High Pressure Crystallization of HDPE Droplets Robert Masirek, † Ewa Piorkowska,* ,† Andrzej Galeski, † Anne Hiltner, ‡ and Eric Baer ‡ Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, 90 363 Lodz, Poland, and Department of Macromolecular Science and Center for Applied Polymer Research, Case Western ReserVe UniVersity, CleVeland, Ohio, 44106-7202 ReceiVed April 25, 2008; ReVised Manuscript ReceiVed August 11, 2008 ABSTRACT: Dispersions of high density polyethylene (HDPE) 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 distribution was controlled by the HDPE initial individual layer thickness: 14, 40 or 120 nm. These unique systems allowed us to demonstrate unequivocally for the first time that the HDPE pseudohexagonal phase formation required nucleation on foreign surfaces or on crystals formed in the orthorhombic form at atmospheric pressure. The dispersions of HDPE particles in the PS matrix were subjected to annealing at high pressure of 480 MPa at 235 °C, in conditions where the pseudohexagonal phase forms in HDPE. To reach the desired conditions the samples were first heated and then subjected to high pressure or they were first pressurized and then heated. The samples treated according to the first route melted and transformed into the pseudohexagonal phase from the molten state whereas the second route allowed for transformation of orthorhombic crystals to the pseudohexagonal phase without previous melting. The lamella thickness formed under high pressure was estimated on the basis of melting temperature. The HDPE particles, annealed under high pressure without previous melting, exhibited high melting temperature, above 140 °C, corresponding to the lamella thickness above 50 nm, that indicated thickening in the pseudohexagonal phase. The HDPE particles that were first melted and then pressurized behaved differently depending on their sizes: thick lamellae were formed under high pressure only in particles large enough to contain heterogeneities able to nucleate HDPE bulk crystallization. These results point out that nucleation, presumably heterogeneous, is advantageous, or even necessary, for the formation of the HDPE pseudohexagonal phase. Introduction Under high pressure, above 300 MPa, polyethylene (PE) forms a two-dimensional pseudohexagonal phase, which also has been claimed to be a precursor phase in crystallization of the usual orthorhombic form even at atmospheric pressure. 1 The conditions of the pseudohexagonal phase stability are defined by the phase diagram; the required temperature for pseudohex- agonal phase stability is also high, usually above 220 °C. Upon cooling under elevated pressure the pseudohexagonal phase transforms to the crystalline orthorhombic form. A metastable pseudohexagonal phase can also form within the orthorhombic- stable region, for instance it is formed when PE crystallizes at high pressure during cooling at a rate 1 °C/min. 1 Based on Raman spectra of the pseudohexagonal phase the presence of a large amount of kink conformational defects of the kind TG(TG( within an otherwise fully trans-planar chain was inferred. 2 Weak interactions between chains within the pseudohexagonal phase enable sliding diffusion that leads to gradual thickening of the ordered domains reaching thicknesses significantly larger than that of lamellae crystallized directly into the orthorhombic form. Melt crystallized orthorhombic crystals transferred into the region of pseudohexagonal phase stability, first by increasing pressure and then temperature, transform into pseudohexagonal domains and thicken considerably. 1,3 The thickness of crystals that crystallized or thickened in the pseudohexagonal phase is larger than that of melt crystallized orthorhombic crystals and usually exceeds 50 nm. 1 However, Bassett and Khalifa 4 demonstrated that film thickness affected the thickness of crystals formed in the pseudohexagonal phase. In 3 μm thick films it was 142 nm, significantly smaller than that in the bulk polymer, subjected to the same treatment, which was 307 nm. Moreover, Khalifa and Bassett 5 reported high pressure annealing of solution grown orthorhombic crystals that resulted in 30 nm thick pseudohexagonal crystals. Such small thickness resulted from severe confinement in 30 nm thick domains. The mesomorphic phase of isotactic polypropylene forms at high supercooling, below 60 °C via homogeneous mechanism. 6,7 The pseudohexagonal phase of PE, having also mesomorphic character, forms at low supercooling that might suggest that heterogeneous rather than homogeneous nucleation is active. The present paper is aimed at answering this question by studying high pressure crystallization in PE droplets. Polymer droplets are long known to solidify via fractionated crystallization 8-14 reflected in the presence of more than one crystallization exothermic peak. The lowest temperature exo- thermic peak with the largest supercooling is usually associated with homogeneous nucleation while exothermic peaks at higher temperatures are attributed to crystallization from nuclei formed on active heterogeneities. Recently Bernal-Lara et al. 15 studied multilayered systems of high density polyethylene (HDPE) and atactic polystyrene (PS) fabricated by means of layer-multiplying coextrusion that uses forced assembly to create thousands of alternating layers of two polymers. 16 Heating films above the melting temperature of HDPE resulted in breakup of HDPE layers into droplets followed by fractionated crystallization during subsequent cooling. The droplet size, hence the crystal- lization behavior, was controlled by the individual HDPE layer thickness. The droplets formed by melting of the thinnest 14 nm layers were numerous enough that the majority did not contain an active heterogeneity and crystallization occurred at about 80 °C, which was interpreted as a result of homogeneous nucleation. We note that the lowest crystallization temperature reported for HDPE is around 70 °C. 13 The differences in reported temperatures of fractionated crystallization of HDPE * Author to whom correspondence should be addressed. E-mail: epiorkow@bilbo.cbmm.lodz.pl. † Polish Academy of Sciences. ‡ Case Western Reserve University. 8086 Macromolecules 2008, 41, 8086-8094 10.1021/ma800933g CCC: $40.75  2008 American Chemical Society Published on Web 10/07/2008