Investigation on Molecular Structures of Electron-Beam-Irradiated Low

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Investigation on Molecular Structures of Electronbeam Irradiated LDPE by Rheology Measurements Hengti Wang, Linfan Li, Jipeng Guan, Haiqing Jiang, Rongfang Shen, Xiaojun Ding, Jingye Li, and Yongjin Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00062 • Publication Date (Web): 09 Mar 2018 Downloaded from http://pubs.acs.org on March 9, 2018

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Industrial & Engineering Chemistry Research

Investigation on Molecular Structures of Electron-beam Irradiated LDPE by Rheology Measurements

Hengti Wang

1, 2, 3

, Linfan Li

1, 2

, Jipeng Guan1, 2, 3, Haiqing Jiang

1, 2

, Rongfang Shen

1, 2

,

Xiaojun Ding 1, 2, Jingye Li * 1, 2, Yongjin Li*3

1 Shanghai Institute of Applied Physics, Chinese Academy of Sciences, No. 2019, Jialuo Road, Jiading District, Shanghai 201800, People’s Republic of China 2 University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China 3 College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, No. 16 Xuelin Rd., Hangzhou 310036, People’s Republic of China

Key words: Electron-Beam irradiated LDPE, Molecular structure, Rheology measurements.

* E-mail: (Y. J. Li) [email protected]; (J. Y. Li) [email protected].

1

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT While the effect of high-energy irradiation on LDPE has been extensively investigated, the molecular structure changes under different irradiation conditions are less well-understood. In this work, we have made the systematic investigations on the molecular structure of electron beam irradiated LDPE under various irradiation conditions. The microstructures have been mainly characterized by rheological measurements using small amplitude oscillatory shear (SAOS) and analyzed by weighted relaxation spectrum and van-Gurp-Palmen plot. It was found that the linear viscoelasticity of LDPE exhibited a strong dependence on radiation atmosphere due to oxidative degradation in the presence of oxygen. An improved/destroyed network structure can be acquired when LDPE was irradiated in nitrogen/oxygen environment, as compared with that in air. Differences became more significant when irradiated at higher temperature (80 ºC) due to the facilitated diffusion of free radicals within polymer matrix. For the first time, the effects of electron-beam on LDPE under various irradiating conditions have been clarified.

2


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1 INTRODUCTION Electric cables are crucial constituents for nuclear power plant (NPP) instrumentation and control systems

1-2

. The safety-related cables must be qualified to maintain their function

against in-service NPP conditions and extreme design basis event (DBE) condition 3-6. Since polymers such as polyolefin are widely used base materials in insulation industry, the irradiation effects on polymer is becoming an important research field from nuclear science to various domains 7-12. LDPE is the most used polymer as insulation materials in NPP because of its superior properties such as physicochemical and mechanical properties under irradiation environment 13-15

. It is well-established that LDPE predominantly undergoes crosslinking upon

high-energy irradiations 16-17, shown to be beneficial for industrial application 18. With regard to this goal, the effects of irradiation on the molecular structure of LDPE have been attracted much attention and comprehensive reviews are available

19-21

. Jehnichen et al

22

have

investigated the network structure and crystallizing behaviors of irradiated LDPE by both sol-gel and thermal analysis. They demonstrated that the crosslink density increased with increasing absorbed doses. Valić et al 23 have studied the impact of high doses γ-ray radiation on oxygen permeability of LDPE films. Ryan et al

24

confirmed that the irradiation

accelerated aging in terms of structural and mechanical properties of LDPE. These results poses a question. As far as the environmental factors on radiation effects are concerned, previous studies have been focused on absorbed doses radiation and

28, 29

the energy

30

25, 26

, dose rate 27, type of

effects on the structure and properties of LDPE. The

influence of radiation temperature as well as atmosphere has not been elucidated, to the best 3



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of our knowledge. This might be helpful in the DBE-simulation of cables testing in NPP. Furthermore, the understanding on the relationship between the molecular structures to properties is relatively unexplored. Traditional characterization techniques of irradiated specimen seemed too difficult to distinguish the slight differences on the microstructures accurately at the different exposure conditions. For another thing, the rheology measurement seemed to be more effective in distinguishing the slight differences. It has been proven as an ideal tool with fingerprints to assess the molecular (such as linear, branch or crosslinked network) topology of polymer chains. According to literature

31-33

, dynamics of highly crosslinked network was dominated by

“Rouse-like diffusion” and arm retraction in a fixed network. The networks, containing both randomly distributed crosslinks and short pendent chains, exhibited enhanced elasticity, longer molecular relaxation and distinct local dynamics in comparison with the linear or low crosslinking counterparts

34-36

. Song et al

37

have investigated the linear rheology properties

of dicumy peroxide (DCP) induced crosslinked LDPE (XLDPE) and assessed the contributions of network structure to viscoelastic responses of XLDPE. It can be inferred that the crosslinked structure led to enhanced plateau modulus and additional contribution to the relaxations of XLDPE. More detailed identification on molecular architectures can be acquired from linear viscoelasticity using network elastic theory

38-40

. Though the crosslink

parameters could be calculated by several methods such as stress-strain

22

or swelling ratio

methods 24, 37, estimation by rheological strategy appeared to be more accurate. For instance, Kramer et al

41

have determined the values of plateau modulus (‫ܩ‬ே଴ ) due to the formation of

transient network structures. They precisely calculated the values of crosslink density (νd) and 4


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molecular mass between network strands (Mc). Hambourger et al

42

have estimated the

relevant parameters of crosslinked poly (dimethyl siloxane) (PDMS) through oscillating shear rheometry. Therefore, the aim of this work seeks to address the change of LDPE’s molecular structure and its reflection to rheology performance under different irradiation condition of electron-beam (EB). The rheology behavior was probed using small amplitude oscillatory shear (SAOS) and analyzed by representative criteria (e.g. van-Gurp-Palmen (vGp) plot, the plot of |η*(ω)| versus |G*(ω)|) and weighted relaxation spectrum, respectively. 1.2 MeV EB irradiation of LDPE sheets with high dose rate (23800 kGy/h) was implemented at different absorbed doses (ranging from 25 kGy to 400 kGy) under various irradiation conditions. We found the discrete differences of the molecular structures when varying the irradiation conditions. To the best of our knowledge, this is the first time to investigate the molecular structure of EB irradiated LDPE under various radiating conditions using rheology measurements.

2 EXPERIMENTAL SECTION 2.1 Materials The study was performed with the commercial LDPE (2426H, Mn=8.1×105 g/mol, Mw/Mn=2.2) purchased from Daqing petrochemical company (China). The density (ρ) and melting flow index (MFI) were 0.924 g/cm3 and 2.1 g/10min at 190 ºC with loading of 2.16 kg, respectively. The melting point (Tm), crystallization temperature (Tc) and the degree of crystallinity (Xc) were calculated to be 110.8 ºC, 99.6ºC and 28% from thermal measurements. 5



The o-xylene was obtained from Sinopharm Group (China). 2.2 EB irradiation LDPE sheets (100×100×1 mm3) without antioxidant were irradiated under 1.2 MeV EB-source in a range of 0-400 kGy with the dose rate of 23800 kGy/h. All specimens were radiated in a sealed box with titanium foil on the top for complete absorption of radiation. The alanine dosimeter (e-scan, Bruker, Germany) was utilized to determine error range of absorbed doses (approx. 4.2%). The radiation under various irradiation conditions (absorbed doses, temperature and atmosphere) was conducted in sequence. The total absorbed doses were 0 kGy (non-radiated LDPE), 25 kGy, 50 kGy, 100 kGy, 200 kGy, 300 kGy and 400 kGy). Two different radiation temperatures including room temperature (RT) of 25 ºC and 80 ºC were chosen for comparison. Variation of irradiation atmosphere involved air, nitrogen (N2) and oxygen (O2) environments. All irradiated specimen were kept in a N2 atmosphere at the temperature of 10 ºC until further experiments. 2.3 Classical Characterization Electron spin resonance (ESR, FA200, JEOL, Japan) measurements were performed at RT immediately after EB irradiation. The microwave field power was 1 mW. Spectroscopic parameters were modulation frequency 100.0 kHz, microwave frequency 9078.29 MHz and central magnetic field 325.41 mT. The relative radical concentration was calculated by the double integration method 43, 44. Fourier transform infrared spectroscopy (FT-IR, VERTEX 70V, Bruker, Germany) measurements were carried out at ambient temperature in vacuum state in the spectra range from 4000 to 400 cm-1 at a resolution of 2 cm-1 with a minimum of 64 scans. All recorded 6


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spectra were normalized for quantitative analysis. The carboxyl index (CI) was calculated from spectra using eq. 1. CI =

஺಴ಹమ ஺಴సೀ

× 100% (1)

Here AC=O and ‫ܣ‬஼ுమ referred to the integral areas of carbonyl groups at 1720 cm-1 and methylene groups at 720 cm-1, respectively. Tensile tests are performed by universal material testing system (model 5966, Instron, Germany) at RT with the stretching speed of 20 mm/min. All irradiated samples are punched out into dumbbell shape with 18×0.5×3 mm3 (L×W×T). Differential scanning calorimeter (DSC, Q2000, TA Instrument, USA) measurement was conducted in N2 atmosphere at heating/cooling rate of 10 °C/min with a temperature range from -50 to 200 °C. Tm and Tc values were determined. The degree of LDPE crystallinity could be estimated using eq. 2. ୼ு

ܺ௖ = ೘బ (2) ୼ு ೘

Here Xc was the crystalizing degree, ∆Hm referred to melting enthalpy of LDPE from first ଴ heating cycle and Δ‫ܪ‬௠ was the melting enthalpy of 100% crystalline LDPE (288 J/g) 45.

The gel-fraction (fgel) was determined by estimating the weight loss of specimen after extraction of soluble fractions by boiling o-xylene for 24h and followed by drying in a vacuum oven at 65 ºC to constant weights. Detailed fgel values of irradiated LDPE were calculated according to eq. 3. ݂௚௘௟ =

௠భ ௠మ

× 100 (3)

Where m1 and m2 were the initial weight and residual weight after swelling in o-xylene and drying of irradiated LDPEs. 7



2.4 Rheological Characterization Dynamic rheology (MCR301, Anton Paar, Austria) measurements of pristine and EB irradiated LDPE were carried out at 160 ºC under nitrogen atmosphere to avoid the degradation. Tests were executed with a parallel plate of 25mm diameter and a gap of 1 mm. The thermal stability of the all specimens at 160 oC can be ensured by thermal gravity analysis as shown in Fig. S-1. 2.4.1 Amplitude scanning tests Strain sweep tests in the amplitude range (γ) from 0.01%-1000% with frequency of 1 Hz were performed first to ascertain the linear viscoelastic region (LVE). A value of 5% of γ was determined and utilized in dynamic frequency sweeping tests. 2.4.2 Small amplitude oscillatory shear (SAOS) tests SAOS tests were conducted in the frequency range of 0.01-500 rad/s with the γ value of 5% to analyze the linear viscoelasticity of irradiated LDPE. Several representations such as vGp plot and the plot of |η*(ω)| versus |G*(ω)| of all specimens were plotted according to SAOS data for better comparison. The molecular weight distributions and the weighted relaxation spectrum were determined using Laplace transform 46 and edge preserving regulation method 47

by software Rheoplus from SAOS data, respectively.

2.4.3 Crosslink density analysis Based on the existence of plateau modulus due to formation of network structure, crosslinking parameters can be calculated from SAOS experiments, as shown in Eq. 4 41. ‫ܩ‬ே଴ = ߥௗ ܴܶ =

ఘோ் ெ೎

(1 −

ଶெ೎ ெ೙

) (4)

Here Mn referred to the number average molecule weight of pristine polymers and the 8


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parenthetical expression was Flory standard correction for influence of dangling chain-ends 41

which do not contribute to ‫ܩ‬ே଴

.

3 RESULTS 3.1 Irradiation effects of EB on LDPE at RT in the air: absorbed dosage dependence 3.1.1 Molecular structures

(b) Transmittance (%)

Intensity (a. u.)

400kGy 300kGy

200kGy 100kGy 50kGy 25kGy

(c)

80

60

60

14

30

20 Carboxyl index Gel content CRadicals

0

0

100

200

300

300kGy 200kGy 100kGy 50kGy 25kGy 0kGy

0

-1

-1

962cm

1500

1000

1720cm

500

Wave number (cm-1)

(d)

40

-(CH2)-n (n>4)

-C=C-

2000

340

B[mT]

Elongation at break (mm/mm)

320

Gel content/Carboxyl index (%)

300

-C=O 400kGy

9

100 8 7 6 50 Elongation at break Yeilding Strength Young's modulus

5 4 3

0 0

400

Yeilding strength/Young's modulus (MPa)

(a)

Cradicals(10 spin/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100

200

300

400

Absorbed dose (kGy)

Absorbed dose (kGy)

Figure 1. Absorbed dosage dependence on EB irradiated LDPEs conducted at RT in air: (a) ESR spectra, (b) FT-IR spectra; (c) Irradiation parameters (CRadicals, CI and fgel) and (d) mechanical properties (Young’s modulus, elongation at break and yielding strength) as a function of absorbed doses.

The amounts of free-radials during EB irradiation were determined by ESR measurement. As 9



Page 10 of 40

illustrated in Fig. 1a, the ESR spectra of EB irradiated LDPE with various absorbed doses, ranging from 25 kGy to 400 kGy, were measured immediately after the radiation process. All spectra exhibited similar shape with complex multiple peaks, which was attributed to the co-existence 21, 48, 49 of alkyl, allyl and peroxide radicals, suggesting similar irradiation effects by EB radiation throughout radiation range. ESR signals increased with increasing absorbed doses and thereby, the 400 kGy specimen demonstrated the highest intensity. Based on ESR data, the relative concentrations of free-radicals (CRadicals) were calculated and shown in Fig. 1c for quantification (Detailed listed in Tab. S-1). It was in consistence with the previous study

6, 22

. Initially, high-energy irradiation produced alkyl radical through release hydrogen

atom from the backbone of LDPE. The alkyl radical gradually shifted to the more stable allyl radicals with increasing absorbed doses

6, 24, 43, 50

. It was confirmed the two radials were

contributed to the formation of crosslinked network by hydrogen abstraction reaction

23, 50

.

Meanwhile, peroxide radials were found due to oxygen diffusion into vicinity of formed radicals or polymer matrix, which facilitated oxidative degradation upon exposure 6, 23. FT-IR spectrum was conducted for probing structural changes upon EB irradiation, as shown in Fig. 1b. Two intensive characteristic peaks at 1470 cm-1 and 720 cm-1 can be seen for all specimens, which were assigned to the deformation of methylene groups. Compared with non-irradiated LDPE, the appearance of weak signals at 1080 cm-1 and 962 cm-1, corresponding to the stretching vibration of C-O-C and trans-vinylene double-bond respectively, was observed in the spectra of irradiated LDPE. The intensity of these bands was proportional to the absorbed doses, implying oxidation of LDPE during radiation. Similar phenomenon was observed from the band of 1720 cm-1, assigned to the stretching 10


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vibration of carbonyl groups. Previous investigations have confirmed the existence of ketones and aldehydes as degradation products in LLDPE against ionizing irradiation

51

. CI values

were calculated by the ratio of integration areas of the band at 720 cm-1 and 1720 cm-1, as listed in Tab. S-1 and illustrated in Fig. 1c. It was observed CI displayed positive correlation towards absorbed doses, providing a direct proof of oxidation on LDPE under EB irradiation. The changes of mechanical properties on dependence of absorbed doses at RT under air environment were plotted in Fig. 1d (detailed in Tab. S-1, Fig. S-2). All samples exhibited typical ductile behaviors throughout the absorbed doses range performed, indicating LDPE’s practicability under irradiation in NPP. As presented, the yielding strength increased as a function of doses from 0 kGy (8.3 MPa) to 100 kGy (10.4 MPa). Similar observations were seen by Oka et al

52

, where they attributed the increase of tensile strength to existence of

crosslinked network. A sequential reduction of strength values were noticed by further increasing absorbed doses from 200kGy (10.2 MPa) to 400 kGy (9.7 MPa), indicating the dominating effect of oxidation which might result in decomposition of LDPE chains in the presence of oxygen. Similar trend was found for the changes of elongation at break, where a slightly increment of elongation value were acquired within 0 kGy to 50 kGy and then, a reduction from 100 kGy (750%) to 400 kGy (370%) were observed. Since crosslinked network was known to suppress strain deformation capacity and consequently diminish the stretching of LDPE chains

24, 53

, it was supposed that LDPE’s chain stretch reached the

saturation value with the absorbed doses of 100 kGy (detailed discussed later). The variation trends of Young’s modulus versus absorbed doses were opposite with that explained before. Young’s modulus dropped sequentially between 0 kGy to 100 kGy, in which a total reduction 11



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of 15.3 MPa were observed. A gradual increase was then observed as doses elevated up to 400 kGy. These corresponded with previous investigations upon γ-ray irradiation

24, 25

, in

which the reduction of stiffness at higher doses was ascribed to the propagation of oxidative degradation under air condition. Tensile properties were relatively sensitive to structural changes induced by EB irradiation and correlated quite well with the gel fraction analysis. As plotted in Fig. 1c (detailed in Tab. S-1), bare of gel was detected at doses less than 25 kGy, while fgel increased from 1.2 % to 45.6 % with increasing absorbed doses from 25 kGy to 100 kGy. At higher doses, fgel values enhanced quite slightly and attained 72.1 % with the doses of 400 kGy. Similar observations could be observed by DSC measurements (shown in Tab. S-2, Fig. S-3)

Scheme 1. The crosslinking and degradation reaction of LDPE under EB irradiation at RT under air environment.

The proposal mechanism of EB irradiation in air atmosphere was depicted in Scheme 1. It 12


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can be concluded that crosslinking reactions (product II, III and IV) and oxidative degradation (product V, VI and VII) coexisted during EB radiation process under air atmosphere at RT. At low doses level (25-200 kGy), crosslinking predominated while the oxidative degradation became dominant with increasing absorbed doses and turned as a leading role at higher dose levels (300-400 kGy). 3.1.2 Rheology behaviors

(a) 10

(b)

5

3

νd (mol/dm )

0.3

10

10

0kGy 25kGy 50kGy 100kGy 200kG 300kGy 400kGy

4

3

10

-1

0

10

1

10

10

5.0x10

0.2

Crosslink density (ν d) Storage moduus G′0.1rad/s

2

0

Angular Frequency, ω (rad/s)

4

G′0.1rad/s (Pa)

G' (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100

200

300

0.0

400

Absorbed dose (kGy)

Figure 2. Absorbed dosage (σ) dependence on the rheological performances of EB irradiated LDPEs at RT in air: (a) Storage modulus (G'(ω, σ)) with angular frequency (ω) from 0.1 to 500 rad/s, (b) The crosslink density (υd) and G'(0.1rad/s) values of LDPEs as a function of σ.

Rheological measurements were performed for better quantification. Fig. 2 showed the linear viscoelastic behaviors of EB irradiated LDPE with different σ by SAOS tests. The critical γ value within linear viscoelasticity region (LVE) of all samples was determined as 5 % by amplitude sweeping tests (shown in Fig. S-4). Fig. 2a showed the storage modulus (G'(ω, σ)) versus angular frequency (ω) for EB irradiated LDPE as a function of absorbed doses (σ). The plots of loss modulus (G''

(ω, σ)),

complex viscosity (|η*(ω, σ)|) and damping factor (Tan δ 13



(ω, σ))

Page 14 of 40

were shown in Fig. S-5.The non-irradiated LDPE showed non-terminal characteristics

and displayed the classical power-law behavior at low ω region (‫ ܩ‬ᇱ ∝ ߱ଶ ) 54. As shown, the plots of G'(ω, σ) for pristine LDPE were almost linear in the log-log coordinate (indicating liquid-like behavior). G'(ω, σ) values of EB-irradiated LDPE increased in magnitude over the whole ω range with increasing σ. A gradual formation of frequency-independent plateau in low ω regime was observed with increasing σ, implying the liquid-to-solid transitions for radiated samples in the terminal region

55, 56

. We speculated that the extra elasticity was

entirely attributed to the formation of crosslinked network which limited the mobility of polymer chains in molten state

5, 24, 37, 38, 41

. The changes of elastic responses were less

profound and even a slight reduction was observed for irradiated LDPEs at higher σ values (σ>200kGy). It can be inferred a large number of LDPE chains were damaged with σ above 200kGy, suppressing the formation of network and resuling in a slower growth rate of elastic responses while a faster rate of viscous performance

5, 24, 37, 54

. The network in 400 kGy

sample was considerably destructed due to the enhancement of chain degradation by radio-oxidation. The terminal values of G'(0.1 rad/s, σ) on dependence of σ were plotted and presented in Fig.2b (detailed listed in Tab. S-3). G'(0.1 rad/s,

σ)

increased from 3090 Pa to 36300 Pa with the

increment of σ from 25 kGy to 200 kGy, suggesting the intensification of crosslinking. It was possible to provide numerical values to quantify the level of crosslinking. υd and molecular weight between network points (Mc) were calculated through rubber-elasticity theory using the plateau modulus (‫ܩ‬ே଴ ) values determined from SAOS measurements

22, 38, 41, 42

. Fig. 2b

showed υd as a function of σ (detailed listed in Tab. S-3). υd increased rapidly with increasing 14


Page 15 of 40

σ from 25 kGy (0.15 mol/dm3) to 200 kGy (0.33 mol/dm3), confirming the development of crosslinked network in the EB irradiated LDPEs. It was noteworthy that while υd elevated quickly from 0.2 mol/dm3 to 0.33 mol/dm3 as rising σ from 100 kGy to 200 kGy, fgel increased quite slightly (shown in Fig. 1i). The crosslinking of LDPE with σ=200 kGy might mainly proceed within the network. And then, the extent of network formation slowed down and even a reduction were found with further increase σ from 300 kGy (0.32 mol/dm3) to 400 kGy (0.30 mol/dm3). 200 kGy was satisfactory for making networks during EB irradiation for LDPE and higher σ though increase the gel content, it can no further enhance network structure since degradation was predominant. This variation seemed to be in agreement with the observations of G'(0.1 rad/s, σ) values, as plotted in Fig. 2b. The changes of G'(0.1 rad/s, σ) values would be employed as analogy of υd for simplification in the Discussion section.

(a)

90

(b) 10

6

80 70

5

10 60 50 40 30 20 10

η∗ (Pa.s)

Phase angle, δ (°)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0kGy 25kGy 50kGy 100kGy 200kG 300kGy 400kGy

4

10

3

10

2

0kGy 50kGy 200kG 400kGy

25kGy 100kGy 300kGy

10 10

3

4

10

∗

10

5

10

Complex modulus, |G | (Pa)

3

4

∗

10

10

5

G (Pa)

Figure 3. (a) van Gurp-Palmen plot (vGp) plot and (b) the plots of |η*(ω, σ)| versus |G*(ω, σ)| of EB irradiated LDPEs.

Two advanced rheology representations

12, 58, 59

, which were sensitive in tracing topological

difference were involved. Fig. 3a showed the van Gurp-Palmentt (vGp) plots 11, 12, 54, 57-59, i.e. a curve of phase angles (δ(ω, σ)) versus complex modulus (|G*(ω, σ)|), of the LDPE samples. It 15



was reported that for typical linear polymers, δ(ω,

σ)

Page 16 of 40

∗ was 90º while |‫(ܩ‬ఠ,ఖ) | → 0 and

∗ ᇱᇱ |‫(ܩ‬ఠ,ఖ) | ≅ ‫(ܩ‬ఠ,ఖ) , showing the completely viscous liquid behavior. In contrast, δ(ω, σ) was 0º ∗ ᇱ while |‫(ܩ‬ఠ,ఖ) | ≅ ‫(ܩ‬ఠ,ఖ) regarding materials with purely elasticity

24, 60

such as the fully

crosslinked polymers. As shown in Fig. 3a, vGp plots were strongly affected by σ. As σ increased up to 100 kGy, δ(ω, σ) reduced systematically. The reduction was more pronounced in low |G*(ω, σ)| region as opposed to high counterpart, signifying that elastic contribution predominant over the loss aspect as a result of the gradual formation of crosslinked network with increasing σ. When σ increased to 200 kGy, δ(ω, σ) increased and the slope became shaper as σ amount to 400 kGy, confirming enhancement of viscous due to considerable chain scission reactions at higher σ. The vGp results were quite different with previous results using classical characterizations and SAOS tests in which continuously increasing values were observed for 200kGy sample as compared with lower σ ones. It can be inferred for the first time that vGp plot showed as a more sensitive indicator of the presence of chain scission even though radio-oxidation was not dominated over crosslinking during EB irradiation of LDPE. The plot of |η*(ω, σ)| versus |G*(ω, σ)|, which was comparable to the plot of steady viscosity towards stress against steady shear was chosen as the second representation 59. Previous study have reported that classical linear polymers displayed a plateau of |η*(ω, σ)| in lower |G*(ω, σ)| regime while the crosslinked or even nano-percolated systems always showed a divergence in terminal area

60, 61

. As shown in Fig. 3b, |η*(ω, σ)| plateau (approx. 23000 Pa.s) was acquired

for non-irradiated LDPE. Significant changes of flow behavior occurred upon EB irradiation on LDPE. For EB irradiated samples the Newtonian plateau in low region vanished while their |η*(ω, σ)| dependence became very steep, signifying the strong flow restrictions due to the 16


Page 17 of 40

crosslinked topologies 59. Similar trends can be found in this plot as a function of σ: initially the gradual deviation was seen from 25 kGy to 200 kGy and then, negative deviations were observed as further increasing σ to 400 kGy. 0kGy 25kGy 50kGy 100kGy 200kGy 300kGy 400kGy

W (log m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10

5

10

6

10

7

Molar mass (g/mol) Figure 4. Molar mass distributions (MWD) of EB irradiated LDPEs with different absorbed doses. The MWD plots were derived from SAOS data using the software Rheoplus.

MWD was used to verify linear rheology characters of all samples (Fig. 4). A unimodal distribution of MWD was observed for the non-irradiated LDPE. As σ values increased from 25 kGy to 200 kGy, the peak of MWD shifted towards the high molecular weight side. It was noteworthy the most significant deviation was found from 100 kGy to 200 kGy. An additional peak at higher molecular weight region was found for all irradiated LDPEs. This peak became more amplitude with increasing σ, which was assigned to the introduction of crosslinked architecture by coupling of macromolecular radials during radiation. When σ=300 kGy, both MMD peaks remained constant, implying that both the scissoring and coupling of macromolecular radicals were dominant. At the highest σ values (400 kGy), 17



MMD peaks somewhat narrowed and moved to lower molecular weight side. It was inferred oxidative degradation determined the overall effects of EB irradiation at relatively high σ values (detailed in Tab. S-3). Although the molecular parameters of the irradiated LDPEs were not truly exact values, the analysis was performed as a quantitative approximation for the purpose of comparison. The rheology measurements were proved as a very effective strategy to evaluate the molecular structure of EB irradiated LDPE. It can be concluded that network structures were gradually formed with increasing σ up to 200 kGy due to the predominant crosslinking reaction at this σ level. For further increasing σ, a slight destruction of network was observed, which was attributed to severe radio-degradation at higher σ. The vGp plot showed as more sensitive indicator of existence of chain scission even though radio-oxidation was not dominated during EB irradiation of LDPE. Systematic investigations on the EB irradiated LDPE were carried out by the rheological analysis and focus was paid on the EB irradiation conditions (irradiation media and temperature) effects in the following sections.

3.2 Rheology of EB irradiated LDPE: irradiation atmosphere dependence The impact of oxygen contents is closely related to oxidative degradation during irradiation. In this section LDPEs were radiated under three media (i.e. N2, air and O2) at RT with the σ up to 200 kGy. Fig. 5 showed the rheology measurements (Section I: linear viscoelasticity in Fig. 5a-c, Section II: the plots of |η*(ω, σ)| versus |G*(ω, σ)| in Fig. 5d-f and Section III: vGp plots in Fig. 5g-h) for the series of EB irradiated LDPEs under various atmospheres.

18


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Figure 5. Rheology behaviors of EB irradiated LDPEs under various atmospheres (N2, air or O2) with σ ranging from 0 to 200 kGy at RT: section (I): G'(ω, σ) of specimen irradiated in (a) N2, (b) air and (c) O2; section (II): van Gurp-Palmen plot of LDPE irradiated in (d) N2, (e) air and (f) O2 and section (III): the plot of |η*(ω, σ)| versus |G*(ω, σ)| of samples irradiated in (g) N2, (h) air and (i) O2, respectively.

G'(ω, σ) curves in a wider ω sweeping range (0.01-500 rad/s) were tested for evaluating more differences on rheological responses in LVE under various atmosphere. Although G'(ω,

σ)

increased by increasing σ values for LDPE irradiated under various media, the increment of 19



G'(ω, σ) was greater for LDPE under N2 (Fig. 5a) while smaller for that under O2 (Fig. 5c), as compared with samples processed in air (Fig. 5b). This phenomenon turned significant when σ was higher than 100 kGy. It was indicated samples irradiated in N2 had the greatest amount of elasticity while that in O2 contained the smallest when LDPE were irradiated with the same σ. Similar trends were observed from the plots of |η*(ω, σ)| versus |G*(ω, σ)|, where the deviation from Newtonian plateau increased sequentially by changing media from N2 (Fig. 5d), air (Fig. 5e) to O2 (Fig. 5f) for LDPEs with identical σ. In other words, flow restrictions of irradiated LDPEs exhibited negative correlations with O2 contents in irradiation media. vGp plots is more sensitive to the variation (shown in Fig. 5g-i). For LDPEs with σ from 0 kGy to 50 kGy, a linear reduction of δ(ω, σ) with increasing |G*(ω, σ)| was observed, irrespective of radiation atmosphere. The decent degree showed a strongly dependent on both σ values and irradiated media: though all the amplitude turned slower with increment of σ, it was more significant for LDPEs under N2 (Fig. 5g) while less for that under O2 (Fig. 5i) as compared with samples irradiated under air with the same σ (Fig. 5h). When LDPEs irradiated at higher σ, the shape of vGp plots differed versus radiating conditions. For LDPE irradiated under N2 (Fig. 5g), an upward tendency of δ(ω, σ) versus |G*(ω, σ)| was observed, implying significantly enhanced elasticity due to gradual formation of crosslinked network; For LDPE irradiated under air (Fig. 5h), the samples with 100 kGy exhibited a maximum at (26800 Pa, 22 º) while the 200 kGy’s one displayed this at (37193 Pa, 37 º). This meant that the viscous was dominated for specimen with 200 kGy irradiation in air. For LDPEs under O2 (Fig. 5i), similar trends were found as compared with that under air: the plots for the 200 kGy samples were shifted to lower |G*(ω, σ)| and higher δ(ω, σ) values. This is a direct evidence that the 20


Page 20 of 40

Page 21 of 40

network structure was considerably damaged when irradiated at 200 kGy under O2 atmosphere. (a)

0kGy 50kGy 200kGy

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Figure 6.Weighted relaxation spectra of EB irradiated LDPEs processed in (a) N2, (b) air and (c) O2 with σ ranging from 0 to 200 kGy at RT.

The weighted relaxation spectrum of pristine and EB irradiated LDPE from 25 kGy to 200 kGy in three different media (N2, air and O2) were derived from SAOS data

47

and shown in

Fig. 6. It is used for further exploring small molecular processes 5, 11, 24, 37, 41. A slight peak at the relaxation time (τ~10 s) was observed for non-irradiated LDPE, associated to relaxation of LDPE segments. Upon irradiation, an additional peak at the τ in the range 100s < τ < 500s was induced by the relaxation of restricted LDPE segments within crosslinked network, since Mc was larger than LDPE’s critical entangle molecular weight (Me,LDPE=1850 g/mol) 54. It was found that with increasing σ, both the two relaxations peaks became more amplitude, being indicative of the retarded dynamics of LDPE segments due to the gradual formation of crosslinked network. The differences among samples (identical σ values) with various irradiation atmospheres were more subtle. For LDPEs irradiated with same σ, the strengths (τH(τ)) of long-term relaxation, which was ascribed to the dangling chains in networks, increased with increasing oxygen contents of radiating atmosphere. For instance, τH(τ) of 21



Page 22 of 40

LDPE under N2 are significant high and even exceeded measurement range (Fig. 6a), suggesting the typical highly crosslinked behaviors where dynamics was dominated by “Rouse like” diffusion and arm-retraction in crosslinked network

31-33

. The long-term

relaxation for samples under O2 showed the lowest amplitude among irradiated LDPEs with identical σ. This can be explained by the existence of high content of non-elastic defects (“loops” or branches) as network imperfections (Fig. 6c), where the dynamic dilution and arm-retraction dominated the viscoelasticity

32

. The direct conclusion from this was that an

improved/destroyed network structure can be acquired when LDPE was irradiated in N2/O2, as compared with that in the air. By virtue of the discrepancies of rheology responses among different atmosphere, we concluded that the irradiating media during EB irradiation played a huge effect on the elasticity changes of LDPEs. The network structure of LDPEs was closely related to σ due to the complex competitions between radio-induced reactions of crosslinking and oxidative degradation. It was indicated that the existence of O2 from atmosphere favored the radio-degradation due to production of peroxide radials (discussed later).

3.3 Rheology of EB irradiated LDPE: irradiation temperature dependence It is well-established that irradiation-induced chemical reaction was driven by the variation of Gibbs free energy and thereby closely related to the temperature where a rise of 10ºC doubled the reaction rates 24 ,62. The impact of irradiating temperature will be estimated in this section. The irradiating temperature of 80 ºC, which was highly above Tg, LDPE while below Tm, LDPE (110.8 ºC) was chosen for eliminating the effect of intermolecular slippage during test 22


Page 23 of 40

process. (a)

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Figure 7. Rheology behaviors of EB irradiated LDPEs under various atmospheres (N2, air or O2) with σ ranging from 0 kGy to 200 kGy at 80ºC: section (I): G'(ω, σ) of specimen irradiated in (a) N2, (b) air and (c) O2; section (II): van Gurp-Palmen plot of LDPE irradiated in (d) N2, (e) air and (f) O2 and section (III): the plot of |η*(ω, σ)| versus |G*(ω, σ)| of samples irradiated in (g) N2, (h) air and (i) O2, respectively.

Fig. 7 (Section I) showed the linear rheology of irradiated LDPEs under various atmosphere at 80ºC where σ values were ranged from 0 kGy to 200 kGy. The variations of nonterminal 23



behaviors among various media were more pronounced as compared with the serious at RT. A much higher (Fig. 7a)/lower (Fig. 7c) plateau values of G'(ω=0.01rad/s, σ) were observed when irradiated under N2/O2 at 80ºC, as compared with their correspondence (Fig. 5a / Fig. 5c) at RT. Advanced criteria were utilized for further comparison. As shown in Fig. 7 (Section II), more steep |η*(ω, σ)| dependence was found when irradiated under N2 at 80ºC while even the |η*(ω, σ)| plateaus at low |G*(ω, σ)| especially irradiated at 200 kGy were observed for LDPEs under O2 at 80 ºC. The vGp plots displayed more distinct divergence for LDPEs irradiated from O2 (Fig. 7g), air (Fig. 7h) to N2 (Fig. 7i) at 80ºC with regard to that at RT from O2 (Fig. 7g), air (Fig. 7h) to O2 (Fig. 7i), respectively. Besides, the weighted relaxation spectra turned more sensitive to the variations. The peak intensity for long-term relaxation of LDPEs within network was further enhanced/decreased for samples irradiated under N2/(air, O2) at 80 ºC, as shown in Fig. 8. 6

(a)

0kGy 50kGy 200kGy

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Page 24 of 40

8.1s 2.9s

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Figure 8.Weighted relaxation spectra of EB irradiated LDPEs in (a) N2, (b) air and (c) O2 with σ ranging from 0 to 200 kGy at 80ºC.

It was confirmed that the irradiation of LDPE at 80ºC under O2 atmosphere mostly resulted in chain-breaking accompanied with the most pronounced oxidation while in the case of LDPE irradiated at 80 ºC under N2 led to the most completed network structure where the 24


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crosslinking dominated. The changes could be attributed to the enhanced diffusion rate of radials as well as polymeric segments when irradiated above Tg, LDPE (discussed later).

4 DISCUSSION The EB irradiation effects on the molecular structure and rheology performance of LDPEs have been investigated. Simultaneous crosslinking and carbon-carbon bond cleavage reactions can be confirmed. The phenomena were possibly induced by formation of alkyl/allyl radicals and the peroxide radicals in the presence of O2 when irradiated under air. At low σ levels (25-100 kGy), crosslinking is predominated, which could rationalized the enhanced fgel, mechanical properties and elastic modulus. With increasing σ from 100 kGy to 200 kGy, chain scission was emergent, evident from the variation on vGp plot (Fig. 3a). At higher σ levels (300-400 kGy), oxidative degradation was dominated while crosslinking also occurred, as verified from the decreased rheological elastic responses with the increased gel content. It should be mentioned that these irradiation induced reactions were mostly occurred within amorphous boundary phases at σ values less than 200 kGy while the crystalline area were destroyed when σ higher than 300 kGy, confirmed from DSC measurements (Fig. S-3). The effect of LDPEs’ EB irradiation under various environments were further explored by means of rheology measurements, in order to clarify radio-induced chain scission by virtue of O2 within amorphous phases. It was confirmed a significantly enhanced/destroyed network structure can be acquired when LDPE was irradiated in N2/O2, as compared with that in air.

25



Page 26 of 40

4.1 Irradiation effects of LDPE under various irradiation conditions The systematic estimation of EB radiation effects on molecular structure and the rheology property of LDPE were reorganized, in terms of irradiating conditions like σ (ranging from 0 kGy to 200 kGy), radiating media (N2, air and O2) and temperatures (RT and 80ºC), as depicted in Fig. 9.

Figure 9. The CRadicals, fgel and G'(ω=0.01rad/s) values of EB irradiated LDPEs as a function of σ (0, 25, 50, 100 and 200 kGy) and irradiation atmosphere (N2, air and O2) irradiating at different radiating temperature: (a) CRadicals at RT, (b) fgel at RT, (c) G'(ω=0.01rad/s) at RT,

and

(d) CRadicals at 80ºC, (e) fgel at 80ºC and (f) G'(ω=0.01rad/s) at 80ºC.

The CRadicals values calculated from ESR spectra were shown in Fig. 9a/9d, as a function of irradiating conditions. CRadicals increased with increasing σ, irrespective of atmosphere, when LDPEs were irradiated at RT. Meanwhile, a positive correlation between oxygen contents within atmosphere and CRadicals was observed and shown in Fig. 9a (CRadicals with identical σ and irradiating temperature: O2>Air>N2), confirming the production of excess peroxide 26


Page 27 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


radials on dependence of O2 contents. The dependence of CRadicals on σ and media became more significant when samples were radiated at 80ºC (i.e. CRadicals with identical σ and irradiating atmosphere: 80 oC>RT) (Fig. 9d). It is in accordance with previous speculations, where the diffusion of radicals were accelerated due to altered Gibbs free energy

24 ,62

at

higher temperatures. The irradiating temperature played as an important factor towards radical chemistry induced by EB irradiation. The fgel values were measured secondly as a function of different processing conditions. As shown in Fig. 9b, 9e, an increment of fgel were found from 100 kGy to 200 kGy while the amplification decreased by further increasing σ up to 400 kGy. On the other hand, although fgel reduced by changing atmosphere from N2, air to O2 for all irradiated LDPEs, an abnormal change was observed when σ=50 kGy. The highest fgel values were acquired for LDPEs irradiated at air, as compared with that under N2 or O2 at 50 kGy. Since crosslinking were predominated at this σ level, the combination of alkyl radicals for LDPEs at σ=50 kGy under N2 might proceed within the network so that fgel was slightly lower than the counterparts under air. Detailed mechanism at lower σ values would be investigated in our further work. The changes of G'(0.01 rad/s, σ) values are presented in Fig. 9c, 9f. It can be regarded as an analogy of υd. The G'(0.01 rad/s, σ) values increased with increment of σ while decreased by increasing O2 content of radiating media (i.e. G'(0.01 rad/s, σ) with identical σ and irradiating temperature: N2>Air>O2) (Fig. 9f). The G'(0.01 rad/s, σ) was much higher in the EB irradiated LDPEs at 80ºC than that with identical σ at RT (G'(0.01 rad/s, σ) with identical σ and irradiating atmosphere: 80oC>RT). This is in agreement with changes of molecular structures discussed before. It is indicated that the irradiating atmosphere played as a decisive factor for network 27



formation while the irradiating temperature and absorbed doses also impacted the crosslinked structures to a great extent.

4.2 Mechanism of EB irradiation on LDPE The differences on the formation of free radicals and LDPE’s networks under different radiating media and temperatures could be clarified, as illustrated in Sch. 2. Since this study mainly focus on LDPE’s crosslinking reaction induced by EB irradiation; besides, the pristine LDPE in this work showed non-terminal characteristics and displayed the classical power-law behavior at low ω region (‫ ܩ‬ᇱ ∝ ߱ଶ , ‫ ܩ‬ᇱᇱ ∝ ߱ ) (shown in Fig. 2a and Fig. S-5a). Thus, the original LCB structure of the non-irradiated LDPE in this work can be neglected.

28


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Scheme 2. EB irradiation on the formation of free radical (Sch. 2a-c) and the crosslinked network (Sch. 2d-i) under various atmospheres at different radiating temperatures: the radicals in (a) N2, (b) air, (c) O2, and the networks formed (d) in N2 at RT, (e) in air at RT, (f) in O2 at RT, (g) in N2 at 80 oC, (h) in air at 80 oC and (i) in O2 at 80 oC.

In absence of O2, abundant production of alkyl free-radicals can be expected (Sch. 2a). The radicals immediately combined with each other to propagate three-dimensional network with crosslinked LDPE chains. LDPEs irradiated under N2 led to a significant increment of crosslink density, which was evident from rheology measurements due to the restricted mobility of macromolecules (shown in Sch. 2d). If O2 was present, peroxide radials were generated because of O2 diffusion into LDPE matrix during irradiating process (Sch. 2b-c). It was reported

23

the peroxide radical favored chain breaking due to its further reaction with

adjacent radicals during radiation. The production of crosslinked network was suppressed when irradiated in air (Sch. 2d), as compared with that under N2. Since the degradation kinetics of EB irradiated LDPE are dependent on oxygen accessibility, further deterioration of network was expected for LDPEs irradiated under O2, as shown in Sch. 2e-f. The differences by adjusting irradiating media became more significant when irradiated at 80 ºC. It is known the irradiation-induced chemical reaction is driven by the Gibbs free energy. Thereby the reaction rate is closely related to the temperature, where a rise of 10ºC doubled the rate

24 ,62

Therefore, both crosslinking (Sch. 2g) and radio radio-oxidation (Sch. 2h-i)

reactions were accelerated during EB irradiation process, which was reasonable with regard to enhanced diffusion rate of both radicals and LDPE segments above Tg, LDPE. 29



These speculations were in agreement with the rheology measurements using SOAS tests, representative criteria and the relaxation spectra. Although the model offered are not truly the exact model in explaining the effects of irradiation, this can be used as an approximation for comparison.

5 CONCLUSION The EB radiation effects on LDPE at different irradiation conditions have been investigated. Based on FT-IR, ESR and the characterizations on fgel and mechanical property, it was concluded that 1.2 MeV-EB irradiated LDPEs in air at RT undergone both crosslinking and degradation throughout σ range from 25, 50, 100, 200, 300, to 400kGy. The SAOS tests accompanied with representative criteria were utilized for quantification. It was found that the competition between crosslinking and degradations during irradiation was closely related to absorbed doses: At low σ level (25-100 kGy), crosslinking predominated as evidenced by the significantly increased fgel, elastic modulus and υd. With increasing σ from 100 kGy to 200 kGy, chain scission was emergent, as evident from vGp plot. At higher σ levels (300-400 kGy), oxidative degradation become dominated while crosslinking also occurred, implying from the decreased rheological elastic responses accompanied with the slowed growth of fgel. The influence of irradiating atmosphere (N2, air and O2) were further explored by means of rheology with σ ranging from 25, 50, 100, to 200 kGy. Significantly enhanced/damaged network structure was detected when LDPE was irradiated in N2/O2, as compared with that in air. These differences turned more pronounced when irradiated at 80 ºC, ascribed to the facilitated diffusion of both radicals and LDPE segments above the temperature of Tg, LDPE. 30


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For the first time, the effects of 1.2 MeV EB irradiation on LDPE under various irradiating conditions are clarified by the rheological measurements combined with the constitutive analysis.

Supporting Information

The supporting information is available free of charge on the ACS Publication website. Additional data including numerical data for the mechanical properties, gen contents (fgel), free radial concentrations (CRadicals) and carboxyl index (CI) (Tab. S-1), Thermal parameters measured from DSC tests (Tab. S-2), Melt characterization from SAOS tests (Tab. S-3), TGA and DTG curves (Fig. S-1), Strain-stress curves (Fig. S-2), DSC thermos-grams (Fig. S-3), Amplitude sweeping tests (Fig. S-4), Frequency sweeping tests (Fig. S-5) and Morphologies (Fig. S-5) of the EB irradiated and non-irradiated LDPE specimens.

ACKOWLEDGEMENTS This work was financially supported by the “Strategic Priority Research Program” of the Chinese Academy of Science (XDA02040300), the National Natural Science Foundation of China (11575277, 21674033, 21374027) and the National Key R&D Program of China (2017YFB0307704).

REFERENCES (1) Gillen, K. T.; Celina, M.; Clough, R. L. Density Measurements as a Condition Monitoring 31



Approach for Following the Aging of Nuclear Power Plant Cable Materials. Radiat. Phys. Chem. 1999, 56, 429-447. (2) Wise, J.; Gillen, K. T.; Celina, M.; Clough, R. L. Time Development of Diffusion-Limited Oxidation Profiles in a Radiation Environment. Radiat. Phys. Chem. 1997, 49, 565-573. (3) Gillen, K. T.; Clough, R. L.; Ganouna-Cohen, G.; Chenion, J.; Delmas, G. The Importance of Oxygen in LOCA Simulation Tests. Nucl. Engin, Des. 1982, 74, 271-285. (4) Jahan, M. S.; King, M. C.; Haggard, W. O.; Sevo, K. L.; Parr, J. E. A Study of Long-Lived Radicals in Gamma-Irradiated Medical Grade Polyethylene. Radiat. Phys. Chem. 2001, 62, 141-144. (5) Berger, K.; Keimel, C.; Helfer, E.; Haar, B.; Mattausch, H.; Riess, G.; Kem, W. The Effects of e-Beam Crosslinking of LDPE on the Permeation of Hydrocarbons. J. Appl. Polym. Sci. 2017, 134, 44968 (1-8). (6) Suljovrujic, E. Post-Irradiation Effects in Polyethylene Irradiated under Various Atmosphere. Radiat. Phys. Chem. 2013, 89, 43-50. (7) Dadbin, S.; Frounchi, M.; Saeid, M. H.; Gangi, F. Molecular Structure and Physical Properties of e-Beam Crosslinked Low-Density Polyethylene for Wire and Cable insulation applications. J. Appl. Polym. Sci. 2002, 86, 1959-1969. (8) Auhl, D.; Stange, J.; Münstedt, H.; Krause, B.; Voigt, D.; Lederer, A.; Lappan, U.; Lunkwitz, K. Long-Chain Branched Polypropylene by Electron Beam Irradiation and Their Rheological Properties. Macromolecules 2004, 37, 9465-9472. (9) Rolón-Garrido, V. H.; Kruse, M.; Wagner, M. H. Size Exclusion Chromatography of Photo-Oxidated LDPE by Triple Detection and Its Relation to Rheological Behavior. Polym. 32


Page 32 of 40

Page 33 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Deg. Stab. 2015, 111, 46-54. (10) Goulas, A. E.; Riganakos, K. A.; Kontominas, M. G. Effect of Ionizing Radiation on Physicochemical and Mechanical Properties of Commercial Monolayer and Multilayer Semirigid Plastic Materials. Radiat. Phys. Chem. 2004, 69, 411-417. (11) Rolón-Garrido, V. H.; Wagner, M. H. Linear and Non-Linear Rheological Characterization of Photo-Oxidative Degraded LDPE. Polym. Deg. Stab. 2014, 99, 136-145. (12) Entezam, M.; Abbasi, M.; Ahmadi, M. Theoretical Correlation of Linear and Non-Linear Rheological Symptoms of Long-Chain Branching in Polyethylene Irradiated by Electron Beam at Relatively Low Doses. Rheo. Acta. 2017, 56, 729-742. (13) Khonakdar, H. A.; Jafari, S. H.; Rasouli, S,; Morshedian, J.; Abedini, H. Investigation and Modeling of Temperature Dependence Recovery Behavior of Shape-Memory Crosslinked Polyethylene. Macromol. Theor. Simul. 2007, 16, 43-52. (14) Chodák, I. Properties of Crosslinked Polyolefin-Based Materials. Prog. Polym. Sci. 1995, 20, 1165-1199. (15) Palmlöf, M.; Hjertberg, T. Crosslinking of Poly (Ethylene-co-1,9-Decadiene) by Electron Beam Irradiation. Polymer 2000, 41, 6481-6495. (16) Singh, A. Irradiation of Polyethylene: Some Aspect of Crosslinking and Oxidative Degradation. Radiat. Phys. Chem. 1999, 56, 375-380. (17) Andersson, L. H. U.; Hjertberg, T. The Effect of Different Structure Parameters on the Crosslinking Behavior and Network Performance of LDPE. Polymer 2006, 47, 200-210. (18) Valentin, J. L.; Posadas, P.; Fernandez-Torres, A.; Malmierca, M. A.; Gonzalez, L.; Chassé, W.; Saalwachter, K. Inhomogeneities and Chain Dynamics in Diene Rubbbers 33



Vulcanized with Different Cure Systems. Macromolecules 2010, 43, 4210-4222. (19) Pamdey, J. K.; Singh, R. P. UV-Irradiated Biodegredability of Ethylene-Propylene Copolymer, LDPE, and I-PP in Compositing and Culture Environments. Biomacromolecules 2001, 2, 880-885. (20) Miličević, D.; Trifunović, S.; Popović, M.; Milić, T. V.; Suljovrujić, E. The Influence of Orientation on the Radiation-Induced Crosslinking/Oxidative Behavior of Different PEs. Nucl. Instrum. Methods Phys. Res., Sect. B 2007, 260, 603-612. (21) Qu, B. J.; Hawthorn, G.; Mau, A. W. H.; Dai, L. M. Photochemical Generation of Polymeric Alkyl-C60 Radicals: ESR Detection and Identification. J. Phys. Chem. B. 2001, 105, 2129-2134. (22) Khonakdar, H. A.; Jafari, S. H.; Wagenknecht, U.; Jehnichen, D. Effect of Electron-Irradiation on Cross-Link Density and Crystalline Structure of Low- and High-Density Polyethylene. Radiat. Phys. Chem. 2006, 75, 78-86. (23) Klepac, D; Ščetar, M.; Baranović, G.; Galić, K.; Valić, S. Influence of High Doses γ-Irradiation on Oxygen Permeability of Linear Low-Density Polyethylene and Cast Polypropylene Films. Radiat. Phys. Chem. 2014, 97, 304-312. (24) Murray, K. E.; Kennedy, J. E.; McEvoy, B.; Vrain, O.; Ryan, D.; Cowman, R.; Higginbotham, C. L. The Effect of High Energy Electron Beam Irradiation in Air on Accelerated Aging and on the Structure Property Relationships of Low Density Polyethylene. Nucl. Instrum. Methods Phys. Res. B 2013, 297, 64-74. (25) Bartoníček, B. Plaček, V.; Hnát, V. Comparison of Degradation Effects Induced by Gamma Radiation and Electron Beam Radiation in Two Cable Jacketing Materials. Radiat. 34


Page 34 of 40

Page 35 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Phys. Chem. 2007, 76, 857-863. (26) Albano, C.; Perera, R.; Silva, P.; Sanchez, Y. Characterization of Irraddiated Pes/PA6 Blends. Polym. Bull. 2006, 57, 901-912. (27) Plaček, V.; Bartoníček, B.; Hnát, V.; Otáhal, B. Dose Rate Effect in Irradiation Degradation of Polymer-based Cable Material. Nucl. Instrum. Methods B. 2003, 208, 448-453. (28) Auhl, D.; Stadler, F. J.; Münstedt, H. Comparison of Molecular Structure and Rheological

Properties

of

Electron-Beam-

and

Gamma-Irradiated

Polypropylene.

Macromolecules 2012, 45, 2057-2065. (29) Nakamura, K.; Ogata, S.; Ikada, Y. Assessment of Heat and Storage Conditions on γ-Ray and Electron Beam Irradiated UHMWPE by Electron Spin Resonance. Biomaterials 1998, 19, 2341-2346. (30) Joshi, R. P.; Hareesh, K.; Bankar, A.; Sanjeev, G.; Asokan, K.; Kanjilal, D.; Dahiwale, S. S.; Bhoraskar, V. N.; Dhole, S. D. Anti-Biofilm Efficacy of 100MeV Gold Ion Irradiated Polycarbonate against Salmonella Typhi, Radiat. Phys. Chem. 2017, 141, 149-154. (31) Curro, J. G.; Pearson, D. S.; Helfand, E. Viscoelasticity of Randomly Crosslinked Polymer Networks. Relaxation of Dangling Chains. Macromolecules 1985, 18, 1157-1162. (32) Chanbari, R.; Assenze, S.; Saha, A.; Mezzenga, R. Diffusion of Polymers through Periodic Networks of Liquid-Based Nanochannels. Langmuir 2017, 33, 3491-3498. (33) Tang, S. C.; Wang, M. Z.; Olsen, B. D. Anomalous Self-Diffusion and Sticky Rouse Dynamics in Associative Protein Hydrogels. J. Am. Chem. Soc. 2015, 137, 3946-3957. (34) Chen, X.; Costeux, C.; Larson, R. G. Characterization and Prediction of Long-Chain 35



Branching in Commercial Polyethylene by a Combination of Rheology and Modeling Methods. J. Rheol. 2010, 54, 1185-1205. (35) van Ruymbeke, E.; Coppola, S.; Balacca, L.; Righi, S.; Vlassopoulos, D. Decoding the Viscoelastic Response of Polydisperse Star/Linear Polymer Blends. J. Rheol. 2010, 54, 507-538. (36) Fedderly, J. J.; Lee, G. F.; Lee, J. D.; Hartmann, B.; Dušek, K.; Dušková-Smrčkova, M.; Šomvársky, J. Network Structure Dependence of Volume and Glass Transition Temperature. J. Rheol. 2000, 44, 961-972. (37) Zhang, X. H.; Yang, H. M.; Song, Y. H.; Zheng, Q. Rehological Behaviors of Randomly Crosslinking Low Density Polyethylene and Its Gel Network. Polymer 2012, 53, 3035-3042. (38) Daniel, W. F. M.; Burdyńska, J.; Vatankhah-Vamosfaderani, M.; Matyjaszewski, K.; Paturej, J.; Rubinstein, M.; Dobrynin, A. V.; Sheiko, S. S. Solvent-Free, Supersoft and Superelastic Bottlebrush Melts and Networks. Nat. Mater. 2016, 15, 183-189. (39) Mott, P. H.; Roland, C. M. Elasticity of Natural Rubber Networks. Macromolecules 1996, 29, 6941-6945. (40) Bergström, J. S.; Boyce, M. C. Deformation of Elastomeric Networks: Relation between Molecular Level Deformation and Classical Statistical Mechanics Models of Rubber Elasticity. Macromolecules 2001, 34, 614-626. (41) Feldman, K. E.; Kade, M. J.; Meijer, E. W.; Hawker, C. J.; Kramer, E. J. Model Transient Networks from Strongly Hydrogen-Bonded Polymers. Macromolecules 2009, 42, 9072-9081. (42) Williams, Z. H.; Burwell, E. D.; Chiomento, A. E.; Demsko, K. J.; Pawlik, J. T.; Harris, S. O.; Schwab, A. D. Rubber-Elasticity and Electrochemical Activity of Iron (II) Tris 36


Page 36 of 40

Page 37 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(bipyridine) Crosslinked Poly (Dimethylsiloxane) Networks. Soft Matter 2017, 13, 6542-6554. (43) Kim, S.; Kang, P. H.; Nho, Y. C.; Yang, O. B. Effect of Electron Beam Irradiation on Physical Properties of Ultrahigh Molecular Weight Polyethylene. J. Appl. Polym. Sci. 2005, 97, 103-116. (44) Jahan, M. S.; Wang, C.; Schwartz, G.; Davidson, J. A. Combined Chemical and Mechanical Effects on Free Radicals in UHMWPE Joints during Implantation. J. Biomed. Mater. Res. 1991, 25, 1005-1017. (45) Krupa, I.; Luyt, A. S. Mechanical Properties of Uncrosslinked and Crosslinked Linear Low-Density Polyethylene / Wax Blends. J. Appl. Polym. Sci. 2001, 81, 973-980. (46) Lang, C. A Laplace Transform Method for Molecular Mass Distribution Calculation from Rheometric Data. J. Rheol. 2017, 61, 947-954. (47) Roths, T.; Maier, D.; Friedrich, C.; Marth, M.; Honerkamp, J. Determination of the Relaxation Time Spectrum from Dynamic Moduli Using an Edge Preserving Regulation Method. Rheol. Acta 2000, 39, 163-173. (48) Zhou, W.; Zhu, S. ESR Study of Peroxide-Induced Cross-Linking of High Density Polyethylene. Macromolecules 1998, 31, 4335-4341. (49) Kuzuya, M.; Yamashiro, T.; Kondo, S. I.; Sugito, M.; Mouri, M. Plasma-Induced Surface Radicals of Low-Density Polyethylene Studied by Electron Spin Resonance. Macromolecules 1998, 31, 3225-3229. (50) Durant, J.; Jahan, M. S. EPR Power Saturation Techniques and Spectral Differentiation Are Used to Isolate and Simulate Radical Species in UHMWPE. Nucl. Instrum. Methods 37



Phys. Res., Sect. B 2005, 236, 160-165. (51) Riganakos, K. A.; Koller, W. D.; Ehelermann, D. A. E.; Bauer, B.; Kontominas, M. G. Effects of Ionizing Radiation on Properties of Monolayer and Multilayer Flexible Food Packaging Materials. Radiat. Phys. Chem. 1999, 54, 527-540. (52) Oka, T.; Kanbe, H.; Yatagai, F.; Hama, Y. Changes in the Chemical Structure and the Mechanical Properties of Single-Site Polyethylene Induced by Ion-Beam Irradiation. Nucl. Instrum. Methods Phys. Res. Sect. B 2003, 208, 181-184. (53) Oshima, A.; Ikeda, S.; Seguchi, T.; Tabata, Y. Improvement of Radiation Resistance for Polytetrafluoroethylene (PTFE) by Radiation Crosslinking. Radiat. Phys. Chem. 1997, 49, 279-284. (54) Wood-Adams, P. M.; Dealy, J. M.; Degroot, A. W.; Redwine, O. D. Effect of Molecular Structure on the Linear Viscoelastic Behavior of Polyethylene. Macromolecules 2000, 33, 7489-7499. (55) Stadler, F. J.; Chun, Y. S.; Han, J. H.; Lee, E.; Park, S. H.; Yang, C. B.; Choi, C. Deriving Comprehensive Structural Information on Long-Chain Branched Polyethylenes from Analysis of Thermo-Rheological Complexity. Polymer 2016, 104, 179-192. (56) Gotsis, A. D.; Zeevenhoven, B. L. F.; Tsenoglou, C. Effect of Long Branches on the Rheology of Polypropylene. J. Rheol. 2004, 48, 895-914. (57) Liu, H.; Fang, Z. P.; Peng, M.; Shen, L.; Wang, Y. C. The Effects of Irradiation Crosslinking on the Thermal Degradation and Flame-Retardant Properties of the HDPE/EVA/Magnesium Hydroxide Composites. Radiat. Phys. Chem. 2009, 78, 922-926. (58) Bazli, L.; Khavandi, A.; Boutorabi, M. A.; Karrabi, M. Correlation between Viscoelastic 38


Page 38 of 40

Page 39 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Behavior and Morphology of Nanocomposites Based on SR/EPDM Blends Compatibilized by Maleic Anhydride. Polymer 2017, 113, 156-166. (59) Song, Y. H. Zheng, Q. Linear Rheology of Nanofilled Polymers. J. Rheol. 2015, 59, 155-191. (60) Taherian, A. R.; Fustier, P.; Ramaswamy, H. S. Effect of Added Oil and Modified Starch on Rheological Properties, Droplets Size Distribution, Opacity and Stability of Beverage Cloud Emulsions. J. Food Eng. 2006, 77, 687-696. (61) Elmendorp, J. J. A Study on Polymer Blending Microrheology. Polym. Eng. Sci. 1986, 26, 418-426. (62) Berejka, A. J.; Cleland, M. R. Industrial Radiation Processing with Electron Beams and X-Rays. Int. At. Energy. Agency. 2011, 34-36.

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Investigation on Molecular Structures of Electron-Beam-Irradiated Low

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