Conformation Selected Direct Formation of Form I ... - ACS Publications

Mar 2, 2018 - Department of Materials and Production, Aalborg University, ... Department of Chemistry and Bioscience, Aalborg University, DK-9220, Aal...
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Conformation Selected Direct Formation of Form I in Isotactic Poly(butene-1) Jingqing Li,† Dong Wang,† Xiaoqian Cai,† Chengbo Zhou,† Jesper de Claville Christiansen,‡ Thomas Sørensen,‡ Donghong Yu,§ Meiling Xue,∥ and Shichun Jiang*,† †

School of Materials Science and Engineering, Tianjin University, Tianjin 300072, People’s Republic of China Department of Materials and Production, Aalborg University, DK-9220, Aalborg, Denmark § Department of Chemistry and Bioscience, Aalborg University, DK-9220, Aalborg, Denmark ∥ School of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, People’s Republic of China ‡

S Supporting Information *

ABSTRACT: Though the transition from metastable to stable crystal (from form II to form I) of isotactic poly(butene-1) (iPB-1) after melt crystallization looks to be unavoidable, form I direct formation in bulk iPB-1 via a bypass of form II is still a charming approach to the crystalline structure control in iPB1. However, the physics behind this transition still remains elusive and there are many arguments. In this work, DSC and WAXS were used to investigate the crystallization behaviors of iPB-1 in detail and direct formation of form I in bulk iPB-1 was found able to occur within a temperature range from the glass transition temperature Tg to a critical temperature Tcr = 35 °C, far below the crystallization temperature Tc of form II. The temperature-dependent 3/1 helix conformation formation within this temperature range was proposed to interpret the further direct formation of form I. Formation of iPB-1 polymorphic structures seem to be controlled by temperature-dependent helix conformation formation. The results may trigger further discussions or shed light on the understanding of the physics behind polymer crystallization and polymorph selection processes as well as the development of crystal structure controlling techniques and applications of iPB-1.

1. INTRODUCTION During the past decades, many researchers have focused on polymer crystallization processes and established various crystallization models.1−3 On the basis of secondary nucleation concepts, Hoffman and Lauritzen established a most widely accepted traditional nucleation−growth model.1 To interpret why the initial SAXS peak moves to smaller angles with the intensity growing exponentially before the emergence of Bragg peaks in WAXS, a spinodal decomposition model was proposed.2 On the basis of the experimental observations indicating that the lamellar crystallites do not form and grow directly from and into the isotropic melt during a one-step process, but rather via transient ordering intermediate states which always appear prior to the appearance of the first crystallite, Strobl3 developed a multistep model. All of these models focused on the controversial start or development of polymer crystallization, while polymorph selection was seldom concerned. The physics behind both polymer crystallization and polymorph selection still remains elusive.4−9 Polymorphic semicrystalline polymers such as isotactic polypropylene (iPP) and isotactic polybutene-1 (iPB-1) are chiral but racemic. Their various crystalline polymorphs display virtually all possible combinations of helical hands, azimuthal © XXXX American Chemical Society

settings, and even nonparallel orientation of helix axes in space.4 A crystal form may correspond to a selected helix conformation of the polymer chain segments packed in a certain geometry, while the conformations of polymer chains in the isotropic coiled melt are randomly trans or gauche (including gauche+ and gauche−) due to the entropy effect according to the second law of thermodynamics.2,10 Though the polymer crystallization process cannot be regarded as a simple inverse melting process of crystals due to the emergence of the transient ordering of intermediate states prior to formation of the crystals,3 analysis of the crystallization process “backwards” via an autopsy of the crystal structures is suggested to be able to elucidate how they form.4 The isotropic coiled chains may need to select correct trans or gauche conformations first to form crystallizable helix chain sequences, which would be packed in a certain geometry to further form a transient ordering intermediate mesophase as proposed in Strobl’s multistep model2 or a local structure as revealed from the work of molecular dynamic simulations5,6 prior to Received: January 20, 2018 Revised: February 27, 2018 Published: March 2, 2018 A

DOI: 10.1021/acs.cgd.8b00119 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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designed heating−cooling thermal protocols, focusing on the influence of temperature. It was found that iPB-1 tends to be able to directly crystallize into form I in bulk samples at temperatures above the glass transition temperature Tg and below the critical temperature Tcr = 35 °C, which is far below the melting temperature Tm of form II. In addition, at temperatures above Tcr, the surviving 3/1 helices or the form I favored mesophase and local structure could result in the formation of a limited amount of form I. It was proposed that the melt crystallization process and polymorph selection of iPB1 are controlled by the formation of the temperature-dependent 3/1 or 11/3 helix conformations of the chain segments and then the crystal favored local structures and crystals. The results could shed light on the understanding of the physics of polymer crystallization and polymorph selection as well as on the development of crystal-controlling techniques of iPB-1.

crystallization into a crystal. If so, with decreasing temperature, more extended chain segments with less gauche conformations and lower free energy would result due to the decreasing entropy effect. It has been proved that the chain conformation changes can be well controlled under various temperature conditions due to entropic effects and there exists a potential barrier at very low temperatures,11 which may result in kinetic trapping of some metastable conformations. Consequently, some helix conformations would be preferred, resulting in corresponding crystallizable helix chain sequences perhaps via a spinoidal decomposition process as proposed in the spinoidal decomposition model.2 This agrees well with Schrodinger’s thought of “order based on order”12 and is consistent with the facts reported in iPP with β-iPP formed from the melt local structure with a 100% right- or left-handed helix ratio.9,13 The helix conformation formation and the subsequent packing process must be the keys to understanding the polymer crystallization as well as the polymorph selection processes. Here, the helix conformation formation should depend on the reasonable conformation transitions from gauche to trans which are thermodynamically determined by the equilibrium between the entropy effect and the enthalpy effect and kinetically influenced by an energy barrier due to the steric hindrance of the side chain groups. iPB-1, known as “gold plastic”,14 is of great industrial importance in comparison with other polyolefins and is also a good model polymer to investigate polymer crystallization processes, which is a matter of great scientific importance. While cooling an iPB-1 melt at atmospheric pressure, one usually obtains kinetically favored metastable form II with 11/3 helices loosely packed in the tetragonal unit cells which spontaneously transit into stable form I with 3/1 helices packed in twinned hexagonal cells over several weeks upon aging at room temperature.15 The difficulty for iPB-1 to directly crystallize into form I may result from the difficulty for the coiled chains to select 3/1 helix conformations and form crystallizable 3/1 helices first. More dense 3/1 helices in comparison to 11/3 helices and the large steric hindrance of ethyl side groups in iPB-1 chains may result in a large energy barrier of 3/1 helix conformation formation. It seems that both the existing energy barrier and the strong entropy effect at high temperatures do not favor 3/1 helix conformation formation. On the basis of the competition between the positive enthalpy effect and the negative entropy effect as well as the negative contribution of the energy barrier, there may exist a temperature window for 3/1 helix conformation and the crystallizable 3/1 helices to form, which may occur at low temperatures due to the relatively weak entropy effect to the chain segments which would not crystallize into form II. Actually, formations of form I in ultrathin iPB-1 films due to the confined conditions and the sample preparation history16 or form I′ also with 3/1 helix conformations but a low melting temperature of about 95 °C under high melt pressure,17 by epitaxial crystallization,18 in iPB-1 with high sterodefect content19 or high content of ethylene20−22 or propylene23−27 comonomer units have been reported to occur under various special conditions, revealing that 3/1 helix conformation formation can be promoted accordingly. In this work, aiming at exploring the physics behind polymer crystallization as well as polymorph selection via experimentally verifying the possibility of direct formation of form I in typical bulk iPB-1 from the viewpoint of helix conformation formation, we examined the crystallization behaviors of iPB-1 in detail with

2. EXPERIMENTAL SECTION 2.1. Materials. Isotactic poly(butene-1) (iPB-1) was obtained from Shandong Oriental Macro Industry Chemical Co., Ltd. The melt flow index is 0.3 g/10 min (190 °C/2.16 kg, Iso 1133), and the isotacticity is 98%. 2.2. Differential Scanning Calorimetry (DSC). DSC measurements were carried out with a DSC Q2000 apparatus (TA Instruments) under a nitrogen atmosphere. The sample amounts are about 10 mg, and the thermal protocols employed are shown in Scheme 1. To erase the thermal history of the samples, they were first

Scheme 1. Illustrations of Thermal Protocols Designed for the iPB-1 Samples

heated at 10 °C/min to Tmelt = 200 °C for tmelt = 2 min and then cooled at 15 °C/min to Tlow,1 = 30 °C for tlow,1 = 2 min. Then the samples were remelted or reheated to Thigh = 125 °C for 2 min at 10 °C/min and cooled again to Tlow,2 for an aging time tlow,2 of 15 °C/ min. Finally, the samples were remelted. 2.3. Wide Angle X-ray Scattering (WAXS). In situ synchrotron WAXS measurements were performed at beamline 1W2A, BSRF, Beijing, People’s Republic of China, with the X-ray wavelength being 0.154 nm.28 The 2D WAXD patterns obtained in the center of the sample were collected with a Mar 165 CCD at a sample to detector distance of 130 mm. 2.4. Fourier Transform Infrared Spectroscopy (FTIR). A compression-molded iPB-1 sheet in form I with a thickness of 0.5 mm was high energy electron irradiated to a calculated dose of 251 KGy. Then a 2 × 2 mm piece was cut from the sheet and compressionmolded at 200 °C to a film with a thickness of 20 μm, which was used for in situ FTIR characterization. The in situ FTIR spectra were collected with infrared spectroscopy and microspectroscopic imaging beamline BL01B equipped with a Bruker HYPERION 3000 microscope coupled with a Bruker IFS 66v FTIR spectrometer at the National Synchrotron Radiation Laboratory (NSRL) of China. B

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Figure 1. (a) DSC melting curves of the iPB-1 samples in forms I and II. The heating rate was 10 °C/min. (b) 1D-WAXS curves of form II and the form I transited from form II. The dashed lines are plotted to guide the eye.

Figure 2. (a) DSC melting curves of the iPB-1 samples after heating to Tmelt = 200 °C for tmelt = 2 min and then aging at Tlow,1 = 30, 0, −30 °C for tlow,1 = 2 min. (b) Enlarged DSC melting peaks at about 130 °C. (c, d) Gaussian peak fitting results. The dashed lines are plotted to guide the eye.

3. RESULTS AND DISCUSSION 3.1. Direct Formation of form I. The melting curves of the iPB-1 samples obtained by DSC (Figure 1a) showed the peak melting temperatures Tm of forms II and I at 115 and 127 °C, respectively. The 1D-WAXS curves of form II and the form I transited from form II (Figure 1b) showed the main diffraction peaks at 2θ values of about 9.9, 17.3, 20.2, and 20.5° corresponding to the crystallographic planes of (110), (300), (220) and (211), of form I and the peaks at 11.9, 16.9, and 18.4° corresponding to the (200), (220), and (301) lattice planes of form II. It should be noted that the melting peak at about 162 °C in the DSC curves (Figure 1a) and the weak diffraction peak at about 2θ = 14° (Figure 1 b) should be attributed to the existence of the α crystal of isotactic polypropylene (iPP) introduced into iPB-1 by the producer.

However, the iPB-1 sample could not crystallize into form I′ while a melt was cooled, as occurred for the copolymers with propylene as a counit23−27 or as occurred for the iPB-1/iPP blends.29 The DSC melting curves of the iPB-1 samples after heating to Tmelt = 200 °C for tmelt = 2 min to remove the thermal histories and then cooling to Tlow,1 = 30, 0, −30 °C for tlow,1 = 2 min are shown in Figure 2a,b as examples. For the sample with Tlow,1 = 30 °C, only the melting peak of form II at 115 °C was observed, while for the other two samples, additional peaks around 130 °C appeared and they both could be fitted into two Gaussian peaks, peak 129 and peak 133, as shown in Figure 2c,d. It is known that the equilibrium melting point Tm0 of form II is about 120−130 °C and that of form I is about 126−139 °C.30 C

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Figure 3. (a) DSC melting curves of iPB-1 samples after thermal history erasing followed by one and nine temperature cycles between Thigh = 125 °C and Tlow,2 = 10 °C to enlarge the melting peaks around 130 °C with heating and cooling rates of 10 and 15 °C/min, respectively. (b, c) Gaussian peak fitting. (d) 1D-WAXS profiles obtained from the in situ collected 2D-WAXS patterns during the melting process of the sample after nine temperature cycles.

Powers reported a slightly higher Tm0 value of 140.9 °C for form I.31 It is reasonable to assign the melting peak at about 130 °C to form I rather than form II. WAXS is a powerful tool to confirm this. However, relative to the melting peak of form II at 115 °C, the peaks at about 130 °C are very weak (Figure 2a). For example, the melting enthalpies ΔHm of the peaks at about 130 °C for the samples with Tlow,1 = −60 and 10 °C were 1.253 and 0.421 J/g, respectively. If the peaks are assigned to form I, the corresponding crystallinity Xc(I) can be determined as 0.89% and 0.30% for the two samples, respectively, according to Xc = ΔHm/ΔHm° with ΔHm°(I) = 141 J/g of the ideal form I.32,33 Though the Xc(I) values may vary on determination by different methods, the estimated Xc(I) values by DSC are below or around the detection limit of WAXS of 1% as reported by Hsiao.34 Before assigning the weak melting peak appearing at about 130 °C to form I by WAXS, we first tried to make it able to be detected conveniently by WAXS. To enhance the WAXS signals of the crystals responsible for the melting peaks at about 130 °C, after the iPB-1 samples were melted to Tmelt = 200 °C for tmelt = 2 min in order to erase their thermal histories and then cooled to Tlow,1 = 30 °C for tlow,1 = 2 min, they were heated again to Thigh = 125 °C for thigh = 2 min and then cooled to Tlow,2 = 10 °C for tlow,2 = 2 min. From Figure 3a, it can be learned that the melting peak around 130 °C was successfully enlarged after nine temperature cycles between Thigh = 125 °C and Tlow,2 = 10 °C were executed. The Gaussian peak fitting results in Figure 3b,c revealed that the ratio of the melting enthalpy ΔHm of peak 129 to that of peak 133 almost remained fixed at 0.42 ± 0.2, indicating that each

repeated temperature cycle almost resulted in the same crystal structures. The iPB-1 sample with nine cycles repeated was then characterized by in situ synchrotron WAXS. The corresponding 1D-WAXS profiles were obtained from the collected 2D-WAXS patterns and are shown in Figure 3d. When the iPB-1 sample was heated to a temperature of around 115 °C, the peaks assigned to form II disappeared. After that, the peaks at 2θ = 9.9, 17.3, 20.2, and 20.5° were observed to exist, showing that they are due to form I rather than form II and form I′. The Tm value of form I′ is about 95 °C, much lower than Tm = 127 °C for form I transited from form II. This confirmed that the fitted peak 129 and peak 133 are both the melting peaks of form I. In Figure 2a,b, the sample with Tlow,1 = 30 °C showed no additional peak around 130 °C. This indicated that no form II could transit into form I while cooling from Tmelt = 200 °C to Tlow,1 = 30 °C for tlow,1 = 2 min followed by melting again. Thus, after the thermal history was erased according to Scheme 1, the iPB-1 sample was only in form II. Figure 1a and Figure 3d showed that if this sample was reheated to Thigh = 125 °C for thigh = 2 min, all of the form II in it could be melted completely. Therefore, when this sample was cooled again to Tlow,2 = 30 °C for tlow,2 = 2 min, the additional melting peak around 130 °C would not appear if it resulted from a form II to form I transition. However, after the iPB-1 was cooled from Thigh = 125 °C to Tlow,2 = 30 °C, as shown in Figure 4a, the DSC melting curves of this sample in Figure 4b−d showed a melting peak at around 130 °C. Further verifications were executed for the iPB-1 samples with Tlow,2 values ranging from 30 to 105 °C. D

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Figure 4. (a) DSC nonisothermal crystallization curves of the iPB-1 samples with cooling from Thigh = 125 °C to Tlow,2 ranging from −60 to 105 °C according to the thermal protocols as shown in Scheme 1. (b) DSC melting curves of the iPB-1 samples after aging at Tlow,2 for tlow = 2 min following the nonisothermal crystallization process. (c, d) Enlarged DSC melting peaks at about 130 °C. The dashed lines are plotted to guide the eye.

The melting peak around 130 °C still appeared, though less time was left for form II to transit into form I relative to the sample with Tlow,2 = 30 °C or even no form II could crystallize from the melt with cooling from Thigh = 125 °C to Tlow,2 = 105 °C. This implied that the melting peaks around 130 °C for the samples with Tlow,2 above 30 °C could be attributed to direct crystallization of iPB-1 into form I. The DSC nonisothermal crystallization curves and melting curves of the iPB-1 samples with Tlow,2 values below 30 °C are also shown in Figure 4a−c. All of the Gaussian fitting results of the resulting DSC melting peaks around 130 °C are shown in Figure 5a−f. For a sample with Tlow,2 above 30 °C, the melting peak around 130 °C could be fitted into one peak 133, while for a sample with a Tlow,2 value below 30 °C, it can be fitted into peak 129 and peak 133. Since peak 133 has been assigned to form I obtained via direct crystallization of iPB-1, peak 129 could be assigned to form I transited from form II. Furthermore, after the iPB-1 samples were heated to Tmelt = 200 °C for tmelt = 2 min and then cooled to Tlow,1 = 30 °C for tlow,1 = 2 min with their thermal histories erased, they were again heated to Thigh = 125 °C for thigh = 2 min and cooled to Tlow,2 = −30, −10, 0, 30 °C for various tlow,2 values ranging from 0 to 300 min. The DSC melting peaks around 130 °C were obtained and are shown in Figure 6a−c. By Gaussian peak fitting, the enthalpy ΔHm(I) of the form I corresponding to peak 129 and peak 133 were obtained and the Xc(I129) and Xc(I133) values were determined according to Xc(I) = ΔHm(I)/ ΔHm°(I) with ΔHm°(I) = 141 J/g of the ideal form I.32,33 The

variations of Xc(I129) and Xc(I133) with tlow,2 are shown in Figure 6d,e. Figure 6d shows that the Xc(I129) values of the samples with Tlow,2 = 0 and 30 °C increased with increasing tlow,2 and the Xc(I129) of the sample with Tlow,2 = −30 °C almost remained constant. Men reported that the maximum nucleation rate of the nucleation process of form II to form I transition appeared at about −10 °C and the maximum rate of the following growth process of the transition occurred at about 40 °C.35 The variations of Xc(I129) with tlow,2 agreed well with form II to form I transition kinetics mainly during the initial nucleation process, proving that peak 129 is the melting peak of form I transited from form II. The variations of Xc(I133) with tlow,2 in Figure 6e were in obviously different ways. Within the first 60 min, the Xc(I133) value of the sample with Tlow,2 = 0 °C increased to a constant value, and for the sample with Tlow,2 = −30 °C, there also appeared a weak increase. For the sample with Tlow,2 = 30 °C, the Xc(I133) value was small but increased slowly within a long period of tlow,2 = 300 min. It can be noted that the increase of Xc(I133) with tlow,2 would finally stop because the amount of the amorphous iPB-1 that can crystallize further is limited in the samples. Once form I crystals directly formed, they contributed to the increase of Xc(I) in spite of later possible reorganizations. This confirmed that peak 129 could be attributed to the form I transited from form II and peak 133 to the form I directly formed in the iPB-1 samples. The melting peaks around 130 °C of the samples after cooling from Tmelt = 200 °C to Tlow,1 = −30, 0, and 30 °C for E

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Figure 5. Gaussian fitting results of the DSC melting peaks around 130 °C for the iPB-1 samples with Tlow,2 of −30 °C (a), 0 °C (b), 30 °C (c), 60 °C (d), 90 °C (e), and 105 °C (f) for tlow,2 = 2 min. The dashed lines are plotted to guide the eye.

Xc(I129) in region C. When Tlow,2 was set above 35 °C but below 95 °C, Xc(I133) remained as a constant value of about 0.02% and Xc(I129) remained as 0 in region D. Once Tlow,2 increased above 95 °C, Xc(I133) increased and Xc(I129) still remained as 0 in region E. The cooling and heating processes of the samples were examined. The glass transition temperature Tg of the iPB-1 sample could be determined as −28.8 °C from the heating curve at a rate of 10 °C/min and the glass transition region (GTR) was from −28.8 to −7.6 °C, as shown in Figure 7b. However, with cooling at 15 °C/min, the GTR was from −44.5 to −35.5 °C. This meant that the sample cooled to a Tlow,2 value in region A was completely in the glass state. Thus, the iPB-1 chains in the amorphous phase would lose their mobility and could not directly crystallize into form I at this Tlow,2. Similarly, the iPB-1 chains in form II also lose their mobility and form II could not transit into form I at this Tlow,2 value in

various tlow,1 values were also examined, as shown in Figure S1a−c. The resulting tlow,1-dependent Xc(I129) and Xc(I133) values are shown in Figure S1d,e. The results were similar to those obtained in Figure 6a−e, and similar conclusions could be drawn. Only in the sample with Thigh = 200 °C for 2 min and Tlow,1 = 30 °C was a long tlow,1 = 30 min needed for peak 133 to appear, which was much weaker than peak 129. 3.2. Temperature Dependence. According to the DSC melting curves shown in Figure 4b−d and the Gaussian peaks fitting results shown in Figure 5 as examples, the dependence on Tlow,2 of the Xc(I133) value as well as Xc(I129) and Xc(Itotal) values can be obtained and are shown in Figure 7a with the curves divided into several regions A−E. With Tlow,2 increasing from −60 to −44.5 °C in region A, both Xc(I133) and Xc(I129) almost remained constant. Then they slightly increased in region B before Tlow,2 reached −7.6 °C. A further increase in Tlow,2 resulted in an obvious decrease in both Xc(I133) and F

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Figure 6. Observed melting peaks at about 130 °C of the iPB-1 samples after aging at different Tlow,2 values of −30 °C (a), 10 °C (b), 0 °C (c), and 30 °C (d) for tlow = 0, 1, 2, 5, 10, 20, 40, 60, 90, 120, 150, 180, 210, 240, 270, 300 min. (e, f) Crystallinity of form I, Xc(I129) and Xc(I133), of the samples cooled from Thigh = 125 °C to Tlow,2 = −30, 0, 30 °C for various tlow,2 values. The solid and dashed lines are plotted to guide the eye.

consistent with the inference of Asada,36 Marigo,37 and Li.38 This implied that direct formation of form I only occurs to the iPB-1 chains with weak mobility. Additionally, Xc(I129) in region C also decreased with increasing Tlow,2, different from the known bell-shaped curve of the II−I transition rate of iPB-1 dependent on temperature with a maximum at room temperature.39 II−I transition kinetics of iPB-1 concern the controlling nucleation process and the growth process.40,41 Men proposed that the nucleation and growth rates were both Gaussian distribution functions of annealing temperature with maximum values at about −10 and 40 °C, respectively, resulting in the maximum total II−I transition rate at room temperature.35 The increase in Xc(I129) with decreasing Tlow,2 in region C as shown in Figure 7a mainly resulted from the nucleation process of the II−I transition. In region D, far from the GTR, only peak 133 appeared and remained almost constant with increasing Tlow,2 from 40 to 90 °C, though it was weak and

region A. The resulting constant Xc(I133) and Xc(I129) values shown in Figure 7a were attributed to the form I formation via direct formation or a II−I transition, respectively, at temperatures above region A during cooling or heating processes at which the chains did not completely lose their mobility. Region B covers both the two GTRs of the sample with cooling and heating. In region B, with increasing Tlow,2 value, the chain mobility of iPB-1 would increase and thus the chains in the amorphous phase could crystallize into form I and a II−I transition could occur when Tlow,2 was kept for 2 min, resulting in the observed slight increase in Xc(I133) and Xc(I129) as shown in Figure 7a. In region C, above and near the GTRs, the Xc(I133) value decreased sharply with increasing Tlow, indicating that high temperatures do not favor the direct formation of form I. This may be attributed to the increasing chain mobility in region C with increasing Tlow,2 above Tg. Such observations were G

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Figure 7. (a) Crystallinity of form I in the iPB-1 samples heated to Thigh = 125 °C for thigh = 2 min and then cooled to various Tlow,2 values for tlow,2 = 2 min. (b) Glass transition regions revealed by DSC cooling and heating curves. (c) Tlow,2-dependent crystallinity of form II Xc(II), the total crystallinity of form II including the form II transited into form I, Xc(II-total) = Xc(II)+ Xc(I129), and the sum crystallinity of both form I and form II, Xc(total) = Xc(II-total) + Xc(I133). (d) Tlow,1-dependent Xc(I133) of the iPB-1 samples after only melting at Tmelt = 200 °C for tmelt = 2 min in comparison with the Tlow,2-dependent Xc(I133) of the sample heated to Thigh = 125 °C for thigh = 2 min and then cooled to various Tlow,2 values for tlow,2 = 2 min. The solid and dashed lines are plotted to guide the eye.

showed only a small Xc(I133) value of about 0.02%. In region D, no peak 129 could be observed, revealing that there was no occurrence of a II−I transition in the samples with Tlow,2 increasing from 40 to 90 °C. Interestingly, with Tlow,2 increasing further to temperatures in region E, more form I was obtained in comparison to that in region D. Furthermore, Figure 7c gives the dependence on Tlow,2 of the Xc(II) value obtained according to Xc(II) = ΔHm(II)/ΔHm0(II) with ΔHm0(II) = 62 J/g.32,33 The Xc(II) almost remained constant in regions A and B, slightly increased in region C, and then reached a constant value followed by a slight decrease in region D. Finally when Tlow,2 increased to temperatures in region E, Xc(II) decreased to very small values since there was no nonisothermal crystallization on cooling from Thigh = 125 °C (Figure 4a) and only minor isothermal crystallization could occur at Tlow,2 above 95 °C. For the sample with Tlow,2 = 105 °C, no melt can crystallize into form II and Xc(II) = 0. Since the crystallinity of the form II crystals could be proposed to be equal to the crystallinity of the resulting form I crystals, the total crystallinity of form II can be obtained as Xc(II-total) = Xc(II) + Xc(I129) and the sum crystallinity of both form I and form II as Xc(total) = Xc(II-total) + Xc(I133) (Figure 7c). The constant Xc(II-total) in regions A−C and part of region D with Tlow,2 below 65 °C showed that the decrease in Xc(II) with decreasing Tlow,2 in region C experimentally equaled the increase in Xc(I129), indicating that no postcrystallization of

form II occurred after its nonisothermal crystallization (Figure 4a). The increasing Xc(total) with decreasing Tlow,2 in regions A−C below 35 °C was due to direct formation of form I. The Xc(I133) values of the samples cooled from Tmelt = 200 °C to Tlow,1 values for tlow,1 = 2 min were obtained from the DSC melting peaks of peak 133, as shown in Figure 2 as examples. The results are shown in Figure 7d in comparison with the Xc(I133) values of the samples which were heated again to Thigh = 125 °C for thigh = 2 min and then cooled to Tlow,2 values for tlow,2 = 2 min. Both of the curves showed the same critical temperatures around Tcr = 35 °C, below which direct crystallization of iPB-1 into form I could occur. Above this Tcr, no direct crystallization of form I could be observed in the samples cooled to Tlow,1 values for tlow,1 = 2 min, while a fixed amount of form I could be obtained in the samples cooled to Tlow,2 values for tlow,2 = 2 min with Tlow,2 below 95 °C and more form I could be obtained in the samples with Tlow,2 above 95 °C if minor iPB-1 melt was permitted to isothermally crystallize into form II at Tlow,2. Additionally, the amount of form I in the samples reheated again to Thigh = 125 °C for thigh = 2 min and then cooled to Tlow,2 values below Tcr = 35 °C for tlow,2 = 2 min was much larger than that in the samples directly cooled from Tmelt = 200 °C to Tlow,1 values for tlow,1 = 2 min. One may argue that, when the iPB-1 sample was cooled from Tmelt = 200 °C to a Tlow,1 = 30 °C for 2 min, some form I nuclei could be formed in the sample but too little to be detected. H

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Figure 8. (a, b) Trans and gauche conformations of C−C bonds in left- and right-handed main chains of iPB-1 with energy barriers for conformation transitions. (c) Temperature-dependent conformation changes of a coiled iPB-1 chain to a helix conformation. (d) Free energy differences ΔG of 3/ 1 and 11/3 helices determined by the competition between the entropy effect and the enthalpy effect with the energy barrier Eb,I for 3/1 helix ∞ ∞ ∞ conformation considered if the differences of T∞ 11/3 and T3/1 from Tm,II and Tm,I can be ignored.

Once the sample was reheated again to a Thigh = 125 °C for thigh = 2 min and then cooled to a Tlow,2 value above Tcr = 35 °C for tlow,2 = 2 min, the form I nuclei grow and accumulate, resulting in the observed formation of form I. If so, more temperature cycles repeated between Thigh = 125 °C and a Tlow,2 value above Tcr = 35 °C could result in further growth of the nuclei and form I accumulation, showing an increase in Xc(I133). However, this did not occur. After nine cycles, more form I could be obtained in the samples with a Tlow,2 value below Tcr = 35 °C in comparison to that in the samples with Tlow,2 values above Tcr = 35 °C, as shown in Figure S2a. More exactly, as revealed from Figure S2b, it is more reasonable that some of the obtained form I after one cycle in the samples with Tlow,2 above Tcr = 60 °C could be destroyed with the number of temperature cycles increasing to 9. In the sample with Tlow,2 = 40 °C, very near to Tcr = 35 °C, slightly more form I was obtained after nine cycles in comparison to that after only one cycle, indicating that the increasing speed of form I formation slightly outweighed the destroying process of the obtained form I during each temperature cycle. This meant that the Tcr = 35 °C determined according to the experimentally detectable form I may be estimated to be lower than the real theoretical value of the critical temperature, below which the iPB-1 chain segments can directly crystallize into form I but are not detectable. 3.3. Proposed Mechanism. Relative to form II, form I of iPB-1 is usually difficult to directly form while a bulk melt is cooled. However, at temperatures below Tcr = 35 °C, which is far below the crystallization temperature of form II, direct crystallization of iPB-1 into form I has been observed in this work. What is the physics behind it? As is known, form II is metastable with 11/3 helices packed in its tetragonal crystal cells and form I is thermodynamically stable with 3/1 helices packed in hexagonal crystal cells. The 3/1 helices are more

extended and stable than the 11/3 helices, which is the major difference of form I from form II. Since form II is usually obtained while an iPB-1 melt is cooled, it is possible to propose that it is much easier for iPB-1 chain segments to select 11/3 rather than 3/1 helix conformations and then to crystallize into form II at temperatures above Tcr = 35 °C. Here, 3/1 and 11/3 conformations of the chain segments are favored by forms II and I, respectively. The form I favored 3/1 helix conformation can only be formed at temperatures below Tcr = 35 °C, above which metastable form II favored 11/3 helix conformation would be trapped. This can interpret the experimental observations of polymorph selection behaviors of iPB-1 with Tcr = 35 °C mainly shown in Figure 7. When an iPB-1 melt is cooled from Tmelt = 200 °C to a lower Tlow,1 below Tcr = 35 °C but above the Tg, more 3/1 helices are preferred and obtained, resulting in further formation of more form I. In the samples with Tlow,1 above Tcr = 35 °C, the lack of 3/1 helix formation accounts for no observations of direct crystallization of iPB-1 into form I. In addition, only a small amount of the iPB-1 chain segments which do not crystallize into form II while nonisothermally cooling are left and have chances to be cooled to temperatures below Tcr = 35 °C to form more stable 3/1 helices and then crystallize directly into form I. Once 3/1 helices rather than 11/3 helices are preferred, direct formation of form I rather than postcrystallization of form II could be observed. In the sample cooled from Tmelt = 200 °C to a Tlow,1 = 30 °C for 2 min, much fewer 3/1 helices may be isolated and surrounded by form II crystals, so that they could not align and crystallize into detectable form I or their alignments may only result in local structures such as mesophase favored by form I which is not able to be detected. If the samples were reheated to a Thigh = 125 °C for thigh = 2 min to have the 3/1 helices or I

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Scheme 2. Formation of Forms I and II from a Bulk iPB-1 Melt

asymmetric main chain, one stable trans and two metastable gauche+ and gauche− conformations, as shown in Figure 8a,b, could be selected and there is an energy barrier for the transition between any two of them due to the steric hindrance of the side ethyl groups. At a high melt temperature, the high thermobility of the chain segments make it easy for an iPB-1 chain to cross the energy barrier and select a coiled conformation state with more gauche conformations of C−C bonds in the main chain, mainly due to the entropy effect according to the second law of thermodynamics.10 From this point of view, more dense and extended 3/1 helices have fewer gauche conformations of C−C bonds in the main chains relative to 11/3 helices. Therefore, 11/3 helices can be regarded as the intermediate conformations between the 3/1 helices and coiled conformations, as shown in Figure 8c. With a decrease in temperature, the thermobility of the iPB-1 chain segments decreases and the entropy effect weakens. Therefore, the coiled chains would tend to extend to have less gauche and more trans conformations because of minimization of their free energies, as approximately shown by the upward triangles and the red line in Figure 8c. For both 3/1 and 11/3 helices transited from the coiled conformations, their formation is determined by the free energy difference ΔG = ΔH − TΔS + Eb. Here, ΔH and ΔS represent the enthalpy and entropy differences for a coiled chain segment to transit into a helix segment, T is the temperature, and Eb is the energy barrier. The free energy difference of both 3/1 and 11/3 helices from the coiled conformations are shown in Figure 8d. The ΔH value of 3/1 helices is smaller than that of 11/3 helices. The slope of the temperature-dependent ΔG of 3/1 helices, −ΔS, is larger than that of 11/3 helices. The energy barrier Eb of 11/3 helices can be ignored, while the Eb value of 3/1 helices makes the ΔG value of 3/1 helices much larger than that of 11/3 helices. Thus, a temperature window from Tcr = 35 °C to Tg is generated for 3/1 helices to form, and a temperature window from T11/3 to Tcr is generated for 11/3 helices to form. Here, the “equilibrium melting temperature” T11/3 of 11/3 helix conformation is supposed to be equal to the equilibrium melting temperature of form II as well as the equilibrium formation temperature of 11/3 helices. The formation

the form I favored mesophase survived but form II melted, some of the survived 3/1 helices would have chances to align together, resulting in local structures of form I favored mesophase which then further develop into form I while the samples were cooled to Tlow,2 above Tcr = 35 °C for tlow,2 = 2 min. If the sample was reheated to Thigh = 125 °C for thigh = 2 min and then cooled to Tlow,2 above 95 °C for tlow,2 = 2 min, only minor melt would crystallize into form II. Thus, less surviving 3/1 helices could be prevented from aligning to form mesophases, resulting in more mesophases which could crystallize further into form I. Importantly, in the presence of the surviving 3/1 helices or the local structures of form I favored mesophases in the iPB-1 samples, form I seemed to be able to crystallize from the bulk iPB-1. In addition, the surviving 3/1 helices should also be responsible for the sharp increase in the amount of the directly obtained form I when the samples are reheated to a Thigh = 125 °C for thigh = 2 min and then cooled to Tlow,2 below Tcr = 35 °C for tlow,2 = 2 min, showing the strong synergistic effects between the surviving 3/1 helices or form I favored mesophases and the 3/1 helices or mesophases formed while cooling from Tmelt = 200 °C to Tlow,1 below Tcr = 35 °C for tlow,1 = 2 min. In Figure 7d, the red dashed line shows the sum Xc(I133) resulting from the contributions of the surviving 3/1 helices or mesophases and that of the formed 3/1 helices or mesophases in the samples while they were cooled from Tmelt = 200 °C to Tlow,1 for tlow,1 = 2 min. The difference of the black line from the red dashed line could be regarded as the contributions of the synergistic effects being much larger than the sum contributions of both the surviving and the directly formed 3/1 helices or the mesophases. This indicated that the presence of the surviving 3/1 helices or mesophases promoted formation of more 3/1 helices or mesophases and then direct formation of more form I. Since direct crystallization of iPB-1 is based on the prior 3/1 helix formation, understanding the formation of the 3/1 helices is the key to understanding the genetic physics of the crystallization and polymorph selection processes. As is known, the iPB-1 chains are chiral but racemic with ethyl groups as side chains.42−44 For each C−C bond in the J

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Figure 9. FTIR spectra collected at different temperatures during the melting process of a high-energy-electron-irradiated iPB-1 sample in form I with a dose of 251 KGy.

attract each other or attract other neighboring chain segments and induce them to select 3/1 helix conformation and then align together to form local structures as a form I favored mesophase, or a preordering “nucleus cluster”,45 with a certain size prior to the appearance of form I crystals.3 Miura proposed that there were large orientation fluctuations in the initial stage and the rigid molecules are uniformly distributed at the initial state.45 The orientation fluctuations might be related to the formation of rigid extended helix chains and their alignments. With crystallization or solidification proceeding, the fluctuations would decrease gradually due to the decreasing concentration of the alignments of the crystallizable helix chains. This indicates that the high concentration of the rigid form I favored 3/1 helix chains and the alignment structure or form I favored mesophase increases the probability of direct formation of form I from the iPB-1 amorphous phase or melt, resulting in its faster crystallization process and finally higher crystallinity of directly obtained form I. As occurs in oligomeric α and β iPP crystals,13 the chain segments soon lose their conformational state kept in crystals with melting. It is difficult to detect the 3/1 helices, the local structure, or the form I favored mesophase prior to the appearance of form I in iPB-1. While melting a high energy electron irradiated iPB-1 with a dose of 251 KGy in form I, we collected the in-situ FTIR spectra and found an FTIR adsorption peak at 912 cm−1 while heating the sample to 125 °C, as shown in Figure 9a. This peak is different from the adsorption peak at 924 cm−1 of form I and at 905 cm−1 of form II. When an iPB-1 sample in form II was melted or an iPB-1 melt was cooled from 150 °C to crystallize into form II, as shown in Figure 9b,c, this peak did not appear. We are

temperature window of 3/1 helices is far below that of 11/3 helices, which accounts for the preferred formation of form II rather than direct crystallization into form I while an iPB-1 melt is cooled. Generally, before the iPB-1 melt was cooled to temperatures below Tcr = 35 °C for formation of 3/1 helices prior to form I formation, the iPB-1 chain segments form 11/3 helices first and then crystallize into form II. Thus, minor chain segments left in the amorphous phase could select 3/1 helix conformations and then crystallize further into form I when the temperature enters the formation temperature window of 3/1 helices. In conclusion, the formation of 3/1 helices is a process driven by the enthalpy effect. Both the entropy effect and the energy barrier attributed to the steric hindrance of side ethyl groups result in difficulty for the isotropic coiled chains to be extended to form 3/1 helices. Crystallization into form II or into form I requires the iPB-1 chain segments to be extended to form less extended form II favored 11/3 helices or more extended form I favored 3/1 helices. For an iPB-1 chain, the formation of 11/3 helices or 3/1 helices is determined mainly by temperature with a critical Tcr = 35 °C. At temperatures above this Tcr, form II favored 11/3 helices are preferred, while at temperatures below this Tcr, form I favored 3/1 helices are preferred. Crystallization of iPB-1 melts into form I or form II is a helix conformation controlled process. Formation of forms I and II from an iPB-1 melt can be proposed as shown in Scheme 2. It should be noted that one single form I favored 3/1 helix chain segment is usually not sufficient to trigger crystallization of form I. The alignment of the 3/1 helix chain segments is required. To be more stable, the 3/1 helices may prefer to K

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convinced that the peak at 912 cm−1 is related to the local structure of form I favored 3/1 helices or a form I favored mesophase concerning direct formation of form I. The FTIR results confirmed the proposed helix conformation controlled crystallization process of iPB-1 and supported the analysis of its polymorph selection behavior.

ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S2 The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.cgd.8b00119.

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REFERENCES

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4. CONCLUSIONS Direct crystallization of iPB-1 into form I was found to occur at temperatures below the critical temperature Tcr = 35 °C and above Tg. The 3/1 helix conformation formation, driven by the positive enthalpy effect which outweighs the negative entropy effect and the negative contribution of the large energy barrier at temperatures below Tcr, was proposed to be the determining step of direct crystallization of iPB-1 into form I. The energy barrier of the 3/1 helix conformation formation is considered to be larger than that of the 11/3 helix conformation formation. At temperatures above Tcr, formation of 3/1 helices is forbidden due to the dominant entropy effect and its energy barrier, while formation of the 11/3 helices could occur and result in the form II favored mesophases or local structures and then form II. The appearance of a form I formation temperature window far below the crystallization temperature of form II interpreted why the metastable form II is usually obtained rather than the stable form I while cooling an iPB-1 melt. Additionally, at temperatures above Tcr, the surviving 3/1 helices or the local structure of the form I favored mesophase could result in direct formation of form I, and at temperatures below Tcr, they could promote direct formation of form I. Too high a temperature could erase the surviving 3/1 helices and the form I favored mesophases or local structures. The results could shed light on a further understanding of the physics behind polymer crystallization and the polymorph selection processes as well as the development of the crystal formation controlling techniques and applications of iPB-1.

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DSC melting peaks and temperature-dependent Xc(I133) values (PDF)

AUTHOR INFORMATION

Corresponding Author

*S.J.: tel, 86-22-85356421; fax, 86-22-27404724; e-mail, [email protected]. ORCID

Jesper de Claville Christiansen: 0000-0001-5501-7019 Shichun Jiang: 0000-0003-3818-0230 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (51673147, 51573131, and 21374077). L

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