Thermomechanical properties of a magnetically and mechanically oriented liquid crystalline copolyester,xydar

Elastic moduli and linear coefficients of thermal expansion (CTE) of a random thermotropic liquid crystalline copolyester, oriented in a magnetic field and by mechanical methods were measured in the directions parallel and perpendicular to the orientation direction. The axial elastic modulus of the magnetically oriented film was lower than that of the uniaxially stretched film. The elastic modulus measured in the transverse direction was higher for the magnetically oriented film. In the axial direction, both the mechanically stretched and magnetically oriented films exhibited shrinkage at low temperatures (CTE ≈ -2 · 10^?5 K^?1) and exhibited expansion at elevated temperatures. In the transverse direction, expansion was observed except for the biaxially stretched film at low temperatures. The magnetically oriented film showed the lowest axial CTE at elevated temperatures.     

Unique orientation of lamellae in film processed poly (phenylene sulfide)

The morphology of poly(phenylene sulfide), PPS, obtained as processed film has been studied using wide angle X-ray scattering (WAXS), scanning electron microscopy (SEM), and differential scanning calorimetry (DSC). A unique morphology has been identified in the original film. The spherulites are very small in size and possess a cylindrical symmetry with their lamellae oriented on the edge. In these processed PPS films, the a-axis of the lattice is preferentially aligned perpendicular to the film plane, while the b and c axes are predominantly in the film plane. In the PPS isothermally crystallized from the melted, original film, there is no such preferential orientation. Thermal analysis of the original and melt crystal-lized PPS shows that while the degree of crystallinity is about the same, the nature of the amorphous phase in the two materials is different. In the original film, we did not observe a heat capacity increment at a glass transition temperature by DSC, indicating that all of the amorphous phase belongs to the category of rigid amorphous phase. In the melt crystallized PPS, a distinct glass transition was seen, though only a portion of the amorphous phase becomes mobile at T_g. The differences in orientation and mobility of the amorphous phase in the original film compared to melt crystallized PPS are explained by the different thermal processing procedures used for the two materials.     

Analysis of the thermal expansion behavior of oriented polyoxymethylene sheets

The applicability of the molecular composite model is extended to include the analysis of the thermal expansion behavior of oriented (anisotropic) polyoxymethylene. The model accounts for changes in the morphological and structural aspects of polyoxymethylene during solid state-induced orientation. The results are reported and analyzed in terms of three-dimensional surfaces. Apart from the effect of temperature, it has been found that the crystalline phase orientation distribution, λ, is the most crucial parameter affecting the thermal expansion behavior of anisotropic polyoxymethylene followed by the volume fraction crystallinity. The size and the geometry of the crystalline phase have negligible effects on the thermal expansion behavior of oriented polyoxymethylene. Above the glass-transition temperature, a minimum in the transverse thermal expansion coefficient is exhibited in the vicinity of λ = 2 for volume fraction crystallinities ≤0.75. It is sufficient to align 75% of the crystallites with their c-axis at ±10° to the draw direction, to attain the ultimate degree of anisotropy in the thermal expansion behavior of oriented polyoxymethylene sheets.     

The effect of orientation on the morphology and kinetics of solvent crazing in polystyrene

Unoriented, uniaxially oriented, and biaxially oriented polystyrene films were crazed by immersing samples in liquid n-hexane at 45°C. The craze morphology and crazing kinetics were studied as a function of the preorientation and thermal histories of the polymer films. The shape of the micropores was related to the degree of orientation of the film. Ellipsoidal microvoids were formed on the surfaces of uniaxially oriented films containing a residual glassy core. Unoriented samples displayed spherical microvoids 0.1 to 3 μ in diameter. The major-to-minor axis ratio of the micropores increased monotonically from 1/1 to 10/1 as the sample orientation was increased from 0% to 200%. The kinetics of the crazing process similarly increased with uniaxial orientation. Surfaces of unannealed films which crazed mainly during the sorption of liquid n-hexane displayed numerous micropores when examined by the SEM. Conversely, films which were extensively annealed crazed mainly during desorption regardless of subsequent orientation. These extensively annealed films exhibited surfaces with many fine cracks and few, if any, micropores.     

A study on the crystallization behavior of poly(β-hydroxybutyrate) thin films on Si wafers

The exact molecular chain orientation of poly(β-hydroxybutyrate) (PHB) in ultrathin films was successfully probed using surface-sensitive, grazing incidence X-ray diffraction techniques. The crystal orientation of spin-coated PHB films was very sensitive to free surface and thermal annealing. In pristine films, the free surface easily exerted its influence on PHB crystallization and caused lamellar orientation with the b-axis perpendicular to the film surface. The effect of the buried interface increased with temperature. With the increase in thermal annealing temperature, the lamellar orientation changed from the b-axis being perpendicular to the film surface to the c-axis becoming perpendicular to the film surface. As film thickness increased, the temperature, at which the lamellae with the b-axes oriented normal to the film surface disappeared, increased. The thickness and temperature dependence of the crystallization behavior of PHB in an ultrathin film could be attributed to the competition between the effects of the free surface and the buried interface.     

Studies on the biaxial stretching of polypropylene film. XI. Type II orientation during [2-2FI]-type stretching and its change by subsequent thermal contraction

In order to study the deformation mechanism of type II stretching, the change in orientation during the restretching and subsequent thermal contraction was investigated by x-ray diffraction method. When a uniaxially oriented film is restretched, the lamellae which are stacked in the stretching direction by the stretching rotate as a whole toward the restretching axis. They rotate backward nearly reversibly during the thermal contraction, unless the restretching exceeds a balancing state, where the orientation in the film plane are equal in all directions. However, when the restretching degree is so high and the film orientation exceeds the balancing state, the lamellar rotation is accompanied by a complex phenomenon. It is considered from the wide-angle and small-angle x-ray diffraction patterns that the lamellar surface becomes indented because of slippage between microfibrils composing the lamellae, and the microfibrils themselves bend at the boundary between the amorphous and crystalline regions within which the tilting of c-axis also occurs. Upon contracting of the film; these changes recover, but even in the last stage of contraction the orientation approaches the symmetrical biaxial orientation but not the uniaxial orientation from which the biaxial orientation is started. These orientation and disorientation behaviors are not affected basically by a slight change in the restretching temperature and the degree of stretching.     

Elimination of orientation in perfluorinated ionomer membranes

An investigation was performed into the effect of processing on microstructural orientation in the perfluorinated ionomer membrane, Nafion® (registered trademark of E. I. du Pont de Nemours and Company). The aim of the study was to explore methods by which such orientation, determined from the azimuthal width of the small‐angle X‐ray “cluster” peak, could be reduced or eliminated. The processing consisted of biaxial deformation, both of the commercial membrane and the precursor resin, as well as high‐temperature treatment of the Nafion membrane. It was found that membranes produced by film blowing were less oriented than commercial membranes produced by flat‐film extrusion of the precursor. Biaxial drawing of commercial membranes at temperatures above the glass‐transition temperature of the fluorocarbon matrix (T_g,m) resulted in a reduction in orientation. However, even the most isotropic of these samples were significantly more oriented than membranes made by film blowing. Orientation was eliminated from commercial membranes by annealing at temperatures above T_g,m for periods in excess of 24 h. POLYM. ENG. SCI., 46:228–234, 2006. © 2005 Society of Plastics Engineers     

Control of molecular orientation

I. Amorphous polymers . The mechanical performance of a glassy amorphous polymer is strongly dependent upon molecular orientation. The pattern of molecular orientation is governed by the kinematics (and temperature) of mechanical forming operations. Three types of controllable orientation are: (a) uniaxial, (b) biaxial, and (c) “crossed.” The optimum pattern of orientation in a part is one which is appropriate for the mechanical stresses encountered in service. For a fiber subjected to tensile and bending loads, uniaxial orientation is appropriate. A shell structure, subjected to multiaxial stresses, requires either biaxial or crossed orientation for maximum performance. As a rule, the maximum achievable multidirectional strength in such a structure is less than the maximum strength of a uniaxially oriented fiber. II. Crystalline polymers . Oriented crystalline polymer structures can be created in two distinct ways. An isotropic polycrystalline polymer can be deformed below the melting point, with extensive reorganization of the crystal morphology, or an oriented amorphous melt can undergo crystallization to yield oriented crystalline polymer. Performance of an oriented semicrystalline polymer depends upon orientation of the amorphous portion as well as orientation of the crystallites. As with amorphous polymers, orientation can be uniaxial, biaxial, or crossed. “Orientation” usually denotes c-axis orientation only, but drawing followed by rolling can result in double orientation—orientation of a-axis, b-axis, and c-axis.     

Studies on the stretching of polypropylene film. II. Polyaxial stretching

Polypropylene film was stretched polyaxially at 100–160°C., and the orientation behavior was studied by means of optical and x-ray method. The molecular chains oriented progressively to the film surface with an increase in stretching area v_A in the range 1–16, and the (040) selective uniplanar orientation developed at the extreme stretching. The plot of orientation versus v_A was less steep when the stretching was carried out at higher temperature, but the final degree of orientation was independent of the temperature, because the final v_A increased with temperature. At 160°C. premelting occured to such a degree that the high stretching and, consequently, the high orientation could not be obtained. The orientation of the amorphous chains was always behind that of the crystalline region. In the initial stage the polyaxial stretching was not as effective in attaining high biaxial orientation as the two-step biaxial stretching, but the final orientation was the same in both types of stretching because v_A reached a value of 16 in the polyaxial stretching while it was only 2 in biaxial stretching.     

Aging characteristics of oriented ply(ethylene terephthalate)

The physical aging characteristics of oriented poly(ethylene terephthalate) (PET), have been studied ad functions of storage time and temperature below the glass transition temperature (T_g) of PET. The free volume relaxation, associated with aging, has been characterized by the enthalpy at T_g, as measured by differential scanning calorimetry. The effects of the free volume relaxation on mechanical properties and the mode of failure have been investigated. It has been determined that a correlation exists between the enthalpy of relaxation and the ductile-to-brittle failure transition. Molecular orientation reduces significantly the enthalpy of relaxation, resulting in the disappearance of the ductile-to-brittle transition when highly oriented samples are aged over time. It has been established that a minimum amount of orientation is required to reduce or eliminate the effects of PET aging. Molecular orientation has also been found to reduce craze formation when oriented PET is exposed to a stress-cracking medium at constant stress.     

Real‐time crystalline orientation measurements during low‐density polyethylene blown film extrusion using wide‐angle X‐ray diffraction

Real‐time wide‐angle X‐ray diffraction studies were successfully used to investigate the effect of film blowing process parameter on the crystalline orientation development during the blown film extrusion of low‐density polyethylene. Azimuthal distribution scans showed the evolution of crystalline orientation in the bubble from an isotropic state to an oriented state as inferred from (110) and (200) planes. These real‐time X‐ray diffraction measurements in a blown film line are consistent with prior observations using polarized Raman spectroscopy (Gururajan and Ogale, J. Raman Spectrosc., 40 , 212 (2009)) and small‐angle light scattering (Bullwinkel et al., Int. Polym. Proc., 16 , 41 (2001)) that significant molecular orientation takes place past the frost‐line height, even after the blown film diameter is locked into place. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

Multiwavelength interferometry and competing optical methods for the thermal probing of thin polymeric films

Multiple‐wavelength interferometry (MWI), a new optical method for the thermal probing of thin polymer films, is introduced and explored. MWI is compared with two standard optical methods, single‐wavelength interferometry and spectroscopic ellipsometry, with regard to the detection of the glass transition temperature (T_g) of thin supported polymer films. Poly(methyl methacrylate) films are deposited by spin coating on Si and SiO₂ substrates. MWI is also applied to the study of the effect of film thickness (25–600 nm) and polymer molecular weight (1.5 × 10⁴ to 10⁶) on T_g, the effect of film thickness on the coefficients of thermal expansion both below and above T_g, and the effect of deep UV exposure time on the thermal properties (glass transition and degradation temperatures) of the films. This further exploration of the MWI method provides substantial insights about intricate issues pertinent to the thermal behavior of thin polymer films. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 4764–4774, 2006     

Influence of molecular orientation direction on the in‐plane thermal conductivity of polymer/hexagonal boron nitride composites

The effect of the molecular orientation direction of a polymer matrix on the in‐plane thermal conductivity (TC) of injection‐molded polymer/hexagonal boron nitride (h‐BN) composites is investigated. In this system, the h‐BN platelets align in the in‐plane direction owing to injection shear flow. Three molecular orientations (perpendicular, random, and parallel to the h‐BN plane) are achieved using liquid crystalline polyesters and the in‐plane TCs are compared. Although a parallel orientation of the polymer chains provides the highest TC of the matrix in the injection direction, the TC of the composites is the lowest of the three systems for this orientation. The highest in‐plane TC is found in the perpendicularly oriented system, irrespective of the in‐plane direction. These results reveal that perpendicularly oriented molecular chains serve as effective heat paths between h‐BN platelets that are arranged one above the other, and consequently, a continuous thermal network is created in the in‐plane direction. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39768.     

Polymer composites with enhanced thermal conductivity via oriented boron nitride and alumina hybrid fillers assisted by 3-D printing

Herein, oriented boron nitride (BN)/alumina (Al₂O₃)/polydimethylsiloxane (PDMS) composites were obtained by filler orientation due to the shear-inducing effect via 3-D printing. The oriented BN platelets acted as a rapid highway for heat transfer in the matrix and resulted in a significant increase in the thermal conductivity along the orientation direction. Extra addition of spherical Al₂O₃ enhanced the fillers networks and resulted in the dramatic growth of slurry viscosity. This, together with filler orientation induced the synergism and provided large increases in the thermal conductivity. A high orientation degree of 90.65% and in-plane thermal conductivity of 3.64 W/(m∙K) were realized in the composites with oriented 35 wt% BN and 30 wt% Al₂O₃ hybrid fillers. We attributed the influence of filler orientation and hybrid fillers on the thermal conductivity to the decrease of thermal interface resistance of composites and proposed possible theoretical models for the thermal conductivity enhancement mechanisms.     

In situ orientation of linear low density polyethylene films subjected to Mode I fracture load

An in situ technique has been developed to study the deformation and orientation near a crack tip during Mode I fracture. The technique employs a pair of rotating crossed polar films positioned on either side of the sample. Images gained from transmitted light show kidney shaped process zones ahead of the crack tip that increase in size with deflection, primarily by isotropic expansion. Extinction bands, which are regions of either unoriented material or material oriented in the direction of one of the two polars, are observed as a function of the angular position of the polars with respect to the loading direction. A set of extinction bands at various crossed polar positions provides orientation direction information within the sample. Continuous flow of the molecules around the crack tip is observed at several stages of deformation and for all films tested. The orientation field around the crack tip evolves with increasing radius of curvature of the crack tip. A transition in orientation is observed when the cracks are coincident with the orientation direction of the film but not when they are normal to the orientation direction. The technique of rotating crossed polars is successfully used to determine the orientation direction in the vicinity of a crack tip for linear low density polyethylene during several stages of Mode I loading. The advantage of the technique is that in situ data may be collected quickly when compared to techniques such as X‐ray scattering, which rely on data collection through scanning and not the parallel data collection utilized herein. The authors acknowledge that this technique is limited to material that transmits light. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 771–777, 2000     

Structure development in biaxially stretched polystyrene film: Part I. Property-orientation correlation

A study of orientation development in polystyrene film by biaxial stretching is described. Stretch ratios up to 7.2 × 7.2 were used. Mechanical properties of polystyrene films were correlated with the level of molecular orientation developed by uniaxial or biaxial stretching. Sensitivity of the mechanical properties to change due to development of orientation varied as follows: Yield strength < Young's modulus < Tensile strength < Elongation to break. Brittle to ductile transition phenomena were observed at certain orientation values in the orientation triangle diagram. The transition occurs when f × f ～ 0.0025 for biaxially oriented and f ～ 0.015 in the machine direction for uniaxially oriented films. SEM photomicrographs show that the fracture surfaces of ductile failures exhibit many fibrils while brittle failures exhibit no fibrils.     

Influence of strain on the thermal behaviour of poly(chlorotrifluoroethylene)

The effects of uniaxial drawing on the free volume of poly(chlorotrifluoroethylene) have been investigated using the positron annihilation lifetime technique. Annealing measurements were made both in unstrained and maximum strained conditions of the polymer to understand the influence of residual stress on the thermal behaviour. The results indicate uncoiling of the molecular chains upon stretching accompanied by orientation of the polymeric chains. The results further indicate an increase of the glass transition temperature of the drawn film and a negative thermal expansion beyond the glass transition temperature, supported by an increase in activation energy for the strained polymer. © 1999 Society of Chemical Industry     

Disclosing the fatigue deformation mechanisms of different types of poly (ethylene terephthalate) industrial fibers on the base of the multiscale structural evolutions

The fatigue behavior of poly (ethylene terephthalate) industrial fibers is a key issue in their long-term service for engineering applications. To have a comprehensive understanding of the fatigue behavior, the high-tenacity (HT) and low-shrinkage (LS) PET fibers were selected to analyze the room temperature dynamic fatigue properties with different stress. Various techniques such as WAXD/SAXS and FTIR were employed to study the multiscale structure changes to disclose the fatigue mechanisms. Although the crystalline structure including orientation and crystallinity did not change, the amorphous structures varied with fatigue stress. The HT fiber exhibited a higher fatigue recovery ratio. The slight increase in amorphous orientation, and amorphous thickness was attributed to the oriented coiled molecular chains during tensile fatigue stress. In contrast, the LS fiber experienced plastic fatigue deformation with a lower recovery ratio. The molecular chains in the large amorphous domain are easily extended and oriented under tensile loading, increasing amorphous orientation and lamellar thickness. The fatigue mechanism for the LS fiber involved the conformation transition from gauche to trans conformers and a higher proportion of irreversible amorphous regions were formed. It is indicated that developing industrial filaments with small amorphous orientation and content is crucial to improving their fatigue resistance.     

Oriented polymer networks obtained by photopolymerization of liquid-crystalline monomers

In this paper a new class of oriented polymers is introduced: highly crosslinked polymer networks obtained by photopolymerization of oriented liquid crystalline (LC) monomers. Orientation of these monomers can be induced in the same manner as usual for conventional LCs applied in LCDs: with the aid of an electric or a magnetic field, or using surface orientation. Photopolymerization offers the advantage that a temperature can be chosen right within the LC temperature region. The resulting polymer networks show a high degree of ordering which is somewhat dependent on the polymerization temperature, as demonstrated by the amount of birefringence. This ordering is maintained over a wide temperature region, contrary to that in LC side-chain polymers. Apart from their optical properties, uniaxially ordered polymer networks show anisotropy also in other physical properties, such as the thermal expansion coefficient and modulus.     

Structures and tensile properties of a magnetically and mechanically oriented liquid crystalline copolyester,Xydar

A wholly aromatic thermotropic liquid crystalline copolyester consisting of p-hydroxybenzoic acid, terephthalic acid, and p,p′-biphenol, one from the Xydar series, was aligned by means of magnetic fields and mechanical methods. The tensile properties of these samples were different depending on the orientation degree and the means used for the orientation. Magnetically oriented films exhibited lower elastic modulus and ultimate tensile strength than mechanically oriented films of the same orientation degree, but the elastic modulus of magnetically oriented films was comparable to that of the mechanically stretched films of lower orientation degrees. This suggests that magnetic fields could be used as an additional means of controlling the orientation of thermotropic liquid crystalline copolyesters during molding or film fabrication. The difference in tensile properties was discussed in relation to the oriented structures examined by scanning electron microscopy, polarizing microscopy, and wide angle X-ray measurement.