Influence of Branch Incorporation into the Lamella Crystal on the

May 21, 2013 - Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200, United States ... Depending on the degree of short cha...
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Influence of Branch Incorporation into the Lamella Crystal on the Crystallization Behavior of Polyethylene with Precisely Spaced Branches Kazuya Matsui,† Shuichiro Seno,† Yoshinobu Nozue,†,* Yuya Shinohara,‡ Yoshiyuki Amemiya,‡ E. B. Berda,§,§ G. Rojas,§,⊥ and K. B. Wagener§,* †

Petrochemicals Research Laboratory, Sumitomo Chemical Co., Ltd., Kitasode, Chiba, Japan Department of Advanced Materials Science, Graduate School of Frontier Sciences, The University of Tokyo, Kashiwa, Chiba, Japan § Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200, United States ‡

S Supporting Information *

ABSTRACT: Depending on the degree of short chain branch (SCB) incorporation, the crystallization behavior and resultant crystalline structure drastically change in polyethylene with precisely spaced branches. In polyethylene with hexyl branches precisely spaced on every 21st carbon (HB21), only crystallization mediated by a transient hexagonal phase without incorporation of the SCB was observed. On the other hand, in polyethylene with ethyl branches precisely spaced on every 21st carbon (EB21), crystallization behavior was strongly dependent on the crystallization temperature. A thin lamella was formed through crystallization mediated by a hexagonal phase and no thickening occurred at 5−8 °C, while thickening of the transient hexagonal lamellae occurred at 10−15 °C, and one SCB seemed to be incorporated into a crystal stem. At 17 °C, no thickening of the hexagonal phase occurred and a hexagonal phase with sufficient lamella thickness was directly formed from the melt. At 21−28 °C, crystallization mediated by hexagonal phase formation was not clearly observed and the crystalline phase was mainly formed by nucleation and growth of a spherulite. Transition between crystallization mediated by a hexagonal phase and that by nucleation and growth of a spherulite is dominated by the degree of SCB incorporation into the crystalline lamellae. At 21 °C or higher, the inclusion of two branches into a stem destabilizes the hexagonal structure, while the free energy of formation of a triclinic phase may be stabilized by tilting the chains and optimizing the packing of the SCB inside the crystal.

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INTRODUCTION Control of the length of short chain branches (SCB) and their intermolecular distribution, i.e., chemical composition distribution, have attracted attention in the research and development of polyethylene, because these structural properties strongly affect mechanical properties, such as a tensile strength and resistance to stress crack.1−5 For further improvement of physical properties, the next property to be controlled is the “intramolecular” distribution of SCB, ultimately with precise placement of SCBs on the main PE chain. Acyclic diene metathesis (ADMET) technology developed by Wagener and co-workers has made it possible to synthesize PEs with precisely spaced branches.6−9 Previous reports have focused on their unique crystallization behavior10 and their very narrow crystal thickness distribution,11,12 as well as the effect of branch length on the degree of branch inclusion10−14 and the effect of spacing interval on crystalline morphology.12,14,15 Branched ADMET PE samples with either ethyl branches precisely spaced on every 21st carbon (EB21) or with hexyl branches on every 21st carbon (HB21) showed that © XXXX American Chemical Society

crystallization is mediated by a transient hexagonal phase (mesophase) under isothermal crystallization conditions when the crystallization temperature is fixed near Tc (∼11 °C in EB21 and ∼1 °C in HB21) determined by DSC cooling scans.10 In mesophase-mediated crystallization, the ethyl branches in EB21 can be incorporated into the crystal, resulting in the formation of thicker lamellae, while the hexyl branches in HB21 are too large for incorporation into the crystal. Furthermore, EB21 crystallizes through nucleation and growth of a spherulite in the range of 21−28 °C and two crystalline forms (form I and form II) appear depending on the crystallization temperature.16 Interestingly, cross nucleation17−20 was observed, in which an early nucleating polymorph (A) does not consume the entire liquid or undergo polymorphic transformation but nucleates another polymorph (B) of higher or lower thermodynamic stability (form I as Received: March 24, 2013 Revised: May 14, 2013

A

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Figure 1. WAXS (a, c) and SAXS (b, d) pattern changes during isothermal crystallization of HB21 at 1 and 5 °C. The scattering intensity in WAXS/ SAXS profiles is expressed by color graduation, where white and blue indicate higher and lower values, respectively. plates, and the sample temperature was controlled as described above for the synchrotron X-ray measurements. The sample compartment was constantly purged with N2 gas in order to prevent frost formation from moisture in the air and the absorption of moisture by the KBr plates. Differential Scanning Calorimetry (DSC). An EXTAR 6000 DSC6220 (SII nanotechnology, Inc.) was used for DSC measurements. Samples in aluminum pans were heated and kept at 100 °C for 5 min by a LINKAM hot stage and transferred to a zip-lock bag and kept at 10 °C in a water bath for the desired crystallization time. After crystallization at 10 °C, the sample pans were quickly moved to the DSC sample stage, already equilibrated at 10 °C, and immediately the samples were heated to 50 °C at a rate of 10 °C/min for the heat flow measurements. Polarized Optical Microscopy (POM). A BX50 BX-CSS (Olympus Co. Ltd.) was used for POM observation. The sample temperature control system mounted on the stage was the same as that described above for the synchrotron X-ray measurements. Samples for POM observation were prepared by the solution casting method. The polymer powder was dissolved in p-xylene at 120 °C for 15 min, and the solution was cast onto a cover glass plate heated at 80 °C. After that, the glass plate was dried in vacuum at 80 °C for 6 h to remove residual solvent completely.

polymorph (A) and form II as polymorph (B)) under isothermal crystallization of EB21 at 25 °C.16 It is interesting to consider why crystallization of EB21 is much more complicated than that of HB21. However, it is not known why a small difference of SCB structure (ethyl vs hexyl branches) induces very large differences in their crystallization behavior. In this work, the crystallization behaviors of EB21 and HB21 are thoroughly observed by SAXS and WAXS measurements over a wide range of isothermal crystallization temperatures. The complicated crystallization behavior of EB21 is discussed based on the effect of SCB numbers incorporated into a crystal stem.

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EXPERIMENTAL METHODS

Small- and Wide-Angle X-ray Scattering. Simultaneous SAXS and WAXS measurements were performed at BL-15A and BL-6A in the Photon Factory of KEK (Tsukuba). In BL-15A and BL-6A, the beam was collimated with the bending mirror and monochromator.21 The X-ray wavelength was 1.50 Å and the beam size at the sample position was about 500 μm × 500 μm. SAXS and WAXS camera lengths were calibrated with silver behenate diffraction rings. The detectors used for data acquisition were PILATUS 300 K and 100 K (DECTRIS Ltd., Switzerland) for SAXS and WAXS measurement, respectively. The sample cell was a temperature-controlling stage (Linkam Scientific Instruments Ltd., THMS-600), which can control the sample heating rate in the range of 0.01−130 °C/min. Samples for X-ray measurement were sandwiched between thin mica plates with a 100-μm spacer. Samples were initially held at 100 °C for 5 min, and then the temperature was dropped to the isothermal crystallization temperature at a rate of 100 °C/min. Isothermal crystallization temperatures examined were 1 and 5 °C and 5, 10, 13, 15, 17, 21, and 25 °C in HB21 and EB21, respectively. Infrared Spectroscopy. For the infrared spectroscopy measurements, a Nicolet 8700 FT/IR Spectrometer (Thermo scientific Co.) equipped with a liquid N2-cooled, single element MCT detector was used. The spectra were obtained with 8 scans at the resolution of 2 cm−1. Samples for IR measurements were sandwiched between KBr

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RESULTS AND DISCUSSION Dependence of Crystallization Behavior on Temperature. SAXS and WAXS pattern changes during isothermal crystallization in HB21 are shown in Figure 1. (The conventional intensity vs q plot with crystallization time is shown in Figure S1 of Supporting Information.) At 1 and 5 °C, the hexagonal phase was transiently formed, and a crystal packing structure with two diffraction peaks gradually grew, while the long period gradually decreased. We suggest that the long period decreased due to the additional formation of the hexagonal phase interpolated between initially formed lamellae. (Note: The details of the one-dimensional scattering pattern changes and the interpretations for SAXS/WAXS profiles are B

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Figure 2. WAXS (a, c, e, g, i) and SAXS (b, d, f, h, j) pattern changes during isothermal crystallization of EB21 at 5−17 °C.

described in our previous report.10) The only difference between crystallization at 1 and 5 °C is the growth rate of the hexagonal phase and the subsequent packing optimization. In HB21, no crystallization was observed above the nominal melting point (about 11−12 °C) determined by DSC.10 As was already reported, HB21 forms a very thin lamellar crystal with thickness (determined by TEM) of about 26 Å,11 which is comparable to the interval length between neighboring SCBs in the trans zigzag conformation (25.4 Å). This indicates that hexyl branches are perfectly excluded from the crystal, in

agreement with DSC measurements of ADMET-PE with various SCB structures.13 Figure 2 shows SAXS and WAXS pattern changes during isothermal crystallization of EB21 at various temperatures from 5 to 17 °C. In this temperature range, crystallization of EB21 is mediated by transient hexagonal phase formation. (The conventional plot for Figure 2 is also shown in Figure S2, Supporting Information.) It is interesting that the long period of some lamellae increases with time during isothermal crystallization at 10 −15 °C, while that of the other lamellae C

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hexagonal phase formation, and direct nucleation of an ordered crystal incorporating two SCBs may occur. It should be mentioned that the phase, hexagonal, or ordered crystal, which is favored to form also depends on the crystallization rate for each phase at the crystallization temperature.24,25 The crystallization rate of the hexagonal phase in a situation where two SCBs exist per stem will be much smaller than that of the ordered crystal phase. Therefore, only the ordered crystal phase was formed in the range 21−28 °C. Form I which initially grows at 21 and 25 °C seems to be a triclinic phase with highly ordered packing along the b-axis direction,16,26 and SCBs may be oriented along the a-axis direction. The experimental results presented above strongly support the view that the degree of ethyl branch inclusion into a lamella crystal is strongly influenced by crystallization conditions such as temperature. Previously, Hosoda investigated the relationship between crystallinity and the degree of SCB per polymer chain in cross fractioned ethylene−butene-1 copolymer with narrow intermolecular chemical composition distribution. He reported that SCBs could be incorporated into a crystalline stem up to two branches and that the degree of the SCB incorporation depended on the degree of SCB per polymer chain: a higher degree of SCB leads to more SCB incorporation into a crystalline stem.23 The crystallization condition employed in Hosoda’s study was the same for all the ethylene−butene-1 copolymers, i.e., rapid cooling by a watercooled press. On the other hand, Inci et al. reported that polyethylene with ethyl branch precisely spaced on every 39th carbon (EB39) exclude the ethyl branch from the crystal for cooling at 10 °C/min.14 In the case of EB39, the CH2 run length is sufficient to form crystals with optimized packing quickly without the necessity of including the ethyl branch into the crystal. It is considered that whether or not the ethyl branch is incorporated into a stem strongly depends on the relationship between the lamellae thickness required for the survival at the given crystallization condition and the interval length between each ethyl branch. In short, if the required lamellae thickness under the given crystallization temperature is larger than the interval length, ethyl branches are incorporated into a crystalline stem. Thus, the degree of ethyl branch inclusion into a crystalline stem is influenced by crystallization temperature, which strongly impacts the crystal structure. The dependence of EB21 crystal structure on crystallization temperature is shown schematically in Figure 4. Kinetics of Hexagonal Phase Formation. Polarized optical microscopy (POM) was used to investigate hexagonal phase formation, as shown in Figure 5. At 21 and 25 °C (Figure 5c,d), the formation of spherulites could be confirmed. On the

decreases (Figure 1(d), (f), (h)). Lamella thicknesses of EB21 formed at 11 °C were observed by TEM imaging (see Figure 3), which showed that there are two populations of lamellae:

Figure 3. TEM image of EB21 crystallized at 11 °C. The areas enclosed by dashed lines show the coexistence of thin and thicker lamellae.

thick lamellae (about 56 Å) and thin lamellae (about 25 Å). These two kinds of lamellae seem to correspond to the two long periods observed by SAXS. Thus, it is plausible to consider that the increase of long period with time corresponds to lamella thickening. Under isothermal crystallization at 10 °C, the population of lamella showing lamella thickening is low, and it becomes higher at higher crystallization temperature. The coexistence of two kinds of lamellae in EB21 at higher temperature can be explained as follows: As stated above for HB21, the nominal melting point of hexagonal phase without involving SCB is about 11−12 °C, which means that thicker lamella or lamellae with more optimized packing and lower free energy are required above 11−12 °C. In EB21, thin crystalline lamellae partially incorporating SCB inside may be originally formed due to the kinetic advantage. This may be followed by thickening or by packing optimization with expulsion of the SCB preferred from the viewpoint of free energy stabilization, in accordance with the Ostwald-ripening law.22 Once an SCB is incorporated into a stem, the most stable lamella thickness will be twice the interval between neighboring SCBs (i.e., just prior to the inclusion of one more SCB), and the lamella thickness may increase to about 53 Å (which corresponds to the length of 42 C−C bonds with trans zigzag conformation) at most. It should also be noted that the high chain mobility in the hexagonal phase may also accelerate the effective thickening until a stable thickness at the crystallization temperature is formed. On the other hand, for thin lamellae, where the SCB is expelled, the most stable lamella thickness will be just the interval length between SCBs. In the case of isothermal crystallization of EB21 at 17 °C (Figure 2j), no increase of long period occurred. It is considered that the lamella thickness formed at the initial stage of hexagonal phase formation is almost comparable to the most stable thickness, and there is thus little or no room for lamella thickening. The formation of thick lamella from the initial stage indicates that the thickness which is required for the survival at 17 °C is very near 53 Å, the largest thickness involving one SCB. On the other hand, for EB21 at 21−28 °C, crystallization mediated by hexagonal phase formation was not observed.16 At 21 and 25 °C, the lamella thicknesses evaluated from long period (SAXS) and crystallinity (WAXS) were 65 and 66 Å, respectively. Since these are thicker than 53 Å, the crystal formed at 21 °C or above must incorporate two SCBs per stem. Existence of two SCBs per stem may destabilize or disturb

Figure 4. Schematic model of branch incorporation into a stem in EB21 at various crystallization temperatures. D

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Figure 6. Time evolution of SAXS integrated intensity during isothermal crystallization of EB21 at various temperatures. The arrows in the graph indicate the beginning of secondary growth.

formation. However, the growth dimension of hexagonal phase formation is evidently less than three-dimensional, judging from the smaller increasing rates at 13 °C, 15 °C, and 17 °C compared to those at 21 and 25 °C. This situation resembles ‘recrystallization from ordered melt’. Carfagna et al. studied the recrystallization kinetics of isotactic polypropylene (iPP) and reported that the Avrami dimension was 1.2−1.6 when the iPP recrystallized after melting at 161−168 °C, where the molten structure has a memory of crystal conformation.29 It should be also mentioned that the curves in 15 and 17 °C are not expressed by single sigmoidal functions, and they seem to have multiple growth steps. The beginning of the secondary growth is indicated by arrows in Figure 6. Under isothermal crystallization at 15 °C, the secondary growth was found to correspond to the lamellae thickening. On the other hand, the secondary growth observed at 17 °C is not due to the lamellae thickening, but rather to the increasing number of thicker lamellae. It is thought that no lamellae thickening occurs at this temperature, as confirmed by the time evolution of the SAXS profile shown in Figure 2(j). Although we do not have a definite answer about what happened between 10 and 100 s at 17 °C, it seems that the kinetics of hexagonal phase formation may vary with isothermal crystallization temperature. To investigate the conformational state of EB21 before the phase transition from melt to hexagonal phase, we performed time-resolved IR measurements while cooling from 100 to 15 °C at 100 °C/min and during subsequent isothermal crystallization at 15 °C. Figure 7a shows spectral changes in the CH2 rocking mode region (720−730 cm−1). The peak absorbance is plotted vs temperature during cooling in Figure 7b and vs time during isothermal crystallization at 15 °C in Figure 7c. It was found that the peak absorbance increased to a greater extent during the cooling process than during isothermal crystallization. It should also be noted that another peak at 721 cm−1, which is the characteristic band of the hexagonal rotator phase in n-alkanes,30 appeared before isothermal crystallization (Figure 7a). This indicates that the number of linkages having the trans conformation increased during the cooling process, so that the molten state just prior to the phase transition was more ordered compared to the equilibrium molten state, and that the conformational state of cooled melt was quite similar to that of the hexagonal phase. This state could be regarded as an ordered molten state with crystal memory, as is the case with the molten state of iPP.27 It is speculated that in the hexagonal phase formation lamellae

Figure 5. POM images during isothermal crystallization of EB21 at 13 °C (a), 17 °C (b), 21 °C (c), and 25 °C (d).

other hand at 13 and 17 °C (Figure 5a,b), where transient hexagonal phase was formed at the early stage of crystallization, bicontinuous-like structures having a characteristic wavelength of about 2 μm were observed as bursts here and there. They did not seem to have a definite nucleation step like that observed in spherulite formation. The bicontinuous-like structure may be analogous to what is observed in spinodal type phase separation. Olmsted et al reported that the conformationdensity coupling could induce liquid−liquid (LL) spinodal phase separation before crystallization.27 However, at this time we cannot conclude whether spinodal LL phase transition occurred in hexagonal phase formation, and more detailed experiments are needed to elucidate this process. At this point, it is fairly certain that the higher-order structure of the hexagonal phase and the growth manner are quite different from those in the formation of spherulites. To study in detail the kinetics of the hexagonal formation in EB21, we calculated integrated SAXS intensities during isothermal crystallization from the data shown in Figure 2 and in Figure 1 of ref 16, and we plotted the data vs crystallization time (see Figure 6). At 21 and 25 °C, the curve had a sigmoidal shape with an obvious induction period. The Avrami constant n at those crystallization temperatures was approximately 3, indicating that crystallization proceeded in accordance with three-dimensional nucleation and growth kinetics. On the other hand, the curve shapes at 17 °C and below were quite different from those at 21 and 25 °C in terms of the increasing rate related to crystal growth dimension. It is difficult to apply Avrami analysis28 to these plots for kinetics information, because lamella thickening and/or packing optimization, which may contribute to the change of scattering intensity, could be occurring in addition to hexagonal phase E

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Figure 7. IR spectra and absorbances during the cooling process (100 to 15 °C at 100 °C/min) and the subsequent isothermal crystallization of EB21 at 15 °C: (a) IR spectrum and (b) temperature dependence of peak absorbance during cooling process, (c) time evolution of absorbance at 721 cm−1 during crystallization. The vertical line in part a indicates the position of 720 cm−1.

may be growing along with a “quasi-hexagonal region”, which may lead to the anomalous one- or two-dimensional crystal growth. On the other hand, in spherulite formation, polymer chains have to reform triclinic crystal nuclei from the ordered molten state due to a difference of packing between the triclinic crystal and the hexagonal phase. Consequently, it is considered that the crystallization kinetics at 21 and 25 °C follow the usual three-dimensional nucleation and growth. Lamella Thickening and Melt-Recrystallization. We also used time-resolved IR measurements to observe changes during lamellae thickening for EB21. Figure 8 shows absorbance changes for a trans band (719 cm−1), two kink bands (1303, 1368 cm−1) and, a gauche band (1352 cm−1) during isothermal crystallization at 10 and 15 °C. At these temperatures, the absorbance of the trans band gradually increased, while the kink and gauche bands decreased with

time. Also, the rate of change of these bands with logarithmic time was lower in the 5−60 s range than in the 60−400 s range, especially at 15 °C. This timing almost corresponds to the time when the lamellae thickening starts (the time is indicated by arrows in Figure 8) and the times when secondary growth in SAXS integrated intensity begins (Figure 6). Furthermore, more lamellae were thickened at higher crystallization temperature, as confirmed by the change of SAXS profile (Figure 2d,h). On the other hand, we also performed the IR measurement under isothermal crystallization at 1 °C for HB21, where no lamellae thickening occurred as mentioned in the previous section. Figure 9 shows absorbance changes for the same bands as in EB21. In HB21, the rate of the change was almost constant, because the changes of the absorbance are attributed only to the formation of thin lamellae. From these

Figure 8. Time evolution of peak absorbance at 719 cm−1, 1368 cm−1, 1352 cm−1, and 1303 cm−1 during isothermal crystallization of EB21 at 10 °C (open circle) and 15 °C (filled circle). The arrows indicate the beginning of the lamellae thickening.

Figure 9. Time evolution of peak absorbance at 719 cm−1, 1368 cm−1, 1352 cm−1, and 1303 cm−1 during isothermal crystallization of HB21 at 1 °C. F

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Figure 10. DSC curve during melting of EB21 isothermally crystallized at 10 °C for various crystallization times (a) and crystallization time dependence of integrated endothermic heat flow (ΔH1 and ΔH2) (b). The definitions of ΔH1 and ΔH2 are shown in part c.

Figure 11. SAXS (left) and WAXS (right) pattern changes during heating process after crystallization at 10 °C for 90 min (a, b) and 1 min (c, d).

formed by recrystallization is form I. Under isothermal crystallization at 25 °C, form I nucleates and grows,16 and the result is reasonable. However, it should be noted that crystallization of form I is faster at 21 °C than at 25 °C due to the greater degree of supercooling. If the memory of the trans zigzag conformation accelerates the melt-recrystallization process, recrystallization should also be observed when the sample formed by several seconds crystallization at 11 °C is melted because it melts at around 22−23 °C (Figure 10a). The recrystallization efficiency increases with time, and the integrated endothermic heat flow of the recrystallized component (ΔH2) after 3 h crystallization is about 2 times that after 1 h crystallization (Figure 10b); ΔH2 after 1 and 3 h crystallization are 8.46 and 17.5 J/g, respectively. However, there is only a very small change in the degree of transformation from the hexagonal phase to the more-packed crystalline phase between 1 and 3 h crystallization (Figure 3c in ref 10). Thus, it is considered that recrystallization is not accelerated by the formation of a more-packed crystalline structure from the transient hexagonal phase, but rather that acceleration is due to lamellae thickening. Furthermore, the

results, it seems that lamella thickening is accompanied by a change from gauche and kink conformations to the trans zigzag conformation. To clarify the effect of crystallization time on the melting point, we measured the heat flow of EB21 isothermally crystallized at 10 °C and compared the results for different crystallization times. Figure 10a shows the DSC heat flows during heating of samples after allowing them to crystallize for the indicated times. The peak melting temperature of original crystal clearly becomes higher when the crystallization time increases. The other interesting point is that recrystallization occurs at around 25 °C after melting of the original crystal (including mesophase) when isothermal crystallization time increases. To specify the crystalline form which is formed through melt-recrystallization and clarify the difference of melting behavior based on the difference of crystallization time, we measured simultaneous SAXS-WAXS patterns during melting of EB21 after 1 and 90 min crystallization at 10 °C. (see Figure 11). Although Figure 11b shows a very weak WAXS pattern at around 25 °C, we concluded that the crystalline form which is G

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increase in the time evolution of ΔH2 is similar to that of the population of thicker lamellae formed by lamellae thickening (Figure 3a in ref 10). As confirmed by the change of IR absorption (Figure 7), lamellae thickening accompanies the increase in the trans zigzag conformation, which will generate longer trans sequences. The longer trans sequence in thicker lamellae is assumed to be more preserved after crystal melting than the shorter trans sequences in thinner lamellae. It is reasonable that the memory structure of the ‘thicker’ hexagonal phase induces faster and more effective recrystallization of thick lamellae including two SCBs. On the other hand, when crystallized for very little time at 10 °C, no recrystallization after melting was confirmed (Figure 11c), as is the case with the DSC measurements. And it is interesting that the peak position of scattering intensity at lower scattering vector (0.12 Å−1 at 10 °C) rapidly increased at 18−19 °C during heating, possibly due to thickening of hexagonal thin lamellae, because the packing has not optimized yet. Therefore, it is considered that the lamellae were thickened during heating resulting in a higher melting point than the initially formed structure at 10 °C.

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Present Addresses §

Department of Chemistry and Materials Science Program, University of New Hampshire, Durham, NH 03824 ⊥ The Colombian Sugar cane Research Center, Calle 58N #3BN-110, Cali, Colombia Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The experiments at Photon Factory were performed under the approval of the Photon Factory Program Advisory Committee (Proposal Nos.: 2010G540 and 2012G663). Preliminary SAXS and WAXS results were acquired at SPring-8 (Affected Facility User Support Proposal, No. 2011A1960). This material is based upon work supported by the National Science Foundation under Grant No. DMR-0703261. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. This material is based upon catalyst work supported by, or in part by, the U.S. Army Research Laboratory and the U.S. Army Research Office under Grant No. W911NF-09-1-0290. And finally K.M. thanks Mr. T. Suzuki for his assistance with the IR measurements.

CONCLUSION

Depending on the degree of SCB incorporation, crystallization behavior and the resultant crystalline structure drastically change in polyethylene with precisely spaced branches. In HB21, only crystallization mediated by a transient hexagonal phase without incorporation of short chain branches (SCB) was observed. On the other hand, in EB21, crystallization behavior was strongly dependent on the crystallization temperature. At 5−8 °C, thin lamellae without incorporation of SCB were formed by crystallization mediated by a hexagonal phase. At 10−15 °C, thickening of the transient hexagonal phase occurred, with apparent incorporation of one SCB into the crystal stem. At 17 °C, no thickening of the hexagonal phase occurred, and hexagonal phase with sufficient thickness was directly formed from the initial stage of crystallization. At 21− 25 °C, crystallization mediated by hexagonal phase formation was not clearly detected, and the crystalline phase was directly formed by nucleation and growth. In EB21, the transition from crystallization mediated by a hexagonal phase to that occurring by nucleation and growth is dominated by the degree of SCB incorporation. The 17 °C crystallization temperature is near the transition temperature between hexagonal phase mediated crystallization and nucleation and growth crystallization, and there is little room for lamella thickening without including one more SCB into the stem. At 21 °C or higher, the inclusion of two branches into a stem destabilizes the hexagonal structure, while the free energy of formation of a triclinic phase may be stabilized by tilting the chains and optimizing the packing of the SCB inside the crystal.

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(1) Hosoda, S.; Uemura, A. Polym. J. 1992, 24, 939. (2) Kennedy, M. A.; Peacock, A. J.; Failla, M. D.; Lucas, J. C.; Mandelkern, L. Macromolecules 1995, 28, 1407. (3) Krishnaswamy, R. K.; Yang, Q.; Fernandez-Ballester, L.; Kornfield, J. A. Macromolecules 2008, 41, 1693. (4) Lustiger, A.; Markham, R. L. Polymer 1983, 24, 1647. (5) Lu, X.; Zhou, Z.; Brown, N. Polym. Eng. Sci. 1997, 37, 1896. (6) Smith, J. A.; Brzezinska, K. R.; Valenti, D. J.; Wagener, K. B. Macromolecules 2000, 33, 3781. (7) Sworen, J. C.; Wagener, K. B. Macromolecules 2007, 40, 4414. (8) Rojas, G.; Wagener, K. B. Macromolecules 2009, 42, 1934−1947. (9) Rojas, G.; Berda, E. B.; Wagener, K. B. Polymer 2008, 49, 2985. (10) Nozue, Y.; Kawashima, Y.; Seno, S.; Nagamatsu, T.; Hosoda, S.; Berda, E. B.; Rojas, G.; Baughman, T. W.; Wagener, K. B. Macromolecules 2011, 44, 4030. (11) Hosoda, S.; Nozue, Y.; Kawashima, Y.; Utsumi, S.; Nagamatsu, T.; Wagener, K. B.; Berda, E.; Rojas, G.; Baughman, T.; Leonard, J. Macromol. Symp. 2009, 282, 50. (12) Hosoda, S.; Nozue, Y.; Kawashima, Y.; Suita, K.; Seno, S.; Nagamatsu, T.; Wagener, K. B.; Inci, B.; Zuluaga, F.; Rojas, G.; Leonard, J. K. Macromolecules 2011, 44, 313. (13) Rojas, G.; Inci, B.; Wei, Y.; Wagener, K. B. J. Am. Chem. Soc. 2009, 131, 17376. (14) Inci, B.; Lieberwirth, I.; Steffen, W.; Mezger, M.; Graf, R.; Landfester, K.; Wagener, K. B. Macromolecules 2012, 45, 3367. (15) Inci, B.; Wagener, K. B. J. Am. Chem. Soc. 2011, 31, 11872. (16) Nozue, Y.; Seno, S.; Nagamatsu, T.; Hosoda, S.; Shinohara, Y.; Amemiya, T.; Berda, E.; Rojas, G.; Wagener, K. B. ACS Macro Lett. 2012, 1, 772. (17) Yu, L. J. Am. Chem. Soc. 2003, 125, 6380. (18) Chen, S.; Xi, H.; Yu, L. J. Am. Chem. Soc. 2005, 127, 17439. (19) Stoica, C.; Tinnermans, P.; Meekes, H.; Vlieg, E.; van Hoof, P. J. C.M.; Kaspersen, F. M. Cryst. Growth Des. 2005, 5, 975. (20) Tao, J.; Yu, L. J. Phys. Chem. 2006, 110, 7098. (21) Igarashi, N.; Watanabe, Y.; Shinohara, Y.; Inoko, Y.; Matsuba, G.; Okuda, H.; Mori, T.; Ito, K. J. Phys.: Conf. Ser. 2011, 272, 012026. (22) Ostwald, W. Z. Phys. Chem. 1897, 22, 289. (23) Hosoda, S. Polym. J. 1988, 20, 383. (24) Cavaollo, D.; Azzurri, F.; Floris, R.; Alfonso, G. C.; Balzano, L.; Peters, G. W. Macromolecules 2010, 43, 2890.

ASSOCIATED CONTENT

S Supporting Information *

Time evolution of WAXS / SAXS profiles. This material is available free of charge via the Internet at http://pubs.acs.org/.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: (Y.N.) [email protected]; (K.B.W) [email protected]fl.edu. H

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dx.doi.org/10.1021/ma400608q | Macromolecules XXXX, XXX, XXX−XXX