Significantly Accelerated Spherulitic Growth Rates for

The layer-by-layer film method infers promising applications, which might be ...... of the layer-by-layer film productions; and (iii) the resulting im...
0 downloads 0 Views 692KB Size
Download PDF
Article pubs.acs.org/JPCC

Significantly Accelerated Spherulitic Growth Rates for Semicrystalline Polymers through the Layer-by-Layer Film Method Yaqiong Zhang,† Hongjun Xu,† Jingjing Yang,† Shouyu Chen,‡ Yunsheng Ding,‡ and Zhigang Wang*,† †

CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui Province 230026, P. R. China ‡ Institute of Polymer Materials & Chemical Engineering, School of Chemical Engineering, Hefei University of Technology, Hefei, Anhui Province 230009, P. R. China S Supporting Information *

ABSTRACT: The influence of a molten liquid polymer layer on the crystallization of the beneath semicrystalline polymer has been seldom considered. In the study, the nucleation and growth of spherulites for the beneath polylactide (PLA) layer in poly(ethylene oxide)/polylactide (PEO/PLA) double-layer films during isothermal crystallization at various temperatures above the melting point of PEO have been investigated by using polarized optical microscopy, with the particular results compared with that for neat PLA and PLA/PEO blend films. It is interesting to find that the top covering molten PEO layer can greatly accelerate the spherulitic growth rate (G) of the beneath PLA layer. Another significant result is that the temperature for the measurable nucleation and spherulitic growth of PLA in the double-layer films can be eventually pushed down close to the glass transition temperature of neat PLA. The changes of glass transition temperature, Tg, for PEO/PLA multilayer films have been measured by using modulated differential scanning calorimetry and dynamic mechanical analysis, which reveal slight decreases of Tg for PLA layer due to the influence of PEO layer. The layer structures of fractured surface of the double-layer films are analyzed on the basis of the observation from scanning electron microscopy, and the existence of interdiffusion areas with irregular boundary between PEO and PLA layers is the key clue to understanding the significant acceleration of G for PLA. The layer-by-layer film method infers promising applications, which might be considered to well replace the blending method.

1. INTRODUCTION Studies on polymer crystallization kinetics have been persistent for several decades; they are substantially related to the resulting structures and morphologies and eventually control the final mechanical and other physical properties of the polymer materials.1−5 Because polymer materials are frequently in contact with some kinds of different forms of surfaces in a variety of applications such as microelectronic devices6 and composites,7 etc., or during industrial processing such as surface coating,8 injection molding,9 and coextrusion,10 etc., surfaceinduced crystallization of polymers has attracted more attention during the past decades, which on the other hand, provides an important way for tailoring polymer crystallization kinetics as necessary.11−13 When semicrystalline polymers are deposited on various types of substrates, heterogeneous nucleation is typically observable for the surface-induced crystallization, and the epitaxial crystallization as the most popular one provides a very useful way to control the crystalline structures and morphologies.11,14,15 For instance, Yan et al. investigated the epitaxial ordering process of poly(ε-caprolactone) (PCL) on the highly oriented polyethylene substrate and claimed that all of the PCL chains could be organized into similar ordered structures at sufficient time.16,17 However, it must be emphasized that for the above-mentioned surface-induced crystallization the research works have mostly focused on the © 2013 American Chemical Society

use of solid substrates, especially oriented solid substrates, although the topology of surface relief with pattern symmetry of amorphous substrates (usually the vacuum-evaporated amorphous carbon layers) was also reported to influence the orientation of the overgrowing polymer materials, a process known as graphoepitaxy or artificial epitaxy.11 From a comprehensive consideration, we sensed recently that the use of some liquid substrates, particularly the use of liquid polymer layers as the substrates for studying the surface-induced crystallization of semicrystalline polymers, might be of great interest. The consideration of the use of liquid polymer layers was also inspired by our recent finding that the liquid layers of PCL or PEO can efficiently tune the self-assembly of copper phthalocyanine (CuPc), an organic molecule, to improve the performance of the organic thin film transistors (OTFTs) based on the assembled CuPc films.18 On the other hand, the consideration of the use of liquid polymer layers can be fundamentally connected with the multilayer coextrusion process of polymers. Hiltner and coworkers have done excellent studies on the surface-confined crystallization of polymers within the multilayer films with their Received: January 17, 2013 Revised: March 1, 2013 Published: March 4, 2013 5882

dx.doi.org/10.1021/jp400580r | J. Phys. Chem. C 2013, 117, 5882−5893

The Journal of Physical Chemistry C

Article

rates of PLA in thin films. For comparison purposes, the films of PLA/PEO blends with different compositions were subjected to similar crystallization kinetics measurements. For providing a reasonable explanation of the surface-induced crystallization kinetics changes, the possible decreases of glass transition temperature of PLA component in PEO/PLA multilayer films were examined by using modulated differential scanning calorimetry (MDSC) and dynamic mechanical analysis (DMA), respectively. PEO/PLA multilayer films were considered to provide enhanced signals for MDSC and DMA measurements to demonstrate the changes of glass transition temperature. Furthermore, the layer structures of fractured PEO/PLA double-layer films were observed by using fieldemission scanning electron microscopy (FE-SEM), from which the possible existence of interdiffusion areas (or layers) consisting of the mixed PEO and PLA components was examined.

main attention paid to the effects of the nanometer-scale confinements and their potentially applicable properties of these types of films such as the significant reduction in gas permeability.19−21 The coextrusion process has been applied to fabricate films with thousands of alternating layers of two polymers with the layer thickness scaled from micrometers down to 20 nm or even lower. One characteristic of their studies is that the constituent polymers of the multilayer films were chosen to create a temperature window, in which a crystallizable polymer could melt and recrystallize within the confinement of an amorphous polymer with high glass transition temperature (considered as a supercooled glass), the so-called nanoconfinement crystallization. Therefore, in other words, the effects of liquid layer of one polymer (with glass transition temperature lower than the measurement temperatures for an amorphous polymer or at the molten liquid state for a semicrystalline polymer) on the crystallization behaviors of another layer of one crystallizable polymer have not been considered much thus far. The other characteristic is that in general the coextrusion process creates multilayer films with relatively sharp interface boundary. However, the interfacial diffusion due to miscibility of the constituent polymers in the multilayer films has basically not been a focus of study so far. The basic consideration in our studies was to pay an attention to the effects of a liquid layer of one polymer on the crystallization kinetics of the layer of another polymer during the isothermal crystallization process. For simplicity doublelayer films with the layer thicknesses of micrometer scale were prepared, which were desired for the observation of spherulitic growths by using a polarized optical microscope. The constituent polymers that we selected are polylactide (PLA, the crystallizable component) and poly(ethylene oxide) (PEO, serving as liquid polymer layer at the measurement temperatures much higher than the melting point of PEO). PLA was selected because PLA as a highly versatile aliphatic polyester possesses excellent properties such as biocompatibility, biodegradability, and availability from renewable sources in comparison with those petroleum-based polymers.22,23 However, PLA products often exhibit an amorphous state due to slow crystallization rate. The resulting physical properties such as poor dimensional accuracy and stability, low modulus and strength, and the rapid hydrolytic degradation rates consequently limit its practical applications.24,25 In order to improve the physical properties of PLA, various chemical modification and blending methods have been used. On the other hand, to provide the guidance for industrial processing it is crucial to understand the crystallization kinetics of PLA at various conditions.26−28 PEO was selected because of its biocompatibility property,29−31 its extensive application as an effective plasticizer (an analogue for poly(ethylene glycol), PEG) for PLA component in the blends,32−35 and more important in this study, its molten liquid layer state at the measured crystallization temperatures for the PLA component (temperatures higher than the melting point of PEO). The isothermal crystallization kinetics of PEO/PLA doublelayer films (PLA layer covered by a molten PEO layer) at various crystallization temperatures was investigated through observations of the nucleation and growth of spherulites of PLA by using a polarized optical microscope. We have observed an obvious effect of a molten PEO layer on the crystallization behaviors of PLA layer; that is to say, covering with a molten layer of PEO can significantly accelerate the spherulitic growth

2. EXPERIMENTAL SECTION Materials. Poly(L-lactide) (PLA, sample code PLA4032D) in pellet form was purchased from NatureWorks China/Hong Kong, Shanghai, China, and had Mw of 1.8 × 105 g/mol, Mn of 1.2 × 105 g/mol, Mw/Mn of 1.5, 98.7 mol % L-isomeric content, and density of 1.24 g/cm3. Poly(ethylene oxide) (PEO) was purchased from the Sigma-Aldrich Company, U.S. and had Mw of 1.5 × 105 g/mol, Mn of 1.0 × 105 g/mol, and Mw/Mn of 1.5. The weight average molecular masses, Mw, and molecular mass distributions, Mw/Mn, of PLA and PEO samples were measured by using the size-exclusion chromatography (SEC) coupled with a DAWN HELEOS II multiangle laser light scattering detector, a refractive index (RI) detector Optilab T-rEX, and a viscometer ViscoStar II. Preparation of PEO/PLA Double-Layer Films and PLA/ PEO Blend Films. PLA in the form of pellets and PEO in the form of flour were dried under vacuum overnight at 60 and 35 °C, respectively, before use. PLA and PEO were dissolved in chloroform to form 2 wt % solutions, respectively. The solutions were stirred at room temperature for 12 h and then were cast on cover glasses, respectively. When chloroform was rapidly evaporated, PLA and PEO films formed on cover glasses, which were further dried under vacuum to constant masses. The film thicknesses were about 15 μm. PEO/PLA double-layer films were prepared in situ for the studies on the nucleation and growth of PLA spherulites by using polarized optical microscope (POM). The PEO/PLA double-layer film preparation procedure can be described as follows: once a PLA film (cast on a cover glass) was melted on a homemade hot stage set at 180 °C (less than 30 s), a PEO film (cast on a cover glass) was then quickly covered on the molten PLA film; thus, a PEO/PLA double-layer film formed with both film surfaces in contact with cover glasses. PLA/PEO blend films were obtained by casting the PLA/PEO blend solutions on cover glasses. Chloroform was used as the solvent for blending. The sample drying procedure for the blend films was same as for drying the neat PLA and PEO films. PLA/PEO blends containing 5, 10, 20, 25, and 30 wt % PEO component were denoted as PLA5, PLA10, PLA20, PLA25, and PLA30, respectively. Nucleation and Growth of PLA Spherulites Observed by POM. A polarized optical microscope (POM, Olympus BX51, made in Japan) was applied to observe the nucleation and growth of PLA spherulites for PEO/PLA double-layer films and PLA/PEO blend films during the isothermal crystallization processes at a wide range of temperatures. The polarized optical 5883

dx.doi.org/10.1021/jp400580r | J. Phys. Chem. C 2013, 117, 5882−5893

The Journal of Physical Chemistry C

Article

microscopy (SEM FEI Sirion200, made in U.S.). PEO/PLA double-layer films were fractured in liquid nitrogen, and the fractured surfaces were sputtered with gold before SEM observation.

micrographs were recorded at appropriate time intervals by using a CCD camera (Tucsen TCC-3.3N, China). The in situ prepared PEO/PLA double-layer films were held at 180 °C for 5 min to erase the thermal histories and then were transferred to another hot stage set at a certain crystallization temperature below the melting point of PLA for the isothermal crystallization observation. PLA/PEO blend films were melted on a hot stage set at 180 °C and then quickly covered by cover glasses. The PLA/PEO blend films sandwiched by two cover glasses were held at 180 °C for 5 min to remove previous thermal histories and then were transferred to another hot stage set at certain crystallization temperatures. The average radial growth rates of PLA spherulites (G) were obtained from the slopes of changes of spherulite radius with crystallization time. Glass Transition Temperatures (Tg) Measured by DSC and MDSC. For obtaining sufficient signals coming from the possible interdiffusion layers, which affected the changes of Tg, PEO/PLA multilayer films were prepared as follows: the dried PLA pellets were compression molded at 180 °C for 3 min in a vacuum laminator (KT-0909, Beijing Future Materials Technology Co., Ltd., China) to obtain rectangular PLA films having length of 18 mm, width of 10 mm, and thickness of approximate 0.13 mm. The dried PEO flour was compression molded at 80 °C for 3 min to obtain the PEO films with the same dimensions. Then, PLA and PEO films were assembled layer-by-layer alternatively (total 8 layers) and pressed at 180 °C for 3 min in the vacuum laminator. DSC heat flow curves of PEO/PLA multilayer film samples (about 7 mg) sealed in aluminum pans were measured by using a differential scanning calorimeter (DSC TA-Q2000, TA Instruments, U.S.). The samples were first melted at 180 °C for 5 min and quenched to −90 °C at the rapid cooling rate of 50 °C/min. The subsequent heating scan step was taken by using a MDSC mode for separating the overlapping signals of melting of PEO crystals from glass transition of PLA, in which a heating rate of 1 °C/ min with modulation amplitude of 0.32 °C in a period of 60 s was applied according to the recommendation of the TA Instruments Manual; thus the total heat flow, reversible heat flow, and nonreversible heat flow curves could be obtained. PLA/PEO blends were subjected to the DSC measurements with the same first two steps; however, in the last step, the MDSC mode was ignored, and the samples were heated up at a heating rate of 20 °C/min. The change of glass transition temperature of the blends (Tg of PLA) with PEO composition could be obtained for the estimation of miscibility between PLA and PEO components. Note that PLA/PEO blends with different compositions were prepared by coprecipitating PLA/ PEO chloroform solutions into a large quantity of cold hexane, filtering, and drying under vacuum at 35 °C for 24 h. Glass Transition Temperatures (Tg) Measured by DMA. Dynamic mechanical analyses (DMA) were performed by using a TA-Q800 DMA (TA Instruments, U.S.) on the molded PEO/PLA multilayer films, which were prepared by using the same procedure for the MDSC sample preparation. The DMA spectra were scanned with a frequency of 1 Hz and a heating rate of 2 °C/min. The temperature range for neat PEO film was from −90 to 50 °C and that for neat PLA and PEO/ PLA multilayer films was from −60 to 120 °C, depending on the glass transition temperatures and melting points of the samples. Layer Structures in Double-Layer Films Observed by SEM. The layer structures of fractured PEO/PLA double-layer films were observed by using field-emission scanning electron

3. RESULTS AND DISCUSSION Isothermal Crystallization at 120 °C for PEO/PLA Double-Layer Film, Neat PLA Film, and PLA/PEO Blend Films. For PLA crystallization spherulites are the most common morphology.36−38 Figure 1 shows the nucleation

Figure 1. Selected POM micrographs for (A) neat PLA film and (B) PEO/PLA double-layer film during isothermal crystallization at Tc of 120 °C. The scale bar in the top left micrograph represents 200 μm and is applied to all other micrographs. The crystallization times are indicated in the micrographs.

and growth of spherulites for PEO/PLA double-layer film (right panel) during isothermal crystallization at Tc of 120 °C, compared with that for neat PLA film (left panel). Note that results at the temperature of 120 °C are selectively shown at first because the fastest spherulitic growth rate is observed for neat PLA films at this temperature. The bell-shaped temperature dependence of spherulitic growth rate will be shown later, and the result is about consistent with other studies on various PLA materials.27,36,38 One typical feature that can be immediately seen from Figure 1 is that PLA spherulites grow much faster in PEO/PLA double-layer film than that in neat PLA film. Another typical feature is that PLA spherulites show the ring-banded textures with band spacing of 23 μm more clearly in PEO/PLA double-layer film than in neat PLA film, even though both film samples show the formation of ringbanded PLA spherulites. The most popular and acceptable explanation for the formation of ring-banded spherulites is the periodic twisting of ribbonlike crystalline lamellae along the radial growth direction of spherulites.39−41 Other mechanisms different from the lamellar twisting in expounding the formation of ring-banded spherulites have also been pro5884

dx.doi.org/10.1021/jp400580r | J. Phys. Chem. C 2013, 117, 5882−5893

The Journal of Physical Chemistry C

Article

PLA/PEO blend films at different isothermal crystallization temperatures for comparison purpose. Figure 3 shows the

posed.42−46 In this article, we will focus more on the differences of the spherulitic growth rates, G, between PEO/PLA doublelayer film and neat PLA film in this article; that is to say, we are more concerned about the effect of the top covering molten PEO layer on accelerating PLA spherulitic growths in the beneath PLA layer, and we will leave the ring-banded spherulitic features for these two film systems to a future publication because these features themselves are sufficiently complicated to deserve a separate presentation and extensive discussion. Figure 2 shows the changes of spherulite radius as

Figure 3. Selected POM micrographs for (a) neat PLA, (b) PLA5, (c) PLA10, (d) PLA20, (e) PLA25, and (f) PLA30 blends during isothermal crystallization at Tc of 120 °C for 15 min. The scale bar in (a) represents 200 μm and is applied to all other micrographs.

Figure 2. Changes of spherulite radius as functions of time for PEO/ PLA double-layer film and neat PLA film during isothermal crystallization at Tc of 120 °C.

selective POM micrographs for neat PLA film and PLA/PEO blend films with different PEO compositions during isothermal crystallization at Tc of 120 °C for 15 min. It is easily seen that the sizes of the spherulites obviously increase with increasing PEO composition for PLA/PEO blends crystallized at the same crystallization temperature with the same crystallization time, which infers that the spherulitic growth rates of PLA can be enhanced by increasing PEO composition in the blends. Figure 4 further shows the changes of spherulite radius as functions of

functions of time for PEO/PLA double-layer film and neat PLA film during isothermal crystallization at Tc of 120 °C. It can be seen that PLA spherulites grow with time linearly and the spherulitic growth rates can be obtained from the slopes of the linearly fitted lines. The spherulitic growth rates for PEO/PLA double-layer film and neat PLA film at 120 °C are evaluated to be 8.9 and 2.2 μm/min, respectively. Therefore, the PLA spherulitic growth rate in PEO/PLA double-layer film is about four times that in neat PLA. This result indicates that the molten PEO layer covering on the PLA film can obviously accelerate the growth rate of PLA spherulites. The mechanism of the acceleration of PLA spherulitic growth rate will be discussed in a later section. Note that the linearly fitted lines in Figure 2 point to the same time in the x-axis, which indicates the similar induction times of nucleation for these two film systems at this temperature. This result can be further confirmed as well by the similar nucleus numbers of PLA spherulites as shown in Figure 1. It is intuitive to consider that the obviously enhanced spherulitic growth rate of PLA in PEO/PLA double-layer film compared with that in neat PLA film is certainly correlated with the top covering molten PEO layer (the liquid layer) by the way that the molten PEO layer and supercooled PLA layer might interdiffuse to form a thin interdiffusion layer, which consists of the mixed PLA and PEO components, similar to a PLA/PEO blend. Poly(ethylene glycol) (PEG) was long known as an efficient plasticizer for PLA, which could obviously increase the spherulitic growth rates of PLA/PEG blends.32,33,35,47,48 PEO is analogous to PEG with the difference that PEO has relatively higher molecular masses than PEG. Therefore, PEO can be also considered as an effective plasticizer to PLA. As a matter of fact, PLA and PEO have been reported to be miscible in the molten and amorphous states,34 and PEO does not cocrystallize with PLA.49 With such a consideration in mind, it would be interesting and necessary to study the nucleation and growth of spherulites of PLA in

Figure 4. Changes of spherulite radius as functions of time for (a) neat PLA, (b) PLA5, (c) PLA10, (d) PLA20, (e) PLA25, and (f) PLA30 blends during isothermal crystallization at Tc of 120 °C.

time for neat PLA film and PLA/PEO blend films during isothermal crystallization at Tc of 120 °C. Again it can be found that the PLA spherulites in the blends all grow linearly with time. The slopes of the fitted lines demonstrate clearly that the growth rates of PLA spherulites in the blends increase with increasing PEO composition. Besides the increasing growth rate of spherulites of the PLA/PEO blends with increasing PEO composition, the evolution of morphology is also apparent. The regular spherulitic morphologies for neat PLA, PLA5, and PLA10 at the early crystallization stage (the left panel of Figure 3) seem different from the ring-banded spherulites for PLA20, 5885

dx.doi.org/10.1021/jp400580r | J. Phys. Chem. C 2013, 117, 5882−5893

The Journal of Physical Chemistry C

Article

Neat PLA Film, and PLA/PEO Blend Films. The results at the crystallization temperature, Tc, of 120 °C show an obviously enhanced PLA spherulitic growth rate in PEO/PLA doublelayer film due to the influence of the top covering molten PEO layer, compared with neat PLA film. Because the temperature of 120 °C corresponds to the highest spherulitic growth rate for neat PLA, a much lower temperature is predicted to correspond to the highest spherulitic growth rate for PEO/PLA doublelayer film due to the influence of molten PEO layer. Figure 6

PLA25, and PLA30. However, the morphologies at the late crystallization stage (the left panel of Figure 5) indicate that the

Figure 5. Selected POM micrographs for (a) neat PLA, (b) PLA5, (c) PLA10, (d) PLA20, (e) PLA25, and (f) PLA30 blends during isothermal crystallization at Tc of 120 °C for the different times when the growing spherulites fill up the film space. The scale bar in (a) represents 200 μm and is applied to all other micrographs.

relatively less regular ring-banded spherulites also form for PLA5 and PLA10 due to the less PEO compositions in the blends. When PEO composition of the blends is 20 wt % or above, the band spacing can be well measured. The band spacing values for PLA20, PLA25, and PLA30 are 39, 22, and 18 μm, respectively. The above results about the spherulitic growth rates and crystalline morphologies of PLA/PEO blends demonstrate that blending with PEO component does exert significant influence on the crystallization kinetics of PLA because the PEO component with much lower Tg (−51 °C, see Figures S1−S3 in the Supporting Information) is an effective plasticizer, similar to PEG, to enhance the segmental mobility of PLA component, which can increase the spherulitic growth rate of PLA. Our DSC measurements on PLA/PEO blends show a significant decrease for the Tg of PLA component in the blends with increasing PEO composition (Tg for neat PLA is about 63 °C and Tg for PLA20 is about 36 °C), and the decrease well follows the Fox equation (at least at PLA compositions above 80%, see Figures S1−S3 in the Supporting Information), indicating the miscibility between PLA and PEO components.32,34 Comparing the results for PLA/PEO blend films with PEO/PLA double-layer film, we can summarize the following two common features: (i) PEO can be used to modify the crystalline morphologies of PLA, i.e., it can assist the formation of the ring-banded spherulites, and (ii) PEO can accelerate the growth rate of PLA spherulites. It is considered that the formation mechanism for ring-banded spherulites in the double-layer film system and the blend system might be the same, which is correlated to the periodical growth of the PLA crystalline lamellae when PEO component is incorporated into the lamellar orientation along the radial growth direction.42−46 More data collection and appropriate analyses should be made in the near future for clearly understanding this issue. Isothermal Crystallization with the Highest Spherulitic Growth Rates for PEO/PLA Double-Layer Film,

Figure 6. Selected POM micrographs taken for (A) neat PLA film and (B) PEO/PLA double-layer film during isothermal crystallization at the two respective temperatures, corresponding to the highest spherulitic growth rates. The scale bar in the top left micrograph represents 200 μm and is applied to all other micrographs. The isothermal crystallization temperatures are indicated in the top micrographs.

displays the selected POM micrographs taken for neat PLA and PEO/PLA double-layer films during isothermal crystallization at two respective isothermal crystallization temperatures, which correspond to the highest spherulitic growth rates. The crystallization temperature with the highest spherulitic growth rate for PEO/PLA double-layer film is 100 °C, about 20 °C lower than the temperature of 120 °C for neat PLA film. The nucleation density in PEO/PLA double-layer film (right panel of Figure 6) is much higher than that in neat PLA film (left panel of Figure 6) at their respective highest spherulitic growth rates. The nucleation density difference is caused by the difference of supercooling (Tmo − Tc, where Tmo is the equilibrium melting point) for the two sample systems, that is to say, the higher supercooling (the lower crystallization temperature) for PEO/PLA double-layer film and the lower supercooling (the higher crystallization temperature) for neat PLA film. The supercooling difference in temperature could be as high as up to 20 °C considering that the equilibrium melting point, Tmo, of PLA layer is not so much decreased by covering 5886

dx.doi.org/10.1021/jp400580r | J. Phys. Chem. C 2013, 117, 5882−5893

The Journal of Physical Chemistry C

Article

Based on the same considerations, the POM observations on neat PLA and PLA/PEO blends at the respective highest spherulitic growth rates were performed for comparison purposes. Figure 8 shows the selected POM micrographs for

with a molten PEO layer. The consideration can be confirmed by the almost same melting peak positions in the MDSC heat flow curves for neat PLA and PEO/PLA double-layer films (see total heat flow curves and nonreversible heat flow curves provided in Figure S4 of the Supporting Information). The consideration is also reasonable from another standpoint that even though the equilibrium melting points of PLA in PLA/ PEO blends can be decreased somewhat by the miscible PEO component, the decreases are relatively slight34 (also see the slightly decreased melting peak positions of about 3 °C for PLA/PEO blends in Figure S1B of the Supporting Information). We further confirm that the nucleation density for neat PLA at 100 °C is relatively higher than that for PEO/ PLA double-layer film at 100 °C (see Figure S5 of the Supporting Information), indicating that the equilibrium melting point of PLA layer does decrease somewhat due to the covering molten PEO layer. On the other hand, at 100 °C the much enhanced growth rate of PLA spherulites in PEO/ PLA double-layer film is obviously seen. It only takes about 6 min for PLA spherulites in PEO/PLA double-layer film to grow and fill up the whole film space. In contrast, at the highest spherulitic growth rate for neat PLA film the filling-up of the whole film space by spherulites takes a much longer time of about 146 min (Figure 1A). This result obviously indicates that the crystallization kinetics of PLA can be greatly accelerated by covering a molten PEO layer on PLA layer. We further quantitatively examine this effectiveness by obtaining the spherulitic growth rates. Figure 7 shows the changes of

Figure 8. Selected POM micrographs for (a) neat PLA, (b) PLA5, (c) PLA10, (d) PLA20, (e) PLA25, and (f) PLA30 blend films during isothermal crystallization at the respective temperatures with the highest spherulitic growth rates. The micrographs were taken at the moments when the PLA spherulites impinge with each other and fill up the film spaces. The scale bar in (a) represents 200 μm and is applied to all other micrographs.

neat PLA film and PLA/PEO blend films during isothermal crystallization with the highest spherulitic growth rates at the respective temperatures. It can be seen from Figure 8 that the nucleation density increases and the time when the film space is filled up with spherulites becomes shorter with increasing PEO composition. At the highest spherulitic growth rates, regular spherulites instead of the ring-banded spherulites are observed for these blends. Figure 9 shows the changes of spherulite radius as functions of time for neat PLA film and PLA/PEO blend films during isothermal crystallization at the respective temperatures, corresponding to the highest spherulitic growth rates. The obtained maximum spherulitic growth rates, Gm, and the corresponding crystallization temperatures, Tcm, for neat

Figure 7. Changes of spherulite radius as functions of time for neat PLA film and PEO/PLA double-layer film during isothermal crystallization at the two respective temperatures, corresponding to the highest spherulitic growth rates.

spherulite radius as functions of isothermal crystallization time for the spherulites shown in Figure 6. The obtained spherulitic growth rates are 2.2 μm/min for neat PLA and 21.3 μm/min for PEO/PLA double-layer film. Thus, covering a molten PEO layer on PLA layer can significantly increase the spherulitic growth rate of PLA up to about 10 times of that for neat PLA film, keeping in mind that the spherulitic growth rate for neat PLA film here refers to its highest rate. We notice that the spherulites formed in PEO/PLA double-layer film look coarser and more open than those formed in neat PLA. In addition, the ring-banded texture of the spherulites formed at 100 °C (Figure 6B) is much less obvious than that formed at the higher crystallization temperatures of 120 °C (Figure 1B), which is related to the highest spherulitic growth rate at 100 °C because the rather rapidly formed crystalline lamellae easily lose arrangement correlation and they cannot be well organized into the ring-banded textures.

Figure 9. Changes of spherulite radius as functions of time for (a) neat PLA, (b) PLA5, (c) PLA10, (d) PLA20, (e) PLA25, and (f) PLA30 blend films during isothermal crystallization at the respective temperatures corresponding to the highest spherulitic growth rates. Inset shows the enlarged portion for (e) PLA25 and (f) PLA30 blend films. Note that the unit of time in the inset is seconds. 5887

dx.doi.org/10.1021/jp400580r | J. Phys. Chem. C 2013, 117, 5882−5893

The Journal of Physical Chemistry C

Article

PLA and PLA/PEO blends are listed in Table 1. These data are visually plotted in Figure 10, which clearly indicates that the Table 1. Maximum Spherulitic Growth Rate, Gm, and Corresponding Crystallization Temperature, Tcm, for Neat PLA Film, PEO/PLA Double-Layer Film, and PLA/PEO Blend Films during Isothermal Crystallization sample

Tcm (°C)

Gm (μm/min)

neat PLA PEO/PLA double-layer PLA5 PLA10 PLA20 PLA25 PLA30

120 100 115 110 105 100 95

2.2 21.3 4.2 6.6 14.8 19.5 22.1

Figure 11. Changes of spherulitic growth rate as functions of isothermal crystallization temperature for (A) neat PLA, PEO/PLA double-layer film, and PLA/PEO blend film with 25 wt % PEO composition (PLA25) and (B) neat PLA and PLA/PEO blend films. The solid lines represent the theoretical curves fitted with the data points on the basis of the Hoffmann−Lauritzen analysis. Figure 10. Changes of maximum spherulitic growth rate, Gm, and the corresponding crystallization temperature, Tcm, with PEO composition for PLA/PEO blend films.

for neat PLA film sample in the studied temperature range. At the lowest studied temperature of 100 °C for neat PLA, we observed the highest nucleation density and the lowest spherulitic growth rate; however, at this crystallization temperature the spherulitic growth rate of PLA becomes almost 20 times higher when the surface of PLA layer is covered with a molten PEO layer (the PEO/PLA double-layer film sample). A more significant feature is that by covering with a molten PEO layer the measurable spherulitic growths in PLA film shift down to much lower temperatures (as low as 85 °C shown in Figure 11A), which indicates that the covering of a molten PEO layer can obviously assist crystallization of PLA. Furthermore, the lowest temperature at which the spherulitic growth rates can be clearly measured for PEO/PLA double-layer film approaches 65 °C (see Figure S6 in the Supporting Information), which is 35 °C lower than that for neat PLA film (100 °C). Figure 11B further shows that for PLA/PEO blends the spherulitic growth rate increases with increasing PEO composition (upward shift of the curves), and the temperature corresponding to the highest spherulitic growth rate for each blend shifts to lower temperatures with increasing PEO composition (left shift of the curves). Note that the solid lines indicate the theoretical curves fitted with the data points on the basis of the Hoffmann−Lauritzen analysis for polymer blends.27,51,52 Fully detailed Hoffmann−Lauritzen analyses and the fitting parameters for PLA/PEO blend films and PEO/PLA double-layer film will be presented in a separate paper in the future. The above results indicate that PEO component in the blends can enhance the spherulitic growth rate of PLA indeed because PEO as an effective plasticizer can enhance the chain segment mobility of PLA component (PEO decreases Tg of PLA in the blends, see Figure S3 in the Supporting

crystallization temperature of the maximum spherulitic growth rate can be linearly shifted to the lower temperatures by increasing PEO composition in the blends. Accordingly the maximum spherulitic growth rate increases exponentially with increasing PEO composition due to the increasing supercooling (corresponding to the decreasing crystallization temperature), consistent with the previous report.50 If comparing the crystalline morphologies of PLA/PEO blend films with PEO/ PLA double-layer film, it can be found that the spherulites formed in PEO/PLA double-layer film (Figure 6B) are much coarser than those in PLA/PEO blend films (Figure 8). The Gm value for PEO/PLA double-layer film listed in Table 1 is slightly higher than that for PLA25 blend film and lower than that for PLA30 blend film. The Tcm value for PEO/PLA double-layer film is same as that for PLA25 blend film. From these POM results, it can be tentatively concluded that the layer-by-layer film preparation method for accelerating PLA crystallization kinetics can be simply used for replacing the conventional blending method. Spherulitic Growth Rates as Functions of Isothermal Crystallization Temperature. The nucleation and growth of PLA spherulites at different isothermal crystallization temperatures for neat PLA, PEO/PLA double-layer film, and PLA/ PEO blend films were completely investigated. Figure 11 illustrates the changes of spherulitic growth rate as functions of isothermal crystallization temperature for neat PLA, PEO/PLA double-layer film, and PLA/PEO blends. It can be seen from Figure 11A that the spherulitic growth rates for PEO/PLA double-layer film samples are obviously much faster than that 5888

dx.doi.org/10.1021/jp400580r | J. Phys. Chem. C 2013, 117, 5882−5893

The Journal of Physical Chemistry C

Article

Information). It is interesting to find that the change of spherulitic growth rate as a function of crystallization temperature for PEO/PLA double-layer film is relatively close to that for PLA25 blend film (Figure 11A). From this similarity we consider that the molten PEO layer on PLA layer plays a significant role in accelerating the spherulitic growth rate of PLA. This suggests that the molten PEO layer can enhance the PLA chain segment mobility somehow through the interdiffusion areas (or layer) between PLA and PEO layers, as will be further revealed by SEM observation shown in the next section. Up to now, we can think about the potential applications from the PEO/PLA layer-by-layer film preparation method. An immediate thought is an application of coextrusion technique to achieve the PEO/PLA multilayer film materials on the basis of our current results that the molten PEO layer can significantly enhance the crystallization kinetics of PLA layer, as an effective way to overcome the disadvantage of slow crystallization kinetics of single PLA component film products in processing. Characterizations on Tg in PEO/PLA Double-Layer Film. If two polymer films are brought into intimate contact, the interface might form, especially when polymers are in the amorphous state above Tg or in the molten state above Tm.53−58 Our results show that the nucleation and growth of PLA spherulites occur at much lower temperatures for PEO/PLA double-layer film than neat PLA film, which indicates the possible existence of an effective interdiffusion layer between PLA and PEO layers. The immediate thought for confirming this is to measure the changes of Tg of PLA and PEO in the double-layer film because the Tg of PLA is predicted to decrease in the interdiffusion layer because of its mixing with PEO component with much lower Tg. Differential scanning calorimetry (DSC) as a conventional method can be used for measuring the Tg of PLA.32,59 The interdiffusion layer between PEO and PLA layers in the double-layer film might be too thin to provide sufficient influences on the signal intensity changes from glass transition; therefore, we prepared the PEO/PLA multilayer film (alternative PEO and PLA layers with a total layer number of eight) to use in the DSC and DMA measurements to enhance the influences of interdiffusion layer on the signal changes of glass transition. We further considered that glass transition is related to the reversible heat flow change of polymers in the modulated differential scanning calorimetry (MDSC) measurements.59 Thus, we applied MDSC rather than the conventional DSC to examine the changes of Tg of PLA due to its surface contact with PEO layer. We clearly observe the Tg of 61 °C for neat PLA film (see Figure S4B in the Supporting Information); however, the Tg of PLA in PEO/PLA multilayer film cannot be measured even through the careful examination on the reversible heat flow curve (Figure S4B) because much stronger heat flow signals from the melting of PEO crystals just overlap with the weak signals from glass transition of PLA. Fortunately, thanks to the DMA technique, we resolve the problem. DMA measurements on PEO/PLA multilayer film were performed for providing some clues about the changes of glass transition for both PEO and PLA layers. Figure 12 shows the changes of storage modulus, E′, the derivative of storage modulus, and loss modulus, E″, with increasing temperature for PEO, PLA, and PEO/PLA multilayer films. The results clearly demonstrate that the glass transitions of PLA and PEO components in either neat polymer films or multilayer films can be well determined. The signal intensities relating to glass transitions are significant.

Figure 12. Changes of storage modulus, E′ (A), the derivative of E′ (B), and loss modulus, E″ (C), with increasing temperature for PEO film, PLA film, and PEO/PLA multilayer film. The curves were obtained by heating scan with frequency of 1 Hz at heating rate of 2 °C/min.

The Tg values of PLA and PEO components can be obtained from the derivative curves of storage modulus (Figure 12B) or from loss modulus curves (Figure 12C). The marked vertical dash lines through the peak positions of the curves indicate qualitatively that the Tg value of PEO shifts to higher temperature and the Tg value of PLA shifts to lower temperature in PEO/PLA multilayer film. The Tg values measured from these films are listed in Table 2. It can be found that Tg decreases about 3 °C for PLA and increases about 3 °C for PEO in PEO/PLA multilayer film compared with neat PLA Table 2. Glass Transition Temperatures, Tg, for PLA and PEO Components in Neat PLA, PEO Films, and PEO/PLA Multilayer Film Obtained from DMA Measurements Tg (°C)

5889

sample

from derivative of E′

from E″ curves

PEO film PLA film PEO/PLA multilayer film

−46 69 −43; 66

−47 68 −44; 66

dx.doi.org/10.1021/jp400580r | J. Phys. Chem. C 2013, 117, 5882−5893

The Journal of Physical Chemistry C

Article

and PEO films, respectively. Even though the changes of Tg (about 3 °C) are much lower than that for PLA/PEO blends (for example, decrease of 27 °C for PLA20 from neat PLA), the changing trend from the DMA measurement is quite convincing, indicating the effectiveness of making such a type of layer-by-layer polymer film. The effectiveness is eventually realized through the significantly enhanced crystallization kinetics of PLA component in PEO/PLA double-layer film, which is inherently related to the change of glass transition temperature of PLA component. The significant enhancement on the crystallization kinetics of PLA is important for improving the application properties of PLA materials such as mechanical properties (tensile modulus and heat distortion temperature, HDT), biodegradable property, and gas permeability property. Observation on Layer Structure of PEO/PLA DoubleLayer Film. To further confirm the existence of interdiffusion layer (or areas) between PLA and PEO layers in PEO/PLA double-layer film, SEM was used to observe the fractured surface of the films. Figure 13 shows the typical SEM micrographs taken from the fractured surface of PEO/PLA double-layer film, which was crystallized at 120 °C for 10 min and then quenched to room temperature. Thanks to the strongly enhanced crystallization of PLA layer by the covering PEO layer, this brings out sufficient morphological contrasts between PEO and PLA layers for valuable SEM observation. Figure 13a clearly shows a double-layer structure: (i) the bottom rough PLA layer containing obvious spherulitic morphology and (ii) the top covering smooth PEO layer containing some tiny spherelike structures, which are considered as tiny PEO spherulites formed during quenching. A large PLA spherulite formed in the beneath PLA layer can be clearly seen from the right side of Figure 13B. Apparently this spherulite grows from a nucleus close to the beneath cover glass surface and has grown up for 10 min during isothermal crystallization at 120 °C. Note that crystallization is not complete for the film sample at 120 °C for 10 min before quenching (referring to POM micrographs shown in the right panel of Figure 1). It is surprising that small coarse PLA spherulites seem to fill up the bottom PLA layer (shown in Figure 13C), and a large area of amorphous PLA cannot be found in the layer. On the other hand, this result is understandable if the function of molten PEO layer on the top of the PLA layer is taken into account. Figure 11 and Figure S6 in the Supporting Information have clearly demonstrated that PEO layer can eventually push down the measurable PLA spherulitic growth temperature to 65 °C. Even at the temperature of 65 °C, the spherulitic growth rate of PLA in the double-layer film is still much higher than the highest spherulitic growth rate of neat PLA (Figure S6 in the Supporting Information). Furthermore, the temperature of 65 °C almost approaches Tg of neat PLA (63 °C in Figure S1 in the Supporting Information). Therefore, the strong enhancement role played by the covering molten PEO layer to the beneath PLA crystallization can be also inferred from the SEM morphologies shown on the fractured surfaces of the doublelayer films. Two more features can be found as follows: (i) the nucleation sites do not particularly locate at the boundary between PLA and PEO layers. Thus, the molten PEO layer does not function to induce the nucleation sites for the beneath supercooled PLA layer. This behavior is different from the surface-induced nucleation for crystallization by solid surfaces such as the added fillers or fibers.11,14,15 As a matter of fact, the

Figure 13. SEM micrographs taken from the fractured surfaces of PEO/PLA double-layer film. The film sample was quenched to room temperature before isothermal crystallization at 120 °C for 10 min from the molten state (180 °C for 5 min). (A) ×5000 showing the whole layer structure; (B) ×8000 mainly showing a large PLA spherulite (right side) and interdiffusion areas (pointed out by orange arrows) between PEO and PLA layers; (C) ×8000 mainly showing the PLA layer containing several spherulites and interdiffusion areas (pointed out by orange arrows).

surface-induced nucleation for polymer crystallization by solid surfaces has been well studied and applied in various fields, especially for the polymer composites applications. As for the layer film systems proposed in this work, the liquid polymer layer at the interfacial surfaces functions to significantly accelerate the crystallization kinetics through the enhancement of spherulitic growth rate rather than through the enhancement of nucleation density. (ii) The boundary between PEO and PLA layers does not look regular and sharp, exhibiting different layer thicknesses at different locations (for example, PLA layer thickness ranges from 12 to 14 μm). The result indicates an existence of some interdiffusion areas containing the mixed PLA and PEO components with thicknesses of about 2−3 μm. Some of the interdiffusion areas are selectively pointed out by orange color arrows shown in Figure 13. The interdiffusion 5890

dx.doi.org/10.1021/jp400580r | J. Phys. Chem. C 2013, 117, 5882−5893

The Journal of Physical Chemistry C

Article

areas (layers) bring out the detectable decreases of Tg of PLA component in PEO/PLA multilayer films (see the DMA result in Figure 12). Although the overall drop of Tg of PLA component is not as significant as the blends, the enhancement of PLA spherulitic growth rate by covering a molten liquid PEO layer on the PLA layer surface is quite astonishing as seen in Figure 11A. On the other hand, the interdiffusion areas (layers) are crucial for the significantly accelerated spherulitic growth rates of PLA as we report in this article because we also found in our experiments that when the PLA layer was laid on a molten PEO layer the effects of the beneath molten PEO layer on accelerating the PLA spherulitic growth rates are quite similar. Therefore, for the processing of the layer-by-layer films consisting of different polymer components, typically from high temperatures down to low temperatures (such as coextrusion or surface coating), if one component can crystallize and the other one cannot, such as the PLA and PEO pairs, then the molten amorphous polymer layer such as the PEO layer, with much lower glass transition temperature, can be used to accelerate the crystallization kinetics of the other semicrystalline polymer layer, such as the PLA layer in this work. Because the interdiffusion areas between the two polymer layers are crucial, our future studies will focus on the following issues: (i) the effects of miscibility between two polymer layer components, such as immiscible polymer layers (PLA/PCL, PLA/PS pairs) and miscible polymer layers (PLA/PEO, PLA/PEG pairs); (ii) the effects of thicknesses of the molten amorphous polymer layer, revealing the critical layer thickness to cause the most significant effect or sufficient effect for reducing cost of the layer-by-layer film productions; and (iii) the resulting improved mechanical properties or other physical properties, such as biodegradability or gas permeability, etc.

much lower temperatures, which eventually approaches the glass transition temperature of neat PLA. Therefore, the layerby-layer film method infers promising applications. A coextrusion technique can be applied to produce PEO/PLA multilayer films, which should provide an effective way to overcome the disadvantage of PLA, the slow crystallization rate in the film processing. In addition, our results also infer that the simple double-layer method for accelerating PLA crystallization can be considered to use to well replace the blending method.

■

ASSOCIATED CONTENT

S Supporting Information *

DSC heating scan curves for PEO/PLA blends; MDSC heating scan curves for PLA, PEO, and PEO/PLA multilayer films; POM micrographs for neat PLA and PEO/PLA double-layer films during isothermal crystallization at 100 °C and changes of spherulitic growth rate as functions of isothermal crystallization temperature for PLA and PEO/PLA double-layer films (Figures S1−S6). This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 0551-63607703. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Z.W. acknowledges the financial support from the National Science Foundation of China with Grant No. 21174139 and National Basic Research Program of China with Grant No. 2012CB025901.

■

4. CONCLUSIONS To overcome the slow crystallization kinetics of the biodegradable PLA material, in this work we considered to enhance the crystallization of PLA by covering a thin layer of an amorphous component on PLA films, which was similar to the layer-by-layer method used in coextrusion processing. PEO was selected as the amorphous component, a perfect candidate for pairing with PLA because it is miscible with PLA and it has a low melting point, keeping in the molten liquid layer state in the isothermal crystallization temperature range for PLA. We prepared PEO/PLA double-layer films and investigated the nucleation and spherulitic growth rates of PLA layer by using polarized optical microscope. Our results clearly indicate that the amorphous molten PEO layer does significantly accelerate the spherilitic growth rates of PLA layer. The results are compared with that of PLA/PEO blends. The glass transition temperature changes in PEO/PLA multilayer films were examined by MDSC and DMA measurements, which show a decrease of about 3 °C for PLA component and an increase of about 3 °C for PEO component in the multilayer films. An interdiffusion area (or layer) with thicknesses of 2−3 μm was found for PEO/PLA double-layer film by SEM observation on the fractured surfaces of the film. The interdiffusion areas (or layer) are considered to be crucial for the significantly accelerated spherulitic growth rates of PLA layer. The temperature dependence of PLA spherulitc growth rate obtained for PEO/PLA double-layer films indicates that not only do the obviously accelerated spherulitic growth rates become achievable but also the temperature range for the formation of measurable spherulites can be pushed down to

REFERENCES

(1) Wunderlich, B. Macromolecular Physics; Academic: New York, 1976; Vol. 2. (2) Schultz, J. M. Polymer Crystallization, 1st ed.; Oxford University Press: Oxford, U.K., 2001. (3) Ward, I. M. Structure and Properties of Oriented Polymers; Wiley: New York, 1975. (4) Keller, A.; Kolnaar, H. W. H. Flow-Induced Orientation and Structure Formation. In Processing of Polymers, Wiley-VCH: Weinheim, Germany, 1997; Vol. 18. (5) Eder, G.; Janeschitz-Kriegl, H. Structure Development during Processing: Crystallization. In Processing of Polymers, Wiley-VCH: Weinheim, Germany, 1997; Vol. 18. (6) Brinkmann, M.; Contal, C.; Kayunkid, N.; Djuric, T.; Resel, R. Highly Oriented and Nanotextured Films of Regioregular Poly(3hexylthiophene) Grown by Epitaxy on the Nanostructured Surface of an Aromatic Substrate. Macromolecules 2010, 43, 7604−7610. (7) Ray, S. S.; Okamoto, M. Polymer/Layered Silicate Nanocomposites: A Review from Preparation to Processing. Prog. Polym. Sci. 2003, 28, 1539−1641. (8) Mellbring, O.; Oiseth, S. K.; Krozer, A.; Lausmaa, J.; Hjertberg, T. Spin Coating and Characterization of Thin High-Density Polyethylene Films. Macromolecules 2001, 34, 7496−7503. (9) Somani, R. H.; Hsiao, B. S.; Nogales, A.; Srinivas, S.; Tsou, A. H.; Sics, I.; Balta-Calleja, F. J.; Ezquerra, T. A. Structure Development during Shear Flow-Induced Crystallization of i-PP: In-Situ Small-Angle X-ray Scattering Study. Macromolecules 2000, 33, 9385−9394. (10) Gupta, M.; Lin, Y. J.; Deans, T.; Baer, E.; Hiltner, A.; Schiraldi, D. A. Structure and Gas Barrier Properties of Poly(propylene-graftmaleic anhydride)/Phosphate Glass Composites Prepared by Microlayer Coextrusion. Macromolecules 2010, 43, 4230−4239. 5891

dx.doi.org/10.1021/jp400580r | J. Phys. Chem. C 2013, 117, 5882−5893

The Journal of Physical Chemistry C

Article

(11) Li, H. H.; Yan, S. K. Surface-Induced Polymer Crystallization and the Resultant Structures and Morphologies. Macromolecules 2011, 44, 417−428. (12) Liu, J.; Wang, J. J.; Li, H. H.; Shen, D. Y.; Zhang, J. M.; Ozaki, Y.; Yan, S. K. Epitaxial Crystallization of Isotactic Poly(methyl methacrylate) on Highly Oriented Polyethylene. J. Phys. Chem. B 2006, 110, 738−742. (13) Yang, J. P.; Liao, Q.; Zhou, J. J.; Jiang, X.; Wang, X. H.; Zhang, Y.; Jiang, S. D.; Yan, S. K.; Li, L. What Determines the Lamellar Orientation on Substrates? Macromolecules 2011, 44, 3511−3516. (14) Zhang, J.; Yang, D.; Thierry, A.; Wittmann, J. C.; Lotz, B. Isochiral Form II of Syndiotactic Polypropylene Produced by Epitaxial Crystallization. Macromolecules 2001, 34, 6261−6267. (15) Wittmann, J. C.; Lotz, B. Epitaxial Crystallization of Polymers on Organic and Polymeric Substrates. Prog. Polym. Sci. 1990, 15, 909− 948. (16) Chang, H. B.; Zhang, J. M.; Li, L.; Wang, Z. H.; Yang, C. M.; Takahashi, I.; Ozaki, Y.; Yan, S. K. A Study on the Epitaxial Ordering Process of the Poly(caprolactone) on the Highly Oriented Polyethylene Substrate. Macromolecules 2010, 43, 362−366. (17) Duan, Y. X.; Zhang, J. M.; Chang, H. B.; Yan, S. K.; Yang, C. M.; Takahashi, I.; Ozaki, Y. Melting Behavior of Epitaxially Crystallized Poly(carprolactone) on a Highly Oriented Polyethylene Thin Film Investigated by in-Situ Synchrotron SAXS and Polarized Infrared Spectroscopy. Macromolecules 2010, 43, 5315−5322. (18) Duan, X. L.; Tang, Q. X.; Qiu, J.; Niu, Y. H.; Wang, Z. G.; Hu, W. P. Polymer Reptation for Molecular Assembly of Copper Phthalocyanine. Appl. Phys. Lett. 2009, 95, 113301. (19) Wang, H. P.; Keum, J. K.; Hiltner, A.; Baer, E.; Freeman, B.; Rozanski, A.; Galeski, A. Confined Crystallization of Polyethylene Oxide in Nanolayer Assemblies. Science 2009, 323, 757−760. (20) Ponting, M.; Lin, Y. J.; Keum, J. K.; Hiltner, A.; Baer, E. Effect of Substrate on the Isothermal Crystallization Kinetics of Confined Poly(epsilon-caprolactone) Nanolayers. Macromolecules 2010, 43, 8619−8627. (21) Liu, R. Y. F.; Ranade, A. P.; Wang, H. P.; Bernal-Lara, T. E.; Hiltner, A.; Baer, E. Forced Assembly of Polymer Nanolayers Thinner than the Interphase. Macromolecules 2005, 38, 10721−10727. (22) Rasal, R. M.; Janorkar, A. V.; Hirt, D. E. Poly(lactic acid) Modifications. Prog. Polym. Sci. 2010, 35, 338−356. (23) Lim, L. T.; Auras, R.; Rubino, M. Processing Technologies for Poly(lactic acid). Prog. Polym. Sci. 2008, 33, 820−852. (24) Liu, H. Z.; Song, W. J.; Chen, F.; Guo, L.; Zhang, J. W. Interaction of Microstructure and Interfacial Adhesion on Impact Performance of Polylactide (PLA) Ternary Blends. Macromolecules 2011, 44, 1513−1522. (25) Pan, P. J.; Zhu, B.; Inoue, Y. Enthalpy Relaxation and Embrittlement of Poly(L-lactide) during Physical Aging. Macromolecules 2007, 40, 9664−9671. (26) Pan, P.; Liang, Z.; Zhu, B.; Dong, T.; Inoue, Y. Roles of Physical Aging on Crystallization Kinetics and Induction Period of Poly(Llactide). Macromolecules 2008, 41, 8011−8019. (27) Kawai, T.; Rahman, N.; Matsuba, G.; Nishida, K.; Kanaya, T.; Nakano, M.; Okamoto, H.; Kawada, J.; Usuki, A.; Honma, N.; Nakajima, K.; Matsuda, M. Crystallization and Melting Behavior of Poly(L-lactic acid). Macromolecules 2007, 40, 9463−9469. (28) Zhang, J. M.; Tsuji, H.; Noda, I.; Ozaki, Y. Structural Changes and Crystallization Dynamics of Poly(L-lactide) during the ColdCrystallization Process Investigated by Infrared and Two-Dimensional Infrared Correlation Spectroscopy. Macromolecules 2004, 37, 6433− 6439. (29) vanderGiessen, W. J.; Lincoff, A. M.; Schwartz, R. S.; vanBeusekom, H. M. M.; Serruys, P. W.; Holmes, D. R.; Ellis, S. G.; Topol, E. J. Marked Inflammatory Sequelae to Implantation of Biodegradable and Nonbiodegradable Polymers in Porcine Coronary Arteries. Circulation 1996, 94, 1690−1697. (30) Schmitt, B.; Alexandre, E.; Boudjema, K.; Lutz, P. Poly(ethylene oxide) Hydrogels as Semi-permeable Membranes for an Artificial Pancreas. Macromol. Biosci. 2002, 2, 341−351.

(31) Rixman, M. A.; Dean, D.; Ortiz, C. Nanoscale Intermolecular Interactions between Human Serum Albumin and Low Grafting Density Surfaces of Poly(ethylene oxide). Langmuir 2003, 19, 9357− 9372. (32) Gaikwad, A. N.; Wood, E. R.; Ngai, T.; Lodge, T. P. Two Calorimetric Glass Transitions in Miscible Blends Containing Poly(ethylene oxide). Macromolecules 2008, 41, 2502−2508. (33) Yang, J. M.; Chen, H. L.; You, J. W.; Hwang, J. C. Miscibility and Crystallization of Poly(L-lactide)/Poly(ethylene glycol) and Poly(L-lactide)/Poly(epsilon-caprolactone) Blends. Polym. J. 1997, 29, 657−662. (34) Nijenhuis, A. J.; Colstee, E.; Grijpma, D. W.; Pennings, A. J. High Molecular Weight Poly(L-lactide) and Poly(ethylene oxide) Blends: Thermal Characterization and Physical Properties. Polymer 1996, 37, 5849−5857. (35) Hu, Y.; Hu, Y. S.; Topolkaraev, V.; Hiltner, A.; Baer, E. Crystallization and Phase Separation in Blends of High Stereoregular Poly(lactide) with Poly(ethylene glycol). Polymer 2003, 44, 5681− 5689. (36) Abe, H.; Kikkawa, Y.; Inoue, Y.; Doi, Y. Morphological and Kinetic Analyses of Regime Transition for Poly[(S)-lactide] Crystal Growth. Biomacromolecules 2001, 2, 1007−1014. (37) Xu, Z. H.; Niu, Y. H.; Wang, Z. G.; Li, H.; Yang, L.; Qiu, J.; Wang, H. Enhanced Nucleation Rate of Polylactide in Composites Assisted by Surface Acid Oxidized Carbon Nanotubes of Different Aspect Ratios. ACS Appl. Mater. Interfaces 2011, 3, 3744−3753. (38) Tsuji, H.; Tezuka, Y.; Saha, S. K.; Suzuki, M.; Itsuno, S. Spherulite Growth of L-lactide Copolymers: Effects of Tacticity and Comonomers. Polymer 2005, 46, 4917−4927. (39) Schultz, J. M. Self-Induced Field Model for Crystal Twisting in Spherulites. Polymer 2003, 44, 433−441. (40) Bassett, D. C.; Hodge, A. M. Lamellar Organization in Banded Spherulites of Polyethylene. Polymer 1978, 19, 469−472. (41) Bassett, D. C. Polymer Spherulites: A Modern Assessment. J. Macromol. Sci. Phys. 2003, B42, 227−256. (42) Wang, Z. G.; Wang, X. H.; Yu, D. H.; Jiang, B. Z. The Formation of Ring-banded Spherulites of Poly(epsilon-caprolactone) in Its Miscible Mixtures with Poly(styrene-co-acrylonitrile). Polymer 1997, 38, 5897−5901. (43) Wang, Z. G.; An, L. J.; Jiang, W.; Jiang, B. Z.; Wang, X. H. RingBanded Spherulite Surface Structure of Poly(epsilon-caprolactone) in Its Miscible Mixtures with Poly(styrene-co-acrylonitrile). J. Polym. Sci., Part B: Polym. Phys. 1999, 37, 2682−2691. (44) Wang, Z. B.; Hu, Z. J.; Chen, Y. Z.; Gong, Y. M.; Huang, H. Y.; He, T. B. Rhythmic Growth-induced Concentric Ring-Banded Structures in Poly(epsilon-caprolactone) Solution-Casting Films Obtained at the Slow Solvent Evaporation. Macromolecules 2007, 40, 4381−4385. (45) Fromsdorf, A.; Woo, E. M.; Lee, L. T.; Chen, Y. F.; Forster, S. Atomic Force Microscopy Characterization and Interpretation of Thin-film Poly(butylene adipate) Spherulites with Ring Bands. Macromol. Rapid Commun. 2008, 29, 1322−1328. (46) Woo, E. M.; Wang, L. Y.; Nurkhamidah, S. Crystal Lamellae of Mutually Perpendicular Orientations by Dissecting onto Interiors of Poly(ethylene adipate) Spherulites Crystallized in Bulk Form. Macromolecules 2012, 45, 1375−1383. (47) Younes, H.; Cohn, D. Phase-Separation in Poly(ethylene glycol)/Poly(lactic acid) Blends. Eur. Polym. J. 1988, 24, 765−773. (48) Nakafuku, C. Effects of Molecular Weight on the Melting and Crystallization of Poly(L-lactic acid) in a Mixture with Poly(ethylene oxide). Polym. J. 1996, 28, 568−575. (49) Nakafuku, C.; Sakoda, M. Melting and Crystallization of Poly(Llactic acid) and Poly(ethylene oxide) Binary Mixture. Polym. J. 1993, 25, 909−917. (50) Piorkowska, E.; Kulinski, Z.; Galeski, A.; Masirek, R. Plasticization of Semicrystalline Poly(L-lactide) with Poly(propylene glycol). Polymer 2006, 47, 7178−7188. 5892

dx.doi.org/10.1021/jp400580r | J. Phys. Chem. C 2013, 117, 5882−5893

The Journal of Physical Chemistry C

Article

(51) Lai, W. C.; Liau, W. B.; Lin, T. T. The Effect of End Groups of PEG on the Crystallization Behaviors of Binary Crystalline Polymer Blends PEG/PLLA. Polymer 2004, 45, 3073−3080. (52) Wang, Z. G.; Jiang, B. Z. Crystallization Kinetics in Mixtures of Poly(epsilon-caprolactone) and Poly(styrene-co-acrylonitrile). Macromolecules 1997, 30, 6223−6229. (53) Klein, J. The Interdiffusion of Polymers. Science 1990, 250, 640−646. (54) High, M. S.; Painter, P. C.; Coleman, M. M. Polymer Polymer Mutual Diffusion Using Transmission FTIR Spectroscopy. Macromolecules 1992, 25, 797−801. (55) Karim, A.; Felcher, G. P.; Russell, T. P. Interdiffusion of Polymers at Short Times. Macromolecules 1994, 27, 6973−6979. (56) Schuman, T.; Stepanov, E. V.; Nazarenko, S.; Capaccio, G.; Hiltner, A.; Baer, E. Interdiffusion of Linear and Branched Polyethylene in Microlayers Studied via Melting Behavior. Macromolecules 1998, 31, 4551−4561. (57) Yeung, C.; Shi, A. C. Formation of Interfaces in Incompatible Polymer Blends: A Dynamical Mean Field Study. Macromolecules 1999, 32, 3637−3642. (58) Tomba, J. P.; Carella, J. M.; Pastor, J. M. Molecular Mechanisms of Interphase Evolution in the Liquid Polystyrene−Glassy Poly(phenylene oxide) System. Macromolecules 2009, 42, 3565−3572. (59) Pyda, M.; Wunderlich, B. Reversing and Nonreversing Heat Capacity of Poly(lactic acid) in the Glass Transition Region by TMDSC. Macromolecules 2005, 38, 10472−10479.

5893

dx.doi.org/10.1021/jp400580r | J. Phys. Chem. C 2013, 117, 5882−5893