Crystallization and morphology of the trigonal form in random propene/1-pentene copolymers

The melting and crystallization behaviour of a series of isotactic propene/1-pentene random copolymers, with 1-pentene contents up to 50 mol%, was investigated by DSC and temperature resolved WAXD/SAXS. The role of the 1-pentene comonomer in the development of the trigonal modification (δ-form) of i-PP was studied and the results were compared with those reported in the literature for PP copolymers with 1-hexene. The crystallizing capability of the δ-form, which develops in the composition range between ca. 10 and 50 mol% of 1-pentene content, only slightly decreases with concentration of 1-pentene. This result is correlated with the limits imposed to cell expansion by the crystal density. The crystallinity degree calculated from the deconvolution of the WAXD patterns is in fair agreement with the results of the DSC analysis, from which the value of the melting enthalpy of the perfect i-PP δ-form has been estimated to be around 140 J/g. The crystallization kinetics of the trigonal modification is characterized by a composition-dependent induction time followed by a relatively fast development of structural order. The sharp WAXD reflections combined with the SAXS data suggest that, notwithstanding the intrinsic intrachain structural disorder, thin and wide lamellae characterize the morphology of the δ-form crystallites.     

i-PP/HDPE blends. III. Characterization and compatibilization at lower i-PP contents

The mechanical properties of high-density polyethylene (HDPE)-rich i-PP/HDPE blends were studied. Two grades of HDPE were investigated, one with a melt viscosity close to that of the polypropylene (PP) and the other having a much lower melt viscosity. Compatibilization of the 10/90 i-PP/HDPE blend with three copolymers (an ethylene/propylene/diene [EPDM] copolymer and two ethylene/vinylacetate [EVA] copolymers, differing in their VA content) was also investigated. Blends of PP with the low melt viscosity HDPE displayed poor mechanical properties. It was not possible to improve these properties sufficiently with EPDM or EVA. In the case where viscosity matching was achieved between PP and HDPE, addition of i-PP (up to 30%) to HDPE resulted in a large drop in the impact strength of the blends, compared to that of the neat HDPE. A large drop (>50%) was also observed in the ultimate tensile elongation. However, the flexural modulus, yield stress, and ultimate tensile strength all increased with the introduction of i-PP into HDPE. Modification of these blends with an EPDM resulted in the return of all properties to values very close to those of the neat HDPE. The ultimate tensile elongation of the EPDM-modified i-PP/HDPE blend even exceeded that of the virgin HDPE. It was also found that although EVAs can be used to compatibilize these blends these additives were not as effective as was the EPDM. © 1996 John Wiley & Sons, Inc.     

Crystallization and phase behavior of crystalline syndiotactic 1,2-polybutadiene/isotactic polypropylene blends in solution-cast thin films

Crystallization and phase behavior in solution-cast thin films of crystalline syndiotactic 1,2-polybutadiene (s-1,2-PB) and isotactic polypropylene (i-PP) blends have been investigated by transmission electron microscopy (TEM), atomic force microscopy (AFM) and field-emission scanning electron microscopy (FESEM) techniques. Thin films of pure s-1,2-PB consist of parallel lamellae with the c-axis perpendicular to the film plane and the lateral scale in micrometer size, while those of i-PP are composed of cross-hatched and single-crystal-like lamellae. For the blends, TEM and AFM observations show that with addition of i-PP, the s-1,2-PB long lamellae become bended and i-PP itself tends to form dispersed convex regions on a continuous s-1,2-PB phase even when i-PP is the predominant component, which indicates a strong phase separation between the two polymers during film formation. FESEM micrographs of both lower and upper surfaces of the films reveal that the s-1,2-PB lamellae pass through i-PP convex regions from the bottom, i.e. the dispersed i-PP regions lie on the continuous s-1,2-PB phase. The structural development is attributed to an interplay of crystallization and phase separation of the blends in the film forming process. With solvent evaporation, s-1,2-PB would crystallize first forming the continuous phase, while the segregated i-PP phase accumulated on s-1,2-PB and crystallized subsequently, forming dispersed i-PP regions.     

Synergistic gelation of solutions of isotactic polypropylene and bis-(3,4-dimethyl benzylidene) sorbitol and its use in gel-processing

We report the synergistic, rapid gelation of solutions of isotactic polypropylene (i-PP) and the nucleating agent 1,3:2,4-bis-(3,4-dimethyl benzylidene) sorbitol (DMDBS) in decalin. Cooling to room temperature of a solution comprising, for instance, 3.0 wt% of a moderately high molecular weight i-PP (M_v=1.3×10⁶ g/mol) and as little as 0.0075 wt% DMDBS (0.25 wt% based on the polymer) resulted in the fast formation of highly ductile gels. In reference experiments without DMDBS, often ‘mud-cracked’, brittle polymer films were obtained, and decaline solutions of DMDBS alone at the above concentration did not form macroscopically coherent gels. In the present work we employed this useful occurrence for gel-processing/drawing of i-PP, yielding material with Young's moduli of 35 GPa, tensile strengths of approximately 1 GPa and melting temperatures measured for constrained samples as high as 228 °C.     

Additives to improve the electret properties of isotactic polypropylene

Substituted benzene-1,3,5-tricarboxylic acid trisamides are known to be efficient nucleating agents and in some selected cases are clarifiers for isotactic polypropylene (i-PP). In this paper we expanded the application range of this class of additives to the area of i-PP electret materials. This paper discusses in particular the relation between charge storage properties and additive concentration. Furthermore, attention is directed towards processing conditions, which were found to play an important role and seemed to be related to the dissolution and crystallization behavior of these additives from the i-PP melt. The formation of isolated nanometer-sized supramolecular structures was established to be important. It was found that, with the addition of benzene-1,3,5-tricarboxylic acid-(N-cyclohexyl)-trisamide, the charge storage properties of i-PP films can be improved at concentrations below 0.02 wt% (200 ppm). At such low concentrations, the additive appears to be present as isolated nano-aggregates, which can, therefore, efficiently act as charge traps. A further improvement in electret characteristics can be achieved by increasing the cooling rate of the polymer/additive blends. A clear correlation between nucleation efficiency and charge storage efficiency could not be revealed.     

Phase behavior, nucleation and optical properties of the binary system isotactic polypropylene/N,N′,N″-tris-isopentyl-1,3,5-benzene-tricarboxamide

The phase behavior of the binary system consisting of isotactic polypropylene (i-PP) and N,N′,N″-tris-isopentyl-1,3,5-benzene-tricarboxamide (1)—a selected member of a class of novel, versatile ‘designer’ nucleating/clarifying agents—was investigated over the entire additive concentration range by means of differential scanning calorimetry (DSC) and optical microscopy. Experimental phase diagrams were constructed from data obtained in melting and crystallization studies, and a simple monotectic is advanced, very similar to the previously studied binary system i-PP/1,3:2,4-bis(3,4-dimethylbenzylidene) sorbitol (DMDBS). In contrast to the latter, the crystallization temperature in the present system i-PP/1 was found to increase to ∼120 °C already at the lowest additive concentration employed and remained constant at further increasing additive concentration. Liquid-liquid phase separation was observed at elevated temperatures for i-PP/1 mixtures comprising more than ∼2 wt% of 1. A study on the optical properties of the i-PP/1 system revealed that the values for haze and clarity of injection-molded plaques progressively decreased and increased, respectively, in the concentration range up to 0.15 wt%. An intermediate region of fairly concentration-independent optical properties was found between 0.15 and 1 wt%, followed by a rapid increase in haze at concentrations exceeding 2 wt%.     

Structural and property changes during uniaxial drawing of ethylene-tetrafluoroethylene copolymer films as analyzed by in-situ X-ray measurements

The structure-property relationship during uniaxial drawing of high-molecular-weight ethylene-tetrafluoroethylene copolymer (ETFE) film was analyzed based on a combination of in-situ wide-angle X-ray diffraction (WAXD) and stress measurements. In-situ WAXD patterns indicated that the hexagonal (100) reflection transformed from the initial un-oriented ring into equatorial spots via split arcs at temperatures both below and above T_g, but that the critical strain when such transition occurred was delayed above T_g. Drawing above T_g produced an equatorial concentration of the spot reflections; thus, the extended chain crystal was enhanced as a result of elongation of the strain-hardening region. In contrast, the draw below T_g remained less oriented even just before breaking. However, the high orientation component also appeared beyond the critical strain; it could be assigned to the so-called “tie molecules” that bind the deformed lamellae. Changes in resultant tensile strength could be interpreted by these extended chain crystals and characteristic tie molecule components.     

Morphology and micromechanical behavior of binary blends comprising block copolymers having different architectures

Morphology and deformation behavior of binary blends comprising styrene/butadiene block copolymers (polystyrene content, Φ_PS∼0.70) having different molecular architectures were studied by means of transmission electron microscopy and tensile testing. In contrast to the binary diblock copolymer blends discussed in literature, the phase separation behavior of the blends investigated was found to be strongly affected by asymmetric molecular architecture. The blends showed macrophase separated grains, in which the structures resembled the microphase morphology of none of the blend components. Unlike the classical rubber-modified or particle-filled thermoplastics, neither debonding at the particle/matrix interface nor the particle cavitation was observed in these nanostructured blends. The microdeformation of the blends revealed plastic drawing of polystyrene lamellae or PS struts dispersed in rubbery matrix and orientation of the whole deformation structures along the strain direction.     

Crystallization and phase separation kinetics in blends of linear low-density polyethylene copolymers

We have systematically studied the crystallization and liquid-liquid phase separation (LLPS) kinetics in statistical copolymer blends of poly(ethylene-co-hexene) (PEH) and poly(ethylene-co-butene) (PEB) using primarily optical microscopy. The PEH/PEB blends exhibit upper critical solution temperature (UCST) in the melt and crystallization temperature below the UCST. The time evolution of the characteristic morphology for both crystallization and LLPS is recorded for blends at various compositions and following a quench from initial homogenous melts at high temperature to various lower temperatures. The crystallization kinetics is measured as the linear growth rate of the super structural crystals, whereas the LLPS kinetics is measured as the linear growth rate of the characteristic length of the late-stage spinodal decomposition. The composition dependence crystallization kinetics, G, shows very different characteristics at low and high isothermal crystallization temperature. Below 116 °C, G decreases with increasing PEB content in the blend, implying primarily the composition effect on materials transport; whereas at above 116 °C, G shows a minimum at about the critical composition for LLPS, implying the influence of the LLPS. On the other hand, LLPS kinetics at 130 °C is relatively invariant at different compositions in the two-phase regime. The length scale at which domains are kinetically pinned, however, depends strongly on the composition. In a blend near critical composition, a kinetics crossover is shown to separate the crystallization dominant and phase separation dominant morphology as isothermal temperature increases.     

Wide angle X-ray diffraction investigation of crystal orientation in miscible blend of poly(ε-caprolactone)/poly(vinyl chloride) crystallized under strain

The orientation of poly(ε-caprolactone) crystals in miscible poly(ε-caprolactone)/poly(vinyl chloride) (PCL/PVC) blends, melt crystallized under strain, has been studied by wide angle X-ray diffraction (WAXD). At low draw ratios or low PVC contents, all the observable (hk0) crystal reflections orient towards the meridional direction in WAXD patterns, indicating the presence of ring-fibre orientation. With the increase of draw ratio or PVC content, additional crystal orientation with the crystal a-axis parallel to the stretching direction is found to superimpose on the WAXD pattern of ring-fibre orientation. Both the ring-fibre orientation, which dominates the WAXD pattern, and the a-axis orientation are characterized by the perpendicular orientation of the crystal c-axis to the stretching direction. The unusual PCL orientation is a consequence of the combined effects of both the stretching and the presence of PVC in the PCL/PVC blends.     

Miscibility in blends of linear and branched poly(ethylene oxide) with methacrylate derivative random copolymers and estimation of segmental χ parameters

Miscibility in the blends of poly(ethylene oxide) (PEO) with n-hexyl methacrylate-methyl methacrylate random copolymers (HMA-MMA) and 2-ethylhexyl methacrylate-MMA random copolymers (EHMA-MMA) was evaluated using glass transition and light scattering methods. EHMA-MMA was more miscible with PEO than HMA-MMA. Both blends of PEO with HMA-MMA and EHMA-MMA showed UCST-type miscibility although homopolymer blends PEO/PMMA were predicted to be of LCST-type. This was attributed to an increase in the exchange enthalpy with increasing HMA or EHMA composition in the random copolymer. From the copolymer composition dependence of miscibility the segmental χ parameters of HMA/MMA, EHMA/MMA, EO/HMA and EO/EHMA were estimated using the Flory-Huggins theory extended to random copolymer systems. Miscibility in the blends of branched PEO with HMA-MMA whose HMA copolymer composition was 0.16 was compared with that in the linear PEO blends. The former blends were more miscible with HMA-MMA than the latter one by about 35 °C at the maximum cloud point temperature.     

Time-resolved FTIR spectroscopic study of the evolution of helical structure during isothermal crystallization of propylene 1-hexene copolymers. Identification of regularity bands associated with the trigonal polymorph

Two different types of regularity bands are identified in a real time FTIR crystallization of a series of random propylene 1-hexene copolymers. The first is akin to the bands observed in the homopolymer, those associated with 3₁ helices of isotactic sequences of different n length (n, number of monomer units). The second type corresponds to vibrational coupling of short sequences of the chain that include the 1-hexene comonomer. Among the latter are absorbances at 910 and 1025 cm⁻¹ which are markers for the formation of a trigonal phase in these copolymers. They remain unchanged prior to and during crystallization in copolymers with the 1-hexene units rejected from the crystallites (<13 mol% 1-hexene) and increase in intensity when the comonomer is an integral part of the crystallites (>13 mol% 1-hexene). Analysis of the real time evolution of IR regularity bands during isothermal crystallization of these copolymers confirms the beginning of crystallization at a critical helical sequence length (n∗) of ∼12 isotactic units (841 cm⁻¹), and enables details of the early and final stages of crystallization. In the homopolymer and copolymers, the intensity of regularity bands with n ≤ 10 is constant in the initial undercooled melt, and increases simultaneously with the appearance of helices with n ≥12, in support of a classical crystallization mechanism of nucleation and growth. Due to density fluctuations in the initial melt, the short helices eventually collapse in aggregates or precursors that spontaneously (within the experimental macroscopic time frame) extend to stable nuclei (n ≥ 10). Stable nuclei further extend and grow cooperatively dragging additional short sequences as inferred by the simultaneous temporal evolution of helices with n = 10 and greater. The intensity of the 998 cm⁻¹ (n = 10) band prior to nucleation, correlates directly with the isotactic sequence length of the copolymer and is independent of the final structure that evolves, either monoclinic or trigonal. This feature infers a nucleation event driven preferentially by the initial steady-state content of short helices in iPP and iPP-based copolymers. The temporal evolution of the 841 cm⁻¹ band is an excellent avenue to study the crystallization kinetics of copolymers, including those with very low crystallinities. Via FTIR, the mechanism of the formation of mesomorphic crystallites in copolymers with ∼10 mol% 1-hexene at low temperatures is contrasted with the formation of alpha crystallites at higher temperatures in the nucleation driven range. The intensity of the 841 cm⁻¹ band (n = 12) at the end of the transformation correlates linearly with the degree of crystallinity obtained by WAXD.     

Effect of comonomer partitioning on the kinetics of mesophase formation in random copolymers of propene and higher α-olefins

The effect of counit type on the kinetics of mesophase formation has been investigated by means of chip-calorimetry in propene/α-olefin random copolymers, containing counits which show large differences in their co-crystallization behavior with propene, i.e. 1-butene and 1-hexene. Non-isothermal crystallization experiments indicated that the minimum cooling rate at which mesophase formation is observed is directly related to the kinetic of α-phase crystallization, which is lower for the copolymer with the bulkier 1-hexene counit. Isothermal structuring was probed in a wide temperature range, revealing that a double bell-shaped curve is required to describe the temperature dependence of crystallization times of the two polymorphs. The ordering kinetics of the mesophase is the fastest in i-PP homopolymer and decreases with increasing comonomer bulkiness, analogous to what happens for the monoclinic phase. The results are discussed by considering the effect of comonomer on the driving force for mesophase formation, also at the light of new WAXD and density evidences, which prove different extents of inclusion of 1-butene and 1-hexene in the ordered phases.     

Functionalization of Isotactic Polypropylene with Citraconic Anhydride

Summary Functionalization of isotactic polypropylene i-PP) with citraconic anhydride (CA) was carried out in 1,2,4-trichlorobenzene solution with dicumyl peroxide as an initiator at 160 °C under nitrogen atmosphere.atmosphere. Chemical and physical structures and thermal behavior of the synthesized graft copolymers with different anhydride units were determined by volumetric titration (acid number), FTIR and ¹H-NMR spectroscopy, X-ray powder diffraction, DSC and TGA thermal analyses. It was shown that the crystallinity and thermal behavior of grafted i-PP’s depend on anhydride unit concentration in grafted i-PP; grafting reaction proceeds selectively which is not accompanied by oligomerization of CA and degradation main chain as in known maleic anhydride/PP system. This fact was explained by inhibition effect of α-methyl group in CA grafted unit on the chain β-scission reactions and no homopolymerization of CA in chosen grafting conditions. Functionalized i-PPs showed high thermal stability in comparison with virgin i-PP.     

Unique orientation textures formed in miscible blends of poly(vinylidene fluoride) and poly[(R)-3-hydroxybutyrate]

The oriented crystallization of poly[(R)-3-hydroxybutyrate] (PHB) in the miscible blends with poly(vinylidene fluoride) (PVDF) was investigated with various compositions. The PVDF/PHB blend films were prepared by solution casting and subsequent melt-quenching in ice water. Oriented films of the blends were prepared by uniaxially stretching the melt-quenched film at 0 °C in ice water using a hand-operated stretching apparatus. The oriented blend films were heat-treated at a fixed length in order to crystallize PHB in the oriented state. The crystal orientation and the lamellar textures of the obtained samples were studied with wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering (SAXS), respectively. The SAXS measurements showed that a considerable amount of molecular chains of PHB are excluded from the lamellar stacks of PVDF and exist in the interfibrillar regions in the oriented films of the blends. The cold crystallization of PHB in the interfibrillar region results in the orientation of PHB crystals, and the type of crystal orientation depends upon the composition of the blends. For the PVDF/PHB=4/6-7/3 blends, the crystal a-axis of PHB is highly oriented parallel to the drawing direction and the crystal c-axis (molecular chain axis) in PHB crystals is perpendicular to the drawing direction, i.e. orthogonal to the chain axis of the crystals of PVDF. It is considered that the a-axis orientation is induced by the confinement of crystal growth in the interfibrillar nano-domains. For the PVDF/PHB=2/8-3/7 blends, however, the crystal c-axis of PHB is primarily oriented in the drawing direction, suggesting that the stressed molecular chains of PHB are crystallized with the molecular orientation retained.     

Copolymerization of Propylene with 1-Hexene and 4-Methyl-1-Pentene in Liquid Propylene Medium. Synthesis and Characterization of Random Metallocene Copolymers with Isotactic Propylene Sequences

Summary Propylene copolymerization with 1-hexene and 4-methyl-1-pentene in liquid propylene medium in presence of MAO-activated C₂-symmetry ansa-zirconocene rac-Me₂Si(4-Ph-2-MeInd)₂ZrCl₂ was studied. Random copolymers of propylene with 1-hexene and 4-methyl-1-pentene content up to 7 mol % were obtained at 60 °C. General kinetic characteristics of propylene/higher α-olefin copolymerization were evaluated. The distinct feature of propylene copolymerization with 1-hexene and 4-methyl-1-pentene in liquid propylene medium – the proximity of comonomer relative reactivity ratios (r₁∼r₂∼1) that indicates azeotropic nature of copolymerization processes in studied conditions. Synthesized copolymers were characterized with the use of IR, ¹³C NMR, GPC, WAXD, DSC techniques, and uniaxial tensile testing.     

Phase morphology development and stabilization in polycyclohexylmethacrylate/polypropylene blends: uncompatibilized and reactively compatibilized blends using two reactive precursors

The phase morphology developed in immiscible polypropylene (PP)/polycyclohexylmethacrylate (PCHMA) blends has been studied using an in situ reactively generated polystyrene-graft-polypropylene compatibilizer from maleic anhydride grafted polypropylene (MA-g-PP) and amine end-capped polystyrene (PS-NH2) reactive precursors during melt-blending. The imidation reaction responsible for the formation of the compatibilizer is similar to the reaction occurring in polyamide/MA-PP (MA-EPR or MA-EPDM) blends which are industrially important. In the present blend PP/PCHMA/(PP-MA-PS-NH2), no undesired reaction occurs between the maleic anhydride groups and the backbone of the PCHMA chain, as is usually the case with polyamide homopolymer. This type of reaction, although considered non significant, has consequences on the phase morphology development as it affects the viscosity of the polyamide matrix when chain scission takes place. PP/PCHMA blends covering the whole range of compositions were prepared. The composition window at which the blends exhibit a droplet-in-matrix phase morphology and that where the two phases are co-continuous were determined using a selective phase extraction in combination with scanning electron microscopy. The generation in situ of the PP-g-PS compatibilizer substantially changed the state of the phase morphology developed. In the blends having a droplet-in-matrix type of morphology, the particle sizes were significantly reduced (by a factor of more than 10). Two types of MA-g-PP reactive copolymers differing in maleic anhydride content (1 and 8 wt%) have been separately employed with the same grade of PS-NH2. Emphasis was put on a detailed investigation of the behaviour and structural stability of the blends exhibiting a co-continuous phase morphology when the compatibilizer is generated. Significant differences were found in relation to the maleic anhydride content of the MA-PP reactive compatibilizer precursor.     

Cocrystallization behavior of poly(hexamethylene terephthalate-co-hexamethylene 2,6-naphthalate) random copolymers

A series of poly(hexamethylene terephthalate-co-hexamethylene 2,6-naphthalate) (P(HT-co-HN)) random copolymers were synthesized by melt polycondensation and characterized using ¹H NMR spectroscopy and viscometry. Their cocrystallization behavior was investigated using differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) method. Even though the P(HT-co-HN) copolymers synthesized are statistically random copolymers, they show a clear melting and a crystallization peak in DSC thermograms over the entire range of copolymer composition and have a minimum melting temperature in the plot of melting temperature versus copolymer composition. WAXD patterns of all the copolymer samples show sharp diffraction peaks and are largely divided into two groups, i.e. PHT type and PHN type crystals. In addition, WAXD patterns of the PHN type crystals are subdivided into two types of PHN α and PHN β according to the copolymer composition. These facts indicate that the P(HT-co-HN) copolymers show isodimorphic cocrystallization. The composition at which the crystal transition between PHT type and PHN type occurs is equivalent to the eutectic composition (22 mol% HN content) for the melting temperature. When the defect free energies were calculated by using the equilibrium inclusion model proposed by Wendling and Suter, the defect free energies in the case of incorporation of HT units in the PHN α and β crystals were higher than the case of incorporation of HN units in the PHT crystal lattice.     

Crystallization analysis fractionation of ethene/1-hexene copolymers made with the MAO-activated dual-site (1,2,4-Me3Cp)2ZrCl2 and (Me5Cp)2ZrCl2 system

Different poly(ethene-co-1-hexene) samples with varying amounts of 1-hexene were characterized by crystallization analysis fractionation (Crystaf). The samples were synthesized with (1,2,4-Me₃Cp)₂ZrCl₂, (Me₅Cp)₂ZrCl₂, and a mixture of these two catalysts in a 1:1 molar ratio. In addition, preparative Crystaf was used to fractionate some of the samples made with the catalyst mixture into 1-hexene-rich and 1-hexene-poor fractions. These fractions were characterized by Crystaf, differential scanning calorimetry (DSC) and gel permeation chromatography (GPC), and compared with copolymers made under similar conditions using the individual catalysts. Both (1,2,4-Me₃Cp)₂ZrCl₂ and (Me₅Cp)₂ZrCl₂ produced copolymers with unimodal distribution of short chain branches (SCBD), as expected for single-site catalysts. The catalyst mixture produced copolymers with bimodal SCBDs when 0.38 mol/l or higher concentrations of 1-hexene were used. The high temperature peak results from crystallization of polymer chains with few comonomer units, and these are attributed to (Me₅Cp)₂ZrCl₂. The low temperature peak results from crystallization of polymer chains made by (1,2,4-Me₃Cp)₂ZrCl₂, and these chains contain many comonomer units. Direct evidence for relative activity enhancement of the (Me₅Cp)₂ZrCl₂ catalyst in the dual-site system was observed.