Surface characteristic of poly(p‐phenylene terephthalamide) fibers with oxygen plasma treatment

Oxygen plasma is widely employed for modification of polymer surfaces. Plasma treatment process is a convenient procedure that is also environmentally friendly. This study reports the effects of oxygen plasma treatment on the surface properties of poly(p‐phenylene terephthalamide) (PPTA) fibers. The surface characteristics before and after oxygen plasma treatment were analyzed by XPS, atomic force microscopy (AFM) and dynamic contact angle analysis (DCAA). It was found that oxygen plasma treatment introduced some new polar groups (O? C?O) on the fiber surface, increased the fiber surface roughness and changed the surface morphologies obviously by plasma etching and oxidative reactions. It is also shown that the fiber surface wettability was improved significantly by oxygen plasma treatment. Copyright © 2008 John Wiley & Sons, Ltd.     

环氧氯丙烷修饰的对位和间位芳纶涂覆剂研究

制备了具有环氧丙基侧链的对位芳纶(PPTA-ECH)和间位芳纶(PMIA-ECH),并将其用做对位芳纶(PPTA)织物/环氧树脂复合材料中PPTA织物的涂覆剂。采用场发射扫描电子显微镜(FE-SEM)及XPS等方法对PPTA织物表面的PPTA-ECH涂层结构进行了表征。考察了PPTA-ECH和PMIA-ECH涂覆的PPTA织物/环氧树脂复合材料的层间剪切强度和面内剪切强度,并与未经涂覆的PPTA织物复合材料的性能作比较。结果表明,PPTA-ECH和PMIA-ECH可显著改善PPTA织物和环氧树脂之间的界面性能。涂覆了PPTA-ECH及PMIA-ECH的PPTA织物/环氧树脂复合材料的层间剪切强度(ILSS)比未经涂覆的复合材料分别提高了26.20%和14.76%,面内剪切强度(ISS)分别提高了26.98%和11.86%。由于PPTA-ECH对PPTA纤维具有更强的亲和能力,因此PPTA-ECH在层间剪切强度和面内剪切强度方面的增强效果均优于PMIA-ECH。对PPTA-ECH在PPTA纤维表面铺展与吸附及对复合材料的增强机理也进行了初步探讨。作为新型涂覆剂,PPTA-ECH在对位芳纶复合材料的开发应用方面具有潜在的应用前景。     

抗紫外老化聚对苯撑苯并二噁唑(PBO)纤维的制备与表征

通过化学添加2,5-二羟基对苯二甲酸(DHTA)共聚,以及添加金红石型纳米TiO2物理共混的方法,制备了聚对苯撑苯并二噁唑(PBO)的抗紫外改性纤维.考察了纤维的力学性能、特性粘度及表面形貌在紫外老化过程中的变化,并结合PBO纤维紫外加速老化后的红外分析,对PBO纤维的光老化机理进行了初步研究.结果表明,本实验所制备的DHPBO纤维以及DHPBO/n-TiO2纤维的抗紫外老化能力明显高于PBO纤维,并且金红石型纳米TiO2对PBO的抗紫外改性效果要优于有机紫外吸收剂(2,2'-(1,2-乙烷二基)双(4,1-亚苯基)双苯并噁唑).     

A study of the surface morphology of poly(p-phenylene terephthalamide) chars using scanning probe microscopy

The objective of this work was to investigate the changes in surface morphology associated with thermal degradation of poly(p-phenylene terephthalamide) (PPTA) into chars. To this end, PPTA samples decomposed at several temperatures up to 800 °C were studied on a local scale using atomic force microscopy (AFM) and scanning tunnelling microscopy (STM). Domains with a diameter of 40-50 nm started appearing among PPTA nanofibrils at about 500 °C. At this temperature and above, a film coating the fibre developed. This layer was much less rigid than PPTA, and remained deposited on the fibres, even at high temperatures. At 800 °C, the STM images showed a surface distribution typical of a carbonaceous material, isotropic although somewhat heterogeneous. When an intermediate isothermal step (500 °C, 200 min) was introduced along with heat treatment of PPTA under a constant rate, the material obtained at the end of this step was conductive enough to be studied by STM. Although the coating over the fibres also remained after the isothermal step, it was less homogeneous than in the absence of isothermal treatment. On further heating, the residue exhibited a surface morphology typical of a carbonaceous material, but much more homogeneous and isotropic than in the absence of the isothermal step.     

Real-time monitoring of polymer swelling on the nanometer scale by atomic force microscopy

The swelling of a polymer surface has been monitored in real time on the nanometer scale by atomic force microscopy (AFM). After modification by oxygen plasma treatment, poly(p-phenylene terephthalamide) (PPTA) displays a characteristic nanostructured surface morphology consisting of high-lying features alternating with topographically depressed areas. Selective swelling of the least cross-linked, depressed areas after the adsorption of ambient water or water from saturated humid atmospheres was observed by tapping mode AFM operated in the attractive interaction regime. The swollen areas could be distinguished from the nonswollen ones by local variations in the sample indentation made by the AFM tip when imaging in the tapping mode repulsive interaction regime. Monitoring the swelling of the plasma-treated polymer surface provided a means to reveal the nanometer-scale heterogeneity that this type of treatment creates on the polymer surface, which is something that would not be possible otherwise. Measurement of AFM tip-sample adhesion forces evidenced rapid water adsorption onto the oxygen plasma-treated surface, supporting the idea of water-induced swelling. This high hydrophilicity was interpreted as arising from the incorporation of polar oxygen functionalities, as demonstrated by X-ray photoelectron spectroscopy (XPS).     

主链含羟基的芳杂环液晶共聚物聚对苯撑苯并二噁唑的 合成及性能研究

通过在聚合过程中添加少量2,5-二羟基对苯二甲酸(DHTA)部分替代对苯二甲酸(TPA)与4,6-二氨基间苯二酚(DAR)盐酸盐进行共聚, 合成了一系列大分子链上含有羟基基团的DHPBO共聚物, 并制得其初生纤维. 利用FTIR、接触角等分析手段对其化学结构和纤维表面性能进行了表征, 通过单丝拔出实验和SEM考察了DHPBO纤维与环氧树脂基体的界面剪切强度, 并采用轴向压缩弯曲实验和紫外光加速老化实验评价了羟基基团的引入对提高纤维压缩性能和抗紫外性能的影响. 结果表明, 羟基基团的引入使得DHPBO纤维的表面亲水性、与环氧树脂的界面剪切强度以及纤维的压缩性能和抗紫外性能都有了显著提高.     

Improvement of interfacial adhesion and nondestructive damage evaluation for plasma-treated PBO and Kevlar fibers/epoxy composites using micromechanical techniques and surface wettability

Comparison of interfacial properties and microfailure mechanisms of oxygen-plasma treated poly(p-phenylene-2,6-benzobisoxazole (PBO, Zylon) and poly(p-phenylene terephthalamide) (PPTA, Kevlar) fibers/epoxy composites were investigated using a micromechanical technique and nondestructive acoustic emission (AE). The interfacial shear strength (IFSS) and work of adhesion, Wa, of PBO or Kevlar fiber/epoxy composites increased with oxygen-plasma treatment, due to induced hydrogen and covalent bondings at their interface. Plasma-treated Kevlar fiber showed the maximum critical surface tension and polar term, whereas the untreated PBO fiber showed the minimum values. The work of adhesion and the polar term were proportional to the IFSS directly for both PBO and Kevlar fibers. The microfibril fracture pattern of two plasma-treated fibers appeared obviously. Unlike in slow cooling, in rapid cooling, case kink band and kicking in PBO fiber appeared, whereas buckling in the Kevlar fiber was observed mainly due to compressive and residual stresses. Based on the propagation of microfibril failure toward the core region, the number of AE events for plasma-treated PBO and Kevlar fibers increased significantly compared to the untreated case. The results of nondestructive AE were consistent with microfailure modes.     

Molecular layer deposition of poly(p-phenylene terephthalamide) films using terephthaloyl chloride and p-phenylenediamine

Ultrathin polymer films can be fabricated using the gas-phase method known as molecular layer deposition. This process typically uses bifunctional monomers in a sequential, self-limiting reaction sequence to grow conformal polymer films with molecular layer control. In this study, terephthaloyl chloride (TC) and p-phenylenediamine (PD) were used as the bifunctional monomers to deposit poly(p-phenylene terephthalamide) (PPTA) thin films. 3-Aminopropyl trimethoxysilane or ethanolamine was used to prepare amine-terminated surfaces prior to the PPTA MLD. The surface chemistry and growth rate during PPTA MLD at 145 degrees C were studied using in situ transmission Fourier transform infrared (FTIR) spectroscopy experiments on high surface area powders of SiO2 particles. PPTA MLD thin film growth at 145 degrees C was also examined using in situ transmission FTIR experiments on flat KBr substrates with an amine-terminated Al2O3 ALD overlayer. The integrated absorbances of the N-H and amide I stretching vibrations were measured and used to estimate the thin film thickness. X-ray reflectivity (XRR) experiments were also employed to measure the film thickness after PPTA MLD at 145 degrees C and 180 degrees C. The experiments revealed that the TC and PD reactions displayed self-limiting surface chemistry. The surface species alternated with sequential TC and PD exposures and the PPTA MLD films grew continuously. However, the growth rates per MLD cycle at 145 degrees C were less than expectations based on the size of the molecules involved in the reaction chemistry and were variable between 0.5 and 4.0 A per TC/PD reaction cycle. The lower growth rates are explained by the growth of a limited number of polymer chains on the substrate. The variability in the growth rate is attributed to the difficulties with the bifunctional monomer precursors. Alternative surface chemistries for polymer MLD are proposed that would avoid the use of bifunctional monomers.     

Adsorption of n-alkanes on plasma-oxidized high-strength carbon fibers

The objective of this work was to characterize the degree of heterogeneity brought about by oxygen plasma treatment of carbon fibers by studying its effects on the adsorption of n-alkanes. Untreated and unsized high-strength carbon fibers were subjected to oxygen plasma treatments with different degrees of severity. A sample of the same material oxidized following a standard industrial method was also studied for comparison. Adsorption of C5-C10n-alkanes at 303-353 K was measured by inverse gas chromatography (IGC). Elution peaks were symmetrical for the fresh and industrially oxidized samples; however, a large extent of asymmetry was observed for the plasma-treated fibers. Differences in surface heterogeneity were quantified in terms of several adsorption thermodynamic magnitudes. Differential heats of adsorption exhibited values similar to those corresponding to the probe-basal plane interaction. The dispersive component of the surface tension of the solids increased clearly upon plasma oxidation, the increase being systematic according to the severity of plasma treatment. It can be concluded that plasma oxidation generates high-surface-energy sites responsible for trapping of n-alkane molecules, this effect being more marked as the chain length increases. The possibility of this effect being associated to creation of micropores was ruled out on the basis of volumetric CO2 adsorption experiments and IGC measurements at finite dilution. Scanning tunneling microscopy observations allowed us to establish a possible connection between fiber surface nanostructure and IGC results. The sites accessible to n-alkane molecules in the industrially oxidized sample seem to be highly disordered, thus leading to a weaker interaction with the adsorbate.     

UV accelerated aging and aging resistance of dihydroxy poly(p‐phenylene benzobisoxazole) fibers

By introducing binary hydroxyl groups into poly(p‐phenylene benzoxazole) (PBO) macromolecular chains, we synthesized dihydroxy poly(p‐phenylene benzobisoxazole) (DHPBO) polymers and then prepared DHPBO fibers by dry‐jet wet‐spinning. Comparative studies were performed between intrinsic PBO fibers and DHPBO fibers. The effects of hydroxyl polar groups on improving the UV aging resistance of PBO fibers were investigated. With the introduction of hydroxyl groups, substantial changes in the chemical structures and surface morphologies of DHPBO fibers were observed. As proved by tensile testing and intrinsic viscosity measurement, the UV resistance of DHPBO fibers is obviously improved compared to that of intrinsic PBO fibers. XRD results indicate that the UV aging of these fibers occurs mainly on the surfaces of fibers. Based on these results, the mechanism of UV aging of PBO fibers was discussed. Copyright © 2009 John Wiley & Sons, Ltd.     

Surface characterization of poly(methyl methacrylate-co-n-butyl acrylate-co-cyclopentylstyryl-polyhedral oligomeric silsesquioxane) by inverse gas chromatography

The surface properties of poly(methyl methacrylate-co-n-butyl acrylate-co-cyclopentylstyryl polyhedral oligomeric silsesquioxane) (poly(MMA-co-BA-co-styryl-POSS)) were studied by means of inverse gas chromatography (IGC) using 10 non-polar and polar solvents as the probes. Thermodynamic parameters of adsorption, e.g., specific retention volume, the dispersive component of the surface free energy, the specific interaction contribution to the free energy of adsorption and the acid/base constants were obtained to investigate the interactions between the surfaces of the copolymers and different solvents. It was found that incorporation of styryl-POSS into polymer resulted in increasing interactions between polymers and solvents, dispersive component of surface free energy of polymer and acidity of the surfaces of the polymers. The more the styryl-POSS were embedded, the stronger the interaction between the polymer surface and solvent, the dispersive component of the surface free energy and the acidity of the polymer surface were.     

连续纤维增强含二氮杂萘酮联苯结构聚芳醚砜酮树脂基复合材料的界面

利用射频感性耦合冷等离子体(ICP)处理技术改性连续纤维表面,分别采用X射线光电子能谱(XPS)、原子力显微镜(AFM)及动态接触角分析(DCA)系统研究了等离子体处理时间、放电气压、放电功率等工艺参数对连续碳纤维、芳纶纤维和对亚苯基苯并二噁唑(PBO)纤维的表面化学成分、表面形貌、表面粗糙度及表面自由能的影响.研究结...     

Synthesis and Characterization of Poly(p-phenylene benzobisoxazole)/Poly(pyridobisimidazole) Block Copolymers

Poly(p-phenylene benzobisoxazole)/poly(pyridobisimidazole) block copolymers (PBO-b-PIPD) were prepared by introducing poly(pyridobisimidazole) (PIPD) moieties into the main chains of poly(p-phenylene benzobisoxazole) (PBO) in order to enhance its photostability. PBO and copolymer fibers were directly prepared from the polymerization solutions by dry-jet wet-spinning. Chemical structures and molecular chains arrangement of the block copolymers were characterized by Fourier transform infrared (FTIR) spectroscopy, solid-state ¹³C-NMR and wide angle X-ray diffraction (WAXD). Thermal stability of the copolymers was investigated by thermogravimetric analysis (TGA) in nitrogen. Thin films of PBO and copolymers were cast from methanesulfonic acid (MSA) solutions. Both the films and fibers were exposed to UV light to determine their photostability. Changes in the chemical structures and surface morphologies of the films were characterized by FTIR spectra and scanning electronic microscopy (SEM), respectively. After UV light exposure, the retention of strength for copolymer fibers is improved compared to PBO fibers. The results revealed that copolymers suffered less photodegradation in comparison with homopolymer. The mechanism for the improved photostability of the copolymers was discussed.     

Morphology of poly(p-phenylene terephthalamide) fibers

Fibers of poly(p-phenylene terephthalamide) (PPTA) have a fibrillar morphology, the individual fibrils having a high proportion of extended chains passing through periodic defect layers. A pleat structure is superimposed. The fibers are fully crystalline (within the limits of determination) with a small fraction of randomly oriented crystalline material. The major distinction between PPTA and conventional fibers lies in the high level of extended chains passing through the defect layers of the former structure. These extended chains result in crystallographic register being maintained between adjacent ordered zones. Quantitatively, a measure of this order is obtained from a comparison of the correlation length, obtained from meridional x-ray peak widths, and the defect spacing. In conventional fibers the defect spacing, i.e., long period, is longer than the correlation length (i.e., crystal size). In PPTA, the analog of the long period, the defect spacing (about 35 nm) is smaller than the correlation length, which is over 80 nm.     

Improvement of surface wettability and interfacial adhesion ability of poly(p-phenylene benzobisoxazole) (PBO) fiber by incorporation of 2,5-dihydroxyterephthalic acid (DHTA)

By introducing 2,5-dihydroxyterephthalic acid (DHTA) into poly(p-phenylene benzoxazole) (PBO) macromolecular chains, dihydroxy poly(p-phenylene benzobisoxazole) (DHPBO) was synthesized and then DHPBO fibers were prepared by dry-jet wet-spinning method. Effects of hydroxyl polar groups on surface wettability and interfacial adhesion ability of PBO fiber were investigated. With the incorporation of double hydroxyl polar groups, contact angle on PBO fiber for water can decrease from 71.4° to 50.70°, and contact angle for ethanol can decrease from 37.2° to 27.40°. The wetting time on DHPBO fibers for water can be as short as 650 ms, which is half of that of PBO fibers. The interfacial shearing strength (IFSS) between DHPBO (10% mol content DHTA) fibers and epoxy resin is 18.87 MPa, 92.55% higher than that of PBO fibers. SEM images indicate that the PBO/epoxy composite failure mode may change from fiber/matrix adhesive failure to partially cohesive failure.     

Quiescent and strain-induced crystallization of poly(p-phenylene terephthalamide) from sulfuric acid solution

Quiescent and strain-induced crystallization of poly(p-phenylene terephthalamide) (PPTA) from sulfuric acid solution has been studied. Negative spherulites (SA-PPTA spherulites) are formed from hot concentrated solutions by cooling. The spherulite consists of radiating fibrous lamellae several hundred angstroms wide. The electron diffraction pattern indicates that PPTA molecules are oriented perpendicular to the long axes of the fibrous lamellae and that the [010] or [110] direction of the modification I crystal and [010] direction of the modification II crystal are parallel to the long axes of the fibrous lamellae. The width of the lamellae is much smaller than the chain length of the starting PPTA. It appears that hydrolysis of PPTA during melting crystallization determines the chain length, i.e., the width of the fibrous lamella. Stacked, lamellar structures like “row structures” are formed under shear. The longer axes of the fibrous lamellae are oriented perpendicular to the shear direction. It is confirmed by electron diffraction studies that the PPTA molecules are oriented parallel to the shear direction. Well-developed fibrils with the PPTA molecules oriented to the fibril axis, are formed by adding the SA-PPTA spherulites to water with vigorous stirring.     

Aging behavior of PBO fibers and PBO‐fiber‐reinforced PPESK composite after oxygen plasma treatment

Aging behavior of poly(p‐phenylene benzobisoxazole) (PBO) fibers and PBO‐fiber‐reinforced poly(phthalazinone ether sulfone ketone) (PPESK) composites after oxygen plasma treatment was investigated. Surface chemical composition, surface roughness and surface morphologies of oxygen‐plasma‐treated PBO fibers before and after aging in air for 1, 3, 5 and 10 days were analyzed by XPS and atomic force microscopy (AFM). The effects of aging on the material were examined by interlaminar shear strength (ILSS) and water absorption measurements. The results indicate that the major aging behavior of the fibers and the composite appeared in the first few days after oxygen plasma treatment, whereas minor aging effects were observed with prolonged aging. Copyright © 2008 John Wiley & Sons, Ltd.     

Surface characterization of industrial fibers with inverse gas chromatography

Inverse gas chromatography (IGC) was applied for the determination of the surface characteristics of Tenax carbon fibers and Akzo Nobel Twaron fibers. Furthermore, IGC procedures for the determination of dispersive and acid-base interactions were validated. The data show that too high values for the dispersive component of the surface energy are obtained when the adsorption area occupied by a single adsorbed n-alkane molecule is estimated from parameters of the corresponding liquid. Comparable values are obtained when the Doris-Gray methodology (area per methylene unit) or measured probe areas are employed. For the fibers studied in this work meaningful Gibbs energy values of the acid-base interaction were only obtained with the polarizability approach. When the dispersive interaction of the polar probes with the fiber surface was scaled to the n-alkane interaction via surface tension, the boiling point, or the vapor pressure of the probes often negative acid-base interaction energies were found. From the temperature dependence of the Gibbs energy, the enthalpy of the acid-base interactions of various probes with the carbon and Twaron aramid fibers was determined. However, from these enthalpy values no meaningful acid-base surface parameters could be obtained. Generally, the limited accuracy with which these parameters can be obtained make the usefulness of this procedure questionable. Also the Gibbs energy data of acid-base interaction can provide a qualitative basis to classify the acidity-basicity of the fiber surface. This latter approach requires only a limited data set and is sufficiently rapid to enable the use of IGC as a screening tool for fibers at a production site. For several polar probes significant concentration effects on carbon fibers were observed. At very low probe loadings the interaction with the fiber surface suddenly increases. This effect is caused by the heterogeneity of the interaction energy of the active sites at the surface. A simple procedure to measure the adsorption isotherm at infinite dilution was developed. The determination of the concentration dependence of the interaction of an n-alkane, an acidic and a basic probe was incorporated in the IGC screening procedure of carbon fibers to monitor this heterogeneity.     

Poly(oxyethylene) based surface coatings for poly(dimethylsiloxane) microchannels

Control of surface properties in microfluidic systems is an indispensable prerequisite for successful bioanalytical applications. Poly(dimethylsiloxane) (PDMS) microfluidic devices are hampered from unwanted adsorption of biomolecules and lack of methods to control electroosmotic flow (EOF). In this paper, we propose different strategies to coat PDMS surfaces with poly(oxyethylene) (POE) molecules of varying chain lengths. The native PDMS surface is pretreated by exposure to UV irradiation or to an oxygen plasma, and the covalent linkage of POE-silanes as well as physical adsorption of a triblock-copolymer (F108) are studied. Contact angle measurements and atomic force microscopy (AFM) imaging revealed homogeneous attachment of POE-silanes and F108 to the PDMS surfaces. In the case of F108, different adsorption mechanisms to hydrophilic and hydrophobic PDMS are discussed. Determination of the electroosmotic mobilities of these coatings in PDMS microchannels prove their use for electrokinetic applications in which EOF reduction is inevitable and protein adsorption has to be suppressed.     

N-substituted poly(p-phenylene terephthalamide)

N-substituted poly(p-phenylene terephthalamide)s (PPTA), such as N-alkylated, N-aralkylated, and N-carboxymethylated poly(p-phenylene terephthalamide), were synthesized from PPTA and the corresponding halides by the polymer reaction via the metalation reaction in a solution of sodium methylsulfinylcarbanion in dimethyl sulfoxide at low temperature. The introduction of various substitutional groups into the amide groups of PPTA increased their solubilities, but decreased their thermal stabilities compared with PPTA. The effects of various substitutional groups on the thermal properties and the solubilities are discussed. Liquid crystal formation was noticed for PPTA substituted with bulky groups such as 9-anthrylmethyl group.