聚四氟乙烯的表面处理与粘接

本文介绍了聚四氟乙烯PTFE的表面处理方法、表面改性剂以及与聚四氟乙烯粘接性能优良的粘接剂     

Paste extrusion of polytetrafluoroethylene (PTFE): Surface tension and viscosity effects

The effects of the lubricant physical properties on the processing of polytetrafluoroethylene (PTFE) fine powder resins are studied. Lubricants having different surface tension and viscosity were used; the two properties changed independently. These effects were studied by using dies of various contraction angle and reduction ratio for resins having a variety of molecular architecture. It was found that the wettability (surface tension) of the lubricant strongly affects the pressure needed to extrude the PTFE pastes. The viscosity of the lubricant was also found to play a significant role in the process since a lubricant with a low viscosity causes the paste to be extruded at a lower pressure. These effects of the physical properties on the extrusion pressure influence significantly the mechanical properties of the final extrudates. The latter are functions of the degree of fibrillation, which is significantly influenced by the wettability and viscosity of lubricants. Finally, the effects of die geometry on extrusion pressure and mechanical properties of extrudates were also assessed in order to determine the geometrical characteristics and operation conditions for the optimization of the process.     

Supercritical CO2 processing and annealing of polytetrafluoroethylene (PTFE) and modified PTFE for enhancement of crystallinity and creep resistance

A method is reported to improve creep resistance in tension for polytetrafluoroethylene (PTFE) and modified PTFE (M-PTFE). PTFE and M-PTFE from different sources were annealed in air, N₂ or supercritical CO₂ (scCO₂) at a range of temperatures, pressures and time intervals. Annealing PTFE in scCO₂ increases crystallinity from 9 to 53%, depending on the material and annealing conditions. No corresponding increase occurs for samples annealed in air or N₂. In comparison to as-received PTFE, significant improvements in tensile creep resistance (18-60%) are observed also dependent upon the material and annealing conditions. For a given temperature and duration, the increase in PTFE tensile creep resistance after annealing in air or N₂ is greater than after annealing in scCO₂ despite the higher crystallinity for post-scCO₂ processed PTFE. Density measurements indicate that the effect of increased crystallinity is counterbalanced by scCO₂-generated microvoids, particularly at higher pressures, leading to smaller creep resistance. In contrast, thermal annealing in air or N₂, which does not significantly change the density or enhance the crystallinity of PTFE or M-PTFE, yields better tensile creep resistance. The detailed morphological origin of improved resistance to tensile creep is unknown, but stress relief by thermal annealing is evident.     

Surface grafting polymerization and modification on poly(tetrafluoroethylene) films by means of ozone treatment

Surface modification on polytetrafluoroethylene (PTFE) films was performed with sequential hydrogen plasma/ozone treatments and surface-initiated polymerization. C-H groups were introduced to the surface of PTFE films through defluorination and hydrogenation reactions under hydrogen plasma treatment. The C-H groups then served as ozone accessible sites to form peroxide groups under ozone treatment. Grafting polymerization initiating from the peroxide groups was performed on the PTFE film surface with using acrylamide, acrylic acid, glycidyl methacrylate and 2-(2-bromoisobutyryloxy)ethyl acrylate (BIEA) as monomers. With utilizing the isobutylbromide groups on the surface of PTFE-g-PBIEA film as initiators, sodium 4-styrenesulfonate (NaSS) was polymerized onto the PTFE film surface via atom transfer radical polymerization, to bring arborescent macromolecular structure to PTFE film surface. The chemical structures of the macromolecules on PTFE film surfaces were characterized with FTIR-ATR, SEM-EDX and XPS. The surface hydrophilicities of modified PTFE films were significantly enhanced with the modification.     

Thermosensitive poly(allylamine)-g-poly(N-isopropylacrylamide): synthesis, phase separation and particle formation

Grafting of poly(N-isopropylacrylamide) (PNIPAAm) having carboxylic groups at one end onto poly(allylamine) (PAH) in the presence of water soluble carbodiimide has yielded PAH-g-PNIPAAm copolymers with grafting ratios of 50, 29 and 18, respectively. These thermosensitive copolymers exhibit a lower critical solution temperature (LCST) at 34 °C at a temperature increase cycle regardless of their grafting ratios, a temperature identical to that of PNIPAAm-COOH oligomers. Temperature cycling reveals completely reversible polymer aggregation and dissolution above and below the LCST, respectively. Much smaller particle sizes are observed by scanning force microscopy and transmission electron microscopy compared to dynamic light scattering. A porous sphere model is suggested to depict the structure of the particles formed above the LCST, by which the dependence of the particle sizes on their grafting ratios is interpreted taking into account the surface tension and the spatial aggregation distance. Finally, to demonstrate the capability of the copolymers being used as thermosensitive polyelectrolytes, assembly onto multilayers is conducted and the increase of layer thickness is confirmed by small angle X-ray scattering and ellipsometry characterizations.     

The use of the J-integral vector to analyse adhesive bonds with and without a crack

The singularity problem in adhesive joints is mentioned in finite element analyses but it is often ignored. Adhesive joints may have cracks and certainly many sources of singularities due to the adherend corners. Cracks are easily dealt with using fracture mechanics concepts. However, angular wedged notches or bi-material wedges are more difficult to treat because the strength of the singularity must be known. Another point that must be born in mind is the fact that crack propagation in adhesive joints occurs in a mixed mode, especially for ductile materials. The present study shows that the use of the J-integral can treat most of the above problems. A very useful fracture mechanics method using the J-integral vector approach is discussed in connection with adhesive bonds but can be generalized to other singular problems. It is shown that the J vector can be used in adhesive bonds where there is always discontinuity in material properties regardless of whether there is a crack or not. One of the advantages of using the J vector in numerical analyses for homogeneous materials is that the accurate stresses (better known) and displacements in the far field can be used to calculate J instead of the not-so-accurate values near the singular point. A method for dealing with the singularity in adhesive bonds, which can be used in engineering design and analysis, has been established in this investigation.     

Influence of alkoxy tail length on the phase behaviour of mesogen-jacketed liquid crystalline polymers with fan-shaped pendants

A series of new monomers of 2, 5-bis [(3, 4, 5-trialkoxy benzyl) oxycarbonyl] styrene (denoted as M-tri-OC_mH_2m + 1, m = 1, 2, 4, 6, 8, 10, 12, where m indicated the number of carbon atoms in the alkoxy group) were designed and synthesized. Then, their corresponding polymers P-tri-OC_mH_2m + 1 (m = 1, 2, 4, 6, 8, 10, 12) were synthesized by free radical polymerization. The chemical structure of the monomers was confirmed by elemental analysis, ¹H NMR and ¹³C NMR. The molecular characterization of polymers was performed with ¹H NMR, gel permeation chromatography (GPC). The thermal stability of polymers was investigated by thermogravimetric analysis (TGA). The phase structure and transition behaviours were studied using differential scanning calorimetry (DSC), polarized light microscopy (PLM), one- and two-dimensional (1D and 2D) wide-angle X-ray diffraction (WAXD). We found that P-tri-OC_mH_2m + 1 (m = 1, 2) with short n-alkoxy substituents as the tail form columnar nematic (Φ_N) phase; that with the increasing length of alkoxy tails, P-tri-OC_mH_2m + 1 (m = 4, 6, 8) can demonstrate the hexagonal columnar (Φ_H) phase; however, when the length of alkoxy tails exceeded a threshold, P-tri-OC_mH_2m + 1 (m = 10, 12) only develop into columnar nematic (Φ_N) phase instead of Φ_H phase.     

Flow and aggregation characteristics of thermo-responsive poly(N-isopropylacrylamide) spheres during the phase transition

To date, a great many researches were focused on improving stimuli-responsive and controlled-release properties of thermo-responsive hydrogel carriers, whereas for the research on flow characteristics during the phase transition, prior reports have not been found. In this paper, poly(N-isopropylacrylamide) (PNIPAM) spheres with thermo-responsive phase transition characteristics were prepared by cross-linked polymerization. In a transparent Pyrex glass pipe with hydrophilic inner wall, flow and aggregation characteristics of PNIPAM spheres during the phase transition from low temperature which was lower than the lower critical solution temperature (LCST) to high temperature (T>LCST) was studied for the first time. Many interesting phenomena about the flow and aggregation behaviors of PNIPAM spheres were found. The velocity of PNIPAM spheres in horizontal pipe decreased from 1.07 cm/s before the phase transition to 0.65 cm/s or even became zero after the phase transition, which is what targeting drug delivery systems desired. When the initial distance was about 5.5 mm at the entrance of testing pipe section, the PNIPAM spheres could aggregate together after the phase transition and subsequently roll forward; but when the initial distance was as large as 8.5 mm, the distance became close at first during the phase transition and then far after the phase transition. Similar results were also found as mentioned above in vertical pipe. When 10 spheres aggregated together, they stopped at a certain position just after the phase transition in horizontal pipe. If the flowrate was more than 40 ml/min, the aggregation configurations such as triangle, tetrahedron, hexahedron and octahedron which formed after the phase transition at flowrate of 20 ml/min disappeared. The results provided valuable information for future applications of thermo-responsive PNIPAM spheres.     

Formation of the crystalline inclusion complex between γ-cyclodextrin and poly(N-acetylethylenimine)

Poly(N-acetylethylenimine) was found to form a crystalline inclusion complex with γ-cyclodextrin (CD). It did not form crystalline inclusion complexes with α-CD or β-CD. It is a hydrophilic, nitrogen atom-containing polymer that forms a crystalline inclusion complex with CD. FT-IR spectroscopy, thermogravimetry analysis, X-ray diffraction, ¹H NMR spectra and ¹³C CP/MAS NMR spectra were used to characterize the structure and property of the crystalline inclusion complex.     

Chain conformations of syndiotactic poly(m-methylstyrene) in the crystalline state

A conformational energy analysis of the isolated chain of syndiotactic poly(m-methylstyrene) under the constraint of a crystalline field is reported. Two different minimum energy conformations having similar energy have been found; the trans-planar conformation with tcm symmetry and the two-fold helical conformation with s(2/1)2 symmetry, according with the observed polymorphic behavior of this polymer. The calculated chain axes are in agreement with the experimental axes of 5.1 and 7.9 Å found for the different polymorphic forms of syndiotactic poly(m-methylstyrene). However, only a metastable disordered modification (form III) having chains in trans-planar conformation has been described. This indicates that, even though the trans-planar conformation is, in the isolated chain as stable as the helical conformation, the packing of the chains in helical conformation is probably more efficient than that of the trans-planar chains.     

Crystallization and phase transformation kinetics of poly(1-butene)/MWCNT nanocomposites

Poly(1-butene)/MWCNT nanocomposites were prepared by simple melt processing technique. Crystallization, crystal-to-crystal phase transformation and spherulitic morphology were studied using differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD) and optical microscopy (OM). The non-isothermal crystallization exhibited higher values of Z_t derived from Avrami theory and lower values of F(T) obtained from Avrami-Ozawa analysis, while the isothermal crystallization revealed a significant increase in crystallization temperatures and lower crystallization half times compared to pristine PB. The observed changes in the crystallization kinetics were ascribed to the enhanced nucleation of PB in the presence of MWCNT. The nucleating activity calculated from the non-isothermal crystallization data revealed that the MWCNTs provide an active surface for the nucleation of PB. The optical micrographs exhibited significantly smaller crystallites with disordered morphology for the nanocomposites compared to the well defined spherulitic morphology for pristine PB. The rate of phase transformation from kinetically favored tetragonal to thermodynamically stable hexagonal form was noticeably enhanced as evidenced by the reduction in the half time for phase transformation from 58 h to 25 h for PB reinforced with 7% MWCNT.     

Thermo-, and pH dual-responsive poly(N-vinylimidazole): Preparation,characterization and its switchable catalytic activity

A monomer, 2-(isobutyramido)-3-methylbutyl methacrylate (IMMA) was synthesized through a two-step reaction. When a few of IMMA (less than 4 mol%) was copolymerized with N-vinylimidazole (VIm) under free radical polymerization condition, water-soluble P(VIm-co-IMMA) copolymers were obtained. Their structural information was verified and interpreted from ¹H NMR, FTIR and GPC. Kinetic analyses from ¹H NMR demonstrated that one-batch addition of IMMA into the polymerization system led to an inhomogeneous distribution of IMMA units in the copolymers, whereas homogeneous distribution of IMMA units in the copolymers could be obtained through the portion-wise addition of IMMA monomer. The thermal properties of such copolymers were measured by DSC. Compared with PVIm homopolymer, the few IMMA units in the P(VIm-co-IMMA) copolymer had little influence on the T_g values. The obtained P(VIm-co-IMMA) copolymers were thermoresponsive in water, and their phase transition temperatures could be efficiently raised through reducing the IMMA content in the copolymers, raising the addition times of IMMA monomers or lowering the pH of media. Dynamic light scattering analysis showed that unlike the traditional thermoresponsive linear polymers, obvious size shrinkage around the phase transition temperature could not be observed in such P(VIm-co-IMMA) copolymers. Such copolymers could be used as smart organocatalysts in the hydrolysis of p-nitrophenyl acetate. Below the phase transition temperature the reaction rate followed the Arrhenius law, but above the phase transition temperature the reaction rate increased much slower than the prediction from the Arrhenius law. Moreover, the catalytic transition temperature could be tuned through utilizing the P(VIm-co-IMMA) copolymers with different phase transition temperature. The mechanism was discussed accordingly.     

Photosensitive poly(benzoxazole) based on precursor from diphenyl isophthalate and bis(o-aminophenol)

A new synthetic method for the preparation of poly(benzoxazole) (PBO) precursor, poly(o-hydroxyamide) (7) from bis(o-aminophenol) (5) and diphenyl isophthalate (6) has been developed. Polymer 7 was prepared by the polycondensation of 5 and 6 in 1-methyl-2-pyrrolidinone (NMP) at 185-205 °C. Model reactions were carried out in detail to elucidate appropriate conditions for the formation of 2-hydroxybenzanilide (3) from o-aminophenol (1) and phenyl benzoate (2). The photosensitive (PBO) precursor based on polymer 7 containing a 22% of benzoxazole unit and 30 wt% 1-{1,1-bis[4-(2-diazo-1-(2H)naphthalenone-5-sulfonyloxy)phenyl]ethyl}-4-{1-[4-(2-diazo-1(2H)naphthalenone-5-sulfonyloxy)phenyl]methylethyl}benzene (S-DNQ) showed a sensitivity of 110 mJ cm⁻² and a contrast of 5.0 when it was exposed to 436 nm light followed by developing with a 2.38 wt% aqueous tetramethylammonium hydroxide solution at room temperature. A fine positive image featuring 8 μm line and space patterns was observed on the film of the photoresist exposed to 200 mJ cm⁻² of UV-light at 436 nm by the contact mode.     

Preparation and characterization of thiocyanate-selective electrodes based on new complexes of copper(II) as neutral carriers

The complexes of hydroxycitronellal (o-aminobenzoic acid) copper(II) (Cu(II)-HXAB) and salicylaldehyde (o-aminobenzoic acid) copper(II) (Cu(II)-SHAB) were used as neutral carriers in PVC-based membrane ion-selective electrodes. The electrode based on Cu(II)-HXAB exhibited near-Nernstian potential response to thiocyanate (SCN⁻) in a linear range of 1.0 × 10^− 6 to 1.0 × 10^− 1 M with a detection limit of 8.5 × 10^− 7 M and a slope of − 57.3 mV/decade in 0.01 M phosphate buffer solution (pH 5.0). The electrode exhibited high selectivity to SCN⁻ over other tested anions with an anti-Hofmeister selectivity sequence. The selectivity behavior might be discussed in terms of UV-Vis spectrum and infrared spectrum. The transfer process of thiocyanate across the membrane interface was investigated by making use of the AC impedance technique. The electrode containing Cu(II)-HXAB could be applied to thiocyanate analysis in waste water with satisfactory results.     

The phase diagram of poly(4-hydroxybenzoic acid) and poly(2,6-hydroxynaphthoic acid) and their copolymers from X-ray diffraction and thermal analysis

Homopolymers and copolymers of 4-hydroxybenzoic acid (HBA) and 2,6-hydroxynaphthoic acid (HNA) have been studied with differential scanning calorimetry and temperature-resolved wide angle X-ray diffraction. All polymers have more than one disordering transition between the glass transition (between 400 and 430 K) and decomposition (between 710 and 750 K). The first transition in PHBA at 616-633 K is from orthorhombic rigid crystals to a conformationally disordered pseudo-hexagonal phase (condis phase). The two higher transitions are first, a further disordering process to a hexagonal condis crystal, and then a change to an anisotropic melt (liquid crystal) at about 800 K, with increasing decomposition above 750 K. In PHNA, orthorhombic crystals change above 600 K to an orthorhombic condis crystal structure, which go to an anisotropic melt at 750 K, and subsequent decomposition. In addition, using empirical entropy rules that account for the changes during the transitions from the crystal to the disordered mobile phases, an effort is made to understand the disorder and mobility, and to arrive at a non-equilibrium phase diagram of the copolymer system. The existence of a single, but up to 200 K wide, glass transition and remaining high crystallinity of the copolyesters, indicate partial solubility of the repeating units in all phases. The new data are compared to and brought into agreement with the large number of prior measurements and often unclear interpretations.     

The effect of interface modification between POM and PTFE on the properties of POM/PTFE composites

A plasma technique was applied to modify the surface of polytetrafluoroethylene (PTFE) fiber to improve the compatibility between PTFE and polyacetal (POM). This technique used argon (Ar) plasma to treat PTFE fiber first and then grafting the fiber with acrylic acid (AAc) by peroxidation. The Ar plasma‐treated PTFE (PPTFE) fiber and AAc‐grafted PPTFE (AAc‐g‐PPTFE) fiber were added into POM to increase the wear resistance and to decrease the friction coefficient of POM. The variables of the experiments were plasma treatment time, monomer concentration of AAc, and grafting time. The graft copolymer was characterized by Fourier transform infrared (FTIR) spectroscopy. The stress–strain behavior, impact strength, Taber wear factor, friction coefficient, and morphology of composites were also investigated. The properties of POM/PTFE composites could be successful modified by surface modification of PTFE in this investigation. The impact strength of POM/AAc‐g‐PPTFE composites was more than twice of that of POM/PTFE composites. The Taber wear factor and friction coefficient of POM/AAc‐g‐PPTFE composites decreased markedly. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 800–807, 2000     

^60Co辐照对PTFE表面结构和表面能的影响

为了改善氟聚合物加工性和二次加工性,研究了PTFE聚合物的加工技术。以不同剂量辐照PTFE,PTFE粉末辐照后表面晶相没有发生变化。模压辐照的PTFE,其密实性较好,表面光滑。同时,辐照PTFE后表面有大量的极性亲水基团出现,使表面极性分量增加;而色散分量变化较小。辐照产生的表面极性基团减小了由于不同模压产生的表面所引起的接触角变化。     

The phase,microstructure evolution and the Nd3+ function in the fabrication process of LuAG transparent ceramics

Nd:LuAG transparent ceramics were fabricated by the solid-state reaction under vacuum sintering using SiO₂ and MgO as sintering aids, commercial Lu₂O₃, Al₂O₃ and Nd₂O₃ as raw materials. The Nd doping concentration was adopted from 0 at. % to 1.3 at. %. The phase transformation and microstructure evolution of 1.3 at. % Nd:LuAG ceramics under different sintering temperature was investigated in detail. Meanwhile, the effects of Nd₂O₃ on the grain growth were surveyed. The results shown that when the samples were sintered at 1780?°C, the 1.3 at. % Nd:LuAG ceramic had clean gain boundary, and the transmittance of it reached 83.8% at 1064?nm.     

Synthesis of laterally attached side-chain liquid crystalline poly(p-phenylene vinylene) and polyfluorene derivatives for the application of polarized electroluminescence

Two series of poly(p-phenylene vinylene) and polyfluorene derivatives (PPV1-PPV4 and PF1-PF5) containing laterally attached penta(p-phenylene) mesogenes were synthesized and characterized. These polymers show nematic liquid crystalline behavior. The optical properties of the polymers were investigated by UV-vis absorption and photoluminescence spectrometers and these polymers were fabricated to form the polarized electroluminescent devices using poly(ethylenedioxythiophene)-poly(styrene sulfonic acid) (PEDOT-PSS) as an alignment layer. In the series of poly(p-phenylene vinylene) derivatives, polymer PPV4 offered the best EL device performance. It emitted yellow light at 588 nm at 4 V. The maximum brightness was about 1337 cd/m² at 9 V with a polarized ratio of 2.6. In another series of polyfluorene derivatives, PF4 offered the best EL device performance with the polarized ratio of 12.4 and a maximum luminescence of 1855 cd/m². In the case of polarized white light, as a consequence of blending small amount of PF4 and PF5 with a host polymer PF2, polarized ratio of up to 10.2 and a maximum brightness of 2454 cd/m² have been attained. The aligned films exhibited pronounced polarized ratio, implying that the polymers exhibit potential for linearly polarized LED application.     

Physical properties of poly(n-alkyl acrylate) copolymers. Part 1. Crystalline/crystalline combinations

The physical properties of n-alkyl acrylate copolymers containing two crystallizeable monomers, including thermal characteristics, structure as determined by small angle X-ray scattering, and gas permeability as a function of temperature, were examined in detail and compared to the corresponding homopolymers. The copolymers exhibit co-crystallization and, thus, for a given average side-chain length have comparable melting temperatures as the corresponding homopolymers. For a given side-chain length, the copolymers have somewhat lower heats of fusion than the corresponding homopolymers because of a reduction in crystallite size as revealed by SAXS. This depression in crystallinity is reflected in the permeability data for the copolymers. Poly(n-alkyl acrylates) exhibit a ‘jump’ in their gas permeability at the T_m of the side-chain lengths that is mainly caused by a switch in the side-chain morphology from crystalline to amorphous upon melting. The depression in crystallinity for the copolymers results in a smaller permeation jump. The jump breadth correlates with the melting endotherms for these polymers as determined by DSC. Ultimately, the melting endotherms for these copolymer systems provide an excellent tool for predicting permeability changes across the melting region.