Water uptake of hydrophilic polymers determined by a thermal gravimetric analyzer with a controlled humidity chamber

Abstract

The moisture uptake of several water-soluble polymers at different humidities was investigated with a thermal gravimetric analyzer equipped with a controlled humidity chamber. The water sorption of poly(acrylic acid) sodium salt, poly(ethylene glycol) and silica, which are known as super absorbers, were examined. In addition, various hydrophilic polymeric materials were selected according to their structural features. These included hydroxyl functions on the side chains (e.g. poly(2-hydroxyethyl methacrylate)), as well as acidic or basic functionalities (e.g. poly (dimethylaminoethyl methacrylate) or poly(vinylimidazole)). In addition, poly(2-methyl-2oxazoline) (P(MeOx)) and poly(2-ethyl-2-oxazoline) (P(EtOx)), which are well-known hydrophilic polymers, were also investigated in this context. More significant weight percent changes were obtained for P(MeOx) (60% at 90% relative humidity (RH)) in comparison to P(EtOx) (35% at 90% RH) as a result of the slight difference in hydrophilicity of the structures. The effect of the chain length on the ability for water uptake was also investigated for both poly(oxazolines). Finally, thermoresponsive polymers with a lower critical solution temperature (LCST) behavior (e.g. poly(N-isopropylacrylamide) and poly(dimethylaminoethyl methacrylate)) were also examined. The measurements for the latter polymers were performed below and above the LCST of each polymer whereby the humidities are varied from 0 to 90% with steps of 10%. Upon increasing humidity, the results revealed relatively high water uptake values (8% and 22% for P(NIPAM) and for P(DMAEMA), respectively) below the LCSTs of the polymers and, contrastingly, a small weight loss above their LCSTs. The present results allow a deeper insight into important structure-property relationships (e.g. the influence of the polymer backbone, functional groups, LCST behavior, etc. on the water-uptake properties), and will in subsequent steps permit the directed design of tailor-made polymers for selected applications.

Study of Water Adsorption in Poly(N-isopropylacrylamide) Thin Films Using Fluorescence Emission of 3-Hydroxyflavone Probes

“…The amount of adsorbed water is potentially considerable, since at 90% RH and at a temperature of 20 ºC, bulk pNIPAM is capable of adsorbing a maximum of ~8% of its own weight in water. 55 Now for all of the probes, we see dramatic changes in the log(I N* /I T* ) ratio (Figure 5a), greater red shift of the N* band (Figure 4), decreases in fluorescence intensity (Figure 2), but no significant change in local heterogeneity experienced by the excited states (Figure 3c) ([fwhm, decreases for N* state, indicating a reduction in heterogeneity of the emitting states]. So this second phase hydration produces spectral effects similar to those produced by the common solvent polarity effect with increasing dielectric stabilization of the N* state causing a greater red shift of the N* band, and a large relative increase in the log(I N* /I T* ) ratio.…”

Advanced microgel-functionalized polyester textiles adaptive to ambient conditions

“…This delayed moisture absorption of PET RCM 1 compared with PET R 1 can be attributed to a similar suggestion made previously about an "incubation" period needed until CM complexes start to absorb water. A similar concept was reported in Thijs et al 2007; there, the formation of a "first hydration shell" on dried hydrophilic polymers, conditioned under various RH values, was conjured to explain sharp increases in the moisture uptakes of these polymers. In the same study it was concluded that there was a certain difficulty in the formation of a first hydration shell, after which additional hydration shells could form more easily.…”

“…Therefore, they could retain moisture for longer periods of time, and could possibly be used in slow release systems. On the other hand, the limited hysteresis of PET RM 1 is not surprising; isothermic curves of similar shape have been reported for pure PNIPAAm (Thijs et al 2007). The lower moisture uptake values of PET RM 1, compared with PET RCM 1, can be attributed to the almost continuous layer that microgel M forms on polyester.…”

“…The decision to test at RH values higher than 65%, rather than lower, was based on the expectation that changes in moisture contents would be more pronounced. In fact, it was reported in Thijs et al 2007 that even dried superabsorbent polymers like poly(acrylic acid)-sodium salt and poly(ethylene glycol) exhibit a steep moisture uptake between 60% and 90% RH, but not at lower RH. The equilibrium moisture uptakes (sorption) or losses (desorption) of PET R 1 and PET RCM 1 were plotted against RH at 20°C, and the corresponding curves are shown in Figure 3.14a.…”

Highly Deliquescent Cationic Polyether with Imidazolium Halide Group

“…Note that poly(acrylic acid) sodium salt and poly(ethylene oxide) were reported to absorb a little moisture under relatively dry conditions (30°C, 40% RH; water uptake <5 wt %); in contrast, poly(ECH-MeIm + Cl ¹ ) is deliquescent at similar conditions (vide supra). 27 With regards to the contribution of the counter anion, poly(N-glycidyl-N¤-methylimidazolium bis(trifluoromethanesulfonyl)imide) (M n(calc) = 11300; DP = 27) (poly(ECH-MeIm + TFSI ¹ )) was hydrophobic (23°C, 40% RH; water uptake <0.2 wt %). To summarize, the deliquescent character of the present cationic polyethers is supposed to be derived from the combination of both the imidazolium halide group and polyether chain.…”