Formation of Mesomorphic Polymorph, Thermal-Induced Phase Transition, and Crystalline Structure-Dependent Degradable and Mechanical Properties of Poly(p-dioxanone)

Abstract

Polymorphic crystalline structure has been a key factor determining the mechanical property and degradation behavior of biodegradable polymers. Herein, we report on the polymorphic crystalline structure, phase transition, and their effects on the mechanical property and degradation behavior of poly­(p-dioxanone) (PPDO), a typical biodegradable and bioabsorbable semicrystalline polymer for biomedical applications. Both the polymorphic crystalline structure and crystallization kinetics of PPDO depend on crystallization temperature (T c) drastically. Melting enthalpy and degree of crystallinity of PPDO first decrease and then increase with increasing T c. PPDO forms a new mesomorphic polymorph (denoted as the α′-form) during crystallization at low T c (≤10 °C), in contrast to the common α crystals generated at high T c (≥60 °C). The α′ crystals have weaker interchain interactions and a shorter long period than the common α crystals, indicating the looser chain packing of α′ crystals. The α′ crystals are metastable, and they transform into the thermodynamically stable α crystals during heating. The α′-form PPDO possesses a slower degradation rate, higher flexibility, but lower strength and modulus than its α counterpart. This would provide a feasible way to tailor the degradation and mechanical properties of PPDO by varying crystal modification.

Polymorphic Crystal Transition and Lamellae Structural Evolution of Poly(p-dioxanone) Induced by Annealing and Stretching

“…Figure a,b shows the DSC heating curves and X c,DSC of α′-form PPDOs annealed at various T a ’s for 20 min. For the unannealed α′-form PPDO crystallized at 5 °C, an exotherm ( P exo , indicated by arrow) was observed prior to the major melting peak ( P m ), which resulted from the α′-to-α transition upon heating . Two endotherms, P m and annealing peak ( P a ), were observed in the melting curves of PPDO annealed at different T a ’s.…”

“…Figure a shows the WAXD patterns of α′-form PPDOs ( T c = 5 °C) after annealing at various T a ’s for 20 min. The α-form PPDO has three characteristic diffraction peaks at 17.8, 19.2, and 23.4° (λ = 0.124 nm), corresponding to the diffractions of (210), (020), and (310) planes, respectively. , Only a diffraction peak was observed at 2θ = 17.8° and the diffraction peak at 2θ = 19.2° was not seen for the α′-form PPDO crystallized at 5 °C . For the α′-form PPDO annealed at a low T a (40, 50 °C), their WAXD patterns changed little compared to the unannealed sample, suggesting the absence of crystalline phase transition.…”

“…Poly­( p -dioxanone) (PPDO) is a typical semicrystalline polymer with good biodegradability, biocompatibility, bioabsorbability, and mechanical properties. PPDO has been widely used in biomedical fields such as surgical suture, drug delivery system, and bone and tissue fixation devices. , Previous report has demonstrated that PPDO can undergo two crystal modifications (α′ and α form) by changing the crystallization temperature ( T c ) . The α′ form is a metastable phase, whereas the α form is thermodynamically stable.…”

“…In addition, the physical properties of PPDO are influenced by its crystal modification. The α′-form PPDO possesses slower hydrolytic degradation, higher flexibility, but lower strength and modulus than its α counterpart …”

“…In spite of recent progress on the crystallization kinetics, the crystalline structure and physical properties of PPDO, ,,, the polymorphic crystal transition of PPDO during stretching and annealing are not yet well understood. In this work, we systematically studied the polymorphic crystal transition and crystalline lamellar evolution of PPDO with annealing and stretching under different conditions.…”