Effect of needle diameter on nanofiber diameter and thermal properties of electrospun poly(methyl methacrylate)

The effect of needle diameter on the resulting electrospun poly(methyl methacrylate) (PMMA) average nanofiber diameter has been evaluated for three different needle gauges. The resulting nanofibers were observed and analyzed by scanning electron microscopy (SEM), suggesting a lack of correlation between the needle diameter used and the resulting average nanofiber diameter. Thermogravimetric analysis (TGA) indicated an increase in the thermal stability of PMMA nanofibers when compared to powdered PMMA, while differential scanning calorimetry (DSC) studies evidenced lower glass transition temperatures (Tg) for PMMA nanofibers in the first heating cycle. Copyright © 2007 John Wiley & Sons, Ltd.     

Enzymatic degradation of poly(L-lactide) and poly(epsilon-caprolactone) electrospun fibers

Poly(L-lactide) (PLLA) and poly(epsilon-caprolactone) (PCL) ultrafine fibers were prepared by electrospinning. The influence of cationic and anionic surfactants on their enzymatic degradation behavior was investigated by measuring weight loss, molecular weight, crystallinity, and melting temperature of the fibers as a function of degradation time. Under the catalysis of proteinase K, the PLLA fibers containing the anionic surfactant sodium docecyl sulfate (SDS) exhibited a faster degradation rate than those containing cationic surfactant triethylbenzylammonium chloride (TEBAC), indicating that surface electric charge on the fibers is a critical factor for an enzymatic degradation. Similarly, TEBAC-containing PCL fibers exhibited a 47% weight loss within 8.5 h whereas SDS-containing PCL fibers showed little degradation in the presence of lipase PS. By analyzing the charge status of proteinase K and lipase PS under the experimental conditions, the importance of the surface charges of the fibers and their interactions with the charges on the enzymes were revealed. Consequently, a "two-step" degradation mechanism was proposed: (1) the enzyme approaches the fiber surface; (2) the enzyme initiates hydrolysis of the polymer. By means of differential scanning calorimetry and wide-angle X-ray diffraction, the crystallinity and orientation changes in the PLLA and PCL fibers during the enzymatic degradation were investigated, respectively.     

Influence of solvents properties on morphology of electrospun polyurethane nanofiber mats

Segmented polyurethane (SPU) nanofiber mats were prepared by electrospinning technique using the combination of four different solvents viz. tetrahydrofuran, N,N′‐dimethyl formamide, N,N′‐dimethyl acetamide, and dimethyl sulfoxide. Morphology of the electrospun nanofibers was examined by field emission scanning electron microscope. Experimental results revealed that the morphologies of polyurethane nanofiber mats have been changed significantly with the solvent selection for the electrospinning. It was observed that the diameters and morphology of the SPU nanofibers were influenced greatly by the use of combination of solvents. The uniform polyurethane nanofibers without beads or curls could be prepared by electrospinning through the selection of combination of good conductive and good volatile solvent viz. 7.5 wt/v% of SPU in N,N′‐dimethyl formamide/tetrahydrofuran (30 : 70 v/v) solutions at 20 kV applied voltages and volume flow rate of 1 ml/min. On the basis of the results obtained from this investigation, it has been established that solvent selection is one of the driving factors for controlling the morphology of the polyurethane electrospun nanofiber mats. The well‐controlled morphology of electrospun polyurethane nanofiber mats could be useful for many potential industrial applications such as in biomedical, smart textiles, nanofiltration, and sensors. Copyright © 2013 John Wiley & Sons, Ltd.     

Processing and characterization of oriented electrospun poly(vinylidene fluoride) mats

Mats of highly oriented poly(vinylidene fluoride) nanofibers were electrospun by means of a conventional electrospinning equipment; the orientation, however, was obtained using a disk collector rotating at a speed of 4000 rpm and a device that reduced the influence of air displacement during nanofiber orientation. Thermal transitions of the mats were determined by differential scanning calorimetry, the predominant crystalline phase by Fourier transform infrared spectroscopy and wide‐angle X‐ray scattering and the nanofiber orientation and morphology by scanning electron microscopy. Relative permittivity, loss index, stable remnant polarization, and coercive field of the mats were also determined and compared with those obtained for a mat electrospun at 2000 rpm and an oriented commercial film. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 000: 000–000, 2012     

Antimicrobial properties of modified and electrospun poly(vinyl phenol)

Antimicrobial polymeric systems were prepared from poly(vinyl phenol) (PVP). Four systems were prepared, two of these based on the modification of the poly(vinyl phenol) by sulfonation with fuming sulfuric acid (SPVP 100k) or by formation of lithium salt of the sulfonated poly(vinyl phenol) brought about by its reaction with lithium hydroxide (LiSPVP 100k). The other two systems were prepared by the electrospinning of poly(vinyl phenol) with molecular weight 20 × 10³ (PVP 20k spun) and 100 × 10³ (PVP 100k spun). The antimicrobial activity of the polymers was examined against different test microorganisms. The plug-cutting technique revealed the potency of SPVP 100k and LiSPVP 100k as antimicrobial agents. SPVP 100k was inhibitory to the growth of gram-negative bacteria (E. coli and Salmonella choleraesius) and gram-positive bacteria (B. subtilis and S. aureus). On the other hand, LiSPVP 100k had antifungal activity against A. niger, T. rubrum and C. albicans. Generally, it was found that polymer morphology and molecular weight affect the activities against test microorganisms. For example, PVP 20k and PVP 100k in their powder form showed no antimicrobial activity. However the results showed that PVP 20k spun has antibacterial activity against B. subtilis, and there is no growth of the tested microorganisms on the electrospun fibers of PVP 100k spun, revealing its property of being a self-sterilizing material (SSM).

Growth inhibition of different concentrations of polymer SPVP. Inoculation: 6 × 10⁴ cells · ml⁻¹, B. subtilis, S. choleraesius, S. aureus and E. coli.     

The triple melting behavior of poly(ethylene terephthalate): Molecular weight effects

The melting behavior of isothermally crystallized PET has been studied using linear heating in a differential scanning calorimeter (DSC). Variables such as crystallization temperature, crystallization time, heating rate, and average molecular weight are the main focus of the study. On the basis of several experimental techniques, a correlation of the melting behavior of PET with the amount of secondary crystallization was found to exist. It was observed that the triple melting of PET is a function of programmable DSC variables such as crystallization temperature, crystallization time, and heating rate. However, in testing the hypothesis that there was a correlation between melting endotherms and secondary crystallization inside spherulites, it was found necessary to use a DSC-independent variable in order to enhance the observed effects. Therefore, on the basis of a crystallization model that involves secondary branching along the edges of parent lamellar structures, it was speculated that an increase in the average molecular weight could affect the triple melting of PET due to an increase of rejected portions of the macromolecules. It was found that the second melting endotherm increased, apparently, at the expense of the third one as the average molecular weight was increased. The second melting endotherm was also found to correlate proportionally with the amount of secondary crystallization inside spherulites. The results support a model of crystallization which basically consists of parent crystals and at least one population of secondary, probably metastable, crystals. This latter structural component must involve excluded portions of the macromolecules that did not crystallize during the isothermal crystallization period of the parent crystals. An increase of molecular weight gives rise to a higher entanglement density which in turn increases the fraction of initially rejected chain sections and therefore the amount of secondary crystallization. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1757–1774, 1997     

Anisotropic properties of poly(ethylene terephthalate) by Brillouin spectroscopy: Anisotropic properties of oriented bulk and nanostructured poly(ethylene terephthalate) determined by Brillouin spectroscopy and birefringence experiments

Using Brillouin spectroscopy (BS), the tensor of the elastic constants of oriented poly(ethylene terephthalate) was determined for a variety of morphologies obtained by different uniaxial drawing procedures. The extreme values of the moduli along the drawing direction at frequencies of a few gigahertz were C₃₃ = 40 GPa and C₄₄ = 1.8 GPa. As a result of the invariants of the single‐phase aggregate model, the oriented state is dominated by the Reuss average even at extreme draw ratios and subsequent to a deformation‐induced crystallization. This is documented in both the BS orientation parameter and the BS mode numbers in comparison with birefringence. Additional spectral lines observed at draw ratios larger than 6 are discussed in relation to the formation of nanostructured phases. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1201–1213, 2002     

Dielectric and conductivity processes in poly(ethylene terephthalate) and poly(ethylene naphthalate) homopolymers and copolymers

Electrical relaxation and conductivity processes in amorphous and semicrystalline poly(ethylene terephthalate) (PET) and poly(ethylene naphthalate) (PEN) homopolymers and certain PET/PEN copolymers have been studied by means of dielectric spectroscopy. Homopolymers and copolymers able to crystallize were subjected to successive thermal runs to investigate the influence of the thermal history upon the morphology and the electrical behavior of the polymeric systems. The morphology of the untreated as well as the heat‐treated specimens was determined by means of Differential Scanning Calorimetry (DSC). All samples exhibit β‐relaxation process, due to local motions of the C?O polar side groups, and α‐relaxation process associated to the glass/rubber transition. In the PEN spectrum an additional, subglass, mode was recorded, most probably attributed to cooperative motions of the naphthalene groups. Finally, the dynamic nature of the crystallization process is expressed via the over glass transition mode and the temperature dependence of dc conductivity recorded in amorphous PET, PEN, and PET/PEN (85/15) (wt/wt) samples. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3078–3092, 2006     

Mammalian cell viability in electrospun composite nanofiber structures

Incorporation of mammalian cells into nanofibers (cell electrospinning) and multilayered cell-nanofiber structures (cell layering) via electrospinning are promising techniques for tissue engineering applications. We investigate the viability of 3T3-L1 mouse fibroblasts after incorporation into poly(vinyl alcohol) nanofibers and multilayering with poly(caprolactone) nanofibers and analyze the possible factors that affect cell viability. We observe that cells do not survive cell electrospinning but survive cell layering. Assessing the factors involved in cell electrospinning, we find that dehydration and fiber stretching are the main causes of cell death. In cell layering, the choice of solvent is critical, as residual solvent in the electrospun fibers could be detrimental to the cells.     

Reorganization of poly(ethylene terephthalate) structures and conformations to alter properties

The solid‐state morphologies, structures, and chain conformations of poly (ethylene terephthalate) (PET) have been reorganized/altered from those normally produced by solution and melt processing. This has been achieved by two distinct methods: (1) formation of a crystalline inclusion compound (IC) between guest PET and host γ‐cylodextrin (γ‐CD), followed by removal of the host γ‐CD and coalescence of the guest PET (c‐PET) and (2) rapid precipitation of PET from a warm trifluoracetic acid solution into a large excess of rapidly stirred acetone (p‐PET). Our prior observations (FTIR, NMR, DSC, X‐ray) demonstrated that c‐PET processed in this manner has a morphology, structure, and non‐crystalline chain conformations that are quite distinct from those of as‐received PET (asr‐PET). Where possible to compare, here we find that c‐ and p‐PETs behave very similarly, but very distinctly from asr‐PET. The reorganized c‐ and p‐PETs were found to be repeatedly rapidly crystallizable from the melt with a high level of crystallinity, and in their non‐crystalline regions to have tightly packed chains predominantly adopting highly extended kink conformations, which evidence no glass‐transition behavior. What is most unusual and somewhat puzzling is that their contrasting structures, morphologies, conformations, and thermal responses were observed to be independent of melt annealing, and persisted even after holding both samples above T_m for extended periods (hours). p‐PET, which can be produced in larger quantities than c‐PET, was utilized to measure additional macroscopic properties, such as melt viscosities, densities, and the stress‐strain and thermal shrinkage of melt‐pressed films, for comparison to those of asr‐PET. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 735–746, 2007     

Characterization of poly(methyl methacrylate) nanofiber mats by electrospinning process

In this study, the aim is to describe the influence of electrospinning parameters on the morphology, the water wetting property and dye adsorption property of poly(methyl methacrylate) nanofiber mats. Specifically, the effects of solution concentration, solvent type, applied voltage, distance between the electrodes and particulate reinforcement on the diameter and shape of the nanofibers were investigated. All poly(methyl methacrylate) nanofiber mats contained beaded nanofiber structures. With increasing the polymer solution concentration, the average fiber diameter also increased. Poly(methyl methacrylate) nanofiber mat electrospun from dimethylformamide solution resulted in thicker fibers when compared with the mat electrospun from acetone solution. Increasing the electric potential difference between the collector and the syringe tip did not increase the average fiber diameter. Besides increasing the distance between the electrodes resulted in a decrease in the average fiber diameter. When compared with PMMA nanofiber mat, thicker fibers were obtained with silica nanoparticles reinforced nanofiber mat. According to the water contact angle measurements, all poly(methyl methacrylate) nanofiber mats revealed hydrophobic surface property. PMMA nanofiber mat with the highest water contact angle gave rise to the highest dye adsorption capacity.     

Strategies toward biocompatible artificial implants: Grafting of functionalized poly(ethylene glycol)s to poly(ethylene terephthalate) surfaces

Epoxide and aldehyde end‐functionalized poly(ethylene glycol)s (PEGs) (M_w = 400, 1000, 3400, 5000, and 20,000) were grafted to poly(ethylene terephthalate) (PET) film substrates that contained amine or alcohol groups. PET‐PAH and PET‐PEI were prepared by reacting poly(allylamine) (PAH) and polyethylenimine (PEI) with PET substrates, respectively; PET‐PVOH was prepared by the adsorption of poly(vinyl alcohol) (PVOH) to PET substrates. Grafting was characterized and quantified by the increase of the intensity of the PEG carbon peak in the X‐ray photoelectron spectra. Grafting yield was optimized by controlling reaction parameters and was found to be substrate‐independent in general. Graft density consistently decreased as PEG chain length was increased. This is likely due to the higher steric requirement of higher molecular weight PEG molecules. Water contact angles of surfaces containing long PEG chains (3400, 5000, and 20,000) are much lower than those containing shorter PEG chains (400 and 1000). This indicates that longer PEG chains are more effective in rendering surfaces hydrophilic. Protein adsorption experiments were carried out on PET‐ and PEG‐modified derivatives using collagen, lysozyme, and albumin. After PEG grafting, the amount of protein adsorbed was reduced in all cases. Trends in surface requirements for protein resistance are: surfaces with longer PEG chains and higher chain density, especially the former, are more protein resistant; PEG grafted to surfaces containing branched or network polymers is not effective at covering the underlying substrate, and thus does not protect the entire surface from protein adsorption; and substrates containing surface charge are less protein‐resistant. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5389–5400, 2004     

Effect of interaction between poly(ethylene terephthalate) and carbon nanotubes on the morphology and properties of their nanocomposites

Poly(ethylene terephthalate) (PET) nanocomposites were prepared by melt‐extruding mixtures of PET and functionalized multiwalled carbon nanotubes (MWNTs) with some interaction with PET molecules. For the functionalization of MWNTs, benzyl isocyanate and phenyl isocyanate with different molecular flexibility were employed on the surface of the MWNTs via chemical modification, respectively. The reaction for functionalization of MWNTs was confirmed by FTIR and transmission electron microscopy (TEM) measurements. TEM observations indicated that both benzyl and phenyl isocyanate groups covered the surface of the MWNTs after functionalization. The PET nanocomposites containing isocyanate groups showed improved mechanical properties, including the tensile strength and tensile modulus, compared with those with pristine and acid‐treated nanotubes. These improvements were ascribed to π–π interactions between the aromatic rings of PET molecules and the isocyanate group in MWNTs. The functionalized MWNTs showed a better dispersion of carbon nanotubes in the matrix polymer and a different fractured cross‐section morphology in scanning electron microscope measurements relative to the pristine MWNTs. The crystallinity of the functionalized MWNT‐PET nanocomposites was significantly higher than that of the pristine and acid‐treated MWNTs. FTIR results indicated that the presence of carbon nanotubes induced trans‐conformation of PET chains, and trans conformation was particularly dominant in PET composites incorporating MWNT‐phenyl. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 900–910, 2008     

Confinement mechanism of electrospun polymer nanofiber reinforcement

The sharp increase in elastic modulus of electrospun polymer nanofibers with decrease in their diameters is now a well-known phenomenon. Unfortunately, up to now, the physical reasons resulting in the above size-dependent behavior are unclear. The proposed explanation is based on the confinement concept. A manifesting mechanism of the confinement effect which provides the size-dependent elastic modulus of electrospun polymer nanofibers is discussed. According to this model, the nanofiber polymer matrix contains anisotropic regions consisting of correlated worm-like subchains, partially oriented along the fiber. A fiber elongation is accompanied by relative rotations of the above regions. Confinement effect is that these rotations are hindered by the fiber boundary. As a result, the elastic modulus depends on the fiber diameter. This restriction is dominant for the thin fibers, is decreasing with increase of the fiber diameter, and becomes negligible for thick fibers. Such a behavior is in good agreement with experimental observations. © 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

Preparation and characterization of poly(butylene terephthalate)/poly(ethylene terephthalate) copolymers via solid‐state and melt polymerization

To increase the T_g in combination with a retained crystallization rate, bis(2‐hydroxyethyl)terephthalate (BHET) was incorporated into poly(butylene terephthalate) (PBT) via solid‐state copolymerization (SSP). The incorporated BHET fraction depends on the miscibility of BHET in the amorphous phase of PBT prior to SSP. DSC measurements showed that BHET is only partially miscible. During SSP, the miscible BHET fraction reacts via transesterification reactions with the mobile amorphous PBT segments. The immiscible BHET fraction reacts by self‐condensation, resulting in the formation of poly(ethylene terephthalate) (PET) homopolymer. ¹H‐NMR sequence distribution analysis showed that self‐condensation of BHET proceeded faster than the transesterification with PBT. SAXS measurements showed an increase in the long period with increasing fraction BHET present in the mixtures used for SSP followed by a decrease due to the formation of small PET crystals. DSC confirmed the presence of separate PET crystals. Furthermore, the incorporation of BHET via SSP resulted in PBT‐PET copolymers with an increased T_g compared to PBT. However, these copolymers showed a poorer crystallization behavior. The modified copolymer chain segments are apparently fully miscible with the unmodified PBT chains in the molten state. Consequently, the crystal growth process is retarded resulting in a decreased crystallization rate and crystallinity. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 882–899, 2007.     

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     

Intrinsic birefringence of poly(trimethylene terephthalate)

Highly oriented poly(trimethylene terephthalate) (PTT) fiber has a low birefringence that is unexpected for an aromatic polyester with a high refractive index. To explain this observation, the intrinsic birefringence Δn of PTT crystal was calculated from its bond polarizabilities to be 0.029. This Δn is almost an order of magnitude smaller than poly(ethylene terephthalate)'s value at 0.22, although both polymers have nearly identical crystal refractive indices. The small Δn is due to the arrangement of PTT's methylene groups in gauche conformations, causing the chain‐repeating unit to be tilted ～53° away from the c axis toward the basal plane. Because of the small Δn, the crystalline‐phase orientation made only a small contribution to the overall birefringence despite the fiber's high crystallinity and orientation. To understand the effect of the number of methylene groups on polyester optical anisotropy, the Δn's of a series of poly(m‐alkylene terephthalates) with m = 2–5 were compared and correlated with ψ: an angle made by the normal of the benzene ring with the crystal's axis. As ψ′ decreases, Δn of the polyesters diminishes. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1513–1520, 2002     

Electrospun chitosan-coated fibers of poly(L-lactide) and poly(L-lactide)/poly(ethylene glycol): preparation and characterization

Fibrous poly(L-lactide) (PLLA) and bicomponent PLLA/poly(ethylene glycol) mats were prepared by electrospinning and then were coated with chitosan. The presence of chitosan coating was proved by scanning electron microscopy and by fluorescence microscopy. On contact with blood, the chitosan coating led to changes in erythrocyte shape and in their aggregation. The haemostatic activity of the mats increased with increasing chitosan content. Microbiological studies against Staphylococcus aureus revealed that the chitosan coating imparts antibacterial activity to the hybrid mats. The combined haemostatic and antibacterial activities render these novel materials suitable for wound-healing applications.     

Unexpected efficiency of cyclic amidine catalysts in depolymerizing poly(ethylene terephthalate)

This article describes studies on the catalytic activity of several nitrogen‐based organic catalysts for the depolymerization of poly(ethylene terephthalate) (PET), in which a few cyclic amidines work more effectively than a potent, bifunctional guanidine‐based catalyst 1,5,7‐triazabicyclo‐[4,4,0]‐dec‐5‐ene (TBD) in the presence of short chain diols that play a role in activation of carbonyl groups through hydrogen bonding. Further studies prove that the catalytic efficiency at the above specific conditions depends only on the extent of activation of a hydroxyl group rather than simply the pK_a of the bases. For glycolysis with excess short‐chain alkanediols, 1,8‐diazabicyclo[5.4.0]undec‐7‐ene is the best catalyst. In contrast, TBD shows outstanding catalytic activity in depolymerizations of PET with mono‐alcohols and longer‐chain diols. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013