Dynamic Studies on the Response to Humidity of Poly (2-hydroxyethyl methacrylate) Hydrogels Produced by Initiated Chemical Vapor Deposition

Abstract

The swelling of 2‐hydroxyethyl methacrylate (HEMA) hydrogels, commonly synthesized by solution polymerization, has been widely studied for its practical and fundamental importance. While a good understanding of polymer‐liquid interactions has been established within the Flory‐Huggins framework, little is known about polymer‐water vapor interactions. In this study, the swelling of pHEMA films in response to water and humidity is investigated by means of X‐ray reflectivity and spectroscopic ellipsometry. The hydrogels are synthesized by initiated chemical vapor deposition, using ethylene glycol dimethacrylate (EGDMA) as a crosslinking agent. In water, copolymers of p(HEMA‐co‐EGDMA) are stable for an EGDMA fraction above 17%. In a humid atmosphere, even samples with no EGDMA are stable. On increasing the EGDMA fraction, both the swellability in water and in humid environments decreases. The swelling in both environments is related to the mesh size and solubility of the hydrogel, using the Flory‐Huggins theory. The mesh size (average length between two crosslinks) decreases with increasing EGDMA fraction, as expected. The interaction parameter between water vapor and the polymer decreases with increasing relative humidity and reaches 1 ± 0.1 at RH > 75%.

Stabilizing the Wettability of Initiated Chemical Vapor Deposited (iCVD) Polydivinylbenzene Thin Films by Thermal Annealing

“…Initiated chemical vapor deposition (iCVD) is a vapor‐phase process for growing conformal polymer thin films, and has been shown as a powerful and versatile technique for modification of surface energy . Hydrophilic thin films synthesized via iCVD can exhibit humidity response, antifouling properties, and hydrophilicity‐enhanced separation performance, while the applications of hydrophobic iCVD polymers include hydrophobicity‐based membrane separation, microfluidic devices, corrosion protection, and icephobic/anti‐hydrate coatings . Recently, an iCVD copolymer of divinylbenzene (DVB) with 1H,1H,2H,2H‐perfluorodecyl acrylate (P(PFDA‐ co ‐DVB)) has been demonstrated with excellent performance for enabling dropwise condensation .…”

Manipulating drug release from tridimensional porous substrates coated by initiated chemical vapor deposition

“…It should be mentioned that these values can be adjusted by the chemical composition of the iCVD layer. In particular, the degree of crosslinking is here of great importance: the more crosslinking is present the smaller is the capability of swelling . Unfortunately, similar in situ ellipsometric measurements are not possible on the fabrics, due to the geometry of the fibers.…”

“…In particular, the degree of crosslinking is here of great importance: the more crosslinking is present the smaller is the capability of swelling. 23 Unfortunately, similar in situ ellipsometric measurements are not possible on the fabrics, due to the geometry of the fibers. An attempt was made of measuring the swelling by gravimetric analysis but the results were not satisfactory since the weight of the substrates was much higher than the weight of the thin hydrogel film which made an estimation error prone.…”

Fast Humidity Sensors for Harsh Environment

“…In a second iteration of the setup the hydrogel got deposited onto the tip of a fiber probe. A detailed description of the deposition technique can be found in the work by Unger et al [3]. In order to apply the sensor also in harsh environment, no parts for the optical detection were placed on the side of the hydrogel thin-film, meaning that the optical setup was installed on the backside of the substrate.…”

“…The presented hygrometer is a first principle measurement system, that is based on an optical measurement of the swelling of a hydrogel thin-film, which is directly related to the relative humidity of the surrounding atmosphere. In former work it was shown, that Poly 2-hydroxyethyl methacrylate (pHEMA), deposited with a method called initiated chemical vapor deposition (iCVD), provides excellent swelling behavior [3]. The correlation of the relative thickness change and the relative humidity RH is described by the Flory-Huggins theory which applies for the used kind of hydrogel [4,5].…”