Halloysite nanotubes as a novel β-nucleating agent for isotactic polypropylene

The nucleating ability of halloysite nanotubes (HNTs) towards isotactic polypropylene (iPP) was investigated by differential scanning calorimetry (DSC), X-ray diffraction (XRD), polarized optical microscopy (POM) and scanning electron microscopy (SEM). HNTs are identified to have dual nucleating ability for α-iPP and β-iPP under appropriate kinetics conditions. The formation of β-iPP is dependent on the HNTs loading in the iPP/HNTs composites. The composite with 20 phr of HNTs is found to have the highest content of β-iPP. Under non-isothermal crystallization the content of β-iPP increases with decreasing of the cooling rate. The maximum β-crystal content is obtained at cooling rate of 2.5 °C/min. The supermolecular structure of the β-iPP is identified as β-hedrites with flower-cup-like and axialite-like arrangements of the lamellae. Under isothermal crystallization the β-crystal can be formed in the temperature range of 115-140 °C. Outside the temperature range, no β-iPP can be observed. The content of β-crystal reaches the maximum value at crystallization temperature of 135 °C. The formation of the β-iPP in the composites is correlated to the unique surface characteristics of the HNTs.     

Finite strain response, microstructural evolution and β→α phase transformation of crystalline isotactic polypropylene

An experimental study of the finite strain response of annealed α and β crystalline isotactic polypropylene (iPP) was conducted over a range of temperatures (25, 75, 110 and 135 °C) using uniaxial compression tests. Uniaxial compression results indicate nearly identical macroscopic stress vs. strain behavior for α-iPP and for β-iPP to true strains in excess of −1.1 at room temperature despite the different initial morphologies. At larger compressive strains (>1.2), β-iPP shows more rapid strain hardening. The orientation of crystalline planes during straining differs at room temperature from that at high temperature, indicating a change of slip mechanisms as temperature increases. In addition, strain-induced crystallization occurred at the highest temperature examined in α-iPP. A continuous transformation of β crystals to α crystals with inelastic deformation at room temperature was observed and it was facilitated at higher deformation temperatures. Scanning electron microscopy (SEM) observations of deformed β-iPP provide strong evidence that the transformation is achieved via a solid-to-solid mechanism despite the different helical hands in α and β crystal structures. Molecular simulations were used to investigate a conformational defect in the 3₁ helical chains of β-iPP, characterized by a 120° helical jump. The propagation of this conformational defect along molecular chains provides the reversal of helical hand required by the solid-to-solid transformation. The β→α phase transformation in iPP is proposed to be accomplished via a solid transformation that includes slip along β(110) and β(120) planes during shear of the crystal lattice.     

The β-nucleating effect of wollastonite-filled isotactic polypropylene composites

Wollastonite (W) with β-nucleating effect (β-W₁₀₀) for iPP crystallization was obtained through reaction between Ca²⁺ in wollastonite and pimelic acid (PA) and the β-iPP composites filled by different content of β-W₁₀₀ were prepared. The effect of PA and wollastonite contents on β-nucleation, crystallization and melting behavior, and crystalline morphology of W and β-W₁₀₀-filled iPP composites was investigated by differential scanning calorimetry (DSC), wide-angle X-ray diffraction, and polarizing optical microscopy. The results indicated that incorporation of W and β-W₁₀₀ increase the crystallization peak temperature of iPP due to its heterogeneous nucleating ability. And iPP/W composites predominantly crystallize in the α-phase iPP and iPP/β-W₁₀₀ composites in the β-phase iPP. The results of DSC multi-scanning in same and different melting temperatures showed that β-W₁₀₀ not only has strong heterogeneous β-nucleating effect but also DSC multi-scanning in same and different melting temperatures has no influence on the heterogeneous β-nucleating effect of β-W₁₀₀. The β-iPP containing high wollastonite content with high β-phase content can be easily prepared.     

Effects of organic nucleating agents and zinc oxide nanoparticles on isotactic polypropylene crystallization

Organic nucleating agents and inorganic nanoparticles, as well as their hybrid composites, affect the crystallization temperature and morphology of the monoclinic α-form of isotactic polypropylene (iPP). Techniques such as differential scanning calorimetry, hot-stage optical microscopy with cross polars, wide angle X-ray diffraction, and transmission electron microscopy were employed. Nanoparticles of zinc oxide function as efficient supports for 1,3,5-benzene tricarboxylic-(N-2-methylcyclohexyl)triamine because the temperature at which the maximum rate of iPP crystallization occurs during 10 °C/min cooling from the molten state increases from 111 °C for the pure polymer to 125 °C at low concentrations of this hybrid nucleating agent. In the absence of zinc oxide, 0.06 wt% of this aliphatic triamine recrystallizes near 165 °C and increases the crystallization temperature of iPP by 7 °C, relative to the pure polymer. Fluorinated aromatic triamines, such as 1,3,5-benzene tricaboxylic-(N-4-fluorophenyl)triamine, are weak nucleating agents that reduce spherulite size in isotactic polypropylene but only increase the crystallization temperature marginally when the polymer is cooled from the molten state. Both micro- and nanoparticles of zinc oxide reduce spherulite size in isotactic polypropylene, but smaller spherulites are observed when the inorganic nanoparticles exhibit dimensions on the order of 40-150 nm relative to micron-size particles. In contrast, 0.06 wt% of the aliphatic triamine in iPP yields a distorted birefringent texture under cross polars that is not spherulitic. Non-spherulitic birefringent textures in iPP are also observed when the aliphatic triamine nucleating agent is coated onto micro- or nanoparticles of zinc oxide. This study demonstrates that the nonisothermal crystallization temperature of isotactic polypropylene increases by an additional 7 °C when an aliphatic triamine is distributed efficiently within the polymeric matrix by coating this nucleating agent onto zinc oxide nanoparticles.     

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.     

Isotactic polypropylene crystallized from the melt. I. Morphological study

The spherulitic structure of isotactic polypropylene (iPP) from the melt was studied by polarized light and scanning electron microscopy. From the crystallization morphology, it can be observed that crystallization of iPP from the melt below 132°C forms two types of spherulites, termed α- and β-spherulites. The structure of iPP isothermally crystallized above 132°C shows α-type only. The α-spherulites have a complex crosshatched array of radial and tangential lamellar structures, while β-spherulites have, to some extent, simpler lamellar morphology with lower crosshatching content compared with α-type. However, in α-spherulites the radial lamellar thickness is greater than that of tangential lamellae, but in β-spherulites the radial and tangential lamellae have approximately the same thickness. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:1259–1265, 1998     

Changes in optical texture of α-spherulites of isotactic polypropylene upon partial melting and recrystallization

Textural changes in -phase spherulites of isotactic polypropylene (iPP) in a sequence of thermal events were examined by means of polarized light microscopy (PLM). This sequence of thermal events involves isothermal crystallization (atT_c = 117 to 140 °C), followed by heating (at 5 °C/min) to nearly complete melting, and then recrystallization upon cooling (at -40°C/min) to T_c During isothermal crystallization, the a-spherulites were of mixed birefringenceat T_c = 117 to 127 °C or of negative birefringence at T_c = 140 °C; upon heating towards melting, the spherulitec birefringence consistently truned negative. More interestingly, after recrystallization during cooling back to Tc from nearcomplate melting, all spherulites exhibited positive birefringence. The recrystallization could also result in speckles of positive birefringence when T_c was high or upon slower cooling. The changes in optical texture are explained in terms of contributions from tangential (or, cross-hatched) subsidiary lamellae which (as compared to the radial dominant lamellae) are relatively low-melting but thicken and recrystallize more readily in the present temperature range.     

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.     

Miscibility and crystallization in crystalline/crystalline blends of poly(butylene succinate)/poly(ethylene oxide)

Miscibility and crystallization behavior have been investigated in blends of poly(butylene succinate) (PBSU) and poly(ethylene oxide) (PEO), both semicrystalline polymers, by differential scanning calorimetry and optical microscopy. Experimental results indicate that PBSU is miscible with PEO as shown by the existence of single composition dependent glass transition temperature over the entire composition range. In addition, the polymer-polymer interaction parameter, obtained from the melting depression of the high-T_m component PBSU using the Flory-Huggins equation, is composition dependent, and its value is always negative. This indicates that PBSU/PEO blends are thermodynamically miscible in the melt. The morphological study of the isothermal crystallization at 95 °C (where only PBSU crystallized) showed the similar crystallization behavior as in amorphous/crystalline blends. Much more attention has been paid to the crystallization and morphology of the low-T_m component PEO, which was studied through both one-step and two-step crystallization. It was found that the crystallization of PEO was affected clearly by the presence of the crystals of PBSU formed through different crystallization processes. The two components crystallized sequentially not simultaneously when the blends were quenched from the melt directly to 50 °C (one-step crystallization), and the PEO spherulites crystallized within the matrix of the crystals of the preexisted PBSU phase. Crystallization at 95 °C followed by quenching to 50 °C (two-step crystallization) also showed the similar crystallization behavior as in one-step crystallization. However, the radial growth rate of the PEO spherulites was reduced significantly in two-step crystallization than in one-step crystallization.     

Comparison of the fracture and failure behavior of injection-molded α- and β-polypropylene in high-speed three-point bending tests

The fracture and failure mode of α- and β-isotactic polypropylene (α-iPP and β-iPP, respectively) were studied in high speed (1 m/s) three-point bending tests on notched bars cut from injection-molded dumbbell specimens and compared. The fracture response of the notched Charpy-type specimens at room temperature (RT) and −40°C, respectively, was described by terms of the linear elastic fracture mechanics (LEFM), namely fracture toughness (K_c) and fracture energy (G_c). K_c values of both iPP modifications were similar, while G_c values of the β-iPP were approximately twofold of the reference α-iPP irrespective of the test temperature. It was demonstrated that β-iPP failed in a ductile and brittle-microductile manner at RT and −40°C, respectively. By contrast, brittle fracture dominated in α-iPP at both testing temperatures. Based on the fracture surface appearance, it was supposed that β-to-α (βα) transformation occurred in β-iPP. The superior fracture energy of β-iPP to α-iPP was attributed to a combined effect of the following terms: morphology, mechanical damping, and phase transformation. Results indicate that their relative contribution is a function of the test temperature. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 2057–2066, 1997     

Isothermal crystallization of isotactic polypropylene blended with low molecular weight atactic polypropylene. Part I. Thermal properties and morphology development

The thermal properties and morphology development of isotactic polypropylene (iPP) homopolymer and blended with low molecules weigh atactic polypropylene (aPP) at different isothermal crystallization temperature were studied with differential scanning calorimeter and wide-angle X-ray scattering. The results of DSC show that aPP is local miscible with iPP in the amorphous region and presented a phase transition temperature at T_c=120 °C. However, below this transition temperature, imperfect α-form crystal were obtained and leading to two endotherms. While, above this transition temperature, more perfect α- and γ-form crystals were formed which only a single endotherm was observed. In addition, the results of WAXD indicate that the contents of the γ-form of iPP remarkably depend both on the aPP content and isothermal crystallization temperature. Pure iPP crystallized was characterized by the appearance of α- and γ-forms coexisting. Moreover, the highest intensity of second peak, i.e. the (0 0 8) of γ-form coexisting with (0 4 0) of α-form, and crystallinity were obtained for blended with 20% of aPP, the γ-form content almost disappeared for iPP/aPP blended with 50% aPP content. Therefore, detailed analysis of the WAXD patterns indicates that at small amount aPP lead to increasing the crystallinity of iPP blend, at larger amount aPP, while decreases crystallinity of iPP blends with increasing aPP content. On the other hand, the normalized crystallinity of iPP molecules increases with increasing aPP content. These results describe that the diluent aPP molecular promotes growth rate of iPP because the diluent aPP molecular increases the mobility of iPP and reduces the entanglement between iPP molecules during crystallization.     

Memory effect of locally ordered α-phase in the melting and phase transformation behavior of β-isotactic polypropylene

Structural changes in β-isotactic polypropylene (β-iPP) during the heating were studied by means of differential scanning calorimetry and real-time in situ X-ray diffraction using a synchrotron source. Crystalline phase transformation and the memory effect caused by residual nuclei of α-iPP were observed during the heating of β-iPP. The memory effect observed in β-iPP during heating and crystallization is believed to be due to the existence of locally ordered α-from in the melt. The effect of local α-form order was probed by studying the behavior under heating of samples with a range of thermal histories. Samples were heated above the equilibrium melting temperature of iPP to remove all residual local order and the memory effect associated with this local order. The samples crystallized isothermally at different temperatures exhibited a significantly different melting and phase transformation behavior during heating. β-iPP is found to be an excellent material for the study of polymorphism, phase transformations, and characteristic memory effects in semicrystalline polymers.     

Isothermal crystallization, melting behavior and crystalline morphology of syndiotactic polystyrene blends with highly-impact polystyrene

Syndiotactic polystyrene (sPS) blends with highly-impact polystyrene (HIPS) were prepared with a twin-screw extruder. Isothermal crystallization, melting behavior and crystalline morphology of sPS in sPS/HIPS blends were investigated by differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and polarized optical microscopy (POM). Experimental results indicated that the isothermal crystallization behavior of sPS in its blends not only depended on the melting temperature and crystallization temperature, but also on the HIPS content. Addition of HIPS restricted the crystallization of sPS melted at 320 °C. For sPS melted at 280 °C, addition of low HIPS content (10 wt% and 30 wt%) facilitated the crystallization of sPS and the formation of more content of α-crystal. However, addition of high HIPS content (50 wt% and 70 wt%) restricted the crystallization of sPS and facilitated the formation of β-crystal. More content of β-crystal was formed with increase of the melting and crystallization temperature. However, α-crystal could be obtained at low crystallization temperature for the specimens melted at high temperature. Addition of high HIPS content resulted in the formation of sPS spherulites with less perfection.     

Anomalous molecular orientation of isotactic polypropylene sheet containing N,N′-dicyclohexyl-2,6-naphthalenedicarboxamide

Small amount of N,N′-dicyclohexyl-2,6-naphthalenedicarboxamide as a β-form nucleating agent is dissolved beyond 280 °C in a molten isotactic polypropylene (iPP) and appears as needle crystals around at 240 °C during cooling procedure. Further, iPP molecules crystallize on the surface of the needle crystals, in which c-axis of the β-form iPP crystals grows perpendicular to the long axis of the needle crystals. Under flow field at extrusion processing, the needle crystals orient to the flow direction prior to the crystallization of iPP. As a result, c-axis of the β-form iPP crystals orients perpendicular to the applied flow direction with a small amount of α-form iPP. Moreover, the vertical molecular orientation of the extruded sheet sample is responsible for unique mechanical anisotropy; the fracture occurs along the transversal direction.     

Analyses of crystal forms in syndiotactic polystyrene intercalated with layered nano-clays

A mixed polymorphic morphology of intercalated/exfoliated structure was observed in syndiotactic polystyrene (sPS)/clay nano-composites, which were successfully prepared by solution intercalation technique using 1,1,2,2-tetrachloroethane (TCE) as a solvent. Furthermore, X-ray diffraction (XRD) and differential scanning calorimetry (DSC) analyses were used to examine the effect of montmorillonite clays (MMT, in pristine or organo-modified forms) in isothermally melt-crystallized sPS at several available crystallization temperatures (T_c) in a competitive environment of coexisting α- and β-crystals. A significant change in polymorphism of sPS was observed by the inclusion of different clays and the temperature regime of the α-crystal formation in sPS was found to increase considerably up to 250 °C by the presence of the organo-clay. Pristine clay (Na-MMT) was found to induce the β-crystal of sPS at all T_c's studied in this work. The overall thermodynamics of crystallization remained unchanged as the β-phases were found in major proportion at higher temperature of crystallization (∼260 °C), irrespective of the nature of the clays. The dispersibility of the clays in sPS matrix is assumed to play the pivotal role in modifying the crystalline structures, which was further corroborated by the polarized optical microscopy (POM). The spherulitic morphology clearly indicates differences in crystallites as affected by the nano-clays. Incorporation of organo-clay with nanoscale dispersibility through the intercalation of sPS molecules into the clay galleries was found to promote rapid formation of α-forms, which develops into spherulites of smaller dimension as compared to those of the β-forms. The alteration in melting behavior of sPS is attributed to the different crystallite structures that lead to formation of different kind of spherulites.     

Crystallization behavior of nano-composite based on poly(vinylidene fluoride) and organically modified layered titanate

To understand the effect of the nano-filler particles on the crystallization kinetics and crystalline structure of poly(vinylidene fluoride) (PVDF) upon nano-composite formation, we have prepared PVDF/organically modified layered titanate nano-composite via melt intercalation technique. The layer titanate (HTO) is a new nano-filler having highly surface charge density compared with conventional layered silicates. The detailed crystallization behavior and its kinetics including the conformational changes of the PVDF chain segment during crystallization of neat PVDF and HTO-based nano-composite (PVDF/HTO) have been investigated by using differential scanning calorimetric, wide-angle X-ray diffraction, light scattering, and infrared spectroscopic analyses. The neat PVDF predominantly formed α-phase in the crystallization temperature range of 110-150 °C. On the other hand, PVDF/HTO exhibited mainly α-phase crystal coexisting with γ- and β-phases at low T_c range (110-135 °C). A major γ-phase crystal coexists with β- and α-phases appeared at high T_c (=140-150 °C), owing to the dispersed layer titanate particles as a nucleating agent. The overall crystallization rate and crystalline structure of pure PVDF were strongly influenced in the presence of layered titanate particles.     

Crystallization behaviour of isotactic polypropylene/linear low density polyethylene blends

Crystallization behaviour of isotactic polypropylene/linear low density polyethylene (iPP/LLDPE) blends has been investigated by optical microscopy and DSC. Crystallization of iPP depends upon blend composition and thermal history. When blended with LLDPE, the crystallization temperature of iPP, T_c, decreased slightly. Crystallinity did not change in the range 0-80wt% LLDPE; there were only slight changes in the crystalline structure, but LLDPE seemed to resist forming the β type of spherulites. Below 80 wt% of LLDPE, iPP was a continuous phase. The iPP spherulite growth rate was almost constant; however, overall crystallization decreased due to decreasing primary nuclei density.     

Thermal, spectroscopy, and morphological studies on polymorphic crystals in poly(heptamethylene terephthalate)

Polymorphism and its influential factors in poly(heptamethylene terephthalate) (PHepT) were probed using differential scanning calorimetry (DSC), Fourier-transform infrared (FTIR) spectroscopy, and wide angle X-ray diffraction (WAXD). PHepT exhibits two crystal types (α and β) upon crystallization at various isothermal melt-crystallization temperatures (T_cs) by quenching from different T_maxs (maximum temperature above T_m for melting the original crystals). Melt-crystallized PHepT with either initial α- or β-crystal by quenching from T_max lower than 110 °C leads to higher fractions of α-crystal, but crystallization from T_max higher than 140 °C leads to higher fractions of β-crystal. In addition to T_max, polymorphism in PHepT is also influenced by crystallization temperature (T_c = 25-75 °C). When PHepT is melt-crystallized from a high T_max = 150 °C (completely isotropic melt), it shows solely β crystal for higher T_c, and solely the α-crystal for T_c < 25 °C; in-between T_c = 25 and 35 °C, mixed fractions of both α- and β-crystals. However, by contrast, when PHepT is melt-crystallized from a lower T_max = 110 °C, it shows α-crystal only at all T_cs, high or low.     

Poly(trimethylene terephthalate) copolyester containing ethylene glycol moieties. Part 1: Crystallization kinetics and melting behavior

A copolyester was characterized to have 91 mol% trimethylene terephthalate unit and 9 mol% ethylene terephthalate unit in a random sequence by using ¹³C NMR. Differential scanning calorimeter (DSC) was used to investigate the isothermal crystallization kinetics in the temperature range (T_c) from 180 to 207 °C. The melting behavior after isothermal crystallization was studied by using DSC and temperature modulated DSC (TMDSC). The exothermic behavior in the DSC and TMDSC curves gives a direct evidence of recrystallization. No exothermic flow and fused double melting peaks at T_c = 204 °C support the mechanism of different morphologies. The Hoffman-Weeks linear plot gave an equilibrium melting temperature of 236.3 °C. The kinetic analysis of the growth rates of spherulites and the morphology change from regular to banded spherulites indicated that there existed a regime II → III transition at 196 °C.     

A highly active novel β-nucleating agent for isotactic polypropylene

A highly active novel β-nucleating agent for isotactic polypropylene (iPP), cadmium bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylate (BCHE30), was found and its effects on the mechanical properties, the content of β-crystal, and crystallization behavior of iPP were investigated, respectively. The results show that the impact strength and crystallization peak temperature of nucleated iPP are greatly increased, while the spherulite size of nucleated iPP is dramatically decreased than that of pure iPP. The content of β-form of nucleated iPP (k_β value) can reach 87% with 0.1 wt% BCHE30. The Caze method was used to study the non-isothermal crystallization kinetics of nucleated iPP and the crystallization active energy was achieved by Kissinger method.