The relaxation process of shear-induced crystal nucleation precursors has been investigated in a temperature range slightly above the melting point using a series of commercial grade isotactic poly(1-butene)s with different molecular weights. Development of transcrystalline morphology from the surface of a fiber pulled through the molten polymer is ascribed to high concentration of ordered clusters promoted by the alignment of chain segments under the high-intensity shear flow at the fiber−melt interface. Sheared melts isothermally crystallized immediately after cessation of flow exhibit a well-pronounced cylindritic morphology, characterized by closely spaced fibrillar branches; on the other hand, prolonged relaxation in the molten state before crystallization leads to classical spherulitic morphology. The lifetime of shear-induced nucleation precursors, t*, has been associated with the complete disappearance of the transcrystalline morphology. It has been found that systems composed of short chains relax much faster than those containing a large fraction of high molecular weight species. When relaxed slightly above the melting point of the tetragonal crystal modification the highest molecular weight samples keep memory of flow-induced structuring during several hours. Temperature has a dominant role on the kinetics of reequilibration of sheared samples: experimental data of t* obtained in wide range of “relaxation” temperatures can be fitted by an Arrhenius-type equation with an apparent activation energy of around 700 kJ/mol. Results can be justified by considering a network of aligned and ordered polymolecular clusters, originated under the high-intensity shear flow field at the solid−melt interface, whose relaxation involves large scale restructuring.
The dissolution of flow-induced nucleation precursors in isotactic polypropylene is investigated indirectly by means of in situ rheo-SAXS measurements. The progress of crystallization and the evolution of crystal orientation are recorded in isothermal conditions after a controlled shear step followed by an annealing step of different duration at various melt temperatures. The results confirm that the survival time of shear-induced nucleation precursors is extremely large compared to typical rheological relaxation times and it is longer for the precursors originated at higher shear rate. Most important, we show that the effect of flow on the development of oriented morphologies is lost much earlier than that on the overall crystallization kinetics. A schematic model for precursors' dissolution involving gradual transformation from row into point-like nuclei is proposed.
Rheooptical experiments on narrow isotactic polystyrene (i-PS) fractions have been carried out to investigate temperature and molar mass dependence of the lifetime of shear-induced nucleation precursors. Similar to i-poly(1-butene) and i-polypropylene, the survival of the flow-induced structures lasts very long, even at temperatures well above the measured melting point of the crystals. It has been observed that the decrease of concentration of nucleation precursors follows a first-order kinetics with a strongly temperature-dependent rate constant. The apparent activation energy of the overall relaxation process is around 400 kJ/mol, of the same order of magnitude previously found for other semicrystalline polymers. A comparative analysis of the apparent activation energy data for different polymers, coupled with morphological evidence, suggests that the rate-determining step in the evolution of the system toward the equilibrium state in the melt is the detachment of stems from the lateral surface of flow-induced oriented bundles, present either as isolated entities, in the case of pointlike nuclei, or as rows alternated by disordered nanodomains, in the long threads of the shish. Thanks to the monodisperse character of the investigated samples, the molar mass dependence of lifetime of oriented nucleation precursors has also been established.
The microindentation hardness technique has been employed to examine the II → I polymorphic transformation of i-PBu1 taking place upon aging at room temperature. The hardness values of form I are shown to be remarkably higher than those of form II due to the denser packing of chains in the hexagonal crystal modification. The kinetics of the II → I transformation has been followed by means of microindentation hardness measurements in real time. The influence of molar mass and crystallization temperature on the kinetics of the polymorphic transformation is examined. Results suggest that the rate of polymorphic transformation is independent of molecular weight. In addition, it is seen that increasing the crystallization temperature (up to 105 °C) notably reduces the time required for a full transformation of form II into form I. The influence of the fraction of amorphous material on the rate of polymorphic transformation is discussed.
No abstract
A simple method to investigate polymer crystallization during fast cooling, based on in situ\ud temperature acquisition and ex-situ structural characterization, is proposed. The approach enables one to\ud obtain the continuous cooling curve (CCC) diagrams, widely used in metallurgy but seldom adopted for\ud semicrystalline polymers. This method is here exploited to gain new insights on polymorphic behavior of\ud quenched polypropylene and its copolymers with ethylene. Experimental CCC diagrams, covering a wide\ud range of crystallization temperatures in the domains of monoclinic structure and mesophase, are obtained for\ud the first time. The role of counits in affecting the development of the mesophase upon fast cooling is assessed:\ud the critical cooling rate above which a predominant fraction of mesomorphic form is generated significantly\ud decreases with increasing comonomer concentration. This is due to the remarkable hindrance of ethylene\ud counits on the crystallization kinetics of the R-form, which indirectly favors the development of the less\ud affected mesophase. We expect that this concept can be extended to any kind of defects that disturbs the\ud structuring of the monoclinic phase
Mesomorphic Phase in Isotactic Polypropylene. Synthetic semicrystalline polymers are generally characterized by a certain amount of disorder along the chains due to defects in the chemical constitution, in the configuration of neighboring repeating units, or in the conformation of a portion of molecule in the solid state. 1 When the polymer crystallizes under certain conditions, all these defects may hinder the packing of the chains, leading to a large amount of structural disorder. The definition of a crystalline unit cell becomes problematic when the degree of disorder is particularly high, and a structure with features intermediate between those of the crystalline and amorphous states is obtained. 2 This solid phase is usually referred to as "mesomorphic form" or "solid mesophase", and it is rather frequently encountered in semicrystalline polymers. 3,4 Because of its industrial relevance, the mesophase of isotactic polypropylene (i-PP), which is easily obtained by quenching the molten polymer, has been extensively investigated. 5-18 Its structure is consistent both with the presence of small bundles of parallel chains in 3/1 helical conformation, with short-range lateral order, 5 and with a conformationally disordered glass, characterized by defects in the handedness of the helices. 6 The typical features of i-PP mesomorphic structure are reflected by its macroscopic properties, e.g., density, 7 intrinsic birefringence, 8 and elastic modulus, 9 which lay between those of the amorphous state and of the monoclinic structure. Formation and thermal stability of the i-PP mesophase have been investigated in detail. [10][11][12][13][14][15][16][17][18] The mesomorphic structure is metastable, and on heating between 40 and 80°C, it undergoes a transformation into the thermodynamically stable crystalline R-form. [10][11][12][13][14] The formation of the mesophase has been extensively studied by Piccarolo et al. [15][16][17][18] For the i-PP homopolymers, they demonstrated that a competition between formation of crystalline and mesomorphic order sets in on increasing cooling rate, and eventually, the latter prevails above ca. 100°C/s. Associated with the development of the different structures, profound changes in the morphology have been reported. The typical cross-hatched lamellae of the R-form spherulites are replaced in the semimesomorphic samples by small nodular morphologies lacking of any superstructure. [15][16][17][18] Kinetic Aspects of Mesophase Crystallization. The kinetic aspects governing the development of the mesomorphic form are less understood, mainly because most of the information is obtained by ex-situ investigation of quenched samples. Formation of mesomorphic phases in semicrystalline polymers takes place only at very high undercooling, which are difficult to attain for a polymer exhibiting relatively high maximum crystallization rate as i-PP. One possibility to circumvent this problem is the cold crystallization of amorphous thin films ultraquenched below the glass transition. Isothermal crystal...
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