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Lamellar Thickness Dependence of Crystal Modification Selection in the Syndiotactic Polystyrene γ‑to-α/β Phase Transition Process Hai Wang, Chunji Wu, Dongmei Cui, and Yongfeng Men* State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, China 130022 ABSTRACT: The syndiotactic polystyrene (sPS) γ-to-α/β phase transition was investigated using temperature-dependent synchrotron wide-angle X-ray diffraction (WAXD) and smallangle X-ray scattering (SAXS) measurement. The sPS γ form samples, which were obtained from acetone solvent induced crystallization followed by annealing at various temperatures (Ta), exhibited a regular manner on the selection of the α or β crystal modification during the phase transition process. As the Ta was increased from 25 to 150 °C, the weight fraction of β form in bulk sPS decreased from 55% to 0%, but that of the α form increased from 45% to 100%. In other words, the α form became the dominating crystalline modification at higher Ta. The lamellar thickness (dc) of the γ form at the initial state (before heating) ranged from 4.4 to 5.8 nm, and the weight crystallinity (Xc) was between 0.52 and 0.64. However, the dc values of the γ form at the final state (before γ-to-α/β phase transition) were very close for the samples pretreated at different Tas, which were in the range 7.0−7.4 nm. Therefore, the most plausible interpretation for the crystal modification selection during the γ-toα/β phase transition was that the nucleation of β form was suppressed totally in the γ form with higher initial dc value (>5.8 nm). With the elevated Ta and increase of initial dc, the promoted nucleation of α form in the γ form resulted in the enhanced growth of the α crystalline phase during the γ-to-α/β phase transition. As a result, after the phase transition the crystalline phase in bulk sPS had a gradually increased weight fraction of the α form and turned out to be pure α form when Ta was increased from 25 to 150 °C. limonene, carvone, and 1-chlorodecane.19 Wang et al. investigated the crystallization behavior of the γ form using the thermodynamic phase diagram, from which a recrystallization line, equilibrium recrystallization line, and equilibrium melting line were obtained.33 Bohje and Tashiro investigated the δ/δe-γ-to-α/β crystal phase transition, from which the effects of the remaining solvent in the bulk on the crystal modification selection were proposed.34−36 However, the initial physical state of the γ form may have an impact on the resulting aggregate structure of the sPS α and β form, especially the preference of the final crystal modification after the γ-to-α/β phase transition, which has not been clarified yet. Therefore, sPS samples with extremely high syndioselectivity ([rrrr] > 99%) were employed in this study with the purpose of clarifying the influence of the initial lamellar structure on the γto-α/β phase transition. The aggregate structure of the sPS γ form that undergone heat treatment at different temperatures was characterized using wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) techniques. The γ-toα/β phase transition was investigated using differential scanning

1. INTRODUCTION Syndiotactic polystyrene (sPS) is one of the most important engineering plastics. It exhibited complicated crystal modifications, including the mesophase,1,2 α,3−10 β,6,11−15 γ,16−19 δ,20−23 and δe.24−27 A number of studies had been carried out detailing the phase transitions between different polymorphs of sPS.28,29 The α and β form consisting of the all-trans conformational (TTTT) chains could be obtained directly from the amorphous sPS by the cold-crystallization or meltcrystallization process, which had hexagonal and orthorhombic unit cell, respectively.3 The γ form was obtained by dipping the amorphous sPS into acetone, which had TTGG conformational chains and monoclinic unit cell similar to the sPS δe form.16,18 The sPS γ form could be considered as an “intermediate state” on the phase transition diagram. The γ form appeared when the sPS ε, δe, and δ forms were heated to 120 °C, which transformed to the α or β form upon further heating.28 Handa et al. investigated the effect of compressed CO2 on the phase transition from the γ form to the α or β form.17 Rudder et al. conducted experiments regarding the role of cyclohexane in the formation of γ form and its transition to the β form.30 Ma et al. reported the direct formation of the γ form induced by supercritical fluid CO2 and its influence on the transition from the γ form to the α or β form.31,32 Rizzo et al. reported the formation of the γ form induced by non-guest solvents, such as © XXXX American Chemical Society

Received: September 8, 2017 Revised: December 1, 2017

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DOI: 10.1021/acs.macromol.7b01943 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules calorimetry (DSC) and temperature-dependent WAXD and SAXS measurements.

2. EXPERIMENTAL SECTION 2.1. Materials. sPS samples (Mn = 11.1 × 105 and Mw/Mn = 1.59) were synthesized using a rare-earth catalyst. The syndioselectivity of the sPS samples determined by 13C nuclear magnetic resonance (NMR) spectroscopy was [rrrr] > 99%, which was the highest in the literature based on our knowledge.37−39 Amorphous sPS films with the thickness of ca. 1 mm were obtained by hot compressing the assynthesized sPS powder at 310 °C followed by quenching into ice water. The as-compressed sPS films were cut into pieces with the width of 2.5 mm and length of 5 mm. The sPS γ form was prepared by immersing the small pieces of amorphous sPS in acetone at room temperature for 48 h followed by drying in a vacuum for 24 h. The asobtained sPS γ form samples were annealed at designated temperatures (Ta) ranging from 25 to 150 °C, which were lower than the phase transition temperature (Ttrans) of the sPS γ form to α or β form, for the completion of lamellar thickening. 2.2. Measurement. 2.2.1. DSC. The DSC measurement was conducted using a Mettler Toledo DSC1 Star system (Mettler Toledo Instrument, Swiss) in order to obtain the melting temperature (Tm) and Ttrans of the sPS γ form. The heating rate was 2.5 and 10 °C/min in all the measurements. 2.2.2. WAXD and SAXS Measurements. The WAXD measurements at room temperature were conducted using a Nano-inXider vertical SAXS/WAXS system of Xenocs SA, France. A multilayer mirror focused Cu Kα X-ray source (λ = 0.154 nm, Genix3D Cu ULD, Xenocs SA, France) was used to generate the incident beam. The detectors used for WAXD measurements were hybrid pixel detector (Pilatus3, Dectris, Swiss). The sample-to-detector distance is 75.3 mm for the WAXD measurement. In situ synchrotron WAXD measurements at the elevating temperatures were performed at beamline 1W2A (λ = 0.154 nm), BSRF, Beijing, China. A piece of sPS γ form samples was heated from crystallization temperature (Tc) until melting by a portable heating device (TST350, Linkam, UK) installed at the beamline to follow the phase transition during heating. The sample-to-detector distance was 121 mm. SAXS measurements with a sample-to-detector distance of 1890 mm were performed at the beamline BL16B (λ = 0.124 nm) at SSRF, Shanghai, China. The SAXS data analyses were carried out according to the standard procedures.40,41 The one-dimensional SAXS profile was obtained by integrating the 2D SAXS patterns. The electron density correlation function K(z) was applied in the analysis of the lamellae structure, which was derived from the inverse Fourier transformation of the integrated intensity distribution I(q) as follows:42

Figure 1. 1D-WAXD profiles of sPS γ form that annealed at various temperatures.

crystalline phase in the 1D-WAXD profiles and listed in Table 1. The temperature-dependent 1D-SAXS profiles with Table 1. Weight Crystallinity (Xc), Long Period (dac), Lamellar Thickness (dc), and Amorphous Thickness (da) of SPS γ Form Annealed at Various Temperatures Ta/°C Xc dac/nm dc/nm da/nm dac/nm dc/nm da/nm

25

60

90

110

130

γ form initial state (before heating) 0.52 0.53 0.56 0.61 0.62 7.5 7.7 8.4 8.7 9.1 4.4 4.6 4.8 5.1 5.3 3.1 3.1 3.6 3.6 3.8 γ form final state (before phase transition) 11.8 12.1 12.5 12.6 12.3 7.0 7.2 7.4 7.4 7.3 4.8 4.9 5.1 5.2 5.0

150 0.64 9.8 5.8 4.0 12.5 7.4 5.1

Lorentz correction of the γ form sample annealed at 25 °C are plotted in Figure 2. The γ-to-α/β form phase transition is a

∞

K (z) =

∫0 I(q)q2 cos(qz) dq ∞

∫0 I(q)q2 dq

where z denotes the location measured along a trajectory normal to the lamellar surfaces, and the multiplication of I(q) with q2 (Lorentz correction) was performed because of isotropically distributed stacks of parallel lamellar crystallites in the sample.

Figure 2. Temperature-dependent 1D-SAXS profiles with Lorentz correction of sPS γ form annealed at 25 °C.

3. RESULTS AND DISCUSSION 3.1. Melting Behavior of SPS γ Form Undergone Different Heat Treatments. The 1D WAXD profiles of the sPS γ form annealed at different temperatures are shown in Figure 1. All the samples were in pure γ form at the designated annealing temperatures from 25 to 150 °C. The strengthened diffraction at 2θ = 10.5°, 14.3°, and 24° indicated a lightly improved crystallinity in the γ form samples. The weight crystallinity (Xc) of all samples was above 50%, which was calculated based on the peak fitting of amorphous and

melting−recrystallization process, during which the melting of γ form and formation of α/β form occurred simultaneously.28 Figure 2 exhibits the growth of γ form (red line), melting of γ form (black line), growth of α/β form (blue line), and the melt of α/β form (orange line). The 1D-SAXS profiles of the γ form annealed at higher temperature showed a similar temperature dependency. In order to avoid the ambiguity resulting from the mixture of the sPS γ and α/β form, the 1D-SAXS profiles corresponding to only the pure γ form were considered. The B

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transition of the γ form. The steps on each thermogram marked by hollow point shifted obviously from around 70 to 170 °C, which was attributed to the reorganization of the γ form.31 The melting peak of sPS γ form was in the range of around 180− 195 °C for the γ form heated at 10 °C/min but ranged from 190 to 195 °C for the γ form heated at 2.5 °C/min. This was due to the different extent of lamellar thickening at high and low heating rate, which was detailed in our previous work.33 The DSC thermograms on the right side were about the melting of the sPS α/β phase. In general, the melting peak shifted toward higher temperature as the Ta was increased at the heating rates of 2.5 and 10 °C/min, during which double peaks were observed. The melting peaks at low and high temperatures correspond to those of sPS β and α form, respectively.43 It was revealed from the relative intensity of the double peaks that the ratio of β and α was changed for the γ form that undergone different heat treatment (Ta). It indicated that there were correlation between the initial lamellar thickness before the γ form phase transition and the phase selection after the melting and recrystallization of the γ form, which was regardless of the heating rates of the DSC thermograms at least in the range of 2.5 and 10 °C/min. This phenomenon would be clarified using the WAXD measurements in the following section. 3.2. Phase Transition of SPS γ-to-α or β Form under Different Heat Treatments. The in situ 1D-WAXD profiles collected in the heating program of sPS γ form that annealed at different temperatures are compared in Figure 5a−d. It indicated that the sPS α form became the dominating phase as the Ta was increased from 25 to 150 °C. It is seen in Figure 5d that the α form was the only crystal modification left in the bulk after the γ-to-α/β phase transition when Ta was not below

evolution of the lamellar thickness of γ form that annealed at different temperatures upon heating was detailed in our previous work.33 Briefly, the γ form initial state (before heating) and the final state (before phase transition) were focused, the long period (dac), lamellar thickness (dc), and amorphous thickness (da) of which were derived from 1DSAXS curves shown in Figure 3a,b and list in Table 1. It was seen that the initial lamellar thickness ranged from 4.4 to 5.8 nm as the Ta was increased. At the heating rate of 2.5 °C/min, the lamellar thickness was increased, which was in the range of 6.9 and 7.4 nm.

Figure 3. 1D-SAXS profiles with Lorentz correction of sPS γ form annealed at various temperatures: (a) initial state (before heating); (b) final state (before phase transition).

Figures 4a and 4b show the DSC thermograms of sPS γ form annealed at different temperatures ranging from 60 to 290 °C, which are heated at 10 and 2.5 °C/min. The DSC thermograms on the left side cover the reorganization, melting, and phase

Figure 4. DSC thermograms of the sPS γ form annealed at different temperatures. C

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Figure 5. Temperature-dependent 1D-WAXD profiles of γ form that annealed at (a) 25, (b) 90, (c) 110, and (d) 150 °C.

150 °C. The relative intensity of the α and β form could be distinguished at 2θ = 5.2° (α form) and 4.9° (β form) or at 2θ = 11.1° (α form) and 11.8° (β form). This also corresponded to the results observed in the DSC thermograms, in which the melting peaks were shifted to higher temperature and merged in one peak. Meanwhile, for the γ form annealed at 25, 90, and 110 °C, the β form appeared at the early stage of the phase transition, followed by the presence of the α form. Upon further heating, β and α form melted sequentially, which was seen from the disappearing of the β and α form diffraction peaks. The lamellar thickness was supposed to be the most probable reason for inducing different amounts of α and β form. On the

basis of the SAXS data, as the initial dc of the γ form increased from around 4.4 to 5.8 nm (Δdc ∼ 1.4 nm) and initial da increased from 3.1 to 4.0 nm (Δda ∼ 0.8 nm), the content of α form became more dominating. Meanwhile, the dc and da of γ form in the final state were very close for samples annealed at different temperatures, which was from 7.0 to 7.4 nm (Δdc ∼ 0.4 nm) and from 4.8 to 5.1 nm (Δda ∼ 0.3 nm), respectively. It indicated that the initial physical state of γ form exhibited relatively large difference compared with that in the final state. In other words, it was most plausible that the crystal modification selection during the γ-to-α/β phase transition was determined by the initial state of the γ form rather than the final state. Based on the above-mentioned phenomena, the D

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of the β form was suppressed finally as the initial lamellar thickness of γ form was increased to about 5.8 nm. De Rosa et al. reported that the crystallization of sPS α and β form did not depend on the crystallization temperature but on the memory of the α form in the melt in the range 240−270 °C.48 By adjusting the melt temperature and holding time, the pure α form was obtained when the sPS melt was kept at 280 °C for 30 s before cooling down. Most researchers also agreed that the β form could be easily formed during slow cooling of the melt, but a very fast cooling was necessary for the formation of the pure α form.48 Although the heating of glassy sPS from pure amorphous state could produce α from it was difficult to avoid β form absolutely in aspect of practical operation. In this particular study, solvent-induced crystallization, which conducted before the melting and recrystallization of γ form, provided a way that the initial lamellar thickness of sPS γ form, if not less than 5.8 nm (1/dc ∼ 0.17 nm−1), would induce the formation of pure α form.

most reasonable speculation was the different nucleation process of the sPS α and β form during the lamellar thickening process of the γ form. The crystallization of semicrystalline polymer involves nucleation and growth stages.44,45 The amount of nuclei with certain crystal modification determined the further growth stage of this crystal. With regard to the form II-to-I phase transition of isotactic polybutene-1 (iPB-1), the predeveloped nuclei of form I were directly related to the amount of form I during the growth stage, which controlled the phase transition process as a result.46 Therefore, it was understood like that the γ-to-α/β phase transition was also a nucleation and growth process. The nucleation of α and β form occurred at the moment of γ form with different initial dcs formed. With the increase of the annealing temperature, the relative amounts of α and β nuclei were changed with the initial dc of the γ form until the totally suppressed β form nucleating at the initial dc of γ form equal to 5.8 nm. Therefore, the relationship between the lamellar thickness and the crystal modification selection is exhibited in Figure 6 schematically.

4. CONCLUSION On the basis of the DSC, SAXS, and temperature-dependent WAXD measurements, an initial dc dependence of crystal modification selection was observed. As the initial dc of the sPS γ form was increased, the α form became the dominating phase after the γ-to-α/β phase transition. This phenomenon was speculated as the impact of the initial dc of the γ form on the α and β nucleating process. As the initial dc was increased to around 5.8 nm, the α nucleating became dominating, resulting in the pure α form after the γ-to-α/β phase transition.

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AUTHOR INFORMATION

Corresponding Author

Figure 6. Summary of the relationship between initial lamellar thickness (dc) and crystal modification selection.

*E-mail: [email protected] (Y.M.). ORCID

Hai Wang: 0000-0001-8902-4014 Dongmei Cui: 0000-0001-8372-5987 Yongfeng Men: 0000-0003-3277-2227

Gowd et al. discussed the effect of solvent on the phase transition of sPS.34,35 They used toluene and norbornadiene as the “guest solvent”, which form sPS−toluene or sPS− norbornadiene complex (sPS δ form). Upon heating, the sPS δ form lost toluene or norbornadiene molecules gradually and transformed into the γ form and then into the α or β form. However, acetone as a “non-guest solvent” was used in this study to induce the crystallization of sPS γ form. There would be no strong interaction between acetone molecules and sPS chains, which is not like the case of sPS δ form. No sharp peak corresponding to the solvent evaporation was observed from DSC thermograms, which meant trace acetone solvent was totally removed from the sPS bulk after long time vacuum treatment. Moreover, the composition of sPS α and β form showed systematically change after the phase transition, which are not plausible determined by the remaining trace solvent. According to previous research, it was very difficult to obtain pure sPS α form from the melt crystallization process. Su et al. reported that the nucleation rate of α form was highly competitive at low temperature but much less at high temperature compared with the β form based on the thermodynamics parameters determined by DSC, WAXD, and SAXS (Tmo-α = 294 °C, Tmo-β form = 306 °C, ΔHf-α = 82 MJ/m3, ΔHf-β = 146 MJ/m3, basal surface energy σe-α = 8.2 mJ/m2, and σe-β = 26.8 MJ/m3).47 But the much higher Tmo-β, ΔHf-β, and σe-β made the crystallization of β form more easily, which resulted in the impurity of the α form during heat treatment. In this particular work, it seemed that the formation

Author Contributions

H.W. and C. W. contributed equally to this work. H.W. conducted the measurements and data analyses; C.W. did the synthesis and sample preparation. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by National Natural Science Foundation of China (21704101, 51525305, and 21134006). We thank Dr. Guang Mo and Prof. Zhonghua Wu for their supports on WAXD experiments at BSRF (2016-BEPC-PT000238) and Dr. Jingyou Lin and Dr. Wenqiang Hua for their assistance on SAXS experiments at SSRF (Z15sr0043).

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REFERENCES

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DOI: 10.1021/acs.macromol.7b01943 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.7b01943 Macromolecules XXXX, XXX, XXX−XXX