Xinrui Zhang

A tough shape memory polymer with triple-shape memory and two-way shape memory properties

In this study, a tough shape memory polymer network based on polydopamine, poly(e-caprolactone) and diisocyanate was synthesized in three steps. Fourier transform infrared spectroscopy was used to confirm the synthesis process. The tensile tests demonstrated the good mechanical properties of the materials with a tensile modulus and tensile strength reaching 362 and 43 MPa, respectively, at room temperature. The thermal properties of the polymer networks were investigated using differential scanning calorimetry and dynamic mechanical analysis. With two broad transition temperatures, the dual-shape memory properties were greatly affected by the deformation temperature, which was investigated in detail. Moreover, the polymers also showed good triple-shape memory and two-way shape memory effects.

Microstructure in a biointerface

The study of the microstructure of nacre has not yet reached maturity [1, 2]. Abalone nacre is traditionally considered as tablet-reinforced composite with a microstructure of “brick and mortar” (BM) arrangement, where bricks refer to flat polygonal crystals of aragonite and the mortar is organic adhesive composed of polysaccharide and protein fibers [3]. Recently, Schaffer et al. [1] observed many nanopores in the interlameller organic matrix layer of nacre, and proposed that interlamellar organic matrix delineates the aragonite tablets but allows the tablets to grow mineral bridges through the nanopores. Based on the mineral bridges, continuous growth of aragonite crystals forms abalone nacre. However, they did not give conclusive evidence of mineral bridges. With the development of high-performance materials, much attention has been given to the microstructure of biomaterials [4, 5]. In particular, nacre, which has elaborate microstructure [1, 2] and a fracture-toughness 3000 times greater than that of the pure mineral [3], has been widely investigated [1–12]. However, neither the microstructure nor the toughening mechanism of nacre is well understood [6–9]. It is generally believed that the BM arrangement of nacre results in the high performance [3, 6, 7], yet the present synthetic biomimetic materials with BM structure do not have the toughness comparable to that of nacre [9, 10]. Here we show the existence of mineral bridges and give the geometrical characteristics of mineral bridges in the organic matrix interface of nacre. To reveal the microstructure of nacre, the samples of abalone nacre were examined by an H-8100 TEM at an accelerating voltage of 200 kV. The testing samples were the nacre from Haliotis iris shell (abalone shell from New Zealand), which may be varied as ceramic composites containing 9 vol% of interlocking aragonite tablets staggered in successive laminae and separated by a 5% organic matrix. The keratin layer of the shell was mechanically worn off and the nacre of the shell was washed with distilled water and air dried at room temperature. Thin films parallel to the cross sectional surface were cut with a diamond saw, mechanically sliced and ground, then thin ion-beam milled at an angle of 10◦ to 50 μm thickness, and finally perforated under a voltage of 5.5 kV. The cross sectional microstructure morphology of nacre reveals an aptitude for traditional BM arrangement (Fig. 1a). However, there also exists many mineral bridges in the interlamellar organic matrix layers (Fig. 1b). The mineral bridges appear to be circular columns and their positions are random. The diameter of mineral bridges is 38–54 nm. And the height of mineral bridges is equal to the thickness of the organic matrix layer, 25–33 nm. Moreover, the thickness of an aragonite tablet on the cross sectional surface is 0.37– 0.43 μm. To obtain the structural characteristics of mineral bridges in the interlamellar organic matrix layers of nacre, first, we randomly chose some cross sectional surfaces from the sample of nacre, and counted the perfect tablets on the surfaces, namely 16 tablets. By measuring the length of these tablets on the cross sectional surfaces, we found that the average length approximately equals 4 μm (Fig. 2a). Secondly, by counting the bridges on each of the chosen tablets, we obtain that total number of bridges was 650 and have the average number on each tablet is approximately 41 (Fig. 2b). Since the microstructure of nacre can be considered to be transverse isotropic, the average bridge-to-bridge spacing and density of mineral bridges is approximately given as 98 nm and 100 μm−2, respectively. Finally, we divide each of the chosen tablets into sixteen equal units along the direction of organic layer, then separately add up the number of bridges (on all chosen tablets) contained by each unit. A distribution of mineral bridges on the tablets can be given (Fig. 2c). Since the average value and standard deviation of the distribution are approximately equal to 8 and 2.6, respectively, the distribution reveals a central feature. The bridges are mainly concentrated in the central region of each tablet where the number of mineral bridges is about 70% of that on the whole tablet, and the diameter of the central region is approximately equal to 1/3 the length of the whole tablet (Fig. 1c). In addition, the surface area of organic matrix on one side of a tablet is estimated to be approximately 16 μm2 while the total cross sectional area of the mineral bridges in the organic matrix is about 2.7 μm2. The area of the bridges is approximately equal to 1/6 that of the organic matrix. So nacre can be seen as a tabletreinforced composite, whereas, its organic matrix layer itself can be considered as a fiber-reinforced composite in which the matrix is organic and the fibers are mineral bridges (Fig. 1c). We take the Young’s modulus of the mineral bridges and the organic matrix as Eb = 100 GPa and Eo = 4GPa, respectively [6], and the volume fractions of the fiber and the matrix of the organic matrix layers as Vb = 1/6 and Vo = 5/6, respectively. According to composite theory, we can estimate the Young’s modulus of the organic layer as

A Synergistic Effect of Graphite and Nano-CuO on the Tribological Behavior of Polyimide Composites

Polyimide (PI) composites modified with nano-CuO and graphite were prepared by means of a hot press molding technique. The effects of nano-CuO and graphite on the tribological properties of the composites were studied using a block-on-ring arrangement. The results revealed that the incorporation of graphite alone improved the friction-reducing and anti-wear abilities of the PI composites, whereas incorporation of nano-CuO alone deteriorated the wear resistance of the PI composite owing to its abrasive effect. A synergistic effect was found for the combination of nano-CuO and graphite, which leads to the best tribological properties. The results showed that the filled PI composites exhibited worse tribological properties under higher load and sliding speed, and showed better friction and wear behavior under oil lubrication and worse tribological properties under water lubrication.

Friction and Wear of Potassium Titanate Whisker Filled Carbon Fabric/Phenolic Polymer Composites

Carbon fabric/phenolic composites modified with potassium titanate whisker (PTW) were prepared by a dip-coating and hot-press molding technique, and the tribological properties of the resulting composites were investigated systematically using a ring-on-block arrangement under different sliding conditions. Experimental results showed that the optimal PTW significantly decreased the wear-rate. The worn surfaces of the composites and the transfer film formed on the counterpart steel ring were examined by scanning electron microscopy (SEM) to reveal the wear mechanisms. The transfer films formed on the counterpart surfaces made contributions to the improvement of the tribological behavior of the carbon fabric composites. The friction and wear of the filled carbon fabric composites was significantly dependent on the sliding conditions. It is observed that the wear-rate increased with increasing applied load and sliding speeds.

Shape memory property of microcrystalline cellulose–poly(ε-caprolactone) polymer network with broad transition temperature

Preparation of shape memory polymers (SMPs) with broad transition temperature was an effective method to realize multishape memory effect. In this study, a novel SMP with a broad glass transition temperature (Tg) based on microcrystalline cellulose was prepared. The structure of the SMP was analyzed by Fourier transform infrared spectroscopy and proton nuclear magnetic resonance, which can prove the successful synthesis of the material. The thermal properties were investigated with differential scanning calorimetry and dynamic mechanical analysis (DMA). The dual- and multishape memory effects were also quantificationally analyzed by DMA. Further, the influence of programming temperature within Tg on dual-shape memory effect was investigated, and a 1D model was built to explain their relationship.

Physical Properties of Micro Hollow Glass Bead Filled Castor Oil-Based Polyurethane/Epoxy Resin IPN Composites

ABSTRACT A series of micro hollow glass beads (HGB) filled castor oil-based polyurethane/epoxy resin graft interpenetrating polymer network (IPN) composites were prepared. The tensile and impact strengths, impact fractured surfaces, damping properties and thermal stability of the IPN composites were studied systematically in terms of composition. Results revealed that the addition of HGB into polyurethane/epoxy IPN can significantly improve not only the tensile strength but also the impact strength. The tensile strength was increased by 61% and at the same time the impact strength was increased by 25% when the HGB content was 1.5%. The damping properties were better than the composition of 0.5% or 2% HGB content when the HGB content was 1% or 1.5%. The thermal decomposition temperature was also slightly improved by the incorporation of HGB. It is suggested that the HGB reinforced polyurethane/epoxy resin IPN composites could be used as structural damping materials.