Toward a viscoelastic modeling of anisotropic shrinkage in injection molding of amorphous polymers

A novel approach to predict anisotropic shrinkage of amorphous polymers in injection moldings was proposed using the PVT equation of state, frozen‐in molecular orientation, and elastic recovery that was not frozen during the process. The anisotropic thermal expansion and compressibility affected by frozen‐in molecular orientation were introduced to determine the anisotropy of the length and width shrinkages. Molecular orientation calculations were based on the frozen‐in birefringence determined from frozen‐in stresses by using the stress‐optical rule. To model frozen‐in stresses during the molding process, a nonlinear viscoelastic constitutive equation was used with the temperature‐ and pressure‐dependent relaxation time and viscosity. Contribution of elastic recovery that was not frozen during the molding process and calculated from the constitutive equation was used to determine anisotropic shrinkage. Anisotropic shrinkages in moldings were measured at various packing pressures, packing times, melt temperatures, and injection speeds. The experimental results of frozen‐in birefringence and anisotropic shrinkage were compared with the simulated data. Experimental and calculated results indicate that shrinkage is highest in the thickness direction, lowest in the width direction, and intermediate in the flow direction. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 2300–2313, 2005     

Enhanced gas barrier and mechanical properties in organoclay reinforced multi-layer poly(amide-imide) nanocomposite film

Multifunctional single and triple-layer films exhibiting flexibility, enhanced modulus and gas barrier properties were developed using a soluble polyamide-imide (PAI) in dimethylacetamide (DMAc) with ammonium-modified montmorillonite (MMT, Cloisite 30B) mineral clay. The drying behavior and associated anisotropy development were determined real-time, using a newly developed real-time measurement system. Out-of-plane birefringence development takes place earlier for thinner neat samples caused primarily by increased depletion rate of solvent. Addition of organoclay content resulted in a decrease in evaporation rate of solvent due to planar orientation of well exfoliated nanoplatelets as shown by TEM images and WAXS. This is in agreement with the out-of-plane anisotropy development observed during drying. Beyond a critical solid wt%, out-of-plane birefringence started to increase earlier with organoclay addition. In the case of multi-layer organoclay reinforced PAI films, the drying behavior of each individual layer was tracked and a complementary drying model is proposed. Planar orientation of nanoplatelets resulted in high helium-barrier properties.     

Anisotropic shrinkage of injection-molded rubber

The anisotropic characteristics of injection-molded flat rubber sheets were investigated. The shrinkage and mechanical properties were measured in two directions: parallel and perpendicular to the flow direction. The results showed that (1) the shrinkage in the parallel direction is larger than that in the perpendicular direction, (2) such anisotropic shrinkage increases with the increase of vulcanization temperature and flow distance, (3) similar anisotropy was also noticed in 300% modulus, tensile strength, and elongation. Two kinds of orientation, “shear orientation” and “expanded orientation,” were observed. The former occurs by plug flow, and the latter by the expansion of the materials. The shrinkage was independent of the expanded orientation but was strongly associated with the shear orientation, while the mechanical properties were affected by the expanded orientation.     

Uniaxially aligned poly[(9,9‐dioctylfluorenyl‐2,7‐diyl)‐co‐bithiophene] thin films characterized by the X‐ray diffraction pole figure technique

Aligned thin films of the liquid‐crystalline polymer poly[(9,9‐dioctylfluorenyl‐2,7‐diyl)‐co‐bithiophene] were prepared, and the correlation between the optical anisotropy and the structural properties was shown. A series of samples with different thicknesses were prepared via a spin‐casting process on rubbed polyimide surfaces. The alignment of the polymer chains was obtained by a temperature treatment just below the clearing temperature. The degree of alignment was investigated with ultraviolet–visible absorption spectroscopy and in‐plane X‐ray diffraction. Independently, each technique revealed Hermans orientation functions in the range of 0.75–0.8. Surprisingly, a layer‐thickness dependence was not observed. In addition, the X‐ray diffraction pole figure technique revealed that the polymer chains were uniaxially aligned along the rubbing direction. The aligned films were in the nematic state, with the director elongated along the rubbing direction. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Preparation and characterization of biaxially oriented films from polybutylene terephthalate based thermoplastic elastomer block copolymers

Film casting and biaxial stretching of a series of polyester thermoplastic elastomers (TPEs) were studied. Biaxial orientation in the stretched films was characterized by wide‐angle X‐ray diffraction and birefringence measurements. Biaxial orientation factors were determined. The X‐ray diffraction and birefringence clearly indicated the development of planar biaxial orientation in the stretched films with biaxial stretching. The phenyl groups in the stretched PBT and TPE films gradually became more parallel to the film surfaces with increasing biaxial orientation. The lower the PBT content in the stretched TPE films, the lower the planar biaxial orientation achieved. The β form of crystalline PBT was found only in the stretched PBT films, but not in the TPE films.     

Block copolymer alignment method for mobility anisotropy and enhancement of poly(3‐hexylthiophene) thin‐film transistors

In this article, a new alignment‐layer approach based on block copolymers (BCPs) is proposed for mobility anisotropy and enhancement of organic thin‐film transistors (OTFTs). In particular, the poly(3‐hexylthiophene) (P3HT) TFT with polystyrene‐block‐poly(methyl methacrylate) alignment layer was studied. With max dichroic ratio 1.23 in polarized absorption spectra of P3HT and corresponding device mobility anisotropy 4.63, taking place at the highest annealing temperature of 230°C, the BCP alignment approach proved to be an effective tool for orientation control of polymers such as the P3HT. However, because of negative effect of thermal annealing that causes twisting in P3HT backbones, the achieved mobility enhancement was only a meager 41%. Nevertheless, with rich varieties in the composition and complexity of BCPs, the proposed BCP alignment design represents an interesting alternative to existing alignment approaches to orientation control of P3HT and mobility anisotropy and enhancement of the resultant OTFTs. Besides, the phase‐alternate surface morphology of the BCP alignment layer was confirmed through a series of atomic force microscopy, X‐ray photoelectron microscopy, and field‐emission scanning electron microscopy. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

The anisotropy of slow crack growth in polyethylene pipes

Slow crack growth was measured in the perpendicular and parallel directions relative to the extrusion direction of the pipe. For five pipes from different manufacturers, the anisotropy factor, the lifetime to fracture in the perpendicular direction divided by lifetime in the parallel direction, varied from 1.2 to 4.7 for complete fracture and 1.4 to 4.0 for crack initiation. The degree of molecular orientation was determined by measuring the shrinkage that occured when a pipe specimen was heated near its melting point. The amount of shrinkage correlated with the anisotropy factor for slow crack growth. The shape change after shrinkage was related to the flow pattern of the resin during extrusion and the cooling rate after extrusion.     

Effects of particle size on the molecular orientation and birefringence of magnetic nanoparticles/polyimide composites

Size dependency of nanoparticles for birefringence and molecular orientation of nanocomposite films have been studied using a prism coupler and near‐edge X‐ray absorption fine structure spectroscopy (NEXAFS). We synthesized two different sizes of magnetic nanoparticles, Ni_0.6Zn_0.4Fe₂O₄. The smaller ones were 6.1 ± 1.3 nm‐diameter nanoparticles showing superparamagnetism and the larger ones were 20.7 ± 6.1 nm‐diameter nanoparticles showing ferrimagnetism. To make nanocomposites, we incorporated these particles into poly(N,N′‐bis(phenoxyphenyl)pyromellitimide) (PMDA‐ODA PI). From the prism coupler study, pristine PI without nanoparticles had higher out‐of‐plane birefringence, which indicated high in‐plane orientation of polyimide. However, the birefringence of PI nanocomposites decreased with the increase of particle content. The birefringence of PI nanocomposite with small nanoparticles was smaller than that of PI nanocomposite with large nanoparticles. The birefringence of PI nanocomposite with 1 wt % of small nanoparticles was reduced to almost half of that of pristine PI due to the decreased orientation of PI molecules. NEXAFS spectra of N K‐edge were the same as the birefringence results. Imide and phenyl rings of pristine PI aligned more parallel to the in‐plane direction, but those of PI with nanoparticles aligned less parallel to the in‐plane direction. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 99: 3433–3440, 2006     

Structure of blown films of polyethylene/polybutene‐1 blends

The structure of blown films of blends of low‐density polyethylene (PE‐LD) and isotactic polybutene‐1 (iPB‐1) with different content of iPB‐1 was investigated using wide‐ and small‐angle X‐ray scattering (WAXS and SAXS), transmission electron microscopy (TEM), and polarizing optical microscopy (POM). TEM proves formation of a matrix–particle phase structure due to immiscibility of the blend components. Within the iPB‐1 particles, needle‐like crystals with c‐axis orientation were observed. The PE‐LD matrix showed two populations of crystals. WAXS data indicate that the majority of crystals were oriented with the c‐axis perpendicular to machine direction (MD), while SAXS data prove additional presence of stacks of lamellae, oriented parallel to MD. Quantitative birefringence measurements showed that the majority of molecule segments were oriented in the direction of the circumference of the film, confirming the WAXS data. The crystal orientation has direct impact on mechanical properties, which was demonstrated by measurement of the anisotropy of the modulus of elasticity. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Study of birefringence character of injection- and compression-molded polycarbonate and its interpretation

Birefringence development in molded polycarbonate is of great importance in the manufacture of optical discs. It is important in characterizing birefringence to realize that we are dealing not with a scalar but with the anisotropy of the refractive index tensor–and with the orientation and shape of the refractive index ellipsoid. This usually varies heterogeneously; i.e., point-to-point through the thickness of molded parts. We present here an experimental study of the shapes and orientations of refractive index ellipsoids in injection- and compression-molded polycarbonate parts. We consider the variations in ellipsoid character as a function of molding conditions such as injection rate and quench. In injection-molded parts, the refractive index ellipsoid is coaxial with the flow direction in the center of the parts, but varies with position in the direction of the mold walls. In some cases, the extinction angle reaches a maximum at the mold wall and in other cases, there is an intermediate maximum. In the central “flow-thickness” direction plane, the transverse axis of the refractive index ellipsoid is perpendicular to this plane. The birefringence (in principal axis coordinates) in the flow-thickness direction plane exhibits an intermediate maximum whose position and magnitude vary with molding conditions. In some cases, multiple maxima exist. In compression-molded parts, a principal axis is normal to the mold wall. The results are interpreted in terms of the Rheo-Optical Law using flow-induced orientation and residual stress mechanisms. If the contribution of the residual stress to the birefringence can be subtracted, we may compute biaxial orientation factors. We do this for a case that has low residual quench stresses.     

Crystal structure and orientation of uniaxially and biaxially oriented PLA and PP nanoclay composite films

Cast films of poly(lactic acid) (PLA) and polypropylene (PP) with 2.5 and 5 wt % organo modified nanoclay were prepared and then uniaxially and biaxially hot drawn at T = 90 and 155°C, respectively, using a biaxial stretcher. The orientation of PLA and PP crystal unit cells, alignment of clay platelets, as well as the extent of intercalation and exfoliation were studied using wide angle X‐ray diffraction (WAXD). The measurement of d‐spacing of the 001 plane (normal to platelets plane) of the clay tactoids indicated the intercalation of the silicate layers for the PLA nanocomposite films, whereas the PP nanofilled films showed only dispersion of the nanoparticles (i.e., neither intercalation nor exfoliation were observed). The intercalation level of the clay platelets in PLA was almost identical for the uniaxially and biaxially drawn films. Our finding showed that the crystallite unit cell alignments are appreciably dependent on uniaxial and biaxial stretching. Moreover, the incorporation of clay to some extent influenced the orientation of the crystal unit cell axes (a, b, and c) of the oriented films. The silicate layers revealed a much higher orientation into the flow direction in the uniaxially stretched films compared to the biaxially drawn samples. In addition, the orientation of the 001 plane of nanoclays was significantly greater in the PLA compared to the PP nanoclay composite films probably due to a better intercalation and stress transfer in the former. Morphological pictograms illustrating the effects of uniaxial and biaxial stretching on the clay orientation are proposed. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Theoretical and experimental studies of anisotropic shrinkage in injection moldings of various polyesters

A novel approach to predict anisotropic shrinkage of slow crystallizing polymers in injection moldings was proposed, using the flow‐induced crystallization, frozen‐in molecular orientation, elastic recovery, and PVT equation of state. In the present study, three different polyesters, polyethylene terephthalate, polybutylene terephthalate, and polyethylene‐2,6‐naphthalate (PEN), are used. The anisotropic thermal expansion and compressibility affected by the frozen‐in orientation function and the elastic recovery that was not frozen during moldings were introduced to obtain the in‐plane anisotropic shrinkages. The frozen‐in orientation function was calculated from the amorphous contribution based on the frozen‐in and intrinsic amorphous birefringence and crystalline contribution based on the crystalline orientation function determined from the elastic recovery and intrinsic crystalline birefringence. To model the elastic recovery and frozen‐in stresses related to birefringence during molding process, a nonlinear viscoelastic constitutive equation was used with the temperature‐dependent viscosity and relaxation time. Occurrence of the flow‐induced crystallization was introduced through the elevation of melting temperature affected by entropy production during flow of the viscoelastic melt. Kinetics of the crystallization was modeled using Nakamura and Hoffman‐Lauritzen equations with the rate constant affected by the elevated melting temperature. Numerous injection molding runs were carried out by varying the packing time, packing pressure, flow rate, melt and mold temperature, and anisotropic shrinkage of moldings were measured. The experimental results were compared with the simulated data and found in a fair agreement. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3526–3544, 2006     

Birefringence of a polymeric cholesteric liquid crystal measured by refractometry

The Abbé refractometer provides a convenient way to measure the refractive indices and birefringence of polymeric liquid crystalline solutions. The cholesteric mesophase formed by (acetoxypropyl) cellulose and its solutions in dibutyl phthalate were examined at 26°C and 50°C. The measured refractive indices were dependent on the direction of shear when the polymer was applied to the refractometer plates. The initial shear-induced nematic-like order relaxed slowly to the equilibrium planar cholesteric structure. The refractive index for the isotropic solutions and the two refractive indices for the anisotropic solutions all varied linearly with polymer concentration, and the width of the intermediate two-phase region was readily determined. The changes in refractive index with polymer concentration and with orientation on the refractometer plate were rationalized with a simple model of the cellulosic mesophase, and an order parameter of 0.7 was estimated. The layer birefringence calculated from optical rotatory dispersion measurements of the cholesteric reflection band intensity was in good agreement with the birefringence measurements.     

Birefringence prediction of optical media

The flow and thermally induced birefringence of injection‐compression molded optical media such as compact discs and digital video discs is predicted by applying a stress‐optical rule to the flow and thermally induced stresses, which are estimated with a viscoelastic material model integrated into a non‐isothermal compressible flow simulation. The resulting model considers flow and cooling induced molecular orientation, and the transient effect of thermal stress and pressure. Contrary to previous research for polystyrene, the validated results indicate that, for polycarbonate, the magnitude of the thermally induced birefringence is comparable to the flow induced birefringence. Simulation results of the flow and thermally induced in‐plane birefringence for compact‐disc‐recordable moldings with an optical grade of polycarbonate compared well with experimental observations at different mold and melt temperatures. Both simulation and experiments indicate that mold and melt temperatures have a significant effect on the level of birefringence; increasing mold or melt temperature significantly reduces the birefringence. Polym. Eng. Sci. 44:814–824, 2004. © 2004 Society of Plastics Engineers.     

Theoretical and experimental studies of anisotropic shrinkage in injection moldings of semicrystalline polymers

A novel approach to predict anisotropic shrinkage of semicrystalline polymers in injection moldings was proposed using flow‐induced crystallization, frozen‐in molecular orientation, elastic recovery, and PVT equation of state. The anisotropic thermal expansion and compressibility affected by the frozen‐in orientation function and the elastic recovery that was not frozen during moldings were introduced to obtain the in‐plane anisotropic shrinkages. The frozen‐in orientation function was calculated from amorphous and crystalline contributions. The amorphous contribution was based on the frozen‐in and intrinsic amorphous birefringence, whereas the crystalline contribution was based on the crystalline orientation function, which was determined from the elastic recovery and intrinsic crystalline birefringence. To model the elastic recovery and frozen‐in stresses related to birefringence during molding process, a nonlinear viscoelastic constitutive equation was used with temperature‐ and crystallinity‐dependent viscosity and relaxation time. Occurrence of the flow‐induced crystallization was introduced through the elevation of melting temperature affected by entropy production during flow of the viscoelastic melt. Kinetics of the crystallization was modeled using Nakamura and Hoffman‐Lauritzen equations with the rate constant affected by the elevated melting temperature. Numerous injection molding runs on polypropylene of various molecular weights were carried out by varying the packing time, flow rate, melt temperature, and mold temperature. The anisotropic shrinkage of the moldings was measured. Comparison of the experimental and simulated results indicated a good predictive capability of the proposed approach. POLYM. ENG. SCI., 46:712–728, 2006. © 2006 Society of Plastics Engineers     

Factors Influencing Anisotropic Sintering Shrinkage in Tape-Cast Alumina: Effect of Processing Variables

The effect of the processing variables shear rate, solids loading, and sintering temperature on the anisotropy of sintering shrinkage of aqueous tape-cast alumina was studied. Higher shear rates and higher solids loading resulted in higher in-plane shrinkage anisotropy, whereas the shrinkage anisotropy in the thickness direction was higher for low solids loadings. The in-plane shrinkage anisotropy was found to be fairly constant above a certain critical shear rate (∼100 s⁻¹) independent of the solids loading. The shrinkage anisotropy through the thickness was higher than in-plane directions. A higher thickness direction sintering rate was observed and attributed to a greater number of interparticle necks in the thickness direction because of the platy nature of alumina particles and the greater thickness direction strains associated with binder removal. The binder did not significantly affect the in-plane sintering shrinkage but significantly affected the shrinkage in the thickness direction. It was suggested that emulsion binder particles occupy sites in between layers of particles in the thickness direction. The degree of anisotropic shrinkage was quantified using edge orientation polarograms and a direct correlation was obtained between the processing variables, shrinkage anisotropy, and the edge orientation index.     

Effect of Particle Shape on Anisotropic Packing and Shrinkage Behavior of Tape-Cast Glass–Ceramic Composites

In this study, the influence of particle shape anisometry and particle alignment in tape-cast green sheets on the shrinkage behavior of low-temperature co-fired ceramics (LTCCs) was investigated quantitatively. A new method for the characterization of particle shape with the use of a particle image analyzer is presented, and its application to real material systems demonstrated. A commercial LTCC system and three developed composite powders with different average particle sizes were analyzed. After tape casting, particle alignment in the green sheets was analyzed using image analysis of SEM micrographs of cross sections. The investigations showed that the degree of particle alignment correlates significantly with the particle shape and size of the materials. A further increase in particle orientation was seen after the lamination process. Additionally, the powder packing of both single layers and laminates was analyzed by mercury porosity. The anisotropic shrinkage behavior during the sintering process was determined by means of optical dilatometry. The data obtained on the particle morphology, particle orientation in the tapes, and their effects on the shrinkage anisotropy will be discussed.     

Orientation of triblock copolymers in planar extension

Three commercial styrenic triblock copolymers were subjected to planar extension in a lubricated channel die, as a model for the orientation that may occur during processing operations. Materials that contain cylinders of polystyrene show a pronounced orientation of the cylinder axis in the flow direction, as revealed by small-angle X-ray scattering in the three orthogonal directions. This anisotropy produces small-strain tensile moduli that differ by a factor of 10 or more in the two in-plane directions. A material that contains lamellae of polystyrene shows pronounced orientation of the lamellae in the plane of the sample; in particular, lamellar normals are strongly forbidden from pointing along the flow direction. For similar strains, planar extension is much more effective in orienting these materials than simple shear.     

Deformation mechanisms of polypropylene/polystyrene multilayered films

Multilayered composites of polypropylene (PP) and polystyrene (PS) fabricated by a layer‐multiplying coextrusion technique are described. The aim of this investigation was to find a correlation between the morphology and the mechanical and micromechanical deformation behavior. The multilayered films had primarily continuous layers, exhibiting only few defects in layer construction and turning into an irregularly layered system when the calculated layer thickness was only 5 nm. The morphology and layer thickness of both the PP and PS layers affected the mechanical and the micromechanical behavior, which was brittle for the films having PS layers thicker than 75 nm and ductile when the PS layers were 50 nm and thinner. Transmission electron microscopy showed crazes in the thicker PS layers and homogeneous deformation in the thinner ones. The molecular orientation during deformation of the ductile films was calculated from rheo‐optical measurements with Fourier transform infrared spectroscopy. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Barrier and mechanical properties of injection molded montmorillonite/polyesteramide nanocomposites

Properties of injection‐molded biodegradable polyesteramide composites containing 5 and 13 wt% octadecylammonium‐treated montmorillonite clay have been studied. Oxygen transmission rates and mechanical properties were measured. X‐ray diffraction was used to assess the degree of intercalation of the clay layer stacks, and transmission electron microscopy (TEM) was used to assess the morphology and degree of layer delamination. A substantial reduction in oxygen permeability was observed when clay was added to the composites. The oxygen permeability of the 13 wt% clay sample was only 20% of that of the pure polymer. The in‐plane stiffness and in‐plane strength of the sheets were greatly improved without any embrittlement. These beneficial effects were probably due to the high degree of clay layer exfoliation and orientation observed by TEM. Heat shrinkage, toughness analysis, and cutting operations suggested that the polymer chains and the clay layers were oriented parallel to the plane of the sheet. TEM and X‐ray showed that stacked layers were still present but that these were significantly intercalated. The clay‐layer periodic spacing increased from 25 Å to approximately 35 Å during processing. POLYM. ENG. SCI. 45:135–141, 2005. © 2004 Society of Plastics Engineers