Using Successive Self-Nucleation and Annealing to Detect the Solid–Solid Transitions in Poly(hexamethylene carbonate) and Poly(octamethylene carbonate)

Abstract

Solid−solid transitions in poly(hexamethylene carbonate) (PC6) and poly(octamethylene carbonate) (PC8), denoted the δ to α transition, have been investigated, using selfnucleation and the successive self-nucleation and annealing (SSA) technique. The SSA protocol was performed in situ for thermal (differential scanning calorimetry (DSC)), structural (wide-angle X-ray scattering (WAXS)), and conformational (Fourier transform infrared spectroscopy (FT-IR)) characterization. The final heating after SSA fractionation displayed an enhanced (compared to a standard second DSC heating scan) endothermic and unfractionated peak signal at low temperatures corresponding to the δ to α transition. The improved (i.e., higher enthalpy and temperature than in other crystallization conditions) δ to α transition signal is produced by annealing the thickest lamellae made up by α and β phase crystals after SSA treatment. As thicker lamellae are annealed, more significant changes are produced in the δ to α transition, demonstrating the transition dependence on crystal stability and, thus, on the crystallization conditions. The ability of SSA to significantly enhance the observed solid−solid transitions makes it an ideal tool to detect and study these types of transitions. In situ WAXS reveals that the δ to α transition corresponds to a change in the unit cell dimensions, evidenced by an increase in the d-spacing. This implies a more efficient chain packing in the crystal, for both samples, in the δ phase (lower d-spacing at low temperatures) than in the α phase (higher d-spacing at high temperatures). The chain packing differences are explained through in situ FT-IR measurements that show the transition from ordered (δ phase) to disordered (α phase) methylene chain conformations.

Challenging Isodimorphism Concepts: Formation of Three Crystalline Phases in Poly(hexamethylene-ran-octamethylene carbonate) Copolymers

“…Figure a compares the WAXS profiles of all materials obtained at 0 °C during heating. At this temperature, according to Figure b and the observations of Pérez-Camargo et al, , the diffraction pattern for both PHC and POC corresponds to the (110) and (200) planes of the δ-phase. According to their results, , the δ-PHC/POC maintain similar crystalline forms as those of α-PHC and α-POC, but with more efficient chain packing, although the specific crystal structure of δ phase has not been reported.…”

“…For POC, Zhao et al first reported a reversible α-β solid phase transition, which is analogous to the Brill transition observed in polyamides. , It was primarily related to the reversible conformational change in methylene sequences, transitioning from a predominantly trans configuration at low temperatures to a coexistence of trans/gauche conformations at high temperatures. Pérez-Camargo et al , disclosed a new reversible α-δ solid phase transition that occurs at a lower temperature than the α-β phase transition. The δ phase is characterized by more efficient packing of methylene groups and ordered chain conformations in comparison to the α phase.…”

“…The δ phase is characterized by more efficient packing of methylene groups and ordered chain conformations in comparison to the α phase. A reversible δ-α solid–solid phase transition has been disclosed in poly­(hexamethylene carbonate) (PHC). , …”

Cyclic Polycarbonates by N-Heterocyclic Carbene-Mediated Ring-Expansion Polymerization and Their Selective Depolymerization to Monomers

“…Differential scanning calorimetry (DSC) analysis was performed which showed that all samples depicted melting peaks except cP5MC due to its amorphous morphology (Figure S4). Both P6MC and P7MC exhibited broad melting peaks with low-temperature shoulders regardless of the topology which can be associated with the reorganization process commonly observed in aliphatic polymers. − Additionally, both cyclic and linear formations of P8MC showed the solid–solid transition of crystal. , The “odd–even” effect of melting temperature ( T m ) was found to be dependent on the number of CH 2 groups between carbonate linkages (i.e., P6MC to P8MC) until a saturation region in which T m of PnMCs correlated to an increasing number of CH 2 groups (P9MC–P12MC) was revealed. However, there existed a minor difference in T m in the region of the “odd–even” effect between cyclic and linear polycarbonates (Figure , left; Figure S4).…”

“…89−91 Additionally, both cyclic and linear formations of P8MC showed the solid−solid transition of crystal. 92,93 The "odd−even" effect of melting temperature (T m ) was found to be dependent on the number of CH 2 groups between carbonate linkages (i.e., P6MC to P8MC) until a saturation region in which T m of PnMCs correlated to an increasing number of CH 2 groups (P9MC−P12MC) was revealed. However, there existed a minor difference in T m in the region of the "odd−even" effect between cyclic and linear polycarbonates (Figure 6, left; Figure S4).…”