Abstract
AbstractThe glass transition temperature Tg of propylene glycol (PG) and poly(propylene glycols) (PPGs) of molecular weight up to 4000 has been measured by differential scanning calorimetry, and the activation energy and change in heat capacity ΔCp have been determined in the glass transition range. The activation energy increases with an increase in the molecular weight of the polymer, and ΔCp measured at a fixed heating rate decreases. The increase in Tg with molecular weight is remarkably more rapid for poly(propylene glycols) than for other polymers, and a limiting value of Tg is reached for a chain containing 20 monomer units. These results are discussed in terms of the Fox‐Flory and the entropy theories. The calorimetric relaxation times are comparable with the extrapolated dielectric relaxation times. The initial increase of ΔCp from PG to PPG 200 is attributed to the decrease of H‐bonding sites from 12 in 3 monomers to 4 on polymerization to PPG 200 and further decrease with increase in molecular weight to an increasingly large amplitude of the β‐process at T < Tg.
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