The composition of the copolymers was determined by infra-red spectral analysis on Perkin-Elmer (Spectrum 100) FTIR spectrophotometer. The copolymers were analyzed as thin films on sodium chloride windows by the method described previously [15
The solubility of the copolymers was studied in various solvents. About 2 mL of each solvent was added to about 0.02 g of the polymer in a screw capped glass vial. The mixture was allowed to stand for 24 h at 23 °C, and a visual observation on the solubility was recorded.
The weight-average molecular weight (MW), the second-virial co-efficient (A2), and the hydrodynamic radius (Rh) of the copolymers was determined using a Zetasizer (Malvern Zetasizer ZS90) with a Helium-Neon laser (λ = 633 nm). The copolymer samples in distilled THF were filtered three times through a syringe filter (Teflon, 0.45 micron) into a clean rectangular quartz cell. For the determination of Mw, the scattered light intensity of six different concentration of the polymer sample was measured at a scattering angle of 90°. The Mw and A2 were determined by the Debye approximation as,
where K, R(θ), and c are the optical constant, Rayleigh ratio, and polymer concentration respectively. The optical constant K is defined as,
where n0, λ, NA, and dn/dc are the refractive index of toluene, wavelength of laser, Avogadro’s constant, and the refractive index increment of the polymer respectively.
For the determination of Rh, the normalized intensity-time auto-correlation function (g2(t) − 1) was measured for 60 s. The auto-correlations were analyzed by Laplace inversion using the model-independent CONTIN program to obtain the average Rh by the Stokes-Einstein relation as,
where kB, T, η, and D are the Boltzmann constant, temperature, viscosity of the solvent, and diffusion respectively.
The LCST of the copolymers were determined both by visual observation and on a UV/Vis spectrophotometer attached with a temperature control. Polymer solutions of 1 wt% were used in this study.
Differential scanning calorimetric measurements were recorded using a Perkin-Elmer diamond DSC. About 10 mg of polymer sample sealed in aluminum pan was used for this measurement. All measurements were carried out under a stream of dry nitrogen with a heating rate of 20 °C∙min−1. The glass transition temperature (Tg) was determined from the mid-point of the inflection of the thermogram.
Micro calorimetric measurements were recorded on a Microcal MC-2 high sensitivity differential scanning calorimeter (Microcal, Northampton, USA). Both the polymer solution and water (reference) were degassed and transferred to the cells using long needle syringes (Hamilton). The enthalpy change for the transition (ΔH) was calculated from the area of the transition peak, and LCST was determined from the maximum of the first derivative of the heat capacity versus temperature plot.
Potentiometric titrations of the polymers were performed using an ABU93 Triburette Titration System (Radiometer, Denmark). The instrument was integrated with a standard RS232C interface and the titration was controlled using the ALIQUOT titration software. The electrode assembly consisted of an Orion pHg201 glass electrode and an Orion REF201 reference electrode. The conductivity was measured using a CDM83 conductivity meter.