Smectic liquid crystal Schlieren texture in rapidly cooled poly(butylene naphthalate)

Abstract

Melt-crystallization of PBN was analyzed as a function of the crystallization conditions. Crystallization above 160 °C leads to formation of crystals and spherulites directly from the melt. Crystallization below 160 °C proceeds via intermediate formation of a smectic liquid crystalline (LC) phase. Cooling faster than 200-500 K/s suppresses crystal formation from the LC phase. The smectic LC phase exhibits a distinct Schlieren texture.

Crystallization of poly(butylene 2,6‐naphthalate) containing diethylene 2,6‐naphthalate constitutional defects

“…Thus, a completely amorphous sample is not obtained. The results gained on PBN, in agreement with our previous work, reveal first formation of a smectic LC phase, which is followed by its transition to α ‐crystals, according to the Ostwald's rule of stages (red curves, Figure ). Crystallization from the mesophase is then kinetically suppressed at higher rates (green curves, Figure ), and eventually only mesophase can form, leading to a smectic LC glass when cooling below T g‐LC (blue curves, Figure )…”

“…FSC results on nonisothermal crystallization at much higher cooling rates, ranging from several tens to thousands of K/s, are reported in Figures and , for the PBN homopolymer and P(B 70 DE 30 N) copolymer, respectively. The series of cooling traces of the homopolymer are in qualitative agreement with previous findings obtained on a commercial sample, although some differences in the values of cooling rates and transition temperatures, due to the different origin of the two polymers, are present. In particular, a double exothermic peak is observed at cooling rates between about 40 and 200 K/s (Figure ).…”

“…The data obtained on the PBN homopolymer show an exothermic crystallization peak in the temperature range from 220 to 205 °C, with the peak temperature decreasing with increasing cooling rate from 0.03 to 0.33 K/s. According to the literature, this peak is related to melt crystallization of β ′‐ and α ‐crystalline phases, whose relative amount depends on the rate of cooling. The DSC cooling scans of P(B 70 DE 30 N) present a more complex behavior, since multiple exothermic events are found.…”

“…PBN is polymorphic, that is, as a function of the crystallization conditions (cooling rate or supercooling of the melt) different crystal modifications may develop, which include α ‐ and β ′‐crystals, and liquid crystals (LC). If the cooling rate is lower than about 0.1 K/min, or if crystallization is performed at temperatures higher than 220 °C, then triclinic β ′‐crystals grow slowly from the isotropic melt, yielding dendritic spherulites the equilibrium melting temperature ( T m o ) of β ′‐crystals is 281 °C; increasing the cooling rate to 10 K/min or decreasing the crystallization temperature to values lower than 200 °C causes spherulitic growth of α ‐crystals with a lower T m o of 261 °C . The α ‐phase is metastable with respect to the β ′‐phase.…”

“…The α ‐phase is metastable with respect to the β ′‐phase. A smectic LC phase, which exhibits a Schlieren texture, is observed if the melt is cooled at rates between 2400 and 24 000 K/min, which, however, transforms at these cooling conditions rapidly into α ‐crystals, following Ostwald's rule of stages . The transition of the LC phase into α ‐crystals is suppressed when cooling the melt faster than 24 000 K/min to below its glass transition temperature T g‐LC of about 65 °C …”