Free Space Makes the Polymer “Dead Layer” Alive

The effect of free space on molecular motion inside the polymer “dead layer” or adsorbed nanolayers on solid surfaces is investigated. Free space is introduced into the nanolayer by choosing a polymer with a relatively big side group, poly n-butyl methacrylate (PnBMA), and polarization-resolved single-molecule fluorescence microscopy is adopted as the method. The rotational motion of the doped fluorescent probes is found to be considerably excited at moderate temperatures, attributed to the free space brought by the side group of the PnBMA. The development of the adsorbed nanolayer by the prolonged annealing of the parent film is carefully monitored, together with the evolution of the molecular motion and the glass transition temperature (Tg). The Tg values of the exposed nanolayers are considerably lower than that of the bulk system, while they become higher than those in the bulk situation when the nanolayer is covered with a polymer top layer. The experimental evidence has demonstrated that the free space made available by the side group and the air–polymer interface has considerably promoted the molecular motion inside the adsorbed nanolayers, even under the situation of overwhelming surface attraction

Counterion Cloud Expansion of a Polyelectrolyte by Dilution

It has long been documented that the reduced viscosity of polyelectrolyte has an anomalous dependence on its concentration, i.e., the Fuoss law. To explore the molecular mechanism, the counterion distribution of sodium polystyrenesulfonate (NaPSS) as a function of concentration is investigated at the single-molecule level. By examination of the fluorescence resonance energy transfer (FRET) between a fluorescence donor on NaPSS chain and an acceptor in the counterions using single-molecule fluorescence spectroscopy, an increase of average counterion–chain distance is discovered upon dilution, indicating the expansion of counterion cloud. By photon counting histogram, an increase of effective charge of the NaPSS chain during dilution is exposed. The variation of these parameters agrees well with that of the reduced viscosity, helping to shed light on the molecular mechanism of the Fuoss law: the expansion of the counterion cloud increases hydrodynamic friction, and the increase of effective charges of NaPSS due to desorption of counterions brings about the stronger interchain coupling

Response of a Permanently Charged Polyelectrolyte Brush to External Ions: The Aspects of Structure and Dynamics

Structure and dynamics inside permanently charged polyelectrolyte brushes, sodium polystyrene sulfonate brushes, during their response to the introduction of external ions (NaCl) are investigated by neutron reflectivity and dielectric spectroscopy. Neutron reflectivity measurements show that the segmental density of the inner part of the brushes decreases and that of the outer part increases when the salt level is tuned from the salt-free condition to a moderate level (<10^–2 M)the brushes swell further compared with the salt-free condition. This is attributed to the breakup of the multiplets formed by dipole–dipole pairs, and by this process, the previously constrained chain segments by the multiplets are released. Dielectric spectroscopy discovers a giant dipole by the charge separation of the adsorbed counterions and the PSS^– chains, induced by electric field. The dynamics of the induced giant dipole is accelerated with the increase of external salt, as a result of the charge regularization by elevated salt level. At high-enough salt level, the screening effect reduces the electrostatic repulsion between the neighboring chains and makes the brushes shrink

Axial Growth and Fusion of Liposome Regulated by Macromolecular Crowding and Confinement

The endomembrane system, including the endoplasmic reticulum, Golgi apparatus, lysosomes, and endosomes, is located in the crowded intracellular environment. An understanding of the cellular structure and functions requires knowledge of how macromolecular crowding and confinement affect the activity of membrane and its proteins. Using negatively charged liposome and the peptide K₃L₈K₃ as a model system, we studied the aggregation behavior of liposome in a matrix of polyacrylamide and hyaluronic acid. Without matrix, the liposomes form spherical aggregates in the presence of K₃L₈K₃. However, they orient in one dimension and fuse into a tube up to 40 μm long in the matrix. The growth of the tube is via end-to-end connection. This anisotropic growth is mainly due to the macromolecular confinement provided by the polymer network. The study of the interactions between liposome and peptide in the crowded environment helps to reveal the mechanism of membrane-related processes in vivo

Resolving the Difference in Electric Potential within a Charged Macromolecule

The difference of the electric potential between the middle and end of polystyrenesulfonate (PSS^–) chain is discovered experimentally. Using a pH-responsive fluorophore attached to these two locations on the PSS^– chain, the local pH value was determined by single molecule fluorescence technique: photon counting histogram (PCH). By the observation of a very high accumulation of proton (2–3 orders of magnitude in concentration) at the vicinity of the PSS^– as a result of the electrostatic attraction between the charged chain and protons, the electric potential of the PSS^– chain is determined. A higher extent of counterion adsorption is discovered at the middle of the PSS^– chain than the chain end. The entropy effect of the counterion adsorption is also discoveredupon the dilution of protons, previously adsorbed counterions are detached from the chain