Effect of polycyclopentene on crystallization of isotactic polypropylene

The application of some polymers as nucleating agents for polypropylene has been examined. Among various polymeric nucleating agents, polycyclopentene was found to be a superior nucleating agent to typical organic nucleating agents. When polycyclopentene was added to polypropylene, the crystallization temperature and the degree of crystallinity of polypropylene increased. In addition, the crystallization rate and the number of spherulites increased whereas the size of spherulites decreased remarkably. As a result of polycyclopentene addition, the transparency of polypropylene film could be improved considerably. © 1994 John Wiley & Sons, Inc.     

Crystallinity morphology and dynamic mechanical characteristics of PBT polymer and glass fiber‐reinforced composites

The crystalline morphologies of PBT (poly butylene terephthalate) and its glass fiber reinforced composite systems were investigated in a thin‐film form by polarized optical microscopy and wide‐angle X‐ray diffraction. Three different types of PBT morphology were identified in the Maltese cross pattern: 45° cross pattern (usual type) by solvent crystallization, 90° cross pattern (unusual type) by melt crystallization at low crystallization temperature, and mixed type by melt crystallization at crystallization temperatures higher than 160°C. The glass fibers increased the number density of spherulites and decreased the size of crystallites acting as crystallization nucleation sites without exhibiting trans‐crystallinity at the vicinity of the glass fiber surfaces. Finally, the storage modulus was analyzed by using a dual‐phase continuity model describing the modulus by the power‐law sum of the amorphous‐ and crystalline‐phase moduli. The crystalline‐phase modulus was extracted out from the PBT polymer and composite systems containing different amount of crystallinity. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 478–488, 2002     

Hot plate welding of polypropylene. Part I: Crystallization kinetics

The crystalline structure formation in the heat affected zone during hot plate welding has a great influence on the performance of the welded semi-crystalline polymers. The quiescent and flow induced crystallization of polypropylene was investigated experimentally. A simplified, phenomenological crystallization model was developed, which can describe the crystal formation from completely melted and partially melted polymer. Predictions obtained from the model were compared with experimental results.     

Physicochemical characteristics of fabrication of fibers from mixtures of polyacrylonitrile-based polymers

The physicochemical principles of fabrication of fibers from mixtures of PAN-based polymers via a common solvent were investigated. The effect of the added polymers on mass exchange and phase separation dynamics was demonstrated. The effect of coprecipitation of the components of stable polymer pairs and separate precipitation of unstable polymer blends from solution was established. The capacity of fibers from polymer blends for orientational drawing and the effect of orientation of the structural elements are determined by the concentration of residual solvent in the fiber, the plasticizing effect, flexibility, and phase state of the added polymers. A quantitative correlation was established between the structure and composition of fibers from polymer blends. A homogeneous (for stable systems) or microheterogeneous (for unstable systems) structure of the fibers is formed with a low concentration of polymer added to PAN. Heterogeneous overall supermolecular (for stable systems) or laminar (for unstable systems) structures occur with a high concentration of the second component. Different supermolecular structures are formed and different physicomechanical properties are obtained as a function of the nature and amount of added polymers and the conditions of fabrication of the fibers.     

Morphological study on the effect of elastomeric impact modifiers in polypropylene systems

The morphological effect of elastomeric impact modifiers has been studied in polypropylene systems by wide-angle and small-angle X-ray diffractometry, small-angle light scattering, light and electron microscopy and differential scanning calorimetry. It was established that the incorporation of an impact modifier altered the superstructure of the polypropylene matrix by decreasing the average size of spherulites through which the impact strength of the composite may be influenced. The changes in the mechanical and thermal properties are probably caused by the interphase between the amorphous elastomeric modifier and the polypropylene spherulites. The particle size of the dispersed elastomer is of vital importance in toughening of the amorphous polymer while in the crystalline resin, the changes in the superstructure also seem to be very important. Above T_g, the amorphous impact modifier acts as an energy absorber which markedly influences the crazing susceptibility of the polypropylene matrix.     

Porous materials from crystallizable polyolefins produced by gel technology

A new method for obtaining porous and porous fiber polymers is presented. This method is based on using gel-type technology (without previously preparing polymer solutions) for crystallizable polymers, preparing polyethylenes, and including polyethylenes of very high molecular mass and isotactic polypropylene. The method consists in swelling crystalline polymer films at elevated temperatures in a proper solvent with subsequent precipitation with a non-solvent at different conditions. In this case, simultaneous or consecutive processes of phase separation of amorphous or/and crystalline type occurs; stretching the sample can also accompany this process. Complete phase diagrams of two- and three-component systems (polymer-solvent and polymer-solvent precipitator) were constructed. Temperature-concentration boundaries of amorphous separartion (binodal) and crystallization (liquidus) are reported for the system polyethylene? o-xylene? dimethyl formamide. Phase transitions of both types influence characteristics of the resultant porous structure. They were prepared by simultaneous (precipitation of a gel by dimethyl formamide at 25°C) or consecutive (precipitation with a hot non-solvent at 138°C and following cooling) phase separation. Studied were the effect of experimental conditions (temperature, times for solvation and precipitation, polymer molecular mass, the thermodynamic quality of solvents and parameters of film stretching) on peculiarities of the structure and quantitative characteristics of final porous and fiber-porous polyolefins. It has been demonstrated that the method proposed allows us to obtain a crystalline and highly porous polymer with open poros, a bimodal size distribution and with a highly developed inner surface. Further high strength and small shrinkage are characteristic of the fiber-porous materials. The method under discussion appears to be universal, it does not require a preliminary preparation of polymer solutions and can be realized within a general technology of polymer films and sheet processing. Highly porous polymers obtained by this technology, primarily based on polyethylenes of very high molecular mass, can be used as neutral supports for multi-functional membranes, polymeric covers, frame systems for implants and other applications.     

Interfacial enhancement by shish-calabash crystal structure in polypropylene/inorganic whisker composites

The polymer matrix structure and the interface are strongly influenced by filler in semi-crystalline polymer composites because the fillers have the potential to nucleate the polymer crystallization. The structure of the nucleated crystalline polymer on filler is of particular interest and is a key to the interfacial enhancement. In this work, whiskers, with a large length/diameter ratio and with a diameter (0.2-2 μm) much larger than that of carbon nanotubes but much smaller than that of common fibers, were used to nucleate crystal morphology in polypropylene (PP)/whisker composites. The crystal morphology, interfacial adhesion and tensile properties of the composites were carefully investigated. A kind of peculiar shish-calabash crystallization morphology, with whisker serves as shish and PP spherulites serves as calabash, was observed for the first time in the thin film via PLM and in the injection molded bars by SEM. The formation mechanism of this shish-calabash structure was attributed to be that only a few nuclei could be induced on the whisker surface, which develop into large PP spherulites without hindrance, and finally stringed by the whisker, forming the shish-calabash structure. As a result, a significant improvement of interfacial interaction and tensile properties has been achieved.     

Nanocomposites of polypropylene/titanate nanotubes: morphology,nucleation effects of nanoparticles and properties

High-quality titanate nanotubes (TiNT) were mixed with modified polypropylene (PP*) by a batch melt-mixing procedure. To improve compatibility between the nanofiller and the matrix, polypropylene (PP) was modified by electron beam irradiation. Effects of TiNT nanoparticles on crystallization, mechanical, thermal and rheological properties of the modified polypropylene were studied and compared with the analogous systems filled with commercial micro- (mTiO₂) and nano- (nTiO₂) titanium dioxide particles. Nucleation effects of the TiO₂-based fillers on PP* crystallization were investigated in detail. The microstructure of the PP*/TiNT nanocomposites shows well-dispersed TiNT sparse aggregates (clouds), penetrated by the polymer. A large-scale structure in the nanocomposite melts confirmed also rheology. In comparison to the matrix characteristics, the stiffness and microhardness of the TiNT nanocomposites increase by 27 and 33 %, respectively. The enhancement in mechanical properties demonstrates that the quality titanate nanotubes can be used as an efficient filler in non-polar polymers using the polymers modified by irradiation. In the case of the nanocomposites containing nTiO₂-anatase particles, the increase in these mechanical characteristics is lower. The investigated changes in the rate of crystallization indicate a marked nucleation effect of the nanotubes. The crystallization kinetics data, processed by the Avrami equation, suggest 3-dimensional crystal growth in the polypropylene matrix. The observed improvement in mechanical properties of the TiNT nanocomposites is induced not only by the nanofiller reinforcement but also by the changes of supermolecular structure of the polymer matrix due to nucleated crystallization.     

Effects of SBS on phase morphology of iPP/aPS blends

The supermolecular structure of binary isotactic polypropylene/poly(styrene‐b‐butadiene‐h‐styrene) (iPP/SBS) and isotactic polypropylene/atactic polystyrene (iPP/aPS) compression molded blends and that of ternary iPP/aPS/SBS blends were studied by optical microscopy, scanning and transmission electron microscopy, wide‐angle X‐ray diffraction and differential scanning calorimetry. Nucleation, crystal growth, solidification and blend phase morphology are affected by the addition of amorphous components (SBS and aPS). As a compatiblizer in immiscible iPP/aPS blends, SBS formed interfacial layer between dispersed honeycomb‐like aPS/SBS particles and the iPP matrix, thus influencing the crystallization process in iPP. The amount of SBS and aPS, and compatibilizing efficiency of SBS, determine the size of dispersed aPS, SBS, and aPS/SBS particles and, consequently, the final blend phase morphologies: well‐developed spherulitic morphology, cross‐hatched structure with blocks of sandwich lamellae and co‐continuous morphology. The analysis of the relationship between the size of spherulites and dispersed particles gave the criterion relation, which showed that, in the case of a well‐developed spherulitization, the spherulites should be about fourteen times larger than the incorporated dispersed particles; i.e. to be large enough to engulf dispersed inclusions without considerable disturbing of the spherulitic structure.     

Supermolecular morphology of polypropylene filled with nanosized silica

The supermolecular morphology of injection‐molded SiO₂/polypropylene (PP) nanocomposites was investigated via thin sections analyzed under polarized light and the systematic development of an appropriate etching technique, which allowed the study of the supermolecular morphologies with light microscopy (LM) and high‐resolution field emission scanning electron microscopy (FESEM). In parallel, information regarding the dispersion, distribution state, and morphology of SiO₂ particles was investigated via transmission electron microscopy (TEM) and scanning electron microscopy (SEM) of the ion‐polished and fractured surfaces of SiO₂‐filled PP. The TEM/SEM results demonstrated an almost homogeneous dispersion and distribution of SiO₂ particle agglomerates in the PP matrix. With polarized transmitting LM, reflecting LM, and FESEM, the spherulitic structure of the nanocomposites could be visualized to obtain information on the nanoparticle influence on the crystallization and structural behavior. The size and size distribution of the spherulites analyzed with transmitting light (thin sections) and reflecting light (etched specimens) showed an excellent correlation. With increasing filler loading, the mean size of the spherulites decrease as did the degree of crystallinity. This was a clear indication that the particles acted as nucleation agents and, on the other hand, hindered the arrangement of the molecules during the crystallization. As a result, the particles were most likely located in three areas: the center of the spherulites, the areas between the highly crystalline branches, and the spherulite boundaries. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39655.     

Annealing of polypropylene–poly(ethylene-co-propylene) blends. I. Thermal and physical properties of blends

Annealing of polypropylene and blends of polypropylene and poly(ethylene-co-propylene) was studied. The structural and physical properties were determined from thermal, mechanical, physicochemical, and spectral investigations. The particular emphasis was on the characteristics of structure and thermal properties of relatively amorphous components segregated from the crystalline region by annealing. Annealing of polypropylene induced an increase in crystallinity resulting in a decrease in impact strength. In contrast, by annealing a blend of polypropylene and poly(ethylene-co-propylene), the impact strength and rigidity were significantly improved with an increase in annealing temperature. The effect of annealing in a binary system was ascribed to the formation of a thicker transitional layer at the interface of the two polymers owing to the increased mobility of amorphous polymer segments. The results of tensile impact strength and brittle temperature were correlated with a deformational mechanism involving the crazing of the matrix.     

Semi‐Crystalline Polymer Blends for Material Extrusion Additive Manufacturing Printability: A Case Study with Poly(ethylene terephthalate) and Polypropylene

Semi‐crystalline polymers are an important class of materials for engineering applications due to their high modulus and barrier properties. Traditional manufacturing methods process semi‐crystalline polymers via rigid molds and well‐controlled temperature and pressure environments to handle the significant change in specific volume occurring during crystallization; however, material extrusion additive manufacturing does not use these features. This often leads to warpage‐induced build failure in fused filament fabrication (FFF). To enable FFF of semi‐crystalline polymers, this work investigates characteristics of immiscible polymer blends (e.g., disparate crystallization behavior and phase separation) to mitigate warping failure during printing. A series of poly(ethylene terephthalate)/polypropylene/polypropylene–graft–maleic anhydride blends are explored and the effect of thermal and morphological characteristics on printability is analyzed. It is shown that these blends can be extruded into filament and printed into a 3D structure. Extrapolations indicate that phase‐separated blends with increased total crystallization half‐time are beneficial for FFF printing.     

Solvent‐induced crystal formation in polymers: Experimental studies and theoretical modeling of poly(vinyl alcohol) based on free‐volume concepts

A model has been developed to describe the simultaneous diffusion and solvent‐induced crystal formation in polymers based on the idea that crystal formation is governed by polymer chain mobility and a thermodynamic driving force. The polymer chain mobility is described based on solvent and polymer physical characteristics using the free‐volume theory of transport. The semicrystalline polymer‐solvent system is treated as a ternary system consisting of crystalline polymer, amorphous polymer, and solvent. The addition of solvent to the amorphous phase is assumed to increase the local free volume and facilitate movement of polymer chains, thereby enabling crystal formation. Diffusion of the solvent is assumed to occur solely in the amorphous polymer phase. The species continuity equations are formulated in volume‐averaged coordinates and give rise to a convective term due to the density change accompanying transformation of the amorphous polymer to the crystalline polymer. Accurate modeling of this problem requires that a moving boundary be considered. The model was tested using gravimetric sorption data for the poly(vinyl alcohol)‐water system. In the experimental studies, the water was initially absorbed and then a high percentage of it was expelled. The proposed model accurately describes this behavior. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45171.     

Studies on the characteristics of microphase separation and stabilization of interphase for blended systems of high performance polymers

In this report, we review and discuss the results of our recent studies on the characteristics of microphase separation behavior and interphase stabilization for high performance polymer blends. The blends investigated include crystalline/crystalline polymers, crystalline/amorphous polymers, liquid crystalline polymer/thermoplastics, and amorphous/amorphous thermoplastics or thermosetting systems. Most of the blends are either immiscible or partially miscible, and are thermodynamically unstable or meta-stable systems. The macro-properties of these blends are controlled by many factors such as the miscibility, phase morphology and structure, crystallinity, kinetics of crystallization or phase separation processing, and interfacial adhesion of the components. Among these, the microphase and interfacial structures are the most significant factors influencing the ultimate properties of the blends. In order to obtain relatively stable blends, formation of semi-IPN in either the bulk or interphase, and/or the occurrence of crosslinking, transesterification and physical entanglement in the interfacial region will be profitable to the stabilization of the blending systems.The project supported by FORD and NSFC No. 09415312     

Dynamically confined crystallization in a soft lamellar space constituted by alternating polymer co-brushes

Crystallization kinetics and behavior of PCL side chains in polymer co-brushes constituted with PCL and PEO side chains alternatively attached on poly(styrene-alt-maleimide) backbones have been determined using in-situ FT-IR and DSC methods. Avrami analysis shows the exponent n increasing from one at 10 °C to two at 30 °C, demonstrating confined crystallization of PCL side chains through homogeneous or heterogeneous nucleation. PLM morphological characterization displays typical spherulites of which size is dependent on the crystallization temperature and further AFM visualization shows typical PCL lamellae at 30 °C and broken lamellae at 10 °C embedded within PEO + backbone matrix inside of spherulites. Such lamellar structure explains the confined crystallization with Avrami exponent n ≤ 2. Formation of the broken lamellae can further clarify the reason why Avrami exponent decreases to n ≈ 1 at 10 °C, that is, homogeneous nucleation in the isolated crystals. Dynamically confined crystallization has been proposed based on their special molecular architecture. Comparing to statically confined crystallization, the construction of confined space and the crystallization process were almost synchronous. The formation of spherulites mesoscopically reveals the entire molecule motion and assembly through a pathway of conventional crystalline polymers and the crystallization of PCL side chains in a space constituted by stiff backbones of poly(styrene-alt-maleimide) plus soft PEO layer microscopically reflects a confined character which has been observed in some conventional block copolymers.     

Die Kristallisation von Mehrkomponentensystemen aus hochpolymeren Stoffen

The differences in the crystallization behaviour between a single component system and a multicomponent system are discussed. Examples for multicomponent systems are homopolymers with a broad distribution in molecular weight, mixtures of different homopolymers, swollen polymers, block copolymers, and statistical copolymers. A distribution in molecular weights manifests itself mainly in extended chain crystallization experiments in that way that a fractionation with respect to the chain length takes place. It causes also a broadening of the melting range. The presence of a second noncrystallizable homopolymer which is miscible with the crystallizable homopolymer leads to a reduction of the melting point and a change in the glass transition temperature. The crystallization remains spherulitic. The noncrystallizable component is expelled from the crystals. If the diffusion rate of this component is large, it is also expelled from the spherulites, otherwise it is incorporated into the spherulites. When the noncrystallizable component is expelled from the spherulites, the growth rate of the spherulites decreases during growth. The temperature range in which crystallization takes place is limited by the melting point of the crystallizable component and by the glass transition temperature of the two-component system. If the crystallizable component is not dissolved completely in the noncrystallizable component, this part which is not dissolved crystallizes much more rapidly than the part which is dissolved. Below the glass transition temperature only the part which is not dissolved crystallizes. This gives a possibility to determine the solubility above the melting point. By swelling, the glass transition temperature and therewith the crystallization temperatures are decreased. When, during swelling of an amorphous sample, the glass transition temperature is decreased below the temperature where swelling is performed one observes a front of spherulites penetrating into the sample simultaneously with the swelling agent. On the other hand, when the glass transition temperature remains above the swelling temperature, one can crystallize the sample after swelling is completed by raising the temperature. As in a pure polymer, one then observes the growth of spherulites from statistically distributed centers; the growth rate of the spherulites increases however with increasing time. Block copolymers of a crystallizable component and a non-crystallizable component sometimes are not able to crystallize. This is the case, if the chains of the noncrystallizable component have a cross section which is larger than that of the chains of the crystallizable component and if, in addition, the latter chains are not so long that they can fold several times in order to compensate the difference in the cross sections. When crystallization takes place, spherulites are formed only if the amount of the crystallizable component exceeds a well defined limit. Otherwise only a diffuse birefringence is developed. In this case a much larger supercooling is necessary to crystallize the sample than in the case of spherulitic crystallization. From long period measurements one can conclude how many times each noncrystallizable chain is folded. From the melting and swelling behaviour one learns whether the noncrystallizable chains form loops or tie molecules. With statistical copolymers consisting of crystallizable units and noncrystallizable units the melting point, the rate of crystallization, the degree of crystallization at the end of the process, and the melting point decrease with increasing amount of noncrystallizable units. The noncrystallizable units are incorporated partly also into the crystals.     

The effect of the structural order of isotactic polypropylene containing magnetically aligned nickel particles on its electrical resistivity

The effect of the high order structure of an isotactic polypropylene (PP) composite on the resistivity of composites containing magnetically aligned Ni particles was studied. Only a small amount of particles needed to be added for the composite material to become conducting after heating while in a magnetic field. The Ni columns formed on applying the field were distorted by the formation of large PP spherulites. Changes to the crystallization process due to the addition of a nucleating agent gave rise to changes in the columnar structure, resulting in large changes in the resistivity of the composite material. Controlling the high order structure of the polymer matrix including its morphology is very important in order to be able to control the magnetically aligned Ni structure.     

Morphology of a melt crystallized iPP/HDPE/hydrogenated hydrocarbon resin blend

The structure, phase structure, morphology, crystallization and melting behavior of isotactic polypropylene (iPP) blended with a master batch (MB), formed by high density polyethylene and hydrogenated hydrocarbon resin (iPP/MB), have been in details investigated by using X-ray diffraction, optical microscopy and differential scanning calorimetry. It was found that the structure and morphology depend on crystallization conditions. A new family of α spherulites of iPP (type I spherulites) can be activated using appropriate crystallization conditions. Nucleation of these spherulites has been explained by using the approach of nucleus migration in polymer blends. Type I spherulites present specific morphological, kinetic and thermal behaviors. In particular it was found that the growth rate of type I spherulites, at a given T_c, is higher than the growth rate of spherulites grown from plain iPP.     

Effect of annealing on the microstructure and mechanical properties of polypropylene with oriented shish‐kebab structure

Annealing is thought to be an effective method to promote chain rearrangement in semicrystalline polymers and improve their physical properties. However, little attention has been paid to the annealing of flow‐oriented semicrystalline polymers despite its importance in polymer processing. In this work, the microstructural evolution of injection‐moulded polypropylene with an oriented shish‐kebab structure upon annealing has been explored with differential scanning calorimetry, small‐angle X‐ray scattering and scanning electron microscopy, Fourier transform infrared spectroscopy and dynamic mechanical analysis. The results show that annealing gives rise to a chain rearrangement in both the crystalline and amorphous phases. Accompanied by the growth and perfecting of the kebabs, relaxation of the initially oriented chains in the amorphous phase is observed. Then, the relationship between the structure and the resulting mechanical properties is established. Copyright © 2011 Society of Chemical Industry     

The influence of rubber-matrix interfaces on the crystallization kinetics of isotactic polypropylene blended with ethylene-propylene-diene terpolymer (EPDM)

Blends of isotactic polypropylene with amorphous and slightly crystalline ethylene-propylene-diene terpolymer (EPDM), prepared by solution blending, have been investigated by optical microscopy and differential scanning calorimetry. Nucleation and crystallization kinetic parameters, such as nucleation rates, nucleation half times, Avrami-exponents and spherulitic growth rates, have been determined. It has been found that the dispersion of crystalline EPDM in iPP is different from that of amorphous EPDM. Both EPDMs are incorporated into the spherulites, causing a decrease of the maximum growth rate of the iPP spherulites. The surface free energy of the iPP crystals is diminished on adding EPDM to iPP and is accompanied by a higher secondary nucleation rate. From the decrease observed in the Avrami exponent with increasing EPDM concentration in the blend, it has been concluded that nucleation becomes predominantly heterogeneous, as there is a proportional increase in the interfacial area between the two components.