Dual Types of Spherulites in Poly(octamethylene terephthalate

Sep 27, 2008 - display solely one type of spherulite or dual types of spherulites (double-ring-banded and ringless ones), depending on Tc or Tmax impo...
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Langmuir 2008, 24, 11880-11888

Dual Types of Spherulites in Poly(octamethylene terephthalate) Confined in Thin-Film Growth Yu-Fan Chen, E. M. Woo,* and Shu-Hsien Li Department of Chemical Engineering, National Cheng Kung UniVersity, Tainan 701-01, Taiwan ReceiVed December 4, 2007. ReVised Manuscript ReceiVed August 26, 2008 Spherulite morphology and growth kinetics of poly(octamethylene terephthalate) (POT), cast on single-side glass or confined between two slides in thin-film forms, were characterized using polarized versus nonpolarized optical microscopy, scanning electron microscopy (SEM), and wide-angle X-ray (WAXD) analysis. POT can simultaneously display solely one type of spherulite or dual types of spherulites (double-ring-banded and ringless ones), depending on Tc or Tmax imposed. Fractions of these two types depend on Tc when quenched from a fixed Tmax ) 160 °C. At lower Tc’s, POT exhibits higher crystallization rates leading to higher fractions of ringless spherulites; at higher Tc’s, POT exhibits lower crystallization rates leading to ring-banded spherulites. At intermediate to high Tc’s where the growth kinetics of POT could be monitored, the ring-band type dominates and the fraction of ringless spherulites is insignificantly small. Both ringless and ring-banded spherulites can be seen in regime III (Tc ) 70-110 °C), with fractions of ringless type of spherulites decreasing with temperature. Thus, growth kinetics for POT was mainly focused on the regime of ring-banded spherulites. In regime III, the ring-band pattern is more orderly concentric with smaller inter-ring spacing (1-2 µm) for lower Tc’s but intermediately larger spacing (3-5 µm) for higher Tc’s. The orderly lamellar orientation in the ring-bands in contrast with the inter-ring valley region is discussed. In regime II (115 °C and above), the ring-band pattern is first distorted to highly zigzag irregularity at higher Tc’s and then eventually disappears at extremely high Tc, with the lamellar crystals eventually turning dendritic with no rings. Apparently, the types of spherulites in polymers are more influenced by the growth rates as determined by Tc and slightly less by Tmax, but not by the substrate surface nucleation.

Introduction Ring-bands and mechanisms of their formation have been widely discussed in the literature.1-11 Interpretation is that rings may be attributed to cooperative twisting of the lamellae along the direction of radial growth; however, relevant mechanisms and relationships between lamellar twisting and rings are still controversial. Poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT) have been studied extensively on their thermal behavior, mechanical properties, and crystal structures.12-16 Recently, poly(trimethylene terephthalate) (PTT) has also attracted much attention due to its excellent mechanical properties.17 More recently, we have observed interesting ringed patterns in poly(trimethylene terephthalate) (PTT) melt-crystallized at temperatures ranging from 150 to 215 °C.18 Ring-bands in spherulites have been reported in PTT or its miscible blends, * Corresponding author. Phone: +886 6 275-7575ext. 62670. Fax: + 886 6 234-4496. E-mail: [email protected]. (1) Lotz, B.; Cheng, S. Z. D. Polymer 2005, 46, 577. (2) Yasuniwa, M.; Tsubakihara, S.; Iura, K.; Ono, Y.; Dan, Y.; Takahashi, K. Polymer 2006, 47, 7554. (3) Okabe, Y.; Kyu, T.; Saito, H.; Inoue, T. Macromolecules 1998, 31, 5823. (4) Jiang, Y.; Zhou, J. J.; Li, L. Langmuir 2003, 19, 7417. (5) Hobbs, J. K.; Binger, D. R.; Keller, A.; Barham, P. J. J. Polym. Sci., Part B: Polym. Phys. 2000, 38, 1575. (6) Hoffman, J. D.; Miller, R. L. Macromolecules 1988, 21, 3038. (7) Wang, Y.; Chan, C. M.; Li, L.; Ng, K. M. Langmuir 2006, 22, 7384. (8) Xu, J.; Guo, B. H.; Zhang, Z. M.; Zhou, J. J.; Jiang, Y.; Yan, S.; Li, L.; Wu, Q.; Chen, G. Q.; Schultz, J. M. Macromolecules 2004, 37, 4118. (9) Beekmans, L. G. M.; Hempenius, M. A.; Vancso, G. J. Eur. Polym. J. 2004, 40, 893. (10) Chuang, W. T.; Hong, P. D.; Chuah, H. H. Polymer 2004, 45, 2413. (11) Singfield, K. L.; Hobbs, J. K.; Keller, A. J. Cryst. Growth 1998, 183, 683. (12) Stein, R. S.; Misra, A. J. Polym. Sci., Polym. Phys. Ed. 1980, 18, 327. (13) Yeh, J. Y.; Runt, J. J. Polym. Sci., Part B: Polym. Phys. 1989, 27, 1543. (14) Ludwig, H. J.; Eyerer, P. Polym. Eng. Sci. 1988, 28, 143. (15) Jakeways, R.; Ward, I. M.; Wilding, M. A.; Hall, I. H.; Desborough, I. J.; Pass, M. G. J. Polym. Sci., Polym. Phys. Ed. 1975, 13, 799. (16) Yokouchi, M.; Sakakibara, Y.; Chatani, Y.; Tadokoro, H.; Tanaka, T.; Yado, K. Macromolecules 1976, 9, 266. (17) Chuah, H. Macromolecules 2001, 34, 6985.

where the stacked lamellae assume a flat-on and an edge-on orientation in the dark and bright regions, respectively, leading to the formation of a ringed morphology.18,19 On the other hand, ring-banded spherulites have rarely been reported in PET or faster-crystallizing PBT, although it has been pointed out that some exceptions are seen in PET, where solution-cast PET can exhibit irregularly ordered ring-bands. Melt-crystallized PET never displays ring-bands in spherulites. By contrast, ringed patterns are distinct in poly(trimethylene terephthalate) (PTT) melt-crystallized at temperatures ranging from 150 to 215 °C.19 For ringless spherulites, the phenomenon of regime transition can also be observed. Lorenzo and Righetti20 have also reported the crystallization regimes of poly(butylene terephthalate) (PBT), which presents a transition of regime III to II. When crystallized at lower temperatures in regime III, the dominant morphology in PBT is unusual spherulites, but “mixed-type” crystals, with eight-arm crosses due to superposition of extinction patterns at 0° and 45°. When crystallized at even higher temperatures, PBT displays a rough shape with less defined borders instead of regular spherulitic patterns. Transition between “mixed-type” spherulites and those without a defined extinction pattern in PBT occurs around 210-212 °C, in correspondence to the regime III-II transition. Aryl polyesters with longer methylene groups, such as poly(pentamethylene terephthalate) (PPT), poly(hexamethylene terephthalate) (PHT), poly(heptamethylene terephthalate) (PHepT), etc., have been less studied due to limited application interests. For PPT, it is known that two distinct crystal forms (R and β) can coexist in oriented forms; in particular, a structure transformation from the R form to β form for PPT has also been 80.

(18) Wu, P. L.; Woo, E. M. J. Polym. Sci., Part B: Polym. Phys. 2003, 41,

(19) Wu, P. L.; Woo, E. M. J. Polym. Sci., Part B: Polym. Phys. 2002, 40, 1571. (20) Lorenso, M. L.; Righetti, M. C. Polym. Eng. Sci. 2003, 43, 1889.

10.1021/la802192w CCC: $40.75  2008 American Chemical Society Published on Web 09/27/2008

Dual Types of Spherulites in POT

found.21,22 Both R and β forms in PPT are characterized by triclinic chain packing, differing only in the c-axis. Yet typically only the R-crystal cell is seen in PPT crystallized/annealed under zero or small tensions. The β-crystal is formed under either high tensions or strained at constant length.21,22 Similar crystal transformation can also be observed in poly(butylene terephthalate) (PBT).16,17 By comparison, PPT, with one more methylene unit than PBT, has been less studied. Little is known about PPT, except that an equilibrium Tm0 value of 149.4 °C of PPT has been reported,23 which is significantly much lower than that of PBT. PBT is not known to crystallize into ring-banded spherulites under most conditions; by contrast, PPT easily crystallizes into ring-banded spherulites.23 Similarly, PHT is not known to crystallize into ring-banded spherulites under most conditions;24 by contrast, PHepT easily crystallizes into ringbanded spherulites.25 By observing these facts, one then wonders whether or not there exist any structural or kinetic factors that may govern the patterns in the crystallized spherulites of aryl polyesters. To answer the intellectually probing questions, in this study we synthesized, examined, and clarified the crystal structures, growth kinetics, and spherulite morphology in poly(octamethylene terephthalate) (abbr.: POT), which has eight methylene units between two terephthalates. The spherulitic and lamellar morphologies in the scales of micrometer- and nanometer-dimensions, respectively, were characterized to probe mechanisms of ringbanded versus ringless spherulites in aryl-polyesters. Earlier, Geil26 has summarized comprehensive subjects on polymer crystalline morphologies, in which spherulite patterns and mechanisms are discussed in detail. However, the coexistence of both ring-banded and ringless spherulites in a same polymer has been rarely reported. By examining several spehrulite features such as the coexisting ring-banded and ringless spherulites in POT, variation of relative fractions of these two patterns with respect to temperature, and their possible correlation with growth regimes or kinetics, this study attempted to unveil further factors controlling polymer spherulite patterns confined in thin-film forms.

Experimental Section Materials and Synthesis. Poly(octamethylene terephthalate) (abbr.: POT), with eight methylene units between terephthalate groups, is not commercially available; thus, it was synthesized inhouse from a glycol monomer of (1,8-octanediol) and dimethyl terephthalate monomer using 0.1% butyl titanate as catalyst by twostep polymerization. The first step was a trans-esterification process, followed by polycondensation. The POT product was purified by reprecipitation and washing three times. The molecular weight and polydispersity of POT determined by GPC (Waters) using polystyrene as a standard were 21 800 g/mol and 1.58, respectively. POT in the amorphous state, by rapid quenching from the melt, was used to obtain Tg ) 3.2 °C and Tm ) 132 °C. Apparatus and Procedures. Thermal Transition and Crystallization Rate. Tg transitions of the blend samples were measured with a differential scanning calorimeter (DSC-7, Perkin-Elmer) equipped with an intracooler (to -60 °C) and data acquisition/ analysis. Additional subambient DSC runs (temperature as low as (21) Hall, I. H.; Pass, M. G.; Rammo, N. N. J. Polym. Sci., Polym. Phys. Ed. 1978, 16, 1409. (22) Hall, I. H.; Rammo, N. N. J. Polym. Sci., Polym. Phys. Ed. 1978, 16, 2189. (23) Wu, P. L.; Woo, E. M. J. Polym. Sci., Part B: Polym. Phys. 2004, 42, 1265. (24) Woo, E. M.; Wu, P. L.; Chiang, C. P.; Liu, H. L. Macromol. Rapid Commun. 2004, 25, 942. (25) Yen, K. C. M.S. Thesis, Dept of ChE. Nat’l Cheng Kung, University Tainan, Taiwan, 2007. (26) Geil, P. H. Polymer Single Crystals; Interscience: New York, 1963.

Langmuir, Vol. 24, No. 20, 2008 11881 -100 °C) were cooled with a liquid nitrogen tank and helium gas purge. Prior to DSC runs, the temperature and heat of transition of the instrument were calibrated with indium and zinc standards. During thermal annealing or scanning, a continuous nitrogen flow in the DSC sample cell was maintained to ensure minimal sample degradation. Morphology Characterization. Polarized-light microscopy POM was used to investigate the spherulitic morphology of isothermally crystallized samples. Thin films for POM were obtained by casting from a 2 wt % solution in CH2Cl2 on the top side of glass slides at 45 °C, with the top side uncovered. For comparison, samples were also prepared with POT confined between two glass slides for OM characterization. Preliminary comparisons showed that both samples (top side uncovered vs two slides confined) yielded the same results of OM spherulite morphology at the same Tmax and Tc. Note that Tmax here in this study is defined as the maximum temperature POT was heated to and held for a period of time to erase all prior thermal history before it was quenched to a Tc for isothermal crystallization. The solvent in the cast-film samples was first vaporized under a hood at a controlled temperature, followed by final solvent removal in a vacuum oven at 80 °C for 5 days. These samples were then melted on the hot stage at Tmax ) 160 °C for 5 min, then quickly quenched to a designated temperature (Tc ) 70-125 °C) for isothermal crystallization. Unless specified otherwise, Tmax was fixed at 160 °C, from which POT was melted and quenched to various Tc’s for crystallization. For investigating the effects of Tmax on the resulting spherulite morphology, other Tmax values ranging from a low 140 to a high 240 °C were also used. Values of Tmax are specified in texts wherever Tmax for samples is different from 160 °C. POT samples were also prepared for scanning electron microscopy characterization (SEM, Philips XL-40FEG) at higher magnifications and better topography for comparisons with the corresponding OM results. The samples were cast on glass slides (top surface open to air), affixed to an aluminum stand with silver glue, then coated with gold vapor deposition using vacuum sputtering prior to SEM characterization. SEM characterization required that the top surface be free; thus, POT film samples were deposited on a single glass slide with the top uncovered. Growth Rate Measurements. Radial growth of spherulites was recorded and rates were measured using a polarized-optical microscope (POM, Nikon Optiphot-2) equipped with a temperaturecontrolled hot stage and a Nikon charge-coupled-device CCD digital camera and automatic image processing software. Magnification and scales were automatically calibrated by the software. Spherulite growth kinetics was based on samples of thin films (5 µm or less) on glass substrates. A drop of a 2% solution of the polymer in chloroform was deposited and uniformly spread on a glass slide, and the solvent was allowed to fully evaporate in an atmosphere. The dried film of sample was heated to the molten/liquid state for a short time and rapidly cooled to the intended crystallization temperature ranging from 106 to 126 °C. The growth rates below 106 °C were found to be too fast to be accurately recorded. Both polarized and nonpolarized graphs of spherulites were recorded for comparison. Especially, nonpolarized light optical microscopy (OM) was used for better discerning inter-ring domains in the presence of crystalline regions of double rings. This is especially critical for estimating the inter-ring bandwidth in spherulites showing ring-bands, as the ring patterns in polarized light may be miscalculated in terms of actual width. WAXD Analysis. For crystal-cell phase analysis, Shimadzu/XRD6000 with a copper KR radiation was used. The scanning angle (2θ) covered a range between 5° and 35° at a rate of 2°/min. POT was dissolved as 4 wt % in solvent, deposited as a thin film on a polyimide film as substrate. Thermal histories were imposed on the POT film samples along with the PI film. The objective was to prepare POT samples for X-ray analysis in ways similar to those of the thin-film spherulites on glass slides for OM analysis. The WAXD data revealing the crystal cell types in the POT samples were then used to correlate with the spherulite patterns via OM characterization.

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Results and Discussion Isothermal Growth Rate of Spherulites. Spherulite growth rates (radial direction along the plane of substrate) of POT in thin films (ca. 3-5 µm) at a series of isothermal temperatures (Tc between 106 and 126 °C) were recorded. With only limited deviation/scattering, the growth rates roughly follow a linear dependence on time, within the time period of measurement. With increasing temperature, the growth rate decreases. The slopes of the straight lines yield rates of two-dimensional growth (µm/ min) of the spherulites at given Tc. Note that the growth rates of POT at Tc below 105 °C were too fast to be accurately monitored by the imaging software in OM; thus, growth kinetics was not available for lower Tc. For Tc above 126 °C, the growth rates became too slow for practical measurements. To analyze the regime behavior of spherulite growth, treatments according to the Lauritzen-Hoffman theory were attempted: 27,28

[

] [

-Kg -U* exp G ) Go exp R(Tc - T∞) Tc(∆T)f

]

(1)

where G is the radial growth rate, Go is an overall constant that depends on molecular weight, and U* is the activation energy. T∞ ) Tg - 30 K is the temperature at which diffusion stops, which denotes the degree of under-cooling, and f ) 2Tc/Tom + Tc. The f factor is a correction coefficient for the temperature dependence of enthalpy of fusion. Kg is the nucleation constant, which can be obtained from Kg ) neboσσeTom/∆Hfoκ. The procedures are classically standard and thus not discussed in detail here. Analysis of equilibrium melting point for POT was performed also using the standard procedures.29,30 The equilibrium melting temperature, Tm°, for POT was found to be 134.34 °C using the classical Hoffman-Weeks method, and this value was used for subsequent calculation of regime behavior. Equation 1 may be rewritten in a logarithmic form for convenience of plotting:

ln G +

Kg U* ) ln Go R(Tc - T∞) Tc∆Tf

(2)

Kg can be estimated from the slopes of suitable plots. With the values of Kg determined, other parameters can be calculated. Figure 1 shows the corresponding Lauritzen-Hoffman plot according to eq 2, where data were obtained at temperature (Tc) of isothermal crystal growth varying from 106 to 126 °C at 2-3 °C step. In this plot, apparently, growth patterns following regime III and regime II of different slopes are present. The intersection of two straight lines corresponds to a discontinuity, which is near Ti ) 117.9 °C. This point agrees reasonably well with the intersection point shown in the previous figure. Kg could be obtained from slopes of plots in this figure for regimes III and II, respectively. Physical constants for POT were needed for such plotting. ∆Hf° is the enthalpy of fusion per unit volume of a perfect and infinitely large crystal lamellae at its Tm. For U* ) 1500 cal/mol, Kg(III)/Kg(II) ) 2.16 is resulted, which turns out to be the closest to the theoretical value of 2.0 according to the predictive requirement of Kg(III) ) 2Kg(II). Temperature at the discontinuity (i.e., transition from regime II to III) is 118.4 °C (TIIfIII). (27) Hoffman, J. D.; Davis, G. T.; Lauritzen, J. I. In Treatise on Solid State Chemistry; Hannay, N. B., Ed.; Plenum: New York, 1976; Vol. 3, Chapter 7. (28) Hoffman, J. D.; Weeks, J. J. J. Res. Natl. Bur. Stand. (U.S.), Sect. A 1962, 66, 113. (29) Xu, J.; Srinivas, S.; Marand, H.; Agarwal, P. Macromolecules 1998, 31, 8230. (30) Marand, H.; Xu, J.; Srinivas, S. Macromolecules 1998, 31, 8219.

Figure 1. Lauritzen-Hoffman plot of POT with U* ) 1500 cal/mol. Insets: ring-band patterns in spherulites melt-crystallized at temperatures from different regimes.

Correlation between Growth Regime and Spherulite Patterns. There is a systematic change in spherulite patterns as the temperature changes in ascending schemes, in which the growth transits from regime III to II. Figure 1 along with inset OM graphs shows the various spherulite patterns corresponding to regimes III and II, respectively. Clear and regular ring-bands with good order are apparent in regime III, from 106 to 117 °C, and the ring bandwidth increases correspondingly. As a matter of fact, dual types of spherulites, ring-banded and ringless, are present in POT films when crystallized. Regime III can be extended further below 106 °C to as low as 70 °C, where dual types of spherulites (ring-banded and ringless ones) coexist with varying fractions of these two types. This interesting phenomenon of dual types of spherulites in POT crystallized at low Tc will be discussed in greater detail later. Within Tc ) 106-117 °C, the majority of spherulites are ring-banded. In regime III, the majority of spherulites are ring-banded; however, the other type, ringless spherulite, being in insignificantly small fraction, was not monitored for kinetic growth. With Tc increasing to enter regime II (120-126 °C), the ring patterns are still recognizable, but the ring-bands become more distorted, rugged or zigzag, and the regularity of the rings also decreases with further temperature increase. The inter-ring space increases significantly and regularity decreases dramatically with increase in Tc within regime II. The intersection point between regime III and regime II is about 118.4 °C. Apparently, in both regimes III and II, POT spherulites are characterized with extinction ring-bands of various interring width (or spacing), but the regularity of the ring-bands transforms from a regularly concentric and closely spaced pattern in regime III to a zigzag pattern with larger inter-ring space in regime II. This type of pattern transition in POT is opposite to that seen in poly(trimethylene terephthalate) (PTT) reported earlier,31 where the growth in regime III correlates with ring-bands, while growth in regime II correlates with ringless Maltese-cross spherulites. That is, coincidence between the temperature of transition from ringless to ring-banded pattern and the temperature of growth regime transition (from II to III) has been reported in poly(trimethylene terephthalate) (PTT). The pattern transition in POT is also different from that reported for poly(ε-caprolactone) (PCL) or PCL blends.32 For semicrystalline polyesters, transition of (31) Hong, P. D.; Chung, W. T.; Hsu, C. F. Polymer 2002, 43, 3335. (32) Chen, Y. F.; Woo, E. M. Colloid Polym. Sci., 2008, 286, 917.

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Figure 2. POM photos for melt-crystallized POT at various Tc’s: (a) 85 °C, (b) 90 °C, (c) 95 °C, (d) 100 °C, (e) 105 °C, (f) 110 °C, (g) 115 °C, (h) 120 °C, and (i) 125 °C. Scale bar ) 20 µm.

Figure 3. OM photos (nonpolarized) for melt-crystallized POT at various Tc’s: (a) 85 °C, (b) 90 °C, (c) 95 °C, (d) 100 °C, (e) 105 °C, (f) 110 °C, (g) 115 °C, (h) 120 °C, and (i) 125 °C. Scale bar ) 20 µm.

ring patterns may not be simply just from ring-banded to ringless ones, or vise versa. For POT, both regimes produce extinctionring spherulites; however, the ring patterns, inter-ring space, and regularity of the ring-bands all exhibit a discontinuity-type shift. Apparently, not all polymers exhibit the same spherulite pattern transition with respect to temperature. WAXD diffraction analysis was performed for POT (in thin films with thickness similar to the POM samples on glass slides) crystallized at 80, 108, and 120 °C. POT samples were prepared in thin films deposited on a PI film substrate that is transparent to X-ray, and the POT samples on the PI substrate for X-ray characterization were subjected to thermal schemes identical to those for OM characterization. The X-ray result shows that the unit cell remains the same regardless of Tc; second, the relative intensity of each of the diffraction peaks remains unchanged. For brevity, a figure of the X-ray diffractograms is not shown here. Changes in Ring Pattern in Different Regimes. Figure 2 shows POM photos (under polarized light) for POT samples melt-crystallized at various higher temperatures: (a) 85 °C, (b) 90 °C, (c) 95 °C, (d) 100 °C, (e) 105 °C, (f) 110 °C, (g) 115 °C, (h) 120 °C, and (i) 125 °C. Interestingly, in POT spherulites crystallized at 85 °C or above, all ring patterns consist of “double rings”, with alternating bright-dark bands [indicated with arrows in (b) and (e)], as shown in micrographs (a) for Tc ) 85 °C to (g) for Tc ) 115 °C. This double-band pattern corresponds to regime III spherulites. Between the bright-dark double bands, there is a narrow “extinction line” separating these two bands. Later, SEM evidence will be discussed showing that the double bright-dark bands are hill-and-valley bands of different height but roughly equal width. In addition to the different height, the crystal texture and/or orientation for these two bands may be different too. At higher Tc (>115 °C), the spherulite pattern is dramatically different, signaling transition into a different regime (regime II). Micrographs (h) for Tc ) 120 °C to (i) for Tc ) 125 °C, in regime II, on the other hand, show no double rings but only widened ring-bands. Ring patterns in regime II differ significantly from those in regime III. In regime II, the inter-ring

spacing and/or ring bandwidth increases significantly, and the pattern of the rings becomes less ordered. There are no double bright-dark bands in regime II; instead, there are only wide bright bands, separated by a narrow dark “extinction line” between the bands. Double ring-bands with two different colors under POM may be difficult for correctly estimating the inter-ring space. Therefore, nonpolarized micrographs were also taken for comparison. Figure 3 shows OM photos (nonpolarized light) for POT melt-crystallized at various temperatures: (a) 85 °C, (b) 90 °C, (c) 95 °C, (d) 100 °C, (e) 105 °C, (f) 110 °C, (g) 115 °C, (h) 120 °C, and (i) 125 °C. Ring patterns are clearly seen in POT when crystallized at Tc ) 85-125 °C. More regular rings are seen in POT crystallized at lower Tc’s (85 °C). For POT crystallized at Tc ) 115 °C or above, the spherulites in POT start to exhibit distorted zigzag rings (graph f), with increasing irregularity at higher Tc’s. Furthermore, the inter-bandwidth increases sharply for POT crystallized at Tc equal to or greater than 115 °C. For POT crystallized at Tc ) 120 °C or higher (graph h), rings are so irregular that measurement of inter-ring spacing was not possible. For POT crystallized at Tc > 90 °C (graphs b-e), the bands are of a “double-ring” pattern, which are composed of alternating bright-dark bands (roughly concentric or spiraling). Optical images of double-bands in spherulites require careful analysis. Figure 4 shows nonpolarized OM photos for the same POT sample crystallized at 115 °C by focusing on different height positions: (a) up, (b) middle, and (c) down. By focusing on different depth, the width of band stripes can vary. The dark band (valley) width is narrower when focus is near the top, and the width becomes slightly larger when focus is brought lower. Its width becomes larger when focus is placed closer to the valley plane. Note that the polarized light induces dual color bands within the double-band rings. That is, the alternating bright and dark rings are actually one single crystal ring when viewed under nonpolarized OM or SEM. Further evidence will be provided by SEM characterization on the POT samples showing double ringbands under polarized light microscopy. Ring spacing reported

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Figure 4. OM photos (nonpolarized) for POT crystallized at Tc ) 115 °C by focusing on different height positions: (a) top, (b) middle, and (c) bottom.

Figure 5. Dual types of spherulites (ring-banded and ringless (circled)) in POT melt-crystallized at low Tc ) 70 °C.

in this work is based on measurement of graphs taken under nonpolarized OM. Later, measurements based on SEM graphs will be discussed too, which would show that agreement in the inter-ring width can be reached between the nonpolarized OM and SEM results. Earlier, it was mentioned that regime III of POT growth can be further down to 70 °C. An interesting phenomenon of dual types of spherulites was found in POT only when crystallized at low Tc. Figure 5 shows dual types of spherulites, that is, ringbanded and ringless types, in POT melt-crystallized at 70 °C. Interestingly, two types of spherulites are clearly seen in POT film-samples crystallized at the low temperature of 70 °C, where the crystallization rate was considerably higher (in comparison to those at 110 °C or higher). Fractions of these two types vary with the Tc imposed on POT. Within the same growth regime III (70-110 °C), a greater fraction of ringless spherulites is present at the low-temperature bound (Tc ≈ 70 °C); oppositely, a greater fraction of ring-banded spherulites exists at the hightemperature bound (Tc ≈ 115 °C) of the regime. OM evidence shows that when crystallized at even lower temperatures (70-30 °C), the ringless spherulites are the increasingly more dominant type present in the POT films, with only very few ring-banded spherulites occluded among mostly ringless ones. The coexistence of dual types of ringless and ringed spherulites at one Tc may be tentatively explained. Figure 6a-f shows POM photos for POT spherulites of different origins: (a) solvent-induced, and melt-crystallized with different Tmax’s of (b) 140 °C, (c) 150 °C, (d) 160 °C, (e) 220 °C, and (f) 240 °C. The two-type spherulite in POT melt-crystallized at a fixed Tc (Tc ) 75 °C) is caused by incomplete melting of the nuclei formed in the sample preparation step, which will grow as ringless spherulites. Figure 6a shows that a POT sample prepared by solvent-casting on a glass slide and crystallized at room temperature displays a sole type of ringless (Maltese-cross) spherulite morphology. Upon heating the initially solvent-cast POT to melt at lower maximum

temperatures (Tmax) (shown in Figure 6b-e), the crystal nuclei (ringless ones) might not be completely erased; thus, both ringless and ring-banded spherulites coexist upon melt-crystallization. However, Figure 6f shows that with increase in Tmax to a high value, only the ring-banded pattern is present upon melt crystallization by quenching from Tmax ) 240 °C. This is perhaps due to completing melting of the crystal nuclei for ringless spherulites; thus, when recrystallized at Tc ) 75 °C from melt held at Tmax of 240 °C, only ring-banded spherulites are formed. In general, fractions of these two spherulite types (ringless vs ring-banded) in POT depend on Tc when quenched from a fixed Tmax ) 160 °C. Alternatively, variation in Tmax at a fixed Tc could lead to effects on POT spherulites similar to those reported earlier for a fixed Tmax with variation in Tc. To summarize, Figure 7 shows a simplified scheme illustrating the changing fractions of two spherulite patterns in POT at a fixed Tc ) 75 °C for Tmax at (A) 160 °C, (B) 220 °C, and (C) 240 °C. POT can simultaneously display solely one type of spherulite or dual types of spherulites (double-ring-banded and ringless ones), depending on Tc or Tmax imposed. General dependence of the spherulite patterns on Tc and Tmax is apparent in the temperature range as discussed above. However, there is a narrow temperature window within which the effects of Tc or Tmax on the POT spherulite patterns do not appear to be of significance. Figure 8 shows POM photos for POT displaying a sole type of ring-banded spherulites at Tc ) 100 °C quenched from different Tmax’s: (a) 160 °C and (b) 220 °C. Interestingly and apparently, when crystallized at Tc )100 °C, variation in Tmax (from 160 to 220 °C) does not seem to influence the types of spherulites (ringless vs ring-banded). Apparently, all spherulites are of the same concentric ring-band pattern with similar interring spacing between bands. Depending on Tmax, there are some portions of the thin films exhibiting variation on the ring-band size as well as varying in the inter-ring spacing and spherulite sizes, but all spherulites are of the same ring-bands when crystallized at Tc ) 100 °C. Thus, discussions on spherulite crystal patterns in polymers will not be complete without taking into account both effects by Tmax and Tc. Thus, by summarizing the above observations, the trend of variation of POT spherulites is clear. At lower temperatures, POT exhibits higher crystallization rates that lead to ringless spherulites; at higher temperatures, POT exhibits lower crystallization rates that lead to ring-banded spherulites. Eventually, at even higher temperatures of regime I (130 °C and above), ring pattern is no longer seen and spherulites are highly dendritic. Both ringless and ring-banded spherulites can be seen in regime III (70-118 °C), where the relative fractions of two types of spherulites depend on temperature, with the former type decreasing with increasing temperature. In regime II (Tc > 118

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Figure 6. POM photos for POT spherulites of different origins: (a) solvent-induced, and melt-crystallized with different Tmax’s of (b) 140 °C, (c) 150 °C, (d) 160 °C, (e) 220 °C, and (f) 240 °C.

Figure 7. Schematic diagrams showing changing fractions of two spherulite patterns in POT at Tc ) 75 °C for various Tmax’s: (A) 160 °C, (B) 220 °C, (C) 240 °C.

°C and above), the ring-band pattern is mostly distorted to zigzag irregularity, with significantly widened inter-ring spacing. It may yield a useful clue to summarize the variation trend of inter-ring spacing in POT with respect to temperature of crystallization. Figure 9 shows bandwidth plotted as a function of Tc between 70 and 120 °C. Inter-ring spacing was estimated using two microscopic techniques (OM and SEM). With increasing temperature, the average bandwidth first stays almost stagnant with only a slight increase between 70 and 85 °C, with a dip cusp at ∼90 °C, but then increases rapidly at Tc higher than 110 to 120 °C. Note that the slight dip from the increasing trend at Tc ) ∼90 °C may be attributed to a sudden increase in the fraction of double-ring spherulite, which exerts a more compact spacing in the single-band spherulite. Experimentally under polarized light, POT crystallized at Tc ) 85 °C or lower exhibited no alternating bright-dark double-rings, while POT samples crystallized at 90 °C (or above) displayed spherulites with distinct alternating bright-dark double-rings. Inter-ring spacings in spherulites grown in different regimes were also measured using the SEM technique. The SEM data are plotted in the same figure, which shows quite good agreement between two sets of data. Transition from Double-Bands to Dendritic Bands at Higher Tc. Figure 9 also shows that a distinct discontinuity in ring width is located at Tc ) 115 °C, where the inter-ring spacing starts to increase almost exponentially with temperature increase beyond 115 °C. As discussed earlier, at this temperature of Tc or above (Tc > 115 °C), the spherulites in POT start to exhibit highly irregular ring patterns; conversely, below this Tc, the spherulites are characterized with regularly concentric rings. Note that this temperature of 115 °C signals a transition from regular rings to irregular ones, which is accompanied by a rapid increase in inter-ring spacing. The temperature of spherulite transition (ca. at ∼115 °C) roughly coincides with the growth regime transition from II to III at T ) 115 °C as discussed in earlier figures. The result in this work on POT further points out that growth regime transition can signal several different transitions in the spherulite patterns, such as inter-ring spacing and ring regularity. Apparently, the growth kinetic regime transition can be accompanied by a variety of pattern transitions in the

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Figure 8. POM micrographs of POT showing a sole type of ring-banded spherulites of the same pattern at Tc ) 100 °C quenched from different Tmax’s: (a) 160 and (b) 220 °C.

Figure 9. Bandwidth (inter-ring distance) in POT spherulites measured using two microscopy techniques: (+) OM and (0) SEM, crystallized at various Tc’s.

spherulites, and not necessarily just a simple transition from ring-banded to ringless patterns (or vise versa). As pointed out earlier, OM evidence shows that at lower temperatures (70-30 °C), the ringless spherulites become the increasingly more dominant type present in the POT films. On the other hand, at higher temperatures of Tc, spherulite species are gradually more of ring-band type. SEM characterization was performed to yield further proofs. Ring types in POT samples crystallized at Tc ) 30-100 °C were compared. Figure 10 shows SEM graphs (5000×) for POT spherulites crystallized at a wide range of low to intermediate Tc ) 80-100 °C. At temperatures of intermediate Tc range of (a) 80 °C, (b) 90 °C, and (c) 100 °C, most spherulites in POT are ring-banded, and the SEM micrographs show that the rings are bigger (d ) ∼20 µm) and assume a much greater depth profile to easy recognition of ringbands. Apparently, a range of medium Tc provideds a more conducive driving force for forming ring-band spherulites in POT. As POT was crystallized at extremely high Tc (>120 °C), the spherulite pattern exhibited further transitions. Evidence showed that the ring-band pattern disappeared and was completely replaced with large straight dendrites in POT crystallzied at Tc equal to or higher than 125 °C. Figure 11 shows SEM graphs for POT spherulites formed at higher Tc: (a) 110 °C, (b) 120 °C, (c) 125 °C, and (d) 128 °C (3000× or 1000×). The crystallization growth rates are increasingly retarded at higher Tc, and the rates at Tc > 120 °C are extremely slow; thus, crystallization for Tc greater than 120 °C was held for at least 12 h. The figure shows

that the spherulites formed at Tc ) 110 and 120 °C are still of a distinct ring-band type. However, by drastic contrast, the spherulites crystallized at either (c) Tc ) 125 °C or (d) Tc ) 128 °C are completely ringless, and the spherulites are filled with straight and thickened lamella radiating out from the center. To view the POT spherulites with double rings or zigzag rings in larger magnifications, SEM characterization was performed on several selected POT samples crystallized at Tc ) 90, 105, and 115 °C, respectively. Note that the phenomenon of doublering-band in OM graphs is an optical interpretation of two concentric stripes (different crystalline constituents or different orientations) of roughly equal width. SEM graphs can reveal better height contrast and lamellar orientation details. Figure 12 shows SEM graphs (3000-5000×) of POT melt-crystallized at (A) Tc ) 90 °C (bandwidth ) 1.19 µm), (B) Tc ) 105 °C (bandwidth ) 2.83 µm), and (C) Tc ) 115 °C (bandwidth ) 10.27 µm). The inter-ring bandwidth can be estimated from these SEM graphs to be: d ) 1.19, 2.83, and 10.27 µm, respectively, for POT crystallized at 90, 105, and 115 °C. These values of inter-ring spacing agree quite well with those shown earlier, where data of inter-ring spacing have been calculated from the nonpolarized OM graphs. Furthermore, finer morphological details about the crystalline lamellar orientation within the ringbands can be discerned in these SEM graphs. Graph A shows POT crystallized at a temperature of regime III (90 °C), where alternating bands of a flat valley and crystalline aggregates are clearly visible. The crystalline aggregates protrude out and are apparently aligned as concentric (or occasionally spiral) ringbands that are surrounded by flat valley regions. The lamellae are single-rod like, with no visible branches. Within the crystalline bands, the lamellae are roughly aligned in the radial direction. The lamellar length in one single ring-band is about 1 µm. Note that the flat valley and the crystalline bands are in general of about the same thickness; thus, measure of either the crystalline ring-band thickness or the inter-ring spacing is roughly the same. Graph B shows POT crystallized at an intermediate temperature of regime III (105 °C), with increased inter-ring spacing. Again, rings are concentric and ordered, but the lamellar stacks in bands are better defined and larger/longer. The lamellae are single-rod like, with no branches, and are all aligned along the radial directions. The lamellar length in one single ring-band in the 105 °C-crystallized POT is about 2-3 µm. Finally, graph C shows POT crystallized at a higher temperature, the intersecting point of regime III to regime II (Tc ) 115 °C), which displays a significantly larger inter-ring spacing, and the pattern becomes more rugged and distorted. Again, alternating bands of flat valley and crystalline aggregates are clearly visible. The lamellar stacks in the ring-bands of the spherulites are highly dendritic (branches

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Figure 10. SEM graphs for POT crystals of dual types (ring-banded and ringless) spherulites at Tc’s of (a) 80 °C, (b) 90 °C, and all ring-banded at (c) 100 °C (3000× and 5000×).

Figure 11. SEM graphs for POT spherulites at high Tc’s: (a) 110 °C, (b) 120 °C, (c) 125 °C, and (d) 128 °C (3000× or 1000×, respectively).

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in one band is roughly 6-10 µm. Graph C shows clearly that the interband region appears featureless with little sign of any crystal species. The valley region may be due to depletion of polymer species, rather than a result of 90° reorientation of lamellae. The valley region is also too wide for the lamellae to remain evenly “flat-on”.

Conclusion The result for POT clarifies that growth regime transition can signal several different transitions in the spherulite patterns, such as inter-ring spacing, ring regularity, or a transition from ringed to ringless types. Apparently, growth kinetic regime transition can be accompanied by a variety of transitions in the spherulites, not necessarily just a simple transition from ring-banded to ringless pattern, or vise versa. POT can simultaneously display solely one type of spherulite or dual types of spherulites (double-ring-banded and ringless ones), depending on Tc or Tmax imposed. In general, fractions of these two types depend on Tc when quenched from a fixed Tmax ) 160 °C. Similarly, at a fixed Tc, fractions of these two types of spherulites are also influenced by Tmax (140-240 °C). At lower Tc’s, POT exhibits higher crystallization rates leading to higher fractions of ringless spherulites; at higher Tc’s, POT exhibits lower crystallization rates leading to ring-banded spherulites. A concise summary of observations of effects of Tc, Tmax, or substrate surface nucleation on the spherulite patterns is stated here. Large and regular ring-bands with diameter of 20-30 µm are seen in POT crystallized at most intermediate Tc values (90-120 °C). Ultimately, at extremely high Tc (125 °C or higher), POT displays only large rough-patterned dendrites with no ring-bands. Alternatively, when compared at a fixed Tc ) 75 °C where crystallization rates are high, variation of Tmax produces a similar effect in that low Tmax (140-160 °C) leads to mixed dual types of spherulites (ringless and ring-banded), while high Tmax (220-240 °C) leads to a single sole type of spherulites (ringbanded). However, interestingly, there exists a narrow window of Tc at which Tmax has no apparent effect on determining the types of spherulites; that is, at Tc ) 100 °C where crystallization rates are low, variation of Tmax does not influence the types of spherulites. Apparently, the types of spherulites in polymers are more influenced by the growth rates as determined by Tc and slightly less by Tmax, but not so much by the substrate surface nucleation.

Figure 12. SEM graphs (3000-5000×) of POT melt-crystallized at (A) Tc ) 90 °C (bandwidth ) 1.19 µm), (B) Tc ) 105 °C (bandwidth ) 2.83 µm), and (C) Tc ) 115 °C (bandwidth ) 10.27 µm).

with needle-like dendrites). The inner rim of the band, where lamella stacks are rooted, is partially smeared with less-defined shapes; by comparison, the outer rim of the band is lined with well-defined needle-like lamella stacks radiating out individually. The lamellar length (radial direction from inner to outer rims)

Acknowledgment. This work was financially supported by a basic research grant (NSC-94 2216 E006 003) in three consecutive years funded by the National Science Council (NSC) of Taiwan. S.-H.L., a Ph.D. student, visited and performed part of the laboratory work for this study at the Department of Nanostructure and Advanced Materials, Kagoshima University, Kagoshima, Japan, via international collaboration/exchange funding kindly provided by Prof. Y. Suda of Venture Business Laboratory (VBL) at Kagoshima University. LA802192W