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A trial to clarify a cause of forming physical gels: Morphology of syndiotacticpolystyrene in n-alkylbenzene

Hideyuki Itagaki*, Tomokatsu Tokami, Jun MochizukiDepartment of Chemistry, Graduate School of Science and Technology and School of Education, Shizuoka University, 836 Ohya, Suruga-ku, Shizuoka 422-8529, Japan

a r t i c l e i n f o

Article history:Received 2 July 2012Received in revised form27 August 2012Accepted 12 September 2012Available online 24 September 2012

Keywords:Physical gelPolymer-solvent molecular compoundMorphology

a b s t r a c t

We dissolved low concentration of syndiotactic polystyrene (SPS) in serial n-alkylbenzenes (NB: C6H5

eCNH2Nþ1 (N ¼ 1e10)) at high temperatures and observed the morphology and structure after cool-ing by SEM, IR, DSC, andWAXD. The morphology of SPS in NB changes with N: (I) N¼ 1e3, SPS/NB formsa gel with fibril structure networking three-dimensionally, (II) N ¼ 4e6, SPS/NB produces paste-likestate where SPS chains with 21-helix conformation gather together to form SPS g crystalline phase,and (III) N ¼ 7e10, SPS/NB produces sherbet-like state where SPS chains with trans zigzag conformationgather together to form SPS b crystalline phase. We have discussed a cause of forming physical gels ofSPS together with the reason why the morphology of SPS/NB depends on N.

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1. Introduction

Syndiotactic polystyrene (SPS), which was first synthesized in1986 by using metallocene catalyst [1], is known to have a complexpolymorphic behavior [2e4]. It has two stable conformations ofpolymer chain, all trans zigzag (TTTT) conformation [5] and 21-helix(TTGG) confomation [6]. Nonsolvated crystalline forms a andb consist of all trans conformation, while the other crystalline formsg, d and ε consist of 21-helix conformation. In particular, the d and ε

forms are quite interesting because they have cavities inside in spiteof a crystalline form: the d-empty form [7] ismonoclinic (spacegroupP21/a;a¼1.74nm;b¼1.18nm; c¼0.78nm;g¼117�) andexhibit perunit cell two identical cavities centered on the center of symmetryand bounded by ten phenyl rings [7a,8]. They can cocrystallize withsome organic molecules and host a functional guest molecule [6].

SPS is known to form thermoreversible gels in quite a fewsolvents, and many papers on SPS gels have been published so farsince Kobayashi et al. [9] In solution or after absorption of solventmolecules, it is established that the 21-helix conformation can beformed [10] and is responsible for the thermoreversible gelation[9]. The detailed phase diagrams of temperature-SPS concentrationhave been obtained for SPS gels with solvents since Guenet et al.[11]. The solvents reported so far are chloroform [11,12], toluene[11,13], benzene [13,14], bromoform [15], chlorobenzene [12], di-,tri-, and tetra-chlorobenzenes [12], cis-decalin [16,17], trans-decalin [16,18,19], benzyl methacrylate [20], cyclohexyl

methacrylate [20], naphthalene [21] and so on. They suggest thatsolvent molecules are staying between SPS chains consisting of 21-helix structure, and that the number of solvent molecules formingpolymer-solvent compounds with SPS is dependent on gelationsolvent itself. In particular, Daniel et al. have clearly demonstratedthat the polymer-rich phase forming the cross-link domains ofthese SPS gels is a d crystalline clathrate phase [22], which was alsoascertained by van Hooy-Corstjens et al. [23] in the same year, andthat the correlation length corresponding to 010 reflection, i.e.,along the direction perpendicular to the ac layers of close-packedenantiomorphous s(2/1)2 helices, is strongly dependent on thechemical nature of the guest solvent molecules [24].

However, cooling of SPS solutions with bulky solvents such asoctadecyl benzoate [17.25] and 1-chlorotetradecane [26,27] resultsin the formation of paste-like opaque states [17] where SPS chainsare in the highly ordered all-trans TT skeletal conformation. Theseare mechanically weak [26,28] while gels with SPS chains of 21-helical structure are strongly elastic. The conformations of SPS arealso reported to depend on the annealing temperature in the case ofcis-decalin solution of SPS resulting that crystallization competeswith gelation [27]. In relation to recrystallization and gelation,some morphologies of SPS have been reported to be spheruliticstructure or lamellar one [26,29]. In any rate, both the paste-likestates and the morphology without three-dimensional networksof fibrils must be excluded from the category of SPS gels [26].

We have been interested in free space among the SPS chainsaggregated in a gel state and have demonstrated that there existsa distribution of free volume in an area where the SPS chainsassociate and into which molecules smaller than 1,5-

* Corresponding author. Tel.: þ81 54 2384627; fax: þ81 54 2373354.E-mail address: edhitag@ipc.shizuoka.ac.jp (H. Itagaki).

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dimethylnaphthalene are able to penetrate [30] by using fluores-cence technique that was successfully established through thestudy on isotactic polystyrene gels [31]. The size of the free volumebetween the SPS chains is consistent with that of the cavity size inthe d-empty crystalline form of SPS solids [30,32]. Thus, theexperiment to observe whether a solvent can form a gel of SPS ornot is a good way of searching for a guest molecule able to getclathrated in SPS d cocrystals.

In the present paper, we have tried to find which parameter ofa solvent is a key to form a gel of SPS and a cause to determinemorphology of SPS after cooling SPS solutions. Since the experi-ments should be systematic, we employed serial n-alkylbenzenes(NB: C6H5eCNH2Nþ1 (N ¼ 1e10)) in order to systematically changemolecular size and the solubility parameter as solvent: we havealready successfully used NB as fluorescent probes for monitoringfree volume in the amorphous regions of poly(methyl methacry-late) (PMMA) films [33]. The present work is a trial of determiningthe condition of a solvent molecule able to form an SPS gel.

2. Experimental

2.1. Materials

Syndiotactic Polystyrene (SPS) used for the measurements waskindly supplied from Idemitsu Kosan Co., Ltd. It is 98% syndiotacticwith an Mw of 152,000 and an Mw/Mn of 1.9. n-Alkylbenzenes (NB:C6H5eCNH2Nþ1 (N ¼ 1e10)) were purchased from Tokyo Kasei,Wako Chemicals, and Aldrich Co. and usedwithout purification. SPSwas dissolved with NB in a screw tube by heating the mixture athigh temperature after influx of dry nitrogen gas for 1 min. Forreference, the dissolving temperatures of SPS in NB (N ¼ 1e10) toprepare each SPS solution of 10 wt% are 120, 153, 179, 206, 210, 215,

224, 224, 224, and 229 �C, respectively. After SPS was perfectlydissolved, the solutions were well stirred by using a vortex mixer,heated again for 2 min, and finally cooled at room temperature formore than 2 days.

2.2. Measurements

Scanning electron microscopy, SEM, pictures were obtainedusing a JEOL JSM-6300 after drying SPS/NB on a glass plate undervacuum for more than 3 days. Dried SPS samples were stuck ona double-sided tape of carbon and coated with platinum or gold.Wide angle X-ray diffraction, WAXD, patterns were measured atroom temperature using a Rigaku MicroMax7HFM with a beam ofCuKa radiation (l ¼ 0.15418 nm) that was finely focused bya confocal mirror and a collimator. The diffraction patterns weredetected by a Rigaku R-axisIVþþ and recorded on an imaging plate.The data were analyzed by using FIT2D program. Diffractionintensities at constant 2q shown in a diffraction pattern were ob-tained by the integration of all the diffraction intensities around thebeam. Infrared absorption, IR, spectra were measured on a PerkineElmer SPECTRUM1000 FT-IR spectrophotometer with resolution of1 cm�1. Thermal behavior of SPS/NB with different concentrationswas monitored by DSC measurements with a PerkineElmer DSC7and/or a PerkineElmer DSC8000. Each sample was packed intolarge-volume capsule.

3. Results and discussion

3.1. Appearance of SPS/NB

SPS was found to be dissolved in NB with N ¼ 1e10 at hightemperatures such as 230 �C, described in detail in the

Fig. 1. Macroscopic views of SPS/NB after dissolving at high temperatures and cooling at room temperature. SPS concentrations are 2.5 wt % for 1B to 3B and 5.0 wt % for 4B to 9B.

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experimental section. Fig. 1 shows the pictures of solutions of SPS/NB after cooling. In the case of 1B, 2B and 3B, SPS solutions turnedout to be of gel form even at concentrations of SPS being 2.5 wt %.They were transparent and elastic. Fig. 1 demonstrates that theshapes of SPS/NB with N ¼ 1e3 did not change by tilting them formonths.

NB with N above 4 did not form a macroscopic gel shape at SPSconcentration of 2.5 wt %. In the case of 4B, 5B, and 6B, the solutionsof SPS with 5.0 wt % apparently looked to be of gel form, but theshapes finally changed by tilting them for a long time. Moreover,when SPS/NB with N ¼ 4e6 was thrust by a spatula, they did notshow any elasticity rather behaved like paste, namely being soft.When the solutions of SPS were cooled in the case of NB whose N islarger than 7, the solutions were of white sherbet form. Theychanged the shape by tilting them for a while, followed by theseparation of NB liquid.

Next, we examined morphology of SPS texture by observingSEM pictures of SPS/NB dried under vacuum (Fig. 2). The initialconcentrations of SPS shown in Fig. 2 were at most 5 wt% when SPSwas dissolved in NB, in order to make all the SPS units contact withsolvent molecules. It is quite interesting that the appearance of SEMpictures coincide with macroscopic characters of SPS/NB shown inFig. 1. Fibril structure of SPS chains was observed in the case of NBwith N¼ 1, 2, and 3: all of themwere of gel form. The morphologiesof SPS dried from NB with N ¼ 7e10 were of lamellar structure,while those of SPS from NB with N ¼ 4, 5, and 6 were of non-lamellar spherulitic structure. Below 20 wt % of SPS, only spheru-litic structures without any textures pleated like Fig. 2 were

observed for 4B, 5B, and 6B. However, when the concentrations ofSPS increased up to 30 wt %, some lamellar spherulitic structureswere found to be observed among non-lamellar spherulitic struc-tures. On the other hand, NB with N � 7 produced morphologieshaving pleats due to lamellar structure. They are not always ofspherulitic structure.

In summary, the appearance of SPS/NB was categorized intothree; (i) N¼ 1e3, a gel form consisting of fibril structure of SPS, (ii)N ¼ 4e6, paste form consisting of non-lamellar spherulitic struc-ture of SPS, and (iii) N ¼ 7e10, sherbet form consisting of lamellarstructure of SPS. We studied each structure of SPS chains by usingspectroscopic method.

3.2. Infrared absorption of SPS/NB

Conformation of SPS chain can be specified by the measure-ments of infrared absorption, since the detection of IR absorptionbands does not require a long range regular order of polymerchains. Thus, we monitored IR spectra of SPS/NB that was driedunder vacuum and dispersed in KBr. Kobayashi et al. first clarifiedthat two infrared absorption peaks at 1223 and 571 cm�1 of SPS gelscorrespond to TT and TTGG [9]. Even recently Torres et al. havemade use that two infrared absorption peaks at 1223 and 571 cm�1

of SPS solids correspond to the trans planar [34] and 21-helix [35]phases, respectively. We chose the concentration of SPS to be5 wt %, because it is good to study on the interaction between SPSand NB under the condition where solvent molecules are muchenough. Fig. 3 demonstrates that the absorption band characteristic

Fig. 2. SEM pictures of SPS/NB (N ¼ 2e10) after drying under vacuum: SPS concentrations are 2.5 wt % for 2B and 3B, and 5.0 wt % for 4B to 10B.

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for 21-helix conformation of SPS chains at 571 cm�1 was observedonly for SPS/NB with N ¼ 1e6.

On the other hand, Fig. 4 exhibits that the absorption bandcharacteristic for trans zigzag conformation of SPS chains atw1223 cm�1 was observed only for SPS/NB with N� 7, when initialconcentrations of SPS was 5 wt %. Thus, for SPS/NB with N � 7, theappearance of the band for trans zigzag conformation of SPS chainscorresponds to the disappearance of the band for 21-helix confor-mation. Needless to say, the change of main conformationsobserved by IR absorption for SPS/NB with N � 7 was found tocorrespond to the change of the morphology observed for SPS/NBwith N � 7 both in Figs. 1 and 2.

In summary, the main conformation of SPS chains in SPS/NB at5 wt % was categorized into two; (i) N ¼ 1e 6, 21-helical confor-mation, and (ii) N ¼ 7e10, trans zigzag conformation.

3.3. Formation of a polymer-solvent molecular compound in SPS/3B

We measured the thermal behavior of some SPS/NB’s at severalSPS concentrations by using DSC. Some DSC charts were found tohave some endothermal and exothermal peaks. In particular, theappearance of a peak turned out to depend on how high we raisedtemperatures of samples. Then, we would like to show in thepresent paper only the DSC data concerning the evidence ofa polymer-solvent molecular compound, though we plan to discussthe thermal behavior of SPS/NB in detail somewhere else.

The temperature-concentration phase diagram for SPS/2B wasalready obtained by Guenet [2] who used the data of Moyses [36].Especially, Tamman’s diagram for the endotherm peak near 100 �Cof SPS/2B clearly showed that this peak corresponded to themelting of a polymer-solvent molecular compound between SPSand 2B. Thus, the molecular compound formed between 21-helixform and 2B was found to produce fibril structure, followed by theformation of three-dimensional network of these fibril structures.

The temperature-concentration phase diagram for SPS/3Bresembles that of SPS/2B [2]. In particular, Tamman’s diagram forthe endotherm peak near 75 �C of SPS/3B demonstrates that thispeak corresponds to the melting of a polymer-solvent molecularcompound between SPS and 3B (Fig. 5): the enthalpy associatedwith the endotherm first increases linearly up to Cp ¼ 23% and then

Fig. 3. Infrared absorption spectra of SPS/NB (N ¼ 1e10) in the range between 400 and800 cm�1. The absorption near 571 cm�1 (demonstrated by broken line) correspondsto TTGG (21-helix) conformation.

Fig. 4. Infrared absorption spectra of SPS/NB (N ¼ 1e10) in the range between 800 and1600 cm�1. The absorption near 1223 cm�1 (demonstrated by gray line with an arrow)corresponds to TTTT (trans zigzag) conformation.

Fig. 5. Tamman’s plot for the endotherm peak near 75 �C of SPS/3B.

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decreases to become zero at about Cp ¼ 50%. The stoichiometricratio of the polymer-solvent molecular compound is given to beabout 2.9 molecules of 3B per styrene monomer unit of SPS. Thestoichiometric ratio of the molecular compound derived from theTamman’s plot may not reflect the correct stoichiometry of theorganized part. It may arise from the degree of solvation of theamorphous phase like some SPS/solvent compounds [37]. So it issupposed to be “thermodynamic” stoichiometry of the polymer-solvent molecular compound. However, it is clear that this “ther-modynamic” stoichiometry reflect the structure of the organizedcompound to some extent.

Fig. 6 shows some DSC charts of SPS/4B at some SPS concentra-tions. All of them are obtained for heating processes, but the chartswere found to be different from each other. We obtained Tamman’splots for each endotherm peak, but all of them turned out to showdifferent change from Fig. 5: namely, even if SPS concentrationincreases, enthalpy values of anyendothermal peakswere foundnotto increase but rather to be almost the same. These changesdemonstrate that the endotherm peaks observed for SPS/4B werenot for the melting of a polymer-solvent molecular complex. It isquite consistent because SPS/NBwithN> 3was not a gel form. Thus,it is concluded so far that (i) for SPS/NB with N � 3, the existence ofa polymer-solvent molecular compound can be ascertained by themeasurements of DSC, but (ii) for SPS/NB with N > 3, any polymer-solvent molecular compounds have not been determined, whichhas given a proof that SPS/NB with N > 3 does not form a gel.

3.4. Wide angle X-ray diffraction (WAXD) of SPS/NB

Fig. 7 shows the WAXD patterns of SPS/NB with N ¼ 1e10 inglass capillaries. Although solvent NB molecules blurred scatteringpeaks of SPS more or less, the X-ray diffraction patterns of SPS/NB

were categorized into three types that were consistent with threetypes of the morphologies: Table 1 summarizes diffraction peaks ofSPS/NB together with those of some reference values of SPS crys-talline forms and gels.

In particular, the diffraction angles seem to be nearly the same inthe case of SPS/NB with N ¼ 7e10: the diffraction peaks at2qz 6.2�, 10.6�, 12.4�, 13.7�, 18.6�, 20.1 and 34.9� for CuKa radiation(l ¼ 0.15418 nm), corresponding to 020, 110, 040, 130, 150, 111, and002, respectively [38]. The peaks are typical of SPS b crystallineform, which consists of trans zigzag conformations. Thus, when SPSmolecules were dissolved in NB with N ¼ 7e10 at high tempera-tures and cooled to room temperature, b crystalline phase of SPSturned out to be formed due to recrystallization process of SPS.

The diffraction patterns of SPS in NB with N ¼ 4e6 appear to bealmost identical with each other but different from those of SPS/NBwith N ¼ 7e10. Note that the deviations among the diffraction

Fig. 6. DSC thermogram for heating processes of SPS/4B: the traces were obtainedonce each sample in a pan was melted at high temperatures and cooled and held at5 �C. Heating rate was 10.0 �C/min.

Fig. 7. X-ray diffraction patterns of SPS/NB (N ¼ 1e10) for CuKa radiation(l ¼ 0.15418 nm): concentrations of SPS were all 10 wt %.

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angles of SPS in NB with N ¼ 4e6 are a bit larger than those of SPS/NB with N¼ 7e10. Now that the conformations of NB with N¼ 4e6are of 21-helix structure determined by the IR measurements, theirWAXD peaks were first compared with those of SPS g crystallineform: the main diffraction peaks of SPS g crystalline form fromreferences are for 200 (at 2q z 9.25� [38], 9.30� [39], 9.38� [40]),020 (10.50�) [38,39], 121 (16.25� [38], 16.30� [39], 16.10� [40]), 031(19.94�) [38], and 132 (28.48�) [38]. The WAXD patterns of SPS inNB with N ¼ 4e6, which contains solvent NB molecules more orless, have similar diffraction peaks corresponding to the peaksobserved for SPS g crystalline form as shown in Table 1.

Thus, it is concluded that the structure observed for SPS in NBwith N ¼ 4e6 is of SPS g crystalline form. The reasons are asfollows: (i) the WAXD patterns of SPS in NB with N ¼ 4e6 arebasically similar to those of SPS g crystalline form by taking intoaccount the reference values themselves being just a little differentfrom each other, (ii) the FT-IR data of these samples have alreadydemonstrated that SPS chains take 21-helical conformations, whichare the same of SPS chains in SPS g crystalline form, and (iii) theDSC measurements have indicated that SPS in NB with N ¼ 4e6does not produce any polymer-solvent molecular compounds,meaning that they are not of d crystalline form.

Next, we have compared the diffraction patterns of SPS in NBwith N ¼ 1e3 with each other. Their WAXD patterns were found tobe similar to each other and be quite different from those of SPS inNB with N � 4. It is no question that SPS in NB with N ¼ 1e3 is ingel form, meaning that they form a polymer-solvent molecularcompound between SPS chain taking 21-helix conformation andNB molecules. We have already proved this by the FT-IR and DSCmeasurements described above. Thus, the WAXD patterns of SPS inNB with N ¼ 1e3 should be similar to those of SPS d clathratecrystalline form. The diffraction peaks of SPS d clathrate crystallineform are known to be dependent on guest molecules [2e4].Nevertheless, the main diffraction peaks observed for SPS d clath-rate crystalline form are for 010, 210, 111, and 321/301/211according to some references. Our WAXD peaks of SPS in NB withN ¼ 1e3 for 010 reflections at 2q are 7.91� (1B), 7.79� (2B), and

7.53� (3B), meaning that the unit cell becomes wider according tothe larger molecular size of NB. The same changes were observedfor 210 and 111 reflections: the diffraction peaks at 2q are 10.36�

(1B), 10.24� (2B), and 10.23� (3B) for 210, and 17.57� (1B), 17.40�

(2B), and 17.00� (3B) for 111. In any rate, fibril structure of SPS in NBwith N ¼ 1e3 observed by SEM is concluded to be a similar crys-talline structure of SPS d clathrate cocrystalline form with an NBmolecule.

3.5. Factors to determine the morphology of SPS in NB

We have determined the structures of SPS in NB with N ¼ 1e10and found that the apparent morphologies corresponds to them.The results can be summarized as follows; (i) N ¼ 1e3, SPS in NBforms gel that consists of fibril structures networking three-dimensionally. The fibril structure is produced by forming a poly-mer-solvent molecular compound between SPS chains with a 21-helical conformation and NB molecules. (ii) N ¼ 4e6, SPS in NBappears to be of a gel but cannot be defined as a gel according toGuenet et al. [26]: the shapes finally changed by tilting them fora long time, and they did not demonstrate any elasticity. Theyrather behaved like paste. The morphology observed by SEM isa spherulitic structure without any pleats, namely non-lamellarstructure of SPS that was found to consist of SPS g crystallinephase having only SPS chains with 21-helix conformation withoutany NB molecules in the crystalline region. (iii) N ¼ 7e10, SPS in NBis not a gel but rather has texture like sherbet. The morphologiesobserved by SEM are lamellar structure that was found to consist ofSPS b crystalline phase having only SPS chains with trans zigzagconformation without any NB molecules in the crystalline region.

Whether SPS can form gel with a solvent is determined bywhether a polymer-solvent molecular compound between SPS andthe solvent can be formed or not. It means that whether a solventmolecule can stay in a free space among SPS chains with 21-helicalconformation is a key to the formation of SPS gel. We have shownthat theWAXD patterns of SPS/NB gel (N¼ 1e3) are quite similar tothose of SPS d clathrate cocrystalline phase with NB: these results

Table 1Diffraction angles of SPS/NB (N ¼ 1e10) together with the reference values and their assignments.a

1B 2B 3B d clathratec hklc 4B 5B 6B gd hkld 7B 8B 9B 10B bd hkld

6.19 6.19 6.19 6.19 6.15 0207.9 7.8 7.5 7.9 010

9.5 9.3 9.3 9.3 20010.4 10.2 10.2 10.2 210 10.2 10.5 10.6 10.5 020 10.6 10.5 10.6 10.5 110

11.8 120b

12.4 12.4 12.4 12.4 12.4 04013.7 13.6 13.7 13.7 13.6 130

14.1 13.9 111=101 14.0 22015.8 030 15.9 140b

17.6 17.4 17.0 17.2 111=121 16.7 16.4 16.5 16.3 12118.6 18.6 18.6 18.5 150

19.9 20.0 321=301=211 19.3 41019.5 19.7 19.8 19.9 031

20.2 200b

20.2 20.1 21.3 040 20.5 20.3 20.1 20.1 20.6 11121.7 231 21.4 04123.0 240 22.2 230b

23.6 23.2 23.4 411=421 23.8 041 23.9 17025.0 202

25.2 08028.2 28.1 28.5 132

34.9 35.0 34.9 34.7 35.5 00237.4 37.5 132

a The diffraction angles of the references are converted to those for CuKa radiation (l ¼ 0.15418 nm).b Reflections typical of the limit ordered b” modification.c Ref. [23].d Ref. [38].

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have supported again the decisive works of Daniel et al. [22] for 1,2-dichloroethane and 1-chloropropane and van Hooy-Corstjens et al.[23] for 1B and benzyl methacrylate: they have determined that thecross-link domains of the SPS gels is a d crystalline clathrate phase.Accordingly, we consider that the key point of SPS gel formation iswhether a solvent molecule can go into and remain in the freespace among SPS chains lining up together. The free space in theaggregated regions of SPS chains would be similar to a cavity of SPSd-empty crystalline form.

In the case of SPS in NB, it is quite obvious that there are threedifferent morphologies that can be defined as fibril, non-lamellar,and lamellar structures. It is important to find the condition ofa solvent determining these three morphologies. If we can definesome appropriate parameters expressing the properties of NB, wewill be able to estimate which morphology SPS in any solventsshows.

We consider that molecular size of solvent and its solubilityagainst SPS are quite important for the formation of an SPS gel orthe morphology of SPS/NB. Thus, we first express each property ofNB by (I) a molecular size and (II) a solubility parameter (SP), andsecond determine a range of these parameters decidinga morphology of SPS, just as a trial.

SP value of each NB can be calculated according to the equationof Small [41].

SP ¼ dSG=M

where SG is the sum for all the atoms grouping in the molecule, d isthe density and M is the molecular weight. SP value of SPS shouldchange with crystalline form, because an SP value based on Small’sequation is calculated by using a value of density. We consider thatthe interaction between SPS chains and solvent molecules insolution state at high temperatures plays an important role in thegel formation of SPS, meaning that the amorphous state of SPSappears to be themost adequate for assuming the SP value of SPS inour case. Thus, the SP value of 9.0 (cal/cm3)1/2 was calculated fromthe density of amorphous SPS solids (1.055 g/cm3, an experimentalvalue of amorphous SPS [7a]). On the other hand, the cavity size ofSPS d-empty crystalline form has been reported to be 120 to 160 Å3

[7c,8].Table 2 summarizes the molecular size and SP value of each NB

together with themorphology of SPS/NB and crystalline form of SPSthat is either identical with or similar to the WAXD pattern of SPS/NB at lower concentrations of SPS. It is needless to say that thesolubility parameters of NB are close to the SP values of SPS,because a benzene ring is compatible with side-chain phenyl

groups of SPS. Since SP values of NB do not change so muchbetween 8.9 (N ¼ 1, toluene) to 8.4 (N ¼ 10, n-decylbenzene), thepresent system using NB is quite suitable for examining the sizeeffect of a solvent molecule against the gelation of SPS.

Let us consider the difference of behavior and structure of SPSone by one. The molecular sizes of 3B and 4B are calculated to be134 and 151 Å3. Thus, we can assume the following: (i) once SPS isdissolved in 3B at higher temperatures, each SPS chain can contactwith 3B molecules well and the good solubility of 3B with SPS urgeSPS chains to take a 21-helix conformation, (ii) when 3B solution ofSPS was cooled to room temperature, SPS molecules aggregatetogether so much, (iii) 3B molecules can remain between the freespace among some SPS chains with a 21-helix conformation liningup together, because 3B molecules are small enough, and (iv)consequently 3B molecules can form a polymer-solvent molecularcompound with SPS, followed by forming fibril structurenetworking three-dimensionally.

On the other hand, the behavior of SPS must change because of4B being larger than the free volume formed among SPS chains thatgather together when 4B solution of SPS is cooling to roomtemperature. We assume as follows; (i) each SPS chain can contactwith 4B molecules well and the good solubility of 4B with SPS urgeSPS chains to take a 21-helix conformation, once SPS is dissolved in4B at higher temperatures, (ii) when 4B solution of SPS was cooledto room temperature, SPS molecules aggregate so much as well,however, (iii) 4B molecules cannot remain among the free spaceamong some SPS chains with a 21-helix conformation lining uptogether because they are larger than the free volume, and (iv)consequently SPS chains with a 21-helix conformation form g

crystalline structure due to the recrystallization process. It isneedless to say that there is no cavity inside the g crystalline phase.The same phenomenon takes place in the case of 5B and 6B due tothe same reason.

Now that 4B cannot remain in the free space among SPS chainsaggregated, the volume of 151 Å3 is assumed to show a sort oflimitation as free volume, though the shape may influence thevalue. It is consistent that this volume is close to the volume of thecavity of SPS d-empty crystalline form. Moreover, the differentphenomenon occurs in the case of NB with N > 6: for example, themolecular size of 7B is calculated to be 202 Å3. Thus, the followingappears to take place because 7B molecules are quite large: (i) eachSPS chain does not contact with 7B molecules well, so SPS chainstake a trans zigzag conformation, even when SPS is dissolved in 7Bat higher temperatures, (ii) when 7B solution of SPS is cooled toroom temperature, SPS molecules aggregate so much as well,however, (iii) since SPS and 7B molecules does not interact witheach other, SPS chains with a trans zigzag conformation formb crystalline structure due to the recrystallization process.

Thus, we conclude that the solvent molecule whose size issmaller than w150 Å3 and whose solubility with SPS is goodenough can form a gel of SPS. Even if the solubility of a solvent withSPS is good, the solvent molecule whose size is larger thanw150 Å3

[42] cannot form a gel. In the present work, NB molecules whosesizes are w150 to w200 Å3 enable to change conformation of SPSchain to 21-helix conformation and to produce g phases. Moreover,if the molecular size of a solvent is larger than 7B, i.e.,w200 Å3, SPSchains do not contact so much with the solvent and spontaneouslychange the conformation of trans zigzag form, which is the moststable. Finally SPS naturally fold the trans zigzag chains to formlamellar b crystalline structure.

In connection with the contact between SPS chains and solventmolecules, we tried to prepare SPS/4B with high concentration ofSPS. At low concentrations of SPS, the morphology of SPS/4Bobserved by SEM was only of non-lamellar spherulitic structurewhose WAXD and IR proved that SPS formed g crystalline phase.

Table 2SP value and molecular volume of NB together with the structural information ofSPS/NB: morphology by SEM, conformation structure by IR, and crystalline form byWAXD.

solvent SP value,(cal/cm3)1/2

Volume, Å3 Morphology Conformation Crystallineform

1B 8.9 100.6 Fibril 21-helix d

2B 8.8 117.5 Fibril 21-helix d

3B 8.7 134.4 Fibril 21-helix d

4B 8.6 151.2 Non-lamellar 21-helix g

5B 8.6 168.1 Non-lamellar 21-helix g

6B 8.6 185.0 Non-lamellar 21-helix g

7B 8.5 201.8 Lamellar Trans zigzag b

8B 8.5 218.7 Lamellar Trans zigzag b

9B 8.5 235.5 Lamellar Trans zigzag b

10B 8.4 252.4 Lamellar Trans zigzag b

SP values were calculated using Small’s equation, and volumes of NB were calcu-lated using winmostar. SP value of SPS is estimated to be 9.0 (cal/cm3)1/2: read thetext.

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However, when the concentration of SPS in 4B was 30 wt %,lamellar structure was observed together with non-lamellarspherulitic structure in the same SEM picture. The WAXD and IRmeasurements clearly demonstrated that there was the mixture ofSPS g crystalline phase consisting of 21-helix conformation and SPSb crystalline phase consisting of trans zigzag conformation. More-over, the SEM picture of SPS/4B with 49 wt % of SPS mainlyexhibited lamellar structure, which was proved to be of SPSb crystalline phase consisting of trans zigzag conformation bymeans of WAXD and IR. Now that the SPS concentration of 30 and49 wt % means that there are only 1.8 molecule and 0.81 moleculeof solvent 4B for one styrene unit of SPS, respectively, we cannotexpect that the contact of solvent with SPS chain is enough formaking an SPS chain to take TTGG conformation. Taking intoaccount all these data, the contact and interaction between a side-chain phenyl group of SPS and a solvent molecule are necessary foran SPS chain to take 21-helix conformation.

4. Conclusions

We employed NB (N ¼ 1e10) as solvents of SPS in order tomonitor whether SPS forms a gel or not. Since molecular size ofNB changes with N little by little, NB can be a good probe tostudy on the boundary condition for SPS to show somemorphologies or textures. Finally, we understand that theincrease in N systematically changes the morphology of SPS afterdissolving SPS in NB at high temperature. The results are asfollows: (I) N ¼ 1e3, SPS/NB forms a polymer-solvent molecularcompound between SPS with 21-helix conformation and NBmolecule, and this fibril structure networks three-dimensionally toform a gel, (II)N¼ 4e6, SPS/NB produces paste-like statewhere SPSchains with 21-helix conformation gather together to form SPS g

crystalline phase, and (III) N ¼ 7e10, SPS/NB produces sherbet-likestate where SPS chains with trans zigzag conformation gathertogether to form SPS b crystalline phase. The change of themorphology is due to either (i) the molecular size of NB comparedwith the free volume among SPS chains with 21-helix conformationthat aggregates together when SPS/NB solution is cooled, or (ii) thesolubility of SPS with NB giving birth to the formation of SPS chainwith 21-helix conformation, which appears to be caused by theassociation of solvent with side-chain phenyl groups. At present,we do not show the limitation value of volume size in detail, butshow faintly that there should be these values: 150 Å3 is near theboundary between fibril structure and non-lamellar spheruliticstructure of SPS, and 200 Å3 is near the boundary between theformation of 21-helix conformation and trans zigzag conformationof SPS chain.

Lastly we would like to comment the cause of the gelation andthe morphology of SPS. By comparing the behaviors of SPS/NB, weunderstand why SPS forms a gel without being recrystallized. Thegelation process of SPS is considered to take place through twostages [23,43], namely a coil-to-helix transition and intermolecularassociations of the helical chains. In the case of SPS/NB for N � 3,SPS chains took the similar structure of SPS/guest clathrate coc-rystals. Each straight chain of SPS having 21-helix conformation isassumed to aggregate together and to get bundled by having somesolvent molecules among them, resulting that one dimensionalfibril structures were formed without getting folded thanks to theexistence of solvent molecules among SPS chains. This concept isbased on a ladder-like model proposed for the gel structure ofisotactic polystyrene by Guenet [44e47], who assumed throughneutron diffraction experiments that polymer chains were bridgedtogether by the solvent molecules and the existence of solventmolecules between chains of isotactic polystyrene preventedchains from folding. Thus, unfolding of polymer chains due to the

solvent molecules staying among them is a key to form a gel. On theother hand, what is the key point to determine morphology oflamellar and non-lamellar? We estimate that the rotation of Gau-che isomer from trans isomer plays an important role in thedetermination of SPS morphology. In the case of solvents that arenot so compatible with SPS, SPS solids are not soluble even at hightemperatures though they may melt. In this case, SPS just take themost stable trans zigzag conformation independently and gathertogether during cooling, resulting the recrystallization of SPSb crystalline phase. Here, they should fold probably due to rotationto gauche conformers and/or some defects. Since the SP values ofNB (N¼ 1e10) are not so different from that of SPS, it is obvious thatSPS is uniformly dissolved in 7B to 10B. Nevertheless because 21-helix conformation was not produced in SPS chains, themolecular contact of a side-chain phenyl group with NB, espe-cially with phenyl part of NB, is assumed not to occur so much,indicating that bulky solvent molecules do not support SPS to takea 21-helix conformation. This is followed by the folding of SPS transzigzag chains together with the formation of lamellar b crystallinephase.

Acknowledgment

This work was supported by Grant-in-aid for Scientific Research(C) (21550206) from the Ministry of Education, Science, Sports andCulture of Japan. The research was partially carried out at theCenter for the Instrumental Analysis, Shizuoka University. Theauthors would like to thank Dr. Tsutomu Asano and Dr. TomohiroSago for their fruitful discussion.

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