Phosphorus-Containing Poly(ethylene terephthalate): Solid-State

Mar 22, 2013 - The sequence distribution of resultant copolyesters was analyzed by 1H ... to Synthesize High Molecular Weight Polyester Appropriate fo...
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Phosphorus-Containing Poly(ethylene terephthalate): Solid-State Polymerization and Its Sequential Distribution Liang-Jie Li, Rong-Tao Duan, Jun-Bo Zhang, Xiu-Li Wang,* Li Chen, and Yu-Zhong Wang* Center for Degradable and Flame-Retardant Polymeric Materials, College of Chemistry, State Key Laboratory of Polymer Materials Engineering, National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), Sichuan University, Chengdu 610064, China S Supporting Information *

ABSTRACT: A series of phosphorus-containing copolyester prepolymers were first sythesized from dimethyl terephthalate (DMT), ethylene glycol (EG), and 2-carboxyethyl (phenyl)phosphinic acid (CEPPA) by molten transesterification and polycondensation, and then solid-state polymerization (SSP) was conducted for different reaction times (tSSP) so as to prepare higher molecular weight copolymers. The intrinsic viscosity was increased with the increase of tssp, but decreased with the increase of CEPPA content. The sequence distribution of resultant copolyesters was analyzed by 1H NMR. It was found that the randomness of copolyesters was decreased after SSP, and the transesterification occurred mainly at the ester bonds formed by CEPPA and EG, resulting in the chain growth. DSC results showed that SSP treatment was favorable to enhance the crystallization ability of the obtained copolyesters.

1. INTRODUCTION Poly(ethylene terephthalate) (PET) has been widely used as textile fiber, technical fiber, film, and bottles because of its excellent properties such as tensile, impact strength, chemical resistance, clarity, processability, colorfastness, and reasonable thermal stability.1,2 However, the flammability of PET restricts its use in many fields.3 In recent years, phosphorus-containing flame retardants have been developed to replace conventional halogen-containing flame retardants to meet the requirements of low smoke and low toxicity in view of environmental protection and public security.4−8 Generally, flame retardants can be incorporated into polyester by blending, melt polycondensation, and fabric finishing through surface treatment.5,9,10 Among these, melt polycondensation with functional flame-retardant monomers was proved to be a suitable method for preparation of flame-retardant polyester fiber materials.9,11,12 As the flame retardants can be chemically bonded to polymers,13 they will not migrate to the surface of polymers during processing, such as extrusion, injection, or spinning, thus the flame retardants can take effect lastingly. However, it is hard to obtain high molecular weight flame-retardant copolyester (intrinsic viscosity >0.8 dL/g) by the general melt polycondensation, because the escape of low molecular weight byproducts becomes more difficult with the increase of viscosity during the reaction. Furthermore, the local overheating leads to the augment of degradation reaction. In general, a long reaction time and a high temperature are necessary for melt polycondensation, which easily induce the increase of acetaldehyde content and yellowing of PET, and the resultant product is not fit for bottle application. Moreover, the regularity of molecular chain is always destroyed during the melt polycondensation, which results in poor crystallization ability. Compared with melt polycondensation, solid-state polymerization (SSP) is a widely used method to produce high molecular weight polyester. SSP occurs at a temperature lower © 2013 American Chemical Society

than melting temperature and higher than glass transition temperature under vacuum, inert gas, or some supercritical fluids.14−22 At SSP temperature, polymer chain has enough mobility, so the terminal carboxyl group and hydroxyl group can take part in the esterification and transesterification reaction,23,24 and thermal degradation can be avoided. Beyond that, after SSP the sequence distribution and thermal behavior of polyesters will change. James et al.25 prepared copolyesters via SSP using PET and poly(ethylene napthalate) (PEN) oligomer blends. They found that the exchange reactions were active during early stages of SSP. Degree of randomness of copolyesters was increased with increasing the reaction time, while the product was a kind of block copolymer. Jasen et al.26−28 prepared other two-block copolyesters with blends of PBT (Mn = 16 kg/mol) and 2,2-bis[4-(2-hydroxyethoxy)phenyl]propane (Dianol 220), as well as blends of PBT (Mn = 16 kg/mol) and bis(2-hydroxyethyl)terephthalate (BHET) via SSP. DSC results confirmed that the incorporation of BHET via SSP resulted in an increase of Tg for PBT−PET copolymers compared to PBT. However, these copolymers showed poor crystallization ability. Although the sequence distribution and crystallization behavior of copolyester before and after SSP have been well discussed, the reaction site where chain growth takes place during SSP has not yet been reported. In our previous research,29,30 a kind of phosphorus-containing copolyester, p o l y ( e t h y l e n e t e r e p h t h a l a t e ) - c o - p o l y ( e t h y l e ne 3 (hydroxyphenylphosphinyl)propionic acid), was synthesized by the melting polycondensation of terephthalic acid, ethylene glycol, and 3-(hydroxyphenylphosphinyl)propionic acid Received: Revised: Accepted: Published: 5326

January 20, 2013 March 18, 2013 March 22, 2013 March 22, 2013 dx.doi.org/10.1021/ie400224z | Ind. Eng. Chem. Res. 2013, 52, 5326−5333

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Scheme 1. Synthetic Route of BHET (a), CEPPE (b), and CxPET (c)

mL three-neck round-bottomed flask with water separator and nitrogen inlet pipe. The mixture was maintained at 180 °C with magnetic stirring for 6 h to ensure high esterification yield (Scheme 1b). Similar with the preparation of BHET, the superfluous EG can make the esterification reaction of CEPPA complete. Then the liquid mixture was distilled at 100 °C by vacuum to remove superfluous EG, and the residual CEPPE was obtained. 2.3. Preparation of High Molecular Weight Copolyester. Two steps including melt polycondensation and SSP were used to prepare copolyester with high molecular weight. First of all, BHET, CEPPE, and catalyst Sb2O3 (5 × 10−4 mol/ mol BHET) were added into a 250-mL three-neck roundbottomed flask equipped with nitrogen inlet and mechanical stirring. The mixtures were kept at 190 °C for 1 h in nitrogen. After the mixtures melted completely, the temperature was raised to 260 °C gradually in nitrogen. Then the reaction was carried out under reduced pressure (40 Pa) for another several hours depending on the CEPPA content (Scheme 1c). Via melt polycondensation, random phosphorus-containing copolyester was obtained. The copolyester was dissolved into HFIP, and then precipitated with methanol. The obtained copolyester was dried in oven at 80 °C for 4 h prior to further use and analysis. Each sample was abbreviated as CxPET, where x represents the molar fraction of CEPPE per hundred mole of BHET. For example, when x = 20, it represents CEPPE takes 20% mole of BHET. To eliminate the effect of the particle size on SSP,31,32 all the samples were ground into powder (about 200 mesh) by mortar and pestle at room temperature for a bigger specific surface area facilitating a higher reaction rate. SSP was carried out in a reaction tube with a diameter of 15 mm and length of 120 mm. A 0.5 g portion of sample was placed in the bottom of the tube, ensuring the cover area as big as possible. SSP reaction was carried out at 20−30 °C below the melting point of corresponding copolyester under vacuum (about 10−20 Pa). SSP reaction time (tssp) was ranged from 0 to 11 h, and the contents of CEPPA were between 0 and 30 mol %. 2.4. Intrinsic Viscosity of Copolyester. Intrinsic viscosities [η] of CxPET were measured with an Ubbelohde

(CEPPA). It was found that CEPPA had high flame retardance efficiency for PET, i.e. only 10% CEPPA was introduced, its LOI was 43.6 and UL-94 vertical test reached V−0 rating. However, via this directly melting polycondensation its intrinsic viscosity was only in the range of 0.6−0.7 dL/g. Therefore, in this paper, this phosphorus-containing copolyester was prepared by transesterification and melt polycondensation. Then SSP was carried out to obtain higher molecular weight copolyester. 1H NMR was used to analyze the phosphorus content and sequential distribution of these copolyesters. Besides this, the chain growth progress during SSP was determined. The thermal transition behaviors of copolyester after SSP were also investigated.

2. EXPERIMENTAL SECTION 2.1. Materials. Dimethyl terephthalate (DMT, CP grade) and antimonous oxide (Sb2O3 99.99%) were provided by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Zinc acetate [Zn(CH3COO)2, AR grade] was obtained from Kelong Chemical Reagent Factory (Chengdu, China). Ethylene glycol (EG, fiber grade) was purchased from Xilong Chemical Co., Ltd. (Chengdu,China). 3-(hydroxyphenylphosphinyl)propionic acid (CEPPA) was received from Chengdu Weili Fire Retardant Chemical Industry Co., Ltd. 1,1,1,3,3,3Hexafluoro-2-propanol (HFIP 99.5%) was obtained from Yancheng Biological Products Company (Jiangsu, China). Other materials were used as received. 2.2. Preparation of BHET and CEPPE. BHET was synthesized through transesterification of DMT and EG (Scheme 1a). DMT (77.6 g), 70.0 mL of EG (ratio of DMT/EG = 1:3), and 0.07 g of transesterification catalyst Zn(CH3COO)2 (1 × 10−3 mol/mol DMT) were added into a 250-mL three-necked round-bottomed flask equipped with water separator and nitrogen inlet. This reaction was carried out at 170 °C under nitrogen with magnetic stirring. After about 4 h, methanol (byproduct of transesterification) was distilled out completely. When the temperature lowered to 80 °C, the mixture was added with cold water to precipitate the product BHET (Scheme 1a). CEPPA (30.0 g) and 31.2 mL of EG (molar ratio of CEPPA/EG = 1:4) were added into a 1005327

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viscometer at 25 °C in phenol/1,1,2,2-tetrachloroethane solution (w/w, 60/40). 2.5. Phosphorus Content Determination. The actual phosphorus content was determined using oxygen flask combustion−inductively coupled plasma atomic emission spectroscopy (ICP-AES) method. The testing sample was prepared according to EN14582:2007. The detailed procedure was as follows: 10 mg of purified and dried copolyester was burned completely in a 1-L flask full of oxygen gas. KOH solution (0.001 mol/L) was used to absorb the yielding gas with 0.1 wt % KMnO4 oxidizing the phosphorus compounds. By measuring the phosphorus contents of the copolyesters, the content of CEPPA could be determined. 2.6. 1H NMR. The 1H NMR spectra were obtained with Bruker AVANCE AV II-600 NMR instrument (600 MHz) at ambient temperature. For the 1H NMR measurements, 5-mg samples were dissolved in 0.6 mL of trifluoroacetic acid-d (TFA), and TFA was used as internal standard with chemical shift at 11.3 ppm. The spectra were acquired using 32 scans. 2.7. Differential Scanning Calorimetry (DSC). The thermal transition behaviors of the SSP were analyzed by a TA Q 200 DSC equipped with a refrigerated cooling system (RCS). About 5 mg of sample was filled into the aluminum crucible. Samples were first heated from 40 to 260 °C at a heating rate of 10 °C/min (first heating scans), isothermal for 3 min, then cooled to 40 °C at 10 °C/min (cooling scans), and finally reheated to 260 °C again at 10 °C/min (second heating scans). The test was carried on with nitrogen flow of 50 mL/ min from beginning to end.

addition, the solid polymerization reaction temperature for copolyester with high CEPPA content decreased due to its lower Tm, and resulted in a lower reacting rate. As we know, during the SSP progress of neat PET, the molecular weight of final products was increased and the reaction rate was decreased with the increase of reaction time.33,34 The same phenomenon was also found in the SSP of CxPET. The reaction rate at initial stage was rapid, and then intrinsic viscosity of CxPET was almost constant from tSSP 7 h to 11 h. This illustrated that with longer reacting time of SSP, the escape of low molecular weight byproducts got more difficult, thus the increase of molecular weight became slow and even stopped. The chemical structure of CPET after SSP was characterized by 1H NMR. Figure 1 shows the 1H NMR spectra of neat PET and C20PET (tSSP = 11 h). The chemical shifts of aliphatic and aromatic protons were in the range of 3.9−4.9 and 7.5−8.4 ppm, respectively. All of the signals of neat PET were found: the peaks at 8.1−8.3 ppm (a) was ascribed to aryl of DMT, while the peak at 4.47 ppm (b) was assigned to the methylene of ester group.35 Besides, the resonance signal of byproduct diethylene glycol produced in the melting polycondensation process appeared at 3.75 (i) and 4.25 (h) ppm. Compared with the spectrum of neat PET, the extra peaks in the C20PET spectrum were supposed to be related to CEPPA. The peaks at 7.6−7.9 and 2.5−3.0 ppm were reasonably assigned to aryl and methylene of CEPPA.11 The signals at 4.55−4.75 ppm were caused by the methylene of EG with different combinations of DMT and CEPPA, while the peaks at 4.35−4.55 ppm were assigned to the methylene of EG with different combinations of CEPPA. The possible combinations of T-E-T, T-E-C, and C-EC were concluded in Scheme S1. Figure S1 shows the resonance peaks of the methylene protons in ethylene glycol unit of CxPET (before SSP) with chemical shift ranging from 4.35 to 4.75 ppm. The resonance intensities of methylene protons of ethylene glycol unit of each sample are listed in Table 2. Comparing the resonance peaks of methylene protons with those of C5PET, C10PET, C20PET, and C30PET, it was revealed that the resonance intensity of proton b was decreased and protons j, k, l, m, and n were increased with the increase of CEPPA content. Thus, the mole fractions of terephthalate (XT) and CEPPA (XC) of CxPET were calculated from the intensities of these methylene protons:

3. RESULTS AND DISCUSSION 3.1. Solid State Polymerization of Cx PET. Four precursors (C5PET, C10PET, C20PET, C30PET) with different CEPPA contents were prepared by melt polycondensation and then subjected to SSP at different temperatures depending on their melting temperatures (shown in Supporting Information Table S1) for several hours. To avoid systematic error, the intrinsic viscosity of all the samples was controlled over 0.4−0.6 by adjusting prepolymerization time. The intrinsic viscosities of the four samples as a function of reaction time (tSSP) are shown in Table 1. As we expected, the intrinsic viscosity of these four Table 1. Intrinsic Viscosity ([η]) of CxPET with Different tSSP sample

0h

1h

3h

5h

7h

9h

11 h

C5PETa C10PETb C20PETc C30PETd

0.55 0.45 0.42 0.48

0.70 0.58 0.51 0.54

0.81 0.70 0.67 0.61

0.90 0.79 0.76 0.66

0.96 0.87 0.88 0.69

1.01 0.95 0.93 0.71

1.06 1.01 0.97 0.72

⎛ ⎞ I X1T = ⎜I TET + TEC + I TEOH⎟ /S ⎝ ⎠ 2 ⎛ ⎞ I j + Ik = ⎜I b + + Ih⎟ /S 2 ⎝ ⎠

The reaction temperature is 200 °C. bThe reaction temperature is 190 °C. cThe reaction temperature is 175 °C. dThe reaction temperature is 160 °C.

a

X 1C =

⎛ I j + Ik ⎞ ⎛ I TEC ⎞ ⎜ + ICEC⎟ /S = ⎜ + Il,m,n⎟ /S ⎝ 2 ⎠ ⎝ 2 ⎠

(1)

(2)

ITET, ITEC, ICEC, and ITEOH represent the integrated intensities of methylene protons resonance signals of T-E-T, T-E-C, C-E-C sequence, and T-E-OH terminal unit, respectively. S is the total integrated intensities of the resonance peaks of methylene protons in the ethylene glycol. The mole fraction of terephthalate (XT) and CEEPPA (XC) can also be obtained from the aliphatic protons of CEPPA by eqs 3 and 4:

samples was increased with the increase of reaction time, which was similar to neat PET. Copolyesters with varied CEPPA content had different final intrinsic viscosities. C5PET containing the least CEPPA content had the highest intrinsic viscosity ([η] = 1.06) after SSP for 11 h. Instead, C30PET having the maximum CEPPA its intrinsic viscosity can only reach 0.72 after the same SSP time. This means more bulky pendent group of CEPPA is introduced, its hindering effect on the intrinsic viscosity of copolyester becomes more obvious. In

X C2 = ICEPPA /S = (Ie + I f )/S 5328

(3)

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Figure 1. 1H NMR spectra of PET (a) and C20PET (b).

Table 2. Resonance Intensities of Methylene Protons of Ethylene Glycol Unit and Molar Fraction Percent of CEPPAa intensity of chemical shifts (%) sample

b

j+k

l+m+n

d

X1C

X2C

XaC

C5PET C10PET C20PET C30PET

90.90 82.20 73.56 64.43

6.76 14.16 18.75 26.46

1.35 2.75 6.73 8.46

0.99 0.89 0.96 0.65

4.96 10.58 17.76 25.00

4.90 10.58 17.77 25.00

6.89 14.05 19.08 26.15

a 1 XC

is the molar fraction calculated by 1NMR intensities of methylene glycol unit. X2C is the molar fraction calculated by 1NMR intensities of aliphatic protons of CEPPA. XaC is the molar fraction calculated by ICP-AES.

X 2T = 1 − X C2

the results are shown in Table 2. The mole fraction of CEPPA determined by ICP-AES was always a little higher than that of obtained by 1H NMR analyses. However, considering systematical error, the mole fraction of CEPPA obtained by these two different analyses was fairly close, which was consistent with its feed ratio. Based on the above results, it can be concluded that almost all the CEPPA had been introduced into the copolyesters.

(4)

ICEPPA represents the integrated intensity of CEPPA aliphatic protons (ICEPPA = Ie + If). The mole fraction of CEPPA calculated by these two methods (X1C and X2C) is also listed in Table 2. It was found that the mole fraction of CEPPA calculated by the above two methods was almost the same. To validate the results, the mole fraction of CEPPA was further determined by the ICP-AES and 5329

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Table 3. Resonance Intensities of Methyl Protons of EG Unit, Number-Average Sequential Length, and Degree of Randomness of C20PET with Different tssp intensity of chemical shifts (%) tSSP

b

j+k

l+m+n

d

PTC

PCT

LnT

LnC

R

0 3 7 11

72.11 74.28 75.00 75.60

18.75 17.89 17.20 16.75

6.73 7.03 7.32 7.66

0.96 0.80 0.48 0

0.11 0.11 0.10 0.10

0.58 0.56 0.54 0.52

8.93 9.35 9.80 10.03

1.71 1.79 1.85 1.90

0.71 0.67 0.64 0.62

Scheme 2. Schematic Representation of Solid-State Polymerization Process of CxPET

3.2. Sequential Distribution Analysis of CxPET. The sequential distribution of CxPET was analyzed by 1H NMR. By taking ethylene glycol unit as a reference point, the probability of finding a C unit next to a T unit can be calculated by the following equation:35−37 PTC = I TEC/(I TEC + I TET + I TEOH) ⎞ ⎛ I j + Ik ⎞ ⎛ I j + Ik =⎜ + Ib + Id⎟ ⎟/⎜ ⎠ ⎝ 2 ⎠ ⎝ 2

were calculated with the above equations. From Table 3, we found that the intensity of b and l + m + n representing T-E-T and C-E-C increased continuously when tSSP increased, as well as j + k and d representing T-E-C and T-E-OH decreased. The increase of T-E-T and C-E-C was partly caused by the translation of T-E-C. Besides, the esterification of end group also made the T-E-T combination increase, which was illustrated by the decrease of ITEOH. PTC and PCT decreased with the increase of tSSP, while LnT and LnC increased. For example, the number-average sequential lengths of the TT unit (LnT) and CC unit (LnC) were increased from 8.93 and 1.71 (before SSP) to 10.03 and 1.90 (after SSP 11 h), respectively. From Table 3, it also can be seen clearly that the degree of randomness R was decreased from 0.71 to 0.62 when SSP was conducted for 11 h. Furthermore, the descent rate of R was getting slow, i.e. for the first 3 h R was decreased from 0.71 to 0.67, and with further increasing SSP time to 7 h R was only dropped to 0.64. As we know, the main reactions leading the extension of molecular chain were esterification and transesterification during SSP process, and both the reactions occurred in the amorphous regions, because the molecular chain or end group located in this area owned enough mobility to take reaction. Because of the asymmetric structure and large steric hindrance, CEPPA was located in the amorphous area. During SSP process, the structure of T-E-C was the most active reaction site, and the esterification or transesterification reactions mostly happened here. As shown in Scheme 2, with the increase of tSSP, some T-E-C structures were translated into T-E-T or C-E-C, which was illustrated by the decrease of peak intensity of j + k (ITEC). Accordingly, the peak intensity of l + m + n (ICEC) was increased. In another words, T-E-T and C-E-C were enriched after SSP, and C20PET became less random as well as tended to transform into block copolymer. As far as the samples containing different CEPPA content are concerned, their resonance intensities of peaks ranging from 4.2 to 5.0 ppm were also investigated by 1H NMR. The detailed peak intensities and the related parameters such as PTC, PCT, LnT, LnC, and R calculated with eqs 5−9 are listed in Table 4. As

(5)

Similarly, the probability of finding a T unit next to a C unit is ⎛ I j + Ik ⎞ ⎛ I j + Ik ⎞ PCT = I TEC/(I TEC + ICEC) = ⎜ + Il,m,n⎟ ⎟/⎜ ⎝ 2 ⎠ ⎝ 2 ⎠ (6)

ITET (Ib), ITEC (Ij, Ik) (ITEC = ICET), ICEC (Il, Im, In), and ITEOH (Id) represent the integrated intensities of methylene protons resonance signals of T-E-T, T-E-C, C-E-C sequence, and T-EOH terminal unit, respectively. The number-average sequential lengths of the TT unit (LnT) and CC unit (LnC) were calculated by the following equation: 1 LnT = PTC (7) LnC =

1 PCT

(8)

The degree of randomness was defined by R = PTC + PCT

(9)

For a total randomness of a copolymer, R equals 1. For an alternative copolymer, R equals 2 and a block copolymer, R is close to zero.38,39 To determine the sequential distribution of CxPET, C20PET with different tSSP was investigated by 1H NMR. The peak intensities of methylene occurring at 4.25−5.10 ppm are listed in Table 3, and related parameters PTC, PCT, LnT, LnC, and R 5330

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Table 4. Resonance Intensities of Methyl Protons of EG Unit, Number-Average Sequential Length, and Degree of Randomness of CxPET before and after SSP with Different Contents of CEPPA intensity of chemical shifts (%) sample

b

j+k

l+m+n

d

PTC

PCT

LnT

LnP

R

C5PET C5PET-SSP11h C10PET C10PET-SSP11h C30PET C30PET-SSP11h

90.90 92.56 82.20 84.03 64.43 66.38

6.76 5.69 14.16 11.75 26.46 23.98

1.35 1.74 2.75 3.92 8.46 9.42

0.99 0 0.89 0.29 0.65 0.21

0.0354 0.0298 0.0745 0.0651 0.169 0.153

0.715 0.620 0.680 0.600 0.610 0.560

28.25 33.56 13.42 15.36 5.917 6.536

1.399 1.613 1.471 1.667 1.639 1.786

0.750 0.649 0.754 0.665 0.779 0.713

Figure 2. DSC cooling (a) and reheating (b) curves of m-C20PET (prepared by melt polycondensation) and SSP-C20PET (prepared by SSP).

Table 4 shows, after SSP for 11 h, R values of C5PET, C10PET, and C30PET, decreased to 0.649, 0.665, and 0.713, respectively. This indicated that their randomness was decreased. However, the degree of randomness decrease was different. The more the CEPPA introduced, the lower the degree of randomness decrease. Because the peak intensity of j + k (ITEC) increased greatly when more CEPPA was introduced, the transesterification was more difficult to happen. Therefore, the reduction level of R values was descended. 3.3. Thermal Transition Behaviors of CxPET. To investigate the effect of SSP on the thermal transition behaviors of C x PET, two kinds of C 20 PET prepared by melt polycondensation (m-C20PET) and SSP (SSP-C20PET), were chosen to test by DSC. The molecular weights of these two samples were almost the same by controlling the reaction condition in order to eliminate the effect caused by molecular weight. Samples were first heated to 260 °C to eliminate their thermal history, then cooled to 40 °C at 5 °C/min, and finally reheated to 260 °C at 5 °C/min. Figure 2 shows the cooling (a) and reheating (b) scans of C20PET, and the detailed data are listed in Table S2. It was found that after SSP, the crystallization ability of C20PET was improved. An obvious endothermic crystalline peak was found in the cooling scans of SSP-C20PET, while it cannot be found for m-C20PET. In their heating scans, the glass transition temperatures (Tg) and melting temperatures (Tm) were found. Tm of SSP-C20PET was increased from 202.9 °C (m-C20PET) to 205.6 °C, and the molten enthalpy increased from 21.0 to 28.1 J/g. This illustrated again that the randomness of C20PET decreased

and its structure became more regular, which made crystallizability of C20PET enhanced. Figure 3 shows the first and second heating DSC curves of C5PET and C20PET after different tSSP. Before SSP, a clear Tm was found for both C5PET and C20PET, i.e. Tm of C5PET was 230.3 °C, and Tm of C20PET was 206.1 °C. When they were processed at tSSP 1 h, double melting peaks were found in their first heating curves (Figure 3a, b). When SSP time was increased to 5 h, these double melting peaks became indistinct. Finally, when tSSP was enhanced to 11 h, only one melting peak was found for both samples. It had been proved that the lower melting temperature was ascribed to the melting and recrystallizing of unstable crystal.40,41 This means when SSP was conducted first, the regularity of copolyester was somewhat enhanced, which resulted in some unstable crystals occurrence. When the SSP time was long enough, these unstable crystals had enough time to form stable crystals. Therefore, only one exothermic melting peak was left. The occurrence of unstable crystal during SSP can also be demonstrated by their second heating scans. From Figure 3c and d, only one melting peak occurred whether the samples were SSP at 1 or 11 h. What’s more, the melting points determined from second heating scans were close to the high one on the first heating curves. This further illustrated that the high melting temperature was due to the stable crystal which was formed by initial unstable crystal via melting− recrystallizing during the DSC heating process. 5331

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Figure 3. DSC heating curves of C5PET and C20PET after different tSSP. First heating curves of C5PET (a) and C20PET (b), second heating curves of C5PET (c) and C20PET (d).

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CONCLUSIONS

ASSOCIATED CONTENT

S Supporting Information *

The phosphorus-containing copolyesters with higher molecular weight and lower randomness have been successfully synthesized by melt polycondensation from BHET and CEPPE followed by solid-state polymerization. The intrinsic viscosity of copolyesters was increased with the increase of reaction time of solid-state polymerization, tSSP, however, the intrinsic viscosity of final obtained copolyesters was decreased when the phosphorus content was higher due to the low reaction activity of CEPPA and low solid-state reaction temperature. The randomness of all the copolyester samples was decreased with the increase of tSSP, and the transesterification reaction only occurs in the amorphous phase of copolyesters, mainly at the ester bonds formed by CEPPA and EG. DSC results confirmed that the crystallizability of copolyesters was increased after SSP.

The characteristic parameters of precursors obtained by melt polycondensation, DSC data of C20PET prepared by two methods, the possible combinations of T-E-T, T-E-C, and C-EC, and expanded NMR spectra of CxPET before SSP. These materials are available free of charge via the Internet at http:// pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (X.-L. Wang) or yzwang@scu. edu.cn (Y.-Z. Wang). Tel. and Fax: +86-28-85410259. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (50933005, 51121001), and 5332

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Program for Changjiang Scholars and Innovative Research Team in University (IRT1026). We also thank the Analysis and Testing Center of Sichuan University for the NMR measurements.

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dx.doi.org/10.1021/ie400224z | Ind. Eng. Chem. Res. 2013, 52, 5326−5333