Complex formation between enantiomeric polyesters

In this study, three different polymers were blended: a racemic PMEPL (PMEPL‐RAC) which consists of a random distribution of repeat units of R or S configurations, an isotactic PMEPL made of repeat units of S configuration [PMEPL‐S(−)], and a second isotactic PMEPL made of repeat units of R configuration [PMEPL‐R(+)]. The acronym PMEPL stands for poly[α‐methyl‐α‐ethyl‐βpropiolactone]. The racemic PMEPL is optically inactive but the isotactic PMEPLs are optically active. In the first part of this study, an equimolar mixture of PMEPL‐S(−) and PMEPL‐R(+) was made. This racemate mixture leads to the formation of stereocomplex between the two polymers which has a melting point roughly 40°C higher than those of the isotactic polymers. The racemate morphology is composed of large spherulites. In the second part of this study, binary blends of PMEPL‐S(−) and PMEPL‐R(+) were prepared with an excess of one of the isotactic PMEPLs. The stereocomplex is preferentially formed between the isotactic polymers and this complex controls almost entirely the morphology of the sample. The enthalpy of fusion of the stereocomplex in the binary blends can be predicted. however, the excess isotactic polymer is trapped between the lamellae of the spherulites of the stereocomplex and its crystallization is hindered by the high viscosity of the partially crystallized medium. In the third part of this study, ternary blends were prepared. It is shown that PMEPL‐RAC does not from a complex with the isotactic polymers. The PMEPL‐S(−)/PMEPL‐R(+) stereocomplex is again preferentially formed and it controls the morphology of the sample. It decreases the degree of crystallinity of both the excess isotactic polymer and the PMEPL‐RAC. This control of the morphology is effective even in samples in which the sterocomplex accounts for only 3% of the total weight of the blend.