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Gamma radiation on poly([epsilon]-caprolactone) in the presence of vinyltrimethoxysilane.


The interest in biodegradable polymers has recently gained exponential momentum, and, within that broad family, polycaprolactone (PCL) constitutes one of the most attractive sources of novel materials because of its flexibility, good biodegradability, and biocompatibility (l-3). However, the low melting point, poor stability, and tendency to rack when stressed limited its wide applications. On the other hand, PCL is easily degraded under high humidity. Because of the easy hydrolysis of ester bonds under high humidity condition, the molecular weight of PCL decreases, which makes tensile strength and thermal degradation temperature of PCL easily decreased. These will greatly limit its certain applications (4).

Cross-linking is a broadly used method for the modification of polymer properties (5), (6). Cross-linking involves the formation of three-dimensional structures-gels-causing substantial changes in material properties. By introducing cross-links into PCL, physical properties such as the crystallinity and the melting point will be influenced. The characteristics of the degradation by hydrolysis will in turn also be influenced. Certain mechanical properties, such as the creep resistance and high-temperature dimensional stability, are, in general, improved by cross-linking. These properties are very important in applications.

Radiation is a very convenient method to modify the properties of polymeric materials by cross-linking, grafting, and degradation (7-9). The molecular changes, induced by radiation, in a polymer may be classified as main-chain bond scission and cross-linking. Main-chain bond scission may result in a decrease in molecular weight and, thus, adversely affect its mechanical properties. Chain cross-linking may lead to an increase in molecular weight and formation of a network structure. It must be emphasized that the two mechanisms, i.e., chain scission and cross-linking, depend on the radiation dose, the radiation temperature, the type of polymer, the radiation medium, and added coupling agent (8). The structure and some physical properties of radiated PCL were investigated by many authors (10-14). For example, Narkis et al. (10) studied the structure and physical properties of [gamma]-irradiated PCL as a function of the irradiation dose level. The studied doses of up to 70 Mrad caused in PCL gel contents of about 35%. Darwis et al. (11) found that the dose required for cross-linking PCL depended on the state of irradiated polymer, which was relatively low when the polymer was irradiated in the molten or super-cooled state. Moreover, cross-linked PCL was also achieved by electron beam irradiation in the presence of polyfunctional monomers (12-14).

Vinyltrimethoxysilane (VTMS) was a widely used functional monomer in silane water-cross-linking polymers (15), (16). VTMS contains functional group of both carbon-carbon double bonds and methoxysilane. The carbon-carbon double bonds are very sensitive to radiation and generally can help to improve the cross-linking efficiency of polymers. On the other hand, under high humidity conditions methoxysilane hydrolysis (--Si--OH) reaction is easy to occur and then forms the siloxane (Si--O--Si) linkage (15), (16). Moreover, because of the bulky side group, it is difficult for VTMS to homopolymerize in free radical reaction (16]. Hence, in view of the structure characteristic of VTMS, it was considered as a good cross-linking agent for cross-linking of PCL, with improved cross-linking efficiency, thermal-mechanical properties, and hydrolysis resistance.

The aim of this study was, therefore, to understand how gamma radiation, together with VTMS as a cross-linking agent, can alter cross-linking efficiency, thermal-mechanical properties, and hydrolysis of PCL. In this study, the effects of cross-linking parameters, such as radiation dose and VTMS concentration, on polymer properties will be investigated.



PCL was a commercial product, CAPA6800 (Solvay Tnterox Ltd.). The weight-average molecular weight of PCL was 68,000 g [mol.sup.-1]. Before melt blending, PCL pellets were dried in a vacuum oven at a vacuum of about 5 Torr at 40[degrees]C for 8 h. VTMS and triallyl isocyanurate (TAIC) were purchased from Aldrich, and used as received.

Sample Preparation

VTMS was blended with PCL at various VTMS contents (1, 3, and 5 wt%) in an internal mixer (rheomix 600p, Haake. Germany) at 90[degrees]C with 50 rpm for 8 min, respectively. TAIC was blended with PCL at 5 wt% content with the same procedure. Then the samples were hot-pressed at 100[degrees]C for 3 min followed by cold-press at room temperature to form the film with different thickness for characterization. For comparison, pure PCL was treated with the same procedure. The film sample was heat-sealed in polyethylene bag in vacuum at a vacuum of about 600 Torr, and radiated by [gamma]-rays (10 k Gy [h.sup.-1]) from a [.sup.60]Co source with various radiation doses.


The gel content of the cross-linked PCL was determined gravimetrically, using a Soxhlet extraction cycle with boiling chloroform as the solvent for 72 h. Approximately 0.2 g of the cross-linked polymer sample was cut into small pieces and placed in a preweighed stainless steel fine mesh. After the extraction, the samples were washed with acetone and vacuum dried to constant weight. The gel fraction was calculated as follows:

Gel fraction = ([W.sub.2]/[W.sub.1]) x 100(%) (1)

where [W.sub.1] is the initial weight of sample and [W.sub.2] is the weight of sample after extraction.

The attenuated total reflectance infrared spectroscopy of the radiated film (about 0.5 mm) was recorded on a Bruker-Tensor 27 spectrometer. The spectra were recorded using a Bio-Rad Win-IR spectrometer in the range of 500-4000 [cm.sup.-1], with a resolution of 4 [cm.sup.-1].

Thermal analysis was performed using a TA Instruments differential scanning calorimetry (DSC) Q20 with a Universal Analysis 2000. Indium was used for temperature and enthalpy calibration. All operations were performed under nitrogen purge, and the weight of the samples varied between 5 and 8 mg. The samples were heated from 0[degrees]C to 70[degrees]C at a heating rate of 10[degrees]C [min.sup.-1] (first heating), held for 2 min to erase the thermal history, and then cooled to 0[degrees]C at a cooling rate of 10[degrees]C [min.sup.-1] (first cooling). The samples were further heated to 70[degrees]C again from Q[degrees]C at a heating rate of 10[degrees]C [min.sup.-1] (second heating) to investigate the subsequent melting behavior. Thermogravimetric analysis was carried out on a Perkin-Elmer TGA 7 thermal analyzer from 35[degrees]C to 600[degrees]C at a heating rate of 10[degrees]C [min.sup.-1] in nitrogen atmosphere. Wide-angle X-ray diffraction (WAXD) patterns were recorded using a Rigaku model Dmax 2500 X-ray diffractometer. The CuKa radiation ([lambda] = 0.15418 nm) source was operated at 40 kV and 200 mA. WAXD patterns were recorded from 5 to 40[degrees] at 4[degrees] [min.sup.-1].

Dynamic mechanical analysis was carried out using a dynamic mechanical thermal analyzer 242 (Netzsch company) to determine the viscoelastic properties in tension mode. Specimens of dimension 20 X 4 X 1 [mm.sup.3] were used. Samples were heated from -90[degrees]C to 0[degrees]C at a heating rate of 3[degrees]C [min.sup.-1] at a frequency of 1 Hz.

The static mechanical properties of samples were determined by using an Instron 1211 testing machine. The test was carried out with a crosshead speed of 100 mm [min.sup.-1] at 25[degrees]C in the tensile mode. The values reported are averages for at least five dumbbell-shaped specimens with necks of 20 mm long and cross-sectional areas of 4 X 1 [mm.sup.2].

The hydrolytic degradation of neat PCL and cross-linked PCL was carried out in sodium hydroxide (NaOH) solution (pH = 12) at 37[degrees]C (17). The degradation of neat PCL and cross-linked PCL was determined by the weight loss as a function of time. The films (about 0.5 mm) were placed in vials filled with 10 mL of NaOH solution at 37[degrees]C for a predetermined period of time. The pH value of the solution was monitored and maintained at 24 h interval. For a given experiment, three replicate specimens were withdrawn from the NaOH solution and washed with distilled water. After wiping, the specimens were dried to a constant weight in a vacuum oven at a vacuum of about 5 Torr at 40[degrees]C and then weighed. The weight-loss coefficient [W.sub.loss](%) = 100 x ([W.sub.0] - [W.sub.[t-dried]])/[W.sub.0] where [W.sub.0] is the initial weight and [W.sub.[t-dried]] is the weight of sample subjected to hydrolytic degradation for time t and drying in vacuum.

The morphology of the film surface before and after hydrolysis experiment was observed using a field emission scanning electron microscopy (XL30 ESEM FEG, FE1 Co.). The film surface was coated with a thin layer of gold before the measurement.


Introducing a few percent of VTMS as a cross-linking agent into the PCL samples led to formation of gel even at the low absorbed dose of 10 kGy (Fig. 1). A high value of 81 wt% gel fraction was obtained in the sample containing 5 wt% VTMS at a dose of 70 kGy, whereas the radiated samples without VTMS completely dissolved in the solvent within the absorption dose range of 10-70 kGy. In fact, the gel cannot be measured until the radiation dose is over 90 kGy for neat PCL. At a definite VTMS level, the extent of gel formation increased with increase in the absorbed dose up to 50 kGy and then seemed to remain nearly constant as shown in Fig. 1. Increasing the percent of cross-linking agent introduced more active sites into the system, which led to formation of tighter network with higher degree of cross-link density at a given absorbed dose. These results show that the efficiency of radiation-induced cross-linking of PCL in the presence of VTMS is higher than that of pure PCL; obviously, VTMS, through its vinyl groups, is acting as a cross-linking agent. The radiation-induced cross-linking behavior of PCL with VTMS was also compared with that of PCL cross-linked with TAIC. TAIC is one of the most widely used polyfunctional monomer for radiation-induced cross-linking of polymers (12-14). It can be clearly seen from Fig. 1 that the gel fraction of PCL radiated with 5 wt% VTMS is very similar to that of PCL radiated with 5 wt% TAIC, indicating that VTMS is also an efficient cross-linking agent to cross-link PCL by radiation. It should be noted here that, although the number of unsaturation of VTMS is less than that of TAIC, the cross-linking efficiency of PCL with 5 wt% VTMS is very similar to that of PCL with 5 wt% TAIC, which may come from the fact that it is difficult for VTMS to homopolymerize in radical cross-linking reaction because of the bulky side group [16]; hence, most of carbon-carbon double bonds of VTMS are active under radiation to experience cross-linking reaction between PCL chains.


According to the classical Charlesby-Pinner equation (18):

S + [S.sup.[1/2]] = [p.sub.0]/[q.sub.0] + 1/([q.sub.0][u.sub.1]D) (2)

where S is the sol fraction, [S.sup.[1/2]] is the square root of the sol fraction, [p.sub.0] and [q.sub.0] are the fractions of the repeat units of the polymer undergoing scission and cross-linking, respectively, [[mu].sub.1] represents the initial number average degree of polymerization, and D is the radiation dose. The relationships between S + [S.sup.[1/2]] and 1/D shown in Fig. 2 are basically linear for all samples. This suggests that radiation-induced cross-linking of PCL in the presence of VTMS follows a random cross-linking law. After an upper extrapolation of these lines to S + [s.sup.[1/2]] = 2, gelation doses of the samples can be obtained, and the intercepts at 1/D - 0 are [p.sub.0]/[q.sub.0]. These results are summarized in Table 1 and, as can be seen, the greater the VTMS content, the lower the gelation dose. The same conclusions are obtained from the ratio of probability of dissociation and cross-linking ([[p.sub.0]/[q.sub.0]]). The fact that [p.sub.0]/[q.sub.0] decreased demonstrates that the probability of radiation-induced cross-linking of PCL in the presence of VTMS increased with the increase in VTMS content. In other words, the efficiency of radiation-induced cross-linking of the PCL in the presence of VTMS was higher than that of neat PCL, and VTMS was able to lower gelation dose by sensitizing the radiation cross-linking of PCL.

TABLE 1. Radiation cross-linking parameters of PCL cross-linked at
different concentrations of VTMS and 5 wt% TAIC.

    Sample      Gelation dose (kGy)  [p.sub.0]/[q.sub.0]

Neat PCL               70                    1.54
PCL/1 wt% VTMS          8.0                  1.38
PCL/3 wt% VTMS          7.7                  1.06
PCL/5 wt% VTMS          7.0                  0.52
PCL/5 wt% TAIC          7.0                  0.44

The reaction degree of VTMS on the radiation cross-linking reaction was investigated by the IR spectra of the sample. As shown in Fig. 3, for PCL mixed with 5 wt% VTMS, the absorbance at 1600 [cm.sup.-1] was due to carbon-carbon double bonds. After irradiation, no trace of carbon-carbon double bonds was found in the sample, indicating the complete consumption of VTMS via grafting or cross-linking on PPC chain upon irradiation.


Thermal Properties

Figure 4a and b shows the first and second scan of DSC for neat and cross-linked PCL samples, respectively. The melting temperature (determined as peak temperature), heat of fusion ([DELTA][H.sub.m]), and degree of crystallinity as determined from DSC measurements for neat and cross-linked PCL with different gel content are summarized in Table 2. The degree of crystallinity has been calculated via the total enthalpy method, according to the equation [X.sub.c] = [DELTA][H.sub.m]/[DELTA][H.sub.m.sup.+], where [X.sub.c] is the degree of crystallinity, [DELTA][H.sub.m] is the specific enthalpy of melting, which has been calculated from the enthalpy of melting normalized to the PCL content, and [DELTA][H.sub.m.sup.+], is the specific enthalpy of melting for 100% crystalline PCL, which is taken as 136 J [g.sup.-1] as reported in the literature (19). Principal differences between the first and second scans can be clearly seen. From the first scan it can be seen that cross-linking has no significant influence on [T.sub.m] and [X.sub.c]. This is because the cross-linking is done well below the melting point under radiation conditions. The radiation-induced cross-linking occurs predominantly in the amorphous regions; hence, [T.sub.m] and [X.sub.c], as measured during the first scan should not be altered with the increasing in cross-linking level. For the second scan, however, [T.sub.m] and [X.sub.c] decrease with the increasing in cross-linking level. The reason of this decrease comes from the fact that, in the second scan, the PCL samples were recrystallized in the presence of crosslinks. During the recrystallization process, formation of cross-links between the polymer chains play the role of defect centers, which restrict chain mobility and hinder the folding and packing of PCL chains. Thus, higher extent of cross-linking should lead to lower [T.sub.m] and [X.sub.c], and this is what is observed in Fig. 4 and Table 2.

TABLE 2. Melting temperature, heat of fusion ([DELTA][H.sub.m]), and
crystallinity ([X.sub.c]) of neat and cross-linked PCL with different
gel contents.

                                      First run

Gel fraction (wt%)    [T.sub.m]   [DELTA][H.sub.m]  [X.sub.c] (%)
                    ([degrees]C)   (J [g.sup.-1])

         0               58.3            68.5            50.4
        30               58.9            70.8            52.1
        65               58.6            69.4            51.0
        81               58.8            70.0            51.5

                                     Second run

Gel fraction (wt%)    [T.sub.m]   [DELTA][H.sub.m]  [X.sub.c] (%)
                    ([degrees]C)   (J [g.sup.-1])

         0              57.1            67.2            49.4
        30              56.0            59.4            43.7
        65              54.7            56.7            41.7
        81              54.0            56.1            41.3

Studies on the crystal structure of neat PCL and cross-linked PCL were also carried out with WAXD. The X-ray diffraction results are shown in Fig. 5. We can see clearly from the WAXD patterns in Fig. 5 that PCL shows two main diffraction peaks at about 21.3[degrees] and 23.8[degrees] as reported in the literature (20). The positions and intensities of the crystalline peaks of cross-linked PCL remain unchanged after the radiation-induced cross-linking, indicating that under the used range of radiation dose, the crystal structures of PCL are not affected. The radiation-induced cross-linking of PCL occurs mainly in the amorphous phase.


The thermal stability of neat PCL and the cross-linked PCL samples with various gel content determined by thermogravimetric analysis in nitrogen is shown in Fig. 6. The thermal decomposition of all the samples experiences a one-stage weight loss as shown in Fig. 6. The onset temperature of thermal degradation of neat PCL is approximately 375[degrees]C, and the degradation completes at about 420[degrees]C. The introduction of cross-linking improves the thermal stability of the PCL, with an increase in both onset thermal degradation temperature and complete degradation temperature. For example, for PCL with 81% gel fraction, the onset degradation and the complete degradation temperature are enhanced to approximately 382[degrees]C and 430[degrees]C, respectively. Formation of more compact three-dimensional network structures can improve thermal stability because it is more stable against formation of gaseous products on heating. This is probably responsible for an increase in thermal stability of cross-linked PCL.


Mechanical Properties

Figure 7a shows storage modulus of neat PCL and the cross-linked PCL with a representative level of gel fraction. The curves of neat PCL and cross-linked PCL exhibit glassy and glass transition. Cross-linking of PCL results in slightly increased storage modulus. Figure 7b shows the tan [delta] of neat PCL and the cross-linked PCL with a representative level of gel fraction. [T.sub.g] was measured from the peak temperature of -transition from the dynamic mechanical analysis spectrum. It can be clearly seen that [T.sub.g] of cross-linked PCL increases compared with neat PCL. Introduction of cross-link junctions into polymer chains restricts segment mobility. The restrictions on motion of the segment of PCL would increase the energy requirements for the transition and, thereby, raise [T.sub.g] (21).


Figure 8 shows the tensile strength and elongation at break of neat PCL and cross-linked PCL as a function of gel fraction. The tensile strength increased with increasing gel fraction before 65 wt% gel fraction, and then it showed a decreasing tendency. The elongation at break decreased with increasing gel fraction. Cross-linking density increases with an increase in gel fraction. With an increase in cross-linking density, the restriction imposed on elongation behavior of the polymer increases. This restriction is due to smaller length of chain available for stretching. Under tensile conditions, the shorter chains reach their maximum extension first, and carry a disproportionate part of the load. The greater the density of cross-linking, the shorter is the chain between such links, and the smaller is its possible extension. Hence, a decrease in elongation at break can be observed with increase in gel content. Cross-linked PCL with 65 wt% gel fraction showed optimal mechanical property; its tensile strength and elongation at break were 48 MPa and 1320%, respectively. The increase of tensile strength and decrease of elongation at break were due to the formation of cross-linked structure, which was consistent with the gel fraction data as shown in Fig. 1.


Hydrolytic Degradation

Hydrolytic stability of PCL is a key property for the durability during applications in high humidity conditions. PCL is very sensitive to humidity, and is easily degraded under high humidity conditions. The poor hydrolysis resistance property of PCL limits many applications. Hence, the effect of cross-linking on the hydrolytic degradation is of great interest. The hydrolytic degradation of neat and cross-linked PCL were investigated by determining the weight loss of PCL versus exposed time in NaOH solution at 37[degrees]C. Figure 9 shows hydrolytic degradation curves of neat PCL and cross-linked PCL. As shown in Fig. 9, hydrolysis degradation properties of PCL are obviously retarded after cross-linking, indicating an improved hydrolytic resistance. Under a definite hydrolytic degradation time, the values of weight loss of cross-linked PCL are lower than that of neat PCL. The ultimate values of the weight loss reach 4.2 wt% for neat PCL, 3.4 wt% for PCL cross-linked in the presence of TAIC, and 2.5 wt% for PCL cross-linked in the presence of VTMS, respectively.


Hydrolytic degradation can be attributed to the hydrolysis of the ester bonds and the subsequent leaching of soluble products from the polymer. A better hydrolytic resistance of the cross-linked samples indicated that a three-dimensional network structure was more resistance to hydrolysis. To form soluble products, more ester bonds had to be hydrolyzed because of a three-dimensional network structure formed through cross-linking. The three-dimensional network structure protects the ester bonds from hydrolysis. However, the more open aliphatic polyesters (noncross-linked) were more susceptible to forming soluble oligomers, which explains their faster hydrolytic degradation. Moreover, it can be clearly seen that, although the gel fraction is very similar, the hydrolysis resistance of PCL cross-linked in the presence of VTMS is better than that of PCL cross-linked in the presence of TAIC. The reason may be that during hydrolysis process, methoxysilane hydrolysis (--Si--OH) reaction in PCL cross-linked in the presence of VTMS is easy to occur and then forms the siloxane (Si--O--Si) linkage (22), which leads to form denser network compared with that of PCL cross-linked in the presence of TAIC. Hence, PCL cross-linked in the presence of VTMS shows better hydrolysis resistance compared with that of PCL cross-linked in the presence of TAIC. The SEM micrographs of the films before and after hydrolysis are shown in Fig. 10. Films before degradation showed good consistency of VTMS or TAIC/polymer blend, exhibiting a smooth morphology after cross-linking. After hydrolysis, neat PCL and the cross-linked films were found to show some degradation on the surface. Nevertheless, from the SEM, both cross-linked films were found to degrade less significantly compared with neat PCL films, which indicated severe surface erosion arising from hydrolysis. Moreover, PCL film cross-linked in the presence of VTMS showed better hydrolysis resistance compared with that cross-linked in the presence of TAIC.



Cross-linked PCL was prepared by gamma radiation in the presence of VTMS. VTMS was efficient to enhance the formation of cross-linking network in PCL. By adding 5 wt% of VTMS, PCL with a gel fraction of 81 wt% was obtained on 70 kGy radiation doese. [T.sub.g] and tensile strength of PCL increased on cross-linking. Radiation-induced cross-linking of PCL in the presence of VTMS was found to retard hydrolytic degradation greatly. Therefore, radiation-induced cross-linking of PCL in the presence of VTMS was a good choice to improve the mechanical and thermal performance and hydrolysis resistance.


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Correspondence to: Changyu Han; e-mail: or Lisong

Dong; e-mail:

Contract grant sponsor: National Science Foundation of China; contract grant numbers: 50703042; contract grant sponsor: Chinese Academy of Sciences Direction Project; contract grant number: KTCX-YW-208.

Published online in Wiley Online Library (

[C]2010 Society of Plastics Engineers

Shusheng Wang, (1), (2), (3) Changyu Han, (1) Lijing Han, (1) Xuemei Wang, (1) Junjia Bian, (1) Yugang Zhuang, (1) Lisong Dong (1)

(1) State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Changchun, People's Republic of China

(2) College of Biology Life, Jilin Agricultural University, Changchun, People's Republic of China

(3) Graduate School of the Chinese Academy of Sciences, Beijing, People's Republic of China

DOI 10.1002/pen.21832
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Author:Wang, Shusheng; Han, Changyu; Han, Lijing; Wang, Xuemei; Bian, Junjia; Zhuang, Yugang; Dong, Lisong
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:9CHIN
Date:Feb 1, 2011
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