Haiying Tan

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.

Melt viscosity behavior of C60 containing star polystyrene composites

The melt viscosity behavior of three-arm and six-arm star polystyrene (SPS)/C60 composites was studied. It was found that the changing trend in the melt viscosity of SPS/C60 composites depended on the molecular weight of the arm chain (Ma) of SPS and the content of C60. When the Ma of SPS was smaller than the critical molecular weight for the entanglement (Mc) of polystyrene (PS), the complex viscosity obtained at 0.05 rad s−1 (η*0.05) of SPS/C60 composites with a low content of C60 was lower than that of pure SPS. When the Ma was larger than the Mc of PS, however, the η*0.05 of the SPS/C60 composites was higher than that of pure SPS. These results were contrary to the previous work on the melt viscosity behavior of linear PS (LPS)/C60 composites (A. Tuteja et al., Macromolecules, 2007, 40, 9427–9434), where the η* at low shear frequencies of LPS/C60 composites was higher than that of pure LPS when the molecular weight (Mw) of LPS was smaller than the Mc of PS, and the η* at low shear frequencies of LPS/C60 was lower than that of pure LPS when the Mw of LPS was larger than the Mc of PS. The possible mechanism behind the melt viscosity behavior of SPS/C60 composites was discussed. It was expected that the topological structure of SPS induced a more heterogeneous dispersion of C60 in the SPS matrix than the dispersion state of C60 in the LPS matrix. When the molecular weight of SPS was different, the contact states between SPS and C60 might also be different, which would make C60 act as a lubricator or barrier, leading to the reduction or increase of the melt viscosity of SPS/C60, respectively.

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.

Insight on the striking influence of the chain architecture on promoting the exfoliation of clay in a polylactide matrix during the annealing process

The influence of the topological structure of polylactide (PLA) chains on the microstructure of PLA–organically modified clay (OMC) nanocomposites was studied. Here, an OMC with a hydroxyl group-containing modifier (I.34TCN) was used as the nanofiller. According to the results from the XRD and TEM measurements, the change in the dispersed states of the OMC with star-shaped PLA (PLA3-x) was totally different from that with linear PLA (PLA1-1) during annealing at high temperature. A transition from the intercalated to the exfoliated structure appeared in the simple PLA3-x/OMC mixture during the annealing process, but the intercalated structure was retained in the PLA1-1/OMC system. The enhanced exfoliation of the OMC in the PLA3-x matrix was attributed to the spatial limitation of the star-shaped PLA3-x, which made the intercalated interlayer swell. As a result, the concentration of PLA3-x chains within the layers of the OMC was lower than that of the outside bulk, and so, more PLA3-x chains diffused into the interlayers of the clay until the clay sheets separated (exfoliation state). Furthermore, the exfoliation extent of the OMC in the star-shaped PLA3-x/OMC composites strongly influenced the glass transition temperature (Tg) of star-shaped PLA3-x.

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 %).

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 and characterization of random or gradient butadiene-p-methylstyrene copolymers via anionic polymerization

Copolymers of 1,3-butadiene and p-methylstyrene (p-MS) were synthesized via anionic polymerization. A benzophenone-potassium complex was added to tune the reactivity ratio of the two monomers, leading to random and gradient composition along the copolymer chain. The overall composition and microstructure could be controlled and well characterized by GPC and 1H-NMR. The p-MS was distributed from gradient to random with increasing the content of the benzophenone-potassium complex, and the 1,2-microstructure in the polybutadiene sequence increased at the same time. The hydrogenation of the copolymer of 1,3-butadiene and p-MS resulted in the corresponding saturated copolymer with welldefined structure and narrow molecular weight distribution.

Controlled Chain‐Scission of Polybutadiene by the Schwartz Hydrozirconation

Controlled chain-scission of polybutadiene (PB), polyisoprene, and poly(styrene-co-butadiene), induced by bis(cyclopentadienyl) zirconium hydrochloride (Cp(2)ZrHCl), was revealed at room temperature. The chain-scission reaction of linear PB was studied by means of GPC, NMR spectroscopy, and MALDI-TOF-MS. It was confirmed that the molecular weights of degraded products were quasi-quantitatively controlled by Cp(2)ZrHCl loading, irrespective of the starting PB, whereas the microstructure of PB chains was crucial to the scission reaction. The hydrozirconation of model molecules indicated that the existence of an internal double bond in compounds with multiple double bonds was essential for chain cleavage. The chain-cleavage mechanism was proposed to involve hydrozirconation of internal double bonds in PB chains and β-alkyl elimination. Furthermore, metallocene-catalyzed chain-scission by a chain-transfer reaction was developed. It is believed that the reported chain scission offers a promising pathway for end-group functionalization by chain cleavage and presents a new application of Schwartzs reagent.