Nucleation and crystallization of isotactic poly(propylene) droplets in an immiscible polystyrene matrix

In this work, isotactic poly(propylene) (iPP) was finely dispersed in immiscible atactic polystyrene (PS) matrices. When the dispersion obtained is fine enough (droplet size of approximately 1–2 μm), the iPP crystallizes in a fractionated fashion at temperatures between 104 and 42°C. By applying a self‐nucleation procedure we were able to corroborate that what causes the fractionated crystallization in most droplets is the lack of highly active heterogeneous nuclei (i. e. those normally active at low supercoolings in the bulk polymer) in every droplet. When a sufficient amount of a compatibilizer is used to obtain very small particle sizes and more homogeneous dispersions, the iPP crystallizes exclusively at a low temperature exotherm that exhibits an onset at 51°C and peaks at 46°C. Wide‐angle X‐ray diffraction measurements indicated that both the iPP in the bulk and in dispersed droplets crystallized in the monoclinic α‐phase, this evidence may rule out the possibility that the crystallization observed at 46°C is due to the formation of another crystal modification or a mesomorphic phase as previously suggested in literature. The results presented in this work indicate that this low temperature exotherm may represent the dynamic crystallization during cooling of heterogeneity‐free droplets that nucleate homogeneously at temperatures close to 51°C.     

Epitaxial crystallization of trans-1,4-polybutadiene on highly oriented isotactic poly(propylene) films

Epitaxial crystallization of trans-1,4-polybutadiene (PBD) on highly oriented isotactic poly(propylene) (iPP) has been investigated at different crystallization temperatures and rates. From electron microscopy and electron diffraction, it is confirmed that epitactic growth of the low-temperature modification (monoclinic) with microcrystals of PBD on the iPP films takes place with their chain directions about ±50° apart. No epitaxial relationship occurs between the high-temperature modification (hexagonal) of PBD and the iPP substrate. Thermal analyses of PBD-PP layered films indicate that the epitaxy has an important effect on the formation of the high-temperature modification of PBD.     

The crystal structure of isotactic poly (propylene oxide)

The results of a detailed investigation on the crystal structure of poly(propylene oxide) are reported. The chain model here proposed differs from the full-planar one only by a deviation of 15° around the C? O bonds. This model is accounted for by both the sensibly better agreement between F_o and F_c and the more justifiable intra-chain contacts with respect to those observed for the model previously studied by STANLEY and LITT , with which it is compared.     

Melting and crystallization behavior of syndiotactic poly(propylene) homopolymer mixtures with atactic poly(propylene)

The melting and crystallization behavior of syndiotactic (sPP) poly(propylene) homopolymer mixtures with atactic (aPP) poly(propylene) are investigated by differential scanning calorimetry (DSC) and wide angle X-ray scattering (WAXS). The tacticity microstructure of sPP homopolymer polymerized with metallocene catalysts varies with polymerization temperature. The resultant sPP microstructure is strongly coupled to the DSC melting behavior. Multiple endotherms are observed which are sensitive to heating rate, consistent with a melting-recrystallization-remelting (MRM) mechanism as a contributing factor to the higher temperature endotherm (Endotherm II). Reduced sPP tacticity and molecular weight promote the reorganization. The multiple melting response is a useful probe of aPP “diluent” segregation within the sPP/aPP blend morphology. The addition of aPP leads to a suppression of crystallization temperature, and promotion of Endotherm II on heating which is sensitive to both the “host” sPP and “diluent” aPP molecular weight. Both behaviors suggest a degree of “local” mixing of diluent within the morphology.     

On the stereoisomerization in thermal degradation of isotactic poly(propylene)

The terminal microstructure of the hydrogenated α,ω-diisopropenyloligo(propylene) prepared by thermal degradation of isotactic poly(propylene) at 370°C was determined by means of ¹³C NMR spectroscopy, and therefrom the stereoisomerization reaction of the reacting polymer segment is discussed in relation to polymer dynamics. With respect to the asterisked carbons of the propyl end group CH₃CH₂^*CH₂ (n-Pr) and of the isopropyl end group (^*CH₃)₂CH (i-Pr) formed by hydrogenation, the mole fractions of triads (m meso, r racemo) F_mm = 0.64, F_mr = 0.09, F_rr = 0.12 of n-Pr-^*CH₂ differ from the microtacticity of the main chain of telechelic oligomers, and the mole fraction of i-Pr-^*CH₃ is as follows: F(δ_tζ_t) + F(δ_eζ_e) = 0.74 (m); F(δ_tζ_e) + F(δ_eζ_t) = 0.26 (r). These results clearly show that the stereoisomerization occurs only at the asymmetric carbon near the end group of the telechelic oligomer. The value of the mole fraction of i-Pr-^*CH₃ is consistent with F_m = 0.73 (F_mm + F_mr) and F_r = 0.27 (F_rr + F_rm) of n-Pr-^*CH₂ at the outer position of the main chain rather than with F_m = 0.79 (F_mm + F_rm) and F_r = 0.21 (F_rr + F_mr) at the inner position. These results could be consistently traced by a simulation based on a reaction model including stepwise intramolecular hydrogen abstraction of the tertiary on-chain macroradical near the center of the main chain before the successive β-scission. This stepwise back-biting is estimated to occur once (20%) and twice (80%) via a quasi six-membered cyclic structure in a transition state, and occurs in a very limited part of the molecular chain.     

Transverse stiffness of myofibrils of skeletal and cardiac muscles studied by atomic force microscopy

The transverse stiffness of single myofibrils of skeletal and cardiac muscles was examined by atomic force microscopy. The microscopic images of both skeletal and cardiac myofibrils in a rigor state showed periodical striation patterns separated by Z-bands, which is characteristic of striated muscle fibers. However, sarcomere patterns were hardly distinguishable in the stiffness distributions of the relaxed myofibrils of skeletal and cardiac muscles. Myofibrils in a rigor state were significantly stiff compared with those in a relaxed state, and in each state, cardiac myofibrils were significantly stiffer compared with skeletal myofibrils. By proteolytic digestions of sarcomere components of myofibrils, it was suggested that cardiac myofibrils are laterally stiffer than skeletal myofibrils because Z-bands, connectin (titin) filament networks, and other components of sarcomere structures for the former myofibrils are stronger than those for the latter.     

Modification of isotactic poly(propylene) by oxygen and helium plasma with reference to thermo-oxidative stability

The modification of isotactic poly(propylene) (PP) foils by reactive (O₂) and inert (He) gas plasma treatment was studied with relevance to surface structure and thermo-oxidative stability for different treatment times. The following effects were investigated: change of non-helical (atactic or amorphous) content in the surface region and formation of vinyl and vinylidene groups by frustrated multiple internal reflection Fourier transform infrared spectroscopy (FMIR-FTIR), formation of peroxyl redicals by electron spin resonance spectroscopy (ESR) and change of the invariant activation energy of degradation by thermogravimetric analysis (TGA). For the latter a new kinetic evaluation method was used based on a selection of multiple step model to determine the invariant Arrhenius constants, e.g., activation energy and preexponential factor, for the weight loss during thermoxidative degradation of untreated and, for the first time, of plasma-treated PP. Several thermogravimetric curves under different temperature scanning conditions have been evaluated with different solid state models as well as with a selection of different multiple step reaction pathways. With a limitation to total weight loss of 60% of the mass for an untreated PP and applying a most probable kinetic model consisting of one consecutive and one parallel reaction step, the activation energies were found to be 84.2 ± 1.0 and 73.2 ± 1.0 kJ/mol for the initial and the consecutive reaction and 137.8 ± 3.0 kJ/mol for the parallel reaction. The corresponding data of the plasma-treated samples are 80 ± 1.0 and 71.0 ± 1.0 kJ/mol for the initial and the consecutive reaction, while the activation energy for the parallel reaction remained almost unchanged. Unexpectedly, we found the He plasma treatment to decrease the activation energies of the first and consecutive reaction nonlinearly and within the first 600 s to a larger extent than O₂ plasma treatment. Based on this kinetic model, a modified mechanism for the thermo-oxidative degradation of PP is proposed in terms of elementary degradation reactions and will be discussed with reference to literature data.     

Crystallization of the γ-form of isotactic poly(propylene)

The pure γ-form of isotactic poly(propylene) can be obtained by melt-crystallization starting from a fraction of a random ethylene-propene copolymer (ethylene content 4 wt.-%) soluble in p-xylene at 28°C. Some structural models previously reported are tested by comparing calculated wide-angle X-ray diffraction patterns with the experimental ones. A method for the quantitative determination of the γ-form crystallinity by X-ray line profiles is proposed. The original copolymer and its fractions are characterized by differential scanning calorimetry and ¹³C NMR techniques.     

Effect of molecular weight on the crystallization kinetics of isotactic poly(1-butene)

Isothermal crystallization of isotactic poly(1-butene) fractions by calorimetry and microscopy was studied for a range of molecular weights from 96 000 to 964000. An Avrami exponent of 2 was found indicating a two-dimensional growth rate of the crystal, following a predetermined nucleation mode, proving that such an exponent is independent of both temperature and molecular weight. The temperature and molecular weight dependence of the growth rate of spherulites and overall crystallization rates was analyzed by means of the theoretical expression given by Hoffman and Lauritzen. The product of the lateral and basal surface free energies of the crystallites, σ·σ_e, increases with molecular weight and approaches its limiting values when a certain molecular weight is attained.     

Solid-state NMR studies of structure and dynamics of erucamide/isotactic poly(propylene) blends

Solid-state NMR spin diffusion experiments and two-dimensional (2D) ¹H-¹³C NMR experiments have been used to study the phase structure, mobility and domain sizes of blends of isotactic poly(propylene) (i-PP) and erucamide (ERU), a low molecular weight slip and antiblock additive for i-PP films. The study of the proton line shapes in pure erucamide and i-PP together with their blends reveals the mobility of the two components. The mobility is higher in the blends than in erucamide and i-PP when they are considered separately. In addition, 2D wideline separation (WISE) NMR spectroscopy indicates that erucamide molecules show both low and high mobilities depending on the erucamide content in the blend. By measuring the proton spin diffusion with ¹H detection, the microdomain dimensions have been estimated quantitatively. The domain sizes range from approximately 13 nm to 60 nm.     

SAXS/WAXS study of the annealing process in quenched samples of isotactic poly(propylene)

The structural rearrangement in samples of quenched isotactic poly(propylene) (iPP) submitted to different annealing treatments has been studied using simultaneous small- and wide-angle X-ray scattering (SAXS/WAXS) at the synchrotron radiation source of DESY, Hamburg. From a quantitative analysis of the WAXS profiles the values of the α-monoclinic, mesomorphic, and amorphous mass fractions coexisting in the material were determined. It is demonstrated that the SAXS patterns were characterized by two different long-period values that are attributed to α- and mesomorphic periodicity, respectively. The related α- and mesomorphic volume phase fractions, calculated from the analysis of the SAXS data, are compared with the corresponding WAXS results. The rearrangement of the initial structure involves i) the thickening of the already existing α-phase lamellae at the expenses of the amorphous regions and ii) the structural rearrangement of the mesomorphic phase leading to its transformation into the α-monoclinic one.     

Synthesis of vinyl-terminated isotactic poly(propylene) using the coupling reaction between Zn-terminated polymer and allyl halides

Propene was polymerized with the TiCl₃/Al(C₂H₅)₂Cl catalyst system using Zn(C₂H₅)₂ as a chain transfer reagent. The resulting Zn-terminated poly(propylene) (PP) was then brought into contact with allyl halides (chloride, bromide and iodide) in the presence of N-methylimidazole to obtain vinyl-terminated PP. Allyl bromide gave the highest conversion among these allyl halides. When the coupling reaction was conducted with allyl bromide at 40°C for 24 h, the conversion to the terminal vinyl groups was over 90% for atactic PP and approximately 60% for isotactic PP. The Zn-polymer bonds of both atactic and isotactic PP were almost quantitatively converted into terminal vinyl groups at 130°C for 3 h.     

Influence of the stretching rate on the transition front structure of uniaxially deformed isotactic poly(propylene)

Recent models on the melting process of polymer crystals allow the study of semicrystalline polymer structures by calorimetric (DSC) measurements. In particular, this work is devoted to the study of those changes occurring during the deformation process of isotactic poly(propylene) samples along the transition front. By observing the evolution of the parameter of intrachain melting cooperativity in comparison with lamellar thickness, it has been possible to characterise the birth and further evolution of double fold molecules, totally extended polymer chains connecting two adjacent crystallites in the fibrillar structure, “taut tie molecules”. Furthermore, those structural changes depend on the stretching conditions. In general, low rate promotes high lamellar fragmentation and the presence of a small number of taut tie molecules. On the contrary, at higher rates crystallites coming from the original lamella show bigger size and a higher number of tie molecules are achieved.     

Time resolved small angle scattering during isothermal crystallization of unoriented poly(ethylene terephthalate) using synchrotron radiation

The time course of the small angle scattering pattern of amorphous poly(ethylene terephthalate), (PET), was measured during isothermal crystallization using synchrotron radiation. The samples were heated to temperatures between 117 and 145°C in an oven placed in the path of the X-ray beam. The scattering curves were measured within 10 s at intervals of 1 s so as to follow the kinetics of the process continuously. It appears that the distance between the centres of mass of the crystallites decreases with increasing crystallization time as in oriented PET. The integrated small angle scattering follows approximately the same time course as the increase in the degree of crystallinity during an isothermal crystallization yielding Avrami exponents between 5 and 7. Guinier plots of the low angle region show that the scattering can be attributed to rod-like particles.     

The influence of regio- and stereoirregularities on the crystallization behaviour of isotactic poly(propylene)s prepared with homogeneous group IVa metallocene/methylaluminoxane Ziegler-Natta catalysts

Poly(propylene)s with narrow molecular weight distributions were prepared with various methylaluminoxane-activated metallocene-based Ziegler-Natta catalysts to study the influence of randomly incorporated regio- and stereoirregularities on the crystallization behavior. As a function of the metallocene type and the polymerization temperature, the molecular weights varied between 11500 < M _n < 63 000, melting temperatures of annealed samples between 125 to 158°C, and the corresponding degrees of crystallinity, as measured by wide-angle X-ray scattering, between 49 and 67%. While the virgin poly(propylene)s exhibited exclusively the α-modification, annealing and melt crystallization favored the development of the γ-modification. The microstructure analysis by ¹³C NMR spectroscopy revealed a linear correlation between the content of the γ-modification and the average length of the isotactic segments.     

Solid-state 13C NMR of microscopic order in transcrystalline films of isotactic poly(propylene)

The microscopic orientational order in a variety of transcrystalline films of isotactic poly(propylene) was investigated by means of two-dimensional solid-state NMR. From the experimental results the orientation distribution function of the microcrystallites relative to the macroscopic growth direction was determined. The NMR measurements were performed at room temperature on samples that had been crystallized at conditions at which the morphology of spherulitic crystallization changes as a function of temperature or composition. For transcrystalline samples of isotactic poly(propylene) crystallized in the α-modification a drastic breakdown of molecular order was observed at a crystallization temperature of 137°C, with the absolute degree of order depending on the molecular mass of the polymer. For samples crystallized in the β-modification a small stepwise decrease of molecular order was observed at a crystallization temperature of 128°C. A continuous decrease of the orientational order of the isotactic part was observed by blending isotactic poly(propylene) with atactic poly(propylene).     

Influence of branching on melting behavior and isothermal crystallization of poly(butylene terephthalate)

The melting behavior and the isothermal crystallization kinetics of linear and randomly branched poly(butylene terephthalate)

1 System. name: poly(tetramethylene terephthalate).

(PBT) samples were investigated by differential scanning calorimetry. The multiple endotherms typical of linear PBT were observed also in branched samples and were found to be influenced both by temperature and degree of branching. The isothermal crystallization kinetic constants, obtained applying Avrami's treatment, were analysed on the basis of Hoffman's secondary nucleation theory. A decrease in the crystallization rate was observed as the content of branching units increased. This result was correlated to surface effects: the product of the surface free energies σσ_e was found to increase with the ramification content. As far as the nucleation process is concerned, the induction time was determined for all the samples at different crystallization temperatures, and the results suggested that in the presence of branching points the nucleation process is favoured.     

In vitro model of annexin A5 crystallization on natural phospholipid bilayers observed by atomic force microscopy

Annexin A5 is a potent anticoagulant protein with a thrombomodulatory function. It is frequently mentioned with systemic inflammatory autoimmune disease, which share higher vulnerability to cardiovascular diseases. The protein has the ability to bind to membranes containing negatively charged phospholipids in a calcium-dependent manner. The potent anticoagulant properties of the protein are a consequence of this crystallization, which forms the lattice of annexin A5 over phospholipid surface, blocking its availability for coagulation reactions. Crystallization of annexin A5 has been proven on homogeneous synthetic phospholipids. However, the crystallization of annexin A5 on inhomogeneous, naturally derived phospholipid surfaces, in p3 and p6 crystal form, has now been reported for the first time. Atomic force microscopy was chosen for the observation of the crystallization of annexin A5 on different solid supported phospholipid bilayers. In this study model, the optimal results were obtained by using: 0.5 mg/ml lipid vesicles suspension (70% phosphatidylcholine, 30% phosphatidylserine) in HEPES buffer saline (HBS) with 2 mM CaCl₂, large unilamellar vesicles with sizes around 200 nm, 41°C of phase transition temperature and 21 μg/ml of native annexin A5 in HBS with 2 or 20 mM CaCl₂. Results were evaluated by imaging and force measurements. Demonstration that native annexin A5 is able to spontaneously crystallize on naturally derived, inhomogeneous phospholipids is supporting the putative role of annexin A5 crystal structures as possible antithrombotic shield. This in vitro system is probably more appropriate for studying the pathogenetic role of antiphospholipid antibodies.     

Critical epitaxial layers of different kinds of polyethylene on highly oriented isotactic poly(propylene) substrates

Epitaxial crystallization of high-density polyethylene (HDPE), linear low-density polyethylene (LLDPE) and low-density polyethylene (LDPE) in the epitaxial system with highly oriented isotactic poly(propylene) (iPP) has been investigated by electron microscopy. The results obtained by bright field electron microscopy and electron diffraction confirmed that all of the three kinds of PE (HDPE, LLDPE and LDPE) have the ability of epitaxy on iPP surfaces. There exists a critical thickness of the PE layer in which the PE can grow epitaxially on iPP substrates. When the PE/iPP layered films are heat-treated at a temperature above the melting temperature (T_m) of PE and below T_m of iPP for 10 min and then quenched to room temperature, the critical layer thickness of epitaxial HDPE, LLDPE and LDPE is about 250, 120 and 30 nm, respectively. For PE further apart from the interface, the epitaxial orientation becomes disturbed due to simultaneous spherulitic nucleation or lamellar twisting of the epitaxially grown lamellae.     

Imaging the surface of Staphylococcus aureus by atomic force microscopy

The surfaces of four strains of Staphylococcus aureus, which differed in their expression of capsular polysaccharides, were examined using atomic force microscopy. The images show that it is possible to get information about surface characteristics of S. aureus using atomic force microscopy (AFM) following simple preparation. Strains Smith Diffuse (serotype 2), Reynolds (serotype 5), Wood-46 (capsule negative) and JL243 (capsule negative) were grown on medium known to promote the expression of capsular polysaccharides. The bacteria were air-dried prior to being imaged using tapping-mode AFM. Differences in the appearance of the bacterial surfaces were evident between the strains. The two capsule-negative strains exhibited a smooth regular surface, as opposed to the mucoid appearance of the two strains having polysaccharide capsules. Moreover, comparison of images of the heavily encapsulated serotype 2 strain and the serotype 5 strain indicates that a type 2 capsule can be distinguished from a type 5 microcapsule.