Characterization of critical gel state of polyamides by viscoelastic, thermal, and IR measurements

We examined the liquid-solid transition for polyamides (PA) 6, 66, and 610 by the Chambon-Winter method under cooling. The polyamides exhibited the transition via the critical gel, for which the critical exponent and the stiffness were consistent with those reported under the isothermal measurements for the other polymers. The DSC measurements showed that, for a few materials, the crystallinity at the gelation φ_gel was very small. This result implies that the hydrogen bonding partially stabilizes the critical gel of such polyamides. The FT-IR measurements demonstrated that the hydrogen bonding was formed cooperatively around the gelation temperature. However, we also found that with increasing the molecular weight, the gelation temperature decreased and φ_gel increased for PA6. Besides, φ_gel was smaller for PA66 and PA610 than that for PA6. The mechanism is unknown for these results that locate in the opposite side to the suggested role of hydrogen bonding.