THERMALLY-INDUCED MIXING IN PARTIALLY MICROPHASE SEGREGATED BINARY N-ALKANE CRYSTALS

In this study we have monitored the chain mixing and conformational di sordering that occur as a partially microphase separated binary n-alka ne mixture, 1:1 C30/C36, is transformed by stepwise heating into a ran domly mixed solid solution. The partially demixed binary mixtures stud ied (1:1 C30H/C36H and C30H/C36D) initially consist of small domains ( percolation clusters) of average lateral dimensions of the order of 5- 8 chains. The samples were prepared by rapid quenching of the melt to a room-temperature solid solution. This mixture, which is unstable, wa s then aged at room temperature to allow some degree of spontaneous de mixing. Raman and infrared spectroscopies were used to monitor the str uctural changes that occur in the mixture as the temperature is increa sed incrementally. The entire transformation to the randomly mixed sta te takes place with the mixture in a crystalline state with orthorhomb ic subcell packing. The mixture is thermally much less stable than its components in their unmixed, pure state, even at room temperature. As the mixture is heated, mixing and conformational disordering both inc rease abruptly at a temperature 10-15 K above room temperature. This t emperature corresponds to the onset temperature of a ''mixing'' endoth erm that is observed in the differential scanning calorimetry heating curve of aged mixtures. The chains disorder mostly in their ends. The temperature behavior of the two component chains is different: the con formational disorder in the longer chains, measured in number of gauch e bonds, is about twice that of the shorter chains; the longer chains disorder at a significantly lower temperature than the shorter chains. The chain mixing and conformational disorder are highly coupled. It i s speculated that the structural transformation that occurs as the sta ble partially demixed mixture at room temperature is heated to a high- temperature stable solid solution occurs also, in reverse, in the earl y stages of room-temperature spontaneous demixing.