Several Methods For Tailoring Cellular Structure Of Microcellular Foams And Its Mechanism Analyses

Microcellular foams are widely used in packaging,household appliances,and automobile parts due to their advantages of high specific strength,good fatigue-resistance,low thermal conductivity and dielectric constant.Microcellular foams with different applications put forward demands for their cellular structure.So,it is important to tailor their cellular structure and analyze its mechanism.In this dissertation,using supercritical carbon dioxide?Sc-CO₂?as physical foaming agent,amorphous polystyrene?PS?and semicrystalline poly?lactic acid??PLA?were foamed in a batch process via two different temperature modes and depressurization.The effects of foaming parameters on the cellular structure,expansion ratio,fractured surface wetting and crystallization peoperties were analyzed.Furthermore,the PLA/PS blends were prepared using the extruder with shear and chaotic flow fields,respectively,which were induced by different screw geometries.The effects of the flow field and screw speed on the phase morphology development and rheological properties of the blend samples collected from different positions along the extruder were analyzed.The flow field-phase morphology-rheological properties relationship was investigated,and then the co-continuous structure was obtained for the blend with low PLA contents.Finally,the PLA/PS blends were foamed in a batch process via two different temperature modes and depressurization.The effects of foaming parameters on the cellular structure,expansion ratio and porosity were analyzed.The formation mechanisms of the open-celled structure for the PLA/PS blends with droplet morphology and co-continuous structure were investigated.For the foaming of PS,a monomodal cellular structure was obtained only through depressurization in the constant-temperature mode?CTM?,whereas a bimodal cellular structure was achieved through the synergistic effect of rising temperature and depressurization in the varying-temperature mode?VTM?.The large and small cells were formed by nucleation in the rising temperature and depressurization stages,respectively.The expansion ratio and the bimodal cellular structure can be controlled by the saturation temperature,foaming temperature,foaming pressure,and second saturation time.The monomodal cellular structure yielded an increase in static contact angle?CA?from the intrinsic value of the PS?87.1°?to 138.8° on the fractured surface of the PS foam;the CA value on the surface with a bimodal cellular structure was increased up to 155.1°,displaying the superhydrophobic wetting properties.For the foaming of PLA,in the CTM,the foamed samples prepared at short saturation time presented nonuniform cellular structure because the CO₂ diffusion front moved ahead of the crystal growth front in the gas saturation stage.The uniformity of cellular structure was much improved with increasing the foaming temperature,foaming pressure and saturation time.In the VTM,the saturation temperature and foaming pressure exerted a significant impact on the crystallinity and size of spherulites in the PLA samples,which were formed in the gas saturation stage,and then on the cellular structure and expansion ratio of the foamed PLA samples.Bimodal and trimodal cellular structures were observed in the foamed samples,and the sample with trimodal cellular structure displayed an extraordinarily high expansion ratio?76.2?.PLA fibers with large length-diameter ratio were achieved by the chaotic flow field.The co-continuous structure was formed in the 25/75 PLA/PS blend sample taken from about 15 times the screw diameter away from the feed inlet,and then broken into fibers and droplets along the extruder with chaotic flow field.High screw speed led to form elongated co-continuous structure and high break-up speed of the co-continuous structure.At constant volumn ratio,compared with the blend with droplet morphology,the blend with co-continuous structure had lower storage modulus?G??in the low-frequency region.The rheological functions of the binary immiscible blend were obtained by the Maxwell model and the Palierne model.The relaxation behaviors of the blend reflected on the G? and the imaginary part of complex viscosity????versus angle frequency???curves were investigated.Three relaxation times for the blend were defined at the ?????curve.It was demonstrated that the reciprocal of the frequency corresponding to the peak in the low-frequency region on the ?????curve was close to the longest relaxation time of the blend obtained from the G????curve.Based on this result,a new method was proposed to calculate the interfacial tension between the two phases in the blend.Using this method,the interfacial tension between the PLA and PS in the blend was calculated,which was 5.3 mN/m.For the foaming of PLA/PS blends,the blends with droplet morphology and co-continuous structure presented different formation mechanisms of open-celled structure.For the former,cell wall moved away from the PLA droplets owing to the poor interfacial compatibility,and then ruptured in the cell growth stage,resulting in the formation of open-celled structure.For the latter,nucleation occurred in the interface between the PLA and PS phases.The nuclei grew and evolved to interconnected cells.The bimodal cellular structure was observed in the foamed 30/70 PLA/PS blend sample when the foaming temperature was higher than 60°C of the saturation temperature.Interestingly,as for the sample prepared at high saturation temperature and low foaming temperature,the cells in the PLA phase mainly displayed open-celled structure,whereas the cells in the PLA phase mainly exhibited close-celled structure,and the former had larger cell diameter than the latter.