Oxidation−Migration Cycle in Polypropylene-Based Nanocomposites

Abstract

Polypropylene (PP) appears to be the most widely investigated polymer for use in the preparation and application of nanocomposites. PP as a highly hydrophobic and nonpolar polymer can be used for the preparation of nanococmposites only after a compatibilzing process in which polar groups are introduced. 1 Among the modifications described in the current literature, the most prevalent consists of grafting maleic anhydride (MA) onto PP. However, this treatment is connected with a number of complications including such side reactions as -scission, chain transfer, coupling, and, above all, severe decrease of the molecular weight. 2-5 One of the aims of this study is to explore the possibility of dispersing organo-layered silicates (OLS) in pristine PP in the presence of air, without pretreatment. Preliminary experiments showed that clay can be dispersed by melt processing in the presence of air as well as by annealing in the presence of air.The interaction of pristine PP and of PP/OLS with oxygen has been studied by several investigators. 6-12 These studies were concerned mainly with the degradation of the polymer by thermal oxidation. The extent of oxidation was monitored by determining the carbonyl groups spectroscopically by ATR-FTIR. The above studies were not concerned with investigating the effect of oxidation on the structure of PP-based nanocomposites. 13,14 A preliminary study on the effect of air oxidation of poly(ethylene-co-vinyl acetate) (EVA) was reported in the literature, and whereas changes in the nanocomposite structure were noticed, they do not pertain to PP nanocomposites due to the differences in chemical structure between the two matrix polymers. 15 A preliminary study of the effect of oxygen on the structure of PP/OLS was recently carried out in these laboratories. 16 It was shown that whereas the structure of PolyOne PP does not change upon annealing with N 2 , profound changes occur in the PP/OLS structure upon annealing under a stream of air. These changes were monitored by XRD and by migration of clay to the surface as determined by ATR-FTIR. 16,17 Additional results on the effect of oxygen during annealing of maleic anhydride (MA) grafted PP were also recently obtained. 18 In this study a limited concentration of air, up to 25% of the purging gas, was used, and effects of two structure modifiers were studied simultaneously, one of the grafted MA and the other of the oxygen. In the present study the effects of increasing concentrations of air up to 100% of the purging gas were studied on pristine PP without any other compatibilizers.The experiments described in the present communication were carried out at 180 and 190°C. These temperatures are below the onset temperature of the decomposition of the surfactant in the clay. This was confirmed by dynamic and isothermal TGA measurements, carried out in these laboratories, and not reported here, which showed a very small weight loss in the temperature range when the TGA was carried out at an atmosphere of N 2 as well as N 2 containing 12.5% air....

Materials engineering for surface-confined flame retardancy

“…The mechanism of flame retardancy can be considered derived from that of bulk nanocomposites where, during combustion, the accumulation on the surface of nanoparticles creates an inorganic barrier that both protects the underlying polymer and favours the char formation [69][70][71][72]. This effect can be mimicked by the layer by layer deposition if these nanoparticles are assembled directly on the surface.…”

Migration and surface modification in polypropylene (PP)/polyhedral oligomeric silsequioxane (POSS) nanocomposites

“…In these studies, the migration of clay entities to the melt air interface of the annealing nanocomposite samples was postulated [20] and experimentally established. [21][22][23][24][25][26][27][28][29][30][31][32] It was determined by attenuated total reflectants Fourier transform infrared spectroscopy (ATR-FTIR) [22,23] as well as by X-ray photoelectron spectroscopy (XPS) [25] and by angle regulated ARXPS. [31] It was found in these studies that neither, pristine clay, nor organically layered clay migrate to the surface from a nanocomposite melt.…”

Release of Engineered Nanomaterials from Polymer Nanocomposites: the Effect of Matrix Degradation

“…47 This last factor, especially, appears to play a dominant role during non-directional heating. The presence or absence of oxygen 49,51,54 and of 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 compatibilizers 50 also impacts the extent of nanofiller relocation to the surface due to mediation of the host matrix polarity, which can in turn create a driving force for exfoliation. Importantly, this relocation of ENMs to the material's surface occurs at temperatures below which the matrix undergoes complete decomposition.…”