1.8 Responsive polymers
1.8.3 CO 2 responsive polymers
One stimulus that has recently garnered a lot of attention is carbon dioxide.76, 164CO 2is an interesting stimulus because it is biocompatible and also possesses good membrane permeability.165An advantage is that repeated applications do not accumulate by-products, whereas repeatedly changing the pH of a system by additions of acid or base may cause salts to accumulate, which may contaminate the system.76 Polymers that respond to CO
2are also of interest due to the potential to use these polymers to trap the gas.166Global emissions of CO2have risen greatly in the last 40 years or so.167Considering that CO2is a key contributor to the greenhouse effect,168the ability to utilise or trap these emissions are of great interest. One way that CO2- responsive polymers can react with CO2 is by the formation of zwitterionic compounds (see Scheme 1.6).166, 169 Endo et al. synthesised a polymer of
4-(1, 4, 5, 6-tetrahydropyrimide-1-yl) methylsytrene (THPS) by free radical polymerisation.166 When a solution of this polymer in DMF was bubbled with carbon dioxide for one hour at room temperature, 73% of the amidine moieties fixed CO2. The fixing efficiency was determined by the weight increase of the reaction mixture.
Scheme 1.6: The formation of the zwitterionic polymer formed upon poly(THPS) reacting with CO2166
A copolymer of THPS and N-vinylacetamide (NVA) was also synthesised by free radical polymerisation and cast as a film. Exposure to CO2at 25 °C for 500 minutes resulted in a fixing efficiency of 25%. Increasing the temperature increased the fixing efficiency (27% at 35 °C and 34% at 45 °C) as a result of higher rates of diffusion of carbon dioxide through the film. Heating the film to 95 °C released the carbon dioxide. The authors demonstrated that the fixing efficiency was not affected over three cycles.166
Carbon dioxide can also react with neutral amidine or amine containing polymers and render them charged.165, 170-173 One area where this has been exploited is in the synthesis of “breathing” vesicles.165, 171, 172 Yan et al. synthesised a diblock copolymer of poly(ethylene oxide) (PEO) and (N-amidino) dodecyl acrylamide (PAD) with narrow dispersity (ÐM = 1.14)viaATRP (see Scheme 1.7).165
The polymers self-assembled in water to form vesicles, evidenced by TEM (Dav= 110 nm) and DLS (Dh = 119 nm). The wall thickness of the vesicles was measured to be 22.5 nm from TEM. After treatment with CO2for 20 minutes the size of the vesicles had increased to 241 nm measured by DLS and 205 nm by TEM (see Figure 1.27). The wall thickness had decreased to 12.5 nm. This is a result of the protonation of the PAD block.
Figure 1.27: TEM images of the PEO-b-PAD vesicles A) before treatment with CO2and B) after treatment with CO2
Analysis of the vesicles by SLS before and after treatment with carbon dioxide, shows that the aggregation number does not significantly change, thereby eliminating the possibility of vesicle fusion as an explanation for the size increase observed. However, the authors noted that should the PAD block be completely protonated, the polymer would be completely hydrophilic and therefore unimers would be formed. Zeta potential measurements confirmed that only 41% of the PAD units are protonated after CO2 treatment. The vesicles could be returned to their original size by treatment with argon. The vesicles were shown to release Rhodamine B from within their central water pools in response to CO2. The amount of Rhodamine B released could be increased by alternating treatment with CO2and argon, as a result of the expansion and retraction movement of the vesicle.165
Zhao and co-workers showed a vesicle to unimer transition in response to carbon dioxide.172 A block copolymer consisting of a relatively short hydrophilic N, N’-dimethylacrylamide (PDMA) block and a longer N, N’-(diethylamino)ethyl methacrylate (PDEAEMA) block
was synthesised by RAFT polymerisation. Self-assembly in water by nano-precipitation formed vesicles (Dh ca. 300 nm). Upon injection of 18 mol% of CO2 into solution, the vesicles dissociated into unimers, evidenced by an increase in transmittance through the solution and a decrease in size to ca. 10 nm observed by DLS. The PDEAEMA block was calculated to be 50% protonated at this concentration of carbon dioxide. Lower mol% of carbon dioxide caused swelling of the vesicles (Dh= 720 nm at 13 mol% CO2), indicating that the vesicles firstly swell before complete dissociation. Purging the unimeric solution with argon removed the CO2 but the vesicles were not reformed and precipitation of the polymer was observed.
Cross-linking the PDEAEMA block by incorporation of coumarin methacrylate (CM) followed by photodimerisation, allowed “breathing” vesicles to be formed. Treatment with carbon dioxide resulted in the vesicle swelling and the degree of swelling could be controlled by the cross-linking density.172 5 mol% of CM was incorporated into the PDEAEMA block and the percentage of photodimerisation controlled by the length of UV irradiation. 90% photodimerised polymers showed a much reduced swelling than polymers that were only 30% dimerised.
Figure 1.28: The amount of swelling of PDMA-b-(PDEAEMA-co-PCM) vesicles is controlled by the percentage of coumarin units that were crosslinked172
hydrophobic PS and PDEAEMA by ATRP.170By varying the length of the middle PS block whilst keeping the PEO and PDEAEMA block length constant, spherical micelles, worm- like micelles or vesicles could be formed upon self-assembly into water. In all cases, the PDEAEMA block was situated in the core (see Figure 1.29).
Figure 1.29: Figure showing the morphologies adopted by the series of PEO-b-PS-b-PDEAEMA triblocks and the morphology deformation upon exposure to CO2170
Purging a solution of spherical micelles with CO2for ten minutes resulted in a size increase from 24 nm to 34 nm, as evidenced by TEM analysis. 30 minutes of bubbling with carbon dioxide resulted in micelles with an average size of 67 nm observed in TEM. The size increase was almost linear with the length of time of carbon dioxide treatment.
The worm-like micelles that formed were observed by TEM to have a large number of curling/curving sites, but after 30 minutes of CO2 exposure the flexible worms had transformed into rigid nanowires (see Figure 1.30).
Figure 1.30: TEM images of worm-like micelles of PEO-b-PS-b-PDEAEMA after a) no exposure to CO2, b) after 15 minutes exposure to CO2, c) after 30 minutes of exposure to CO2and d) the number of curving sites observed in TEM images after different time lengths of CO2exposure170
The self-assembled vesicles were also found to undergo a deformation in response to CO2. The size of the structures did not change upon bubbling with CO2, but the vesicles appeared to have smaller sacs situated within them. All the shape changes were reversible upon bubbling the polymer solutions with nitrogen.
CO2-responsive polymers can also be used to tune the LCST cloud points of thermo- responsive polymers.173, 174Theato and co-workers synthesised a series of doubly-responsive copolymers by firstly synthesising a homopolymer of PFPA by RAFT.173 Three different copolymers (PI – PIII) were then made by substituting the PFP groups with functional primary amines. PI contained isopropyl amine (NIPAM) and 3-N, N-(dimethylamino) propylamine (DMPA), PII contained NIPAM and L-arginine and PIII contained cyclopropylamine (CPA) andL-arginine (see Figure 1.31).
Figure 1.31: The different polymers synthesised by reacting a PFP homopolymer with various amines173
The LCST cloud point of PIwas determined to be 44.8 °C, which is higher that than seen for homopolymers of PNIPAM (31 °C). The LCST cloud point of a PNIPAM homopolymer was shown to be unaffected by bubbling with CO2 but the cloud point of PI increased to 51.1 °C after 25 minutes of purging with CO2. Purging the solution with argon resulted in the cloud point decreasing to that observed before CO2 exposure. PII was not soluble in water, indicating that incorporation of the L-arginine had dramatically decreased the LCST cloud point of the NIPAM block. PIII had the opposite response to CO2 than PI. After exposure to CO2the cloud point was reduced from 54.7 °C to 39.9 °C (see Figure 1.32).
Figure 1.32: The change in LCST cloud point for copolymer 3 after bubbling with carbon dioxide or argon173