Dendritic Morphology Composed of Stacked Single Crystals in Poly

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Dendritic Morphology Composed of Stacked Single Crystals in Poly(ethylene succinate) Melt-Crystallized with Poly(p‑vinyl phenol) Hikmatun Ni’mah and Eamor M. Woo* Department of Chemical Engineering, National Cheng Kung University, Tainan, 701-01, Taiwan S Supporting Information *

ABSTRACT: Dendritic morphology with stacked single crystals of poly(ethylene succinate) (PESu) melt-crystallized in the presence of amorphous poly(p-vinyl phenol) (PVPh) were observed and analyzed using optical microscopy (OM), atomic force microscopy (AFM), and transmission electron microscopy (TEM). At higher temperatures, the fluffy dendritic morphology was a transition in melt-crystallized PESu/PVPh from originally compact crystals at lower crystallization temperature. The addition of amorphous and strongly interacting PVPh polymer, confinement in thin films, and high crystallization temperature were considered as the main factors of the crystals transition from compact crystals to dendritic crystals. Single crystals of PESu were proven by AFM in the PESu/PVPh blend especially when confined in films of above 500 nm, melt-crystallized at Tc = 70 °C or above. The electron diffraction pattern of TEM result further confirmed the formation of PESu single crystals. The combination of thin film thickness, high crystallization temperature, and strong interaction between two polymers were believed to be the main factors for melt-crystallized PESu single crystals. In the single crystals with spiral arrays, the PESu chains, though achiral, tend to exhibit paired-up crystal entities resembling “identical twins” but with opposite helical directions. The single-layer thickness is almost equal between the paired-up lozenge lamellae screwing clockwise or counterclockwise. The PESu single crystals in PESu/PVPh blend were melt-grown, and electron diffraction patterns proven in this study are in agreement with the literature for neat PESu single crystals grown in dilute solutions.

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isotactic polystyrene (iPS),18−20 poly(ethylene oxide) (PEO),21 poly(caprolactone),22,23 poly-2-vinylpyridine-poly(ethylene oxide) (P2VP-PEO) block copolymers,24 and poly(ethylene oxide) (PEO)/poly(methyl methacrylate) (PMMA) blends.25−27 All these studies showed that the dendritic crystals were formed in the samples with the film thickness of below 150 nm. Morphological transition from dendritic crystals to faceted crystals occurred as the crystallization temperature increases, indicating that the growth was controlled by a diffusion field.18−21 Dendritic growth has attracted some interest because of its variety of patterns and its relation to the properties of crystalline polymers. Moreover, the mechanisms of dendritic growth remain unclear. Okerberg et al.28 have reported the mechanism of dendrite formation in the PEO/PMMA blends thin films, in which they suggested that “noise” of temperature fluctuations plays an important role for the side branches formation. However, they did not report about the lamellar arrangement in the dendrite crystals formation.

INTRODUCTION Crystalline polymers can crystallize into various kinds of crystalline morphologies depending on the conditions such as the nature of polymers, blending or copolymerization condition, crystallization conditions, and other factors. There are two kinds of crystalline morphologies according to their terminology, such as noncrystallographic branching and crystallographic branching grown crystals. The symmetrically rounded crystal, called Spherulite, is the crystalline morphology whose growth is determined by noncrystallographic branching.1 Dendrite morphology is another crystalline morphology with apparent crystallographic branching.1 Most bulk polymeric materials crystallize in the form of spherulite. High polymers, for example, hydrocarbons such polyethylene, polypropylene,2 polyisobutylene, poly(butene-1),3 iso-poly(4-methylpentene1),4 and polystyrene,5 crystallize to form spherulite. Some polyesters also form spherulite such as poly(ethylene terephthalate),6 poly(heptamethylene terephthalate),7,8 poly(octamethylene terephthalate),9 poly(nonamethylene terephthalate),10−12 poly(R-3-hydroxyvalerate),13 poly(R-3-hydroxybutyrate),14 poly(lactic acid),15 poly(vinylidene fluoride),16 etc. In reverse, dendritic crystallization has been reported for a number of polymers, polymer blends, or copolymers, meltcrystallized in ultrathin films, including polyethylene (PE),17 © XXXX American Chemical Society

Received: September 23, 2013 Revised: December 13, 2013

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single-crystal like PESu lamellae could stack and assemble into dendrites. Formation of dendritic morphology, including the mechanism of lamellar assembly leading to dendritic crystals in spherulites, were the focus of this study. Melt-crystallized single crystals of PESu in the blend, and hierarchical structures from single crystal to lamellae in spherulites, were characterized and confirmed.

Single crystals in polymers were commonly and conventionally obtained from dilute solutions. Many studies in the literature have reported that poly(ethylene succinate) (PESu) single crystals were mostly solution-grown from dilute solutions.29,30 In an earlier study,31 we found that a total of nine different types of crystalline spherulite morphology can be identified in PESu/PVPh blend with amorphous PVPh contents from 10 to 35 wt % and melt-crystallized at Tc = 40−70 °C confined in ultrathin film thickness (ca. 200 nm), and that these multiple types of PESu crystalline morphology are not seen in neat PESu but only occur in PESu/PVPh blend with amorphous PVPh higher than 20 wt %. A few of these multiple spherulites and lamellae therein PESu/PVPh blend are likely associated with a seemingly single-crystal assembly. Gan et al.32,33 reported that neat PESu single crystals could also be obtained by melt-crystallization but from ultrathin films of about 100 nm, at crystallization temperature (Tc) = 85 °C. Recently, our previous study34 also found that the PESu single crystals could be observed in medium thin films (around 800 nm) of polymer blends by melt-crystallization. The shape of PESu single crystals was similar regardless of either origins from solution growth or melt crystallization. More often, meltcrystallization single crystals, however, were identified as lozenge-shape multilayers. The PESu single crystal shape was similar to the single crystal shape of poly(4-hydroxybutyrate) (PHB),35 poly(ethylene glycol) (PEG),36 or poly(L-lactic acid) (PLLA).37−41 The monolayer lozenge-shaped single crystal usually has an average thickness of around 7−8 nm.29,30 Below the lamellar scales, the crystalline lattice structure of the α-form unit cell in PESu was first studied by Fuller and Erickson.42 They proposed a monoclinic unit cell with dimensions of a = 0.905 nm, b = 1.109 nm, c (fiber axis) = 0.832 nm. Another modified crystal structure of PESu was found by Ueda et al.43 They have suggested that the α-form PESu has an orthorhombic unit cell with the lattice parameters of a = 0.760 nm, b = 1.075 nm, c (fiber axis) = 0.833 nm. From the cdimension in the PESu unit cell as reported, a simple calculation indicates that the monolayer lamella thickness of the single crystal contains about 10 crystal lattice units (or ca. 10 monomer units) between the folds. Single crystal morphology of PESu has also been observed in the blend system of PESu/tannic acid (TA).34 The seaweedlike dendritic morphology in PESu/TA (80/20) at Tc = 70 °C was found to be composed of lozenge-shaped single crystals. It was suggested that such single-crystal-like morphology was induced by the strong hydrogen-bonding interactions between PESu and TA.34 The lozenge-shaped single crystals from melt crystallization in PESu/TA blend were equivalent to those from solution-grown single crystals in neat PESu with a single lamella thickness of 7−8 nm.29,30 However, this single crystal morphology induced by melt-crystallization in blends of PESu with a strongly interacting diluent polymer (TA) has not been characterized yet by transmission electron microscopy (TEM)electron diffraction (ED) patterns. Moreover, the lamella direction of single-crystals arrangement in packing into dendritic spherulite morphology has yet to be expounded. This present study probed in greater depth observations of single crystals, and analyses were performed in hierarchical structure scales for more details in PESu blended with a strongly interacting PVPh. In addition to the interesting multiple types of spherulite patterns in PESu crystallized with interacting PVPh as proven in a concurrent study,31 this study further expanded and probed the mechanisms of how the

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EXPERIMENTAL SECTION

Materials and Preparation. Poly(ethylene succinate) was purchased from Scientific Polymer Products (SP2), Inc. (USA) with a Mw of 10 000 g/mol and PDI = 1.90, glass transition temperature (Tg) = −19 °C and melting temperature (Tm) = 102 °C. Poly(p-vinyl phenol) (PVPh) was purchased from Polysciences, Inc. (USA) with Mw = 22 000 g/mol and Tg = 114 °C. All polymer materials were used as received. Samples of PESu/PVPh blend with a composition range from 80/ 20 to 75/25 were prepared by solution-casting using p-dioxane as a common solvent with concentration of 4 wt % (polymers/solvent). A drop of solution of the polymer mixtures was uniformly spin-coated on a glass slide, and the solvent was allowed to fully evaporate in atmosphere. The sample thickness was kept at around above 500 nm. The samples were then crystallized without a top cover (for AFM characterization). Samples of blends were first heated to melt on a hot stage to a maximum melt temperature (Tmax = 130 °C) held for 2 min for erasing the prior crystals or thermal histories and then rapidly replaced to another hot stage preset at a designated isothermal Tc = 40−70 °C. Apparatus and Procedures. An optical microscopy (OM, Nikon Optiphot-2), equipped with a digital camera charge-coupled device (CCD) and a microscopic hot stage (Linkam THMS-600 with TP-92 temperature programmer), was used for analyzing the crystalline morphology. To investigate the spherulites morphology of the blends, the blend samples were held at temperatures above melting temperature (Tmax = 130 °C) for 2 min and then quickly transferred to hot stage equilibrated at the desired crystallization temperatures (Tc) at which the spherulitic morphology of the blends was observed. Atomic-force microscopy (AFM) investigations were made in intermittent tapping mode of AFM (diCaliber, Veeco Co., Santa Barbara, USA) with a silicon-tip ( f = 70 kHz, r = 10 nm). AFM measurements were carried out to observe the crystalline lamellar arrangement and the height profiles of various types of spherulite in PESu/PVPh blend. The largest scan range was 150 μm × 150 μm, but the smallest range could be down to 5 μm × 5 μm for larger magnifications on selected areas of interest. Thin films were deposited on substrates of glass slides, and the top surfaces of the film samples were exposed with no top cover glass. TEM and ED pattern (TEM 1400, JEOL Ltd., Munich, Germany) instruments were used to observe in detail the PESu single crystals morphology and crystal structure. Only the samples, melt-crystallized at Tc = 70 °C, were analyzed. The samples, spin-coated on a glass slide, were immersed in the 1 wt % of hydrofluoro acid solution until the polymer films were separated from the substrate and float on the solution’s surface. These sections were then transferred to 200-mesh Cu grids. The 200-mesh Cu grids, contained polymer films, were then carbon-coated prior to the characterization. TEM image and ED pattern were obtained at an accelerated voltage of 100 kV and camera length of 40 cm, respectively.

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RESULTS AND DISCUSSION Crystalline Morphology and Lamellar Arrangement. Multiple types of crystalline morphology have been found and reported in the thin films of PESu/PVPh blends. In this study, focus was on the detailed analysis of dendrite morphology, and we further sought to expound the mechanisms of lamellae/ crystals arrangement in forming a dendrite morphology. Figure 1 shows the regular well-round spherulite and dendrite B

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spherulite is filled with closely parallel and radiating lamellae with little or no space for developing side branches, while the dendritic morphology (with perpendicularly oriented branches) appears as the temperature increases. The detailed lamellar arrangement of well-round spherulites and dendrite crystals, as preliminarily revealed in the POM characterization, were further analyzed using higher-resolution AFM, as shown in Figure 2. Some of the crystalline

Figure 1. OM images of well-round spherulite and dendrite morphology for PESu/PVPh (80/20) melt-crystallized at different temperatures: (a) 40 °C; (b) 50 °C; (c) 60 °C; (d) 70 °C.

morphologies of PESu/PVPh (80/20) blend in thin films, which had been melt-crystallized at crystallization temperatures (Tc) of 40−70 °C. At low range of Tc = 40 °C, the well-round spherulite crystals in the melt-crystallized blend are in the shape of a sphere (with zigzag peripheral edge) and have no apparent optical birefringence, owing to thin thickness. The spherulite morphology/pattern apparently differs with respect to variation in Tc. However, in this study, we only focused on the singlecrystal-like dendritic morphology for analyses and discussion. When Tc is increased to 50 °C, dendrite crystals start to show up. These dendrite crystals show two dramatically different shapes, which are lozenge-like (diamond-like) and flower-like shapes. These dendrite crystals appear to be a transition form between lozenge-shape and flower-shape crystals at intermediate crystallization temperatures. The detailed observation of the lamellar pattern will be discussed in the next figure for explanation. The dendrite crystals have main stalks and primary side branches, with the branches growing perpendicularly to the main stalks. The secondary side branches further grow perpendicularly (90° orientation) to the primary side branches. When the temperature is increased to 60 °C, the dendrite morphology still appears in PESu/PVPh blend. The morphology of dendrite crystals at this temperature (Tc = 60 °C) is more obvious than that at Tc = 50 °C, and the primary side branches and secondary side branches at 60 °C are more pronounced than those at 50 °C. These dendrite crystals also show two different shapes, which are lozenge-shaped and flower-like dendrite. At an even higher temperature Tc = 70 °C, the crystalline morphology shows big dendrite crystals with longer and more obvious primary side branches and secondary side branches. Similar to the dendrite crystals formed at previous Tc’s, the dendrite morphology at this high temperature is also formed in lozenge-shaped and flower-like shaped patterns. The morphology observation shows that the wellround spherulite morphology exists at lower Tc, and the round

Figure 2. AFM height and phase images of well-round spherulite morphology and dendrite morphology in PESu/PVPh (80/20) blend melt-crystallized at different Tc: (a) 40 °C, (b) 50 °C, (c) 60 °C, (d) 70 °C.

morphologies (well-round spherulite vs dendrite) show a morphological transition as the crystallization temperature and/ or blend composition is changed, which will be explained in the following description. The AFM micrographs clearly show that the well-round spherulites have some wrinkles in most of the area, as displayed in Figure 2a. Height image zoomed-in to the center part, as shown in Figure 2a(1), shows that the center of spherulites also has some small wrinkles. The phase image zoomed-in to the center part shows that the lamellar pattern of the center part is composed of small spherical flat-on lamellae, as displayed in Figure 2a(2). Other parts of the round spherulite crystal are also arranged by small spherical flat-on lamellae. Otherwise, the dendrite morphology has main stalks, primary side branches, and secondary side branches. Tertiary side branches are allowed to form only when there is available space around the increasingly jammed secondary side branches. C

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shows that the IR absorbance peak of carbonyl (CO) shifts to lower wavenumber as the PVPh content increases. The apparent shifting in the carbonyl-stretching peaks in the IR spectra is also distinctly clear evidence for strong interactions between PESu and PVPh supporting the miscibility and blend phase homogeneity. From the observation of morphology and lamellar patterns, this study shows that the high Tc and high PVPh contents are the two most influencing factors for the formation of dendrite morphology, which is arranged by lozenge single crystals. Dendrite morphology always shows a branching pattern. As earlier reported by Okerberg et al.,28 the crystallization temperature plays an important role in the side branch formation. Effect of PVPh has a critical fraction in the PESu/ PVPH blend to develop into such patterns. The addition of PVPh at low concentrations (i.e., 10 wt % PVPh) to PESu does not show the formation of dendrite morphology even at high Tc, as shown in Figure S2, Supporting Information. The presence of amorphous polymer may fill the available space for crystal growth. Therefore, the crystal formed was not a compact well-rounded form but a dendritic pattern. Moreover, the addition of PVPh to PESu caused a dilution effect in the polymer chains. In addition, concentration fluctuations also play an important role for side branch formation.28 Therefore, the addition of amorphous polymer (PVPh), further assisted with high Tc, could dramatically induce the formation of dendritic morphology, which is composed of lozenge single crystals. When the amorphous PVPh content in the PESu/PVPh blend was increased to 25 wt %, the crystalline morphology of the blend also showed multiple spherulite types. The blend composition of PESu/PVPh (75/25), when melt-crystallized at lower Tc, exhibits a seaweed-like morphology. Figure 3 shows

The dendrite morphology appears in two different shapes, which are lozenge-shaped and flower-like-shaped. The lozengeshaped dendrite crystals have four main stalks; by contrast, the flower-like shaped dendrite crystals have more than four main stalks. The number of main stalks radiating from the nucleus center at early stage of growth tends to determine the final shapes of dendritic spherulite. The detail morphology of dendrite crystals at Tc 50 °C is shown in Figure 2b. The height images show some short primary side branches, secondary side branches, and even tertiary side branches. The fast growth of the crystals corresponds to the growth of the branches. The branches in all regions grow much faster so that their growth is impeded by other dendrite’s branches growth, which cause some short-branch formation. At this low temperature, the dendrite growth competes with the growth of other spherulite types. Therefore, at lower Tc, the dendrite crystal cannot grow too big, and the branches are also short. Crystal packing in the PESu/PVPh blend at increasingly higher Tc (greater than 40 °C) was also examined and compared. The phase image in Figure 2b shows that the dendrite is arranged by small flat-on lamellae crystallized at Tc = 50 °C. The size of the lamellae at this temperature is a little bigger than that at Tc = 40 °C. In general, composition or temperature influences the formation and/or patterns of dendrite morphology. At Tc = 60 °C, the morphology of dendrite crystals becomes more obvious. The primary side branches grow perpendicularly to the main stalks, and the secondary side branches grow perpendicularly to the primary side branches. The tapping AFM phase image of the main stalks shows that they are composed of lozenge-like flat-on lamellae, as displayed in Figure 2c(1). The secondary side branches are arranged by mixed patterns of spherical and lozenge-like flat-on lamellae, as seen in Figure 2c(2). It shows that the spherical flat-on lamellae become lozenge-like flat-on lamellae as the growth time and temperature increase. At a high temperature (Tc = 70 °C), the morphology of dendrite crystal becomes more obvious with more pronounced primary side branches and secondary side branches, as shown in Figure 2d. The branches are longer and wider, so that the dendrite crystals are also bigger. The height and phase images of zoomed-in center part show that the lamellar pattern of these crystals is arranged by lozenge single crystal flat-on lamellae, as displayed in Figure 2d(1,2). The lozenge single crystals are apparent in the PESu/ PVPh blend. A literature report34 has earlier revealed that a single crystalline morphology could be formed in the blend of PESu/TA at several specific conditions, which is caused by strong interactions between the two polymers via hydrogen bonding. In this study, PESu is strongly interacting with PVPh via hydrogen bonding between CO groups in PESu and O− H groups in PVPh. A literature report44 indicates that PESu is miscible with PVPh as shown by the existence of single and composition-dependent glass transition temperature over the whole composition range. Figure S1 (Supporting Information) shows FTIR spectra in the PVPh hydroxyl-stretching (a) and PESu carbonyl-stretching region (b) for PESu/PVPh blend of various compositions as indicated. Figure S1a shows two IR absorbance peaks at 3206 cm−1 and 3362 cm−1 for PVPh−OH. Those IR absorbance peaks correspond to self-bounded −OH (intramolecular hydrogen bonding) and free hydroxyl group respectively. With increasing PESu contents in the blend, the intensity of the IR peak of self-bounding −OH decreases, indicating a corresponding increase in strength of intermolecular hydrogen bonding between PVPh and PESu. Figure S1b

Figure 3. OM images of seaweed-like and dendrite morphology for PESu/PVPh (75/25) melt-crystallized at different temperatures: (a) 40 °C; (b) 50 °C; (c) 60 °C; (d) 70 °C.

the crystalline morphology of PESu/PVPh (75/25) blend meltcrystallized at different temperatures. The crystalline morphology of PESu/PVPh (75/25) blend is seen to vary with respect to the crystallization temperature (Tc). A seaweed morphology (with branches of irregular shapes at irregular angles to the four main stalks) is found at Tc = 40 °C, as shown in Figure 3a. This morphology shows some wide branches growing in the radial direction. The previous blend composition with a lower PVPh content [PESu/PVPh (80/20)], at this same low Tc, shows compact crystals. However, the blend compositions with higher PVPh contents [e.g., PESu/PVPh (75/25)] tend to show seaweed crystal with a branching feature. It is clearly seen that D

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the PVPh amorphous content may help in formation of branches in leading to dendritic patterns; similarly, as the crystallization temperature is increased, the crystal shows dendrite morphology. That is, both amorphous PVPh contents and high Tc have similarly equivalent effects on PESu crystals in packing into highly branched dendrites, composed by singlecrystalline building blocks. This dendritic morphology has many branches, which all grow perpendicularly to their parental lamellae. Especially, the branches become more obvious as the Tc increases, as displayed in Figure 3b−d. The collective results are displayed and discussed in Figure 3, which shows clearly that the seaweed-like crystals (irregularly radiating lamellar bundles) are formed at lower Tc (40 °C or lower), but as the Tc increases to 50 °C or higher, the lamellae tend to be packed to form dendrite crystals (with branches at exactly 90° to the arms of main stalks). The detailed lamellar arrangements of seaweed and dendrite crystals, which appear in the PESu/PVPh blend composition of (75/25) as revealed in the OM graphs in the previous figure, were further analyzed using high-resolution atomic-force microscopy (AFM). Figure 4 shows that the seaweed crystals in the PESu/PVPh blend composition of (75/25) have a branched morphology, in which the branches grow in a direction angle lower than 90 degrees (or irregular angles) to the main stalks, as shown in Figure 4a. The lamellar pattern of seaweed crystals is composed of spherical flat-on lamellae, as

shown in Figure 4a(2). The dendrite morphology, found in this composition of PESu/PVPh (75/25), is similar to that in the previous composition of PESu/PVPh (80/20). Again, the dendrites are composed of main stalks, primary side branches, and secondary side branches. Tertiary side branches are possibly formed when there is available space between the secondary side branches. The dendrite morphology in this composition also appears in two different shapes, which are lozenge shaped and flower-like shaped, respectively. The morphology of the dendrite crystals (primary or secondary branches at exactly 90° to main stalks becomes more obvious and the sizes of the lamellae increase with the increasing Tc, as shown in Figure 4b−d. The branches (including primary side branches and secondary side branches) become wider, longer, and clearer, with respect to increasing Tc. The lamellae in spherulites display a pattern from initially spherical flat-on, then small plate flat-on, finally lozenge-shaped flat-on lamellae, as the Tc increases from Tc = 50 °C, 60 °C, to Tc = 70 °C, respectively. At Tc = 70 °C, the dendrite crystals are also composed of lozenge-shaped lamellae. Thus, we can conclude that the PESu lozenge-shaped single crystals are obtained in PESu/PVPh (80/20) and PESu/PVPh (75/25) blends, meltcrystallized at Tc = 70 °C. Lozenge-Shaped Single Crystals by Melt Crystallization. The low-magnification AFM images in previous figures clearly have shown that the lozenge single-crystal lamellae are present in the blend of PESu/PVPh (80/20) and (75/25), when crystallized at Tc = 70 °C. Zoom-in AFM analyses were performed to fully characterize the detailed morphology in larger magnifications. Figure 5 shows AFM observation of

Figure 5. AFM height and phase images of lozenge single crystal formed in PESu/PVPh (80/20) blend melt-crystallized at Tc = 70 °C.

lozenge single crystal formed in PESu/PVPh (80/20) blend, melt-crystallized at Tc = 70 °C, as a representative sample. The height and phase images show that the single crystal morphology display a multilayer lozenge shaped plates, in which two paired crystals screw in clockwise and counterclockwise direction, respectively. These PESu single crystals lamellae grow from screw dislocation forming multilayered lozenge single crystalline morphology. This type of growth mechanism, as induced by screw dislocation, was also observed in several polymers single crystals such as poly(4-hydroxybu-

Figure 4. AFM height and phase images of seaweed-like morphology and dendrite morphology in PESu/PVPh (75/25) blend meltcrystallized at different Tc: (a) 40 °C, (b) 50 °C, (c) 60 °C, (d) 70 °C. E

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tyrate) (P4HB),35 poly(ε-caprolactone) (PCL),36 poly(Llactide acid) (PLLA),39 etc. Since the dendrite crystals are composed of lozenge shaped flat-on lamellae, the dendrite crystals occur as multilayered lozenge shaped crystals lamellae with spiral growth. PESu in this study is an achiral polymer, in which the lamellae were revealed to pack in paired-up twins with opposite screw directions. For chiral polymers, earlier, Iwata and Doi29,30 have concluded that the direction of screw dislocation is determined by the helical handedness of polymer chain instead of the chirality of the polymers such as polylactides. The AFM height profile for PESu shows that one layer plate has thickness of around 8 nm. It has been reported that the thickness of single crystal of PESu is around 7−8 nm.29,30 Therefore, the result of this study is in agreement with the literature in terms of single-crystal lamellae thickness. There is no difference in the thickness between the lozenge lamellae screwing clockwise or counterclockwise (shown by dash line and black line), indicating the paired crystal entities resemble “identical twins” but with opposite helical directions. The detail observation of PESu single crystal will be discussed in the following section. TEM and ED characterization were carried out to prove and analyze the single-crystal lamellae of PESu in the blends. These characterizations were a possible approach for observing these single crystals structures in detail. The PESu single crystals from melt-crystallized thin-film samples of PESu/PVPh (80/20) or PESu/PVPh (75/25) at Tc = 70 °C were prepared. The electron micrographs of lozenge-shaped multilayered crystals of PESu/PVPh (80/20) blend grown at Tc = 70 °C is shown in Figure 6a,b. The PESu single crystals show flat-on lamellae

Table 1. Comparison of Calculated and Measured d-Spacing for Poly(ethylene succinate) index

dcalc (nm)

dobs (nm)

020 200

0.538 0.380

0.539 0.379

PESu single crystals formed in PESu/PVPh (80/20) or PESu/ PVPh (75/25), at Tc = 70 °C, which have the same orthorhombic unit cell as reported for neat PESu earlier by Ueda et al.43 The ED diffraction for PESu did not show (110) diffraction, like that in PLLA single crystals. Thus, the PESu single crystals tend to pack differently from the PLLA single crystals, which are packed with 60° orientation to the main stalks. PESu single crystals display only pronounced (020) and (200) diffraction, which helps to support that packing of single crystals is perpendicular (90°) to boundary planes. In addition, in an earlier study by this lab,34 the single crystal formation was first conceived and demonstrated to be feasible in meltcrystallization conditions (rather than the conventional dilutesolution growth or confinement in ultrathin nanometer films) when PESu interacts with strong H-bonding tannic acid (TA), where the PESu single crystals were characterized only with AFM imaging and height profiling. In this study, we further and more directly advanced to confirm the lattice forms of the single crystals in PESu/PVPh blend with ED patterns, in addition to the evidence of AFM images. Mechanism of Lamellar Arrangement in Dendritic Crystal. Figure 7 shows a proposed mechanism for the branch

Figure 6. Electron micrographs of multilayered lozenge single crystals of PESu/PVPh (80/20) screwing (a) counter-clockwise and (b) clockwise; and (c) electron diffraction pattern of PESu single crystals. Figure 7. (a) AFM images of seaweed-like crystals appearing at low Tc (Tc = 40 °C) of PESu/PVPh (75/25) blend; (b) scheme of branching formation mechanism at low Tc.

morphology, which consist of several lozenge-shaped lamellae, which grow counter clockwise and clockwise from screw dislocation, as seen in Figure 6, panels a and b, respectively. The lozenge-shaped single crystals have long and short axes, as mentioned previously. Figure 6c shows the selected-area electron diffraction diagram obtained from lozenge-shaped multilayered single crystals. The strong reflections represent the smallest dimension in the unit cell of single crystal. The a* and b* axes are along the 200 and 020 plane directions, respectively. Ueda et al.43 have found that the α-form PESu crystal has an orthorhombic unit cell with the lattice parameters of a = 0.760 nm, b = 1.075 nm, c (fiber axis) = 0.833 nm. On the basis of these parameters, the d-spacing obtained from the calculation is similar to that obtained from the electron diffractograms. Table 1 shows the detail value of the d-spacing obtained from calculation and observation. Therefore, we suggest that the

formation in the seaweed-like crystals appearing at low Tc (Tc = 40 °C) of PESu/PVPh (75/25) blend, as an example. At low Tc, the morphology of dendrite crystals shows a branching morphology with the branching angle being less than 90 deg relative to the main stalk, as displayed in the AFM height images in Figure 7a. The mechanism of the branch growth is described schematically in Figure 7b. First, the nuclei grow to form spherical flat-on lamellae. The AFM phase image in Figure 4a shows that the seaweed-like crystals, formed at low Tc, are composed of spherical flat-on lamellae. The lamellae then grow to form four main stalks. Then, the lamellae of the side branches along the edge of main stalks start to grow perpendicularly to the main stalks. The lamellae at the edge F

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Crystal Growth & Design

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special orientation of the arranged lamellae to form a branch. Figure 8a shows the AFM images of dendrite morphology of PESu/PVPh (80/20) at Tc = 70 °C. The AFM height image shows that the dendrite morphology has primary side branches and secondary side branches, which are perpendicular to the main stalks and primary side branches, respectively. The overall shape of that dendrite assumes a diamond shape, which has a long axis and a short axis. AFM characterization could not cover a wide enough scanner range for showing the whole shape of the dendrite morphology; thus the dash lines were used to indicate the expected long and short axes of the entire crystals. The AFM phase image shows that the lozenge single crystal flat-on lamellae in the center region are arranged with the long axis parallel to the long axis of the diamond-shaped dendrite crystal, as seen in Figure 8a(1). The short axis of main stalks is also arranged by lozenge single-crystal flat-on lamellae whose long axis is parallel to the long axis of dendrites. The scheme in Figure 8a describes the lamellar arrangement in the center region of the dendrite crystal. Then, the primary side branches grow at the edge of the main stalks. As discussed previously, the primary side branches grow perpendicularly to the main stalks, and the secondary side branches grow perpendicularly to the primary side branches. Interestingly, the long axis of the lozenge single crystal flat-on lamellae in the primary and secondary side branches are arranged parallel to the long axis of the diamond-shaped dendrite crystal. Zoom-in images to specific spots were further inspected for morphology details in these lamellae and their structural assemblies. Figure 8b shows the lozenge single crystal with flaton lamellae arrangement in the main stalks, primary and secondary side branches in PESu/PVPh (80/20) blend crystallized at Tc = 70 °C. The phase images in Figure 8b show clearly the direction of lamellae arranged in the dendrites. The lozenge-shape lamellae in main stalks, primary side branches, and secondary side branches are arranged in the direction of their long axis parallel to the long axis of big dendrite morphology. Supporting evidence of Figure S3(b) shows that the growth rate of dendritic crystals at high Tc is 0.023 μm/s. This figure shows that the crystal growth rate at high Tc is slower than that of at low Tc, as shown in Figure S3a. To simplify the description of the mechanism of lamellar arrangement to form a dendrite crystal morphology, we illustrate the proposed mechanism in a graphic schematic diagram as shown in Figure 9. The lozenge flat-on lamellae first

of the main stalks near the center will impinge with other lamellae at the edge of other main stalks, which cause the lamellae to not be able to grow in straight lines as described in Figure 7b(1). Those lamellae then continue to grow since the growth rate of lamellae at low Tc is high, which cause them to grow in the direction of less than 90 deg to the main branches. The crystal growth rate at Tc = 40 °C is 0.046 μm/s, as shown in Figure S3a, Supporting Information. The same mechanism is also responsible for the occurrence of the secondary or tertiary side branches, which grow from the edges of the primary side branches. According to that mechanism, as the Tc increases, the growth direction moves to be perpendicular to the main stalks and to the primary side branches for the primary side branches and for the secondary side branches, respectively. Therefore, at an intermediate Tc, for example, Tc = 50 °C, the growth direction of the branches is not exactly perpendicular to the other branches. However, at high Tc = 70 °C, the growth direction of the branches is perpendicular to their parental branches from which they grow outward. As mentioned previously, at high Tc, for example, Tc = 70 °C, the growth direction of the branches is perpendicular to the other branches. The branch formation mechanism at high Tc is also similar to that at low Tc. Because the growth rate of the lamellae is low at high Tc, the branches will not continue to grow in another direction when they impinge with other branches so that the branches formed are perpendicular to the other branches, as seen in Figure 8. The growth mechanisms and patterns at higher Tc are different from the growth at lower Tc, where the branch formation mechanism at low Tc is characterized with no special arrangement of the lamellae on the branch formation, because the lamellar pattern is only spherical flat-on lamellae. By contrast, at high Tc, we found a

Figure 9. Schematic illustration of stacked multilayered lozenge crystals in dendrite morphology formation in PESu/PVPh blend.

grow to form the main stalks with four directions as already mentioned previously. Then, the lamellae grow to form primary side branches, and then the secondary side branches grow perpendicularly to the primary branches. Tertiary side branches grow from the secondary branches if space allows. These packing procedures repeat themselves until forming the final

Figure 8. AFM images of dendrite morphology and lamellar arrangement for PESu/PVPh (80/20) at Tc = 70 °C in (a) center area, (b) main stalks, primary and secondary side branch as indicated by red squares in part (a). G

dx.doi.org/10.1021/cg401413f | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

dendrite crystal morphology in the spherulites. The fluctuation (noise) of concentration and temperature have an influence for the side branch formation and growth direction.28 That is why they seem to be arranged in a such well-defined way. The combination between the fluctuation of concentration and temperature and screw dislocation in lamella growth may lead to the formation of side branch. A study45 by other investigators (cited as follows) has reported that the screw dislocation could cause branching formation. They have reported that in poly(3-hydroxybutyrate-co-3-hydroxyhexanoate), new lamellae can grow from a mother lamellae via screw dislocations. These new lamellae form branches. The lamellar arrangement is all the same in a perpendicular orientation in the main stalks, primary or secondary side branches. The long axis of the lamellae is parallel to the long axis of the diamond-shaped dendrite crystal morphology. The formation of lozenge-shape (i.e., diamond-shape) dendrites is more favorable since the lozenge crystals have four stalks. Therefore, the stacking process follows those straight-line stalks, which themselves were packed of monolayer lozengeshaped single crystals. As it can be seen in the scheme, the most compact packing from center develops, and the single crystals tend to pack into a mutually perpendicular cross pattern. As shown earlier in the ED patterns, PESu single crystals display only pronounced (020) and (200) diffraction, which helps to support that packing of single crystals is perpendicular (90°) to the crystal boundary planes. In the figure, the main stalks are shown by the red arrow, the primary side branches are indicated by the blue arrow, and the secondary side branches are shown by the green arrow. It is interesting and critical to point out here that the dendrites in PESu/PVPh blend have four distinct main stalks and the branches, which are apparently arranged in a cross pattern, with the single crystals being arranged either in straight lines or branching out in 90° perpendicular direction. From the AFM graphs on the lozenge-shaped single crystals in PESu/ PVPh blend, the basal angle (at the tip) is about 75−80°, which is greater than 60° for the basal angle in the single crystals of poly(L-lactic acid) (PLLA).46 For such PESu single crystals in packing into dendrites, they usually align themselves in straight lines in growth with a number of main stalks and branch out at fixed angles. Depending on the basal angle of the single crystals, the dendrites packed by single crystals can assume different shapes and various numbers of main stalks and branching angles. This type of growth and branching has also been similarly reported in an earlier work from our lab on PLLA blended with poly(butylene adipate) (PBA).46 However, the dendrites in PLLA/PBA blend are six stalks with a branching angle equal to 60°. In poly(L-lactic acid) (PLLA), the basal angle of the single crystals is 60°; in PESu, it is 75−80° (from AFM graphs). When the basal angle of the single crystals is much greater than 60° (but lower than or equal to 90°), as in the case for PESu, they cannot be packed into six-stalks dendrites, owing to space exclusion, but instead they tend to be more conveniently packed into four-arms dendrites with distinctly 90° cross pattern and branching angle equal to 90°. It should be emphasized, however, when spherulites are not packed by single-crystal like plates, then such a straight habit is no longer valid. The lamellae can usually bend, turn, or flip, etc., for lamellae or lamellar bundles in thicker bulk polymers. To sum up the discussion, the dendritic spherulite morphology under melt-crystallization condition of PESu/ PVPh thin films was found to be sensitive as a function of PVPh

contents and crystallization temperatures. Compact crystals in round-shaped spherulites were formed in the PESu/PVPh blend of lesser PVPh contents (