Equilibrium melting temperature and crystallization kinetics of α- and β′-PBN crystal forms

Abstract

The melting behavior and crystallization kinetics of PBN-PDEN and PBN-PTDEN copolymers were investigated using differential scanning calorimetry. Multiple endotherms were observed in all of the copolymers under investigation, originating from melting and recrystallization processes. By applying the Hoffman-Weeks method, the T m 1 of the a and b¢-PBN phases were derived. The T m 1 value of the b¢-form, which has not been determined before, is significantly higher, as expected, because the b¢-phase is thermodynamically favored and more tightly packed. The isothermal crystallization kinetics were analyzed according to the Avrami treatment. The presence of either oxygen or sulfur atoms in the PBN polymeric chain was found to reduce its crystallizability. In particular, the crystallization rate regularly decreased as the co-unit content was increased. Lastly, the a-PBN phase was found to crystallize faster than b¢-one, which is expected, as it the more kinetically favored phase. Polymer Journal (2012) 44, 174-180; doi:10.1038/pj.2011.112; published online 9 November 2011Keywords: equilibrium melting temperature; melt isothermal crystallization; random copolymers of poly(butylene naphthalate) INTRODUCTION Naphthalene-containing thermoplastic polyesters have attracted an increasing degree of interest in recent years. Poly(ethylene naphthalate), poly(butylene naphthalate) and poly(propylene naphthalate) are the best known members of this family of thermoplastics. These newly developed high-performance polymers that contain a rigid naphthalene ring and a flexible alkylene group in the repeat unit exhibit physical and mechanical properties superior to the widely used corresponding phthalate-based polyesters. In particular, poly(butylene 2,6-naphthalate) (PBN) is characterized by very good gas barrier properties, high UV resistance, high solvent-resistance, long-term electrical properties and thermostability. To date, two crystalline structures for PBN, denoted as a-and b-forms, have been acknowledged. The transition between these two forms can take place reversibly by mechanical deformation. 1 Ju et al. investigated the crystalline forms of PBN samples obtained by different thermal treatments in bulk. The a-form was produced by annealing a quenched sample in the solid state or by crystallizing PBN from the static melt at temperatures lower than 225 1C. An exclusive b¢-form was generated by performing non-isothermal crystallization from the melt at an extremely low cooling rate (0.1 1C per min). This thermally prepared b¢-form is characterized by a WAXD profile similar to that of the b-form obtained by mechanical deformation, except for the substantial d-spacing deviation in the (0-11) and (010) planes. 2 Nevertheless, if PBN is melt crystallized at high T c , both the a and b¢-forms are obtained simultaneously, and their relative ratio is dependent on the adopted crystallization temperature. Additionally, changes in the crystalline form never occur in the solid state. Lastly,

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

“…The PBN homopolymer is semicrystalline and displays scattering peaks diagnostic of both α ‐ and β ′‐phases. In particular, samples cooled at the lowest rate crystallize predominantly in the β ′‐phase, which is the thermodynamically stable and kinetically favored structure at low supercooling . However, the α ‐phase dominates the crystalline product already at cooling rates of 0.17 K/s or higher, consistent with previous findings.…”

“…In particular, samples cooled at the lowest rate crystallize predominantly in the β 0 -phase, which is the thermodynamically stable and kinetically favored structure at low supercooling. [6] However, the α-phase dominates the crystalline product already at cooling rates of 0.17 K/s or higher, consistent with previous findings. As expected, no signature of smectic mesophase periodicity at low 2θ angles is found in the homopolymer cooled at these low rates, since the observation of the LC phase at room temperature would require sufficiently high cooling rate to enable both its formation from the melt (at high supercoolings) and the suppression of the subsequent LC-to-α-crystal transformation.…”

“…The repeat unit of PBN contains a stiff naphthalene ring and a flexible alkyl group, [2][3][4] allowing crystallization on supercooling the melt. Since the semicrystalline morphology and therefore the properties of PBN are largely controlled by the crystallization process, research efforts include studies of the effect of the crystallization conditions on the crystallization kinetics, [5][6][7] on the crystal structure and polymorphism, [8][9][10] and on the stability of crystals. [11][12][13] The crystallization rate of PBN is relatively high, as the critical cooling rate to avoid crystallization and to obtain a completely amorphous state is several thousand K/s.…”

“…[15] 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 αand β 0 -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 0 -crystals grow slowly from the isotropic melt, yielding dendritic spherulites [10,16] the equilibrium melting temperature (T m o ) of β 0 -crystals is 281 C [6] ; 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. [6] The α-phase is metastable with respect to the β 0 -phase. A smectic LC phase, which exhibits a Schlieren texture, [16] 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.…”

Synthesis, characterization and isothermal crystallization behavior of poly(butylene succinate)-b-poly(diethylene glycol succinate) multiblock copolymers

“…Recently, novel copolymers of PBS with ether‐oxygen atoms have attracted considerable attention as the incorporation of ether linkages in the polymer chain could increase the hydrophilicity and biodegradability, as reported by Cao et al . and Soccio et al . In our recent study, we introduced diethylene glycol (DEG) unit into PBS chain to synthesize P(BS‐ co ‐DEGS) random copolymers with improved properties compared with neat PBS .…”