dynamic study of polymer sol-gel transition

• Main Authors: Hsiu- Li Lin, 林秀麗
• Other Authors: Chien-Hong Cheng
• Format: Others
• Language: zh-TW
• Published: 2000
• Online Access: http://ndltd.ncl.edu.tw/handle/93846887676937088442

博士 === 國立清華大學 === 化學系 === 88 === Abstract The thesis studied the sol-gel transition of polymers using dynamic light scattering (DLS) and dynamic viscoelastic (DVE) measurements. Two polymer solution systems were studied. One was an irreversible crosslink polymer, i.e. unsaturated polyester/styrene thermoset resin. The other was a thermo-reversible crosslink polyelectrolyte solution,i.e. poly (vinyl alcohol)/ borax aqueous solution. The crosslinking reaction of polymer in solvents causes the formation of clusters. The clusters continuously grow up via crosslinking and finally forms a macro-network structure with a extremely high molecular weight. The DLS detects the relaxation time (τs) of clusters and it is found that the τs increases with crosslinking density while the polymer is in the solution state. At the sol-gel transition point, the τs→ ∞. The DVE results show G’(ω) > G”(ω) in the solution state. It predicts that the results of log G’(ω) and log G”(ω) versus log ω (ω=frequency) could be either parallel or overlap during sol-gel transition. The research results of this thesis show that the sol-gel transition points of a polymer solution predicted by DLS and DVE are consistent with each other. The curing of unsaturated polyester/styrene is a chemical covalent bond crosslinking reaction which is irreversible. At the sol-gel transition, the data of log G’(ω) versus log ω revealed a negative slope at low frequency (ω→0) which was smaller than that at high frequency (table II-3, α=9.0%). These results looked similar to the plateau region of a vulcanized rubber. On the other hand, the crosslinking of the poly (vinyl alcohol)/borax solution is thermo-reversible. The ionic bond is in the equilibrium state, it fluctuates with temperature and changeover between crosslink bonding and breaking. While the detection period (τ=1/ω) of DVE was longer than the life time of crosslink-bonds of polymers, the G’(ω) data appeared to be similar to the flow region of polymer solutions. However, when the detection period (τ=1/ω) of DVE was shorter than the lift time of crosslink bond of polymers, the G’(ω) data were similar to the plateau region of a vulcanized rubber. The data of log G’(ω) versus log ω revealed a negative slope at low frequency (ω→0) which was larger than that at high frequency while the polymer solution were at sol-gel transition state (table III-11, n1’>n2’). The conformation of poly (vinyl alcohol)/borax complex in aqueous solution was controlled by the following factors : 1.the extruded volume effect of poly (vinyl alcohol). 2.the intra-molecular and inter-molecular borate-didiol crosslink reaction. 3.the effect of electrostatic charge repulsion of negative charges borate ions bound on PVA polymers. 4.the shield effect of free sodium ions to the negative charges of PVA/borax complexs. These factors resulted in the size expansion/shrinkage reentrant phenomenon of poly (vinyl) alcohol/borax complexs in water while borax concentration increased. The study of DLS and DVE in this thesis showed the reentrant phenomenon of the poly (vinyl) alcohol/borax system in semi-dilute aqueous solution.