Spontaneous Crystallization of the Planar Zigzag Form of Syndiotactic Polypropylene at 0 °C

Abstract

Introduction.Recently, syndiotactic polypropylene (sPP) with high stereoregularity was synthesized by Ewen et al. 1 Whereas early investigations carried out by using less syndiotactic samples had difficulties in precise characterization, the success of highly stereoregular polymerization made the detailed analysis of the crystal structure possible. A large number of investigations on the molecular chain conformation and molecular packing in crystals have also been reported for sPP. As for the polymorphs, four crystalline forms are known: the planar zigzag, 2-5 t 2 g 2 t 6 g 2 , 6-8 and two forms with the t 2 g 2 conformation. 9-22 Of these, the crystals with the planar zigzag conformation have been believed to be formed only by cold drawing after quenching from the melt into an ice-water mixture. [2][3][4][5] Very recently, we have found that the crystallization of the planar zigzag form is spontaneously induced at 0 °C without any mechanical stress, although the growth rate is very slow. In this communication, we report briefly the first results to confirm such spontaneous crystallization of the form at 0 °C, which have been obtained by CP/MAS 13 C NMR and wide-angle X-ray scattering methods.Experimental Section. A highly syndiotactic polypropylene sample with an rr fraction of 0.96, provided by Sumitomo Chemical Co. Ltd., was used without further purification. CP/MAS 13 C NMR spectra were measured at room temperature on a Bruker MSL 200 spectrometer under a static magnetic field of 4.7 T. Magic angle spinning at 3 kHz was achieved with the double air bearing system. The contact time for the cross polarization process was 1 ms and the pulse delay time was 4 s. 13 C chemical shifts were expressed as values relative to tetramethylsilane (Me 4 Si) by using the CO carbon line at 176.03 ppm of glycine crystals as an external reference. Wide-angle X-ray diffraction patterns were obtained at room temperature on an automatic RIGAKU diffractometer with Ni-filtered Cu KR radiation.Results and Discussion. Figure 1 shows CP/MAS 13 C NMR spectra of sPP films quenched in ice-water from the melt at 170 °C, which were measured at room temperature. The times shown in this figure indicate the periods for which each film was kept in ice-water. As shown in Figure 1a, the film taken out immediately after quenching in ice-water gives a spectrum similar to the previously reported one; 23 two resonance lines assignable to the methylene carbons clearly appear at 47.7 and 39.1 ppm. Although the downfield line at 47.7 ppm seems to be composed of multiple components

Correlation between crystallinity and some infrared bands in the spectra of syndiotactic poly(propylene)

“…Highly syndiotactic poly(propylene) (sPP), recently synthesised with new metallocene homogeneous catalysts 1) , shows a very complex polymorphism [2][3][4][5][6][7][8][9][10][11][12] . A stable modification characterised by chains in the s(2/1)2 helical conformation can be packed in different crystalline cells (named Form I and II in ref.…”

“…9) , depending on the degree of stereoregularity and thermal history of the samples. A different form (Form III) can be obtained by stretching the compression moulded samples or by crystallization at 0 8C 7,12) , and it is characterised by chains in zigzag planar conformation. Form IV was obtained by Chatani et al 8) by exposing fibre specimens in the zigzag conformation to organic solvents.…”

Evolution of a Metallocenic sPP with Time: Changes in Crystalline Content and Enthalpic Relaxation

“…An efficiently quenched and rapidly analyzed sPP sample, sPP-h0, is completely amorphous, as seen by the appearance of its characteristic amorphous halo similar to that exhibited by an atactic poly(propylene) [35] (diffractogram represented at bottom of this figure). It is worth noting that preparation of amorphous sPP is not simple and the quenching at 0 8C induces formation of its mesomorphic form [36][37][38][39][40][41][42][43][44] for specimens with relatively high syndiotacticity. The moderately low syndiotactic degree exhibited by this sPP under study delays its crystallization rate and allows its preparation in a complete amorphous state, as seen for sPP-h0 diffractogram.…”

“…Form III can be obtained by stretching the most stereoregular sPP samples and, as occurs with Form IV, upon solvent treatments. Recently, the crystallization of the trans-planar Form III was reported for samples kept at 0 8C for a long time without any mechanical stress [24] though other authors have affirmed that the structure developed under those conditions do not correspond to the crystalline Form III but to a mesophase, or also paracrystalline, disordered phase. [23] When sPP, rapidly quenched from the melt in a cold bath, is brought to room temperature it is elastic due to a very low crystallinity, but the elastic behavior is soon lost due to a relatively rapid crystallization phenomenon and to the aging of the amorphous component.…”

Crystallization Behavior of Syndiotactic Poly(propylene) Freeze-Dried from Toluene at Very Dilute Concentration

“…[11 -13, 20] The trans-planar conformation of s-PP had been found [11, 20 -22] to be induced by cold drawing after quenching from the melt into an ice/water mixture. More recently, Nakaoki et al [12,23,24] reported that the crystallization of the trans-planar form is induced spontaneously at 0 8C in a period of about 20 h without any mechanical stress. In our studies, the formation of the trans-planar conformation may be explained by the interactions between polymer chain and solvent molecules.…”