bk-2000-0739.ch010

Chapter 10

Downloaded by GEORGE MASON UNIV on July 10, 2014 | http://pubs.acs.org Publication Date: July 30, 1999 | doi: 10.1021/bk-2000-0739.ch010

Simultaneous In-Situ SAXS and WAXS Study of Crystallization and Melting Behavior of Metallocene Isotactic Poly(propylene) 1

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Patrick S. Dai , Peggy Cebe , Malcolm Capel , Rufina G. Alamo , and Leo Mandelkern 3

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Department of Physics and Astronomy, Tufts University, Medford, MA 02155 Department of Biology, Brookhaven National Laboratory, Upton, NY 11973 Department of Chemical Engineering, Florida Agricultural and Mechanical and Florida State University, Tallahassee, FL 32310 Institute of Molecular Biophysics and Department of Chemistry, Florida State University, Tallahassee, FL 32306 2

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The isothermal crystallization and subsequent melting behavior of a metallocene isotactic poly(propylene) (m-iPP) was studied by simultaneous in-situ wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS), and differential scanning calorimetry (DSC). The m-iPP chosen was one which is known to produce large amounts of the γ modification under normal isothermal crystallization conditions. Both DSC and WAXS data show that during crystallization at 117°C, α and γ modifications appear at about the same time. Thermal analysis by immediate rescan after partial crystallization shows that m-iPP exhibits dual melting endotherms. WAXS scans show that during melting, the γ modification melts first at lower temperature, followed by α modification at higher temperature. Once γ crystals begin to melt, they do not undergo recrystallization, nor do they convert into α phase crystals. We also report changes in the SAXS parameters corresponding to the events observed in DSC and WAXS. Systematic changes in the scattering invariant and Bragg long period are seen during isothermal crystallization and melting of miPP.

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Corresponding author.

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© 2000 American Chemical Society

In Scattering from Polymers; Cebe, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

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Introduction Isotactic poly (propylene), iPP, exists in several different crystal structures, depending upon the packing of the helical chains [1-6]. The three modifications are monoclinic (a) [2], trigonal (β) [3], and orthorhombic (γ) [4-6]. The formation of these modifications depends on several factors, such as the thermal treatment conditions, mechanical conditions, the molecular weight, and content of molecular chain defects. Of all the structures, the α modification is the most common, observed in melt crystallization at atmospheric pressure, in commercial-type poly(propylenes). The β and γ modifications in Ziegler-Natta synthesized iPP usually can only be formed under highly specific conditions. The β modification usually forms under conditions of high cooling rates [7,8], high crystallization temperature [8,9], or stress applied under melt conditions [10], and only in very small amounts. In the presence of selective β-nucleating agents [11-13], the content of β can be significantly increased. The β and α modifications may occur together forming complex spherulitic structures, and at certain temperatures the growth rate of β is larger than that of α [14]. The γ modification occurs even more rarely, but may form in degraded, low molecular weight iPP, or under high pressure conditions [15-18]. The relevance of the γ phase of iPP is that it is "the first and so far unique example of a polymer structure with non-parallel chain stems."[l] Here we report a study of the isothermal melt crystallization and subsequent melting of an isotactic poly(propylene) (m-iPP), synthesized with metallocene-type catalysts [19]. A material with relatively low isotacticity has been chosen for this study. Characteristics of this material, and several prior studies which include this material, have been reported [20-23]. In this m-iPP, the γ modification occurrence does not require any special crystallization conditions as previously found for conventional iPP, and it can form a significant fraction of the total crystal population [21,23]. Formation of γ-iPP may be attributed to the existence of high contents of stereo and regio defects in the chain [23]. α modification is favored by rapid cooling and low crystallization temperature, while γ is favored by higher crystallization temperature. This m-iPP offers the chance to study the crystallization and melting behaviour of γ phase crystals developed under ambient pressure conditions, in an undegraded material. Using simultaneous SAXS and WAXS we follow structure development during isothermal crystallization and subsequent melting. SAXS is a widely used method for the investigation of lamellar structure in the two phase systems. To obtain structural parameters, such as the average crystal separation and crystal thickness, the one dimensional electron density correlation function is often used. In syndiotactic poly(propylene) the one dimensional model calculation [24] can be applied since the amorphous phase and crystalline phase form one-dimensional stacks of crystalline lamellae. But in the case of monoclinic iPP, electron microscopy reveals the existence of unique cross-hatched lamellar structure [25-30], and the applicability of the one dimensional model has been questioned by Albrecht and Strobl [31]. These researchers used SAXS and dilatometry to study structure development in PP, and presented a scheme to check for the failure of the



154 one dimensional model. In the present work, we calculate SAXS parameters that depend only on the position of the intensity maximum, reserving the correlation function analysis for a later publication [32]. The origin of the multiple endothermic response in iPP is also an important issue. Contributions to multiple endotherms may arise for many different reasons including melting and recrystallization during DSC scanning [33-35], different levels of perfection, such as primary and secondary crystals [36,37], or different crystal modifications [30,35]. In iPP with a low concentration of defects in the chain, the double melting of monoclinic crystals has also been directly associated with the presence of cross-hatching [38]. Here, we show that the α and γ phase melt at quite different temperatures, and create the dual endothermic response seen in this m-iPP. Contrary to previous findings [39] once the γ crystals have melted, they do not recrystallize and they do not transform into α phase crystals.

Experimental Section The metallocene isotactic poly(propylene) (m-iPP) used in this study is an experimental product of Hoechst. Characterization of the material shows that the fractional content of isotactic pentads (mmmm) is low at 0.908 mol-%, the M is 335,500 g/mol, and the polydispersity is 2.3 [20]. Defect content was assessed by C-NMR [20] and the stereo and regio defects are 1.68mol% and 0.67mol%, respectively. The material, received as pellets, was formed into films by compression molding at 200°C, then was quenched to room temperature in cold water. Differential scanning calorimetry (DSC) study was carried out on the TA instruments model 2920-DSC. The heat flow and temperature were calibrated using Indium as a standard. Nitrogen was used as a protection gas (30 ml/min) and no thermal degradation was detected. Thin films were encapsulated in A l pans and the sample mass was about 9 mg. The m-iPP was melted at 200°C for 1 minute, then cooled to 117°C at 10C°/min for isothermal crystallization. The crystallization time at 117°C varied from 1 to 40 minutes, and the sample was immediately re-heated at 5°C/min (without cooling) to observe the development of melting endotherms after partially crystallizing the material. Real-time small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS) were performed at beam line X12B of National Synchrotron Light Source at Brookhaven National Laboratory. Monochromatic X-radiation with a wavelength λ = 1.54Å was used. The sample was located inside the Mettler FP80 hot stage between two layers of Kapton™ tape. The SAXS data were collected with a twodimensional position sensitive histogramming detector. The sample to detector distance was 180.0 cm. SAXS data were taken continuously during the experiment, and each scan was of 20 seconds or 60 seconds duration. All SAXS data were first corrected for absorption, beam line intensity fluctuation and background. Then the Lorentz corrected intensity, I(s)s (where s=2sinθ/λ), w

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passed a low pass zero-phase digital filter, which was used to eliminate the high frequency noise (comprising more than 90% of the frequency distribution). The filter was run in both forward and reverse directions, to ensure there was no phase shift introduced in the process. The filtered Lorentz corrected intensity was smoothed by an area conserving four-point smoothing function, and finally interpolated by Fourier transformation methods, and extrapolated to scattering vector s = oo by Porod's law. In this paper, we calculate quantities which are independent of the one dimensional model assumption. Here, the scattering invariant, Q, is found from:

Q is also proportional to:

where x is the linear stack crystallinity, x is the spherulite volume fraction, and Δρ is the electron density difference between crystals and amorphous phase. The average Bragg spacing, L , is found from the peak of the Lorentz corrected intensity. The WAXS data were collected with a Braun 7cm one-dimensional position sensitive wire detector. The detector operated at 3 kV, with Argon/Methane (90/10) gas flowing at 1 ml/min. The d-spacings were calibrated by reference to NaCl and KC1 powders. The 20 angular range covered by the wide angle detector was 9.4° to 41.2°. Scans were collected for 20 seconds or 60 seconds simultaneously with the SAXS scans. A l l WAXS data were first corrected for background. The resulting WAXS data contain high frequency noise, especially during the initial stage of isothermal crystallization, when the crystal content is low. We use a low pass zerophase digital filter to remove the high frequency noise. Finally, for calculation of the relative area under the α and y peaks, the amorphous scattering curve from the completely melted state was scaled and subtracted from the WAXS data. In the WAXS diffractograms of iPP [18], many peaks of α, β, and γ crystals are in similar 20 locations. However, each modification has a distinctive reflection peak, which is well defined in our experiment. The α and γ modifications are distinguished by their own characteristic scattering angle 20 and Miller indices (hkl), at 18.5° (130) for a, and 20.2°C (117) for y. No β phase was observed in our study. To quantify the relative amount of α and y crystals, we calculate the area under the characteristic peak for the i - crystal type, Sj (i=l, 2 for α and y, respectively). The ratio: 0

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reflects the relative proportion of the ill* modification. This calculation was performed for both the WAXS peak areas, and the endothermic heat flow areas. The heat of fusion for the α and y forms have been reported to be similar [40].


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Results and Discussion DSC and WAXS Results Compared to the conventional Ziegler-Natta synthesized iPP, the m-iPP used in this study has a lower crystallization temperature and a low melting temperature. Figure 1 shows the DSC exothermic heat flow during isothermal crystallization at 117°C. At this temperature, the crystals develop fairly quickly, with a crystallization half-time of about 4 minutes. Little exothermic heat flow is seen past 15 minutes. Figure 2a shows the DSC melting curves, immediately after crystallization at 117°C for 1, 2, 3, 4 and 5 minutes. In the case of the 1 minute scan, only one very tiny endotherm at T = 142°C is observed. In the case of the 2 minutes scan, two endotherms appear: one is at the same position of T , and the other appears at lower temperature Tj = 132°C. The curve is nearly flat, and its area compared to the higher temperature one is quite small. For the 3, 4 and 5 minutes scans, there are clearly two growing endotherms. The locations of the first and second endotherms are in about the same positions, but the ratio of area of the lower Tj endotherm to that of the higher T endotherm is significantly larger than in the case of 2 minutes. When the crystallization time is prolonged beyond the half time to 10, 20 and 40 minutes, as shown in Figure 2b, the double endotherms are barely changed. Both melting temperatures are shifted in the higher temperature direction. This suggests that there are two populations of crystals developed in the m-iPP sample during isothermal crystallization. At the initial stage of crystallization, for the times less than the half time, the two populations grow nearly simultaneously. When the crystallization continues beyond the half-time, both populations remain relatively stable, and the ratio of endothermic peak heights is nearly constant. Using the data from Figures 2a,b, we calculate the areas underneath the lower and upper endothermic peaks. A flat baseline was drawn underneath the two peaks, and the total area calculated. The peaks were then simply divided by a straight line through the valley between the peaks. The relative area under the lower peak is shown in Figure 3 (solid squares) as a function of original crystallization time. The crystal population forming the lower endotherm, which later on will be shown to be γ phase, increases rapidly at first, and then levels off after 10 min. The γ phase at the end of crystallization represents 0.65 of the total. To verify the origin of these two endotherms, we must look at the WAXS data. Figure 4 is a sequence of WAXS intensity curves taken during the initial 8 minutes of isothermal crystallization at 117°C. Each scan is taken for 1 minute. The distinctive scattering peak positions for α and γ phase are shown by arrows. Both types of crystals begin to grow within the first 2 minutes. Then follows the rapid growth of γ crystals. By the end of 4 minutes, the γ peak is nearly as high as α peak, and by the end of 5 minutes, the γ peak becomes even higher than the α peak. The small peak at 16° is a combination of a γ reflection and a small amount of noise. At the initial stage, the amount of α crystals is significantly larger than the γ crystals. As time goes on, the γ crystals catch up, and eventually form an even larger amount of


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Time (min) Figure 1. DSC exothermic heat flow of m-iPP during isothermal crystallization 117°C.



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Figure 2. DSC heat flow melting endotherms, immediately after m-iPP crystallized at 117°C for the times indicated. Heating rate of 5°C/min. a.) 1, 2, 3, 4 and 5 min.; b.) 5, 10, 20 and 40 min.


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Crystallization Time (min) Figure 3. The time development of γ crystals of m-iPP during isothermal crystallization at 117°C from equation 3. Relative γ fraction using data taken from area of the immediate rescan endotherms shown in Figure 2 ( • ), and from area under the WAXS scattering curves ( Ο ).



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15 20 25 30 Scattering Angle 2Θ (°)

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Figure 4. WAXS intensity vs. scattering angle taken during the initial eight minutes of m-iPP isothermal crystallization at 117°C.



161 crystals. Using equation 3, the relative area under the γ characteristic WAXS peak is plotted in Figure 3 (open circles) where it may be directly compared to the lower endothermic peak area. The agreement between the two data sets is very close, though the similarity of time scales is a qualitative one. Figure 5a is a sequence of WAXS intensity curves taken during melting, in the temperature range from 121°C to 140°C. After isothermal crystallization at 117°C for 60 minutes, we quenched the m-iPP sample to room temperature, and then heated at l°C/min from 100°C to 170°C. We took each scan for one minute, so the scans in Figure 5a are separated by 1°C intervals. The crystal scattering intensity begins to drop at about 124°C, and this trend continues until finally γ completely disappeared by 140°C. Even after complete γ melting, a small noise peak remains in the WAXS spectrum at 16°. The intensity of α crystals remains nearly constant within this temperature range. But the subsequent melting of α crystals happens at a faster pace, as indicated in Figure 5b which shows the WAXS intensity curves in the temperature range of 141 °C to 150°C. The α crystal intensity begins to drop at 141°C, and by 148°C, has completely disappeared. Comparing the above DSC and WAXS observations, during the initial isothermal crystallization at 117°C, the α and γ crystals seem to form simultaneously. During subsequent melting, the γ crystals melt first at Ti, and the α crystals melt second at T . There is no recrystallization of γ crystals, nor do they convert into α crystals, once they have melted, as indicated by the constant intensity of the α phase reflection during the melting of the γ phase. u

SAXS Results Figure 6a shows the time development of the scattering invariant Q and Bragg long spacing L during isothermal crystallization. During the initial stage of crystallization, the invariant develops very fast, and after 6 minutes, the increase becomes smaller, and then holds steady for the remainder of crystallization. The long spacing L decreases from an initial value of 20.9 nm, then decreases steadily with time at a much slower pace. After 60 minutes, L is reduced to 17.1 nm. Figure 6b shows the temperature variation of the scattering invariant Q and long spacing L during subsequent melting after crystallization. In this test, the sample was first quenched from 117°C to room temperature and then reheated at l°C/min. From 100°C to about 124°C, both Q and L increase together. The change in long period is far greater than can be accounted for on the basis of thermal expansion. The linear coefficient of thermal expansion of PP is given as 14.6 χ 10* /°C (from 3060°C) [41] while here the change in L is 5.3 χ 10"3/°C (from 100°C to 124°C). The changes in Q and L can be explained on the basis of crystals perfecting and/or melting during heating. The quenched sample contains a small population of very imperfect crystals which can become perfected through mechanisms such as melt-recrystallization, fold surface smoothing, or rejection of defects from the crystal. These would tend to increase the electron density difference between the crystal B

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Scattering Angle 2Θ (°)

Scattering Angle 2Θ (°)

Figure 5. WAXS intensity vs. scattering angle taken during melting of m-iPP after isothermal crystallization at 117°C for 60 minutes. Heating rate of 1 °C/min. a.) 121°C to 140°C; b.) 141°C to 150°C



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Figure 6. Scattering invariant, Q, and Bragg long period, L , of m-iPP: a.) vs. time during isothermal crystallization at 117°C; b.) vs. temperature during subsequent melting at heating rate of 1 °C/min. B


164 phase and the amorphous interlayer in the temperature interval between 100°C and 120°C. Since Q is proportional to the square of the electron density difference (see eqn. 2), small changes in Δρ are amplified. Regarding changes in L , crystal perfecting should leave L unchanged. The fact that L increases further suggests that some imperfect crystals melt at these lower temperatures. Once the main population of y phase crystals begins to melt, at about 124°C, Q decreases while L increases. The slope of either Q or L vs. Τ smoothly changes as γ melting ends and α melting begins. At 145°C, L seems to decrease, a result of the inability to properly calculate these Bragg long periods: the Bragg peak is moving into the beam stop region at this point, and the intensity is quite weak. B

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Conclusions We have performed simultaneous SAXS and WAXS experiments on an m-iPP known to crystallize in α and y modifications. Our results show that: 1. During isothermal crystallization at 117°C, α and y crystals grow simultaneously, within the time resolution of our experiment. The fraction of y crystals is larger than α at the end of the crystallization. 2. Dual endotherms in m-iPP are caused by the melting of different crystal modifications, γ crystals melt at a lower temperature than α crystals. 3. Once γ crystals have melted, they do not recrystallize, nor do they transform into α crystals.

Acknowledgments Research was supported by the U.S. Army Research Office, Grant DAAH04-961-0009. The work performed at Florida State University was supported by NSF Polymer Program (DHR-94-19508).

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