Study On The High-strength High-modulus Polyvinyl Alcohol Fiber And The Chain Entanglement During The Preparation Process

Polyvinyl alcohol(PVA) is a water-soluble, biodegradable polymer, which is prepared by an alcoholysis reaction with polyvinyl acetate(PVAc). The molecular backbone of PVA is a carbon chain with each repeat segment containing a hydroxyl. Because hydroxyls are small and high polar, they easily form cross-linked networks in the polymer melt or polymer solution by hydrogen bonding intra-molecularly and inter-molecularly. Therefore, the melting point(Tm) of PVA can reach 228-240℃, the theoretical tensile strength and modulus of PVA fiber can reach 208cN/dtex and 1988cN/dtex, respectively. However, until now, tensile strength of the PVA fibers prepared using PVA with conventional polymerization degree, can only reach 14cN/dtex, which is far below the strength standard( ≥ 17.7cN/dtex) of high performance fiber. In comparison with other synthetic fibers, high-strength high-modulus PVA fiber has the advantages of high elastic modulus, high bonding strength, acid and alkali resistance, abrasion resistance and ultraviolet resistance, which make it an excellent material for resin matrix and cement composite reinforcement.In recent years, there has been significant amount of research on high strength PVA fibers. Because of the special structure of a PVA molecule, researchers mainly studied from three aspects. First, molecular weight is increased and molecular weight distribution of PVA is decreased as much as possible. This can reduce the fiber break caused by stress concentration at the end of molecule chains; Secondly, the syndiotactic degree of PVA is increased as much as possible. In this case, molecule chains can more easily form inter-molecular hydrogen bonds, thus the PVA molecule chains can be stretched more effectively during the drawing process. Thirdly, chain entanglement structure between molecule chains of PVA is reduced during the spinning process as much as possible. This is an effective way to increase the maximum draw ratio, degree of orientation and crystallization of the fiber.In order to improve the strength and modulus of PVA fiber using PVA with conventional polymerization degree, the followings were studied in detail: optimization of the dry-jet wet spinning process; measurement and research of chain entanglement of PVA; effect of spinning parameters on the chain entanglement of PVA fiber; preparation of high strength high modulus PVA fibers under entanglement control. The following are investigated in this study:1. The dry-jet wet spinning process of PVA/DMSO system is studied in detail. Effects of jet stretch, air drawing, hot drawing, heat setting on the strength, modulus, orientation and crystallinity of PVA fibers are investigated. The technical parameters are optimized. PVA fiber with tensile strength of 16.5cN/dtex and initial moulus of 185cN/dtex is achieved. In addition, hot water resistance property of PVA fiber is improved after acetalation using terephthalaldehyde(TDA). Results show that the tensile strength of the fiber is slightly decreased and hot water resistance property of the fiber is greatly improved after modification. The ratios of weight retention and tensile strength retention reached 97% and 91% respectively in boiling water after 15 minutes.2. The entanglement structure of PVA is investigated using the swelling differential scanning calorimetry(swelling DSC) method. A specific mixture consisting of a good solvent(DMSO) and a poor solvent(EG) in a volume ratio of 5:5 is used as a suitable swelling agent for PVA. When the ratio between PVA and swelling agent is 1:5, well-resolved thermogram peaks are obtained. The swelling and dissolution process of PVA fiber in swelling agent is observed using an optical polarizing thermomicroscope, and effects of the molecular weight and molecular structure on the swelling DSC curves are studied. It is evident that the protrusion peak on the tail part of the swelling DSC thermogram of PVA is closely associated with the chain entanglement structure.3. The entanglement changes of PVA fiber during the dry-jet wet spinning process are measured by swelling DSC method. It shows that the entanglement structure of PVA fiber is greatly dependent on the parameters such as spinning dope concentration, jet stretch ratio, extruding shear stress and draw ratio of the entire spinning process: The degree of entanglement in the spinning dope increases with the concentration level. PVA molecule chains are entangled very weakly when the concentration is equal to or less than 8wt%; When the jet stretch ratio is below 1.15, the entanglement structure increases a little with the increase of jet stretch ratio. When the jet stretch ratio is above 1.15, the entanglement structure decreases with increasing jet stretch ratio; The entanglement structure decreases enormously with increasing shear stress; The entanglement structure can not be disentangled through drawing in air at room temperature.4. The kinetics of chain entanglement and disentanglement of PVA is studied. Non-entangled and high-entangled PVA film samples are concentrated from PVA/DMSO solutions with concentration of 5wt% and 22wt%, respectively. Isothermal entanglement and isothermal disentanglement process of the samples at 220℃(hot drawing temperature) are analyzed. It indicates that, when the degree of entanglement is higher than a certain extent, molecule chains are disentangled when being heated. On the contrary, when the degree of entanglement is below a certain extent, molecule chains are entangled when being heaed. Using the Avrami equation, the rates of entanglement and disentanglement at 220 ℃ are calculated: for non-entangled PVA, half-time(1?2) of all molecule chains entangled is 6.4 minutes; for high-entangled PVA, half-time(1?2) of all molecule chains disentangled is 0.62 minutes. Using the Kissinger method, the disentanglement activation energy of PVA is calculated as 1010kJ/mol.5. The chain entanglement of PVA during the spinning process is controlled properly through increasing the extruding shear stress, increasing the jet stretch ratio, cancelling the air drawing process and increasing the hot drawing time. Thus, PVA fiber with higher tensile strength and modulus can be obtained. Nano-silica(nano-SiO2) and graphene oxide(GO) are added to the spinning dopes respectively. Then, PVA/nano-SiO2 composite fiber and PVA/GO composite fiber are prepared by dry-jet wet spinning technique. Results show that, cross-linked structures form between PVA molecule chains when nano-SiO2 is added. The tensile strength and initial modulus of PVA/nano-SiO2 fiber are 14.3c N/dtex and 202cN/dtex respectively. The degree of entanglement decreases greatly when GO is added. The tensile strength and initial modulus of PVA/GO fiber reach 17.9 cN/dtex and 296cN/dtex respectively.