Synthesis and aqueous solution properties of new hydrophilic/hydrophilic diblock copolymers.
Well-defined block copolymers of oligo (ethylene glycol) methacrylate (OEGMA)/2-(dimethylamino)ethyl
methacrylate (DMA) and OEGMAl2-(diethylamino)ethyl methacrylate (DEA) have been synthesised via group
transfer polymerisation (GTP). The crude diblock copolymers were contaminated with either DMA or DEA
homopolymer which could be removed by selective precipitation of the copolymers into n-hexane. Polymers
with a range of molecular weights and block compositions were synthesised and characterised by gel
permeation chromatography (GPC) and nuclear magnetic resonance spectroscopy (NMR), which indicated that
well-defined copolymers with narrow molecular weight distributions had been produced.
DMAlOEGMA copolymers were derivatised with either methyl iodide or 1,3-propanesultone in order to
produce copolymers with a permanent cationic or betaine block. The degree of quaternisation was greater than
95 %, as calculated from NMR and / or elemental analyses.
Photon correlation spectroscopy (PCS) was used to examine micellisation of the copolymers in aqueous
solution. DMAlOEGMA copolymers with relatively high DMA contents reversibly formed micelles with DMA
cores and OEGMA coronas above the lower critical solution temperature (LCS1) of the DMA bloc~ cooling
these micellar solutions caused the micelles to completely dissolve. IH NMR spectra indicated that the degree
of hydration of the hydrophobic block did not change as the solution was heated to micellisation temperature, in
other words, the micelle cores were highly hydrated, even at high temperature. 'Anomalous' aggregation of
certain copolymers was also observed; this phenomenon may be due to copolymer compositional heterogeneity.
DMAlOEGMA copolymers were surface active even at room temperature, where the DMA is still hydrophilic;
increasing the solution temperature above the LeST does not appear to affect the surface activity significantly.
The aqueous solution properties of a DMAlOEGMA copolymer were compared to those of a DMAlethylene
oxide (EO) copolymer with the same DMA block length and ethylene glycol content and micellisation was only
observed for the DMAIPEO copolymer. It is thought that that the PEO block is able to stabilise the hydrophobic
micelle cores because the linear arrangement of the ethylene glycol residues allows the formation of a closepacked,
uniform micelle corona. In contrast, when the ethylene glycol residues were arranged into short,
branched chains (OEGMA), stabilisation of the micelle did not occur due to poor packing efficiency of
OEGMA blocks in a micelle corona. Micellisation of the quaternised copolymers was observed in concentrated
salt solution at high temperature. Under these conditions the positively charged DMA block remained soluble
and the OEGMA residues were 'salted out'. A reduction in the solvation and mobility of the OEGMA block in
the micelle core was indicated by a decrease in the NMR signals due to these residues.
At pH 3 the DEAlOEGMA copolymers were fully soluble due to protonation of the amine residues. As the pH
was raised, the DEA residues became deprotonated, and therefore insoluble, and micelles with a DEA core and
OEGMA corona were formed. NMR spectroscopy indicated that the DEA micelle cores were significantly, but
not completely, dehydrated at pH 9. This micellisation was also reversible: addition of acid caused micellar
dissolution. The surface activity of these copolymers is strongly dependent on pH: at low pH the surface activity
is negligible but the copolymers are very surface active at pH 7, with limiting surface tensions as low as 35 mN
mol for I % copolymer solutions.
In both DMAlOEGMA and DEAlOEGMA copolymer micelles the degree of hydration of the micelle core, as
seen by NMR. is higher than might be expected. Micelles with both DMA and DEA cores have been shown by
other authors to be significantly more dehydrated than observed here. It can therefore be concluded that the
presence on OEGMA corona affects the degree of hydration of the micelle core. In particular, it is thought that
the highly branched corona-forming OEGMA block prevents the close packing of the hydrophobic chains
which is required to form a completely dehydrated micelle core.
Standard industrial tests were performed on DMAlOEGMA and DEAlOEGMA copolymers to assess their
performance as novel polymeric surfactants. Both copolymers were found to be poor wetters over a range of
temperatures and pH values. The foaming capacity of DMAlOEGMA was poor, but the DEAlOEGMA
copolymer has high foaming capacity at certain pH and temperatures.