Aliaksandr Zhuk

Hydrogen-bonded layer-by-layer temperature-triggered release films.

A hydrogen-bonded layer-by-layer (LbL) technique was used to build multilayers of neutral, temperature-responsive polymers such as poly(N-isopropylacrylamide) (PNIPAM), poly(N-vinylcaprolactam) (PVCL), poly(vinyl methyl ether) (PVME), or poly(acrylamide) (PAAm) with a polycarboxylic acid such as poly(acrylic acid) (PAA), poly(methacrylic acid) (PMAA), or poly(ethacrylic acid) (PEAA). For all multilayers involving temperature-responsive polymers, the temperature used during or after self-assembly had a significant effect on film stability with pH changes. The proximity of the self-assembly or post-self-assembly temperature to the critical temperature of phase separation of a neutral polymer from solution resulted in a higher pH stability of multilayers. However, for polymers with a lower critical solution temperature (LCST) such as PNIPAM, PVCL, or PVME within PNIPAM/PMAA, PVCL/PMAA, or PVME/PMAA multilayers, the critical pH of film disintegration (pH(crit)) increased in the temperature range from 10 to 37 degrees C, whereas for polymer films with an upper critical solution temperature (UCST), such as PAAm within PAAm/PMAA, the film showed the opposite trend. Using a hydrogen-bonded polyvinylpyrrolidone (PVPON)/PMAA system, which is not responsive to temperature changes, we constructed hybrid films with lower [PNIPAM/PMAA](n) and higher [PVPON/PMAA](m) strata and obtained free-floating [PVPON/PMAA](m) films by temperature-triggered dissolution of the PNIPAM/PMAA layers at a constant pH value. The kinetics of [PVPON/PMAA](m) film release was strongly dependent on the number of bilayers within the PNIPAM/PMAA stratum, indicating significant interpenetration between PNIPAM/PMAA and PVPON/PMAA bilayers. Importantly, the use of PEAA instead of PAA or PMAA in film assembly enabled the construction of hydrogen-bonded LbL films that can be released by applying temperature as a trigger at near-physiological pH values. This feature makes such release layers attractive candidates for future tissue engineering applications.

Multiresponsive Clay-Containing Layer-by-Layer Films

We report on polymer/clay layer-by-layer films responsive to multiple stimuli. Temperature- and salt-responsive films were constructed using assembly of poly(N-isopropylacrylamide) (PNIPAM) and montmorillonite clay nanosheets. An additional pH response was achieved by depositing and cross-linking hybrid, dual-network PNIPAM/clay/PNIPAM/poly(methacrylic acid) (PMAA) multilayers. Both types of films remained stable in a wide pH range and were highly swollen. For example, PNIPAM/clay films swelled up to ~14.5 times their dry film thickness in low-salt solutions at 25 °C, as shown by laser scanning confocal microscopy. At temperatures higher than PNIPAMs lower critical solution temperature (LCST) of 32 °C, or in 0.3 M Na(2)SO(4) solutions at room temperature, both PNIPAM/clay and PNIPAM/clay/PNIPAM/PMAA films reversibly deswelled as a result of collapse of PNIPAM chains. Films of both types showed a decrease in permeability to fluorescein-tagged dextrans of various molecular weights. Importantly, film permeability to dextrans was decreased at temperatures above PNIPAMs LCST, and the effect could be reversed by lowering the temperature. Inclusion of PMAA within multilayers provided an additional pH response to film swelling and permeability. Hybrid PNIPAM/clay/PNIPAM/PMAA films showed drastic deswelling at low pH values due to the onset of hydrogen bonding between PNIPAM and PMAA, and the diffusion of 70 kDa dextran through multilayers at acidic pH was completely blocked. These multiresponse features of clay-containing films make them promising candidates for applications in sensing, actuation, and controlled delivery.

Hydrogen-bonded layer-by-layer films of block copolymer micelles with pH-responsive cores.

We report on construction of hydrogen-bonded monolayers and multilayers of micelles of the poly(2-(diethylamino)ethyl methacrylate)-block-poly(N-isopropyl acrylamide) (PDEA-b-PNIPAM) with PNIPAM-corona and polybasic PDEA cores. Films were constructed at pH 7.5 and 25°C to assure the deposition of PDEA-b-PNIPAM in the micellar form. When monolayers of block copolymer micelles (BCMs) were exposed to moderately acidic pH values, micellar cores dissolved, while PDEA-b-PNIPAM remained adsorbed at the surface as unimers. In contrast to reversible micellization of PDEA-b-PNIPAM in solution, micelle-to-unimer transition was irreversible at the surface. Adsorption of a layer of tannic acid (TA) or polyethacrylic acid (PEAA) on top of BCM monolayers inhibited pH-triggered morphological changes within adsorbed BCMs. By taking advantage of the high pK(a) values of TA and PEAA, we were also able to construct multilayers of PDEA-b-PNIPAM micelles through hydrogen bonding interactions between micellar PNIPAM coronas and TA or PEAA. Similar to BCM monolayers coated with TA or PEAA, dissolution of BCMs was also inhibited when incorporated within hydrogen-bonded multilayers. Such inhibition of dissolution is due to enhanced hydrogen bonding interactions between coronal PNIPAM chains and protonated TA molecules or PEAA chains at decreasing pH values restricting the pH-induced conformational changes of the micellar core chains within the adsorbed layer. Films of responsive BCMs are attractive coatings for controlled delivery of functional molecules from surfaces due to a combination of stimuli-response properties with the relatively high loading capacity for functional molecules.

Stimuli-responsive layer-by-layer nanocomposites

The fast-growing area of materials prepared with the layer-by-layer (LbL) technique has become firmly established over the last three decades. Responsive LbL assemblies present one attractive type of LbL materials, useful in sensing, controlled delivery and actuation. However, stimuli-responsive LbL films made exclusively from polymers or organic molecules are somewhat restricted in the types of responses that can be applied to a material, and often suffer from poor mechanical strength and limited durability. Assembly of inorganic nanoparticles (INPs), including spherical nanoparticles, nanorods, nanotubes, and nanosheets, among others, within multilayer films brings additional benefits by not only improving mechanical, electronic, magnetic and optical properties of these materials, but also endowing them with the capability to transform external signals and transfer them to other film components. Combining INPs with temperature-, pH-, or light-responsive polymers within LbL films yields novel materials with unique characteristics and properties easily controllable by external stimuli. In this paper, we discuss strategies for designing various types of INP–organic LbL assemblies, the nature of their stimuli response and potential applications.

Tunable pH and temperature response of weak polyelectrolyte brushes: role of hydrogen bonding and monomer hydrophobicity

We report on weak polyacid brushes with highly tunable pH and temperature response characteristics. This was achieved by synthesizing a series of homo- and copolymers which contain 2-alkylacrylic acids (aAAs) of increased hydrophobicity, i.e. acrylic acid (AA), methacrylic acid (MAA), or 2-ethylacrylic acid (EAA), using surface-initiated reversible addition–fragmentation chain transfer (SI-RAFT) polymerization. As revealed by contact angle measurements, in situ ellipsometry and AFM studies of brush swelling, the pH-response of PAA and PMAA brushes was similar, with brushes remaining highly swollen (swelling ratio 2.5–3.0) at low pH values. The PEAA brush, however, was unique as it showed low degrees of water uptake ( 70°) contact angles in the entire pH region from 2 to 8. Copolymer brushes of aAAs with N-isopropylacrylamide (NIPAM), denoted as P(AA-co-NIPAM), P(MAA-co-NIPAM) and P(EAA-co-NIPAM), demonstrated dual pH and temperature response, which was strongly dependent on the type of aAA co-monomer. P(AA-co-NIPAM) and P(MAA-co-NIPAM) brushes underwent large-amplitude pH-induced changes in brush swelling and water contact angle in the range of pH from 3 to 6, and were only weakly responsive to temperature in the transition region. In contrast, more hydrophobic P(EAA-co-NIPAM) brushes demonstrated both pH and temperature responses at physiologically relevant neutral/basic pH values even when the content of EAA units in the copolymer was as high as ∼50%. We discuss the role of inter- and intra-molecular hydrogen bonding and monomer hydrophobicity and ionization (quantified by FTIR) in determining pH ranges for brush response. These findings might enable control of molecular/cellular adhesion and flow at interfaces potentially useful in microfluidic and biomedical applications.

Chain Conformation and Dynamics in Spin-Assisted Weak Polyelectrolyte Multilayers

We report on the effect of the deposition technique on film layering, stability, and chain mobility in weak polyelectrolyte layer-by-layer (LbL) films. Ellipsometry and neutron reflectometry (NR) showed that shear forces arising during spin-assisted assembly lead to smaller amounts of adsorbed polyelectrolytes within LbL films, result in a higher degree of internal film order, and dramatically improve stability of assemblies in salt solutions as compared to dip-assisted LbL assemblies. The underlying flattening of polyelectrolyte chains in spin-assisted LbL films was also revealed as an increase in ionization degree of the assembled weak polyelectrolytes. As demonstrated by fluorescence recovery after photobleaching (FRAP), strong binding between spin-deposited polyelectrolytes results in a significant slowdown of chain diffusion in salt solutions as compared to dip-deposited films. Moreover, salt-induced chain intermixing in the direction perpendicular to the substrate is largely inhibited in spin-deposited films, resulting in only subdiffusional (<2 Å) chain displacements even after 200 h exposure to 1 M NaCl solutions. This persistence of polyelectrolyte layering has important ramifications for multistage drug delivery and optical applications of LbL assemblies.

Selective water uptake within micelle-containing layer-by-layer films of various architectures: a neutron reflectometry study†

Swelling of micelle-containing layer-by-layer (LbL) films of various architectures has been studied by neutron reflectometry (NR). Multilayers of the first type were constructed using poly(2-(dimethylamino)ethyl methacrylate)-block-poly(N-isopropylacrylamide) (PNIPAM-b-PDMA) block copolymer micelles (BCMs) alternately assembled with poly(4-styrene sulfonate) (PSS). NR data showed that the films maintained their layered structure, but deuterated PSS (dPSS), deposited within every 5th layer as a marker, was highly interdiffused into neighboring layers. In situ NR measurements demonstrated that the films swelled homogeneously by ∼35% in an aqueous environment. The second type of multilayer contained three zones: bottom and top stacks consisting of PDMA/dPSS homopolymer assemblies, and BCMs deposited in the middle stack as (BCM/dPSS)n, where n is the number of deposition cycles, equal to 1 or 2. The individual micellar layer deposited in a single deposition step (n = 1) only partially covered the surface, whereas a complete layer of micelles was achieved after two deposition cycles. In situ NR study of these stacked films revealed different degrees of water uptake by film internal strata. While layers of assembled micelles took up ∼38% water by volume when dry films were exposed to an aqueous environment at 25 °C, the bottom homopolymer stack was able to take up only 11–20% water. In addition, the film architecture and the degree of surface coverage by BCMs were found to be important factors enabling the study of selective swelling of film strata. NR-enabled observations of selective swelling of assembled amphiphilic BCMs allow one to correlate film swelling on the nanoscale with internal structure, and present a powerful approach for future studies of BCM-containing systems, which are useful in actuation, sensing, and controlled delivery applications.