Reverse Thermal Organogel

Abstract

Organogels have been extensively studied during the last decade because of their potential as an oil-recovery medium, sensors, media for electrochemistry, templates for the fabrication of nanostructures, drug-delivery, and rheological modifiers, as well as because of their interesting supramolecular assembly patterns. [1][2][3][4][5] Typically, the gel-forming material, called an organogelator, has marginal solubility in organic solvents because of the presence of a solubilizing alkyl group and strongly interacting groups of peptide, sugar, and aromatic derivatives in the molecule. [6][7][8] The gelation mechanism is not fully understood, and the electron microscopy image of most organogels shows a fibrous network structure in the gel. [5][6][7][8][9][10] Hydrogen bonding, van der Waals interactions, dipole interactions, p-p interactions, and stacking of the secondary structure of peptides have been suggested as driving forces for the gel formation. [8,11] However, most organogels, except for a recently reported cobalt-based coordination compound, [12] have specific melting points and the gel networks disintegrate as the temperature increases. Here, we report the opposite case of an organogel, that is, the poly(ethylene glycol)-poly(L-alanine-co-L-phenyl alanine) diblock copolymer (PEG-PAF), which forms a freeflowing solution in chloroform (1.0 wt. %) at low temperature; however, it forms a gel as the temperature increases above 20°C. The sol-to-gel transition temperature can be controlled by varying the polymer composition or the concentration of the polymer in chloroform. The PEG-PAF was prepared by the ring-opening polymerization of the N-carboxy anhydride of L-alanine and N-carboxy anhydride of L-phenylalanine in the presence of a-methoxy-x-amino poly(ethylene glycol). Two reverse thermal PEG-PAF organogels with different PAF block lengths and the same PEG block were synthesized. Supporting Information (Fig. S1) shows the bell-shaped electrospray mass spectra of the PEG-PAF (M n ∼ 1590 Da, M w / M n ∼ 1.04; 1 Da ≈ 1.66 × 10 -27 kg), PEG-PAF-R (M n ∼ 1530 Da, M w /M n ∼ 1.03), and starting a-methoxy-x-amino poly(ethylene glycol) (M n ∼ 890 Da, M w /M n ∼ 1.02); M n and M w are the number-and weight-average molecular weights of the polymer, respectively. M w /M n is a measure of the polydispersity of the polymer. The phase behavior of both polymers was similar and the following discussion will be focused on PEG-PAF. The dynamic mechanical analysis on PEG-PAF-R will be added only as Supporting Information for this paper. The methyl group of alanine at 1.3-1.7 ppm and the phenyl group of phenylalanine at 7.1-7.3 ppm in the 1 H NMR spectra in deuterated acetic acid (Supporting Information, Fig. S2) were used to calculate the composition of PAF. The mole ratio of alanine to phenyl alanine was 4.2. Based on the electrospray mass spectra (M n ) and NMR data, the actual composition (by mole) of the PEG-PAF was calculated to be 1.0:6.7:1.6 (PEG/alanine/phenyl alanine). The 1 H NMR spectra showed more collapsed alanine ...

Significance of secondary structure in nanostructure formation and thermosensitivity of polypeptide block copolymers

“…19,20 The molecular weight of the polymer was determined by the peak area at 3.7-4.1 ppm (-CH 2 CH 2 O) and 1.5-2.0 ppm (-COCH(CH 3 )NH-) in 1 H-NMR (CF 3 COOD). The molecular weights of the polymers were 1710 ((EG) 21 -(A) 10 ) and 1780 ((EG) 21 -(A) 11 ) for mPEG-L-PA and mPEG-DL-PA, respectively.…”

“…19,20 To prepare mPEG-L-PA, a-methoxy, u-amino poly(ethylene glycol) (2.5 g, 2.5 mmole; MW 1,000 daltons) was dissolved in anhydrous toluene (50 mL) and the residual water was removed by azeotropic distillation to a final volume of about 5 mL. Anhydrous chloroform/N,Ndimethyl formide (30 mL; 2/1 v/v), N-carboxy anhydrides of L-alanine (4.0 g, 34.8 mmole) were added to the reaction mixtures.…”

Rheology of associative polymer solutions

“…Tang et al [28] observed gelation for aqueous solutions of PMMA-b-PNIPAM induced by aggregation of the PNIPAM corona above its critical temperature. Similarly Jeong et al [29] observed gelation of PEO-bpoly(L-alanine-co-L-phenylalanine) in chloroform with increasing temperature due to the increasing attractions between the PEO coronas. Grandjean et al [30] studied the aggregation and gelation of PS-P(AA/EA) in water and found that attraction between the coronas increased if they contained a larger fraction of EA.…”

Injectable biodegradable materials

“…When the PEG-polypeptide was dissolved in chloroform, the polymer solution also underwent sol-to-gel transition as the temperature increased. 78,79 The desolvation of the chloroform from the PEG is responsible for the transition. When the polypeptides have a mixed conformation of -sheets and -helixes in chloroform, the gel morphology tends to be a spherical micelles or short tubular structure, whereas the gel morphology tends to be a fibrous structure when the polypeptides dominantly form a -sheet conformation in chloroform.…”