Crystalline morphology of isotactic polypropylene (iPP) in injection molded poly(ethylene terephthalate) (PET)/iPP microfibrillar blends

The poly(ethylene terephthalate) (PET)/isotactic polypropylene (iPP) in situ microfibrillar blends have been prepared through a “slit die extrusion-hot stretch-quenching” process, in which PET assumes microfibrils with 0.5-15 μm in diameter depending on the hot stretching ratios (HSR, the area of the transverse section of the die to the area of the transverse section of the extrudate). The injection molded specimens of virgin iPP and the PET/iPP blends were prepared by conventional injection molding (CIM) and by shear controlled orientation injection molding (SCORIM), respectively. The effect of shear stress and PET phase with different shape on superstructures and their distribution of injection molded microfibrillar samples were investigated by means of small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS). The shear (or elongational) flow during CIM and SCORIM can induce oriented lamellae (i.e. kebabs induced by shish). The shish-kebab structure appears not only in the skin and intermediated layers of CIM samples, but also in the whole region of SCORIM samples. For the neat iPP samples, a more “stretched” shish-kebab structure with higher orientation degree can be obtained in the interior region (intermediate and core layers) by the SCORIM method; moreover, the SCORIM can result in the growth of β-form crystal both in intermediate layer and in core layer, which only appears in intermediate layer of the neat iPP samples obtained by CIM. For the PET/iPP blends, interestingly, the addition of microfibrils as well as their aspect ratios can affect the orientation degree of kebabs only in the intermediate layers, and the addition of microfibrils with a low aspect ratio can bring out a considerable increase in the orientation degree of kebabs along the flow direction. However, for the SCORIM, the addition of microfibrils seems to be a minor effect on the orientation degree of kebabs, and it tends to hamper the formation of a more “stretched” shish-kebab structure and suppresses the growth of β-form crystal distinctly. Furthermore, It appears from experiment that γ-form crystals can grow successfully in this oriented iPP melt with the synergistic effect of shear and pressure only when the growth of β crystals can be restrained by some factors, such as the PET dispersed phase and thermal conditions (cooling rate).     

Ductile-brittle transition controlled by isothermal crystallization of isotactic polypropylene and its blend with poly(ethylene-co-octene)

The correlation between crystalline morphology development and tensile properties of isotactic polypropylene (iPP) and its blend with poly(ethylene-co-octene) (PEOc) was investigated to study the ductile-brittle transition (DBT) in fracture modes. The sample processing strategy and the scientific observations have never been reported previously. The samples were first isothermally crystallized at 130 °C, 123 °C or 115 °C for a wide range of crystallization times, and then quenched to 35 °C for characterization. It was found that the crystallization conditions including crystallization temperature and time governed the crystalline morphology and even the tensile properties of iPP and the iPP/PEOc (80/20) blend. The lower the crystallization temperature, the shorter the crystallization time was needed for the occurrence of DBT, and the sharper the transition would be. The addition of the elastomer component delayed the DBT occurrence for the iPP/PEOc blend in terms of the crystallization time, owing to the fact that the existence of PEOc domains between the iPP lamellar stack regions or at the iPP spherulitic boundaries enhanced the ductility of the blend. The X-ray diffraction results displayed the oriented and destroyed crystalline structure characterizing the ductile fracture, while unoriented structure describing the brittle failure. The DBT is closely related to the crystal perfection, and factors such as the crystallization temperature and time and the compositions have been proven to be significant variables in determining the DBT occurrence.     

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     

Carbon nanotubes induced crystallization of poly(ethylene terephthalate)

We have investigated the crystallization characteristics of melt compounded nanocomposites of poly(ethylene terephthalate) (PET) and single walled carbon nanotubes (SWNTs). Differential scanning calorimetry studies showed that SWNTs at weight fractions as low as 0.03 wt% enhance the rate of crystallization in PET, as the cooling nanocomposite melt crystallizes at a temperature 10 °C higher as compared to neat PET. Isothermal crystallization studies also revealed that SWNTs significantly accelerate the crystallization process. WAXD showed oriented crystallization of PET induced by oriented SWNTs in a randomized PET melt, indicating the role of SWNTs as nucleating sites.     

In situ poly(ethylene terephthalate) microfibers- and shear-induced non-isothermal crystallization of isotactic polypropylene by on-line small angle X-ray scattering

In situ microfibrillar reinforced blend (MRB) based on poly(ethylene terephthalate) (PET) and isotactic polypropylene (iPP) was elaborated by a slit die extrusion, hot stretching, and quenching process. The scanning electronic microscopic images show well-developed PET microfibers in the blends. The on-line small angle X-ray scattering (SAXS) test shows that PET microfibers have high nucleation for iPP crystallization. At the same time, after shear, neat iPP and microfibrillar blend both can faster crystallization rate. Three nucleation origins are proposed in microfibrillar reinforced blends under shear flow field: (a) the classical row nuclei model, (b) fiber nuclei and (c) nuclei induced by fiber assistant alignment. The polarized optical microscopic images indicate that, during the non-isothermal crystallization at a cooling rate of 10 °C/min from 200 °C to room temperature, the neat iPP forms common spherulites, while the diluted microfibrillar blend with 1 wt% of PET has a typical transcrystalline structure.     

Epitaxy growth and directed crystallization of high-density polyethylene in the oriented blends with isotactic polypropylene

Various lamellar orientations of high-density polyethylene (HDPE), due to competition between bulk nucleation and interfacial nucleation, have been realized in its melt drawn blends with isotactic polypropylene (iPP) upon cooling after subjected to 160 °C for 30 min. Directed crystallization, with heterogeneous nucleation in the bulk (within domains), is defined as lamellar growth along boundary of anisotropic domains and is favored in larger domains at higher temperature (slow cooling), since overgrowth of lamellae can feel the interface rather than impingement with neighbor ones as a result of scare nuclei at higher temperature. Moreover, lamellar growth caused by directed crystallization is dependent of dimension of confinement. Due to 2D confinement of cylindrical domains, lamellae can only grow along the axis of cylinder and thus b-axis orientation is formed. While in the layered domains with 1D confinement, however, lamellae grow with the normal of (110) plane along the melt drawn direction. On the other hand, epitaxial growth of HDPE chains onto iPP lamellae is related to the surface-induced crystallization and dominated by the interfacial nucleation. Only interfacial nucleation is preferred can epitaxial growth occur. Therefore, retarded crystallization, realized by either strong confinement in finer domains or rapid cooling or both, is favorable for it.     

Oxygen solubility and specific volume of rigid amorphous fraction in semicrystalline poly(ethylene terephthalate)

The existence of rigid amorphous fraction (RAF) in semicrystalline poly(ethylene terephthalate) (PET) is associated with the lamellar stack crystalline morphology of this polymer, the regions where several crystalline lamellas are separated by very thin (20-40 Å) amorphous layers. In contrast, regular or mobile amorphous fraction is associated with much thicker interstack regions. The oxygen transport properties of PET isothermally crystallized from the melt (melt-crystallization) or quenched to the glassy state and then isothermally crystallized by heating above T_g (cold-crystallization) were examined at 25 °C. Explanation of unexpectedly high solubility of crystalline PET was attributed to the formation of RAF, which in comparison with mobile amorphous phase is constrained and vitrifies at much higher than T_g temperature thus developing an additional excess-hole free volume upon cooling. Measurements of crystallinity and jump in the heat capacity at T_g were used to determine the amount of mobile and rigid amorphous fractions. Overall oxygen solubility was associated with the solubility of mobile and rigid amorphous fractions. The oxygen solubility of the RAF was determined and related to the specific volume of this fraction. The specific volume of the RAF showed a direct correlation with the crystallization temperature. It was shown that upon crystallization from either melt or glassy state, the constrained between crystalline lamellas PET chains consisting of the RAF, vitrify at the crystallization temperature and resemble the glassy behavior despite high temperature. When cooled to room temperature, the RAF preserves a memory about the melt state of polymer, which is uniquely defined by the crystallization temperature.     

Epitaxial crystallization of olefin block copolymers (OBCs) on uniaxially oriented isotactic polypropylene and high-density polyethylene films

The epitaxial crystallization behavior of olefin block copolymers (OBCs) on uniaxially oriented isotactic polypropylene (iPP) and high-density polyethylene (HDPE) films has been investigated by transmission electron microscopy (TEM). The crystallizable blocks of the OBCs under investigation were epitaxially nucleated by both iPP and HDPE substrates and epitaxial growth of OBC lamellae was observed. Epitaxial crystallization of the OBCs has been found for slow and fast cooling conditions from the melt which pointed to the strong interaction between the polyolefin substrates and the OBCs. However, the epitaxial morphology of the OBCs strongly depends on their octene concentration difference (Δ_C8) between crystallizable and non-crystallizable blocks, which probably is related to the OBC segregation strength in the melt. With high Δ_C8 the development of epitaxial crystallization of the OBC was restricted within isolated crystalline domains surrounded by the amorphous phase. In contrast, with low Δ_C8 the oriented lamellae of the OBC were distributed homogeneously on iPP but formed separated crystalline domains on HDPE, which has a stronger nucleation capability than iPP on the crystalline OBC blocks because of its similar molecular architecture. Our study points to epitaxy as another reason for the strong interaction between OBC and polyolefins which causes the advanced compatibilization behavior of OBCs when compared with conventional random copolymers.     

Shear-induced crystallization of isotactic polypropylene within the oriented scaffold of noncrystalline ultrahigh molecular weight polyethylene

Shear-induced crystallization of isotactic polypropylene (iPP) within the oriented scaffolds of noncrystalline ultrahigh molecular weight polyethylene (UHMWPE) was investigated by means of in situ synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD). The study was carried out using iPP/UHMWPE blends under isothermal crystallization at 145 °C (i.e., above the melting point of polyethylene) and step shear (shear rate=60 s⁻¹, duration=5 s) conditions. The oriented and isotropic iPP crystalline phases were extracted from the 2D WAXD pattern, and their kinetics data were evaluated with the Avrami equation. The dominant component in the oriented iPP phase was a kebab structure, whose nanostructure dimensions were determined by a novel SAXS analysis scheme. The minor non-crystalline but oriented UHMWPE component played a key role in the nucleation of iPP, which could be explained in terms of mutual diffusion at the interface, resulting in a significant increase in the relaxation time of iPP chains. As a result, after shear, the interfacial iPP chains could also retain their orientation and formed oriented nuclei to initiate the kebab growth.     

增容剂对PP/PET原位微纤化共混物的影响

通过"熔融挤出-热拉伸-淬冷"的方法制备了原位微纤化共混物。采用扫描电镜、差示扫描量热仪和力学性能测试等方法研究了增容剂PP-g-GMA含量对共混物微观形态、力学性能和结晶性能的影响。结果表明,增容剂的加入可明显提高两相相容性,改善界面效果,明显降低拉伸前初始粒子的尺寸,但同时使拉伸后形成的微纤呈现一定的损坏,长径比有所降低。增容剂可以明显改善微纤化共混物力学性能,当其含量为2 %（质量分数,下同）时拉伸强度比未增容试样提高了11.0 %,弯曲强度都提高了11.3 %;当其含量为6 %时冲击强度也比未增容共混物提高了34.5 %。此外,PET微纤对PP有很好的异相成核作用,使其结晶温度提高了16.3 ℃,结晶时间为纯PP的32 %左右,而增容剂的加入使共混物中PP的结晶时间延长。     

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

l ‐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 (T_cp) 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.     

Structure and thermal stability of polyethylene nanolayers

Confinement of the crystallizable polymer chain to the lamellar size scale is expected to affect nucleation and growth habit to the extent that new crystalline structures might be created. In this study, films with hundreds of extremely thin layers of high density polyethylene (HDPE) sandwiched between thicker polystyrene (PS) layers were fabricated by ‘forced assembly’ using layer multiplying coextrusion. Thermal analysis showed that as the HDPE layers became thinner, the crystallinity decreased from about 60% to almost 30%. Decreased crystallinity was accompanied by a change in morphology from banded discoids in HDPE microlayers (>100 nm) to long bundles of edge-on lamellae in HDPE nanolayers (<100 nm) as shown by atomic force microscopy and wide angle X-ray diffraction. Changes in crystallinity and crystalline morphology were responsible for an increase in oxygen permeability of the HDPE layer by a factor of 3 as the layer thickness decreased from 1.1 μm to 20 nm. It is inherent to the concept of forced assembly that nanolayers may not be stable when they are heated into the melt state. Heating films above the melting temperature of HDPE resulted in fractionated crystallization as indicated by two crystallization exotherms in thermograms. The lower temperature exotherm at 80 °C was identified with homogeneous nucleation. The droplets responsible for fractionated crystallization resulted from instability and breakup of the layers when they were taken into the melt. The number of nanodroplets formed by breakup of nanolayers was large enough that the majority did not contain an active heterogeneity and crystallization occurred primarily by homogeneous nucleation.     

Shear amplification and re-crystallization of isotactic polypropylene from an oriented melt in presence of oriented clay platelets

In this study, we first prepared isotactic Polypropylene (iPP)/organoclay nanocomposite specimens via twin-screw extruder and by adding compatibilizer (maleic anhybride grafted PP). Then PP and the composites were subjected to dynamic packing injection molding, in which the melt was firstly injected into the mold then forced to move repeatedly in a chamber by two pistons that moved reversibly with the same frequency as the solidification progressively occurred from the mold wall to the molding core part. The dispersion and orientation of layered organoclay in the nanocomposite were estimated by transmission electron microscopy (TEM) and 2d-wide angle X-ray scattering (2d-WAXS). A much higher degree of orientation of PP was found in the composites compared with the pure PP. This was explained by so called shear amplification in that a great enhancement of local stress occurred in the small interparticles region of two adjacent layered tactoids with different velocities. Furthermore, re-crystallization of isotactic polypropylene (iPP) by melting the dynamic packing injection molded samples has been investigated by polarizing light microscopy (PLM). A highly oriented threadlike crystallites was observed for the first time when crystallization occurs by melting the dynamic packing injection molded samples at 180 °C. However, spherulitic morphology is always obtained once PP crystallizes from an isotropic melt by melting the samples at 200 °C. The shear amplification mechanism and the formation mechanism of oriented threadlike crystallites have been discussed in detail.     

Morphology and mechanical behavior of isotactic polypropylene (iPP)/syndiotactic polypropylene (sPP) blends and fibers

The crystallization morphologies and mechanical behaviors of iPP/sPP blends and the corresponding fibers were investigated in the present work. For all the investigated iPP/sPP blends, the starting crystallization temperature of sPP during cooling process was significantly increased with increasing iPP content. The iPP/sPP blends are strongly immiscible at the conventional melt processing temperatures, in consistence with the literature results. As isothermally crystallized at 130 °C, sPP still keeps melt state, while iPP component is able to crystallize and the spherulites become imperfect accompanied by decreasing of the crystallite size as sPP content increases. The addition of sPP decreases the crystallinity of iPP/sPP blends and fibers. The storage modulus, E′, of the iPP/sPP blends is higher than that of sPP homopolymer in the temperature range from −90 to 100 °C. The iPP/sPP fibers can be prepared favorably by melt-spinning. As sPP content exceeds 70%, the elastic recovery of the iPP/sPP fibers is approximately equal to that of sPP homopolymer fiber. The drawability of the as-spun fiber of iPP/sPP (50/50) is better than that of sPP fiber, which improves the fiber processing performance and enhances the mechanical properties of the final product. The drawn fiber of sPP presents good elastic behavior within the range of 50% deformation, whereas the elastic property of the iPP/sPP (50/50) fiber slightly decreases, but still much better than that of iPP fiber.     

Transcrystallization in fiber-reinforced isotactic polypropylene composites in a temperature gradient

Transcrystallization of isotactic polypropylene (iPP) on different fibers (carbon fiber, glass fiber, and aramid fiber) was conducted in a temperature gradient. The Ultra-High-Module carbon fiber (UHMCF), the High-Module carbon fiber, and the aramid fiber (Twaron) showed sufficient nucleation ability to form transcrystallization of iPP in certain temperature ranges. Among them, the UHMCF showed the best nucleation ability. On the contrary, the Intermediate-Module carbon fiber, the High-Tenacity carbon fiber, and the E-glass fiber showed too low nucleation ability to form transcrystallization of iPP. One efficient way to induce transcrystallization on these fibers was proved by pulling the fibers in supercooled iPP melts. The interface shear between fiber and supercooled matrix melt on crystallization and the interface temperature gradient between fiber and supercooled matrix melt on crystallization are considered to be two very important factors for the formation of transcrystallization. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 67–75, 1997     

Spherulite nucleation in blends of isotactic polypropylene with isotactic poly(butene-1)

The phase morphology and the influence of composition on the primary nucleation of isotactic polypropylene in isotactic polypropylene/isotactic poly(butene-1) (iPP/iPB) blends were investigated by electron and light microscopy and small-angle light scattering. It was found that iPP and iPB are miscible but the thermal treatment induces partial phase separation of components and the formation of iPP-rich and iPB-rich phases. The complete phase separation needs high temperatures and/or a long time of melt annealing. In samples crystallized isothermally at low undercooling the heterogeneous primary nucleation in blends is depressed as compared to plain iPP. In blends the less active heterogeneities lose their activity because of an increase of the energy barrier for critical size nucleus formation due to phase separation of blend components during crystallization. For the same reason the rate of homogeneous nucleation in blends decreases, as observed in samples crystallized at very high undercooling. At very high undercooling iPP and iPB are able to crystallize with similar rates, which results in the formation of a fraction of iPB spherulites in addition to iPP spherulites. Consequently the number of spherulites in the blend is larger than that in plain iPP, in spite of the decrease in the homogeneous nucleation rate of iPP in the blend. © 1994 John Wiley & Sons, Inc.     

Crystallization kinetics and morphology of poly(trimethylene terephthalate)

In this work, the isothermal crystallization kinetics of polytrimethylene terephthalate (PTT) was first investigated from two temperature limits of melt and glass states. For the isothermal melt crystallization, the values of Avrami exponent varied between 2 and 3 with changing crystallization temperature, indicating the mixed growth and nucleation mechanisms. Meanwhile, the cold crystallization with an Avrami exponent of 5 indicated a character of three-dimensional solid sheaf growth with athermal nucleation. Through the analysis of secondary nucleation theory, the classical regime I→II and regime II→III transitions occurred at the temperatures of 488 and 468 K, respectively. The average work of chain folding for nucleation was ca. 6.5 kcal mol⁻¹, and the maximum crystallization rate was found to be located at ca. 415 K. The crystallite morphologies of PTT from melt and cold crystallization exhibited typical negative spherulite and sheaf-like crystallite, respectively. Moreover, the regime I→II→III transition was accompanied by a morphological transition from axialite-like or elliptical-shaped structure to banded spherulite and then non-banded spherulite, indicating that the formation of banded spherulite is very sensitive to regime behavior of nucleation.     

Surface crystallization of poly(ethylene terephtalate) studied by atomic force microscopy

Poly(ethylene terephtalate) (PET) crystallization was shown by atomic force microscopy (AFM) to occur at 85 °C in the first few nanometers near the polymer-air interface. The surface was fully transformed into spherulites after 30 min, while no signs of bulk crystallization were observed by FTIR. All the observed spherulites presented a nucleation centre, indicating that the crystallization process started at the surface of the film. Tapping mode AFM confirmed that the spherulites were not covered by an amorphous layer. The most probable explanation is a decrease of T_g near the surface. Due to the poor crystallization conditions, the constitutive units of the spherulites were small crystalline blocks. By changing the annealing time, it was possible to produce PET surfaces with different surface fractions consisting of semi-crystalline material (spherulites) and amorphous matrix. This provided a controlled surface heterogeneity on the submicrometer scale, with a contrast in terms of stiffness, roughness and swelling by organic solvents.     

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.     

Fast isothermal calorimetry of modified polypropylene clay nanocomposites

Calorimetric experiments at cooling rates comparable to those during injection molding, as an example, are needed to study phase transitions under conditions relevant for processing. Ultra fast scanning calorimetry is a technique which provides a means to analyze the materials of interest under rapid cooling conditions and it is a promising technique by which the crystallization behavior of composite systems based on fast crystallizing polymers like isotactic polypropylene (iPP) can be studied. By combining conventional DSC and ultra fast chip calorimetry isothermal crystallization experiments were performed in the whole temperature range between glass transition and melting temperature of iPP. Because of the very small time constant of the calorimeter, isothermal crystallization processes with peak times down to 100 ms were investigated after cooling the sample from the melt at 2000 K/s. iPP grafted with maleic anhydride (PPgMA) - montmorillonite clay nanocomposites were studied. The influence of various clay loadings on the crystallization behavior of PPgMA at different temperatures was followed by ultra fast isothermal calorimetry. PPgMA clay nanocomposites showed a variation in crystallization peak times with different clay loadings at crystallization temperatures between 70 °C and 100 °C. No influence of clay loading was observed at lower crystallization temperatures. At these temperatures, where the mesophase is formed and homogeneous nucleation is expected, the contribution of the clay as a nucleating agent is negligible. For crystallization at about 80 °C, where the α-phase is formed, the nucleating effect of the clay is observed yielding complex crystallization kinetics. In the temperature range 75-85 °C in some nanocomposites a double peak during isothermal crystallization was observed corresponding to a fast and a slow crystallization processes occurring simultaneously. At higher temperatures, above 120 °C, the clay slightly retards the crystallization process.