Isothermal crystallization of P3HT:PCBM blends studied by RHC

Defining appropriate annealing temperatures and times is vitally important for increasing the efficiency of bulk heterojunction solar cells by favoring the crystallinity of the polymer-fullerene blend components. In order to better understand the annealing process, the isothermal crystallization of poly(3-hexyl thiophene) (P3HT) and [6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM) blend investigated by means of rapid heating cooling calorimetry (RHC). Isothermal crystallization experiments at temperatures in between the glass transition and melting, within the temperature range of 70–150 °C, can successfully be performed since RHC permits cooling at a sufficiently high rate in order to prevent crystallization during cooling. Crystallization isotherms were determined from the subsequent melting behavior of the blend. They were measured for a wide set of annealing temperatures and times, and the evolution of the crystallization rate with temperature is compared for annealing from the glassy state and from the melt state.     

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     

Infrared dichroism of glassy polycarbonate at small strains

The dichroism of the 889, 1364, and 3063 cm^?1 infrared absorption bands of glassy, amorphous polycarbonate has been measured as a function of the strain in the range 0 to 2% at 23°C. The data obtained for these three bands superpose rather well over this strain range. Negligible dichroism is observed up to about 0.6% strain; above this level, the dichroic ratio increases in an approximately linear manner. Independent mechanical data, obtained under comparable conditions of time-scale and temperature, are cited which show that a transition from approximately linear to marked non-linear viscoelastic behavior occurs with glassy polycarbonate in the range 0.7 to 1.0% strain. The coincidence on the strain axis of a relatively abrupt increase in optical absorption anisotropy with a distinct change in Young's modulus is discussed in terms of a recent molecular theory of deformation of glassy polymers. It is suggested that the data are consistent with the view that the transition from linear to nonlinear viscoelastic behavior in glassy polycarbonate is marked by the onset of significant rotation around backbone bonds.     

Molecular orientation of aromatic polycarbonates containing fluorene side chains by polarized infrared spectroscopy and birefringence analysis

The molecular orientation of an aromatic polycarbonate containing fluorene side chains was investigated by polarized infrared spectroscopy and birefringence analyses. The copolymers were synthesized from 2,2‐bis(4‐hydroxyphenyl)propane (BPA), 9,9‐bis(4‐hydroxy‐3‐methylpheny)fluorene (BMPF), and phosgene by interfacial polycondensation. The 1449‐cm^?1 band of the uniaxially oriented films, stretched at the glass‐transition temperature (T_g) plus 5 °C, was assigned to various combinations of CC stretching and CH in‐plane bending vibrations in the fluorene ring, and the transition moment angle was estimated to be 90°. The intrinsic birefringence of aromatic polycarbonate films with BMPF molar ratios ranging from 0.5 to 1 was obtained with the 1449‐cm^?1 band. The copolymer was estimated to show zero intrinsic birefringence at the BMPF molar ratio of 0.75, and the BMPF homopolymer showed negative intrinsic birefringence. A linear relationship between the volume fraction of BMPF units and the intrinsic birefringence indicated that the two monomer units of BPA and BMPF in each copolymer were not independent, and an intrinsic birefringence could be defined even in the copolymer. The sign of the photoelastic coefficient in the homopolymer with BMPF units was positive. The different signs of the photoelastic coefficient and the intrinsic birefringence suggest that the fluorene side‐chain orientation induced by stress in the glass state is quite different from the orientation of the uniaxially oriented films stretched at T_g + 5 °C. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1554–1562, 2003     

A comparison of spin relaxation and local motion between symmetrically and asymmetrically ring-substituted bisphenol units in dissolved polycarbonates

Carbon-13 and proton spin-lattice relaxation times were measured at two field strengths on solutions 10% by weight of two polycarbonates in C₂D₂Cl₄ from ?20 to +120°C. The first polycarbonate is an asymmetrically substituted form with one chlorine on one of the two phenylene aromatic rings of the bisphenol unit, whereas the second polycarbonate is symmetrically substituted with two chlorines on each of the two rings. The nuclear spin relaxation data are interpreted in terms of several local motions likely in these polymers. Segmental motion was described by the Hall–Helfand correlation function. Segmental motion in the monosubstituted polycarbonate is somewhat slower than in unsubstituted polycarbonate, whereas segmental motion in the tetrasubstituted polycarbonate is considerably slower. Phenylene ring rotation is observed in unsubstituted polycarbonate and in the monosubstituted polycarbonate above 40°C. Below 40°C in the monosubstituted species, and at all temperatures in the tetrasubstituted species, ring rotation is replaced by ring libration as the predominant motion contributing to spin lattice relaxation. In addition, the rotational motion of the two types of rings in the asymmetric monosubstituted form are very similar although not identical. The substituted ring is slightly less mobile than the rings of unsubstituted polycarbonate. This indicates a strong coupling of ring motion, although the coupling leads to less than synchronous motion. Methyl group rotation is present in both polymers and is little affected by the various structural modifications.     

Annealing effects on the solid state properties of transition-metal coordination complexes and networks based on diene polymers with palladium chloride

This study focuses on the thermal and mechanical properties of 1,2-polybutadiene and 3,4-polyisoprene with an inorganic salt, bis(acetonitrile)dichloropalladium (II). Upon mixing in THF, effective crosslinks are formed because the acetonitrile ligands of the palladium salt are displaced by olefinic pendant groups of the polymers. Using a simple nth-order irreversible kinetic rate model, the palladium-catalyzed Heck-like exothermic reaction in solid films was characterized via isothermal and nonisothermal DSC. Thermal energy and mass balances appropriate to a batch reactor are developed from first principles and applied to the isothermal DSC output curve to calculate the time dependence of reactant conversion. Relevant kinetic parameters, such as the order of the reaction, the characteristic time constant for the chemical reaction, and the activation energy, have been determined. The kinetic data suggest that the palladium-catalyzed crosslinking reactions are diffusion controlled in the solid state because the reaction order is very close to unity. Higher glass transition temperatures (T_g) are measured by dynamic mechanical thermal analysis (DMTA) and differential scanning calorimetry (DSC) when (i) palladium concentration, (ii) annealing (heat treatment) time, and (iii) annealing temperature increase. After 2 h of annealing at 80°C, which corresponds to a temperature below the first exothermic crosslinking reaction (≅ 115°C) during nonisothermal DSC kinetic studies, rubbery materials containing very low concentrations of PdCl₂ (i.e., 0.5 mol %) exhibit reinforced ductile stress-strain response. When annealing is performed at the peak temperature of the first exothermic event, the rubbery materials are transformed into glasses. Transition-metal compatibilization of atactic 1,2-polybutadiene and 3,4-polyisoprene via PdCl₂ is demonstrated by monitoring the glass transition obtained from dynamic mechanical tan δ profiles. The effect of annealing this ternary reactive “blend” produces a glassy material exhibiting an elevated T_g and synergistic mechanical properties. © 1996 John Wiley & Sons, Inc.     

The influence of annealing on thermal transitions in a nematic copolyester

Thermal transitions of a glassy, main chain, liquid crystalline, random copolyester, HIQ‐40, have been characterized. HIQ‐40 is made from 40 mol percent p‐hydroxybenzoic acid (HBA) and 30 mol % each of p‐hydroquinone (HQ) and isophthalic acid (IA). This polymer is soluble in organic solvents, permitting the preparation of thin, solution‐cast films that are in a glassy, metastable, optically isotropic state. On first heating of an isotropic HIQ‐40 film in a calorimeter, one glass transition is observed at low temperature (approximately 42°C), and is ascribed to the glass/rubber transition of the isotropic polymer. A cold crystallization exotherm centered near 150°C is observed. This is associated with the development of low levels of crystalline order. A broad melting endotherm is centered at about 310°C; this endotherm marks the melting of crystallites and the transformation to a nematic fluid. A nematic to isotropic transition was not observed by calorimetry. After quenching from the nematic melt, a T_g is observed in the range of 110–115°C and is associated with the glass/rubber transition of the nematically ordered polymer. Annealing optically isotropic films at temperatures above the isotropic glass transition results in the systematic development of axial order. In these annealed samples, T_g increases rapidly until it is near the annealing temperature, then T_g increases more slowly at longer annealing times. In as‐cast films annealed at 120–135°C, the light intensity transmitted through a sample held between crossed polarizers in an optical microscope (a qualitative measure of birefringence and, in turn, axial order) initially increases rapidly and uniformly throughout the sample and, at longer annealing times, approaches asymptotic values that are higher at higher annealing temperatures. The increase in transmitted intensity is ascribed to the development of axial order. The uniform increase in transmitted intensity suggests that ordering occurs by a rather global process and not via a nucleation and growth mechanism. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 505–522, 1999     

Crystallization kinetics of a thermotropic liquid crystalline polyimide derived from 1,3-bis[4-(4′-aminophenoxy) cumyl] benzene

We have studied the nonisothermal and isothermal crystallization kinetics of an aromatic thermotropic liquid crystalline polyimide synthesized from 1,2,4,5-benzenetetracarboxylic dianhydride (PMDA) and 1,3-bis[4-(4′-aminophenoxy) cumyl] benzene (BACB) by means of differential scanning calorimetry (DSC). Polarized light microscopy (PLM) and wide-angle X-ray diffraction (WAXD) results confirm that this polyimide exhibits a smectic texture. Nonisothermal crystallization showed two strong and one weak exothermic peaks during cooling. The phase transition from isotropic melt to liquid crystalline state is extremely fast which completes in several seconds. The mesophase transition has a small Avrami parameter, n, of approximate 1. The isothermal crystallization of 253–258°C has been examined. The average value n is about 2.6 and the temperature-dependent rate constant k changes about two orders of magnitude in the crystallization temperature range of 6°C. The slope of ln k versus 1/(T_cΔT) is calculated to be −2.4 × 10⁵, which suggests nucleation control, via primary and/or secondary nucleation for the crystallization process. During the annealing process, a new phase (slow transition) is induced, which grows gradually with annealing time. At lower annealing temperatures (220–230°C), the slow transition process seems not to be influenced by the crystals formed during cooling process and its Avrami parameter n is ca. 0.3–0.4. However, the slow transition was hindered by the crystals formed during cooling process when annealed at higher temperature (230–240°C). © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1679–1694, 1998     

Preparation and phase transition characterization of VO2 thin film on single crystal Si (100) substrate by sol–gel process

Vanadium dioxide (VO₂) thin films were fabricated on single crystal Si (100) substrates by sol–gel method, including a process of annealing a vanadium pentoxide (V₂O₅) gel precursor at different temperatures. The crystalline structure and morphology of the films were investigated by XRD, FE-SEM and AFM, indicating that the films underwent the grain growth, agglomeration and grain refinement process with increased annealing temperatures. The film annealed at 500 °C exhibits the formation of VO₂ phase with a strong (011) preferred orientation and high crystallinity, the surface of the film is uniform and compact with a grain size of about 120 nm. Meanwhile, the film exhibits excellent phase transition properties, with a decrease of transmittance from 35.5 to 2.5% at λ = 25 μm and more than 3 orders of resistivity magnitude variation bellow and above the phase transition temperature. The phase transition temperature is evaluated at 60.4 °C in the heating transition and 55.8 °C in the cooling transition. Furthermore, the phase transition property of the VO₂ film appears to be able to remain stable over repetitive cycles 100 times.     

Dielectric absorption in oriented poly(vinylidene fluoride)

The dielectric behavior of poly(vinylidene fluoride) is affected by orientation and crystal modification. The loss peak caused by molecular motion of the molecules in crystalline regions appears at about 70°C (110 Hz) (α₁ absorption) for the α form, and at about 110°C (110 Hz) (α₂ absorption) for the β form. Orientation significantly affects the magnitude of the β absorption which appears at about ?40°C. The very high value of the dielectric constant for stretched film is believed to be due to the orientation effect. The γ absorption, which is assumed to be local-mode absorption, is not so much affected by orientation. An additional loss peak has been found at around 0°C in dynamic mechanical measurements, but the molecular mechanism is unknown.     

The effect of synthesis conditions on the structure of compounds formed in the Dy<Subscript>2</Subscript>O<Subscript>3</Subscript>–TiO<Subscript>2</Subscript> system

Crystallization and phase transition processes taking place in the Dy₂O₃–TiO₂ system during the isothermal annealing of the precursors synthesized by co-precipitation were studied. The phase composition of the obtained powders is determined by not only the chemical composition of the precursor (Dy₂O₃: TiO₂ ratio) and annealing temperature but also by the procedure of precursor synthesis determining the uniformity of Dy and Ti distribution in the precursor precipitate. Higher homogeneity of precursor particles provides the formation of almost single-phase nanocrystaline dysprosium titanate (Dy₂TiO₅, Dy₂Ti₂O₇) powders at annealing temperatures of 800–1000°C.     

Thermally stimulated discharge current studies on the effect of thermal treatment on the strength of polycarbonate

It is well known that polycarbonate annealed at 80–130°C undergoes gradual changes in mechanical properties. Annealing below T_g (ca. 150°C) results in a decrease in impact resistance and an increase in strength. Polycarbonate has three single relaxation processes and some distributed relaxation processes in the temperature range between 100 and 250°K (the β transition region). The effect of thermal pretreatment on the relaxation has been investigated by the thermally stimulated discharge current technique. Partial heating, peak cleaning, and theoretical fitting have also been performed and the activation parameters associated with the relaxation processes have also been calculated to assist in the analysis of the relationship between effects of annealing and structural motions in polycarbonate.     

Rapid mechanical deformation of poly(ethylene terephthalate) fibers at temperatures above the glass transition

The mechanical properties of poly(ethylene terephthalate) (PET) fibers at temperatures above the glass transition are investigated by means of a specially constructed device. Measurements of the deformation rate and of the “dynamic” stress-strain curves of the fibers are performed in nearly isothermal regime (after initial rapid heating) in a temperature interval 100–200°C. The results reported in the present work demonstrate that the high-temperature mechanical characteristics of rapidly crystallizing polymers can be deduced to a satisfactory precision, while keeping the crystallinity development at low level. Our investigations indicate that if the high-temperature deformation is sufficiently fast, the polymer behavior is similar to the deformation at sub-T_g temperatures. Based on this similarity, a qualitative model of the deformation in the high-temperature region is proposed. The proposed model is fundamentally equivalent to the models describing mechanical deformation of glassy polymers at temperatures below the glass transition. ©1995 John Wiley & Sons, Inc.     

Kinetic and equilibrium phenomena in the system: Acetone vapor and polycarbonate film

Amorphous films of Lexan polycarbonate have been exposed to acetone vapor at controlled temperatures and partial pressures in order to study sorption kinetics and thermodynamics and polymer crystallization behavior. Sorption isotherms show a discontinuity is slope at or near the depressed glass transition, which itself was identified by torsion pendulum measurements. Crystallization abruptly begins to occur at partial pressures equal to or slightly above that of the solubility transition and is manifested by delayed desorption and whitening phenomena. In this process 20% crystallinity is usually developed, as measured by calorimetry which, however, produces a 40% drop in acetone solubility. Although the depressed glass temperature is near 0°C. in saturated atmospheres—a drop of 145°C.—the melting point is only depressed 60 or 70°C. Such disparity probably accounts for the enhanced polycarbonate crystallization rate in acetone over that in the dry bulk polymer above the normal T_g.     

Swelling and sorption in polymer–CO2 mixtures at elevated pressures

Experimental data on gas sorption and polymer swelling in glassy polymer—gas systems at elevated pressures are presented for CO₂ with polycarbonate, poly(methyl methacrylate), and polystyrene over a range of temperatures from 33 to 65°C and pressures up to 100 atm. The swelling and sorption behavior were found to depend on the occurrence of a glass transition for the polymer induced by the sorption of CO₂. Two distinct types of swelling and sorption isotherms were measured. One isotherm is characterized by swelling and sorption that reach limiting values at elevated pressures. The other isotherm is characterized by swelling and sorption that continue to increase with pressure and a pressure effect on swelling that is somewhat greater than the effect of pressure on sorption. Glass transition pressures estimated from the experimental results for polystyrene with CO₂ are used to obtain the relationship between CO₂ solubility and the glass transition temperature for the polymer. This relationship is in very good agreement with a theoretical corresponding-states correlation for glass transition temperatures of polystyrene-liquid diluent mixtures.     

Physical aging of an amorphous polyimide: Enthalpy relaxation and mechanical property changes

The physical aging behavior of an isotropic amorphous polyimide possessing a glass transition temperature of approximately 239°C was investigated for aging temperatures ranging from 174 to 224°C. Enthalpy recovery was evaluated as a function of aging time following sub‐T_g annealing in order to assess enthalpy relaxation rates, and time‐aging time superposition was employed in order to quantify mechanical aging rates from creep compliance measurements. With the exception of aging rates obtained for aging temperatures close to T_g, the enthalpy relaxation rates exhibited a significant decline with decreasing aging temperature while the creep compliance aging rates remained relatively unchanged with respect to aging temperature. Evidence suggests distinctly different relaxation time responses for enthalpy relaxation and mechanical creep changes during aging. The frequency dependence of dynamic mechanical response was probed as a function of time during isothermal aging, and failure of time‐aging time superposition was evident from the resulting data. Compared to the creep compliance testing, the dynamic mechanical analysis probed the shorter time portion of the relaxation response which involved the additional contribution of a secondary relaxation, thus leading to failure of superposition. Room temperature stress‐strain behavior was also monitored after aging at 204°C, with the result that no discernible embrittlement due to physical aging was detected despite aging‐induced increases in yield stress and modulus. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1931–1946, 1999     

Mesophase transition-induced reorientation in a stretched side-chain liquid crystalline polymer

The orientation relaxation behavior of a stretched side-chain liquid crystalline polymer (SCLCP) on a poly(vinyl alcohol) (PVA) film under strain was investigated through infrared dichroism at temperatures near its phase transitions. We found a reorientation of the aligned mesogens over the smectic to nematic transition of the SCLCP, changing the alignment from an initially, mechanically induced perpendicular orientation to a parallel orientation with respect to the film-stretching direction. This reorientation was found to be irreversible during subsequent nematic to smectic transition, with the parallel orientation preserved. We show that it is possible to stop the reorientation process by cooling the SCLCP back to its smectic phase just before the change in the alignment direction. Moreover, this interruption can result in a stable, zero macroscopic orientation of the mesogens in the stretched SCLCP, and a subsequent heating to the smectic-nematic transition allows the reorientation process to restart and to be completed. We discuss the possible mechanisms for this mesophase transition-induced reorientation and the factors that could influence the process. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1491–1499, 1997     

Dielectric studies of tetraaryl and triaryl polycarbonates and comparisons with bisphenol A‐polycarbonate

The relative permittivity, loss, and breakdown strength are reported for a commercial sample of bisphenol A‐polycarbonate (comm‐BPA‐PC) and a purified sample of the same polymer (rp‐BPA‐PC) as well as for two new polycarbonates having low molecular cross‐sectional areas, namely a copolymer of tetraaryl polycarbonate and BPA‐PC (TABPA‐BPA‐PC) and a triaryl polycarbonate homopolymer (TriBPA‐PC). The glass transition temperatures of the new polymers are higher than the T_g of BPA‐PC (187 and 191 °C vs. 148 °C). Relative permittivity and loss measurements were carried out from 10 to 10⁵ Hz over a wide temperature range, and results for the α‐ and γ‐relaxation regions are discussed in detail. For the α‐relaxation, the isochronal peak position, T_α, scales approximately with T_g. On the other hand, the peak temperature for the γ‐relaxation is approximately constant, independent of T_g. Also, in contrast to what is observed for α, γ exhibits a strong increase in peak height as temperature/frequency increases and a significant difference is found between Arrhenius plots determined from isochronal and isothermal data analyses. Next, the γ‐relaxation region for comm‐BPA‐PC and associated activation parameters show strong history/purity effects. The activation parameters also depend on the method of data analysis. The results shed light on discrepancies that exist in the literature for BPA‐PC. The shapes of the γ loss peaks and hence glassy‐state motions for all the polymers are very similar. However, the intensities of the TriBPA‐PC and TABPA‐BPA‐PC γ peaks are reduced by an amount that closely matches the reduced volume fraction of carbonate units in the two new polymers. Finally, for comm‐BPA‐PC, the breakdown strength is strongly affected by sample history and this is assumed to be related to volatile components in the material. It is found that the breakdown strengths for TriBPA‐PC and TABPA‐BPA‐PC are relatively close to that for rp‐BPA‐PC with the value for TriBPA‐PC being slightly larger than that for rp‐BPA‐PC or the value usually reported for typical capacitor grade polycarbonate. Finally, it is shown that the real part of the relative permittivity remains relatively constant from low temperatures to T_g. Consequently, based on the dielectric properties, TriBPA‐PC and TABPA‐BPA‐PC should be usable in capacitors to at least 50 °C higher than BPA‐PC. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

The dependence of slow crack growth in a linear polyethylene on test temperature and morphology

The slow crack growth behavior of a linear polyethylene with different morphologies was studied by using three point bending with a single edge notched specimen at testing tem-peratures from 30 to 80°C. The morphology was varied by annealing the quenched material at temperatures from 86°C to 135°C. It was found that at test temperatures of 60°C or less, the changes in failure time with annealing temperature are very similar to the change in density with a maximum at 130°C. At testing temperatures above 60°C, the relationship of between failure time and annealing temperature is altered when the test is in the range of the α transition temperature. These results indicate that with respect to slow crack growth in the case of a homopolymer the strength of the crystals is relatively more important than the number of tie molecules. © 1993 John Wiley & Sons, Inc.     

Influence of the crystalline phase on the molecular mobility of PVDF

Dielectric relaxation and pyrocurrent of PVDF were studied by thermostimulated current spectroscopy. The transition spectrum of the material was investigated by differential scanning calorimetry. Two well-resolved relaxation peaks have been observed in the temperature range [?100–100°C]. The molecular mechanisms of these phenomena have been discussed, based on a comparative study of α-PVDF. and β-PVDF. The β relaxation mode is located at ?41°C in α-PVDF and is slightly shifted toward higher temperatures in the stretched material. This mode has been ascribed to the dielectric manifestation of the glass transition (T_g) of PVDF. It is comprised of two components corresponding to the free and constrained amorphous phases, respectively, in the order of increasing temperatures. The α_c transition/relaxation has been associated with molecular motions in the crystalline/amorphous interphase. At higher temperatures, a compensation phenomenon corresponding to cooperative movements liberated at the Curie transition has been observed in β-PVDF. © 1993 John Wiley & Sons, Inc.