Study of Strain-Induced Crystallization and Enzymatic Degradation of

Article pubs.acs.org/Macromolecules

Study of Strain-Induced Crystallization and Enzymatic Degradation of Drawn Poly(L‑lactic acid) (PLLA) Films Deepika Rangari and Nadarajah Vasanthan* Department of Chemistry, Long Island University, One University Plaza, Brooklyn, New York 11201, United States ABSTRACT: Poly(lactic acid) (PLLA) melt-pressed films with low crystallinity were crystallized by stretching at a constant strain rate. The strain-induced crystallization and enzymatic degradation of drawn PLLA films in the presence of proteinase K at 37 °C was investigated using weight loss measurements, differential scanning calorimetry (DSC), and Fourier transform infrared (FTIR) spectroscopy. The results show that drawing has a significant effect on the crystallinity, molecular orientation, and enzymatic degradation. The absorbance ratio of the bands at 921 and 956 cm−1 (A921/A956) was chosen to determine the structural changes during strain-induced crystallization and hydrolysis. The DSC crystallinity and A921/A956 showed an increase with the draw ratio. Since we were unable to obtain the transition moment angle for the bands at 921 and 956 cm−1, the dichroic ratios were compared. It was found that the crystalline orientation develops rapidly at lower draw ratios whereas the amorphous orientation develops much more slowly. The enzymatic degradation of annealed PLLA films was reported, and surprisingly, quite different results were observed for the enzymatic degradation of oriented PLLA films. The extent of degradation was lower for the drawn PLLA film than for the undrawn melt-pressed PLLA film. The DSC crystallinity and A921/A956 of drawn PLLA films increased with the degradation time, suggesting an increase in the crystalline phase with degradation. This reveals that degradation occurs in both the free and the restricted amorphous region in the case of drawn PLLA films, whereas it occurs only in the free amorphous region in annealed unoriented PLLA films.

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by melt or by cold crystallization; the β form, by high-speed spinning and drawing to high draw ratios at high drawing temperature; and the γ form, by epitaxial crystallization. Amorphous PLLA has lower strength and dimensional stability, and therefore, stretching or annealing above the glass transition temperature is required to increase the molecular orientation and crystallinity for commercial applications. The development of the structure of PLLA has been studied by X-ray diffraction26,27 and differential scanning calorimetry,28,29 and it has been characterized by IR spectroscopy as a complementary technique.30−32 Many studies have investigated the degradability of PLLA.33−49 It has been found that the degradability can be modified significantly by changing the microstructure of the polymer. Many studies have also investigated the effect of structural parameters on the hydrolysis of PLLA in different media such as phosphate buffered solution (pH = 7.4), alkaline media, acidic media, and enzymes.33−49 Our group studied the hydrolytic degradation of PLLA films in the presence of 0.1 M NaOH and proteinase K.48,49 It has been found that the degradation of PLLA films in the presence of a base increased with increasing crystallinity. On the other hand, degradation in the presence of an enzyme decreased with increasing crystallinity.

INTRODUCTION Poly(L-lactic acid) (PLLA) is a biodegradable and biocompatible semicrystalline polymer. It has been widely studied as an alternative to conventional commercially available polymers because it is biodegradable, compostable, and nontoxic.1−7 Studies have shown that the mechanical properties of PLLA are comparable to those of other polymers such as polyethylene and polystyrene, and therefore, it can be used for manufacturing packaging films and fibers.1−7 PLLA can be processed into transparent films or fibers, or it can be injection-molded into bottles in the same way as poly(ethylene terephthalate) (PET).8 Poly(lactic acid) has two stereoisomeric forms: Dlactide and L-lactide. It can also exist as a racemic equimolar mixture of D- and L-lactides, designated as DL-lactide.9,10 PLLA has a glass transition temperature (Tg) of ∼60 °C, which is relatively higher than that of other thermoplastics, and a crystalline melting temperature (Tm) ranging from 130 to 180 °C, which is relatively lower than that of other thermoplastics.11,12 PLLA can be crystallized either by thermally induced crystallization or by strain-induced crystallization.13−15 The mechanical and physical properties of semicrystalline PLLA have been widely reported to depend on their morphology and crystal structure.16−20 In this light, the development of the structure of PLLA during thermally induced crystallization and strain-induced crystallization has been extensively studied.13−15,21−25 PLLA has been found to crystallize into one of three crystal forms, α, β, or γ, depending on the crystallization conditions. The α form is formed either © 2012 American Chemical Society

Received: July 17, 2012 Revised: August 24, 2012 Published: September 4, 2012 7397

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stretched films before and after degradation were obtained. All experiments were carried out under a constant nitrogen flow rate of 20 mL/min. The degree of crystallinity, χc, was calculated from the heat of fusion and obtained from the DSC scans by the equation

Although the degradation of PLLA has been widely studied, the influence of the microstructure on degradation has not yet been clarified. This study aims to investigate the effect of straininduced crystallization on the enzymatic degradation of PLLA using weight loss measurements, DSC, and FTIR spectroscopy. The microstructural changes in PLLA after degradation are also studied. Furthermore, PLLA subjected to strain-induced crystallization and thermally induced crystallization is compared. This study aims to provide some new insights into how microstructural properties such as the crystallinity and molecular orientation affect enzymatic degradation.

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% χc =

ΔHm − ΔHc × 100 ΔH100%

where ΔHm is the enthalpy of melting; ΔHc, the enthalpy of crystallization; and ΔH100%, the heat of fusion of 100% crystalline PLLA. The crystallinity of PLLA was determined from the heat of fusion of 100% crystalline PLLA for all samples (93 J/g). Fourier Transform Infrared Spectroscopy (FTIR). The infrared spectra of the melt-pressed, stretched, and degraded films were obtained using a Nicolet Magna 760 spectrometer. The spectra were collected in the mid-IR region from 4000 to 500 cm−1 in the transmission mode with a resolution of 2 cm−1. The absorbance of various infrared bands was determined using Omnic software. Two different transmission spectra were collected for each sample for orientation studies by using an incident beam parallel and perpendicular to the fiber axis using a polarizer. The infrared dichroic ratio was determined using the following equation: D = A∥/A⊥, where A∥ is the absorbance parallel to the draw direction and A⊥ that perpendicular to the stretching direction.

EXPERIMENTAL SECTION

Materials. Poly(L-lactic acid) (PLLA) with a molecular weight of 300 000 was obtained from Polysciences, Inc. The enzyme proteinase K from Tritirachium album in the form of lyophilized powder, 1 M Tris-HCl buffer solution with pH 8, and sodium lactate 60% (w/w) with a density of 1.3 g/mL were obtained from Sigma-Aldrich Chemical Co. The pH of the buffer solution was adjusted to 8.5, and the solution was stored in a refrigerator until further use. Preparation of PLLA Films. PLLA films were prepared by solvent-casting followed by melt-pressing using a Carver press. PLLA pellets (0.5 g) were dissolved in a 25 mL 50:50 mixture of dichloromethane and chloroform. The resulting solution was poured into a glass Petri dish, and it was left to stand overnight at room temperature for evaporation. Solvent-cast PLLA films were placed between the preheated platens at 200 °C for 5 min, and a pressure of 10 000 lb was applied. The films were then removed and quickly quenched in ice-cold water to prevent further crystallization. These films were essentially had low crystallinity and had a thickness of 50− 60 μm. Rectangular films with dimensions of 4 × 3 cm2 were cut from the undrawn films, and they were uniaxially stretched to different draw ratios of 1 to 3 at a constant strain rate of 8.333 × 10−3 s−1. The various films obtained were labeled as PLLA1, PLLA1.5, PLLA2, PLLA2.5, and PLLA3, with the numeral in the label indicating the draw ratio. Enzymatic Degradation. PLLA films with dimensions of 2 × 1.5 in.2 were immersed in a 100 mL beaker containing 2.0 mL of the enzyme solution and 18.0 mL of the Tris-HCl buffer solution. The enzyme solution was prepared by dissolving 1 mg of proteinase K enzyme in 1 mL of e-pure water. The 0.05 M Tris-HCl buffer solution with pH 8.5 was prepared by taking 50 mL of 1 M Tris-HCl solution and adjusting the pH to 8.5 using 1 M NaOH. The solution was then transferred to a 1 L volumetric flask, and its volume was made up to 1 L with e-pure water. Enzyme solutions with films were maintained at 37 °C in a constant-temperature bath. The degradation data were measured every day for 10 days. To do so, every day, the film was removed, washed with ice-cold water, and dried in a vacuum oven. The weights of the films were measured before and after degradation using an analytical balance. The degradation studies were carried out for at least two sets of films. The degraded film solutions were collected and stored in the refrigerator for further use. Weight Loss Measurements. The percentage weight loss of the degraded film was calculated using the mass of PLLA films before and after degradation by the equation

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RESULTS AND DISCUSSION Strain-Induced Crystallization Studies. The meltpressed PLLA films (PLLA1) were stretched to different draw ratios to obtain PLLA films with varying crystallinity and molecular orientation. Figure 1 shows DSC scans of PLLA1,

Figure 1. DSC scans of melt pressed and PLLA films stretched to different draw ratios.

PLLA2, and PLLA3. The DSC scan of the unoriented meltpressed film shows three transitions: melting temperature (Tm), glass transition temperature (Tg), and cold crystallization temperature (Tc). DSC scans of the PLLA film drawn to different draw ratios suggest a significant difference from the thermal behavior of the undrawn film, and they only show the melting temperature (Tm). Tm increases with the draw ratio, and this increase is often accompanied by increasing lamellar thickness. For example, PLLA1, PLLA2, and PLLA3 melt at 179, 182, and 184 °C, respectively. Tc and Tg disappear completely for films that are stretched to different draw ratios, suggesting that all of the stretched films have a significant amount of crystallinity. As the crystallinity develops, the mobility of polymer chains in the noncrystalline region is expected to be significantly reduced, resulting in a slower rate of crystallization. The thermal properties of PLLA films drawn at a strain rate of 0.17 s−1 have been reported previously,15 and it has been shown that Tc appears in DSC scans for PLLA films

Wloss (%) = 100 × (Wbefore − Wafter)/Wbefore where Wloss (%) is the percentage weight loss of the degraded PLLA film; Wbefore, the weight of the dried PLLA film before degradation; and Wafter, the weight of the dried PLLA film after degradation. Differential Scanning Calorimetry (DSC). A Perkin-Elmer DSC 7 differential scanning calorimeter was used to investigate the thermal properties of PLLA films before and after degradation. Samples weighing ∼4−6 mg were used for the DSC experiments. The instrument was calibrated for the temperature and heat of fusion using an indium standard (Tm = 156.6 °C and ΔH = 28.5 J/g). The instrument was heated from 30 to 200 °C at a constant heating rate of 10 °C/min. Thermal properties such as Tg, Tm, and ΔH of the 7398

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crystallizes into one of three possible forms: α, β, or γ. The α form, which is most common, is obtained either by melt or solution crystallization, whereas the β form is obtained by drawing to high draw ratios at high drawing temperatures. Infrared band assignments were reported for both forms. The bands at 697, 739, 921, and 1293 cm−1 were attributed to the α crystalline phase and those at 710, 757, 895, 956, and 1302 cm−1 to the amorphous phase. It was found that the absorbance of crystalline bands increases relative to that of amorphous bands with increasing draw ratios. This observation further confirms that crystallinity increases with the draw ratio. It has been reported that bands at 921 and 908 cm−1 can be used to identify the α and β crystalline phases, respectively. Figure 3 shows no band at 908 cm−1, suggesting that there is no β crystal form present in both drawn and undrawn PLLA films. Vasanthan and Ly used FTIR spectroscopy to investigate the structural changes in PLLA films after thermally induced crystallization and hydrolytic degradation.48,49 The bands at 921 and 956 cm−1 were chosen to determine the structural changes that occur during crystallization at different annealing temperatures and after hydrolytic degradation because these are relatively isolated from other bands. These bands were attributed to the coupling of C−C stretching with CH3 rocking vibration. In this study, these bands were used to examine the structural changes in PLLA during stretching as well as enzymatic degradation. Figure 4 shows the infrared bands at

drawn up to draw ratio of 5. This is different from our observation in which Tc was not observed at all for drawn PLLA films. It should be noted that the PLLA films used in this study were drawn at a much lower strain rate (0.00833 s−1) compared to that in the previous study, and therefore, the different thermal behavior may be attributed to the different strain rate used for stretching. The crystallinity of the PLLA films was measured using DSC. Figure 2 shows the development of the crystallinity with the

Figure 2. DSC crystallinity as a function of draw ratio for all PLLA films drawn to different draw ratios.

draw ratio. The crystallinity increases with the draw ratio, which is consistent with the previously reported data for other semicrystalline polyesters such as PET 50,51 and poly(trimethylene terephthalate) (PTT)52,53 drawn at temperatures above Tg. The crystallinity increases rapidly from the unstretched PLLA film to the PLLA film stretched to a draw ratio of 2, and the crystallinity increases more gradually for a draw ratio above 2. For example, the crystallinity of an unoriented PLLA film was ∼20%; it increased to 40% for the film drawn to a draw ratio of 2 and 43% for one drawn to a draw ratio of 3. Figure 3 shows the FTIR spectra of PLLA1, PLLA2, and PLLA3 in the region from 1000 to 650 cm−1. An obvious spectral difference can be seen between the FTIR spectra of drawn and undrawn PLLA films. As mentioned above, PLLA

Figure 4. IR spectra of melt pressed and PLLA films stretched to different draw ratios in the region of 1000−900 cm−1 .

921 and 956 cm−1 for all drawn PLLA films. It is seen that the absorbance of the band at 921 cm−1 increases relative to that of the band at 956 cm−1. To measure the changes in the crystalline phase quantitatively, the spectra of unoriented films are required. We have measured the spectra of uniaxially oriented films under parallel and perpendicular polarization. Figure 5 shows the FTIR spectra of a PLLA film drawn to a draw ratio of 3 measured under parallel and perpendicular polarization. The polymer chain usually orients parallel to the drawing direction. The absorption of infrared radiation depends on the transition moment vector and the applied electric vector. In the case of solids and crystals, each molecule has a fixed direction. Therefore, the absorbance of a vibrational mode depends on the transition moment vector of the normal vibration (M) and the applied electric vector (E). The quantity M·E determines the molecular transition moment. Maximum absorption will occur when M and E are parallel and minimum absorption when they are perpendicular. Parallel and perpendicular bands

Figure 3. FTIR spectra of PLLA films stretched to different draw ratios in the region of 1000−650 cm−1: (a) DR 1, (b) DR 2, and (c) DR 3. 7399

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identify infrared bands that have an absorbance lower than ∼1 in order to satisfy the Beer−Lambert law. On the basis of previous studies, we have identified the bands at 921 and 956 cm−1 as being suitable for investigating the crystalline and amorphous orientation of PLLA because their absorbance is less than 1. The transition moment angles of these vibrations are not known, and therefore, the dichroic ratio is used to compare the molecular orientation. The dichroic ratio (D) of a perfectly oriented polymer parallel to the draw axis is given by D0 = 2 cot2 α, where α is the transition moment angle of a particular vibration. The dichroic ratio of a partially oriented polymer is given by D = A /A⊥

where A∥ is the absorbance parallel to the draw direction and A⊥ that perpendicular to the stretching direction. Figure 7 Figure 5. FTIR spectra of PLLA film drawn to draw ratio of 3 measured under parallel and perpendicular polarization in the region of 1000−650 cm−1: (a) perpendicular polarization; (b) parallel polarization.

respectively become stronger and weaker in absorbance when the polarized IR light is parallel to the draw direction. On the other hand, perpendicular bands become stronger when the IR light is perpendicular to the draw direction. Figure 5 suggests that the bands at 1384, 1360, 1214, 921, 870, and 751 cm−1 are perpendicular, whereas those bands at 1370, 1188, 956, 739, and 710 cm−1 are parallel. The unoriented absorbance of an IR band was then calculated using the spectra obtained under parallel and perpendicular polarization by the formula A un = (A + 2A⊥)/3

Figure 7. Dichrotic ratio of bands at 921 and 956 cm−1 as a function of draw ratios: (●) 956 cm−1 and (○) 921 cm−1.

where Aun is the absorbance of the unoriented film; A∥, the absorbance under parallel polarization; and A⊥, the absorbance under perpendicular polarization. Figure 6 shows a plot of the

shows the changes in the dichroic ratio of the bands at 921 and 956 cm−1 as a function of the draw ratio of the PLLA films. The dichroic ratio of the respective bands decreases and increases as the draw ratio increases, indicating that both the crystalline and the amorphous orientation increases with the draw ratio. The orientation of the crystalline phase appears to reach a maximum at a draw ratio of 2, whereas that of the amorphous phase continues to increase until a draw ratio of 3. The rapid development of the crystalline orientation to an almost full fiber axis compared with the slower development of the amorphous orientation is commonly observed in semicrystalline polymers. Enzymatic Degradation and Morphological Changes after Degradation. The undrawn and drawn melt-pressed PLLA films were subjected to enzymatic degradation using proteinase K enzyme for 10 days. The percentage weight loss of the oriented and the unoriented PLLA films was calculated from the mass of dried PLLA films before and after degradation by the equation

Figure 6. Absorbance ratio (A921/A956) changes as a function of draw ratio for drawn PLLA films.

Wloss (%) = 100 × (Wbefore − Wafter)/Wbefore

unoriented absorbance ratios of 921 and 956 cm−1 against the draw ratio. It is apparent that the absorbance ratio (A921/A956) increases with the draw ratio. The absorbance ratio increases rapidly up to a draw ratio of 2, and it increases more gradually for a draw ratio of 2−3; this is in strong agreement with our DSC results. Polarized infrared studies were used to obtain information about the molecular orientation of the crystalline and the amorphous phase of PLLA. One of the practical difficulties is to

where Wbefore is the initial weight of dried PLLA films before degradation and Wafter the weight of dried PLLA films after degradation. Figure 8 shows the percentage weight loss as a function of degradation time for PLLA1, PLLA1.5, PLLA2, and PLLA3, which have different crystallinity and molecular orientation. The percentage weight loss increases with the degradation time for both undrawn and drawn films. It is not easy to completely 7400

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Figure 9. DSC scans of PLLA melt pressed PLLA films before and after enzymatic degradation.

Table 1. Thermal Properties and Crystallinity Obtained by DSC of Drawn PLLA Films before and after Enzymatic Degradation at Various Times

Figure 8. Percentage weight loss as a function of degradation time (days) for melt pressed and drawn PLLA films: (●) DR1, (○) DR1.5, (▲) DR2, and (△) DR3.

sample PLLA1

analyze how the microstructure affects the enzymatic degradation of PLLA films because the crystallinity as well as the molecular orientation changes with the draw ratio. To investigate the effect of PLLA crystallinity on enzymatic degradation, an undrawn PLLA film and one drawn to a draw ratio of 2 were compared. The highest percentage weight loss was observed in the former. Furthermore, the crystallinity of the latter (40%) was ∼2 times larger than that of the former (20%). It is well-known that the degradation of semicrystalline polymers first occurs in the amorphous phase and then in the crystalline region, and therefore, our observation should be expected. This is also consistent with results reported by others who investigated the effect of the crystallinity of an undrawn PLLA film on enzymatic degradation. The degradation rate for an unoriented PLLA film is found to be much higher compared to that for a PLLA film stretched to draw ratios of 2 and 3. It should be noted that the molecular orientation, which is specific to the drawn PLLA film, might affect enzymatic degradation. The molecular orientation includes the crystalline and the amorphous orientation. These can respectively be measured using the IR bands at 921 and 958 cm−1. The difference between the crystallinity of PLLA2 and PLLA3 is very small (40% and 43%, respectively), and therefore, PLLA films drawn to draw ratios of 2 and 3 are compared to determine the effect of amorphous orientation. The degradation rate and extent of degradation are similar for both PLLA2 and PLLA3, suggesting that the amorphous orientation plays a minimal role in the degradation. This in turn suggests that the crystallinity plays a dominant role in determining the extent of degradation relative to the molecular orientation. Figure 9 shows the DSC scans of undrawn and drawn PLLA films before and after enzymatic degradation. The onset values were taken as Tg, Tc, and Tm (Table 1). Tg, Tc, and Tm clearly show a significant increase with degradation for the undrawn PLLA film. For example, Tg of neat PLLA is ∼54 °C, but it increases to 62 °C after 10 days of degradation. Similarly, Tc of neat PLLA is ∼89 °C, but it increases to 96 °C. The Tg and Tc depend primarily on chain flexibility, molecular weight, and cross-linking, suggesting that segmental interaction increased as a function of degradation time for the undrawn PLLA film. Tg and Tc could not be detected for PLLA films drawn to different draw ratios before and after degradation. Table 1 shows Tm of

PLLA1.5

PLLA2

PLLA2

degradation time (days)

Tg (°C)

Tc (°C)

Tm (°C)

crystallinity (x)

0 4 7 10 0 4 7 10 0 4 7 10 0 4 7 10

54 58 59 62 57 58 60 62

89 90 95 96 92 93 95 97

178 178 179 179 180 181 183 183 182 183 183 184 183 183 187 185

19.7 20.0 20.6 23.2 32.0 33.2 35.5 40.1 40.9 41.0 43.0 48.0 42.6 43.6 45.6 52.6

undrawn and drawn films as a function of degradation time. Tm was compared for all PLLA films before and after degradation. Tm increased with the draw ratio; however, Tm for all PLLA films showed a small increase and stayed relatively constant during degradation. Figure 10 shows the crystallinity of PLLA1, PLLA 1.5, PLLA2, and PLLA3 as a function of degradation time. The crystallinity values of both drawn and undrawn PLLA films increase with the degradation time. It has been shown previously that the crystallinity of annealed PLLA film increased with degradation time in the phosphate buffer; this was attributed to the selective removal of PLLA chains from the free and restricted amorphous region.43,44 Recently, we studied the effect of crystallinity on the enzymatic degradation of annealed PLLA film and found that there is no significant change in crystallinity with degradation time; this was attributed to the selective removal of PLLA chains from the free amorphous region.49 Furthermore, we observed an increase in the crystallinity of a drawn PLLA film with degradation time; this was attributed to the selective removal of PLLA chains from the free and restricted amorphous region. The FTIR spectra of degraded PLLA films were obtained and compared with those of the original film. The crystallinity changes as a function of degradation time were determined using FTIR spectroscopy. FTIR spectra taken for degraded 7401

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phasedecreased with increasing draw ratio. The bands at 921 and 956 cm−1 were chosen to determine the structural changes during strain-induced crystallization. The IR band ratio (921/956) increased with the draw ratio, which was in agreement with the DSC results. The dichroic ratio at 921 and 956 cm−1 was determined using polarized FTIR spectroscopy. The dichroic ratio at 921 cm−1 decreased and that at 956 cm−1 increased with increasing draw ratio. The extent of degradation was lower for the drawn PLLA film than for the undrawn melt-pressed PLLA film. The DSC crystallinity as well as A921/A956 increased with degradation time, suggesting an increase in the crystalline phase with degradation. This reveals that degradation occurs in both the free and the restricted amorphous region, whereas it occurs only in the free amorphous region in annealed unoriented PLLA films.

Figure 10. DSC crystallinity of undrawn and drawn PLLA films as a function of degradation time: (●) DR1, (○) DR1.5, (▲) DR2, and (△) DR3.

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AUTHOR INFORMATION

Corresponding Author

PLLA films showed that the band at 956 cm−1 decreased and that at 921 cm−1 increased with increasing degradation time, suggesting an increase in crystallinity with degradation; this is in agreement with our DSC observation. Furthermore, Figure 11

*E-mail [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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Figure 11. IR band ratio (A921/A956) of undrawn and drawn PLLA films as a function of degradation time: (●) DR1, (○) DR1.5, (▲) DR2, and (△) DR3.

shows a plot of absorbance ratio, A921/A956, against the degradation time. The plot shows an increase in absorbance ratio with the degradation time, suggesting an increase in crystallinity with time. The dichroic ratio of the bands at 921 and 956 cm−1 was calculated for all degraded samples using the spectra obtained under parallel and perpendicular polarization. No significant change was observed in the dichroic ratio with time for both drawn and undrawn PLLA films.

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CONCLUSIONS The effect of strain-induced crystallization on enzymatic degradation was studied using weight loss measurements, DSC, and FTIR spectroscopy, and it was compared with the enzymatic degradation of annealed PLLA films. Undrawn PLLA films showed three transitions, Tg, Tc, and Tm, whereas drawn films showed only Tm. Tm showed a slight increase with the draw ratio. The crystallinity showed a rapid increase from the case of an undrawn film to a draw ratio of 2 and a more gradual increase from a draw ratio of 2 to 3. The absorbance of the bands at 697, 739, 921, and 1293 cm−1attributed to the crystalline phaseincreased and that of the bands at 710, 757, 895, 956, and 1302 cm−1attributed to the amorphous 7402

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dx.doi.org/10.1021/ma301482j | Macromolecules 2012, 45, 7397−7403