Mixing Assisted Direct Formation of Isotactic Poly(1-butene) Form I

Feb 19, 2016 - The experimental results reveal the possibility to modify the crystallization pathway of iPB-1 in iPB-1/iPP blend through the mixing de...
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Mixing Assisted Direct Formation of Isotactic Poly(1-butene) Form I′ Crystals from Blend Melt of Isotactic Poly(1-butene)/Polypropylene Youxin Ji, Fengmei Su, Kunpeng Cui, Ningdong Huang,* Zeming Qi, and Liangbin Li* National Synchrotron Radiation Lab and College of Nuclear Science and Technology, CAS Key Laboratory of Soft Matter Chemistry, University of Science and Technology of China, Hefei, China S Supporting Information *

ABSTRACT: The influence of mixing of iPB-1/iPP blend on the polymorphism of iPB-1 under processing-relevant conditions is studied with emphasis on the competition between the thermodynamically stable form I′ crystal and the kinetically favored form II. In situ optical microscopy measurements reveal that the upper critical solution temperature (UCST) of iPB-1/iPP blend locates in the range of 180− 200 °C. Unexpectedly, by quenching mixed iPB-1/iPP melt down to temperatures below UCST and melting point, form I/I′ can be produced directly which is further identified as form I′ by temperaturedependent WAXS and DSC. The formation of form I′ is promoted by increasing the annealing time above UCST, while is suppresses by raising the quenching temperature. In addition, the crystallization of iPP also displays a similar trend as iPB-1 does. The correlated crystallization of each constituent with dependence on the initial mixing degree suggests that the crystallization behavior of the binary blends is determined by the interplay between simultaneous processes concomitant with the liquid−solid transition. The experimental results reveal the possibility to modify the crystallization pathway of iPB-1 in iPB-1/iPP blend through the mixing degree which is initially controlled by annealing but is subject to evolve during the subsequent thermal treatment. Possible mechanisms are discussed including the roles of phase separation and concentration fluctuation in crystallization.

1. INTRODUCTION Isotactic poly(1-butene) (iPB-1) is a widely used semicrystalline polyolefin material benefiting from its excellent physical and mechanical properties, for instance, relatively high heat distortion temperature, low tendency to creep, and high stress cracking resistance.1 The key feature of iPB-1 is its complex polymorphic behavior,2−26 which has been a subject attracting continuing attention for its commercial interest and scientific importance but also has limited its commercial development. iPB-1 exhibits four crystal modifications including I, II, III, and I′ depending on the formation condition.2−6,10 Form I crystal, the most stable one characterized by chains in 3/1 helical conformation packed in a twined hexagonal unit cell, is usually obtained from the spontaneously transition from the unstable form II crystal.10,11,16−18 Form II crystal, the metastable form with chains in 11/3 helical conformation packed in a tetragonal unit cell,3,19,20 can be obtained by melt crystallization and will slowly transform into stable form I crystals on aging at room temperature. Form III crystal, orthorhombic with a 4/1 helix, is generally obtained by crystallization from dilute solutions.6,12,21,23 A further crystal modification, form I′ crystal with the same chain conformation of 3/1 helix as form I whereas packs in an untwined hexagonal unit cell, can be obtained by crystallization from solution or from melt under peculiar conditions.5,22 Form I′ is similar to © XXXX American Chemical Society

form I in helical conformation and crystallographic structure but differs from the latter in melting temperature. Form I melts at 120−135 °C whereas form I′ melts at about 90−100 °C. The melt temperatures of form II and form III are 110−120 and 90−100 °C, respectively.3,11,13−15 The existence of spontaneous solid phase transformation from kinetically favored form II to stable form I causes a time dependence of mechanical properties and shrinkage of iPB-1 products,1,9−11,18,25−28 which considerably limited the application and commercial diffusion of iPB-1. A great deal of research has been oriented toward finding solutions to accelerate or avoid the phase transformation. Direct formation of form I′ from the melt is considered to be advantage for industrial because of the bypassing of the disadvantageous transient stage of form II formation. The condition to generate form I′ is rather stringent requiring high pressure29−31 or particular treatment under ambient pressure conditions, such as crystallization onto suitable substrates,13,32 in ultrathin samples,33,34 by self-seeding,34−36 from memorized ordered melt,37 in polymers with stereodefects,38−40 or in copolymers with 1-alkenes.41−46 Those special treatments not Received: October 1, 2015 Revised: January 21, 2016

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Figure 1. Schematic illustration of sample preparation processes of (a) quenching and (b) isothermal crystallization after annealing.

form II, form I′ or concomitant crystallization of two forms can be observed. The revealed direct formation of iPB-1 form I′ crystals and its connection to the thermal treatment can help to understand the blend system better and can also provide new guidance to improve the performance of material in industrial processing.

only complicate the processing with higher cost but also obscure the exact mechanism of direct formation of form I′ crystals from molten state of iPB-1 and its copolymers. Polymer blending provides a simple and effective approach for the preparation of materials with new desirable properties as well as reducing the basic costs and improving the processability.47,48 The properties of polymer blends depend not only on the constituents’ individual properties but also on their mixing degree. Recently, the crystalline/crystalline polymer blends have received much attention, as they can produce a wide variety of superstructures, which is more helpful in tailoring the properties.49−51 As most polymer pairs are thermodynamically immiscible or partially miscible, two types of phase transitions, liquid−liquid phase separation (LLPS), and crystallization may occur upon cooling the blend melt.52−58 The final morphology and property of the polymer blends are controlled by the pathways determined by the competition and interaction between these phase transitions. However, up to now, the complexity of the interplay between LLPS and crystallization remains largely unexplored.52,54,55,59−68 As one kind of widely used polyolefin, isotactic polypropylene (iPP) is similar to iPB-1 in chemical structure and helical conformation in molecular chains. The investigation of these two homologue crystalline polyolefine iPB-1/iPP blend system has been a subject of continuing interest for its confused miscibility and complicated crystallization behavior. The miscibility of iPB-1/iPP blend has been debated for several decades and has no definite answer yet. 60,69−72 The crystallization behaviors of iPB-1/iPP blend system such as kinetics, crystallinity, and morphology of this crystalline− crystalline blends have been widely studied.60,69−74 It is noticeably that the addition of iPP component has been found to accelerate the crystalline transformation from form II to form I for iPB-1, similar to the effect of propylene as a comonomer with butylene.44,75 Based on the important roles of iPP in the crystallization behaviors of iPB-1 in the iPB-1/iPP blend, a systematic investigation on the phase behavior of the blend and the crystallization polymorphism of iPB-1 following various thermal treatments under atmosphere pressure is carried out by combining optical microscope (OM), Fourier transform infrared spectroscopy (FTIR), wide-angle X-ray diffraction (WAXD), and differential scanning calorimetry (DSC). The sample treating procedure mimics processing relevant conditions by fast cooling which is followed by sample characterization with emphasis placed on the detection of formation of form I′ crystals. In situ OM measurements reveal the upper critical solution temperature (UCST) feature of iPB-1/iPP blend with a critical temperature located in the range of 180− 200 °C. By modifying the thermal treatment, formation of pure

2. EXPERIMENTAL SECTION The iPB-1 pellets were kindly supplied by LyondellBasell Industries with a trade name of PB0110M. The weight-average molecular weight (Mw) is 711 kg/mol, and the melt flow index is 0.4 g/10 min (190 °C/ 2.16 kg, ISO 1133). The melting point of form I and form II crystals are about 127 and 115 °C, respectively. The isotacticity of iPB-1 is 92.9 and 97.4% [mmmm]% on the whole sample and of the isotactic part, respectively.76 The iPP granules prepared with metallocene catalysts were also supplied by LyondellBasell Industries. It has a melt flow index of 30 g/10 min (230 °C/2.16 kg, ASTM D) and Mw of about 212 kg/mol. The melting point is about 145 °C. [mr]% and [rr]% are 3.09 and 0.86, respectively, and [mm]% is 96.05.77 The iPB1/iPP blends were prepared by the solution-precipitation method. A desired amount of each component was dissolved in xylene to form a homogeneous solution at a total polymer concentration of about 3 wt %. The solution was held at 130 °C for 90 min with continuous stirring under a nitrogen atmosphere to ensure that the two components were mixed sufficiently and then poured into cool methanol. The white precipitates taken from the mixture were dried at room temperature and then dried under vacuum for more than 3 days at 60 °C. The two pure polymers were also prepared by the same method. The film samples with a thickness of about 70 μm were obtained by hot-pressing using a homemade compression molder. iPB1/iPP (50/50 wt/wt) samples were chosen in this study. For samples of different compositions the FTIR experimental results also show direct formation of form I/I′ crystals (Figure S1, Supporting Information). The 50/50 (w/w) sample was chosen mainly because the larger phase domains and the higher phase contrast facilitate the experimental observation of the phase evolution for this composition than others. Besides, the kinetics of mixing process upon annealing is faster with the particular composition in comparison to other ones, which can shorten the annealing time at high temperature to reach a fully mixed state and prevent the degradation of the sample. As a follow-up, investigation on more compositions will be carried out in the future. The main thermal protocols are presented in Figure 1. The blend samples were heated to 220 °C on a homemade hot stage (with a temperature uncertainty of ±0.1 °C) and stayed for varying time periods from 10 to 120 min under a nitrogen atmosphere to erase thermal history and simultaneously to study the phase evolution over time of the blend melt by OM (BX51 Olympus). After annealing, the samples were immediately cooled down to different temperatures for quenching or isothermal crystallization. The cooling temperatures were chosen as 0, 25, 60, 90, and 115 °C. For low cooling temperatures (≤90 °C), samples were put into water with desired temperatures for quenching. The homemade hot stage was used to monitor the temperature of the samples at high temperatures (>100 B

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Macromolecules °C). Samples cooled to 90 and 115 °C were isothermally crystallized at each temperature for 60 min and then put into 0 °C ice/water. Immediately after quenching or isothermal crystallization, the samples were characterized by FTIR, WAXD, and DSC in turns. FTIR observations were carried out using a TENSOR 27 (Bruker) FTIR spectrometer. The measured spectrum wavenumber range was 3900−700 cm−1 with a resolution of 4 cm−1, and 36 scans were taken for averaging. Each FTIR test was finished in less than 1 min. The baseline of the spectrum was carefully adjusted uniformly using OPUS 5.5 package. Peak area was considered as integral intensity of the characteristic absorption bands in this study. WAXD were measured by using an in-house setup with the aid of an image plate detector (Mar 345, 3072 × 3072 pixels with pixel size of 150 μm) attached to a micro Cu Kα X-ray source (Incoatec, GmbH, λ = 0.154 nm).78 Each WAXD pattern was collected within 15 min with a sample-to-detector distance of 364 mm. Fit2D software from the European Synchrotron Radiation Facility was used to analyze the WAXS data. DSC measurements were conducted using a TA Q2000 differential scanning calorimeter (TA Instruments) under a nitrogen atmosphere with the sample weight of about 5−8 mg. The as-prepared samples described above were heated from room temperature to 220 °C at a rate of 10 °C/min, held at 220 °C for 5 min, and then cooled to 0 °C at a rate of 20 °C/min. Melting and crystallization temperatures were determined from the heating and cooling runs using Universal Analysis 2000 software (TA Instruments). All the measurements were performed in turns immediately after the thermal treatment, during which the solid phase transformation was almost negligible.

°C in comparison to that at 220 °C. However, when annealed at a lower temperature (180 °C) but still much higher than the melting point of iPP (about 142 °C for iPP in 50/50 (w/w) samples, see Figure 9), no obvious changes of phase

Figure 9. DSC thermograms of the first heating scan of SA, SB, and SC. The DSC thermograms of pure iPB-1 and iPP samples prepared with the same solution−precipitation method are also plotted for reference (the values of pure samples are multiplied by a constant to adjust to the same order of magnitude).

morphology are detected. Thus, the iPB-1/iPP blends are partially miscible with an UCST type of phase diagram which is consistent with the result of Marand et al.60 Besides, the experiment results show that the critical temperature should be located in the range of 180−200 °C for the samples investigated in this study. For blends exhibiting UCST behavior, the degree of mixing can be improved when annealed a phase-separated sample at temperature higher than the critical point, with annealing time as shown in Figure 2. Cooling down the annealed samples to temperatures below UCST and the melting point, LLPS and crystallization will take place spontaneously. The influence of mixing degree and the effect of LLPS on the polymorphic modifications upon followed quenching or isothermal crystallization of iPB-1 is first studied by FTIR. The characteristic absorption bands in the fingerprint region of FTIR spectroscopy can reflect chain arrangement and helical conformation and thus can be used to distinguish crystal forms. The FTIR spectra of these two homopolymers are shown in Figure 3f. For iPB-1, 923 and 905 cm−1 bands are used as the signs of from I/I′ and form II crystals, respectively, which is commonly believed though the crystalline nature needs to be confirmed by WAXD. Figure 3a− e shows the FTIR spectra obtained immediately after the thermal treatments, in which two absorption peaks around 923 and 900 cm−1 arise. Note that the absorption peak around 900 cm−1 is a superposition of iPB-1 from II 905 cm−1 and iPP 899 cm−1 band as indicated in Figure 3a, which are too close to be resolved. Surprisingly, the 923 cm−1 band as a fingerprint of iPB-1 form I or I′ appears in all the cases, indicating the direct formation of form I or I′ crystals. The profile of the spectrum curve, i.e., the relative intensities of these absorption bands, varies with the annealing time at 220 °C as well as with the subsequent quenching or isothermal crystallization temperature. The absorption peak at 923 cm−1 evolves from a strong peak in Figure 3a to a shoulder in Figure 3e apparently, indicating its intensity decreases with the increase of the quenching or crystallization temperature. Regardless of the quenching or isothermal crystallization temperature, the absorption peak at 923 cm−1 enhances with the increase of annealing time at 220 °C, accompanied by the reduce of the other one. The broad peak around 900 cm−1 shifts toward to 899 cm−1 gradually, indicating the reduction of the 905 cm−1

3. RESULTS Figure 2 shows the time-resolved optical micrographs of the blend melt annealing at 220 °C. The granular heterogeneous

Figure 2. Time-resolved optical micrographs of blend melt kept at 220 °C for (a) 0, (b) 10, (c) 30, (d) 60, (e) 120, and (f) 240 min.

structure appears immediately when the temperature reached 220 °C, with small characteristic length and high contrast of refractive index as observed in Figure 2a, meaning that the experiments start from a highly phase separated and heterogeneous blend melt. With the increase of annealing time, the size of domains grows up gradually. Meanwhile, the interface gradually fades and the contrast of refractive index between two phase domains weakens, as shown in Figure 2b−e. The refractive index is associated with the concentration of each component. Therefore, upon annealing, the adjacent iPPrich and iPB-1-rich domains mix together, leading to the increase of the domain size and the reduction of the concentration contrast between adjacent new-merged phase domains. When the annealing time is long enough, the blend melt fully merges into a uniform melt, presenting a homogeneous structure in Figure 2f. Annealing experiments at 200 and 250 °C are also performed, showing very similar evolution of the phase morphology of the blend melt to that at 220 °C. The kinetics is much slower at 200 °C but faster at 250 C

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Figure 4. (a) Dependence of calculated relative content of I or I′ to form II on the annealing time at 220 °C and (b) the subsequent quenching or isothermal crystallization temperature (b). The solid (quenched) or dashed lines (isothermally crystallized) in (a) serve as guidance of view.

Figure 4a. For samples annealed at 220 °C for the same time with equivalent degree of mixing in the melt state, χI(FTIR) reduce significantly from more than 0.5 to nearly 0.15 with raised subsequent quenching or isothermal crystallization temperature as shown in Figure 4b, except for the case of isothermally crystallized at 90 °C. Whether the components form into crystal or not and the crystallization polymorphism are further studied by WAXD. The I/I′(110) and II(200) diffraction peaks at 10.1° and 11.9° are the indicators of the iPB-1 form I or I′ and form II, respectively. WAXD is carried out immediately after the FTIR measurement for each sample. The integrated 1D X-ray diffraction profiles are plotted in Figure 5a−e. Obviously, the diffraction peak at 10.1° is detected in all the cases, confirming that form I or I′ crystals are generated directly from the iPB-1/ iPP blend melt at atmospheric pressure. The integrated 1D Xray diffraction profiles exhibit superposition of crystalline diffractions of iPB-1 (from form I or I′ and form II with variable relative intensity) with the signals of iPP mesophase (quenched) or iPP α-form (isothermal crystallized). Characteristic diffraction peaks corresponding to different crystal forms are indicated. Regardless of the quenching or crystallization temperatures, the diffraction peak of I/I′(110) around 10.1° enhanced significantly with the annealing time at 220 °C, in contrast to the diminishment of nearby diffraction peaks around 11.9° from II(200). For samples quenched to 0 °C after annealing at 220 °C for 120 min, almost only the diffraction peaks of form I or I′ appear. When quenched to low temperature, no crystalline signal from iPP α-forms, but the halos of the iPP mesophase around 14.8° and 21.8° are present as shown in Figure 5a−c. On the other hand, when isothermal crystallized at higher temperature, obvious diffraction peaks from iPP α-form can be observed. The diffraction peak of iPP α(110) around 14.1° grows with the annealing time at 220 °C similar to iPB-1 I/I′(110) does as shown in Figure 5c−e. Besides, no obvious diffraction peaks from iPB-1 form III or iPP β- or γ-form crystals is observed. The absence of new peaks or appreciable shift in peak positions in comparison to the pure components suggests no cocrystallization occurs for iPB-1/iPP blend and is consistent with the studies of Siegmann69,70 and Geil et al.71 Note that for iPB-1 homopolymer no detection of I/I′(110) around 10.1° upon quenching or isothermal crystallization as shown in Figure 5f, which means only form II was generated in melt crystallized iPB-1 homopolymer samples. The high overlapping of diffraction peaks of the iPB-1/iPP blend makes it impossible to do quantitative analysis on all peaks; thus, only the two major peaks of iPB-1 I/I′(110) and

Figure 3. FTIR spectra obtained immediately after quenched to (a) 0, (b) 25, and (c) 60 °C and isothermally crystallized at (d) 90 and (e) 115 °C after annealing at 220 °C for 10, 30, 60, and 120 min. (f) FTIR spectra of these two homopolymers.

band. Besides, when quenched to low temperatures (0 and 25 °C), the two absorption peaks go through an intensity inversion with increasing the annealing time at 220 °C. On the other hand, the absorption peak around 900 cm−1 always dominated, and only moderate changes of 923 cm−1 peak occur for the case of 60, 90, and 115 °C. Quantitative analysis of the spectrum is conducted to demonstrate the thermal treatment dependence of the relative content of from I or I′ to form II. For each spectrogram, the characteristic absorption bands of form I/I′ (923 cm−1), form II (905 cm−1), and characteristic absorption band of iPP (899 cm−1) are carefully fitted using Origin software (Figure S2). The relative content of form I/I′ to form II (χI(FTIR)) is calculated using the equation χI(FTIR) = A923/(A923 + A905) where A923 and A905 are the integrated peak areas representing the contents of form I or I′ and form II, respectively. The dependence of χI(FTIR) on the annealing time at 220 °C and the subsequent quenching or isothermal crystallization temperature are summarized in Figure 4. Some inaccuracy may be introduced into the calculation of peak area and consequently in the derived χI(FTIR) because of the highly overlapped characteristic absorption bands. However, the variation tendency of χI(FTIR) is credible and obvious by comparing the spectra. Figure 4a shows that under the same quenching or crystallization temperature χI(FTIR) increase with the annealing time at 220 °C, that is, with the increase of mixing degree of the blend melt according to the results observed by OM. When quenched to low temperatures (see the solid line in Figure 4a), χI(FTIR) are quite high and show an obvious increment of about 0.35, while only discernible increment is observed in the case of isothermal crystallization as guided by the dashed line in D

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of χI(WAXD) with annealing time at 220 °C is more prominent in the case of quenching, especially quenched to low temperature (0 °C). Under some peculiar conditions, χI(WAXD) reach almost 1.0, indicating the sole formation of iPB-1 form I or I′. In contrast, only a moderate increment is observed in the case of isothermal crystallization as shown by the dashed line in Figure 6a. χI(WAXD) show a decreasing trend with the elevation of quenching or crystallization temperature though samples are annealed in the same way. It is consistent with the studies of Cavallo et al. that in random butane-1/propylene the relative fraction of form I′ on the total crystallinity steadily increases with decreasing temperature.44 An exception occurs for the case of isothermally crystallized at 90 °C, which is also observed by FTIR but more evident in the WAXD results. Possibly it can be attributed to the simultaneous occurrence of LLPS and crystallization of these two components at this temperature as discussed later. Both FTIR and WAXD results strongly manifest that iPB-1 form I or I′ crystals can be generated directly from the iPB-1/ iPP blend melt. Form I′ crystals can form solely under some peculiar conditions or concomitant with form II crystals, indicating the competitive formation of these two forms. Further, the formation of form I′ depends not only on the initial state of blend melt but also on the subsequent thermal treatments. It is promoted by increasing the annealing time above UCST but suppressed by raising the quenching or isothermal crystallization temperature. In addition to confirming crystal forms and exploring the dependence of χI(WAXD) on the thermal treatment, another novel and noteworthy phenomenon is derived from the WAXD results. Figure 5c shows that with the increase of annealing time at 220 °C the diffraction patterns of iPP change from halos of mesophase to discernible iPP α(110) diffraction peak around 14.1° and eventually to clear α(110) diffraction peak. For samples isothermally crystallized at high temperatures, iPP α(110) diffraction peaks are also enhanced with increase of annealing time at 220 °C (Figure 5d,e). The intensity of iPP α(110) is calculated by the same approach as iPB-1 I/I′(110) and II(200) for the case of cooling to 60, 90, and 115 °C (the same as Figure S3 and eq S1, Supporting Information), as when quenched to low temperatures (0 and 25 °C) mesophase of iPP formed and no diffraction peaks of α-form appear. Figure 7

Figure 5. Integrated 1D X-ray diffraction profiles of samples quenched to (a) 0, (b) 25, and (c) 60 °C and isothermally crystallized at (d) 90 and (e) 115 °C after annealing at 220 °C for 10, 30, 60, and 120 min. The respective (110), (300), and (220) + (211) reflections of form I/ I′ at 2θ = 10.1°, 17.4°, and 20.5°, and the (200), (220), and (213) reflections of form II at 2θ = 11.9°, 16.9°, and 18.5°, are labeled in panel b. The (110), (040), (130), (111), and (131) + (041) reflections of iPP α-form at 2θ = 14.1°, 16.9°, 18.5°, 21.3°, and 21.9°, respectively, and the halos of iPP mesophase around 2θ = 14.8° and 21.8° are indicated in panels c and e. (f) Integrated 1D X-ray diffraction profiles of the iPB-1 homopolymer quenched to 0 °C and isothermally crystallized at 90 °C after annealing at 220 °C for 120 min.

II(200) are considered for simplification (Figure S3 and eq S1, Supporting Information). The corresponding integral peak area is used to present the content of each crystalline form. The relative content of form I/I′ to form II (χI(WAXD)) obtained by WAXD is calculated using the simplified calculation equation χI(WAXD) = AI(110)/(AI(110) + AII(220)) and is summarized in Figure 6. χI(WAXD) increase with the annealing time at 220 °C, which means that the relative content of form I or I′ increases with the degree of mixing of iPB-1/iPP blends and is in line with the results by FTIR. In addition, the increase

Figure 7. Intensity evolution of iPB-1 I/I′(110) and iPP α(110) diffraction peaks of samples (a) quenched to 60 °C and (b) isothermal crystallized at 90 °C and (c) 115 °C after annealing at 220 °C for 10, 30, 60, and 120 min.

displays the calculated intensity evolution of iPB-1 I/I′(110) and iPP α(110) diffraction peaks, which show a positive correlation between each other. It means that with the improvement of the mixing degree of the constituents the formation of iPB-1 form I crystals is promoted; meanwhile, the ability of crystallization of the iPP component is also improved. Both the FTIR and WAXD results provide direct evidence that form I or I′ can be generated from the melt of iPB-1/iPP blend directly at atmospheric pressure. However, whether the

Figure 6. (a) Dependence of calculated relative content of I or I′ to form II on the annealing time at 220 °C and (b) the subsequent quenching or isothermal crystallization temperature. The solid (quenched) or dashed lines (isothermal crystallized) in (a) serve as guidance of view. E

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iPP blends should be defined as form I′ solely. The second endothermic peaks around 110 °C are due to the melting of the form II crystals. SA shows little WAXD signal of form II crystals (Figure S4) but obvious melting peak of form II crystal upon DSC heating, which could be attributed to the recrystallization into form II crystals. The recrystallization is also observed by WAXD in Figure 8 and actually explains the exothermic peak around 100 °C following the melting of the form I′ crystals in DSC, which is rather prominent for SA but also discernible for SB and SC. The major endothermic peaks around 142 °C are due to the melting of iPP crystals. The DSC cooling thermograms right after the heating ones are also measured for various samples. The results are given in Figure S5 and summarized in Table S1. Although the status of the samples measured during cooling have already been altered by the precedent heating in comparison to the annealed blends, original information regarding the as-prepared samples can be more or less preserved in the melted crystals. Indeed, distinctive shifts of crystallization temperatures for iPB-1 and iPP are observed during the cooling. For samples with low degree of mixing (see SC), the crystallization temperature Tc reduces for iPP but increases for iPB-1 compared to the pure counterpart. In contrast, for SA and SB where two components are highly mixing or even miscible at molecule level, Tc(iPP) and Tc(iPB1) both decrease, suggesting suppression of the crystallization of both components. In addition, two weak exothermic peaks of iPB-1 can be perceived for SA and SB by the inset in Figure S5, which could indicate two steps or two types of crystallization in those two samples. The difference in the change of crystallization behaviors measured by DSC cooling reveal hints on the difference in the crystallization pathways in the samples originally prepared with various mixing degree, which could shed light on the direct formation of form I′.

crystal is form I or I′ cannot be distinguished directly by FTIR or WAXD because of their identical helical conformation and crystallographic structure. While the differences in their melting temperature makes it possible to identify these two forms by DSC testing or following the melting behavior during heating with WAXD. The sample was quenched to 0 °C after being annealed at 220 °C for 120 min and then step-heated to 100 and 125 °C. WAXD was performed at each stage to monitor the evolution of diffraction patterns. The integrated 1D X-ray diffraction profiles are shown in Figure 8, and characteristic

Figure 8. Integrated 1D X-ray diffraction profiles of sample quenched to 0 °C after annealing at 220 °C for 120 min, followed by reheating to 100 and 125 °C.

diffraction peaks corresponding to different crystal forms are indicated. The diffraction peaks corresponding to iPB-1 form I or I′(110), (220) + (211), and (300) can be clearly identified for the curve right after quenching before reheating while no signal from other crystal is detected, indicating that only form I or I′ is formed for iPB-1 while iPP is solidified to mesophase. By reheating the quenched sample to 100 °C, the diffraction peaks of I/I′ vanishes completely. If form I existed, there should leave at least some residual signal of the (110) feature as its melting temperature is much higher than 100 °C. Therefore, the unexpected crystal form should be form I′ which is characterized by lower melting temperature. Characteristic diffraction peaks corresponding to iPB-1 form II and iPP αform emerge upon heating, indicating the occurrence of melting and recrystallization. Similar thermal behavior has been detected in DSC testing which will be shown in the following. When the temperature goes up to 125 °C, iPB-1 form II crystals melt and only diffraction peaks of iPP α-form are detected. A more systematic investigation to identify the crystal form is carried out to further confirm the exclusive formation of iPB form I′ without form I by measuring DSC of samples treated with various thermal procedures. For convenience, the samples were named as SA, SB, and SC, respectively. SA and SC were quenched to 0 °C directly after annealing at 220 °C for 120 and 10 min, respectively. SB was isothermally crystallized at 115 °C for 60 min after annealing at 220 °C for 120 min and then put into 0 °C water. WAXD and DSC measurements were performed immediately after the thermal treatment. Figure 9 shows the DSC thermograms of the first heating scan for the samples obtained after WAXD measurement, in comparison to those of the pure iPB-1 and iPP samples prepared with the same solution−precipitation method. Since the Tm of forms I′ and I are 90−100 and 120−135 °C, respectively, the DSC endothermic peak around 90 °C for each case is ascribed to the melting of form I′ crystals. In addition, no endothermic peak is detected in the temperature range of 120−135 °C, excluding the existence of form I. Thereby, the unexpected iPB-1 crystalline form obtained directly from the melt of the iPB-1/

4. DISCUSSION On the basis of OM study on the real space phase morphology evolution at high temperature of iPB-1/iPP blend melt and FTIR, WAXD, and DSC measurements on the polymorphism of iPB-1, some conclusions are drawn from the results. (i) The UCST feature of iPB-1/iPP blend with a critical temperature located in the range of 180−200 °C is manifested. (ii) Form I′ crystal of PB-1 is generated directly from the iPP/PB-1 blend melt at atmospheric pressure. (iii) The formation of form I′ is significantly promoted by enhancing the degree of mixing of the constituents resulting from the increase of the annealing time above UCST. (iv) The raise of subsequent quenching or isothermal crystallization temperature suppresses the formation of form I′. In this study, the directly formation of form I′ crystal of iPB-1 from the iPB-1/iPP blend melt at atmospheric pressure should be ascribed to the addition of iPP component, and obviously it depends not only on the initial mixing degree of the constituents in molten state but also on the subsequent thermal treatments. The mixing as well as the phase separation upon cooling is determined by the miscibility of the iPB-1/iPP blend. Though it has been studied for several decades, the miscibility of the iPB-1/iPP blend remains debatable. Experiments by Siegmann et al.69,70 and Geil et al.71 suggest that the iPP/iPB blend melt is compatible or miscible although it is difficult to achieve the miscible state by ordinary melt mixing. Chen et al.72,74 calculated the polymer−polymer interaction parameters in the blend and found negative values at various compositions, suggesting that iPP and iPB are compatible in the F

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crystallization behavior of the two-component polyolefin blend system and proposed the “fluctuation-assisted crystallization” mechanism. Besides, they found that LLPS plays a role in the crystal morphology.61,64 Similar results are also reported by the simulation work performed by Hu and Frenkel.65,66 The strength of LLPS is mainly controlled by the thermodynamic deriving force, which comes from the concentration difference between the concentration at melt state before cooling and the equilibrium coexistent concentrations given by the binodal curve at that certain temperature. The assistant effect of LLPS on crystallization is certainly linked with the strength of LLPS. Thus, the higher mixing degree of the melt state and the lower the cooling temperature, the stronger the assistant effect. In our study, it is obvious that the formation of form I′ is intimately linked with the mixing degree of the constituents as well as the subsequent quenching or isothermal crystallization. Thus, the relationship between formation of form I′ and the melt state as well as cooling condition is in line with the foregoing description of the fluctuation-assisted crystallization mechanism. We propose that the fluctuation-assisted crystallization mechanism may explain our experimental findings. It is plausible that the concentration fluctuations caused by LLPS may alter the state of chain segments, which may not only assist nucleation but also alter the crystallization pathway, promoting the formation of iPB-1 form I′ instead of form II. Moreover, the adjacent domains with different concentrations caused by concentration fluctuation may lead to similar confinement effects as that proposed in partial melt and copolymer samples. Based on this hypothesis, the experimental results can be well explained as follows. With the extension of annealing time at 220 °C, the content of iPB-1 form I′ show a large increase, since the concentration fluctuation assisted crystallization is enhanced with the improvement of the initial mixing degree. The promotion of iPP α-form (at suitable conditions, see Figures 5c−e and 7) is another evidence to prove the existence of fluctuation-assisted crystallization. For samples annealed at 220 °C for the same time with equivalent degree of mixing in the melt state, the blend system enters into the deep unstable region if quenched to low temperatures (0 °C). LLPS takes place strongly and rapidly in a very short period of time, resulting in more effective assistant effect on crystallization. Consequently, the more amount of form I′ is generated. The assistant effect weaken gradually with the raising of quenching temperature, leading to the reduced amount of form I′ (25 and 60 °C). On the other hand, if samples are isothermally crystallized at a high temperature (115 °C), with time going on, iPP component crystallizes and iPB-1/iPP phase separates. The concentration of the blend melt approached the coexistent compositions determined by this temperature. After quenching to 0 °C, the fluctuation-assisted nucleation can still happen only with the suppressed strength, leading to the formation of small amounts form I′. When isothermally crystallized at a moderate temperature (90 °C), LLPS and iPB-1 crystallization may occur simultaneously, accordingly facilitating the assistant effect. Thus, the relative content of form I′ for the case of 90 °C appears abnormally high (see Figures 4 and 6).

amorphous region. On the other hand, a liquid−liquid demixing phenomenon of the iPP/iPB blend melt has been observed by Marand et al.,60 indicating the UCST behavior of the iPP/iPB blend. In our study, when annealed at sufficiently high temperatures, the phase morphology of the blend melt goes through the granular heterogeneous structure to bicontinuous interconnected structure and finally to a homogeneous melt with annealing time above UCST. The partially miscible iPB-1/iPP blends observed by OM are consistent with a UCST type of phase diagram.60 The critical temperature located between 180 and 200 °C for the 50/50 (w/w) samples investigated in this study, however, disagrees with the description of Marand et al.60 that the critical temperature is above their respective degradation temperatures. This divergence may be attributed to the lower isotacticity of iPP used in this study and the difference in the molecular weights of components from the previous studies. For blends exhibiting UCST behavior, the kinetics of the mixing process as well as the inverse process, LLPS, is controlled by the viscosity of the constituents and the thermodynamics driving force. The direct formation of form I′ by cooling the bulk sample melts is unusual and the formation conditions need to be rather particular and stringent. Several possible explanations of the direct formation of form I′ have been provided.35,37,38,45 In our case, miscibility between iPB-1 and iPP is found to play an important role in the crystallization pathway and to directly associate with the formation of iPB-1 form I′ crystals in the blend. As a typical compatible polyolefine pair, iPB-1 and iPP can be miscible at molecular level, which will alter the thermodynamic state of the blend melt as well as the crystallization pathway. It is likely that the crystallization kinetics of form I′ becomes competitive with or more favorable than that of form II, promoting the formation of the former with even no presence of the latter. Besides, the molecular chain conformation of iPB-1 may be affected by the inherent 3/ 1 helical conformation of iPP chains, which is similar to that of iPB-1 form I′ crystals. In addition, the framework of the presolidified iPP formed during cooling may cause strong confinement effects on the subsequent crystallization of iPB-1, which is similar to the case of partially melted or ultrathin samples. What is more, for crystalline−crystalline iPB-1/iPP blend with UCST behavior, the situation is much more complicated. Upon cooling the blend melt, two types of phase transitions, liquid−liquid phase separation and liquid−solid transition, take place simultaneously. Actually, the liquid−solid transition includes the crystallization of iPB-1 and iPP, which may be affected by each other. Besides, as polymorphism polymers, the competition formation between different crystal forms may also exist. Indeed, it is formidable to provide an unequivocal mechanism underlying the phenomena observed in the polymer blend system which is complicated by many factors and procedures. However, how the mixing affects the crystallization and vice versa deserves more discussion and further exploration. So we attempt to provide one plausible mechanism by coupling the liquid−liquid phase separation and liquid solid transitiontwo processes known to take place during quenching the blend melt. As already mentioned, when the UCST type blend is cooled from a temperature above the critical point to a temperature below both the critical and melting points, the blend system enters into the metastable or unstable region, upon which two coupled phase transitions, LLPS and crystallization, both will take place. Han et al.62,63,67,68 studied the effect of LLPS on the

5. CONCLUSION The phase evolution of the blend melt at high temperature and the polymorphism of iPB-1 in the iPB-1/iPP blend have been studied. FTIR, WAXD, and DSC results demonstrate that form I′ crystal can be generated directly at atmospheric pressure when cooling the iPB-1/iPP blend melt. The formation of form G

DOI: 10.1021/acs.macromol.5b02161 Macromolecules XXXX, XXX, XXX−XXX

Macromolecules

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ACKNOWLEDGMENTS The authors thank Dr. Zhe Ma (TJU) for the initial discussion. We acknowledge Prof. Zhigang Wang’s group (Hefei) for their kindly help on DSC measurement. This work is supported by the National Natural Science Foundation of China (51325301 and 51227801), the fund of Chinese Academy of Sciences (2015SRG-HSC026), the Fundamental Research Funds for the Central Universities and the Project supported by NPL, CAEP (2013BB05). The experiment is partially carried out in National Synchrotron Radiation Lab (NSRL).

I′ is intimately linked with the mixing degree of the constituents and the subsequent thermal treatment, increasing with the increase of annealing time above UCST but decreasing with raised temperature of quenching or isothermal crystallization. By altering the state of blend melt and the following cooling condition, the crystallization pathway can be governed. The obtained crystal forms can be tuned from ordinary form II, to concomitant crystallization of forms II and I′, or to pure form I′, depending on the thermal treatments. The results reported in this paper provide a novel experimental observation of the crystallization of the form I′ crystals from the iPB-1/iPP blend melt at atmospheric pressure pertinent to industrial processing. This result may be of interest because it can provide a practical approach to avoid the spontaneous solid phase transformation of iPB-1a formidable problem that stands in the way of the industrial development of iPB-1. The focus of this article is to report the novel experimental discovery that the formation of the iPB-1 form I′ and also the crystallization of iPP are closely associated with the initial mixing degree of the blend melt as well as the subsequence thermal treatment. It is generally recognized that the mixing degree of the blend system is significantly affected by the phase transition processes including LLPS and crystallization. However, this study demonstrated the other direction of the connection between mixing and phase transitions. In particular, the crystallization pathway is found to be altered by the mixing degree between the biconstituents, which however is not stationary but involves concomitantly other procedures, for instance liquid−liquid phase separation, as the crystallization goes on. Based on these results, one plausible mechanism taking the two types of phase transitions into consideration is proposed to account for the experimental phenomena. Other possibilities are not necessarily ruled out, and more detailed works are needed to give an explicit explanation of these phenomena. Further studies may shed light on a better understanding of the interplay between liquid−liquid phase separation and crystallization, the two coupled phase transitions in polymer blends, in terms of the scientific viewpoint.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02161. FTIR spectra of samples with different compositions quenched to 0 °C after annealing at 200 °C for 60 min; example of peak fitting for FTIR spectrogram by Origin software; method of approximate calculated of the integral peak area from the integrated 1D X-ray diffraction profiles; the integrated 1D X-ray diffraction profiles of SA, SB, and SC; DSC cooling data of SA, SB, and SC and the homopolymers and the related discussion (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (N.H.). *E-mail [email protected] (L.L.). Notes

The authors declare no competing financial interest. H

DOI: 10.1021/acs.macromol.5b02161 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.5b02161 Macromolecules XXXX, XXX, XXX−XXX