Micellar transitions in solvent-annealed thin films of an amphiphilic block copolymer controlled with tunable surface fields

We investigated the response of symmetric poly(styrene-block-4vinylpyridine) P(S-b-4VP) diblock copolymer micelles to surface fields of variable strength at free surfaces and substrate interfaces when the micelles as spun were subjected to solvent annealing. Free surface interactions were controlled with solvent annealing in solvents of varied selectivity. On exposure to vapors of a solvent strongly selective for PS, the micelles retained their spherical shape but grew into cylindrical micelles or lamellar nanostructures via fusion on exposure to slightly selective or neutral solvent vapors. Giant 2D disks that completely wetted PS-grafted substrates resulted when spherical micelles were exposed to vapors of a highly selective solvent for P4VP. The interfacial interactions were controlled through subjecting them to UV/ozone (UVO) substrates initially coated with an end-grafted layer of short PS chains, with which the grafted PS chains became oxidized, degraded, or totally removed through UVO treatment for a controlled duration. When thin films were annealed in vapors of THF, the structural transition from spherical to cylindrical micelles depended on the interfacial field. On applying selective UVO exposure of optimal duration, we fabricated a substrate with two interfacial chemistries that promoted varied micellar species (spherical and cylindrical micelles) with a sharp boundary developed within thin films through solvent annealing for a controlled duration.     

Engineering a sharp physiological transition state for poly(n‐isopropylacrylamide) through structural control

Poly(N‐isopropylacrylamide) (pNIPAAm), a well‐studied, biologically inert polymer that undergoes a sharp aqueous thermal transition at 32 °C, has been a subject of widespread interest for possible biological applications. A major hindrance to its successful application is due to the difficulty of maintaining a sharp transition when the polymer is modified for a physiological transition temperature, especially in isotonic solutions. Current copolymer blends raise the transition temperature but also make the transition significantly broader. We have combined the use of reversible addition‐fragmentation chain transfer (RAFT) polymerization with tacticity control to synthesize well‐defined pNIPAAm that demonstrates sharp transitions under physiological conditions. By selecting a RAFT agent with appropriate end groups, controlling molecular weight, and increasing the racemo diad content, we were able to increase the thermal transition temperature of pure pNIPAAm to a sharp transition at 37.6 °C under isotonic conditions. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Thermal degradation of lyocell/poly-N-isopropylacrylamide graft copolymers gels

In the present work Lyocell fibers were subjected to graft copolymerization of poly-N-isopropylacrylamide (pNIPAAm) thermosensitive polymer. The thermal degradation and stability of lyocell/pNIPAAm copolymers gels were characterized by differential scanning calorimetry (DSC) and thermogravimetric analysis (TG). pNIPAAm/lyocell copolymers are thermally stable and more resistant to temperature than lyocell fibres. Thermal characterization was analyzed as a function of percentage by mass of the pNIPAAm grafted. It has been shown that for pNIPAAm/lyocell copolymers, degradation occurs at higher temperature when increasing the degree of grafting.     

Kinetics of latex formation of PBMA latex in the presence of alkali soluble resin using atomic force microscopy

 The effect of alkali-soluble resin (ASR), poly(ethylene-co-acrylic acid), EAA, postadded to emulsifier-free monodisperse poly(butyl methacrylate) (PBMA) latexes on the kinetics of film formation was investigated using atomic force microscopy (AFM). Corrugation height of latex particles in films was monitored at various annealing temperatures as a function of annealing time. Enhanced polymer diffusion was found in a latex film containing ASR regardless of anneal-ing temperature. With increasing annealing temperature, a much higher rate of polymer diffusion was found in latex films containing ASR. These results can be interpreted that the low molecular weight and low Tg EAA resin adsorbed at the particle surface is more susceptible to diffusion than that of the PBMA in the film formation stage, thus it enhances the mobility of PBMA polymer. Received: 30 October 1997 Accepted: 20 March 1998     

Anti-biofouling properties of amphiphilic phosphorylcholine polymer films

Surfaces of amphiphilic phosphorylcholine polymer (PC1036) prepared by spin-coating were characterized by spectroscopic ellipsometry, water contact angle and atomic force microscopy. The antifouling properties of the PC1036 films to marine benthic diatom Nitzschia closterium MMDL533 were also investigated. The results showed that the dry PC1036 film promoted the adhesion of N. closterium MMDL533 because the hydrophobic lauryl groups were present in the film surface. The 2 h-swelled PC1036 films had excellent anti-fouling properties with extremely low attachment densities and retention densities no matter what the annealing temperature was. The thickness of the coated films lower than 147 Å had a profound effect on the film anti-fouling properties. Otherwise, when the film thickness was higher than that value, there was no more improvement of diatom cell reduction observed. The annealing temperature had only a little effect on the film resistant to diatom adhesion, which might be attributed to two factors including the PC group packing densities in the outer PC layer and the equilibrated water volume fraction in the 2 h-swelled PC1036 films.     

Thermoresponsive terpolymers based on methacrylate monomers: Effect of architecture and composition

A series of amphiphilic thermoresponsive copolymers was synthesized by group transfer polymerization. Seven copolymers were prepared based on the nonionic hydrophobic n‐butyl methacrylate (BuMA), the ionizable hydrophilic and thermoresponsive 2‐(dimethylamino)ethyl methacrylate (DMAEMA) and the nonionic hydrophilic poly(ethylene glycol)methyl methacrylate (PEGMA). In particular, one diblock copolymer and six tricomponent copolymers of different architectures and compositions, one random and five triblock copolymers, were synthesized. The polymers and their precursors were characterized in terms of their molecular weight and composition using gel permeation chromatography and proton nuclear magnetic resonance spectroscopy, respectively. Aqueous solutions of the polymers were studied by turbidimetry, hydrogen ion titration, and light scattering to determine their cloud points, pK_as, and hydrodynamic diameters and investigate the effect of the polymers' composition and architecture. The thermoresponsive behavior of the copolymers was also studied. By increasing the temperature, all polymer solutions became more viscous, but only one polymer, the one with the highest content of the hydrophobic BuMA, formed a stable physical gel. Interestingly, the thermoresponsive behavior of these triblock copolymers was affected not only by the terpolymers' composition but also by the terpolymers' architecture. These findings can facilitate the design and engineering of injectable copolymers for tissue engineering that could enable the in situ formation of physical gels at body temperature. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 775–783, 2010     

Experimental Investigations on Highly Conducting Solid Bio-Polymer Electrolytes Based on Latex of Calotropis gigantea Plant

The study has prepared highly conducting polymer electrolyte films using solution cast technique with poly(vinylidene fluoride-co-hexafluoropropylene) PVDF-HFP, mixture of ethylene carbonate (EC), and propylene carbonate (PC) as plasticizer and latex of Calotropis gigantea (CGL) as an ionic source. In this study, four films are prepared using PVDF-HFP:CGL in ratio 1:1 with the increasing concentration of EC+PC as 1, 2, 3, and 4 M named as 1:1:1, 1:1:2, 1:1:3, 1:1:4. The prepared polymer electrolyte is examined by polarized optical microscopy (POM), elemental dispersive X-ray technique (EDX), and complex impedance spectroscopy. EDX and POM are studied for the surface morphology of all prepared samples and to investigate the porous nature of films. The enhancement in ionic conductivity occurs due to CGL and increasing amount of EC-PC. Conductivity of highest composition (1:1:4) polymer electrolyte film is found to be ≈10⁻³ S cm⁻¹. The optimized polymer electrolyte film is considered as a promising candidate for application in supercapacitors.     

Effect of filler networking on the response of thermosensitive composite hydrogels

Several composite hydrogels of poly(N-isopropylacrylamide) (pNIPAAm) with sodium montmorillonite (NaMM) have been synthesized using a fixed polymer/NaMM ratio (4:1 wt./wt.), but various monomer concentrations, in order to obtain hydrogels with different degrees of swelling, and thus different clay contents in the swollen state. For comparison, unfilled pNIPAAm gels have been also prepared at the same concentrations. The equilibrium swelling behaviour of the gels has been studied both in the swollen and in the shrunk state. In the swollen state, the polymer volume fraction increases with the initial monomer concentration C₀. In the shrunk state, the polymer fraction in pNIPAAm hydrogels is dependent on the specimen size and on C₀, whereas in the composite gels a constant polymer content is observed. When subjected to stepwise heating from 25 to 45 °C, unfilled gels undergo only poor deswelling. By contrast, complete deswelling takes place in composite gels. The latter show half-shrinking times varying over two orders of magnitude, depending on the monomer concentration and on the procedure followed to disperse NaMM, which determine the overall dispersion state of the filler, as evidenced by transmission electron microscopy (TEM). In particular, TEM observations show clay networking above a percolation threshold near 2.5 wt.% of NaMM. The effect of the incorporation of clay on the response to thermal stimuli is discussed in terms of the ability of NaMM to hinder the hydrophobic association of pNIPAAm segments and in terms of its dispersion state. It is suggested that, above the percolation threshold, NaMM forms a hydrophilic, physical network, through which water can flow also above the volume transition temperature, where pNIPAAm acquires a hydrophobic character.     

Roughness correlation and interdiffusion in thin films of polymer chains

The surface morphology of thin bilayer polymer films on top of glass substrates was investigated. The bilayer consists of a blend film of protonated and deuterated polystyrene and an underlying deuterated polystyrene film. Choosing the thickness of the top film larger than 8 times and smaller than 2 times the radius of gyration of the chains enables the determination of film thickness and confinement effects. With diffuse neutron scattering at grazing incidence in the region of total external reflection, a depth sensitivity and a contrast even at the internal polymer–polymer interface was achieved. The underlying film is conformal to the substrate, and depending on the thickness of the top film two different types of roughness correlations are observed. Thin confined films nestle to the underlying polymer films, while the stiffness of thicker bulky films provides an independent morphology. In both cases, annealing above the glass-transition temperature yields an interdiffusion at the internal polymer–polymer interface, and the polymer–air surface remains essentially unchanged with respect to roughness correlations. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2862–2874, 1999     

Structural optimization of highly branched thermally responsive polymers as a means of controlling transition temperature

Highly branched “smart” polymers have emerged as a unique class of polymers with wide‐ranging applications. Poly(N‐isopropylacrylamide) (pNIPAAm) is at the forefront of stimuli‐responsive polymers; however, few transition temperature‐modification methods of linear pNIPAAm have been explored in highly branched systems. In this study, the three primary techniques of transition temperature modification of linear pNIPAAm are investigated for their efficacy on highly branched polymers. Of these techniques, cosolvent‐mediated tacticity control demonstrates an effect opposite of that which is expected. Temperature transition control via end‐group modification shows a marked decrease in efficacy in highly branched systems, despite highly branched systems having more end groups per polymer. Copolymerization with hydrophilic comonomers exhibits varying changes in efficacy compared to linear analogs, lending insights into the specific effects on the structured water surrounding the copolymer. While copolymerization proved to be most versatile in changing the transition temperature, all of the techniques showed interesting secondary effects. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Retention Behavior of Neutral and Positively and Negatively Charged Solutes on an Immobilized‐Artificial‐Membrane (IAM) Stationary Phase

The retention behavior of neutral, positively charged, and negatively charged solutes on the IAM.PC.DD2 stationary phase was investigated and compared. A set of monofunctional compounds and complex drugs (steroids, nonsteroidal anti‐inflammatory drugs, and β‐blockers) were selected for this study, i.e., neutral solutes and solutes with acidic or basic functionalities which are positively charged or negatively charged at pH 7.0. The correlation between the retention factor log k_w at pH 7.0 on the IAM.PC.DD2 stationary phase and the partition coefficient log P_oct or the distribution coefficient log D_7.0 showed that the retention mechanism depends on the charge state and structural characteristics of the compounds. The neutrals were least retained on the IAM.PC.DD2 stationary phase, and positively charged solutes were more retained than negatively charged ones. This implies that the retention of the charged solutes is controlled not only by lipophilicity but also by the electrostatic interaction with the phospholipid, with which positively charged solutes interact more strongly than negatively charged ones.     

Compatibility studies of polystyrene and poly(vinyl acetate) blends using electrostatic force microscopy

The effect of thermal treatment on the phase separation process of the components of a polymer blend was investigated using electrostatic force microscopy (EFM). EFM technique is an advance on conventional atomic force microscopy, which enables us to measure locally the dielectric properties of the samples under investigation providing compositional information. In this work, we studied the phase separation process of the polymer blend thin films made of polystyrene and poly(vinyl acetate) (PS/PVAc) (75/25 weight fraction). The samples were subjected to different thermal treatments. It was found that at low annealing temperature, PVAc forms many small islands within PS matrix. As the annealing temperature increases, the number of PVAc islands decreases with an increase in the size of the islands. These islands take spherical‐like shape when annealed at a temperature well above the glass transition temperatures of both the component polymers. Despite these morphological/topographical changes, EFM images evidence that there is no interdiffusion which was further confirmed by quantitatively measuring the value of the dielectric permittivity across the interphase. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1332–1338, 2011     

Interdiffusion of polymer chains during latex coating

The fluorescence method was used to study the interdiffusion of polymer chains during annealing of latex above its glass transition temperature (T_g). The latex film was prepared from mixture of naphthalene (N) (donor) and pyrene (P) (acceptor)-labeled poly(methyl methacrylate) (PMMA) latex particles. Heptane was used as the coalescent agent. A steady-state fluorescence technique was employed to measure the amount of direct nonradiative energy transfer from N to P during the interdiffusion of polymer chains across the particle–particle junction. Various latex films with different latex contents were used to measure the percentage critical occupation for the reliable steady-state fluorescence measurements. Diffusions activation energies in these latex films were measured and found to be around 30 kcal/mol, which was attributed to the backbone motion of PMMA chains.     

Thermal diffusivity of polycaprolactone/poly(bisphenol A carbonate) blends by flash radiometry

This article presents thermal diffusivity (D) measurement by flash radiometry for the polymer blend of polycaprolactone (PCL) and poly(bisphenol A carbonate) (PC) with a lower critical solution temperature (LCST) phase diagram. The dependence of D on temperature is significantly changed by annealing above the cloud point. D is largely increased by the growth of PCL crystallite in the blends. In the miscible state, D is smaller than that predicted by the rule of mixtures, whereas the dependence of D on the weight fraction of PC for the immiscible (phase‐separated) state is well expressed by the rule of mixtures. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 745–749, 1999     

Temperature-dependent interfacial properties of hydrophobically end-modified poly(2-isopropyl-2-oxazoline)s assemblies at the air/water interface and on solid substrates

We describe herein the properties at the air/water (A/W) interface of hydrophobically end-modified (HM) poly(2-isopropyl-2-oxazoline)s (PiPrOx) bearing an n-octadecyl chain on both termini (telechelic HM-PiPrOx) or on one chain end (semitelechelic HM-PiPrOx) for different subphase temperatures and spreading solvents using the Langmuir film balance technique. The polymer interfacial properties revealed by the π–A isotherms depend markedly on the architecture and molecular weight of the polymer. On cold water subphases (14 °C), diffusion of PiPrOx chains onto water takes place for all polymers in the intermediate compressibility region (5 mN m⁻¹). At higher subphase temperatures (36 and 48 °C), the HM-PiPrOx film exhibited remarkable stability with time. Brewster angle microscopy (BAM) imaging of the A/W interface showed that the polymer assembly was not uniform and that large domains formed, either isolated grains or pearl necklaces, depending on the polymer structure, the concentration of the spreading solution and the subphase temperature. The Langmuir films were transferred onto hydrophilic substrates (silica) by the Langmuir–Blodgett (LB) technique and onto hydrophobic substrates (gold) by Langmuir–Schaefer (LS) film deposition, resulting in the formation of adsorbed particles ranging in size from 200 to 500 nm, depending on the polymer architecture and the substrate temperature. The particles presented “Janus”-like hydrophilic/hydrophobic characteristics.     

Phase transition properties of vanadium oxide films deposited by polymer-assisted deposition

Vanadium dioxide (VO₂) films were synthesized on mica substrates by a polymer-assisted deposition method, followed by rapid annealing with different annealing temperatures. The crystalline structure and morphology of the films were investigated by XRD and FE-SEM, and their phase transition properties were studied by in situ FT-IR. The results indicated that the annealing temperature affected the crystalline structure and morphology of the films remarkably, which then resulted in varied phase transition properties. In particular, the films annealed at higher temperature showed more polycrystalline structure, increased particle size and reduced phase transition intensity. But the films exhibited the similar hysteresis temperature width with increasing annealing temperature.     

Viscoelastic properties of ultrathin polycarbonate films by liquid dewetting

A liquid dewetting method for the determination of the viscoelastic properties of ultrathin polymer films has been extended to study thickness effects on the properties of ultrathin polycarbonate (PC) films. PC films with film thicknesses ranging from 4 to 299 nm were placed on glycerol at temperatures from below the macroscopic glass transition temperature (T_g) to above it with the dewetting responses being monitored. It is found that the isothermal creep results for films of the same thickness, but dewetted at different temperatures can be superposed into one master curve, which is consistent with the fact of PC being a thermorheologically simple material. Furthermore, the results show that the T_g of PC thin films is thickness dependent, but the dependence is weaker than the results for freely standing films and similar to literature data for PC films supported on rigid substrates. It was also found that the rubbery plateau region for the PC films stiffens dramatically, but still less than what has been observed for freely standing polycarbonate films. The rubbery stiffening is discussed in terms of a recently reported model that relates macroscopic segmental dynamics with the stiffening. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1559–1566     

Salt‐Induced Depression of Lower Critical Solution Temperature in a Surface‐Grafted Neutral Thermoresponsive Polymer

Summary: Quartz crystal microbalance with dissipation monitoring (QCM‐D) is employed to determine the effect of salt on the volume phase transition of thermoresponsive polymer brushes. Changes in mass and viscoelasticity of poly(N‐isopropylacrylamide) (PNIPAM) layers grafted from a QCM‐D crystal are measured as a function of temperature, upon contact with aqueous solutions of varying salt concentrations. The phase‐transition temperature of PNIPAM brushes, T_C,graft, quantified from the QCM‐D measurements is found to decrease as the concentration of salt is increased. This phenomenon is explained by the tendency of salt ions to affect the structure of water molecules (Hofmeister effect). However, in contrast to the linear decrease in phase‐transition temperature upon increasing salt concentration observed for free PNIPAM, the trend in T_C,graft for PNIPAM brushes is distinctively non‐linear.

Schematic representation of the effect of salt concentration on the phase transition behavior of thermoresponsive polymer brushes.     

Using 2-isopropyl-2-oxazine to explore the effect of monomer distribution and polymer architecture on the thermoresponsive behavior of copolymers

Poly(cyclic imino ether)s (PCIE) have emerged as a highly promising class of polymer for use in biomedical applications with their value being derived from their excellent biocompatibility, diverse chemistry, and tunable hydrophilicity. Here, we investigate the properties of poly(2-isopropyl-2-oxazine) (PiPrOz), a relatively unexplored PCIE, determining it to have a cloud point temperature (T_cp) below physiological temperature, not to crystallize from bulk or in solution, and to be highly biocompatible. Furthermore, a series of copolymers consisting of iPrOz and 2-methyl-2-oxazine (MeOz) was investigated with regard to the effect of monomer distribution and polymer architecture on thermoresponsive properties. To this end, linear block and statistical co-poly(2-oxazine)s (co-POz), along with three comb-shaped POz with block or statistical POz side chains were prepared. Each of the five polymers showed distinct thermoresponsive behavior, with the linear block co-POz undergoing micelle formation and the other polymers macroscopic phase-separation at different T_cps. The variety observed in response to heating clearly highlights the importance of monomer sequence and polymer architecture when designing thermoresponsive polymers. We anticipate that our findings will prove useful to polymer chemists seeking to prepare novel thermoresponsive biomaterials.     

Self-organization of polymer particles on hydrophobic solid substrates in aqueous media. I. Self-organization of cationic polymer particles on alkylated glass plates

Monodisperse, cationic polymer particles bearing quaternary ammonium groups effectively self-organized on hydrophobic solid substrates such as alkylated glass plates and polymer films to form particle monolayers. With an increase of the particle surface charge density, the surface coverage decreased and the morphology of particle monolayers changed from aggregated type to dispersed type. The dispersed type of particle monolayers having a relatively regular particle distance was formed at higher temperature. The self-organization behaviors on alkylated glass plates were different from those on unmodified glass plates through electrostatic interaction. The formation of particle monolayers on alkylated glass plates occurred only over a certain latex concentration range in contrast with that on unmodified glass plate. The adhesive strength of particle monolayers was enhanced by annealing at temperatures above the glass transition temperature (T _g) of the particles. Lens-shaped particle monolayers were fabricated by annealing the dispersed type of particle monolayers.