Photochemically cross-linked poly(ε-caprolactone) with accelerated hydrolytic degradation

Accepted Manuscript Photochemically cross-linked poly(ε-caprolactone) with accelerated hydrolytic degradation Csaba Kósa, Michal Sedlač ík, Agnesa Fiedlerová, Štefan Chmela, Katarína Borská, Jaroslav Mosná ček PII: DOI: Reference:

S0014-3057(15)00176-7 http://dx.doi.org/10.1016/j.eurpolymj.2015.03.041 EPJ 6831

To appear in:

European Polymer Journal

Received Date: Revised Date: Accepted Date:

20 October 2014 24 February 2015 7 March 2015

Please cite this article as: Kósa, C., Sedlač ík, M., Fiedlerová, A., Chmela, Š., Borská, K., Mosná ček, J., Photochemically cross-linked poly(ε-caprolactone) with accelerated hydrolytic degradation, European Polymer Journal (2015), doi: http://dx.doi.org/10.1016/j.eurpolymj.2015.03.041

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Photochemically cross-linked poly(ε-caprolactone) with accelerated hydrolytic degradation

Csaba Kósa,a Michal Sedlačík, b Agnesa Fiedlerová,a Štefan Chmela,a Katarína Borská,a Jaroslav Mosnáčeka,*

a

Polymer Institute, Slovak Academy of Sciences, Dúbravská cesta 9, 842 36 Bratislava,

Slovak Republic b

Centre of Polymer Systems, University Institute, Tomas Bata University in Zlin, Nad Ovcirnou

3685, 760 01 Zlin, Czech Republic * Corresponding author: [email protected]; phone: 00421 2 3229 4353

Abstract Irradiation of benzophenone (BP)-doped poly(ε-caprolactone) (PCL) films at >350 nm alters the molecular characteristics, viscoelastic properties and hydrolytic degradation rate. The influence of BP concentration on changes caused by irradiation of BP-doped PCL film was investigated using gel permeation chromatography along with analyses of gel content, DSC and rheology. Changes in the rate of basic hydrolysis of irradiated BP-doped PCL films were also investigated. Irradiation of PCL film doped with at least 0.5% w/w BP led to cross-linking and gel formation, even though simultaneous degradation was also observed. Polar groups formed during the irradiation of BP-doped PCL films enhanced the hydrolysis of cross-linked PCL films compared with untreated films.

Keywords: biodegradable polymer, photochemical cross-linking, viscoelastic properties, benzophenone

1

Introduction Recently, possible medical applications and increasing demand for environmentally friendly polymeric materials have evoked growing interest in biodegradable polymers. Polycaprolactone (PCL) is a degradable aliphatic polyester that can be degraded enzymatically by bacteria and fungi and non-enzymatically in the body [1-5]. Properties of PCL such us high solubility, low melting point and exceptional blend compatibility have inspired prolonged, intensive research into its biomedical applications [1]. During the 1970s and 1980s, PCL and its copolymers were used in several drug-delivery systems, especially for the long-term encapsulation of molecules [5,6]. For drug delivery, research is focused on the application of biodegradable PCL microparticles and nanospheres, which are highly permeable to small molecules and have little tendency to generate an acidic environment [1]. Some studies have also been performed to utilize PCL in biodegradable implants. However, PCL does not possess the required mechanical properties for load-bearing applications [1]. Currently, PCL is most intensively exploited in tissue engineering, with excellent biodegradability and biocompatibility, easy processing and good rheological and viscoelastic properties compared with those of other resorbable polymers [7]. The main drawback of PCL in some applications is its slow degradation, both in vivo and in vitro, due to high crystallinity and hydrophobicity [8,9]. To overcome this slow degradation, copolymers of PCL with other lactones or glycolides/lactides have recently been used [9,10]. Such copolymers usually have impaired mechanical properties comparison with those of PCL homopolymers. Photochemical treatment of biodegradable polymers can lead to materials with modified properties and different degradation rates. Photochemical processes in PCL films have been studied upon exposure to various radiation sources. Gamma irradiation of PCL led to dosedependent chain scission and cross-linking [11]. The critical dose for measurable gel

2

formation (cross-linking) was approximately 26 Mrad under air or vacuum [11]. To reduce the dose, polyfunctional monomers can be used as cross-linking accelerators [12]. Unlike gamma radiation, the irradiation of PCL with UV light (full-spectrum mediumpressure mercury arc) produces only photodegradation via a Norrish II cleavage mechanism [13,14]. Rates of PCL photodecomposition are similar in air or inert atmospheres [13]. UVinitiated cross-linking is obtained only upon modification of PCL with acrylic monomers using phosphine [15] or ketone photoinitiators [16-18]. Among ketone photoinitiators, derivatives of benzophenone (BP) have been widely used [17]. BP and its derivatives are well known to be effective photoinitiators in radical polymerization and in UV-induced surface graft polymerization, especially in the presence of free or linked amines as coinitiators [1922]. However, they have also been investigated as promoters in the photocross-linking of polymers such as polyethylene (PE), polystyrene (PS) or poly(methyl methacrylate) (PMMA) upon irradiation with UV (300 – 400 nm) [23-25]. Surprisingly, no changes were observed by Bei et al. during the irradiation of PCL in a Xenotest unit under air with a low concentration (0.3% w/w) of BP [26]. However, a detailed study of the photoreaction of BP in a PCL matrix has not been reported. Recently, we showed that the photochemical transformation of benzil (1,2-diphenylethan1,2-dione) dopant in PCL films led to significant changes in molecular characteristics and in the rate of hydrolytic degradation of PCL [27]. Therefore, here, we study in detail the influence of the photochemical transformation of BP dopant on the molecular characteristics and properties of PCL. We show that the irradiation of PCL in the presence of at least 0.5% w/w BP leads to changes in the molecular characteristics of PCL, accompanied by the crosslinking of the PCL matrix. Despite the cross-linking, the hydrolytic degradation of photochemically treated PCL was faster than that of the untreated material.

3

Experimental section Materials Poly(ε-caprolactone) (CAPA® 6800, SOLVAY Caprolactones, GB; Mw = 80,000 g·mol-1, ρ =1.1 g·L-1, crystallinity 46.8%, Tm = 58 – 60 °C), benzophenone (Schuchardt, Munich, Germany) and chloroform p.a. (Penta, Chrudim, Czech Republic) were used as received. Tetrahydrofuran p.a. (THF) (POCH s.a. Gliwice, Poland) was stored over KOH overnight and then distilled from CaH2.

Preparation of polymer films PCL films were prepared by casting 1 mL of a chloroform solution of PCL (50 mg) and BP (0.25 – 4 mg) on a 10 cm2 glass plate. To ensure slow evaporation and to obtain a transparent film, the glass plates were covered with Petri dishes. The self-supporting PCL films were separated from the glass plate by dipping into distilled water, and then the films were dried at room temperature under vacuum. The PCL films were placed in aluminum foil frames, and the uncovered section (approximately 6 – 7 cm2) was irradiated for all studies. The amount of BP in this section of the PCL film was confirmed by UV-Vis spectroscopy. For rheological experiments and crystallinity determination using differential scanning calorimetry, PCL films were prepared by casting 8 mL of a chloroform solution of PCL (50 mg·mL–1) and BP in a Petri dish (6 cm diameter). After slow evaporation of the solvent, films were dried under high vacuum and melt pressed at 75 °C (1 min heating without pressure, followed by 30 s at 20 kN and 30 s at 45 kN).

Irradiation and measurements Irradiation of polymer films with UV-VIS light at λ > 350 nm (i.e., irradiation at λ = 366, 405, 408, 436 and 546 nm) was carried out using a medium pressure mercury lamp in a

4

Spectramat apparatus (Ivoclar AG, Schaan, Liechtenstein). Samples were inserted into the glass finger and cooled with water to prevent heating of the sample during irradiation. The distance of each sample from the arc was approximately 10 cm. The intensity of the light on the sample was approximately 20 mW·cm-2. Photochemical reactions of BP in the PCL films were followed by FT-IR using an IMPACT 400 (Nicolet, Germany) Fourier transform infrared spectrophotometer in the range 600 – 4000 cm-1 (64 scans for each measurement). The concentration of BP in the studied middle part of the PCL films was determined by UV-VIS spectroscopy using a Schimadzu UV – 1650 PC spectrophotometer (Shimadzu, Japan). The molar mass and dispersity of the PCL films were estimated using a gel permeation chromatography (GPC) equipment (Shimadzu, Japan) equipped with refractive index detector and 3 x PSS SDV 5 μm columns (d = 8 mm; length = 300 mm; 100 Å + 500 Å +105 Å) using THF eluent and polystyrene standards for calibration. For gel content determination, the irradiated parts of the samples were cut from the aluminum frames, weighted and stirred in THF at ambient temperature for 18 h. The soluble part was used for GPC. The insoluble part of the film was refluxed in chloroform for 24 h, dried and weighed. Gel content was calculated as the ratio of the weight of the dried gel to the initial weight of the irradiated part of the film. Hydrolysis was performed in 1 M sodium hydroxide at 25 °C for predetermined times. After hydrolysis, the films were washed with distilled water at room temperature 3 times, followed by drying under vacuum. The content of extractable product was determined from the change in the weight of the polymer films before (M0) and after (M) hydrolysis (%extractable. products

= 100 × (M0 – M/M0). After washing out the extractable products, the residual film was

used for gel content determination.

5

A Mettler-Toledo DSC821e differential scanning calorimeter was used for determining the melting point and crystallinity. Samples were heated (10 °C·min-1) from 0 to 90 °C under nitrogen atmosphere. Crystallinity was determined from heat of fusion using a reference value of ΔHm = 157 J·g-1 for 100% crystalline PCL [28]. Indium was used for temperature and heat fusion calibration.

Rheological characterization Real-time curing rheological experiments were performed with an Anton Paar modular compact Rheometer MCR 502 (Austria) interconnected with an OmniCure Series 1000 UVVIS mercury lamp curing system (Lumen Dynamics, Canada). Light emitting diodes within the fixture emitted UV-VIS radiation (λ = 334, 366, 405, 408, and 436 nm) through a clear acrylic UV-transparent window. It is not expected that using a slightly different wavelength range in rheological tests would dramatically affect the results because PCL matrices do not absorb at 334 nm. Thus, BP was the only photoactive compound absorbing the radiation. Radiation intensity was fixed during each experiment at 256 mW cm-2. Temperature was controlled using an Anton Paar TC 30 temperature control unit with a heating chamber and a CTD600 convection temperature device with a UV-VIS light accessory. A parallel-plate measuring system with a diameter of 25 mm (PP25) and a gap of 0.5 mm was used. All measurements were performed at 70 °C to provide sufficient melting without thermal decomposition. Oscillatory shear tests were carried out to study the dynamic characteristics of microstructures during UV-VIS irradiation. First, strain sweep was performed under 0.1–10% applied strain () at a fixed frequency of 10 Hz to determine the linear viscoelastic region (LVR). The strain value within the LVR (1.58% in all experiments) was used for frequency sweep measurements; i.e., the frequency, f, was changed from 0.1 to 10 Hz on a log scale with

6

10 pts/decade. Furthermore, the sample was exposed to irradiation for 1 hour at 3.16 Hz and

 = 1.58% for the real-time monitoring of viscoelastic moduli development. After this, the frequency sweep described above was performed once again to provide additional evidence of sample curing. The same testing and set up times were used for all samples to guarantee equal conditions.

Results and discussions Changes in FT-IR spectra during the irradiation of PCL/BP films Consumption of BP in PCL matrices during irradiation was monitored using FT-IR spectroscopy. Usually, an ideal absorption band for following BP consumption is the carbonyl stretching band. As shown in Fig. 1, the irradiation of the PCL film containing approximately 7% w/w BP reduced the characteristic stretching band of the carbonyl groups in the 1650 – 1670 cm-1 region. However, at BP concentrations below 2% w/w, the absorbance of carbonyl groups was too low to follow the photochemical process. BP consumption during the irradiation of BP-doped PCL films can also be followed by a decline in the absorption band at 640 cm-1 from the BP phenyl ring (Fig. 1). PCL does not absorb in this region; therefore, changes in absorption can be used for PCL films at all studied BP concentrations. BP consumption was accompanied by a small increase in absorption at 640 – 660 cm-1 from benzpinacol. This, together with benzhydrol, is usually formed from BP after hydrogen abstraction from donors [29]. In addition, a small broad peak formed in the 3360 – 3560 cm-1 region corresponding to the oxidation of PCL chains [30] and/or the formation of benzhydrol and benzpinacol. Changes in the carbonyl absorption region (1690 – 1750 cm-1) could not be observed due to PCL absorption peaks in this region. No changes were observed in FT-IR spectra after the irradiation of pure PCL for 1.5 hours.

7

1.6

0 min 10 min 20 min 30 min 40 min 50 min 60 min 120 min

Absorbance

1.4 1.2 1.0

0.9

0.6

0.8 0.6

0.3 benzpinacol in PCL

0.4 1680

1640

660

640

620

-1

Wavenumber [cm ]

Fig. 1 FT-IR absorption spectra of 7% w/w BP in a PCL matrix irradiated for various times at λ> 350 nm. Changes in the BP carbonyl absorption at 1660 cm-1 and phenyl ring –CH absorption at 640 cm-1 are shown. The spectrum of benzpinacol, one of the products of the photochemical transformation of BP, in PCL is shown as well.

Effect of irradiation on the molecular characteristics of the PCL matrix The main photochemical reaction of BP in the presence of hydrogen donors is hydrogen abstraction by BP in its excited triplet state, leading to the formation of ketyl radicals [31,32]. Depending on the structure, polymer matrices can serve as hydrogen donors as well [33]. The macroradical formed can react with another macroradical to form a cross-linked polymer or react with oxygen to form a peroxide, which ultimately leads to PCL chain scission (Scheme 1). These two competitive processes can proceed simultaneously.

8

Scheme 1 Formation of a macroradical after the abstraction of hydrogen from PCL by benzophenone in its excited state, and subsequent reactions of the macroradical leading to cross-linking and/or oxidation, accompanied by degradation of PCL chains

Because the cross-linking and/or degradation of PCL would be expected during the photochemical transformation of BP in PCL films, changes in the molecular characteristics of irradiated PCL/BP films were investigated by gel permeation chromatography (GPC) and gel content analysis. In pure PCL, no changes in molecular characteristics were observed by GPC after irradiation for 1.5 hours, but significant changes were observed in the presence of BP. Typical changes in GPC traces after various irradiation times are shown in Fig. 2 for PCL doped with 1% w/w BP. Shifts in the GPC traces to both higher and lower molar masses could be observed due to two competitive reactions, namely, the recombination of macroradicals and degradation. Extension of GPC traces to higher molar masses (formation of a shoulder) after as little as 10 min of irradiation indicated that the recombination process was preferred. With prolonged irradiation times (more than 20 min) the GPC traces started to shift to lower molar masses. The evolution of molar masses during irradiation are shown in Figure 3 for all samples. As observed in Figure 3, for all samples, the molar mass first increased due to recombination reactions. With prolonged irradiation, molar masses decreased. It should be

9

mentioned that in all cases, gel formation was observed after 10 or 20 min of irradiation, and GPC traces showed only the soluble portion of PCL. A more detailed investigation of gel formation during the irradiation of PCL/BP samples showed increased gel content with prolonged irradiation time (from 10 to 90 minutes), reaching a maximum gel content of 77 – 85% depending on BP concentration (Figure 3b). Note that prolonged irradiation (120 min) led to a slight decrease in gel content.

0 min 10 min 20 min 40 min 60 min 90 min degradation recombination

4

5

6

logMw

Fig. 2 GPC traces of PCL with 1% w/w BP, irradiated for the indicated times at λ >350 nm.

90k

100

a)

80k

b) 80

Gel content [%]

70k

Mn

60k 50k 40k

0.5 wt% BP 1 wt% BP 2 wt% BP 4 wt% BP

30k 20k 10k

60

40

0.5 wt% BP 1 wt% BP 2 wt% BP 4 wt% BP

20

0

0

0

10

20

30

40

50

60

70

80

90

100

Irradiation time [h]

0

20

40

60

80

100

Irradiation time [min]

Fig. 3 Evolution of a) the molar mass (Mn) of the soluble portion of PCL and b) gel content against irradiation time for PCL films containing the indicated concentrations of BP.

10

Real-time rheological measurements To follow changes in PCL structure and their impact on material properties, a real-time rheological properties were monitored during the irradiation of PCL and PCL/BP films. First, amplitude sweep measurements were performed to determine the region in which the material’s viscoelastic moduli are independent of applied strain (at constant frequency). Figure 4 presents the storage modulus (G’), representing elastic system behavior, and the loss modulus (G”), representing viscous behavior, versus strain for PCL films containing various concentrations of BP in the absence of UV irradiation. This result reveals the linear viscoelastic region (LVR). G’ was greater than G” for all samples, reflecting their rather elastic character in the observed strain range. As can also be observed in Fig. 4, LVR slightly decreased with increasing concentrations of BP. Viscoelastic moduli became dependent on applied strain at higher values. Based on the LVR, a strain of 1.58% was used in all subsequent rheology measurements.

Fig. 4 Storage (G’, solid symbols) and loss (G”, open symbols) moduli versus strain (γ) at 10 Hz for PCL/BP films containing various concentrations of BP. Symbols for BP concentration (% w/w): 0 (,), 0.5% (,), 1% (,), 2% (,), 4% (,), and 8% (,).

11

For practical applications, it is important to know the values of G’ and G” in the LVR and their strain frequency-dependence. This is because viscoelastic moduli measured at low frequencies correspond to slow processes, while those at high frequencies correspond to fast processes. Figure 5a shows a frequency sweep in the range 2.5 – 10 Hz (only the narrow range of frequencies where G’ is higher than G’’ is presented) for pure PCL, before and after 1 hour of irradiation. In the non-irradiated sample, G” was slightly greater than G’ at low applied frequencies, indicating more liquid-like character. However, the viscous response of the sample could not adapt to higher frequencies. That is, G’ became greater than G”, resulting in relatively solid-like behavior in the non-irradiated sample. Frequency sweep is an effective tool to monitor curing reactions because less flexible macromolecular chains hinder movement in the molecular range. Hence, the domination of G’ over G” moved to lower frequencies as the degree of curing increases. Some changes occurred in pure PCL as the intersection of G’ and G” (an increase in elastic over viscous behavior) moved to slightly lower frequencies after 1 hour of irradiation due to the increase in G’. However, it is not possible to confidently assign this change to the cross-linking process. Cross-linking undoubtedly occurred for all samples containing BP (Figs. 5b–d). In the original samples, G’ and G” intersected in the range 0.1 – 10 Hz. Nevertheless, after 1 hour of irradiation G’ increased over G” across the whole frequency range. That is, all samples evinced significantly greater elastic properties due to cross-linking. It is also evident that the increase of G’ over G” increased with BP concentration up to 2% w/w. In addition, the slope of G’ fell with increasing BP concentrations, indicating the formation of a 3D internal network. For PCL films containing 4 or 8% w/w BP, moduli after 1 hour of irradiation were similar to those at 2%.

12

13

Fig. 5 Storage (G’, solid symbols) and loss (G”, open symbols) moduli versus frequency (f) for PCL containing a) 0%; b) 0.5%; c) 1%; and d) 2% w/w BP before irradiation (,) and after 1 hour of irradiation (,).

Figure 6 shows real-time monitoring of the effect of photochemical processes on the properties of PCL containing various concentrations of BP. From the change in G’ with irradiation time, it is evident that the degree of curing was dependent on the concentration of BP, while G’ was almost unchanged during irradiation of pure PCL. Furthermore, at 3.16 Hz, the intersection of G’ and G” caused by curing occurred after approximately 800 s of irradiation. The time to intersection of G’ and G” decreased with BP concentration. A slight

14

increase or even a slight decrease of G’ was observed after increasing BP from 2 up to 4 or 8% w/w.

Fig. 6 Storage (G’, solid symbols) and loss (G”, open symbols) moduli versus time of irradiation (t) at strain 1.58% and frequency 3.16 Hz for PCL films with the indicated concentrations of BP. Symbols for BP concentration (% w/w): 0 (,), 0.5% (,), 1% (,), 2% (,), 4% (,), and 8% (,).

Effect of irradiation on crystallinity of PCL and PCL/BP films Changes in crystallinity of PCL during irradiation were strongly dependent on the concentration of BP (Table 1). In pure PCL, neither crystallinity nor melting temperature showed any notable change during irradiation. However, the irradiation of PCL containing 2 or 4% w/w BP led to a significant decrease in crystallinity. The crystallinity of PCL fell with increases in both irradiation time and BP concentration. That is, increasing cross-link density via higher concentrations of BP reduced the crystallinity of PCL. The formation of cross-link junctions may disturb reorganization and chain folding during crystallization, resulting in the formation of imperfect crystallites of smaller size. The same behavior was reported previously [29,34] for the cross-linking of PE and PCL by peroxides.

15

Table 1 Changes in melting temperature and crystallinity of PCL films with irradiation time and BP concentration. Irradiation time

Melting temperature (°C) / Crystallinity a) (%)

(min)

a)

pure PCL

2% w/w BP

4% w/w BP

0

57.6 / 52.8 ± 0.6

56.2 / 52.0 ± 1.6

53.4 / 51.3 ± 2.0

30

57.5 / 51.6

55.3 / 45.9

55.7 / 41.1

60

57.6 / 52.0

54.8 / 43.3

56.0 / 38.5

90

57.5 / 52.5

54.5 / 40.5

57.7 / 34.5

120

57.8 / 52.3

54.1 / 39.9

57.2 / 34.5

100% crystallinity represents 157 J/g

Hydrolysis of irradiated PCL/BP films Both changes in molecular characteristics and increases in polar groups (FTIR) for PCL/BP films after irradiation might also affect the rate of hydrolytic degradation of PCL/BP films. Therefore, we investigated the basic hydrolysis of PCL films irradiated for 1.5 hour at λ > 350 nm in the presence of 0.5, 1, 2 and 4% w/w BP. The results were compared to the hydrolysis of non-irradiated pure PCL films. Hydrolysis was followed by the determination of extractable products and changes in gel content. The amount of extractable products was determined from the difference in the weight of the polymer film before and after hydrolysis. Figure 7a shows that surprisingly, the rate of weight loss in irradiated PCL/BP films increased with BP concentration. Moreover, despite the cross-linking of PCL/BP films after irradiation, weight loss during hydrolysis was faster than for pure PCL. These results may be due to the faster penetration of water into the PCL film due to polar groups formed during irradiation. Figure 7b shows that the gel content of irradiated PCL/BP films decreased with hydrolysis time. Moreover, a faster reduction was observed in PCL containing more BP, most 16

likely due to greater oxidation of PCL chains during the photochemical transformation of BP. This result is consistent with faster weight loss during the hydrolysis of irradiated PCL/BP

100

100

a)

b)

80

Gel content (%)

Extractable products (%)

films containing higher initial BP concentrations.

60

40

pure PCL 0.5 wt% BP 1 wt% BP 2 wt% BP 4 wt% BP

20

0

0.5 wt% BP 1 wt% BP 2 wt% BP 4 wt% BP

80

60

40

20

0 0

20

40

60

80

100

Time of hydrol ysis (days)

0

20

40

60

80

100

Time of hydrolysis (days)

Fig. 7 Changes in extractable products (a) and gel content (b) during the hydrolysis of pure non-irradiated and irradiated PCL films containing BP. PCL/BP films were irradiated for 90 min at λ > 350 nm. Hydrolysis was performed in 1 M NaOH solution at 25 °C.

Conclusions The effect of irradiation (λ > 350 nm) of PCL films doped with BP on molecular characteristics was studied. GPC spectra showed significant changes in the molecular characteristics of PCL doped with BP, including both chain scission and macroradical recombination, after a short irradiation time. In PCL doped with BP, recombination prevailed over chain scission, leading to gel formation. Real-time rheological monitoring of photochemically induced processes under oscillatory shear measurements proved the formation of a 3D internal network in PCL films doped with BP. Increases in cross-linking with BP concentration observed in frequency sweeps were confirmed in real-time experiments because the elastic portion increased proportionally. However, almost no changes occurred in

17

pure polymer samples during irradiation. The crystallinity of PCL films doped with BP decreased with irradiation time. Despite cross-linking, the rate of hydrolytic degradation of photochemically treated PCL doped with BP was higher than for pure non-irradiated material. Thus, the doping of PCL with 0.5% w/w BP and irradiation at λ > 350 nm can give a crosslinked material with good viscoelastic properties even at high temperatures, retaining good hydrolytic degradability for a wide range of biomedical and/or ecofriendly applications.

Acknowledgements We are grateful for the financial support of the Slovak Research and Development Agency through grant APVV-0109-10, the Grant Agency VEGA through Grant 2/0112/13 and the Centre of Excellence SAS for Functionalized Multiphase Materials (FUN-MAT). This research was also supported by Operational Program Research and Development for Innovations, co-funded by the European Regional Development Fund (ERDF) and the Czech Republic,

within

the

framework

of

the

Centre

of

Polymer

Systems

project

(CZ.1.05/2.1.00/03.0111).

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FIGURE CAPTIONS Fig. 1 FT-IR absorption spectra of 7% w/w BP in a PCL matrix irradiated for various times at λ> 350 nm. Changes in the BP carbonyl absorption at 1660 cm-1 and phenyl ring –CH absorption at 640 cm-1 are shown. The spectrum of benzpinacol, one of the products of the photochemical transformation of BP, in PCL is shown as well. Fig. 2 GPC traces of PCL with 1% w/w BP, irradiated for the indicated times at λ >350 nm. Fig. 3 Evolution of a) the molar mass (Mn) of the soluble portion of PCL and b) gel formed against irradiation time for PCL films containing the indicated concentrations of BP. Fig. 4 Storage (G’, solid symbols) and loss (G”, open symbols) moduli versus strain (γ) at 10 Hz for PCL/BP films containing various concentrations of BP. Symbols for BP concentration (% w/w): 0 (,), 0.5% (,), 1% (,), 2% (,), 4% (,), and 8% (,). Fig. 5 Storage (G’, solid symbols) and loss (G”, open symbols) moduli versus frequency (f) for PCL containing a) 0%; b) 0.5%; c) 1%; and d) 2% w/w BP before irradiation (,) and after 1 hour of irradiation (,). Fig. 6 Storage (G’, solid symbols) and loss (G”, open symbols) moduli versus time of irradiation (t) at strain 1.58% and frequency 3.16 Hz for PCL films with the indicated concentrations of BP. Symbols for BP concentration (% w/w): 0 (,), 0.5% (,), 1% (,), 2% (,), 4% (,), and 8% (,). Fig. 7 Changes in extractable products (a) and gel content (b) during the hydrolysis of pure non-irradiated and irradiated PCL films containing BP. PCL/BP films were irradiated for 90 min at λ > 350 nm. Hydrolysis was performed in 1 M NaOH solution at 25 °C.

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Graphical abstract

Photochemically crosslinked poly(ε-caprolactone) with accelerated hydrolytic degradation

Csaba Kósa,a Michal Sedlačík,b Agnesa Fiedlerová,a Štefan Chmela,a Katarína Borská,a Jaroslav Mosnáčeka

a

Polymer Institute, Slovak Academy of Sciences, Dúbravská cesta 9, 842 36 Bratislava,

Slovak Republic, E-mail: [email protected] b

Centre of Polymer Systems, University Institute, Tomas Bata University in Zlin, Nad

E x t ra c t a b l e p r o d u c t s [ % ]

Ovcirnou 3685, 760 01 Zlin, Czech Republic 100

photochemically crosslinked benzophenone-doped PCL

80

60

40

pure PCL

20

0 0

20

40

60

Time of hydrolysis [days]

24

80

Highlights 

PCL film doped with benzophenone was cross-linked using irradiaton at λ > 350 nm



Photochemical cross-linking of PCL films was proved by gel content and rheology



Photochemical cross-linking of PCL films accelerates its hydrolytic degradation

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