Branch-Induced Heterogeneous Chain Motion in Precision Polyolefins

Dec 4, 2015 - Lucas Caire da Silva†, Robert Graf‡, Clifford R. Bowers†, and Kenneth B. Wagener†. † The George and Josephine Butler Polymer R...
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Branch-Induced Heterogeneous Chain Motion in Precision Polyolefins Lucas Caire da Silva,† Robert Graf,‡ Clifford R. Bowers,† and Kenneth B. Wagener*,† †

The George and Josephine Butler Polymer Research Laboratory, Department of Chemistry and Center for Macromolecular Science and Engineering, University of Florida, Gainesville, Florida 32611-7200, United States ‡ Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany S Supporting Information *

ABSTRACT: The effect of branching on the chain dynamics of model branched polyethylene was studied by solid state 2H and 13C NMR. Methyl branched polyethylene models with branches at every 15th (PE15) and 21st (PE21) chain carbon were synthesized with deuterons placed either at the carbon alpha to the branching point or in the middle of the chain between branches. Line shape analysis of the temperaturedependent 2H spectra revealed that the distribution of motional amplitudes in the solid state is homogeneous along the crystalline chain but heterogeneous in the less constrained amorphous phase. The 13C and 2H longitudinal spin relaxation data obtained at different positions along the chain are explained in terms of the segmental dynamics as a function of branch incorporation into the crystals. A detailed analysis of the chain dynamics in the conformationally disordered phase exhibited in PE15 is also provided.

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INTRODUCTION Chain branching is important in structure−property relationships in macromolecules. Branching modifies a wide range of mechanical and thermal properties that are essential in defining a polymer’s scope of application. Expansion of the crystalline lattice, crystallization dynamics, melting temperatures, heats of fusion, mechanical relaxation, and melt flow are among the properties that show a strong correlation with branch identity and distribution.1−5 A well-known example is the fine-tuning of polyethylene properties through Ziegler−Natta and metallocene catalysis to introduce branches with different degrees of control over branch identity and distribution.6 This determines not only morphological features but also the complex chain dynamics governing relaxation phenomena. Branches can be viewed as permanent point defects, as opposed to the shortlived dynamical defects responsible for the mechanical αrelaxation observed in linear polyethylene.7 Thus, the study of how these permanent defects affect conformational dynamics as a function of precise branch identity and frequency is essential for a complete description of structure−property relationships. In semicrystalline polyethylene, inclusion or exclusion of branches from the lamellae is a function of branch identity. Nonequilibrium conditions allow inclusion of short chain branches, such as methyl and ethyl branches, while longer branches are excluded.8−11 The consequences of branch inclusion are manifold. In terms of structure, studies on methyl branched PE show a lateral expansion of the lamellae to accommodate the branch, while the c-axis shortens as a consequence of the chain disorder imposed by the defect.12 This lattice expansion is accompanied by creation of a branch© XXXX American Chemical Society

controlled local motion which becomes cooperative through inter- and intramolecular interactions. As shown in a study combining 13C and 2H solid state NMR, branched PE with methyl branches at every 21st carbon shows defect-promoted local axial motion, while collective motion is detected only in PE with methyl branches at every 15th carbon along the chain.13 In other words, localized motion can become cooperative if the frequency of defects in the crystal is higher than a threshold value. In this work we focus on the distribution of segmental dynamics caused by permanent branch-promoted defects along different points on the main chain of methyl branched PE. In order to eliminate structural variables, model polymers with exact branch frequency, distribution, and identity were synthesized via acyclic diene metathesis (ADMET) polycondensation. Site selective deuteration at the carbons adjacent to the branching carbon and at the midpoint between branches allowed the local molecular dynamics at different positions along the chain to be probed. Line shape analysis of the temperature-dependent 2H quadrupole echo spectra and T1 relaxation data are used to determine the relative amplitude and frequency of motions at different positions along the chain. These two different NMR techniques probe motions on very different time scales. While the T1 measurement probes the spectra density of motions near the Larmor frequency (i.e., 76.8 MHz for 2H at 11.74 T and 75 MHz for 13C at 7 T), the Received: August 26, 2015 Revised: November 22, 2015

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Figure 1. Model polyethylene with methyl branches at every 15th and 21st main-chain carbon showing deuterium labels at the carbon adjacent to the branch (PE15-D4 and PE21-D4) and in the middle of the chain (PE15-D2 and PE21-D2).

breakdown of the fast motional averaging regime for the 2H quadrupole echo line shape occurs on a much slower time scale that is determined by the quadrupole coupling constant (120 kHz). By combining the T1 and line shape measurements at the D2 and D4 positions in the chain, we are able to study the dependence of motional amplitudes and rates as a function of position relative to the branching group in PE15 and PE21.

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Table 1. Polymer Characterization Data polymer

Mwa (kg mol−1)

PDIb

Tmc (°C)

ΔHmd (J g−1)

PE15-D2 PE15-D4 PE21-D2 PE21-D4

66.9 101 59.7 94.8

2.3 2.6 2.2 1.6

38.8 38.5 61.7 62.7

76.8 76.6 100.8 103.3

a

Mass average molecular weight determined from gel permeation chromatography with polystyrene standards. bPolydispersity index. c Melting temperature from differential scanning calorimetry (DSC). d Heat of fusion from DSC.

EXPERIMENTAL SECTION

Model Polymer Synthesis and Characterization. The structures of the four isotopically labeled model polymers used in this work are shown in Figure 1. These model ethylene/α-olefin polymers, with well-defined alkyl branches precisely placed along the main-chain carbon, were synthesized via acyclic diene metathesis.14−17 Monomers were prepared via a two-step alkenylation18/reductive decyanation19 yielding symmetric α,ω-dienes. The isotopically labeled premonomers 1a and 1b (Figure 2) were prepared by double

angle between consecutive sites (i.e., the jump angle) is defined by φ. The 2-site model is an approximation of a classical harmonic oscillator with distributions peaked near the turning points defined by the two sites. Motion correlation times were assumed to be 10 MHz).

two sites (p1 and p2) are assumed equal (p1 = p2 = 0.5), and the time between jumps is considered infinitesimally small compared to the residence time in each site. To better describe the dynamics experienced by the C−D bond, simulations were performed using a Gaussian distribution of jump angles. This same axial-jump motional model, with θj = 90°, was used previously to fit the 2H quadrupole echo NMR line shapes of the CP spectra in PE15-CD3.24 The simulated CP spectra are overlaid on the experimental spectra in Figure 4, where the standard deviation of the normal distribution about the mean angle φ is indicated in parentheses. Excellent agreement with the experimental data was obtained in cases where the segmental motion is well-defined, i.e., cases in which the crystalline spectra are described by a single component. This well-defined regime is observed at 0 °C for PE21 and below at −30 °C for PE15. Additional narrow spectral features are observed beyond these limits, suggesting the coexistence of at least two dynamically distinct crystalline components. This effect is clear in the spectra of PE15-D4 at 0 °C, where a low-intensity component with lower anisotropy is superposed on the broad crystalline signal. The line shape of PE15-D4 at 0 °C could be simulated by assuming that the final spectrum is the sum of two components with distinct mobilities. The crystalline component shows moderate jump angles of ca. 35° corresponding to constrained segments within the crystal. The second component is characterized by significantly larger jump angles (≈111°) and is assigned to illdefined regions within the interphase between the crystalline and the fully amorphous phase. It must be noted that the 2-site jump model is only an approximation of the actual motion of segments in noncrystalline domains. Therefore, the 111° jump angle observed in PE15-D4 should be interpreted as a

qualitative indication that the mobility of segments in the noncrystalline is higher compared to segments in the crystalline phase, where jump angles of 35° are observed. Bowers et al. reported a similar behavior in a study of PE15/21-CD3 (PE with −CD3 groups at every 15th and 21st chain carbon).24 Information on how the oscillation amplitudes vary along the crystalline chain was gained by comparing the corresponding line shapes from D2 (middle of the chain) and D4 positions (branching unit) at 0 °C. This temperature corresponds to an intermediate point between the static solid and the molten phase, which facilitates the data analysis for the study of the heterogeneity of motion along the polymer chain. As shown in Figure 4, the geometry and amplitudes of the axial motion at crystalline D2 positions are similar to the geometry/amplitude at D4 positions in both PE15 (≈35° at D2 and D4) and PE21 (≈15° and 20° at D2 and D4, respectively). However, the separation of the spectra into CP and AP components is imperfect in the case of PE15-Dx at 0 °C, as is evident by the residual intensity in the center of the pattern. Therefore, additional spectra of these samples were also collected at −30 °C. The lower temperature significantly improved the quality of separation, affording clean CP and AP line shapes. The simulations of the spectra at −30 °C confirm the conclusion obtained at 0 °C, i.e., that the mobilities at D2 and D4 in the crystalline phase are very similar in terms of the oscillation amplitudes. The mobility at both positions is dramatically higher than in nonbranched PE.25 For example, 2H NMR shows that the C−D bonds in high density PE undergo fast reorientation with amplitudes of ca. 4° at 22 °C.26 Mean oscillation amplitudes of ≈15° and ≈28° are found for D2 positions in PE21 and PE15, respectively, at 0 °C (PE21) and −30 °C (PE15).13 These numbers illustrate the drastic increase D

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Macromolecules of the molecular dynamics of the methylene chains in the crystals caused by the incorporation of branches. Departure of the polar angle θ from 90° was observed in PE21-D4 at 0 °C, where good agreement with the experimental line shape could be obtained with θ = 70°. This angle is consistent with the formation of local kinks at the branching site caused by the incorporation of the methyl branch in the crystal. PE15-D4 could also be simulated with θ = 70°. However, the simulation of PE15-D4 is complicated by the twocomponent nature of the CP signal at 0 °C, which does not allow a clear distinction between θ = 90° and θ = 70°. Independent evidence for chain kinks at the branching sites of the crystals has been provided by previous 13C NMR, molecular dynamics, and X-ray diffraction studies.12,13,22 These studies showed that localized gauche defects are introduced at the branching site due to branch inclusion into the crystalline phase. These permanent defects cause the lattice to expand and create conformationally distorted regions with higher free volume as the molecular packing becomes less than ideal. The degree of disorder introduced in the crystalline phase is proportional to the branch content in the lattice. As a consequence, the motional profile along the crystalline chain changes significantly as the short branch content increases from 48 branches/1000 chain carbons in PE21 to 67/1000 in PE15. Two conclusions can be drawn from the 2H NMR line shape analysis: (1) in the collective regime in PE15, the oscillation amplitudes in the middle of the crystalline chain are similar to those at the branching carbon, and (2) the amplitude of motions at the D2 position in PE21 demonstrates that segments halfway between branches are not static but experience motion amplitudes similar to the branching units. This homogeneous distribution of motional amplitudes throughout the crystals in PE15 and PE21 is a consequence of the way torsional stress is dissipated. Molecular dynamics and molecular mechanics calculations show that inclusion of short branches such as −CH3 and −CH2CH3 dramatically increases the intermolecular interaction between adjacent chains.27,28 This allows the pressure created by conformational changes during creation, propagation, and annihilation of dynamic defects to be efficiently distributed to adjacent chains, allowing the entire crystal lattice respond homogeneously to local conformational stress. Therefore, given sufficient time, and assuming a high concentration of branches, the average conformational space spanned by the C−D vectors at D2 and D4 positions will be equivalent, leading to a homogeneous distribution of oscillation amplitudes observed in PE15 and PE21. Conformationally Disordered Phase in PE15. A particularly interesting line shape is observed at 30 °C, but only for PE15-D4. At this temperature, 10 °C below the melting point, PE15-D4 displays a new Pake pattern with about one-half the splitting of the static Pake pattern observed at −20 °C (Figure 5). This line shape is attributed to a fast largeamplitude oscillation of the C−D bond vector around the chain axis, and it can be simulated by considering either a full rotation of the C−D bonds around the chain axis or a 2-site jump with a 90° jump angle. Both models produced the same line shape shown in Figure 5a. The same motionally averaged pattern was observed in deuteriomethyl branched PE and was said to be consistent with a rotator phase.13 Rotator phases are commonly observed in short linear alkyl chains, which undergo a transition to a nearly hexagonal lattice with important consequences for crystalliza-

Figure 5. Motionally averaged line shape of PE15-D4 at 30 °C. Top: (a) simulated and experimental spectra of PE15-D4 at 30 °C. The same simulated line shape is obtained by assuming either a full rotation or a 90° jump about the axis perpendicular to the C−D bond vector. (b) Experimental PE15-D2 spectrum acquired at 30 °C.

tion.29,30 Similar transitions have also been observed in the high-pressure phase of PE,31,32 ultradrawn PE fibers,33 and ethylene−propylene copolymers.34,35 The classification of a rotator phase for PE15, however, should be distinguished from rigid-body rotator phase of short alkanes. Molecular dynamics studies of linear and branched PE have shown that the introduction of defects, such as branches and/or chain ends, prevents the free 360° rotation of the chain stems as a rigid body. Instead, the pseudohexagonal phase formed in PE15 (confirmed by X-ray) is characterized by a high degree of conformational disorder along the crystalline chain.12 Remarkably, the characteristic motionally averaged Pake pattern obtained for PE15-D4 at 30 °C was not reproduced by PE15-D2, indicating that the amplitudes of the motions in the middle of the chain are smaller than those of the branchcontaining segments. Additional NMR line shape simulations were performed to investigate these differences. Figure 6 shows simulated line shapes assuming three different idealized models for the motion in PE15-D2. Model a considers a full rotation around the chain axis with simultaneous oscillation of the C−D bond vector relative to the chain axis given by a Gaussian distribution (2σ) around θ = 90°. Models b and c simulate the axial jump φ around the chain axis with different fixed θ angles. In model b the C−D bond is perpendicular to the chain axis (θ = 90°) while in model c θ = 70°. Models b and c in Figure 6 both satisfactorily reproduce the dynamically averaged asymmetry parameter (η = 1) and line width (ca. 150 kHz) of the PE15-D2 powder pattern at 30 °C (Figure 6b) with φ = 70°. While the idealized models b and c are not distinguished by the experimentally observed spectrum of PE15-D2 at 30 °C, it can be concluded from the combined results for PE15-D4 and PE15-D2 that the conformationally disordered phase of PE15 is characterized by largeamplitude axial oscillations of ca. 90° at the branching site and somewhat smaller axial oscillations of ca. 70° in the middle of the chain. 2 H NMR Longitudinal Relaxation. A second aspect to be considered is how the short branch content influences the distribution of motional rates along the crystalline chain. The variation of motion correlation times along the crystalline chain can be qualitatively assessed by the positional dependence of the 2H longitudinal relaxation times (T1D). Figure 7 shows T1D of the crystalline (T1D[CP]) and amorphous (T1D[AP]) phases of PE15 and PE21 measured at positions D2 and D4. E

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position of the C−D probe. In PE15, the small difference between relaxation times observed in both positions (D2 and D4) suggests a more homogeneous distribution of motional rates along the chain than that of PE21. Contrary to PE15, PE21 exhibits a slower relaxation at position D2 (2.0 s [D2] vs 0.6 s [D4]), suggesting that motional rates are significantly smaller in the middle of the chain than at the branching unit. The longer relaxation time observed for PE21-D2 is consistent with the more constrained dynamics due to the dense packing of the segments in the middle of the chain compared to the less constrained branching sites. The shorter relaxation times obtained for PE15 compared to PE21 are consistent with the increased disorder of PE15 due to its higher branch content. 13 C NMR Longitudinal Relaxation. To further study the heterogeneity in the mobility of the methylene segments along the crystalline phase, 13C T1 relaxation times of the methylene segments in the crystalline phase (T1C) were obtained at different temperatures (Table 2). Relaxation times were Table 2. 13C Spin−Lattice Relaxation Times of All-Trans Segments (10 kHz MAS) T (°C) PE15-D4

PE21-D4

10 20 30 20 30 40

T1C,1 (s)

T1C,2 (s)

± ± ± ± ± ±

n/a n/a n/a 18 ± 2 14 ± 1 11 ± 1

0.730 0.628 0.52 1.42 1.22 1.32

0.005 0.005 0.11 0.19 0.08 0.10

T1A (s) 0.64 0.58 0.58 0.87 0.87 0.86

± ± ± ± ± ±

0.01 0.01 0.06 0.14 0.1 0.07

obtained at the isotropic chemical shift corresponding to extended all-trans methylene chains in the crystallites (Figure 8).13,36 The last column includes T1 of the amorphous component (T1A) in each sample. The parameters in Table 2 were obtained by fitting the normalized experimental magnetization decay (Figure 9) to exponential functions A1e−(t−t0)/T1 or A1e−(t−t0)/T1 + A2e−(t−t0)/T2. The magnetization decay of PE21-D4 could be fit to a biexponential with parameters indicated in Table 2. Since T1 corresponds to segments in the crystalline phase, the observation of two different relaxation times T1C,1 and T1C,2 in PE21 is consistent with the heterogeneous chain mobility caused by variations of the local molecular packing. The presence of a “short” and “long” 13C T1C has also been observed in linear PE.36 Therefore, we attribute the biexponential relaxation in PE21 to the presence of regions with different mobility along the crystalline chain. The 13C relaxation of PE15 is dominated by a monoexponential decay characterized by a single T1C whose values are similar to the corresponding T1A. Consistent with the deuterium spin relaxation results, this result also reflects the high mobility of the entire chain in PE15 due to the incorporation of branches into the lamellae. The magnetization follows a monoexponential decay between 10 and 20 °C but shows a small departure from a monoexponential decay at 30 °C as the conformationally disordered phase becomes important (Figure 9). Chain Motion in the Amorphous Phase. Amorphous phase signals obtained by the saturation-recovery pulse sequence are displayed in Figure 4. Signals from PE15-D4 and PE21-D4 are equivalent to the corresponding line shapes exhibited by PE15-CD3 and PE21-CD3, which have been discussed in previous works, except that the effective quadru-

Figure 6. Simulated axially symmetric oscillation of C−D in the fastmotion regime. (a) Rotation model were θ ± σ indicates the variation of the angle C−D makes with the chain axis. The width of the variation is determined by a Gaussian distribution with full width 2σ. (b) Two-sites jump model with θ = 90° and jump angle between sites 1 and 2 given by φ. (c) Two-sites jump model with θ = 70° and jump angle φ. The quadrupolar coupling constant was 123 kHz, and the (static) asymmetry parameter was η = 0 for all cases.

Figure 7. 2H spin−lattice relaxation times. Crystalline (CP) and amorphous phase (AP) values were obtained at 0 °C and represent averages over the quadrupole line shape.

The results in Figure 7 show that the relaxation time at D2 is strongly dependent on the short branch content and the F

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Figure 9. Magnetization decay plots of 13C located in all-trans segments in PE21-D4 and PE15-D4. Plots show the decay of the normalized NMR Signal (S/S0) following initial cross-polarization under 10 kHz MAS. (a) Biexponential fits to the magnetization decays for PE21-D4 at three different temperatures. (b) Monoexponential fit to the magnetization decays for PE15-D4.

Figure 3 shows that the narrow component, which arises from large amplitude, more isotropic-like motions in the amorphous phase, is already present at D2 positions at temperatures as low as −20 °C. At this temperature, the same narrow component is not visible at the D4 positions. Therefore, the onset of large amplitude, isotropic-like dynamics in the amorphous phase is observed at lower temperature at D2 than D4 for both PE21 and PE15.

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CONCLUSION Solid state 2H NMR line shape analysis and spin−lattice relaxation measurements on model methyl branched polyethylenes were used to study the molecular dynamics in the crystalline and amorphous phases at two different branch spacings. Our results show that incorporation of methyl branches in the crystals has two main effects: (1) Apart from the overall increase in chain mobility with increasing short chain branch content, branch inclusion introduces contact points between adjacent chains that significantly enhance the propagation of conformational stress throughout the crystals. As a consequence, our 2H quadrupolar echo line shape simulations show that the conformational space spanned by any given segment displays no significant dependence on its position relative to the branching site, and the axial oscillation amplitudes at the D2 and D4 positions are similar in both PE15 and PE21. (2) The 2H spin relaxation time is highly sensitive to fluctuations of the electric quadrupole interaction with spectral density near the Larmor frequency. In PE21, the 2H relaxation time is found to be significantly longer at the D2 position, suggesting a longer correlation time of the chain motion in the middle of the chain. In contrast, the 2H spin relaxation times at

Figure 8. Solid state 13C CP MAS (10 kHz) spectrum of PE15-D4 at 10 °C and PE21-D4 at 20 °C. The chemical shifts of the marked peaks corresponds to signals from alkyl segments in the crystalline (32.54 and 33.29 ppm) and amorphous phase (30.10 and 30.47 ppm).

pole coupling constant in the −CD3 groups is reduced by a factor of 3 due to dynamic averaging by fast methyl rotation.24 No single motional model or combination of models could offer a satisfactory fit to the line shapes of PE15-Dx and PE21Dx. This result illustrates the high degree of conformational disorder as well as the complexity of the amorphous phase in terms of the number of dynamically distinct domains that are active at a given temperature. The line shapes in Figure 4 indicate that the amorphous phase is in fact a collection of mobile components, each with a distinct motional profile. Although simple models are not sufficient to describe details of the complex conformational dynamics in the amorphous phase, the existence of a nonhomogeneous distribution of motion amplitudes along the chain can be inferred by the extent of averaging of the quadrupolar interaction observed at different C−D positions. G

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Foundation (DMR-1203136), the US Army Research Laboratory, and the US Army Research Office (W911NF-13-10362).

the D2 and D4 position in PE15 are similar, suggesting homogeneous reorientation dynamics throughout the crystalline phase. The 13C spin relaxation data in the all-trans segments in between branches are consistent with this picture. The observation of multiexponential 13C relaxation for PE21 indicates a distribution of motional rates, while the singleexponential behavior in PE15 provides evidence for homogeneous chain dynamics It should be emphasized that the variations in the dynamics that affect T1 are not necessarily manifested as a changes in the 2 H quadrupolar echo line shape if the motion remains in the fast motional averaging regime. While the line shape analysis shows that branch content has no significant impact on the distribution of the axial oscillation amplitudes along the polymer chain, the distribution of motion rates clearly depends on the local molecular packing, with slower motions becoming apparent in the middle sections between the branching points in PE21. The dependence of the relative motion rates on the position along the chain decreases with increasing branch content. In PE15, the concentration of defects is sufficiently high to affect the entire chain dynamics, which becomes more homogeneous (i.e., site-independent) compared to PE21. Our study has also provided further details regarding the unique Pake pattern observed only at the D4 position in PE15 at 30 °C, which was described by large amplitude axial oscillations of the crystalline chain. Branching also has an important effect on the amorphous chain dynamics, as demonstrated by the line shape analysis of the low temperature PE15/21 spectra. We conclude that the chain dynamics of amorphous segments in the middle of the chain tends to adopt a more isotropic character compared to the more constrained anisotropic axial oscillations at branching sites. In conclusion, selective deuteron labeling of model methyl branched PE enabled the effects of branch spacing on the chain dynamics at different positions along the chain to be studied by solid state NMR. The basic concepts and conclusions derived by this approach advance the understanding of how branching affects the chain dynamics in polymeric materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01882. Detailed reaction procedures and characterization; solid state NMR acquisition parameters (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]fl.edu (K.B.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We kindly acknowledge the Max Planck Institute Research School for Polymer Materials Science at the Max Planck Institute for Polymer Research, Mainz. We also thank Paul Blom and his research group for all the support and Hans W. Spiess for advice and discussions on solid state NMR. This material is based upon work supported by the National Science H

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DOI: 10.1021/acs.macromol.5b01882 Macromolecules XXXX, XXX, XXX−XXX