Competition between crystal growth and intracrystalline chain diffusion determines the lamellar thickness in semicrystalline polymers

Published in Materials
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Idea and motivation

Due to the massive increase in the usage of plastics in the last decades, fundamental research in polymer science is essential to develop more efficient and more sustainable polymeric materials. The mechanical properties of a polymer and with that the utilizability is largely defined by the morphology. The exact formation mechanism of the semi-crystalline structure in polymers has been the subject of research for decades. Various theories and approaches exist, but without providing a unified description of this complex process. Therefore, we started to re-investigate this question from a phenomenological perspective to develop a general classification for different polymer systems. For this purpose, we used a variety of complementary experimental methods and our experimental results add up to a consistent picture of polymer crystallization. Here, we would like to refer you to our publication and focus on the following -what we think- is the exceptionality in our work

Cooperation and multi method-approach

The road to success was for us clearly a strong collaborative work of two Ph.D. students, which are affiliated in different research groups but study strongly related topics. Within the framework of the SFB TRR 102, a long-term Transregional Collaborative Research Centre, this collaborative research was enabled and lasted already for nearly a decade. The third generation of Ph.D. students continues our investigation of polymer crystallization by utilizing the developed multi-method approach used in this publication:

With Nuclear Magnetic Resonance (NMR) it is possible to measure the timescale of the defect movement within the crystal, while polarisation microscopy yields the timescale of the lamellar growth. Setting these timescales into relation we developed our classification scheme and could prove the influence on the semicrystalline morphology directly with small angle X-ray scattering (SAXS). We achieved a systematic comparison for three polymer systems; Polycaprolactone (PCL), Polyoxymethylene (POM), and polyethylene oxide (PEO) with non-existent, moderate, and fast Intracrystalline Chain Diffusion (ICD). 

The results of only one method without the information of the others would have only given us an incomplete picture of the studied polymer systems. Thus, a productive teamwork of us (two Ph.D. students and the respective professors) was necessary. This included agreements on the experimental procedure and data analysis, which had to be suitable for all applied methods. This was especially important in the case of POM, which turned out to be a rather complex sample. The strong degradation in the ambient atmosphere at elevated temperatures required a well-defined procedure for all experiments. Otherwise, a comparison of the obtained results would not be possible. Additionally, we applied several other techniques (DSC, FSC, Rheology, and TGA) as control experiments but many of the results are not shown in the final paper.

Ups and Downs

Our real Ups and Downs started, when the scientific work was already completed. Besides methodic adaptations and sample problems, it turned out that especially the publication process took nerves and time.  First, we tried to publish our work in a lower-rated journal than Nature Communications. We got 5 reviews, rating our work quite differently. From there a stressful review process followed, where we had the impression that the importance of our work was either not understood or the publication of our results was deliberately hindered/delayed. We were confronted with the decision: How much do we believe in our work and how much effort do we want to spend? Well, the result can be seen now: luckily, we decided to go one step higher and decided for Nature Communications, where we experienced a fair and in fact very constructive review process.

To sum up:

First of all, we think that such strategic long-term cooperation between different groups, especially in the form of two Ph.D.-students working in parallel is quite efficient. Independent of the concrete project we could observe, that the cooperation was widening and the groups started to interact more and more – which was and still is productive and extended even to other research topics. Additionally, the same starting time of our Ph.D. enabled us to grow and develop together and to motivate and support each other, which was – from our personal perspective – quite helpful.

Also, from a scientific perspective, our collaboration was very fruitful. We were able to describe the characteristic morphologies for crystal-fixed and crystal-mobile polymers with a well-defined crystalline thickness for polymers with no or very slow ICD and well-defined amorphous thickness for crystal-mobile systems. In the latter case, the timescale of the ICD has a strong influence on the final semicrystalline structure.

Mareen Schäfer & Martha Schulz

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