Semicrystalline polymers are employed in a variety of applications in view of their good stability, mechanical and barrier properties. In spite of their widespread use, the fundamental knowledge about their fluid transport and sorption behavior is somehow poor and limited to approximate models. As far as their thermodynamic properties of their mixtures with gases and vapors are concerned, it is generally accepted that semicrystalline polymers absorb lower amounts of fluid than the corresponding, wholly amorphous polymers. This behavior is due to the fact that the crystalline domains are practically impermeable to fluids and can hardly accommodate any host molecule, and furthermore, to a reduced sorptive capacity of the amorphous phase with respect to the pure amorphous polymer. The equation of state models, which accurately represent the experimental solubility of fluids in molten polymers, often overestimate the same property below Tm, even accounting for the negligible penetrant uptake in the crystalline phase. In this work we attribute the significant overestimation of solubility to the constraining effect that the rigid crystallites impose on the amorphous phase [1,2]. The present approach follows a similar one applied by Memari et al. to the same case, which made us of a Montecarlo technique to calculate the polymer amorphous phase solubility [3]. The constraining effect is accounted by considering that a constraint pressure, pc, acts on the amorphous solid phase in addition to the one prevailing in the fluid phase, p [4]. In particular we use the Sanchez Lacombe Equation of State (SL EoS) [5], as it describes accurately the behavior of amorphous polymer phases. The binary parameter for the fluid-polymer energetic interactions, kij, and the constraint pressure pc are adjusted on the experimental solubility data above and below the polymer melting point Tm, respectively. The approach is applied to the description of the solubility of different solutes in a variety of polymers above and below their melting points. In spite of its simplicity, the model is effective and accurate in representing the thermodynamic behavior of several fluid/polymer systems: n-C4, i-C4, N2 and CO2 in LDPE, CO2 in HDPE, 1-hexene in LLDPE, CO2 in i-PP, CO2 and C3H8 in PEO in wide ranges of temperature. In all those cases, the binary energetic parameter of the model, kij, is adjusted on the solubility data above Tm, where pc is zero by default, as no crystallites are present in the polymer phase, and then extrapolated below the melting point, where the only adjusted parameter is pc. Interestingly, the values of pc do not depend on the penetrant type but only on the polymer type, degree of crystallinity, and on the temperature, as it is consistent with the physical meaning of such parameter. In particular, the values of pc for polyolefins increase exponentially with the crystalline mass fraction, up to about 80 MPa at a crystalline fraction of 0.73, and decrease exponentially with increasing temperature, vanishing across the melting point.
An equation of state (EoS) based model for the fluid solubility in semicrystalline polymers
MINELLI, MATTEO;DE ANGELIS, MARIA GRAZIA
2014
Abstract
Semicrystalline polymers are employed in a variety of applications in view of their good stability, mechanical and barrier properties. In spite of their widespread use, the fundamental knowledge about their fluid transport and sorption behavior is somehow poor and limited to approximate models. As far as their thermodynamic properties of their mixtures with gases and vapors are concerned, it is generally accepted that semicrystalline polymers absorb lower amounts of fluid than the corresponding, wholly amorphous polymers. This behavior is due to the fact that the crystalline domains are practically impermeable to fluids and can hardly accommodate any host molecule, and furthermore, to a reduced sorptive capacity of the amorphous phase with respect to the pure amorphous polymer. The equation of state models, which accurately represent the experimental solubility of fluids in molten polymers, often overestimate the same property below Tm, even accounting for the negligible penetrant uptake in the crystalline phase. In this work we attribute the significant overestimation of solubility to the constraining effect that the rigid crystallites impose on the amorphous phase [1,2]. The present approach follows a similar one applied by Memari et al. to the same case, which made us of a Montecarlo technique to calculate the polymer amorphous phase solubility [3]. The constraining effect is accounted by considering that a constraint pressure, pc, acts on the amorphous solid phase in addition to the one prevailing in the fluid phase, p [4]. In particular we use the Sanchez Lacombe Equation of State (SL EoS) [5], as it describes accurately the behavior of amorphous polymer phases. The binary parameter for the fluid-polymer energetic interactions, kij, and the constraint pressure pc are adjusted on the experimental solubility data above and below the polymer melting point Tm, respectively. The approach is applied to the description of the solubility of different solutes in a variety of polymers above and below their melting points. In spite of its simplicity, the model is effective and accurate in representing the thermodynamic behavior of several fluid/polymer systems: n-C4, i-C4, N2 and CO2 in LDPE, CO2 in HDPE, 1-hexene in LLDPE, CO2 in i-PP, CO2 and C3H8 in PEO in wide ranges of temperature. In all those cases, the binary energetic parameter of the model, kij, is adjusted on the solubility data above Tm, where pc is zero by default, as no crystallites are present in the polymer phase, and then extrapolated below the melting point, where the only adjusted parameter is pc. Interestingly, the values of pc do not depend on the penetrant type but only on the polymer type, degree of crystallinity, and on the temperature, as it is consistent with the physical meaning of such parameter. In particular, the values of pc for polyolefins increase exponentially with the crystalline mass fraction, up to about 80 MPa at a crystalline fraction of 0.73, and decrease exponentially with increasing temperature, vanishing across the melting point.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
