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Effect of poly(L-lactide)/poly(D-lactide) block length ratio on crystallization behavior of star-shaped asymmetric poly(L(D)-lactide) stereoblock copolymers.


The demand for bioplastic will create a new market in the near future, not only because of the decline of oil reserves but also because of the environmental problems caused by the petro-based plastic [1, 2]. As a bio-based and biodegradable plastic, poly(lactic acid) (PLA) attracted much interest in various application fields [3-5] and accounted for a large component of the total installed capacity of bioplastic [2], It is being considered as an alternative for petro-based polymers to reduce the environmental impact [6, 7], However, the application of PLA is still limited by several drawbacks, such as its poor thermal resistance and brittleness.

The crystallization rate, crystalline thickness, and spherulite size affect the thermomechanical properties of PLA articles significantly. Therefore, a promotion in crystallization rate and decrease in spherulite size are desired to enhance the thermal resistance and deformability of PLA materials [8-15], especially, in an attempt to use PLA as an engineering plastic. One of the most common routes is blending poly (D-lactic acid) (PDLA) with poly (L-lactic acid) (PLLA) to form stereocomplex poly (lactic acid) (sc-PLA) [13, 15, 16]. Tsuji et al. reported that the stereocomplex crystallization was an effective and promising method to improve the mechanical and thermal properties of the PLA-based materials [17], However, in most cases, a coexistence of homocrystallites and stereocomplex crystallites will appear, when using high-molecular-weight (over 1 x [10.sup.5] g [mol.sup.-1] [18]) enantiomeric PLA components. The existence of homocrystallites will decrease the heat deflection temperature of PLA materials. As a result, the thermal properties of the blend of high-molecular-weight PLLA and PDLA are poor. However, in most application fields, high-molecular-weight PLA with good thermal resistance is required [17, 19], Numerous efforts have been made on the synthesis of stereoblock poly (lactic acid) (sb-PLA), which will form more stable stereocomplex crystallite of high-molecular-weight PLA [14, 20-22],

Li et al. [23] reported the stereocomplex crystallization behaviors of poly(L-lactide)-b-poly(D-lactide) diblock oligomers ([M.sub.n] = 7.4-7.8 x [10.sup.3] g [mol.sup.-1]) with various enantiomeric PLA block length ratios. The enantiomeric PLA block length had an evident effect on the crystal structure: in the symmetric PLLA/ PDLA situation, only the stereocomplex crystallites were obtained over the whole range of crystallization temperatures; while in the strongly asymmetric form (PLLA/PDLA block length ratio = 80/20), only homocrystallites formed. Tsuji et al. [24] reported the stereocomplex crystallization and spherulite growth behavior of symmetric poly (L-lactide)-b-poly (D-lactide) with the molecular weights from 3.9 x [10.sup.3] to 1.1 x [10.sup.4] g [mol.sup.-1], which contained only stereocomplex crystallites as crystalline species. They found that the radial growth rate of the spherulite was the highest at molecular weights of 9.3 x [10.sup.3] g [mol.sup.-1].

As is known to all, different molecular weight could bring different physical properties. Higher molecular weight enhances the mechanical properties of polymer materials. The investigation of the crystallization behaviors of high-molecular-weight polymers is necessary. However, to the best of our knowledge, the crystallization behavior of the asymmetric PLA stereoblock copolymer with high molecular weight has not been explored in detail.

In our previous research, we synthesized a series of star-shaped PPO-PDLA-PLLA copolymers among which the highest molecular weight is more than 1 x [10.sup.5] g [mol.sup.-1] [25]. In the crystallization of those copolymers, only stereocomplex crystallites formed. Moreover, the stereocomplex crystallites were fully reversible in the process of crystallization-melt-recrystallization [26], The purpose of this work is to investigate the effects of different PLLA/ PDLA block length ratio on the isothermal melt crystallization and spherulite growth behavior of star-shaped PPO-PDLA-PLLA stereoblock copolymers. It is expected that this research can provide essential data on the crystallization behavior of this star-shaped PPO-PDLA-PLLA stereoblock copolymers.



The star-shaped PPO-PDLA-PLLA stereoblock copolymers were synthesized by ring-open polymerization of lactide according to the method previously reported [25], In brief, three-arm poly (propylene oxide) ([M.sub.n] = 3.6 x [10.sup.3] g [mol.sup.-1], supplied by Shanghai Research Institute of Petrochemical Technology) was used as a macroinitiator to synthesize PPO-PDLA copolymers by ring-open polymerization of D-lactide (purchased from Purac). PPO-PDLA copolymers were then functionalized by tin (II)-alkoxide groups. The functionalized PPO-PDLA copolymers were used as initiator in polymerization of L-lactide to synthesis PPO-PDLA-PLLA stereoblock copolymers. Synthesized copolymers were purified by reprecipitation using a mixture of chloroform and 1,1,1,3,3,3-hexafluoro-2-propanol (purchased from J&K) (vol/vol = 95/5) as the solvent and alcohol as the nonsolvent, respectively. Then the samples were dried in vacuo for 7 days to remove the solvent. The block length ratios of PPO, PDLA, and PLLA of the synthesized samples were 1:7:7, 1:7:14, and 1:7:28, respectively. Those samples will be denoted as TB7, TB14, and TB28 in this article.


The [sup.1]H NMR spectra was recorded using a Bruker DMX500 spectrometer (BRUKER, Germany), and the chemical shifts were referred to the internal standard of tetramethylsilane (TMS). For the preparation of [sup.1]H NMR specimens, mixture of chloroform-d and 1,1,1,3,3,3-hexafluoro-2-propanol (vol/vol = 95/5) was used as the solvent. The [M.sub.n] (NMR) values of the synthesized copolymers were estimated by the following equation [27]: [M.sub.n] (PPO-PDLA-PLLA) = [M.sub.n] (PPO) + 72 x [DP.sub.PPO] x ([n.sub.LA]/[n.sub.PO]), where [M.sub.n] (PPO) = 3.6 x [10.sup.3] g mol-1, [DP.sub.PPO] is the degree of polymerization of PPO with a constant of 53, [n.sub.LA]/[n.sub.PO] is the mole ratio of the lactide and propylene oxide units in the PPO-PDLA-PLLA copolymers, which can be estimated from the [sup.1]H NMR spectra. For reference, the molecular weight of PPO-PDLA-PLLA stereoblock copolymers were evaluated in chloroform at 25[degrees]C utilizing Perk-Elmer Series 200 GPC system (Perk-Elmer, USA) using polystyrene standards. Therefore, the [M.sub.n] (GPC) is relative to the polystyrene. In the following sections, we use only [M.sub.n] (NMR) values as the [M.sub.n] values.

To estimate the D-lactyl unit contents, the specific optical rotation values of the copolymers were measured using aforementioned mixed solvent at a concentration of 1 g/dL and 25[degrees]C.

D-lactyl unit content (%)

= 100 x {[[alpha].sup.589.sub.25] + [[[alpha]].sup.589] (PDLA)}/{2 x [[[alpha]].sup.589.sub.89] (PDLA)}

where [[alpha].sup.589.sub.25] (PDLA) is the [[alpha].sup.589.sub.25] value for PDLA homopolymer. The evaluated molecular characteristics of PPO-PDLA-PLLA stereoblock copolymers are shown in Table 1.

The isothermal crystallization kinetics of PPO-PDLA-PLLA stereoblock copolymers were tested using the Perk-Elmer 8500 differential scanning calorimeter (DSC) (Perk-Elmer, USA). All the measurements were carried out under dry nitrogen atmosphere and the weight of samples were about 5 mg for each DSC scanning. The samples were first heated and equilibrated at 225[degrees]C for 2 min to erase the thermal history. Then the samples were cooled from 225[degrees]C to crystallization temperature ([T.sub.m]) between 156 and 140[degrees]C at 150[degrees]C [min.sup.-1] and held until the crystallization completed to monitor the isothermal crystallization process.

The wide-angle X-ray diffraction (WAXD) measurements were taken on a Bruker D8 Advance X-ray diffractometer (BRUKER, Germany) with Ni-filtered Cu Ka radiation ([lambda] = 0.15418 nm). The scattering angle ranged from 2[theta] = 5-40[degrees] with a scanning rate of 4[degrees] [min.sup.-1] at room temperature. The specimens for WAXD experiment were prepared by the temperature program described above.

The spherulite growth of the specimens during the isothermal crystallization was observed employing Leica DM2500 P polarized optical microscope (Leica, Germany) equipped with a LINKAM THMS 600 heating-cooling stage (Linkam, UK). The specimens were placed between a microscope slide and a cover slip, melted at 225[degrees]C for 2 min, and then quenched to [T.sub.c] between 180 and 150[degrees]C to observe the spherulite growth behavior.


Thermal Properties of Star-Shaped PPO-PDLA-PLLA Stereoblock Copolymers

To study the thennal properties of PPO-PDLA-PLLA copolymers, the PPO-PDLA-PLLA copolymers were heated from amorphous state to 225[degrees]C at 10[degrees]C [min.sup.-1]. Obtained thermograms are given in Fig. 1. For all the samples, only one cold crystallization peak and one melting peak present at about 90 and 193[degrees]C, respectively. The results indicate that only stereocomplex crystallites formed during heating from amorphous state. As shown in Fig. 1, the melting temperature only slightly increased (ca 4[degrees]C) as the PLLA/PDLA block length ratio increased. This result indicates that the PLLA block length has no significant impact on the thickness of stereocomplex crystal.

The nonisothermal crystallization of PPO-PDLA-PLLA stereoblock copolymers from melt at 5[degrees]C [min.sup.-1] was also traced by DSC, and the thermograms are given in Fig. 2. As shown in Fig. 2, the crystallization started at about 140[degrees]C. As the PLLA/ PDLA block length ratio increased, the [T.sub.c] decreased slightly. This is because the movement of the longer stereocomplex segments is difficult than short ones.


Generally, the melt crystallization of polymer could be divided into nucleation and crystal growth. It is applicable to carry isothermal melt crystallization experiment in the nucleation process, the temperature range of which is around the onset temperature of melt crystallization. According to Fig. 2, we chose the 140-156[degrees]C as the isothermal crystallization temperature range. All the thermal properties data are summarized in Table 1.

Wide-Angle X-ray Diffraction

To investigate the crystal structure of PPO-PDLA-PLLA stereoblock copolymers at different crystallization temperature ([T.sub.c]), WAXD measurement were performed. Figure 3 shows the WAXD profiles of TB28 crystallized at various [T.sub.c]. As shown in Fig. 3, three crystalline diffraction peaks are observed at 20 values of about 12, 21, and 24[degrees]. TB7 and TB14 have the same profiles which are not given here. These 2[theta] values are in agreement with those reported for PLA stereocomplex crystallites [24, 28, 29], The results reveals that the PLLA/PDLA block length ratio has no effect on the crystal structure, and that all the PPO-PDLA-PLLA stereoblock copolymers only fold into stereocomplex crystalline during the isothermal crystallization processes. The results of our research are different from those reported by Li et al in which homocrystallites could be observed in the asymmetric PLLA/PDLA situation. They attributed the formation of homocrystallites to the relatively short PDLA chain length of poly(L-lactide-b-D-lactide) diblock copolymers, as a result, of which thicker homocrystallites (12 nm) is more stable than thinner stereocomplex crystallites (6 nm) [23].



Isothermal Crystallization Kinetics

As is known, crystallization behavior plays an important role in the physical properties of polymers, especially in the thermal and mechanical properties of polymers. For PLA materials, the isothermal crystallization behavior has an important effect on the performance, thus an understanding of the crystallization behavior of PPO-PDLA-PLLA stereoblock copolymers is essential. In this study, the isothermal crystallization kinetics of PPOPDLA-PLLA stereoblock copolymers was studied utilizing the DSC with various [T.sub.c] from 156 to 140[degrees]C.

Figure 4 shows the plots of relative degree of crystallization ([X.sub.t]) versus the crystallization time (t) for PPO-PDLA-PLLA stereoblock copolymers at different crystallization temperature. The relative degree of crystallinity [X.sub.t] is defined as


where the dH/dt is the instantaneous rate of enthalpy change at time t. As shown in Fig. 4, for all the samples, as the PLLA/ PDLA block length ratio decreased, it took shorter time to finish the crystallization. At 156[degrees]C, it took about 33 min to complete the crystallization for TB7, but for TB14 and TB28, it took about 45 and 50 min, respectively. The results indicate that the crystallization rate decreases as the PLLA/PDLA block length ratio increases.


The half-time of crystallization ([t.sub.1/2]) was also estimated from the plots, and the values are plotted as a function of [T.sub.c] in Fig. 5. As shown in Fig. 5, at the same [T.sub.c], the [T.sub.1/2] values of copolymers increase as TB7 < TB14 < TB28. Generally, [t.sub.1/2] value can indicate the crystallization rate of polymers. In this study, the [t.sub.1/2] values of all samples are much smaller than [t.sub.1/2] values of PLLA (41.7 min at 140[degrees]C [30] and 27.8 min at 120[degrees]C [31]). This can be attributed to the fact that the process of stereocomplex crystallization is faster than that of homocrystallization. In addition, the [t.sub.1/2] values of PPO-PDLA-PLLA stereoblock copolymers are also lower than values of multiblock PLAs (5.3-107 min for different samples) reported by Tsuji et al. [32], The disparity of tm values can be attributed to the small molecular weights ([M.sub.w] = 1.25-1.52 x [10.sup.4] g [mol.sup.-1]) of multiblock PLAs used in Tsuji's research.

1 - [X.sub.t] = exp(-[kt.sup.n]) (2)

Generally, the isothermal crystallization kinetics traced by DSC can be analyzed by the Avrami equation [33, 34], according to which the X, can be expressed as:

1 - [X.sub.t] = exp(-[kt.sup.n]) (2)

where n is the Avrami exponent, the values of which depend on the crystallization growth mechanism and crystallites morphology, and k is the crystallization rate constant. The equation can be rewritten as a liner form as follows:

log[-ln(1 - [X.sub.t])] = log k + n log t (3)

So the Avrami parameters can be obtained from the slope and intercept of Eq. 3. Lorenzo et al. [35] have proved that the conversion range of the [X.sub.t] employed for the fitting had an important influence on the error assessment of the Avrami equation because the secondary crystallization process produced nonlinearity in the Avrami plot. Therefore, we chose the [X.sub.t] range from 3 to 20% as the primary crystallization range to estimate the n and k values.

Figure 6 shows the Avrami plots of stereoblock copolymers. In all cases, the isothermal crystallization data were well described by the Avrami equation. The Avrami parameters obtained from the plots are summarized in Table 2. The n values are in the ranges of 2.55-2.84 for TB7, 2.41-2.72 for TB14, and 2.34-2.84 for TB28. For all the stereoblock copolymers, the n values are around 3, suggesting that all the growth of crystallization is three dimensional. The PLLA/PDLA block length ratio had no effect on the crystallization growth mechanism of PPO-PDLA-PLLA stereoblock copolymers crystallization. The crystallization rate constant k for all the polymer was also calculated using the following equation:

[k.sub.Cal.] = ln2/[t.sup.n.sub.1/2] (4)

The values of [k.sub.Cal.] obtained are also summarized in Table 2. As shown in Table 2, the [k.sub.Cal.] values agree well with the k values obtained from Fig. 6.



Polarized Optical Microscope

In this study, the growth behavior and morphology of PPO-PDLA-PLLA stereoblock copolymers spherulite were also monitored by the polarized optical microscope. Figure 7 shows the micrographs of PPO-PDLA-PLLA stereoblock copolymers obtained during the crystal growth at [T.sub.c] = 165[degrees]C at different time. For all the copolymers, well-defined Maltese crosses were observed, which indicates that the crystal aggregates are three-dimensional spherulites, and this result is consistent with the Avrami results mentioned above. As can be seen form Fig. 7, the spherulite size decreased as TB7 > TB14 > TB28. Obviously, the spherulites size became smaller with the increase of PLLA/PDLA block length ratio. The diameter of spherulite of TB7 reached up to 117 pm can be observed eventually, as for TB28, the diameter reduced to 60 pm or less. Moreover, with the increase in PLLA/PDLA block length ratio, the density of spherulites increased sharply as shown in Fig. 7. The increase in nucleation density of spherulites implies an enhancement of the mechanical properties of PLA [36, 37], this will be further studied.

Furthermore, the radial growth rate of the spherulite (G) of the PPO-PDLA-PLLA stereoblock copolymers was estimated based on the polarized optical photomicrographs taken at different crystallization time. The obtained G values are plotted in Fig. 8 as a function of [T.sub.c]. As shown in Fig. 8, the G value decreases as the PLLA/PDLA block length ratio and the [T.sub.c] increased, which is in accordance with the aforementioned DSC results.

Lauritzen-Hoffman Analysis and Correlation of Crystal Morphology with Crystallization Regime

The Lauritzen-Hoffman (LH) [38] theory can provide a molecular-level description of the crystallization from the melt. In this study, the LH theory was used to analyze the crystallization data from Fig. 8 and isothermal crystallization. The nucleation constant [K.sub.g] and the front constant [G.sub.0] can be estimated by the LH theory, in which G can be expressed as following equation:

G = [G.sub.0]exp[-[U.sup.*]/R([T.sub.c] - [T.sub.[infinity]])]exp[-[K.sub.g]/[T.sub.c][DELTA]Tf)] (5)

The equation can also be rewritten as following:

lnG + [U.sup.*]/R([T.sub.c] - [T.sub.[infinity]]) = ln[G.sub.0] - [K.sub.g]/([T.sub.c][DELTA][T.sub.f]) (6)

where the [U.sup.*] is the activation energy for the transportation of polymer chain segments from the melt to the crystallization site, R is the gas constant, [T.sub.[infinity]] is the hypothetical temperature where all motion associated with viscous flow ceases, [DELTA] [T.sup.0.sub.m] - [T.sub.c] is the degree of supercooling, and [T.sup.0.sub.m], is the equilibrium [T.sub.m], f = 2[T.sub.c]/[T.sup.0.sub.m] + [T.sub.c]) is a correction factor that accounts for the change in heat of fusion as the temperature decreases below [T.sup.0.sub.m]. Here, for sc-PLA, [U.sup.*] = 1500 cal/mol, [T.sub.[infinity]] = [T.sub.g] - 30 K, and [T.sup.0.sub.m] = 552.15 K is used as the equilibrium Tm, all the values are used in previous research [24, 39, 40], and [T.sub.g] values are obtained from the melt-quenched polymers in this study. Figure 9 shows the LH plots constructed form the G data in Fig. 8. The [K.sub.g] and [G.sub.0] values can be calculated from the slopes and intercepts, respectively. The values are listed in Table 3.


As shown in Fig. 9, TB7, TB14, and TB28 all could be fitted into one line. LH theory divides crystallization into three regimes, Regime I, Regime II, and Regime III depending on the degree of supercooling, and [K.sub.g] in Regime I and Regime III is twice as much as that in Regime II. Therefore, different [K.sub.g] values indicate the difference in the crystalline growth mechanism. The [K.sub.g] values of PPO-PDLA-PLLA stereoblock copolymers from 8.42 to 8.93 X [10.sup.5] [K.sup.2], are similar to those previously reported for PLA stereodiblock copolymers [24]. As there is only one [K.sub.g] value for each copolymer, we could not specify the regime of the PPO-PDLA-PLLA stereoblock copolymers. However, according to the temperature range used and [K.sub.g] values reported for branched PLA blends (8.22-11.1 X [10.sup.5] [K.sup.2]) [41], all PPO-PDLA-PLLA stereoblock copolymers in this study should be crystallized in Regime I. This result indicates that the PLLA/PDLA block length ratio has no effect on the crystal growth mechanism of PPO-PDLA-PLLA stereoblock copolymers. Similarly, the [K.sub.g] values for copolymers decrease with the increase of PLLA/PDLA block length ratio.


Since the [K.sub.g] is the thermodynamic nucleation barrier, the result indicates that the formation of nucleation in TB28 may be easier than it in TB7, which agrees with the results observed in POM (Fig. 7). In LH theory, [K.sub.g] can be expressed as following:


Where [b.sub.0] is the thickness of a polymer stem, [sigma] is the lateral-surface free energy, [[sigma].sub.c] is the fold-surface free energy, [DELTA][H.sup.0.sub.m] is the heat of fusion of perfect crystal, and k is the Boltzmann constant. The lateral-surface free energy a can be expressed as:

[sigma]=[alpha][DELTA][H.sup.0.sub.m][([a.sub.0][b.sub.0]).sup.1/2] (8)

Here for Regime 1 and Regime III, n = 4, for Regime II, n = 2, [a.sub.0] = 5.97 [Angstrom], [b.sub.0] = 5.17 [Angstrom], [DELTA][H.sup.0.sub.m] = 208 J/[cm.sup.3], and [alpha] = 0.25 [42]. Utilizing Eqs. 7 and 8, we can calculate [[sigma].sub.c]. The obtained [[sigma].sub.c] values are listed in Table 3.


As shown in Table 3, the [[sigma].sub.c] value decreases as the PLLA/PDLA block length ratio increased from 7:7 to 28:7. The result indicates that the nucleation formation becomes easier with the increase of PLLA/PDLA block length ratio. This result agrees with POM observations well, where nucleation sites density increase with the increase of PLLA/PDLA block length ratio (Fig. 7).


Isothermal crystallization kinetics of star-shaped PPO-PDLAPLLA stereoblock copolymers from the melt state was investigated in detail. The overall crystallization rate decreased with the increase of PLLA/PDLA block length ratio in the crystallization temperature range at 140-156[degrees]C. The minimum [t.sub.1/2] value is 2.85 min for TB7 at 140[degrees]C. In addition, for all the copolymers, the [T.sub.1/2] values decrease as the [T.sub.c] increases. Avrami analysis indicated that the PLLA/PDLA block length ratio did not affect the crystallization growth mechanism, and all copolymers crystallites in three dimensions with the Avrami exponent n around 3.

By the POM observation, the size of spherulite decreased in the following order: TB7 > TB14 > TB28, and the radial growth rate of the spherulite (G) had the same trend. The nucleation density increased as the PLLA/PDLA block length ratio increased, and the nucleation density of TB28 was the highest among all the copolymers.

LH theory analysis indicated that all copolymers exhibited one crystallization regime. The fold-surface free energy increased from 73.3 to 77.7 erg/[cm.sup.2] as the PLLA/PDLA block length ratio decreased from 28:7 to 7:7.


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Wei Li, Yan Ma, Zhongyong Fan

Department of Materials Science, Fudan University, Shanghai, 200433, People's Republic of China

Correspondence to: F. Zhongyong; e-mail:

Contract grant sponsor: National Nature Science Foundation of China (Project No. 51373041).

DOI 10.1002/pen.24144

Published online in Wiley Online Library (
TABLE 1. Characterization of PPO-PDLA-PLLA stereoblock copolymers

             [M.sub.n] (NMR) (a)          [M.sub.n] (GPC) (b)
Samples   ([10.sup.4]g [mol.sup.-1])   ([10.sup.4]g [mol.sup.-1])

TB7                  6.2                          2.1
TB14                 9.6                          4.1
TB28                 14.0                         5.8

          (deg [dm.sup.-1] [g.sup.-1]                [T.sub.g]
Samples           [cm.sup.3])           D-LA (%)   (([degrees]C)

TB7                  -6.4                 48.0         45.0
TB14                 -69.4                28.3         49.6
TB28                -116.5                13.6         53.8

          [] (c)    [T.sub.m]     [] (c)
Samples    ([degrees]C)    ([degrees]C)    ([degrees]C)

TB7            82.0           192.9           134.5
TB14           84.2           196.6           133.1
TB28           89.2           197.1           131.5

(a) Number-average molecular weight was calculated form the [sup.1]H
NMR spectroscopy.

(b) Number-average molecular weight was determined by GPC.

(c) [] and [] are onset cold crystallization and
onset melt crystallization temperature, respectively.

TABLE 2. Avrami exponent (n), kinetic rate constant (k), half-time of
crystallization ([t.sub.1/2]) for copolymers.

Samples  ([degrees]C)   [t.sub.1/2] (min)    n      k ([min.sup.-n)

TB7          140               2.85         2.79   3.46 x [10.sup.-2]
             144               3.28         2.70   2.63 x [10.sup.-2]
             148               4.76         2.75   1.04 x [10.sup.-2]
             152               5.99         2.74   4.89 x [10.sup.-3]
             156               6.78         2.55   5.75 x [10.sup.-3]
TB14         140               2.95         2.67   3.71 x [10.sup.-2]
             144               3.58         2.45   2.95 x [10.sup.-2]
             148               4.78         2.42   1.66 x [10.sup.-2]
             152               7.33         2.41   6.02 x [10.sup.-3]
             156              12.79         2.72   5.01 x [10.sup.-4]
TB28         140               5.31         2.57   8.71 x [10.sup.-3]
             144               6.93         2.57   3.02 x [10.sup.-3]
             148               8.37         2.57   4.78 x [10.sup.-3]
             152              11.52         2.56   1.51 x [10.sup.-3]
             156              13.46         2.84   5.12 x [10.sup.-4]

Samples  ([degrees]C)   [k (a).sub.Cal]. ([min.sup.-n])

TB7          140               3.73 x [10.sup.-2]
             144               2.81 x [10.sup.-2]
             148               9.49 x [10.sup.-3]
             152               5.14 x [10.sup.-3]
             156               5.26 x [10.sup.-3]
TB14         140               3.86 x [10.sup.-2]
             144               3.05 x [10.sup.-2]
             148               1.57 x [10.sup.-2]
             152               5.70 x [10.sup.-3]
             156               6.76 x [10.sup.-4]
TB28         140               9.49 x [10.sup.-3]
             144               4.79 x [10.sup.-3]
             148               2.95 x [10.sup.-3]
             152               1.33 x [10.sup.-3]
             156               4.31 x [10.sup.-4]

(a) The value was estimated form Eq. 4.

TABLE 3. Nucleation constant ([K.sub.g]) and front constant
([G.sub.0]) values for copolymers.

                [K.sub.g]             [G.sub.0]
Samples   ([10.sup.5][K.sup.2])    ([micro]m/min)     Regime

TB7               8.93            3.91 x [10.sup.7]     I
TB14              8.89            4.72 x [10.sup.6]     I
TB28              8.42            1.30 x [10.sup.6]     I

Samples   [[sigma].sub.e] (erg/[cm.sup.2])

TB7                     77.7
TB14                    77.3
TB28                    73.3
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Author:Li, Wei; Ma, Yan; Fan, Zhongyong
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:4EUGE
Date:Nov 1, 2015
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