Advanced Rheological, Dynamic Mechanical and Thermal Characterization of Phase-Separation Behavior of PLA/PCL Blends
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe paper is well written and takes into account an interesting topic related to the manufacturing field: namely the rheological characterization of PLA/PCL blends. Such characterization is surely helpful for future researchers based on eco-friendly materials. My only very minor comment on this research is to better clarify why such a characterization can be helpful AM-wise. My advise is that your outcome could be used in mathematical models capable to predict process parameters such. Here some works https://doi.org/10.1016/j.addma.2020.101791, https://doi.org/10.1016/j.addma.2020.101368 , https://doi.org/10.1016/j.addma.2021.102208
Author Response
Author's Reply to the Review Report (Reviewer 1)
We thank the reviewer for their thoughtful comments and suggestions, which have helped to improve our manuscript. Below are our detailed responses:
Point 1
My only very minor comment on this research is to better clarify why such a characterization can be helpful AM-wise.
Answer to the Reviewer comment to Point 1
We appreciate this suggestion to elaborate on the application of our study to additive manufacturing (AM). To address this, we propose adding the following paragraph in the Conclusions, marked in green (Lines 491-497):
"Furhermore, the rheological characterization of PLA/PCL blends is critical for understanding their flow behavior, which directly impacts their suitability for additive manufacturing (AM) applications. In extrusion-based AM processes, material properties such as viscosity, phase separation, and crystallization behavior govern the printability, mechanical performance, and interlayer adhesion of fabricated components. By optimizing these rheological properties, this research provides insights into tailoring PLA/PCL blends for eco-friendly and high-performance AM applications."
Point 2
My advice is that your outcome could be used in mathematical models capable to predict process parameters such. Here some works https://doi.org/10.1016/j.addma.2020.101791, https://doi.org/10.1016/j.addma.2020.101368 , https://doi.org/10.1016/j.addma.2021.102208.
Answer to the Reviewer comment to Point 2
We acknowledge the reviewer's suggestion and the recommended references on mathematical modeling for prediction of printing process parameters. While the development of specific mathematical models is beyond the scope of this study, we highlight that in our prior works (https://doi.org/10.1007/s11043-021-09503-2 and https://doi.org/10.3390/nano13050835) we have applied modeling of rheological behavior and have predicted a flow instability during 3D printing of PLA nanocomposites with graphene and carbon nanotubes. We had demonstrated the integration of rheological properties into process simulation to optimize printability and material performance for the design and optimization of printable nanocomposites. In our future works, we will expand further the predictive modeling approaches considering the suggested references by the reviewer.
The following text is included in the manuscript at the end of 3.2. Rheology characterization section Lines 282-285 marked in green.
„ Тhe rheological results from this study will be used in our further works applying mathematical modeling and simulation to predict 3D printing process parameters [39] and the flow instability [40] of PLA/PCL blends and nanocomposites, towards the design of 3D printable materials.”
New references were added in Lines 599-602
[39] Percoco, G.; Luca Arleo, L.; Gianni Stano, Bottiglione, F. Analytical model to predict the extrusion force as function of the layer height, in extrusion based 3D printing, Additive Manufacturing, 2021, 38, 101791, [CrossRef]
[40] Kotsilkova, R.; Tabakova, S. Exploring effects of graphene and carbon nanotubes on rheology and flow instability for designing printable polymer nanocomposites. Nanomaterials 2023, 13(5), 835 [CrossRef]
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThe present study deals with the characterization of PLA/PCL blends. The English language is comprehensible and there are almost no typos, which makes the text easy readable.
However, there is unfortunately one main reason and several smaller reasons why I suggest the rejection of the paper.
The main reason for rejecting the paper is the lack of novelty. There is no real gain in scientific knowledge related to the conducted work. All of the addressed morphology-structure-property-relationships were already previously revealed and explained in Wachirahuttapong et al. [Effect of PCL and Compatibility Contents on the Morphology, Crystallization and Mechanical Properties of PLA/PCL Blends, Energy Procedia, Volume 89, 2016, 198-206] and in Bouakaz et al. [Organomontmorillonite/graphene-PLA/PCL nanofilled blends: New strategy to enhance the functional properties of PLA/PCL blend, Applied Clay Science, Volume 139, 2017, 81-91] and recently discussed in the review by Matumba et a. [Morphological Characteristics, Properties, and Applications of Polylactide/Poly(ε-caprolactone) Blends and Their Composites—A Review. Macromol. Mater. Eng. 2024, 309, 2400056]. So the question arises of what are actually the new findings that surpass the current state of knowledge?
Additionally, several content-related questions arise while reading the paper:
1. What the average molecular weight of PLA in the research?
2. Line 228. Authors mention only here "cross-linking"? What type of cross-linking they expect to observe and, most importantly, what results indicate the presence of cross-linking in their blends?
3. Figure 5. I would suggest to insert a table with temperature values discussed in the text for clarity of interpretation.
4. Why there is this large difference in Tg (~10+ °C) when evaluated from DMTA and DSC?
5. Lines 372-373. Authors claim the suppressing effect of PCL on the crystallization of PLA, while data in Figure 9 indicates a percentage of PLA crystalline phase almost constant. Please revise accordingly.
Author Response
Author's Reply to the Review Report (Reviewer 2)
We thank the reviewer for the thorough evaluation and valuable feedback. We acknowledge the concerns regarding the novelty and scientific contribution of our study. Below, we address the issues raised and clarify the novel aspects of our research as well as respond to specific content-related questions.
Point 1
The main reason for rejecting the paper is the lack of novelty. There is no real gain in scientific knowledge related to the conducted work. All of the addressed morphology-structure-property-relationships were already previously revealed and explained in Wachirahuttapong et al. [Effect of PCL and Compatibility Contents on the Morphology, Crystallization and Mechanical Properties of PLA/PCL Blends, Energy Procedia, Volume 89, 2016, 198-206] and in Bouakaz et al. [Organomontmorillonite/graphene-PLA/PCL nanofilled blends: New strategy to enhance the functional properties of PLA/PCL blend, Applied Clay Science, Volume 139, 2017, 81-91] and recently discussed in the review by Matumba et a. [Morphological Characteristics, Properties, and Applications of Polylactide/Poly(ε-caprolactone) Blends and Their Composites—A Review. Macromol. Mater. Eng. 2024, 309, 2400056]. So the question arises of what are actually the new findings that surpass the current state of knowledge?
Answer to the Reviewer comment in Point 1
Unlike the previous works cited by the reviewer, our study combines advanced structural, rheological, dynamic mechanical, and thermal characterization methods to provide a holistic understanding of the structure-property relationships of immiscible PLA/PCL blends without addition of compatibilizer. Additionally, the detailed rheological analysis and Cole-Cole plots highlight viscoelastic behavior under shear, which is essential for processing considerations, particularly in additive manufacturing. Our findings are tailored to applications such as smart and responsive materials, emphasizing functional properties like thermal stability and shape memory, which are not comprehensively addressed in the cited studies. The novelty in our paper lies in its integration of multidisciplinary characterization methods and the focus on the phase-separation behavior and thermal-mechanical properties of PLA/PCL blends.
Compared to the cited by the reviewer 3 papers, the following key differences and advancements are evident:
1) A multidisciplinary approach of combined characterization methods, like SEM, rheology, DMTA, DSC and TGA were applied in our study which surpasses earlier studies typically focused primarily on basic properties like tensile strength, crystallization and morphology;
2) Our study discusses the phase inversion behavior at varying PLA/PCL ratios and its impact on viscoelastic, thermal and thermomechanical properties, providing insights into applications like shape memory and additive manufacturing. Earlier studies of Wachirahuttapong et al. and Matumba et al., highlight properties towards packaging, biomedical, and structural applications.
3) Our study utilizes melt mixing as a scalable and sustainable approach, optimizing blend ratios for industrial applications. This processing method and its relation to morphology and property enhancement have not been as comprehensively addressed in the cited papers. It also explores varying PLA/PCL ratios systematically (e.g., 5–70% PCL), identifying specific transitions such as droplet-matrix to co-continuous morphologies.
Answering the reviewer comment above, we have added a new text in the Introduction and Conclusions, marked in yellow in Lines 64–79 and Lines 490–497:
Lines 64–79 “Wachirahuttapong et al. [25] investigated the effects of PCL on morphology, crystallization behavior and mechanical properties of PLA/PCL blends in the presence of Pluronic as plasticizer. They found that PLA and PCL are immiscible, with PCL forming droplets within the PLA matrix, and their size increasing with higher PCL content. The addition of Pluronic led to improving ductility and elongation at break, but higher concentrations negatively impact mechanical properties due to larger PCL particles. The crystallization behavior of PLA was altered by both PCL and Pluronic. Bouakaz et al. in [26] explored the use of organomontmorillonites (Cloisite®15A and Cloisite®30B) combined with epoxy-functionalized graphene to enhance the compatibility of a PLA/ PCL blend. The study demonstrated significant improvements of thermal and barrier properties due to the synergistic effect of these nanofillers. The effectiveness of combining classical melt blending techniques with advanced filler combinations for creating high-performance biodegradable composites was highlighted. In the comprehensive review, Matumba et al in [27] examined the morphological, mechanical, and thermal characteristics of PLA/PCL blends, highlighting the roles of various fillers and compatibilizers in optimizing these properties for diverse applications in packaging, biomedical, and industrial fields.”
Lines 490–497: “Furthermore, the rheological characterization of PLA/PCL blends is critical for under-standing their flow behavior, which directly affects their suitability for additive manufacturing (AM) applications. In extrusion-based AM processes, material properties such as viscosity, phase separation, and crystallization behavior govern the printability, mechanical performance, and interlayer adhesion of fabricated components. By optimizing the rheological properties, this research provides insights into tailoring PLA/PCL blends for eco-friendly and high-performance AM applications.”
Additionally, the three papers suggested by the reviewer have been incorporated into the References section:
- Wachirahuttapong, S.; Thongpin, C.; Sombatsompop, N. Effect of PCL and compatibility contents on the morphology, crystallization and mechanical properties of PLA/PCL blends. Energy Procedia, 2016, 89, 198-206. [CrossRef]
- Bouakaz, B.S.; Habi, A.; Grohens, Y.; Pillin, I. Organomontmorillonite/graphene-PLA/PCL nanofilled blends: New strategy to enhance the functional properties of PLA/PCL blend. Applied Clay Science, 2017, 139, 81-91. [CrossRef]
- Matumba, K.I.; Mokhena, T.C.; Ojijo, V.; Sadiku, E.R.; Ray, S.S. 2024. Morphological Characteristics, Properties, and Applications of Polylactide/Poly (ε‐caprolactone) Blends and Their Composites — A Review. Macromolecular Materials and Engineering, 2024, 1-14. [CrossRef]
Point 2
What the average molecular weight of PLA in the research?
Answer to the Reviewer comment in Point 2
Polylactic acid Ingeo™ 3D870 (NatureWorks LLC, USA) with a molecular weight of 110,000 g/mol was used. This information is added to the "Materials and Methods" section Line 101
Point 3
Line 228. Authors mention only here "cross-linking"? What type of cross-linking they expect to observe and, most importantly, what results indicate the presence of cross-linking in their blends?
Answer to the Reviewer comment in Point 3
Thank you for this insightful comment. The term "cross-linking" in our manuscript refers to enhanced physical interactions between PLA and PCL phases. The following text have been added in Lines 251-256, marked in yellow.
“The "cross-linking" refers here to enhanced physical interactions between PLA and PCL phases. Evidence for this includes increased complex viscosity at low frequencies, enhanced storage modulus (G'), and deviations from liquid-like behavior in Cole-Cole plots, particularly in blends with phase inversion or intermediate PCL content and these observations reflect network-like structures arising from phase morphology and interfacial interactions.”
Point 4
Figure 5. I would suggest to insert a table with temperature values discussed in the text for clarity of interpretation.
Answer to the Reviewer comment in Point 4
We do not find necessary to insert a table with temperature values related to Figure 5 (a-g), as the graphs compared the G’ & G” modulus vs. temperature determined by DMTA at two heating rates, presenting separately for each composition, as well as the combined curves comparing all compositions. The temperature values of Тg,PLA across the blends were presented in Figure 6 (a,b) at heating rates 3 oC/min (a) and 5 oC/min and additionally the values were summarized in Table 2.
Point 5
Why there is this large difference in Tg (~10+ °C) when evaluated from DMTA and DSC?
Answer to the Reviewer comment in Point 5
The observed difference in Tg when evaluated from DMTA and DSC is attributed to the distinct principles of the two techniques. DMTA measures mechanical transitions associated with viscoelastic properties under dynamic loading, while DSC detects thermal transitions based on heat flow. The heating rate also plays a role in shifting Tg values. The discussion in the Sections 3.3 (Lines 377 to 385) clarify these differences, together with relevant literature which support our explanation:
“In Table 2, the Tg,PLA values, as determined by both the DMTA at two heating rates and the DSC tests (in section 3.4.1) were compared. The observed difference in the Tg,PLA determined by both tests for the neat PLA can be attributed to the distinct physical properties measured by each technique. The DMTA measured mechanical changes, the variations in storage and loss moduli as a response to temperature, while the DSC analysis detected calorimetric changes associated with the heat flow during the glass transition [42-44]. These transitions for one and the same sample were manifested herewith at different temperatures, as affected by the heating rate and the differing sensitivities and principles of the two methods.”
Point 6
Lines 372-373. Authors claim the suppressing effect of PCL on the crystallization of PLA, while data in Figure 9 indicates a percentage of PLA crystalline phase almost constant. Please revise accordingly.
Answer to the Reviewer comment in Point 6
In Lines 413-414 (in the revised file), the suppressing effect of PCL on the PLA crystallization refers to the absence of a visible melt crystallization peak for PLA in DSC thermograms in Figure 8(b) at PCL content of 30, 40 and 70%. While, the overall crystallinity of PLA across the blends was calculated from the melting enthalpy of the DSC curves in Figure 8(a) with contributions from the cold crystallization using Eq. (1) (Line 164) and it appears relatively constant.
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for AuthorsIn the paper," Advanced rheological, dynamic mechanical and thermal characterization of phase-separation behavior of PLA/PCL blends", the authors investigate the effect of relative concentration of constituents on the final thermal, mechanical and morphological characteristics.
The results of this study can be used to tune the properties of PLA/PCL blends for specific applications. The blends are created by mixing pellets by weight and compounded in a twin screw extruder. Samples were then fabricated for various characterization methods. The following are the comments to the authors:
1) Section 2.2 - Characterization methods can be organized better. Each characterization method can have its own sub heading.
2) Sample sizes should be mentioned clearly. Were more than 1 sample used for measurements such as DMTA, TG, Crystallinity? If multiple samples were used, how does variation within a blend compare with variation across blends?
3) Section 3.2 and 3.3: Need to mention sample size clearly. Was the same sample used for 3 C/ min and 5C/min study. Also, Tg at 5C per min seems to have a clear inverse relationship with %PLA but Tg at 3C/min has outliers. Using low sample sizes can lead to this behavior and can be mentioned in the paper. Same comments apply to the section on Crystallinity and other experimental results.
4) The conclusions and implication of research is presented clearly.
Overall, the paper is acceptable for publication with minor changes as indicated above. The scientific method is adequate, however, it can be improved if the authors can include multiple samples for each test.
Author Response
Author's Reply to the Review Report (Reviewer 3)
We thank the reviewer for their constructive feedback and appreciation of our study. We have addressed the specific points raised to improve the clarity and comprehensiveness of the manuscript. Below are our detailed responses and the corresponding revisions made to the paper.
Point 1
Section 2.2 - Characterization methods can be organized better. Each characterization method can have its own sub heading.
Answer to the Reviewer comment to Point 1
We have reorganized Section 2.2 to provide distinct subheadings (highlithed in yellow) for each characterization method (e.g., “2.2.1. Scanning electron microscopy”; “2.2.2. Rheological measurements”; “2.2.3. Dynamic mechanical thermal analysis”; “2.2.4. Differential scanning calorimetry” and “2.2.5. Thermogravimetric analysis”). This reorganization improves readability and makes it easier for readers to locate specific experimental details.
Point 2
Sample sizes should be mentioned clearly. Were more than 1 sample used for measurements such as DMTA, TG, Crystallinity? If multiple samples were used, how does variation within a blend compare with variation across blends?
Answer to the Reviewer comment to Point 2
The sample sizes are outlined in the Materials and Methods section and highlighted in blue to each characterization method in the subsection 2.2. For each test, at least three samples were measured to ensure reproducibility. The average values within a blend of standard error around ± 1 % at confidence level 95% were presented in the results. The variation across the blends are commented for the DMTA, DSC, Crystallinity and TG results, when they are above the standard error of ± 1 % (the variation within a blend).
The corrections regarding variation across the blends are made in the manuscript text, marked in blue in Lines 366-367; Lines 416-417, Lines 420-430, Lines 431-434, Lines 463-465:
Lines 137-140: “Three samples with a thickness of 2 mm and a diameter of 20 mm were measured for each blend and neat polymer, and the average results with a standard error around ±1.0 % were presented herewith.”.
Lines 145-148: “The test samples were plates of size 60 x 12 x 2.5 mm and at least three samples were tested of each composition, presenting here the average results, with a standard error around ± 1 % at confidence level 95%.”.
Lines 155-158: “Multiple samples (n=3) were used for each blend composition to ensure reproducibility and the average results with a standard error below ±1% at a confidence level of 95% were presented here.”.
Lines 175-177: “Multiple samples (n=3) were tested for each blend composition and the average results with a standard error below ±1% were presented here.”.
Lines 416-417: “The variation in crystallization temperature of the PC, T_(c PCL) was ~2 % across the blends by increasing the PCL content, …”.
Lines 428-430: “… by increasing the PCL content with variation across the blends of 2-3%, with an exception of the 60PLA/40PCL blend with ?c PLA ~ 22%.”.
Lines 431-434: “… increased to ?c PCL = 67% with variation of 30% for the co-continuous blends 70PLA/30PCL. In contrast, the crystallinity of the blends with sea-island morphology decreased to ?c PCL = 37-40%, compared to the neat PLA with variation across the blends around 5 %.”.
Lines 463-465: “… with variation about 3%, while further increase of PCL content to 70% slightly increased the thermal stability with variation across the blends around 1% .”.
Point 3
Section 3.2 and 3.3: Need to mention sample size clearly. Was the same sample used for 3 C/ min and 5C/min study. Also, Tg at 5C per min seems to have a clear inverse relationship with %PLA but Tg at 3C/min has outliers. Using low sample sizes can lead to this behavior and can be mentioned in the paper. Same comments apply to the section on Crystallinity and other experimental results.
Answer to the Reviewer comment
The sample sizes and experimental conditions are detailed in the "Materials and Methods" section of the paper highlighted in blue. For dynamic mechanical thermal analysis (DMTA), test samples with dimensions of 60 x 12 x 2.5 mm were used for all experiments, including the studies conducted at 3°C/min and 5°C/min heating rates. While the same fabrication methodology was followed, new samples with the same size were used for testing each heating rate to maintain consistency and avoid potential thermal history effects.
Regarding the reviewer comment: “Tg at 5 oC/min seems to have a clear inverse relationship with %PLA but Tg at 3 oC/min has outliers”, we have added the following corrections marked in blue in Lines 363-376:
“Moreover, as the PLА content decreased in the PLA/PCL blends, the Tg,PLA gradually increased for both heating rates. The variation across the blends is small (1-2%) for the 95PLA/5PCL blend compared to the neat PLA, but it increased to 6-8% at 30PLA/70PCL. However, at 5°C/min, the Tg,PLA shows a consistent inverse relationship, with the de-creasing percentage of PLA in the blend, which aligns with the expected influence of PCL content and the phase morphology. This trend may be associated with the PCL effects on the segmental dynamics and reduced mobility of PLA chains, likely due to the phase interactions and possible suppression of PLA crystallization, as well as to the formation of phase-separated to co-continuous morphologies, particularly at higher PCL content. While, at 3°C/min, the variations in Tg,PLA, including apparent outliers, could be influ-enced by the increased sensitivity to molecular relaxation at a slower heating rate. The slower heating rate allows more time for phase-specific interactions, leading to potential deviations depending on the morphology and crystallinity of individual samples.“.
Point 4
The conclusions and implication of research is presented clearly.
Answer to the Reviewer comment
We appreciate the positive feedback regarding the clarity of the conclusions.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
Comments and Suggestions for AuthorsDear Authors,
I would like to express my appreciation for taking my feedback into account and revising your work. Based on the improvements you have made, I am pleased to recommend the publication of your article in its current form.
