Kinetic Control of Chlorine Packing in Crystals of a Precisely

Dec 24, 2013 - In summary, all experimental data obtained from DSC, X-ray diffraction, and vibrational and NMR spectroscopy support the suggested pack...
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Kinetic Control of Chlorine Packing in Crystals of a Precisely Substituted Polyethylene. Toward Advanced Polyolefin Materials Papatya Kaner,† Carolina Ruiz-Orta,† Emine Boz,§ Kenneth B. Wagener,§ Masafumi Tasaki,‡ Kohji Tashiro,‡ and Rufina G. Alamo*,† †

Department of Chemical and Biomedical Engineering, FAMU-FSU College of Engineering, 2525 Pottsdamer St., Tallahassee, Florida 32310-6046, United States ‡ Graduate School of Engineering, Toyota Technological Institute, 2-12, Hisakata, Tempaku-ku, Nagoya-shi, Aichi-ken 468-8511, Japan § Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200, United States S Supporting Information *

ABSTRACT: The crystallization of a polyethylene with precise chlorine substitution on each and every 15th backbone carbon displays a drastic change in crystalline structure in a narrow interval of crystallization temperatures. The structural change occurs within one degree of undercooling and is accompanied by a sharp increase in melting temperature, a change in WAXD patterns, and a dramatic increase in TG conformers around the Cl substitution while the main CH2 sequence remains with the all-trans packing. These changes correlate with the formation of two different polymorphs characterized by a different packing and distribution of Cl atoms in the crystallites. Under fast crystallization kinetics, the chains assemble in an all-trans planar packing (form I) with a layered Cl distribution that presents some longitudinal disorder, while slower crystallization rates favor a more structured intermolecular halogen staggering consistent with a herringbone-like nonplanar structure (form II). The drastic change in morphology is enabled by the precise halogen placement in the chain and appears to be driven by the selection of the nucleus stem length in the initial stages of the crystallization. Exquisite kinetic control of the crystallization in novel polyolefins of this nature allows models for generating new materials based on nanostructures at the lamellar and sublamellar level not feasible in classical branched polyethylenes.

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INTRODUCTION Macroscopically, polymer crystallization is a thermodynamic first-order transition where the enthalpic change involved in packing polymer chain segments from their liquid random coils surmounts the loss of entropy in the process. However, it is well-known that polymer crystallization is never complete. Due to the long chain nature of polymers, the process is controlled by kinetic factors rather than by equilibrium thermodynamics. The classical view of the transition from the isotropic melt to the solid is that it evolves via nucleation and growth steps. The rate-limiting step is nucleation since the free energy of forming a stable nucleus (ΔG), via statistical fluctuations of molecular clusters in the metastable undercooled melt, must first be surmounted.1,2 ΔG depends on the dimensions of the nucleus. As the dimensions of the initial embryo increase, the free energy change first increases due to a large surface-to-volume ratio, but eventually ΔG reaches a maximum at the critical dimensions of barrier height (ξn*) and further decreases, becoming negative. At this point the nucleus becomes stable (ξns), and further growth is spontaneous. Therefore, the overall polymer crystallization rate is nucleation controlled. The connectivity of long-polymer molecules adds a segmental © 2013 American Chemical Society

transport barrier that counteracts the negative temperature coefficient of the kinetics of nucleation rate. Because of the long-chain nature of polymers, the same molecule can participate in more than one growing crystallite, resulting in a complex, hierarchically ordered metastable structure. Polymer crystallites are usually lamellar-like, connected by disordered amorphous regions, and arranged in spherical domains with diameters of 1−200 μm.3 There are many examples that point to a selection of the first nucleus stem as driving the final crystal morphology. One of the most prominent is the crystallization of long-chain n-alkanes. Long-chain n-alkanes (>150 carbons) crystallize as extended or folded chains depending upon the lowest nucleation barrier at each undercooling (ΔT). For example, C198H398 crystallized from solution at large undercooling forms folded crystallites, while extended-chain crystals are favored when crystallized from the melt at temperatures close to the observed melting.4,5 Using the free energy of fusion to form a stable nucleus for Received: July 22, 2013 Revised: December 17, 2013 Published: December 24, 2013 236

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polyethylene with finite chain length, it was found that at the smallest undercooling the calculated thickness of the critical size nucleus was close to the extended chain length while the thickness of the nucleus falls to less than half the extended length at high undercoolings in correspondence with the experimental observations.6 The crystallizations of low molecular weight poly(ethylene oxide) and isotactic polypropylenes7 are additional examples of the role of the selection of the initial stem length in the final crystalline structure. The data for these systems are conclusive in that the selection of the segment length that is needed to initiate stable nuclei controls the thickness and major characteristics of the mature polymer crystallites. In this work, we report an unusual crystallization behavior of a novel polyolefin that could also be explained by the length dependence of the stable nuclei concept. The polyolefin studied belongs to the special families recently synthesized via acyclic diene methatesis polymerization (ADMET).8 They are characterized by the stereoirregular (atactic) placement of pendant groups at a precise distance along a polyethylene-like chain. Systems with a large variety of side groups have been synthesized, including alkyl branches,9 acid and ionomer groups,10 halogens,11 and other functional groups.12 Structurally, they are a new class of polyolefins with the repeating unit −[(CH2)i−CHX]n− where i changes from 8 to 74, X is the branch, halogen, or functional group, and molar masses are 30K−150K. However, in spite of being attractive structures as analogues of linear low-density polyethylenes or as ethylene copolymers with a regular and precise placement of the side group, their crystalline properties that are the major drive in the material performance have been scarcely studied.11,13−17 Using solid-state 13C NMR, AFM, and X-ray scattering, it was demonstrated in prior works that polyethylenes with precise halogen substitution crystallize as homopolymers. In other words, there is no preferential partitioning of the halogen between crystalline and amorphous regions.11,13,14 These prior studies were undertaken in samples that were taken relatively rapidly from the melt to room temperature; hence, the crystallization was uncontrolled and fast for most samples.11,14 We have now studied the morphology and conformational changes under controlled isothermal crystallization conditions and report here a unique sharp change in crystalline structure with decreasing undercooling in one of the members of the series, PE15Cl (i = 15, X = Cl). The structure of the two polymorphs is also inferred from X-ray patterns and FTIR spectra.

Figure 1. Melting endotherms of PE15Cl after isothermal crystallization at the temperatures indicated.

of the total transformation. The situation is different in the range of Tc > 53 °C when crystallization kinetics become more pronounced. The half-time crystallization at 53 °C is about 30 min and increases to ∼300 min for a Tc of 58 °C. Hence, the sharp increase in melting is associated with crystallization kinetics. By melting crystallites formed at Tc = 53 °C at progressively higher heating rates, we ruled out the possibility that the double melting found at Tc = 53 °C or the high temperature melting endotherm observed at higher Tc is associated with a melting− recrystallization−melting during the heating scan (Figure S1 of Supporting Information). Similar double melting peaks, as those found for Tc = 53 °C, were observed for isothermally crystallized n-alkanes and explained as isothermal thickening or a solid−solid transformation between once-folded and extended crystal structures.18,19 To demonstrate that this is not the case for PE15Cl and that crystallites formed at Tc do not undergo isothermal thickening, the crystallization of PE15Cl was halted at different times, and melting recorded starting from Tc. The endotherms, and temporal variation of the heat of fusion are shown in Figure 2 in the range of Tc where both forms coexist. For Tc below or above the range shown, basically single melting peaks are obtained during transformation, indicative of formation of pure forms (Figure S2). The increasing area of both endotherms with time rules out any isothermal solid−solid transformation of the low melting to the higher melting temperature crystallites. Consequently, the precise placement of the chlorine in the backbone leads to two drastic crystallization regimes where crystalline structures are enabled with marked differences in thermal stability. The formation of these two structures, which only coexist in a very narrow Tc range, is controlled by undercooling. We term form I the crystallites that develop at Tc < 53 °C and form II those formed at higher temperatures. At Tc = 53 °C, the crystallization is dominated by form I even before temperature equilibration, but it takes ∼20 min for form II to start developing. Conversely, at Tc = 54 °C form I grows first but saturates at a low transformation after ∼25 min, while form II continues to grow up to much higher levels. At Tc = 54.5 °C, only form II develops, taking ∼15 min to initiate. It is hence

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RESULTS AND DISCUSSION PE15Cl was isothermally crystallized in a range of increasing temperatures (Tc) for sufficiently long times to ensure complete transformation at Tc and further taken at room temperature for spectroscopic, X-ray, and DSC melting analyses. The melting behavior analyzed by DSC at 10 °C/ min is given in Figure 1. The behavior is unique in that crystals formed below 52 °C melt at a constant temperature of 61 ± 0.5 °C, while crystals formed at Tc > 53 °C melt at about 10 °C higher temperatures. These data point to a sharp increase in melting temperature with just one degree increase in Tc. The data of Figure 1 also indicate that within a very narrow Tc range (Tc = 53 ± 0.5 °C) the melting is bimodal with peak temperatures corresponding to the low and high melting temperatures. In the range of Tc from 23 to 52 °C the crystallization is very fast, taking less than 2 min to develop half 237

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Figure 2. Temporal variation of melting endotherms of PE15Cl after isothermal crystallization at the temperatures and times indicated. The bar adjacent to the Y-axis of the endotherms denotes 2 W/g. The change in heat of fusion with crystallization time is given in the lower plots. Pure form I develops at Tc ≤ 52 and pure form II at Tc ≥ 54.5 °C.

clear from Figure 2 that form I develops prior to the formation of form II, pointing to a kinetically favored low melting structure. Although a full analysis of the kinetics of both forms is out of the scope of the present work, the fact that the initiation of form I requires less than 5 min at any Tc while the induction time of form II is much longer suggests a large difference in nucleation barrier between both forms. Also notable is the decrease in total heat of fusion with increasing crystallization temperature, from ∼80 J/g at room temperature to ∼60 J/g at Tc = 55 °C and to ∼50 J/g at a temperature of 58 °C, in agreement with the kinetic restrictions to develop form II. Coupled with the sharp melting transition, there is a change in crystallographic packing as indicated by the wide-angle X-ray patterns of Figure 3a. At Tc < 53 °C, the WAXS patterns of form I display two sharp and strong reflections at ∼19° and ∼22° also observed after relatively fast cooling, which on the basis of similar patterns for a methyl branched system could be associated with the (100) and (010) planes of a triclinic lattice.14,20 The crystallites formed at Tc > 53 °C (form II) give three additional WAXS reflections at 2θ of 19.7°, 20.9°, and 23.1°, thus indicating a substantial difference in chain packing. The diffractograms of Figure 3a were collected at room temperature and hence, for Tc > 53 °C, contain a contribution of crystallites in form I formed during quenching. In order to establish the reflections of pure form II, a diffractogram was collected at an isothermal temperature of 56 °C after full transformation. The diffractogram is given in Figure 3b together with the diffractogram of the same isothermally crystallized sample and further cooled to room temperature for comparison. Using the diffractogram of the melt at 56 °C

Figure 3. Wide-angle X-ray diffraction patterns of PE15Cl after isothermal crystallization at the temperatures indicated. (a) Diffractograms taken at room temperature of isothermally crystallized samples. (b) Diffractograms obtained at Tc = 56 °C after crystallization for 15 h and on cooling after crystallization at Tc = 56 °C.

(Figure S3), the level of crystallinity changes from 0.42 to 0.52 (±4%) on cooling, but the reflections of form II are basically unchanged after quenching. Hence, reflections at 19°, 19.7°, 20.9°, and 23.1° are characteristic of form II. An additional feature in the WAXS patterns is the single reflection at intermediate angles (2θ = 5.9°) for Tc < 53 °C and the appearance of diffraction orders (2θ = 5.6° and 11.2°) at Tc > 53 °C. These reflections are absent in the melt and correspond to distances of 14.9 and 15.8 Å, which are close to 238

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Figure 4. (a) Room temperature FTIR spectra of PE15Cl after isothermal crystallization at the temperatures indicated. (b) Percentage of TG conformers with increasing crystallization temperature. Data collected at room temperature (ATR) and under transmission at the isothermal temperature are shown.

but below the all-trans extended length of the repeat unit, 19.1 Å. They thus reflect intramolecular periodicity between crystalline chlorines. The difference in periodicity with respect to the length between CHCl units in the chain suggests that the Cl are arranged in the crystalline regions of PE15Cl in layers, tilted with respect to the chain axis by about 55°. In other words, in a more conventional terminology, the chains are tilted at 35° to the normal of the Cl layers, which remarkably is the same tilt found between chain and normal of the crystal layer for lamellar crystallites of polyethylenes and long chain nalkanes crystallized at high temperature.5 Tilted chains for the n-alkanes are associated with more ordered crystal surfaces than those with perpendicular chains. The chain tilt is believed to arise from the need to increase the surface area to better accommodate the end methyl groups in an ordered layer. The similarity in tilt angle suggests the same origin for the chain tilt in crystallites of PE15Cl. Furthermore, the fact that the secondorder reflection is prevalent only at Tc > 53 °C where form II develops is a strong indication that in form II the crystalline chlorines are arranged in ordered layers with highly correlated symmetry. Conversely, at Tc < 53 °C, the arrangement of crystalline chlorines, albeit layered, lacks high symmetry, as the second-order diffraction is absent. Hence, a major difference between forms I and II is in the packing of chlorines inside the lamellar crystallites. The change in conformation around the substitution point that induces this unique change in packing was further probed by vibrational spectroscopy. Room temperature FTIR spectra for the isothermally crystallized samples are given in Figure 4. FTIR has been proven useful to characterize the backbone conformation around the Cl substitution in ethylene−vinyl chlorides (EVC).21,22 In the 500−700 cm−1 absorbance, characteristic of the C−Cl stretching region, the vibration frequency of the C−halogen bond depends on the conformation of the adjacent C−C bonds.23−25 The band at 612 cm−1 corresponds to the stretching absorbance of C−Cl side groups with vicinal C−C backbone carbons in an all-trans (TT) conformation, while the absorbance at 665 cm −1

corresponds to C−Cl stretching when the side group is adjacent to backbone carbons in the gauche (TG) conformation. Spectra of EVC in this stretching region are complex due to the presence of diads, vicinal, and other mixed substitutions in addition to the isolated one, thus increasing the number of conformations around the C−CHCl unit.21,25 Only for EVC with a relatively low ( 53 °C, indicating that as form II is enabled, the content of gauche conformers around the substitution increases substantially in a narrow Tc transition range. Hence, although a similar change in conformation was noticed in a small extent in EVC copolymers,25 the precise nature of the Cl substitution in PE15Cl induces at Tc > 53 °C macromolecular packing characterized by a very high concentration of TG bonds adjacent to the CH−Cl unit. The area of the 665 cm−1 band, characteristic of TG conformers, over the total TT plus TG area serves to quantify the content of gauche conformers around the substitution as a function of undercooling (Figure 4b). This analysis assumes that the extinction coefficients of the TT and TG absorptions are equal, an assumption supported by the same drastic change obtained from the Raman spectra (Figure S4), and by the equal 239

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51 °C), and form II (Tc = 56 °C) can be found in Figure S5. The ratio of TG/(TT + TG) area for form I and form II follows the same trend as for samples crystallized isothermally and taken at room temperature (see isothermal data added to Figure 4b), but the content of TG conformers is a little higher, as expected, since the spectra recorded at Tc are free of form I crystallites that develop on quenching at room temperature. With reliable contents for TG conformers for pure form II at Tc = 56 °C (0.65) and for the degree of crystallinity from the WAXD pattern at the same isothermal crystallization temperature (0.42 from Figure 3b), we are in a good position to calculate the content of gauche conformers around the substitution in the crystalline regions from the relation

ratio of TT over TG area absorbance obtained from two KBr pellets with a different concentration of PE15Cl prepared in form I at 45 °C. For crystallization temperatures up to 53 °C (form I) the crystalline packing is mainly in the all-trans since only ∼10% are TG conformers. At the transitional temperature of Tc = 53 °C where both crystal types coexist, the % of gauche conformers increases to 30% and reaches 50−55% when form II develops at Tc > 53 °C. Such a dramatic increase in TG conformers for Tc ≥ 53 °C can be explained by kinks caused by C−Cl side groups in a close proximity and resulting from staggering layers of Cl atoms inside the lamellar crystallites. The general feature that emerges from the WAXS and FTIR studies is that PE15Cl adopts packing in two drastically different crystalline structures controlled by crystallization kinetics. Under relatively rapid crystallization, or for Tc < 53 °C (the onset of the exotherm at a cooling rate from the melt of 2 °C/min), form I crystals develop with an all-trans C−C conformation and a somewhat disordered, albeit intermolecularly layered, positioning of Cl inside the crystallites. Conversely, under slower crystallization (Tc > 53 °C) van der Waals intermolecular interactions of the methylene runs prevail, thus forcing C−C bonds adjacent to the substitution to adopt bends, or kinks, for a better accommodation and close packing of Cl in a parallel chain registry. The kinks are possibly of the ...TTGGTT....TTG′G′TT.. type to perpetuate lamellar packing. The result is the formation of form II crystallites with a nonplanar, herringbone-like structure with chain tilt similar to that for form I and Cl atoms ordered in layers with high symmetry. On this basis, we propose the schematic models of Figure 5 for form I and form II crystallites. Form II crystals are structurally more ordered than those of form I; they hence melt at much higher temperatures.

(%TGoverall ) = (Xc(form II))(%TG(form II)) + (Xam)(%TGam) (1)

where Xc(form II) is the crystallinity level in form II, %TG(form II) the percentage of crystalline TG conformers in form II, Xam the amorphous content, and %TGam the percentage of TG conformers in the amorphous regions. The amorphous content is Xam = 1 − Xc, and the percentage of gauche conformers in this region is %TGam = 0.43. This is the TG value, around the Cl substitution, obtained from the spectrum of the melt at 56 °C prior to crystallization. Bringing these data to eq 1, % TG(form II) = 0.96, thus confirming that the crystallites of PE15Cl formed isothermally at 56 °C adopt a unique nonplanar packing, characterized by gauche bonding to both sides of the methine group. The progression of methylene rocking bands in the spectral region between 700 and 1100 cm−1 is particularly helpful to probe if the majority of the CH2 in the sequence between Cl remain in the all-trans conformation as proposed in the models of Figure 5. This region is expanded in Figure 6 for

Figure 5. Schematics of the crystalline packing in form I and form II. For clarity, chains protruding from the crystals to the amorphous regions are omitted.

Nonplanar structures such as that suggested for form II in Figure 5 will require the majority, if not all, of the crystalline −CHCl units gauche bonded to the adjacent CH2 units, while the rest of the methylene sequence remains in a basically trans conformation. Support for the proposed types of packing is found by additional analysis of transmission FTIR spectra. The spectra of Figure 4a were collected with ATR to provide conformational data on the same set of samples prepared for DSC, X-ray, and NMR. These samples were too thick for transmission FTIR spectra which are usually the preferred configuration for a quantitative analysis. Hence, to quantify the content of crystalline gauche conformers in form II from the stretching C−Cl region, we collected transmission FTIR spectra at the isothermal crystallization temperature. Transmission FTIR spectra of the melt (as reference), form I (Tc =

Figure 6. FTIR absorption spectra of PE15Cl recorded at Tc = 51 °C (form I) and Tc = 56 °C (form II) on an expanded scale from 500 to 1000 cm−1. The methylene progression rocking bands are indicated (K = 1−12).

transmission spectra of form I (Tc = 51 °C) and form II (Tc = 56 °C). In this rocking−twisting region, the vibrational spectra of extended all-trans n-paraffins display a series of progression bands that are analyzed considering the CH2 sequence as a linear array of m identical oscillators, each having one degree of freedom.26,27 This simple oscillator model predicts a mode frequency that is only a function of the phase difference between adjacent oscillators (ϕ(K)) given as 240

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Kπ m+1

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(K = 1, 2, 3, ...)

(2)

m is the number of CH2 in the n-alkane and K the mode order. For infrared-active vibrations, K must be an odd integer. Frequency versus phase angle for progression modes of nalkanes in the rocking, twisting, and wagging regions are well established,27,28 with the rocking modes displaying the highest intensities. Hence, if a progression of absorbances in the rocking region follows the dictates of the n-paraffinic all-trans packing, identifying the sequence length accounts merely to counting bands starting from the lowest well-known 720 cm−1 absorbance (K = 1, 3).26,27 Albeit simple, the coupled oscillator model provides a useful method to test periodic n-alkane-like structures. One example is the frequency versus phase angle curve of FTIR absorbances of a series of fatty acids with 12−16 methylene sequences. Fatty acids may assemble in planar and nonplanar polymorphs;29−31 however, independently of the type of packing, the methylene rocking progression bands of their IR spectra fell in the same dispersion curve, indicating the same all-trans methylene packing.29 Weak bands belonging to forbidden (even K) modes were also found in the spectra of fatty acids and short n-alkanes and explained as the effect of the chain ends or carboxylic acid in reducing the symmetry of the sequence of oscillators. Frequencies associated with odd and even modes fall on the same dispersion curve.27,31 The array of methylene rocking modes of PE15Cl in forms I and II is identified with the corresponding K value in Figure 6. Clearly, local packing around the Cl substitution either in trans or gauche conformers does not suppress the methylene rocking progression bands. However, there are notable differences between the bands of both spectra. As in the spectra of fatty acids, odd and even modes are prominent in the spectrum of form I PE15Cl due to some distortion exerted by the Cl at both ends of the methylene sequence. Even modes, if present, are too weak to be discernible in the spectrum of form II, which is consistent with the higher symmetry of this form. The effect of the nonplanar gauche conformations at the end of the methylene sequence of form II on the rocking vibration is observed by a shift to lower frequencies of the absorbances corresponding to the higher order progression modes (K = 9, 11, ...). Interestingly, only when the phase angle (ϕ(K)) of the coupled oscillator model for the crystals of PE15Cl is calculated for a sequence of 12 oscillators (m = 12) is the dispersion curve for forms I and II equivalent to the all-trans dispersion curve of n-alkanes. This equivalence is documented in Figure 7 where the observed frequency for the different methylene rocking absorbances is plotted against the phase angle for the two forms. Experimental frequency data for the n-alkane with m = 12 (C14H30) from Snyder’s work27 are also added in Figure 7 as reference of the corresponding all-trans sequence. Except the point at the highest frequency for form II, all points fall on the n-alkane dispersion curve, thus confirming that the methylene packing for both structures has the same all-trans conformation. A sequence of 12 all-trans methylenes is the expected one for the suggested conformation of crystalline form II as gauche conformations at the sequence’s ends leave 13 bonds with the all-trans conformation as for the C14H30 n-alkane. However, the frequencies of the all-trans planar form I structure also conform to a 12 CH2 all-trans sequence, indicating that the local disorder around the Cl substitution in form I, inferred from the WAXD patterns, does not translate far beyond the methylene

Figure 7. Dispersion curve for the methylene rocking progression bands of PE15Cl crystallites in forms I and II and for n-alkane C14H30 as reference of the all-trans crystal packing.27 A sequence of 12 CH2 oscillators is considered for the three systems. Since the points of the three crystals fall on the same curve, all three have CH2 sequences with the same all-trans conformation.

closest to the substitution. This disorder may be accounted for by the atactic nature of the Cl substitution11,32 and by some strain to the (trans) torsional angles adjacent to the CHCl unit, the latter incurred by interchain staggering of Cl in form I. It is feasible that within the Cl layer of form I the halogens are displaced longitudinally by one unit above and below the methine unit of nearby chains, thus introducing some disorder in the layer. This is the local disorder represented in the model for form I of Figure 5. In further support of the TT and TG conformations of PE15Cl around the substitution for crystalline forms I and II, respectively, we present in Figure 8 room temperature solid

Figure 8. Room temperature solid state 13C NMR cpMAS spectra of PE15Cl crystallized at 45 °C (form I) and 55 °C (form II). Resonances of crystalline (cr) and amorphous (am) CH2, methine carbons, and methylenes α and β to the methine are identified.

state 13C NMR cpMAS spectra of PE15Cl crystallized at 45 °C (form I) and 55 °C (form II). Carbon resonances have been previously identified.14 The major differences between the spectra of forms I and II are a 2−3 ppm upfield shift of the crystalline CH (from ∼68 to 66 ppm) and crystalline CH2 adjacent to the methine (from ∼42 to 39 ppm). Following prior work,33,34 the upfield shifts conform to a γ-gauche effect or shielding of CH and α CH2 by the CH2 that are gamma (or three bonds apart) to them. Shielding takes place only when the CH−C bond is in a gauche conformation.33,34 As it is clear from the spectra of Figure 8 that the βCH2 (27.7 ppm) and other CH2 further from the CH unit have equivalent resonances for forms I and II, the 13C NMR spectra provide further evidence of the gauche conformation around the crystalline CH−Cl unit. Moreover, even if cp MAS spectra are not quantitative, they also provide useful qualitative informa241

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Figure 9. (a) Small-angle X-ray scattering patterns collected at the isothermal crystallization temperature (red profiles), and at room temperature after isothermal crystallization (Tc + Q, black profiles). (b) Long spacing evaluated from Lorentz-corrected SAXS patterns (Iq2 vs q) as a function of Tc.

Figure 10. Polarized optical micrographs of PE15Cl isothermally crystallized in form I (Tc = 51 °C) and form II (Tc = 54.5 and 62 °C). Form I develops on cooling after crystallization at 62 °C as seen in the lower right micrograph. The scale bar corresponds to 100 μm.

tion about the amount of C−CH gauche bonding from the pattern of the CH resonances of forms I and II. Notice that while the resonance of amorphous CH is unchanged at 64.2 ppm, basically all crystalline CH resonance of form II undergoes the ∼2 ppm upfield shift, indicating that the majority of the crystalline CH are gauche bonded to adjacent CH2, as found from the FTIR spectra and in consonance with the schemes of Figure 5. If crystallites in form II had a significant content of all-trans conformers, the crystalline CH resonance of form II would have spanned a much broader ppm range than observed. In summary, all experimental data obtained from DSC, X-ray diffraction, and vibrational and

NMR spectroscopy support the suggested packing models of Figure 5. The lamellar structure of PE15Cl crystallites in forms I and II is next analyzed by the variation of the long period, assessed by small-angle X-ray scattering (SAXS), in Figure 9a. The black profiles in this figure are room temperature SAXS patterns of isothermally crystallized samples, and the red are SAXS collected at the isothermal Tc after complete transformation. The scattering peak of form I crystallites (those formed up to Tc = 53 °C) displays a small change to lower q with increasing temperature, consistent with a small increase in crystal thickness expected with decreasing the undercooling. The drastic shift of the room temperature SAXS peak toward a 242

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longer ones to the intercrystalline regions.36 When the branch is rejected, the crystal thickness is about the distance between side branches.16 With ethyl side branches every 21 backbone carbons, crystallites were found to accommodate the branch at different levels and to generate different crystal structures depending on crystallization kinetics.17 A pseudohexagonal form was associated with crystals under fast cooling, while triclinic crystallites were suggested under isothermal crystallizations accommodating either one or two branches with increasing crystallization temperature. However, contrary to the present findings in PE15Cl, the transition temperature between the different crystal forms does not appear to be sharp, and the distribution of the ethyl branch inside the crystallites was not evaluated.17 While it is likely that the ethyl branch may be too flexible to set in crystalline layers within the lamellae, one could also envision, based on the present data, that this restriction will be relaxed with smaller side groups, such as methyl branches or in precision systems with better control of tacticity.35 The experimental evidence for the formation of two drastically different crystalline structures in polyethylene with precise chlorine substitution (PE15Cl) is compelling from the thermal, structural, morphological, and spectroscopic evidence. However, the question remains of what features at the molecular level induce the formation of form II with one degree change in undercooling and to a chain folding possibly at the same location, thus leading to a layered sublamellar structure. Because of the sharpness of the Tc range for the transition between forms I and II, we posit that the formation of these two crystal morphologies is associated with the initial critical step in the nucleation process. This step is the selection of the critical stem length (ξn*) required to form a stable nucleus at a given Tc. When the required critical stem length is below the distance between chlorines, the initial nucleus bundle is formed most likely by staggering backbone sequences at any location along the methylene run, as van der Waals attraction between methylenes is the most favorable packing. The bundle further extends quickly to the mature crystallite with the alltrans arrangement. However, if the required critical stem length is approximately or slightly above the distance between chlorines, the nucleus may already include a close packing of chlorines and likely the kinked (TGGT) structure, a structure energetically more costly, as morphologically observed by a dramatic decrease in number of nuclei. Once the nucleation energy barrier is surmounted for form II, the proposed nonplanar herringbone-like motif will further extend to the length of the stable nucleus and to the mature crystallite. In order to test this hypothesis, we used classical nucleation theory to compute the critical nucleus stem length (ξn*) of PE15Cl as a function of Tc. Molecular simulations of the nucleation of n-alkanes describe the stable nucleus as a molecular bundle that closely resembles a cylindrical shape.37,38 On this basis, we used the cylindrical nucleus geometry in the calculations. The critical stem length for a three-dimensional nucleus is given as1

higher scattering vector (lower periodicity) when form II is enabled at Tc > 53 °C is intriguing. It appears as if the crystallites formed in form II were thinner than those for form I. However, this is not the case as SAXS collected at the isothermal crystallization temperature (red profiles) reveal a similar position of the SAXS peak for both forms. A vertical dashed line is added in the figure to emphasize the latter. The long period (L), extracted from the peak value of Lorentz corrected patterns, is plotted vs Tc in Figure 9b for data collected before and after quenching at room temperature. The decrease of L from ∼210 to ∼160 Å upon quenching for samples crystallized at Tc > 53 °C is associated with “infilling”, or crystallites in form I that develop on quenching and to a large extent in the interlamellar regions of the spherulites of form II, as shown by AFM (Figure S6). It was earlier documented by WAXD that the level of crystallinity increases by an additional 5−20% upon quenching. Polarized optical micrographs also demonstrate that there are large differences in morphology between forms I and II (Figure 10). Crystallization in form II develops large, bright double banded spherulites, another feature that points to a strong biaxial orientation of the herringbone packing, especially at the highest Tc as seen in the images, while profuse nucleation of form I leads to very small spherulites or irregular aggregates at the lowest Tc. Aided by large differences in nucleation rates, and the conformational change requirements to propagate the herringbone-like structure, form II develops fewer and more open spherulites than those of form I, as shown in the micrographs. On quenching after isothermal crystallization, kinetically favored form I develops readily in the interspherulitic region and in the interlamellar regions of form II, shifting the SAXS peak to lower values. Lamellar thicknesses were evaluated from the SAXS periodicity applying the one-dimensional electron correlation function in the direction normal to the lamellar stacks (for details see Figure S7). In this analysis the AFM images of samples crystallized at 45 °C were used to distinguish between the thicknesses of amorphous and crystalline layers and thus overcome this limitation of the correlation function analysis.14 Crystal thicknesses evaluated from SAXS are basically unchanged at 140 ± 5 Å for forms I and II in the Tc range evaluated and correspond closely to crystal thicknesses from room temperature AFM measurements (125 ± 8 Å). When account is made for the 52° in form I and 56° for form II between chain and lamellae surface, the crystal thickness corresponds to 9−10 repeating units inside the crystal layer. Comparing the present results with data from recent works on the crystallization of analogue polyolefins with alkyl branches16,17 or acid side groups15,35 placed at a precise regular distance, it appears that controlling the development of crystalline structures with ordered or less ordered distribution of the substituent in the crystals is unique to the nature of the substituent. Precision ethylene-based systems with halogen substitution crystallize as homopolymers, as shown here by thick lamellae crystallites that accommodate multiple contiguous repeating units and by the equivalence on content of chlorine inside and outside of the crystallites measured by 13C NMR in prior work.11,13,14 The same crystallization mechanism appears to hold for isotactic precision systems with acetate pendant groups synthesized via ROMP (ring-opening metathesis polymerization).35 However, crystallites formed from precision polyethylenes with flexible alkyl branches only accommodate branches shorter than a butyl group, rejecting

ξn* =

4σen ΔGu

(3)

where σen is the interfacial free energy of the end surfaces and ΔGu the free energy of fusion per mole of pure crystalline unit. For relatively small undercooling, ΔGu is approximated by (ΔHuΔT)/Tm°; here Tm° is the equilibrium melting temperature, ΔHu the heat of fusion at Tm°, and ΔT the undercooling 243

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(Tm° − Tc). In order to test if values of ξn* for form I at increasing Tc approach the all-trans length corresponding to 15 CH2 at the Tc where form II is observed to develop, the relevant thermodynamic parameters to compute ξn* are those corresponding to form I. These values were previously estimated as Tm° = 338 K and ΔHu = 204 J/g.14 From density measurements, we estimated a crystal density of 0.987 g/cm3, and σen was taken as 4 erg/cm2, which is the value obtained from a molecular dynamic simulation of the homogeneous nucleation of n-eicosane,38 the n-alkane with 20 carbons that closely resembles the length between chlorines in PE15Cl. The calculated stem length of the critical cylindrical nucleus from eq 2 was converted to number of CH2 units using the C− C length for the all-trans packing (1.27 Å) and is given as a function of increasing crystallization temperature in Figure 11.

ordered (form II) crystal structure takes place within one degree of undercooling and is driven by the initial nucleus stem length, as described. This exquisite kinetic control of the distribution of “defects” in crystallites of novel polyolefins of this nature allows strategies for generating new polyolefin materials based on nanostructures at the lamellar and sublamellar level not yet feasible in classical branched polyethylenes obtained via coordination catalysis.39 This finding invites further screening of the crystallization behavior of other precision systems synthesized by ADMET and/or feasible via ROMP.35 It also challenges investigating in more detail the crystallization of polyethylenes with a more equisequential, rather than random, distribution of counits. Presently, the generality of this unique feature is being investigated by us in other precision halogen-substituted polyethylenes.

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EXPERIMENTAL METHODS

The ADMET synthesis of PE15Cl was detailed in a previous work.32 In reference to a calibration with linear polyethylene fractions, using dichlorobenzene as solvent, the GPC gives Mw = 51 400 and Mn = 28 600 Da. Room temperature FTIR, WAXD, and SAXS measurements were conducted on samples that were brought to ambient conditions subsequent to their isothermal crystallization. Depending on the crystallization temperature, two different procedures were applied for isothermal crystallizations. For Tc < 50 °C, the initial powder was melted on a thin glass coverslip and then very quickly transferred to a water bath maintained at the corresponding Tc with ±0.5 °C control. For slow crystallizations, the film was placed in an aluminum pan without cover, melted in the DSC at 120 °C for 2 min, and then cooled to the selected Tc at 40 °C/min. The required crystallization times were assessed by the leveling off of the DSC exothermic peak of crystallization. Once the required crystallization times had elapsed, each sample was removed from the water bath or from the DSC and brought to ambient conditions for further testing. Selected diffractograms and spectra were also collected in situ under isothermal conditions. We used a PerkinElmer DSC7 calibrated with indium and operating under nitrogen flow. The sample mass used was about 4 mg and the heating rate 10 °C/min. Room temperature FTIR spectra of relatively thick samples also prepared for X-ray diffraction and SS 13C NMR were collected in ATR mode using a Thermo Scientific Nicolet 6700 spectrometer equipped with a TE cooled DTGS detector. The spectra were collected using the instrument’s OMNIC software at a resolution of 2 cm−1 in a wavenumber range between 500 and 900 cm−1. Isothermal in-situ spectra were collected under transmission using zinc selenide windows in a Linkam FTIR 600 stage with TMS94 for temperature control. A ∼100 μm thick film of PE15Cl was placed on a ∼0.2 mm KBr disk for isothermal measurements. The Linkam’s stage temperature was calibrated using a microthermocouple and by recording the change in light intensity under cooling and heating in a polarized optical microscope in reference to the DSC enthalpic change. Wide- and small-angle X-ray diffractograms (WAXD and SAXS) were obtained at room temperature or at Tc using a Bruker Nanostar diffractometer with Iμs microfocus X-ray source and equipped with a HiStar 2D Multiwire SAXS detector and a Fuji Photo Film image plate for WAXS detection. The Peltier attachment of the Bruker Nanostar diffractometer was used for temperature control. The plates were read with a Fuji FLA-7000 scanner. SAXS profiles were calibrated with silver behenate and WAXD patterns with corundum; both standards were obtained from Bruker. The wavelength of the radiation source was λ = 0.1541 nm (Cu Kα) and the scattering vector, q (q = (4π sin θ)/λ) (with 2θ denoting the scattering angle). Background-corrected SAXS intensities were used to obtain the normalized one-dimensional correlation function of the electron density fluctuations perpendicular to the lamellar stacks according to

Figure 11. Critical nucleus stem length, ξn*, calculated for a threedimensional nucleus, as a function of crystallization temperature. The length at Tc = 53 °C corresponds to 15 CH2 units.

In the range of Tc between 20 and 50 °C, ξn* increases from 5 to 14 CH2 and, remarkably, it reaches a length of 15 units at Tc ∼ 53 °C, the temperature of the transition from form I to form II. Hence, although the thermodynamic parameters of form II would be needed to probe the nucleus dimensions for this form, the result is in good agreement with the initial hypothesis that the ordered form II is enabled when the length of the critical nucleus for form I equals the distance between chlorines. A close packing of chlorines near the nucleus basal surface induces kinks in the structure of the nucleus of form II that further propagate as a repeat motif in the structure of the mature crystallite. Chain folding in this structure must be preferential, possibly near the substitution point to perpetuate the proposed herringbone structural order.

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CONCLUSIONS We have demonstrated in this work that it is feasible to encode unique crystal microstructures using a precise placement of halogens, rather than the classical random distribution, in polyethylene-based polyolefin materials. Two drastically different crystalline forms are enabled by controlling crystallization kinetics. Under relatively fast crystallization, the chains assemble in the all-trans packing accommodating ∼9 repeating units. In this form, the interchain arrangement of the Cl atoms is layered but with some longitudinal disorder. Slower crystallization rates favor a highly ordered structure by intermolecular staggering of crystalline chlorines in highly symmetric layers with a kinked or herringbone-like conformation. The nonplanar chain conformation of form II is supported by ∼100% of crystalline C−CHCl units adopting a TG conformation and by a trans packing of the CH2 sequence between Cl. The formation of a less ordered (form I) or 244

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∞

γ(r ) =

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∫0 q2Ic(q) cos(qr) dq ∞

∫0 q2Ic(q) dq

(4)

The raw intensity was corrected for its value at the pixel location at the beam stop by a linear extrapolation of the initial recorded intensity data. Solid state cp MAS 13C NMR spectra were acquired at room temperature on an 11.75 T (500 MHz 1H NMR frequency) Bruker Advance III NMR system with a 2.5 mm MAS probe. Spectra were referenced to the 13C chemical shift of TMS. Cp MAS were attained at 25 kHz MAS. Cross-polarization was applied with a constant 50 kHz radio-frequency field applied to the 13C channel during the 1H−13C spinlock (2 ms contact time) and 1H radio-frequency fields ramping linearly around 75 ± 25 kHz. A recycle delay of 5 s was used for all NMR scans. The 1H nutation frequency applied for decoupling was 110 kHz. The mass of sample used to collect spectra of PE15Cl in forms I and II was ∼5 mg. Polarized optical micrographs were obtained with 40 ± 10 μm films using an Olympus BX51 optical microscope fitted with an Olympus digital camera (Type DP72) and a Linkam Scientific Instruments hot stage for temperature control (Type TMS94). AFM images were obtained using the environmental Jeol 4210 scanning probe microscope operating under ambient conditions. Topographic and phase images were simultaneously collected in noncontact ac mode at 256 × 256 standard resolution using Olympus single side coated silicon cantilevers with resonant frequency at ∼300 kHz.

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ASSOCIATED CONTENT

* Supporting Information S

DSC endotherms of PE15Cl (Tc = 53 °C) at different heating rates; temporal evolution of DSC melting endotherms at 52 ≤ Tc ≤ 54.5 °C; deconvolution of WAXD patterns for crystalline and noncrystalline contents; room temperature Raman spectra and % TG conformers as a function of Tc; FTIR spectra collected at Tc; AFM images at 45 and 55 °C; Lorentzcorrected SAXS patterns and correlation function analysis. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (R.G.A.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation under Grant DMR1105129. We acknowledge Laura Santonja, Xiaoshi Zhang, and Al Mamun for collecting optical microscopy images and SAXS patterns and Dr. A. Paravastu for collecting NMR spectra. K.T. acknowledges support from MEXT “Strategic Project to Support the Formation of Basic Research at Private Universities”. We are also indebted to the High Performance Materials Institute of Florida State University for access to X-ray and AFM instrumentation.

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REFERENCES

(1) Mandelkern, L. In Crystallization of Polymers; Cambridge University Press: London, 2004; Vol. 2, p 69. (2) Cheng, S. Z. D. In Phase Transitions in Polymers. The Role of Metastable States: Elsevier: Amsterdam, 2008; p 32. (3) Bassett, D. C. In Principles of Polymer Morphology; Cambridge University Press: London, 1981. 245

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