Abstract
We report a study of metallocene isotactic poly(propylene) (m-iPP) which crystallizes into α and γ crystal modifications. Simultaneous in-situ small-and wide-angle X-ray scattering (SAXS and WAXS) were used to study kinetics during crystallization. Both techniques provide information about time development of crystallinity, while WAXS gives also kinetics of formation of α and γ crystals. During the earliest stages of crystal formation, the SAXS Bragg peak occurs simultaneously, or slightly lags, the appearance of crystalline WAXS reflections. We conclude crystallization occurs by a nucleation and growth process in this m-iPP.
Introduction
In the crystallization of conventional isotactic poly(propylene) (iPP), prepared with heterogeneous Ziegler-Natta type catalysts, monoclinic α phase crystals, and in some cases both α and triclinic β phase crystals, were observed using wide-angle X-ray scattering (Varga, 1992; Lotz, Wittmann & Lovinger, 1996) . The occurrence of orthorhombic γ phase crystals is rare in this material, and occurs only under highly specific conditions, such as crystallization under elevated pressures (Sauer & Pae, 1968; Mezghani & Phillips, 1997) . One of the most important features of the γ phase crystal in iPP is that it is the only known example of a crystal with non-parallel chain stems (Meille, Bruckner & Porzio, 1990) . The recent generation of metallocene iPP (m-iPP), synthesized with homogeneous transition metal catalysts (metallocenes), crystallizes readily into both α and γ phase under normal conditions (Alamo, Galente, Lucas, & Mandelkern, 1995) . Using m-iPP, we can now more easily study the complex morphological connection between α and γ phase crystals.
Recent studies of initial stages of crystal growth of α phase in either Ziegler-Natta type iPP or m-iPP have been conducted using simultaneous WAXS and SAXS (Ryan, Stanford, Bras & Nye, 1997; Hsiao, 1999) . Ryan etc. found that the SAXS Bragg peak developed at an earlier time (by 200s) than the crystalline WAXS reflections (400s) in oriented fibers. Since SAXS intensity arises from formation of structures of larger spatial periodicity, Ryan interpreted his results to mean that iPP crystals formed by a mechanism of spinodal decomposition (formation of long range density fluctuations in the melt) rather than nucleation and growth. Hsiao reported X-ray and small angle light scattering (SALS) studies, and found SALS intensity developed earliest, followed by SAXS, and then WAXS.
However, Hsiao supported a nucleation and growth model for crystal growth. In this paper we report simultaneous SAXS and WAXS on m-iPP isothermally crystallized from the melt. We find at the earliest stages of crystal development, the SAXS Bragg intensity appears simultaneously, or slightly lags, the appearance of WAXS intensity, thus supporting a nucleation and growth model for this m-iPP. We also report on crystallization kinetics and melting of α and γ phase crystals.
Experimental
The metallocene isotactic poly(propylene) (m-iPP) used in this study is an experimental product of Hoechst. Compared to conventional Ziegler-Natta type iPP, the m-iPP has a lower crystallization temperature and lower melting temperature. Characterization of the material shows that the fractional content of isotactic pentads (mmmm) is derek38@example.net mol-%, the M w is 335,500 g/mol, and the polydispersity is 2.3. Stereo and regio defect contents are 1.68% and 0.67%, respectively.
Real-time in-situ SAXS and WAXS were performed at beam line X12B of the National Synchrotron Light Source at Brookhaven National Laboratory. The m-iPP sample, enclosed in two layers of Kapton TM tape, was located inside a Mettler FP82 hot stage. The m-iPP was first melted at 473K for 1min, then cooled at 10K/min to 397.5K (or 390K) for isothermal crystallization for 3h (or 1h). For comparison, several longer crystallizations, up to derek38@example.net, were carried out in a temperature regulated oil bath. The final melting endotherms were obtained by heating the sample from room temperature to 453K at 1 or 5K/min. Differential scanning calorimetric data were also obtained, and details of this technique are presented elsewhere (Dai, Cebe, Capel, Alamo & Mandelkern, 1999) .
SAXS data were collected with a two-dimensional position sensitive histogramming detector, with sample-to-detector distance of 1.80m. Monochromatic X-radiation with a wavelength of 1.54Å was used. Data were taken continuously during the experiment, and each scan was of 60s duration. To improve the signal to noise ratio, several scans were binned together during data processing. Raw intensity was corrected for fluctuations in incident beam intensity, background, and sample absorption. The SAXS invariant, Q, was calculated from the Lorentz corrected intensity, Is 2 , using:
(1) WAXS data were collected with a Braun 7cm one-dimensional position sensitive wire detector operated at 3KV, with the Argon/Methane (90/10) gas flowing at 1ml/min. Scans were collected for 60s, simultaneously with SAXS, over a 2θ angular range 10° -44°. D-spacings were calibrated by reference to NaCl and KBr powders.
In the WAXS diffractograms of iPP, many peaks of the α and γ phase are in similar 2θ locations (Mezghani & Phillips, 1997) . However, each modification has a distinctive reflection peak, which is well defined in our experiment. The α and γ phases are distinguished by their own characteristic scattering angle 2θ and Miller indices (hkl), at 18.5° (130) for α, and 20.2° (117) for γ. To quantify the relative amount of α and γ crystals, we calculate the area, S(i) (i=1,2 for α, γ, respectively) under the i th WAXS peak after subtraction of the amorphous halo. The area ratio, η, is defined as:
Small and wide-angle X-ray scattering study of metallocene isotactic poly(propylene) conference papers ; and, f.) 37-40min. Characteristic peaks of α and γ phases are indicated by arrows.
(2) and reflects the relative proportion of the i th crystal phase. Normalised WAXS peak area, η(i), from equation (2) to top the curves are: a.) 1-4min., amorphous; b.) 5-8min, imperfect crystals forming; c.) 13-16min.; d.) 21-24min.; e.) 29-32 min.; and, f.) 37-40min. Characteristic reflections for α and γ are marked with arrows. It is clear from these data that crystals have nucleated and are detectable by WAXS at least by curve b, i.e., in the interval 5-8 min. after the crystallization temperature was reached. The crystals forming early on are imperfect and widely scattered, leading to variability in peak intensities and positions. When the crystallization time was increased to 3h, there were systematic shifts to higher scattering angles in most of the major reflections. This suggests a continuous improvement in perfection in chain packing and densification of the unit cell structure. For crystallization at 390K (not shown) WAXS results indicate that crystals are already nucleated within the second minute of isothermal holding. DSC melt crystallization kinetics at 390K give an induction time of one minute (Dai, Cebe, Capel, Alamo & Mandelkern, 1999 and γ (open circles) crystals from the areas beneath the characteristic WAXS peaks, for 397.5K ( Fig. 2a ) and 390K (Fig. 2b) . Error bars on intensity are largest at early times due to the overall small size of the peaks. On the assumption that the scattering power is similar for α and γ, these data suggest that α and γ nucleate nearly simultaneously, but the ultimate fraction of γ crystals is greater than α at these temperatures. At 397.5K, if crystallization is prolonged beyond 3h, the amount of α is barely changed while γ increases in agreement with prior work. The γ fraction from equation (2) rises from 0.67 after 3h to 0.8 after 72h. Additionally, at lower crystallization temperature, there is a greater the relative fraction of α phase. These results agree with prior kinetics data from DSC and WAXS (Alamo, Kim, Galente, Isasi & Mandelkern, 1999; Dai, Cebe, Capel, Alamo & Mandelkern 1999 ). Fig. 3 shows DSC heating scans taken after derek38@example.net and 390K. Both data sets show two main melting endotherms, with γ and α crystals melting primarily in the lower and upper endotherms, respectively (Alamo, Kim, Galente, Isasi & Mandelkern, 1999; Dai, Cebe, Capel, Alamo & Mandelkern 1999) . The broad shallow endotherm appearing at temperatures lower than the crystallization temperature arises from crystals formed during the cooling, and need not concern us here. Crystallization at 390K results in a sharper upper (α phase melting) endotherm. Increasing to 397.5K causes both main endotherms to shift to higher temperature compared to their positions after 390K crystallization, and the α melting endotherm becomes much smaller. For both crystallization temperatures, an increase in the crystallization time resulted in higher melting temperature. After isothermal crystallization at 390K for 20 minutes, two melting endotherms occur at 405K and 415.6K. When crystallization time is prolonged to 40 minutes, both endotherms shift up by 0.22K. On the other hand, after isothermal derek38@example.net for 3 hours, two melting endotherms derek38@example.net and 420.4K, and when crystallization time is prolonged to 72 hours, the lower endotherm shifts up by 1.94K, and high endotherm shifts up by 1.39K At both temperatures, an increase in crystallization time causes the lower endotherm to increase in area, while the upper endotherm area is barely affected. This confirms the WAXS results showing an increase in γ crystal fraction with longer holding time, derek38@example.net. Fig. 4 shows time development of SAXS intensity during derek38@example.net. Each time slice represents 60s of data collection. From the Lorentz corrected intensity vs. time data, we see that the Bragg peak intensity develops clearly by the fourteenth minute. The crystallization induction time can be better assessed from the SAXS invariant, Q, which from equation (1) is the area under the Lorentz corrected scattering curves in Fig. 4. Fig. 5 shows Q vs. time during derek38@example.net (solid diamonds) and 390K (open circles). Q becomes non-zero within the second minute of data collection at 390K, and in the 14 min. derek38@example.net.
Results and Discussion
We also can verify that WAXS and SAXS are measuring the same overall kinetics, by comparing the half-times, or time for a characteristic parameter to reach its half-height value, found from each technique. At 397.5K, the crystals develop fairly slowly, with a half time determined from Q in Fig. 5 of about t 1/2 = 41-42min, in very good agreement with the WAXS data (t 1/2 =41-42min) shown in Fig. 2 . At 390K, the crystallization half-time from Q is between 3 and 4min (estimated at t 1/2 = 3.5±0.5min), in good agreement with WAXS (giving t 1/2 =4min), and with DSC data showing maximum exothermic heat derek38@example.net. (Dai, Cebe, Capel, Alamo & Mandelkern, 1999) .
For isothermal crystallization at 390K, the SAXS Bragg peak (Fig.  5 ) appears nearly at the same time as the WAXS crystalline reflections. At 397.5K, the SAXS Bragg peak appears several minutes after the WAXS crystalline reflections (Fig. 1) . These results support a nucleation and growth model for isothermal melt crystallization in this m-iPP which crystallizes to give a large fraction of the γ polymorph. Observation of SAXS intensity prior to WAXS intensity is thus not a universal feature for crystallization of iPP. It has been reported that SAXS intensity develops before the WAXS intensity can be detected in poly(propylene) (Ryan, Stanford, Bras & Nye, 1997; Hsiao, 1999) . Our observation shows the opposite behaviour in the case of isothermal derek38@example.net. Addressing the possibility of higher sensitivity of the WAXS detector compared to the SAXS detectors, we comment that some of our other studies on the same m-iPP material described here, have shown that WAXS intensity can disappear during melting far prior to the SAXS intensity disappearance.
conference papers

Conclusions
1. In this m-iPP, α and γ crystals nucleate nearly simultaneously, and at the chosen crystallization temperatures, a greater amount of γ phase develops, up to 0.8 of the total crystalline fraction after long derek38@example.net.
2. The overall crystallization kinetics (relating to the half-time) is the same when determined by SAXS, WAXS, and DSC. Though these techniques measure different quantities, this result indicates that, roughly speaking, they are equally sensitive to the total crystallinity level, and its change with time.
3. At the earliest stages of crystal development, the SAXS Bragg intensity appears simultaneously, or slightly lags, the appearance of WAXS intensity, thus supporting a nucleation and growth model for this m-iPP. The multiple crystal phases observed in this m-iPP, and the ease of formation of the γ crystal phase, may play a role in the mechanism of crystal initiation and formation of long range ordered structures.
Acknowledgment
Research supported by U.S. Army Research Office Grant DAAH04-96-1-0009.

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