Photoswitchable Hydrogel Surface Topographies by Polymerisation‐Induced Diffusion

Poly(acrylic acid-co-N,N'-methylenebisacrylamide) hydrogel films were synthesized by copolymerizing acrylic acid (AAc) with N,N'-methylenebisacrylamide (MBA) as a cross-linker via photo polymerization in the spacing confined between two glass plates. NMR spectroscopy was utilized to determine the cross-linking density. We found that the cross-linking density determined by NMR is higher than that expected from the feed concentrations of cross-linkers, suggesting that MBA is more reactive than AAc and the heterogeneous nature of the cross-linking. In addition to the swelling tests, indentation tests were performed on the hydrogel films under water to investigate effects of the cross-linking density on the adhesion and mechanical properties of the hydrogel films in terms of adhesive pull-off force and Hertz-type elastic modulus. As the cross-linker concentration increased, the effective elastic modulus of the hydrogel films increased dramatically at low cross-linking densities and reached a high steady-state value at higher cross-linking densities. The pull-off force decreased with increasing cross-linker concentration and reached a lower force plateau at high cross-linking densities. An optimal "trade-off" cross-linking density was determined to be 0.02 mol fraction of MBA in the hydrogel, where balanced elastic modulus and adhesive pull-off force can be obtained.

Electron-beam-initiated grafting of trimethylolpropane triacrylate (TMPTA) onto the bulk ethylene propylene diene monomer (EPDM) was carried out with varying concentrations of TMPTA at a constant irradiation dose of 100 kGy and over a wide range of irradiation doses (0–500 kGy) at a fixed concentration (10%) of TMPTA. The rubber was also modified in the bulk by tripropylene glycol diacrylate (TPGDA, 10%) and tetramethylol methane tetraacrylate (TMMT, 10%) at an irradiation dose of 100 kGy. The modified rubbers were characterized by IR spectroscopy, crosslinking density measurements, and mechanical, dynamic mechanical, and electrical properties. The IR studies indicated increased peak absorbances at 1730, 1260, and 1019 cm−1 due to increased 〉CO and COC concentrations up to certain levels of TMPTA and irradiation dose. These are accompanied by an increase in the crosslinking density. The tensile strength of the samples increases gradually with increasing both the concentration of the monomer and radiation dose up to a certain level. The values of the modulus also increase at the expense of the elongation at break. An increase in the number of double bonds from two in the case of the diacrylate to four in the case of the tetraacrylate also brings about an increase in the tensile strength and moduli values. The elongation at break, however, decreases. The DMTA measurements indicate changes in the glass transition temperature, Tg, and tan δmax on modification. The Tg shifts to a higher temperature with a simultaneous lowering of the tan δmax values as the TMPTA level is increased. A similar trend is observed when the irradiation dose is increased and the nature of the monomer changes from di- to tetraacrylate. The dielectric loss tangent registers an increase on modification by irradiation of TMPTA while the permittivity is decreased. All the results could be explained on the basis of the structural modification and crosslinking density. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 323–337, 2000

Critical transition of epoxy resin from brittleness to toughness by incorporating CO2-sourced cyclic carbonate

The mechanical properties of biomaterials have profound consequences on cellular and host responses, however, the underlying mechanisms remain poorly understood. Presented are findings that confirm a clear relationship between the elastic modulus, the bulk-to-bound water ratio and the adaptive attachment of attachment dependent cells. We show that biomimetic hydrogels possessing no specific integrin binding motifs but that are of lower elastic modulus and lower bulk-to-bound water ratio, preferentially support cell attachment. Anchorage-dependent human muscle fibroblasts (RMS 13) were cultured on tetraethylene glycol (TEGDA) cross-linked poly(2-hydroxyethyl methacrylate) [poly(HEMA)]-based biomimetic hydrogels containing phosphorylcholine (PC) (1 mol%) and dimethylamino amino ethyl methacrylate (DMAEMA), a 3° amine (5 mol%), as well as on silicone and agarose controls. Changes in the cross-link density (1 to 12 mol%) of the hydrogel produced a monotonic reduction in the glass transition temperature, Tg (131.8 °C at 1 mol% TEGDA to 110.4 °C at 12 mol% TEGDA), but an exponential increase in the bulk-to-bound water (4.25 at 1 mol% to 27.04 at 12 mol%) that exactly parallels the increase in elastic modulus as measured by nano-indentation AFM (152 ± 62 kPa at 1 mol% TEGDA to 1777 ± 1152 kPa at 12 mol% TEGDA). Enumeration, MTT assay and fluorescence microscopy following 4, 8 and 12 days of culture confirms that short-term cell viability and long term proliferation were favored on low cross-link density, low modulus hydrogels and that cells were preferentially attached to low cross-link density hydrogels. Bound water is central to the adaptive attachment of attachment dependent cells.

Poly(N-isopropylacrylamide) microgels prepared without exogenous crosslinker are extremely "soft" as a result of their very low crosslinking density, with network connectivity arising only from the self-crosslinking of pNIPAm chains. As a result of this extreme softness, our group and others have taken interest in using these materials in a variety of bioengineering applications, while also pursuing studies of their fundamental properties. Here, we report deswelling triggered structural changes in poly (N-isopropylacrylamide-co-acrylic acid) (ULC10AAc) microgels prepared by precipitation polymerization. Dynamic light scattering suggests that the deswelling of these particles not only depends on the collapse of the pNIPAm chains but is also influenced by the ionization state of the acrylic acid moieties present in the copolymer. The ULC10AAc microgel behaves like a traditional crosslinked pNIPAm microgel at pH 3.5, showing a sharp decrease in the hydrodynamic diameter around the lower critical solution temperature (LCST) of pNIPAm. As the pH is increased to 4.5 we observe multiple transitions in the deswelling curve, suggesting inhomogeneity in the structure and/or composition of the microgels. At pH 6.5 the microgels cease to be thermoresponsive over the studied temperature range due to increased charge repulsion between the fully deprotonated AAc groups and an increase in gel osmotic pressure due to solvated counterion ingress. Atomic force microscopy images of particles deposited at different temperatures reveal a temperature induced morphological change, with punctate structures forming inside microgels at pH 4.5 and 6.5 and temperature above the gel volume phase transition temperature (VPTT).

Spherical particles confined to liquid interfaces generally self-assemble into hexagonal patterns. It was theoretically predicted by Jagla two decades ago that such particles interacting via a soft repulsive potential are able to form complex, anisotropic assembly phases. Depending on the shape and range of the potential, the predicted minimum energy configurations include chains, rhomboid and square phases. We recently demonstrated that deformable core-shell particles consisting of a hard silica core and a soft poly(N-isopropylacrylamide) shell adsorbed at an air/water interface can form chain phases if the crosslinker is primarily incorporated around the silica core. Here, we systematically investigate the interfacial self-assembly behavior of such SiO2-PNIPAM core-shell particles as a function of crosslinker content and core size. We observe chain networks predominantly at low crosslinking densities and smaller core sizes, whereas higher crosslinking densities lead to the formation of rhomboid packing. We correlate these results with the interfacial morphologies of the different particle systems, where the ability to expand at the interface and form a thin corona at the periphery depends on the degree of crosslinking close to the core. We perform minimum energy calculations based on Jagla-type pair potentials with different shapes of the soft repulsive shoulder. We compare the theoretical phase diagram with experimental findings to infer to which extent the interfacial interactions of the experimental system may be captured by Jagla pair-wise interaction potentials.

Controlled uptake and release of lysozyme from glycerol diglycidyl ether cross-linked oxidized starch microgel.

The microstructure of the poly(N-isopropylacrylamide-co-acrylamido-2-methyl-1-propane sulphonic acid) gel, poly(NIPA-co-AMPS), was investigated as a function of temperature and cross-link density using the small angle neutron scattering technique. The sample temperature was varied in the range 30 to 55C. Two different behaviours of poly(NIPA-co-AMPS) gels were observed. At low temperature (30C), the magnitude of the scattered intensity increased with cross-link density suggesting that additional cross-links introduced more inhomogeneities in the gel network. At high temperatures the trend was reversed; that is the lower cross-link density, the higher the scattered intensity. Therefore, the role of cross-links at high temperature was to suppress microphase separation. The fitting of the experimental data with the Rabin and Panyukov theory indicated qualitative agreement.

The expulsion of hydrocarbons from kerogen is the initial step in the primary migration process, during which the composition of the expelled petroleum is enriched in saturated and aromatic hydrocarbons while the retained bitumen is enriched in polar compounds. The physical and chemical principles responsible for this chemical fractionation are not well understood, and numerous theories have been proposed to explain the expulsion process. A multicomponent equilibrium likely exists between the kerogen matrix and the expelled fluid during petroleum generation and expulsion. To test whether such equilibrium can explain the nature and extent of chemical fractionation, an extended Flory−Rehner Regular Solution Theory model was developed and applied to a series of kerogens of varying structure, generative potential, and maturity. Thermodynamic parameters for immature Type II and IIIC kerogens (solubility parameter, cross-link density, and native swelling) were derived experimentally and extended to higher maturity. Mixtures of model compounds with well-defined properties were created to reflect the composition of primary generated products of kerogen thermal decomposition. Multicomponent equilibrium then was calculated under closed system conditions. The amount and composition of modeled expelled products are most sensitive to the generative potential and cross-link density of the kerogen. In general, lower source richness and cross-link density is associated with bitumen retention and a relative enrichment of the aliphatic components in the expelled fluid. Higher source richness and cross-link density result in earlier expulsion of fluids that are enriched in polar components. The predicted compositions of expelled fluids correspond well with the compositional range observed for produced petroleum. The predicted bitumen (kerogen-retained, soluble organic matter) compositions are uniformly >50% C14+ NSOs at all levels of maturity for all modeled kerogens. A chemically driven equilibrium mechanism based on Regular Solution Theory can explain almost completely the nature and extent of chemical fractionation that takes place during expulsion.

D&W Tool Inc.

We discuss a new class of polymer absorbents called “petrogels”, polyolefin-based hydrophobic absorbents that demonstrate selective absorption of hydrocarbon (oil) molecules in water, counterpart to “hydrogels” that are known for absorbing aqueous solutions. To understand the effects of the molecular structure (i.e., polymer composition, cross-linking density, and morphology) to the absorption capacity with various hydrocarbons and mixtures, a systematical study was conducted involving polyolefin copolymers that exhibit a highly swellable network but no dissolution in hydrocarbons at ambient temperature. They include a set of semicrystalline poly(ethylene-co-1-octene) thermoplastics (metallocene-LLDPEs) with low crystallinity, melting temperature, and various morphologies (i.e., pellets, films, and foams) and a set of cross-linked amorphous poly(1-decene-co-divinylbenzene) (x-D-DVB) elastomers with low cross-linking densities. The absorption study involves pure hydrocarbons (toluene and heptane), refined ...

The effect of cross-link density on the visco-elastic performance of an advanced polymer matrix (LaRC™-PETI-SI) was investigated through creep compliance tests. Testing consisted of short-term isothermal creep and recovery with the creep segments performed under constant load. The tests were conducted at three temperatures below the glass transition temperature of each material with different cross-link density. Through the use of time-aging-time superposition procedures, the material constants, material master curves and aging-related parameters were evaluated at each temperature for a given cross-link density. It was shown that the high cross-link density materials have increased creep compliance and creep compliance rate, and are less sensitive to physical aging than the low cross-link density materials. The temperature sensitivity of the time-temperature superposition (TTSP) shift factor was shown to increase monotonically with increasing cross-link density. Furthermore, at the highest temperature, a separation in the TTSP shift factors was apparent between different cross-link densities. This separation occurred at a molecular weight ( Mc) of ~3321 g/mol.

Epoxy systems EPN/BA and EPN/DDS having significantly different cross-link densities have been modified using core–shell rubber particles. The toughening mechanisms have been investigated and the results show that cavitation and particle–matrix debonding play different roles in the low and high cross-link density epoxy resins.

Effect of the ladder-like aminopropyl silsesquioxane on the curing and properties of epoxy composition

Dynamic polyimine-amide networks with exceptional properties, including high flexibility, excellent thermal stability and dual closed-loop circularity were designed by combining dynamic covalent imine-functionalities with amide-chemistry. The solvent free up-scalable synthesis started from carboxyl-functionalization of lignin-derivable aldehydes followed by melt polycondensation with a triamine to form two dynamic networks (PIAX1 and PIAX2, respectively). While previously reported vanillin-derived polyimine thermosets were typically non-flexible and brittle, our polyimine-amides are flexible with elongation at break 380 % for PIAX1 and 65 % for PIAX2, where the higher flexibility of PIAX1 is deduced to the lower glass transition temperature and crosslinking density. Both materials illustrate fast stress relaxation even at low temperature, down to 50 °C in the case of PIAX1. Retained and even improved mechanical properties were observed after several cycles of thermal reprocessing, e.g., after three reprocessing cycles by hot pressing, PIAX1 recovered 116 % of original tensile stress and 126 % of original elongation at break, while chemical recycling under acidic conditions at room temperature yielded repolymerizable trialdehydes and triamines. Furthermore, rapid self-healing and shape memory behaviour at low temperature were demonstrated for PIAX1. A promising molecular design, further tuneable by choice of aldehyde and triamine, is demonstrated enabling high performance and dual closed-loop circularity.