iPP Chains Research Articles

Direct formation of form I’ crystals in polybutene-1/polypropylene blend enhanced by cold crystallization

During commercial processing of isotactic polybutene-1 (PB-1), crystallization kinetics favor form II while thermodynamics favor the more desired form I/I′, and once the former is adopted, transformation to the latter is extremely slow. In this work, the cold crystallization behavior of PB-1/iPP (isotactic polypropylene) blend (90/10) is investigated with emphasis on the competition between the PB-1 polymorphs. The PB-1/iPP blend is quenched from the melt, annealed first at a low temperature and then at a high temperature, with the ensuing crystallization studied by in-situ Fourier transform infrared spectroscopy, differential scanning calorimetry, and wide-angle X-ray diffraction. It is found that upon annealing at low temperature, form II formation is suppressed and form I′ nuclei prevail, which then develop into crystalline PB-1 dominated by form I′ in the subsequent annealing at room temperature or above. The formation of form I′ as the major polymorph in PB-1/iPP blend containing only a small amount of iPP is attributed mainly to the confinement on PB-1 imposed by the dispersed iPP chains, which hinders form II nucleation.

The Influence of Space Restriction on the Mechanical Properties of Isotactic Polypropylene/Reduced Graphene Oxide Nanocomposite Injection Bars

Isotactic polypropylene (iPP)/reduced graphene oxide (RGO) nanocomposite bars were prepared by injection molding and the effects of RGO on the iPP matrix were investigated via differential scanning calorimetry, two dimensional wide angle X-ray diffraction (2D WAXD) and small angle X-ray scattering (2D SAXS) techniques. It is demonstrated that RGO is an effective nucleation agent for iPP and the incorporation of RGO can enhance the degree of orientation of iPP crystals at flow direction. Furthermore, the orientation of iPP chains at flow direction intensifies with the increase of RGO contents, which can be attributed to that RGO obstructed the motion of orientated polymer chains. Although the enhancement effect is weaker than that of High density polyethylene (HDPE)/RGO nanocomposites with epitaxial crystallization, the mechanical properties of iPP/RGO nanocomposites can also be improved apparently.

Influence of β-nucleating agent dispersion on the crystallization behavior of isotactic polypropylene

In this article we report an in-situ catalytic strategy for homogeneous dispersion of β-nucleating agent N,N′-dicyclohexyl-terephthalamide (DCTPA) into isotactic polypropylene (iPP) as well as morphology control of the DCTPA/iPP composites. This strategy is based on catalyst to polymer morphology replication approach, in which by embedding Ziegler-Natta catalyst species on DCTPA surface, the fresh iPP chains first grew on the DCTPA tiny crystals and then wrapped about them; thus the interaction between DCTPA crystals and iPP matrix was enhanced, and the thermodynamic barrier of dispersing DCTPA in the inert iPP was avoided. The dispersion morphology and the self-assembly/recrystallization morphology of DCTPA in iPP were investigated; the relationship between DCTPA/iPP composites micro-structures and their crystallization behaviors was compared with that of the melting-blended DCTPA/iPP systems. Due to the improvement of DCTPA dispersion and the enhancement of interaction between DCTPA and iPP melts, the synthesized samples presented characteristics of fast crystallization initiation and slow spherulites growth rate, which is different from the general melting-blended composites.

Oriented Overgrowths of Poly(l-Lactide) on Oriented Isotactic Polypropylene: A Sequence of Soft and Hard Epitaxies.

The crystallization behavior of an amorphous poly(l-lactide) (PLLA) layer deposited on uniaxially oriented isotactic polypropylene (iPP) substrate is been studied by atomic force microscopy (AFM) and electron microscopy combined with electron diffraction. A patterned PLLA structure with two fixed lamella and chain orientations is observed. Electron diffraction demonstrates that the major lamellar set is oriented with molecular chains perpendicular to the chain direction of the iPP. The minor lamellar set is inclined at ≈64° to both the iPP chain axis direction and the lamellae of the major set as judged from both the bright field electron micrograph and the AFM image. The orientation of the main set is explained in terms of "soft" epitaxy or graphoepitaxy, in which PLLA chains oriented parallel to the ditches of the iPP substrate caused by alternatively arranged crystalline and amorphous regions. The minor set is due to a homoepitaxy of PLLA with parallelism of the helical paths. The orientation of this minor set of lamellae therefore depends on and can help determine the chirality-l or d-of the PLA investigated.

Effect of Self-Assembly of Oxalamide Based Organic Compounds on Melt Behavior, Nucleation, and Crystallization of Isotactic Polypropylene.

We report on the effect of an aliphatic oxalamide based nucleating agent (OXA3,6) on the melt and crystallization behavior of isotactic polypropylene (iPP) under defined shear conditions. Through polarized optical microscopy, we demonstrate that OXA3,6 self-assembles from the iPP melt into rhombic crystals whereas their size and distribution proved highly dependent on the employed cooling rates. The presence of 0.5 wt % of OXA3,6 in iPP results in a significant suppression in iPP melt viscosity, which could not be explained via molecular modeling. A possible cause for the drop in viscosity in the presence of OXA3,6 is attributed to the interaction (absorption) of high molecular weight iPP chains with the nucleating agent, thereby suppressing their contribution to the viscoelastic response of the melt. This proposed mechanism for the suppression in melt viscosity appears similar to that encountered by the homogeneous distribution of nanoparticles such as CNTs, graphene, and silica. Shear experiments, performed using a slit flow device combined with small-angle X-ray diffraction measurements, indicate that crystallization is significantly enhanced in the presence of OXA3,6 at relatively low shear rates despite its lowered sensitivity to shear. This enhancement in crystallization is attributed to the shear alignment of the rhombic OXA3,6 crystals that provide surface for iPP kebab growth upon cooling. Overall, the suppression in melt viscosity in combination with enhanced nucleation efficiency at low as well as high shear rates makes this self-assembling oxalamide based nucleating agent a promising candidate for fast processing.

Effect of polystyrenes with different architectures on the β‐nucleating efficiency and toughening of isotactic polypropylene

AbstractThe effect of polystyrenes (PSs) with different architectures (three‐arm star‐shaped polystyrene (sPS), comb‐like branched polystyrene (cPS) and linear polystyrene) on their β‐nucleating efficiency for isotactic polypropylene (iPP) during crystallization and final impact and the tensile properties of iPP/PS blends were investigated by dynamic rheological measurements, SEM, DSC, polarized optical microscopy, wide angle X‐ray diffraction and mechanical property measurements. The results show that the architecture of PS has marked influence on its dispersibility in iPP and β‐nucleating efficiency. For iPP/cPS blend, plenty of short side chains reduce the probability of cPS chain entanglements, facilitating the interdiffusion between iPP and cPS chains. A favorable interfacial interaction results in good dispersibility, high β nucleating efficiency and an excellent toughening effect of cPS on iPP. However, the relatively high chain entanglement degree of sPS may not be in favor of chain diffusion between iPP and sPS and therefore relatively poor dispersibility and toughening effect are obtained. The elongation at break and impact strength of iPP were dramatically improved, especially with the addition of 1 wt% cPS. The toughening mechanism of PS on iPP is the dissipated energy caused by cavitation and the β‐nucleating effect of PS. © 2018 Society of Chemical Industry

Effect of benzoic acid surface modified alumina nanoparticles on the mechanical properties and crystallization behavior of isotactic polypropylene nanocomposites.

The effect of benzoic acid (BA) surface modified alumina (Al2O3) nanoparticles (NPs) on the mechanical properties and crystallization behavior of isotactic polypropylene (iPP) nanocomposites was studied. Characterization of the modified Al2O3 NPs (BA-Al2O3) by FTIR and XRD analyses confirmed that benzoic acid molecules chemisorb on the surface of the NPs, forming benzene groups-rich microstructures. A considerable increase in the tensile strength, flexural modulus, and toughness was observed for the nanocomposites with only 0.2 wt% BA-Al2O3. Enhanced interfacial adhesion with the matrix was achieved, which enabled effective reinforcement of the nanocomposites. The higher crystallization temperature along with shorter crystallization halftime indicated the higher nucleation activity of BA-Al2O3. Furthermore, the interchain conformational ordering of iPP was significantly accelerated in the presence of the BA-Al2O3 NPs. The CH–π interaction between the polymer and BA-Al2O3 NPs was considered to facilitate the attachment of the iPP chains and stimulate conformational ordering, crystallization, as well as mechanical properties of nanocomposites.

Nonisothermal crystallization of isotactic polypropylene in carbon nanotube networks

Isotactic polypropylene (iPP) composites with carbon nanotube (CNT) networks at relatively high loadings could have various applications such as electromagnetic interference shielding and thermal conductivity. The crystallization behavior of iPP inside CNT networks could be very much related to the above properties, which was found to be quite different from that of neat iPP in this work. In CNT networks, CNTs not only act as effective heterogeneous nucleating agents to noticeably increase the onset temperature (more than 18°C) of iPP crystallization but also bring strong confinement on the mobility of iPP chains and then reduce the overall crystallization rate of iPP matrix. It is interesting to find that CNT networks, especially in the case of ultrahigh loading (90 wt%), have remarkable confinement effect on crystallization of iPP, overcoming the heterogeneous nucleation of CNTs, resulting in a decline of crystallization rate of iPP. The nonisothermal crystallization kinetics of iPP in the dense CNT network quite fits to the modified Avrami mode by Jeziorny, even more satisfactorily than the case of neat iPP. When confinement effect is dominated during crystallization, we found that the perfection and size of crystallites are extremely decreased at ultrahigh loading, leading to a very low melting point of iPP (approximately 140°C).

Non-isothermal crystallization analysis of isotactic polypropylene filled with titanium dioxide particles modified by a dicarboxylic acid

In this work, the non-isothermal crystallization behavior of polypropylene (iPP), induced by a dicarboxilic acid linkage at the surface of commercial titanium dioxide (TiO2), was investigated. The modification of TiO2 particles with pimelic acid (c-TiO2) played an important role in improving the crystallization kinetic parameters, as observed by an increase in the temperature of the crystallization peak during the incubation period (Δtc). Jeziorny analysis suggested that the dominant secondary crystallization process occurs by nucleation on pre-existing crystals and is mainly the two-dimensional extension of the lamellar crystals formed during primary crystallization. As shown by the model of Mo, composites with c-TiO2 particles exhibited a high crystallization rate when the relative crystallinity exceeded the earliest crystallization stage. Activation energies were also determined and higher values were found for the studied composites, compared to neat iPP. These results show how the ability to nucleate into iPP is improved by c-TiO2 particles, which also increase the affinity to iPP chains, minimizing the energy required to reach the crystalline state.

Influence of thermal history on crystalline morphologies of isotactic polypropylene in its miscible blends with polybutene‐1

ABSTRACTThe miscibility behaviors in blends of isotactic polypropylene (iPP) and polybutene‐1 (PB) have been studied using in‐situ FTIR imaging. The heterogeneous melt of 3/7 iPP/PB blends were formed at 250, 220, and 180°C and then quenched to the same crystallization temperature of iPP at 125°C, respectively. Evolution processes of composition distribution during crystallization were monitored according to their characteristic peaks, and the results suggest a trend from local concentration to uniform dispersion of PB fraction. Further studies of the PB fraction as the distance from the growth front of iPP spherulite indicate an irreversible phase behavior with the progress of thermal history. The cyclic melting and crystallization favor the mixing of iPP/PB blend. Meanwhile, the nonlinear growth rate of iPP spherulite is mainly responsible for compatible promotion of iPP/PB blend, which hinders the transportation of iPP chains to its growth front. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43282.

Unfolding of Isotactic Polypropylene under Uniaxial Stretching.

Despite numerous investigations on polymer processing, understanding the deformation mechanisms of semicrystalline polymer under uniaxial stretching is still challenging. In this work, 13C-13C Double Quantum (DQ) NMR was applied to trace the structural evolution of 13C-labeled isotactic polypropylene (iPP) chains inside the crystallites stretched to an engineering strain (e) of 21 at 100 °C. DQ NMR based on spatial proximity of 13C labeled nuclei proved conformational changes from the folded chains to the locally extended chains induced by stretching. By combining experimental findings with literature results on molecular dynamics, it was concluded that transportation of the crystalline chains plays a critical role to achieve large deformability of iPP.

Folding of Polymer Chains in the Early Stage of Crystallization.

Understanding the structure formation of an ordered domain in the early stage of crystallization is one of the long-standing issues in polymer science. In this study, we investigate the chain trajectory of isotactic polypropylene (iPP) formed via rapid and deep quenching, using solid-state NMR spectroscopy. Comparisons of experimental and simulated 13C-13C double quantum (DQ) buildup curves demonstrated that individual iPP chains adopt adjacent re-entry sequences with an average folding number ⟨n⟩ = 3-4 in the mesomorphic form, assuming an adjacent re-entry fraction ⟨F⟩ of 100%. Therefore, long flexible polymer chains naturally fold in the early stage of crystallization, and folding-initiated nucleation results in formation of mesomorphic nanodomains.

Determination of Chain-Folding Structure of Isotactic Polypropylene in Melt-Grown α Crystals by 13C–13C Double Quantum NMR and Selective Isotopic Labeling

Chain-folding (CF) patterns, successive CF number ⟨n⟩ , and fraction ⟨F⟩ of 13C CH3 15% labeled isotactic polypropylene (iPP) in melt-grown crystals at two crystallization temperatures (Tcs) of 100 and 150 °C were investigated using 13C–13C Double quantum (DQ) NMR and spin-dynamics simulation. The former provided the α1 packing structure while the latter lead to the α2-rich sample with an α2 fraction of ca. 57%. It was demonstrated that the iPP chains adopt small cluster structures consisting of 6–8 stems connected via adjacent re-entry as mean structures in both α1 and α2 phases. There was no kinetic effect on ⟨F⟩ and ⟨n⟩ values, while kinetics did influence the folding directions of the chains under the assumptions of configurational and packing constraints. It was concluded that the CF process occurs at the growth front and dominates the chain-packing process of iPP α phases in a wide Tc.

The Crystallization Behavior of Isotactic Polypropylene Induced by a Novel Antinucleating Agent and Its Inhibition Mechanism of Nucleation

The addition of foreign substances is well-known to be able to improve the crystallization of isotactic polypropylene (iPP) following a nucleation-promoting mechanism. In the present work, however, we found that zinc salt of 1, 4, 5, 6, 7, 7-hexachlorobicyclo [2.2.1] hept-5-ene-2, 3-dicarboxylic acid (HCHD-Zn) unexpectedly decreased the crystallization temperature (Tcp) and crystallization rate of iPP, generating remarkable antinucleation effects. The results of differential scanning calorimetry (DSC) and polarized optical microscopy (POM) showed Tcp of iPP decreased more than 4 °C and nucleation densities of iPP decreased nearly 70% upon the addition of the “anti-nucleating agent” HCHD-Zn. Isothermal crystallization kinetics indicated HCHD-Zn lowered the crystallization kinetic constant k of iPP but had similar Avrami exponent with neat iPP, which revealed HCHD-Zn hindered the crystallization of iPP but did not change the type of nucleation and growth geometries of iPP. We proposed a mechanism of antinucleation of iPP upon the addition of HCHD-Zn from a structural perspective. Hydrogens on the tertiary carbons in the melt of iPP may react with the chlorine in the HCHD-Zn particles dispersed within. The removal of hydrochloride left a 3D network of iPP chains cross-linked by HCHD-Zn particles. Therefore, the restricted movement of polymer chains inhibited the nucleation of iPP.

Three dimensional molecular orientation of isotactic polypropylene films under biaxial deformation at higher temperatures

We investigated the in-situ molecular orientation behavior of isotactic polypropylene (iPP) films under simultaneous and sequential biaxial deformation by polarized infrared spectroscopy at near melting temperature. The three-dimensional orientation functions in two stretching directions (in-plane of the film specimen) and its thickness directions were obtained from the dichroic ratios of two bands with different transition moments. Equi-biaxial stretching was found to cause iPP chains in both crystalline and amorphous regions to rotate towards the stretching plane. However, under sequential biaxial elongation, the iPP chains in both regions first oriented towards the first stretching direction and then rotated to the second stretching direction during the second stretching process.

Measuring the orientation of chains in polypropylene welds by infrared microscopy: A tool to understand the impact of thermo-mechanical treatment and processing

The superstructures developed in the welds between injection molded plates made from α-nucleated and non-nucleated isotactic polypropylene (iPP) were analyzed by infrared (IR) microscopy, polarized light microscopy (PLM) and differential scanning calorimetry (DSC). Layers of distinct structures and size formed during welding in the welds were identified by PLM. Deformation of spherulites occurred in the weld seams due to the sheared melt flow. The orientation of the polymer chains in the welds, which is the result of melting and recrystallization, was quantified by IR microscopy for the first time: In the injection molded plates the chains were oriented along machine direction (MD), whereas this pattern changed to normal direction (ND) at the weld seams. Two positions at the welds were inspected to study the orientation of the chains: In one of these the PLM image depicts two well defined weld seams, while at the other, the weld seams were absent. DSC analysis of microtomed sections revealed the presence of both iPP polymorphs in the injection molded plates, whereas in the weld seam and weld core the α- and β-phase, respectively, were found. The degree of crystallinity (Xc) of the welds of iPP calculated using DSC and IR microscopy showed a homogeneous profile across the welds.The antioxidants (AOs) and the nucleating agent were profiled across the weld using IR-microscopy. A significant consumption of AOs during welding could be shown, and, using the principle of IR-dichroism, thermo-oxidative degradation of iPP chains was observed, even in the presence of AOs. The nucleating agent was homogeneously distributed across the weld.

Dynamics of polypropylene chains in their binary blends of different stereochemical sequences studied by Monte Carlo simulations

The conformational and dynamic properties of polypropylene (PP) for both pure melts and blends with different chain tacticity were investigated by Monte Carlo simulation of isotactic (iPP), atactic (aPP) and syndiotactic (sPP) polypropylenes. The simulation of coarse-grained PP models was performed on a high coordination lattice incorporating short- and long-range intramolecular interactions from the rotational isomeric state (RIS) model and Lennard-Jones (LJ) potential function of propane pairs, respectively. The dynamics of chains in binary PP/PP mixture were investigated with the composition of C150H302 with different chain taciticity. The diffusion rates of PP with different stereochemistry are generally in the order as: iPP > aPP ≫ sPP. For PP/PP blends with 50:50 wt% binary mixtures, immiscibility was observed when sPP was introduced into the mixtures. The diffusion rate of iPP and aPP became slower after mixing, while sPP diffuses significantly faster in the binary mixtures. The mobility of PP chains depends on both intramolecular (molecular size and chain stiffness) and intermolecular (chain packing) interactions. The effect of intramolecular contribution is greater than that of intermolecular contribution for iPP and aPP chains in binary mixtures. For sPP chain, intermolecular interaction has greater influence on the dynamics than intramolecular contribution.

Melting processes of oligomeric α and β isotactic polypropylene crystals at ultrafast heating rates

The melting behaviors of α (stable) and β (metastable) isotactic polypropylene (iPP) crystals at ultrafast heating rates are simulated with atomistic molecular dynamics method. Quantitative information about the melting processes of α- and β-iPP crystals at atomistic level is achieved. The result shows that the melting process starts from the interfaces of lamellar crystal through random dislocation of iPP chains along the perpendicular direction of lamellar crystal structure. In the melting process, the lamellar crystal gradually expands but the corresponding thickness decreases. The analysis shows that the system expansion lags behind the crystallinity decreasing and the lagging extents for α- and β-iPP are significantly different. The apparent melting points of α- and β-iPP crystals rise with the increase of the heating rate and lamellar crystal thickness. The apparent melting point of α-iPP crystal is always higher than that of β-iPP at differently heating rates. Applying the Gibbs-Thomson rule and the scaling property of the melting kinetics, the equilibrium melting points of perfect α- and β-iPP crystals are finally predicted and it shows a good agreement with experimental result.

Novel approach to tune mechanics of β-nucleation agent nucleated polypropylene: Role of oriented β spherulite

In this study, a novel approach has been developed to introduce orientation to bundle-like β spherulite in iPP samples. The addition of UHMWPE phase, which plays the role of maintaining orientation of iPP chains, into β-nucleation agent nucleated iPP samples leads to the formation of oriented β spherulite in the inner region of injection molded samples. Accordingly, both the tensile strength and impact strength of the sample have been improved. It proves that the arrangement of crystallites is another important factor to determine the toughness in addition to β-crystal content, especially when the content is relatively high. Based on the investigated results, a schematic model about the formation of orientated β crystals via the addition of UHMWPE is proposed. It's the first time that oriented β spherulite is observed and investigated. Our study suggested an alternative approach to achieve good mechanics in polypropylene via introducing orientation to β spherulite.

Morphology and mechanical properties of polypropylene-POSS hybrid nanocomposites obtained by reactive blending

The polypropylene-polyhedral oligomeric silsesquioxane (PP-POSS) organic–inorganic hybrids were obtained and studied. The hybrids were prepared by grafting of POSS on PP chains during a reactive melt-blending of polypropylene (iPP), maleic anhydride functionalized PP (PP-g-MA), and amine-functionalized POSS (aminopropylheptaisobutyl-POSS, ambPOSS, aminopropylheptaisooctyl-POSS, amoPOSS, or aminoethylaminopropylheptaisobutyl-POSS, am2bPOSS), taking advantage of the high efficiency of amino-anhydride reaction in the molten state. The structure, morphology, and physical properties of the obtained hybrids and blends were studied by means of wide- and small-angle X-ray scattering, dynamic scanning calorimetry, scanning electron microscopy, dynamic mechanical thermal analysis, as well as tensile and impact experiments. The influence of POSS chemical structure and grafting degree on the morphological characteristics and mechanical properties was investigated. It was found that grafting of POSS cages on PP chains leads to the POSS dispersion on the molecular level. On contrary, when POSS was mixed with plain iPP any grafting of POSS on iPP chains was impossible, which resulted in phase-separated blend with crystallites of POSS dispersed in iPP matrix. The mechanical tests revealed that modification of polypropylene by grafting with POSS molecules does not affect significantly its mechanical properties, both static and dynamic, except ultimate strain, which is markedly lower in hybrids and their blends than in plain iPP. Also the impact properties of PP were practically not improved by modification with POSS. POLYM. COMPOS., 34:929–941, 2013. © 2013 Society of Plastics Engineers