Transformation of isotactic polypropylene droplets from the mesophase into the α‐phase

Abstract

The structural transformation of homogeneously nucleated metastable mesophase of polypropylene (PP) particles was investigated in this study. We demonstrated the formation of heterogeneity‐free mesophase by slow cooling of the droplets unlike mesophase formation by quenching of the PP melt, which contained large number of bulk nuclei. Submicron size PP droplets were produced by thermal break up of PP and polystyrene layered film assembly. When cooled from melt, the PP droplets crystallized into mesophase at 44 °C revealing granular morphology. Subsequent heating thermogram of the PP particles showed a broad exotherm, which was attributed to the transformation of mesophase into α‐phase. This transformation was investigated during heating by annealing the PP particles at different temperatures. Annealed PP particles were analyzed by means of thermal, morphological and structural properties measurements. Results revealed a two step process for the transformation process. In the first step, the internal rearrangement of PP chains, as against melting and recrystallization of the mesophase, was observed. Since granular morphology was not affected significantly up to 120 °C, it was suggested that translational and rotational motions of PP helices produced ordered α‐phase. In the second step, increment in grain size distribution was observed, when the droplets were annealed at 140 °C. The results were attributed to enhanced chain mobility and merging of the grain boundaries. Annealing at 160 °C revealed the formation of short lamellar structures. Crystal thickening, melting and recrystallization of α‐phase were suggested at high temperature annealing. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011

Nucleation‐controlled semicrystalline morphology of bulk polymers

“…The latter is demonstrated with the two additional star symbols in the iPP data set, labeled “nanophase.” These data were observed on nanometer‐sized samples free of heterogeneous nuclei, with the preparation scheme described in more detail in the literature . Most important, however, is the observation that in such samples, crystallization always occurs at low temperature, even on slow cooling.…”

Density of heterogeneous and homogeneous crystal nuclei in poly (butylene terephthalate)

“…Submicron size heterogeneity-free droplets of iPP [29,30] and PA 6 [31] were slowly cooled from the melt and then crystallized only at high supercooling, leading to a similar granular morphology as was observed in quenched samples which contained heterogeneous nucleators and which were crystallized close to T g . As such, homogeneous crystal nucleation either may be enforced by quenching the melt, required to avoid crystallization via a heterogeneous nucleation mechanism at high temperature, or by absence of heterogeneous nucleators, as was achieved in droplet experiments due to the drastic decrease of the crystallizing volume, excluding presence of heterogeneities.…”

Fast Scanning Calorimetry

“…[121]. Moreover, the density of primary crystal nuclei in such heterogeneity-free droplets and bulk, impurities-containing samples when crystallized at high supercooling seemingly is identical [139,[154][155][156][157].…”