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

Differential scanning calorimetry (DSC) is a useful tool to examine the structural variations in irradiated polymeric materials. Two basic types of data can be obtained from DSC curves, melting or crystallization temperature (*T*m or *T*c) and melting or crystallization enthalpy (Δ*H*<sup>m</sup> or Δ*H*c). The former is connected with the size of the crystalline units, whereas the last corresponds to the crystalline portion. The larger the crystalline unit, the higher *T*m observed. Similarly, the more the crystalline phase in the polymer, the higher the melting enthalpy measured. In general, for most polymers fully or partially crosslinkable by electron beam irradiation, the dependences of both melting temperature and melting enthalpy on the dose measured from the first DSC heating reflect immediate changes in the original crystalline structure occurring during irradiation. The data from the second melting demonstrates the overall changes particularly due to irradiation-induced crosslinking and partial degradation, both occurring during irradiation mainly in the amorphous phase. The original crystalline structure melts within first heating. The following cooling leads to the crystallization of the melt of the irradiated polymer, so that the effect is related to the overall degree of crosslinking, representing defects hindering the crystalline structure formation after melting during the first heating [2]. The kinetic parameters of processes linked to the heat exchange can be determined as well.

Because PAs are semicrystalline polymers, DSC is used often to characterize them. As mentioned previously, the crosslinking mainly occurs in the amorphous phase. The effects of dose on the *T*m determined for virgin PA-6 in the first and second runs (melting *T*m1 and remelting *T*m2) are displayed in **Figure 3**, whereas **Figure 4** shows the dependence of melting and remelting enthalpies. A monotonous decrease in the *T*<sup>m</sup> is observed over the 0 to 500 kGy dose range and the second run (*T*m2) is lower than the first run (*T*m1) [7,16].

crosslinking agents. The filler increases the viscosity of the matrix, which leads to a retardation of the cross-bond formation when compared to virgin PA due to slower macroradical recom‐

Concerning gel formation, the application of electron beam irradiation on incompatible polymeric blends or layers in laminates to generate free radicals and the following interaction, the grafting, between the components to improve their compatibility, can vary from case to case. Gel point shifts depending on the character of the polymeric components as well as possible crosslinking agent addition are shown in several works involving PA-6 [19–21]. Rare data on electron beam effect on gel creation in PA-12 can be found for ethylene propylene diene monomer (EPDM)/PA-12/maleated EPDM blend and these do not show any PA-in origin gel

It is worth mentioning that crosslinking becomes easier as the number of methylene groups between the amide groups increases [23,24]. This finding is based on the gel measurement on PA-6, PA-610, and PA-12 irradiated under the same conditions. This conclusion is under‐ standable because, as mentioned above, the initiation starts through a hydrocarbon sequence. The longer the hydrocarbon sequence, the more probable the hydrogen abstraction occurs.

Differential scanning calorimetry (DSC) is a useful tool to examine the structural variations in irradiated polymeric materials. Two basic types of data can be obtained from DSC curves, melting or crystallization temperature (*T*m or *T*c) and melting or crystallization enthalpy (Δ*H*<sup>m</sup> or Δ*H*c). The former is connected with the size of the crystalline units, whereas the last corresponds to the crystalline portion. The larger the crystalline unit, the higher *T*m observed. Similarly, the more the crystalline phase in the polymer, the higher the melting enthalpy measured. In general, for most polymers fully or partially crosslinkable by electron beam irradiation, the dependences of both melting temperature and melting enthalpy on the dose measured from the first DSC heating reflect immediate changes in the original crystalline structure occurring during irradiation. The data from the second melting demonstrates the overall changes particularly due to irradiation-induced crosslinking and partial degradation, both occurring during irradiation mainly in the amorphous phase. The original crystalline structure melts within first heating. The following cooling leads to the crystallization of the melt of the irradiated polymer, so that the effect is related to the overall degree of crosslinking, representing defects hindering the crystalline structure formation after melting during the first heating [2]. The kinetic parameters of processes linked to the heat exchange can be determined

Because PAs are semicrystalline polymers, DSC is used often to characterize them. As mentioned previously, the crosslinking mainly occurs in the amorphous phase. The effects of dose on the *T*m determined for virgin PA-6 in the first and second runs (melting *T*m1 and remelting *T*m2) are displayed in **Figure 3**, whereas **Figure 4** shows the dependence of melting and remelting enthalpies. A monotonous decrease in the *T*<sup>m</sup> is observed over the 0 to 500 kGy

dose range and the second run (*T*m2) is lower than the first run (*T*m1) [7,16].

bination, leaving more time for disproportionation.

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

up to 200 kGy [22].

256 Radiation Effects in Materials

as well.

**Figure 3.** Variation of the first and second melting temperatures with absorbed electron beam dose for virgin PA-6 [7]. With permission of Elsevier.

**Figure 4.** Variation of the first and second melting enthalpy with absorbed electron beam dose for virgin PA-6 [7]. With permission of Elsevier.

Such behavior is typical for the second run of irradiated crosslinkable polymers after melting and following the crystallization of a previously crosslinked material. The decrease of *T*m1 points to the thinning of the lamellae as a consequence of crystalline phase disruption within irradiation. The decrease in Δ*H*1 indicates this fact, too. Consequently, it can be implicated that, during irradiation also, a considerable degradation occurs along with crosslinking. Similar facts are observed, irradiating PA-6 at temperatures above *T*g with doses of 0 to 1200 kGy. *T*<sup>m</sup> decreased from 224°C for 0 kGy up to 213°C for 1000 kGy. However, irradiation at RT leads to a decrease of *T*m up to 600 kGy only with a subsequent mild increase [10]. One possible explanation may be the formation of larger crystallites due to a higher generation of smaller fragments from PA chains at higher doses and those being more mobile incorporate into existing crystallites more easy. This suggestion is in compliance with the finding that the samples irradiated above *T*g showed a predominance of the crosslinking over scission (Sec‐ tion 2.1).

The effect of irradiation on crystallinity is linked with the values of the melting enthalpy directly proportionally, and some decrease is observed for virgin PA-6 with rising dose (**Figure 4**). The higher value of Δ*H*m2 than Δ*H*m1 for PA-6 (**Figure 4**) is a consequence of different cooling rates between cooling the testing specimens after preparation by injection molding (using a higher initial cooling rate from the processing temperature of ~290°C) and cooling of the sample after first heating in the DSC cell (applied cooling rate of 10°C/min). Therefore, a lower crystalline portion is the result for rapidly cooled materials. The irradiation effect on PA-6 is also observable on the shape of the DSC melting curves (**Figure 5**).

**Figure 5.** DSC melting endotherms of first melting (150–250°C) for PA-6 irradiated with electron beam.

The endotherms of irradiated samples become wider with increasing dose as a consequence of the progressive lamella thinning and amorphization that took place under electron beam irradiation. Additionally, the enlargement in the peaks indicates that the distribution of crystallite size becomes broader while the unit endotherm surface (Δ*H*m) decreases.

PA-6 included in a multilayer film and irradiated in nitrogen applying a dose in the range of 0 to 150 kGy shows only a small total decrease in melting temperature at approximately 1°C and the crystallinity is diminished from 25.3% to 21.5% [18].

The influence of the crosslinking agent TAC on PA-6 irradiated with relatively low doses up to 100 kGy demonstrates a more pronounced effect on *T*m and crystallinity than present in virgin PA-6. The decrease in both *T*<sup>m</sup> and crystallinity for the doped PA is clearly observable already for the 40 kGy dose, whereas, for the virgin PA-6, the decrease is moderate, if any. The decrease in *T*<sup>m</sup> seems to be dependent on TAC content (the more TAC, the lower the Tm), but the measure of the decrease in crystallinity is not affected by TAC content in the range of 1% to 3% [6].

The development of thermal characteristics with dose can somewhat vary when PA is part of a blend or composite. PAs are often used as reinforced composites or filled with various fillers, with GF being the most common reinforcing additive. There is not much information on the crosslinking of reinforced PAs (PA/GF) in the scientific literature. Concerning the changes in the melting temperature (**Figure 6**) and enthalpy of PA-6/GF (**Figure 7**) with rising dose, they are much less pronounced for PA-6/GF composite compared to virgin PA-6 (Figures **3** and **4**), indicating that the presence of GF partially eliminates the irradiation effects on PA. This conclusion arises from the dependences in **Figure 2** as well.

The effect of irradiation on crystallinity is linked with the values of the melting enthalpy directly proportionally, and some decrease is observed for virgin PA-6 with rising dose (**Figure 4**). The higher value of Δ*H*m2 than Δ*H*m1 for PA-6 (**Figure 4**) is a consequence of different cooling rates between cooling the testing specimens after preparation by injection molding (using a higher initial cooling rate from the processing temperature of ~290°C) and cooling of the sample after first heating in the DSC cell (applied cooling rate of 10°C/min). Therefore, a lower crystalline portion is the result for rapidly cooled materials. The irradiation effect on PA-6 is

also observable on the shape of the DSC melting curves (**Figure 5**).

258 Radiation Effects in Materials

**Figure 5.** DSC melting endotherms of first melting (150–250°C) for PA-6 irradiated with electron beam.

crystallite size becomes broader while the unit endotherm surface (Δ*H*m) decreases.

and the crystallinity is diminished from 25.3% to 21.5% [18].

to 3% [6].

The endotherms of irradiated samples become wider with increasing dose as a consequence of the progressive lamella thinning and amorphization that took place under electron beam irradiation. Additionally, the enlargement in the peaks indicates that the distribution of

PA-6 included in a multilayer film and irradiated in nitrogen applying a dose in the range of 0 to 150 kGy shows only a small total decrease in melting temperature at approximately 1°C

The influence of the crosslinking agent TAC on PA-6 irradiated with relatively low doses up to 100 kGy demonstrates a more pronounced effect on *T*m and crystallinity than present in virgin PA-6. The decrease in both *T*<sup>m</sup> and crystallinity for the doped PA is clearly observable already for the 40 kGy dose, whereas, for the virgin PA-6, the decrease is moderate, if any. The decrease in *T*<sup>m</sup> seems to be dependent on TAC content (the more TAC, the lower the Tm), but the measure of the decrease in crystallinity is not affected by TAC content in the range of 1%

The development of thermal characteristics with dose can somewhat vary when PA is part of a blend or composite. PAs are often used as reinforced composites or filled with various fillers,

**Figure 6.** Variation of the first and second melting temperatures with absorbed electron beam dose for PA-6/GF(30%) composite [7]. With permission of Elsevier.

**Figure 7.** Variation of the first and second melting enthalpy with absorbed electron beam dose for PA-6/GF (30%) com‐ posite [7]. With permission of Elsevier.

The examination of (PP/PA-6+talc) composite with or without a compatibilizer or a crosslink‐ ing agent TAIC, applying a dose in range of 0 to 200 kGy [25], reveals a decrease in *T*m for PA-6 component only when doses 50 and 100 kGy are used. A dose of 200 kGy provokes already a minor *T*m growth and that is evident for the formulation containing TAIC. However, all *T*ms are lower than the *T*ms for the corresponding unexposed composite. Concerning the degree of PA-6 crystallinity, the percentage for (PP/PA-6+talc) formulation varies mildly being lower for the initial crystallinity; for the formulation involving the compatibilizer and TAIC, the decrease is definite.

The behavior of PA-66 is similar to PA-6 in principle. Injection-molded PA-66 samples irradiated with 200 and 500 kGy doses show the decrease in initial *T*<sup>m</sup> regardless of the addition of TAC or the exposure at RT or 120°C. Nor does the water annealing affect this tendency, but the decrease for the samples with no TAC irradiated at RT is a little less when compared to the others. The same can be said about the crystallinity [15]. Thus, the dependences of the crystallinity on dose for PA-66 films [8] and for the injection-molded pieces are the same essentially regardless of the mode of sample preparation.

Data on PA-12 melting characteristics are found for EPDM/PA-12/maleated EPDM blend after being irradiated with 25 to 100 kGy doses [22]. Melting temperature does not show any change and the crystallinity is more or less also the same. However, such a narrow dose range does not allow the estimation of further development under higher doses.
