Effect of nucleating agent on the brittle–ductile transition behavior of polypropylene/ethylene–octene copolymer blends

In the present work, α‐form nucleating agent 1,3:2,4‐bis (3,4‐dimethylbenzylidene) sorbitol (DMDBS, Millad 3988) is introduced into the blends of polypropylene/ethylene–octene copolymer (PP/POE) blends to study the effect of the nucleating agent on the toughness of PP/POE blends through affecting the crystallization behavior of PP matrix. Compared with the PP/POE blends, in which the toughness of the blends increases gradually with the increasing content of POE and only a weak transition in toughness is observed, addition of 0.2 wt % DMDBS induces not only the definitely brittle‐ductile transition at low POE content but also the enhancement of toughness and tensile strength of the blends simultaneously. Study on the morphologies of impact‐fractured surfaces suggests that the addition of a few amounts of DMDBS increases the degree of plastic deformation of sample during the fracture process. WAXD results suggest that POE induces the formation of the β‐form crystalline of PP; however, DMDBS prevents the formation of it. SEM results show that the addition of DMDBS does not affect the dispersion and phase morphologies of POE particles in PP matrix. DSC and POM results show that, although POE acts as a nucleating agent for PP crystallization and which enhances the crystallization temperature of PP and decreases the spherulites size of PP slightly, DMDBS induces the enhancement of the crystallization temperature of PP and the decrease of spherulites size of PP more greatly. It is concluded that the definitely brittle–ductile transition behavior during the impact process and the great improvement of toughness of the blends are attributed to the sharp decrease of PP spherulites size and their homogeneous distribution obtained by the addition of nucleating agent. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 577–588, 2008     

Formation of β-crystal from nonisothermal crystallization of compression-molded isotactic polypropylene melt

The crystalline structures of isotactic polypropylene (iPP) subjected to compression-molding were studied by means of differential scanning calorimeter (DSC), optical microscope (OM) and wide-angle X-ray diffraction (WAXD). β-crystal can be formed from nonisothermal crystallization of the sample compression-molded at the molten state and the β-phase content increased with increasing pressure of molding. Thermal treatment of the molten sample at 200 °C could eliminate the effect of compression-molding on crystalline structure. It was suggested that the compression-molding of iPP melt plays an important role in improving the nucleation ability of β-crystal.     

成核剂含量对β晶相聚丙烯结晶与熔融行为的影响

用DSC研究了β成核剂含量对β聚丙烯在等温与非等温结晶条件下的结晶与熔融行为的影响,发现当成核剂含量为0.005％时,结晶焓△H_c、β晶的熔融焓△H_(mβ)及熔点T_(mβ)均为最大,而α晶的相对含量最小.广角X-衍射数据表明,成核剂含量高的试样的(301)衍射峰的相对强度下降,反映分子链排列的纵向有序性降低.根据聚丙烯分子在β成核剂上附生结晶的成核机理解释了上述结果.     

Effects of clay on polymorphism of polypropylene in polypropylene/clay nanocomposites

The effects of clay on polymorphism of polypropylene (PP) in PP/clay nanocomposites (PPCNs) under various thermomechanical conditions were studied. In extruded PP and PPCN pellet samples, only α-phase crystallites existed, as they were prepared by rapidly cooling the melt extrudates to room temperature. Under compression, β-phase crystallites can develop in neat PP under various thermal conditions, of which isothermal crystallizing at 120 °C gave the highest content of β-phase crystallites. In contrast, no β-phase crystallite was detected in the PPCN samples prepared under the same conditions. This indicated that clay significantly inhibits the formation of β-phase crystallites. The likely reason is that the presence of clay in PPCNs greatly sped up the crystallization process of the α phase, whereas it had an insignificant effect on the crystallization rates of the β phase. The results also showed that clay may slightly promote the formation of γ-phase PP crystallites in PPCNs. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1810–1816, 2004     

On the crystallization of γ-isotactic polypropylene: A high pressure study

The non-parallel chain structure determined for γ-phase isotactic polypropylene (γ-iPP) is confirmed by Rietveld analysis for highly isotactic high molecular mass iPP crystallized at 200 MPa. The new refinement shows that: (i) stereoregularity or crystallization pressure do not significantly influence the lattice dimensions; (ii) defect inclusion in γ-iPP crystals is unlikely. The α and γ forms have nearly identical bulk internal energy and density, but α-iPP should be normally kinetically favored over γ-iPP which may in turn predominate due to its greater ability to host defects at the crystalline-amorphous interface.     

Crystal phases,structure, and orientation in isotactic polypropylene after isothermal crystallization under oscillatory shear as a function of nucleation agent

2D wide-angle X-ray diffraction (2D-WAXD) measurement was performed to investigate the effects of both oscillatory shear and the nucleating agent on the crystalline structure distribution and orientation of isotactic polypropylene (iPP). 1,3:2,4-bis(p-methylbenzylidene) sorbitol (MDBS) and 1,3:2,4-di(3,4-dimethylbenzylidene) sorbitol (DMDBS) can induce α-PP and β-PP simultaneously. The presence of MDBS (or DMDBS) and oscillatory strain (oscillatory frequency is fixed) exhibits a synergistic interaction on increasing the content of β-crystals of iPP. Under the oscillatory shear field at the fixed oscillatory strain, the β-crystal content and the orientation of iPP with and without MDBS (or DMDBS) change slightly with the increase of the oscillatory frequency. Comparing with MDBS (or DMDBS) nucleated iPP crystallization under shear field, the periodically changed flow direction of the oscillatory shear field leads to the shorter α-row nuclei, weaker orientation but more β-crystals of the nucleated iPP.     

Nonisothermal crystallization behaviors of polypropylene with α/β nucleating agents

In this work, the nonisothermal crystallization and subsequent melting behaviors of polypropylene (PP) nucleated with different nucleating agents (NAs) have been studied. α‐phase NA 1,3:2,4‐bis (3,4‐dimethylbenzylidene) sorbitol (DMDBS, Millad 3988), β‐phase NA aryl amides compound (TMB‐5), and their compounds were introduced into PP matrix, respectively. The results show that the nonisothermal crystallization behaviors and crystalline structures of PP with compounded NAs are dependent on the composition of NAs. In the sample of PP with 0.1 wt % DMDBS and 0.1 wt % TMB‐5, the nucleation efficiency (NE) of TMB‐5 is much higher than that of DMDBS and PP crystallizes mainly nucleated by TMB‐5, and in this condition, β‐phase PP is the main crystallization structure. For the sample of PP with 0.2 wt % DMDBS and 0.2 wt % TMB‐5, 0.2 wt % DMDBS has higher NE than 0.2 wt % TMB5, and α‐phase is the main crystalline structure. The cooling rate is proved to be very important in controlling the nonisothermal crystallization behavior and the final crystalline structure of nucleated PP. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1853–1867, 2008     

Multiple melting behavior of isotactic polypropylene and poly(propylene-co-ethylene) after stepwise isothermal crystallization

The multiple melting behavior of several commercial resins of isotactic polypropylene (iPP) and random copolymer, poly(propylene-co-ethylene) (PPE), after stepwise isothermal crystallization (SIC) were studied by differential scanning calorimeter and wide-angle X-ray diffraction (WAXD). For iPP samples, three typical melting endotherms appeared after SIC process when heating rate was lower than 10 °C/min. The WAXD experiments proved that only α-form crystal was formed during SIC process and no transition from α1- to α2-form occurred during heating process. Heating rate dependence for each endotherm was discussed and it was concluded that there were only two major crystals with different thermal stability. For the PPE sample, more melting endotherms appeared after stepwise isothermal crystallization. The introduction of ethylene comonomer in isotactic propylene backbone further decreased the regularity of molecular chain, and the short isotactic propylene sequences could crystallize into γ-form crystal having a low melting temperature whereas the long sequences crystallized into α-form crystal having high melting temperature.     

Crystallization and melting behavior of isotactic polypropylene nucleated with individual and compound nucleating agents

In this study, α-phase nucleating agent (NA) 1,3:2,4-bis(3,4-dimethylbenzylidene) sorbitol (DMDBS), β-phase rare earth NA (WBG), and their compound NAs were introduced into isotactic polypropylene (iPP) matrix, respectively. Crystallization kinetics and subsequent melting behavior of the nucleated iPPs were comparatively studied by differential scanning calorimetry (DSC) under both isothermal and nonisothermal conditions. For the isothermal crystallization process, it is found that the Avrami model successfully described the crystallization kinetics. The active energy of nonisothermal crystallization of iPP was determined by the Kissinger method and showed that the addition of nucleating agents increased the activation energy. Melting behavior and crystalline structure of the nucleated iPPs are dependent on the nature of NAs and crystallization conditions. Higher proportion of β-phase can be obtained at higher content of β-nucleating agent and lower crystallization temperature or lower cooling rate.     

γ–α transformation in isotactic polypropylene

A γ-phase to α-phase transformation in a specimen of isotactic polypropylene crystallized under conditions of high pressure was induced by drawing at 100°C. X-ray studies showed that the unoriented component remained in the γ-phase, and that the oriented component was found only in the α-phase. This evidence supports a previous suggestion that the phase transformation is martensitic in character. The consequences of such an assumption are discussed. The role of dislocations in polymeric systems is generally believed to be not too significant, but since martensitic reactions involve cooperative movements of atoms, an exception in this case is suggested. A possible mechanism for the phase transformation is suggested.     

A novel polypropylene composite filled by kaolin particles with β-nucleation

Kaolin-filled polypropylene (PP) composites generally form α-crystal due to the effect of kaolin with α-nucleation. The transition from α- to β-nucleation of kaolin has been investigated, and a novel kaolin with β-nucleation (β-kaolin) and kaolin-filled PP composites with high β-crystal content were prepared first. The DSC and WAXD results indicated that the β-kaolin exhibits stronger β-nucleating ability than CaPA as β-nucleating agent for PP crystallization. It is found that the β-crystal content has been influenced little by filler contents in β-kaolin-filled PP composites. Mechanical properties and spherulitic morphology of filled PP composites was characterized. The synergistic effect of filler and β-crystal significantly improved impact strength of kaolin-filled PP composites.

     

Effects of temperature and water on the γ → α crystalline transition of nylon 6 caused by stretching in the chain direction

The effects of temperature and water on the crystal transition of nylon 6 from the γ phase to the α phase, which is caused by stretching along the chain direction, were investigated. The γ-phase fibers with high crystallite orientation were stretched at constant load under various conditions. An inversion of the effect of water on the transition occurs at about ?60°C. Stretching in the wet state is more effective for the transition at higher temperature. In contrast, at low temperatures water in the crystalline regions actsasa cohesive agent for the chains and increases the activation energy for the transition. Thus, dry stretching is more effective than wet stretching at very low temperature. The fraction of transformed α-phase crystallites increases abruptly over a narrow range of stress. Thus the critical stress can be determined for the transition. The critical stress changes appreciably with temperature; the higher the temperature, the lower the critical stress. The relation between stretching temperature and critical stress was analyzed by Flory's equation for the shift of transition temperature by stress. About 220°C. was estimated as the zero-stress transition temperature. The heat content of the γ-phase crystal was estimated to be smaller by 500 cal./mole than that of the α-phase crystal. This result suggests that the free energy of the γ-phase crystal is lower than that of the α-phase crystal at temperatures lower than the transition point. The irreversible strain of a sample in which the crystal transition has taken place is very small at low temperature. This small extension of the sample is evidence that the γ → α transition produced by stretching along the chain axis is a crystal-crystal transition.     

制备方法对iPP/EPR合金晶相结构的影响

用示差扫描量热仪(DSC)和广角X射线衍射仪(WAXD)研究了溶液共混法和熔融共混法制备的等规聚丙烯/二元乙丙橡胶(iPP/EPR)(85:15,W/W)合金的晶相结构.发现溶液共混法制得的iPP/EPR合金晶相中仅存在α-iPP,而熔融共混样品中则同时生成了α-iPP和β-iPP.这一结果表明,EPR并不是iPP/EPR合金中β-iPP生成的关键因素.考察了结晶温度和熔体热处理对iPP/EPR合金晶相结构的影响,发现通常的热处理并不能消除合金中β-iPP的生成.     

The variable role of clay on the crystallization behavior of DMDBS-nucleated polypropylene

The effect of clay on the nucleating behavior of 1,3:2,4-bis(3,4-dimethylbenzylidene) sorbitol(DMDBS) in cryatallization of isotactic polypropylene(iPP) was investigated by means of differential scanning calorimetry(DSC), dynamic rheology and polarized light microscopy(PLM).It is interesting to note that the incorporation of layered clay nanoparticles into DMDBS-nucleated iPP may induce a synergetic nucleation effect while the DMDBS content is below 0.1 wt%,otherwise it restricts the crystallization rate prominently as the DMDBS content increases up to 0.3 wt%,which has exceeded the content threshold to yield a nucleating agent(NA) network.As shown by dynamic rheological investigations, the clay nanoparticles demonstrate an obstructive effect of disturbing the consistency of DMDBS fibrils network.Moreover, to further demonstrate the importance of NA network formation in the crystallization of iPP,we used another NA named HPN-20e,which can not form network structure at all over the concentration studied,for comparison.In this case,the nucleated-crystallization rate is independent on the addition of clay nanoparticles,as the nucleating mechanism is an individual nuclei manner without NA network forming.     

Isothermal crystallization of poly(vinylidene fluoride) in the presence of high static electric fields. II. Effect of crystallization temperature and electric field strength on the crystal phase content and morphology

The melt crystallization of poly(vinylidene fluoride) in a static electric field was studied for different fields strengths and undercooling conditions. The γ-phase nucleation process was examined directly by polarized optical microscopy and indirectly by small-angle light scattering. The crystal phase content was assessed by differential scanning calorimetry. It is shown that the γ-phase nucleation density and γ-phase content increase with field strength and that the higher the crystallization temperature, the larger the effect of the field. These experimental results confirm the predictions of the model of nucleation in an electric field that we published previously. It is also noted that the degree of crystallinity and the perfection of crystal orientation along the γ-phase spherulite radical direction decrease with field strength. The homogeneity of morphology resulting from the crystallization in the field is also examined by polarized optical microscopy on specimens microtomed across their thickness. When the crystallization is carried out under high field (E ≈ 0.1 MV/cm) and high temperature (T > 166°C) a nonuniform morphology results, characterized by a higher nucleation density at the positive electrode than at the negative electrode. Near the negative electrode very large disklike spherulites are seen to grow parallel to the substrate.     

Morphological and structural characterization of polypropylene/conductive graphite nanocomposites

The structure and morphology of polypropylene/conductive graphite (PP/CG) composites were studied by wide angle X-ray diffraction, small-angle X-ray scattering and scanning electron microscopy. An effect of graphite on the crystallization behavior was observed and the opposite influences of enhanced thermal conductivity and hinder of chain mobility on the formation of the γ-phase of PP were discussed.     

Oriented γ-isotactic polypropylene crystallized at atmospheric pressure

A method for crystallizing oriented samples of the γ-phase of isotactic polypropylene (iPP) under atmospheric pressure in the presence of nucleating fibers has been developed. The technique uses iPP grades of high molecular weight and high isotacticity and produces a mixture of α and γ-phase crystals within the matrix of both pitch-based carbon and Kevlar®-reinforced composites. Two-dimensional wide-angle X-ray scattering (WAXS) patterns from these samples show that the content of the γ-phase decreases as the fiber loading in the composites decreases, suggesting that the γ-phase is directly nucleated by the fibers. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2821–2827, 1998     

Effect of Polyhedral Oligomeric Silsesquioxane and Sorbitol on Properties of Isotactic Polypropylene

Isotactic polypropylene(iPP) was modified by the introduction of polyhedral oligomeric silsesquio- xanes(POSS) and 1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol(DMDBS). Chemical combination of (3-mercapto)- propyl-heptaisobutyl POSS with DMDBS(POSS-DMDBS), and physical mixing of DMDBS with octaisobutyl POSS (iso-POSS/DMDBS) or trisilanolisobutyl POSS(tri-POSS/DMDBS) were applied respectively to modifying iPP, and the effects of POSS and DMDBS on crystallization, rheological and mechanical properties of iPP were systematically investigated. The results indicate that iso-POSS/DMDBS and tri-POSS/DMDBS were more effective than POSS-DMDBS on the improvement of the crystallization behavior of iPP due to the higher crystallization temperature, while the crystallinity of iPP containing POSS-DMDBS was enhanced, approximately approached to that of iPP containing tri-POSS/DMDBS. The tensile strength of iPP with POSS-DMDBS was significantly increased from 34 MPa to 40 MPa, as high as that of iPP with iso-POSS/DMDBS. The different effects caused by the specific interaction between POSS and DMDBS could possibly be applied in the modification of iPP.     

Enhanced <Emphasis Type="Italic">αγ</Emphasis>′ Transition of Poly(vinylidene fluoride) by Step Crystallization and Subsequent Annealing

Poly(vinylidene fluoride) (PVDF) exhibits pronounced polymorphs.Its γ phase is attractive due to the electroactive properties.The γ-PVDF is however difficult to obtain under normal crystallization condition.In a previous work,we reported a simple melt-recrystallization approach for producing y-phase rich PVDF thin films through selective melting and subsequent recrystallization.We reported here another approach for promoting the αγ'phase transition to prepare γ-phase rich PVDF thin films.To this end,a stepwise crystallization and subsequent annealing process was used.The idea is based on a quick generation of a large amount of α-PVDF crystals with some of their γ-PVDF counterparts at suitable crystallization temperature and then annealing at a temperature above the crystallization temperature for enhancing the molecular chain mobility to overcome the energy barrier of phase transition.It was found that crystallizing the PVDF melt first at 152 ℃ for4 h,then quenching to room temperature and finally annealing the sample at 160 ℃ for 100 h was the most efficient to produce γ-PVDF rich films.This is related to the melting and recrystallization of the α-PVDF crystals produced during quenching in the annealing process at 160 ℃,which favors the formation of γ-PVDF crystals for triggering the αγ'phase transition.     

超细CaCO_3的粒子尺寸对PP结晶行为的影响

The effects of CaCO3 on the crystallization behavior of polypropylene (PP) were studied by means of DSC and WAXD.The average sizes of the CaCO3 powders used were 0.1μm (UC) and 0.5μm (GC),respectively.The PP/CaCO3 composites at compositions of 1phr and 10 phr were investigated.The results showed that the addition of CaCO3 reduced the supercooling,the rate of nucleation and the overall rate of crystallization (except for the 10phr UC/PP sample).The crystallinity of PP was increased and the size distribution of the crystallites of α-PP was broadened.On the other hand,the crystallization rate of 10phr UC/PP is 1.5 times higher than that of neat PP.It has an overall rate of crystallization 2 times as much as that of the neat PP and has the maximum crystallinity.The sizes of crystallites and the unit cell parameters of α-PP were varied by the addition of CaCO3.β-PP was formed by addition of GC and was not detected by addition of UC.The differences of crystallization behaviors of PP might be attributed to the combined effects of the content and size of CaCO3 filled.