Heterogeneity and its Influence on the Properties of Difunctional Poly(ethylene glycol) Hydrogels: Structure and Mechanics

Difunctional polymer hydrogels, such as those prepared from poly­(ethylene glycol) diacrylate (PEGDA) macromers, are widely used for a number of potential applications in biotechnology and advanced materials due to their low cost, mild cross-linking conditions, and biocompatibility. The microstructure of such hydrogels is known to be heterogeneous, yet little is known about the specific structure itself, how it is impacted by the molecular parameters of the macromer, or its impact on macroscopic gel properties. Here, we determine the structure of PEGDA hydrogels using small-angle neutron scattering over a significant range of macromer molecular weights and volume fractions. From this, we propose a structural model for PEGDA hydrogels based on self-excluded, highly branched star polymers arranged into a fractal network. The primary implication of this structure is that heterogeneity arises not from defects in the cross-linking network, as is commonly assumed, but rather from a heterogeneous distribution of polymer concentration. This structural model provides a systematic explanation of the linear elasticity and swelling of PEGDA hydrogels

Structure and Dynamics of Networks in Mixtures of Hydrophobically Modified Telechelic Multiarm Polymers and Oil in Water Microemulsions

The structural and dynamical properties of oil-in-water (O/W) microemulsions (MEs) modified with telechelic polymers of different functionality (e.g., number of hydrophobically modified arms, <i>f</i>) were studied by means of dynamic light scattering (DLS), small-angle neutron scattering (SANS), and high frequency rheology measurements as a function of the polymer architecture and the amount of added polymer. For this purpose, we employed tailor-made hydrophobically end-capped poly­(<i>N,N</i>-dimethylacrylamide) star polymers of a variable number of endcaps, <i>f</i>, of different alkyl chain lengths, synthesized by the reversible addition–fragmentation chain transfer method. The addition of the different end-capped polymers to an uncharged ME of O/W droplets leads to a large enhancement of the viscosity of the systems. SANS experiments show that the O/W ME droplets are not changed upon the addition of the polymer, and its presence only changes the interdroplet interactions. The viscosity increases largely upon addition of a polymer, and this enhancement depends pronouncedly on the alkyl length of the hydrophobic sticker as it controls the residence time in a ME droplet. Similarly, the high frequency modulus <i>G</i>₀ depends on the amount of added polymer but not on the sticker length. <i>G</i>₀ was found to be directly proportional to <i>f</i> – 1. The onset of network formation is shifted to a lower number of stickers per ME droplet with increasing <i>f</i>, and the network formation becomes more effective. Thus, the dynamics of network formation are controlled by the polymer architecture. The effect on the dynamics seen by DLS is even more pronounced. Upon increasing the polymer concentration, slower relaxation modes appear that become especially pronounced with increasing number of arms. The relaxation dynamics are correlated to the rheological relaxation, and both are controlled by the polymer architecture

Amphiphilic Polymer Conetworks Studied by SANS: Effect of the Type of Solubilizate and Molecular Architecture on the Swollen Gel Structure

Amphiphilic polymer conetworks (APCNs) are hydrogels with hydrophobic regions synthesized by cross-linking well-defined copolymers. Due to their amphiphilicity, they have oil solubilization ability. In this paper, we present a small-angle neutron scattering (SANS) study of the oil solubilization at the mesoscopic level in APCNs swollen in D2O, where for better contrast conditions, the hydrophobic monomer (M) was deuterated. The study was carried out on a series of APCNs where we systematically varied the mol fraction of the hydrophobic methyl methacrylate (M) monomer repeating units (from 0.1 to 0.9) with respect to the hydrophilic 2-(dimethylamino)ethyl methacrylate (D) monomer repeating units as well as the general block copolymer architecture (MDM vs DMD). First, the structure of the D2O-swollen APCNs was characterized by means of SANS, which showed a well-defined structure with a repeat spacing of the domains, d, that scales directly with the architecture of the building blocks of the APCNs. In the second step, the solubilization of oils of different polarities (octane, toluene, eugenol, and 1-hexanol) was probed, and a clear correlation of oil solubilization with the oil polarity was observed. The most unpolar oil, octane, did not solubilize at all, while the much more polar toluene and 1-hexanol were incorporated very well but in a markedly different fashion. Toluene completely swelled the M part, while 1-hexanol appeared to be much more associated with the amphiphilic interface. This demonstrates that the studied APCNs are very selective with respect to their solubilization properties and efficient for distinguishing different types of oils

Synthesis of Oil-Laden Poly(ethylene glycol) Diacrylate Hydrogel Nanocapsules from Double Nanoemulsions

Multiple emulsions have received great interest due to their ability to be used as templates for the production of multicompartment particles for a variety of applications. However, scaling these complex droplets to nanoscale dimensions has been a challenge due to limitations on their fabrication methods. Here, we report the development of oil-in-water-in-oil (O₁/W/O₂) double nanoemulsions <i>via</i> a two-step high-energy method and their use as templates for complex nanogels comprised of inner oil droplets encapsulated within a hydrogel matrix. Using a combination of characterization methods, we determine how the properties of the nanogels are controlled by the size, stability, internal morphology, and chemical composition of the nanoemulsion templates from which they are formed. This allows for identification of compositional and emulsification parameters that can be used to optimize the size and oil encapsulation efficiency of the nanogels. Our templating method produces oil-laden nanogels with high oil encapsulation efficiencies and average diameters of 200–300 nm. In addition, we demonstrate the versatility of the system by varying the types of inner oil, the hydrogel chemistry, the amount of inner oil, and the hydrogel network cross-link density. These nontoxic oil-laden nanogels have potential applications in food, pharmaceutical, and cosmetic formulations

Controlling Complex Nanoemulsion Morphology Using Asymmetric Cosurfactants for the Preparation of Polymer Nanocapsules

Complex nanoemulsions, comprising multiphase nanoscale droplets, hold considerable potential advantages as vehicles for encapsulation and delivery as well as templates for nanoparticle synthesis. Although methods exist to controllably produce complex emulsions on the microscale, very few methods exist to produce them on the nanoscale. Here, we examine a recently developed method involving a combination of high-energy emulsification with conventional cosurfactants to produce oil–water–oil (O/W/O) complex nanoemulsions. Specifically, we study in detail how the composition of conventional ethoxylated cosurfactants Span80 and Tween20 influences the morphology and structure of the resulting complex nanoemulsions in the water–cyclohexane system. Using a combination of small-angle neutron scattering and cryo-electron microscopy, we find that the cosurfactant composition controls the generation of complex droplet morphologies including core–shell and multicore–shell O/W/O nanodroplets, resulting in an effective state diagram for the selection of nanoemulsion morphology. Additionally, the cosurfactant composition can be used to control the thickness of the water shell contained within the complex nanodroplets. We hypothesize that this degree of control, despite the highly nonequilibrium nature of the nanoemulsions, is ultimately determined by a competition between the opposing spontaneous curvature of the two cosurfactants, which strongly influences the interfacial curvature of the nanodroplets as a result of their ultralow interfacial tension. This is supported by a correlation between cosurfactant compositions that produces complex nanoemulsions and those that produce homogeneous mixed micelles in equilibrium surfactant–cyclohexane solutions. Ultimately, we show that the formation of complex O/W/O nanoemulsions is weakly perturbed upon the addition of hydrophilic polymer precursors, facilitating their use as templates for the formation of polymer nanocapsules