Diffusion in microstructured block copolymer solutions

This work explores the effect of heterogeneity of chemical composition on tracer diffusion, when the characteristic size of the heterogeneities approaches that of the diffusing molecule. A heterogeneous environment is created by the self-assembly of diblock copolymers in solution. The system chosen for this study is polystyrene-polyisoprene diblock copolymers in toluene, which is a common solvent for the two blocks. Above a certain critical concentration, these systems are known to microphase separate into swollen domains of polystyrene and polyisoprene. Diffusion of homopolystyrene through the microstructure is measured in this work. The characteristics of the microstructure are varied by studying block copolymers of different molecular weights and compositions. The tracer diffusion coefficients of the labelled polystyrenes are measured by forced Rayleigh scattering, while the microstructure of the matrix is inferred from small angle X-ray scattering measurements. In this paper, we report results for the diffusion of polystyrenes (molecular weights 3.2 × 10⁴ and 9.0 × 10⁴) in microstructured solutions of three copolymers. Two copolymer samples forming lamellae of alternating polystyrene and polyisoprene microphases and one sample forming polystyrene cylinders embedded in a polyisoprene matrix have been examined. The data indicate that the tracer diffusion coefficient of 3.2 × 10⁴ molecular weight polystyrene in lamellar and cylindrical solutions is comparable to that of the homopolymer in a homogeneous solution of the same concentration. In contrast, the diffusion of 9.0 × 10⁴ molecular weight polystyrene is substantially slower in the structured solutions than in unstructured solutions of the same concentration. For example, the diffusion coefficient of 9.0 × 10⁴ molecular weight polystyrene in a 47% block copolymer solution with cylindrical microstructure is lower than its value in a homogeneous solution of the same concentration by a factor of 30.     

Templated biological synthesis of polymers of abiological monomers

A general method for the in vivo incorporation of amino acid analogues into artificial proteins is described. The method involves the construction of an artificial gene encoding the sequence of interest (with the corresponding natural amino acid encoded in place of the analogue), transformation of a bacterial host strain that cannot synthesize the natural amino acid, and induction of protein synthesis in a host culture enriched in the analogue. Results are described for the amino acid analogues selenomethionine, p-fluorophenylalanine, trifluoroleucine and 3-thienylalanine.     

Three-dimensional Full-field Measurements of Large Deformations in Soft Materials Using Confocal Microscopy and Digital Volume Correlation

A three-dimensional (3-D) full-field measurement technique was developed for measuring large deformations in optically transparent soft materials. The technique utilizes a digital volume correlation (DVC) algorithm to track motions of subvolumes within 3-D images obtained using fluorescence confocal microscopy. In order to extend the strain measurement capability to the large deformation regime (>5%), a stretch-correlation algorithm was developed and implemented into the Fast Fourier Transform (FFT)-based DVC algorithm. The stretch-correlation algorithm uses a logarithmic coordinate transformation to convert the stretch-correlation problem into a translational correlation problem under the assumption of small rotation and shear. Estimates of the measurement precision are provided by stationary and translation tests. The proposed measurement technique was used to measure large deformations in a transparent agarose gel sample embedded with fluorescent particles under uniaxial compression. The technique was also employed to measure non-uniform deformation fields near a hard spherical inclusion under far-field uniaxial compression. Introduction of the stretch-correlation algorithm greatly improved the strain measurement accuracy by providing better precision especially under large deformation. Also, the deconvolution of confocal images improved the accuracy of the measurement in the direction of the optical axis. These results shows that the proposed technique is well-suited for investigating cell-matrix mechanical interactions as well as for obtaining local constitutive properties of soft biological materials including tissues in 3-D.