Guangchun Zhang

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.

One-pot approach to prepare high-performance graphene-reinforced poly(vinyl chloride) using lithium alkyl as covalent bonding agent

A novel one-pot approach has been demonstrated for the preparation of poly(vinyl chloride) (PVC)/graphene nanocomposites based on the grafting reaction of PVC chains to graphene using n-BuLi as an initiator. The grafting reaction was successfully confirmed by FT-IR, XPS, TGA, TEM and ICP-OES. Raman measurements showed that the structural integrity of graphene framework was retained after surface functionalization, leading to the development of homogeneous nanocomposites with well-dispersed graphene in PVC matrix. Because of a strengthened interfacial interaction and improved dispersion of graphene in the resultant PVC/graphene nanocomposites, the graphene platelets could change their random alignment into oriented alignment parallel to the stretching direction with the movement of the matrix during deformation. As expected, the PVC matrix filled with very low loading of graphene showed significantly improved mechanical properties and electrical conductivity over their counterparts without n-BuLi treatment. The results show that it is very important to rationally and simultaneously control dispersion state, structural integrity of graphene and interfacial interaction between graphene and polymer matrix to fabricate high-performance polymer nanocomposites.

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.

Enhancement of sinterability and mechanical properties of B4C ceramics using Ti3AlC2 as a sintering aid

Boron carbide (B4C) ceramics are currently the leading control rod materials in fast nuclear reactors and promising high temperature structural materials. However, several drawbacks such as poor sinterability and low toughness seriously limit their wide applications. In order to enhance the sinterability and mechanical properties of B4C ceramics, titanium aluminum carbide (Ti3AlC2) was chosen as a new efficient sintering aid to densify B4C ceramics using spark plasma sintering. Fully dense B4C ceramics were obtained at a lower sintering temperature of 1500 °C by adding a small amount of Ti3AlC2. Meanwhile, the mechanical properties were enhanced remarkably. For B4C ceramics sintered with 15 vol% Ti3AlC2, optimized mechanical properties were obtained with Vickers hardness of 40.2 GPa and indentation fracture toughness of 4.7 MPa m1/2. These results indicate that Ti3AlC2 can be used as a novel sintering aid for the densification of B4C ceramics.

Synthesis of polystyrene-based Y-shaped asymmetric star by the combination of ATRP/RAFT and its thermal and rheological properties

A2A′-type asymmetric stars and A2B-type miktoarm star polymers were prepared by the combination of atom transfer radical polymerization (ATRP) and reversible addition-fragmentation chain transfer polymerization (RAFT) using the designed initiator. The first step involved the preparation of linear polystyrene with a hydroxyl group (LPSOH) by ATRP using the synthesized initiator 4,4′-di(bromomethyl)benzhydrol. Styrene was polymerized in bulk at 110 °C in the presence of Cu(I)Br and 2,2′-bipyridyl (Bipy) as a catalytic system. Next, the hydroxyl group in the resulting LPSOH chains was esterified to obtain LPS containing thiocarbonylthio (LPSCS2) chains. The last step consisted of growing the third PS chain or poly-(n-butyl acrylate) chain by RAFT. This methodology enabled us to synthesize A2A′ triarm PS stars with asymmetry in the molar mass of their branches and A2B stars with chemically different PS and PBA arms. It also provided us a facile way to synthesize Y-shaped polymers. The effects of the length of the backbone and branched chain on the thermal properties and the rheology of the synthesized asymmetric polystyrene were studied. This method provided a way to obtain well-defined polymers with fixed backbone but different branch length. On using this method grams of sample can be obtained for melt behavior study.

Synthesis of star-like polybutadienes by a combination of living anionic polymerization and “click” coupling method

Three-arm and four-arm star-like polybutadienes (PBds) were synthesized via the combination of living anionic polymerization and the click coupling method. Kinetic study showed that the click reaction between the azido group terminated PBd-t-N3 and the alkyne-containing multifunctional linking reagent was fast and highly efficient. All coupling reactions were fully accomplished within 40 min at 50 °C in toluene in the presence of the reducing agent Cu(0), proven by 1H-NMR, FTIR and GPC measurements. For the coupling reactions between the PBd-t-N3 polymer and dialkyne-containing compound, the final conversion of the coupled PBd-PBd polymer was ca. 97.0%. When a PBd-t-N3 polymer was reacted with trialkyne-containing or tetraalkyne-containing compound, the conversion of three-arm or four-arm PBd was around 95.5% or 87.0%, respectively. Several factors influencing the coupling efficiency were studied, including the molecular weight of the initial PBd-t-N3, arm numbers and the molar ratio of the azido group to the alkynyl group. The results indicated that the conversion of the target products would be promoted when the molecular weight of the PBd-t-N3 was low and the molar ratio of the azido to alkynyl groups was close to 1.

High-temperature hydrogenation behaviour of bulk titanium silicon carbide

Thermal stability of Ti3SiC2 was investigated at 1200–1400°C in hydrogen atmosphere for 3 hours. The hydrogenation mechanism was clarified by a combination of X-ray diffraction, scanning electron microscope, Raman spectroscopy and first principles calculation. At 1200°C, a dense and uniform TiSi2 layer formed on the sample surface, which originated from both the preferable lose of silicon from the Ti3SiC2 substrate and the dissociation of Ti3SiC2. As temperature increased to 1300°C, TiSi2 layer began to scale off and presented laminated Ti3SiC2 grains beneath this layer, which indicated preferential hydrogenation occurred along the basal planes. This phenomenon was ascribed to the fact that the introduction of H interstitial atom weakened the combination between titanium and silicon interface layer, which was confirmed by first principles calculations. In addition, the formation of TiSi2 owing to the dissociation of Ti3SiC2 caused the volume expansion after hydrogenation, resulting in that majority of TiSi2 layer spelled off at 1400°C.

Synthesis of well-defined long chain branched polyethylene via anionic polymerization combined with graft-onto method

Comb-like polyethylene(PE) was prepared via anionic polymerization combined with “graft-onto” process. The polybutadiene(PB) backbone underwent hydroxylation at 1,2-vinyl groups to obtain a controlled number of hydroxyl groups along the main chain. After the translation of hydroxyl groups to tosyl groups, a nucleophilic substitution by living anionic PB chains was achieved. The comb PE was finally obtained by the hydrogenation of the obtained unsaturated comb polymer. Since the living anionic polymerization was used to prepare the backbones and the branch chains, molecular weight to molecular weight distribution(Mw/Mn<1.5) can be well-controlled in the final comb polymer, including the average number and length of branches.