Đang chuẩn bị liên kết để tải về tài liệu:
ORGANIC AND PHYSICAL CHEMISTRY OF POLYMERS phần 10

Không đóng trình duyệt đến khi xuất hiện nút TẢI XUỐNG Tải xuống

hạn chế. Các tính di động cao của chuỗi PBT và điểm nóng chảy thấp hơn của PET làm cho polymer này dễ dàng hơn để xử lý hơn PET, nó là chủ yếu xử lý bằng cách tiêm ở nhiệt độ PBT thường được trộn lẫn với sợi thủy tinh ngắn, | 562 LINEAR MONODIMENSIONAL SYNTHETIC POLYMERS limited. The high mobility of PBT chains and the melting point lower than that of PET make this polymer easier to process than PET it is mainly processed by injection at a temperature 250 C. PBT is often mixed with short glass fibers which simultaneously allows an increase of its stress at break tenacity and its elastic modulus. Its annual world production is approximately 200 000 tons and its growth rate is high. 15.5.2.3. Poly bisphenol A carbonate PC O Among the various diols that were tested for polycarbonate synthesis the one resulting from bisphenol A BPA was found to be a material exhibiting interesting mechanical characteristics. Thus it corroborates the general idea that the introduction of bisphenol A groups into a macromolecular chain largely improves the mechanical properties of the resulting material. Among the various methods that can be utilized to produce PC processing through interfacial polycondensation is widely used. The hydrosoluble precursor is the disodic salt of bisphenol A. It reacts with phosgene Cl2CO solubilized in a hydrophobic solvent that is generally a chlorinated solvent CH2Cl2 CHCl3 C6H5Cl etc. n Cl-CO-Cl 2n NaCl O Another method for the polycondensation involves the reaction between COCl2 and bisphenol A in methylene chloride solution in the presence of pyridine to trap the HCl produced. The transesterification with phenyl carbonate is also possible but is more difficult to transpose to an industrial level than the preceding methods. Depending upon the method being used it is possible to obtain PC whose molar masses vary from 2 X 104 to 2 X 105 g-mol-1. However for injection molding the best adapted molar masses are in the range 2-3 X 104g-mol-1. LINEAR CONDENSATION POLYMERS 563 Poly bisphenol A carbonate shows a perfectly regular structure. However it is unable to crystallize spontaneously. Indeed the rigidity of the chains due to the presence of phenylene rings restricts their .