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Infrared Spectroscopy and X-ray Diffraction Characterization of Dimorphic Crystalline Structures of Polyethylenes with Halogens Placed at Equal Distance Along the Backbone Xiaoshi Zhang, Laura Santonja-Blasco, Kenneth B Wagener, Emine Boz, Masafumi Tasaki, Kohji Tashiro, and Rufina G Alamo J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b08877 • Publication Date (Web): 10 Oct 2017 Downloaded from http://pubs.acs.org on October 15, 2017

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The Journal of Physical Chemistry

Infrared Spectroscopy and X-ray Diffraction Characterization of Dimorphic Crystalline Structures of Polyethylenes with Halogens Placed at Equal Distance Along the Backbone Xiaoshi Zhang1, Laura Santonja-Blasco1, Kenneth B. Wagener2, Emine Boz2, Masafumi Tasaki3, Kohji Tashiro3, Rufina G. Alamo1* 1

Department of Chemical and Biomedical Engineering, FAMU-FSU College of Engineering, 2525 Pottsdamer St, Tallahassee, FL, 32310-6046. USA

2

The George and Josephine Butler Polymer Research laboratory, Department of Chemistry. University of Florida, Gainesville, Florida 32611-7200. USA

3

Department of Future Industry-Oriented Basic Science and Materials, Graduate School of Engineering, Toyota Technological Institute, Tempaku, Nagoya 468-8511. Japan

Abstract Polyethylenes with halogens placed on each and every 21st, 15th or 9th backbone carbon display crystallization patterns enabled by the size of the halogen and by changing crystallization kinetics. The different structures have been identified from X-ray patterns combined with a detailed analysis of the infrared spectra of series containing F, Cl or Br atoms that were either fast or isothermally crystallized from the melt. Under both crystallization modes, all specimens develop layered crystallites that accommodate 5-9 repeating units along the chain’s axis. The size of the halogen and intermolecular staggering to maximize packing symmetry, are responsible for striking structural differences observed between the series and between the two modes of crystallization. While the small size of the F atom causes a small perturbation to the crystal lattice and the orthorhombic structure is maintained for all members of the series either fast or isothermally crystallized, each Cl or Br-containing system presents dimorphism. Under fast crystallization, Cl and Br containing samples adopt the all-trans conformation (planar Form I), while in slowly crystallized samples gauche conformers set for bonds of the backbone carbons adjacent to the carbon with the halogen due to a close intermolecular staggering of halogens (herringbone Form II). In both forms the methylene sequence between halogen maintains the all-trans conformation. The structural details are extracted from the analysis of the C-halogen stretching region of the IR spectra, and from adherence to the n-alkane behavior of CH2 rocking, CH2 wagging, and C-C stretching progression modes. 1 ACS Paragon Plus Environment


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*Corresponding author ([email protected])

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Introduction The fundamental principles that allow for the choice of microstructures and processes to suit specific properties are usually formulated based on the behavior of models of polyethylenes with well-defined microstructures. To this end, products synthesized with metallocene catalysts have played a major role.1,2 Materials have been developed ranging from statistical random copolymers to stereo-blocky elastomeric types exemplifying the ability of new synthetic strategies to tailor structures and properties of polyolefins by a suitable choice of the coordination catalyst precursor.1,3,4 Polyethylene-like molecules with equidistant placement of a co-unit are of special interest as suitable models to study the effect of size and polarity of the counit on chain folding and on the various modes of inter-molecular staggering during crystallization.5 Special sets of these models of precision polyethylenes are available for these studies as they have been recently synthesized via olefin metathesis polycondensation.6–9 Packing of the methylene sequences of polyethylene molecules with halogens or other types of moieties placed at a precise equal distance along the methylene backbone is a major factor in determining their three-dimensional structure and related properties.5,10 In prior works it was demonstrated that precision polyethylenes of this nature containing halogens crystallize like homopolymers, accommodating the halogen inside the crystals, in spite of the atactic configuration of the pendant halogen in the chain.11–15 Recently, the molecular packing and crystalline morphology of a polyethylene with a chlorine atom placed on each and every 15th backbone carbon (PE15Cl) were analyzed as a function of increasing isothermal crystallization temperature. It was found that this precision polyethylene develops two distinctive polymorphs (Form I and Form II) depending on crystallization kinetics.16 The two polymorphs differ in the conformation of the backbone adjacent to the substitution. Crystallized at relatively large undercooling, the chains assemble in all-trans planar packing with layered intermolecular Cl arrays that present some longitudinal mismatch (Form I). Slower crystallization rates favor Form II, a structure enabled by a better intermolecular staggering of the methylene sequences and of the halogen resulting in kinks and gauche bonding around the methine while conserving the all-trans packing of the methylene sequence.16 Consequently, the corresponding structure of Form II is a herringbone-like, nonplanar structure with methylene sequences bending back and forth around the carbon with the halogen. The crystal structures of forms I and II were further extracted from 2D WAXD fiber 3 ACS Paragon Plus Environment


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patterns of PE21Br, a precision polyethylene with a bromine atom on each 21st backbone carbon that also develops Form I at high undercooling or Form II after isothermal crystallization. The non-planar structure is unprecedented in the realm of classical branched polyethylenes and is only enabled by the regularity of the substitution.16,17 Evidence of PE15Cl crystal transitioning from packing in Form I to Form II with changing undercooling was given by large differences in melting points, a change in the X-ray diffraction pattern, and a change in conformation of the backbone bonds vicinal to the halogen substitution point. The change from a planar all-trans conformation to the non-planar herringbone type by changing crystallization kinetics was extracted from quantitative FTIR analysis of the content of crystalline gauche conformers around the substitution and by a quantitative analysis of the rocking progression modes in reference to the spectra of all-trans nalkanes of equivalent length.16 Taking as reference the unique structural behavior found in PE15Cl, it is likely that other precision polyethylenes may develop more than one solid phase, or display “polytypes” differing from one another only by different stacking arrangements of otherwise identical layers, which are common features observed in n-alkanes and in most fatty acid compounds.18–21 Polytypes are of special significance in evaluating folding or self-assembly of precision polyethylenes with moieties that can be easily incorporated into a crystalline lattice, because the continuity of a chain-like molecule will favor certain stacking arrangements, for example those with higher symmetry, over others that result in more defective crystallites. The latter can be probed evaluating the structure and conformation of other precision polyethylenes with different types and content of co-units. Infrared spectroscopy has been highly instrumental in analyzing the conformation of crystalline continuous methylene sequences, based on rocking, twisting and wagging vibrational bands for a large number of systems, including n-alkanes,22,23 fatty acids,24,25 lipid bilayers,21,26,27 phospholipid gels,20 intercalated surfactant bilayers,28 polyamides,29,30 and arylate polyesters,31 among others. By treating n-alkanes as a system of many oscillators (i.e., methylene groups, CC bonds) and using GF method,32 Snyder successfully associated the frequencies of methylene rocking, twisting and wagging, as well as C-C stretching modes to the phase difference (φ) in motion between adjacent methylene groups of n-alkanes.33,34 In brief, for a particular vibration of


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a system of m coupled oscillators, i.e. the methylene sequence of the n-alkane molecule, the solution of the secular equation gives, (,) = + 2 φ(,) [1]

Where ao, a1, a2 are the eigenvectors or coefficients of the matrix,33 and the phase angle is given by,

φ(,) =

( = 1, 2, 3, … ) [2] +1

The integer K characterizes each normal mode; for infrared active vibrations even K modes are forbidden.33 Frequency versus phase angle for progression modes of n-alkanes in the methylene rocking, twisting, and wagging regions are well-established dispersion curves independent of chain length, as predicted by eq. 2.33,34 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 progression bands starting from the lowest wellknown 720 cm-1 absorbance (K = 1).33,34 The phase angle, φ(,) corresponding to a given K mode is calculated with equation 2. Albeit simple, the coupled oscillator model provides a useful method to test periodic n-alkane-like self-assembled structures. For example, adherence to the nalkane dispersion curve of the progression of infrared methylene rocking and wagging bands has been used successfully to identify the same all-trans packing of methylene sequences of myristate salts that crystallize in two different polymorphs.21 Trans packing of methylene sequences and conformational disorder has been extracted for other molecules as well from the analysis of progression bands in reference to the n-alkane behavior.17–21 The effect of the CH3 ends in n-alkanes or of the acid end group in fatty acids is to reduce the symmetry of the sequence of oscillators and leads to observable, albeit weak, forbidden (even) K modes in their infrared spectra, and to deviations from the predicted frequencies of the n-alkane dispersion curve at the highest K values. These shifts were found in aliphatic polyamides29 and polyesters31 with long methylene sequences that also conform to the all-trans n-alkane packing. Hence, the analysis of the progression frequency modes is instrumental in extracting the conformation of long methylene sequences in the crystalline phase, and to deduce distortions inferred by moieties placed at the end of the sequence or within a polyethylene-like chain. 5 ACS Paragon Plus Environment


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The availability of sets of similarly constituted precision polyethylenes with a fluorine, chlorine or bromine atom placed on each 9th, 15th or 21st backbone carbon allows models of polyolefin-like materials to test the effect of the halogen size, and distance along the backbone, on the generality of the polymorphic transformation observed in PE15Cl. The challenge is to determine the various structures as a function of halogen distance, to analyze and understand the delicate interplay of forces that lead to a given phase, and then to relate the structure to the properties of the novel materials. Questions to address are, for example, if strain to the lattice by the accommodation of large sized halogens, such as the bromine placed at close backbone distances, is too large to sustain the required lattice symmetry for Form II. In precision molecules with long methylene sequences, folding within the sequence will suppress the formation of layered crystallites, as one may predict from the behavior of the random systems or of the defectfree chain. These features are analyzed in the present manuscript for samples crystallized under rapid and under slow conditions. The two crystallization modes are chosen to favor the conditions for which Form I and Form II were found in PE15Cl. The different crystalline structures are first identified by X-ray diffraction and the conformation of the methylene sequence, and around the substitution, is extracted by a detailed analysis of their FTIR spectra. Combining the information from X-ray data and the FTIR analysis, the molecular packing in one, or two major polymorphs, of these systems with precision placement in the backbone of F, Cl or Br is predicted.

Experimental Section Materials The precision polyethylenes with halogen atoms placed on each and every 9th, 15th, and 21st backbone carbons studied here were synthesized via acyclic diene metathesis (ADMET) followed by exhaustive hydrogenation. Details of the ADMET synthesis and chain characterization were given in prior works.11–14 These polymers are labeled PE9X, PE15X, and PE21X, where the number corresponds to the backbone carbon number for the precise location of halogen atom, and X corresponds to the type of halogen atom (X = F, Cl, or Br). The repeated structural unit is –[(CH2)m-1-CHX]n–, where m is 9, 15 or 21. The weight average molecular mass (Mw), distribution (Mw/Mn), and thermal characterization by DSC of these precision halogencontaining polyethylenes are listed in Table 1. 6 ACS Paragon Plus Environment

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Table 1. Molecular mass and thermal characterization of precision halogen-containing polyethylenes. Mol% Mw ·103 Tc peak Tm peak ∆Hm Sample Mw/Mn (Halogen) (g/mol) (°C) c (°C) c (J/g) c PE9F 11.11 8.9a 2.0 114 123 137 a PE15F 6.67 10.4 2.2 114 124 174 a PE21F 4.76 7.6 1.8 113 124 205 a PE9Cl 11.1 48.7 1.8 15 41 27 a PE15Cl 6.67 51.4 1.8 54 63 87 b PE21Cl 4.76 31.1 4.4 70 81 111 b PE9Br 11.1 23.6 1.8 -22 -13 18 b PE15Br 6.67 27.6 1.7 27 49 45 b PE21Br 4.76 94.1 2.2 53 70 70 a b c GPC vs PE in DCB. GPC vs PS in THF. Crystallization peak (Tc peak), second melting peak Tm peak, and heat of fusion (∆Hm) obtained by DSC at 10°C/min.

Measurements FTIR spectra were collected at controlled crystallization temperatures (Tc) using a Thermo Scientific Nicolet 6700 spectrometer equipped with a TE cooled DTGS TEC detector. Instrumental control and peak analysis were carried out using the OMNIC software provided with the instrument. The spectra were collected on absorption mode in a wavenumber range between 4000 and 400 cm-1 at a resolution of 2 cm-1. The number of scans varied from 16 to 128 per spectrum depending on the crystallization kinetics and quality requirements. Thin films suitable for infrared measurements were prepared by melting a ~100 µm thick film between two ~0.2 mm thick KBr pellets. The KBr sandwich was placed on a Linkam FTIR 600 stage with a TMS94 temperature controller. The temperature was raised to about 30°C above the observed melting and held for 5 minutes to erase the previous thermal history. The sample was then cooled to the crystallization temperature either at 40°C/min, for relatively low Tc, or at a rate of 100°C/min to crystallize at the lowest Tc avoiding crystallization prior reaching a low Tc. The latter was achieved with a N2 environment in the sample compartment and using liquid nitrogen in the cooling jacket of the Linkam stage. The temperature of the sample in the Linkam FTIR 600 stage was calibrated by recording the change in light intensity during cooling and during heating in a polarized optical microscope in reference to the DSC enthalpic change under identical cooling and heating runs. 7 ACS Paragon Plus Environment


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Wide-angle X-ray diffractograms (WAXD) were obtained using a Bruker Nanostar diffractometer with IµS microfocus X-ray source, and equipped with a Fuji Photo Film image plate. The plate was read with a Fuji FLA-7000 scanner. WAXD profiles were calibrated in reference to the pattern of corundum provided WAXDby Bruker. The incident X-ray beam used was the Cu Kα line with a wavelength λ=1.5418 Å. The samples were isothermally crystallized from the melt using a TA Q2000 differential scanning calorimeter or using thermostatic baths with temperature controlled with ice or with a mixture of dry ice and isopropanol, depending on the crystallization temperatures. The crystallization time at Tc was sufficiently long so as to ensure high crystallinity levels in each crystalline form. After isothermal crystallization, WAXD patterns of all but PE9Cl and PE9Br precise polyethylenes were collected at room temperature. PE9Cl (Tc = -15°C) and PE9Br were isothermally crystallized at different crystallization temperatures in a Peltier device placed inside the WAXS instrument. The diffractograms were collected after full transformation.

Results and discussion It has been well-documented that the halogen of random or precision ethylene vinyl halides is partially discriminated if the halogen is randomly distributed, or not discriminated against entering the crystalline regions if the halogen is placed at an equal distance along the backbone.11,14,15 In

fact,

precision

halogen-containing

polyethylenes

crystallize

like

homopolymers, and their path to fold during crystallization is only hampered by the need to accommodate the halogen in the lamellar crystal. Their crystalline structure and thermodynamic properties depend on the size of the halogen, and on the amount of halogen to stagger in layered crystallites.15–17 It was also found from detailed studies of one of these systems, PE15Cl, that the crystalline packing can be drastically changed by controlling the extent to which the melt is undercooled prior to crystallization.16 Planar all-trans Form I crystals develop under fast crystallization and a non-planar, herringbone type of structure, Form II, is formed under slow crystallization conditions. To test if other precision halogen-containing samples display such polymorphic behavior, we prepared the 9 precision samples listed in Table 1 under similar fast or slow crystallization. The crystallization peak temperature of the exotherm obtained by DSC on cooling from the melt at 10°C/min (Tc peak listed in Table 1) was taken as reference of a relatively fast cooling. With this reference, all precision samples were crystallized from the melt at two 8 ACS Paragon Plus Environment

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temperatures, one well below Tc peak for fast crystallization, and a second well above Tc peak for slow or isothermal crystallization conditions. The WAXD patterns of the three series are comparatively shown in Figure 1. The patterns on the left correspond to fast crystallized samples, and those on the right are for samples isothermally crystallized. Each panel displays WAXD for precision samples with F, Cl or Br placed at the same distance. A clear effect of the size of halogen and crystallization kinetics on packing emerges from the WAXD patterns. For both crystallization conditions, the patterns of all specimens with fluorine conform to the orthorhombic packing of the unsubstituted polyethylene as shown by the two major reflections at 21.5° and 23.8° corresponding to (110) and (200) planes respectively, and less intense ones at higher angles. The fact that the crystal packing of polyethylene is preserved in all fluorine substituted polyethylenes, even in those with F placed on every 9th backbone carbon, indicates that the F atom is a small perturbation to the orthorhombic unit cell. Conversely, the WAXD patterns of all precision polyethylenes with Cl or Br differ from the orthorhombic pattern, and for a given molecule, they also differ with crystallization mode. The drastic change in unit cell packing compared to the orthorhombic pattern of F containing samples indicates that the higher van der Waals radius of Cl and Br strains the crystal lattice to levels such that symmetry is found in a different packing structure. Under fast crystallization, Cl and Br containing systems spaced by 21 or 15 backbone carbons display two major reflections at ~19° and 22°. As shown in our previous work, the fiber patterns obtained from oriented fast crystallized PE21Br are consistent with a triclinic form that includes two repeats in the c axis.17 From the equivalent number of reflections between the patterns of PE21Br and those of PE21Cl, PE15Cl and PE15Br (Figures 1a and 1b), the same triclinic packing arrangement is inferred for all these samples under fast crystallization. Compared to the orthorhombic packing of precision polyethylenes with F substitution, the triclinic packing is degeneration in the scale of symmetry and explains the need for a reduced order to facilitate minimum spatial requirements to accommodate the bulkier Cl and Br atoms between adjacent molecules in the crystal.



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a)

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d) Tc

0

5

10

15

20 2θ (°)

25

Tc

PE21F 50 °C

PE21F 110 °C

PE21Cl 65 °C

PE21Cl 74 °C

PE21Br 47 °C

PE21Br 62 °C

30

35

40 0

b)

5

10

15

20 2θ (°)

25

30

35

e) Tc

5

10

Tc

PE15F 50 °C

*

0

15

20 2θ (°)

25

PE15F 111 °C

*

PE15Cl 38 °C

PE15Cl 56 °C

PE15Br 0 °C

PE15Br 35 °C

30

35

40 0

c)

5

10

15

20 2θ (°)

25

30

*

40

Tc

*

PE9F 50 °C

PE9F 110 °C

PE9Cl 36 °C

PE9Cl -15 °C

PE9Br 0 °C

PE9Br -20 °C

5

35

f) Tc

0

40

10

15

20 2θ (°)

25

30

35

40 0

5


10

15

20 2θ (º)

25

30

35

40

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Figure 1. Wide-angle X-ray patterns of precision polyethylenes with fluorine (blue), chlorine (red), and bromine (green) after fast crystallization (a, b, c), and after isothermal crystallization at the temperature indicated (d, e, f). The reflection marked with asterisk is an impurity.

The reflection at ~22° associated with the lattice spacing of the (100) plane, shifts appreciably to lower angles with increasing halogen size (Figures 1a and 1b), a clear signature of the expansion of the a axis or of the inter-planar distance that contains the halogen. The lattice expansion is also prominent with increasing content of Br in the chain, as shown in Figure 2, where the d spacing corresponding to the 22° - 23° reflections is plotted vs. content of halogen. The spacing for Cl and Br containing series extrapolate for the chain free of halogen to a periodicity close to 3.8 Å, which is the value reported for triclinic polyethylene, as expected.35 The periodicity of samples with F is closer to the value corresponding to the (200) plane of orthorhombic polyethylene (3.71 Å).36 Comparing patterns within the Cl or Br series, the intensity of the reflections decreases with increasing halogen content due to a drastic decrease in the level of crystallinity.5 This feature is quite relevant, and defines a unique aspect of these precision systems. While the crystallization is akin to that of a homopolymer, the halogen is acting as a defect for crystallization; therefore, chains with higher content of Cl or Br have more restrictions to crystallize. Not only the crystallization and melting points are drastically lowered by the formation of more defective crystallites with increasing halogen content (Table 1), but the content of crystallites that are formed is also reduced. 4.7

PEmF Low Tc PEmCl Form I

4.5

d spacing (Å)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PEmBr Form I 4.3 4.1 3.9 3.7 3.5 0

2

4

6

8

10

12

Halogen content (mol%)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 2. d-spacing of orthorhombic (200) and triclinic (100) planes, 2θ ~ 22 - 23°, as a function of increasing halogen for fast crystallized precision polyethylenes. Lines are added to guide the behavior of each series. The plane corresponding to the reflection at ~19° of fast crystallized systems with Cl or Br is less affected by the incorporation of the halogen in the crystal; as a consequence, with increasing halogen content and decreasing degree of crystallinity, the two main reflections merge as seen in the patterns of fast crystallized PE9Cl and PE9Br. The single broad reflection of PE9Br resembles the pattern of a highly mobile lattice, pointing to a more disordered structure when a higher number of Br atoms are incorporated in rapidly formed crystallites. The reflections observed in the middle angle range (2θ between 2 and 10°) are absent in the patterns of the molten systems, and correspond to distances that are close to but below the all-trans extended length of the repeat units. They thus reflect crystalline halogen layers in the lamellar crystallites. Second and third order reflections of the layer peak are found in the patterns of slowly crystallized Cl and Br systems, a signature of a structure with higher symmetry than for fast crystallized samples. In the later, second order reflections are only found in the patterns of PE21Br and PE15Br, or for samples containing the halogen with the strongest diffraction. Due to the small size of F, the layer peak is weak in the patterns of these systems obtained in the Nanostar diffractometer (Figure 1). Moreover, the layered crystal packing of the fluorinated systems was confirmed in patterns recorded at Argonne National Laboratory using synchrotron radiation (Figure SI 1). Thus, all X-ray patterns indicate a clear affinity for these precision systems to pack the halogens in layers, even if the halogen is of a small size like fluorine. Layer reflections and calculated periodicities for fast and slowly crystallized samples are listed in Table 2. The diffractograms of samples crystallized isothermally are shown in panels d, e and f of Figure 1. Except for the F series that maintain the orthorhombic pattern, the patterns of all Cl and Br precision samples display multiple reflections, and hence a different packing arrangement than under fast crystallization. Thus, it appears from the X-ray patterns that the sudden change in crystallographic packing with decreasing undercooling that was observed in PE15Cl, from an all-trans planar packing (Form I) to a non-planar herringbone-type molecular packing (Form II), may be a universal feature for precision polyethylenes with halogens other than fluorine. If this is the case, the change in conformation that enables Form II must be present in all isothermally 12 ACS Paragon Plus Environment

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crystallized samples. We test this hypothesis next by the characteristic change in the FTIR spectra from trans to gauche conformations of the backbone carbons vicinal to the substitution point, and by a detailed analysis of progression modes of the methylene sequences between consecutive halogens for conformance with the all-trans behavior of n-alkanes of equivalent length.

Table 2. WAXD layer reflections and calculated chain tilt. Sample

Tc (°C)

Halogen content, mol %

2θ (°) 1st order

PE21F

50 110 50 111 50 110

4.76 4.76 6.67 6.67 11.1 11.1

3.53 3.57 5.00 4.98 8.36 8.50

65 74 38 56 -15 36

4.76 4.76 6.67 6.67 11.1 11.1

4.50 4.25 6.20 5.69 8.68 9.11

47 62 0 35 -20 0

4.76 4.76 6.67 6.67 11.1 11.1

4.42 4.36 6.19 5.63 9.37 9.26

PE15F PE9F

PE21Cl PE15Cl PE9Cl

PE21Br PE15Br PE9Br

2θ (°) 2nd order

Observed layer distance, (Å) 25.07 24.78 17.69 17.75 10.58 10.40

all-trans extended repeat unit, (Å)a 26.67 26.67 19.05 19.05 11.43 11.43

Chain tilt (°)b

19.36 20.79 14.26 15.55 10.19 9.71

26.67 26.67 19.05 19.05 11.43 11.43

42.6

19.99 20.29 14.27 15.70 9.44 9.56

26.67 26.67 19.05 19.05 11.43 11.43

9.12 8.25 11.31

8.93 8.66 12.56 11.39

19.8 21.4 21.7 21.3 22.2 24.4

41.6 26.9

41.5 41.5 34.3

(a) The planar-zigzag length of the repeating unit is calculated with C-C bond length 1.54Å and CCC

bond angle 110° 37,38 (b) Estimated chain tilt with respect to the lamellae normal.

FTIR spectra, collected at the indicated crystallization temperatures, are given in Figure 3 for the spectral range between 500 and 1000 cm-1. Following the arrangement of the X-ray patterns in Figure 1, spectra in panels a, b and c are for fast crystallized samples, while those in panels d, e and f were obtained under isothermal crystallization. 13 ACS Paragon Plus Environment


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7

a) 13 S818 17

15

12

S8 16

1000

14

13

7

d) 13

11 10 9

12

11

900

10

S818 17

15

PE21F 118 °C 7

7

9 PE21Cl 65°C

15

13

11

15

13

11

PE21Cl 76 °C 7

7

9

S8

PE21Br 0°C

700

5

8

600

500

1000

900

9

PE21Br 58 °C

800

700

600

500

Wavenumber (cm-1)

1-3

8

e)

7

S6

10 9

PE21F 30°C

800

9

11 12

Wavenumber (cm-1)

b)

1-5

Tc

Tc

15 13 11 9 S8 16 14 12 10

15

1-5

Page 14 of 38

9

1-3

5

7

S6

11

Tc

11

Tc

PE15F 123 °C

PE15F 30 °C

11 S6

9

7

10 11

S6

1000

8

10

9

8

900

S6

5 6

7

9

11

S6

6 5

9

11

700

600

500

PE15Br 35 °C

1000

900

800

600

500

5

f)

1 Tc

3 4

700

Wavenumber (cm-1)

5

S4

6

5

7

PE15Br 0 °C

800

7

5 PE15Cl 58 °C

Wavenumber (cm-1)

c)

7

PE15Cl 0 °C

S4 7

PE9F 30 °C

6

3

4

1 Tc PE9F 110 °C

3

5

3

6 3

5

PE9Cl 42 °C 3

5

6

1000

5

PE9Cl 0 °C

PE9Br 10 °C

PE9Br -20°C

900

800

700

600

500

1000

Wavenumber (cm-1)

900

800

700

Wavenumber (cm-1)


600

500

Page 15 of 38

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Figure 3. FTIR spectra in the region of C-halogen stretching and for the CH2 rocking modes of precision polyethylenes with fluorine (blue), chlorine (red), and bromine (green) after fast crystallization (a, b, c), and after isothermal crystallization at the temperature indicated (d, e, f). The numbers indicate the methylene rocking mode order. S4, S6 and S8 near 1000 cm-1 are C-C stretching modes.

We first analyze the C-halogen stretching bands in the region between 500 and 700 cm-1 where striking differences are observed in the spectra of Cl and Br containing samples between fast and slow crystallization. In contrast, all the spectra of samples with F are basically flat in this region. The absorption of C-Cl and C-Br stretching is found below 750 cm-1, hence well removed from the region of CH2 rocking modes.33,39 In this region, the vibration frequency of the C-halogen bond depends on the conformation, trans or gauche, of the C-C backbone bonds adjacent to the carbon with the halogen substitution. Gauche conformers absorb at higher frequencies. Gauche and trans conformers are easily identified in all the spectra of Cl and Br containing polyethylenes of Figure 3 in reference to extensive past work on ethylene vinyl chloride and ethylene vinyl bromide copolymers.39–41 The C-Cl band at 614 cm-1 corresponds to vicinal trans conformers, and the band at 665 cm-1 is associated with the gauche conformations. Equivalent bands for bromine containing polyethylenes appear at frequencies of 538 cm-1 for trans conformers and at 612 cm-1 for those with the gauche conformation. As seen in all spectra for Cl and Br-containing samples of Figure 3, the intensity of the trans conformers of fast crystallized samples is very high compared to the absorption of gauche conformers, while the opposite is seen in the spectra of samples that develop relatively high levels of crystallinity after isothermal crystallization (panels c, d and f of Figure 3). Hence, one of the major differences between slow and fast crystallized precision Br and Cl containing polyethylenes is the conformation of the carbons adjacent to the carbon with the halogen. Using the degree of crystallinity at the temperature the FTIR spectra were recorded, and following quantitative analysis described for PE15Cl,16 we find that for samples rapidly crystallized all crystalline backbone bonds vicinal to the methine have the trans conformation, while the same analysis applied to the spectra of samples slowly crystallized gives gauche bonding around the substitution in the crystalline regions. Hence, the feature that emerges from the FTIR stretching region, combined with the X-ray patterns for all Cl and Br samples analyzed, is one of crystalline structures equivalent to the two structures found in PE15Cl and further 15 ACS Paragon Plus Environment


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Page 16 of 38

indexed from fiber patterns of PE21Br.16,17 Conversely, all precision systems with F form layered crystallites with the all-trans orthorhombic packing. For the latter, the calculated orthorhombic pattern is basically identical to the experimentally observed one (Figure SI 2). Schematics of the crystal structures in reference to the unit cells of systems with halogens spaced by 21 carbons, are given in Figure 4. Under fast crystallization, the molecules containing Cl or Br pack in layered Form I crystals with the all-trans molecular conformation (Figure 4b). A reduction in number or lack of the second order layer reflection in the WAXD patterns is attributed to some longitudinal mismatch in the registry of the intermolecular halogen layer. Furthermore, weak or loss of the reflections in the patterns of fast crystallized PE9Cl and PE9Br most probably reflect conformational disorder in the lattice by the need to stagger halogens in nearby layers within the core of lamellae crystals. We recall that core lamellar crystals of these systems are 90 – 180 Å thick, corresponding to 5 – 9 chain repeats, as found by AFM and small angle X-ray scattering.5 These relatively thick crystallites suggest that in non-oriented specimens long segments of the chain fold back and forth in lamellae crystallites and the shorter periodicity of the layer reflection (4º < 2θ

Infrared Spectroscopy and X-ray Diffraction ... - ACS Publications

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