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DOI: 10.1039/C5RA13285A (Paper) RSC Adv., 2015, 5, 70473-70481

Effect of the content and distribution of ultraviolet absorbing groups on the UV protection and degradation of polylactide films

Feijie Ge, Yanyun Ding, Long Yang, Yun Huang, Long Jiang* and Yi Dan*
State Key Laboratory of Polymer Materials Engineering of China, Polymer Research Institute of Sichuan University, Chengdu 610065, P. R. China. E-mail: jianglong@scu.edu.cn; danyichenweiwei@163.com; Fax: +86-2885402465; Tel: +86-28-85407286

Received 7th July 2015 , Accepted 3rd August 2015

First published on 10th August 2015

Abstract

Introduction of a UV absorbing group onto a polymer chain through covalent bonding greatly reduced the UV light transmittance of the resulting polylactide (PLA) film and preserved the high transparency to visible light. Compared to simply blending a UV absorber with a PLA matrix, covalent bonding of the UV absorbing group with polylactide enabled the prepared films to have better dispersion of the UV absorbing group, stable solvent resistance, better protection effect against UV damage and a slower rate of UV irradiated degradation, while the distribution of the UV absorbing group made no difference to the aforementioned properties. The degradation of both the PLA film covalently bonded with a UV absorbing group and the PLA/UV-absorber blended film followed the same mechanism as pure PLA films, in that the alkyl-oxygen bond broke first to produce acyl-oxygen and secondary carbon radicals, which then captured hydrogen to form carboxyl groups and alkyl groups at the end of the fractured polymer chain. More UV absorbing groups at the chain end were favorable for reducing UV transmittance, providing a better protection effect on the packaged probe, and slowing the UV irradiated degradation of PLA film. The thermal stability was dependent on the molecular weight of polylactide and was hardly affected by the introduction of the UV absorber or UV absorbing group.

1. Introduction

Due to concerns of an energy crisis and pollution of waste plastics, green polymers1 that are bio-renewable and degradable have attracted massive attention in world-wide research. Polylactide (PLA) has been regarded as the most promising of these because of its renewable resources2,3 and biodegradability.4–6 Polylactide is naturally brittle, with elongation at break less than 6%,7 hence the rich research on polylactide toughening7–9 so that PLA might be applied in many fields such as vehicles, packaging, agricultural mulch films and medical materials.10 In addition to poor flexibility, the hydrolysis of PLA11,12 in humid environments is also a threat to the utility of PLA materials, thus several references13–16 have reported modification to delay the hydrolytic behavior of PLA. Another disadvantage of PLA in urgent need of modification lies in its high transmittance of ultraviolet (UV) light17 and poor resistance to UV irradiated degradation,18–21 which is unfavourable for the protection of packaged goods and the lifespan of the material itself used either as packaging or in the medical field.19,22,23 However, some inorganic particles have the function of blocking or absorbing UV light. The side effects of incorporating such particles into a PLA matrix are decreased transparency,24 poor compatibility and accelerated degradation25 under UV irradiation. Resveratrol, a naturally occurring polyphenol, was used to dope PLA26 to enhance the photostability of the resulting material, and inhibited the thermal oxidation and UV light irradiated degradation. However, resveratrol is not resistant to extraction by solvents or food as it is soluble in many compounds. To make a strong combination between the UV absorber and polymer matrix, grafting technology27 was applied to modify the surface of wood so that the material can be well protected against damage from UV light, but this technology was not favorable for PLA as on the one hand, it lacks a reactive group on the main chain, and on the other hand, it easily degrades if exposed to UV grafting.13,28–30 Given that polylactide is synthesized by ring opening polymerization (ROP) of lactide with the initiation of an initiator, and that a residual group of the initiator remains in the resulting polylactide, the idea of utilizing a UV absorber as an initiator will provide the resulting PLA with UV absorption functionality. In our previous work31 we successfully designed and synthesized polylactide with a UV absorbing group covalently bonded to the end or middle of the polymer chain, i.e. PLA-B (terminated by one 2-hydroxy benzophenone group), PLA-DB (terminated by two 2-hydroxy benzophenone groups) and PLA-HB-PLA (with one 2-hydroxy benzophenone group in the middle of the polymer chain as a pendant group), but the number average molecular weight (Mn) obtained was not high except for PLA-DB. Besides, the UV protection effect and UV irradiated degradation behavior of the preceding films remain unknown. In this work, films of PLA-B, PLA-DB, PLA-HB-PLA, PLA-C (polylactide initiated by cetyl alcohol) and PLA-C/UV-0 (2,4-dihydroxy benzophenone) blended films with Mn higher than 50[thin space (1/6-em)]000 are prepared. The effect of content and distribution of the UV absorbing groups on the UV-vis transmittance, solvent extraction resistance, surface morphology, crystallization, thermal stability, UV stability and UV protection effect of the prepared films are investigated so that they can be evaluated as UV-resistant packaging materials. By comparing the aforementioned properties of the above PLA films with covalently bonded UV absorbing groups and the PLA-C/UV-0 blended films, a new type of polylactide material that can transmit in the range of visible light but resist ultraviolet light will be developed.

2. Experimental

2.1. Materials and preparation of PLA films

β-Carotene (96.5%) and 2,4-dihydroxy benzophenone (UV-0, 99%) were commercially available from Shanghai Jinchun Biochemical Technology Co., Ltd. (China). L-Lactide (99.7%, with D-lactide content less than 1%) was purchased from Shenzhen Brightchina Industrial Co., Ltd. (China). Tin(II) 2-ethylhexanoate (Sn(Oct)2, 95%) was purchased from Sigma-Aldrich (Japan). Chloroform (99%), ethanol (99.7%) and n-hexane (99%) were provided by Kelong Chemical Reagent Factory of Chengdu (China). All chemicals were used as received.
Synthesis of polylactides. Scheme 1 shows the polylactides linked to UV absorbing groups that were synthesized according to the references and characterized therein.31 The difference was that the monomer to initiator ratio ([LA]0/[I]0) varied for polylactides with different Mn. In brief, three UV absorbers named 2-hydroxy-4-(3-methacryloxy-2-hydroxylpropoxy)benzophenone (BPMA), 2,2′-dihydroxy-4,4′-(2-hydroxylpropoxy)dibenzophenone (DHDBP), and 2-hydroxy-4-(2,3-dihydroxy propoxy)benzophenone (HPBP) were synthesized and used as initiators respectively, and stannous octoate (Sn(Oct)2) was used as a catalyst in the ring opening polymerization (ROP) of L-lactide. For example, PLA-B350 (with 350 being the monomer to initiator ratio) was synthesized as follows: 2 g L-lactide and 0.014 g BPMA were weighed and added into a flame-dried glass tube at room temperature, then 0.0028 g Sn(Oct)2, 1/2000 of the molar amount of L-lactide, was added with a syringe. The tube was subsequently evacuated by an oil pump and filled with argon (99.999%) repeatedly for at least three cycles. After it was evacuated and sealed with an alcohol blow lamp, the tube was immersed in an oil bath at 145 °C for 5 h under magnetic stirring, and then the solid products were dissolved in 40 ml chloroform, precipitated in cold ethanol, filtered and dried in vacuum at 40 °C to give a constant weight. Likewise, PLA-C350, PLA-C400, PLA-DB400 and PLA-B2000 were defined, synthesized and purified.
image file: c5ra13285a-s1.tif
Scheme 1 Schematic presentation of the chain structures of polylactides.
Film preparation. Films were prepared by a solution casting method. PLA-B350, for example, was prepared by the following procedure: first, 0.3 g of PLA-B350 powder was weighed and added into a 10 ml glass bottle previously filled with 6 ml chloroform, followed by magnetic stirring for an hour to form a 5% (w/v) solution. Then the solution was cast into a flat bottom Petri dish with a diameter of 90 mm and allowed to evaporate at room temperature overnight. Finally the Petri dish was put into a vacuum oven at room temperature to give a constant weight. Films of PLA-C350, PLA-C400, PLA-DB400, PLA-HB350-PLA and PLA-B2000 were prepared likewise. For the PLA-C350/UV-0 film, the only difference was that 0.0013 g of UV-0 was weighed and added together with 0.3 g of PLA-C350, while for the PLA-C400/UV-0 film the added amount of UV-0 was 0.0022 g. The PLA-C350/UV-0 film had a similar molecular weight and similar amount of the UV absorbing group as PLA-B350 and PLA-HB350-PLA did, and was used for comparison with the latter. Similarly, PLA-C400/UV-0 corresponds to PLA-DB400. See Table 1 for information about molecular weight and its distribution, and the UV absorber content of the samples. The films were peeled off and measured (with a thickness gage, manufactured by Shanghai Liuling Instrument Factory, China) to be 42 μm in average thickness.
Table 1 Monomer/initiator ratio, UV absorber content, molecular weight and its distribution of prepared PLA films
Sample [LA]0/[I]0a UV absorber content (%) Mnb (g mol−1) PDIc
a Monomer to initiator ratio.b Number average molecular weight determined by GPC.c Polydispersity index, determined by GPC.
PLA-B350 350 0.59 50[thin space (1/6-em)]000 1.19
PLA-HB350-PLA 350 0.57 52[thin space (1/6-em)]000 1.48
PLA-C350/UV-0 350 0.53 50[thin space (1/6-em)]000 1.37
PLA-C350 350 0 50[thin space (1/6-em)]000 1.37
PLA-DB400 400 0.80 63[thin space (1/6-em)]000 1.60
PLA-C400/UV-0 400 0.73 58[thin space (1/6-em)]000 1.46
PLA-C400 400 0 58[thin space (1/6-em)]000 1.46
PLA-B2000 2000 0.12 137[thin space (1/6-em)]000 1.33

2.2. Tests

2.2.1 Test of the UV protection effect on the probe. To investigate the UV protection effect of the prepared films, a simulation protection test of the PLA film on packaged goods was conducted using a hexane solution of β-carotene as a probe and two UV-C lamps (PHILIPS T8, Netherlands) with length of 450 mm, main wavelength at 254 nm and power of 25 W each, as the irradiation source. Probes of equal volume were placed in silica cells and each cell was covered with different PLA films with surfaces perpendicular to the irradiating UV light with intensity of 9.8–10 W m−2 (measured by a UV-C irradiatometer, Photoelectric Instrument Factory of Beijing Normal University, China) as shown in Fig. 1.
image file: c5ra13285a-f1.tif
Fig. 1 Schematic presentation of the UV protection test of the PLA films.
2.2.2 Test of solvent extraction resistance. The films were cut into small pieces with sizes of 20 mm × 8 mm. Each sample piece was immersed in ethanol (6 ml) in a separate glass bottle for the same period of time, after which the sample was taken out for determination of UV transmittance as well as the UV absorbance of residual ethanol. The total time of extraction was 66 h.
2.2.3 Test of exposure to UV irradiation. The films labelled PLA-C350, PLA-B350, PLA-HB350-PLA, PLA-C350/UV-0, PLA-C400, PLA-DB400, PLA-C400/UV-0 and PLA-B2000 were exposed to the same UV irradiation as illustrated in Section 2.2.1, with air flow for a total of 34 h, during which samples were taken out after different intervals for characterization and analysis.

2.3. Characterization

2.3.1 UV-vis spectroscopy. UV-vis transmittance of the PLA films and UV absorbance of the residual extraction solvent were determined by a UV-2300 UV-vis spectrophotometer (Shanghai Techcomp Ltd., China). Data were collected from 190–800 nm in the transmission mode for the films and from 220–800 nm in the absorbance mode for the extraction solvent.
2.3.2 Scanning electron microscopy (SEM). SEM (Quanta-250, FEI company, USA) was used to observe the surface of the PLA-DB400 film and PLA-C400/UV-0 blended film at a voltage of 5 kV, with a power of magnification of 80[thin space (1/6-em)]000 (1 μm) and 20[thin space (1/6-em)]000 (5 μm) respectively.
2.3.3 Gel permeation chromatography (GPC). Variation of molecular weights (number average molecular weight Mn) and polydispersity index (PDI) of polylactide films during UV irradiation were monitored by a Shimadzu GPC instrument with tetrahydrofuran (HPLC grade, Sigma-Aldrich Co., USA) as mobile phase at a flow rate of 1 ml min−1 and oven temperature of 40 °C. Polystyrene standards with narrow molecular weight distributions were used for calibration.
2.3.4 Differential scanning calorimetry (DSC). A differential scanning calorimeter (NETZSCH 204, Germany) was employed to determine the melting temperature (Tm), cold crystallizing temperature (Tcc) and degree of crystallization (Xc). 5 g of each film sample was sealed in an aluminium pan and heated twice from 30 °C to 200 °C at a rate of 10 K min−1 under the protection of nitrogen. The degree of crystallization was calculated according to the following eqn (1):
 
image file: c5ra13285a-t1.tif(1)
where ΔHm is the enthalpy of fusion, ΔHc stands for the enthalpy of cold crystallization and ΔH0 is enthalpy of fusion of 100% crystalline PLA, considered as 93 J g−1.36
2.3.5 Thermal gravimetric analysis (TGA). A thermal gravimetric analyzer (NETZSCH TG 209F1 Iris, Germany) was used to measure the thermal stability of the PLA films. Samples sealed in an Al2O3 pan were heated from 30 °C to 500 °C at 30 K min−1 under nitrogen flow with a rate of 60 ml min−1.
2.3.6 Attenuated total reflection infrared spectroscopy (ATR-IR). PLA films before and after UV irradiation were characterized by ATR-IR (Thermal Fisher NICOLET IS10, USA) in absorption mode. Data were collected in the range of 4000–400 cm−1 with a resolution of 4 cm−1, after 32 scans of samples.
2.3.7 Hydrogen nuclear magnetic resonance (1H NMR). The PLA films were characterized by 1H NMR spectra to investigate changes of their structures after UV irradiation for different numbers of hours. 5 mg of each sample was dissolved in 0.6 ml CDCl3 (99.8%) containing 0.03% tetramethylsilane (TMS) as an internal standard. A Varian INOVA-400 spectrometer (Bruker, U.S.A.) was used to record the spectra at 600 MHz.

3. Results and discussion

3.1 UV protection effect of PLA films on probe

In order to study the UV protection effect of PLA films on the probe, we investigated first the UV-vis transmission of the prepared PLA films to understand their photo-characteristics. Fig. 2 demonstrates that all films show high transparency in the visible light region, indicating that the transparency of polylactide films is independent of the introduced UV absorber/absorbing group originating from UV-0, which is beneficial for the use of PLA as packaging material that requires clear observation of the packaged goods.
image file: c5ra13285a-f2.tif
Fig. 2 UV-vis transmittance spectra of the solution-cast polylactide films covalently bonded with UV absorbing groups (PLA-B350, PLA-HB350, PLA-DB400, and PLA-B2000) and blended with UV-0 (PLA-C350/UV-0 and PLA-C400/UV-0).

In terms of UV light transmittance, films containing similar amounts of the UV absorbing group or UV absorber, for example the series of PLA-B350, PLA-HB350-PLA, PLA-C350/UV-0 and series of PLA-DB400, PLA-C400/UV-0, exhibit similar transmittance of UV light in the range of 400 nm and below, irrespective of the distribution of the UV absorbing group on the polymer chain or how the UV absorber is incorporated into polylactide. PLA-DB400 and PLA-C400/UV-0 films show the lowest transmittance of UV light, less than 5%, due to the highest content (0.85%) of the UV absorber, while PLA-B2000 had the highest UV transmittance that reached 60% due to a tiny content (0.12%) of the UV absorbing group. In spite of a slight deviation in UV transmittance caused by differences in thickness of films, the results are convincing that the UV transmittance of the PLA films is dependent on the content of UV absorber/UV absorbing group whilst this is not affected by the distribution of the UV absorbing group nor by covalent bonding or physical blending.

By using a UV radiometer (Photoelectric Instruments Factory of Beijing Normal University, China) and UV-C (λmax = 254 nm) light as the source of irradiation, we measured the intensity of UV light that was transmitted through the prepared films, and results are shown in Fig. 3. As presented, the transmitted light intensity is in accordance with the UV transmittance demonstrated in Fig. 2, where higher content of UV absorber/absorbing group leads to less transmission of UV light.


image file: c5ra13285a-f3.tif
Fig. 3 Intensity of UV-C light transmitted through PLA films.

Since many vegetables, food and medicine are rich in β-carotene, and this is very sensitive to UV light, it is chosen as probe and subjected to UV-C exposure to simulate the irradiation of UV light on packaged goods. Variation of the maximum absorbance at 450 nm (A450, see UV-vis absorption spectrum in Fig. 4) with irradiation time was recorded to evaluate to what extent the packaged probes were damaged under the protection of different films.


image file: c5ra13285a-f4.tif
Fig. 4 Absorbance of β-carotene in hexane (c = 10−5 M).

As presented in Fig. 5, the probe under the protection of PLA-C400 decreases the most in absorbance over time as no UV absorber was introduced into PLA-C400. But PLA-DB400 and PLA-C400/UV-0 films show the best protection effect on the irradiated probes as the absorbances of the probes under their cover decrease the least, which results from the fact that PLA-DB400 and PLA-C400/UV-0 have the same and highest content of UV absorber among all the prepared films, greatly reducing the intensity of the transmitted UV-C light as seen in Fig. 3. Compared with PLA-DB400 and PLA-C400/UV-0, PLA-B350 and PLA-HB350-PLA have a worse protection effect due to a lower content of a UV absorbing group. PLA-B350 and PLA-HB350-PLA films containing the same content of UV absorbing group give similar protection to the probe regardless of the distribution of the UV absorbing group. In addition, Fig. 5 also reveals that PLA-DB400, PLA-B350 and PLA-HB350-PLA films have better protection effects than PLA-C400/UV-0 and PLA-C350/UV-0, respectively, with the same content of UV absorber or UV absorbing group, the reason for which is estimated to be the poor dispersion of the free UV-0 blended into PLA-C. Aggregation of UV-0 in the PLA-C matrix leads to an uneven distribution of the UV absorber, thus more severe damage of the covered probe is caused. The difference in protection effect of the PLA film (covalently bonded with UV absorbing group) and PLA/UV-0 blended film that contains a similar amount of the UV absorbing group is of interest, and thus the dispersion of the UV absorber in the PLA matrix will be investigated and analyzed in the following section, mainly by surface observations of the PLA-DB400 film and PLA-C400/UV-0 blended film.


image file: c5ra13285a-f5.tif
Fig. 5 A450 (absorbance at λ = 450 nm) of β-carotene in hexane under the protection of PLA films as a function of irradiation time.

3.2 Surface morphology of the PLA-DB400 and PLA-C400/UV-0 films

As observed in Fig. 6(a), the surface of PLA-C400/UV-0 film is severely rough due to aggregated UV-0 buried in the PLA-C matrix. The aggregation indicates poor compatibility and dispersion of UV-0 in PLA, and based on the power of magnification, the size of the particles is estimated to be in the range of 50–300 nm. On the contrary, the PLA-DB400 film presents a flat and even surface morphology, as shown in Fig. 6(b), because the residual group of the initiator is covalently bonded at the chain end of polylactide so that the UV absorbing group can not migrate or aggregate. In fact, the above analysis also supports the conclusion of the previous section that covalent bonding of the UV absorbing groups to PLA enables the films to have a better UV protection effect than PLA-C/UV-0 blended films do.
image file: c5ra13285a-f6.tif
Fig. 6 SEM photographs of (a) PLA-C400/UV-0 blended film and (b) PLA-DB400 film before UV irradiation (resolution: left, 5 μm; right, 1 μm).

3.3 Resistance to solvent extraction

Though the PLA-C400/UV-0 film can prevent UV damage to the probe to an extent as shown in Fig. 5, the physically blended UV-0 can not withstand solvent extraction or heat migration,32 thus the long-term resistance of the blended film to UV light irradiation is limited. Like many UV absorbers, UV-0 is soluble in organic solvents such as ethanol, acetone and so on. The weak physical combination between the UV-0 and PLA matrix makes it susceptible to solvent extraction when UV-0 is blended into the PLA.

As demonstrated in Fig. 7(a), the UV absorbance of residual ethanol increases with extraction time, indicating that UV-0 migrates gradually from PLA matrix into the ethanol. As a result, the UV transmittance of the PLA-C/UV-0 film elevates to 80% after an extraction time of 66 h, as shown in Fig. 7(b), indicating that only a trace amount of residual UV absorber is left in the blended films. On the other hand, PLA-B2000, PLA-B350, PLA-HB350-PLA and PLA-DB400 films remain unchanged with respect to transmittance of UV light, proving that covalent bonding between the polylactide and UV absorbing group is strong enough to resist solvent extraction.


image file: c5ra13285a-f7.tif
Fig. 7 UV-vis absorption spectra of residual ethanol solvent (a) and transmittance (b) (the total extraction time is 66 h) of ethanol-extracted PLA films.

The polylactide films with covalently bonded UV absorbing groups have shown advantages over the PLA/UV-0 blended films in their protection against UV damage on packaged probes and resistance to solvent extraction, but it remains unknown whether introducing a UV absorbing group/UV absorber to polylactide has an influence on the degradation of the resulting films. Therefore, the subsequent section will deal with effect of the distribution and content of the UV absorbing groups on the UV irradiated degradation of the prepared films.

3.4 Effect of UV irradiation on the properties of the PLA films

3.4.1 Molecular weight changes. As seen in Fig. 8, the PLA-C400 film degrades faster than PLA-C350 during the process of UV irradiation, indicating that a pure PLA film with higher molecular weight is more sensitive to UV irradiation. This could also be supported by the degradation of PLA-B2000 as this has the highest molar mass and tiny content (0.12%) of UV absorbing group but the fastest rate of degradation. Noteworthy is that with a similar content of UV absorbing group, PLA-B350, PLA-HB350-PLA and PLA-DB400 degrade slower than PLA-C350/UV-0 and PLA-C400/UV-0, respectively, evidencing that covalent bonding between the UV absorbing group and PLA leads to better UV stability than blending does.
image file: c5ra13285a-f8.tif
Fig. 8 Variation of Mn and PDIs of the PLA films with irradiation time (a), 0.5–34 h; (b), 0–5 h.

In terms of covalent bonding, PLA-B350 and PLA-HB350-PLA show a similar rate of degradation, and this proves that the distribution of the UV absorbing group on the polymer chain has no influence on the UV stability with the same content of UV absorbing group and molar mass of PLA. Moreover, PLA-DB400 shows the best UV stability among all the films due to both a higher content of UV absorbing group and its covalent bonding with polylacitde. For all films, the PDIs tend to increase first and decrease later, but the inflection point comes later for those films containing more UV absorber or UV absorbing group. This again proves that covalent bonding makes PLA films with better UV stability than blending does. After 5 hours of UV irradiation, the degradation rates of all films based on molecular weight tend to slow down with irradiation time, and the differences among the films in degradation rate are not obvious thereafter. As irradiation time extends to 34 h, the molecular weights of all the tested film samples decrease to less than 20[thin space (1/6-em)]000, and this shows that in the long run the polylactides covalently bonded to the UV absorbing group on the polymer chain are photodegradable, which is beneficial for degradation of packaging materials after usage.

3.4.2 Thermal behavior. As the intensity of the UV-C lamps used was relatively strong (about 10 W m−2), two hours of UV irradiation caused fast degradation of some of the tested films with a low content of UV absorbing group at different rates based on their molecular weights.

PLA-DB400 and PLA-C400/UV-0 have relatively high molecular weights remaining as shown in Fig. 8 since they contain the largest amounts of the UV absorbing group or UV absorber, while the others decreased so much in molecular weight that they were not practically viable any longer, thus only PLA-DB400 and PLA-C400/UV-0 are further investigated by DSC and TGA instruments.

As a result of the mild damage that resulted in a relatively high molecular weight being preserved, PLA-DB400 and PLA-C400/UV-0 films both show a slight increase in crystallinity and slight decrease in Tm values with exposure time, which are presented in Fig. 9 and Table 2. The enhanced degree of crystallization is related to the chain scission of the amorphous region and the rearrangement of the released polymer chain,33 explained as chemi-crystallization34 while a decrease in Tm is the opposite to the reference result where increased Tm was observed and speculated to be caused by crystallite thickening.34 Anyway, the cold crystallization temperature (Tcc) remained almost the same for each film as a consequence of a less decreased molecular weight, and the cooling and the second scanning traces revealed that the two films preserve an approximate degree of crystallization respectively to what had been attained before UV irradiation. This means that the UV absorber and UV absorbing group actually have a protection effect on polylactide.


image file: c5ra13285a-f9.tif
Fig. 9 DSC first heating (a), cooling (b), and second heating (c) curves of PLA-DB400 and PLA-C400/UV-0 films before and after UV irradiation for 2 h.
Table 2 Thermal parameters obtained from DSC
Sample Irradiation time (h) Tcca (°C) Tmb (°C) ΔHccc (J g−1) ΔHcd (J g−1) ΔHmd (J g−1) Xce (%)
a Tcc is cold crystallization temperature.b Tm is peak temperature of melting during the second heating.c ΔHcc is cold crystallization enthalpy.d ΔHc and ΔHm are crystallization enthalpy and melting enthalpy, respectively, with the data in the first line obtained from the first heating, and the data in the next line obtained from the second heating.e Xc is degree of crystallization obtained from the first heating (previous line) and second heating (next line).
PLA-C400/UV-0 0 98.7 175.9 −20.98 −10.81 41.87 33.4
0 47.08 50.6
2 100.0 174.2 −22.5 −11.43 46.52 37.7
−5.55 49.99 47.8
PLA-DB400 0 101.3 175.5 −10.86 −21.66 43.6 23.6
−9.98 46.58 39.4
2 101.3 173.9 −13.65 −24.98 51.93 29.0
−9.80 58.07 51.9

The thermal stability of polylactide is highly relevant to the molecular weight, and this correlation has also been proven by the TGA results of the PLA-DB400, PLA-C400/UV-0 and PLA-C400 films as shown in Fig. 10 and Table 3. The weight loss before 250 °C observed is probably caused by evaporation of water adsorbed on the films before decomposition. As observed, the onset decomposition temperature of the films decreases obviously due to UV irradiated degradation that causes a loss of molecular weight, and this is most obvious for PLA-C400. As PLA-DB400 has a higher molecular weight than PLA-C400/UV-0 after UV irradiation for two hours, it also exhibits higher decomposition temperatures (Td-5%, Td-max, and Td-end) than PLA-C400/UV-0. In conclusion, the thermal behavior of the two-hour irradiated PLA-DB400 and PLA-C400/UV-0 films reveals that the thermal stability of polylactide is neither affected by the addition of the UV absorber nor by the covalent bonding of the UV absorbing group (due to low content), but is dependent on the molecular weight of the polylactide.


image file: c5ra13285a-f10.tif
Fig. 10 Thermal gravimetric curves of PLA-DB400 and PLA-C400/UV-0 films before and after UV irradiation for 2 h.
Table 3 Thermal parameters obtained from TGA
Sample Irradiation time (h) Td-5%a (°C) Td-maxb Tendc
a Td-5% is onset decomposition temperature (at 5% weight loss).b Td-max refers to the temperature at the highest rate of decomposition.c Tend refers to the temperature at the end of decomposition.
PLA-C400 0 252.4 280.2 293.1
2 234.5 271.9 297.9
PLA-C400/UV-0 0 257.7 280.0 293.8
2 252.7 282.7 306.9
PLA-DB400 0 272.4 295.1 313.4
2 265.2 296.9 325.6

3.4.3 Mechanism of UV irradiated degradation of PLA films. To further understand the effect of UV irradiation on the evolution of the chemical structure of the PLA-DB400 and PLA-C400/UV-0 films, 1H NMR spectra are collected and presented in Fig. 11. As seen therein, the hydrogen bonding that originates from 2-hydroxyl benzophenone still exists because the peaks at δ = 12.68 are observed after even 5 hours of irradiation. In addition, peaks at δ = 1.26 and δ = 2.35 arise after UV irradiation, and they are respectively assigned to the methyl (CH3–C–O–) and methylene (C–CH2–COO–R) hydrogens. The two peaks both show higher intensity in Fig. 11(A) than in Fig. 11(B), indicating that the PLA-C400/UV-0 film is more severely damaged than the PLA-DB400 film. This might be due to low content or loss of newly formed products. No other new emerging peaks are observed in the 1H NMR spectra.
image file: c5ra13285a-f11.tif
Fig. 11 1H NMR spectra of the (A) PLA-C400/UV-0 and (B) PLA-DB400 films before and after UV irradiation for different numbers of hours.

In order to get more clear and detailed information on the chemical structure of the degraded films, the irradiation time was prolonged to 50 hours, and then the films were characterized by ATR-IR spectra. In Fig. 12, the absorption peaks of the spectra of the irradiated PLA-C and PLA-C/UV-0 films at 3140 cm−1, 3050 cm−1 and 918 cm−1 are ascribed to hydroxyl stretching vibrations and hydroxyl bending vibrations, respectively, of the newly formed carboxyl group. Obviously, the PLA-C films are severely degraded while the PLA-C/UV-0 and PLA-DB400 films are less degraded, and the newly emerging group is hardly observed in the spectrum of PLA-DB400 even after prolonged UV irradiation. This agrees with the results obtained from the 1H NMR spectra. As a consequence of the observed alkyl groups and newly formed carboxyl groups, the degradation mechanism of PLA-DB and PLA-C/UV-0 films is proposed and shown in Scheme 2. It shows that during the process of UV irradiation on PLA films, the alkyl-oxygen bond dissociates first to produce acyl-oxygen and secondary carbon radicals, both of which then capture hydrogen to form carboxyl groups and alkyl groups at the end of the broken polymer chains. This mechanism of UV irradiated degradation of polylactide agrees with what has been proposed in ref. 19 and 35.


image file: c5ra13285a-f12.tif
Fig. 12 ATR-IR spectra of the PLA films before and after UV irradiation.

image file: c5ra13285a-s2.tif
Scheme 2 Proposed photodegradation mechanism of PLA under UV-C light irradiation.

4. Conclusions

The introduction of UV absorbing groups onto polymer chains by covalent bonding greatly reduced the UV light transmittance of polylactide films, and the transmittance was dependent on the content of the UV absorbing group. As a consequence of UV irradiation, the thermal behavior, including the degree of crystallization and thermal decomposition temperature, was reduced with decrease of the molecular weight. In terms of the mechanism of UV irradiated degradation of polylactide films, the breakage of the alkyl-oxygen bond occurs first to produce acyl-oxygen and secondary carbon radicals, both of which then capture hydrogen to form carboxyl groups and alkyl groups at the end of the broken polymer chains. As to the films that have UV absorbing groups covalently bonded, the degradation rate and protection effect are not affected by the distribution of the UV absorbing group on the polymer chain but by the content of the UV absorbing group, and the more UV absorbing group the better for resistance to UV irradiation and UV protection. Addition of UV-0 into the PLA matrix may reduce the UV light transmittance, delay the degradation of polylactide and provide protection to packaged goods, but the free UV absorber has poor dispersion in the matrix and is not resistant to solvent extraction. In contrast, covalent bonding of the UV absorbing group with the polymer chain enables the resulting PLA film to have a slower rate of UV irradiated degradation and a better protection effect on packaged probe against UV damage. This makes polylactide that has a UV absorbing group covalently bonded—especially at the end of the polymer chain—a promising candidate for packaging materials that need UV light blocking and resistance to UV accelerated degradation as well as medical materials that require UV-C disinfection and sterilization, while in the long run they can be photodegradable.

Acknowledgements

The authors appreciate the financial support of the National Natural Science Foundation of China (no. 50873067 and no. 51133005) and the Research Fund for the Doctoral Program of Higher Education of China (no. 20110181110032).

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