Nuclear Science and Technology,^ • No. 2(2013), 1
Mechanical properties and thermal stability of poly (L-lactic acidl treated by Co-60 gamma radiation
Iran Minh Quynh^'*, Nguyen Van Binh', Pham Duy Duong\
Pham Ngoc Lan^ Hoang Phuong Thao', Le Thi Mai Linh"
' Hanoi Irradiation Center, Vietnam Atomic Energy Institute, No.S. Minli Khai, Tu Liem. Hano
^ Hanoi University of Science, Vietnam National .Umversity. Nguyen Trai. ThanhXuan, Hanoi 'Email: tmgthuguynh<'S)yahoo. com
(Received 27 June 2013, accepted 24 September 2013)
Abstract: Poly (L-lactic acid) (PLLA) was mixed with 5 wt% polyethylene glycol 1000 g.mol"' , .^
as a plasticizer and 3 wt% U^allyl isocyanurate JTAIC) as a crosslinking agent for preparation ofthe plasticized PLLA films. The crosslinking p l M ^ ^ d materials were prepared from the plasticized PLLA by irradiation with various radiation doses under the Cobalt-60 gamma radiation source ai Hanoi Irradiation Center. The crosslinking stmctures were introduced in different formulations of PLLA, and the crosslinking density increased with radiation dose and seemed to be saturated al 50 kGy. The resulting stable crosslinking network inliibited the mobility for crystallization of PLLA chains. As a result, thennal stability of the crosslinking plasticized PLLA increased, and the plasticized PLLA crosslinked with TAIC at 50 kGy become much higher than that of initial PLLA with a very small endothermic peak at its melting temperature in the DSC thermogram. The stress- strain curves ofthe crosslinking plasticized PLLA showed the toughness ofthe materials reduced but still higher than that of initial PLLA, whereas its tensile strength was much improved by radiation crosslinking.
Keywords: L-lactic acid, polyethylene, crosslinking, thermogram.
I. ESITRODUCnON
By the end of the last century, biodegradable polyesters have been attracted great attention from scientists and managers as the promising candidates to replace for synthetic plastics and polymers, which are usually none or less degraded for long time after disposal to the envirormient. Among these, poly (L-lactic acid) (PLLA) is a polyester, which can be produced from renewable resource has been much studied [1].
Since PLLA is a thermoplastic polymer with good biocompatibility, non-toxic and having relative high tensile and performance, it has been applied in various fields, from medicine, agriculture, biotechnology, industry to environment [2]. However, the number of PLLA application is still limited because its poor thermal stability as well as its low tensile strength and modulus, which are not met requirements of industrial processing. PLLA
can be processed using injection-ino'' I^DiTipiession-molding, extrusion
^feermoforming etc. but some drav,
^BiLluding high cost, brittleness, toughnes Jjjjj^w thermal distortion temperature limit applications. The material properties processibility of PLLA have to be imp
^ ^ r expansion its applications. Many dil methods were applied to improve not oi thermal stability but also other propertie ' Ss copolymerization, blending with monomers or polymers having high tl
•Stability, stereocoinplexation between 1 D-enantiomers, annealing trea plasticization. and crosslinking [3].
Modification of PLA by copolymerization or physical blending is useful tool for decreasing its brittleness, and heat distortion temperature. Various additives such as plasticizer, toughening agents, reinforcing^
fillers and compatibilizers were incorporated
©2013 Viemam Atomic Energy Society and Vietnam Atomic Energy Institute
MECHANICAL PROPERTIES AND THERMAL STABILITY OF POLY (L-LACTIC ACID)..
into PLLA matrix [4]. Recently, radiation crosslinking was also proved to be a usefiil method for enhancing the mechanical and thermal stability of PLLA [5]. Ionizing radiation can be applied as an initiation agent replacing for the chemical initiators in polymerization reactions. Radiation degradation is applied to prepare shorter segments with the same characteristics of the origin materials. Radiation crosslinking and radiation grafting are also applied to create new materials with improved properties [6-9].
In recent studies, triallyl isocyanurate (TAIC) has been proved as the best crosslinking agent for preparation of the crosslinked PLLA, and the gel fraction of the radiation-induced crosslinking PLLA increased with the ratio of TAIC to 3 % and leveled off, suggested that the 3 % TAIC was enough for the radiation crosslinking PLLA samples with high crosslinking density [10]. Our previous results also revealed that the heat resistance of the radiation crosslinking PLLA materials is much improved, but the crosslinked materials become harder and more brittle.
Recent studies of PLLA plasticized with polyethylene glycol (PEG) have indicated that the efficiency of plasticization increased with decrease of PEG molecular weight [ U ] . In a previous study, we found that 5 wt% of PEG 1000 is suitable to improve the toughness of the radiation crosslinking PLLA [12].
Therefore, in the present study, the crosslinking PLLA films were prepared from P L L A / 5 % P E G 1 0 0 0 / 3 % T A I C by gamma irradiation, and their thermal stability and mechanical properties were investigated with radiation dose.
n. EXPERIMENTAL A. Materials
PLLA pellet (4042D grade, melting point of about 160°C) was purchased from
NatureWorks (Malaysia branch). PEG 1000 and TAIC were bought from Sigma Aldrich (United State) and Tokyo Chemical Inc.
(Japan), respectively.
B. Sample preparation and Irradiation treatQiQit
PLLA (92%), PEG 1000 (5%) and TAIC (3%) were melt-mixed at 180 ± 5°C in the plastic mixer (Brabender, Haake, Germany) into homogenous blend. About 14 g blend were put between 2 stainless steel molder, preheated to 180°C for 3 min, hot-pressed at the same temperature imder 150 kg/cm"
pressure for other 2 min, then cold-pressed using water circulation. The resulting PLLA films were sealed in PE bag, and irradiated in air at the same dose rate of about 4.3 kGy per hour with various radiation doses under Cobalt-60 gamma source at Hanoi Irradiation Center.
C. Characterization
The radiation crosslinking PLLA samples were characterized by the crosslinking density and structure. In this study, the crosslinking densities obtained in the crosslinking samples were measured by gel fraction in chloroform according to following equation:
Gel Section (%) = (W/Wo) ^100 (1) where W^ is weight (dry) of the crosslinked PLLA, W. is the weight remaining (dry gel component) of the crosslinked film after dissolved in chloroform at RT for 24 h.
The structure of the crosslinking gels formed in irradiated polymers determined their capacity in adsorption of solvent.
Therefore, the dried PLLA gels were immerged in chloroform and their swelling degree (time) was calculated by the following equation:
D^ree of swelling (time) - (W^ - WJAV. (2)
TRAN MINH where W^ is the weight of the dried gel extracted from the crosslinking PLLA, W^ is the weight ofthe gel swollen in chloroform at RT for 48 h. pp and PCHCD are densities of PLLA and chloroform, respectively.
About 5 mg of PLLA was put in the aluminum pan, sealed and set in the sample holder of a differential scanning calorimeter (DSC). The sample was heated ftom room temperature to 200°C in air, then cooled with the same heating and cooling rate of lO^C per min. Mehing point ( r „ ) . glass transition temperature (7"g) and enthalpy of melting (AHm) of each sample from DSC thermogram. And its degree of crystallization was calculated as follow;
Xc(%) = I00x. {AH^ + 4 / 4 ) / 135 (3) where AH^ and 4 / 4 are enthalpies of melting and crystallization, respectively. Heat of flision of PLLA crystal (AH^ is 135 J.g' as determined by Miyata and Masuko [13].
Thermal degradation behavior of PLLA was investigated by a thermo gravimetric analysis using a TG/DTA {Institute of Chemistry). About 10 mg sample was put on
QUYNH et al.
sample holder, heated ftom room temperature to 500°C with a heating rate of 10°C per min under nitrogen flow of 30 mL per min, and the amount and rate of change in the weight of a material were recorded with temperature.
Dynamic mechanical analyses (DMA) of the typical crosslinking PLLA samples were carried out with a DMA-7e (Perkin Elmer, Malaysia Nuclear Agency). The film was cut into a rectangular specimens of 20 x 12 x 0.5 mm. Measurements were performed at a frequency of 1 Hz under nitrogen atmosphere ftom 30 to 200°C with heating rate of 5°C per min.
PLLA sheets were cut into dumbbell samples of Type V according to ASTM D 638. The sample was fixed in the gauges form the top, provided that the length between 2 gauges was kept at a determined distance.
Mechanical properties of PLLA samples were measured using a tensile with a 10 kN load, 5 mm.min'' in crosshead rate. Sttess-strain curve was recorded with time at room temperature and the mechanical properties were determined by the film thickness. At least 3 samples were tested for each material.
Table I. Gel traction and swelling degree ofthe radiation-induced crosslinking PLLA/5%PEG/3%TAIC with radiation dose
Radiation dose (kGy) Non-irradiated
10 20 30 50 100 ND: Non-detected
Gel Fraction
(%)
ND ND 9.04 67.30 85.42 88.64
Degree of Swelling (time)
35.26 26.86 22.70 10.53
MECHANICAL PROPERTIES AND THERMAL STABILITY OF POLY (L-LACTIC ACID)...
m . RESULTS AND DISCUSIONS A. Gel behavior of the radiation oosslinking PIXA san:q>Ies
Gel fractions of the irradiated samples were determined with radiation dose as presented in Table I. The results indicated that the crosslinking networks was not produced in the PLLA/PEG/TAIC by gamma irradiation with the dose below 10 kGy, though TAIC has been proved as a good crosslinking agent for PLLA [5, 10]. It may be due to the presence of oxygen during irradiation for the PLLA films in our present study accelerated oxidation and prevented the formation of crosslinking sites at low radiation dose. Also, the presence of PEG may inhibit the recombination of macromolecular radicals formed during gamma irradiation.
A significant insoluble gels were observed in other samples irradiated with dose higher than 20 kGy. The gel fractions of PLLA/PEG/TAIC are 9.04, 67.3, 85.43 and 88.64% by irradiation at 20, 30, 50 and 100 kGy, respectively. The gel fraction quickly increased with radiation dose to 50 kGy and leveled off with fiarther increasing ofradiation dose up to 100 kGy. These results suggested that the dose of 50 kGy is enough for
introduction ofthe crosslinking network in the plasticized PLLA samples. Figure 1 shows one possibility of the crosslinking network produced in the plasticized PLLA by gamma irradiation.
Table I also revealed the swelling behavior of the crosslinked gels. It is interesting that the swelling degree quickly increased with increasing of radiation dose to 30 kGy, then significantly decreased with higher radiation doses, though the gel fraction almost the same. This was attributed to the change of the crosslinking structure. In generally, radiation degradation and crosslinking coin concurtently occurred in the plasticized PLLA during irradiation, but the presence of crosslinker, TAIC speeded up the crosslinking between the polymer chains as observed from Figure 1. With increasing of radiation dose, the number of radicals increased. As results, the probability of crosslink between polymer and crosslinker also increased, and tighter crosslinking networks with higher crosslinking point were formed in the samples irradiated at higher dose, and their degree of swelling decreased.
X
•y Irradiation
- PLLA chain _ PEG 1000 chain
TAIC molecules
Fig. 1. Possible crosslinking network formed in the irradiated PLLA/PEG/TAIC 24
TRAN MINH QUYNH et al.
Table E. Thermal properties of PLLA/PEG/TAIC with radiation dose Radiation dose T, (°C) TK (°C) T.(°C) AH„(Jg-') Xc (%)
Neat PLLA Non-iiradiated
10 kGy 20 kGy 30 kGy SO kGy 100 kGy
58.01 37.88 38.24 39.73 42.83 44.07 39.69
128.40 112.63 112.53 11489
•
152.40 148.70 147.35 146.99 144.24 144.56 146.43
21.11 20.46 18.54 14.91 0.49 0.37 5.78
16.02 15.16 13.73 11.04 0.36 0.28 4.28
Temperature (°C)
Fig. 2. Thermo gravimetry thennographs of PLLA (a); plasticized PLLA (b); the plasticized PLLA inflated at 10 (c); 20 (d); 30 (e) and 50 kGy (f).
B. Thetmsl propeities and stability of tbe crosslmking PLLA
DSC thermograms of neat PLLA and crosslinking plasticized PLLA samples were recorded with temperature, and their thermal properties were presented in Table II. As one can see, the glass transition, cold crystallization and melting temperature of initial PLLA were much reduced by adding 5% PEG and 3 % TAIC. This may due to the plasticization effect of PEG for PLLA.
However, the glass transition temperature (Tg)
of the plasticized PLLA recovered by gamma irradiation. The stable crosslinking networks introduced to PLLA restrained the mobility for crystallization of polymer chains. As results, degree of crystallization of the crosslinked PLLA reduced with radiation dose. The higher the radiation dose, the lower is the crystallization degree. Even the plasticized PLLA crosslinked at dose higher than 30 kGy showed no crystallization and very small melting peak. These results suggested that the crosslinking samples become more stable when temperature rises.
25
MECHANICAL PROPERTIES AND THERMAL STABILITY OF POLY (L-LACTIC ACID)...
This result is entirely suitable with our As on can see from Table III, the initial previous studies on the thermal properties of decomposition temperature for plasticized the radiation induced crosslinking PLLA [10]. PLLA was 276.2°C, and about 87.4%
TGA curves for different PLLA samples PLLA/PEG/TAIC were thermal degraded at were showed in Figure 2. PLLA started to be around 300°C, its remaining mass exhibited a pyrolysed at around 285°C and its weight higher heat resistance and seemed to be rapidly reduced with temperature to 350°C. completely degraded at around 400°C. The While the plasticized PLLA displayed two- two-step degradation may be due to the step degradation with heating, the crosslinking crystallization domains of PLLA or PEG did PLLA showed single-step decomposition not be plasticized, still kept thermal stability similar with initial PLLA. Thermal stability of like initial PLLA. All crosslinking PLLA PLLA much reduced by adding of PEG and samples become more stable with heating. The TAIC, but it recovered by gamma irradiation. temperature where 50% sample mass was It suggested that the crosslinking networks pyrolysed for the crosslinking PLLA increased produced in polymer made it become harder to and its weight loss decreased with radiation be melted and thermal degraded. dose.
Table M. Thermo gravimetric data ofthe crosslinking plasticized PLLA Samples T „ „ ("C) T „ ^ ( ° C ) Weight Loss (%) Initial PLLA
PLLA/PEG/TAIC PLLA/PEG/TAIC-lOkGy PLLA/PEG/rAIC-20 kGy PLLA/PEG/rAIC-30 kGy PLLA/PEG/rAIC-50 kOy
285.7 276.2 273.5 293.1 311.2 325.2
309.9 298.9 304.7 318.2 344.6 357.7
98.4 994 92.9 97.0 96.4 96.0 Table IV. Mechanical properties of PLLA, plasticized and crosslinking PLLA samples
Samples Tensile strength Young's modulus Elongation at break (MPa) (MPa) (%) Initial PLLA
Plasticized PLLA/PEG/TAIC PLLA/PEG/TAIC-10 kGy PLLA/PEG/TAIC-20 kGy PLLA/PEG/rAIC-30 kGy PLLA/PEG/rAlC-50 kGy PLLA/PEG/TAIC-100 kGy
51.76 38.63 32.62 42.32 47.54 53.11 58.18
804.77 523.63 579.21 679.57 737.61 841.53 777.04
3.5 256.8 200.1 68.2 28.6 13.7 10.3
26
TRAN MINH Q U Y N H et al.
-Initial PLLA -Plasticized PLLA -CrosslinkingPLLA-50
30 40 50 60 70 80 90 K T e m p e r a t u r e ("C)
Fig. 3. Tan 8 ofdifferent PLLA samples with temperature Stress-strain curves ofthe different PLLA
films were recorded using a tensile tester, and their mechanical properties were presented in the Table IV. Initial PLLA shows rather high modulus and tensile strength meet the requirements of many applications, but its toughness is not enough for the application in industry with low elongation at break of 3.5%
only. After mixing with PEG, the elongation at break of plasticized PLLA was about 80 times of the value of initial PLLA, whereas it's tensile and modulus reduced. The tensile strength and young's modulus of the plasticized PLLA were recovered by gamma irradiation, while their elongation at break gradually reduced, but still higher than that of initial PLLA. These results suggested that the radiation induced crosslinking PLLA become harder and tougher, namely is more stable in mechanical aspect.
Dynamic mechanical analyses (DMA) of typical PLLA samples were investigated for clarification their miscibility and glass transition temperature. Figure 3 shows the tan delta of initial PLLA. plasticized and crosslinked PLLA as functions of temperature.
The peak of tan 6 revealed as the glass
transition temperature (Tg) of the polymer samples. The temperature and intensity ofthe tan delta peak of PLLA were obviously decreased by plasticization effect of PEG, suggested that the presence of plasticizer retarded the segmental motion of polymer matrix during the transition. However, their mobility might be somewhat recovered by gamma irradiation. Thus, the crosslinking network inhibited the motion of PLLA chains for crystallization, but not for transition.
IV. CONCLUSION The crosslinking network was introduced into the PLLA/PEG/TAIC during gamma irradiation. The gel fraction ofthe crosslinking samples increased and their degree of swelling decreased with radiation dose. PEG of 5 wt%
and radiation dose of 50 kGy are considered to be optimal condition to prepare the crosslinking plasticized material with crosslinking density of about 85 %.
Plasticization effect of PEG was much reduced the thermal properties of PLLA, but the crosslinking network made the materials become more stable with heating and the crosslinked at dose higher than 30 kOy 27
MECHANICAL PROPERTIES AND THERMAL STABILITY OF POLY (L-LACTIC ACID)..
required the higher temperature for pyrolysis of 50 % initial mass, even Tmjdsei of the PLLA/PEG/TAIC-50 kGy about 50°C higher than that of initial PLLA. The flexibihty of PLLA much increased by plasticization, while it's tensile and modulus reduced. The mechanical stability of the plasticized PLLA was recovered by radiation crosslinking.
Elongations at break ofthe crosslinking PLLA samples were reduced, but still higher than that of initial PLLA. Thus, mechanical properties of PLLA were significantly improved by radiation crosslinking.
[1]
[2]
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