production of selected secondary metabolites in transformed ...

Temperature (°C)
45
40
35
30
25
20
PLGA/PEG/PLGA
ITA-PLGA/PEG/PLGA-ITA
white viscous
suspension
suspension
white gel
white viscous cloudy gel
cloudy viscous
cloudy viscous
amber sol
amber sol
15
0 5 10 15 20 25
Concentration (w% )
white gel
amber viscous
cloudy gel
amber viscous
amber gel
amber gel
Fig. 2: The sol-gel transition phase diagram of the triblock copolymers PLGA-PEG-PLGA
and ITA-PLGA-PEG-PLGA-ITA
The diagram demonstrates three basic areas - sol, gel and suspension (precipitate). The
phase diagram of PLGA-PEG-PLGA shows the critical gel concentration (CGC) of 19.2 w%
and the critical gelation temperature (CGT) of 34.5 °C. For the next study, the solution of 23
%wt was used because of forming the amber hydrogel at around 37°C. As for the copolymer
modified by ITA, CGC and CGT were determined to be 15.3 w% and 25.0 °C, respectively.
The values were shifted below the curves of the polymer without the itaconic acid which
pointed that the presence of hydrophilic –COOH groups causes better interaction between the
polymer and water molecules. Therefore, CGC and CGT were observed to be lower than
those of the copolymer without ITA.
The change from sol to gel, which occurred by increasing the temperature, was not sharp.
At the temperature much lower than the critical gel temperature; unimers, individual micelles,
and grouped micelles coexisted in the sol state (Fig. 2 amber sol). The unimer fraction
decreased with the temperature increasing (Fig. 2 amber viscous). At the same time, the
grouped micelle size grew rapidly resulting in sol-gel transition (Fig. 2 amber gel). The
aggregation and packing interactions between micelles increased to form denser gel with the
raising temperature (Fig. 2 cloudy viscous state, cloudy gel). When the temperature was
further raised, the hydrophobic chains in the micelle core shrank tightly. Also, the hydrophilic
PEG block underwent dehydration and the second gel-sol transition arisen (Fig. 2 white
viscous state, white gel, suspension). The over shrunk micelle groups precipitated in water
and the solution separated into two phases of water and precipitated polymer (Fig. 2
precipitation) [8].
Sborník soutěže Studentské tvůrčí činnosti Student 2006 a doktorské soutěže O cenu děkana 2005 a 2006
Sekce DSP 2006, strana 220