High-Oxygen Barrier Materials Based on Hyperbranched Aliphatic

Chapter 2 High-Oxygen Barrier Materials Based on Hyperbranched Aliphatic Polyesters

Downloaded by STANFORD UNIV GREEN LIBR on July 30, 2012 | http://pubs.acs.org Publication Date: January 1, 2009 | doi: 10.1021/bk-2009-1004.ch002

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Jason D. Pratt , Brian G. Olson , Justin P. Brandt , Mohammad K. Hassan , Jo Ann Ratto , Jeffrey S. Wiggins , James W. Rawlins , and Sergei Nazarenko 1

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School of Polymers and High Performance Materials, Department of Polymer Science, The University of Southern Mississippi, Box 10076, Hattiesburg, MS 39406 U.S. Army Natick Research, Development and Engineering Center, Combat Feeding Directorate, Kansas Street, Natick, MA 01760 2

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This work explored commercially available Boltorn dendritic hydroxylated aliphatic esters to provide a new material platform for development of high oxygen barrier biodegradable films and coatings. Improved mechanical behavior was achieved via covalent linking of dendritic molecules using 1,6-hexamethylene diisocyanate. This report encompasses the information on structure, thermal, mechanical, oxygen barrier, and biodegradation behavior of pure and network systems.

© 2009 A m e r i c a n Chemical Society

In Polymer Degradation and Performance; Celina, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.

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Introduction


The recognized lack o f biodegradability o f most commercial polymers, in particular those used for food packaging, focused public attention on the potential global environmental problem associated with plastic waste build-up in the countryside and on the seashore. Therefore, there is a growing demand for sea-biodegradable polymers. Biodegradability is promoted by enzymes and it is generally accepted that only some hetero-chain polymers, i.e., aliphatic polyesters are truly biodegradable, although in practice the bio-assimilation step normally proceeds first v i a abiotic hydrolysis which results in monomeric and oligomeric products that are more readily consumed by microorganisms in the environment (1-3). W h i l e most commercially available biodegradable synthetic polyesters used for food packaging applications today, for instance poly(lactic acid) ( P L A ) , polycaprolactone ( P C L ) , poly(3-hydroxybutyrate-co-3-hydroxyvalerate) ( P H B V ) etc., have reasonably good water barrier, they unfortunately exhibit a mediocre oxygen barrier which is not high enough to satisfy the current market for packaging o f oxygen-sensitive food (4, 5). The oxygen permeability o f these biodegradable polyesters is somewhat comparable or in most instances higher than that exhibited by poly(ethylene terephthalate) ( P E T ) . A search for new biodegradable aliphatic esters with more advanced properties, in particular oxygen barrier, is ongoing. Dendritic macromolecules exhibit compact globular structures w h i c h lead to their low viscosity in the melt or in solution. Furthermore, dendritic macromolecules are characterized by a very large number o f available functional groups, which lead to unprecedented freedom for changing/tuning/tailoring the properties o f these multivalent scaffolds via complete or partial derivatization with other chemical moieties. A l l these features have contributed to multidisciplinary applications o f these unique macromolecular structures in recent years (6, 7). The development o f efficient synthetic routes in recent years has given rise to a virtually unlimited supply o f commercially available dendritic polymers, at very affordable price. The transport properties o f hyperbranched and dendritic polymers have recently attracted attention as potentially new barrier and membrane materials (8-9). Commercially available hyperbranched aliphatic polyesters ( B o l t o r n ™ polyols) attracted our attention as potentially very interesting candidates for the aforementioned applications because o f their degradable aliphatic ester nature and the presence in the structure o f multiple hydroxyl groups which is an important attribute for the polymer to exhibit high oxygen barrier due to hydrogen bonding. The pseudo-one-step, divergent synthesis o f these aliphaticester dendritic polymers, first described by M a l m s t r o m et al. in 1995, involves sequential addition o f monomer during synthesis; each addition corresponds to the stoichiometric amount o f the next generation (10).


19 Although characterized by imperfect branching and significant polydispersity, these polymeric structures preserve the essential features o f dendrimers, namely, high end-group functionality and a globular architecture. A n imperfect dendrimer structure ( 4 generation) o f hydroxylated polyester based on 2,2-bis-methylopropionic acid ( b i s - M P A ) with an ethoxylated pentaerytriol (PP50) core is shown in Figure 1. Fourth generation B o l t o r n ™ polyols contain 64 hydroxyl groups per molecule. Imperfect branching, which leads to incorporation o f linear b i s - M P A units, naturally introduces hydroxyl groups not only in the periphery as can be expected in the case o f ideal dendrimers but also within the dendritic shell structure.


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In contrast to linear polymers, the lack o f chain entanglements between dendritic units and the overall globular architecture make dendrimers fairly weak when in the bulk. Thus to make materials based on dendrimers flexible and mechanically strong, the dendritic units must be covalently linked to form a network as shown in the insert in Figure 1. Considering the polyol nature o f B o l t o r n ™ polyesters, using diisocyantes for linking dendritic molecules together was a natural step, as it is well known that isocyanates undergo rapid reactions with compounds containing active hydrogen.

Experimental Materials n d

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Hydroxyl-functional dendritic (hyperbranched) polyestera o f 2 and 4 generation, Boltorn® H 4 0 were obtained from Perstorp Specialty Chemicals A B , Sweden, in the form o f pellets. A b o u t 100 um thick films were prepared v i a compression m o l d i n g at 130°C in a Carver press followed by cooling under pressure by flowing water through the press platens. Prior to molding, as received pellets were dried in vacuum at 5 5 ° C for at least 24 hrs.

B o l t o r n ™ H 4 0 dendritic molecules were covalently linked in this work to make a network with aliphatic 1,6-hexamethylene diisocyanate ( H D I ) . The molar N C O / O H ratio was varied for the reactants from 10 to 5 0 % to prepare networks with different degrees o f connectivity o f dendritic units. The network samples are designated as H 4 0 / Z where Z stands for N C O / O H ratio expressed as a percentage. The network formation reaction o f H 4 0 with H D I was carried out in N,N-dimethylformamide ( D M F ) o f 99.8% purity at 90°C. N o catalyst was added. Isocyanates react readily with moisture to form urea linkages, therefore special precautionary measures were implemented to prevent moisture uptake either by H D I or D M F . Observation o f the reaction vessel was maintained over the course o f the reaction to monitor the viscosity o f the solution. A s viscosity o f the solution increased to the desired level, suggesting that the gelation point was near, the solution was cast onto a glass plate which was immediately placed



HO HO

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VV«

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H 0 ^ V OH Q

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>V -HNOCO OH

H0

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Figure 1. Schematics of Boltorn™ hyperbranched aliphatic esters. Insert shows the network formed by covalent linking of hyperbranched molecules with diisocyanate.

H0ln

OH

H (

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High-Oxygen Barrier Materials Based on Hyperbranched Aliphatic

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