Study of Non-Isothermal Crystallization of Polydioxanone and Analysis of Morphological Changes Occurring during Heating and Cooling Processes

Abstract

Non-isothermal crystallization kinetics of polydioxanone (PDO), a polymer with well-established applications as bioabsorbable monofilar suture, was investigated by Avrami, Mo, and isoconversional methodologies. Results showed Avrami exponents appearing in a relatively narrow range (i.e., between 3.76 and 2.77), which suggested a three-dimensional spherulitic growth and instantaneous nucleation at high cooling rates. The nucleation mechanism changed to sporadic at low rates, with both crystallization processes being detected in the differential scanning calorimetry (DSC) cooling traces. Formation of crystals was hindered as the material crystallized because of a decrease in the motion of molecular chains. Two secondary nucleation constants were derived from calorimetric data by applying the methodology proposed by Vyazovkin and Sbirrazzuoli through the estimation of effective activation energies. In fact, typical non-isothermal crystallization analysis based on the determination of crystal growth by optical microscopy allowed secondary nucleation constants of 3.07 × 10 5 K 2 and 1.42 × 10 5 K 2 to be estimated. Microstructure of sutures was characterized by a stacking of lamellae perpendicularly oriented to the fiber axis and the presence of interlamellar and interfibrillar amorphous regions. The latter became enhanced during heating treatments due to loss of partial chain orientation and decrease of electronic density. Degradation under various pH media revealed different macroscopic morphologies and even a distinct evolution of lamellar microstructure during subsequent heating treatments.

Nonisothermal crystallization of polymers: Getting more out of kinetic analysis of differential scanning calorimetry data

“…Here, we only mention a couple of newly obtained results. In agreement with isothermal microscopy studies, this technique has allowed for detecting a change between the regimes II and III in crystallization of polydioxanone . Similarly, a change in the crystallization mechanism between the regimes I and II has been detected for blends of Nylon 66 with liquid crystalline polymer as well as for poly(ɛ‐caprolactone) and its graphene oxide nanocomposites .…”

“…The plot is built for a single β and is usually reasonably linear. The resulting values of m typically vary with varying β . Most importantly, simulations reveal that the Jeziorny technique retrieves an incorrect value of the Avrami exponent.…”

“…Ideally, n and m should be identical, and a in Equation should be 1. Indeed, experimentally estimated values of a are typically close to 1 . Although F ( T ) and a do not provide any direct insights into the crystallization process, Song et al indicate that the appearance of the log β versus log t plot may hint at the crystallization mechanism change.…”

Influence of thermal annealing on mechanical properties andin vitrodegradation of poly(p‐dioxanone)

“…Its semicrystalline characteristics create additional capability to fine tune final products' performance including mechanical strength, for example, by increasing crystallinity through a thermal annealing process, or heat treatment . Obtaining optimum mechanical and biological properties for various applications require an overall understanding of the semicrystalline polymer morphology development, which largely relies on the crystallization process .…”

“…As PDO is emerging as an attractive biodegradable polymer; however, there is still a lack of understanding in its structure–property relationships. For example, although both isothermal and nonisothermal crystallization kinetics of PDO have been studied , there is no report on its effect on mechanical and degradation properties as of yet. Furthermore, to our best knowledge, there are few reports on molecular weight measurements, mainly because of limited instrumentation capable of accommodating hexafluoroisopropanol (HFIP), which is the best solvent for high‐molecular‐weight PDO .…”