Interdiffusion versus crystallization at semicrystalline interfaces of sintered porous materials

Abstract

Sintering process at temperature intervals close to the melting point of polymers is greatly important due to its role in synthesizing porous materials. During sintering, particles of polymeric materials coalesce throughout a process called interdiffusion. On the contrary, crystallization strongly affects the interdiffusion process at temperature intervals below and close to the melting point. Apparently, the outcome of the contention between these two factors would determine the interfacial width. Therefore, the current study presents a model, which takes both crystallization and interdiffusion into account, to predict sintering kinetic. Consequently, interfacial strength was assessed with respect to the following influencing mechanisms, “reentanglement” relying on mutual interpenetration distance and “cocrystallization” determined by interfacial lamellar thicknesses. Based on the results of the present study, by changing sintering temperature of high molecular weight high density polyethylene nascent powder from 125 to 129 °C, the mutual interpenetration distance changes from 4.8 to 52.9 nm and interfacial lamellae thicknesses also vary from L ≈ 0–35.4 nm. On the other hand, porosity measurements revealed the reverse dependency to the sintering temperature. Interfacial lamellae thicknesses were calculated by means of differential scanning calorimetry (DSC) and also scanning electron microscope (SEM) micrographs. Eventually, the results of the shear punch test clearly demonstrated the role of sintering temperature in interfacial strength. Accordingly, the maximum bearable load in the samples sintered at 125 and 129 °C, increases from 25 to 315 N, respectively, which was attributed to change in interfacial volume of two sintered particles in a simplified model. Resultantly, the present study indicates that even a degree centigrade temperature variation would significantly affect the interfacial strength and porosity of the samples due to its effect on crystallization; which in consequence might transform a porous material into a dense one.