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Polymer Degradation and Stability 96 (2011) 1430e1437

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Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab

Influence of electron beam irradiation on physicochemical properties of poly(trimethylene carbonate) Joanna Jozwiakowska a, Radoslaw A. Wach a, *, Bozena Rokita a, Piotr Ulanski a, Sameer P. Nalawade b, Dirk W. Grijpma b, c, Jan Feijen b, Janusz M. Rosiak a a

Institute of Applied Radiation Chemistry, Technical University of Lodz, Wroblewskiego 15, 93-590 Lodz, Poland Institute for Biomedical Technology (BMTI), Department of Polymer Chemistry and Biomaterials, Faculty of Science and Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands c Department of Biomedical Engineering, University Medical Center Groningen, University of Groningen, P.O. Box 196, 9700 AD Groningen, The Netherlands b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 April 2011 Accepted 11 May 2011 Available online 19 May 2011

Electron beam (EB) irradiation of poly(trimethylene carbonate) (PTMC), an amorphous, biodegradable polymer used in the field of biomaterials, results in predominant cross-linking and finally in the formation of gel fraction, thus enabling modification of physicochemical properties of this material without significant changes in its chemical structure. PTMC films (Mw: 167e553 kg mol1) were irradiated with different doses using an electron accelerator. Irradiation with a standard sterilization dose of 25 kGy caused neither significant changes in the chemical composition of the polymer nor significant deterioration of its mechanical properties. Changes in viscosity-, number-, weight-, and z-average molecular weights of PTMC for doses lower than the gelation dose (Dg) as well as gelesol analysis and swelling tests for doses above Dg indicate domination of cross-linking over degradation. EB irradiation can be considered as an effective tool for increasing the average molecular weight of PTMC and sterilization of PTMC-based biomaterials. Ó 2011 Elsevier Ltd. All rights reserved.

Keywords: Degradation Crosslinking Electron beam irradiation Molecular weight changes Poly(trimethylene carbonate)

1. Introduction For decades polymers have been used in medicine, not only for the production of disposable medical products but also as biomaterials and materials for biomedical devices. Biodegradable polymers have several advantages for biomedical applications because they do not have to be removed after implantation. Ideally, the materials are fully degradable in the body and degrade into natural metabolites under physiological conditions by simple hydrolysis or by the action of enzymes. After serving its function the remaining material and degradation products should not cause severe inflammatory or toxic reactions. Beside the most common biodegradable polymers widely applied as biomaterials, poly(lactic acid) (PLA) [1], poly(glycolic acid) (PGA) and their copolymers there is increasing interest in other synthetic biodegradable polymers, including poly(trimethylene carbonate) (PTMC) [2e5]. PTMC belongs to the family of poly(alkylene carbonate)s and is prepared from the cyclic monomer trimethylene carbonate [5]. It is a linear, amorphous polymer with a glass transition temperature of approximately 15  C. At room temperature it is a rubber-like, flexible material. PTMC has been investigated for potential biomedical applications, * Corresponding author. Tel.: þ48 426313164; fax: þ48 426840043. E-mail address: [email protected] (R.A. Wach). 0141-3910/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2011.05.010

for instance in soft tissue engineering [2], as a system for nerve regeneration [3] or in drug delivery systems [4]. Typical synthetic methods yield PTMC of relatively low molecular weight, which in turn results in materials with poor mechanical properties. Therefore trimethylene carbonate is often copolymerized with other monomers like 3-caprolactone or lactic acid, a process resulting in biodegradable polymers of higher strength, sufficient for applications such as soft tissue engineering [2,3]. On the other hand, an increase in molecular weight improves the mechanical properties of the polymer, thus PTMCs with molecular weights higher than 300 kg mol1 possess mechanical properties that allow their utilization in several applications [6]. This triggered further efforts to synthesize PTMC of high molecular weight. Biomaterials have to be sterile at the time of implantation to eliminate the risks of infection, and the chosen method of sterilization should not deteriorate the properties of the biomedical material or a device. Ionizing radiation is being used by the medical industry for the sterilization of devices comprising polymer components. This method is very efficient, reliable and relatively inexpensive. While high energy radiation, like gamma rays or electron beam, effectively destroys microorganisms and viruses, it may also affect the properties of the sterilized polymeric material. Irradiation may either improve the properties (e.g. in the sense of reinforcing the

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Table 1 Intrinsic viscosities, average molecular weights, polydispersities PD and glass transition temperatures Tg of poly(trimethylene carbonate)s used in this study. PTMC

[h] [dm3 g1]

SD

Mv [kg mol1]

1 2 3 4 5

0.532 0.401 0.266 0.249 0.229

0.013 0.006 0.010 0.003 0.011

733 500 287 263 235

Tg [ C]

GPC Mn [kg mol1]

SD

Mw [kg mol1]

SD

Mz [kg mol1]

SD

PD

433 289 159 233 147

29 30 15 24 5

553 420 245 307 167

11 33 33 19 13

707 604 371 407 192

25 36 41 14 3

1.27 1.45 1.54 1.32 1.14

material due to the crosslinks formed between polymer chains) or, as in the case of some synthetic biodegradable polyesters commonly used as biomaterials, may cause deterioration of some critical parameters such as toughness or biodegradation profile [7]. It has been found that during irradiation crosslinking and chain scission frequently occur side-by-side, and that their yields are influenced by factors like the polymer structure and crystallinity, the presence of oxygen, sample shape and/or thickness as well as irradiation conditions (for instance the dose rate, i.e., rate at which radiation energy is introduced in a mass unit of the material). In the case of degradation, the molecular weight decreases and, typically, as a consequence the material loses its mechanical strength, while crosslinking causes increase in the molecular weight usually resulting in tensile strength improvement [7,8]. When considering the use of ionizing radiation for sterilization of biomaterials or a biomedical devices, one should also investigate the effects of gamma rays or electron beam on the properties of the polymers involved. Recently, Grijpma et al. have found that gamma irradiation of PTMC with a sufficiently high dose leads to the formation of a gel fraction, i.e., an insoluble crosslinked structure [6]. This indicates that for PTMC radiation-induced crosslinking dominates over chain scission. This important finding may have a two-fold impact on the application of PTMC as a biomaterial. First of all, it would allow using irradiation as a sterilization method without parallel deterioration of properties. Furthermore, it implies that electron beam or gamma irradiation can be used for increasing the molecular weight of PTMC, providing an interesting alternative to the classical synthetic methods where reaching high molecular weights of PTMC usually poses problems [5,9]. Since the process of PTMC crosslinking by ionizing radiation has not been investigated in detail, and so far only been mentioned for gamma irradiation while a significant part of industrial irradiation plants uses another kind of radiation, i.e., electron beam (EB), the aim of our work is to explore the influence of ionizing radiation in the form of electron beam on PTMC in some detail. Viscosity and GPC tests were conducted to establish the influence of irradiation on the molecular weight of this polymer below the gelation dose. Additionally, solegel analysis and swelling tests in chloroform were performed with the crosslinked PTMC polymer gels. These experiments have been complemented with DSC and FTIR tests to detect any pronounced side effects which could influence the polymer structure. The influence of irradiation on the tensile properties of PTMC was also studied. 2. Materials and methods 2.1. Materials Polymer grade 1,3-trimethylene carbonate (TMC) was obtained from Boehringer Ingelheim, Germany and used without further purification. Stannous octoate (SnOct2) (stannous 2-ethylhexanoate) was used as received from Sigma, USA. Chloroform, dichloromethane (Chempur, Poland) and other solvents (Biosolve, The Netherlands) were of analytical grade.

16.6 16.2 16.4 16.6 16.6

2.2. Polymer synthesis In an argon atmosphere, TMC monomer was charged into freshly silanised (Serva, Boehringer Ingelheim Bioproducts Partnership, Germany) and dried glass ampoules and 2  104 mol of stannous octoate per mol of monomer was added as a solution in sodium-dried pentane. The pentane was removed afterwards by evacuation. The ampoules were purged three times with dry argon and heat-sealed under vacuum. Several PTMC batches were prepared; their characteristic (Table 1) includes values of intrinsic viscosities [h], viscosity-average molecular weights Mv, numberaverage molecular weights Mn, weight-average molecular weights Mw, z-average molecular weights Mz, polydispersities PD ¼ Mw/Mn and glass transition temperatures Tg. The polymerizations were conducted at 130  C for 3 days. By exposing the monomer to air for different periods of time, different molecular weight PTMC batches were prepared. The polymers were purified by dissolution in chloroform and precipitation in ethanol, washing with fresh ethanol and drying at room temperature under vacuum. 2.3. Polymer processing Compression moulding of purified polymers was done on a Fontijne laboratory press THB008 (The Netherlands) in 500 mm thick stainless steel moulds. The films were melt-pressed at 140  C and subsequently cooled to 15  C under pressure using cold water. 2.4. Polymer characterization 2.4.1. Gel permeation chromatography (GPC) Gel permeation chromatography (GPC) measurements were done using a system equipped with a P580 pump (Dionex), two columns of 10 mm and 5 mm pore size (Knauer) and three detectors: Viscotec Ralls Detector (static light scattering at 90  C at a wavelength of 670 nm) and Viscotec Dual Detector 250 (refractometer/ viscometer). Dichloromethane was used as an eluent at 30  C at a flow rate of 0.8 ml min1. Sample concentrations in the range 2e10% (wt/vol) and injection volumes of 100 ml were used. Solutions were filtered prior to injection into the GPC through 5 mm PTFE membrane filters (Sartorius). 2.4.2. Intrinsic viscosity Intrinsic viscosities of poly(trimethylene carbonate) in chloroform were determined at 25  C with an AVS-310 automatic viscometer (Schott Geräte) equipped with a 01/0a type Ubbelohde viscometer. Prior to the analysis, the samples were filtered through 0.45 mm pore size filters (Sartorius) except for the samples irradiated with doses just before the gelation point was reached, that were filtered through to 5.0 mm filters. In the calculation of viscosity-average molecular weights (Mv), the following MarkeHouwink coefficients (based on weight-average molecular weights) were applied: K ¼ 2.43  105 dm3 g1 and a ¼ 0.74 [6].

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2.4.3. DSC Glass transition temperatures of the poly(trimethylene carbonate)s were determined by differential scanning calorimetry (Q200 M-DSC) of TA Instruments. Samples of 5e8 mg sealed in aluminium pans were analyzed in the temperature range -60 to 50  C at a heating rate of 10  C min1. The data presented were collected during the first heating scan. The glass transition temperature Tg was taken as the midpoint of the heat capacity change. 2.4.4. FTIR Infrared spectra were obtained on an Avatar TM 330 FTIR Spectrometer (Nicolet) in the HATR mode. Approximately 0.4 ml of 0.1% polymer solution in chloroform was cast onto the IR transmitting windows (80  10 mm ZnSe plates) to form a uniform layer. The plates were dried for 24 h to remove traces of solvent before spectra acquisition. Spectra were collected with a resolution of 4 cm1 and average of 64 scans. For analysis, Omnic software was used. 2.4.5. NMR Proton nuclear magnetic resonance (1H NMR) spectra were recorded at 300 MHz on Varian Innova spectrometer, using CDCl3 as a solvent (Merck, Germany). 2.4.6. Mechanical tests Tensile tests were performed with polymer strips of 5 mm width, approximately 0.5 mm thick and with 50 mm in length between the clamps at a head speed of 50 mm min1 on the tensile tester Instron 5582 equipped with a 2 kN load cell. Experiments were run at room temperature. 2.5. Electron beam irradiation Polymer films in vacuum-sealed bags were irradiated by an electron beam from the ELU-6 linear accelerator. Pulses of 6 MeV electrons at a frequency of 20 Hz (single pulse duration: 4 ms) were applied. The average dose rate, determined by calorimetry, was 5.8 kGy min1. 2.6. Gel fractions and equilibrium degrees of swelling Gel fractions and equilibrium degrees of swelling were determined gravimetrically after removing the sol fraction by extraction with chloroform at room temperature. The samples (approximately 0.4 g) were soaked in 100 cm3 of chloroform, which was replaced every 3 days. Under these conditions, equilibrium swelling was reached within 2 weeks. After weighing the swollen samples, they were dried at 25  C to constant weights. The gelation dose was evaluated by the solegel method known from the literature [10] (see section 3.3 below). Equilibrium degrees of swelling (DSN) were calculated using the following formula:

DSN ¼

msN  md md

Fig. 1. FTIR spectra of poly(trimethylene carbonate) e PTMC 4, unirradiated and irradiated with a dose of 30 kGy.

3.2. PTMC characteristics as a result of irradiation by electron beam with doses lower than the gelation dose Irradiation with a sterilization dose (25 kGy) should not influence the structure and physical properties of the sterilized polymer to an extent which would compromise its application. This was partially verified for PTMC by DSC, FTIR, NMR and mechanical testing (see below). The average glass transition temperature (Tg) of the original PTMC samples of various molecular weights is 16.5 C  0.2 (Table 1). Irradiation with a dose of 25 kGy did not influence the glass transition temperature that was determined to be 16.4 C  0.3. From the NMR (data not presented here) and FTIR spectra (Fig. 1) of PTMC and PTMC irradiated with a dose of 30 kGy one can conclude that no significant changes in the chemical composition of PTMC are induced by irradiation of the polymer. The results of those tests indicated that no major changes in the chemical structure of the polymer due to irradiation had occurred. Irradiation may cause crosslinking of the polymer and/or chain scission. Various biodegradable polymers commonly used as materials for implants and devices react differently to ionizing radiation, e.g. poly(lactic acid) degrades with accompanying decrease of the molecular weight since chain scission predominates over crosslinking [11e13]. An example of a crosslinkable biodegradable polymer is poly(3-caprolactone), for which an increase in the average molecular weight and formation of a gel has been observed [8,14,15]. In a study on the effect of gamma rays on PTMC it has been assumed that the molecular weight of PTMC increased during irradiation up to the gelation dose, above which a gel, i.e., a three-dimensional insoluble polymer network, was formed [6].

(1)

where msN stands for the mass of swollen gel in equilibrium and md is the mass of the dried gel. 3. Results and discussion 3.1. Polymer synthesis and characterization High molecular weight PTMC polymers were synthesized by ring opening polymerisation of TMC (Table 1). The conversion of TMC for all samples was higher than 95%.

Fig. 2. Viscosity-average molecular weight of various PTMC samples irradiated by EB, estimated on the basis of the intrinsic viscosity. The filtrate fraction does not contain microgels. Formed microgels were possibly removed by filtration.

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Fig. 3. Example GPC traces of PTMC 1 original and after irradiation with 6 kGy.

a

b

1433

Fig. 5. Changes in polydispersity of various PTMC irradiated by EB.

In the present study, changes in the molecular weight of PTMC after irradiation with an electron beam were monitored by intrinsic viscosity measurements and by gel permeation chromatography. The intrinsic viscosity, and consequently, the Mv of various PTMCs initially decreased and reached a minimum value before a steady increase was observed (Fig. 2). Samples irradiated with doses close to but slightly lower than the gelation dose were only partially soluble in chloroform. This was already evident for example for PTMC 1, where at a dose of 8 kGy microgel formation occurred, which was visible by the naked eye. Even below the gelation dose an insoluble fraction is separated by the 5mm filter. This implies that the molecular weight determined for this sample might have been underestimated due to the removal of a very high molecular weight fraction. The points for these filtered fractions are also given in Fig. 2. Similarly to the viscosity data, the gel permeation chromatography results also show eventually an increase of the molecular weight. Exemplary GPC traces are shown in Fig. 3, and the molecular weights determined for the three PTMC samples are shown in Fig. 4. Irradiation of PTMC causes significant broadening of the molecular weight distribution, which was observed as an increase in the early eluting (high molecular weight) fractions and as a formation of a shoulder at longer elution times representing lower molecular weight fractions. Fig. 5 depictures changes of the PDs of three PTMC samples with various initial molecular weights. A continuous increase in the PD of the samples with irradiation is observed, and the PD increases substantially at doses close to the gelation point for a particular PTMC sample. In other words, the PD increases with the dose e Mn decreases somewhat initially but Mw tends to increase, and Mz rises significantly. This effect indicates that in irradiated PTMC intermolecular

c

Fig. 4. GPC-based number-, weight- and z-average molecular weights [kg mol1] of PTMC irradiated by EB; PTMC: a/1, b/2, c/5.

Fig. 6. Gel fraction after extraction in chloroform at room temperature as a function of absorbed dose for various PTMC samples.

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Fig. 7. Equilibrium degree of swelling in chloroform at room temperature for various PTMC samples as a function of absorbed dose.

crosslinking and chain scission occur simultaneously, however the crosslinking reaction dominates. Broadening of molecular weight below the gelation dose may suggest that the mechanism of crosslinking involves formation of Y-links rather than classical H-inkages [16]. Y crosslinking points are generated when the radical on the macromolecule reacts with a radical located at the end group of the other macromolecule that was probably formed after a scission reaction of the initial chain. It is worth mentioning that changes in the number-average molecular weight for polymers undergoing simultaneous crosslinking and scission are very limited and it is assumed that Mn at the gelation dose is only 1.33 of its initial value if the initial polymer weight distribution was statistical [17]. 3.3. Effect of electron beam irradiation on PTMC gel formation Continuous irradiation of PTMC at doses over the Dg (this value varies depending on the initial molecular weight of the polymer), results in formation of an insoluble polymer fraction. Figs. 6 and 7 illustrate the percentage of gel fraction and equilibrium degree of swelling, respectively, as a function of absorbed dose for various PTMC samples. The gel fraction increases with irradiation dose. The equilibrium degree of swelling in chloroform decreases with the dose. It reaches a maximum value of several hundreds at low irradiation doses, only slightly above the Dg. This is due to a low concentration of intermolecular crosslinks that allows for high solvent uptake. With an increase in irradiation dose, the crosslinking density rises and a pronounced decrease in equilibrium swelling is observed, which further levels off. Such tendencies in the gel fraction and equilibrium degrees of swelling are typical for polymers that undergo crosslinking during irradiation [8]. When comparing data for different PTMC samples it is observed that the shape of the curves remains similar, but individual plots differ from each other in terms of maximum gel fraction and the amount of solvent that can be absorbed and retained in the gel at a particular dose of irradiation. These values are directly related to the initial molecular weight of the polymer used and to the gelation dose. The dependence of the gel fraction g on the irradiation dose was used for the determination of the gelation dose Dg and of the ratio of chain scission density to intermolecular crosslinking density, p0/q0. Table 2 Results of solegel analysis evaluated by the modified CharlesbyePinner Equation (2): compression moulded samples were EB-irradiated in vacuum-sealed bags. PTMC

1

2

3

4

5

p0/q0 Dg [kGy]

0.71 10.5

0.79 21.7

0.94 50.6

0.72 38.8

0.74 54.4

Fig. 8. Values of gelation doses, calculated by the modified CharlesbyePinner equation, against initial number- and weight-average molecular weights of PTMC polymers.

This can be done by the commonly used CharlesbyePinner equation [17]. However this equation only holds for polymers with a statistical initial molecular weight distribution (polydispersity 2) and when crosslinking occurs by an H-link mechanism and when crosslinking and scission occurs with random spatial distribution in the polymer. In order to avoid inaccuracies due to, for instance, a non-statistical initial molecular weight distribution, a modified version of the CharlesbyePinner equation [10] may be applied (Equation (2))



   pffiffi Dv þ Dg p p s ¼ 0þ 2 0 q0 q0 Dv þ D

(2)

where s is the sol fraction (s ¼ 1  g), D is the actual absorbed dose and Dv is the virtual dose, which is the dose that would be needed to transform a virtual sample of the same polymer of an initially random molecular weight distribution (where Mw0/Mn0 ¼ 2) into the real polymer sample under consideration. In other words, Dv is equal to the difference between a gelation dose of the polymer of a random distribution and the gelation dose of the polymer under study. While the Dv values themselves are rarely of practical interest, Equation (2) is a convenient tool allowing to determine Dg and p0/q0 for any irradiated polymer samples regardless of their initial molecular weight distribution [8,10,18e20]. Data on our samples resulting from calculations based on Equation (2), i.e., values for gelation doses and p0/q0 ratios are listed in Table 2. As predicted by the theory developed by Carothers [21], Flory [22,23], and Stockmayer [24], and later adapted to radiationinduced crosslinking of polymers by Charlesby [17], a general rule for polymers with the same chemical structure and with a statistical molecular weight distribution is that the gelation dose decreases with increasing initial molecular weight (see for instance ref. [25]). This is also observed for PTMC (Fig. 8). The decrease in the gelation dose with increasing PTMC molecular weight is due to the

Table 3 Radiation-chemical yields [107 mol J1] of intermolecular crosslinking and scission of various PTMC irradiated by EB. PTMC

4Gx  Gs

1 2 3 4 5 Average

3.43 2.23 1.67 2.79 2.04 2.43  0.69

Radiation yields Gx

Gs

1.33 0.92 0.79 1.09 0.81 0.99  0.23

1.89 1.45 1.48 1.57 1.20 1.52  0.25

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crosslink per chain. That is why the initial average molecular weight of the polymer has a direct influence on the gelation dose. In case of chain scission and intermolecular crosslinking reactions occurring side-by-side (which is the case for most of the polymers under irradiation), it is advisable to estimate the radiationchemical yields of these reactions. Radiation-chemical yield of scission, Gs, is defined as the number of scission events (bonds broken in the polymer backbone) per unit of absorbed energy, while the yield of intermolecular crosslinking, Gx, denotes the number of crosslinks formed between previously separate chains per unit of absorbed energy. On the basis of molecular weight changes in the pre-gelation dose range, it is possible to estimate the difference between the yields of intermolecular crosslinking and scission Gs, i.e., expression 4Gx  Gs, by Equation (3), [17,26,27]. Fig. 9. Stressestrain curves for PTMCs 1 and 4, unirradiated and irradiated with 25 kGy.

fact that at the gelation dose, statistically one crosslink is formed per one initially existing polymer chain in the system. The lower the molecular weight, the more chains are present in a mass unit of a polymer sample, thus a higher dose is needed to form one

4Gx  Gs ¼

  2 1 1  D Mw0 Mw

(3)

in which Mw0 and Mw are the initial weight-average molecular weight and the weight-average molecular weight after the dose D was absorbed, respectively. Because of gel formation, at the gelation dose, when D corresponds to Dg, Mw tends to infinity, thus the second term in brackets in Equation (3), the reciprocal Mw, can be

a

b

c

d

e

f

Fig. 10. Tensile properties of PTMC samples (1e5).

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omitted. This simplifies the expression to Equation (4). Taking into account Equation (5) [17], for which values of p0/q0 are estimated from solegel analysis, respective yields were calculated and are included in Table 3.

4Gx  Gs ¼

2 Dg MW0

Gs p ¼ 0 2Gx q0

Table 4 Sterilization dose of 25 kGy as a percentage of Dg for various PTMC. PTMC

1

2

3

4

5

25 kGy/Dg [%]

238

115

49

64

46

(4)

(5)

The fundamental requirement of gel formation is that Gs/Gx is less than 4, and in this case it is fulfilled e it is about 1.5. Calculated Gx and Gs values are in a similar range as those determined for PCL irradiated at room temperature, for which Gx was about 1 and Gs was equal to 1.77  107 mol J1 [8]. The slightly lower Gs for PTMC might be related to the fact that this is an amorphous polymer, thus there are no radicals trapped in a crystalline phase, which usually leads to chain scission. 3.4. Effect of electron beam irradiation on the mechanical properties of PTMC Films of poly(trimethylene carbonate)s used for the mechanical tests were prepared by compression moulding. Typical stresse strain curves are presented in Fig. 9. The mechanical properties of PTMC films irradiated by electron beam with 25 kGy were compared to those of unirradiated material. The results are presented in Fig. 10. First of all, it should be mentioned that the initial mechanical properties of the PTMC films depend on the molecular weight. The yield values and the modulus are somewhat reduced with a decrease of molecular weight, while stress at break, which is related in the case of this polymer to the maximum stress, decreases rapidly for lower molecular weight PTMC. Some of the mechanical properties underwent changes upon irradiation. The yield stress decreased for all samples, the most pronounced change was by 33% (for sample 1). On the other hand irradiation has no real impact on the yield strain. After irradiation, the Young’s Modulus of all samples decreased by about 20e25%. The changes in the stress at break values differed for samples with various molecular weights. After irradiation this parameter decreased slightly (by ca. 9% for samples 1 and 3) or remained stable (for samples 4 and 5). A radical decrease of 70% was observed for sample 2. After irradiation. the strain at break increased significantly for PTMC with a relatively high molecular weight (sample 1) and slightly for sample 2 with a somewhat lower molecular weight, while for the lower molecular weight polymers an opposite behaviour was observed, i.e., the value of stress at break decreased. Some differences in the tensile properties of PTMC after irradiation may be explained by the fact that the high molecular weight sample 1 underwent relatively strong crosslinking, yielding about 45% of gel at 25 kGy, compared to only about 20% of gel for sample 2. On the other hand an irradiation dose of 25 kGy caused no gel formation but only broadening of the molecular weight for initially lower molecular weight samples of PTMC (in this case the Dg’s were 40e50 kGy). This is the reason for the decrease in the yield stress and modulus of these samples after irradiation. The initial values for some parameters for the mechanical properties of low molecular weight PTMC samples were already low and no essential changes for the stress at break values were observed after irradiation. In Table 4 the typical sterilization dose of 25 kGy is shown as a percentage of the gelation dose for each PTMC sample (25 kGy/Dg). An irradiation dose of 25 kGy is still insufficient to obtain PTMC material with improved mechanical properties, yet its behaviour is different from that of other common synthetic polymers used as

biomaterials, for example that of PLA, for which some reduction of tensile properties after irradiation is related to a dramatic decrease in molecular weight [28]. The present study is directed to the influence of the standard sterilization dose of 25 kGy on the properties of PTMC. One may expect that irradiation of PTMC material with higher doses, by further increasing the average molecular weight and the gel fraction, could lead to an improvement of its functional properties. In this case a simple sterilization practice would turn into technological processing by radiation. Besides changes in molecular weight and mechanical properties, one should also consider the effect of the introduction of crosslinks on the degradation profile of radiation sterilized PTMC-based biomaterial. Studies related to these issues are currently under way. 4. Conclusions The effects of electron beam irradiation on 5 different batches of poly(trimethylene carbonate) with different molecular weights were studied. The thermal characteristics and FTIR spectra of PTMC before and after irradiation with a standard sterilization dose of 25 kGy did not change. Therefore no significant changes in the chemical structure of the polymer occurred by irradiation to 25 kGy. Scission and crosslinking occurred simultaneously under EB irradiation leading to a pronounced broadening of the molecular weight distribution: Mn initially decreased while Mw and especially Mz increased significantly up to the gel point. The gelation dose and radiation-chemical yields of intermolecular crosslinking and scission were determined on the basis of sol-gel analysis for PTMC with different molecular weights. The gelation dose decreased with increasing PTMC initial molecular weights. The tensile properties of PTMC samples before and after irradiation with the standard sterilization dose did not differ much. It is concluded that ionizing radiation by an electron beam can be used efficiently as a sterilization technique for PTMC-based biomaterials and biomedical devices as far as their basic physicochemical and mechanical properties are concerned. Moreover, irradiation may be used as a tool to effectively increase the molecular weight and therefore to modify the properties of PTMC. Acknowledgements This work has been financed in part by the European Commission, projects NMP3-CT-2005-STRP 517070 PROTEC, and MIGR-CT2007-206269 Nerve Regeneration and by the Ministry of Education and Science, Poland (Project 6/6.PRUE/2006/7). References [1] Zhang X, Espiritu M, Bilyk A, Kurniawan L. Morphological behaviour of poly(lactic acid) during hydrolytic degradation. Polym Degrad Stab 2008; 93(10):1964e70. [2] Pego AP, Poot AA, Grijpma DW, Feijen J. Biodegradable elastomeric scaffolds for soft tissue engineering. J Control Release 2003;87(1e3):69e79. [3] Pego AP, Poot AA, Grijpma DW, Feijen J. Copolymers of trimethylene carbonate and 3-caprolactone for porous nerve guides: synthesis and properties. J Biomater Sci Polymer Ed 2001;12(1):35e53. [4] Zhang Z, Grijpma DW, Feijen J. Trimethylene carbonate-based polymers for controlled drug delivery applications. J Control Release 2006;116(2):e28e9. [5] Rokicki G. Aliphatic cyclic carbonates and spiroorthocarbonates as monomers. Prog Polym Sci 2000;25(2):259e342.

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