Isothermal melt crystallization kinetics, melting behavior, and spherulitic morphology of novel biobased poly(hexylene oxalate)

Abstract

The isothermal melt crystallization kinetics, melting behavior, and spherulitic morphology of poly(hexylene oxalate) (PHO), a novel semicrystalline polyester derived from biobased resources, were investigated in detail with differential scanning calorimetry and polarizing optical microscopy in a wide crystallization temperature (T_c) range. The isothermal melt crystallization kinetics of PHO was well described by the Avrami equation in the investigated T_c range. With increasing T_c, the crystallization rate of PHO was reduced; however, the crystallization mechanism remained unchanged. Double melting endotherms were observed for PHO, which was explained by the melting, recrystallization, and remelting mechanism. Through the Hoffman–Weeks equation, the equilibrium melting point of PHO was determined to be 86.6 °C. Depending on T_c, PHO exhibited different spherulitic morphologies. Banded spherulites were observed in a wide range of T_c; moreover, the band spacing increased with increasing T_c. The spherulitic growth rates of the PHO spherulites decreased with increasing T_c; furthermore, the PHO spherulites exhibited a crystallization regime transition from regime II to regime III at about 50 °C, according to the secondary nucleation theory.

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Introduction

Polyoxalates are a family of biobased polymers, which can be produced mostly from grass.^1–10 As polyoxalates have the characteristic –O(CO)₂O– functional group in their chemical structures, they are not only hydrophobic but also have a relatively high degradability in aqueous media. Therefore, polyoxalates may find appropriate practical application in medicine field, such as drug delivery.¹ Some of polyoxalates possess good mechanical properties; thus, these biobased polyesters have the potential to make low cost and recyclable thermoplastics.²

Polyoxalates can be synthesized from a two-step reaction based on oxalic acid or derivatives.^3–10 Firstly, the ester interchange reaction happened when oxalic acid and diols were heated, forming a prepolymer. Secondly, condensation polymerization continued to occur under low pressure, resulting in the final polymer product. The synthesis of polyoxalates were preliminarily reported by Carothers et al., who prepared the polyalkylene oxalates with methyl oxalate or ethyl oxalate and the corresponding glycols. They also found that monomeric ethylene oxalate was macrocrystalline; moreover, poly(ethylene oxalate) (PEO) hydrolyzed rapidly like monomeric ethylene oxalate.³ Recently, Miller et al. have developed a modified synthetic route of polyoxalates for the purpose to make packaging thermoplastics with excellent thermal properties. They synthesized polyalkylene oxalates and polyarylene oxalates by means of an oxalate metathesis polymerization (OMP) that adopted acid-catalyzed ester interchange of dimethyl oxalate and a diol in a 1 : 1 ratio; furthermore, they also successfully prepared the copolymers of aliphatic diols and aromatic diols with dimethyl oxalate. The linear polyalkylene oxalates –[(CO)₂O(CH₂)_nO]– were crystalline when n was even; nevertheless, those polymers were amorphous when n was odd.⁴

PEO and poly(butylene oxalate) (PBO) are the two most investigated members of polyoxalates. Ueda et al. successfully prepared PEO through condensation polymerization from ethylene glycol and oxalic acid. They reported the molecular and crystal structures of PEO as follows. The chain conformation of PEO was T5GT5Ḡ, and the crystal structure of PEO was orthorhombic with the unit cell dimensions of a = 0.644 nm, b = 0.622 nm, c (fiber axis) = 1.193 nm.⁵ Ballistreri et al. synthesized PEO by the same method and found intramolecular exchange reactions predominated in the primary thermal fragmentation process.⁶ Sheng et al. reported the synthesis of copolyesters – poly(ethylene terephthalate-co-oxalate-co-sebacate) (PETOXS) via a melting polycondensation and found that the existence of PEO units promoted the formation of crystalline entities but hindered the rearrangement due to slower segment mobility. In addition, the Young's modulus and the maximum tensile stress improved with increasing the PEO content in the copolyesters, while the elongation at break substantially decreased.⁷ Similar to PEO, PBO has also attracted considerable concerns.^8–10 Finelli et al. successfully synthesized PBO and its random copolymers with azelaic acid and discussed the thermal properties as well as crystal structures of the copolyesters. They discovered that the addition of azelaic acid reduced the glass transition temperature and melting point values but hardly affected the thermal stability and crystal structures of the copolyesters, with respect to PBO.⁸ Lo et al. studied the crystallization and microstructure of PBO with several techniques. They found that PBO crystallized according to the triclinic crystal structure with the unit cell dimensions of a = 0.852 nm, b = 0.475 nm, c = 0.761 nm, α = 105.8°, β = 113.7°, and γ = 87.4°. The spherulites morphology was observed for PBO; moreover, PBO crystallized in regime II with a nucleation parameter (K_g) of 97 264 K² and a lateral surface free energy of 17.68 erg cm⁻². On the basis of the small angle X-ray scattering results, they provided a quantitative description of the morphological parameters along with crystallization temperature. Both the long period and lamellar thickness slightly increased with crystallization temperature, indicating that thicker crystalline layers formed at high crystallization temperature.⁹ Lo et al. also blended PBO with ploy(L-lactide) (PLLA) to improve the poor toughness and low crystallization rate of PLLA and found that PBO/PLLA blends were partially miscible.¹⁰

Similar to PEO and PBO, poly(hexylene oxalate) (PHO) is also a member of biobased polyoxalates, which can be synthesized from dimethyl oxalate and 1,6-hexanediol.⁴ Unlike the most investigated PEO and PBO, PHO has received little attention and has not been systematically investigated until now. In this work, we not only synthesized PHO but also investigated the isothermal melt crystallization kinetics, melting behavior, and spherulitic morphology of PHO in detail. To our knowledge, it is the first work that reports the crystallization kinetics and morphology of PHO. The significance of this research is as follows. First, it would be important to get a better understanding of the crystallization kinetics and morphology of a novel biobased polyoxalate, i.e., PHO. Second, it would be interesting and important to better understand the structure and properties relationship of polyoxalates by comparing the results of PHO with those of other members of polyoxalates. Third, the results reported herein may be of interest and help from the viewpoints of polymer crystallization and the potential application of biobased polyoxalates.

Experimental

The monomers dimethyl oxalate and 1,6-hexanediol were purchased from Sinopharm Chemical Reagent Company, and the catalyst para-toluene sulfonic acid was bought from Guangdong Fine Chemical Engineering Technology Center. PHO was synthesized through a typical ester interchange reaction from dimethyl oxalate and 1,6-hexanediol. The detailed synthesis process was similar to the method described by Miller et al.⁴ For brevity, it was not shown here. The obtained sample had an average molecular weight of 13 400 g mol⁻¹. The chemical structure of PHO is shown in Fig. 1.

Fig. 1 Chemical structure of PHO.

The basic thermal properties, isothermal melt crystallization kinetics, and melting behavior of PHO were analyzed with a TA instrument differential scanning calorimeter (DSC) Q100. The sample was first annealed at 110 °C for 3 min to erase any previous thermal history before the subsequent thermal analysis process. For the basic thermal behavior study, the sample was cooled from 110 °C to −70 °C and heated to 110 °C with cooling and heating rates of 10 °C min⁻¹. The nonisothermal melt crystallization peak temperature (T_cc) was obtained from the DSC cooling trace, while the glass transition temperature (T_g) and melting point temperature (T_m) were measured from the DSC heating trace. For the isothermal melt crystallization kinetics study, PHO was cooled at 60 °C min⁻¹ to the desired crystallization temperature (T_c) from the crystal-free melt, held for 2 h to ensure complete crystallization, and heated to the melt again at 10 °C min⁻¹. Temperature modulated DSC (TMDSC) experiments were also performed with the same TA Q100 instrument in this work. The following experimental conditions were used for the TMDSC measurements, i.e., a heating rate of 2.5 °C min⁻¹ with the oscillation amplitude of 0.5 °C and the oscillation period of 40 s.

The wide-angle X-ray diffraction (WAXD) patterns of PHO were obtained with a Rigaku X-ray diffractometer RINT 2100 at 40 kV and 200 mA with a scanning rare of 5° min⁻¹ at room temperature. The samples for the WAXD experiments were isothermally crystallizing at 40 and 60 °C, respectively, in a vacuum oven for 2 h after erasing any previous thermal history.

The spherulitic morphology and growth of PHO were investigated by an Olympus BX51 polarizing optical microscope (POM) with a Linkam THMS600 temperature controller. The sample was first heated to 110 °C for 3 min to eliminate previous thermal history and then cooled to the desired T_c at 60 °C min⁻¹. Spherulite growth rate (G) was obtained from the variation of radius (R) with time (t), i.e., G = dR/dt.

Results and discussion

Basic thermal behavior

Like other polyesters, PHO is also a semicrystalline polymer. It is essential to study the basic thermal properties of PHO. As described in the Experimental section, the basic thermal parameters, including T_g, T_cc, and T_m, were investigated with DSC. Fig. 2 displays the cooling and subsequent heating DSC curves of PHO at 10 °C min⁻¹. As shown in Fig. 2a, PHO displayed a relatively narrow and sharp crystallization exotherm, from which T_cc of PHO was determined to be 35.5 °C with a crystallization enthalpy of 73.6 J g⁻¹. As illustrated in Fig. 2b, PHO exhibited a meting endotherm at 73.5 °C with a heat fusion (ΔH_m) of 68.9 J g⁻¹, which were very close to the data (a T_m of 76 °C with ΔH_m of 65 J g⁻¹) of PHO reported by Miller et al.⁴ In addition, PHO also showed a small crystallization exothermic peak just prior to its large melting peak, which may be related to the recrystallization of the unstable crystals formed during the previous cooling process. Similar phenomena were often found for other aliphatic polyesters, such poly(ethylene succinate) (PES) and poly(butylene succinate) (PBS).^11,12 Fig. 2c displays the enlarged glass transition region, from which T_g of PHO was estimated to be about −35 °C. It was interesting to compare the basic thermal results of PHO with those of PBO, as they had the similar chemical structures. The difference in the chemical structures between PHO and PBO was that the number of the CH₂ groups of the former was six while that of the latter was four. Lotti et al. once reported that PBO possessed a T_g of −32 °C, a T_m of 99 °C, and a T_cc value of 51 °C.⁸ It was clear that all the characteristic thermal parameters of PHO were smaller than those of PBO. Such reduction should be attributed to the difference in the chemical structures, as the more CH₂ groups of the backbone of PHO rendered more chain mobility and flexibility than those of PBO.

Fig. 2 DSC curves of PHO showing (a) nonisothermal melt crystallization exotherm, (b) melting endotherm, and (c) enlarged glass transition region.

Isothermal melt crystallization kinetics

The isothermal melt crystallization kinetics of PHO was further investigated with DSC. Fig. 3a demonstrates the plots of relative crystallinity (X_t) versus crystallization time (t) of PHO at different T_c values ranging from 40 to 60 °C, showing the typical S shaped feature. As shown in Fig. 3a, crystallization time became gradually longer with increasing T_c, especially at higher T_c, indicating that the crystallization process of PHO was decelerated with an increase in T_c.

Fig. 3 (a) Plots of relative crystallinity versus crystallization time and (b) Avrami plots of PHO at indicated T_c values.

By analyzing the data of X_t and t shown in Fig. 3a with the well-known Avrami equation, the isothermal melt crystallization kinetics of PHO at different T_c values was further investigated. The Avrami equation is as follows:

1 − X_t = exp(−ktⁿ) (1)

where n is Avrami exponent, reflecting crystallization mechanism, and k is the crystallization rate constant, relating to crystallization rate.^13,14 The related Avrami plots are shown in Fig. 3b. A set of almost parallel straight lines were found in Fig. 3b, when PHO was isothermally crystallized at different T_c values, suggesting that the Avrami equation could suitably describe the isothermal melt crystallization process of PHO. Meanwhile, the almost parallel lines also indicated that PHO may crystallize through the same crystallization mechanism within the investigated wide range of T_c.

The obtained Avrami parameters are listed in Table 1. In the investigated T_c range, the n values were almost the same and only varied slightly between 2.4 and 2.6, indicating a three-dimensional spherulitic growth with athermal nucleation mechanism.¹⁵ Both Fig. 3b and Table 1 clearly suggested that the crystallization mechanism of PHO did not change within the investigated T_c range, because the Avrami plots were almost parallel and the variation of the n values was rather slight. Table 1 also shows that the k values were reduced with an increase in T_c. For instance, when PHO was crystallized at 45, 55, and 60 °C, PHO had the same n value of 2.4; moreover, the k values were 3.56 × 10⁻¹, 1.47 × 10⁻³, and 6.04 × 10⁻⁵ min^−2.4, respectively. Such result clearly demonstrated that the isothermal melt crystallization process of PHO was significantly decelerated. In addition, crystallization half-time (t_0.5), the time corresponding to 50% of the final crystallinity of the sample, was used in this work to discuss the crystallization kinetics of PHO within the studied T_c range, which was calculated by the following equation:

Table 1 Isothermal melt crystallization kinetics parameters of PHO based on the Avrami equation

Tc (°C)	n	k (min^−n)	t0.5 (min)	1/t0.5 (min^−1)
40	2.5	1.89	0.7	1.43
45	2.4	3.56 × 10^−1	1.3	7.69 × 10^−1
50	2.6	2.79 × 10^−2	3.5	2.86 × 10^−1
55	2.4	1.47 × 10^−3	12.9	7.75 × 10^−2
60	2.4	6.04 × 10^−5	51.5	1.94 × 10^−2

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On the basis of the n and k values listed in Table 1, the t_0.5 and 1/t_0.5 values were calculated and are also included in Table 1. As shown in Table 1, t_0.5 gradually increased and 1/t_0.5 correspondingly decreased with increasing T_c, suggesting a slower crystallization rate at higher T_c.

It is well known that equilibrium melting point temperature (T⁰_m) is an important and basic thermal parameter of semicrystalline polymers. T⁰_m can usually be conveniently obtained through the Hoffman–Weeks equation, which is shown as follows:

where T_m is the apparent melting temperature of the crystals formed at T_c, and β represents the lamellar thickness ratio related to the stability of crystals.¹⁶ T⁰_m can thus be obtained from the intersection of the line with the T_m = T_c equation.

In this work, T⁰_m of PHO was determined through the thermal analysis study. As introduced in the Experimental section, the subsequent melting behavior of PHO was further investigated with DSC after it finished isothermal melt crystallization at different T_c values. Fig. 4 illustrates the subsequent melting behavior of PHO after isothermally crystallizing at indicated T_c values ranging from 40 to 60 °C. Depending on T_c, two melting endotherms or one main melting endotherm with an unapparent shoulder were observed for PHO after crystallization at indicated T_c values. The two melting endotherms, denoted as T_m1 and T_m2 from low to high temperature, varied differently with T_c. With an increase in T_c from 40 to 60 °C, T_m1 gradually shifted upward from 60.4 to 71.6 °C, while T_m2 remained almost unchanged at about 73 °C. In addition, the shape and intensity of the two melting endotherms also exhibited T_c dependence. When PHO was isothermally crystallized at 40 and 45 °C, T_m2 was the dominant while T_m1 was the minor. At a T_c of 50 °C, the two meting endothermic peaks were comparable. When PHO was isothermally crystallized at 55 °C, T_m1 became the dominant while T_m2 was the minor. At a T_c of 60 °C, only T_m1 could be clearly detected, while T_m2 became an unobvious shoulder and merged into with that of T_m1, showing one main melting endotherm.

Fig. 4 Melting behavior of PHO after isothermally crystallizing for 2 h at indicated T_c values.

It is of great interest to investigate the origin of the double melting behaviors of PHO after isothermally crystallizing at different T_c values. It is well known that the presence of different crystal modifications (polymorphism) may result in the double melting behaviors of semicrystalline polymers.¹⁷ In the present work, the WAXD patterns of PHO after isothermally crystallizing at low and high T_c values were studied. Fig. 5 illustrates the WAXD patterns of PHO after isothermally crystallizing at 40 and 60 °C for 2 h. As shown in Fig. 5, PHO displayed almost the same diffraction peaks at the similar locations. The almost unchanged WAXD patterns indicated that the crystal structure of PHO did not modify with the variation of T_c. Therefore, the origin of the double melting behaviors of PHO from different crystal modification should be excluded.

Fig. 5 WAXD patterns of PHO after crystallizing at indicated T_c values.

Such double melting endotherms were often reported for semicrystalline polymers, which were well explained by the famous crystallization, recrystallization, and remelting mechanism.^17–19 According to the this mechanism, T_m1 corresponded to the melting of the crystals developed during the isothermal crystallization process at indicated T_c, while T_m2 was attributed to the melting of the crystals formed through the recrystallization of the imperfect crystals during the heating process.^17–19 To confirm the melting, recrystallization, and remelting mechanism, different heating rates were used for the PHO samples after isothermally crystallizing at 50 °C for 2 h. If the meting, recrystallization, and remelting model was valid, T_m1 should slightly shift upward to a high temperature range, while T_m2 should slightly downward to a low temperature range with increasing heating rate, because the sample did not have enough time to melt and recrystallize during a fast heating process. In this case, the recrystallization process would be restricted at a faster heating rate than at a slower heating rate. As a result, the remelting corresponding to T_m2 should be reduced. Fig. 6 displayed the melting behaviors of PHO at indicated heating rates after isothermal crystallization at 50 °C for 2 h. With increasing heating rate from 5 to 30 °C min⁻¹, T_m1 slightly increased from 64.4 to 66.0 °C, while T_m2 slightly decreased from 73.4 to 71.8 °C. In addition, the ratio of the area of T_m1 to that of T_m2 increased with increasing heating rate, indicating again that the recrystallization process was restricted at a faster heating rate than at a slower heating rate.

Fig. 6 Melting behaviors of PHO at indicated heating rates after crystallizing at 50 °C for 2 h.

TMDSC was also used in this work to support the melting, recrystallization, and remelting mechanism of PHO after isothermally crystallizing at different T_c values. Fig. 7 illustrated the TMDSC traces of the melting behavior of PHO after isothermally crystallizing at 50 °C for 2 h. As shown in Fig. 7, the three curves from the top to the bottom were reversible heat flow (R), total heat flow (T), and nonreversible heat flow (NR), respectively. In the total heat flow, PHO displayed two melting peaks (T_m1 and T_m2) and a small recrystallization peak (T_rc). In the reversible heat flow, PHO only presented T_m1 and T_m2. However, in the nonreversible heat flow, PHO exhibited an obvious T_rc between the small T_m1 and the large T_m2, indicating that the recrystallization process really occurred during the heating process for the PHO sample after isothermal crystallization. In brief, the double melting behaviors of PHO after isothermal melt crystallization at different T_c values may be well explained by the melting, recrystallization, and remelting mechanism. With increasing T_c, the crystals formed during the isothermal crystallization process became more stable; therefore, the recrystallization process was restricted, resulting in a large fraction of T_m1 and a small fraction of T_m2.

Fig. 7 TMDSC traces of the melting behavior of PHO after isothermally crystallizing at 50 °C for 2 h.

As a result, T_m1 was used for the Hoffman–Weeks equation. The Hoffman–Weeks plot is displayed in Fig. 8, from which T⁰_m of PHO was determined to be 86.6 °C.

Fig. 8 Hoffman–Weeks plot of PHO for the estimation of T⁰_m.

Spherulitic morphologies and growth kinetics

In this section, the spherulitic morphology and growth of PHO were further studied with POM in a wide T_c range, because they were directly related to the final physical properties. Fig. 9 shows the spherulitic morphologies of PHO, after it was crystallized at indicated T_c values and filled the whole space. As illustrated in Fig. 9, different spherulitic morphologies were observed for PHO, depending on T_c. When T_c was at 56 °C and below, PHO formed characteristic banded spherulites. Banded spherulites were often observed in some semicrystalline polymers, such as poly(ethylene adipate) (PEA), poly(butylene succinate) (PBS), poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx), etc.^20–22 These polyesters usually formed featured banded spherulites in a certain range of T_c and exhibited different spherulites morphology at other T_c values. It should be noted that the origin of the banded spherulites is still an open and unsolved issue in polymer crystallization. Some mechanisms for the formation of banded spherulites of semicrystalline polymers have been proposed in literature.^23–29 For instance, some researchers have proposed the following different mechanisms, including surface stress unbalance leading to twisting, crystal growth and periodical concentration effects in blends, and defects formed during crystal growth, etc.^23–29 In most cases, the occurrence of banded spherulites was usually attributed to the cooperative lamellar twisting in the direction of radical growth.^23–29 As far as the banded spherulites of PHO in this work, the origin of the formation of the ring bands was still uncertain, which needed further investigation with different techniques such as POM, transmission electron microscope, and atomic force microscopy under different crystallization conditions. Such work was still underway and would be reported in a forthcoming research. In this work, banded PHO spherulites disappeared when T_c was increased up to 60 °C. Only typical Maltese cross extinction could be seen at a T_c of 60 °C and above. In addition, as shown in Fig. 9, PHO spherulites became obviously smaller with decreasing T_c, suggesting an increased spherulites nucleation density at large degree of supercooling.

Fig. 9 Spherulitic morphologies of PHO crystallized at (a) 60, (b) 56, (c) 52, (d) 48, (e) 44, and (f) 40 °C.

It should also be noted that PHO spherulites displayed similar size and almost equal band spacing at a given T_c, indicating an athermal nucleation mechanism.³⁰ Fig. 9 clearly demonstrates that the band spacing of PHO spherulites was affected by T_c. Fig. 10 summarizes the T_c dependence of band spacing of PHO spherulites, from which we could see the band spacing increased gradually with increasing T_c. For instance, the band spacing was only 3.9 μm at a T_c of 44 °C, while it was significantly increased to be 22.1 μm at a T_c of 58 °C. Similar result was also reported for PHBHHx spherulites, which also displayed an obvious increase in the band spacing with an increase in T_c.³¹ The exact origin of the increased band spacing of PHO spherulites with increasing T_c was still unclear. One possible reason may be related to the relationship between the size of spherulites and the degree of supercooling. Generally, at high T_c, the degree of supercooling was small; therefore, the size of spherulites became larger. Accordingly, the banding spacing may also increase for the banded spherulites of PHO crystallized at high T_c in this work.

Fig. 10 Variation of band spacing of PHO spherulites with T_c.

The spherulitic growth rates were measured in a wide T_c range. PHO spherulites showed linear growth with crystallization time before they impinged with each other during the isothermal crystallization process, indicating that growth rate was independent of crystallization time. The variation of G with T_c is shown in Fig. 11. As displayed in Fig. 11, G decreased significantly with increasing T_c. For instance, PHO displayed a great G value of 13.8 μm min⁻¹ at a T_c of 40 °C, while it presented a small G value of 9.6 × 10⁻¹ μm min⁻¹ at a T_c of 60 °C. The trend may result from the weaker thermodynamic driving force required for the spherulites growth with an increase in T_c.

Fig. 11 Plot of G of PHO spherulites versus T_c.

It is well known that the crystal growth kinetics of semicrystalline polymers may be described by the secondary nucleation theory proposed by Lauritzen and Hoffman.³² In this work, the Lauritzen–Hoffman equation was applied to analyze the spherulitic growth rates of PHO in a wide range of T_c. The equation is described as follows:

where G₀ is a pre-exponential factor, U* is the activation energy for transporting the polymer chain segments to the crystallization site, R is the gas constant, ΔT is the degree of supercooling (T⁰_m − T_c), T_∞ is the temperature below which the polymer chain movement ceases, f is a correction factor accounting for the variation in the enthalpy of fusion, given as f = 2T_c/(T⁰_m + T_c), and K_g is the nucleation constant.³² Generally, the values of U* = 17 246.3 J mol⁻¹ and T_∞ = T_g – 51.6 K were used for the kinetics analysis of the spherulite growth.³³ For practical convenient use, eqn (4) was rewritten as follows:

Fig. 12 displays the Lauritzen–Hoffman plot for PHO. From Fig. 12, all the data could be well fitted with two straight lines with different slopes. Such result indicated that a crystallization regime transition occurred, according to the secondary nucleation theory described by Lauritzen and Hoffman.³² In other words, there existed a crystallization regime transition between regime II at high T_c range and regime III at low T_c range.

Fig. 12 Crystallization regime transition analysis of PHO based on the Lauritzen–Hoffman equation.

From Fig. 12, the nucleation constants for regime II (K^II_g) and regime III (K^III_g) were determined to be 8.69 × 10⁴ and 1.39 × 10⁵ K², respectively. The K^III_g/K^II_g value was about 1.6, which was slightly smaller than the theoretical value of 2; moreover, the regime transition temperature (T_tr) of PHO was about 50 °C.

Conclusions

In this work, we first successfully synthesized PHO from the monomers of 1,6-hexanediol and dimethyl oxalate, which had an average molecular weight of 1.34 × 10⁴ g mol⁻¹. PHO had a glass transition temperature of −35 °C, a nonisothermal melt crystallization peak temperature of 35.5 °C, and a melting point of PHO of 73.5 °C when it was cooled and heated at a rate of 10 °C min⁻¹. We further investigated the isothermal melt crystallization kinetics, melting behavior, and spherulitic morphology of PHO in detail in a wide T_c range with DSC, WAXD and POM. The experimental results revealed that the Avrami equation could describe the overall isothermal melt crystallization kinetics of PHO when it was isothermally crystallized in a T_c range from 40 to 60 °C. The Avrami plots were almost parallel in the investigated T_c range, suggesting PHO was crystallized through the same crystallization mechanism, regardless of T_c; moreover, with an increase in T_c, the crystallization rates became gradually slower. The subsequent melting behavior of PHO was further investigated after PHO finished isothermal crystallization at indicated T_c values. PHO exhibited two melting endotherms, which were well explained by the melting, recrystallization, and remelting model. The equilibrium melting point of PHO was determined to be 86.6 °C on the basis of the Hoffman–Weeks equation. Different spherulitic morphologies were observed for PHO, depending on T_c. It displayed characteristic banded spherulites in a wide range of T_c; furthermore, PHO exhibited larger band spacing with an increase in T_c. The spherulitic growth rates of PHO were also measured. With increasing T_c, the spherulitic growth rates decreased obviously. The spherulitic growth kinetics of PHO was further analyzed by the Lauritzen–Hoffman equation. Consequently, PHO spherulites illustrated a crystallization regime transition at about 50 °C between regimes II to III. The results may be of interest and help for the better understanding of the structure and properties relationship and the wider application of PHO.

Acknowledgements

Part of this research was supported by the National Natural Science Foundation, China (51573016, 51373020 and 51221002).

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