Microphase Separation and Crystallization in H-Bonding End

Article pubs.acs.org/Macromolecules

Microphase Separation and Crystallization in H‑Bonding EndFunctionalized Polyethylenes Ian German,† Franck D’Agosto,*,† Christophe Boisson,*,† Sylvie Tencé-Girault,‡ and Corinne Soulié-Ziakovic*,‡ †

CPE Lyon, CNRS UMR 5265, Laboratoire de Chimie Catalyse Polymères et Procédés (C2P2), Equipe LCPP Bat 308F, Université de Lyon 1, 43 Bd du 11 Novembre 1918, F-69616 Villeurbanne, France ‡ Matière Molle et Chimie, UMR 7167 CNRS-ESPCI, Ecole Supérieure de Physique et Chimie Industrielles de la Ville de Paris, 10 rue Vauquelin, 75005 Paris, France S Supporting Information *

ABSTRACT: Well-defined, crystalline, low molar mass polyethylene PEx (where x is the molar mass 1300 and 2200 g mol−1) bearing thymine (Thy) or 2,6-diaminotriazine (DAT) end groups have been synthesized from amino-terminated PE. Either double-layer or monolayer solid-state morphologies were attained depending on the nature of the end-group(s). PE1300-NH2, PE1300-DAT, and the equimolar blend PE1300Thy/DAT-PE1300 all organized into double-layer structures composed of extended PE chains sandwiched between Hbonding chain-ends. The double-layered morphology arose from the microphase separation of the polar end-groups and the nonpolar PE chains and was frozen by the crystallization of the PE domains. The regularity of the PE lamellar stacking was higher for the stronger and more directional associated pair Thy/DAT compared with samples of either PE-NH2 or PE-DAT. For PE1300-Thy, the mesoscopic organization was driven by the crystallization of Thy domains prior to crystallization of the PE chains, forcing the small proportion of nonfunctionalized PE chains to segregate and crystallize separately to the PE-Thy chains. The confinement of PE chains between Thy domains lead to a conventional monolayer form in which extended PE chains were interdigitated. The volume fraction of Thy or DAT end-groups was a key parameter in the organization in all these systems: the PE crystallinity was higher with longer PE chains (i.e., a low volume fraction of Thy or DAT units), but the mesoscopic organization of the supramolecular PE was less regular.

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INTRODUCTION Polyethylenes (PE) display unique mechanical, optical, and chemical properties that have positioned them as the most heavily produced industrial polymers. For the majority of PE’s commodity uses, relatively high and broadly distributed molar masses with no extra functionality are appropriate. The introduction of chain-end groups brings new properties and functionality to conventionally apolar, inert PE, exposing potential routes to high molar mass polymers and to block copolymers. For more specialty applications such as imaging, inks, coatings, personal care, oils, and plastic additives,1 however, PE displaying low and narrowly distributed molar masses and added functionalities are highly desirable. Provided that crystallization is still possible in the PE domains, the end-functionalized PE retains the desirable properties of PE while presenting exciting possibilities for additional functionality including supramolecular ordering and stimuli-responsiveness. Supramolecular chemistry is a powerful tool for the creation of stimuli-responsive functional materials.2−7 The incorporation of functional groups capable of reversibly forming noncovalent bonds can impart useful and potentially tunable properties in © XXXX American Chemical Society

self-assembled polymeric structures. For example, end-functionalized homopolymers bearing functional groups capable of noncovalent assembly, e.g. by hydrogen bonding, have been shown to form such self-assembled high order structures.8−18 To encourage more selective interactions, for instance in the design of block copolymers in which the two blocks are linked by a reversible bond, complementary host and guest interaction sites may be introduced on respective components to then induce supramolecular assembly. In order to attain tunable material properties in the self-assemblies, the host/guest interaction must be sufficiently strong for the associated state to predominate, while weak enough (i.e., weaker than that of a covalent bond) to be stimuli-responsive (i.e., to dissociate under specific conditions). The goal of the present work is to produce novel endfunctionalized PEs capable of supramolecular interactions. It was envisioned that such systems could behave like high molar mass PE at low temperatures but show behavior typical of low Received: November 14, 2014 Revised: March 31, 2015

A

DOI: 10.1021/ma502304k Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Figure 1. Supramolecular end-functionalized PEs through (a) Thy/Thy self-association, (b) DAT/DAT self-association, and (c) Thy/DAT complementary association.

exhibiting the crystallinity (melting temperature above 110 °C) and entanglement (critical entanglement of linear crystalline PE is around 1000 g mol−1) of true PE despite their short chain length.51 We investigate how the volume fraction and the association mode of the polar end-groups (directional or not, crystalline or not) influence the mesoscopic organization.

molecular mass PE at elevated temperatures (i.e., temperature above which the end-group interactions cease). The highly desirable properties of high molecular mass PE would thereby be retained while introducing an enormous processability advantage. Furthermore, adding a competitor of the reversible interaction (e.g., a solvent) would help to recycle the materials. Few articles report the introduction of hydrogen bonding sites into polyolefins, and those which do typically feature noncrystalline materials,19−23 or semicrystalline materials attained when weakly associative units (carboxylic acids,24 primary amines,25 phosphonic acid26) are precisely introduced and sufficiently spaced along the polymer backbone. We propose in this work the utilization of the complementary trivalent hydrogen-bonding system based on 2,6diamino-1,3,5-triazine (DAT) and thymine (Thy). While weaker than systems which form four (or more) hydrogen bonds per pair, such as ureidopyridinone used by Meijer et al. to prepare supramolecular polymers assemblies27 and multiblock copolymers,19,21,28 the DAT/Thy pair has proven highly suitable for inducing order in functionalized polymers. Notably, Binder et al. stabilized microphase separations of poly(isobutene) (PIB) with poly(ether ketone) (PEK)29 and PIB with poly(caprolactone) (PCL)30 via directional and selective Thy/DAT interaction. A recent study has illustrated that directional interactions of Thy and DAT groups influence the overall structure less than the crystallization of thymines.31 In this paper, we present well-defined crystalline PE chains with Thy or DAT end groups (PE-Thy and PE-DAT, respectively), synthesized by simple chemical steps and able to form supramolecular associations: Thy/Thy, DAT/DAT, or Thy/DAT (Figure 1). The aim of these syntheses was the targeted incorporation of end groups into crystalline PE that can facilitate supramolecular assembly and effectively adjust the polymer’s physical properties. Many existing strategies for the end-functionalization of polyolefins such as conventional coordination−insertion polymerization,32−35 anionic polymerization of butadiene, or ring-opening metathesis polymerization of cycloolefins using chain transfer agents36−38 suffer from excessive reaction sequences or costly reagents, limiting their potential industrial applicability. We instead exploited a recently developed protocol for the in situ end-functionalization of low-mass (Mn = 1000−5000 g mol−1) PE following catalyzed chain growth (CCG) polymerization of ethylene39−43 to access linear, reactive PE-X with very high or quantitative incorporation of reactive X end-groups.44 These functional PE-X units may then be employed as building blocks in further reactions,45−50 while

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EXPERIMENTAL SECTION

Materials. Toluene (SDS) and N,N-dimethylformamide (DMF; Sigma-Aldrich) were used as purchased. Tetrachloroethylene (SigmaAldrich) was purified by distillation and dried over 3 Å molecular sieves. Deuterated benzene (Eurisotop) was dried over 3 Å molecular sieves. 2-Chloro-4,6-diaminotriazine (DAT-Cl), thymine-1-acetic acid, O-(benzotriazol-1-yl)-N,N,N′,N′-tetramethyluronium tetrafluoroborate (TBTU), and triethylamine were all purchased from SigmaAldrich and used as received. Amine end-functionalized PEs PEx-NH2 (Mn = 1300 and 2200 g mol−1; Đ = 1.3 and 1.4; functionality 93% and 84%) were obtained according to a previously published procedure.49,50 Analytical Techniques. High-resolution liquid NMR spectroscopy was carried out with a Bruker DRX 400 spectrometer operating at 400 MHz for the 1H nucleus and 100 MHz for 13C. Spectra were recorded at 363 K using a 5 mm QNP probe for 1H NMR and a PSEX 10 mm probe for 13C and 2-dimensional NMR. Polymer samples were examined as 5−15% (w/v) solutions. A mixture of tetrachloroethylene (TCE) and deuterated benzene (C6D6) (2/1 v/v) was used as solvent. Chemical shift values (δ) are given in ppm with reference internally to tetramethylsilane (TMS). Fourier transform inf rared (FTIR) analyses were carried out in a Nicolet 460 FT-IR spectrometer at room temperature over 32 scans (range 650−4000 cm−1). MALDI-TOF mass spectrometry analyses were recorded using a Voyager DE-STR (ABSciex) in linear and reflection modes equipped with a nitrogen laser emitting at 337 nm. The ions were accelerated under a potential of 20 kV. The positive ions were detected in all cases. The spectra were the sum of 300 shots, and an external mass calibration was used (mixture of peptides standards, Sequazyme kit). A PE solution was prepared by dissolving the polymer in toluene at 1 g L−1 at 70 °C. Then, samples were prepared by mixing 1 μL of the matrix 2,4,6-trihydroxyacetophenone (THAP, Sigma, at 10 g L−1 in toluene/ THF (1/1, v/v)) with 1 μL of the PE solution and 1 μL of an AgTFA solution (Sigma-Aldrich, at 1 g/L in THF). The addition of metal ions such as AgTFA is required to enhance the cationization of PE samples for analysis by MALDI-TOF mass spectrometry. Resulting mixtures (1 μL) were spotted on the MALDI sample plate and air-dried. Dif ferential scanning calorimetry (DSC) experiments were performed under helium on a TA Q1000 instrument. All samples were melted at 200 °C for 2 min to erase all thermal history, then cooled to 0 °C, and heated to 200 °C at a rate of 10 °C/min. The degree of crystallinity Xc(DSC) was calculated by the equation B


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Macromolecules Scheme 1. (i) DAT and (ii) Thy End-Functionalization of PE Chains

Xc(DSC) =

ΔHm ϕ × ΔHm100%

mixture was heated to 110 °C, and triethylamine (10 equiv) was added. The flask was fitted with a reflux condenser, and the colorless solution was stirred under a gentle argon flow at 110 °C for 24 h. The reaction mixture was then cooled and diluted with methanol (300 mL), and the resulting suspension was filtered. The solids collected were then washed with methanol (100 mL). The resulting colorless solid was added to toluene (250 mL), and the suspension was heated to 90 °C and then filtered at 90 °C. The filtrate collected was cooled, diluted with methanol (300 mL), and filtered. The solid collected was then washed thrice with methanol (3 × 100 mL) and dried to a colorless powder. For PE1300-DAT: yield = 88%. Functionality (PE1300DAT as mole fraction of total polymer; estimated from 1H NMR spectrum) = 0.85. 1H NMR (400 MHz, 90 °C, TCE/C6D6 2/1 v/v, ppm) δ: 4.59 (1H, br s), 4.36 (4H, br s), 3.23 (2H, q, J = 6.4 Hz), 1.25 (200H, br m), 0.84 (3H, t, J = 6.8 Hz). 13C NMR (100.6 MHz, 90 °C, TCE/C6D6 2/1 v/v, ppm) δ: 168.37, 167.93, 41.14, 32.45, 30.25, 29.85, 27.51, 23.13, 14.26. Elemental analysis: Predicted (85% PE1300DAT, 10% PE1300-NH2, 5% PE1300): C 81.25, H 13.55, N 5.16%. Obtained: C 80.64, H 13.42, N 4.54%. Synthesis of PEx-Thy (PEx; x = 1300 and 2200 g mol−1). In a round-bottomed flask, PEx-NH2 (1 equiv), thymine-1-acetic acid (2 equiv), and O-(benzotriazol-1-yl)-N,N,N′,N′-tetramethyluronium tetrafluoroborate (5 equiv) were suspended in a mixture of toluene (250 mL) and N,N-dimethylformamide (125 mL). The reaction mixture was heated to 110 °C, and triethylamine (10 equiv) was added, triggering a color change in the solution from colorless to yellow/ brown. The flask was fitted with a reflux condenser, and the pale brown solution was stirred under a gentle argon flow at 110 °C for 24 h. The reaction mixture was then cooled and diluted with methanol (300 mL), and the resulting suspension was filtered. The solids collected were then washed thrice with methanol (3 × 100 mL) and dried to a pale brown powder. For PE1300-Thy: yield = 85%. Functionality (PE1300-Thy as mole fraction of total polymer; estimated from 1H NMR spectrum) = 0.80. 1H NMR (400 MHz, 90 °C, TCE/ C6D6 2/1 v/v, ppm) δ: 8.78 (1H, br s), 6.74 (1H, s), 6.27 (1H, br s), 3.94 (2H, s), 3.09 (2H, q, J = 6.5 Hz), 1.70 (3H, s), 1.25 (200H, br m), 0.84 (3H, t, J = 6.8 Hz).13C NMR (100.6 MHz, 90 °C, TCE/C6D6 2/1 v/v, ppm) δ: 166.19, 163.37, 151.57, 140.45, 111.12, 51.30, 40.17, 32.46, 30.25, 29.84, 27.44, 23.14, 14.27, 12.18. Elemental analysis: Predicted (80% PE1300-Thy, 15% PE1300-NH2, 5% PE1300): C 81.57, H 13.32, N 2.46%. Obtained: C 80.39, H 13.23, N 2.52%. PE1300-Thy/DAT-PE1300 Blend. A mixture of PE1300-DAT (0.12 g, 72 μmol) and PE1300-Thy (0.13 g, 72 μmol) was suspended in toluene (5 mL). The suspension was heated to 90 °C to dissolve all polymers, and the resulting solution was left under a saturated toluene atmosphere at 75 °C until all toluene evaporated. The blend was annealed under vacuum at 100 °C for 3 h. 1H NMR (400 MHz, 90 °C, TCE/C6D6 2/ 1 v/v, ppm) δ: 12.93 (1H, br s), 6.73 (1H, s), 6.20 (1H, br s), 5.86 (1H, br s), 5.42 (4H, br s), 3.96 (2H, s), 3.27 (2H, q, J = 6.4 Hz), 3.08 (2H, q, J = 6.5 Hz), 1.71 (3H, s), 1.25 (400H, br m), 0.84 (3H, t, J = 6.8 Hz). 13C NMR (100.6 MHz, 90 °C, TCE/C6D6 2/1 v/v, ppm) δ: 167.63, 166.93, 166.50, 166.12, 153.36, 141.39, 111.46, 51.14, 41.29, 40.29, 32.49, 30.28, 29.89, 27.70, 27.60, 14.35, 12.31.

× 100%

Here, ΔHm and ΔH100% are respectively the measured melting m enthalpy and the calculated melting enthalpy for the 100% crystalline PE. ϕ is the weight fraction of PE in the sample, taking into account the DAT and Thy weight fraction. Polarized optical microscopy (POM) was performed in transmission using a Leica Leitz DM RD light microscope. Scattering Techniques. X-ray scattering experiments were performed in transmission mode by using Cu Kα radiation (λ = 1.54 Å) from an X-ray generator (XRG3D Inel) operating at 40 kV and 25 mA. Experiments were performed at room temperature on samples, of thickness around 1.5 mm, slowly cooled from the melt in a polytetrafluoroethylene (PTFE) holder. WAXS patterns were collected with a curve position sensitive detector (CPS120 Inel) and fitted using the Fityk 0.9.8 software52 for crystal phase identification and crystalline ratio measurements. The degree of crystallinity Xc(WAXS) of samples was calculated using the equation

Xc(WAXS) =

Ac × 100% ϕ(Ac × A a )

in which Ac and Aa are the areas under fitted crystalline and amorphous halo, respectively. ϕ is the weight fraction of PE in the sample, taking into account the DAT and Thy weight fraction. The crystallite size (D) was estimated using the Scherrer formula:53 D=

0.9λ Δ(2θ) cos θ

where λ is the Cu Kα radiation wavelength, 2θ is the scattering angle, and Δ(2θ) is the peak width at the half-maximum. WAXS patterns at high temperature were also acquired on the PE1300-Thy film between two mica sheets enclosed in a Linkam heating stage HFS600. SAXS curves were acquired with a linear detector (LPS50 Inel). Two configurations were used to explore a larger range of scattering angles differing only by the sample-to-detector distance, which were chosen at 70.5 and 40.5 cm. The q range covered by these two configurations encompassed 0.015−0.35 Å−1. Standard data corrections were applied for both SAXS and WAXS measurements, normalized by transmission, thickness, and intensity of the incident beam and acquisition duration (50 000 s for SAXS and 3000 s for WAXS), and then corrected for background scattering. The SAXS scattering profiles were plotted as a function of the scattering vector q defined as

q=

4π sin θ 2π = λ d

where d is the Bragg distance. Synthesis. Synthesis of PEx-DAT (PEx; x = 1300 and 2200 g mol−1). In a round-bottomed flask, PEx-NH2 (1 equiv) and 2-chloro4,6-diaminotriazine (2 equiv) were suspended in a mixture of toluene (250 mL) and N,N-dimethylformamide (125 mL). The reaction C


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Figure 2. 1H NMR spectra (400 MHz, 2:1 TCE:C6D6, 90 °C) of (A) PE1300-DAT, (B) PE1300-Thy, and (C) PE1300-Thy/DAT-PE1300 (∗: signal of residual C6D5H in NMR solvent).

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RESULTS AND DISCUSSION Synthesis and Chemical Characterization of PE EndFunctionalized with H-Bonding Units Thy (PEx-Thy) and DAT (PEx-DAT). Crystalline, low molar mass, amino endfunctionalized PEs (PEx-NH2, where x corresponds to the molar mass of the chains = 1300 or 2200 g mol−1) were prepared via our previously reported three-step procedure involving ethylene polymerization in the presence of a dialkylmagnesium chain-transfer agent, followed by chain-end functionalization to give iodo and azido intermediates which were then converted to amines.50 To attach DAT and Thy moieties, the nucleophilic amino chain-end of PEx-NH2 was exploited in a similar strategy to that which we previously reported for noncrystalline polymers.31,54 In adapting this synthetic pathway, we found a way to overcome the processing and solubility restrictions imposed by PE which have limited the prevalence of literature on reactions at PE chain-ends. PE end-functionalized with DAT was obtained by the nucleophilic aromatic substitution of 4-chloro-DAT by PExNH2 (Scheme 1i).55 PEx-Thy were obtained via the catalyzed amidation of thymine-1-acetic acid by PEx-NH2 (Scheme 1ii).56 In both cases, synthetic conditions were adapted to the PE-X requirements using a mixture of toluene and N,N-dimethylformamide (DMF) at 110 °C, with the hot toluene ensuring dissolution of the PE chains and DMF facilitating solubility of DAT-Cl and thymine-1-acetic acid. PEx-DAT and PEx-Thy were characterized by NMR and infrared spectroscopy and MALDI-TOF mass spectrometry (see Figures S1−S6 in Supporting Information for PE1300-X series). From the 1H NMR spectra of PE1300-DAT and PE1300-Thy, the integral ratio of the PEx methyl chain-end peak (δ = 0.84 ppm) and the α-methylene chain-end peak CH2NH−X (δ = 3.23 ppm for X = DAT and δ = 3.09 ppm for X = COCH2Thy) gave DAT and Thy grafting efficiencies of 0.85 and 0.80, respectively (Figure 2A,B). FTIR analysis confirmed the presence of amide functionalities (νCO 1750−1600 cm−1) in PEx-Thy (Supporting Information, Figure S6), and MALDITOF confirmed that the molar masses of the dominant peak

pattern correspond to chains with the desired end groups (see Supporting Information for PE1300-X series). After successfully functionalizing the PE with complementary H-bonding moieties in high yield, the selective interaction of the complementary chain ends was validated. The equimolar blend PE1300-Thy/DAT-PE1300 obtained by solvent casting was analyzed by NMR spectroscopy (see Figure 2C for 1H NMR spectrum) and FTIR (Supporting Information, Figure S8). Although Thy and DAT are both capable of H-bonding with themselves, Thy/DAT complementary association is known to be much stronger than either Thy/Thy or DAT/DAT selfassociation, as revealed by the different thermodynamic binding constants: KThy‑DAT = 890 M−1 versus KDAT‑DAT = 2.2 M−1 and KThy‑Thy = 4.3 M−1 (determined by 1H NMR spectroscopy in CDCl3).57 In our system, confirmation that this complementary pairing predominated in the 1:1 blend was provided by 1H NMR spectroscopy, which demonstrated dramatic shifts in the end-group proton resonances (compared with those of the pure end-functionalized polymers). Specifically, the thymine imino proton NH (w in Figure 2B,C) was shifted downfield from 8.78 to 12.93 ppm, and the DAT amino protons NH2 (a in Figure 2A,C) were shifted downfield from 4.36 to 5.42 ppm. In the 13 C NMR spectrum (Supporting Information, Figure S7), resonances of the annular thymine carbonyls of PE1300-Thy/ DAT-PE1300 were shifted downfield from 163.37 and 151.57 ppm to 166.11 and 153.40 ppm, respectively, providing further proof of strong interaction. Structural Characterization. With the solution NMR having confirmed that complementary H-bonding occurs between the Thy/DAT end-groups, detailed analysis of this blend as well as PEx-Thy and PEx-DAT alone was conducted on samples slowly crystallized from the melt. Analyses were performed by DSC, POM, WAXS, and SAXS to determine the degree and nature of the structural organization. To assess the influence of functional units on the crystal structure and morphology of the PE chains, amino end-functionalized PExNH2 was also analyzed for comparison. Structural Characterization of the Amino End-Functionalized PEx-NH2. The thermogram resulting from DSC of D


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Figure 3. Structural characterization of PE1300-NH2: (A) DSC experiment and (B) WAXS and SAXS (inset) patterns (∗: signal of PTFE holder).

nonfunctionalized PE of similar molar masses.58 The supercooling is the temperature difference between melting (on heating) and crystallization (on cooling). It can easily be deduced from the Tm and Tc values reported in Table 1 and illustrates how easily or not the organization (crystal, mesophase) can form from the melt. These results indicate that amino end-groups do not perturb the crystallization of PE chains. Furthermore, even though the amino end-groups are Hbonded (broad absorption at 3450 cm−1 in the FTIR spectrum, Supporting Information Figure S1) and should lead to a double layered crystalline PEx-NH2 (as observed for long chain fatty acids63), the Tm data do not suggest this supramolecular assembly since, in this were case, the PE chains length would be double and Tm would be higher (calculated Tm(184) = 127 °C for a double-layered PE1300-NH2). Thus, PE-NH2 samples rather consist of extended-chain crystals, as found for nparaffins and short PE chains.58 Using the calculated heat of fusion ΔH100% m (n) (Table 2) of 100% crystalline PE chains with specific molar mass,60,62 degrees of crystallinity of 68% and 89% were calculated for PE1300-NH2 and PE2200-NH2, respectively, from DSC results (cf. 83% and 89% for nonfunctionalized PE of similar molar masses).58 These results indicate that polymer crystallinity is highly sensitive to the chain length in endfunctionalized polymers, since the shorter PE1300-NH2 was considerably less crystalline than its nonfunctionalized literature analogue, while there was no difference observed for PE2200NH2. The diminished crystallinity in the shorter PE1300-NH2 was also verified by WAXS analysis. The WAXS diffraction pattern (Figure 3B) indicates a crystallinity of 76% and exhibits two intense Bragg peaks at 2θ = 21.5° (d110 = 4.12 Å) and at 23.8° (d200 = 3.73 Å), corresponding to the (110) and (200) reflections in the orthorhombic lattice of PE (a = 7.42 Å, b = 4.95 Å, and c = 2.55 Å).64 The analysis of the Bragg peak widths using the Scherrer formula shows that the extent of the crystalline domains perpendicular to the chains is rather large: D110 ∼ 280 Å and D200 ∼ 230 Å. In the SAXS pattern (Figure 3B, inset), a very small peak is observed, indicating the existence of the characteristic distance, LNH2, of around 140 Å, the same order of magnitude of the calculated elongated chain length (∼119 Å in Table 2). It should be noted that no direct comparison can be drawn between crystallinity estimated by DSC (by comparing the melting enthalpy with that of a 100% crystalline PE) and the crystallinity determined by WAXS since the latter is performed at room temperature without calibration. The crystalline organization of PE1300-NH2 is also revealed by a small-scale birefringent texture observed by polarized optical microscopy (POM, Supporting Information Figure S9). This

PE1300-NH2 is presented in Figure 3A, while experimental melting temperatures and enthalpies of PEx-NH2 at each molar mass are compared in Table 1. Table 1. Experimental Crystallization (Tc), Melting Temperature (Tm), Melting Enthalpy (ΔHm), and Crystallinity (Xc) of End-Functionalized PEs Alone and of the PE1300-Thy/DAT-PE1300 Equimolar Blend PEx-X PE1300-NH2 PE1300-DAT PE1300-Thy

PE1300-Thy/DATPE1300 PE2200-NH2 PE2200-DAT PE2200-Thy

Tc (°C)

Tm (°C)

ΔHm (J g−1)

Xc(DSC) (%)

Xc(WAXS) (%)

111.0 109.8 100.0 108.0 172.9 106.8

118.4 118.5 107.0 118.0 179.0 118

203 184 130 27 8 192

68 67 50 10

76 63 58

72

67

114.3 112.3 111.7 143.3

124.3 122.2 122.4 153.3

267 238 210 5

89 85 77

63

As expected, a greater melting temperature was observed for higher molar mass PE chains (124 °C for PE2200-NH2 compared with 118 °C for PE1300-NH2), with values consistent with those found in the literature for nonfunctionalized PE of comparable molar masses (Tm = 110.5 and 122.5 °C for molar masses of 1150 and 2150 g mol−1, respectively).58 Molar masses of PE chains, estimated from SEC on PExNH2, allowed the calculation of their lengths that could be found lower than the upper critical limit of 370 Å for chain folding.59 Consequently, PE chains could be considered as completely extended and Broadhurst’s equation,60,61 which relates the equilibrium melting temperature Tm(n) to the number n of methylene units in the PE chain could be applied (Table 2). We obtained calculated equilibrium melting temperatures Tm(n) close to the experimental values (Table 1). The supercoolings of PE1300-NH2 and PE2200-NH2 were 7 and 10 °C, respectively, compared to 2 and 6 °C for Table 2. Average Number of Methylene Groups (n) and Calculated Values for Stretched Chain Length (lc), for Melting Temperature (Tm(n)), and for Enthalpy (ΔHm100%(n))60−62 Mn of PE (g mol−1)

n

lc (Å)

Tm(n) (°C)

−1 ΔH100% m (n) (J g )

1300 2200

92 156

119 200

113.5 124.5

299.8 298.4 E


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Figure 4. DSC thermograms of (1) PE1300-Thy, (2) PE1300-DAT, and (3) PE1300-Thy/DAT-PE1300 blend: (A) first cooling from 200 °C; (B) second heating.

Figure 5. WAXS and SAXS (inset) patterns of (A) PE1300-DAT, (B) PE1300-Thy, (C) PE1300-Thy/DAT-PE1300 blend, and (D) PE2200-Thy (∗: signal of PTFE holder).

180 Å) and reduced crystallinity (63%) compared to PE1300NH2 (76%). SAXS experiments revealed a well-defined peak corresponding to a characteristic distance, LDAT, of 190 Å (Figure 5A, inset), which is larger than the value for PE1300NH2 (140 Å, Figure 3B). The intensity of the SAXS signal of PE1300-DAT is higher than the one of PE1300-NH2. This is explained by the higher electronic contrast between the DAT end-group and the PE chain compared with the contrast between the NH2 and PE chain of PE1300-NH2. Since the decrease in crystallinity of PE1300-DAT (compared with PE1300-NH2) was only minor while the increase in the characteristic distance was significant, we can conclude that DAT units only slightly disturb the crystallization between the PE chains (perpendicular to the chain direction) but significantly alter the organization along the PE chain (parallel to the chain direction).

birefringent texture vanishes above the melting temperature (120 °C). Structural Characterization of PE1300-DAT, PE1300-Thy, and Their Equimolar Blend. Thermograms of PE1300-DAT, PE1300Thy, and their equimolar blend are compared in Figure 4, and WAXS and DSC features for all species are summarized in Table 1. Comparison of the Tm values for PEx-DAT and PExNH2 at both molar masses shows that the presence of the DAT chain end has no effect on the melting temperature, indicating no significant supramolecular assembly of the DAT homologues. In contrast, the crystallization temperature and the degree of crystallinity are slightly decreased at each molar mass (Table 1 and Figure 4), showing that DAT end-groups disrupt the PE crystallization to a small extent. The WAXS diffraction pattern of PE1300-DAT (Figure 5A) reveals the same Bragg peaks (d110 = 4.09 Å and d200 = 3.73 Å) as PE1300-NH2, with slightly smaller crystalline domains (D110 = 250 Å and D200 = F


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is estimated at 108 Å, and only the higher orders, 3q, 4q, 5q, and 6q, are observed on the WAXS pattern. On the broad amorphous signal, very small peaks (marked with + in Figure 6) appear at ∼11, ∼ 4.6, and ∼4.0 Å and could be attributed to the crystalline Thy planes.31 This high-temperature pattern shows that a lamellar order with a period of 108 Å exists above the melting of the PE chains and is driven by the Thy crystallization. The DSC thermogram of the equimolar blend PE1300-Thy/ DAT-PE1300 features a single endothermic peak at 118 °C (Figure 4) which is characteristic of PE1300 melting and was also observed for PE1300-NH2 and PE1300-DAT. The crystallinity of 72% calculated by DSC (Table 1) is higher than the crystallinities observed for the components, PE1300-DAT and PE1300-Thy (and closely matches the 67% obtained by WAXS analysis). The absence of additional endotherms indicates that PE1300-Thy does not self-assemble into crystalline Thy microdomains in the blend. As observed in solution by 1H and 13C NMR spectroscopy (Figure 2 and Supporting Information Figure S7), Thy/DAT associations are strongly preferred over those of DAT/DAT and Thy/Thy, preventing the formation of crystallizable Thy/Thy domains. POM images have features consistent with the DSC results: a homogeneous isotropic phase is observed at temperatures above 118 °C, whereas birefringence is observed at lower temperatures (Supporting Information, Figure S9). The WAXS profile of PE1300-Thy/DAT-PE1300 (Figure 5C) features the two intense Bragg peaks characteristic of PE (4.10 Å (110) and 3.73 Å (200)). The lateral extensions of the crystalline domains, D110 = 210 Å and D200 = 140 Å, are intermediate between those of PE1300-DAT (250 and 180 Å) and those of PE1300-Thy (170 and 100 Å). The SAXS profile shows an intense long-range ordering reflection at q ≈ 0.031 Å−1, with further low-intensity reflections corresponding to 66 and 42 Å (Figure 5C, inset). These three peaks, corresponding respectively to q, 3q, and 5q, are consistent with a lamellar order period, LThy‑DAT, of approximately 200 Å. Volume Fraction of Supramolecular Bonding Units. The longer PE2200-X polymers were analyzed in a similar fashion to the PE1300-X series to determine the effect of chain length (which can also be expressed as end-group volume fraction) on the crystallization behavior. The crystallinity of PE, estimated from DSC experiments, was significantly higher for the PE2200 series than for PE1300 series (Table 1) and tended to be close to the literature value of 89% for nonfunctionalized PE of similar mass.58 These results show that the crystallization of the PE chains depends on the volume fraction of the polar H-bonding chain end-groups (−NH2, −Thy, −DAT): in the longer polymers the end-group volume fraction was sufficiently low as to not affect the degree of crystallinity, whereas in the shorter polymers crystallinity was suppressed to some extent. The influence of Thy moieties on PE crystallization was studied with additional WAXS and SAXS experiments. The WAXS diffraction pattern of PE2200-Thy (Figure 5D) shows the same Bragg peaks at d110 = 4.12 Å and d200 = 3.72 Å as in PE1300-Thy, but with higher crystalline domains extensions (D110 = 220 vs 170 Å and D200 = 165 vs 100 Å) and crystallinity (63% vs 58%). In the SAXS spectrum (inset of Figure 5D) a single peak is evident, associated with a characteristic distance L2200 Thy ∼ 168 Å, indicating less regular mesoscopic organization than in the short analogue. Further evidence that the mesoscopic organization is less regular is provided by the

The crystallinity of PE is perturbed to a significantly greater extent by grafting a Thy end-group, as shown by the multiple endotherms present in both the cooling and heating thermograms for PE1300-Thy (Figure 4, A and B, respectively). In the heating thermogram (Figure 4B) two peaks are evident in the PE melting region of PE1300-Thy, between 95 and 125 °C. The lower enthalpy peak is close to the temperature measured for the PE1300-DAT and PE1300-NH2 (Tm2 = 118 °C) and is attributed to melting of non-thymine-functionalized PE chains (15 mol % as determined by 1H NMR). The more significant endotherm occurs at a lower temperature (Tm1 = 107 °C in Figure 4B) and corresponds to the melting temperature of thymine-functionalized PE chains (85 mol % by 1H NMR). The PE crystallinities estimated from these two endotherms are 10% and 50%, respectively, for the non-thymine-functionalized PE chains (Tm2 = 118 °C) and the PE functionalized with Thy groups (Tm1 = 107 °C). A third endotherm is also present at high temperature (179 °C) with a small melting enthalpy. As thymine derivatives have been shown to readily crystallize31 (in contrast to glass-forming DAT derivatives65,66), this feature can be attributed to the melting of crystallized Thy units. WAXS analysis (Figure 5B) confirms the relatively low crystallinity of PE1300-Thy (58%). The two Bragg peaks corresponding to PE (4.11 Å (110) and 3.73 Å (200)) are observed but with significant broadening, leading to smaller correlation lengths (D110 = 170 Å and D200 = 100 Å) compared to PE1300-DAT (and PE1300-NH2). The SAXS profile (Figure 5B, inset) shows long-range ordering reflections with q ratios as 1:2:3, and higher ordered scattering peaks are found in the lowangle region of the WAXS pattern. Peaks corresponding to 91 Å (q), 45 Å (2q), 30 Å (3q), 23 Å (4q), and 19 Å (5q) indicate a lamellar order with period, LThy, of 91 Å. The intensity of the first peak is low compared to the others. Compared to the patterns of PE1300-DAT and PE1300-NH2 (Figures 5A and 3B, respectively) showing a single peak, the PE1300-Thy pattern is notable for the high number of diffraction orders, typical of a well-defined lamellar structure (Figure 5B). This WAXS pattern was acquired at room temperature, i.e., below all the melting temperatures. A WAXS acquisition (Figure 6) was performed at 130 °Ca temperature above the

Figure 6. WAXS pattern of PE1300-Thy acquired at 130 °C.

two melting temperatures of the PE and below the melting point of the Thy units. On this WAXS pattern, we observed small well-defined peaks in the low-angle region at 36, 27, 22, and 18 Å and a broad peak around 2θ ≈ 19° (d ≈ 4.6 Å). While the broad peak is attributed to the PE phase, amorphous at this temperature, the small sharp peaks are interpreted as the highorder Bragg diffractions of a lamellar phase. The lamellar period G


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Figure 7. Proposed model of a long-range lamellar order of PEx-Thy comprising of interdigitated, extended-chain crystalline PE (PEc) and amorphous PE (PEa) layers confined between crystalline Thy layers (Thy/Thy). For the sake of clarity, the drawing does not take into account the intrinsic crystalline and amorphous distribution of PE chains.

Thy planes as observed for confined PA domains in polyamide−polyethylene copolymers.71 The resulting confined PE domains had half the lateral extension of PE1300-DAT or PE1300-NH2, leading to lower melting temperature and crystallinity. For PE2200-Thy, the concentration (volume fraction) of Thy end-groups was too small to allow their rigid crystalline planes to efficiently confine the PE chains and thereby expel PE2200-NH2 and nonfunctionalized PE from the PE2200-Thy chains. A single crystallization peak was therefore observed. The SAXS patterns of PE1300-Thy and PE2200-Thy, featuring 2200 the respective lamellar spacing distances L1300 Thy = 91 Å and LThy = 168 Å, lower than the calculated PE chain lengths (Table 2), indicate that the PE chains were tilted, partially extended, interdigitated, and confined between thin Thy planes. For PE1300-Thy, the numerous diffraction orders indicate a high regularity in lamellar stacking and thin interfaces between the crystalline layers. This regularity diminished with longer PE chains in PE2200-Thy. We can estimate the molecular length of PE1300-Thy in this lamellar structure by considering the ratio of crystalline and amorphous PE and the Thy crystalline plane thickness. The overall layer thickness is given by

DSC analysis (Table 1), in which the small Thy melting endotherm occurs at a lower temperature (153 °C vs 179 °C). Similar results were observed in our previous work on telechelic amorphous PPO oligomers equipped with terminal Thy units, which could be explained by considering the distance between Thy units (or their volume fraction). Indeed, longer PPO chains increased the lamellar parameter but at the expense of mesoscopic ordering,31 which disappeared almost completely in the longest 4000 g mol−1 polymer. In contrast, very short oligomers (e.g., 230 g mol−1) showed no crystallization since the Thy units were not mobile enough to position themselves into ordered domains. Other researchers have reported similar behavior in PE copolymers containing precisely positioned polar pendant groups along the backbone.26 Layered morphologies were obtained when there were sufficiently long PE segments between the polar pendant groups to allow arrangement of the polar groups into ordered domains and alignment of the PE segments to form crystalline lamellae (typically 15 or 21 methylene units). At higher volume fractions of polar pendants groups no ordering was observed. In another system where self-assembled pendants formed rigid sheets,67 the morphological parameter was shown to increase with the PE segment length, and the layered morphologies persisted after melting of the PE segments68 as we observed for PE1300Thy in the present work. The volume fraction of Thy units must therefore be sufficient to promote Thy−Thy interactions and facilitate their crystallization. In the present system, it can be anticipated that even longer PE chains may completely suppress the Thy crystallization and therefore the mesoscopic organization. Indeed, the crystallinity of other long, crystalline supramolecular polymers has been reported as unperturbed compared to the corresponding nonfunctionalized polymer.69 Monolayer Interdigitated Morphology of PEx-Thy (x = 1300 and 2200 g mol−1). From the melt, the crystallization of Thy end-groups into rigid planes occurred prior to the crystallization of PE chains, confining the PE chains between lamellar interfaces. Because of the rigidity of the interfaces, the significant residues of PE1300-NH2 and nonfunctionalized PE chains (ca. 15% in total) were expelled from the confined PE lamellae and formed separated regions as shown for copolymers.70 Notably, PE1300-Thy presented two distinct PE crystallization temperatures108 °C for nonfunctionalized chains and 100 °C for Thy-functionalized chains with a lower crystallinity compared to pure PE1300-NH2 (68%)reflecting the topological confinement of the PE chains between the rigid

d = dPE,c + dPE,a + e Thy−Thy ⎛ c ⎞ = ⎜nc PE ⎟ cos(ϕPE) + nalPE,a + e Thy−Thy ⎝ 2 ⎠

where nc and na are the number of carbons in the crystalline and the amorphous parts of the PE1300 chain, respectively (n = 92, nc = 53, na = 39), cPE is the c-parameter of the orthorhombic PE (cPE = 2.55 Å), ϕE is the average tilting angle of the PE chains from the normal layer (ϕE = 22−35°),62,72−74 and lPE,a is the thickness per carbon in the amorphous PE layer (lPE,a = 0.712 Å). eThy−Thy is the thickness of crystalline Thy planes normal to the PE lamella, in which Thy were associated by two parallel hydrogen bonds, i.e., roughly the length of two H-bonded Thy. Dimensions of two H-bonded thymines have been minimized with a MM2 field, and the length was found to be around 11 Å, which is compatible with the b-axis of the crystalline 1-(2cyanoethyl)thymine, 11.27 Å.75 Assuming a tilt angle of 35° (found in the literature for both PE62 and a supramolecular fatty acid73), the expected lamellar order spacing d is 94 Å, which is close to the experimental value of 91 Å (Figure 7). For PE2200-Thy, d is 155 Å, which is also in good agreement with the experimental value of 168 Å. H


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Figure 8. Proposed model of a long-range lamellar order of PE1300-DAT and the equimolar blend PE1300-Thy/DAT-PE1300: lamellae of nonfunctionalized PE chains (green) and functionalized PE chains (blue) between noncrystalline layers of H-bonded polar units. In the case of PE1300-Thy/DAT-PE1300, the stronger and more directional association between Thy and DAT moieties of different lamellae ensured the regularity in lamellar stacking. For clarity, the drawing does not take into account the intrinsic crystalline and amorphous distribution of PE chains.

by taking into account the crystalline PE chain segment tilted at an angle ϕPE to the normal layer, dPE,c, the amorphous PE chain segment dPE,a, and the length dX−X of the H-bonded units DAT/DAT or Thy/DAT (estimated to 11 Å by minimization with a MM2 field). The overall layer thickness is expressed as

The PE chain extension required to accommodate the lamellar morphology is determined by the crystal packing of the 2-dimensional crystallized Thy plane. Assuming an incompressible system, the volume V of one PE chain is equal to L·Σ, where Σ is the area of the Thy plane occupied by one chain and L the average end-to-end length of PE in the ordered lamellar state. This area is related to the distance Δ between two crystallized Thy by Δ = √Σ = √(V/L). The volume V of one PE chain of n methylene units can be estimated from the PE molar volume (VCH2 = 28.9 cm3 mol−1) as V = n·VCH2, giving V = 2659 cm3 mol−1 for PE1300 and V = 4510 cm3 mol−1 for PE2200. By taking L = d − eThy/Thy, Δ is estimated to 5.76 and 5.36 Å, respectively, consistent with Thy microphase separation in the crystallized state. Double-Layer Morphology of PE1300-DAT and PE1300-Thy/ DAT-PE1300. The SAXS profile of PE1300-DAT (Figure 5A, inset) has a low-intensity peak at d = 190 Å, whereas that of the equimolar blend PE1300-Thy/DAT-PE1300 (Figure 5C, inset) reveals two extra peaks at 66 and 42 Å. The thermogram of PE1300-Thy/DAT-PE1300 features only a single endotherm at 118 °C corresponding to the PE crystalline domains, which indicates that each Thy unit is associated with a corresponding DAT unit (rather than another Thy unit) and that the Thy/ DAT associations are not crystalline. The same lamellar distance of 200 Å determined by SAXS corresponds to twice the length of extended PE chains inclined at an angle to the plane of the layer (Figure 8). Such double-layered structures have been observed for shortchain n-alkanoic acids58,76 and even a very long chain analogue (C191H383COOH) crystallized from the melt.63 Their formation was attributed to the pairing of molecules through hydrogen bonding between the carboxylic acid groups, which is analogous to the bonding in our system. As for the PEx-Thy system, the proposed lamellar order was validated by calculating the expected layer thickness and comparing it with the experimentally obtained value. The calculation was performed

d = 2(dPE,c + dPE,a) + dX−X ⎛ c ⎞ = 2⎜nc PE cos(ϕPE) + nalPE,a⎟ + dX−X ⎝ 2 ⎠

The crystallinity of PE1300-DAT was estimated at 63%. Considering the length of the DAT/DAT unit to be 11 Å and a tilting angle of 35°,62,72−74 the lamellar parameter was calculated as 181 Å. For PE1300-Thy/DAT-PE1300 with estimated crystallinity of 67%, the lamellar parameter was calculated as 183 Å. While higher orders were not observed in the SAXS analysis of PE1300-DAT, the PE1300-Thy/DAT-PE1300 pattern (Figure 5C, inset) clearly shows two peaks at 66 Å (3q) and 42 Å (5q), consistent with a double-layered order of spacing d = 200 Å. The fact that higher orders are observed, indicating more regularity in lamellar stacking, is probably due both to higher electron density of Thy (more contrasted planes) and to stronger and more directional complementary H-bonding for Thy/DAT association than for DAT/DAT association (illustrated in Figure 8). Furthermore, while DAT units are known to pack poorly, 65,66 the directional Thy/DAT interactions may be accompanied by weak lateral associations, via H-bonds (of amide linkages anchoring the Thy end-group to the PE chain) or π−π interactions (between lateral Thys and DATs), that induce an additional directional order of Thy/ DAT pairs. The fact that only odd diffraction orders are observed is due to a specific symmetry of the electronic density. Indeed, it is possible to express the differential one-dimensional electron density profiles Δρexp(x) along the layer normal in I


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Δρexp (x) =

∑ n=1

In cos(qn + ϕ), ϕ = 0, π ;

qn =

to freeze the double-layered morphology that formed in the melt state. In the higher molecular weight PE2200 series, greater overall crystallinity was observed, attributed to the longer chains and therefore lower volume fractions of the Thy or DAT end-groups. The regularity of solid-state structures was strongly dependent on the volume fraction of the associative units and on their incompatibility with the polymer chain: lower endgroup volume fractions and lower incompatibility between endgroups and PE both strongly reduced regularity of solid state structures. In our PE systems, the crystallization of PE chains enhanced the long-range order by freezing the microphase separation. Thus, by controlling the balance between directional associations, dispersion interactions, and polymer chain length, different mesoscopic organizations could be achieved. Finally, the efficient synthetic routes for Thy- and DATterminated PE, along with advances in telechelic PE formation,79,80 offer the possibility to generate telechelic PE end-functionalized with Thy and/or DAT. Such polymers with chain lengths tailored to optimize both crystallization and mesoscopic organization can be envisaged to possess properties of low molecular weight PE at high temperature, such as the fluidity required for their processing, while exhibiting key properties such as enhanced toughness of high molecular weight PE at room temperature.

2πn L

From this expression, we deduce that, if only odd diffraction orders are considered, then Δρexp(x) is an odd function of x about origins at x = L/4 and 3L/4.63 So, if the electronic density is maximum in the Thy-DAT layer (x = 0), then it is minimum at x = L/2 in amorphous end chains layer (see Figure 8). The observed lamellar order of PE1300-DAT and PE1300-Thy/ DAT-PE1300 was thus driven not by the crystallization of the polar supramolecular units (as for PE1300-Thy) but by the dispersion forces between polar H-bonding units (DAT, Thy/ DAT) and the apolar PE chains. In this case, there was no confinement and no subsequent expulsion of nonfunctionalized PE chains. Since, contrary to PE-Thy, crystallization does not drive the order, it is only dispersion interactions (defined by the Flory−Huggins parameter) which are responsible for microphase separation. The Flory−Huggins parameter χ which reflects the affinity between two molecules can be estimated from the Hildebrand’s parameters determined by Fedor’s method.78 For the PE/DAT and PE/Thy pairs, it yields χPE/DAT = 3.0 and χPE/Thy = 8.5, showing clearly that the PE chain has weaker affinity for Thy than for DAT. Despite similar values being obtained for analogous PPO/DAT and PPO/Thy pairs (3.6 and 10.8, respectively), no mesoscopic order can be observed for the equimolar blend Thy-PPO-Thy/DAT-PPODAT.54 The additional order observed for PE1300-DAT and PE1300-Thy/DAT-PE1300 is likely due to the crystallization of PE chains freezing the microphase separationan effect not possible for amorphous PPO chains.

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ASSOCIATED CONTENT

S Supporting Information *

FTIR and POM images of PE1300 derivatives, HMBC spectra of PE1300-DAT and PE1300-Thy, MALDI-TOF of PE1300-DAT, 13C NMR of PE1300-Thy/DAT-PE1300. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ma502304k.

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AUTHOR INFORMATION

Corresponding Authors

CONCLUSION Low molar mass linear PE chains (Mn 1300 and 2200 g mol−1) were efficiently end-functionalized with the H-bonding units thymine (Thy) and 2,6-diaminotriazine (DAT). In the lower molecular weight series, PE1300-Thy crystallized in a monolayer structure with long-range lamellar order composed of alternating crystalline Thy planes and crystalline lamellae of interdigitated PE chains. This lamellar organization was driven by the crystallization of the Thy units, since it was preserved after the melting of PE chains. Because of the rigidity of the Thy planes that crystallized at higher temperature than the PE chains, the Thy functionalized PE chains were confined and their crystallization was delayed and altered compared to the non-Thy-containing systems. This confinement also resulted in the expulsion of the minor fraction of nonfunctionalized PE chains displaying less polar end-groups (15%), which crystallize separately at a higher temperature than the PE1300-Thy chains. PE1300-DAT and the equimolar blend PE1300-Thy/DAT-PE1300 presented double-layered solid-state structures, in which chainend crystallization did not occur and the disparity between the polar H-bonding chain-end units and nonpolar PE chains was instead sufficient to drive microphase separation. The equimolar blend PE1300-Thy/DAT-PE1300 showed significant, selective Thy/DAT triple H-bonding interactions in both solution and solid states. Thy/DAT pairs were strongly and directionally associated, forming soft polar interfaces between crystalline PE lamellae that allowed greater regularity in lamellar stacking. The crystallization of PE chains was believed

*(F.D.) E-mail [email protected]. *(C.B.) E-mail [email protected]. *(C.S.-Z.) E-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Jessalyn Cortese and Dr. Alexandre Prevoteau for their help and skills in the synthesis and characterization of the supramolecular PEx derivatives. We thank Prof. Ludwik Leibler (MMC) and Dr. Samuel Pearson (C2P2) for very helpful discussions. We acknowledge the LEM laboratory (Arkema Cerdato Serquigny) for facilitating X-ray scattering experiments and especially Sylvie Lebreton for her technical help. We thank the NMR Polymer Center of Institut de Chimie de Lyon (FR5223) for assistance and access to the NMR facilities. The financial support from the French National Agency for Research (ANR 13-BS08-0006) is acknowledged.

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REFERENCES

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