Rigid polymer, poly(p-phenylene benzobisthiazole), formed lamellar crystals where the molecular chains were oriented perpendicular to the lamellae. It was supposed that, because of wide distribution in the chain length, the lamellar surface bristled with the chain cilia among which many voids were included. Crystallographically, this region
afforded us a transitional structure from full to deficient packings of chains. The structure was analyzed using the scanning probe microscope. In the course the method for imaging one molecular chain end was developed. From the images it was concluded that an isolated long cilius did not move so violently at room temperature

Inoue, Tomohiro

Shimamura, Kaoru

Uchida, Tetsuya

Okayama University Scientific Achievement Repository

Memoirs of the Faculty of Engineering, Okayama University, Vo1.35, Nos.1&2, pp.35.-40, March 2001Scanning Probe Microscopy ofPoly(p-phenylene benzobisthiazole}Lamellar CrystalKaoru Shimamura, Tetsuya Uchida and Tomohiro Inoue(Received December 21, 2000)Rigid polymer, poly(p-phenylene benzobisthiazole), formed lamellarcrystals where the molecular chains were oriented perpendicular to thelamellae. It was supposed that, because of wide distribution in thechain length, the lamellar surface bristled with the chain cilia amongwhich many voids were included. Crystallographically, this regionafforded us a transitional structure from full to deficient packings ofchains. The structure was analyzed using the scanning probemicroscope. In the course the method for imaging one molecular chainend was developed. From the images it was concluded that an isolatedlong cilius did not move so violently at room temperature.1. INTRODUCTIONFlexible crystalline polymers form lamellar crystals when crystallized from dilutesolution. Experimental studies have shown that in the crystals each molecular chain foldsback and forth several times with constant fold length which does not depend on molecularchain length [1,2]. Arrangement of the fold on the lamellar surface was examined by thescanning probe microscopy (SPM) [3,4,5].Poly (p-phenylene benzobisthiazole) (PBZT) is a rigid crystalline polymer and themolecular chains can not fold. The so-called PBZT single crystals have been preparedfrom dilute solution [6,7]. Their morphologies are strongly dependent on the molecularlength and its distribution. Shorter PBZT chains crystallize into a lozenge lamellar crystalwhere the rigid chains of different length are accommodated perpendicular to the lamella. Aquestion of what happens to the chain end portions in this case remains as an essentiallyunsolved problem. In the work reported here a modified SPM method will be presentedwhich enables the chain-end positions to be identified, suggesting a possibility ofcharacterization of chain end motions.Department of Applied Chemistry3536 Kaoru Shimamura, Tetsuya Uchida and Tomohiro Inoue2. EXPERIMENTAL2.1 MaterialsPBZT polymer was synthesized according to the method of Wolfe et al. [8]. Intrinsicviscosity of the product was 3.4 dllg from which average molecular length was estimated to be54 nm from Berry's equation [9]. The solvent used for crystallization was 92% sulfuric acid.PBZT was put into the solvent (at concentration of 0.1 %), heated slowly to 105°C, thencrystallized by slow cooling (cooling rate:O.1 °C/min). This procedure was carried out underdry nitrogen to ensure an inert atmosphere.2.2 Transmission electron microscopy (TEM)The precipitates were roughly separated from H2S04 by decantation and rinsed withdistilled water several times. PBZT crystal suspension in water dropped onto carbon gridsfor TEM observation allowing water to evaporate. TEM image and selected area electrondiffraction (SAED) were taken on JEM 200EX (JEOL, Tokyo, Japan) operated at 200kV andrecorded on FUJI FG film. SAED pattern was taken at a camera length of 30 cm from area of1-2 JJ. m in diameter. The camera constant was calibrated by measuring d-spacing of goldevaporated onto selected specimens.2.3 Scanning probe microscopy (SPM)SPM image was taken in air on Nanoscopema (Digital Instruments, Inc.). The probeand cantilever used were microfabricated silicon single crystals (Nano Probes, DigitalInstruments, Inc., cantilever length: 450 JJ. m, spring constant: 0.02-0.1 N/m, resonancefrequency: 6-20 kHz, nominal tip radius of curvature: 5-20 nm). In this instrument,deflection of the probe was measured by change in direction of a laser beam reflected from thecantilever. Two different ways were adopted for imaging. One was the ordinary tappingmode where the piezo (the sample holder) responds to root-mean square (RMS) of thecantilever deflection signals by moving the sample up and down to keep the cantilevervibration amplitude constant (servomechanism). Another way was that the scanning wascarried out without operating the servomechanism, Le., with keeping the sample holder atconstant height and constant input energy for cantilever vibration, while the deflection signals(RMS of cantilever deflection) were directly sent to the display for imaging. Heresignificance of this image exists only at area recorded by less energy transfer between thecantilever tip and the specimen. Several tens scannings with consecutive input energywere necessary for detecting many chain ends at different height for constructing a 3-dimensional image of the chain end distribution.Imaging was reliable only when the scanning was slow enough for the tip to catch up withprotrusions and dents. Actually by observation with the speed faster than 30Hz,resultant image smeared along the scanning direction. So we adopted 20Hz as thescanning speed.SP~I of PBZT Lamellar Crystal3. RESULTS and DISCUSSION3-1 Crystalline core in PBZT lamella observed by TEMTransmission electron microgragh of a lozenge PBZT lamellar crystal is shown in Fig.l.At the center of the lamellar crystal there is a protruded region less than 15 nm in diameter.In addition, a protruded line ea.8nm wide extended along the diagonal. It is plausible thatthe protrusions acted as nuclei during the crystallization. Thickness of the lamellar edge, asdetermined from the width of platinum-palladium shadowing, was 54 nm, which was slightlyless than the average chain length estimated from intrinsic viscosity.Analysis of SAED pattern revealed that the lamella was composed of twin crystallites andthat molecular chains oriented perpendicular to the lamella. This, coupled with theknowledge that the molecular chains were completely rigid and the chain length waspolydisperse, suggested that the lamellar surface bristled with rigid chain cilia of differentlength.SOOnmb* b*'" *, 'I'I ~J101 110~~1011f :I I, ,, II"1--+-t--1~f---._-+-.2o::00'-t---+':::OO. *a- aFig.1 TEM of lozenge PBZT lamellar crystal and its SAED withreciprocallattice.3-2 Structure of transitional region from the crystalline core to the lamellar surfaceThe lamellar surface was observed with SPM in the ordinary tapping mode. Averageheight variation was measured along a line (A-B) drawn on Fig.2. Bumps and dents on3738 Kaoru Shimamura. Telsuya L'chida and Tomohiro InoueFig.3 Schematic illustration of rigidchains deformed by the tip of thecantilever during observation by theordinary tapping mode]Ar::::3o 0.50 1.00 1.5011mFig.2 SPM height image of PBZTlamellar crystal surface taken inthe ordinary tapping mode. Thecross-sectional height profilealong the line A-B is shown belowthe image.the lamella on a scale of from several to a few tensnanometers across and from less than one to a few tensnanometers high were evident. It should be noted thatrows of bumps developed from the diagonal line at anangle of 60 degree, appearing like a fish bone. Itseems that the rows grew dendrically along 310direction then the crevices were filled up later.Here we have to examine the meaning involved inthis image. By the tapping mode, the cantileverdeflection signals expressed as RMS was fed back tothe piezo controller in order to compensate thedifference from the prescribed value. Voltage appliedto the piezo controller for the compensation wasmonitored as height information and sent to the display.The drawback of this process was that the piezocontroller responded slower with smaller theprescribed value (concomitantly RMS).Consequently we usually set a large value as theprescribed value in order to take a clear image.Thus RMS for a clear image was obtained only whenmore than tens cilia interacted simultaneously with thetip because the tip of the cantilever was much thickerand much heavier than a cilius. So the imagerepresented a layer of fairly populated cilia. Heremost chains were inevitably deformed by the tipduring detection (Fig.3). If we control thetapping force, the layers with various chain densitywill be imaged. Small stimulus to the cantilevertip such as that from one chain cilius was too small forthe piezo to respond promptly for imaging in thisordinary tapping mode.nm 500 nm3-3. Observation of sparse cilia on PBZT lamellaIn order to image one cilius with weaker deflection signals, we had better send theoriginal RMS directly on the display as a function of X and Y. Then the delay inresponse through the servomechanism can be eliminated. Fig.4 illustrates how thecilia can be detected as a function of the cantilever vibration amplitude. With a smallvibration amplitude (Fig.4a), the vibration is deflected only by a longer cilius B, butnot by another cilius A, thus only B can be recorded on the map. With increase inthe amplitude (Fig.4b), the cilius A also begins to interfare the vibration, i.e., both arerecorded on the map. Fig.Sa shows one of the maps. For comparison, the image ofthe same area taken in the ordinary tapping mode is also shown in Fig.Sb. It is clearSPM of PBZT Lamellar Crystal 39Fig.4 Schematic illustration of geometry between theamplitude of cantilever vibration and cilia on PBZT lamellaCVA is large.Both cilia A and Baredetected.CVA is small.Only cilius B is detected.(a) vibrating cantilevert~Icon...ntrN:~~rl(b)(!~'-""'I"""__con...ntb~{!H•Specimen height is fixed.•Detectability depends on cantilevervibration amplitude (CVA).Probe radius5 .... 10 nmDiameter of onecilius ca 0.6 nmCantilever deflection isdirectly detected andrecorded.that islands such as A, B etc.in Fig.5a are observed tolocate at positions of thecorresponding bumps.Broad area B came from alonger cilius (or it might comefrom several cilia). On theother hand, Point A is aposition where the tip slightlytouched a cilius. By changinginput energy for the cantilevervibration, several tensconsecutive images wereobtained. Reproducibilitywas ascertained by the factthat the features appeared atthe same positions with thesame sizes by decreasing andagain by increasing amplitudeof the cantilever vibration.Fig.5c is a bird's-eye viewof Fig.5a where A looks like abamboo shot. The formresulted from convolution ofshapes of the chain end andthe probe tip, so significant information is only limited to the top area of the bamboo shot.*Thus height difference among the molecular chains were determined resulting in the 3-dimensional chain end distribution. Another important information is capability ofdetection of chain end motion as an image. For example, the cilius A, which was atleast 15 nm long and standing solitarily, might rotate or vibrate thermodynamically atroom temperature. But as the point A was observed to be less than 3 nm in diameter, itwas concluded that this cilius was not moving so violently at 20°C. In addition, there isthe possibility of detecting what chemical groups exist at the chain ends, ,by chemicalmodification of the tip surface for increasing or decreasing in affinity between the tip and thechain ends.* Here it should be examined whether the bamboo shot really constructed of only one cilius ora few cilia. As PBZT was prepared by polycondensation, it has broad distribution inmolecular length. So two or more cilia seldom meet side by side at the same height.]40(a)Kaoru Shimamura, Tetsuya Uchida and Tomohiro Inoue(b)200 nmFig.S SPM height image of rigid cilia of different length on a PBZT lamellar crystalobserved (a) by direct RMS mode, (b) by the ordinary tapping mode. The contrast coversheight variations in the 0 - 2 nm in (a) and in the 0 - 30 nm range in (b).(c) and (d) are bird's-eye views of (a) and (b) respectively.4. CONCLUSIONSA rigid polymer crystal, which has no chain folding but rigid cilia on the surface,was studied by TEM and SPM. In the analysis a new SPM mode was developed forimaging an isolated chain cilius. By combining the ordinary tapping mode with thismode, less perfect structural region with deficiency of molecular chains near the crystalsurface was characterized.REFERENCES1) A.Keller; Phil.Magazine, 2 (1957), 1171. ,2) P.H.Geil "Polymer Single Crystals", John Wiley & Sons, Inc, New York (1963).3) R.Patil, D.H.Reneker; Polymer, 35 (1994), 1909.4) K.D.Jandt, M.Buhk, M.J.Miles and J.Petermann; Polymer, 35 (1994), 2458.5) T.Kajiyama, LOhki and A.Takahara; Macromolecules, 28 (1995), 4768.6) K.Shimamura, H.Kobayashi and T.Ikawa; Sen-I Gakkaishi, 49 (1993), 26.7) K.Shimamura and T.Uchida; J. Macromol. Sci. Phys. B39 (2000), 667.8) J.F.Wolfe, B.H.Loo, F.E.Arnold; Macromolecules, 14 (1981), 915.9) G.C.Berry, P.C.Metger, P.C.Metger, S.Venkatram and D.C.Cotts, Polym.Prepr., 20 (1979), 42.

Scanning Probe Microscopy of Poly(p-phenylene benzobisthiazole) Lamellar Crystal

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