Effect of linkage groups on motional cooperativity in the secondary relaxations of some glassy polymers

The secondary relaxation of bisphenol A polycarbonate (BPA-PC) is due to a cooperative motion that includes several repeat units. We believe that the linkage between neighboring BPA segments plays an important role in requiring this long-range cooperativity. It is expected that a more flexible linkage would require less motional cooperativity. To demonstrate this idea, polyformals based on the comonomer BPA, tetramethylbisphenol A (TMBPA), and methylene chloride with controlled sequence structure were synthesized, and DMA studies were conducted. The results show that the secondary relaxation behavior of copolyformals is not dependent on the block length of BPA segments, which suggests that the molecular motion does not require in-chain cooperation. The lower in-chain motional cooperativity of BPA-PF is attributed to the lower rigidity of the formal linkage.