Triggering the Thermosensitive Properties of Hydrophobically Modified Poly(N‐isopropylacrylamide) by Complexation with Cyclodextrin Polymers

Abstract

Summary: Hydrophobically modified poly(N‐isopropylacrylamide) (PNIPAM) containing either an adamantyl or a dodecyl group were prepared and characterized. Self‐association in aqueous solutions was evidenced by fluorescence measurements using pyrene as a probe. The lower critical solution temperatures (LCST) were determined from cloud point measurements. They strongly depended on the hydrophobic group and the substitution level. The association between hydrophobically modified PNIPAM and β‐cyclodextrin (monomers and polymers) was investigated by cloud point and viscosity measurements. The presence of β‐cyclodextrin monomers generally shifted the LCST to a higher temperature, the complexation increasing the solubility of PNIPAM chains. β‐Cyclodextrin polymers mixed with hydrophobically modified PNIPAM generated supramolecular assemblies. This was evidenced by viscosity measurements of the mixtures at temperatures lower than the LCST. Moreover, depending on the substitution level of the PNIPAM, the LCST was increased (1% hydrophobic groups) or decreased (4% hydrophobic groups) by more than 10 °C upon β‐cyclodextrin polymer addition.

Physical Gelation of Amphiphilic Poly(N‐isopropylacrylamide): Influence of the Hydrophobic Groups

“…A previous study with the same copolymers has shown that the adamantyl-substituted PNIPAM-Ada do not show any self-association tendency until concentrations larger than several wt.-% whereas the two dodecyl-substituted PNIPAM-C12 form hydrophobic microdomains at concentrations lower than 0.1 wt.-%. [24] The results of Table 1 show that the sizes are correlated to the copolymer molecular weight (with a proportionality to M 1=2 V ) and not to the substitution nature and degree. Thus the selfassociation of the hydrophobic groups (in the case of PNIPAM-C12) should not involve multichain aggregation.…”

“…[24,34] The feed ratio N-alkylacrylamide/N-isopropylacrylamide was low (around 1-5 mol-%) in order to obtain copolymers still soluble in water. The copolymer composition was determined either by quantitative 13 C NMR for the copolymers with the highest hydrophobicto-N-isopropylacrylamide unit ratio or by viscosity measurements of copolymer/pbCD mixtures.…”

“…The copolymer composition was determined either by quantitative 13 C NMR for the copolymers with the highest hydrophobicto-N-isopropylacrylamide unit ratio or by viscosity measurements of copolymer/pbCD mixtures. [24] Briefly, the specific viscosity is measured for mixtures at fixed total concentration (10 g Á L À1 ) with different ratio copolymer/pbCD. A maximum of viscosity is observed when a maximum of interactions occurs between the b-cyclodextrin polymer and the PNIPAM copolymer.…”

“…Presence of salts, such as NaCl or NaBr, lowers the phase transition temperature of PNIPAM and consequently decreases the temperature at which the physically crosslinked network is formed. Introduction of hydrophobic groups on PNIPAM chains also modified the physical properties of PNIPAM [22][23][24] but to the best of our knowledge no report on rheological properties has been published for these compounds. Bokias et al [25] have studied the shear-induced thickening of positively charged amphiphilic PNIPAMs.…”

Hidrogeles de ciclodextrinas para administración de fármacos

“…La HP-β-CD incrementó la velocidad de cesión de los tres fármacos, mientras que la γ-CD no solo no alteró la liberación de ibuprofeno, que tiene una baja tendencia a formar complejos, las de los hidrogeles físicos, en los que las interacciones son reversibles, y las de los hidrogeles reticulados químicamente que son estables frente a la dilución. Este fenómeno se ha observado al mezclar: i) un polímero con CDs "colgantes" con un polímero con cadenas laterales hidrofóbicas de 4-tert-butilanilida [77]; ii) un conjugado de quitosano-CD con quitosano con grupos adamantilo o polietilenglicol [78]; iii) poli(acrilamida)-CD con poli(acrilamida) con anillos aromáticos [79]; y iv) polímeros de β-CD con poli(N-isopropilacrilamida) con grupos adamantilo o dodecilo [80]. Por ejemplo, se produce un autoensamblaje espontáneo entre polímeros de β-CD (poli-β-CD) y dextranos con cadenas alquílicas o grupos adamantilo colgantes (Figura 1.5).…”