Aliphatic polyesters as models for relaxation processes in crystalline polymers: 4. Dielectric relaxation in oriented specimens

To investigate further the effect of the crystal phase on amorphous-phase relaxation, samples of 6-6, 5–7, and 6–10 polyesters have been oriented by extrusion in the solid-state through a tapered die and the dielectric constant and loss measured both parallel and perpendicular to the extrusion direction. The data have been analysed quantitatively in terms of the Cole—Cole phenomenological equation and relaxation times, width parameters and relaxation strengths determined. For the β (glass—rubber relaxation) the relaxation times are shifted 2–3 orders of magnitude to longer times and the relaxation is broadened slightly compared to the unoriented polymers. There also tends to be reduction of overall parallel and perpendicular relaxation strength for the β process on orientation. The γ processes show little effect of orientation on relaxation time and width and do not show reduction of overall relaxation strength. These observations are consistent with crystal displacement further restricting amorphous-phase longer-range segmental motion (β process) but having little effect on the locallized motions associated with the γ process. Both the γ and β processes show anisotropy in relaxation strength. The anisotropy is analysed with a newly developed theory to separate inherent phase anisotropy from apparent anisotropy due to composite or form effects. The average angle made by the lamellar surface normal to the extrusion direction enters as a parameter in the theory. Measurements of relaxed specimen $\text{ε}{}_{\mathrm{}}$ and ε_⊥ values for both the γ and β processes serve to determine both f_a the amorphous-phase orientation function and the average tilt angle for the specimens here. This is possible because for the γ process relaxation strength is small and specimen anisotropy is dominated by inherent phase anisotropy but for the γ + β processes larger relaxation strength leads to specimen anisotropy being strongly influenced by crystal—amorphous phase composite effects. Values of f_a are in the range of 0.05–0.20 for specimens with f_c in the range 0.5–0.9. Indicated lamellar tilt angles are in the range of 24°–80°.     

Melt‐electrospinning of poly(ethylene terephthalate) and polyalirate

Rodlike polymer samples were made from three kinds of poly(ethylene terephthalate) (PET) pellets with different intrinsic viscosities (IV), and from polyalirate (Vectra) pellets. PET and Vectra fibers were produced using a melt‐electrospinning system equipped with a CO₂‐laser melting device from these rodlike samples. The effects of IV value and laser output power on the fiber diameter of PET were investigated. Furthermore, the effect of the laser output power on the fiber diameter of Vectra was investigated. The crystal orientation of these produced fibers was also investigated by X‐ray photographs. The following conclusions were reached: (i) the diameter of PET fiber decreases with increasing laser output power; (ii) the minimum average diameter of PET fibers is scarcely influenced by the value of IV; (iii) the electrospun PET fibers show isotropic crystal orientation; (iv) fibers having an average fiber diameter smaller than 1 μm cannot be obtained from PET and Vectra using the system developed; and (v) preferred liquid crystal orientation can be seen in electrospun Vectra fibers. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Rotating frame 1H n.m.r. spin-lattice relaxation studies of modified isotactic polypropylene

Rotating frame ¹H n.m.r. spin-lattice relaxation times T_1ϱ have been used to study relaxation processes in partially crystalline polypropylene and polypropylene modified by a copolymer of ethylene and alkylaminoacrylate. Measurements were carried out within the temperature range of 250–420 K. Three relaxation times T_1ϱ were detected over the whole temperature range. α_c relaxation in the crystalline regions of the polymer, relaxation processes β(U) and β(L), associated with an apparent double glass transition, and α_a relaxation process, which was ascribed to a free reorientation of the whole chains in the amorphous regions, were observed by temperature dependences of these relaxation times. In addition to the α_c relaxation, another relaxation process with relatively low molecular mobility was found in crystalline regions. The influence of the polymer modifier on the observed relaxation processes greatly depends on its amount.     

Correlation of dielectric and visco-elastic relaxation in polymer solutions

The interpretation of studies of dielectric relaxation in polymers is often restricted by ignorance of the mode of motion responsible for dipole orientation in an applied field. Relevant information can be drawn from studies of visco-elastic relaxation, since the visco-elastic relaxation time is comparable with the dielectric relaxation time when the latter requires some ‘whole molecule’ mode of motion involving simultaneous movement of many chain segments. The dielectric and visco-elastic relaxation times are expected to differ when the former involves a localised segmental motion. The dielectric relaxation process is shown to involve a localised segmental motion in acrylic polymers, but involves a ‘whole molecule’ rotation in low molecular weight poly(N-vinylcarbazole). Poly(p-substituted phenyl acetylenes) exhibit two dielectric relaxation processes. A low frequency relaxation comparable with the visco-elastic relaxation is ascribed to the backbone double bonds being polarised unidirectionally along the chain, and a high frequency process is ascribed to a localised segmental motion of the substituted phenyl moiety.     

Strongly entangled polymer chains in a melt. Description of n.m.r. properties associated with a submolecule model

A model is proposed to illustrate properties of the transverse magnetic relaxation function, G(t), of proton pairs linked to strongly entangled polymer chains in a melt. According to this model, any polymer molecule is described as a freely jointed chain and it is divided into submolecules of equal contour length L^v_e. Every link is supposed to carry a proton pair; dipolar spin couplings between different proton pairs are neglected. The disentanglement relaxation time is supposed to be much longer than any characteristic time of the spin system; consequently, any submolecule observed on an n.m.r. time scale is supposed to have fixed ends. It is considered that the residual spin-coupling energy resulting from such a constraint governs the magnetic relaxation process. The free induction decay is expressed as a contour length function; its time evolution is shown to exhibit two ranges, which might be characterized by two relaxation times. The model is easily extended to rotating methyl groups. Theoretical results are compared with magnetic relaxation properties observed on entangled real chains: polydimethylsiloxane (PDMS) and cis-1,4-polybutadiene (PB). An attempt to adjust the contour length value to experimental results leads to the determination of average submolecule molecular weights M^v_e equal to 8200 and 2000 for PDMS and PB, respectively; the values usually obtained from viscoelastic plateau modulus measurements are 8100 and 1900, respectively.     

Experimental investigation of the governing parameters in the electrospinning of polymer solutions

In the electrospinning process, polymer nanofibers with submicron-scale diameters are formed when a droplet of a viscoelastic polymer solution is subjected to a high voltage electrostatic field. We report the experimental work on the electrospinning process in which the influence of different process parameters on the electric current and volume and surface charge density in the polymer jet was measured. Shear viscosity, surface tension, relaxation time, and the electric conductivity and permittivity were measured as well. For this purpose different solutions of polyethylene oxide (PEO), polyacrylic acid (PAA), polyvinyl alcohol (PVA), polyurethane (PU) and polycaprolactone (PCL) were prepared and underwent electrospinning. The governing parameters investigated were the applied voltage (V), the solution flow rate (Q), the polymer weight concentration (C), the molecular weight of the polymer (M), the nozzle-to-ground distance (H) and, in some solutions, the concentration of ethanol (C_et).     

Nonlinear optical materials by electrospinning technique

The possibility of using electrospinning to deposit, and simultaneously electrically pole, polymeric mats containing nonlinear optical (NLO) molecules is studied. Disperse Red 1 (DR1) chromophore dispersed into polymethyl methacrylate (PMMA) fibers is chosen as test system. Aligned fibers of DR1/PMMA are deposited by means of the rotating collector electrospinning technique. The processing parameters are optimized to obtain an oriented texture of fibers and the NLO activity is characterized by means of second harmonic generation measurements. Comparison with spin coating and corona poling samples of the same material is discussed. The results point toward a low efficiency of chromophore orientation for the electrospun samples in comparison to the poled ones. An improvement in the electrospinning apparatus is proposed to enhance the chromophore orientation in the polymeric fibers. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40913.     

Fabrication of SiO2-ZrO2 composite fiber mats via electrospinning

(1 ? x)SiO₂-(x)ZrO₂ (x = 0.1, 0.2) composite fiber mats were prepared by electrospinning their sol-gel precursors of zirconium acetate and tetraethyl orthosilicate (TEOS) without using a polymer binder. The electrospun composite fibers were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), fourier transform infrared spectroscopy (FT-IR) and mercury porosimetry. The composite fibers having a tetragonal crystalline ZrO₂ were obtained by calcining the electrospun composite fibers at high temperatures. The results show that the structure and crystallization of ZrO₂ in the composite fibers can be controlled by sintering temperature, while the porosity and morphology of the fiber mats did not depend on the sintering temperature.     

Preparation of photochromic poly(vinylidene fluoride-co-hexafluoropropylene) fibers by electrospinning

Photochromic poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-co-HFP) fibers were prepared by electrospinning from a solution of copolymer and ester-functionalized nitrospiropyran (SPEST) molecules. The surface and internal features of the electrospun (ES) fibers were characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and wide-angle X-ray diffraction (WAXD). The strong segregation of fluorine-rich groups on the fiber surface, which occurs during and/or after the electrospinning process, is driven by the lower surface tension for fluorine-rich groups and leads to encapsulation of SPEST predominantly near the core of the fibers, as confirmed by both X-ray photoelectron spectroscopy (XPS) and dynamic water contact angle (CA) measurements. The photochromic behavior of the spiropyran is preserved in the polymeric fibers, as confirmed by steady-state absorption and emission spectroscopy. Both isomeric forms of the photochrome in SP-PVDF-co-HFP were emissive, an effect that is thought to be due to the steric and/or electrostatic restrictions on the ring-opening reaction imposed by the fiber.     

聚乙交酯纤维的结构及其降解性能

纺制了聚乙交酯(PGA)纤维,并对其进行了不同温度和时间的热处理。对处理后PGA纤维的结构进行了测试,并将其放置于恒温箱的磷酸盐缓冲液中进行两周降解处理,研究了不同热处理工艺下PGA纤维的结构及降解性能。试验结果表明:随着热处理温度的提高,PGA纤维材料的晶区取向度变化不大,结晶度逐渐增大,晶粒尺寸也相应发生变化;在试验条件下,经过恒温缓冲液两周时间的降解处理,纤维的强度保持率随热处理温度提高呈现先增大后降低的趋势,纤维的强度保持率在110℃出现最大值;同时,在热处理温度110℃时,随着热处理时间的延长,PGA纤维的强度保持率增加。     

Scaling law on particle-to-fiber formation during electrospinning

The development of various morphologies such as beads, beaded fibers, pure fibers and their scaling as a function of solution properties and processing variables in electrospinning is reported. Polyvinyl pyrrolidone (PVP), at various molecular weights and concentrations dissolved in a mixture of water and ethanol, was used to prepare different morphologies and sizes. The morphology of beads and fibers was predicted and measured based on an entanglement number diagram and rheological measurements. A constant-current electrospinning system was employed to control the processing variables. Scaling laws related to solution properties and processing variables (voltage, current and flow rate), and their effect on the fiber/bead diameter, were discussed. Viscosity (η), flow rate (Q), and current (I) were found to play significant roles in the control of morphology during electrospinning. Processing variables involved in electrospinning followed a power scaling that was in agreement with the model. The dependence of fiber diameter (d_f) on the Q/I for different molecular weights and concentrations also followed a power law, and the scaling varied between 0.11-0.29 for beaded fiber and 0.36-0.51 for pure fiber. In addition, the relationship between viscosity and fiber diameter followed scaling laws: d_f ∼ η^0.98.     

Molecular motion of syndiotactic poly(α-methylstyrene) in solution studied by carbon-13 n.m.r. relaxation measurements

The carbon-13 n.m.r. spin-lattice relaxation times, nuclear Overhauser enhancement factor (NOE), and line widths have been measured for a syndiotactic poly(α-methylstyrene) in solutions in toluene-d₈ and o-dichlorobenzene-d₄ as a function of temperature from 40° to 163δC. The single correlation time model of relaxation is inadequate to explain the data of spin-lattice relaxation time and NOE. But, within experimental error, these relaxation data in two solvents over a full temperature range can be interpreted in terms of either the Cole-Cole or the log-χ² distributions of correlation times, or a conformational jump model proposed by Monnerie et al. The internal rotation of the methyl group is relatively rapid, while that of the phenyl ring is slow and practically overshadowed by the backbone segmental reorientation over the temperature range examined. The solvent dependence of relaxation data was discussed.     

Synthesis, characterization, and electrospinning of zwitterionic poly(sulfobetaine methacrylate)

Poly(sulfobetaine methacrylate) (PSBMA) can be potentially utilized in filtration and wound dressing applications for which nanofibers structures are highly desirable. In this work, a series of PSBMAs with different molecular weights were synthesized, characterized, and electrospun into nanofibers. The polymer molecular weight was controlled by varying the amount of redox initiators in the free radical polymerization of SBMA, with the highest molecular weight achieved at an intermediate initiator concentration. From the intrinsic viscosity measurements, the Mark-Houwink parameters for PSBMA (at 21 °C in 0.2 M NaCl solution) was determined as a = 0.4071 and k = 2.06 × 10⁻³. Thermogravimetric (TGA) analysis shows that the PSBMAs were thermally stable up to at least 250 °C. Fourier transform infrared (FTIR) spectra indicate major structural changes of both polymer backbone and pendant groups by thermal degradation. Results from differential scanning calorimetry (DSC), TGA, and FTIR characterizations all demonstrate the existence of water strongly bound in PSBMA. DSC analysis also indicates different degrees of crystallinity for the PSBMAs of different sizes. Viscosity of the PSBMA solutions, a critical parameter for electrospinning, increased with the solution concentration and the polymer molecular weight. For the electrospinning of PSBMA, it was found that high solution concentration and high molecular weight favored the formation of smooth fibers while low solution concentration or low molecular weight led to the formation of beaded fibers or beads. Fiber diameters ranging from 200 to 570 nm were achieved by controlling solution concentration and polymer molecular weight. The characterization data and electrospinning results were finally correlated to explore the relationships between fiber formation, viscosity, molecular weight, and concentration.     

Dual fiber behavior of polyvinyl alcohol/zirconium n-propoxide composite fibrous mats prepared via electrospinning

The electrospinning technique has been used to prepare composite fibrous mats of polyvinyl alcohol and zirconium n-propoxide. A halide-free synthesis route was adopted to prepare sol of zirconium n-propoxide. The fiber mats were deposited under different process parameters (applied voltage, needle–collector distance and flow rate). Two distinct regions have been noticed in the each fibrous mat for every specific process parameter. The microstructural change in the outer as well as in the inner regions of the composite fibers has been characterized by scanning electron microscopy. The process parameters significantly affect the fiber morphology of both the distinct regions. Outer region fibers showed least beaded morphology and more uniformity in the fiber diameter throughout their length. The average fiber diameter in the inner and outer regions was found to be in the range of 130–225 nm & 115–180 nm, respectively, under the different process parameters.     

Structure and properties of melt‐spun PET/MWCNT nanocomposite fibers

Polyethylene terephthalate (PET) melt‐spun fibers were modified with multiwall carbon nanotubes (MWCNT) to obtain conductive microfibers smaller than 90 μm in diameter. Physical properties such as crystallinity and orientation of as‐spun fibers were studied by X‐ray diffraction, Raman spectroscopy, and microscopy techniques at different draw ratios (DR) and MWCNT concentrations. Morphological and orientation analysis of MWCNT after melt‐spinning process showed agglomerates formation and highly oriented CNTs. The study of the orientation of PET crystalline phase in drawn fibers proved that the addition of nanoparticles decreases the orientation of crystalline units inside the fibers. The orientation of MWCNT as well as that of PET chains was studied using Raman spectroscopy at different DR and a high degree of CNT orientation was observed under high DR conditions. Mechanical and electrical properties of as‐spun fibers were also investigated. Our results showed that it was possible to achieve conductive fibers at a MWCNT concentration of 2% w/w, and more conductive fibers using higher DR were also obtained without increasing the MWCNT concentration. Mechanical properties results showed interestingly high value of maximum tensile strain at break (ε_max) of nanocomposite fibers, up to three times more than pure PET fibers. POLYM. ENG. SCI., 50:1956–1968, 2010. © 2010 Society of Plastics Engineers     

Simulation of mechanical properties of oriented glassy polystyrene

Mechanical properties of processed polymers depend sensitively on their microstructure. In order to understand how different processing conditions affect the mechanical properties of polymers, one needs a means to describe the process-induced microstructure. Because the characteristic relaxation times of processed polymer chains often span several orders of magnitude, it is commonly the case that partial relaxation of the chains is frozen into the final product. We report results of molecular simulations by the Semi-Grand Canonical Monte Carlo (SGMC) method to study the orientation-dependent elasticity of glassy polystyrene as a function of both the system-average degree of orientation and the degree of relaxation of chain ends at a constant average orientation, in accord with the tube model of Doi and Edwards. Our simulations reproduce quantitatively the experimentally observed trends in the tensile modulus E₁₁ as a function both of the system-average orientation and of the inhomogeneity of the orientation along the chain due to rapid relaxation of chain ends. The results show that the partial relaxation of the polymer chains is sufficient to explain the observed variation of mechanical properties for samples that differ in processing history, yet have the same observed birefringence.     

Construction of hierarchical structures by electrospinning or electrospraying

To expand the application of electrospun fibers or electrosprayed beads, micro-nano hierarchical structures of polystyrene (PS) have been constructed through the adjustment of solvent, polymer concentration, environment humidity, electrospinning temperature, etc. Primary structures, such as fibers, beads and bead-on-string structure, as well as secondary structures, such as nanopores, nanopapilla and net-work structure, have been constructed. Solvent plays an important role in the construction of both primary structures and secondary structures. By using N,N-dimethylformamide (DMF), tetrahydrofuran (THF) and mixed solvent of DMF/THF, the micro-nano hierarchical structures can be controlled. Humidity is a key factor to the construction of secondary structures. The obtained fibers or beads have smooth surface at low humidity. While at high humidity, secondary structures tend to appear. For the PS/DMF system, vapor-induced phase separation may be the most pertinent mechanism to explain the formation of secondary structures. While for the PS/THF system, breath figure theory can explain the formation of uniform nanopores properly.     

Stress relaxation of poly(1,4-dimethylene-trans-cyclohexyl suberate)

Stress relaxation experiments were conducted on poly(1,4-dimethylene-trans-cyclohexyl suberate) (MCS) as a function of preparation condition and temperature. Deconvolution of the stress relaxations provides relaxation times, which can be plotted as a function of temperature to obtain an activation energy for the relaxation process. For an MCS sample of M_n=24.8 K, MWD=2, the activation energy varies from 12.7-5.0 kcal mol^?1 with forming temperatures varying from 45–90°C. These activation energies are associated with different populations of tie molecules between lamellae. We believe that these activation energies reflect the reorientation process in the amorphous segments of the polymer during stress relaxation.     

Structure,properties, and phase transitions of melt‐spun poly(vinylidene fluoride) fibers

Three different experimental techniques were used to study structural phase transitions in melt‐spun poly(vinylidene fluoride) fibers, which were produced with different process parameters and processed in the draw‐winding process at different temperatures and draw ratios. The fibers are examined with the help of wide‐angle X‐ray diffraction at elevated temperatures, differential scanning calorimetry with stochastic temperature modulation, and dynamic mechanical analysis. An oriented mesophase and deformed crystal structures can be observed in all fibers and assigned to the mechanical stress occurring in the processes. Furthermore, several phase transitions during melting and two mechanical relaxation processes could be detected. The observed transitions affect the crystal geometry, the orientation distribution, anisotropic thermal expansion, and the mechanic response of the fiber samples. The relaxation processes can be related with an increasing amount of crystalline β‐phase in fibers drawn at different temperatures. The detailed information about phase transitions and the related temperatures are used to produce fibers with an extended amount of β‐phase crystallites, which are responsible for piezoelectric properties of the material. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011