Controlling Rheology of Fluid Interfaces through Microblock Length of Sequence-Controlled Amphiphilic Copolymers.

Previous studies have demonstrated that films of sequence-controlled amphiphilic copolymers display contact angles that depend on microblock size. This suggests that microblock length may provide a means of tuning surface and interfacial properties. In this work, the interfacial rheology of a series of sequence-controlled copolymers, prepared through the addition of bicyclo[4.2.0]oct-1(8)-ene-8-carboxamide (monomer A) and cyclohexene (monomer B) to generate sequences up to 24 monomeric units composed of (A m B n ) i microblocks, where m, n, and i range from 1 to 6. Interfacial rheometry is used to measure the mechanical properties of an air-water interface with these copolymers. As the microblock size increases, the interfacial storage modulus, G', increases, which may be due to an increase in the size of interfacial hydrophobic domains. Small-angle X-ray scattering shows that the copolymers have a similar conformation in solution, suggesting that any variations in the mechanics of the interface are due to assembly at the interface, and not on solution association or bulk rheological properties. This is the first study demonstrating that microblock size can be used to control interfacial rheology of amphiphilic copolymers. Thus, the results provide a new strategy for controlling the dynamics of fluid interfaces through precision sequence-controlled polymers.

Techniques to characterize dynamics in biomaterials microenvironments: XPCS and microrheology of alginate/PEO-PPO-PEO hydrogels.

Many recent studies have highlighted the timescale for stress relaxation of biomaterials on the microscale as an important factor in regulating a number of cell-material interactions, including cell spreading, proliferation, and differentiation. Relevant timescales on the order of 0.1-100 s have been suggested by several studies. While such timescales are accessible through conventional mechanical rheology, several biomaterials have heterogeneous structures, and stress relaxation mechanisms of the bulk material may not correspond to that experienced in the cellular microenvironment. Here we employ X-ray photon correlation spectroscopy (XPCS) to explore the temperature-dependent dynamics, relaxation time, and microrheology of multicomponent hydrogels comprising of commercial poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymer F127 and alginate. Previous studies on this system have shown thermoreversible behavior in the bulk oscillatory shear rheology. At physiological temperatures, bulk rheology of these samples shows behavior characteristic of a soft solid, with G' > G'' and no crossover between G' and G'' over the measurable frequency range, indicating a relaxation time >125 s. By contrast, XPCS-based microrheology shows viscoelastic behavior at low frequencies, and XPCS-derived correlation functions show relaxation times ranging from 10-45 s on smaller length scales. Thus, we are able to use XPCS to effectively probe the viscoelasticity and relaxation behavior within the material microenvironments.

Temperature-dependent structure and compressive mechanical behavior of alginate/polyethylene oxide-poly(propylene oxide)-poly(ethylene oxide) hydrogels.

We report the structural and mechanical behavior of multicomponent hydrogels comprising the commercial poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymer F127 and alginate. Previous studies on this system have shown thermoreversible behavior in shear rheology. Here we explore the properties of these materials under compression and large deformations, relevant to applications such as wound dressings that require mechanical robustness. For gels with lower F127 concentration, we find that the stiffness of the gels can be ascribed to the alginate network, and that the Young's modulus and fracture stress do not strongly depend on temperatures. However, for gels with an F127 concentration of 30 wt %, the Young's modulus is enhanced at higher temperatures. Under large deformations, the fracture stress and fracture strain of the materials can be independently varied using the alginate and F127 concentrations, respectively; without the trade-off in these properties that is often observed in rigid polymer networks. Small-angle X-ray scattering shows a power-law dependence scattering intensity on q arising from the alginate network and scattering peaks consistent with rearranging micelles. For gels with lower F127 concentrations, we find a disordered-body-centered cubic (BCC)-face-centered cubic (FCC) progression of states with temperature, and a BCC/FCC mixture for gels with higher F127 concentrations.

Hierarchical assembly in PLA-PEO-PLA hydrogels with crystalline domains and effect of block stereochemistry.

Understanding the development of microstructure (e.g., structures with length scales roughly 0.5-500 µm) in hydrogels is crucial for their use in several biomedical applications. We utilize ultra-small-angle neutron scattering (USANS) and confocal microscopy to explore microstructure of poly(lactide)-poly(ethylene oxide)-poly(lactide) (PLA-PEO-PLA) triblock copolymer hydrogels with varying l/d-lactide ratio. We have previously found that these polymers self-assemble on the nanoscale into micelles. Here, we observe large-scale structures with diverse morphologies, including highly porous self-similar networks with characteristic sizes spanning approximately 120 nm-200 µm. These structural features give rise to power-law scattering indicative of fractal structures in USANS. Mass fractal and surface fractal structures are found for gels with l/d ratios of 80/20 and 50/50, respectively. Confocal microscopy shows microscale water-filled channels and pores that are more clearly evident in gels with a higher fraction of l-lactide in the PLA block as compared to the 50/50 hydrogels. Tuning block stereochemistry may provide a means of controlling the self-assembly and structural evolution at both the nanoscale and microscale, impacting application of these materials in tissue engineering and drug delivery.

Impact of stereochemistry on rheology and nanostructure of PLA-PEO-PLA triblocks: stiff gels at intermediate l/d-lactide ratios.

We report rheology and structural studies of poly(lactide)-poly(ethylene oxide)-poly(lactide) (PLA-PEO-PLA) triblock copolymer gels with various ratios of l-lactide and d-lactide in the PLA blocks. These materials form associative micellar gels in water, and previous work has shown that stereoregular triblocks with a l/d ratio of 100/0 form much stiffer gels than triblocks with a 50/50 l/d ratio. Our systems display an unexpected maximum in the storage modulus, G', of the hydrogels at intermediate l/d ratio. The impact of stereochemistry on the rheology is very striking; gels with an l/d ratio of 85/15 have storage moduli that are ∼1-2 orders of magnitude higher than hydrogels with l/d ratios of 100/0. No stereocomplexation is observed in the gels, although PLLA crystals are found for gels with l/d ratios of 95/5 and 90/10, and SANS results show a decrease in the intermicellar spacing for intermediate l/d ratios. We expect the dominant contribution to the elasticity of the gels to be intermicellar bridging chains and attribute the rheology to a competition between an increase in the time for PLA endblocks to pull out of micelles as the l/d ratio is increased and PLLA crystallization occurs, and a decrease in the number of bridging chains for micelles with crystalline PLA domains, as formation of bridges may be hindered by crowded crystalline PLA domains. These results provide a new strategy for controlling the rheology of PLA-based hydrogels for potential applications in biomaterials, as well as fundamental insights into how intermicellar interactions can be tuned via stereochemistry.