Thermo-responsive polymers transition

Poly(7V-isopropylacrylamide) (PNlPAAm) is a well-known thermo-responsive polymer and exhibits a lower critical solution temperature (LCST) of 32°C in water. It assnmes a random coil structure (hydrophilic state) below the LCST and a collapsed globnlar stractnre (hydrophobic state) above. Because of this sharp reversible transition, this polymer finds a vast array of applications,... 

Thermo-responsive polymers have been used to release encapsulated drugs in response to temperature changes. These polymers typically undergo a rapid and reversible hydration-dehydration at lower critical solution temperature (LCST, from soluble to insoluble) or upper critical solution temperature (UCST, from insoluble to soluble) upon heating. [71] For example, a thermo-responsive polymer, poly (2-isopropyl- 2-oxazoline) (PiPrOx), was conjugated to pH-responsive poly(benzyl ether)dendrons, which exhibited sharp thermal transitions as well as pH-dependent behaviors (Fig. 4a).[105]... 

It would be of interest to add some functionalities such as properties in response to changes in pH, temperature, electrical potential and so on as well as hydrophilic properties to PTFE surfaces by the combined technique just described. Poly(AT-isopropylaoylamide) (PNIPA ) is a well-known thermo-responsive polymer whose hydrogel undergoes a volume change around 31 C in an aqueous solution. And further, poly 2-(dimethylamino)ethyl methacrylate (PDMA) in a pH 10 buffer solution exhibits a drastic decrease in the transmittance around 28 °C, which is considered to come from a phase transition of PDMA chains (ii). Therefore, we have tried to prepare JV -isopropylacrylamide (NIPAAm) or 2-(dimethylamino)ethyl... 

Figure 1.1 Curves showing phase transition phenomenon, (a) Lower critical solution temperatnre (LCST) and (h) npper critical solution temperature (UCST) phase transition behaviors of thermo-responsive polymers in solution.

Figure 1.2 Temperature-induced phase transitions in thermo-responsive polymer chains. Extended chains-to-globules-to-aggregate transition is shown.

Spectroscopic ellipsometry allows to detect small shifts of the phase transition temperature introduced either by a variation of the thermo-responsive polymer or the liquid environment. The latter dependency was demonstrated by Schmaljohann et al. (2003) for buffer solutions and cell culture medium and by Cordeiro et al. (2009) for artificial sea water. In both cases, the electrolyte shifts the transition temperature toward lower values. 

However, the development of in vivo applications for PNIPAM is limited by its non-biodegradability and the presence of amide moieties that reduce its biocompatibility. For this reason, other thermo-responsive polymers have been investigated in recent years. Poly(N-vinylcaprolactam) is a promising alternative. This polymer has a LCST between 35 and 38°C, again close to the temperature of the human body, and is characterized by high biocompatibility and low toxicity (Konak et al, 2007 Medeiros et al, 2010 Shtanko et al, 2003 Yanul et al, 2001). Additionally, amphiphilic copolymers such as Pluronics and Tetronics have been developed, based on copolymers of polyethylene oxide and polypropylene oxide. These copolymer systems exhibit a solution-gel transition at close to human body temperature that permits their application as injectable implants (Samchenko et ai,2011). 

The most commonly studied thermo-responsive polymers are poly (N-isopropylacrylamide), poly(2-alkyl-2-oxazoline)s, poly(vinyl methyl ether), poly(N-vinyl caprolactam), and polymers of oligoethylene glycol (meth)acrylates. Each of them is characterized with specific LCST, mechanism of heat-driven phase transition, structure and properties of the resulting mesoglobules, as well as with varying extents of reversibility and reproducibility of the process of mesoglobule formation [58]. The most preferable characteristics of the thermo-responsive polymers and mesoglobules used as templates for nanocapsule preparation have been formulated below [58] ... 

Orakdogen N, Okay O (2006) Reentrant conformation transition in poly(N, N-dimethylacryla-mide) hydrogels in water-organic solvent mixtures. Polymer 47 561-568 Panayiotou M, Freitag R (2005) Influence of the synthesis conditions and ionic additives on the swelling behaviour of thermo-responsive polyalkylacrylamide hydrogels. Polymer 46 6777-6785... 

The low recovery stress (<10 MPa) of thermo-responsive SMPs originates from their intrinsically low modulus, in the order of 0.1-1 GPa below the thermal transition temperature. CNTs have proven to enhance the mechanical properties, particularly modulus, of various polymers. ... 

Uny et also reported the chemical synthesis of protein polymers based on the (Val-Pro- Ala-Val-Gly) repeat sequence in which glycine is replaced by the D-alanine residue. The hetero-chiral Pro- Ala diad would be erqrected on the basis of stereochemical considerations to adopt a type-II p-tum conformation. Stmctural analyses of small-molecule "Pro- Ala turn models support the formation of the type-II p-mm conformation in solution and the solid state. Polymers based on the (Val-Pro- Ala-Val-Gly) repeat sequence display a thermo-reversible phase transition similar to the corresponding polypeptides derived from the parent (Val-Pro-Gly-Val-Gly) sequence, albeit with a shift of the Tt to approximately 5-10 ° G below the latter due to a slight inaease in hydrophobic character due to the presence of the alanine residue. NMR spectroscopic analyses of the (Val-Pro- Ala-Val-Gly) polymer suggest that the repeat unit retains the p-tum stmcture on the basis of comparison to the corresponding behavior of the (Val-Pro-Gly-Val-Gly) polymer. Stress-strain measurements on cross-linked matrices of the (Val-Pro- Ala-Val-Gly) polymer indicate an elastomeric mechanical response in which the elastic modulus does value in comparison to the (Val-Pro-Gly-Val-Gly) polymer. These smdies of glycine suhstitution support the hypothesis that type-II p-tum formation can he associated with the development of elastomeric behavior with native elastins and elastin-derived polypeptide sequences. Several investigators have proposed that the (Val-Pro-Gly-Val-Gly) pentapeptide represents the minimal viscoelastic unit... 

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