Isothermal crystallization kinetics and morphology development of isotactic polypropylene blends with atactic polypropylene

The crystallization kinetics and morphology development of pure isotactic polypropylene (iPP) homopolymer and iPP blended with atactic polypropylene (aPP) at different aPP contents and the isothermal crystallization temperatures were studied with differential scanning calorimetry, wide‐angle X‐ray diffraction, and polarized optical microscopy. The spherulitic morphologies of pure iPP and larger amounts of aPP for iPP blends showed the negative spherulite, whereas that of smaller amounts of aPP for the iPP blends showed a combination of positive and negative spherulites. This indicated that the morphology transition of the spherulite may have been due to changes the crystal forms of iPP in the iPP blends during crystallization. Therefore, with smaller amounts of aPP, the spherulitic density and overall crystallinity of the iPP blends increased with increasing aPP and presented a lower degree of perfection of the γ form coexisting with the α form of iPP during crystallization. However, with larger amounts of aPP, the spherulitic density and overall crystallinity of the iPP blends decreased and reduced the γ‐form crystals with increasing aPP. These results indicate that the aPP molecules hindered the nucleation rate and promoted the molecular motion and growth rate of iPP with smaller amounts of aPP and hindered both the nucleation rate and growth rate of iPP with larger amounts of aPP during isothermal crystallization. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1093–1104, 2007     

SIMULATION OF CRYSTALLIZATION KINETICS AND MORPHOLOGICAL DEVELOPMENT DURING ISOTHERMAL CRYSTALLIZATION OF POLYMERS: EFFECT OF NUMBER OF NUCLEI AND GROWTH RATE

The effect of number of nuclei and growth rate on crystallization kinetics and detailed morphological development during isothermal crystallization of a polymer was investigated using a stochastic simulation. The results show that number of nuclei significantly affects both crystallization kinetics and polymer morphology. An increase in the number of nuclei hastens the crystallization process by speeding up the impingement phenomenon and increasing the levels of impingement. Growth rate has a stronger impact on crystallization kinetics, but it only helps speed up the impingement phenomenon without increasing the level of impingement. Although growth rate influences an average spherulite size and distribution of spherulite size during crystallization, it has no effect on final morphology. The quantitative understanding of morphological development obtained from this work will be a key element for constructing quantitative morphology-property relationships.     

Isothermal crystallization behavior of polypropylene catalloys

The isothermal crystallization behavior of polypropylene (PP) catalloys and neat PP were studied with differential scanning calorimetry and polarized optical microscopy (POM). The crystallization kinetics of the samples were described with the well‐known Avrami equation. The crystallization rate depended remarkably on the content of the ethylene component in the PP catalloys. The crystallization half‐time increased obviously with the increase of the ethylene component in the PP catalloys. We also observed by POM that in isothermal crystallization, there were many more nuclei in the PP catalloys than that in neat PP and with an increase of the ethylene component, the average size of the spherulites decreased obviously. Even when ethylene content was as high as 27%, the crystallization rate still increased apparently, and this was quite different from common PP melting blends, in which the crystallization rate decreased when the ethylene content was relatively high because of the obstruction effect of dispersed droplets to the spherulite growth of the PP matrix. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 877–882, 2004     

Crystallization behavior and mechanical properties of polypropylene/halloysite composites

In this work, halloysite nanotubes (HNTs), a new type of inexpensive filler, were used for the modification of polypropylene (PP). HNTs were first surface treated by methyl, tallow, bis-2-hydroxyethyl, quaternary ammonium, then melt mixed with PP. Scanning electron microscope (SEM) was used to examine the dispersion of HNTs in PP matrix. Differential scanning calorimetry (DSC), polarized light microscope (PLM), dynamic melt rheometry and wide angle X-ray diffraction (WAXD) were employed to investigate the crystallization behavior of the prepared PP/HNT composites. The mechanical properties were evaluated by Instron and impact tests. SEM results revealed that HNTs could be well-dispersed in PP matrix and had a good interfacial interaction with PP, even up to a high content of 10 wt%. DSC data indicated that HNTs could serve as a nucleation agent, resulting in an enhancement of the overall crystallization rate and the non-isothermal crystallization temperature of PP. PLM showed a constant spherulite growth rate and a decreased spherulite size at given isothermal crystallization temperature, suggesting that nucleation and growth of a spherulite are two independent processes. The result obtained by dynamic melt rheometry indicated that HNTs mainly promoted nucleation and had not much influence on the growth of PP crystallization. Nevertheless, by fast cooling the samples, almost constant spherulite size can be obtained for both pure PP and PP/HNT composites due to the limited nucleation effect of HNTs on PP crystallization. WAXD showed that HNTs mainly facilitated α-crystal form of PP. Though a good dispersion of HNTs in PP matrix was observed, out of our expectation, not much enhancement on mechanical properties of PP/HNT composites had been achieved, and this could be mainly ascribed to the constant crystallinity and spherulite size of PP as well as the small length/diameter ratio of HNTs.     

Nucleation‐controlled dual semicrystalline morphology of polyamide 11

Crystallization of polyamide 11 at low supercooling of the melt proceeds via heterogeneous nucleation and spherulitic growth of lamellae, while at temperatures close to the glass transition homogeneous nucleation prevails, preventing spherulite formation and leading to formation of a large number of nanometer‐sized mesophase domains. It is shown that spherulitic and non‐spherulitic crystallization at low and high supercooling of the melt, respectively, can be enforced by tailoring the cooling conditions, causing a twofold semicrystalline morphology at ambient temperature. Analysis of non‐isothermal crystallization as a function of the cooling rate, using fast scanning chip calorimetry, reveals that in the case of polyamide 11 such twofold semicrystalline morphology is predicted when cooling at rates between about 20 and 200 K s^?1, since then two separate crystallization events are observed. The prediction has been confirmed by preparation of films crystallized during ballistic cooling at different rates which then were analyzed regarding their structure using optical microscopy, X‐ray diffraction and calorimetry. The study is completed by discussion of implications of twofold non‐isothermal crystallization for structure evolution in polymer processing, as well as by providing information that such behavior is not only typical for polyamide 11 but also for isotactic polypropylene or poly(butylene terephthalate) as two further examples. © 2018 Society of Chemical Industry     

Banded spherulites in poly(L‐lactic acid): Effects of the crystallization temperature and molecular weight

The spherulitic morphology of pure poly(L ‐lactide) (PLLA) was investigated with polarized optical microscopy as a function of the crystallization temperature and molecular weight. After being melted at 210°C for 3 min, samples were cooled quickly to designated temperatures for isothermal crystallization. It was shown for the first time that a clear banding‐to‐nonbanding morphological transition took place at a critical temperature for PLLA with a number‐average molecular weight of 86,000. With the increasing molecular weight of the material, the spherulite growth rates decreased notably, and the band spacing decreased significantly. On the basis of the main‐chain chirality in PLLA and the observation of a nonbanded spherulitic morphology in a certain temperature region, it was suggested that the crystallization temperature might have an effect on the relationship between the sense of lamellar twisting and the main‐chain chiral structure in PLLA. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Thermal properties and crystallization behavior of polyolefin ternary blends

Crystallization behaviors, spherulite growth and structure, and the crystallization kinetics of polypropylene (PP)/ethylene‐α‐olefln copolymer (mPE)/high‐density polyethylene (HDPE) ternary blends and of mPE/HDPE binary blends have been studied using polarizing optical micrography (POM) and differential scanning calorimetry (DSC). In mPE/HDPE blends, large pendant groups of mPE disturbed spherulite growth of HDPE, leading to a different crystallite morphology and isothermal kinetics. Non‐isothermal properties, morphology, and isothermal crystallization kinetics of PP in ternary blends were significantly influenced by the composition and crystallization behavior of the mPE/HDPE binary blends as well as the crystallization condition. Polym. Eng. Sci. 44:1858–1865, 2004. © 2004 Society of Plastics Engineers.     

Effects of processing on the microstructure,melting behavior,and equilibrium melting temperature of polypropylene

Polypropylene (PP) was extruded and injection-molded several times to mimic the effect of recycling procedures on PP. Differential scanning calorimetry (DSC) was used to follow crystallization rates under isothermal conditions in a temperature range of 120–150°C. Melting behavior and equilibrium melting temperatures were studied using the Hoffman-Weeks method of extrapolation. Optical microscopy combined with a hot stage was also used to follow the spherulite microstructure and crystal phase upon recycling of PP. Wide-angle X-ray spectroscopy identified the crystal phase at different isothermal crystallization temperatures. Twin melting peaks obtained for PP melting following isothermal crystallization were associated with crystal rearrangement during fusion. PP spherulite size and equilibrium melting temperatures were seen to increase with processing events, whereas reprocessing decreased nuclei density. © 1996 John Wiley & Sons, Inc.     

Blends of thermoplastic polyurethane and polypropylene. II. Thermal and morphological behavior

Neat thermoplastic polyurethane (TPU), polypropylene (PP), and TPU/PP blends with different weight ratios that were prepared in a twin‐screw extruder were investigated with differential scanning calorimetry and light and scanning electron microscopy. The results confirmed PP matrix to TPU matrix phase inversion in the concentration region between 60/40 and 80/20 TPU/PP blends. The total degree of crystallinity of the blends and the crystallization temperature of PP decreased with increasing TPU content. On the other hand, the addition of elastomeric TPU to PP significantly increased the spherulite size of PP. The TPU melt islands in the PP matrix prolonged the crystallization of PP during solidification, and this enhanced the growth of spherulites. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Crystallization kinetics of polypropylene composites filled with nano calcium carbonate modified with maleic anhydride

The subject of this study was the crystallization behavior and thermal properties of polypropylene (PP)/maleic anhydride (MAH) modified nano calcium carbonate (nano‐CaCO₃) composites. In this study, 5 wt % nano‐CaCO₃ modified with different contents of MAH was filled into a PP matrix. X‐ray diffraction and differential scanning calorimetry were used to characterize the crystal morphology and crystallization kinetics of a series of composites. The results demonstrate that the nano‐CaCO₃ modified with MAH had an important effect on the thermal and morphological properties of the nanocomposites. The Avrami exponent of the pure PP was an integer, but those of the composites were not integers, but the crystallization rate constant decreased as the content of MAH in the nano‐CaCO₃ filler increased in isothermal crystallization. In nonisothermal crystallization, the kinetic parameter F(T) and the degree of crystallinity of pure PP were compared with those of the PP composites filled with nano‐CaCO₃. We suggest that heterogeneous nucleation existed in the PP composites and that the transformation and retention of the β‐form crystal into the α‐form crystal took place in the composite system and the β‐form crystal had a higher nucleation rate and growth process than the α‐form crystal in the PP composites. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Effects of crystallization behavior on morphological change in poly(trimethylene terephthalate) spherulites

In poly(trimethylene terephthalate) (PTT) spherulites during isothermal crystallization, the morphological changed from an axialite/or elliptical banded spherulite to banded spherulite and then non-banded spherulite with temperature decreasing were studied by following the lamellar growth behaviors. We report lamellar growth mechanism on varied crystallization temperature, which explicitly probes the link between microscopic structure and macroscopic morphology in the development of patterns. Fibrillation of the edge-on lamellae was observed on the surfaces of axialite and the convex bands of banded spherulite. Terrace-like lamellae were observed on the surface of the non-banded spherulite and the concave bands of banded-spherulite. In thin film crystallization, PTT banded spherulite exhibits a texture of alternate edge-on and flat-on lamellae, wavy-like surface and rhythmic growth. The deceleration of growth rate takes place in convex bands with a growth habit of fibrillation of the edge-on lamellae for emerging ridge surface. On the other hand, the acceleration of growth rate appears in concave bands with a growth habit of terrace-like lamellae for emerging valley surface. The alternating growth mechanism of the lamellae was considered to be related with the formation of spatiotemporal self-organization patterns far from equilibrium. In order to explain the rhythmic growth and periodic growth of the lamellae, we may conjecture that the emergence of PTT banded spherulite in thin film crystallization is associated with an oscillatory dynamics of the spherulite growth front driven by latent heat diffusion. We present some tentative ideas on the possibility of band-to-nonband (BNB) morphological transition, which might be analogous with the second order transition in non-equilibrium phase transition.     

Growth process of homogeneously and heterogeneously nucleated spherulites as observed by atomic force microscopy

A poly(bisphenol A octane ether) (BA-C8) was synthesized. The isothermal spherulitic growth process was studied in situ using atomic force microscopy (AFM) at room temperature. For spherulites formed by homogeneous nucleation, the growth process includes the birth of a primary nucleus, the development of a founding lamella and the growth of the founding lamella into a spherulite. An embryo below a critical size is unstable. A stable embryo grows into a founding lamella. There is only one founding lamella in each spherulite. All other lamellae originate from this founding lamella. Two eyes can be seen at the center of a spherulite. For spherulites formed through heterogeneous nucleation, many lamellae grow at the nucleus surface and propagate outward radially. The spherulites acquire spherical symmetry at the early stage of crystallization. No eyes are found for this kind of spherulites.     

Effects of spatial confinement and selective distribution of CB particles on the crystallization behavior of polypropylene

The effects of selective distribution of carbon black (CB) particles and spatial confinement on the crystallization behavior of isotactic polypropylene (iPP)/Polystyrene (PS)/CB composite were studied. The crystallization behaviors and the morphologies of the composite were studied by differential scanning calorimetry (DSC), polarized light microscope (PLM), and scanning electron microscopy (SEM). The results indicated the typical cocontinuous structure appeared in PP/PS/CB (55/45/1) composite, and CB particles are distributed in PS phase, which follows the theory of interfacial tension. Compared with PP/CB composite, the nucleation effect of CB particles on the crystallization process of PP in PP/PS/CB was greatly weakened by selective distribution. Moreover, the morphologies of cocontinuous structure, which means that the crystallization process of PP had to take place in the micron‐scale spatial confinement formed by continuous PS phase, greatly influenced the crystallization behavior of PP in PP/PS/CB composite. The spherulite radial growth rate of PP in spatial confinement was lower than that of neat PP during isothermal crystallization processes, and the results of the total crystallization activation energy (ΔE) and the nucleation parameter (K_t) implied that in comparison to neat PP, the activation energy of PP chain segments arranged into crystal was higher in composite with cocontinuous structure. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Effect of nano-nucleating agent addition on the isothermal and nonisothermal crystallization kinetics of isotactic polypropylene

Using differential scanning calorimetry (DSC) technique, a comparative study has been made of the isothermal and nonisothermal crystallization kinetics of nonnucleated isotactic polypropylene (iPP) and of nucleated iPP with 0.5 wt% of single-walled carbon nanotubes (SWCNTs) as a nucleating agent. The Avrami exponents (n) of iPP and nucleated iPP are close to 3.0 for isothermal crystallization. These results indicate that the addition of nucleating agents did not change the crystallization growth patterns of the neat polymer and that crystal growth was heterogeneous three-dimensional spherulitic. The results show that the addition of SWCNTs can shorten the crystallization half-time (t _1/2) and increase the crystallization rate of iPP. In the nonisothermal crystallization process, the Ozawa model failed to describe the crystallization behavior of nucleated iPP. The Cazé–Chuah model successfully described the nonisothermal crystallization process of iPP and its nanocomposite. A kinetic treatment based on the Ziabicki theory is presented to describe the kinetic crystallizability, in order to characterize the nonisothermal crystallization kinetics of iPP and nucleated iPP. Polarized light microscopy (PLM) experiments reveal that SWCNTs served as nucleating sites, resulting in a decrease of the spherulite size.     

Deformation and pore formation mechanism under tensile loading in isotactic polypropylene

Four different polypropylene (PP) samples were prepared through isothermally crystallizing at 0 °C (PP‐Q), 80 °C (PP‐80), 100 °C (PP‐100) and 120 °C (PP‐120). The results of differential scanning calorimetry, wide‐angle X‐ray diffraction, polarized light microscopy and tensile testing indicate that the spherulite structure gradually improves with increasing isothermal crystallization temperature. Meanwhile, the interface between spherulites becomes more obvious due to the larger dimension and the higher strength of spherulites. Therefore, the trend of interfacial debonding during stretching is enhanced distinctly. In addition, based on the structural characterization of samples at different draw ratios, two completely distinct morphological changes are demonstrated. There are no defects generated after longitudinal stretching within PP‐Q, because intra‐spherulitic deformation predominates, which is caused by the imperfect spherulites of PP‐Q. As a result, no microporous structure is produced after sequential biaxial stretching. And the improvement of the crystalline structure makes interfacial debonding more likely to occur. Therefore, fully developed crazes and cracks disperse between microfibril structures after longitudinal stretching. Furthermore, numerous microporous structures are produced through debonding of fully developed crazes and cracks after sequential biaxial stretching. Meanwhile, the quantity, dimension and uniformity of the microporous structures and the porosity are gradually improved. © 2017 Society of Chemical Industry     

Crystallization and microstructure in quenched slabs of various molecular weight polypropylenes

The purpose of this study was to accumulate material data for use in a simulation of morphology and crystallization in quenched slabs of isotactic polypropylenes (i-PP's). The molecular weight effect on the crystallization kinetics of various i-PP's were studied using isothermal and nonisothermal experiments. An experimental setup was constructed to measure the spherulite growth rate of i-PP's using isothermal and nonisothermal melt crystallization experiments. The molecular weight effect on spherulite growth rate of i-PP's was determined. Molecular weight effect on nuclei concentrations as a function of temperature was determined from growth and crystallization rate constants. The morphology and crystallization in quenched slabs of i-PP's were determined from experiments and compared to computer simulations. The molecular weight effect on the degree of crystallinity and morphology was determined for i-PP during quenching.     

Crystallization behavior of biodegradable poly(L‐lactic acid) filled with a powerful nucleating agent: N,N′‐bis(benzoyl) suberic acid dihydrazide

N,N′‐Bis(benzoyl) suberic acid dihydrazide (NA) as nucleating agent for poly(L ‐lactic acid) (PLLA) was synthesized from benzoyl hydrazine and suberoyl chloride, which was deprived from suberic acid via acylation. PLLA/NA samples were prepared by melt blending and a hot‐press forming process. The nonisothermal and isothermal crystallization, spherulite morphology, and melting behavior of PLLA/NA with different contents of NA were investigated with differential scanning calorimetry, depolarized‐light intensity measurement, scanning electron microscopy, polarized optical microscopy, and wide‐angle X‐ray diffraction. With the incorporation of NA, the crystallization peak became sharper and shifted to a higher temperature as the degree of supercooling decreased at a cooling rate of 1°C/min from the melt. Nonisothermal crystallization indicated that the presence of NA accelerated the overall PLLA crystallization. In isothermal crystallization from the melt, the presence of NA affected the isothermal crystalline behaviors of PLLA remarkably. The addition of NA led to a shorter crystallization time and a faster overall crystallization rate; this meant that there was a heterogeneous nucleation effect of NA on the crystallization of PLLA. With the addition of 0.8% NA, the crystallization half‐time of PLLA/NA decreased from 26.5 to 1.4 min at 115°C. The Avrami theory was used to describe the kinetics of isothermal crystallization of the PLLA/NA samples. Also, with the presence of NA, the spherulite number of PLLA increased, and the spherulite size decreased significantly. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Overall crystallization behavior of polypropylene–clay nanocomposites; Effect of clay content and polymer/clay compatibility on the bulk crystallization and spherulitic growth

The effect of clay nanoparticles on the overall crystallization (isothermal crystallization, spherulitic growth, and nonisothermal crystallization) behavior of polypropylene (PP) was studied by means of differential scanning calorimetry and polarized light optical microscopy. In addition, the changes produced by the compatibility between the filler and the matrix were analyzed by using more hydrophobic clays or incorporating PP grafted with maleic anhydride (PP‐g‐MA). Different models were used to predict the relative degree of crystallinity and several parameters were analyzed. A clear nucleating effect of clay nanoparticles was found on the experimental behavior (induction time, half‐crystallization time, and overall crystallization time) and also deducted from the models parameters (Avrami exponent, rate constant, nucleation activity, activation energy). The effect was also related with the matrix/clay compatibility. In addition, the polarized light optical microscopy showed that the number of spherulites increased and their size decreased when clay was incorporated, which is also an indication of the heterogeneous nucleating behavior of such particles. We also noted faster spherulitic growth and increasing K_g (the model parameter). © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Linear growth rates of random propylene ethylene copolymers. The changeover from γ dominated growth to mixed (α+γ) polymorphic growth

The spherulitic linear growth rates of a homo-poly(propylene) and a series of propylene-ethylene copolymers, all synthesized with the same type of metallocene catalyst were analyzed. The inter-chain distribution of comonomer content is uniform in these copolymers and the intra-chain distribution adheres to the random behavior. Furthermore, the concentration of stereo and regio defects is constant for all copolymers. Thus, with these polymers it was possible to investigate the influence of ethylene content on the crystallization kinetics, as extracted from their linear growth rates. All iPPs investigated display mixed polymorphic behavior during isothermal crystallization and major emphasis was given to integrate the simultaneous development of the α and γ polymorphs, and their intimate structural relations during growth, in the analysis of the crystallization kinetics. A sharp break in spherulitic growth is found between times domains of mixed α+γ growth and growth of pure γ crystals reflecting a drastic change in growth mechanisms at the point where α development ceases. The rates corresponding to α growth are significantly higher than those characteristics of γ growth. In addition, growth data in the domain of mixed α+γ growth, which, following the structural models for α and γ branching, reflect the growth pattern of α crystals, display a discontinuity in the temperature gradient at the changeover from growth of mixed α+γ to γ dominated growth. This behavior correlates with that found in other systems that undergo a similar extended chain to folded change in crystallization mechanisms. The temperature coefficient of the linear growth rates is analyzed according to regime theory for both domains of growth. The results allow a quantitative framework for discussion of the interfacial free energies of α and γ crystals obtained from this analysis.