The origin of memory effects in the crystallization of polyamides: Role of hydrogen bonding

The effect of hydrogen bonding stability on the memory effects in the crystallization of long chain polyamides have been investigated by the self-nucleation calorimetric technique. Self-nucleation is characterized by three domains in decreasing temperature order: complete melting or Domain I, exclusive self-nucleation or Domain II and, self-nucleation and annealing or Domain III. The memory effect is observed in the high temperature range of Domain II (when all crystals are molten, or in Domain IIa). In the low temperature range of Domain II, crystal remnants act as self-seeds (i.e., Domain IIb). The hydrogen bonds between amide groups were detected with FTIR, and a ratio of the content of hydrogen bonded vs. free amide groups could be calculated. The energy needed to break the hydrogen bonds decreases as the self-nucleation temperature (Ts) increases. This means that hydrogen bonds become weaker (and their amount decrease), while the crystalline memory disappears upon crossing from Domain IIa to Domain I. Comparing the widths of Domain IIa in different polyamides, we found for the first time a clear correlation with the relative content of amide groups with respect to methylene groups in the repeat units. In conclusion, we have demonstrated that memory in polyamides is a strong function of hydrogen bonding between chain segments.