Structure and properties of naphthalene-containing polyesters. 4. New insight into the relationship of transesterification and miscibility

It is generally agreed in the literature that transesterification leads to or enhances miscibility; that is, transesterification is a necessary condition for miscibility in the polyester blends. We present here, for the first time, indirect and direct experimental evidence to prove that this statement is not completely correct. The experiments have been designed on the basis of three considerations: (1) to avoid sample artifacts, we prepared binary blend systems by coprecipitating the blend from the solution; (2) blends were melt pressed at different times in vacuum; (3) both solution and solid state NMR were simultaneously used to study the same sample sets. For many liquid crystal polyester and crystal polyester blends, 45 min melt-press times show neither transesterification nor miscibility. Based on the solid state NMR results, the PEN-d(10)/PET blend becomes miscible after melt press time greater than 0.5 min. Based on solution NMR results, there is no transesterification for the blends that were melt pressed for 30 s and 2 min. The transesterification kinetic rate constant of 0.028 37 min(-1) and a 1.5 min retardation time are obtained. During this 1.5 min retardation time the blend system becomes miscible. After that time the polyester blend undergoes transesterification following conventional kintics. This is the strong and direct evidence to prove that transesterification is not a necessary condition for miscibility.