Description of the turbidity measurements near the phase transition temperature of poly(N-isopropyl acrylamide) copolymers: the effect of pH, concentration, hydrophilic and hydrophobic content on the turbidity

Absorbance values between 300 and 800 nm of aqueous solutions of poly(N-isopropyl acrylamide-co-itaconic acid-9.80), poly(N-isopropyl acrylamide-co-itaconic acid-52.05) and poly(N-isopropyl acrylamide)s containing Tegomer H-Si 2111 end groups and/or blocks were measured using a Shimadzu 160-A UV-visible spectrometer. Turbidities obtained from these absorbance values were used to interpret the macromolecular phase transition from a hydrophilic to a hydrophobic structure of the polymers. The effects of comonomer type and content, concentration of the solutions, pH and temperature on the coil-globule transition were discussed in terms of turbidity form factor, β related to size and shapes of particles and calculated by using the simplified form of Debye equation.The results presented in this work show that the presence of Tegomer H-Si 2111 (Si containing end groups and/or blocks) or high amount of itaconic acid (IA) in the chains prevent a collapse transition from hydrated extended coils to hydrophobic globules, which aggregate and form a separate phase (β<2). Furthermore, it was observed that in the case of concentrated solutions intermolecular hydrophobic interactions between isopropyl groups overcame the repulsive forces resulting from the ionized carboxylic acid groups of IA or surface active nature of Si containing hydrophobic groups (β>2). This stage of the transition corresponds to macroscopic phase separation after an intramolecular process.     

Aqueous solution behavior of p(N-isopropyl acrylamide) in the presence of water-soluble macromolecular species

We report here the polymerization of N-isopropyl acrylamide (NIPAM) via the reversible addition fragmentation chain transfer (RAFT) process. Two trithiocarbonates (S,S′-bis(α,α′-dimethyl-α″-acetic acid)-trithiocarbonate and 2-dodecylsulfanylthiocarbonylsulfanyl-2-methyl propionic acid) were used as the chain transfer agents in conjunction with 4,4′-azobis(4-cyanovaleric acid) and 2,2′azobis(2-methylpropionamidine) dihydrochloride as the initiating species. Poly(NIPAM) is a thermo-responsive polymer that has a sharp lower critical solution temperature (LCST). Herein, we investigated the aqueous solution behaviour of well defined p(NIPAM) prepared by the RAFT process as a function of molecular weight (degree of polymerization: 50, 100 and 200) and temperature. Furthermore, we examine the influence of varying concentrations of macromolecular species (neutral polyethylene glycol (M_n - 3400 g/mol) and ionic bovine serum albumin (M_n - 63 000 g/mol)) on the LCST of p(NIPAM). The aqueous solution behaviour was assessed by spectrophotometry, dynamic light scattering and surface tensiometry. The macromolecular additives was found to have a significant effect on the coil to globular transition of the lower molecular weight p(NIPAM).     

Composite membranes based on gellan and poly(N-vinylimidazole). Synthesis and characterization

Composite membranes based on gellan and poly(N-vinylimidazole) were prepared by using the method of solvent evaporation. In acid medium ionic interactions are established and evidenced by measurements of FTIR spectroscopy. The properties of these membranes were investigated by using elemental analysis, thermogravimetric analysis, atomic force microscopy and dynamic swelling experiments. The thermic behavior of the membranes depends on the nature of interactions and swelling degree is highly dependent on pH and composition. The properties of these membranes suggest potential applications in the field of biodegradable materials or as supports for drugs in view of their controlled release.     

Thermosensitive poly(N-isopropylacrylamide-co-acrylamide) hydrogels: Synthesis, swelling and interaction with ionic surfactants

Thermosensitive hydrogels were prepared by free-radical polymerization in aqueous solution from N-isopropylacrylamide (NIPA) and acrylamide (AAm) monomers. N,N-Methylenebis(acrylamide) (MBAAm) was used as a crosslinker. A kinetic study of the absorption determined the transport mechanism. The diffusion coefficients of these hydrogels were calculated for the Fickian mechanism. It was shown that the swelling behavior of the P(NIPA-co-AAm) hydrogels can be controlled by changing the amount of MBAAm. The swelling equilibrium of the P(NIPA-co-AAm) hydrogels was also investigated as a function of temperature in aqueous solutions of the anionic surfactant sodium dodecyl sulfate (SDS) and the cationic surfactant dodecyltrimethylammonium bromide (DTAB). In SDS and DTAB solutions, the equilibrium swelling ratio of the hydrogels increased, this is ascribed to the conversion of non-ionic P(NIPA-co-AAm) hydrogel into polyelectrolyte hydrogels due to binding of surfactant molecules through the hydrophobic interaction. Additionally, the amount of free SDS and DTAB ions was measured at different temperatures by a conductometric method, it was found that the electric conductivity of the P(NIPA-co-AAm)—surfactant systems depended strongly on both the type and concentration of surfactant solutions.     

Poly(N-isopropylacrylamide)/poly[(N-acetylimino)ethylene] thermosensitive block and graft copolymers

Block and graft copolymers with poly(N-isopropylacrylamide) and poly[(N-acetylimino)ethylene] (PNAI) sequences were synthesized via PNAI derivatives (macroinitiators or macromers). The polymerization yields for block copolymers synthesized in ethanol, using the PNAI macroinitiator, were low (<10%), except where photochemical polymerization was applied. By contrast, for the copolymerizations of N-isopropylacrylamide with the PNAI macromers, performed in alcoholic solution, quite high polymerization yields, around 80-90%, were reached. ¹H-NMR and IR spectral and differential scanning calorimeter thermal data confirmed the copolymer formation. Thermosensitivity of the copolymers was investigated by means of turbidimetric technique as a function of their nature, average molecular weight and composition. It was found that the length of the chain of the PNAI macromer and the content in hydrophilic PNAI units of the resulted copolymer affected this behavior.     

Synthesis and characterization of fluorinated poly(imide siloxane) block copolymers

Five new block copoly(imide siloxane)s have been prepared by reacting two different diamines, 4,4″-bis(p-aminophenoxy)-3,3″-trifluoromethyl terphenyl (APTTFT) and amino-propyl terminated polydimethylsiloxane (APPS), separately with 4,4′-(4,4′-isopropylidenediphenoxy)bis(phthalic anhydride); BPADA. The reactions were conducted by a two pot solution imidization technique. The diamine APTTFT and the dianhydride BPADA composed the hard block segment while APPS and BPADA composed the soft block segment. The soft and hard blocks of different block lengths were generated by different stoichiometric imbalance in two different flasks and the final polymers were obtained by reacting both the blocks together. Different block copoly(imide siloxane)s were prepared on increasing the hard block lengths (DP) from 7 to 12, 18, 23 and 28 and the soft block lengths (DP) from 4 to 6, 8, 10 and 12, respectively. The resulting polymers have been well characterized by NMR, DSC and DMA techniques. The properties of the block copolymers were compared with the analogous random copolymers and homopolyimide prepared without APPS.     

Thermoresponsive poly(N-isopropylacrylamide) nanogels/poly(acrylamide) nanostructured hydrogels

Here we report the preparation and characterization of nanostructured thermo-responsive poly(acrylamide) (PAM)-based hydrogels. The addition of slightly crosslinked poly(N-isopropylacrylamide) (PNIPA) nanogels to AM reactive aqueous solution produces nanostructured hydrogels that exhibit a volume phase transition temperature (T_VPT). Their swelling kinetics, T_VPT's and mechanical properties at the equilibrium-swollen state (H_eq) are investigated as a function of the concentration of PNIPA nanogels in the nanostructured hydrogels. Nanostructured hydrogels with PNIPA nanogels/AM mass ratios of 20/80 and above exhibit higher H_eq and longer time to reach the equilibrium swelling than those of the conventional PAM hydrogels. However, the PNIPA nanogels possess thermo-responsive character missing in conventional PAM hydrogels. The T_VPT of nanostructured hydrogels depends on PNIPA nanogel content but their elastic and Young moduli are larger than those of conventional hydrogels at similar swelling ratios. Swelling kinetics, T_VPT, and mechanical properties are explained in terms of the controlled in-homogeneities introduced by the PNIPA nanogels during the polymerization.     

Synthesis and characterization of (AB)n-type poly(l-lactide)-poly(dimethyl siloxane) multiblock copolymer and the effect of its macrodiol composition on urethane formation

An (AB)_n-type multiblock copolymer containing alternating poly(l-lactide) (PLLA) and poly(dimethyl siloxane) (PDMS) segments was synthesized by chain extension of hydroxyltelechelic PLLA-PDMS-PLLA triblock copolymers, which were prepared by the ring-opening polymerization of l-lactide initiated by α,ω-functionalized hydroxyl poly(dimethyl siloxane), using 1,6-hexamethylene diisocyanate as a chain extender. The triblock and the multiblock copolymers were characterized by FT-IR, ¹H NMR and GPC. From the results of thermal analysis, two glass transition temperatures which were measured by DSC showed the occurrence of phase separation phenomena in the triblock and multiblock copolymers because of the difference of solubility parameters between PLLA and PDMS segments. The effect of the chemical composition of the triblock copolymers, including the Mw and the constitutive segment chain length of the macrodiol, on the development of the Mw of the multiblock was discussed based on diffusion effect. Furthermore, the consumption of the isocyanate groups was determined by FT-IR to investigate the dependence of the reaction kinetics of the urethane formation on the chemical composition of the triblock copolymer. The results reveal that the order of the chain extension reaction depended on the Mw of the triblock copolymer: a second order reaction was transformed into a third reaction as the Mw of the triblock copolymer increased from 7000 to 25,000 (g/mol) perhaps because of the inhibition of the formation of an active complex involved in the catalyzed-urethane reaction by the polymer chain aggregation. Finally, the mechanical properties of the multiblock copolymers demonstrated that the introduction of the extremely flexible PDMS segment substantially improved the elongation at breakage, and the tensile strength and the tensile modulus declined due to the intrinsic elasticity of such segments.     

Complexation between poly(N,N-diethylacrylamide) and poly(acrylic acid) in aqueous solution

The complexation between poly(N,N-diethylacrylamide) (PDEA) and poly(acrylic acid) (PAA) in aqueous solution was studied by viscometric, potentiometric, and fluorescence techniques. It was found that an interpolymer complex formed between the two polymers through hydrogen bonding interactions with the stoichiometry of r=0.6 (r is unit molar ratio of PAA/PDEA), and the complex formation show the dependence on pH values. The phase behaviour studies showed that the lower critical solution temperature of the PDEA-PAA aqueous solution gradually increased with the increasing of r from 0.01 to 0.15, until a soluble system in the whole temperature region was obtained, which remained in the range of r=0.15-0.3. At higher PAA concentrations, when r is above 0.3, the system appeared phase separation, and almost no temperature dependence was observed. Based on these conclusion and structure characteristics of PDEA and PAA, a model containing only short sequences of monomer residues was proposed for the structure of PDEA-PAA complex.     

Swelling and network parameters of crosslinked thermoreversible hydrogels of poly(N-ethylacrylamide)

Samples of Poly(N-ethylacrylamide) (PEA) have been synthesized by free radical polymerization in water using N,N′-methylene bis-acrylamide (BIS) as crosslinker. Hydrogels obtained by swelling them in water, 15 wt% KCl and 1 wt% sodium dodecyl sulphate (SDS) were examined by gravimetric, dimensional and compression-strain measurements to afford values of swelling ratio, polymer-solvent interaction parameters, elastic moduli and effective crosslinking density ν_e. Crosslinking inefficiency is evidenced by the low value (0.23) of ν_e relative to the theoretical crosslinking density ν_t based on the content of BIS in the synthesis. A small but finite extrapolated value of ν_e at ν_t = 0 is indicative of hydrophobic physical interactions. In water at 298 K increasing the content of BIS leads to a decrease in swelling and increases in values of elastic moduli and polymer-water interaction parameter. At a fixed content of BIS the values of ν_e and the elastic moduli exhibit an unusual increase with temperature, the crosslinking thus being thermally reversible. It is proposed that this results from a balance between hydrophobic interaction and breakage of hydrophilic hydrogen bonding. Although KCl in the medium decreases the swelling compared with that in water, the opposite effect occurs on incorporation of SDS, which is assumed to confer some polyelectrolyte character to the PEA chains. The finding, that these two swelling media reduce the values of ν_e and elastic moduli cf the value in water, has not been resolved satisfactorily.     

The synthesis, characterisation, phase behaviour and swelling of temperature sensitive physically crosslinked poly(1-vinyl-2-pyrrolidinone)/poly(N-isopropylacrylamide) hydrogels

Physically crosslinked complexes of polyvinyl pyrrolidinone-poly (N-isopropylacrylamide) (PVP-PNIPAAm) were prepared by photopolymerisation from a mixture of the monomers 1-vinyl-2-pyrrolidinone and N-isopropylacrylamide. IR spectroscopy and calorimetry were used to characterise the resulting xerogels. By alternating the monomer feed ratio, copolymers were synthesised to have their own distinctive lower critical solution temperature (LCST). The transition temperature of the gels was established using cloud point measurement and modulated differential scanning calorimeter (MDSC). This ability to shift the phase transition temperature of the copolymers provides excellent flexibility in tailoring transitions for specific uses. Swelling experiments were performed on the copolymer disks in distilled water at varying temperatures to establish the behaviour of the gels above and below phase transition temperature. The results obtained show that below transition temperature, the gels are water soluble but above this temperature they are slightly less water soluble; significantly less water soluble; or water insoluble; depending on the composition and LCST of the gel.     

Synthesis,characterization, and mechanical properties of PMMA/poly(aromatic/aliphatic siloxane) semi-interpenetrating polymer networks

Semi-interpenetrating polymer networks (IPNs) composed of poly(methyl methacrylate) (PMMA) and aromatic/aliphatic siloxanes have been made via sequential and simultaneous polymerizations. As the percentage of aliphatic siloxane increases, flexibility and, in general, toughness of the IPNs increases and clarity is reduced. This loss in clarity is due to the mismatch of refractive indices (1.49 form PMMA vs. 1.43 for aliphatic siloxane). PMMA is quite transparent. On the other hand, in making aromatic siloxane/PMMA IPNs clarity is retained as aromatic siloxane is increased due to better matching refractive index (1.49 for PMMA and −1.49 for poly(diphenyl siloxane)). Gel permeation chromatography (GPC) indicates slightly crosslinked IPNs with the THF soluble portions having number-average molecular weight, M¯_n, of 10⁵–10⁶. NMRs of IPNs essentially show peaks for the components, PMMA and the siloxane, which make up the respective IPNs. ²⁹Si-NMRs indicate cross-linking and grafting. Mechanical properties show increased toughness of IPNs versus PMMA as percentage of siloxane and crosslinker increases, but with a corresponding loss in tensile strength. © 1996 John Wiley & Sons, Inc.     

Preparation and characterization of sodium carboxymethylcellulose/poly(N-isopropylacrylamide)/clay semi-IPN nanocomposite hydrogels

A new kind of pH-/temperature-responsive semi-interpenetrating polymer network hydrogels based on linear sodium carboxymethylcellulose (CMC) and poly(N-isopropylacrylamide) (PNIPA) cross-linked by inorganic clay (CMC/PNIPA/Clay hydrogel) was prepared. The temperature- and pH-responsive behaviors, the mechanical properties of these hydrogels were investigated. The CMC/PNIPA/Clay hydrogels exhibited a volume phase transition temperature around 32 °C with no significant deviation from the conventional PNIPA hydrogels. The swelling ratio of the CMC/PNIPA/Clay hydrogels gradually decreased with increasing the contents of clay. The influence of pH value on swelling behaviors showed that there is a maximum swelling ratio at pH 5.9. Moreover, the CMC/PNIPA/Clay hydrogels exhibited excellent mechanical properties with high tensile stress and elongation at break in excess of 1200%.     

Preparation and characterization of poly(imide siloxane) (PIS)/titania(TiO2) hybrid nanocomposites by sol-gel processes

Poly(imide siloxane)(PIS)/titania(TiO₂) hybrid nanocomposites with organic-inorganic covalent bonding have been successfully synthesized by sol-gel processes. The PIS copolymer synthesized in this study was characterized by the observed coexisting two segments: the polyimide (PI) segment and polydimethyldiphenylsiloxane segment, and the latter were specially featured with the introduction of a diphenyl group for improved homogeneity.The obtained TiO₂ networks in PIS matrix were well dispersed and their average diameter was less than 50 nm. Meanwhile, the PIS/TiO₂ hybrid nanocomposite films exhibited good optical transparency at 20 wt% of TiO₂ content. The thermal stability, tensile strength and elongation of the nanocomposites decreased with increasing TiO₂ content. The glass transition temperature (T_g) and Young’s modulus increased with increasing TiO₂ content. The chemical structure and morphologies of PIS/TiO₂ hybrid nanocomposites was characterized by Fourier transform infrared spectroscope (FT-IR), X-ray photoelectron spectroscopy (XPS), and transmission electron microscope (TEM). The T_g and thermal stability were measured by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA), respectively. The mechanical properties were examined by dynamic mechanical analysis (DMA) under controlled force mode.     

Aging and thermal degradation of poly(N-methylaniline)

The conductivity aging and thermal stability of poly(N-methylaniline) are reported. Poly(N-methylaniline) doped with chloride ion was electrochemically synthesized. The conductivity data obtained in the temperature range between 118 and 483 K are analysed by Arrhenius and Mott models to elucidate the conduction mechanism. The thermal degradation of both doped and dedoped samples of poly(N-methylaniline) in air and nitrogen atmosphere has been followed using thermogravimetric and differential thermal analysis techniques. The polymer is heat-aged at various temperatures and the aged samples are analysed by FT-IR, SEM and XRD. The thermogravimetric data are further analysed by three different methods: Horowitz and Metzger [Anal. Chem. 35 (1963) 1464], Coats and Redfern [Nature 201 (1964) 68], Chan et al. [Synth. Met. 31 (1989) 95] to evaluate the energy of activation. The applicability of the three methods for the evaluation of kinetic parameters is discussed.     

Thermo-sensitive colloidal crystals of silica spheres in the presence of large spheres with poly (N-isopropyl acrylamide) shells

Thermo-sensitive colloidal crystals are prepared simply by mixing colloidal silica spheres and large thermo-sensitive gel spheres. The thermo-reversible change in the lattice spacing of colloidal crystals of monodisperse silica spheres (CS82, 103 nm in diameter) depends on the size of the admixed temperature-sensitive gel spheres. For spheres with sizes less and greater than that of the silica spheres, the lattice spacing upon temperature increase above the lower critical solution temperature of poly(N-isopropyl acrylamide) decreases (cf. Okubo et al. Langmuir 18:6783, 2002) and increases, respectively. A mechanism, which is able to explain these experimental findings, is proposed. Moreover, crystal growth rates and the rigidities of the thermo-sensitive colloidal crystals are studied.     

Synthesis and phase separation of amine-functional temperature responsive copolymers based on poly(N-isopropylacrylamide)

Free radical copolymerizations of N-isopropyl acrylamide (NIPAM) and cationic N-(3-aminopropyl) methacrylamide hydrochloride (APMH) were investigated to prepare amine-functional temperature responsive copolymers. The reactivity ratios for NIPAM and APMH were evaluated in media of different ionic strength (r_NIPAM = 0.7 and r_APMH = 0.7-1.2). Phase separation behavior of the random copolymers with only 5 mol% of the APMH was found to be suppressed in pure water at temperatures up to 45 °C due to electrostatic repulsion among the cationic amine groups randomly distributed along the copolymer chain. Alternate sequential addition of PNIPAM/APMH mixtures and pure NIPAM was used to provide increased control of the location of APMH units along the chain. Consequently (close to) homo-PNIPAM block(s) were formed as evidenced by its characteristic phase transition at 33 °C. The influences of the monomer feeding time and feeding interval time to the APMH distribution were investigated to prepare copolymers with thermo-induced phase separation under physiologically relevant temperature and to determine the extent of conjugation to poly(ethylene oxide).     

Electroactivity, electrochemical stability and electrical conductivity of multilayered films containing poly(3,4-ethylendioxythiophene) and poly(N-methylpyrrole)

Multilayered systems of poly(3,4-ethylendioxythiophene) and poly(N-methylpyrrole) have been prepared using a layer-by-layer electrodeposition technique. The electrochemical and electrical properties of films formed by 3, 5, 7 and 9 layers have been characterized and compared with those of pure polymers and copolymers prepared from mixtures of 3,4-ethylendioxythiophene and N-methylpyrrole with various concentration ratios. Results indicate that the electroactivity and electrical stability of the multilayered systems are higher than those of both poly(3,4-ethylendioxythiophene) and copolymers. Furthermore, these electrochemical properties improve when the number of layers increases. On the other hand, the electrical conductivity of the multilayered systems is slightly lower than that of pure poly(3,4-ethylendioxythiophene), and significantly higher than those of poly(N-methylpyrrole) and copolymers.     

Novel poly(N‐isopropylacrylamide)/clay/poly(acrylamide) IPN hydrogels with the response rate and drug release controlled by clay content

Novel interpenetrating network (IPN) hydrogels (PNIPAAm/clay/PAAm hydrogels) based on poly(N‐isopropylacrylamide) (PNIPAAm) crosslinked by inorganic clay and poly(acrylamide) (PAAm) crosslinked by organic crosslinker were prepared in situ by ultraviolet (UV) irradiation polymerization. The effects of clay content on temperature dependence of equilibrium swelling ratio, deswelling behavior, thermal behavior, and the interior morphology of resultant IPN hydrogels were investigated with the help of Fourier transform infrared spectroscopy, differential scanning calorimeter (DSC), scanning electron microscope (SEM). Study on temperature dependence of equilibrium swelling ratio showed that all IPN hydrogels exhibited temperature‐sensitivity. DSC further revealed that the temperature‐sensitivity was weakened with increasing amount of clay. Study on deswelling behavior revealed that IPN hydrogels had much faster response rate when comparing with PNIPAAm/clay hydrogels, and the response rate of IPN hydrogels could be controlled by clay content. SEM revealed that there existed difference in the interior morphology of IPN hydrogels between 20 [below lower critical solution temperature (LCST)] and 50 °C (above LCST), and this difference would become obvious with a decrease in clay content. For the standpoint of applications, oscillating swelling/deswelling behavior was investigated to determine whether properties of IPN hydrogels would be stable for potential applications. Bovine serum albumin (BSA) was used as model drug for in vitro experiment, the release data suggested that the controlled drug release could be achieved by modulating clay content. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 96–106, 2009     

Monodisperse and isolated microspheres of poly(N-methylaniline) prepared by dispersion polymerization

Monodisperse and isolated microspheres of poly(N-methylaniline) were successfully prepared through chemical polymerization of N-methylaniline by in adipic acid containing poly(vinylpyrrolidone) (PVP). Mean diameters of the microspheres with smooth surfaces changed from 320 to 100 nm by increasing the reaction temperature from 25 to 75 °C. The concentration of PVP did not affect much the size of microspheres, but the increased PVP concentration led to longer induction times for the onset of dispersion polymerization.