Thermally Responsive Amphiphilic Conetworks and Gels Based on Poly(N‑isopropylacrylamide) and Polyisobutylene

Novel amphiphilic conetworks (APCN) consisting of thermoresponsive poly(N-isoproplyacrylamide) (PNiPAAm) cross-linked by hydrophobic methacrylate-telechelic polyisobutylene (MA-PIB-MA) were successfully synthesized in a broad composition range. The resulting PNiPAAm-l-PIB conetworks (“l” stands for “linked by”) were obtained by radical copolymerization of NiPAAm with MA-PIB-MA in tetrahydrofuran, a cosolvent for all the components. Low amounts of extractables substantiated efficient network formation. The composition dependent two glass transition temperatures (Tg) by DSC analysis indicate microphase separation of the cross-linked components without mixed phases. It was found that the PNiPAAm-l-PIB conetworks are uniformly swellable in both water and n-hexane; i.e., these new materials behave either as hydrogels or as hydrophobic gels in aqueous or nonpolar media, respectively. The uniform swelling in both polar and nonpolar solutes indicates cocontinuous (bicontinuous) phase morphology. The equilibrium swelling degrees (R) depend on composition, that is, the higher the PIB content, the lower the R in water and the higher in n-hexane. The PNiPAAm phase keeps its thermoresponsive behavior in the conetworks as shown by significant decrease of the swelling degree in water between 20 and 35 °C. The lower critical solubility temperature (LCST) values determined by DSC are found to decrease from 34.1 °C (for the pure PNiPAAm homopolymer) to the range of 25–28 °C in the conetworks, and the extent of the LCST decrease is proportional with the PIB content. Deswelling-swelling, i.e., heating–cooling, cycle indicates insignificant hysteresis in these new thermoresponsive materials. This indicates that PNiPAAm-l-PIB conetworks with predetermined and thermoresponsive swelling behavior can be designed and utilized in several advanced applications on the basis of results obtained in the course of this study

Double Networks Based on Amphiphilic Cross-Linked Star Block Copolymer First Conetworks and Randomly Cross-Linked Hydrophilic Second Networks

This study presents the preparation and characterization of double networks (DN) based on a first amphiphilic polymethacrylate conetwork (APCN) and a second polyacrylamide network. The APCN first network comprised interconnected "in-out" star copolymers of 2-(dimethylamino)ethyl methacrylate (DMAEMA, hydrophilic ionizable monomer) and 2-ethylhexyl methacrylate (EHMA, hydrophobic comonomer) or lauryl methacrylate (LauMA, second hydrophobic comonomer), synthesized using group transfer polymerization, following one-pot, sequential, monomer, and hydrophobic cross-linker (ethylene glycol dimethacrylate, EGDMA) additions. The second network was prepared by the aqueous photopolymerization of acrylamide (AAm) at two different concentrations, 2 and 5 M, and N,N′-methylenebis(acrylamide) cross-linker in the presence of the fully ionized (via HCl addition) APCN. After synthesis, all DNs and single (first and second) (co)networks, equilibrium-swollen in water, were characterized in terms of their mechanical properties in compression. The DNs exhibited improved mechanical properties (stress and strain at break, and elastic modulus) compared to the corresponding single networks. Better reinforcement was achieved in the DNs whose APCN first networks bore a lower hydrophobic content and whose hydrophobic monomer was EHMA rather than LauMA. The best DN exhibited stress at break above 8 MPa and strain at break nearly 80%, close to the values of the best DNs in the literature. Nanoindentation studies were also performed on selected DNs which proved again the enhanced mechanical properties of the present DNs, manifested as high resistance to penetration and low creep displacement. Small-angle X-ray scattering (SAXS) indicated a broad correlation peak for all APCN first networks, suggestive of microphase separation with short-range order, arising from the presence of the hydrophobic segments. The single correlation peak was preserved in the SAXS profiles of the DNs, which was, however, shifted to lower q-values, consistent with further network swelling. Despite the SAXS evidence for only weak phase separation on the nanoscale in the DNs, half of the water-swollen DNs (the ones with a 5 M AAm concentration in the second network) exhibited strong birefringence which probably arose from the stretching of the charged DMAEMA segments rather than the presence of anisotropic nanophases

Effect of pH on the Dynamics and Structure of Thermoresponsive Telechelic Polyelectrolyte Networks: Impact on Hydrogel Injectability

We report the effect of pH on the dynamics and structure of 3D networks formed by the temperature-dependent hydrophobic association of the end blocks of P(nBuMA18-co-TEGMA15)-b-PDMAEMA159-b-P(nBuMA18-co-TEGMA15) where PDMAEMA and P(TEGMA-co-nBuMA) stand for poly(N,N-dimethylamino ethyl methacrylate) and poly(tri-ethylene glycol methyl ether methacrylate/n-butyl methacrylate respectively) triblock terpolymer in aqueous media. Thanks to the design of the sticky end blocks that can be independently controlled by temperature, it was possible to explore the pH effect, influencing the charge density and chain conformation of the hydrophilic midblock, on the network dynamics and structure by oscillatory rheology and small-angle neutron scattering. Three pH regimes are observed: At low pH ( 6, an appreciable increase of τ (about 6-fold) was observed, likely due to the enhancement of the network connectivity arising from the decrease of the charge density and the increase of the chain flexibility of the poly(2-dimethylamino ethyl methacrylate) (PDMAEMA) block that promote crosslinking, as reflected in GN. Finally, at even higher pHs, the network connectivity decreases substantially, owing to bridge-to-loop transitions, leading eventually to abrupt decrease of τ and GN, thus revealing a gel-to-sol transition. These results can be utilized to fine-tune the injectability of the thermoresponsive telechelic polyelectrolyte hydrogels based on their pH responsiveness.Fil: Lencina, María Malvina Soledad. University of Patras; Grecia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Física del Sur. Universidad Nacional del Sur. Departamento de Física. Instituto de Física del Sur; ArgentinaFil: Ko, Chia-Hsin. Universitat Technical Zu Munich; AlemaniaFil: Jung, Florian A.. Universitat Technical Zu Munich; AlemaniaFil: Schweins, Ralf. Institut Laue Langevin; FranciaFil: Rikkou-Kalourkoti, Maria. University fo Cyprus; ChipreFil: Patrickios, Costas S.. University fo Cyprus; ChipreFil: Papadakis, Christine M.. Universitat Technical Zu Munich; AlemaniaFil: Tsitsilianis, Constantinos. University of Patras; Greci

Amphiphilic conetworks and gels physically cross-linked via stereocomplexation of polylactide

10.1021/la403432yLangmuir294614307-14313LANG

Thermoresponsive Hydrogels Based on Telechelic Polyelectrolytes: From Dynamic to “Frozen” Networks

A novel thermoresponsive gelator of (B-<i>co</i>-C)-<i>b</i>-A-<i>b</i>-(B-<i>co</i>-C) topology, comprising a poly­(2-(dimethyl­amino)­ethyl methacrylate) (PDMAEMA) weak polyelectrolyte as central block, end-capped by thermosensitive poly­(triethylene glycol methyl ether methacrylate/<i>n</i>-butyl methacrylate) [P­(TEGMA-<i>co</i>-<i>n</i>BuMA)] random copolymers, was designed and explored in aqueous media. The main target of this design was to control the dynamics of the stickers by temperature as to create an injectable hydrogel that behaves as a weak gel at low temperature and as a strong gel at physiological temperature. Indeed, at low temperatures, the system behaves like a viscoelastic complex fluid (dynamic network), while at higher temperatures, an elastic hydrogel is formed (“frozen” network). The viscosity increases exponentially upon heating, about 5 orders of magnitude from 5 to 45 °C, which is attributed to the exponential increase of the lifetime of the self-assembled stickers. The integration of thermo- and shear responsive properties in the gelator endows the gel with injectability. Moreover, the gel can be rapidly recovered upon cessation of the applied stress at 37 °C, simulating conditions similar to those of injection through a 28-gauge syringe needle. All these hydrogel properties render it a good candidate for potential applications in cell transplantation through injection strategies

Multiple Network Hydrogels: A Study of Their Nanoindentation Hardness

Nanoindentation is employed to investigate at the nanoscale the mechanical properties of multiply-interpenetrated N,N-dimethylacrylamide hydrogels cross-linked using N,N′-methylenebis(acrylamide). The main result from these measurements is the determination of the hardness of the materials, that is, their resistance to penetration by the nanoindenter, which increases with network multiplicity, arising from the increase in network compactness with multiplicity. In addition to hardness, the nanoindentation elastic modulus and the percentage of recoverable energy are also determined, and both are found to increase with network multiplicity as well. A general conclusion from this study is that nanoindentation is a facile and fast method for the characterization of a number of mechanical properties of hydrogels, with the important advantage of small amount of sample required for the measurements