Plastic deformation of spherulitic semi-crystalline polymers: An in situ AFM study of polybutene under tensile drawing

The plastic deformation of semi-crystalline polybutene (PB) has been studied at the micrometric and nanometric scales by Atomic Force Microscopy (AFM). Owing to a movable tensile drawing stage, capturing images from the same locus of the sample allowed for quasi in situ observations of the plastic processes. In the case of PB films having an average spherulite diameter of about 20 μm, the macroscopic deformation was homogeneous over the whole gauge length of the sample, up to rupture. In parallel, the local deformation at the scale of the spherulites was very close to homogeneous and obeyed an affine deformation law over the whole strain range: the shape of the deformed spherulites was kept roughly elliptical up to rupture without clues of fibrillar transformation. The inter-spherulitic boundaries displayed very high cohesion. Fragmentation of the crystalline lamellae proved to be a predominant process, while crystal slip could not be detected at the scale of the AFM resolution, i.e. a few nanometers. Wide-angle and small-angle X-ray scattering yet revealed the occurrence of crystal plastic shear. Similar observations have been made in the case of PB films having an average spherulite diameter of about 5 μm. In the conclusion, a comparison is made with a previous study regarding the deformation mechanisms of a PB sample having 200 μm wide spherulites which displayed brittle behavior.     

A structural study of the tensile drawing behaviour of poly(ethylene terephthalate)

A study of single-stage and two-stage drawing of poly(ethylene terephthalate) has been undertaken. Measurements of the modulus of the drawn films were combined with a range of structural measurements, including refractive index, X-ray diffraction and infra-red spectroscopy. The development of molecular orientation during drawing is discussed in terms of the deformation of a molecular network, and reasons for the differences between single stage and two-stage drawing are proposed. The relationships between different measures of molecular orientation are considered with the aim of obtaining an understanding of the factors which influence the modulus values. It appears that the modulus relates primarily to the molecular chains which are in the extended trans conformation, irrespective of crystallinity.     

聚乳酸纤维拉伸性能研究

介绍了不同纺丝工艺路线、拉伸倍率、拉伸温度对聚乳酸纤维(PLA)拉伸性能的影响,不规则拉伸的成因以及拉伸工艺对纤维结构性能参数的影响。     

Conformational development of polylactide films induced by uniaxial drawing

Uniaxially drawn polylactide (PLA) films were prepared using various draw ratios (2, 3, 4 and 4.5) at a constant draw rate and temperature. It was confirmed that the conformational structure of the PLA films prepared using the uniaxial drawing process was composed of the α′‐phase form. The conformational structure deformation of α′‐phase PLA films, according to the various draw ratios used, was investigated in terms of strain‐induced crystalline behavior and molecular orientation analysis. It is of utmost importance to confirm the α′‐phase structural deformation caused by the uniaxial drawing process because it directly relates to the characteristics of PLA films. The conformational structure deformations of the α′‐phase, created by uniaxial drawing, led to improved mechanical properties, as evident from mechanical testing and dynamic mechanical analyses results. © 2013 Society of Chemical Industry     

In situ AFM investigation of crazing in polybutene spherulites under tensile drawing

This report deals with the study of the plastic deformation processes of semi-crystalline polymers at the micrometric and nanometric scales by atomic force microscopy. Capturing images from the same locus of the sample as a function of strain allows in situ observation of the processes. New experimental findings regarding initiation, growth and coalescence of crazes in poly(1-butene) are reported. The benefits of the technique are emphasized in comparison with previous studies carried out by transmission electron microscopy on ultra-thin films of various semi-crystalline polymers. The present situation is claimed to be closely representative of bulk deformation owing to the much greater sample thickness in comparison with the characteristic craze size. The occurrence of crazes oblique to the principal tensile stress is discussed in terms of triaxiality of the local stress field within the spherulites.     

Glass transition and structural relaxation in semi-crystalline poly(ethylene terephthalate): a DSC study

The aim of this work is to determine the relaxation times of the cooperative conformational rearrangements of the amorphous phase in semi-crystalline poly(ethylene terephthalate) (PET) and compare them with those calculated in amorphous PET. Samples of nearly amorphous polymer were prepared by quenching and samples with different crystallinity fractions were prepared from the amorphous one using cold crystallisation to different temperatures. The differential scanning calorimetry (DSC) thermograms measured on samples rapidly cooled from temperatures immediately above the glass transition show a single glass transition which is much broader in the case of high-crystallinity samples than in the amorphous or low-crystallinity PET. To clarify this behaviour, the samples were subjected to annealing at different temperatures and for different periods prior to the DSC measuring heating scan. The thermograms measured in samples with low crystallinity clearly show the existence of two amorphous phases with different conformational mobility, these are called Phases I and II. Phase I contains polymer chains with a mobility similar to that in the purely amorphous polymer, while Phase II shows a much more restricted mobility, probably corresponding to conformational changes within the intraspherulitic regions. The model simulation allows to determine the temperature dependence of Phase II relaxation times, which are independent from the crystallinity fraction in the sample and around two decades longer than those of the amorphous polymer at the same temperature.     

Cavitation during tensile drawing of annealed high density polyethylene

Annealing of semicrystalline polymers usually leads to refinement and thickening of crystals. It appears that also cavitation is affected during tensile drawing. In the uniaxially drawn high density polyethylene massive cavitation was detected by X-ray scattering in the samples previously annealed at 125 °C. The number of voids depends on the annealing time, while their size and orientation depends on the local strain. Cavitation resulted in 30% increase in volume for the annealed samples, strained to 4-5. Cavitation and volume increase were not observed for small and intermediate strains if polyethylene samples were not annealed. The decrease in the drawing rate results in the reduction of cavitation and void stability - at the low strain rate voids were detected during tensile drawing, though they disappeared after unloading the sample.     

Polyesters with small structural variations improve the mechanical properties of polylactide

Improving the properties of biodegradable polymeric materials is needed to obtain materials competitive with current bulk plastics. Low‐molecular weight polyesters with small differences in their backbone were synthesized using a straight‐forward method and were subsequently blended with polylactide (PLA). The materials showed an improved ductility of up to 100% points and otherwise retained material properties. The changes in mechanical properties were shown to match the miscibility range of the materials and can be predicted by the solubility parameters of the materials up to a polyester content of roughly 10% w/w. The thermal stability of all the low‐molecular weight polyesters was higher than that of PLA, and most 25% w/w blends showed a thermal degradation behavior similar to that of neat PLA. Low‐molecular weight polyesters were demonstrated as being potential enhancers of the properties of PLA, while the materials degradability was maintained. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Real‐time FTIR and WAXS studies of drawing behavior of poly(ethylene terephthalate) films

The development of molecular orientation and crystallization was studied during uniaxial drawing of poly(ethylene terephthalate) (PET) films, which was immediately followed by subsequent taut annealing at the drawing temperature. The behavior was monitored in real time throughout the drawing and annealing using dynamic FTIR spectroscopy and in situ WAXS measurements using the Daresbury Synchrotron Radiation Source. Films were drawn at 80 and 85°C at varying strain rates (0.001–0.7 s⁻¹). The true stress–strain behavior was determined at each of the drawing conditions and the density and optical anisotropy of unloaded samples was measured. The IR spectra were analyzed using curve reconstruction procedures developed previously, and they showed that orientation of the phenylene groups and the trans glycol conformers occurred before significant gauche–trans conformational changes could be seen. The onset of crystallization, defined as the point that the crystalline 1 05 reflection could be first observed using WAXS, was not found to correlate with any specific change in the proportions of trans and gauche isomers nor with any feature on the stress–strain curve. However, it was clear that, for these comparatively low strain rates, crystallization occurred during the drawing process while the crosshead was moving and the draw ratio was increasing. The orientation of the crystallites was calculated from the 1 05 reflection observed in a tilted film, transmission geometry. The crystallites were found to form at a draw ratio of about 2.5 with high orientation values (P₂ > 0.8) that increased during drawing and annealing to P₂ values of 0.95, irrespective of the drawing conditions. Semiquantitative measurements of crystallinity showed that the fraction of crystalline material that developed during drawing decreased with increasing strain rate. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1825–1837, 2001     

Crystallization studies of semi-crystalline polymers under oscillatory shear using the rheometrics mechanical spectrometer

A method has been developed on the Rheometrics mechanical spectrometer using the eccentric rotating disks mode to study the crystallization kinetics of different semi-crystalline polymers (polyethylene, polypropylene, poly(butylene terephthalate) and Nylon 11) under oscillatory shear. Dynamic shear moduli (storage G′ and loss G″), loss tangent (tan δ), and dynamic viscosity (η′) were simultaneously, monitored during the crystallization process. The onset and completion of crystallization were characterized by the initial rise and final levelling off of G′, while the peak time, of crystallization (t_p) is calculated from the time elapsed between the onset and peak of crystallization which is indicated by the G″ or η′ maximum. In the case of polypropylene, going from a low frequency of ?0.1 rad/s, to higher frequencies of up to 10 rad/s, there is a monotonic decrease in peak time of crystallization (t_p) together with a progressive decrease in spherulitic morphology. The observed acceleration in crystallization is due predominantly to the increase in nucleation rate and orientation of chains in melt crystalline aggregate. The progressive disappearance of the spherulitic morphology is attributed to the disruption of the spherulite superstructure at higher frequencies of shear.     

Study of the production of ultra-high modulus polyoxymethylene by tensile drawing at high temperatures

The tensile drawing behaviour of polyoxymethylene has been studied, with particular reference to the production of ultra-high modulus oriented material. The influence of molecular weight and draw temperature, and the comparative effectiveness of single-stage and two-stage drawing processes have also been examined. In addition to the molecular weight range it appears that both draw temperature and drawing rate must be specified within very narrow limits, if ultra-high modulus material is to be produced. It is tentatively suggested that this is because effective high draw requires a suitable coincidence of rate processes involving both the crystalline and non-crystalline regions of the polymer.     

Elastic contact stress analysis of semi-crystalline polymers under normal and tangential loading

Stress distribution in a polymeric subsurface under the asperity contact is investigated assuming the well known Hertzian contact stress distribution. Elastic stress analyses for three different materials are performed using a finite element method: (1) isotropic elastic solid, (2) isotropic elastic solid with a soft layer, and (3) isotropic elastic solid with a hard layer. Highly linear polymers as high density polyethylene (HDPE), poly(tetrafluoroethylene) (PTFE), and polyoxymethylene (POM) which transfer thin wear films are modeled as the elastic solid with a soft layer. Gammaray irradiated highly linear polymers and the other ordinary semi-crystalline polymers which transfer massive lumpy wear debris are considered as the elastic solid without any heterogeneous surface layer. Helium plasma treated polymers are modeled as the elastic solid with a hard layer. Octahedral shear stress and equivalent strain contours in the subsurface are obtained for each case. The octahedral shear stress and equivalent strain distributions are examined to explain various wear behaviors of semicrystalline polymers based on the Mises yield criterion and the delamination theory of wear.     

Influence of iron onto the structural reorganization process during the sintering of kaolins

Reference kaolins KGa-1b, KGa-2 and KF are used as starting materials to examine the effect of chemically added iron on their structural transformations (near 950–980 °C) during sintering. The equivalent total Fe₂O₃ content is varied from 0.21 to 11.72 mass%. Iron addition leads to strong modification of the structural reorganization path of metakaolinite. Iron ions start to diffuse into metakaolinite network at 900 °C and lead to an early breakdown of the system. The corresponding exothermic peak is shifted from 970 °C to 900 °C. Regarding the heating rate, this peak appears to result from three overlapping reactions. The peak temperatures of these reactions are respectively 925 °C, 950 °C and 970 °C. A low heating rate enhances iron diffusion, promoting the structural reorganization process.     

Manufacturing of polypropylene laminates and related structural reorganization in the crystalline phase

The conditions for obtaining recrystallization in more ordered modifications are described for an uniaxially oriented sample of isotactic polypropylene, annealed with fixed ends. These recrystallization conditions are related to the conditions of a lamination process.     

Flow‐induced crystallization of polylactide stereocomplex under pressure

Although shear and pressure fields always coexist in practical polymer processing, their combined influence on the crystallization behavior of polylactide stereocomplex (SC) is ambiguous due to the limit of experiment device. In that case, a homemade device was employed to prepare samples under the coexistence of shear and pressure and explore the crystallization behavior of SC. Differential scanning calorimetry and synchrotron radiation were used to investigate the combined effect of shear flow and pressure on SC crystallization. The results show that shear flow was helpful for SC formation. Shear flow promoted the phase mixing of the polymer blends and improved the nucleation efficiency of SC. Pressure had a negative effect on SC formation because of the decrease in free volume. Regard to polylactide homogeneous (HC), pressure played a positive role on HC formation. Pressure suppressed the formation of SC network which could impede HC generation. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46378.     

Mechanical and structural properties of polylactide/chitosan scaffolds reinforced with nano-calcium phosphate

In this study, chitosan/polylactide scaffolds reinforced with nano-calcium phosphate (average crystallite size of 16.5?nm) (CP) were fabricated to create a material with excellent properties for bone tissue engineering applications via freeze-casting method. The structural and mechanical properties of nanocomposite scaffolds were studied by increasing amount of chitosan/poly lactide ratio and nano-CP content in both dry and hydrate states, which reflected the exact status of scaffolds in a real biomechanical environment. The morphologies of the nanocomposite scaffolds were viewed using scanning electron microscopy (SEM) and all the scaffolds exhibited a high porosity (up to 92?%) with open pores of 38?C387???m average diameters, which decreased with increased chitosan/polylactide ratio and nano-CP content. Also, SEM photograph of the cross-sectional area of the scaffold showed nano-CP was dispersed all over the polymer matrix thoroughly. The results of mechanical tests showed that the compressive modulus (E) and compressive stress (??) enhanced, when chitosan and nano-CP increased. X-ray diffraction analysis indicated typical chitosan, polylactic acid and nano-CP peaks and showed that the increase in nano-CP weight percentage increased its peak intensities. In addition, the effect of pore-size distributions of the scaffolds with the same composition was studied in relation to mechanical properties. The results showed substantial differences in the pore-size distributions of scaffolds with the same composition prepared, which have no effect on their dry states.     

Crystallization behavior of polylactide matrix under the influence of nano‐magnetite

Iron oxide nanoparticle (Fe₃O₄) has gained remarkable research interest for various applications, from environmental to biological, because of its superparamagnetic properties and good biocompatibility. In this work, the nucleation effect of Fe₃O₄ in a polylactide (PLA) matrix under an influence of an alternating magnetic field was studied. The nanocomposite films that is, containing different concentrations of Fe₃O₄ (~8 nm) which were uniformly dispersed in the PLA matrix, were prepared via a solution casting method. The amplitude and frequency of the magnetic field impose great effects on the morphology and nucleation rate of PLA crystallization. Thermogravimetric analysis was used to study the thermal stability of Fe₃O₄/PLA and it showed that the thermal stability of Fe₃O₄/PLA was affected by the Fe₃O₄ content. Fourier transform infrared spectroscopy and X‐ray diffractometry indicated that Fe₃O₄ shows impeding effect to the crystallization of PLA. Based on the differential scanning calorimetry results, composite with 1% of Fe₃O₄ content could promote the crystallization of PLA but it would become an obstacle when 3% of Fe₃O₄ was added. The result of polarized optical microscopy also showed a good agreement that the incorporation of Fe₃O₄ could act as an effective nucleation regent to the composite film. POLYM. ENG. SCI., 59:608–615, 2019. © 2018 Society of Plastics Engineers     

Photo-DSC I: A new tool to study the semi-crystalline polymer accelerated photo-ageing

Photo-DSC was used, to study, in situ, the photo-ageing of polycyclo-octene which is a semi-crystalline elastomer. The ‘crystallizability’, which is the ability of the polymer to crystallize, was tightly dependent on the exposure time and was used to follow the photo-ageing.The irradiation system of photo-DSC was also compared with a usual accelerating device and no difference was detected in the chemical alteration of the polymeric matrix. Meanwhile, with photo-DSC, the photo-ageing was accelerated compared to a usual accelerating device. By using photo-DSC, atmosphere, exposure time, light intensity and ageing temperature were accurately controlled.