Two-stage collapse of unimolecular micelles with double thermoresponsive coronas

Phase transition behavior of unimolecular dendritic three-layer nanostructures with dual thermoresponsive coronas is studied. Successive reversible addition-fragmentation transfer (RAFT) polymerizations of N-isopropylacrylamide (NIPAM) and 2-(dimethylamino)ethyl methacrylate (DMA) were conducted using fractionated fourth-generation hyperbranched polyester (Bolton H40) based macroRAFT agent. At lower temperatures (<20 degrees C), dendritic macromolecules H40-poly(N-isopropylacrylamide)-poly(2-(dimethylamino)ethyl methacrylate) (H40-PNIPAM-PDMA) exist as unimolcular core-shell-corona nanostructures with hydrophobic H40 as the core, swollen PNIPAM as the inner shell, and swollen PDMA as the corona. PNIPAM and PDMA homopolymers undergo phase transitions at their lower critical solution temperatures (LCST), which are found to be 32 degrees C for PNIPAM and 40-50 degrees C for PDMA, respectively. Upon continuously heating through the LCSTs of PNIPAM and PDMA, such dendritic unimolecular micelles exhibit two-stage thermally induced collapse. This process is reversible with a two-stage reswelling upon cooling. Laser light scattering, micro-differential scanning calorimetry, and excimer fluorescence measurements are used to investigate the double phase transitions.     

Tailoring thermoresponsive nanostructured poly(N-isopropylacrylamide) hydrogels made with poly(acrylamide) nanoparticles

The thermoresponsive behavior and mechanical properties of nanostructured hydrogels, which consist of poly(acrylamide) nanoparticles embedded in a cross-linked poly(N-isopropylacrylamide) hydrogel matrix, are reported here. Nanostructured hydrogels exhibit a tuned volume phase transition temperature (T _VPT), which varies with nanoparticle content in the range from 32 up to 39–40 °C. Moreover, larger equilibrium water uptake, faster swelling and de-swelling rates, and larger equilibrium swelling at 25 °C were obtained with nanostructured hydrogels compared with those of conventional ones. Elastic and Young’s moduli were larger than those of conventional hydrogels at similar swelling ratios. The tuned T _VPT and the de-swelling rate were predicted with a modified Flory–Rehner equation coupled with a mixing rule that considers the contribution of both polymers. These behaviors are explained by a combination of hydrophilic/hydrophobic interactions and by the controlled inhomogeneities (nanoparticles) introduced by the method of synthesis.     

Synthesis of thermoresponsive polystyrene-based comb-type grafted poly(N-isopropylacrylamide)

Narrowly distributed polystyrene-g-p(N-isopropylacrylamide) (PSt-g-PNIPAM) was prepared by atom transfer radical polymerization (ATRP) of N-isopropylacrylamide using the brominated polystyrene as macroinitiator and CuCl combined with hexamethyltriethylenetetramine as catalyst. Fourier transform infrared (FT-IR) spectroscopy and nuclear magnetic resonance (NMR) spectroscopy confirmed the structure of PSt-g-PNIPAM. The gel permeation chromatography (GPC) showed that the graft copoly- mer had a single distribution peak with molecular weight, Mn (g/mol) of 19815 g/mol (using polystyrene as the standard). Differential scanning calorimetry (DSC) revealed that due to both effects of hydro- phobic isopropyl groups and hydrogen bonds in the amide group, the glass transition temperature (Tg) of PSt-g-PNIPAM enhanced 16.0 ℃ compared to the Tg of the polystyrene.     

Surface friction of thermoresponsive poly(N-isopropylacrylamide) gels in water

In this paper, we report the experimental results of surface friction between thermoresponsive poly(N-isopropylacrylamide) gels in water. The static friction force was found to depend on the waiting period prior to slider movement after contact between gel surfaces, which was a result of two relaxation mechanisms: the stress decay process due to macroscopic deformation under a normal load and the microscopic conformational change in the real contact area of polymer networks. The sliding velocity and the normal load dependence of the kinetic friction force were extensively measured. The results suggested that the following two mechanisms depended on the sliding velocity: the friction force generated by direct contact of the solid-like behavior and the viscous resistance of the liquid-like behavior. The strong temperature dependence of kinetic friction was observed, which was a result of a change in the balance between hydrophobic and hydrophilic interactions. The experimental results are discussed in terms of the multi-asperity contacts between the swollen gel/gel interfaces (solid friction, depending on the waiting period) and the viscous resistance and lubricating effect between the gel/water interfaces (fluid friction, depending on the sliding velocity).     

Structural properties of thermoresponsive poly(N-isopropylacrylamide)-poly(ethyleneglycol) microgels

We present investigations of the structural properties of thermoresponsive poly(N-isopropylacrylamide) (PNiPAM) microgels dispersed in an aqueous solvent. In this particular work poly(ethyleneglycol) (PEG) units flanked with acrylate groups are employed as cross-linkers, providing an architecture designed to resist protein fouling. Dynamic light scattering (DLS), static light scattering (SLS), and small angle neutron scattering (SANS) are employed to study the microgels as a function of temperature over the range 10 °C ≤ T ≤ 40 °C. DLS and SLS measurements are simultaneously performed and, respectively, allow determination of the particle hydrodynamic radius, R(h), and radius of gyration, R(g), at each temperature. The thermal variation of these magnitudes reveals the microgel deswelling at the PNiPAM lower critical solution temperature (LCST). However, the hydrodynamic radius displays a second transition to larger radii at temperatures T ≤ 20 °C. This feature is atypical in standard PNiPAM microgels and suggests a structural reconfiguration within the polymer network at those temperatures. To better understand this behavior we perform neutron scattering measurements at different temperatures. In striking contrast to the scattering profile of soft sphere microgels, the SANS profiles for T ≤ LCST of our PNiPAM-PEG suspensions indicate that the particles exhibit structural properties characteristic of star polymer configurations. The star polymer radius of gyration and correlation length gradually decrease with increasing temperature despite maintenance of the star polymer configuration. At temperatures above the LCST, the scattered SANS intensity is typical of soft sphere systems.     

Structural characterization of nanoparticles from thermoresponsive poly(N-isopropylacrylamide)-DNA conjugate

Poly(N-isopropylacrylamide) (PNIPAAm) grafted with single-stranded (ss) DNA conjugate (PNIPAAm-g-DNA) self-assembles above its lower critical solution temperature to form colloidal particles. When the ssDNA within the particle hybridizes with its complementary DNA, the particles aggregate above a certain threshold of salt concentration with drastically increased turbidity in solution. Detailed structural information of the particle was obtained mainly by small-angle X-ray scattering. The influence of copolymer composition on the morphology of particle and non-crosslinking aggregation was examined. The particle consists of hydrophobic PNIPAAm core surrounded by hydrophilic DNA strands. The increase in DNA fraction brought about a significant decrease in core size, whereas the shell thickness little changed and corresponded to the length of DNA. A structural model with a sticky potential was applied to the analysis of particle aggregate. This analysis provided that the particles aggregate while the coronal layers interpenetrate each other. The interaction between the particles was quantified in terms of the sticky potential and showed a trend to be influenced by the particle size rather than the graft density of DNA strands on the particle.     

Aggregation behavior of poly(N-isopropylacrylamide) microspheres

Electrophoretic mobility and aggregation in suspensions of three types of microspheres (Ms 1, Ms 2 and Ms 3) are studied at different pH, ionic strengths and temperatures of the medium. Here Ms 1 is a core particle composed of poly(N-isopropylacrylamide-co-styrene). Ms 2 is a core-shell microsphere consisting of Ms 1 as the particle core covered with a surface layer of poly(N-isopropylacrylamide) hydrogel. Ms 3 is also a core-shell microsphere composed of MS-1 covered with a surface layer of poly(N-isopropylacrylamide-co-acrylic acid) hydrogel. The charge density zN and the softness parameter 1/λ of the microspheres were obtained from the electrophoretic mobility data on the basis of an electrokinetic theory of soft particles. It is shown that when zN is large, suspensions of microspheres are always stable, showing no aggregation. When zN is small, the suspensions are stable for large 1/λ but show strong aggregation for small 1/λ.     

Coil-to-globule transition of poly(N-isopropylacrylamide) doped with chiral amino acidic comonomers

By combined light scattering and circular dichroism measurements (CD), we have investigated the coil-to-globule transition of the thermosensitive polymer poly(N-isopropylacrylamide) (pNIPAAm) copolymerized with a 1/10 fraction of valine- or leucine-derived groups randomly positioned along the chains. The comonomers provide the pNIPAAm chains with chirality, electric charge, and increased hydrophobicity. For valine-derived copolymers, the coil-globule transition is basically unmodified with respect to pNIPAAm, whereas doping with leucine-derived groups significantly lowers the transition temperature and makes the transition discontinuous. We find the CD signal of the chiral comonomers to cleanly depend on the local chain density. We interpret this behavior as an effect of the whole chain conformation on the conformations accessible to the chiral groups.     

Phase separation in semidilute aqueous poly(N-isopropylacrylamide) solutions

The phase separation mechanism in semidilute aqueous poly(N-isopropylacrylamide) (PNIPAM) solutions is investigated with small-angle neutron scattering (SANS). The nature of the phase transition is probed in static SANS measurements and with time-dependent SANS measurements after a temperature jump. The observed critical exponents of the phase transition describing the temperature dependence of the Ornstein-Zernike amplitude and correlation length are smaller than values from mean-field theory. Time-dependent SANS measurements show that the specific surface decreases with increasing time after a temperature jump above the phase transition. Thus, the formation of additional hydrogen bonds in the collapsed state is a kinetic effect: A certain fraction of water remains as bound water in the system. Moreover, H-D exchange reactions observed in PNIPAM have to be taken into account.     

Coil-to-globule-type transition of poly(N-isopropylacrylamide) adsorbed on colloidal silica particles

 The temperature dependence of the dimensions of poly(N-isopropylacrylamide) (PNIPAM) adsorbed on two different colloidal silica particles was studied with dynamic light scattering. The hydrodynamic diameter was measured when the temperature was varied stepwise from 10 to 60 °C. PNIPAM molecules free in solution undergo a conformational transition at the θ temperature. We have found that PNIPAM adsorbed onto silica particles also undergoes a transition below the θ temperature. When a small amount of polymer was adsorbed the coil-to-globule transition at the θ temperature did not occur. Potentiometric titrations showed that the surface charge of the silica particles was not affected by the polymer adsorption. Sodium dodecyl sulfate (SDS) (100–1200 mg/l) was added to improve the stability. The particles with a higher zeta potential required a smaller addition of SDS to prevent coagulation compared to the particles with a smaller surface potential. For low additions of SDS the transition curves of adsorbed PNIPAM were unaffected. For larger additions of SDS the collapse of PNIPAM was shifted to higher temperatures. When as much as 1200 mg/l SDS was added, two regions with weak transitions were observed before the collapse. It was also observed that the presence of SDS results in a smaller adsorption of PNIPAM onto the particles. The addition of SDS strongly increased the magnitude of the electrophoretic mobility of the polymer–particle unit. From the electrophoretic measurements an electrokinetic layer thickness was calculated and it was found to be smaller than the corresponding hydrodynamic layer thickness, as obtained by dynamic light scattering. Received: 14 December 1999/In revised form: 22 February 2000/Accepted: 6 March 2000     

Fluorescence studies on volume phase transition in poly(acrylamide) and poly(N-isopropylacrylamide) gels

The changes in microenvironments during the volume phase transition of poly(acrylamide) and poly(N-isopropylacrylamide) gels induced by pH change or the change in solvent composition were studied by using dansyl or pyrenyl fluorescent probes.     

Temperature-induced phase transition in hydrogels of interpenetrating networks poly(N-isopropylmethacrylamide)/poly(N-isopropylacrylamide)

The combination of ¹H NMR spectroscopy, DSC, dynamic mechanical spectroscopy, and optical microscopy was used to investigate temperature-induced volume phase transition in hydrogels of interpenetrating networks (IPNs) poly(N-isopropylmethacrylamide)/poly(N-isopropylacrylamide) (PNIPMAm/PNIPAm) with various PNIPMAm content. In these IPNs, both networks are thermosensitive; such systems were not examined so far. All methods showed phase transition starting at 307 K, which is the volume phase transition temperature of PNIPAm, the major network component. Only the sample with the lowest content of PNIPAm (~54 %) shows two-step collapse transition, other samples with higher PNIPAm content show a single transition in NMR and DSC which indicates enhanced mutual entanglement of both components. In all samples, the phase transition results in substantial increase of both components of the shear modulus. Although the properties of all samples change with temperature in similar way, differences in dependence on the PNIPMAm content and the shape of the sample can be seen.     

Potential application of poly(N-isopropylacrylamide) gel containing polymeric micelles to drug delivery systems

We have investigated rapidly thermo-responsive NIPA gel containing polymer surfactant PMDP (NIPA-PMDP gel) as a potential drug carrier using (+)-l-ascorbic acid as a model drug. In the NIPA-PMDP gel system micelles of polymer surfactant PMDP are trapped by the entanglement of polymer chains inside the gel networks. Therefore, in principle the gel system tightly stores targeted drug in the micelles and rapidly releases controlled amount of the drug by switching on-off of external stimuli such as temperature or infrared laser beam. In our investigation on release profile, the NIPA-PMDP gel system showed completely different releasing behavior from that of the conventional NIPA gel. The NIPA-PMDP gel released rapidly all loaded (+)-l-ascorbic acid above the phase transition temperature (ca. 34 degrees C), while slowly released the corresponding amount of the drug below the temperature. In contrast, the conventional NIPA gel released more slowly limited amount of the drug above the phase transition temperature while similarly did to the NIPA-PMDP gel below the temperature. The release profile of the NIPA-PMDP gel seems to be governed by only kinetics of volume phase transition of the gel network but not by the hydrophobic domains of the micelles probably because of too hydrophilic nature of (+)-l-ascorbic acid.     

Effect of urea on the conformational behavior of poly(N-isopropylacrylamide)

The effect of urea on the conformational behavior of poly(N-isopropylacrylamide) (PNIPAM) in dilute aqueous solution has been investigated using fluorescence spectroscopy, fluorescence quenching and fluorescence anisotropy measurements via pyrene (Py) probe and acenaphthylene (ACE) label studies. It was demonstrated that urea promotes the partitioning of the hydrophobic probe, Py, towards the bulk aqueous phase at temperatures above the lower critical solution temperature (LCST) of the polymer due to swelling of the compact coil conformation. However, the compact coil structure of the polymer at temperatures greater than its LCST is not completely destroyed, even for urea concentrations up to 3 M, at which the phase transition is hardly observed. As expected, urea has little effect on the conformational behavior of PNIPAM at temperatures below its LCST. Received: 9 February 2000/Accepted: 13 June 2000     

"On-demand" control of thermoresponsive properties of poly(N-isopropylacrylamide) with cucurbit[8]uril host-guest complexes

The chain end complexation of a functional PNIPAM by a cucurbit[8]uril-viologen complex causes a shift in its lower critical solution temperature (LCST) by over 5 °C. An instantaneous phase change of the thermally responsive polymer beyond its LCST can be induced by addition of the aqueous cucurbituril host-guest complex. Subsequent decomplexation upon addition of a competitive guest releases the PNIPAM terminus and triggers complete reversibility.     

Phase transition behavior of a poly(oxyethylene-b-butylene adipate) ionomer with shape memory properties

A series of biodegradable poly(oxyethylene-b-butylene adipate) ionomers (POBAi) were prepared by two-step in situ polymerization using adipic acid, 1,4-butanediol and mixed monomers of bis(poly(oxyethylene)) sulfonated dimethyl fumarate. The chemical composition of these POBAi was ascertained by ¹H NMR spectroscopy. The objective of this study was to investigate the shape memory effect of POBAi containing ionomer compared to non ionic POBA. It was observed that POBA5.0i showed a good shape memory effect than that of POBA 2.5 mol% or none of ionic group due to much physical cross-linking point by rich ionic group. Stress-induced phase transition was investigated during the shape deformation and recovery process using a wide-angle X-ray diffractometer (WAXD). The POBA crystal phase transition from β- to α-form was observed in all POBA samples by either thermal treatment or physical drawing. The α-form crystal did not recover to the initial β-form during the recovery process because the monoclinic α-form crystal is structurally more stable than the orthorhombic β-form crystal.