main line;A polymer is a large molecule (大分子 macromolecule) composed of repeating structural units结构单元 typically connected by covalent chemical bonds共价键. The word polymer is derived from the Greek words πολυ- - poly- meaning "many"; and μ?ρο? - meros meaning "part". ;Typical Polymer;While polymer in popular usage suggests plastic, the term actually refers to a large class of natural and synthetic materials with a wide variety of properties, including properties typically associated with plastics.;Natural polymeric materials such as shellac虫胶, amber琥珀, and natural rubber天然橡胶 have been in use for centuries. Biopolymers 生物高分子such as proteins蛋白质 and nucleic acids核酸 play crucial roles in biological processes. A variety of other natural polymers exist, such as cellulose纤维素, which is the main constituent of wood and paper.;Polymers are studied in the fields of polymer chemistry, polymer physics, and polymer science.;Starting in 1811, Henri Braconnot did pioneering work in derivative cellulose compounds, perhaps the earliest important work in polymer science. The development of vulcanization硫化 later in the nineteenth century improved the durability of the natural polymer rubber, signifying the first popularized semi-synthetic polymer. ;In 1907, Leo Baekeland created the first completely synthetic polymer, Bakelite酚醛树脂, by reacting phenol and formaldehyde at precisely controlled temperature and pressure. Bakelite was then publicly introduced in 1909.;Despite significant advances in synthesis and characterization of polymers, a correct understanding of polymer molecular structure did not emerge until the 1920s. Before then, scientists believed that polymers were clusters of small molecules (called colloids胶体), without definite molecular weights, held together by an unknown force, a concept known as association theory连结理论. ;In 1922, Hermann Staudinger proposed that polymers consisted of long chains of atoms held together by covalent bonds, an idea which did not gain