Petr Stepanek

Static and dynamic scattering from ternary polymer blends: Bicontinuous microemulsions, Lifshitz lines, and amphiphilicity

Ternary polymer blends, comprising two homopolymers and the corresponding diblock copolymer, have been examined by small-angle neutron scattering (SANS) and dynamic light scattering (DLS). Two chemical systems have been employed: one consisting of polyethylethylene, polydimethylsiloxane, and poly(ethylethylene-b-dimethylsiloxane), and another containing polyethylenepropylene, polyethyleneoxide, and poly(ethylenepropylene-b-ethylene oxide). The molecular weights and compositions were chosen to emphasize the region of the phase prism dominated by the bicontinuous microemulsion (BμE) phase; the homopolymer molecular weights and volume fractions were approximately equal. The SANS intensity was compared quantitatively with the Teubner–Strey structure factor, and interpreted via the amphiphilicity factor fa. The transition from a fully disordered mixture at higher temperatures to a well-developed BμE upon cooling did not correlate well with either the disorder line (fa=+1) or the total monomer Lifshitz line (fa...

pH-triggered block copolymer micelles based on a pH-responsive PDPA (poly[2-(diisopropylamino)ethyl methacrylate]) inner core and a PEO (poly(ethylene oxide)) outer shell as a potential tool for the cancer therapy

The potential of a novel pH-triggered block copolymer as a promising drug delivery platform for the cancer therapy has been explored. The block copolymer poly(ethylene oxide)-b-poly(glycerol monomethacrylate)-b-poly[2-(diisopropylamino)ethyl methacrylate] herein referred to as PEO113-b-PG2MA30-b-PDPA50 upon dissolution in ethanol followed by single-step nanoprecipitation in phosphate buffered saline (PBS) self-assembled into highly regular spherical micelles whose structure was characterized in detail by static (SLS), dynamic (DLS) and electrophoretic (ELS) light scattering, small angle X-ray scattering (SAXS), fluorescence spectroscopy and transmission electron microscopy (TEM). The micellar size (2RH = 42 nm) and micellar molecular weight (Mw(micelles) > 106 kDa) were found to be in the range to avoid renal clearance providing a long blood circulation time. Their size is below the cut-off size of the leaky pathological vasculature (DH < 200 nm), making them candidates for the use in cancer therapy based on the EPR effect. The pH-responsive PDPA core could be loaded with the poorly water-soluble anti-cancer drug paclitaxel (PTX) with encapsulation efficiency ∼70% and drug loading content ∼7% wdrug/wpolymer. The pKa of the diisopropylamino group of the PDPA block was determined as pKa = 6.8 in the simulated physiological condition, which is remarkably close to the pH microenvironment of tumoral cells. The release experiments evidenced that approximately 90% of the encapsulated PTX was sustained at the PDPA micellar core within the first 9 h at pH 7.4 whilst only 18 h were required for complete drug release at pH 5.0. These results suggest that the micellar dissociation might be triggered at the slightly acid tumoral extracellular environments (pH < pKa(PDPA)). The nanostructures were further placed in contact with human plasma or human serum albumin (HSA) diluted in PBS. The DLS experiments revealed that the micelles are especially stable for up to at least 48 h in such conditions, attesting the possibly long blood circulation time of the nanoparticles at serum environments which is a pre-requisite for the drug delivery applications. The cell viability experiments demonstrated that the drug-free block copolymer micelles are non-toxic and the number of viable cells is always greater than 85% compared to the survival number of a control group.

Critical dynamics of polymer blends

The critical behavior of six polyisoprene/poly(ethylene–propylene) binary blends has been examined, primarily by dynamic light scattering. Specifically, the decay rate (Γ) of fluctuations in composition has been measured as a function of scattering vector (q) and temperature, above the stability limit. The results are interpreted in the context of mode‐coupling corrections to the dynamics. Theoretically predicted crossovers between mean‐field and nonclassical regimes, and between mode‐coupled and mode‐decoupled dynamics, are clearly seen. The mean‐field to nonclassical crossover, or Ginzburg criterion, is localized, and the mode‐coupled to mode‐decoupled transition in the dynamics occurs well into the static mean‐field regime. In addition, geometric crossovers between Γ∼q2 (‘‘diffusive’’) and Γ∼q3 or Γ∼q4 (‘‘critical nondiffusive’’) regimes are in quantitative agreement with theory. The applicability of the Kawasaki–Stokes relationship between Γ and the dynamic correlation length is also demonstrated.

pH-triggered reversible sol–gel transition in aqueous solutions of amphiphilic gradient copolymers

We demonstrate the possibility of reversible pH-controlled sol–gel transition in aqueous solution of associating amphiphilic triblock copolymer poly(styrene-grad-acrylic acid)-b-poly(acrylic acid)-b-poly(styrene-grad-acrylic acid), (PS-grad-PAA)-b-PAA-b-(PS-grad-PAA), synthesized vianitroxide-mediated (NM) radical copolymerization. The presence of pH-sensitive co-monomer units of the acrylic acid in the terminal blocks ensures the dynamic nature of the styrene-rich hydrophobic nano-domains which are formed at low pH. At small polymer concentrations the association triggered by lowering the pH gives rise to flower-like micelles stabilized by partially ionized PAA coronae. The pH-controlled association was monitored by DLS-titration and manifested in the evolution of a correlation peak in the SANS spectra. The resulting copolymer aggregates were visualized by TEM, which confirmed the spherical shape of the dense styrene-rich domains. Above the micelle overlap concentration a decrease in pH provokes macroscopic gelation. Here the styrene-reach domains perform as cross-links in the transient network. The pH-triggered sol-to-gel transition is manifested in an abrupt and strong (up to 3 orders of magnitude) increase in the zero-shear viscosity and in a characteristic change in the frequency dependence of the storage and loss moduli. The discovered effect can be used for efficient pH-control of the rheological properties of aqueous solutions.

Synthesis and pH- and salinity-controlled self-assembly of novel amphiphilic block-gradient copolymers of styrene and acrylic acid

A novel type of amphiphilic ionic copolymer comprising a hydrophilic poly(acrylic acid) (PAA) block and an amphiphilic poly(acrylic acid)-grad-poly(styrene) (PAA-grad-PS) copolymer block was synthesized using a one step direct nitroxide-mediated polymerization (NMP). A strong influence of the macroinitiator on the values of the reactivity ratios of the co-monomers is confirmed by 1H NMR. The aggregation behaviour of the copolymers in the aqueous medium was studied by small-angle neutron scattering (SANS) and dynamic light scattering (DLS) and by transmission electron microscopy (TEM) in a wide range of pHs and ionic strengths. It has been demonstrated that PAA-b-(PAA-grad-PS) copolymers are soluble in alkaline water at room temperature without the special experimental procedures (addition of co-solvent, heating, etc.) that are usually required for solubilisation of classical PAA-b-PS diblock copolymers. The self-assembly of the PAA-b-(PAA-grad-PS) copolymers into nano-scale aggregates at low/moderate pH and/or high ionic strength was demonstrated by SANS and DLS experiments. The SANS spectra for the copolymer solution exhibit a correlation peak pointing to the formation of micelles with repulsive coronae. TEM images indicate that the micelles have an approximately spherical shape and exhibit a wide size distribution. Our results prove, that in contrast to “frozen” aggregates formed by PAA-b-PS copolymers in aqueous media, the micelles of PAA-b-(PAA-grad-PS) amphiphilic copolymers exhibit “dynamic” pH-responsive properties, i.e. they can reversibly change their aggregation number upon a variation in the pH or salinity of the solution.

Macromolecular HPMA-based nanoparticles with cholesterol for solid-tumor targeting: detailed study of the inner structure of a highly efficient drug delivery system.

We report a rigorous investigation into the detailed structure of nanoparticles already shown to be successful drug delivery nanocarriers. The basic structure of the drug conjugates consists of an N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer bearing the anticancer drug doxorubicin (Dox) bound via a pH-sensitive hydrazone bond and a defined amount of cholesterol moieties that vary in hydrophobicity. The results show that size, anisotropy, and aggregation number N(aggr) of the nanoparticles grows with increasing cholesterol content. From ab initio calculations, we conclude that the most probable structure of HPMA copolymer-cholesterol nanoparticles is a pearl necklace structure, where ellipsoidal pearls mainly composed of cholesterol are covered by a HPMA shell; pearls are connected by bridges composed of hydrophilic HPMA copolymer chains. Using a combination of techniques, we unambiguously show that the Dox moieties are not impregnated inside a cholesterol core but are instead uniformly distributed across the whole nanoparticle, including the hydrophilic HPMA shell surface.

Effect of hydrophobic interactions on properties and stability of DNA-polyelectrolyte complexes.

Polyplexes are polyelectrolyte complexes of DNA and polycations, designed for potential gene delivery. We investigated the properties of new polyplexes formed from cholesterol-modified polycations and DNA. Three complexes were tested; their cholesterol contents were 1.4, 6.3, and 8.7 mol %. UV spectroscopy and fluorescence assay using ethidium bromide proved the formation of polyplexes. The kinetics of turbidity of polyplexes solutions in physiological solution showed that the colloid stability of polyplexes increases with increasing content of cholesterol in polycations. Dynamic, static, and electrophoretic light scattering, small-angle X-ray scattering, and atomic force microscopy were used for characterization of polyplexes. The observed hydrodynamic radii of polyplexes were in the range of 30-60 nm; they were related to the polycation/DNA ratio and hydrophobicity of the used polycations (the cholesterol content). The properties of polyplex particles depend, in addition to polycation structure, on the rate of polycation addition to DNA solutions.

Coating of Vesicles with Hydrophilic Reactive Polymers

Vesicles bearing either cationic (amino) groups or zwitterionic (amino acid) groups on the surface were coated with a reactive multivalent hydrophilic N-(2-hydroxypropyl)methacrylamide polymer (PHPMA) and its positively charged analogue (3 mol % quaternary ammonium groups), both having reactive thiazolidine-2-thione (TT) groups randomly distributed along the polymer chain. The vesicles were dispersed in water at a concentration of 1 mg/mL. The effect of surface charges of model vesicles on the surface coating efficiency was evaluated. The changes in the weight-average molecular weight, in the hydrodynamic size, and in the zeta-potential of model vesicles were tested using light scattering methods. The most effective coating of vesicles was observed for the zwitterionic vesicles coated with the positively charged hydrophilic PHPMA-TT copolymer at a concentration of reactive polymer cp = 2 mg/mL. The coating efficiency was more than 1 order of magnitude higher than that obtained for positively charged vesicles coated by the uncharged hydrophilic polymer at the same cp.

pH sensitive polymer nanoparticles : effect of hydrophobicity on self-assembly

The influence of hydrophobicity on formation, stability, and size of pH-responsive methacryloylated oligopeptide-based polymer nanoparticles has been studied by dynamic light scattering (DLS), transmission electron microscopy (Cryo-TEM), and NMR. Different polyanions/surfactant systems have been studied at constant polymer concentration and within a broad range of surfactant concentrations. The two newly synthesized pH-sensitive hydrophobic polyanions, poly(N(ω)-methacryloyl glycyl-L-leucine) and poly(N(ω)-methacryloyl glycyl-L-phenylalanyl-L-leucinyl-glycine), and three nonionic surfactants (Brij97, Brij98, and Brij700) have been investigated. The surfactants were different in the length of hydrophilic poly(ethylene oxide) (PEO) chain. In surfactant-free solution at basic pH, the polyanions form hydrophobic domains. In the presence of a surfactant, our results prove the complex formation at high pH between the nonionic surfactant and the polyelectrolyte; a pearl-necklace structure is formed. At low pH below critical pH (pH(tr)), reversible nanoscale structures occur in solutions for all systems. The detailed mechanism of the formation of pH-sensitive nanoparticles from polymer-surfactant complex with varying pH is established. Our results suggest that the polymer hydrophobicity is of primary importance in pretransitional behavior of the complex. Once preliminary nanoparticle nuclei are formed, the hydrophobicity of the polymer plays a minor role on further behavior of formed nanostructures. The subsequent transformation of nanoparticles is determined by the surfactant hydrophilicity, the length of hydrophilic tail that prevents further aggregation due to steric repulsions.

Hydrolytically Degradable Polymer Micelles for Drug Delivery: A SAXS/SANS Kinetic Study

We report kinetic studies of therapeutically highly potent polymer-drug conjugates consisting of amphiphilic N-(2-hydroxypropyl) methacrylamide (HPMA)-based copolymers bearing the anticancer drug doxorubicin (Dox). Highly hydrophobic cholesterol moieties as well as the drug were attached to the polymer backbone by a pH-sensitive hydrazone bond. Moreover, the structure of the spacer between the polymer carrier and the cholesterol moiety differed in order to influence the release rate of the hydrophobic moiety, and thus the disintegration of the high-molecular-weight micellar nanoparticle structure. We performed time-dependent SAXS/SANS measurements after changing pH from a typical blood value (pH 7.2) to that of tumor cells (pH 5.0) to characterize the drug release and changes in particle size and shape. Nanoparticles composed of the conjugates containing Dox were generally larger than the drug-free ones. For most conjugates, nanoparticle growth or decay was observed in the time range of several hours. It was established that the growth/decay rate and the steady-state size of nanoparticles depend on the spacer structure. From analytical fitting, we conclude that the most probable structure of the nanoparticles was a core-shell or a core with attached Gaussian chains. We concluded that the spacer structure determined the fate of a cholesterol derivative after the pH jump. Fitting results for 5α-cholestan-3-onecholestan-3-one and cholesteryl-4-oxopentanoate (Lev-chol) implied that cholesterol moieties continuously escape from the core of the nanoparticle core and concentrate in the hydrophilic shell. In contrast, cholest-4-en-3-one spacer prevent cholesterol escaping. Dox moiety release was only observed after a change in pH. Such findings justify the model proposed in our previous paper. Lastly, the cholesteryl 4-(2-oxopropyl)benzoate (Opb-Chol) was a different case where after the release of hydrophobic Opb-Chol moieties, the core becomes more compact. The physicochemical mechanisms responsible for the scenarios of the different spacers are discussed.