Diblock Poly(ester)-Poly(ester-ether) Copolymers: I. Synthesis

Article pubs.acs.org/IECR

Diblock Poly(ester)-Poly(ester-ether) Copolymers: I. Synthesis, Thermal Properties, and Degradation Kinetics Nowsheen Goonoo,† Archana Bhaw-Luximon,† Gary L. Bowlin,‡ and Dhanjay Jhurry*,† †

ANDI Centre of Excellence for Biomedical and Biomaterials Research, MSIRI Building, University of Mauritius, Réduit, Mauritius Department of Biomedical Engineering, Virginia Commonwealth University, Richmond, Virginia, United States

‡

S Supporting Information *

ABSTRACT: The synthesis and characterization of polycaprolactone (PCL) and poly(dioxanone-methyl dioxanone) (P(DX-coMeDX)) block copolymers in a range of compositions of the two segments and with varying methyl dioxanone units is herein reported. The thermal properties of the copolymers were studied by differential scanning calorimetry (DSC) which revealed that copolymers exhibited two melting transitions ranging between 48 and 53 °C for the PCL segment and 71−79 °C for the P(DXco-MeDX) segment. Copolymers exhibited only one crystallization exotherm which decreased as the MeDX content of the copolymer increased, thereby increasing miscibility of PCL and P(DX-co-MeDX) segments, a result also confirmed by scanning electron micrographs (SEM). Lastly, the kinetics of thermal degradation of PCL-b-P(DX-co-MeDX) copolymers were investigated by thermogravimetric analysis (TGA). Thermal degradation was shown to proceed in three distinct steps with the P(DX-co-MeDX) segment degrading in the first stage followed by the PCL segment in the last two stages most likely via unzipping and random polymerization mechanisms. The activation energies of copolymer degradation were determined and were found to decrease with increasing MeDX content of the copolymer. Overall, increasing MeDX content influenced both thermal properties and degradation kinetics through phase mixing of segments in the copolymers.

1. INTRODUCTION Polydioxanone (PDX), a biodegradable poly(ester-ether), was first introduced by Ethicon in 1981 and it was marketed as an FDA approved biodegradable monofilament suture PDS (polydioxanone suture).1 In 2006, EU-approved Kalangos Biodegradable Ring based on polydioxanone was developed for pediatric mitral and tricuspid heart valve repair.2,3 The processability of PDX on account of its thermal degradability has been one of the barriers to its development and continues to attract researchers’ attention. Recently, Zeng et al.4 reported on the isothermal and nonisothermal cold crystallization kinetics of PDX. The development of this poly(ester-ether) as biomaterials has thus remained a virgin area for a long time, except for a few applications. New materials based on combinations of PDX with other biodegradable polymers such as PCL have been elaborated by a few groups to improve the processability or to alter the mechanical or biological properties of PDX. For instance, the degradation rate of block copolymers of PDX and PCL under in vitro conditions decreased with increasing PCL content on account of a synergistic increase in the PDX block stability.5 Langer et al.6 patented the use and preparation of shape-memory PDX-PCL block copolymers or blends. As shown by the authors, objects made of this material can recover their original shape after being subjected to the following cycle: heated above their shape recovering temperature (Ttrans), cooled below Ttrans, and reheated to Ttrans. Those materials were designed for applications such as sutures, bone screws, stents, catheters, drug delivery devices, and implants.7 PCL-b-PDX copolymers have been electrospun and assembled into scaffolds which also possessed excellent shape memory capabilities as confirmed by cyclic mechanical tests.8 Mimicking the glycolide-lactide © 2012 American Chemical Society

polyester family, we have reported on the successful synthesis and homopolymerization of methyl-substituted dioxanone, DL3-methyl-1,4-dioxan-2-one, referred to as 3-MeDX as well as the preparation of a range of random copoly(ester-ether)s by the nonsequential polymerization of 1,4-dioxan-2-one with DL3-methyl-1,4-dioxan-2-one.9,10 Electrospun copoly(ester-ether)s fibers with small amounts of 3-MeDX (5−10%) exhibited lower melting transitions up to 15 °C as compared to PDX homopolymer which translates to a decrease in the stiffness of the copolymer chains.11 In this Article, we report on the synthesis of a range of diblock copolymers consisting of poly(dioxanone-methyl dioxanone) and PCL segments in various proportions and lengths. The influence of the presence of methyl dioxanone units on the thermal properties of the copolymers has been analyzed in depth. Thermogravimetric analysis has shown that copolymer degradation occurs in three distinct stages and is dependent on the weight ratio of the components. The activation energy of thermal degradation was determined. The possibility to modify the crystallinity of the copolymers via the incorporation of MeDX units and the likely impact on degradation behavior opens up new perspectives for such materials.

2. EXPERIMENTAL SECTION 2.1. Materials. Solvents were purchased from Aldrich Chemicals or Fischer Scientific and were subjected to Received: Revised: Accepted: Published: 12031

June 27, 2012 August 17, 2012 August 24, 2012 August 24, 2012 dx.doi.org/10.1021/ie301703j | Ind. Eng. Chem. Res. 2012, 51, 12031−12040

Industrial & Engineering Chemistry Research

Article

Scheme 1. Synthesis of (PCL)n-OH

Scheme 2. Preparation of (PCL)n-b-P(DX-co-MeDX)x+ya

a

Where n = 50 and 12 ≤ (x + y) ≤ 44.

thermal analyzer. All copolymers were heated from 30 to 120 °C, cooled to −30 °C, and reheated to 120 °C at 3 K/min. Copolymers were subjected to thermogravimetric analysis in an inert atmosphere of nitrogen. TG 209 F3 Tarsus analyzer was used to measure and record the sample mass change with temperature over the course of the pyrolysis reaction. Thermogravimetric curves were obtained at three different heating rates (5, 10, and 15 K/min) between 25 and 1000 °C. Nitrogen was used as an inert purge gas to displace air in the pyrolysis zone, thus avoiding unwanted oxidation of the sample. The sample mass used in this study was approximately 10 mg. Surface morphology characterization was accomplished using scanning electron micrographs (SEM, Zeiss EVO 50 XVP). Copolymer samples were mounted on an aluminum stub and sputter coated with gold for imaging.

purification prior to use in polymerization. Butan-1-ol (99% Aldrich) and tin(II) octanoate (Alfa Aeser, 97%) were used as received. 1,4-Dioxan-2-one and D,L-3-methyl-1,4-dioxan-2-one were synthesized according to procedures previously described by us.9 Synthesis of PCL-OH. The monomers ε-CL (1.14 g, 0.01 mol), butan-1-ol (18.3 μL, 0.2 mmol), and stannous octanoate (2.28 mg) were added to a quick fit tube in a glovebox. Bulk polymerization was carried out at 110 °C for 24 h. Residual monomers were removed by dissolving the crude sample in chloroform followed by precipitation in diethyl ether. Copolymerization of 1,4 Dioxan-2-one and MeDX Using PCL-OH as Macroinitiator (80/20). A typical copolymerization is hereby described. PCL-OH (1.1748 g, 0.010 mol) was weighed in a quick fit tube in a glovebox. A solution of Sn(Oct)2 was prepared by dissolving 0.1 g of Sn(Oct)2 in toluene (5 mL). This solution (12.75 μL) was transferred to the quick fit tube containing the macroinitiator. The tube was then placed in a preheated oil bath at 110 °C for 24 h to allow complexation between PCL-OH and Sn(Oct)2 to take place. DX (1,4 dioxan-2-one) ((0.714 g, 7 mmol) and MeDX (0.348 g, 3 mmol) were then injected with a syringe via the septum. The temperature was reduced to 80 °C prior to the addition of the monomers. After the desired polymerization time, the reaction was quenched and the crude sample was characterized by 1H and 13C NMR. The crude sample was then dissolved in chloroform followed by precipitation in diethyl ether. The precipitate was isolated, dried under vacuum, and then characterized by 1H and 13C NMR. 2.2. Measurements. 1H and 13C NMR spectra were recorded on a 250 MHz Bruker Electrospin spectrometer in CDCl3 at room temperature. Differential scanning calorimetry (DSC) analysis was carried out using a DSC 200F3Maia

3. RESULTS AND DISCUSSION 3.1. Synthesis of PCL-b-P(DX-co-MeDX). In a first step, hydroxyl end-capped poly(caprolactone) was synthesized via bulk ring-opening polymerization of ε-caprolactone at 110 °C using tin(II) octanoate/n-butyl alcohol (Scheme 1). The resulting purified homopolymer was used as macroinitiator for the copolymerization of DX and MeDX at 80 °C in the presence of tin(II) octanoate (Scheme 2). The crude copolymer was then dissolved in chloroform and precipitated in diethyl ether to remove any residual monomers, followed by drying under vacuum. 1H NMR determination of the PCL length after polymerization confirmed that no thermal degradation had occurred during complexation and polymerization. The purified diblock copolymers were characterized by 1H (Figure 1) and 13C NMR (Figure 2). (See Figure S1 in Supporting Information for the 13C NMR of PCL-OH and 12032

dx.doi.org/10.1021/ie301703j | Ind. Eng. Chem. Res. 2012, 51, 12031−12040


Article

Figure 1. 1H NMR (CDCl3) of (A) PCL-OH; (B) PCL-b-PDX; (C) PCL-b-P(DX-co-MeDX).

MeDX (171.1 ppm), DX-DX-MeDX + MeDX-DX-DX (170.8 ppm), and DX-DX-DX (170.3 ppm). Calculation of DX to MeDX Mole Ratio in the Copolymer. Copolymers with weight percentages of PCL and P(DX-coMeDX) varying in the range of 56−82% and 18−44%, respectively, were synthesized (Table 1). The copolymers also contained varying MeDX content up to 43 mol %. PCL-b-

PCL-b-PDX.) The signals due to P(DX-co-MeDX) were assigned as reported by Lochee et al.10 The carbonyl region of the 13C NMR spectrum for the copolymer indicated the presence of six peaks assigned to PCL (176 ppm) and to the triads MeDX-MeDX-MeDX + DX-MeDX-MeDX (173.0 ppm), MeDX-MeDX-DX + DX-MeDX-DX (172.9 ppm), MeDX-DX12033



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Figure 2. 13C NMR (CDCl3) of (A) PCL-OH and (B) PCL-b-P(DX-co-MeDX).

Table 1. Summary of Block Contents (wt %) and Mole (%) Incorporation of DX and MeDX in the Copolymers Synthesized

block content (wt %) in purified copolymer sample 1 2 3 4 5 6

PCL50-b-PDX20 PCL50-b-P(DX8-coMeDX3) PCL50-b-P(DX8-coMeDX4) PCL50-b-P(DX8-coMeDX6) PCL50-b-P(DX23-coMeDX8) PCL50-b-P(DX34-coMeDX10)

percentage incorporation of DX =

mole % of comonomeric units in P(DXco-MeDX) segment of purified copolymer

PCL

PDX or P(DX-coMeDX)

DX

MeDX

74 83

26 17

100 70

30

82

18

65

35

80

20

57

43

64

36

73

27

56

44

77

23

Ix 2 Ix 2

percentage incorporation of MeDX =

+ Iy

× 100%

Iy Ix 2

+ Iy

(1)

× 100% (2)

Thermal Analysis of Copolymers. Analysis of the purified PCL-b-P(DX-co-MeDX) copolymers by DSC (3 K/min) revealed the presence of two melting transitions ranging between 45−53 °C and 71−79 °C, corresponding to PCL and P(DX-co-MeDX) polymer segments, respectively (Figure 4, Table 2). This is indicative of poor miscibility of copolymer chains. As the number of MeDX repeat units increases, a decrease in crystallinity is expected and this is confirmed by lower Tm values for both polymer blocks. Comparing PCL50-bPDX20 and PCL50-b-(PDX23-co-MeDX8) with an approximately equivalent number of PCL and PDX repeat units, it can be observed that incorporation of MeDX lowers the Tm (80.2 to 78.1 °C). The melting transition of the PCL segment was also reduced in a similar manner (46.8, 52.3 °C to 42.2, 49.1 °C). A splitting of the melting endotherm was observed for PCL as opposed to a single peak for PCL homopolymer. Similar results were reported by Brito et al.12 for PCL/PDX blends. This was accounted for by increasing physical interactions in the case of low molecular weight PCL and PDX segments. Comparing the melting endotherms of PCL50-b-P(DX8-co-MeDX3), PCL50-bP(DX8-co-MeDX4), and PCL50-b-P(DX8-co-MeDX6) at the higher temperature, a change in PCL ΔHm values from 7.09 to 16.19 to 22.64 J/g was noted with increasing MeDX. It was therefore concluded that greater MeDX content probably

P(DX-co-MeDX) copolymers were soluble in both chloroform and THF while PCL-b-PDX was soluble only in chloroform. The percentage incorporation of DX and MeDX in the copolymer was determined from eqs 1 and 2 using the signal intensities of peaks due to −COCH2O− of PDX and −COCH(CH3)O− of PMeDX in the 1H NMR spectrum of the purified copolymer (Figure 3). 12034



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Figure 3. 1H NMR (CDCl3) of PCL-b-P(DX-co-MeDX).

Table 2. DSC Derived Thermal Data of Homopolymers and Diblock Copolymers block content (wt %) in purified copolymer

Figure 4. DSC scan of PCL50 and PCL50-b-P(DX23-co-MeDX8) at 3 K/min.

brings about enhanced miscibility and physical interaction between PCL and P(DX-co-MeDX). As demonstrated, the crystallinity of the PCL-b-P(DX-coMeDX) copolymers could be modulated by the weight ratio of the two blocks and the MeDX content. Copolymer crystallinity (χ) (Table 3) was calculated from melting enthalpy values using as reference the value of 139.5 J/g for a 100% crystalline PCL.13 Indeed, the melting enthalpies of the copolymers were found to be dependent on the weight % of the two segments (Table 3). As the weight % of P(DX-co-MeDX) versus PCL increased, the enthalpy of the poly(ester-ether) also increased while that of the polyester decreased. DSC results (Table 2) show that only one crystallization exotherm is observed regardless of the diblock compositions, indicating that coincident crystallization of the PCL and P(DXco-MeDX) segments occurred from the melt at temperatures slightly lower than that of PCL homopolymer. Similar observations were made by Albuerne et al.14 whereby the PDX block in PCL-b-PDX copolymer crystallized at lower

sample

PCL

PCL50 PDX 1 2 3 4 5 6

100 74 83 82 80 64 56

2nd heating scan

cooling scan

PDX or P(DX-coMeDX)

Tc (°C)

ΔHc (J/g)

100 26 17 18 20 36 44

36.1 45.7 25.6 25.2 28.7 23.2 21.4 19.3

82.23 62.14 61.26 66.05 70.56 53.55 58.53 42.00

PCL

PDX or P(DX-coMeDX)

Tm (°C)

Tm (°C)

54.7 46.8, 45.6, 48.5, 43.2, 42.2, 41.4,

52.3 51.8 53.4 49.6 49.1 48.3

81.2 80.2

78.8 78.1 70.9

Table 3. Variation of % Crystallinity with Weight Percent of P(DX-co-MeDX) in the Copolymer sample

weight % PCL

ΔHm (2nd scan) PCL

χ % (2nd scan) PCL

weight % P(DX-coMeDX)

ΔHm (2ndscan) P(DX-coMeDX)

2 3 4 1 5 6

83 82 80 74 64 56

66.90 75.72 62.33 54.35 49.47 41.90

47.96 54.28 44.68 38.96 35.46 30.04

17 18 20 26 36 44

1.06 7.80 12.13 32.66

temperatures than expected and its crystallization exotherm overlapped with that of the PCL block. This confirms the diblock character of our copolymers, since in the case of mixtures of PDX and PCL homopolymers, two distinct crystallization exotherms are obtained.12 12035



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Figure 5. (a) SEM images (150× ) of (A) PCL50, (B) PCL50-b-PDX33, and (C) PCL50-b-P(DX34-co-MeDX10). (b) SEM image (500× ) of PCL50-bP(DX23-co-MeDX8). (c)SEM image (500× ) of PCL50-b-P(DX8-co-MeDX3).

Figure 6. TGA of PCL50, PDX, PCL50-b-PDX20, and PCL50-b-(DX34-co-MeDX10).

can be noted for the copolymers as depicted by the presence of irregular disk-like structures. Moreover, distinct phases separated by cracks are clearly visible on the copolymer surface as in Figure 5b, showing a degree of immiscibility in agreement with the conclusions of previous thermal analysis. With increasing mol % of MeDX in the copolymer, enhanced degree of miscibility is observed (Figure 5c). Thermal Stability of the Copolymers. Onset degradation temperatures for PDX and PCL homopolymers were recorded at 80.1 and 302.6 °C, respectively, while those of the diblock PCL-b-P(DX-co-MeDX) copolymers lie between the two (119.7 °C) but closer to the PDX homopolymer and lower than a PCL-b-PDX copolymer (150.8 °C), as shown by thermogravimetric (TG) curves (Figure 6). TG curves for PCL-b-PDX and PCL-b-P(DX-co-MeDX) show similar trends

We also observed a decrease in the crystallization temperature with an increase in the weight percent of P(DX-coMeDX) block to as low as 19 °C. It is known that, for PDX, the best nucleating agent is its own self-nuclei.15 “Antinucleating” effect in PDX is characterized by a decrease in its Tc and ΔHc as a result of a decrease in crystallinity. A significant drop in Tc and ΔHc was observed when MeDX was incorporated in PCLb-PDX (Table 2) showing its antinucleating effect on PDX. Surface Morphology of Copolymers. Block copolymers with immiscible blocks result in microphase separation which leads to nanoscale ordered morphology with structures such as spheres, gyroid, cylinders, and lamellae, depending on the volume fraction of the phases.16 Figure 5a shows a comparison of SEM images of PCL50, PCL50-b-PDX33, and PCL50-bP(DX34-co-MeDX10). A perturbation in the crystalline structure 12036



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degradation steps were calculated, and these were found to be in accordance with the weight percentage of PCL and P(DX-coMeDX) in the diblock copolymers as determined by 1H NMR. This also further confirms that the decomposition in the first stage was primarily due to the P(DX-co-MeDX) block and the second and third stages were due to the PCL block. The stepwise degradation suggested the existence of a phase separated system. The onset degradation temperature decreased with increasing P(DX-co-MeDX) block content. To confirm the stepwise degradation, one copolymer sample, PCL50-b-P(DX23-co-MeDX8), was subjected to FTIR and 1H NMR analysis. FTIR spectra of the copolymer were recorded before and after degradation at 300 °C. The carbonyl stretching at 1731 cm−1 and CH2 rocking at 850.51, 873.59, 930.6, and 969.85 cm−1 of the P(DX-co-MeDX) segment are no longer present in the spectrum after degradation. (See Figure S2 in Supporting Information for FTIR spectra.) This confirms preliminary degradation of the poly(ester-ether) segment. The residue after thermal degradation at 300 °C was analyzed by 1H NMR. In addition to absence of signals due to the poly(esterether) segment, the spectrum (Figure 8) also depicts the presence of peaks at δ = 5.0 ppm and 5.8 ppm assigned to hexenoic acid17 as well as peaks due to PCL segment, thus showing initial degradation of the latter. The stepwise degradation of the copolymer is here confirmed. The success of any implant or medical device depends greatly on precise control over the processing and processing conditions used during its manufacture. Langer et al.6 prepared PCL-b-PDX based shape memory objects using injection molding, blowing, extrusion, and laser ablation. It has been reported for polyesters that molding procedures involving thermal treatments such as melt encapsulation,18 extrusion,19 and thermal sterilization20 are known to induce degradation with a decrease of molecular weight.21 Hence, investigation of the thermal stability of our poly(ester-ether)s will enable one to choose the appropriate thermal processing method. 3.3. Kinetic Methods for the Calculation of Activation Energy. A more in-depth analysis of the nonisothermal degradation of PCL-b-P(DX-co-MeDX) copolymers was attempted by means of Friedman (FR),22 Kissinger− Akahira−Sunose (KAS), 23,24 and Flynn−Wall−Ozawa (FWO)25,26 isoconversional methods. A description of the kinetic methods used is provided in the Supporting Information. 3.4. Calculation of the Activation Energy. The activation energies were estimated using all three methods for comparison purposes. In the Friedman method, the activation

except for the fact that the incorporation of MeDX causes a displacement in the curve to lower temperatures. The thermogravimetric analysis (TGA) profiles for PCL-b-PDX and PCL-b-P(DX-co-MeDX) suggest that degradation occurs in three stages. The onset degradation temperatures of the first stage for PCL-b-PDX and PCL-b-P(DX-co-MeDX) were 150.8 and 125 °C, respectively. However, both copolymers showed similar onset degradation values for the second/third stages. This suggested that PCL segment degrades most probably during the second and third stages while the P(DX-co-MeDX) block degrades in the first stage. 3.2. Thermal Degradation. Figure 7 shows the DTG curves for PCL50 and PCL50-b-P(DX34-co-MeDX10). Similarly

Figure 7. DTG curve of PCL and PCL-b-P(DX-co-MeDX).

to the TGA profile, the presence of three peaks in the DTG also indicated that degradation proceeds in three stages. Persenaire et al.17 highlighted the two-step thermal degradation of PCL which proceeds first by rupture of the polyester bonds followed by an unzipping depolymerization process. Thus, the first stage of the degradation profile was assigned to the P(DXco-MeDX) segment and the last two stages to the PCL segment. Copolymers of varying weight percent of PCL and P(DX-coMeDX) were heated at a rate of 10 K/min, and the TG curves were recorded. The mass change, onset temperatures, and inflection temperatures were recorded for all copolymer samples. The results are summarized in Table 4. Weight losses ΔWb (first stage) and ΔWb′ (second and third stages) of the

Table 4. Summary of Results Obtained from Thermogravimetric Analysis of Copolymersa 1st stage decomposition

block content sample PCL50−OH PDX 1 3 4 5 6

PCL (wt %) PDX or P(DX-co-MeDX) (wt %) 100 0 74 82 80 64 56

0 100 26 18 20 36 44

2nd/3rd stage decomposition

T5% (°C)

Tonset (°C)

Tinflection (°C)

ΔWb (wt %)

258 119 151 163 110 133 98

302.6 80.1 150.8 140.5 134.4 125.0 119.7

402.9 212.4 203.0 168.1 177.5 177.1 174.7

45 98 24 14 21 35 50

Tonset (°C)

Tinflection (°C)

ΔWb′ (wt %)

386.6

402.9

49

373.0 377.5 375.4 376.1 372.0

406.0 403.6 403.5 402.0 378.3

75 85 73 63 46

a

T5% (°C): Temperature at which 5 wt % of polymers have decomposed. Tonset (°C): Onset temperature of degradation. Tinflection (°C): Inflection point temperature. 12037



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Figure 8. 1H NMR (CDCl3) of residue after thermal degradation at 300 °C, depicting the presence of alkene double bonds due to hexenoic acid.

activation energy of PDX was 127 KJ/mol while Ding et al.28 and Yang et al.29 obtained 81 and 89 KJ/mol, respectively. The higher value obtained by Nishida et al.27 can be attributed to the high molar mass of their PDX and the use of diethyl zinc as catalyst compared to stannous octanoate. Yang et al.29 also proposed that the difference between their calculated value and that of Nishida et al.27 was also due to the different chain ends resulting from the use of different catalysts. Figures 9 and 10

energies were calculated from the average gradient of ln (β(dx/ dT)) against 1/T for conversions 0.04−0.2 and 0.4−0.85 for the PCL-b-P(DX-co-MeDX) (100/0) copolymer (see Figures S3 and S4 in Supporting Information for graphs). In the Kissinger−Akahira−Sunose method, activation energy was obtained from the average gradient of ln (β/T2) against 1/ T for conversions 0.04−0.2 and 0.4−0.85 for the PCL-b-P(DXco-MeDX) (100/0) copolymer. (See Figures S5 and S6 in Supporting Information for graphs.) Activation energy from the Flynn−Wall−Ozawa method was obtained from the average gradient of ln β against 1/T for conversions 0.04−0.2 and 0.4−0.85 for the PCL-b-P(DX-coMeDX) (100/0) copolymer. (See Figures S7 and S8 in Supporting Information for graphs.) The mean values of activation energies (Ea) determined using the FR, KAS, and FWO methods for the P(DX-coMeDX) (100/0) segment were 68, 61, and 65 kJmol−1, respectively, while the Ea values for the PCL block were 168, 157, and 160 kJmol−1, respectively. Correlation coefficients between 0.90 and 0.999 show the applicability of the methods in the chosen conversion range. Table 5 summarizes the activation energies obtained using the FR, KAS, and FWO methods. The value of activation energy obtained for the PDX block was compared to literature. Nishida et al.27 found that the

Figure 9. Dependence of activation energy on conversion, x, for thermal degradation of PCL-b-P(DX-co-MeDX) (100/0) according to FR, KAS, and FWO methods.

Table 5. Activation Energies of PCL-b-P(DX-co-MeDX) (100/0) Calculated by FR, KAS, and FWO Methods 2nd/3rd stage degradation

1st stage degradation conversion

EFWO

EFR

EKAS

conversion

EFWO

EFR

EKAS

0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2

63 59 61 63 63 65 67 71 71

66 62 64 66 67 68 71 74 75

59 55 56 59 59 61 63 66 67

0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85

143 134 145 156 159 162 162 182 171 182

150 141 153 164 167 170 170 191 180 191

140 130 142 153 156 159 159 180 169 180

160

168

157

mean

65

68

61

Figure 10. Dependence of activation energy on conversion, x, for thermal degradation of PCL-b-P(DX-co-MeDX) (57/43) according to FR, KAS, and FWO methods. 12038



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catalyst. Raquez et al.34 proposed that degradation of PCL-bPDX occurred in two consecutive steps involving a first unzipping depolymerization of the PDX blocks followed by the degradation of the PCL blocks via both ester pyrolysis and unzipping reactions. As far as the thermal degradation mechanism of PCL is concerned, Persenaire et al.17 proposed that the thermal degradation of PCL proceeds in two steps whereby there is statistical rupture of the polyester chains followed by the formation of ε-caprolactone as a result of an unzipping depolymerization process. On the basis of 1H NMR analysis of one copolymer sample subjected to thermal degradation at 200 °C whereby CH2OH end groups are observed, it can be deduced that the P(DX-co-MeDX) segment also degrades by an unzipping process.

show the variation of the activation energies with the conversion x for the thermal degradation of PCL-b-P(DX-coMeDX) (100/0) and (57/43), respectively, based on the three isoconversional methods. The values of the apparent activation energies obtained by the FR method are slightly higher but more reliable than those obtained by the KAS and FWO methods.30 Irrespective of the methods used, the activation energy remains constant in two ranges of values of x. For instance, in the case of the PCL-bP(DX-co-MeDX) (57/43) copolymer, the activation energy remains practically constant in the 0.1−0.4 range, then increases, and stays almost constant in the 0.65−0.9 range. This is proof that pyrolysis proceeds in two major degradation stages, i.e., a first step involving the pyrolysis of the P(DX-coMeDX) segment followed by the thermal degradation of PCL. In fact, the two mechanisms involved in the thermal degradation of PCL occurred so closely on the temperature scale that it was impossible for us to determine the activation energies of the steps separately. In other words, the activation energy obtained for the PCL segment is the combined activation energy for both PCL thermal degradation stages.17 Activation energies were determined for PCL-b-P(DX-coMeDX) copolymers with varying DX/MeDX ratios. A linear relationship (linear regression parameter 0.995) was found to exist between the % incorporation of MeDX in the PCL-bP(DX-co-MeDX) copolymers and the activation energies of degradation of the P(DX-co-MeDX) block in the copolymers (Figure 11).

4. CONCLUSIONS Diblock poly(caprolactone)-b-poly(1,4-dioxan-2-one-co-3methyl-1,4-dioxan-2-one) copolymers with varying weight compositions of PCL and P(DX-co-MeDX) and varying mole % of DX to MeDX were successfully synthesized and fully characterized by NMR. DSC analysis further confirmed the block structure and poor miscibility of the two polymer sequences by the presence of two melting transitions. Tm values for both blocks decreased with increasing MeDX content in the copolymer, as a result of decreased crystallinity. The single crystallization exotherm obtained suggested that the P(DX-coMeDX) segment cocrystallized with the PCL block. The decrease in Tc and ΔHc showed the antinucleating effect of MeDX units on the PDX segment in the copolymer. The incorporation of MeDX in the copolymer reduced its thermal stability. The use of Kissinger−Akahira−Sunose, Friedman, and Flynn−Wall−Ozawa methods showed a linear decrease in the activation energy of degradation of the P(DX-co-MeDX) segment with increasing % incorporation of MeDX. It is expected that copolymers with low PCL contents and hence lower crystallinity will degrade faster, given that hydrolytic degradation generally occurs first in the amorphous regions followed by the crystalline segments. More in-depth studies on the hydrolytic degradation of the block copolymers are currently underway.

■

ASSOCIATED CONTENT

* Supporting Information

Figure 11. Graph showing linear relationship between Ea of P(DX-coMeDX) block and % incorporation of MeDX.

S

Figure S1: 13C NMR (CDCl3) of (A) PCL-OH and (B) PCLb-PDX. Figure S2: FTIR spectra of PCL50-b-P(DX23-coMeDX8) before (A) and after (B) thermal degradation at 300 °C. Figure S3: Friedman plots for PCL-b-P(DX-co-MeDX) (100/0) for conversions 0.04−0.2. Figure S4: Friedman plots for PCL-b-P(DX-co-MeDX) (100/0) for conversions 0.4−0.85. Figure S5: KAS plots for PCL-b-P(DX-co-MeDX) (100/0) for conversions 0.04−0.2. Figure S6: KAS plots for PCL-b-P(DXco-MeDX) (100/0) for conversions 0.4−0.85. Figure S7: FWO plots for PCL-b-P(DX-co-MeDX) (100/0) for conversions 0.04−0.2. Figure S8: FWO plots for PCL-b-P(DX-co-MeDX) (100/0) for conversions 0.4−0.85. Brief description of the kinetic methods. This information is available free of charge via the Internet at http://pubs.acs.org/.

3.5. Mechanism of Degradation. Nishida et al.27 proposed a scheme for the degradation process of PDX pyrolysis, according to which the latter proceeds mainly by the unzipping depolymerization initiated from chain ends. Random degradation also proceeds competitively. Unzipping depolymerization may be influenced by inter- and intramolecular transesterification as well as random degradation. Aliphatic poly(esters) are prone to inter- and intramolecular transesterifications, causing a broadening of the molecular weight distribution and also the formation of cyclic oligomers.30−33 The active species involved in the transesterification are mainly the terminal hydroxyl and carboxyl groups and their anions. Transesterifications occur in the pyrolysis of PDX, with the polymerization catalyst favoring the unzipping reaction. Kopinske et al.31 also claimed that depolymerization of PLA starts with the free hydroxyl groups at the chain ends of PLA, with selective lactide production reaction and tin acting as

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(13) Crescenzi, V.; Manzini, G.; Calzolari, G.; Borri, C. Thermodynamics of fusion of poly-β-propiolactone and poly(εcaprolactone). Comparative analysis of the melting of aliphatic polylactone and polyester chains. Eur. Polym. J. 1972, 8 (3), 449. (14) Albuerne, J.; Marquez, L.; Muller, A. J. Nucleation and crystallization in double crystalline poly(p-dioxanone)-b-poly(εcaprolactone) diblock copolymers. Macromolecules 2003, 36, 1633. (15) Sabino, M. A.; Ronca, G.; Muller, A. J. Heterogeneous nucleation and self-nucleation of poly(p-dioxanone). J. Mater. Sci. 2000, 35, 5071. (16) Bates, F. S.; Fredrikson, G. H. Block copolymers - Designer soft materials. Phys. Today 1999, 52, 32. (17) Persenaire, O.; Alexandre, M.; Degee, P.; Dubois, P. Mechanisms and kinetics of thermal degradation of poly(εcaprolactone). Biomacromolecules 2001, 2, 288. (18) Ranchandani, M.; Pankaskie, M.; Robinson, D. The influence of manufacturing procedure on the degradation of poly(lactidecoglycolide)-85/15 and poly(lactide-co-glycolide)-50/50 implants. J. Controlled Release 1997, 43, 161. (19) Widmer, M. S.; Gupta, P. K.; Lu, L. C.; Meszlenyi, R. K.; Evans, G. R. D.; Brandt, K.; Savelt, T.; Gurlek, A.; Patrick, C. W.; Mikos, A. G. Manufacture of porous biodegradable polymer conduits by an extrusion process for guided tissue regeneration. Biomaterials 1998, 19, 1945. (20) Gogolewski, S.; Mainil-Varlet, P. Effect of thermal-treatment on sterility, molecular and mechanical-properties of various polylactides. 2. Poly(l/d-lactide) and poly(l/dl-lactide). Biomaterials 1997, 18, 251. (21) Ramkumar, D. H. S.; Bhattacharya, M. Steady shear and dynamic properties of biodegradable polyesters. Polym. Eng. Sci. 1998, 38, 1426. (22) Friedman, H. Kinetics of thermal degradation of char-forming plastics from thermogravimetry. Application to a phenolic plastic. J. Poly. Sci., Part C: Polym. Lett. 1964, 6, 183. (23) Kissinger, H. E. Reaction kinetics in differential thermal analysis. Anal. Chem. 1957, 29, 1702. (24) Akahira, T.; Sunose, T. Trans joint convention of Four Electrical Institutes, paper no. 246, 1969 research report; (Chiba Institute of Technology) Sci Technol, 1971; Vol. 16, p 22. (25) Flynn, J.; Wall, L. A. Quick direct method for the determination of activation energy from thermogravimetric data. Polym. Lett. 1966, 4, 323. (26) Ozawa, T. A new method of analyzing thermogravimetric data. Bull. Chem. Soc. Jpn. 1965, 38, 1881. (27) Nishida, H.; Yamashita, M.; Hattori, N.; Endo, T.; Tokiwa, Y. Thermal decomposition of poly(1,4-dioxan-2-one). Polym. Degrad. Stab. 2000, 70 (3), 485. (28) Ding, S. D.; Wang, Y. Z. Enhanced thermal stability of poly(1,4dioxan-2-one) in melt by adding a chelator. Polym. Degrad. Stab. 2006, 91, 2465. (29) Yang, K. K.; Wang, X. L.; Wang, Y. Z.; Wu, B.; Jin, Y. D.; Yang, B. Kinetics of thermal degradation and thermal oxidative degradation of poly(p-dioxanone). Eur. Polym. J. 2003, 39, 1567. (30) Vyazovkin, S. Modification of the integral isoconversional method to account for variation in the activation energy. J. Comput. Chem. 2001, 22, 178. (31) Kopinske, F. D.; Remmler, M.; Mackenzie, K.; Möder, M.; Wachsen, O. Thermal decomposition of biodegradable polyesters-II. Poly(lactic acid). Polym. Degrad. Stab. 1996, 53, 329. (32) Hofman, A.; Slomkowski, S.; Penczek, S. Polymerization of εcaprolactone with kinetic suppression of macrocycles. Macromol. Chem. Rapid Commun. 1987, 8, 387. (33) Melchiors, M.; Keul, H.; Höcker, H. Depolymerization of poly[(R)-3-hydroxybutyrate] to cyclic oligomers and polymerization of the cyclic trimer : An example of thermodynamic recycling. Macromolecules 1996, 29, 6442. (34) Raquez, J. M.; Degée, Ph.; Narayan, R.; Dubois, Ph. Diblock copolymers based on 1,4-dioxan-2-one and ε-caprolactone: Characterization and thermal properties. Macromol. Chem. Phys. 2004, 205 (13), 1764.

The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript and have contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the Tertiary Education Commission (Mauritius) for providing an MPhil/PhD scholarship to N.G.

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ABBREVIATIONS PDX = polydioxanone FDA = Food and Drug Administration PCL = polycaprolactone DX = 1,4-dioxan-2-one MeDX = 3-methyl-1,4-dioxan-2-one THF = tetrahydrofuran PLA = polylactides COPOL = copolymer HOMOPOL = homopolymer TGA = thermogravimetric analysis FR = Friedman KAS = Kissinger−Akahira−Sunose FWO = Flynn−Wall−Ozawa

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REFERENCES

(1) Ray, J. A.; Doddi, N.; Regula, D.; Williams, J. A.; Melveger, A. Polydioxanone (PDS), a novel monofilament synthetic absorbable suture. Surg. Gynecol. Obstet. 1981, 153 (4), 497. (2) Kalangos, A. Polydioxanone polymers for annuloplasty in valve repair. Macromol. Symp. 2005, 231 (1), 81. (3) Kalangos, A.; Sierra, J.; Vala, D.; Cikirikcioglu, M.; Walpoth, B.; Orrit, X.; Pomar, J.; Mestres, C.; Albanese, S.; Jhurry, D. Annuloplasty for valve repair with a new biodegradable ring: An experimental study. J. Heart Valve Dis. 2006, 15, 783. (4) Zeng, J. B.; Srinivansan, M.; Li, S. L.; Narayan, R.; Wang, Y. Z. Nonisothermal and isothermal cold crystallization behaviors of biodegradable poly(p-dioxanone). Ind. Eng. Chem. Res. 2011, 50, 4471. (5) Albuerne, J.; Marquez, L.; Muller, A. J.; Raquez, J. M.; Degee, P.; Dubois, P. Hydrolytic degradation of double crystalline PPDX-b-PCL diblock copolymers. Macromol. Chem. Phys. 2005, 206, 903. (6) Langer, R. S.; Lendlein, A. Shape memory polymers. US Patent 0055198A1, April 13, 2004. (7) Langer, R.; Tirrell, D. A. Designing materials for biology and medicine. Nature 2004, 428, 487. (8) Kratz, K.; Habermann, R.; Becker, T.; Richau, K.; Lendlein, A. Shape-memory properties and degradation behavior of multifunctional electro-spun scaffolds. Int. J. Artif. Organs 2011, 34 (2), 225. (9) Lochee, Y.; Jhurry, D.; Bhaw-Luximon, A.; Kalangos, A. Biodegradable poly(ester-ether)s: ring-opening polymerization of D,L-3-methyl-1,4-dioxan-2-one using various initator systems. Polym. Int. 2010, 59, 1310. (10) Lochee, Y.; Bhaw-Luximon, A.; Jhurry, D.; Kalangos, A. Novel biodegradable poly(ester-ether)s: Copolymers from 1,4 dioxan-2-one and D,L-3-methyl-1,4-dioxan-2-one. Macromolecules 2009, 42 (19), 7285. (11) Wolfe, P. S.; Lochee, Y.; Jhurry, D.; Bhaw-Luximon, A.; Bowlin, G. L. Characterization of electrospun novel poly(ester-ether) copolymers: 1,4-dioxan-2-one and D,L-3-methyl-1,4-dioxan-2-one. J. Eng. Fibers Fabr. 2011, 6 (4), 60−69. (12) Brito, Y.; Sabino, M. A.; Gladys, S.; Albuerne, J.; Muller, A. J. Study of the miscibility, morphology and crystallization of the mixtures of poly(p-dioxanone)-poly(ε-caprolactone). Rev. Latinoam. Metal. Mater. 2006, 26, 1. 12040


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