**3. Results and Discussion**

### *3.1. Grafting of Hydrogels onto PET-Fabrics*

Grafts of HEMA or HEMA/PEGMA were introduced by the photochemical or the direct gamma irradiation methods; grafting percentage was calculated using the Equation (1). Tables 1 and 2 show the grafting conditions and the hydrogel percentage obtained by the photochemical and the direct gamma irradiation methods, respectively. Results showed that when using the photochemical method, the grafting yield was between 49% and 96%, whereas using the gamma radiation method, it was between 210% and 400%, both related to the weight of the unmodified PET-fabric. The high grafting percentages obtained by gamma irradiation are not good for the application of PET as fabric owing to their increased thickness; even at 50 kGy of irradiation dose, the percentage of grafted hydrogel was too high (285%). These high values may have resulted due to the higher monomer concentrations used, as well as owing to the high radiochemical yield of the monomers and the solvent used. One option to tailor down the grafting percentage could be the use of a mold with a spacer of adequate thickness or the use of a modified grafting method: The so-called oxidative pre-irradiation method previously described [41].

An important step in the synthesis of hydrogels on the PET fabric is the initiation of the polymerization. In this stage, with the photochemical method, free radicals are generated by UV radiation evolving from the photoinitiator (Irgacure ® 651), since this dissociates after the absorption of photons, generating benzoyl and dimethoxybenzoyl radicals, the latter tending to form a methyl radical; both benzoyl radicals and methyl radicals are involved in the polymerization reaction, however the benzoyl radical is more stable than the methyl radical [42].

Scheme 1 shows the reaction mechanism for the hydrogels cross-linked with EGDMA, initiated by the photochemical dissociation of the photoinitiator. In the direct gamma irradiation method, the PET backbone can produce two types of free radicals under γ-ray radiation, but –[CH2\_CHOOCC ˙ 6H4COO]– is the predominant free radical [25,43]. The main procedure possibly involved in the graft copolymerization induced by γ-rays can be described as shown in Scheme 2.

The reactions were carried out in aqueous medium to favor a rapid formation of free radicals on the reagents used, where the reaction is initiated by the formation of free radicals by the radiolysis of deionized water, after the formation of radicals, the coupling of the di fferent species of radicals is favored, forming grafts of polymer networks attached to the PET fabric. Please note that in this case, EGDMA crosslinker was not added.

**Scheme 1.** Mechanism of photochemical cross-linking of 2-hydroxyethyl methacrylate (HEMA) and poly (ethylene glycol) methacrylate (PEGMA) onto PET fabric.

**Scheme 2.** Possible mechanism of grafting HEMA and PEGMA onto PET fabric by γ-ray induced graft copolymerization.

### 3.1.1. Characterization of Hydrogel-Grafted PET-Fabrics
