Crystal-to-crystal transitions in nylon M5T

Abstract

Crystal-to-crystal transitions of a high-temperature nylon, poly(2-methtyl pentamethylene terephthalamide, nylon M5T), were studied by conventional and modulated temperature DSC (DSC and MT-DSC, respectively), as well as modulated thermomechanical analysis (MT-TMA), wide-angle X-ray scattering (WAXS), dynamic mechanical analysis (DMA) and hot stage microscopy. This polymer can easily be quenched to the completely amorphous phase at a cooling rate as low as 80 °C min⁻¹. This polymer has three different crystal forms plus a smectic phase, and these crystal forms change into each other during a continuous heating of the amorphous polymer as evidenced by MT-DSC, MT-TMA, DMA and WAXS. The crystal-to-crystal transitions of two vastly different molecular masses were investigated: One was prepared by polycondensation followed by chain extension (see Menczel et al. J Thermal Anal 46:753, 1996), and the other sample was made by simple polycondensation. T_g of the amorphous polymer (including the as-spun fiber) is 143 °C, which is about 25 °C higher for the drawn fiber. The crystal-to-crystal transitions take place between 170 and 290 °C. MT-DSC and DMA measurements (the reversing heat flow signal and the tensile storage modulus of the as-spun fiber) proved that these crystal-to-crystal transitions take place through melting. Efficient nucleating agents (carbon nanofibers) do affect the temperature of these transitions as well as the melt-to-crystal transition during cooling of the melt and also during heating of the amorphous polymer. Polarization optical microscopy observations suggest that the highest temperature crystal form obtainable in high-temperature melt isothermal crystallization is probably a smectic, and this phase changes to some spherulitic structure during further cooling. Non-isothermal and isothermal crystallization measurements let us conclude that the two high-temperature crystal forms (smectic and crystal form C) are thermodynamically stable (enantiotropic), while the two low-temperature crystal forms are metastable (monotropic). The Hoffman–Weeks method gave 339 °C for the equilibrium melting point of this polymer. DMA experiments gave three secondary transitions. A transition at ~ − 120 °C was assigned to a local mode relaxation of the methylene groups. The transition at  − 65 °C corresponds to the onset of rotation of the amide groups, while another secondary transition at 63 °C is the onset of rotation of the phenylene groups.