Microphase separation in copolymers of hydrophilic PEG blocks and hydrophobic tyrosine-derived segments using simultaneous SAXS/WAXS/DSC.
Hydration- and temperature-induced microphase separations were investigated by simultaneous small- and wide-angle X-ray scattering (SAXS and WAXS) and differential scanning calorimetry (DSC) in a family of copolymers in which hydrophilic poly(ethylene glycol) (PEG) blocks are inserted randomly into a hydrophobic polymer made of either desaminotyrosyl-tyrosine ethyl ester (DTE) or iodinated I(2)DTE segments. Iodination of the tyrosine rings in I(2)DTE increased the X-ray contrast between the hydrophobic and hydrophilic segments in addition to facilitating the study of the effect of iodination on microphase separation. The formation of phase-separated, hydrated PEG domains is of considerable significance as it profoundly affects the polymer properties. The copolymers of DTE (or I(2)DTE) and PEG are a useful model system and the findings presented here may be applicable to other PEG-containing random copolymers as well. In copolymers of PEG and DTE and I(2)DTE, the presence of PEG depressed the glass transition temperature (T(g)) of the copolymer relative to the homopolymer, poly(DTE carbonate), and the DTE/ I(2)DTE segments hindered the crystallization of the PEG segments. In the dry state, at large PEG fractions (> 70 vol%), the PEG domains self-assembled into an ordered structure with 14-18 nm distance between the domains. These domains gave rise to a SAXS peak at all temperatures in the iodinated polymers, but only above the T(g) in non-iodinated polymers, due to the unexpected contrast- match between the crystalline PEG domains and the glassy DTE segments. Irrespective of whether PEG was crystalline or not, immersion of these copolymers in water resulted in the formation of hydrated PEG domains that were 10-20 nm apart. Since both water and the polymer chains must be mobile for the phase separation to occur, the PEG domains disappeared when the water froze, and reappeared as the ice began to melt. This transformation was reversible, and showed hysteresis as did the melting of ice and freezing of the water incorporated into the polymer. PEG-water complexes and PEG-water eutectics were observed in WAXS and DSC scans, respectively.
Murthy
N S
NS
New Jersey Center for Biomaterials, Rutgers University, Piscataway, NJ 08854.
Wang
W
W
Kohn
J
J
J Biol Chem. 1977 Jun 10;252(11):3578-81
405385
Nat Mater. 2005 Jan;4(1):19-31
15689991
Soft Matter. 2005 Aug 5;1(3):238-242
32646080
J Mater Sci Mater Med. 2003 Jan;14(1):9-15
15348533
Soft Matter. 2005 Sep 26;1(4):253-258
32646115
Expert Rev Med Devices. 2005 Nov;2(6):667-71
16293093
J Biomater Sci Polym Ed. 2005;16(5):629-41
16001721
Science. 2010 Feb 5;327(5966):672-5
20133568
Biophys J. 1994 Aug;67(2):706-12
7948683
J Colloid Interface Sci. 2001 Aug 15;240(2):630-635
11482974
Biomaterials. 1999 Feb;20(3):253-64
10030602
Biomaterials. 2004 Jan;25(2):247-58
14585712
J Biomater Sci Polym Ed. 2002;13(12):1337-52
12555900
Biopolymers. 1988 May;27(5):865-81
3260114
Langmuir. 2007 Apr 24;23(9):5039-44
17397197
Biophys J. 1994 Oct;67(4):1724-32
7819504
Science. 1991 Feb 22;251(4996):898-905
17847383
Biophys J. 1999 Nov;77(5):2850-5
10545382
J Biomed Mater Res A. 2003 Dec 15;67(4):1294-304
14624516