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Positron annihilation lifetime spectroscopy for miscibility investigations of styrene-butadiene-styrene copolymer/polystyrene blends.

The free volume parameters of styrene-butadiene-sty -rene copolymer/polystyrene (SBS/PS) blends were investigated with positron annihilation lifetime spectroscopy (PALS) in this study. The behaviors of free volume distribution, average free volume, and relative fractional free volume revealed the difference of inter-facial miscibility. Based on different models, inter-chain interaction parameter [i, geometric factor -y, and hydro-dynamic interaction parameter a obtained from free volume data were employed to further determine the effect of molecular architecture and styrene content on the miscibility. The results suggest the better miscibility in star-shaped SBS/PS blends than that of corresponding linear SBS/PS systems, even than that of systems containing more styrene unit. In addition, differential scanning calorimetry, dynamic mechanical analysis, and scanning electron microscopy, which are sensitive to heterogeneities in larger domain size, give different results of miscibility from free volume data. It should be attributed the difference of characterization scale. The mechanical property corroborates the results of miscibility. POLYM. ENG. SCI.. 54:785-793. 2014. 2013 Society of Plastics Engineers

INTRODUCTION

Polymer blending is an extensive and effective method of improving the material properties or developing the new performances. As a typical system, the blends of sty-rene-buladiene-slvrene copolymer (SBS)/polystyrene (PS) are sustained attended |1-6). in which ihe britlleness of PS is significantly ameliorated. It is a consensus ihat the phase morphology and miscibility are the critical factors affecting ihe properties of blends. Therefore, the microstruclures and miscibility of SBS/PS blends are deeply investigated via the various characterization methods, such ax DMA, DSC. TEM. AFM. NMR, SAXS. and so on (5-8). DSC and DMA are sensitive to heterogeneities larger than approximately 50 nm in domain size. NMR has been reported lo be capable of a more refined a.ssurance of miscibility al the segmental level However, these methods are limited in characterizing the properties in smaller scale.

In recent years, positron annihilation lifetime spectroscopy (PALS) is a successful technique lo delect the angstrom scale changes of free volume in polymers. When constituent polymers are not miscible in molecular level, ihe interphases or interlaces between two phases will affect the positronium formation and annihilation probabilities. Therefore. PALS can be used to research the micmstructures and miscibility of blends |[degrees]-15) or composites 116-19). Camilla el al. (9) investigated the positron lifetime distribution and free volume parameters of PEO/PMMA blends, and they found that ihe width of positron lifetime distribution was a function of blend composition and temperature. Liu et al. |I0| discussed ihe free volume properties of two types of blends, a miscible blends of tetrameihv l-bisphenol A polycarbonate and PS. and an immiscible blend of bisphenol A polycarbonate and PS. No dispersion in the free volume distributions was appeared in miscible blends and a large broadening in immiscible blends, lndose el al. (20) researched ihe free volume of polydrimethylene tcrephthalatel/polycar-bonale blends, and their results suggested that the free volume became smaller when the iransesterification reaction occurred at the interface, indicating the partial miscibilily between two components. Dlubek el al. |I2| thought that PALS is a useful technique for studying inter-diffusion in blends of chemically different polymers. Due to the diffusion of polymers containing positronium (Ps) inhibitors from one phase inio another, the integral orlho-Ps (o-Psi intensity of the hlend is lower compared wilh the totally demixed stale. However, they considered thai the limitation of PALS is unknown concentration dependence of the mutual diffusion coefficient and therefore difficult lo consider correctly during analysis. In addition, they proposed that PALS technique may also be used to research the demixing of initially mixed blends, and determine hi nodal and spinodal decomposition temperature. Gunther et al. |211 located that PALS seems to be a powerful method to detect both binodal and spinodal phase separation temperature of blends because the intensity of the longest lifetime showed continuous and steplike changes, respectively, al ihe binodal and spinodal separation lemix*ratures.

Based on free volume data, some models were constructed, and some parameters were used lo investigate the miscibilily in polymer blends. A parameter p* obtained via free volume fraction was used lo delennine the simple hinary inter-chain interaction (10, 22). A negative deviation was observed in miscible system, and a complicated variation in immiscible blend. The negative deviation can be interpreted as a result of favorable interactions of segmental confonnation and packing between dissimilar molecules. Ranganathaiah (23) thought thai B was limited lo predict the concentration-dependent degree of miscibilily, and constructed a new model for miscibility delennina-tion based on the free volume which provided two parameters: a geometric factor (*y) and a hydrodynamic interaction parameter (a), "y depended on the molecular architecture, whereas a accounted for excess friction at the interface between the constituents of the blend, and ihe authors inferred that a was perhaps an appropriate parameter for determining the concentration-dependent probability of miscibility in binary blend systems.

In summary. PALS technique is an effective method for investigating the microsiructure and miscibility of blends. Therefore, we introduced PALS lo analyze the free volume parameters of SBS/PS blends. The free volume hole size, free volume fraction, and the free volume distribution were dissected. Based on different models, relative fractional free volume, inter-chain interaction parameters B. geometric factor y and hydrodynamic interaction parameter a were used to examine the effect of molecular architecture and styrene contenl of SBS on miscibilily.

EXPERIMENTAL

Materials and Sample Preparation

A general purpose polystyrene, 666D (density is 1.05 g cm \ was supplied by Yanshan Petrochemical Co.

China. Three kinds of styrene-butadienc-slyrene copolymer, 411. 501. and 604. were from LG Chemical Co., and the ratios of slyrene are 31. 31. and 41 wt7r (density-is 0.94. 0.94. and 0.96 g cm '). SBS 411 is a star-shaped, and the others are linear.

The blends of SBS/PS were prepared with a Brabender Mixer with a rate of 40 rpm al 200 C for 10 min. The SBS/PS blends were mixed with 10/9(1. 30/70. and 40/60 weighl ratio, and signed with 411-10, 411-30, 411-40. and so on.

Characterization

Differential scanning ealorimeirv (DSC) measurements were run on a NETZSCH DSC 204 Ft. and a heating rate of 10" C/min was used. Dynamic mechanical analysis (DMA) was performed wilh NETZSCH DMA 242C using two point bending mode, at the frequency of 10 Hz. with oscillation amplitude of 120 urn and a heal rale of 2[degrees]C/ min. The morphologies of the blends were analyzed using a scanning electron microscope (Quanla 200F).

The PALS measurements were carried out by a fasl-slow coincidence spectrometer with a time resolution 197ps at ambient lemperaiure. The positron source "Na uas sandwiched in between iwo identical pieces of sample, and two million counts were recorded for each spectrum. The average lifetime parameters and positron lifetime distribution were obtained by MELT4. The PALS spectrum is usually consisted of ihree lifelimes. t|, X2.and Ti. corresponding to different annihilation processes, and each lifetime has a relative intensity. f% and 1% [\2\. t] is the lifetime of ortho-Ps (o-Ps) localized in a free volume hole, so the radius of free volume (R) can be estimated via Eq.,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

where &Jt is ihe thickness of a homogeneous electron layer surrounding the free volume, and A/? = 0.166 nm is usually assumed for polymers. The free volume fraction was evaluated using Eq.2.

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

where A is an empirical constant (A is set as 1 in this study). V - \nR:.The probability density function (PDF) for free volume can be detennined wilh Eq.

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

Materials and Sample Preparation

in which "( 1/T) is the positronium annihilation rate probability distribution function.

The static mechanical properties were measured with a tensile testing machine, and ihe crosshead rate was set at 50 mm min '.

RESULTS AND DISCUSSION

The shift in the glass transition temperature (Tj.) of the components is normally taken as a sign of some degree of miscibility. Therefore, differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) were respectively performed to characterize [T.sub.y] of SBS/PS blend. Figure 1 shows DSC curves of different systems. All the blends show two individual 7,, and hoih 7',.s of SBS and PS remain unchanged, which indicates ihe blends are the phase-separated systems. In SBS 604/PS systems, the glass transition of SBS is relatively unob-vious comparing with pure SBS 604 system. Il should be attributed to the higher content of styrene in SBS 604 improves miscibility between two components.

Loss tangent {tan 5) obtained from DMA is presented as a function of temperature in Fig. 2. The results show that all spectra contain two pans, and lower temperature pan below zero degree is attributed lo the relaxation of polybutadiene phase ([T.sub.rii]).and higher temperature region above 100[degrees]C reflects the relaxation of polystyrene phase (Tps).In SBS 411/PS and SBS 501/PS blends, the higher lemperaiure part also involves two distinguishable peaks, and the peak at about I17'C corresponds to ihe pure PS phase relaxation (7"i^|) and the olher peak al aboul 106^ is for the PS phase in SBS (Tps;). With increasing the content of SBS in blends, the peaks at T,S weaken gradually, and opposiiely the peaks al 7*PB and "S: strengthen. However, the SBS 6O4-40/PS blend presents only one broad peak al higher temperature region, which suggests that the higher styrene contenl of SBS 604 promotes the miscibility between two components. The result is consistent with DSC analysis.

The phase morphology and the adherent domain between the components can be inluilively observed in SEM image, the miscibilily of blend can also be determined from ihe results of SEM. The SEM images of SBS/PS blends are shown in Fig. 3. The size of dispersion phase reduces wilh decreasing the weight percent of SBS. But the morphologies have no obvious difference among these SBS/PS blends.

As mentioned earlier, DSC and DMA technology are sensitive to heterogeneities larger than approximately 50 nm in domain size. However, these methods are limited in characterizing the properties in smaller scale (9J. Positron annihilation lifetime spectroscopy (PALS) is a successful technique to detect the angstrom scale changes of free volume in polymers, and it has been widely used lo research the microstructures and miscibility of blends. In most of blends, polymers are not miscible in the molecular level, and ihere is interphase or interface between ihe two phases. The discrepancy of miscibilily will result in the different interfacial structures, thereby the positronium formation and annihilation probability can be also probably affected. The results of PALS may nol necessarily be ihe same as those from macroscopic measurements. Since ihe Ps atom probes the local environment, the free volume from PALS should be more appropriate for understanding interactions between dissimilar chains. Therefore, PALS is applied to investigale the miscibility of SBS and PS in this study.

All ihe measured lifetime spectra were resolved into three lifetime components [T|, r2. and T.0 with corresponding intensities. with the MELT4.0 program, and lifetime distributions of the samples were given in Fig. 4. Generally, the shortest lifetime component. T|, with intensity is attributed to the annihilations from p-Ps and free positron annihilations. The intennedi-ate lifetime component. T2. is mainly due to annihilation of positrons trapped at ihe defects present in the crystalline regions or in the crystalline-amorphous inlerface regions. The longest component, [T.sub.l] is often ascribed to pick-off annihilation of o-Ps in the free volume hole present mainly in the amorphous regions as well as the interface regions of the polymer matrix or in Ihe blends (23L Therefore, tj was used lo calculate free volume parameters in this study.

The probability density funclion-free volume radius curves can be obtained via Eq. 3.Figure 5 shows the free volume radius distributions changing from pure PS to pure SBS at room lemperaiure. Comparing with pure PS, three kinds of SBS exhibit more board distribution, and the distribution shifts lowards larger radius wilh SBS content increasing. For SBS 411-30/PS and SBS 411-40/PS system, the peak width obviously becomes narrower than those of pure polymers. The previous research suggests ihat the formation of inlerfacial regions between incompatible polymers in the immiscible blends will cause the broadening of free volume distribution |9). So the phenomenon in SBS 411/PS systems probably reveals a belter miscibility at interface of star-shaped SBS and PS molecules. In order to investigate the miscibilily al interface, the free volume data are in-depth analyzed.

PALS technology predicts only one new mean o-Ps lifetime belween the values of the pure constiiuenis. In ihe case of miscible blends, a reduction in the mean free volume is usually observed versus that predicted by a simple additivity rule, which corresponds to a negative deviation in volume due lo favorable interactions or closer packing al ihe interface of components. A positive deviation from the additivity rule corresponds to an immiscible blend (9|. In Fig. 6, the complicated variation of average free volume size (Vg) and relative free volume fractions ([f.sub.v]) in immiscible blends observed by PALS is a result of the high sensitivity of the o-Ps not only to free volume holes bul also to interfacial structures. It can be observed that even in a completely immiscible blend, the mean free volume hole size as determined with PALS cannot be expressed by a simple rule of mixtures. The dashed line represents values predicted by the additivity rule. In these systems, both V, and F, increase as SBS contenl increases. For SBS 411 /PS composites. V, and [f.sub.v]show a negative deviation from the linear additivity rule throughout the composition range. For others systems, [V.sub.r] and F\ basically exhibit ihe positive deviation. When the miscibility between two polymers is belter, the polymer chains at the interface can pack closely, and it results in the reducing of free volume. Therefore, the results of free volume suggest thai SBS 41 I has better miscibility al the interface wilh PS.

Based on the theory of Liu |I0). F, in a simple binary blend can be expressed as:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

where iP| and 2 correspond to the volume fractions of constituent polymer 1 and 2, respectively. B is considered as an inter-chain interaction parameter between dissimilar chains. B has been observed to be negative in the case of miscible blends and positive or zero for immiscible blends. The plots of B as a function of weighl percentage of SBS are shown in Fig. 7. In SBS 4I l/PS system, 3 is negative for all ranges of composition, and has a larger negative value when 10 wl% SBS 411 is introduced. It suggests that SBS 411 has better miscibility with PS. and 10 wt% SBS 411 produces better miscibility. Parameter S shows positive deviation in SBS 504/PS and SBS 60I/PS systems, which reveals the immiscibility.

However. Ranganathaiah (23) thought [F.sub.v] or B was limited to predict the concentration-dependent degree of miscibility. and constructed a new model for miscibility determination on the basis of free volume results. In this study, this model is also employed to investigate the mis-cihilii) al the interface of SBS/PS blends. The relation can be expressed as following equation:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

In Eq.5. the left-hand-side quantity AF,is calculated as follows:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

And 8 is defined as

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

The molecular architecture and styrene contenl had a direct effect on molecular arrangement and packing at ihe interface, which can be reflected from free volume data. Parameter -y is geometric factor and is used to research the effecl of molecular architecture (linear and siar) and styrene content of SBS on the miscibility of blends. The plots of y as a function of SBS concentration are shown in Fig. 8. The behavior of 7 is similar to that of B in SBS 41 l/PS and SBS 504/PS systems. However, a negative deviation is observed in SBS 604-40/PS blend.

In addition, the hydrodynamic interaction parameter a in Eq.5 by definition quantifies the deviation of friction from ideality belween components I and 2 of the blend, and ihe changes at the interface of constitute polymers influence the parameter a. For a miscible blend, ihe chains of polymer 1 are evenly distributed in polymer 2. and the interaction sites are large. This implies that a good amount of friction is generated in Ihis system and parameter a attains large negative value. The plots of a as a function of SBS weight percent are shown in Fig. 9. Throughout the range of SBS 411 concentration, a exhibits negative deviation, and has a large negative value at 4M SBS 411, which suggests that larger friction al interface between SBS 411 and PS. especially higher concenlralion SBS is incorporated. For SBS 501/PS and SBS 604/PS systems, a also exhibits negative deviation, bul the values arc very close to zero. It indicates that the hydrodynamic interactions in the Iwo systems are smaller, and there is no friction or much less friction at the interface of the dissimilar chains of ihe constituent polymers.

The behaviors of parameter B. -y. and a indicate lhal star-shaped SBS 411 has better miscibility with PS. These results are consistent with previous theoretical and experimental researches, which investigated the miscibility of star/linear binary blends based on thermodynamic interaction parameter Xcit- Theodorakis et al. (24| compared the effective interactions of linear/star, linear/linear, and star/star polymer blends by Monte Carlo simulations. They found that linear/star blends are more miscible than the corresponding linear/linear blends, and linear/star mixtures are less miscible than star/star blends. Shen et al. [ 251 found that the star poly( 2-methyl-2-oxazolinc) (POMx) was miscible with poly(vinylidene fluoride) (PVDF). bul ihe blends of linear POMx with PVDF were phase-separated. In our work, star-shaped SBS 411 has wider free volume distribuiion (Fig. 5) which is attributed to ineffectively pack of molecular chains. At ihe interface, the larger free volume provides more space filled by PS chains, ihus ihe distribuiion of free volume become narrower, and parameter B. y. and a exhibit negative deviation which suggests belter miscibilily at interlace in star-shaped SBS 41 l/PS systems.

Ihe mechanical properties of blends were perfonned lo further examine the miscibility belween SBS and PS. With the increase of SBS content, the elongations at break of the SBSs/PS blends improve significantly, bin the tensile stresses have the conspicuous diminutions (Fig. 10). Apparently SBS 501-40/PS blend has a lower elongation al break comparing with SBS 411-40/PS and SBS 604-40/PS. It should be attributed lo the poor miscibility between two components of SBS 501-40/PS blend, which revealed by DSC. DMA, and PALS results.

CONCLUSIONS

In this study, DSC, DMA. and PALS technology were employed to investigation the miscibility of SBS/PS blends. Owing lo the difference of characterization scale, these technologies exhibit the discrepancy in ihe respect of determining the miscibility of binary blend. The results of DSC and DMA suggest lhal SBS b ()4 containing more sivrene unit has heller miscibilily with PS phase. However, parameter p. y. and a oblained from free volume-data reveal thai star-shaped SBS 401 has belter miscibilily al the interface of SBS and PS phase in spite of less styrene proportion. The superior mechanical properties were achieved when components show better miscibility.

ACKNOWLEDGMENTS

Xiaoyu Meng designed the project, analyzed and interpreted data, dratted the paper. Xu Liu carried out all experiments in this study except for positron annihilation lifetime spectroscopy measurements. Zhuoxi Li carried out positron annihilation lifetime spectroscopy measurements. Qiong Zhou analyzed data and revised the manuscript. All authors discussed the results and commented on the manuscript.

ABBREVIATIONS

AFM          Atomic Force Microscopy
DMA          Dynamic Mechanical Analysis
DSC          Differential Scanning Calorimelry
[F.sub.v]  Relative Free Volume Fractions
NMR          Nuclear Magnetic Resonance
PALS         Positron Annihilation Lifetime Spectroscopy
PEO          Polyethylene Oxide
PMMA         Poly methyl Methacrylatc
PS           Polystyrene
Ps           Positronium
o-Ps         Ortho-positronium
SAXS         Small Angle X-ray Scattering
SBS          Styrene-Buladiene-Styrenc Copolymer
SEM          Scanning Electron Microscopy
TEM          Transmission Electron Microscopy
[V.sub.f]  Average Free Volume Size


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Ciirn-\piunlcncf to: X. Meng: email: XmengK004IK@ayahoo.com.en

Contract gran! sponsor: Research Funds Provided to New Recruitments ot China University of Petroleum. Beijing: contract grant number: QD-2010-10: contract grant sponsor: Research Fund of Beijing Key Labora-tory: contract grant number: Z12I104002812044.

DOI UI.I(X)2/pen.236l4

Pubhshed online in Wiley Online Library (wdeyonlinclibrary.com).

2013 Society of Plastics Engineers

Xiaoyu Meng,(1)(2) Xu Liu,(1) (2) Zhuoxi Li,(3) Qiong Zhou(1) (2)

(1) College of Science, China University of Petroleum (Beijing), Beijing 102249, China

(2) Beijing Key Laboratory of Failure, Corrosion and Protection of Oil/gas Facilities, Beijing 102249, China

(3) Key Laboratory of Nuclear Analysis Techniques, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China
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