Real-Time WAXD Detection of Mesophase Development during Quenching of Propene/Ethylene Copolymers

Abstract

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...

Polymer crystallization studies under processing-relevant conditions at the SAXS/WAXS DUBBLE beamline at the ESRF

“…about 130 K s À1 , the pure isotactic polypropylene crystallizes in a mixture of and mesomorphic phases Cavallo, Portale et al, 2010), as deduced from the simultaneous presence of the monoclinic structure reflections together with two superimposed broad halos. The addition of a commercial nucleating agent (NA-11) without selectivity towards a particular crystalline modification accelerates the crystallization of the i-PP phase, which indeed occurs at earlier times/higher temperature with respect to the pure polymer (compare Figs.…”

“…Recently, we reported for the first time real-time ultrafast WAXS measurements of mesophase structuring during quenching for i-PP homopolymers and propylene/ethylene copolymers (Cavallo, Portale et al, 2010). The whole cooling from the melt lasted a few seconds, with the structuring process taking place in fractions of a second.…”

Crystallization of a Polyamide 6/Montmorillonite Nanocomposite at Rapid Cooling

“…WAXS experiments were performed at the BM26B DUBBLE beamline at ESRF,19 with the specific experimental setup for simultaneous recording of cooling curves, sample temperature as a function of time, and series of WAXS patterns at high frequency of 20 Hz described in detail elsewhere 20, 21. The experiments performed provided information about the X‐ray structure of PA 6 and the PA 6/OMMT nanocomposite formed as a result of a specific cooling history, including rapid cooling at rates up to about 200 K · s −1 .…”

Studying nanostructure gradients in injection-molded polypropylene/montmorillonite composites by microbeam small-angle x-ray scattering

“…First, the structure formation in PP appears highly dependent on its cooling rate [69] and on comonomers in the chain [70]. Studies that monitor fast solidification benefit from a concept of metallurgy that has been introduced to polymer science by Spruiell and White [71], the so-called continuous cooling transformation and related plots [69, 70]. As long as the NC components are joined by melt-mixing, it may remain difficult to avoid the introduction of nucleating grainy clay besides the desired exfoliated sheets.…”