On the physical behavior of isotactic polypropylene fibers extruded at different draw‐down ratios: II. Structure and properties

Microinterferometry (MIF), wide‐angle X‐ray scattering (WAXS), differential scanning calorimetry (DSC), and an Instron tensile tester (ITT) were used to determine the correlation between optical and structural properties of polypropylene (PP) fibers. For the purpose of the study a set of as‐spun isotactic PP fibers were extruded by melt spinning at different draw‐down ratios (DDR). The birefringence (Δn), degrees of orientation, degree of crystallinity (χ), Young's modulus (E_e), and tenacity (τ) were determined for PP fibers at the different DDR. An equiangular orientation of PP fibers at particular DDR was predicted experimentally, and the transverse modulus (E_t) was estimated for the tested fibers. Empirical formulae were developed for correlating the fiber birefringence with some of the studied structural properties of PP fibers. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

Processing–mechanical property relationships in injection moldings of polybutylene

Two unfilled nonpigmented extrusion grades of polybutylene have been injection-molded into a tensile bar mold under a wide range of barrel and mold temperatures. The overall structure of the moldings has been determined and correlated with processing conditions. The short term tensile mechanical properties of the moldings have been ascertained and correlated with molding structure. For low mold temperatures, the Young's modulus and tensile strength of injection moldings of polybutylene are controlled by the extent of and structure within the highly oriented skin. Low barrel temperatures can give rise to highly crystalline thick skins that treble the Young's modulus and fracture stress, when compared to high barrel temperature moldings. Increasing the mold temperature introduces a brittle response in polybutylene injection moldings. Modulus is controlled, at the high mold temperatures, by the skin thickness and by the crystallinity of the material comprising the core of the molding.     

Defining the physical structure and properties in novel monofilaments with potential for use as absorbable surgical sutures based on a lactide containing block terpolymer

Monofilaments of a block terpolymer of l-lactide, ?-caprolactone and glycolide have been melt spun for potential use as absorbable surgical sutures. As-spun fibres of the terpolymers produced by melt spinning were elastic, amorphous and isotropic. A two-stage process involving hot drawing was employed to enhance their mechanical properties. WAXS and SAXS results coupled with DSC demonstrated that hot drawing leads to an orientated amorphous matrix containing small highly aligned crystals. Hot drawing was carried out at a range of temperatures using the highest possible draw rate commensurate with maintaining continuity of the fibre. A novel WAXS analysis based on a spherical harmonic analysis allowed a separation of the scattering into three components: oriented crystalline, oriented amorphous, and an isotropic amorphous. There is a steady increase in the fraction of oriented crystalline material with increasing hot draw temperature, although the level of crystallinity is ultimately limited by the statistical nature of the terpolymer. The material shows highly promising potential properties for use as a monofilament suture.     

Structure and properties of injection‐molded polypropylenes with different molecular weight distribution and tacticity characteristics

Two series of polypropylenes with different molecular weight distribution and tacticity characteristics were injection molded into flexural test specimens by varying cylinder temperature and the effects of the molecular weight distribution and tacticity on the structure and properties of the moldings were studied. Measured propertied were flexural modulus, flexural strength, heat distortion temperature, Izod impact strength, and mold shrinkage and structures studied were crystallinity, the thickness of skin layer, a*‐axis‐oriented component fraction and crystalline orientation functions. The relations between the structures and properties were also studied. It was found that the molecular weight distribution and tacticity characteristics affect the properties mainly through the molecular orientation and crystallinity, respectively. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 2142–2156, 2002     

Characterization of molecular orientation in injection-molded thermoplastics by transmission and reflection infrared spectroscopy

Orientation in injection moldings of polypropylene (PP), polyethylene (PE), polyamide 6 (PA-6), and polystyrene (PS) was investigated by transmission and reflection infrared spectroscopy. Orientation of the surface was measured by the reflection method, the depth profiles of orientation and of the fraction of the crystalline phase were measured by the transmission spectroscopy of microtome sections. The maximum of orientation of PP lies in the subsurface layer (ca. 250 μm); the crystalline phase is oriented more than the amorphous one. The maximum of the depth profile of orientation corresponds to the minimum of the fraction of the crystalline phase in PP. The profile of orientation of PE Is similar; at the beginning (to a depth of about 500 μm) the parallel orientation of the c axis of the crystalline phase is the most distinct one, towards the center the orientation of the a axis passes from the perpendicular to the parallel one. Under the described molding conditions PA-6 is not significantly oriented, the fraction of the crystalline phase increases towards the center of the molding. Unoriented PA-6, the surface layer of which was removed by milling, has a highly oriented surface due to its mechanical treatment. No pronounced orientation of PS was observed under the molding conditions used.     

Structure and properties of injection moldings of polypropylene/polystyrene blends

A homoisotactic polypropylene (PP) was melt blended with 0–30 wt % of three kinds of polystyrene (PS) with melt flow indexes lower than, similar to, and higher than that of PP. The blends were injection molded at cylinder temperatures of 200–280°C, and the structure and properties of the injection moldings were studied. With PS blending, although the PP molding whitened, no surface defect such as layer peeling and pearl-like appearance occurred. The rigidity and dimensional accuracy of the molding improved without much deterioration in impact strength and heat resistance. At the same time the fluidity also improved. The injection moldings of PP/PS blends did not show clear skin/core structure under a polarizing microscope. The degrees of crystallinity and crystalline c-axis orientation decreased with PS blending. PS particles were the smallest when the ratio of the viscosity of the PS to that of PP at molding shear rate was slightly lower than unity. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 1015–1027, 1997     

Effects of acid neutralization on the morphology and properties of organoclay nanocomposites formed from K+ and Na+ poly(ethylene-co-methacrylic acid) ionomers

Nanocomposites were prepared by melt blending various sodium (Na⁺) and potassium (K⁺) ionomers formed from poly(ethylene-co-methacrylic acid) and the M₂(HT)₂ organoclay formed from montmorillonite (MMT). The effects of the neutralization level of the acid groups and the precursor melt index on the morphology and properties of the nanocomposites were evaluated using stress-strain analysis, wide angle X-ray scattering (WAXS), and transmission electron microscopy (TEM) coupled with particle analysis. The aspect ratio generally increases as the neutralization level increases, except for Na⁺ ionomer nanocomposites with neutralization levels >50%. It appears from both WAXS and TEM analyses that Na⁺ ionomer nanocomposites have higher levels of MMT exfoliation and particle orientation in the flow direction than K⁺ ionomer nanocomposites. DSC results indicate that the level of crystallinity in the Na⁺ ionomers generally increases slightly with MMT addition, while the crystallinity in the K⁺ ionomers decreases slightly with MMT addition. The relative modulus of K⁺ ionomer nanocomposites increases as the degree of neutralization increases. The relative moduli of Na⁺ ionomer nanocomposites are higher than the relative modulus of K⁺ ionomer nanocomposites, likely due to the increased crystallinity of the Na⁺ ionomers and the decreased crystallinity of the K⁺ ionomers upon addition of MMT, the higher exfoliation levels measured by the aspect ratios and the particle densities, and the higher particle orientation indicated by TEM and WAXS. The relative modulus generally increases as the aspect ratio increases. The elongation at break generally decreases as the MMT content increases and as the neutralization level increases for both ionomer types. The fracture energy of most of the ionomers increases with the addition of MMT, reaches a maximum between 2.5 and 5 wt% MMT, and then decreases upon further MMT addition.     

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     

Effect of melt spinning and cold drawing on structure and properties of poly (aryl ether ketone) (PAEK) fibers

The effects of melt spinning and cold drawing on structure development and resulting properties of poly (aryl ether ketone) (PAEK) have been investigated. Melt spun and subsequently cold drawn fibers were characterized by differential scanning calorimetry, wide angle X-ray diffraction, small angle X-ray diffraction, and birefringence techniques. At low take-up speeds, essentially amorphous fibers are produced. High take-up speeds result in development of crystallinity in the as-spun fibers. Cold drawing above the, T_g of PAEK causes further increase of crystallinity. Wide angle X-ray patterns indicate progressive alignment of chains along the fiber axis in as spun as well as in cold-drawn fibers with the draw down ratio and cold draw ratio. However, cold drawing was observed to broaden the WAXS peaks. SAXS patterns of cold drawn and fixed annealed fibers changed from two-point to four-point patterns indicating mosaic pattern formation of crystalline and amorphous regions. Mechanical properties including tensile strength, elongation at break, modulus, and yield strength were measured and correlated with fiber structure. Fracture surfaces of tensile tested fibers were observed using scanning electron microscopy and correlated with developed fiber structure.     

Structure development during melt spinning of linear polyethylene fibers

Apparatus has been developed for studying the development of crystallinity and orientation during the melt spinning of synthetic fibers. Tension in the fiber and temperature, diameter, and x-ray diffraction patterns are measured as a function of distance from the spinneret for a running monofilament. Measurements are presented for linear polyethylene over a range of spinning variables together with other investigations carried out on the final as-spun fibers. These data indicate that the development of crystallinity in polyethylene is controlled by a balance between increased crystallization kinetics caused by the stress in the fiber and a tendency for increased supercooling with change in any spinning variable that increases cooling rates in the fiber. The type of crystalline orientation observed, its development during the spinning process, and the changes observed with changes in spinning conditions suggest a model for the as-spun fiber structure in which varying amounts of row nucleation and twisting of lamellar, folded-chain crystal overgrowths occur depending on the spinning conditions. As-spun fiber birefringence was shown to depend primarily on the crystalline orientation. Mechanical properties correlated well with c-axis crystalline orientation function and spinline stress.     

The crystallinity of poly(phenylene sulfide) and its effect on polymer properties

The crystallinity and crystallizability of poly(phenylene sulfide) have been examined by a number of common techniques. Several provided qualitative information, but only one, x-ray diffraction, was considered sufficiently reliable and reproducible to allow quantitative comparisons. Based on x-ray measurements, an approximate degree of crystallinity, termed crystallinity index (C_i), could be readily assigned. According to this method, virgin polymer possesses significant crystallinity (C_i ≈ 65%). Curing (crosslinking) the resin below its melting point did not change the crystallinity but did affect the crystallizability. Lightly cured resin suitable for molding and film extrusion was easily quenched from the melt to give amorphous polymer. The amorphous samples crystallized rapidly when heated to temperatures > 121°C (250°F). At mold temperatures below 93°C (200°F), moldings with very low surface crystallinity were produced. Annealing (204°C, 400°F) caused rapid crystallization of such moldings, and changes in crystallinity were correlated with observed changes in physical properties. The resin crystallizes so rapidly that these quenched moldings possessed a crystallinity gradient, the internal crystallinity being substantially greater. At high mold temperatures (121–204°C, 250–400°F), moldings very similar to fully annealed specimens were obtained.     

Chain orientation in melt-extruded samples of vectra A,vectra B,and blends in relation to the mechanical properties

The profile of molecular orientation within injection-molded tensile bars of the liquid crystalline copolyesters Vectra A, Vectra B, and Vectra C, as well as blends of these polymers, was investigated by means of wide-angle X-ray scattering (WAXS) using synchrotron radiation (HASYLAB, Hamburg). The local variation of chain orientation was resolved into steps of 100 μm. An even higher resolution was obtained by using the microfocus camera (focal spot 2 μm) at the European Synchrotron Radiation Facility (ESRF) in Grenoble. In Vectra A and in the blend of Vectra A and Vectra B, a smooth variation of the orientation was found being almost zero at the surface and showing its maximum at a distance of 0.6 mm from the surface. The orientation in Vectra B was rather fluctuating. The average chain orientation in the blend samples processed under the same conditions was higher than in samples of the pure liquid crystalline copolyesters. The mechanical properties of the different layers within the injection moldings were determined by cutting the samples into slices and measuring the stress-strain curves. For specimens of comparable orientation, it turned out that the blend samples had the largest values of Young's modulus and tensile strength. The synergism of orientation and mechanical strength was also found for different blend compositions, as well as in blends of Vectra B and Vectra C. Annealing the injection moldings above the melting point resulted in a rapid relaxation of the orientation, whereas the chain alignment persisted at lower temperatures. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67: 531–545, 1998     

High modulus polypropylene fibers. II. Influence of fiber preparation upon structure and morphology

The influence of drawing on the limiting draw ratio upon formation of the morphological structure of fibers spun from binary polypropylene (PP) blends was studied. Fibers were spun from a fiber‐grade CR‐polymer and from the blends of a fiber‐grade CR‐polymer with a molding‐grade polymer in the composition range of 10–50 wt % added. As‐spun fibers were immediately moderately and additionally highly drawn at the temperature of 145°C. The structure and morphology of these fibers were investigated by small‐angle X‐ray scattering, wide‐angle X‐ray scattering, differential scanning calorimetry, scanning electron microscopy, density, birefringence, and sound velocity measurements. It was shown that continuously moderately drawn fibers are suitable precursors for the production of high tenacity PP fibers of very high modulus, because of so called oriented “smectic” structure present in these fibers. With drawing at elevated temperature, the initial metastable structure of low crystallinity was disrupted and a c‐axis orientation of monoclinic crystalline modification was developed. Hot drawing increased the size of crystallites and crystallinity degree, the orientation of crystalline domains, and average orientation of the macromolecular chains and resulted in extensive fibrillation and void formation. It was found that the blend composition has some influence on the structure of discontinuously highly drawn fibers. With increasing the content of the molding‐grade polymer in the blend, the size of crystalline and amorphous domains, density and crystallinity, as well as amorphous orientation decreased. Relationship has been established between the mechanical properties, crystallinity, and orientation of PP fibers. It was confirmed that by blending the fiber‐grade CR‐polymer by a small percentage of the molding‐grade polymer, maximization of elastic modulus is achieved, mainly because of higher orientation of amorphous domains. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1067–1082, 2006     

Injection molding of glass fiber reinforced phenolic composites. 2: Study of the injection molding process

This study of injection molding of glass fiber reinforced phenolic molding compounds examines fiber breakage and fiber orientation with key material and processing variables, such as injection speed, fiber volume fraction, and the extent of resin pre-cure. The fiber orientation, forming discrete skin-core arrangements, is related to the divergent gate to mold geometrical transition, the extent of pre-cure and injection speed functions of the melt viscosity. Transient modifications to the melt viscosity during mold filling produce variations in skin/core structure along the flow path, which are correlated to the mechanical properties of injection moldings. The melting characteristics of the phenolic resin during plasticization impose a severe environment of mechanical attrition on the glass fibers, which is sequentially monitored along the screw, and during subsequent flow through runners and gates of various sizes. Differences found between the processing characteristics of thermosets and thermoplastics raise questions concerning the applicability of thermoplastic injection molding concepts for thermosets.     

Blends of thermoplastic polyurethane and polypropylene. I. Mechanical and phase behavior

Pure thermoplastic polyurethane (TPU), polypropylene (PP), and TPU/PP blends with different weight ratios prepared in a twin‐screw extruder were investigated by dynamic mechanical analysis (DMA), the universal tester for mechanical investigation, and by wide‐angle X‐ray diffraction (WAXD). The addition of PP above 20 wt % to the TPU stepwise changed the ductility and Young's modulus, i.e., apparently a kind of ductile → brittle transition occurred between TPU/PP 80/20 and TPU/PP 60/40 blends. This fact and the result of analysis of WAXD curves indicated matrix → dispersed phase inversion in this concentration region. TPU melt enabled easier migration of the PP chains and prolonged crystallization of PP matrix during solidification process affecting thus crystallite size, orientation, and crystallinity. In accordance to this fact, DMA results indicated partial miscibility of PP with polyurethane in the TPU/PP blends due to the lack of interfacial interaction and adhesion between the nonpolar crystalline PP and polar TPU phases. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104, 3980–3985, 2007     

Birefringence in injection‐compression molding of amorphous polymers: Simulation and experiment

The influence of the processing variables on the residual birefringence was analyzed for polystyrene and polycarbonate disks obtained by injection‐compression molding under various processing conditions. The processing variables studied were melt and mold temperatures, compression stroke, and switchover time. The modeling of flow‐induced residual stresses and birefringence of amorphous polymers in injection‐compression molded center‐gated disks was carried out using a numerical scheme based on a hybrid finite element/finite difference/control volume method. A nonlinear viscoelastic constitutive equation and stress‐optical rule were used to model frozen‐in flow stresses in moldings. The filling, compression, packing, and cooling stages were considered. Thermally‐induced residual birefringence was calculated using the linear viscoelastic and photoviscoelastic constitutive equations combined with the first‐order rate equation for volume relaxation and the master curves for the Young's relaxation modulus and strain‐optical coefficient functions. The residual birefringence in injection‐compression moldings was measured. The effects of various processing conditions on the measured and simulated birefringence distribution Δn and average transverse birefringence <n_rr?n_θθ> were elucidated. Comparison of the birefringence in disks manufactured by the injection molding and injection‐compression molding was made. The predicted and measured birefringence is found to be in fair agreement. POLYM. ENG. SCI., 2013. © 2013 Society of Plastics Engineers     

Formation and transformation of hierarchical structure of β-nucleated polypropylene characterized by X-ray diffraction, differential scanning calorimetry and scanning electron microscopy

Commercial grade isotactic polypropylene has been modified with a specific β-nucleant (N,N′-dicyclohexylnaphthalene-2,6-dicarboxamide) in two concentrations (0.03 and 0.10 wt%). Specimens for structural characterization have been prepared by injection moulding, subsequent melting and re-crystallization or solid-state drawing at 100 °C. Individual levels of hierarchical structure, including molecular orientation, have been characterized by a combination of wide angle X-ray scattering (WAXS), differential scanning calorimetry and scanning electron microscopy. Based on the analysis of the azimuthal reflections (110) and (300), the Hermans orientation functions have been calculated separately for the crystalline phases α and β. Besides the longitudinal orientation along the injection-moulding direction, β-crystallites tilted to the injection-moulding direction have been found. Upon thermal treatment the fraction of the crystalline β-phase has decreased and molecular alignment within the crystalline regions has improved. During solid-state drawing the fraction of the crystalline β-phase was markedly decreasing with increasing draw ratio, while the overall crystallinity has not changed but slightly. The experiments have also revealed a disruption of molecular alignment at the beginning of the drawing process and subsequent distinct improvement of molecular orientation along the draw direction in crystallites α and β. The Hermans orientation functions provided by the WAXS analysis have been compared with recently published data obtained with similar specimens by polarized photoacoustic spectroscopy.     

Controlling of threadline dynamics via a novel method to develop ultra‐high performance polypropylene filaments

A detailed study was conducted to investigate the effects of horizontal isothermal bath (hIB) on the production of ultra‐high performance polypropylene filaments. Two different commercial PP polymers were used with the melt flow rate of 4.1 and 36 g/10 min. The optimum process conditions depended on polymer molecular weight. Fibers showed distinct precursor morphology for each at each optimum process condition. However, two sets of filaments demonstrated similar fiber tenacity and modulus of about 7 and 75 g d^?1, respectively, for as‐spun and more than 12 g d^?1 for tenacity and more than 190 g d^?1 for modulus values of drawn fibers after just 1.49 draw ratio. The mean value for the modulus after the drawing process for the high melt flow rate was 196 g d^?1. The theoretical modulus of PP is 35–42 GPa¹⁹, (275–330 g d^?1), shows the hIB fiber's modulus performance is approaching its theoretical maximum value. Fibers had greatly improved thermal properties, degree of crystallinity, crystalline and amorphous orientation factors. The hIB spinning system produced highly oriented and predominantly amorphous structure for as‐spun fibers and a well‐defined, highly oriented crystalline fibrillar and amorphous structure after drawing process with the draw ratios lower than 1.5. POLYM. ENG. SCI., 55:327–339, 2015. © 2014 Society of Plastics Engineers