Crystallization of Isotactic Polypropylene under High Pressure (γ Phase)

Abstract

Isothermal crystallization of isotactic polypropylene at high pressures (from 200 to 250 MPa) have been carried out using a high-pressure dilatometer leading to a pure γ-form, as revealed by WAXD. The melting temperatures at the same pressures have been determined to assess the pressure effect on the equilibrium melting temperature by the Hoffman-Weeks analysis. The crystallization times have been analyzed according to the Hoffman-Lauritzen theory and corrections introduced to take into account the pressure effects on both the equilibrium melting temperature and the diffusion process. This analysis showed a transition from crystallization regime II to regime III for an undercooling of 53.7 K. The lateral and fold surface free energies were evaluated. Furthermore, the morphology of the samples investigated by polarized light microscopy, density measurements, and SAXS depends only on the crystallization undercooling, as a result of the influence of pressure on the equilibrium melting temperature. Moreover, the lamellar thickness experimental data agree well with the theoretical calculation.

The prediction of mechanical performance of isotactic polypropylene on the basis of processing conditions

“…In agreement with Angelloz et al [23] and Iijima et al [58] the equilibrium melting temperature T 0 m is chosen at 193 C, and used to describe the experimental data sets shown in Fig. 7.…”

“…The solution of these equations in isothermal conditions, and with a constant nucleation and growth rate, again results in Eq. (23). In this work the number of nuclei and the growth rate are temperature and pressure dependent and described by the expressions (25) and (26) (26) where N ref is the reference number of nuclei at the reference temperature T Nref .…”

Application of a multi-phase multi-morphology crystallization model to isotactic polypropylenes with different molecular weight distributions

“…The β modification is usually formed using specific nucleating agents [2][3][4] or from oriented α-crystals created by moderately strong flow [5]. Elevated pressure and high crystallization temperatures induces the γ-phase [6,7], which is found also in iPPs with stereo-and/or regio-errors [8][9][10] or small amount of α-olefines comonomer [11]. Elevated cooling rates (>100°C/s) inhibit the stable monoclinic modification in favour of a mesomorphic form [1,12].…”

Crystallization behavior of isotactic polypropylene induced by competition action of β nucleating agent and high pressure

“…For β-iPP, although the shear condition and temperature gradient provoke the nucleation and growth of β-iPP, addition of an active β-phase nucleating agent (β-NA) is proved to be the most efficient way to obtain iPP articles with high content of β-iPP [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]. For γ-iPP, it is already known that crystallization under high pressure (>200 MPa) is the main way to achieve high content of γ-iPP [25][26][27][28] in common iPP. Under atmospheric pressure, γ-iPP also can be provoked in a low molecular weight iPP [29] or PP copolymer with the presence of comonomer unit in the main chain [6,30,31], and in this condition, the relative fraction of γ-iPP is relatively smaller than that in iPP obtained under high pressure.…”