**3.1. Variations in chemical structure**

γ-Ray radiation differs from electron beam mainly by a much slower rate of dose due to γ-rays being always of less energy than 10 MeV. Therefore, γ-irradiation requires a longer period to supply the same dose as an electron beam. When a polymer is γ-irradiated in air, enough time is available for generated radicals to react with oxygen. Therefore, considerable oxidative degradation can be expected along with crosslinking. In contrast, an inert atmosphere during irradiation as well as after the irradiation suppresses oxidative degradation and supports crosslinking. However, the oxidative degradation cannot be excluded absolutely due to some portion of oxygen (and possibly also humidity) present in the polar PA matrix. These antici‐ pations were confirmed and both chain scission and crosslinking are observed to occur in PA-6 under γ-irradiation in either air or inert atmosphere, with chain scission prevailing over crosslinking if irradiation proceeds in air [30].

A thorough comparison is provided by a recent study [31] examining PA-6 and GF-reinforced PA-6 (30% GF) irradiated with different γ-ray doses in the range of 0 to 500 kGy in either air or inert atmosphere. As displayed in **Figure 14**, it can be seen that the irradiation in air generates a small amount of gel in PA-6 only, whereas no gel is found in the PA-6/GF composite.

**Figure 14.** Dependence of gel content in PA-6 and PA-6/GF composite on absorbed dose when γ-irradiated in air or inert atmosphere.

In the PA-6, the crosslinked portion irradiated in air increases slightly with the rising absorbed dose. Concerning the gel point, a calculation following the Charlesby-Pinner equation [14] using the experimental data and taking into consideration the experimental errors gives an estimation that the gel point is to be in the vicinity of a dose of 300 kGy for the PA-6 irradiated in both atmospheres. Such gel point is somewhat higher compared to 200 kGy dose value

determined for the same materials irradiated with electron beam in air [7]. The irradiation of PA-6 in argon atmosphere produced considerably more gel in comparison to exposure in air. It demonstrates oxygen influence. The absence of oxygen (or low content of it) affords better facilities to form crosslinks from generated macroradicals because the macroradicals are not attacked. In argon atmosphere, the gel point for PA-6 as well as PA-6/GF is observed to be approximately 300 kGy. However, also in inert atmosphere, PA-6 shows more gel (65%) than PA-6/GF (45%) for the ultimate dose of 500 kGy. The lower gel content in the composite confirms the retarding action of the filler on the networking of the PA matrix. Simultaneously measured solution viscosity for PA-6 increases up to 200 kGy, and above this dose, the viscosity could not already be measured correctly due to the incomplete dissolution of the PA-6 polymer indicating incoming gel point. The increase of the PA-6 solution viscosity below 200 kGy indicates growth in the molecular mass via the recombination of the secondary macroradicals formed on PA-6 chains, which leads to branching as pre-crosslinking stage up to the gel point [5,6]. Whereas the irradiation of PA-6/GF in air did not generate any gel within the applied dose, viscosity increased (see **Table 3**), illustrating some recombination of the macroradicals and relating growth in molecular mass. The solubility of the PA-6/GF matrix was observed within all doses corresponding with no gel content. The reduction of the viscosity beyond 350 kGy is attributed to the continuing branching of already branched chains. A consequence is that the amount of the particles and the corresponding gyration radius of the macromolecules decrease, reducing the viscosity.

**3. γ-Irradiation of PAs**

268 Radiation Effects in Materials

**3.1. Variations in chemical structure**

crosslinking if irradiation proceeds in air [30].

inert atmosphere.

In polymer chemistry, as for electron irradiation, γ-irradiation is employed to initiate chemical reactions also in the solid phase without the addition of initiator. The purpose can be to initiate the crosslinking of individual polymers or the grafting of various monomers onto PA chains. Basically with γ-irradiation of PAs, the same processes run as those with electron irradiation.

γ-Ray radiation differs from electron beam mainly by a much slower rate of dose due to γ-rays being always of less energy than 10 MeV. Therefore, γ-irradiation requires a longer period to supply the same dose as an electron beam. When a polymer is γ-irradiated in air, enough time is available for generated radicals to react with oxygen. Therefore, considerable oxidative degradation can be expected along with crosslinking. In contrast, an inert atmosphere during irradiation as well as after the irradiation suppresses oxidative degradation and supports crosslinking. However, the oxidative degradation cannot be excluded absolutely due to some portion of oxygen (and possibly also humidity) present in the polar PA matrix. These antici‐ pations were confirmed and both chain scission and crosslinking are observed to occur in PA-6 under γ-irradiation in either air or inert atmosphere, with chain scission prevailing over

A thorough comparison is provided by a recent study [31] examining PA-6 and GF-reinforced PA-6 (30% GF) irradiated with different γ-ray doses in the range of 0 to 500 kGy in either air or inert atmosphere. As displayed in **Figure 14**, it can be seen that the irradiation in air generates a small amount of gel in PA-6 only, whereas no gel is found in the PA-6/GF composite.

**Figure 14.** Dependence of gel content in PA-6 and PA-6/GF composite on absorbed dose when γ-irradiated in air or

In the PA-6, the crosslinked portion irradiated in air increases slightly with the rising absorbed dose. Concerning the gel point, a calculation following the Charlesby-Pinner equation [14] using the experimental data and taking into consideration the experimental errors gives an estimation that the gel point is to be in the vicinity of a dose of 300 kGy for the PA-6 irradiated in both atmospheres. Such gel point is somewhat higher compared to 200 kGy dose value

However, some differences occur due to variable conditions.


**Table 3.** Viscosity number of PA-6 and PA-6/GF irradiated with γ-rays in air (solvent formic acid, dose rate of 9.5 kGy/h).

Gupta and Pandey [30] presented a similar fact when, irradiating in air, the chain scission of PA-6 prevailed over crosslinking. Because γ-irradiation took a longer time (from 5 to 52 h), during this period, oxygen could attack the produced macroradicals. The result was the overbalance of scission due to oxidation over crosslinking. In addition, the presence of GF hindered the diffusion of the macroradicals in the matrix, so that the branching was supported and crosslinking was suppressed.

An opposite effect can occur as reported by Aytaç et al. [13] for PA-6 and PA-6,6 tyre cords being calendered and then γ-irradiated within 0 to 75 kGy in air. As already mentioned about the electron exposure of those materials (Section 2.1), also the γ-irradiation of those PAs led to the reduction of the limiting viscosity number with increasing dose. This demonstrates how any pretreatment of material subsequently subjected to the irradiation can be important. In general, calendering itself can start mechano-oxidative degradation leading to the decrease in molecular mass as well as in the corresponding viscosity. In addition, during the calendering, the diffusion of oxygen into the PAs is more facile, as the molecular movement in matrix is increased at higher temperature. From this point of view, the decrease in the limiting viscosity number is expected. The decrease is larger than that under comparable exposure to electron beam, indicating more deteriorative effect of γ-irradiation in comparison to electron beam. Corresponding dependences of the breaking strength on dose show a lower strength, too, in conformity with the viscosity results.

PAs are combined with other polymers in various microfiltration membranes especially to enhance the mechanical properties. The membranes are exposed to γ-irradiation to be sterilized. Such a membrane involving PA-66 as reinforcing part was put in Pyrex glass, purged with argon, and, after adding deionized water again, purged with argon and sealed. The glass with the membrane was γ-irradiated in the range of 0 to 100 kGy and then several character‐ istics were tested [32]. At first sight, this is different from the PA-6 and PA-6,6 cords mentioned previously [13]. However, also in this case, the reduced viscosity as a function of dose displays a decrease with dose (**Figure 15**).

**Figure 15.** Evolution of reduced viscosity of PA-66 as a function of γ-irradiation doses [32]. With permission of Elsevi‐ er.

The downward trend of the reduced viscosity indicates the decrease in molecular mass due to chain scission despite the exposure being conducted in argon purged deionized water. The authors [32] suggested that the viscosity reduction could be due to the chains connecting different lamellae. However, the water medium is a rich source of oxygen and could be the main reason in supporting oxidative degradation. Free radicals generated by γ-rays induce the cleavage of water molecules. In addition, water penetrates PA relatively easy acting as a plasticizer, and the applied dose within 10 to 100 kGy is sufficient to attain the dissociation energy for water producing oxygen (HO-H ~498 kJ mol-1 and O-H ~428 kJ mol-1) and that is comparable to the dissociation energy for H-C cleavage (H-C ~339 kJ mol-1, H-CH ~452 kJ mol-1, and H-CH2 ~473 kJ mol-1) [33]. That is why corresponding tensile properties (Young's modulus, stress at break, elongation, and energy at break) [32] mirrors the course of the decreasing viscosity.

Radicals generated during irradiation are capable of surviving in the polymer matrix for a long time as found by Menchaca et al. [34,35] after 6 years from exposure of PA-6,12 crystalline fibers with applied low dose of 1 to 25 kGy at ambient conditions. The survival of frozen free radicals in the matrix demonstrated itself by the changes in some properties as thermal and morphology characteristics; the melting temperature decreased and the crystallinity increased with the period of storing. It indicates that formed shorter chains generated thinner lamellae and integrated into the crystalline phase.

Applying γ-irradiation can also lead to an improvement in the compatibilization of immiscible polymers in a blend. An example is the blend of PA-6 with LDPE either irradiated or nonir‐ radiated [36]. The structure and properties of the blends with γ-irradiated LDPE differ significantly compared to PA-6 blends with the nonirradiated materials. The difference was ascribed to the formation of functionalized groups on the polyethylene chain during irradia‐ tion in air and these interact with PA. However, when analogous blends of LDPE/PA-6 are irradiated in vacuum [37], crosslinking is achieved mainly in the PE component, whereas the main effect on PA-6 is chain branching.
