Thermal shrinkage of electrospun PVP nanofibers

This paper outlines the shrinkage of electrospun polyvinylpyrrolidone (PVP) fiber mats during thermal treatment. The thermal behavior and phase changes within the fibers were investigated by DSC and TGA/DTA. Five precursors with different PVP loading in ethanol were electrospun. The mats shrinkage as function of temperature was measured in the RT–200 °C range. Shrinkage rate drastically increased above the polymer glass transition point, T_g (150–180 °C), due to increase in polymer chain mobility. Mats shrinkage at 200 °C as function of PVP concentration showed a minimum at ∼10%wt. Below 10% PVP the mats morphology is non‐uniform, consisting of beads and fibers. Above 10% PVP, only flat and uniform fibers were observed. This paper outlines the dominant mechanism governing the mats shrinkage during heating. In addition, the effect of PVP concentration on the expansion of fibers diameter was investigated and found to be consistent with the linear shrinkage observing a minimum at ∼10% PVP. The effect of applied voltage on mat shrinkage was investigated, and showed a minimum at 12 kV. Understanding the interplay between fibers morphology and thermal shrinkage allows precursor composition and system optimization needed for minimizing shrinkage negative effects on the structure and properties of electrospun fiber mats. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 248–254     

Epoxy/polyhedral oligomeric silsesquioxane nanocomposites from octakis(glycidyldimethylsiloxy)octasilsesquioxane and small‐molecule curing agents

Epoxy/polyhedral oligomeric silsesquioxane (POSS) nanocomposites were obtained from octakis(glycidyldimethylsiloxy)octasilsesquioxane (OG) and diglycidyl ether of bisphenol A cured with small‐molecule curing agents of diethylphosphite (DEP) and dicyandiamide (DICY). An increase in the POSS contents of the nanocomposites and an improvement in the nanocomposite homogeneity were observed with the use of the small‐molecule curing agents. Phosphorus in DEP and nitrogen in DICY also performed synergism with POSS for thermal stability enhancement and flammability improvement in the nanocomposites. The nanocomposites possessing high OG contents exhibited good thermal stability, improved flammability, and high storage moduli. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3825–3835, 2006     

Free radical photopolymerization process for fiber‐reinforced polymer: Effect on the mechanical properties

Glass–fiber‐reinforced polymers were manufactured either through a room temperature thermal curing or under ultraviolet (UV) light from a LED. The thermal system yields high performances when a post‐curing process at 65°C is applied. The photochemical curing leads to a composite in a faster timescale, albeit at the extent of the mechanical properties. It is found that in this case, impregnation and vacuum steps are too fast to allow a good wetting of the fibers, thereby leading to mechanical weaknesses and larger void volume. However, when applying longer vacuum and impregnation steps, the mechanical properties of the photochemically cured sample match the best thermally cured one. As a conclusion, it is shown that photochemical curing of glass–fiber‐reinforced polymer can lead to high performance composite provided that the preparation steps are well controlled.     

Novel approach to preparing epoxy/polyhedral oligometric silsesquioxane hybrid materials possessing high mass fractions of polyhedral oligometric silsesquioxane and good homogeneity

Epoxy/polyhedral oligometric silsesquioxane (POSS) hybrid materials, containing 50 wt % POSS and exhibiting good homogeneity, were obtained in a two‐step preparation. Monoamine‐functionalized POSS was first reacted with diglycidyl ether of bisphenol A to form an epoxy POSS precursor, which was then cured. Curing agents such as 4,4′‐diaminodiphenylmethane, dicyandiamide (DICY), and diethylphosphite (DEP) were used for the synthesis of the epoxy–POSS hybrid materials. The use of small‐molecule curing agents, such as DICY and DEP, efficiently avoided macrophase separations and enhanced the thermal properties of the hybrid materials. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1869–1876, 2006     

Preparation of polybenzoxazole fibers via electrospinning and postspun thermal cyclization of polyhydroxyamide

Polybenzoxazole (PBO) fibers with a submicron diameter were successfully prepared by electrospinning its precursor, polyhydroxyamide (PHA), solutions to obtain the PHA fibers first, followed by appropriate thermal treatments for cyclization reaction. BisAPAF‐IC PHA with two different molecular weights (MWs) were synthesized from a low temperature polymerization of 2,2′‐bis(3‐amino‐4‐hydroxyphenyl) hexafluoropropane (BisAPAF) and isophthaloyl chloride (IC). Using dimethylacetamide (DMAc) and tetrahydrofuran (THF), solvent effects on the electrospinnability of PHA solutions were investigated. For balancing the solution properties, it was found that DMAc/THF mixture with a weight ratio of 1/9 was the best cosolvent to prepare smooth PHA fibers; uniform PHA fibers with a diameter of 325–720 nm were obtained by using 20 wt % PHA/(DMAc/THF) solutions. For a fixed PHA concentration, solutions with a lower MW of PHA yielded thinner electrospun fibers under the same electrospinning condition. After obtaining the electrospun BisAPAF‐IC PHA fibers, subsequent thermal cyclization up to 350 °C produced the corresponding thermally stable BisAPAF‐IC PBO fibers with a diameter of 305–645 nm. The structure of the precursor fibers and the fully cyclized fibers were characterized by FTIR. For the cyclized BisAPAF‐IC PBO fibers, thermogravimetric analysis showed a 5% weight loss temperature at 523 °C in nitrogen atmosphere. The interconnected fiber structure in the BisAPAF‐IC PBO fiber mats was irrelevant to the curing process, but resulted from the jet merging during the whipping process as revealed by the high speed camera images. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 8159–8169, 2008     

Synthesis,characterization, and thermal properties of ladderlike polyepoxysiloxanes

Starting from trichlorosilanes and using 1,4‐phenylenediamine as a template, we have synthesized some ladderlike poly(glycidyl‐co‐alkyl/aryl)siloxanes (polyepoxysiloxanes or polyepoxies for short). The structures of copolymers were confirmed through IR, ¹H NMR, elemental analyses, and gel permeation chromatography. Curing behaviors of these polyepoxies in the absence and presence of a curing agent have been studied with DSC. It was shown that these epoxies could be cured without any curing agent. Copolymers having aromatic groups showed higher curing reactivity than those having alkyl groups. The experimental results also demonstrate that the curing reaction occurred solely via epoxy functionality, not via the condensation reaction of the hydroxy groups located at the end of polymer main chains. The thermal stability of the cured polymers was examined by thermogravimetric analysis. The results confirm that polyepoxies with aromatic groups had better thermal stability than those with alkyl groups. It was also found that polyepoxies cured with a diamine have a higher thermal stability than those cured in the absence of a curing agent. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2215–2222, 2001     

Synthesis and characterization of a novel liquid‐crystalline epoxy resin combining biphenyl and aromatic ester‐type mesogenic units

A novel liquid‐crystalline epoxy resin combining biphenyl and aromatic ester‐type mesogenic units, diglycidyl ether of 4,4′‐bis(4‐hydroxybenzoyloxy)‐3,3′,5,5′‐tetramethyl biphenyl, was synthesized. Its spectroscopic structure, thermal properties, and phase structures were investigated with NMR, differential scanning calorimetry (DSC), and polarized light microscopy (PLM), respectively. The curing agent, diaminodiphenylsulfone, was chosen to investigate the curing behavior by means of DSC and PLM during isothermal and nonisothermal processes. Only one exothermal peak appeared in the isothermal DSC curves. Birefringence was also observed during the curing processes and preserved after postcuring. Compared with short rigid‐rod and flexible epoxies, the cured liquid‐crystalline epoxy resin that was obtained displayed special thermal stability according to thermogravimetric analysis because of its long rigid‐rod mesogenic unit and bulky methyl groups. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 727–735, 2007     

Synthesis and properties of a cyanate ester containing dicyclopentadiene(II)

A 2,6‐dimethyl phenol‐dicyclopentadiene novolac (DCPDNO) was synthesized from dicyclopentadiene and 2,6‐dimethyl phenol, and the resultant DCPDNO was reacted with cyanogen bromide into 2,6‐dimethyl phenol‐dicyclopentadiene cyanate ester (DCPDCY). The structures of the novolac and cyanate ester were confirmed with Fourier transform infrared spectroscopy, elemental analysis, mass spectrometry (MS), and nuclear magnetic resonance. For the purpose of increasing the mobility of residual DCPDCY during the final stage of curing and achieving a complete reaction of cyanate groups, a small quantity of a monofunctional cyanate ester, 4‐tert‐butylphenol cyanate ester (4TPCY), was added to DCPDCY to form the cyanate ester copolymer. The synthesized DCPDCY was then cured with 4TPCY at various molar ratios. The thermal properties of the cured cyanate ester resins were studied with dynamic mechanical analysis, dielectric analysis, and thermogravimetric analysis. These data were compared with those of the commercial bisphenol A cyanate ester system. Compared with the bisphenol A cyanate ester system, the cured DCPDCY resins exhibited lower dielectric constants (2.52–2.67 at 1 GHz), dissipation factors (0.0054–0.0087 at 1 GHz), glass‐transition temperatures (261–273 °C), thermal stability (5% degradation temperature at 406–450 °C), thermal expansion coefficients (4.8–5.78 × 10^?5/°C before the glass‐transition temperature), and moisture absorption (0.8–1.1%). © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 671–681, 2005     

Preparation of submicron‐scale,electrospun cellulose fibers via direct dissolution

Cellulose nonwoven mats of submicron‐sized fibers (150 nm–500 nm in diameter) were obtained by electrospinning cellulose solutions. A solvent system based on lithium chloride (LiCl) and N,N‐dimethylacetamide (DMAc) was used, and the effects of (i) temperature of the collector, (ii) type of collector (aluminum mesh and cellulose filter media), and (iii) postspinning treatment, such as coagulation with water, on the morphology of electrospun fibers were investigated. The scanning electron microscopy (SEM) and X‐ray diffraction studies of as‐spun fibers at room temperature reveal that the morphology of cellulose fibers evolves with time due to moisture absorption and swelling caused by the residual salt and solvent. Although heating the collector greatly enhances the stability of the fiber morphology, the removal of salt by coagulation and DMAc by heating the collector was necessary for the fabrication of dry and stable cellulose fibers with limited moisture absorption and swelling. The presence and removal of the salt before and after coagulation have been identified by electron microprobe and X‐ray diffraction studies. When cellulose filter media is used as a collector, dry and stable fibers were obtained without the coagulation step, and the resulting electrospun fibers exhibit good adhesion to the filter media. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1673–1683, 2005     

Surface methacrylation and graft copolymerization of ultrafine cellulose fibers

Ultrafine fibrous (? from 100 to 450 nm) cellulose membranes were generated by electrospinning of cellulose acetate [degree of substitution (DS): 2.45, weight‐average molecular weight: 30,000 Da], followed by alkaline deacetylation. Reaction of these ultrahigh surface‐area cellulose fibers with methacrylate chloride (MACl) produced activated surfaces without altering the fiber morphology. Surface methacrylation of these fibers was confirmed by the acquired hydrophobicity (θ_water = 84°) as compared to the originally hydrophilic (θ_water = 56°) cellulose. Changing the MACl:OH molar ratios could vary the overall DS of methacrylation. The very low overall DS values indicate the surface nature of the methacrylation reaction. At a DS of 0.17, the thermal properties of the surface methacrylated cellulose resemble those of cellulose derivatives at much higher DS values, an unusual behavior of the ultrafine fibers. The methacrylated cellulose could be further copolymerized with vinyl monomers (methyl methacrylate, acrylamide, and N‐isopropylacrylamide) as linear grafts or three‐dimensional (3D) networks. The morphology of cellulose fibers and the interfiber pore structure were not altered at 15–33% graft levels. This study demonstrates that either linear or 3D networks of vinyl polymers could be efficiently supported on ultrafine cellulose fibrous membranes via surface methacrylation. Through these surface reactions the chemical, thermal, and liquid wetting and absorbent properties of these ultrafine fibrous membranes were significantly altered with no change to the fiber dimensions or interfiber pore morphology. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 953–964, 2003     

Thermoplastic polyurethane/silica nanocomposite fibers by electrospinning

Thermoplastic polyurethane/silica nanocomposite fibers with good mechanical properties were prepared by electrospinning, using colloidal silica as the source of silica and dimethyl formamide as the solvent. The fiber morphology was examined by field emission scanning electron microscopy. The average fiber diameter is about 0.8 μm with 0–10 wt % silica, and silica nanoparticles were observed on all fiber surfaces. X‐ray photoelectron spectroscopy analysis of Si in combination with transmission electron microscopy observation suggest that silica nanoparticles have a fairly uniform distribution in the fibers rather than enriching on the fiber surfaces. Tensile tests show that the incorporation of silica nanoparticles can bring about a significant reinforcing effect without decreasing the ductility. The reinforcing effect is further confirmed by dynamic mechanical analysis. The thermoplastic polyurethane/silica composite fiber mats can adsorb gold nanoparticles after further treatment with 3‐aminopropyltriethoxysilane, demonstrating that the composite fibers could be used as functional fibers by using the properties of silica nanoparticles. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Preparation of conductive polybenzoxazines by oxidative polymerization

Soluble and thermally curable conducting high molecular weight polybenzoxazine precursors were prepared by oxidative polymerization 3‐phenyl‐3,4‐dihydro‐2H‐benzo[e][1,3] oxazine (P‐a) alone and in the presence of thiophene (Th) with ceric ammonium nitrate in acetonitrile. The structure of the precursors was confirmed by FTIR, ¹H NMR, and DSC measurements, indicating the presence of a cyclic benzoxazine structure, together with small but varying amount of a ring opened phenolic structure. The resulting polymers exhibit conductivities around 10^?2 S cm^?1 and undergo thermal curing at various temperatures. Attempts to copolymerize P‐a with another electroactive monomer, pyrrole (Py), by a similar redox process were unsuccessful, which was attributed to the unfavourable oxidation potential of Py. The cured products exhibited high thermal stability but lower conductivity, than those of the precursors. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 999–1006, 2007     

Thermal and mechanical properties of a dendritic hydroxyl‐functional hyperbranched polymer and tetrafunctional epoxy resin blends

Blends of a tetrafunctional epoxy resin, tetraglycidyl‐4,4′‐diaminodiphenylmethane (TGDDM), and a hydroxyl‐functionalized hyperbranched polymer (HBP), aliphatic hyperbranched polyester Boltorn H40, were prepared using 3,3′‐diaminodiphenyl sulfone (DDS) as curing agent. The phase behavior and morphology of the DDS‐cured epoxy/HBP blends with HBP content up to 30 phr were investigated by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and scanning electron microscopy (SEM). The phase behavior and morphology of the DDS‐cured epoxy/HBP blends were observed to be dependent on the blend composition. Blends with HBP content from 10 to 30 phr, show a particulate morphology where discrete HBP‐rich particles are dispersed in the continuous cured epoxy‐rich matrix. The cured blends with 15 and 20 phr exhibit a bimodal particle size distribution whereas the cured blend with 30 phr HBP demonstrates a monomodal particle size distribution. Mechanical measurements show that at a concentration range of 0–30 phr addition, the HBP is able to almost double the fracture toughness of the unmodified TGDDM epoxy resin. FTIR displays the formation of hydrogen bonding between the epoxy network and the HBP modifier. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 417–424, 2010     

Polyaniline/poly(methyl methacrylate) coaxial fibers: The fabrication and effects of the solution properties on the morphology of electrospun core fibers

Electrically conductive polyaniline (PANi)/poly(methyl methacrylate) (PMMA) coaxial fibers were prepared through the chemical deposition of PANi onto preformed PMMA fibers via in situ polymerization. PMMA fibers were prepared as core materials via electrospinning. Spectral studies and scanning electron microscopy observations indicated the formation of PANi/PMMA coaxial fibers with a diameter of approximately 290 nm and a PANi layer thickness of approximately 30 nm. The conductivity of the PANi/PMMA coaxial fibers was significantly higher than that of electrospun fibers of PANi/poly(ethylene oxide) blends and blend cast films of the same PANi composition. To reproducibly generate uniform‐core polymer fibers, the organic solution properties that affected the morphology and diameter of the electrospun fibers were investigated. The polymer molecular weight, solution concentration, solvent dielectric constant, and addition of soluble organic salts were strongly correlated to the morphology of the electrospun fiber mat. In particular, the dielectric constants of the solvents substantially influenced both the fiber diameter and bead formation. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3934–3942, 2004     

Synthesis and characterization of self‐catalyzed imide‐containing pthalonitrile resins

Some new amino‐ and imide‐containing phthalonitrile compounds with 1:1 molar ratio of amino group to pthalonitrile unit were successfully synthesized. The molecular structures were characterized by spectroscopic techniques. They were thermally polymerized under nitrogen/air, even in the absence of curing additives. The thermal properties of the cured products were characterized by thermogravimetric analysis and differential scanning calorimetry. The 5% weight loss temperatures ranged from 525 to 528 °C and 513 to 520 °C under nitrogen and air, respectively. Char yields (900 °C) were in the range of 62–70%. Rheometric measurements showed that the rate of the cure reaction differs for all the three monomers. The glass transition temperature advances with increasing extent of cure and disappears on postcure at 375 °C. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Modeling of downstream heating in melt electrospinning of polymers

In this study, both modeling and experimental approaches are used to demonstrate that downstream volumetric heating of electrospun fibers during melt electrospinning can result in markedly decreased fiber diameters. Previous melt electrospinning techniques were limited to production of micron‐sized fibers. This is because high viscosity and low electrical conductivity of the polymer melt coupled with rapid heat loss to the surroundings resulted in solidification of the jet before it had been significantly stretched by the electric field. In our study, we utilize a model for non‐isothermal melt electrospinning in the presence of a volumetric heat source. Our simulation results demonstrate that downstream heating does reduce the fiber diameter, and is therefore a feasible approach for resolving the limitations of melt electrospinning. In addition, our model has also been used to capture the effect of the surrounding temperature, which affects the thinning of the fiber through surface rather than volumetric interactions. Finally, melt electrospinning experiments are utilized to validate the model predictions for downstream heating. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1393–1405     

Synthesis and curing kinetics of cardanol‐based curing agents for epoxy resin by in situ depolymerization of paraformaldehyde

Three novel cardanol‐based phenalkamines with good stability have been successfully prepared by Mannich reaction using phenolic compounds with paraformaldehyde and hexamethylenediamine (or its mixture with other amines). The structure of the prepared phenalkamines has been analyzed using liquid chromatography‐mass spectrometry, nuclear magnetic resonance, and Fourier transform infrared spectroscopy. The curing kinetics of the prepared epoxy resin/phenalkamine systems has been investigated using differential scanning calorimetry (DSC), and determined by Kissinger, Flynn–Wall–Ozawa, and Crane methods. Furthermore, the thermal properties of the cured materials have been evaluated using DSC and thermogravimetric analysis, and the mechanical properties of the cured materials have been analyzed systematically. The results demonstrate that the phenalkamine 1 (PAA1) had a lower reactivity and better toughness than phenalkamine 2 (PAA2) and phenalkamine 3 (PAA3). In addition, PAA1 is a solid curing agent, while PAA2 and PAA3 are liquid curing agents, which were more convenient for practical usage. Results indicate that the properties of the prepared phenalkamines strongly depend on the structures. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 472–480     

The effect of the curing agent content on the curing and liquid‐crystalline phase of liquid‐crystalline epoxy resin

The effect of the curing agent content on the curing behavior and liquid‐crystalline (LC) phase of the liquid‐crystalline epoxy (LCE) resin 4,4′‐di(2,3‐epoxypropyloxy)phenyl benzoate was studied. Diaminodiphenylester (DDE) was used as a curing agent. The curing behavior was observed via differential scanning calorimetry, and the LC phase was investigated with a polarized optical microscopy. The LC phase in the LCE/DDE mixture with a high DDE content was developed during curing. The onset time was inversely proportional to the DDE content. The mesophase stability of LCE/DDE was enhanced by the addition of large amounts of DDE. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 374–379, 2001     

Synthesis and curing of poly(glycidyl methacrylate) nanoparticles

Glycidyl‐functional polymer nanoparticles [poly(glycidyl methacrylate) (PGMA)] were fabricated with microemulsion polymerization. The successful fabrication of PGMA nanoparticles was confirmed by Fourier transform infrared spectroscopy and transmission electron microscopy (TEM). A TEM image showed that the average diameter of the PGMA nanoparticles was approximately 10–28 nm and was fairly monodisperse. As the surfactant concentration increased, the average size of the nanoparticles decreased and approached an asymptotic value. A significant reduction of the nanoparticle size to the nanometer scale led to an enhanced number of surface functionalities, which played an important role in the curing reaction. The PGMA nanoparticles were cured with a low‐temperature curing agent, diethylene triamine, to produce ultrafine thermoset nanoparticles. The low‐temperature curing process was performed below the glass‐transition temperature of PGMA to prevent the coagulation and deformation of the nanoparticles. A TEM image indicated that the cured PGMA nanoparticles did not exhibit interparticle aggregation and morphological transformation during curing. The average size of the cured PGMA nanoparticles was consistent with that of the pristine PGMA nanoparticles © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2258–2265, 2005     

Synthesis and curing of liquid crystalline epoxy resin based on naphthalene mesogen

Aromatic liquid crystalline epoxy resin (LCE) based on naphthalene mesogen was synthesized and cured with aromatic diamines to prepare heat‐resistant LCE networks. Diaminodiphenylester (DDE) and diaminodiphenylsulfone (DDS) were used as curing agents. The curing reaction and liquid crystalline phase of LCE were monitored, and mechanical and thermal properties of cured LCE network were also investigated. Curing and postcuring peaks were observed in dynamic DSC thermogram. LCE network cured with DDE displayed liquid crystalline phase in the curing temperature range between 183 and 260°C, while that cured with DDS formed one between 182 and 230°C. Glass transition temperature of cured LCE network was above 240°C, and crosslinked network was thermally stable up to 330°C. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 419–425, 1999