Ana C. F. Moreira

The compatibilization of SBR/EVA by mercapto-modified EVA

Abstract Styrene–butadiene copolymer/ethylene–vinyl acetate copolymer (SBR/EVA) blends with different ratios were prepared by using a Haake internal mixer. These blends were compatibilized with mercapto-modified EVA (EVALSH), which promotes bonding between the SBR phase and the EVALSH through a chemical reaction between the mercapto groups of the reactive compatibilizing agent and the double bond of the unsaturated rubber. The presence of EVALSH resulted in an improvement of the tensile strength of the unvulcanized SBR/EVA blends and also a more uniform dispersion of the EVA phase inside the SBR matrix for blends containing higher amount of the rubber phase. The formation of insoluble material in the blends containing EVALSH indicates reactive compatibilization, which was confirmed by Fourier transform infrared analysis and also by dynamic mechanical analysis. In this case, a decrease in damping properties of the SBR phase was observed as a consequence of decreasing chain mobility. EVALSH also caused a decrease in degree of crystallinity of the EVA phase as observed by differential scanning calorimetry analysis.

Thermal degradation studies of poly(EVAL-g-methyl methacrylate)

The thermal stability of poly[(ethylene-co-vinyl alcohol)-g-methyl methacrylate] (EVAL-g-PMMA) was investigated by thermogravimetric analysis. The graft copolymers were prepared by free-radical polymerization of MMA in the presence of mercapto-modified ethylene-vinyl alcohol copolymers (EVALSH) as transfer agent. The thermal stability of PMMA segments in the graft copolymers is usually higher than that of the corresponding PMMA homopolymers and is related to the amount of free SH groups in the EVALSH backbone after the graft copolymerization.

The influence of long chain branches of LLDPE on processability and physical properties

Two polyethylene-based on single-site metallocene catalyst (mLLDPE) were selected to characterize the effect of long chain branching (LCB) on blown film processability, optical and mechanical properties. 13C NMR and parallel plate rheology were used to identify LCB presence on LLDPEs. Blown films were produced from 100% LLDPEs using three different machine direction (MD) stretch ratios. When the same processing conditions for the two LLDPEs grades were used, better processability was observed for LLDPE with LCB. In relation to mechanical and physical properties, Elmendorf tear and optical properties were highly influenced by the presence of LCB. Tear resistance is affected by film orientation and is inversely proportional to the level of LCB in the polymer. It was observed a reduction of 50% in the MD tear strength when comparing with the polymer without LCB. However, haze decreases significantly with the presence of LCB, about 40%.

Morfologia Co-Contínua na mistura poliestireno/copolímero de etileno-acetato de vinila

Polystyrene/ethylene ¾ vinyl acetate (EVA) copolymer blends were prepared in an internal mixer with the purpose of investigating the cocontinuous range. The effect of the processing temperature and the chemical composition of the EVA component was studied by scanning electron microscopy (SEM), selective extraction experiments and dynamic mechanical analyses (DMTA). A decrease of mixing temperature and an increase of vinyl acetate content in the EVA component resulted in a shift of the percolation threshold of dispersed PS to lower concentrations. The range where cocontinuous structure can be found depends upon the processing temperature and the EVA nature. The results obtained from selective extraction experiments are in agreement with those obtained from SEM. The changes in phase morphology with the composition and temperature processing was also confirmed by DMTA results.

Poly(ethylene-co-vinyl acetate-g-styrene) from macro-transfer agent: synthesis and use in polymer blend

Abstract A systematic study involving the free-radical graft polymerization of styrene in the presence of ethylene–vinyl acetate (EVA) copolymer functionalized with mercapto groups was investigated in terms of monomer, mercapto group and azo-bis-isobutyronitrile (AIBN) concentrations. The mercapto-modified EVA copolymer (EVALSH) acts as a macro-transfer agent, giving rise to graft copolymers with a relatively high grafting efficiency. High molecular weight of grafted chains and high grafting efficiency can be achieved by using low AIBN content and high amount of styrene. The amount of SH groups in the EVALSH backbone does not exert strong influence on the molecular weight of the grafted segments but increases the conversion of the monomer as graft. The addition of 5 wt.% of this copolymer in polystyrene/EVA (70:30 and 60:40 wt.%) blends resulted in better mechanical performance as compared to that of uncompatibilized blends. In addition, by employing scanning electron microscopy, it was possible to observe that the compatibilization decreased the size of the EVA phase, which became thinner and more elongated, contributing to the formation of a physical interpenetrating network.