Crystallization Kinetics and Spherulitic Morphologies of

Jan 5, 2013 - Poly(butylene succinate-co-diethylene glycol succinate) (P(BS-co-DEGS)) random copolymer was synthesized and characterized in a previous...
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Crystallization Kinetics and Spherulitic Morphologies of Biodegradable Poly(butylene succinate-co-diethylene glycol succinate) Copolymers Guang-Chen Liu, Jian-Bing Zeng,* Cai-Li Huang, Ling Jiao, Xiu-Li Wang, 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 ABSTRACT: Poly(butylene succinate-co-diethylene glycol succinate) (P(BS-co-DEGS)) random copolymer was synthesized and characterized in a previous paper [Ind. Eng. Chem. Res. 2012, 51, 12258−12265]. In this paper, we focus on the isothermal crystallization behaviors of P(BS-co-DEGS). The isothermal crystallization kinetics, spherulitic morphology, and growth kinetics of P(BS-co-DEGS) were investigated by differential scanning calorimetry and polarized optical microscopy and compared with those of neat poly(butylene succinate) (PBS). The results suggest that the crystallization rate of P(BS-co-DEGS) was much slower than that of neat PBS and decreased with increase of DEGS content, while the crystallization mechanism remain unchanged. P(BS-co-DEGS) showed banded morphology, and the band spacing decreased with increase of DEGS content at a given supercooling. The spherulitic growth rate of P(BS-co-DEGS) decreased with increase in DEGS content. A transition from crystallization regime II to crystallization regime III occurred for all samples, and the transition shifted to lower temperatures with increase in DEGS content.

1. INTRODUCTION Biobased and biodegradable aliphatic polyesters, such as poly(lactic acid) (PLA), poly(butylene succinate) (PBS), and bacterial polyesters, have attracted considerable attention recently due to their biodegradability, renewability, excellent thermal processability, and mechanical properties.1−3 PBS with mechanical properties comparable to polyethylene (PE) and polypropylene (PP) is regarded as one of the most promising substitutes for nonbiodegradable commodity plastics in the application of packaging, mulching film, shopping and trash bags, and disposable food containers.4 PBS can be synthesized by condensation polymerization of 1,4-butanediol and succinic acid. Recently, succinic acid has been produced via bioconversion of renewable resources such as starch, which would further stimulate the development of the PBS industry.5−8 However, due to the inherent high degree of crystallinity, PBS shows a slowed biodegradation rate compared to other aliphatic polyesters, for example, PLA and bacterial polyesters.9−11 In addition, the ductility of PBS products would be reduced during service or storage because of the cold crystallization of the high regular molecular chains.12 Those results suggest that the crystallization behavior plays an important role in the properties of PBS. Therefore, the crystal structure, crystallization kinetics, spherulitic morphology, and growth rate of PBS have been investigated extensively in recent literature.13−20 The results provide very valuable fundamental data for further property modification of PBS to widen its application. The most widely reported methods to modify properties of PBS are physical blending and chemical copolymerization. Physical blending provides a simple and convenient way to develop new materials with desired properties. Therefore, many thermoplastic polymers, such as poly(ε-caprolactone), poly(lactic acid), poly(ethylene succinate), poly(hydroxybutyrate), © 2013 American Chemical Society

poly(ethylene oxide), poly(vinyl phenol), and poly(3-hydroxybutyrate-co-hydroxyvalerate), have been applied to blend with PBS.21−27 Unfortunately, most of those polymers are immiscible with PBS; thus, modified materials with excellent properties may not be anticipated by simple blending, since phase separation occurred for the blends. In contrast, chemical copolymerization provides an efficient way to synthesize copolymers with acceptable properties, because phase separation of the different components can be avoided by this way. 28−31 Many comonomers including adipic acid, terephthalic acid, ethylene glycol, and propanediol have been incorporated into the PBS polymer chain.32−41 The incorporation of the comonomers indeed makes it possible to prepare novel biodegradable copolymers with controllable degree of crystallinity, improved ductility, and accelerated degradation rates compared with neat PBS. In our previous study, we incorporated a small amount of diethylene glycol (DEG) into the polymer chain of PBS to form poly(butylene succinate-co-diethylene glycol succinate) (P(BSco-DEGS)) copolymers to modify the properties of PBS, and investigated the effect of the DEG content on the crystal structure, mechanical properties, thermal stability, and biodegradation of the copolymers.42 It was found that the ductility and degradation rate were effectively improved while the crystal structure did not change and only the degree of crystallinity decreased after incorporation of DEG. The crystallization kinetics, melting behavior, and spherulitic morphology are also important both theoretically and practically for new copolyesters Received: Revised: Accepted: Published: 1591

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microscopic cover glasses at 140 °C. The samples were first melted at 140 °C for 3 min to eliminate the 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.

with potential general-purpose applications. Therefore, we studied the effects of DEGS content on the isothermal crystallization kinetics, equilibrium melting temperature, spherulitic morphology, and growth rate of P(BS-co-DEGS) and compared them with those of neat PBS in the present paper. To the best of our knowledge, no results have been reported on the crystallization properties of such a copolymer in previous literature.

3. RESULTS AND DISCUSSION 3.1. Basic Properties of Neat PBS and P(BS-co-DEGS) Copolymers. As described in the Experimental Section, both neat PBS and P(BS-co-DEGS) copolymers were synthesized by a two-step condensation polymerization technique. Scheme 1 shows the synthetic procedure of P(BS-co-DEGS) copolymers. Typically, predetermined amounts of 1,4-butanediol, succinic acid, and diethylene glycol were charged into a three-necked round-bottomed flask which was equipped with an agitator, water separator, and N2 inlet pipe. The esterification was carried out at 180 °C for 4 h, and then catalyst tetrabutyl titanate (0.1 wt % of total reactants) was introduced into the flask and the polycondensation was carried out at 220 °C with a vacuum of 30 Pa for 4 h. The obtained products were purified by dissolving in chloroform and then precipitating in excessive methanol. The detailed information for the characterization of the samples is reported elsewhere.42 For brevity, it is not shown in this paper. Neat PBS and three copolymers with DEGS molar fractions of 7, 14, and 29% were prepared. The basic properties of neat PBS and the copolymers have been investigated systemically in our previous study,42 and the important parameters were collected and are summarized in Table 1. It can be seen that all the samples showed comparable molecular weights and polydispersity indexes (PDIs). The glass transition temperature (Tg) increased while the melting temperature (Tm) decreased with increasing DEGS content. The cold crystallization temperature (Tcc) shifted to higher temperature range with incorporation of DEGS, suggesting that the crystallizability of the copolymers decreased with increase in DEGS content. The degree of crystallinity (Xc) of P(BS-co-DEGS) copolymers decreased with increase in DEGS content. 3.2. Isothermal Melt Crystallization Kinetics. In order to investigate the effect of ether linkage on the crystallization behavior of PBS in detail, the isothermal melt crystallization kinetics of P(BS-co-DEGS) copolymers were studied by DSC and compared with that of neat PBS. Parts a and b of Figure 1 show the development of relative crystallinity with crystallization time for neat PBS and P(BS-co-14 mol % DEGS), respectively. It can be seen that more time is required to accomplish crystallization with increasing crystallization temperature for both neat PBS and P(BS-co-14 mol % DEGS), suggesting that the crystallization was retarded with increasing temperature. P(BS-co-7 mol % DEGS) and P(BS-co-29 mol % DEGS) also show similar behaviors. For brevity, the plots are not shown in

2. EXPERIMENTAL SECTION 2.1. Materials. Both neat PBS and P(BS-co-DEGS) copolymers were synthesized through a two-step procedure of esterification and subsequent polycondensation with different molar ratios of succinic acid, 1,4-butanediol, and diethylene glycol. Scheme 1 shows the synthetic route for P(BS-co-DEGS) Scheme 1. Synthesis of Poly(butylene succinate-co-diethylene succinate) Random Copolymer

copolymers. The detailed procedures for the synthesis were reported in our previous study. Three P(BS-co-DEGS) copolymers with molar ratios of BS unit to DEGS unit of 93:7, 86:14, and 71:29 determined by 1H NMR analysis were prepared and named as P(BS-co-7 mol % DEGS), P(BS-co-14 mol % DEGS), and P(BS-co-29 mol % DEGS), respectively. Values for the molecular weight, polydispersity index (PDI), and general thermal properties of neat PBS and P(BS-co-DEGS) copolymers, reported in our previous study,42 were selected and are summarized in Table 1. 2.2. Isothermal Crystallization Kinetics. The isothermal crystallization kinetics of neat PBS and P(BS-co-DEGS) were performed on a TA Instrument differential scanning calorimeter (DSC) Q200 with a Universal Analysis 2000 program. The samples were first melted at 140 °C for 3 min to erase the thermal history and cooled to the predetermined crystallization temperature (Tc) at a 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 10 °C/min to investigate the melting behaviors and calculate the equilibrium melting temperatures. All experiments were carried out under nitrogen atmosphere with 50 mL/min flow rate. 2.3. Polarized Optical Microscopy (POM). The spherulitic morphologies and growth rates of neat PBS and P(BS-co-DEGS) copolymers were studied with a polarized optical microscope (POM; NIKON ECLIPSE LV100POL) equipped with a hot stage (HSC621V). Sample films were pressed between two

Table 1. Composition and Molecular Weight of Neat PBS and P(BS-co-DEGS) sample

Mna (×10−4 g/mol)

PDIa

Tgb (°C)

Tccb (°C)

Tmb (°C)

ΔHmb (J/g)

Xc,WAXDc (%)

neat PBS P(BS-co-7 mol % DEGS) P(BS-co-14 mol % DEGS) P(BS-co-29 mol % DEGS)

6.67 8.50 6.24 8.33

2.55 2.84 3.13 2.86

−38.1 −36.6 −35.0 −32.9

−5.6 −3.0 −2.1 11.0

113.5 107.7 101.2 88.0

58.1 57.3 50.7 40.6

57.1 47.7 45.8 39.5

a

Determined by gel permeation chromatography with chloroform and polystyrene as solvent and standard. bObtained from the heating scan of meltquenched sample at heating scan rate of 10 °C/min. cCalculated from wide angle X-ray diffraction (WAXD) patterns. 1592

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Figure 1. Plots of relative crystallinity versus crystallization rate at various crystallization temperatures for (a) neat PBS and (b) a typical copolymer P(BS-co-14 mol % DEGS).

Figure 2. Avrami plots of (a) neat PBS and (b) P(BS-co-14 mol % DEGS).

the nucleation and growth process.43,44 Equation 1 can be rewritten as

this study. It is more interesting to study the effect of the content of ether linkage on the crystallization of the samples. Neat PBS finished crystallization in only 2 min when it was isothermally crystallized at Tc of 96 °C. The crystallization of P(BS-co-14 mol % DEGS) at Tc of 96 °C was not observed. Even at Tc of 82 °C, P(BS-co-14 mol % DEGS) required about 70 min to finish crystallization, while P(BS-co-7 mol % DEGS) finished crystallization in only 2 min at 82 °C. From the above results, we can conclude that incorporation of DEGS would result in retarded crystallization of P(BS-co-DEGS) compared with that of neat PBS, and that the crystallization rate of P(BS-co-DEGS) slowed down with increasing DEGS content. However, it is worth noting that the supercooling is not the same for neat PBS and P(BS-co-DEGS) copolymers even at the same given Tc, because the incorporation of DEGS would depress the equilibrium melting temperature of the samples. The detailed effect of supercooling on the crystallization of the samples will be discussed with equilibrium melting temperature in section 3.3. 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 1 − X t = exp( −kt n)

log[− ln(1 − X t )] = log k + n log t

(2)

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. Parts a and b of Figure 2 illustrate the Avrami plots of neat PBS and P(BS-co-14 mol % DEGS), respectively. A series of parallel straight lines of both neat PBS and P(BS-co-14 mol % DEGS) were obtained for various crystallization temperatures, and similar parallel straight lines were also obtained when using the Avrami equation to treat the isothermal melt crystallization kinetics of P(BS-co-7 mol % DEGS) and P(BS-co-29 mol % DEGS), indicating that the Avrami equation could be reasonably used to describe the isothermal melt crystallization kinetics of neat PBS and P(BS-coDEGS) copolymers. The Avrami parameters k and n of all samples were obtained from the slopes and intercepts of the Avrami plots and are shown in Table 2. The n values for the four samples were between 2.1 and 2.8 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 nucleation20,45 and incorporation of DEGS would not change the crystallization mechanism.

(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 1593

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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 DEGS content, which can be ascribed to the two factors caused by the incorporation and increase of DEGS component. First, the DEGS component has a diluent effect on the crystallization of copolymers, since it does not crystallize; the crystallization of the copolymers was caused by the BS unit. Therefore, the content of crystallizable component was diluted with increase in the DEGS component, which would slow down the crystallization rate. Second, incorporation of DEGS would depress the equilibrium melting point temperatures (Tm0) of the copolymers compared with that of neat PBS. Thus, the supercooling (Tm0 − Tc), known as the driving force of isothermal crystallization, for the same given Tc decreased with increase in DEGS content, which would also result in a retarded crystallization rate. 3.3. 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 a crystal, can only be determined on the basis of the Tm0. In the present study, the effect of content of DEGS 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 as46

Table 2. Isothermal Crystallization Kinetics of Neat PBS and the Copolymers sample neat PBS

P(BS-co-7 mol % DEGS)

P(BS-co-14 mol % DEGS)

P(BS-co-29 mol % DEGS)

Tc (°C)

n

k (min−n)

96 100 104 106 108 82 86 90 94 98 66 70 74 78 82 50 54 58 62 66

2.8 2.5 2.6 2.5 2.4 2.6 2.4 2.7 2.8 2.6 2.4 2.5 2.6 2.6 2.5 2.7 2.6 2.6 2.7 2.7

1.5 1.5 × 10−1 3.7 × 10−3 6.4 × 10−4 1.5 × 10−5 6.3 × 10−1 1.7 × 10−1 1.3 × 10−2 7.0 × 10−4 7.0 × 10−1 5.9 × 10−1 9.1 × 10−2 9.2 × 10−3 1.1 × 10−3 1.1 × 10−4 1.2 1.7 × 10−1 1.8 × 10−2 1.3 × 10−3 1.2 × 10−4

Since the values of n vary with composition, it is inappropriate to compare the overall crystallization rates of neat PBS and the copolymers directly from the values of rate constants. The halftime 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:

t1/2

⎛ ln 2 ⎞1/ n ⎟ =⎜ ⎝ k ⎠

⎛ 1⎞ T Tm = Tm 0⎜1 − ⎟ + c γ⎠ γ ⎝

(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. The Tm0 can then be obtained from the intersection of this line with the Tm = Tc line. As mentioned in the Experimental Section, the subsequent heating curves of PBS and the copolymers after isothermal crystallization at various Tc’s were recorded for equilibrium melting point temperature analysis. As an example, Figure 4 shows the melting curves of P(BS-co-14 mol % DEGS) after isothermal crystallizing at various temperatures. The samples showed two melting peaks at relatively lower and higher

(3)

The 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 1/t1/2 were calculated and are graphically shown in Figure 3. It can be seen that 1/t1/2 decreased with increase of Tc for all samples, suggesting that the crystallization rates of the samples decreased with Tc, which is very common in the isothermal crystallization of polymers and

Figure 3. Values of 1/t1/2 of neat PBS and copolymers at various crystallization temperatures.

Figure 4. Melting curves of P(BS-b-14 mol % DEGS) after crystallization at various temperatures. 1594

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and the copolymers. The Tm0 of neat PBS was calculated to be 135.8 °C, which approached the results reported by Qiu20 and Gan.47The Tm0 values of the copolymers are 125.0, 111.6, and 106.3 °C for P(BS-co-7 mol % DEGS), P(BS-co-14 mol % DEGS), and P(BS-co-29 mol % DEGS), respectively. The results suggest that incorporation of DEGS segments would reduce the equilibrium melting temperature of PBS. Similar results were also obtained when another comonomer such as ethylene glycol was incorporated into the PBS molecular chain.20 3.4. Crystalline Morphology and Spherulitic Growth Kinetics. The spherulitic morphology and spherulite size could significantly affect the physical properties and biodegradation behavior of biodegradable crystalline polymers. Therefore, the effects of crystallization temperature and copolymer composition on the crystalline morphology and spherulitic growth of the samples have been studied. Figure 6 shows the crystalline morphology of P(BS-co-14 mol % DEGS) crystallized at various supercoolings observed by POM. The spherulites showed banded morphology with characteristic “Maltese cross” patterns regardless of crystallization temperature. The number of spherulites decreased and the size of spherulites increased with decreasing supercooling (i.e., increasing Tc), which was caused by the fact that the nucleation becomes difficult and thus the number of nuclei decreased with decreasing supercooling. It is worth noting that the band spacing of the spherulites increased with 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 is observed for many polymers, and the banded morphology may even disappear at high temperatures. Similar results were also obtained for neat PBS, P(BS-co-7 mol % DEGS), and P(BS-co-29 mol % DEGS) after isothermal crystallization at various crystallization temperatures. The effect of the weight fraction of DEGS on the crystalline morphology of the samples was further analyzed. To study the

temperatures, i.e., Tm1 and Tm2, after crystallizing at 66, 70, 74, 78, and 82 °C. The values of Tm1 increased from 87.5 to 97.1 °C and those of Tm2 almost remain unchanged around 102 °C as the crystallization temperature increased. 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 of the sample.20 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 the heating process in DSC measurement. Thus, Tm1 was employed for calculating Tm0 with the Hoffman−Weeks equation. Figure 5 shows the Hoffman−Weeks plots of neat PBS

Figure 5. Hoffman−Weeks plots for calculating Tm0 of neat PBS and copolymers.

Figure 6. Crystalline morphology of P(BS-co-14 wt % DEGS) formed at supercooling of (a) 60, (b) 50, (c) 40, and (d) 30 °C. 1595

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Figure 7. Crystalline morphologies of (a) neat PBS, (b) P(BS-co-7 wt % DEGS), (c) P(BS-co-14 wt % DEGS), and (d) P(BS-co-29 wt % DEGS) formed at the same supercooling of 30 °C.

crystallization temperatures for neat PBS and the copolymers. It was found that the G value increased with decrease of Tc for all samples within the observed temperature range. For different samples, the G value decreased with the increase in the weight fraction of DEGS at a given Tc, which was ascribed to the diluent effect of the DEGS segment to PBS segments, since the crystallization of the copolymer originated from PBS segments. The spherulitic growth kinetics of the samples were further analyzed by the Lauritzen−Hoffman (LH) method,48 known as the secondary nucleation theory, in which the spherulitic growth rate G at a crystallization temperature Tc can be represented by

effect of the weight fraction of DEGS on the crystalline morphology of the samples, the crystallization of the samples was carried out at the same supercooling. Figure 7 shows the spherulitic morphologies of neat PBS and the copolymers formed at supercooling of 30 °C. Neat PBS shows fibrous morphology while the copolymers show banded morphology, and the band spacing decreases with increase of DEGS content. The results suggest that the periodic twisting of lamellae occurred more easily with increasing content of DEGS at a given crystallization temperature. In addition, we measured the spherulitic growth rate (G) of the samples during POM observation. At a given Tc, the plot of spherulite radius versus crystallization time gives a straight line, from the slope of which the G value can be obtained. Figure 8 shows the dependence of spherulitic growth rate on the

⎡ ⎤ ⎡ Kg ⎤ U* G = G0 exp⎢ − ⎥ exp⎢ − ⎥ ⎣ R(Tc − T∞) ⎦ ⎣ Tc(ΔT )f ⎦

(5)

where G0 is a temperature-independent pre-exponential factor, 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, 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 described by Kg =

nb0σσeTm 0 ΔHf k

(6)

The value of n is dependent on the crystallization regime, σ and σe are the lateral and end-surface free energies, respectively, b0 is the molecular thickness, ΔHf is the fusion enthalpy per unit volume, and k is the Boltzmann constant. After taking the logarithmic form, eq 5 can be rewritten as

Figure 8. Dependence of spherulitic growth rate G on crystallization temperature for neat PBS and copolymers. 1596

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Figure 9. Lauritzen−Hoffman plots of (A) neat PBS and (B) P(BS-co-14 mol % DEGS).

Table 3. Spherulitic Growth Kinetic Data of PBS and Copolyesters parameters empirical

WLF

ln G +

sample neat PBS P(BS-co-7 mol % DEGS) P(BS-co-14 mol % DEGS) P(BS-co-29 mol % DEGS) neat PBS P(BS-co-7 mol % DEGS) P(BS-co-14 mol % DEGS) P(BS-co-29 mol % DEGS)

Kg U* = ln G0 − Tc(ΔT )f R(Tc − T∞)

Tm0 (°C)

KgIII (K2)

KgII (K2)

135.8 125.0 111.6 106.3 135.8 125.0 111.6 106.3

2.28 × 10 1.54 × 105 1.32 × 105 1.73 × 105 2.59 × 105 1.81 × 105 1.63 × 105 2.19 × 105

1.22 × 10 7.60 × 104 6.86 × 104 7.32 × 104 1.41 × 105 9.15 × 104 8.54 × 104 1.02 × 105

5

5

KgIII/KgII

Ttr (°C)

1.89 2.02 1.92 2.36 1.84 1.98 1.91 2.15

94 88 82 62 94 88 82 62

slight changes in the line slopes. The values of KgIII/KgII for neat PBS, P(BS-co-7 wt % DEGS), P(BS-co-14 wt % DEGS), and P(BS-co-29 wt % DEGS) were 1.89, 2.02, 1.92, and 2.36 with empirical values and 1.84, 1.98, 1.91, and 2.15 with WLF values. The transition temperatures from regime II to III for neat PBS, P(BS-co-7 mol % DEGS), P(BS-co-14 mol % DEGS), and P(BSco-29 mol % DEGS) were 94, 88, 82, and 62 °C regardless of the values of U* and T∞. The results suggest that the transition temperature of the samples decreased with the increase in the mole fraction of DEGS, which was attributed to the depression of the equilibrium melting temperature with incorporation of the DEGS component.

(7)

In order to understand the effect of the 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.46 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 next nucleus in this regime. In regime II, the rate of secondary nucleation is comparable with that of diffusion. 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. Prior to the kinetics analysis, the values of U* and T∞ should be determined to obtain the 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;49 the other is the Williams− Landel−Ferry (WLF) values of U* = 4200 cal/mol and T∞ = Tg − 51.6 K.48 Figure 9 shows the Lauritzen−Hoffman plots of neat PBS and P(BS-co-14 mol % DEGS) as examples. The two sets of parameters of U* and T∞ are able to allow the good fitting of two straight lines with different slopes in 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 3. The temperature at which the regime transition occurs is defined as the transition temperature. The use of different U* and T∞ values resulted in

4. CONCLUSION The crystallization kinetics and spherulitic morphology of P(BSco-DEGS) were investigated and compared with those of neat PBS. The crystallization rate of P(BS-co-DEGS) was slower than that of neat PBS and decreased with increase of DEGS content, while the crystallization mechanism remained regardless of the composition of the polymer. The equilibrium melting temperature gradually decreased with increase of DEG content, and the values were 135.8, 125.0, 111.6, and 106.3 °C for neat PBS, P(BSco-7 mol % DEGS), P(BS-co-14 mol % DEGS), and P(BS-co-29 mol % DEGS), respectively. The spherulites of neat PBS showed fibrous morphology while those of P(BS-co-DEGS) showed banded morphology, and the band spacing decreased gradually with increase of DEGS content. Both neat PBS and the copolymers showed crystallization regime transitions from II to III, and the transition temperature decreased with increase in content of the DEGS component. 1597

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel./fax: +86-28-85410755. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by National Natural Science Foundation of China (20904034 and 51121001), the Excellent Youth Foundation of Sichuan (2011JQ0007), the Specialized Research Fund for the Doctoral Program of Higher Education (20110181130008), the Program of International S & T Cooperation (2011DFA51420), and the Program for Changjiang Scholars and Innovative Research Teams in University of China (IRT 1026).

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