Introduction

Biodegradable polymers have received increasing attention, and are widely used in various applications, especially in biomedical, packaging, agriculture and environmental fields. Polylactic acid (PLA) is among the most popular biodegradable polymers, as this not only possesses good mechanical properties, but is also biocompatible and biodegradable. Nonetheless, low thermal stability, brittleness, low gas-barrier and lack of functionality are its shortcomings. To overcome these drawbacks, many approaches have been performed, such as stereo-complexation, copolymerization, molecular design and blending with other polymers.1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14

Copolymerization of lactide with specific functional comonomers is of interest for introducing functionality into PLA chains. Louis et al.14 reported the synthesis of a copolymer of lactide and glycidol by ring-opening polymerization. The resulting hyper-branched poly(lactic acid-co-glycidol) copolymer contained hydroxyl groups, which served as reactive sites for further modifications, such as a curing reaction to generate thermoset copolyester. However, content and placement of these hydroxyl functional groups were not well defined, as these are also correlated with other factors, that is, Tg, molecular weight, mechanical properties, structures and architectures of the copolymer. It is a challenge to control molecular weight, structures and chain topology of a copolymer.

In our attempt to overcome this challenge, we have developed a synthesis process of random poly(lactic acid-co-glycidol) copolymer, whose molecular weight and hydroxyl content are adjustable by varying the molar ratio of lactyl to glycidol units in feed.15 Incorporating of methacrylate groups is also of interest. Glycidyl methacrylate (GMA) has been employed as a reactive compatibilizer in polymer blends.16, 17, 18 Methacrylic anhydride has been used in preparation of crosslinked poly(ɛ-caprolactone) and polylactide oligomers.19, 20 Nonetheless, these reactive functional groups have rarely been incorporated and distributed along PLA polymer chains.

In this work, a synthesis route of a new functionalized PLA copolymer, poly(lactic acid-co-glycidyl methacrylate), P(LLA-co-GMA), is developed. Effects of polymerization conditions, that is, temperature, time, comonomers feed ratio and catalyst type on structures and properties of the copolymer are investigated. Curing behavior of the copolymer is evaluated by employing photo- and thermo-crosslinking processes. Mechanical properties of the resulting cured copolymer are also examined in terms of compressive stress.

Experimental procedure

Materials

L-lactide monomer was prepared in our laboratory, and purified by recrystallization in ethyl acetate. Tin octoate, Sn(Oct)2, was purchased from Wako (Tokyo, Japan). Ethyl acetate, chloroform, ethanol and toluene solvents were purchased from Lab Scan (Bangkok, Thailand). GMA, magnesium ethoxide, Mg(OEt)2, and benzoyl peroxide thermo-initiator were purchased from Sigma-Aldrich (St Louis, MO, USA). Camphorquinone photo-initiator was purchased from ESSTECH (Essington, PA, USA), and 2-dimethylaminoethyl methacrylate was purchased from Fluka (St Louis, MO, USA). All chemicals were used as received.

Synthesis of P(LLA-co-GMA)

P(LLA-co-GMA) was synthesized by ring-opening polymerization using a 90/10 LLA/GMA molar feed ratio. Effect of catalyst types on reaction efficiency and properties of the copolymer was investigated by employing Sn(Oct)2, Mg(OEt)2 and (C6H5)4Sn. The reaction was conducted in a bulk process using 0.3 mol% catalyst, with the polymerization temperature and time set at 100 °C and 24 h, respectively. Effect of polymerization conditions on properties and structures of the copolymers was also examined by varying reaction temperatures and times at 80, 90 and 100 °C for 4, 10 and 24 h. For the study on effect of GMA feed contents, the reactions were carried out in tetrahydrofuran (THF) solution at 100 °C for 24 h, with the GMA content ranging from 2, 5, 10, 20 and 30 mol%.

Crosslinking process of P(LLA-co-GMA)

Curing behaviors of P(LLA-co-GMA) copolymers containing various GMA contents were studied by photo- and thermo-crosslinking processes. Samples in a disk form with a diameter of 1.0 cm and thickness of 0.5 cm were prepared. In the photo-crosslinking process, the copolymer was first dissolved in chloroform. Camphorquinone photo-initiator (1 wt% of copolymer) and 2-dimethylaminoethyl methacrylate reactive monomer (1:1 by wt on Camphorquinone basis) were then added. The solution was dry-cast to form disk specimens. The samples were then irradiated under visible blue light at a wavelength of 468 nm using 3M ESPE Elipar 2500 (3M ESPE Dental Supplies, St Paul, MN, USA) at various crosslinking times. In a thermo-crosslinking process, P(LLA-co-GMA) was first dissolved in chloroform. Benzoyl peroxide (2 wt%) was then added to the solution. The mixture was stirred at room temperature and dry-cast as sample discs. The crosslinking process was then conducted in a vacuum oven by setting appropriate temperatures and times.

Characterizations

Average molecular weights and molecular weight distributions (Mn, Mw and PDI) were measured by gel permeation chromatography using a Waters 150-CV gel permeation chromatograph and employing a mono-disperse polystyrene standard. The glass transition (Tg) and melting temperatures (Tm) of the copolymer were determined by differential scanning calorimeter (DSC) on a DSC822e mettler Toledo. The sample was scanned twice from −20 to 200 °C with a heating and cooling rate of 20.0 °C min−1. All data were taken from the second scan. Chemical structures of the copolymer were examined using an AVENCE 300 MHz digital nuclear magnetic resonance spectrometer (NMR, Bruker Biospin; DPX-300, Rheinstetten, Germany) and fourier transform infrared spectrometer (FTIR, EQUINOX55, Bruker, Optics, Ettlingen, Germany).

Compressive stress of the cured samples was measured on a universal testing machine (UTM, Instron model 55R4502, Instron Corp., Norwood, MA, USA) with a crosshead speed of 50 mm min−1. Sample disk specimens with 1.0 cm diameter and 0.5 cm thickness were prepared according to McCabe’s method.21 Gel content of the cured products from both thermal- and photo-crosslinking processes was determined by extracting with THF for 24 h and vacuum drying the insoluble fraction at 50 °C for overnight before weighing. The gel content was calculated from the weight ratio of the dried THF-insoluble fraction and the original sample. Degree of swelling was examined by immersing disk specimens in THF at room temperature. The specimen was removed at equilibrium swelling time, the time when its solvent uptake reached a constant value, wiped with filter paper and weighed. The degree of swelling was then calculated, as follows:

where Wo=original weight of dried sample (g)

Weq=weight of swollen sample at equilibrium swelling time (g)

Results and discussion

Chemical structures and compositions

Chemical structures and compositions of the P(LLA-co-GMA) copolymer were examined by 1H NMR spectrum, as shown in Figure 1. Characteristic peaks of lactyl units were observed at 5.1–5.2 (Hb) and 1.5–1.7 p.p.m. (Ha), which corresponded to methine proton (–O=C–CH–(CH3)–O) and methyl protons (–O=C–CH–(CH3)–O), respectively. The signal located at 4.3–4.4 p.p.m. (Hc) was assigned to methine proton at chain end (–O=C–CH–(CH3)–OH) of lactyl unit. Peaks associated with GMA appeared at 6.1 and 5.2 p.p.m. (Hh), assigned to protons next to methacrylic double-bond (–O=C–C(CH3)=CH2–). The signal at 3.7 p.p.m. (He) was due to methylene protons of (–O–CH2–CH–O) in the backbone, whereas those located at 4.1–4.3 p.p.m. (Hd, Hf) were assigned to methine proton (–O–CH–CH2–O) and methylene protons (–O–CH–CH2–O) on side chains. The peak at 2.0 p.p.m. (Hg) represented methyl protons located next to the double-bond of comonomer unit (–(CH3)C=CH2).

Figure 1
figure 1

1H NMR spectrum of P(LLA-co-GMA) copolymer.

Effect of catalyst types

Various catalysts, that is, Sn(Oct)2, Mg(OEt)2 and (C6H5)4Sn, were used in the synthesis of P(LLA-co-GMA) using a 10-mol% GMA feeding content in a bulk process. Chemical structures and properties of the resulting copolymers are presented in Table 1. The incorporation of GMA in the copolymer chains was 7.2 and 6.5 mol% when Sn(Oct)2 and Mg(OEt)2 were employed, respectively. However, a relatively-low GMA composition (3.4 mol%) in the copolymer chains was obtained when (C6H5)4Sn was used. This is likely owing to the sterically bulky (C6H5)4Sn groups obstruct the insertion of incoming comonomers. This is also reflected by the molecular weight of the resulting copolymers, where those with higher Mw were produced from Sn(Oct)2 (9131 g mol−1) and Mg(OEt)2 (6566 g mol−1) compared with that of (C6H5)4Sn (5930 g mol−1).

Table 1 Effect of catalyst types on structures and properties of P(LLA-co-GMA) copolymers

All copolymers showed lower Tm than that of PLA homopolymer (178 °C). Effect of catalyst types on Tm and ΔHm of the copolymers in descending order is as follows; Sn(Oct)2>Mg(OEt)2>(C6H5)4Sn. Tg of the copolymers were observed in the range of 21–36 °C, which were also lower than that of PLLA (55.9 °C) and PGMA (84 °C). A copolymer with the lowest Tg was obtained when Mg(OEt)2 catalyst was employed, probably because this material consists of large fractions of low MW chains with high MW distribution (1.6 times higher than other samples). The decrease of Tm and ΔHm of the copolymers is largely because the incorporated comonomer units act as defects that interrupt the long crystalline sequence of PLA. The low MW nature of the copolymers also has a major role in the reduction of Tm and crystallinity of the copolymers, where apparent correlation in these values is observed. Nonetheless, the results conclusively indicate that Sn(Oct)2 is the most appropriate catalyst for this reaction.

Effects of polymerization temperature and time

P(LLA-co-GMA) were synthesized using 20 and 30 mol% GMA in feed, at different polymerization temperatures and times, that is, 80, 90 and 100 °C for 4, 10 and 12 h. Properties of the resulting copolymers are shown in Table 2. With the use of 30 mol% GMA feeding and a 12-h reaction time, a low GMA content (8.5 mol%) was incorporated into the copolymer chain when the reaction was carried out at 80 or 90 °C. In contrast, a crosslinked copolymer network was directly obtained upon increasing the polymerization temperature to 100 °C. This is likely due to a long polymerization time. As a result, the reaction time was shortened to 10 and 4 h. It was found that at 10 h polymerization time, the auto-crosslinking reaction still occurred, despite the reduction of the mol% GMA feeding. However, a further decrease in polymerization time to 4 h generated a copolymer that was completely soluble in THF. Moreover, the molecular weight of the resulting copolymer was also higher than that synthesized at lower polymerization temperature.

Table 2 Effects of polymerization temperature and time on properties of P(LLA-co-GMA)

Effect of comonomer feed ratios

Effect of comonomer composition on the properties of P(LLA-co-GMA) was investigated by varying LLA:GMA molar feeding ratios at 98:2, 95:5, 90:10, 80:20 and 70:30. Figure 2 shows 1H NMR spectra of the resulting P(LLA-co-GMA) copolymers as a function of comonomer feed ratios. Integration of GMA signals at 6.1 and 5.2 p.p.m., due to methylene protons next to the double-bond, clearly increased with GMA content in the copolymer chains. The copolymer compositions were calculated from the ratio of integration of peaks at 6.1 or 5.2 p.p.m. to that at 5.3 p.p.m. (methine proton of L-lactide monomer units). The results, as summarized in Table 3, indicated that the GMA composition in the copolymer chains were lower than those in the feed, and the difference increased with the GMA feed contents. The molecular weights of the copolymers decreased with an increase of GMA feed compositions. This reflects that the introduction of GMA retarded the polymerization of LLA.

Figure 2
figure 2

1H NMR spectra of P(LLA-co-GMA) copolymers at various GMA contents (a) 0 mol% GMA, (b) 1.7 mol% GMA, (c) 2.3 mol% GMA and (d) 9.5 mol% GMA.

Table 3 Effects of comonomer feed ratios on properties of P(LLA-co-GMA)

FTIR spectra, in the 1850–1550 cm−1 region, of P(LLA-co-GMA) copolymers are shown in Figure 3. Characteristic bands at 1760 and 1720 cm−1 are due to C=O stretching modes of PLLA and GMA, whereas that at 1640 cm−1 was assigned to the C=C stretching mode of GMA. A unique band at 815 cm−1 is due to a −CH2 deformation mode of GMA. The band ratio of the two C=O stretching modes can be employed to determine the GMA content in the copolymer chains. Also, the 1640 and 815 cm−1 bands are conveniently used in quantitative measurements of the C=C content, that is, the copolymer’s GMA content, as the band was clearly separated from other vibrational modes. Results on band area of these modes were strongly correlated with the GMA contents calculated from 1H NMR spectra.

Figure 3
figure 3

FTIR spectra of P(LLA-co-GMA) copolymers at various GMA compositions: (a) 0, (b) 9.5, (c) 14.2, and (d) 19.2 mol% GMA.

DSC thermograms of P(LLA-co-GMA) copolymers are shown in Figure 4, and the correlation of copolymer’s chain structures and thermal properties are summarized in Table 3. Melting temperature (Tm) of copolymers varied with the GMA content in the chains from 154–125 °C. The melting peak, associated with PLLA domains, shifted to lower temperature with an increase of GMA content, and disappeared when the content was higher than 14%. This indicates the disruption of crystal formation of PLLA segments upon introduction of GMA units. Glass transition temperature (Tg) of the copolymers varied from 29–37 °C, compared with 56 °C for pure PLLA. The decrease of Tg with an increase in the GMA content was likely because the presence of side chains from comonomer units facilitated the segmental movements. The low molecular weight nature of the copolymers may also lead to a reduction in their Tg values. However, upon further increasing the GMA content >19 mol%, the Tg of the copolymer started to increase, probably because of a domination of GMA characteristics, which has a high Tg.

Figure 4
figure 4

DSC thermograms of P(LLA-co-GMA) copolymers with various GMA contents.

Crosslinking behaviors of P(LLA-co-GMA)

Curing behaviors of P(LLA-co-GMA) with various GMA content were investigated by photo- and thermo-crosslinking processes. The detailed results are as follows:

Photo-crosslinking process

Effects of irradiation time and the initiator content on crosslinking efficiency were examined via the measurement of gel content. The initiator and the copolymer’s GMA contents were first fixed at 0.5 wt% and 9.5 mol%, whereas various crosslinking times at 2, 4, 6, and 8 min were employed. Table 4 indicates that the gel content of all cured products derived from various times was significantly similar (72–75%). The shortest curing time of 2 min was, therefore, chosen for further study to avoid deterioration of samples due to over-irradiation. When the initiator content was increased from 0.5 to 1.0 wt%, the gel content increased from 72 to 86%. A further increase in the content to 1.5 and 2 wt%, however, led to no significant difference in the gel content. Therefore, a 1.0-wt% initiator content was employed as a proper parameter, where the gel content of 86% was obtained. Given these results, it is apparent that the crosslink density of the cured products cannot be further increased by changing the irradiation conditions, unless the copolymer’s GMA content is increased. Curing of the copolymers consisting of higher GMA compositions was then investigated by employing the pre-determined curing conditions. The gel content increased to 89 and 95%, when copolymers constituting of 14.2 and 19.2 mol% GMA were employed.

Table 4 Effects of GMA content, irradiated time and the initiator content on gel content (%) of cured P(LLA-co-GMA) products

Thermo-crosslinking process

Effects of curing conditions on the gel content of crosslinked products derived from P(LLA-co-GMA) copolymer with 9.5 mol% GMA were investigated by varying curing temperatures from 80–120 °C and crosslinking times from 15 min to 6 h. Figure 5 shows that the gel content values were strongly dependent on both parameters. The results indicate that the optimum conditions for thermo-crosslinking of this copolymer were 120 °C and 15 min, as this provided the highest gel content value at the shortest curing time. The use of higher temperature led to higher gel contents in shorter curing time. This is likely to be because at higher temperatures, unsaturated chain-pendants of the copolymer units had a high degree of movement that increased the efficiency of the crosslinking reaction. Chains translation may also have a role on the kinetics of this curing reaction. Given that these are related to the viscoelasticity of the copolymer, the time-temperature equivalence principle22 is applicable to this curing behavior. Our attempt to preliminarily apply the principle to the data obtained from the curing reactions at different temperatures is shown in Figure 6, where master curves correlating the gel content and the curing time were constructed at three different temperatures by shifting of the curing time scale. The predicted values are beneficial in extrapolating the curing behaviors beyond the practical temperature range, either at low temperatures, which require long curing time, or at high temperatures where thermal degradation may compete with the curing reaction.

Figure 5
figure 5

Gel content (%) of crosslinked P(LLA-co-GMA) as a function of crosslinking temperature and time.

Figure 6
figure 6

Predicted gel content of crosslinked P(LLA-co-GMA) as a function of curing time at: (a) 120, (b) 100, and (c) 80 °C, calculated from the time-temperature equivalence principle.

Mechanical properties

Compressive stress and degree of swelling of the thermally cured products with various gel contents were measured. The results, as summarized in Table 5, indicate that compressive stress of copolymer samples increased with the gel (and GMA) contents. Up on introducing GMA units to PLLA chains, a drastic drop in the compressive stresses is observed (from 19.4 MPa for neat uncured PLLA to 3.0 MPa for the copolymer containing 9.5 mol% GMA). This is due to the much lower MW and crystalline content (ΔHm) of the copolymer sample (Table 3). An increasing trend of the compressive stress as a function of GMA content, that is, the crosslink density, is observed in P(LLA-co-GMA) cured samples, despite a decrease in their original MW and crystalinity (Table 3). This indicates a dominating effect of the crosslink density on the compression stress, which represents viscoelastic behavior of the cured samples. It was observed that Tg of the cured products increased by 15–20 °C, compared with that of the original copolymer samples, as shown in Figure 7, indicating movement restriction of polymer segments, especially chain-ends, due to the formation of crosslink points. It is concluded that the thermal and mechanical properties of the copolymers can be enhanced by the crosslinking process. As expected, the degree of swelling of the cured samples decreased with an increase in the crosslink density (gel content).

Table 5 Compressive stress and degree of swelling of cured P(LLA-co-GMA) products
Figure 7
figure 7

DSC thermograms of cured (dash line) and original (solid line) copolymers. consisting of various GMA compositions: (a) 9.5 mol%, (b) 14.2 and (c) 19.2 mol%.

Conclusion

Curable P(LLA-co-GMA) copolymers, which possess biocompatible and degradable properties, were synthesized via ring-opening polymerization. Effect of catalyst types and their content on efficiency of the reaction was examined, with tin octoate observed as the most effective catalyst. GMA composition and chain structures of the copolymers are controllable by adjusting the GMA monomer feed composition and reaction conditions. Thermal properties and curing behaviors of the resulting copolymers are strongly dependent on GMA content in the copolymer chains, and also curing processes and conditions. The photo-crosslinking process is proven as a practical method in the curing of the copolymers, because the reaction is almost complete within 2 min, as indicated by gel content results. Mechanical and thermal properties of the cured products are enhanced, with the degree of improvement being dependent on the content of the curable GMA units. The resulting copolymers have high potential for use in biomedical applications, where biocompatibility, degradability and high strength are required.