Crystallization and Morphology of Immiscible Double Crystalline Poly

Sep 29, 2014 - For the immiscible double crystalline block co-polymers, the ..... Innovative Research Team in University of Ministry of Education of C...
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Crystallization and Morphology of Immiscible Double Crystalline Poly(p‑dioxanone)−Poly(tetramethylene ether)glycol Multiblock Co-polymers Jingjing Zhang, Miqin Zhan, Min Wei, Hui Xie, Keke Yang,* and Yuzhong Wang Center for Degradable and Flame-Retardant Polymeric Materials (ERCEPM-MoE), National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610064, China ABSTRACT: The crystallization behavior and intriguing crystal morphologies of double crystalline poly(p-dioxanone)-poly(tetramethylene ether)glycol (PPDO−PTMEG) multiblock co-polymers were investigated via differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and transmission electron microscopy (TEM). The effects of composition and microphase morphology on the crystallization of each segment were also explored. The results indicated that PPDO segments and PTMEG segments were immiscible. Composition and unique microphase morphology affected the crystallization ability and crystal morphology of PPDO and PTMEG segments. The kinetics of isothermal crystallization was studied calorimetrically and microscopically. It was found that crystallization rate of PPDO segment was slower than that of PPDO-H, which should be responsible for the dilution of PTMEG segments. In the case of PTMEG segments, the crystallization kinetics were highly correlated with the microphase morphology.

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INTRODUCTION Block co-polymers involve a wide variety of polymeric material, which has one type of repeat unit joined to another repeat unit at one or both ends.1−5 Compared to its parent polymer, this spaced chain structure of block co-polymer make it show significant difference in properties, such as crystallization behaviors.6−10 Depending on the segregation strength in the melt, either crystallization can drive structure formation for miscible systems as well as some weakly segregated co-polymers, or crystallization can be confined within the segregated microdomain for immiscible systems with strongly segregated co-polymers. So the block co-polymer that involves at least one crystalline polymer has attracted great interest for their crystallization behaviors and crystalline morphologies.10−15 In double crystalline block co-polymers, the situation is even more complicated.1,3,13,16−22 When cooling from the melt, both of the blocks can crystallize at different temperatures within different crystallization times: one crystallized block can produce a specific morphology and affect the crystallization and morphology of another.11,23 If the difference in melting temperature between the two segments is large enough, intriguing crystallization morphologies may occur.3,16,24 Therefore, the crystallization condition plays an important role in the crystallization behavior of double crystallization co-polymers. Besides, the miscibility of the two segments also affects the crystallization process. For the miscible double crystalline co-polymer, spherulites of one segment continue to grow in those of the other segment when they are crystallized from a homogeneous melt; also, the difference in the melting temperature between the two segments can influence the formation process. In the case of a small difference in Tm, both segments develop spherulites simultaneously; however, when the difference is an intermediate value, the spherulites of the higher-Tm segment first fill the entire volume, and the lower-Tm segment then nucleates and shows spherulitic growth inside those of the higher-Tm © 2014 American Chemical Society

segment. For the immiscible double crystalline block copolymers, the crystallization behaviors are much more complex, because the microphase morphology also plays an important role. The great diversity of microphase morphology is formed by the strong repulsive interaction accompanied by the limitation of the covalent bond between the blocks. Over the past decades, the crystallization behavior of immiscible diblock or triblock co-polymers has been extensively investigated, and the effect of composition, segregation strength, and microphase morphology are profoundly explored.1,5,6,18,20 Recently, much more attention has focused on the more-complicated multiblock co-polymer systems.2,20,21 Compared to the immiscible diblock or triblock co-polymers, the segregation strength between blocks in this system will be weakened by the existence of more-covalent bonding; therefore, the microphase morphology is not only dominated by the composition, but also is highly dependent on the block length. In fact, there are still many issues have not been completely understood. In our previous work, we developed a series of poly(p-dioxanone)−poly(tetramethylene ether)glycol multiblock co-polymers (PPDO−PTMEG) based on poly(p-dioxanone) (PPDO) and poly(tetramethylene ether) glycol (PTMEG).25 In this co-polymer, the PPDO segments with high Tm serve as “hard” segments, while PTMEG segments with low Tm act as “soft” segments. The immiscible double crystalline PPDO− PTMEG shows good shape-memory effect. Specially, the crystallization temperature (Tc) of the PPDO segments is quite closed to the Tm of PTMEG segments; meanwhile, Tc of the PTMEG segments is just slightly higher than the glass-transition temperature (Tg) of PPDO segments. Undoubtedly, the particular Received: Revised: Accepted: Published: 16793

July 31, 2014 September 25, 2014 September 29, 2014 September 29, 2014 dx.doi.org/10.1021/ie503051c | Ind. Eng. Chem. Res. 2014, 53, 16793−16802

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S-TWIN electron microscope at an acceleration voltage of 200 kV. For these measurements, the samples were first microtome in ultrathin slices, and then the droplets of uranyl acetate were deposited onto the cast copper grids with ultrathin slices of samples to stain. Excess solvent was wiped with filter paper, and then dried under room temperature. Polarizing Optical Microscopy (POM). A POM microscope (Model ECLIPSE LV100POL, Nikon, Japan) equipped with a temperature controller (Model HSC621 V, Instec, USA) was used to probe the miscibility and crystal morphology of PPDO−PTMEG, PPDO-H and PTMEG-H grown in films. Each sample was heated to 140 °C and held for 5 min, then quenched to the predetermined crystallization temperature immediately and maintained enough time to complete the isothermally crystalline process. The crystallization process was monitored by taking photomicrographs, using a computer that was equipped with a CCD camera.

interaction between those two blocks should affect the crystallization behavior. Therefore, it is necessary to take a deep insight of the relationship among the composition, microphase morphology, and crystallization behavior of PPDO−PTMEG to get a full understanding of the shape-memory mechanism. In the present work, the immiscibility between PPDO segments and PTMEG segments was confirmed by differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) observation. The influence of morphologies and composition on the crystallization behaviors of PPDO−PTMEG were investigated via DSC, temperature-modulated differential scanning calorimetry (TMDSC), and POM.

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EXPERIMENTAL SECTION Materials and Co-polymers Preparation. Dihydroxyl terminated poly(tetramethylene ether)glycol (PTMEG diol) (Mn = 2900 g/mol, reagent-grade) was supplied by Aldrich Company, and p-dioxanone (PDO) was obtained from the pilot plant of the center for degradable and flame-retardant polymeric materials (Chengdu, China). 1,6-Hexamethylene diisocyanate (HDI) (AR-grade) from Sigma−Aldrich was used without further purification. Preparation of PPDO−PTMEG Multiblock Co-polymers. Dihydroxyl terminated poly(p-dioxanone) (PPDO diol) and was synthesized by ring-opening polymerization of p-dioxanone (PDO)26 PPDO−PTMEG multiblock co-polymers were prepared by using HDI as a coupling agent to combine PPDO diol and PTMEG diol, according to our previous work.25 Characterization. 1H NMR spectra were obtained at 400 MHz using a Bruker Model 400 system (Bruker, Switzerland), with deuterated chloroform as a solvent. Gel permeation chromatography (GPC) was performed on a Waters instrument equipped with a Model 1515 pump, a Waters Model 717 autosampler, and a Model 2414 refractive index detector CHCl3 and polystyrene were used as the eluent and standard, respectively. The intrinsic viscosity ([η]) of the resulting polymers was measured in phenol/1,1,2,2-tetrachloroethane (1:1 v/v) solution using an Ubbelohde viscometer maintained at 30 °C. Given that the co-polymers that contain PPDO segments with high molecular weight has poor solubility in the solvents commonly employed to perform gel permeation chromatography (GPC) at ambient temperature, only low-molecular-weight PPDO prepolymer could be measured by GPC; the co-polymers with high molecular weight can simply be evaluated by intrinsic viscosity ([η]). Differential Scanning Calorimetry (DSC). Both conventional DSC and TMDSC measurements were performed on a TA Instruments Model DSC-Q200 system under a nitrogen purge; the sample weight was ∼5 mg. For nonisothermal crystallization, all samples were first heated to 140 °C to erase thermal history, and then scanned between 140 °C and −50 °C at 10 °C/min to obtain the thermal properties. In the case of isothermal crystallization, samples were also melted at 140 °C and kept for 3 min to eliminate thermal histories and quenched to the crystallization temperature. The samples were crystallized at the isothermal crystallization temperature until saturation. Then, the samples were heated to 140 °C at 10 °C/min. With regard to the TMDSC measurements, we chose a heating rate at 2 °C/min with an oscillation amplitude of 0.3 and an oscillation period of 110 s by experimentation. Transmission Electron Microscope Observations (TEM). Miscibility and phase morphology of PPDO−PTMEG co-polymers were investigated, using a Tecnai Model G2F20

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RESULTS AND DISCUSSION Synthesis and Characterization of PPDO−PTMEG Copolymers. PPDO−PTMEG multiblock co-polymers with different compositions were prepared by coupling the PTMEG diol and PPDO diol with HDI (Scheme 1). For comparison, PPDO-H and PTMEG-H were also prepared via chain-extending PPDO diol and PTMEG diol with HDI, respectively. The basic data for all samples employed in this work are tabulated in Table 1. In order to characterize the chemical structure of PPDO diol, PTMEG diol, and PPDO−PTMEG multiblock co-polymers, 1 H NMR analysis was employed; the spectra are illustrated in Figure 1. In the PPDO diol spectrum (Figure 1a), three signals with high intensity at 4.16 ppm (δHa), 3.78 ppm (δHb), 4.33 ppm (δHc) are attributed to the three methylene protons in PPDO diol repeat units, respectively. In addition, the weak signals at 3.69 ppm (δHb′) and 3.61 ppm (δHc′) are assigned to the corresponding protons in terminal group of PPDO diol. The spectrum of PTMEG diol is showed in Figure 1b, the resonances ascribing to the methylene protons of repeat units appear at 1.61 ppm (δHi) and 3.41 ppm (δHj), and the resonances corresponding to the protons in terminal group appear at 1.66 ppm (δHi′) and 3.58 ppm (δHj′). To determine the chemical structure of PPDO−PTMEG copolymers, PPDO−PTMEG-50/50 was chosen as an example (Figure 1c). There are ten characteristic resonances: 4.14 ppm (δHe) and 1.75 ppm (δHd) belong to the residue methylene from 1,4-butanediol. The resonances at 4.11 ppm (δHa), 3.73 ppm (δHb), and 4.28 ppm (δHc) are assigned to the three methylene protons in PPDO diol repeat units, respectively, while the peak positions corresponding to the methylene protons situate at 1.62 ppm (δHi) and 3.41 ppm (δHj) belong to repeat units of PTMEG diol. It makes sense that all characteristic signals for PTMEG diol and PPDO diol repeat units remained, but the peaks ascribing to the end groups of the two prepolymers disappear. The further evidence for the occurring the chainextending reaction is the presence of new signals at 1.51 ppm (δHg), 3.16 ppm (δHh), and 1.34 ppm (δHf), which are attributed to three types of methylene protons derived from HDI. Miscibility and Microphase Morphology of PPDO− PTMEG Co-polymers. Although both PPDO and PTMEG segments has crystalline ability, the miscibility of two segments plays an important role to dominate the crystallization behavior. In order to clarify this point, the DSC and TEM were employed to detect the change in glass-transition temperature (Tg) and the micromorphology of the co-polymer. 16794

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Scheme 1. Synthetic Route the PPDO−PTMEG Co-polymers

Table 1. Composition of Co-polymers

a

sample

PPDO diola content (wt %)

PTMEG diolb content (wt %)

[η] (dL/g)

PPDO-H PPDO−PTMEG-75/25 PPDO−PTMEG-50/50 PPDO−PTMEG-25/75 PTMEG-H

100 75 50 25 0

0 25 50 75 100

2.00 2.64 1.94 2.21 1.18

n(PDO):n(BD) = 70:1, Mn, PPDO diol = 7700 g/mol, PDI = 1.42. bMn, PTMEG diol = 2900 g/mol.

Figure 1. 1H NMR of (a) PPDO diol, (b) PTMEG diol, and (c) PPDO−PTMEG-50/50 co-polymer.

bases to ensure the miscibility of a binary co-polymer. In the present system, PPDO-H exhibits an apparent Tg,2 at −10.9 °C, while the Tg value of PTMEG-H is not observed because it is below the limitations of the instrument. For PPDO−PTMEG co-polymers, all show one distinct Tg value in the second heating

Figure 2 presents the standard DSC traces of PPDO-H, PTMEG-H, and co-polymers with various compositions recorded by the cooling scan (Figure 2a) and the heating scan (Figure 2b). The relevant information is summarized in Table 2. As is well-known, the shifting trend of Tg is one of important 16795

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Figure 2. DSC curves of PPDO-H, PTMEG-H, and PPDO−PTMEG co-polymers: (a) cooling run after melt quenching at rate of 10 °C/min and (b) subsequent heating run following the procedure described for panel “a” at a rate of 10 °C/min.

Table 2. Results from DSC Cooling and Second Heating Scans of Multiblock Co-polymers, PPDO-H, and PTMEG-H Cooling Runa sample PPDO-H PPDO−PTMEG-75/25 PPDO−PTMEG-50/50 PPDO−PTMEG-25/75 PTMEG-H a

Tc1 (°C) −5.2 −9.1 −12.5 −11.4

ΔHc1 (J/g) 16.2 29.5 35.9 54.8

Second Heating Runa Tm1 (°C)

ΔHm1 (J/g)

25.8 27.1 26.2 26.2

Tc2 (°C)

ΔHc2 (J/g)

Tm2 (°C)

ΔHm2 (J/g)

Tg2 (°C)

43.4 29.2 24.7

43.1

98.1 95.5 94.7 94.3

44.0 33.7 21.9 10.7

−10.9 −10.6 −11.4 −10.5

55.9

The subscript “1” denotes the PTMEG segments, and the subscript “2” denotes the PPDO segments.

Figure 3. TEM micrographs of PPDO−PTMEG co-polymers: (a) PPDO−PTMEG-25/75, (b) PPDO−PTMEG-50/50, and (c) PPDO−PTMEG75/25.

scan, at approximately −11 °C, which is constant and close to that of PPDO-H. No distinct shift of Tg,2 to lower temperature is observed. The two independent Tg values were also identified by DMA test, which were detected at −58.97 °C and 5.81 °C, respectively. The almost-unchanged Tg value and the melting point over the entire composition range indicated that PPDO− PTMEG co-polymers are immiscible.27 For a more-direct proof of this view, TEM micrographs of the co-polymers with different composition are supplied in Figure 3, in which the stained PPDO phase presents in dark color, contrasting with the PTMEG domains in light color. All samples show microphase separation morphology, but the structure and the scale of the microdomains varied with composition. For the PTMEG-rich sample (PPDO−PTMEG-25/75), the PTMEG segments trended to form adjacent globular microdomains with a diameter of ∼30−50 nm but not a continuous phase; otherwise, the PPDO segments filled in the interstitial space of the PTMEG microdomains. For PPDO−PTMEG-50/50, PTMEG acted as a dispersed phase in a diameter of ∼50 nm, distributed uniformly in the PPDO continuous phase. For

PPDO−PTMEG-75/25 (a PPDO-rich sample), a sausage-shaped phase structure was observed. PPDO acted as both a continuous phase for the PTMEG dispersed phase, and as a dispersed phase in each PTMEG domain. Based on a full understanding of the morphological structure, we investigated the crystal morphology and crystallization kinetics to explore their effect on crystallization behavior in the following section. Nonisothermal Crystallization Behaviors of PPDO− PTMEG Co-polymers. Beside the information on the Tg value of the co-polymers, the DSC curves in Figure 2 also illustrates the crystallization and melting process of PPDO-H, PTMEG-H, and co-polymers. Since the crystallization rate of PPDO segments is quite slow,13 PPDO-H does not crystallize during the cooling scan at a rate of 10 °C/min (Figure 2a): it only exhibits a cold-crystallization peak (Tc,2) at 43.4 °C, and a corresponding melting peak (Tm,2) at 98.1 °C (Figure 2b). However, PTMEG-H shows an sharp exothermic crystallization peak (Tc,1) at −11.4 °C during the cooling scan and an endothermic melting peak (Tm,1) at 26.2 °C during the subsequent heating scan. 16796

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As a double crystalline co-polymer, the crystallization behavior of one segment will be affected by the other more or less. In this system, the crystallization temperature of PTMEG segments (Tc,1) decreases slightly in PPDO−PTMEG-25/75, and then increases with the addition of PPDO segments. On the other hand, the cold-crystallization temperature of PPDO segments (Tc,2) decreases from 43.4 °C to 24.7 °C as the PTMEG segments content increases from 0% to 50%. It seems that the PTMEG segment has a nucleating effect for the crystallization of PPDO segments. Since the crystalline temperature of the PPDO segment is closed to the melting temperature of the PTMEG segment, there is mutual interference between the cold-crystallization exothermic peak of the PPDO segments and the melt endothermic peak of the PTMEG segments. In order to dissociate these two opposite thermal signals from the overlap curve, TMDSC was used with a low scan rate of ∼2 °C/min.28 As shown in Figure 4, the cold-crystallization

Table 3. Results from TMDSC Heating Scans of Co-polymers sample

Tc2a (°C)

ΔHc2a (J/g)

Tm1a (°C)

ΔHm1a (J/g)

PPDO−PTMEG-75/25 PPDO−PTMEG-50/50 PPDO−PTMEG-25/75

15.96 14.65 16.22

26.23 20.3

25.79 27.01 26.11

15.82 19.44

The subscript “1” denotes PTMEG segments, and the subscript “2” denotes PPDO segments; Tc2 is obtained from the nonreversing heat flow, and Tm1 is obtained from the reversing heat flow. a

co-polymers are highly related to the weight fraction of the two segments. In order to understand this influence quantitatively, the isothermal crystallization kinetics of PTMEG segments in co-polymers with different composition was studied by differential scanning calorimetry (DSC) with the isothermal crystallization temperature ranged from −11 °C to −3 °C. Considering that the crystallization rate of samples varies markedly with the composition, it is difficult to choose a temperature range to conduct the isothermal crystallization of all of the samples with a suitable crystallization rate, which is the limitation of the instrument resolution. Specifically, if the crystallization rate is too slow, the amount of heat evolved per unit time is too small to be registered; on the other hand, if the crystallization rate is too fast, the experiments are limited by crystallization during cooling to Tc (i.e., the sample could have already crystallized by a small amount before the isothermal DSC run starts), which may lead to incomplete isothermal runs.13 In order to avoid crystallization of the material during the previous cooling ramp to the selected crystallization temperature, we chose a reasonable crystallization temperature for a certain sample to explore the isothermal crystallization behavior. Hence, not all of the co-polymers cover the same temperature range in this work. Figure 5 displays the plotted curves of relative crystallinity (Xt) as a function of time (t) for different compositions and crystallization temperatures. For each sample, it can be seen that Xt increases rapidly with time at a lower temperature. While at the same crystallization temperature, such as −7 °C, it should be more interesting to investigate crystallization behaviors of the PTMEG segments: less time is required for PPDO− PTMEG-50/50 and PPDO−PTMEG-75/25 to finish the crystallization, compared to PTMEG-H. However, in the case of PPDO−PTMEG-25/75, the crystallization rate is slower. Indication of the crystallization of PTMEG segments is suppressed within PPDO−PTMEG-25/75, but is enhanced in PPDO− PTMEG-50/50 and PPDO−PTMEG-75/25. This trend is consistent with the previous DSC analysis. The explanations for such special behavior should be discussed in detail. As we know, the Avrami exponent, which is derived from isothermal crystallization, reflects the nucleation mechanism and growth dimension of the crystals.29 In addition, analysis of the crystallization kinetics is assumed as the classical Avrami equation (eq 1) with the development of the relative degree of crystallinity and crystallization time (t).

Figure 4. TMDSC traces of PPDO−PTMEG-50/50 nonisothermally crystallized from the amorphous state at 2 °C/min. Traces that represent the reversing heat flow (“Reversing”), nonreversing heat flow (“Nonreversing”), and total heat flow (“Total”) are shown.

exothermic peak of PPDO segments exhibits in nonreversible heat flow, while the melting endothermic peak of PTMEG segments in reversible heat flow. It is worth to be mentioned that the overlap pattern was changed in the total superposition curve, showing as the Tcc of PPDO segment shift to the lower temperature. As we known, viscously driven diffusion process plays the leading role during the cold crystallization. While the heating rate is 10 °C/min, there is not enough time for the motion of PPDO segments. However, at 2 °C/min, it will take less time for chains to move to the growing crystal front, so the cold crystallization shifts to lower temperature. The relevant information is summarized in Table 3. Isothermal Crystallization of PPDO−PTMEG Copolymers. According to the previous studies of block copolymers, during the cooling scan, two segments crystallized in well-separated temperature regimes: one segment crystallized first, and then the crystallization of another segment is affected.16,28 Hence, the crystallization is physically constrained by the microphase morphology of the crystalline phase. However, when PPDO−PTMEG multiblock co-polymers were quenched to the designed crystallization temperature, PPDO segments cannot crystallize immediately; however, the microdomain created by PPDO pre-existing morphologies also may restrict the crystallization of PTMEG segments. According to previous TEM observation, the microphase morphologies of

1 − X(t ) = exp[ − Kt n]

(1)

where K is a composition rate constant involving nucleation and growth rate parameters; n is the Avrami exponent, depending on the nature of nucleation and growth geometry of crystals. Traditionally, K and n can be extracted from a line fitted to the following double logarithmic plot: 16797

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Figure 5. Crystallization isotherms at the indicated temperature for (a) PTMEG-H, (b) PPDO−PTMEG-25/75, (c) PPDO−PTMEG-50/50, and (d) PPDO−PTMEG-75/25.

Figure 6. Avrami plots of the isothermal crystallization at indicated temperature for (a) PTMEG-H, (b) PPDO−PTMEG-25/75, (c) PPDO− PTMEG-50/50, and (d) PPDO−PTMEG-75/25.

log[− ln(1 − X t )] = log K + n log t

In the Avrami expression, the parameter n provides qualitative information on the nature of nucleation and the growth process. Table 4 shows that the average exponent n for PTMEG-H, PPDO−PTMEG-50/50, and PPDO−PTMEG-75/25 is ∼2.5; however, for PPDO−PTMEG-25/75, n has a comparatively low value, ranging from 1.95 to 2.16. Sakruai et al. discussed a small n as

(2)

Figure 6 shows the Avrami plots of co-polymers at different crystallization temperatures. We only focus on the kinetics of primary crystallization. The crystallization of co-polymers fit eq 2 very well, and the kinetic parameters obtained are summarized in Table 4. 16798

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and nucleation mechanisms. Generally, an n value of ∼3 indicates an athermal nucleation progress, followed by threedimensional crystal growth, while an n value of ∼2 is an athermal nucleation progress followed by two-dimensional crystal growth.31,32 In general, the parameter K is controlled by nucleation and diffusion mechanisms.30 In the present system, K was found to be very sensitive to temperature, and decreased significantly as Tc increased in all of the samples (see Table 4), suggesting that the crystallization rate was decreased. For a more precise description, the crystallization rate, which represents another important parameter (half-time, t1/2), was introduced. Specifically, t1/2 is the time required to achieve 50% of the final crystallinity of the samples, which is calculated by the following equation:

Table 4. Isothermal Crystallization Kinetics of PTMEG-H and Co-polymers Based on the Avrami Equation sample PTMEG-H

PPDO−PTMEG25/75

PPDO−PTMEG50/50

PPDO−PTMEG75/25

Tc (°C)

n

K (min−n)

t1/2a (min)

t1/2b (min)

−3 −5 −7 −9 −11

2.32 2.85 2.58 2.43 2.52

0.0043 0.014 0.19 0.88 4.47

8.97 3.92 1.65 0.91 0.48

9.00 3.90 1.65 0.90 0.47

−7

1.98

0.044

4.04

4.06

−9 −11

1.95 2.16

0.63 2.25

1.05 0.58

1.05 0.57

−5

2.85

0.26

1.42

1.40

−7 −9 −11

2.44 2.59 2.29

1.90 11.75 14.45

0.66 0.34 0.27

0.66 0.32 0.26

−3

2.85

7.24

0.44

0.44

−5 −7 −9

2.37 2.19 2.25

18.20 20.89 34.67

0.25 0.21 0.18

0.25 0.21 0.17

t1/2 =

⎛ ln 2 ⎞1/ n ⎜ ⎟ ⎝ K ⎠

(3)

where the K and n have the same definitions as those given for the Avrami equation. The values of t1/2 at different Tc, calculated from eq 3, are tabulated in Table 4. It is clear that, at a given crystallization temperature, PTMEG segments show a significant difference in the value of t1/2. Compared to PTMEG-H, PPDO−PTMEG25/75 has a larger t1/2 value, but PPDO−PTMEG-50/50 and PPDO−PTMEG-75/25 have smaller t1/2 values. We also found that the value of t1/2 obtained directly from the conversion curves matched that calculated from the Avrami parameters well, which indicated that the Avrami equation analysis is suitable for the crystallization of this system. Crystal Morphology of PPDO−PTMEG Co-polymers. The intriguing crystal morphologies of PPDO−PTMEG during the isothermal crystallization were recorded by POM. As expected (Figure 7a), PPDO exhibits well-defined banded spherulite with a typical maltese cross-extinction pattern. In the case of multiblock co-polymers, the crystal morphology of PPDO

a

Calculated from the Avrami parameters by eq 3. bObtained from the time of 50% conversion, from Figure 5.

follows: a lower geometric dimensionality in the microdomain should give a higher spatial constraint.30 For PPDO−PTMEG25/75, it may be caused by the following reasons: first, the microdomain is smaller as discussed previously; on the other hand, the smaller microdomains are surrounded by PPDO segments. As for a nonintegral n value, it may be resulted from the crystal branching, two-stage crystal growth, mixed growth,

Figure 7. Polarized optical microscopy (POM) micrographs of PPDO segments crystal morphology after completely crystallized at 30 °C: (a) PPDO-H, (b) PPDO−PTMEG-75/25, (c) PPDO−PTMEG-50/50, and (d) PPDO−PTMEG-25/75. 16799

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of PPDO segments in co-polymers and PPDO-H can be determined from the slope of the related line. It may be concluded that the spherulites growth rate decreases with the increasing of PTMEG fraction. This should be ascribed to the crucial dilution role of PTMEG, which reduces the number of crystallization units at the crystal growth front.32 Figure 9 shows the intriguing morphology of PTMEG-H. Distinctly, there are two different types of crystal morphology: one is the typical spherulite, and the other is a crystal in axial symmetry. This phenomenon presumably results from the differences in growth category, since the early stage of crystal growth strongly influences the late-stage crystal morphology. To identify this speculation, we record the growth process of these two different types of crystal. An admixture of category A and category B is obtained in Figure 10, which were obtained at the same crystallization condition. From a mathematical perspective, category A grain initially form its structure as slender threadlike fibers, then secondary fibrils nucleate at the growth front to form crystal sheaves. Lastly, it successively branches to form space-filling patterns. However, for category B, the initial crystal has a 4-fold symmetric structure, and the associated branching lead to isotropic growth. For an immiscible double crystalline co-polymer system, the microphase morphology and the microdomain size affects the crystallization behavior and crystalline morphology significantly. Just as we mentioned previously, varying the composition of the PPDO−PTMEG co-polymers resulted in different microdomain sizes. Take PPDO−PTMEG-75/25 as an example. Figure 11 illustrates the variation of crystalline morphology before and after the melting of PTMEG segments. At 30 °C, which is higher than the Tm of PTMEG, we can observe imperfect spherulites with several small dark dots. As we know, PPDO−PTMEG75/25 exhibits a sausage-shaped phase structure, which was observed by TEM (recall Figure 3). As the dominant component, the PPDO segment is exhibited not only as the continuous phase corresponding to PTMEG disperse phase, but it also exists as a smaller disperse phase spread in each PTMEG phase. Therefore, the imperfect spherulites are formed by PPDO segment in continuous phase, and the small dark dots may be attributed to the PTMEG phase contained PPDO amorphous phase. When cooling the sample to a lower temperature 5 °C (T ≈ Tc1), the dark dots brighten, because of the formation of new crystals from the PTMEG segment, and the dispersed PPDO phase cannot crystallize in such small domains. Heating the sample to 30 °C again, the newly formed crystals melted; only the banded PPDO

Figure 8. Spherulite diameter of PPDO−PTMEG co-polymers and PPDO-H as a function of time of growth (isothermally at 30 °C).

Figure 9. POM micrographs of PTMEG-H crystal morphology.

segments transformed with its weight fraction. As shown in Figure 7b, PPDO segments in PPDO−PTMEG-75/25 show well-defined banded spherulite morphology with uniform holes, but a granular aggregate morphology for PPDO−PTMEG-50/50, as evidenced in Figure 7c. The granular aggregates that correspond to PPDO segments are still three-dimensional for the maltese cross-pattern. With regard to PPDO−PTMEG-25/75 (Figure 7d), the PPDO segments exhibits irregular crystals, which are dispersed in the dark matrix of the PTMEG segment melt. Crystallization rate may also be observed microscopically by measuring the growth of the spherulites as a function of time. At an isothermal crystallization temperature of 30 °C, the spherulite diameter of co-polymers and PPDO-H shows a linear relationship with growth time before spherulites impinge on their neighbors (see Figure 8). The spherulites growth rate

Figure 10. Crystal morphology category of PTMEG-H. 16800

dx.doi.org/10.1021/ie503051c | Ind. Eng. Chem. Res. 2014, 53, 16793−16802

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Figure 11. Crystalline morphology of PPDO−PTMEG-75/25 formed at different temperatures: (a, a′) 30 °C, (b, b′) 5 °C, and (c, c′) 30 °C again (panels a, b, and c represent optical micrographs, and panels a′, b′, and c′ represent POM micrographs).

spherulites still existed. In such a case, it is easier to conclude that PTMEG segments can also crystallize just inside the dark region and coexist with the crystalline PPDO segments. Similar coexistence is also found in PPDO−PTMEG-50/50 and PPDO− PTMEG-25/75, which can only crystallize at a lower temperature (i.e., 0 °C), but the crystal size of PTMEG segments is smaller than that of PPDO−PTMEG-75/25. Considering the previous observation of isothermal crystallization, it can be confirmed that the PTMEG segments is a 3D spherical assembly of 2D lamellar crystals.

separate crystallization temperature, and microphase morphology. In addition, the composition of PPDO−PTMEG co-polymers evidently affects the crystal morphology of PPDO and PTMEG segments. • Second, for the PPDO segment, its cold-crystallization peak temperature was found to shift to the lower temperature range due to the incorporation of PTMEG segments. • Third, for the PTMEG segment, its crystallization behaviors were highly dependent on the microphase morphology of the co-polymer. As compared to PTMEG-H, the crystallization ability of PTMEG segments in PPDO−PTMEG-25/75 was depressed, because of the restriction of PPDO domain, which causes a coincident crystallization phenomenon, such as lower crystallization temperature, longer half-time (t1/2), and smaller n. However, in PPDO−PTMEG-50/50 and PPDO−PTMEG-75/25, the crystallization rate of the PTMEG segments accelerates. • Lastly, the composition of PPDO−PTMEG co-polymers evidently affects the crystal morphology of PPDO, but no obvious influence is found in the crystal morphology of PTMEG segments.

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CONCLUSIONS The phase separation, crystallization behavior, and crystal morphology of PPDO−PTMEG multiblock co-polymers have been studied by DSC, TMDSC, POM, and TEM. PPDO-H and PTMEG-H were also discussed, for the sake of comparison. The following conclusions were obtained: • First, within PPDO−PTMEG multiblock co-polymers, PPDO segments were immiscible with PTMEG segments as evidenced by the composition-independent glass transition, 16801

dx.doi.org/10.1021/ie503051c | Ind. Eng. Chem. Res. 2014, 53, 16793−16802

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-28-85410755. Fax: +86-28-85410284. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported financially by the National Science Foundation of China (Nos. 51273120, 51473096), the 863 Program (No. 2012AA062904), and the Program for Changjiang Scholars and Innovative Research Team in University of Ministry of Education of China (No. IRT1026).

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