Year: 2014
Type: dissertation
Abstract: Le poly(3-hydroxybutyrate), ou PHB, est un polymere biodegradable synthetise par voie bacterienne. L'interet porte a ce polymere biodegradable est motive par ses proprietes mecaniques, qui sont fort similaires a celles du polypropylene, un polymere non-biodegradable avec plusieurs applications commerciales. Actuellement, la substitution du polypropylene par du PHB est limitee par les proprietes physiques (?) de ce dernier qui le rendent difficile a traiter. Ces limitations sont liees au fait que le PHB est presque isotactique, ce qui signifie que sa cristallisation est fort favorisee. Cette cristallinite elevee rend le polymere rigide mais fragile, et augmente son point de fusion a 170oC. Par comparaison, le point de fusion de poly(4-hydroxybutyrate) est de 53oC. Ce point de fusion eleve complique le traitement du PHB, qui commence a se degrader spontanement au-dessus de 170oC, c'est-a-dire des qu’il est fondu. Etant donne que le polymere doit etre fondu afin d'etre melange ou moulu, il est impossible d’eviter sa degradation. Dans cette etude, le PHB est melange avec differents produits chimiques concus specialement pour augmenter le poids moleculaire des polyesters tels que le PHB. Ces produits chimiques sont appeles « chain extender », ou « allongeur de chaine », car ils reagissent avec les embouts des chaines polymeriques pour produire une chaine allongee. Quatre allongeurs de chaine (le Joncryl ® ADR 4368-C, le dianhydride pyromellitique, le diisocyanate d’hexamethylene, et le polycarbodiimide) ont ete melanges avec un copolymere aleatoire de PHB (98 mol%) et de poly(3-hydroxyvalerate) (2 mol%). La concentration d’allongeur de chaine dans le polymere a ete variee entre 0.25% et 4% massique. L’evolution du poids moleculaire du polymere resultant de ces differents melanges est deduite de la viscosite complexe, determinee par rheologie a plan parallele. Il a ete observe qu'aucun des allongeurs n'a permis de reduire le taux de degradation du PHB, et que seul le Joncryl a entraine une augmentation du poids moleculaire du polymere. Le PHB a egalement ete melange avec du poly(L-acide lactique), ou PLA, pour des rapports PHB/PLA de 100/0, 75/25, 50/50, 25/75, et 0/100. Comme precedemment, la stabilite thermique a ete deduite de la viscosite complexe. Lors de ces experiences, il a ete observe que les melanges riches en PHB (100% PHB/0% PLA, 75% PHB/25% PLA) ont continue a se degrader au meme taux que le PHB pur. Le melange 50% PHB/50% PLA, quant a lui, a presente un gain initial de viscosite complexe, mais a neanmoins continue a se degrader au meme taux que le PHB pur. Par contre, la viscosite complexe du melange de 25% PHB/75% PLA etait egale a celle du PLA pur. Ces cinq melangesviii
ont ensuite ete reproduits mais en presence de 1% massique de Joncryl dans le but d'etudier l’effet de l’allongement des chaines sur les caracteristiques du polymere. Il en a resulte que les melanges 50% PHB/50% PLA, 25% PHB/75% PLA et 100% PLA presentaient une augmentation de viscosite complexe de plus de dix fois celle de leur homologue sans Joncryl. Cependant, le melange 75% PHB/25% PLA n’a presente une augmentation de sq viscosite complexe que de deux fois celle du melange sans Joncryl. L’effet du Joncryl sur les proprietes mecaniques des melanges de PHB et PLA a aussi ete etudie par essai de traction uniaxial. Les resultats ont indique une augmentation de la contrainte au seuil d’ecoulement et d’allongement a la rupture avec l'ajout de Joncryl, mais une diminution du module de Young. En conclusion, l'ajout d'allongeurs de chaines n’a qu’un effet marginal sur la degradation du PHB.
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Poly(3-hydroxybutyrate), referred to as PHB, is a bacterially-synthesized and biodegradable polymer which is being considered as a substitute for non-biodegradable bulk polymers like polypropylene. PHB is naturally extremely isotactic and naturally has a very high degree of crystallinity, resulting in a stiff but brittle material. The stability of PHB crystals also means that the melting point of the polymer is approximately 170oC, high with respect to similar polymers. For instance, the melting point of poly(4-hydroxybutyrate) is only 53oC (Saito, Nakamura, Hiramitsu, & Doi, 1996). Above 170oC, PHB is subject to a thermomechanical degradation mechanism, meaning that the polymer cannot be melted without degrading. One possible solution to the problem of degradation is to add a chain extender to the molten polymer to increase average molecular weight to counteract the molecular weight lost to degradation. In this work, a variety of chain extenders (Joncryl® ADR 4368-C, pyromellitic dianhydride, hexamethylene diisocyanate, polycarbodiimide) were compounded with a random copolymer of 98 mol% 3-hydroxybutyrate and 2 mol% 3-hydroxyvalerate (referred to as PHB) in concentrations ranging from 0.25% to 4%, to determine which chain extender functionality worked best with PHB. Molecular weight change was inferred from torque monitored during compounding, and from complex viscosity determined from parallel-plate rheology. None of the chain extenders changed the rate of degradation of PHB, although Joncryl increased the complex viscosity of the polymer. PHB was also blended with Poly(L-lactic acid), referred to as PLLA in PHB/PLLA ratios of 100/0, 75/25, 50/50, 25/75 and 0/100, to determine the effect of blending on the thermal stability of PHB. Again, thermal stability was determined by monitoring torque during compounding and by measuring complex viscosity through parallel-plate rheology. Blends in which PHB was the more abundant phase, as well as the 50% PHB/50% PLA blend continued to degrade, and the PLLA did not in these cases significantly increase complex viscosity. By contrast, the 25/75 PHB/PLLA blend had a complex viscosity equal to the neat PLLA blend, and both of the blends remained stable. All five blends were also produced with 1% Joncryl to observe the effect of Joncryl on the blends. In the 50/50 blend and the blends in which PLLA was the major component, complex viscosity increased by at least an order of magnitude, while in the 75/25 PHB/PLLA blend and the neat PHB blend, the effect of Joncryl was to increase complex viscosity only by a factor of 2. The effect of blending and of Joncryl on PHB-PLA blends was further investigated through uniaxial tensile stress testing of compression-
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moulded samples of the blends, neat and with 1% Joncryl. The results showed an increase in tensile stress at yield and tensile strain at break for blends with the addition of Joncryl, although Young’s modulus was somewhat diminished for these blends. In conclusion, chain extenders were not effective in reversing the effect of thermomechanical degradation, possibly because they do not change the resistance to bond rotation in PHB chains, or because they are not reactive with acrylates, although the exact cause has not been determined.
Author: Glenn Bousfield
Language: English
Subfield: Biomaterials
Field: Materials Science
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W2154300066