iPP Films Research Articles

Stepwise crystallization method combining isotactic polypropylene-based plastic waste film to prepare high-performance polypropylene random copolymer

Upcycling of waste plastic film is increasingly important at the background of energy shortage and environmental crisis. However, direct reuse of the waste plastics as reworked material inevitably downgrades the mechanical properties. Herein, a stepwise crystallization method was used to enable the recycling of waste isotactic polypropylene (iPP) film value-added in iPP-based homocomposites. In detail, the effect of β-nucleating agent (β-NA) and iPP on crystallization behaviors of polypropylene random copolymer (PPR) was investigated at first. The stepwise crystallization procedure was further introduced to promote the β-crystallization of PPR/iPP/β-NA composites in which the crystal structures, polymorph compositions and phase morphologies were systematically researched. At last, impact behaviors of PPR/iPP/β-NA composites were characterized and a penitential toughening mechanism was proposed. It was found that the stepwise crystallization boosted β-crystallization and growth of rubbery phase during the isothermal procedure, resulting in an increase of relative β-crystal content and low temperature impact toughness from 0 % and 2.02 kJ/m2 of neat PPR to 33.4 % and 3.06 kJ/m2 of PPR/iPP/β-NA composites. This work provides a simple, implementable, and effective method for the recycling of waste iPP films and the low-temperature toughening of PPR.

Yielding behavior of isotactic polypropylene at elevated temperature understood at the spherulite level

It is of great significance to study the deformation mechanism of isotactic polypropylene (iPP) which differs considerably from that of traditional polymers in consideration of its unique cross-hatched structure of α crystal. In this work, the yielding mechanism of three iPP precursor films prepared via crystallizing in different supercooling during stretching under elevating temperature had been investigated systematically. The results show that three mechanisms of deformation had been aroused in the change of yielding stress with stretching temperature named as inter-spherulitic, intra-spherulitic and mix deformation, respectively. Moreover, the transition temperature (60 °C) is in close correlation with the α2 relaxation of iPP, where the chain diffusion within the crystal blocks is activated. The competition between inter-spherulitic deformation and intra-spherulitic deformation is not only determined by the spherulite structures, but also the stretching temperatures. Further studies indicate that, larger spherulitic size, more daughter lamellae and lower stretching temperature are dominant issues to induce inter-spherulitic deformation. This work is expected to provide a guideline for the preparation of iPP films with desired structures and functionalities.

Real-time structural characterization of isotactic polypropylene during cast film extrusion

Understanding the crystallization behavior of semi-crystalline polymers during processing is key to achieve the desired material performances. Isotactic polypropylene characteristics are highly tunable and easy to adjust by changing the processing conditions thanks to its structural versatility and polymorphic behavior. Hereby, we performed for the first time ever a challenging real-time X-ray characterization of isotactic polypropylene during the cast film extrusion process by interfacing a compact cast film extruder machine with a synchrotron line.We report data of excellent quality recorded from the contact point between the just extruded polymer melt and the water-refrigerated cooling roll further down to the cooling roll line by using a grazing incidence scattering geometry. The explored cooling roll temperatures (from 70 °C to 15 °C) allowed to produce iPP films with defined structural composition, from α to mesophase.Evolution of the phase crystallinity as a function of the contact time between the polymer melt and the cooling roll clearly shows the presence of three crystallization zones: 1) an amorphous zone, 2) a frostline zone and lastly 3) a high-crystallinity zone. In the end, the experimental data have been matched with simulated data using a simple crystallization kinetics model, finding a good agreement in terms of crystalline development over the investigated time scale.As a closing remark, we hope that our effort will spark new interest in the real-time characterization of semi-crystalline materials and that will ultimately be utilized as a starting point to optimize and expand the characterization of these extremely important materials.

Competition of shearing and cavitation effects on the deformation behavior of isotactic polypropylene during stretching

Owing to the unique cross-hatched structure of α crystal, the deformation behavior of isotactic polypropylene (iPP) differs considerably with the traditional polymers. The influence of crystalline structures on the cavitation and shearing effects of α-iPP under tensile loading is still not well established. In this work, various iPP samples were prepared via crystallizing from 0 °C to 130 °C, after which the crystalline structures were characterized via differential scanning calorimetry, polarized optical microscopy and scanning electron microscopy. The lamellar thickness and crystallinity of α-iPP precursor films change slightly, while the content of tangential lamellae in α-spherulite and the spherulite size increase significantly with the decreasing supercooling. On the other hand, the morphological evolution of α-iPP during stretching was tracked by in-situ two-dimensional small angle X-ray scattering. It is found that cavitation effect become dominant instead of shearing effect when the size of α-spherulite exceeds 25 μm. Furthermore, we disclose four typical deformation behaviors of α-iPP resulting from the competition between intra-spherulitic deformation and inter-spherulitic deformation, namely the shearing without cavitation, shearing then localized cavitation, homogeneous cavitation with concomitant shearing, and cavitation with absence of shearing. Accordingly, we construct a route for the structural evolution of α-iPP with different crystalline structures during stretching, which provides an effective guidance to produce diverse iPP films with desired structures and tunable functionalities.

Visualization of small-angle X-ray scattering datasets and processing-structure mapping of isotactic polypropylene films by machine learning

With the rapid development of the synchrotron radiation X-ray characterization techniques, the preprocessing of large small-angle X-ray scattering (SAXS) datasets and the data mining become urgent requirements for researchers. In this work, we apply the variational autoencoder (VAE) and the conditional variational autoencoder (cVAE) to visualize a large SAXS dataset of hard-elastic isotactic polypropylene (iPP) films in 2- and 1-dimensional latent spaces. The low-dimensional representations enable us to capture key features of the dataset rapidly, such as the similarity among SAXS patterns and the structural evolution trends. The preprocessing of the dataset points out the further direction of data analysis so that researchers can focus on the most valued regions in the dataset. Then, we develop a hybrid VAE-multilayer perceptron (MLP) neural network to realize the processing-structure mapping of iPP films. The robustness of the hybrid VAE-MLP network is verified. Finally, SAXS patterns in the temperature-strain space are generated, which allows us to explore the processing parameter space not involved by previous experiments. These capabilities indicate that the developed machine-learning methods are valuable artificial intelligence toolset to assist in the preprocessing of large-scale SAXS datasets and the establishment of comprehensive processing-structure relationship of hard-elastic iPP films.

Synthesis of Anion Electrolyte Membrane through Radiation-induced Graft Polymerization of Poly(4-vinylbenzyl chloride) onto Isotactic Polypropylene Film

Radiation grafting, a widely used process for preparing different functional materials, allows the facile amalgamation of desirable properties from two or more polymers without altering the inherent properties of the base substrate. In this study, an anion electrolyte membrane (AEM) was successfully prepared through radiation grafting of poly(vinylbenzyl chloride) (PVBC) from isotactic polypropylene (iPP) film using gamma-ray irradiation. It was shown that the amount of grafted PVBC increased with increasing absorbed dose, which yielded iPP-g-PVBC with 127% degree of grafting at 50 kGy. The iPP-g-PVBC was reacted with aqueous trimethylamine hydrochloride solution to introduce quaternary ammonium ions, thereby producing the AEM membrane in chloride form (iPP-g-PVBC-TMA-Cl), which was further reacted with potassium hydroxide (KOH) to convert it into hydroxide form (iPP-g-PVBC-TMA-OH). The pristine, grafted and functionalized iPP films were characterized using attenuated total reflectance – Fourier transform infrared spectrometer, thermogravimetric analyzer and scanning electron microscopy – energy dispersive X-ray spectrometer. The effect of degree of grafting on the ionic conductivity, ion exchange capacity and water uptake were evaluated in both AEM’s chloride and hydroxide forms. Results showed that a higher degree of grafting films achieved higher ionic conductivity, ion exchange capacity and water uptake for both chloride and hydroxide forms. The synthesized AEM with a degree of grafting of 70% (IEC = 1.87 meq/g) obtained a conductivity of 129.34 mS/cm, which is higher than the AEMs reported in previous works.

Environmental degradation and formation of secondary microplastics from packaging material: A polypropylene film case study

A model study on isotactic polypropylene (iPP) films as commonly used in packaging was performed for simulating the process of microplastics formation from large-scale objects like littered packaging material,. Accelerated ultraviolet (UV) ageing in a Xenotest device resulted in chemical degradation and oxidation of the polymer, together with fragmentation into sub-mm particles, within less than 48 h. Combining the results with an independent Arrhenius study allowed an estimate of the lifetime until sub-mm particle formation between 9 months and 3.2 years, depending on location and climate. The findings offer a possible explanation for the mass loss at the lower end of the particle size distribution of environmental microplastics and the divergence between input estimates and actually detected amounts of microplastics.

Micropore formation and crystalline evolution during biaxial stretching process of iPP film constructed of ordered and continuous β-transcrystallinity

Since biaxial stretched polypropylene (PP) microporous membrane was increasingly popular and crucial in scientific research and industrial production, the problems of less porosity and uneven micropore distribution urgently awaited solution. To further explore these issues, two kinds of iPP films, one constructed of continuous and ordered β transcrystallinity and the other constructed of β spherulite, were introduced into synchronous biaxial stretching process in this work. The experiments demonstrated that the direction of β lamellae arrangement along the force field played a key role in the micropore formation and crystalline evolution. As a result, the lamellae in β transcrystallinity were easier to be separated to form micropores and the crystalline transformation process was steadier, hence contributing to the higher porosity, less stems and much more uniform distribution. In practice, the microporous membrane stretched from the β transcrystallinity film achieved considerable enhancement in separation performance as well as the permeation property. Furthermore, an interpretation model concerning the relevance between the initial lamellae arrangement and microporous membrane property was proposed, providing fresh insights on how to produce microporous membrane with better performance.

Simultaneous Enhancement of Toughness and Strength of Stretched iPP Film via Tiny Amount of β-Nucleating Agent under “Shear-free” Melt-extrusion

Simultaneous Enhancement of Toughness and Strength of Stretched iPP Film via Tiny Amount of β-Nucleating Agent under “Shear-free” Melt-extrusion

Markedly improved photo-oxidation stability of α form isotactic polypropylene with nodular morphology

Photo-oxidation degradation behaviors of isotactic polypropylene (iPP) films with different crystalline morphology were investigated by artificially accelerated aging test. The α form iPP with nodular morphology was obtained by quenching the melt and subsequent annealing process, and its photo-oxidation degradation behavior was compared with that of the melt-crystallized α form iPP sample. Fourier transform infrared spectroscopy (FTIR), gel permeation chromatography (GPC), differential scanning calorimetry (DSC), X-ray scattering, dynamic mechanical analysis (DMA) and atomic force microscopy (AFM) were utilized to characterize the microstructural variations of these two kinds of iPP films during photo-oxidation degradation process. The results showed that iPP film of nodular morphology exhibits improved photo-oxidation stability compared with that of lamellar and spherulitic structure. UV-Vis spectra and O2 diffusivity test of unexposed and aged samples demonstrated that the iPP film of nodular morphology has higher transparency and lower O2 diffusivity than that of lamellar morphology. Combining the microstructural analysis and surface morphology observation, the improved photo-oxidation stability of “nodular” iPP was correlated with its higher UV light transparency, lower O2 diffusion coefficient as well as the slower segmental mobility in the amorphous phase.

Preparation of isotactic polypropylene (iPP) films with different top and bottom surface parts via solution cast method using decahydronaphthalene as the diluent

ABSTRACT iPP films with different top and bottom surface parts were prepared via solution cast method. The structure and mechanism of iPP films prepared using this method were analyzed. When the evaporation temperature was not enough to form a homogeneous film, a top surface part was formed due to thermally induced phase separation (TIPS) and the crystallization, a bottom surface part was created due to the adsorption of carrier and both these parts were temperature sensitive. When the evaporation temperature was 60°C, particles with lotus leaf structure appeared in the top surface part, resulting in a quasi superhydrophobic surface.

Probing into the microstructural evolution of isotactic polypropylene during photo-oxidation degradation

Photo-oxidation degradation behavior of isotactic polypropylene (iPP) has been extensively investigated, however, there are still some ambiguous points, e.g., the origin of improved crystallinity at the early stage of photo-oxidation, the evolution of condensed state structure during the process of UV light irradiation, etc. In the present work, iPP film samples, unstabilized and stabilized with a specific reactive hindered amine light stabilizer (RHALS), were exposed to UV irradiation for accelerated aging tests. The microstructural evolution of the samples was systematically investigated by combination of Fourier transform infrared spectroscopy (FTIR), gel permeation chromatography (GPC), differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS) and atomic force microscopy (AFM). The results show that chemi-crystallization exists both in unstabilized and stabilized iPP, but the chemi-crystallization extent is much lower in the stabilized iPP. In addition to chemi-crystallization, the loss of amorphous region induced by “UV light etching” is another probable factor contributing to the increase of crystallinity at the early stage of photo-oxidation. AFM results provide direct evidence that with the progress of photo-oxidation, the degradation gradually propagates from amorphous region to crystalline region, thus inducing the destruction of lamellar structure. On the basis of above results, a comprehensive model is summarized for the structure evolution of iPP film during photo-oxidation and will be of meaningful reference to the degradation investigation of other semi-crystalline polymeric materials.

Preparation of highly oriented single crystal arrays of C8-BTBT by epitaxial growth on oriented isotactic polypropylene

Device arrays are fabricated with organic single crystals epitaxially grown on highly oriented iPP films by solvent vapor annealing.

Direct Comparison of Crystal Nucleation Activity of PCL on Patterned Substrates

A sample containing different regions with poly(e-caprolactone) (PCL), oriented polyethylene (PE), and oriented isotactic polypropylene (iPP) films in contact with glass slide has been prepared to be observed in the same view field in an optical microscope and the crystallization of PCL in different regions during cooling from 80 °C down to room temperature at a rate of 1 °C·min−1 was studied. The results showed that the crystallization of PCL started first at the PE surface and then at the iPP surface, while its bulk crystallization occured much later. This indicates that though both PE and iPP are active in nucleating PCL, the nucleation ability of PE is stronger than that of iPP. This was due to a better lattice matching between PCL and PE than that between PCL and iPP. Moreover, since lattice matching existed between every (hk0) lattice planes of both PCL and PE but only between the (100)PCL and (010)iPP lattice planes, the uniaxial orientation feature of the used PE and iPP films resulted in the existence of much more active nucleation sites of PCL on PE than on iPP. This led to the fact that the nucleation density of PCL at PE surface was so high that the crystallization of PCL at PE surface took place in a way like the film developing process with PCL microcrystallites happened everywhere with crystallization proceeding simultaneously. On the other hand, even though iPP also enhanced the nucleation density of PCL evidently, the crystallization of PCL at iPP surface included still a nucleation and crystal growth processes similar to that of its bulk crystallization.

Direct Evaluation of Local Dynamic Viscoelastic Properties of Isotactic Polypropylene Films Based on a Dynamic μ-Beam X-ray Diffraction Method.

The local mechanical properties of crystalline polymer were evaluated using synchrotron radiation X-ray diffraction with 10 μm lateral resolution. A nonoriented isotactic polypropylene (iPP) film with isolated spherulites in a crystallized matrix was used as a model sample. In situ wide-angle X-ray diffraction (WAXD) measurement was performed on the iPP film using a microbeam synchrotron radiation X-ray under sinusoidal strain. The lattice spacing of the crystal planes increased and decreased in response to the applied sinusoidal strain. Local dynamic viscoelastic functions (dynamic storage and loss moduli (E' and E″)) were calculated at room temperature from the relationship between the calculated applied stress and the response strain obtained by dynamic μ-beam WAXD measurement inside and outside of the spherulites. The E' values inside and outside of spherulite obtained from the change in spacing of the (110) plane were 1.8 and 1.1 GPa, respectively. Furthermore, the E' value inside of spherulite obtained from the change in spacing of the (1̅13) plane was 6.0 GPa. These values can be explained by the deformation of crystallite, which depends on the direction of crystal planes. The results obtained here revealed that synchrotron radiation X-ray diffraction measurement gives not only structural information but also the local mechanical properties of the materials E'.

Nanoindentation mapping of multiscale composites of graphene-reinforced polypropylene and carbon fibres

The local mechanical properties of hierarchically reinforced laminates of graphene-isotactic polypropylene (iPP) and carbon fibres are investigated by nanoindentation. Carbon fibre fabrics are alternated with films of iPP reinforced with different graphene contents and types (pristine and modified with iPP brushes). The distribution of the mechanical data obtained around one isolated carbon fibre at the boundary with the polymer layer is compared with a similar analysis far away from any fibre. In the case of neat iPP, a high E′ tail can be identified with a micrometre-sized interphase showing distinct properties from those of the polymer matrix. With increasing amount of graphene, the E′ distribution including the high E′ tail progressively shifts to higher values. It is suggested that graphene platelets accumulate at the front of the ply due to the filtering effect of the carbon fibres giving rise to a transition region, superimposed on the interphase, with enhanced properties. Nanoindentation studies reveal that the chemical modification of graphene by short-chain iPP lowers the reinforcement of the nanofiller, both near and far away from the fibres front. On the other hand, indentation push-in tests suggest that modified graphene increases the resistance of the laminate under shear stress conditions. However, from the point of view of overall performance, it lowers the electrical conductivity across the laminate. The work offers a discussion of the role of graphene type and content on the properties of the multiscale composite.

Stress-induced microphase separation of interlamellar amorphous phase in hard-elastic isotactic polypropylene film

Lamellar stacks, arranged alternatively with hard crystal and soft amorphous nano-layers, are the basic building units to determine the intrinsic mechanical properties of semi-crystalline polymeric materials. The mechanical instability of hard-elastic isotactic polypropylene (iPP) films with highly parallel lamellar stacks is studied with in-situ synchrotron radiation small- and wide-angle X-ray scattering (SAXS/WAXS) techniques during cyclic tensile deformation. Unexpectedly, the micro-strain, deduced from the relative variation of lamellar periodicity, shows an accelerated increase at the onset of instability and reaches values larger than the corresponding macro-strain after yield, during which no irreversible plastic destruction of crystal is observed. Combining the unpredictable increase of long period and other structural information obtained with in-situ SAXS/WAXS measurements, we propose that stress-induced microphase separation of interlamellar amorphous phase is responsible for the yield behavior and the hyperelasticity of hard-elastic iPP films, which stems from the heterogeneous distribution of tie chain/trapped entanglement in interlamellar amorphous nano-layers. This reversible stress-induced non-equilibrium phase transition of interlamellar amorphous phase is different from current plastic deformation models with crystal destruction in semi-crystalline polymers but in line with the nearly 100% elastic recovery ratio of hard-elastic films.

Effect of Flowing Preformed Spherulites on Shear-Induced Melt Crystallization Behaviors of Isotactic Polypropylene

The effects of flowing preformed spherulites on shear-induced melt crystallization behaviors of isotactic polypropylene (iPP) had been investigated by using a Linkam shearing cell coupled with an optical microscope, atomic force microscope, and rheometer. By quenching iPP melt to 130 °C for a short time, preformed spherulites were intentionally obtained. While iPP films were heated up to temperatures below the nominal melting point of iPP (164 °C), various shear conditions were applied, and then the crystalline morphologies during crystallization at the isothermal crystallization temperature of 135 °C were traced by alternately using phase contrast optical microscopy and polarized optical microscopy modes. Two distinct highly orientated thread- and cylinder-like crystalline morphologies were induced by shearing with preformed spherulites present, which were dependent on the applied shear rate and temperature. The distinct crystalline morphologies were attributed to relative movements between shear flowing...

Oxidative degradation and morphological properties of gamma‐irradiated isotactic polypropylene films

Isotactic polypropylene (iPP) films of 25 μm thickness were subjected to gamma-ray irradiation for different doses (30, 60, 90 and 100 kGy) at ambient atmosphere and both molecular modifications and surface properties have been investigated by Fourier transform infrared spectroscopy (FTIR) and atomic force microscopy (AFM), respectively. The occurrence of the oxidation process is identified and quantified by the FTIR. In addition, it is found that the carbonyl index increases with the absorbed dose which confirms that the irradiation process induces the diffusion of the oxygen into the iPP matrix. The AFM image (for virgin and higher dose irradiated samples) showed a dramatic change in the surface topography. The gamma radiation induces the formation of the nanostructured iPP surface. The roughness of the polymer surface, measured by AFM, increases after gamma irradiation. These results revealed that the oxidative degradation may be correlated with the morphological properties.

Evaluation of thermal degradation in isotactic polypropylene films used in power capacitors

Power capacitors are widely used by electric power utilities in electrical systems to improve power quality for their consumers. However, problems with physical installation, ambient temperature and electrical disturbances can raise the internal operating temperature above the ideal designed for the component, which can accelerate the degradation of the dielectric material. Since polypropylene film is the main part of the capacitor dielectric system, it is important to know about the effects of thermal aging on the physicochemical properties of the material. This work presents results of physical–chemical characterization tests of thermal aged isotactic polypropylene (iPP) film used in power capacitors. Tensile strength tests, dynamic mechanical thermal analysis, differential scanning calorimetry, X-ray diffraction (WAXS) and dielectric strength were performed. The results showed that the thermal aging causes morphological changes and reduction in the dielectric breakdown voltage of the iPP film.