Inelastic neutron scattering from isotactic polypropylene

We used inelastic neutron scattering to probe the low‐energy excitations in semicrystalline isotactic polypropylenes with different degrees of crystallinity. The contributions from the amorphous and crystalline regions to the total scattering intensity were extracted under the assumption of a weighted linear contribution of the two regions in a simplified two‐phase system. The resulting intensity from the amorphous region showed a peak at 1.2 meV that was in good agreement with the previously determined boson peak characteristic of atactic polypropylene. The possibility of a contribution to the boson peak region by longitudinal acoustic mode modes that are characteristic of semicrystalline polymers and appear in the same low‐frequency region is discussed. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2852–2859, 2001     

Nucleation of isotactic polypropylene crystallization by gold nanoparticles

Nucleation of isotactic polypropylene (iPP) crystallization by gold (Au) nanoparticles was studied. Regardless of their size, 4.3, 8.8, 28.3, and 84.5 nm, all particles were able to nucleate spherulites when deposited on the iPP surface. However, when added and melt‐mixed with iPP, only the smallest particles affected significantly the iPP bulk crystallization. Au nanoparticles larger than 4.3 nm, at the concentration of 0.001 wt %, did not influence the crystallization of iPP. Contrary to this, 0.001 and 0.005 wt % of Au nanoparticles having the size of 4.3 nm increased crystallization temperature of the iPP by 7–8 °C and decreased markedly the sizes of polycrystalline aggregates. Aggregation of Au nanoparticles in the polymer matrix was evidenced by electron microscopy and contributed to their decreased effectiveness in the nucleation of iPP crystallization. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 469–478, 2010     

The thermal expansion of the monoclinic unit cell of isotactic polypropylene

The lattice parameters of a highly stereoregular metallocene polypropylene crystallized at 145°C were obtained after cooling and heating cycles in a temperature interval between 25°C and 165°C. The b dimension undergoes a large thermal expansion with temperature (0.6 Å) while the change of the a axis is relatively small (0.1 Å). The unit cell dimension along the molecular (c) axis appears less sensitive to temperature than are the intermolecular distances. The difference in dimensions between the a and c axis at low and high crystallization temperatures is small, varying from 2.3 to 3.5%. This small difference allows the formation of daughter, crosshatched lamellae in the complete interval of crystallization temperatures. The thermal expansion coefficient of the unit cell specific volume is also reported. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2945–2949, 1997     

Homogeneously Dispersed Poly(propylene)/SiO2 Nanocomposites with Unprecedented Transparency

Summary: Physico‐chemical interactions between hydrophobic polyolefin materials and hydrophilic inorganic nanoparticles such as surface‐hydroxylated SiO₂ have not been well understood so far. In this study, the effects of particle size and content of SiO₂ nanoparticles on isothermal growth rate of PP spherulites in various PP/SiO₂ nanocomposites were investigated by polarized optical microscopy. It was unexpected to find that hydrophilic SiO₂ nanoparticles can be homogeneously dispersed in a PP matrix. Spherulite growth rates of PP in PP/SiO₂ nanocomposites decrease significantly with increasing SiO₂ content and decreasing particle size. Most interestingly, the spherulite growth rate was zero for PP/16 nm‐SiO₂ nanocomposites with SiO₂ content above 2.5 wt.‐% resulting in a highly transparent film.

Photographs of 200 µm‐thick PP and PP/16 nm‐SiO₂ (5 wt.‐%) sheets in front of a graphic pattern.     

Synthesis of polypropylene block copolymers from brominated styrene-terminated isotactic polypropylene

Isotactic polypropylene block copolymers, isotactic-polypropylene-block-poly (methyl methacrylate) (i-PP-b-PMMA) and isotactic-polypropylene-block-polystyrene (i-PP-b-PS), were prepared by atom transfer radical polymerization (ATRP) using a brominated styrene-terminated isotactic polypropylene macroinitiator synthesized from bromination of styrene-terminated isotactic polypropylene. The styrene-terminated isotactic polypropylene can be obtained by polymerization of propylene in the presence of styrene and hydrogen chain transfer agents using a rac-Me₂Si[2-methyl-4-(1-naphyl)Ind]₂ZrCl₂ as catalyst. The molecular weights of isotactic polypropylene block copolymers were controlled by altering the amount of hydrogen used in the polymerization of propylene and the amount of monomer used in the blocking reaction. The effect of i-PP-b-PS block copolymer on PP-PS blends and that of i-PP-b-PMMA block copolymer on PP-PMMA blends were studied by scanning electron microscopy.     

Step‐like melting mechanisms of isothermally crystallized isotactic polypropylene

The complex melting behavior of isotactic polypropylene, after isothermal crystallization, was studied within the context of step‐like melting mechanisms which were previously proposed for high temperature polymers. The morphological characteristics of the melting process were also studied as a function of molecular weight, and close similarities were observed with respect to high temperature polymers. Positive birefringence crystals of low molecular weight samples developed double melting behavior in three steps. The first melting step was assigned to continuous melting of secondary crosshatch reversing lamellae, together with recrystallization of the remaining isothermal crystals. In the second melting step (first melting endotherm), crystals tended to lose their original coarse negative birefringence due to melting of secondary reversing branching. This effect rendered new, finer texture, but still negative birefringence crystals. In the third melting step (second melting endotherm), there was a combination of melting of two crystal populations, one consisting of the remaining fraction of reversing primary crystals, and the other consisting of nonreversing primary crystals. A crosshatch secondary branching model was therefore proposed to explain the overall results. Mixed birefringence spherulites of high molecular weight samples displayed similar, although proportional, behavior under identical crystallization and melting conditions corroborating the proposed melting mechanism. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2188–2200, 2008     

Exclusion effect of carbon dioxide on the crystallization of polypropylene

We investigated the crystallization growth of isotactic polypropylene under carbon dioxide (CO₂) at various CO₂ pressures and temperatures by in situ observation with a digital high‐fidelity microscope and a specially designed high‐pressure visualized cell. The fibrils within the spherulite were distorted and branched by crystallization under CO₂ at pressures higher than 2 MPa, and this suggested the exclusion of CO₂ from the growth front of the fibrils. The spherulite growth rate (G) at 140 °C increased with the CO₂ pressure, attained a maximum value around 0.3 MPa, and then decreased. Above 6 MPa, it became slower than that under air at the ambient pressure. An analysis of the crystallization kinetics by the Hoffman–Lauritzen theory revealed that the pressure dependence of G could be ascribed to the change in the transportation rate of crystallizable molecules (β_g) with pressure; that is, β_g increased and then decreased with pressure. The increase in β_g at a low pressure was caused by the plasticizing effect of CO₂, whereas the decrease in β_g at a high pressure was due to the exclusion of CO₂ from the crystal growth front. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1565–1572, 2004     

Specificity and versatility of nucleating agents toward isotactic polypropylene crystal phases

Nucleating agents with an ≈6.5 Å lattice parameter induced the α phase of isotactic polypropylene (iPP, α‐iPP). A 6.5 Å periodicity is also involved in the nucleating agents for the β phase of iPP (β‐iPP). The similarity in substrate periodicities suggests that some nucleating agents may induce either the α or β phase under different crystallization conditions. 4‐Fluorobenzoic acid, dicyclohexylterephthalamide, and γ‐quinacridone (the latter two are known as β‐iPP nucleators) were tested over a wide range of crystallization temperatures [up to crystallization temperature (T_c) > 145 °C]. The two former nucleating agents induce exclusively α‐iPP and β‐iPP, respectively. γ‐Quinacridone on the contrary is a versatile agent with respect to the crystal phase generated. More specifically, the same crystal face of γ‐quinacridone induces either β‐iPP or α‐iPP when T_c is below or above ≈140 °C. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2504–2515, 2002     

Isothermal crystallization behaviors of isotactic polypropylene nucleated with α/β compounding nucleating agents

Sodium 2,2′‐methylene‐bis(4,6‐di‐tert‐butylphenyl) phosphate (NA40) and N,N‐dicyclohexylterephthalamide (NABW) are high effective nucleating agents for inducing the formation of α‐isotactic polypropylene (α‐iPP) and β‐iPP, respectively. The isothermal crystallization kinetics of iPP nucleated with nucleating agents NABW, NA40/NABW (weight ratio of NA40 to NABW is 1:1) and NA40 were investigated by differential scanning calorimetry (DSC) and Avrami equation was adopted to analyze the experimental data. The results show that the addition of NABW, NA40/NABW and NA40 can shorten crystallization half‐time (t_1/2) and increase crystallization rate of iPP greatly. In these three nucleating agents, the α nucleating agent NA40 can shorten t_1/2 of iPP by the largest extent, which indicates that it has the best nucleation effect. While iPP nucleated with NA40/NABW compounding nucleating agents has shorter t_1/2 than iPP nucleated with NABW. The Avrami exponents of iPP and nucleated iPP are close to 3.0, which indicates that the addition of nucleating agents doesn't change the crystallization growth patterns of iPP under isothermal conditions and the crystal growth is heterogeneous three‐dimensional spherulitic growth. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 590–596, 2007     

Transport properties of organic vapors in nanocomposites of isotactic polypropylene

Isotactic polypropylene nanocomposites were obtained by the melt blending of polypropylene‐graft‐maleic anhydride and organophilic layered silicate (OLS) consisting of synthetic fluorohectorite modified by cation exchange with protonated octadecylamine. The composition of the inorganic clay was varied between 2.5 and 10 wt %, and films of the composites were obtained via hot‐press molding. X‐ray analysis showed that nanocomposites in which silicate layers were either delaminated or ordered as in an intercalated structure were obtained. The elastic modulus of the samples was higher than that of the pure polymer over a wide temperature range and increased with increasing inorganic content. The transport properties, sorption and diffusion, were measured for two organic vapors, dichloromethane and n‐pentane. For both vapors, the sorption was not very different from that of the pure polymer, whereas the zero‐concentration diffusion parameter strongly decreased with increasing OLS content. Therefore, the permeability, that is, the product of sorption and diffusion, decreased for both vapors as a result of the decreased value of the diffusion parameter. The decrease was higher for the less interacting n‐pentane. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1798–1805, 2003     

Melting behavior in blends of isotactic polypropylene and a liquid crystalline polymer

The influence of low contents of a liquid crystalline polymer on the crystallization and melting behavior of isotactic polypropylene (iPP) was investigated using electron and optical microscopy, differential scanning calorimetry, and X-ray diffraction. In pure iPP, the α modification was found, whereas for iPP/Vectra blends at Vectra concentration <5%, both α and β forms were observed. The amount of β phase varied from 0.23 to 0.16. Optical microscopy showed that Vectra was able to nucleate both α and β forms. Non-isothermal crystallization produces a material with a strong tendency for recrystallization of the α and β forms (αα′ and ββ′ recrystallization) leading to double endotherms for both crystalline forms in DSC thermograms. Melting thermograms after isothermal crystallization at low temperatures showed a similar behavior. At values of T_c > 119 °C for the α form and T_c > 125 °C for the β form, only one melting endotherm was observed because enough perfect crystals, not susceptible to recrystallization, were obtained. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1949–1959, 2004     

Changes in the melting behavior with the radial distance in isotactic polypropylene spherulites

The melting of highly tactic i‐polypropylene occurs in two stages even for crystallization at 145 °C, a temperature at which reorganization during scanning is negligible. A comparison of two such polypropylenes, one nucleated and the other not nucleated, together with fractions from the latter, has been made with electron microscopy following permanganic etching, in addition to differential scanning calorimetry. This has allowed the two melting stages to be assigned to two components of the lamellar morphology, with progressive changes in both occurring with increasing radial distance within a spherulite. The highest melting temperature is for dominant radial lamellae far from a spherulite center. The lowest melting regions are the evenly crosshatched spherulite centers and a narrow peripheral band. Lower melting is attributed to the suppression of isothermal lamellar thickening paralleling recent direct demonstrations in polyethylene. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2342–2354, 2003     

The discovery of isotactic polypropylene and its impact on pure and applied science

This article recalls some aspects of the fascinating history of the discovery by Giulio Natta and his research group of the stereoselective polymerization of propylene and the understanding of the relationships between structure and properties of semicrystalline polymers. The impact of the discovery of isotactic polypropylene and stereoregular polymers on pure and applied science is briefly outlined. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 391–395, 2004     

Crystallinity changes in plastically deformed isotactic polypropylene evaluated by x‐ray diffraction and differential scanning calorimetry methods

The crystallinity of isotactic polypropylene (iPP), when deformed with plastic plane‐strain compression, was studied with wide‐angle X‐ray scattering (WAXS) and differential scanning calorimetry (DSC) techniques. A comparison of the obtained crystallinity data with annealed iPP samples was performed. The material used in this study was commercial iPP (weight‐average molecular weight = 117.400 g/mol; number‐average molecular weight = 17.300 g/mol). A significant decrease in the crystallinity was observed with increasing deformation pressure when the X‐ray method was employed, whereas only a small decrease was registered when the DSC method of crystallinity determination was used. However, the annealed iPP samples demonstrated a slight crystallinity increase when evaluated by both techniques. The reason for the difference between WAXS and DSC crystallinity results is discussed. This study of iPP specimens subjected to large deformation led us to the conclusion that the WAXS method provides accurate crystallinity values for the deformed material, whereas the values obtained by the DSC method do not reproduce the real crystallinity of the deformed material. This is due to the inherent heating process of the method, which causes a relaxation process and a significant change in the crystallinity of the deformed material, providing values nearer to its intrinsic equilibrium state. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 896–903, 2002     

Effect of a sorbitol nucleating agent on fractionated crystallization of polypropylene droplets

The effect of a sorbitol nucleating agent on crystallization of polypropylene (PP) in droplets was studied. Layer‐multiplying coextrusion was used to fabricate assemblies of 257 layers, in which PP nanolayers alternated with thicker polystyrene (PS) layers. The concentration of a commercial nucleating agent, Millad 3988 (MD) in the layers was varied up to 2 wt %. When the assembly was heated into the melt, interfacial driven breakup of the 12 nm PP layers produced a dispersion of submicron PP particles in a PS matrix. Analysis of optical microscope images and atomic force microscope images indicated that the particle size was not affected by the presence of MD. The crystallization behavior of the particle dispersion was characterized by thermal analysis. In the absence of a nucleating agent, the submicron particles crystallized almost exclusively by homogeneous nucleation at about 40 °C. Addition of a nucleating agent to the PP layers offered a unique opportunity to study the nature of heterogeneous nucleation. Nucleation by MD resulted in fractionated crystallization of the submicron PP particles. The concentration dependence of the multiple crystallization exotherms was interpreted in terms of the binary polypropylene‐sorbitol phase diagram. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1788–1797, 2007     

Synthesis of polypropylene graft copolymers by the combination of a polypropylene copolymer containing pendant vinylbenzene groups and atom transfer radical polymerization

This article discusses a facile and inexpensive reaction process for preparing polypropylene‐based graft copolymers containing an isotactic polypropylene (i‐PP) main chain and several functional polymer side chains. The chemistry involves an i‐PP polymer precursor containing several pendant vinylbenzene groups, which is prepared through the Ziegler–Natta copolymerization of propylene and 1,4‐divinylbenzene mediated by an isospecific MgCl₂‐supported TiCl₄ catalyst. The selective monoenchainment of 1,4‐divinylbenzene comonomers results in pendant vinylbenzene groups quantitatively transformed into benzyl halides by hydrochlorination. In the presence of CuCl/pentamethyldiethylenetriamine, the in situ formed, multifunctional, polymeric atom transfer radical polymerization initiators carry out graft‐from polymerization through controlled radical polymerization. Some i‐PP‐based graft copolymers, including poly(propylene‐g‐methyl methacrylate) and poly(propylene‐g‐styrene), have been prepared with controlled compositions. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 429–437, 2005     

Transient and athermal effects in the crystallization of polymers. I. Isothermal crystallization

The isothermal crystallization of poly(propylene) and poly(ethylene terephthalate) was investigated with differential scanning calorimetry and optical microscopy. It was found that the induction time depends on the cooling rate to a constant temperature. The isothermal crystallization of the investigated polymers is a complex process and cannot be adequately described by the simple Avrami equation with time‐independent parameters. The results indicate that crystallization is composed of several nucleation mechanisms. The homogeneous nucleation occurring from thermal fluctuations is preceded by the nucleation on not completely melted crystalline residues that can become stable by an athermal mechanism as well as nucleation on heterogeneities. The nucleation rate depends on time, with the maximum shortly after the start of crystallization attributed to nucleation on crystalline residues (possible athermal nucleation) and on heterogeneities. However, the spherulitic growth rate and the exponent n do not change with the time of crystallization. The time dependence of the crystallization rate corresponds to the changes in the nucleation rate with time. The steady‐state crystallization rate in thermal nucleation is lower than the rate determined in a classical way from the half‐time of crystallization. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1835–1849, 2002     

In‐Reactor Compounding Metallocenic Isotactic Poly(propylene)/Nucleation Agent Compositions by Employing the Nucleation Agent as a Catalyst Support

A nucleation agent, 1,3,2,4‐dimethylbenzylidene sorbitol (DMBS), was used as a support for C₂‐symmetric metallocene rac‐Me₂Si[2‐Me‐4‐Naph‐Ind]₂ZrCl₂. Propylene polymerization with the supported catalyst resulted in i‐PP polymers with granular morphology. The role of a catalyst support ensures a good dispersion of the nucleation agent in the formed i‐PP matrix. The employment of a nucleation agent as catalyst support provides a unique solution to in‐reactor compounding metallocenic i‐PP/nucleation agent compositions.

     

Synthesis of octahedral zirconium complex bearing [NHC?O] ligands,and its behavior as catalyst in the polymerization of olefins

The synthesis and characterization of a zirconium complex, having two alkoxide functionalized N‐heterocyclic carbene ligands, and its behavior as catalyst in the polymerization of ethylene and propylene, have been reported. NMR analysis showed that more than one species was obtained during synthesis. These data were confirmed by ethylene polymerization that gave rise to a linear polyethylene having a high Molecular weight and a polydispersity index (MDI) > 2 and often bimodal. The same catalytic system was able to produce highly isotactic polypropylene together with an atactic fraction. DFT studies on the complex stereoisomer stability gave indications on the species possibly involved in the polymerizations. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011