**3.2. Thermal properties related to supermolecular structure**

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

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

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

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

conformity with the viscosity results.

270 Radiation Effects in Materials

a decrease with dose (**Figure 15**).

er.

A comparison of DSC characteristics for PA-6 and composite PA-6/GF after being γ-irradiated in air or in inert [31] within 0 to 500 kGy can provide some framework observations. Similar to Section 2.2, both first and second heating runs give complementary information. First, the effect of irradiation dose on *T*m1 for PA-6 is qualitatively the same if irradiated in air or inert atmosphere; however, the absolute values for each particular dose indicate lower *T*m1 values for samples irradiated in air (**Figure 16**).

**Figure 16.** Variation of the first melting temperature of PA-6 and composite PA-6/GF (30%) with absorbed dose of γirradiation in air and inert atmosphere. Adapted from Porubská et al. [31].

The effect of GF presence consists of a certain soft increase of sensitivity towards irradiation, which is in certain contradiction with lower gel formation. It may mean that the presence of GF can act as a nucleating agent. If so, the crystallites are formed especially around the fiber surface. However, during exposure, the fiber surface is more heated than the matrix and some destruction in crystalline portion occurs in this area. That is why the noticeable decrease in *T*m1 is observed for the composite irradiated in inert atmosphere.

The figures of melting heat Δ*H*<sup>1</sup> (**Figure 17**), directly proportional to crystallinity, do not show any tendency with increasing dose in air, changing only within variance of the experimental results.

**Figure 17.** Variation of the first melting enthalpy of PA-6 and composite PA-6/GF (30%) with absorbed dose of γ-irra‐ diation in air and inert atmosphere. Adapted from Porubská et al. [31].

It can be seen that the values of Δ*H* of both PA-6 and composite PA-6/GF samples irradiated in air are higher compared to those irradiated in inert atmosphere. The course of changes of Δ*H* measured in the second DSC run, Δ*H*2 (**Figure 18**), depending on the absorbed dose is in conformity with expectation, consisting of a gradual decrease of Δ*H*2 values with rising dose. Irradiation in air results in higher melting heat values than irradiation in inert atmosphere. In this case, perhaps the important role consists of heating the samples during irradiation and the extent of heating depends on the absorbed dose.

All samples show a decrease in remelting temperature *T*m2 with rising dose (**Figure 19**), which is conformable with expectation. The reason is that the crystallization of polymer remelted after being irradiated is hindered due to the defects generated in polymer within irradiation. However, the same decrease in *T*m1 would not be expected if the irradiation impose structural changes in amorphous phase first. Because *T*m and Δ*H* vary, the crystalline phase also is affected by irradiation, inducing the lamellae thinning. It is evident that the process runs already from the beginning of irradiation.

The effect of GF presence consists of a certain soft increase of sensitivity towards irradiation, which is in certain contradiction with lower gel formation. It may mean that the presence of GF can act as a nucleating agent. If so, the crystallites are formed especially around the fiber surface. However, during exposure, the fiber surface is more heated than the matrix and some destruction in crystalline portion occurs in this area. That is why the noticeable decrease in *T*m1

The figures of melting heat Δ*H*<sup>1</sup> (**Figure 17**), directly proportional to crystallinity, do not show any tendency with increasing dose in air, changing only within variance of the experimental

**Figure 17.** Variation of the first melting enthalpy of PA-6 and composite PA-6/GF (30%) with absorbed dose of γ-irra‐

It can be seen that the values of Δ*H* of both PA-6 and composite PA-6/GF samples irradiated in air are higher compared to those irradiated in inert atmosphere. The course of changes of Δ*H* measured in the second DSC run, Δ*H*2 (**Figure 18**), depending on the absorbed dose is in conformity with expectation, consisting of a gradual decrease of Δ*H*2 values with rising dose. Irradiation in air results in higher melting heat values than irradiation in inert atmosphere. In this case, perhaps the important role consists of heating the samples during irradiation and

All samples show a decrease in remelting temperature *T*m2 with rising dose (**Figure 19**), which is conformable with expectation. The reason is that the crystallization of polymer remelted after being irradiated is hindered due to the defects generated in polymer within irradiation. However, the same decrease in *T*m1 would not be expected if the irradiation impose structural changes in amorphous phase first. Because *T*m and Δ*H* vary, the crystalline phase also is affected by irradiation, inducing the lamellae thinning. It is evident that the process runs

is observed for the composite irradiated in inert atmosphere.

diation in air and inert atmosphere. Adapted from Porubská et al. [31].

the extent of heating depends on the absorbed dose.

already from the beginning of irradiation.

results.

272 Radiation Effects in Materials

**Figure 18.** Variation of the second melting enthalpy of PA-6 and composite PA-6/GF (30%) with absorbed dose of γirradiation in air and inert atmosphere. Adapted from Porubská et al. [31].

**Figure 19.** Variation of the second melting temperature of PA-6 and composite PA-6/GF (30%) with absorbed dose of γ-irradiation in air and inert atmosphere. Adapted from Porubská et al. [31].

Similar to electron beam irradiation, the presence of crosslinking agents can modify the crystallinity of γ-irradiated PAs, too. Electrospun PA-66 fibers exposed in nitrogen atmosphere with 20 and 50 kGy dose do not display any change in crystallinity, whereas the addition of TAC raises the crystallinity and this increases with applied dose [38]. This fact is interpreted as the consequence of a TAC-induced crosslinking and formation of a tighter network. Thermal stability measured as weight loss depending on temperature is lower with TAC than without it. After irradiation, the stability with TAC improves, however, without achievement of stability for nonadditive PA-66, whereby the dose of 20 kGy provides better result than 50 kGy.

The exposure of PA-66 to various doses of γ-rays ranging from 100 to 1250 kGy shows an increase in the crystalline nature of the polymer at higher doses as a result of significant decrease in the peak width of X-ray diffraction (XRD) patterns [39]. Higher doses induce more macromolecular fragments of higher mobility and these can integrate easily into crystalline phase modifying supermolecular structure.

Waste PA finds exploitation in various material combinations. Hassan et al. [40] studied the effect of γ-irradiation on blends containing waste PA-6/PA-66 copolymer and ground rubber from tires with various ratios of these incompatible components. The blends irradiated in the range of 0 to 200 kGy give the melting temperature and crystallinity decreasing with increasing dose due to the crosslinking at interphase. The visible side shoulder in the endotherm for 100 kGy is missing in the 200 kGy endotherm and microphotographs show a relatively smooth fracture surface. Thermal stability measured by thermogravimetry is a little worse after irradiation. Montmorillonite clay is then added into the blend to formulate nanocomposite [41]. The composite after being γ-irradiated between 0 and 200 kGy obtains a markedly magnified thermal stability. When 12% montmorillonite is present in the mixture, the DSC data indicate the increase in the melting temperature with dose with reverse order of the onset in melting endotherm. The crystallinity is observed to be highest for the 100 kGy dose and the corresponding endotherm outlines multiplicity. The increase of montmorillonite portion to 18% leads to a decrease in the melting temperature. Also, the temperature onset falls. In this case, the highest crystallinity belongs to the nonirradiated composite and is followed by the 200 kGy dose. The dose of 100 kGy corresponds to the lowest crystallinity with the most structured melting endotherm. The multiplicity of the endotherm indicates a new element in the supermolecular structure as a consequence of γ-irradiation. Another composite consisting of the same polymer components PA-6/PA-6,6 copolymer and ground rubber but with added carbon black was examined by the same authors [42], applying the same doses of 0 to 200 kGy. As reported, the content of carbon black within 6% to 24% improves the thermal stability in both cases without and with γ-irradiation. The melting temperature and crystallinity of the composite with 12% carbon black decrease with rising dose slightly more when compared to 18%. The melting endotherm becomes smoother and the composite irradiated with 200 kGy presents a homogeneous fracture surface. Such studies are useful in optimizing a filler portion regarding other required properties. Usually, some compromise is necessary.
