Composite banded core and non-banded shell transition patterns in stereocomplexed poly(lactide acid) induced by strongly interacting poly(p-vinyl phenol)

Abstract

Banded cores and non-banded shells with transitional patterns in stereocomplexed poly(lactic acid) sc-PLA (PLLA : PDLA = 1 : 1) interacting with amorphous poly(p-vinyl phenol) (PVPh) (20–30 wt%) were investigated by using polarized optical (POM), atomic force (AFM), and scanning electron microscopy (SEM). The central core of the spherulites is ring-banded, which transitions into non-banded patterns on the outer peripheral in sc-PLA/PVPh blends crystallized at T_c = 160–180 °C. This composite crystal morphology is distinctly different from that of the neat sc-PLA, which is a Maltese-cross without any ring patterns in spherulites. The bands in the core region are also dramatically different from those bands with birefringence contrasts observed in classical ring-bands. The bands in the core show only crystal topology and no birefringence difference. Both regions of ring-banded and non-banded crystals are composed of edge-on lamellae, but the central-core lamellae undergo periodical waving up and down. The specific stage-wise and preferential interactions between PVPh and sc-PLA and glass substrate, which simultaneously occur during amorphous PVPh rejection and growth processes, were evaluated as some plausible kinetic reasons for the special composite crystal morphology of sc-PLA complex induced by blending with PVPh.

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Introduction

After the first finding and reporting of stereo-complexes between chiral poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) by Ikada et al.,¹ numerous studies have been conducted on the formation of the stereo-complexes as well as their crystalline morphology, crystallization behavior and physical properties.^2–4 A stereocomplexed PLA (sc-PLA) can be obtained from equi-molar (1 : 1) racemic mixtures of PLLA and PDLA. Physical properties of sc-PLA are significantly different from those of homopolymeric PLLA or PDLA, in four aspects: (a) the melting temperature (T_m) of sc-PLA is 50 °C higher than that of PLLA or PDLA,^2,5,6 (b) sc-PLA crystallizes into trigonal unit cells with 3₁-helical conformation,^7–9 (c) it is more compact and has higher mechanical properties than PLLA or PDLA,⁹ and (d) unlike PLLA or PDLA, sc-PLA spherulites are not ring-banded when PDLA/PLLA blends are crystallized at any T_c. The crystalline morphology of a melt-crystallized sc-PLA spherulite typically shows a Maltese-cross pattern, which may become disordered and less distinct under nonequimolar conditions owing to a lesser extent of complexation between the two chiral polymers.^1,5,10,11

Unlike the homopolymers PLLA or PDLA that can form ring-banded spherulites at a specific crystallization condition, sc-PLA (stereo-complexed PLLA/PDLA: 1/1 mixture) has never been reported to form ring-banded spherulites at any T_c. Neat PLLA was reported to form ring-banded spherulites at a narrow range (ΔT = 5 °C) of T_c = 125–130 °C when crystallized with or without top cover.¹² However, inclusion of miscible amorphous polymers to PLLA usually does not change the ring-banding nature, but may shift the range of T_c within which ring-banded spherulites appear. PLLA can show ring-banded patterns at a wider range of T_cs (110–130 °C) when it is blended with poly(hydroxy butyrate) (PHB).¹² The PLLA/poly(ethylene oxide) (PEO) blend melt-crystallized at T_c = 110 °C, at which the PEO component in the blend, being in the molten amorphous state, also exhibited a ring-banded pattern.^13–15 The miscible PEO component was believed to induce re-orientation of lamellae, resulting in ring-banded spherulites, as shown in a previous study.¹⁶ In the miscible PEO/PLLA blends, a greater fraction of the amorphous polymer can be trapped between the lamellae of spherulites to form interference rings. On the other hand, PDLA has been reported to a lesser extent to form ring-banded spherulites either in neat conditions or while blending with another component. A study has reported the formation of ring-banded patterns in the blends of PDLA 75/25 poly(lactide)/poly(ethylene glycol) (PEG) films crystallized at 120 °C after annealing 1 h at 160 °C.¹⁷ In contrast to homopolymeric PLLA or PDLA or their blends with other polymers, equi-molar sc-PLA has never been reported to form ring-banded spherulites even after the addition of another component. Several studies have investigated several instances of sc-PLA blended with another polymer such as PHB,¹⁸ poly(d,L-lactide) (PDLLA),¹⁹ poly(methyl methacrylate) (PMMA),²⁰ and poly(vinyl phenol) (PVPh).²¹ The addition of another polymer to sc-PLA was reported to influence the crystallization behavior^19,21 and improved the processability and thermomechanical properties of sc-PLA.²⁰ The crystalline morphology of stereocomplexed PLA (sc-PLA) could change from the original nicely rounded Maltese-cross spherulites to the dendritic form after being blended with high PHB contents (50 wt% or higher) and crystallized at high T_c (130 °C or above), at which PHB was in the molten state or acted as an amorphous species.¹⁸ That study shows that the addition of amorphous species could affect some changes on the crystalline morphology of sc-PLA, but none have reported findings of banded spherulites in sc-PLA or its blends.

A previous study²¹ has reported the blend of sc-PLA, from a 1 : 1 mixture of high molecular weight PLLA (HM_w-PLLA) and PDLA, with amorphous PVPh and pointed out that PVPh influences the crystallization behavior of sc-PLA by acting as a nucleating agent instead of affecting the sc-PLA crystalline morphology, which is negative-type Maltese-cross spherulites. It is generally known that, although neat PLLA or PDLA may crystallize into ring-banded spherulites within a certain T_c range, their racemic 1 : 1 mixture forms a stereo-complex that does not form ring-banded spherulites at any T_c. Our previous study²² has reported the crystal morphology of LM_w-PLLA/PVPh blends, which show three types of spherulite morphologies. No ring-banded morphology was observed in the LM_w-PLLA/PVPh blends, suggesting that PVPh can induce the morphology changes in different respects. In this study, we further demonstrated that a ring-banded pattern could be induced in the stereocomplex of PLLA/PDLA mixture, as long as differential interactions (among PVPh–polymer–substrate) were created. This study used low-molecular weight PLLA (LM_w-PLLA) to mix with PDLA and obtain sc-PLA from an equi-molar mixture of PLLA and PDLA. Then, amorphous PVPh was added in order to observe its effects on the crystalline morphology and lamellar assembly of sc-PLA. The role of PVPh addition in affecting the morphology of sc-PLA was also evaluated.

Experimental section

Materials and preparation

Poly(L-lactide) (PLLA), with a relatively low molecular weight, was purchased from Polysciences, Inc. (USA). PLLA (code name: LM_w-PLLA) was characterized using gel permeation chromatography (GPC, Waters) to reveal M_w = 11 000 g mol⁻¹ (PDI = 1.1). LM_w-PLLA has T_g = 45.3 °C and T_m = 155 °C measured from DSC characterization. Low-molecular-weight PLLA was used for forming stereo-complex because it has a stronger complexing capacity. Poly(D-lactide) (PDLA) was obtained from Fluka, Inc. (Switzerland), with weight-average molecular weight of 124 000 g mol⁻¹, T_g(onset) = 54 °C and T_m = 170 °C. Poly(4-vinyl phenol) (PVPh) with M_w = 22 000 g mol⁻¹ and T_g = 148 °C, was obtained from Polysciences, Inc. (USA).

Film samples of stereocomplexed PLLA/PDLA and amorphous PVPh with various weight fractions were prepared by solution blending using p-dioxane as a solvent with the concentration of 4 wt% (film thickness = 6–9 μm). The sc-PLA was obtained from equi-molar (1 : 1) mixture of PLLA and PDLA, which was further blended with amorphous PVPh. Thus, in the blend of sc-PLA/PVPh (70/30), the composition of each component in the ternary composition is PLLA/PDLA/PVPh (35/35/30). A drop of the polymer solution (sc-PLA/PVPh) was deposited and uniformly spread on a micro glass slide at 45 °C and the solvent was allowed to fully evaporate in atmosphere and dried in vacuum ovens. Prior to characterization, the dried film samples were thermally treated by the following steps: samples were heated on a hot plate at a maximum melting temperature (T_max = 240 °C) held for t_max = 1 min to erase the prior crystals, and then rapidly moved to another hot stage pre-set at a designated isothermal T_c = 110 °C (or other T_cs) for full crystallization. Such procedures ensured rapid quenching at a designated temperature for isothermal crystallization with minimum temperature lag or fluctuation. For comparison purposes, the time held at T_max = 240 °C was also varied to a value different from t_max = 1 min such as 3 or 5 min; however, T_max, T_c, or other thermal treatments were kept the same.

Apparatus and procedures

A polarized optical microscopy (POM, Nikon Optiphot-2), equipped with a Nikon NFX-DX exposure and Nikon NFX-35 digital camera, and a microscopic hot stage (Linkam THMS-600 with TP-92 temperature programmer), was used to characterize the optical birefringence and crystalline morphology of the crystallized samples. POM was also used for in situ monitoring of spherulite growth. Growth rate measurements of spherulites at selected T_cs were performed on a temperature-controlled microscope hot stage with digital camera and automated software capable of monitoring/recording in situ images of spherulites in a fixed time interval.

The glass transitions (T_g) of the blend samples were measured with a differential scanning calorimeter (Diamond DSC, Perkin-Elmer) equipped with an intracooler for sub-ambient temperatures down to −70 °C. For determining the T_g transition temperatures, cold-crystallization temperatures, and melting transitions of the crystallized sc-PLA/PVPh blend, DSC characterizations were made at a heating rate of 20 °C min⁻¹. During thermal annealing or scanning, a continuous nitrogen flow in the DSC sample cell was maintained to ensure minimal sample degradation.

Atomic-force microscopy (AFM, diCaliber, Bruker Corp.) investigations were performed in intermittent tapping mode with a silicon-tip (f_o = 70 kHz, r = 10 nm) installed. The largest scan range was 150 × 150 μm, and the scan was maintained at 0.5 Hz for the overview scan and zoom-in regions (10 × 10 μm). AFM samples were prepared in the same thermal treatments as those for POM samples. AFM measurements were also carried out to determine the height profiles and phase images for mapping the morphology and lamellar patterns of the crystals in spherulites.

Scanning electron microscopy (SEM) (FEI Quanta-400F, Hillsboro, OR, USA) was performed to reveal the surface morphology of the spherulites. The film samples intended for SEM characterization in this study were prepared without top cover. Prior to the SEM characterization, the top surfaces of the exposed film samples were coated with gold by vacuum sputtering.

Results and discussion

Phase behavior and crystalline morphology

Phase homogeneity in sc-PLA/PVPh was first investigated. Although it is known that PVPh is miscible with either PLLA or PDLA in each of the binary mixtures, but phase behavior in ternary PLLA/PDLA/PVPh mixtures was yet to be demonstrated. Fig. 1 shows the DSC thermograms for neat PLLA-LMw, PDLA, and a ternary blend of (PLLA/PDLA)/PVPh (i.e., sc-PLA/PVPh) of several compositions with increasing PVPh content. The sc-PLA/PVPh blends (with 10–50 wt% PVPh) show a single glass transition temperature (T_g), which increases with higher PVPh content, as shown by the black-line plots in Fig. 1. The apparently single T_g with composition dependence indicates that the ternary blends are miscible over this composition range. In addition, the high T_g of PVPh component in the blend induces a significant shift in T_g of sc-PLA to higher temperatures. The endothermic melting peak of sc-PLA/PVPh blends is found to be constant as the PVPh content increases. The endothermic peak of the blends at ca. 220 °C can be assigned as melting of the stereocomplex crystallites. As shown in Fig. 1 (blue and pink-line plots), the melting peak of neat LM_w-PLLA and HM_w-PDLA are around 145 °C and 170 °C, respectively. These melting peaks of homopolymers PLLA and PDLA do not appear in the DSC thermograph of the sc-PLA/PVPh blends, indicating that the stereocomplex crystallites were completely generated from the equi-molar mixture of PLLA and PDLA. Although no apparent down-shift of melting peak can be found upon the addition of PVPh to sc-PLA, the enthalpy of melting of sc-PLA/PVPH slightly decreases as the PVPh content increases. Furthermore, the cold-crystallization temperature of the stereocomplex PLLA/PDLA shifts to a significantly higher temperature with PVPh content, suggesting interactions between sc-PLA and PVPh. The single T_g and up-shift of cold crystallization temperature are the typical thermal behaviors for a miscible blend of sc-PLA with PVPh.

Fig. 1 DSC thermograms (2nd scans) for sc-PLA/PVPh blend of different compositions as labeled on traces.

Fig. 2 shows spherulite morphologies of sc-PLA at various crystallization temperatures (T_c). No ring-band pattern can be observed as the T_c increases. The ringless Maltese-cross sc-PLA spherulites of the LM_w-PLLA/PDLA blend exhibit negative-type birefringence. The negative-type Maltese-cross spherulite found in sc-PLA is similar to the crystalline morphology of sc-PLA from an equi-molar mixture of HM_w-PLLA and PDLA, as reported in a previous study,²¹ which showed that the typical crystalline morphology for neat sc-PLA is ringless and negative type when crystallized at any T_c; i.e., regardless of the M_w of PLLA, the spherulite patterns of sc-PLA are similar, although lower-M_w PLLA does have better complexing capacity with PDLA.

Fig. 2 POM and OM images showing the crystalline morphology of neat sc-PLA at various crystallization temperatures (T_cs).

The morphology of sc-PLA blended with PVPh was compared to that of neat sc-PLA. After the addition of 30 wt% PVPh to sc-PLA, the crystalline morphology crystallized at a range of T_cs (160–175 °C) significantly changed with the crystallization temperatures (T_cs). Fig. 3 shows POM and OM images for the crystalline morphology of the sc-PLA/PVPh (70/30) blend at various crystallization temperatures (T_cs). The composite spherulites show ring-banded patterning in the central core eventually transforming into a non-banded pattern in the outer peripheral of the spherulite. The ring-band pattern in the center of the spherulite becomes more pronounced as the T_c increases. As a matter of fact, the sizes of the banded core and width of each band steadily increase with T_c from 165 to 180 °C. There are reasons why both POM and OM micrographs for crystallized sc-PLA/PVPh were characterized and discussed here. Interestingly, the bands in the central core do not show obvious birefringence differences between the ridges and valleys, in contrast to the obvious blue/orange birefringence colours seen in classical banded spherulites. In POM images (with a tint plate), the ring-band pattern in the core region is not really obvious, masked by the stronger birefringence of other dendritic crystals. However, in the OM images, the bands in the core are clear and regular enough with only crystal topology but with low or nearly non-birefringence. This fact also suggests that these ring bands in the core region of sc-PLA/PVPh may be due to only periodical height variation of the crystals and no re-orientations in the corresponding ridge/valley bands.

Fig. 3 POM (upper row) and OM (bottom row) images showing the crystalline morphology of sc-PLA/PVPh (70/30) blend at various crystallization temperatures (T_cs = 160–175 °C).

Such unique and complex features with transition from banded core to non-banded outer shell in spherulites would be later further justified by AMF analysis on the central core regions. Interestingly, this special composite morphology in sc-PLA/PVPh has never been observed before in sc-PLA blends with other amorphous polymers. In the present study, this unique morphology can also be seen in the sc-PLA/PVPh blend with lower PVPh content (20 wt%), as shown in the ESI of Fig. S1.† However, the ring-banded pattern in the central region of the sc-PLA/PVPh (80/20) blend composition is less obvious than that of the sc-PLA/PVPh (70/30) blend composition. Therefore, in this present study, we chose the composition of sc-PLA/PVPh (70/30) blend melt-crystallized at T_c = 170 °C for further detailed characterization. When the sc-PLA was blended with smaller amounts of PVPh (5 and 10 wt%), the typical crystalline morphology of sc-PLA, with distinct Maltese-cross non-banded spherulites, did not change at all but exhibited all the features of neat sc-PLA; i.e., sc-PLA/PVPh compositions with PVPh less than 20 wt% still show negative-type Maltese-cross spherulites with no ring-band pattern, which is similar to that of neat sc-PLA.

ESI in Fig. S2† shows the crystalline morphology of (a) sc-PLA/PVPh (95/5) and (b) sc-PLA/PVPh (90/10) blends at various crystallization temperatures (T_cs). From this observation, it is clear that the addition of PVPh of equal to or more than 20 wt% into sc-PLA starts to influence the crystalline morphology of sc-PLA in order to display such a composite ring-banded/ringless spherulite pattern. Fewer amounts of PVPh (<20 wt%) in the blends do not show any effect on the stereocomplex morphology.

Fig. 4 shows POM and OM images of whole spherulite of sc-PLA/PVPh (70/30) at T_c = 170 °C (a), the AFM height image of ring-banded pattern (b), and AFM height profile of some parts in ring-banded pattern as indicated by the yellow-dashed line (c). The OM image in Fig. 4(a) clearly shows that the spherulite morphology is composed of a ring-banded pattern in the center and non-banded pattern in the outer region of the center. The AFM height image in Fig. 4(b) displays in detail the ring-banded pattern in the center of the spherulite. It clearly shows that the bright and dark regions corresponded to the ridge and valley regions, respectively. The AFM height profile quantitatively shows the height of ridge and valley region in the ring-banded pattern. The distance between two ridges is around 15–20 μm, and the height difference of the highest ridge is around 350 nm from the valley. The AFM height image in Fig. 4(b) also shows there is an intermediate ridge between the high ridge and valley. The AFM height profile shows that the intermediate ridge is lower than the high ridge. The height of lower intermediate ridge is around 200 nm, about half the height of the high ridge (350 nm). Thus, each band is composed of three hierarchical lamellae of high ridge-half ridge-valley, and this is repeated from the banded core until transition into ringless regions.

Fig. 4 (a) POM and OM images of whole spherulite of sc-PLA/PVPh (70/30) at T_c = 170 °C, (b) AFM height image of ring-banded core-region, and (c) height profile along yellow-dashed line of image-(b).

Detailed lamellar arrangement by AFM

The AFM tapping-phase characterization was carried out to observe the detailed lamellar assembly in three different regions (high ridge, intermediate ridge, and valley) of each band in the ring-banded core of the blend morphology. Fig. 5 shows the phase images of the three regions (high ridge, int. ridge, and valley) as indicated by squares in the AFM height image. The lamellae in the high ridge, low ridge and valley regions are composed of mainly edge-on lamellae; i.e., all these regions in each band are composed of the same lamellae pattern of no twists or re-orientation, although the three regions have different height profiles that are responsible for the ring-banded pattern in the core of the composite spherulites. AFM graphs clearly demonstrate that the ring-banded pattern in the core of the composite spherulite is not caused by gradually twisting or spiraling lamellae from edge-on to flat-on. In some polymers, the ring bands in banded spherulites create vivid birefringence colour contrasts such as crystallized poly(butylene adipate) (PBA)²³ or poly(ethylene adipate) (PEA).²⁴ In these cases, the ridge part of PBA or PEA is composed of lamellae of orientations differing significantly from lamellae in the valley. For another example of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV),²⁵ there has been a proposal of twisting lamellae based on observations and analyses on crystallized thin films of PHBV. However, the vivid birefringence contrast of ring bands have also been claimed to be due to lamellae being radially (in valley) and longitudinally (in ridge) oriented in interior dissection into bulk 3D spherulites.²⁶ In this present study on sc-PLA/PVPh, the bands in the core region are dramatically different from bands with birefringence contrasts in PBA, PEA or PHBV; i.e., the ridge band is composed of not single- but double-peaks with high- and intermediate-ridges. Both intermediate and high ridges in composite sc-PLA/PVPh spherulites are packed by all edge-on lamellae; furthermore, even the valley lamellae are also fibrous edge-on, similar to those in the high ridge or intermediate ridge. In the double ridges (high and intermediate), the arrangement of edge-on lamellae is less tidy and more fanned out, with more space in comparison to that in the valley part, in which the lamellae are collectively bent into a concave “V”-shape and become more compact in the limited space. Lamellae in radial growth are not single-stalk crystal plates, as many early investigators claimed in monotonously proposing twists or spirals for the purpose of providing ground for the bands. Crystal lamellae, by nature, branch out as they grow outward from the center; however, they cannot remain single-stalk plate-like crystals as these melt-crystallized lamellae are polycrystalline with branches and not single crystals at all. These AFM graphs on sc-PLA/PVPh clearly reveal and demonstrate that most lamellae in the ridge region spray out lamellae branches and these branches bend in either right or left orientations with minor angles with respect to the original stalks. The fibrous edge-on lamellae even bend in the vertical direction along the radial growth to form periodical ridges and valley in the core banded region of the spherulites.

Fig. 5 The AFM phase images of three regions (high ridge, low ridge, and valley) in ring-banded pattern of the sc-PLA/PVPh (70/30) blend morphology at T_c = 170 °C, as indicated by squares in the AFM height image.

As mentioned in previous discussion, the morphology of the sc-PLA/PVPh (70/30) blend shows a ring-banded pattern in the center and non-banded pattern in the outer region of the spherulite center. Fig. 6 shows the scheme of whole composite spherulites and the detailed lamellar arrangement in four different regions: (I) ridge, (II) valley in central banded core in comparison to (III) lamellar bundles and (IV) inter-lamellar junction in non-banded outer peripheral region of the spherulite. The ridge and valley regions are both composed of edge-on lamellae as discussed in detail previously. The non-banded region consists of lamellar bundles and an inter-lamellar junction, as illustrated in the scheme in Fig. 6. Interestingly, the non-banded region is also arranged by edge-on lamellae, either in lamellar bundles or inter-lamellar junctions. Therefore, both ring-banded and non-banded regions are composed of the same lamellar pattern of edge-on lamellae. The arrangement of edge-on lamellae in the lamellar bundle part is slightly different from that in inter-lamellar junction parts. In the peripheral part, some edge-on lamellae are arranged in the radial direction and the others are in the tangential direction, as shown by the green and yellow arrows. In contrast, the arrangement of edge-on lamellae in the inter-lamellar junction region [Region-IV in the outer peripheral of the spherulite] is along the radial direction with slight bending to right and left, as shown in ESI Fig. S3.† From observing the detailed lamellae patterns in core vs. peripheral of spherulites, it has become more interesting to know the mechanism of such morphology formation. The mechanism will be proposed in the next discussion, after the SEM and kinetic observation in the following section.

Fig. 6 POM, AFM micrographs and scheme of whole spherulite. (I) Ridge, (II) valley in banded core, in comparison to (III) lamellar bundles and (IV) inter-lamellar junction in non-banded peripheral region of the spherulite.

Fig. 7 displays SEM images of the surface of the spherulite, which consists of a ring-banded pattern in the center (a) and non-banded pattern in the edge of spherulite (b). The morphology of ring-banded and non-banded patterns is similar, as already explored from AFM images in the previous discussion. Fig. 7(a) shows a wavy texture on the top surface of ring-banded central core region of the composite spherulite. However, a straight fibrous texture is pronounced on the top surface of non-banded pattern in the outskirt region of the spherulite, as shown in Fig. 7(b). The AFM and SEM characterizations both show that the lamellar arrangement in ring-banded and non-banded patterns is similar, but the top surface textures are different between the core and outskirt regions of the spherulite. Such patterns occur only after the addition of a certain amount of PVPh to sc-PLA. The inclusion of PVPh should be the main factor for the formation of the wavy texture in the ring-banded core in the sc-PLA spherulites.

Fig. 7 SEM images of composite banded/non-banded spherulites (a) ring-banded pattern in the center, (b) and non-banded pattern in the edge of composite spherulite.

Spherulitic growth rate

Before evaluating the effect of PVPh in the formation of ring-banded patterns, kinetic observation was necessarily carried out to measure the spherulitic growth rate of the sc-PLA spherulite in the presence of amorphous PVPh as a diluent. Fig. 8 shows the change of radius as function of time for ring-band and non-band regions (a) and in situ observation of crystal growth with increasing time (b). The growth rate of ring-banded patterns is higher than that of non-banded patterns. It is supposed that in the early growth stage, the crystals grow faster, leading to the formation of a ring-banded pattern. After that, the crystals grow more slowly than the previous growth, resulting in non-banded pattern formation. The existence of amorphous PVPh might influence the kinetic behavior of the spherulites.

Fig. 8 (a) Graph of crystal radius vs. crystallization time of sc-PLA/PVPh (70/30) melt-crystallized at T_c = 170 °C. (b) In situ POM images displaying the detailed growth process of unique spherulite.

Proposed mechanism of special spherulites formation

Fig. 9 shows an illustration of the mechanism for lamellar arrangement inside the ring-banded pattern in the crystalline morphology of sc-PLA/PVPh (70/30), which suggests the procedures for crystal formation of banded cores are followed by crystal plates of the non-banded peripheral. After nucleation, in the early growth, the sc-PLA materials diffuse to the growth front to form edge-on lamellae. During the growth process, the non-crystallizable materials (mainly PVPh) were rejected to the growth front of domains mostly underneath the sc-PLA crystals. It is well known that PVPh, with a –OH group, has better interaction with the glass substrate than does sc-PLA; therefore, PVPh chains would prefer to go more closely to the glass substrate. Moreover, although the PVPh was rejected to the underneath of the sc-PLA lamellae, the PVPh chains also maintain a strong interaction with sc-PLA via hydrogen-bonding interactions. However, PVPh has a hydroxyl group (–OH), which can strongly interact with the carbonyl group (C=O) of PLLA and PDLA via a hydrogen-bonding interaction. Fig. S4† shows the FTIR spectra of the sc-PLA/PVPh blends with the molecular interaction between PVPh and sc-PLA. With increasing sc-PLA content in the blend, the intensity of the IR peak of self-bonding –OH decreases, showing a corresponding increase in strength of intermolecular hydrogen bonding between PVPh and sc-PLA. The interaction between PVPh and glass substrate and sc-PLA, simultaneously induces the wavy texture (ring-band) in the sc-PLA morphology. The non-banded region appears after the complete formation of the ring-banded region in the core. The slower growth rate of this non-banded region causes more diffuse PVPh in sc-PLA lamellae, leading to the formation of non-banded patterning.

Fig. 9 Illustrative mechanism of lamellar arrangement inside the ring-banded core in crystalline spherulite of sc-PLA/PVPh (70/30) melt-crystallized at T_c = 170 °C.

Moreover, it should be emphatically pointed out that this special morphology (banded central core in transition to non-banded outer shell) could be found only in a specific conditions, such as amorphous PVPh content in the blends being more than 20 wt% and a fixed range of T_c (160–180 °C), suggesting that favourable kinetics might be a governing factor. For the sc-PLA/PVPh (70/30) blend, the composite core–shell ringed/ringless morphology was observed in the blend sample, which was melted for 1 min (t_max) at T_max = 240 °C before quenching to T_c. By varying the t_max, the blend of sc-PLA/PVPh (70/30) did not show the ring-banded and non-banded morphology at all. ESI in Fig. S5† shows the crystal pattern of the sc-PLA/PVPh (70/30) blend at T_c = 170 °C and various t_max. The crystalline pattern of the blend shows a dendritic pattern (flower-like pattern in OM image) at melting time (t_max) longer than 1 min.

Conclusions

Introduction of amorphous PVPh (20–30 wt%) into sc-PLA (1 : 1 equi-molar mixture of LM_w-PLLA and PDLA) was used as a model for investigating effects of strong inter-molecular interactions on the morphology of originally stabilized and stereo-complexed sc-PLA crystals. The ternary blend was first proven to be a miscible mixture as sc-PLA was blended up to 50 wt% of PVPh. A composite spherulite morphology (core–shell), composed of a ring-banded pattern in the center region gradually transforming into a non-banded pattern in the outer region, was found in sc-PLA/PVPh blends (70/30) melt-crystallized at a specific range of T_c (160–180 °C). Such unique morphology was only observed when the amorphous PVPh content in the blends was more than 20 wt%, suggesting that dilution with a critical amount of strongly interacting and amorphous PVPh is a pre-requisite for re-assembly of originally stabilized sc-PLA spherulites. This composite (core–shell) crystal morphology is distinctly different from that of the crystallized neat sc-PLA (no PVPh), which is a Maltese-cross ringless pattern without ring bands anywhere in the spherulites.

In the composite spherulites of the sc-PLA/PVPh blend, both regions of ring-banded and non-banded crystals are composed of edge-on lamellae, but the central-core lamellae undergo periodical waving up and down. AFM analyses reveal that each of the bands is actually composed of two ridges and two valleys, i.e., there is an intermediate ridge between the high ridge and valley. The AFM height profile shows that the intermediate ridge is lower than the high ridge. The unique pattern in the banded core of crystallized sc-PLA/PVPh blend is very different from the conventional single ridge-valley band normally seen in other crystallized polymers. Each band in the central core is composed of two ridges and two valleys, and the height of the lower intermediate ridge is around 200 nm, about half the height of the high ridge (350 nm). Thus, each band in the central core of the spherulites is composed of three hierarchical lamellae of high ridge-half valley-half ridge-valley, and this lamellar assembly is repeated from the banded core until transition into ringless regions. The inter-band spacing between two high ridges is about 15–20 μm. As all lamellae in hierarchical assemblies of high ridge-half valley-half ridge-valley are edge-on and pointing radially, there is no optical birefringence contrast between ridges and valleys, except for topological up-and-down profiles. The specific stage-wise and preferential interactions between PVPh and sc-PLA and glass substrate, which simultaneously occur during amorphous PVPh rejection and growth processes, were evaluated as the reason for the special composite crystal morphology of sc-PLA.

Acknowledgements

This work has been financially supported by a basic research grant (NSC-99-2221-E-006-014-MY3) for three consecutive years from Taiwan's National Science Council (NSC), now Ministry of Science and Technology (MOST), to which the authors express their gratitude. This research was also partially supported by the Ministry of Education, Taiwan, R.O.C., through the Aim for the Top University Project to the National Cheng Kung University (NCKU).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: The crystalline morphology of sc-PLA/PVPh (95/5), (90/10), and (80/20) blends at various T_cs. AFM micrograph of inter-lamellar junction in non-banded region. The FTIR spectra of sc-PLA/PVPh at various blend compositions. The crystalline morphology of sc-PLA/PVPh (70/30) blend at T_c = 170 °C and various t_max. See DOI: 10.1039/c4ra09859e

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