PAL study of graft polymerization of MMA on polyethylene

PAL measurements were used to study the process of graft polymerization of methyl methacrylate (MMA) to low density polyethylene (LDPE). When the monomer was just sorbed to LDPE, both I, and z, were increased. When MMA was grafted to LDPE, on the other hand, they were decreased. These contrasting results suggests usefulness of the PAL method to study the micro-structures of polymers. Introduction PAL is important since it brings forth information about the free volume of polymers. We have started a collaborative work of using PAL to study the structure of graft-polymerization. The first outcome was the study of vapor sorption in polymers(l~. In that paper it was shown that PAL is useful to distinguish different sorption states, i.e. Langmuir-type and Henry-type sorptions. This is mainly due to the particular nature of positronium (Ps) which reflects the size and density of the free volume holes. In this paper we present more recent results, where sorption and grafting of methyl methacrylate (MMA) to low density polyethylene (LDPE) are compared. Experimental Fig.1 The sample tube for the sorption The PAL spectrometer was the standard fast-fast and desorpt ion experiments. coincidence method. Several sheets of LDPE films were cut and stacked together with a positron source in the sandwich geometry. For the sorption experiments, this stack of polymer and source was put into a glass tube as shown in Fig.1. MMA liquid was put in the other arm of the tube, and after evacuating the whole system the stop cock connecting the both arms were opened. PAL spectra were repeatedly recorded at a regular time interval (sorption experiment). After sufficient duration of these measurements, the whole system was evacuated and PAL measurements were continued (desorption experiment). This desorpt ion measurements were sample and positron source performed in order to confirm that we are exactly Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1993446 288 JOURNAL DE PHYSIQUE IV F i g . 2 E f f e c t s o f s o r p t i o n o f MMA i n LDPE 10 20 30 40 50 60 70 TIME (H) TIME (H) R e s u l t s The data of sorption and desorption measurements are shown in Fig.2 and Fig.3, respectively. In the sorption measurements both I, and r , were increased. This agrees with our previous report(') for sorption of various kinds of vapor molecules in LDPE. According to the discussion there, the increase of I, and r, is due to plasticization of LDPE caused by MMA sorbed by the Henry-type sorption. In the desorption measurements both I, and r, are seen to restore their initial values. Thus it is concluded that the observed effects are due to the sorbed MMA and not due to poly-MMA that might have formed during the PAL measurements. The yield of the graft polymerization is shown in Fig.4. The graft-polymers are obtained up to about 30%. The variations of the o-Ps parameters are shown in Fig.5 and Fig.6. decreases with the graft yield and I, decreases sharply at first and then increases gradually. These changes are quite different from the case of the sorption (Fig.2). The structure of the 33 E F i g . 3 E f f e c t s o f d e s o r p t i o n o f MMA f rom LDPE 3 . 5 3 5 -