3.1. Soft Confined Crystallization and Microphase Separation-Determined Morphologies of PEO-b-PCLA Diblock Copolymers [7]
PEO-
b-PLA diblock copolymer films several microns thick were completely melted at 180 °C, and then quickly cooled to the temperatures (
Tc) for isothermal crystallization. After complete crystallization, the films were cooled to room temperature rapidly. The morphologies were observed using polarized optical microscope at room temperature. General spherulites were observed in the copolymer films isothermally crystallized at 90 °C and 100 °C as shown in
Figure 18a,b.
Figure 18c showed that banded spherulite was formed, when crystallized at 110 °C from the disordered state. An interesting morphology of which the center is spherulite and the outer is dendrite was formed at
Tc = 115 °C shown in
Figure 18d. When crystallized at a higher temperature,
Tc > 115 °C, the crystallization behavior and the crystalline morphologies are absolutely different from those at low
Tc [
7,
44,
45,
46,
47]. Dense dendrites were formed at 120 °C, as shown in
Figure 18e,f; the morphologies composed by the main branches were star-shaped, and the second branches grew along the main branches. At even higher
Tc, the dendrite and fractal dendrite were formed, as shown in
Figure 18g at 125 °C and
Figure 18h at 130 °C. The morphologies composed by the main branches are still star-shaped, but the number of the main branch is much smaller than that of dense branches. Furthermore, the morphologies formed at 130 °C are more regular, which are composed of six main branches, and the second branches grow along the main branches, the second branches along one main branch are parallel with each other, then the third branches grow in the same way as the second branches. The peculiar dendrite formed in high temperature range can be explained with nucleation, diffusion, growth, and their competition with each other and the addition of PEO block [
7,
44,
48].
Figure 18.
Morphologies of PLLA
16k-
b-PEO
5k copolymer films isothermally crystallized at (
a) 90 °C; (
b) 100 °C; (
c) 110 °C; (
d) 115 °C; (
e) 120 °C; (
f) 120 °C; (
g) 125°C;and (
h) 130 °C. The bar corresponds to 100 μm. Reprinted with permission from Wiley, 2008 [
7].
Figure 18.
Morphologies of PLLA
16k-
b-PEO
5k copolymer films isothermally crystallized at (
a) 90 °C; (
b) 100 °C; (
c) 110 °C; (
d) 115 °C; (
e) 120 °C; (
f) 120 °C; (
g) 125°C;and (
h) 130 °C. The bar corresponds to 100 μm. Reprinted with permission from Wiley, 2008 [
7].
As shown in
Figure 18, the morphologies of the diblock copolymer are crystallization temperature (
Tc) dependent. The morphologies evaluate from radial spherulite, banded spherulite at low
Tc, to dense branches, dendrite and fractal dendrite at high
Tc. It has been reported that the crystallization behavior of PLA can be classified into the low and high temperature ranges based on the crystallization modification, nucleation, and growth rate [
7,
44,
47,
49,
50,
51,
52,
53,
54], but such distinguished morphological evolution with
Tc has never been investigated. We speculated that the
Tc-dependent morphological behavior of the asymmetric copolymers is the result of soft confinement crystallization. It has been confirmed that the PEO-
b-PLA copolymers are disordered in the melt [
6,
10,
55]; the two components, PLA and PEO blocks, are weakly segregated. Microphase separation of the copolymers is driven by the crystallization of PLA block, which will influence the diffusion PLA component, growth and orientation of PLA lamellae. Consequently, dendrites are formed at high crystallization temperature range [
10]. However, the state of PEO block is melted during the whole crystallization process, and the volume fraction of PEO block is small (<25%). As a consequence, the confinement of the microdomains formed by microphase separation on the crystallization of PLA is weak. Although the amorphous PEO phase influences the diffusion of the PLA component, growth, and orientation of the PLA lamellae, the PLA component is still able to diffuse across the amorphous PEO layer to the growth front. Consequently, the crystallization of the PLA block overwrites the former structure formed by microphase separation, and the morphological features are of
Tc dependence, not dominated by structure formed by microphase separation.
A synchronous SAXS was employed to explore the structural change with various annealing temperatures from the melt. SAXS data was obtained via scanning the samples at the temperature with 30 s exposure annealed from the disordered melt. All of the data were corrected for background scattering before analysis and treated with software Fit 2D.
Figure 19a shows the SAXS patterns of PLA
16k-
b-PEO
5k copolymers isothermal at 130, 120, 110 and 100 °C from the disordered state. Scattering peaks can be seen in SAXS patterns, indicating that disorder to order transition occurs. That is microphase separation driven by crystallization of PLA block. The positions of scattering peak for PEO-
b-PLA copolymer shift to smaller q, indicating the long periods in the copolymers are larger than that in PLA homopolymer. Furthermore,
Figure 19a also shows that the long period for PLA
16k-
b-PEO
5k copolymer becomes larger with
Tc increasing in the temperature range from 110 to 130 °C. Meanwhile, the long period at 100 °C is larger than that at 110 °C and the value of the long period at 100 °C is nearly the same with that at 120 °C. In the high crystallization temperature range (110–130 °C), the crystallization of PLA block predominates the crystal structure, the lamellar thickness increases with
Tc, which results in the increase of the long period with
Tc. In the low crystallization temperature range, the influence of microphase separation on the crystal structure becomes stronger. Consequently, the long period at 100 °C is larger than that at 110 °C.
Figure 19.
SAXS patterns of PLLA16k-b-PEO5k copolymer (a) isothermally crystallized at 130, 120, 110, 100 °C from the melt (b) the isothermally process at 100 °C annealed from the melt.
Figure 19.
SAXS patterns of PLLA16k-b-PEO5k copolymer (a) isothermally crystallized at 130, 120, 110, 100 °C from the melt (b) the isothermally process at 100 °C annealed from the melt.
Figure 19b shows the SAXS patterns of PLA
16k-PEO
5k copolymer in the annealing and isothermal processes. The copolymer is melted at 180 °C, and then cooled to 100 °C; there is a small scattering peak in the SAXS pattern labeled 100 °C 0 min in
Figure 19b, indicating that disordered to order transition occurs, which is driven by crystallization of PLA block. This has been confirmed by SAXS, as shown in
Figure 19a. The following isothermal process shows the structural change in the process of crystallization. The increase of the intensity and the long period indicate the crystallization growth of PLA.
On the basis of the morphological features in
Figure 18 and the SAXS data in
Figure 19, the microstructure during phase transition in a soft environment can be schematically illustrated in
Figure 20. The copolymer system is disordered in the melt state shown in
Figure 20a. Crystallization of PLA block drives microphase separation of the copolymer, and then alternate amorphous–crystalline phase structure is formed, the microstructure can be shown in
Figure 20b. Because of the weak confinement of the microdomain on the diffusion and crystallization growth, the crystallization will breakout the confinement, overwrite the microstructures formed by microphase separation, and a new morphology is formed. The amorphous PEO and PLA blocks are distributed between the interfaces of PLA crystals and the surfaces, as schematically shown in
Figure 20c.
Figure 20.
A schematic illustration of phase transition of the asymmetric PLLA-b-PEO copolymers in a soft environment: (a) disordered state; (b) micro-structures formed by microphase separation and the crystallization of PLLA block; and (c) the final morphological structure.
Figure 20.
A schematic illustration of phase transition of the asymmetric PLLA-b-PEO copolymers in a soft environment: (a) disordered state; (b) micro-structures formed by microphase separation and the crystallization of PLLA block; and (c) the final morphological structure.
3.2. Hard Confined Crystallization and Microphase Separation-Determined Morphologies of PEO-b-PCLA Diblock Copolymers [7]
Figure 21 shows the morphologies of PLA
16k-
b-PEO
5k copolymer films formed with cold crystallization and in a hard environment. For cold crystallization, the copolymer films were cooled from the melt state, and then isothermally crystallized at very low temperature. Dense-distributed small spherulites are observed at 55, 60, 70 °C and 80 °C as shown in
Figure 21a–d, respectively. Microphase separation of the copolymer may occur driven by the immiscibility of PLA and PEO blocks at low temperatures. However, the microdomains formed by microphase separation are not stabilized, because the
Tc is higher than the glass transition temperature of PLA block and the melting point of PEO block. As a result, the PEO block is in melt state, and the chain mobility of PLA block is relatively strong. Meanwhile, the inducement time for crystallization at low temperature is quite short, homogeneous nucleation occurs in quite a short time, then a number of small spherulites grow up within a small domain rapidly.
A distinct annealing process named process B was carried out. PEO-b-PLA copolymer films were completely melted, and then cooled to 30 °C, which is below the glass transition temperature of PLA block. Five min later, it is heated to Tc for isothermal crystallization.
Figure 21e–i and
Figure 22 show the morphologies of PEO-
b-PLA copolymers crystallized in hard environment observed at room temperature. Star-shaped (or flower-shaped) morphologies were observed in PLA
16k-
b-PEO
5k and PLA
30k-
b-PEO
5k copolymers shown in
Figure 21e–i and
Figure 22. No Maltese cross can be seen in the polarized optical microscopy images, and the boundary lines of the two morphologies are not linear but irregularly curvilinear, which both confirm that the morphologies are not spherulites, but a new kind of morphology which has never been reported for PLA and its copolymers.
In order to get detailed information about the peculiar star-shaped morphology, AFM is used, and the typical height and the phase images are shown in
Figure 23. It is found that the morphological features are absolutely different from those in melt crystallization shown in
Figure 18. The morphologies which are tens of microns or even a hundred microns in size are composed of lozenge-shaped PLA crystals shown in
Figure 23. The size of the lamellae is at the nanometer scale, which is similar to that of the PLA single crystal. The lozenge-shaped lamellae with abundance of screw dislocation were also observed via AFM (shown in
Figure 23c). The single crystals orientated along the radical direction and the size of the lozenge-shaped crystals become larger when the
Tc increases (in comparison with those in
Figure 23a–c).
Figure 21.
Morphologies in PLA16k-b-PEO5k copolymer films crystallized at (a) 55 °C; (b) 60 °C; (c) 70 °C; (d) 80 °C from melt state; (e) 110 °C; (f) 115 °C; (g) 120 °C; (h) 125°C and (i) 130°C from Tg of PLA block with heating after quenching from annealing temperature. The bar corresponds to 100 μm.
Figure 21.
Morphologies in PLA16k-b-PEO5k copolymer films crystallized at (a) 55 °C; (b) 60 °C; (c) 70 °C; (d) 80 °C from melt state; (e) 110 °C; (f) 115 °C; (g) 120 °C; (h) 125°C and (i) 130°C from Tg of PLA block with heating after quenching from annealing temperature. The bar corresponds to 100 μm.
Figure 22.
Morphologies in PLA30k-b-PEO5k copolymer films crystallized at (a) 110 °C (b) 120 °C from Tg of PLA block with heating after quenching from annealing temperature. The bar corresponds to 100 μm.
Figure 22.
Morphologies in PLA30k-b-PEO5k copolymer films crystallized at (a) 110 °C (b) 120 °C from Tg of PLA block with heating after quenching from annealing temperature. The bar corresponds to 100 μm.
Figure 23.
AFM images of detailed morphological structures in asymmetric PLA-b-PEO copolymer films. PLA16k-b-PEO5k: isothermally crystallized at (a) and (a′) 130 °C, (b) and (b′) 110 °C, (c) and (c′) 125 °C; PLA30k-b-PEO5k: (d) and (d′) 110 °C from Tg of PLA block with heating after quenching from annealing temperature.
Figure 23.
AFM images of detailed morphological structures in asymmetric PLA-b-PEO copolymer films. PLA16k-b-PEO5k: isothermally crystallized at (a) and (a′) 130 °C, (b) and (b′) 110 °C, (c) and (c′) 125 °C; PLA30k-b-PEO5k: (d) and (d′) 110 °C from Tg of PLA block with heating after quenching from annealing temperature.
Furthermore, lamellar thickness (d) of PLA16k-b-PEO5k copolymer crystallized in a hard environment increases from 18.6 nm at 110 °C, 19.3 nm 125 °C, to 20.5 nm 130 °C. The values of lamellar thickness (d) of the copolymers crystallized in a soft confined environment are about 4~6 nm larger than those formed in melt crystallization. Alternate amorphous–crystalline phase structure is formed in both soft and hard confined environments. As a consequence, the lamellar thickness (d) evaluated by AFM is the sum of PLA lamellar thickness (dPLA,c) and the amorphous layer thickness (da). In a soft confined environment, the PLA block and the PEG block are homogeneous or weakly segregated (miscible) in the melt state. As the temperature is decreased, crystallization is the dominating driving force leading to extensive rearrangement of the morphology when the PLA block crystallizes first. During its crystallization, the lamellar thickness increases. After the PLA block crystallizes completely, the lamellar thickness is 12–16 nm, which consists of the lamellar crystal thickness of the PLA block and the thickness of the amorphous PEG layer if the thickness of the amorphous PLA layer is neglected. In a hard confined environment, the immiscibility of PLA and PEO blocks, and vitrification of PLA block during the annealing from the disordered state and isotheral process at 30 °C (even part of PEO block is probably to be crystallized) form a relatively stable layer–layer structure. During the crystallization of PLA after heating from 30 °C, despite the PEO block being completely melted, the microdomain formed by microphase separation confines the crystallization of PLA block and diffusions of PLA and PEO blocks. After PLA block crystallizes completely, the lamellar thickness is 18–20 nm, which consists of the lamellar crystal thickness of the PLA block and the thickness of the amorphous PEG and PLA layer, and the thickness of the amorphous PLA layer cannot be neglected. In a soft confined environment, most of the PEO component may distribute on the surfaces of the crystalline morphologies driven by the competition between crystallization of PLA and microphase separation. In a hard confined environment, the stabilized microdomains make PEO blocks distribute between the PLA lamellae. As a result, the increase of the lamellar thickness is probably to be ascribed to the increase of thickness of amorphous PEO and PLA layer.
3.3. Dendritic Superstructures and Structure Transitions in PEO-b-PCL Diblock Copolymers [48]
Optical microscopic images of PLA
16k-
b-PEO
5k copolymer thin films isothermally crystallized at different temperatures for very long times are shown in
Figure 24. Dendritic morphologies are formed. At 90 °C shown in
Figure 24a, dendritic morphologies with discal contour were observed, which were formed with many radial branches. Dendritic superstructures with hexagonal contour were observed in thin films crystallized above 100 °C, as seen in
Figure 24b–d. The images show that the dendritic morphology is composed of six sectors with some overgrowth lamellae. The change of the morphology contour with crystallization temperature shown in
Figure 1 is ascribed to the temperature dependence of nucleation and growth, the addition of PEO block, and phase separation between the two unlike blocks during the crystallization of PLA.
Figure 24.
Morphologies of PLA
16k-
b-PEO
5k copolymer thin films crystallized at (
a) 90 °C; (
b) 100 °C; (
c) 110 °C; and (
d) 120 °C. The thickness of the films is ~220 ± 30 nm. The bar corresponds to 100 μm. Reprinted with permission from American Chemical Society, 2009 [
48].
Figure 24.
Morphologies of PLA
16k-
b-PEO
5k copolymer thin films crystallized at (
a) 90 °C; (
b) 100 °C; (
c) 110 °C; and (
d) 120 °C. The thickness of the films is ~220 ± 30 nm. The bar corresponds to 100 μm. Reprinted with permission from American Chemical Society, 2009 [
48].
To visualize the detailed crystalline morphologies of PLA
16k-
b-PEO
5k copolymer, tapping mode AFM was employed. The AFM images are shown in
Figure 25. For the thin films crystallized at lower temperatures, the number of the main branch every crystalline morphology is larger than that of identical films crystallized at higher temperatures. Lozenge-spiral dislocation (right arrowed in the phase images at 120 °C), lozenge multilayer (arrowed in the images at 100 °C), or truncated-lozenge multilayer (left arrowed in the images at 120 °C) was formed in the growth direction perpendicular to the lamellae. The size of lamellae increases with crystallization temperature. The d spacing of lamellae 10–12 nm thick is identical with that of the PLA single crystal. The crystalline morphology is formed by stacking flat-on growing lamellae in the thin film. Lozenge-shaped single crystal with screw dislocation of PLA has been reported in thin film of PLA or its copolymers that the thickness is less than 100 nm [
6,
7,
12,
55,
56,
57]. However, the special crystalline morphology of hexagonal dendrite stacked with PLA single crystals has never been reported. The dependence of crystalline morphology on
Tc can be explained by supercooling and phase separation. For PLA homopolymer thin films with identical thickness, spherulitic morphologies are formed in the temperature range from 90 to 120 °C. For PEO-
b-PLA copolymer thin films, microphase separation driven by the crystallization of PLA block formed a layer–layer structure (that is to say alternate PLA- and PEO-rich domains). Furthermore, the size of the PLA layer is quite smaller to the film thickness, which is tens or a hundred nanometers, as determined by AFM. At lower
Tc, the supercooling is larger and many nucleus are formed rapidly. Subsequently, many small PLA single crystals grow and star-shaped dendritic morphologies are observed at 90 °C in
Figure 24a and
Figure 25a. While at a higher
Tc, the number of elementary nucleuses is relatively small; secondary nucleation is therefore important in the crystallization process. As a consequence, dendritic morphologies composed with main-branches, secondary side-branches and tertiary side-branches were formed shown in
Figure 24b–d and
Figure 25b–d. Furthermore, the structure in the areas of the films that appear not to contain dendritic structures was amorphous PEO and PLA blocks. At high crystallization temperatures, it is difficult for nucleation, especially for homogeneous nucleation. Once the nucleus was formed, crystallization procedures begin by diffusion and ordering of PLA chains. In the crystallization process, the PLA block can preferentially attach to the growth front, and then the accumulation of amorphous components in the vicinity of the dendrites will prevent the nucleation and crystallization in the areas.
Figure 26 shows typical optical microscope (OM) images of PLA
16k-
b-PEO
5k thin films with different thicknesses crystallized at 110 °C. Spherulites are formed in ~10 μm thick films as shown in
Figure 26a, while for the films in the thickness range from 1 to ~200 nm, dendritic morphologies were observed, as seen in
Figure 26b–e. Maltese cross extinction was not found in the center of any crystalline morphology in the films with the thickness range from 1 to ~200 nm, indicating that the optic axis (the chain axis) is normal to the plane of the film. The morphologies in
Figure 26b–e are discal dendritic as confirmed from AFM observations. Most of their fringe frame of the morphologies formed in thin films with the thickness from 200 to 400 nm are hexagonal as shown in
Figure 26c–e, and the size of the hexagonal morphologies is about 100 μm. The hexagonal dendritic morphologies were composed of six sectors. In the film with 115 nm thickness (
Figure 26f), lozenge-shaped lamellae were formed. The stack and the growth of PLA single crystals are oriented, as shown in
Figure 26f. The diffusion and crystallization processes of PLA block drove microphase separation between the unlike blocks, and the formation of concentration gradient of PLA. Moreover, the average growth rate was nearly linear, and it was easy to form dendritically and not spherulitically in thin film. As a result, the hexagonal contour morphology was likely formed. The formation of the dendritic morphologies stacked with PLA single crystals in PEO-
b-PLA thin films ranging from 1 to 200 nm thick is related with the phase structure and its evaluation during the crystallization process, and the mechanism will be further discussed in the following paragraph.
Figure 25.
AFM height (
left column) and phase (
right column) images of PLA
16k-
b-PEO
5k copolymer thin films crystallized at (
a) and (
a′) 90 °C; (
b) and (
b′) 100 °C; (
c) and (
c′) 110 °C; (
d) and (
d′) 120 °C; and (
e) and (
e′) 110 °C, A: the main branch; B: the secondary side-branch; C: the tertiary side-branch. Reprinted with permission from American Chemical Society, 2009 [
48].
Figure 25.
AFM height (
left column) and phase (
right column) images of PLA
16k-
b-PEO
5k copolymer thin films crystallized at (
a) and (
a′) 90 °C; (
b) and (
b′) 100 °C; (
c) and (
c′) 110 °C; (
d) and (
d′) 120 °C; and (
e) and (
e′) 110 °C, A: the main branch; B: the secondary side-branch; C: the tertiary side-branch. Reprinted with permission from American Chemical Society, 2009 [
48].
Figure 26.
Morphologies of PLA
16k-
b-PEO
5k copolymer thin films crystallized at 110 °C. Film thickness: (
a) ~10 μm; (
b) 1 μm; (
c) ~400 nm; (
d) ~300 nm; (
e) 220 nm; and (
f) 115 nm. The bar corresponds to 100 μm. Reprinted with permission from American Chemical Society, 2009 [
48].
Figure 26.
Morphologies of PLA
16k-
b-PEO
5k copolymer thin films crystallized at 110 °C. Film thickness: (
a) ~10 μm; (
b) 1 μm; (
c) ~400 nm; (
d) ~300 nm; (
e) 220 nm; and (
f) 115 nm. The bar corresponds to 100 μm. Reprinted with permission from American Chemical Society, 2009 [
48].
Figure 27 shows the effect of the component on crystalline morphology. It can be seen that the morphology changes from spherulite (
Figure 27a) to dendrite (
Figure 27b), to dendrite with hexagonal contour (
Figure 27c) with the volume fraction of PEO increasing. The effect of PEO blocks on the crystallization and morphological behaviors of PLA blocks is multiple. The melting points of PLA block in PEO-
b-PLA copolymers decrease with its fraction decreased, so the degree of supercooling (Δ
T =
Tm −
Tc) decreases with PEO fraction increase. Subsequently, at 110 °C, it is a low temperature range of crystallization for PLA homopolymer, and a spherulite is formed (
Figure 27a), while for PLA
30k-
b-PEO
5k and PLA
16k-
b-PEO
5k copolymers, it is a high temperature range of crystallization, and dendritic morphologies are observed (
Figure 27b,c). It is common to form a dendritic morphology in thin film, and this has been previously discussed. Meanwhile, the volume fractions of PLA block and the total degree of polymerization (N) are different in the three samples in
Figure 27a–c; the segregated strength of microphase separation between the two blocks is different. As a result, the number of nucleus of every area, and the density of the branches in PLA
30k-
b-PEO
5k copolymer thin film, are larger than that in PLA
16k-
b-PEO
5k. That is to say, as the volume fraction of PEO block increases, the effect of PEO on the crystalline morphology also increases. It affects the crystallization of PLA block in three ways. First, the number of PLA blocks decreases in thin film of the same thickness. Second, the diffusion of PLA block becomes more difficult because of the increase of the volume fraction of PEO. Third, the size of microstructure driven by phase separation may be larger. As a consequence, we obtained the evolution of crystalline morphology in copolymer thin films with a different component in
Figure 27. The morphologies were observed after annealed to room temperature. As a result, the PEO blocks might also crystallize, but the wide-angle X-ray diffraction (WAXD) research has confirmed that it is quite difficult for PEO block to crystallize in such asymmetric block copolymers because of the strong confinement from microphase separation.
Figure 27.
Morphologies of (
a) PLA
31k homopolymer; (
b) PLA
30k-
b-PEO
5k; and (
c) PLA
16k-
b-PEO
5k copolymer films with the thickness of ~220 ± 30 nm crystallized at 110 °C. The bar corresponds to 100 μm. Reprinted with permission from American Chemical Society, 2009 [
48].
Figure 27.
Morphologies of (
a) PLA
31k homopolymer; (
b) PLA
30k-
b-PEO
5k; and (
c) PLA
16k-
b-PEO
5k copolymer films with the thickness of ~220 ± 30 nm crystallized at 110 °C. The bar corresponds to 100 μm. Reprinted with permission from American Chemical Society, 2009 [
48].
It is not certain whether the crystalline morphologies observed by OM and AFM in
Figure 24,
Figure 25,
Figure 26 and
Figure 27 are those created during crystallization of PLA block. It is possible that some of the patterns are formed by rearrangements after crystallization (or rearrangements of already crystallized parts while new material is still crystallizing. In order to confirm whether the morphologies observed are those formed during crystallization or not,
in situ OM experimentation for isothermal crystallization was carried out.
Figure 28 shows the polarized optical microscopic images of morphological growth of PLA
16k-PEO
5k copolymer thin film isothermally crystallized at 110 °C. At 12 min, as shown in
Figure 28a, the similar dendritic morphologies with hexagonal contour as those in
Figure 24 and
Figure 26 are observed. From
Figure 28b–d, the thickness and the size of the crystalline morphologies grew thicker and larger, but the crystalline morphology observed by OM is remains the same the entire time, indicating the morphologies observed are created during crystallization.
The thickness of copolymer film in the experiment is 1 μm~200 nm; as a result, the substrate may influence the crystalline morphology. It is possible that one of the blocks adsorbs strongly to the substrate. If this is the case, then the strong attachment of one block to the substrate can control the morphology. PEO-b-PLA copolymers and PLA/PEO blends belong to a weakly segregated system, PEO and PLA could not be strongly segregated or one of them strongly adsorbed to the substrate. Meanwhile, the film thickness range from 1 μm to 200 nm is much larger than the size of the extended copolymer chain, so the effect of substrate on the morphology is negligible.
Figure 28.
Polarized optical microscopic images of PLA
16k-PEO
5k thin films crystallized at 110 °C at (
a) 12 min; (
b) 18 min; (
c) 28 min; and (
d) 49 min after being completely melted at 180 °C. The bar corresponds to 100 μm. Reprinted with permission from American Chemical Society, 2009 [
48].
Figure 28.
Polarized optical microscopic images of PLA
16k-PEO
5k thin films crystallized at 110 °C at (
a) 12 min; (
b) 18 min; (
c) 28 min; and (
d) 49 min after being completely melted at 180 °C. The bar corresponds to 100 μm. Reprinted with permission from American Chemical Society, 2009 [
48].
Figure 29 shows the time-dependent changes in surface morphology of PEO-
b-PLA thin films with 220 nm thickness during isothermal crystallization at 110 °C. At 125.9 °C, the sectors labeled by arrows in
Figure 29b started melting, when the temperature reached 144 °C, as shown in
Figure 29c. The sectors next to the labeled ones began to melt, and at 148.9 °C in
Figure 29d, the sectors labeled with arrows obfuscated the crystalline morphology, while clarity in the sectors next to the labeled ones indicated that the thermal-stability of the sectors were different.
For PLA crystals grown in spin-coated thin films on solid surfaces, four symmetrically disordered sectors in the hexagonal superstructure are formed. They can be classified into two sectors, (100) and (110), in terms of chain folding and crystal growth directions. From the growth process in
Figure 29 and the dendritic morphologies with hexagonal contour formed in thin film with ~200–400 nm thickness in
Figure 24,
Figure 25 and
Figure 26, the hexagonal superstructure is composed of two different sectors shown in
Figure 30a,b.
Figure 30c’s schematic represents the hexagonal morphology. In the sector (110) growth plane, the chain-folding direction is the same as the crystal growth direction. On the other hand, the chain-folding direction in the sectors with a (100) growth plane alternates between (110) sectors. In the case of PLA thin film, the sectors with (100) growth plane showed some striations, they are along the main-branches and parallel with each other, the striations are nearly perpendicular to the crystal growth face, suggesting that the disordered chain (amorphous PLA and PEO phase) packing exists in addition to the chain-folding instability.
Figure 29.
Optical micrographs of the melting process of PLA
16k-
b-PEO
5k thin film (~220 nm thick, isothermally crystallized at 110 °C). Reprinted with permission from American Chemical Society, 2009 [
48].
Figure 29.
Optical micrographs of the melting process of PLA
16k-
b-PEO
5k thin film (~220 nm thick, isothermally crystallized at 110 °C). Reprinted with permission from American Chemical Society, 2009 [
48].
Figure 30.
Two types of sectors, (110) and (100), were classified in terms of chain-folding and crystal growth directions in hexagonal superstructure of PLA-
b-PEO copolymer thin films isothermally crystallized at (
a) 110 °C and (
b) 125 °C. (
c) Schematic representation of the hexagonal superstructure of PLA-
b-PEO copolymer thin film. Reprinted with permission from American Chemical Society, 2009 [
48].
Figure 30.
Two types of sectors, (110) and (100), were classified in terms of chain-folding and crystal growth directions in hexagonal superstructure of PLA-
b-PEO copolymer thin films isothermally crystallized at (
a) 110 °C and (
b) 125 °C. (
c) Schematic representation of the hexagonal superstructure of PLA-
b-PEO copolymer thin film. Reprinted with permission from American Chemical Society, 2009 [
48].