- Email: [email protected]
Morphological development of melt crystallized poly(propylene oxide) by in situ AFM: formation of banded spherulites
European Polymer Journal 40 (2004) 893–903 www.elsevier.com/locate/europolj
Morphological development of melt crystallized poly(propylene oxide) by in situ AFM: formation of banded spherulites L.G.M. Beekmans, M.A. Hempenius, G.J. Vancso
*
Materials Science and Technology of Polymers, MESAþ Institute for Nanotechnology, and Faculty of Science and Technology, University of Twente, NL 7500 Enschede, The Netherlands MESAþ Research Institute, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands Received 15 December 2003; received in revised form 20 January 2004; accepted 21 January 2004
Abstract The morphology of poly(propylene oxide) (PPO) crystals grown from the melt was investigated. The spherulites of the optically pure S polymers displayed a regular pattern of concentric rings as observed by polarizing optical microscopy, while the stereocopolymer developed irregularly banded, or non-banded spherulites depending on the degree of undercooling. The organization of the lamellar crystals within the spherulites was examined by means of atomic force microscopy (AFM). For all cases, the lamellar structures appeared to adopt an alternating flat or edge-on orientation. Examination of the morphology of single crystals in the melt of the stereocopolymer revealed truncatedlozenge crystals, which were elongated in shape. Results from crystallization kinetics, obtained by in situ AFM observations, showed that the elongated habit is related to differences in the growth rates of the {2 0 0} and {1 1 0} facets. Interestingly, the melt-grown RS-PPO crystals developed a curved asymmetrical three-dimensional shape. Based on these observations it can be proposed that the chiral nature of the chain is transmitted to higher structural levels of ordering in the crystal aggregates. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Atomic force microscopy; In situ crystallization; Polymer morphology; Banded spherulites; Chirality; Poly(propylene oxide)
1. Introduction Spherulitic textures are morphological features commonly observed in semi-crystalline polymers. The generation of a spherulite starts from a precursor structure, which quite often consists of an incipient lamellar crystal. This precursor then develops into a spherically symmetrical structure. In certain polymers, spherulites exhibit a concentric pattern of periodic banding when examined by polarized optical microscopy (OM). Twisting of the orientation of the lamellar
*
Corresponding author. Tel.: +31-53-489-2974/2967; fax: +31-53-489-3823. E-mail address: [email protected] (G.J. Vancso).
crystals in the radial direction, and hence the refractive index tensor elements, generates these bands with the bandwidth increasing with temperature [1]. Two possible models have been postulated to describe the mechanism that leads to this lamellar twisting organization within spherulites. These models, which share some common elements, have been put forward by Bassett and coworkers [2,3] and by Keith and Padden [4,5], and their descriptions have been recently reviewed [6,7]. In essence, lamellar twisting is considered to be the result of the non-orthogonal relation between the crystal faces and the polymer chain axis. According to Bassett and coworkers [2,3], the handedness of screw dislocations in multilayer crystals is linked to the direction of chain tilt in the lamella. At isochiral screw dislocations adjacent lamellae will splay apart, presumably due to
0014-3057/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2004.01.025
894
L.G.M. Beekmans et al. / European Polymer Journal 40 (2004) 893–903
cilia pressure, and hence initiate twisting of the spherulitic building blocks. Another possibility, as proposed in the second model, is that chain tilting will create unequal stresses at opposing fold surfaces, which ultimately results in the twisting of the lamellae [4,5]. For chiral polymers, the possibility that the handedness of lamellar twisting is related to the chirality of the polymer chain has been put forward. Indeed, direct experimental evidence from atomic force microscopy (AFM) by Singfield et al. [8] of melt-crystallized chiral poly(epichlorohydrin) (PECH) spherulites has shown that a chirality-twisting relationship may exist for this particular case. The correlation between the handedness of lamellar twisting in the banded spherulite and chain chirality has been also reported from tilting studies under OM [9,10]. Detailed examination of the three-dimensional morphology of lamellar crystals provides important information about the mechanism by which the chirality of the chain is transmitted to higher structural levels. Direct observation of lamellar crystal habit has been achieved by electron microscopy of solution-grown single crystals [11–13]. Whereas the above mentioned spherulitic melt-crystallization studies [4,5,8–10] proposed that a correlation exists between the sense of twisting and chain chirality, the study of solution-grown crystals of chiral polymers did not give definite support to this origin of twisting at the lamellar level [11–13]. Lamellar single crystals of optically active polymers such as poly(hydroxybutyrate) (PHB) [11], PECH, poly(propylene oxide) (PPO) [12] and poly(hydroxyvalerate) (PHV) [12,13] grown from dilute solution are flat in shape and display no helical morphology. These results indicate that in this particular case chain chirality has not been transferred to the lamellar morphology. On the other hand, twisted-ribbon-like single crystals have been obtained from a liquid-crystalline chirally pure polymer although planar single crystals were also grown in the melt [14]. At higher temperatures of crystallization the planar single crystals become more dominant, indicating that the influence of chirality on the morphology can be suppressed by decreasing the undercooling [15]. In order to shed further light onto these controversies, a new study was initiated to study the morphological habit at the lamellar level in the melt. The aim has been to examine the morphology of lamellar crystals in a state that more closely approaches the complexity within the lamellar organization of melt-crystallized spherulites. This approach was expected to reveal the possible existence of configurational factors influencing the structure at the lamellar level. This study, therefore, reports the results of a detailed investigation of the morphology of melt-crystallized lamellar crystals of racemic poly((R,S)-polypropylene oxide) (RS-PPO), linking spherulitic to single crystals. The most important argument for choosing racemic RS-PPO for this study was its ability to crystallize from the melt as banded
spherulites at relative low undercooling like chiral pure S- or R-PPO. It is anticipated that such an AFM crystallization study at lamellar resolution shall provide new insights into the lamellar arrangement in this chiral polymer yielding results of the morphological development in real time [16–18], which would enable us to test the applicability of the above-mentioned models. The crystal morphology and crystallization kinetics of PPO have not received much attention since the discovery of RS-PPO in 1955. Magill et al. studied the meltcrystallization of the stereocopolymer RS-PPO and of optically pure S-PPO by optical microscopy [19]. Solution-grown PPO single crystals were reported recently [12]. The lenticular crystals were planar and showed curved {2 0 0} facets, which extended towards the apex under an acute angle. 2. Experimental (R,S) and (S)-propylene oxide were obtained from Aldrich and used after purification by vacuum distillation from calcium hydride. The polymerization was carried out according to the method of Price and Osgan [20]. The crude product was fractionated by crystallization from an acetone solution at 0 °C (1 g/200 ml). The characteristics of the samples used in this work are given in Table 1. 13 C-NMR spectra were recorded on a Bruker AC 250 spectrometer at 250.1 MHz. Deuterated chloroform was used as solvent without an internal standard. Fig. 1A shows expanded 13 C-NMR spectra of the methyl region of both RS-PPO and S-PPO. The methyl side-carbons display chemical NMR sensitivity according to the configuration of the b-carbon in the polymer chain. The peaks at d ¼ 18:39 and 18.13 ppm are assigned to region defects (head-to-tail defects). The signals at d ¼ 18:05, 17.93 and 17.86 ppm, only observed in the racemic samples were assigned to stereo defects (RS defects) of RS-PPO which amount to no more than 1.5% of the polymer chain [21]. A Perkin–Elmer calorimeter Pyris 1, used for DSC measurements, was calibrated with gallium–indium prior to the experiments. All DSC scans were recorded at a heating rate of 10 °C/min, and the sample chamber was kept under a constant flow of nitrogen. Isothermal crystallization experiments were performed by quenching the samples from the melt at 100 °C to a selected Table 1 Characteristics of the PPO samples used in this study
RS-PPO S-PPO
Fraction
Stereo defects [%]
Regio defects [%]
3 2
1.5 –
1.7 4.5
Defects were estimated from the
13
C-NMR spectra.
L.G.M. Beekmans et al. / European Polymer Journal 40 (2004) 893–903
Fig. 1. (A) 13 C-NMR spectra of the methyl region of RS-PPO and S-PPO. (B) DSC melting thermograms of S-PPO, R-PPO and RS-PPO isothermally crystallized at 45 °C for 1 h, subsequently quenched to 30 °C and heated.
crystallization temperature of 45 °C. When crystallization was complete, the samples were quickly cooled to 30 °C and the melting thermograms were recorded. Fig. 1B contains representative DSC heating traces of the optically pure S-PPO and the RS-PPO, respectively. Under these conditions all the polymers exhibit endotherms with three melting peaks, suggesting multiple degrees of crystal size, perfection, and reorganization. However the positions of these melting peak maxima
895
coincide for all polymer samples, indicating that the samples produced crystals of similar thermal stability with a wide range of melting points. Optical microscope observations were carried out on an Olympus BX60 polarizing microscope equipped with a Mettler FP80 hot-stage. The samples were prepared by first pressing the polymer between two glass cover slides at 100 °C, were the polymer was first melted and then cooled to the temperature of crystallization within approximately 1 min. The spherulites were photographed between crossed polarizers. For the AFM experiments the glass cover slides were taken out from the Mettler hot-stage and the top cover slide was removed, leaving a polymer film of an estimated thickness of 30–80 lm. The experiments were performed under a nitrogen atmosphere using a NanoScope III setup (Digital Instruments). The instrument was equipped with a J-scanner (maximum scan size 100 lm2 ). Commercially available Si-cantilevers were used with force constants of 27–83 N/m as stated by the manufacturer (Nanoprobes). Measurements were carried out in the tapping mode, using always the smallest possible set-point to minimize interaction forces between the tip and sample [22]. Height and phase images were acquired simultaneously. The heating stage used for AFM observations was a variation of a previously published design [23].
3. Results and discussion 3.1. Spherulitic morphology The two PPO fractions crystallized from the melt formed spherulites with the characteristic Maltese extinction cross as it was clearly visible under the polarizing optical microscope. The spherulites were
Fig. 2. (A–C) Optical micrographs of (A) S-PPO, (B) R-PPO and (C) RS-PPO isothermally crystallized at 60 °C for 2 days.
896
L.G.M. Beekmans et al. / European Polymer Journal 40 (2004) 893–903
obtained by isothermal crystallization at 60 °C, which implies an undercooling of about 20 °C. In addition, the spherulites of the optically active S-PPO exhibited periodic birefringent extinction bands. Conversely, the racemic RS-PPO displays irregular banded spherulites with weak signs of extinction effects. The polarized light photomicrographs in Fig. 2 illustrate these features. The presence of banding in polymer spherulites is generally ascribed to a twisting of crystallographic orientation along the radial direction of the spherulite. It apparently reflects a cooperative twisting of radiating lamellae and implies a degree of coordination in the packing of the lamellae during crystallization [1,5]. It is known that the banding pattern of the spherulites is even more apparent when the sample is crystal-
lized with a free surface [1,8,24]. The topological images of a corresponding surface obtained by AFM, as can be seen in Fig. 3, show a periodic pattern of ridges and valleys. The micrograph in Fig. 3A is a low magnification AFM height image of a spherulite of the S-PPO grown at 55 °C. The periodicity of the surface contour corresponds to the birefringent extinction rings observed by OM. Higher magnification height and phase images of the upper right side of this S-PPO spherulite are presented in Fig. 3B and C, respectively. The height image still reveals clear radial bands separated from one another by thinner, dark regions (i.e. lower height). The picture in Fig. 3D shows an even more detailed image of the spherulite corresponding to the zone as indicated by the added box in (B) and (C). In this image the indi-
Fig. 3. (A–D) AFM images of a S-PPO spherulite isothermally crystallized at 55 °C for 1 day. The images were obtained at room temperature. (A) Is a low magnification height image, while (B) and (C) are high magnification height and phase images, respectively, of the zone arrowed in (A). (D) Phase image at higher magnification from the zone framed in (B) and (C).
L.G.M. Beekmans et al. / European Polymer Journal 40 (2004) 893–903
897
Fig. 4. (A–C) AFM images of RS-PPO spherulite isothermally crystallized at 55 °C for 1 day. The images are obtained at room temperature. (A) Is a low magnification height image, while (B) and (C) are high magnification height and phase images, respectively, of the zone arrowed in (A).
vidual lamellar crystals are clearly resolved. It can be seen on this phase image that the elevated regions in the height image are regions in which the lamellar crystals are oriented edge-on while the dark regions seem to correspond to lamellae with a closer to flat-on orientation. The effect of polymer microstructure on the spherulitic morphology is illustrated in the images shown in Fig. 4. It is clearly seen that the spherulite formed from the stereoblock RS-PPO at the same temperature of crystallization at which the chirally pure polymer exhibits banding (Fig. 3), does not display a regular array of concentric, banded pattern. The height and phase images of higher magnification, respectively, afford a more detailed look at the lamellar organization within the racemic spherulites (Fig. 4B and C). The lamellae seem to possess only short-range order which is visible as bunches of edge-on seen lamellar crystals distributed in a matrix consisting of predominantly closerto flat-on crystals. Only at small undercooling, the morphology of the RS-PPO shows sign of longer range directional lamellar order as it is illustrated in the image of Fig. 5, and also seen in the optical micrograph of Fig. 2C. The size of these regions should depend on the extent to which chains of opposite chirality are excluded from the growing crystals. At high growth rates, i.e. observed at the lower crystallization temperatures such as 55 °C, the exclusion of chains of opposite chirality may be incomplete. The concentration of antipole enantiomeric chains at the growing front would hamper a regular lamellar twisting, and even could, in certain cases, represent points of twist reversal [8]. This behavior is anticipated to yield spherulites with a coarse structure of the type seen in Fig. 4A. Similar optical microscopy results for PPO were published by Magill [19]. Two different types of spherulites, coarse and banded, were described for the
Fig. 5. AFM height image of RS-PPO spherulite isothermally crystallized at 60 °C for 2 days.
racemic RS-PPO stereoblock. The crystal lattice of PPO has been determined to be P21 21 21 [25], which corresponds to a structure in which the unit cell is occupied by polymer chains of the same chirality, i.e. no stereocomplexes are present [26]. The extent to which chains of opposite chirality could be accepted in the crystal without catastrophically disturbing the lattice remains unknown. The observation of different crystal orientations in zones corresponding to the concentric bands raises the question of the possibility of twisting at the lamellar level. Indeed, OM studies of PPO have assigned the observed extinction bands present in banded spherulites to a twisting orientation of the crystals along the radial growth direction [9,10]. In our case, the complex morphology of spherulitic aggregates was difficult to
898
L.G.M. Beekmans et al. / European Polymer Journal 40 (2004) 893–903
examine by AFM. The main reason is that following a particular lamellar crystal through a series of flat and edge-on orientations is confusing since the growth direction of the radiating crystal does not remain parallel to the viewing direction. Because of this complex spherulitic morphology, our examination was focused on isolated lamellar PPO crystals grown in the melt. These crystals provide the basis for the lamellar packing and arrangement in the PPO spherulites. Such simpler crystalline morphologies can be obtained under the slower crystallization rates, which occur at lower degrees of undercooling. 3.2. Lamellar single crystals Recently, Saracovan et al. obtained single crystals of PPO grown from solution [12]. These crystals exhibited a lenticular shape with {2 0 0} growth faces as schematically shown in Fig. 6A. The main characteristics of the solution-grown crystals were the curved lateral habit on the growth faces extending towards the apexes of the lamellae under an acute angle. The single crystals are reported to possess a planar, three-dimensional shape and a high axial ratio. The axial ratio is defined as the ratio between the length along the b-axis and that along the a-axis. The lamellar morphology of melt-grown single crystals of RS-PPO is shown in the height and phase images reproduced in Fig. 7A and B, respectively, which correspond to flat-on views. The height image displays poorer lateral resolution but allows for a characterization of the actual height. It can be seen that the crystal is growing parallel to the melt surface. The basic growth habit of the lamellar crystals is of the lozenge-shape type
Fig. 6. Schematic representation of PPO single crystals. (A) Lenticular crystals observed in solution crystallization. (B) Truncated-lozenge crystals as obtained here. The c-axis is normal to the plane of the Figure. The lateral growth faces and sector lines are indicated.
with pronounced roundness on presumably the {2 0 0} growth faces. A complete schematic reconstruction of the crystal shape is shown in Fig. 6B, which is superimposed in the image of Fig. 7A. The crystal shows clear evidence of flat growth faces at the growing tip in the (0 1 0) direction with an obtuse angle, a feature that is not observed for the solution-grown crystals. Identification of these growth faces was accomplished by analyzing the apex angle (a), as defined in Fig. 6B. The value of a was measured from truncated crystals with straight faces in the b-axis direction. An average value of 66 ± 2° was estimated between the growth faces. Additional confirmation for this interpretation was gained by analyzing the growth rates along different crystallographic directions. The growth rate in the direction of the crystallographic b-axis of a single crystal is determined by the growth rate of the growth face (G ¼ Gb sin a) [27]. On the basis of the orthorhombic unit cell for PPO [25], the lozenge-shaped crystals are considered to occur with {1 1 0} (a ¼ 66°) as the growth faces border the tips along the (0 1 0) axes. Growth rates for the two basic crystallographic directions of the meltgrown RS-PPO crystals are estimated from the initial series of phase images shown in Fig. 7 and are plotted in Fig. 8. From the slope of the length versus time data, linear growth rates are estimated to be 27 and 25 nm/ min along the b-axis and for the {1 1 0} growth face, respectively. These measurements resulted in an apex angle of 68 ± 4°. Furthermore, the initial linear growth rate along the (1 0 0) direction, Gf200g , was measured to be 6 nm/min (Fig. 8). These results indicate that the crystal growth directions are highly anisotropic in the PPO lamellar single crystals when grown from the melt, which implies a noticeable influence of kinetic factors. The {1 1 0} growth face grows about 4.6 times faster than the {2 0 0} faces. A dominant feature that is observed in the phase images of Fig. 7 is the preferential orientation of ridges at the surface of the crystal. Different alignment directions are observed in agreement with the sectorization of the crystals. Three sector lines, dividing the crystal into three pairs of sectors are seen, as schematically drawn in Fig. 7A. The ridges seem to consist of elongated features of 76 ± 20 nm in width and are oriented with their long axis normal to the growth face of the corresponding sector in the crystal. Similar surface decoration patterns are normally obtained by the widely known polymer decoration method based on the condensation–crystallization of PE upon the fold surface of the lamellar crystal [28]. In such cases, the PE molecular fragments are deposited as rod-like crystals, which align on the fold surface. Hence, the ridges observed in the present case could be interpreted as a ‘‘soft decoration mechanism’’. Note that the basal crystal surface of the individual RS-PPO crystal shows overgrowth by lamellae later in time (Fig. 7B–F) which seems to result from a frac-
L.G.M. Beekmans et al. / European Polymer Journal 40 (2004) 893–903
899
Fig. 7. (A–F) AFM tapping mode phase and height images of an RS-PPO single crystal growing at a temperature of 66 °C. (A) and (B) the simultaneously recorded height and phase image. The successive phase images in (C), (D), (E) and (F) correspond to elapsed times of 24, 38, 78, and 132 min, respectively, with respect to the images in (A) and (B).
900
L.G.M. Beekmans et al. / European Polymer Journal 40 (2004) 893–903
3.3. Three-dimensional morphology
Fig. 8. Growth rates as measured at 66 °C for different crystallographic directions as indicated.
tionation/segregation process during crystal growth. It has been suggested that the longer, more regular polymer chains tend to be located in the first-forming dominant lamellae and that the excluded chains, which have less chemical regularity, will crystallize within this framework [29]. Evidence for the occurrence of such a fractionation/segregation process on the basis of chain perfection selection during crystallization was sought for by analyzing the growth rates of these subsidiary lamellae. Linear growth rates of 6 and 1.3 nm/min were estimated from the slope of the length versus time data plots for the crystallographic b and a directions, respectively. The reason for this decrease in growth rates as compared to the dominant crystal is associated with the additional thermodynamic penalty that results from the incorporation or rejection of chain defects into the crystal lattice. Although it is to be expected that the fractionation process is influenced by the presence of the melt/air interface in our experiments, the growth kinetics of the individual lamellae seems to suggest that the RS-PPO crystal aggregates consist of lamellar crystals differing in the perfection of the chains they are made of. The relative decrease in growth rate for the different growth faces is of similar magnitude resulting in an axial ratio of 4.6 which is identical to that found for the genuine lamella. Their basic growth habit and orientation is a continuation of the underlying crystal. It is of interest to note that the lateral profile of the melt-grown PPO crystals displays dissimilarities from the PPO crystals grown from solution. Similar differences have been reported for PE crystallization studies and seem to reflect the effect of the crystallization temperature on the crystal structure [27,30]. The lenticularshaped PE single crystals were obtained at low degrees of undercooling while at higher undercooling predominantly truncated, lozenge-shaped crystals were observed. Although the chain of PE does not possess chirality, spherulites may also exhibit a banded texture. For PE, it was concluded that there must be a definite association between the truncated-lozenge shape of the crystals and the banding of the spherulitic structures [4,30]. A similar interrelation can be thought to happen in the case of PPO under study.
Having identified the lamellar habit of PPO crystals grown from the melt, our study was then directed to characterize the three-dimensional morphology of the PPO lamellar crystals. The stereocopolymer RS-PPO was selected for this study since it is also able to crystallize from the melt, forming banded spherulites, at low degrees of undercooling, where the growth kinetics are accessible by hot-stage AFM [23]. Although the initial observations suggest that the RS-PPO crystals are planar (t ¼ 0 min, Fig. 7A and B), the sequence of images taken at increasing crystallization times indicates a tendency of the crystal to bend. Indeed a careful inspection of the lamellar growth faces reveals asymmetrical bending along the different crystallographic axes. This bending is also indicated by the apparent slowing down of growth rates of the {2 0 0} and {1 1 0} faces occurring at higher crystallization times due to projection effects [17,18,31]. The crystal observed in Fig. 7 seems to develop a curved cross-sectional profile when viewed down the baxis. However, this lamellar profile is best determined when the crystal is viewed along its long axis, that is, when the lamellae are oriented with the a–c plane lying at the surface. The series of images displayed in Fig. 9 contains such views as could be estimated from the growth rates of the edge-on seen crystals. The lamellar crystal growth rate as observed at the surface can be used to determine this tilt angle (/) of the crystals with respect to the crystallographic a-axis. The relationship between observed rate and crystal orientation: 0° < / < 76°, G ¼ Gf200g = cos c and 76° < / < 90°, G ¼ Gf110g = sinð24 þ /Þ where / is the tilt angle of the crystal as defined in Fig. 6B. When / ¼ 76°, the captured edge corresponds to the sector line between the {2 0 0} and {1 1 0} fold sectors. So if / < 76:1°, the {2 0 0} faces are observed at the surface. Average growth rates of 16 and 13 nm/min are estimated for the lamellar crystals below and above the center of the crystal aggregates as indicated by the arrows in Fig. 9A, resulting in corresponding / angles of 68° and 63°, respectively. Since / < 76:1°, it seems that the crystal aggregate is observed with its {2 0 0} growth faces at the surface of observation. It should be noted that the crystal, as shown in Fig. 9, is not viewed exactly down the b-axis (G 6¼ Gf200g ). From this cross-sectional view of the crystal it can be seen that the lamellae in melt-crystallized RS-PPO develop an S-shaped nonplanar habit when viewed along the radial direction within a spherulite. A schematic representation of the crystal habit of this edge-on view is added in the image of Fig. 9B. S-shape profiles have been reported previously for polyethylene (PE) [32] and poly(vinylidene fluoride) (PVF) [6,33] crystals grown from the melt, and more
L.G.M. Beekmans et al. / European Polymer Journal 40 (2004) 893–903
901
Fig. 9. (A–D) AFM tapping mode phase images of RS-PPO lamellar crystals growing at a temperature of 66 °C. The successive images in (B), (C) and (D) correspond to elapsed times of 23, 45, and 68 min, respectively, with respect to the image in (A).
recently for the lamellar structures comprising hedritic aggregates of solution-crystallized chiral poly((S)-epichlorohydrin) at high undercooling [12]. Lustiger et al. proposed that the observation of this non-planar crystal habit along the axial axis is related to a twisting lamellar orientation within spherulites [24] and is accompanied by asymmetrical development along the radial crystallographic direction (b-axis). Indeed, the lamellar crystal in Fig. 7 seems to develop an asymmetrical curvature along the radial b-axis. The initial planar {1 1 0} sectors develop different degrees of curvature. It can be seen that the upper initial planar {1 1 0} growth faces have continued growth into the melt as indicated by the black arrows in Fig. 7C and D. It curves away from the surface while the lower {1 1 0}
continues to grow parallel to the images’ surface. This { 1 1 0} sector develops a non-planar habit later in time by again curving away from the surface as indicated by the white arrows in Fig. 7E and F. The degree of bending differs in magnitude for the opposing {1 1 0} fold sectors and this asymmetry would eventually result in a left-handed twisting behavior of the crystal on a lamellar crystal level. The image obtained later in time in Fig. 7 shown in Fig. 10, suggests that the asymmetrical curvature eventually will result in a change in orientation of the crystals (black arrows in Figs. 7F and 10). These subsidiary lamellae, which are a crystallographic continuation of the underlying dominant crystal, posses a closer to edge-on view. This indicates a tendency of lamellae to bend asymmetrically which will ultimately
902
L.G.M. Beekmans et al. / European Polymer Journal 40 (2004) 893–903
in crystal orientation at the lamellar level. In contrast to the planar lenticular morphology observed in solutiongrown crystals, the single crystals in the melt are truncated-lozenges with a complex non-planar habit. An understanding of the PPO single crystal shape was developed by evaluating the growth rates and geometry of the crystals formed. The single crystals possessed planar {1 1 0} growth faces in the radial direction and curved {2 0 0} in the axial direction of the PPO spherulites. In addition, distinct boundaries between the {2 0 0} and {1 1 0} growth faces were observed during AFM imaging of the crystal growth process. By monitoring the three-dimensional morphology development in the melt, it was established that the RSPPO single crystals developed a non-planar S-shaped habit along the axial direction in the spherulite. Along the radial direction, the opposing {1 1 0} fold sectors developed different degrees of bending, which eventually will result in a twisting orientation of the individual crystals. The non-planarity of the PPO single crystals seems to suggest that the chirality of the polymer chain is transferred to the higher structural lamellar level.
Acknowledgements L.G.M. Beekmans would like to thank the Dutch Foundation for Chemical Research (NWO-CW) for a graduate research fellowship. The authors are grateful to Prof. S. Mu~ noz-Guerra (Universitat Politecnica de Catalunya) and Prof. Toda (Hiroshima University) for helpful discussions and revision of the manuscript.
References Fig. 10. (A,B) AFM tapping mode phase images of an RS-PPO single crystal growing at a temperature of 66 °C. The images in (A) and (B) correspond to elapsed times of 115 and 198 min, respectively, with respect to the image in Fig. 7A.
result in a twisting orientation of the crystal on a lamellar level. This behavior suggests that the chiral character of the polymer chains can be transferred to the higher levels of structural hierarchy when the crystallization conditions more closely resemble the conditions under which the polymer eventually develops a banded spherulitic structure.
4. Conclusions The present morphological study of the optically pure and stereoblock PPOs has shown that the concentric bands within spherulites can be assigned to changes
[1] Keller A. J Polym Sci 1959;39:151–73. [2] Bassett DC. Philos Trans Royal Soc London A 1994;348: 29–43. [3] Bassett DC, Olley RH, Al Raheil IAM. Polymer 1988;29: 1539–43. [4] Keith HD, Padden Jr FJ. Polymer 1984;25:28–42. [5] Keith HD, Padden Jr FJ. Macromolecules 1996;29:776– 7786. [6] Toda A, Arita T, Hikosaka M. Polymer 2001;42:2223–33. [7] Keith HD. Polymer 2001;42:9987–93. [8] Singfield KL, Klass JM, Brown GR. Macromolecules 1995;28:8006–15. [9] Saracovan I, Keith HD, Manley RSJ, Brown GR. Macromolecules 1999;32:8918–22. [10] Singfield KL, Hobbs JK, Keller A. J Cryst Growth 1998; 183:683–9. [11] Birley C, Briddon J, Sykes KE, Barker PA, Organ SJ, Barham PJ. J Mater Sci 1995;30:633–8. [12] Saracovan I, Cox JK, Revol J-F, Manley RSJ, Brown GR. Macromolecules 1999;32:717–25. [13] Iwata T, Doi Y. Macromolecules 2000;33:5559–65.
L.G.M. Beekmans et al. / European Polymer Journal 40 (2004) 893–903 [14] Li CY, Ge JJ, Bai F, Calhoun BH, Harris FW, Cheng SZD, et al. Macromolecules 2001;34:3634–41. [15] Li CY, Yan DH, Cheng SZD, Bai F, He TB, Chien LC, et al. Macromolecules 1999;32:524–7. [16] Pearce R, Vancso GJ. Macromolecules 1997;30:5843–8. [17] Hobbs JK, McMaster TJ, Miles MJ, Barham PJ. Polymer 1998;39:2437–46. [18] Beekmans LGM, Vancso GJ. Polymer 2000;41:8975–81. [19] Magill JH. Makromol Chem 1965;86:283–8. [20] Price CC, Osgan M. J Am Chem Soc 1956;78:4787. [21] Schilling FC, Tonelli AE. Macromolecules 1986;19:1337. [22] Beekmans LGM, Vancso GJ. Eur Polym J, in press. [23] Beekmans LGM, vanderMeer DW, Vancso GJ. Polymer 2002;43:1887–95. [24] Lustiger A, Lotz B, Duff TS. J Polym Sci: Polym Phys 1989;27:561–79.
903
[25] Cesari M, Perego G, Marconi W. Makromol Chem 1966; 94:194–204. [26] Okihara T, Tsuji M, Kawaguchi A, Katayama K, Tsuji H, Hyon SH, et al. J Macromol Sci: Phys 1991;B30:119–40. [27] Toda A. Colloid Polym Sci 1992;270:667–81. [28] Wittmann JC, Lotz B. Polymer 1985;30:27–34. [29] Vaughan AS, Bassett DC. Crystallization and morphology. In: Booth C, Price C, editors. Comprehensive polymer science: polymer properties, vol. 2. Oxford: Pergamon; 1989. p. 415–57. [30] Toda A, Keller A. Colloid Polym Sci 1993;271:328–42. [31] Beekmans LGM, Vallee R, Vancso GJ. Macromolecules 2002;35:9383–90. [32] Bassett DC, Hodge AM. Proc Royal Soc London A 1981; 377:25–37. [33] Vaughan AS. J Mater Sci 1993;28:1805–13.
Morphological development of melt crystallized poly(propylene oxide) by in situ AFM: formation of banded spherulites L.G.M. Beekmans, M.A. Hempenius, G.J. Vancso
*
Materials Science and Technology of Polymers, MESAþ Institute for Nanotechnology, and Faculty of Science and Technology, University of Twente, NL 7500 Enschede, The Netherlands MESAþ Research Institute, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands Received 15 December 2003; received in revised form 20 January 2004; accepted 21 January 2004
Abstract The morphology of poly(propylene oxide) (PPO) crystals grown from the melt was investigated. The spherulites of the optically pure S polymers displayed a regular pattern of concentric rings as observed by polarizing optical microscopy, while the stereocopolymer developed irregularly banded, or non-banded spherulites depending on the degree of undercooling. The organization of the lamellar crystals within the spherulites was examined by means of atomic force microscopy (AFM). For all cases, the lamellar structures appeared to adopt an alternating flat or edge-on orientation. Examination of the morphology of single crystals in the melt of the stereocopolymer revealed truncatedlozenge crystals, which were elongated in shape. Results from crystallization kinetics, obtained by in situ AFM observations, showed that the elongated habit is related to differences in the growth rates of the {2 0 0} and {1 1 0} facets. Interestingly, the melt-grown RS-PPO crystals developed a curved asymmetrical three-dimensional shape. Based on these observations it can be proposed that the chiral nature of the chain is transmitted to higher structural levels of ordering in the crystal aggregates. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Atomic force microscopy; In situ crystallization; Polymer morphology; Banded spherulites; Chirality; Poly(propylene oxide)
1. Introduction Spherulitic textures are morphological features commonly observed in semi-crystalline polymers. The generation of a spherulite starts from a precursor structure, which quite often consists of an incipient lamellar crystal. This precursor then develops into a spherically symmetrical structure. In certain polymers, spherulites exhibit a concentric pattern of periodic banding when examined by polarized optical microscopy (OM). Twisting of the orientation of the lamellar
*
Corresponding author. Tel.: +31-53-489-2974/2967; fax: +31-53-489-3823. E-mail address: [email protected] (G.J. Vancso).
crystals in the radial direction, and hence the refractive index tensor elements, generates these bands with the bandwidth increasing with temperature [1]. Two possible models have been postulated to describe the mechanism that leads to this lamellar twisting organization within spherulites. These models, which share some common elements, have been put forward by Bassett and coworkers [2,3] and by Keith and Padden [4,5], and their descriptions have been recently reviewed [6,7]. In essence, lamellar twisting is considered to be the result of the non-orthogonal relation between the crystal faces and the polymer chain axis. According to Bassett and coworkers [2,3], the handedness of screw dislocations in multilayer crystals is linked to the direction of chain tilt in the lamella. At isochiral screw dislocations adjacent lamellae will splay apart, presumably due to
0014-3057/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2004.01.025
894
L.G.M. Beekmans et al. / European Polymer Journal 40 (2004) 893–903
cilia pressure, and hence initiate twisting of the spherulitic building blocks. Another possibility, as proposed in the second model, is that chain tilting will create unequal stresses at opposing fold surfaces, which ultimately results in the twisting of the lamellae [4,5]. For chiral polymers, the possibility that the handedness of lamellar twisting is related to the chirality of the polymer chain has been put forward. Indeed, direct experimental evidence from atomic force microscopy (AFM) by Singfield et al. [8] of melt-crystallized chiral poly(epichlorohydrin) (PECH) spherulites has shown that a chirality-twisting relationship may exist for this particular case. The correlation between the handedness of lamellar twisting in the banded spherulite and chain chirality has been also reported from tilting studies under OM [9,10]. Detailed examination of the three-dimensional morphology of lamellar crystals provides important information about the mechanism by which the chirality of the chain is transmitted to higher structural levels. Direct observation of lamellar crystal habit has been achieved by electron microscopy of solution-grown single crystals [11–13]. Whereas the above mentioned spherulitic melt-crystallization studies [4,5,8–10] proposed that a correlation exists between the sense of twisting and chain chirality, the study of solution-grown crystals of chiral polymers did not give definite support to this origin of twisting at the lamellar level [11–13]. Lamellar single crystals of optically active polymers such as poly(hydroxybutyrate) (PHB) [11], PECH, poly(propylene oxide) (PPO) [12] and poly(hydroxyvalerate) (PHV) [12,13] grown from dilute solution are flat in shape and display no helical morphology. These results indicate that in this particular case chain chirality has not been transferred to the lamellar morphology. On the other hand, twisted-ribbon-like single crystals have been obtained from a liquid-crystalline chirally pure polymer although planar single crystals were also grown in the melt [14]. At higher temperatures of crystallization the planar single crystals become more dominant, indicating that the influence of chirality on the morphology can be suppressed by decreasing the undercooling [15]. In order to shed further light onto these controversies, a new study was initiated to study the morphological habit at the lamellar level in the melt. The aim has been to examine the morphology of lamellar crystals in a state that more closely approaches the complexity within the lamellar organization of melt-crystallized spherulites. This approach was expected to reveal the possible existence of configurational factors influencing the structure at the lamellar level. This study, therefore, reports the results of a detailed investigation of the morphology of melt-crystallized lamellar crystals of racemic poly((R,S)-polypropylene oxide) (RS-PPO), linking spherulitic to single crystals. The most important argument for choosing racemic RS-PPO for this study was its ability to crystallize from the melt as banded
spherulites at relative low undercooling like chiral pure S- or R-PPO. It is anticipated that such an AFM crystallization study at lamellar resolution shall provide new insights into the lamellar arrangement in this chiral polymer yielding results of the morphological development in real time [16–18], which would enable us to test the applicability of the above-mentioned models. The crystal morphology and crystallization kinetics of PPO have not received much attention since the discovery of RS-PPO in 1955. Magill et al. studied the meltcrystallization of the stereocopolymer RS-PPO and of optically pure S-PPO by optical microscopy [19]. Solution-grown PPO single crystals were reported recently [12]. The lenticular crystals were planar and showed curved {2 0 0} facets, which extended towards the apex under an acute angle. 2. Experimental (R,S) and (S)-propylene oxide were obtained from Aldrich and used after purification by vacuum distillation from calcium hydride. The polymerization was carried out according to the method of Price and Osgan [20]. The crude product was fractionated by crystallization from an acetone solution at 0 °C (1 g/200 ml). The characteristics of the samples used in this work are given in Table 1. 13 C-NMR spectra were recorded on a Bruker AC 250 spectrometer at 250.1 MHz. Deuterated chloroform was used as solvent without an internal standard. Fig. 1A shows expanded 13 C-NMR spectra of the methyl region of both RS-PPO and S-PPO. The methyl side-carbons display chemical NMR sensitivity according to the configuration of the b-carbon in the polymer chain. The peaks at d ¼ 18:39 and 18.13 ppm are assigned to region defects (head-to-tail defects). The signals at d ¼ 18:05, 17.93 and 17.86 ppm, only observed in the racemic samples were assigned to stereo defects (RS defects) of RS-PPO which amount to no more than 1.5% of the polymer chain [21]. A Perkin–Elmer calorimeter Pyris 1, used for DSC measurements, was calibrated with gallium–indium prior to the experiments. All DSC scans were recorded at a heating rate of 10 °C/min, and the sample chamber was kept under a constant flow of nitrogen. Isothermal crystallization experiments were performed by quenching the samples from the melt at 100 °C to a selected Table 1 Characteristics of the PPO samples used in this study
RS-PPO S-PPO
Fraction
Stereo defects [%]
Regio defects [%]
3 2
1.5 –
1.7 4.5
Defects were estimated from the
13
C-NMR spectra.
L.G.M. Beekmans et al. / European Polymer Journal 40 (2004) 893–903
Fig. 1. (A) 13 C-NMR spectra of the methyl region of RS-PPO and S-PPO. (B) DSC melting thermograms of S-PPO, R-PPO and RS-PPO isothermally crystallized at 45 °C for 1 h, subsequently quenched to 30 °C and heated.
crystallization temperature of 45 °C. When crystallization was complete, the samples were quickly cooled to 30 °C and the melting thermograms were recorded. Fig. 1B contains representative DSC heating traces of the optically pure S-PPO and the RS-PPO, respectively. Under these conditions all the polymers exhibit endotherms with three melting peaks, suggesting multiple degrees of crystal size, perfection, and reorganization. However the positions of these melting peak maxima
895
coincide for all polymer samples, indicating that the samples produced crystals of similar thermal stability with a wide range of melting points. Optical microscope observations were carried out on an Olympus BX60 polarizing microscope equipped with a Mettler FP80 hot-stage. The samples were prepared by first pressing the polymer between two glass cover slides at 100 °C, were the polymer was first melted and then cooled to the temperature of crystallization within approximately 1 min. The spherulites were photographed between crossed polarizers. For the AFM experiments the glass cover slides were taken out from the Mettler hot-stage and the top cover slide was removed, leaving a polymer film of an estimated thickness of 30–80 lm. The experiments were performed under a nitrogen atmosphere using a NanoScope III setup (Digital Instruments). The instrument was equipped with a J-scanner (maximum scan size 100 lm2 ). Commercially available Si-cantilevers were used with force constants of 27–83 N/m as stated by the manufacturer (Nanoprobes). Measurements were carried out in the tapping mode, using always the smallest possible set-point to minimize interaction forces between the tip and sample [22]. Height and phase images were acquired simultaneously. The heating stage used for AFM observations was a variation of a previously published design [23].
3. Results and discussion 3.1. Spherulitic morphology The two PPO fractions crystallized from the melt formed spherulites with the characteristic Maltese extinction cross as it was clearly visible under the polarizing optical microscope. The spherulites were
Fig. 2. (A–C) Optical micrographs of (A) S-PPO, (B) R-PPO and (C) RS-PPO isothermally crystallized at 60 °C for 2 days.
896
L.G.M. Beekmans et al. / European Polymer Journal 40 (2004) 893–903
obtained by isothermal crystallization at 60 °C, which implies an undercooling of about 20 °C. In addition, the spherulites of the optically active S-PPO exhibited periodic birefringent extinction bands. Conversely, the racemic RS-PPO displays irregular banded spherulites with weak signs of extinction effects. The polarized light photomicrographs in Fig. 2 illustrate these features. The presence of banding in polymer spherulites is generally ascribed to a twisting of crystallographic orientation along the radial direction of the spherulite. It apparently reflects a cooperative twisting of radiating lamellae and implies a degree of coordination in the packing of the lamellae during crystallization [1,5]. It is known that the banding pattern of the spherulites is even more apparent when the sample is crystal-
lized with a free surface [1,8,24]. The topological images of a corresponding surface obtained by AFM, as can be seen in Fig. 3, show a periodic pattern of ridges and valleys. The micrograph in Fig. 3A is a low magnification AFM height image of a spherulite of the S-PPO grown at 55 °C. The periodicity of the surface contour corresponds to the birefringent extinction rings observed by OM. Higher magnification height and phase images of the upper right side of this S-PPO spherulite are presented in Fig. 3B and C, respectively. The height image still reveals clear radial bands separated from one another by thinner, dark regions (i.e. lower height). The picture in Fig. 3D shows an even more detailed image of the spherulite corresponding to the zone as indicated by the added box in (B) and (C). In this image the indi-
Fig. 3. (A–D) AFM images of a S-PPO spherulite isothermally crystallized at 55 °C for 1 day. The images were obtained at room temperature. (A) Is a low magnification height image, while (B) and (C) are high magnification height and phase images, respectively, of the zone arrowed in (A). (D) Phase image at higher magnification from the zone framed in (B) and (C).
L.G.M. Beekmans et al. / European Polymer Journal 40 (2004) 893–903
897
Fig. 4. (A–C) AFM images of RS-PPO spherulite isothermally crystallized at 55 °C for 1 day. The images are obtained at room temperature. (A) Is a low magnification height image, while (B) and (C) are high magnification height and phase images, respectively, of the zone arrowed in (A).
vidual lamellar crystals are clearly resolved. It can be seen on this phase image that the elevated regions in the height image are regions in which the lamellar crystals are oriented edge-on while the dark regions seem to correspond to lamellae with a closer to flat-on orientation. The effect of polymer microstructure on the spherulitic morphology is illustrated in the images shown in Fig. 4. It is clearly seen that the spherulite formed from the stereoblock RS-PPO at the same temperature of crystallization at which the chirally pure polymer exhibits banding (Fig. 3), does not display a regular array of concentric, banded pattern. The height and phase images of higher magnification, respectively, afford a more detailed look at the lamellar organization within the racemic spherulites (Fig. 4B and C). The lamellae seem to possess only short-range order which is visible as bunches of edge-on seen lamellar crystals distributed in a matrix consisting of predominantly closerto flat-on crystals. Only at small undercooling, the morphology of the RS-PPO shows sign of longer range directional lamellar order as it is illustrated in the image of Fig. 5, and also seen in the optical micrograph of Fig. 2C. The size of these regions should depend on the extent to which chains of opposite chirality are excluded from the growing crystals. At high growth rates, i.e. observed at the lower crystallization temperatures such as 55 °C, the exclusion of chains of opposite chirality may be incomplete. The concentration of antipole enantiomeric chains at the growing front would hamper a regular lamellar twisting, and even could, in certain cases, represent points of twist reversal [8]. This behavior is anticipated to yield spherulites with a coarse structure of the type seen in Fig. 4A. Similar optical microscopy results for PPO were published by Magill [19]. Two different types of spherulites, coarse and banded, were described for the
Fig. 5. AFM height image of RS-PPO spherulite isothermally crystallized at 60 °C for 2 days.
racemic RS-PPO stereoblock. The crystal lattice of PPO has been determined to be P21 21 21 [25], which corresponds to a structure in which the unit cell is occupied by polymer chains of the same chirality, i.e. no stereocomplexes are present [26]. The extent to which chains of opposite chirality could be accepted in the crystal without catastrophically disturbing the lattice remains unknown. The observation of different crystal orientations in zones corresponding to the concentric bands raises the question of the possibility of twisting at the lamellar level. Indeed, OM studies of PPO have assigned the observed extinction bands present in banded spherulites to a twisting orientation of the crystals along the radial growth direction [9,10]. In our case, the complex morphology of spherulitic aggregates was difficult to
898
L.G.M. Beekmans et al. / European Polymer Journal 40 (2004) 893–903
examine by AFM. The main reason is that following a particular lamellar crystal through a series of flat and edge-on orientations is confusing since the growth direction of the radiating crystal does not remain parallel to the viewing direction. Because of this complex spherulitic morphology, our examination was focused on isolated lamellar PPO crystals grown in the melt. These crystals provide the basis for the lamellar packing and arrangement in the PPO spherulites. Such simpler crystalline morphologies can be obtained under the slower crystallization rates, which occur at lower degrees of undercooling. 3.2. Lamellar single crystals Recently, Saracovan et al. obtained single crystals of PPO grown from solution [12]. These crystals exhibited a lenticular shape with {2 0 0} growth faces as schematically shown in Fig. 6A. The main characteristics of the solution-grown crystals were the curved lateral habit on the growth faces extending towards the apexes of the lamellae under an acute angle. The single crystals are reported to possess a planar, three-dimensional shape and a high axial ratio. The axial ratio is defined as the ratio between the length along the b-axis and that along the a-axis. The lamellar morphology of melt-grown single crystals of RS-PPO is shown in the height and phase images reproduced in Fig. 7A and B, respectively, which correspond to flat-on views. The height image displays poorer lateral resolution but allows for a characterization of the actual height. It can be seen that the crystal is growing parallel to the melt surface. The basic growth habit of the lamellar crystals is of the lozenge-shape type
Fig. 6. Schematic representation of PPO single crystals. (A) Lenticular crystals observed in solution crystallization. (B) Truncated-lozenge crystals as obtained here. The c-axis is normal to the plane of the Figure. The lateral growth faces and sector lines are indicated.
with pronounced roundness on presumably the {2 0 0} growth faces. A complete schematic reconstruction of the crystal shape is shown in Fig. 6B, which is superimposed in the image of Fig. 7A. The crystal shows clear evidence of flat growth faces at the growing tip in the (0 1 0) direction with an obtuse angle, a feature that is not observed for the solution-grown crystals. Identification of these growth faces was accomplished by analyzing the apex angle (a), as defined in Fig. 6B. The value of a was measured from truncated crystals with straight faces in the b-axis direction. An average value of 66 ± 2° was estimated between the growth faces. Additional confirmation for this interpretation was gained by analyzing the growth rates along different crystallographic directions. The growth rate in the direction of the crystallographic b-axis of a single crystal is determined by the growth rate of the growth face (G ¼ Gb sin a) [27]. On the basis of the orthorhombic unit cell for PPO [25], the lozenge-shaped crystals are considered to occur with {1 1 0} (a ¼ 66°) as the growth faces border the tips along the (0 1 0) axes. Growth rates for the two basic crystallographic directions of the meltgrown RS-PPO crystals are estimated from the initial series of phase images shown in Fig. 7 and are plotted in Fig. 8. From the slope of the length versus time data, linear growth rates are estimated to be 27 and 25 nm/ min along the b-axis and for the {1 1 0} growth face, respectively. These measurements resulted in an apex angle of 68 ± 4°. Furthermore, the initial linear growth rate along the (1 0 0) direction, Gf200g , was measured to be 6 nm/min (Fig. 8). These results indicate that the crystal growth directions are highly anisotropic in the PPO lamellar single crystals when grown from the melt, which implies a noticeable influence of kinetic factors. The {1 1 0} growth face grows about 4.6 times faster than the {2 0 0} faces. A dominant feature that is observed in the phase images of Fig. 7 is the preferential orientation of ridges at the surface of the crystal. Different alignment directions are observed in agreement with the sectorization of the crystals. Three sector lines, dividing the crystal into three pairs of sectors are seen, as schematically drawn in Fig. 7A. The ridges seem to consist of elongated features of 76 ± 20 nm in width and are oriented with their long axis normal to the growth face of the corresponding sector in the crystal. Similar surface decoration patterns are normally obtained by the widely known polymer decoration method based on the condensation–crystallization of PE upon the fold surface of the lamellar crystal [28]. In such cases, the PE molecular fragments are deposited as rod-like crystals, which align on the fold surface. Hence, the ridges observed in the present case could be interpreted as a ‘‘soft decoration mechanism’’. Note that the basal crystal surface of the individual RS-PPO crystal shows overgrowth by lamellae later in time (Fig. 7B–F) which seems to result from a frac-
L.G.M. Beekmans et al. / European Polymer Journal 40 (2004) 893–903
899
Fig. 7. (A–F) AFM tapping mode phase and height images of an RS-PPO single crystal growing at a temperature of 66 °C. (A) and (B) the simultaneously recorded height and phase image. The successive phase images in (C), (D), (E) and (F) correspond to elapsed times of 24, 38, 78, and 132 min, respectively, with respect to the images in (A) and (B).
900
L.G.M. Beekmans et al. / European Polymer Journal 40 (2004) 893–903
3.3. Three-dimensional morphology
Fig. 8. Growth rates as measured at 66 °C for different crystallographic directions as indicated.
tionation/segregation process during crystal growth. It has been suggested that the longer, more regular polymer chains tend to be located in the first-forming dominant lamellae and that the excluded chains, which have less chemical regularity, will crystallize within this framework [29]. Evidence for the occurrence of such a fractionation/segregation process on the basis of chain perfection selection during crystallization was sought for by analyzing the growth rates of these subsidiary lamellae. Linear growth rates of 6 and 1.3 nm/min were estimated from the slope of the length versus time data plots for the crystallographic b and a directions, respectively. The reason for this decrease in growth rates as compared to the dominant crystal is associated with the additional thermodynamic penalty that results from the incorporation or rejection of chain defects into the crystal lattice. Although it is to be expected that the fractionation process is influenced by the presence of the melt/air interface in our experiments, the growth kinetics of the individual lamellae seems to suggest that the RS-PPO crystal aggregates consist of lamellar crystals differing in the perfection of the chains they are made of. The relative decrease in growth rate for the different growth faces is of similar magnitude resulting in an axial ratio of 4.6 which is identical to that found for the genuine lamella. Their basic growth habit and orientation is a continuation of the underlying crystal. It is of interest to note that the lateral profile of the melt-grown PPO crystals displays dissimilarities from the PPO crystals grown from solution. Similar differences have been reported for PE crystallization studies and seem to reflect the effect of the crystallization temperature on the crystal structure [27,30]. The lenticularshaped PE single crystals were obtained at low degrees of undercooling while at higher undercooling predominantly truncated, lozenge-shaped crystals were observed. Although the chain of PE does not possess chirality, spherulites may also exhibit a banded texture. For PE, it was concluded that there must be a definite association between the truncated-lozenge shape of the crystals and the banding of the spherulitic structures [4,30]. A similar interrelation can be thought to happen in the case of PPO under study.
Having identified the lamellar habit of PPO crystals grown from the melt, our study was then directed to characterize the three-dimensional morphology of the PPO lamellar crystals. The stereocopolymer RS-PPO was selected for this study since it is also able to crystallize from the melt, forming banded spherulites, at low degrees of undercooling, where the growth kinetics are accessible by hot-stage AFM [23]. Although the initial observations suggest that the RS-PPO crystals are planar (t ¼ 0 min, Fig. 7A and B), the sequence of images taken at increasing crystallization times indicates a tendency of the crystal to bend. Indeed a careful inspection of the lamellar growth faces reveals asymmetrical bending along the different crystallographic axes. This bending is also indicated by the apparent slowing down of growth rates of the {2 0 0} and {1 1 0} faces occurring at higher crystallization times due to projection effects [17,18,31]. The crystal observed in Fig. 7 seems to develop a curved cross-sectional profile when viewed down the baxis. However, this lamellar profile is best determined when the crystal is viewed along its long axis, that is, when the lamellae are oriented with the a–c plane lying at the surface. The series of images displayed in Fig. 9 contains such views as could be estimated from the growth rates of the edge-on seen crystals. The lamellar crystal growth rate as observed at the surface can be used to determine this tilt angle (/) of the crystals with respect to the crystallographic a-axis. The relationship between observed rate and crystal orientation: 0° < / < 76°, G ¼ Gf200g = cos c and 76° < / < 90°, G ¼ Gf110g = sinð24 þ /Þ where / is the tilt angle of the crystal as defined in Fig. 6B. When / ¼ 76°, the captured edge corresponds to the sector line between the {2 0 0} and {1 1 0} fold sectors. So if / < 76:1°, the {2 0 0} faces are observed at the surface. Average growth rates of 16 and 13 nm/min are estimated for the lamellar crystals below and above the center of the crystal aggregates as indicated by the arrows in Fig. 9A, resulting in corresponding / angles of 68° and 63°, respectively. Since / < 76:1°, it seems that the crystal aggregate is observed with its {2 0 0} growth faces at the surface of observation. It should be noted that the crystal, as shown in Fig. 9, is not viewed exactly down the b-axis (G 6¼ Gf200g ). From this cross-sectional view of the crystal it can be seen that the lamellae in melt-crystallized RS-PPO develop an S-shaped nonplanar habit when viewed along the radial direction within a spherulite. A schematic representation of the crystal habit of this edge-on view is added in the image of Fig. 9B. S-shape profiles have been reported previously for polyethylene (PE) [32] and poly(vinylidene fluoride) (PVF) [6,33] crystals grown from the melt, and more
L.G.M. Beekmans et al. / European Polymer Journal 40 (2004) 893–903
901
Fig. 9. (A–D) AFM tapping mode phase images of RS-PPO lamellar crystals growing at a temperature of 66 °C. The successive images in (B), (C) and (D) correspond to elapsed times of 23, 45, and 68 min, respectively, with respect to the image in (A).
recently for the lamellar structures comprising hedritic aggregates of solution-crystallized chiral poly((S)-epichlorohydrin) at high undercooling [12]. Lustiger et al. proposed that the observation of this non-planar crystal habit along the axial axis is related to a twisting lamellar orientation within spherulites [24] and is accompanied by asymmetrical development along the radial crystallographic direction (b-axis). Indeed, the lamellar crystal in Fig. 7 seems to develop an asymmetrical curvature along the radial b-axis. The initial planar {1 1 0} sectors develop different degrees of curvature. It can be seen that the upper initial planar {1 1 0} growth faces have continued growth into the melt as indicated by the black arrows in Fig. 7C and D. It curves away from the surface while the lower {1 1 0}
continues to grow parallel to the images’ surface. This { 1 1 0} sector develops a non-planar habit later in time by again curving away from the surface as indicated by the white arrows in Fig. 7E and F. The degree of bending differs in magnitude for the opposing {1 1 0} fold sectors and this asymmetry would eventually result in a left-handed twisting behavior of the crystal on a lamellar crystal level. The image obtained later in time in Fig. 7 shown in Fig. 10, suggests that the asymmetrical curvature eventually will result in a change in orientation of the crystals (black arrows in Figs. 7F and 10). These subsidiary lamellae, which are a crystallographic continuation of the underlying dominant crystal, posses a closer to edge-on view. This indicates a tendency of lamellae to bend asymmetrically which will ultimately
902
L.G.M. Beekmans et al. / European Polymer Journal 40 (2004) 893–903
in crystal orientation at the lamellar level. In contrast to the planar lenticular morphology observed in solutiongrown crystals, the single crystals in the melt are truncated-lozenges with a complex non-planar habit. An understanding of the PPO single crystal shape was developed by evaluating the growth rates and geometry of the crystals formed. The single crystals possessed planar {1 1 0} growth faces in the radial direction and curved {2 0 0} in the axial direction of the PPO spherulites. In addition, distinct boundaries between the {2 0 0} and {1 1 0} growth faces were observed during AFM imaging of the crystal growth process. By monitoring the three-dimensional morphology development in the melt, it was established that the RSPPO single crystals developed a non-planar S-shaped habit along the axial direction in the spherulite. Along the radial direction, the opposing {1 1 0} fold sectors developed different degrees of bending, which eventually will result in a twisting orientation of the individual crystals. The non-planarity of the PPO single crystals seems to suggest that the chirality of the polymer chain is transferred to the higher structural lamellar level.
Acknowledgements L.G.M. Beekmans would like to thank the Dutch Foundation for Chemical Research (NWO-CW) for a graduate research fellowship. The authors are grateful to Prof. S. Mu~ noz-Guerra (Universitat Politecnica de Catalunya) and Prof. Toda (Hiroshima University) for helpful discussions and revision of the manuscript.
References Fig. 10. (A,B) AFM tapping mode phase images of an RS-PPO single crystal growing at a temperature of 66 °C. The images in (A) and (B) correspond to elapsed times of 115 and 198 min, respectively, with respect to the image in Fig. 7A.
result in a twisting orientation of the crystal on a lamellar level. This behavior suggests that the chiral character of the polymer chains can be transferred to the higher levels of structural hierarchy when the crystallization conditions more closely resemble the conditions under which the polymer eventually develops a banded spherulitic structure.
4. Conclusions The present morphological study of the optically pure and stereoblock PPOs has shown that the concentric bands within spherulites can be assigned to changes
[1] Keller A. J Polym Sci 1959;39:151–73. [2] Bassett DC. Philos Trans Royal Soc London A 1994;348: 29–43. [3] Bassett DC, Olley RH, Al Raheil IAM. Polymer 1988;29: 1539–43. [4] Keith HD, Padden Jr FJ. Polymer 1984;25:28–42. [5] Keith HD, Padden Jr FJ. Macromolecules 1996;29:776– 7786. [6] Toda A, Arita T, Hikosaka M. Polymer 2001;42:2223–33. [7] Keith HD. Polymer 2001;42:9987–93. [8] Singfield KL, Klass JM, Brown GR. Macromolecules 1995;28:8006–15. [9] Saracovan I, Keith HD, Manley RSJ, Brown GR. Macromolecules 1999;32:8918–22. [10] Singfield KL, Hobbs JK, Keller A. J Cryst Growth 1998; 183:683–9. [11] Birley C, Briddon J, Sykes KE, Barker PA, Organ SJ, Barham PJ. J Mater Sci 1995;30:633–8. [12] Saracovan I, Cox JK, Revol J-F, Manley RSJ, Brown GR. Macromolecules 1999;32:717–25. [13] Iwata T, Doi Y. Macromolecules 2000;33:5559–65.
L.G.M. Beekmans et al. / European Polymer Journal 40 (2004) 893–903 [14] Li CY, Ge JJ, Bai F, Calhoun BH, Harris FW, Cheng SZD, et al. Macromolecules 2001;34:3634–41. [15] Li CY, Yan DH, Cheng SZD, Bai F, He TB, Chien LC, et al. Macromolecules 1999;32:524–7. [16] Pearce R, Vancso GJ. Macromolecules 1997;30:5843–8. [17] Hobbs JK, McMaster TJ, Miles MJ, Barham PJ. Polymer 1998;39:2437–46. [18] Beekmans LGM, Vancso GJ. Polymer 2000;41:8975–81. [19] Magill JH. Makromol Chem 1965;86:283–8. [20] Price CC, Osgan M. J Am Chem Soc 1956;78:4787. [21] Schilling FC, Tonelli AE. Macromolecules 1986;19:1337. [22] Beekmans LGM, Vancso GJ. Eur Polym J, in press. [23] Beekmans LGM, vanderMeer DW, Vancso GJ. Polymer 2002;43:1887–95. [24] Lustiger A, Lotz B, Duff TS. J Polym Sci: Polym Phys 1989;27:561–79.
903
[25] Cesari M, Perego G, Marconi W. Makromol Chem 1966; 94:194–204. [26] Okihara T, Tsuji M, Kawaguchi A, Katayama K, Tsuji H, Hyon SH, et al. J Macromol Sci: Phys 1991;B30:119–40. [27] Toda A. Colloid Polym Sci 1992;270:667–81. [28] Wittmann JC, Lotz B. Polymer 1985;30:27–34. [29] Vaughan AS, Bassett DC. Crystallization and morphology. In: Booth C, Price C, editors. Comprehensive polymer science: polymer properties, vol. 2. Oxford: Pergamon; 1989. p. 415–57. [30] Toda A, Keller A. Colloid Polym Sci 1993;271:328–42. [31] Beekmans LGM, Vallee R, Vancso GJ. Macromolecules 2002;35:9383–90. [32] Bassett DC, Hodge AM. Proc Royal Soc London A 1981; 377:25–37. [33] Vaughan AS. J Mater Sci 1993;28:1805–13.