Search

Injectable hydrogels show substantial promise for use in minimally invasive tissue engineering and drug delivery procedures. A new injectable hydrogel material, developed from recombinant telechelic proteins expressed in E. coli, demonstrates shear thinning by 3 orders of magnitude at large strains. Large-amplitude oscillatory shear illustrates that shear thinning is due to yielding within the bulk of the gel, and the rheological response and flow profiles are consistent with a shear-banding mechanism for yielding. The sharp yielding transition and large magnitude of the apparent shear thinning allow gels to be injected through narrow gauge needles with only gentle hand pressure. After injection the gels reset to full elastic strength in seconds due to rapid re-formation of the physical network junctions, allowing self-supporting structures to be formed. The shear thinning and recovery behavior is largely independent of the midblock length, enabling genetic engineering to be used to control the equilibrium modulus of the gel without loss of the characteristic yielding behavior. The shear-banding mechanism localizes deformation during flow into narrow regions of the gels, allowing more than 95% of seeded cells to survive the injection process. [ABSTRACT FROM AUTHOR]

The Flory−Huggins interaction is measured for a model rod−coil block copolymer system, poly(alkoxyphenylenevinylene- b-isoprene), by fitting the interfacial segregation of block copolymer to a homopolymer interface and by using the random phase approximation (RPA) for block copolymers. The measured interfacial segregation of a block copolymer to the interface between homopolymers, fit with a self-consistent field theory (SCFT) simulation using χ as a variable parameter, gives a functional form χ = 34.8/ T− 0.091. When RPA is applied to neutron scattering curves for the rod−coil system above the order−disorder transition, the theoretical structure factors are inconsistent with observed scattering curves due to complex aggregated structures formed in the nematic and isotropic states. Using the Flory−Huggins parameter and a previously measured value of the Maier−Saupe parameter, the PPV- b-PI phase diagram may be converted from system-specific variables to dimensionless parameters. Under the assumptions that the rods are ideal nematogens, interaction strengths are composition-independent, and rod−coil and rod−rod interactions are local, this yields the first quantitative universal phase diagram for rod−coil block copolymers. [ABSTRACT FROM AUTHOR]

Unlike the rounded grains that are well known to form in most soft materials, square grains of microphase-separated lamellae are observed in thin films of a rod−coil block copolymer because of hierarchical structuring originating from the molecular packing of the rods. The square grains are oriented with lamellar layers parallel to the film interface and result from growth along orthogonal low-surface-energy directions as a result of the effects of the tetragonal crystalline lattice that forms within the rod-rich lamellar nanodomains of poly(2,5-di(2‘-ethylhexyloxy)-1,4-phenylene vinylene)-b-polyisoprene (PPV-b-PI). These grain shapes form only for a narrow range of coil volume fractions around 72% as a result of kinetic barriers at lower coil fractions and disordering of the lattice at higher coil fractions, and the polydisperse grain size suggests that growth is nucleation-limited. The grains form in both weakly and moderately segregated polymers at all annealing temperatures below the order−disorder transition, and they are observed for all thicknesses at which parallel-oriented grains are grown. [ABSTRACT FROM AUTHOR]

The rod orientation and crystalline packing of a model semiconducting rodlike polymer, poly(2,5-di(2‘-ethylhexyloxy)-1,4-phenylenevinylene) (DEH−PPV), is shown to affect the self-assembly of weakly segregated rod−coil block copolymers. The in-plane packing of DEH−PPV rods in lamellar poly(2,5-di(2‘-ethylhexyloxy)-1,4-phenylenevinylene-b-isoprene) (DEH−PPV-b-PI) diblock copolymers is nearly identical to that observed in DEH−PPV homopolymers for compositions ranging from 0.42 to 0.82 vol % coil block. The crystal structure of DEH−PPV, characterized by grazing incidence X-ray diffraction and electron diffraction, consists of a tetragonal unit cell having c0.665 nm with ab1.348 nm. The polymer chain axis is aligned along the 001 direction, and the nearest neighbor rod−rod spacing along 〈110〉 is 1.0 nm. As-cast thin films of DEH−PPV homopolymer demonstrate chain alignment primarily perpendicular to the substrate in 5100 g/mol homopolymer, while for 3500 g/mol homopolymer the chains align both perpendicular and parallel to the substrate. For the DEH−PPV-b-PI block copolymers, a sharper 001 reflection is observed due to the effect of microphase separation, improving alignment and stacking of the rods. The lamellar phases have a smectic A-like packing structure with the rods oriented parallel to the lamellar normal regardless of coil fraction; however, at coil fractions above about 0.8 the crystalline lattice of the rods becomes rapidly disordered as evidenced by loss of all but the two strongest Bragg reflections. This suggests that the constraints of packing into the unit cell outweigh the chain stretching and segment−segment repulsion energies that are predicted to lead to a transition from normal (smectic A) to tilted (smectic C) rod orientation within the lamellae at high coil fraction; increasing coil fraction breaks apart the crystalline lattice rather than distorting it into a tilted polymorph. [ABSTRACT FROM AUTHOR]

A new phase consisting of rectangular rod nanodomains packed onto a hexagonal lattice is observed in rod−coil block copolymers at the limit of both large volume fraction asymmetry and large geometrical asymmetry between the rod and coil. In moderately segregated poly(alkoxyphenylenevinylene-b-isoprene) (PPV-b-PI), an order−order transition is observed between hexagonal and lamellar phases for polymers near the phase boundary, and the lamellar phase is observed at high temperatures. The domain spacings of polymers in the lamellar phase collapse on to a simple scaling relationship where domain spacing is proportional to molecular weight. The proportionality constant is equal to the statistical segment length of the PPV rod block, suggesting that the angle between the rod director and the lamellar interface is nearly 90°. At higher temperatures, the block copolymers transition from ordered to nematic to isotropic states, with the intermediate nematic phase being observed for all coil fractions studied. A three-dimensional phase diagram shows the microphase and liquid crystalline transitions in rod−coil block copolymers as a function of temperature, geometrical asymmetry, and coil fraction. [ABSTRACT FROM AUTHOR]

The addition of homopolymers to a self-assembling rod−coil block copolymer is demonstrated to be a flexible route toward domain size control. Molecular weight matched rod-like homopolymers interdigitate with the rod-blocks within their respective lamellae. As a result of the interdigitation, the coil blocks must rearrange to occupy more interfacial area resulting in an unprecedented decrease in domain spacing with increasing rod homopolymers. Conversely coil homopolymers were locally solubilized within the coil microdomain resulting in an increase of domain spacing with increasing coil homopolymers. The mechanisms of homopolymers solubilization are in qualitative agreement with predictions made by self-consistent mean-field theory (SCFT) calculations. [ABSTRACT FROM AUTHOR]

Poly(alkoxyphenylenevinylene-b-isoprene) (PPV-b-PI) rod−coil block copolymers demonstrate novel structures due to the rodlike PPV block. Thin films of the polymers self-assemble into lamellar microphases upon thermal annealing with the lamellae oriented primarily parallel to the substrate. The parallel lamellae show symmetric wetting of PI at both the substrate and vacuum interfaces. Grains of lamellae with parallel orientation are characterized by irregular polygon shapes and are bounded by defect regions where the lamellae are oriented out of the plane of the film. Grazing-incidence small-angle X-ray scattering (GISAXS) shows that these out-of-plane lamellae are strongly oriented perpendicular to the film. The perpendicular lamellae are much straighter than those typically observed in coil−coil block copolymers due to the high bending energy of the liquid crystalline rod nanodomains. Islands or holes form in the films, and domain spacings estimated from the island/hole heights are equal to the bulk domain spacing. The perpendicular “defect” lamellae mediate the change in thickness required to transition between islands or holes and the surrounding region. Increasing film thickness results in an increasing fraction of the surface covered by perpendicular lamellae, presumably due to limited penetration of the substrate orienting field into the film. At great enough thickness total reorientation of the lamellar structure from parallel to perpendicular orientation at the vacuum interface is observed. [ABSTRACT FROM AUTHOR]