High field conduction in poly(p-phenylene vinylene)

We have shown that in polyacetylene, for electric fields above 10⁶ V/cm polarons are dissociated because the electron or hole can move much faster than the lattice distortion. Similar results should hold for poly(p-phenylene vinylene), PPV. To determine whether the free carriers thus created can be heated by the field sufficiently to allow impact ionization, we investigated free-carrier scattering in PPV. The important scatterers are longitudinal acoustic modes, molecular vibrations, and librational torsional modes. The coupling constants and important frequencies are known approximately for the first two of these processes in PPV. Using these values, some reasonable values for librons, and the calculated band structure of PPV, we conclude that at least some of the carriers could be heated to 2 eV at a field of ∼10⁶ V/cm, and that they could acquire energy required for impact ionization at not much higher fields.     

Thermal oxidation of poly(1-pentene): 1. Indentification of products and mechanisms

Isotactic poly(1-pentene) was degraded in the presence of pure oxygen at 115°C. The functional groups present in the non-volatile products were identified using infra-red spectroscopy. The volatile products formed were identified by means of a relatively new technique which combines thermal and mass chromatography. In this study, fourteen volatile products were detected and identified and their relative abundance estimated. The results obtained could readily be reproduced. Various oxidation mechanisms for these products are postulated.     

Synthesis and characterization of poly(1-methyl-1-silabutane), poly(1-phenyl-1-silabutane) and poly(1-silabutane)

Summary Anionic ring opening polymerization of 1-methyl-1-silacyclobutane, 1-phenyl-1-silacyclobutane and 1-silacyclobutane co-catalyzed by n-butyllithium and hexamethylphosphoramide (HMPA) in THF at-78°C yields poly(1-methyl-1-silabutane), poly(1-phenyl-1-silabutane) and poly(1-silabutane) respectively. These saturated carbosilane polymers possess reactive Si-H bonds. They have been characterized by ¹H, ¹³C and ²⁹Si NMR as well as FT-IR and UV spectroscopy. Their molecular weight distributions have been determined by gel permeation chromatography (GPC), thermal stabilities by thermogravimetric analysis (TGA) and glass transition temperatures (T_g) by differential scanning calorimetry (DSC).     

A mechanism of ionic conduction of poly (vinylidene fluoride)-lithium perchlorate hybrid films

Polymeric solid electrolytes were prepared by the hybridisation of poly(vinylidene fluoride) and lithium perchlorate. These were obtained as films of about 0.1 mm thickness, and showed high lithium ionic conductivity of about 10^?5 (S cm^?1). The conductivity depended on the contents of lithium perchlorate and polar additives having high boiling temperature. The amount of lithium perchlorate and the logarithm of the conductivity showed linear relation up to a certain (critical) amount of lithium perchlorate. Beyond the critical value, crystals of lithium perchlorate grew in polymer matrix, and the conductivity was not increased so much. Viscosity and dielectric constant of the additives were potent factors to increase the conductivity of the hybrid film. Organic polar materials with lower viscosity such as N,N-dimethyl formamide or γ-butyrolactone strongly contributed to the improvement of lithium ionic conductivity. The activation energy of conduction decreased dramatically by increasing mole ratio of additive and lithium perchlorate. The lithium ions were considered to migrate in the conduction path which was formed by polymer matrix with organic additive molecules.     

Electrical conduction in poly(ethylene-2,6-naphthalate) films of varying crystallinity

Electrical conductivity in poly(ethylene-2,6-naphthalate) (PEN) film was investigated. Electronic conduction was found to exist at temperatures below 333 K and an increase in the crystallinity was found to enhance the photocurrent of PEN. However, a further increase in crystallinity which was followed by the formation of excimer sites caused a decrease in the photocurrent.     

Investigations of the electrical conduction mechanisms of polyaniline‐DBSA/poly(acrylonitrile‐butadiene styrene) blends

The electrical properties of Al/PANI‐DBSA/ABS/Au blend with PANI (5%) w/w have been investigated by using of current‐voltage (I‐V) measurements, in a temperature range of 100–313 K. The analysis of I‐V characteristics in the forward direction was based on thermionic emission mechanism for applied electrical field till ～3 × 10² V/cm. The thickness dependence of the current‐voltage relationship, clearly demonstrates that the electrical current for larger fields is space charge limited current (SCLC). Temperature dependences of the ideality factor, barrier height, and series resistance have been calculated. The mobility of carriers which is temperature dependent was calculated using the trap free SCLC as 1.53 × 10^?4 cm² V^?1 s^?1 at room temperature. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40688.     

Effects of pressure on poly(ether-ether-ketone) (PEEK) sintering mechanisms

This study deals with the understanding of the sintering mechanisms that occur during consolidation of an ultra-high-performance polymer: poly(ether-ether-ketone). In particular, we investigated the effects of uniaxial pressure during spark plasma sintering (SPS) processing. Glass-transition temperature (T_g) measurements under loading, stress–strain curves and scanning electron microscopy analysis allowed us to determine the role of pressure intensity and temperature of application on macromolecular chain mobility in both the bulk and the surface of the particles. First, a loss of chain mobility in the bulk of the particles was observed under high pressure. Second, it was shown that high pressure applied at low temperature leads to friction effects between the particles which enhances chain mobility in the particle shell through a local melting phenomenon. These experimental conditions favor the healing of particles and high cohesion was then observed in the resulting sintered samples. The level of friction is enhanced when pressure is applied at a low temperature since the powders are still in a glassy state. Finally, the achievement of cohesive grain boundaries was found to be related to the location and conformation of chain ends. Good healing between particles can only occur if a sufficient number of chain ends are available at the surface of the particle shell. We showed that the native powder state plays a significant role. The direct use of as-received powder leads to final material exhibiting good cohesion whereas pretreatments of the native powder are highly detrimental. It should be noted that this processing does not affect the high initial degree of crystallinity because no bulk melting is observed during consolidation by sintering. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47645.     

Infra-red study of poly(1-pentene) and poly(4-methyl-1-pentene)

The infra-red (i.r.) spectra of the two polymorphous forms (modification 1 and 2) of isotactic poly(1-pentene) are reported. Several absorption bands characteristic of the two modifications have been detected and attributed to the helical conformation of the polymer chains. The differences between the two modifications are reproducible, thus enabling their identification from i.r. spectroscopy. The infra-red spectra of isotactic poly(4-methyl-1-pentene) is also reported, and compared with that of the atactic polymer. General assignment of absorption bands is presented.     

Identification of charge carriers in the conduction mechanism of an alternated copolymer of poly(aniline) and poly(phenylene sulfide)

In this paper, we have investigated the electrochemical behavior of a soluble copolymer of poly(aniline) (PANI) and poly(phenylene sulfide) in organic media. By using ‘in situ’ UV–vis and Raman spectroscopies, it was proved that during the oxidation of the first cycle, polarons and bipolarons are formed consecutively, due to the loss of electrons from the nitrogen and sulfur, respectively. In addition, it was verified that the formation of polarons is reversible while the formation of bipolarons is irreversible. In the second and subsequent cycles, only one reversible redox process is observed. This process corresponds to the transformation of polarons to bipolarons and vice versa. The ‘in situ’ resistance measurements have indicated that bipolarons are the charge carriers for doped PPSA, distinctly than it was observed for PANI.     

Degradation mechanisms of poly(ethylene terephthalate) tire yarn

In order to clarify the fatigue mechanism of PET tire cords in rubber focusing on chemical degradation, hydrolysis in saturated steam atmosphere, ammonolysis in ammonia gas, and the degradation of the cords embedded in rubber have been studied at high temperature conditions. It was found that the strength loss of polyester tire cords in rubber has linear correlation with the amount of chemically induced chain scissions mainly caused by hydrolysis catalyzed by amines.     

Synthesis mechanisms of poly(oxybenzoate) liquid crystal polymers

Summary Evidence is presented for the growth of bilayered lamellae during the polymerization ofp-acetoxybenzoic acid to form uniform thickness poly(p-oxybenzoate) lamellar crystals. Fracturing one preparation of these crystals lead to the formation of fibers connecting the fracture faces, reminiscent of those formed during fracture of folded chain flexible polymer single crystals. The results raise questions concerning previously proposed polymerization — crystal growth mechanisms.     

DC electrical conduction in solution-grown pure and iodine-doped poly(butylene terephthalate) films

DC electrical conduction in poly(butylene terephthalate) films sandwiched between planar aluminium electrodes was investigated. The films were grown by the isothermal immersion technique. The conduction process was studied in the temperature range 300–450K, with an applied electric field of 0.22 × 10⁷?6.66 × 10⁷ Vm^?1. It was found that the Richardson-Schottky type of conduction mechanism was dominant throughout the temperature range of study. The values of the Schottky field lowering constant β, the effective metalinsulator potential barrier and the activation energy were evaluated. The value of the activation energy (U = 0.7 eV) indicates the predominance of electronic conduction. The effect of impurities like iodine on electrical conduction was also investigated.     

Influence of DGEBA crosslinking on Li ion conduction in poly(ethyleneimine) gels

We characterize the DC conductivity (σ₀) of solution electrolytes prepared by adding LiCF₃SO₃ (LiTf) salt to a (50/50 w/w) solution of branched poly(ethyleneimine) (PEI) in N,N-dimethylformamide (DMF). The value of σ₀ increases with decreasing LiTf concentration over the range of compositions studied due to the formation of contact ion pairs at higher LiTf concentrations, with the highest value of σ₀ exceeding 10⁻³ S cm⁻¹ at 20 °C. Rubber-like gel electrolytes are prepared by epoxide-amine crosslinking of selected solutions by addition of diglycidyl ether of bisphenol A (DGEBA). Holding the [N]:[Li] mole ratio fixed, increasing the crosslink density dramatically decreases σ₀ at all temperatures studied. The decrease in σ₀ cannot be attributed to an increase in the glass transition temperature, as little variation in T_g is noted amongst the samples due to their high solvent content. Rather, we propose that the decrease in conductivity is due to loss of fast segmental motions associated with chain ends, which become tethered to the network upon crosslinking.     

Aging of poly(lactide)/poly(ethylene glycol) blends. Part 1. Poly(lactide) with low stereoregularity

Poly(lactide) (PLA) is rapidly gaining interest as a biodegradable thermoplastic for general usage in degradable disposables. To improve mechanical properties, a PLA with low stereoregularity was blended with polyethylene glycol (PEG). Blends with up to 30 wt% PEG were miscible at ambient temperature. Blending with PEG significantly decreased the T_g, decreased the modulus and increased the fracture strain of PLA. However, the PLA/PEG 70/30 blend became increasingly rigid over time at ambient conditions. The mechanism of aging primarily under ambient conditions of temperature and humidity was studied. Changes in mechanical properties, thermal transitions and solid state morphology were examined over time. Aging was caused by slow crystallization of PEG. Crystallization of PEG depleted the amorphous phase of PEG and gradually increased the T_g. As T_g approached the aging temperature, reduced molecular diffusivity slowed the crystallization rate dramatically. Aging essentially ceased when T_g of the amorphous phase reached the aging temperature. The increase in matrix T_g and the reinforcing effect of the crystals produced a change in mechanical properties from elastomer-like to thermoplastic-like.     

Multiple transitions in poly(allylbenzene): 1. Thermophysical properties

Polyallylbenzene is a semicrystalline material the thermal behaviour and physical properties of which reveal several transitions. Thermal and differential calorimetric analysis have shown the influence of the history of the material about the glass transition and melting. Several techniques such as thermomechanical analysis, dilatometry and inverse gas chromatography give additional information for the unidimensional and volume behaviour of the polymer versus temperature.     

Delamination failure mechanisms in microlayers of polycarbonate and poly(styrene-co-acrylonitrile)

The peel strength and delamination failure mode of coextruded microlayer sheets consisting of alternating layers of polycarbonate (PC) and poly(styrene-co-acrylonitrile) (SAN) were studied with the T-peel test. Four delamination modes were observed: two modes where the crack propagated along the PC–SAN interface and two other modes where the crack propagated through crazes in the SAN. The SAN layer thickness determined whether crack propagation was interfacial or through crazes. Crazing and crack propagation through crazes were observed only if the SAN layer was thicker than 1.5 μm. As the thickness of the SAN layer increased, the amount of crazing in front of the crack tip and the amount of craze fracture gradually increased; the peel strength increased accordingly. If the SAN layers were thinner than 1.5 μm and the PC layers were relatively thick, the crack propagated along a single interface. The peel strength for this delamination mode was the lowest and equal to about 90 J/m², independent of layer thicknesses. This delamination mode came closest to providing a ”real” measure of the adhesive toughness of PC to SAN. With both interfacial and craze delamination, the crack could move from layer to layer if the PC was thin enough. Tearing of the relatively thin PC layers increased the peel strength of the multiple-layer delamination modes. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68:793–805, 1998     

Kinetics and mechanisms of fatigue damage in rubber-toughened poly(methyl methacrylate) (RTPMMA)

Specimens of RTPMMA were subjected to fully reversed tension–compression cycling under load control, at a frequency of 0.5 Hz and an ambient temperature of 21°C. Modulus, hysteresis, and temperature of the polymer were monitored continuously. Significant changes were observed at stress amplitudes greater than half the short-term yield stress. Temperature rises of between 5°C and 20°C account for the initial increases in compliance and hysteresis, which subsequently continue to increase, at approximately constant temperature. It is concluded that shear yielding is responsible for this second stage of the softening process. The rate of shear yielding increases exponentially with stress amplitude.     

Miscibility of poly(1-vinylimidazole)/ poly(p-vinylphenol) blends

Summary Blends of poly(1-vinylimidazole) (PVI) and poly(p-vinylphenol) (PVPh) were cast from ethanol or N,N-dimethylformamide (DMF). All the blends are miscible based on the single glass transition temperature (T_g) criterion. The T_g values of the blends are higher than those calculated from linear additivity rule. Blends cast from ethanol show a larger positive deviation than those cast from DMF. The T_g-composition curves can be fitted by the Kwei equation. FTIR studies show the existence of a strong hydrogen-bonding interaction between PVI and PVPh.     

D.s.c. investigation of the states of water in poly(vinyl alcohol) membranes

D.s.c. analysis shows that heat absorbed in the heating process of water in poly(vinyl alcohol) membranes is higher than that evolved in the cooling process. A remarkable discrepancy is found between these heats when the water retention, H, of the membrane is lower than 0.5 g H₂O/g swollen membrane. The amounts of freezable water in the membranes were estimated by use of the relation observed between the temperature and the enthalpy of the crystallization of super-cooled water. The amounts estimated from the heats absorbed or evolved agree within 2% when H was above 0.6. When H is below 0.5, however, the ratio of these two amounts was greater than 2. This difference is explained by the heat capacity of the water below 250 K increasing anomalously compared with that of bulk water.