Jian Qiu

Effect of polystyrene long branch chains on melt behavior and foaming performance of poly(vinyl chloride)/graphene nanocomposites

Several poly(vinyl chloride)-g-polystyrene graft copolymers (PVC-g-PS) with well defined molecular structures were synthesized via atom transfer radical polymerization (ATRP) from the structural defects of PVC. The effects of PS branch chains on the shear and extensional rheology as well as foaming properties were investigated. Compared to linear PVC, the introduction of PS branches results in increased complex viscosity, an elevated value of storage modulus at low shear frequencies, more pronounced shear-thinning behavior, more significant upshifted deviation from linear behaviour and a strain hardening phenomenon. Under the same foaming conditions, most of the resulting PVC-g-PS foams exhibit a closed cell structure, increased cell density and uniform cell size distribution while the linear PVC foam has serious cell coalescence. Moreover, graphene nanosheets could be well dispersed in the PVC-g-PS matrix due to the π–π stacking with PS relative to the PVC without PS branch chains. As expected, both the nucleation effect and increased melt viscosity from well-dispersed graphene sheets significantly improve the foaming behavior of PVC-g-PS/graphene nanocomposites, in comparison with the poor foamability of PVC/graphene composites due to the non-uniform dispersion of graphene.

Interplay between the composition of LLDPE/PS blends and their compatibilization with polyethylene- graft -polystyrene in the foaming behaviour

Polyethylene-g-polystyrene (PE-g-PS) copolymers, which were prepared by the combination of the ROMP and ATRP method, were utilized to compatibilize LLDPE/PS blends. On one hand, the effect of PE-g-PS on the morphologies of LLDPE/PS blends was investigated. On the other hand, the influences of branch length and added amount of PE-g-PS on the cell morphology of foamed LLDPE/PS blends with different compositions were studied using supercritical CO2 as a physical foaming agent in a batch foaming process. It was found that the presence of PE-g-PS in the LLDPE/PS blends showed different influences on the foaming behaviour, strongly depending on the composition of the blends (i.e. the weight ratio of LLDPE and PS). How the interplay of compatibilization and composition of the LLDPE/PS blends affected the foaming behaviour of the LLDPE/PS blends was studied. A reasonable explanation was ascribed to consecutive states of the interfacial region, resulting from different phase structures of the blends. Compared to pristine LLDPE and PS, the blends with a sea-island phase structure showed the improved foam morphology, but the presence of PE-g-PS did not strongly influence the foaming behaviours of these blends. In contrast, the presence of PE-g-PS dramatically promoted the foaming ability of LLDPE/PS blends with a co-continuous phase structure. It was ascribed to the strengthened interfacial adhesion blocking the channel between two components through which CO2 was released, and the viscoelasticity of the blends was not the key factor to determine the foaming behaviour under the same foaming conditions in this work.

Dependence of Melt Behavior of Star Polystyrene/POSS Composites on the Molecular Weight of Arm Chains

Rheological behavior of three-arm and six-arm star polystyrene (SPS) with a small amount of polyhedral oligosilsesquioxane (POSS) was studied. Both linear oscillatory frequency sweep and steady state shear results of SPS/POSS composites showed the reduction of melt viscosity in the unentangled SPS matrix and the increase of viscosity in the entangled SPS matrix. In particular, when molecular weight of the arm (Ma) of SPS was smaller than the critical molecular weight for entanglement (Mc) of PS, the melt viscosity of SPS/POSS composites with low content of POSS was lower than that of pure SPS. The abnormal phenomenon of reduced melt viscosity in SPS/POSS composites was in coincidence with the melt viscosity behavior of SPS/C60 composites reported in our previous work ( Soft Matter 2013 , 9 , 6282 - 6290 ), although the diameters of two nanoparticles and their interaction with SPS matrix were different. A possible mechanism behind the melt viscosity behavior was discussed. Furthermore, the time-temperature superposition principle (TTS) was applied in SPS and SPS/POSS composites. The Cox-Merz empirical relationship was verified to be valid for SPS/POSS composites when the content of POSS was low (1 wt %).

Novel Method for Preparing Auxetic Foam from Closed-Cell Polymer Foam Based on the Steam Penetration and Condensation Process

Auxetic materials are a class of materials possessing a negative Poissons ratio. Here, we established a novel method for preparing auxetic foam from closed-cell polymer foam based on the steam penetration and condensation (SPC) process. Using polyethylene (PE) closed-cell foam as an example, the foams treated by the SPC process presented a negative Poissons ratio during stretching and compression testing. The effect of steam-treated temperature and time on the conversion efficiency of negative-Poissons ratio foam was investigated, and the mechanism of the SPC method for forming a reentrant structure was discussed. The results indicated that the presence of enough steam within the cells was a critical factor for the negative Poissons ratio conversion in the SPC process. The pressure difference caused by steam condensation was the driving force for the conversion from conventional closed-cell foam to the negative-Poissons ratio foam. Furthermore, the applicability of the SPC process for fabricating auxetic foam was studied by replacing PE foam by polyvinyl chloride foam with a closed-cell structure or replacing water steam by ethanol steam. The results verified the universality of the SPC process for fabricating auxetic foams from conventional foams with a closed-cell structure. In addition, we explored the potential application of the obtained auxetic foams by the SPC process in the fabrication of shape-memory polymer materials.