Thermoresponsive Behavior Of Poly Research Articles

A Spontaneous In Situ Thiol-Ene Crosslinking Hydrogel with Thermo-Responsive Mechanical Properties.

The thermo-responsive behavior of Poly(N-isopropylacrylamide) makes it an ideal candidate to easily embed cells and allows the polymer mixture to be injected. However, P(NiPAAm) hydrogels possess minor mechanical properties. To increase the mechanical properties, a covalent bond is introduced into the P(NIPAAm) network through a biocompatible thiol-ene click-reaction by mixing two polymer solutions. Co-polymers with variable thiol or acrylate groups to thermo-responsive co-monomer ratios, ranging from 1% to 10%, were synthesized. Precise control of the crosslink density allowed customization of the hydrogel's mechanical properties to match different tissue stiffness levels. Increasing the temperature of the hydrogel above its transition temperature of 31 °C induced the formation of additional physical interactions. These additional interactions both further increased the stiffness of the material and impacted its relaxation behavior. The developed optimized hydrogels reach stiffnesses more than ten times higher compared to the state of the art using similar polymers. Furthermore, when adding cells to the precursor polymer solutions, homogeneous thermo-responsive hydrogels with good cell viability were created upon mixing. In future work, the influence of the mechanical micro-environment on the cell's behavior can be studied in vitro in a continuous manner by changing the incubation temperature.

Effect of the macromolecular architecture on the thermoresponsive behavior of poly(N-isopropylacrylamide) in copolymers with poly(N,N-dimethylacrylamide) in aqueous solutions: Block vs random copolymers

Three samples from each type of poly(N-isopropylacrylamide)-block-poly(N,N-dimethylacrylamide) (PNIPAM-b-PDMA) block copolymers and poly(N-isopropylacrylamide-co-N,N-dimethylacrylamide) (P(NIPAM-co-DMA)) random copolymers with similar chemical composition were synthesized by RAFT polymerization. The effects of DMA on the thermoresponse of its two kinds of copolymers with NIPAM were comparatively investigated by turbidity analysis, variable-temperature 1H NMR and dynamic light scattering with PNIPAM homopolymer as a control. It was discovered that the phase transition temperature, the temperature response window and the hysteresis values of the PNIPAM-b-PDMA and P(NIPAM-co-DMA) counterpart were significantly different. In particular, the temperature response window, which represents the sensitivity of response, is smaller for random copolymers than that of block copolymers in aqueous solution, indicating that the temperature response of random copolymers is more sensitive than that of block copolymers. The possible reasons leading to different thermo-responsive phase transition characteristics between block copolymers and random copolymers are discussed, and the formation of different hydrophobic structures with the temperature increasing is ascribed.

Effect of Dexamethasone on Thermoresponsive Behavior of Poly(2-Oxazoline) Diblock Copolymers.

Thermoresponsive polymers play an important role in designing drug delivery systems for biomedical applications. In this contribution, the effect of encapsulated hydrophobic drug dexamethasone on thermoresponsive behavior of diblock copolymers was studied. A small series of diblock copoly(2-oxazoline)s was prepared by combining thermoresponsive 2-n-propyl-2-oxazoline (nPrOx) and hydrophilic 2-methyl-2-oxazoline (MeOx) in two ratios and two polymer chain lengths. The addition of dexamethasone affected the thermoresponsive behavior of one of the copolymers, nPrOx20-MeOx180, in the aqueous medium by shifting the cloud point temperature to lower values. In addition, the formation of microparticles containing dexamethasone was observed during the heating of the samples. The morphology and number of microparticles were affected by the structure and concentration of copolymer, the drug concentration, and the temperature. The crystalline nature of formed microparticles was confirmed by polarized light microscopy, confocal Raman microscopy, and wide-angle X-ray scattering. The results demonstrate the importance of studying drug/polymer interactions for the future development of thermoresponsive drug carriers.

Synthesis, self-assembly and thermoresponsive behavior of Poly(lactide-co-glycolide)-b-Poly(ethylene glycol)-b-Poly(lactide-co-glycolide) copolymer in aqueous solution

Thermoresponsive block copolymer (BCP) micelles have attracted increasing attentions due to their potential in biological applications. Many studies have investigated facile strategies to tune the cloud point (Tcp) of thermoresponsive polymer, such as changing hydrophobic/hydrophilic block ratio, micelle concentration and addition of salt. Considering the dependence of properties and functions of BCPs on thermal-induced structural transition, it is necessary to understand the universal mechanism of structural changes during temperature-induced phase transition; whereas it is challenging to achieve precise structure and properties of BCP micelles with Tcp changes. Herein, we choose thermoresponsive poly (L-lactide-co-glycolide)-poly (ethylene glycol)-poly (L-lactide-co-glycolide) (PLGA-PEG-PLGA) block copolymer as a model system and utilize synchrotron-radiation small-angle X-ray scattering (SAXS) technique to systematically investigate the assembled structure of PLGA-PEG-PLGA and the mechanism for thermally induced structural transition of BCP micelle. With the increase of PLGA hydrophobic block, the Tcp significantly decreases, ascribed to the enhancement of the core size, aggregation number, as well as the packing density of PLGA in micelle core. Moreover, the increase of micelle concentration and the addition of salt also decreases the Tcp. Furthermore, temperature-induced drug release is also investigated, the release rate is much faster at above Tcp.

Effect of Host-Guest Complexation on the Thermoresponsive Behavior of Poly(oligo ethylene glycol acrylate)s Functionalized with Dialkoxynapththalene Guest Side Chains.

The combination of thermoresponsive polymers with supramolecular host-guest interactions enables accurate tuning of the phase transition temperature, while also providing additional response mechanisms based on host-guest complexation. Most studies focused on a single thermoresponsive polymer to demonstrate the effect of host-guest complexation on the responsive behavior. In this work, the effect of the polymer structure on the host-guest complexation and thermoresponsive behavior is reported. Therefore, different poly(oligoethylene glycol acrylate)s, namely, poly(2-hydroxyethylacrylate) (PHEA), poly(methoxy diethylene glycol acrylate), poly(methoxy triethylene glycol acrylate), and poly(methoxy tetraethylene glycol acrylate), are synthesized functionalized with 1,5-dialkoxynaphthalene guest molecules in the side chain. Their complexation with the cyclobis(paraquat-p-phenylene) tetrachloride host is studied to understand the effect of polymer structure on the supramolecular association and the polymer phase transition, revealing that the oligoethylene glycol side chains lead to weaker host-guest complexation and also have a smaller increase in the cloud point temperature compared to PHEA.

Specific Anion Effects on Charged-Neutral Random Copolymers: Interplay between Different Anion-Polymer Interactions.

The study of ion specificities of charged-neutral random copolymers is of great importance for understanding specific ion effects on natural macromolecules. In the present work, we have investigated the specific anion effects on the thermoresponsive behavior of poly([2-(methacryloyloxy)ethyl trimethylammonium chloride]-co-N-isopropylacrylamide) [P(METAC-co-NIPAM)] random copolymers. Our study demonstrates that the anion specificities of the P(METAC-co-NIPAM) copolymers are dependent on their chemical compositions. The specific anion effects on the copolymers with high mole fractions of poly(N-isopropylacrylamide) (PNIPAM) are similar to those on the PNIPAM homopolymer. As the mole fraction of PNIPAM decreases to a certain value, a V-shaped anion series can be observed in terms of the anion-specific cloud point temperature of the copolymer, as induced by the interplay between different anion-polymer interactions. Our study also suggests that both the direct and the indirect anion-polymer interactions contribute to the anion specificities of the copolymers. This work would improve our understanding of the relationship between the ion specificities and the ion-macromolecule interactions for naturally occurring macromolecules.

Thermoresponsive behavior of poly(DEGMA)-based copolymers. NMR and dynamic light scattering study of aqueous solutions

We studied temperature behavior of diblock copolymers of poly[2-(2-methoxyethoxy)ethyl methacrylate] (PDEGMA) with poly(N-(2-hydroxypropyl)methacrylamide and a statistical copolymer PDEGMA-co-N-propargylmethacrylamide in aqueous solutions. NMR spectra and dynamic light scattering showed that the copolymers exhibit a reversible phase transition which depends on the composition of the block copolymer and slightly on polymer concentration. In contrast to statistical copolymer, different behavior of proton groups in polymer backbone (and in the nearest side chain OCH2 group) on the one hand and in other side chain groups of the PDEGMA on the other hand was observed for diblock copolymers. Additionally, two types of water molecules were detected in solutions of the diblock copolymers above the phase transition –“free” with long and “bound” with short spin-spin relaxation times T2. NOESY spectra indicate that a core- to- shell morphology is unlikely in these block copolymers. This report emphasizes the importance of understanding of the self-association of copolymers in solution on the molecular level and provides important information for the design of “smart” thermoresponsive polymer-based drug delivery systems.

Thermoresponsive behavior of poly[trialkyl-(4-vinylbenzyl)ammonium] based polyelectrolytes in aqueous salt solutions

A systematic method to induce thermoresponsive behavior for polycations with salts from the reversed Hofmeister series is introduced.

Visualization of the Stimuli-responsive Surface Behavior of Functionalized Wood Material by Chemical Force Microscopy

The hierarchical and porous wood structure provides a stable scaffold to design functionalized lignocellulosic materials with extended properties by chemical modification techniques. However, proper nanoscale characterization methods for these novel materials are needed to confirm the presence of the added functionality and to locate the introduced functional groups with high spatial resolution. Chemical force microscopy is a suitable characterization method to distinguish chemical surface characteristics by scanning the samples surface with a functionalized tip. We report the application of this nanotechnology method on both, unmodified and functionalized wood samples to confirm the thermo-responsive behavior of poly(N-isopropylacrylamide) (PNIPAM) modified spruce wood. By performing force measurements on ultra-microtomed surfaces, adhesion force differences on the analysed structure are monitored and reveal the location and functionality of introduced functional groups. The modified samples are scanned below and above their lower critical solution temperature with a hydrophobic tip in aqueous media to observe adhesion changes. Additionally, confocal Raman microscopy support the chemical force microscopy measurements by revealing the success of the modification and the distribution of PNIPAM across the sample cross-sections. The results show that PNIPAM is mainly located in wood cell wall areas close to the lumen in early- and transitionwood.

Thermoresponsive Behavior of Poly(acrylic acid-co-acrylonitrile) with a UCST

Poly(acrylic acid) is water soluble but can be converted into a thermoresponsive polymer possessing an upper critical solution temperature (UCST) by copolymerizing with acrylonitrile. The copolymer...

Thermoresponsive behavior of poly(3-methylene-2-pyrrolidone) derivatives

The thermoresponsive behavior of a 3-methylene-2-pyrrolidone (3M2P), its copolymer with N-butyl methacrylamide (N-BuMAAm), a hydrophobic monomer, and 3M2P’s N-alkylated derivatives was investigated. P3M2P did not display a cloud point within the measurable temperature range, even in the presence of high concentrations of strongly kosmotropic Hofmeister salts. The copolymer of 3M2P with N-BuMAAm, and the homopolymers of P3M2P N-alkylated with ethyl and propyl groups demonstrated thermoresponsive behavior, whilst N-methylation did not introduce a sufficient hydrophobic character, to impart thermoresponsivity to the P3M2P system. The reversibility of the thermal phase transition, and the influence of experimental conditions, i.e. (co)polymer concentration, heating rate and Hofmeister salts, on the thermoresponsivity of the (co)polymers was also investigated, to assess the potential of the P3M2P derivatives as thermoresponsive polymers.

Thermoresponsive behavior of poly(N-isopropylacrylamide)s with dodecyl and carboxyl terminal groups in aqueous solution: pH-dependent cloud point temperature

It was recently reported that poly(N-isopropyl acrylamide) (PNIPAm) polymers synthesized by RAFT polymerization using S-1-dodecyl-S′-(α,α′-dimethyl-α′′-acetic acid)trithiocarbonate as a chain transfer agent form micelles in aqueous solutions with the core of hydrophobic terminal dodecyl groups and the corona of PNIPAm chains with carboxylic groups at the periphery, the ionization of which prevents the micelles from phase separation above the lower critical solution temperature of PNIPAm in water (Langmuir 30:7986–7992). In this paper, we study the pH- and ionic strength-dependence of the aggregation behavior of two HOOC-PNIPAm-C12 polymers, differing in the degree of polymerization, in aqueous solutions. We show that the cloud point temperature (CPT) of HOOC-PNIPAm-C12 can be shifted up to several tens of K by changing pH of the solution. The aggregation of the PNIPAms above the CPT can be efficiently accelerated by screening electrostatic repulsion between PNIPAm micelles by changing ionic strength of the solution.

Tunable, concentration‐independent, sharp, hysteresis‐free UCST phase transition from poly(N‐acryloyl glycinamide‐acrylonitrile) system

ABSTRACTPoly(N‐acryloylglycinamide‐co‐acrylonitrile) (poly(NAGA‐AN)) copolymers were synthesized using reversible‐addition‐fragmentation transfer polymerization. In contrast to poly(NAGA) the thermoresponsive behavior of poly(NAGA‐AN) shows a narrow cooling/heating hysteresis in water with a tunable cloud point, depending on the acrylonitrile amount in polymer. Furthermore, we showed that there is no significant effect of the solution concentration on the cloud point and stable phase transition behavior in electrolyte solutions, which is presumable controlled by forming stable micellar like structures as a result of the block/graft‐copolymer structure. This is in contrast to poly(NAGA) which shows a strong concentration dependent cloud point in aqueous solution with a broad cooling/heating hysteresis. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 274–279

Hydrogen bonding induced UCST phase transition of poly(ionic liquid)-based nanogels

In this study, novel thermo-responsive nanogels were facilely fabricated via ternary cross-linking copolymerization of ionic liquid (IL)-based monomers and cross-linker in methanol. To achieve the thermo-responsive performance of nanogels, monomers with hydroxyl or carboxyl group which can contribute to hydrogen (H)-bonding interaction is the key point of this strategy. The thermo-responsive behavior of poly(ionic liquid)s (PIL)-based nanogels were testified using dynamic light scattering (DLS), turbidimetric method, and differential scanning calorimetry (DSC) measurements. The results demonstrated that these novel nanogels presented reversible upper critical solution temperature (UCST) phase transition with the temperature changing. And their UCSTs could be tuned via the feed ratio of IL-based monomers and cross-linker. The nanogels were also measured using scanning electronic microscopy (SEM), thermo-gravimetric analyses (TGA) and Fourier transform infrared spectroscopy (FT-IR) techniques. In addition, a possible mechanism of thermo-responsive performance was suggested based on the above results. As a result, our findings provide an efficient and applicable platform for the fabrication of thermo-responsive polymeric nanomaterials.

Surface Interaction Forces of Cellulose Nanocrystals Grafted with Thermoresponsive Polymer Brushes

The colloidal stability and thermoresponsive behavior of poly(N-isopropylacrylamide) brushes grafted from cellulose nanocrystals (CNCs) of varying graft densities and molecular weights was investigated. Indication of the grafted polymer brushes was obtained after AFM imaging of CNCs adsorbed on silica. Also, aggregation of the nanoparticles carrying grafts of high degree of polymerization was observed. The responsiveness of grafted CNCs in aqueous dispersions and as an ultrathin film was evaluated by using light scattering, viscosimetry, and colloidal probe microscopy (CPM). Light transmittance measurements showed temperature-dependent aggregation originating from the different graft densities and molecular weights. The lower critical solution temperature (LCST) of grafted poly(NiPAAm) brushes was found to decrease with the ionic strength, as is the case for free poly(NiPAAm) in aqueous solution. Thermal responsive behavior of grafted CNCs in aqueous dispersions was observed by a sharp increase in dispersion viscosity as the temperature approached the LCST. CPM in liquid media for asymmetric systems consisting of ultrathin films of CNCs and a colloidal silica probe showed the distinctive effects of the grafted polymer brushes on interaction and adhesive forces. The origin of such forces was found to be mainly electrostatic and steric in the case of bare and grafted CNCs, respectively. A decrease in the onset of attractive and adhesion forces of grafted CNCs films were observed with the ionic strength of the aqueous solution. The decreased mobility of polymer brushes upon partial collapse and decreased availability of hydrogen bonding sites with higher electrolyte concentration were hypothesized as the main reasons for the less prominent polymer bridging between interacting surfaces.

Unperturbed Volume Transition of Thermosensitive Poly-(N-isopropylacrylamide) Microgel Particles Embedded in a Hydrogel Matrix

The thermoresponsive behavior of poly-(N-isopropylacrylamide) (PNiPAM) microgels embedded in a covalently cross-linked polyacrylamide hydrogel matrix was investigated using ultraviolet-visible (UV-vis) spectroscopy, small-angle neutron scattering (SANS), and confocal laser scanning microscopy. The hydrogel synthesis was performed at two different temperatures, below and above the volume phase transition temperature of PNiPAM, resulting in highly swollen or fully collapsed PNiPAM microgel particles during the incorporation step. UV-vis spectroscopy experiments verify that the incorporation of thermosensitive microgels leads to temperature-sensitive optical properties of the composite materials. SANS measurements at different temperatures show that the thermosensitive swelling behavior of the PNiPAM microgels is fully retained in the composite material. Volume and structure criteria of the embedded microgel particles are compared to those of the free microgels in acrylamide solution. To visualize the temperature responsive behavior of larger PNiPAM particles, confocal fluorescence microscopy images of PNiPAM beads, of 40-microm size, were taken at two different temperatures. The micrographs also demonstrate the retained temperature sensitivity of the embedded microgels.

Microstructure and thermo-responsive behavior of poly( N-isopropylacrylamide) brushes grafted in nanopores of track-etched membranes

We report on the surface-initiated polymerization by aqueous free radical polymerization (FRP) and atom transfer radical polymerization (ATRP) of N-isopropylacrylamide (NIPAM) from nanopore walls of poly(ethyleneterephthalate) (PET) membranes. By combining atomic force microscopy and transmission electron microscopy coupled with an electron energy loss detector, we show that grafting polymers from the walls is an efficient process leading to functional and switchable membranes. The thickness of PNIPAM brushes grafted within small and large nanopores could be measured by a template-based method, indicating that confinement impacts strongly the polymerization process. Finally, the temperature-modulated permeability properties of these responsive polymeric systems are evaluated. Depending on the membrane pore size, two types of permeation control mechanisms are observed. For large pore membranes ( Φ ∼ 330 nm), expanded PNIPAM chains conformations (at T < LCST) result in reduced effective pore size and therefore lower permeabilities relative to collapsed macromolecules chain conformations. In contrast, for narrow pore membranes ( Φ ∼ 80 nm), the PNIPAM layer grafted at the surface of the membrane becomes the rate-controlling factor; the expanded PNIPAM brush represents greater degrees of hydration in the surface layer and therefore gives rise to higher permeabilities than the collapsed conformation. In this situation, the overall permeability is thus comparable to that of a hydrogel membrane.

Thermoresponsive Behavior of Poly(N-Isopropylacrylamide) Hydrogels Containing Gold Nanostructures

We report the changes in the structure and thermoresponsive behavior of poly(N-isopropylacrylamide) (PNIPAm) hydrogels when gold nanostructures are synthesized in situ within the hydrogel matrix. Cross-linked PNIPAm hydrogels were synthesized using NIPAm and 0.00-3.50% (w/w versus NIPAm) of N,N'-methylenebisacrylamide (MBAm) and/or N,N'-cystaminebisacrylamide (CBAm) as cross-linking agents. The hydrogels were soaked in potassium tetrachloroaurate to introduce gold ions. The hydrogels containing Au3+ were then immersed in a sodium borohydride solution to reduce the gold ions. Infrared spectroscopy, UV-visible spectroscopy, and equilibrium swelling were used to examine the structural/physical differences between gels of different compositions; UV-visible spectroscopy and mass measurements were used to observe the kinetics and thermodynamics of the hydrogel volume phase transition. These studies revealed several differences in the physical characteristics and thermoresponsive behavior of hydrogels based on cross-linker identity and the presence or absence of gold nanostructures. Hydrogels with gold nanostructures and high CBAm and low MBAm content have equilibrium swelling masses 3-20 times their native analogues. In comparison, gold-containing hydrogels with high MBAm and low CBAm content have swelling masses that are equal to their native analogues. Additionally, the gold-containing PNIPAm hydrogels cross-linked with only CBAm have a deswelling temperature of approximately 40 degrees C, approximately 8 degrees C above the samples cross-linked with only MBAm. Varying the CBAm content and introducing gold enables tuning of the deswelling temperature.