Evidence of a Heterogeneous Nucleus in a Polypropylene Spherulite Using Electron Probe Microanalysis

Abstract

ABSTRACT:The behavior of nucleating agents in the crystallization of polypropylene (PP) wasinvestigated by an electron probe microanalyzer (EPMA). Aluminum benzoate (AlB) and sodium benzoate (NaB) were used as the nucleating agents. In a PP spherulite containing NaB, sodium Ka was detected in the neighborhood of the spherulitic center. This result indicates that NaB granules act as nuclei for PP spherulites. It is also confirmed that the AlB granule exists at the spherulitic center.KEY WORDS Polypropylene I Spherulite I Nucleating Agent 1 Electron Probe Microanalyzer I Heterogeneous nuclei in polypropylene and several other polymers considerably change the crystallization kinetics, morphology and physical properties of the base polymers. 1 -3 Various Materials, such as catalyst residues, 4 organocarboxylic acid salts 5 and inorganic compounds 6 have been reported as heterogeneous nucleating agents for polymer crystallization. In most cases the function of the nucleating agent was assessed by measuring its effect on the polymer supercooling 5 ' 7 and the crystallization mechanism of the PP-nucleating agent compound was explained using the modified Avrami equation 8 on the assumption that nucleating agents act as heterogeneous nuclei for polymer crystallization.However, few papers have been published on a microscopic description of the compounds. The present study was undertaken to elucidate the behavior of AlB and NaB granules in PP spherulites during the crystallization process by comparison of EPMA data with those of thermal analysis. EXPERIMENTAL Materials and PreparationNoblene w 101 produced by Sumitomo Chemical Co. Ltd. (M.=7X10 4 ) was used as base PP without further purification. AlB and NaB were used as nucleating agents for PP because both chemicals were reported as good 600 nucleating agents, 5 • 8 and both chemicals were reagent grade commercial materials used without further purification. AlB and NaB were passed through a sieve of 325 mesh to obtain fine granules. A proper amount of the granules was vigorously stirred with PP in xylene at about l30°C until the granules were homogeneously dispersed. After being dried to remove xylene, the AlB-and NaB-PP compounds were pressed at 200°C to a film of about 20 ,urn in thickness.In order to make the morphological difference between the AlB-or NaB-PP compound and the base PP visible, the film sandwiched between glass slides, was isothermally crystallized for 1 hr at 150±3°C after being melted for 10 min at 220°C, and was then quenched in ice-water solution. Films of AIB-PP, NaB-PP and base PP, were observed using a polarizing microscope. For scanning electron microscopy (SEM), the films stripped from the glass slides were etched in a solution containing 20-% sulfuric acid saturated with chromium trioxide for 5 min at 70 oc. 9,to DSC and EPMA MeasurementsAfter being maintained for 10 min at 220oc to exclude thermal history, the peak crystallization temperatures of the compounds were measured on a Perkin-Elmer DSC 1 B with a constant cooling rate of 10°Cjmin...