Birefringence in polycarbonate: Molecular orientation induced by cooling stresses. II. Effects of sectioning

Birefringence distributions in cross-sections of quenched polycarbonate disks are investigated. The effects of the introduction of new surfaces by sectioning are studied by using different cross-section widths, by quenching rod-shaped specimens, and by measuring the birefringence with oblique incident light. The experimental results are confirmed by numerical calculations of stress distributions in rods with varying aspect ratios. Stress release by appropriate sectioning is used to distinguish between stress and orientation contribution to birefringence. It is found that cutting cross-sections leads to a major stress redistribution in the width direction. This does not affect the birefringence measurement, but the decrease of the stress level in the long direction needs to be taken into account. The molecular orientation in the quenched samples cannot be explained only by the effect of transient thermal stresses above the glass transition. There is another orientation contribution, most likely related to stress relaxation below T_g. © 1993 John Wiley & Sons, Inc.     

Tearing energy study of “oriented and relaxed” polystyrene in the glassy state

To study the effect of processing history, molecular weight/molecular weight distribution, and thermal history on solid state properties (in particular fracture properties and orientation), carefully characterized polydisperse and monodisperse polystyrene samples were drawn above T_g and the orientation frozen in. The objective was to simulate the incidental orientation of polymer chains after processing, molding, and so forth (e.g., injection or compression, blow molding) as a result of melt flow. A series of polystyrene samples was produced by hot drawing at temperatures of 113 and 148 °C, followed by a relaxation period, and then a quench to below T_g. The level of segmental orientation imposed in the samples was determined by birefringence measurements. The tear energy of the sheets was measured at 20 °C by tearing along the draw direction, ultimately giving a value for the fracture energy, G_3C. Samples of high draw ratio and low segmental orientation were unexpectedly found to have highly anisotropic fracture properties despite the low level of optical anisotropy. The fracture properties also depended significantly on whether the samples were drawn with or without lateral constraint. The results are compared with measurements of isotropic samples and the findings of a previous investigation utilizing SANS and birefringence. Modeling the drawing conditions at the chain level using a recent nonlinear tube theory explains how birefringence alone is an inadequate measure of molecular orientation. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 377–394, 2007     

Miscibility and orientation behavior of poly(vinyl alcohol) / poly(vinyl pyrrolidone) blends

The miscibility of poly(viny1 alcohol)/poly(vinyl pyrrolidone) (PVA/PVP) blends is investigated by differential scanning calorimetry (DSC) and wide-angle x-ray diffraction (WAXD). The molecular orientation induced by uniaxial stretching of the blends is also examined by WAXD and birefringence measurements. It is shown by the DSC thermal analysis that the polymer pair is miscible, since a single glass transition temperature (T_g) is situated between the T_gs of the two homopolymers at every composition. The T_g versus composition curve does not follow a monotonic function but exhibits a cusp point at a PVP volume fraction of a little under 0.7, as in a case predicted by Kovacs' theory. The presence of a specific intermolecular interaction between the two polymers is suggested by an observed systematic depression in the melting point of the PVA component. A negative value of the polymer-polymer interaction parameter, χ₁₂ = 0.35 (at 513 K), is estimated from a thermodynamic approach via a control experiment using samples crystallized isothermally at various temperatures. The extent of optical birefringence (Δn) of the drawn blends decreases drastically with increasing PVP content up to 80 wt %, when compared at a given draw ratio, and ultimately Δn is found to change from positive to negative at a critical PVP concentration of a little over 80 wt %. Discussion of the molecular orientation behavior takes into consideration a birefringence compensation effect in the miscible amorphous phase due to positive and negative contributions of oriented PVA and PVP, respectively.     

The relationship between the physical structure and the mechanical properties of polycarbonate

The room-temperature tensile mechanical properties and fracture topographies of polycarbonate are reported as a function of strain rate, sample preparation, and thermal history above and below T_g. The bulk physical structural changes produced by various thermal treatments were monitored by density, yield stress, and differential scanning calorimetry observations. Ordered regions do not form in bulk polycarbonate at or below 145°C. The changes produced in the mechanical properties of polycarbonate on annealing below T_g, relative to a quenched or 145°C equilibrium-state glass, are caused by liquidlike packing changes in free volume. In room-temperature tensile a 125°C–6 day annealed glass exhibits transitional behavior from shear free volume, such as quenched and 145°C equilibrium-state glasses, this transition occurs at higher strain rates. Polycarbonate embrittles as a result of the cessation of shear yielding and reversion to a crazing failure mode with a corresponding decrease in molecular flow and energy to failure. Density measurements indicate that ordered regions do start to grow immediately above 145°C in bulk polycarbonate. This phenomenon allows precrystalline and/or crystalline entities to grow below the bulk T_g in thin films and on the free surfaces of thick films where mobility restrictions are less severe than in the bulk. From bright-field transmission electron micrographs of thin films and carbon–platinum surface replicas of etched thick films it is suggested that the observed spherical precrystalline structures are aggregates of 50–60 Å ordered molecular do mains.     

Anisotropy in the thermal shrinkage of polyimide film

Anisotropy in a polyimide film was investigated in a quantitative manner by a nonlinear regression of the thermal shrinkage data obtained from thermomechanical analysis. The thermally induced shrinkage of this pyromellitic dianhydride–oxydianiline polyimide film at 573 K was directionally anisotropic in the film plane by as much as about 0.4%. The direction of maximum thermal shrinkage was inclined by about π/6 rad from the machine direction of the film. The thermal shrinkage behavior of the polyimide films in the vicinity of the glass‐transition temperature (T_g) showed an unusual anisotropic response. On the basis of a correlation between the anisotropy in the thermal shrinkage of the films and the molecular orientation of the polyimide, this characteristic thermal shrinkage behavior around T_g is suggested to be due to a recovery of the free volume lost by a physical aging process. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3222–3229, 2000     

Thermal conductivity of poly(ether ether ketone) and its short-fiber composites

The effects of crystallinity, orientation, and short-fiber filler on the thermal diffusivity D and thermal conductivity K of poly (ether ether ketone) (PEEK) have been studied. Below the glass transition, D increases by less than 10% as the crystallinity increases from 0 to 0.3. For amorphous PEEK, there is an abrupt drop in D at the glass transition (T_g ? 420 K). The drop is less prominent for the 30% crystalline sample and occurs at 20 K higher. At a draw ratio of 2.5, the axial thermal conductivity is 2.3 times higher while the transverse thermal conductivity is 30% lower than that of the unoriented material. For an injection-molded bar of carbon fiber reinforced PEEK, the variation of D with position along the width or thickness direction is found to correlate well with the fiber orientation. By regarding the injection-molded bar as a multidirectional laminate comprising a large number of unidirectional plies, the thermal conductivities along the longitudinal and transverse direction are calculated and found to agree closely with the experimental data. © 1994 John Wiley & Sons, Inc.     

Mechanism of generation of photoelastic birefringence in methacrylate polymers for optical devices

We clarified the birefringence properties of poly(methyl methacrylate), poly(ethyl methacrylate), poly(isobutyl methacrylate), poly(cyclohexyl methacrylate), poly(isopropyl methacrylate), and poly(tert‐butyl methacrylate). We demonstrated that the conformational change in polymer molecules that causes orientational birefringence differs from that causing photoelastic birefringence. Orientational birefringence depends mainly on the orientation of the main chains of the methacrylate polymers above T_g. On the other hand, photoelastic birefringence in elastic deformation below T_g depends mainly on the orientation of the side chains while the main chains are scarcely oriented. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 2029–2037, 2010     

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     

Birefringence in polycarbonate: Molecular orientation induced by cooling stresses. I. Free quenching

The residual birefringence distributions in polycarbonate disks quenched in a liquid bath under various conditions are determined. Unbalanced distributions of approximately parabolic shape are obtained in the cross-sections in general. A systematic influence of the initial quench temperature on the imblance of the birefringence distributions is observed, with higher initial temperatures leading to a strong shift of birefringence in the positive direction. The systematic variation of the birefringence distribution with quenching conditions is interpreted in terms of a stress and an orientation contribution. The distributions are predicted correctly by a numerical program making use of the linear stress-optical rule with a different stress-optical coefficient above and below the glass-transition temperature. The contribution of molecular orientation is distinguished experimentally from the stress contribution by annealing experiments. © 1993 John Wiley & Sons, Inc.     

Residual stresses and birefringence in injection molding of amorphous polymers: Simulation and comparison with experiment

A physical modeling and a two‐dimensional numerical simulation of the injection‐molding of a disk cavity by using a hybrid finite element method (FEM) and finite difference method (FDM) are presented. Three stages of the injection‐molding cycle––filling, packing, and cooling––are included. The total residual stresses are taken to be a sum of the flow stresses calculated using a compressible nonlinear viscoelastic constitutive equation and the thermal stresses calculated using a linear viscoelastic constitutive equation. The total residual birefringence is taken to be the sum of the flow birefringence related to the flow stresses through the stress–optical rule, and the thermal birefringence related to the thermal stresses through the photoviscoelastic constitutive equation. The Tait equation is used to describe the P‐V‐T relationship. The simulation shows that without packing the birefringence in the surface layer of moldings, with its maximum near the surface, is caused by the frozen‐in flow birefringence (flow stresses) and in the core region by the frozen‐in thermal birefringence (thermal stresses). With packing, a second birefringence maximum appears between the center and the position of the first maximum due to flow in the packing stage. The predicted birefringence profiles and extinction angle profiles are found to be in fair agreement with corresponding measurements in literature for disk moldings. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 622–639, 2006     

Nanostructure reorganization in a thermotropic copolyester. A simultaneous WAXS and SAXS study

Thermally induced Angstrom and nanometer‐scale reorganization in thermotropic liquid crystalline polymer based on (1,4)‐hydroxybenzoic acid (B) and (2,6)‐hydroxynaphthoic acid (N) was investigated by simultaneous wide‐angle and small‐angle X‐ray scattering (SAXS, respectively). Extruded tapes 50 µm thick were annealed at 240°C under dry air conditions. The as‐received tape exhibited fiber‐like structure with crystalline order, whereas the SAXS patterns exhibited diamond‐shaped diffuse scattering elongated along the equatorial axis elucidating nanovoid morphology oriented along the extrusion axis. Guinier analyses showed that the radius of gyration R_g of nanovoids were ca. 17 nm along the extrusion axis. Heat treatment produced a sharpening of the 002 meridional reflection and the 110 equatorial reflection suggesting an improvement of molecular register and packing. The molecular alignment, as quantified by the order parameter , increased as well as the degree of crystallinity χ. On the other hand, SAXS intensity along the equatorial axis decreased evidencing reduction of R_g, i.e. lateral compression of the nanovoids and better molecular packing. Thermal treatment increased the thermal stability and the uniaxial tensile Young's modulus, E, along extrusion axis. However, the tapes exhibited microhardness anisotropy and the indentation anisotropy, ?H, gradually decreased suggesting reduction of elastic recovery in the molecular chain direction. Scanning electron microscopy evidenced an outer skin with an internal layered morphology that transformed into sheet‐like morphology with meandering fibrils. This investigation evidenced microstructure and morphology reorganization correlating with improved thermal and mechanical properties. Copyright © 2015 John Wiley & Sons, Ltd.     

Solid-state 13C NMR characterization of molecular orientation of hot drawn nylon 6

Two-dimensional cross polarization (CP), magic angle spinning (MAS) rotor synchronization NMR spectroscopy has been used to determine quantitatively the molecular orientational distribution function on hot-drawn Nylon 6. Both films and fibers are studied that had been thermally deformed at temperature above T_g, from 60 to 175°C at draw ratios in the range of 1-5.5. In the two-dimensional NMR spectrum, the sidebands that intrinsically originate from the chemical shift anisotropy reveal the degree of molecular orientational order. No preferential orientational order is detected for the sample without thermal deformation, and the highest degree of order is observed for samples which have been hot drawn above T_g at ratios ca. 5. Based on the aggregate model the maximum achievable order parameters are determined. © 1994 John Wiley & Sons, Inc.     

FOURIER TRANSFORM INFRARED STUDIES OF POLY (ETHYLENE TEREPHTHALATE) FILM IN THE GLASS TRANSITION REGION

Three specimens from a solution-cast poly (ethylene terephthalate) (PET) film, one being liquid-N_2 quenched from 92℃(Q), one being slowly cooled from 92℃(SC) and one being quenched and sub-T_g annealed at 67℃(AN), have been studied by specimen difference spectra Q-SC and AN-Q and temperature difference spectra T-70 and T_2-T_1 for every 2℃steps on heating to 90℃at 2℃/min. SC and AN showed more gauche conformers than Q. That means that the PET chain has more trans conformers at higher temperatures and some of these are frozen during quenching through T_g. A band at 1340 cm~(-1) has been found to be complex containing overlapping bands reflecting trans in crystalline regions and trans in amorphous regions. The temperature difference spectra on heating through T_g showed that the spectral changes in Q are gradual while a rather abrupt change occurs in AN at 80—82℃for the bands at 1340, 1042 and 1020 cm~(-1). No new conformational structure or new vibrational mode is involved. A kind of locking mechanism is suggested which hinders the molecular vibrational changes in AN below T_g until a sudden release occurs at T_g. These locking sites can be nothing else than sites of tighter local packing of chain segments. Consequently it is believed that inter-chain van der Waals attraction energy plays a dominating role in the volume relaxation and sub-T_g annealing of quenched amorphous polymers.     

Influence of the polymer structure on the achievement of polar orientation in high glass transition temperature nonlinear optical polyimides by photo-assisted poling

We have used combinations of light, heat, and electrostatic fields to investigate the orientation of nonlinear azo-chromophores chemically incorporated into high glass transition temperature (T_g) polyimides. A number of nonlinear optical polyimides have been synthesized in which the interaction between the nonlinear optical chromophore and the polymer main chain was systematically altered to determine to what extent this steric interaction influences the orientation of the nonlinear chromophore. Chromophores in polymers may be oriented by a number of methods: (a) polarized light at room temperature (i.e., photo-induced orientation or PIO), (b) polarized light and electric fields (i.e., photo-assisted poling or PAP) at temperatures ranging from room temperature to the polymer T_g, and (c) electric fields at T_g (thermal poling). While thermal poling and PIO are usually possible, PAP depends strongly on the molecular structure of the polymer. Previously we have shown that PIO can be accomplished at room temperature in a system where the nonlinear chromophore is embedded into the polyimide main chain via the donor substituent, and this orientation can only be thermally erased at temperatures approaching T_g. In this article we show that, whereas photoisomerization can efficiently depole donor-embedded polyimides in a matter of few minutes at room temperature, PAP does not induce any polar order. This behavior is in marked contrast to a structurally related, side-chain, nonlinear polyimide, in which the azo chromophore is tethered via a flexible linkage to the polymer backbone. In this case some PAP occurs even at room temperature, while no PAP is observed for a donor-embedded system with a similar T_g. We suggest that the orientation during PAP below T_g in the side-chain polyimide is primarily due to the movement of the azo side chains, and there is a very little coupling of this motion to the main chain. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1669–1677, 1998     

Subglass-transition-temperature annealing of poly(ethylene terephthalate) studied by FTIR

Using FTIR spectroscopy we have examined conformational changes in the quenched and slowly cooled amorphous PET films during physical aging process. It was observed that the amount of trans conformers for quenched sample decreased upon sub-T_g annealing. For the slowly cooled sample that corresponds to a state closer to equilibrium, the amount of trans conformers hardly decreased, but increased gradually during sub-T_g annealing process. The conformational populations of these two samples tend to be identical with annealing time. These results demonstrate that sub-T_g annealing will lead to closer interchain packing and result in the formation of new cohesional entanglements along the chains. In situ FTIR studies on the conformational changes of these samples were also carried out during heating up of the sample through the glass transition region. The results showed that incremental changes of the amount of trans conformers in Samples Q and SC were gradual, while an abrupt change of trans conformers occurred in the sub-T_g annealed samples. These results were in agreement with the formation of the interchain cohesional entanglement due to sub-T_g annealing. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 783–788, 1998     

Thermally induced conformational changes in poly(vinyl chloride)

Fourier transform infrared spectroscopy has been used for the detection of thermally induced conformational changes in atactic poly(vinyl chloride) (PVC). Difference spectra emphasize changes in the distribution of gauche defects in the chains as functions of temperature and annealing. Specific bands in the ν(CCl) and δ(CH₂) regions varied with inverse temperature, allowing a calculation of the activation energies of the rotamer species. Conformational changes were also detected in quenched PVC films as a result of annealing below the glass transition temperature, T_g.     

Birefringence of high-pressure injection-molded high-density polyethylene at 70.5 μm wavelength

A newly constructed far-infrared laser interferometer was used to measure the birefringence of 1-mm-thick injection-molded high-density polyethylene test bars, manufactured at unusually high molding pressures. The long wavelength used, 70.5 μm, allowed nondestructive tests to be made, demonstrating the usefulness of far-infrared techniques for probing crystalline polymers. The birefringence was shown to increase with increasing molding pressure, supporting the belief that molecular orientation increases with increasing molding pressure. The tensile modulus was also measured and was found to increase linearly with molding pressure.     

Electric‐field‐induced molecular alignment of side‐chain liquid‐crystalline polyacetylenes containing biphenyl mesogens

Electric‐field‐induced molecular alignments of side‐chain liquid‐crystalline polyacetylenes [? {HC?C[(CH₂)_mOCO‐biph‐OC₇H₁₅]}? , where biph is 4,4′‐biphenylyl and m is 3 (PA3EO7) or 9 (PA9EO7)] were studied with X‐ray diffraction and polarized optical microscopy. An orientation as high as 0.84 was obtained for PA9EO7. Furthermore, the molecular orientation of PA9EO7 was achieved within a temperature range between the isotropic‐to‐smectic A transition temperature and 115 °C, and this suggested that the orientational packing was affected by the thermal fluctuation of the isotropic liquid and the mobility of the mesogenic moieties. The maximum achievable orientation for PA9EO7 was much greater than that for PA3EO7. This was the first time that the electric‐field‐induced molecular orientation of a side‐chain liquid‐crystalline polymer with a stiff backbone was studied. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1333–1341, 2004     

Orientation of polymer molecules during melt spinning. II. Orientation of crystals in as-spun polyolefin fibers

Three different polyolefins, a linear polyethylene, an isotactic polypropylene, and an isotactic polybutene-1, were melt-spun into filaments. The degree of orientation of the filaments was measured by polarized-light microscopy, x-ray diffraction, and a retraction technique, and the results were then related to the melt-draw ratio. The increase in the elastic deformation ratio of polymer chains by spin-stretching, estimated by thermal retraction at a temperature above T_m, was monotonic with respect to the melt-draw ratio. On the other hand, as-spun filaments of polyethylene and polypropylene were characterized by a plateau in birefringence over the range of melt-draw ratios from 8 to 80. The change in orientation functions for crystals in these filaments was similar to the change of birefringence. On the other hand, the birefringence and the crystalline orientation functions for polybutene-1 increased smoothly with increasing melt-draw ratio. The most highly melt-drawn filaments of these polymers had a strongly oriented structure, corresponding to that in highly cold-drawn specimens.