Synthesis and physicochemical characterization of a well-defined poly(butadiene 1,3)-block-poly(dimethylsiloxane) copolymer

For the first time, order-order and order-disorder transitions were detected and characterized in a model diblock copolymer of poly(butadiene-1,3) and poly(dimethylsiloxane) (PB-b-PDMS). This model PB-b-PDMS copolymer was synthesized by the sequential anionic polymerization (high vacuum techniques) of butadiene 1,3 (B) and hexamethylciclotrisiloxane (D₃), and subsequently characterized by nuclear magnetic resonance (¹H and ¹³C NMR), size exclusion chromatography (SEC), Fourier Transform infrared spectroscopy (FTIR), Small-Angle X-ray scattering (SAXS) and rheology. SAXS combined with rheological experiments shows that the order-order and order-disorder transitions are thermoreversible. This fact indicates that the copolymer has sufficient mobility at the timescale and at the temperatures of interest to reach their equilibrium morphologies.     

Investigation of the relationships between the chain organization and rheological properties of atactic poly(vinyl alcohol) hydrogels

The structure and rheological behavior of atactic poly(vinyl alcohol) (a-PVA) hydrogels prepared by freeze/thaw cycles were investigated as a function of polymer concentration and number of freeze/thaw cycles. The presence of phases with different mobilities was observed using ¹³C CP/MAS and DP/MAS NMR experiments. The degree of crystallinity of the a-PVA-rich phase was determined by ¹H NMR free decay experiments. Measurements of the shear storage and loss modulus were performed at a fixed frequency of 1 Hz and a strain value of 0.1%, i.e. under conditions where the deformation imposed on the gel structure is entirely reversible. Results thus obtained showed that an increase in the number of freeze/thaw cycles induces an increase in the degree of crystallinity in the polymer-rich phase together with an increase in the storage modulus. The a-PVA hydrogels became more fragile as the number of freeze/thaw cycles was increased. Moreover, both the percentage of protons in a rigid environment measured by ¹H NMR and the storage modulus values tended to a limiting value after six freeze/thaw cycles. These results show that the first five or six freeze/thaw cycles play a very important role in determining the hydrogel structure and rheological properties. A more detailed comparison of NMR and rheological data led to the conclusion that the storage modulus is mainly controlled by the a-PVA crystallinity while the hydrogen bond interactions have a much smaller contribution.     

Effect of fluoropolymer antidripping agent on the microstructure and properties of polyesters

Poly(ethylene terephthalate) (PET) is of excellent mechanical properties and melt processability and is widely used as raw material for textile fibers. However, the flame retardant properties of PET were rather poor, and both reactive and additive phosphorus- and halogen-containing compounds have been employed to enhance the reaction-to-fire properties while the meltdripping behaviour during burning hasn’t been handled properly with the flame retardants. In this work, fluoropolymer was blended with both pure PET (pPET) and reactive phosphorus-containing flame retarded copolyester (fPET), and the flame retardance and char formation and mechanical properties of the resulted pPET and fPET blends were investigated. The tensile strengths of modified pPET samples were worsen whereas those of modified fPET samples were improved at low concentrations. The initial thermal degradation in nitrogen was accelerated remarkably for the two polyesters with fluoropolymer. The oxygen indices of the all modified samples were reduced while char formation and meltdripping suppression were encouraged. The apparent melt viscosity and elasticity for the two polyesters were gained much with antidripping agent. Therefore, fluoropolymer improved char formation of the two polyesters based on the gaseous phase mechanism while the partial suppression of meltdripping behavior and the decrease of mechanical properties mainly originated from the increase of melt viscosity via fibrillation for pPET.     

Synthesis and applications of heterobifunctional poly(ethylene oxide) oligomers

Poly(ethylene oxide) (PEO) oligomers are employed extensively in pharmaceutical and biomedical arenas mainly due to their excellent physical and biological properties, including solubility in water and organic solvents, lack of toxicity, and absence of immunogenicity. PEO can be chemically modified and reacted with, or adsorbed onto, other molecules and surfaces. Sophisticated applications for PEO have increased the demand for PEO oligomers with tailored functionalities, and heterobifunctional PEOs are often needed. This review discusses the synthesis and applications of heterobifunctional PEO oligomers possessing amine, carboxylate, thiol, and maleimide functional groups.     

Shear-induced crystallization of poly(phenylene sulfide)

The crystalline morphology of poly(phenylene sulfide) (PPS) isothermally crystallized from the melt under shear has been observed by polarized optical microscope (POM) equipped with a CSS450 hot-stage. The shish–kebab-like fibrillar crystal structure is formed at a higher shear rate or for a longer shear time, which is ascribed to the tight aggregation of numerous oriented nuclei in the direction of shear. The crystallization induction time of PPS decreases with the shear time, indicating that the shear accelerates the formation of stable crystal nuclei. Under shear, the increase of spherulite growth rate results from highly oriented chains. The melting behavior of shear-induced crystallized PPS performed by differential scanning calorimetry (DSC) shows multiple melting peaks. The lower melting peak corresponds to melting of imperfect crystal, and the degree of crystal perfection decreases as the shear rate increases. The higher melting peak is related to the orientation of molecular chains. These oriented molecular chains form the orientation nuclei which have higher thermal stability than the kebab-like lamellae that are developed later. A new model based on the above observation has been proposed to explain the mechanism of shish–kebab-like fibrillar crystal formation under shear flow.     

Scaling laws and internal structure for characterizing electrospun poly[(R)-3-hydroxybutyrate] fibers

Using chloroform/dimethylformamide (CF/DMF) co-solvent, electrospinning of poly[(R)-3-hydroxybutyrate] (PHB) solutions was carried out at ambient temperature. The effects of the applied voltage (V), flow-rate (Q), and solution viscoelastic properties on the Taylor cone, electrified jet, and fiber morphology were investigated. In addition, the electric field developed by the needle-plate electrode configuration was calculated using a finite element analysis to reveal the tip-to-collector (H) effect. Among the processing parameters (V, Q and H), it was found that Q played a key role in determining the jet diameter (d_j) and electrospun fiber diameter (d_f), and scaling laws existed between them, i.e., d_j-Q^0.61 and d_f-Q^0.33. The diameter reduction ratios of D_o/d_j (D_o is the needle diameter) and d_j/d_f were measured as 50-120 and 5-10, respectively; it suggested that major jet stretching took place in the straight electrified jet region, and further chain orientation could be gained by the subsequent process of jet whipping. By changing PHB concentrations from 5 to 15 wt%, the solution viscosity (η_o) was increased from 100 to 4900 cP, whereas the surface tension and solution conductivity remained unchanged; it provided a good model solution to exclusively reveal the η_o effect on the electrospinning process. Our results showed that the η_o-dependence of d_j and d_f also followed simple scaling laws: d_j-η_o^0.06, and d_f-η_o^0.39, with a prefactor depending on the processing variables, mainly the flow-rate. Regardless of the PHB concentrations used, the obtained PHB fibers showed a similar crystallinity fraction of ca. 0.63 and possession of major α-crystals together with a small amount of β-crystals with zigzag chain conformation.     

Morphological changes in poly(?-caprolactone) in dense carbon dioxide

Morphological changes that take place in poly(?-caprolactone) upon exposure to carbon dioxide at high pressures have been explored as a function of pressure and temperature. SEM and DSC results point to a competition between CO₂-modulated crystallization and pressure-induced phase separation which leads to unique morphologies. At 293 K, exposure to CO₂ at pressures up to 45 MPa leads to recrystallization resulting in higher level of crystallinity and higher melting temperatures. Highest crystallinity levels along with distinct crystal morphology were observed after exposure to CO₂ at 308 K and 21 MPa. At a higher pressure at this temperature (308 K/34 MPa) polymer undergoes melting, and foaming is achieved during depressurization prior to solidification. At 323 K, the polymer is found to display unique crystal morphology with concave crystal geometry as well as porous domains. The results are discussed in terms of the crystallization and phase separation paths that are followed during exposure to CO₂ and the depressurization stages.     

Determination of the solubility parameter of poly(ethylene oxide) at 25°C by gas-liquid chromatography

The solubility parameter of poly(ethylene oxide) at 25°C has been determined using the method developed by DiPaola-Baranyi and Guillet, by the extrapolation of the values of the interaction parameter χ, at high temperatures, ranging from 70°–90°, 90°–110° and 110°–130°C down to 25°C. The values of the solubility parameter obtained, depending on the temperature ranges employed, are 9.8, 9.9 and 10.1, respectively.     

淀粉丙烯酸接枝共聚吸水性树脂的研究进展

基于淀粉丙烯酸接枝共聚作为吸水性树脂具有环保和可生物降解的优良特性 ,目前 ,人们普遍关注的是如何提高其吸水性 ,抗温性 ,控制水份释放 ,压力下保水 ,以及电解液的吸收等。这些特性的改进将使产品的用途更为广泛 ,可以预见 ,淀粉丙烯酸接枝共聚作为吸水性树脂具有广阔的应用和发展前景。