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 morphology of the γ (triclinic) phase of isotactic polypropylene and its relation to the α phase

γ-phase crystals of isotactic polypropylene (iPP) obtained from low-molecular-weight extracts of pyrolyzed polymers are examined by electron microscopy and electron diffraction. γ-phase crystals differ from α-phase crystals in three important respects: (i) they are elongated along the b^* rather than the a^* axis, (ii) the chain axis is inclined at 50° to the lamellar surface (indexed as 101) rather than normal to it, and (iii) they show screw dislocations, while α crystals do not. γ crystals are nucleated on the lateral (010) faces of a α crystals; the b_α and b axes are parallel. Virtually no nucleation of the α phase takes place on the γ phase, which is therefore not involved in the repetitive lamellar branching leading to iPP quadrites. Crystallization of the γ phase appears to be favored by or linked to the absence of chain folds and may be involved in the macroscopic curvature of iPP branches.     

Structure and mechanical properties of talc‐filled blends of polypropylene and styrenic block copolymers

The structure–property relationships of isotactic polypropylene (iPP)/styrenic block copolymer blends filled with talc were examined by optical and scanning electron microscopy, wide‐angle X‐ray diffraction, and tensile‐ and impact strength measurements. The composites were analyzed as a function of the poly(styrene‐b‐ethylene‐co‐propylene) diblock copolymer (SEP) and the poly(styrene‐b‐butadiene‐b‐styrene) triblock copolymer (SBS) content in the range from 0 to 20 vol % as elastomeric components and with 12 vol % of aminosilane surface‐treated talc as a filler. Talc crystals incorporated in the iPP matrix accommodated mostly plane‐parallel to the surface of the samples and strongly affected the crystallization process of the iPP matrix. The SBS block copolymer disoriented plane‐parallel talc crystals more significantly than the SEP block copolymer. The mechanical properties depended on the final phase morphology of the investigated iPP blends and composites and supermolecular structure of the iPP matrix because of the interactivity between their components. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1255–1264, 2004     

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     

The β‐crystalline form of isotactic polypropylene in blends of isotactic polypropylene and polyamide‐6/clay nanocomposites

Blends of isotactic polypropylene and polyamide‐6/clay nanocomposites (iPP/NPA6) were prepared with an internal batch mixer. A high content of the β‐crystalline form of isotactic polypropylene (β‐iPP) was observed in the injection‐molded samples of the iPP/NPA6 blends, whereas the content of β‐iPP in the iPP/PA6 blends and the iPP/clay composite was low and similar to that of neat iPP. Quiescent melt crystallization was studied by means of wide‐angle X‐ray diffraction, differential scanning calorimetry, and polarized optical microscopy. We found that the significant β‐iPP is not formed during quiescent melt crystallization regardless of whether the sample used was the iPP/NPA6 blend or an NPA6 fiber/iPP composite. Further characterization of the injection‐molded iPP/NPA6 revealed a shear‐induced skin–core distribution of β‐iPP and the formation of β‐iPP in the iPP/NPA6 blends is related to the shear flow field during cavity‐filling. In the presence of clay, the deformation ability of the NPA6 domain is decreased, as evidenced by rheological and morphological studies. It is reasonable that the enhanced relative shear, caused by low deformability of the NPA6 domain in the iPP matrix, is responsible for β‐iPP formation in the iPP/NPA6 blends. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3428–3438, 2004     

Molecular weight dependency of crystallization and melting behavior of β‐nucleated isotactic polypropylene

Compounds of isotactic polypropylene (iPP) and β‐nucleating agent were used to investigate the relationship between the development of β phase and molecular weight in iPP under quiescent crystallization conditions by using wide angle X‐ray diffraction and differential scanning calorimetry techniques. In all cases, the dependency of the formation of β phase in iPP on molecular weight of iPP at a defined crystallization temperature range was found. The iPP with high molecular weight possessed a wide range of crystallization temperature in inducing rich β phase. However, poor or even no β phase was obtained for the samples with low molecular weight in the same range. In addition, an upper critical crystallization temperature of producing dominant β phase was found at 125 °C. Beyond this temperature, a phenomenon of prevailing α phase became obvious. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1301–1308     

Effects of microparticles on isotactic polypropylene: Thermomechanical and thermal properties

Five types of melamine-formaldehyde microcapsules, that is, with shells of different compositions [melamine formaldehyde shell or a melamine formaldehyde-poly(hexamethylene adipate glycol) shell] and containing or not a flame retardant, diammonium hydrogen phosphate (DAHP), have been prepared by an in situ polymerization method and have been added to an isotactic polypropylene matrix (iPP) by melt blending at 5 wt %. Wide-angle X-ray diffraction and differential scanning calorimetry were employed to investigate the crystallization behavior of the prepared iPP/microparticles composites. The tensile properties and the thermal stability were also evaluated. It was stated that the morphology and the shape of the microparticles not only influence the crystallization behavior but also the thermomechanical properties of these composites. Thus, rougher microparticles act as a nucleating agent for the iPP, and in the presence of microparticles containing DAHP, the α- and the β-crystals are formed. Moreover, the presence of microparticles improves the thermal stability of the iPP. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2566–2576, 2008     

Morphology evolution of polypropylene in immiscible polymer blends for fabrication of nanofibers

Immiscible blends of cellulose acetate butyrate (CAB) and isotactic polypropylenes (iPPs) with different melting index were extruded through a two‐strand rod die. The extrudates were hot‐drawn at the die exit at different draw ratios by controlling the drawing speed. The morphologies of iPP fibers extracted from the as‐obtained extrudates after removal of CAB by acetone were investigated by scanning electron microscopy. The influences of draw ratio, viscosity ratio, and composition ratio of CAB/iPP on the morphology evolution of iPP phase into nanofibers in the immiscible blends were studied. It was found that the thermoplastic iPP nanofibers were formed from the elongation of iPP ellipsoids, end‐to‐end merging of elongated iPP microfibers, and the size decrease of iPP microfibers in the processes of extrusion and drawing. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 921–931, 2010     

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     

Nylon 6 9 can crystallize with hydrogen bonding in two and in three interchain directions

Nylon 6 9 has been shown to have structures with interchain hydrogen bonds in both two and in three directions. Chain-folded lamellar crystals were studied using transmission electron microscopy and sedimented crystal mats and uniaxially oriented fibers studied by X-ray diffraction. The principal room-temperature structure shows the two characteristic (interchain) diffraction signals at spacings of 0.43 and 0.38 nm, typical of α-phase nylons; however, nylon 6 9 is unable to form the α-phase hydrogen-bonded sheets without serious distortion of the all-trans polymeric backbone. Our structure has c and c^* noncoincident and two directions of hydrogen bonding. Optimum hydrogen bonding can only occur if consecutive pairs of amide units alternate between two crystallographic planes. The salient features of our model offer a possible universal solution for the crystalline state of all odd–even nylons. The nylon 6 9 room-temperature structure has a C-centered monoclinic unit cell (β = 108°) with the hydrogen bonds along the C-face diagonals; this structure bears a similarity to that recently proposed for nylons 6 5 and X3. On heating nylon 6 9 lamellar crystals and fibers, the two characteristic diffraction signals converge and meet at 0.42 nm at the Brill temperature, T_B · T_B for nylon 6 9 lamellar crystals is slightly below the melting point (T_m), whereas T_B for nylon 6 9 fibers is ≅ 100°C below T_m. Above T_B, nylon 6 9 has a hexagonal unit cell; the alkane segments exist in a mobile phase and equivalent hydrogen bonds populate the three principal (hexagonal) directions. A structure with perturbed hexagonal symmetry, which bears a resemblance to the reported γ-phase for nylons, can be obtained by quenching from the crystalline growth phase (above T_B) to room temperature. We propose that this structure is a “quenched-in” perturbed form of the nylon 6 9 high-temperature hexagonal phase and has interchain hydrogen bonds in all three principal crystallographic directions. In this respect it differs importantly from the γ-phase models. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1153–1165, 1998     

Synthesis and ferromagnetic properties of composites of a water-soluble polyaniline copolymer containing iron oxide

Composites of water-soluble conducting polyaniline copolymers, poly(aniline-co-aminobenzenesulfonic acid) (PAOABSA), containing γ-Fe₂O₃magnetic particles with nanometer size, were synthesized by a chemical method. The ferromagnetic properties of the resulting PAOABSA composites were measured as a function of the pH value of the reaction solution, the sulfonated degree of the copolymer, and the concentration of FeCl₂. The structure of the composites was characterized by means of elemental analysis, FTIR, XPS, and X-ray diffraction. It was found that increasing the pH value of the reaction solution and the concentration of FeCl₂ is favorable for an enhancement of the saturated magnetization. As high as 33.2 emu/g of saturated magnetization for the PAOABSA composites was observed. No hysteresis loop (i.e. H_c = 0) was observed, which is independent of the preparation conditions. Structure characterizations show that iron oxide existing in the composite is mainly γ- Fe₂O₃, which is responsible for the ferromagnetic properties of the PAOABSA composites, whereas γ- Fe₂O₃ magnetic particles nanometer in size (∼85 nm) may be attributed to a lower coercive force (i.e. H_c = 0) of the composites. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 2749–2755, 1998     

Transition of nylon 6 γ-phase crystals by stretching in the chain direction

A crystal transition was found in nylon 6 fibers from the γ-phase to α-phase on stretching in the chain direction. The γ-phase fiber prepared by iodine treatment was stretched under constant load and the crystal deformation was observed by an x-ray method. The critical stress for the transition was estimated as 4 × 10³ kg./cm.² at room temperature. For this crystal transition the following conditions must be satisfied: (1) extension of the γ-phase chain to untwist the chain around the amide groups, (2) translational mobility of the chain to change the stacking in the crystallite. At the critical stress, the chain in the crystal is extended to nearly the same length as that of α-phase. The translational movement occurs under stress higher than about 3 × 10³ kg./cm.², and the pseudohexagonal cell is deformed into a monoclinic form. However, the monoclinic crystallites present at a stress lower than the critical value estimated above are unstable and may be brought back to the original form by head treatment at 100°C. No crystal transition occurs at low temperature.     

Crystallization behavior of sorbitol derivative nucleated polypropylene block copolymer under high pressure

The preparation of γ-phase in polypropylene is still an interesting issue in a long-term. In this work, we introduced a highly effective α-phase nucleating agent 1,3:2,4-bis(3,4-dimethylbenzylidene) sorbitol derivatives (DMDBS) into polypropylene block copolymer (PPB) and investigated the crystallization behaviors of nucleated PPB sample under high pressure. The crystallization and melting behaviors of samples were characterized by differential scanning calorimetry, and the crystalline structure as well as the relative fraction of γ-phase in the sample was characterized by wide-angle X-ray diffraction (WAXD) and two-dimensional WAXD. Scanning electron microscope was used to characterize the supermolecular structure of samples. The results show that the effects of DMDBS and high pressure on crystallization behavior of PPB were dependent on the content of DMDBS. When the content of DMDBS is smaller than the critical value, there is a synergistic effect between DMDBS and high pressure to promote the formation of γ-phase. Specifically, the critical pressure under which γ-phase dominates completely is also decreased dramatically. When the content of DMDBS is higher than the critical value, there is a competition between the effect of DMDBS, which promotes the formation of α-phase and the effect of high pressure which promotes the formation of γ-phase.     

Nucleation characteristics of the α/β compounded nucleating agents and their influences on crystallization behavior and mechanical properties of isotactic polypropylene

Nucleation characteristics of isotactic polypropylene (iPP) nucleated by the α/β compounded nucleating agents (NAs) were investigated by wide‐angle X‐ray scattering, differential scanning calorimetry and mechanical testing. The results showed that the nucleation effect of the α/β compounded NAs depends on not only nucleation efficiency (NE) of individual β and α NAs and their ratios but also the processing conditions, especially the cooling rates. The nucleating characteristics of the α/β compounded NAs can be illustrated by competitive nucleation. The NA with high NE played a leading role during iPP crystallization even at a low weight ratio and at different cooling rates. The stiffness and toughness of iPP can be simultaneously improved by using suitable compositions at the appropriate ratios. Finally, the nonisothermal crystallization kinetics of iPP nucleated with the α/β compounded NAs was described by Caze method and the crystallization activation energy of nucleated iPP was calculated by Kissinger equation. The result indicated that the crystal growth pattern of nucleated iPP was heterogeneous nucleation followed by three‐dimension spherical growth. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 653–665, 2010     

Isotactic polypropylene foams crystallized from compressed propane solutions

Crystallization of isotactic polypropylene (iPP) from homogeneous solution in supercritical propane yields open-cell foams of high surface area (120–150 m²/g). Their morphology usually consists of microspheres with a dense core and a porous periphery of radiating fibrils. Pore radii covering the mesopore range (2–50 nm), making their largest contribution at 10–20 nm, were calculated from nitrogen adsorption isotherms. Surface areas of the correct order of magnitude are obtained by assuming that gas adsorption takes place on the surfaces of lamellar crystals. Crystallization of iPP from n-butane and n-heptane generates foams of lower mesoporosity and smaller surface area. These more “liquid-like” solvents do not allow the formation of an open network of mesopores or they promote its collapse upon their removal. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 617–627, 1998     

Probing into the epitaxial crystallization of β form isotactic polypropylene: From experimental observations to molecular mechanics computation

Compared with the most stable crystalline form of isotactic polypropylene (α‐iPP), β‐iPP shows superior impact strength and high temperature performance, though the mechanism of how the frustrated structure of β‐iPP is formed still remains unclear. In present work, the single crystal structure of a traditional β‐iPP nucleating agent, N,N′‐dicyclohexylterephthalamide (DCHT), was obtained for the first time and correlated with the epitaxial crystallization of β‐iPP on the surface of DCHT crystal. The combination of synchrotron radiation X‐ray microdiffraction and molecular chain packing model confirmed that a two dimensional match of chain‐axis and inter‐chain direction coexists between β‐iPP (110) plane and DCHT (001) plane. It was further found that an epitaxial model is helpful to understand the formation of the frustrated structure of 3₁ helices packing in β‐iPP. The molecular mechanics computation showed that as the (001) plane of DCHT is fixed, the packing mode of β‐iPP (110) plane on the substrate surface is more stable than that of α‐iPP (010) plane. This work clarifies the epitaxial crystallization mechanism of β‐iPP on DCHT by employing both experimental and computational evidences. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 418–424     

A comparison study on the homogeneity and heterogeneity fiber induced crystallization of isotactic polypropylene

The crystallization behavior of iPP in composites with PET, Nylon-6 and its own fibers under various conditions was studied using an optical microscope equipped with a hot stage. The results show that the nucleation capacity of PET and Nylon-6 fibers towards the iPP matrix is mainly controlled by the shear flow of the iPP matrix during sample preparation. When the composites were prepared at a temperature where the iPP was kept in its supercooled state, the nucleation of iPP on the PET and Nylon-6 fiber surfaces was enhanced due to the shearing of the iPP melts caused by introduction of the fibers. The nucleation was markedly reduced by keeping the composites at the fiber introduction temperature for a short time to relax the shear flow of the iPP matrix. The nucleation of iPP on its own fiber, however, is mainly related to the nature of the iPP fiber itself. No detectable morphological change of iPP on its own fiber can be identified under all thermal conditions used in this study.     

Correlations between microstructure of α‐row nuclei and polymorphism of shear‐induced iPP/carbon fiber cylindrite

Herein, we reported the formation mechanism of hybrid crystalline (cylindrite) in isotactic polypropylene (iPP)/carbon fiber (CF) via pulling a CF within the iPP melt. The α‐row nuclei layer closely attached to the surface of CF acts as a self‐nucleation site, rather than a heterogeneous nucleation one, to grow cylindrites. As a result, the polymorphic feature of iPP/CF cylindrite is significantly influenced by the microstructure of α‐row nuclei. With decreasing crystallization temperature (T_c), the polymorphic cylindrite changes from pure α‐form to mixed α‐/β‐form and to β‐rich form. The main characteristics of this change include: (a) the outlines of α‐row nuclei layer correspond to wave‐like, saw‐like, and straight lines; (b) the orientation level of iPP molecules in the α‐row nuclei layer become higher; (c) the α‐lamellae rearrange from loose to compact; and (d) the distance between the growth sites of β‐sectors and the surface of CF is evidently longer than in the case of α‐sectors. Moreover, this study provides a guideline for developing the interfacial enhanced iPP/CF composites through manipulation of polymorphic structure in cylindrites. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 368–377     

Effect of γ-irradiation on slow crack growth in a copolymer of polyethylene

The effect of γ-irradiation on slow crack growth (SCG) in a medium density polyethylene (MDPE) was measured and compared with behavior of high density polyethylene (HDPE) and a recrystallized HDPE (RCHDPE). The three materials exhibited the same dependence on dose up to 3 Mrd. The HDPE became brittle above 50 Mrd. The resistance to SCG of MDPE and RCHDPE increased very rapidly with dose above 3 Mrd, until at 50–80 Mrd their resistance to SCG became extraordinarily high. This high resistance to SCG was accompanied by a transition from crazing to shear deformation at the root of a notch. It was found that for the same concentration, crosslinks are more effective than short chain branches for increasing resistance to SCG. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2349–2354, 1998     

Nucleation effect of cyclodextrin inclusion compounds on the crystallization of polypropylene

The β‐cyclodextrin (β‐CD) and γ‐cyclodextrin (γ‐CD) inclusion compounds (ICs) with two different molecular weight isotactic polypropylene (iPP) were prepared. The ICs with high molecular weight iPP as guest molecule had lower inclusion rate. The crystallization behavior of iPP blended with the CDs and ICs was investigated by differential scanning calorimetry, polarized optical microscopy, and light scattering. The iPP blended with the ICs was found to exhibit higher crystallization temperature (T_C), smaller spherulites, and faster crystallization rate than those of neat iPP. These results indicate that the ICs play a role of nucleating agent on the crystallization of iPP and induce the accelerated crystallization. Both β‐CD‐iPP ICs and γ‐CD‐iPP ICs with longer iPP molecular chains had better nucleation effect than the ICs with shorter iPP molecular chains. This suggested that the nucleation effect of these ICs was affected by the inclusion rate of ICs. The lower inclusion rate could result in better nucleation effect, due to the interaction of extended iPP molecules inside the CD cavity and iPP molecules in the matrix. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 130–137, 2009