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ORGANIC AND PHYSICAL CHEMISTRY OF POLYMERS phần 9

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Thí nghiệm sau đó quan sát thấy rằng sự đóng góp của năng lượng nội bộ là không đáng kể so với dữ liệu ngẫu nhiên Lý thuyết thống kê của cao su đàn hồi Như đã chỉ ra trước đây, đàn hồi có thể được xác định với một hội đồng của các chuỗi Nυ kết nối thông qua Nμ phân phối ngẫu nhiên liên kết chéo được ngăn cách với nhau bởi một kết thúc bậc trung bình | 498 NATURAL AND ARTIFICIAL POLYMERS In spite of the presence of many highly reactive functional groups such as hydroxyls cellulose is poorly reactive. Interchain molecular interactions hydrogen bonds are strong and ensure the main part of the cohesive properties while preventing the penetration of reagents. The breaking of these interactions is the precondition of any reaction. The ways to achieve such a breaking are given in Section 14.2.2 which deals with cellulose derivatives. Cellulose is not water-soluble but is strongly hydrophilic. This property is responsible for the great comfort exhibited by cellulose-based fibers and by the corresponding fabrics. Under normal conditions of use cellulose may contain up to 70 of loosely bound water. The partial replacement of polymer-polymer interactions by hydrogen bonds between cellulose and water causes a plasticization of the resulting material and thus a lowering of its mechanical characteristics. Whereas the tensile strength of highly crystalline and dry cellulose fibers can reach 700 MPa it can lose up to 30 of its value in wet atmosphere. Still due to the strong cohesion of this material cellulose is insoluble in most organic solvents. Only some highly polar mixtures such as N N -dimethylacetamide lithium chloride N-methylmorpholine water Cu OH 2 ammonia trifluoroacetic acid alkyl chloride calcium thiocyanate water and ammonium thiocyanate liquid ammonia are solvents of cellulose. In spite of the potential applications of such solutions they are exploited relatively little due to their high cost. The high degree of crystallinity of cellulose makes difficult the measurement of its glass transition temperature. The latter is located beyond 200 C but is impossible to measure accurately since cellulose degrades thermally above 180 C. Obviously the melting point is not accessible since its value is much higher than the degradation temperature. As all polymers that contain oxygen atoms in the main chain cellulose is .