Small Angle Neutron Scattering Study of Conformation of Oligo(ethylene glycol)-Grafted Polystyrene in Dilute Solutions: Effect of the Backbone Length
Gang Cheng,1 Yuri B. Melnichenko,1 George D. Wignall,1
Fengjun Hua,2 Kunlun Hong,2 and Jimmy W. Mays2
1Neutron Scattering Sciences Division, Oak Ridge National Laboratory
2Center for Nanophase Materials Sciences, Oak Ridge National Laboratory
The cooperative interactions among functional segments of biopolymers have led to attempts to create novel synthetic polymers, which are environmentally responsive to various stimuli, such as temperature or pH, in a controlled manner. Understanding the nanoscale conformational changes and phase behavior upon exposure of these polymers to external stimuli is crucial to the development of a fundamental science foundation that will ultimately lead to novel biosensors able to detect nanoliter and picoliter concentrations, biometric actuators, and tunable surfaces with reversible nanopatterned hydrophobic and hydrophilic properties.
Using small angle neutron scattering (SANS) data, we have discovered that the length of the backbone is a key factor in controlling the structure of amphiphilic comb-like polymers in various solvents. With increasing length of the backbone, the initially rigid polymer starts to behave as a flexible chain, the persistence length of which is mainly determined by the solvent-mediated interactions between oligo(ethylene glycol) side chains. The influence of the end groups on polymer-polymer interactions becomes especially pronounced for short chains. That is, small chain lengths correspond to high relative concentrations of hydrophobic phenyl end groups, which can lead to a tendency for polymer-polymer aggregation on the nanoscale, especially in poor solvents like D2O. As a result, clustering gradually increases in the D2O solutions of these amphiphilic polymers with decreasing degree of polymerization (DP). Strong nano-clustering is observed in all three solvents for polymers with a DP of 8, and the sizes of the nano-clusters are between 100 and 200 Å. The hydrophobic polystyrene backbone of the polymers is in a partially collapsed state in D2O, whereas it expands in toluene and methanol. SANS data also suggest that the individual polymers assume the shape of a rigid cylinder with a relatively sharp interface in D2O and with a rather diffuse interface in toluene and methanol.
Oligoethylene glycol (OEG)-grafted hydrophobic polymers are particularly interesting due to the special solution properties of EG chains, i.e. they are soluble in both water and organic solvents under appropriate conditions. We have thus synthesized both homopolymers as well as block copolymers based on the general structure shown in Scheme 1. These polymers are soluble in water when x³3 and, moreover, we have discovered that they exhibit distinct lower critical solution temperatures (LCST) and their block copolymers exhibit reversible micellization behavior, as shown in Figure 1. Such unique phase behavior is the result of balanced interactions between the hydrophobic main chain, hydrophilic side chain, and the solvent (water). Moreover, the LCST is tunable by changing the side chain length (x).
Small-angle neutron scattering (SANS) is a powerful tool to understand the conformation and phase behavior of polymers. SANS was used to study the global conformation of P(OEG)4Sn with different degree of polymerization (DP) in various deuterated solvents (methanol, water, and toluene). As shown in Figure 2a, in toluene-d8 the polymers assume ellipsoidal, cylindrical or worm-like conformations as the DP of the polymer was increased. In contrast, in D2O (Fig. 2c), their conformations can be described as rigid cylinders. The solvent quality of toluene, methanol and D2O may be categorized as good, marginal and poor, respectively, for this polymer. Consequently, the PS backbone (DP = 85) is in a partially collapsed state in D2O in comparison to that in toluene and methanol, which explains the different global conformations. On the other hand, polymers with a backbone DP of 8 were found to aggregate into clusters in all three solvents, with a characteristic cluster size of 100 to 200 Å.
This work was published in Macromolecules, 2008, 41, 9831. A portion of this research at Oak Ridge National Laboratory's Center for Nanophase Materials Sciences was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.
|Scheme 1 Structure of polystyrene densely grafted with oligo(ethylene glycol) (OEG)x|
Figure 1. (A) Temperature-dependent absorbencies of a 0.5 wt% of P(OEG)2S41-b-P(OEG)4S77 block copolymer in water during heating (■) and cooling (●) cycles (B) Photographs of 0.5 wt% aqueous solution of P(OEG)2S41-b-P(OEG)4S77 at 25o C (bluish) and 37o C (white). The color change is reversible and reflects a transition from a colloidal state to a precipitated state as temperature is increased.
Figure 2. SANS curves of 1.0 wt% PTrEGS homopolymers with various degrees of polymerization (DP) in (a) toluene-d8, (b) methanol-d4 and (c)D2O (the data were shifted by various factors for clarity). The inset in Figure 2a shows the schematic diagram of the shape transition of the polymers with increasing DP. The solid lines are fitted data.
Publication: Gang Cheng, Yuri B. Melnichenko, George D. Wignall, Fengjun Hua, Kunlun Hong, and Jimmy W. Mays “Small Angle Neutron Scattering Study of Conformation of Oligo(ethylene glycol)-Grafted Polystyrene in Dilute Solutions: Effect of the Backbone Length”, Macromolecules, 2008, 41, 9831.