Article

Comparison between the LCST and UCST Transitions of Double Thermoresponsive Diblock Copolymers: Insights into the Behavior of POEGMA in Alcohols

Macromolecules (Impact Factor: 5.93). 04/2012; 45(7):3221-3230. DOI: 10.1021/ma300374y

ABSTRACT Doubly thermoresponsive polymers consisting of a poly[oligo(ethylene glycol) methyl ether methacrylate] (POEGMA) block displaying UCST behavior in alcohols and a block of poly(N-isopropylacrylamide) (PNIPAM) or poly(N,N-diethylacrylamide) (PDEAM), each of which has an LCST in water, were synthesized using RAFT polymerization followed by simultaneous activated ester/amine and nucleophilic thiol?ene postpolymerization conversions. Upon heating aqueous solutions of POEGMA-b-PNIPAM, 1H NMR spectroscopy confirmed a sudden decrease of the PNIPAM signals at the LCST, indicating dehydration and chain collapse. Dynamic light scattering (DLS) and turbidity measurements observed the macroscopic phase separation of the PNIPAM block at the same temperature. In 2-propanol, 1H NMR spectroscopy showed a gradual decrease of the POEGMA signals over a range of more than 30 °C during its UCST transition, indicating early stages of chain crumpling up to 20 °C above the macroscopic phase separation. The OEG side chains were found to collapse onto the backbone starting at the ester linkages, indicating the most unfavorable enthalpic polymer?solvent interactions occur adjacent to the ester group. Although the diblock copolymers displayed a strong concentration-dependent cloud point, 1H NMR spectroscopy revealed a concentration-independent desolvation, indicating the potential for applications that are not based on phase separation but on changes of polymer conformation. The phase separation occurred within a narrow temperature range of ?6 °C as evidenced by turbidity and DLS. This transition could be exploited to self-assemble POEGMA-b-PDEAM into micellar structures with POEGMA cores in 1-octanol. Cooling to ?15 °C below the cloud point was necessary to produce compact structures. Upon heating, the aggregates remained compact until redissolving entirely within a range of 1 °C, making the UCST of POEGMA in alcohols a valuable tool for reversible self-assembly applications.

Download full-text

Full-text

Available from: Peter J. Roth, Apr 30, 2014
0 Followers
 · 
151 Views
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The upper critical solution temperature (UCST) behaviour of poly[oligo(ethylene glycol) monomethylether methacrylate] (POEGMA) in 1-octanol was exploited to self-assemble and crosslink micellar aggregates. Four diblock copolymers, POEGMA-block-poly(N-isopropylacrylamide) (POEGMA-b-PNIPAM), POEGMA-b-poly(N,N-diethylacrylamide) (POEGMA-b-PDEAM), and two examples of POEGMA-b-[PNIPAM-co-poly(pentafluorophenyl acrylate)] (POEGMA-b(PNIPAM-co-PPFPA)), containing different amounts of activated PFP esters, were found to reversibly self-assemble into well-defined spherical micelles upon cooling in 1-octanol, as evidenced by dynamic light scattering (DLS) and electron microscopy. Transition temperatures, measured by turbidity and DLS, were around room temperature and the PNIPAM and PDEAM blocks did not significantly influence the critical temperature of the POEGMA block compared to homo POEGMA. The aggregates exhibited an inverted morphology compared to PNIPAM core POEGMA shell micelles accessible through a thermally triggered self-assembly in water exploiting the lower critical solution temperature (LCST) behaviour of PNIPAM. Inverted micelles with PNIPAM-co-PPFPA coronae in 1-octanol were shell crosslinked using a diamine and subsequently transferred into water. This procedure yielded cage-like structures with swollen POEGMA cores and crosslinked PNIPAM shells. The shells reversibly collapsed onto the cores when heated above the LCST of PNIPAM, providing particles with novel architecture and the potential to host and release guest molecules by a thermal trigger.
    01/2012; 3(8-8):2228-2235. DOI:10.1039/c2py20204b
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Two series of random copolymers of oligo(ethylene glycol) phenyl ether acrylate (OEGPhA) with either oligo(ethylene glycol) methyl ether acrylate (OEGMeA) or oligo(ethylene glycol) methyl ether methacrylate (OEGMeMA) with varying molar fractions of OEGPhA comonomer, comparable molecular weights (similar to 14 kg mol(-1)) and low molecular weight distributions (1.19-1.36) were prepared by the RAFT process. In aqueous solution copolymers exhibited a lower critical solution temperature (LCST) which decreased linearly with an increasing content of phenyl ethers. In a similar trend of decreasing solubility, the upper critical solution temperature (UCST) of the OEGMeA/OEGPhA series in 2-propanol increased linearly from 11.8 degrees C to 73.6 degrees C for (co)polymers containing from 0 to 71 mol% of OEGPhA units. Variable temperature NMR measurements in 2-propanol-d(8) performed on a diblock copolymer containing a p(OEGMeA-co-OEGPhA) block and a soluble p(dimethyl acrylamide) block revealed the phenyl ethers to remain solvated to a large extent at temperatures 30 degrees C below the cloud point of the p(OEGMeA-co-OEGPhA) block. The effect of the phenyl groups decreasing solubility in alcohols was attributed to a promotion of favourable polymer-polymer interactions and a decreased contribution to mixing entropy due to rigidity. For high OEGPhA content a phase diagram in ethanol-water mixtures showed cononsolvency in the LCST regime (high water content) followed by a miscibility gap. The UCST regime, showing a solubility maximum at approx. 80 vol% ethanol extended to ethanol concentrations as low as 55 vol%. UCST transitions in intermediate ethanol-water mixtures, unknown for OEGMe(M)A homopolymers, are believed to increase the applicability of non-linear PEG analogues.
    Soft Matter 01/2013; 9(6):1825-. DOI:10.1039/c2sm27427b · 4.15 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Dynamic thermally reversible hydration behavior of a well-defined thermoresponsive copolymer P(MEO2MA-co-OEGMA475) in D2O synthesized by ATRP random copolymerization of 2-(2-methoxyethoxy)ethyl methacrylate (MEO2MA) and oligo(ethylene glycol) methacrylate (Mn = 475 g/mol) was studied by means of IR spectroscopy in combination with perturbation correlation moving window (PCMW) technique and two-dimensional correlation spectroscopy (2DCOS). Largely different from poly(N-isopropylacrylamide) (PNIPAM), P(MEO2MA-co-OEGMA475) exhibits a sharp change below LCST and a gradual change above LCST due to the absence of strong intermolecular hydrogen bonding interactions between polymer chains, and the apparent phase transition is mainly arising from the multiple chain aggregation without a precontraction process of individual polymer chains. Additionally, the self-aggregation process of P(MEO2MA-co-OEGMA475) is found to be mainly dominated or driven by the conformation changes of oxyethylene side chains, which collapse first to get close to the hydrophobic backbones and then distort to expose hydrophilic ether oxygen groups to the “outer shell” of polymer chains as much as possible. On the other hand, PCMW easily determined the phase transition temperature to be ca. 32.5 °C during heating and ca. 31 °C during cooling as well as the transition temperature range to be 28.5–37 °C. 2DCOS was finally employed to discern the sequence order of all the group motions during heating and cooling. It is concluded that during the phase transition P(MEO2MA-co-OEGMA475) chains successively experience “hydrated chains–dehydrated chains–loosely aggregated micelles–densely aggregated micelles” four consecutive conformation changes. The results were further confirmed by temperature-variable 1H NMR analysis and molecular dynamics simulation.
    Macromolecules 01/2013; 46(1):236-246. DOI:10.1021/ma3022376 · 5.93 Impact Factor