Subscriber access provided by RYERSON UNIV
Article
Robust and biodegradable elastomers based on corn starch and polydimethylsiloxane (PDMS) Luca Ceseracciu, José Alejandro Heredia-Guerrero, Silvia Dante, Athanassia Athanassiou, and Ilker S. Bayer ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/am508515z • Publication Date (Web): 26 Jan 2015 Downloaded from http://pubs.acs.org on February 2, 2015
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 30
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Materials & Interfaces
Robust and biodegradable elastomers based on corn starch and polydimethylsiloxane (PDMS) Luca Ceseracciu†, José Alejandro Heredia-Guerrero†, Silvia Dante‡, and Athanassia Athanassiou†, Ilker S. Bayer†,* †
Smart Materials, Istituto Italiano di Tecnologia, Via Morego 30, 16163, Genova, Italy
‡
Nanophysics, Istituto Italiano di Tecnologia, Via Morego 30, 1663, Genova, Italy
Abstract Designing starch-based biopolymers and biodegradable composites with durable mechanical properties and good resistance to water is still a challenging task. Although thermoplastic (destructured) starch has been an emerging alternative to petroleum based polymers, its poor dimensional stability under humid and dry conditions extensively hinders its use as a biopolymer of choice in many applications. Unmodified starch granules, on the other hand, suffer from incompatibility, poor dispersion and phase separation issues when compounded into other thermoplastics once the concentration levels exceed 5%. Herein, we present a facile biodegradable elastomer preparation method by incorporating large amounts of unmodified corn starch, exceeding 80% by volume, in acetoxy- polyorganosiloxane thermosets to produce mechanically robust and hydrophobic bio-elastomers. The naturally adsorbed moisture on starch surface enables auto-catalytic rapid hydrolysis of the polyorganosiloxane forming Si‒O‒Si networks. Depending on the amount of starch granules, the mechanical properties of the bioelastomers can be easily tuned with high elastic recovery rates. Moreover, starch granules lowered the surface friction coefficient of the polyorganosiloxane network considerably. Stress relaxation measurements indicated that the bio-elastomers have lower strain energy dissipation factors than conventional rubbers rendering them promising green substitutes for plastic mechanical energy dampeners. The corn starch granules also have excellent compatibility with addition-cure polysiloxane chemistry that is used extensively in micro-fabrication. Regardless of the starch concentrations, all the developed bio-elastomers have hydrophobic surfaces with low friction coefficient and much less water uptake capacity than thermoplastic starch. The bioelastomers are biocompatible and are estimated to biodegrade in the Mediterranean Seawater within three to six years. Keywords: Starch, Polydimethylsiloxane, Biodegradable elastomer, biopolymer
ACS Paragon Plus Environment
ACS Applied Materials & Interfaces
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 30
Introduction Starch is a highly abundant and renewable agricultural resource whose industrial production exceeds 7 million tons a year in Europe only1. Half of this production is used for non-food applications such as paper sizing, adhesives, biofuels and bioplastics2. The molecular structure of starch consists of a combination of linear macromolecules known as amylose and branched macromolecular chains known as amylopectin. Starch granule is a continuous structure that is made up of regions of amorphous and highly and moderately crystalline zones. The crystalline area is formed by linear fractions of amylopectin, whereas branch points and amylose are the main components of the amorphous zones3,4. Starch-based biopolymers are among the most widely studied biomaterials5. The most common form of starch in biopolymer technology is known as thermoplastic starch (TPS) or destructured starch6. To obtain TPS, starch can be destructured by gelatinizing in hot water that can be cast into films upon drying. Alternatively, its original structure can be disrupted by heating and shearing simultaneously resulting in a thermoplastic melt. This can be accomplished with conventional extruders (or similar equipment) used for processing thermoplastics7. Unfortunately, TPS is highly susceptible to dimensional instability under both low and high humidity environments. Plasticization with various additives such as glycerol is very common but plasticized TPS has still very low resistance to water. In a typical plasticized TPS model system, the diffusion of plasticizer out of TPS when exposed to low humidity conditions and diffusion of water into the product under high humidity conditions is inevitable and results in poor stability. This causes brittleness due to loss of plasticizer under low humidity environment, and shape and texture retention problems due to excess absorbed water under high humidity environment8. In order to circumvent this problem, starch is generally destructured in the presence of other biodegradable polymers such as vinyl alcohol copolymers, poly(lactic acid)s, aliphatic polyesters and cellulose derivatives to name a few9-11. Particularly, thermoformed blends of TPS with biodegradable polyesters such as polycaprolactone and poly(lactic acid)s are highly popular and some of them are already commercialized12. Use of compatibilizers, however, is still needed in order to enhance TPS dispersion and to minimize interfacial failure13. Unmodified (granular) starch on the other hand, has been proposed as sustainable filler material for petroleum based polymers14. However, use of unmodified starch as a component for polymer compounding is quite challenging due to its large particle size (~ 20 µm), inherent hydrophilicity and moisture uptake. Griffin15 was the first to introduce biodegradability to 2 ACS Paragon Plus Environment
Page 3 of 30
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Materials & Interfaces
synthetic commodity polymers such as polyethylene by adding 7-20 % granular corn starch without destructuring but by modifying the starch surface with a silane coupling agent. The main problem associated with introducing unmodified starch into hydrophobic non-degrading polymers, such as low density polyethylene, is the formation of water bubbles within the polymer matrix degrading the composite mechanical properties considerably once the starch concentration levels exceed 15%16. These bubbles originate from naturally adsorbed moisture in starch. Bubbles can form during extrusion while melting of the polymer matrix or after the formation of the composite during storage or use. Therefore, blending starch granules with hydrophobic petroleum thermoplastics generally requires complete particle drying and surface functionalization or modification in the form of grafting or coupling agents15,16. A very recent work by Kim and Peterson17 compounded ethyl cyanoacrylate (ECA) monomer with unmodified starch granules by up to 60 % and molded them into various highly rigid shapes. The natural moisture on the surface of starch initiated the polymerization of ECA monomer into poly(ethyl cyanoacrylate). The mechanical strength of the biodegradable composites was comparable to non-degrading petroleum based polymers such as polypropylene, polyethylene and polystyrene17. In this study, we report fabrication and characterization of biodegradable elastomers (bioelastomers) by incorporating large amounts of unmodified corn starch granules into polyorganosiloxane matrices crosslinking either by condensation (acetoxy cure) or by addition (platinum cure) in the presence of starch. The cross-linked polyorganosiloxane networks are known to biodegrade in soil, through abiotic hydrolysis to monomeric dimethylsilanediols18-20 and via other mechanisms such as microbiotic degradation and volatilization21,22 into CO2 and silica. Polydimethylsiloxane (acetoxy-PDMS) networks formed in this way are known as room temperature vulcanizing silicone elastomers that are used in a wide range of applications from electrical insulation to medical prosthetics and plastic surgery23. Fabrication requires no complicated equipment or tools and takes place under ambient conditions. The bio-elastomers demonstrate high hydrophobicity, low friction coefficient compared to standard silicone rubbers, excellent cell biocompatibility and very good mechanical energy damping properties. Bioelastomers are also shown to start biodegradation in the Mediterranean Seawater, which was chosen as a model environment.
3 ACS Paragon Plus Environment
ACS Applied Materials & Interfaces
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 30
Materials and Methods Unmodified regular corn starch containing approximately 73% amylopectin and 27% amylose and reagent grade heptane were purchased from Sigma Aldrich and used as received. Acetoxypolysiloxane (Acetoxy-PDMS; Elastosil E43) was purchased from Wacker Chemie AG and used as received. It is a proprietary mixture of hydroxyl end-blocked polydimethylsilxoane, also known as hydroxyl terminated polydimethylsiloxane (PDMS), triacetoxy(methyl)silane (
