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Influence of Thermal Annealing on Mechanical Properties and In Vitro Degradation of Poly(p-Dioxanone).

INTRODUCTION

Unlike biodegradable polyesters such as polylactides, poly(glycolic acid) or poly(e-caprolactone), poly(p-dioxanone) (PDO) is a poly(ester-ether) synthesized by ring-opening polymerization of p-dioxanonc. The ether bond and methylene bonds on the PDO repeat unit provide the polymer with great flexibility, yet with relatively low mechanical strength, which has hindered its use in applications requiring high mechanical strength. Its semicrystalline characteristics create additional capability to fine tune final products' performance including mechanical strength, for example, by increasing crystallinity through a thermal annealing process, or heat treatment [1]. Obtaining optimum mechanical and biological properties for various applications require an overall understanding of the semicrystalline polymer morphology development, which largely relies on the crystallization process [2, 3].

Since the commercialization of PDO as a biodegradable monofilament in 1981, the polymer has received only limited interest until recently. It has been recognized that PDO is a biocompatible material that is used in general surgery for sutures as well as in cardiovascular procedures due to the in vivo degradability, lower inflammatory response, prolonged higher strength retention, softness, and flexibility [4, 5], PDO can be completely absorbed in about 6 months, with no significant foreign body reaction [6], Despite the unique advantages that PDO can offer, it does come with certain disadvantages as well. Shape memory characteristic of the PDO, for some applications, offers disadvantages. For example, when used as a suture, the PDO often coils to retain its spooled shape and, thus, can be difficult to use. PDO also has low surface friction allowing it to glide through tissues easily, but the lack of friction coupled with shape memory makes knot retention difficult [7], Other downsides of PDO sutures include the degradation rates that are too rapid to provide a durable closure for wounds with abnormal healing. While the disadvantages of PDO provide challenges as a suture, its biocompatibility, flexibility, degradation rate, and mechanical property retention, on the other hand, could provide advantages for tissue engineering, vascular conduits, and some other medical devices. Additionally, the low surface friction of PDO could prove advantageous for reduced bacterial infection. In fact, PDO was also the first polymer used for bioresorbable blood vessel ligating clips as alternatives to metallic clips. Other biomedical applications include bone or tissue fixation devices, fasteners, stents, and drug delivery systems, since the polymer can be easily thermally processed such as injection-molding [8-10].

As PDO is emerging as an attractive biodegradable polymer; however, there is still a lack of understanding in its structure-property relationships. For example, although both isothermal and nonisothermal crystallization kinetics of PDO have been studied [3, 11-14], there is no report on its effect on mechanical and degradation properties as of yet. Furthermore, to our best knowledge, there are few reports on molecular weight measurements, mainly because of limited instrumentation capable of accommodating hexafluoroisopropanol (HFIP), which is the best solvent for high-molecular-weight PDO [3]. Additionally, there is currently no report on PDO absolute molecular weight. In addition, despite the increasing application fields of PDO, information of its property correlation with structural manipulation remains insufficient [15]. This study aims to explore the influence of annealing PDO at various temperatures on mechanical properties, in vitro degradation as well as molecular weight change.

EXPERIMENTAL

Materials

The PDO with trademark RESOMER[R] X 206 S was provided by Evonik Nutrition & Care GmbH, Darmstadt, Germany. The inherent viscosity of this polymer is in a range of 1.5-2.2 dL/g, which is measured at 0.1% w/v in HFIP at 30[degrees]C with an Ubbelohde size Ob glass capillary viscometer.

Injection Molding

Tensile specimens of PDO following ISO 527 -IBB were made using an HAAKE MinJet Pro piston injection molding system. To prevent thermal degradation, the PDO raw material was dried in a Dri-Air nitrogen gas micro dryer (Model CNFM) at 40[degrees]C for 8 h. The melting and mold temperatures were 155[degrees]C and 25[degrees]C, respectively. After melting, the material was injected into the mold with an injection pressure of 65 MPa for 8 s and a hold pressure of 40 MPa for 4 s.

Thermal Characterization

Dynamic thermal characteristics of dried PDO pellets were characterized using differential scanning calorimetry (DSC822, Mettler Toledo, Greifensee, Switzerland) from -30[degrees]C to 130[degrees]C at a ramp rate of 10[degrees]C/min under nitrogen flow. The second DSC scan at the same condition after removing prior thermal history was used to characterize the thermal transition of PDO. The crystallinity of PDO was calculated according the following equation:

x = [DELTA][H.sub.m]/[DELTA][H.sup.0.sub.m] x 100% (1)

in which [DELTA][H.sub.m] is the enthalpy of melting of the PDO and [DELTA][H.sup.0.sub.m] is the enthalpy of melting for 100% crystalline PDO, which was taken as 141.18 J/g [16].

Annealing

To derive the annealing conditions for this study, PDO specimens were first subjected to a full DSC cycle including heating up to 130[degrees]C to eliminate prior heat history followed by quenching to 20[degrees]C at 20[degrees]C/min. The annealing consisted of heating the specimens in the DSC to the desired temperature at 20[degrees]C/min and held for 8 h followed by cooling to room temperature. The annealed specimens were then subjected to a DSC cycle up to 130[degrees]C and the thermograms were analyzed.

Based on the analysis of the DSC thermograms, 50[degrees]C and 80[degrees] C were chosen as annealing temperatures for injection molded tensile bar specimens. To prevent thermal degradation during annealing, all PDO specimens were annealed under vacuum. After putting the injection molding specimens inside a vacuum oven, the chamber was degassed for 30 min and then heated to the designated temperature for 8 h.

In Vitro Degradation

Phosphate-buffered saline (PBS) lx solution (Fisher Bio-Reagents, Pittsburgh, PA) with pH 7.4 [+ or -] 0.1 was used in this study to investigate the influence of annealing on in vitro degradation of PDO. This PBS solution contains 11.9 raM phosphates, 137 mM sodium chloride, and 2.7 raM potassium chloride. The nonannealed and annealed PDO tensile bars were placed in the PBS solution at 37[degrees]C in an incubator for 2 weeks, 1 month, and 2 months. Every week the specimens were washed and replaced with fresh PBS solution. Upon reaching designated degradation time, the tensile bars were allowed to cool down to room temperature prior to mechanical testing. The same specimens were used for DSC analysis. The surface morphology of selected specimens was examined using a scanning electronic microscope (TM3030, Hitachi, Schaumburg, IL).

Mechanical Testing

Yield strength, yield strain, ultimate tensile strength, elongation at break, and elastic modulus of each sample were determined using an Instron testing machine (Model 3366, Norwood, MA) at room temperature with a crosshead rate of 20 mm/min per ISO 527. Young's modulus was measured between 0.05% and 0.3% linear elongation. Six replicates were tested, and average values were reported.

Gel Permeation Chromatography

The absolute molar mass distributions of PDO specimens before and after in vitro degradation were measured using size exclusion chromatography equipment with LC-20AD pump (Shimadzu, Columbia, MD) with a multiangle light scattering detector (miniDAWN TREOS, Wyatt, Santa Barbara, CA) and a differential refractometer detector (Optilab T-rEX, Wyatt, Santa Barbara, CA). PDO samples were dissolved in HFIP containing 0.01 M tetraethylammonium nitrate to make 2 mg/mL solution. After dissolving overnight, the sample solutions were filtered through a 0.45pm polytetrafluoroethylene syringe filter. Each sample solution was analyzed on the SEC system with triplicate injections.

Wide Angle X-Ray Scattering

Wide-angle X-ray scattering (WAXS) measurements for as-molded and annealed PDO specimens at various conditions were performed using a Xeuss 2.0 laboratory beamline (Xenocs Inc, Sassenage, France) at University of Southern Mississippi, with an X-ray wavelength of 1.54 [Angstrom] and a sample-to-detector distance of -30 cm. Diffraction images were recorded on a Pilatus 1 M Detector (Dectris, Inc.) with an exposure time of 1 h, then analyzed using the Nika software package. Measurements were collected in sample in transmission geometry at room temperature.

RESULTS AND DISCUSSION

PDO Thermal Transition

The thermal behavior of PDO was investigated by melting and eliminating its thermal history, then subsequent heating from room temperature to 130[degrees]C at a 10[degrees]C/min heating rate. This revealed a [T.sub.g] at - 10[degrees]C, a prominent cold-crystallization peak at 48[degrees]C, a minor cold-crystallization peak appearing at 83[degrees]C, and a melting peak at 106[degrees]C, as shown in Fig. 1. The appearance of cold-crystallization peaks indicates PDO chain folding for crystallization under favorable circumstances. Well-controlled conditions providing the required energy at or adjacent to cold-crystallization temperatures facilitate the crystallization process, which typically defines a thermal annealing treatment in semicrystalline polymers to fine tune crystallinity [15].

Annealing

Isothermal crystallization was performed via DSC under nitrogen flow for 8 h to ensure heat flow was leveled off indicating a completion of crystallization. Upon annealing, the less stable regions were melted and redistributed between the more stable regions [17]. Local molecular reorganization processes allowed to partially remove chain folds in the less stable regions with increasing annealing time or temperature. Annealing of semicrystalline polymers effectively increases the percentage of crystallinity and can occur during heating or cooling of the polymer. For example, if a polymer is heated to a temperature above [T.sub.g], but below [T.sub.m], annealing will occur. Cooling polymers from [T.sub.m] to some temperature below [T.sub.m] but above [T.sub.g] can also incur annealing. The former is referred to cold crystallization, which can ultimately influence mechanical properties during practical applications, while the latter usually determines product properties postmanufacturing [18, 19]. This study chose to remove thermal histoiy by reaching the melt temperature of PDO, followed by quenching to room temperature, followed by increasing temperature to the desired annealing temperature.

Reheating the annealed PDO from room temperature to 130[degrees]C revealed a unique thermal transition behavior (Fig. 2) compared with the one shown in Fig. 1. The previous cold-crystallization peak around 47.5[degrees]C disappeared on all annealed specimens. Even annealing at 40[degrees]C for 8 h diminished the cold-crystallization peak, instead a small melting peak appeared on all samples about 10[degrees]C higher than the annealing temperature. This melting peak might represent the reorganized regions resulting from different crystallite sizes and/or lattice imperfection fueled by the annealing temperature [20], However, they are far from perfection to maintain integration and are dissociated upon heating.

Starting at 60[degrees]C anneal temperature, the small melting peak described above begins to overlap with the recrystallization peak at approximately 83[degrees]C (see Fig. 1). Andjelic et al [15] observed dissimilar crystallization behavior of PDO at different annealing temperatures, whereby a secondary crystallization happened during isothermal process only when the temperature was higher than 60[degrees]C. However, in our study, the secondary crystallization occurred at all annealing temperatures but is unclear for those at high annealing temperatures such as 80[degrees]C and 85[degrees]C, since the melting transition dominated this secondary transition as seen by the entirety of these transitions being below the baseline. The difference might be that the annealing process in the current work involves prior thermal history removal and cooling down at a controlled rate of 20[degrees]C/min and then reheated to the annealing temperature. It is worth noting here that annealing temperature below 50[degrees] C only induced a small exothermic peak, which is closely associated with the annealing temperature. A clear correlation can be observed between the temperature of this exothermic peak shifting and the associated annealing temperature.

Annealing in fact is a dynamic process that removes the less stable crystalline parts leaving the more stable ones intact. Polymer crystals usually grow under conditions far away from a thermodynamic equilibrium. Smaller or less-ordered regions melt at lower temperatures than larger or better ordered ones. The capability of crystallization control leads to reduced undesired effects during thermal processing such as excessive anisotropy leading to shrinkage, warpage, and dimensional instability [8], To prevent specimen warpage at high temperature in current study, all specimens were restricted inside vacuum oven during annealing.

For a better understanding of the influence of annealing on PDO crystalline behavior, WAXS measurements were performed. The WAXS patterns in the range of 2[THETA] 5[degrees]-45[degrees] for nonannealed and annealed PDO specimens under various conditions are shown in Fig. 3. All PDO specimens show the diffraction peaks at 2[THETA] of 22.3[degrees], 23.9[degrees], and 29.4[degrees]. The corresponding spacings between the planes in the crystal were calculated according to Bragg's equation [21] are 0.398 ([d.sub.210]), 0.372 ([d.sub.020]), and 0.305 ([d.sub.310]) nm, respectively, indicating no significant change in crystalline structure after annealing.

The diffraction peak intensity increases with annealing temperature increase. Scherrer's equation can be used to calculate crystallite size as follows [22]:

L = [k.sub.s][lambda]/[tau] cos [theta] (2)

where L is the mean size of the crystalline domains, [k.sub.s] is the shape factor in a range of 0.8-1.2 (typically is 0.9), [lambda] is the X-ray wavelength, [tau] is the full width at half maximum diffraction peak, and [THETA] is the Bragg angle.

At a given specific diffraction peak, the measurement of the crystallite size is a function of the width at half-maximum diffraction peak in a manner of:

L [varies] 1/[tau] (3)

with the increased peak intensity, the subsequent narrow full-width at half-maximum indicates an increased crystallize site. These WAXS results are in agreement with the DSC measurements for the increased crystallinity.

Considering the bigger crystallite size and the increased crystallinity confirmed from this WAXS study, the small melting peaks appeared on the DSC curves (Fig. 2) may indicate sparsely packed and less perfect crystalline structure formed during annealing. It speculates that these defective crystalline regions may attribute to a lamellar reorganization process that leads to lamellar thickening during annealing. However, the reorganized lamellar structure seems less thermally stable and melts a slightly lower temperature than the original and more perfect spherulites.

To further compare the effect of annealing on specimens' mechanical and degradation behavior, 50[degrees]C and 80[degrees]C were thereby selected for annealing injection-molded specimens for 8 h.

Mechanical Properties

From a medical implant design standpoint, once the yield point is reached, deformation of the material after that becomes permanent and irreversible to its original shape. In certain medical device applications, the yield point becomes the upper limit even though the material does not break. Under this circumstance, it is necessary to compare the mechanical properties of PDO at yield.

As-molded PDO without annealing showed 27.7 MPa yield strength, 10.4% yield strain, elastic modulus of 593.5 MPa, and elongation at break of 215.7% tested under dry condition (namely without contact with PBS solution), as shown in Table 1. The annealing process dramatically enhanced yield strength. For example, annealing at 50[degrees]C generated yield strength at 36.2 MPa corresponding to a 31% enhancement, while there was no change to yield strain compared with specimens without annealing. Annealing at higher temperature of 80[degrees]C further enhanced yield strength to 38.3 MPa, corresponding to a 38% improvement from unannealed specimens. An annealing process at appropriate temperature not only promotes crystallinity, but also can relax internal stress. The degrees of improvement might be due to the difference in crystalline morphology [23, 24], e.g., the size and density of the spherulites. The enhanced yield strength and modulus are expected from the synergy of these physical processes.

Comparing the overall mechanical properties of the specimens being annealed at the two investigated temperatures with those without annealing, a general trend was found that yield strength increased with annealing temperature, as did the elastic modulus while maintaining yield strain. Elongation at break, on the other hand, decreased at higher annealing temperature, although this is not a significant finding considering medical device implants are not expected to perform in this region. The more significant mechanical characteristic of yield strength is shown to be increased in Fig. 4. Of all the three investigated conditions, strain hardening after yield point leads to much higher ultimate failure strength than yield strength. Ultimate tensile strength has some utility in the design of medical implants, more from a safety standpoint in that during a failed procedure, the device needs to have adequate strength to be pulled out of the anatomy without failure. Depending on the design and nature of the medical device, it may be possible to stretch the sample during manufacturing (as in the case of sutures) for the purpose of increasing the tensile strength. Based on results shown in Fig. 3, it may be possible to further increase yield strength and ultimate tensile strength of this type of device further by annealing at 80[degrees]C.

In an attempt to compare the performance of PDO as tested in dry conditions with that in wet conditions, specimens were immersed in PBS solution overnight. The as-molded PDO specimens exposed to PBS overnight had a yield strength reduction of 11.5%, an elastic modulus reduction of 18.6%, and an elongation increase of 3.5%; compared with dry condition test results. The 50[degrees]C annealed specimens exposed to PBS overnight had a yield strength reduction of 20.4%, an elastic modulus reduction of 17.8%, and an elongation increase of 23.5%; compared with dry condition test results. The 80[degrees]C annealed specimens exposed to PBS overnight had a yield strength reduction of 9.9% and elastic modulus reduction of 13.6%, and a negligible elongation decrease of only 1.9%; compared with dry condition test results. This is an indication that annealing at 80[degrees]C minimizes the 24 h damage to mechanical properties when exposed to PBS solution. It is noteworthy that the ultimate tensile strength of the PDO material, regardless of annealing condition or whether wet or dry largely remained the same in a range of 41.3-45.3 MPa.

Water molecules diffused into PDO and acted as plasticizer to reduce the yield strength, soften the modulus, and extend the elongation at break. The dramatic reduction in mechanical strength or stiffness of the materials after being in contact with water could lead to catastrophic event in medical applications once implanted. The capability of annealed specimens retaining initial high yield strength and modulus in wet conditions opens up opportunities for using PDO as implantable medical devices when appropriate mechanical properties are required.

In Vitro Degradation

In vitro degradation of PDO being annealed at various conditions was investigated in a time span of 2 months. As an important measure of material performance, certain mechanical property retention indicates service life of fabricated products. The mechanical properties of three groups of PDO specimens were compared after being immersed in PBS solution for overnight, 2 weeks, 1 month, and 2 months [Fig. 5(a-c)].

At the two-week checkpoint, both yield strength and elastic modulus for all three groups of PDO increased and reached similar levels regardless of initial PDO annealing conditions. For example, yield strength reached to 35.2, 35.2, and 39.0 MPa for as-molded PDO, annealed PDO at 50[degrees]C, and annealed PDO at 80[degrees]C, respectively. Concurrently, their elastic moduli further enhanced to 647.3 as-molded, 607.3 for 50[degrees]C, and 664.5 MPa for 80[degrees]C. Their corresponding yield strains maintained similar level at 11.4 [+ or -] 0.3%, 11.4 [+ or -] 0.5%, and 11.0 [+ or -] 0.1 %, respectively. These yield strain results are at comparable levels with those of annealed PDO dry specimens listed in Table 1.

After 1 month immersion in PBS solution at 37[degrees]C, within this short period of time, an abrupt drop of yield strength was observed. Regardless of PDO initial conditions, three groups of the investigated specimens exhibited similar yield strength at 20.6, 20.7, and 20.2 MPa, respectively. The previous strain hardening phenomenon disappeared; thus, the yield strength became the highest tensile strength. Elastic modulus, however, was further improved to 781.4, 795.2, and 811.8 MPa, respectively. The initially flexible PDO displayed a brittle breakage upon testing evidenced by the dramatically reduced elongation at break only at 4.0%, 3.7%, and 3.4% for the PDO initially being as-molded, annealed at 50[degrees]C, and annealed at 80[degrees]C. Further extending immersion time to 2 months, although all specimens retained the tensile bar shape inside the PBS solution, they were extremely delicate and broken into several pieces before mounting to the tensile grips.

The process of immersion of PDO specimens in PBS solution at 37[degrees]C is a continuous annealing treatment in aqueous condition due to its low [T.sub.g] at -10[degrees]C. Water plasticizing the PDO specimen leads to an increase in free volume and a decrease in polymer chain interaction leading to higher chain mobility, as can be seen in the first 24-h time-point results (Table 1). This increased chain mobility favors further crystallization at 37[degrees]C, which takes place simultaneously with water plasticization in a dynamic fashion. The crystallization process tends to dominate plasticization leading to the changes seen in yield strength and elastic modulus (Fig. 5). It appears that 39.0 MPa is the highest yield strength the PDO can achieve.

Although hydrolytic degradation takes place simultaneously with annealing in PBS under body temperature, the initial 2 weeks of annealing caused by exposure to 37[degrees]C surpasses the effects of the degradation process, which leads to improvement in strength and modulus (Fig. 5). With an additional 2 weeks of exposure, elastic modulus increased to over 800 MPa, which is the highest value for the investigated specimens. Although PDO annealed under vacuum and dry condition alone is not able to reach the highest yield strength and elastic modulus, the results from this study imply that there is still room to optimize conditions in order to achieve the highest mechanical performance. Annealing in solvent appears to be one of the options. Low-molecular-weight organic solvent was found to induce crystallization in glassy polyethylene terephthalate) by plasticizing the material [25]. Annealing in solvent enables the polymer chains to better self organize in nanodomains that can dramatically alter materials or product performance [26], but does not suffer the consequences of hydrolytic degradation. Although this is not within the scope of this paper, it is an interesting topic to further investigate. In either case, the increase in crystallinity with time has a limit where most amorphous regions have been depleted [27].

PDO degrades in the presence of water or moisture by means of hydrolysis. Hydrolysis degradation in initial high-molecular-weight PDO takes place randomly along the chains similar to other polyester degradation behaviors [28]. Once the monomer p-dioxanone that is a hydroxyl acid exists as one of the degradation products, it acts as catalyst to accelerate the hydrolysis process in a dominant chain-end cleavage mechanism [29]. In the current study, PBS buffer solution was changed every week and solution pH value was also monitored to be around 7.4. This procedure removes excess degradation products being released into the PBS buffer solution, even though the degradation products within the specimens remain trapped. The monomer p-dioxanone inside the specimen still accelerates the hydrolysis degradation. Absolute molecular weight of PDO reduces substantially against in vitro degradation time as listed in Table 2. Two-week PBS solution immersion reduces the PDO molecular weight by about 38% in terms of Mw from initial 75,200 to 47,200. One-month immersion further reduces molecular weight dramatically to 26,100, about one-third of initial molecular weight. It is noteworthy that the polydispersity, which is a ratio between Mw and Mn, increases with degradation time from initial 1.83 to 1.94 at 2 weeks, and 2.06 at 1 month indicating increased chain length distribution. Further immersion of the specimens for a total of 2-month in vitro degradation, the PDO can only maintain molecular weight (Mw) of 10,700 corresponding to 14.2% of initial PDO molecular weight. The polydispersity, however, reduces to 1.68 indicating relatively uniform chain length. With the catalyst effect of monomer p-dioxanone, certain initial or degradation induced short chains can be completely degraded into oligomer or monomers.

Being immersed in PBS solution at body temperature for 1 month and 2 months, the thermal transition behavior of the degraded PDO is of interest, since no related report has been found so far. The same heating rate of 10[degrees]C/min was used for the DSC temperature sweep from -30[degrees]C to 130[degrees]C. All DSC traces showed sharp melting peaks during the first heating [Fig. 6(a)], Corresponding crystallinity of the 1-month PBS immersion specimens is 54.9% and 51.9% for PDOs without initial annealing and being annealed at 50[degrees]C for 8 h respectively, which remains highly crystalline with no significant difference. Although specimens after 2-month immersion in PBS are not able to be tested for mechanical strength, they were subjected to DSC analysis. Compared with 1-month PBS immersion, a more prominent cold-crystallization peak appeared at 80[degrees]C, which might be due to the larger lamellae requiring higher temperature and more energy for recrystallization [30]. Thermal behavior of these specimens further confirms an annealing process by immersion of PDO at body temperature. After removing the 1-month thermal history by heating to 130[degrees]C, subsequently all specimens displayed the same thermal transition [Fig. 6(b)] as the neat PDO did in Fig. 1.

Combining the mechanical and DSC results, loss of mechanical integration and retention of high crystallinity furthermore indicate that connecting tie chains within the PDO amorphous regions were lost or reduced by hydrolysis after 2-months of PBS immersion. Similar results were previously reported of in vitro degradation without changing the buffer solution or distilled water where the acidic degradation byproducts could accelerate degradation [27].

Surface morphology of the specimens before and after immersion in PBS solution was examined. The specimen with no contact of PBS solution showed an integrated surface pattern transferred from the injection mold. Sparse cracks started to appear on specimen surface after 2-week PBS immersion (Fig. 7). With the same magnification and field-of-view size, apparently there are more surface cracks on the nonannealed specimen after 2-week PBS immersion than those annealed specimens. Cracks continuously emerge and propagate as time evolves. After 2-month immersion, many cracks propagate and meet with other crack growth paths. Interestingly, there are fewer cracks appearing on the 80[degrees]C annealed specimens than for the 50[degrees]C annealed and nonannealed specimens. No doubt, these surface defects dramatically reduced specimen mechanical integrity.

CONCLUSION

Thermal annealing at elevated temperatures increased the crystallinity of PDO. Consequently, the thermal annealing at appropriate temperature and time is an effective process to improve the polymer's yield strength. Annealing at 50[degrees]C for 8 h improved the yield strength by 31%, and annealing at 80[degrees]C for 8 h enhanced yield strength by 38.3% compared with those without annealing. The PDO polymer became more rigid after thermal annealing assisted by the increased crystallinity. Mechanical and thermal properties changed by immersing PDO polymer in PBS solution at 37[degrees]C. These conditions initially reduce the yield strength, soften the elastic modulus, and extend the elongation at break by acting as plasticizer. However, after 2-week immersion, the PDO specimens exhibited higher elastic modulus and decreased elongation compared with 24 h. Due to the low glass transition temperature of the PDO, immersion at body temperature in PBS solution in fact can be understood a long-term annealing process. The yield strength and elastic modulus can further be enhanced by this effect even under wet conditions. All PDO specimens started to lose flexibility at 1-month immersion, and completely lose mechanical strength at 2-month immersion. Annealing improves PDO yield strength and elastic modulus and enables them retaining initial high yield strength and modulus in wet conditions. This process opens up opportunities for using PDO as implantable medical devices such as vascular closure devices, ligating clips, or any other implantable device involving short-term service life.

ACKNOWLEDGMENT

The authors are grateful to Ryan Lawson for assisting in injection molding, Dr. Rafael Gentsch for proving raw materials, Dr. Brigitte Skalsky, and Dr. Markus Glaenzel for valuable discussion and approval for publishing of this work. The authors also thank SGS Polymer Solution Inc (Christiansburg, VA) for the help in absolute molecular weight measurement, and University of Southern Mississippi for the help in WAXS measurement.

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Jian-Feng Zhang, (1) Scott Jones (iD), (1) Donghui Wang, (1) Andrew Wood, (1) Tommy Washington, (1) Kevin Acreman, (1) Brian Cuevas, (1) ([dagger]) Andreas Karau ([dagger])

(1) Medical Device Competence Center, Evonik Corporation, Birmingham, Alabama, 35211

(2) Business Line Health Science, Evonik Nutrition & Care GmbH, Darmstadt, 64293, Germany

Correspondence to: S. Jones; e-mail: scott.jones@evonik.com

([dagger]) B. Cuevas is currently at University of Southern Mississippi, Hattiesburg, Mississippi 39406.

DOI 10.1002/pen.25169

Published online in Wiley Online Library (wileyonlinelibrary.com).

Caption: FIG. 1. DSC trace of PDO after removing prior thermal history. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 2. DSC curves of reheating annealed PDO. The inserted texts indicate annealing temperatures where specimens being annealed for 8 h before reheating. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 3. WAXS patterns of PDO at various annealing conditions. Annealing temperatures from top to bottom are 85[degrees]C, 80[degrees]C, 70[degrees]C, 60[degrees]C, 50[degrees]C, 45[degrees]C, 40[degrees]C and without annealing. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 4. Typical strength-elongation plots of PDO being annealed at various conditions. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 5. Mechanical properties of PDO under in vitro degradation at 37[degrees]C. (a) Yield strength; (b) elastic modulus; (c) elongation at break. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 6. DSC of the PDO specimens being immersed in PBS solution at 37[degrees]C for 1 month and 2 months: (a) first heating and (b) subsequent second heating. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 7. Surface of PDO specimens before and after being immersed in PBS solution at 37[degrees]C for different times.
TABLE 1. Mechanical properties of PDO annealed at various
temperatures and tested at both dry and wet conditions
(immersion in PBS solution overnight).

Sample                  Yield strength     Yield strain (%)
conditions                   (MPa)

As molded        Dry   27.7 [+ or -] 0.6   10.4 [+ or -] 0.3
                 Wet   24.5 [+ or -] 0.4   11.4 [+ or -] 0.1
Annealed at      Dry   36.2 [+ or -] 3.4   10.4 [+ or -] 0.2
  50[degrees]C   Wet   28.8 [+ or -] 2.1   10.7 [+ or -] 0.5
Annealed at      Dry   38.3 [+ or -] 0.6   12.4 [+ or -] 0.4
  80[degrees]C   Wet   34.5 [+ or -] 0.3   13.3 [+ or -] 1.2

Sample                 Tensile strength      Elastic modulus
conditions                   (MPa)                (MPa)

As molded        Dry   41.3 [+ or -] 2.0   593.5 [+ or -] 46.6
                 Wet   43.0 [+ or -] 1.4   483.1 [+ or -] 39.8
Annealed at      Dry   42.4 [+ or -] 0.7   682.7 [+ or -] 31.2
  50[degrees]C   Wet   43.1 [+ or -] 2.3   561.1 [+ or -] 39.8
Annealed at      Dry   45.3 [+ or -] 2.2   698.7 [+ or -] 20.5
  80[degrees]C   Wet   42.7 [+ or -] 1.3   603.4 [+ or -] 31.7

Sample                     Elongation
conditions                at break (%)

As molded        Dry   215.7 [+ or -] 35.4
                 Wet   223.2 [+ or -] 15.1
Annealed at      Dry   172.7 [+ or -] 6.4
  50[degrees]C   Wet   213.3 [+ or -] 35.7
Annealed at      Dry   165.3 [+ or -] 9.8
  80[degrees]C   Wet   162.2 [+ or -] 13.3

TABLE 2. Absolute molecular weight of PDO as molded
and degraded for various times in PBS solution.

Sample conditions            Mn

As molded           41,500 [+ or -] 4,710
2 weeks in PBS      24,300 [+ or -] 1,080
1 month in PBS       12,700 [+ or -] 760
2 months in PBS      6,400 [+ or -] 668

Sample conditions            Mw             Polydispersity
                                               CMw/Mn)

As molded           75,200 [+ or -] 1,390        1.83
2 weeks in PBS       47,200 [+ or -] 660         1.94
1 month in PBS       26,100 [+ or -] 260         2.06
2 months in PBS      10,700 [+ or -] 280         1.68
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Author:Zhang, Jian-Feng; Jones, Scott; Wang, Donghui; Wood, Andrew; Washington, Tommy; Acreman, Kevin; Cuev
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:1USA
Date:Aug 1, 2019
Words:5966
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