Unidirectional solidification of isotactic polypropylene

Thin films of isotactic polypropylene have been solidified unidirectionally by pulling specimens through a fixed temperature gradient of 5.5°/mm at rates ranging from 0.07 to 7 mm/hr. At pulling rates lower than 1.7 mm/hr, polypropylene spherulites grew preferentially in the direction of solidification with a tendency to the generation of more asymmetric spherulites at lower rates of solidification.     

Low and high-yield isotactic polypropylene

The crystallization and melting behaviour of isotactic polypropylene (iPP) samples synthetized with different catalyst systems (low and high-yield) have been studied by differential scanning calorimetry and optical microscopy. The isothermal crystallization rates from the melt have been found to depend on the catalyst system employed and on the isotacticity index of the sample. Moreover, for low-yield iPP, Avrami analysis of the overall kinetics has provided evidence of the presence of secondary crystallization phenomena. The values of the equilibrium melting point, energy of nucleation and surface energy of folding of iPP lamellar crystals have been calculated according to the ‘Kinetic theories’ of polymer crystallization. The observed variation of such thermodynamic parameters for the various iPP samples has been accounted for by the amount and type of configurational irregularities present along the chains and by the differences in the molecular weight distribution.     

Crystallization of oxidized isotactic polypropylene

The effect of processes accompanying thermal oxidation of the polymer on the characteristics of its isothermal crystallization has been revealed. It has been shown that crystallization decelerates with a rise in the degree of PP oxidation. The higher the concentration of functional groups, the stronger the deceleration. The energy of nucleation increases when passing from virgin to oxidized PP samples. The higher the concentration of carbonyl groups accumulated in polymer chains, the more pronounced this effect, although the degradation of the chains must lead to a reduction in this parameter. It has been concluded that the kinetic and thermodynamic parameters of the isothermal crystallization are applicable to investigation of processes accompanying thermal oxidation of the crystallizable polymer.     

Melting temperature and enthalpy of isotactic polypropylene

The fusion process, melting temperature and heat of fusion of carefully crystallized fractions of isotactic polypropylene (molecular weights from 5·4 × 10⁴ to 2·2 × 10⁵) have been investigated. The equilibrium melting temperature, obtained by the extrapolation method in experiments involving very low degrees of crystallinity, corresponds to 208°. The fusion enthalpy was found to be 1·386 cal/mole. This low value and the high depression of Tm^o suggest a relatively high interfacial energy; a value of 285 erg/cm² is estimated for σ_ec.     

Partial melting and recrystallization of isotactic polypropylene

<正>A relatively high predetermined crystallization temperature(135℃) was chosen to grow well developed iPP spherulites,then the partial melting was carried out at a temperature of 165℃,where the preformed spherulites were seen to only decrease their size but not completely melted.The crystallization behavior of partially melted isotactic polypropylene (iPP) has been carefully examined by different scanning calorimetry(DSC) and polarized light microscopy(PLM).The experimental results show that at a special annealing temperature(165℃) the melting behavior of iPP includes two parts with different mechanism,one part is the melting of iPP spherulite outside,another is the partial lamellae perfection during longer annealing time in the unmelted spherulite.The conformational orders of the iPP melt decrease with the increase of the annealing temperature.     

Structural-mechanical phase diagram of isotactic polypropylene

The polymorphic transformations associated with the plastic deformation of isotactic polypropylene samples of different stereoregularity, prepared with different metallocene catalysts, have been studied. Crystals of alpha or gamma forms, present in the unstretched samples, transform into the mesomorphic form by stretching. The formation of the mesophase facilitates successive further deformation up to very high strains and produces development of outstanding unusual properties of high flexibility and elasticity. The phase diagram of isotactic polypropylene, where the regions of stability of the different polymorphic forms are defined as a function of stereoregularity and degree of deformation, is reported.     

High-pressure crystallization of isotactic polypropylene droplets

Dispersions of isotactic polypropylene (PP) particles in polystyrene (PS) were produced by interfacially driven breakup of nanolayers in multilayered systems that were fabricated by means of layer-multiplying coextrusion. The droplet size was controlled by the individual PP layer thickness ranging from 12 to 200?nm. In addition, PP was melt blended with PS to produce PP droplets larger than those formed by breakup of nanolayers. The dispersions of PP particles in the PS matrix were melted and annealed under high pressure of 200?MPa. Only the largest PP droplets, with average sizes of 170?μm, crystallized predominantly in the γ form. In the 42-μm droplets obtained by breakup of 200?nm layers, a minor content of the γ form was found whereas the smaller droplets obtained by breakup of the thinner nanolayers contained the α form and/or the mesophase. The results showed that the γ phase formed only in the droplets sufficiently large to contain the most active heterogeneities nucleating PP crystallization under atmospheric pressure. It is concluded that the presence of nucleating heterogeneities is necessary for crystallization of PP in the γ form under high pressure.     

Thermal oxidation of ester-modified isotactic polypropylene

The relationship between the thermal oxidation of isotactic PP samples modified by esters miscible and immiscible with the polymer and the structure of these samples has been studied. An analysis of kinetic features of oxygen uptake, buildup of oxidation products, and changes in the mechanical properties and structural parameters of PP in the course of oxidation has shown that the effect of a modifier on the kinetics of thermal oxidation of the polymer depends on compatibility of an additive and a polymer matrix. The addition of ester that is partially miscible with PP accelerates oxidation. In the case of an immiscible ester, the effect is quite the reverse. This phenomenon is rationalized by the fact that the phase state of the system determines changes in the initial structure of the polymer matrix and, hence, manifestation of structural effects in the reaction kinetics and the participation of additives in chain reactions of cooxidation with PP.     

Drawing behavior of cross-linked isotactic polypropylene

The drawing behavior of cross-linked isotactic polypropylene was analysed. The crystallinity and the composition of the crystalline phase are affected by cross-linking. Structural and topological features play a significant role in the drawing behavior. The yielding, the necking phenomenon, and the ultimate properties, i.e., stress and strain at the fracture, can be correlated with the sample structure. The general picture agrees well with Peterlin's model of cold drawing.     

Modeling of the crystallization of isotactic polypropylene chains

The mechanical properties of polypropylene depend critically on the crystallizability of the chains, which depends in turn on their stereochemical structures. These dependences were investigated using Monte Carlo methods to generate chains having various stereochemical sequences and then scrutinizing parallel arrays of these chains to look for matches in isotactic stereochemistry that could lead to the formation of crystallites. The fraction of such units in matches, for example, gives a direct measure of the degree of crystallinity expected for the specified degree of isotacticity. Other quantities of interest obtainable in this way were the natures of the sequence distributions themselves, melting points, free energies of fusion, interfacial free energies, and moduli (calculated on the basis of the crystallites acting as crosslinks within an elastomeric network structure). © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2757–2764, 1997     

Nuclear magnetic resonance investigation on isotactic,atactic and deuterated isotactic polypropylene

Ohne ZusammenfassungProf. Dr.Alfons Krause, Pozna (Polen), Grunwaldzka 6.     

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.     

Stability and phase transformations of the mesomorphic form of isotactic polypropylene in stereodefective polypropylene

A study of the thermodynamic stability and the related polymorphic transformations induced by thermal treatments of the mesomorphic form that crystallizes in stereodefective metallocene isotactic polypropylene (iPP) is presented. We show that the mesomorphic form of the more isotactic samples is stable at room temperature, whereas the mesomorphic form crystallizing in the more stereoirregular sample is unstable and crystallizes at room temperature in the crystalline α form. In any case, the mesomorphic form transforms during heating or by annealing at temperatures higher than 60–80 °C always in the α form, regardless of the stereoregularity, even in the case of stereoirregular samples generally crystallizing from the melt in the γ form. These data confirm the proposed model of structure of the mesomorphic form as small aggregates of chains in three-fold helical conformation packed with lateral correlations similar to the α form of iPP.     

Peroxides of isotactic powdered polypropylene as the initiator of emulsion polymerization in redox system

Hydroperoxides of oxidized powdered isotactic polypropylene have an unfavourable influence on the modification of polypropylene by styrene in the presence of chelate Fe²⁺-triethylenetetramine in emulsion at 30°. Elimination of hydroperoxides from the polypropylene results in increased bonding of styrene to polypropylene while the rate of homopolymerization decreases. The production of homopolymer depends on the presence of hydroperoxides of polypropylene; the production of modified polymer is connected with the decomposition of other more stable peroxides of polypropylene, probably dialkylperoxides, which are decomposed by the chelate. For polypropylene modification, oxidation of polypropylene by oxygen containing ozone is better than oxidation by oxygen at 110° or oxidation under γ-irradiation. The thermal stability of peroxides of powdered isotactic polypropylene is rather low; hydroperoxides begin to decompose in an inert atmosphere at temperature above 30°; other peroxides are stable up to 40°.     

Flow-induced crystallization regimes and rheology of isotactic polypropylene

We investigated the effect of flow in combination with molecular architecture on crystallization for linear and branched polypropylenes by means of dynamical mechanical spectroscopy. Compared to the linear polymer, the branched one exhibits much slower relaxation after deformation, causing higher levels of molecular orientation and molecular stretch for the same flow conditions. Different regimes of flow-induced crystallization are observed as a function of an increased level of molecular orientation and stretch. These regime changes cause the structure development in the material to vary from 3-dimensional spherulitical growth to nearly 1-dimensional fiber-like growth. Intermediate behavior is observed when the flow is strong (for the linear polymer) or weak enough (for the branched polymer), or in time when the effect of flow is exhausting. It is important to note that dynamical mechanical measurements can be used to probe different aspects of the (flow induced) crystallization process, such as a relative level of crystallinity and timescales of the evolution, but do not give absolute values. A correlation is found with the level of crystallinity obtained from real-time WAXS measurements and the kinetics of linear viscoelastic properties, but this correlation is not unique and depends on the pre-shear conditions applied.     

Block copolymers of isotactic polypropylene and 1,4-polybutadiene

Two block copolymers of isotactic polypropylene and 1,4 polybutadiene were synthesized using techniques involving a transformation from anionic to Ziegler–Natta polymerization mechanisms. The yield of block copolymer was about fifteen percent (weight basis) in both polymerizations, the remainder being unreacted polybutadiene from the first block synthesis. Molecular characterization experiments and model reactions were consistent with a block-like structure for the copolymers; definitive evidence for the proposed molecular structure was obtained through transmission electron microscopy which clearly revealed microphase-separated morphologies characteristic of block copolymers.     

Miscibility of isotactic polypropylene with atactic polystyrene

The miscibility of isotactic polypropylene-atactic polystyrene blends using dilatometric, TOA, DAT and stress-strain methods were studied. It appears that the blends are heterogeneous, having two glass transition temperatures. However, some interactions between components are evident to change theTgs of their amorphous phases and theTm of isotactic polypropylene crystals. The results are discussed on the basis of the blends' thermal and tensile properties.     

Crystallization behavior of isotactic polypropylene based nanocomposites

The effect of clay dispersion on the crystallization behavior of isotactic polypropylene (iPP)-based nanocomposites is reported. The T _m⁰ of the materials was calculated by the method proposed by Marand, the kinetics of crystallization was evaluated by the Avrami analysis and also the Hoffman-Lauritzen theory of crystallization regimes was applied. Montmorillonite was found to depress T _m⁰, to enhance the rate of crystallization and to ease the chain folding of macromolecules. These effects were magnified if clay was exfoliated, rather than intercalated.     

Effect of components of a sulfur-containing vulcanizing system on the structure and properties of isotactic polypropylene

With the use of methods of X-ray diffraction analysis and differential scanning calorimetry, the structure and melting and crystallization of isotactic polypropylene are studied in the presence of components of a sulfur-containing vulcanizing system composed of sulfur, zinc oxide, stearic acid, Altax, and thiuram, which is used for synthesis of thermoplastic vulcanizates. It is found that the addition of zinc oxide results in the formation of the β phase of polypropylene, and the content of this product depends on the concentration of a nucleating agent. The heat pretreatment of samples has a strong effect on the formation of β -polypropylene. The addition of residual compounds to polypropylene at the studied concentrations has practically no effect on its structure. With an increase in the content of the β phase, the elastic modulus and elongation at neck formation of polypropylene increase, whereas its tensile strength, elongation at yield, and elongation at break decrease.     

On the development of special positive isotactic polypropylene spherulites

A new type of positive α‐iPP spherulites has been developed by self‐seeding process. The growth process of these positive α‐iPP spherulites is just like “photographic development process,” which is very different from the conventional growth process of polymer spherulites. Scanning electron microscopy (SEM) was used to explore the morphologies of these positive α‐iPP spherulites on a lamellar level. The results show that these spherulites are composed of a large number of lamellae having interwoven structures, which result in different optical character, special melting behavior, and different contrast under SEM as compared with the conventional melt‐crystallized spherulites. The development of these interwoven lamellar structures has been considered because in the sites of the original spherulites, a large number of self‐nuclei are formed because of the incomplete melting of the original spherulites and these induce nearly equal number of radial and tangential lamellae at rather high temperatures. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1114–1121, 2006