Synthesis Of High Molecular Weight PVA And Preparation Of High Strength And High Modulus PVA Fiber

Development of high strength and high modulus fiber has became the most important issue in the high technical fiber area due to its superior performance and widely use. Polyvinyl alcohol (PVA) with its specific structure has become another important research model for preparation of high strength and high modulus fiber from flexible-chain polymer. The theoretical strength and modulus of PVA are 210 and 1900 cN/dtex, while the highest strength and modulus of the commercial PVA are just 21.1cN/dtex ( K-II) and 600 cN/dtex, which are just 10% and 30% around of the theoretical strength and modulus. Laboratory PVA fiber's maximal strength and modulus are 39 cN/dtex and 915 cN/dtex, which just 15% and 50% of theoretical limit. So the high strength and high modulus PVA fiber still has a great developing space.It is well known that in order to obtain the high strength and high modulus PVA fiber the necessary conditions are the high molecular weight PVA with high linear degree and as high as possible drawing times in the fiber formation.In this thesis, an emulsifier-free emulsion polymerization is adopted to synthesize the high molecular weight PVAc, using redox initiator system at room temperature (14～20℃). The PVAc was then hydrolyzed to the high molecular weight PVA. The influence of polymerization condition, such as the polymerization temperature, concentration of initiator and monomer, on molecular weight and distribution of PVAc is studied. The controlling method on structure of linear high molecular weight PVA is discussed. Results show that the PVA with DP=9899 and high linear degree can be gotten via adjusting polymerization condition and procedure, and the polymerization process has notable influence on the molecular weight and structure of PVA. The high molecular weight PVA with narrow distribution and linear structure is obtained in constant propagation period of polymerization, while the branching and cross-linkage of PVAc become prominent during autoacceleration period, which causes an adverse effect on linear degree of PVA. Therefore, the control of molecular weight and chain structure of PVA can be achieved by manipulating polymerization process.PVA with different structure and conversion is spun into the fiber via a gel-spinning method. The results show that the PVA hydrolyzed from the low conversion PVAc is more suitable to be the raw material for high strength and high modulus PVA fiber than that hydrolyzed from the high conversion PVAc. The tensile strength and Young's modulus of PVA fiber hydrolyzed from low conversion PVAc are 19.96 cN/dtex and 385.2 cN/dtex, respectively, which are much higher than that of PVA fiber hydrolyzed from the high conversion PVAc. For PVA fiber spun from PVA with higher molecular regularity, the crystalline band is observed by optical microscope and polarized microscope. The fiber with crystalline band generally has the higher mechanical property than common fiber. The regular equidistant crystalline bands reflect a regular crystal structure existed in the fiber. The structure analysis of the regular crystalline band is made by POM, SEM, DSC. The results show that such crystal structure is a shashlik bunch crystal structure which has a remarkable effect on the mechanical properties of fiber. It is found that the suitable preparation conditions are : negative stretch -25%～-60%, initial drawing time 2.5, drawing temperature 240～250℃, thermo-setting temperature 220～230℃in order to obtain high strength and modulus PVA fiber.