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Effect of electron beam radiation on tensile and viscoelastic properties of styrenic block copolymers.

INTRODUCTION

Electron beam (E-beam) radiation as an efficient way to crosslink polymers has advantages of short residence time and initiation without use of initiator [l-3]. Commercial interest has led to extensive research on the effects of E-beam radiation on polymeric materials and in particular rubbers [4-6]. E-beam radiation technology has been widely applied in various industries including wire, cable, coatings, microelectronics, and the tire industries [7-11]. Generally, the E-beam modifies polymeric materials either by chain scission or crosslinking, and sometimes both. This is due to a series of chemical reactions, which are caused by the highly energetic E-beam [ 12-14].

The industrial application of E-beam has been predominantly focused on the crosslinking of elastomers [15-17], Rubbers are vulcanized by heating at high temperature with adding of sulfur or peroxide [18-22]. The disadvantage is that the long time heating may cause side reactions, which will affect the final properties of the materials, plus it is not energy efficient. While radiation curing, on the other hand, is a process, which is performed at room temperature and under controlled conditions, such as radiation dose, atmosphere, and penetration depth [1]. In addition, the radiation crosslinking process can lead to an improvement of desired physical properties of rubbers, such as tensile strength, modulus, and abrasion resistance, at a competitive cost [10, 23].

Styrenic block copolymers (SBCs) as a class of thermoplastic elastomers (TPEs) contain phase separated polystyrene block and rubbery block. Poly (styrenc-block-butadiene-block-styrene) (SBS) and poly(styrene-block-isoprene-block-styrene) (SIS) are two types of widely used SBCs [24-27]. The poly(styrene-block-isoprene/butadiene-block-styrene) (SIBS) is a newly developed SBC with a hybrid (isoprene/butadiene) midblock. There haven't been any reports on the effect of E-beam radiation on properties these SIBS, thus far. Because of the phase separation, SBCs form physically cross-linked structure due to the thermodynamic incompatibility of the styrene endblock and the rubber midblock [28, 29], Like other TPEs, SBCs possess the mechanical properties of vulcanized rubbers, and the processing characteristics of thermoplastics [25], However, SBCs suffer from poor chemical resistance, lower strength, and modulus, etc. Much effort has been put to improve the properties and performance of SBCs by either modifying the end or midblock. Fetters and Morton replaced polystyrene with poly-[alpha]-methylstyrene to improve the tensile strength of SIS [30], Yu et al. used poly(methyl methacrylate) as endblock to modify the properties of SBS [31]. These approaches require sophisticated synthesis process. Because of the presence of polydiene midblocks, SBS, SIS, and SIBS are capable of crosslinking. This crosslinking process could provide a more efficient way to improve the chemical resistance, high temperature stability, tensile properties, and barrier properties of SBCs.

Although there have been some articles published on thermal initiated crosslinking of SBCs [9, 19, 20, 23, 32-37], little research has been performed on the effects of E-beam radiation initiated crosslinking on SBCs. The objective of this article is to investigate the effect of E-beam radiation on mechanical and viscoelastic properties of SBCs. Three types of SBCs with different midblocks were investigated and compared. Tensile properties of SBCs under uniaxial stretching were evaluated. Crosslink density of each copolymer before and after E-beam radiation was calculated based on dynamic mechanical analysis (DMA). Stress relaxation behaviors of SBCs were also studied as functions of E-beam radiation dose and temperature. Before and after E-beam exposure, molecular weight was determined for each SBCs and homopolymer of polyisoprene and polybutadiene using size exclusion chromatography (SEC) to ascertain whether chain scission or crosslinking were the predominate reaction pathway.

EXPERIMENTAL

Materials

SBCs synthesized by anionic polymerization were provided by Kraton Polymers, Houston, TX. The detailed information of SBCs is listed in Table 1. Polybutadiene (~20% vinyl; ~80% c/s-and trans-1, 4, [M.sub.n] ~6 k) and polyisoprene (cis-1,4, [M.sub.n] ~17 k) were purchased from Sigma-Aldrich. Polystyrene was obtained from Americas Styrenics, Marietta, OH with product No. of D4030.01. Solvents were purchased from Sigma-Aldrich and used as received. Cyclohexane (ACS reagent, [greater than or equal to] 99%), tetrahydrofuran (THF) (HPLC grade, [greater than or equal to] 99.9%). Alanine films were manufactured by Kodak BioMax with lot number B0308A. Polytetrafluoroethylene (PTFE) was purchased from Fluoro-Plastics, Inc.

Film Fabrication

Films for E-beam crosslinking studies were prepared by solution casting method. All the SBCs materials are soluble in cyclohexane. A low solid content (15 wt%) and slow evaporation process were used for casting, so that the residual stress did not develop in the films. The procedure is as follows: SIBS (50 g) was dissolved in cyclohexane (283 g) for 5 h until a homogeneous solution with 15 wt% solid content was obtained. The solution was poured onto a polytetrafluoroethylene substrate and kept in a solvent evaporation chamber at room temperature. The film was first kept in closed chamber for 3 days, and then put in the hood for two more days. Finally, the film was dried in vacuum oven for additional 2 days at room temperature to remove any residual solvent. All the films were prepared following the same procedure.

Electron Beam Radiation Condition

E-beam exposure was performed at NEO beam crosslinking facility in Middlefield, OH. The E-beam radiation was conducted under following conditions: voltage: 4.48 mV, current: 30.0 mA, atmosphere: air, line speed: 15 feet/min, radiation time: ~30 s. Four E-beam radiation doses were investigated (60, 120, 190, and 240 kGy). The dose was measured by dosimeter consists of alanine films. The dosimeters are measured in a Bruker e-scan ESR spectrometer. Alanine film dosimeters are commonly used in the radiation dose measurement [1, 38, 39]. Alanine, a crystalline amino acid, can form a stable free radical when subjected to ionizing radiation. The alanine free radical can yield an electron spin resonance (ESR) signal that is dose dependent.

Characterizations

Tensile Test. Tensile test was performed on Instron universal electromechanical tester 5567 at extension speed of 500 mm/min (20 in/ min). According to ASTM D-412, five dumbbell shape specimens were used to perform the tensile test. Averages were reported.

Molecular Weight. SEC was used to determine polymer molecular weight and polydispersity index (PDI). The SEC used THF as the mobile phase, and was equipped with a Waters 1515 isocratic pump, three waters styragel columns (HR 3, HR 4, HR 4E) and a Waters 2414 refractive index (RI) detector. The columns were calibrated with narrow distribution polystyrene (PS) standards.

Viscoelastic Properties. Dependence of dynamical modulus on temperature was obtained on DMA Q-80 (TA Instruments) in multi-frequency strain mode. Films were tested under following conditions: tension mode, at heating rate of 3[degrees]C/min, and frequency of 1 Hz. The maximum of tan delta was used to determine the glass transition temperature, while the crosslink density was calculated from the storage modulus (E') in the rubbery plateau [40],

[v.sub.e] = E' / 3RT (T [much less than] [T.sub.g]) (1)

where E' is the storage modulus in rubbery plateau, [v.sub.e] is the crosslink density, R is the gas constant, T is the temperature.

Stress relaxation tests were also performed on DMA Q-80 (TA Instruments). Test was performed at various temperatures (30, 40, 50,and 60[degrees]C) with 1% strain for 30 min.

RESULTS

The SBCs with different midblock were investigated and compared with respect to E-beam exposure. The midblocks were polyisoprene, polybutadiene, and copolymer of isoprene and butadiene, respectively. It was anticipated that the different midblocks would result in changes in crosslink density, and thus tensile properties upon E-beam radiation. Four E-beam radiation doses, 60, 120, 190, and 240 kGy were investigated. The E-beam doses were selected based on the industrial cost and safety consideration.

Effect of E-Beam Dose on Tensile Properties

The midblock of SIBS is a copolymer of isoprene and butadiene. Uniaxial tensile test was performed and the representative stress-strain behavior of SIBS is shown in Fig. 1. The tensile properties derived from stress-strain measurement are shown in Table 2. The modulus of E-beam irradiated SIBS was increased gradually with increasing of E-beam radiation dose. The tensile strength decreased by ~10% at lower E-beam dose (e.g., 60 and 120 kGy). Although at higher E-beam dose (e.g., >190 kGy), the tensile strength increased by 16% compared to the tensile strength of SIBS before E-beam radiation. This means the tensile strength of SIBS decreased first and then started to increase as the E-beam radiation dose increasing. The 300% modulus of SIBS was observed to increase with increasing of E-beam radiation dose. At 240 kGy, the 300% modulus of SIBS increased 64%, and the elongation-at-break also decreased by 26% compared to the initial state of SIBS.

The change of tensile properties of SBS as a function of E-beam radiation dose was also studied, as shown in Table 2. With increasing of E-beam dose, tensile strength was slightly increased at lower E-beam dose. However, it decreased at high E-beam radiation dose (e.g., 190 and 240 kGy). SBS with the highest E-beam radiation dose (240 kGy) had the lowest tensile strength. Elongation-at-break decreased with increasing of E-beam radiation dose. Previous researchers also found the similar results for radiation cross-linked SBS |32]. It is curious that after E-beam radiation, an enhancement in modulus and hence presumably crosslink reaction did not translate into an increase in tensile strength of SBS. The elongation-at-break of SBS without E-beam exposure was 1518%, whereas the SBS films with E-beam radiation dose of 240 kGy was 988%, which was 65% of the initial value. High E-beam radiation dose significantly decreased the elongation-at-break of SBS.

Regarding the modulus, the 300% modulus of SBS increased continuously with increasing of E-beam radiation dose. After E-beam exposure with radiation dose of 60 kGy, the 300% modulus increased by 14%, compared to the initial value. It was found that after each 60-70 kGy E-beam dose the 300% modulus increased by 14%, which was true for SBS with E-beam dose of 60, 120, and 190 kGy. For SBS with 240 kGy exposure, the 300% modulus only increased 5% in comparison with SBS with E-beam radiation dose of 190 kGy. This was attributed to the more extensive chain scissions at higher dosage.

The tensile behavior of SIS with different E-beam radiation doses is shown in Table 2. The modulus of the SIS appeared to be increased as the E-beam radiation dose increasing. The change of tensile strength for SIS after E-beam radiation appeared to be different from SBS. The tensile strength of SIS after E-beam radiation was lower compared to SIS before E-beam radiation, which means that E-beam radiation did not increase the tensile strength of SIS. The trend was similar to the SIBS. Similar behavior was found in the peroxide cross-linked SIS copolymers. The decrease in tensile strength and increase in modulus at 300% strain of SIS were observed after vulcanization [25].

Both tensile modulus and modulus at 300% strain of SBCs are listed in Table 2. Even at the small strain region, the tensile modulus of SBCs also increased with increasing of E-beam radiation dose. The tensile modulus of SBCs after irradiated with dose of 240 kGy increased 82, 145, and 370% for SIBS, SBS, and SIS, respectively. Due to the high extensibility of SBCs, the stress-strain curves in small regions almost overlapped, as shown in Fig. 1. In the stress-strain curves, the modulus at 300% strain could more effectively reflect the change of the stiffness of SBCs after E-beam radiation.

Effect of E-Beam Radiation on Dynamic Mechanical Properties of SBCs

To find the reason for the different response of SBCs with E-beam radiation, the DMA was performed. Storage modulus of SBCs before E-beam radiation is compared in Fig. 2. The crosslink density calculated based on the storage modulus in the rubbery plateau is listed in Table 3. The crosslink density of SBS is 10 times larger than SIS and S1BS. This data proved that the difference in crosslink density was the reason for the different change trend of tensile strength upon E-beam exposure.

SBCs with different E-beam radiation doses were also analyzed by DMA. The representative DMA measurement of storage modulus and tan delta of SIBS as a function of temperature is shown in Fig. 3. And the data are listed in Table 3. It was found that the storage modulus in rubbery plateau was increased with increasing of electron beam radiation dose. According to the rubber elasticity theory, the storage modulus in rubbery plateau indicated the change of crosslink density of elastomers. The higher value of storage modulus in rubbery plateau indicated the higher crosslink density of materials. Therefore, the crosslinking reaction should be predominating during the E-beam radiation. Glass transition temperature of polydiene phase slightly increased after E-beam radiation, which is consistent with Kanbara's results [34], This was due to the formation of the crosslinks upon E-beam exposure.

From the tan delta versus temperature curves, as shown in Fig. 3b, it was found that the width of transition increased after the SBCs were irradiated with dose of 60 kGy. This indicated the decrease of homogeneity of the material. This might be the reason for the decrease of the tensile strength of the SIBS. The width of the transition tended to narrow at higher radiation dose, which indicated the homogeneity of the system increased at high radiation dose. This could also cause the increase of the tensile strength of SIBS. The similar behavior was also observed in SIS and SBS. However, the tensile strength of SBS continued to decrease was mainly due to the high crosslink density induced low extensibility of the material.

Effect of E-Beam Radiation on Stress Relaxation Behaviors of SBCs

Viscoelastic property is an important characteristic of block copolymers [41, 42]. The E-beam radiation introduced chemical crosslinking network to the system. The change of macroscopic mechanical properties is due to the consequences of molecular motion, which are generally difficult to measure directly. However, it can be readily observed in stress relaxation experiment [43, 44], For SBCs, there are three processes causing stress relaxation: physical flow of the polydiene phase, physically trapped confinements, and conformational change of the phenyl ring side group in the polystyrene phase. It has been reported that physical flow is the fastest relaxation process, while the relaxation of domains is the slowest [45, 46].

The stress relaxation behaviors of SBCs before and after E-beam radiation are shown in Fig. 4. Controlled temperature (30[degrees]C) and strain (1%) were chosen to compare the stress relaxation process of SBCs with various E-beam radiation doses. In Fig. 4a, SBS relaxed to 50% of initial stress fast before E-beam radiation, while after E-beam crosslinking at 240 kGy, SBS relaxed to 75% of the initial stress after 30 min. This was attributed to the increased crosslink density of SBS, which slowed down the relaxation process of the polymer chain and maintained a higher stress after holding for 30 min. The stress relaxation behaviors of SIS are shown in Fig. 4b. SIS relaxed to almost zero within 30 min before E-beam radiation. After E-beam crosslinking at 60 kGy, SIS relaxed to 67% of initial stress after 30 min. This was attributed to the increased crosslink density of isoprene block, which slowed down the relaxation of the polymer chain and maintain a higher stress after holding for 30 min with strain of 1%.

Influences of both E-beam radiation dose and temperature on stress relaxation behaviors of SIBS were also investigated. The effect of E-beam dose on stress relaxation behavior was studied at controlled temperature, 30[degrees]C, as shown in Fig. 4c. With increasing of E-beam dose, the relaxation rate was slowed down; higher stress was observed after 30min. The effect of temperature on stress relaxation behavior was studied at a controlled E-beam dose and a series of temperature, ranged from 30 to 60[degrees]C, as shown in Fig. 5. Higher temperature enhanced the stress relaxation process due to the high mobility of the polymer chain. With increasing of the temperature, SIBS relaxed at higher speed, and relaxed to lower stress level compared to the initial stress.

Effect of E-Beam Dose on Molecular Weight

Studying the molecular weight characteristic of polymer is a direct way to know whether chain scission or chain crosslinking reaction was occurred during E-beam radiation. Copolymers with E-beam radiation dose of 60 kGy were able to be dissolved in THF and the molecular weight was measured by SEC. This provided the information about what reaction was occurred and what was the initial change of the polymer chain during the E-beam radiation process.

At 60 kGy, the entire sample was soluble in THF. Samples irradiated with greater than 60 kGy resulted in sol and gel with respect to THF. For the 60 kGy samples, both the molecular weight and PDI increased for SBCs. It suggested that the crosslinking reaction was predominating even at low dosage over chain scission based on the SEC chromatograms in Fig. 6. Molecular weight information before and after E-beam radiation is listed in Table 4.

Molecular weight of homopolymers of polybutadiene, polyisoprene, and polystyrene were also compared. This information will help elucidate the behavior of each block during E-beam radiation. For polyisoprene, there is a shoulder on SEC chromatogram at the shorter elution time, as shown in Fig. 7a. This indicated the formation of high molecular weight polyisoprene after E-beam radiation and the crosslinking reaction was predominating in polyisoprene block. In addition, no significant amount lower molecular weight polyisoprene was observed after radiation. Effects of E-beam radiation on molecular weight of polybutadiene was also studied. Molecular weight of polybutadiene with ~80% cis-and trans-1, 4 structures are shown in Fig. 7b. Peak position moved to shorter elution time side, which indicated the formation of higher molecular weight polybutadiene. For the PS homopolymer, no higher molecular weight polystyrene was observed from SEC chromatogram after E-beam radiation. This means polystyrene block are relatively stable to E-beam radiation [47]. Slight chain scission may occur, as shown in Fig. 8. The aromatic group has a protective effect because the resonant structure of the aromatic ring is able to absorb a considerable amount of energy without rupture of the bonds [48].

DISCUSSION

Because the E-beam radiation can initiate both chain scission and crosslinking reaction on polymeric materials [49, 50], knowing what reaction is predominating as a function of radiation dose is quite important. In addition, for block copolymers, different blocks may have different reaction upon E-beam radiation, which makes it a more complicated system than homopolymers. There are several possible scenarios. It is likely that chain scission and crosslinking reactions may take place simultaneously, with one reaction more predominate than the another [10]. Because macro-properties of materials are dependent on the chemical structure of polymer chains [51], the ratio of chain scission to bond formation, albeit crosslinking, will control the polymeric properties.

Viscoelastic property is an important characteristic of block copolymers [41, 42], The E-beam radiation introduced chemical crosslinking network to the system. The change of macroscopic mechanical properties is due to the consequences of molecular motion, which are generally difficult to measure directly. However, it can be readily observed in stress relaxation experiment [43, 44]. For SBCs, there are three processes causing stress relaxation: physical flow of the polydiene phase, physically trapped confinements, and conformational change of the phenyl ring side group in the polystyrene phase. It has been reported that physical flow is the fastest relaxation process, while the relaxation of domains is the slowest [45, 46], E-beam radiation cross-linked the polydiene phase. Consequently, the relaxation process was slowed down. The relaxation speed of SBCs decreased with increasing of E-beam radiation dose. This indicated that the physical flow and disentanglement of polydiene phase was restricted after E-beam exposure. The stress relaxation of SBCs also had temperature dependence. The higher temperature might increase the mobility of polymer chains. The higher relaxation rate as the temperature increasing might be attributed to physical flow of both phases.

The comparison of the molecular weight as a function of midblock and E-beam exposure shed some light on which chemical process was dominate. At lower E-beam radiation dose (<60 kGy), the SBCs were still soluble in the solvent. In this case, the molecular weight characteristics reflected the initial response of the materials to E-beam radiation. On the basis of the molecular weight characterization of three types of SBCs, it was found that these copolymers all undergo crosslink reaction after E-beam radiation with E-beam dose of 60 kGy. Because lower molecular weight polyisoprene (17 k) and polybutadiene (<6 k) were selected, the materials were still soluble at higher E-beam dose (>120 kGy). The molecular weight characterization showed that the E-beam radiation (<240 kGy) predominately initiated crosslinking reactions in the polyisoprene and polybutadiene blocks. The E-beam radiation did not initiate significant crosslink reactions on polystyrene phase, as the SEC chromatogram of polystyrene did not show any shoulders. Polystyrene is known to be radiation resistant and relatively stable under E-beam radiation [12-14]. So the property change of SBCs after E-beam radiation mainly depends on the polydiene phase. In addition, the crosslink densities of SIBS, SIS, and SBS showed the increase of the modulus in rubbery plateau. This indicated the increase of overall crosslink density after E-beam radiation.

To elucidate the effect of low dosage of E-beam radiation on tensile properties of SBCs, it is essential to relate the microstructure to the failure mechanism of SBCs under uniaxial stretch. The typical morphology of styrenic triblock copolymers, such as SBS and SIS are illustrated graphically in Scheme 1. Due to the incompatibility of the polystyrene and polydiene chain, a two-phase morphology is formed [28]. Polystyrene domain acts as physical crosslinker [25, 27, 28, 52], which makes the SBCs have high tensile strength and similar properties as vulcanized rubbers [25]. The molecular weight between corsslink is much lower than the molecular weight of midblock, which means there are chain entanglements in the rubbery phase [26, 53]. Therefore, there are two physical crosslinking networks in the materials. One is the polystyrene domain, which aggreates and acts as crosslinker and reinforcing fillers [27], the other is the chain entanglement of rubbery phase. These two factors are both important for the high tensile strength of SBCs. Before E-beam crosslinking, the slippage of entanglement in polydiene chain helps to distributate the stress more evenly and delay the failure of the bulk material [54, 55]. After E-beam crosslinking, another network was introduced to the system, which was chemically crosslinked polydiene, as shown in Scheme 1. This changed the microstructure structure and the tensile properties of the copolymers.

The crosslinked network regualarity changed as a function of E-beam radiation dose due to the crosslinking reaction in polydiene segment. Before E-beam exposure, the physically crosslinked network formed by phase separation of two blocks had a relatively high regularity. This is important for distribution of the stress evenly during the uniaxial stretching. After E-beam exposure, new crosslinking junctions were introduced into the system. At lower E-beam radiation dose, the regularity of the network decreased compared to the initial state due to the formation of a few crosslinks. The network became heterogenerous, as shown in Scheme 2b. In this case, the highly crosslinked cluster would have stress concentration under unaxial stretching and might be broken first during the tensile test, as shown in Scheme 2c. This was the reason for the decrease of tensile strength for S1BS and SIS at 60 and 120 kGy. However, with increasing of the E-beam dose, a new network was formed with higher crosslink density and network regularity, as shown in Fig. 9a. The network regularity plays an important role in the uniaxial tensile stretch. Basically, crosslinked materials with homogeneous network has higher tensile strength under same condition. In Fig. 9b, the initial decrease in of the tensile strength (60 kGy) could be attributed to a decrease in network regularity, and the decrease at higher dosage is attributed to the high crosslink density, which reduced the ductility of the materials.

The variation of tensile strength for SIBS,S1S, and SBS are summerized in Fig. 9b. The tensile strength of SIBS decreased first and then increased at higher E-beam dose. Because the midblock of SIBS is copolymer of butadiene and isoprene, with larger content of isoprene than butadiene, the tensile properties of SIBS was more like the SIS. The tensile strength of SIS also decreased first at lower E-beam dose, and then increased with increasing of E-beam dose. This phenomenon was different from the vulcanized rubbers [56], For nonblock rubbers, there are no physical crosslinks in the materials before vulcanization. In this case, the tensile strength was usually found to pass through a maximum with increasing crosslinking. This maximum is due primarily to changes in viscoelastic properties with crosslinking [57]. However, the SIBS copolymer has a baseline amount of physical crosslinks before E-beam radiation. Therefore, the behavior of the tensile strength of SIBS was different from the nonblock copolymer rubbers.

The tensile strength of SBS increased a little at lower E-beam dose, and then it continueously decreased with increasing of E-beam dose. This trend is more like a traditional vulcanized rubber. Actually, only the influence of the E-beam dose on the tensile strength were taken into account when the comparison was made. However, the crosslinking density of each material before E-beam exposure and with same radiation dose was different (See Table 3). These three types of SBCs fell into different ranges of phsyical crosslink density. For S1BS and SIS, the crosslink densities are 111.12 and 80.19 mol/[m.sup.3], respectively. These values were much lower than the crosslink density of SBS (1610.38 mol/[m.sup.3]). The differences in initial physical crosslink density and the crosslinking efficiency of the SBC control the effect of E-beam radiation on the tensile properties at least at low dosage. More generally, if a process has both bond breaking and bond making capabilities (as in E-beam), the degree of physical entanglements for polymeric chains, which can not be disentangled other than bond breaking is important.

This is the first study to investigate the effect of E-beam radiation on properties of SBCs by comparing three types of most widely used SBCs, including a newly developed S1BS. During the past, studies focused on either one of the block copolymer [17, 34, 58] or homopolymers of elastomers [44, 59, 60]. The molecular weight change of SBCs under E-beam radiation has not been investigated thus far. Because in the SBCs, sytrene as a common block with different midblocks, it was helpful to perform a systematic study of three types of SBCs and find out the key factors that controlled the properties of SBCs upon E-beam radiation. The block copolymers systems were found to be more complicated than the parent homopolymers. The E-beam crosslinking process highly depends on radiation dose and the initial structure of the materials. Although, the initial physical crosslink density plays an important role in determining the tensile strength of copolymers upon E-beam radiation, the properties of SBCs can by adjusted with higher the E-beam dose.

CONCLUSION

The Effect of E-beam radiation on SBCs, including SBS, SIS, and SIBS were investigated as a function of E-beam radiation dose. It was found that the crosslinking reaction was predominating in all copolymers investigated. Molecular weight of both SBCs and polydiene homopolymers were increased after E-beam exposure. Tensile moduli of SBS, SIBS, and SIS were significantly increased with increasing of E-beam radiation dose. This indicated that the stiffness of materials increased as the E-beam radiation dose increased, which was attributed to the E-beam induced increase of overall crosslink density of SBCs. The tensile strength of SBCs highly depended on both crosslink density and network regularity of the copolymers. The decrease of network regularity could prevent the uniform stress distribution during uniaxial stretching and cause the decrease of tensile strength. However, at higher E-beam dose, the increase of the tensile strength was attributed to the increase of the both crosslink density and the network regularity. After passing a maximum point, the tensile strength started to decrease as the E-beam radiation dose increasing. This was attributed to the high crosslink density induced low extensibility. Therefore, knowing the crosslink density before E-beam radiation was quite important in modifying the tensile properties of the SBCs. The overall crosslink density increased as E-beam radiation dose increased. From the stress relaxation measurement, it was observed that the relaxation rate of the cross-linked SBCs decreased with increasing of E-beam radiation dose and increased with increasing of temperature. This indicated that the crosslinks introduced by E-beam radiation slowed down the disentanglement of the polydiene segments. And higher temperature increased the overall physical flow of material.

ACKNOWLEDGMENT

We thank Kathryn Wright at KRATON Polymers LLC for providing the styrenic block copolymers used in this study and discussions on the results.

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Jinping Wu, Mark D. Soucek, Mukerrem Cakmak

Department of Polymer Engineering, The University of Akron, Ohio 44325

Correspondence to: M.D. Soucek; e-mail: msoucek@uakron.edu

DOI 10.1002/pen.23859

Published online in Wiley Online Library (wileyonlinelibrary.com).

TABLE 1. Composition information of styrenic block copolymers.

Polymer                D1171P         D1102K    D1114P

Type                    SIBS            SBS       SIS
Styrene content          20%            28%       20%
Structure              Linear         Linear    Linear
Midblock            Copolymer of      100% Bd   100% Ip
                  Bd (a) and Ip (b)

(a) Butadiene.

(b) Isoprene.

TABLE 2. Tensile properties of SBCs with four E-beam radiation doses.

                    Tensile           Elongation-
Sample           strength/MPa          at-break/%

SIBS-OkGy     12.50 [+ or -] 0.18   1700 [+ or -] 30
SIBS-60kGy    11.41 [+ or -] 0.10   1500 [+ or -] 20
SIBS-120kGy   11.53 [+ or -] 0.67   1380 [+ or -] 45
SIBS-190kGy   14.54 [+ or -] 0.75   1460 [+ or -] 60
SIBS-240kGy   14.41 [+ or -] 1.12   1250 [+ or -] 81
SBS-OkGy      34.46 [+ or -] 1.60   1518 [+ or -] 60
SBS-60kGy     35.64 [+ or -] 2.58   1453 [+ or -] 64
SBS-120kGy    34.83 [+ or -] 4.09   1330 [+ or -] 54
SBS-190kGy    27.45 [+ or -] 6.78   1127 [+ or -] 90
SBS-240kGy    21.77 [+ or -] 2.66    988 [+ or -] 91
SIS-OkGy      24.28 [+ or -] 0.76   2019 [+ or -] 74
SIS-60kGy     22.93 [+ or -] 0.69   1995 [+ or -] 67
SIS-120kGy    15.26 [+ or -] 0.86   1564 [+ or -] 87
SIS-190kGy    19.07 [+ or -] 0.57   1766 [+ or -] 58
SlS-240kGy    21.09 [+ or -] 0.79   1850 [+ or -] 80

                     300%               Tensile
Sample           Modulus/MPa          modulus/MPa

SIBS-OkGy     1.70 [+ or -] 0.05   0.28 [+ or -] 0.03
SIBS-60kGy    1.92 [+ or -] 0.03   0.31 [+ or -] 0.01
SIBS-120kGy   2.27 [+ or -] 0.06   0.38 [+ or -] 0.02
SIBS-190kGy   2.44 [+ or -] 0.07   0.47 [+ or -] 0.03
SIBS-240kGy   2.79 [+ or -] 0.08   0.51 [+ or -] 0.04
SBS-OkGy      3.72 [+ or -] 0.05   0.57 [+ or -] 0.04
SBS-60kGy     4.25 [+ or -] 0.06   0.69 [+ or -] 0.07
SBS-120kGy    4.90 [+ or -] 0.09   0.87 [+ or -] 0.13
SBS-190kGy    5.66 [+ or -] 0.11   1.26 [+ or -] 0.45
SBS-240kGy    5.96 [+ or -] 0.03   1.40 [+ or -] 0.43
SIS-OkGy      1.28 [+ or -] 0.04   0.23 [+ or -] 0.01
SIS-60kGy     1.46 [+ or -] 0.05   0.30 [+ or -] 0.15
SIS-120kGy    1.63 [+ or -] 0.05   0.39 [+ or -] 0.07
SIS-190kGy    1.72 [+ or -] 0.04   0.81 [+ or -] 0.06
SlS-240kGy    1.79 [+ or -] 0.04   1.08 [+ or -] 0.09

TABLE 3. Glass transition temperature and
crosslink density of SBCs derived front DMA.

               [T.sub.g]                   [V.sub.e]
Sample        ([degrees]C)   E' (MPa)   (mol/[m.sup.3])

SIS-0             -43          0.56          80.28
SIS-60kGy         -42          1.16         165.53
SIS-120kGy        -40          1.45         205.58
SIS-190kGy        -38          1.54         217.08
SIS-240kGy        -38          1.68         236.99
SBS-0             -82          9.68         1610.38
SBS-60kGy         -76         17.30         2810.75
SBS-120kGy        -75         17.60         2846.92
SBS-190kGy        -74         18.00         2907.51
SBS-240kGy        -73         22.10         3549.33
SIBS-0            -56          0.74         111.12
SIBS-60kGy        -50          0.80         117.94
SIBS-120kGy       -47          0.99         143.90
SIBS-190kGy       -48          1.21         176.59
SIBS-240kGy       -48          2.21         322.42

TABLE 4. Molecular weight characteristics of SIBS, SBS, and SIS.

Materials    [[bar.M].sub.n]   [[bar.M].sub.w]   PDI

SIBS             156,000           187.000       1.2
SIBS-60kGy       170,000           237,000       1.4
SBS              106,000           123,000       1.2
SBS-60kGy        116,000           191,000       1.6
SIS              169,000           188,000       1.1
SlS-60kGy        170,000           250.000       1.5
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