Special Issue "Thermoresponsive Polymers"

A special issue of Polymers (ISSN 2073-4360). This special issue belongs to the section "Polymer Processing and Performance".

Deadline for manuscript submissions: closed (20 November 2020).

Special Issue Editors

Prof. Andrzej Dworak
Website
Guest Editor
Polish Academy of Sciences, Centre of Polymer and Carbon Materials, Poland
Interests: living polymerizations; precise synthesis of polymers; amphiphilic polymers; self-organization of polymers in solution; stimuli-responsive polymers; polymers for biomedical applications
Prof. Barbara Trzebicka

Guest Editor
Polish Academy of Sciences, Centre of Polymer and Carbon Materials, Poland
Interests: physicochemical studies of stimuli sensitive polymers; amphiphilic polymers and their self-organization; polymers for biomedical applications; polymeric bioconjugates and carbon materials

Special Issue Information

Dear Colleagues,

Any sensible study of properties of thermoresponsive polymer ask for well-defined materials. In the last two decades or so, the development of controlled polymerizations (radical or living) and their widespread acceptance in polymer research institutions have made well defined polymers easily (or easier, in any case) available.

Stimuli-responsive polymers and thermoresponsive polymers are not merely the subject of fundamental research. Even though a lot of problems remain to be explained, the studies of the fundamentals remain very up-to date. Research groups investigate, vigorously and with great success, any possible or envisaged applications of such polymers, in biomedical fields as carriers of medically active species, injectable gels, sensors, actuators, and many more. This indicates that the area is living and actual. It also indicates that the field is enjoying a growing interest from researchers.

Of course, the fate of each Special Issue depends upon how well it will be received by the scientific community and how many people will respond by submitting reviews, research reports, short communications, and other communications.

The list of keywords below is not exhaustive, but it may help in preparing your submission. Please do not hesitate to go beyond it or to submit a paper whose topic is not explicitly mentioned in this list.

Best regards

Prof. Andrzej Dworak
Prof. Barbara Trzebicka
Guest Editors

Manuscript Submission Information

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Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Polymers is an international peer-reviewed open access semimonthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2200 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • Thermoresponsive polymers
  • Transition mechanism
  • Thermodynamics of transition
  • Controlled synthesis
  • Molecular design
  • Structure–property relationship
  • LCST
  • UCST
  • Water-soluble polymers
  • Mesoglobules
  • Self-assembly
  • Thermoresponsive surfaces
  • Smart materials
  • Polymer nanoparticles
  • Thermoresponsive nanogels
  • Microcarrier
  • Drug delivery
  • Molecular brushes
  • Self-healing materials
  • Sensors
  • Stimuli responding hydrogels
  • Microfluidics and actuators

Published Papers (21 papers)

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Open AccessArticle
Thermal Behaviour of Common Thermoresponsive Polymers in Phosphate Buffer and in Its Salt Solutions
Polymers 2021, 13(1), 90; https://doi.org/10.3390/polym13010090 - 28 Dec 2020
Abstract
Thermoresponsive polymers are a promising material for drug nanocarrier preparation, which makes the study of their aggregation in physiological conditions very important. In this paper, the thermal behaviour of the thermoresponsive polymers poly(N-isopropylacrylamide), poly(2-isopropyl-2-oxazoline-co-2-n-propyl-2-oxazoline) and poly[(2-hydroxyethyl methacrylate)-co-oligo(ethylene [...] Read more.
Thermoresponsive polymers are a promising material for drug nanocarrier preparation, which makes the study of their aggregation in physiological conditions very important. In this paper, the thermal behaviour of the thermoresponsive polymers poly(N-isopropylacrylamide), poly(2-isopropyl-2-oxazoline-co-2-n-propyl-2-oxazoline) and poly[(2-hydroxyethyl methacrylate)-co-oligo(ethylene glycol) methyl ether methacrylate] were studied in phosphate buffer (PBS) and solutions of its salts in concentration as in PBS. The thermal response of the polymers was measured using UV-Vis and dynamic light scattering (DLS). The salts shifted the cloud point temperature (TCP) of the (co)polymers to higher values compared to the TCP of aqueous polymer solutions. In PBS and NaCl solutions, all polymers exhibited an unexpected and previously unreported transmittance profile. During heating, an additional aggregation of polymers appeared above the TCP accompanied by the formation of a precipitate. In monosodium phosphate solutions and pure water, the studied polymers showed lower critical solution temperature (LCST-type) behaviour. DLS measurements showed that a salt influenced the size of the resulting polymer particles. The sizes and stability of particles depended on the heating rate. In PBS and NaCl solutions, the size of particles in the dispersion decreased above 60 °C, and the precipitate appeared on the bottom of the cuvette. The additional aggregation of polymer and its falling out of solution may hinder the removal of carriers from the body and has to be taken into account when preparing nanocarriers. Full article
(This article belongs to the Special Issue Thermoresponsive Polymers)
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Open AccessArticle
Amphiphilic Molecular Brushes with Regular Polydimethylsiloxane Backbone and Poly-2-isopropyl-2-oxazoline Side Chains. 2. Self-Organization in Aqueous Solutions on Heating
Polymers 2021, 13(1), 31; https://doi.org/10.3390/polym13010031 - 23 Dec 2020
Abstract
The behavior of amphiphilic molecular brushes in aqueous solutions on heating was studied by light scattering and turbidimetry. The main chain of the graft copolymers was polydimethylsiloxane, and the side chains were thermosensitive poly-2-isopropyl-2-oxazoline. The studied samples differed in the length of the [...] Read more.
The behavior of amphiphilic molecular brushes in aqueous solutions on heating was studied by light scattering and turbidimetry. The main chain of the graft copolymers was polydimethylsiloxane, and the side chains were thermosensitive poly-2-isopropyl-2-oxazoline. The studied samples differed in the length of the grafted chains (polymerization degrees were 14 and 30) and, accordingly, in the molar fraction of the hydrophobic backbone. The grafting density of both samples was 0.6. At low temperatures, macromolecules and aggregates, which formed due to the interaction of main chains, were observed in solutions. At moderate temperatures, heating solutions of the sample with short side chains led to aggregation due to dehydration of poly-2-isopropyl-2-oxazoline and the formation of intermolecular hydrogen bonds. In the case of the brush with long grafted chains, dehydration caused the formation of intramolecular hydrogen bonds and the compaction of molecules and aggregates. The lower critical solution temperature for solutions of the sample with long side chains was higher than LCST for the sample with short side chains. It was shown that the molar fraction of the hydrophobic component and the intramolecular density are the important factors determining the LCST behavior of amphiphilic molecular brushes in aqueous solutions. Full article
(This article belongs to the Special Issue Thermoresponsive Polymers)
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Open AccessArticle
Thermoresponsive Shape Memory Fibers for Compression Garments
Polymers 2020, 12(12), 2989; https://doi.org/10.3390/polym12122989 - 15 Dec 2020
Abstract
Their highly deformable properties make shape memory polymers (SMP) a promising component for the development of new compression garments. The shape memory effect (SME) can be observed when two polymers are combined. In here, polycaprolactone (PCL) and thermoplastic polyurethane (TPU) were melt spun [...] Read more.
Their highly deformable properties make shape memory polymers (SMP) a promising component for the development of new compression garments. The shape memory effect (SME) can be observed when two polymers are combined. In here, polycaprolactone (PCL) and thermoplastic polyurethane (TPU) were melt spun in different arrangement types (blend, core-sheath, and island-in-sea), whereas the best SME was observed for the blend type. In order to trigger the SME, this yarn was stimulated at a temperature of 50 °C. It showed a strain fixation of 62%, a strain recovery of 99%, and a recovery stress of 2.7 MPa. Full article
(This article belongs to the Special Issue Thermoresponsive Polymers)
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Open AccessArticle
Low-Temperature-Meltable Elastomers Based on Linear Polydimethylsiloxane Chains Alpha, Omega-Terminated with Mesogenic Groups as Physical Crosslinker: A Passive Smart Material with Potential as Viscoelastic Coupling. Part II—Viscoelastic and Rheological Properties
Polymers 2020, 12(12), 2840; https://doi.org/10.3390/polym12122840 - 29 Nov 2020
Cited by 1
Abstract
Rheological and viscoelastic properties of physically crosslinked low-temperature elastomers were studied. The supramolecularly assembling copolymers consist of linear polydimethylsiloxane (PDMS) elastic chains terminated on both ends with mesogenic building blocks (LC) of azobenzene type. They are generally and also structurally highly different from [...] Read more.
Rheological and viscoelastic properties of physically crosslinked low-temperature elastomers were studied. The supramolecularly assembling copolymers consist of linear polydimethylsiloxane (PDMS) elastic chains terminated on both ends with mesogenic building blocks (LC) of azobenzene type. They are generally and also structurally highly different from the well-studied LC polymer networks or LC elastomers: The LC units make up only a small volume fraction in our materials and act as fairly efficient physical crosslinkers with thermotropic properties. The aggregation (nano-phase separation) of the relatively rare, small and spatially separated terminal LC units generates temperature-switched viscoelasticity in the molten copolymers. Their rheological behavior was found to be controlled by an interplay of nano-phase separation of the LC units (growth and splitting of their aggregates) and of the thermotropic transitions in these aggregates (which change their stiffness). As a consequence, multiple gel points (up to three) are observed in temperature scans of the copolymers. The physical crosslinks also can be reversibly disconnected by large mechanical strain in the ‘warm’ rubbery state, as well as in melt (thixotropy). The kinetics of crosslink formation was found to be fast if induced by temperature and extremely fast in case of internal self-healing after strain damage. Thixotropic loop tests hence display only very small hysteresis in the LC-melt-state, although the melts show very distinct shear thinning. Our study evaluates structure-property relationships in three homologous systems with elastic PDMS segments of different length (8.6, 16.3 and 64.4 repeat units). The studied copolymers might be of interest as passive smart materials, especially as temperature-controlled elastic/viscoelastic mechanical coupling. Full article
(This article belongs to the Special Issue Thermoresponsive Polymers)
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Open AccessArticle
Synthesis, Structure, Hydrodynamics and Thermoresponsiveness of Graft Copolymer with Aromatic Polyester Backbone at Poly(2-isopropyl-2-oxazoline) Side Chains
Polymers 2020, 12(11), 2643; https://doi.org/10.3390/polym12112643 - 10 Nov 2020
Cited by 1
Abstract
New thermoresponsive graft copolymers with an aromatic polyester backbone and poly(2-isopropyl-2-oxazoline) (PiPrOx) side chains are synthesized and characterized by NMR and GPC. The grafting density of side chains is 0.49. The molar masses of the graft-copolymer, its backbone, side chains, and the modeling [...] Read more.
New thermoresponsive graft copolymers with an aromatic polyester backbone and poly(2-isopropyl-2-oxazoline) (PiPrOx) side chains are synthesized and characterized by NMR and GPC. The grafting density of side chains is 0.49. The molar masses of the graft-copolymer, its backbone, side chains, and the modeling poly-2-isopropyl-2-oxaziline are 74,000, 19,000, 4300, and 16,600 g·mol−1, respectively. Their conformational properties in nitropropane as well as thermoresponsiveness in aqueous solutions are studied and compared with that of free side chains, i.e., linear PiPrOx with a hydrophobic terminal group. In nitropropane, the graft-copolymer adopts conformation of a 13-arm star with a core of a collapsed main chain and a PiPrOx corona. Similarly, a linear PiPrOx chain protects its bulky terminal group by wrapping around it in a selective solvent. In aqueous solutions at low temperatures, graft copolymers form aggregates due to interaction of hydrophobic backbones, which contrasts to molecular solutions of the model linear PiPrOx. The lower critical solution temperature (LCST) for the graft copolymer is around 20 °C. The phase separation temperatures of the copolymer solution were lower than that of the linear chain counterpart, decreasing with concentration for both polymers. Full article
(This article belongs to the Special Issue Thermoresponsive Polymers)
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Open AccessArticle
Formation and Stability of Smooth Thin Films with Soft Microgels Made of Poly(N-Isopropylacrylamide) and Poly(Acrylic Acid)
Polymers 2020, 12(11), 2638; https://doi.org/10.3390/polym12112638 - 10 Nov 2020
Abstract
In this work, soft microgels of Poly(N-Isopropylacrylamide) (PNIPAm) at two different sizes and of interpenetrated polymer network (IPN) composed of PNIPAm and Poly(Acrylic Acid) (PAAc) were synthesized. Then, solutions of these different types of microgels have been spin-coated on glass substrates [...] Read more.
In this work, soft microgels of Poly(N-Isopropylacrylamide) (PNIPAm) at two different sizes and of interpenetrated polymer network (IPN) composed of PNIPAm and Poly(Acrylic Acid) (PAAc) were synthesized. Then, solutions of these different types of microgels have been spin-coated on glass substrates with different degrees of hydrophobicity. PNIPAm particles with a larger diameter form either patches or a continuous layer, where individual particles are still distinct, depending on the dispersion concentration and spin speed. On the other, PNIPAm particles with a smaller diameter and IPN particles form a continuous and smooth film, with a thickness depending on the dispersion concentration and spin-speed. The difference in morphology observed can be explained if one considers that the microgels may behave as colloidal particles or macromolecules, depending on their size and composition. Additionally, the microgel size and composition can also affect the stability of the depositions when rinsed in water. In particular, we find that the smooth and continuous films show a stimuli-dependent stability on parameters such as temperature and pH, while large particle layers are stable under any condition except on hydrophilic glass by washing at 50 °C. Full article
(This article belongs to the Special Issue Thermoresponsive Polymers)
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Open AccessArticle
Low-Temperature Meltable Elastomers Based on Linear Polydimethylsiloxane Chains Alpha, Omega-Terminated with Mesogenic Groups as Physical Crosslinkers: A Passive Smart Material with Potential as Viscoelastic Coupling. Part I: Synthesis and Phase Behavior
Polymers 2020, 12(11), 2476; https://doi.org/10.3390/polym12112476 - 25 Oct 2020
Cited by 2
Abstract
Physically crosslinked low-temperature elastomers were prepared based on linear polydimethylsiloxane (PDMS) elastic chains terminated on both ends with mesogenic building blocks (LC) of azobenzene type. They are generally (and also structurally) highly different from the well-studied LC polymer networks (light-sensitive actuators). The LC [...] Read more.
Physically crosslinked low-temperature elastomers were prepared based on linear polydimethylsiloxane (PDMS) elastic chains terminated on both ends with mesogenic building blocks (LC) of azobenzene type. They are generally (and also structurally) highly different from the well-studied LC polymer networks (light-sensitive actuators). The LC units also make up only a small volume fraction in our materials and they do not generate elastic energy upon irradiation, but they act as physical crosslinkers with thermotropic properties. Our elastomers lack permanent chemical crosslinks—their structure is fully linear. The aggregation of the relatively rare, small, and spatially separated terminal LC units nevertheless proved to be a considerably strong crosslinking mechanism. The most attractive product displays a rubber plateau extending over 100 °C, melts near 8 °C, and is soluble in organic solvents. The self-assembly (via LC aggregation) of the copolymer molecules leads to a distinctly lamellar structure indicated by X-ray diffraction (XRD). This structure persists also in melt (polarized light microscopy, XRD), where 1–2 thermotropic transitions occur. The interesting effects of the properties of this lamellar structure on viscoelastic and rheological properties in the rubbery and in the melt state are discussed in a follow-up paper (“Part II”). The copolymers might be of interest as passive smart materials, especially as temperature-controlled elastic/viscoelastic mechanical coupling. Our study focuses on the comparison of physical properties and structure–property relationships in three systems with elastic PDMS segments of different length (8.6, 16.3, and 64.4 repeat units). Full article
(This article belongs to the Special Issue Thermoresponsive Polymers)
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Open AccessArticle
Thermoresponsive Polymers of Poly(2-(N-alkylacrylamide)ethyl acetate)s
Polymers 2020, 12(11), 2464; https://doi.org/10.3390/polym12112464 - 24 Oct 2020
Abstract
Thermoresponsive poly(2-(N-alkylacrylamide) ethyl acetate)s with different N-alkyl groups, including poly(2-(N-methylacrylamide) ethyl acetate) (PNMAAEA), poly(2-(N-ethylacrylamide) ethyl acetate) (PNEAAEA), and poly(2-(N-propylacrylamide) ethyl acetate) (PNPAAEA), as well as poly(N-acetoxylethylacrylamide) (PNAEAA), were synthesized by solution RAFT [...] Read more.
Thermoresponsive poly(2-(N-alkylacrylamide) ethyl acetate)s with different N-alkyl groups, including poly(2-(N-methylacrylamide) ethyl acetate) (PNMAAEA), poly(2-(N-ethylacrylamide) ethyl acetate) (PNEAAEA), and poly(2-(N-propylacrylamide) ethyl acetate) (PNPAAEA), as well as poly(N-acetoxylethylacrylamide) (PNAEAA), were synthesized by solution RAFT polymerization. Unexpectedly, it was found that there are induction periods in the RAFT polymerization of these monomers, and the induction time correlates with the length of the N-alkyl groups in the monomers and follows the order of NAEAA < NMAAEA < NEAAEA < NPAAEA. The solubility of poly(2-(N-alkylacrylamide) ethyl acetate)s in water is also firmly dependent on the length of the N-alkyl groups. PNPAAEA including the largest N-propyl group is insoluble in water, whereas PNMAAEA and PNEAAEA are thermoresponsive in water and undergo the reversible soluble-to-insoluble transition at a critical solution temperature. The cloud point temperature (Tcp) of the thermoresponsive polymers is in the order of PNEAAEA < PNAEAA < PNMAAEA. The parameters affecting the Tcp of thermoresponsive polymers, e.g., degree of polymerization (DP), polymer concentration, salt, urea, and phenol, are investigated. Thermoresponsive PNMAAEA-b-PNEAAEA block copolymer and PNMAAEA-co-PNEAAEA random copolymers with different PNMAAEA and/or PNEAAEA fractions are synthesized, and their thermoresponse is checked. Full article
(This article belongs to the Special Issue Thermoresponsive Polymers)
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Open AccessArticle
Micellar Organocatalysis Using Smart Polymer Supports: Influence of Thermoresponsive Self-Assembly on Catalytic Activity
Polymers 2020, 12(10), 2265; https://doi.org/10.3390/polym12102265 - 01 Oct 2020
Abstract
Micellar catalysts with a switchable core are attractive materials in organic synthesis. However, little is known about the role of the shell forming block on the performance of the catalyst. Thermoresponsive block copolymers based on poly(N-isopropylacrylamide-co-vinyl-4,4-dimethylazlactone) attached to different [...] Read more.
Micellar catalysts with a switchable core are attractive materials in organic synthesis. However, little is known about the role of the shell forming block on the performance of the catalyst. Thermoresponsive block copolymers based on poly(N-isopropylacrylamide-co-vinyl-4,4-dimethylazlactone) attached to different permanently hydrophilic blocks, namely poly(ethylene glycol), poly(N,N-dimethylacrylamide), and poly(2,3-dihydroxypropyl acrylate), were successfully synthesized via reversible addition/fragmentation chain transfer radical polymerization (RAFT). Post-polymerization attachment of an amino-functionalized L-prolineamide using the azlactone ring-opening reaction afforded functionalized thermoresponsive block copolymers. Temperature-induced aggregation of the functionalized block copolymers was studied using dynamic light scattering. It was shown that the chemical structure of the permanently hydrophilic block significantly affected the size of the polymer self-assemblies. The functionalized block copolymers were subjected to an aldol reaction between p-nitrobenzaldehyde and cyclohexanone in water. Upon temperature-induced aggregation, an increase in conversion was observed. The enantioselectivity of the polymer-bound organocatalyst improved with an increasing hydrophilic/hydrophobic interface as a result of the different stability of the polymer aggregates. Full article
(This article belongs to the Special Issue Thermoresponsive Polymers)
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Open AccessArticle
Microfluidic Fabrication of Monodisperse Microcapsules for Thermo-Triggered Release of Liposoluble Drugs
Polymers 2020, 12(10), 2200; https://doi.org/10.3390/polym12102200 - 25 Sep 2020
Abstract
Here, we report a novel thermo-triggered-releasing microcapsule for liposoluble drug delivery. Monodisperse microcapsules with a poly(N-isopropylacrylamide-co-methacrylic acid) hydrogel shell and an oil core were successfully fabricated by a double coaxial microfluidic device. Fluorescent dye Lumogen Red F300 as a model liposoluble [...] Read more.
Here, we report a novel thermo-triggered-releasing microcapsule for liposoluble drug delivery. Monodisperse microcapsules with a poly(N-isopropylacrylamide-co-methacrylic acid) hydrogel shell and an oil core were successfully fabricated by a double coaxial microfluidic device. Fluorescent dye Lumogen Red F300 as a model liposoluble drug was dissolved in the oil core with controllable loading capacity. The volume phase transition temperature (VPTT) of the microcapsule was adjusted by copolymerizing with the hydrophilic methacrylic acid. The in vitro release study demonstrates that the shells shrink, leading to the thermo-triggered release of the model drug from the microcapsules at the environmental temperature above the VPTT, while the swollen hydrogel shells can protect the encapsulated drug from leakage and contamination below the VPTT. The proposed microcapsule is a promising liposoluble drug delivery system with controllable loading and smart thermo-triggered release. Full article
(This article belongs to the Special Issue Thermoresponsive Polymers)
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Open AccessArticle
Different Strategies for the Preparation of Galactose-Functionalized Thermo-Responsive Nanogels with Potential as Smart Drug Delivery Systems
Polymers 2020, 12(9), 2150; https://doi.org/10.3390/polym12092150 - 21 Sep 2020
Abstract
Different synthetic strategies were tested for the incorporation of galactose molecules on thermoresponsive nanogels owing to their affinity for receptors expressed in cancer cells. Three families of galactose-functionalized poly(N-vinylcaprolactam) nanogels were prepared with the aim to control the introduction of galactose-moieties [...] Read more.
Different synthetic strategies were tested for the incorporation of galactose molecules on thermoresponsive nanogels owing to their affinity for receptors expressed in cancer cells. Three families of galactose-functionalized poly(N-vinylcaprolactam) nanogels were prepared with the aim to control the introduction of galactose-moieties into the core, the core-shell interface and the shell. First and second of the above mentioned, were prepared via surfactant free emulsion polymerization (SFEP) by a free-radical mechanism and the third one, via SFEP/reversible addition-fragmentation chain transfer (RAFT) polymerization. Synthetic recipes for the SFEP/free radical method included besides N-vinylcaprolactam (NVCL), a shell forming poly(ethylene glycol) methyl ether methacrylate (PEGMA), while the galactose (GAL) moiety was introduced via 6-O-acryloyl-1,2,:3,4-bis-O-(1-methyl-ethylidene)-α-D-galactopiranose (6-ABG, protected GAL-monomer): nanogels I, or 2-lactobionamidoethyl methacrylate (LAMA, GAL-monomer): nanogels II. For the SFEP/RAFT methodology poly(2-lactobionamidoethyl methacrylate) as GAL macro-chain transfer agent (PLAMA macro-CTA) was first prepared and on a following stage, the macro-CTA was copolymerized with PEGMA and NVCL, nanogels III. The crosslinker ethylene glycol dimethacrylate (EGDMA) was added in both methodologies for the polymer network construction. Nanogel’s sizes obtained resulted between 90 and 370 nm. With higher content of PLAMA macro-CTA or GAL monomer in nanogels, a higher the phase-transition temperature (TVPT) was observed with values ranging from 28 to 46 °C. The ρ-parameter, calculated by the ratio of gyration and hydrodynamic radii from static (SLS) and dynamic (DLS) light scattering measurements, and transmission electron microscopy (TEM) micrographs suggest that core-shell nanogels of flexible chains were obtained; in either spherical (nanogels II and III) or hyperbranched (nanogels I) form. Full article
(This article belongs to the Special Issue Thermoresponsive Polymers)
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Open AccessArticle
Thermo-Responsive Shape Memory Effect and Conversion of Porous Structure in a Polyvinyl Chloride Foam
Polymers 2020, 12(9), 2025; https://doi.org/10.3390/polym12092025 - 04 Sep 2020
Abstract
In this paper, a thermo-responsive shape memory effect in a polyvinyl chloride thermoset foam is characterized. Excellent shape recovery performance is observed in foam samples programmed both at room temperature and above their transition temperature. The conversion of porous structures in the foam [...] Read more.
In this paper, a thermo-responsive shape memory effect in a polyvinyl chloride thermoset foam is characterized. Excellent shape recovery performance is observed in foam samples programmed both at room temperature and above their transition temperature. The conversion of porous structures in the foam from closed-cell to open-cell after a shape memory effect cycle is revealed via a series of specially designed oil-dripping experiments and optical images of the micro pores. Programming the strain higher than 20% results in an apparent increase in open-cell level, whereas programming temperatures have almost no influence. Full article
(This article belongs to the Special Issue Thermoresponsive Polymers)
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Open AccessArticle
Thermoresponsive Poly(glycidyl ether) Brush Coatings on Various Tissue Culture Substrates—How Block Copolymer Design and Substrate Material Govern Self-Assembly and Phase Transition
Polymers 2020, 12(9), 1899; https://doi.org/10.3390/polym12091899 - 24 Aug 2020
Abstract
Thermoresponsive poly(glycidyl ether) brushes can be grafted to applied tissue culture substrates and used for the fabrication of primary human cell sheets. The self-assembly of such brushes is achieved via the directed physical adsorption and subsequent UV immobilization of block copolymers equipped with [...] Read more.
Thermoresponsive poly(glycidyl ether) brushes can be grafted to applied tissue culture substrates and used for the fabrication of primary human cell sheets. The self-assembly of such brushes is achieved via the directed physical adsorption and subsequent UV immobilization of block copolymers equipped with a short, photo-reactive benzophenone-based anchor block. Depending on the chemistry and hydrophobicity of the benzophenone anchor, we demonstrate that such block copolymers exhibit distinct thermoresponsive properties and aggregation behaviors in water. Independent on the block copolymer composition, we developed a versatile grafting-to process which allows the fabrication of poly(glycidyl ether) brushes on various tissue culture substrates from dilute aqueous-ethanolic solution. The viability of this process crucially depends on the chemistry and hydrophobicity of, both, benzophenone-based anchor block and substrate material. Utilizing these insights, we were able to manufacture thermoresponsive poly(glycidyl ether) brushes on moderately hydrophobic polystyrene and polycarbonate as well as on rather hydrophilic polyethylene terephthalate and tissue culture-treated polystyrene substrates. We further show that the temperature-dependent switchability of the brush coatings is not only dependent on the cloud point temperature of the block copolymers, but also markedly governed by the hydrophobicity of the surface-bound benzophenone anchor and the subjacent substrate material. Our findings demonstrate that the design of amphiphilic thermoresponsive block copolymers is crucial for their phase transition characteristics in solution and on surfaces. Full article
(This article belongs to the Special Issue Thermoresponsive Polymers)
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Open AccessArticle
Temperature Behavior of Aqueous Solutions of Poly(2-Oxazoline) Homopolymer and Block Copolymers Investigated by NMR Spectroscopy and Dynamic Light Scattering
Polymers 2020, 12(9), 1879; https://doi.org/10.3390/polym12091879 - 20 Aug 2020
Abstract
1H NMR methods in combination with dynamic light scattering were applied to study temperature behavior of poly(2-isopropyl-2-oxazoline) (PIPOx) homopolymer as well as PIPOx-b-poly(2-methyl-2-oxazoline) (PMeOx) and poly(2-ethyl-2-oxazoline) (PEtOx)-b-PMeOx diblock copolymers in aqueous solutions. 1H NMR spectra showed a [...] Read more.
1H NMR methods in combination with dynamic light scattering were applied to study temperature behavior of poly(2-isopropyl-2-oxazoline) (PIPOx) homopolymer as well as PIPOx-b-poly(2-methyl-2-oxazoline) (PMeOx) and poly(2-ethyl-2-oxazoline) (PEtOx)-b-PMeOx diblock copolymers in aqueous solutions. 1H NMR spectra showed a different way of phase transition for the main and side chains in PIPOx-based solutions. Additionally, the phase transition is irreversible for PIPOx homopolymer and partially reversible for PIPOx-b-PMeOx copolymer. As revealed by NMR, the phase transition in PEtOx-based copolymers solutions exists despite the absence of solution turbidity. It is very broad, virtually independent of the copolymer composition and reversible with some hysteresis. 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 revealed information about conformational changes observed already in the pre-transition region of PIPOx-b-PMeOx copolymer solution. Full article
(This article belongs to the Special Issue Thermoresponsive Polymers)
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Open AccessArticle
Thermoresponsive Nanogels of Modified Poly((di(ethylene glycol) methyl ether methacrylate)-co-(2-aminoethyl methacrylate))s
Polymers 2020, 12(8), 1645; https://doi.org/10.3390/polym12081645 - 24 Jul 2020
Abstract
A series of copolymers of di(ethylene glycol) methyl ether methacrylate (D) and 2-aminoethyl methacrylate (A) (P(D-co-A)) with variable ratios of comonomers were synthesized using atom transfer radical polymerization. Then, the amino groups of obtained copolymers were modified to clickable azide or prop-2-yn-1-yl carbamate [...] Read more.
A series of copolymers of di(ethylene glycol) methyl ether methacrylate (D) and 2-aminoethyl methacrylate (A) (P(D-co-A)) with variable ratios of comonomers were synthesized using atom transfer radical polymerization. Then, the amino groups of obtained copolymers were modified to clickable azide or prop-2-yn-1-yl carbamate groups. A thermoresponsive copolymers were obtained with the value of cloud point temperature (TCP) dependent on the type and number of functional groups in the copolymer and on the concentration of solutions. For P(D-co-A) copolymers, the TCP increased with increasing content of 2-aminoethyl methacrylate comonomer. The presence of azide and prop-2-yn-1-yl carbamate groups caused the changes of TCP of modified copolymers. All studied copolymers in dilute aqueous solutions aggregated above TCP to nanoparticles with sizes dependent on the solution concentration, heating procedures, and types and numbers of functional groups present in a copolymer chain. The presence of hydrophilic elements in the chain and the increase in the copolymer concentration led to the enlargement of the particle sizes. Aggregates were crosslinked using click reaction between an azide and prop-2-yn-1-yl carbamate groups that led to stable thermoresponsive nanogels. A systematic study of the behavior of copolymers allowed the determination of the chains useful for possible application in drug delivery. Full article
(This article belongs to the Special Issue Thermoresponsive Polymers)
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Open AccessArticle
Complex Temperature and Concentration Dependent Self-Assembly of Poly(2-oxazoline) Block Copolymers
Polymers 2020, 12(7), 1495; https://doi.org/10.3390/polym12071495 - 04 Jul 2020
Cited by 1
Abstract
The effect of polymer concentration on the temperature-induced self-association of a block copolymer comprising a poly(2-ethyl-2-oxazoline) block and a random copolymer block consisting of 2-ethyl-2-oxazoline and 2-n-propyl-2-oxazoline (PEtO80-block-P(EtOxx-stat-PropO40-x) with x = [...] Read more.
The effect of polymer concentration on the temperature-induced self-association of a block copolymer comprising a poly(2-ethyl-2-oxazoline) block and a random copolymer block consisting of 2-ethyl-2-oxazoline and 2-n-propyl-2-oxazoline (PEtO80-block-P(EtOxx-stat-PropO40-x) with x = 0, 4, or 8 were investigated by dynamic light scattering (DLS) and transmittance measurements (turbidimetry). The polymers reveal a complex aggregation behavior with up to three relaxation modes in the DLS data and with a transmittance that first goes through a minimum before it declines at high temperatures. At low temperatures, unassociated polymer chains were found to co-exist with larger aggregates. As the temperature is increased, enhanced association and contraction of the aggregates results in a drop of the transmittance values. The aggregates fragment into smaller micellar-like clusters when the temperature is raised further, causing the samples to become optically clear again. At high temperatures, the polymers aggregate into large compact clusters, and the samples become turbid. Interestingly, very large aggregates were observed at low temperatures when the polymer concentrations were low. The formation of these aggregates was also promoted by a more hydrophilic copolymer structure. The formation of large aggregates with an open structure at conditions where the solvent conditions are improved is probably caused by depletion flocculation of the smaller aggregates. Full article
(This article belongs to the Special Issue Thermoresponsive Polymers)
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Open AccessArticle
Synthesis, Phase-Transition Behaviour, and Oil Adsorption Performance of Porous Poly(oligo(ethylene glycol) Alkyl Ether Acrylate) Gels
Polymers 2020, 12(6), 1405; https://doi.org/10.3390/polym12061405 - 23 Jun 2020
Abstract
To probe the effects of pendant side-chain structures on the properties of porous thermoresponsive polymer gels, oligo(ethylene glycol) alkyl ether acrylates were polymerised in an aqueous medium under radical-mediated phase-separation conditions. The monomer structures varied according to the lengths and termini of [...] Read more.
To probe the effects of pendant side-chain structures on the properties of porous thermoresponsive polymer gels, oligo(ethylene glycol) alkyl ether acrylates were polymerised in an aqueous medium under radical-mediated phase-separation conditions. The monomer structures varied according to the lengths and termini of their ethylene glycol side chains. The porous poly(oligo(ethylene glycol) alkyl ether acrylate) (POEGA) gels exhibited variable lower critical solution temperatures (LCSTs) but similar and rapid swelling–deswelling behaviours. Although the LCST of the poly(tri(ethylene glycol) monomethyl ether acrylate) (PTEGA) gel decreased with increasing aqueous NaCl or CaCl2 concentration, PTEGA showed excellent thermosensitivity in highly concentrated salt solutions, recommending its application in saline environments. Examination of PTEGA adsorption performance in an oil–water emulsion showed that n-tridecane adsorption increased with temperature. Although n-tridecane was effectively adsorbed at 70 °C, its release from the fully adsorbed PTEGA gel was difficult despite a temperature reduction from 70 to 20 °C. Full article
(This article belongs to the Special Issue Thermoresponsive Polymers)
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Open AccessArticle
Synthesis and Characterization of Stiff, Self-Crosslinked Thermoresponsive DMAA Hydrogels
Polymers 2020, 12(6), 1401; https://doi.org/10.3390/polym12061401 - 22 Jun 2020
Cited by 1
Abstract
Stiff thermosensitive hydrogels (HG) were synthesized by self-crosslinking free radical polymerization of N,N-dimethylacrylamide (DMAA) and N-isopropylacrylamide (NIPAAm), adjusting the degree of swelling by carboxylate-containing sodium acrylate (NaAc) or a 2-oxazoline macromonomer (MM). The formation of hydrogels was possible due to the self-crosslinking [...] Read more.
Stiff thermosensitive hydrogels (HG) were synthesized by self-crosslinking free radical polymerization of N,N-dimethylacrylamide (DMAA) and N-isopropylacrylamide (NIPAAm), adjusting the degree of swelling by carboxylate-containing sodium acrylate (NaAc) or a 2-oxazoline macromonomer (MM). The formation of hydrogels was possible due to the self-crosslinking property of DMAA when polymerized with peroxodisulfate initiator type. The MM was synthetized by the ring-opening cationic polymerization of 2-methyl-2-oxazoline (MeOxa) and methyl-3-(oxazol-2-yl)-propionate (EsterOxa), and contained a polymerizable styryl endgroup. After ester hydrolysis of EsterOxa units, a carboxylate-containing MM was obtained. The structure of the hydrogels was confirmed by 1H high-resolution (HR)-MAS NMR spectroscopy. Suitable conditions and compositions of the comonomers have been found, which allowed efficient self-crosslinking as well as a thermoresponsive swelling in water. Incorporation of both the polar comonomer and the macromonomer, in small amounts furthermore allowed the adjustment of the degree of swelling. However, the macromonomer was better suited to retain the thermoresponsive behavior of the poly (NIPAAm) due to a phase separation of the tangling polyoxazoline side chains. Thermogravimetric analysis determined that the hydrogels were stable up to ~ 350 °C, and dynamic mechanical analysis characterized a viscoelastic behavior of the hydrogels, properties that are required, for example, for possible use as an actuator material. Full article
(This article belongs to the Special Issue Thermoresponsive Polymers)
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Open AccessArticle
Effects of Chemical Modifications on the Thermoresponsive Behavior of a PDMAEA-b-PNIPAM-b-POEGA Triblock Terpolymer
Polymers 2020, 12(6), 1382; https://doi.org/10.3390/polym12061382 - 19 Jun 2020
Cited by 1
Abstract
In this work, the synthesis, selective chemical modifications, and self-assembly behavior in aqueous media of a novel poly(2-(dimethylamino)ethyl acrylate)20-b-poly(N-isopropylacrylamide)11-b-poly(oligo ethylene glycol methyl ether acrylate)18 (PDMAEA20-b-PNIPAM11-b-POEGA18) dual-responsive (pH and temperature) and triply hydrophilic [...] Read more.
In this work, the synthesis, selective chemical modifications, and self-assembly behavior in aqueous media of a novel poly(2-(dimethylamino)ethyl acrylate)20-b-poly(N-isopropylacrylamide)11-b-poly(oligo ethylene glycol methyl ether acrylate)18 (PDMAEA20-b-PNIPAM11-b-POEGA18) dual-responsive (pH and temperature) and triply hydrophilic amino-based triblock terpolymer are reported. The amine functional triblock terpolymer was synthesized by sequential reversible addition fragmentation chain transfer polymerization (RAFT) polymerization and molecularly characterized by size exclusion chromatography (SEC) and 1H-NMR spectroscopy that evidenced the success of the three-step polymerization scheme. The tertiary amine pendant groups of the PDMAEA block were chemically modified in order to produce the Q1PDMAEA20-b-PNIPAM11-b-POEGA18 as well as the Q6PDMAEA20-b-PNIPAM11-b-POEGA18 quaternized triblock terpolymers (Q1 and Q6 prefixes show the number of carbon atoms (C1 and C6) attached on the PDMAEA groups) using methyl iodide (CH3I) and 1-iodohexane (C6H13I) as the quaternizing agents and the SPDMAEA20-b-PNIPAM11-b-POEGA18 sulfobetainized triblock terpolymer using 1,3 propanesultone (C3H6O3S) as the sulfobetainization agent. The self-assembly properties of the triblock terpolymers in aqueous solutions upon varying temperature and solution pH were studied by light scattering and fluorescence spectroscopy experiments. The novel triblock terpolymers self-assemble into nanosized aggregates upon solution temperature rise above the nominal lower critical solution temperature (LCST) of the temperature-responsive PNIPAM block. The remarkable stimuli-responsive self-assembly behavior of the novel triblock terpolymers in aqueous media make them interesting candidates for biomedical applications in the fields of drug and gene delivery. Full article
(This article belongs to the Special Issue Thermoresponsive Polymers)
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Open AccessArticle
Thermoresponsive Polymer Gating System on Mesoporous Shells of Silica Particles Serving as Smart Nanocontainers
Polymers 2020, 12(4), 888; https://doi.org/10.3390/polym12040888 - 11 Apr 2020
Abstract
Spherical silica nanoparticles with solid cores and mesoporous shells (SCMS) were decorated with thermoresponsive polymer brushes that were shown to serve as macromolecular valves to control loading and unloading of a model dye within the mesopores. Thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) brushes were grafted from [...] Read more.
Spherical silica nanoparticles with solid cores and mesoporous shells (SCMS) were decorated with thermoresponsive polymer brushes that were shown to serve as macromolecular valves to control loading and unloading of a model dye within the mesopores. Thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) brushes were grafted from the surfaces of both solid core (SC) and SCMS particles of similar size using surface-initiated atom transfer radical polymerization. Both systems based on porous (SCMS-PNIPAM) and nonporous (SC-PNIPAM) particles were characterized using cryo-TEM, thermogravimetry and elemental analysis to determine the structure and composition of the decorated nanoparticles. The grafted PNIPAM brushes were found to be responsive to temperature changes enabling temperature-controlled gating of the pores. The processes of loading and unloading in the obtained systems were examined using a model fluorescent dye—rhodamine 6G. Polymer brushes in SCMS-PNIPAM systems were shown to serve as molecular valves enabling significant adsorption (loading) of the dye inside the pores with respect to the SC-PNIPAM (no pores) and SCMS (no valves) systems. The effective unloading of the fluorescent cargo molecules from the decorated nanoparticles was achieved in a water/methanol solution. The obtained SCMS-PNIPAM particles may be used as smart nanocontainers or nanoreactors offering also facile isolation from the suspension due to the presence of dense cores. Full article
(This article belongs to the Special Issue Thermoresponsive Polymers)
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Review

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Open AccessReview
Thermoresponsive Polyoxazolines as Vectors for Transfection of Nucleic Acids
Polymers 2020, 12(11), 2609; https://doi.org/10.3390/polym12112609 - 06 Nov 2020
Abstract
Poly(2-oxazoline)s (POx) are an attractive platform for the development of non-viral gene delivery systems. The combination of POx moieties, exhibiting excellent biocompatibility, with DNA-binding polyethyleneimine (PEI) moieties into a single copolymer chain is a promising approach to balance toxicity and transfection efficiency. The [...] Read more.
Poly(2-oxazoline)s (POx) are an attractive platform for the development of non-viral gene delivery systems. The combination of POx moieties, exhibiting excellent biocompatibility, with DNA-binding polyethyleneimine (PEI) moieties into a single copolymer chain is a promising approach to balance toxicity and transfection efficiency. The versatility of POx in terms of type of substituent, copolymer composition, degree of polymerization, degree of hydrolysis, and chain architecture, as well as the introduction of stimuli-responsive properties, provides opportunities to finely tune the copolymer characteristics and physicochemical properties of the polyplexes to increase the biological performance. An overview of the current state of research in the POx–PEI-based gene delivery systems focusing particularly on thermosensitive POx is presented in this paper. Full article
(This article belongs to the Special Issue Thermoresponsive Polymers)
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