Page 125 - Biodegradable Polyesters

4.4 Mathematical Modeling of the Synthesis of Aliphatic Polyesters 103
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IV with temperature was observed with the value at 245 C almost double that
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at 210 C. This is a result of higher esterification and transesterification reaction
rates obtained at increased temperatures, as well as higher diffusion rates of by-
products produced (i.e., water and ethylene, propylene, or butylene glycol). The
same effect of temperature on the reaction was observed in all polyesters, that
is, PESu, PPSu, and PBSu. An increase in polycondensation time increases the
IV at each temperature and polyester produced. This increase of IV with time is
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smoother at low temperatures (e.g., 210 C), while more abrupt at higher temper-
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atures (e.g., 245 C).
Results of the theoretical simulation curves are presented as continuous lines in
Figure 4.14. As can be seen, although the theoretical model used was very simple,
the simulation model fits very well the experimental data at all different tempera-
tures and polyesters produced.
Comparing theeffect of thetypeofglycolusedonthe time evolutionofthe poly-
mer intrinsic viscosity and number average molecular weights at different temper-
atures, it was observed that in the PPSu samples, at all temperatures and reaction
times, the intrinsic viscosity and, as a result, the average molecular weight of the
polymer formed showed lower values compared to PESu or PBSu. The higher IV
values of PESu samples may be explained either by increased reaction rates or by
an increased diffusion of condensates. As regards the later, indeed higher diffusion
rates of by-products from PESu synthesis (i.e., ethylene glycol and water) would
be expected as EG is more volatile compared to butanediol or propanediol. How-
ever, this would result in higher IV values of PPSu compared to PBSu, which is
not the case. Therefore, it appears that synthesis of PPSu is somehow decelerated
compared to PBSu.
In order to clarify this effect on the reactions taken place during polyconden-
sation, end-group analysis was performed and the variation of the carboxyl and
hydroxyl end groups of PPSu with time are illustrated in Figure 4.15.
Carboxyl end groups decrease constantly with time at all temperatures with
higher rates at increased temperatures. A similar reduction was also observed in
PESu and PBSu. Since carboxyl end groups are reducing only by esterification, it
is thus confirmed that this reaction is accelerated by increased temperatures. It
is interesting to note that after the first hour of polycondensation, a plateau in
the carboxyl end-group values is almost reached. These values, although small
enough, are not zero. In addition, the constant COOH end groups after 1 h of
polycondensation would result also in constant IV values. However, this was not
observed experimentally. Therefore, the effect of temperature on the transesteri-
fication reaction was examined by calculating the hydroxyl end groups in terms of
the number-average molecular weight and carboxyl content. From Figure 4.15b,
an initial abrupt reduction of hydroxyl groups was realized during the first 30 min
of polycondensation, while afterward this reduction leveled off. This reduction
was more pronounced at higher temperatures.
From the estimated values of the kinetic parameters, it was clear that the val-
ues of k , are higher in the case of PESu followed by PBSu and then by PPSu.
1
Thus, it seems that the transesterification reaction is favored in PESu synthesis