Effect of crystallite size on the deformation and fracture of poly(vinylidene fluoride‐trifluorethylene) ferroelectric copolymer films under uniaxial tension

The deformation and fracture behavior is characterized for poly(vinylidene fluoride‐trifluorethylene) 75/25 mol% copolymer films under uniaxial tension. The films are prepared by solution casting and then annealed at 135°C for 2 h, 6 h, and at 140°C for 2 h, respectively. The results demonstrate that all the copolymer films show the typical polymeric brittle fracture behavior. The copolymer film annealed at 135°C for 2 h shows the best combination of ductility and strength, with the fracture strain reaching 9.6%. Scanning electron microscope observation and X‐ray diffraction analysis are carried out to examine the morphology and microstructure change during uniaxial tension. It is revealed that the average size of the polymer crystallites increases while the content of the crystallites hardly changes as the annealing time or temperature increases. Reduction in the crystallite size leads to enhanced fracture strain and strength for the copolymer film under uniaxial tension. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Interferometric determination of the physical properties of annealed isotactic polypropylene fibers

Automated variable wavelength interference microscope was used to study the effect of annealing process on the physical properties of isotactic polypropylene fibers (4:1 draw ratio, 515 tex, Bolton UK). The isotactic polypropylene (iPP) fibers were annealed at temperatures ranging from 60°C to 140°C. The spectral dispersion curves of refractive indices and birefringence of iPP fibers were determined at different annealing temperatures. The resulting data were used to determine the optical orientation function, orientation angle, degree of crystallinity, and dispersion parameters of the annealed samples. The study indicates that, the measured birefringence, orientation function, and the dispersion of crystallinity of iPP fibers have been improved with the increasing of annealing temperature. POLYM. ENG. SCI., 59:35–41, 2019. © 2018 Society of Plastics Engineers     

Real time synchrotron SAXS and WAXS investigations on temperature related deformation and transitions of β-iPP with uniaxial stretching

In-situ synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) were carried out to investigate the uniaxial drawing-induced deformation and structure transitions of β form isotactic polypropylene (iPP) at varying temperatures (30 °C, 60 °C, 80 °C, 100 °C and 120 °C). The WAXS results indicated that the initial strain for the strain-induced β–α transformation decreases with the tensile temperature according to the engineering stress–strain curves. The SAXS data showed that the long period increased along the direction perpendicular to the tensile force and changed little along the tensile direction with increasing strain in the elastic deformation stage before the yield point. The analysis of the obtained scattering results indicated that the angle between parent and daughter lamella rotates from initial 40° or 140° to close to 90° accounts for the lateral expansion of the samples with tension, which matches the essential auxetical behavior. A structure deformation and transition mechanism was proposed for β form iPP with uniaxial drawing. The initialization of the crystalline structure transition is after the yield point, then the mechanical loading-induced β–α transition seems to be a gradual process with lamella slippage and breaking which triggers the β–α polymorphic transition.     

Annealing‐induced structural rearrangement and its toughening effect in injection‐molded isotactic polypropylene

To improve the toughness of isotactic polypropylene (iPP), by blending modification or special processing techniques, is an ongoing pursuit in the polymer community. In this study, the toughness of injection‐molded iPP bars has been enhanced by about four times upon annealing at 140°C, in parallel with slight increasing of stiffness. Through various structural characterizations it has been confirmed that the remarkable toughening effect in the annealed iPP bars mostly results from the enhanced molecular mobility in the amorphous phase due to lamellar perfection, rather than from the change of molecular orientation, crystal modification, crystallinity, and so on. Moreover, it is deduced that the gain of mobile amorphous fraction in the hierarchical iPP bars upon annealing follows the order of skin > intermediate > core, same to that contributing to the toughness enhancement. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

Phase separation induced morphology evolution andcorresponding impact fracture behavior of iPP/PEOc blends

In this study, the influence of phase separation on impact toughness of isotactic polypropylene (iPP)/poly(ethylene‐co‐octene) (PEOc) blends was investigated. For the typical toughened polymeric system, three iPP/PEOc compositions (80/20, 70/30, and 60/40) were selected. When the polymeric blends were annealed at 200°C, the coarsening of phase domains was more prominent for the blend containing higher content of PEOc, and the scale of its morphological evolution was increased as well. The impact test showed that the impact strength variation trend as a function of annealing time was closely related to morphological evolution. It was believed that the sharpening of phase boundary and coarsening of phase domains were responsible for the depression of impact toughness, and the probable fracture mode alteration from shear banding to crazing and voiding. Structure evolution induced by phase separation showed an important effect on impact toughness, and it was also affected by the environmental conditions. Proper temperature was required to catch the tough‐brittle transition induced by phase separation. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Structural evolution of β – iPP during uniaxial stretching studied by in–situ WAXS and SAXS

Polymorphism and crystal transition are of great significance for property mediation in polymer materials. Isotactic polypropylene (iPP) with β – crystal has been widely utilized for the preparation of high performance plastics or films. In the present work, the structural evolution of initially isotropic β – nucleated iPP (β – iPP) during uniaxial stretching at different temperatures was investigated by in–situ X – ray scattering using synchrotron radiation. The wide – angle X – ray scattering (WAXS) results confirmed that the β – crystal transformed either to the mesophase at lower temperature (30 °C) or to the α – crystal at higher temperature (60, 100 and 120 °C) during stretching. An interesting orientation of β – crystal with molecular chains perpendicular to the tensile direction was identified. As revealed by small – angle X – ray scattering (SAXS), cavitation took place in β – iPP stretched at temperatures lower than 120 °C. The size and shape of the cavities were observed by scanning electron microscope. A deformation mechanism of β – iPP combining the crystal transition, cavitation and orientation was proposed.     

Dispersed microfibril‐dominated deformation and fracture behaviors of linear low density polyethylene/isotactic polypropylene blends

Linear low density polyethylene/isotactic polypropylene (LLDPE/iPP) blends, with oriented microfibrils of iPP dispersed in the nearly isotropic LLDPE matrix, has been prepared via melt extrusion drawing and subsequent thermal treatment at 160°C to melt LLDPE matrix. The presence of oriented microfibrils of iPP in the LLDPE/iPP blends not only promotes the homogenous deformation, with no drop of nominal stress around yield point, but also enhances the fracture toughness significantly. The specific Essential Work of Fracture w_e, which is a pure crack resistance parameter per ligament area unit, is 24.7 and 33.6 N/mm for the blends with 15 and 30 wt % microfibrils of iPP, respectively. Moreover, with the deduced deformation parameters, such as true yield stress and strain hardening modulus, the relationship between deformation parameters and fracture toughness is explored. It is demonstrated that the fracture toughness can be well correlated with the ratio of true yield stress to strain hardening modulus σ_ty/G, and either a decrease in yield stress or an increase in strain hardening can improve fracture toughness. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1291–1298, 2007     

A study on multiple melting of isotactic polypropylene

Multiple melting characteristics of a highly isotactic polypropylene (iPP) were studied by means of differential scanning calorimetry (DSC). Double melting characteristics were observed on melting iPP crystallized isothermally at temperatures ranging from 110 to 140°C. iPP crystallized below and above 125°C exhibited different double melting characteristics from each other. For iPP crystallized below 125°C, the single melting peak split into two peaks during slow DSC heating scans without changing the total crystallinity in the polymer. On the other hand, the double melting endotherm of iPP crystallized above 125°C seemed to come from two preexisting crystal fractions having different T_m. There existed an optimum annealing temperature range where the five-minute annealing of iPP raised T_m of the polymer significantly. The treatment also increased the crystallinity of iPP crystallized isothermally at 110°C by 12%.     

Properties of syndiotactic-rich poly(vinyl alcohol) thin films in water. VI. Elastic behavior of undrawn/annealed thin films by repeated elongation/contraction in water

The elastic behavior of undrawn/annealed swollen thin films of syndiotactic-rich poly(vinyl alcohol), derived from vinyl trifluoroacetate, was studied by repeated elongation/contraction in water. For the films annealed at temperatures below 175°C, the characteristic of deformation was divided into low-drawn and high-drawn regions. The elastic deformation was dominant over the plastic deformation in the low-drawn region and vice versa in the high-drawn region. The effect of heat treatment on the elastic behavior of the swollen films in water almost independent of annealing temperatures below 125°C. The elastic deformation in water at 70°C was especially remarkable in the low-drawn region for the films annealed at temperatures below 125°C and at 80°C for the films annealed at 150°C. For the films annealed at 200°C, considerable plastic deformation occurred in addition to elastic deformation from the initial drawing; the films were broken in the low-drawn region.     

Thermal annealing‐induced superior toughness in polypropylene/poly(ethylene glycol) blend and its structural origin

In this study, a water‐soluble crystallizable polymer, poly(ethylene glycol) (PEG), was compounded with isotactic polypropylene (iPP), and a subsequent thermo‐annealing process was added to improve the toughness of iPP/PEG blend. By adding a small amount of PEG (5 wt%) into iPP, only a mild increment of 40% in toughness was achieved. However, the toughness of iPP/PEG (95/5) blend could be improved remarkably when the postprocessing procedure, thermal annealing, was utilized. For example, the notched impact strength of iPP/PEG blend annealed under 120°C for 12 h was five times of that of neat iPP. In addition, the tensile strength of annealed blend was slightly changed, compared with neat iPP. To ascertain the origin of toughening, various crystallographic and morphological/structural characterizations, including X‐ray diffraction, electron microscopy, and calorimeter were employed. A specific structural change, in which more and more amounts of micrometer/sub‐micrometer voids, yielded within the amorphous iPP region during the annealing process, was responsible for the prominent toughening behavior. POLYM. ENG. SCI., 53:2053–2060, 2013. © 2013 Society of Plastics Engineers     

Microstructural development of liquid-phase-sintered silicon carbide during annealing with uniaxial pressure

β-SiC powders containing 1.1 wt.% α-SiC particles as seeds were hot-pressed at 1800 °C and then annealed at 2000 °C under 25 MPa uniaxial pressure to enhance grain growth. Microstructural development during annealing with pressure was investigated quantitatively and statistically using image analysis. The bimodal grain-thickness distribution in samples annealed with pressure was obtained due to abnormal grain growth of some grains. In situ-toughened microstructure has been developed after 3-h annealing. The grain-thickness and aspect ratio of large grains increase with annealing time, but grain growth comes mainly from increases in thickness after 3-h annealing, owing to the impingement of large gains. Typical flexural strength and fracture toughness of 4-h annealed sample were ∼500 MPa and ∼7.5 MPa m^1/2, respectively.     

Study on structure and property relations of α‐iPP during uniaxial deformation via in situ synchrotron SAXS/WAXS and POM investigations

Micro‐ and meso‐scale structure changes of α‐form isotactic polypropylene (α‐iPP) during uniaxial stretching is studied by time‐resolved synchrotron small‐angle X‐ray scattering (SAXS) and wide‐angle X‐ray scattering (WAXS). The structure/property relations are investigated at different temperatures, and the effects of isothermal crystallization are also studied with POM. The X‐ray scattering results show that the long period increased and the lamellar oriented along the stretching direction in the elastic deformation stage. The lamellar and crystals start destructing after yielding. And from it POM images it can be seen that with higher crystallization temperature the spherulites connected to form a crystalline network, on which the stress is mainly loaded. It turns out different environment temperatures affect mostly the amorphous domains. And samples exhibit different yielding mechanisms with different thermal histories. A hypothetical structural mechanism is proposed based to explain the observed relationship between the processing parameters, thermal history and the structure/property relations of α‐iPP. POLYM. ENG. SCI., 58:160–169, 2018. © 2017 Society of Plastics Engineers     

Enhanced spherulite growth rates of isotactic polypropylene in incompatible blends

Isothermal radial growth rates (G) of isotactic polypropylene (iPP) spherulites in immiscible blends with a polyethylene based ionomer (1) partially neutralized with Zn⁺², (2) completely acidified, and (3) completely esterified have been determined at crystallization temperatures between 118 and 124°C by time-lapse photomicroscopy. In these blends iPP spherulites grow with increased G as compared to that of pure iPP. This behavior is attributed to increases in the rate of transport, due to limited miscibility induced by mechanical mixing. However, subtle differences in the melting behavior suggest that changes in Spherulite morphology may also be a contributing factor.     

Mechanical Characterization of Annealed ZrB2–SiC Composites

Composites consisting of 70 vol% ZrB₂ and 30 vol% α‐SiC particles were hot pressed to near full density and subsequently annealed at temperatures ranging from 1000°C to 2000°C. Strength, elastic modulus, and hardness were measured for as‐processed and annealed composites. Raman spectroscopy was employed to measure the thermal residual stresses within the silicon carbide (SiC) phase of the composites. Elastic modulus and hardness were unaffected by annealing conditions. Strength was not affected by annealing at 1400°C or above; however, strength increased for samples annealed below 1400°C. Annealing under uniaxial pressure was found to be more effective than annealing without applied pressure. The average strength of materials annealed at 1400°C or above was ~700 MPa, whereas that of materials annealed at 1000°C, under a 100 MPa applied pressure, averaged ~910 MPa. Raman stress measurements revealed that the distribution of stresses in the composites was altered for samples annealed below 1400°C resulting in increased strength.     

Nodular structure of isotactic polypropylene crystallizes from the melt

This article highlights the melt crystallization behavior of different grades of isotactic polypropylene (iPP) using a hot‐stage polarizing optical microscopy. iPP samples were heated up at a heating rate of 10°C/min passing the melting temperature and then kept for 3 min at a temperature range of 175–200°C before they cooled rapidly at 40°C/min to crystallize isothermally at a range of 130–145°C. It has been found that the temperature at which the samples were kept has a strong effect on the crystallization mode; for samples heated up and kept at temperatures below 190°C, the crystallization started with thin and long rods or nodules, which grew in the circumferential direction only while their lengths remain unchanged as the time passed. The shape of the nodules can be straight, circular, branched, or entangled, and they can grow parallel to each other or they can be crossed or in a random way. This phenomenon disappeared completely for samples melted and kept at temperatures above 195°C. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Untersuchung der ursachen für das auftreten mehrerer schmelzpeaks in den DSC-thermogrammen von hochverstreckten und getemperten folien aus isotaktischem polypropylen

Isotactic polypropylen films were highly stretched and annealed during 20 s under stretching tension at temperatures up to 206°C at maximum. The melting behaviour of these films was investigated by using a differential scanning calorimeter (DSC — 1 B). Two melting peaks at about 165 and 174°C respectively, were observed. The heat of fusion of the former peak predominates, if the film temperature during annealing exceeds 190°C. Its position is independent of the stretching ratio. This melting peak is assigned to crystalline domains, which show a long identity period of 140 to 160 Å and corresponds to an amorphous orientation factor of 0 to ?0.2. The higher melting peak appears during stretching. This peak is assigned to crystalline domains, which show a long identity period longer than 200 Å and corresponds to an amorphous orientation factor of 0.4 to 0.5. This fraction refers obviously to the paracrystalline building blocks of the microfibrils, which are transformed at a film temperature higher than 190°C to a greater part into smaller lamellaforming paracrystallites. The transformation of the fibrillar into a lamellar texture was visualised by transmission electronmicroscopy.     

Doubly oriented β-form films of poly(vinylidene fluoride)

PVDF sheets, rapidly quenched, were (1) two-step transversely stretched at various temperatures and (2) stretched at various temperatures, rolled at room temperature and then annealed. The orientation patterns of the β-form crystal (which contains the polar b-axis) in these films were analysed on the basis of X-ray diffraction photographs taken with flat and cylindrical cameras. In the case of (1), when both of the two-step transversely stretching temperatures were below 100°C, a doubly oriented film with the plar b-axis oriented parallel to the film surface was obtained. In the case of (2), when the stretching temperature was below 100°C, the sheets then rolled without annealing, another doubly oriented film with the polar b-axis preferentially oriented at 30° to the film surface was obtained. On the other hand, when these films were annealed above 100°C, or the stretching temperatures were above 100°C, orientation patterns in which the polar b-axis was partially rotated through 60° were obtained. The orientation mechanisms of these films are discussed using the measurements of the lattice spacings of the β-form crystal.     

Effect of the composition ratio of pimelic acid/calcium stearate bicomponent nucleator and crystallization temperature on the production of β crystal form in isotactic polypropylene

The influence of the composition ratio of pimelic acid/calcium stearate bicomponent nucleator on the β crystal form content of isotactic polypropylene (iPP) had been studied at the crystallization temperature of 120°C and duration of 30 min. It was found that the β crystal form content increased continuously with increasing amount of calcium stearate at the constant amount of 0.15% pimelic acid. High β crystal form content polypropylene could be produced when the amount of calcium stearate was greater than 0.30% (the mass composition ratio of pimelic acid/calcium stearate was less than 1/2, the mole ratio was less than 1.89/1). It was shown that pimelic acid and calcium stearate could react to produce a high effective β nucleator (calcium pimelate) “in situ” during the melt‐mixing of iPP and the bicomponent nucleator. The influence of crystallization temperatures (100–140°C) on the β crystal form content of iPP had also been studied at the constant composition ratio of 0.15% pimelic acid/0.5% calcium stearate (the calcium pimelate produced in situ was 0.16%, which was calculated from stoichiometry). It was found that the β crystal form content increased continuously with increasing crystallization temperature and it maximized at 130°C. β Crystal form content decreased sharply at the crystallization temperature of 140°C. It was shown that β → α modification transformed between 130 and 140°C. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Trans- and Dimethyl quinacridone nucleation of isotactic polypropylene

Isotaclic polypropylene (iPP) was nucleated specifically for structural and mechanical studies, especially of impact resistance. Nucleation was accomplished by two red pigments: a linear trans-quinacridone dye (as β-phase nucleator) and a dimethyl quinacridone resulting in the formalion of a pure α-phase structure. This procedure allowed the comparison of both the crystal structure and the morphology of the polymer. For the β-nucleated iPP, by means of the WAXS measurements in the transmission mode, a maximum of the k-value (fraction of β-phase) was found for a concentration of 5 × 10^?5 wt% of the trans-quinacridone. By means of polarized light microscopy (PLM), the mold temperature and pigment concentration dependent spherulites distribution on the cross section of the samples was observed. DSC permitted confirmation of the WAXS measurements, giving a characteristic double melting peak for the β polypropylene. By isothermal WAXS measurements the temperature of 135°C as the upper limit of the β phase was detected. For both nucleating agents (linear trans-quinacridone and dimethyl quinacridone) the dependence of the mechanical properties (modulus of elasticity and brittleness by impact resistance) on the pigment concentration was observed. For the β-phase iPP the highest ductility by impact testing corresponds to the highest values of the k parameter. The explanation is based on the SEM observations, which permitted the detection of spherulites with cross-hatched lamellae (for the α-spherulites) and with simple radial growth lamellae for the β-spherulites.     

Effect of high loading rate and low temperature on mode I fracture toughness of ductile polyurethane adhesive

The mode I fracture toughness of an adhesive at low temperatures under high loading rates are studied experimentally. Typical R-curves of the polyurethane adhesive under different loading rates (0.5?mm/min, 50?mm/min, 500?mm/min) at different temperatures (room temperature, ?20?°C, ?40?°C) respectively are obtained. From the experimental results, the mode I fracture toughness of this adhesive is extremely sensitive to the high loading rates and low temperatures. With the increase of the loading rate and decrease of temperature, the mode I fracture toughness of this adhesive decreases significantly. Under the loading rate of 500?mm/min at ?40?°C, the mode I fracture toughness of adhesive is 15% of the value at room temperature (RT) under quasi-static conditions. Through the experiment, the relationship between mode I fracture toughness of this adhesive, nominal strain rate and temperature is obtained.