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.

Fluorescence recovery after photobleaching has been applied to determine, to our knowledge for the first time, the molecular weight (M w) dependence of lateral diffusion of polymer chains within layer-by-layer (LbL) films. As shown by neutron reflectometry, polyelectrolyte multilayers containing polymethacrylic acid (PMAA, M w/M n < 1.05) of various molecular weights assembled from solutions of low ionic strengths at pH 4.5, where film growth was linear, showed similar diffusion of PMAA in the direction perpendicular to the film surface. At a salt concentration sufficient for unfreezing electrostatically bonded chains, layer intermixing remained almost unaffected (changes <1.0 nm), while the lateral diffusion coefficient (D) scaled with the PMAA molecular weight as D ∼ M w –1±0.05.

We report on the correlation of polyelectrolyte chain dynamics in polyelectrolyte complexes (PECs) with the deposition mode and chain mobility of polyelectrolytes (PEs) within layer-by-layer-assembled (LbL) films. The study was performed using two polyelectrolyte systems: poly(2-(dimethylamino)ethyl methacrylate)/poly(methacrylic acid) (PDMA/PMAA) and completely quaternized PDMA (Q100M)/PMAA. Hydrodynamic sizes of PDMA/PMAA and Q100M/PMAA complexes in solution were followed by fluorescence correlation spectroscopy (FCS), while three different techniques were applied to probe the structure and dynamics of the same PE pairs within LbL films. Specifically, deposition of PEs at surfaces was monitored by phase-modulated ellipsometry, film internal structureby neutron reflectometry (NR), and diffusion of assembled chains in the direction parallel to the substrateby fluorescence recovery after photobleaching (FRAP). By applying these complementary techniques to PDMA/PMAA and Q100M/PMAA systems in solution and at surfaces at various pH values, we found that the dynamics of polyelectrolyte chains within PECs underwent a prominent pH-dependent transition, and that this transition in chain dynamics was closely correlated with the transition between linear and exponential film growth modes. Neutron reflectometry results confirm that, at the transition point, film structure changed from layered for linearly depositing films to highly intermixed for exponentially depositing LbLs. Moreover, FRAP indicated a several-fold difference in PE lateral diffusion coefficient for the two different film growth modes. In addition, the pH transition point was affected by steric restrictions to ionic pairing, and the pH range of exponential growth and higher chain mobility was wider for Q100M/PMAA as compared with the PDMA/PMAA system, due to the presence of a methyl spacer at the amino group, resulting in weaker ionic pairing.

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 PNIPAM's 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 PNIPAM's 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.

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 (<10%) at pH < 5 due to insolubility of this polyelectrolyte in acidic solutions, moderate swelling at high pH and relatively high (>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 pHinduced 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.