Solution Behavior of Poly(n-Isopropylacrylamide) in Water: Effect of Additives

Cosolvent incorporation modulates the thermal and structural response of PNIPAM/silyl methacrylate copolymers

Polymers functionalized with inorganic silane groups have been used in wide-ranging applications due to the silane reactivity, which enables formation of covalently-crosslinked polymeric structures. Utilizing stimuli-responsive polymers in these hybrid systems can lead to smart and tunable behavior for sensing, drug delivery, and optical coatings. Previously, the thermoresponsive polymer poly(N-isopropyl acrylamide) (PNIPAM) functionalized with 3-(trimethoxysilyl)propyl methacrylate (TMA) demonstrated unique aqueous self-assembly and optical responses following temperature elevation. Here, we investigate how cosolvent addition, particularly ethanol and N,N-dimethyl formamide (DMF), impacts these transition temperatures, optical clouding, and structure formation in NIPAM/TMA copolymers. Versus purely aqueous systems, these solvent mixtures can introduce additional phase transitions and can alter the two-phase region boundaries based on temperature and solvent composition. Interestingly, TMA incorporation strongly alters phase boundaries in the water-rich regime for DMF-containing systems but not for ethanol-containing systems. Cosolvent species and content also alter the aggregation and assembly of NIPAM/TMA copolymers, but these effects depend on polymer architecture. For example, localizing the TMA towards one chain end in 'blocky' domains leads to formation of uniform micelles with narrow dispersities above the cloud point for certain solvent compositions. In contrast, polydisperse aggregates form in random copolymer and PNIPAM homopolymer solutions - the size of which depends on solvent composition. The resulting optical responses and thermoreversibility also depend strongly on cosolvent content and copolymer architecture. Cosolvent incorporation thus increases the versatility of inorganic-functionalized responsive polymers for diverse applications by providing a simple way to tune the structure size and optical response.

Nonionic Microgels Adapt to Ionic Guest Molecules: Superchaotropic Nanoions

Microgels are commonly applied as solute carriers, where the size, density, and functionality of the microgels depend on solute binding. As representatives for ionic solutes with high affinity for the microgel, we study here the effect of superchaotropic Keggin polyoxometalates (POMs) PW12O403- (PW) and SiW12O404- (SiW) on the aqueous swelling and internal structure of nonionic poly(N-isopropylacrylamide) (pNiPAM) microgels by light scattering techniques and small-angle X-ray scattering. Due to their weak hydration, these POMs bind spontaneously to the microgels at millimolar concentrations. The microgels thus become charged and swell at low POM concentration, surprisingly without strongly increasing the volume phase transition temperature, and deswell at higher POM concentration. The swelling arises because of the osmotic pressure of dissociated counterions of the POMs, while the deswelling is due to POMs acting as physical cross-links in the microgels under screened electrostatics in NaCl or excess POM solution. This swelling/deswelling transition is sharper for PW than for SiW related to the lower charge density, weaker hydration, and stronger binding of PW. The POMs elicit qualitatively and quantitatively different swelling effects from ionic surfactants and classical salts. Moreover, the network softness and topology govern the swelling response upon POM binding. The softer the microgel, the stronger is the swelling response, while, inside the microgel, regions of high polymer density swell/contract more upon electric charging/cross-linking than regions with low polymer density. POM binding thus enables fine-tuning of microgel properties and highlights the role of network topology in microgel swelling. Because POMs decompose at an alkaline pH, these POM/microgel systems also exhibit pH-responsive swelling in addition to the typical temperature responsiveness of pNiPAM microgels.

Comparing polymer-surfactant complexes to polyelectrolytes

HYPOTHESIS

Understanding the complex interactions between polymers and surfactants is required to optimise commercially relevant systems such as paint, toothpaste and detergent. Neutral polymers complex with surfactants, forming 'pearl necklace' structures that are often conceptualised as pseudo-polyelectrolytes. Here we pose two questions to test the limits of this analogy: Firstly, in the presence of salt, do these polymer-surfactant systems behave like polyelectrolytes? Secondly, do polymer-surfactant complexes resist geometric confinement like polyelectrolytes?

EXPERIMENTS

We test the limits of the pseudo-polyelectrolyte analogy through studying a poly(N-isopropylacrylamide) (PNIPAM) brush in the presence of sodium dodecylsulfate (SDS). Brushes are ideal for interrogating pseudo-polyelectrolytes, as neutral and polyelectrolyte brushes exhibit distinct and well understood behaviours. Spectroscopic ellipsometry, quartz crystal microbalance with dissipation monitoring (QCM-D), and neutron reflectometry (NR) were used to monitor the behaviour and structure of the PNIPAM-SDS system as a function of NaCl concentration. The ability of the PNIPAM-SDS complex to resist geometric confinement was probed with NR.

FINDINGS

At a fixed SDS concentration below the zero-salt CMC, increasing NaCl concentration <100 mM promoted brush swelling due to an increase in osmotic pressure, not dissimilar to a weak polyelectrolyte. At these salt concentrations, the swelling of the brush could be described by a single parameter: the effective CMC. However, at high NaCl concentrations (e.g., 500 mM) no brush collapse was observed at all (non-zero) concentrations of SDS studied, contrary to what is seen for many polyelectrolytes. Study of the polymer-surfactant system under confinement revealed that the physical volume of surfactant dominates the structure of the strongly confined system, which further differentiates it from the polyelectrolyte case.

Swelling of Thermo-Responsive Gels in Aqueous Solutions of Salts: A Predictive Model

The equilibrium degree of swelling of thermo-responsive (TR) gels is strongly affected by the presence of ions in an aqueous solution. This phenomenon plays an important role in (i) the synthesis of multi-stimuli-responsive gels for soft robotics, where extraordinary strength and toughness are reached by soaking of a gel in solutions of multivalent ions, and (ii) the preparation of hybrid gels with interpenetrating networks formed by covalently cross-linked synthetic chains and ionically cross-linked biopolymer chains. A model is developed for equilibrium swelling of a TR gel in aqueous solutions of salts at various temperatures T below and above the critical temperature at which collapse of the gel occurs. An advantage of the model is that it involves a a small (compared with conventional relations) number of material constants and allows the critical temperature to be determined explicitly. Its ability (i) to describe equilibrium swelling diagrams on poly(N-isopropylacrylamide) gels in aqueous solutions of mono- and multivalent salts and (ii) to predict the influence of volume fraction of salt on the critical temperature is confirmed by comparison of observations with results of numerical simulation.

Study on preparation, compression strength and theophylline/diclofenac sodium release ability of NIPAAm/DMAPMAAm hydrogels

The present study was undertaken to investigate the effect of the composition of the polymerization medium and the type of drug/drug loading process on the mechanical strengths and release profiles of poly(N-isopropylacrylamide-co-N-[3-(dimethylamino)propyl] methacrylamide) P(NIPAAm-co-DMAPMAAm) hydrogels. In line with this goal firstly, the temperature- and pH-responsive hydrogels of NIPAAm and DMAPMAAm were synthesized in three different media at 60 °C: pH 7.4 phosphate-buffered saline (PBS), pH 7.4 phosphate buffer without NaCl/KCl (PB), and distilled-deionized water (pH ≈ 5.5 DDW). The result is that the presence of anionic species such as phosphate (HPO₄^2-/H₂PO₄^-) and chloride (Cl^-) ions in the solution affects on their basic network properties such as volumetric swelling ratio and compression modulus. To evaluate their intermolecular interactions with protonated DMAPMAAm units and drug molecules, depending on composition, type of loading process and drug structure, each of the hydrogels was loaded with diclofenac sodium (DFNa) and theophylline (Thp) by using both diffusion and in situ loading methods. DFNa and Thp release profiles in pH 7.4 PBS at 37 °C were analysed by using zero-order, first-order, Higuchi, Korsmeyer-Peppas, and Peppas-Sahlin models. It has been observed that for the first 60% of DFNa and Thp releases from P(NIPAAm-co-DMAPMAAm) hydrogels synthesized in PB at 60 °C, the contribution of the chain relaxation for the copolymer hydrogels loaded during gelation process was higher than the ones loaded by diffusion process.

SALTING-IN AND SALTING-OUT EFFECTS OF POLYPHENOLS, AROMATIC COMPOUNDS, AND AMINO ACIDS ON POLY (N-ISOPROPYLACRYLAMIDE) AND EGG WHITE AQUEOUS SOLUTIONS

Introduction. Understanding the biophysical phenomena related to the Hofmeister series or cosmotropic/chaotropic properties of ions requires experimental data on specific ion effects of large organic molecules.Problem Statement. Specific ion effects are of significant importance for biophysics and medicine. It is interesting to find out if additives with biologically relevant anions can interact with proteins and avoid aggregation.Purpose. The purpose of this research is to study the stabilizing/destabilizing effects in Poly(N-isopropylacrylamide) (PNIPAM)/water, hen egg white/water systems under influence of substances of various classes of different hydrophobia.Materials and Methods. Materials: sodium salts: salicylate, ferulate, benzoate, vanillate, cinnamate; humic acid sodium salt, hydroxy-sodium benzoate, glycine, L-alanine, sodium L-glutamate, D-(—)-quinnic acid, PNIPAM, egg white. Methods: measurement of the transition temperature (TT) of PNIPAM/water and the denaturationtemperature of the egg white / water systems.Results. Ion-specific effects have been studied with the use of models based on the research of the transition temperature (TT) evolution of binary PNIPAM / water mixtures (for heating from 0 to 35°C) and the denaturation temperature of the egg white / water (for heating from 48 to 65°C). The dependences of these temperatureson the content of substances that occur in live nature have been received at pH = 7.4.Conclusions. The results have shown the tendencies of the additive solubilizing effects on PNIPAM and egg white. The majority of substances studied has the salting-out effect on PNIPAM / water in the order NaBz ~ NaCinn < L-NaGlu ~ NaFer. For NaSal and NaHum; the salting-in effect has been established. For the egg white, all additives show the salting-in effect. In PNIPAM and egg white systems, NaBz and L-NaGlu demonstrate the opposite effects. The results have been compared with the ones for binary mixtures of water/di-propylene glycolpropyl ether (DPnP).

Synthesis, characterization and in vitro cytotoxicity studies of poly-N-isopropyl acrylamide gel nanoparticles and films

In this work, we show synthesis that leads to thermoreponsive poly-N-isopropyl acrylamide (pNIPAM) nanogels with sizes below 100 nm, irrespectively of the surfactant to crosslinker ratio. We also show that in many environments the temperature induced pNIPAM collapse at Lower Critical Solution Temperature (LCST) of 32.5 °C is accompanied by gel nanoparticles' aggregation. Thus, the proper information on the nanoparticle (NP) structure and deswelling can be obtained only if the routinely measured hydrodynamic radius is supplemented by information on the molecular weight, which can be obtained from the intensity of scattered light. We measured the dynamics and reversibility of the deswelling and subsequent aggregation processes. Furthermore, we show that the highly concentrated pNIPAM gel NPs reversibly form bulk hydrogel networks of varied interconnected porous structure. We show, that in case of drying pNIPAM gel NPs above the LCST, it is possible to obtain films with 20-fold increase in storage modulus (G') compared to hydrogel networks measured at room temperature. They exhibit temperature hysteresis behavior around LCST of 32.5 °C similar to pNIPAM films. Finally, we show that these hydrogel films, lead to extended proliferation of cells across three different types: fibroblast, endothelial and cancer cells. Additionally, none of the films exhibited any cytotoxic effects. Our study brings new insights into physicochemical characterization of pNIPAM gel NPs and networks behavior in realistic conditions of in vitro measurements, especially by means of dynamic light scattering as well as final unique properties of both gel NPs and formed porous films for possible tissue engineering applications.

The role of osmolytes in the temperature-triggered conformational transition of poly(N-vinylcaprolactam): an experimental and computational study

Biomedical industries are widely exploring the use of thermo-responsive polymers (TRPs) in the advanced development of drug delivery and in many other pharmaceutical applications. There is a great need to investigate the use of less toxic and more (bio-)compatible TRPs employing several additives, which could modify the conformational transition behavior of TRPs in aqueous solution. To move forward in this aspect, we have chosen the less toxic bio-based polymer poly(N-vinylcaprolactam) (PVCL) and three different methylamine-based osmolytes, trimethylamine N-oxide (TMAO), betaine and sarcosine, in order to investigate their particular interactions with the polymer segments in PVCL and therefore the corresponding changes in the thermo-responsive conformational behavior. Several biophysical techniques, UV-visible spectroscopy, fluorescence spectroscopy, dynamic light scattering (DLS) and laser Raman spectroscopy, as well as classical computer simulation methods such as molecular dynamics are employed in the current work. All the studied methylamines are found to favor the hydrophobic collapse of the polymer thus stabilizing the globular state of PVCL. Sarcosine is observed to cause the maximum decrease in lower critical solution temperature (LCST) of PVCL followed by TMAO and then betaine. The differences observed in the LCST values of PVCL in the presence of these molecules can be attributed to the different polymer-osmolyte interactions. The less sterically hindered N atom in the case of sarcosine causes a significant difference in the phase transition temperature values of PVCL compared to betaine and TMAO, where the nitrogen atom is buried by three methyl groups attached to it.

Self-assembly of oppositely charged polyelectrolyte block copolymers containing short thermoresponsive blocks

The assembly of oppositely charged block copolymers, containing small thermoresponsive moieties, was investigated as a function of salt concentration and temperature. Aqueous solutions of poly-[N-isopropylacrylamide]-b-poly[dimethylaminoethyl methacrylate] (NIPAM₄₄-b-DMAEMA₂₁₆) and PNIPAM-b-poly[acrylic acid]-b-PNIPAM (NIPAM₃₅-b-AA₂₀₀-b-NIPAM₃₅) were mixed in equal charge stoichiometry, and analysed by light scattering (LS), NMR spectroscopy and small angle X-ray scattering (SAXS). At room temperature, two different micelle morphologies were found at different salt concentrations. At NaCl concentrations below 0.75 M, complex coacervate core micelles (C3M) with a PNIPAM corona were formed as a result of interpolyelectrolyte complexation. At NaCl concentrations exceeding 0.75 M, the C3M micelles inverted into PNIPAM cored micelles (PCM), containing a water soluble polyelectrolyte corona. This behavior is ascribed to the salt concentration dependence of both the lower critical solution temperature (LCST) of PNIPAM, and the complex coacervation. Above 0.75 M NaCl, the PNIPAM blocks are insoluble in water at room temperature, while complexation between the polyelectrolytes is prevented because of charge screening by the salt. Upon increasing the temperature, both types of micelles display a cloud point temperature (T_cp), despite the small thermoresponsive blocks, and aggregate into hydrogels. These hydrogels consist of a complexed polyelectrolyte matrix with microphase separated PNIPAM domains. Controlling the morphology and aggregation of temperature sensitive polyelectrolytes can be an important tool for drug delivery systems, or the application and hardening of underwater glues.

The assembly of oppositely charged block copolymers, containing small thermoresponsive moieties, was investigated as a function of salt concentration and temperature.

Structural transitions and encapsulation selectivity of thermoresponsive polyelectrolyte complex micelles

Polyelectrolyte complex micelles containing thermoresponsive coronas can exhibit varying morphologies and encapsulate multivalently charged therapeutics for drug delivery applications.

Clouding phenomenon in amphiphilic systems: A review of five decades

Phase separation in amphiphilic systems is an important phenomenon. The temperature at which an amphiphilic solution phase separates is known as Cloud Point (CP). This article reviews in detail the process of phase separation in various amphiphiles (surfactants, polymers and drugs) and effect of different classes of additives on the CP of these amphiphilic systems. Ions affect the CP of drugs in a different way: kosmotropes and hard bases decrease while chaotropes and soft bases increase the CP of nonionic and cationic surfactants. Anionic surfactants show CP in presence of quaternary salts only. Thus, depending upon the nature and concentration of additive, the CP of an amphiphilic system gets increased or decreased and, hence, properties of the system may be tuned as per the need and use. A system with CP at high concentration can be made to phase separate at lower concentration by simply introducing an appropriate additive in it. This makes the system cost effective. On the other hand, if not required, a low CP can be enhanced with the help of another type of a suitable additive.

New endeavours involving the cooperative behaviour of TMAO and urea towards the globular state of poly(N-isopropylacrylamide)

Studies have provided evidence for the destruction of the hydrogen bonds of poly(N-isopropylacrylamide) (PNIPAM) in the presence of osmolytes such as trimethylamine N-oxide (TMAO) and urea.

Specific ion modulated thermoresponse of poly(N-isopropylacrylamide) brushes

The influence of specific anions on the equilibrium thermoresponse of poly(N-isopropylacrylamide) (pNIPAM) brushes has been studied using in situ ellipsometry, quartz crystal microbalance with dissipation (QCM-D) and static contact angle measurements between 20 and 45 °C in the presence of up to 250 mM acetate and thiocyanate anions in water. The thickness and changes in dissipation exhibited a broad swelling transition spanning approximately 15 °C from collapsed (high temperatures) to swollen conformation (low temperatures) while the brush surface wettability changed over approximately 2 °C. In the presence of the kosmotropic acetate anions, the measured lower critical solution temperature (LCST) by the three techniques was very similar and decreased linearly as a function of ionic strength. Conversely, increasing the concentration of the chaotropic thiocyanate anions raised the LCST of the pNIPAM brushes with variation in the measured LCST between the three techniques increasing with ionic strength. The thickness of the pNIPAM brush was seen to progressively increase with increasing thiocyanate concentration at all temperatures. It is proposed that specific ion binding of the chaotropic thiocyanate anion with pNIPAM amide moieties increases the electrostatic intra- and intermolecular repulsion within and between pNIPAM chains. This allows the brush to begin to swell at higher temperatures and to an overall greater extent.