Solid‐phase extrusion of polyamide‐6 by using combined deformation schemes

Potentialities of new combined deformation schemes, including the solid state extrusion through conical die (ED) and equal‐channel multiple angle extrusion (ECMAE) implemented in different sequence to modify structure and properties of semicrystalline polymers, have been studied for polyamide‐6 as an example. It is shown that deformation by the ED‐ECMAE scheme gives the best complex of physical and mechanical properties. A significant improvement in elastic and strength properties of polyamide‐6 with the conserved high level of plastic characteristics has been observed. There was only a slight anisotropy and dispersion of microhardness across the extrudates. A more uniform oriented structure with lamellae orientation along extrudate's axis has been formed in semicrystalline polymer because the ED‐ECMAE scheme implementation. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

Mechanical investigation of confined amorphous phase in semicrystalline polymers: Case of PET and PLA

The effect of confinement onto the mechanical properties of the amorphous phase of Polyethylene terephthalate (PET) and poly(lactic acid) (PLA) was investigated. These polymers have the advantage of being in bulk amorphous or in semicrystalline state allowing mechanical and physical investigation of the amorphous phase on bulk and confined configuration. Based on small angle X‐ray scattering (SAXS), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA) experiments, the micro‐structural arrangement of the amorphous and crystalline phase, the rigid amorphous fraction, and the visco‐elastic mechanical properties of the different semicrystalline samples were investigated. DSC results help quantifying the rigid amorphous fraction dependence on the crystallinity. DMA measurements lead to quantify the viscoelastic properties of the free and confined amorphous phases for PET and PLA polymers. Indeed, based on the DMA tests, where the maximum of tan(δ) peak is usually related to the glass transition temperature, shifts upon crystallization, the mechanical properties of the restricted and mobile amorphous phase were determined. This result was correlated along with the amorphous phase thickness distribution determined by SAXS results. This observation was bolstered based on literature results about geometrical confinement configurations and their effect on the glass transition temperature of polymeric materials. POLYM. ENG. SCI., 55:397–405, 2015. © 2014 Society of Plastics Engineers     

Puncture deformation and fracture mechanism of oriented polymers

In this study, we investigated the effect of orientation by solid‐state cross‐rolling on the morphology, puncture deformation, and fracture mechanism of an amorphous TROGAMID material and three semicrystalline polymers: high‐density polyethylene (HDPE), polypropylene (PP), and nylon 6/6. In amorphous TROGAMID, it was found that orientation preferentially aligned polymer chains along the rolling deformation direction and reduced the plastic deformation of TROGAMID in a low‐temperature puncture test. The decrease of ductility with orientation changed the fracture mechanism of TROGAMID from ductile hole enlargement failure in the unoriented control to a more brittle delamination failure in TROGAMID cross‐rolled to a 75% thickness reduction. For semicrystalline polymers HDPE, PP, and nylon 6/6, the randomly oriented crystalline lamellae in the controls were first oriented into an oblique angle to the rolling direction (RD) before the lamellae became fragmented and preferentially oriented with the chain axis parallel to the RD. The morphological change resulted in the decrease of ductility in HDPE in the low‐temperature puncture test. In PP and nylon 6/6, the brittle fracture of unoriented controls was changed into ductile failure when they were cross‐rolled to a 50% thickness reduction. This was attributed to the tilted crystal lamellae morphology, which permitted chain slip deformation of crystals with the chain axis parallel to the maximum shear stress direction. With further orientation of PP and nylon 6/6 to a 75% thickness reduction, the failure mechanism changed back to brittle fracture as the morphology transformed into a layered discoid structure with the chain axis of the fragmented crystal blocks parallel to the RD; this prevented chain slip deformation of the crystals. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Properties of a semi-crystalline and an amorphous thermotropic liquid crystalline polymer

The physical properties, thermal stability, rheology and tensile properties of a commercial semi-crystalline and an amorphous thermotropic liquid crystalline polymer (TLCP) have been investigated. Analysis by differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA) confirm the presence of a small melting endotherm and a glass transition in the former material. The as-received amorphous TLCP exhibits no obvious melting endotherm and a strong glass transition is detected. The flow and tensile properties of the semicrystalline polymer are dominated by the presence of the crystalline to nematic transition temperature. The properties of the amorphous TLCP appear to be governed by increasing mobility afforded by increasing temperature. Based on flow behaviour and further DSC analysis it has been shown that under appropriate annealing conditions the as-received amorphous TLCP can develop solid crystalline order.     

Solvent‐ and thermal‐induced crystallization of poly‐L‐lactic acid in supercritical CO2 medium

The effect of different annealing treatments with supercritical carbon dioxide (SCCO₂) on the structural and mechanical properties of semicrystalline poly‐L ‐lactic acid (L ‐PLA) was investigated. 2000, 27,000, 100,000, and 350,000 g mol^?1 molecular weight L ‐PLA polymers were used in the study. The solid‐state processing of L ‐PLA at temperatures lower than the effective melting point led to solvent‐ and thermal‐induced crystallization. Solvent‐induced and isothermal crystallization mechanisms could be considered similar regarding the increase of polymer chain mobility and mass‐transfer in the amorphous region; however, quite different microstructures were obtained. SCCO₂ solvent‐induced crystallization led to polymers with high crystallinity and melting point. On the contrary, SCCO₂ thermal‐induced crystallization led to polymers with high crystallinity and low melting point. For these polymers, the hardness increased and the elasticity decreased. Finally, the effect of dissolving SCCO₂ in the molten polymer (cooling from the melt) was analyzed. Cooling from the melt led to polymers with high crystallinity, low melting point, low hardness, and low elasticity. Distinctive crystal growth and nucleation episodes were identified. This work also addressed the interaction of SCCO₂‐drug (triflusal) solution with semicrystalline L ‐PLA. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

On‐line measurement of polymer orientation using ultrasonic technology

The propagation velocity of an ultrasonic shear wave can be used to detect anisotropic behavior in the mechanical properties of a solid. Thus, an ultrasonic shear transducer imbedded in an injection mold produces a signal that is sensitive to polymer orientation. This results in a non‐invasive, on‐line technique for monitoring the orientation of polymer in an injection mold cavity during part cooling and solidification. The technique is shown to be quite sensitive for semicrystalline polymers, but much less effective for amorphous polymers. Sensor results are compared to mechanical tests.     

Linear viscoelastic behavior of poly(ethylene terephtalate) above Tg amorphous viscoelastic properties Vs crystallinity: Experimental and micromechanical modeling

Linear viscoelastic behavior of amorphous and semicrystalline poly(ethylene terephtalate), (PET), was experimentally investigated. PET’s samples with different crystallinities (Xc) were prepared and viscoelastically characterized. Based on our experimental results (properties of the amorphous PET and semicrystalline polymers), micromechanical model was used to, first predict the viscoelastic properties of the semicrystalline polymers and second predict the changes on the viscoelastic properties of the amorphous phase when the crystallinity increases. For the micromechanical modeling of semicrystalline material’s viscoelastic properties, difficulties lie on the used numerical methods (Laplace-Carson transformation) and also on the actual physical and mechanical properties of the amorphous phase. In this paper we tried to simplify the Laplace-Carson-based method by using a pseudo-elastic one that avoids the numerical difficulties encountered before. The time-dependant problem is so replaced by a frequency-dependant set of elastic equations. Good agreement with low crystallinity fraction was found however large discrepancies appear for medium and high crystallinity. The poor agreement raises the issue of which amorphous mechanical properties should be taken as input in the micromechanical model? According to the dynamic mechanical analysis (DMA) experimental data, multiple amorphous phases with different glass transition temperatures were observed for each tested semicrystalline sample. For each sample, new glass transition temperature related to an equivalent amorphous phase was determined. DMA tests done at 1 Hz help estimating the mechanical properties of the new amorphous phase based on its new glass transition temperature. Using the new micromechanical approach developed in this paper, the changes occurring on the viscoelastic behavior of the amorphous phase upon crystallization were estimated. Good agreement was found after comparing the micromechanically estimated amorphous behavior with the experimentally estimated one leading to believe that the physical and mechanical properties of the amorphous phase change upon crystallization and taking on account this phenomenon is a key to a good prediction of the semicrystalline behavior using micromechanical models.     

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.     

Equal‐channel multiangular extrusion of semicrystalline polymers

Potentialities of a new simple shear‐based scheme of the solid‐phase extrusion of polymers, named the equal channel multiple angular extrusion (ECMAE), for modification of semicrystalline polymer structure have been investigated by the example of low‐ and high‐density polyethylene (LDPE, HDPE), polyamide‐6 (PA‐6), and polytetrafluoroethylene (PTFE). The effects of velocity and extrusion temperature, plastic deformation intensity, and accumulated equivalent plastic strain value on properties of a number of crystallizing polymers have been studied. It has been shown that the highest strength characteristics are attained for the extrusion temperatures of (0.8–0.95) of the melting temperature and deformation velocity of (0.6–1.1) mm s^?1. For the ECMAE‐processed specimens, the density, the enthalpy, and the melting temperature have become higher. In the oriented structure of semicrystalline polymers formed by the ECMAE, the lamellae are oriented along the extrudate axis. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

Crystallization behavior and mechanical properties of poly(aryl ether ether ketone)/poly(ether imide) blends

Crystallinity and mechanical properties of blends with different amounts of semicrystalline poly(aryl/ ether ether ketone) (PEEK) and amorphous poly(ether imide) (PEI) polymers have been studied. The blends, prepared by melt mixing, have been investigated by differential scanning calorimeter (DSC) to analyze the miscibility between the components and the final crystalline content. Moreover, for the 20/80 PEEK/PEI blend, crystallization in dynamic and isothermal conditions has been carefully investigated in order to find proper conditions for maximum development of crystallinity. Mechanical tests (static and dynamic) have been performed to evaluate the properties of the as-molded and crystallized blends and to compare them with those of crystalline PEEK and amorphous PEI neat resins. Finally, a few SEM observations have been performed to compare the fractured surface of the blend with those of the pure constituents.     

Effect of physical aging on the microstructure and dynamic mechanical properties of poly(ether sulfone) copolymer

Effects of physical aging on the submicroscopic structure and dynamic mechanical properties of amorphous poly(ether sulfone) copolymer film were studied by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and electron‐microscopy measurements. Heat flow responses were measured after annealing the amorphous samples obtained, by quenching the melt into an ice‐water bath close to but below the glass transition temperature. The extent of aging is related to the supercooling from the glass transition temperature and to the aging time. The activation energy of the aging process was estimated by the Williams–Watt expression (Williams and Watts, Trans Faraday Soc 1970, 66, 80). A systematic study of the influence of aging on the dynamic mechanical properties of poly(ether sulfone) copolymer has also been made. During isothermal annealing, the increase of the temperature of tan δ peak for the α and β′ relaxation with aging time has been observed. The aging in the zone of the β peak has also been investigated and an interpretation of the results was proposed on the basis of foregone theories. The result of electron‐microscopy investigation indicates that poly(ether sulfone) copolymer has formed a local order structure during the physical aging. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 530–534, 2006     

Ultrasonic determination of mechanical moduli of oriented semicrystalline polymers

Ultrasonics has been used for the determination of the mechanical properties of oriented semicrystalline polymers through time-of-flight measurements of elastic waves propagating in various directions within the material. While being nondestructive, such a method allows one to obtain more mechanical moduli with a better accuracy than the conventional tensile tests, especially regarding the shear properties and the Poisson's coefficients. Until now, the approach used to interpret the data was approximate and not rigorous. We present here a self-consistent rigorous approach for interpreting time-of-flight data based on the group velocity including allowance for lateral displacement of the transmitted beam. Results are presented for roll-drawn PET with various draw ratios. These samples are considered to have transversely isotropic symmetry. For the Young's moduli, comparisons are made with conventional tensile tests and the differences observed are interpreted in terms of viscoelastic efforts considering both the amorphous and crystalline phases.     

Effect of Molecular Weight on the Strength of Nylon 6 and PET Fibers

Abstract

The mechanical properties of polymers depend on their morphological characteristics. With many systems and especially with semicrystalline polymers, it is possible to achieve significant changes in structure and properties by varying their thermal and stress histories. Considering the broad range of polymer applications it would, therefore, be desirable to have a quantitative understanding of the effects of structure on the properties of polymers in the solid state. The morphology of polymers is, however, often so complex that the interpretation of the mechanical responses must be restricted to qualitative considerations. In particular, the analysis of strength of un-oriented semicrystalline polymers is quite complex because of large plastic deformations which usually precede the rupture. However, by orienting the polymers in the form of fibers, it is possible to achieve structures which can be ruptured in tension with minimum plastic deformation. Under these conditions, the analysis of mechanical properties is greatly facilitated for fibers. Nevertheless, we still lack a quantitative understanding of the mechanical responses of polymeric fibers mainly because their structure has not yet been described with a precision allowing rigorous treatments.     

Characterization of biaxial strain of poly(l‐lactide) tubes

Poly(l ‐lactide) in its l ‐form has promising mechanical properties. Being a semicrystalline polymer, it can be subjected to strain‐induced crystallization at temperatures above T_g and can thereby become oriented. Following a simultaneous (SIM) biaxial strain process or a sequential (SEQ) biaxial strain process, the mechanical properties of biaxially strained tubes can be further improved. This study investigated these properties in relation to their morphology and crystal orientation. Both processes yield the same mechanical strength and modulus, yet exhibit different crystal orientation. Through further wide angle X‐ray scattering analysis it was found that the SEQ biaxial strain yields larger interplanar spacing and distorted crystals and looser packing of chains. However, this does not influence the mechanical properties negatively. A loss of orientation in SEQ biaxially strained samples at high degrees of strain was detected, but was not seen for SIM biaxial strain and did not correlate with mechanical performance in either case. However, post‐annealing reduced the orientation to the same level in both cases, and the modulus and strength decreased for both SIM and SEQ biaxial strain. It is therefore concluded that mechanical properties after biaxial strain are related to strain‐induced amorphous orientation and the packing of crystals, rather than strain‐induced crystallinity. © 2015 Society of Chemical Industry     

Effect of liquid isothermal bath position in modified poly(ethylene terephthalate) PET melt spinning process on properties and structure of As-spun and annealed filaments

The ability to produce as-spun poly(ethylene terephthalate) (PET) filaments that possess previously unsurpassed levels of as-spun orientation and tensile properties was achieved through the implementation of a device described as a liquid isothermal bath (LIB). Although much has been published regarding the general effect of the LIB on various properties and structural features, the results of the present study further contribute to the continued development of this unique technology by investigating the positional dependence of the device, as well as the effect of a subsequent annealing process. Characterization methods employed in the present study included birefringence, percent crystallinity, tensile properties, loss tangent temperature dependence, DSC melting behavior, and wide-angle and small-angle X-ray scattering. Strong inferences drawn from the loss tangent temperature dependence indicate that all of the as-spun and annealed LIB filaments possess a more rigid amorphous phase than that present in either the as-spun or annealed no LIB filament and that the extent of rigidness appears to become more profound as the bath is operated at a position more distant from the spinneret. DSC melting endotherms of the as-spun LIB filaments consist of dual overlapping peaks, one component of which is believed to represent the presence of a novel extended chain type of crystalline structure. Application of a simple two phase model allowed for the quantitative evaluation of an amorphous orientation factor, which was found to range, depending on the bath position, from 1.7 to 3.9 times higher in the as-spun LIB filaments than that present in the as-spun no LIB filament. The annealing process was found to play an important role in facilitating the transformation from an as-spun highly oriented and predominantly amorphous structure to a well-defined semicrystalline fibrillar structure. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 2051–2068, 1998     

Solid-phase flow behavior of polymers

The solid-phase flow behavior of polymers is very important in the mechanical performance and testing of solid polymers and in solid-phase forming. This paper includes an extensive characterization of the solid-phase flow curve for a wide range of commercially important polymers. Rigid semicrystalline, ductile semicrystalline, tough ductile amorphous, and two-phase ductile amorphous resins were studied in both tension and compression. It is clearly shown that semicrystalline polymers normally exhibit a load drop upon yielding due only to geometrical strain softening while the amorphous polymers exhibit yield drops due to material strain softening. New flow equations are given that closely model the observed behavior for all types of materials, over the entire range of strain.     

Development of polyurethane engineering thermoplastics. II. Structure and properties

Rigid, tough thermoplastic polyurethanes were synthesized by a two–step technique. These materials presented a complex structure consisting of a continuous, rigid, amorphous, or semicrystalline matrix with a dispersed soft phase. The influence of soft–segment content upon rigidity modulus, heat–distortion temperature, and fracture behavior has been deter-mined on model amorphous polyurethanes. The influence of crystallinity upon final prop-erties has been studied on two materials: a preparation of ours and a commercial product. Various degrees of crystallinity have been induced in the samples by annealing. Phase structure was characterized by differential scanning calorimetry and wide–angle X–ray scat-tering. Some of the polyurethane samples assayed in this paper exhibited similar properties to those reported in respect of more conventional engineering thermoplastics, such as ABS or nylon. © 1993 John Wiley & Sons, Inc.     

Synthesis and characterization of biodegradable non-toxic poly(ester-urethane-urea)s based on poly(ε-caprolactone) and amino acid derivatives

Non-toxic biodegradable linear poly(ester-urethane-urea)s with different hydrophilic character were synthesized from poly(ε-caprolactone) as macrodiol, l-lysine diisocyanate (LDI) and ethyl ester l-lysine or l-ornithine as chain extenders. Linear segmented polymers were synthesized by the prepolymer method with a tertiary amine playing a critical role in the chain extension in heterogeneous conditions. The prepared segmented polymers were fully chemically and physically characterized, including water uptake and hydrolytic stability measurements. Depending on the poly(ε-caprolactone) length, the segmented polymers were amorphous or semicrystalline. The crystallinity degree strongly affected the mechanical properties and water uptake behaviour.     

Generation of a fibrillar core in PET fiber by cold drawing

Cold draw processing of poly(ethylene terephthalate) yarns has been discovered that yields a sheath/fibrillar core (s/fc) microstructure in the each fiber of a yarn. When aged, unoriented, noncrystalline spun yarns were cold drawn, high (>5.7 : 1) draw ratios could be achieved and an s/fc microstructure resulted. Cold drawing also generated a high oriented amorphous content. The tensile and shrinkage properties of yarns with the s/fc microstructure and high oriented amorphous content were examined as function of processing (drawing, annealing, and relaxing). The microstructure was examined by scanning electron microscopy, transmission electron microscopy, small‐angle X‐ray scattering, wide‐angle X‐ray scattering, and optical microscopy. The fibrillar microstructure survives all processing. Management of the oriented amorphous component under heat and tension contributes to the greater stiffness (modulus) and dimensional stability of processed s/fc yarns. The properties of yarns with an s/fc microstructure are compared with more conventional hot drawn yarns. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2335–2352, 1999     

Transparent semi-crystalline polymeric materials and their nanocomposites: A review

Optical transparency is an important property for a material, especially in certain fields like packaging, glazing, and displays. Existing commercial transparent polymeric materials are mostly amorphous. Semicrystalline polymers have often-superior chemical resistance and mechanical properties particularly at elevated temperatures or after solid-state drawing but they appear opaque or white in most cases. This review describes the present state-of-the-art of methodologies of fabricating optically transparent materials from semicrystalline polymers. A distinction is made between isotropic, biaxially stretched, and uniaxially stretched semicrystalline polymers. Furthermore, some functionalities of transparent nanocomposites based on semicrystalline polymers are also discussed. This review aims to provide guidelines regarding the principles of manufacturing transparent high-performance semicrystalline polymers and their nanocomposites for potential applications in fields like packaging, building, and construction, aerospace, automotive, and opto-electronics.