Lamellar Thickness of Poly(ethylene oxide) Film Crystallized from the

Sep 25, 2018 - The lamellar thickness of poly(ethylene oxide) (PEO) film established from a gel state was investigated in an effort to understand the ...
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Lamellar Thickness of Poly(ethylene oxide) Film Crystallized from the Gel State Jungju Ryu,†,‡ Hoik Lee,‡,§ Hyosin Kim,‡ and Daewon Sohn*,‡ †

Neutron Science Center, Korea Atomic Energy Research Institute, Daejeon 34057, Korea Department of Chemistry and Research Institute for Convergence of Basic Science, Hanyang University, Seoul 04763, Korea § Nano Fusion Technology Research Group, Division of Frontier Fibers, Institute for Fiber Engineering (IFES), Interdisciplinary Cluster for Cutting Edge Research (ICCER), Shinshu University, Nagano 386-8567, Japan

Macromolecules Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 09/28/18. For personal use only.

‡

S Supporting Information *

ABSTRACT: The lamellar thickness of poly(ethylene oxide) (PEO) film established from a gel state was investigated in an effort to understand the crystalline structure which forms from the cross-linked network. Crystalline films were prepared with PEO solutions irradiated by γ-rays. The structural aspect of the films was revealed in terms of the exposed radiation energy and the polymer concentration by using differential scanning calorimetry, X-ray diffraction, and small-angle X-ray scattering. The long-period distance, lamellar thickness, and amorphous thickness in the crystalline films were analyzed based on the small-angle X-ray scattering patterns. The wide distribution and nonuniformity of the lamellar stacks were observed in the elongated and the swollen films. The lamellar thickness was compared to the chain lengths between the cross-links, and the possible presence of cross-links in the lamellar stacks was discussed. The results revealed the lamellar thickness influenced by the cross-links and demonstrated the structural changes associated with both the crystalline phase and cross-linking.

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INTRODUCTION Polymer crystallization is established under nonequilibrated conditions with the requirements of thermodynamic equilibrium and kinetic factors. The structural variables of semicrystalline polymers are governed by the crystallization factors such as the type of chain segments,1,2 the polymer− diluent mixture,3,4 and the presence of cross-links.5−7 In particular, cross-links depress the rate of crystallization and reduce the crystallinity due first to the restriction of reptation movement to develop distances exceeding the nanometer scale and second to the exclusion of cross-links from the crystalline phase.6−8 As part of the fundamental understanding of crystallization, it has been reported that the lamellar thickness depends on the degree of cross-links.6,7,9 Observations of the crystallization of cross-linked polyethylene suggest that there are three important regions.6 First, the thickness increases in rarely cross-linked polymers for thermodynamic reasons, allowing the incorporation of cross-links into the crystalline registers. Second, it is decreased by the sequence lengths in cases of cross-link exclusion, as increases in cross-linking © XXXX American Chemical Society

density. Third, short length between cross-links causes the thickness to level off.7 These regions are described in light of the results of competition between secondary nucleation and surface spreading. The latter two regions indicate a relatively reduced rate of surface spreading.7 These building blocks of lamellae directly influence the functional properties of materials. In particular, several issues have arisen due to the endowment of reinforcement and flexibility of the materials.10,11 For example, the stiffness and flexibility of poly(ethylene oxide) (PEO), which is widely used in biomimetic artificial materials, have been reported as important factors to regulate cellular fate processes for tissue engineering and biological applications.12−14 Recently, we reported specific phenomena pertaining to the crystalline state and diffusive chains of PEO networks. Crosslinked PEO film exhibits reattachment of the pieces (damaged Received: June 13, 2018 Revised: September 10, 2018

A

DOI: 10.1021/acs.macromol.8b01232 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules sides) of film during wetting−drying triggering processes.15 This reattached film recovers more than 80% of the storage modulus.15 It was predicted that the polymer chains diffuse and interpenetrate at interfaces bridged with a good solvent on the wetted pieces, after which the interface recovers to a crystalline state during the drying process, playing a key role in reaching the original strength. The present study investigates the crystalline lamellar thickness of cross-linked PEO films and demonstrates phenomena related to crystallization by drying from a gel state. The networks of PEO were formed from an aqueous solution by means of γ-ray radiation. The polymer chains activated by γ-rays produce chain segments containing radicals such as R−O• and R−C•, resulting in a three-dimensional network through reactions between radicals on neighboring chain segments.10,16 This process indicates that the formation of cross-linking is influenced by the amount of the radiation energy that produces radicals and by the degree of chain mobility that provides opportunities to encounter the radicals on different segments. The latter depends on the sample state under the given conditions, i.e., the specimen state (powder, melt, or solution) and the polymer concentration in the solution.17,18 Therefore, more effective cross-links are formed upon a reduction in the concentration of the polymer solution before radiation and with increases in the radiation energy. The PEO cross-links in this study were prepared with various concentrations of polymer solutions exposed at a relatively low dose (∼30 kGy) and were dried as a film (thickness of 18 MΩ cm at several concentrations (5−20% (w/v)); this was accomplished through vigorous stirring at room temperature over 3 days in a tightly sealed bottle. The polymer solutions were poured into a polystyrene Petri dish to a depth of 3 mm. The solutions were irradiated by 10−30 kGy of 60Co γ-rays at KAERI, Jeongeup, Korea, under ambient conditions for 2 h with a dose rate of 5−15 kGy/h. The crystalline films were established from the gel state processed by the above radiation method. The irradiated samples were dried slowly at room temperature and 40% relative humidity

∞

γ(r ) =

B

∫0 I(q)q2 cos(qr) dq ∞

∫0 I(q)q2 dq

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Figure 1. FE-SEM images of the surfaces of PEO films: the films from the 20% solution exposed to different doses of (a) 10 kGy and (b) 30 kGy and (c) the films from the 10% solution exposed to 30 kGy. (d) is the swollen morphology of (b). where I(q) is the scattering intensity at the scattering vector and r is the distance in real space. The SAXS intensity was corrected by subtracting the background from thermal density fluctuations (Figure S3). The structural changes were revealed in the deformed and the swollen films. For the deformation experiments, the samples were loaded onto a sample stage which could impart strain onto the sample. The films were cut into rectangular pieces (1 cm wide × 2 cm long) and were then elongated with the ratio (Λ) defined as the measured sample length (l) divided by the original sample length (l0). The 1D scattering profiles were obtained by circular averages along the azimuthal direction from the 2D scattering patterns of the film under a given magnitude of strain. To identify the anisotropic patterns, the sector averaged intensity was taken from the azimuthal angle span of ±15° at the beam center along directions both vertical and horizontal to the stretching directions of the samples. The annular scattering intensity was also determined at the selected scattering vector of the respective lamellar and amorphous regions (i.e., 0.026 and 0.015 Å−1 of the film, 20%−30 kGy). The data reduction was performed using the GISAXS-shop macro for the Igor program.23 The magnitude of anisotropy of the scattering intensity was analyzed with the Hermans parameters (f) as the following equations:24,25 f=

3 1 ⟨cos2 Φ⟩ − 2 2

generator operated at 40 kV and 30 mA. The samples were loaded on the stage with the windows of Kapton films, and the melt-crystallized films were stretched up to Λ = 2 on the elongation stage. The scattering intensity was collected by subtraction of air scattering including empty cells. The 1D circular averaged scattering patterns were analyzed using Equations S1 and S2 in the analysis package of the Igor program, as distributed by the National Institute of Standards and Technology (NIST).28 Internal Structure of PEO Solution. PEO solutions at several concentrations were measured at the 4C beamline of PAL. The scattering profiles were obtained at a sample-to-detector distance of 5 m using a beam energy of 13.6 keV (ΔE/E = 2 × 10−4). The q range was 0.005 Å−1 < q < 0.12 Å−1. The scattering angle was calibrated with a standard sample of polyethylene-b-polybutadiene-b-polystyrene (SEBS) block copolymer (q = 0.19165 nm−1).29,30 The temperature was controlled on the cooling sample stage by means of cryo-compact circulation. The samples loaded onto a holder with mica windows were measured at 20 °C. The scattering intensity of PEO in solution was obtained after the subtraction of water scattering from the data collected with a 2D detector (Rayonix SX165).31

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RESULTS AND DISCUSSION Cross-Linking of PEO Film. Cross-linked networks of PEO were formed by γ-ray irradiation of concentrated solutions. PEO solutions exposed to 30 kGy exhibited typical characteristics that appear in hydrogels, allowing the swelling behavior and the gel fraction shown in Figure S1. A continuous network is ensured in the microstructure of the swollen gel of Figure 1d. However, the sample exposed to 10 kGy results in an incomplete network that could pass through a 75 μm sieve after swelling. With regard to the dry films, the surface morphologies of 20%−10 kGy and 20%−30 kGy films were observed, as shown in Figures 1a and 1b. The films were composed of polymer domains with voids among them. The sample irradiated at 30 kGy gives rise to densely packed domains and fewer voids compared to that irradiated at 10 kGy. These morphologies may indicate the results that rarely cross-linked chains induce localized aggregations due to finite connections while sufficient cross-links produce infinite networks. The formation of the network of the film was also influenced by the PEO concentration. The swelling ratios and the gel fractions depending on the PEO concentration at 30 kGy are shown in Figure S1. Upon an increase in the initial concentration in the solution before gelation (radiation), the

(4)

2π

2

⟨cos Φ⟩ =

∫0 Iq(Φ) cos2 Φ sin Φ dΦ 2π

∫0 Iq(Φ) sin Φ dΦ

(5)

where Iq is the scattering intensity at the azimuthal angle Φ. The values of the Hermans parameters can vary from −0.5 to 1. A negative value (−0.5) describes an orientation along the parallel direction (vertical anisotropy), while the values of 1 and 0 indicate the perpendicular orientation (horizontal anisotropy) and the random orientation, respectively. The further investigations were performed to examine the structural change at different magnitudes of water invasion into the film. Three conditions were applied: One group of PEO films was swollen for 12 h in a container of RH > 95% using wet gauze, and another group was wetted with a 0.1 mL water droplet. The other group was fully swollen for 48 h in a solution. The scattering profiles were obtained by subtracting the air scattering, and their characteristic scattering intensities were analyzed. The fully swollen films and meltcrystallized films were measured by using a SAXS instrument (NANOPIX, Rigaku Co., Japan) installed at HANARO, KAERI. The covered q range was 0.004−0.1 Å−1 using a wavelength of 1.54 Å of Cu Kα radiation, which is generated from a rotating anode C

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films as a function of the radiation dose. The crystalline films were prepared with identical polymer concentrations of 20% as the initial solutions. The highest degree of crystallinity was observed in the nonirradiated film, and this value decreased in the films which underwent γ-ray radiation. The crystallinity was slightly reduced with an increase in the radiation dose in the range 10−30 kGy. These results indicate that higher crosslinking depending on the radiation energy leads to lower crystallinity. Figure 3 shows the DSC results of PEO films

gel fraction was reduced but the swelling ratio was increased. The values of Mc, which were calculated using eq 1, were 13 and 53 kg/mol for the samples of 10%−30 kGy and 20%−30 kGy, respectively. These results indicate that effective crosslinking occurred in the gels prepared at lower concentrations; consequently, the highly cross-linked gel could not readily swell. It should be noted that gelation using a high-energy source such as γ-rays or electron beams depends on the sample conditions when they increase the occurrence of covalent bonds between macroradicals generated by irradiation.17 Specifically, the networks were formed by reaction between the segments containing macroradicals. To meet the reaction opportunities, it is advantageous for solutions to be dilute. These conditions have been demonstrated with chain mobility, as confirmed in the literature, which introduces the radiationinduced cross-links in molten states and in solutions.16−18 Thus, our observations of effective cross-links in a diluted solution are correlated to the results of cross-linking formation depending on the chain mobility. Crystallization of PEO Film. The internal structure of the PEO film established in the cross-linked state reflects the crystallization as well as the cross-links. The films were subjected to DSC measurements to investigate the crystallinity considering the exposed energy and the PEO concentrations before the irradiation process. Figure 2 shows the degree of crystallinity and the crystallization temperatures (Tc) of the

Figure 3. DSC results of the crystalline films prepared from different concentrations of PEO solution (5−20%) exposed to 30 kGy and the results of the film from 20% solution with no radiation treatment. (a) DSC cooling scans at 10 °C/min after heat−cooling cycle. (b) Plot of the crystallization temperature (Tc) and crystallinity obtained from heat of fusion.

prepared at different concentrations with an identical dose of 30 kGy. Tc and the crystallinity both increased with the PEO concentration, as shown in Figure 3b. Here, we note that a diluted PEO solution leads to a higher cross-linking density, as described in the previous section. Therefore, the concentration-dependent results of Figure 3 imply cross-link exclusion in the crystalline phase. Figure 4 shows the XRD patterns of PEO films established from the gel state, which was attained in the irradiated solution with different concentrations, and from the nonirradiated solution. After the drying process, the PEO chains form a crystalline structure with typical Bragg peaks at 19.1° and 23.2°. The interlayer spacing from the peak at 19.1° is 4.5 Å, corresponding to the (120) plane. That from the peak at 23.2° assigned to the (112) and (032) planes is 3.8 Å.32 The (120) direction is aligned parallel to the extended chain in the crystal,

Figure 2. DSC results of the crystalline films prepared from 20% PEO solution exposed to different radiation energy (0−30 kGy). (a) DSC cooling scans at 10 °C/min after heat−cooling cycle. (b) Plot of the crystallization temperature (Tc) and crystallinity obtained from heat of fusion. D

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Figure 4. X-ray diffraction results of the films prepared from different concentrations of PEO solution (5−20%) exposed to 30 kGy and the results of the film from the 20% solution with no radiation treatment.

which is known for the fastest growth direction.33,34 There was no shift of the peaks noted in the measured samples; thus, the films establish the crystalline state of the PEO chains even in a cross-linking network. Lamellar Stacks of PEO Film. The interlamellar distances of the dried films were investigated in an effort to determine the internal structure, consisting of both a crystalline state and a cross-linked network. A typical lamellar structure was observed in the SAXS patterns of the 20%−30 kGy film, showing multiple peaks (q*, 2q*, 3q*) in Figure S2. The intensity of lamellar peaks becomes weak in the films that received a relatively low energy. Accordingly, the weakest intensity was observed in the film exposed to no radiation, which exhibited the highest crystallinity among all of the samples. In a two-phase system, the scattering intensity is described with the invariant Q; Q = Δρ2φ1(1 − φ1) ∼ ∫ ∞ 0 I(q) q2 dq, where ρ is the scattering length density and φ is the volume fraction.26 Therefore, the weak intensity of the peaks in the samples exposed to a lower energy level explains the presence of a high degree of local crystallinity. Otherwise, the strong intensity of peaks was observed in the films prepared from a gel state involving cross-links. Figure 5 shows Lorentz-corrected SAXS patterns of the films according to the polymer concentrations, which generate different magnitudes of cross-linking by identical radiation doses of 30 kGy. It is evident that the interlamellar distance changes with the concentration. The highly cross-linked film, which was developed from lower concentrations, exhibits a shorter interlamellar distance as well as a less crystallinity observed in Figure 3. The same trend was observed in the melt-crystallized films, as shown in Figure S4. The concentration-dependent scattering patterns show at least two peaks of the lamellar phase, and the values of d-spacing at q* considering the interlamellar thickness (L) were 24, 22, and 19 nm (q* = 0.026, 0.028, and 0.032 Å−1) for the samples 20%− 30 kGy, 10%−30 kGy, and 5%−30 kGy, respectively. As another estimated distance for the crystalline structure, the long-period distance (Lmax), the crystalline thickness (lc), and amorphous thickness (la) were obtained from the normalized 1D correlation functions plotted in Figure 5b and Figure S4b. In a two-phase model, Lmax is comparable to the Bragg distance, L, at q*. The values of Lmax, lc, and la are shown in Table 1. Here, lc should be greater than la because the crystallinity of the samples is high (Xc > 50%).35 Thus, the thickness of amorphous phase, la, was estimated through the initial slope and the plateau at the minimum position in Figure

Figure 5. (a) Lorentz-corrected SAXS results and (b) normalized 1D correlation functions of the crystalline films prepared from different concentrations of PEO solution (5−20%) exposed to 30 kGy.

Table 1. Estimated Distances in PEO Films Established from a Gel State According to the Polymer Concentrations at Identical Radiation Dosesa sample

Lmax (Å)

lc (Å)

la (Å)

Lct (Å)

ξ (Å)

20%−30 kGy 10%−30 kGy 5%−30 kGy

239 224 181

175 166 131

64 59 51

320 98

51 56

a Long-period distance (Lmax) and crystalline and amorphous layer thicknesses (lc and la) were obtained from 1D-correlation functions, and the extended chain length (Lct) was calculated from molecular weight between cross-links.

5b. The Lmax decreases from 24 nm (20%−30 kGy) to 22.4 nm (10%−30 kGy) in the cast films. The same trend was noted in the melt-crystallized films (Figure S4); Lmax in the 10% film is 16.6 nm, which is smaller than Lmax = 18.9 nm in the 20% film. The linear crystallinity obtained from lc/Lmax is 73%, which is similar to that of bulk crystallinity, Xc, in the 20%−30 kGy film. The linear crystallinity in the 10%−30 kGy film was 74%, which is larger than Xc = 65%. It is estimated that the homogeneous lamellar stacks are presented in the 20%−30 kGy with lower cross-linking density, while lamellar stacks may not fill in the 10%−30 kGy with higher cross-linking density. The obtained long-period distances (L, Lmax) of the films created from the 20% and 10% solutions are covered in the lengths of the totally extended chain between the cross-links (Lct) as calculated from Mc, showing the shorter values of Lmax E

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Figure 6. 2D SAXS patterns of the films prepared from the 20%−30 kGy and 10%−30 kGy films, during stretching from Λ = 1 to Λ = 2. The Herman parameters (f) at the first lamellar peaks (q*) show negative values.

Figure 7. Annular and sector averaged scattering intensity of the 20%−30 kGy film with Λ = 2. (a) Annular intensity at q = 0.015 Å−1 and q = 0.026 Å−1 along the azimuthal angle defined in (b). (d) Scattering patterns of the film from the sector averaged intensity at a 30° angle span from the vertical and horizontal directions in (c), the 2D scattering pattern.

the lamellar phase were shown along the specific q values, with the first peak becoming stronger on the vertical intensity compared to the horizontal intensity results. The vertically anisotropic scattering describes the parallel orientation with the stretching direction, corresponding to real space.38 If polymer chains in the gels slip off along the elongation direction, vertically anisotropic patterns would emerge in the high q regions.39 The 2D patterns in this experiment did not indicate the slipping off of the polymers under the applied deformation. Instead, the intensity of the lamellar peaks appeared stronger along the vertical direction. Therefore, these results suggest the lamellae are oriented along the elongation direction. To confirm the anisotropy of the SAXS patterns, the annular intensity was obtained as a function of the azimuthal angles (Φ), which was defined as a value of 0 in the stretching direction. Anisotropy of the 2D patterns appears in the lamellar peaks; thus, the intensity along the azimuthal angle was obtained from the positions of q* at 0.026 and 0.028 Å−1 for

than those of Lct, where Lct = 320 and 98 nm in the 20%−30 kGy and 10%−30 kGy films, respectively. The estimated distances are summarized in Table 1. This comparison shows that the cross-linking points are frequently positioned in lamellar stacks of the film prepared from a highly cross-linked state. Lamellar Stacks of PEO Film under Deformation. Lamellar stacks of the crystalline films were observed under uniaxial deformation on an elongation stage. Stable elongation was realized up to Λ = 2 (200% elongation against the original length) in the case of the 20%−30 kGy film, which exhibits higher modulus compared to the films from lower concentrations.15 This is due to the major role of crystalline structure to enhance the modulus. Crystalline regions are closely packed in ordered alignments governed by extensive secondary forces, such as van der Waals force. Figure 6 shows the 2D SAXS patterns of the films, 20%−30 kGy and 10%−30 kGy, under the elongation. The stretching triggers were applied to the horizontal direction on the images. The scattering patterns of F

DOI: 10.1021/acs.macromol.8b01232 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules the films 20%−30 kGy and 10%−30 kGy, respectively. As shown in Figure 7a for the film 20%−30 kGy, distinct variation in intensity is observed at 0.026 Å−1 of the crystalline peak, while a nearly constant intensity appears at 0.015 Å−1 along the azimuthal angle. In the same manner, the film of 10%−30 kGy shows similar propensity (Figure S5). These anisotropic patterns were also observed in the results of the meltcrystalline films (Figure S6). The results indicate that the deformation of the film generates the structural orientation of the lamellar stacks. The magnitude of orientation was stronger at the 20%−30 kGy film than the 10%−30 kGy film. The Hermans parameters, which were calculated using eqs 4 and 5 from the annular intensity, show more negative values in the 20%− 30 kGy film as noted in Figure 6. These negative values indicate a parallel orientation along the stretching direction.25 For the two orthogonal directions in Figure 7d, the results of sector averaged intensity for the 20%−30 kGy sample show that the intensities at q* are relatively diminished in the horizontal intensity on the 2D scattering pattern as a result of the vertical anisotropy of lamellar intensity. The differences of the intensities between two orthogonal directions are stronger in the 20%−30 kGy film, which gives rise to a distinct orientation compared to the 10%−30 kGy film in Figure S5. There was no shift of the lamellar distance between the orthogonal directions to the elongation. Thus, the orientation of the lamellae was observed as the main response against deformation. The circular averaged scattering intensities are plotted in Figure 8 according to the magnitude of elongation. The interlamellar distances were estimated at the scattering vectors

of the lamellar peaks in Table S1. The 20%−30 kGy sample in Figure 8a maintains almost identical q values of the lamellar peaks, and the 10%−30 kGy sample in Figure 8b proves the slight shift toward the higher q range. Lamellar Stacks of PEO Film in a Swollen State. The changes of the interlamellar distances of the films were determined in the swollen state. SAXS measurements were performed for the three groups of samples according to the magnitude of swelling. The first group, swelling-1, was stored in a humid chamber at RH > 95%, and the second group, swelling-2, was prepared by wetting with a 0.1 mL water droplet. The third group, swelling-3, was fully swollen and equilibrated. Figure 9 shows the change of the long-period distance, L, evaluated from the Bragg distance during swelling process. In

Figure 9. SAXS patterns of the (a) 20%−30 kGy and (b) 10%−30 kGy films under the different magnitudes of swelling. The samples were the dry film, differently swollen film, and the initial PEO solution before radiation.

an ideal two-phase system, the parameter value L corresponds with the distance at the maximum and minimum positions (L = Lmax = Lmin/2) in the 1D correlation function.42 In Figure 10, the plot of the dry film illustrates the approximation of the distance that describes the typical two-phase model, showing the identical distances of L and Lmax, such as L = Lmax = 24 nm in the case of the 20%−30 kGy film. However, when the films were swollen, the complicated correlation functions were obtained. This originated from the wide distribution of thicknesses of the crystalline and amorphous layers.43,44 The correlation functions of the swollen film (swelling-1) exhibit distortions in the curves which indicate the nonuniformity of the thickness of lamellae and amorphous phase. These curvatures were more complicated in the film from the less concentrated solution. The symmetric maximum peaks also

Figure 8. Circular averaged SAXS patterns of the (a) 20%−30 kGy and (b) 10%−30 kGy films under deformation from Λ = 1 to Λ = 2. The insets are Lorentz-corrected scattering patterns. G

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periodic spacing. First, Lmax increased in the swollen film of 20%−30 kGy. The variation of amorphous region was observed from the change of the initial slope as shown in Figure 10a. Second, in the swollen film of 10%-30 kGy, the change of Lmax is ambiguous, but the change of the initial slope and skewing of the neighbor peak are observed in Figure 10b. Therefore, we can suggest the approximated models. The swollen film of 20%−30 kGy exhibits variations in thickness of amorphous region and the thick long-period distance. The swollen film of 10%−30 kGy contains nonuniformity of crystalline thickness at almost constant long period distance. These lamellar stacks were disrupted during the swelling process of swelling-2 and swelling-3, as shown in Figure 9. In the 10%−30 kGy film by the swelling-2 process, the peaks of the lamellar phase disappeared. The fully swollen films (swelling-3) are described with inhomogeneities and polymer regime. Mostly, gels can be fitted with the Debye−Bueche function and Lorentz-type function.41,42 This model describes the system where the cross-links are introduced to a polymer solution in semidilute regime, providing the interpretation for inhomogeneities of network and polymer chains. For the fully swollen film, the patterns were examined with this model and alternative models, a two correlation model with different correlation lengths. The reasonable fit results were obtained by using a two correlation model due to the exponent parameters, which do not obey two terms with Lorentzian and squared Lorentzian terms of Debye−Bueche and Lorentz-type functions. The obtained parameter values are presented in Table S2. The analysis of the high q region describing the polymer chains results in ξ = 5.3 and 5.9 nm in the 20%−30 kGy and 10%−30 kGy films, respectively, showing exponents between 3 and 4 indicating a surface fractal structure. Similar structural aspects were observed in the results of PEO solutions which are not cross-linked. The scattering patterns of the PEO solutions are described as a concentrated system associated with two different behaviors of large clusters (the low q region) and polymer chains (the high q region),43,44 as presented in Figure 9 and Table S2. The analysis of the profiles provided correlation lengths of around 5−5.6 nm of the entangled polymer chains, allowing q−3−q−4 behavior in all of the examined samples, as shown in Table S3.

Figure 10. 1D correlation functions of the (a) 20%−30 kGy and (b) 10%−30 kGy film by swelling process at swelling-1.

became asymmetric,37,38 as shown in Figure 10b, which reveals the variation of the long-period distance of the film from lower concentration solutions. Several cases that exhibit deviations from an ideal system observed in the simulation results.37 (i) The variations of amorphous thickness (constant interdomain spacing) induced the change of the initial slope. (ii) The initial slope is not affected by long-period distance. (iii) The variation of crystalline thickness (constant domain spacing) led to the first-neighbor peak skewing.37 The 1D-correlation functions of the swollen gels contain the features of the variations of

Figure 11. Schemes of long-period distance, Lmax compared to correlation length, ξ, end-to-end distance, Dend, and contour length, Lct, between cross-links in the (a) 20%−30 kGy and (b) 10%−30 kGy films during drying-swelling process. The red circles indicate cross-links. H

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Macromolecules The effects of the cross-linking density and the concentration should be considered with regard to the crystalline state of the examined samples, as they were established from the gel state. The films were cast from a concentrated solution above the overlap concentration (c*), at which PEO (Mw 100 kDa) concentration is 0.78% (w/v).45 For concentrated solutions, entanglements are one of the parameters that contribute to a crystalline state. They prevent crystalline growth due to the reasons described with the following theoretical concepts: long-range repulsion by the connectivity of the entanglements between crystalline clusters11 or sliding diffusion that the chains are disentangled upon crystallization.46 Here, the concentration-dependent restriction of the crystalline growth was not totally elucidated in the examined films, thereby decreasing the lamellar thickness in the films prepared at lower concentrations. In semidilute solution, mesh size follows the relation with concentrations from ∼c−0.75 to c−1. The weak concentration dependence of the examined solution (Table S3) indicates local concentration is almost constant. Meanwhile, concentration induced the different cross-linking density in the samples. As another important factor, the cross-linking density influences the crystalline state. It eliminates reptation behavior describing a specific motion, which require the free chain ends to permit a translation.7 The bulky cross-links reduce the secondary nucleation rate and control the lamellar thickness on the assumption that cross-links are excluded.6 In particular, when increasing the cross-linking density more, the lamellar thickness decreases due to the reduced chain lengths between the cross-links, as the case of cross-link exclusion.6,7 The results showed that the lamellar thickness, Lmax, was varied between 18 and 24 nm depending on the polymer concentrations, which produce effective cross-links, in the range 5−20%. The extended chain length between cross-links, Lct, covers Lmax including the thickness of the crystalline layer. The length of Lct of the 20%−30 kGy sample is 320 nm, which is 3 times that of 10%−30 kGy, as shown in Table 1. These results take into account the possible presence of cross-links in the lamellar stacks, as presented in Figure 11. The estimated Lmax was compared to Lct, the end-to-end distance between the cross-links (Dend), and ξ in the swollen film (swelling-3). Dend was calculated using the relation (bLct)1/2 = bN1/2, where b is the segment length and N is the number of repeating segments. Dend indicates that the behavior of chains under the assumption of ideal chains in the network with equally distributed crosslinks. However, the analysis of the swollen state found that the network consists of the entangled chains with ξ ∼ 5−6 nm, showing behavior between q−3 and q−4 indicating a surface fractal structure. Compared to the lengths of ideal chains between cross-links Dend, the correlation length ξ of the entangled chains is longer (ξ > Dend) in the 20%−30 kGy film, whereas it becomes shorter (ξ < Dend) in the 10%−30 kGy film. These results suggest that the relatively frequent presence of cross-links can lead to structural changes corresponding to elongation or swelling, showing smaller crystallites with wide distributions. On the other hand, lamellae apart from the crosslinks present relatively weak destruction of the compositions inside the lamellar stacks, as observed in the 20%−30 kGy film.

when a polymer solution was exposed to γ-ray radiation. The networks with different cross-linking density were formed in terms of the radiation energy and the polymer concentration. The crystalline films were established from the cross-linked network. The resulting crystallinity and crystalline structure were explained by the fact that the cross-links, as a type of defect incorporated into the chains, prevent the crystalline growth.6,7 The behavior of lamellar stacks was determined by the SAXS measurements under various conditions, and the lamellar thickness was observed with the 1D correlation functions of SAXS profiles. Our findings illustrate that the cross-linking influences the crystalline structure. The results were discussed in terms of (i) the lamellar thickness influenced by cross-links, (ii) the comparison between Mc and the lamellar thickness, and (iii) the structural change influenced by cross-links in crystalline state. First, the results presented examples of cross-link exclusion for crystallization. The increases in cross-links reduce the crystallinity and generated smaller distance of the lamellar and amorphous phases. The resulting scattering intensity presented obvious periodic distances by the volume fractions of the crystalline and amorphous regions. Second, the parameters of lamellar structure, Lmax, lc, and la, were considered in a comparison with Lct calculated from Mc, resulting in Lct > Lmax. The value of Lct of the 20%−30 kGy film was 3 times that of the 10%−30 kGy film. These results imply that cross-links are incorporated frequently in the lamellar stacks of the film with a higher cross-linking density. Third, these observations support the structural change of lamellar stacks in the given conditions. The crystalline lamellar structure was arranged by the elongation, and it is disrupted in swollen state showing complicated curves in 1D correlation functions, which imply nonuniformity. The cross-links distributed rarely in the crystalline state (the film from the higher concentration) led to an effective orientation along the elongation direction, resembling the behavior of reinforcing binders in a matrix.45,47 The results suggest that the structural aspects of the lamellar stacks are influenced by the possible presence of cross-links around crystalline layers. The research is based on the idea that semicrystalline polymer chains achieve nanometer-sized ordering from a gel state, where the chains are loosely cross-linked. The results represent a fundamental understanding of the phenomenology of PEO films that can recover their original properties after intended defects, as reported in our previous work. On the damaged side of the crystalline film, the chains are unfolded and entangled during swelling process. When it becomes dry, the entanglements of the interpenetrated chains were folded back again to form lamellar stacks, which are responsible for the recovery of the mechanical property. The observations here suggest the concept by which the stiffness of a crystalline polymer matrix is controllable by simple triggers such as the humidity. The results provide knowledge by which to develop an application strategy for the PEO matrix that can lead to advanced products in the biomedical and bioengineering fields.48−50

CONCLUSION The lamellar thicknesses of PEO films established from a gel state were investigated, and the structural characteristics of the lamellar stacks were revealed. Cross-links were introduced

S Supporting Information *

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ASSOCIATED CONTENT

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01232. I

DOI: 10.1021/acs.macromol.8b01232 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

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Swelling ratio and gel fraction of PEO films, SAXS patterns of the films prepared with different radiation energy, background correction for 1D correlation functions, SAXS results of the melt-crystallized films, anisotropy on SAXS patterns of the 10%−30 kGy film, table of the interlamellar distances of the films by deformation and swelling process, the fitting parameter values of the fully swollen films, SAXS analysis and the fitting parameter values of PEO solutions before gelation (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (D.S.). ORCID

Daewon Sohn: 0000-0002-7200-9683 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Bio & Medical Technology Development Program of the National Research Foundation (NRF) funded by the Ministry of Science & ICT (No. 2017M3A9G8084539). The authors acknowledge the support of the Pohang Accelerator Institute (PAL) in providing X-ray beamline. We thank the Korea Atomic Energy Research Institute (KAERI) in operating γ-ray facility (KAERI, Jeongeup) and X-ray instrument (KAERI, HANARO).

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ABBREVIATIONS DSC, differential scanning calorimeter; FE-SEM, field emission scanning microscope; PEO, poly(ethylene oxide); SEBS, polyethylene-b-polybutadiene-b-polystyrene; SAXS, smallangle X-ray scattering; XRD, X-ray diffraction.

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