Unique isodimorphism and isomorphism behaviors of even-odd poly(hexamethylene dicarboxylate) aliphatic copolyesters

Abstract

Three series of even-odd aliphatic copolyesters, poly(hexamethylene gluarate-co-hexamethylene pimelate) (PHGP), poly(hexamethylene pimelate-co-hexamethylene azelate) (PHPA), and poly(hexamethylene gluarate-co-hexamethylene azelate) (PHGA), were firstly synthesized derived from even diol of 1,6-hexanediol with odd diacids of glutaric acid, pimelic acid or azelaic acid, respectively. Then, their cocrystallization behaviors were systematically investigated by DSC, WAXD, and POM techniques. It is found that both PHGP and PHPA showed isodimorphism with a characteristic eutectic behavior observed from the plot of melting point versus composition, however, PHGA exhibited strict isomorphism in the whole composition range without a eutectic point. WAXD studies also confirmed the unique isodimorphism and isomorphism behaviors for even-odd copolyesters. The only slight change of the methylene number in alkyl unit of comonomers could generate so large difference in crystallization behavior, which exhibits unique physicochemical properties and structure-properties correlations in such even-odd copolyesters. Furthermore, the difference in cocrystallization miscibility effect of comonomers was analyzed by thermodynamics based on the defect Gibbs energy calculated from Wendling-Suter model. For PHGP and PHPA, the comonomer units have better miscibility in the case of the incorporation of shorter unit into the longer unit crystal than the opposite case. By contrast, in the case of PHGA, the HG and HA units are perfectly miscible in a same crystal lattice with only a little difference in crystal cell dimensions due to the identity of chain conformation of HG and HA units during packing into the crystal lattice. Finally, a model of crystal lattice structure of copolyesters was proposed to better understand the difference in cocrystallization miscibility effect of comonomers between isodimorphism and isomorphism.

Crystallization of isodimorphic aliphatic random copolyesters: Pseudo-eutectic behavior and double-crystalline materials

“…These changes can be identified by tracking the crystallographic spacings (dspacing) of the various planes as a function of copolymer composition. Examples of the three different situations are reported in Figure 5 for different systems [4]. both an increase or decrease in the distance between a given family of planes, see P(HG-ran-HP) and P(HP-ran-HAz), Figure 5a and b, respectively.…”

“…As a result, a total inclusion of both comonomers in the crystalline lattice occurs, and the composition of the crystal perfectly reflects the one of the polymer chain. Because of the specific conditions to be met, as far as the author are aware, only four random copolyesters have been reported to exhibit isomorphic crystallization: poly (hexamethylene gluarate-ranhexamethylene azelate) (P(HG-ran-HA)) [4], poly (ε-caprolactone-ran-2-oxepane-1,5dione) (P(CL-ran-OPD)) [5], poly (ε-caprolactone-ran-ω-pentadecalactone) (P(CL-ran-PDL)) [6], and poly (butylene succinate-ran-butylene fumarate) (P(BS-ran-BF)) [7]. isomorphic behavior, isodimorphic behavior for copolymers with small amount of comonomer exclusion, isodimorphic behavior with a large amount of comonomer exclusion, copolymers with total exclusion of second comonomer.…”

“…For instance, a homopolymer with a large unit cell is expected to include more easily a comonomer whose respective homopolymer presents a smaller unit cell [8]. Also, a comonomer with an even number of methylene groups in the repeating unit 6 will most likely include similar even CH 2 comonomers in its unit cell, and exclude those with odd number of carbon atoms in their chemical structure [4,9]. These empirical rules, related to the miscibility of the comonomers in the crystalline state are not always strictly obeyed.…”

“…The pseudo-eutectic behavior of isodimorphic polymers is manifested by a decrease in the transition temperatures with composition departing from those of the two homopolyesters, until a minimum value is reached. The comonomer concentration at which this minimum is found is generally located around the equimolar composition [20,21], however, some systems show pseudo-eutectic points in significantly different concentration interval (e.g., 60:40, 80:20) [4,11,13,16,19,23,36,37]. For instance, in poly (hexamethylene dicarboxilate) copolymers the eutectic composition shifts from a co-unit concentration of 50 to 20 %, with increasing the length of aliphatic diacid units from 4 to 8 CH 2 units [11].…”

“…For instance, in poly (hexamethylene dicarboxilate) copolymers the eutectic composition shifts from a co-unit concentration of 50 to 20 %, with increasing the length of aliphatic diacid units from 4 to 8 CH 2 units [11]. In most studies [4,11,16,19,21,36], the minimum of crystallization/melting temperature is located at compositions shifted towards higher contents of the commoner whose respective homopolyester has the lower melting point.…”