Influence of pH on Morphology and Structure during Hydrolytic Degradation of the Segmented GL-b-[GL-co-TMC-co-CL]-b-GL Copolymer
Abstract
:1. Introduction
2. Experimental Section
2.1. Materials
2.2. Hydrolytic Degradation
2.3. Measurements
3. Results and Discussion
3.1. Hydrolytic Degradation of GL-b-[GL-co-TMC-co-CL]-b-GL in Different pH Media
3.2. Morphological Changes during Hydrolytic Degradation of GL-b-[GL-co-TMC-co-CL]-b-GL in Different pH Media
- (1)
- Fibers are constituted by oriented lamellae and different amorphous regions which will be more susceptible to the hydrolysis process. In fact, oriented crystallites are embedded in an amorphous matrix, being possible to distinguish between interlamellar and interfibrilar amorphous regions (Figure 8a), according to previously postulated models [40,41]. The interlamellar domains alternate with lamellae in the direction of the fiber and possess the lowest molecular orientation and density since they are formed by molecular folds, tie chain segments between adjacent lamellar structures, and free chain ends. Therefore, these domains appear to be the most susceptible to hydrolysis. On the contrary, the interfibrilar domains may have a partial orientation and correspond to the regions placed on lateral sides of lamellae arranged in a fibrillar way. Degradation may therefore follow two different pathways: longitudinal and transversal depending on whether the hydrolysis proceeds mainly through the interfibrilar or interlamellar regions, respectively.
- (2)
- The hydrolysis mechanism is different as previously indicated for acid and basic media. Differences not only concern the kinetic mechanism but also the capability to solubilize degradation products. In this way, retention may be significant in both the interlamellar and interfibrillar regions when an acidic medium is employed and therefore lateral and longitudinal diffusion of water molecules could be hindered. On the contrary, rapid solubilization of degradation products may enhance the surface erosion of exposed fibers in a basic medium.
- (3)
- Morphology of fibers is not completely homogeneous due to a spinning process where the fiber surface is cooled faster than the core. Therefore, the shell layer should have different degree of molecular orientation and probably is constituted by a slightly different lamellar architecture (e.g. size of crystalline and amorphous domains).
- (1)
- Degradation in a neutral pH proceeds through three well differentiated steps. The first one corresponds to the development of straight and longitudinal cracks that evolve through the detachment of the skin layer. Microcracks that propagate circumferentially around the fiber axis are characteristic of the second step although they initially contribute to the peeling out process of the outermost skin. In this step microcracks are irregularly distributed along the fiber and are not completely extended along the circumferential perimeter. In fact, optical micrographs show the presence of irregular cracks that appear inclined with respect to the fiber axis. The last step is associated to the deep propagation of the initiated circumferential cracks through the cross-sectional planes where interlamellar amorphous domains more susceptible to the hydrolysis exist. At the end of this step, regularly distributed discs perpendicular to the fiber axis are evident in the optical and SEM micrographs.
- (2)
- Peeling was not observed when the pH of the degradation medium was lower than 6 and furthermore the formation of longitudinal cracks was clearly hindered in these acidic media (Figure 10). Logically, degradation became slower as the pH decreased and in fact unaltered surfaces were observed after 21 days of exposure even for the irradiated sample. Circumferential and highly spaced cracks perpendicular to the fiber axis were detected at the first stages of degradation in the most acidic media (pHs 2 and 3) whereas irregular and inclined cracks were observed in the optical microscopy images taken at pHs between 4 and 6. Nevertheless, SEM micrographs revealed that these cracks had a zig-zag appearance. In all cases, additional cracks appeared at longer exposures, giving rise to irregular fissures that ultimately lead to the thinner and regular discs that were perfectly oriented perpendicular to the fiber axis.
- (3)
- Degradation in basic media showed the formation of deep longitudinal cracks at the beginning of exposure that lead to peeling only when pH was lower than 9. Circumferential cracks were also observed at the earlier degradation steps together with a clear erosion of the fiber surface, which can already be detected in the irradiated samples exposed for only 21 days in a pH 10 medium. SEM micrographs revealed also the formation of such circumferential cracks and the resulting wrinkled fiber surface. In fact, degradation in the high basic media was peculiar since deep transversal cracks that led to narrow discs were practically formed at the beginning of exposure. Note also that the high solubilization of degradation products caused the lineal and smooth fiber profile rapidly to evolve towards highly tortuous surfaces.
3.3. Changes on the Lamellar Parameters of GL-b-[GL-co-TMC-co-CL]-b-GL during Hydrolytic Degradation in Different pH Media
pH | Time (Days) | LB (nm) | Lγ (nm) | lc (nm) | la (nm) | XcSAXS |
---|---|---|---|---|---|---|
- | 0 | 8.3 | 7.3 | 5.3 | 2.0 | 0.73 |
4 | 12 | 7.8 | 6.5 | 5.2 | 1.3 | 0.80 |
4 | 35 | 7.7 | 6.5 | 5.2 | 1.3 | 0.80 |
4 | 88 | 5.4 | 5.2 | 4.2 | 1.0 | 0.79 |
7 | 35 | 7.3 | 6.4 | 5.1 | 1.3 | 0.80 |
9 | 35 | 7.4 | 6.4 | 5.1 | 1.3 | 0.80 |
11 | 28 | 7.6 | 6.6 | 5.2 | 1.4 | 0.80 |
3.4. Thermal Annealing of Degraded GL-b-[GL-co-TMC-co-CL]-b-GL Samples in Different pH Media: Repercussions on the Lamellar Morphology
- (1)
- Samples degraded in acidic media basically showed the characteristic meridional reflections associated with lamellar stacking. Firstly, these reflections had the previously indicated evolution when temperature was increased that led to an increase of both lamellar thickness and breadth. However, at the highest temperatures and especially for the most degraded samples (e.g., see patterns of samples degraded for 35 and 88 days), reflections were elongated in the meridional direction and extended towards the center of the pattern as presumable for a decrease of the crystalline domain size (i.e., the number of stacked lamellae). It is interesting to note that extrameridional reflections with a practically meridional orientation could also be detected during the heating of the less degraded samples (e.g., see arrow for the sample exposed for 12 days at pH 4). These spots suggest the sporadic formation and tilting of lamellae with a slightly greater thickness (i.e., 17.6 nm respect to 14.0 nm) that cannot be well accommodated in the surrounding and compact amorphous phase (Figure 8b). Note that solubilization of degradation products in the acidic medium was scarce at low exposure times.
- (2)
- An equatorial reflection is enhanced during heating of samples degraded at a neutral pH where the development of longitudinal cracks is characteristic. This reflection can be associated to the interfibrillar spacing (Figure 8b) which is greater than the interlamellar one (e.g., ca. 10.4 nm respect to ca. 12.3 nm as can be measured in the patterns shown in Figure 15). The regularity of interfibrillar domains was lost at the highest temperatures and consequently the corresponding reflection clearly disappeared before lamellar stacking was affected by partial fusion. Figure 15 also shows as equatorial reflections could still be detected at pH 9.
- (3)
- Patterns of samples exposed to basic pHs were more complicated since solubilization of degradation products allowed a greater readjustment/reorientation of constitutive lamellae. For example, the meridional reflection was split in the samples exposed to pH 9, suggesting a slight tilting of lamellae according to opposite directions. Note that optical micrographs revealed the development of tortuous fiber morphologies during exposure to basic pHs (Figure 11). This lamellar tilting seems not appropriate to justify the regular six spot pattern detected at higher pHs, being a plausible explanation the enhancement during degradation of a macrolattice arrangement where lamellar domains were disposed at different levels along the fiber axis as displayed in Figure 8.
3.5. Change of Lamellar Parameters of Degraded GL-b-[GL-co-TMC-co-CL]-b-GL Samples during Subsequent Non-Isothermal Crystallization and Reheating Processes
4. Conclusions
Acknowledgments
Author Contributions
Conflicts of Interest
References
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Márquez, Y.; Martínez, J.C.; Turon, P.; Franco, L.; Puiggalí, J. Influence of pH on Morphology and Structure during Hydrolytic Degradation of the Segmented GL-b-[GL-co-TMC-co-CL]-b-GL Copolymer. Fibers 2015, 3, 348-372. https://doi.org/10.3390/fib3030348
Márquez Y, Martínez JC, Turon P, Franco L, Puiggalí J. Influence of pH on Morphology and Structure during Hydrolytic Degradation of the Segmented GL-b-[GL-co-TMC-co-CL]-b-GL Copolymer. Fibers. 2015; 3(3):348-372. https://doi.org/10.3390/fib3030348
Chicago/Turabian StyleMárquez, Yolanda, Juan Carlos Martínez, Pau Turon, Lourdes Franco, and Jordi Puiggalí. 2015. "Influence of pH on Morphology and Structure during Hydrolytic Degradation of the Segmented GL-b-[GL-co-TMC-co-CL]-b-GL Copolymer" Fibers 3, no. 3: 348-372. https://doi.org/10.3390/fib3030348