Article pubs.acs.org/Langmuir
Crystal Morphology of Poly(3-hydroxybutyrate) on Amorphous Poly(vinylphenol) Substrate Xiaoli Sun,† Nan Gao,† Quan Li,† Jidong Zhang,‡ Xiaoqiu Yang,§ Zhongjie Ren,† and Shouke Yan*,† †
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, China § Basic Research Service, MOST, Beijing 100862, China ‡
S Supporting Information *
ABSTRACT: The crystalline morphology and orientation of poly(3-hydroxybutyrate) (PHB) thin film on the poly(vinylphenol) (PVPh) sublayer with different thickness was studied by atomic force microscopy, X-ray diffraction, and infrared spectroscopy. PVPh sublayer influences the morphology of PHB greatly. Although edge-on lamellae form on both Si and PVPh surfaces at relatively lower crystallization temperature, the morphology of them is quite different. It appears as sheaflike edge-on lamellar morphology on PVPh sublayer. In addition, the edge-on lamellae prefer to form on the PVPh sublayers at much higher crystallization temperature compared with that on Si wafer. The PVPh layer thickness also influences the crystalline morphology of PHB. On a 30 nm thick PVPh layer, sheaflike edge-on lamellae form in a wide range of crystallization temperatures. When the PVPh thickness increases to 65 nm, fingerlike morphology is observed when the crystallization temperature is lower than 95 °C. The fingerlike morphology is caused by a diffusion-limited aggregation process, and it requires an optimum condition. Thickness ratio between PHB and PVPh sublayer and temperature are two key factors for the formation of fingerlike morphology.
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INTRODUCTION Due to the rapid development of coating, adhesion, nanoscience, and nanotechnology, the design and fabrication of ultrathin polymer layers are of increasing importance. The macromolecular structures in the confined geometry play an important role in the final properties of ultrathin films. Therefore, understanding the impact of nanoconfinement on the structure of polymer thin films has drawn intense interest. The structure, especially the crystalline structure, of polymers can be affected by many factors such as free surface, interface, film thickness, crystallization temperature, and so forth. The study of polymer thin films on solid substrates has attracted much attention. The morphologies,1−8 the crystallization kinetics,9−18 and the transformation between crystals with various metastabilities for thin film on solid substrate19−22 have been well studied, and some common views have been obtained. For example, film thickness plays an important role in the orientation of crystals. In thin films with hundreds of nanometers, edge-on lamellae are predominantly found. And flat-on lamellae are preferred in ultrathin films and monolayer films. A general trend is always observed: decrease of film thickness gradually shifts the favor of edge-on orientation to flat-on orientation. The more the film thickness decreases, the more easily flat-on lamellae can be observed. The transition thickness is different for different polymers. The crystallization temperature also affects the lamellar orientation. At higher crystallization temperature, flat-on lamellae are induced by heterogeneous nucleation. By contrast, at lower crystallization © XXXX American Chemical Society
temperature, edge-on lamellae are observed due to the homogeneous nucleation. Several models have been proposed by the researchers to explain the orientation of crystals. Wang et al. have explained the preference of lamellae orientation by using a simple thermodynamic model.23 By assuming a specific shape for edge-on and flat-on nucleus, the critical energy for forming both types of nucleus was calculated using critical dimensions. This model can explain the thickness dependence of crystal orientation. But it is insufficient to describe the decrease of edge-on lamellae with decrease of film thickness and the coexistence of edge-on and flat-on lamellae. Recently, Wang et al. proposed a three layer model.8 The three layers, that is, the surface layer, middle layer, and polymer/substrate interface layer, have a different glass transition temperature (Tg). By combining the nucleation and kinetic effects, they predicted thickness and crystallization temperature dependence of crystal orientation. Both heterogeneous nucleation, which can be identified by observation of lamellae growing in all direction normal to the surface of the nucleus, and homogeneous nucleation, which can be identified by observation of lamellae growing in two direction, can be observed in thin films.24,25 Although the crystallization behavior of polymers on solid substrates has been studied extensively, the crystallization behavior of a polymer on another polymer Received: January 8, 2016 Revised: February 27, 2016
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And two-dimensional X-ray diffraction and IR are used as complementary methods to characterize the orientation of PHB crystals and intermolecular interaction. By using these techniques, the temperature and thickness dependent crystalline morphology of PHB is well discussed.
substrate is rarely explored. With the research direction of the polymer materials developing to high performance and special function, the layered composite film material is becoming an important research direction. The condensed state structure of polymer thin film in the interface layer plays an important role in the performance of thin layered composite material. Changing interface or surface provides an important way for tailoring crystalline morphology of polymer and kinetic, and so forth. For example, Wang et al. have employed layermultiplying coextrusion technology to create thin films with hundreds or thousands nanometers of alternating layers of poly(ethylene oxide) (PEO) and poly(ethylene-co-acrylic acid) (EAA) and have done excellent studies on the surface-confined crystallization of polymers within multilayers.9 By changing the layer thickness of PEO, the crystalline morphology of PEO has been well controlled and consequently the permeability for this kind material to oxygen is well controlled. When the PEO layer thickness decreases to 20 nm, extremely large lamellae form. Such kind of crystalline structure decreases the oxygen permeability significantly, almost 2 orders of magnitude less than its bulk value. Recently, Wang et al. also prepared the double layered films with the layer thickness of micrometer scale selecting the PEO as the liquid polymer substrate.26 They studied the influence of molten PEO layer on the crystallization of the polylactide (PLA) and found that the spherulite growth rate of PLA has been accelerated by the molten PEO layer. For the layered polymer composites, the interaction of two polymers and interdiffusion layers at the interface are the key factors to regulate the condensed structure of the polymers. Previously, we have prepared poly(vinylphenol) (PVPh)/ poly(3-hydroxybutyrate) (PHB) double layers and studied the crystallization and melting behavior of PHB on PVPh sublayers.27,28 The crystal structure of PHB is orthorhombic, P212121−D42, with lattice parameters a = 5.76 Å, b = 13.20 Å, and c = 5.96 Å (fiber repeat distance).29,30 The strong interaction between PHB and PVPh occurs since the hydroxyl groups (OH) of PVPh can form hydrogen bonds with the carbonyl groups (CO) of PHB. We have found that the fraction of CO groups connected with OH groups of PVPh increases with the PVPh thickness which leads to the PVPh thickness dependent cold crystallization and melting behavior of spin-coated PHB thin film. Moreover, the amount of intermolecular hydrogen bonds between PVPh and PHB increases significantly in the heating process above a certain temperature. The PVPh sublayer does not significantly affect the crystallization behavior of PHB in solution-cast samples, while it inhibits the crystallization of PHB after melt recrystallization and an interdiffusion layer of PVPh and PHB formed.27 Due to the thickness confinement and interdiffusion of PVPh and PHB, a concentration gradient of intermolecular hydrogen bonds exists at the interface of PVPh and PHB. The PHB layer can be divided into two layers. In the inactive layer, the fraction of intermolecular hydrogen bonding between C O and H−O is very high and stable amorphous state always forms. In the interactive layer, the crystallization of PHB is sensitive to temperature. The existence of inactive layer accounts for the formation of crystals oriented with b-axes perpendicular to substrate. Although, crystallization of PHB on PVPh substrate has been studied by using X-ray and Infrared spectroscopy (IR), the crystalline morphology and orientation of PHB thin film have been rarely explored. So in the present study, the crystalline morphology is investigated by atomic force microscopy (AFM).
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EXPERIMENTAL SECTION
Material and Preparation Procedures. Bacterially produced PHB with a weight-averaged molecular weight of 6.0 × 105 g mol−1 was obtained from the Procter & Gamble Corp. Its melting temperature is measured by DSC to be 172 °C. To remove impurities, the polymer was dissolved in hot chloroform, then precipitated in methanol and finally vacuum-dried at 60 °C. PVPh with Tg at approximately 104 °C was purchased from Aldrich Corp. The PVPh covered Si wafers were prepared by spin-coating a uniform PVPh layer from its tetrahydrofuran solution onto the Si wafers. The obtained PVPh layer was then dried in a vacuum oven at 130 °C for 6 h. The PHB thin layers, either on the Si wafer or on the PVPh covered on Si wafer, were spin-coated from its chloroform solution with different concentrations to regulate the thicknesses of the PHB layers, and then dried at 60 °C for 24 h under vacuum prior to measurements and other thermal treatments. The spin-coating process was performed at room temperature with a rotating speed of 4000 rpm. It should be pointed out that, during the spin-coating of PHB onto PVPh surface, the PVPh layer was not disturbed since it does not dissolve in chloroform. To study the influence of PVPh on the melt crystallization behavior of PHB, the samples were first heat-treated at 185 °C for 3 min and then immediately moved to another hot plate setting at desired temperatures for a 48h isothermal crystallization, which is long enough for the completion of PHB crystallization on Si and thick PVPh sublayers of 30 and 65 nm. The crystallization temperatures were set at 25, 53, 72, 95, and 112 °C. After that, the samples were moved to room temperature for measurement. It should be noted here that interdiffusion layers formed at the interface of PHB and PVPh because the thermal treatment temperature is much higher than the Tg of PVPh, which has been discussed previously in ref 27. Thickness Measurement and Morphology. Thickness measurements of the polymer films were performed with a Nanoscope III Multi Mode atomic force microscope using Si cantilever tips (TESP) with a resonance frequency of 300 kHz and a spring constant of 40 N/ m. The scan rate varied from 0.5 to 1.0 Hz. PVPh film thickness was determined from AFM height profiles by partially removing the PVPh film from the substrate and then evaluating the distance between the substrate surface and the sample surface. PHB film thickness was obtained by subtracted PVPh film thickness from the thickness of the PHB/PVPh double layers. To minimize experimental error, the film thickness was averaged through several height profiles recorded at different locations and along different directions on the sample surface. Tapping mode AFM images were measured under the same condition. AFM−IR instrument produced by American company Ansys Inc. was used to characterize the topography and spectra information on samples simultaneously. IR Measurement. Infrared spectra were measured with 2 cm−1 resolution using a Fourier transform infrared (FTIR) spectrometer. A total of 128 scans were coadded for each IR spectral measurement to ensure a high signal-to-noise ratio. GIXD Measurement. The X-ray diffraction measurements were carried out on the diffractometer at the beamline BL14B1 of Shanghai Synchrotron Radiation Facility (SSRF). The wavelength of the radiation was 1.24 Å. A two-dimensional CCD detector, Rayonix MX 225 with the pixel size of 73 × 73 μm2 (3072 × 3072 pixels), was used to collect the diffraction patterns. The sample-to-detector distance was 293 mm (calibrated by Lanthanum Boride, LaB6). All the scattering patterns were corrected for background scattering and air scattering. The grazing incident angle is set at 0.2° which is slightly larger than the critical angle of PHB and PVPh (0.17°). B
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Figure 1. AFM images of the 120 nm thick PHB thin films on Si melt recrystallized at (a) 25, (b) 53, (c) 72, (d) 95, and (e) 112 °C. Images (b), (c), and (e) have the same magnification as that of (a).
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RESULTS The Crystallization Morphology of PHB on Si and PVPh Layers. The AFM images of PHB crystals on Si wafers are shown in Figure 1. It can be seen that edge-on lamellae form at 25 °C, Figure 1a. The PHB lamellar morphology does not change much when the crystallization temperature is enhanced to 53 °C (compare Figure 1b with a), although the lamellar density is a little bit larger at higher temperature. To further indentify the lamellar orientation, the GIXD measurement is employed. The GIXD patterns for PHB on Si are shown in Figure 2. For the samples crystallized at 25 and 53 °C, the GIXD patterns (Figure 2a and b) reveal intense reflections of (020) and (110) lattice planes at q = 0.96 Å−1 and q = 1.20 Å−1, respectively. Other weaker reflections of the (021), (101), and (111) lattice planes are also seen, which are located at q = 1.43, 1.52, and 1.59 Å−1, respectively. The appearance of reflection arcs suggests the existence of crystal orientation in the film. The GIXD pattern for PHB crystallized at 25 °C is similar to that at 53 °C. To precisely determine the crystal orientation, Cerius2 program was used. Three different sets of single crystal diffraction patterns are simulated along three directions, which are ⟨101⟩, ⟨001⟩, and ⟨100⟩ (see Supporting Information Figure S1). After comparing the experimental and simulated results, it is found that the GIXD patterns for the PHB melt recrystallized below 53 °C are superimposed by the diffraction patterns along ⟨101⟩, ⟨ 001⟩, and ⟨100⟩ directions. This suggests that PHB crystals prefer to orient with the b-axis perpendicular to the substrate. The a- and c-axes are randomly oriented in film plane (see Figure 2f). When the PHB is crystallized at 72 °C, we cannot see clear lamellar structure anymore from AFM image, see Figure 1c. GIXD pattern shows the (020) diffraction ring with maximum intensity along the out-of-plane direction (see Figure 2c), suggesting that the b-axis oriented randomly in the plane perpendicular to the beam axis with somewhat preferred
direction along the out-of-plane. The coexistence of (020) diffraction maximum and (110) reflection arcs in the horizontal direction demonstrates the existence of also flat-on PHB crystals, which will be discussed in detail later. If the crystallization temperature is higher than 95 °C, the X-ray diffraction patterns (Figure 2d and e) have close resemblance to the fiber pattern of PHB with obvious layer line in the vertical direction, indicating a fiber orientation of PHB with the c-axis arranged vertically while the a- and b-axes rotated randomly about c-axis (as sketched in Figure 2g), that is, the formation of randomly oriented flat-on PHB crystals. The flaton lamellar morphology has also been recognized in the AFM images presented in Figure 1d and e. These results indicate that the lamellar orientation of PHB crystallized on Si wafer changes from edge-on lamellae to flat-on lamellae with the increase of isothermal crystallization temperature. Figure 3 shows the AFM images of the 120 nm thick PHB thin films on a 30 nm thick PVPh sublayer melt recrystallized at different temperatures. As can be seen from Figure 3, AFM images illustrate clearly edge-on lamellar structures of the samples crystallized below 95 °C. The morphology of lamellae is similar to the sheaflike morphology. The sheaflike morphology shows more open structures in large scales with the increase of crystallization temperature. Under the same crystallization condition, the lamellar thickness of PHB crystals on PVPh is much thicker than that on Si wafer. The edge-on lamellar structure disappears when the sample was crystallized isothermally at 112 °C (Figure 3e). This implies that the lamellar orientation of PHB crystals on a 30 nm PVPh layer changes from edge-on to flat-on lamellae at a higher crystallization temperature as compared to that on the Si wafer. The corresponding GIXD results confirm the above conclusion. One can see from Figure 4a−c that the samples isothermally crystallized at temperatures below 95 °C show essentially the same X-ray diffraction patterns with sharp (020) C
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Figure 2. 2D GIXD patterns of the 120 nm thick PHB thin films on Si melt recrystallized at (a) 25, (b) 53, (c) 72, (d) 95, and (e) 112 °C. The sketch for the lamellar orientation of (f) edge-on and (g) flat-on.
diffraction spot along the out-of-plane direction, demonstrating that b-axis is perpendicular to the substrate. For the sample crystallized at 95 °C, except for the strong (020) diffraction spot in the vertical direction, a very weak (020) diffraction ring can be recognized with close inspection, indicating the deviation of b-axis of some PHB crystals from the vertical direction. Moreover, the appearance of the (110) diffraction spot in the horizontal direction implies the existence of flat-on PHB crystals. This can be actually identified also in the AFM image shown in Figure 3d. It is clear that, while the edge-on lamellae are dominated, some flat-on lamellae are observed between the lamellar stacks, as indicated by the arrows. When the crystallization temperature is enhanced to 112 °C, the GIXD profile shows the typical flat-on lamellar diffraction pattern. By comparing the crystallization behavior of PHB on Si wafer and 30 nm thick PVPh sublayer, two differences can be identified. (1) The temperature for producing flat-on lamellae of PHB rises on PVPh sublayer. In other words, the presence of PVPh sublayer favors the formation of edge-on lamellae with baxis perpendicular to the substrate. (2) The morphology of the edge-on lamellae on PVPh is also different from that on Si
substrate. The PHB crystal on PVPh sublayer prefers to appear as the sheaflike morphology with thicker lamellae. It was found that the thickness of PVPh sublayer also affects the crystallization behavior of PHB significantly. When PVPh sublayer is increased to 65 nm, a very different morphology is observed for the samples crystallized below 72 °C (Figure 5a− c). Fingerlike morphologies with the height of several nanometers appear (see the insets of the height profiles along the dashed lines in Figure 5b). X-ray results indicate that the crystallinity of the fingerlike structure is very low. It is even in amorphous state at 25 °C (see Figure 6a). Very weak (020) diffraction spot is observed along the out-of-plane direction at 53 and 72 °C (Figure 6b and c), suggesting that the crystals comprising the fingerlike morphology adopt the edge-on orientation. The formation mechanism for fingerlike structure will be discussed later. If the crystallization temperature is enhanced to 95 °C, the fingerlike morphology disappears. Now edge-on lamellae are clearly observed. The (020) Bragg diffraction intensity is very strong in the out-of-plane direction (see Figure 6d), suggesting the crystallinity increases significantly and the edge-on lamellae with b-axis orientation D
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Figure 3. AFM images of the 120 nm thick PHB thin films on 30 nm thick PVPh melt recrystallized at (a) 25, (b) 53, (c) 72, (d) 95, and (e) 112 °C. Images (b), (c), and (d) have the same magnification as that of a.
Figure 4. 2D GIXD patterns of the 120 nm thick PHB thin films on 30 nm thick PVPh melt recrystallized at (a) 25, (b) 53, (c) 72, (d) 95, and (e) 112 °C.
are formed. At 112 °C, (020) Bragg spots appear both in the out-of-plane and in-plane directions with weak reflection ring, implying a mixture of edge-on and flat-on lamellae. This can also be proved by AFM images; see Figure 5e. Clearly, both PVPh thickness and crystallization temperature play a
significant role in the crystallization of PHB including crystallization degree, crystalline morphology, and crystal orientation. When the PVPh sublayer is increased to 100 nm, amorphous PHB films are always obtained, no matter what crystallization E
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Figure 5. AFM images of the 120 nm thick PHB thin films on 65 nm thick PVPh melt recrystallized at (a) 25, (b) 53, (c) 72, (d) 95, and (e) 112 °C.
Figure 6. 2D GIXD patterns of the 120 nm thick PHB thin films on 65 nm thick PVPh melt recrystallized at (a) 25, (b) 53, (c) 72, (d) 95, and (e) 112 °C.
temperature was chosen. The AFM images and GIXD results are shown in the Supporting Information (see Figures S2 and S3, respectively). Similar amorphous PHB/PVPh bilayer is also obtained in our previous studies in which the 175 nm thick PHB spin-coated on 165 nm thick PVPh. These results demonstrate that the amorphous PHB/PVPh bilayer can always
be obtained if the thickness ratio between PHB and PVPh layer is properly adjusted. The crystallization behavior and related morphology of PHB crystals on different substrates are summarized in Table 1. Clearly, PVPh substrate alters the crystalline ability and morphology of PHB significantly. Flat-on lamellae are relatively difficult to form on the PVPh substrate. F
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Figure 7 shows the spectra of PHB on PVPh or Si substrate with different crystallization temperature. It can be seen that the peak intensity of f ree CO becomes stronger with the thickening of PVPh sublayer. By contrast, the peak intensity of intra CO becomes weaker with the thickening of PVPh sublayer, implying the crystallization degree of PHB decreases. For the PHB on Si, only two components are detected, that is, the f ree and the intra CO groups. To make a quantitative analysis, a curve fitting procedure is employed to decompose the spectra in the CO stretching region. The detailed fitting method can be seen in the literature of 27. After curve fitting procedure, the fractions of f ree and intra COs as a function of crystallization temperature are obtained (see Figure 8). Obviously, the fraction of intra CO increases with crystallization temperature with the sacrifice of f ree CO fraction, implying more and more PHB molecular chains from amorphous phase pack into crystal lattice with increased crystallization temperature. The spectra of PHB on a 30 nm PVPh layer at different crystallization temperature are shown in Figure 7b. And their corresponding fractions of f ree, intra, and inter CO, as a function of crystallization temperature, are shown in Figure 8b. The inter component starts to appear with very small fraction, which shows a slightly increase first with crystallization temperature and thereafter decreases above 72 °C. The change of f ree and intra with crystallization temperature is similar to that on Si except for that the value of intra decreases slightly on PVPh substrate. On 65 nm thick PVPh sublayer, the change rule of the three fractions is quite different. The intra fraction increases gradually while the f ree fraction decreases gradually with temperature when the crystallization temperature is lower than 72 °C. The intra fraction increases sharply above 72 °C along with the decrease of f ree and inter fractions. Obviously, on 65 nm thick
Table 1. Temperature and Substrate Dependence of Crystal Orientationa recrystallization temperature (°C) substrates
25 °C
53 °C
72 °C
Si
E()
E()
F(∥)
F(∥)
30 nm PVPh 65 nm PVPh 100 nm PVPh
E()
E()
E()/ F(∥) E()
E()/F(∥)
F(∥)
E()/F(∥)
E()/F(∥)
amorphous phase
amorphous phase
95 °C
112 °C
E() and F(∥) represent edge-on and flat-on lamellae; represents fingerlike structure.
a
The crystallization temperature at which edge-on lamellae disappears increases with the thickness of PVPh sublayer, while the crystallization of PHB is inhibited when the PVPh layer is thicker than 100 nm. Quantitative Analyses of Temperature and Substrate Dependence of CO Stretching Vibration Bands. To better understand the structure evolution of PHB layer on the PVPh surface at different crystallization temperatures, IR study is carried out. The IR spectra in the CO stretching vibration region of the PHB/PVPh double layers have been found to consist of three elemental spectra, f ree CO (none hydrogen bonded CO group), intra CO (intramolecular hydrogen bonded CO group), and inter CO (the CO group of PHB hydrogen bonded with the OH group of PVPh). The previously established procedure was employed to decompose the spectra into the elemental spectra around 1740 cm−1 (f ree), 1723 cm−1 (intra), and 1713 cm−1 (inter), respectively.31−33
Figure 7. Infrared spectra in the CO stretching region for the PHB on (a) Si, (b) 30 nm PVPh, (c) 65 nm PVPh, and (d) 100 nm PVPh. G
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Figure 8. Change of the fractions for the elemental CO stretching bands with crystallization temperature. 120 nm PHB on (a) Si, (b) 30 nm PVPh, (c) 65 nm PVPh, and (d) 100 nm PVPh.
Figure 9. Multifunctional measurements performed with the AFM−IR on a fingerlike structure. Measurements show topography (A), representative IR absorption spectra at point “a” and “b” (B), and an IR absorption image at 1728 cm−1 (C).
PVPh sublayer, most molecular chains of PHB are in amorphous state at low temperature, while only a small part
of PHB molecular chains connects with PVPh chains through hydrogen bonding (f inter = 15%) or crystallizes (f intra = 10%) at H
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Figure 10. Sketch for the crystallization of PHB on Si and PVPh sublayer.
degree decreases due to the increased fraction of inter CO. Further thickening of the PVPh layer, only the amorphous layer is obtained, implying that the thickness ratio of PHB and PVPh is the key factor to determine the formation of fingerlike structures. To get the thickness value of PHB which can form fingerlike structures intuitively, the PVPh sublayer thickness is fixed (65 nm) and the PHB layer thickness is changed. It is found that on the 65 nm thick PVPh layer, when the PHB is thinner than 105 nm, the amorphous layer is observed irrespective of crystallization temperature (see Supporting Information Figure S4). For the PHB with the thickness of 120 nm, the fingerlike morphology can be observed in a wide crystallization temperature range (lower than 95 °C). And when the PHB thickness is equal to 138 nm, the fingerlike morphology is observed only at 25 °C. Further thickening of PHB layer, fingerlike morphology disappears. So the optimum condition for the presence of fingerlike morphology on 65 nm PVPh sublayer is that the thickness of PHB layer is around 120 nm and the crystallization temperature is lower than 95 °C. Thickness ratio between PHB and PVPh sublayer and crystallization temperature are two key factors for the formation of fingerlike morphology.
lower crystallization temperature. The decrease of intermolecular hydrogen bonds at temperatures higher than 72 °C indicates that the intermolecular hydrogen bonds break up at high temperature, which enables the molecular chains packing in crystal lattice. Combining with the morphology of PHB on PVPh sublayer, it can be seen that fingerlike morphology has very low crystallization degree and changes to flat-on or edgeon lamellae along with the increase of intra fraction at higher crystallization temperature. On 100 nm thick PVPh sublayer, the intra CO peak is very difficult to be discerned in the IR spectra and a very small fraction of intra CO is obtained through curve fitting. Considering that no Bragg reflection is observed for PHB on 100 nm PVPh sublayer, we speculate that the molecular chains with this kind of conformation do not pack into crystal lattice or, if they are crystallized, the amount of the formed crystals are not enough for X-ray detection. The fraction of intra increases slightly with crystallization temperature. The fractions of inter and f ree are much larger for the PHB on 100 nm PVPh sublayer than that on thinner PVPh sublayer implying that most molecular chains of PHB are in amorphous state or connecting with PVPh chains. The fraction of inter still displays a sharp decrease when the crystallization temperature is set above 72 °C, which further confirms that the intermolecular hydrogen bonds of PHB and PVPh always prefer to break up at high temperature. The Formation of Fingerlike Morphology. From above description, one phenomenon worth to be noted is the fingerlike morphology formed under specified conditions. When the 120 nm thick PHB on 65 and 100 nm PVPh melt recrystallized at temperature lower than 95 °C, the fingerlike morphology is observed. AFM−IR is employed to detect the difference between the raised and hollow places, and it is found that the intra CO peak intensity is much higher than the f ree CO peak in the raised places (see Figure 9A and B). By contrast in the hollow places, the intra CO peak intensity is much weaker than the f ree peak. The height images of fingerlike morphology and their corresponding IR absorption images at 1728 cm−1 are shown in Figure 9A and C. The red and blue colors demonstrate the strong and weak peak intensity at 1728 cm−1, respectively. It further demonstrates that the raised places in the fingerlike structures have higher intra CO peak intensity suggesting that the crystallization degree of PHB for the raised places is higher than that for the hollow places. The fingerlike morphology of PHB is also observed on 100 nm thick PVPh sublayer, which is similar to that on 65 nm thick PVPh sublayer. The only difference is that the crystallization
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DISCUSSION According to the above obtained experimental results, it is clear that the crystallization degree, crystalline morphology and crystal orientation of PHB are very sensitive to both the thickness ratio between PVPh and PHB layers and crystallization temperature. Several aspects can be discussed here. (1) The effect of PVPh sublayer on the crystal orientation and morphology. (2) The temperature and thickness dependence of intermolecular hydrogen bonds between PVPh and PHB. (3) A new way to obtain the fingerlike morphology. Let us first discuss the effect of PVPh sublayer. In the present study, we chose two factors to control the crystallization behavior of PHB: thickness of PVPh sublayer and crystallization temperature. A sketch is drawn and displayed in Figure 10 to describe the structure change of PHB on Si or PVPh sublayer. On Si substrate, edge-on lamellae are observed in a wide crystallization temperature range. With the increase of temperature, the crystallization degree increases gradually. A critical temperature exists at which edge-on lamellae disappear. Such critical temperature for PHB on Si is 95 °C. The critical temperature for PHB on PVPh sublayer rises significantly. For example, on the 30 nm thick PVPh sublayer, the critical temperature is 112 °C. On the 65 nm thick PVPh sublayer, I
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Langmuir pure flat-on lamellae cannot be obtained and a mixture of flaton and edge-on lamellae is observed at very high crystallization temperature. The reason that the flat-on lamellae are difficult to form on PVPh sublayer can be caused by the temperature dependence of nucleation type. It is reported that when homogeneous nucleation occurs at a film’s surface, the edge-on orientation is preferred because of the lower surface free energy of the lateral surface compared with the fold surface.25 On the other hand, when heterogeneous nucleation occurs at the polymer/substrate interface of polymer films, the flat-on orientation is favored.23 At low temperatures, the homogeneous nucleation is the quickest at the surface of the film and edge-on lamellae develop consequently. At high temperatures, the heterogeneous nucleation rate at the polymer/substrate is higher than the homogeneous nucleation rate at the surface. Consequently, more flat-on lamellae are observed at the surface. Considering the edge-on lamellae are observed in a very wide crystallization temperature range for the PHB on PVPh sublayer, it can be predicted that heterogeneous nucleation is hard to form on PVPh sublayer. The difficult formation of heterogeneous nucleation can be attributed to the intermolecular hydrogen bonds between PVPh and PHB. The above prediction is based on the assumption that heterogeneous nucleation always occurs on the surface of substrate and it can only induce flat-on lamellae. The crystallization behavior on the substrate is different in a dynamic Monte Carlo simulation. Ma et al.14 predicted that edge-on lamellae exist predominantly at the interface between a polymer and a slippery wall due to crystal nucleation near the wall, while flaton lamellae are dominant when there are strong interactions between the polymer and substrate. From this simulation, it can be seen that both edge-on and flat-on lamellae can form on the substrate interface depending on the surface properties of substrate. If we also suppose that both edge-on and flat-on lamellae can be induced by the substrate in the present case, it can be found that edge-on lamellae are preferred to be induced by PVPh substrate which has strong interaction with PHB. This result is against with the simulation result which indicates that flat-on lamellae prefer to develop on strong interacting substrate. Considering the presence of free surface in the present experiment, the edge-on lamellae are more likely induced by free surface rather than the substrate and homogeneous nucleation occurs on the free surface. Thus, the heterogeneous nucleation is depressed on PVPh sublayer at high crystallization temperature and edge-on lamellae developed consequently. Second, the hydrogen bonds between PVPh and PHB should be discussed. The reason that PVPh sublayer plays a significant role in the crystallization of PHB is the formation of hydrogen bonds between PVPh and PHB. In the melt recrytallization process, an interdiffusion layer is developed along with the formation of intermolecular hydrogen bonds. On the premise of fixed thickness of PHB layer, the interdiffusion layer thickens with the thickening of PVPh layer. This leads to the fraction of CO groups of PHB connected to the O−H groups of PVPh (intermolecular hydrogen bonds) increase with the thickening of PVPh layer. The increased fraction of intermolecular hydrogen bonds subsequently reduces the crystallization degree of PHB and also alters the crystalline morphology, as can be seen from the sketch in Figure 10. Thinner PVPh sublayer (e.g., 30 nm) produces thinner interdiffusion layer and smaller f inter, producing a similar lamellar morphology and orientation as on Si substrate except for a slightly increased lamellar thickness
and the critical temperature at which the edge-on lamellae disappear. Thicker PVPh layer (e.g., 65 nm) makes the interdiffusion layer thickens and f inter increases. It leads to extremely low molecular chain mobility and a very thin crystallizable surface layer of PHB. Thus, diffusion-limited morphologies are observed. When the PVPh layer is increased further to 100 nm, because of the extremely thick interdiffusion layer and high f inter, PHB is unable to crystallize. The f inter is not only determined by the thickness of PVPh, but also exhibits the dependence on temperature. With the increase of crystallization temperature, the f inter increases gradually with temperature and reaches its maximum value at 72 °C. Thereafter, it decreases evidently due to the higher mobility of molecular chains at higher temperature. Finally, the formation of fingerlike morphology is concerned. Generally speaking, the fingerlike, or seaweed or dendrite morphologies are frequently observed in ultrathin films of various polymers. In such kind of ultrathin film, the crystallization usually occurs by a diffusion-limited aggregation process (DLA) and grows in typical diffusion-limited morphologies.34−37 In the DLA process, the diffusing of molecular chains to the crystal growth front cannot meet the demand of the ever-increasing stack of lamellae and the lamellae growth sequentially stops, which creates protruded fingers. Newly molecular chains, which cannot pack onto the growth front of lamellae, form a depletion zone. On the basis of the observations described above for PHB on PVPh, it can be surmised that the DLA process is only effective for the PHB/ PVPh double layer with proper thickness ratio. It is our conjecture that DLA process occurring in the PHB layer on PVPh is primarily due to the slowing down of diffusion rate of PHB molecular chains caused by the formation of hydrogen bonds between PHB and PVPh. It may be argued that the PHB layer is very thick here and the fingerlike structure is usually observed in ultrathin film with the thickness of tens of nanometers. Here, although the initial thickness of the PHB layer is not very thin (120 nm), diffusion between PHB and PVPh layer occurs. The interdiffusion layer with high fraction of intermolecular hydrogen bonds depresses the mobility of PHB molecular chains. An inactive layer and an interactive layer form, as presented in previous study.27 In the inactive layer, the amorphous state is always obtained and it is independent of crystallization temperature. In the interactive layer, crystallization degree increases with the crystallization temperature due to the increased molecular chain mobility. Under the condition that the PHB thickness is the same, the fraction of inter and the thickness of inactive layer will increase with the thickening of PVPh layer. It can be easily figured out that the PHB cannot crystallize when the PVPh thickness is above a certain critical value. When the PVPh thickness is around the critical value, the PHB molecular chains in the surface layer is not completely bonded by PVPh. The mobility of PHB molecular chains in the topmost thin layer is relatively higher, and they can diffuse and pack into crystal lattice. But due to such kind of layer being very thin and also the existence of thick inactive or interactive layer with high concentration of intermolecular hydrogen bonds, its diffusion is very difficult. Consequently, the fingerlike morphology at the surface induced by the limited diffusion is observed. The diffusion rate in the surface layer will be increased with crystallization temperature due to increased molecular mobility. Thus, a big crystallization degree difference between the lower and higher crystallization temperature is observed. With the thickening of the J
DOI: 10.1021/acs.langmuir.6b00058 Langmuir XXXX, XXX, XXX−XXX
Langmuir
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crystallizable surface layer, the diffusion rate is increased and fingerlike morphology disappears above a certain crystallization temperature. Thickening PHB layer and/or thinning PVPh layer causes the thickening of the surface layer. It is easy to understand that the thickness ratio between PHB and PVPh sublayer and crystallization temperature are two key factors for controlling the formation of fingerlike morphology.
CONCLUSION The crystallization behavior and resultant morphology of PHB on the PVPh sublayer with different thicknesses were studied by using AFM, X-ray, and IR. PVPh sublayer influences the morphology of PHB greatly. Although edge-on lamellae form on either Si or PVPh sublayer at relatively lower crystallization temperature, their morphologies are quite different. Sheaflike edge-on lamellar structure of PHB was observed on PVPh sublayer. In addition, the edge-on lamellae prefer to form on PVPh sublayers at much higher crystallization temperature compared with that on Si wafer. The orientation change of PHB on PVPh sublayer is caused by the difficulty of heterogeneous nucleation on substrate. PVPh layer thickness also affects the crystalline morphology of PHB. On a 30 nm thick PVPh layer, sheaflike edge-on lamellae form in a wide range of crystallization temperature and flat-on lamellae are observed at 112 °C. When the PVPh thickness increases to 65 nm, the fingerlike morphology is observed when the crystallization temperature is lower than 95 °C. Above this temperature, edge-on and flat-on lamellar structures form. The fingerlike morphology has also been observed on 100 nm PVPh sublayer at lower crystallization temperature (such as 25 °C). The fingerlike morphology is caused by a diffusion-limited aggregation process. The low diffusion rate of molecular chains can be attributed to the presence of interdiffusion layer with high fraction of intermolecular hydrogen bonds. Thickness ratio between PHB and PVPh sublayer and temperature are two key factors for the formation of fingerlike morphology. ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b00058.
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Simulated single crystal diffraction patterns, AFM images, and GIXD patterns (PDF)
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was financially supported by the National Natural Science Foundations of China (No. 21274010 and 21434002). The BL14B1 beamline at SSRF is acknowledged for kindly providing the beam time. The AFM−IR measurement was supported by the long-term subsidy mechanism from the Ministry of Finance and the Ministry of Education of PRC. K
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