Dynamics of Water in Partially Crystallized Solutions of Glass …
175
temperature for these solutions including the dry oligomer. A linear dependence is
observed up to c w = 50 wt% with a maximum variation of T g of 8 K between the dry
and the 50 wt% sample. This variation indicates that water acts as plasticizer of 3PG
molecules. Plasticization [36] increases the flexibility of the molecules (whether
synthetic or biological molecules), allowing internal motions as well as shielding
of solute-solute interactions. Because of plasticization, both the viscosity and glass
transition temperature of the compounds decrease compared with those values of the
dry system.
In the following, we analyze the variation of the T g value under isothermal crystallization. In this case, the measurements were performed in the temperature-modulated
mode because it allows separation of overlapping transitions and more accurate
measurements of initial crystallinity. In this case, a different heating rate (5 K/min)
compared with the measurements showed in Fig. 2 was used. Water crystallization
takes place above T g on the heating cycle due to the presence of some nucleation,
which grows below the melting region during the cooling cycle. If the crystallization
temperature (T cryst ) is close to T g , the crystallization occurs more slowly than at
higher temperatures.
The isothermal experiments were carried out over a series of crystallization
temperatures (T cryst ) and crystallization times (t cryst ) in order to obtain different
amounts of ice in each sample (see Table 1). Figure 3 shows the reversing heat flow
for 3PG with a water content of 50 wt% at different T cryst and t cryst as indicated in the
figure. The first scan (red trace in Fig. 3) shows the T g value before crystallization.
After this scan, the sample was maintained at a certain temperature (T cryst ) during
some time (t cryst ) also indicated in the figure. After this time, we have measured again
the glass transition of the semi-crystalline sample (green trace in Fig. 3) as well as
the melting of the ice produced at higher temperatures. Table 1 shows the values of
T g for the first and second scans (i.e., before and after crystallization.
From Fig. 3, we observe that when T cryst is very low (i.e., T cryst is approximately
20° above the T g value), the crystallization proceeds slowly and the T g value does
not appreciable change before and after crystallization (the portion of the water
crystallized is very small). Accordingly, in the second scan, this small fraction of
water is not participating in the glass transition phenomenon, and therefore the heat
flow step decreases. However, in the case of the samples crystallized at the highest
temperature (228 K for 50 wt% or 223 K for 40 wt%), the T g variation between the
amorphous and semi-crystalline states is about 2–4° (and much bigger for PVME or
PVP solutions). If an appreciable part of the water crystallizes, the residual solution
becomes freeze-concentrated and therefore the glass transition temperature increases.
We will get back to these results after we have analyzed the dynamics of water in
amorphous and semi-crystalline environments.
In addition to 3PG solutions, we also analyze the response of two polymers: poly
(vinyl methyl ether) (PVME) and poly (vinyl pyrrolidone) (PVP) in solution with c w
= 50 and 55 wt%, respectively. As for 3PG, PVME shows a linear T g concentration
dependence whereas PVP shows a stronger T g concentration dependence, as seen in
Fig. 4.
175
temperature for these solutions including the dry oligomer. A linear dependence is
observed up to c w = 50 wt% with a maximum variation of T g of 8 K between the dry
and the 50 wt% sample. This variation indicates that water acts as plasticizer of 3PG
molecules. Plasticization [36] increases the flexibility of the molecules (whether
synthetic or biological molecules), allowing internal motions as well as shielding
of solute-solute interactions. Because of plasticization, both the viscosity and glass
transition temperature of the compounds decrease compared with those values of the
dry system.
In the following, we analyze the variation of the T g value under isothermal crystallization. In this case, the measurements were performed in the temperature-modulated
mode because it allows separation of overlapping transitions and more accurate
measurements of initial crystallinity. In this case, a different heating rate (5 K/min)
compared with the measurements showed in Fig. 2 was used. Water crystallization
takes place above T g on the heating cycle due to the presence of some nucleation,
which grows below the melting region during the cooling cycle. If the crystallization
temperature (T cryst ) is close to T g , the crystallization occurs more slowly than at
higher temperatures.
The isothermal experiments were carried out over a series of crystallization
temperatures (T cryst ) and crystallization times (t cryst ) in order to obtain different
amounts of ice in each sample (see Table 1). Figure 3 shows the reversing heat flow
for 3PG with a water content of 50 wt% at different T cryst and t cryst as indicated in the
figure. The first scan (red trace in Fig. 3) shows the T g value before crystallization.
After this scan, the sample was maintained at a certain temperature (T cryst ) during
some time (t cryst ) also indicated in the figure. After this time, we have measured again
the glass transition of the semi-crystalline sample (green trace in Fig. 3) as well as
the melting of the ice produced at higher temperatures. Table 1 shows the values of
T g for the first and second scans (i.e., before and after crystallization.
From Fig. 3, we observe that when T cryst is very low (i.e., T cryst is approximately
20° above the T g value), the crystallization proceeds slowly and the T g value does
not appreciable change before and after crystallization (the portion of the water
crystallized is very small). Accordingly, in the second scan, this small fraction of
water is not participating in the glass transition phenomenon, and therefore the heat
flow step decreases. However, in the case of the samples crystallized at the highest
temperature (228 K for 50 wt% or 223 K for 40 wt%), the T g variation between the
amorphous and semi-crystalline states is about 2–4° (and much bigger for PVME or
PVP solutions). If an appreciable part of the water crystallizes, the residual solution
becomes freeze-concentrated and therefore the glass transition temperature increases.
We will get back to these results after we have analyzed the dynamics of water in
amorphous and semi-crystalline environments.
In addition to 3PG solutions, we also analyze the response of two polymers: poly
(vinyl methyl ether) (PVME) and poly (vinyl pyrrolidone) (PVP) in solution with c w
= 50 and 55 wt%, respectively. As for 3PG, PVME shows a linear T g concentration
dependence whereas PVP shows a stronger T g concentration dependence, as seen in
Fig. 4.