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). 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.     

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.     

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     

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     

Changes in superstructure and mechanical properties of poly(ethylene‐2,6‐naphthalate) fibers with critical necking tension

Poly(ethylene‐2,6‐naphthalate) fibers were zone‐drawn under a critical necking tension (σ_c) defined as the minimum tension needed to generate a necking at a given drawing temperature (T_d). In the zone drawing under σ_c, the neck was observed from 110 to 160 °C. The superstructure in a neck zone induced at each T_d was studied. The σ_c value decreased exponentially with increasing T_d and dropped to a low level at a higher T_d. The draw ratio increased rapidly with T_d increasing above 90 °C, but the birefringence and degree of crystallinity decreased gradually. To study the molecular orientation in the neck zone, we measured a dichroic ratio (A_‖/A_?) of a C? O band at 1256 cm^?1 along a drawing direction in the neck zone with a Fourier transform infrared microscope. A_‖/A_? at T_d = 110 °C increased rapidly in the narrow neck zone, and that at T_d = 140 °C increased in the edge of the wide neck zone. Wide‐angle X‐ray diffraction patterns of the fibers obtained at T_d = 130 °C and lower showed three reflections due to an α form, but those at T_d = 140 and 150 °C had reflections due to the α form and a β form. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1629–1637, 2001     

Equibiaxial deformation of poly(4-methyl-1-pentene) by a forging process

Poly(4-methyl-1-pentene) (PMP) has been uniaxially compressed by a forging (equibiaxial) process. The rheology of the process has been examined for this semicrystalline polyolefin, melting point about 235°C. The yield energy, area under the compressive stress-strain curve up to the yield point, as a function of temperature was found to consist of two linear components of different slope. These two linear relations arise from the glassy and crystalline phases of PMP. The intercept temperature (T_i) at zero yield energy for the glassy phase has been evaluated. The attainable maximum compression ratio without sample rupture (CR_max) increased steadily on increasing forging temperature above T_i, and below T_m. In this range, the crystalline relaxation temperature (T_c), evaluated from an Arrhenius plot of yield stress was 160°C. Above T_c, a CR_max of 240 was reached. This value is five times higher than that attained for isotactic polypropylene (i-PP). However, the draw efficiency evaluated by elastic recovery in the plane direction of PMP (0.76) is lower than for i-PP (0.97). Differential scanning calorimetry analyses showed that the melting peak became a complex doublet on increasing compression ratio ( > 100). The drawing and stress-strain behavior of PMP are compared with i-PP. © 1994 John Wiley & Sons, Inc.     

Effects of amorphous poly(vinyl acetate) on crystalline morphology of poly(3-hydroxybutyric acid-co-3-hydroxyvaleric acid)

The amorphous and crystalline phase behavior, spherulite morphology, and interactions between amorphous poly(vinyl acetate) (PVAc) and poly(3-hydroxybutyric acid-co-3-hydroxyvaleric acid) (PHBV) were examined using differential scanning calorimetry, polarized-light optical and scanning electron, atomic-force microscopy (DSC, POM, SEM, AFM), and small-angle X-ray scattering (SAXS). The PHBV/PVAc blend was found to be miscible with an almost linear T _g-composition relationship, indicating perfect homogeneity. Interaction parameter by melting point depression is a negative value of χ = −0.32, suggesting quite favorable interaction strength. With the intimate interaction between the amorphous PVAc and crystalline PHBV polymers, effects of PVAc on the spherulitic morphology of PHBV are quite significant. Owing to the higher T _g of PVAc (than that of PHBV), the spherulite growth rate of PHBV was depressed by increasing PVAc content in blends. Neat PHBV exhibits ring-banded spherulites when crystallized at T_c = 60 ~ 110^° C {T_{\rm{c}}} = {6}0\sim {11}0^\circ {\hbox{C}} ; however, with increasing PVAc content in the blends, the temperature range at which the PHBV/PVAc blends exhibit ring-banded spherulites remains similar but the regularity increases, and the inter-ring spacing significantly decreases. In addition, the spherulite size and ring-band patterns therein are strongly dependent on T _max (190 vs. 220 °C, respectively, for erasing prior nuclei), from which the blends were quenched to a T _c (60–110 °C) for crystallization. For PHBV/PVAc blends crystallized at the same T _c from different T _max, higher T _max tends to erase nuclei, leading to larger spherulites. However, such larger spherulites owing to higher T _max are not necessarily packed with thicker lamellae.     

Cold crystallization and thermal shrinkage of uniaxially drawn poly(ethylene 2,6-naphthalate) by solid-state coextrusion

Solid-state coextrusion has been used to prepare uniaxially drawn films from isotropic poly(ethylene 2,6-naphthalate) (PEN) of a minimum degree of crystallinity (ca. 5%) both below and above its glass transition temperature T_g. The onset of cold crystallization (T_c) of the drawn films has been studied as a function of the extrusion temperature (ET) and the draw ratio (EDR). It has been shown that T_c decreases markedly on draw, as much as 95°C, and, at constant draw ratio T_c goes through a minimum in the T_g region. For undrawn PEN, annealing below 153°C has no significant effect on T_c. To evaluate the crystallization rate constant (k) and the activation energy (E_a) of the drawn specimens, a nonisothermal DSC procedure has been used. With increasing EDR, k increases markedly and E_a goes down over threefold compared with the undrawn polymer. At high ET, strain-induced crystallization has also been shown to play an important role in lowering E_a for cold crystallization. Thermal shrinkage above T_m indicates a high elastic recovery, underlining the efficiency of deformation, ca. 93%, achieved by solid-state coextrusion.     

Isothermal crystallization of concentrated amorphous starch systems measured by modulated differential scanning calorimetry

The slow isothermal crystallization of concentrated amorphous starch systems is measured by Modulated Differential Scanning Calorimetry (MDSC). It can be followed continuously by the evolution (stepwise decrease) of the MDSC heat capacity signal (Cp), as confirmed with data from X-ray diffractometry, Dynamic Mechanical Analysis, Raman spectroscopy, and conventional Differential Scanning Calorimetry. Isothermal MDSC measurements enable a systematic study of the slow crystallization process of a concentrated starch system, such as a pregelatinized waxy corn starch with 24 wt % water and 76 wt % starch. After isothermal crystallization, a broad melting endotherm with a bimodal distribution is observed, starting about 10°C beyond the crystallization temperature. The bulk glass transition temperature (T_g) decreases about 15°C during crystallization. The isothermal crystallization rate goes through a maximum as a function of crystallization time. The maximum rate is characterized by the time at the local extreme in the derivative of Cp (t_max), or by the time to reach half the decrease in Cp (t_1/2). Both t_max and t_1/2 show a bell-shaped curve as a function of crystallization temperature. The temperature of maximum crystallization rate, for the system studied, lies as high as 75°C. This is approximately 65°C above the initial value of T_g. Normalized Cp curves indicate the temperature dependence of the starch crystallization mechanism. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2881–2892, 1999     

The temperature dependence of the viscoelastic softening and terminal dispersions of linear amorphous polymers

Thermorheological simplicity is shown to hold for poly(vinyl acetate) in the temperature range extending from T_g + 25°C to T_g + 80°C. Between T_g and T_g + 25°C the softening (glass to rubberlike) viscoelastic dispersion exhibits time-scale shift factors a_T different from those of the terminal (rubberlike to steady-state) dispersion. The a_T values calculated from zero-shear viscosities coincide with those from the terminal dispersion in the temperature range 60–154°C (T_g ? 35°C). The a_T shifts obtained from the response in the terminal dispersion can be fitted to the Williams, Landel, and Ferry equation over the entire temperature range 42–154°C. The a_T obtained from the softening dispersion is shown to exhibit a different functionality. An empirical modification of the Doolittle equation yields a very flexible relation which can be fitted to some a_Ts which cannot be represented by the usual Doolittle free-volume expression.     

Drawing behavior of linear polyethylene. II. Effect of draw temperature and molecular weight on draw ratio and modulus

The drawing behavior of linear polyethylene homopolymers with weight-average molecular weights (M?_w) from 101,450 to ca. 3,500,000 has been studied over the temperature range 75°C to the melting point. In all cases 1-cm gauge length samples were drawn in an Instron tensile testing machine at a constant cross-head speed of 10 cm/min. With the exception of the lowest molecular weight polymer, it was found that increasing the draw temperature led to substantial increases in the maximum draw ratio which could be achieved, and that this increased monotonically with increasing draw temperature. Measurements of the Young's modulus of the drawn materials showed, however, that the unique relationship between modulus and draw ratio previously established for drawing at 75°C was not maintained to the highest draw temperatures. The highest draw temperature at which this relation held was found to be strongly molecular weight dependent, increasing from ca. 80 to ca. 125°C when M?_w increased from 101,450 to 800,000. In all cases conditions could be found for drawing samples to draw ratios of 20 or more with correspondingly large values of the Young's modulus.     

Daily Maximum Erythemal Dose Rates in the Tropics

The maximum erythemal dose rate (EDR_max) at the Earth's surface tends to occur at local noon. However, clouds can make the timing of EDR_max significantly away from local noon. In fact, EDR_max and its time of occurrence depend mainly on the solar zenith angle at noon (SZA_n), site's altitude, the total ozone column (TOC), cloud cover, cloud genera and aerosols. This work depicts the daily incidence of EDR_max for Belo Horizonte (19.92°S, 43.94°W, 858 m a.s.l.) in the Southern Hemisphere tropics for a period of five years (2005–2010). Daily values of EDR_max ranged from 0.063 W m^?2 (1σ > 6.9%, Moderate UV‐Index of 3, winter) to 0.486 W m^?2 (Extreme UVI of 19, summer). Indicative values of EDR_max for cloudless days were 0.336 W m^?2 (summer, TOC = 258 Dobson Units), 0.311 W m^?2 (fall, 260 DU), 0.253 W m^?2 (spring, 274 DU) and 0.143 W m^?2 (winter, 246 DU). Radiation enhancement events by clouds made EDR_max up to 45% higher than the reference EDR_max for cloudless summer skies at a time resolution of a few minutes. The main cloud genera to be associated with such events are Cumulus, Altocumulus, Altostratus and Stratocumulus. The EDR can also be significantly affected by aerosols, which attenuated on average 0.031 W m^?2 (22%) of the erythemal UV in a case study at the site.     

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     

Orientation of uniaxially stretched poly(ethylene naphthalene 2,6‐dicarboxylate) films by polarized infrared spectroscopy

Poly(ethylene naphthalene‐2,6‐dicarboxylate) has been uniaxially stretched at different draw ratios and at two different temperatures below and above its glass transition (T_g ～ 120 °C) respectively, at 100 and 160 °C. Crystallinity has been evaluated from calorimetric analyses and compared to the values deduced by FTIR spectroscopic data. As expected, the obtained results are quite similar and show that films stretched at lower temperature (100 °C) are more crystalline than those stretched at 160 °C. Optical anisotropy associated with orientation has been evaluated by birefringence and show that films stretched at 100 °C are more birefringent than those stretched at 160 °C as a result of a higher chain relaxation above T_g. Polarized FTIR was also performed to evaluate the individual orientation of amorphous and crystalline phases by calculating dichroic ratios R and orientation functions 〈P₂(cos θ)〉 and also show that amorphous and crystalline phases are more oriented in the case of films stretched below T_g. Nevertheless, the orientation of the amorphous phase is always weaker than that of the crystalline phase. Films stretched at 100 °C show a rapid increase in orientation (and crystallinity) with draw ratio and 〈P₂(cos θ)〉 reaches a limit value when draw ratio becomes higher than 3.5. Films drawn at 160 °C are less oriented and their orientation is increasing progressively with draw ratio without showing a plateau. A careful measurement of the IR absorbance was necessary to evaluate the structural angles of the transition moments to the molecular chain axis. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1950–1958, 2007     

Preparation of oriented β‐form poly(L‐lactic acid) by solid‐state extrusion

Melt‐crystallized, low molecular weight poly(L ‐lactic acid) (PLLA) consisting of α crystals was uniaxially drawn by solid‐state extrusion at an extrusion temperature (T_ext) of 130–170 °C. A series of extrusion‐drawn samples were prepared at an optimum T_ext value of 170 °C, slightly below the melting temperature (T_m) of α crystals (～180 °C). The drawn products were characterized by deformation flow profiles, differential scanning calorimetry (DSC) melting thermograms, wide‐angle X‐ray scattering (WAXD), and small‐angle X‐ray scattering as a function of the extrusion draw ratio (EDR). The deformation mode in the solid‐state extrusion of semicrystalline PLLA was more variable and complex than that in the extensional deformation expected in tensile drawing, which generally gave a mixture of α and β crystals. The deformation profile was extensional at a low EDR and transformed to a parabolic shear pattern at a higher EDR. At a given EDR, the central portion of an extrudate showed extensional deformation and the shear component became progressively more significant, moving from the center to the surface region. The WAXD intensities of the (0010)_α and (003)_β reflections on the meridian as well as the DSC melting thermograms showed that the crystal transformation from the initial α form to the oriented β form proceeded rapidly with increasing EDR at an EDR greater than 4. Furthermore, WAXD showed that the crystal transformation proceeded slightly more rapidly at the sheath region than at the core region. This fact, combined with the deformation profiles (shear at the sheath and extensional at the core), indicated that the crystal transformation was promoted by shear deformation under a high pressure rather than by extensional deformation. Thus, a highly oriented rod consisting of only β crystals was obtained by solid‐state extrusion of melt‐crystallized, low molecular weight PLLA slightly below T_m. The structure and properties of the α‐ and β‐form crystals were also studied. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 40: 95–104, 2002     

Poole-Frenkel effect in amorphous poly(p-phenylene sulfide)

The conductivity of poly(p-phenylene sulfide) (PPS) amorphous samples sandwiched between metallic electrodes has been studied as a function of applied voltage, temperature, and electrode material. The voltage (U) dependence of the currents for electric fields within the range 10³–10⁶ V/cm exhibits exp βU^1/2 behavior with β = β_Schottky below the glass transition temperature (T_g ≊ 90°C), and β = β_{Poole-Frenkel} above T_g. Coordinated temperature measurements of dc currents with different metallic contacts and thermally stimulated currents (TSC) indicate, however, that the conductivity at T < T_g is consistent with the so-called “anomalous” Poole-Frenkel effect rather than the Schottky effect. Consequently, the p-type conductivity in amorphous PPS is proposed to be a bulk-limited process due to ionization of two different types of acceptor centers in the presence of neutral hole traps. © 1996 John Wiley & Sons, Inc.     

Studies of the amorphous region of polymers. II. Relationship between change of structure and glass-transition temperature in polycarbonate

A study of the effect of drawing on the glass transition temperature T_g of amorphous polycarbonate was carried out. The T_g attains a maximum at a draw ratio in the range from 1.6 to 2.0. The relationship between the change of structure and T_g is discussed in terms of the configurational entropy and the rate of molecular motion in local mode relaxation. The variation of T_g with drawing may be explained by using the iso-entropy theory. The frequency of the maximum in the dielectric loss f_max and the dielectric relaxation strength Δε of the β mechanism changed with drawing in the same way as that of T_g. Based on the parallel behavior in these results, it is reasonable to consider that the β process of local mode relaxation of polyesters such as PC and PET reflects the structure of the glassy state.     

Thermal analysis and microscopical characterization of cholesterol in gallstones

Summary Cholesterol constitutes the major component of most gallstones. It was identified and determined in gallstones by thermal analysis technique (DSC and TG-DTA), mainly by the use of the melting temperature (T_onset=145°C and T_max=149°C) and by DTG peak decomposition (T_max=364°C). Cholesterol anhydrous (ChA), which showed endothermic polymorphic peak, T_max=40°C, without mass loss, was differentiated from cholesterol monohydrate (ChH), which showed a broad endothermic peak, T_max=59°C, attributed to loss of water of crystallization (theoretical 4.45%). Morphological studies of gallstones were performed by optical microscopy and scanning electron microscopy (SEM). The stones consisted of a pigmented core with a variably-sized irregular central cavity, surrounded by a radially arranged deposits of plate-like ChH. The outer part of the stones showed ChA crystal arborescences. X-ray microanalysis gave a typical spectrum rich in C and O, and in some instances the presence of P, which was attributed to the presence of phospholipids. CaCO₃ was easily characterized by TG with the use of DTG decomposition peak at 674°C.     

Ultimate tensile properties of elastomers. VII. Effect of crosslink density on time–temperature dependence

Data on tensile strength and elongation at break for a series of Viton A-HV vulcanizates are discussed. The data were obtained at various extension rates at temperatures from ?5 to 230°C (25 ? T — T_g ? 260°C) on seven vulcanizates having crosslink densities v_e (estimated from C₁ in the Mooney-Rivlin equation) from 0.46 × 10^?5 to 24.4 × 10^?5 mole/cm³. At an extension rate of 1 min^?1, an increase in v_e affects the tensile strength σ_b (based on the undeformed cross-sectional area) and the true tensile strength σ_bσ_b (based on the cross-sectional area of a deformed specimen) as follows: σ_b is essentially constant at a low temperature; it passes through a decided maximum at intermediate temperatures; and it increases to a plateau at elevated temperatures. In contrast, λ_bσ_b decreases markedly at all temperatures, an exception being the most lightly crosslinked vulcanizate(s). Application of time—temperature superposition to the ultimate-property data gave log a_T; its temperature dependence is that typical of nonpolar rubbery polymers. Data on the vulcanizates were compared in corresponding temperature states by plotting log 273σ_b/T, log 273λ_bσ_b/T, and (λ_b — 1)/(λ_b — 1)_max against logt_b/(t_b)_max, where t_b is the temperature-reduced time to break and (t_b)_max is the value at which the ultimate extension ratio λ_b attains its maximum, (λ_b)_max. Except for the most lightly crosslink vulcanizate, the comparison shows that 273λ_bσ_b/T and (λ_b — 1)/(λ_b — 1)_max are substantially independent of (or only weakly dependent on) crosslink density, that 273λ_b/T increases with v_e, and that 273λ_b/T ∝? v_e^0.6 and λ_b ∝? v_e^?0.4 at a large value of t_b/(t_b)_max.