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Article

The Influence of the Molecular Weight of Poly(Ethylene Oxide) on the Hydrolytic Degradation and Physical Properties of Polycaprolactone Binary Blends

by
Maurice Dalton
1,
Farnoosh Ebrahimi
1,
Han Xu
1,
Ke Gong
1,
Gustavo Fehrenbach
1,
Evert Fuenmayor
1,
Emma J. Murphy
1,2 and
Ian Major
1,*
1
PRISM Research Institute, Athlone Campus, Technological University of Shannon: Midland and Midwest, N37 HD68 Athlone, Ireland
2
LIFE—Health and Bioscience Research Institute Midwest Campus, Technological University of the Shannon, V94 EC5T Limerick, Ireland
*
Author to whom correspondence should be addressed.
Macromol 2023, 3(3), 431-450; https://doi.org/10.3390/macromol3030026
Submission received: 23 March 2023 / Revised: 27 April 2023 / Accepted: 2 June 2023 / Published: 3 July 2023
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Abstract

:
The use of biodegradable polymers in tissue engineering has been widely researched due to their ability to degrade and release their components in a controlled manner, allowing for the potential regeneration of tissues. Melt blending is a common method for controlling the degradation rate of these polymers, which involves combining these materials in a molten state to create a homogenous mixture with tailored properties. In this study, polycaprolactone (PCL) was melt blended with hydrophilic poly (ethylene oxide) (PEO) of different molecular weights to assess its effect on PCL material performance. Hydrolytic degradation, thermal and viscoelastic properties, and surface hydrophilicity were performed to contrast the properties of the blends. DSC, DMA, and FTIR were performed on selected degraded PCL/PEO specimens following mass loss studies. The results showed that adding PEO to PCL reduced its melt viscosity-torque and melt temperature while increasing its hydrophilicity, optimizing PCL/PEO blend for soft tissue engineering applications and could contribute to the development of more effective and biocompatible materials for soft tissue regeneration.

1. Introduction

Synthetic biodegradable polyesters are a widely used type of polymer in tissue engineering due to their biodegradability and cost-effectiveness. Among them, aliphatic polyesters have been extensively investigated for their mechanical properties and ability to control biodegradation rates through chemical modifications [1,2,3,4,5,6,7,8]. However, creating a scaffold that mimics the extracellular matrix of the target tissue while maintaining appropriate mechanical properties, porosity, and biocompatibility remains challenging [9,10]. This calls for the exploration of new materials and manufacturing techniques. In this context, aliphatic polyesters such as poly (butylene succinate), poly (lactic acid), their copolymer poly(lactic-co-glycolic acid), and polycaprolactone (PCL) are promising alternatives for tissue engineering applications [2,6,7,11,12].
PCL is semi-crystalline at room and human body temperatures, with a relatively low melting point of about 60 °C and glass transition of −60 °C [13]. This polymer has a desirable potential for blending with other polymers, leading to differentiation in biodegradation behavior and mechanical strength [14]. Due to its long degradation time, PCL has been widely used as a replacement for hard tissues where healing requires a long extended period which favors its hydrophobic nature. PCL degradation can be tailored by copolymerization, and creating composite and blends [13]. The frequent use of PCL across a range of medical applications including drug delivery [15,16,17,18], orthopedic implants [19,20,21], and nerve regeneration [22,23]. In addition, PCL has been widely used as a scaffold material for tissue engineering due to its biocompatibility, biodegradability, and mechanical properties. PCL scaffolds can be fabricated using a variety of techniques such as additive manufacturing [12,18,19,20,24,25,26], electrospinning [22,27,28,29,30], solvent casting [31,32], and melt extrusion [33,34,35] to create a range of structures with controlled pore size and mechanical strength. However, the stiffness of PCL is unfavorable for soft tissue scaffold applications such as cardiac, nerve, and muscle repair [36].
Melt blending is the most preferred method for mixing thermoplastics and has the highest relevance for industrial applications [37], including the production of medical devices [38,39,40,41], drug delivery [19,20,21,31,39,40,41], scaffold [42,43,44], and tissue engineering [43,44,45,46], including the creation of aliphatic polyester blends for tissue engineering [29,47], drug delivery [15,16,17], and scaffolds [41]. In specific to melt blending production methods, hot melt extrusion (HME) has widespread acceptance not only in the production of many pharmaceutical products, but also in the production of implants and medical devices [48]. HME enables the selection of desirable properties from various polymers, which can then be combined via melt blending, creating a far superior material to the conventional polymers currently available. Melt blending polymers without the use of a solvent via HME is an approach that results in improved polymer properties suitable for biomedical applications [49]. Furthermore, blending can help tailor the degradation rate and drug release properties of the new material enabling the creation of new polymeric blends with unique and valuable properties broadening the range of commercially available polymers and their potential applications.
Poly(ethylene oxide) (PEO) is a non-ionic, hydrophilic, water-soluble semi-crystalline polymer widely used for drug delivery [15,16,42,50,51], scaffold fabrication [52,53,54,55], and tissue engineering [27,54,56,57,58]. However, in order to meet the requirements of each application, PEO has to have appropriate thermodynamic parameters and crystallization kinetics, as well as appropriate molecular weight (Mw) [59]. PEO with a low relative molecular weight can biodegrade within the body and its degradation products are later excreted. However, if the PEO’s relative molecular weight exceeds a certain threshold, polymer degradation in the body is impossible [57]. Therefore, since PCL has a low melting temperature and good blending capabilities, melt blending with a polymer such as PEO would be a simple way to improve its properties and extend its use into a broader range of tissue engineering applications [36,60]. PCL is a biodegradable polyester with good mechanical properties, while PEO is a hydrophilic polymer that is water-soluble and biocompatible. PCL and PEO polymer compositions can exhibit improved mechanical strength, flexibility, and surface properties, as well as enhanced biocompatibility due to the presence of PEO [47].
The aim of this study is to develop novel PCL/PEO blends with tunable hydrolytic degradation kinetics for potential soft tissue engineering applications. While PCL is widely used in biomedical applications, its hydrophobic nature and slow degradation rate limit its use in soft tissue engineering. In this study, we employed hot melt extrusion (HME) to create PCL/PEO blends with varying molecular weight PEO and investigated the effect of blending on the physicochemical properties of the resulting polymers. The novelty of our study lies in the use of PEO as a hydrophilic component to enhance the hydrolytic degradation rate of PCL, and the creation of PCL/PEO filaments suitable for fused filament fabrication (FFF) to fabricate scaffolds for tissue engineering. We characterized the blends using Fourier-transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and rheology and evaluated the potential of each blend for scaffold fabrication based on established criteria. Our findings have important implications for the optimization of PCL/PEO blends for soft tissue engineering applications, which may ultimately lead to the development of more effective and biocompatible materials for tissue regeneration.

2. Materials and Methods

2.1. Materials

Poly(ε-caprolactone) and CAPA 6500 (Mw 50,000) were received from Perstrop (Cheshire, UK). Poly (ethylene oxide) (average Mw 60,000 and 300,000) was obtained from Sigma-Aldrich, Ireland, Cheshire, UK. Both polymers were received in powder form and were used according to the manufacturing instructions. Sample compositions are displayed in Table 1. Equal amounts of PCL and PEO (powder form) were measured in various quantities according to Table 1, transferred into an LDPE bag, and mixed by inverting several times until both polymers were equally distributed.

2.2. Hot Melt Extrusion Conditions

All melt compounding detailed herein was carried out using an MP 19 TC 25 laboratory-scale co-rotating twin-screw extruders (APV Baker, Newcastle-under-Lyme, UK) with 16 mm diameter screws and a length-to-diameter ratio of 25/1. APV co-rotating extruder screws are designed and manufactured in modular construction using the geometry of the kneader and conveyer zones. The compounding temperature profile was established on the APV extruder by means of six temperature controllers placed along the length of the barrel. A seventh temperature controller was used to regulate the temperature at the die. The rpm of the screws was maintained at such a rate to ensure that the materials were starve-fed into the feed zone of the extruder. This ensured that in all cases throughput was independent of the screw rpm. The resultant melt was extruded through a die to form a strand. Extrudate filaments were collected. Torque data was also monitored and recorded because it would be useful when processing blends with high viscosity to prevent the extruder from exceeding allowable torque.

2.3. Attenuated Total Reflectance Fourier-transform Infrared Spectroscopy

Attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) was carried on a Perkin Elmer Spectrum fitted with a universal ATR sampling accessory. All data were recorded at ambient temperature, in the spectral range of 4000–650 cm−1, utilizing a 16 scan per sample cycle and a fixed universal compression force of 80 N. Subsequent analysis was carried out using Perkin Elmer Spectrum 10™ instrument Software.

2.4. Differential Scanning Calorimetry

Differential scanning calorimetry (DSC) analysis was carried out using the TA Instruments DSC 2920 DSC (TA Instruments). Samples were prepared by weighing out dry samples ranging from 8–12 mg using a Sartorius scale having a resolution of 0.01 mg. Empty aluminum hermetic pans were used as reference samples. Before thermal analysis was carried out, the cell was cleaned using a glass fiber brush and burned off at 400 °C for 5 min. The instrument was then calibrated using indium as standard. Samples in sealed hermetic pans were then carefully placed into the cell and scanned at a rate of 10 °C/min from 20 to 200 °C. All DSC analysis was carried out under a nitrogen atmosphere to prevent oxidation of the samples.

2.5. Melt Flow Index

Melt flow index samples were measured using Melt Flow Index at 130 °C (load 2.16 kg) using the Melt Flow Quicker Index.

2.6. Macrostructure Observation

The macrostructures of all fabricated filaments were examined using a Nikon ShuttlePix P-MFSC Firmware Digital Microscope with a range of 20× magnitude Please note that the timeline was represented in weeks as (T) for the data representation.

2.7. Scanning Electron Microscopy (SEM)

Scanning electron microscopy (SEM) was performed on a MIRA SEM (Tescan Oxford Instruments, Cambridge UK) to identify miscibility and effect changes induced by accelerated degradation. A freeze fracture method was implemented in order to observe these changes in the material foam structure. The experiment included the analysis of the HDPE foam samples and virgin HDPE. Prior to testing, the samples were placed in liquid nitrogen for 10 min before being fractured by a Charpy impact machine. After returning to room temperature the samples were prepared for SEM by ensuring that the orientation of the fractured side was correctly placed onto an aluminum pin mount adapter using double-sided carbon tape. All sample specimen’s gold coated using a Baltec SCD 005 vapor (Capovani Brothers Inc., New York, NY, USA) deposition coating to increase the electrical conductivity. Subsequently, Mira FE-SEM was used in high vacuum mode with an acceleration voltage of 15 kV and a magnification of 2.00 k×, to visualize particle dispersion, utilizing a backscattered electron detector.

2.8. Dynamic Mechanical Analysis

Dynamic mechanical analysis (DMA) was performed on filaments of all formulations using TA Instruments DMA Q800 (Dublin, Ireland). The test was performed using tensile test mode at a frequency of 3 Hz and an amplitude of 15 µm. The temperature range between −100 °C to 60 °C with a 3 °C/min rate was used to determine the storage modulus, the loss modulus, and the glass transition temperature (tan δ) for all polymeric formulations.

2.9. Rheological Analysis

Dynamic rheological tests were conducted on PCL/PEO extrudates using an Advanced Rheometer Trios v 3.3.1 (TA Instruments) fitted with an environment test chamber. The instrument was calibrated for inertia and mapped before use. The geometry used in this analysis was a 4 cm diameter cone steel plate. ±2.0 g of filament was weighed out and placed onto the ETC base. The sample gap was set at 0.5 mm for the studies carried out. Amplitude sweeps were conducted first in order to obtain an appropriate strain value that lies within the linear viscoelastic region. To determine the viscoelastic region of the blended material, frequency sweeps tests were performed. In this investigation, storage modulus (G′) and loss modulus (G″) were examined with a constant temperature of 100 °C. All tests were conducted at an angular frequency of 1 Hz and 1% controlled strain.

2.10. Accelerated Degradation Studies

Degradation studies were measured as percentage weight over extended periods of time. Filament strands were cut 1 cm and measured at ± ~0.55 g and placed into tubes containing 25 mL of sodium hydroxide (5 M NaOH). Each filament was submerged into individual tubes. The tubes were transferred into a shaking incubator, maintaining a temperature of 37 °C at a shaking rate of 9.0 rpm. The filament was removed at weekly time points of (0–8 weeks) blot dried and rinsed using deionized water and dried in an oven at 37 °C. The samples were then weighed using a Sartorius scale.

2.11. Contact Angle

The surface properties of blends and base polymers were assessed using contact angle analysis. The contact angles of water on the thin film polymers were measured using a ramé-hart goniometer (Succasunna, NJ, USA) at room temperature (20 °C). The drops were made using a Gilson Pipetman (Middleton, WI, USA) set to 0.1 mL.

3. Results and Discussion

3.1. Filament Manufacturing and Melt Flow

Table 2 presents the batches that were extruded using HME twin-screw extrusion, with constant screw speed, feed rate, and haul-off speed, to ensure uniform processing conditions and mechanical stresses during manufacturing. Torque and MFI values were measured during and post processing, respectively, to investigate the effect of PEO concentration and molecular weight (Mw) on PCL melt viscosity. Torque is used to determine the relative viscosity of polymer melts and is a proxy for shear stresses during the extrusion of the material. The torque data revealed that due to its highly viscous nature, PEO had the highest torque values, and higher viscosity melts exert more pressure than lower viscosity melts. The viscosity behavior of PEO can be controlled by screw speed, with increasing screw speed resulting in decreased viscosity due to shear force and a shorter transit time through the extruder [61,62].
Furthermore, torque analysis revealed that PCL was having a plasticizing effect on the PEO blends. Melt fracture occurs when polymer chains are forced into the die and recoil into random reconfiguration upon exit, resulting in an increase in torque, which means that the likelihood of this happening increases proportionally with the length of the polymer chain. Evidence of this effect was observed when torque readings were taken for batches containing higher molecular weight PEO, as more mechanical effort is necessary to extrude these. Extruder torque reached its peak when processing 100 PEO 300, reflecting the mechanical effort required for the melt extrusion of this material, which can incur polymer degradation due to augmented shear stresses.
Increasing the concentration of PCL reduced torque and made the blends easier to melt process, whereas increasing the concentration of PEO resulted in extrudate surging due to material adhesion to the screws and insufficient filling of the screw’s metering section. The melt flow index (MFI) is a useful quantitative method to evaluate the flow properties of a polymer melt at a particular temperature. This study investigated the influence of PEO 60 and PEO 300 on the flow properties of PCL, with the goal of using these blends for fused filament fabrication (FFF) applications. To achieve proper FFF printability, the melt viscosity should be at least 10 g/10min [19]. The results showed that increasing the PEO content in PCL/PEO blends resulted in lower MFI values, and higher PEO molecular weight (Mw) led to even lower MFI values. Therefore, increasing the PEO content in PCL/PEO blends would result in poor printability, which is a critical factor for FFF. Table 2 displays the data for all batches extruded using HME twin-screw extrusion, along with their respective torque and MFI values. These results highlight the effects of varying PEO concentration and Mw on PCL melt viscosity.
At the MFI testing conditions, only two batches had MFI values above the FFF printability threshold of 10 g/10 min. It is possible to increase the hot nozzle temperature during printing to compensate for the low MFI values. However, this could result in inconsistent melt flow since PCL/PEO blends are binary, and as the temperature increases, the melt flow characteristics of each of the phases would further deviate from one another. Therefore, it is best to limit the potential blends for FFF studies to those with MFI values above 4 g/10 min, as shown in Table 2. The remaining batches would require excessive increases in nozzle temperature, resulting in inconsistent melt volume for FFF fabrication. Overall, these findings suggest that careful consideration of PEO content and Mw is necessary when developing PCL/PEO blends for FFF applications.

3.2. Fourier-transform Infrared (FTIR) Spectroscopy

Attenuated total reflectance FTIR spectroscopy is used to generate a molecular fingerprint of both polymers and monomers. It was used in this study to investigate the vibrational differentiation within the chemical bonds of blended ratios compared with pure PCL and PEO, thus indicating any potential interactions. Figure 1 and Figure 2 show FTIR spectra overlay of all blends post extrusion. The spectrum for virgin PCL displayed prominent characteristic bands for aliphatic methylene stretching at 2943.70 cm−1 and 2866.98 cm−1. Additional bands include carbonyl stretching at 1722.81 cm−1, and C-O and C-C backbone stretching attributed to the amorphous phase at 1163.35 cm−1. Peaks at 1293.69 cm−1 and 1240 cm−1–1106.23 cm−1 are assigned to crystalline phases and C-C-O stretching. These findings have been previously reported [63]. Figure 1 also displays the spectrum of virgin 100 PEO 60 displaying an asymmetric C-H at 2881.19 cm−1. The peak at 1144.98 cm−1 corresponds to the stretching vibration of PEO C-O-C, attributing to the amorphous content of PEO. Swinging vibrations of C-H and CH2 were also detected within the amorphous region of PEO at 1300–1466.50 cm−1. Additional peaks at 954.63 and 841.41 cm−1 can be assigned CH2 rocking vibrations of the methylene group attributing to PEO’s helical molecular morphology [39].
Evaluation of the miscibility of PCL/PEO displayed a peak at approximately 1100 cm−1, identified as the C–O–C group, which is only present in PEO. Essentially, as the concentration of PEO increases the intensity of the C-O-C band increases in conjunction. In addition, the C=O group peak at 1730 cm−1 also only existed in PCL, and its intensity decreased with an increased ratio of PEO. This IR spectrum illustrates that PCL and PEO were successfully compounded and that no chemical reactions occurred during melt blending. However, it must be presumed the PCL/PEO blends are immiscible due to the absence of new chemical bonds.

3.3. Differential Scanning Calorimetry (DSC)

Both virgin PCL and PEO samples exhibited a well-defined endothermic peak (Figure 3a,b). In relation to the blended material, by increasing the concentration of PCL, the Tm of both PEO 60 and 300 K blends decreased. The higher the percentage of PEO the higher the fusion and crystallization enthalpies of the blends. PCL displayed a melting peak around ±58 °C, whilst PEO 60 displayed a melting peak at ±65 °C and ±69 °C for PEO 300. With the inclusion of PEO, the intensity of the peak increases and broadens. Due to the presence of a shoulder on the 75 PCL 60, 50 PCL 60, 75 PCL 300, and PCL 300, it could be argued that the blends are immiscible. Due to the similar thermal properties of PCL and PEO, it would be difficult to decipher if the blends were miscible and there is mounting evidence from FTIR and SEM to assume they are only a physical blend. To this effect, research conducted by Qiu et al. [40] used contrast microscopy and DSC in conjunction with the Avrami equation. They determined a biphasic separation within PCL and PEO blends, indicating immiscibility. In addition, Jiang et al. confirmed the immiscibility of PCL and PEO blends via crystallization peaks [41].

3.4. Dynamic Mechanical Analysis (DMA)

DMA is a widely used thermal analysis technique that allows for the detection of phase transitions and the viscoelastic responses of polymer materials under sinusoidal loads. DSC and DMA were utilized to investigate the miscibility of polymer blends and their responses to dynamic mechanical forces. However, the glass transition temperature (Tg) of PCL and PEO blends could not be determined using the DSC, due to the similarity of PCL and PEO’s Tg values. DMA analysis was utilized to further elucidate if the PCL and PEO blends were miscible and to investigate the effect PEO had on the properties of PCL. DMA is a technique that can provide information on the polymer–polymer interaction and interphase mixing, whilst DSC only provides information on the change in heat capacity when a polymer transition changes from glassy to rubbery [45]. Analyzing the glass transition is usually the most conventional method used to ascertain the miscibility of polymeric blends. A single Tg is usually an indicator of whether the blends have been successfully blended. However, as Figure 4 illustrates, a single transition was observed ±45–50 °C. However, due to PCL and PEO’s similar thermal characteristics, it would be difficult to decipher if the blends were blended successfully. There has been an abundance of research investigating the miscibility of PCL and PEO blends. Kuo et al. [46] investigated the miscibility of PCL and PEO via DSC and FTIR. They reported the miscibility of the two polymers in the amorphous region. As a result, it must be presumed that the PCL and PEO blends are heterogeneous.
The mechanical transition of the blends depicted in Figure 4 was analyzed in terms of storage modulus (green line), and loss modulus (blue line) and their ratio was presented as tan δ (maroon). The DMA thermograms illustrate a clear increase in both storage modulus and tan δ as the concentration of PEO increases, representing an increase in plastic behavior. Similar observations were also detected for stiffness (N/m), essentially increased concentration of PEO increased both stiffness and tan δ. It is also important to note that the testing temperature was limited to 60 °C since DMTA testing is only compatible with samples in a solid state. The proximity of the melting transition at 60 °C is observed as a sharp decrease in storage modulus. Results were similar to those by Douglas et al. who utilized PCL-PEG instead of PEO [64].
The viscoelastic properties of hydrolytically degraded samples were also studied using DMA, which showed a significant reduction of the loss and storage modulus in conjunction with time and respect to temperature. This reduction of loss modulus of the specimen decreased with increasing time in vitro via accelerated NaOH hydrolysis for all samples. No significant difference was observed for 100 PEO 60 and 100 PEO 300. Thus, the damping ability, and energy absorption/dissipation capacity of PEO, improved with the addition of PCL. Attributing to PCL’s higher molecular mobility and low melting point dispersed within PEO’s matrix.

3.5. Rheometry

Oscillatory strain sweeps were conducted on all blend samples. The composition-dependent complex viscosity is depicted as a non-linear curve in Figure 5. The influence of varying PCL and PEO ratios on the blend’s complex viscosity is illustrated in Figure 5, where the x-axis represents the angular frequency (ω rad/s) and the y-axis denotes complex viscosity (η* Pa·s). The non-linearity observed is a common rheological characteristic of immiscible polymer blends due to distinct phase structures under weak shear conditions.
At 120 °C, PCL/PEO blends exhibited the highest complex viscosities for 100 PEO 300, followed by 25 PCL 300 and 50 PCL 300. Previous analyses suggested immiscibility in PCL/PEO blends; however, small amounts of PCL chains appeared to interact with the PEO domain, resulting in increased thermal mobility of PEO chains. Figure 5 demonstrates that the complex viscosity increases as the concentration of PEO rises. It is worth noting that PCL’s viscosity remains nearly constant across all shear rate ranges due to its Newtonian plateau. In contrast, PEO exhibits shear-thinning behavior, making it more viscous than PCL at frequencies below 3 rad/s, while reducing viscosity in proportion to increased shear rate.
An increase in PEO molecular weight also raised blend viscosity, which corresponds well with melt flow index (MFI) measurements. Notably, the 75 PCL 60 blend exhibited significantly different rheological behavior compared to other blends. Although no definitive conclusion was reached for this phenomenon, we propose two possible explanations: (1) the disparity in viscosities of the two polymers may contribute to the observed behavior, where the lower viscosity PCL tends to encapsulate the more viscous PEO, resulting in rheological behaviors closely resembling pure PCL rather than a combination of PCL and PEO; and (2) alternatively, the occurrence of dead zones—stagnant regions in which molten material accumulates and deteriorates during the extrusion process—may adversely impact the viscoelastic region of the polymeric blend.

3.6. Degradation

Accelerated degradation tests were conducted on each specimen to investigate the degradation kinetics of both virgin and blended samples over a 12-week duration. Accelerated degradation was achieved using alkaline media, which facilitated hydrolytic degradation of the mechanical properties of the blends. Sodium hydroxide (NaOH) was selected for its molecular similarity to water and its abundance of hydroxyl groups (OH−). The principle behind accelerated degradation is to simulate physiological conditions while increasing the concentration of OH− ions to hasten the hydrolysis reaction. Lam et al. studied accelerated degradation processes on PCL and PCL-TCP scaffolds to complete the degradation process in a shorter time frame [61]. This enables researchers to investigate morphological and chemical changes during the degradation process more efficiently.
Mass loss results are displayed in Figure 6 as mean weight change percentage values for the filament (n = 3) for all blends. The gross morphology and architecture of 100% PCL progressively diminished over time, resembling a bulk degradation kinetic after two weeks [61]. Unfortunately, blends with 100 PEO 300 did not achieve significant mass loss (± 20%), attributed to the sodium salts exhibiting higher affinity for water molecules than the ethylene oxide monomers. However, a significant degradation interaction between 100 PCL and 100 PEO 60 occurred within the first three weeks. In summary, Figure 6 shows the highest weight loss for 100% PCL, followed by 75% PCL and 50% PCL, due to PCL’s susceptibility to OH- ions in solution. The blended material experienced a 60% mass loss for 75 PCL 60, 45% for 50 PCL 60, and 30% for 25 PCL 60. Each blend also saw a 10% reduction after week 2, but mass loss gradually decreased to only 5% at week 4 and remained consistent through weeks 7–8.
Although hydrophobic, PCL degraded faster than the water-soluble PEO in each blend. This result can be explained by PEO’s inhibition of the hydrolysis of OH− ions responsible for breaking down polymer chains in the solution. This effect is more pronounced at higher salt concentrations, where PEO chain solubility in solution decreases. Blends with a higher concentration of PCL exhibited surface erosion. It should be noted that using NaOH to assess the degradation kinetics of PCL/PEO blends reflects the need to pretreat PCL scaffolds in an alkaline environment to render them more hydrophilic for in vivo studies. It can be hypothesized that if PCL-PEO blends were studied in a neutral medium, such as phosphate-buffered saline (PBS), the effects would be inversed in vitro due to PEO’s hydrophilicity, causing it to solubilize in a water-based medium significantly faster than PCL.

3.7. Filament Morphology

We utilized conventional light microscopy to assess the morphological changes and reduction of the width of the polymer filament resulting from exposure to NaOH media during mass loss studies. Our observations indicated that the diameter of virgin PCL filaments consistently decreased over time, whereas no diameter reduction was observed for 100 PEO 60 and 100 PEO 300 due to their insolubility in NaOH media. As depicted in Figure 7A, the 50 PCL 60 filament at T0 displayed a smooth and uniform surface. However, at T3, a loss of structural integrity and roughness was observed. By T5 and T6, extensive breakage and roughness were observed, indicating a high degree of degradation. During the lateral stage of the accelerated degradation analysis, filament blends with a high concentration of 100 PEO 300 exhibited a high degree of stiffness, with similar observations noted for 100 PEO 60, albeit to a lesser extent. The exposure of polymer filaments to NaOH media results in morphological changes and reduction of width, with PCL filaments demonstrating greater susceptibility to degradation than PEO blends. These observations may have important implications for the development of biodegradable materials in tissue engineering applications.
Morphological analysis of the filaments was also conducted using SEM analysis. Figure 8A provides a clear visualization of the 50 PCL 60 filament at T0, displaying a smooth and polished surface morphology with the exception of sea-island structures resulting from the co-continuous phase of immiscibility between PCL and PEO. At T1, surface roughness was observed on the filament. T3 exhibited pit-like structures and coarse corrugations that appeared randomly distributed along the filament surface. By T6, extensive damage was observed due to prolonged exposure to NaOH, resulting in increased surface area roughness, pitting, and breaking that was clearly visible throughout the filament’s surface. This was due to the PCL regions degrading in the media, leaving behind insoluble PEO domains. The SEM analysis provides detailed information about the morphological changes occurring during the degradation process, which can be essential for understanding the underlying mechanisms of degradation. These findings can inform the design of more effective and biocompatible materials for tissue engineering applications.
During the six-week degradation period, larger pits and crevices appeared randomly over the filament surfaces. At T2 and T3, pits and crevices increased in size and number. By T5 and T6, some of the PCL/PEO filaments exhibited tunneling or were completely broken into half by degradation. The diameter of higher concentration PCL filaments consistently shrunk over time, in tandem with increased roughness of the corrugated surface (Figure 9). At the T6 time point, the internal architecture showed extensive pores and pit-like structures throughout its internal surface. Similar observations were made by Hou, who reported that with the addition of more PEO, elongated or fibrous structures were formed, creating channels and voids in the blend [62]. As the samples were submerged in the media for an increasing amount of time, the morphology underwent changes that reflected the blend of PCL/PEO. Specifically, the dissolution of the PCL regions resulted in the formation of a PEO-only scaffold, indicating a homogenous integration of the PCL and PEO phases in bulk form. Notably, no significant crevices, voids, or agglomerates were observed, suggesting that the blending of the PCL and PEO components was successful across all ratios and molecular weights. These observations were made through SEM analysis, providing valuable insight into the morphological changes occurring during the degradation process of PCL/PEO blends.

3.8. Contact Angle

Evaluation of PCL/PEO hydrophilicity matrices provides information if PEO interacts with PCL by increasing hydrophilicity. Thus, by increasing the hydrophilicity of PCL, biological interactions between cells and the polymeric matrix are enhanced. Table 3 illustrates the surface wettability of the scaffolds, as determined by contact angulation; 100 PCL 50 exhibits the highest hydrophobic properties followed by 75 PCL 300, 50 PCL 300, and 75 PCL 60. Essentially, increasing the concentration of PEO increased the hydrophilicity of the filament in comparison with PCL filaments. In summary, PEO decreased the contact angulation of the scaffolds ranging from 54°–50°. The observed differences in surface hydrophilicity between PEO 60 and PEO 300 blends could be attributed to the difference in molecular weight and the associated changes in polymer chain mobility. PEO 60 is a lower molecular weight polymer, which is expected to have higher chain mobility compared to PEO 300, a higher molecular weight polymer. The increased chain mobility of PEO 60 may have allowed for better interaction with water molecules, leading to higher surface hydrophilicity in the 50 PCL 60 sample. On the other hand, the higher molecular weight of PEO 300 may have reduced chain mobility, leading to less interaction with water molecules and resulting in lower surface hydrophilicity in blends containing PEO 300. It is important to note that the hydrophilicity observed in each blend is also influenced by the concentration of the polymer components and their respective degradation rates. Therefore, it is possible that the concentration of PEO 60 in the 50 PCL 60 blend contributed to its increased hydrophilicity, while the concentration of PEO 300 in the pure 100 PEO 300 sample resulted in its higher surface hydrophilicity.
Additionally, contact angle measurements were used to assess the impact of degradation on polymer specimens. The results indicated that the angulation decreased from 62.45 to 44.73 degrees for the 50 PCL 60 blend, with similar observations made for all batches. However, after four weeks, the filaments were no longer able to support droplets on their surfaces due to the increased porosity of the blends caused by the hydrolyzing effect of NaOH. Consequently, contact angle measurements could not be conducted post T3. This suggests that NaOH penetrated through the polymer matrix, affecting the polarity of the polymer by enhancing hydrophilicity through OH- ions. Notably, the introduction of PEO considerably increased the hydrophilicity of the blends in comparison to the homopolymer. However, the degradation of varying Mw sequences did not significantly enhance the hydrophilicity of PCL due to phase separation between the two polymers in aqueous media.

4. Conclusions

In conclusion, the results of this study demonstrate the successful creation of PCL/PEO blends suitable for subsequent 3D printing of scaffolds for soft tissue engineering applications. By incorporating different molecular weight PEO into the blends, we investigated their effect on the thermal, mechanical, and hydrolytic degradation of the materials. Surface morphology, thermal, and mechanical analysis were used to characterize the interaction of PCL and PEO. PEO 60 blends demonstrated the lowest shear torque and the highest MFI during the fabrication process, while PEO 300 blends had significantly higher torque and lower MFI. DSC thermal analysis revealed immiscibility between PCL and PEO blends, with separate endothermic peaks. Accelerated degradation analysis revealed extensive filament degradation both internally and externally for both PEO 60 and PEO 300 blends, with SEM imaging revealing unexpected pore-like structures within the samples. Furthermore, the incorporation of PEO into hydrophobic PCL materials increased their hydrophilicity and demonstrated enhanced properties that highlight the potential of binary blended PCL/PEO for soft tissue engineering applications. The next step for this work is to fabricate scaffolds for cell culture testing from the best blends identified in this study.

Author Contributions

Conceptualization, M.D. and I.M.; methodology, M.D., K.G., H.X. and F.E.; investigation, M.D., K.G., H.X. and F.E.; data curation, K.G., M.D., H.X. and F.E.; writing—original draft preparation, M.D., K.G., H.X. and F.E.; writing—review and editing, E.J.M., G.F., E.F., K.G. and I.M.; supervision, I.M.; project administration, I.M.; funding acquisition, I.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The data presented in this study are contained within the article.

Conflicts of Interest

The authors declare no conflict of interest.

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Macromol 03 00026 g001
Figure 1. IR spectrum of PCL 60 blends.
Figure 1. IR spectrum of PCL 60 blends.
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Figure 2. IR spectrum of PCL 300 blends.
Figure 2. IR spectrum of PCL 300 blends.
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Figure 3. Overlaid thermograph displaying melt peaks of (a) PCL 60 blends and (b) PCL 300 blends.
Figure 3. Overlaid thermograph displaying melt peaks of (a) PCL 60 blends and (b) PCL 300 blends.
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Figure 4. DMA thermograms for PCL/PEO blends formulations displaying storage (E′), loss modulus (E″), and tan (α): (a) 100 PCL 50; (b) 75 PCL 60; (c) 50 PCL 60; (d) 100 PEO 60; (e) 25 PCL 300; and (f) 100 PEO 300.
Figure 4. DMA thermograms for PCL/PEO blends formulations displaying storage (E′), loss modulus (E″), and tan (α): (a) 100 PCL 50; (b) 75 PCL 60; (c) 50 PCL 60; (d) 100 PEO 60; (e) 25 PCL 300; and (f) 100 PEO 300.
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Figure 5. Rheometry measurement in terms of complex viscosity versus frequency of all blends.
Figure 5. Rheometry measurement in terms of complex viscosity versus frequency of all blends.
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Figure 6. Mass loss (%) of virgin and blended blends. Higher concentration of PCL decreased in mass, due to the higher affinity for the ethyl oxide groups to water, enabling it to degrade at a susceptible rate until week 12. (a) 100% and 75% blends; (b) 50% and 25% blends.
Figure 6. Mass loss (%) of virgin and blended blends. Higher concentration of PCL decreased in mass, due to the higher affinity for the ethyl oxide groups to water, enabling it to degrade at a susceptible rate until week 12. (a) 100% and 75% blends; (b) 50% and 25% blends.
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Figure 7. Filament macrostructure of 50 PCL 60 as degradation time increases: (A) T1, (B) T3, (C) T5, and (D) T6 (×20).
Figure 7. Filament macrostructure of 50 PCL 60 as degradation time increases: (A) T1, (B) T3, (C) T5, and (D) T6 (×20).
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Figure 8. SEM view of 50 PCL 60 external surface at the following description: (A) T0, (B) T1, (C) T3, and (D) T6 undergone accelerated degradation by 5 M NaOH. Micrographs are at 2.00×.
Figure 8. SEM view of 50 PCL 60 external surface at the following description: (A) T0, (B) T1, (C) T3, and (D) T6 undergone accelerated degradation by 5 M NaOH. Micrographs are at 2.00×.
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Figure 9. Internal and external SEM view of 75 PCL 60 (A,B) and 50 PCL 60 (C,D) at (T6). Micrographs 3.00×.
Figure 9. Internal and external SEM view of 75 PCL 60 (A,B) and 50 PCL 60 (C,D) at (T6). Micrographs 3.00×.
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Table 1. Sample ID and compositions.
Table 1. Sample ID and compositions.
NamePCL 50,000 (wt %)PEO 60,000 (wt %)PEO 300,000 (wt %)
100 PCL 50100--
75 PCL 607525-
50 PCL 605050-
25 PCL 602575-
100 PEO 600100-
75 PCL 30075-25
50 PCL 30050-50
25 PCL 30025-75
100 PEO 3000-100
Table 2. Melt flow indices (MFI) for each blend and the torque (N/m) produced during extrusion.
Table 2. Melt flow indices (MFI) for each blend and the torque (N/m) produced during extrusion.
BlendMFI (g/10 min)Torque N/m
100 PCL16.00 ± 18
75 PCL 6015.00 ± 18
50 PCL 609.00 ± 18
25 PCL 604.5 ± 19
100 PEO 601.500 ± 111
75 PCL 3003.5 ± 113
50 PCL 3001.2 ± 114
25 PCL 3000.8 ± 111
100 PEO 3000 16
Table 3. Water contact analysis of PCL/PEO scaffolds, in which the standard deviations have been shown.
Table 3. Water contact analysis of PCL/PEO scaffolds, in which the standard deviations have been shown.
BatchContact Angulation (°)
100 PCL 5074.1 ± 0.9
75 PCL 6054.1 ± 3.5
75 PCL 30069.4 ± 1.2
50 PCL 6044.9 ± 1.1
50 PCL 30066.9 ± 1.5
25 PCL 6049.0 ± 4.6
25 PCL 30050.6 ± 1.0
100 PEO 6051.9 ± 2.4
100 PEO 30037.4 ± 2.8
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Dalton, M.; Ebrahimi, F.; Xu, H.; Gong, K.; Fehrenbach, G.; Fuenmayor, E.; Murphy, E.J.; Major, I. The Influence of the Molecular Weight of Poly(Ethylene Oxide) on the Hydrolytic Degradation and Physical Properties of Polycaprolactone Binary Blends. Macromol 2023, 3, 431-450. https://doi.org/10.3390/macromol3030026

AMA Style

Dalton M, Ebrahimi F, Xu H, Gong K, Fehrenbach G, Fuenmayor E, Murphy EJ, Major I. The Influence of the Molecular Weight of Poly(Ethylene Oxide) on the Hydrolytic Degradation and Physical Properties of Polycaprolactone Binary Blends. Macromol. 2023; 3(3):431-450. https://doi.org/10.3390/macromol3030026

Chicago/Turabian Style

Dalton, Maurice, Farnoosh Ebrahimi, Han Xu, Ke Gong, Gustavo Fehrenbach, Evert Fuenmayor, Emma J. Murphy, and Ian Major. 2023. "The Influence of the Molecular Weight of Poly(Ethylene Oxide) on the Hydrolytic Degradation and Physical Properties of Polycaprolactone Binary Blends" Macromol 3, no. 3: 431-450. https://doi.org/10.3390/macromol3030026

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