PHB/PEO blend

NPT MD simulations are often performed to calculate the volume-temperature properties of the studied system. Figures 6.4 and 6.5 show the specific volume versus temperature plots of miscible poly(hydroxybutyrate) (PHB)/PEO blend (1 2 blends in terms of repeated units) and PHB/PE blend (1 2 blends in terms of repeated units) from the literature [34,36]. The intersect point of the two straight lines segments marks the Tg value. For the miscible PHB/PEO blend, only one Tg (about 258) could be found, and it is a little higher than the experimental values, 230 to 252 K. For the immiscible PHB/PE blend, two Tg (249 K and 187 K) were found. Comparing to the Tg of pure PHB (268 to 278 K) and PE (140 to 170 K), the two transition temperatures correspond to PHB-rich (249 K) and PE-rich domains (187 K) Tg, respectively. 

In 2004, our group calculated the solubility parameters for two polymers (PHB and PEO). The solubility parameters of the two polymers are similar and are consistent with the literature values. This means that PHB may be compatible with PEO. Then the volume-temperature curve of PHB/PEO blend system (12 blends in terms of repeated units) is simulated. From the curve, we got only one Tg for the blend system. The calculated Tg is consistent well with the experimental results. Based on the above two points, we concluded that PHB/PEO is a miscible blend. To confirm this conclusion further, the MD simulation was also carried out for an immiscible PHB/PE blend. Two Tg are observed for the immiscible blend. This is qualitatively in agreement with the experiment and supports our conclusion. 

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