Well-Dispersed Chitosan/Graphene Oxide Nanocomposites

Nanocomposites of chitosan and graphene oxide are prepared by simple self-assembly of both components in aqueous media. It is observed that graphene oxide is dispersed on a molecular scale in the chitosan matrix and some interactions occur between chitosan matrix and graphene oxide sheets. These are responsible for efficient load transfer between the nanofiller graphene and chitosan matrix. Compared with the pure chitosan, the tensile strength, and Young’s modulus of the graphene-based materials are significantly improved by about 122 and 64%, respectively, with incorporation of 1 wt % graphene oxide. At the same time, the elongation at the break point increases remarkably. The experimental results indicate that graphene oxide sheets prefer to disperse well within the nanocomposites

Soluble Main-Chain Azobenzene Polymers via Thermal 1,3-Dipolar Cycloaddition: Preparation and Photoresponsive Behavior

Two soluble polymers containing azobenzene chromophore in main chain were successfully synthesized from α-azide, ω-alkyne A−B type azobenzene monomers, 3′-ethynylphenyl[4-hexoxyl(2-azido-2-methylpropionate)phenyl]azobenzene (EHPA) and 3′-ethynylphenyl[4-(4-azidobutoxy)phenyl]azobenzene (EAPA), via thermal 1,3-dipolar cycloaddition in bulk. Compared to the polymers obtained from Cu(I)-catalyzed 1,3-dipolar cycloaddition (“click” chemistry), the polymers obtained from thermal 1,3-dipolar cycloaddition showed good solubility in common solvents like CHCl3 and THF and good film-forming ability. The polymers were thermally stable up to 330 °C. The structures of the main-chain azobenzene polymers were characterized by gel permeation chromatography (GPC), 1H NMR, UV−vis, and FT-IR spectra. The photoinduced trans−cis isomerization of the polymers in chloroform (CHCl3) solution was examined. With illumination of linearly polarized Kr+ laser beam at 413.1 nm, surface relief gratings formed on PEHPA2 spin-coating films were investigated

Helical Crystals in Aliphatic Copolyesters: From Chiral Amplification to Mechanical Property Enhancement

We demonstrate herein a bottom-up strategy for achieving helical crystals via chiral amplification in copolyesters by incorporating a small amount of (d)-isosorbide into semicrystalline polyester, poly(ethylene brassylate) (PEB). During bulk crystallization of poly(ethylene-co-isosorbide brassylate)s, the molecular chirality of isosorbide in the amorphous region is transferred to PEB crystal chirality and amplified by the formation of right-handed helical crystals. Increasing isosorbide content or reducing crystallization temperature leads to thinner PEB lamellae crystals, strengthening chiral amplification by forming superhelices with a smaller helical pitch. Moreover, the superhelices with smaller helical pitch (larger chiral amplification) endow aliphatic copolyesters with enhanced modulus, strength, and toughness without sacrificing elongation-at-break. The principle outlined here could apply to the design of strong and tough materials

Iron-Mediated AGET ATRP of Styrene in the Presence of Catalytic Amounts of Base

The first example of atom tramsfer radical polymerization using activators generated by electron transfer (AGET ATRP) of styrene in bulk and solution was investigated in the presence of catalytic amounts of NaOH or Fe(OH)3, using FeCl3·6H2O as the catalyst, (1-bromoethyl)benzene (PEBr) as the initiator, vitamin C (VC) as the reducing agent, and a cheap and commercially available tetrabutylammonium bromide (TBABr) or tetra-n-butylphosphonium bromide (TBPBr) as the ligand. It was found that both the polymerization rate and controllability over molecular weights and molecular weight distributions (∼1.2) of the resultant polymers could be enhanced in the presence of the catalytic amounts of base as compared with those without base. For example, the polymerization rate of bulk AGET ATRP with a molar ratio of [St]0/[PEBr]0/[FeCl3·6H2O]0/[TBABr]0/[VC]0/[NaOH]0 = 250/1/1/2/2/1.5 using NaOH as the additive was much faster than that without NaOH. The former was 3.5 times the latter. Furthermore, the polymerization of styrene could be successfully carried out even in the conditions when the amount of iron salts, FeCl3·6H2O as the catalyst, reduced to ppm level

Thermoresponsive Brushes Facilitate Effective Reinforcement of Calcium Phosphate Cements

Calcium phosphate ceramics are frequently applied to stimulate regeneration of bone in view of their excellent biological compatibility with bone tissue. Unfortunately, these bioceramics are also highly brittle. To improve their toughness, fibers can be incorporated as the reinforcing component for the calcium phosphate cements. Herein, we functionalize the surface of poly­(vinyl alcohol) fibers with thermoresponsive poly­(N-isopropylacrylamide) brushes of tunable thickness to improve simultaneously fiber dispersion and fiber–matrix affinity. These brushes shift from hydrophilic to hydrophobic behavior at temperatures above their lower critical solution temperature of 32 °C. This dual thermoresponsive shift favors fiber dispersion throughout the hydrophilic calcium phosphate cements (at 21 °C) and toughens these cements when reaching their hydrophobic state (at 37 °C). The reinforcement efficacy of these surface-modified fibers was almost double at 37 versus 21 °C, which confirms the strong potential of thermoresponsive fibers for reinforcement of calcium phosphate cements

Effect of Coordination Sacrificial Bond Strength on Toughening Properties of Polyesters

By the coordination of dangling terpyridine (TPY) ligands with transition metal ions to form supramolecular cross-linking points as sacrificial bonds (SBs), a double-cross-linked system is constructed from the physically cross-linked semicrystalline polyesters. The mechanical properties are greatly enhanced, and the effect of coordination interaction strength on the toughening effect has been investigated by using different ions (Fe2+, Co2+, Ni2+, Cu2+, and Zn2+). It shows that stronger coordination bonds provide a larger Young’s modulus, but not for strength and toughness, which increase first and then decrease after achieving a maximum value at an optimal SB strength. This is due to the properties estimated at different strains, where the supramolecular and physically cross-linking points behave differently. Our results demonstrate that SB strength is not the higher, the better, which should be useful for the design of strong and tough materials

Temperature-Induced Reversible Morphological Changes of Polystyrene-<i>block</i>-Poly(ethylene Oxide) Micelles in Solution

Temperature-induced reversible morphological changes of polystyrene-block-poly(ethylene oxide) micelles with degrees of polymerization of 962 for the PS and 227 for the PEO blocks (PS962-b-PEO227) in N,N-dimethylformamide (DMF)/water, in which water is a selective solvent for the PEO block, were observed. For a system with 0.2 wt % copolymer concentration and 4.5 wt % water concentration in DMF/water, the micelle morphology observed in transmission electron microscopy changed from vesicles at room temperature to worm-like cylinders and then to spheres with increasing temperature. Mixed morphologies were also formed in the intermediate temperature regions. Cooling the system back to room temperature regenerated the vesicle morphology, indicating that the morphological changes were reversible. No hysteresis was observed in the morphological changes during heating and cooling. Dynamic light scattering revealed that the hydrodynamic radius of the micelles decreased with increasing temperature. Combined static and dynamic light scattering results supported the change in morphology with temperature. The critical micellization temperatures and critical morphological transition temperatures were determined by turbidity measurements and were found to be dependent on the copolymer and water concentrations in the DMF/water system. The morphological changes were only possible if the water concentration in the DMF/water system was low, or else the mobility of the PS blocks would be severely restricted. The driving force for these morphological changes was understood to be mainly a reduction in the free energy of the corona and a minor reduction in the free energy of the interface. Morphological observations at different time periods of isothermal experiments indicated that in the pathway from one equilibrium morphology to another, large compound micelles formed as an intermediate or metastable stage

Controlling Blend Film Morphology by Varying Alkyl Side Chain in Highly Coplanar Donor–Acceptor Copolymers for Photovoltaic Application

A series of varied length alkyl substituted donor–acceptor (D–A) conjugated copolymers with benzo[1,2-b:4,5-b′]dithiophene (BDT) as donor and thiophene rings attached to both sides of the benzothiadiazole (TBT) moieties as acceptors were designed and synthesized. The optical and electrochemical properties showed that the absorption spectrum, the band gaps, and the energy levels of the copolymers were not affected by the varied substituted alkyls, and all these copolymers showed low band gaps around 1.75 eV. In addition, the morphologies of the blend film between copolymers and PCBM can be fine-tuned by increasing the length of substituted alkyl of the copolymers, changing from pea-like aggregation to interpenetrating network to grain-like aggregation. Bulk heterojunction photovoltaic devices were fabricated by using the copolymers as donors and (6,6)-phenyl C61-butyric acid methyl ester (PC61BM) or (6,6)-phenyl C71-butyric acid methyl ester (PC71BM) as acceptors. The optimized photovoltaic performances showed the stable open-circuit voltage (Voc) in the range of 0.68 to 0.74 eV, and dramatically increasing short circuit current density (Jsc) by optimizing the blending morphologies of copolymer and PCBM films. The optimized photovoltaic performance with a Voc of 0.70 V, Jsc of 7.19 mA/cm2, a fill factor (FF) of 0.52, and a power conversion efficiency (PCE) of 2.88%, was obtained by the copolymer PBDT-TBT-C8 (PBDT-TBT-C8:PC61BM, 1:3 w/w, in CB solution). This is due to its low band gap and interpenetrating network morphology of PBDT-TBT-C8:PC61BM blend film. The photovoltaic device based on PBDT-TBT-C8:PC71BM showed a Jsc of 8.6 mA/cm2 and a PCE of 3.15%

Zero-valent Iron/RAFT Agent-Mediated Polymerization of Methyl Methacrylate at Ambient Temperature

In this work, zero-valent iron powder (Fe(0)) was used to catalyze the polymerization of methyl methacrylate (MMA) in the presence of a reversible addition−fragmentation chain transfer (RAFT) agent, 2-cyanoprop-2-yl 1-dithionaphthalate (CPDN) without any ligand at ambient temperature. The polymerization behavior complied with the features of typical “living”/controlled radical polymerizations. The number-average molecular weights of poly(methyl methacrylate) increased linearly with monomer conversion, while maintaining narrow molecular weight distributions (Mw/Mn 0:[CPDN]0:[Fe(0)]0 = 200:1:0.2, the polymerization was also controllable; however, it presented a depressed polymerization rate and a prolonged induction period (about 12 h). The polymerization rate also decreased with increasing of CPDN concentration. From experimental results, it was deduced that the initiating species were derived from the cooperative reaction of Fe(0) and CPDN, in which CPDN acted as a pseudohalide alkyl initiator. The control process was supposed to proceed via a synactic mechanism. One mechanism was the synergic mediation by Fe(0) and CPDN, in which Fe(III) formed in situ acted as an deactivator, however, this deactivation was supposed to be ineffective. The other was the RAFT mechanism with CPDN as the RAFT agent, which may dominate the whole control