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Growth of banded spherulites of poly(∈-caprolactone) from the blends: An examination of the modeling of spherulitic growth
Polymer 53 (2012) 1765e1771
Contents lists available at SciVerse ScienceDirect
Polymer journal homepage: www.elsevier.com/locate/polymer
Growth of banded spherulites of poly(˛-caprolactone) from the blends: An examination of the modeling of spherulitic growth Akihiko Toda*, Ken Taguchi, Hiroshi Kajioka Graduate School of Integrated Arts and Sciences, Hiroshima University, Higashi-Hiroshima 739-8521, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 October 2011 Received in revised form 15 February 2012 Accepted 19 February 2012 Available online 28 February 2012
For the banded spherulites of poly(˛-caprolactone), PCL, grown from the blends with miscible polymers of polyvinyl butyral and poly(styrene-co-acrylonitrile), the effects of blended amorphous polymers on the band spacing have been examined experimentally. The results reconfirmed the strong influence of the second components even with small amount (c.a. 0.09 wt%). For the crystallization under the strong influence of the second components probably on the lamellar surface, we have examined the applicability of our modeling of spherulitic growth and its limit. Important findings in this paper are the followings: 1) On the confirmation of the applicability of the modeling for the amount of the second component small enough and the band spacing long enough. 2) On the violation of the predicted relationship of the modeling with increasing amount of the second component, which caused sharp decrease in the band spacing. 3) On the observation of the lower bound of the band spacing, to which the band spacing approached with the increase in the second component. With approaching the lower bound, the band spacing eventually became independent of other growth conditions such as crystallization temperature. 2012 Elsevier Ltd. All rights reserved.
Keywords: Band spacing Poly(˛-caprolactone) Blends
1. Introduction For the inner structure of spherulites formed by aliphatic polyesters, blending with a second amorphous polymer favors the appearance of concentric rings (Fig. 1): e.g. poly(˛-caprolactone) (PCL) blended with polyvinyl butyral (PVB) [1] or poly(styrene-coacrylonitrile) (SAN) [2] and poly(L-lactide) blended with atactic poly(D,L-lactide) [3,4] or atactic poly[(R,S)-3-hydroxybutyrate] [4e6]. Those types of spherulites are called banded spherulites, and for the emergence of concentric bands, the amount of second component can be quite small [1], c.a. less than 0.1 wt% (Fig. 1c). In addition, for PCL crystallization from the blend with PVB, the addition of PVB has a strong effect of suppression of primary nucleation [1], as clearly indicated by the difference in the size of spherulites in Fig. 1a and c. Owing to the effect, large banded spherulites can be easily obtainable by blending, so that the method is often utilized to examine the inner structure of banded spherulites by micro-beam X-ray diffraction and scattering [7] and by three-dimensional electron tomography [8]. Both of the emergence of bands and the suppression of nucleation density clearly suggest strong interaction of PVB molecules
* Corresponding author. Tel.: þ81 82 424 6558; fax: þ81 82 424 0757. E-mail address: [email protected] (A. Toda). 0032-3861/$ e see front matter 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2012.02.030
with PCL crystallites [1]. In banded spherulites, lamellar crystals show twisting correlation along the radial direction with the phase angle of twist coherent along the tangential direction [9]. The most probable physical origin of lamellar twist will be the unbalanced surface stresses caused by steric hindrance of chain folding on the folding surfaces, as extensively discussed and reviewed by Lotz and Cheng [10]. Therefore, it is also most probable that the added PVB molecules selectively adsorb on the folding surfaces of PCL lamellar crystallites and cause additional surface stresses to enhance the twisting. In addition, strong reduction of nucleation density can also be expected by the deactivation with PVB molecules adsorbed on active sites of heterogeneous nucleation, which is the most dominant process of nucleation in polymer crystallization from bulk melt. In terms of the formation mechanism of polymer spherulites and the emergence of bands in them, the strong interaction of PVB molecules with folding surfaces reasonably indicates continuous twist of lamellae in the banded spherulites of PCL. On the twisting mechanism, besides the continuous twist, we can also think of discontinuous large re-orientation on the occasion of branching of lamellae. Both of those processes are supported in theoretical modeling and experimentally observed by microscopy in other polymers [10e13]. In our prior studies on polymer crystallization from bulk melt [14e19], we have proposed a modeling of spherulitic crystallization
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potential spontaneously formed at the growth front in the viscous media of bulk melt. The gradient can be caused by the density gap between the crystal and melt, which must be compensated by the melt flow toward the growth front driven by “pressure gradient” in the viscous media [20]. The original proposal by Keith and Padden [21,22] of “compositional gradient” is also possible. The instability driven by the gradient field sets the critical lamellar width of branching, l, given as,
lfðhVÞ1=2
(1)
for the pressure gradient with the growth rate, V, and the viscosity of the media, h [14]. For the compositional gradient, similar relationship with diffusion coefficient, D, can be derived. We have experimentally confirmed the dependences of the lamellar width at the growth front, l, on V and h (or D) for polyethylene (PE) [14,15], poly(vinylidene fluoride) (PVDF) [16], isotactic poly(butene-1) [17,18], and isotactic polystyrene [19]. The results of the dependences on molecular weight [15,18] and on temperature near the glass transition [19] suggested the instability determined by viscosity, not by diffusion coefficient. For the banded spherulites of PE and PVDF, we have also confirmed that the inner structure of spherulites, such as the band spacing, L, is determined by the size of the building blocks, i.e. the width of lamellar crystals at the growth front, l, namely,
Lfl
(2)
It means that the strength of twist in PE and PVDF is determined by the lamellar width: larger angle of twist with narrower width and vice verse. In comparison with the behaviors confirmed for PE and PVDF, we think the crystallization of aliphatic polyesters from the blends with second amorphous component will give us unique opportunities to examine the applicability of the modeling and categorize the spherulitic crystallization of polymers. In the present paper, we examine the manner of twist in PCL crystallization from bulk melt with second amorphous component of PVB or SAN, especially the effect of the second component on the band spacing of the spherulites under various conditions. The results are analyzed on the basis of our modeling of spherulitic growth to examine the applicability limit of the modeling. 2. Experimental
Fig. 1. POM images of PCL49K spherulites crystallized at 38.5 C: (a) pure PCL, (b) PCL/ SAN ¼ 90/10, (c) PCL/PVB ¼ 99.91/0.09. The bars represent 100 mm.
with branching and re-orientation of lamellar crystals, and experimentally examined the predictions. The modeling is based on an instability driven branching and spontaneous re-orientation of branches subsequently. The spontaneous re-orientation is reasonably expected as a natural consequence of the excess surface stresses on the folding surfaces. We suppose the branching caused by a fingering instability under a gradient field of chemical
Poly(˛-caprolactone), PCL, samples with various Mw and Mw/Mn are tabulated in Table 1. The sample of 49K was purchased from Scientific Polymer Products, Inc., 27K and 46K were from Polymer Source, Inc., and 14K, 89K and 134K were from SigmaeAldrich Co. The molecular weight and the distribution of 49K, 89K and 134K were determined by GPC to check possible degradation due to aging of aliphatic polyesters. Those of the others are from the suppliers. Polyvinyl butyral, PVB, with a hydroxyl content of 11 wt%, acetate content of 1 wt%, butyral content of 88 wt% and Mw ¼ 70,000, and poly(styrene-co-acrylonitrile), SAN, with an acrylonitrile content of 25 wt% and Mw ¼ 165,000 were purchased from Scientific Polymer Products, Inc. The blend was prepared by dissolving the desired ratio of PCL and PVB and/or SAN in tetrahydrofuran at room temperature. The Table 1 Molecular characteristics of PCL samples. Sample
Mw 103
Mw/Mn
Sample
Mw 103
Mw/Mn
14K 27K 46K
14.0 27.3 46.4
1.4 1.4 2.0
49K 89K 134K
48.8 88.8 134
1.6 1.7 1.7
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solution was then cast on a glass cover, removed solvent by evaporation, and kept at 40 C under vacuum at least over night. The films on glass cover were melted at 150 C for 3 min, isothermally crystallized, and observed by using a hot stage (THMS600 controlled by LK-600, Linkam) and a polarizing optical microscope, POM, (BX51, Olympus). 3. Results and discussion 3.1. Mixture with PVB Fig. 2 shows typical banding patterns in spherulites of PCL mixed with PVB. Fig. 3 shows the growth rate, V, and the band spacing, L, plotted against crystallization temperature of PCL for the mixture with various concentrations of PVB. Here, it is noted that, for the irregularly deteriorated patterns of long band spacing (e.g., inner part of Fig. 2a), the spacing was determined by the spacing of patchy dark regions along the radial direction (shown by the double arrows in Fig. 2a); the typical error was 10% as the bars shown in Fig. 3b. For the additive of PVB in the range of 0.09e16 wt%, the growth rate and the inner structure of the spherulites, i.e. the band spacing, showed strong influence of the additives. The effects are also verified in Fig. 4 as the plots of log[V/V0.09] and log[L/L0.09], Fig. 3. (a) Linear growth rate, V, and (b) band spacing, L, plotted against crystallization temperature, Tc, of PCL49K from the blend with PVB of 0.09 (C), 0.25 (:), 1.0 (-), 5.0 (;), 16.0 wt% (A) in (a) and (b) and of pure PCL49K (B) in (a).
Fig. 2. POM images of PCL49K spherulites crystallized (a) from PCL/PVB ¼ 99.75/0.25 at 42.5, 38.5 and 34.4 C and (b) from PCL/SAN ¼ 84/16 at 40.5, 32.4 and 42.5 C, subsequently. The double arrows in (a) indicate the spacing read from the image. The bars represent 100 mm.
Fig. 4. Relative changes in (a) V, log[V/V0.09], and (a) L, log[L/L0.09], and (c) L2VhT of the data in Fig. 3. The meaning of the symbols is the same as in Fig. 3.
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where V0.09 and L0.09 are the values at 0.09 wt% of PVB, to visualize the relative changes with the increase in the composition of additives. It is clearly seen that the degrees of the changes in V and L are in the same order in magnitude. On the other hand, in terms of the temperature dependence of the band spacing, for the small amount of additive of 0.09 wt% (C) in Fig. 3b, the change became close to one order in magnitude. In the temperature range, it has been reported that the lamellar thickness undergoes at most 20% change [23]. On the basis of the elastic torsion with surface stresses, the pitch of twist is theoretically predicted to be in proportion to the second power of the lamellar thickness [24]. Therefore, this large change in L with temperature rules out the possibility that the band spacing is solely determined by the elastic torsion with surface stresses for the smallest amount of additive. As seen in Fig. 3b, the decrease in L with the additive became much stronger at higher temperatures. It means that the temperature dependence became weaker with increasing the concentration of additive, and eventually at 16 wt% (A) L became almost independent of crystallization temperature. In our modeling of the formation of inner structures, it is assumed that the inner structure is determined by the size of the building blocks, i.e. lamellar width as Eq. (2). The temperature dependence of L has been explained by the combination of Eqs. (1) and (2) with the strong dependence on temperature through those of V and h. The plots of Fig. 4c show the test of the applicability of the modeling, which predicts the following from Eqs. (1) and (2),
L2 V hT fh1 0
Owing to this effect, the relationship of Eq. (3), namely of Eq. (1) and/or Eq. (2), is violated, and the evaluated L2VhT in Fig. 4c deviated from horizontal lines. 3.2. Mixture with SAN In the same way as PVB, for the mixture with various concentrations of SAN (and 0.05 wt% PVB), Fig. 5 shows typical banding patterns and Fig. 6 shows the growth rate, V, the band spacing, L, and L2VhT plotted against crystallization temperature. Here, the addition of small amount of PVB (0.05%) was for reducing the number density of spherulites to observe longer band spacing in the spherulites. As for the mixture with PVB, the change in L in Fig. 6b amounts to almost one order in magnitude, so that the band spacing cannot be solely determined by the elastic torsion with surface stresses. With SAN, it seems that there is also a lower bound of the band spacing, L, at about 10 mm. The lower bound limits the applicability of the mechanism described by Eqs. (1) and (2). Above the lower bound for the smallest amount of SAN, the plots of L2VhT (C) shown in Fig. 6c are close to horizontal line, as predicted by Eq. (3), and for the data points near the lower bound of L around 10 mm the plots deviate from the horizontal lines even with the smallest amount. With the increase of the second components, L decreases
(3)
where hT and h0 represent the temperature dependence of viscosity and a coefficient independent of temperature, respectively. The temperature dependence of viscosity is given as [25],
hT ¼ exp
U=R Tc Tg TK
(4)
where Tg represents the glass transition temperature, R is the gas constant, and the constants, U and TK, are set at U ¼ 4120 cal/mol and TK ¼ 51.6 K as “universal” constants [25]. Here, Tg is influenced by the composition, and the dependence can be roughly evaluated by the Fox equation [26],
X wi 1 ¼ Tg Tgi i
(5)
where wi and Tgi represent the weight fraction and the glass transition temperature of the component i, respectively. The Tg of PCL is 64.15 C [27] and those of PVB and SAN were 53.9 and 102.6 C determined by DSC (Q100, TA Instruments) at the heating and cooling rates of 10 K/min. Based on Eq. (3), the plots of L2VhT against Tc in Fig. 4c should be on horizontal lines. The experimental results in Fig. 4c showed that the plots of the lowest PVB concentration (C) are close to horizontal line. However, with the increase of the additive of PVB, the plots significantly deviate from horizontal lines. The present results hence suggest that the basic assumptions of the modeling are violated by the increase in composition of PVB. In addition to the effects on the band spacing, PVB has a strong effect to reduce nucleation density of spherulites, as has been mentioned in Introduction. Both of the behaviors strongly suggest the essential role of PVB on the structural evolution of PCL crystallization. The most probable effect will be the direct interaction of PVB molecules with the lamellar surfaces by physical adsorption. With this effect, the lamellar crystals will be more stressed. Then, the band spacing will be eventually independent of growth conditions and kept constant (the minimum spacing w 10 mm), as seen in Fig. 3b for 16 wt% PVB.
Fig. 5. POM images of PCL49K spherulites crystallized (a) from PCL/SAN/PVB ¼ 97.95/ 2/0.05 at 42.5, 46.6 and 50.7 C and (b) from PCL/SAN/PVB ¼ 89.95/10/0.05 at 42.5, 46.6 and 50.7 C, subsequently. The bars represent 100 mm.
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Fig. 8. AFM images of PCL14K crystallized at (a) 37.0 and (b) 50.0 C from PCL/ SAN ¼ 98/2, quenched, and chemically etched.
Fig. 6. (a) Linear growth rate, V, (b) band spacing, L, and (c) L2VhT plotted against crystallization temperature, Tc, of PCL49K from the blend with PVB of 0.05 wt% and SAN of 2 (C), 5 (:), 10 (-), 22 wt% (;) in (a) and (b) and of pure PCL49K (B) in (a).
and approaches to the lower bound, and the plots in Fig. 6c systematically deviate from the horizontal lines. In terms of the influence of the additives of PVB and SAN on the band spacing, the effects are schematically summarized in Fig. 7. In both cases of PCL/PVB and PCL/SAN, there seems to be a lower bound of band spacing at around 10 mm. Near the lower limit, the
Fig. 7. Schematic summary of the dependences of L and L2VhT on the compositions of PCL/PVB (a and c) and PCL/SAN (b and d), respectively. The arrows indicate the changes with increasing the second components. The broken lines in a and b represent the lower bound of L w 10 mm.
band spacing became independent of growth conditions. For the limiting behavior, we can think of two possible scenarios based on the mechanism of Eqs. (1) and (2). Firstly, the relationship of Eq. (1) is violated due to enhanced branching by the interaction between additives and the lamellar crystals at the growth front. In this case, the lamellae are segmentalized by the effect. Secondly, the proportional relationship of Eq. (2) is lost, and the torsion of PCL lamellar crystals is solely determined by the continuous twist with the enhanced stresses under the influence of the additives. In order to examine the effects on the instability driven branching predicted by Eq. (1), we need to directly examine the lamellar width at the growth front. For that purpose, we have tried chemical etching of bulk samples quenched after isothermal crystallization with sodium hydroxide 5% w/v aqueous solution [28]. Probably because of densely packed lamellae of thickness thinner than 10 nm [23], we have not succeeded in observing the detailed morphology of flat-on lamellar crystals at the growth front of spherulites. The bundle of edge-on fine lamellae at low Tc and a large multi-layered single crystal at high Tc of the lowest Mw sample with the smallest amount of SAN could be viewed clearly, as seen in Fig. 8. The significant change in morphology with Tc suggests systematic change in lamellar width with Tc at least for the smallest amount of SAN in accordance with the prediction of Eq. (1) associated with Tc dependence of V. Because of the unsuccessful observation of lamellar width at the growth front and the quantitative examination under various growth conditions with increasing amount of additives, the physical origin of the lower bound of L and the different behaviors with PVB and SAN summarized in Fig. 7 could not be clarified, but should be related to the manner of adsorption on the crystal surfaces and the differences with PVB and SAN. It is noted that due to large difference in Tg of PCL (w 64 C) and SAN (z100 C), the variation of Tg with compositions is not negligible in the plots of Fig. 6c; based on the Fox equation of Eq. (5), Tg ¼ 62.3 to 41.5 C for 2.0e22 wt% of SAN, respectively. If the variation is neglected in the estimate of hT, the plots vertically
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The above results of PCL/SAN blend indicate that, with less amount of SAN and for the band spacing substantially longer than the lower bound of w10 mm, the plots of L2VhT in Fig. 6c are close to horizontal lines. It suggests that the formation of inner structures of
spherulites gets closer to the mechanism proposed by us on the basis of Eqs. (1) and (2) for the inner structure evolution. In order to further confirm the relationship given by Eqs. (1)e(3), we have examined the molecular weight dependence of the band spacing; from Eq. (3), L2VhT should be independent of Tc and represents the molecular weight dependence of chain mobility in the melt, i.e. a with a w 3.4e3.6. L2 V hT fMw Based on the expectation that the formation mechanism gets closer to the mechanism represented by Eqs. (1) and (2) with less amount of SAN, we have reduced the concentration of SAN to the minimum for the emergence of periodic bands. Fig. 10 shows typical banding patterns of PCL spherulites of (a) the lowest and (b) highest Mw, respectively. Fig. 11 shows the experimental results of V, L, and L2VhT. For the lowest molecular weight in Fig. 11c (C), L2VhT actually remains to be almost constant over the examined temperature range, probably because the band spacing is much longer than the lower bound of 10 mm. For higher molecular weights, on the other hand, L2VhT deviates from horizontal lines at lower temperatures again probably due to proximity to the lower bound of L. The values of L2VhT then level off at higher temperatures with the increase in L. The approach to constant L2VhT of higher molecular weights can also be confirmed by the results with further lower composition of SAN, shown in Fig. 11c as open symbols. If the L2VhT values of the horizontal broken lines in Fig. 11c are plotted against Mw, the plots shown in Fig. 12 suggest the power of 2.8 close to the expected value.
Fig. 10. POM images of spherulites of (a) PCL14K and (b) PCL134K crystallized from PCL/SAN/PVB ¼ 97.95/2/0.05 at 42.5 C. The bars represent 100 mm.
Fig. 11. (a) Linear growth rate, V, (b) band spacing, L, and (c) L2VhT plotted against crystallization temperature, Tc, of PCL14K (C), 27K (:), 89K (;), 134K (A) from the blend with PVB of 0.05 wt% and SAN of 2 wt%, of PCL 27K (6), 89K (7) with PVB of 0.05 wt% and SAN of 1 wt%, and of PCL134K (>) with PVB of 0.05 wt% and SAN of 0.5 wt%.
Fig. 9. The plots same as Fig. 6c with fixed Tg ¼ 64.15 C in the estimate of hT.
shift to each other, as seen in Fig. 9. Similar observation of the important role of the variation in Tg with composition has been reported for the same system of PCL/SAN [29]. 3.3. Molecular weight dependence
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bound, below which the band spacing eventually became independent of other growth conditions such as crystallization temperature. Those confirmation and findings will provide valuable information clarifying the mechanism of spherulitic growth and the emergence of bands in them. Acknowledgment The authors acknowledge helpful discussions of Prof. S. Tanaka of Hiroshima University and Prof. Y. Yamazaki of Waseda University. The authors wish to thank Mr. T. Ohashi of Western Industrial Research Institute of Hiroshima for the GPC measurements. This work was supported by KAKENHI (Grant-in-Aid for Scientific Research) on Priority Area “Soft Matter Physics” from the Ministry of Education, Culture, Sports, Science and Technology of Japan. Fig. 12. Double logarithmic plots against molecular weight, Mw, of L2VhT shown in Fig. 11 as the broken lines. The slope of the fitting straight line is 2.8.
4. Conclusions The band spacing of spherulites has been examined of PCL blended with small amount of PVB and SAN. The growth rate of spherulites and the band spacing showed significant effects of the second component especially with PVB. It is known that the addition of PVB also reduces the nucleation density of spherulites significantly. Both of the effects suggest the strong interaction of PVB with PCL lamellar crystallites and with the active surfaces of heterogeneous nuclei. The interaction probably causes additional excess surface stresses on the folding surfaces of PCL, and decreases the band spacing with strong torsion. For the growth under the strong influence of the second components, the predictions of the modeling of spherulitic growth have been critically examined. If the composition of additives is small enough and the band spacing is long enough, the change in band spacing is in accordance with the prediction of Eq. (3), which is based on the instability driven branching represented by Eq. (1) associated with the proportional relationship of Eq. (2); the prediction has been experimentally confirmed with PE and PVDF. Under this condition, the dependence on molecular weight suggested the branching influenced by the viscosity of the media, as confirmed in other polymers forming spherulites. With increasing the amount of the second components, the interaction overrides the mechanism represented by Eqs. (1)e(3). With the effect, the band spacing approaches a lower
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]
Keith HD, Padden Jr FJ, Russell TP. Macromolecules 1989;22:666. Li W, Yan R, Jiang B. Polymer 1992;33:889. Tsuji H, Ikada Y. Polymer 1996;37:595. Xu J, Guo B-H, Zhou J-J, Li L, Wu J, Kowalczuk M. Polymer 2005;46:9176. Focarete ML, Ceccorulli G, Scandola M, Kowalczuk M. Macromolecules 1998; 31:8485. Ohkoshi I, Abe H, Doi Y. Polymer 2000;41:5985. Nozue Y, Kurita R, Hirano S, Kawasaki N, Ueno S, Iida A, et al. Polymer 2003; 44:6397. Ikehara T, Jinnai H, Kaneko T, Nishioka H, Nishi T. J Polym Sci B Polym Phys 2007;45:1122. Geil PH. Polymer single crystals. New York: John Wiley; 1963. Lotz B, Cheng SZD. Polymer 2005;46:577. Bassett DC, Hodge AM. Proc Roy Soc 1981;A377:61. Bassett DC, Olley RH. Polymer 1984;25:935. Bassett DC, Vaughan AS. Polymer 1985;26:717. Toda A, Okamura M, Taguchi K, Hikosaka M, Kajioka H. Macromolecules 2008; 41:2484. Toda A, Taguchi K, Kajioka H. Macromolecules 2008;41:7505. Toda A, Taguchi K, Hikosaka M, Kajioka H. Polym J 2008;40:905. Kajioka H, Hikosaka M, Taguchi K, Toda A. Polymer 2008;49:1685. Kajioka H, Yamada K, Taguchi K, Toda A. Polymer 2011;52:2051. Kajioka H, Yoshimoto S, Taguchi K, Toda A. Macromolecules 2010;43:3837. Schultz JM. Polymer crystallization. Oxford: Oxford University Press; 2001. Chap. 10. Keith HD, Padden Jr FJ. J Appl Phys 1963;34:2409. Keith HD. J Polym Sci 1964;A2:4339. Hocquet S, Draye AC, Dosière M, Koch MHJ. HASYLAB Annual Report 2001. Okano K. Jpn J Appl Phys 1964;3:351. Ferry JD. Viscoelastic properties of polymers. 3rd ed. New York: Wiley; 1980. Fox TG. Bull Am Phys Soc 1956;2:123. Gaur U, Lau S-F, Wunderlich B. J Phys Chem Ref Data 1983;12:65. Shabana HM, Olley RH, Bassett DC, Jungnickel B-J. Polymer 2000;41:5513. Wang Z, Wang X, Yu D, Jiang B. Polymer 1997;38:5897.
Contents lists available at SciVerse ScienceDirect
Polymer journal homepage: www.elsevier.com/locate/polymer
Growth of banded spherulites of poly(˛-caprolactone) from the blends: An examination of the modeling of spherulitic growth Akihiko Toda*, Ken Taguchi, Hiroshi Kajioka Graduate School of Integrated Arts and Sciences, Hiroshima University, Higashi-Hiroshima 739-8521, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 October 2011 Received in revised form 15 February 2012 Accepted 19 February 2012 Available online 28 February 2012
For the banded spherulites of poly(˛-caprolactone), PCL, grown from the blends with miscible polymers of polyvinyl butyral and poly(styrene-co-acrylonitrile), the effects of blended amorphous polymers on the band spacing have been examined experimentally. The results reconfirmed the strong influence of the second components even with small amount (c.a. 0.09 wt%). For the crystallization under the strong influence of the second components probably on the lamellar surface, we have examined the applicability of our modeling of spherulitic growth and its limit. Important findings in this paper are the followings: 1) On the confirmation of the applicability of the modeling for the amount of the second component small enough and the band spacing long enough. 2) On the violation of the predicted relationship of the modeling with increasing amount of the second component, which caused sharp decrease in the band spacing. 3) On the observation of the lower bound of the band spacing, to which the band spacing approached with the increase in the second component. With approaching the lower bound, the band spacing eventually became independent of other growth conditions such as crystallization temperature. 2012 Elsevier Ltd. All rights reserved.
Keywords: Band spacing Poly(˛-caprolactone) Blends
1. Introduction For the inner structure of spherulites formed by aliphatic polyesters, blending with a second amorphous polymer favors the appearance of concentric rings (Fig. 1): e.g. poly(˛-caprolactone) (PCL) blended with polyvinyl butyral (PVB) [1] or poly(styrene-coacrylonitrile) (SAN) [2] and poly(L-lactide) blended with atactic poly(D,L-lactide) [3,4] or atactic poly[(R,S)-3-hydroxybutyrate] [4e6]. Those types of spherulites are called banded spherulites, and for the emergence of concentric bands, the amount of second component can be quite small [1], c.a. less than 0.1 wt% (Fig. 1c). In addition, for PCL crystallization from the blend with PVB, the addition of PVB has a strong effect of suppression of primary nucleation [1], as clearly indicated by the difference in the size of spherulites in Fig. 1a and c. Owing to the effect, large banded spherulites can be easily obtainable by blending, so that the method is often utilized to examine the inner structure of banded spherulites by micro-beam X-ray diffraction and scattering [7] and by three-dimensional electron tomography [8]. Both of the emergence of bands and the suppression of nucleation density clearly suggest strong interaction of PVB molecules
* Corresponding author. Tel.: þ81 82 424 6558; fax: þ81 82 424 0757. E-mail address: [email protected] (A. Toda). 0032-3861/$ e see front matter 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2012.02.030
with PCL crystallites [1]. In banded spherulites, lamellar crystals show twisting correlation along the radial direction with the phase angle of twist coherent along the tangential direction [9]. The most probable physical origin of lamellar twist will be the unbalanced surface stresses caused by steric hindrance of chain folding on the folding surfaces, as extensively discussed and reviewed by Lotz and Cheng [10]. Therefore, it is also most probable that the added PVB molecules selectively adsorb on the folding surfaces of PCL lamellar crystallites and cause additional surface stresses to enhance the twisting. In addition, strong reduction of nucleation density can also be expected by the deactivation with PVB molecules adsorbed on active sites of heterogeneous nucleation, which is the most dominant process of nucleation in polymer crystallization from bulk melt. In terms of the formation mechanism of polymer spherulites and the emergence of bands in them, the strong interaction of PVB molecules with folding surfaces reasonably indicates continuous twist of lamellae in the banded spherulites of PCL. On the twisting mechanism, besides the continuous twist, we can also think of discontinuous large re-orientation on the occasion of branching of lamellae. Both of those processes are supported in theoretical modeling and experimentally observed by microscopy in other polymers [10e13]. In our prior studies on polymer crystallization from bulk melt [14e19], we have proposed a modeling of spherulitic crystallization
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potential spontaneously formed at the growth front in the viscous media of bulk melt. The gradient can be caused by the density gap between the crystal and melt, which must be compensated by the melt flow toward the growth front driven by “pressure gradient” in the viscous media [20]. The original proposal by Keith and Padden [21,22] of “compositional gradient” is also possible. The instability driven by the gradient field sets the critical lamellar width of branching, l, given as,
lfðhVÞ1=2
(1)
for the pressure gradient with the growth rate, V, and the viscosity of the media, h [14]. For the compositional gradient, similar relationship with diffusion coefficient, D, can be derived. We have experimentally confirmed the dependences of the lamellar width at the growth front, l, on V and h (or D) for polyethylene (PE) [14,15], poly(vinylidene fluoride) (PVDF) [16], isotactic poly(butene-1) [17,18], and isotactic polystyrene [19]. The results of the dependences on molecular weight [15,18] and on temperature near the glass transition [19] suggested the instability determined by viscosity, not by diffusion coefficient. For the banded spherulites of PE and PVDF, we have also confirmed that the inner structure of spherulites, such as the band spacing, L, is determined by the size of the building blocks, i.e. the width of lamellar crystals at the growth front, l, namely,
Lfl
(2)
It means that the strength of twist in PE and PVDF is determined by the lamellar width: larger angle of twist with narrower width and vice verse. In comparison with the behaviors confirmed for PE and PVDF, we think the crystallization of aliphatic polyesters from the blends with second amorphous component will give us unique opportunities to examine the applicability of the modeling and categorize the spherulitic crystallization of polymers. In the present paper, we examine the manner of twist in PCL crystallization from bulk melt with second amorphous component of PVB or SAN, especially the effect of the second component on the band spacing of the spherulites under various conditions. The results are analyzed on the basis of our modeling of spherulitic growth to examine the applicability limit of the modeling. 2. Experimental
Fig. 1. POM images of PCL49K spherulites crystallized at 38.5 C: (a) pure PCL, (b) PCL/ SAN ¼ 90/10, (c) PCL/PVB ¼ 99.91/0.09. The bars represent 100 mm.
with branching and re-orientation of lamellar crystals, and experimentally examined the predictions. The modeling is based on an instability driven branching and spontaneous re-orientation of branches subsequently. The spontaneous re-orientation is reasonably expected as a natural consequence of the excess surface stresses on the folding surfaces. We suppose the branching caused by a fingering instability under a gradient field of chemical
Poly(˛-caprolactone), PCL, samples with various Mw and Mw/Mn are tabulated in Table 1. The sample of 49K was purchased from Scientific Polymer Products, Inc., 27K and 46K were from Polymer Source, Inc., and 14K, 89K and 134K were from SigmaeAldrich Co. The molecular weight and the distribution of 49K, 89K and 134K were determined by GPC to check possible degradation due to aging of aliphatic polyesters. Those of the others are from the suppliers. Polyvinyl butyral, PVB, with a hydroxyl content of 11 wt%, acetate content of 1 wt%, butyral content of 88 wt% and Mw ¼ 70,000, and poly(styrene-co-acrylonitrile), SAN, with an acrylonitrile content of 25 wt% and Mw ¼ 165,000 were purchased from Scientific Polymer Products, Inc. The blend was prepared by dissolving the desired ratio of PCL and PVB and/or SAN in tetrahydrofuran at room temperature. The Table 1 Molecular characteristics of PCL samples. Sample
Mw 103
Mw/Mn
Sample
Mw 103
Mw/Mn
14K 27K 46K
14.0 27.3 46.4
1.4 1.4 2.0
49K 89K 134K
48.8 88.8 134
1.6 1.7 1.7
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solution was then cast on a glass cover, removed solvent by evaporation, and kept at 40 C under vacuum at least over night. The films on glass cover were melted at 150 C for 3 min, isothermally crystallized, and observed by using a hot stage (THMS600 controlled by LK-600, Linkam) and a polarizing optical microscope, POM, (BX51, Olympus). 3. Results and discussion 3.1. Mixture with PVB Fig. 2 shows typical banding patterns in spherulites of PCL mixed with PVB. Fig. 3 shows the growth rate, V, and the band spacing, L, plotted against crystallization temperature of PCL for the mixture with various concentrations of PVB. Here, it is noted that, for the irregularly deteriorated patterns of long band spacing (e.g., inner part of Fig. 2a), the spacing was determined by the spacing of patchy dark regions along the radial direction (shown by the double arrows in Fig. 2a); the typical error was 10% as the bars shown in Fig. 3b. For the additive of PVB in the range of 0.09e16 wt%, the growth rate and the inner structure of the spherulites, i.e. the band spacing, showed strong influence of the additives. The effects are also verified in Fig. 4 as the plots of log[V/V0.09] and log[L/L0.09], Fig. 3. (a) Linear growth rate, V, and (b) band spacing, L, plotted against crystallization temperature, Tc, of PCL49K from the blend with PVB of 0.09 (C), 0.25 (:), 1.0 (-), 5.0 (;), 16.0 wt% (A) in (a) and (b) and of pure PCL49K (B) in (a).
Fig. 2. POM images of PCL49K spherulites crystallized (a) from PCL/PVB ¼ 99.75/0.25 at 42.5, 38.5 and 34.4 C and (b) from PCL/SAN ¼ 84/16 at 40.5, 32.4 and 42.5 C, subsequently. The double arrows in (a) indicate the spacing read from the image. The bars represent 100 mm.
Fig. 4. Relative changes in (a) V, log[V/V0.09], and (a) L, log[L/L0.09], and (c) L2VhT of the data in Fig. 3. The meaning of the symbols is the same as in Fig. 3.
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where V0.09 and L0.09 are the values at 0.09 wt% of PVB, to visualize the relative changes with the increase in the composition of additives. It is clearly seen that the degrees of the changes in V and L are in the same order in magnitude. On the other hand, in terms of the temperature dependence of the band spacing, for the small amount of additive of 0.09 wt% (C) in Fig. 3b, the change became close to one order in magnitude. In the temperature range, it has been reported that the lamellar thickness undergoes at most 20% change [23]. On the basis of the elastic torsion with surface stresses, the pitch of twist is theoretically predicted to be in proportion to the second power of the lamellar thickness [24]. Therefore, this large change in L with temperature rules out the possibility that the band spacing is solely determined by the elastic torsion with surface stresses for the smallest amount of additive. As seen in Fig. 3b, the decrease in L with the additive became much stronger at higher temperatures. It means that the temperature dependence became weaker with increasing the concentration of additive, and eventually at 16 wt% (A) L became almost independent of crystallization temperature. In our modeling of the formation of inner structures, it is assumed that the inner structure is determined by the size of the building blocks, i.e. lamellar width as Eq. (2). The temperature dependence of L has been explained by the combination of Eqs. (1) and (2) with the strong dependence on temperature through those of V and h. The plots of Fig. 4c show the test of the applicability of the modeling, which predicts the following from Eqs. (1) and (2),
L2 V hT fh1 0
Owing to this effect, the relationship of Eq. (3), namely of Eq. (1) and/or Eq. (2), is violated, and the evaluated L2VhT in Fig. 4c deviated from horizontal lines. 3.2. Mixture with SAN In the same way as PVB, for the mixture with various concentrations of SAN (and 0.05 wt% PVB), Fig. 5 shows typical banding patterns and Fig. 6 shows the growth rate, V, the band spacing, L, and L2VhT plotted against crystallization temperature. Here, the addition of small amount of PVB (0.05%) was for reducing the number density of spherulites to observe longer band spacing in the spherulites. As for the mixture with PVB, the change in L in Fig. 6b amounts to almost one order in magnitude, so that the band spacing cannot be solely determined by the elastic torsion with surface stresses. With SAN, it seems that there is also a lower bound of the band spacing, L, at about 10 mm. The lower bound limits the applicability of the mechanism described by Eqs. (1) and (2). Above the lower bound for the smallest amount of SAN, the plots of L2VhT (C) shown in Fig. 6c are close to horizontal line, as predicted by Eq. (3), and for the data points near the lower bound of L around 10 mm the plots deviate from the horizontal lines even with the smallest amount. With the increase of the second components, L decreases
(3)
where hT and h0 represent the temperature dependence of viscosity and a coefficient independent of temperature, respectively. The temperature dependence of viscosity is given as [25],
hT ¼ exp
U=R Tc Tg TK
(4)
where Tg represents the glass transition temperature, R is the gas constant, and the constants, U and TK, are set at U ¼ 4120 cal/mol and TK ¼ 51.6 K as “universal” constants [25]. Here, Tg is influenced by the composition, and the dependence can be roughly evaluated by the Fox equation [26],
X wi 1 ¼ Tg Tgi i
(5)
where wi and Tgi represent the weight fraction and the glass transition temperature of the component i, respectively. The Tg of PCL is 64.15 C [27] and those of PVB and SAN were 53.9 and 102.6 C determined by DSC (Q100, TA Instruments) at the heating and cooling rates of 10 K/min. Based on Eq. (3), the plots of L2VhT against Tc in Fig. 4c should be on horizontal lines. The experimental results in Fig. 4c showed that the plots of the lowest PVB concentration (C) are close to horizontal line. However, with the increase of the additive of PVB, the plots significantly deviate from horizontal lines. The present results hence suggest that the basic assumptions of the modeling are violated by the increase in composition of PVB. In addition to the effects on the band spacing, PVB has a strong effect to reduce nucleation density of spherulites, as has been mentioned in Introduction. Both of the behaviors strongly suggest the essential role of PVB on the structural evolution of PCL crystallization. The most probable effect will be the direct interaction of PVB molecules with the lamellar surfaces by physical adsorption. With this effect, the lamellar crystals will be more stressed. Then, the band spacing will be eventually independent of growth conditions and kept constant (the minimum spacing w 10 mm), as seen in Fig. 3b for 16 wt% PVB.
Fig. 5. POM images of PCL49K spherulites crystallized (a) from PCL/SAN/PVB ¼ 97.95/ 2/0.05 at 42.5, 46.6 and 50.7 C and (b) from PCL/SAN/PVB ¼ 89.95/10/0.05 at 42.5, 46.6 and 50.7 C, subsequently. The bars represent 100 mm.
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Fig. 8. AFM images of PCL14K crystallized at (a) 37.0 and (b) 50.0 C from PCL/ SAN ¼ 98/2, quenched, and chemically etched.
Fig. 6. (a) Linear growth rate, V, (b) band spacing, L, and (c) L2VhT plotted against crystallization temperature, Tc, of PCL49K from the blend with PVB of 0.05 wt% and SAN of 2 (C), 5 (:), 10 (-), 22 wt% (;) in (a) and (b) and of pure PCL49K (B) in (a).
and approaches to the lower bound, and the plots in Fig. 6c systematically deviate from the horizontal lines. In terms of the influence of the additives of PVB and SAN on the band spacing, the effects are schematically summarized in Fig. 7. In both cases of PCL/PVB and PCL/SAN, there seems to be a lower bound of band spacing at around 10 mm. Near the lower limit, the
Fig. 7. Schematic summary of the dependences of L and L2VhT on the compositions of PCL/PVB (a and c) and PCL/SAN (b and d), respectively. The arrows indicate the changes with increasing the second components. The broken lines in a and b represent the lower bound of L w 10 mm.
band spacing became independent of growth conditions. For the limiting behavior, we can think of two possible scenarios based on the mechanism of Eqs. (1) and (2). Firstly, the relationship of Eq. (1) is violated due to enhanced branching by the interaction between additives and the lamellar crystals at the growth front. In this case, the lamellae are segmentalized by the effect. Secondly, the proportional relationship of Eq. (2) is lost, and the torsion of PCL lamellar crystals is solely determined by the continuous twist with the enhanced stresses under the influence of the additives. In order to examine the effects on the instability driven branching predicted by Eq. (1), we need to directly examine the lamellar width at the growth front. For that purpose, we have tried chemical etching of bulk samples quenched after isothermal crystallization with sodium hydroxide 5% w/v aqueous solution [28]. Probably because of densely packed lamellae of thickness thinner than 10 nm [23], we have not succeeded in observing the detailed morphology of flat-on lamellar crystals at the growth front of spherulites. The bundle of edge-on fine lamellae at low Tc and a large multi-layered single crystal at high Tc of the lowest Mw sample with the smallest amount of SAN could be viewed clearly, as seen in Fig. 8. The significant change in morphology with Tc suggests systematic change in lamellar width with Tc at least for the smallest amount of SAN in accordance with the prediction of Eq. (1) associated with Tc dependence of V. Because of the unsuccessful observation of lamellar width at the growth front and the quantitative examination under various growth conditions with increasing amount of additives, the physical origin of the lower bound of L and the different behaviors with PVB and SAN summarized in Fig. 7 could not be clarified, but should be related to the manner of adsorption on the crystal surfaces and the differences with PVB and SAN. It is noted that due to large difference in Tg of PCL (w 64 C) and SAN (z100 C), the variation of Tg with compositions is not negligible in the plots of Fig. 6c; based on the Fox equation of Eq. (5), Tg ¼ 62.3 to 41.5 C for 2.0e22 wt% of SAN, respectively. If the variation is neglected in the estimate of hT, the plots vertically
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The above results of PCL/SAN blend indicate that, with less amount of SAN and for the band spacing substantially longer than the lower bound of w10 mm, the plots of L2VhT in Fig. 6c are close to horizontal lines. It suggests that the formation of inner structures of
spherulites gets closer to the mechanism proposed by us on the basis of Eqs. (1) and (2) for the inner structure evolution. In order to further confirm the relationship given by Eqs. (1)e(3), we have examined the molecular weight dependence of the band spacing; from Eq. (3), L2VhT should be independent of Tc and represents the molecular weight dependence of chain mobility in the melt, i.e. a with a w 3.4e3.6. L2 V hT fMw Based on the expectation that the formation mechanism gets closer to the mechanism represented by Eqs. (1) and (2) with less amount of SAN, we have reduced the concentration of SAN to the minimum for the emergence of periodic bands. Fig. 10 shows typical banding patterns of PCL spherulites of (a) the lowest and (b) highest Mw, respectively. Fig. 11 shows the experimental results of V, L, and L2VhT. For the lowest molecular weight in Fig. 11c (C), L2VhT actually remains to be almost constant over the examined temperature range, probably because the band spacing is much longer than the lower bound of 10 mm. For higher molecular weights, on the other hand, L2VhT deviates from horizontal lines at lower temperatures again probably due to proximity to the lower bound of L. The values of L2VhT then level off at higher temperatures with the increase in L. The approach to constant L2VhT of higher molecular weights can also be confirmed by the results with further lower composition of SAN, shown in Fig. 11c as open symbols. If the L2VhT values of the horizontal broken lines in Fig. 11c are plotted against Mw, the plots shown in Fig. 12 suggest the power of 2.8 close to the expected value.
Fig. 10. POM images of spherulites of (a) PCL14K and (b) PCL134K crystallized from PCL/SAN/PVB ¼ 97.95/2/0.05 at 42.5 C. The bars represent 100 mm.
Fig. 11. (a) Linear growth rate, V, (b) band spacing, L, and (c) L2VhT plotted against crystallization temperature, Tc, of PCL14K (C), 27K (:), 89K (;), 134K (A) from the blend with PVB of 0.05 wt% and SAN of 2 wt%, of PCL 27K (6), 89K (7) with PVB of 0.05 wt% and SAN of 1 wt%, and of PCL134K (>) with PVB of 0.05 wt% and SAN of 0.5 wt%.
Fig. 9. The plots same as Fig. 6c with fixed Tg ¼ 64.15 C in the estimate of hT.
shift to each other, as seen in Fig. 9. Similar observation of the important role of the variation in Tg with composition has been reported for the same system of PCL/SAN [29]. 3.3. Molecular weight dependence
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bound, below which the band spacing eventually became independent of other growth conditions such as crystallization temperature. Those confirmation and findings will provide valuable information clarifying the mechanism of spherulitic growth and the emergence of bands in them. Acknowledgment The authors acknowledge helpful discussions of Prof. S. Tanaka of Hiroshima University and Prof. Y. Yamazaki of Waseda University. The authors wish to thank Mr. T. Ohashi of Western Industrial Research Institute of Hiroshima for the GPC measurements. This work was supported by KAKENHI (Grant-in-Aid for Scientific Research) on Priority Area “Soft Matter Physics” from the Ministry of Education, Culture, Sports, Science and Technology of Japan. Fig. 12. Double logarithmic plots against molecular weight, Mw, of L2VhT shown in Fig. 11 as the broken lines. The slope of the fitting straight line is 2.8.
4. Conclusions The band spacing of spherulites has been examined of PCL blended with small amount of PVB and SAN. The growth rate of spherulites and the band spacing showed significant effects of the second component especially with PVB. It is known that the addition of PVB also reduces the nucleation density of spherulites significantly. Both of the effects suggest the strong interaction of PVB with PCL lamellar crystallites and with the active surfaces of heterogeneous nuclei. The interaction probably causes additional excess surface stresses on the folding surfaces of PCL, and decreases the band spacing with strong torsion. For the growth under the strong influence of the second components, the predictions of the modeling of spherulitic growth have been critically examined. If the composition of additives is small enough and the band spacing is long enough, the change in band spacing is in accordance with the prediction of Eq. (3), which is based on the instability driven branching represented by Eq. (1) associated with the proportional relationship of Eq. (2); the prediction has been experimentally confirmed with PE and PVDF. Under this condition, the dependence on molecular weight suggested the branching influenced by the viscosity of the media, as confirmed in other polymers forming spherulites. With increasing the amount of the second components, the interaction overrides the mechanism represented by Eqs. (1)e(3). With the effect, the band spacing approaches a lower
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