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Structural Relaxation Behaviour in Polymers; A Molecular Weight Dependence in the Hypersonic Properties of Low Molecular Weight Poly(Propylene Glycol)

A molecular weight dependence in the hypersonic properties of low molecular weight poly(propylene glycol) (PPG) (400-4000) has been measured. Results for PPG 2000 are emphasized. Sound velocity increases with decreasing molecular weight and the maximum of the hypersonic loss peak shifts to higher temperatures. We ascribe this effect to a decrease in the flexibility of the polymer chain backbone as the molecular weight decreases. Hydrogen bonding involving the "OH" terminating groups causes an increase in the gauche/trans ratio and stiffens the chain.