Analysis of the isothermal crystallization of isotactic polypropylene nucleated with sorbitol derivatives

The isothermal crystallization kinetics of isotactic polypropylene (iPP) and iPP nucleated with the sorbitol derivatives 1,3:2,4‐bis(4‐methyldibenzylidene)sorbitol and 1,3:2,4‐bis(3,4‐dimethylbenzylidene)sorbitol was studied, along with the subsequent melting behavior, as a function of the nucleating agent concentration. The influence of the agents on the crystallization rate, crystallization temperature, and crystallization range was examined. The isothermal crystallization temperature increased, along with the crystallization rate, with increasing nucleating agent concentration. The maximum effect of the additives occurred at concentrations of 0.3% or greater. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2261–2274, 2003     

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     

Flow‐induced crystallization during isotactic polypropylene film casting

In this work, a previously proposed model for flow‐induced crystallization of polymers was slightly modified and successfully applied in describing film casting experiments performed working with isotactic polypropylene. In this process, the melt experiences flowing conditions, during the cooling. The effect of flow on crystallization kinetics was accounted for by the entropic increase of melting point because of the orientation effect of the flow. The orientation of the macromolecules was described by means of a nonlinear dumbbell model, and the entropic shift was calculated on the basis of the rubber elasticity theory. The flow‐induced crystallization model, using reasonable values for relaxation time, was proved able to quantitatively predict the enhancement effect of the flow as an increase of spherulite nuclei density and growth rate, in presence of low‐level flows (Deborah number <1–10). The morphology changes due to higher level of flow (fine grained, fibrils, shish‐kebab) were not predicted by this approach. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

Melting and crystallization behaviors of injection‐molded polypropylene

The melting and crystallization behaviors of the skin layer in an injection‐molded isotactic polypropylene (PP) have been studied, mainly in comparison with those of the core layer and subsidiarily in comparison with those of a compression‐molded PP and a nucleator (talc)–added PP. The skin layer contains about 5% crystals, which have a high melting point of up to 184°C. They thermally vanish by melting once. The subsequent melting history will scarcely affect the melting behaviors. On the other hand, crystallization behaviors are strongly affected by the melting history. The skin layer crystallizes in a wide temperature range at high temperature. This tendency weakens with increasing melting temperature, approaching a constant and that of the core layer above 230°C, which suggests that the memory effect of the residual structure of PP vanishes by melting above 230°C. In explaining these experimental results, it is assumed that the residual structure substance is a melt orientation of molecular chains that works as crystallization nuclei and that the vanishing of the residual structure is nothing but a relaxation of the melt orientation. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1751–1762, 2000     

聚丙烯复合材料的非等温结晶动力学

采用差示扫描量热仪研究了β成核剂和水滑石(LDH)/β成核剂复配的成核剂对聚丙烯(PP)非等温结晶动力学及熔融行为的影响。结果表明:加入成核剂后,PP中晶体分布不均匀且分散度增大。莫志深方法采用F(θ)表征聚合物在单位时间内达到某一结晶度时所需的冷却(或加热)速率,结晶度达到40%时,纯PP的F(θ)为3.82,加入β成核剂的PP的F(θ)为3.30,加入LDH/β成核剂的PP的F(θ)为2.49。与纯β成核剂相比,LDH/β成核剂能更好地提高PP的结晶温度、结晶速率,增强PP的β晶熔融峰,减弱β晶和α晶共存熔融峰和α晶熔融峰。     

Rheological and structure investigation of shear‐induced crystallization of isotactic polypropylene

In the present work, the influence of well‐defined simple shear flow histories on the isothermal crystallization of an isotactic polypropylene (i‐PP) has been investigated. At first, the research of the flow conditions in terms of temperature, shear rate ( ) and shear strain (γ) has been performed by means of the rheological technique. The continuous shearing analysis enabled us to build the flow curve at 144°C showing a Newtonian region as well as a shear‐thinning zone. Indeed, for above a critical value, the molecular orientation occurring during flow provides a kinetic promotion of the crystallization process. In the rheological step‐shear flow analysis, an increase of the flow sensitivity parameter, k_S/k_Q, with increasing the shear rate at a constant strain (γ = 150) is observed. The structure of the crystallized samples has been investigated by differential scanning calorimetry (DSC) and wide angle X‐ray scattering (WAXS) methods. In agreement with the DSC, the WAXS results show that crystals with a certain bimodal distribution are generated in the samples crystallized under step‐shear flow conditions. A small orientation of the (110) plane of the i‐PP α‐phase crystals is also detected. POLYM. ENG. SCI., 45:153–162, 2005. © 2005 Society of Plastics Engineers.     

Some crystallization kinetics of isotactic polypropylene

Infra-red evidence is presented that the building of helices in the crystallization process from the glassy (or smectic) phase or from the melt are second order or zero-th order respectively. The observation coupled with data from density and X-ray diffraction measurements is interpreted in molecular structural terms.     

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     

Character of crystallization nuclei in isotactic polypropylene

Examination of the isothermal crystallization and the effect of melting conditions on samples of isotactic polypropylene and its composite with talc, combined with electron-microscopic observation, has shown two types of heterogeneous nuclei effective in the crystallization process: (1) metastable nuclei, representing the unmelted crystalline phase of polypropylene, stabilized by solid heterogeneities, and operating after melting at relative low melting temperature and/or short melting time; and (2) stable crystallization nuclei, associated with solid heterogeneities, being probably catalyst residues. On the surface of these nuclei isotactic polypropylene tends to crystallize in an ordered fashion.     

Cavitation during isothermal crystallization of isotactic polypropylene

Cavitation during isothermal crystallization of thin films of isotactic polypropylene was investigated systematically by light microscopy. Cavitation results from the negative pressure buildup due to density change during crystallization in the pockets of melts occluded by impinging spherulites. The morphology of such areas was also studied by SEM. The value of the negative pressure at the moment of cavitation was calculated from the drop of the spherulite growth rate. It was shown that the process of cavitation and the value of the negative pressure causing cavitation depend on the crystallization temperature. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 2439–2448, 2001     

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.     

Effect of polycyclopentene on crystallization of isotactic polypropylene

The application of some polymers as nucleating agents for polypropylene has been examined. Among various polymeric nucleating agents, polycyclopentene was found to be a superior nucleating agent to typical organic nucleating agents. When polycyclopentene was added to polypropylene, the crystallization temperature and the degree of crystallinity of polypropylene increased. In addition, the crystallization rate and the number of spherulites increased whereas the size of spherulites decreased remarkably. As a result of polycyclopentene addition, the transparency of polypropylene film could be improved considerably. © 1994 John Wiley & Sons, Inc.     

Substrate effect on the crystallization of isotactic polypropylene

The evolution of storage modulus measured by a rotational rheometer shows that the isothermal crystallization of isotactic polypropylene (iPP) melts in contact with aluminum plates (PP-Al) are considerably faster than that with stainless-steel plates (PP-SS). The difference is bigger at higher temperatures, and this behavior is opposite to that expected by our numerical simulation considering uniform bulk phase transition and substrate's ability to remove the latent heat. Polarized optical observations and surface energy evaluations via contact angle measurement indicate that surface energy of the substrates, including the effects of submicrometer morphology and roughness, should be the key factor to affect the crystallization of iPP. Transcrystallization zones, in which the nucleation density is controlled by the surface energy of substrates, were observed to grow toward the bulk with the thickness of about 0.2 mm for iPP to affect the global crystallization behavior. The critical value of surface energy of substrate to promote the interfacial crystallization of a polymer melt is derived, in terms of which the aluminum and stainless steel as well as optical glass, promote the surface nucleation with respect to the bulk nucleation of iPP. As a consequence, the conventional differential scanning calorimetry measurement mainly gives the heat fluxes of interfacial crystallization rather than the bulk crystallization due to the large surface-to-volume ratio of the specimen and the aluminum pan used which is a high surface energy substrate. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

The effect of bicyclo[2.2.1]hept‐5‐ene‐2,3‐dicarboxylate on the mechanical properties and crystallization behaviors of isotactic polypropylene

The mechanical and optical properties of iPP nucleated with bicyclo[2.2.1]heptenedicarboxylate salts (BCHED) have been investigated. The results showed that aluminum bicyclo[2.2.1]heptenedicarboxylate (BCHE13) is the most effective nucleating agent to improve the mechanical and optical properties. Then the effects of the BCHE13 concentration on mechanical and optical properties and crystallization behaviors were studied. The results indicated that the saturated concentration of BCHE13 is about 0.2 wt %, at which nucleated iPP showed the better comprehensive mechanical and optical properties and high crystallization peak temperature. Nonisothermal crystallization kinetics of iPP nucleated with different BCHE13 contents have been investigated by Caze method. The results indicated Avrami exponents of nucleated iPP gradually increased with the increasing of BCHE13 concentration. The results can be explained that crystallization and growth model of nucleated iPP is heterogeneous nuclei followed by more than three‐dimension spherical growth during nonisothermal crystallization, which can be proved by agglomeration of BCHE13 in melt iPP. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Morphology,crystallization, and thermal behavior of isotactic polypropylene/propylene‐g‐styrene blends

Optical microscopy, differential scanning calorimetry, and small angle X‐ray scattering techniques were used to study the influence of the crystallization conditions on morphology and thermal behavior of samples of binary blends constituted of isotactic polypropylene (iPP) and a novel graft copolymer of unsaturated propylene with styrene (uPP‐g‐PS) isothermally crystallized from melt, at relatively low undercooling, in a range of crystallization temperatures of the iPP phase. It was shown that, irrespective of composition, no fall in the crystallinity index of the iPP phase was observed. Notwithstanding, spherulitic texture and thermal behavior of the iPP phase in the iPP/uPP‐g‐PS materials were strongly modified by the presence of copolymer. Surprisingly, iPP spherulites crystallized from the blends showed size and regularity higher than that exhibited by plain iPP spherulites. Moreover, the amount of amorphous material located in the interspherulitic amorphous regions decreased with increasing crystallization temperature, and for a given crystallization temperature, with increasing uPP‐g‐PS content. Also, relevant thermodynamic parameters, related to the crystallization process of the iPP phase from iPP/uPP‐g‐PS melts, were found, composition dependent. The equilibrium melting temperature and the surface free energy of folding of the iPP lamellar crystals grown in the presence of uPP‐g‐PS content up to 5% (wt/wt) were, in fact, respectively slightly lower and higher than that found for the lamellar crystals of plain iPP. By further increase of the copolymer content, both the equilibrium melting temperature and surface free energy of folding values were, on the contrary, depressed dramatically. The obtained results were accounted for by assuming that the iPP crystallization process from iPP/uPP‐g‐PS melts could occur through molecular fractionation inducing a combination of morphological and thermodynamic effects. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 2286–2298, 2001     

Shear‐induced crystallization of isotactic polypropylene studied by simultaneous light intensity and rheological measurements

We investigate the effects of high shear rate and a melt‐insensitive, organophosphate nucleating agent on shear‐induced crystallization of four isotactic polypropylenes using a sliding plate rheometer under isothermal, low‐supercooling conditions. We used a bifurcated optical fiber probe to measure light intensity and a shear stress transducer to monitor the simultaneous viscoelastic response to small‐amplitude oscillatory shear. The two techniques complement each other; at early times of crystallization, large attenuation in the light intensity is observed, whereas during the later stages, a major change in the viscoelastic response occurs due to the growing volume fraction of spherulites. In contrast to quiescent crystallization, the nucleation pathway of nucleated polymers after a brief, strong shear is little influenced by the nucleating agent but strongly affected by molecular weight. The early kinetics of non‐nucleated polymers is more strongly enhanced by shear than that of nucleated polymers. Increasing either shear rate or strain accelerates crystallization, and we found the product of shear rate and strain to be useful for correlating our data. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

Investigation of the crystallization behavior of isotactic polypropylene polymerized with different Ziegler‐Natta catalysts

Detailed characterization of the crystallization behavior is important for obtaining better structure property correlations of the isotactic polypropylene (iPP), however, attributed to the complexity in ZN‐iPP polymerization, the relationship between crystallization behavior and the stereo‐defect distribution of iPP is still under debate. In this study, the crystallization kinetics of the primary nucleation, crystal growth and overall crystallization of two iPP samples (PP‐A and PP‐B) with nearly same average isotacticity but different stereo‐defect distribution (the stereo‐defect distribution of PP‐B is more uniform than PP‐A) were investigated. The results of isothermal crystallization kinetics showed that the overall crystallization rate of PP‐A was much higher than that of PP‐B; but the analysis of self‐nucleation isothermal crystallization kinetics and the polarized optical microscopy (POM) observation indicated that the high overall crystallization rate of PP‐A was attributed to the high primary nucleation rate of the resin. The stereo‐defect distribution plays an important role in determining both the nucleation kinetics and crystal grow kinetics, and thus influence the overall crystallization kinetics. A more uniform distribution of stereo‐defects restrains the crystallization rate of iPP, moreover, it has more influence on nucleation kinetics, comparing with the crystal growth. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Influence of a sorbitol‐based nucleating agent modified with silsesquioxanes on the non‐isothermal crystallization of isotactic polypropylene

This article investigates the effect of modifying the polypropylene (iPP) α‐phase nucleating agent 1,3:2,4‐bis(3,4‐dimethylbenzylidene) sorbitol (DMDBS) with tetrasilanolphenyl silsesquioxane (phPOSS). It has been proven that an increasing amount of silsesquioxane leads to differences in the crystallization behavior. What is more, it has been observed that the nucleation effect that results from the addition of sorbitol derivatives is suppressed by phPOSS activity. To understand the influence of phPOSS addition on the crystallization kinetics of PP/DMDBS/phPOSS composites that have been prepared by melt processing in a twin screw extruder, differential scanning calorimetry, rotational rheometry and Fourier transform infrared spectroscopy are performed. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40131.     

Effect of chain disentanglement on melt crystallization behavior of isotactic polypropylene

Isotactic polypropylene (iPP) with “disentangled” chains was generated through crystallization of iPP from its mineral oil solution. TGA test assured complete removal of mineral oil from iPP precipitates. Time sweep rheological measurements showed the modulus build-up with time indicating the formation of “disentangled” chains in iPP after the sample disentanglement treatment. The “disentangled” chains could preserve for a certain time before completely re-entangled during melting. Crystallization kinetics of iPP with “disentangled” chains was studied by using polarized optical microscope. The growth rate of spherulites in “disentangled” iPP was faster than that in the entangled one.     

间规聚苯乙烯/等规聚丙烯共混物的热性能

用差示扫描量热法(DSC)、动态力学分析(DMA)和热重法(TGA)等研究了间规聚苯乙烯/等规聚丙烯(sPS/iPP)、sPS与聚-1-丁烯(B30)组成的嵌段共聚物和sPS/iPP/B30共混体系的热性能。结果表明sPS/iPP是不相容体系,B30与iPP可相容,B30可作为sPS/iPP共混物的相容剂。DSC和DMA分析结果表明,加入适量的B30时,sPS/iPP/B30共混物中iPP的玻璃化转变温度随B30加入量的增加而逐渐升高,而sPS的玻璃化转变温度则随B30加入量的增加而逐渐降低;TGA分析结果表明共混体系的热失重温度高于300℃,比iPP的高。