Durability of Shape Memory Polymer Composite Laminates under Thermo-Mechanical Cycling
Round 1
Reviewer 1 Report
The manuscript entitled "Durability of Shape Memory Polymer Composite Laminates under Thermos-Mechanical Cycling" presents the results of single thermo-mechanical cycles at increasing strains and multiple cycling on shape memory polymer composites. Results allow to analyze the durability and shape memory performance of the composite.
The topic is interesting. The manuscript can be accepted if the following comments are addressed:
- The authors talk about space applications. However, it is known that numerous requirements restrict the range of materials for space applications. Is the analyzed material suitable for this type of applications? This should be commented in the manuscript.
- Lines 188-195, page 6: the authors report that shape fixity and shape recovery indices are not suitable for the analyzed composite. It is not clear which are the indices used to verify shape memory performances. These indices should be clearly stated, instead of generally saying "a new instrumented procedure has been defined".
- line 214, page 7: the shape ratio is mentioned, but in the result and discussion sections the measured values are not clear.
- Table 4: it would be appropriate to analyze the differences between the first and the last cycle (maybe with a proper index).
- Please correct some language typos.
Author Response
Durability of Shape Memory Polymer Composite Laminates under Thermos-Mechanical Cycling
Answers to reviewers
The authors are grateful to the reviewers for their comments and suggestions. They tried to incorporate all of them into the revised manuscript even if some of them would request a full restructuration which is not possible in few days. Nevertheless, some comments have been added to discuss related issues. The authors hope that these changes can meet the reviewers’ expectations, and that this study can be positively evaluated in the end. The revised manuscript is proposed with the track change mode, as requested. In the following, single details of the reviewers’ analyses are discussed.
Reviewer 1
- The authors talk about space applications. However, it is known that numerous requirements restrict the range of materials for space applications. Is the analyzed material suitable for this type of applications? This should be commented in the manuscript.
The authors are aware of all the restrictions for space use. At present, these materials are under consideration in research projects for space use founded by space agencies and Defense (mainly for debris capturing and solar sails). Moreover, those materials have been used in a study with NASA for exposing SMPCs in open space (on the MISSE platform of ISS). Results of these exposure tests are not yet published. In order to stress this concept, it has been also added: “Despite of numerous requirements which restrict the range of materials for space applications, SMPCs seem to comply them”.
- Lines 188-195, page 6: the authors report that shape fixity and shape recovery indices are not suitable for the analyzed composite. It is not clear which are the indices used to verify shape memory performances. These indices should be clearly stated, instead of generally saying "a new instrumented procedure has been defined".
There was a misunderstanding on the discussion about of the shape fixity and shape recovery indexes. These parameters are fundamental but not sufficient for designing structures as information about loads are absent. In order to highlight this point, the entire section has been reviewed in this way: “. In the case of SMPC, these two parameters are important, as well, but not sufficient to describe the full SM behaviour. First of all, due to the fibre contribution, 100% of Rf and Rr is not possible. Fibres do not show any SM property, and partially damage SM behaviour of the matrix, but fibres are added to increase material stiffness during transition, and to provide higher recovery loads. Recovery loads and speeds are important data for designing SMPC actuators or deploying structures. For this reason, a new instrumented procedure has been defined to extract load values during shape recovery”.
- line 214, page 7: the shape ratio is mentioned, but in the result and discussion sections the measured values are not clear.
There are 2 typing errors, the first time “shape ratio” was written instead of “shape fixity”, the second time instead of “shape recovery”. Sorry for these errors which have been corrected.
- Table 4: it would be appropriate to analyze the differences between the first and the last cycle (maybe with a proper index).
The composites behaved in very similar way during the 10 cycles apart from the first which was influenced by the different initial transient phase. Now it is also reported: “It is not possible to identify any degradation effect by comparing the first with the last loading peaks”.
- Please correct some language typos.
Several typo errors have been corrected in the revised manuscript.
Reviewer 2 Report
The manuscript presented for review describes the results of research on the properties of the thermally induced shape memory of epoxy laminate. The work is interesting but requires many additions before publishing. As it stands it is largely engineering rather than scientific work.
Introduction Part
The Introduction part is quite exhaustive, correctly introducing to the issue. Unfortunately, there is no short description of the causes and course of the phenomenon of thermally induced shape memory in polymer materials, as well as references to relevant pioneering works on such research. There is also a clear lack of literature references to works describing composites with shape memory, relatively new and significant, very similar in terms of topics to those described in the reviewed work, e.g. such as; ACS Appl. Mater. Interfaces 2020, 12, 58295 - 58300., Materials 2019, 12, 1107. https://doi.org/10.3390/ma12071107 and many more.
Materials and Methods
In scientific work, it is necessary to characterize the materials used in detail. And so, in the case of laminates, a detailed description of the prepregs used is necessary - the type and chemical structure of the resin used (polyester or epoxy resin, if so, the chain structure, the degree of filling of the composite - ratio polymer/fibres, the type of fibres, filler, the composition of the cross-linking system, etc.). Which means prepregs have very little shape memory??? Whether it is material before the final cross-linking, the thermomechanical parameters should be provided that determine the memory properties of the prepreg after its full cross-linking (post-curing).
The same comments apply to the characteristics of the epoxy resin used by the authors - not only the technical name, but also the chemical structure, degree of cross-linking, and the type of cross-linking system used. Why does this epoxy resin used have shape memory properties? Describe the cause of this phenomenon in this type of polymers, what is the chemical structure of the chains of this polymer, what is the importance of the degree of cross-linking of the resin on the shape memory properties?
SMPC test device
The applied procedure of heating the sample causing the shape recovery effect is unnecessarily complicated and hinders the interpretation of the obtained results. In this case, a testing machine with a thermostated heat chamber should be used. Based on the experiments carried out by the authors, due to the heat flow that is difficult to determine, the lack of a constant temperature during the experiments, the heterogeneity of the heat distribution, it is difficult to determine the basic thermal parameters of the process, such as minimal recovery temperature, temperature window, changes in mechanical properties (modulus) depending on temperature - which is of colossal importance in measuring a very important parameter - the force of return to the permanent shape. These parameters are sorely lacking.
I believe that at least some of the most important experiments should be performed under isothermal conditions to obtain important data characterizing the material described.
The SMP thermo-mechanical cycle
This part lacks a detailed description of the applied cold deformation procedure to the temporary shape (what is the ratio of the applied deformation stress to the breaking stress, was the deformation value - sample deflection arrow the same in all samples and what was it?)
The applied procedure, due to the variability of the mechanical parameters of the tested composite (changes in the structure of the laminate), influences the final properties to a different extent - after deformation. Therefore, it is difficult to compare the properties of these laminates in the next stages of shape memory research.
Data included in tables in many cases are difficult to analyze. It seems that at least some of them could be presented on the appropriate charts. All tables can be included in an additional parallelly published file; supported materials.
What stresses arise during the phenomenon of automatic return to permanent shape?
Conclusions
In summary, there lack general and practical conclusions from the research, describing the composition of the optimal composite and the way of its application, as well as the extended possibilities of its application related to the shape memory effect.
Author Response
Durability of Shape Memory Polymer Composite Laminates under Thermos-Mechanical Cycling
Answers to reviewers
The authors are grateful to the reviewers for their comments and suggestions. They tried to incorporate all of them into the revised manuscript even if some of them would request a full restructuration which is not possible in few days. Nevertheless, some comments have been added to discuss related issues. The authors hope that these changes can meet the reviewers’ expectations, and that this study can be positively evaluated in the end. The revised manuscript is proposed with the track change mode, as requested. In the following, single details of the reviewers’ analyses are discussed.
Reviewer 2
Introduction Part
The Introduction part is quite exhaustive, correctly introducing to the issue. Unfortunately, there is no short description of the causes and course of the phenomenon of thermally induced shape memory in polymer materials, as well as references to relevant pioneering works on such research. There is also a clear lack of literature references to works describing composites with shape memory, relatively new and significant, very similar in terms of topics to those described in the reviewed work, e.g. such as; ACS Appl. Mater. Interfaces 2020, 12, 58295 - 58300., Materials 2019, 12, 1107. https://doi.org/10.3390/ma12071107 and many more.
The introduction section has been modified to take into account this suggestion. The two papers are now mentioned as references and used to discuss the mechanism of the SM behavior. Initially, those contributions were not considered as the introduction focused on SMPC with long carbon fibers. Nor it is reported: “Shape memory polymer composites (SMPCs) and polymers (SMP) have been extensively studied in the last years, and many potential uses have been found in many fields covering aerospace engineering, biomedical devices, flexible electronics, soft robotics, shape memory arrays, and 4D printing [1]. Recent studies have shown the importance of molecular mobility on the definition of the SM behaviour as in the case of nanoparticle filling [2] or optimized cross-linking [2]. Carbon fibres (CFs) have an important impact on molecular mobility, as well, but their use is essential for space”.
Materials and Methods
In scientific work, it is necessary to characterize the materials used in detail. And so, in the case of laminates, a detailed description of the prepregs used is necessary - the type and chemical structure of the resin used (polyester or epoxy resin, if so, the chain structure, the degree of filling of the composite - ratio polymer/fibres, the type of fibres, filler, the composition of the cross-linking system, etc.). Which means prepregs have very little shape memory??? Whether it is material before the final cross-linking, the thermomechanical parameters should be provided that determine the memory properties of the prepreg after its full cross-linking (post-curing). The same comments apply to the characteristics of the epoxy resin used by the authors - not only the technical name, but also the chemical structure, degree of cross-linking, and the type of cross-linking system used. Why does this epoxy resin used have shape memory properties? Describe the cause of this phenomenon in this type of polymers, what is the chemical structure of the chains of this polymer, what is the importance of the degree of cross-linking of the resin on the shape memory properties?
Authors apologize because of few information provided for the adopted Aeronautical prepreg. Some data have been added in the revised manuscript according to the datasheet. The sentence on the SM behavior of prepreg has been revised as it was misleading. Now, the “thermoset matrix” of the prepreg “after curing” in mentioned. About the SM resin, several references have been added as it has been extensively studied in previous works.
SMPC test device
The applied procedure of heating the sample causing the shape recovery effect is unnecessarily complicated and hinders the interpretation of the obtained results. In this case, a testing machine with a thermostated heat chamber should be used. Based on the experiments carried out by the authors, due to the heat flow that is difficult to determine, the lack of a constant temperature during the experiments, the heterogeneity of the heat distribution, it is difficult to determine the basic thermal parameters of the process, such as minimal recovery temperature, temperature window, changes in mechanical properties (modulus) depending on temperature - which is of colossal importance in measuring a very important parameter - the force of return to the permanent shape. These parameters are sorely lacking. I believe that at least some of the most important experiments should be performed under isothermal conditions to obtain important data characterizing the material described.
This is a very important point which has been discussed in several studies, already published. Unfortunately, it was not clearly mentioned in the manuscript. Now, this point has been highlighted: “Conventional heating chambers cannot be used because of the long set-up time and low heating rate”.
The SMP thermo-mechanical cycle
This part lacks a detailed description of the applied cold deformation procedure to the temporary shape (what is the ratio of the applied deformation stress to the breaking stress, was the deformation value - sample deflection arrow the same in all samples and what was it?)
The tests were made in a range far from the failure in the optic of comparison in terms of applied strains. This point has been now clarified in the manuscript: “%). The idea was not comparing the SM behaviour in terms of maximum allowable stresses but in the same condition of maximum strain”.
The applied procedure, due to the variability of the mechanical parameters of the tested composite (changes in the structure of the laminate), influences the final properties to a different extent - after deformation. Therefore, it is difficult to compare the properties of these laminates in the next stages of shape memory research.
This important consideration has been inserted in the conclusion section: “Even if the effect of the laminate structure can influence the comparison of the SM behaviour, provided data are available for possible use in designing of this kind of structures”.
Data included in tables in many cases are difficult to analyze. It seems that at least some of them could be presented on the appropriate charts. All tables can be included in an additional parallelly published file; supported materials.
Unfortunately, graphic representation of SMP data is not effective. For this reason, average values have been chosen in some cases. Tables are used as a support for other studies. This point now is discussed in the manuscript: “Tables are preferred for data representation as trends are not always clearly visible”.
What stresses arise during the phenomenon of automatic return to permanent shape?
Stresses are experimentally measured and derive from polymer stretching. They are the same kind of frozen stresses in moulded polymers. This point is now discussed: “Stresses arise due to polymer stretching in the memory step”.
Conclusions
In summary, there lack general and practical conclusions from the research, describing the composition of the optimal composite and the way of its application, as well as the extended possibilities of its application related to the shape memory effect.
Thanks to these suggestions, the conclusion session has been partially reviewed.
Author Response File: 
Author Response.pdf
Round 2
Reviewer 2 Report
Most of my questions and comments have been explained by the authors of the manuscript. Appropriate additions to the text were introduced.
I believe that the manuscript in its current form is acceptable for publication.
