The purpose of this research was to evaluate the effect and nature of hydration on the glass transition temperature (Tg) of poly(D,L-lactide-co-glycolide) and investigate the physical state of water within the polymer during hygrothermal aging. The polymer was incubated in water at 23, 30, 37 and 55 degrees C, while the vapor sorption studies were carried out at 37 degrees C using saturated salt solutions. The water content and the thermal behavior of PLGA-water system were assessed by Karl Fischer titration and modulated differential scanning calorimetry, respectively, the hygrothermal aging was monitored by gel permeation chromatography. Water depressed reversibly the Tg by about 15 degrees C regardless of the incubation conditions. The Tg then remained constant at approximately 30 degrees C for five days, except when degradation occurred. A broad ice melting peak was detected around 0 degrees C. In the sorption studies, a linear correlation (r2 0.9837) between the Tg and the moisture content was observed in the range of 0.3-2.6% w/w, but there was no discernible endothermic event associated with the melting of ice. Data were found to fit reasonably well to the Gordon-Taylor/Kelley-Bueche equation. There were no differences between bulk and vapor water aging. It is proposed that the water responsible for plasticizing the polymer was non-freezable (bound) water and the small fraction of such water which was absorbed at high relative humidity caused polymer degradation in the same manner as bulk water.
"As reported, moisture ingress into FRPs may degrade the mechanical properties and thermal properties (e.g., glass transition temperatures) due to plasticization . Water uptake may also lead to swelling of the composite structures , resulting in micro-cracking inside the composites, and in evolution of localized stress and strain fields in the fibrous composites  . Stress caused by swelling of matrix due to water ingress deteriorate the interface between the fiber and the resin   . "
[Show abstract][Hide abstract] ABSTRACT: In the present article, a pultruded basalt fiber-reinforced polymer (BFRP) plate was thermally aged at 135 °C and 300 °C for 4 h, and the aged BFRPs were sequentially immersed in distilled water and strong alkaline solution (pH = 13.5) at 20, 40 and 60 °C for six months. The moisture uptake behaviors due to thermal aging were investigated. As found, thermal aging at 300 °C brought in many voids, and obvious degradation of the cross-linking density (ρ) of the epoxy matrix due to thermal decomposition. Further immersion in alkaline solution leads to additional cracks and more serious hydrolysis of the matrix. Aging at 135 °C causes post-curing of the resin matrix, but deteriorates the bonding behavior between fiber and resin matrix, which is indicated by the enhanced coefficient of diffusion (D) along fiber directions. Two-stage water diffusion model can describe the water uptake process for aged and un-aged BFRPs efficiently. D along and perpendicular to the fibers are determined theoretically and experimentally. Due to the cracks, voids as well as fiber debonding occurred in the thermal aged BFRPs, much increased Ds, lowered activation energy for diffusion and even mass loss (in high temperature alkaline solution) were found.
"Pore formation and pore closure are two important processes that control the release of the encapsulated macromolecular drugs in poly(D,L-lactide-co-glycolide)based drug delivery systems . It has previously been shown that the glass transition temperature of PLGA based systems in aqueous medium is around 10–15 °C and lower than in the dry state due the plasticizing effect of absorbed water  . Furthermore, it has been also observed that addition of plasticizing agents to PLGA microspheres exposed to relative high humidity led to pore closure . "
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