Correlation Between Microscale Indentation Creep and Macroscale Tensile Creep of PLA/PCL Polymer Blends

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Recommendation: Minor Revisions needed

Summary

The manuscript extends the authors' prior homopolymer work (ref. 5) to PLA/PCL blends across the full composition range, showing that the ISO 14577 indentation creep parameter (CIT) and the Power Law exponent (n) both yield physically inverted trends, while total-deformation descriptors corrected for initial elastic deformation (Δh², Δε) behave consistently at both scales. The negative result on CIT/n is a genuine and useful contribution. The paper is methodologically sound, but the central predictive claim needs stronger quantitative support, and several mechanistic points need clarification.

Major Comments

1. (Fig. 8, lines 432–459) The paper's central claim — that Δh² predicts Δε — is supported only by visually similar bar charts across composition, not by a direct regression between the two quantities. Figs. 3c/d provide an actual E–EIT and Y–HIT cross-correlation with R²; Fig. 8 should include the analogous Δh²-vs-Δε (and h²-vs-ε) cross-plots with fitted correlation and R². Without this, "correlation" in the title is not yet quantitatively demonstrated.
2. Indent size vs. domain size. SEM (Figs. 1–2) shows substantial phase coarsening near 40/60–60/40, including co-continuity and phase inversion. The manuscript does not address whether individual indents (at 50/300 gf) sample a representative volume of both phases at these compositions, or whether indents may preferentially probe one phase. This should be discussed, at minimum qualitatively, given its direct bearing on whether microscale measurements reflect bulk blend behavior.
3. Terminology inconsistency — "physical aging" vs. secondary creep (line ~477). The Conclusions attribute the macroscale n anomaly to "physical aging of PLA," but Sections 3.3/4.1 (lines 291–312, 401–408) attribute it to a distinct secondary creep regime. These are different mechanisms (structural relaxation below Tg vs. viscoplastic flow under load). Please reconcile terminology throughout.
4. Missing Tg context. At the 22 °C test temperature, PCL (Tg ≈ −60 °C) is well above its Tg while PLA (Tg ≈ 55–60 °C) is glassy. This large difference in physical state relative to Tg plausibly underlies both the creep-magnitude difference and PLA's secondary/stepwise creep behavior but is never discussed. If DSC data already exist from the authors' prior work (refs. 10, 15), adding this context would substantially strengthen the mechanistic interpretation at low cost.
5. Single tensile stress level (lines 140–148). Tensile creep was measured at only one load (2 kg). Without at least one additional stress level, linearity of the viscoelastic response cannot be confirmed, which is a precondition for applying the PL/EVP formalisms. Please state this as a limitation, or justify the single-stress design.
6. Replication criteria asymmetry. Microindentation: ≥30 measurements averaged (line 119–121). Tensile creep: "at least twice," repeated only if results "differed substantially" (lines 268–270), with no stated threshold. Please provide a quantitative repeatability criterion for the tensile creep protocol.

Minor Comments

• Line 204: Unresolved cross-reference error visible in text ("错误!未找到引用源。") — must be fixed before publication.
• Line 6: "Czech Academic of Sciences" → "Czech Academy of Sciences."
• Line 314: Section 4.1 heading has a stray word: "...of in polymer systems" → "...in polymer systems."
• Fig. 6c: Error bars appear to overlap across compositions; the claimed "small but detectable" decrease in n would benefit from a stated statistical test rather than visual assessment alone.
• Appendix Table A5: Several R²(all) values for PLA-rich blends are negative (e.g., PLA/PCL-100/0, EVP3: −3.36), indicating the long-term extrapolation performs worse than the mean. This striking result supports the paper's core argument and deserves explicit mention in the main text (Section 3.3 or 4.1), not just the appendix table.

 

Author Response

General introductory comment: The manuscript extends the authors' prior homopolymer work (ref. 5) to PLA/PCL blends across the full composition range, showing that the ISO 14577 indentation creep parameter (CIT) and the Power Law exponent (n) both yield physically inverted trends, while total-deformation descriptors corrected for initial elastic deformation (Δh², Δε) behave consistently at both scales. The negative result on CIT/n is a genuine and useful contribution. The paper is methodologically sound, but the central predictive claim needs stronger quantitative support, and several mechanistic points need clarification.

Answer: We would like to thank the reviewer for their comments that helped us to improve our manuscript. All changes in the manuscript are marked with red font.

Comment 1: (Fig. 8, lines 432–459) The paper's central claim — that Δh² predicts Δε — is supported only by visually similar bar charts across composition, not by a direct regression between the two quantities. Figs. 3c/d provide an actual E–EIT and Y–HIT cross-correlation with R²; Fig. 8 should include the analogous Δh²-vs-Δε (and h²-vs-ε) cross-plots with fitted correlation and R². Without this, "correlation" in the title is not yet quantitatively demonstrated.

Answer: We modified Fig. 8 as the reviewer suggested. We would like to thank for a useful hint. Indeed, the h²-ε correlations are quite illustrative. When adding the correlations to Fig. 8, we decided to split it in two parts. The modified Fig. 8 now contains bar plots and correlations showing h², Δh², ε and Δε, while the compliances C0 (former Fig. 8c) are given as self-standing Fig. 9. The text of the manuscript and figure legends were modified accordingly. All changes are in section 4.3. and are marked with red font.

Comment 2: Indent size vs. domain size. SEM (Figs. 1–2) shows substantial phase coarsening near 40/60–60/40, including co-continuity and phase inversion. The manuscript does not address whether individual indents (at 50/300 gf) sample a representative volume of both phases at these compositions, or whether indents may preferentially probe one phase. This should be discussed, at minimum qualitatively, given its direct bearing on whether microscale measurements reflect bulk blend behavior.

Answer: The following text was added at the end of the second paragraph of Section 3.1: The standard deviations for microindentation measurements were reasonably low in the whole concentration range, which documented both the precision of the measurements and the fact that the indents were large enough to probe the whole blend rather than its individual phases. The indent size for the roughest PLA/PCL blends with composition 40/60 and 60/40 was >90 μm, which was well above the characteristic dimensions of structural inhomogeneities observed in SEM (Figs. 1 and 2). This can be verified using the approximate formula H_IT » 1.854⋅F/d², where F is the maximum force (50 or 300 gf) and d is the diagonal length of the indent on the polymer surface [Balta-Calleja 2000].

Comment 3: Terminology inconsistency — "physical aging" vs. secondary creep (line ~477). The Conclusions attribute the macroscale n anomaly to "physical aging of PLA," but Sections 3.3/4.1 (lines 291–312, 401–408) attribute it to a distinct secondary creep regime. These are different mechanisms (structural relaxation below Tg vs. viscoplastic flow under load). Please reconcile terminology throughout.

Answer: The terminology in the submitted manuscript was inconsistent and the term physical aging was wrong (this was a mistake in the original draft that escaped my attention and I would like to thank the reviewer for catching this). We wanted to say that PLA aging (in general) caused partial PLA degradation that was connected with chain scissions and the resulting lower molecular weight promoted the development of secondary creep. The relation between PLA degradation and secondary creep development was reported elsewhere [Woo 2025, Chen 2026]. Therefore, the terminology was corrected, the wrong term “physical aging” was removed, and a short explanation (citing the two abovementioned papers) was added at the end of 3rd paragraph of Section 3.3.

Comment 4: Missing Tg context. At the 22 °C test temperature, PCL (Tg ≈ −60 °C) is well above its Tg while PLA (Tg ≈ 55–60 °C) is glassy. This large difference in physical state relative to Tg plausibly underlies both the creep-magnitude difference and PLA's secondary/stepwise creep behavior but is never discussed. If DSC data already exist from the authors' prior work (refs. 10, 15), adding this context would substantially strengthen the mechanistic interpretation at low cost.

Answer: The reviewer is right here. For the researchers working with the two polymers this context is clear, but it is useful to mention it explicitly for other potential readers. The information about glass transition temperatures of both polymers was added to Experimental part (Section 2.1) and the relation between T_g of the components and blend properties was briefly mentioned in Results and discussion (the beginning of the 2nd paragraph of section 3.1.2).

Comment 5: Single tensile stress level (lines 140–148). Tensile creep was measured at only one load (2 kg). Without at least one additional stress level, linearity of the viscoelastic response cannot be confirmed, which is a precondition for applying the PL/EVP formalisms. Please state this as a limitation, or justify the single-stress design.

Answer: In fact, we performed tensile creep experiments at multiple stress levels and proved that we are in LVE range, but this fact was not mentioned in the original manuscript. Nevertheless, we acknowledge that this fact is important and worth mentioning. Therefore, we added the information about in the substantially extended Section 2.3.4 in the Experimental part. All changes are marked with red font.

Comment 6: Replication criteria asymmetry. Microindentation: ≥30 measurements averaged (line 119–121). Tensile creep: "at least twice," repeated only if results "differed substantially" (lines 268–270), with no stated threshold. Please provide a quantitative repeatability criterion for the tensile creep protocol.

Answer: The quantitative repeatability criterion was added in the modified Section 2.3.4 in the Experimental part (see also the answer to the previous comment). The end of the modified final paragraph of the section reads: If the two creep curves differed by more than 20 %, another two measurements were per-formed. Therefore, the final tensile creep curves in this study are averages of at least two measurements.

Additional minor comments (copy-pasted) with answers:

• Line 204: Unresolved cross-reference error visible in text ("错误!未找到引用源。") — must be fixed before publication.

Answer: This was a formatting error during editing. The editor mentioned this in his decision letter and apologized for the mistake. Now it is corrected.

• Line 6: "Czech Academic of Sciences" → "Czech Academy of Sciences."

Answer: Corrected.

• Line 314: Section 4.1 heading has a stray word: "...of in polymer systems" → "...in polymer systems."

Answer: Corrected.

• Fig. 6c: Error bars appear to overlap across compositions; the claimed "small but detectable" decrease in n would benefit from a stated statistical test rather than visual assessment alone.

Answer: The reviewer is right that the statistical test is better than visual assessment. Nevertheless, in this case the main conclusion was that the creep exponent, n, is not a good creep descriptor anyway, and so the statistical tests confirming that the observed trend is statistically significant were not necessary. We decided not to include them for the sake of brevity.

• Appendix Table A5: Several R²(all) values for PLA-rich blends are negative (e.g., PLA/PCL-100/0, EVP3: −3.36), indicating the long-term extrapolation performs worse than the mean. This striking result supports the paper's core argument and deserves explicit mention in the main text (Section 3.3 or 4.1), not just the appendix table.

Answer: This fact is briefly mentioned at the end of Section 3.3. as the reviewer suggested. The inserted sentence is marked with red font and contains reference to both Fig. 5 and Appendix C, where the R²(all) values are listed for all fits.

Reviewer 2 Report

Comments and Suggestions for Authors

The study investigates the relationship between short-term microindentation creep and macroscale tensile creep of PLA/PCL blends with different compositions. The authors compared the material response at both length scales and analyzed the data using the Power Law model and elasto-visco-plastic (EVP) models. Particular attention was paid to evaluating the suitability of different creep descriptors for describing and comparing the behavior of PLA/PCL blends.

I have the following questions/comments/suggestions:

1 The authors repeatedly use the term “prediction”; however, the presented results primarily demonstrate a similar ranking of the investigated compositions. It should be clarified on what basis the agreement between the observed trends can be considered evidence of the actual predictive capability of the proposed method.

2 The macroscale creep test duration was only 6000 s (100 min). Referring to it as “long-term creep” appears unjustified and must either be revised or supported by substantially longer experiments.

3 All materials were tested under the same macroscale load despite considerable differences in their stiffness and yield strength. It would be beneficial to discuss how differences in the relative stress level may have affected the comparison of the creep responses.

4 I have concerns regarding the applied pre-conditioning procedure, in which the specimen was subjected to a strain greater than the expected final strain during the subsequent creep test. The authors need to justify this approach, as this treatment may significantly alter the subsequent viscoelastic response of the material.

5 The exact stress and strain applied during pre-conditioning should be explicitly stated. Furthermore, it must be demonstrated that the one-hour recovery period was sufficient to obtain a reproducible material state prior to the actual creep test.

6 How many independent material specimens were used for the microindentation measurements? Multiple indentations performed on the same specimen cannot be treated as independent experimental replicates and this should be clearly stated in the methodology.

7 Considering the very small strains observed for the PLA-rich specimens, the authors should clarify whether the macroscale results were corrected for machine compliance, grip displacement, and possible specimen slippage.

8 The claim that the proposed descriptors may also be applicable to copolymers, crosslinked polymers, and composites goes beyond the scope of the present study. This should be presented strictly as a hypothesis requiring further experimental validation.

9 The Abstract needs to be revised to more clearly present the main assumptions of the study, the specific research gap, and the principal conclusions.

10 A more in-depth discussion of the obtained results is required in the main text.

11 The Conclusions should be made more specific and directly linked to the key findings of the study, rather than relying on general statements.

Author Response

General introductory comment:  The study investigates the relationship between short-term microindentation creep and macroscale tensile creep of PLA/PCL blends with different compositions. The authors compared the material response at both length scales and analyzed the data using the Power Law model and elasto-visco-plastic (EVP) models. Particular attention was paid to evaluating the suitability of different creep descriptors for describing and comparing the behavior of PLA/PCL blends.

Answer: We would like to thank the reviewer for their comments that helped us to improve our manuscript and clarify some items. All changes in the manuscript are marked with red font.

Comment 1: The authors repeatedly use the term “prediction”; however, the presented results primarily demonstrate a similar ranking of the investigated compositions. It should be clarified on what basis the agreement between the observed trends can be considered evidence of the actual predictive capability of the proposed method.

Answer: This is an important and useful comment. We agree that the term “prediction” could be interpreted as quantitative prediction of the macroscale creep response, which was not the aim of this study. In this work, prediction refers specifically to the ability of short-term microindentation creep to correctly rank the investigated PLA/PCL compositions according to their macroscale creep behavior, rather than to quantitatively predict the macroscopic creep curves.

We have therefore clarified the meaning of “prediction” at the very beginning of the Introduction, specifically at the beginning of the second paragraph of Introduction, which now reads: In our previous work [5], several analyses were conducted to evaluate the potential of microindentation creep as a predictive tool for macroscopic tensile creep of individual polymers. It is worth noting that in our studies, the term prediction refers to the ability of microindentation creep to correctly rank different materials according to their macroscopic creep behavior, rather than to quantitatively predict the macroscopic creep response.

Moreover, we revised the Abstract and Conclusions accordingly (see also the answers to comments 9 and 11 and made various minor clarifications throughout the whole manuscript. All changes are marked with red font.

Comment 2: The macroscale creep test duration was only 6000 s (100 min). Referring to it as “long-term creep” appears unjustified and must either be revised or supported by substantially longer experiments.

Answer: The reviewer is right that the typical creep experiments are longer than 100 min. Therefore, we removed the adjective “long-term” from the description of our tensile creep measurements in the whole manuscript. It may be worth mentioning that our parallel results have shown that prolonged tensile creep experiments do not change the final results from the point of view of ranging the studied systems according to their total creep deformation. Nevertheless, these parallel experiments are not part of the manuscript and so the omission of “long-term” is the best solution.

Comment 3: All materials were tested under the same macroscale load despite considerable differences in their stiffness and yield strength. It would be beneficial to discuss how differences in the relative stress level may have affected the comparison of the creep responses.

Answer: In fact, the same load for macroscale tensile experiments was selected intentionally after careful consideration. We wanted to verify if the microscale indentation experiments can predict the macroscale tensile creep experiments in the sense that both macro- and microscale tests rank the investigated materials in the same order. For that, we needed some standard, well defined experimental conditions that could be applied to the whole series of samples. The load 2 kg (which corresponded to sigma = 4.91 MPa) was still low enough to be within linear viscoelasticity range for the softest PLA/PCL(0/100) blend and high enough to result in measurable creep in the stiffest PLA/PCL(100/0) blend. This is explained in the modified Section 2.3.4 in the Experimental part. All changes in this thoroughly re-written section are marked with red font.

Comment 4: I have concerns regarding the applied pre-conditioning procedure, in which the specimen was subjected to a strain greater than the expected final strain during the subsequent creep test. The authors need to justify this approach, as this treatment may significantly alter the subsequent viscoelastic response of the material.

Answer: The description of the pre-conditioning procedure was substantially improved in the revised manuscript. The modified text is in the 3rd  paragraph of Section 2.3.4 in Experimental part. Important fact is that our creep measurements were based on previously described, well-established and thoroughly tested methodology of Kolarik et al.; the key references [Kolarik 2024, Kolarik 2005] are cited in the revised manuscript (the first paragraph of section 2.3.4) together with our recent work focused on homopolymers [Slouf 2023].

Comment 5: The exact stress and strain applied during pre-conditioning should be explicitly stated. Furthermore, it must be demonstrated that the one-hour recovery period was sufficient to obtain a reproducible material state prior to the actual creep test.

Answer: The stress and strain during the pre-conditioning are described in the thoroughly modified section 2.3.4 of the revised manuscript (see also the answer to the previous comment). The 1h recovery period was sufficient for the recovery as the pre-conditioning comprised rather brief 1 min loading at high stress (which was still below the yield point) followed by much longer, 60 min relaxation. Again, this protocol is not new or experimental. It is based on the well-established methodology of Kolarik et al. that was employed in many previous studies, as mentioned in the answer to the previous comment.

Comment 6: How many independent material specimens were used for the microindentation measurements? Multiple indentations performed on the same specimen cannot be treated as independent experimental replicates and this should be clearly stated in the methodology.

Answer: We used three independent cut surfaces cut from three different dogbone specimens employed in the macroscale creep testing. More precisely, we used only the central part of the dogbone specimen for the creep testing, while the edges could be employed in microindentation testing. This experimental detail was added at the beginning of the third paragraph of Section 2.3.3.

Comment 7: Considering the very small strains observed for the PLA-rich specimens, the authors should clarify whether the macroscale results were corrected for machine compliance, grip displacement, and possible specimen slippage.

Answer: No corrections for machine compliance, grip displacement and possible specimen slip were performed. Nevertheless, each specimen was measured at least twice and if the creep curves differed by more than 20 %, the measurement was repeated. This is described in the last paragraph of section 2.3.4 of the revised manuscript. The above-described approach resulted in quite reproducible results in both previous study [Slouf 2003; cited in the revised manuscript] and the current study, where all creep curves looked reasonable. The somewhat unexpected secondary creep observed for PLA-rich specimens could be attributed to the aging of this biodegradable polymer, which is well documented in the literature (references [Chen 2026 and Woo 2025], which are cited in the revised manuscript.

Comment 8: The claim that the proposed descriptors may also be applicable to copolymers, crosslinked polymers, and composites goes beyond the scope of the present study. This should be presented strictly as a hypothesis requiring further experimental validation.

Answer: The reviewer is right that this is just a speculation, even if it looks reasonable. Therefore, we exchanged the phrase “…these parameters may be applicable to other polymer systems …” for “… these parameters might be applicable to other polymer systems …” This modification was applied both in the last paragraph Discussion section and in the Conclusion.

Comment 9: The Abstract needs to be revised to more clearly present the main assumptions of the study, the specific research gap, and the principal conclusions.

Answer: We have revised the Abstract to make it clearer and more concise, with particular emphasis on the main assumptions, research gap, and principal conclusions. In the revised Abstract, we clarify that microindentation creep is used to predict specifically the ranking of macroscale creep behavior, rather than the macroscale creep curves themselves. The changes are marked with red font.

Comment 10: A more in-depth discussion of the obtained results is required in the main text.

Answer: The main text describing and discussing the creep results was modified and/or clarified in multiple locations based on the comments of both reviewers. All changes are marked in red. The most important changes are in Section 4.3, where we added correlations between macroscale and microscale creep deformations and split the original Fig. 8 into the new Figs. 8 and 9. The new Fig. 8 demonstrates that alternative creep descriptors, such as the total creep deformation, not only exhibited the same trends at the micro- and macroscale but also showed strong linear correlations. The new Fig. 9 further demonstrates that the initial compliances estimated by fitting the EVP3 model to the experimental creep data at both the micro- and macroscale correspond well to the independently measured elastic moduli, considering that the initial compliance from creep experiments is the reciprocal of the elastic modulus.

Comment 11: The Conclusions should be made more specific and directly linked to the key findings of the study, rather than relying on general statements.

Answer: The Conclusions have been revised to make them more specific and directly linked to the key findings of the study. In particular, we now explicitly distinguish between the demonstrated ability of microindentation creep to correctly rank the investigated blends according to their macroscale creep behavior and the potential applicability of the identified creep descriptors to other polymer systems. All changes in Conclusions are marked with red font.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have satisfactorily answered my questions, and the manuscript has been properly revised. Based on this, I accept the manuscript for further publication procedures.