Investigation of the Molecular Mechanisms of Melting and Crystallization of Isotactic Polypropylene by in Situ Raman Spectroscopy

In situ Raman spectroscopy has been used to reveal the molecular behavior of isotactic polypropylene during melting and crystallization. The 400 cm–1 band assigned to C–C–C bending shows a blue-shift during heating, suggesting an increase in conformational disorder, whereas the 1330 cm–1 band assigned to CH2 twisting shows a red-shift owing to an increase in the interchain distance. We suggest that interchain expansion proceeds even in the melt state, while the chain conformation becomes sufficiently disordered at the melting point. During the cooling process, the peak shifts at given temperatures are essentially the same as those during heating, except for the supercooled region where the peak shifts show obvious hystereses. By extrapolating the linear correlation between the peak shift of the 400 cm–1 band and the characteristic length along the chain axis, the critical stem size for crystallization is estimated to be ∼2.4 nm.


INTRODUCTION
Isotactic polypropylene (iPP) is a typical semicrystalline polymer, and it is ubiquitously used for a variety of daily commodities. The crystalline structures and superstructures that govern the mechanical properties of polymeric solids are spontaneously formed during the cooling process from the melt. The crystallization and melting behaviors of iPP have long been investigated by various methods, such as differential scanning calorimetry (DSC) and polarized optical microscopic observations, and the mechanisms of crystal growth have been proposed. [1][2][3][4][5][6][7][8] Recent in situ X-ray experiments have revealed the crystallization behavior at the mesoscopic scale, such as the kinetics of the disappearance of the crystalline lattice 9 and domain formation of the lamellar precursor during crystallization. [10][11][12] The growth front of spherulites has been investigated by combining microscopic X-ray and infrared (IR) spectroscopy, and the radial growth of lamellar crystals has been directly observed in situ. 13 Vibrational spectroscopy is advantageous for molecular investigation of melting and crystallization mechanisms because it is sensitive to changes in the microstructure of polymers and/or conformation of polymer chains. The temperature dependence of the Raman peak shifts for iPP and polyethylene terephthalate (PET) have been investigated, and the shifts can be interpreted by thermal expansion of the unit cell. 14,15 The increase in the number of trans chains of polyethylene in the cooling process has been monitored in situ by the C-C stretching modes. 16 Raman spectroscopy of iPP has been used to determine the crystallinity [17][18][19] and the length of the helical sequences. [20][21][22] Low-frequency Raman spectroscopy below ~100 cm −1 has also been used to characterize the changes in the lamellar crystalline structure of semicrystalline polymers. It has been found that the frequency of the longitudinal acoustic mode (LAM) around 10 cm −1 is sensitive to the lamellar thickness and long period. [23][24][25] According to observation of the LAM, rod-like structures (length ~10 nm) consisting of helical chains form in glassy iPP. 26 It has been suggested that the conformational changes of iPP can be detected by the 400 cm −1 band assigned to the C-C-C bending mode. 27,28 The intensity of the 400 cm −1 band decreases during melting, which can be explained by reduction of the sequence length of the helical chains. 19 It is well known that iPP shows polymorphism (α, β, γ, and mesomorphic forms) depending on the crystallization conditions and additives. [29][30][31][32][33][34] The monoclinic α form, which is composed of 3 1 helical chains of iPP, is commonly observed when cooling from the melt. 35,36 Several regularity bands have been reported, which correspond to a specific number (n) of monomeric units forming helical chains. [20][21][22] IR spectroscopy has revealed that short helical chains (n≤10) persist even above the melting temperature, and the length and number of the chains gradually decrease with increasing temperature. [37][38][39][40] The critical number of monomeric units at the onset of the crystallization of iPP is 12 (n=12) 37 according to Raman spectroscopic observation. 15 In this work, in situ Raman spectroscopy is used to investigate the molecular aspects of the melting and crystallization processes of iPP. The temperature dependences of the crystallinity and the intensity of the regularity bands are evaluated. The peak shifts of the 400 and 1330 cm −1 bands are related to the structural changes in the directions parallel and perpendicular to the molecular chain axis in the solid phase. The molecular mechanisms of melting and crystallization of iPP are discussed by extrapolating these correlations to the melt state.

EXPERIMENTAL SECTION
Pellets of iPP (M w = 56 × 10 4 , M w /M n = 5.8) were provided by Japan Polychem Corporation (Yokkachi, Japan). The iPP pellets (2 g) were dissolved in 120 mL of xylene and then precipitated in 2 L of methanol three times to remove additives that would act as nucleation agents. The iPP samples were dried in an oven at 120 °C for 12 h. The additive-free iPP was melted in a laboratory hot press for 5 min at 230 °C and 20 MPa. It was then quenched in boiled water to prepare a sheet with a thickness of about 1 mm or 100 µm. For comparison, an iPP sample with lower molecular weight (M w = 22 × 10 4 , M w /M n = 4.2) was also prepared using the same procedure.
The DSC measurements were performed with a Perkin-Elmer Diamond differential scanning calorimeter. A portion of the sample (3 mg) was cut out of the 100 µm thick sheet and packed in an aluminum pan. The pan was heated from 25 to 230 °C at a rate of 2 or 15 °C/min in a nitrogen atmosphere. The sample was maintained at 230 °C for 10 min before cooling to 25 °C at the same rate used for heating. The crystallinity χ DSC (T) at a given temperature T was determined with the following equation: (1) where ∆H(T) and ∆H total are the enthalpies of the melting or crystallization transitions integrated to temperature T and over the entire transition, respectively. The relative crystallinity was determined by normalizing by the value at room temperature.
The apparatus used for in situ Raman spectroscopy is shown schematically in Fig. 1. A hot stage (FP-82, Mettler Toledo) was installed in the Raman spectroscopic apparatus developed in our laboratory. 41 Laser light from a DPSS laser (LASOS, Jena, Germany) with a wavelength of 637.9 nm and laser power of 200 mW was irradiated into a 1 cm square specimen cut out of the additive-free iPP sheet (~1 mm thick) and placed between two cover glasses. The backwardscattered light was detected by a charge-coupled device (CCD) camera equipped with a monochromator (PIXIS 100 and SpectraPro 2300i, Princeton Instruments, Trenton, NJ, USA).
The sample temperature was controlled with the hot stage, where the temporal change of the sample temperature was set to be the same as that of the DSC measurements. The Raman spectra were accumulated 50 times with an exposure time of 1 s.  Table 1. The regularity bands at 973, 998, 841, and 1220 cm −1 are assigned to the helical chains of 5, 10, 12, 14 monomeric units, respectively. [20][21][22] The 973 cm −1 band is used as the internal reference. 21,42,43 While many of the bands assigned to the crystalline chains disappear upon melting, Raman bands assigned to the amorphous chains, such as the C-C stretching modes at 830 cm −1 , are clearly observed in the melt state. The 400 and 1330 cm −1 bands in the spectra of both the solid and melt states are assigned to the C-C-C bending and CH 2 twisting modes, respectively. 44   Table 1. Vibrational and phase assignment of the Raman spectrum of iPP. [20][21][22]44 As shown in Fig. 3, each Raman band can be successfully fitted by the sum of Voigt functions, except for the 830 and 841 cm −1 bands ( Fig. 3(b)). The three Raman bands at 808, 830, and 841 cm −1 were fitted by the sum of two Voigt functions, because the 830 cm −1 band is observed as a shoulder of the strong 841 cm −1 band. The intensity of the 808 cm −1 band was used to determine the crystallinity of iPP by dividing the intensity of this band by the sum of the intensities of the three bands at 808, 830, and 841 cm −1 : 17 (2) The relative crystallinity at given temperature T was determined by normalizing by the value at a sufficiently low temperature where the crystallinity is sufficiently high: Raman is the crystallinity at room temperature. The relative intensities of the regularity bands [20][21][22] were determined by dividing the intensities of the 998 and 1220 cm −1 bands by that of the internal reference band at 973 cm −1 . The peak shift ∆ν(T) was defined by the deviation of the peak position from that at room temperature: where ν(T) and ν 0 are the peak positions of the Raman band at temperature T and room temperature, respectively. of cartridge heaters and a thermocouple. The temperature was maintained at each temperature for 10 min before the measurement. The two-dimensional WAXD patterns were collected with an exposure time of 5 min and the one-dimensional profiles were obtained by integration over the azimuthal angles. The lattice plane distance d hkl was calculated by the Bragg equation: where λ (=1.54 Å) is the wavelength of the X-ray and θ is the diffraction angle. For the monoclinic crystal form, the lattice constants were calculated with the following equation: 45 (6) where a, b, c, and β are the lattice constants of the monoclinic iPP crystal. Raman spectra were also measured with the same setup used for the WAXD measurements to compare the lattice constants and Raman shifts at high temperatures.

RESULTS AND DISCUSSION
The relative crystallinities of iPP during heating and cooling determined by DSC and Raman spectroscopy are compared in Fig. 4  The 1220 cm −1 band disappears in the melt state, while the 998 cm −1 band merges with the nearby 1030 cm −1 band and persists as a broad band at ~1100 cm −1 . Although quantitative analysis of the 998 cm −1 band in the melt is difficult, the persistence of the 998 cm −1 band suggests the existence of helical chains up to n=10 in the melt state, which is consistent with an IR study. 40 Fig. 6 shows the temperature dependence of the relative intensities of the regularity bands at 998 and 1220 cm −1 , which correspond to helical chains with monomeric units of n=10 and 14, 20-22 respectively. In the heating process, the relative intensities of the regularity bands decrease with increasing temperature, suggesting a decrease of the length of the helical sequences. The slope becomes steeper above 100 °C, which is consistent with the decrease of . The relative intensities of these regularity bands sharply decrease at the melting temperature, and the 1220 cm −1 band completely disappears above the melting temperature. In the cooling process, both the 998 and 1220 cm −1 bands show a drastic increase at 122 °C, indicating rapid growth of the helical structure at the crystallization temperature followed by a gradual increase of the helical sequences with decreasing temperature in the solid phase, which has also been found by IR spectroscopy. 15     constant c corresponding to the period along the chain axis (l // =c) shows a linear correlation with ∆ν 400 , as shown in Fig. 10(b). It then follows that the value of l // (in Å) can be expressed as a linear function of ∆ν 400 : Although the linear relationships of Eqs. (7) and (8)  The 400 cm −1 band is sensitive to conformational disorder. 27,28 It has been reported that the 400 cm −1 band is essentially reproduced by skeletal vibration, where the hydrogen motions are omitted in the calculation. 28 The blue shift of the 400 cm −1 band with increasing temperature can then be explained by the increase in the conformational disorder of the crystalline chains leading to an increase of the natural frequency of the coupled oscillators of the helical chain, which is similar to the disordered longitudinal acoustic mode (D-LAM). 53 This interpretation is consistent with normal coordinate analysis 27 of iPP, which predicts that helical chains shorter than n=6 give a shoulder peak on the blue side of the 400 cm −1 band. There are good correlations between ∆ν 400 and the relative intensities of the regularity bands at 998 and 1220 cm −1 in the whole solid phase, as shown in Fig. 11. It is likely that distortion of the crystalline stems is closely related to the decrease of the helical sequences. Combining with the WAXD results in Fig. 10(b), we suggest that conformational disorder also results in contraction along the c axis. The temperature dependences of ∆ν 400 , ∆ν 1330 , and are shown in Fig. 12. In the solid phase during heating, while remains one, the 400 and 1330 cm −1 bands show gradual blue and red shifts, respectively. This suggests that conformational disorder and the interchain distance increase at the molecular level, even though no significant change is detected by thermal analysis. Above about 120 °C, the slope of ∆ν 400 shows a slight increase, which is consistent with the decrease of the helical sequences in Fig. 6. At the melting point of 167 °C, these shifts show abrupt increases, suggesting that the chain conformation becomes highly disordered and the interchain distance significantly increases owing to the melting transition. In the melt state, where the crystalline structure has disappeared, the blue shift of the 400 cm −1 band occurs more gradually, suggesting that the chain conformation is highly disordered and it gradually approaches a random conformation at a sufficiently high temperature. Even in the melt  During the cooling process, the values of the peak shifts are identical to those during the heating process, except for the supercooled region between 167 and 122 °C where the peak shifts show obvious hystereses (see Fig. 12). The hysteresis loops of these Raman shifts are consistent with the results of a molecular dynamics simulation of helical polymers, which predicted the hysteresis of the average helix length, as well as the global order parameter. 54 In the melt state, the slope of the 1330 cm −1 band is significantly steeper than that of the 400 cm −1 band, suggesting that the interchain approach is predominant, while the chain conformation is still significantly disordered before crystallization. In the supercooled region, the molecular structural changes continue to proceed at the same rates as those at relatively high temperatures, and abrupt changes in the inter-and intrachain structures simultaneously occur at the crystallization temperature. Even after the drastic change at the crystallization temperature, microscopic structural changes still occur owing to the decrease of the interchain distance and conformational ordering of the helical chains.
As shown in Fig. 13, at faster heating/cooling rates of ±15 °C/min, the crystallinity significantly changes, the melting transition begins at a lower temperature, and the crystallization temperature is ~10 °C lower. Despite these differences in the thermal analysis, the trends of   The molecular mechanism of iPP crystallization is shown in Fig. 15. Although the linear relations of Eqs. (7) and (8) 37 The increase of the stem length when approaching the crystallization temperature may lead to formation of the mesomorphic phase. 56 The sequence length of the helix conformation in the supercooled state is determined by the freeenergy balance between the energy loss, which is proportional to the length of the crystalline sequence, and the loss of conformational entropy under thermal fluctuation. The stiffened stems can align parallel to reduce the excluded volume interaction of rod-like segments, 57 and the orientation of chains may also promote crystallization. 58 At the crystallization temperature, a stem consisting of a long helical chain forms accompanied by a slight decrease of the interchain distance (~1%). Fluctuation in the crystalline structure occurs down to ~100 °C, where the interchain distance decreases by ~1% and the crystalline axis elongates by ~2%. At room temperature, the lamellar crystalline thickness is 10 nm (determined by a small-angle X-ray scattering measurement) and the average interchain distance is 0.57 nm.

CONCLUSIONS
In situ Raman spectroscopy has been used to investigate the molecular mechanisms of iPP during melting and crystallization. The crystallinity was determined with the intensities of the 808, 830, and 840 cm −1 bands. The crystallinity gradually decreases with increasing temperature above ~100 °C followed by a sharp decrease at the melting temperature in accordance with the decrease of the n=10 and 14 helical chains determined from the relative intensities of the 998 and 1220 cm −1 bands, respectively. During the heating process, the 400 cm −1 band (∆ν 400 ) assigned to the C-C-C bending mode shifts to higher wave number, suggesting an increase in the conformational disorder of the crystalline stem. The 1330 cm −1 band (∆ν 1330 ) assigned to the CH 2 twisting mode shifts to lower wave number, suggesting enhancement of the CH 2 vibration owing to an increase of the interchain distance. At the melting temperature, ∆ν 400 shows a marked blue shift and the temperature dependence becomes weaker in the melt, suggesting that the chain conformation becomes disordered during melting. Conversely, ∆ν 1330 shows a significant red shift at the melting point and the slope becomes steeper in the melt, suggesting enhancement of CH 2 vibration even in the melt, presumably because of an increase in the interchain distance. During the cooling process, the values of ∆ν 400 and ∆ν 1330 lie on the same curves as those during heating, except for the supercooled region where each peak shift shows clear hysteresis. The microscopic structural changes along and perpendicular to the molecular chains were evaluated by combining the Raman spectroscopy results with the results of WAXD analysis. Extrapolation of the linear relationship to the melt state suggests that the interchain distance significantly decreases with cooling before crystallization (by ~3%), and the critical stem size for crystallization is estimated to be ~2.4 nm. The results also suggest that the interchain distance decreases by ~1% and the crystalline axis elongates by ~2% during cooling in the solid state.

Funding Sources
This work was financially supported by JSPS KAKENHI (Grant Number 26410221). for