In vitro hydrolysis of poly(l-lactide) crystalline residues as extended-chain crystallites: II. Effects of hydrolysis temperature.
The effects of hydrolysis temperature on the hydrolysis behavior and mechanism of poly(l-lactide) crystalline residues or extended-chain crystallites were investigated in phosphate-buffered solution (50-97 degrees C), using gel permeation chromatography and differential scanning calorimetry (DSC). The hydrolysis of the crystalline residues proceeded from their surface composed of very short chains with a free end along the chain direction, irrespective of hydrolysis temperature, but the hydrolysis from their lateral surface could not be traced. The activation energy of hydrolysis for the crystalline residues (extended-chain crystallites) was evaluated to be 18.0 kcal mol(-1) (75.2 kJ mol(-1)). The monotonic melting temperature (T(m)) and crystallinity decreases occurred after their initial very small increases, excluding the monotonic crystallinity decrease at 97 degrees C with no initial increase. The T(m) decrease reflects the decreased thickness of the crystalline residues. The equilibrium T(m) of the crystalline residues (extended-chain crystallites) was estimated to be 464.5-464.9 K. The free energy values for the surface composed of very short chains with a free end, which are neighboring the air (or nitrogen during DSC scanning), were calculated to be 55.6-56.4 erg cm(-2) for heat of fusion per unit mass = 135 J g(-1). The obtained surface free energy values are significantly higher than that for the surface composed of folding chains, tie chains, and the chains with a free end, which are neighboring the same kind of amorphous chains (39.9 erg cm(-2)).
Tsuji
Hideto
H
Department of Ecological Engineering, Faculty of Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi, Aichi 441-8580, Japan. tsuji@eco.tut.ac.jp
Ikarashi
Kensaku
K
D002152:Calorimetry, Differential Scanning; D002850:Chromatography, Gel; D003460:Crystallization; D006868:Hydrolysis; D011091:Polyesters / Q000737:chemistry*; D013696:Temperature; D013816:Thermodynamics
D011091:Polyesters; C033616:poly(lactide)