Rheological properties of RAFT-mediated poly(styrene-co-butyl acrylate)-clay nanocomposites [P(S-co-BA)-PCNs]: Emphasis on the effect of structural parameters on thermo-mechanical and melt flow behaviors

RAFT-mediated random poly(styrene-co-butyl acrylate)-[N-(4-((((dodecylthio)-carbonothioyl)thio)methyl)benzyl)-N,N-dimethylethanammonium]-clay nanocomposites (P(S-co-BA)-DCTBAB-PCNs) and poly(styrene-co-butyl acrylate)-[N,N-dimethyl-N-(4-(((phenylcarbonothioyl)thio)-methyl)benzyl)ethanammonium]-clay nanocomposites (P(S-co-BA)-PCDBAB-PCNs) were prepared by miniemulsion free-radical polymerization. The RAFT agents (i.e. DCTBAB and PCDBAB) were anchored onto the clay layers prior to polymerization, and were able to control the polymerization process, as evident from the decreasing molar mass and polydispersity index (PDI) values as the concentration of the RAFT agent in the system increased. The efficiency of the anchored RAFT agents increased as the RAFT agent concentration in the system increased, i.e. as the clay loading increased. The nanocomposites that were prepared were found to have a partially exfoliated morphology at low clay loadings, as determined by SAXS and TEM, whereas, at high clay loadings the morphology changed to become predominantly intercalated. The thermo-mechanical properties of the nanocomposites were found to be a function of the molar mass, PDI, PCN morphology, and clay loading. In the glassy state, the storage modulus was seen to effectively decrease as clay loading increased, whereas the opposite was true for the loss modulus and tan delta. At low clay loadings the melt rheological properties were dominated by the matrix effects, whereas at high clay loadings the effect of the clay filler dominated, resulting in pseudo solid-liquid-like behavior. © 2008 Elsevier Ltd. All rights reserved.Articl

Encapsulated clay particles in polystyrene by RAFT mediated miniemulsion polymerization

RAFT grafted montmorillonite (MMT) clays [i.e., N,N-dimethyl-N-(4- (((phenylcarbonothioyl)thio)methyl)benzyl)ethanammonium-MMT (PCDBAB-MMT) and N-(4-((((dodecylthio)carbonothioyl)thio)methyl)benzyl)-N,N- dimethylethanammonium-MMT (DCTBAB-MMT)] of various loadings were dispersed in styrene (S) monomer and the resultant mixtures emulsified and sonicated in the presence of a hydrophobe (hexadecane) into miniemulsions. The stable miniemulsions thus obtained were polymerized to yield encapsulated polystyrene-clay nanocomposites (PS-CNs). The molar mass and polydispersity index (PDI) of the PS-CNs depended on the amount of RAFT agent in the system, in accordance with the features of the RAFT process. The morphology of the PS-CNs ranged from partially exfoliated to an intercalated morphology, depending on the percentage clay loading. The thermomechanical properties of the PS-CNs were better than those of the neat PS polymer, and were dependent on the molar mass, PS-CN morphology and clay loading. The similarities and differences of the PS-CNs prepared here by miniemulsion polymerization were compared to those prepared using the same RAFT agents and polymer system by bulk polymerization (as reported by us in a previous article). © 2008 Wiley Periodicals, Inc.Articl

Field Irrigation Practice and the Performance of Smallholder Irrigation in Zimbabwe: Case Studies from Chakohwa and Mpudzi Irrigation Schemes

Developing economies have often regarded smallholder irrigation as a means to ensure food security, create employment and promote agro-based industries in rural areas. However, of late smallholder irrigation has come under scrutiny as its performance has fallen short of expectations. This has prompted the need for an understanding of irrigation practices and their impact on performance so that remedial measures can be implemented. An analysis of smallholder irrigation practices and the subsequent impact on performance for two irrigation schemes located in the drier parts of Zimbabwe are presented. One is a surface irrigated scheme (Chakohwa scheme) measuring 90 ha while the other is drag hose sprinkler irrigated scheme (Mpudzi scheme) measuring 48 ha. Farmer practices were assessed through observation and use of structured interviews. Technical performance measures relating to the efficiency, adequacy and uniformity of irrigation events were assessed using standard field evaluation methods. To a large extent, the farmers\' practices were determined by their perceptions and understanding of irrigation events. An assortment of irrigation scheduling methods was practised, water application approaches were not as per system design and there was a general tendency to over irrigate. The performance of both schemes was generally poor as the technical performance measures did not match the expected irrigation standards. At Chakohwa irrigation scheme, application efficiency averaged a low 24.7%, deep percolation losses a high 75%, and requirement efficiency a high of 87.3%. At Mpudzi drag hose irrigation scheme, the distribution uniformity was 44.9%, application efficiency of the lower quarter was low at 26.4%, and Christiansen\'s uniformity coefficient averaged 62.6%. These results indicated that the effectiveness of irrigation was low to medium implying an inefficient utilisation of resources, especially water, in irrigation. This has implications on sustainability of smallholder irrigation. Despite these technical performance shortcomings the irrigation schemes contributed significantly to the livelihoods of the irrigators. J. agric. sci. technol. Vol.5(1) 2003: 1-2

Use of acrylic based surfmers for the preparation of exfoliated polystyrene-clay nanocomposites

Two polymerizable cationic surfactants, (11-acryloyloxyundecyl)dimethyl(2-hydroxyethyl)ammonium bromide (hydroxyethyl surfmer) and (11-acryloyloxyundecyl)dimethylethylammonium bromide (ethyl surfmer), were used for the modification of montmorillonite (MMT) clay. The modification of MMT dispersions was carried out by ion exchange of the sodium ions in Na+-MMT by surfactants in aqueous media. Modified MMT clays were then dispersed in styrene and subsequently polymerized in bulk by a free-radical polymerization reaction to yield polystyrene-clay nanocomposites. An exfoliated structure was obtained using the ethyl surfmer-modified clay, whereas a mixed exfoliated/intercalated structure was obtained using the hydroxyethyl surfmer-modified clay. Nanocomposite structures were confirmed by small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). The nanocomposites exhibited enhanced thermal stability and an increase in glass transition temperature, relative to neat polystyrene. The nanocomposites also exhibited enhanced mechanical properties, which were dependent on the clay loading. Intercalated polystyrene-clay nanocomposites were obtained using the non-polymerizable surfactant-modified clay (cetyltrimethylammonium bromide). Nanocomposites made from mixtures of surfmer-modified and CTAB-modified clays were also prepared, showing intermediate properties. However, when the nanocomposites were prepared in solution only intercalated morphologies were obtained. This was attributed to the competition between the solvent molecules and monomer in penetrating into clay galleries. These nanocomposites also exhibited enhanced thermal stability relative to the virgin polystyrene prepared by the same method. Similar temperatures of degradation (at 50% decomposition) were found for these nanocomposites relative to those prepared by bulk polymerization. © 2006 Elsevier Ltd. All rights reserved.Articl