**1. Introduction**

Polyamides (PAs) are linear semicrystalline polymers containing amide groups -CO-NH- in the chains. The nomenclature applied for PA uses numbers to describe the number of carbons between acid and amine function groups including the carbon of the carboxylic acid. Two types of polyreactions are used for their preparation. The first, polyaddition, is based on a lactam as the starting reactant. After opening the lactam ring breaking peptide bond, the created end groups -C(O)- and -N(H)- are coupled with -N(H)- and -C(O)- groups originating from other cleaved molecules of the lactam. This is the case of PA-6 when ε-caprolactam is used.

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$$\text{Zn} \left[ (\text{CH}\_{2})\_{\text{s}} - \text{CO} - \text{NH} \right] \xrightarrow{\text{H}\_{2}\text{O}} \left[ -\text{HN} - \left( \text{CH}\_{2} \right)\_{\text{s}} - \text{CO} - \right]\_{\text{a}} \tag{1}$$

The same result can be reached using the corresponding amino acid. However, the cleavage of water molecules is required, which is the characteristic for a polycondensation mechanism. Therefore, polycondensation is the second polyreaction used to prepare PAs. The most commonly applied reaction is the polycondensation of dicarboxylic acids with diamines (e.g., adipic acid with hexamethylene diamine giving PA-66).

$$\text{CH}\_2\text{N}-(\text{CH}\_2)\_6-\text{NH}\_2 + \text{HOOC}-(\text{CH}\_2)\_4-\text{COOH} \rightarrow \text{[}-\text{HN}-(\text{CH}\_2)\_6-\text{NH}-\text{OC}-(\text{CH}\_2)\_4-\text{CO}-\}\tag{2}$$

In such a case, the structure of the PA corresponds to the repetition of structure unit consisting of one of each monomer, so that they alternate in the chain unlike for a PA chain synthesized from a monomer as single starting reactant. Many demanding applications require a careful control of the synthesis as well as processing conditions considering the resulting molecular mass.

Variation of the starting reactants enables a large scale of PAs differing in properties from hard and tough PA to soft and flexible to be acquired. Depending on the type, PAs absorb different amounts of moisture, which affect the mechanical as well as dimensional characteristics. In general, PAs are characterized by high rigidity, hardness, abrasion resistance, thermal stability, good sliding properties, stress cracking resistance, barrier properties against oxygen, smells, and oils. The disadvantages of PA types involve a weak stability in the presence of UV radiation, oxidizing agents, strong acids, and bases, high shrinkage in molten section, degra‐ dation in electrical and mechanical properties due to high moisture absorptivity, and high notch sensitivity. The various particular properties of PAs enable various PAs to be processed into various items. They are most frequently used in the textile industry, electrotechnics, and automotive industry as engineering plastics either virgin or composites with modifying fillers. Depending on the type of the product application, the manufacturers can adjust the properties of initial materials to some extent. Besides physical modification, involving admixture of proper additives, the chemical modification of PA can also be applied. According to PA type, a high melting temperature within 170°C up to 290°C occurs. Therefore, the homogenous admixture of intended agents into the melt is possible only for sufficiently thermally stable substances. When the chemical modification of PA requires the generation of radicals (e.g., grafting various functional groups or crosslinking initiated by organic peroxides), then this modification cannot be always carried out in melt due to a weak thermal stability of some substances participating in the process. In particular, peroxides will be thermally destroyed well before the PA is melted. This handicap can be overcome using a proper radiation technology enabling such a modifying process in the solid state. Presenting a technology of one step, the radiation technologies have been largely applied during the last decades and have brought a wider spectrum of properties or design variations.

Because PA-6 and PA-66 comprise most of the world's market (~80%) [1], the effects of radiation modification involving them have been frequently investigated. Concerning some conflicting reports on electron beam effect on polymers found in scientific sources, in general, it could be said that variations in gel content in PAs irradiated with identical doses may not be the same. The reason is that the competing reactions, chain scission and recombination, can be affected by several factors, among others polymer type with its molecular characteristics. However, under irradiation, the most decisive factors are time and dose rate. The presence of oxygen provokes some side reactions and the consequence is a complex process. The oxygen effect can be hardly eliminated completely even if the irradiation is carried out in an inert atmosphere. Within the simultaneous process of recombination and scission macroradicals, the most important factor is the stationary concentration of the macroradicals.

Therefore, although the doses are the same, the results may differ if different electron beam sources and PA types are used. From this aspect, every comparison can be of framing character only. The same can be said regarding γ or proton exposure.
