Mechanical Behavior of Bulk Poly(Ethylene Terephthalate) Subjected to Simple Shear

Abstract

Mechanical behavior of bulk semicrystalline poly(ethylene terephthalate) (PET) processed through simple shear is investigated. The equal channel angular extrusion (ECAE) process was used to achieve the simple shear condition. The PET samples were processed in one and two ECAE passes in the same direction, with the sample rotated 180° about the extrusion axis for the second pass. Microstructural features at the nanometer and micrometer scales were studied by small‐angle x‐ray scattering (SAXS) and scanning electron microscopy (SEM). SAXS results showed that at the nanometer scale, two types of lamellar orientations are induced in both samples, but with different extents of orientation. In the ECAE‐oriented PET structures on the micrometer scale, as revealed by SEM, are well‐defined macrofibrils. However, the fibrillar structures in the sample extruded once are more oriented than those in the sample extruded twice. Fractography investigations suggest that the ECAE‐induced fibrillar structure and stretched amorphous chains are responsible for the change in mechanical properties.     

Study on Properties and Structure of Near Melt Point Extruded High‐Density Polyethylene

The effect of extrusion temperature on the mechanical properties of high‐density polyethylene (HDPE) was examined using solid‐state extrusion (SSE) and melt‐state extrusion (MSE) techniques. Differential scanning calorimetry (DSC) and scanning electron microscopy (SEM) investigations were employed to provide evidence for explaining the relationship between mechanical properties and morphology of extrusion moldings. Extruded from a convergence‐divergence die, compared with samples obtained by MSE, the yield strength of samples obtained by SSE was enhanced in both longitudinal and transverse directions with a ductile failure. The yield strength decreased sharply with increasing extrusion temperature. The maximum longitudinal yield strength of samples extruded at 112°C was 181 MPa with an 87% elongation at break; the corresponding values were 28 MPa and 800% for samples extruded at 140°C (MSE); in the transverse direction the yield strength was 27 MPa with a 101% elongation at break for samples extruded at 140°C, while the maximum yield strength was 51 MPa with a 45% of elongation at break for samples extruded at 116°C. Compared with sheets extruded at 140°C, DSC data shows a 5.3°C increase in melting point, a 9.5°C decrease in melt point width, and a 7.1% decrease in crystallinity for sheets extruded at 112°C. SEM indicates that spherulites predominate in MSE samples, while a preferred orientation of the lamellae along the extrusion direction were mainly produced by SSE.     

CO2-induced Crystallization of Isotactic Polypropylene by Annealing Near Its Melting Temperature

CO₂-induced crystallization of isotactic polypropylene (iPP) by annealing had been studied using differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS). The iPP before annealed was in α-form and amorphous states. At lower temperatures by CO₂ isothermal treatments, iPP chains crystallized from the amorphous phase and only one crystal form, i.e., α-form, was observed. At higher temperatures by CO₂ isothermal treatments, both crystallization from the amorphous phase and thickening of existing crystal lamellae were observed. Moreover, light γ-form crystal appeared in the treated iPP. The crystalline lamellar thickness of iPP annealed at different CO₂ pressures had been determined. Using the Gibbs–Thomson plot method, the equilibrium melting temperature was found to be 187.6°C.     

Morphological Distribution in Micro-Injected Polypropylene Parts in the Presence of β-Nucleating Agent

Micro-injection molding is attracting much attention nowadays. Characterization of the morphological distribution in parts prepared by micro-injection molding is thus of growing importance. The morphological features of micro-parts may strongly differ from those of the macro-parts prepared by conventional injection molding, resulting in specific physical properties. In the present study, β-nucleated isotactic polypropylene micro-parts (μPPB) with 200 μm thickness, as well as macro-parts (PPB) with 2000 μm thickness, were prepared. Polarized light microscopy (PLM), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and wide-angle X-ray diffraction (WAXD) were used to investigate their morphological features. The results show that the morphology distribution in μPPB had many differences from that of the PPB. The one-dimensional WAXD and DSC analysis showed similar results; the degree of crystallinity of the μPPB was higher than that of the PPB. However, the content of β-crystals of μPPB was lower than that of the PPB. This can be explained by the restraining effect for the formation of β-crystals in β-nucleated isotactic polypropylene (iPP) under the strong shear field. The through-the thickness-morphology of both μPPB and PPB exhibited a “skin-core” structure from PLM observations, but the former had a large fraction of shear layer in comparison to the latter implied. The SEM observations showed that the shear layer of μPPB consisted of a highly oriented shish-kebab structure, while that of the core layer consisted of deformed spherulites structure. The two-dimensional WAXD pattern of the core layer of PPB, showing full Debye rings, indicated an overall random orientation of the iPP chains, while the arcing indicated a pronounced orientation in the shear layer. The more pronounced arcing of the μPPB indicated a more pronounced orientation.     

A Novel Strategy for Achieving High Melt Strength Polypropylene and an Investigation of Its Foamability

A novel and feasible strategy for preparing high melt strength polypropylene (HMSPP) was developed by a melt grafting reaction in the presence of macro-monomer vinyl polydimethylsiloxane (VS), co-monomer styrene (St), and initiator benzoyl peroxide through a one-step reactive extrusion. The rheological behaviors, melt strength, and foaming ability of HMSPP were studied. The results showed that VS and St were successfully grafted onto polypropylene (PP), and the weight average molecular weight, molecular weight distribution, and melt strength of HMSPP were dramatically increased compared with those of the virgin isotactic polypropylene (iPP). Especially, the melt strength of HMSPP increased from 0.022 N for the virgin iPP to 0.29 N, which made the foamability of HMSPP significantly improved when supercritical carbon dioxide was used as the blowing agent. The foaming condition was optimized to 160°C and 14 MPa, for which HMSPP foams with a high expansion ratio of 66 times and a high cell density of about 5.8×10⁷ cell/cm³ were obtained, while the virgin iPP did not yield foams with good cell structure. Moreover, the resultant HMSPP could be foamed in a remarkably wide temperature range from 145°C to 165°C, which would be of great significance for industrial application.     

Skin Thickness and β-Crystals Development in Injection-Molded iPP Along the Flow Direction

In this study, iPP was injection molded at 180°C, 200°C, and 220°C. According to polarization optical microscopy (POM) results, for a given part, the skin thickness steadily decreases along the flow direction. However, at the same distance from the gate, the skin thickness of the parts molded at lower melt temperature is larger than that molded at higher melt temperature. It is found that flow time (here, the time taken for melt to pass the specific position along the flow direction) and melt temperature are two significant factors leading to this phenomenon, while the gate size is another one.

The DSC and WAXD results show that the relative fraction of β-form crystals, for a specific part, decreases along the flow direction, which is mainly determined by flow time. However, for the parts molded at different molding temperatures, the fraction of the β-form crystals is mainly determined by the molding temperature, though this influence is very complex.     

Large Strain Tensile Deformation and Failure in Oriented Isotactic Polypropylene/Clay Nanocomposite: Role of Voiding and Molecular Orientation

Plane strain compression of isotactic polypropylene iPP)/clay nanocomposite in a channel die at 140 and 160°C, respectively, has been adopted to prepare oriented samples with well-controlled structure for comparative studies. Molecular orientation in the amorphous phase, independent of clay loadings, decreases with increasing preparation temperature, whereas crystallographic orientation is nearly the same for all oriented samples. Severer voiding and void coalescence during stretching, mostly induced by the crystals and inter-chain sliding in the amorphous phase, respectively, is suggested to be responsible for higher volume dilatation and lower failure strain in the oriented samples prepared at higher temperature (e.g., 160°C). Fracture toughness is well correlated with the molecular orientation and crystal-dependent voiding in the oriented samples with respect to preparation temperatures. Furthermore, debonding of clay in the iPP matrix, especially in the oriented samples prepared at 140°C, is another contributor to the enhanced toughness.     

A Comparison of the Effects of Calcium Glutarate and Pimelate on the Formation of β Crystalline Form in Isotactic Poly(propylene)

The effect of calcium glutarate (Cagt) and calcium pimelate (Capt) on the formation of β crystalline form in isotactic poly(propylene) in the crystallization temperature range of 110–130°C has been investigated. The content of β phase crystals increase with the addition of calcium glutarate. K (relative content of β crystalline form in the iPP sample) attains its maximum value for iPP doped with 0.3 wt.% Cagt isothermally crystallized at 110°C (26.71%) or 120°C (30.27%), and for iPP doped with 0.2 wt.% Cagt isothermally crystallized at 130°C (31.97%), respectively. Compared with the K values of iPP doped with 0.1 wt.% Capt (78.33–94.76%), the β nucleation ability of Cagt is inferior to that of Capt. The spherulite size of iPP doped with Capt is smaller than that of iPP doped with Cagt. The difference in the β nucleation ability between Cagt and Capt is explained by the difference between their crystal structure parameters and those of β‐iPP.     

Micro and Macro Injection Molded Parts of Isotactic Polypropylene/Polyethylene Blends: Shear-Induced Crystallization Behaviors and Morphological Characteristics

The present study focused on the importance of scale effect (micro- and macro-injection molded parts) and iPP content to the formation of epitaxial crystallization and crystal structure formed in injection-molded bars of high-density polyethylene (HDPE)/isotactic polypropylene (iPP) blends. After making the blends with different iPP content via melt mixing, the injection-molded bars were prepared via both micro and conventional injection molding. Hot stage polarized light microscopy (HS-PLM), scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) were used to investigate their morphological and crystal features. The results indicated that an appropriate matching of micro-part and relative high iPP content was most favorable for epitaxial crystallization. The micro-parts had a large fraction of shear layer in comparison with macro-parts. The SEM observations showed that the shear layer of the former consisted of a highly oriented shish-kebab structure. The memory effect of the crystalline structure of the micro-parts and macro-parts at high temperature, investigated in detail through HS-PLM experiments, showed that micro-part had a relatively high memory effect of the preceding crystallization process.     

Observation of transcrystalline growth of PLA crystals on sisal fibre bundles and the effect of crystal structure on interfacial shear strength

Hot-stage microscopy was used to characterise crystal growth at the interface between sisal fibre bundles and a polylactic acid (PLA) matrix in order to better understand the mechanical properties of sisal fibre–PLA composites. Cooling rates and crystallisation temperatures and times were varied to influence crystalline morphology at the interface. Single sisal fibre bundles were evaluated in their as received state or treated with 6 wt.% caustic soda solution for 48?h at room temperature. A microbond shear test was used to characterise the shear strength of the interface as a function of fibre surface treatment. These tests were performed on sisal fibre bundles carefully embedded in flat films of PLA supported on card mounts. Fibre bundles in a PLA matrix were cooled from 180?°C at rates from 2 to 9?°C/min and then crystallised isothermally. For as received fibre bundles uneven growth of PLA spherulites occurred at all cooling rates and crystallisation temperatures. For caustic soda treated fibres, uneven spherulitic growth was observed at crystallisation temperatures at and above 125?°C. In contrast, transcrystalline growth was observed for samples cooled to 120?°C at cooling rates from 2 to 6?°C/min and then allowed to crystallise. The microbond shear strengths of untreated and caustic soda treated fibre bundles were evaluated using Weibull statistics and the caustic soda treated fibres exhibited higher interfacial shear strengths in comparison to untreated fibres, reflecting the development of a transcrystalline layer at the fibre to matrix interface.     

Effect of Die Temperature on Morphology and Performance of Polyethylene Pipe Prepared via Mandrel Rotation Extrusion

The effect of die temperature on the mechanical performance and morphology of polyethylene (PE) pipe prepared via mandrel rotation extrusion is described. The experimental results showed that during the rotation extrusion of PE pipe, the hoop flow caused by the mandrel rotation was superimposed on the axial flow to deviate the formed shish-kebab from the axial direction which was favorable to improve the hoop strength of the PE pipe. However, high die temperature caused relaxation of most of the oriented molecular chains and the consequent formation of isotropic crystals in the PE pipe, whereas too low a die temperature led to imperfections in the PE pipe. As a result, there was an optimum temperature range for the enhancement of the hoop strength. When the mandrel rotated at 6 r/min, the hoop strength of the PE pipes prepared at the die temperature of 170°C reached the maximum value, 31.8 MPa, 22% and 29% higher than that at 150°C and 210°C, respectively.     

Formation of β-iPP in Isotactic Polypropylene/Acrylonitrile–Butadiene–Styrene Blends: Effect of Resin Type,Phase Composition,and Thermal Condition

The formation of β-iPP (β-modification of isotactic polypropylene) in the iPP/ABS (acrylonitrile–butadiene–styrene), iPP/styrene–butadiene (K resin), and iPP/styrene–acrylonitrile (SAN) blends were studied using differential scanning calorimery (DSC), wide angle X-ray diffraction (WAXD), and scanning electron microscopy (SEM). It was found that α-iPP (α-modification of isotactic polypropylene) and β-iPP can simultaneously form in the iPP/ABS blend, whereas only α-iPP exists in the iPP/K resin and iPP/SAN blend samples. The effects of phase composition and thermal conditions on the β-iPP formation in the iPP/ABS blends were also investigated. The results showed that when the ABS content was low, the ABS dispersed phase distributed in the iPP continuous phase, facilitating the growth of β-iPP, and the maximum amount of β-iPP occurred when the composition of iPP/ABS blend approached 80:20 by weight. Furthermore, it was found that the iPP/ABS blend showed an upper critical temperature T _c * at 130°C for the formation of β-iPP. When the crystallization temperature was higher than the T _c *, the β-iPP did not form. Interestingly, the iPP/ABS blend did not demonstrate the lower critical temperature T _c ** previously reported for pure iPP and its blends. Even if the crystallization temperature decreased to 90°C, there was still β-iPP generation, indicating that ABS has a strong ability to induce the β-iPP. However, the annealing experiments results revealed that annealing in the melt state could eliminate the susceptibility to β-crystallization of iPP.     

Solid-state morphology of aromatic-aliphatic polyester crystallized from smectic liquid crystal

The solid-state morphology of poly(hexamethylene p,p'-bi-benzoate) (BB-6) and its dependence on the texture of the smectic liquid crystal state from which it crystallized were studied using electron microscopy, polarizing optical microscopy, and small-angle x-ray diffractometry. Thin films of BB-6 suitable for the microscopy were prepared by casting a hot solution of the polymer in tetrabromoethane onto mica at 150°C. The as-cast film of BB-6 was first heated to 260°C, maintained in the smectic range (210°C), and then cooled gradually to room temperature. A number of types of defect structures—such as Dupan cyclides, edge dislocation, tilt walls, and disclinations — were observed in the solid BB-6 formed from the smectic melt. On the basis of the electron microscopy and small-angle x-ray diffractometry, it is suggested that folded-chain crystals are formed during the crystallization from the smectic melt. Electron microscopic observations of the solid BB-6 and its orientation behavior under shear can be reasonably explained by assuming the presence of a precursory chain-folded layer structure in the smectic liquid crystal of BB-6.     

Study of Metallocene-Catalyzed Linear Low-Density Polyethylene Solid-State Extrusion: Processing and Mechanical Properties

The effect of extrusion temperature and extrusion drawing ratio (EDR) on the die swell ratio (DSR) and mechanical properties of metallocene-catalyzed linear low-density polyethylene (m-LLDPE) was examined with the application of solid-state extrusion (SSE). Scanning electron microscopy (SEM) was employed to characterize the microstructure and morphology of the extrudates. Extruded from a convergence-divergence die, compared with samples obtained by melt-state extrusion (MSE), the DSR decreases for SSE samples prepared at low extrusion temperature and high EDR. Mechanically strong SSE samples were also obtained at low extrusion temperatures and high EDR. Mechanically strong SSE samples were also obtained at low extrusion temperatures and high EDR. SEM indicates that the microstructures of the MSE samples consist primarily of ring-banded spherulites; the microstructure of the SSE samples was microfibers oriented along the direction of extrusion. The highly oriented microfibers contribute to the improved mechanical strength of the SSE samples.     

Properties and Crystallization Behaviors of Isotactic Polypropylene Under Action of an Effective Nucleating Agent

The calcium salt of hexahydrophthalic acid (Hyperform HPN-20E) is an effective nucleating agent for polyethylene which was developed by Milliken Chemical Co., (USA) in recent years. In this paper, the properties and crystallization behaviors of isotactic polypropylene (iPP) in the presence of Hyperform HPN-20E were investigated by using differential scanning calorimetry and polarized optical microscopy. Addition of Hyperform HPN-20E improved the tensile, flexural and optical properties of iPP significantly and increased the crystallization rate of iPP greatly. The nucleation effects were comparable to the nucleation efficiency of a highly effective commercial iPP nucleating agent Hyperform HPN-68. When the addition amount of Hyperform HPN-20E in iPP was 0.2 wt.%, the tensile strength, tensile modulus, flexural strength, and flexural modulus of iPP were increased by 10.81%, 8.65%, 16.67%, and 11.96%, respectively, compared to those of pure iPP; the haze value was decreased by 42.44% and the crystallization peak temperature was increased by 11.2°C. In addition, incorporation of Hyperform HPN-20E in iPP greatly reduced the spherulite size of iPP.     

Synthesis and Effect of Ammonium 2,2′-Methylene-bis-(4,6-di-t-Butylphenylene) Phosphate as Nucleating Agent of Poly(Propylene)

A nonmetallic organophosphate salt, ammonium 2,2′- methylene-bis-(4,6-di-t-butylphenylene) phosphates (An), was synthesized via a simple method, and its application in iPP as a nucleating agent was investigated. Differential scanning calorimetry (DSC) result showed the melting temperature of An was 262°C, and SEM (scanning electron microscopy) observations indicated that its crystallization morphology was lamellar shape with a quite smooth surface. The crystallization behavior of nucleated isotactic poly(propylene) (iPP) containing An demonstrated that An can effectively raise the crystallization peak temperature and reduce the spherulite size, resulting in obviously improved strength and transparency. The tensile and flexural strength of iPP nucleated with An was increased by 11% and 32%, respectively. The haze value of iPP/An was decreased from 36.7% to 15.5%. These results revealed that this organophosphate ammonium salt can be used as an effective nucleating agent of iPP and provides a new organophosphate salt type nucleating agent for polymers.     

On the molecular mechanism of the crystal transformation (tetragonal-hexagonal) in polybutene-1

The phase transformation from the tetragonal to the hexagonal crystal modification in highly oriented lamellae of poly-butene-1 has been followed by transmission electron microscopy (TEM). It is found that the reaction-controlling step is the nucleation process. No lattice orientation relationship (besides the [001]-direction, which is parallel in both crystal modifications) exists between non-transformed and transformed crystals. The nucleation is strongly enhanced by thermal or external stresses. Crystal growth, nucleated by external stresses, was observed at temperatures as low as — 150°C. The molecular mechanisms of the transformation are discussed.     

Synthesis of Core-Shell Latex Particles Through One-Step Emulsion Polymerization

Polystyrene/polyacrylate acid (PSt/PAA) core-shell latex particles were obtained through a one-step emulsion polymerization by making use of controlled distribution of the reaction monomer at different temperatures. The polymerization conversion and stability were characterized to find appropriate synthetic conditions. The results show that the mixed solution of certain proportions of St, AA, and distilled water were inhomogeneous at lower temperature, e.g. 25°C, but the mixed solution would become more and more homogeneous with increasing temperature. The mixed solution became completely homogeneous at 75°C. After this, according to the results of uniformity degree change, the pre-emulsion temperature was set at 75°C, and the polymerization temperature was 65°C. α,α-Azobisisobutyronitrile was used as initiator. Stable latex and latex particles, with obvious core-shell structure, were obtained. The composition of the mixed solution was characterized by UV-vis spectrophotometry. The morphology of latex particles was observed using transmission electron microscopy and scanning electron microscopy. The research provides a new method to obtain polymer particles with a core-shell structure.     

Nature of Shear-Induced Primary Nuclei in iPP Melt

Although observations of molecular processes in the formation of primary nuclei prior to actual crystallization are beyond the detection limits of current instrumentation, we attempted to probe the nature of primary nuclei in sheared isotactic polypropylene (iPP) polymer melt. In situ rheo-SAXS (small-angle X-ray scattering) and -WAXD (wide angle X-ray diffraction) experiments using synchrotron radiation were carried out to evaluate the effects of an addition of a high molecular weight atactic polypropylene (aPP) (5 wt%), which is compatible with the iPP matrix but does not crystallize, on the evolution of oriented structures in the sheared iPP melt and its crystallization kinetics. It is unlikely that the aPP chain segments can be incorporated into iPP nuclei or crystal; hence, its addition effects, if any, would be seen only in the amorphous melt prior to crystallization. The results showed stonger orientation and improved crystallization kinetics in the iPP/aPP blend compared to pure iPP. Observations that the presence of long chains of an amorphous polymer aid in nucleation and crystallization kinetics of iPP, combined with our previous synchrotron results of sheared iPP melts at high temperature (165°C), lead us to conclude that primary nuclei in iPP most likely consist of liquid-crystalline or mesomorphic bundles of aligned chain segments prior to the formation of crystals.