iPP Melt Research Articles

A combined melt-stretching and quenching setup for experimental studies of polymer crystallization under complex flow-temperature environments.

A combined melt-stretching and quenching setup is designed and developed to allow experimental investigations of polymer crystallization under the complex flow-temperature environments comparable to those encountered in the actual industrial processing. The melt-stretching proceeds by two drums rotating in the opposite directions with simultaneous recording of a stress-strain curve, where the Hencky strain and strain rate (≤233s-1) are adjustable over a large range. After stretching, liquid N2 is used as a cooling medium to quench the free-standing melt, which is sprayed directly to the deformed melt driven by an electric pump. To ensure a high cooling efficiency, a three-way solenoid valve is employed to execute a sequential control of the liquid N2 flow direction to reduce the boil-off of liquid N2 before entering the sample chamber. The melt cooling rate depends on the liquid N2 flow rate controlled by a flow valve, which is up to 221 °C/s when quenching the isotactic polypropylene (iPP) melt with a thickness of 0.28mm at 150 °C. Two independent temperature control modules are designed to meet the requirements of different stages of melt-stretching and quenching. To verify the capability of the setup, we have performed the melt-stretching and quenching experiments on iPP samples. The setup is demonstrated to be a valuable new tool to study polymer crystallization under coupled flow-cooling fields.

Reversible nucleation behavior of N, N′-dicyclohexylterephthalamide to polypropylene with the aid of polyamide 12

Through the hydrogen bonding between polyamide 12 (PA12) and N, N′-dicyclohexylterephthalamide (DCHT) nucleator, the intentionally controlled self-assembly of DCHT and the crystallization behavior of isotactic polypropylene (iPP) matrix are investigated. The introduction of PA12 could severely postpone the condensation of DCHT vapor in iPP matrix due to the reduced vapor concentration. As a result, the nucleating efficiency of DCHT at lower concentration for example iPP/PA12/0.05DCHT even failed at a relatively higher final heating temperature Tf. Such a failure of nucleating efficiency could be recoverable after a step-cycle treatment at lower Tf, where the sublimation of DCHT can hardly occur. The changes from a cluster-like structure to a network structure driven by hydrogen bonding between DCHT and PA12 could be a reason of nucleation failure and recovery. This means that the nucleating efficiency of DCHT and crystallization of iPP could be tuned by controlling the absorption and release of DCHT vapor at the surface of PA12 particles. For iPP/PA12/0.5DCHT with a higher concentration of DCHT, the absorption of PA12 could only reduce the DCHT vapor concentration in the iPP melt and thus, the dendrite structure of DCHT shows more and thinner branches, which results in better nucleating efficiency of DCHT than that in iPP/0.5DCHT.

Organogelators with dual β- and α-nucleating ability in isotactic polypropylene

Nucleation efficiency and the special supermolecular structure formed in the presence of novel nucleating agents, N,N′-dicyclohexylsuberoylamide and N,N′-dicyclohexylsebacoylamide are presented in this work. The nucleation effect is studied in isotactic polypropylene (iPP). The melting and crystallization processes as well as the polymorphic composition are studied using calorimetric and thermo-optical techniques, while the solubility of the nucleating agents is studied by rheology. The properties of the iPP products nucleated by the novel compounds are characterized by conventional tensile and impact tests and the optical properties are measured by standardized haze measurements. The results indicated clearly that the studied nucleating agents are partially soluble in the iPP melt and possess dual nucleating ability. The formation of β-phase is evidenced by the calorimetric and thermo-optical measurements. The morphology of the nucleated samples shows similarity to iPP nucleated by well-known soluble “organogelators”; however, the nucleating agents introduced in this work are the first “organogelators” with β-nucleating efficiency.

Fundamental studies on shear-induced nucleation and beta-phase formation in the isotactic polypropylene\u2014effect of the temperature

The complex and incompletely understood phenomenon of shear-induced crystallization of polymers may be nowadays analysed via the in situ POM-shear stage methodology. In this research, the two main issues were investigated with the use of the Linkam CCS450 shear stage connected with POM microscope. It was found that the secondary nucleation in the tree well-known temperature regimes plays the greater role in the overall crystallization kinetics than the shear induced primary nucleation. Furthermore, it was found that the tendency towards β-phase formation in shear conditions is dependent on the temperature value during shear treatment. It may be concluded that the temperature is the key parameter in the primary and secondary nucleation process and beta-phase formation in the iPP melts.

Investigation of the Ordered Structure in Partially Melted Isotactic Polypropylene.

The ordered structure of partially melted isotactic polypropylene (iPP) was investigated using polarized optical microscopy (POM) and small-/wide-angle X-ray scattering (SAXS/WAXS) measurements. The crystalline morphology was first examined by means of pulling a glass fiber through the iPP melt, which was generated by partially melting a preformed spherulite. The results from the POM experiments indicated that, even at a minimal pulling rate, the surviving ordered structure could also relocate along the direction of fiber pulling and grow into cylindrites eventually. In addition, during the quiescent crystallization from the partially melted sample, which had the same thermal history of fiber-pulling experiments, the obvious memory effect of melt was also observed from the results of X-ray experiments. Moreover, the SAXS profile derived from the partially melted iPP at 170 °C was fitted by the theory of scattering amplitude with the cylindrical form factor. The fit result implied that the surviving ordered structure is of cylindrical nanocrystals with a diameter D ≈ 30 ± 3 nm and height h ≈ 45 ± 3 nm, which can significantly influence the crystallization morphology and kinetics during the subsequent crystallization process.

Synergistic Effect of Pressurization Rate and β-Form Nucleating Agent on the Multi-Phase Crystallization of iPP.

Using a homemade pressure device, we explored the synergistic effect of pressurization rate and β-form nucleating agent (β-NA) on the crystallization of an isotactic polypropylene (iPP) melt. The obtained samples were characterized by combining small angle X-ray scattering and synchrotron wide angle X-ray diffraction. It was found that the synergistic application of pressurization and β-NA enables the preparation of a unique multi-phase crystallization of iPP, including β-, γ- and/or mesomorphic phases. Pressurization rate plays a crucial role on the formation of different crystal phases. As the pressurization rate increases in a narrow range between 0.6–1.9 MPa/s, a significant competitive formation between β- and γ-iPP was detected, and their relative crystallinity are likely to be determined by the growth of the crystal. When the pressurization rate increases further, both β- and γ-iPP contents gradually decrease, and the mesophase begins to emerge once it exceeds 15.0 MPa/s, then mesomorphic, β- and γ- iPP coexist with each other. Moreover, with different β-NA contents, the best pressurization rate for β-iPP growth is the same as 1.9 MPa/s, while more β-NA just promotes the content of β-iPP under the rates lower than 1.9 MPa/s. In addition to inducing the formation of β-iPP, it shows that β-NA can also significantly promote the formation of γ-iPP in a wide pressurization rate range between 3.8 to 75 MPa/s. These results were elucidated by combining classical nucleation theory and the growth theory of different crystalline phases, and a theoretical model of the pressurization-induced crystallization is established, providing insight into understanding the multi-phase structure development of iPP.

Rheological response of entangled isotactic polypropylene melts in strong shear flows: Edge fracture, flow curves, and normal stresses

Isotactic polypropylene (iPP) melts are industrial semicrystalline polymers whose processing typically involves strong shear flows. The study of the rheological response of iPP melts, well beyond the linear viscoelastic limit, is limited by edge fracture, which manifests in rotational rheometers. In this work, we used a reflection polariscope under shear to detect the onset shear rate at which edge fracture is observed for various rotational rheometry fixture diameters. The onset shear rate for edge fracture was found to correlate with the zero-shear viscosity, thereby enabling the prediction of edge fracture by only knowing the zero-shear viscosity; a quantity that is easier to measure compared to the second normal stress difference. Edge fracture is then mitigated by using a cone-partitioned plate, which enabled the study of the first normal stress difference, and in combination with capillary rheometry, allowed the measurement of flow curves with a very well-resolved shear thinning region. For strongly polydisperse iPPs at high shear rates, we found that viscosity scales as the −0.7 power of the shear rate, while primary normal stress difference scales as the square root of the shear rate. The dependence of the shear thinning of iPPs on polydispersity was then unravelled, offering a broad set of data to develop and test molecular models.

The Synergistic Effect of Rare‐Earth Complex Nucleating Agent and Graphene Oxide on the Non‐Isothermal Crystallization Behavior of iPP Originating From the Diverse Self‐Assembly Morphology

AbstractRare‐earth nucleating agent (WBG‐II) is a kind of additive for isotactic polypropylene (iPP), which will dissolve into iPP melt during heating and re‐aggregate into diverse morphologies during cooling through self‐assembly. However, up to now, little work about the influence of graphene oxide (GO) on the self‐assembly of WBG‐II can be found. Results in this study suggest that via the hydrogen‐bond interaction, GO can provide additional nuclei for WBG‐II during self‐assembly. Consequently, during non‐isothermal crystallization, WBG‐II and GO present a synergistic effect rather than a superposition effect on the crystallization onset temperature (Tc‐on), the crystallization peak temperature (Tc‐peak), the crystallization half time (t1/2), and the relative content of β‐iPP (Kβ), especially when the final heating temperature (Tf) is higher than 240 °C. Specifically, as Tf is lower than 220 °C, Tc‐on, Tc‐peak, t1/2, and Kβ are insensitive to the weight concentration of GO‐ODA (ϕG), since that little WBG‐II is dissolved into iPP melt and the morphology of WBG‐II is changed only very slightly; as Tf is higher than 240 °C, Tc‐on, Tc‐peak, and Kβ first decrease to a minimum value with increasing of ϕG, and then increase when ϕG exceeded 0.2%.

Correlations between microstructure of α‐row nuclei and polymorphism of shear‐induced iPP/carbon fiber cylindrite

ABSTRACTHerein, we reported the formation mechanism of hybrid crystalline (cylindrite) in isotactic polypropylene (iPP)/carbon fiber (CF) via pulling a CF within the iPP melt. The α‐row nuclei layer closely attached to the surface of CF acts as a self‐nucleation site, rather than a heterogeneous nucleation one, to grow cylindrites. As a result, the polymorphic feature of iPP/CF cylindrite is significantly influenced by the microstructure of α‐row nuclei. With decreasing crystallization temperature (Tc), the polymorphic cylindrite changes from pure α‐form to mixed α‐/β‐form and to β‐rich form. The main characteristics of this change include: (a) the outlines of α‐row nuclei layer correspond to wave‐like, saw‐like, and straight lines; (b) the orientation level of iPP molecules in the α‐row nuclei layer become higher; (c) the α‐lamellae rearrange from loose to compact; and (d) the distance between the growth sites of β‐sectors and the surface of CF is evidently longer than in the case of α‐sectors. Moreover, this study provides a guideline for developing the interfacial enhanced iPP/CF composites through manipulation of polymorphic structure in cylindrites. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 368–377

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.

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.

Shear‐Induced Skin‐Core Structure of Molten Isotactic Polypropylene and the Formation of β‐Crystal

AbstractThe study of crystallization behavior and crystalline morphology of polymer melt under shear flow is of great interest due to the strong effect of flow field on the final properties of polymer products in the practical processing. In this respect, the shearing hot stage provides a unique tool which monitors sensitively the changes in crystalline structure induced by precise experimental conditions. Herein, the impacts of both melting temperature and shear rate on the crystallization behavior of isotactic polypropylene (iPP) melt are investigated. Under static conditions, there are only random spherulite structures. Once shear is involved, the cylindrite‐layers appear near both surfaces of the sample, which is consistent with the skin‐core structure in the injection molded parts. Meanwhile, the β‐crystals can be developed and are related to the molecular orientation, depending on the applied melting temperatures and shear rates. More interestingly, the crystallinity of β‐crystal in the pure iPP can reach 15%. The above results indicate that the melting temperature and shear rate are important factors in determining the β‐form crystal development of iPP matrix.

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...

Transparent Low Electrostatic Charge Films Based on Carbon Nanotubes and Polypropylene. Homopolymer Cast Films.

Use of multi-wall carbon nanotubes (MWCNTs) in external layers (A-layers) of ABA-trilayer polypropylene films was investigated, with the purpose of determining intrinsic and extrinsic factors that could lead to antistatic behavior of transparent films. The incorporation of 0.01, 0.1, and 1 wt % of MWCTNs in the A-layers was done by dilution through the masterbatch method. Masterbatches were fabricated using isotactic polypropylene (iPP) with different melt flow indexes 2.5, 34, and 1200 g/10 min, and using different ultrasound assist methods. It was found that films containing MWCNTs show surface electrical resistivity of 1012 and 1016 Ω/sq, regardless of the iPP melt flow index (MFI) and masterbatch fabrication method. However, electrostatic charge was found to depend upon the iPP MFI, the ultrasound assist method and MWCNT concentration. A percolation electron transport mechanism was determined most likely responsible for this behavior. Optical properties for films containing MWCNTs do not show significant differences compared to the reference film at MWCNT concentrations below 0.1 wt %. However, an enhancement in brightness was observed, and it was attributed to ordered iPP molecules wrapping the MWCNTs. Bright transparent films with low electrostatic charge were obtained even for MWCNTs concentrations as low as 0.01 wt %.

Unique crystallization behavior of isotactic polypropylene in the presence of l‐isoleucine and its inhibition and promotion mechanism of nucleation

ABSTRACTl‐Isoleucine (l‐Ile) was identified as an efficient anti‐nucleating agent for isotactic polypropylene (iPP). At 0.08 wt %, l‐Ile could significantly decrease the peak crystallization temperature (Tcp) of iPP by up to 8 °C at a cooling rate of 20 °C/min. Furthermore, l‐Ile exhibited both anti‐nucleation and pro‐nucleation abilities; i.e., a low content of l‐Ile inhibited iPP crystallization, whereas a high content promoted iPP crystallization. The unique crystallization behavior of iPP in the presence of l‐Ile was investigated by differential scanning calorimetry, polarized optical microscopy (POM), and rheological measurement. According to POM, a low content of l‐Ile completely dissolved in the iPP melt, whereas a high content of l‐Ile did not. Therefore, a mechanism by which l‐Ile inhibits and promotes the nucleation of iPP was proposed. Dissolving l‐Ile molecules in the iPP melt hindered the homogeneous nucleation of iPP as a “dilution effect”; however, as the content increases, l‐Ile could not be completely dissolved in molten iPP, and the residual crystals of l‐Ile thus provided heterogeneous nucleation sites for iPP and further promoted its crystallization. Experimental evidence from rheology and POM supported this mechanism. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45956.

The formation of interfacial morphologies of iPP derived from transverse flow during multi-penetration in secondary melt flow

A growth mechanism of various oriented structures of isotactic polypropylene (iPP) has been investigated in multiple penetrated multi-melt multi-injection molding (MPM3IM), in which multiple high density polyethylene (HDPE) melts as the second injection melts to rapidly penetrate the first iPP melt. The crystal forms of iPP skin layer at different positions were characterized by differential scanning calorimetry (DSC) and 2D-WAXD. And the evolution of hierarchical crystal morphologies in MPM3IM samples from MN1 to MN2 were characterized using polar light microscopy (PLM) and scanning electron microscopy (SEM), respectively. The MN1was the position at which the weld line between two core melts situated and the MN2 was the position of the maximum diameter of single core melt. From the results, on account of the temperature field and complex flow field consisted of penetration flow along the machine direction (MD) and transverse flow (TF) along the transverse direction (TD), the oriented structure, shish-kebab-like structure which was adjacent to interface in iPP skin layer was relaxed to turn into banded fibrillary row nuclei and confined β-cylindrites, at the same time the banded row nuclei receded from interface with its width decreased from MN2 to MN1. Nevertheless, there was only one simplex highly oriented structure formed near the interface at cross section in skin layer of single penetrated multi-melt multi-injection molding (SPM3IM) sample, for instance shish-kebab, which can be attributed to the absence of TF.

Mechanistic insights into the shear effects on isotactic polypropylene crystallization containing β-nucleating agent

The crystalline structures and crystallization behaviors of iPP containing β nucleation agent TMB-5 (iPP/TMB-5) were investigated by synchrotron radiation wide angel X-ray diffraction (SR-WAXD), differential scanning calorimeter (DSC) and polarized light microscope (PLM). It was found that α-crystallization lagged behind β-crystallization at normal temperatures, but the discrepancy reduced with increasing temperature. TMB-5 could not induce β-iPP when the nucleation agent is wrapped up with α-crystal that crystallized at high temperatures. The polymorphic composition of iPP/TMB-5 was susceptible to the introductory moment of shear. New crystallization process of β-nucleated iPP was proposed to understand the experimental phenomena which could not be explained by those reported in the literature. It was supposed that polymer crystallization initiated from mesophase, and the formations of iPP crystals involved the organization of helical conformation ordering within mesophase. It was proposed that the iPP melt contained mesophases with stereocomplex-type ordering of right-handed and left-handed helical chains which could be disturbed by shear or TMB-5, leading to different polymorphic structures.

Crystallization, melting, supermolecular structure and properties of isotactic polypropylene nucleated with dicyclohexyl-terephthalamide

Dicyclohexyl-terephthalamide (DCHT) is an efficient and partially soluble nucleating agent for isotactic polypropylene. Its properties, nucleating efficiency and selectivity were characterized using infrared spectroscopy, calorimetry and polarized light microscopy techniques with special attention to the dissolution and recrystallization of DCHT in the iPP melt. The mechanical properties of the nucleated samples were studied using standard tensile tests. It was found that DCHT possesses dual (α and β) nucleating ability. DCHT is soluble in polypropylene melt and recrystallized from the melt during the cooling. It was observed that a special supermolecular structure is formed in its presence, where micron-sized α-iPP is dispersed finely in the β-iPP matrix. This unique structure promoted good stiffness and toughness simultaneously.

Hydrophilic nonwovens by Forcespinning™ of isotactic polypropylene blended with amphiphilic surfactants

As a widely applied polymer, hydrophilic modification of polypropylene (PP) has been an important research field. Forcespinning™ uses centrifugal force to fabricate nanofibers rather than electrostatic force as in conventional electrospinning. The absence of electric fields provides a board opportunity for low dielectric materials and the utilization of centrifugal forces significantly increases the yield compared to electrospinning. Hydrophilic nonwovens from isotactic polypropylene (iPP) melt blended with amphihilic surfactant TWEEN20 and TWEEN60 were obtained by Forcespinning™. The factors were investigated by SEM and DSC, which determined fiber diameter distribution and crystallinity. The meshes showed continuous homogeneity along the length of the fiber when the system was employed at 225 °C and the rotational speed at 14,000 rpm. Blending with surfactants bearing ethoxylate groups led to increased oxygen content of the meshes surface as confirmed by ATR-FTIR spectroscopy and X-ray photoelectron spectroscopy (XPS). XPS results indicated preferential distribution of additives on surfaces. The forcespun meshes exhibited excellent hygroscopicity even when the load of TWEEN20 was 10 %.

Study on variable nucleation efficiency of N,N ′-Dicyclohexyl-2,6-naphthalenedicarboxamide on isotactic polypropylene

N, N′-Dicyclohexyl-2,6-naphthalenedicarboxamide (NU100), a commercial β-nucleating agent (NA) for isotactic polypropylene (iPP), is found dissolvable in iPP melt. Various heating temperatures ( Ths) of iPP/NU100 cause different solubilities of NA, resulting in various aggregation morphologies of NU100 in the melt. Moreover, nucleation efficiency of NA NU100 on iPP changes with the Th of iPP/NU100. In this article, the effects of Th on nucleation behaviors of iPP nucleated with different concentrations of NU100 were systematically studied. The results indicated nucleation efficiency of NU100 depended not only on concentration of NA but also on Th of nucleated iPP. With the increase of Th, crystallization peak temperature ( Tcp) of nucleated iPP decreased remarkably. The whole process that Tcp shifted from 126.8°C to 120.8°C was obtained by investigating the nucleation behavior of iPP/0.1 wt% NU100. Polarized optical microscopy observation confirmed lower Tcp, representing the crystallization of iPP nucleated by dissolved–recrystallized NA NU100. Further, diverse aggregation morphologies of NA in iPP melt during crystallization were observed. A schematic diagram was proposed illustrating the morphologies formed during recrystallization process of NU100 melted at different Ths.