The effect of specific nucleation on tensile mechanical behaviour of isotactic polypropylene

Commercial-grade isotactic polypropylene was modified with a specific β-nucleation agent NJ-Star (N,N^′-dicyclohexylnaphthalene-2,6-dicarboxamide) in concentrations 0.03, 0.10 wt.% and with a specific α-nucleating agent Millad 3988 (1,2:3,4-bis-O-(3,4-dimethylbenzylidene)sorbitol) in a concentration of 1.0 wt.%. Specimens for mechanical studies were prepared by injection moulding. Two types of tensile mechanical testing were performed at room temperature: (1) stress-strain test encompassing the plastic behaviour well behind the yield point and (2) tensile creep in the region of non-linear viscoelasticity. The results derived from the stress-strain traces show a distinct decrease in Young's modulus and yield stress for samples containing the crystalline β-phase as compared with non-nucleated and α-nucleated samples. This decrease was more pronounced with samples containing the lower β-nucleant concentrations (0.03 wt.%). Higher compliance of specimens containing the β-phase was also manifested in their creep behaviour. However, the creep rate of the specimen with the higher nucleant content (0.10 wt.%) did not rise with time so that its creep curve intersected the creep curves of non-nucleated and α-nucleated samples. Thus, at creep times longer than 1000 min, the sample with 0.1 wt.% of the β-nucleant showed a lower compliance than non-nucleated polypropylene and at 10 000 min reached the compliance of the α-nucleated sample. The different softening effect of the β-phase in the high-strain and low-strain regions has been ascribed to a specific structure of the amorphous interlayer induced by the presence of the β-crystallites.     

Tensile behaviour of isotactic polypropylene modified by specific nucleation and active fillers

Commercial-grade polypropylene was modified with a specific nucleation agent based on an amide of dicarboxylic acid, which promotes crystallization predominantly in the β-phase. The resulting material was used as a matrix for composites containing 10%, 20%, and 30% (by weight) of different calcium carbonate fillers. These fillers differed in particle size and surface treatment. The β-phase content, morphology and tensile mechanical properties were investigated. A distinct β-nucleation activity was found with surface-treated fillers; nevertheless, to obtain stiff and reasonably ductile composite materials, a matrix containing a critical nucleant concentration (0.03 wt%) was necessary.     

Rules of supermolecular structure formation in sheared isotactic polypropylene melts

Shear-induced crystallization of isotactic polypropylene (iPP) homo-, block, and random copolymers was studied and compared to that in quiescent melt. It was evidenced by means of the thermo-optical technique that melt-shearing, caused by fiber pulling, is associated with the development of α-row-nuclei. The surface of the in situ formed α-row-nuclei may induce the growth of the β-modification of iPP resulting in a cylindrite of polymorphous composition. The polymorphous composition is controlled by the temperature-dependent relative growth rate of the α- and β-iPP for which a model explanation was given. The β-nucleation ability of the α-row-nuclei is lost by melt-shearing at high temperature or remelting. This was attributed to a coverage of the β-nuclei by the α-phase. The structural memory of the supermolecular structures was studied in repeated melting and crystallization cycles and discussed. It was found that the quality of the fiber did not influence the mechanisms concluded. The shear-induced crystallization of propylene block copolymers was highly analogous to the homopolymers. In case of the random copolymers, however, crystallization in sheared melt resulted in an α-cylindritic structure, because for propylene random copolymers the growth rate of the α-modification is always higher than that of the β. It was also demonstrated that the mechanism of shear-induced crystallization was unaffected when the crystallizing PP melt contained selective β-nucleants. © 1996 John Wiley & Sons, Inc.     

The relation between chemical structure of dicarboxylic dihydrazide compounds and nucleation effect in isotactic polypropylene

Journal of Thermal Analysis and Calorimetry - A series of dicarboxylic dihydrazide compounds (DCDH-R-n) were prepared and used as nucleating agents for isotactic polypropylene (iPP). To investigate...     

The relation between chemical structure of branched amide nucleating agents and nucleation effect in isotactic polypropylene

Journal of Thermal Analysis and Calorimetry - The relation between chemical structure of branched amide compounds as nucleating agents and the nucleation effect of them in isotactic polypropylene...     

The role of specific nucleation in polypropylene photodegradation

The paper focuses on the effects of polymorphism on photodegradation of isotactic polypropylene. The starting polymer was modified by a specific α-nucleating agent, 1,3;2,4-bis(3,4-dimethylbenzylidene)sorbitol, by a specific β-nucleating agent, N,N′-dicyclohexylnaphthalene-2,6-dicarboxamide, or their combination. Samples prepared by compression moulding were then exposed to UV-irradiation in the interval from 0 to 240 h. The differences in morphology were reflected in different photooxidative behaviour. Infrared spectroscopy showed that neat polypropylene was the most sensitive to photooxidation and the sample modified solely by the β-nucleating agent was the least sensitive. The remaining two samples exhibited an intermediate sensitivity. Differential scanning calorimetry revealed that the UV-exposure led to gradual changes in crystallization mechanism specifically asserting in individual materials. This behaviour was ascribed to homogeneous nucleation of partly degraded macromolecules. Possible changes of the nucleating agent itself during UV-exposure were also discussed.     

Crystalline phase orientation in biaxially stretched isotactic polypropylene films

The different kinds of crystalline orientations which can occur in industrially relevant isotactic polypropylene (i-PP) films are compared with those generally observed for other semicrystalline polymers. The peculiar bimodality typical of the uniplanar-axial orientation which is present in two-step biaxially stretched i-PP films is rationalized in terms of the two alternative stackings on the (0k0) face of bilayers of macromolecules of the monoclinic unit cell.     

Heterogeneous nucleation and self-nucleation of isotactic polypropylene with addition of nano-ZnO

Journal of Thermal Analysis and Calorimetry - The influence of nano-zinc oxide (nano-ZnO) on the crystallization and melting behaviors of isotactic polypropylene (iPP) was investigated by...     

A real-time AFM study on crystal nucleation and growth in nanodroplets of isotactic polypropylene

Isotactic polypropylene (iPP) nanodroplets were prepared by using the classical droplet method in this study. The formation of nanodroplets allowed the controlled observation of polymer nucleation as well as access to crystal growth at exceptionally high supercooling in iPP. Three cases including the heterogeneous nucleation and fast crystallization in iPP droplets, the formation of multiple independent homogeneous nuclei within a single droplet and a single nucleus within a single droplet were detected by using atomic force microscopy (AFM) during gradually cooling after remelting the nanodroplets. Moreover, it is found that when the volume of droplet is larger than the value of ca. 130000 nm3, the first case was observed. Otherwise, the latter two cases appeared. The temperature at which the onset of nucleation was observed in individual droplets was found to be mainly dependent on height of the droplets when the size scale of the droplet is comparable to the size of the critical nucleus in at least one dimension, which indicates the nucleation behavior under confinement.     

Non-isothermal crystallization kinetics and nucleation behavior of isotactic polypropylene composites with micro-talc

Journal of Thermal Analysis and Calorimetry - The current investigation has presented a new synthesis technique to prepare pentaethylene glycol-treated graphene nanoplatelets (PEG-GnP) and...     

The effect of morphology on thermal stability of isotactic polypropylene in air

Summary The studies on thermooxidative degradation of isotactic polypropylene films with different morphological structure were carried out in the air in temperature range 85–145 °C. The various crystallinity degrees and morphologies of PP films were obtained by cooling the melt and crystallization at different temperatures. The studies on structural changes going together with degradation and on the kinetics of chain scission have shown that this reaction occurs in two stages. The first is connected with consumption of oxygen dissolved in the initial films and the second is controlled by its diffusion into the films. The rates of degradation and structural changes occurring simultaneously depend not only on the initial crystallinity but also on morphology of the systems under study.With 8 figures and 1 tableDedicated to Prof. G. Rehage on occasion of his 60th birthday     

The effect of annealing on the elastoplastic response of isotactic polypropylene

Four series of tensile loading-unloading tests are performed on isotactic polypropylene in the sub-yield domain of deformations at room temperature. In the first series, injection-molded specimens are used as produced, whereas in the other series the samples are annealed for 24 h at 120, 140 and 160 °C, which covers the low-temperature region and an initial part of the high-temperature region of annealing temperatures. A constitutive model is developed for the elastoplastic behavior of a semicrystalline polymer. The stress-strain relations are determined by five adjustable parameters that are found by fitting the experimental data. The effect of annealing is analyzed on the material constants.     

The effect of gamma-irradiation on the morphology of quenched isotactic polypropylene

Wide-angle X-ray diffraction, differential scanning calorimetry and gel-permeation chromatography measurements have been used to investigate the effect of low absorbed doses of γ-radiation, up to 100 kGy, on isotactic polypropylene (iPP). The bimodal endotherm for the sample quenched in iced water was attributed to the presence of both “smectic” and monoclinic crystalline phases. The changes in the morphology, molecular weight distribution, decrease of melting temperature and heat of fusion with absorbed dose are related to the radiation processes in pure iPP.     

Crystal phases,structure, and orientation in isotactic polypropylene after isothermal crystallization under oscillatory shear as a function of nucleation agent

2D wide-angle X-ray diffraction (2D-WAXD) measurement was performed to investigate the effects of both oscillatory shear and the nucleating agent on the crystalline structure distribution and orientation of isotactic polypropylene (iPP). 1,3:2,4-bis(p-methylbenzylidene) sorbitol (MDBS) and 1,3:2,4-di(3,4-dimethylbenzylidene) sorbitol (DMDBS) can induce α-PP and β-PP simultaneously. The presence of MDBS (or DMDBS) and oscillatory strain (oscillatory frequency is fixed) exhibits a synergistic interaction on increasing the content of β-crystals of iPP. Under the oscillatory shear field at the fixed oscillatory strain, the β-crystal content and the orientation of iPP with and without MDBS (or DMDBS) change slightly with the increase of the oscillatory frequency. Comparing with MDBS (or DMDBS) nucleated iPP crystallization under shear field, the periodically changed flow direction of the oscillatory shear field leads to the shorter α-row nuclei, weaker orientation but more β-crystals of the nucleated iPP.     

Transcrystallization in nanofiber bundle/isotactic polypropylene composites: effect of matrix molecular weight

Polyamide 66 (PA 66) nanofiber bundles were first electrospun and then introduced into isotactic polypropylene (iPP) melts to prepare nanofiber bundle/iPP composites. To reveal the influences of matrix molecular weight (M _n ) on the transcrystalline layer, three kinds of iPP with different M _n were adopted. Polarized optical microscope was employed to investigate the transcrystallinity. In the presence of PA 66 nanofiber bundle, the heterogeneous nucleation distinctly happened in iPP melts. Moreover, the higher the iPP M _n , the denser the nuclei. Both a decrease in matrix M _n and an increase in isothermal crystallization temperature led to an increase in the induction time. The maximum temperature at which the transcrystalline layer can be optically observed increased with the increase of M _n . The growth rate of transcrystallinity decreased with the increasing M _n and crystallization temperature. Moreover, selective melting of the transcrystalline layers confirmed that it was merely composed of α form crystal for all composites.     

Preparation and nucleation effect of a novel compound nucleating agent carboxylated graphene/calcium pimelate for isotactic polypropylene

Journal of Thermal Analysis and Calorimetry - A highly efficient compound nucleating agent of carboxylated graphene/calcium pimelate (Gra/CaPi) for isotactic polypropylene (iPP) was prepared. The...     

The effect of crosslinking on the reactivity of free radicals in isotactic polypropylene

Isotactic polypropylene of various degrees of crosslinking was prepared using high concentrations of dicumyl peroxide as the crosslinking agent. Free radicals were generated by γ irradiation of crosslinked iso-PP with a dose of 2.4 Mrad. It was found that the rate constant for free-radical decay is independent of crosslinking in the temperature range from ?30 to +20°C. The result is discussed from the point of view of degradation processes taking place simultaneously with crosslinking of iso-PP.     

Study of nucleation induced structure modification in isotactic polypropylene by DMTA and solid state NMR

Isotactic polypropylene (iPP) modified by heterogeneous nucleation and molten state drawing was investigated using the DMTA and NMR methods. The nucleation was realized by specific α and β nucleating agents, 1,3,2,4-bis(3,4-dimetylobenzylideno) sorbitol (Millad 3988, Miliken) leading to the creation of the α phase, and N,N′-dicyklohexylo-2,6-naftaleno dikarboxy amid (NJ100) as the β phase promoter. The processing induced modification was performed by molten state drawing during an extrusion in the range between the die exit and the calibration unit. An increase of the glass transition temperature of iPP was found to be drawing independent for the β-nucleated samples, and dependent in the case of the α-iPP. Changes in the macromolecular mobility, depending on the α/β iPP structure and molten state drawing, was found by NMR lineshape and second moment measurements.     

Influence of isotacticity and molecular weight on the properties of metallocenic isotactic polypropylene

The relationships between structure and properties have been established for isotactic polypropylene, iPP, homopolymers synthesized by metallocene catalyst systems. These iPPs exhibit different isotacticity degrees and molecular weights, and several thermal treatments during their processing have been applied. The most important factor affecting the structure and properties of these polymers is the isotacticity content. The thermal treatment, i.e., the rate of cooling from the melt, is also important and a clear molecular weight effect has been also found for the sample with lowest M_w. These factors affect the thermal properties, the degree of crystallinity and, therefore, the structural parameters and the viscoelastic behavior. A slow cooling from melt favors the formation of the γ phase instead of the α modification. The storage modulus, Young modulus and microhardness values increase as crystallinity does, independently of the origin of this increase: higher isotacticity or application of a slow crystallization from the melt.