Structural and Morphological Evolution upon Heating of Quenched Polyamide 1012

The pseudohexagonal γ′ phase of polyamide 1012 can be obtained by melt-quenching and gradually transformed into the γ form during heating, in which the trans methylene segments away from the amide group are further twisted. What happens during uniaxial stretching? In this work, the multistage structural evolution of the γ′ phase during uniaxial stretching has been studied by using in situ wide-angle X-ray scattering (WAXS) and Fourier transform infrared spectroscopy (FTIR). The multistage conformational evolution of the γ′ phase during stretching was studied by quantitative analysis of the infrared progression bands and was found to be closely correlated with the mechanical response during tensile deformation. In the yield stage, the twisted methylene segments in the initial γ′ phase undergo conformational ordering under stress activation, and the γ′ phase spontaneously transforms into a defective α phase. Furthermore, in the plateau stage, strain induces significant conformational disorder and the transient α phase transforms into the most conformationally disordered γ phase. Finally, at the strain hardening stage, the reversible crystal form transition from the γ phase to the regular γ′ phase is induced by stress, accompanied by an obvious conformational ordering process. The stress- and strain-induced conformational torsional motions are found to be completely opposite, with the former inducing conformational order and the latter inducing conformational disorder.

Multistage Crystal Transformation and Mechanism of PA1012 γ′ Phase by Uniaxial Stretch

Yang,

Li,

Yuan

et al. 2024

Mixed Isodimorphic/Isomorphic Crystallization in Aliphatic Random Copolycarbonates

Liao,

Pérez-Camargo,

et al. 2024

In this work, we investigate the crystallization behavior of two series of novel aliphatic random copolycarbonates: poly(heptamethylene-ran-octamethylene carbonate) (PC7/PC8) and poly(octamethylene-ran-dodecamethylene carbonate) (PC12/ PC8). Both copolymers display apparent isodimorphic behavior as they crystallize over the entire composition range, exhibiting pseudoeutectic points at 45 and 76 mol % of PC8 content for PC7/ PC8 and PC12/PC8, respectively. However, the evolution in melting enthalpies (ΔH m ) and crystallinities (X c ) deviate from the expected pseudoeutectic behavior, indicating mixed isodimorphic/ isomorphic crystallization, a behavior reported for the first time recently by us in poly(hexamethylene-ran-octamethyelene carbonate) (PC6/PC8). Further understanding of this behavior was obtained through structural and conformational characterization employing in situ synchrotron radiation wide-and small-angle X-ray scattering (WAXS/SAXS) and Fourier-transform infrared spectroscopy (FT-IR). For PC7/PC8 copolymers, a new third crystalline phase, from now on named the γ phase, different from PC7-and PC8-type crystalline phases (or any of their polymorphs: δ, α, and β), emerged for intermediate compositions, i.e., 34−45 mol % PC8, in line with the atypical variations of ΔH m and X c . For PC12/ PC8 copolymers, a coexistence of the γ/PC8 type phase was found at PC8-rich contents. According to FT-IR results, the γ phase adopted a polyethylene-like conformation in all cases despite the presence of C�O groups. However, compared with those compositions where homopolymer-like phases dominate the crystallization, there is a stronger dipole−dipole interaction in the γ phase, evidenced by a shift of absorption band associated with C�O stretching. This evidence aligns with our previous work in PC6/PC8 copolymers, demonstrating that the mixed isodimorphic/isomorphic crystallization found in PC8-based random copolycarbonates is likely a general case. Additionally, as the number of methylene groups in the repeating unit of the second comonomer in the PC8-based copolymers increases, the composition range where the γ phase is observed narrows. At the same time, the position of the pseudoeutectic point shifts toward compositions rich in PC8, demonstrating how the chemical structure affects the exact location of the pseudoeutectic point in these random copolycarbonates.

Thermal-Induced Structural Evolution of Melt-Stretched PA11: Direct Evidence for the Preservation of Hydrogen-Bonded Sheets Above the Brill Transition Temperature

Ma,

Zhang,

Zhang

et al. 2025

Despite extensive research into the thermally induced structural transitions of polyamides (PAs), the stability of hydrogen-bonded (H-bonded) sheets above the Brill transition temperature (T B ) remains a contentious issue. Herein, we investigated the structural development of melt-stretched PA11 during heating and cooling cycles, utilizing in situ synchrotron wide-angle X-ray diffraction (WAXD) and Fourier transform infrared (FTIR) spectroscopy. By leveraging the unique twisted and oriented lamellar morphology created during melt stretching, we successfully identified and monitored, for the first time by WAXD, the H-bonded sheets across a broad temperature range up to the melting point (T m ). Our findings demonstrate that the Hbonded sheets are well maintained above T B until the sample fully melts, exhibiting distinct evolutionary trends in interplanar spacing, diffraction azimuth and orientation in response to temperature and melt-stretch ratio, compared to other crystallographic planes. The preserved H-bonded sheets have stronger interchain interactions, which impart a high anisotropy of thermal expansion to the high-temperature δ-phase. Further analysis of the FTIR data indicates that lattice variations below T B are driven by significant conformational twisting around the amide groups in the α′-phase, while thermal expansion predominantly dictates the variations in the δ-phase above T B . Additionally, the absence of notable alterations in H-bond interactions supports the continued stability of Hbonded sheets below T m . This study enhances our understanding of the molecular mechanisms underlying thermally induced crystalline structural evolution in polyamide systems.