Enthalpy Relaxation, Crystal Nucleation and Crystal Growth of Biobased Poly(butylene Isophthalate)

Abstract

The crystallization behavior of fully biobased poly(butylene isophthalate) (PBI) has been investigated using calorimetric and microscopic techniques. PBI is an extremely slow crystallizing polymer that leads, after melt-crystallization, to the formation of lamellar crystals and rather large spherulites, due to the low nuclei density. Based upon quantitative analysis of the crystal-nucleation behavior at low temperatures near the glass transition, using Tammann’s two-stage nuclei development method, a nucleation pathway for an acceleration of the crystallization process and for tailoring the semicrystalline morphology is provided. Low-temperature annealing close to the glass transition temperature (Tg) leads to the formation of crystal nuclei, which grow to crystals at higher temperatures, and yield a much finer spherulitic superstructure, as obtained after direct melt-crystallization. Similarly to other slowly crystallizing polymers like poly(ethylene terephthalate) or poly(l-lactic acid), low-temperature crystal-nuclei formation at a timescale of hours/days is still too slow to allow non-spherulitic crystallization. The interplay between glass relaxation and crystal nucleation at temperatures slightly below Tg is discussed.

Stability of Crystal Nuclei of Poly (butylene isophthalate) Formed Near the Glass Transition Temperature

“…The PBI was synthesized as described in detail elsewhere [27]. The number average molar mass and polydispersity of the polymer were 33.150 g/mol and 2.0, respectively.…”

“…The recorded melting peak is largest if the nuclei are directly transferred to the growth temperature of 85 °C, while it becomes smaller on in-between heating to a higher temperature of up to 140 °C, with the decrease of the melting peak area/crystallization enthalpy caused by a lowered number of nuclei. [27]. Second, when reading the individual data sets as a function of the spike temperature, it appears that the enthalpies of crystallization remain constant at an upper plateau and then decrease above a critical value.…”

“…Figure 3 shows with the left plot the temperature-time profile for the analysis of the thermal stability of crystal nuclei isothermally formed at 50 • C during annealing for different times between 100 and 5000 s (blue segment). Prior research revealed that the nucleation rate is highest at around 50 • C, [27] and variation in the nucleation time may provide information on whether it affects the cluster size distribution. The stability of the nuclei was examined by heating the system at 1000 K/s to different maximum spike temperatures between 85 and 140 • C (orange segments) before probing the nuclei number as a function of the spike temperature by an analysis of the crystal fraction forming within 1000 s at 85 • C, that is, at fixed growth conditions (red segment).…”

“…For example, annealing PBI at 50 °C for 5000 s (orange diamonds) produces populations of crystal nuclei which all survive heating to about 105 °C. However, heating to higher temperatures causes "melting" of populations of crystal [27]. Second, when reading the individual data sets as a function of the spike temperature, it appears that the enthalpies of crystallization remain constant at an upper plateau and then decrease above a critical value.…”

“…In order to accommodate the rather novel information about important effects of the transfer heating rate and growth temperature in Tammann's experiment on characterizing the nucleation characteristics, the initial crystallization study of PBI [27] is expanded with this work in order to further quantify its nucleation behavior. In the first part, this paper provides information about the critical rates of temperature change, above which the non-isothermal formation of nuclei and crystals is suppressed.…”