Resulting Polymer Networks Research Articles

Marine Fouling Release Performance of Amphiphilic Carboxybetaine Perfluoropolyether Methacrylate Polymer Coatings

AbstractAmphiphilic polymer systems have been demonstrated to be an effective and environmentally friendly approach to combat the challenges of biofouling, especially in the real ocean environment. A variety of polymer compositions containing fluorophilic perfluoropolyether (PFPE) urethane dimethacrylate and zwitterionic carboxybetaine methacrylate (CBMA) are in situ copolymerized across a broad concentration range and anchored to chemically functionalized substrates by grafting‐through polymerization. Dynamic accumulation assays with the marine diatom Navicula perminuta and the marine bacterium Cobetia marina revealed an enhanced antifouling performance in most of the amphiphilic polymer mixtures when zwitterionic units are combined with hydrophobic PFPE compounds. It is noticed that the resulting polymer networks still possess enough orientational freedom to quickly rearrange in the presence of water. It is further observed that several of the very hydrophilic formulations and some of the moderately hydrophobic formulations exhibit low fouling against the marine fouling organisms Navicula perminuta and Cobetia marina. This correlated with increased surface roughness and the ability for underwater rearrangement. Furthermore, it is discussed whether the probability that highly hydrated hydrogels, which appear sponge‐like, may create a diffuse interphase which promotes the incorporation of silt. Latter is counteracted by the hydrophobic PFPE building blocks in the amphiphilic coatings.

Polyampholyte Hydrogels with pH-Dependent Swelling for Controlled Catch and Release of Model Dyes

Polyampholyte hydrogels with tunable charge are synthesized by a facile two-step approach including the free-radical crosslinking copolymerization of tert-butoxycarbonylaminomethylacrylate (tBAMA) with N,N′-methylenebisacrylamide and subsequent deprotection. Thermal, photo- and redox-initiating methods were utilized in the synthesis of crosslinked PtBAMA and the resulting polymer networks swell during deprotection in a mixture of trifluoro acetic acid/water. While the crosslinked PtBAMA forms organogels in various organic solvents such as chloroform, acetone and DMSO, polydehydroalanine (PDha) networks after deprotection form hydrogels with pH-dependent swelling and oscillatory swelling/deswelling depending on pH value and salinity. The tunable charge of the developed hydrogels was employed for a catch-and-release platform controlled by pH, in which methylene blue as a cationic model was adsorbed at pH 11 and desorbed at pH 2, whereas methyl blue as an anionic model dye was adsorbed at pH 2 and desorbed at pH 11.

Mechanical properties of polymer networks with polyrotaxane crosslinkers with different molecular structures based on polyethylene glycol and α-cyclodextrin

The modification of vinyl groups on polyrotaxane derivatives, obtained with polyethylene glycol as the axial molecule and α-cyclodextrin as the cyclic molecule, allows the production of crosslinkers that react with various monomers, facilitating the formation of polymer networks. In contrast to conventional polymer crosslinked networks using typical crosslinking agents, polymer networks employing polyrotaxane crosslinking agents allow for mobile crosslinking points. Consequently, the resulting polymeric crosslinked network may exhibit increased toughness, as external stress is uniformly distributed throughout the polymeric network. This distribution helps to avoid excessive stress concentration in specific areas. To optimize the mechanical properties of polymer networks formed using polyrotaxane crosslinkers, we investigated the impact of the molecular weight of the axial molecules and the number of crosslinking points of the cyclic molecules in the polyrotaxane crosslinkers on the mechanical properties of the resulting polymer networks. Our results show that the fracture strain, fracture stress, and fracture energy of the resultant polymer networks increase with increasing molecular weight of polyethylene glycol. We also observed that the fracture stress and toughness of the polymer networks increased when there was less than one modified vinyl group on α-cyclodextrin, compared to the case with multiple vinyl groups.

Effect of localized control of cross-link density on mechanical properties of bicontinuous cubic lyotropic networks via copolymerization with different singly-polymerizable monomers

Two singly-polymerizable analogs of a bicontinuous cubic (Q) phase, lyotropic liquid crystal (LLC) (i.e., amphiphilic) cross-linking monomer were designed and synthesized as phase-compatible, non-cross-linkable co-monomers. These were copolymerized with the parent, single-head/single-tail cross-linking monomer to locally control the cross-link density in either the hydrophobic tail or hydrophilic headgroup regions of the Q phase and study their effect on the mechanical properties of the resulting polymer networks. Structure-property relationships were elucidated: (1) Blends with smaller amounts of the parent cross-linking monomer relative to non-cross-linkable analog exhibit progressively lower endpoint stress and Young's modulus values. (2) A decrease in cross-link density also influences the pliability of each sample as there appears to be a threshold (i.e., minimum) cross-link density that must be exceeded before stretchability is significantly affected. (3) Each LLC-based structural motif tolerates different levels of non-cross-linkable analog, shifting the stress-strain behavior of these cross-linked Q-phase materials.

Low-intensity mixing process of high molecular weight polymer chains leads to elastomers of long network strands and high fatigue threshold.

Many polymer networks are prepared by crosslinking polymer chains. The polymer chains and crosslinkers are commonly mixed in internal mixers or roll mills. These intense processes break the polymer chains, lower viscosity, and ease mixing. The resulting polymer networks have short chains and a fatigue threshold of ∼100 J m-2. Here, we show that a low-intensity process, a combination of kneading and annealing, preserves long chains, leading to a network of polybutadiene to achieve a fatigue threshold of 440 J m-2. In a network, each chain has multiple crosslinks, which divides the chain into multiple strands. At the ends of the chain are two dangling strands that do not bear the load. The larger the number of crosslinks per chain, the lower the fraction of dangling strands. High fatigue threshold requires long strands, as well as a low fraction of dangling strands. Once intense mixing cuts chains short, each short chain can only have a few crosslinks; the strands are short and the fraction of dangling strands is high-both lower the fatigue threshold. By contrast, a low-intensity mixing process preserves long chains, which can have many crosslinks; the strands are long and the fraction of dangling strands is low-both increase the fatigue threshold. It is hoped that this work will aid the development of fatigue-resistant elastomers.

Role of breaking physical crosslinks on safeguarding polymer networks

Polymer networks that are crosslinked jointly by covalent bonds and reversible physical bindings usually offer superior mechanical strength and toughness compared to their permanently crosslinked counterparts. By introducing short oligomers as stickers that bind physically to different polymer chains, the resulting polymer networks show a unique feature of dissipating stretching-induced energy, which is absent in permanent networks. We show that large sized clusters of physical crosslinks in a transient network break into smaller pieces when the transient network is stretched at strain rates sufficiently high that the transiently crosslinked structure is not given enough time to relax. After deducting the part of the stress that is generated solely by a thermal motion-driven structural readjustment of the transient network, the stress enhancement arising from a stretching-enforced breakup of physical crosslinks is quantitatively verified, demonstrating the sacrificial role played by these physical crosslinks in safeguarding the unrelaxed structure.

Toughening of Polymer Networks by Freezing-induced Monomer Insertion

In solvent freezing, polyurethane gels cross-linked with tetraarylsuccinonitrile (TASN) moieties which can be cleaved to afford pink-colored and yellow-fluorescent radicals by mechanical stress, show mechano-activated fluorescence based on dissociated TASN radicals and provide a consistent reaction field with vinyl monomers. We herein propose a toughening of a polymer network via freezing-induced mechanochemistry. Through the consistent reaction field, the toughness of resulting polymer networks can be changed by the number of repeated freeze-thaw cycles and the concentration of dissociated TASN radicals.

Tough combinatorial poly(urethane-isocyanurate) polymer networks and hydrogels synthesized by the trimerization of mixtures of NCO-prepolymers

The development of tough hydrogels is an essential but challenging topic in biomaterials research that has received much attention over the past years. By the combinatorial synthesis of polymer networks and hydrogels based on prepolymers with different properties, new materials with widely varying characteristics and unexpected properties may be identified. In this paper, we report on the properties of combinatorial poly(urethane-isocyanurate) (PUI) type polymer networks that were synthesized by the trimerization of mixtures of NCO-functionalized poly(ethylene glycol) (PEG), poly(propylene gylcol) (PPG), poly(ε-caprolactone) (PCL) and poly(trimethylene carbonate) (PTMC) prepolymers in solution. The resulting polymer networks showed widely varying material properties.Combinatorial PUI networks containing at least one hydrophilic PEG component showed high water uptakes of >100 wt%. The resulting hydrogels demonstrated elastic moduli of up to 10.1 MPa, ultimate tensile strengths of up to 9.8 MPa, elongation at break values of up to 624.0% and toughness values of up to 53.4 MJ m−3. These values are exceptionally high and show that combinatorial PUI hydrogels are among the toughest hydrogels reported in the literature. Also, the simple two-step synthesis and wide range of suitable starting materials make this synthesis method more versatile and widely applicable than the existing methods for synthesizing tough hydrogels. An important finding of this work is that the presence of a hydrophobic network component significantly enhances the toughness and tensile strength of the combinatorial PUI hydrogels in the hydrated state. This enhancement is the largest when the hydrophobic network component is crystallizable in nature. In fact, the PUI hydrogels containing a crystallizable hydrophobic network component are shown to be semi-crystalline in the water-swollen state. Due to their high toughness values in the water-swollen state together with their water uptake values, elastic moduli and ultimate tensile strengths, the developed hydrogels are expected to be promising materials for biomedical coating- and adhesive applications, as well as for tissue-engineering. Statement of significanceThe development of tough hydrogels is a challenging topic that has received much attention over the past years. At present, double network type hydrogels are considered state-of-the-art in the field, demonstrating toughness values of several tens of MJ m−3. However, in terms of ease and versatility of the synthesis method, the possibilities are limited using a double network approach.In this work, we present combinatorial poly(urethane-isocyanurate) type polymer networks and hydrogels, synthesized by the trimerization of mixtures of NCO-functionalized prepolymers. The resulting hydrogels demonstrate exceptionally high toughness values of up to 53 MJ m−3, while the synthesis method is versatile and widely applicable. This new class of hydrogels is therefore considered highly promising in the future development of load-bearing biomaterials.

Expanding the stoichiometric window for metal cross-linked gel assembly using competition

Polymer networks with dynamic cross-links have generated widespread interest as tunable and responsive viscoelastic materials. However, narrow stoichiometric limits in cross-link compositions are typically imposed in the assembly of these materials to prevent excess free cross-linker from dissolving the resulting polymer networks. Here we demonstrate how the presence of molecular competition allows for vast expansion of the previously limited range of cross-linker concentrations that result in robust network assembly. Specifically, we use metal-coordinate cross-linked gels to verify that stoichiometric excessive metal ion cross-linker concentrations can still result in robust gelation when in the presence of free ion competing ligands, and we offer a theoretical framework to describe the coupled dynamic equilibria that result in this effect. We believe the insights presented here can be generally applied to advance engineering of the broadening class of polymer materials with dynamic cross-links.

Correlating synthesis parameters with physicochemical properties of poly(glycerol sebacate)

Poly(glycerol sebacate), PGS, is an elastomeric biodegradable polyester increasingly proposed in a variety of biomedical applications. It is prepared by polycondensation of sebacic acid and glycerol in a first stage in which a prepolymer is obtained, followed by a curing to conveniently crosslink it. In this work, synthesis parameters such as the curing temperature and time, and the molar ratio between reactants, were systematically varied to correlate them with the physicochemical properties of the resulting polymer networks. The efficiency of each manufacturing process was quantified through the relative mass effectively crosslinked and insoluble in tetrahydrofuran. Infrared spectra gave an estimation of the ratio of non-condensed polar terminal groups. These results were correlated with swelling results, which in turn provided the means to calculate the chains density through Flory-Rehner equilibrium swelling equation for lightly crosslinked polymers. The role of the synthesis parameters on the physical state of the resulting polymers, as well as their proneness to hydrolyze, were followed. The results obtained highlight the relevance of rinsing them following synthesis, to remove non-crosslinked chains that easily diffuse to the surrounding medium. Curing under mild conditions equimolar mixtures of sebacic acid and glycerol proved to lead to poorly crosslinked swellable networks, which hydrolyze easily in bulk mode. Alternative molar ratios yield sticky and difficult to handle materials at higher polyol fractions in the reactive mixture, whilst an excess of acid terminal groups leads to a faster mass loss by hydrolysis in aqueous media together with surface salts deposition, concomitant with a lesser cell viability in in vitro culture. PGS synthesized from an equimolar ratio between reactants and cured at 130°C or higher, for 48h or longer, show suitable features for their use in tissue engineering applications where hydrophobic surface-degradable rubbers are required, without significant differences among them.

Thermal, mechanical and conductive properties of imidazolium-containing thiol-ene poly(ionic liquid) networks

Abstract A series of imidazolium-based bis(trifluoromethylsulfonyl)imide [NTf2] poly(ionic liquid)s (PILs) were prepared by conducting the solventless thiol-ene ‘click’ photopolymerization of bisallylimidazolium [NTf2] and pentaerythritol tetrakis(3-mercaptopropionate) (PTMP). The thiol:ene molar ratio was varied in order to examine changes in the thermal, mechanical and conductive properties of the resulting polymer networks. The 1.0:2.0 thiol-ene PIL network exhibited the highest glass transition temperature (Tg of −5.0 °C) and storage modulus (E′ of 9.24 MPa at 100 °C) values. Temperature-dependent ionic conductivities were found to rely on both the Tg/crosslink density as well as the IL content, with the highest ionic conductivity observed for the 1.0:3.0 thiol-ene PIL network (1.42 × 10−5 S/cm at 25 °C). Application of Vogel-Fulcher-Tammann and Williams-Landel-Ferry theories revealed the impact of both Tg/crosslink density and free ion concentration on temperature-dependent conductivity data, and indicated lower free volumes and fragilities for the thiol-ene networks relative to other non-network PILs.

Static and dynamic properties of biodegradable poly(antioxidant β-amino ester) networks based on incorporation of curcumin multiacrylate

Poly(antioxidant β-amino esters) (PAβAE) are a recently-developed class of biodegradable polymeric hydrogels that have shown promise in their ability to control cellular response by reducing oxidative stress and thereby improving biocompatibility. The degradation and resulting antioxidant properties of these biomaterials are closely related to the composition and architecture of the networks established during polymerization. Therefore, in order to efficiently design PAβAE for specific biomedical applications, it is important to understand the impact of synthesis conditions on the thermomechanical properties of the resulting polymer networks. In this work, we examine a series of hydrogel networks formed by polymerization of a commercial diacrylate, polyethylene glycol diacrylate (PEG400DA), and a primary diamine, 4,7,10-trioxa-1,13-tridecanediamine (TTD), in combination with acrylate-functionalized curcumin. Swelling response of these hydrogels in acetonitrile (ACN) and aqueous degradation at 37 °C were evaluated as a function of composition. Network structure and thermomechanical response properties were examined using dynamic mechanical analysis and broadband dielectric spectroscopy. An increase in curcumin content led to an overall increase in the glass transition temperature and the corresponding network degradation time.

Electron Beam Irradiation of LDPE Filled with Calcium Carbonate and Metal Hydroxides

This article deals with the effect of electron beam irradiation and flame-retardant loading on the performances of LDPE-based formulations for wire and cable applications. In this study the influence of electron beam irradiation on different blends of low density polyethylene (LDPE) filled with aluminum trihydrate (ATH), magnesium hydroxide (MH), and calcium carbonate (CaCO3) were studied. The mechanical, thermal, and burning properties of the resulting polymer networks have been analyzed and discussed. Addition of all non-halogenated fillers to LDPE deteriorated the mechanical properties. Addition of MH to LDPE presented a significant increase on adhesion forces inside polymer matrices and acted more efficiently than similar ATH/LDPE and CaCO3/LDPE compounds. LDPE flame retardancy improved significantly by a carbonaceous, non-flammable coating formation with high plasticity of CaCO3 addition at high temperature. The resulting MH blends were more efficient thermally and burned more stably than similar ATH blends. It was also concluded that electron beam irradiation had improved effects on thermal stability and mechanical properties for all the polymeric samples in this research.

ABA triblock copolymer based hydrogels with thermo-sensitivity for biomedical applications

ABSTRACTOligo(ethylene glycol)-oligo(propylene glycol)-oligo(ethylene glycol) (OEG-OPG-OEG) triblock copolymers are hydrogel forming and extensively investigated in the field of drug release due to their biocompatibility and thermo-sensitivity. Here the synthesis and characterization of OEG-OPG-OEG based polymer networks from methacrylated oligomers by photo-irradiation are reported. Two precursors were selected to have comparable hydrophilicity (80 wt% OEG content) but different molecular weights of Mn = 8400 g·mol-1 and 14600 g·mol-1. The precursor solutions were prepared in concentration 10 to 30 wt%. The resulting polymer networks prepared from high Mn precursors exhibited higher swellability at equilibrium (up to 3400%) and mechanical properties in the range of G’ ∼ 0.1 to 1 kPa at 5 °C compared to networks based on low Mn precursors. A more significant thermo-sensitive behavior in terms of swellability, volumetric contraction and mechanical transition, starting at 30 °C could also be observed for the networks based on high Mn precursors, thus promoting future application in the field of drug release.

Formation of nanoscale networks: selectively swelling amphiphilic block copolymers with CO2-expanded liquids

Polymeric films with nanoscale networks were prepared by selectively swelling an amphiphilic diblock copolymer, polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP), with the CO(2)-expanded liquid (CXL), CO(2)-methanol. The phase behavior of the CO(2)-methanol system was investigated by both theoretical calculation and experiments, revealing that methanol can be expanded by CO(2), forming homogeneous CXL under the experimental conditions. When treated with the CO(2)-methanol system, the spin cast compact PS-b-P4VP film was transformed into a network with interconnected pores, in a pressure range of 12-20 MPa and a temperature range of 45-60 °C. The formation mechanism of the network, involving plasticization of PS and selective swelling of P4VP, was proposed. Because the diblock copolymer diffusion process is controlled by the activated hopping of individual block copolymer chains with the thermodynamic barrier for moving PVP segments from one to another, the formation of the network structures is achieved in a short time scale and shows "thermodynamically restricted" character. Furthermore, the resulting polymer networks were employed as templates, for the preparation of polypyrrole networks, by an electrochemical polymerization process. The prepared porous polypyrrole film was used to fabricate a chemoresistor-type gas sensor which showed high sensitivity towards ammonia.

Real-Time Observation of Polymer Network Formation by Liquid- and Solid-State NMR Revealing Multistage Reaction Kinetics

The reaction rate for the end-cross-linking process of vinyl-terminated poly(dimethylsiloxane) by a cross-linker with four Si-H functionalities in the presence of solvent was studied by (1)H liquid-state NMR in dependence of the reaction temperature. The properties of the resulting polymer networks, i.e., the gel-point and the formation of the elastically effective network, were monitored in situ during the reaction by single-evolution-time (1)H double-quantum (SET-DQ) low-field NMR. It was found that the cross-linking kinetics shows no uniform reaction order for the conversions of the functional groups before the topological gelation threshold of the polymer network. The two NMR methods are combined to investigate the formation of the elastically effective network in dependence of the conversion of the functional groups of the precursor polymers and the cross-linker. The high chemical and time resolution of the experiments enabled an in-depth analysis of the reaction kinetics, allowing us to conclude on a multistage model for PDMS network formation by hydrosilylation-based end-linking in the presence of solvent. We found that the nonuniform network formation kinetics originates from a dependence of the apparent reaction rate on the number of the Si-H groups of the cross-linker that have already reacted during the progress of the reaction. The fastest overall reaction rate is observed in a range until each cross-linker has reacted once on average, and a uniform apparent overall reaction order of unity with respect to cross-linker concentration is only found at a later stage, when multiply reacted cross-linker molecules with similar reactivity dominate.

Fractal analysis of heterogeneous polymer networks formed by photopolymerization of dental dimethacrylates

ObjectivesIn this work the influence of the dimethacrylate monomer chemical structure on structural heterogeneity and physico-mechanical properties of the resulting polymer networks was investigated. Rigid aromatic dimethacrylate (Bis-GMA), triethylene glycol dimethacrylate (TEGDMA) and flexible aliphatic urethane-dimethacrylate (UDMA) were chosen for room-temperature homopolymerizations and copolymerizations induced by camphorquinone/N,N-dimethylaminoethyl methacrylate photoinitiating system. MethodsAtomic force microscopy (AFM) was used for visualizing the morphology of poly(dimethacrylate)s, which was described by: the fractal dimension (DF), the generalized fractal dimensions (Dq and ΔD) as well as the modified fractal dimension (Dβ). Estimated fractal characteristics were correlated with polymer density, hardness and impact strength. ResultsAFM images of fractured surfaces revealed the highly complex morphology of dimethacrylate polymer networks. They were found to possess the fractal character. The fractal parameters were observed to be proportional to the density, hardness and impact resistance of investigated polymers. ΔD appeared to be a good indicator of the structural heterogeneity of dimethacrylate networks. The results suggest that the fracture behavior of poly(dimethacrylate) matrix of dental materials can be controlled by the fractal morphology. SignificanceCorrelating the morphological studies with the mechanical tests would be beneficial in defining the role of morphology in the mechanical behavior of dimethacrylate networks and consequently, lead to the development of a reliable method for identifying the cause of dental material failures under stress. Thus, fractal analysis could become one of the key elements in designing and developing dental materials.

Two-Dimensional Polyamide Networks with a Broad Pore Size Distribution on the Ag(111) Surface

Covalently bonded, two-dimensional polyamide networks were prepared by vacuum deposition polymerization under ultra-high-vacuum conditions on the Ag(111) surface. By a careful choice of the annealing conditions, ordered domains of the corresponding oligomers could be also achieved. The layers were characterized by scanning tunneling microscopy. We used the step-growth polymerization between a trifunctional acid chloride and a bifunctional amine, taking place directly on the surface. The resulting polymer networks are nonuniform and show an open porous structure with a broad distribution of pore sizes. Besides large pores of varying shape, also small, 8- to 12-membered polymer rings with a regular shape are formed. A detailed analysis of the pore size distribution gives insight into the influence of the kinetic preference and the role of conformational flexibility for the formation of disordered covalent networks.

Polymers in Biomedicine and Electronics

The versatility of polymeric materials originates from the broad variety in their chemical structure and in their hierarchical organization as well as from the possibility to realize a wide spectrum of functions. Often such functions result from a combination ofamoleculararchitecture/morphology with a physical process and therefore require integration in a suitable system (e.g. medical device, electric circuit) or environment (e.g. physiological environment). Frequently, research in the area of functional polymers is motivated by applications. This special issue focuses on two application areas, in which polymer research as well as product development are progressing rapidly: Biomedicine and Electronics. Here (multi)functional polymers are an enabling technology and in this way create substantial and sustainable value. The significance of this topic is also highlighted at the conference ‘‘Polymers in Biomedicine and Electronics’’, which takes place from Oct 3 to 5, 2010 in Berlin combining the biannual meetings of the Gesellschaft Deutscher Chemiker (GDCh) Division ‘‘Macromolecular Chemistry’’ and of the Berlin-Brandenburgischer Verband fur Polymerforschung (BVP). Topics of the conference cover the design and the synthesis of functional polymers, physical principles underlying the function, the characterization of such functionswithin a system as well as product-oriented industrial research. Smart biomaterials, biocompatible coatings and their applications in controlled drug delivery systems or in regenerative therapies will be discussed in the biomedicine-oriented sessions of the conference. Photovoltaic, charge transport, light emission aswell as sensors andactuatorswill be subjects in the area of electronics.

Selective retention of some polyaromatic hydrocarbons by highly crosslinked polymer networks

AbstractHighly crosslinked polymer networks were characterized in terms of structural differences based on the crosslinked network structures with their chromatographic molecular retentivity for some polyaromatic hydrocarbons (PAHs). Because PAHs and some sterically bulky solutes were used in the chromatographic characterization, tiny differences in the crosslinked polymer networks were observed in terms of the chromatographic molecular retentivity. Ethylene dimethacrylate afforded molecular retentivity for anthracene, and this recognition ability changed with the polymerization time. In addition, 1,4‐butanediol dimethacrylate afforded molecular retentivity for pyrene, and this retentivity was larger than that for anthracene. The polymerization methods also affected the resulting polymer networks drastically. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2556–2566, 2005