Dynamics and Rheology in Aqueous Solutions of Associating Diblock and Triblock Copolymers of the Same Type

Influence of Li-Salt on the Mesophases of Pluronic Block Copolymers in Ionic Liquid

We study the complex mixture of a polyethylene oxide-b-polypropylene oxide-b-polyethylene oxide triblock copolymer (Pluronic F127) with ionic liquid (IL) and Li-salt, which is potentially interesting as an electrolyte system with decoupled mechanical and ion-transport properties. Small-angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC) are employed to scrutinize the phase structures and elucidate the ternary phase diagram. These data are combined with the ion diffusivities obtained by pulsed field gradient (PFG) nuclear magnetic resonance (NMR). Analyzing the partial ternary phase diagram of F127/LiTFSI/Pyr₁₄TFSI, hexagonal, lamellar, and micellar mesophases are identified, including two-phase coexistence regions. While the PPO block is immiscible with the liquid, and forms the backbone of the mesostructured aggregates, the PEO blocks are not well miscible with the IL. Poorly solvated, the latter may still crystallize. At a higher IL content, PEO is further solvated, but a major solvation effect occurs due to addition of Li-salt. Li ions promote solubilization of the PEO chains in the IL, since they coordinate to the PEO chains. This was identified as the mechanism of a transition of the mesostructures, with increasing Li-salt content changing from a hexagonal to a lamellar and further to a micellar phase. In summary, both, the amount of IL and its compatibility with the PEO block, the latter being controlled by the Li-salt amount, influence the compositions of the formed mesophases and the ion diffusion in their liquid regions.

Rapid and Quantitative De-tert-butylation for Poly(acrylic acid) Block Copolymers and Influence on Relaxation of Thermoassociated Transient Networks

The synthesis of charged polymers often requires the polymerization of protected monomers, followed by a polymer-analogous reaction to the polyelectrolyte product. We present a mild, facile method to cleave tert-butyl groups from poly(tert-butyl acrylate) blocks that yields poly(acrylic acid) (pAA) blocks free of traces of the ester. The reaction utilizes a slight excess of HCl in hexafluoroisopropanol (HFIP) at room temperature and runs to completion within 4 h. We compare deprotection in HFIP with the common TFA/DCM method and show that the latter does not yield clean pAA. We show the effect of complete tert-butyl cleavage on a ABA triblock copolymer, where poly(N-isopropylacrylamide) (pNIPAM) is A and pAA is B, by means of viscosimetry, DLS, and SAXS on solutions above overlap. The pNIPAM blocks dehydrate, and their increased self-affinity above the lower critical solution temperature (LCST) results in network formation by the triblocks. This manifests itself as an increase in viscosity and a slowing down of the first-order correlation function in light scattering. However, this stickering effect manifests itself exclusively when the pAA block is tert-butyl-free. Additionally, SAXS shows that the conformational properties of tert-butyl-free pAA copolymers are markedly different from those with residual esters. Thus, we illustrate a surprising effect of hydrophobic impurities that act across blocks and assert the usefulness of HCl/HFIP in pAA synthesis.

Colloidal aggregate and gel incubated by amorphous conjugated polymer in hybrid-solvent medium

A practical valuable amorphous conjugated polymer, poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene (MEH-PPV), has been revealed to foster an abundance of micrometer-sized colloidal aggregates at relatively low concentration (below 1 wt %) in a hybrid-solvent medium that contains a nonsolvent, and the solution turned into gel by colloidal bridging after one-day aging at 30 °C. In contrast with typical polymer gels fostered by (anisotropic) chain cross-linking or planar packing on selective interacting sites, the MEH-PPV gel has been revealed (via dynamic light scattering, small-angle light scattering, time-sweep dynamic modulus and optical microscope) to first develop featureless aggregate clusters in solution and, as the solvent quality worsens with reduced system temperature, bridge themselves to form gel through a one-dimensional (1-D) to three-dimensional (3-D) kinetic pathway. Combined dynamic/static light scattering analyses, along with supporting scanning electron microscope image and molecular dynamics simulation, indicated a concomitant structural reorganization within the colloidal aggregates, where spontaneous chain packing was perceived to form local fiber-like materials that are elastic by nature (i.e., a q-independent decay rate). The near coincidence of the above-mentioned microscopic and macroscopic phase alterations led us to contend that similar fibrous materials have served as the exterior bridging agent to fabricate colloidal strands upon gelation. The present findings clarify previously enigmatic, much speculative, gelation phenomena of MEH-PPV, and shed light on the prospect of capitalizing on specific polymer-solvent interactions to incubate desirable colloidal aggregates and gels in room-temperature processing of practical valuable conjugated polymers.

Slow dynamics in transient polyelectrolyte hydrogels formed by self-assembly of block copolymers

Transient polyelectrolyte hydrogels were formed by self-assembly of triblock copolyelectrolytes with a central hydrophilic block, poly(acrylic acid) (PAA), and two hydrophobic end blocks, poly(n-butyl acrylate(50%)-stat-acrylic acid(50%)) [P(nBA(50%)-stat-AA(50%))]. The relaxation of the concentration fluctuations was investigated by dynamic light scattering as a function of the concentration, the pH, the temperature, and the ionic strength. A relatively fast mode was observed at all polymer concentrations caused by cooperative diffusion of the polymers. Above the critical percolation concentration a second slow relaxation mode was observed caused by a linear displacement of small heterogeneities in the network with constant velocity. The relative amplitude of the slow mode was determined by the strength of the electrostatic repulsion. The velocity of the displacement in the transient network is shown to be directly correlated to the terminal relaxation time of the shear modulus and has the same Arrhenius temperature dependence. Both the velocity of the displacement and the mechanical relaxation strongly slow down with decreasing degree of ionization below 0.7 and increasing ionic strength above 0.5 M. A ballistic relaxation process has been reported earlier for colloidal gels, and the present study shows that it can also occur in polymeric networks.

Interchain coupled chain dynamics of poly(ethylene oxide) in blends with poly(methyl methacrylate): coupling model analysis

Quasielastic neutron scattering and molecular dynamics simulation data from poly(ethylene oxide) (PEO)/poly(methyl methacrylate) (PMMA) blends found that for short times the self-dynamics of PEO chain follows the Rouse model, but at longer times past t(c) = 1-2 ns it becomes slower and departs from the Rouse model in dependences on time, momentum transfer, and temperature. To explain the anomalies, others had proposed the random Rouse model (RRM) in which each monomer has different mobility taken from a broad log-normal distribution. Despite the success of the RRM, Diddens et al. [Eur. Phys. Lett. 95, 56003 (2011)] extracted the distribution of friction coefficients from the MD simulations of a PEO/PMMA blend and found that the distribution is much narrower than expected from the RRM. We propose a simpler alternative explanation of the data by utilizing alone the observed crossover of PEO chain dynamics at t(c). The present problem is just a special case of a general property of relaxation in interacting systems, which is the crossover from independent relaxation to coupled many-body relaxation at some t(c) determined by the interaction potential and intermolecular coupling/constraints. The generality is brought out vividly by pointing out that the crossover also had been observed by neutron scattering from entangled chains relaxation in monodisperse homopolymers, and from the segmental α-relaxation of PEO in blends with PMMA. The properties of all the relaxation processes in connection with the crossover are similar, despite the length scales of the relaxation in these systems are widely different.

Microrheology and dynamics of an associative polymer

We study the microscopic viscoelastic properties and relaxation dynamics of solutions of a side-chain associative polymer, hydrophobically modified hydroxyethyl cellulose (hmHEC). Dynamic light scattering from small tracer particles suspended in the polymer solutions is used to determine their viscous and elastic moduli on the scale of the particles. Bulk-scale viscoelastic properties are measured by shear rheometry. The motion of the tracer particles in hmHEC is diffusive at short times and subdiffusive at intermediate and long times. The long-time subdiffusive motion was not observed in parallel experiments on unmodified HEC solutions, and is explained in terms of hindered reptation of the hydrophobically modified polymer chains in the associative network. Dynamic light scattering from the polymer molecules themselves shows that chain relaxation in hmHEC is dominated by slow concentration-dependent processes due to the large-scale associative network structure, while that in HEC is dominated by fast concentration independent Rouse-like dynamics.

Characterization of the chemical degradation of hyaluronic acid during chemical gelation in the presence of different cross-linker agents

Dynamic light scattering (DLS) and rheological experiments have been performed on semidilute aqueous hyaluronic acid (HA) solutions during the chemical cross-linking process with a water-soluble carbodiimide (WSC) to produce a hydrogel. The formation and destruction of the gel are characterized. The results suggest that the gel is cross-linked via ester linkages and at later stage in the process, the omnipresent hydrolysis of interpolymer ester linkages and glycosidic bonds prevails, leading to disruption of the gel. The process of forming and breaking the gel is affected by the cross-linker concentration and pH. The cross-linking of HA with WSC in the presence of L-lysine methyl ester produced a gel with a longer time of gelation and the degradation of the gel was prolonged because of the more stable amide bond formation as the cross-link. By using the Ugi multicomponent condensation reaction, interpolymer cross-linking occurs via the formation of amide linkages and a stable gel evolves, which is only slightly degraded over an extended time window. DLS measurements on HA solutions with WSC show the emergence of a long-time power-law tail in the correlation functions at conditions both before and beyond the viscosity maximum. At a late stage in the gel-breakage regime, the power-law profile of the decay disappears and the long-time tail of the correlation function can be portrayed by a stretched exponential. The findings indicate that the power-law feature is associated with the confinement of chain dynamics and anomalous diffusion in the system. At later times, the connectivity is lost due to fragmentation of the network, and the long-time stretched exponential decay in the correlation function reflects the relaxation of clusters of various sizes.

Self-assembly of poly(ethylene oxide)-b-poly(epsilon-caprolactone) copolymers in aqueous solution

The associative behavior of monodisperse diblock copolymers consisting of a hydrophilic poly(ethylene oxide) block and a hydrophobic poly(epsilon-caprolactone) or poly(gamma-methyl-epsilon-caprolactone) block has been studied in aqueous solution. Copolymers have been directly dissolved in water. The solution properties have been studied by surface tension, in relation to mesoscopic analyses by NMR (self-diffusion coefficients), transmission electron microscopy, and small-angle neutron and X-ray scattering. The experimental results suggest that micellization occurs at low concentration (approximately 0.002 wt %) and results in a mixture of unimers and spherical micelles that exchange slowly. The radius of the micelles has been measured (ca. 11 nm), and the micellar substructure has been extracted from the fitting of the SANS data with two analytical models. The core radius and the aggregation number change with the hydrophobic block length according to scaling laws as reported in the scientific literature. The poly(ethylene oxide) blocks are in a moderately extended conformation in the corona, which corresponds to about 25% of the completely extended chain. No significant modification is observed when poly(gamma-methyl-epsilon-caprolactone) replaces poly(epsilon-caprolactone) in the diblocks.

Protein partitioning in thermoseparating systems of a charged hydrophobically modified ethylene oxide polymer

The phase behavior of a thermoseparating cationic hydrophobically modified ethylene oxide polymer (HM-EO) containing tertiary amines has been investigated at different pH, salt and sodium dodecyl sulfate (SDS) concentrations, in order to find a water/HM-EO two-phase system suitable for protein partitioning. The used polymer forms micellar aggregates that can be charged. By changing pH and SDS concentrations the netcharge of the SDS/HM-EO aggregate can be shifted from positive to negative. Bovine serum albumin (BSA) and lysozyme were partitioned in the thermoseparated two-phase systems of the cationic polymer at different pH, salt and SDS concentrations. The dominant attractive interactions between the polymer aggregates and the studied proteins were shown to be of electrostatic (Coulomb) nature rather than hydrophobic interaction. At low ionic strength the positively charged polymeric aggregates attracted negatively charged BSA and repelled positively charged lysozyme. Upon addition of SDS the negatively charged aggregates attracted lysozyme and repelled BSA. Thus, it was possible to direct proteins with different charges to the polymeric phase and redirect them to a polymer-depleted phase by changing the netcharge of the polymeric aggregates. The effect of different salts on the partitioning of BSA in a system of slightly positively charged HM-EO was studied. NaCl and KBr have a significant effect on driving the BSA to the polymer-depleted phase, whereas KF and K2SO4 have a smaller effect on the partitioning. The cloud point temperature of the charged polymer decreased upon addition of SDS near the isoelectric molar ratio of SDS to polymer and also upon salt addition. In the latter case the decrease was smaller than expected from model calculations based on Flory-Huggins theory, which were performed for a charged thermoseparating polymer at different charges and salt concentrations.

Primary recovery of a genetically engineered Trichoderma reesei endoglucanase I (Cel 7B) fusion protein in cloud point extraction systems

Here we present data to demonstrate how partitioning of a hydrophilic enzyme can be directed to the hydrophobic detergent-enriched phase of an aqueous two-phase system by addition of short stretches of amino acid residues to the protein molecule. The target enzyme was the industrially important endoglucanase I, EGI (endo-1,4-beta-D-glucan-4-glucanohydrolase, EC 3.2.1.4, Cel7B) of Trichoderma reesei. We investigated the partitioning of three EGI variants containing various C-terminal peptide extensions including Trp-Pro motifs of different lengths and localizations. Additionally, a recently developed system composed of the thermoseparating copolymer HM-EOPO was utilized to study the effects of fusion tags. The addition of peptides containing tryptohan residues enhanced the partitioning of EGI to the HM-EOPO-rich phase. The system composed of a nonionic detergent (Agrimul NRE1205) resulted in the highest partition coefficient (K = 31) and yield (90%) with the construct EGI(core-P5)(WP)(4) containing (Trp-Pro)(4) after a short linker stretch. A recombinant strain of T. reesei Rut-C30 for large-scale production was constructed in which the fusion protein EGI(core-P5)(WP)(4) was expressed from the strong promoter of the cellulase gene cbh1. The fusion protein was successfully expressed and secreted from the fungus during shake-flask cultivations. Cultivation in a 28-L bioreactor however, revealed that the fusion protein is sensitive to proteases. Consequently, only low production levels were obtained in large-scale production trials.

Purification of protein and recycling of polymers in a new aqueous two-phase system using two thermoseparating polymers

In this study we present a new aqueous two-phase system where both polymers are thermoseparating. In this system it is possible to recycle both polymers by temperature induced phase separation, which is an improvement of the aqueous two-phase system previously reported where one of the polymers was thermoseparating and the other polymer was dextran or a starch derivative. The polymers used in this work are EO50PO50, a random copolymer of 50% ethylene oxide (EO) and 50% propylene oxide (PO), and a hydrophobically modified random copolymer of EO and PO with aliphatic C14H29-groups coupled to each end of the polymer (HM-EOPO). In water solution both polymers will phase separate above a critical temperature (cloud point for EO50PO50 50 degrees C, HM-EOPO, 14 degrees C) and this will for both polymers lead to formation of an upper water phase and a lower polymer enriched phase. When EO50PO50 and HM-EOPO are mixed in water, the solution will separate in two phases above a certain concentration i.e. an aqueous two-phase system is formed analogous to poly(ethylene glycol) (PEG)/dextran system. The partitioning of three proteins, bovine serum albumin, lysozyme and apolipoprotein A-1, has been studied in the EO50PO50/HM-EOPO system and how the partitioning is affected by salt additions. Protein partitioning is affected by salts in similar way as in traditional PEG/dextran system. Recombinant apolipoprotein A-1 has been purified from a cell free E. coli fermentation solution. Protein concentrations of 20 and 63 mg/ml were used, and the target protein could be concentrated in the HM-EOPO phase with purification factors of 6.6 and 7.3 giving the yields 66 and 45%, respectively. Recycling of both copolymers by thermoseparation was investigated. In protein free systems 73 and 97.5% of the EO50PO50 and HM-EOPO polymer could be recycled respectively. Both polymers were recycled after aqueous two-phase extraction of apolipoprotein A-1 from a cell free E. coli fermentation solution. Apolipoprotein A-1 was extracted to the HM-EOPO phase with contaminating proteins in the EO50PO50 phase. The yield (78%) and purification factor (5.5) of apolipoprotein A-1 was constant during three polymer recyclings. This new phase system based on two thermoseparating polymers is of great interest in large scale extractions where polymer recycling is of increasing importance.

Thermoseparating water/polymer system: a novel one-polymer aqueous two-phase system for protein purification

In this study we show that proteins can be partitioned and separated in a novel aqueous two-phase system composed of only one polymer in water solution. This system represents an attractive alternative to traditional two-phase systems which uses either two polymers (e.g., PEG/dextran) or one polymer in high-salt concentration (e.g., PEG/salt). The polymer in the new system is a linear random copolymer composed of ethylene oxide and propylene oxide groups which has been hydrophobically modified with myristyl groups (C(14)H(29)) at both ends (HM-EOPO). This polymer thermoseparates in water, with a cloud point at 14 degrees C. The HM-EOPO polymer forms an aqueous two-phase system with a top phase composed of almost 100% water and a bottom phase composed of 5-9% HM-EOPO in water when separated at 17-30 degrees C. The copolymer is self-associating and forms micellar-like structures with a CMC at 12 microM (0.01%). The partitioning behavior of three proteins (lysozyme, bovine serum albumin, and apolipoprotein A-1) in water/HM-EOPO two-phase systems has been studied, as well as the effect of various ions, pH, and temperature on protein partitioning. The amphiphilic protein apolipoprotein A-1 was strongly partitioned to the HM-EOPO-rich phase within a broad-temperature range. The partitioning of hydrophobic proteins can be directed with addition of salt. Below the isoelectric point (pI) BSA was partitioned to the HM-EOPO-rich phase and above the pI to the water phase when NaClO(4)was added to the system. Lysozyme was directed to the HM-EOPO phase with NaClO(4), and to the water phase with Na-phosphate. The possibility to direct protein partitioning between water and copolymer phases shows that this system can be used for protein separations. This was tested on purification of apolipoprotein A-1 from human plasma and Escherichia coli extract. Apolipoprotein A-1 could be recovered in the HM-EOPO-rich phase and the majority of contaminating proteins in the water phase. By adding a new water/buffer phase at higher pH and with 100 mM NaClO(4), and raising the temperature for separation, the apolipoprotein A-1 could be back-extracted from the HM-EOPO phase into the new water phase. This novel system has a strong potential for use in biotechnical extractions as it uses only one polymer and can be operated at moderate temperatures and salt concentrations and furthermore, the copolymer can be recovered.