Critical crystallization temperature for the occurrence of epitaxy between high-density polyethylene and isotactic polypropylene

The crystallization behavior of high-density polyethylene (HDPE) on highly oriented isotactic polypropylene (iPP) at elevated temperatures (e.g., from 125 to 128°C), was studied using transmission electron microscopy and electron diffraction. The results show that epitaxial crystallization of HDPE on the highly oriented iPP substrates occurs only in a thin layer which is in direct contact with the iPP substrate, when the HDPE is crystallized from the melt on the oriented iPP substrates at 125°C. The critical layer thickness of the epitaxially crystallized HDPE is not more than 30 nm when the HDPE is isothermally crystallized on the oriented iPP substrates at 125°C. When the crystallization temperature is above 125°C, the HDPE crystallizes in the form of crystalline aggregates and a few individual crystalline lamellae. But both the crystalline aggregates and the individual crystalline lamellae have no epitaxial orientation relationship with the iPP substrate. This means that there exists a critical crystallization temperature for the occurrence of epitaxial crystallization of HDPE on the melt-drawn oriented iPP substrates (i.e., 125°C). © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 2029–2034, 1997     

Crystallization of oriented isotactic polypropylene (iPP) in the presence of in situ poly(ethylene terephthalate) (PET) microfibrils

The present article reports the nonisothermal crystallization process and morphological evolution of oriented iPP melt with and without in situ poly(ethylene terephthalate) (PET) microfibrils. The bars of neat iPP and PET/iPP microfibrillar blend were fabricated by shear controlled orientation injection molding (SCORIM), which exhibit the oriented crystalline structure (shish-kebab), especially in the skin layer. The skin layer was annealed at just above its melting temperature (175 °C) for a relatively short duration (5 min) to preserve a certain level of oriented iPP molecules. It was found that the existence of ordered clusters (i.e. oriented iPP molecular aggregates) leads to the primary nucleation at higher onset crystallization temperature, and formation of the fibril-like crystalline morphology. However, the overall crystallization rate decreases as a result that the relatively high crystallization temperature restrains the secondary nucleation. With the existence of PET microfibrils, the heterogeneous nucleation distinctly occurs in the unoriented iPP melt and results in the increase of crystallization peak temperature and overall crystallization rate, for the first time, we observed that the onset crystallization temperature has been enhanced further with addition of PET microfibrils in the oriented iPP melt, indicating the synergistic effect of row nucleation and heterogeneous nucleation under quiescent condition.     

Foaming of linear isotactic polypropylene based on its non-isothermal crystallization behaviors under compressed CO2

The non-isothermal crystallization behaviors of isotactic polypropylene (iPP) under ambient N₂ and compressed CO₂ (5–50 bar) at cooling rates of 0.2–5.0 °C/min were carefully studied using high-pressure differential scanning calorimeter. The presence of compressed CO₂ had strong plasticization effect on the iPP matrix and retarded the formation of critical size nuclei, which effectively postponed the crystallization peak to lower temperature region. On the basis of these findings, a new foaming strategy was utilized to fabricate iPP foams using the ordinary unmodified linear iPP with supercritical CO₂ as the foaming agent. The foaming temperature range of this strategy was determined to be as wide as 40 °C and the upper and lower temperature limits were 155 and 105 °C, which were determined by the melt strength and crystallization temperature of the iPP specimen under supercritical CO₂, respectively. Due to the acute depression of CO₂ solubility in the iPP matrix during the foaming process, the iPP foams with the bi-modal cell structure were fabricated.     

Nodular structure of isotactic polypropylene crystallizes from the melt

This article highlights the melt crystallization behavior of different grades of isotactic polypropylene (iPP) using a hot‐stage polarizing optical microscopy. iPP samples were heated up at a heating rate of 10°C/min passing the melting temperature and then kept for 3 min at a temperature range of 175–200°C before they cooled rapidly at 40°C/min to crystallize isothermally at a range of 130–145°C. It has been found that the temperature at which the samples were kept has a strong effect on the crystallization mode; for samples heated up and kept at temperatures below 190°C, the crystallization started with thin and long rods or nodules, which grew in the circumferential direction only while their lengths remain unchanged as the time passed. The shape of the nodules can be straight, circular, branched, or entangled, and they can grow parallel to each other or they can be crossed or in a random way. This phenomenon disappeared completely for samples melted and kept at temperatures above 195°C. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

A novel β‐nucleating agent for isotactic polypropylene

The nucleating ability of p‐cyclohexylamide carboxybenzene (β‐NA) towards isotactic polypropylene (iPP) was investigated by differential scanning calorimetry, X‐ray diffraction, polarized optical microscopy and scanning electron microscopy. β‐NA is identified to have dual nucleating ability for α‐iPP and β‐iPP under appropriate kinetic conditions. The formation of β‐iPP is dependent on the content of β‐NA. The content of β‐phase can reach as high as 96.96% with the addition of only 0.05 wt% β‐NA. Under non‐isothermal crystallization the content of β‐iPP increases with increasing cooling rate. The maximum β‐crystal content is obtained at a cooling rate of 40 °C min^–1. The supermolecular structure of the β‐iPP is identified as a leaf‐like transcrystalline structure with an ordered lamellae arrangement perpendicular to the special surface of β‐NA. Under isothermal crystallization β‐crystals can be formed in the temperature range 80–140 °C. The content of β‐crystals reaches its maximum value at a crystallization temperature of 130 °C. © 2012 Society of Chemical Industry     

Isothermal crystallization kinetics and subsequent melting behavior of β‐nucleated isotactic polypropylene/graphene oxide composites with different ordered structure

The effects of ordered structure on isothermal crystallization kinetics and subsequent melting behavior of β‐nucleated isotactic polypropylene/graphene oxide (iPP/GO) composites were studied using differential scanning calorimetry. The ordered structure status was controlled by tuning the fusion temperature (T_f). The results showed that depending on the variation of crystallization rate, the whole T_f range could be divided into three regions: Region I (T_f > 179 °C), Region II (170 °C ≤ T_f ≤ 179 °C) and Region III (T_f < 170 °C). As T_f decreased from Region I to Region III, the crystallization rate would increase substantially at two transition points, due to the variation of the ordered structure status. Calculation of Avrami exponent n indicated that the ordered structure induced the formation of two‐dimensional growing crystallites rather than three‐dimensional growing crystallites. Moreover, in the case of isothermal crystallization, the ordered structure effect (OSE) can also greatly increase the relative content of β‐phase (β_c). In Region II, OSE took place, resulting in evident increase of β_c, achieving 92.4% at maximum. The variation of the isothermal crystallization temperature (T_iso) had little influence on the T_f range (Region II) of the OSE. The higher T_f in Region II was more favorable for the formation of higher β_c. The ordered structure was favorable for the improvement of the nucleating efficiency of β‐nucleating agent (β‐NE), and was more effective for the improvement of lower β‐NE. © 2018 Society of Chemical Industry     

Crystallization of polymer melts under fast cooling. II. High-purity iPP

Samples of a high-purity isotactic polypropylene (iPP) were quenched from the melt so as to monitor cooling history. A continuous variation of morphology and crystal structure was obtained with cooling rate. This is discussed in relation to sample thermal history evidencing that cooling history relevant to quenched samples is in the neighborhood of 90°C. In particular the samples are essentially mesomorphic when at this temperature cooling rates larger than 80°C/s were adopted, while below a few tens of °C/s only α-monocline form is obtained. Densities of quenched samples were compared with predictions of an isokinetic extrapolation of Avrami model of polymer crystallization kinetics.     

β-Crystal formation of isotactic polypropylene induced by N, N’-dicyclohexylsuccinamide

N, N’-dicyclohexyl succinamide (DCS) was found to be a new β-nucleating agent for isotactic polypropylene (iPP) by means of wide angle X-ray diffraction (WAXD) and polarized light microscopy (PLM) measurements for the first time. The maximum proportion of β-form within iPP specimen was 79.1% with addition of 0.05% DCS. With increasing crystallization time, the proportion of β-form changed slightly when the nucleated iPP specimens crystallized at 120°C and 130°C for more than 20 min; but at 140°C, the content of β-form markedly decreased because β-α solid to solid transformation occurred. The analysis of the cell parameters of DCS and β-form iPP showed good lattice matching relationship between them.     

Effects of melt structure on crystallization behavior of isotactic polypropylene nucleated with α/β compounded nucleating agents

In this study, the melt structure of isotactic polypropylene (iPP) nucleated with α/β compounded nucleating agents (α/β‐CNA, composed of the α‐NA of 0.15 wt % Millad 3988 and the β‐NA of 0.05 wt % WBG‐II) was tuned by changing the fusion temperature T_f. In this way, the role of melt structure on the crystallization behavior and polymorphic composition of iPP were investigated by differential scanning calorimetry (DSC), wide‐angle X‐ray scattering (WAXD) and scanning electron microscopy (SEM). The results showed that when T_f = 200°C (iPP was fully molten), the α/β‐CNA cannot encourage β‐phase crystallization since the nucleation efficiency (NE) of the α‐NA 3988 was obviously higher than that of the β‐NA WBG‐II. Surprisingly, when T_f was in 179–167°C, an amount of ordered structures survived in the melt, resulting in significant increase of the proportion of β‐phase (achieving 74.9% at maximum), indicating that the ordered structures of iPP played determining role in β‐phase crystallization of iPP nucleated with the α/β‐CNA. Further investigation on iPP respectively nucleated with individual 3988 and WBG‐II showed that as T_f decreased from 200°C to 167°C, the crystallization peak temperature T_c of iPP/3988 stayed almost constant, while T_c of iPP/WBG‐II increased gradually when T_f < 189°C and became higher than that of iPP/3988 when T_f decreased to 179°C and lower, which can be used to explain the influence of ordered structure and α/β‐CNA on iPP crystallization. Using this method, the selection of α‐NA for α/β‐CNA can be greatly expanded even if the inherent NE of β‐NA is lower than that of the α‐NA. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41355.     

Influence of the quenching temperature on the crystallization of the trans-planar mesomorphic form of syndiotactic polypropylene

The crystallization from the melt of the trans-planar mesomorphic form of syndiotactic polypropylene is investigated at different quenching temperatures. The formation of the trans-planar mesomorphic form at −5, 0 and 6 °C is followed as a function of the residence time at these temperatures by X-ray diffraction and FTIR spectroscopy. The quenching temperature influences the rate of formation of the mesomorphic form as well as the maximum amount of the obtained mesomorphic form. By increasing the quenching temperature, in the examined range between −5 and +6 °C, an increase in the rate of formation of the mesomorphic form is observed. The maximum amounts of mesomorphic form obtained at 6 and −5 °C are lower than the amount achieved at 0 °C, which corresponds to nearly 100% of the total crystalline phase.     

Influence of structural organization on tensile properties in mesomorphic isotactic polypropylene

The effects of annealing on the structure and mechanical properties of mesomorphic isotactic polypropylene have been investigated using wide-angle and small-angle X-ray scattering and rheo-optics in addition to tensile tests. Young's modulus of mesomorphic phase was estimated to be 5 GPa using Takayanagi model. The α-crystallitic iPP prepared by annealing the quenched mesomorphic iPP was transparent because of the absence of spherulitic structure. It was found that the mechanical yielding of α-crystallitic iPP is dominated by the plastic flow of crystalline structural units whereas the yield process of α-spherulitic iPP quenched at 80 °C is caused by the fracture or fragmentation of crystalline structural units.     

Morphology, reorganization and stability of mesomorphic nanocrystals in isotactic polypropylene

The morphology and thermodynamic stability of crystals of isotactic polypropylene (iPP) were analyzed as a function of the path of crystallization by atomic force microscopy (AFM) and differential scanning calorimetry (DSC). Samples were melt-crystallized at different rates of cooling using a “controlled rapid cooling technique”, and subsequently annealed at elevated temperature. Mesomorphic equi-axed domains with a size less than 20 nm were obtained by fast cooling from the melt at a rate larger about 100 K s⁻¹. These domains stabilize on heating by growing in chain direction and cross-chain direction, to reach a maximum size of about 40-50 nm at a temperature of 433 K, with the quasi-globular shape preserved. Annealing at 433 K additionally triggers formation of different types of lamellae. It is suggested that these lamellae either develop by coalescence of nodules, or by recrystallization from the melt. The transition from the disordered mesomorphic structure, evident at ambient temperature after fast crystallization, to monoclinic structure on heating at about 340 K occurs at local scale within existing crystals, and cannot be linked to complete melting of mesomorphic domains and recrystallization of the melt. The temperature of melting of initial mesomorphic domains, after reorganization at elevated temperature, is identical to the temperature of melting of rather perfect lamellae, obtained by initial slow melt-crystallization, followed by annealing. The close-to-identical temperatures of melting of these crystals of largely different shapes are confirmed by model calculations, using the Gibbs-Thomson equation. Modeling of the melting temperature reveals that nodular crystals, stabilized by annealing at high temperature, exhibit a similar fold-surface as lamellar crystals.     

Crystallization behavior and crystallization kinetic studies of isotactic polypropylene modified by long‐chain branching polypropylene

In this article, we discuss the crystallization behavior and crystallization kinetics of isotactic polypropylene (iPP) modified by long‐chain‐branching (LCB) high‐melt‐strength iPP over a wide composition range, that is, LCB‐iPP from 10 to 50 wt %. Over the entire range we investigated, the presence of LCB‐iPP accelerated crystallization in both the isothermal crystallization process and nonisothermal crystallization process, even when the LCB‐iPP content was as low as 10%, and both crystallization processes were enhanced more significantly as the LCB‐iPP content increased. Hoffman–Lauritzen theory analysis revealed that the fold‐free energy decreased effectively with the occurrence of the LCB structure, although the growth rate of spherulites was depressed, as shown by polarized optical microscopy. Meanwhile, the regime III–regime II transition temperature was about 15° higher for all of the LCB‐iPP compositions than that of iPP because the LCB structure reduced the mobility of the polypropylene chains. Furthermore, the γ‐form crystal structure was favored by LCB compared to the β form, which was supported by wide‐angle X‐ray diffraction. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Isothermal crystallization behavior of metallocene‐catalyzed isotactic polypropylene

Isothermal crystallization behavior of isotactic polypropylene (iPP) synthesized using metallocene catalyst was investigated in this work. The isotacticity of the polypropylene was characterized by ¹³C‐NMR spectroscopy. It was found that the melting temperature (T_m) of the iPP is 123.51°C and the crystallization temperature (T_c) is 93°C. The iPP synthesized in this work did not show a general increase of T_m with an increase of crystallization temperature T_c, due to the short crystallization time of 20 min and low molecular weight (number average molecular weight = 6,300). The iPP showed a tendency of increasing heat of fusion (ΔH_f) with decreasing crystallization temperature. All the spherulites of iPP samples showed negative birefringence. For the iPP sample crystallized at the highest T_c (= 123°C, just below T_m), the spherulite showed a pronounced Maltese Cross and a continuous sheaf‐like texture aligning radially, which suggests that R‐lamellaes are dominant in this spherulite. The crystalline structure of the iPP was also investigated by X‐ray diffraction. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 231–237, 2005     

Conformation transition and crystalline phase variation of long chain branched isotactic polypropylenes (LCB-iPP)

The changes of conformation and crystalline structure of long chain branched isotactic polypropylene (LCB-iPP) under different crystallization temperatures and the effects of their special molecular architecture on the crystallization behavior were investigated by a combination of Fourier transform infrared spectroscopy (FT-IR), wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). In these polymers, long chain branching was introduced via in situ polymerization of polypropylene and an asymmetric diene monomer using the metallocene catalyst technology. Through the characterization of the specific IR band variation, it was proved that the conformational orders of helical sequences of LCB-iPP show great changes in different crystallization temperature ranges. In lower crystallization temperature range (100-130 °C), the intensities of all regular helical conformation bands of LCB-iPP increase with the increasing crystallization temperature and the regular helical conformation bands with more monomer units increase faster than that with less monomer units. In higher crystallization temperature range (130-150 °C), the intensities of all regular helical conformation bands of LCB-iPP decrease with the increasing crystallization temperature and the regular helical conformation bands with more monomer units decrease faster than that with less monomer units. The results of WAXD and DSC showed that LCB-iPP crystallizes from the melt as a mixture of α and γ forms. The content of the γ form increases with the increasing crystallization temperature, reaches a maximum value at 130 °C, and then decreases with a further increase of the temperature. At the same time, the crystallization of γ form is favored by the presence of the LCB structure of iPP. Moreover, the transitional temperatures of different helical conformations and crystallization structures of LCB-iPP show obvious correlations.     

Crystallization and thermal behaviour of isotactic polypropylene blended with an alicyclic random co-oligomer

A fully saturated, alicyclic hydrocarbon resin (IST) was blended with isotactic polypropylene (iPP). IST is a random co-oligomer of low average molecular weight obtained by oligomerization of indene, α-methyl styrene and vinyl toluene, followed by hydrogenation. The crystallization and thermal behaviour of iPP/IST blends were analyzed by microscopy and differential scanning calorimetry (DSC). The influence of blend composition on the spherulite growth rate and on the overall crystallization rate suggests that the two components form a miscible blend in the amorphous phase, as confirmed by the following observations: All blends show a single glass transition temperature (Tg); its value, depending on composition, lies between the iPP and IST Tg values (–14°C and 82°C, respectively), and is in good agreement with the theoretical values calculated by the Fox equation. The equilibrium melting temperature calculated for pure iPP was equal to 187°C; this value decreases with blending to 175°C for the iPP/IST 50/50 (w/w) blend. The χ₁₂ parameter of the iPP/IST system was equal to ?0.435, the negative value should suggest that the two components can form a compatible mixture which is thermodynamically stable above the equilibrium melting temperature.     

In situ studies on the temperature‐related deformation behavior of isotactic polypropylene spherulites with uniaxial stretching: The effect of crystallization conditions

The deformation behavior of isotactic polypropylene (iPP) spherulites with uniaxial stretching was investigated at different drawing temperatures via in situ polarized optical microscope (POM) observation. The iPP spherulites were prepared by two procedures: cooled to the room temperature from melt and annealed at 135, 140, and 145°C for 3 h. It was found that the crystallization conditions dominate the crystalline morphology and even the tensile properties of iPP. For iPP which crystallized during cooling progress, the spherulites were imperfect and the boundaries of the spherulites were diffuse, displaying good toughness at various drawing temperatures. For iPP annealed at high temperatures displayed the brittle fracture‐modes and the crack happened between spherulites, which due to the large and perfective spherulites have thick lamellas and weak connection at interspherulitic boundary. The shape and size of the iPP spherulites formed at 140 and 145°C are affected with uniaxial stretching till to the fracture of the samples at different drawing temperatures. The spherulites obtained at 135°C are deformed along the drawing direction at 100°C but not affected at low drawing temperatures, indicating the toughness increased with the increase of the drawing temperatures. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

Highly active thermally stable β‐nucleating agents for isotactic polypropylene

Calcium salts of suberic (Ca‐Sub) and pimelic (Ca‐Pim) acids were synthesized and implemented as in different grades of isotactic polypropylene (iPP). Propylene homopolymer, as well as random and block copolymers containing these additives, crystallized iPP into pure or nearly pure β modification in the isothermal and nonisothermal crystallization experiments. Recently, Ca‐Sub proved to be the most effective β‐nucleating agent of iPP. The Ca‐Sub nucleating agent widens the upper crystallization temperature range of pure β‐iPP formation up to 140°C. In this study the effect of the these additives on the crystallization, melting characteristics, and structure of the PP were studied. The degree of crystallinity of β‐iPP was markedly higher than that of α‐iPP. A widening in the melting peak of the samples crystallized in a high temperature range was first observed and discussed in regard to literature results of the same phenomenon for α‐iPP. The morphology of the β‐iPP samples was revealed by scanning electron microscopy. Independent of the type of polymer or nucleating agent, hedritic structures were found in the early stages of growth of the β‐spherulites. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2357–2368, 1999     

Effect of the composition ratio of pimelic acid/calcium stearate bicomponent nucleator and crystallization temperature on the production of β crystal form in isotactic polypropylene

The influence of the composition ratio of pimelic acid/calcium stearate bicomponent nucleator on the β crystal form content of isotactic polypropylene (iPP) had been studied at the crystallization temperature of 120°C and duration of 30 min. It was found that the β crystal form content increased continuously with increasing amount of calcium stearate at the constant amount of 0.15% pimelic acid. High β crystal form content polypropylene could be produced when the amount of calcium stearate was greater than 0.30% (the mass composition ratio of pimelic acid/calcium stearate was less than 1/2, the mole ratio was less than 1.89/1). It was shown that pimelic acid and calcium stearate could react to produce a high effective β nucleator (calcium pimelate) “in situ” during the melt‐mixing of iPP and the bicomponent nucleator. The influence of crystallization temperatures (100–140°C) on the β crystal form content of iPP had also been studied at the constant composition ratio of 0.15% pimelic acid/0.5% calcium stearate (the calcium pimelate produced in situ was 0.16%, which was calculated from stoichiometry). It was found that the β crystal form content increased continuously with increasing crystallization temperature and it maximized at 130°C. β Crystal form content decreased sharply at the crystallization temperature of 140°C. It was shown that β → α modification transformed between 130 and 140°C. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Effects of melt structure on non‐isothermal crystallization behavior of isotactic polypropylene nucleated with α/β compounded nucleating agents

In this study, the effects of melt structure (tuned by controlling the fusion temperature T_f) on non‐isothermal crystallization and subsequent melting behaviors of isotactic polypropylene (iPP) nucleated with α/β compounded nucleating agents (α/β‐CNAs) have been further investigated. The results show that under all cooling rates studied (2–40°C/min), the crystallization temperature on cooling curves increased gradually with decrease of T_f, meanwhile, when T_f was in temperature range of 166°C–179°C where ordered structures survived in the melt (defined as Region II), crystallization activation energy ΔE was found to be evidently lower compared with that when T_f > 179°C or T_f < 166°C. The results of subsequent heating showed that occurrence of Ordered Structure Effect can be observed at all the cooling rates studied; the location of the Region II was constant when cooling rate varied; Low cooling rate encouraged formation of more β‐phase triggered by ordered structure. Moreover, the role of ordered structure on β‐α recrystallization was comparatively studied by tuning the end temperature of recooling (T_end) after held at T_f, and it was found that ordered structure encouraged the formation of β‐phase with high thermal stability at low temperature part of Region II, while enhanced the β‐crystal with relatively low thermal stability at high temperature part of Region II. POLYM. ENG. SCI., 57:989–997, 2017. © 2016 Society of Plastics Engineers