Development of oriented structure and properties on drawing of poly(vinylidene fluoride) by solid‐state coextrusion

Gel films of poly(vinylidene fluoride) (PVDF) consisting of α‐form crystals were drawn uniaxially by solid‐state coextrusion to extrusion draw ratios (EDR) up to 9 at an optimum extrusion temperature of 160 °C, about 10°C below the melting temperature (T_m). The development of an oriented structure and mechanical and electrical properties on coextrusion drawing were studied as a function of EDR. Wide‐angle X‐ray diffraction patterns showed that the α crystals in the original gel films were progressively transformed into oriented β‐form crystals with increasing EDR. At the highest EDR of 9 achieved, the drawn product consisted of a highly oriented fibrous morphology with only β crystals even for the draw near the T_m. The dynamic Young's modulus along the draw direction also increased with EDR up to 10.5 GPa at the maximum EDR of 9. The electrical properties of ferroelectricity and piezoelectricity were also markedly enhanced on solid‐state coextrusion. The D–E square hysteresis loop became significantly sharper with EDR, and a remanent polarization P_r of 100 mC/m² and electromechanical coupling factor along the thickness direction k_t of 0.27 were achieved at the maximum EDR of 9. The crystallinity value of 73–80% for the EDR 9 film, estimated from these electrical properties, compares well with that calculated by the ratio of the crystallite size along the chain axis to the meridional small‐angle X‐ray scattering (SAXS) long period, showing the average thickness of the lamellae within the drawn β film. These results, as well as the appearance of a strong SAXS maximum, suggest that the oriented structure and properties of the β‐PVDF are better explained in terms of a crystal/amorphous series arrangement along the draw axis. Further, the mechanical and electrical properties obtained in this work are the highest among those ever reported for a β‐PVDF, and the latter approaches those observed for the vinylidene fluoride and trifluoroethylene copolymers. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1371–1380, 2001     

Solid-state coextrusion of poly(ethylene terephthalate). I. Drawing of amorphous PET

Films of uniaxially oriented poly(ethylene terephthalate) (PET), M _v = 81,000, have been drawn by solid-state coextrusion in the range 40–100°C surrounded by polyethylene. This is well below the PET melting temperature and in some cases below its glass transition temperature. Properties of the extrudates, such as degree of crystallinity, mechanical and thermal properties, were investigated as a function of coextrusion temperature and draw ratio (EDR ≤ 4.4). The results show that the percent crystallinity depends strongly on draw ratio, whereas its sensitivity to extrusion temperature is limited only to the highest draw ratio (4.4). On the other hand, Young's modulus was sensitive to both extrusion temperature and draw ratio, exhibiting a maximum at EDR = 4.4 and T_ext = 65°C. Above this temperature, moduli decrease apparently because of increased chain mobility, resulting in dissipation of chain orientation. Furthermore, changes in yield and tensile strength followed the changes in mechanical properties, suggesting that they are dominated by the same factors. The cold-crystallization temperature T_CC also revealed information about the morphological changes occurring during the extrusion drawing. For samples of EDR = 4.4, T_CC increased with extrusion temperature, suggesting again dissipation of orientation by thermal motions. On the other hand, T_CC decreases with EDR, and a ΔT_CC as high as 73°C was found. Conventional drawing of amorphous PET has been widely reported. To our knowledge this is the first time oriented PET has been prepared using the advantages of solid-state coextrusion.     

Uniaxial drawing of polytetrafluoroethylene virgin powder by extrusion plus cold tensile draw

Polytetrafluoroethylene (PTFE) virgin powder was ultradrawn uniaxially by a two-stage draw. A film, compression molded from powder below the melting temperature (T_m), was initially solid-state coextruded to an extrudate draw ratio (EDR) of 6–20 at an established optimum extrusion temperature of 325°C, near the T_m of 335°C. These extrudates from first draw were found to exhibit the highest ductility at 45–100°C for the second-stage tensile draw, depending on the initial EDR and draw rate. The maximum achievable total draw ratio (DR_{t, max}) was 36–48. Such high ductility of PTFE, far below the T_g (125°C) and T_m, is in sharp contrast to other crystalline polymers that generally exhibit the highest ductility above their T_g and near T_m. The unusual draw characteristics of PTFE was ascribed to the existence of the reversible crystal/crystal transitions around room temperature and the low intermolecular force of this polymer, which leads to a rapid decrease in tensile strength with temperature. The structure and tensile properties of drawn products were sensitive to the initial EDR, although this had no significant influence on DR_t,max. The most efficient and highest draw was achieved by the second-stage tensile draw of an extrudate with the highest EDR 20 at 100°C, as evaluated by the morphological and tensile properties as a function of DR_t. The efficiency of draw for the cold tensile draw at 100°C was a little lower than that for solid-state coextrusion near the T_m. However, significantly higher tensile modulus and strength along the fiber axis at 24°C of 60 ± 2 GPa and 380 ± 20 MPa, respectively, were achieved by the two-stage draw, because the DR_t,max was remarkably higher for this technique than for solid-state coextrusion (DR_t,max = 48 vs. 25). The increase in the crystallite size along the fiber axis (D₀₀₁₅), determined by X-ray diffraction, is found to be a useful measure for the development of the morphological continuity along the fiber axis of drawn products.© 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2551–2562, 1998     

Solid-state coextrusion of poly(ethylene terephthalate). II. Drawing of semicrystalline PET

The drawing of semicrystalline (33 and 50%) poly(ethylene terephthalate) (PET) films has been studied by solid-state coextrusion. Because of its brittleness and opacity, isotropic and semicrystalline PET film is of little practical use. Early attempts to cold-draw crystalline films led to fracture in contrast to deformation of amorphous PET. However, we have succeeded in systematically preparing films with extrusion draw ratios ≤4.4 from semicrystalline PET. In many cases, the properties of the drawn extrudates, as a function of extrusion temperature T_ext and extrusion draw ratio EDR, were similar to those prepared from amorphous PET. However, some remarkable differences have also been found. In the case of coextrudates prepared from isotropic 50% crystalline PET, we found that the larger the deformation, the lower the apparent resulting crystallinity. In the extreme, a 34% reduction in crystallinity after deformation was observed. For the coextrudates drawn from initially 33% crystalline PET, slightly different behavior occurred. For T_ext ≤ 90°C, all extrudates showed crystallinities lower than the original isotropic film, with a minimum at EDR = 3; for T_ext ≥ 110°C, crystallinities were slightly greater than in the original film and increased with EDR. Qualitative measurements of heats of fusion were in agreement with density gradient results for PET crystallinity. In contrast is our previous finding that extrudates from initially amorphous PET always increase in crystallinity with EDR, because of stress-induced crystallization. The results now suggest that in the T_ext range investigated, the initial spherulitic structure is at least in part destroyed on drawing. In addition, the percent crystallinity is revealed to be dependent on T_ext, with lower values at lower temperatures. Mechanical tests show that the extrudates are similar or sometimes higher in tensile modulus when compared to amorphous PET drawn under the same conditions.     

Uniaxial Draw Of Poly(aryl-ether-ether-ketone) by solid-state extrusion

Poly(aryl-ether-ether-ketone) (PEEK) films and rods have been solid-state extruded at 154 and 310°C, respectively. The crystal orientation functions, melting behavior, density, and tensile properties of the drawn PEEK films (EDR ≤ 3.7) and rods (EDR ≤ 5.5) have been measured. As extrusion draw ratio (EDR) was increased, the c-axis orientation function, melting temperature, and tensile modulus and strength increased. Moduli up to 6.5 GPa and the strengths up to 600 MPa, 3 and 6 times the values of undrawn films, respectively, were obtained for the drawn films. The thermal expansivities along (α_‖) and perpendicular (α_?) to the draw direction of PEEK rods were measured from ?40 to +10°C. As EDR was increased, α_? increased, but α_‖ decreased. At EDRs of 3.8 and 5.5, α_‖ even exhibited negative values (about ?5 × 10^?6°C^?1), probably due to reversible contraction of elongated tie-molecules.     

Superdrawing of polytetrafluoroethylene nascent powder by solid‐state coextrusion

A film of nascent powder of polytetrafluoroethylene (PTFE), compacted below the ambient melting temperature (T_m, 335 °C), was drawn by two‐stage draw techniques consisting of a first‐stage solid‐state coextrusion followed by a second‐stage solid‐state coextrusion or tensile draw. Although the ductility of extrudates was lost for the second‐stage tensile draw at temperatures above 150 °C due to the rapid decrease in strength, as previously reported, the ductility of extrudates increased with temperature even above 150 °C when the second‐stage draw was made by solid‐state coextrusion, reflecting the different deformation flow fields in a free space for the former and in an extrusion die for the latter. Thus, a powder film initially coextruded to a low extrusion draw ratio (EDR) of 6–20 at 325 °C was further drawn by coextrusion to EDRs up to ～?400 at 325–340 °C, near the T_m. Extremely high chain orientation (f_c = 0.998 ± 0.001), crystallinity (96.5 ± 0.5)%, and tensile modulus (115 ± 5 GPa at 24 °C, corresponding to 73% of the X‐ray crystal modulus) were achieved at high EDRs. Despite such a morphological perfection and a high modulus, the tensile strength of a superdrawn tape, 0.48 ± 0.03 GPa, was significantly low when compared with those (1.4–2.3 GPa) previously reported by tensile drawing above the T_m. Such a low strength of a superdrawn, high‐modulus PTFE tape was ascribed to the low intermolecular interaction of PTFE and the lack of intercrystalline links along the fiber axis, reflecting the initial chain‐extended morphology of the nascent powder combined with the fairly high chain mobility associated with the crystal/crystal transitions at around room temperature. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3369–3377, 2006     

Crystal Transformation and Development of Tensile Properties upon Drawing of Poly(L-lactic acid) by Solid-State Coextrusion: Effects of Molecular Weight

Melt-crystallized films of poly(L -lactic acid) (PLLA) with M_v in the range of 3.8 ∼ 46 × 10⁴ consisting of α-form crystals were uniaxially drawn by solid-state coextrusion. The effects of M_v, extrusion draw ratio (EDR), and extrusion temperature (T_ext) on the crystal/crystal transformation from α- to β-form crystals and the resultant tensile properties of drawn products were studied. The crystal transformation proceeded with EDR and more rapidly for the higher M_v's. Furthermore, the crystal transformation proceeded most rapidly with EDR at a T_ext around 130 °C, independently of the M_v's. As a result of the optimum combination of processing variables influencing the the crystal transformation (M_v, T_ext, and drawability), highly oriented films consisting of β-form crystals alone were obtained by coextrusion of higher M_v samples at T_ext's slightly below the melting temperature (150 ∼ 170 °C) and at higher EDR's > 11. Both the tensile modulus and strength increased rapidly with EDR. The modulus at a given EDR was slightly higher for the samples with higher M_v's. In contrast, the strength at a given EDR was remarkably higher for the higher M _v's. The highest tensile modulus of 8.0 GPa and strength of 500 MPa were obtained with the sample of the highest M_v of 46 × 10⁴ coextruded at 170 °C to the highest EDR of 14.     

Crystallization kinetics of chalcogenide glass Se0.8 Te0.2

The crystallization kinetics of the chalcogenide glass Se_0.8Te_0.2 was studied by means of differential scanning calorimetry. The variation in partial area (X) with temperature (T) revealed that the transition from the amorphous to the crystalline phase occurs in two dimensions.Activation energies were determined for both the glass transition (E _t) and the crystallization (E _c).E _t was calculated from the variation inT _g with the heating rate (a).E _c was determined by three different methods: (i) variation inX withT, (ii) variation inT _p witha, and (iii) variation inT _c witha.E _t andE _c have values of 161.01±2.75 and 84.75 ±8.21 kJ/mol, respectively.

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Zusammenfassung Mittels DSC wurde die Kristallisierungskinetik des Chalkogenidglases Se_0.8Te_0.2 untersucht. Eine Änderung partieller Gebiete (X) mit der Temperatur (T) zeigte, daß der Übergang von der amorphen zur kristallinen Phase zweidimensional verläuft.Es wurde die Aktivierungsenergie sowohl für den Glasübergang (E _t) als auch für die Kristallisierung (E _c) bestimmt.E _t wurde mittels der Abhängigkeit vonT _g von der Aufheizgeschwindigkeit (a) ermittelt.E _c wurde auf drei verschiedene Wege bestimmt: (i) Änderung vonX in Abhängigkeit vonT, (ii) Änderung vonT _p in Abhängigkeit vona und (iii) Änderung vonT _c in Abhängigkeit vona. Die Werte vonE _t undE _c betragen 161.01±2.75 bzw. 84.75±8.21 kJ/mol.

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This work was partly supported by a Grant-in-Aid for Scientific Research from the GTZ GmbH and DAAD, W. Germany.     

Non-isothermal crystallization kinetics of poly(trimethylene terephthalate)/poly(ethylene 2,6-naphthalate) blends

The glass-transition temperature and non-isothermal crystallization of poly(trimethylene terephthalate)/poly(ethylene 2,6-naphthalate) (PTT/PEN) blends were investigated by using differential scanning calorimeter (DSC). The results suggested that the binary blends showed different crystallization and melting behaviors due to their different component of PTT and PEN. All of the samples exhibited a single glass-transition temperature, indicating that the component PTT and PEN were miscible in amorphous phase. The value of T_g predicted well by Gordon-Taylor equation decreased gradually with increasing of PTT content. The commonly used Avrami equation modified by Jeziorny, Ozawa theory and the method developed by Mo were used, respectively, to fit the primary stage of non-isothermal crystallization. The kinetic parameters suggested that the PTT content improved the crystallization of PEN in the binary blend. The crystallization growth dimension, crystallization rate and the degree of crystallinity of the blends were increased with the increasing content of PTT. The effective activation energy calculated by the advanced iso-conversional method developed by Vyazovkin also concluded that the value of E_a depended not only on the system but also on temperature, that is, the binary blend with more PTT component had higher crystallization ability and the crystallization ability is increased with increasing temperature. The kinetic parameters U^* and K_g were also determined, respectively, by the Hoffman-Lauritzen theory.     

Application of zone-drawing and zone-annealing method to poly(p-phenylene sulfide) fibers

A zone-drawing and zone-annealing treatment was applied to poly(p-phenylene sulfide) fibers in order to improve their mechanical properties. The zone-drawing (ZD) was carried out at a drawing temperature of 90°C under an applied tension of 5.5 MPa, and the zone-annealing (ZA) was carried out at an annealing temperature of 220°C under 138.0 MPa. The differential scanning calorimetry (DSC) thermogram of the ZD fiber had a broad exothermic transition (T_c = 110°C) attributed to cold-crystallization and a melting endotherm peaking at 286°C. The T_c of the ZD fiber was lower than that (T_c = 128°C) of the undrawn fiber. In the temperature dependence of storage modulus (E′) for the ZD fiber, the E′ values decreased with increasing temperature, but increased slightly in the temperature range of 90–100°C, and decreased again. The slight increase in E′ was attributable to the additional increase in the crosslink density of the network, which was caused by strain-induced crystallization during measurement. The resulting ZA fiber had a draw ratio of 6.0, a degree of crystallinity of 38%, a tensile modulus of 8 GPa, and a tensile strength of 0.7 GPa. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1731–1738, 1998     

Studies of miscibility,transesterification and crystallization in blends of poly(ethylene terephthalate) and Poly(ethylene-2,6-naphthalene dicarboxylate)

Blends of poly(ethylene terephthalate) (PET) and poly(ethylene-2,6-naphthalene dicarboxylate) (PEN) were obtained by coprecipitation from solution followed by melt-pressing for different timest _mand quenching in iced water. When the melt-pressing time was 0.2 and 0.5 min, two glass transition temperaturesT _gwere observed by means of dynamic mechanical analysis (DMA), indicating that there are two phases present, a PEN-rich phase and a PET-rich phase. The differential scanning calorimetry (DSC) curves show two crystallization peaks and two melting peaks which, according to wide-angle x-ray scattering (WAXS) measurements, can be attributed to PET and PEN, respectively. In the case oft _m=2 min or longer, a single value ofT _gand thus a single phase is found to exist. Fort _m=10 min and 45 min no crystallization and melting at all is observed during heating with 10°C/min, indicating that a copolyester of PET and PEN has been formed by transesterfication during melt-pressing.Time-resolved WAXS measurements during isothermal crystallization show that, in the blend, the half-time of crystallization of PET is different from that of PEN, and not the same as that which is found in the pure polymer.Dedicated with best wishes to Prof. Dr. E.W. Fischer on the occasion of his 65th birthday     

Drawing of poly(p-phenylene sulfide) by solid-state coextrusion

The effects of drawing temperature on the physical and mechanical properties of poly(p-phenylene sulfide) have been studied. A melt-quenched film was drawn by solid-state coextrusion both below (75°C) and above (95 and 110°C) the glass transition temperature T_g (85°C) of PPS. The maximum extrusion draw ratio (EDR_max) increased from 3.4 to 5.6 with increasing extrusion temperature T_e from 75 to 110°C. It was found that extrusion drawing just above the T_g of PPS (95°C) produced more stress-induced crystals. A high efficiency of draw in the amorphous region was achieved by extrusion at T_e-75°C. The tensile modulus at EDR_max decreased from 5.1 to 3.5 GPa with increasing T_e from 75 to 110°C. The low efficiency of draw for the samples extruded at 110°C is explained in terms of disentanglement and chain slippage during drawing due to a less effective network.     

Superdrawing of polytetrafluoroethylene virgin powder above the static melting temperature

A new two‐stage draw technique was successfully applied to the superdrawing of polytetrafluoroethylene (PTFE) virgin powder. A film, compression‐molded from powder below the melting temperature (T_m = 335 °C), was initially solid‐state coextruded to an extrusion draw ratio (EDR) of 6–20 at 325 °C, about 10 °C below the T_m. These extrudates from the first‐stage draw were further drawn by a second‐stage pin draw in the temperature (T_d) range of 300–370 °C that covers the static T_m. The maximum achievable total draw ratio was ～60 at a T_d = 300 °C and increased rapidly with increasing T_d, reaching a maximum of 100–160 at a temperature window between 340 and 360 °C, depending on the initial EDRs. At yet higher T_d's, the ductility was lost as a result of melting. The high ductility of the PTFE extrudates at such high temperatures was ascribed to the improvement of interfacial adhesion and bonding between the deformed powder particles upon the first‐stage extrusion combined with the rapid heating of only a portion of the extrudate followed by the elongation at a high rate. The highly drawn fibers were highly crystalline (χ_c ≤ 87%) and showed high chain orientation (f_c ≤ 0.997) and a large crystallite size along the chain axis (D₀₀₁₅ ≤ 160 nm). The molecular draw ratio, estimated from the entropic shrinkage above the T_m, was close to the macroscopic deformation ratio independently of the initial EDRs. These results indicate that the draw was highly efficient in terms of chain extension, orientation, and crystallization. Thus, the maximum tensile modulus and strength achieved in this work were 102 ± 5 and 1.4 ± 0.2 GPa, respectively, at 24 °C. These tensile properties are among the highest ever reported on oriented PTFE. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1995–2004, 2001     

A pulsed NMR study of the effects of initial morphology on the structure of extrusion-drawn poly(ethylene terephthalate)

Proton spin–spin relaxation times and the Weibull coefficient have been measured as functions of temperature for poly(ethylene terephthalate) (PET) drawn at 50°C in both the amorphous and the semicrystalline (50%) states. Two relaxation times T_2a (long) and T_2c (short) are observed for all samples. They are ascribed, respectively, to the relaxation of the amorphous and of the crystalline components including highly strained noncrystalline segments. Effects of initial morphology are found for chain mobility in the noncrystalline regions and for the crystal perfection, evaluated from T_2a and the Weibull coefficient μ_c of the T_2c-component, respectively. For all draw ratios, T_2a for extrudates prepared from the semicrystalline polymer (C-50) is short compared to that for preparations from the amorphous (A-50) polymer. In the A-50 samples, the perfection of stress-induced crystals increase with increasing draw ratio. In the C-50 samples, the crystal orientation increases, whereas the perfection decreases with increasing draw ratio. To improve the crystal perfection, annealing at higher temperature or longer time is required for C-50 as compared with A-50. The value of μ_c correlates well with the change in crystal perfection during deformation and annealing.     

Effect of in impurity on crystallization kinetics of (Se.7Te.3)100−xInx system

The effect of In impurity on the crystallization kinetics and the changes taking place in the structure of (Se₇Te₃) have been studied by DTA measurements at different heating rates (α=5 deg·min^?1, 10 deg·min^?1, 15 deg·min^?1 and 20 deg·min^?1). From the heating rate dependence of the values ofT _g,T _c andT _p, the glass transition activation energy (E _t) and the crystallization activation energy (E _c) have been obtained for different compositions of (Se₇Te₃)_100?xIn_x (0≤×≤20). The variation of viscosity as a function of temperature has been evaluated using Vogel-Tamman-Fulcher equation. The crystallization data are analysed using Kissinger's and Matusita's approach for nonisothermic crystallization. It has been found that for samples containing In=0, 10, 15, 20 at%, three dimensional nucleation is predominant whereas for samples containing In=5 at%, two dimensional nucleation is the dominant mechanism. The compositional dependence ofT _g and crystallization kinetics are discussed in terms of the modification of the structure of the Se?Te system.     

Study of formation of nano-quasicrystals and crystallization kinetics of Zr-Al-Ni-Cu metallic glass

The formation of nano-quasicrystals on isothermal annealing of melt-spun ribbons of Zr_69.5Al_7.5Ni₁₁Cu₁₂ metallic glass has been investigated using transmission electron microscopy (TEM). The crystallization study of this metallic glass has been carried out using differential scanning calorimetry (DSC) in non-isothermal (linear heating) mode. It exhibits two-stage crystallization where the first stage corresponds to the precipitation of icosohedral nano-quasicrystalline phase. This has been confirmed with the help of TEM investigations. The crystallization parameters like the activation energy (E _c) and frequency factor (k ₀) have been derived using the Kissinger peak shift analysis. The activation energies for the first and second crystallization peak are found to be 278 and 295 kJ mol^–1, respectively. The frequency factors obtained for the two peaks are respectively 7.16·10¹⁹ and 1.42·10²⁰ s^–1. E _c, k ₀ and the Avrami exponent (n) have also been derived by fitting the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation for the transformed volume fraction (x) to the crystallization data. JMAK results of E _c for the first and second crystallization peak turn out to be 270 and 290 kJ mol^–1 respectively. However, k ₀ and n are found to be heating rate dependent as reported in similar studies. The values of n for the first crystallization stage ranges between 1.66 and 2.57 indicating diffusion-controlled transformation in agreement with earlier reports.     

Activation energy of Se2Ge0.2Sb0.8 chalcogenide glass by differential scanning calorimetry

Results of differential scanning calorimetry (DSC) at different heating rates on Se₂Ge_0.2Sb_0.8 chalcogenide glass are reported and discussed. As the heating rate () changed, also the glass transition temperature (T _g) and onset temperature of crystallization (T _c) changed. As the value of the transition activation energyE _t changed, the crystallization fraction (), heat flow (_q and the crystallization peak temperature (T _p) also changed. The value of the effective activation energy of crystallizationE _c was calculated by means of six different methods. The Se₂Ge_0.2Sb_0.8 chalcogenide glass has two crystallization mechanisms, a one-dimensional and an other surface crystallization growth. The average value ofE _t for Se₂Ge_0.2Sb_0.8 is equal to 194.95±3.9 kJ·mol^–1 and the average value ofE _c is equal to 164±3.3 kJ·mol^–1.     

Physical aging in poly(ethylene naphthalene-2,6-dicarboxylate) in relation to sorbed water,enthalpic relaxation,and mechanical properties

Amorphous poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) films (～ 220 μm thick), stored in ambient atmosphere for different periods of time and after annealing at different temperatures below T_g = 123°C, for different times, have been investigated by infrared spectroscopy (FTIR), microhardness, and differential scanning calorimetry (DSC). FTIR spectroscopy and weight measurements reveal the presence of water which is easily removed by annealing of the films. Films again recover their initial weight and absorption bands after 1-2 days storage in ambient atmosphere. Samples annealed at different temperatures T_a for different times t_a show an increasing microhardness for relatively short times of t_a. The microhardness passes through a maximum at an annealing time depending on T_a, and it decreases toward values somewhat larger than the initial ones. The changes observed in the microhardness and in the values of the excess enthalpy with storage time of the samples at room temperature depend on the physical aging as well as on the content of water of PEN films. © 1995 John Wiley & Sons, Inc.     

Differential Pulse Polarography for a First‐Order EC Process and Its Diagnostic Parameters

Theoretical expressions for differential pulse polarography (DPP) for a reversible electron transfer coupled with an irreversible follow‐up first‐order chemical reaction (E_rC_i) is derived approximately. The peaks as given by the current expressions are analyzed in terms of several parameters such as a ratio of anodic‐to‐cathodic peak‐currents (i_p^a/i_p^c), a separation of peak‐potentials (E_p^c?E_p^a), and a ratio of anodic‐to‐cathodic half‐peak‐widths (W_1/2^a/W_1/2^c) in order to characterize the E_rC_i process and distinguish it from other types of electrode processes. The anodic peak is found to be more susceptible to the post kinetics than the cathodic peak. The new parameter of W_1/2^a/W_1/2^c ratio is much more sensitive to the post kinetics than the peak separation (E_p^c?E_p^a). The peak current ratio (i_p^a/i_p^c) and the peak‐width ratio (W_1/2^a/W_1/2^c) have comparable sensitivities to the kinetics. Hence, W_1/2^a/W_1/2^c ratio is a better diagnostic parameters than (E_p^c?E_p^a) which has a poor sensitivity. This phenomenon is different from cyclic voltammetry (CV) in which E_p^c?E_p^a is as sensitive as i_p^a/i_p^c. The new criteria for EC with DPV is tested and successfully applied to several Co(III) complex systems, including coenzyme B₁₂. The homogeneous rate constant (k) for the follow‐up step is estimated from the measurements of the experimental values of the parameters. The present treatment is valid quantitatively at lower values of k, yielding relatively larger errors for higher k values (k>10 s^?1).     

NMR observations of drawn polymers. V. Sorption into drawn and undrawn polyethylene

NMR measurements on undrawn polyethylene (PE) samples in contact with a solvent such as C₂Cl₄ indicate an increase in the mobility of the mobile chain segments as compared to dry samples. Highly drawn PE shows no such effect. This is because S_a, the sorption per unit mass of noncrystalline material present, decreases from 20.9 wt.-% (dry basis), found for undrawn quenched PE, to 0.63 wt.-% after drawing (S_a determined at 25°C. and 0.80 vapor activity). Drawing also reduces the segment mobility according to the NMR spectrum. It is shown that these effects are caused by considerable structural changes occurring in the noncrystalline regions of PE upon drawing. Annealing of drawn PE samples at successively higher temperatures leads to a gradual relaxation of the noncrystalline regions towards the state characteristic of undrawn PE. With increasing annealing temperature S_a as well as the mobility approach values found with undrawn PE.