Isothermal Crystallization Behavior of Biodegradable P (BS-b-PEGS

Article pubs.acs.org/IECR

Isothermal Crystallization Behavior of Biodegradable P(BS-b-PEGS) Multiblock Copolymers Xi Lu, Jian-Bing Zeng,* Cai-Li Huang, and Yu-Zhong Wang Center for Degradable and Flame-Retardant Polymeric Materials (ERCPM-MoE), 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: Biodegradable multiblock copolymers poly(butylene succinate-b-poly(ethylene glycol) succinate) (P(BS-bPEGS)) were prepared by direct polycondensation. The thermal and crystallization behavior of P(BS-b-PEGS) with weight fraction of PEGS component from 6 to 26 wt % were comparatively investigated with those of neat poly(butylene succinate) (PBS) by wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), and polarized optical microscopy (POM). P(BS-b-PEGS) showed the same crystal structure as neat PBS, but the degree of crystallinity of copolymers was lower than that of neat PBS and decreased with increase in PEGS content. The isothermal crystallization kinetics study suggests that incorporation of PEGS component did not change the crystallization mechanism, but reduced the crystallization rate of the samples, and that increasing crystallization temperature decreased the crystallization rate of all the samples. The spherulites of neat PBS and P(BS-b-PEGS) showed banded morphologies. The spherulitic growth rate of the samples also decreased with increase of PEGS content and crystallization temperature. A transition from crystallization regime II to III occurred for all the samples, and the transition shifted to lower temperatures with increase in PEGS content.

1. INTRODUCTION As a biodegradable polymer, poly(butylene succinate) (PBS) has attracted increasing attention in recent years due to its excellent mechanical properties, biodegradability, and potential renewability.1−4 The biodegradation rate of PBS is relatively slow in comparison with that of other aliphatic polyesters such as poly(trimethylene succinate) (PTS),5 poly(ε-caprolactone) (PCL), and bacterial polyesters.6,7 The biodegradabilities of polymers are dependent on their chemical structures, and the biodegradation rates of biodegradable polymers are significantly influenced by their crystallization behavior such as degree of crystallinity, crystal structures, and crystalline morphologies.8−18 In addition, crystallization behavior also affects the mechanical properties of polymeric materials considerably.19,20 Therefore, the crystallization behavior of PBS including crystal structure, crystalline morphology, spherulite size, crystallinity, and crystallization kinetics have been widely investigated in recent literature.21−28 It is reported that the degree of crystallinity affects the degradation rate of PBS more than other factors.29 Thus methods, such as incorporation of a third monomer, which are able to reduce the degree of crystallinity of PBS, can be applied to accelerate its biodegradation rate. Several co-monomers such as ethylene glycol,30−33 trimethylene glycol,34,35 cyclic carbonate,36,37 and adipic acid38−41 have been reported to copolymerize with succinic acid and 1,4-butanediol. The degree of crystallinity of copolymers decreased with incorporation of the comonomers, and the copolymers showed faster degradation rates than neat PBS. The crystalline morphology could also affect the biodegradation rate of PBS. It was reported that the isothermally crystallized PBS showed faster hydrolytic degradation rate than © 2012 American Chemical Society

the melt-quenched PBS although the two samples have similar degree of crystallinity. The difference in biodegradation rates of the two samples was caused by the different internal spherulite morphology formed under different crystallization conditions. The spherulites of isothermally crystallized sample showed coarse and loosely packed fibrils whereas those of the meltquenched sample displayed finer and tightly packed fibrils. The spherulites with the former morphology are more easily attacked by water molecules during hydrolytic degradation than the ones with the latter morphology.42 Hydrophilic/hydrophobic balance also affects the biodegradation rates of polymers especially in enzymatic and hydrolytic degradation.34 The molecular chain of PBS is known to be hydrophobic. Its biodegradation rate may be increased by incorporation of hydrophilic molecular chains to increase its hydrophilicity. It is reported that a polyurethane prepared using PBS and poly(ethylene glycol) (PEG) as the prepolymers showed accelerated degradation rate in comparison with the polyurethane prepared with PBS as the only prepolymer, which was partially caused by increased water affinity after incorporation of the PEG hydrophilic segments.42 Another copolymer (P(BS-b-PEGS) synthesized by condensation polymerization of PEG, succinic acid, and 1,4-butanediol also showed similar degradation behaviors with increase of PEG content.43,44 Although the synthesis and degradation of this copolymer have been reported, the important crystallization behavior of the copolymers, as described above being very Received: Revised: Accepted: Published: 8262

February 2, 2012 May 23, 2012 June 1, 2012 June 1, 2012 dx.doi.org/10.1021/ie300289b | Ind. Eng. Chem. Res. 2012, 51, 8262−8272

Industrial & Engineering Chemistry Research

Article

Figure 1. 1H NMR spectrum and peak assignments of P(BS-b-26 PEGS).

was introduced into the flask, and the polycondensation was continued at 220 °C with vacuum of 30 Pa for 5 h. The obtained products were purified by dissolving in chloroform and precipitating in excessive methanol. The precipitate was washed with methanol several times and then dried to constant weight in vacuum at 40 °C. 2.3. Proton Nuclear Magnetic Resonance (1H NMR) Spectroscopy. 1H NMR spectra of the polymers were recorded on a Bruker AC-P 400 MHz spectrometer at ambient temperature in CDCl3 with tetramethylsilane as the internal reference. 2.4. Gel Permeation Chromatography (GPC). GPC was performed on a Waters instrument, equipped with a model 1515 pump, a model 717 autosampler, and a model 2414 refractive index detector. CHCl3 and polystyrene were used as the eluent and standard, respectively. The flow rate of eluent and the concentration of samples were 1.0 mL/min and 0.25 mg/mL, respectively. The experiments were carried out at 35 °C. 2.5. Wide-Angle X-ray Diffraction (WAXD). WAXD patterns of the polymers were recorded by an X-ray diffractometer (Philips X’Pert X-ray diffractometer) with Cu Kα radiation at room temperature with a scan rate of 2°/min scanning from 10° to 40°. The purified product powders were used for the WAXD experiments. 2.6. Differential Scanning Calorimetry (DSC). Thermal analysis was performed on a TA Instruments differential scanning calorimeter (DSC) Q200 with a Universal Analysis 2000 program. For measuring glass transition temperatures, 5mg samples in aluminum pans were first held at 140 °C for 3 min to eliminate any thermal history then quenched to −70 °C with liquid nitrogen, and finally the samples were heated to −20 °C at a rate of 20 °C/min. To determine the crystallization and melting temperatures, the samples were first annealed at 140 °C for 3 min to erase thermal history, then cooled to −20 °C at a rate of 10 °C/min, and finally heated to 140 °C at the same rate. Both the cooling and heating scans were recorded for analysis. The isothermal crystallization was carried out with the same DSC instrument. The samples were first melted at

important to the biodegradation behavior, have not yet been reported. Thus, it is essential to study the crystallization behavior of these copolymers which would be helpful to provide predictive information on their degradation behavior. In the present study, we prepared neat PBS and P(BS-bPEGS) multiblock copolymers with various content of PEGS component and systematically investigated the effects of polymer composition and crystallization temperatures on the crystallization kinetics, crystal structures, crystalline morphology, and spherulitic growth kinetics of a series of P(BS-bPEGS) copolymers. The crystallization behavior of PBS-based random copolymers has been widely investigated; however, not much attention has been focused on PBS-based multiblock copolymers. The results reported in this paper are supposed to be meaningful for further understanding the relationships among the chemical structure, crystallization behavior, and physical properties of biodegradable polymers, especially of the biodegradable multiblock copolymers.

2. EXPERIMENTAL SECTION 2.1. Materials. Neat PBS and the P(BS-b-PEGS) copolymers were synthesized by condensation polymerization of succinic acid (SA) with 1,4-butanediol (BDO) and/or poly(ethylene glycol) (PEG, Mn = 1000 g/mol). SA, BDO, and PEG were all procured from Kelong Chemical Corporation (Chengdu, China) and used without any pruification. Tetrabutyl titanate (Ti(OBu)4 solution, Sinopharm Chemical Reagent Corporation, Shanghai, China) with concentration of 0.2 g/mL was prepared by dissolving in anhydrous toluene. All other chemicals with reagent grade were used as received. 2.2. Synthesis of PBS and Its Multiblock Copolymers. The polymers were synthesized via a conventional two-step bulk condensation polymerization method, i.e., esterification and subsequent polycondensation. Predetermined amounts of SA, BDO, and PEG were charged into a 250-mL three-necked round-bottom flask equipped with mechanical stirrer, water separator, and nitrogen inlet pipe. The molar ratio of (BDO + PEG)/SA was 1.05/1. The esterification was carried out at 180 °C for 4 h. Then the catalyst with 0.1 wt % of the total reactants 8263

dx.doi.org/10.1021/ie300289b | Ind. Eng. Chem. Res. 2012, 51, 8262−8272


Article

140 °C for 3 min to erase thermal history and cooled to the predetermined crystallization temperature (Tc) at cooling rate of 60 °C/min. The exotherms were recorded for analysis. After the crystallization was complete, the samples were heated to 140 °C at a heating rate of 20 °C/min to investigate the melting behaviors and calculate the equilibrium melting temperatures. All the experiments were carried out under nitrogen atmosphere with 50 mL/min flow rate. 2.7. Polarized Optical Microscopy (POM). Spherulitic growth kinetics and morphology of the polymers were studied with a polarized optical microscope (POM) (Nikon Eclipse LV100POL) equipped with a hot stage (HSC621 V). Sample films were pressed between two microscopic cover glasses at 140 °C. The samples were first melted at 140 °C for 3 min to eliminate thermal history and quenched to a given temperature to observe the spherulitic growth. The radii of spherulites during the crystallization process were recorded and the spherulitic growth rates were obtained from the slopes of the plots of spherulitic radii versus crystallization time.

3.2. Thermal Behavior and Crystal Structure. The glass transition temperature (Tg) was obtained from the DSC heating scans of the melt-quenched sample with a heating rate of 20 °C/min. The results are summarized in Figure 2 and

3. RESULTS AND DISCUSSION 3.1. Chemical Structure and Compositions. PBS and its copolymers were synthesized through a two-step procedure as described in the Experimental Section. The chemical structure and composition of the polymers were confirmed by 1H NMR. Taking P(BS-b-26 wt % PEGS) for an example, Figure 1 shows its chemical structure, 1H NMR spectrum, and signal assignments. The signal at 2.66 (δHa) ppm was assigned to the methylene proton of SA unit in the polymer, and those at 1.72 (δHb) and 4.12 (δHc) ppm were caused by the two different methylene protons of BDO unit. The peaks of methylene protons of PEG repeating unit were observed at 3.64 ppm (δHd). The two small peaks at 3.82 (δHd′) and 4.25 (δHd″) ppm were ascribed to the two methylene protons of PEG terminus linked with SA. The compositions of the polymers could be deduced from the peak areas of Hc and Hd. The results are listed in Table 1. It is obvious that the weight

Figure 2. DSC heating scans of the polymers for Tg analysis.

Table 2. Thermal Properties and Degree of Crystallinity of Neat PBS and Its Copolymers Xc (%)

sample neat PBS P(BS-b-6 wt % PEGS) P(BS-b-12 wt % PEGS) P(BS-b-26 wt % PEGS)

BS/PEGS weight ratio in copolymer

Mn (g/mol)

PDI

55.5/44.5/0 53/42/5

100/0 94/6

5.44 × 104 3.78 × 104

1.84 1.95

50.1/39.5/10

88/12

4.55 × 104

2.03

45.4/34.6/20

74/26

3.41 × 104

1.98

Tg (°C)

Tc (°C)

ΔHc (J/g)

Tm (°C)

ΔHm (J/g)

DSC

WAXD

neat PBS P(BS-b-6 wt % PEGS) P(BS-b-12 wt % PEGS) P(BS-b-26 wt % PEGS)

−37.7 −39.0

80.5 70.2

69.9 56.7

114.1 112.3

70.9 62.9

64.2 57.0

62.3 54.9

−40.1

66.3

52.1

110.5

54.8

49.6

39.6

−41.4

58.2

48.8

104.6

48.7

44.1

33.6

Table 2. Neat PBS showed a Tg at −37.7 °C, which decreased to −39.0, −40.1, and −41.4 °C for the copolymers with weight fraction of PEGS increasing from 6% to 12% and 26%. The result is expected since PEGS is a flexible segment with lower glass transition. The crystallization peak temperature (Tc) and melting temperature (Tm) of the polymers were measured at a cooling and heating rate of 10 °C/min, and the DSC heating and cooling thermograms are shown in Figure 3. All the copolymers showed single crystallization and melting peaks, which were close to that of neat PBS but shifted to lower temperature with increase of PEGS component. The results suggest that the crystallization of the copolymers was mainly arisen from the PBS segment. PEG itself is able to crystallize, but in the present study its crystallization was not observed mainly owing to the suppression effect caused by copolymerization and its small content in the copolymers. The value of Tc for neat PBS was 80.5 °C, and it gradually decreased to 70.2, 66.3, and 58.2 °C for P(BS-b-6 wt % PEGS), P(BS-b-12 wt % PEGS), and P(BS-b-26 wt % PEGS). The crystallization enthalpy (ΔHc) of PBS was 69.9 J/g, which decreased to 56.7, 52.1, and 48.8 J/g with increasing PEGS from 6% to 26%. The value of Tm for neat PBS was 114.1 °C, which shifted to lower temperatures with increase of PEGS component in the copolymer. Tm decreased to 112.3, 110.5, and 104.6 °C as PEGS weight fraction increased from 6 to 12 and 26%, which

Table 1. Composition and Molecular Weight of Neat PBS and Its Copolymers SA/BD/PEG weight ratio in feed

sample

fractions of PEGS in the copolymers were higher than those in the feed; this is reasonable since SA and BD deducted more amount of water during polycondensation due to their low molecular weights compared with PEG, and for the relatively low boiling point, BD is easier to be evaporated from the reaction system during the reaction. The molecular weights of the polymers were measured by GPC and the results are also listed in Table 1. The block lengths of PBS segment, calculated by the method reported in our previous study,45 in P(BS-co-6 wt % DEGS), P(BS-co-12 wt % DEGS), and P(BS-co-26 wt % DEGS) were 90, 42, and 16, respectively. 8264



Article

Figure 3. DSC cooling (A) and heating (B) scans of the polymers.

and PEGS did not change the crystal structure but reduced the crystal size, crystallization rates, and degree of crystallinity of the polymers. 3.3. Isothermal Crystallization Kinetics. To investigate the effects of the weight fraction of PEGS on the crystallization rates and mechanisms of the multiblock copolymers in detail, the isothermal crystallization kinetics of neat PBS were comparatively investigated with the multiblock copolymers. Figure 5a and b show the plots of relative crystallinity (Xt) as a function of crystallization time at various crystallization temperatures (Tc) for neat PBS and P(BS-b-26 PEGS), respectively. It can be seen that more time was required for the polymer to crystallize completely as crystallization temperature increased, suggesting that the crystallization rates decreased with increase of Tc. It should be more interesting to investigate the effect of PEGS weight fraction on the crystallization behavior of the copolymers. In this sense, the plots of Xt versus crystallization time at Tc of 86 °C for neat PBS and the copolymers are graphically shown in Figure 6. It can be seen the time needed to complete crystallization increased from 6.7 to 10.1, 20.5, and 88 min for neat PBS and the copolymers with increasing weight fraction of PEGS, suggesting the crystallization of the polymers was retarded with incorporation and increase of PEGS component. The results are probably caused by the two effects resulted from incorporation of PEGS: first, PEGS plays a dilute effect to the crystallization of PBS segments; second, for crystallization at the same temperature, the supercooling (Tm − Tc) decrease with increase in PEGS content since the melting temperature of copolymer decreased with PEGS content. Higher supercooling usually causes higher crystallization rate. The Avrami equation is the most conventional method to study the isothermal crystallization kinetics of crystalline polymers, which assumes that the relative crystallinity develops with crystallization time t as

was attributed to the increasing disturbance of PBS molecular chain by increase in PEGS content. And the fusion enthalpy (ΔHm) of neat PBS was 70.9 J/g, which also decreased with increase of PEGS, being 62.9, 54.8, and 48.7 J/g for P(BS-b-6 wt % PEGS), P(BS-b-12 wt % PEGS), and P(BS-b-26 wt % PEGS). The degree of crystallinity (Xc) can be calculated by: Xc = ΔHm/ΔHm0, where ΔHm0 is the theoretical value for perfectly (100%) crystalline polymer. For neat PBS, ΔHm0 is 110.4 J/g.46 The same value is also used for the copolymers due to the limited crystallization of PEGS in the copolymer. The Xc of the polymers decreased with increase in PEGS content, as shown in Table 2. The crystal structures of neat PBS and the copolymers were analyzed by WAXD. Figure 4 shows the WAXD patterns of the

Figure 4. WAXD patterns of neat PBS and the copolymers.

polymers. Neat PBS shows three strong diffraction peaks located at 2θ values of 19.6, 21.9, and 22.6° corresponding to (020), (021), and (110) planes.30 The copolymers display diffraction peaks similar to that of neat PBS, however, the peak intensities of the copolymers decreased with increase in PEGS content. The results suggest that incorporation of PEGS to PBS did not change the crystal structures but reduced the crystallite size and Xc of the polymer. The values of Xc could be calculated through deconvolution of crystalline and amorphous peaks in the WAXD pattern using the peak separation software. The results are lower than those obtained by DSC analysis, as shown in Table 2; however, they showed the same trend with increase in PEGS content. All the results indicate that the crystallization of the copolymers originated from PBS block,

1 − X t = exp( −kt n)

(1)

where Xt is the relative crystallinity at time t, k is a crystallization rate constant depending on nucleation and crystalline growth rate, and n is the Avrami exponent which denotes the nature of the nucleation and growth process.47,48 Equation 1 can be rewritten as log[− ln(1 − X t )] = log k + n log t 8265

(2)



Article

Figure 5. Plots of relative crystallinity versus crystallization time at various crystallization temperatures for neat PBS (A) and P(BS-b-26 wt % PEGS) (B).

crystallization temperatures, indicating that the Avrami equation could be reasonably used to describe the isothermal crystallization kinetics of neat PBS and P(BS-b-26 wt % PEGS). The use of the Avrami equation to analyze the isothermal crystallization of P(BS-b-6 wt % PEGS) and P(BS-b-12 wt % PEGS) also generated parallel straight lines. The Avrami parameters k and n of all samples were obtained from the slopes and intercepts of the Avrmai plots. The n values (as shown in Table S1 of Supporting Information) for the four samples were between 2.1 and 2.7 within the crystallization temperature range involved in the present study, which suggests that the crystallization kinetics of PBS and the copolymers might correspond to three-dimensional truncated spherulitic growth with athermal nucleation32,49 and incorporation of PEGS component would not change the crystallization kinetics mechanism. Since the values of n are different with different compositions, it is inappropriate to compare the overall crystallization rates of neat PBS with the copolymers directly from the values of rate constants. The half-time of crystallization (t1/2), defined as the time needed to achieve 50% of the final crystallinity, is thus calculated to describe isothermal crystallization kinetics. The value of t1/2 can be deduced from the following equation:

Figure 6. Plots of relative crystallinity versus crystallization time at 86 °C for neat PBS and the copolymers.

A plot of log[−ln(1 − Xt)] versus log t would give a straight line from which both the rate constant and the Avrami exponent can be derived. In the present study, the isothermal crystallization kinetics of neat PBS and the copolymers were also analyzed by the Avrami equation. Figure 7A and B illustrate the Avrami plots of neat PBS and P(BS-b-26 wt % PEGS), respectively. A series of parallel straight lines of both neat PBS and P(BS-b-26 wt % PEGS) was obtained for various

Figure 7. Avrami plots of neat PBS (A) and P(BS-b-26 wt %PEGS) (B). 8266



Article

Figure 8. (A) t1/2 and (B) 1/t1/2 of neat PBS and the copolymers at various crystallization temperatures.

t1/2 =

⎛ ln 2 ⎞1/ n ⎜ ⎟ ⎝ k ⎠

As mentioned in the Experimental Section, the subsequent heating curves of PBS and the copolymers after isothermal crystallization at various Tc were recorded for equilibrium melting point temperatures. Figure 9 shows the melting curves

(3)

Reciprocal value of t1/2, i.e., 1/t1/2 is usually employed to represent the overall crystallization rates of polymers. According to the values of n and k, the values of t1/2 and 1/ t1/2 were calculated and are summarized in Figure 8. It can be seen that t1/2 increased while 1/t1/2 decreased with increase of Tc for all the samples, suggesting that the crystallization rates of the samples decreased with Tc, which is very common in the isothermal crystallization of polymer and can be ascribed to the fact the isothermal crystallization of the samples is a nucleation controlled process within the crystallization temperature range involved in the present study. It is worth noting that the crystallization rates of the samples decreased with increasing PEGS content, which can be ascribed to the two factors caused by the incorporation and increase of PEGS component. First, PEGS component is excluded to the amorphous phase, thus it works as a diluting factor for the crystallization of PBS chains. Second, the equilibrium melting point temperatures (Tm0) of the copolymers decreased with increased content of PEGS, as discussed in the next section. Thus, the supercooling (Tm0 − Tc), known as the driving force of isothermal crystallization, for the same given Tc decreased with increase of PEGS, which would also result in slowing of crystallization rates. 3.4. Melting Behavior and Equilibrium Melting Point Tm0. The equilibrium melting temperature Tm0 is of great importance for a polymer in investigating crystallization kinetics since the supercooling ΔT, which is a crucial parameter for the growth rate of crystal, can only be determined on the basis of the Tm0. In the present study, the effect of content of PEGS on the equilibrium melting temperature of the copolymers was investigated by DSC. The Hoffman−Weeks method was employed to calculate the Tm0 of the samples. The method is given as50 ⎛ 1⎞ T Tm = Tm0⎜1 − ⎟ + c γ⎠ γ ⎝

Figure 9. Melting curves of P(BS-b-26 wt % PEGS) after crystallization at various temperatures.

of P(BS-b-12 wt % PEGS) after isothermally crystallizing at various temperatures as an example. The samples showed two melting peaks at relatively lower and higher temperatures, i.e., Tm1 and Tm2, after crystallizing at 78, 82, 86, 90, and 94 °C. The values of Tm1 shifted from 96.6 to 105.9 °C and those of Tm2 almost remain unchanged at around 110.3 °C with various crystallization temperatures. From the peak areas of Tm1 and Tm2, we can see that Tm2 was the dominant peak at relatively lower Tc, and Tm1 was dominant at relatively higher Tc, which might be ascribed to the mechanism of melting, recrystallization, and remelting behaviors of the sample.32 Peak 1 was the melting of the crystals formed during the primary crystallization at Tc, and peak 2 was the melting of the crystals formed by reorganization of crystals during heating process in DSC measurement. Thus, Tm1 was employed for calculating Tm0 with the Hoffman−Weeks equation. Figure 10 shows the Hoffman−Weeks plots of neat PBS and the copolymers. It can be seen that at the same crystallization temperature, the melting temperature of the samples decreased with the weight fraction of PEGS units, implying that the incorporation of PEGS units causes the decrease of observed melting temperature. The Tm0 of neat PBS was calculated to be

(4)

where Tm is the observed melting temperature of a crystal formed at crystallization temperature Tc, γ is the ratio of final to initial lamellar thickness. Plotting the Tm as a function of Tc can give a straight line. And the Tm0 can then be obtained from the intersection of this line withthe Tm = Tc line. 8267



Article

crystallization temperatures observed by POM. The spherulites showed banded morphology with characteristic “Maltese Cross” extinction patterns regardless of crystallization temperature. The number of spherulites decreased and the size of spherulites increased with increasing Tc, which was caused by the fact that the nucleation becomes difficult and thus the number of nuclei decreased with increase of Tc. It is worth noting that the band spacing of the spherulites increased with the Tc. Banded spherulites are thought to result from the periodic twisting of lamellae along the growth direction.32 The increase in band spacing with Tc was observed for many polymers, and the banded morphology may even disappear at high temperatures. Similar results were also obtained for neat PBS and the other two copolymers after isothermal crystallization at various crystallization temperatures. The effect of weight fraction of PEGS on the crystalline morphology and spherulitic growth of the samples was further analyzed. Figure 12 shows the crystallization morphology of the samples formed at the same crystallization temperature of 80 °C. The four samples formed well-defined banded spherulites with Maltese Cross extinction. The spherulitic sizes increased and spherulitic numbers decreased with increase in weight fraction of PEGS in the polymers, suggesting that the nucleation effects decreased with increase of PEGS weight fraction, which is reasonable since the molecular chain regularity was increasingly disturbed and the concentration of crystalline chains was diluted with increase of PEGS. The decreased supercooling caused by the depression of equilibrium melting temperatures by incorporation of PEGS is another possible reason for the results. In addition, we can find the band spacing increased with weight fraction of PEGS, which could also be attributed to the decrease of supercooling at a given Tc with increase of PEGS.

Figure 10. Hoffman−Weeks plots for calculating Tm0 of neat PBS and the copolymers.

126.2 °C, which approached to the results reported by Gan30 and Qiu.32 And the Tm0 values of the copolymers are 124.8, 121.5, and 117.2 °C for P(BS-b-6 wt % PEGS), P(BS-b-12 wt % PEGS), and P(BS-b-26 wt % PEGS) (as shown in Table S2 of Supporting Information), respectively. The results suggest that incorporation of PEGS segments would reduce the equilibrium melting temperature of PBS. 3.5. Crystalline Morphology and Spherulitic Growth. For biodegradable crystalline polymers, the crystalline morphology and size are of importance in determining the physical properties and biodegradation rate and mechanisms. Thus, a lot of work has been done to investigate the crystalline morphology of biodegradable polymers. In the present study, the effects of crystallization temperature and composition on the crystalline morphology and spherulitic growth of the samples have been studied. Figure 11 shows the crystalline morphology of P(BS-b-26 wt % PEGS) crystallized at various

Figure 11. Crystalline morphology of P(BS-b-26 wt % PEGS) formed at various crystallization temperatures: (A) 65; (B) 70; (C) 75; (D) 80 °C. 8268



Article

Figure 12. Crystalline morphology of the samples formed at 80 °C: (A) neat PBS; (B) P(BS-b-6 wt % PEGS); (C) P(BS-b-12 wt % PEGS); (D) P(BS-b-26 wt % PEGS).

The effects of crystallization temperature and polymer composition on the crystallization kinetics of the samples were investigated in the isothermal crystallization section. These effects on the spherulitic growth kinetics of the polymers were also investigated in the present study using POM measurements. The spherulitic growth rates of isothermally melt-crystallized samples at a given Tc were measured by the increase of radius with time. The spherulitic growth rates were obtained from the slopes of the plots of crystal radius vs crystallization time. Figure 13 shows the dependence of spherulitic growth rate on the crystallization temperature for neat PBS and the copolymers. It is found that the spherulitic growth rate of all the samples increased with decrease of Tc and the spherulitic growth rate decreased with the increase in the weight fraction of PEGS at a given Tc. The results are in accordance with the change in the crystallization rates obtained

from Figure 8B. The decrease of spherulitic growth rates with increase of PEGS was ascribed to the diluent effect of the PEGS units in the copolymers. The detailed information on the spherulitic growth rates of neat PBS and the copolymers was further analyzed by the Lauritzen−Hoffman (LH) method,51 known as the secondary nucleation theory, according to which the spherulitic growth rate G at a crystallization temperature Tc can be represented by the following equation: ⎡ ⎤ ⎡ Kg ⎤ U* G = G0exp⎢ − ⎥exp⎢ − ⎥ ⎣ R(Tc − T∞) ⎦ ⎣ Tc(ΔT )f ⎦

(5)

where G0 is a pre-exponential factor practically independent of temperature, U* is the activation energy of the molecular transfer through the melt-crystal interface, R is the gas constant, T∞ is the temperature below which polymer chain motion stops, ΔT is the degree of supercooling defined as Tm0 − Tc where Tm0 is the equilibrium melting temperature, f is a correction factor for the temperature dependence of the enthalpy of fusion, described as 2Tc/(Tm0 + Tc), and Kg is the nucleation constant, as depicted by Kg =

nb0σσeTm0 ΔHf k

(6)

where n value is dependent on the crystallization regime, σ and σe are the lateral and end-surface free energies, respectively, b0 is molecular thickness, ΔHf is the fusion enthalpy per unit volume, and k is the Boltzmann constant. After taking the logarithmic form, eq 4 can be rewritten as ln G +

Figure 13. Dependence of spherulitic growth rate G on the crystallization temperature Tc for neat PBS and the copolymers. 8269

Kg U* = ln G0 − Tc(ΔT )f R(Tc − T∞)

(7)



Article

Figure 14. Lauritzen−Hoffman plots of neat PBS (A) and P(BS-b-12 wt % PEGS) (B).

the weight fraction of PEGS, which was attributed to the depression of equilibrium melting temperature with incorporation of PEGS component.

To realize the effect of nucleation process on the crystalline morphology, two important processes, the diffusion of polymer segment and secondary nucleation, and their rates should be taken into account. Three crystallization regimes proposed by Hoffman during isothermal crystallization at various temperatures are regimes I, II, and III.50 The rate of diffusion is much faster than that of secondary nucleation in regime I, and thus only one nucleus is formed and diffuses the layer before the formation of the next nucleus in this regime. In regime II, the rate of secondary nucleation is comparable with that of diffusion. And in regime III, the rate of secondary nucleation is much faster than that of diffusion. The n values are 4 for regimes I and III and 2 for regime II. Regime transitions usually happen to polymers and characteristic morphologies might be observed due to the different crystal packing in the different regimes, for example, spherulitic and axialitic textures are the typical morphologies for regimes II and I, respectively.13 Prior to the kinetics analysis, the values of U* and T∞ should be determined to obtain best fitting plots of the experimental data. Two sets of parameters are usually employed for this calculation. One is the empirical universal values with U* = 1500 cal/mol and T∞ = Tg − 30 K;52 the other is the Williams− Landel−Ferry (WLF) values of U* = 4200 cal/mol and T∞ = Tg − 51.6 K.51 Figure 14 shows the Laurizen−Hoffman plots of neat PBS and P(BS-b-12 wt % PEGS) as examples. The two sets of parameters of U* and T∞ are able to allow the fitting of two straight lines with different slopes at a wide Tc range. According to the LH theory, this discontinuity should be ascribed to a regime transition from III at low Tc (large supercooling) to II at high Tc (small supercooling). The Kg values for crystallization regimes II and III of neat PBS and the copolymers were obtained from the line slopes (as shown in Table S2 in Supporting Information). The temperature at which the regime transition occurs is defined as the transition temperature. The use of different U* and T∞ values resulted in slight changes in the line slopes. The values of KgIII/KgII for neat PBS, P(BS-b-6 wt % PEGS), P(BS-b-12 wt % PEGS), and P(BS-b-26 wt % PEGS) were 1.94, 2.18, 1.84, and 1.34 with empirical values and 1.96, 2.16, 1.86, and 1.34 with WLF values. The transition temperatures of regime II to III for neat PBS, P(BS-b-6 wt % PEGS), P(BS-b-12 wt % PEGS), and P(BS-b-26 wt % PEGS) were 95, 94, 92, and 81 °C regardless of the values of U* and T∞. The results suggest that the transition temperatures of the samples decreased with the increase in

4. CONCLUSION Thermal transition, crystallization kinetics, crystal structure, crystalline morphology, and spherulitic growth of P(BS-bPEGS) with different compositions were comparatively investigated with those of neat PBS by the techniques of DSC, WAXD, and POM. All the glass transition, crystallization, melting, and equilibrium temperatures decreased with introduction and increase of PEGS component. The incorporation of PEGS component did not change the crystal structures but reduced the degree of crystallinity of the samples. The isothermal crystallization mechanism remained unchanged regardless of PEGS content and crystallization temperature while the overall crystallization rates decreased with increase of PEGS component and crystallization temperature. The spherulitic growth rates decreased with increasing PEGS content due to its diluent effect to the crystallization of BS unit. All the samples showed crystallization regime transitions from II to III, and the transition temperature decreased with increasing PEGS component.

■

ASSOCIATED CONTENT

S Supporting Information *

Data of isothermal crystallization and spherulitic growth of neat PBS and P(BS-b-PEGS). This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel. and Fax: +86-2885410755. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by Natural Science Foundation of China (20904034 and 51121001), Program of International S & T Cooperation (2011DFA51420), and the program for Changjiang Scholars and Innovative Research Teams in University of China (IRT 1026). 8270



■

Article

(22) Miyata, T.; Masuko, T. Crystallization behaviour of poly(tetramethylene succinate). Polymer 1998, 39, 1399−1404. (23) Yoo, E. S.; Im, S. S. Morphological Study on Hydrolytic Degradation of Poly(tetramethylene succinate) Single Crystals and Spherulites. J. Polym. Environ. 1998, 6, 223−231. (24) Dong, T.; Shin, K.; Zhu, B.; Inoue, Y. Nucleation and Crystallization Behavior of Poly(butylenes succinate) Induced by Its rCyclodextrin Inclusion Complex: Effect of Stoichiometry. Macromolecules 2006, 39, 2427−2428. (25) Liu, X.; Li, C.; Zhang, D.; Xiao, Y. Melting behaviors, crystallization kinetics, and spherulitic morphologies of poly(butylene succinate) and its copolyester modified with rosin maleopimaric acid anhydride. J. Polym. Sci., Part B: Polym. Phys. 2006, 44, 900−913. (26) Ichikawa, Y.; Kondo, H.; Igarashi, Y.; Noguchi, K.; Okuyama, K.; Washiyama, J. Crystal structures of α and β forms of poly(tetramethylene succinate). Polymer 2000, 41, 4719−4727. (27) Papageorgiou, G. Z.; Achilias, D. S.; Bikiaris, D. N. Crystallization Kinetics of Biodegradable Poly(butylene succinate) under Isothermal and Non-Isothermal Conditions. Macromol. Chem. Phys. 2007, 208, 1250−1264. (28) Papageorgiou, G. Z.; Bikiaris, D. N. Crystallization and melting behavior of three biodegradable poly(alkylene succinates). A comparative study. Polymer 2005, 46, 12081−12092. (29) Mochizuki, M.; Mukai, K.; Yamada, K.; Ichise, N.; Murase, S.; Iwaya, Y. Structural Effects upon Enzymatic Hydrolysis of Poly(butylene succinate-co-ethylene succinate)s. Macromolecules 1997, 30, 7403−7407. (30) Gan, Z.; Abe, H.; Kurokawa, H.; Doi, Y. Microstructures, Thermal Properties, and Crystallization of Biodegradable Poly(butylene succinate) (PBS) and Its Copolyesters. Biomacromolecules 2001, 2, 605−613. (31) Gan, Z.; Abe, H.; Doi, Y. Crystallization, Melting, and Enzymatic Degradation of Biodegradable Poly(butylene succinate-co14 mol % ethylene succinate) Copolyester. Biomacromolecules 2001, 2, 313−321. (32) Yang, Y.; Qiu, Z. Crystallization kinetics and morphology of biodegradable poly(butylenes succinate-co-ethylene succinate) copolyersters: Effects of comonomer composition and crystallization temperature. CrystEngComm 2011, 13, 2408−2417. (33) Cao, A.; Okamurac, T.; Nakayamab, K.; Inoued, Y.; Masuda, T. Studies on syntheses and physical properties of biodegradable aliphatic poly(butylene succinate-co-ethylene succinate)s and poly(butylene succinate-co-diethylene glycol succinate)s. Polym. Degrad. Stab. 2002, 78, 107−117. (34) Papageorgiou, G. Z.; Bikiaris, D. N. Synthesis, Cocrystallization, and Enzymatic Degradation of Novel Poly(butylene-co-propylene succinate) Copolymers. Biomacromolecules 2007, 8, 2437−2449. (35) Xu, Y.; Xu, J.; Liu, D.; Guo, B.; Xie, X. Synthesis and Characterization of Biodegradable Poly(butylene succinate-co-propylene succinate)s. J. Appl. Polym. Sci. 2008, 109, 1881−1889. (36) Yang, J.; Hao, Q.; Liu, X.; Ba, C.; Cao, A. Novel Biodegradable Aliphatic Poly(butylenes succinate-co-cyclic carbonate)s with Functionalizable Carbonate Building Blocks. 1. Chemical Synthesis and Their Structural and Physical Characterization. Biomacromolecules 2004, 5, 209−218. (37) Yang, J.; Tian, W.; Li, Q.; Li, Y.; Cao, A. Novel Biodegradable Aliphatic Poly(butylenes Succinate-co-cyclic carbonate)s Bearing Functionalizable Carbonate Building Blocks: II. Enzymatic Biodegradation and in Vitro Biocompatibility Assay. Biomacromolecules 2004, 5, 2258−2268. (38) Tserkia, V.; Matzinosa, P.; Pavlidoub, E.; Vachliotisc, D.; Panayiotou, C. Biodegradable aliphatic polyesters. Part I. Properties and biodegradation of poly(butylene succinate-co-butylene adipate). Polym. Degrad. Stab. 2006, 91, 367−376. (39) Tserkia, V.; Matzinosa, P.; Pavlidoub, E.; Panayiotou, C. Biodegradable aliphatic polyesters. Part II. Synthesis and characterization of chain extended poly(butylene succinate-co-butylene adipate). Polym. Degrad. Stab. 2006, 91, 377−384.

REFERENCES

(1) Xu, J.; Guo, B. H. Poly(butylene succinate) and its copolymers: Research, development and industrialization. Biotechnol. J. 2010, 5, 1149−1163. (2) Fujimaki, T. Processability and properties of aliphatic polyesters,‘BIONOLLE’, synthesized by polycondensation reaction. Polym. Degrad. Stab. 1998, 59, 209−214. (3) Ray, S. S.; Okamoto, K.; Okamoto, M. Structure−Property Relationship in Biodegradable Poly(butylene succinate)/Layered Silicate Nanocomposites. Macromolecules 2003, 36, 2355−2367. (4) Tachibana, Y.; Masuda, T.; Funabashi, M.; Kunioka, M. Chemical Synthesis of Fully Biomass-Based Poly(butylenes succinate) from Inedible-Biomass-Based Furfural and Evaluation of Its Biomass Carbon Ratio. Biomacromolecules 2010, 11, 2760−2765. (5) Bikiaris, D. N.; Papageorgiou, G. Z.; Achilias, D. S. Synthesis and comparative biodegradability studies of three poly(alkylene succinate)s. Polym. Degrad. Stab. 2006, 91, 31−43. (6) Kasuya, K.; Takagi, K.; Ishiwatari, S.; Yoshidab, Y.; Doi, Y. Biodegradabilities of various aliphatic polyesters in natural waters. Polym. Degrad. Stab. 1998, 59, 321−332. (7) Yang, H. S.; Yoon, J. S.; Kim, M. N. Dependence of biodegradability of plastics in compost on the shape of specimens. Polym. Degrad. Stab. 2005, 87, 131−135. (8) Doi, Y.; Abe, H. Structural effects on biodegradation of aliphatic polyesters. Macromol. Symp. 1997, 118, 725−731. (9) Mochizukiw, M.; Hirami, M. Structural Effects on the Biodegradation of Aliphatic Polyesters. Polym. Adv. Technol. 1997, 8, 203−209. (10) Koyama, N.; Doi, Y. Effects of Solid-State Structures on the Enzymatic Degradability of Bacterial Poly (hydroxyalkanoic acids). Macromolecules 1997, 30, 826−832. (11) Li, S.; McCarthy, S. Influence of Crystallinity and Stereochemistry on the Enzymatic Degradation of Poly(lactide)s. Macromolecules 1999, 32, 4454−4456. (12) Focarete, M. L.; Ceccorulli, G.; Scandola, M. Further Evidence of Crystallinity-Induced Biodegradation of Synthetic Atactic Poly(3hydroxybutyrate) by PHB-Depolymerase A from Pseudomonas lemoignei. Blends of Atactic Poly(3-hydroxybutyrate) with Crystalline Polyesters. Macromolecules 1998, 31, 8485−8492. (13) Gan, Z.; Abe, H.; Doi, Y. Biodegradable Poly(ethylene succinate) (PES). 1. Crystal Growth Kinetics and morphology. Biomacromolecules 2000, 1, 704−712. (14) Gan, Z.; Abe, H.; Doi, Y. Biodegradable Poly(ethylene succinate) (PES). 2. Crystal Morphology of Melt-Crystallized Ultrathin Film and Its Change after Enzymatic Degradation. Biomacromolecules 2000, 1, 713−720. (15) Kurokawa, K.; Yamashita, K.; Doi, Y.; Abe, H. Structural Effects of Terminal Groups on Nonenzymatic and Enzymatic Degradations of End-Capped Poly(l-lactide). Biomacromolecules 2008, 9, 1071−1078. (16) Zhu, B.; He, Y.; Nishida, H.; Yazawa, K.; Ishii, N.; Kasuya, K.; Inoue, Y. Crystalline-Structure-Dependent Enzymatic Degradation of Polymorphic Poly(3-hydroxypropionate). Biomacromolecules 2008, 9, 1221−1228. (17) Iwata, T.; Doi, Y. Morphology and Enzymatic Degradation of Poly(l-lactic acid) Single Crystals. Macromolecules 1998, 31, 2461− 2467. (18) Kikkawa, Y.; Abe, H.; Iwata, T.; Inoue, Y.; Doi, Y. Crystallization, Stability, and Enzymatic Degradation of Poly(l-lactide) Thin Film. Biomacromolecules 2002, 3, 350−356. (19) Maiti, P.; Nam, P. H.; Okamoto, M. Crystallization on Intercalation, Morphology, and Mechanical Properties of Polypropylene/Clay Nanocomposites. Macromolecules 2002, 35, 2042−2049. (20) Gagnon, K. D.; Lenz, R. W.; Farris, R. J.; Fuller, R. C. Crystallization behavior and its influence on the mechanical properties of a thermoplastic elastomer produced by Pseudomonas oleovorans. Macromolecules 1992, 25, 3723−3728. (21) Ihn, K. J.; Yoo, E. S.; Im, S. S. Structure and Morphology of Poly(tetramethylene succinate) Crystals. Macromolecules 1995, 28, 2460−2464. 8271



Article

(40) Nikolic, M. S.; Djonlagic, J. Synthesis and characterization of biodegradable poly(butylenes succinate-co-butylene adipate)s. Polym. Degrad. Stab. 2001, 74, 263−370. (41) Zhao, J. H.; Wang, X. Q.; Zeng, J.; Yang, G.; Shi, F. H.; Yan, Q. Biodegradation of poly(butylene succinate-co-butylene adipate) by Aspergillus versicolor. Polym. Degrad. Stab. 2005, 90, 173−179. (42) Cho, K.; Lee, J.; Kwon, K. Hydrolytic Degradation Behavior of Poly(butylenes succinate)s with Different Crystalline Morphologies. J. Appl. Polym. Sci. 2001, 79, 1025−1033. (43) Lee, S. I.; Yu, S. C.; Lee, Y. S. Degradable polyurethanes containing poly(butylene succinate) and poly(ethylene glycol). Polym. Degrad. Stab. 2001, 72, 81−87. (44) Pepic, D.; Zagar, E.; Zigon, M.; Krzan, A.; Kunaver, M.; Djonlagic, J. Synthesis and characterization of biodegradable aliphatic copolyesters with poly(ethylene oxide) soft segments. Eur. Polym. J. 2008, 44, 904−917. (45) Huang, C. L.; Jiao, L.; Zhang, J. J.; Zeng, J. B.; Yang, K. K.; Wang, Y. Z. Poly(butylene succinate)-poly(ethylene glycol) multiblock copolymer: Synthesis, structure, properties and shape memory performance. Polym. Chem. 2012, 3, 800−808. (46) Zhu, Q. Y.; He, Y. S.; Zeng, J. B.; Huang, Q.; Wang, Y. Z. Synthesis and characterization of a novel multiblock copolyester containing poly(ethylene succinate) and poly(butylene succinate). Mater. Chem. Phys. 2011, 130, 943−949. (47) Avrami, M. Kinetics of Phase Change. I General Theory. J. Chem. Phys. 1939, 7, 1103−1112. (48) Zeng, J. B.; Zhu, Q. Y.; Li, Y. D.; Qiu, Z. C.; Wang, Y. Z. Unique Crystalline/Crystalline Polymer Blends of Poly(ethylene succinate) and Poly(p-dioxanone): Miscibility and Crystallization Behaviors. J. Phys. Chem. B 2010, 114, 14827−14833. (49) Wunderlich, B. Macromolecular Physics, Vol. 2; Academic Press: New York, 1976. (50) Hoffman, J. D.; Weeks, J. J. Melting process and the equilibrium melting temperature of polychlorotrifluoroethylene. J. Res. Natl. Bur. Stand. 1962, 66A, 13−28. (51) Hoffman, J. D.; Davis, G. T.; Lauritzen, J. I. Vol. 3: Crystalline and Noncrystalline Solids. In Treatise on Solid State Chemistry; Hannay, N. B., Ed.; Plenum Press: New York, 1976. (52) Williams, M. L.; Landel, R. F.; Ferry, J. D. The Temperature Dependence of Relaxation Mechanisms in Amorphous Polymers and Other Glass-forming Liquids. J. Am. Chem. Soc. 1955, 77, 3701−3707.

8272


Isothermal Crystallization Behavior of Biodegradable P (BS-b-PEGS

Recommend Documents