Chengzhen Geng

Design and Preparation of a Unique Segregated Double Network with Excellent Thermal Conductive Property

It is still a challenge to fabricate polymer-based composites with excellent thermal conductive property because of the well-known difficulties such as insufficient conductive pathways and inefficient filler-filler contact. To address this issue, a synergistic segregated double network by using two fillers with different dimensions has been designed and prepared by taking graphene nanoplates (GNPs) and multiwalled carbon nanotubes (MWCNT) in polystyrene for example. In this structure, GNPs form the segregated network to largely increase the filler-filler contact areas while MWCNT are embedded within the network to improve the network-density. The segregated network and the randomly dispersed hybrid network by using GNPs and MWCNT together were also prepared for comparison. It was found that the thermal conductivity of segregated double network can achieve almost 1.8-fold as high as that of the randomly dispersed hybrid network, and 2.2-fold as that of the segregated network. Meanwhile, much higher synergistic efficiency (f) of 2 can be obtained, even greater than that of other synergistic systems reported previously. The excellent thermal conductive property and higher f are ascribed to the unique effect of segregated double network: (1) extensive GNPs-GNPs contact areas via overlapped interconnections within segregated GNPs network; (2) efficient synergistic effect between MWCNT network and GNPs network based on bridge effect as well as increasing the network-density.

Mechanically reinforced chitosan/cellulose nanocrystals composites with good transparency and biocompatibility

Chitosan/cellulose nanocrystals (CS/CNCs) composites were prepared with different contents of CNCs. Due to the homogeneous dispersion of CNCs and the strong interfacial interactions resulting from hydrogen bonding between CS chains and CNCs, the transparency of CS is well retained and the overall mechanical properties of CS are significantly improved. Furthermore, because both CS and CNCs are biocompatible materials, cell proliferation test shows that the obtained composites are noncytotoxic and can potentially meet safety requirements of biomedical applications. These advantages pave the way of potential applications of CS in the field of commercial plastics and encourage the use of CS as environment-friendly material and biomedical material.

Interfacial strength and mechanical properties of biocomposites based on ramie fibers and poly(butylene succinate)

In this work, a single ramie fiber was firstly treated with various surface modification agents including alkali, silane, maleic anhydride and acetic anhydride. Then single fiber fragmentation was carried out to evaluate the interfacial interaction between the fiber and poly(butylene succinate) (PBS) matrix by means of interfacial shear strength (IFSS). The highest IFSS was found for the fiber treated with alkali. In this simple way, alkali was easily selected as a good surface modification agent for the preparation of PBS/ramie fibers biocomposites. In order to verify that the single fiber fragmentation is an effective method for the selection of a good surface modification agent, the interfacial interaction and mechanical properties of the prepared PBS/alkali treated ramie fibers composites were investigated. It was found that alkali treated ramie fibers could be well dispersed and had better interfacial adhesion with the matrix than untreated fibers, as indicated by scanning electron microscope (SEM) observation and dynamic mechanical analysis (DMA). The tensile strength and modulus of PBS were greatly increased by adding the alkali treated fibers, while only a slight increase of tensile strength and modulus was observed for those compounded with untreated fibers. These results indicate that alkali is indeed a good coupling agent for the improvement of the interfacial strength of the composites, which is well consistent with that obtained via the IFSS measurement. Therefore, single fiber fragmentation test can be used as an effective and simple method to evaluate the surface modification and interfacial enhancement as well.

Strong and conductive double-network graphene/PVA gel

Two kinds of graphene gel are prepared from graphene oxide (GO) by a hydrothermal method. The average sheet size of graphene gel with smaller GO (sG gel) is 0.75 μm2, while that of graphene gel with larger GO (lG gel) is 12.5 μm2. Both the sG gel and lG gel have a three dimensional graphene framework. The sG gel has better mechanical and electrical properties than the lG gel because of the smaller pore size and higher graphene density. Furthermore, graphene/polyvinyl alcohol double-network hydrogels (G/PVA gel) were fabricated by introduction of PVA chains into graphene hydrogels followed by freezing and thawing for the first time. The compressive fracture strength could reach as high as 213 kPa for sG/PVA gel and 104 kPa for lG/PVA gel, which were 245% and 243% higher than the corresponding sG gel and lG gel, respectively. This could be due to the good interaction, entanglement and support of each other between the PVA network and graphene network. G/PVA gel was conductive because of the well-preserved intact graphene network. Cyclic compressing and recovery tests demonstrated that the structure and mechanical properties of the gels were stable. And the conductivity was strain-sensitive, which increased with strain increasing when compressing and decreased with strain recovery. The novel properties make the G/PVA gel attractive in a wide variety of applications, such as cartilage, sensors and high performance nanocomposites.

Synergetic effects of a matrix crystalline structure and chain mobility on the low temperature toughness of polypropylene/ethylene–octene copolymer blends

It is believed that the good toughness of β-modified polypropylene (PP) is due to its easier lamellar slippage compared with that of α-modified PP, while the improvement in toughness of PP caused by annealing is due to increased chain mobility in the amorphous part of PP. The aim of this work is to reveal the combined effects of the matrix crystalline structure and amorphous chain mobility on the low temperature toughness of PP/ethylene–octene copolymer (POE) blends by β-modification and annealing. An impact test was performed over a wide range of temperatures (room temperature, 0 °C, −20 °C and −40 °C) to verify the enhancement in toughness, and various characterizations were carried out to inspect the structural evolution and toughening mechanism. The results show that β-modification and annealing will work synergistically to toughen the blend and reduce the POE content necessary for effective toughening over the temperature range tested, due to the synergetic enhancement in the matrix cavitation ability. Besides, the tensile properties will not be lowered by β-modification and annealing. This work not only provides a more efficient way to improve the impact resistance of polymers with stiffness–toughness balance, but also demonstrates the vital role of matrix microstructures on the toughness of the material.

Simultaneous improvements of thermal stability and mechanical properties of poly(propylene carbonate) via incorporation of environmental-friendly polydopamine

Inspired by the photoprotection, radical scavenging of melanin together with versatile adhesive ability of mussel proteins, polydopamine (PDA) nanoparticles were successfully prepared and incorporated into environmentally friendly polymer, poly(propylene carbonate) (PPC) via solvent blending. The prepared composites exhibited excellent thermal stability in air and nitrogen atmosphere and extraordinary mechanical properties. The composites displayed eminent increase of temperature at 5% weight loss (T5%) by 30–100 K with 0.3 wt%–2.0 wt% loadings, meanwhile, the tensile strength and Young’s modulus were significantly improved from 11.5 MPa and 553.7 MPa to 40.5 MPa and 2411.2 MPa, respectively. The kinetic calculation indicated that improvement of T5% is presumably derived from suppressing chain-end unzipping. The glass transition temperature (Tg) of the PPC/PDA composites increased by 8–10 K. This is probably due to hydrogen bonding interaction since the abundant proton donors along PDA chains would interact with proton acceptors like C=O and C-O-C in PPC which would cause restriction of segmental motion of PPC chains.

Size Distribution and Anisotropy of the Minor Phase Droplets in Polypropylene/Ethylene-Octene Copolymer Blends： Effects of Shear and Component Miscibility

In this work, polypropylene (PP)/octene-ethylene copolymer (POE) blends were injection-molded using the socalled dynamic packing injection technique, which imposed oscillatory shear on the gradually cooling melt during the packing solidification stage. In this way, the effect of shear on the size distribution and anisotropy of the minor phase droplets could be investigated. Besides, by using two kinds of POE with different octene contents, the effect of component miscibility was also studied. The results show that the droplet size is mainly determined by composition and miscibility, and droplet anisotropy is mainly determined by droplet size and shear. Most importantly, under the same processing condition, droplet anisotropy increases with droplet size, and there seems a linear fit between them, disregarding the miscibility factor. These results may provide guidance for preparing polymer blends with desired properties by tailoring their phase morphologies.