Polybenzoxazole Research Articles

Mechanical Reinforcement of Polybenzoxazole by Carbon Nanotubes through Noncovalent Functionalization

Polybenzoxazole (PBO) reinforced with carbon nanotubes (CNTs) is of great interest in the field of polymer composites because PBO and CNTs possess the highest-level mechanical properties among polymers and filler materials, respectively. However, the insolubility of PBO and the CNTs makes it difficult to develop a CNT/PBO composite in a solvent except for strong acids. In this study, we describe a novel strategy to design and prepare a CNT/PBO composite; namely, a method using a PBO precursor having an excellent solubility in organic solvents. We discovered that a PBO precursor wraps CNTs though a strong interfacial interaction between the CNT surfaces and the PBO precursor that enables the dispersion of the CNTs in organic solvents. By this method, a CNT/PBO precursor composite film was easily obtained from the CNT/PBO composite solution, and by heating to 400 °C, we obtained a CNT/PBO composite film. Mechanical property measurements revealed that the CNT/PBO films exhibited a high tensile strength (151 ...

The effect of purge environment on thermal rearrangement of ortho-functional polyamide and polyimide

The effects of nitrogen and air purge during thermal rearrangement of an ortho-functional polyamide (o-PA) and an ortho-functional polyimide (o-PI) towards a polybenzoxazole (PBO) structure have been investigated in terms of physicochemical changes and gas transport properties. The o-PA polymer was prepared from 2,2-bis(3-amino-4-hydroxyphenyl) hexafluropropane (BisAPAF) and 4,4′-biphenyl-dicarbonyl chloride (BPDC) while the o-PI polymer was derived from 4,4-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) and 3,3′-dihydroxybenzidine (HAB). Experimental results show that the purge environment for the conditions used does not affect the thermal rearrangement of the o-PA film but significantly affects the thermal conversion of the o-PI film. Nearly identical chemical structures and pure gas permeability values are observed for o-PA films thermally treated at 300 °C under air or N2. These properties become different in the o-PA films treated at 425 °C, which is presumed to be attributed to the influence of oxygen on the thermal stability of the derived PBO and probably the various degrees of thermal crosslinking reaction induced at a high temperature. The o-PI film was thermally rearranged at 425 °C because its thermal conversion takes place at a higher temperature range of 300 °C–450 °C. The o-PI film thermally rearranged in air exhibits improved gas permeation properties but significantly deteriorated mechanical properties. The air purge interrupts the thermal conversion of the ortho-functional imide to benzoxazole by oxidatively degrading the imide structure and forming the imine structure. As a result, both polymer structure and film properties change.

Synthesis and photoluminescence properties of polybenzoxazoles containing perylenebisimide functionalized graphene nanosheets via stacking interactions

The stable acidic dispersion of reduced graphene oxide (RGO) is prepared by a non-covalent functionalization method using perylenebisimide-modified fluorinated poly(hydroxyamide) (PTCDA-6FPHA). Then a series of RGO–polybenzoxazole (PBO) composites were prepared by a solution blending method in MSA. The change of perylenebisimide absorption peaks in the UV-Vis absorption spectra indicates that π–π stacking indeed occurs between graphene and perylene rings. The conjugation between functionalized RGO and PBO molecular chains increases the conjugation degree of the composite, since the PTCDA-6FPHA-containing perylene rings have a high π-electron delocalization degree. The energy gap of RGO–PBO composite is 2.72 eV with the absorption edge at 456 nm. The corresponding data of pure PBO film are 2.78 eV and 446 nm, respectively. Moreover, the intensity of the two emission peaks at 442 and 467 nm in the PL emission spectra of RGO–PBO decreases rapidly along with the increase of RGO–PBO ratio. The intensity of the weak emission peak at 622 nm increases with the increase of the RGO–PBO ratio. This behavior can be attributed to the energy transfer between benzoxazole chromophore units and functionalized RGO. Meanwhile, the RGO–PBO composite has outstanding thermal stability.

Synthesis and characterization of thermally rearranged (TR) polymers: effect of glass transition temperature of aromatic poly(hydroxyimide) precursors on TR process and gas permeation properties

Soluble aromatic polyimides containing ortho-positioned hydroxy groups were synthesized as precursors for thermal rearrangement (TR) to polybenzoxazoles (PBOs). Fully imidized polyimides with high-molecular-weights were afforded via a ‘one-pot’ solution imidization technique (i.e., ester-acid method). The poly(hydroxyimide)s were designed to vary in their glass transition temperatures (Tg) by carefully selecting dianhydride–bisaminophenol combinations to introduce various levels of chain rigidity. TR conversion (imide-to-benzoxazole conversion) occurred in solid-state films only under inert atmosphere and over a temperature range of 300–450 °C, depending on the chemical structure (chain rigidity) of precursors. The effect of the precursor Tg on TR conversion was studied using TGA, DSC, FTIR and gel fraction measurements. The TR conversion temperature of imide-to-benzoxazole rearrangement strongly depended on the precursor Tg. Thus, for example, the feasible TR temperature was successfully reduced by ∼100 °C by lowering the precursor Tg by using a bisphenol A type dianhydride in the polymer synthesis. Gas permeation properties of representative TR systems are also reported. The TR process significantly increased gas permeabilities while maintaining good selectivities. By correlating the TR conversion degree with gas transport properties, there appears to be an optimal TR conversion degree that can maximize both gas permeability and selectivity. Systematic studies on TR polymers derived from low Tg precursors were suggested to further explore this correlation.

주사슬에 Trifluoromethyl 그룹을 갖는 Poly(benzoxazole imide)의 제조 및 난연 특성

2,2-Bis(3-amino-4-hydroxyphenyl)hexafluoropropane 와 방향족 디이미드-디카르복실산을 thionyl chloride, triethyl amine 및 NMP 하에서 직접 중축합하여 trifluoromethyl group을 포함하는 일련의 poly(hydroxyamide)s(PHAs)를 제조 하였다. PHAs은 <TEX>$35^{\circ}C$</TEX> DMAc 용액에서 0.54-0.96 dL/g의 고유점도를 보였다. 모든 PHAs은 여러 종류의 유기용매에 쉽게 용해되었으나, PBOs은 황산에 부분적으로 용해되는 것을 제외하고, 전혀 용해되지 않았다. PHAs은 열적고리화 반응에 의해서 흡열과 함께 PBOs로 전환되었다. PHAs의 최대 중량손실온도는 <TEX>$559-567^{\circ}C$</TEX>의 영역을 보였고, poly(benzoxazole imide)s(PBOs)은 47-59%의 비교적 높은 char 수득율을 보였다. PBOs의 Pyrolysis Combustion Flow Calorimeter(PCFC) 결과에서 12-19 W/g의 HRR 과 2.7-3.6 kJ/g의 total HR을 보였다. PBO 1의 HRR은 PBO 3 의 HRR (19 W/g)보다 37 % 낮은 12 W/g의 가장 낮은 값을 보였다. A series of poly(hydroxyamide)s (PHAs) having trifluoromethyl group were prepared by direct polycondensation of aromatic diimide-dicarboxylic acids with 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane by thionyl chloride and triethyl amine in N-methyl-2-pyrrolidinone (NMP). The PHAs exhibited inherent viscosity in the range of 0.54-0.96 dL/g at <TEX>$35^{\circ}C$</TEX> in DMAc solution. All PHAs were readily soluble in a variety of organic solvents, whereas the polybenzoxazoles (PBOs) were quite insoluble except partially soluble in sulfuric acid. PHAs were converted to PBOs by thermal cycling reaction with heat of endotherm. The maximum weight loss temperature of the PHAs occurred in the range of <TEX>$559-567^{\circ}C$</TEX>. The PBOs showed relatively high char yields in the range of 47-59%. Pyrolysis Combustion Flow Calorimeter (PCFC) results of the PBOs showed 12-19 W/g heat release rate (HRR), and 2.7-3.6 kJ/g total heat release (total HR). The HRR of PBO 1 showed the lowest value of 12 W/g, which was 37% lower than that of PBO 3 (19 W/g).

Benzoxazole Resin: A Novel Class of Thermoset Polymer via Smart Benzoxazine Resin

Among the wide list of known high performance polymers, polybenzoxazoles (PBOs) have gained a prominent position as the most heat-resistant polyheterocyclic polymer. Nonetheless, PBOs have found applications in a rather restricted variety of technologies, mainly in the form of fibers. Herein, we report our pioneering work for producing cross-linked polybenzoxazole via a novel route using recently developed smart class of benzoxazine resins as precursors. This class of benzoxazines incorporates multiple smart features all in one molecule. The most attractive feature is its structural transformation into a more thermally stable cross-linked polybenzoxazole without the harmful consequences of traditional polybenzoxazole synthesis, such as the use of poly(phosphoric acid) as solvent. By this smart conversion, the flame-retardant oxazole moieties are successfully incorporated into the network structure. Further advantages of this new route for cross-linked polybenzoxazoles include outstanding flexibility in mo...

방향족 폴리히드록시아미드의 합성과 열적 고리화 거동(I) -벤젠고리 치환구조의 영향-

We have synthesized polyhydroxyamides (PHAs), a possible precursor which could be converted to polybenzoxazole (PBO) through a thermal cyclization reaction, by low temperature solution polymerization of 3,3`-dihydroxybenzidine with terephthaloyl chloride or isophthaloyl acid. Structural characteristics, solubility, thermal cyclization, and thermal decomposition of the PHAs were investigated by the FT-IR, DSC and TGA in order to understand the effect of chemical structure of acyl chlorides on the thermal properties of PHAs. The FT-IR study reveals that two types of PHAs can cyclize on heating and be transformed into PBOs. The meta-type PHA (m-PHA) shows better solubility in N,N-dimethylformamide and dimethyl sulfoxide than the para-type PHA (m-PHA). DSC and TGA results demonstrate that the m-PHA can cyclize at lower temperature than p-PHA, which is due to the difference in activation energy of thermal cyclization between m-PHA and p-PHA. TGA results reveal that the p-PHA has better thermal stability than m-PHA while in flame.

Composite porous membrane for protecting high‐performance fibers from ultraviolet–visible radiation

AbstractHigh‐strength fibers are used to produce high‐strength‐to‐weight‐ratio materials for applications such as composites, soft and hard body armor, bulletproof vests, and tendons for scientific balloons. Unfortunately, these fibers degrade when they are exposed to ultraviolet–visible (UV–vis) radiation. The objective of this research was to develop systems to improve the UV resistance of such fibers. Composite porous membranes from a polyurethane (PU) matrix loaded with rutile titanium dioxide (TiO2) nanoparticles were developed to protect a braid made of polybenzobisoxazole (PBO) yarns. The PU membranes loaded with TiO2 nanoparticles were prepared by a phase‐inversion technique. The effects of the amount of TiO2 nanoparticles on the composite membrane morphological structure and UV–vis light transmission were evaluated. The results show that when the concentration of TiO2 nanoparticles was increased, the porosity of the membrane and its UV–vis blocking effectiveness increased. The UV–vis protection was evaluated by the wrapping of the PBO braid with the composite membranes and exposed to UV–vis radiation. The strength loss of the PBO fiber due to exposure was decreased from 75% for the unprotected sample to 7.8% for the protected sample in the PU loaded with 4% TiO2 nanoparticles. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

A prospective study on the application of thermally rearranged acetate-containing polyimide membranes in dehydration of biofuels via pervaporation

Dehydration of biofuels (bioalcohols) derived from fermented products via distillation process is always energy intensive and often limited by the formation of azeotropic mixtures between alcohols and water. We have explored the feasibility of thermally rearranged polybenzoxazole (PBO) membranes in dehydration of biofuels via pervaporation process. The polyimide based precursor was synthesized by polycondensation of 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) and 3,3′-dihydroxybenzidine diamine (HAB) and cast into dense membranes. The resultant polyimide membranes were then subjected to thermal rearrangement reaction to initiate the solid-state chemical reconstruction into PBO. The effects of heating temperature and duration on pervaporation performance were systematically investigated. In general, the thermally rearranged PBO membranes showed an impressive enhancement in permeation flux with reasonable separation performance as compared to its polyimide precursor. In addition, the pervaporation performance was found to be greatly affected by the thermal rearrangement temperature as compared to the dwell duration. Furthermore, the stability of the thermally rearranged PBO membrane has been proven by continuously monitoring the pervaporation performance at 80°C for 250h. The long-term data revealed that there was no significant changes in separation performance that signifies the feasibility of the thermally rearranged PBO membrane in the separation and purification of biofuels via pervaporation process in biorefineries.

Improvement of interfacial shear strengths of polybenzobisoxazole fiber/epoxy resin composite by n‐TiO2 coating

AbstractSmooth polybenzobisoxazole (PBO) fiber has limited interfacial interaction with resin matrix. In this article, nano‐TiO2 coating on PBO fiber is applied to improve the interfacial adhesion between PBO fiber and epoxy resin. The test results suggest that the PBO fiber had good interaction with epoxy resin matrix after its treatment with n‐TiO2 sol. Nano TiO2 particle embedded onto PBO fiber surface, acting as a chock, which made fiber implanted into the resin better. This greatly improved the shear strengths (IFSS) of PBO fiber/epoxy resin composite. It has been found that a 56% increase in interfacial IFSS has achieved without sacrificing mechanical properties of fiber. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Formation of thermally rearranged (TR) polybenzoxazoles: Effect of synthesis routes and polymer form

This study demonstrates that thermal treatment of ortho-hydroxy containing polyimides (HPIs) in the solid state certainly leads to the formation of polybenzoxazoles (PBOs). Thermal conversion protocol, including temperature and dwell time, and the form of the sample (film or powder) determine the rearrangement reaction rate of HPI into PBO. Thermal rearrangement kinetics seems to be faster for film samples rather than for powder ones. Also, mild thermal treatment conditions (i.e., low temperatures and short times) seem to result in negligible or low degrees of conversion to PBO. Moreover, synthetic routes of HPI do not alter in any way the thermal conversion pathway. These findings validate the widely reported imide-to-benzoxazole thermal rearrangement mechanism, and contradict the alternative rearrangement pathway, proposed recently, of HPI into poly(biphenylene bisimide) polymers.

Micro-drawing of glassy polybenzoxazole rigid rods and the molecular interactions with carbon nanotubes

The high-temperature (Tg > 650°) wholly aromatic polybenzoxazoles (PBO) polymer chains in thin films underwent elastic energy release via local deformation of crazing when stretched beyond a critical strain around 0.5%. The strain localization in the ultra-rigid polymer was quickly superseded by craze fibril breakdown, triggering catastrophic fracture at low extensions below ∼3%. Although the drawing stress of craze fibrillation, determined to be ∼3 GPa, was insufficient to separate chains in PBO crystallites, it forced the chains in the amorphous regions to flow into large molecular deformations (∼300% strain) at room temperature. The poor craze fibril stability of the rigid-rod chains was enhanced dramatically when surface-functionalized single-walled carbon nanotubes (SWCNTs) were dispersed into the polymer. No toughening effects were observed, however, for multi-walled carbon nanotubes (MWCNTs) although the elastic enhancement leading to increase of strain delocalization was still operative. The toughening selectivity was attributed to the PBO/CNT load transfer coupling during nanoplastic flows in which only the CNTs of compatible bending moments permitting fibril drawing were allowed to participate.

The evolution of poly(hydroxyamide amic acid) to poly(benzoxazole) via stepwise thermal cyclization: Structural changes and gas transport properties

The thermally induced structure transformation and polymer chain rearrangement of a thermally rearranged (TR) material for gas separation has been explored in this work. A tremendous enhancement as high as 215 folds in CO2 permeability has been achieved by converting the pristine poly(hydroxyamide amic acid) (PHAA) to the final polybenzoxazole (PBO) via thermal cyclization at 400 °C for 2 h. The evolution of the pristine polymer PHAA derived from 2,2-bis(3-amino-4-hydroxyphenyl)hexafluropropane (BisAPAF) and trimellitic anhydride chloride (TAC) by thermal treatment from 130 °C to 400 °C has been examined by various characterization techniques including elemental analysis, TGA, TGA-IR, DSC, ATR-FTIR, XPS, XRD and Positron Annihilation Lifetime Spectroscopy (PALS). The stepwise cyclization process commences with cyclodehydration followed by cyclodecarboxylation. At the first step, PHAA transforms to poly(imide benzoxazole) (PIBO) up to 300 °C, while at the second step, the final structure of PBO is formed at 400 °C. Following the changes in the cyclization process, gas transport properties also show the stepwise changes. The significant enlargements of polymer inter-chain distance and free volume cavity radius provide the fundamental understanding for the changes of gas transport properties at the molecular level.

Highly Soluble Poly (Aryl-Ether Benzoxazole) Random Copolymers by Aromatic Nucleophilic Polycondensation: Synthesis and Properties

Aromatic polybenzoxazoles (PBOs) are very promising materials for civil, aerospace and electronics industries due to their outstanding mechanical properties, good thermooxidative stability and chemical resistance. However, the feasibility of the wide applications is hindered due to the difficulty of their processing derived from the very rigid structure. Poly(aryl-ether benzoxazole) random copolymers (PAEXBOx) were prepared by nucleophilic polycondensation. Ary-ether ketone and aryl-ether sulfone segments along with flexible linkages and bulky groups were introduced to provide PAEXBOx with good solubility in organic solvents. PAEXBOx exhibit the improved Tgs, and intermediate properties between pristine PBOs and poly(acryl ether)s (PAEs). The structure and properties of the polymers were systematically characterized.

Wafer level package with thermal-stress-absorbing interface structure and elongated pad

The increasing demand for high performance, cost-effective, and smallest packages has led to wafer level package (WLP). One of the biggest concerns about this technology is the interconnection reliability during thermal cycling. To improve solder joint reliability in WLP a thermal-stress-absorbing interface structure consisting of two stress-buffering polymer layers and elongated pad design was investigated in this study. To efficiently help absorbing thermal stresses during thermal cycling, the second polymer layer, polybenzoxazole (PBO), was constructed to extend substantially over the entire first polymer layer, epoxy contained polysiloxane. For the reliability test, the board-mounted chips without underfill were subjected to temperature cycling tests, with temperature ranging from −25 °C to 125 °C. The results of this experiment show a significant improvement in solder joint reliability when using the thermal-stress-absorbing interface structure together with the elongated pad design.

Synthesis of fluorinated polybenzoxazoles with low dielectric constants

AbstractTwo fluorinated aromatic bis(o‐aminophenol)s, 1,1‐bis(3′‐amino‐4′‐hydroxyphenyl)‐1‐(3′‐trifluoromethylphenyl)‐2,2,2‐trifluoroethane (6FAP) and 1,1‐bis(3′‐amino‐4′‐hydroxyphenyl)‐1‐(3′,5′‐ditrifluoromethylphenyl)‐2,2,2‐trifluoroethane (9FAP) were synthesized, which were allowed to polycondense with aromatic diacyl chlorides to afford the fluorinated aromatic polybenzoxazoles (PBOs) via a conventional two‐step procedure in which the low‐temperature solution polycondensation process was first performed to yield poly(o‐hydroxyamide)s (PHAs) followed by the thermal cyclodehydration to give the PBOs. Experimental results indicated that the PHAs had inherent viscosities in the range of 0.29–0.68 dL/g, showing excellent solubility in organic solvents. The PHAs could be thermally cyclodehydrated into the corresponding PBOs at 260–370 °C. The obtained PBOs exhibited enhanced glass‐transition temperature but decreased solubility with respect to the PHAs. The PBOs showed glass‐transition temperatures in the range of 315–337 °C and excellent thermal stabilities with 5% original weight‐loss temperatures (T5) of &gt;513 °C. Additionally, the PBO films had average refractive index of 1.5298–1.5656, birefringence of 0.0051–0.0092, and low dielectric constants of 2.57–2.70. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010

Wafer Level Processing of Integrated Passive Components Using Polyimide or Polybenzoxazole/Copper Multilayer Technology

Focus of this work was the development and optimization of process techniques for the application of Polyimide (PI) or Polybenzoxazole (PBO) to act as dielectric material together with semi-additive copper as conductive material in a thin film multilayer buildup, which is suitable for the integration of all three types of passive components. A thin film set up including seven layers was used to create integrated coils with values between 1 and 80 nH , integrated resistors with values between 100 ¿ and 100 k¿ as well as integrated radio-frequency (RF) filters for application between 2 and 5 GHz. As substrate material both low resistivity silicon and glass (Borofloat 33) was used, which influences the quality factor of the coils as well as the filter performance substantially. The fabricated structures performed well in thermal cycling tests, where no critical failure could be observed after 3000 thermal cycles for all examined components. The RF components have shown no critical deviations between initial measurements and measurements performed after thermal cycling. The resistor values have shown no deviations between the measurements, which have been taken before and after thermal cycling. Thus Polyimide/Polybenzoxazole/copper multilayer wiring represents a reliable and versatile base technology for the fabrication of passive components at wafer level.

Highly permeable and selective poly(benzoxazole-co-imide) membranes for gas separation

Abstract A series of copolymer membranes was prepared using polyimide (PI) and hydroxyl-containing polyimide (HPI) precursors. Thermal conversion of the hydroxyl-containing imide group into a benzoxazole structure was performed at the solid state to increase rigidity of the polymer backbone, and thus generate free volume elements to improve gas separation performance of the resultant copolymer membrane. Free volume cavities produced during thermal conversion were easily controlled by varying HPI composition in the copolymer. Evidence of thermal conversion was confirmed using spectroscopic and thermogravimetric analysis. O 2 permeability of copolymer membranes varied from 0.17 Barrer (1 Barrer = 1 × 10 −10 cm 3 (STP) cm/cm 2 s cmHg) to 220 Barrer depending on membrane composition without a significant loss in selectivity. Also, fully converted polybenzoxazole (PBO) membranes showed high CO 2 permeability (1014 Barrer) with a CO 2 /CH 4 selectivity of 24. The copolymer membranes presented here easily overcome the conventional polymeric upper bound limit, and are comparable to the gas separation performance of superior membrane materials such as carbon molecular sieves. The copolymer membrane was also expected to improve the shape properties of the polymer membrane.

Heat and Solvent Resistant Electrospun Polybenzoxazole Nanofibers from Methoxy‐Containing Polyaramide

Polybenzoxazole (PBO) nanofibers were prepared via electrospinning and thermal conversion from its precursor, methoxy‐containing polyaramide (MeO‐PA). The MeO‐PA was synthesized from low‐cost monomers. The structures and properties of polymers (MeO‐PA and PBO) were characterized by FT‐IR, TGA, DSC, OPM, SEM, and so forth. It was found that the PBO nanofibers had a diameter ranging from 300 nm to 350 nm. The PBO nanofiber mats with high porosity provided a promising prospect in high‐temperature filter materials, heat insulation materials, and high‐temperature protection materials for its outstanding heat and solvent resistance and in preparing carbon nanofibers for its high char yield percentage.

Synthesis and characterization of novel fluorinated polybenzoxazoles derived from 2,5-difluoroterephthalic acid

AbstractA new serious of fluorinated polybenzoxazoles (PBOs) were prepared from 2,2-bis(3-amino-4-hydroxyphenyl) hexafluoropropane together with 2,5- difluoroterephthalic acid (DFTA) and terephthalic acid by the one-pot polycondensation in polyphosphoric acid. The resulting PBOs showed good solubility in many common organic solvents and the thermal stability of PBOs was improved by the introduction of DFTA monomer.