Glass Transition Temperature and Î² Relaxation Temperature around

Article pubs.acs.org/JPCB

Glass Transition Temperature and β Relaxation Temperature around Chain End of Polystyrene Determined by Site Specific Spin Labeling Yohei Miwa,*,† Osamu Urakawa,‡ Akinari Doi,‡ Katsuhiro Yamamoto,§ and Shogo Nobukawa∥ †

Analytical Technology Laboratory, R & D Center, Mitsubishi Chemical Corporation, 1 Toho-cho, Yokkaichi, Mie 510-8530, Japan Department of Macromolecular Science, Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan § Department of Materials Science and Technology, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan ∥ School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan ‡

ABSTRACT: A glass transition temperature, Tg, and a β relaxation temperature, Tβ, of spin-labeled polystyrene (PS) having a number average molecular weight (Mn) of ca. 25 kDa were determined by the microwave power saturation (MPS) method of electron spin resonance (ESR). Spin labeling was selectively carried out at chain ends or midchain segments. This method allowed us to determine the local Tg and the local Tβ around the spinlabeled sites, selectively. The Tg determined by the ESR, Tg,ESR, was in good agreement with that determined by differential scanning calorimetry, Tg,DSC; the Tg,ESR decreased with decreasing Mn with blending oligomers as well as the Tg,DSC. The Tg,ESR for the end-labeled PS (PS-E) was equal to that for the midchain-labeled PS (PS-M) irrespective of the Mn. However, we previously reported that the PS-E showed distinctly higher segmental mobility than the PS-M in the temperature range 423−463 K (above Tg). Therefore, we conclude that the chain ends intrinsically have higher segmental mobility than midchain segments due to the discontinuity of repeat units; however, the mobilities of chain ends and midchain segments are averaged out in the vicinity of Tg due to the cooperativities with neighboring numerous chain segments. Concerning the β relaxation, the Tβ determined by the MPS was in good agreement with those determined by dielectric and dynamic mechanical spectroscopies and dilatometry. The Tβ of the PS-E was the same with that of the PS-M within experimental uncertainties; the Tβ was insensitive to the Mn in contrast to the Tg. Therefore, we conclude that the effect of chain end is little on the β relaxation of PS due to its local character. In addition, the effect of annealing at 353 K was found to be the same for the Tβs of the PS-E and PS-M.

1. INTRODUCTION All polymers have chain ends; the molecular weight dependences in some physical properties are attributed to effects of chain ends. For example, it has been widely recognized that a glass transition temperature, Tg, decreases with increasing chain end concentration.1−3 Robertson et al. recently showed that the Tg and fragility index of mixtures of monodispersed PS were a single function of the average number of the chain ends, independent of the nature of the molecular weight distribution.4 Moreover, recent researches revealed that the effects of chain ends on the dynamic behaviors of polymers were often emphasized at surfaces and interfaces of polymeric materials because the chain ends tend to segregate there.5−8 Kajiyama et al. suggested that the segregation of chain ends at the surface of the PS film was one of the causes of the decrease in Tg at the surface.7 Additionally, Tanaka et al. recently measured the Tg at the PS/solid substrates using fluorescence lifetime measurements; the Tg at the interfacial region showed less marked molecular weight dependence compared to the bulk Tg. This behavior was interpreted by the restriction of the chain end mobility at the solid substrate surface.9 These examples clearly show that understanding dynamic properties around the chain ends is important not only for bulk materials but also for © 2011 American Chemical Society

advanced polymeric materials such as nanocoatings, nanocomposites, multilayer devices, etc. However, polymer dynamics around chain ends is still a controversial problem because of the insufficiency of experimental and theoretical studies. In this work, we examine the glass transition (α relaxation) and β relaxation processes around chain ends of PS using the spinlabel method of electron spin resonance (ESR) to obtain a better understanding of chain end dynamics. The pioneering work on the α relaxation process of the chain end of PS (Mn = ca. 11 kDa) around Tg was carried out by Mansour et al. using dielectric spectroscopy (DS).10,11 In their work, PS terminated with a p-cyanobenzyl group was synthesized to highlight the dynamics of the chain end in the dielectric measurement. Their result showed that the mobility of the chain end was equal to that of the other segments just above the Tg. More recently, the author developed a novel method for the detection of the Tg of the spin-labeled polymers applying a microwave power saturation (MPS) of ESR.12 This method allows us to detect whether Tg varies between the Received: November 5, 2011 Revised: December 28, 2011 Published: December 29, 2011 1282

dx.doi.org/10.1021/jp210630m | J. Phys. Chem. B 2012, 116, 1282−1288

The Journal of Physical Chemistry B

Article

different sites in the polymer chain that are addressed by the spin labels. Using this method, it was shown that the difference between the Tgs for end-labeled poly(cyclohexyl acrylate) (PCHA, Mn = ca. 15 kDa) and that of the midchain-labeled PCHA (Mn = ca. 15 kDa) was little. Lund et al. also carried out the DS measurement for oligomer PS (Mn = 2.2 kDa) endfunctionalized with a polar group to study the chain end mobility over an extensive temperature range (ca. 110− 413 K).13,14 In their work, the mobility of the end-functionalized PS was almost equal to that of the nonfunctionalized PS around the Tg being similar to the Mansour’s result; however, higher mobility of the end-functionalized PS than the nonfunctionalized PS became more distinct with increasing temperature. Recently, our ESR work also showed the higher mobility of the chain ends of PS (Mn = ca. 25 kDa) than that of midchain segments in the temperature range 423−463 K.15 The present work more extensively studied the local Tg around the chain ends in the PS using the MPS method with the site-specific spin labeling. Compared to the DS, the MPS method allows us to determine the local Tg around chain ends even for high molecular weight polymers thanks to the better sensitivity and contrast of the ESR. Therefore, the first aim of this work is to verify the influence of the average molecular weight on the local Tg around chain ends. The monodispersed PS having a number average molecular weight (Mn) of ca. 25 kDa were spin-labeled at the chain ends or midchain segments. In order to examine the effects of Mn and the concentration of chain ends on the local Tg around chain ends, the binary mixtures with various Mn were prepared by blending the spin-labeled PS and nonlabeled oligomer PS (Mn = 0.8 kDa and 2.4 kDa). The second aim of this work is to clarify the effect of the chain end on a β relaxation process. The β relaxation appears at lower temperature than Tg and is attributable to more local and less cooperative motions compared to the α relaxation (glass transition). Reportedly, PS shows β process at about 300−310 K measured in both dielectric and mechanical spectroscopies16−20 and dilatometry.21 The β relaxation has been identified as a local mode involving phenyl group motion coupled with backbone libration or oscillation.16,17,22,23 In this work, the β relaxation temperature (Tβ) around chain ends of PS was determined using the MPS method and the effect of the chain end on the β relaxation as well as the α relaxation was studied.

Chart 1

respectively. First, the spin-labeled PS and oligomer PS were dissolved in chloroform (Kishida Chemical Co., Ltd., Extra Pure Reagent) with the concentration of 5 wt %. The solution was casted on a Teflon plate and dried in the hood for one day, and then the residual solvent in the sample film was completely removed in a vacuum at 363 K for 24 h. The Mn of the spinlabeled PS/PS2.4k (50/50) blend is ca. 4.4 kDa. The Mns of the spin-labeled PS/PS0.8k (50/50) and (70/30) blends are ca. 1.5 kDa and 2.5 kDa, respectively. 2.2. ESR Measurement. Spectra were recorded with JEOL X-band (ca. 9 GHz) FA200 spectrometers with 100 kHz field modulation and the modulation amplitude of 0.2 mT. The magnetic field width, sweep time, and time constant were typically 15 mT, 60 s, and 0.03 s, respectively. The tuning parameters of the ESR spectrometer and sample position in the cavity were kept constant, and only temperature was varied for measurement. Samples were encapsulated into 5 mm o.d. quartz tubes and sealed under a vacuum. The temperature control unit could not heat or cool the sample at a constant rate; therefore, quenched condition was applied for the Tg,ESR measurement to dominate the same thermal history. The sample contained in an ESR sample tube was heated at 393 K for 15 min by a heater and quenched into an ESR cavity kept at 323 K. After the sample was kept at 323 K for 5 min, the ESR measurement was performed stepwise from 323 to 398 K. The samples were allowed to equilibrate for at least 5 min after reaching the desired temperature. However, the Tβ measurements were carried out for quenched and annealed conditions: for the quenched condition, the sample contained in an ESR sample tube was heated at 393 K for 30 min by a heater and quenched into an ESR cavity kept at 283 K, and then, after the sample was kept at 283 K for 5 min and cooled to 243 K, the ESR measurement was performed stepwise from 243 to 353 K. For the annealed condition, the sample was annealed at 353 K for 10 h after heating at 393 K for 30 min. The sample was quenched to 283 K for 5 min, and then, the ESR measurement was carried out from 243 to 353 K. 2.3. Other Measurements. Differential scanning calorimetry (DSC) measurement was carried out using Q10 differential scanning calorimeter manufactured by TA Instruments and calibrated with an indium standard. For the cooling of samples, a quench cooler accessory (TA Instruments) was used. The DSC cell was purged with dry nitrogen gas during the measurement with the flow rate of 50 mL min−1. Samples were heated from a room temperature to ca. Tg + 30 K at a rate of 20 K min−1, kept for 5 min, cooled to ca. Tg − 50 K at a rate of 10 K min−1, and heated to ca. Tg + 30 K at a rate of 10 K min−1. The data collection was carried out on the second heating

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Two PS having the same molecular weight but labeled at different sites, the chain end or the midchain, with a nitroxide via a short tether were prepared (Chart 1). The notations for the end-labeled and midchainlabeled PS are defined to be PS-E and PS-M, respectively. Synthesis of the PS-E and PS-M by the atom transfer radical polymerization (ATRP) was described in our previous article.24 The α end of the PS-M and PS-E is 1-phenylethyl group because 1-phenylethyl bromide was used as an initiator. The Mn and the molecular weight distribution (Mw/Mn) for the PS-E determined by gel permeation chromatography (GPC) were 26.1 kDa and 1.09, respectively. However, the Mn and Mw/Mn for the PS-M were 24.9 kDa and 1.15, respectively. The preparation of binary mixtures of the spin-labeled PS and oligomer PS is as follows. The oligomer PS (Mn = 2.4 kDa; Mw/Mn = 1.05) (Mn = 0.8 kDa; Mw/Mn = 1.10) purchased from Tosoh Co., Ltd. are denominated PS2.4k and PS0.8k, 1283



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P = 0.02 mW, VR/VR,0.02mW, is proportional to P0.5 because of no saturation when P is small enough. However, the deviation from the linear relationship appears with the increase in P0.5. This deviation means the increase in the S value. This phenomenon is called microwave power saturation. The S was determined from the saturation curve as follows: at the same P0.5, the ratio of the saturated VR value, VR,S, to the unsaturated one, VR,US, is written as

process. The Tg,DSC was determined to be the midpoint, i.e., the temperature corresponding to half of the endothermic shift and included ±2 K of experimental uncertainties. Gel permeation chromatography (GPC) was carried out with four polystyrene gel columns (Tosoh TSK gel GMH (beads size is 7 μm), G4000H, G2000H, and G1000H (5 μm)) connected to a Tosoh CCPE (Tosoh) pump and an ERC-7522 RI refractive index detector (ERMA Inc.) and with tetrahydrofuran as an eluent at 313K in order to determine the weight and number average molecular weights of labeled PS samples. The column set was calibrated by using standard PS (Tosoh) samples with narrow molecular weight distributions. 2.4. Determination of Tg and Tβ by Microwave Power Saturation (MPS) Method. In general, the mobility of spinlabeled polymer is determined by the ESR spectral shape analysis. However, the X-band ESR spectral shape is only sensitive on time scales in the range 10−11−10−7 s; therefore, detection of Tg is hard by this method because the correlation time, τc, of molecular tumbling motion of nitroxides bonded to polymers is longer than 10−7 s around Tg.25 Instead, the author discovered that the ESR signal intensity measured with an overloaded microwave power supply (typically more than 1 mW) significantly increased above Tg.12 This phenomenon is based on a decrease in the degree of microwave power saturation above the Tg, which is attributed to enhanced molecular tumbling motion of nitroxides. Briefly, the MPS method determines a local Tg around spin-labeled sites from monitoring the temperature dependence of the ESR signal intensity. The principle of the MPS method has been described in detail in our previous article.12 The relative signal intensity VR from an X-band ESR spectrometer is given by

VR =

VR , S VR , US

1 1+S

(2)

Therefore, the S is given by

S=

VR , US VR , S

−1 (3)

The relationship of VR,S and VR,US at P = 9 mW is schematically shown in Figure 1. When the microwave power, P, is fixed at each temperature, the S is a function of the T1 and T2. In the range where the τc of molecular tumbling motion of nitroxides is longer than 10−7 s, the T1 is proportional to the τc.27 However, the effect of the T2 was discussed in our previous work based on experimental results.12 In the previous work, we prepared three spin-labeled PCHAs having different nitroxide concentrations, 1.3 × 10−7, 4.8 × 10−7, and 9.6 × 10−7 mol g−1, to evaluate the effect of T2. The T2 is expected to decrease with an increase in the nitroxide concentration. However, the nitroxide concentration little affected the temperature dependence of the S. From this result, we concluded that the nitroxide concentration less than ca. 1.0 × 10−6 mol g−1 was low enough to ignore the effect of the T2.12 Therefore, the S is proportional to the τc of the motion of nitroxide around the Tg. The Arrhenius plot of the S determined at P = 9 mW, S9mW, for the PS-M and PS-E is shown in Figure 2. The plots were vertically shifted to avoid

γH1(T1T2)0.5 1 + γ2H12T1T2

=

(1)

where γ, H1, T1, and T2 are the gyromagnetic ratio, microwave magnetic field, spin−lattice relaxation time, and spin−spin relaxation time, respectively.26 The γ2H12T1T2 is defined as a saturation factor, S, in this work. Without the saturation, the VR is proportional to H1 because the S is small enough to be ignored. In experiments, the VR is usually plotted against the square root of the microwave power, P0.5, because the H1 is proportional to the P0.5. As an example, the plots for the PS-M at 323 and 383 K are shown in Figure 1 where the VR is

Figure 2. Temperature dependence of S9mW for PS-M (Mn = 24.9 kDa) and PS-E (Mn = 26.1 kDa). The plots are vertically shifted to avoid overlapping. The temperature at the inflection in the temperature dependence of S9mW is defined as Tg,ESR. The Tg,ESRs for the PS-M and PS-E are 375 ± 2 K and 374 ± 2 K, respectively. The Tg,DSC is also shown.

overlapping. When nitroxide is attached to a polymer segment, its mobility is strongly affected by that of the polymer segment and the local free volume. Therefore, it is reasonably considered that the temperature dependence of the S changes in the vicinity of Tg because the polymer segmental mobility and the free volume size are largely different in the glassy and rubbery states. The temperature at the inflection in the temperature dependence of S9mW is defined as Tg,ESR (see Figure 2). The

Figure 1. Plots of VR normalized by VR,0.02mW against P0.5 for PS-M at 323 and 383 K. Here, the Tg,DSC of PS-M is 375 K. Saturated VR value, VR,S, and unsaturated one, VR,US, are shown with arrows.

obtained from a double integration of a spectrum. Here, the Tg,DSC of PS is about 375 K. The VR normalized by the value at 1284



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500.42 Namely, one unit at the chain end moves cooperatively with hundreds of midchain repeat units at Tg. In this situation, the reduction of Tg in the local region around chain ends must be little because the molecular motions of the chain end and hundreds of midchain units are averaged out. Therefore, it is concluded that polymer chain ends act as a plasticizer by reducing the degree of cooperativities between neighboring segments because of the discontinuity of repeat units; however, the local reduction of Tg around chain ends is little due to the intersegmental cooperativities. We previously showed that the chain ends of PS had distinctly higher segmental mobility than the midchain segments in the temperature range 423−463 K (above Tg) where the mobility was determined from computer simulations of ESR spectra.15 The higher segmental mobility of chain ends were attributed to lesser intersegmental hindrance around chain ends. We consider that the intrinsic higher mobility of chain end is pronounced above Tg because the intersegmental cooperativities reduce with increasing temperature. In fact, recent experiments showed that the number of dynamically correlated repeat units of polymers decreased dramatically with increasing temperature above Tg.41,43,44 One may expect the aggregation of spin-labeled sites in PS because the nitroxide is polar. In particular, the influence on the behavior at the chain end may be expected to be greater than that at the middle of the chain; this should affect the mobility of the chain end. The ESR spectrum is sensitive to the aggregation of spins. If the spin-labeled chain ends were aggregated, the ESR spectrum should show significant broadening due to the spin−spin interactions. However, the ESR spectrum of the PSE measured at 77 K in ref 15 did not show the significant broadening. This is the direct evidence for that the aggregation of the spin-labeled chain ends in PS is little. 3.2. Molecular Weight Dependence of Local T g around Chain End. In this section, we show the molecular weight dependence of Tg around chain ends. In Figure 3, the

Tg,ESR is in good agreement with the Tg,DSC. To verify the experimental uncertainty in the Tg,ESR, the measurements were separately carried out more than three times for each sample. It should be noted that the absolute value of the S determined by the experiment is strongly influenced by many factors, such as the tuning parameters of the ESR spectrometer, the insert position and the shape of samples in the cavity, etc; therefore, the direct comparison of absolute S values is hard so far. However, we confirmed that the Tg,ESR was not influenced by such factors when the tuning parameters of the ESR spectrometer and sample position in the cavity were kept constant and only temperature was varied. Determination of the β relaxation temperature, Tβ, by the MPS method is completely the same with the case of Tg measurement. In contrast to the obvious change in the temperature dependence of S at Tg, the change around Tβ is slight (see Figure 4) because the magnitude of the β relaxation is much smaller than the glass transition.

3. RESULTS AND DISCUSSION 3.1. Local Tg around Chain Ends of PS. When approaching Tg from the higher temperature, some physical properties like a viscosity and a relaxation time increase dramatically. The formation of collectively and dynamically rearranging regions near the Tg was proposed as an explanation for the observed slowing down of the dynamics a long time ago.28,29 This idea was strongly supported by recent computer simulations.30,31 Adam and Gibbs called this spatial aspect of the dynamically correlated assembly of molecules a “cooperative rearranging region (CRR)”.28 As a modern model, Ngai proposed the coupling model (CM) taking into account the importance of the strength in intermolecular couplings. His theory gives a viable description for the α-relaxation dynamics in which coupled relaxation dynamics of many molecules is involved.32,33 One may expect smaller CRR and/or weaker strength in intersegmental couplings around the chain ends because of the reduced local molecular packing induced by the discontinuity of repeat units. Indeed, the larger free volume around the chain ends of PS has been suggested based on the result that photoisomerization kinetics of azobenzene chromophores labeled at the chain ends was faster than those at midchain.34,35 Based on the CM, Rizos and Ngai suggested reduced intersegmental coupling around chain ends from comparison of PS having high and very low Mn.36 From these results, one may expect local decrease of Tg around chain ends. However, the Tg,ESR for the neat PS-M (Mn = 24.9 kDa) and PS-E (Mn = 26.1 kDa) are 375 ± 2 K and 374 ± 2 K, respectively (Figure 2); therefore, the decrease of the local Tg around the chain ends is little. This result is in good agreement with the DS works on end-functionalized PS.10,11,13,14 We consider that taking into account the size of the CRR and/or the strength in intersegmental couplings around Tg is important to understand quantitatively this result. The length scale of the CRR for bulk glass formers around Tg has been determined by various methods.37−40 Although the length scale is material dependent, the values are on the order of 1−4 nm. Recently, numbers of repeat units or molecules dynamically correlated around Tg in various polymers and low-molecular-weight glass formers have been evaluated by the four−point susceptibility method.39,41−44 These results showed that hundreds of molecules or repeat units dynamically correlated around Tg. In particular, Capaccioli et al. evaluated the number of dynamically correlated repeat units at Tg in the PS to be ca.

Figure 3. Plots of inverses of Tg against Mn−1 for spin-labeled PS/ oligomer PS blends.

inverses of the Tg,DSC and the Tg,ESR for the spin-labeled PS/ oligomer PS blends are plotted as a function of Mn−1 as proposed by Ueberreiter and Kanig (UK)2

Tg −1 = Tg ∞−1 + A /M n

(4)

Here, Tg∞ is a value of the Tg at infinite molecular weight, and A is a constant. It is known that this UK plot gives better linearity in the low Mn region than the plot of Tg vs Mn−1 suggested by Fox and Flory.1,3 The inverses of the Tg,ESR and Tg,DSC linearly increased with an increase in the Mn−1, independent of the nature of the molecular weight distribution. 1285



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This result is in good agreement with the result previously shown by Robertson and Roland in which the Tg of binary mixtures of monodispersed PS was determined by the dynamic mechanical measurement.4 The Tg∞ and A for the Tg,DSC are 378 ± 3 K and 0.78 ± 0.04, respectively. However, the Tg∞ and A for the Tg,ESR of the PS-M and PS-E are 380 ± 3 K and 0.73 ± 0.04 and 378 ± 3 K and 0.74 ± 0.04, respectively. Namely, the molecular weight dependence of the local Tg around chain ends is concluded to be the same with that around midchain segments because of the intersegmental cooperativities as described in the section 3.1. 3.3. Effect of Chain End on β Relaxation Temperature. More than 40 years ago, Johari and Goldstein (JG) showed, even in rigid molecular glass-formers, the universal existence of a β relaxation process, proving that this process does not involve intramolecular degrees of freedom but instead some motions involving essentially all parts of the molecule.45 This β relaxation process is called JG β relaxation and is distinguished from other secondary relaxations such as partial motions of pendant groups because of the importance for the glass transition mechanism. Ngai et al. pointed out that the α and JG β relaxations are not independent processes in the view of the many correlations existing between the α and JG β relaxations.32 According to his CM, global many-body α relaxation originates from elementary and local one-body relaxation (i.e., the primitive relaxation); the JG β relaxation is identified as the primitive relaxation.32,33,46,47 The Tβ of PS is generally detected at about 300−310 K in both dielectric and mechanical spectroscopies16−20 and dilatometry.21 The β relaxation merges with the α relaxation and has been identified as a local mode involving phenyl group motion coupled with backbone libration or oscillation.16,17,22,23 The coupling of motions of side groups and main chain is one criterion for the JG β relaxation. Moreover, Kudlik et al. found a relationship between the Tg and the activation energy of the JG β relaxation, Eβ, written as48

Eβ /RTg = 24

Figure 4. (a) Temperature dependence of S9mW for PS-M and PS-E with quenched condition. The plots are vertically shifted to avoid overlapping. Tβs are shown with arrows. (b) Temperature dependence of S9mW for PS-M/oligomer PS (Mn = 2.4 kDa) (50/50) with quenched condition.

volume around the spin-labeled sites induced by the β relaxation of neighboring PS segments. Namely, the Tβ of the PS-E reflects the β relaxation of PS segments neighboring with the spin-labeled chain end. Therefore, our ESR result suggests that the intermolecular effect such as the large free volume and/or reduced intersegmental coupling around chain ends does not significantly affect the β relaxation process of PS. To evaluate the intramolecular effect, a nonlabel method or a label method with an extremely small agent such as deuterium may be expected. In addition, we examined the effect of the chain end concentration on the Tβ of PS. In Figure 4b, the Arrhenius plot of S9mW for the even blend consisting of the PS-M and PS2.4k measured with the quenched condition is shown. The Mn of the blend is ca. 4.4 kDa, and the Tg,ESR is 360 ± 2 K as shown in Figure 3. However, the Tβ of this blend, 320 ± 3 K, was almost the same with that of the neat PS-M, 319 ± 4 K within experimental uncertainties. This result suggested that the Tβ was insensitive to the chain end concentration in contrast to the Tg. As a similar result, Reissig et al. first measured the molecular weight dependence of the JG β relaxation rate for poly(ethyl methacrylate) (PEMA) using the DS.51,52 They showed that the JG β relaxation rate was little affected by the molecular weight, while the Tg decreased with decreasing molecular weight; an additional relaxation process associated with the JG β relaxation of chain end segment was not reported even for very short PEMA with the Mn of 1650 Da. Similarly, the additional relaxation process was not detected by the DS for short poly(methyl methacrylate) (PMMA) having the Mn of 1720 Da.53 These results demonstrated that the JG β relaxation rate of the chain end segment was not distinctly faster than that of inner segments for these polymers; the effect of chain end concentration on the JG β relaxation was very small. This result is in good agreement with our result. In addition, no shift of the Tβ of PS was observed in miscible polymer blends with poly(vinyl methyl ether) or poly(phenyl oxide) even though the Tg varied depending on the composition.18,20 Namely, the β relaxation of PS is insensitive to intermolecular interactions. It

(5)

The accuracy of this relationship was reexamined by Ngai and Capaccioli; they found that there were notable large deviations in a few glass-formers, and the values of (Eβ/RTg) for non-JG β relaxations were significantly smaller than 24.49 However, the activation energy for the β relaxation of PS was reported to ca. 126 kJ mol−1, and the (Eβ/RTg) is calculated to ca. 40.16 Therefore, we consider that the β relaxation process in PS may be identified to the JG β relaxation process. The Arrhenius plot of S9mW for the PS-M and PS-E measured with quenched condition in the temperature range 243 K−353 K is shown in Figure 4a. The Tβs for the PS-M and PS-E were determined to 319 ± 4 K and 318 ± 4 K, respectively, and almost no difference was found within experimental uncertainties. One might expect lower Tβ for the PS-E than the PS-M because the conformation transition of the main chain may occur easily at the chain end due to the smaller steric hindrance. Unfortunately, this intramolecular effect is not evaluated from the PS-E because the chain end of the PS is terminated with the spin-label unit. Moreover, in the glassy state, the motion of the nitroxide spin label is not strongly coupled with that of the polymer segment because the libration is the main motion of the nitroxide; the mobility of the nitroxide spin label is strongly affected by the local free volume size.50 Therefore, the change in the temperature dependence of S9mW at the Tβ is attributed to an increase in the local free 1286



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ESR. Although distinctly higher segmental mobility of the PS chain end compared to the midchain segments in the temperature range 423−463 K (above Tg) was shown in our previous work, no difference in the Tg around the chain end and midchain segment was observed irrespective of the Mn. In conclusion, while chain ends intrinsically have higher segmental mobility than midchain segments at high temperatures, in the vicinity of Tg, chain ends are involved in cooperative motion with neighboring numerous midchain segments, and thus, the mobilities of the chain end and midchain segment are averaged out. Namely, polymer chain ends act as a plasticizer because of the discontinuity of repeat units; however, the local reduction of Tg around chain ends is little because of the intersegmental cooperative motion. Concerning the β relaxation, the Tβ of the PS-E was the same as that of the PS-M within experimental uncertainties; moreover, the Tβ was not affected significantly by an increase in the chain end concentration, while the Tg distinctly decreased. Therefore, it was concluded that the β relaxation of PS was not affected significantly by the large free volume and/or reduced intersegmental coupling around chain ends due to its less cooperative and local character. In addition, the effect of annealing at 353 K on the Tβs of the PS-M and PSE was found to be almost the same.

was also reported that essentially no effects by mixing diluents to glass formers, such as polybutadine,54 sorbitol,55 and xylitol,56 were found. Taking into account these reported results and our result, we conclude that the intermolecular effect of the chain end such as the large free volume and/or reduced intersegmental cooperativity is little on the β relaxation of PS in contrast to the α relaxation. We consider this is caused by the less cooperative and local nature of the β relaxation of PS. 3.4. Effect of Physical Aging on Tβ of PS. In Figure 5, the Arrhenius plot of S9mW for the PS-M and PS-E measured with

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Figure 5. Temperature dependence of S9mW for PS-M and PS-E with annealed condition. The plots are vertically shifted to avoid overlapping. Tβ is shown with arrows.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

annealed condition in the temperature range 243 K−353 K is shown. The result of dilatometry measurement showed an increase in the density with the annealing at 353 K.21 The annealing experiment is generally called physical aging. The Tβs for the annealed PS-M and PS-E were 311 ± 3 K and 312 ± 3 K, respectively. Compared to the quenched samples, the Tβ was slightly decreased by the physical aging at 353 K. Effects of the physical aging on the β relaxation have been reported for some glass-formers including polymers.19,57−61 In the case of dipropyleneglycol dibenzoate, the peak tops of the JG β and non-JG β relaxations in a dielectric spectrum largely and slightly moved to lower frequency, respectively, during the physical aging.57,59 However, the peak maximum frequency of the JG β relaxation in a dielectric spectrum of polyvinylethylene moved to higher frequency, while the β peak became narrower on both the high and low frequency sides during the physical aging.60 In the cases of PS,19 PMMA,53,58 and telmisartan,61 although the peak maximum frequency (or peak maximum temperature) of the JG β relaxation was constant, the β peak became narrower at the low frequency side (or high temperature side), and the separation of α relaxation from the β relaxation increased during the physical aging. Consequently, the average JG β relaxation rate (or Tβ) became faster (or lower). Similar to these reported results, the change in temperature dependence of S9mW at Tβ became more distinct by the annealing. We consider the slight decrease in the Tβ in our result is also attributed to the reduction of higher temperature side of the β relaxation during the physical aging. As a result, the effect of the annealing for the PS-E and PS-M was found to be almost the same. We are planning further experiments on the physical aging effect on the chain end of PS.

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REFERENCES

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4. CONCLUSIONS The glass transition temperature, Tg, and the β relaxation temperature, Tβ, of PS spin-labeled at the chain end or midchain segments were determined by the MPS method of 1287



Article

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Glass Transition Temperature and Î² Relaxation Temperature around

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