Solid State Phase Transitions in Poly (ethylene oxide) Crystals

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Solid State Phase Transitions in Poly(ethylene oxide) Crystals Induced by Designed Chain Defects Muhammad Haris Samiullah,† Martin Pulst,† Yury Golitsyn,† Karsten Busse,† Silvio Poppe,† Hazrat Hussain,‡ Detlef Reichert,† and Jörg Kressler*,† †

Faculty of Natural Sciences II, Martin Luther University Halle-Wittenberg, D-06099 Halle (Saale), Germany Department of Chemistry, Quaid-i-Azam University, Islamabad 45320, Pakistan

‡

S Supporting Information *

ABSTRACT: We have used Cu(I)-catalyzed azide−alkyne cycloaddition to synthesize a new series of poly(ethylene oxide)s having in the center of their chains two 1,2,3-triazole (TR) rings separated by (CH2)n spacers with 2 ≤ n ≤ 4 (PEO11-TR-(CH2)n-TR-PEO11). The degree of crystallinity obtained by temperature-dependent WAXS measurements indicates that only one out of the two PEO11 chains of the three polymers forms a 72 helix upon cooling to −10 °C and crystallizes into a monoclinic unit cell known from PEO homopolymer. A solid-state phase transition occurs for all samples during heating below their melting temperature. Solid-state 13 C MAS cross-polarization and single-pulse NMR spectroscopy indicate the complete incorporation of the chain defects into the PEO crystals (PEO-TR phase) during this transition. The 2D WAXS pattern of an oriented PEO11-TR-(CH2)2-TR-PEO11 sample generates a structural model where the crystal lattice of the initial PEO phase becomes highly distorted during the solidstate phase transition due to C−H···π interactions of the aromatic TR rings. Furthermore, an additional phase transition occurs for PEO11-TR-(CH2)4-TR-PEO11 after melting of the PEO-TR phase. This phase has complex characteristics; i.e., the typical 72 helix of PEO forms, but the two TR rings and the methylene groups of the alkyl spacer are in different chemical environments.

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INTRODUCTION Being a water-soluble and biocompatible polymer, poly(ethylene oxide) (PEO) is one of the most important synthetic polymers with a plethora of pharmaceutical and biomedical applications.1−3 It usually crystallizes with the characteristic 72 helix conformation arranged in a monoclinic unit cell with P21/ a symmetry.4,5 However, a planar zigzag (all-trans) chain conformation has also been found for pure PEO systems, e.g., during freeze-drying6,7 or by stretching of a necked PEO sample,8 where the chains are arranged in a triclinic unit cell having the space group P1̅.8 The ability of PEO chains to pack together with guest molecules into one unit cell has been investigated in the past. Because of the noncovalent interactions, it forms clathrates with several low molar mass organic molecules,9,10 hydrates with water,11 and cocrystals with some lithium salts.12−15 However, an interesting behavior is expected when these molecular entities are covalently connected to the PEO chain and act as a well-defined defect for the PEO crystal. Chen et al. have investigated these systems by introducing terephthalate, phthalate, and isophthalate units to the center of the PEO chain.16−18 These midchain defects are found to be excluded from the crystal lamellae without affecting the monoclinic crystal lattice of PEO. Slightly different results were obtained in our previous study, when the disubstituted benzene ring has been introduced via an ether bond between the two PEO chains. 19 Only the 1,3disubstituted benzene defect is incorporated into the PEO © XXXX American Chemical Society

crystals caused by reduced spatial requirements of the ether bonds and minimum packing constrains of its substitution pattern, as compared to the polymers containing the benzene defect with 1,4- and 1,2-substitution pattern, respectively. A similar study is also performed by Song et al. on polyethylene (PE), where disubstituted benzene and naphthylene defects were either included or excluded from the PE crystal, depending on their substitution pattern.20 Because of the tremendous importance of the Cu(I)-catalyzed Huisgen-type 1,3-dipolar cycloaddition (CuAAC, “click” reaction)21,22 for polymeric systems, we have investigated the effect of one 1,4disubstituted 1,2,3-triazole (TR) midchain defect on the crystallization of the low molar mass PEO model system (PEO11-TR-PEO11).23 The TR defect incorporates into the crystals with tilted PEO chains19 near to the crystal surface upon heating below Tm which completely suppresses the mobility (helix jumps) of the PEO chains in these crystals.24 In this work, we continue to study the influence of TR defects on the crystallization of PEO by introducing different sizes of TR containing defects in the PEO chain. For this purpose, two 1,4-disubstituted 1,2,3-triazole (TR) rings, separated by n-alkyl groups of various lengths (CH2)n with 2 ≤ n ≤ 4, are placed in between two PEO chains with 11 Received: March 9, 2018 Revised: May 18, 2018

A

DOI: 10.1021/acs.macromol.8b00508 Macromolecules XXXX, XXX, XXX−XXX

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Small-Angle X-ray Scattering. Temperature-dependent smallangle X-ray scattering (SAXS) measurements employed a Rigaku Rotaflex rotating anode equipped with a pinhole collimator using Cu Kα radiation. The q-scale was calibrated with silver behenate. The samples were heated above their melting temperatures and filled in glass capillaries with a diameter of 1 mm. Then, they were placed in a LINKAM TMS 94 hot stage attached to the SAXS setup. The samples were cooled by evaporation of liquid nitrogen to T = −20 °C, and the SAXS traces were recorded in steps of ΔT = 5 K until reaching the melting temperature of the sample. The heating and cooling rates were 10 K min−1. The acquisition times varied from 1200 to 3600 s depending on the scattering intensity to obtain a good signal-to-noise ratio. The measured SAXS intensities were corrected for background scattering (simple exponential decay and a constant background for air scattering are subtracted from the signal) and evaluated with the onedimensional scattering function25,26

monomer units (PEO11-TR-(CH2)n-TR-PEO11; cf. Scheme. 1). For the “click” reaction, the catalytic system CuSO4 and sodium Scheme 1. Synthesis of the PEO11-TR-(CH2)n-TR-PEO11 via CuAAC

ascorbate have been used in water/t-BuOH (50% v/v) at room temperature. Temperature-dependent wide-angle and smallangle X-ray scattering (WAXS, SAXS) measurements are employed to observe the crystallization and phase transition behavior of these polymers. Simultaneously, solid-state 13C magic angle spinning (MAS) cross-polarization (CP) and single-pulse (SP) nuclear magnetic resonance (NMR) spectroscopy measurements allow for the investigation of these phase transitions in more detail. Differential scanning calorimetry (DSC) and polarized optical light microscopy (POM) further characterize the solid-state phase transition.

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I=

2 2 2 2 a ∑ (F(n)e−(q − nq*) /2w )e−q u w n

(1)

where q is the scattering vector, I is the intensity, a and w are the respective amplitude and width of the first order maximum, q* is the scattering vector of the first maximum, n is the order of the peak, F(n) = sin2(nπΦ)/n2 is the form factor of a single lamella for the simple 2 2

two-phase model at each peak position, and e−q u is the exponential Debye−Waller factor. The long period d can be calculated from the position of the first-order peak q* (d = 2π/q*).27 The parameter Φ is either the volume fraction of the crystalline phase Φc or of the amorphous phase Φa which were assigned using additional DSC or WAXS data. The lamella thickness was calculated by lc = dΦc.27 Solid-State NMR Spectroscopy. A BRUKER AVANCE 400 NMR spectrometer with a standard 4 mm VT-MAS probe was used to perform 13C MAS single-pulse (SP) and cross-polarization (CP) experiments. 1H decoupling (TPPM) was applied to remove dipolar broadening and typical MAS rates were 10 kHz. The sample temperature was controlled by a standard BRUKER VT-controller and calibrated with methanol.28 After cooling of the samples from T = 30 °C to T = −20 °C, the spectra were taken upon reheating in steps of ΔT = 5 °C. The measurement time per step was approximately 4 h collecting 1024 scans. Differential Scanning Calorimetry. A Mettler Toledo DSC 822e module recorded the DSC traces under continuous flow of nitrogen (40 mL min−1). The samples were heated to T = 100 °C and held there for 5 min, followed by cooling to T = −40 °C with a rate of 1 K min−1. After 5 min of holding at this temperature, the samples were heated to T = 60 °C with a heating rate of 1 K min−1. The melting temperature (Tm) was determined from the maximum of the endotherm in the second heating trace. Polarized Optical Microscopy. An Axioplan 2 microscope from Zeiss Jena equipped with a LINKAM THMS 600 hot stage provided polarized optical microscopy (POM) images using an AxioCam MRc camera. A constant stream of dry nitrogen prevents any moisture. PEO11-TR-(CH2)4-TR-PEO11 was first heated to and held at T = 80 °C for 5 min and then cooled to T = −10 °C with a rate of 5 K min−1. After complete crystallization, the images were taken upon heating with a rate of approximately 5 K min−1. In a second experiment, PEO11-TR-(CH2)4-TR-PEO11 was crystallized at T = 5 °C, heated to 18 °C, and then recrystallized at 5 °C.

EXPERIMENTAL PART

Materials. The PEO11-TR-(CH2)n-TR-PEO11 samples were synthesized via CuAAC as shown in Scheme 1 (for details see Supporting Information on pages S-2 to S-7; this is especially important since some steps involve toxic and explosive chemicals). The number-average molar masses of the polymers were calculated from 1H NMR spectroscopy data which were also used together with the corresponding 13C NMR spectra to confirm the structure and the purity of the compounds (all spectra are depicted in the Supporting Information, Figures S1−S4). Complete removal of the Cu catalyst after successful CuAAC reaction was confirmed by ICP mass spectrometry for an analogue polymer as described in our previous study.23 Wide-Angle X-ray Scattering. A PANalytical Empyrean diffractometer equipped with a position sensitive detector (PIXcel3D) was used to perform temperature-dependent wide-angle X-ray scattering (WAXS) measurements in Bragg−Brentano geometry (reflective mode). The diffraction patterns were recorded using Cu Kα radiation in a scan range of 7° ≤ 2θ ≤ 60° with a step size of Δ2θ = 0.053° and a counting time per step of 44 s. The samples were placed on zero background silicon substrates in a TTK 450 temperature chamber from Anton Paar. At first, the polymers were heated to T = 80 °C and held there for about 5 min, followed by cooling to T = −40 °C with a cooling rate of 5 K min−1. The WAXS diffractograms were recorded in steps of ΔT = 2 K from T = −10 °C until the complete melting of the sample, during the second heating. To record the 2D diffraction pattern of PEO11-TR-(CH2)2-TRPEO11, a diffractometer with a 2D detector (Vantec 500, Bruker, AXS, Madison, WI) along with pinhole collimated (Ni-filtered) Cu Kα radiation was used. The calibration was carried out using silver behenate. The exposure time was 180 min, and the sample-to-detector distance was 9.85 cm. Glass capillaries (diameter of 1 mm, purchased from Hilgenberg GmbH, Malsfeld, Germany) were filled with the sample. Orientation of PEO11-TR-(CH2)2-TR-PEO11 was achieved by cooling the sample in the capillary to T ≈ 5−10 °C and then moving the sample to about 1 cm away from its initial position in the capillary by an airflow. Afterward, the sample was quenched using liquid nitrogen in order to preserve the induced orientation in PEO11-TR(CH2)2-TR-PEO11 and stored at −20 °C. The 2D diffraction pattern was recorded after heating the sample to T = 18 °C.

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RESULTS AND DISCUSSION Phase Transitions in PEO11-TR-(CH2)2-TR-PEO11. Temperature-dependent WAXS measurements of PEO11-TR(CH2)2-TR-PEO11 provide details about the influence of the designed chain defect on the crystallization behavior of PEO (see Figure 1a). The WAXS diffraction pattern at T = −10 °C clearly reveals the presence of two prominent reflections at 2θ = 19.4° and 2θ = 23.5°. They correspond to the (120) and (032)* Miller planes (indexed as (032)* means an overlap of B

DOI: 10.1021/acs.macromol.8b00508 Macromolecules XXXX, XXX, XXX−XXX

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23.8° are observed (marked by black dashed vertical lines) until the final melting of PEO11-TR-(CH2)2-TR-PEO11 at Tm ≈ 29 °C (cf. Figure S5). Only the chain defect can be responsible for the change of the WAXS diffraction pattern as a function of temperature since PEO homopolymers do not show this phase transition. Solid state 13C magic angle spinning (MAS) cross-polarized (CP) and single pulse (SP) NMR experiments are carried out to characterize the phase transformation in more detail (see Figure 2). The 13C MAS CP NMR spectrum of PEO11-TR-(CH2)2-

Figure 1. (a) Temperature-dependent WAXS diffraction pattern of PEO11-TR-(CH2)2-TR-PEO11 during heating from −10 to 30 °C in temperature steps of ΔT = 2 K. For the assignment of Miller indices, see the text. (b) Normalized integral intensities (top) of the reflection at 2θ = 17.1° (PEO-TR phase) and 2θ = 19.4° (PEO phase) and their respective lattice spacing (bottom) as a function of the temperature.

Figure 2. Comparison of the CP (T = 15 °C) and SP (T = 30 °C) 13C MAS NMR spectra of PEO11-TR-(CH2)2-TR-PEO11 with assignment of the resonances. The red dashed lines indicate the observed shift of the signals.

(032), (1̅32), (112), (2̅12), (1̅24), (2̅04), and (004) reflections) of the monoclinic unit cell with four 72 helices known from PEO homopolymers5 (called PEO phase). The crystallinity Xc is calculated by comparing the integral of all Bragg reflections with the integral of the total diffractogram after subtraction of the background scattering. It should be noted that the contributions of incoherent and diffuse scattering are neglected in this relatively simple approach.29 A value of Xc ≈ 44% is determined from the WAXS pattern at T = −10 °C. Thus, one can conclude that one PEO11 chain belongs to the crystal and the other PEO11 chain of the polymer is in the amorphous state. The crystallization of the second PEO11 chain is prohibited for two reasons: (i) Low molar mass PEO forms exclusively extended chain crystals and the number of methylene groups of the spacer is obviously not sufficient to form a chain fold that the second PEO11 chain can reenter the crystal (at least 10 CH2 groups (five ethylene units of PE) are necessary to form a fold30). (ii) The crystallization starts with the random packing of one of the two PEO11 chains into the monoclinic unit cell and lamellae form. The TR-(CH2)2-TR group and the second PEO11 chain are dangling ends, and they locate statistically distributed on both sides of the lamella. The chain density relative to the crystal surface area fixes the tilt angle of the chains leaving the crystal into the amorphous phase.19 Thus, they have a distance, which does not match the helix−helix distance necessary for the crystallographic packing. However, the Bragg reflections of the monoclinic unit cell of PEO gradually disappear during heating and new reflections of a different crystal system at 2θ = 17.1°, 20.4°, 21.7°, 22.7°, and

TR-PEO11 which results from less mobile nuclei shows two sharp resonances at δ = 70.4 ppm and δ = 56.9 ppm, indicating that PEO11 chains at T = 15 °C belong to the newly formed crystal system. Additionally, sharp signals at δ = 120.2 ppm and δ = 146.2 ppm corresponding to the resonances of the TR rings are observed. The signal of the CH2 groups in between the two TR rings appears at δ = 21.0 ppm. However, when comparing the chemical shifts of the 13C MAS SP NMR signals of molten PEO11-TR-(CH2)2-TR-PEO11 (T = 30 °C) and the 13C MAS CP NMR signals at T = 15 °C, a shift of the resonances of the TR rings and the CH2 groups in-between them is noticed. This is a clear indication that the TR-(CH2)2-TR group is completely located in the crystalline phase named PEO-TR, and π−π interactions between the TR rings in the crystal of the PEO-TR phase can contribute to this shift. Such behavior is also discussed for thiophene rings in poly(3-hexylthiophene) by Russell and co-workers.31 The incorporation of the complete defect into the crystal structure upon heating below Tm is also demonstrated when looking to the temperature-dependent intensity of the (120) reflection of the monoclinic PEO crystal and the reflection at 2θ = 17.1° of the PEO-TR crystal as shown in Figure 1b. The intensity of the (120) reflection of the PEO phase starts to decrease at T = −8 °C, indicating the incipient melting of the PEO phase. The intensity of the Bragg reflection belonging to the PEO-TR modification starts to increase at −2 °C. SAXS measurements as shown in Figure 3a also indicate the incorporation of the TR-(CH2)2-TR defect into the crystal of C

DOI: 10.1021/acs.macromol.8b00508 Macromolecules XXXX, XXX, XXX−XXX

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polymer chains during crystallization. Fitting results with crystallinities above 50% are neglected. The form factor F(n) of this model is given by

((ρ − ρ ) sin(nπΦ ) + 2ε F(n) = c

a

a

n2

1 − cos(n π Φa ) n

2

)

(2)

with the density decrease between ρa and the density in the center of the amorphous region ε. Free fit factors are Φa (volume fraction of amorphous phase), ε, and from the structure factor q*, width w, amplitude a, and Debye−Waller factor u2. With this model, the measured SAXS trace can excellently be described (cf. the inset of Figure 3a), and the thickness of the crystalline layer is determined to lc = 30.7 Å (T = 15 °C), which corresponds to 44 vol % or approximately 46% crystallinity, which is slightly less than 47% obtained by WAXS, at T = 16 °C. With the results obtained by 13C MAS CP and SP NMR spectroscopy that the TR-(CH2)2-TR part belongs to the crystal, it is assumed that the thinner PEO crystals melt first, and then PEO recrystallizes again with TR groups included in the crystal. It should be noted that a characteristic disordered transition phase is usually formed during a solid state phase transition33 while the crystals do not melt in a simple cold crystallization.34 However, the SAXS trace of the molten PEO11-TR-(CH2)2-TR-PEO11 shows no peak, indicating the miscibility of PEO and the TR-(CH2)2-TR chain defect which is also reported for PEO and 4,4′-bi(1,2,3-triazole).35 Structure Model of PEO11-TR-(CH2)2-TR-PEO11. Since the WAXS diffraction patterns of the PEO phase and the PEOTR phase of PEO11-TR-(CH2)2-TR-PEO11 are completely different (cf. Figure 1a), a change of the 72 helix conformation upon heating cannot be excluded per se. However, the Fouriertransform infrared (FT-IR) spectra of the PEO-TR phase show the characteristic bands of the 72 helix conformation36 (cf. Figure S6). Thus, we propose a crystallographic model based on the 2D diffraction pattern of an oriented PEO11-TR-(CH2)2TR-PEO11 sample shown in Figure 4a. This pattern allows for extraction of the azimuthal angle profiles as depicted in Figure 4b−e. The reflections in the range of small 2θ values at the center on the meridian of the WAXS pattern (cf. Figure 4b) indicate that the c-direction of the lamellae aligns nearly parallel to the meridian. The PEO-TR phase has also a monoclinic symmetry since every reflection appears four times with similar intensity on a reflection ring (except these in shear direction (hk0) which appear twice). Because of the presence of characteristic reflections, it can be assumed that the space group P21/a37 of PEO is also not changed during the phase transition upon heating since a chiral sorting of right- and lefthanded PEO helices during the incorporation of the TR(CH2)2-TR group into the crystal is highly unlikely. Comparing both the angle of the reflections with respect to the meridian and their intensities, the typical PEO reflections (see Figure 4a) occur also in the PEO-TR phase. It is a strongly deformed PEO lattice in agreement with FT-IR and NMR spectroscopy results discussed above. However, there are also additional reflections (especially on the equator (hk0)) which do not belong to the PEO crystal system. The additional electron density in the PEO-TR phase is then the result of the incorporation of the TR-(CH2)2-TR group into the crystal. Using these reflections, we can assign a unit cell for the PEO-TR phase. The lattice parameter c is calculated according to the method described by De Rosa and Auriemma,38 and the corresponding lamella thickness lc = c sin(β) = 31.5 Å is in good agreement with the

Figure 3. (a) Temperature-dependent background corrected SAXS traces of PEO11-TR-(CH2)2-TR-PEO11 during heating −20 °C ≤ T ≤ 35 °C with temperature steps of ΔT = 5 K. The inset shows the fit of the SAXS trace of the PEO-TR phase at T = 15 °C with the scattering function (red line). (b) Schematic representation of a two-phase electron density model of PEO11-TR-(CH2) 2-TR-PEO11 with amorphous and crystalline regions, depending on the lateral direction x normalized to the long period d.

PEO11-TR-(CH2)2-TR-PEO11 upon heating. After crystallization at T = −20 °C, the first-order peak appears at q* = 0.101 Å−1 corresponding to a long period of d = 62.2 Å. Because of the absence of higher order peaks at nq*, no proper stacking of lamellae is established. During heating of the sample, the formation of the new crystallographic structure also induces significant changes in the SAXS trace between 10 and 30 °C. The first-order peak at q* shifts to smaller values, indicating an increase of the long period during the phase transition to d = 69.6 Å at 15 °C. Several higher order scattering maxima appear up to the fifth order at 5q* with the characteristic absence of the second-order peak at 2q*. This confirms that a lamellar structure of the high temperature modification forms. In particular, the observation of well-defined 3q*, 4q*, and 5q* scattering maxima indicates a highly ordered structure probably caused by π−π interactions of the aromatic triazole rings when they belong to the crystals. As a simple rectangular two-phase model is not sufficient to describe the data, a slightly modified electron density profile is employed consisting of a layer of crystalline material (high electron density) which is covered by an amorphous region (lower electron density) with additional slightly decreasing density to its center (see Figure 3b). A constant electron density in the amorphous part is not sufficient to describe the data. The absolute values of the electron density of crystalline (ρc = 0.405 e−/Å3) and amorphous parts (ρa = 0.369 e−/Å3) are the values of neat PEO32 and not varied during the fitting procedure. The obtained variation ε of the electron density is ≈2% of the value for neat amorphous PEO and might be based on the packing stress near to the crystalline layer or depletion of the inner part due to tensile forces on the D

DOI: 10.1021/acs.macromol.8b00508 Macromolecules XXXX, XXX, XXX−XXX

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Figure 5. (a) Structure model of the crystalline part of PEO11-TR(CH2)2-TR-PEO11. (b) Arrangement of the TR-(CH2)2-TR groups; the C−H···π interactions between the aromatic TR rings are marked as purple dotted lines.

reflections result from the intermolecular distances of the TR(CH2)2-TR groups. In contrast, the (130), (216̅), and (126̅) reflections are caused by intramolecular distances of the TR(CH2)2-TR group as visualized in Figure S8. However, intermolecular C−H···π interactions, one of the three known subtypes of π−π interactions,39,40 are also observed for the PEO-TR phase of PEO11-TR-(CH2)2-TR-PEO11 as shown in Figure 5b which explain the strong deformation of the initial PEO lattice. These attractive interactions are the driving force for the observed phase transition, but there are many difficulties to overcome during recrystallization after melting of the PEO phase. The monoclinic angle of β = 125.4° in the initial PEO lattice needs to be reduced so that the TR rings can interact optimally. This causes the large deformation of the PEO lattice, and it is obviously a slow process as discussed above. However, the question arises if the PEO-TR phase can also occur when the length of the spacer, i.e., the number of CH2 groups between the two TR rings, is increased, or the above-mentioned packing constraints might prevent this solid state phase transition. Phase Transitions in PEO11-TR-(CH2)3-TR-PEO11 and PEO11-TR-(CH2)4-TR-PEO11. In order to gain a better understanding for the influence of the alkyl spacer length on the appearance of the PEO-TR phase, PEO11-TR-(CH2)3-TRPEO11 and PEO11-TR-(CH2)4-TR-PEO11 are also investigated by WAXS. Figure 6a provides the temperature-dependent WAXS diffractograms of PEO11-TR-(CH2)3-TR-PEO11. This polymer undergoes also the characteristic phase transition from the PEO phase to the PEO-TR phase upon heating below Tm. Thus, the complete TR-(CH2)3-TR group can also be incorporated into the PEO crystals which is in analogy to the polymer with n = 2. The normalized integral intensities of the reflection at 2θ = 17.1° (PEO-TR phase) and 2θ = 19.4° (PEO phase) which are shown in Figure S9 reveal that the transition starts at T = −8 °C, which is marginally lower than in the respective polymer with n = 2. However, the melting temperature of PEO11-TR-(CH2)3-TR-PEO11 of Tm = 22 °C (see Figure S5) is significantly lower than the respective value of PEO11-TR-(CH2)2-TR-PEO11. Obviously, the length of the n-alkyl group between the two TR rings has a remarkable influence on Tm of the PEO-TR phase. Figure 6b depicts temperature-dependent WAXS diffractograms of PEO11-TR(CH2)4-TR-PEO11. In agreement with the other two PEO11TR-(CH2)n-TR-PEO11, the known PEO modification is observed in the WAXS diffractogram after crystallization at T

Figure 4. (a) 2D diffraction pattern of the PEO-TR phase of the oriented PEO11-TR-(CH2)2-TR-PEO11 sample at T = 18 °C. The Miller indices are included for the observed reflections. (b−e) Azimuthal intensity profiles of different reflections obtained from the 2D WAXS pattern.

value from SAXS data (see discussion of Figure 3). The other unit cell parameters are refined using BIOVIA Materials Studio Reflex software. The unit cell parameters are a = 10.11 Å, b = 13.02 Å, c = 36.34 Å, and β = 119.8°. The additional reflections at 2θ = 21.7° and 2θ = 22.8° belong to the (210) and (130) Miller planes, respectively (see Figure 4e). These reflections appear in a similar diffraction angle range as compared to the observed reflection in PEO11-TR-PEO11 caused by intermolecular C−H···π interactions of the TR rings.23 This is also valid for PEO11-TR-(CH2)2-TR-PEO11 since these interactions of the aromatic TR rings are confirmed by 13C MAS CP NMR spectroscopy (see discussion above). The Bragg reflection at 2θ ≈ 20.4° is assigned to the (200) Miller plane (at the equator, cf. Figure 4d) as well as from the (216̅) or (126̅) Miller planes (at approximately 55° with respect to the equator; see also Figure 4d). Thus, the position of the TR-(CH2)2-TR group is refined to the intensities of these reflections using the same software. Figure 5a shows the structure received. The simulated diffraction pattern is in good agreement with the measured data (see Figure S7). As assumed above, the (210) and (200) E

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(CH2)4-TR-PEO11 since the helix−helix distance calculated from the (120) reflection at 2θ = 16.7° is significantly larger as compared to the PEO-TR phase of the other PEO11-TR(CH2)n-TR-PEO11 (see Figures 1b and 7a as well as Figure S9). This means that the largest deformation of the initial PEO lattice occurs for PEO11-TR-(CH2)4-TR-PEO11, indicating tremendous problems during packing of the TR-(CH2)4-TR group into the crystals. However, a second phase transition is observed at T = 18 °C. The Bragg reflections of both the remaining PEO phase and the formed PEO-TR phase disappear, and new reflections appear gradually in the diffractogram. It resembles this of the initial PEO phase, but the Bragg reflections shift to lower scattering angles (called the PEO′ phase; see Figures 6b and 7a). This is in perfect agreement with the DSC trace where an endotherm at T = 17 °C followed by an exothermal peak is noticed (cf. Figure 7a). The 13C MAS CP NMR spectrum at T = 15 °C gives information on the PEO′ phase (cf. Figure 7b and Figure S10). The CP spectrum shows pairs of resonances for the TR rings at δ = 121.6 and 123.6 ppm as well as δ = 145.9 and 147.4 ppm. This is a clear indication that there are two populations of TR rings in different chemical environments. The chemical shifts of the upfield signals (marked by red arrows in Figure S10) are in perfect agreement with the corresponding values of the molten sample, indicating that these TR rings belong to the amorphous regions. In contrast, the downfield shifted resonances of the TR rings in the CP spectrum (marked by blue arrows in Figure S10) have significantly different chemical shifts revealing that they belong to the crystals. Thus, it can be assumed that the chain defect is only partially inserted in the PEO′ phase of PEO11-TR-(CH2)4-TR-PEO11. This conclusion is also supported by the relatively complex line shape of the resonances of the four CH2 groups between the two TR rings in the chemical shift range between 20 ppm < δ < 35 ppm with sharp resonances and broad components which cannot be unequivocally assigned to the respective methylene groups. To characterize the second phase transition in more detail, polarized optical microscopy (POM) is employed. After cooling to T = −10 °C, the growth of spherulites is observed (see

Figure 6. Temperature-dependent WAXS diffraction patterns of PEO11-TR-(CH2)n-TR-PEO11 where (a) n = 3 and (b) n = 4 with temperature steps of ΔT = 2 °C. The color code of the Miller indices is identical with the assignment of the phases in Figure 7a.

= −10 °C. During heating, two phase transitions are observed. The first transition occurs at T = 0 °C where the PEO phase begins to convert into the PEO-TR modification (cf. Figure 7a) as known for the other two PEO11-TR-(CH2)n-TR-PEO11 under investigation. However, this phase transition is not complete as can be seen from the normalized intensity profiles of the two characteristic WAXS reflections depicted in Figure 7a. The integral intensity of the (120) reflection of the lowtemperature PEO modification is not further decreasing with increasing temperature, and simultaneously, the intensity of the reflection at 2θ = 16.7° of the PEO-TR phase remains nearly constant. This is also in agreement with the 13C MAS CP NMR data (see Figure 7b) since the broad signal of the TR rings in the amorphous phase (δ = 121 ppm and δ = 147 ppm, marked by black arrows) is also remaining together with sharp resonances of the partially incorporated TR-(CH2)4-TR groups with increasing temperature. The reason for the incomplete phase transition is caused by the PEO-TR phase of PEO11-TR-

Figure 7. (a) Combined plot of the DSC trace (top), normalized integrated intensity of the (120) reflections of the PEO and PEO-TR phases (middle), and the distance calculated from the diffraction angle of the (120) reflections as the function of temperature of PEO11-TR-(CH2)4-TRPEO11 (bottom). (b) Temperature-dependent 13C MAS CP NMR spectrum in the temperature range of −10 °C ≤ T ≤ 20 °C in steps of ΔT = 5 K. (c−g) Polarized optical microscopy (POM) images of PEO11-TR-(CH2)4-TR-PEO11 (c) during isothermal crystallization at T = −10 °C and (d) after complete crystallization at this temperature. (e) Cracks appear in the center of the spherulithes (marked by orange arrows) after the phase transition into the PEO-TR phase at T = 10 °C. (f) Dark regions indicating an isotropic disordered transition phase are observed in the spherulites at T = 18 °C, and (g) a new spherulite morphology is observed after the phase transition into the PEO′ phase at T = 22 °C. F

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Macromolecules Figure 7c). After forming a space filling morphology (Figure 7d), the spherulites undergo a phase transition from the PEO into the PEO-TR phase, and cracks appear in the center of the spherulites at T = 10 °C (indicated by orange arrows in Figure 7e). This is a clear indication that the density of the PEO-TR phase is larger than the respective value of the initial PEO phase, which is also in good agreement with the WAXS data where the largest deformation of the lattice parameters is observed during this transition, and it supports also the given explanation of the incomplete solid state phase transition. Furthermore, the second phase transition can be observed by POM since dark areas appear in the spherulites at T = 18 °C as depicted in Figure 7f which belong to the characteristic amorphous/liquid intermediate phase of the solid state phase transitions.23,33 After further heating, the dark areas vanish and spherulithes of the PEO′ phase remain with a different color and morphology at T = 22 °C (see Figure 7g). In a second experiment, PEO11-TR-(CH2)4-TR-PEO11 is crystallized at T = 5 °C (the coexistence temperature range of the PEO and PEO-TR phases, cf. Figure 7a), and the growth of two types of spherulites is observed (see Figure S11a) which have a completely different morphology. After complete crystallization, the spherulites impinge and curved impingement lines are observed between them (marked by white arrows in Figure S11b), indicating different growth rates of the two spherulite types. During heating, the melting of one spherulite type occurs, and dark areas appear in the POM image at T = 18 °C as depicted in Figure S11c. This is in perfect agreement with the melting temperature of the PEO-TR phase. Spherulites of the other type (PEO′ phase) remain at this temperature. After cooling back to T = 5 °C, the dark areas become again bright (see Figure S11d), indicating the crystallization of PEO11-TR(CH2)4-TR-PEO11 in the PEO′ modification which is in agreement with additional WAXS measurements indicating the irreversibility of the phase transition (for details see Figure S12).

phase with complex characteristics. It might be interesting for future studies to further increase the length of the n-alkyl spacer.

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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.8b00508. General experimental procedures and characterization (1H and 13C NMR) as well as additional DSC, WAXS, SAXS, POM, IR, and 13MAS NMR data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.K.). ORCID

Martin Pulst: 0000-0001-5957-9895 Detlef Reichert: 0000-0002-6876-1901 Jörg Kressler: 0000-0001-8571-5985 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Deutsche Forschungsgemeinschaft (DFG), projects KR 1714/9-1 (J.K.) and RE 1025/19-1 (D.R.) as well as the SFB TRR 102, project B07 (J.K.) for financial support. We thank also Wolfgang H. Binder and Diana Döhler for support with the IR measurements within the cooperation of SFB TRR 102. Helpful discussions with Jens Balko and his support with the WAXS measurements within the cooperation of the SFB TRR 102 (project B03, Thomas Thurn-Albrecht) are also gratefully acknowledged. H.H. thanks the Alexander von Humboldt Foundation − Germany (Georg Forster Research Fellowship).

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CONCLUSIONS A series of PEO11-TR-(CH2)n-TR-PEO11 which have in the center of the chain two TR rings separated by n-alkyl spacers of different lengths were prepared and studied with respect to their crystallization behavior. WAXS, SAXS, DSC, and solidstate NMR spectroscopy measurements revealed solid-state phase transitions of all samples under investigation prior to final melting. After crystallization, the WAXS diffractograms at T = −10 °C reveal that one PEO11 chain of each PEO11-TR(CH2)n-TR-PEO11 crystallized in the known monoclinic 72 helix structure, the TR-(CH2)n-TR group, and the second PEO11 chain remain amorphous. Upon heating the complete TR-(CH2)n-TR defect can be incorporated into the PEO crystal structure, and a new crystal system (PEO-TR phase) was formed. The melting temperatures of the PEO-TR phase showed a decreasing tendency with an increasing number of CH2 groups (29, 22, and 17 °C for n = 2, 3, and 4, respectively). Obviously, the melting temperatures were dominantly influenced by π−π interactions of the TR rings as revealed by 13C MAS CP and SP NMR spectroscopy. With increasing number of CH2 groups of the TR-(CH2)n-TR chain defect, the crystal stabilizing C−H···π interactions were reduced due to packing difficulties which results into a lower melting temperature. However, PEO11-TR-(CH2)4-TR-PEO11 showed a more complicated behavior; i.e., an additional phase transition was noticed where the polymer was converted into the PEO′

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REFERENCES

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