Segmented polyurethanes based on poly(l-lactide), poly(ethylene glycol) and poly(trimethylene carbonate): Physico-chemical properties and morphology
Graphical abstract
Introduction
Polyurethanes (PU) represent an important class of polymers characterized by a broad spectrum of compositions, molecular architectures, properties and applications (such as PU foams, footwear soles, adhesives, textiles, automotive parts and varnish). The first reports concerning polyurethanes synthesis, by Oto Bayer in 1937, consists of polyaddition of diisocyanates with low molar mass diols or diamines [1], [2]. Polyurethanes are typically characterized by the carbamate (NHCOO) or an urea (NHCONH) groups in the backbone as well as by other groups like ethers, esters and amides. Segmented polyurethanes (SPU) are usually obtained by the reaction of a polyfunctional isocyanate with a polyol, polyamine or others poly functional reactants in a one step reaction. A two step method consisting in pre-polymerization using non-stoichiometric ratios of isocyanates (A) and polyols (B) followed by chain growth using a low molecular mass extensor agent (C), is also applicable and allows greater control of architecture and composition [3]. The nature and proportions of components A, B and C allow the PU properties to be controlled. By choosing bifunctional components, non-crosslinked segmented polyurethanes (SPU) can be obtained.
The reaction mechanism between diisocyanates and alcohols is quite complex and dependent on the catalysts, reactants structures, solvents, etc. Therefore, generalizations must be avoided. However, the basis of urethanization relies on the high activity of the isocyanate group (-NCO); therefore, the nucleophilic hydroxyl group is added to the electrophilic carbonyl group with the simultaneous transfer of the proton to the nitrogen atom [3], [4].
Recently, there has been increasing interest in the use of uncrosslinked polyurethanes as substitutes for crosslinked ones because of their versatility. They can be used in designing materials with numerous applications, such as thermoplastic-elastomers, biomedical materials and coatings. SPUs based on polyester, polyether and polycarbonate blocks have been studied due to their mechanical properties and potential biomedical applications, with significant interest in research into hybrid or multiblock polyurethanes, which are capable of combining antagonistic properties of each block, such as soft and rigid segments, hydrophilic and hydrophobic moieties. Hyper-branched and linear SPUs based on poly(ethylene glycol), poly(propylene glycol) and poly(caprolactone) (PEG, PPG and PCL, respectively) were studied by Jun Li [5] and coworkers. The physical and chemical properties of these SPUs as well as their biocompatibility and biodegradability in hydrolytic conditions were dependent on both their compositions and architectures. Aqueous solutions of these polyurethanes undergo thermal-induced phase inversion (lower-critical solution temperature behavior) due to the amphiphilic characteristics achieved by the varying composition. Wagner [6] and coworkers synthesized elastomeric segmented polyurethanes based on PCL and poly(trimethylene carbonate), PTMC, whose mechanical and thermal properties are directly related to the composition; the PTMC blocks are responsible for the elastomeric behavior. Polyurethanes based on PLLA (poly(l-lactide)) and PCL triblock copolymers were investigated by Kenny [7]; these fully polyester-based SPUs showed synergic increases in the mechanical properties and thermal shape-memory behavior.
Although a wide range of precursors and their combinations have been applied in SPU synthesis, there are still no reports of SPUs based simultaneously on polyethers, polyesters and polycarbonates in special ternary SPUs based on PEG, PLLA and PTMC, while some binary systems have been reported [8], [9]. PEG is an uncharged and non-immunogenic hydrophilic polyether with a wide range of chemical, biomedical and industrial applications [10], [11]. The incorporation of PEG in copolymers increases their hydrophilicity and minimizes cell and protein adhesion when used in drug delivery systems [12], [13]. Therefore, PEG is the most common option for hydrophilic constituents in block copolymers and polyurethanes [14], [15]. Lactide derived polymers stand out among the most used hydrophobic polyesters for biomedical applications because of their biocompatibility and biodegradability [16]. Poly(lactide) presents different stereoisomers, i.e., PLLA, PDLA and PLDLA, derived from L, D or an L/D mixture. PTMC is obtained from the cyclic monomer trimethylene carbonate (1,3-dioxan-2-one) via ring opening polymerization, resulting in an amorphous polymer with a glass transition temperature around −20 °C [17]. Despite being biocompatible, its poor mechanical properties make PTMC unsuitable for most applications. Therefore, it is often modified or combined with other polymers, such as PEG, PLLA and PCL [18], [19], [20], [21].
Our interest is in the development of uncrosslinked multicomponents, segmented polyurethanes based on low molar mass PEG, PLLA and PTMC homopolymers in special ternary systems. To evaluate the relationship between the composition and properties of these SPUs, sixteen different compositions were synthesized, resulting in 3 monocomponent, 9 binary and 4 ternary SPUs. Investigating monocomponent and binary SPU properties allows for better descriptions of and justifications for the use of the three precursors. Our goal is to obtain ternary SPUs with a set of properties not found in binary systems.
Section snippets
Experimental methods
Homopolymer PEG, 2,4-toluene diisocyanate (2,4-TDI), 3,6-dimethyl-1,4-dioxan-2,5-dione (LLA), 1,4-butanediol (BDO) and the catalysts tin(II)-2-ethylhexanoate (SnOct2) and dibutyltin dilaureate (DBTDL) were purchased from Sigma Aldrich, Germany. Trimethylene carbonate (1,3-dioxan-2-one - TMC) was purchased from Boehringer Ingelheim, Germany. LLA and TMC were lyophilized prior to use. Toluene was dried by stirring with CaCl2, followed by filtration and distillation in the presence of metallic
Results and discussion
In general, an excess of diisocyanate and high temperatures are typically used for the synthesis of crosslinked polyurethanes due to the formation of allophanate, urea and amine groups. The dimerization of diisocyanates occurs at temperatures just higher than 43 °C, yielding yellow-colored uretidione structures (dissociation of the dimer is expected only at temperatures above 175 °C) [3], [24]. In this work, side reactions were avoided via rigorous control of the stoichiometry and by keeping the
Conclusions
High-molar-mass, uncrosslinked and segmented polyurethanes based on PEG, PLLA and PTMC were successfully synthesized via a two-step route. In this work, side reactions, such as diisocyanate dimerization and trimerization and allophanate formation, were prevented by rigorous control of the temperature (maintained at 40 °C) and the stoichiometry of the polymerization.
The reactivity of the hydroxyl groups in the macrodiols decrease in the order PEG ≈ PTMC > PLLA, leading to binary and ternary SPUs
Acknowledgments
The authors acknowledge FAPESP - Brazil (Process n° 2010/02098-0, 2010/17804-7 and 2011/09479-1) for financial support and the technical support from the Laboratory for Surface Science (LCS) (project AFM-NSIIIa-16677) at the Brazilian Nanotechnology National Laboratory (LNNano) in the Brazilian Center for Research in Energy and Materials (CNPEM).
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