CHEM2090-LAB-EXP9-2010

CHEM2090-LAB-EXP9-2010 - EXPERIMENT 9 Polymer Cross-Linking...

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9-1 EXPERIMENT 9 Polymer Cross-Linking and Viscosity Objective: Devise and perform a procedure capable of determining the effect cross-linking has on polymer solution viscosity. Borate ion [B(OH) 4 ] will be used to cross-link strands of the polymer poly(vinyl alcohol). Introduction: Polymers are compounds in which chains or networks of small repeating units form very large molecules. Polymers are composed of small repeating units owing to the manner in which they are formed. A polymerization reaction involves the repetitive bonding together of many smaller molecules, called monomers. For example, polyethylene, one of the simplest polymers, is formed from ethylene monomers: C C H H H H n Polymerization C H H C H H n Ethylene Polyethylene (1) where n represents a very large number (usually several thousand). The polymer to be examined in this experiment is poly(vinyl alcohol), which is often referred to by its initials, PVA. Interestingly, PVA is not made by polymerizing vinyl alcohol C C H H H OH Vinyl alcohol Figure 9.1 because vinyl alcohol is unstable.
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Experiment 9: Polymer Cross-Linking and Viscosity 9-2 Rather, PVA is made by first polymerizing vinyl acetate, yielding poly(vinyl acetate): C C H H H O Vinyl acetate C CH 3 O Polymerization C H H C H O n Poly(vinyl acetate) C H 3 C O n (2) Poly(vinyl acetate) is then hydrolyzed in basic solution to give acetate ion (CH 3 CO 2 ) and PVA, which has the structure shown in Figure 9.2. C H H C H OH n Poly(vinyl alcohol) (PVA) Figure 9.2 The physical properties of polymers are determined by factors such as the average chain length, the strength of the intermolecular forces between polymer strands, and the efficiency with which polymer chains pack together. For example, the mechanical strength of a polymer is affected by all three of these factors. The mechanical strength of a polymer increases as the strength of the interactions between chains increases. Increasing the average chain length increases the accumulated intermolecular forces between chains, resulting in greater polymer strength. For polymers of the same average chain length, greater intermolecular forces lead to greater mechanical strength. Polymer chains that interact via dipole–dipole forces, for example, are stronger than those which interact by London dispersion forces. Lastly, chain packing arrangements that maximize intermolecular contact also maximize intermolecular forces, resulting in greater strength. Unbranched polymer chains better maximize intermolecular contact than do branched polymer chains.
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