Better Understanding of the Polymeric Irradiation Using Physico-Electrochemical Characteristics

Poly (ester-urethane) urea (PEUU) based on ethanolamine as a chain extender contain hydroxyl and amino groups was chemically synthesized using 1,1'-methylenebis(4-isocyanatobenzene) MDI diisocyanates, and castor oil. One step polymerization procedure has been used to complete the PEUU formation and this polymeric target was irradiated with different doses (100, 250, 400 and 600 kGy) of gamma-rays. A full physico-electrochemical characterization package was performed on the solid surfaces, radiated vs non-irradiated, for the better understanding of the structure changes. To that end, Fourier transform infrared spectroscopy (FTIR), and the morphological features were characterized by the scanning electron microscope (SEM). Thermal stability was investigated using the differential scanning calorimetry (DSC), while the crystallinity and electrochemical properties were explored by the X- ray diﬀraction (XRD), and cyclic voltammetry (CV), respectively. Eventually, swelling, crosslinking density, mechanical strength, water absorption and contact angle measurements were discussed. Ultimately, the crosslinking density was discovered to be irradiated dependent. Worth mentioning here, this kind of study is recommended as a protocol that could be applied on other polymeric targets exposed to electromagnetic radiations.


Introduction
Segmented polyurethane-urea linear block copolymers are consisted of two incompatible blocks alternating "hard" and "soft" segments along their macromolecular chain due to their two-phase morphology, formed by the reaction of diisocyanate and diamine extender which yields a urea linkage, while the soft segment is made up from the polyol [1][2][3][4]. From the structural point of view, segmented PUU could be further distributed into polyether polyolbased and polyester polyol-based PUU [5,6]. The use of natural plant oils to replace the use of petroleum oils for the development of polymers has attracted much attention, this is attributed to widespread availability, ease of chemical modification of their backbone, low cost and highly promising applications in several fields [7]. Among natural plant oils, castor oil represents a promising raw material and can be used directly as a starting material for the synthesis of polyurethanes or polyurethane-urea [8]. The major constituent of castor oil, ricinoleic acid (12-hydroxy-cis-9-octadecenoic acid), is the hydroxyl-containing fatty acids [9]. Due to their significant biocompatibility and biodegradability, the polyurethanes-based castor oils have been used in biomedical applications such as wound dressing, bio-adhesives, bone tissue engineering, and drug delivery systems [10][11][12][13][14]. It is known that the PUU properties are strongly deponent on the type of the diisocyanate, polyol and chain extender, reaction conditions, degree of microphase separation, hydrogen bonding properties, copolymer structure, hard and soft phases content. Accordingly, the PUU exhibited excellent properties of high elasticity, huge tensile strength and extra toughness [15][16][17][18][19]. Thus, the PUUs are among the most important polymeric materials gained considerable attention in the last decades, exhibiting functional properties that make them convenient to be exploited in many different fields whether industries or everyday life such as coatings and paints, footwear, textile, elastic fibers, electronics, machinery, biomedical applications like drug delivery, biomedical therapies, biological and membrane science, as well as, many of the biomedical devices such as insulation for prosthetic heart valves, pacemakers, blood pumps, and vessels [20][21][22]. Insulation is an important property of PUU and it is the major focus nowadays because of their chemical constituents yet highly promising applications [6].
Ionizing radiation can be used to improve the properties of polymers, change the physical and mechanical properties [23]. Radiation can increase polymerization reaction, initiation of the polymerization process as well as cross-linking [24,25]. Radiation crosslinking is one of the most important research fields of radiation polymerization and has been great development since Charles [26] found polyethylene radiation effect and predicted these radiation effects can be used in industrial applications. Radiation crosslinking (RC) is one of the approaches of physical functionalization of synthetic polymers [27]. The RC leads to detaching hydrogen atoms from the surface of polymer chain because of the effect of high-energy radiation, and usually the RC is taking place in a solid phase and in a direct contact with the target polymeric surfaces. Hence the RC has the advantage over the other chemical crosslinking that no catalysts or other additives are needed, and therefore, no by-products are generated, no heat treatment, and the formation of crosslinks in the solid state and complete control of density of crosslinking [28,29]. Here, we have synthesized ETA-based PEUU elastomer, and we studied the effect of γ-irradiation with different doses on the physico-electrochemical properties of the PEUU. Understanding the radiation influence on the polymer features changed was achieved.
CO -MDI -ETA was synthesized by a typical one step polymerization procedure as is shown in Scheme 1 and their feed ratios were 1:0.6:0.4. Briefly, three solutions were placed in 250 ml polypropylene beaker, stannous octoate (0.03 wt%, with respect to the reactant) was added to the solution as a catalyst and stirred vigorously. The resulting viscous (PEUU) was then poured into a silica mold and heated at 50 °C for 100 h in an oven to obtain a complete polymerization. Then, the samples were conveyed to a roll mill mixer for few minutes to remove air bubbles through the casting process. Finally, the product was undergone to hot press, and compression molded into 1 mm plates at 175 °C for 45 min at pressure of 62.05 MPa, and then cooled to room temperature. The composition, sample code, HS and SS content for PEUU are presented in Table 1. The HS and SS contents are calculated using equation (1) and (2) respectively.

Irradiation of the samples
Poly(ester-urethane) urea films were irradiated by gamma-rays at doses 100, 250, 400 and 600 kGy with a dose rate 5kGy/h. The irradiation process was performed at ambient room temperature (25°C), where a cooling system was used in the irradiation chamber to avoid heating of samples during irradiation. Gamma-irradiation was carried out using Cobalt-60 gamma cell source available in National Center for Radiation Research and Technology  The cyclic voltammetric measurements were carried out using a computer-controlled Gamry potentiostat/galvanostat/ZRA G750 (Gamry, Pennsylvania, USA).

Mechanical measurements
Mechanical testing was performed by the tensile testing machine (Mecmesin, UK, MultiTest 25-I model). Five samples were cut out from the polymeric sheets in a dumbbell-shape using a steel die of standard width (4 mm) with 1 mm thickness. A benchmark of 1.5 cm and crosshead speed of 500 mm /min was set for carrying out each part of the test specimen for elongation estimation.

Swelling measurements
All Specimens of known weight were immersed in DCM solvent for 72 h at 25 C.
Subsequent to this, the swollen samples were removed from the DCM and weighed again before being placed in a vacuum oven. The vacuum oven was preheated to 60 C and the swollen samples were dried until a constant weight was achieved. This procedure was repeated four times for each sample. The swollen polymers were calculated according to the following equation:

Swelling ratio = Ws+Wi/Wi (3)
Where Ws is the weight of the swollen gel and Wi is the initial polymer weight.

Cross linking density measurements
It has been shown that the true stress in simple extension can be considered as a sum of two contributions as follows: σ is the true stress, λ is the extension ratio, the value of σo depends on the chemical nature of the rubber but not on the crosslink density. Ge depends on the degree of cross-linking. The average molecular weight between cross-links Mc, which is directly related to the crosslink density, can be estimated from the value of Ge as follows:

Mc = AoρRT/Ge (5)
Mc is the average molecular weight of the polymer between crosslinks, Ao is the a prefactor equal to 1, ρ is the polymer density, R is the gas constant equal to 8.3 × 10 6 cm -1 pa mol -1 , T is the absolute temperature [32, 33].

Water absorption measurements
The water absorption testing was determined as previously described [34]. The samples were first cut into 1 cm × 1 cm square shapes and weighed. Next, samples were placed in an oven of 50 o C for 24 h. Then the samples were transferred into a desiccator and cooled to room temperature. Eventually, the samples were immersed in boiling deionized water for 30 min.
cooling for 15 min until the water in the surface of the samples was dried up with filter paper, the weight of the samples was measured again and recorded. The absorptivity (Wm) was finally calculated from the equation:

Water absorption (Wm) = (6)
Where m1 is the weight of samples before soaking in water and m2 is the weight of samples after absorbing water.

Contact angle measurements
The contact angle was measured after 10s by dropping a water drop on the PEUUs surface.
Three samples of each material were measured and three measurements were carried out for each sample, and the angle between the liquid surface and PEUUs was then calculated using Image-J program.

Results and discussion
The FTIR spectra of pristine (un-irradiated) and the irradiated polyurethane-urea solid surfaces were exposed to gamma irradiation at various doses from 0 to 600 kGy, as illustrated in Figure 1, and frequencies for characteristics peaks are tabulated in

Mechanical properties
Mechanical properties of PEUU are extremely important for its polymeric performance, particularly in biomedical application. Therefore, changes in mechanical features such as tensile strength, elongation, hardness and modulus have been investigated and plotted in the  showed the lowest hardness value (77 ± 2 MPa) and increased very slowly with 100, 400 and 600 kGy (79.5 ± 1.5, 96 ± 3.9 and 101 ± 5.1) while with the 250 kGy the highest value (93 ± 3.02) was obtained.

Morphological characterization using the electron microscopy (SEM)
Changes in the surface morphology of the PEUU was studied before and after irradiation.
Hence, scanning electron microscopy (SEM) images were conducted on the pristine (0 kGy) and gamma irradiated (600 kGy), as shown in Figure 3. As result, the surface of PEUU appeared to be very rough before the irradiation, while the irradiated surfaces were smoother break, which may be attributed to the formation of the crosslinking by the irradiation process.
The increase in the smooth-surface by gamma ray could be correlated to the formation of oxidation on the surface of the irradiated PEUU, which, in turn, modified the properties of irradiated sample [42,43].

Differential Scanning Calorimetry (DSC) Analysis
The thermal properties of PEUU activated by different gamma ray doses were determined by the DSC analysis, as shown in Figure 4. The glass transitions temperature (Tg) of hard segment and melting transition temperature (Tm) of soft segment are represented in Table 3.
The Tg values of the hard segment were observed from 53 to 74 °C for all PEUU samples with unsettled changes in the Tg when the samples were irradiated. This is accredited to branching and chain separation achieved by the irradiation process, which in turn leads to increase the free volume of the samples [44].

X-ray diffraction (XRD)
Since the XRD is the principal technique to assess the degree of crystallinity in polymers, crystallinity phase of all PEUU surfaces were studied, as shown in Figure 5, and Table 4.
The XRD diffractograms for the non-irradiated and irradiated PEUU disclosed the crystallinity degree which is evidenced by the sharp diffraction peaks allocated at several 2θ values, where PEUU0 (2θ =20.46°), PEUU100 (2θ =20.84°), PEUU250 (2θ =20.4°), PEUU400 (2θ =20.12°), and the PEUU600 (2θ =20.32°). This indicates that the crystallinity of PEUU elastomers is provided by the soft segments and is related to a certain degree of structural ordering [53]. On the other hand, as the crystallite size is another important parameter could be fetched out the XRD analysis, the crystalline size of the PEUU surfaces were calculated by cherrer's equation [54]. As can been seen in Table 3, there is no significant changes in the crystalline size of PEUU after the irradiation, whereas the crystalline size average was about 12 Å, expect the last sample (PEUU600) with a value of 13 Å. These results confirm that, gamma irradiation had no effect on the materials crystalline size.

Swelling and Crosslink density
Basically, swelling study gives an indication of the extent of crosslink density in polymer chains [57]. The swelling value is usually decreases with the increase of crosslink density

Water uptake performance
In order to study the ability of water uptake during the network formation, degree of water absorption of the PEUU was measured for the irradiated samples while the pristine sample was considered as the positive control. The values calculated based on equation (6) as shown in Table 5, all PEUU irradiated surfaces absorbed volumes of water with a large degree of variation in the water absorption capability. The pristine PEUU, as the control, exhibited the maximum volume followed by a decrease in the water content with increasing irradiation dose. The water uptake occur as a result of swelling (water uptake of the polymer matrix material) or hydration (entry of water into accessible spaces into the porous matrix) [59].
Hence, the irradiated PEUU surfaces exhibited a resistance of water absorption with increasing crosslink density between polymer chains.

Contact angle
In order to evaluate the surface hydrophilicity of the PEUU films after exposed to gamma rays, water contact angles were measured and presented in Figure 7. As a result, water contact angle was increased from 0 to 250 kGy, and then reduced with irradiation dose 400 and 600 kGy to 69.7±3.07 and 72.7±1.6, respectively. According to the formation of crosslinking between polymer chains, the contact angle was increased and the hydrophilicity was decreased at the lower doses 250 and 400 kGy. Moreover, a notable reduction of the contact angle values was observed with 400 and 600 kGy, this might be attributed to the free radicals produced upon radiolysis with oxygen and/or surface degradation [60, 61].

Electrochemical characterization
Electrochemical properties of the target polymers were studied here using the cyclic voltammetric technique, as shown in finding is a strong evidence that the insulating electric feature is there for the PEUU, and it was not affected by the irradiation even at the highest dose.

Conclusions
PEUU elastomers were successfully synthesized using ethanolamine as a chain extender, the hard segment constituted of 1,1'-methylenebis(4-isocyanatobenzene) was formed by the castor oil as a soft segment. FTIR spectra confirmed the formation of the urethane-urea group. Cross-linking by gamma irradiation, at different doses, affected the thermomechanical properties of the PEUU but did not show any effect on the electrical performance of the irradiated surfaces. Diffraction patterns demonstrated the crystalline phase segments.
The prominent effect with crosslink density was determined by swelling measurements, further information was obtained by water absorption and contact angle. Eventually, this study provided a wide over view on the effect of electromagnetic radiations on the physicoelectrochemical properties of synthetic polymers. This will support a better understanding of the polymer future applications.        Table 5. Comparison of water absorption of PEUU before and after irradiation.