Radiation Effects on Polymer-Based Systems

Abstract

The applications of the radiation processing involve the deep modifications in the irradiated materials starting with molecular scissions. The specific answers illustrate the progress of chemical changes onto the foreseen goals, which are related to the exposure parameters: dose, dose rate, environment. The type of bonds influences the radiation resistance by their energies and the reactivities of intermediates are the main criterion on which the selection of technological purpose is based. The initial structure of radiation processed material, the radiation stability of main component, the proposed formulation or the manufacture conditions are responsible for the amplitude of conversion or for the process yields. The essential aspects of radiochemical behavior of processed polymers are related to the followed mechanisms, which have particular features provided by material functionality. The main processes involving radiation effects: crosslinking and grafting as well as degradation are analyzed on different polymer structures, on various irradiation parameters, on the material availabilities to high modification levels under high energy radiation exposure. The radiation processing of polymers by their exposure to different types of irradiation sources is presented for the illustration of general possibilities offered by industrial applications. This review is relevant for the extension of applications which can be adapted to several conditions. The presented examples are start points for further studies in which raw materials can be changed as well as blend formulations. The open directions are available based on the provided information.

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1 Introduction

High energy radiation induces the “clean” modifications and efficient modification source of many applications. Along the time, several books and reviews presenting the effects induced during high energy irradiation were issued [1–11]. The energy transferred from incident radiation onto macromolecules allows appearing a high concentration of radical on the track. The radiochemical yield is a measure of stability; its value describes the number of events occurred as the result of the absorption of 100 eV energy. In the radiation field the polymer structures and reactivity are the feature connected to initial molecular configuration. New formed radicals are involved in further complex processes, which define the final radiolysis products.

The resistance of polymers against the action of ionizing radiation places them in a stability sequence, which depicts the material capacity to be modified. The successful irradiation processes present several advantages relative to thermal and chemical processes: high productivity without overloading reagents, reduced energy consumption, nonsignificant environmental pollution, sudden break of process under strict control of product quality, the lack of wastes. The concept of “high energy radiation” is based on the large amount of energy that is deposited on macromolecules by incidental rays (X, γ, accelerated electrons). This transferred energy exceeds many times the values of ionizing potentials and bond energies (10–15 eV). This received energy will be localized and the scission of involving bond occurs. Because during the ionization or excitation these processes take place on radiation track, several spurs appear forming cylindrical columns (Fig. 1).

Fig. 1

The distribution of primary events along the radiation track

The analysis of the advantages and disadvantages of EB accelerators and different γ-irradiators reveals various aspects that are taken into consideration:

• the penetrations of these two types of radiation are different and the LET is related to the mass of rays (Fig. 2).

  Fig. 2

  

  Penetration of accelerated electrons versus energy

• the electron accelerators adjust irradiation beams at desired intensity and energy, while gamma irradiators allows the exposure at certain energy, higher incidental energy is provided by ⁶⁰Co,

• the nuclide sources with which irradiators are provided are consumed according to the half life of source isotope,

• the construction of gamma irradiators presumes large protection walls, that means higher investment,

• the electron accelerators assures convenient high dose rate shortening processing times,

• the maintenance of accelerators is cheaper than the price of source replacement,

• the rate of conveyer movement for exposure in accelerators can be automatically changed modifying the dose range as required.

Radiation processing is largely applied for various purposes, when the purpose of further operation requires an advanced wear in the limited conditions. The large areas of activities are fitted to radiation processing, which long term usage is combined with high performances of material. The specific dose values characterize the limitation of overdose that induces degradation. The type of processing is associated to the certain dose range, whose frame must be respected (Table 1).

Table 1 Some principal application areas associated with characteristic dose ranges [10]

There are some empirical relationships with which penetration ranges can be calculated [12]:

• optimal penetration L (the material thickness whole outlet dose equals inlet dose):

  $$ {\text{L}}\;({\text{optimal}}) = 0. 404\;{\text{E}} - 0.161 $$

• demi penetration L 50 (the distance on which the final dose is half of inlet dose):

  $$ {\text{L}}\; ( 50 )= 0. 435\;{\text{E}} - 0.152 $$

• demi penetration L50e (the distance on which local dose is half of inlet dose):

  $$ {\text{L}}\;( 50{\text{e}}) = 0. 458\;{\text{E}} - 0.152 $$

The choice of irradiation conditions is determined by many factors:

• (1) Nature of materials. The existence of a vulnerable sites on backbone, they are primordially modified either by bond splitting or by removing as primary radicals. An illustrative example can be found in the domain of ethylene-propylene elastomers [13]. While ethylene propylene copolymer (EPR), which is manufactured by the copolymerization of ethylene with propylene, it presents a saturated molecular structure. In contrast, ethylene-propylene terpolymer (EPDM) is produced by copolymerization of ethylene with propylene in the presence of a third diene component (for example ethylidene 2-norbornene), which brings a certain unsaturation level.

• (2) Chemistry of environment. The presence of oxidant atmosphere (usually air) determines the start of oxidative degradation from the start of irradiation. An inert atmosphere like vacuum or nitrogen does not provide any source of secondary reactions with radicals that appear during radiolysis. The diffusion of oxygen into irradiation material influences the distribution of oxidation products that takes a parabolic shape with the maximum amount at the both external sides and minimum is placed on the symmetry axe [14]. The penetration of molecular oxygen allows the reactions of free radicals with it and the peroxyl radical are formed. They are the initiators of further oxidation, which advances as a chain process.

  The availability of molecules to scission and, consequently, for the competitive oxidation in air imposes the decision concerning the selection of irradiation conditions, which will lead to different final state of processed material. Moreover, if the irradiation is performed in aqueous surrounding, the intermediates resulted from water radiolysis (e ⁻_aq , H, HO, HO₂, HO⁻, H₂O₂ [15]) specifically react with polymer substrate and the result significantly differs from the product obtained during the irradiation at similar dose condition, but in air.

• (3) procedure of synthesis. The density of energy transfer is an important factor, which determines the direction of modifications. If the applied dose rate is low, the competition between oxidation and crosslinking is favorable to degradation, because the local concentration of free radicals is low and the amount of oxygen diffuses into polymer is sufficient high for reaction with radicals placed at long distance to each other. In these cases, it is preferable the polymer processing by electron beam irradiation in stead of γ-irradiation al low dose rates. The saturated elastomers like ethylene-propylene copolymer (EPR) are crosslinkable during high dose rate exposure, but on the low dose rate range they are oxidized [16].

  The formation of radical intermediates influences not only the chemical state of irradiated polymers, but it brings also about physical characteristics: solubility, fusion enthalpy, heat capacity, crystallinity, mechanical features, electrical properties: permittivity, dielectric loss, breakdown tension, thermal conductivity. The new formed structures may be unlike in comparison with initial material, because intermolecular bridges, new organic functions appeared on macromolecular backbones, general molecular order or free volumes attain other values.

  The formation of radical intermediates influences not only the chemical state of irradiated polymers, but it brings also about physical characteristics: solubility, fusion enthalpy, heat capacity, crystallinity, mechanical features, electrical properties: permittivity, dielectric loss, breakdown tension, thermal conductivity. The new formed structures may be unlike in comparison with initial material, because intermolecular bridges, new organic functions appeared on macromolecular backbones, general molecular order or free volumes attain other values.

• (4) dose conditions. The choice of dose values must be the result of material stability. According with the formulation, with the state of stabilization, with the chemistry of irradiation atmosphere, with the sample composition, with the type of pretreatments, with the dose rate, the modifications occurred in polymer substrate attains certain level, which is the consequence of simultaneous actions of experimental factors. Figure 3 proves the differences that exist between the effect of dose rate. The increase of crosslinking starts from a certain dose named gel dose. It keeps characteristic values. For example, LDPE presents start gelation under irradiation at about 10–15 kGy, while ethylene-propylene elastomers have measurable gel fraction from 5–8 kGy. The presence of additives or crosslinkers modifies essentially the progress in the radiolytical modification of polymers.

  Fig. 3

  

  Accumulation of the main oxygenated functions during g-irradiation of EPR. Total dose: 50 kGy

• (5) sample composition. Two blending components presenting different radiation stability plays opposite roles during the radiolysis of their mixtures. The component with a lower stability becomes a source of radicals for the other couple partner. On the low total dose range, a crosslinking process is possible, because the concentration of oxygen is not sufficient for feeding oxidative degradation. But, the progress in irradiation makes possible a significant oxidation and the ageing is accelerated, when the concentration of low stability component is significant. Figure 4 exemplifies the contribution of polypropylene to the evolution of chemical states of EPDM/PP blends [17].

  Fig. 4

  

  Modification in oxidation rates of polymer systems (EPDM:PP), (white) EPDM, (light grey) EPDM:PP = 80:20, (dark grey) EPDM:PP = 60:40, (black) EPDM:PP = 40:60, (hachured) EPDM:PP = 20:80

The peculiar factor that determines the behavior of polymer under irradiation is radiochemical yield. It depicts the number of events that occur as the result of the absorption of 100 eV, G. The greater the values of G, the less stable the polymer [12]. The consequences of irradiation conditions related to the tremendous radiochemical yields place various macromolecular materials in different groups of stability: mainly degradable type and manly crosslinkable type [12, 18].

The directions of radiation processing are analysed by various reports [19–25] for the emphasizing the great potential of this technique in the development of economy and science including the present and promising applications.

The energy required for the modification of exposed materials can be provided by two types of sources: irradiators provided with radioisotopes, usually ⁶⁰Co or ¹³⁷Cs, on one side and electron accelerators, on the other side. These two categories of facilities are designed for the radiation processing of great amounts of materials, which are destined to further handling. The effects of dose rate on the efficiency in the chemical modifications are different, if the same material is exposed to γ-rays of electron beam, because the throughput of EB exposure produces high productivity. The high dose rate available by the irradiation with accelerators, about 100 kGy s⁻¹ reduces drastically the oxidative degradation that it would occurred during γ-irradiation, even the dose rate is at an acceptable level (10 kGy h⁻¹) [26]. Though electron accelerators suppose a large investment for the start, the further utilization satisfies the technological requirements.

The great troublesome problem in the electron beam (EB) irradiation is the depth of penetration [27], which depends not only by the incident particle energy, but also on the material density. This problem can be solved by double face irradiation, when exposure dose can be considered uniform along profundity [28]. The main problem in the selection of the type of accelerator is the nominal power, which must be rigorously correlated with its applications [29]. The factors that influence the choice of accelerators are output power, energy of EB, room temperature for accomplishment of process, number of passes and rate of conveyer [30]. A variety of industrial electron accelerators can now provide electron energies from 0.3 MeV to more than 10 MeV with average beam power capabilities up to 300 kW.

The monitoring of dose can be performed with different techniques: chemical systems (Fricke and ceric solutions [31]), radiochromic compounds (dyes [32, 33]), polymeric tapes (polyethylene [34], poly(vinyl chloride) [35, 36], poly(methyl methacrylate) [37–40], epoxy resin [41]), radiation thermoluminescence phosphors [42, 43]. Several requirements are imposed for a proper dosimeter: similarity with processed material in respect with linear energy transfer, reproducibility, sensitivity, lack of the influence of humidity, stability after irradiation, easy to calibrate, appropriate dose range and dose rate, linearity and independency on the type of radiation, the response being constant in time, lack of post-irradiation modification.

Each dosimeter has a certain dose range, on which the specificity recommends it as proper system for accurate dose measurement [44]. Due to their easy availability, handling and processing, polymers may be preferred [45]. For comparison, some practical aspects of X-rays and gamma rays, such as product penetration, dose uniformity, utilization efficiency and processing capacity were reported [46].

Industrial scale of radiation processing is accomplished also on the sterilization of polymeric medical wear [47], food packaged in polymer bags [48] or electrical insulation of wires and cables [49, 50]. The radiation technologies are based on the possibility of attaining improved properties and extension of service life is attained.

2 Improvement in Thermal Properties of Polymeric Materials/Composites

2.1 Crosslinking

Starting from the crosslinking of polyethylene [51] the improvement in the functional characteristics of polymers by crosslinking keeps various aspects. The general overview on the involvement of free radicals that are formed during radiolysis concerns the reactions of intermediates to each other or with oxygen [52]. The accumulation of cured fraction or oxygen-containing products depends on the reactivity of radicals. The accumulation of insoluble fraction, which consists of three dimensions network built up by new intermolecular bridges, turns processed materials onto desired behavior exemplified by the mechanical properties, namely tensile strength, scratch resistance, the performances at higher temperatures, the chemical strength by the diminution of rate diffusion of organic solvents, the reduction in the amount of penetrated gases, memory shape retention, electrical properties.

The basic equation which describes the decreasing in soluble fraction of irradiated polymers was reported by Charlesby and Pinner as linear function of S + S^1/2 on reciprocal value of dose [52]:

$$ S + \sqrt S = \frac{{P_{0} }}{{Q_{0} }} + \frac{1}{{Q_{0} .P_{n,0} .D}} $$

where S is the soluble fraction that can be calculate for the abstraction of gelated mass from total sample weight, P₀ and Q₀ represent the radiochemical yields of scission and crosslinking, respectively, P_n,0 is the initial polymerization degree and D is irradiation dose expressed in kGy. The ratio between radiochemical yield of scission and radiochemical yield of crosslinking illustrates the availability of polymers for crosslinking or degradation [52].

The content of gel fraction will influence material properties. The theoretical approach starts with the the evaluation of radiochemical crosslinking yield:

$$ {\text{G}}_{\text{n}} ({\text{crosslinked}}\;{\text{units}}) = 0. 9 6\times 10^{ 6} \,{\text{q}}_{0} /{\text{w}} $$

where G_n(crosslinked units) represents the number of crosslinked units per 100 eV absorbed energy, q₀ is susceptibility of polymeric system to be crosslinked and w is the average gravimetric weight of polymer. It is defined as the proportion of crosslinked monomer for dose unit. Because the bridging takes place involving two units, the radiochemical crosslinking yield is:

$$ {\text{G(crosslinking)}} = 0. 4 8\times 10^{ 6} \,{\text{q}}_{0} /{\text{w}} $$

Consequently, the dependency of molecular weight on absorbed dose is:

$$ {\text{M}}_{\text{c}} = 0. 4 8\times 10^{ 6} /{\text{G}}.{\text{D}} $$

The last relationship can be used for the calculation of radiochemical yield of crosslinking based on the measurement of average weight molecular weight.

The most commercialized polymer is polyethylene, which present a very good stability under irradiation. Because there are manufactured different sorts of polyethylene, they present unlike radiochemical strength.

The processability of polyolefins can be improved by the irradiation in the presence of crosslinker, for example TAIC [53]. Many other polymers can form efficiently increased amounts of insoluble fraction in the presence of various functional crosslinkers: ethylene–vinyl alcohol copolymer with TAIC, DEGDA, NPGDA and TMPTA [54] at maximum 65 % gel fraction at 200 kGy, poly(ε-caprolactone) and poly(butylene succinate-co-adipate) with TMAIC [55] forming insoluble content around 60–80 % at 100 kGy, polyamide 6 with TAC [56] generating a crosslinked phase of 95 % at 80 kGy, poly(L-lactic acid) with TAIC, TMAIC, TMPTA, TMPTMA, HDDA, EG [57] at different gelation degrees between 25 and 85 % obtained at 100 kGy. The addition of TAIC and SiO₂ in the formulation of poly(L-lactic acid) allows to attain a gel fraction of 100 % at 40 kGy [58].

Low density polyethylene is the most frequently used because of its low branching degree. The radiation processing increases the stability by the continuous increasing of the crosslink density from zero in pristine material to 0.204, 1.022 and 4.807 mol dm⁻³ at 100, 200 and 400 kGy, respectively [59]. Of a great importance for the effect of irradiation is the orientation of molecules that determines the differences in the repartition of crystalline phase in polyethylenes. The gel fraction in LDPE, LLDPE and HDPE depends not only on the received dose, but also on the drawing ratios [60]. The differences in the accumulation of insoluble fraction are explained by the unlike values of crystallinities.

The irradiation environment plays an important role in the evolution of polymer stability. While unsaturated hydrocarbon like acetylene [61] or divinyl benzene [62] is present in the material surrounding and provides radicals for the formation of intermolecular bridges, oxidative atmosphere, oxygen or air, promotes oxidation as the result of diffusion inside the polymer matrix. The distribution profile for carbonyl products that generated during irradiation takes a parabolic form [63]. The source of radicals may be one of the components of blends, which presents a lower stability. This case can be illustrated by various blends, EPDM/PP [64], EPDN-NR [65]. These polymer mixture show the maximum level of crosslinking at about 120–150 kGy.

The blends consisting of SBR and EPDM in different proportions are compatibilised by crosslinking under γ-irradiation [66]. The viscosimetric measurements and the swelling investigations gave revealed the appearance of a new phase by the connections of molecular chains with chemical bridges, much stronger that physical interactions. Even though structural differences exist, the crosslinked fraction consists of different morphologic components, graft copolymer, block copolymer and spatial copolymer. The increase in the concentration of EPDM from 0 to 100 %, the molecular masses and crosslinking densities become 13,564 and 449, and 3.69 and 111, respectively, obtained for irradiated mixtures at total dose of 400 kGy.

The compatibilisation of different polymers by irradiation can be achieved by EB irradiation starting from latex components, for example, natural rubber and acrylonitrile-butadiene rubber in the presence of EPTA [67]. The variation of the normalized absorbances at 1625 cm⁻¹ (vinylene C=C) and 1450 cm⁻¹ (–CH₂–) scissor against the absorbance at 2223 cm⁻¹ (–CN stretch) with the radiation dose (0–500 kGy) has demonstrated that at 100 kGy the process reaches the steady state. In the Charlesby-Pinner representations straight lines could be drawn at each of three concentrations (0, 2 and 4 phr) of crosslinking promoter.

The effect of TMPTA on the crosslinking of PET and HDPE by γ-irradiation was described by the accumulation of gel fraction in their blends that keeps the intermediate maximum value, 10 % obtained at 30 kGy [68]. Even the total dose increases at 70 kGy, the amount of insoluble fraction remained constant for the studied formulation, PET/HDPE = 80/20. The best mechanical properties were measured at this limit dose, 30 kGy. It was supposed that further irradiation of crosslinked PET/HDPE systems the degradation by scission of new formed bonds occurred. This behaviour can be found in the cases of components with different structures and polarities.

The radiation technology applied for the modification of crystalline or semi-crystalline polymers allows manufacturing crosslink shape-memory products [69]. The exposure to high energy radiation promotes crosslinking of cylindrical tubes at certain dimensions. After heating the stain appears. This radiation processing can be applied successfully to natural rubber [70, 71], polyethylenes [72–74], ethylene vinyl acetate copolymer [75], poly(ε-caprolactone) [76]. The efficient crosslinking of this kind of smart materials is attained, if the formulation contains a sensitizer [77] or a multifunctional additive [78]. The applied dose in the systems consisting of modifying polymer and additive is significantly lower than the nonmodified compound, because the higher gel content is obtained much easier and the number of crosslinks is higher. Consequently, the charge with which material presses inner body is corresponding greater. The applications of SMPs cover different areas, for example: medical treatment (tight immobilisation of tissues or bones, sterilisation), electrical engineering (joint cables for continuous connection, electro-active sensors, encapsulation of electronic parts), mechanical engineering (temperature markers, assembling different equipment parts, fabrication of geometric structures), handling and preservation of food, smart textiles [79, 80]. The shrinkability, the accumulation of gel fraction, the convenient mechanical properties, the chemical stability against oxidation are some features that must be shown by an appropriate SMP [81].

In the medical praxis, UHMWPE have been used successfully as one half of the bearing couple (against metallic alloys or ceramics) in total hip and total knee joint replacements crosslinked under irradiation [82]. Even γ-irradiated UHMWPE at doses higher than 100 kGy presents the incipient fracture development [83], the alkyl macroradicals are involved in crosslinking and in a smaller proportion they promote oxidation [84, 85]. The prosthesis manufactures by the irradiation of UHMWPE have long durability, because the application of radiation treatment induces an increased crystallinity and promotes sterilization in the whole volume of material.

The functional properties of polymers can be ameliorated by the irradiation of their nanocomposites [86, 87]. The addition of MWCNT to low density polyethylene increases the radiation resistance in comparison to the pure LDPE, which was dependent on the MWCNT content [88]. WCNT nanocomposites were gamma irradiated at 90 kGy to improve the interaction between MWCNTs and the polymer matrix [89]. The irradiation produced a 38 % decrease in the toughness of neat UHMWPE. The incorporation of MWCNTs did not significantly affect the melting point of the neat UHMWPE but decreased the degree of crystallinity of the raw UHMWPE, which was related to a reduction in the UHMWPE lamellar density. The tensile tests showed a 38 % increase in the Young’s modulus in the reinforced nanocomposites and a small decrease in toughness (5 %). The addition of carbon nanotubes in polypropylene brings about conductivities of the order of 10⁻² S/cm, but its scavenger effect reduces the number of radicals generated by irradiation, lessening the strain hardening effect [90]. The presence of CB and silica in the formulation of NBR/CSM blends at maximum concentration of 30 phr changed satisfactorily polymer properties: 152 % increase in tensile strength, 116 %, in elongation at break and 142 % modulus at 100 % elongation according to synergistic effect between the fillers [91]. Ethylene-vinyl acetate copolymer (EVA) flame retarded by a combination of intumescent flame retardants (IFR) and organically modified montmorillonite (OMMT) crosslinked by EB irradiation shows a synergistic effect of IFR and OMMT on the flame retardancy. With the addition of 1 wt% OMMT and 24 wt% IFR, the LOI value of EVA/IFR/OMMT nanocomposite increased from 30.5 to 33.5 % [92]. The reinforcement of polypropylene with 5 % POSS after g-irradiation at 50 kGy showed improved thermal stability due to the formation of crosslinked network between PP and POSS by radiation [93]. The mixing carboxyl-terminated butadiene-acrylonitrile (CTBN) with nano-clay to improve the toughness and mechanical strength of bisphenol A type epoxy followed by further γ-irradiation at high doses (500, 1000 and 1500 kGy) has pointed out the contribution of nanofiller to the stabilization by intercalation of macromolecules into clay layers [94].

Nanocomposites of two different kinds of rubber (acrylonitrile-butadiene rubber NBR and styrene butadiene rubber SBR)/organo-montmorillonite nanocomposites modified by hexadecyltrimethyl ammonium bromide have a remarkable resistance when they are radiochemically processes in the presence of TMPTMA [95]. Similar results were reported on epoxy resin modified with titania presents a very good resistance to the thermal oxidation after receiving 50 kGy γ-dose [96]. The oxidation in neat material starts quickly and progresses with a great oxidation rate. By the addition of titania nanoparticles, the host resin presents a decreased oxidation rates at filler concentrations up to 10 %. A very smooth oxidation takes place in irradiated epoxy resin/titania hybrids because inorganic phase acts as efficient adsorbent agent relative to the radicals formed during radiolysis.

One largely commercialized elastomer, EPDM, can attain improved tensile strength, when it is irradiated as hybrid composites with nanoclay particles [97]. The reinforcement of filler in the case of ATH induced a de-cohesion inside polymers with direct consequences on [98]. Moreover, at room temperature, i.e. below the melting temperature, all the consequences of ageing by gamma irradiation are strongly attenuated by the presence of a semi-crystalline microstructure, the morphology of which is not too strongly modified by irradiation. The content of silica nanoparticles modifies the kinetics of the degradation of EPDM substrate through the complex modification of the filler–filler and filler–matrix interactions involved in the mechanical properties of the filler network.

The differences that exist between polyolefins, namely LDPE, PP and EPDM in the presence of various concentrations of silica demonstrate the structural contributions of branching and unsaturation to the durability of this kind of nanocomposites [99]. An opposite effect for the thermal stability of irradiated PP was observed in the presence of titania, which promotes oxidation initiated by radiation processing [100].

Polylactide (PLA) and PLA-Cloisite 30B (C30B) nanocomposites under g-irradiation present significant modifications with dose. The thermal stability is higher for the nanocomposite, even the decrease of oxidation temperature is noticed for the both formulations [101].

The progress in the oxidation of LDPE modified with maleic anhydride and alumina proceeds somewhat similar at various doses, because oxygen diffusion is hindered by filler nanoparticles [102]. The noticeable difference between pristine and modified LDPE consists of the presence of maleic anhydride, which interacts with molecular chains due to the electronegativity of oxygen atoms. The same radiation dose affects differently the dielectric behavior of the nanocomposites depending on the filler content. The dose of 50 kGy applied on LDPE-g-AM filled with 5 wt% nano-Al₂O₃ leads to a relative permittivity smaller than unfilled LDPE. γ-Radiation can lead to a decrease in the dielectric losses of LDPE Al₂O₃ nanocomposites for properly chosen combination dose–filler content.

The γ-irradiation of natural rubber (Malaysian type) in the presence of nanotubes does not modified the accumulation of gel content, which reaches an upper limit (98 %) at about 25 kGy, but mechanical properties are influenced by the nanofiIler loading [103]. The uniform crosslinking in homogenous distribution of CNT confirms the high quality of radiation processed NR.

The radiation exposure makes possible the crosslinking of poly(ε-caprolactone)/polyhedral silsesquioxane nanocomposites by electron beam irradiation [104]. The addition of POSS nanoparticles gathers the PCL in a hard fraction, which explains the highest values in tensile strength at 100 kGy and the sharp decrease in the elongation at break after receiving 50 kGy, equivalent with the gel dose. In comparison to the virgin PCL with a tensile strength of about 20 MPa, the tensile strength of the crosslinked PCL/POSS nanocomposites increased to 25.8 MPa with an increasing POSS content and absorbed dose to 100 kGy, whereas their elongation-at-break was considerably reduced. As the result of intermolecular strength the swelling of composites goes down as the content of POSS increases.

For the manufacture of long term products, such as automobile and aerospace components, sporting goods, etc., it is recommended the grafting of nanoparticles on polymer backbones. This goal is accomplished in the case of PP [105] and PU [106] by the exposure of hybrid materials to the action of γ-radiation.

Vulcanization of rubber blend of acrylonitrile butadiene rubber (NBR) and ethylene-propylene diene monomer (EPDM) rubber (50/50) loaded with increasing contents, up to 100 phr, of reinforcing filler, namely, high abrasion furnace (HAF) carbon black and further subjected to gamma radiation doses up to 250 kGy was performed [107]. Mechanical properties, namely, tensile strength, tensile modulus at 100 % elongation (M100), and hardness have been followed up as a function of irradiation dose and degree of loading with filler.

The radiation treatment of polymer materials, even they are as receives or modified with nanofillers converts them into engineering products with high economical values [29, 108].

The necessity of new form of polymers, which would be behaved friendly, was solved by hydrogels, three-dimensional networks of covalently bound hydrophilic polymer chains. Typical simple materials applied for general-purpose hydrogels are ethylene oxide, vinyl alcohol, vinylpyrrolidone, hydroxyethyl methacrylat, crosslinked natural polymers, mainly polysaccharides. The propagation of crosslinking takes place through the activities of different radicals. The accumulation of gel during irradiation follows the Rosiak, Olejniczak and Charlesby, similar with the law Charlesby-Pinner [109]. By the application of Charlesby-Pinner equation on the results obtained for the radiation processing of PVA/PVP blends it has demonstrated that the gel fraction of blend is placed between the similar values of components. Gel doses were measured for the polymers and they are 11 kGy for PVA, 3.7 kGy for PVP and 4.6 kGy for a mixture of PVA and PVP with a mole fraction of PVP of 0.19 [110]. The addition of a crosslinking agent, for example acrylic acid, into the PAA/PVP irradiated blends stimulates gelation, even though values of their gel doses increase smoothly [111].

The preparation of hydrogels can be achieved by various procedures. The general manner through which hydrogels are prepared is polymerization. The double bonds represent the start points of radical polymerization and crosslinking. One example is N-isopropylacrylamide, which generates hydrogels in aqueous solutions [112]. During irradiation the water radiolysis intermediates (hydrated electrons, hydroxyl radicals and hydrogen atoms) react with monomer producing α-carboxyalkyl radicals. They become the initiators of polymerization.

During radiolytic exposure a complex chain of reactions occurs. The primary intermediates appear either due to the scission of molecules by energy depositing on components, or by transfer of radicals [113]. The attack of hydroxyl radicals followed by the abstraction of proton becomes a source of crosslinking precursors. The most vulnerable position is tertiary carbon, which generates the highest proportion of radicals. They act as precursors of final products according with Fig. 5.

Fig. 5

Transfer reactions caused by radiolytic hydroxyl radicals which are the start of hydrogel formation

PVME microgel particles can be prepared by irradiation of a phase separated diluted aqueous PVME solution above their lower critical solution temperature (37 °C) following the mechanism described in Fig. 5. The macroradicals can appear by the scission of C–H bonds of carbon atoms composing vinyl methyl ether monomer. When system receives 10.5 kGy the average weight molecular weight increases from 57.10³ g mol⁻¹ to about 10⁶ g mol⁻¹ [114].

The applications of hydrogels in the production of medical items, resulting materials must have several features, which recommends them: non-toxicity, functionability, sterilizability, biocompatibility [115]. These characteristics are requires for wound dressings, drug delivery systems, transdermal systems, injectable polymers, implants, dental and ophthalmic applications, stimuli-responsive systems, hydrogel hybrid-type organs.

Radiation processed PVA–polysaccharides hydrogels can be prepared at maximum dose of 30 kGy [116]. Using concentration of polysaccharides as low as 0.5–2 % resulted in increase of tensile strength from 45 to 411 g cm⁻², elongation from 30 to 410 % and water uptake from 25 to 157 % with respect to PVA gel without polysaccharides. Formulations containing 7–9 % PVA, 0.5–1.5 % carrageenan and 0.5–1 % agar gave highly effective usable hydrogel dressings.

A series of antibacterial hydrogels were fabricated from an aqueous solution of AgNO₃, gelatine and carboxymethyl chitosan (CM-chitosan) by radiation-induced reduction and crosslinking at ambient temperature [117]. For the total dose of 30 kGy, the gel fraction was situated around 75 % invariant relative to the concentration of AgNO₃, and diameter of particles was between 2 and 5 nm. The swelling ratio is higher (90–100 %) at the concentrations of silver nitrate exceeding 1 %.

Hydrogel wound dressings have been prepared using the gamma rays irradiation technique. The dressings are composed of poly(vinyl pyrrolidone) (PVP), poly(ethylene glycol) (PEG) and agar [118, 119]. At 50 kGy the stress at break increases with 40 %, while swelling degree is enhanced significantly with the concentration of PEG and with the time of immersion. Interpenetrating polymer networks (IPNs) can be synthesized by irradiating acrylonitrile solutions of poly(N-vinyl-2-pyrrolidone) with Co-γ rays [120]. Thermal gravimetric analysis of resulting IPNs has proved the enhancement in the stability of materials, which can be related to the intermolecular bonds formed during irradiation. Polyvinyl alcohol and polyvinyl pyrrolidone (PVA–PVP) blended hydrogel for wound dressing has been prepared by using gamma rays irradiation. WVTR values of PVA–PVP blended hydrogel are around 80–200 g m⁻² h⁻¹, which place these radiation processed systems qualitatively above commercial products [121].

The hydrogelmatrix can be obtained by γ-irradiation, which induces crosslinking simultaneously with the in situ reduction of Ag⁺ ions initiated by the products of water radiolysis (e ⁻_aq , OH⁻, H^., H⁺, H₂, H₂O₂). For the radiochemical gelation of vinyl pyrrolidone two radical entities are involved in different proportion (Fig. 6).

Fig. 6

Radical intermediates promoting crosslinking in irradiated vinyl pyrrolidone

The stress/strain representations are linear PVP and PVP/Ag⁺ hydrogels. Obtained values of the effective crosslink density in the range of 52.8–54.0 mol m⁻³ and those corresponding to the molar mass between crosslinks in the range of 15.5–15.9 kg mol⁻¹ for pure PVP hydrogel prove the development of good hydrogel material. Similar results were reported for the hydrogel containing simultaneous silver and gold ions for the increasing in the antibacterial activity [122].

The polyphenol trans-resveratrol is a natural phytoalexin, which is found in red wine and in a wide variety of plant species. Resveratrol displays a wide array of biological activities, such as modulation of lipid metabolism, anti-inflammatory and antioxidant activities. The results of gel fraction and swelling degree were approximately 90 and 1600 %, respectively for applied dose not exceeding 1 kGy [123]. The swelling degree attains maximum value after about 6.5 h. These information concerning the resveratrol stability suggest that it has showed no structural decomposition by the primary and secondary radicals of water radiolysis. The polymeric matrix composed by PVP, PEG and agar showed suitable physical and chemical characteristics to resveratrol immobilization to compose a hydrogel dressing for dermatological use.

Protein-crosslinking whether done by enzymatic or chemically induced pathways increases the overall stability of proteins. Papain, a proteolytic enzyme (EC 3.4.22.2) of biotechnological and biomedical relevance was selected for the delivery of papain under γ-irradiation as globular crosslinked protein [124, 125]. The size of papain particles enlarges due to the radiation processing from 172 nm in the case of nonirradiated ethanol solution to 910 nm after receiving 40 kGy. The use of ethanol combines the protein aggregation, essential for the involved process and radioprotection due to its intrinsic scavenger property, capturing specific radicals generated by water radiolysis, known to cause a deep impact on protein integrity.

The applications of hydrogel in the retention of metal ions were intensively studied [126]. pH-sensitive hydrogels based on poly(N-vinyl-2-pyrrolidon) (PVP), acrylic acid (AAc) and styrene (Sty) were prepared by gamma irradiation [127]. PVP/(AAc-co-Sty) hydrogels were subjected to radiation modification to use them as adsorbent materials for removal of heavy metal ions from aqueous solution. Effect of functionalization of hydrogels by sulfonation (Sf), partial hydrolysis with alkaline solution (NaOH) and treated with the two processes (NaOH/Sf) on metal ion uptake was evaluated, and it results in appreciable uptake of Co²⁺, Cu²⁺ and Fe³⁺ ions from aqueous solution.

In the same category of reports the drug delivery hydrogels can be considered [128–133]. They must show some characteristics, which get them proper for foreseen goals [134, 135]: the lack of homopolymer, the highest absorption capacity (maximum equilibrium swelling) in saline, desired rate of absorption (preferred particle size and porosity) depending on the application requirement, the highest absorbency under load and reasonable remote rate the highest durability and stability in the swelling environment and during the storage., the highest biodegradability without formation of toxic species following the degradation, pH-neutrality after swelling in water, colorlessness, odorlessness, and absolute non-toxic, photo stability, re-wetting capability (if required) the hydrogel has to be able to give back the imbibed solution or to maintain it; depending on the application requirement (e.g., in agricultural or hygienic applications), low production price. The adequate usage of these hydrogels in the post-operatory application is the adhesive features for easy withdrawing from surgery intervention site [136].

The recycling of polymers receives a large attention because it solves two problems, one is the economy of raw materials, and the other is the decrease of environmental pollution. A review on the contribution of radiation processing to the reclaiming polymer wastes were published previously [137]. The effects of an electron radiation dose (up to 300 kGy) and compatibilizer on the Charpy impact strength (σ _c) and tensile-impact strength (σ _t) of composites made of the following recycled polymers: low-density polyethylene (LDPE), high-density polyethylene (HDPE), polypropylene (PP), polystyrene (PS), and poly(ethylene terephthalate) (PET) indicate the processing potential of radiation treatment applied to polymers [138]. Styrene–ethylene/butylene–styrene elastomer grafted with maleic anhydride (SEBS-g-MA) and trimethylol propane trimethacrylate (TMPTA) were used as the compatibilizers, added at 10 and 1 wt%, respectively. It was found that, under the influence of SEBS-g-MA, the values of σ _c and σ _t for the studied composites increased by over three and almost five times, respectively. The electron radiation and TMPTA had no noticeable effect on these quantities, which might be due to the protective properties of aromatic rings included in the macromolecules of PS and PET. It was demonstrated that the inclusion of SEBS-g-MA enabled achievement of satisfactory impact strength of composites made of recycled polyolefins, PS and PET, while it was not necessary to use the electron radiation and TMPTA.

Basically, the recovery of polymer wastes is based on the compatibilization, when radiation acts as the radical formatting agent [64]. The radiation compatibilization of high-density polyethylene (HDPE)/ethylene–vinyl acetate (EVA) copolymer blends can be achieved, because the vinyl acetate content of EVA was beneficial to radiation crosslinking [139]. The gel content has a maximum value at 30 kGy for all relative concentrations of components. The compatibility checked by TGA investigations as well as mechanical properties showed that the two plastics coexist homogeneously due to the intermolecular bridges appeared during irradiation. At higher doses like 500 kGy, degradation of blends takes place.

Recycled and pristine low-density polyethylene samples were crosslinked by ⁶⁰Co gamma rays in the presence of two commercial sensitizers: trimethylolpropanetriacrylate (TMPTA) and trimethylolpropane trimethacrylate (TMPTMA), and another laboratory-synthesized sensitizer, hexakisalylaminocyclotriphosphazatrine (HAAP) [140]. The measurement of gel fraction revealed a light difference between virgin and recycled LDPE, this discrepancy being duet to the accumulation of degrading compounds. PET/HDPE blends can be processed by γ-ray irradiation combined with using a cross-linking agent, TMPTA [68]. The specificity of this process consists of the new structure, HDPE-g-PET proved by FTIR records. When the weight ratio of PET/HDPE blend was 80/20, the content of TMPTA was 1 wt% and the absorbed dose was 30 kGy, the tensile strength, elongation at break and impact strength of irradiated blends were improved greatly compared with non-irradiated blends. Blends of high-density polyethylene (HDPE) can be crosslinked by radiation processing with recycled and pristine polyethylene terephthalate (PET) [141]. PET contains aromatic groups, which are effective at dissipation of the energy of the ionizing radiation during γ-radiolysis forming a copolymer capable of improving the compatibility of the blend HDPE/PET. The gel content differs for one composition to the other, when PET loading varies from 50 to 30 %. The initial lower amount of PET in the raw blend favorizes an advanced crosslinking level.

Radiation-induced compatibility behaviour of SBR–EPDM blends of different composition was studied for intimate compatibilizing components [66]. The results have shown that at a threshold dose of 10 kGy exists and good interaction between the components of the blends was achieved. Higher radiation doses lead to formation of crosslinked three-dimensional copolymer network, whose swelling depends on the radiation dose imparted. The anomalous diffusion of solvent into the gels was confirmed by rigorous treatment of the swelling data. Permeation data agreement with the series model indicated that in SBR–EPDM blends EPDM exists as continuous phase and SBR as dispersed phase.

The γ-processing of IIR gives the possibility of an efficient recycling of this material. The sharp drop of weight average molecular weight (Fig. 7) is an economical way for the manufacture of rubber ingredient used for toughening tire raw materials [17].

Fig. 7

Change of weight average molecular weight in irradiated IIR

The EB treatment of the GTR containing LDPE and EVA matrix blends showed significant benefits. The 200 kGy EB absorbed dose (in air) resulted in a better tensile strength and increased elongation at break, without changing the tensile modulus, which provides more rubber-like properties [142]. The modest change in hardness proves the cross-linking effect caused by the EB treatment in all cases. The dynamic mechanical analysis confirmed the compatibilization effect of EVA and EB between the GTR and (thermoplastic) polymer matrix. Recycling of gamma irradiated inner tubes made of butyl rubber in butyl based rubber compounds was studied for the reclaming waste tires [143]. Gamma irradiated inner tube wastes and commercial butyl rubber crumbs devulcanized by conventional methods were replaced with butyl rubber up to 15 phr in the compound recipe. The rheological and mechanical properties and carbon black dispersion degree for both types of compounds were measured and then compared to those of virgin butyl rubber compound. The results lead to the procedure through which tires can be reinforced. γ-Irradiation is used for the in situ compatibilisation of blends of recycled high density polyethylene (rHDPE) and ground tyre rubber (GTR) powder [144]. The expected compatibilisation mechanism involves the formation of free radicals, leading to chain scission within rubber particles, crosslinking of polyethylene matrix and co-crosslinking between the two blend components at the interface. While uncompatibilised rHDPE/GTR blends show poor mechanical properties, especially for elongation at break and Charpy impact strength, irradiation leads to a significant increase of these mechanical performances.

The improvement in the quality of recycled polymers can be achieved by the accomplishment of high energy irradiation in hydrocarbon atmospheres [62, 145, 146], because the scission of π component of double bonds or the opening of cycles provides radicals, which crosslink polymer molecules. An example of mechanistic scheme applied in the irradiation of unsaturated structures is presented in Fig. 8.

Fig. 8

Scheme for the reaction mechanism involving unsaturated monomer

There are other alternatives for the improvement of polymer stability starting with polymer wastes [147, 148]. The presence of sulphur in irradiated IIR increases the crosslinking level up to 100 kGy. The addition of stabilizers is a solution for the protection of materials for further oxidation [149].

2.2 Grafting

The association of new monomer onto macromolecules, which allows it, represents the grafting process [150]. The great advantages of radiation processing in grafting polymers are the initiation without catalyst and the simplicity of treatment, which can be conducted in connection with the reactivities of monomer and substrate. The connection of this outside structure brings different functional properties, with which the application area can be enlarged. The incidental radiation provides energy for the generation of reactive sites. Synthetic materials obtained by grafting are proper alternative to blended polymers, which may satisfy the practice requirements imposed in any certain service. High energy radiation substitute chemical reagents, which creates active positions on macromolecules. Some example of this kind of reactivity stimuli are ferrous ions in redox medium, cereous ions, manganate ions in acid environment and many others. Their involvement in redox processes causes the transfer of one electron for each ion from polymer molecule and, consequently, the formation of free radicals, with carry on the attachment of grafting entity.

Grafted copolymers can be obtained by several ways [1, 151–153]:

• the polymerization by addition of vinyl monomer M on polymer RR′,

• the reaction of two macroradicals appeared from two different polymer components RR′ and R₁R₂,

• polycondensation of two macromolecules RR′ and R₁R₂, which have functional groups that can react:

  • direct grafting of vinyl monomer on polymer macromolecule,

  • grafting catalysed by peroxyl structure appeared during radiolysis,

  • initial grafting of trapped radicals,

  • co-crosslinking of two different polymers by linking one type of macromolecule to other molecule from the second component.

Synthetic view on these routes is presented in Fig. 9. The application of grafting procedure, one polymer substrate gains supplementary features, which allow it to be used in particular conditions of operation. The new structure is regarded as a combination between the two initial configurations, but the basic material exhibits predominant characteristics. The grafted units correct some of undesirable properties, which are turned to the favourable behaviour. If grafting takes place in solution, the participation of solvent molecules increases the conversion yield, because they act as transporters of energy between radiation and irradiation environment [154]. The pre-irradiation plays also an important role, because it forms radicals, which become the process initiators.

Fig. 9

View of possible grafting routes

The modification of polyethylene and cellulose by grafting of styrene and acrylonitrile simultaneously present in polymer bulk allows the attaining of maximum grafting percentage at about 30 kGy by γ-irradiation [155]. In the samples grafted with styrene no crosslinking were obtained. It could be concluded that when the yield of grafting is very high in acrylonitrile, its termination occurs via the combination of two growing chains of grafted branches and leads to a three-dimensional network. In contrast, cellulose samples after extraction with acetone showed decrease in the size of birling fragments that occurred at low dose rate and the cellulosic sheets broke into short fragments and particles. This could be explained by, polyethylene being a polymer of crosslinking type and cellulose being a polymer of degrading type.

The polymers compatibilization of different nonmiscible materials requires the grafting one of them with maleic structures. LLDPE can be modified by g-irradiation [156], when grafting degree increases as the concentration of DEM in the reaction mixture and the absorbed doses are increased up to 100 kGy. The evaluation of MFI values of irradiated samples up to 100 kGy proves the progress of grafting (Fig. 10), as well as spectroscopic records do. The highest grafting degree does not exceed 0.3 mol%.

The interaction between various compounding phases can be optimised by the grafting of base polymer with maleic anhydride ans other polar structures [157]. EB treatment is proper way for the modification of polyethylene, that be further subjected to addition of mineral filler, for example magnesium oxide (MgO).

Radiation-induced grafting of glycidyl methacrylate (GMA) onto high-density polyethylene (HDPE) and the radiation lamination of HDPE by bulk grafting of GMA can be accomplished for the production of medical items [158]. The γ-ray irradiation induced grafting of GMA onto HDPE occurs very easily the because of the existence of double bond in monomer (Fig. 10). The extent of grafting was higher in 2 M GMA than in 1 M GMA at the same irradiation dose. The extent of grafting initially increased quickly with the total irradiation dose and then remained almost constant. The maximum amount of GMA linked on polyethylene was obtained at 15 kGy. This low dose is explained by the radiation instability of monomer, which affects the radiation stability of grafted substrate. For various medical applications, the surface properties of low-density polyethylene (LDPE) can be modified radiochemically by the grafting of 2-hydroxyethyl methacrylate (HEMA) [159] (Fig. 10). LDPE films were grafted at 14.0 and 268.0 % using an initial concentration of [HEMA]_i at 15 % (v/v) by irradiation in absence of air, during 10 and 30 h, respectively, at a dose-rate of 0.3 kGy h⁻¹.

Fig. 10

Mechanistic scheme for the grafting of HEMA, GMA and DEM

The service of membranes for fuel cells asks grafting of appropriate monomers for assuring certain electrical conductivity and stability [160–166]. The radiation processing of these systems is accomplished by the initiation of radical generation at low and moderate doses, which interact with proton releasing units and provide suitable concentration of charge carriers. The increase in the polymer stability and functionality can be achieved by the grafting of various monomers, which induce a delocalization of electrons to create new structures with less reactivities. The grafting of styrene on the molecular configuration of polypropylene by gamma exposure [167] or EB treatment [168], starch [169], PTFE [170], cellulose [171] makes possible to enlarge application ranges onto the most aggressive conditions, because the benzene scavenge energy and does not release it on the entire molecules. The initial concentration of styrene influences the rate of grafting. The preirradiation or the presence of monomer capable to bind more efficiently styrene leads to high conversion of neat polymer, which must contain an optimal concentration of 30 % styrene. The localization of energy on benzene ring is also favourable to the compatibilization with different polymer structures that are less miscible with pristine polymer.

The high thermal stable polymers, fluorinated compounds, can be modified by grafting under irradiation [172, 173]. The modification of PTFE, FEP and PFA with styrene can be achieved at room temperature by the EB exposure in two steps. The first irradiation was accomplished at high temperatures (385 °C for PTF, 350 °C for PFA and 290 °C for FEP) for initiating grafting, where polymers were brought in liquid state in nitrogen atmosphere. The next step was EB irradiation at 50 kGy for the accomplishment of process. The maximum grafting yields were attained for high preirradiation dose (500 kGy). The order of the increasing efficiency for these polymers was PFA > PTFE > FEP because of their structures. For the proton exchange membranes (PEM) PTFE was grafted by styrene under γ-irradiation in air at room temperature. After the immersion in styrene, the grafting process was performed at different temperatures. The grafting yields reached different levels after 20 h of reaction: 76 % for 60 °C, 57 % at 80 °C, 22 % at 100 °C and 17 % at 120 °C. It seems that the lower temperature hinders the formation of homopolymer.

3 Accelerated Degradation by Radiation Exposure

High energy radiation produces ionization and excitation in polymer molecules. These energy-rich species undergo dissociation, abstraction and addition reactions in a sequence of reactions leading to chemical changes. Scission and crosslinking of the polymer molecules, formation of small molecules and modification of the chemical structures of radiation processed polymers are responsible for the changes in material properties at macroscale level. The susceptibility of polymers to degradation is tightly related by the molecular structures, which may indicate the sensible sited of scission. Apart of the formation of radicals, the radiation degradation is controlled by oxygen diffusion through which the feeding of molecular oxygen is defined. The distribution of final oxidation products has a parabolic shape because the gradient of oxygen concentration exists [63]. The degradation mechanisms under radiation are based on the processed model compounds, n-hexadecane and squalene [174], whose structures. The main criterion for the evaluation of radiation effects on polymers is the ratio between the two main radiochemical yields: scission and crosslinking. The values exceeding unity denotes stabilization of polymers by crosslinking. Its values less than unity define the degradable polymers [10]. The first process after scission is the formation of peroxyl radicals, which are the precursors of final oxidation products. The contribution of these structures may be found in the oxidation rates, which are reflected by the accelerated oxidation initiated even at moderate temperatures starting from 50 °C (Fig. 11) [99, 175].

Fig. 11

The nonisothermal chemiluminescence spectra recorded on ethylene-propylene-diene terpolymer modified with silica

The oxidation of high energy irradiated polymers progresses in relation with their basic component structure and formulation, which is initiated by the scission of backbones. The activation energy (E_a) required for oxidative degradation is the key of stability evaluation. The values of activation energies are placed on the range between 100 and 120 kJ mol⁻¹ for different classes of insulation materials (polyethylene, ethylene-propylene copolymer, poly(vinyl chloride) used in the manufactures of cables for nuclear power plants [176]. The life time allows the prediction of the susceptibility to oxidation directly connected to the degradation mechanism and the service requirements [177]. The temperature regime of oxidation determines the ageing progress under different mechanisms, which modifies the diffusion rate of oxygen and, consequently, the Arrhenius dependency. The alteration of oxidation rate is also occurred due to the change in the molecular weight [178, 179], which characterizes the start of oxidation through the radical routes. The accumulations of carbonyl and hydroxyl containing compounds follow different rates because of the intermediate reactions involving hydroperoxides (Fig. 12) [180].

Fig. 12

Changes in absorbances of carbonyl () and hydroxyl () bands recorded for EPDM/PP blends. Blending ratios: a 80:20, b 60:40 and c 40:60

The evidences of existing differences between radiation degradation of polymers confirm the interaction between intermediates, which are the precursors of new structures [181]. The presence of additives like antioxidants or fillers turns the degradation onto specific alterations. An illustrative example for the contribution of nanostructured filled has been reported on EPDM/clay hybrids [97]. Apart of the improvement in the mechanical properties caused by radiation crosslinking, the irradiated nanocomposites show higher tensile strength than that of conventional composite and unfilled EPDM, inasmuch nanoclay dispersed in the matrix plays the role of reinforced filler which protects the matrix against degradation. The tensile strength of nanocomposites at 500 KGy irradiation dose is about 51 % higher than that of conventional composite, and is subsequently 78 % higher than that of unfilled EPDM.

The desired features that must be obtained with polymer nanocomposites are planned before their synthesis, because the intimate distribution of nanoparticles in polymer matrix is attained during the in situ crosslinking [104, 182–188].

The presence of nanofiller changes the probability of interradical reactions. The crosslinking of nanocomposites is somewhat diminished (Fig. 13, [189]), because the radical density is modified by inorganic component molecules. According with the FTIR spectral records, the oxidation of LDPE runs somewhat faster, because in the presence of OMMT, some additional reactive sites come from the Hoffman elimination of the organo-modifier of the clay nano-particles. These results are related to different levels of modification in the values of polymer free volume accompanying to the intimate interaction between ionized molecules and organoclay particles. The electron irradiation of PP/OMMT enhances the compatibility between the two components of the composite ensuring better cohesion in between [190].

Fig. 13

Development in the insoluble fraction of EB-irradiated LDPE

The vicinity of polymer molecules and nanoparticles is an auspicious factor for the formation of bridges between components [92], so that the thermal and radiation stabilities are improved and their lifetime is significantly longer.

The mechanical properties of polymer nanocomposites are also influenced by the chemical treatment of nanoparticles due to the different neighborhood in the material. The free volume that characterizes the density of material is modified and, consequently, the penetration of fluids (solvents, oxygen) is rather favorable to degradation. The diffusion of xylene in ethylene-propylene diene terpolymer is unlike, if material presents different consistency (Fig. 14, [191]). The competitive radiochemical processes, crosslinking of polymer and degradation of covering layer are the most important reasons responsible for the different shapes of swelling curves.

Fig. 14

Changes in the swelling degree of γ-irradiated of various EPDM samples

Apart from the guiding the evolution of oxidative degradation by the adsorption of radicals on the enormous interphase surface, nanoparticle component of hybrid materials influences the modification induced in the molecular weights [192]. The ratios between weight and numerical average molecular weights are sharply increased and the macroscale properties are consequently modified. The activation energies involved in radiation degradation follow similar trend, because the process enthalpies are directly correlated with molecular sizes (Fig. 15).

Fig. 15

Changes in the polydispersity of PLLA nanocomposites. (Square) before irradiation, (Circle) after exposure to 10 kGy

The addition of nanoparticles in the material formulations represents the key of polymer modification [193], which brings about the attaining upgraded performances simultaneously with the compatibilization of various blends. The features of polymer nanocomposites can be converted by high energy exposure and their application can be extended due to the favorable adjustment.

4 Conclusion

Polymeric nanocomposites show useful properties related to their stability. Their processing by exposure to high energy irradiation induces modifications, which enlarge the application limits. The potential applications can be found in various fields: automotive industry, from interior and exterior car parts to tyres, sporting goods, packaging, coatings, wire and cables, fuel cells, biomedical. The common feature is the long term stability investigated by accelerated degradation under irradiation. The interaction between basic polymer material and nanofiller component turns the functional characteristics onto the foreseen behavior, but the level of reinforcement depends strongly on the radiation stability of macromolecules.

Radiation processing is a tremendous way on which the filler is tightly incorporated in cured material. The main challenge in fabrication of these polymer nanocomposites for industrial applications is uniform dispersion of nanoparticles in the polymer matrix. For synthesis procedures, the scission of monomer promotes the promising adjustment of stabilization. In contrast, the decomposition/degradation of polymer phase creates new structures, which are able to be adsorbed onto the nanoparticle surface or they change the material properties to the unpredicted shift. However, the radiation treatment allows the proper preparation of ameliorated material in respect with conventional practice. During irradiation the basic aspects of stability and functionality are the dose thresholds, over which the proportion of degradation becomes evident.

The investigation achieved on the effect of nanofillers on the material durability must consider the impact of degradation on environmental health and safety. The post-irradiation stage of polymer hybrids pursues the long term operation under optimal parameters. The research effort on radiation processed polymer nanocomposites emerges to beneficial applications even for nuclear industry that develops crucial diminishment of degradation.