Solid-State NMR and Small-Angle X-ray Scattering Study of Microphase Structure of Amorphous and Semicrystalline Poly(styrene−ethylene oxide) Diblock Copolymers

Synthesis and characterization of novel dumbbell shaped copolymers poly(ethylene oxide)x-b-polystyrene-b-poly(ethylene oxide)x with tunable side arms by combination of efficient thiol-ene addition reaction with living polymerization mechanism

A series of well-defined dumbbell shaped copolymers consisting of PEO and PS segments were successfully synthesized by LAP and ROP mechanisms and the efficient thiol-ene addition reaction. This versatile synthetic route might be used to synthesize other similar copolymers.

Surfactant or block copolymer micelles? Structural properties of a series of well-defined n-alkyl-PEO micelles in water studied by SANS

Here we present an extensive small-angle neutron scattering (SANS) structural characterization of micelles formed by poly(ethylene oxide)-mono-n-alkyl ethers (Cn-PEOx) in dilute aqueous solution. Chemically, Cn-PEOx can be considered as a hybrid between a low-molecular weight surfactant and an amphiphilic block copolymer. The present system, prepared through anionic polymerization techniques, is better defined than other commercially available polymers and allows a very precise and systematic testing of the theoretical predictions from thermodynamical models. The equilibrium micellar properties were elaborated by systematically varying the n-alkyl chain length (n) at constant PEO molecular weight or increasing the soluble block size (x), respectively. The structure was reminiscent of typical spherical star-like micelles i.e. a constant core density profile, ∼r(0), and a diffuse corona density profile, ∼r(-4/3). Through a careful quantitative analysis of the scattering data, it is found that the aggregation number, Nagg initially rapidly decreases with increasing PEO length until it becomes independent at higher PEO molecular weight as expected for star-like micelles. On the other hand, the dependency on the n-alkyl length is significantly stronger than that expected from the theories for star-like block copolymer micelles, Nagg ∼ n(2) similar to what is expected for surfactant micelles. Hence the observed aggregation behavior suggests that the Cn-PEOx micelles exhibit a behavior that can be considered as a hybrid between low-molecular weight surfactant micelles and diblock copolymer micelles.

Effect of interfaces on the melting of PEO confined in triblock PS-b-PEO-b-PS copolymers

Block copolymers form nanostructures that have interesting physical properties because they combine, for a single compound, the complementary features brought by each block. However, in order to fully exploit these properties, the physical state of each kind of domain must be precisely controlled. In this work, triblock PS-b-PEO-b-PS copolymers consisting of a central poly(ethylene oxide) (PEO) block covalently bonded to polystyrene (PS) blocks were synthesized by Atom Transfer Radical Polymerization. Their morphology was investigated by X-ray scattering and TEM experiments whereas their thermodynamic behavior was characterized by DSC. A strong decrease of both the melting temperature and the degree of crystallinity of PEO, due to its confinement between the PS domains, was observed and analyzed with a modified Gibbs-Thomson equation, following the approaches used for fluids confined in porous media. The existence of an amorphous bound layer, a few nanometers thick, at the PEO/PS interface, that does not undergo any phase transition in the temperature range investigated, accounts for both the melting temperature depression and the decrease of crystallinity upon confinement. This interfacial layer may significantly affect the mechanical and transport properties of these block copolymers that find applications as solid polymer electrolytes in batteries for example. Moreover, the value obtained for the solid PEO/liquid PEO surface tension is lower than those previously published but is thermodynamically consistent with the surface tensions of polymers at the solid/vapor and liquid/vapor interfaces.

Properties of small molecular drug loading and diffusion in a fluorinated PEG hydrogel studied by H molecular diffusion NMR and F spin diffusion NMR

R(f)-PEG (fluoroalkyl double-ended poly(ethylene glycol)) hydrogel is potentially useful as a drug delivery depot due to its advanced properties of sol-gel two-phase coexistence and low surface erosion. In this study, (1)H molecular diffusion nuclear magnetic resonance (NMR) and (19)F spin diffusion NMR were used to probe the drug loading and diffusion properties of the R(f)-PEG hydrogel for small anticancer drugs, 5-fluorouracil (FU) and its hydrophobic analog, 1,3-dimethyl-5-fluorouracil (DMFU). It was found that FU has a larger apparent diffusion coefficient than that of DMFU, and the diffusion of the latter was more hindered. The result of (19)F spin diffusion NMR for the corresponding freeze-dried samples indicates that a larger portion of DMFU resided in the R(f) core/IPDU intermediate-layer region (where IPDU refers to isophorone diurethane, as a linker to interconnect the R(f) group and the PEG chain) than that of FU while the opposite is true in the PEG-water phase. To understand the experimental data, a diffusion model was proposed to include: (1) hindered diffusion of the drug molecules in the R(f) core/IPDU-intermediate-layer region; (2) relatively free diffusion of the drug molecules in the PEG-water phase (or region); and (3) diffusive exchange of the probe molecules between the above two regions. This study also shows that molecular diffusion NMR combined with spin diffusion NMR is useful in studying the drug loading and diffusion properties in hydrogels for the purpose of drug delivery applications.

Domain sizes in heterogeneous polymers by spin diffusion using single-quantum and double-quantum dipolar filters

1H spin-diffusion experiments employing a double-quantum (DQ) dipolar filter were performed for the characterization of the microdomain structure of heterogeneous samples. For this purpose the NMR spin-diffusion process was analysed based on a model morphology of three different domains with arbitrary sizes, diffusivities, and filter efficiency. General analytical solutions for z magnetization source and sink were obtained valid for a one-dimensional lamellar morphology in the full range of spin-diffusion times. These solutions of the spin-diffusion equations were used for determining the crystalline, interface, and amorphous domain sizes in polystyrene-poly(ethylene oxide) (PS-PEO) and poly(hydroxyethylmethacrylate)-poly(ethylene oxide) (PHEMA-PEO). The DQ dipolar filter has a good efficiency for PS-PEO but is only partially efficient in filtering the signal of the mobile domains in the PHEMA-PEO diblock copolymer. The domain sizes measured by the DQ filter method are compared to those obtained using the traditional dipolar filter creating z magnetization in the mobile domains.