Block Copolymers as Stabilisers in Emulsion Polymerisation

Most aqueous emulsion and dispersion polymerisation that have been reported are based on a few commercial products with a broad molecular weight distribution and varying block composition. The results obtained from these studies could not establish the effect that the structural features of the block copolymer would have on their stabilising ability and effectiveness in polymerisation. Fortunately, model block copolymers with well-defined structures can be synthesised, and their roles in emulsion polymerisation have been determined using model polymers and model latexes. 

A series of well-defined A-B block copolymers of polystyrene-block-polyethylene oxide (PS-PEO) were synthesised [6] and used for the emulsion polymerisation of styrene. These molecules are ideal as the polystyrene (PS) block is compatible with the PS formed, and thus it forms the best anchor chain. The PEO chain (the stabilising chain) is strongly hydrated with water molecules and extends into the aqueous phase where it forms the steric layer necessary for stabilisation. 

However, the PEO chain can become dehydrated at high temperatures (due to breakage of hydrogen bonds), which reduces the effective steric stabihsation. Thus, emulsion polymerisation should be carried out at temperatures well below the theta (0)-temperature of PEO. 

The results showed clearly the effect of these factors on the resulting latex. For example, when using a block copolymer with 400 PMMA and 750 PEO (i.e., containing 65wt% PEO), the resulting latex had fewer particles when compared to the other surfactants. The most dramatic effect was obtained when the of PMMA was increased to 900 while keeping the of PEO unchanged (750). This block 

Four graft copolymers were synthesised with a PMSt backbone with 4540, while increasing the wt% of PEO (68, 73, 82 and 92 wt%, corresponding to 4.8, 6, 

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