Thermal micro ATR/FT-IR spectroscopic system for quantitative study of the molecular structure of poly(N-isopropylacrylamide) in water
Introduction
Recently, poly (N-isopropylacrylamide) (PNIPAAM) as an intelligent polymer has received growing interest in drug delivery systems and in biotechnology [1], [2], [3], [4], [5], [6], due to its thermal-responsive function. PNIPAAM has not only hydrophilic groups (NH, CO) but also a hydrophobic group (isopropyl), thus the hydrophilic interaction as well as hydrophobic interaction may play a dominant role in the thermo-shrinking transition [7]. It is well known that PNIPAAM is insoluble in water above the lower critical solution temperature (LCST), about 32°–33°C [7], [8], and is reversibly solubilized below that temperature when PNIPAAM exists as flexible but extended coils in aqueous solution. Above the LCST, it shrinks as aggregates. The driving force for this coil-to-globule transition is associated with the temperature-dependent molecular interactions, mainly hydrogen bonding and hydrophobic interaction [7], [9], [10]. However, there has been much debate as to whether hydrophobic effect or hydrogen bonding effect is a dominant driving force in PNIPAAM aqueous solution below and above LCST. Moreover, how much quantitative contribution that both effects can afford to molecular interactions is hardly studied.
Several evidences have demonstrated the usefulness of attenuated total reflection/Fourier transform infrared (ATR/FT-IR) spectroscopy in the non-destructive analysis of biological tissues and biomaterials, quantitative determination of polymer coating, and reaction kinetics and structure development in polymer processing [11], [12], [13], [14], [15], [16], [17], [18]. We have previously explored a series of studies by using the reflectance or transmission microscopic FT-IR/DSC system to simultaneously investigate the polymorphic and glass transition temperature of drugs, polymers and liquid crystal, the lipid thermotropic transition and protein conversion in skin, and the kinetics of thermal-dependent condensation of polymer [19], [20], [21], [22], [23], [24], [25], [26]. In this study, we report the first use of a newly developed thermal micro attenuated total reflection/Fourier transform infrared (ATR/FT-IR) spectroscopic system with curve-fitting program to quantitatively investigate the molecular structure of PNIPAAM in water below and above their LCST.
Section snippets
Polymerization
N-isopropylacrylamide (NIPAAM, Eastman Kodak Co., USA) was recrystallized from n-hexane. NIPAAM (3 g) was dissolved in 30 ml of deionized, distilled water containing 0.6 g of ammonium persulfate and bubbled with nitrogen to remove the dissolved oxygen. This solution was poured into 150 ml of paraffin oil containing 1% of pluronic L-61, which was previously purged with nitrogen. The agitation speed was 500 rpm in a three-necked reaction bottle, and nitrogen was continously supplied in the course of
Results and discussion
The cloud point was determined by spectrophotometric detection of the changes in the turbidity of sample solution heated at different temperatures. We have found that the turbidity of PNIPAAM aqueous solution changed gradually with the increase of temperature but increased deeply after 33°C, which may be related to LCST, and the onset of turbidity was not so sharp. Moreover, a clear endothermic peak at 34.5°C was obviously observed in the DSC thermogram in which the onset temperature was also
Conclusions
A newly developed thermal micro ATR/FT-IR spectroscopic system has been successfully and quantitatively applied to investigate the LCST and the molecular structure of PNIPAAM in water. Below the LCST, the molecular structure of PNIPAAM in water exhibited a predominantly intermolecular hydrogen bonding (about 50%–70% of total molecular interaction estimated from amide I band) between PNIPAAM and water. However, above the LCST the intramolecular hydrogen bonded conformation was about 70% in total
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