Form Of Isotactic Polypropylene Research Articles

Crystallization Behavior of Isotactic Propene-Octene Random Copolymers

The crystallization behavior of random propene-octene isotactic copolymers (iPPC8) prepared with a homogeneous metallocene catalyst has been studied. Samples of iPPC8 with low octene content up to about 7 mol% were isothermally crystallized from the melt at various crystallization temperatures. The samples crystallize in mixtures of the α and γ forms of isotactic polypropylene (iPP). The relative amount of γ form increases with increasing crystallization temperature, and a maximum amount of γ form (fγ(max)) is achieved for each sample. The crystallization behavior of iPPC8 copolymers is compared with the crystallization from the melt of propene–ethylene, propene–butene, propene–pentene, and propene–hexene copolymers. The results show that the behavior of iPPC8 copolymers is completely different from those described in the literature for the other copolymers of iPP. In fact, the maximum amount of γ form achieved in samples of different copolymers of iPP generally increases with increasing comonomer content, while in iPPC8 copolymers the maximum amount of γ form decreases with increasing octene content. The different behaviors are discussed based on the inclusion of co-monomeric units in the crystals of α and γ forms of iPP or their exclusion from the crystals. In iPPC8 copolymers, octene units are excluded from the crystals giving only the interruption effect that shortens the length of regular propene sequences, inducing crystallization of the γ form at low octene concentrations, lower than 2 mol%. At higher octene concentration, the crystallization of the kinetically favored α form prevails.

Crystallization of Propene–Pentene Isotactic Copolymers as an Indicator of the General View of the Crystallization Behavior of Isotactic Polypropylene

The crystallization from the melt in isothermal conditions of metallocene random propene–pentene isotactic copolymers (iPPC5) has been studied. All samples with pentene concentration between 0.5 and 10 mol % crystallize at any crystallization temperature in mixtures of α and γ forms of isotactic polypropylene (iPP) and the amount of γ form increases with increasing crystallization temperature up to a maximum (fγ(max)), which depends on pentene concentration. Pentene defects produce a shortening of the regular propene sequences that in turn induces crystallization of the γ form. At concentrations higher than 6–7 mol %, pentene units are incorporated to a high extent in the crystals of α and trigonal forms, which are stabilized over the γ form, and fγ(max) decreases. The maximum fraction of γ form is, therefore, related to the average length of regular propene sequences and the degree of incorporation of defects in the crystals of α and γ forms. The values of fγ(max) that develop in iPPC5 copolymers have been compared with those that develop in copolymers of iPP with ethylene (iPPC2), butene (iPPC4), and hexene (iPPC6) and in stereoirregular iPPs reported in the literature. Stereoirregular iPPs and iPPC2 copolymers give the same relationship between fγ(max) and the average length of regular propene sequences (LiPP), whereas iPPC4, iPPC5, and iPPC6 copolymers show different behaviors. In particular, iPPC5 copolymers exhibit a behavior intermediate between those of iPPC4 and iPPC6 copolymers. The relationship between fγ(max) and LiPP of iPPC5 copolymers fits perfectly between the relationships found for iPPC4 and iPPC6 copolymers, in agreement with the different types and sizes of comonomers and the different efficiencies of their interruption and inclusion effects. These data give evidence of the general view of the crystallization behavior of iPP, based on the definition of a double role exerted by defects, the interruption effect that shortens the regular propene sequences and favors crystallization of γ form, and the effect of incorporation of defects into the crystalline unit cells of α and γ forms, which favors crystallization of the form that better accommodates the defect into crystals. The relative efficiency of these two effects depends on the type and size of the defect. The different relationships between fγ(max) and LiPP are a result of the equilibrium between interruption and inclusion effects achieved by each defect and confirm that the crystallization of γ form is a perfect indicator of the length of the regular propene sequences and may provide very detailed information on the molecular structure of iPP.

High-Pressure Crystallization of iPP Nucleated with 1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol.

1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol (DMDBS) is highly effective in nucleation of the α- form of isotactic polypropylene (iPP). However, its role in high-pressure crystallization of iPP, facilitating the formation of the γ- polymorph, has not been explored. The present paper focuses on the influence of DMDBS on nucleation of high-pressure crystallization of iPP. iPP with 0.2–1.0 wt.% of the DMDBS was crystallized under elevated pressure, up to 300 MPa, in various thermal conditions, and then analyzed by PLM, WAXD, SEM, and DSC. During cooling, crystallization temperatures (Tc) were determined. It was found that under high-pressure DMDBS nucleated crystallization of iPP in the orthorhombic γ- form. As a consequence, Tc and the γ- form content increased for the nucleated iPP, while the size of polycrystalline aggregates decreased, although the effects depended on DMDBS content. The significant increase of Tc and the decrease of grain size under high pressure of 200–300 MPa required higher content of DMDBS than the nucleation of the α-form under lower pressure, possibly due to the effect of pressure on crystallization of DMDBS itself, which is a prerequisite for its nucleating activity.

Effects of different metal salts of aliphatic dicarboxylic acids on the formation of β-crystalline form in isotactic polypropylene

The effects of barium, calcium and zinc salts of the aliphatic dicarboxylic acids on the nucleation ability and formation of β-crystalline form in isotactic polypropylene (iPP) were investigated. The results showed that barium, calcium and zinc salts of aliphatic dicarboxylic acid were related to the number of carbon atoms in the acids, and carbon atomic numbers of 5–8 had good nucleation ability for iPP except calcium adipate and zinc pimelate, in which barium glutarate, calcium suberate and zinc suberate were highly efficient β-nucleating agents for iPP. In addition, the optimal addition amounts of barium glutarate, calcium suberate and zinc suberate were 0.1 mass%, 0.1 mass% and 0.15 mass%, respectively, and the relative contents of β-crystals reached 0.9561, 0.9351 and 0.9447, respectively. The results of mechanical and thermal properties with the optimal addition amount showed that barium glutarate, calcium suberate and zinc suberate improved the toughness and thermal properties but decreased the rigidity of iPP.

Mechanical Properties and Morphology of Propene–Pentene Isotactic Copolymers

The mechanical behavior of propylene–pentene isotactic copolymers synthesized with a single-center organometallic catalyst in a broad interval of pentene (Pe) concentration, from 3 to 50 mol %, is analyzed. All copolymers show flexible behavior with very high deformation at breaking. The presence and entrance of Pe counits in α and δ forms of isotactic polypropylene (iPP) transform a strong and fragile iPP homopolymer into ductile and flexible materials. The stress at yield decreases as the Pe content increases, but a different behavior has been observed for samples in the α form or in the δ form. In samples in the α form, from 3 to 8.8 mol % of Pe, the stress at yield increases as the thickness of crystalline lamellae increases, while in samples in the δ form containing higher Pe counits, it decreases as the thickness of lamellae increases and Pe content increases. Phase transition of the α form in the mesophase and crystallization of the δ form, in samples with 8–11 mol % of Pe, occur upon deformation a...

Relationships among lamellar morphology parameters, structure and thermal behavior of isotactic propene-pentene copolymers: The role of incorporation of comonomeric units in the crystals

The correlations between the thermal behavior and the crystal morphology of isotactic propene-pentene copolymers were studied through wide-angle (WAXS) and small-angle (SAXS) X-ray diffraction. Copolymers with pentene concentration lower than 11 mol% crystallize in the α form of isotactic polypropylene (iPP) and a concomitant decrease of melting temperature and of the thickness of crystalline lamellae with increasing pentene concentration has been observed. At higher pentene concentrations the trigonal form of iPP crystallizes and a neat increase crystalline lamellar thickness and of the long period, with a slower decrease of melting temperature and crystallinity have been observed. These results have been treated in the general framework of copolymer crystallization theories, using a method proposed by Crist and correlated with the different level of inclusion of pentene co-units in the crystals of α and trigonal forms. For copolymers with pentene concentration lower than 11 mol% pentene co-units are in part incorporated in the crystals of α form. For higher pentene concentrations the decrease of melting temperature coupled with the increase of lamellae thickness with increasing comonomer content is the hallmark of the almost total inclusion of pentene co-units in the crystals of the trigonal form.

The “Nodular” α Form of Isotactic Polypropylene: Stiff and Strong Polypropylene with High Deformability

A study of the nucleating effects of isotactic poly(trimethylallylsilane) (iPTMAS) and poly(vinylcyclohexane) (iPVCH) for isotactic polypropylene (iPP) and their influence on the mechanical behavior of iPP is reported. The particles of catalyst are covered by poly(trimethylallylsilane) or poly(vinylcyclohexane) by polymerization of the corresponding monomers before polymerization of propylene. The method guarantees that the nucleating agents are perfectly dispersed into the iPP samples, with their consequent high efficiency. Isotactic poly(vinylcyclohexane) and poly(trimethylallylsilane) favor crystallization from the melt of α and β forms, respectively. Both nucleating agents affect the morphology greatly reducing the size of shperulites. This in turn affects the mechanical properties improving ductility and flexibility. All nucleated samples crystallize by rapid cooling from the melt in the α form and not in the mesophase, as occurs in the pure iPP. The resulting α form presents nodular crystals and beh...

Solid-State NMR Characterization of the Chemical Defects and Physical Disorders in α Form of Isotactic Poly(propylene) Synthesized by Ziegler–Natta Catalysts

The order–disorder phenomenon and spatial heterogeneity of chain packing, partitions of stereodefects, and molecular dynamics of α form of isotactic polypropylene (iPP) samples, which are synthesized by Zieglar–Natta catalysts, are investigated by solid-state (SS) NMR. High-resolution 13C NMR under high-power TPPM decoupling at field strengths of 110 kHz allows observation of the order–disorder phenomenon in the chain-packing structures of α form. High isotacticity samples (isotacticity at pentad level, ⟨mmmm⟩ = 99.4%) give a maximum ordered packing (α2) fraction of 66% at crystallization temperature (Tc) of 155 °C while low stereoregularity samples (⟨mmmm⟩ = 91.0%) have only 47% at the same Tc. However, Mw (58.7–982 kg/mol) does not play a significant role in ordered packing formation. Using 13C-labeled CH3 of iPP, direct spatial correlations between the α2 and α1 structures are investigated by 13C detection of two-dimensional (2D) 1H–1H spin-diffusion (CHHC) experiments. The time dependence of the spin-diffusion polarization transferred signal intensities determines the average domain size of the α1 and α2 structures of iPP crystallized at 150 °C, which was found to be 40 nm under an assumption of 2D spin diffusion. Additionally, the 13C filter CPMAS NMR spectrum on 13C CH3-labeled iPP demonstrates that chemical defect is almost excluded from the crystalline region at Tc = 150 °C (defect free crystal) while ca. 2% is in melt quench sample. Moreover, 13C centerband-only detection of exchange experiments on α2-rich sample with highest ⟨mmmm⟩ = 99.4% indicate that crystalline dynamics follows a single Arrhenius plot with an activation energy of 116 kJ/mol across reported order–disorder transition temperatures (157–159 °C).

Effect of Malonic Acid/Calcium Stearate Bicomponent Nucleator on the β-Crystalline Formation in Isotactic Poly(propylene)

Abstract The effect of a new bicomponent β-nucleator composed of malonic acid (MA) and calcium stearate (CaSt) on the formation of the β-crystalline form in isotactic polypropylene (iPP) has been investigated. It has been found that the relative content of the β-crystalline form (k value) of iPP notably increases with the addition of the MA/CaSt mixture, and it attains a maximum value of 0.94 at 0.15% MA/0.30% CaSt. For iPP nucleated with the bicomponent nucleator, the spherulitic size decreases continuously with increasing CaSt dosage. FTIR analysis has shown that an “in situ” reaction may occur between MA and CaSt during melt blending, yielding an effective β-nucleator (calcium malonate, CaMt). The β-nucleation efficiency of an MA/CaSt mixture is higher than that of CaMt itself; this may be ascribed to greater dispersion of the “in situ” produced CaMt in the iPP matrix through the assistance of the CaSt residue and the stearic acid.

Effect of Metallic Salts of Glutaric Acid on the Formation of β-crystalline Form in Isotactic Polypropylene

The effects of glutaric acid and the lithium, sodium, potassium, magnesium, calcium, strontium, barium, zinc, and cadmium salts of glutaric acid on the formation of β-crystalline form in isotactic polypropylene (iPP) at the crystallization temperatures 120 and 130°C have been investigated. It is found that glutaric acid and the sodium, potassium, and zinc salts of glutaric acid inhibit the formation of β-crystalline form in polypropylene. Lithium glutarate has a limited positive effect on the formation of β-crystalline form. Magnesium, calcium, strontium, barium salts of glutaric acid exhibit increased β-nucleation abilities with the increasing atomic radii of the combined metals. Cadmium glutarate is a good β-nucleating agent and the formation of β-crystalline form is favored at the crystallization temperature of 120°C. The increases in the orientation of b-axes of the nucleated iPP samples are due to the presences of sodium and zinc salts of glutaric acid.

Effect of a Novel Family of N,N′-Diphenyl Bisamides on the Formation of β Crystalline Form in Isotactic Polypropylene

In this paper, a family of eight N,N′-diphenyl bisamide compounds were synthesized and characterized by fourier transform infrared (FT-IR) and 1H nuclear magnetic resonance (NMR) technologies. The influence of different compounds on the formation of β crystalline form in iPP had been investigated by means of wide angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC) and polarized light microscopy (PLM). N,N′-diphenyl succinamide (DPS), N,N′-diphenyl glutaramide (DPG) and N,N′-diphenyl adipamide (DPA) are β-nucleating agents for iPP among these compounds. The results also indicate that DPS and DPA have higher ability to induce the β-form than DPG. The analysis of the cell parameters of three efficient β-nucleating agents shows that they all have approximate periodic length of b-axis of 0.65 nm. And it is also suggested that two-dimensional lattice matching relationships exist between the crystal structures of the β-nucleating agents and that of β-iPP.

Stress-Induced Polymorphic Transformations and Mechanical Properties of Isotactic Propylene-Hexene Copolymers

A study of the relationships between the stress-induced phase transitions and the mechanical properties of isotactic propylene-hexene copolymers with hexene concentration in the range 1−26 mol%, prepared with metallocene catalysts, is reported. Hexene units are included in crystals of α form of isotactic polypropylene (iPP) and produce large disturbance of the crystalline lattice and a consequent decrease of melting temperature, degree of crystallinity, crystallite size, and plastic resistance of the crystals. Defective crystals of α form rapidly transform by stretching into the mesomorphic form of iPP that, in turn, facilitates further stretching up to very high values of deformation of nearly 900−1000%, resulting in high flexibility. This explains the experimental observation that the presence of hexene comonomeric units induces a strong enhancement of ductility, flexibility, and toughness, compared to the highly stereoregular homopolymer prepared with the same catalyst. These copolymers show mechanical properties of highly flexible materials with values of the tensile strength, elastic modulus, and resistance to the plastic deformation that depend on the degree of crystallinity and the occurrence of phase transitions during deformation and can be easily tailored by changing the hexene concentration.

Effect of N,N′‐diphenyl adipamide on the formation of the β‐crystalline form in isotactic polypropylene

AbstractThe polymorphic compositions and mechanical properties of isotactic polypropylene (iPP) samples nucleated by a selective β‐nucleating agent [N,N′‐diphenyl adipamide (DPA)] were investigated with wide‐angle X‐ray diffraction, polarized light microscopy, scanning electron microscopy, and mechanical tests. It was found that β‐phase crystals emerged with the addition of DPA, and the relative proportion of the β‐crystalline form reached the maximum value of 0.97 with the addition of 0.1 wt % DPA. The curved lamellae in the β spherulites were like flowers. The β spherulites were etched more easily than α spherulites because amorphous regions were distributed inside the β spherulites. The Izod notched impact strength increased sharply with the addition of DPA and attained the maximum value of 7.30 kJ/m2 (the value of blank iPP was 3.13 kJ/m2) with the addition of 0.1 wt % DPA. An analysis of the misfit factors between DPA and β‐iPP showed that β‐iPP could epitaxially crystallize on the DPA crystal well. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Effect of Metallic Salts of Malonic Acid on the Formation of β Crystalline Form in Isotactic Polypropylene

The effects of malonic acid and the lithium, sodium, potassium, zinc, magnesium, calcium, strontium, and barium salts of malonic acid on the formation of β crystalline form in isotactic polypropylene at the crystallization temperatures 120 and 130°C have been investigated. It was found that malonic acid and the lithium, sodium, and potassium salts of malonic acid inhibit the formation of β crystalline form in polypropylene. Zinc malonate has a limited positive effect on the formation of β crystalline form, while magnesium, calcium, strontium, and barium salts are β nucleating agents, in descending order. The decreased β nucleation abilities of the alkaline earth metallic salts of malonic acid are attributed to the increasing atomic radii of the combined metals and decreasing crystallization temperatures.

The β - and γ -Form Distributions of Isotactic Polypropylene Injection Molding Controlled via Melt Vibration

The effect of melt vibration on the crystal form of isotactic Polypropylene (iPP) samples prepared by low-frequency Vibration-Assisted Injection Molding (VAIM) was investigated. The distribution of crystal forms through the thickness was investigated under different VAIM processing conditions, with Conventional Injection Molding (CIM) for comparison. Wide-Angle X-ray Diffraction (WAXD) showed that the content of β -form crystals decreases from the surface to the core region in the CIM and VAIM samples. The β -form distribution through the thickness is controlled by the VAIM processing conditions. The occurrence of γ -form crystals was found in the VAIM samples produced at relatively low vibration frequency and relatively large pressure vibration amplitude.

Effect of Calcium Salts of Glutaric Acid and Pimelic Acid on the Formation of β Crystalline Form in Isotactic Polypropylene

The effect of calcium glutarate (Cagt) and calcium pimelate (Capt) on the formation of β crystalline form in isotactic polypropylene (iPP) after isothermal crystallization at 130°C for 30 min has been investigated. The maximum KWAXD and KDSC values were 31.97% and 38.06%, respectively, for iPP doped with 0.2 wt.% Cagt. The maximum values of KWAXD and KDSC were 73.60% and 68.44% for iPP doped with 0.3 wt.% and 0.15 wt.% Capt, respectively. The crystal size of iPP doped with Capt is finer than that of iPP doped with Cagt. It is shown that the β nucleation ability of Capt is superior to that of Cagt. The difference in the β nucleation ability between Cagt and Capt is explained by the structural difference of the nucleators with β-iPP.

Effect of calcium malonate on the formation of β crystalline form in isotactic poly(propylene)

AbstractCalcium malonate (Camt) is used to induce the β crystalline form in isotactic polypropylene (iPP) for the first time. The relative content of the β crystalline form (K value) increases markedly with the addition of Camt and attains the maximum value of 73.48% at 0.40% Camt. The amounts of bright and colorful β spherulites increase and the spherulitic sizes decrease with the increase in Camt contents. The tensile strength and the Izod notched impact strength of the nucleated iPP samples increase with the addition of Camt; in particular, the latter almost doubles at 0.40% Camt compared to that of the blank iPP sample. The β nucleation mechanism of Camt is analyzed by the dimensional lattice match criterion. Copyright © 2008 John Wiley & Sons, Ltd.

Effect of magnesium malonate on the formation of the β crystalline form in isotactic polypropylene

AbstractMagnesium malonate (MgMt) is used to induce the β crystalline form in isotactic polypropylene (iPP) for the first time. It is shown by wide angle X‐ray diffraction, differential scanning calorimetry, and polarized light microscopy that the content of the β crystalline form increases markedly with the addition of MgMt and attains the maximum value at an MgMt content of 0.10wt%. The tensile strength and flexural modulus of the doped iPP sample decrease somewhat; the tensile strain at break increases slightly; and the Izod notched impact strength increases significantly with the addition of MgMt, as compared with the values for the blank iPP sample. The β‐nucleation mechanism of MgMt is analyzed by the dimensional lattice match criterion. J. VINYL ADDIT. TECHNOL., 2008. © 2008 Society of Plastics Engineers.

Temperature Dependence of Crystal Growth Rate for α and β Forms of Isotactic Polypropylene

The crystal growth rate of the α1, α2 and β forms of isotactic polypropylene (iPP) and their morphological changes were studied in a wide range of crystallization temperatures. The temperature dependence of the crystal growth rate of the α1 form showed a bell-shaped curve with the maximum growth rate (Gmax) at 70 °C. The crystal growth rate of the β form induced by calcium pimelate also showed a bell-shaped curve with Gmax at 79 °C. Two crossover points on the growth rate curve from the α1 form to the β form or vice versa were observed at 90 °C and 133 °C, respectively. Tear-drop shaped spherulites were observed at temperatures between the two crossover points. Electron diffraction patterns showed the high ordered structure of the α2 form for thin films crystallized at 140 °C. The α2 fraction became detectable in X-ray diffraction pattern for temperatures above 110 °C. The α2 fraction increased with crystallization temperature and saturated to 100% at about 140 °C. In the temperature region from 110 °C to 140 °C, several crystal growth rates were found at the same crystallization temperature. The growth rate variations could be associated with the co-crystallization of the α1 and α2 forms. Above 140 °C, the growth rate variations disappeared, since the fraction of the α2 reached to 100%. The extrapolated Gmax of the α2 showed at 72 °C.

Effect of the composition ratio of pimelic acid/calcium stearate bicomponent nucleator and crystallization temperature on the production of β crystal form in isotactic polypropylene

AbstractThe influence of the composition ratio of pimelic acid/calcium stearate bicomponent nucleator on the β crystal form content of isotactic polypropylene (iPP) had been studied at the crystallization temperature of 120°C and duration of 30 min. It was found that the β crystal form content increased continuously with increasing amount of calcium stearate at the constant amount of 0.15% pimelic acid. High β crystal form content polypropylene could be produced when the amount of calcium stearate was greater than 0.30% (the mass composition ratio of pimelic acid/calcium stearate was less than 1/2, the mole ratio was less than 1.89/1). It was shown that pimelic acid and calcium stearate could react to produce a high effective β nucleator (calcium pimelate) “in situ” during the melt‐mixing of iPP and the bicomponent nucleator. The influence of crystallization temperatures (100–140°C) on the β crystal form content of iPP had also been studied at the constant composition ratio of 0.15% pimelic acid/0.5% calcium stearate (the calcium pimelate produced in situ was 0.16%, which was calculated from stoichiometry). It was found that the β crystal form content increased continuously with increasing crystallization temperature and it maximized at 130°C. β Crystal form content decreased sharply at the crystallization temperature of 140°C. It was shown that β → α modification transformed between 130 and 140°C. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008