Single fibre deformation studies of poly(p-phenylene benzobisoxazole) fibres

Changes in crystal strain and crystallite orientation of three varieties of PBO fibre (namely PBO AS, HM and HM+) have been investigated during deformation from the analysis of diffraction patterns obtained across single filaments, using a synchrotron X-ray source. Crystal strain was measured from the positions of the meridional reflections and orientation calculated from azimuthal broadening of the equatorial reflections. It has been demonstrated that no difference in crystal strain across the fibre exists, with the calculated strain being equal between fibre skin and core at a given level of stress. Further skin-core crystallite orientation analysis (calculation of the orientation parameter ) proved that the AS fibre was the only PBO variety with a significant difference in orientation across the fibre, with the core region being less oriented due to the processing conditions. The skin and core orientation of all three fibres was found to improve with deformation, with the core of the AS fibre showing a significantly higher rate of improvement. This resulted in a similar level of orientation for both skin and core regions of the PBO AS fibre at high levels of stress. The fibre modulus was found to increase with the increasing initial degree of crystallite orientation. Furthermore, improvement in orientation with external stress was related to _{= 0}, with higher values resulting in greater shear forces on the crystallites and therefore a greater rate of orientation improvement.     

The adhesive behaviour of poly(p-phenylene benzobisthiazole) (PBT)/epoxy composites

A modified scarf joint specimen was developed for characterizing the adhesive behaviour of poly (p-phenylene benzobisthiazole) (PBT) film/epoxy composites. This method subjected samples to varying amounts of normal stress (tensile or compressive) and shear stress. This resulted in the determination of two adhesive strengths; one in the absence of shear stress and one in the absence of normal stress. As a result, the dependence of the adhesive strength on the degree of normal stress was determined. The adhesive behaviour of PBT/epoxy composites was investigated at cure temperatures of 55, 85, 115 and 215°C. Adhesive strengths of 3.5 and 8.2 MPa were measured in the absence of shear and normal stress, respectively, for samples cured at 55° C. A decrease in adhesive strength with increasing cure temperature was attributed to residual cure and thermal stresses. The fracture of these composites was predominantly adhesive, resulting in a clean delamination of the PBT film from the epoxy surface. A modified Tsai-Wu failure criterion is suggested for these composites.     

Single fibre deformation studies of poly(p-phenylene benzobisoxazole) fibres part I Determination of Crystal Modulus

This paper constitutes the first part of a study to assess the influence of processing conditions on the final properties of poly(p-phenylene benzobisoxazole) PBO fibres. Three different samples were selected: as-spun (AS), high-modulus (HM), and ultra-high modulus (HM+) fibres. Synchrotron radiation was used to obtain single-fibre diffraction patterns. It is the first time this approach is taken to estimate the effects of deformation on the crystal properties of PBO fibres. The crystal modulus of the different types of fibre was calculated from the variation with stress of the c-spacing determined from the shift of the (005) and (006) reflections. The HM fibre was found to have the highest crystal modulus of the three fibres, with AS and HM+ PBO being lower. In comparison with tensile data, none of the fibres were found to have a Young's modulus near to the crystal modulus value, although the HM+ fibre was closest due to its production route. These results could be compared to previous diffraction experiments, where the crystal modulus of PBO fibres were determined using fibre-bundles, assuming homogeneous stress in the bundle. Also, Raman spectroscopy experiments were carried out to examine the differences in Raman bandshift rates in response to both stress and strain. The Raman results showed both the HM and HM+ fibres to have stress-induced bandshifts of approximately –4 cm^–1/GPa. The AS fibre value was significantly lower, this being attributed to the non-uniformity of the fibre cross-section. The strain-induced Raman bandshifts were found to be dependent on the tensile modulus of the fibre.     

Hierarchical poly(p-phenylene benzobisoxazole)/graphene oxide reinforcement with multifunctional and biomimic middle layer

A new hierarchical reinforcement developed by coating biomimic polydopamine (PDA) on the surface of poly(p-phenylene benzobisoxazole) (PBO) fibers, which served as a platform for the graphene oxide (GO) grafting, using branched polyethyleneimine (b-PEI) as a bridging agent. The surface morphologies and chemical structures of PBO fibers were characterized for confirming the formation of covalent bond between GO and PBO fibers. The surface roughness (R_a) and wettability of the obtained fibers, denoted as PBO@PDA-PEI-GO, were obviously increased in comparison with those of untreated one. The reinforcement offered a 68.8% enhancement in the interfacial shear strength (IFSS) without degrading the base fiber. The PDA layer on the PBO fiber surface led to improved UV resistance. The hydrothermal aging resistance of PBO/epoxy composite was also greatly improved. This biomimic surface modification approach is facile to prepare, highly efficient to enhance interface, adaptable to all high-performance fibers, and meaningful in multifunctional applications.     

The structure and deformation behaviour of poly(p-phenylene benzobisoxazole) fibres

The relationship between structure and mechanical properties in as-spun and heat-treated high modulus poly (p-phenylene benzobisoxazole) (PBO) fibres has been examined using a combination of electron microscopy, mechanical testing, and Raman microscopy. The structure of the fibres has been determined by obtaining longitudinal sections, and electron diffraction has shown that skin regions are significantly more oriented than the fibre cores. Heat treatment of the fibres at elevated temperatures produces an improvement in the level of crystallinity especially in core regions. Heat treatment also produces an increase in fibre modulus but for fibres heat treated at 650° C there is a significant decrease in strength compared with ones heat treated at 600° C. Well-defined intense Raman spectra were obtained from individual fibres and three main bands at 1280, 1540 and 1615cm^–1 have been identified. All three bands are sensitive to the level of applied strain with the 1280cm^–1 being the most sensitive, shifting by-7.9cm^–1% strain for PBO fibres heat treated at 600° C. The dependence of the sensitivity of the position of the 1615cm^–1 band to strain upon fibre structure has been examined in detail. The rate of shift of band position with strain increases with fibre modulus. It is shown that these shifts in Raman bands are a direct reflection of molecular deformation within the fibres.     

多壁碳纳米管/聚亚苯基苯并二噁唑复合材料的微结构与性能

以甲基磺酸(MSA)为溶剂通过溶液共混法制备了不同多壁碳纳米管(MWNTs)含量的多壁碳纳米管/聚亚苯基苯并二噁唑(MWNTs/PBO)复合材料, 用扫描电镜(SEM)对热处理前后复合材料的微结构进行了分析, 并对其导电、力学和耐热性能进行了研究。结果表明: MWNTs能均匀地分散在聚合物基体中, 并能形成一定的网络结构, 热处理后的复合材料较热处理前的结构更致密, 导电性能和力学性能都有所改善, 其中MWNTs质量分数为10％的热处理后复合材料与纯PBO聚合物相比, 体积电阻率降低约9个数量级, 而拉伸强度和拉伸模量分别提高了95％和53％, 耐热性能也有一定的提高。      

Crack bridging and fibre pull-out in polyethylene fibre reinforced epoxy resins

An investigation has been undertaken of the stress distributions in high-performance polyethylene fibres bridging cracks in model epoxy composites. The axial fibre stress has been determined from stress-induced Raman band shifts and the effect of fibre surface treatment has been followed using untreated and plasma-treated polyethylene fibres. It is found that when the specimen is cracked, the fibres do not break and stress is transmitted from the matrix to the fibre across the fibre/matrix interface. A debond propagates along the fibre/matrix interface accompanied by friction along the debonded interface. The axial stress distributions in the fibres can be analysed using a partial-debonding model based upon shear-lag theory and it is found that the maximum interfacial shear stress at the bond/debond transition is a function of the debond length. The debonding process has been modelled successfully in terms of the interfacial fracture energy-based criterion developed by Hsueh for the propagation of a debond along a fibre/matrix interface accompanied by constant friction along the interface.     

Molecular simulation of compressive failure in poly(p-phenylene teraphthalamide) crystals

Molecular mechanics simulations are carried out on crystals of poly(p-phenylene teraphthalamide) (PPTA) as a function of an applied axial compressive stress. The vibrational frequencies of the long wavelength acoustic modes which propagate along the chain axis and are polarized perpendicular to the plane of the hydrogen-bonded sheets are found to become imaginary when the imposed stress exceeds the modulus for shear between hydrogen-bonded sheets. The imaginary frequencies denote an elastic buckling instability. This instability occurs at a compressive stress of 0.3 GPa, in good agreement with the experimental result for the stress which causes material failure in PPTA fibres. It is suggested that previous overestimates of compressive strengths based on elastic buckling models occurred due to the use of the torsion modulus as the relevant shear modulus; however, the torsion modulus is not relevant because it represents an average of shear moduli in different directions, while elastic buckling takes place along the direction of easiest shear.     

Interfacial dispersion of poly(N-isopropylacrylamide)/ gold nanocomposites

The nanoparticle dispersity and interfacial property could be considered as a basis of their further application in the nanostructured materials. In this paper, the dispersity and interfacial phenomena of poly(N-isopropylacrylamide) modified gold nanoparticles were investigated. Firstly, such polymer/gold nanocomposites were demonstrated to have a good dispersity in water, tetrahydrofuran, alcohols and also chloroform, so they were used to entrap fluorescent dye-labelled lipids in chloroform as nanocontainers and subsequently delivery the fluorescent lipids into water as nanocarriers. Secondly, when the nanocomposites in water/chloroform mixture were heated above 35 degrees C, the nanocomposite particles could be partially transferred from water into chloroform across the interface, and they would come back into water again as cooling, displaying a reversible thermal response. Moreover, such polymer/gold nanocomposites at the immiscible water/toluene fluids preferred to assembly into 2-dimensional membranes with variable density at the water/oil interface. The special dispersion properties of the poly(N-isopropylacrylamide)/gold nanocomposites provide many potentials in the future.     

离子基团对PBO 纤维的表面性能及其界面粘结性能的影响

PBO 作为增强纤维存在与环氧树脂基体界面粘结性能差的问题。通过在聚合过程中添加少量5-磺酸钠2间苯二甲酸部分替代对苯二甲酸与4 , 6-二氨基间苯二酚盐酸盐进行共聚, 合成了大分子链上含有离子基团的SPBO 共聚物, 并制得SPBO 初生纤维。通过接触角测试和XPS 研究了纤维的表面性能, 通过微脱粘实验和SEM评价了纤维与环氧树脂基体的界面粘结性能。结果表明: 与PBO 纤维相比, SPBO 纤维表面浸润性能提高, 表面含氮、氧量均增加, 与环氧树脂的界面剪切强度从8. 2 MPa 提高到10. 1 MPa , 提高了23 %。      

Effects of thermal treatment on the mechanical properties of poly(p-phenylene benzobisoxazole) fiber reinforced phenolic resin composite materials

Thermal degradation behaviors of the poly(p-phenylene benzobisoxazole) (PBO) fiber and phenolic resin matrix were investigated. The unidirectional PBO fiber reinforced phenolic resin composite material laminates were fabricated and exposed in a muffle furnace of 300 °C, 550 °C, 700 °C, and 800 °C for 5 min, respectively, to study the effects of thermal treatment on mechanical properties of the composites. After undergone thermal treatments at 300 °C, 550 °C and 700 °C for 5 min, the flexural strength was reduced by 17%, 37% and 80%, respectively, the flexural modulus was decreased by 5%, 14% and 48%, respectively, and the interlaminar shear strength (ILSS) was lowered by 12%, 48% and 80%, respectively. Thermal treatment at 300 °C, the phenolic resin began to pyrolyze and shrink resulted in the irreversible damage of the composites. After 550 °C thermal treatment, the phenolic resin pyrolyzed mostly but the PBO fiber had no obvious pyrolyze, the interface had sever broken. After 700 °C thermal treatment, the phenolic resin formed amorphous carbonaceous and PBO fiber pyrolyzed mostly so the mechanical properties dropped dramatically. At being heated at 800 °C for 5 min, the fiber was nearly totally pyrolyzed and and kept fibrous carbonaceous although the specimen became too brittle to stand any load thereon.     

离子基团对PBO纤维的表面性能及其界结性能的影响

PBO作为增强纤维存在与环氧树脂基体界面粘结性能差的问题.通过在聚合过程中添加少量5-磺酸钠-间苯二甲酸部分替代对苯二甲酸与4,6-二氨基间苯二酚盐酸盐进行共聚,合成了大分子链上含有离子基团的SPBO共聚物,并制得SPBO初生纤维.通过接触角测试和XPS研究了纤维的表面性能,通过微脱粘实验和SEM评价了纤维与环氧树脂基体的界面粘结性能.结果表明: 与PBO纤维相比,SPBO纤维表面浸润性能提高,表面含氮、氧量均增加,与环氧树脂的界面剪切强度从8.2MPa提高到10.1Mpa,提高了23%.     

The extraction of uranium from seawater by poly(amidoxime)/poly(hydroxamic acid) resins and fibre

The uranium concentration in seawater is 3 μg/l with an estimated 5 × 10⁹ tonnes of uranium in the oceans, in solution as the tricarbonato complex. Any extraction process will encounter the problems attendant on this high dilution, the only feasible methods currently being ion exchange on chelating resins or sorption onto inorganic materials.Poly(amidoxime)/poly(hydroxamic acid) chelating resin has been produced with high uranium sorption from neutral solutions containing the metal as the tricarbonato complex, and the results of a study of the behaviour of this resin towards seawater are given. High chemical and biochemical stability and fast sorption kinetics are properties of the resin which can sorb 68 per cent of the uranium present using a 24 s resin to seawater contact time. Poly(amidoxime)/poly(hydroxamic acid) fibre, prepared by the oximation of poly(acrylonitrile) fibre, is able to sorb 12.5 per cent of uranium in seawater with a 2 s contact time and has the advantage of being in a form capable of weaving into a chelating cloth. Sorbing 1.8 mg uranium per gramme fibre per 10 days, the cloth can be produced as an endless belt, for a continuous process for uranium extraction. A theoretical model indicates that uranium production could be possible at 6 tonnes per annum.     

Electrorheology of undoped poly(p-phenylene) particle-based suspension

Electrorheological (ER) properties of synthesized, pristine poly(p-phenylene) (PPP) particles without any dopant treatment was investigated via flow curves including shear viscosity and yield stress. The ER characteristics were examined as functions of both particle concentration and applied electric field strengths. This undoped, PPP based suspension exhibited normal ER characteristics displayed by conventional semi-conducting polymeric ER materials with doping. The yield stress, which is an important design parameter for ER fluids, satisfied a universal scaling function. We found that the critical electric field strength for the undoped PPP suspension was much higher than the doped PPP suspension. These undoped particles can be used as a model system to test the doped system at low concentration via a universal scaling function.     

Structural studies of poly(p-phenylene benzbisthiazole) films

Poly(p-phenylene benzbisthiazole) (PBT) is one member of a new class of highly-rigid, linear, thermally-stable aromatic heterocyclic polymers. The role of heat-treatment in the improvement of the perfection of crystallinity and mechanical properties of oriented films is discussed. Part of the heat-treatment process seems to be to increase the conjugation length of the polymer chain by increasing the planarity of the molecule, as revealed by visual colour changes and by differential scanning calorimetry. This may in turn account for the improved quality of crystallinity. Considerable detail can be seen in the electron diffraction patterns of heat-treated films. With the exception of the equatorial diffraction peaks this scatter can be accounted for by the detailed molecular transform of the PBT polymer, suitably cylindrically averaged, indicating that the crystal structure is essentially two-dimensional, that is the chains while closely and regularly packed lack longitudinal register. A two-dimensional unit cell with the corresponding molecular packing is proposed which can satisfactorily account for the observed density and for the equatorial diffraction peaks.     

聚多巴胺修饰聚对苯撑苯并双噁唑纤维增强其抗老化性与功能性

高性能纤维聚对苯撑苯并双噁唑(PBO)容易受紫外光照射而导致其性能显著下降。为了提高PBO材料的抗老化性能,文中通过简易方法在纤维表面直接修饰富含酚羟基的纳米厚度的聚多巴胺薄层,并比较研究了初始纤维与多巴胺修饰纤维在紫外光照射前后的力学性能、热稳定性能、结晶取向等性能的变化。结果表明,未经修饰的PBO纤维的力学等性能在紫外光照射后显著下降,如拉伸强度下降了67%;而表面经过修饰的PBO纤维在经过紫外光照射后,其性能变化显著减小,如拉伸强度仅下降了33%。并且,聚多巴胺修饰的PBO纤维对污染物罗丹明B染料等具有一定的吸附性能,吸附24 h的吸附容量达0.25 mg/g。因此,在材料表面修饰聚多巴胺层可有效改善PBO纤维的抗紫外光老化性能,提高了材料的功能性。     

复合抗紫外剂对PBO纤维光稳定性的影响

通过在聚合过程中同时添加有机紫外吸收剂UVA和UVP的方法制备了PBO聚合物,并通过干喷-湿纺工艺制备了含紫外吸收剂(UVA+UVP)的PBO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,结果表明,单独添加紫外吸收剂UVA能改善PBO纤维的紫外稳定性,而添加紫外吸收剂(UVA+UVP)则使PBO纤维的紫外稳定性进一步提高,并且都未给聚合和纺丝带来不利影响。