Tengfei Shen

Influence of blocked polyisocyanate on the mechanical, thermal and interfacial properties of bamboo/hydroxyl-terminated polyurethane composites

In this study, chemical modification of bamboo powder (BP)/hydroxyl-terminated polyurethane (HPU) composites obtained by hot press was investigated using blocked polyisocyanate synthesized from reaction of isophorone diisocyanate with 1,1,1-trimethylolpropane (TMP) followed by addition of methyl ethyl ketoxime (MEKO). The modified surface of MEKO-blocked polyisocyanate (M-bp)-treated BP/HPU composites was identified by Fourier transform infrared spectroscopy from the appearance of CO bands absorbance and the reduction in relative intensity of OH, with respect to bamboo. The effects of M-bp as a cross-linker on the properties of the products were studied. Mechanical testing results showed that the tensile and flexural properties of the composites were improved by addition of such a cross-linker. The morphology analysis revealed that the M-bp-modified composites exhibited better compatibility and homogeneous morphologies in comparison with the unmodified composites. Moreover, the thermogravimetric analysis demonstrated that thermal resistance of the composites was enhanced by addition of M-bp. The moisture absorption characteristics of the products were also studied and discussed, because it is a major influencing factor for natural fibre composites. The results showed that better water resistance of modified composites was obtained due to stronger interfacial adhesion. Based on the findings in this work, they all indicated that the chemical modification occurred by the cross-linking effect of M-bp, which induced the strong bonds among the composite components.

Modification of the Properties of Polylactide/ Polycaprolactone Blends by Incorporation of Blocked Polyisocyanate

This work investigates and discusses the thermal, mechanical, thermo-mechanical properties and the morphology of solvent cast products from polylactide (PLA) and polycaprolactone (PCL), as well as their blends containing ethyl cellosolve blocked polyisocyanate (EC-bp), which was introduced as a crosslinker. Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) are used to analyze the effects of the addition of EC-bp and PCL on the thermal and crystallization behaviors. The results show that the glass transition temperature (Tg) and crystallization rate is increased and decreased by the addition of EC-bp, respectively, whereas the opposite tendency is observed in the presence of PCL. Tensile testing shows much better tensile properties of the EC-bp modified blends in comparison with unmodified ones. Dynamic mechanical analysis (DMA) reveals that the storage modulus and thermal stability of the PLA/PCL blends with EC-bp are enhanced with respect to the ones without. Environmental scanning electron microscopy (ESEM) is used to characterize the morphology and evaluate the interfacial adhesion. The result demonstrates that much better dispersion (PCL domains) and homogeneity morphology is achieved for EC-bp modified blends, which indicates that the addition of EC-bp can improve the interfacial adhesion.

Synthesis and properties of biodegradable polyurethane crosslinkers from methyl ethyl ketoxime-blocked diisocyanate

AbstractA series of novel biodegradable blocked polyurethane crosslinkers (BPUCs) were synthesized from the reaction of toluene 2,4-diisocyanate (TDI), isophorone diisocyanate (IPDI), polycaprolactone (PCL), 1,1,1-trimethylolpropane (TMP), and methyl ethyl ketoxime (MEKO). Synthesis of the accurate kind of BPUCs was confirmed by Fourier transform infrared spectroscopy (FTIR), proton nuclear magnetic resonance spectroscopy (1H NMR), and gel permeation chromatography (GPC). Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were used to determine the deblocking temperatures of the BPUCs. The thermal analysis revealed that both TDI- and IPDI-based BPUC had the proper initial deblocking temperature (<160 °C) and the maximum deblocking temperature (<200 °C) for practical applications. Compared to IPDI-based BPUC, the TDI-based BPUC had a lower thermal dissociation temperature and a faster deblocking rate. Hydroxyl polyurethane (HPU) was introduced to study the crosslinking effect of prepared BPUCs. The reaction proceeded at various deblocking temperatures in different curing times. It was noticed that the elastic modulus and tensile strength of the HPU sample increased whereas elongation at break decreased with the addition of BPUC in comparison with pure HPU, which suggested better interfacial adhesion due to the strong chemical reaction between the released NCO groups from BPUC and hydroxyl groups from HPU. In addition, improvement on water resistance of the BPUC modified HPU samples compared to pure HPU samples also demonstrated the good crosslinking effect of prepared BPUCs.

Synthesis and Characterization of Novel Liquid Crystalline Epoxides with Lateral Substituent Showing Imposing Shape Memory Properties

In this work, a series of novel shape memory liquid crystalline (LC) epoxides with lateral substituent were prepared and characterized. Dynamic mechanical analysis (DMA) and shape memory testing were used to investigate on the thermo-mechanical properties and shape memory effects of prepared samples. The results showed that the modulus in the rubbery region (ER ) decreased as the length of lateral alkyl group increases. And more than 95% of the deformation can be fixed as the sample was cooled down below the glass transition temperature (Tg), which was determined by differential scanning calorimetry (DSC) measurement, and can be recovered completely as heating. Effects of the curing agent and substituent on the shape memory behavior were also studied. We found that LC epoxides cured by aromatic diamine have a higher recovery speed. The recovering speeds of the shape memory decreased as the length of lateral alkyl group inscreased. The local ordered structures and the lateral substituent played an important role in shape memory effects.

Reversible Addition-Fragmentation Chain Transfer Polymerization of Methyl Methacrylate in Microemulsion: The Influence of Reaction Conditions on Polymerization

The reversible addition-fragmentation chain transfer (RAFT) polymerization of methyl methacrylate (MMA) using cetyltrimethylammonium bromide (CTAB) as surfactant and a difunctional RAFT agent S,S′-bis (α, α′-dimethylacetic acid) trithiocarbonate (BDAT) as chain transfer were conducted in microemulsion. The influence of polymerization temperature and concentration of RAFT agent on the polymerization were investigated, respectively. The results showed that the molecular weight of products increased linearly with conversion, the polydispersity indexes remained low value, and the polymerization processes were totally under control with increasing concentration of RAFT agent, the polymerization behavior exhibited living polymerization characters. In addition, the influence of RAFT concentration on the particle size was investigated by TEM. The results indicated that the particles were highly monodispersed and the particle size increased with increasing concentration of RAFT agent.

Design and characterization of novel conductive nanocomposites based on carbon nanotube modified liquid crystalline epoxy

In this work, a series of novel isotropically conductive adhesives (ICAs) based on multiwalled carbon nanotube (MWCNT)-modified liquid crystalline epoxy (LCE) resin was prepared. Different loadings of MWCNTs were used to produce LCE composites. Transmission electron microscope, scanning electron microscope, differential scanning calorimetry, polarized light optical microscopy, X-ray diffraction, electrical conductivity measurement, and tensile and flexural measurements were used to characterize the morphology, thermal, phase change, electrical, and mechanical properties of the prepared composites. The results showed that the added MWCNTs were dispersed well in the LCE matrix. The introducing MWCNTs did not show any major influence on typical curing peaks of LCE. Electrical conductivity at room temperature of the composites was increased significantly compared with neat LCE. The conductivity value of 10−2 S cm−1 was obtained at a weight concentration of only 0.5% MWCNT. In addition, tensile strength, Young’s modulus, flexural strength, and flexural modulus of the composites were increased clearly compared with neat LCE matrix. These produced LCE composites with very low MWCNT loadings showed a great prospect of being used as ICAs.

Preparation and Properties of Non-ionic Polyurethane Crosslinkers from 2-Ethoxyethanol/ϵ-caprolactam-Blocked Diisocyanate

A series of novel water-based non-ionic blocked polyurethane crosslinker (n-BPUC) dispersions have been synthesized by the reaction of toluene 2,4-diisocyanate (TDI), isophorone diisocyanate (IPDI), polyethylene glycol (PEG), 1,1,1-trimethylolpropane (TMP), 2-Ethoxyethanol (2-Et) and ϵ-caprolactam (CL). The physical properties of prepared n-BPUC dispersions such as viscosity, pH, and storage stability are measured and compared. The chemical structure of the prepared n-BPUC dispersions is confirmed by Fourier transform infrared spectroscopy (FTIR) and gel permeation chromatography (GPC). Deblocking temperatures of the n-BPUC dispersions are analyzed by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) techniques. The thermal analysis reveals that deblocking temperature obtained by DSC and TGA techniques is compared and found to be in the order DSC < TGA. Based on DSC and TGA data, it is shown that deblocking of n-BPUC dispersions based on 2-Et start at lower temperatures compared to that of the ones based on CL. The TDI-based n-BPUCs show higher reactivity than the ones based on IPDI. Hydroxyl-terminated polyurethane (HPU) is introduced to estimate the crosslinking effect of the prepared n-BPUCs. The better tensile properties and water resistance of n-BPUC modified HPU films compared to pure HPU film demonstrate the good crosslinking effect of the prepared n-BPUCs.