This work describes the melting and polymorphic behavior of poly(decamethylene terephthalamide) (PA 10T). Both solution-crystallized (SC) and melt-crystallized (MC) PA 10T show double melting endotherms in DSC. The SC crystal form melts at 260–300°C giving the first melting endotherm, and meanwhile undergoes a polymorphic transition forming the MC crystal form. The subsequent melting of the MC crystal form gives the second melting endotherm at 300–325°C. This irreversible polymorphic transition is confirmed by variable-temperature WAXD and IR. Dynamic mechanical thermal analysis (DMTA) shows a glass transition temperature (T _g ) at 127°C and the presence of an α′ transition at 203°C (0.1 and 1 Hz). This transition could be confirmed by DSC and variable-temperature WAXD experiments. The α′ transition correlates with a reversible thermal process and a sudden change in intersheet spacing.

We have explored semicrystalline poly(decamethylene terephthalamide) (PA 10T) based thermosets as single-component high-temperature (>200 °C) shape memory polymers (SMPs). The PA 10T thermosets were prepared from reactive thermoplastic precursors. Reactive phenylethynyl (PE) functionalities were either attached at the chain termini or placed as side groups along the polymer main chain. The shape fixation and recovery performance of the thermoset films were investigated using a rheometer in torsion mode. By controlling the M_n of the reactive oligomers, or the PE concentration of the PE side-group functionalized copolyamides, we were able to design dual-shape memory PA 10T thermosets with a broad recovery temperature range of 227-285 °C. The thermosets based on the 1000 g mol^-1 reactive PE precursor and the copolyamide with 15 mol % PE side groups show the highest fixation rate (99%) and recovery rate (≥90%). High temperature triple-shape memory behavior can be achieved as well when we use the melt transition (T_m ≥ 200 °C) and the glass transition (T_g = ∼125 °C) as the two switches. The recovery rate of the two recovery steps are highly dependent on the crystallinity of the thermosets and vary within a wide range of 74%-139% and 40-82% for the two steps, respectively. Reversible shape memory events could also be demonstrated when we perform a forward and backward deformation in a triple shape memory cycle. We also studied the angular recovery velocity as a function of temperature, which provides a thermokinematic picture of the shape recovery process and helps to program for desired shape memory behavior.

We have prepared semi-crystalline polyamide (PA) thermosets using reactive side-group functionalized copolyamides as precursors. Reactive meta- and para-based phenylethynyl diacid chlorides (IPE and TPE) were synthesized and incorporated in poly(decamethylene terephthalamide) (PA 10T) using a low temperature solution polymerization method. The phenylethynyl-based comonomers disrupt crystallization of the final copolyamides and lower the onset of melting. Copolyamides containing 5, 10 and 15 mol% of the reactive comonomer could be cured at 350 °C into freestanding PA thermoset films. All thermoset films are stable up to 400 °C, as confirmed by DMTA, which is the result of network formation. The thermosets exhibit both a crystalline phase and a crosslinked amorphous phase. Depending on the concentration of the side-groups, the degree of crystallinity of the final thermosets can be controlled and suppressed by 52–76% compared to the PA 10T reference polymer. Most notable is the fact that the IPE-15 thermoset film exhibits outstanding stress–strain behavior, i.e. elongation at break (∼17%) and toughness (766 MJ·m⁻³).

We have explored the synthesis of semi-crystalline polyamide (PA) thermosets via a reactive PA oligomer precursor route. Poly(decamethylene terephthalamide) (PA 10T) oligomers with a target M_n of 1 K and 3 K g·mol⁻¹ were synthesized via solution polymerization and end-capped with reactive phenylethynyl (PE) groups. The reactive oligomers could be cured in a stepwise manner using successive high temperature treatment steps. Depending on the concentration of PE end-groups, the degree of crystallinity of the final thermosets can be controlled and depressed by 60% for the crosslinked 3 K precursor and completely eliminated for the crosslinked 1 K precursor. From the calculated molecular weight between crosslinks, we estimated that 35–41% of the PE end-groups form crosslinking functionalities and the remainder of the PE end-groups contribute to chain extension. All thermoset films are stable up to ∼400 °C in DMTA experiments due to network formation, effectively increase the maximum use temperature of PA 10T by 100 °C.