Elsevier

Polymer Degradation and Stability

Volume 108, October 2014, Pages 175-181
Polymer Degradation and Stability

Biodegradable polyurethanes from crystalline prepolymers

https://doi.org/10.1016/j.polymdegradstab.2014.06.017Get rights and content

Abstract

Polyurethanes in the solid state are commonly used in many structural and medical applications. They are obviously obtained by the polyaddition reaction, where usually ethylene glycol is used as the chain extender. This paper deals with the fabrication of polyurethanes from crystalline prepolymers where water is used as a chain extender. Biodegradable polyurethanes were synthesized from: poly(ε-caprolactono)diol (PCL), 4,4′methylenebis(cyclohexyl) isocyanate (HMDI) and distilled water (w) as a chain extender. The prepolymer was crystallized at three different temperatures: 7, 22 and 30 °C, respectively. The materials were subjected to degradation in a solution of phosphate buffered saline (PBS) at the temperature of 37 °C, where the solution was changed every week. The results of the physical, mechanical and thermal properties, as well as sample's surface observations after the degradation are presented in the comparison to polyurethane obtained in a polyaddition reaction with ethylene glycol as a chain extender. Biodegradable polyurethanes obtained from crystalline prepolymers extended by water exhibit better mechanical properties and higher degradation rate in a solution of phosphate buffered saline (PBS) at the temperature of 37 °C than biodegradable polyurethanes obtained by the polyaddition reaction with the application of ethylene glycol as a chain extender.

Introduction

History of medicine shows that interest in using of synthetic materials in medicine is constantly growing due to an increasingly broad range of available materials, as well as the rapid development of modern surgical techniques.

Polymeric biomaterials, due to its properties, quickly replacing other materials, such as metals and theirs alloys or ceramics in medical applications. In 2003, sales of polymeric biomaterials exceeded 7 billion US dollars, which nearly reached 88% of all biomaterials sold this year [1].

Polymers used as biomaterials are characterized by comparable mechanical properties and degradation time required for a given application [2]. Ideal polymer used in medicine should exhibit the following properties [2], [3]: i) doesn't induce inflammation, toxic reactions and allergic diseases in the body, ii) enable readiness to obtain the final product, iii) be easy to sterilize, iv) doesn't change the properties after sterilization, v) to be biocompatible, vi) to be durable, functional and reliable.

Synthetic polymers used in medical applications can be divided into two groups [3]: (1) biostable (not degradable), (2) biodegradable and bioresorbable. Bioresorbable and biodegradable materials degrade in the environment biologically active to harmless products. However, the degradation products of bioresorbable polymers occur naturally in the body and take part of metabolic process. In contrary products formed in the biodegradable polymers degradation are not necessarily naturally occurring in the human body. In cases of biodegradable polymers, the degradation products are excreted from the body [3]. Among the biodegradable synthetic polymers, polyesters, polyamides, polyurethanes, polypeptides and polysaccharides should be distinguished. The most commonly used are aliphatic polyesters, such as poly(ε-caprolactone), polylactide and poly(butylene succinate) [4], [5].

Polyurethanes (PUs) are a group of polymers with the most versatile properties and the broadest range of industrial applications [6]. The first PUs were produced and examined by Dr. Otto Bayer in 1937 [7], [8]. Its chemical structure can comprise not only urethane groups but many others such as: ether, ester, urea, allophanate, biuret, carbodiimide, hydrocarbon aromatic rings, ionic groups, etc. It is mainly driven by substrates used, additives and manufacturing technology (synthesis conditions) [9].

The main substrates for the preparation of polyurethanes are polyol, isocyanate and chain extender [7], [10], [11]. Polyol creates in the polyurethane so-called flexible segment while the isocyanate and chain extender – rigid segment [7], [12], [13]. On the border of the flexible and rigid segments urethane group are formed by the reaction of –NCO groups (from isocyanate) and –OH group (from polyol). In the reaction of the isocyanate with the chain extender a urethane or urea groups can be formed. Usually, the urethane groups are formed when the chain extender terminated with –OH groups is used (e.g., glycols, glycerol) and the urea groups are created by using the chain extender terminated with –NH2 groups (e.g., dicyandiamide) [7], [10]. Also in the reaction of –NCO groups with water the urea group is formed and CO2 as the reaction by-product. Depending on the type and the molar ratio of substrates materials with different properties can be obtained [14].

Most of the bulk polyurethanes are synthesized by polyaddition reaction, while polyurethane foams are produced by the polycondensation reaction in which, in the addition to the final product, the low molecular by-product is formed (most commonly carbon dioxide – CO2).

A team from the Faculty of Materials Science and Engineering of Warsaw University of Technology developed a method for the fabrication of solid polyurethanes in the bulk using a polycondensation reaction of crystalline prepolymers with water [15]. It is commonly known that usually carbon dioxide, which is formed during the reaction is accumulated in the reactive mixture of substrates resulting in the formation of discontinuities in the material, which due to the high viscosity of mixture are impossible to eliminate [16], [17]. This can be avoided by reacting with water the prepolymer in its crystalline form. Polycondensation of crystalline prepolymers with water, as distinct from traditional polycondensation lies in the fact that the prepolymer is reacted with water after crystallization at the given temperature [17], [19]. The prepolymer is formed in the reaction of an isocyanate with an oligodiol. Polyurethanes obtained from the crystalline prepolymers extended by water are characterized by the spherulitic structure with different morphology and properties dependently on the crystallization temperature of the prepolymer [18], [19]. The higher the crystallization temperature of the prepolymer the higher is the Young's modulus and wear resistance of the polyurethane [20].

This paper describes polyurethanes prepared from crystalline prepolymers based on substrates used for biomedical applications. Biodegradable properties of developed materials were tested and presented in this paper.

Section snippets

Materials

For the preparation of biodegradable polyurethanes, the following substrates were used:

  • poly(ε-caprolactono)diol (PCL) with molecular weight of Mn = 2000 g/mol, supplied by Sigma–Aldrich.

  • 4,4′-methylenebis(cyclohexyl)isocyanate (H12MDI), supplied by Sigma–Aldrich.

  • chain extenders:

    • distilled water (w)

    • ethylene glycol (EG), supplied by POCH Gliwice,

  • dibutyltin dilaureate, supplied by Sigma–Aldrich, used as a catalyst for accelerating the reaction of isocyanate with hydroxyl groups.

Synthesis of polyurethanes

The molar ratio of H12

Results and discussion

The results of density measurements, as well as mechanical properties obtained under the static loading of biodegradable polyurethanes are shown in Table 2.

The results shown in Table 2 proved that the polyurethane obtained from crystalline prepolymers extended by water (PCL/w7, PCL/w22 and PCL/w30), before their amorphization, are characterized by comparable Young's Modulus E and the elongation at break εr to the polyurethanes extended with ethylene glycol (PCL/EG). However, they exhibit higher

Conclusions

Polyurethanes made of crystalline prepolymers extended by water are characterized by interesting properties with the perspective of their application in medicine. Biodegradable polyurethanes from crystalline prepolymers extended by water produced in this work are competitive for materials obtained by polyaddition reaction with ethylene glycol.

The mechanical properties of biodegradable polyurethanes obtained from crystalline prepolymers in the reaction with water are higher in comparison to the

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