Dear Stéphane Bruzaud, generally speaking, chain flexibility is the most deciding factor for good adhesion joint. This chain flexibility is the consequence of chain structure/architecture, which in turn define the spatial arrangement and morphology. Chain side groups influence both morphology and adhesion (depending on the mechanism). Side groups may or may not take part of the adhesion. Chain lenght (MW) has also it influence in that chain mobility is oppositly related to MW. My Regards

Dear Stéphane Bruzaud ,

as Abdelkader BOUAZIZ mentioned, chain mobility helps the adhesion. In crystalline parts, the polymer chains are packed dendely and not very mobile. In amorphous parts, the polymer chains are more mobile and the material can deform. You can take a polymer as polyethylene as an example: Above the melting temperature, the polymer obtains it's stability from the crystalline parts. Without the crystalline parts, it's just a polymer melt, because it's glass transition is < -100°C, and can adept any shape.

The ability to adapt to an external stress is related to the relaxation time and thus to the molecular weight, as Abdelkader BOUAZIZ , mentioned.

There are chemical and physical way to surpress crystallinity: Chemically you might introduce side chains, which hinder the polymer chain to form crystals (as an example HDPE has a higher degree of crystallinity than LDPE, which has more branches). Copolymerization with additional monomers may also influence the tacticity of the polymer and may decrease the crystallinity.

In some syntheses you can control the incorporation of monomers by the reaction conditions. If you conduct the polymerization comparably fast, the incorporation of the monomers is more random, which may influence tacticity and thus crystallinity.

A physical approach to reduce crystallinity may be quenching from the polymer melt. The side groups influence the kinetics of the crystallization (i.e. PE crystallizes in seconds whereas isotactic PS needs several ten minutes to crystallize) and if you cool fast enough, there is only a little amount of crystals or the crystals do not have time to grow a lot until the polymer chains are frozen in their position as the glass transition temperature is reached.

Thanks Matthias and Abdelkader!

Dear Prof. Bruzaud, Dear RG colleagues,

Following the great concern for biopolymers/i.e., PHAs, I found the topic and discussion of real interest, whereas I must agree with most of the previous comments.

I assume that following the tests performed in your laboratory was observed that generally (amorphous) PHAs have stronger adhesive (“stickiness”) properties compared to other biopolymers. Unfortunately, there are no details about the nature of PHAs (type of homopolymer, or copolymer), polarity of surfaces of contact that show stickiness /adhesion, type of processing, etc., therefore I will refrain from more comments.

“PHA is also the stickiest. Does anyone have a solution to remove this stickiness?”

I was involved in a scientific collaboration regarding the processing and modification of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx), a highly versatile PHA.

In the first compression molding tests performed to obtain thin or thicker films, PHBHHx showed a strong adhesion on the polyimide films/Kapton(NB: used on both sides of the rectangular mold to obtain films with surfaces of good quality). This was strange for us, considering that other biopolyester traditionally used in our laboratory (i.e., PLA, even amorphous grade) can be nicely processed using a similar procedure. Why are these differences, PLA vs PHA (PHBHHx) ? (differences in polarity/hydrophobicity/rates of crystallization, lower dynamic viscosity in the molten state in the case of PHAs or the presence of some degradation products, stronger compatibility/bounds with the surface of contact, and so on). Some experimental progress was obtained using PET films for slight /easier demolding, whereas in literature are few examples regarding the use of PTFE films (e.g., to process PHB by compression molding).

Lastly, films of good quality were finally obtained with the following procedure (doi: 10.1002/app.43808): To prepare films of 150 and 500 mm thickness, the samples of PHBHHx were compressed and molded at 140 °C using a hydraulic press. The polymer was preheated for 90 s without pressure, after which it was pressed at 30 bar for 150 s, followed by three degassing cycles. Finally, the films were pressed at high pressure of 150 bar for 120 s, followed by slow cooling at 50 bar for 20 min at 60 °C (on a second compression molding machine) to allow film crystallization and easy demolding. Maybe, a similar/adapted procedure could be tested in your laboratory. (NB: A higher temperature of crystallization can also be considered, or the additional post-crystallization in a drying/vacuum oven to allow the easier demolding.) This agrees with the one key conclusion of the discussion, i.e., a higher crystallinity is helpful in the decrease of the adhesion properties of PHAs.

However, between the other methods which could show interest (e.g., chemical modification), I will not exclude the use of some additives/modifiers such as nucleating agents, lubricants/slipping agents (12-hydroxystearate, erucamide, stearamide, EBS, etc.), plasticizers, fillers, or the blending with other polymers (e.g., PBAT), can be of interest to improve the processing of PHAs.

Finally, I will add that in some reviews is mentioned that the PHAs are “less ‘sticky’ than traditional polymers when melted” (DOI: 10.3144/expresspolymlett.2014.82; DOI: 10.1039/d1ra02390j ), maybe is the case of some homopolymers.

Good luck to your students in the R&D projects involving the processing of PHAs and my best regards to all of you,

Marius

For additional information:

Vandewijngaarden, J., Murariu, M., Dubois, P., Carleer, R., Yperman, J., D'Haen, J., Peeters, R. and Buntinx, M. (2016), Effect of ultrafine talc on crystallization and end-use properties of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate). J. Appl. Polym. Sci., 133, 43808. doi: 10.1002/app.43808

Vandewijngaarden, J., Wauters, R., Murariu, M. et al. Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)/Organomodified Montmorillonite Nanocomposites for Potential Food Packaging Applications. J Polym Environ 24, 104–118 (2016). https://doi.org/10.1007/s10924-016-0751-1

Poly(lactic acid) (PLA) and polyhydroxyalkanoates (PHAs), green alternatives to petroleum-based plastics: a review

DOI: 10.1039/D1RA02390J (Review Article) RSC Adv., 2021, 11, 17151-17196

Hazer, Baki , "Amphiphiles from Poly(3-hydroxyalkanoates)" , in The Handbook of Polyhydroxyalkanoates ed. Martin Koller (Boca Raton: CRC Press, 14 oct. 2020 ), accessed 02 nov. 2023 , Routledge Handbooks Online.

https://www.routledgehandbooks.com/pdf/doi/10.1201/9781003087663-4/1698917620971

Stéphane Bruzaud