Fully Isotactic Poly(p-methylstyrene): Precise Synthesis via Catalytic

Article Cite This: Macromolecules 2019, 52, 4839−4846

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Fully Isotactic Poly(p‑methylstyrene): Precise Synthesis via Catalytic Polymerization and Crystallization Studies Tianyu Wu,† Luis Valencia,‡,§,# Thomas Pfohl,† Barbara Heck,† Günter Reiter,†,∥,⊥ Pierre J. Lutz,‡,§ and Rolf Mülhaupt*,‡,∥,⊥ †

Physikalisches Institut, University of Freiburg, Hermann-Herder-Str. 3, D-79104 Freiburg, Germany Institut für Makromolekulare Chemie, University of Freiburg, Stefan-Meier-Str. 31, D-79104 Freiburg, Germany § Institute Charles Sadron, CNRS UPR 22, University of Strasbourg, 23, rue du Loess, F-67034 Strasbourg, France ∥ Freiburg Materials Research Center FMF, University of Freiburg, Stefan-Meier-Str. 21, D-79104 Freiburg, Germany ⊥ Freiburg Center for Interactive Materials and Bioinspired Technologies FIT, University of Freiburg, Georges-Köhler-Allee 105, D-79110 Freiburg, Germany

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‡

S Supporting Information *

ABSTRACT: The synthesis of stereoregular styrenic polymers on single-site catalysts has received great attention over the last decades; however, little is known with respect to tailoring fully isotactic poly(p-methylstyrene) (iPpMS). Here, we demonstrate that isospecific coordination polymerization of p-methylstyrene can be achieved in the presence of the catalyst dichloro[1,4-dithiabutanediyl-2,20-bis(4,6-di-tert-butyl-phenoxy)] titanium activated by methylaluminoxane. Moreover, iPpMS has been assumed to be noncrystallizable for decades, as X-ray diffraction and differential scanning calorimeter measurements of powder samples failed to reveal a well-defined crystal structure. However, we successfully generated dendritic crystals of iPpMS from dilute solutions by carefully controlling the evaporation rate of the solvent. The crystal morphology was studied by optical microscopy and atomic force microscopy. In situ heating experiments of dendritic crystals allowed us to measure the melting temperature of iPpMS crystals. Moreover, a vapor annealing process was carried out to prepare multilamellar crystals for small-angle X-ray scattering measurements to characterize the spacing and orientation of the formed crystalline lamellae.

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INTRODUCTION Over 60 years after their initial discovery by Natta,1 ZieglerNatta catalysts2,3 and later stereospecific single-site (post) metallocene catalysts4−6 keep revealing new capabilities regarding the precise control of stereoregularity of different polymer families, making them one of the most remarkable technological breakthroughs in the field of polymer chemistry, as stereoregularity exerts a significant influence over the thermal behavior, solubility, crystallinity, and other physical properties of polymers. The synthesis of stereoregular syndiotactic styrenic polymers has received great attention over the last few decades due to their wide range of applications.7−9 For instance, Ishihara et al.10 used a trivalent mono(cyclopentadienyl)titanium catalyst to prepare syndiotactic polystyrene that exhibited high heat and solvent resistance, a high melting temperature (around 265 °C), improved dielectric constant and crystallinity, and low gas permeability. Isotactic polystyrene (iPS) has received less attention mainly because it has a much lower crystallization rate, which is disadvantageous in processing by extrusion or injection molding techniques.11−14 Since the pioneer work of Natta,15 among the stereospecific polymerization of styrenic monomers, methyl monosubstituted styrenes, such as m-methylstyrene and p-methylstyrene, have © 2019 American Chemical Society

been extensively studied by means of different catalyst systems. For instance, the catalyst systems CpTiCl3/methylaluminoxane (MAO) and Ti(benzyl)4/MAO16 have been reported to yield both poly(m-methylstyrene) and poly(p-methylstyrene) (PpMS) with high degree of syndiotacticity. The relevance of methyl-substituted styrenic polymers arises from the benefits that the methyl group in the monomer’s benzene ring confers over the general purpose of polystyrene, such as lower density and melt viscosity.17 Furthermore, it provides versatility for chemical modification by means of halogenation, metalation, or oxidation, which is of some importance when the interaction with other materials is paramount or when adhesion or printability is desired. Functional polyolefins characterized by a block of grafted structures represent typical examples.18 Isotactic poly(p-methylstyrene), hereinafter referred as iPpMS, although it has been studied for decades since early reports by Natta et al.,1 has been considered as a noncrystalline polymer, and to date, no synthetic route has been established to produce a crystallizable fully isotactic PpMS; therefore, the properties and capabilities of this material have never been Received: March 29, 2019 Revised: May 29, 2019 Published: June 19, 2019 4839

DOI: 10.1021/acs.macromol.9b00640 Macromolecules 2019, 52, 4839−4846

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was terminated by addition of acidified methanol and the product was precipitated in methanol, filtered, and dried under vacuum at room temperature until a constant weight was received (see Scheme 1).

fully explored. In recent studies, some attempts to synthesize fully isotactic PpMS have been undertaken using cationic polymerization;19−21 however, isotactic pentad fractions of only around 40% were observed; therefore, the attributed properties to iPpMS, such as crystallinity, are questionable. Möller et al.22 and later De Carlo et al.23 reported the synthesis of highly isotactic PS and substituted styrenes in the presence of the catalyst dichloro[1,4-dithiabutanedyi-2,2′bis(4,6-di-tert-butyl-phenoxy)]titanium, together with a great number of studies on its application for the isospecific coordination polymerization of styrene including functional iPS.24−26 Recently, Nakata et al.27 reported the development of [OSSO]-type bis(phenolate) catalysts based on a trans-1,2cyclooctanediyl platform associated with several early transition metals. These different catalysts have been extensively studied in further studies, researchers have efficiently polymerized monomer pairs such as styrene−isoprene, 4methyl-1,3-pentadiene and styrenic monomers,28,29 and ethylene−styrene30 or even used them for the synthesis of stereoregular block copolymers31 using mainly MAO as activator. However, very little have been reported in the literature on the use of these catalysts for the coordination (co)polymerization of p-substituted styrenes.32 In this work, we report on the isospecific polymerization of p-methylstyrene (isotactic pentad fraction > 95%) performed in the presence of the catalyst dichloro[1,4-dithiabutanediyl2,20-bis(4,6-di-tert-butyl-phenoxy)]titanium activated by modified-methylaluminoxane (MMAO). A major part of the work was devoted to a detailed study on the crystallization of selected iPpMS. To investigate its crystallization behavior, we performed a series of experiments that enable us to form crystalline structures in a thin polymer film. The results demonstrate that dendritic crystals can be formed from dilute solutions by carefully controlling the evaporation rate of the solvent from solution.

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Scheme 1. Schematic Representation of the Isospecific Polymerization of pMS

Characterization Technique. The 13C NMR spectra of the iPpMS samples were measured on an ARX 300 MHz spectrometer (300 MHz for 1H and 75 MHz for 13C) at 25 °C using 5 mm NMR tubes and 40 mg of polymer samples dissolved in CDCl3 (0.7 mL) and analyzed at 16 000 scans. Chemical shifts were referenced to tetramethylsilane and calculated by the residual isotopic impurities of the deuterated solvent (δ = 7.26 for CHCl3). Data analysis was carried out using Mestrenova software. The proportions of mmm pentads, mm triads, and m dyads were determined based on the integrals of characteristic signals at 143.6, 143.2, and 142.8 ppm, respectively, according to the values reported in the literature,33 as follows p(mmmm) =

p(mm) =

A143,6 A143,6 + A143,2 + A142,8 A143,6 + A143,2

A143,6 + A143,2 + A142,8

× 100

× 100

The number-average molecular weight (Mn), weight-average molecular weight (Mw), and dispersity (Đ) were determined by SEC/RI (or SEC/LS) at room temperature on a Waters apparatus equipped with five PL gel columns (Polymer Laboratories Ltd.), a Waters WISP 717 autosampler, a Shimadzu RID 6A differential refractometer, and a Wyatt DAWN DSP multiangle laser light scattering (LS) detector (laser: λ = 632.8 nm). Tetrahydrofuran was used as an eluent at a flow rate of 1.0 mL/min. Standard linear PSs (Mn: 1350 to 2 × 106 g/mol) were used for calibration. The melting temperature and glass transition temperature for the powder samples were studied using a Perkin-Elmer differential scanning calorimeter (DSC) 7 instrument operated under a nitrogen atmosphere. The specimens were sealed in an aluminum pan and heated from 0.0 to 300.0 °C at 10.0 °C/min. To identify the structure of the iPpMS crystals and the orientation of the lamellae on a substrate, X-ray scattering measurements were performed using a modified Siemens D500 X-ray diffractometer with a conventional Cu Kα X-ray source of wavelength, λ, of 0.1542 nm. A θ-2θ scan was taken from a film prepared on a silicon wafer in reflection mode. In this mode, the sample stage moves by the angle θ, whereas the detector simultaneously moves by the angle 2θ. The scattering vector is defined by the following equation: qz = 4π/λ sin θ. The measurement range was from 1.4° (qz = 1 nm−1) to 10° (qz = 7.1 nm−1); thus, we covered the small-angle X-ray scattering (SAXS) and the near wide-angle X-ray scattering ranges. The surface morphology of the samples was analyzed by means of optical microscopy (Olympus, Japan) and atomic force microscopy (JPK, Germany) in tapping mode. The original optical images were inverted into gray value images and enhanced in contrast by ImageJ software. Crystallization Method. iPpMS (Mw = 60 × 103 g/mol; Đ = 1.71) was carefully purified to remove traces of the catalyst and residual monomer by repeated precipitation and drying to a constant weight. Then, it was dissolved in dry toluene at a concentration of 1.0

EXPERIMENTAL SECTION

Materials. Reactants and solvents were purchased from SigmaAldrich. Toluene was purified by conventional methods (1,1-diphenyl ethylene/sec-butyllithium) and kept under an argon atmosphere. The toluene solution of modified-methylaluminoxane (MMAO) (SigmaAldrich) (10 wt %) was purified by distillation under vacuum whereupon toluene and most of trimethylaluminium (TMA) could be removed. MMAO was thus dried under vacuum to a constant weight (3 h at 60 °C). With this treatment, the content of TMA in solidpurified MMAO is less of 3 mol % (determined by 1H NMR). 1,9Decadiene (98%, ABCR GmbH) was dried over CaH2 and distilled. p-Methylstyrene (pMS) was purified by distillation from a sodium wire under reduced pressure before use and kept under an argon atmosphere. The catalyst dichloro[1,4-dithiabutandiyl-2,20-bis(6-tertbutyl-4-methylphenoxy)] titanium was obtained from MCAT GmbH (see ref 25). Schlenk flasks and glovebox techniques were used for the “preparation” of the catalyst and the purification of MMAO. The polymerization reactions were conducted in classical glass vessels, Schlenk flasks, or in a “PicoClave” pressure reactor under a slight argon pressure. Polymerization Procedure. The polymerization of pMS was carried out following a standard procedure under an argon atmosphere in 50 mL Schlenk flasks equipped with a magnetic bar and immersed in a thermostat water bath at 0, 20, 40, or 60 °C for 1 h. The glass flask was sequentially loaded with 20 mL of dry toluene and 3 mL of purified pMS, followed by the proper amount of MMAO and catalyst solution in toluene ([Al]/[Ti] = 300). After equilibration of the temperature, the reaction was initiated by addition of the catalyst precursor in 5 mL of toluene solution with a syringe and the reaction was sampled each 15 min for 1 h. Then, the polymerization 4840


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Macromolecules mg/mL. The solution was kept on a hot stage at 60 °C for 0.5 h to permit complete dissolution. A silicon wafer (1.5 × 1.5 cm2) was cleaned under UV radiation for 1 h in a water vapor-saturated chamber and used as a substrate. Toluene (2 mL) in an open vial (diameter 2 cm, depth 1 cm) was placed inside a sealed dish (diameter 9 cm, depth 1.5 cm) as a reservoir for 30 min to create a toluene vapor atmosphere. The cover was opened for a few seconds for the deposition of 100 μL of solution on the silicon wafer by a pipette. Then, a small orifice of a certain size (from 0.5 to 3 cm2) was made on the cover, as shown in Figure 1. The diluted solution on the

Around 80% of monomer conversion can be achieved after only 15 min in contrast to 15% of conversion achieved at 0 °C during the same polymerization time. The catalytic activity increased considerably as a function of the reaction temperature (see Table 1), in the way that at 60 °C almost 3 times higher (25.8 kg iPpMS/mol Ti h) catalytic activity was observed, as compared to that of the reaction carried out at 0 °C (7.95 kg iPpMS/mol Ti h). The polymerizations at 40 and 60 °C showed similar catalytic activities, as in both cases full monomer conversion was achieved after 1 h of reaction. The kinetics of the polymerization reactions was analyzed by looking at the timeconversion relations at different temperatures, which could be described as a first-order kinetics relation with respect to the monomer (see Figure 2b), and the calculated apparent kinetic constant (ka) values are presented in Table 1. The stereoregularity and configuration of the synthesized PpMS examined by 13 C NMR spectroscopy and the representative spectra are given in Figure 3. The stereoregularity can be determined by the intensity ratios of the characteristic peaks of the aromatic C1 carbon in the region 142−144 ppm for isotactic, atactic, and syndiotactic sequences, correspondingly. The 13C NMR spectra in Figure 3 display the main characteristic resonance at δ of 143.6, 143.2, and 142.8 ppm. The resonance at 143.6 ppm is assigned to the mmmm pentad fraction of isotactic sequence. The resonance at 143.2 is for rmmr triad fraction and at 142.8 ppm is for mmrm, mmrr triad or rmrm rrmr dyad.33 The proportions of mmmm pentad and mm triad for PpMS were determined to be around 95 and 99%, independently of the polymerization time and temperature. Furthermore, to study the crystallization of iPpMS, we synthesized low-molecular-weight polymers by utilizing 1,9decadiene as a transfer agent, considering that relatively short polymer chains form crystals more readily than long chains, simply because long chains tend to be more entangled. The polymer properties are described in Table 2, wherein it can be observed that the molecular weight decreased dramatically upon addition of the chain transfer agent, keeping a narrow dispersity and slightly increasing the isotacticity of the polymers. Crystallization Study of Isotactic Poly(p-methylstyrene). Isotactic polystyrene (iPS) is a common model for crystallization studies. Due to the presence of bulky phenyl groups, a low nucleation probability and a slow growth rate allow the formation of highly ordered structures.35 A variety of crystalline morphologies from faceted single crystals to dendritic structures has been previously reported.36−38 The formation of nonequilibrium patterns is strongly related to

Figure 1. Schematic of the experimental setup. The setup is designed to control the evaporation rate. The small orifice on the cover allows for slow evaporation of the solvent from the dilute solution film. silicon wafer was monitored during the evaporation process. The time from depositing the solution to the time at which no liquid was visible on the substrate anymore was defined as the evaporation time. Based on the evaporation time, we could calculate the corresponding evaporation rate. In our experiments, we could tune the evaporation rate from 1.8 to 11.1 μL/min. In this setup, solvent molecules are evaporating from the solution and reservoir and are effusing through the orifice, generating a steadystate vapor pressure inside the sealed dish. Evaporation leads also to an increase of the polymer concentration in the deposited droplet, which is forming a thin film on the substrate. The rate of loss of the solvent from the deposited film of the dilute solution is proportional to the surface area of the substrate that is exposed to a nonsaturated vapor atmosphere.34 In this regard, for a system with fixed positions and constant temperature, the evaporation rate is only controlled by the size of the opening.

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RESULTS AND DISCUSSION Polymerization of p-Methylstyrene. We carried out a series of experiments related to the synthesis of iPpMS, which was catalyzed by dichloro[1,4-dithiabutanediyl-2,20-bis(4,6ditert-butyl-phenoxy)]titanium and activated by MMAO in toluene. The Al/Ti molar ratio was 300. The polymerization time was 1 h. Four reaction temperatures (0, 20, 40, and 60 °C) were chosen for this polymerization system, as shown in Table 1. The evolution of the conversion for each temperature is presented in Figure 2a. As expected, the polymerization system has a higher catalytic activity at higher temperatures and a higher polymerization rate was observed at 60 °C.

Table 1. Reaction Conditions and General Features of Synthesized iPpMSa temp. (°C)

catalytic activityb

kac (L/mol/min)

0 20 40 60

7.95 17.14 23.27 25.88

45.70 143.01 426.57 496.85

P(mmmm)d (%) 97.00 95.84 96.78 97.51

P(mm)d (%)

Mw (×105)e (g/mol)

Đf

97.85 98.26 99.31 99.63

1.7 2.2 5.2 3.7

1.15 1.37 1.26 1.67

a n(Ti) = 5.84 mmol; n(Al)/n(Ti) = 300; n(p-methylstyrene) = 22.77 mmol; V(toluene) = 10 mL; reaction time = 60 min. bkg iPpMS/mol Ti/h, calculated after 30 min of reaction. cThe apparent first-order rate constant ka was calculated considering the kinetic law d[M]/dt = ka[Ti][M] and from plots ln(1 − x)−1 = f(t), where x is the conversion. dProportions of pentad and triad isotactic fraction, calculated by 13C NMR after 60 min of reaction. eWeight-average molecular weight (g/mol) determined after 30 min of reaction by size-exclusion chromatography (SEC) with PS standards. fDispersity, Mw/Mn.

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Figure 2. Rate of conversion (a) and first-order kinetics (b) of the polymerization of iPpMS.

comparison to that of iPS and that it is almost impossible to form iPpMS crystals in the melt, possibly because of the impact of the additional methyl group. For many polymers, especially when they are rather rigid (semiflexible), it is difficult to obtain crystalline structures when trying to crystallize them from the melt. However, in many cases, this difficulty is due to a slow nucleation rate and slow growth kinetics in the melt. Nonetheless, from a thermodynamic point of view, even such polymers can form well-defined and large crystals, as we have demonstrated. For example, Crossland et al. reported the formation of poly(3hexylthiophene) (P3HT) spherulites by CS2 vapor annealing, in which the nucleation density and spherulite size are precisely controlled.41 Rahimi et al. achieved large single crystals of P3HT by performing self-seeding in polymer solutions.42 Researchers have tried to improve the crystallinity of iPpMS using solution experiments rather than melts. In Gunesin’s toluene/isopropanol system,39 toluene is a good solvent for iPpMS at room temperature. With the addition of a nonsolvent, isopropanol, the solubility of the polymer in the mixed solvents has been decreased. The created supersaturation in the solution forced the polymer chains to crystallize into ordered structures. The major advantage of crystallization from solution is that the polymer chains have a high mobility both for backbone and side groups, which reduces the energy barrier for crystallization. However, to form ordered crystals of large sizes, a low number of nuclei and a slow growth rate are also needed. The direct mixing of a good solvent and a poor solvent is not sufficient to form crystals. In this study, we attempt to grow crystals by slowly evaporating the solvent from a dilute solution. Since the intrinsic driving force of polymer crystallization is supersaturation, which is related to the concentration of the polymer in solution, the whole system reaches a nonequilibrium state once the solubility limit has been overcome. Well-separated chains are the initial state of polymers in a dilute solution. Because the initial polymer concentration is below the solubility limit (at the temperature of the experiment), there is no crystallization occurring unless this limit has been overcome, either by decreasing the temperature or increasing the concentration. The concentration of polymers will gradually increase with the removal of solvent by slow evaporation. Once the solubility limit is reached, polymer chains will prefer to form aggregates due to the presence of van der Waals forces. However, the formation of stable nuclei is time dependent; nuclei can survive only when their sizes become bigger than a critical size, which is determined by the excess free energy. In fact, the possibility

Figure 3. 13 C NMR spectra (CDCl 3 ) of isotactic poly(pmethylstyrene).

Table 2. Reaction Conditions and General Features of LowMolecular-Weight iPpMS Polymers [pMS]/[CTA]a

temperature (°C)

60 80

40 40 40

P(mmmm) P(mm) b b (%) (%) 97.5 98.0 98.2

99.3 99.4 99.9

Mw (×104) c (g/mol)

Đd

6.5 0.6 1.0

1.24 1.72 2.2

a

Molar ratio of the monomer/chain transfer agent (1,9-decadiene). Proportions of pentad and triad isotactic fractions, calculated by 13C NMR. cWeight-average molecular weight (g/mol) determined after 60 min of reaction by SEC (aPS standards). dDispersity, Mw/Mn. b

crystallization conditions. To form large size single crystals, one needs a low nucleation probability and a low rate of crystallization. The isotactic poly(p-methylstyrene) (iPpMS) has an extra methyl group in the para position of the benzene ring, which may cause a lower crystallization rate than iPS.39 So far, only a few studies on crystallization of iPpMS have been published;19,39 however, the growth kinetics and crystal morphology are still unknown. The determination of the melting point of iPpMS has been difficult. Natta even claimed that the polymer obtained was noncrystalline and could not crystallize1 as no melting peak was observable in DSC. Gunesin et al.40 proposed a sample treatment using toluene/isopropanol solutions for dissolving iPpMS allowing to increase the degree of crystallinity. DSC measurements and X-ray diffraction (XRD) scans for powder samples with molecular weights ranging from 60 × 103 to 620 × 103 g/mol did not show strong evidence of the presence of iPpMS crystals (see Figures S2 and S3). Therefore, we carried out isothermal crystallization experiments on iPpMS films aiming to form crystals in the melt (at 180 and 200 °C). During the annealing process, we could not observe any change in the morphology of the film after more than 48 h. It seems that iPpMS has a different crystallization behavior in 4842


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thicker than the main branches, less dark in the micrographs, and the main branches are thicker than secondary branches.45 A typical atomic force micrograph (AFM) showing the structural details of iPpMS crystal is presented in Figure 5a. One can clearly identify the morphology of dendritic structure; a series of side branches is connected to the main branch. As shown in the corresponding height profile, the thickness of the side branch is rather uniform as 3 nm, which is quite thin compared to the underlying 50 nm-thick film (see the scheme in Figure 5c). The measured thickness corresponds to the distance from the surface of lamella to the lowest position of the depletion zone. To test the thermal behavior of the dendritic structures in the film, we carried out in situ heating experiments using optical microscopy. The shapes and sizes of the crystals remained unchanged during heating from 25 to 225 °C, as shown in Figure 6a,b. The side branches disappeared once the temperature reached 235 °C (Figure 6c). As the temperature was further increased, the main branches started to melt. At 239 °C, the crystal was almost completely molten (Figure 6d). This different melting behavior may be correlated to the different thickness of the lamellae; therefore, the range of melting temperatures may indicate the variation of the lamellar thickness.46 This observation is consistent with our analysis of the difference in gray-scale values of the dendritic crystal in optical images (see Figure 4). To determine the lamellar spacing and lamellar orientation of iPpMS, we have grown a large number of periodic crystalline domains for small-angle X-ray scattering (SAXS) measurements. Thus, we carried out solvent vapor annealing for monolayer dendritic crystals at room temperature. iPpMS films, which experienced slow evaporation, were placed in saturated toluene vapor overnight for vapor annealing at room temperature. During this process, toluene molecules diffuse into the film that reduces the glass transition temperature of the polymer.47 The resulting swollen polymer chains exhibit mobility enhancement, leading to an improvement of the crystal structures. White halos can be observed around the dendritic structure in the optical micrograph (Figure 7a), being an indication of a depletion zone, which may result from the vertical growth of the crystals. AFM images show that after solvent vapor annealing crystal branches have a height of more than 10 nm. A new layer of lamella can be found on top of crystal branch, as shown in Figure 7c.

of nucleation in solution is related to the degree of supersaturation; the closer to the equilibrium limit, the more difficult to form nuclei. By controlling the evaporation rate, we are varying the rate of concentration change; therefore, the number density of the crystals decreases as the evaporation rate decreases. Optical micrographs of an iPpMS film prepared by slow evaporation of solvent (evaporation time 55 min, evaporation rate 1.8 μL/min) are presented in Figure 4. We found a thin

Figure 4. Optical micrographs (contrast enhanced) of the iPpMS film obtained by slow evaporation of toluene (1.8 μL/min) at room temperature. The size of the images: (a) 400 μm × 400 μm and (b) 80 μm × 80 μm.

polymer film with faint structures, which can be visualized in a more detail after enhancing the contrast of the optical micrographs. The original color of polymer film is brown, which indicates that the film has a thickness of around 50 nm. It can be clearly seen that polymers formed branched lamellar structures, i.e., dendritic structures that are a typical characteristic pattern of a single crystal resulting from a diffusion-limited aggregation process.43,44 Most of the crystals consisted of 5−6 main branches, and the prevailing angle was about 60°, initiating from a single point, which is regarded as the nucleus. Side branching can be observed in some cases. Since growth of crystals needs continuous supply of polymer chains, as shown in Figure 4a, the growth of the crystals was terminated once they interfere with neighboring crystals, leaving “distinctive” depletion zones around the dendritic crystal. We provide the high-magnification view of a dendritic crystal in Figure 4b. The center regions, which are slightly darker in the micrographs, are

Figure 5. (a) Typical atomic force micrograph showing dendritic crystals of iPpMS in a thin film of about 50 nm. (b) Corresponding cross section of the dashed line in (a). (c) Side view scheme shows a lamella on an amorphous film supported by a silicon substrate. The size of image (a) is 50 × 40 μm2. 4843


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Figure 6. Optical micrographs showing the in situ melting of a dendritic crystal in a polymer film. The temperature was increased in steps of 1 °C from 220 to 250 °C, and the time interval remaining at each step was 5 min. The size of the images is 75 × 75 μm2.

fixed 2θ positions of the detector, thus establishing a twodimensional map of the Bragg peak at qz of 5.00 nm−1. Five different qz values (4.41, 4.69, 4.98, 5.26, and 5.55 nm−1) were chosen for the rocking scans in Figure 8b, showing a narrow peak, demonstrating that the sample is highly oriented. In a reflection experiment for such a highly oriented sample, lamellar peaks will only emerge if the lamellae are oriented parallel to the substrate. Overall, we determined the spacing and orientation of iPpMS lamellae, which verified that a multilayer lamellar structure has been generated in the thin film after solvent vapor annealing.

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CONCLUSIONS In the present work, we were able to prepare fully isotactic poly(p-methylstyrene) via catalytic polymerization of pMS on MMAO-activated dichloro[1,4-dithiabutanediyl-2,20-bis(4,6ditert-butyl-phenoxy)]titanium. As expected from the type of catalyst used, high activity was observed and narrow dispersity could be obtained. The proportion of mmmm pentads was close to 99%, independent of the polymerization time and temperature. Although the additional methyl group makes crystallization difficult, we were able to grow dendritic structures from the dilute solution by slow evaporation of the solvent, which is the direct evidence that single crystals of iPpMS can be grown. Moreover, the temperature range of crystal melting of 235−239 °C could be determined by in situ heating experiments using optical microscopy. The melting behavior suggested that the dendritic crystals were most likely composed of a single lamella. Using vapor annealing, crystal growth and development could be enhanced. Based on combined AFM studies on the morphology and on X-ray experiments on the structure, we assume that the dendritic crystals showed a transformation from a single layer to a multilayer with the aid of solvent vapor annealing. The obtained lamellar spacings were about 5 nm, and all lamellae were oriented parallel to the substrate.

Figure 7. Multilayer crystal of iPpMS in a film obtained by solvent vapor annealing after slow evaporation. (a) Optical micrograph (70 × 70 μm2) showing the depletion zone (white halo) around the dendritic structure. (b) Typical atomic force micrograph of crystal branches (15 × 15 μm2). (c) Typical morphology showing the crystal surface terraces on a smaller size scale (6 × 2 μm2).

Small-angle X-ray scattering (SAXS) measurements for the iPpMS film after vapor annealing show clear evidence of the existence of a lamellar structure. The peak positions of peak I (qz = 1.23 nm−1) and peak II (qz = 2.52 nm−1 ≈ 2 × 1.23 nm−1) indicate an ordered lamellar structure (qz scan in Figure 8a). Peak III at qz of 5.00 nm−1 is related to a reflection of the crystal lattice in chain direction.39,48,49 The long period (d) can be calculated as d = 2π/qz and has a value of 5.1 nm. Each crystalline branch was composed of more than two layers of lamellae. A sequence of rocking curves was recorded by scanning the sample angle, ω, at fixed scattering vector, qz, i.e.,

Figure 8. (a) SAXS results for the iPpMS film after solvent vapor annealing. (b) Rocking curve measurement showing that lamellae were oriented parallel to the substrate. 4844


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Macromolecules

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00640. Molecular weight characteristics of the PpMS, 13C NMR spectra of iPpMS, first heating DSC traces for iPpMS powder samples, XRD for iPpMS powder samples, corresponding XRD values of iPpMS powder samples, evaporation of solvent, evaporation of the solvent on silicon substrate, schematic pathway of polymer crystallization, evaporation time dependence of solution concentration, and wide-angle X-ray diffraction results for the iPpMS film

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(PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Luis Valencia: 0000-0001-6572-7460 Günter Reiter: 0000-0003-4578-8316 Present Address #

Division of Materials and Environmental Chemistry, University of Stockholm, Frescativägen 8, Stockholm 10691, Sweden (L.V.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank, especially, the DFG (IRTG Soft Matter Science 1642) and the China Scholarship Council (CSC) for financial support; Wenbing Hu, Jun Xu, and Bin Zhang for discussions; A. Collard (ICS/Strasbourg) for his help in the preparation of the samples; and M. Legros and C. Foussat (ICS/Strasbourg) for their strong support with the solution characterization of the polymer.

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REFERENCES

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