Crystallization of Poly(ethylene oxide) with a Well ... - ACS Publications

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Crystallization of Poly(ethylene oxide) with a Well-Defined Point Defect in the Middle of the Polymer Chain Martin Pulst, Muhammad H. Samiullah, Ute Baumeister, Marko Prehm, Jens Balko, Thomas Thurn-Albrecht, Karsten Busse, Yury Golitsyn, Detlef Reichert, and Jörg Kressler* Faculty of Natural Sciences II, Martin Luther University Halle-Wittenberg, D-06099 Halle (Saale), Germany S Supporting Information *

ABSTRACT: Poly(ethylene oxide) (PEO) is a polymer of great interest due to its prevalence in biomedical, pharmaceutical, and ion conductive systems. In this study, the crystallization behaviors of a PEO with 22 monomer units (PEO22) and a PEO having the same degree of polymerization but with an additional 1,4-disubstituted 1,2,3-triazole ring in central position of the chain (PEO11-TR-PEO11) are investigated. PEO11-TR-PEO11 shows one type of lamella crystal after cooling to T = 0 °C, but structural changes during heating below their final melting are detected by WAXS, DSC, POM, and solid-state NMR spectroscopy. The lamella thickness increases, but simultaneously the helix−helix distance decreases and an additional Bragg reflection appears at 2θ = 22.1°. A model is proposed which explains these structural changes by incorporation of the TR ring into the crystals which are additionally stabilized by attractive C−H···π interactions of the TR rings. Additionally, two different types of extended chain lamella crystals are found in PEO22 by SAXS which are discussed in the context of fractionation caused by the molar mass distribution obtained from MALDI-ToF data.

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monomethyl ether with the respective acid chlorides.20−22 There is not any report on the influence of well-defined 1,2,3triazole (TR) defects on the physical properties of PEO. But this seems to be important since Cu(I)-catalyzed Huisgen-type 1,3-dipolar cycloaddition (“click” reaction)23−26 has frequently been employed for modifying the polymer architecture of PEO.27−29 It is known that a huge number of 3-alkyl-1,2,3triazolium groups (one defect per three EO units) may suppress the crystallization of PEO completely, but these studies are not directly comparable with 1,2,3-triazole containing polymers due to the ionic character of the triazolium groups.30−33 The incorporation of defects into PEO might reduce the crystallinity which is one approach to improve the ion conductivity of PEO based polymer electrolytes.34,35 Here, we introduce a TR based point defect to the middle of the PEO chain. Two end-group-modified PEO chains with 11 monomer units are joined so that they contain in the newly formed polymer a 1,4-disubstituted 1,2,3-triazole ring exactly in the center of the chain (PEO11-TR-PEO11, cf. Scheme 1a). The crystallization and melting behavior of PEO11-TR-PEO11 is studied in detail by differential scanning calorimetry (DSC),

INTRODUCTION Poly(ethylene oxide) (PEO) is an important synthetic polymer due to its plethora of applications. The water solubility and the tight shielding of the polyether chain by water molecules make PEO a biocompatible polymer with applications in nanomedicine or for pharmaceutical formulations.1,2 Since many applications of PEO utilize rather oligomers (also called poly(ethylene glycol) (PEG)) than polymers, the physical properties depend strongly on the end-group functionality.3−6 The tailoring of physical properties of PEO can also be achieved by variation of PEO architecture as e.g. star-PEO,7,8 branched-PEO,9,10 or PEO-networks.11,12 Especially block copolymers of PEO with hydrophobic blocks have been synthesized exploiting then their amphiphilic character13−15 or their self-assembly in thin films.16,17 Since PEO is a semicrystalline polymer, the melting temperature Tm and the degree of crystallinity X are important variables to modify properties. Both Tm and X are influenced by chain defects inserted deliberately into polymer chains. Generally, two types can be distinguished. First, randomly distributed chain defects are obtained by copolymerization,18,19 and second, well-defined chain defects can be introduced by reacting two or more polymer blocks with functional end groups. The latter has been done by inserting terephthalate, phthalate, or isophthalate units to the center of the polymer chain after reacting PEO © XXXX American Chemical Society

Received: May 24, 2016 Revised: July 11, 2016

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DOI: 10.1021/acs.macromol.6b01107 Macromolecules XXXX, XXX, XXX−XXX

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with silver behenate. The heating and cooling rate was about 1 K min−1. PEO11-TR-PEO11 was crystallized at T = 0 °C to avoid freezing of humidity, and the traces are taken in steps of ΔT = 8 K. PEO22 was measured at room temperature after cooling from T = 70 °C with a heating rate of 1 K min−1. The 2D diffraction patterns were analyzed over the complete azimuthal angle range (0° ≤ χ ≤ 360°) for PEO22 or in the cross section where the highest intensity is measured for the oriented PEO11-TR-PEO11 sample. The background-corrected SAXS traces (primary beam signal with simple exponential decay and a constant background for air scattering are subtracted) are fitted with a one-dimensional scattering function37

Scheme 1. (a) Synthesis of PEO11-TR-PEO11 Containing the 1,4-Disubstituted 1,2,3-Triazole Ring in the Middle of the Polymer Chain; (b) Structure of PEO22

I=

⎛ −(q − nq*)2 /2w2 2 ⎞ e sin (nπϕ) ⎟ −q2u2 a ∑ ⎜⎜ ⎟e 2 w n ⎝ n ⎠

(2)

where I is the Intensity, q is the scattering vector, a and w are measures for amplitude and width of the first maximum, q* is the scattering 2 2

vector of the first peak maximum, n is the order of the peak, and e−q u is the exponential Debye−Waller factor. The long period d is calculated by d = 2π/q*.38 Since ϕ is either the volume fraction of the crystalline phase ϕc or of the amorphous phase ϕa, it is necessary to use additional information from DSC (eq 1) or WAXS to assign the respective volume fractions. The lamella thickness Lc is then the product of ϕc with the long period (Lc = dϕc).38−40 Wide-Angle X-ray Scattering. Wide-angle X-ray scattering (WAXS) experiments were performed in Bragg−Brentano geometry, using a PANalytical Empyrean diffractometer, equipped with a position-sensitive detector (PIXcel-3D). The samples were placed on silicon zero background substrates in a TTK 450 temperature chamber from Anton Paar. The measurements were performed under dry nitrogen with nickel-filtered Cu Kα radiation (λ = 1.5418 Å). PEO11TR-PEO11 was cooled to T = −4 °C held there for 20 min and then reheated in steps of ΔT = 4 K until final melting. The diffractograms of PEO22 were taken in a similar temperature range but in steps of ΔT = 5 K due to the higher melting point. The spectra were recorded at these temperatures in a scan range of 6° < 2θ < 60° with a step size of Δ2θ = 0.053° and a counting time per step of 93 s. MALDI-ToF. The MALDI-ToF measurements were carried out with a Bruker Autoflex III system where the ratio of the THF solutions of the matrix to polymer to salt is fixed to 100:10:1 (matrix: c = 20 mg mL−1; polymer: c = 20 mg mL−1; salt: c = 10 mg mL−1). For PEO22, trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB) was used as matrix and silver(I) trifluoroacetate (AgTFA) as salt. The matrix−salt system was changed to 1,8,9anthracenetriol (dithranol) and sodium trifluoroacetate (NaTFA) for PEO11-TR-PEO11 since triazoles can react with silver salts.41,42 Polarized Optical Microscopy. Polarized optical microscopy (POM) was studied with an Axioplan 2 imaging microscope from Carl Zeiss Jena equipped with a LINKAM THMS 600 hot stage. The images were taken using an AxioCam MRc camera from Carl Zeiss Jena. The samples were placed on a cover glass into the hot stage. The measurement chamber was purged with dry nitrogen gas. PEO11-TRPEO11 was cooled to T = −4 °C and reheated with a heating rate of about 1 K min−1. Solid-State NMR Spectroscopy. 13C MAS cross-polarization (CP) and single-pulse (SP) experiments were carried out on a BRUKER AVANCE 400 with a standard 4 mm VT-MAS probe. Typical MAS rates were 11 kHz, and 1H decoupling (TPPM) was applied to remove dipolar broadening. The line assignment was carried out with the solution 2-D 1H−13C gHSQCAD NMR spectrum which is given in the Supporting Information (Figure S9a). The sample temperature was controlled by a standard BRUKER VT-controller and calibrated with methanol.43 PEO11-TR-PEO11 was heated in the NMR probe at T = 30 °C and rapidly cooled by a stream of cold gas to T = −15 °C where it was kept for at least 30 min for crystallization. Typically, 512 scans were accumulated, resulting in a time frame for every experiment of approximately 1 h at a given temperature. Experiments to determine the second moments of the 1H NMR wideline spectra M2 were carried out on a BRUKER AVANCE 200 using a

small-angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS), polarized optical microscopy (POM), and solidstate NMR spectroscopy. Additionally, the crystallization behavior of the triazole free PEO22 as shown in Scheme 1b is investigated as a control experiment.

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EXPERIMENTAL SECTION

Materials. The structures of the polymers under investigation are shown in Scheme 1. PEO22 dimethyl ether (Mn ≈ 1000 g mol−1) and PEO11 monomethyl ether (Mn ≈ 500 g mol−1) were purchased from Sigma-Aldrich. The number-average molar masses were calculated from solution 1 H NMR spectroscopy data (see Supporting Information, Figures S1 and S3). The molar mass distributions of the polymers under investigation were obtained by MALDI-ToF measurements as shown below. All details of the syntheses and characterization by NMR spectroscopy, MALDI-ToF mass spectrometry, ICP mass spectrometry, and FT-IR spectroscopy are given in the Supporting Information (see pages S-2 to S-7). Differential Scanning Calorimetry. Differential scanning calorimetry (DSC) measurements were performed under continuous nitrogen flow using a Mettler Toledo DSC 822e module. Aluminum pans were filled with about 5−15 mg of sample. The samples (PEO11TR-PEO11 and PEO22) were held for 10 min in the melt at T = 70 °C and then cooled to T = −40 °C with a cooling rate of 1 K min−1. The heating trace was recorded after 10 min with a heating rate of 1 K min−1 up to T = 70 °C. Additional isothermal crystallization experiments of PEO11-TR-PEO11 were also performed under a nitrogen atmosphere. The sample was cooled from T = 70 °C with the fastest possible cooling rate (between 30 and 50 K min−1) to the respective crystallization temperature and held there for 20 min. The following heating trace was recorded with a heating rate of 1 K min−1. It should be noted that the DSC trace of PEO11-TR-PEO11, recorded from T = −40 °C to T = 70 °C, is not shown since the observed transitions and the corresponding temperatures are identical with the traces obtained after isothermal crystallization at Tc = −4 °C. The crystallinity X of PEO22 is determined from the melting enthalpy Δhm X=

Δhm × 100% Δhm0

(1)

where Δh0m is the melting enthalpy of a 100% crystalline PEO. A value of Δh0m = 197 J g−1 is used for the calculation.36 Small-Angle X-ray Scattering. Small-angle X-ray scattering (SAXS) experiments of PEO22 and PEO11-TR-PEO11 were performed in transmission, using a 2D VÅNTEC-500 area detector from Bruker. Nickel-filtered Cu Kα radiation (λ = 1.5418 Å) was used. The samples were sucked in glass capillaries with a diameter of 1 mm and placed on a LINKAM THMS 600 hot stage, and the calibration was carried out B


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Figure 1. (a) Temperature-dependent and background-corrected SAXS traces of PEO11-TR-PEO11 after crystallization at Tc = 0 °C; the green line is the best fit of the data at T = 16 °C with the scattering function (eq 2). The 2D-diffraction pattern of PEO11-TR-PEO11 is shown in (b) and the corresponding pattern of PEO22 in (c).

below Mn ≤ 3000 g mol−1.48,49 It should be noted that there is one set of lamellae with uniform lamella thickness observed in PEO11-TR-PEO11, but the triazole-free PEO22 crystallizes in two types of lamella crystals (see section below) which might be caused by the more narrow molar mass distribution (cf. Figure S5b, Supporting Information) of PEO11-TR-PEO11 compared to PEO22 or by the TR ring in the center of the PEO chain. The long period of PEO11-TR-PEO11 does not change significantly during heating, but the intensities of the second- and third-order maxima are increasing, indicating that the crystallinity is changing. Compared with DSC and WAXS data (see below), an increase in the degree of crystallinity can be expected, which is confirmed by the thickening of the crystalline layer obtained from fitting of the SAXS trace with the scattering function (eq 2) to Lc = 40.4 Å at T = 16 °C. The value of Lc is significantly larger than the theoretical length of one arm of the PEO11-TR-PEO11 (Lc,th = 30.9 Å),15 indicating that the triazole ring is incorporated into the polymer lamellae upon heating. Theoretically, this might be possible due to the small size of a triazole ring of 2.12 Å50 (without substituents) compared to the helix−helix distance of about 4.6 Å in the unit cell of PEO. This result is completely new since previous results indicate only that phthalic acid derivative defects might be arranged perpendicular to the lamellar basal surface of a folded PEO crystal.51 The SAXS trace of molten PEO11-TR-PEO11 at T = 30 °C shows no remaining peak, suggesting the absence of a possible phase separation between the PEO chains and triazole rings. This is in agreement with the fact that liquid 1,2,3-triazole is a good solvent for PEO which is additionally verified experimentally using PEO22 and liquid 1,2,3-triazole. The mixture is homogeneous in the melt, and upon cooling a eutectic behavior is observed. (cf. Figure S6). Additionally, the 2D SAXS pattern of PEO11-TR-PEO11 (see Figure 1b) does not show uniform diffraction circles but broadened reflections centered approximately on the meridian of the pattern, indicating an alignment of PEO crystals with the long period nearly parallel to the meridian. Normally, such oriented PEO patterns are only attainable investigating large single crystals,52 stretched/sheered samples,53 or thin PEO films with microbeam radiation.54−56 Here, the degree of orientation f can be calculated using Hermans orientation function55,57−59

special wide-line NMR probe featuring a very short dead time of 2.5 μs to avoid signal loss during the initial parts of the free-induction decay (FID). The wide-line spectra are governed by the homonuclear dipole−dipole interactions among all protons and provide an integral information about the mutual intra- and intermolecular distances between the protons. Since the spectra do not provide chemical resolution, the data are commonly evaluated in the time domain. To obtain reliable data, a number of experiments (FID, MSE, MAPE) were combined, and phase decomposition was applied to obtain a clear signal of the immobile protons.44−46 From the resulting signal, M2 is calculated by M 2 = aA 2 +

1 2 bA 3

(3)

The parameters aA and bA are obtained from the parameter fitting of the first t = 200 μs of the NMR signal using the Abragam function 2 sin(b t ) I(t ) A = e−0.5(aA t ) I(0) bA t

(4)

where I(t)/I(0) is the normalized intensity of the NMR signal of the immobile protons (for details, see ref 47). An example of the fit with eq 4 to the data is shown in the Supporting Information (Figure S9c). The value of M2 and its temperature dependence carry qualitative information about the mutual distances between neighboring protons, and thus, its intermolecular contribution can be used to yield information about the density of molecules in the crystals where larger values of M2 generally mean a tighter packing of the chains.

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RESULTS AND DISCUSSION Crystallization of PEO11-TR-PEO11. In order to understand the influence of the TR point defect in the middle of the polymer chain of PEO, PEO11-TR-PEO11 was investigated by SAXS and the background corrected SAXS traces are shown in Figure 1a. After crystallization at T = 0 °C a peak at q* = 0.0875 Å−1 is observed together with its second and third order at 2q* and 3q*. The corresponding long period (d = 2π/q*) is determined to d = 71.8 Å. Using the scattering function (eq 2) yields a lamella thickness of Lc = 32.6 Å, which is in good agreement with the fact that the almost missing second-order peak is an indication for similar dimensions of the amorphous and crystalline regions (La ≈ Lc). This thickness corresponds to a PEO chain with 11−12 monomer units in a 72-helix,15 indicating the crystallization of one PEO11 chain of PEO11TR-PEO11 in an ECC as observed for PEOs with molar masses C


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Figure 2. (a) Temperature-dependent WAXS diffractograms of PEO11-TR-PEO11 during heating from T = −4 °C to T = 24 °C in steps of ΔT = 4 °C. Based on the monoclinic PEO unit cell, the Miller indices are given for the two reflections with the highest intensity. The arrow indicates the appearance of a new reflection during heating. (b) WAXS diffractogram of PEO22 at T = 25 °C. (c) Changes of the distances with temperature, calculated from the diffraction angle of (120) and (032)* reflections and the reflection at 2θ = 22.1° (full symbols) and their normalized integral intensities (open symbols) as a function of the temperature of PEO11-TR-PEO11. (d) Changes of the distances with temperature, calculated from the diffraction angle of (120) and (032)* reflections of PEO22. (e) Second moment M2 of 1H NMR wide-line spectra for PEO11-TR-PEO11 and PEO22.

f=

3⟨cos2 χ ⟩ − 1 2

Structural Changes of PEO11-TR-PEO11. In order to gain a better understanding for the influence of the TR ring on the PEO crystallization, temperature-dependent WAXS measurements were carried out as can be seen in Figure 2a. After crystallization at T = −4 °C, the diffractogram of PEO11-TR-PEO11 (Figure 2a) shows the characteristic PEO reflections (cf. Figure 2b). However, the (120) reflection appears at 2θ = 19.37°, which is a slightly higher value than the value of PEO22 (2θ = 19.11°), indicating that the 72 helices in the crystal have a slightly lower distance to each other.53 This is confirmed by a similar shift observed for the (032) reflection at 2θ = 23.52° which is overlapped by the (1̅32), (112), (2̅12), (1̅24), (2̅04), and (004) reflections54 (here we call it the (032)* reflection). The determination of the crystallinity, i.e., comparing the area of the total diffractogram with the area of the Bragg reflections after subtracting the amorphous halo with X’Pert HighScore software, yields a value of X = 41%, which is slightly lower compared to the value obtained by SAXS. This might be caused by contributions of diffuse and incoherent scattering to the total scattering intensity which is not corrected in our calculation.60,61 It should be noted that the crystallinity cannot be calculated from the DSC melting enthalpy since the

(5)

with π

2

⟨cos χ ⟩ =

∫0 I(χ ) cos2 χ sin χ dχ π

∫0 I(χ ) sin χ dχ

(6)

where χ is the azimuthal angle. It should be noted that f = 0 is calculated for a completely disordered sample; f = 1 and f = −0.5 are the limiting values for a perfect orientation in vertical or horizontal alignment, respectively. The azimuthal intensity profile of PEO11-TR-PEO11 is depicted in the Supporting Information (Figure S7a). Evaluation of the data according to eqs 5 and 6 yields a value of f = 0.92, confirming the large degree of orientation. Since the orientation effect has never been observed for PEO22 (f = −0.02; cf. Figure 1c and Figure S7b), it seems reasonable that the TR ring is responsible for this effect either by interacting with the glass surface of the capillary or by attractive intermolecular π−π interactions as will be discussed below. D


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Figure 3. (a) DSC heating traces of PEO11-TR-PEO11 after isothermal crystallization at temperatures between −4 °C ≤ Tc ≤ 8 °C, recorded with a heating rate of 1 K min−1. The dotted line represents the baseline and should guide the eye. The inset shows the heating traces after crystallization at T = −4 °C (full line) and the respective reheating curve without the exothermal peak after isothermal annealing at T = 15 °C and cooling to T = −20 °C (dotted line). (b)−(e) Spherulite morphology of PEO11-TR-PEO11 after crystallization at (b) T = −4 °C and during reheating at (c) T = 6 °C, (d) T = 12 °C, and (e) T = 16 °C obtained by POM. The white arrows in (c) and (d) indicate the appearing black areas (liquid/amorphous phase).

dependent WAXS diffractograms where the shift of the Bragg reflections belonging to the PEO crystal system as well as the appearing reflection at 2θ = 22.1° upon heating is noticed. The final melting of PEO11-TR-PEO11 is observed at Tm = 22.8 °C. This type of DSC trace is typical for structural changes in polymers, e.g., known for poly(1-butene) where a 41 helix (orthorhombic crystals system) transforms into a 113 helix in a tetragonal unit cell64 or for paraffins were a chain defolding is observed during heating.65 The latter can be excluded since the PEO11-TR-PEO11 crystallizes in ECCs (see above). Thus, further isothermal crystallization experiments are carried out to gain a deeper understanding of the structural changes during heating (Figure 3a). The first endothermal transition during reheating does not shift if the crystallization temperature is varied below Tc ≤ −4 °C, but the transition temperature shifts to higher temperatures if the crystallization temperature is increased above Tc ≥ −4 °C. In complete analogy to this observation, the exothermal peak is also shifted with increasing Tc, indicating that both processes are related to each other; in other words, the incipient melting is a requirement for the observed structural changes. This is also supported by the systematic change of the melting and recrystallization enthalpy which is in perfect agreement with the assumption of structural changes as observed by WAXS rather than a simple cold crystallization66 since an amorphous/liquid intermediate phase seems to be necessary for these rearrangement processes. The structural changes are irreversible within the time scale of the DSC experiments since only the final melting of the transferred structure is observed in the DSC reheating curve (see inset of Figure 3a) if the sample is heated and annealed at temperatures between 13 °C ≤ T ≤ 17 °C and afterward cooled down. The structural changes can also be observed by POM. After isothermal crystallization at Tc = −4 °C spherulites can be observed (cf. Figure 3b). In analogy to the DSC curve, black areas are noticeable in the spherulites at T = 6 °C (Figure 3c), indicating an amorphous/liquid isotropic intermediate phase. The POM images also change during further heating. The formerly black areas grow and become colored (Figure 3d), indicating the recrystallization after the rearrangement processes. After impingement of the dark areas, the spherulites have the same appearance as prior to the structural change, but

C−H···π interactions (see structure model below) have also a contribution to the melting enthalpy (some kJ mol−1)62 which could not be considered quantitatively in eq 1. Two different changes in the WAXS diffractograms of PEO11-TR-PEO11 can be observed with increasing temperature. First, a shift of the (120) and (032)* reflections to higher diffraction angles indicating a reduction of the distances of the respective Miller planes of Δd ≈ 0.5% (cf. Figure 2c) is observed which is not detected in PEO22 (see Figure 2d). Second, an additional reflection in the diffractogram appears at 2θ = 22.1° (marked by an arrow in Figure 2a) during heating above T ≥ 8 °C. Its intensity increases until melting at T = 24 °C (cf. open symbols in Figure 2c). This additional reflection can be explained neither by the 72 helix based monoclinic unit cell of PEO (see discussion in the section below) nor by the planar zigzag modification of PEO in a triclinic unit cell.63 It should be noted that the tighter packing of the chains of PEO11-TR-PEO11 in the transformed phase and its increasing volume fraction with temperature is also confirmed by the temperature dependence of M2 (Figure 2e). Recalling that a decrease in M2 means qualitatively a less tight packing of the molecules (increased intermolecular distances between neighboring chains), the decrease in M2 for temperatures below T ≤ −10 °C is thus a result of the thermal expansion. On the opposite, its increase for temperatures above T ≥ 0 °C means a more tightly packed transformed phase, as discussed above. For comparison, the data for PEO22 are also shown in Figure 2e which indicate the absence of the transition discussed above. However, the observed structural changes can be studied in more detail by DSC and POM as shown in Figure 3. After isothermal crystallization at Tc = −4 °C, the DSC heating trace (Figure 3a) shows an endothermal peak upon heating at T = 2.0 °C which is related to the incipient melting of the crystalline PEO chains of PEO11-TR-PEO11. The lamella thickness at T = 0 °C is equal to the length of one PEO11 arm; i.e., the TR rings have to be arranged on the surface of the PEO lamellae. Because of these facts, rearrangement processes become easily possible to stabilize the lamellae by attractive π−π interactions of the TR rings combined with a larger lamella thickness. We correlate these structural changes to the exothermal peak of the DSC trace with a minimum at T = 6.4 °C, which is in perfect agreement with the temperatureE


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increase. However, the structural changes appear in the 13C solid-state NMR experiments at slightly lower temperatures compared to the other experimental techniques caused by the fact that the NMR data acquisition time is significantly longer (see Experimental Section). It should be noted that most of the data shown in Figure 4 are recorded using cross-polarization (CP) which preferably excites the less mobile nuclei only; i.e., 13 C MAS CP NMR spectroscopy is ideally suited for investigations of the crystalline regions in polymers.67 In contrast, 13C MAS single-pulse (SP) NMR experiments detect exclusively mobile (liquid) molecules. An example recorded at T = 0 °C (at this temperature the CP experiment shows pairs of lines) is shown as the upper spectrum in Figure 4. The peaks are not split and thus caused by the mobile (liquid) molecules observed as dark areas in POM experiments (Figure 3c,d). The irreversibility of the structural changes upon the second cooling as described above in the DSC experiments (inset of Figure 3a) is additionally confirmed by 13C MAS solid-state NMR spectroscopy as shown in the Supporting Information (Figure S9b). In the 13C MAS CP NMR spectra, the resonances of the high-temperature structure do not change during cooling from T = 10 °C to T = −5 °C, and simultaneously the splitting does not occur. Structure Model of PEO11-TR-PEO11. After observation of the structural changes of PEO11-TR-PEO11, they should be discussed in more detail. As already mentioned, the WAXS diffractogram at low temperatures exhibits exclusively Bragg reflections which belong to the 72 helix of PEO packed into the monoclinic unit cell. Thus, the cell parameters of this lowtemperature modification are calculated by refinement of the diffractogram using BIOVIA Materials Studio Reflex software using the cell parameters of the monoclinic PEO unit cell as input. The results of the refinement are shown in Figure 5a. The calculated cell parameters are additionally summarized in Table 1. It should be noted that a separate assignment and refinement of the WAXS reflections of an oriented sample in the glass capillary is not possible since an overlapping of the reflections is noticed which is also simulated by Zhu et al. (cf. ref 68 and Figure S8).

their color has changed slightly (see Figure 3e). A short movie can be seen in the Supporting Information. Further support of the structural changes is derived from 13C solid-state MAS NMR spectra (Figure 4). At T = −10 °C, the

Figure 4. 13C MAS NMR spectra of PEO11-TR-PEO11 at different temperatures after isothermal crystallization at T = −15 °C. For line assignment see Supporting Information, Figure S9a. Spectra were acquired using cross-polarization (CP) while the spectrum on top is a single-pulse (SP) spectrum (see text). The intensity of the SP NMR spectrum is multiplied by a factor of 4 for visualization.

two resonances of the triazole rings at about δ = 125 ppm and δ = 145 ppm and those of the adjacent EO groups at δ = 65 ppm and δ = 50 ppm split into pairs. Upon temperature increase, the upfield peaks of the triazole rings decrease in intensity while those of the downfield peaks increase. The ratios develop from about 1:4 at T = −10 °C and 1:2 at T = −5 °C to 4:1 at T = 0 °C. A qualitatively similar behavior is observed for the abovementioned EO signals. The observed resonance pairs are caused by structural changes; i.e., the TR rings and the adjacent EO groups are in a different chemical environment prior to the structural changes compared to the structure at higher temperatures. We thus assign the low-field peaks to the structure after the observed changes during temperature

Figure 5. Refinement of the WAXS diffractogram of PEO11-TR-PEO11 at (a) T = −4 °C with the 72 helix of the PEO chain in the monoclinic unit cell, the inset showing the zoom in the range of 21° ≤ 2θ ≤ 24°. (b) T = 16 °C with the 72 helix structure containing additionally the TR defect in the monoclinic unit cell (see text). (c) Zoom of (b) in the range of 21° ≤ 2θ ≤ 24° (top) and refinement of the WAXS diffractogram at T = 16 °C with the defect free 72 helix structure in the monoclinic unit cell (bottom). The range of the additional reflection at 2θ ≈ 22.1° and possible corresponding Bragg positions are highlighted. The amorphous halo of all diffractograms is subtracted before refinement (see text), and the positions of the calculated Bragg reflections are additionally shown as vertical bars on the x-axis for all diagrams. F


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defined by the turn of the helix (7 EO units due to the 72 helix) for the defect-free PEO structure at lower temperatures. In contrast, this value has to be expanded to the full length of the crystalline part of the chain for the TR defect containing crystal structure at higher temperatures, resulting in a significant difference in the values of c at different temperatures as shown in Table 1. The space group and symmetry are not changed in our model since the (120) and (032)* reflections are only slightly shifted and a chiral sorting of right- and left-handed helices is unlikely, which means four PEO chains have to be packed into the unit cell. The chain length of the PEO which is packed into the unit cell is calculated according to the SAXS lamella thickness of Lc = 40.4 Å. Considering the length of a 1,4-disubstituted triazole ring of about 5 Å (TR diameter of 2.12 Å plus twice the typical bond lengths of 1.45 Å to the two substituents), it yields a value of 13 EO units with an average length of 2.78 Å53 in the 72 helix conformation. It should be noted that the substituents in 1- and 4-positions of the TR ring have an angle of approximately (2/5)·360° = 144° to each other, which explains that the helix cannot be extended to the full length of the PEO11-TR-PEO11 molecule and only two more EO units of the second chain (less than one turn of the 72 helix) are able to pack into the crystal. This is in perfect agreement with the 13C solid-state NMR spectra (Figure 4) where the splitting of the resonances is observed for carbon atoms of the heterocycle and the directly adjacent EO groups to the TR ring, indicating that the adjacent EO groups are also incorporated into the crystal. The structure which is packed into the unit cell is depicted in Figure 6b, and the refinement of the calculated WAXS trace of this structure to the measured diffractogram is shown in Figure 5b. The calculated WAXS diffractogram fits to the measured trace which is additionally

Table 1. Cell Parameters Obtained by Refinement of the WAXS Diffractograms for PEO11-TR-PEO11 cell parameters

T = −4 °C

T = 16 °C

a (Å) b (Å) c (Å) β (deg)

7.981 12.985 19.586 125.41

7.909 12.964 40.678 125.41

In contrast to the known PEO structure observed at low temperatures, the WAXS diffractogram at higher temperatures (e.g., T = 16 °C) exhibits in addition to the (120) and (032)* reflections a reflection at 2θ = 22.1° (cf. Figure 2a). Since the (120) and (032)* reflections are remaining (see Figure 2c), it is clear that the monoclinic crystal system of PEO based on the 72 helix is only slightly changed. However, the appearing reflection above T ≥ 8 °C cannot be explained with the ordinary PEO unit cell which is demonstrated in Figure 5c. It is clearly visible that the diffraction conditions (vertical bars) in the range of angles are not fulfilled where the additional reflection is observed. Thus, our explanation for the additional Bragg reflection is the incorporation of the TR ring with a periodic spacing into the PEO crystal system. For checking this assumption, a unit cell is build where the input values of a and b are taken from the monoclinic PEO unit cell and the c value is obtained from the 2D WAXS pattern (Figure S8) of the oriented PEO11-TR-PEO11 sample according to the method described by De Rosa and Auriemma,69 which is also in good agreement with the calculated SAXS lamella thickness (see discussion of Figures 1a and 6a). This procedure has been established for crystal structure determination of macromolecules.70,71 It should be noted that the unit cell should be chosen as small as possible; i.e., the length of the fiber axis c is

Figure 6. (a) Schematic model of amorphous and crystalline regions of PEO11-TR-PEO11 at T = 0 °C (top) and after the structural changes at T = 16 °C (bottom). (b) Crystalline part of a PEO11-TR-PEO11 chain at T = 16 °C which is packed into (c) the unit cell with orange (122) Miller planes. The purple highlighted unit cell axes indicate the position of the unit cell in the schematic model depicted in (a). (d) Top view of a PEO helix and a TR ring with their respective diameters. (e) Arrangement of the TR rings in this crystal structure (PEO atoms are removed for better visualization). (f) Herringbone packing of the TR rings within the unit cells (the green stripes should guide the eye). G


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offset π−π stacking alignment. Thus, the observed reduction of the helix−helix distance in PEO11-TR-PEO11 after the incorporation of the TR ring into the crystal structure can be explained by attractive C−H···π interactions in a T-shaped geometry. The distance between two centroids is dcent = 5.04 Å (cf. Figure 6e), which is in the typical range for these interactions.62,75,76 The overall packing type of the TR rings, neglecting the PEO chains, is the typical herringbone structure as depicted in Figure 6f. Fractional Crystallization of PEO22. The crystallization behavior of PEO22 was used as a control experiment for the data of PEO11-TR-PEO11 described in the sections above. However, there is an interesting observation which deserves brief discussion. It has been known that PEO crystallizes exclusively into extended chain crystals (ECCs) when the degree of polymerization is smaller than 68 (Mn ≤ 3000 g mol−1).49 Owing to the fact that the polydispersity of PEO22 is rather small, i.e., Đ = 1.03, one would expect intuitively that crystals with a uniform lamella thickness are formed upon crystallization. However, as can be seen in the SAXS trace of Figure 7a, PEO22 forms two types of lamella crystals upon crystallization indicated by the occurrence of two sets of first-, second-, and third-order peaks at q*i , 2q*i , and 3q*i . This is also supported by the occurrence of two exothermal crystallization processes measured by DSC as shown in the inset of Figure 7a. It should be noted that also the DSC heating trace shows two melting endotherms since the melting point of PEO depends on the lamella thickness Lc.77−79 However, chain folding effects can be excluded as explanation (which is assumed erroneously by Machado et al.80), but fractional crystallization of extended PEO chains with different lengths into two populations of stacks of lamellae with different average thicknesses (and also different long periods) seems to be reasonable as discussed by Song and Krimm.81 This is schematically drawn in Figure 7b. Analysis of the SAXS data using eq 2 yields a lamella thickness of Lc,1 = 46.4 Å for the thick lamellae and Lc,2 = 38.6 Å for the thin lamellae; i.e., the determined crystallinity of ϕc = 62% is identical for both sets of lamellae. It should be noted that the WAXS diffractogram shows exclusively the pattern of the monoclinic unit cell which contains four PEO 72-helices which has been discussed above. A quantitative assignment of the chain lengths comprising the two different lamellae can be done by comparing SAXS results with MALDI-ToF spectra (Figure 7c). The ratio of the peak areas belonging to the q2*series relative to the q1* series of the SAXS data yields a value of 58 vol % for the thin lamellae and 42 vol % for the thick lamellae. Since MADLI-ToF measures the molar mass distribution, the values have to be converted into volume units using fractional van der Waals volumes reported by Bondi.82 Thus, the volume fractions are calculated for the MALDI-ToF peaks for each chain length and summed up. 59 vol % is reached after summation up to a chain length of 23 EO units. Thus, it can be assumed that the thin lamellae consist mainly of PEO chains with a length of n ≤ 23 EO units, and the longer chains (n ≥ 24) are predominantly packed into the thicker lamellae. Two possible molar mass distributions which belong to the two different types of crystal thicknesses are also plotted in Figure 7c by fitting the data with two Gaussian functions which are usually used for fitting MALDIToF data of narrowly dispersed polymers.83,84 Now, the mechanism of fractionated crystallization can be analyzed in detail. During cooling, the longer chains have a higher thermodynamic driving force for crystallization

supported by the low R-factors (Rp = 10.47%, Rwp = 4.36%) and the finally calculated cell parameters are summarized in Table 1. To get more accurate information about the arrangement of the triazole rings in the crystals, it is necessary to know the Miller indices of the reflection in the range of 2θ ≈ 22.1°. The reflections (008), (033), (03̅3), (122), (12̅2), (1̅28), (1̅2̅8), and (206̅) of the structure are observed in the diffraction angle range of 21.4° < 2θ < 22.5°. A list of the reflections with the corresponding diffraction angles is given in the Supporting Information, Table S1. Four out of the eight listed (hkl) reflections with high values of l (|l| ≥ 6) can be excluded since the arrangement of the TR rings would be nearly perpendicular to the helix direction, but this is prohibited due to the necessary angle of ≈180° of the substituents to the TR ring plane. Compared to the crystal structure of defect free PEO, the appearing reflection means that an additional electron density appears in at least one of these Miller planes ((033) or (122) and their symmetry analogues, respectively) which would be the most probable case if the TR rings arrange in these planes since the helix structure is not changed. The refinement shows that the electron density is in perfect agreement with measured data if the TR ring is located in one of the two Miller planes (cf. Figures 5b and 5c). Because of the fact that the cell parameters a and b as well as the intermolecular helix−helix distance in the crystal at T = 16 °C (cf. Table 1 and Figures 2c,e) are slightly decreased compared to the low-temperature modification, it must be concluded that attractive interactions between the TR rings and/or between PEO and the TR rings are present. In this case, the TR rings have to arrange in the (122) Miller plane since interactions are unlikely if the TR ring is located in the (033) Miller plane (for details see the Supporting Information, Figure S10). The proposed structure where the TR rings are arranged in the (122) Miller plane of PEO11-TR-PEO11 is obtained by packing of four polymer chains as shown in Figure 6b into the unit cell with the space group P21/a, which is shown in Figure 6c. A schematic model of the crystalline and amorphous regions summarizing the structural changes during heating of PEO11-TR-PEO11 is additionally depicted in Figure 6a. It can be seen that the size of a TR ring is smaller than the diameter of a PEO 72 helix, which means that it can be excellently incorporated into the PEO helix (cf. Figure 6d) which is additionally supported by the diameter of an unsubstituted TR ring of 2.12 Å.50 Simultaneously, it is shown that two TR rings are arranged into the (122) Miller plane; the other two are arranged into the (12̅2) Miller plane due to the symmetry of the space group of the unit cell. However, the remaining question is the reason for the reduced cell parameters a and b in this modification. The answer is given in Figures 6e,f. It can be seen clearly that the aromatic triazole rings can interact with each other via attractive C− H···π interactions in a T-shaped π−π stacking geometry. It should be noted that there are two possible π−π stacking arrangements of aromatic molecules with attractive interactions which are described by the rules of Hunter and Sanders:62,72,73 (i) an offset π−π stacking geometry which is stabilized by π−σ interactions and (ii) a T-shaped π−π stacking alignment which is stabilized by attractive C−H···π interactions. The predominant arrangement of the triazole rings in the two known crystal structures of 1,2,3-triazole is the offset π−π stacking, but Tshaped geometries with attractive C−H···π interactions are also known.50,74 However, the helix−helix distance in PEO11-TRPEO11 of d ≈ 4.6 Å is too large for attractive interactions in an H


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Figure 7. (a) Background-corrected SAXS trace of PEO22 at room temperature. The two colored lines are the best fits using the scattering function (eq 2). The inset shows the DSC cooling trace of PEO22. (b) Schematic presentation of the two different lamella stacks. (c) MALDI-ToF mass spectrum of PEO22; the inset shows a zoom for two chain lengths differing in one monomer unit (all peaks contain the mass of silver (M = 107.9 g mol−1)). The spectrum is composed of two Gaussian functions which are representative for the molar mass distributions which fractionate into the two different lamella stacks (see text). The colors for the two detected species are chosen identical in all images.

pattern, and the rearrangement process to incorporate the TR rings into the helical PEO structure can take place in regions close to the crystal surface. 13C MAS cross-polarization NMR spectra showed a splitting of the resonances of the TR ring and the adjacent EO groups at low temperatures caused by beginning structural changes. In contrast, a single set of NMR lines was observed after structural changes at higher temperatures. Furthermore, the liquid/amorphous phase which occurred during the structural changes was detected by 13C MAS single-pulse NMR experiments. Finally, a structure model was generated which could explain the reduced PEO helix− helix distance by attractive C−H···π interactions of the triazole rings in a T-shaped geometry after incorporation into the lamella. Additionally, two types of extended chain crystals were found in PEO22 at room temperature which had a different lamella thickness. Analyzing the MALDI-ToF spectra gave quantitative values for the distribution of the chain lengths in the two sets of lamellae; i.e., the thicker lamellae consist mainly of PEO chains with 24 or more EO units, and the thinner lamellae are chiefly formed by polymer chains with 23 or fewer monomer units. This fractionation was caused by the higher thermodynamic driving force of the longer chains for crystallization at lower undercooling; i.e., the longer chains crystallize at first, and the shorter chains were enriched in the remaining melt phase and can crystallize separately in thinner extended chain crystals at lower temperatures. This study showed that 1,2,3-triazole rings can strongly influence the crystallization behavior of PEO; i.e., possible interactions have to be investigated and considered in future studies since azide−alkyne based “click” chemistry is often used for modification of polymers due to the facile reaction and good yields. Furthermore, the incorporated 1,2,3-triazole defects might be of great scientific interest; e.g., the induced dipole moment in the PEO helix can be used to study the αc-relaxation process of PEO (“helix jumps”) by dielectric spectroscopy which was done for PE by inserting carbonyl groups into the chain due to partially oxidation of the CH2 groups.87,88 Further studies on triazolium salts of PEO11-TR-PEO11 would be of great interest since self-assembly was observed in PEO diblock copolymers containing a triazolium group in the middle of two polymer blocks.89 Ion conductivity measurements of these polymer based ionic liquids might support their potential for

compared to shorter polymer chains. This is the direct result of the dependence of Tm on the molar mass which leads to a larger undercooling for longer PEO chains compared to the shorter ones at identical crystallization temperatures.49 Only the long chains can crystallize at higher temperatures; the remaining short chains are enriched in the melt phase. During further cooling the remaining short chains can form also crystals with a smaller lamella thickness. This fractionated crystallization explains also the SAXS trace of PEO22 since the observation of q*1 and q*2 together with their second- and thirdorder peaks indicate the high order of the two crystal populations. A similar observation is reported for PEO with slightly higher molar mass.48 Fractional crystallization has been observed for polyolefins by temperature rising elution fractionation (TREF) or by crystallization analysis fractionation (CRYSTAF) experiments.85,86

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CONCLUSIONS The crystallization behavior of PEO22 and PEO11-TR-PEO11 containing a 1,2,3-triazole point defect in the center of the PEO chain was studied. One set of lamella crystals was formed in PEO11-TR-PEO 11, and the calculated lamella thickness indicated that in average one PEO11 arm was crystalline at T = 0 °C. The lamella thickness was increased during heating until final melting at Tm = 22.8 °C. Temperature-dependent WAXS measurements showed that the PEO helix−helix distance is reduced when the sample was heated above T ≥ 8 °C, and an additional Bragg reflection appeared which could not be explained by the ordinary monoclinic unit cell of PEO. The tighter packing of PEO11-TR-PEO11 at these temperatures was confirmed by an increase of the second moments of the 1H wide-line NMR spectra. Furthermore, an exothermal transition was noticed in the DSC trace during heating below Tm, which was in agreement with the temperature range of the structural changes in the WAXS diffractogram. This transition was preceded by a small endotherm indicating a liquid/amorphous phase which was confirmed by POM where black areas appeared within the spherulites at these temperatures. The experimental data could only be explained when the 1,2,3triazole ring is incorporated into the PEO helix during heating of PEO11-TR-PEO11 above T ≥ 8 °C. This seems to be plausible since a preordering of the aromatic rings at lower temperatures on the lamellar surface is supported by the SAXS I


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(10) Pfefferkorn, D.; Pulst, M.; Naolou, T.; Busse, K.; Balko, J.; Kressler, J. Crystallization and Melting of Poly(glycerol Adipate)Based Graft Copolymers with Single and Double Crystallizable Side Chains. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1581−1591. (11) Samiullah, M. H.; Reichert, D.; Zinkevich, T.; Kressler, J. NMR Characterization of PEG Networks Synthesized by CuAAC Using Reactive Oligomers. Macromolecules 2013, 46, 6922−6930. (12) Zhou, H.; Schön, E.-M.; Wang, M.; Glassman, M. J.; Liu, J.; Zhong, M.; Díaz Díaz, D.; Olsen, B. D.; Johnson, J. A. Crossover Experiments Applied to Network Formation Reactions: Improved Strategies for Counting Elastically Inactive Molecular Defects in PEG Gels and Hyperbranched Polymers. J. Am. Chem. Soc. 2014, 136, 9464−9470. (13) Kyeremateng, S. O.; Busse, K.; Kohlbrecher, J.; Kressler, J. Synthesis and Self-Organization of Poly(propylene Oxide)-Based Amphiphilic and Triphilic Block Copolymers. Macromolecules 2011, 44, 583−593. (14) Kyeremateng, S. O.; Henze, T.; Busse, K.; Kressler, J. Effect of Hydrophilic Block-A Length Tuning on the Aggregation Behavior of α,ω-Perfluoroalkyl End-Capped ABA Triblock Copolymers in Water. Macromolecules 2010, 43, 2502−2511. (15) Pfefferkorn, D.; Kyeremateng, S. O.; Busse, K.; Kammer, H.-W.; Thurn-Albrecht, T.; Kressler, J. Crystallization and Melting of Poly(ethylene Oxide) in Blends and Diblock Copolymers with Poly(methyl Acrylate). Macromolecules 2011, 44, 2953−3963. (16) Bang, J.; Kim, B. J.; Stein, G. E.; Russell, T. P.; Li, X.; Wang, J.; Kramer, E. J.; Hawker, C. J. Effect of Humidity on the Ordering of PEO-Based Copolymer Thin Films. Macromolecules 2007, 40, 7019− 7025. (17) Bang, J.; Kim, S. H.; Drockenmuller, E.; Misner, M. J.; Russell, T. P.; Hawker, C. J. Defect-Free Nanoporous Thin Films from ABC Triblock Copolymers. J. Am. Chem. Soc. 2006, 128, 7622−7629. (18) Alamo, R. G.; VanderHart, D. L.; Nyden, M. R.; Mandelkern, L. Morphological Partitioning of Ethylene Defects in Random PropyleneEthylene Copolymers. Macromolecules 2000, 33, 6094−6105. (19) De Rosa, C.; Auriemma, F.; Ruiz de Ballesteros, O.; Resconi, L.; Camurati, I. Crystallization Behavior of Isotactic Propylene-Ethylene and Propylene-Butene Copolymers: Effect of Comonomers Versus Stereodefects on Crystallization Properties of Isotactic Polypropylene. Macromolecules 2007, 40, 6600−6616. (20) Lee, S.-W.; Chen, E.; Zhang, A.; Yoon, Y.; Moon, B. S.; Lee, S.; Harris, F. W.; Cheng, S. Z. D.; von Meerwall, E. D.; Hsiao, B. S.; et al. Isothermal Thickening and Thinning Processes in Low Molecular Weight Poly(ethylene Oxide) Fractions Crystallized from the Melt. 5. Effect of Chain Defects. Macromolecules 1996, 29, 8816−8823. (21) Chen, E.-Q.; Weng, X.; Zhang, A.; Mann, I.; Harris, F. W.; Cheng, S. Z. D.; Stein, R.; Hsiao, B. S.; Yeh, F. Primary Nucleation in Polymer Crystallization. Macromol. Rapid Commun. 2001, 22, 611− 615. (22) Chen, E.-Q.; Lee, S.-W.; Zhang, A.; Moon, B.-S.; Honigfort, P. S.; Mann, I.; Lin, H.-M.; Harris, F. W.; Cheng, S. Z. D.; Hsiao, B. S.; et al. Isothermal Thickening and Thinning Processes in Low Molecular Weight Poly(ethylene Oxide) Fractions Crystallized from the Melt 6. Configurational Defects in Molecules. Polymer 1999, 40, 4543−4551. (23) Huisgen, R. Kinetics and Mechanism of 1,3-Dipolar Cycloadditions. Angew. Chem., Int. Ed. Engl. 1963, 2, 633−696; Angew. Chem. 1963, 75, 742−754. (24) Huisgen, R. 1,3-Dipolar Cycloadditions Past and Future. Angew. Chem., Int. Ed. Engl. 1963, 2, 565−632; Angew. Chem. 1963, 75, 604− 637. (25) Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless, K. B. A Stepwise Huisgen Cycloaddition Process: Copper(I)-Catalyzed Regioselective “Ligation” of Azides and Terminal Alkynes. Angew. Chem., Int. Ed. 2002, 41, 2596−2599; Angew. Chem. 2002, 114, 2708− 2711. (26) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Click Chemistry: Diverse Chemical Function from a Few Good Reactions. Angew.

applications in solar cells, fuel cells, light-emitting diodes, batteries, electrochromic devices, or catalysis.31

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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.6b01107. General experimental procedures and characterizations of the polymers as well as additional X-ray scattering and NMR data (PDF) A short movie of the structural changes in the spherulites (AVI)

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.K. thanks the Deutsche Forschungsgemeinschaft (DFG) for financial support KR 1714/9-1 and the SFB 102, project B07. D.R. thanks the Deutsche Forschungsgemeinschaft (DFG) for financial support RE 1025/19-1. The authors thank Wolfgang H. Binder and S. Tanner for the support with the MALDI-ToF measurements as well as Kay Saalwächter for helpful discussions of the NMR data. We are also thankful to Martin Herzberg and Dietrich H. Nies for providing the ICP-MS measurements.

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REFERENCES

(1) Greenwald, R. B. PEG Drugs: An Overview. J. Controlled Release 2001, 74, 159−171. (2) Knop, K.; Hoogenboom, R.; Fischer, D.; Schubert, U. S. Poly(ethylene glycol) in Drug Delivery: Pros and Cons as Well as Potential Alternatives. Angew. Chem., Int. Ed. 2010, 49, 6288−6308; Angew. Chem. 2010, 122, 6430−6452. (3) Cammas, S.; Nagasaki, Y.; Kataoka, K. Heterobifunctional Poly(ethylene Oxide): Synthesis of α-Methoxy-ω-amino and αHydroxy-ω-amino PEOs with the Same Molecular Weights. Bioconjugate Chem. 1995, 6, 226−230. (4) Thierry, A.; Skoulios, A. Cristallisation D’un Poly(oxyéthylène) α,ω-Dioctadécylé. Colloid Polym. Sci. 1977, 255, 334−342. (5) Zhang, B.; Zhang, H.; Myers, B. K.; Elupula, R.; Jayawickramarajah, J.; Grayson, S. M. Determination of Polyethylene Glycol End Group Functionalities by Combination of Selective Reactions and Characterization by Matrix Assisted Laser Desorption/ ionization Time-of-Flight Mass Spectrometry. Anal. Chim. Acta 2014, 816, 28−40. (6) Dong, X.-H.; Van Horn, R.; Chen, Z.; Ni, B.; Yu, X.; Wurm, A.; Schick, C.; Lotz, B.; Zhang, W.-B.; Cheng, S. Z. D. Exactly Defined Half-Stemmed Polymer Lamellar Crystals with Precisely Controlled Defects’ Locations. J. Phys. Chem. Lett. 2013, 4, 2356−2360. (7) Stowe, M. K.; Liu, P.; Baker, G. L. Star Poly(ethylene Oxide) as a Low Temperature Electrolyte and Crystallization Inhibitor. Chem. Mater. 2005, 17, 6555−6559. (8) Marzantowicz, M.; Dygas, J. R.; Krok, F.; Florjańczyk, Z.; Zygadło-Monikowska, E.; Lapienis, G. Ionic Conductivity of Electrolytes Based on Star-Branched Poly(ethylene Oxide) with High Concentration of Lithium Salts. Solid State Ionics 2011, 192, 137−142. (9) Hawker, C. J.; Chu, F.; Pomery, P. J.; Hill, D. J. T. Hyperbranched Poly(ethylene Glycol)s: A New Class of IonConducting Materials. Macromolecules 1996, 29, 3831−3838. J


Article

Macromolecules Chem., Int. Ed. 2001, 40, 2004−2021; Angew. Chem. 2001, 113, 2056− 2075. (27) Binder, W. H.; Sachsenhofer, R. “Click” Chemistry in Polymer and Materials Science. Macromol. Rapid Commun. 2007, 28, 15−54. (28) Donahoe, C. D.; Cohen, T. L.; Li, W.; Nguyen, P. K.; Fortner, J. D.; Mitra, R. D.; Elbert, D. L. Ultralow Protein Adsorbing Coatings from Clickable PEG Nanogel Solutions: Benefits of Attachment under Salt-Induced Phase Separation Conditions and Comparison with PEG/Albumin Nanogel Coatings. Langmuir 2013, 29, 4128−4139. (29) Malkoch, M.; Vestberg, R.; Gupta, N.; Mespouille, L.; Dubois, P.; Mason, A. F.; Hedrick, J. L.; Liao, Q.; Frank, C. W.; Kingsburye, K.; et al. Synthesis of Well-Defined Hydrogel Networks Using Click Chemistry. Chem. Commun. 2006, 2774−2776. (30) Colliat-Dangus, G.; Obadia, M. M.; Vygodskii, Y. S.; Serghei, A.; Shaplov, A. S.; Drockenmuller, E. Unconventional Poly(ionic Liquid)s Combining Motionless Main Chain 1,2,3-Triazolium Cations and High Ionic Conductivity. Polym. Chem. 2015, 6, 4299−4308. (31) Mudraboyina, B. P.; Obadia, M. M.; Allaoua, I.; Sood, R.; Serghei, A.; Drockenmuller, E. 1,2,3-Triazolium-Based Poly(ionic Liquid)s with Enhanced Ion Conducting Properties Obtained through a Click Chemistry Polyaddition Strategy. Chem. Mater. 2014, 26, 1720−1726. (32) Obadia, M. M.; Mudraboyina, B. P.; Allaoua, I.; Haddane, A.; Montarnal, D.; Serghei, A.; Drockenmuller, E. Accelerated Solventand Catalyst-Free Synthesis of 1,2,3-Triazolium-Based Poly(Ionic Liquid)s. Macromol. Rapid Commun. 2014, 35, 794−800. (33) Obadia, M. M.; Drockenmuller, E. Poly(1,2,3-Triazolium)s: A New Class of Functional Polymer Electrolytes. Chem. Commun. 2016, 52, 2433−2450. (34) Robitaille, C. D.; Fauteux, D. Phase Diagrams and Conductivity Characterization of Some PEO -LiX Electrolytes. J. Electrochem. Soc. 1986, 133, 315−325. (35) Fullerton-Shirey, S. K.; Maranas, J. Effect of LiClO4 on the Structure and Mobility of PEO-Based Solid Polymer Electrolytes. Macromolecules 2009, 42, 2142−2156. (36) Buckley, C. P.; Kovacs, A. J. Melting Behaviour of Low Molecular Weight Poly (Ethylene-Oxide) Fractions. Prog. Colloid Polym. Sci. 1975, 58, 44−52. (37) Vonk, C. G. Synthetic Polymers in the Solid State. In Small Angle X-ray Scattering; Glatter, O., Kratky, O., Eds.; Academic Press: London, 1982; pp 433−437. (38) Strobl, G. The Physics of Polymers, 3rd ed.; Springer: Berlin, 2007. (39) Strobl, G. R.; Schneider, M. Direct Evaluation of the Electron Density Correlation Function of Partially Crystalline Polymers. J. Polym. Sci., Polym. Phys. Ed. 1980, 18, 1343−1359. (40) Vonk, C. G.; Kortleve, G. X-Ray Small-Angle Scattering of Bulk Polyethylene. Colloid Polym. Sci. 1967, 220, 19−24. (41) Rajeswaran, M.; Blanton, T. N.; Giesen, D. J.; Whitcomb, D. R.; Zumbulyadis, N.; Antalek, B. J.; Neumann, M. M.; Misture, S. T. Azine Bridged Silver Coordination Polymers: Powder X-Ray Diffraction Route to Crystal Structure Determination of Silver Benzotriazole. J. Solid State Chem. 2006, 179, 1053−1059. (42) Su, P. Y. S.; Tseng, J. C. W.; Lee, K.-M.; Wang, J.-C.; Lin, I. J. B. Tetranuclear Silver(I) Clusters Showing High Ionic Conductivity in a Bicontinuous Cubic Mesophase. Inorg. Chem. 2014, 53, 5902−5910. (43) Ammann, C.; Meier, P.; Merbach, A. E. A Simple Multinuclear NMR Thermometer. J. Magn. Reson. 1982, 46, 319−321. (44) Papon, A.; Saalwächter, K.; Schäler, K.; Guy, L.; Lequeux, F.; Montes, H. Low-Field NMR Investigations of Nanocomposites: Polymer Dynamics and Network Effects. Macromolecules 2011, 44, 913−922. (45) Meyer, H. W.; Schneider, H.; Saalwächter, K. Proton NMR Spin-Diffusion Studies of PS-PB Block Copolymers at Low Field: Two- vs Three-Phase Model and Recalibration of Spin-Diffusion Coefficients. Polym. J. 2012, 44, 748−756. (46) Kim, S. Y.; Meyer, H. W.; Saalwächter, K.; Zukoski, C. F. Polymer Dynamics in PEG-Silica Nanocomposites: Effects of Polymer

Molecular Weight, Temperature and Solvent Dilution. Macromolecules 2012, 45, 4225−4237. (47) Bärenwald, R.; Champouret, Y.; Saalwächter, K.; Schäler, K. Determination of Chain Flip Rates in Poly(ethylene) Crystallites by Solid-State Low-Field 1 H NMR for Two Different Sample Morphologies. J. Phys. Chem. B 2012, 116, 13089−13097. (48) Arlie, J. P.; Spegt, P.; Skoulios, A. E. Etude de La Cristallisation Des Polymères I. Structure Lamellaire de Polyoxyéthylènes de Faible Masse Moléculaire. Makromol. Chem. 1966, 99, 160−174. (49) Godovsky, Y. K.; Slonimsky, G. L.; Garbar, N. M. Effect of Molecular Weight on the Crystallization and Morphology of Poly(ethylene Oxide) Fractions. J. Polym. Sci., Part C: Polym. Symp. 1972, 38, 1−21. (50) Goddard, R.; Heinemann, O.; Krüger, C. Pyrrole and a CoCrystal of 1H- and 2H-1,2,3-Triazole. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1997, 53, 1846−1850. (51) Chen, E.-Q.; Xue, G.; Jin, S.; Lee, S.-W.; Mann, I.; Moon, B.-S.; Harris, F. W.; Cheng, S. Z. D. Defect Orientation on the Chain Folded Surfaces of Two-Arm Poly(ethylene Oxide) Lamellar Crystals. Macromol. Rapid Commun. 1999, 20, 431−434. (52) Monobe, K.; Yamashita, Y. Giant Single Crystals of Poly (Ethylene Oxide). Mem. Fac. Eng. 1967, 2, 84−87. (53) Takahashi, Y.; Tadokoro, H. Structural Studies of Polyethers, (-(CH2)m-O-)n. X. Crystal Structure of Poly(ethylene Oxide). Macromolecules 1973, 6, 672−675. (54) Lai, C.-Y.; Hiltner, A.; Baer, E.; Korley, L. T. J. Deformation of Confined Poly(ethylene Oxide) in Multilayer Films. ACS Appl. Mater. Interfaces 2012, 4, 2218−2227. (55) Wang, H.; Keum, J. K.; Hiltner, A.; Baer, E. Confined Crystallization of PEO in Nanolayered Films Impacting Structure and Oxygen Permeability. Macromolecules 2009, 42, 7055−7066. (56) Wang, H.; Keum, J. K.; Hiltner, A.; Baer, E. Crystallization Kinetics of Poly(ethylene Oxide) in Confined Nanolayers. Macromolecules 2010, 43, 3359−3364. (57) Liu, C.-L.; Lin, M.-C.; Chen, H.-L.; Müller, A. J. Evolution of Crystal Orientation in One-Dimensionally Confined Space Templated by Lamellae-Forming Block Copolymers. Macromolecules 2015, 48, 4451−4460. (58) Miyazaki, T.; Hoshiko, A.; Akasaka, M.; Sakai, M.; Takeda, Y.; Sakurai, S. Structure Model of a Poly(vinyl Alcohol) Film Uniaxially Stretched in Water and the Role of Crystallites on the Stress-Strain Relationship. Macromolecules 2007, 40, 8277−8284. (59) Somani, R. H.; Yang, L.; Hsiao, B. S.; Sun, T.; Pogodina, N. V.; Lustiger, A. Shear-Induced Molecular Orientation and Crystallization in Isotactic Polypropylene: Effects of the Deformation Rate and Strain. Macromolecules 2005, 38, 1244−1255. (60) Ruland, W. X-Ray Determination of Crystallinity and Diffuse Disorder Scattering. Acta Crystallogr. 1961, 14, 1180−1185. (61) Balko, J.; Lohwasser, R. H.; Sommer, M.; Thelakkat, M.; ThurnAlbrecht, T. Determination of the Crystallinity of Semicrystalline Poly(3- Hexylthiophene) by Means of Wide-Angle X-Ray Scattering. Macromolecules 2013, 46, 9642−9651. (62) Hunter, C. A.; Sanders, J. K. M. The Nature of π-π Interactions. J. Am. Chem. Soc. 1990, 112, 5525−5534. (63) Takahashi, Y.; Sumita, I.; Tadokoro, H. Structural Studies of Polyethers. IX. Planar Zigzag Modification of Poly(ethylene Oxide). J. Polym. Sci. Polym. Phys. Ed. 1973, 11, 2113−2122. (64) Lee, K.-H.; Snively, C. M.; Givens, S.; Chase, D. B.; Rabolt, J. F. Time-Dependent Transformation of an Electrospun Isotactic Poly(1Butene) Fibrous Membrane. Macromolecules 2007, 40, 2590−2595. (65) Ungar, G.; Stejny, J.; Keller, A.; Bidd, I.; Whitting, M. C. The Crystallization of Ultralong Normal Paraffins: The Onset of Chain Folding. Science 1985, 229, 386−389. (66) Wunderlich, B. Theory of Cold Crystallization of High Polymers. J. Chem. Phys. 1958, 29, 1395−1404. (67) Shen, X.; Hu, W.; Russell, T. P. Measuring the Degree of Crystallinity in Semicrystalline Regioregular Poly(3-Hexylthiophene). Macromolecules 2016, 49, 4501−4509. K


Article

Macromolecules (68) Zhu, L.; Cheng, S. Z. D.; Calhoun, B. H.; Ge, Q.; Quirk, R. P.; Thomas, E. L.; Hsiao, B. S.; Yeh, F.; Lotz, B. Crystallization Temperature-Dependent Crystal Orientations within Nanoscale Confined Lamellae of a Self-Assembled Crystalline-Amorphous Diblock Copolymer. J. Am. Chem. Soc. 2000, 122, 5957−5967. (69) De Rosa, C.; Auriemma, F. Crystals and Crystallinity in Polymers: Diffraction Analysis of Ordered and Disordered Crystals; Wiley: Hoboken, NJ, 2014. (70) De Rosa, C.; Auriemma, F.; Santillo, C.; Di Girolamo, R.; Leone, G.; Boccia, A. C.; Ricci, G. Crystal Structure of Isotactic Poly((R,S)-3-Methyl-1-Pentene). Macromolecules 2015, 48, 5251− 5266. (71) Tashiro, K.; Hu, J.; Wang, H.; Hanesaka, M.; Saiani, A. Refinement of the Crystal Structures of Forms I and II of Isotactic Polybutene-1 and a Proposal of Phase Transition Mechanism between Them. Macromolecules 2016, 49, 1392−1404. (72) Hunter, C. A. Meldola Lecture. The Role of Aromatic Interactions in Molecular Recognition. Chem. Soc. Rev. 1994, 23, 101−109. (73) Janiak, C. A Critical Account on π−π Stacking in Metal Complexes with Aromatic Nitrogen-Containing Ligands. J. Chem. Soc., Dalt. Trans. 2000, 3885−3896. (74) Pulst, M.; Balko, J.; Golitsyn, Y.; Reichert, D.; Busse, K.; Kressler, J. Proton Conductivity and Phase Transitions in 1,2,3Triazole. Phys. Chem. Chem. Phys. 2016, 18, 6153−6163. (75) Desiraju, G. R.; Gavezzotti, A. From Molecular to Crystal Structure; Polynuclear Aromatic Hydrocarbons. J. Chem. Soc., Chem. Commun. 1989, 621−623. (76) Cox, E. G. Crystal Structure of Benzene. Rev. Mod. Phys. 1958, 30, 159−162. (77) Tammann, G. Uber Eine Methode Zur Bestimmung Der Abhängigkeit Des Schmelzpunktes Einer Kristallamelle von Ihrer Dicke. Zeitschrift für Anorg. und Allg. Chemie 1920, 110, 166−168. (78) Spegt, P. A.; Terrisse, J.; Gilg, B.; Skoulios, A. Étude de La Cristallisation Des Polymeres. Makromol. Chem. 1967, 107, 29−38. (79) Buckley, C. P.; Kovacs, A. J. Melting Behaviour of Low Molecular Weight Poly (Ethylene-Oxide) Fractions I. Extended Chain Crystals. Prog. Colloid Polym. Sci. 1975, 58, 44−52. (80) Machado, J. C.; Silva, G. G.; de Oliveira, F. C.; Lavall, R. L.; Rieumont, J.; Licinio, P.; Windmö ller, D. Free-Volume and Crystallinity in Low Molecular Weight Poly(ethylene Oxide). J. Polym. Sci., Part B: Polym. Phys. 2007, 45, 2400−2409. (81) Song, K.; Krimm, S. Raman Longitudinal Acoustic Mode (LAM) Studies of Folded-Chain Morphology in Poly(ethy1ene Oxide) (PEO). 3. Chain Folding in PEO as a Function of Molecular Weight. Macromolecules 1990, 23, 1946−1957. (82) Bondi, A. Van Der Waals Volumes and Radii. J. Phys. Chem. 1964, 68, 441−451. (83) Mourey, T. H.; Hoteling, A. J.; Balke, S. T.; Owens, K. G. Molar Mass Distributions of Polymers from Size Exclusion Chromatography and Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry: Methods for Comparison. J. Appl. Polym. Sci. 2005, 97, 627−639. (84) Schnöll-Bitai, I.; Hrebicek, T.; Rizzi, A. Towards a Quantitative Interpretation of Polymer Distributions from MALDI-TOF Spectra. Macromol. Chem. Phys. 2007, 208, 485−495. (85) Pasch, H.; Malik, M. I. Crystallization-Based Fractionation Techniques. In Advanced Separation Techniques for Polyolefins; Springer: Heidelberg, 2014; pp 11−73. (86) Thomann, Y.; Sernetz, F. G.; Thomann, R.; Kressler, J.; Mülhaupt, R. Temperature Rising Elution Fractionation of a Random Ethene/styrene Copolymer. Macromol. Chem. Phys. 1997, 198, 739− 748. (87) Tuijnman, C. A. . The Dielectric Behaviour of Oxidized HighPressure Polyethylene I. Polymer 1963, 4, 259−268. (88) Mansfield, M.; Boyd, R. H. Molecular Motions, the α Relaxation, and Chain Transport in Polyethylene Crystals. J. Polym. Sci., Polym. Phys. Ed. 1978, 16, 1227−1252.

(89) Luo, Y.; Montarnal, D.; Treat, N. J.; Hustad, P. D.; Christianson, M. D.; Kramer, E. J.; Fredrickson, G. H.; Hawker, C. J. Enhanced Block Copolymer Phase Separation Using Click Chemistry and Ionic Junctions. ACS Macro Lett. 2015, 4, 1332−1336.

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