Thermal stability and optical properties of radiation-induced grafting of methyl methacrylate onto low-density polyethylene in a solvent system containing pyridine

Samera Ali Al-Gahtany

doi:10.1515/epoly-2022-0081

Article Open Access

Thermal stability and optical properties of radiation-induced grafting of methyl methacrylate onto low-density polyethylene in a solvent system containing pyridine

Samera Ali Al-Gahtany

Published/Copyright: November 23, 2022

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From the journal e-Polymers Volume 22 Issue 1

Abstract

In this study, the grafting of methyl methacrylate (MMA) in a solvent system containing nitrogen of pyridine onto LDPE films was performed using the post-irradiation technique in nitrogen at different gamma doses. The DG% obtained in MMA grafting was 71.0% at 10 kGy of γ dose was increased to 90% in (MMA/Py) (80/20 v/v%) system, indicating the existence of Py enhancement in the grafting % of MMA. The addition of pyridine (Py) into MMA matrix increases the molecular weight of the matrix due to the plasticizing effect of Py on the system. Morphological and structural changes in optical properties and thermogravimetric analysis were performed for the films. According to Fourier transform infrared data, a reaction may be placed between Py and MMA molecules. Furthermore, the effect of Py molecules on the optical properties of LDPE films is studied. The optical transition upon the grafting process increased, indicating the movement of the electrons due to intramolecular hydrogen bonds between MMA and Py molecules. The Urbach energy and the optical band gab, E _g, were investigated and found to depend mainly on the grafting degree. The results obtained from E _g calculations recommended using an irradiation dose of 15 kGy to get LDPE-g-MMA/Py films with suitable optical properties.

Keywords: radiation; grafting; optical properties; kinetics of thermal decomposition; activation energy

1 Introduction

Radiation-induced grafting of monomers onto a polymeric matrix is a process where monomer molecules are covalently bonded to polymer chains through active species and the active substrate generated by ionizing radiation (1). In this one-step grafting process, irradiation of polymer and monomer simultaneously leads to free radicals forming. The subsequent free radicals form chemical bonds on the surface of the base polymer (2). The graft distribution may occur throughout the base matrix due to the radiation activation of the substrate across its thickness. This means that desirable properties are introduced into the polymer by introducing functional groups without much affecting its original properties (3). In other words, the radiation-induced grafting technique is a useful tool for modifying the surface and the available properties of polymeric materials (1). The irradiation techniques are used widely to prepare advanced materials due to their ecofriendly methods (2,4,5).

Poly(methyl methacrylate) (PMMA) is created by initiating the polymerization of methyl methacrylate (MMA) monomer via gamma irradiation techniques. It is widely employed in templates, optoelectronic devices, detection, and separation, thanks to its monodisperse diameter, high biocompatibility, outstanding transparency, and various chemical groups originating from the ester group (6). Radiolysis reactions induce gamma irradiation, the impact of radiolysis on numerous kinds of polymers, polymer blends, and copolymers’ compatibilization, physicochemical, mechanical, and other properties. It was discovered that radiolysis affects a variety of qualities, including compatibility, phase adhesion, crystallinity, hardness, tensile strength, and yield stress (7,8,9,10,11).

The applications of polyolefin, such as high-density polyethylene, low-density polyethylene (LDPE) and polypropylene, are limited due to their weak hydrophilicity (12,13). After grafting using a polar monomer, the surface and functional properties developed and the hydrophilicity of the membranes is improved and achieves unique properties. Ionizing radiation grafting of LDPE improves properties, such as electrical conductivity, biocompatibility, and adhesion, depending on the graft yield and the number of functional groups, which can be dominated by appropriate grafting conditions (9,14,15). The grafting conditions include the temperature, kind, and composition of solvent and monomer, as well as the irradiation type and dose. The properties of the grafted LDPE depend on the type and length of grafted polymer chains created, its density in the whole grafted layer, the location of grafting, and so on (16). Considerable studies were reported on the grafting degree induced by pre-irradiation, post-irradiation, or simultaneous irradiation as well as grafting conditions of LDPE using one or binary mixture of monomers (17–19). The advantage of the pre-irradiation method is to increase the penetration depth and is recommended for crystalline base polymers, while the disadvantage is to prevent the formation of a homogeneous polymer/gel in the solution (20). The “direct” graft method and the “post” method are performed when the polymer immersed in the monomer solution is irradiated under vacuum, inert gas, or in air. Grafting induced by irradiation of the polymer in the air or inert gas is faster and characterized by its low chain scission, simplicity, and high potential in industrial applications (21).

In this study, the grafting of MMA in the presence of pyridine onto LDPE films was performed using the post-irradiation technique in nitrogen. The aim is to improve the surface of LDPE by grafting through gamma irradiation. The grafting process enhances the grafted LDPE films’ thermal stability and optical properties. The effect of irradiation dose on graft yield was studied at room temperature. The grafted films were characterized using Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), thermo gravimetric analysis, and UV-Vis spectroscopy.

2 Experimental

2.1 Materials

LDPE films (thickness ∼ 50 μm) were supplied by El-Nasr Co. for Medical Supplies, Cairo, Egypt. MMA monomer (C₅H₂O₂ = 100.12) and pyridine (Py, C₅H₅N = 79.1) were obtained from Sigma Aldrich, Germany. Chemical reagents and solvents were of a laboratory scale and were used as received without purification.

2.2 Irradiation induced grafting onto LDPE

Strips of LDPE films 10 cm × 10 cm were washed with acetone, dried at 70°C in a vacuum oven, weighed = W _o, and then rolled and put one by one into a glass tube containing a monomer solution. The composing solution in each tube was 80/20 MMA/Py (v/v) monomer and pyridine/methanol as a co-solvent. Monomer concentration was 30 wt% of methanol. The components in the glass tubes were flushed with nitrogen several times to drive away the air in the tubes. The irradiation of the tubes was carried out using ⁶⁰Co-γ-source at a dose rate of 9.23 kGy·h⁻¹ for different doses of (5–30 kGy). The irradiated tubes were stored in a dark chamber at room temperature for 3 days. Afterward, grafted films were extracted and washed by hot water and then acetone to extract traces of any homopolymer or residual monomers accumulated on the film surface. The grafted samples were then dried under a vacuum at 50°C for 24 h until constant weight = W _g, gravimetrically.

The grafting degree (DG%) was calculated gravimetrically as follows:

(1) DG % = ( ( W g – W o ) / W o ) × 100

2.3 Characterization

FTIR spectroscopy of the prepared samples was recorded in the absorbance mode (IRAfinity- Shimadzue, Japan) in the wavelength range from 400 to 3,500 cm⁻¹. The optical property (UV–Vis) absorbance of the obtained samples was achieved in the wavelength ranges from 200 to 900 nm using a Shimadzu-UV2600 spectrophotometer. An SEM (Jeol, JSM at a voltage of 20 kV) was used to monitor the structural–morphological characteristics of the prepared samples. Thermogravimetric analysis (TGA) was carried out in nitrogen from room temperature up to 600°C using thermogravimetric analyzer (TGA-50, Shimadzu). The nitrogen gas flow rate was 50 mL·min⁻¹ and the heating rate was 20°C·min⁻¹.

3 Results and discussion

3.1 Effect of gamma dose on grafting yield

Gamma ray-induced grafting of the MMA/Py (80/20) mixture was performed on LDPE films with different irradiation doses. Gamma ray-induced grafting of MMA onto LDPE film with a single γ dose equal to 10 kGy was prepared in methanol for comparison. The effect of irradiation dose on the grafting yield is presented in Table 1. It is clear that DG% within the irradiation dose ranging from 5 to 30 kGy gradually increases with increasing irradiation dose. The increases in γ dose increase the number of the formed free radicals, leading to increased DG%. This is in agreement with the same results obtained in ref. (22); the increase of the absorbed dose leads to an increase in grafting%. The degree of MMA grafting % was 71.0% at 10 kGy of γ dose was lower compared to DG% (90%) obtained in MMA/Py exposed to similar irradiation dose, indicating that the existence of Py enhancement the degree of MMA grafting. Graft polymerization of many monomers onto low-density polyethylene by ionization effect of groups is a very effective method leads to reducing the mutual repulsion between the hydrophobic polyethylene and the ionized monomer, in addition, helps to the growing the macromolecular radicals (23). The irradiation-induced polymerization of MMA in the percent of Py was reported. The addition of pyridine into MMA matrix increases the molecular weight of the matrix due to the plasticizing effect of Py on the system (24). Plasticization of the polymer matrix leads to more chain mobility and reduces the glass transition temperature (25). The degree of grafting depends mainly on the plasticizing activity of the grafting mixture solution (26). The variation in Hildebrand solubility parameters of the grafting solution and base polymer substrate might also disrupt the secondary interactions in the LDPE chain and consequently increase the degree of grafting (27).

Table 1

Effect of irradiation dose on DG% of the grafted LDPE films

Sample	Monomer	Irradiation dose (kGy)	Degree of grafting (%)
SM10	MMA	10	71.0
SMP5	MMA/Py (80/20)	5	35.0
SMP10	MMA/Py (80/20)	10	90.0
SMP15	MMA/Py (80/20)	15	118.5
SMP20	MMA/Py (80/20)	20	132.0
SMP30	MMA/Py (80/20)	30	140.0

Monomer concentration: 30 wt% in methanol.

3.2 FTIR

The FTIR analysis of LDPE, LDPE-g-MMA, and LDPE-g-MMA/Py, grafted at 10 kGy, are shown in Figure 1a–c, respectively. The LDPE spectra (Figure 1a) are characterized by C–H symmetry stretching beak at 2,899 cm⁻¹ and CH₂ stretching and bending deformation beaks at 1,474 and 737 cm⁻¹, respectively (28). The LDPE-g-MMA (Figure 1b) showed additional bands at 1,736 cm⁻¹, which attributed to C═O ester groups of PMMA (29,30). The bands at 1,448, 987, and 755 cm⁻¹ were consistent with CH₂ scissoring, wagging, and rocking modes of PMMA. The band corresponding to CH stretching at 2,899 cm⁻¹ has a lower intensity compared to LDPE blank sample. This could attribute to the intermolecular hydrogen bonding formed during grafting of MMA onto LDPE. The LDPE-g-MMA/Py (Figure 1c) exhibited an additional band at 1,580 cm⁻¹ due to the stretching of aromatic C═C of Py moieties. Also, it could be shown that the band at 755 cm⁻¹ observed in Figure 1b is no longer present, indicating a reaction between Py and MMA molecules.

Figure 1

IR spectra of (a) LDPE, (b) LDPE-g-MMA, and (c) LDPE-g-MMA/Py, grafted at 10 kGy.

3.3 SEM

The SEM of original LDPE and grafted LDPE at different irradiation doses was performed and is presented in Figure 2a–f. The surface of original LDPE in Figure 2a is smooth with no observable discontinuities. The grafted LDPE films at irradiation doses of 5, 10, 15, 20, and 30 kGy are presented in Figure 2b–f, respectively. The surfaces of the grafted films are observed to be rough compared to the pure LDPE sample. Significant white clusters with a relatively narrow size distribution were observed and uniformly dispersed along the surface of the grafted films, indicating that a grafting polymerization is placed on the surface of LDPE. The observed increase in the number of white clusters with increasing radiation dose indicated that the grafting degree is significantly increased (an increase of DG%) and confirms the morphological and structural changes due to the grafting process onto LDPE.

Figure 2

SEM images of (a) LDPE, (b) LDPE-g-MMA/Py, 35% graft, (c) 90%, (d) 118.5%, (e) 132%, and (f) 140%.

3.4 UV-Vis spectroscopy

In most grafted polymers, the base chain is linked with a function group to enhance a significant property, such as electrical and optical conductivity. The achieved developed property depends on the type and how the function group interacts with the host matrix and its density. Investigating the optical absorption of polymeric materials is a useful technique to investigate the optical transition in the matrix, which is considered the key for determining the band gap structure and gives information about lattice fluctuations (31). The optical transition occurs through the direct transition of the electrons in the valence band, across the energy band gap, to the conduction band, or through the indirect transition of these electrons that move to the conduction band after interacting with the phonons resulting from the lattice vibration 3. The smaller the energy gap, the higher the electrical conductivity.

The UV-Vis absorption coefficient (α) of LDPE and grafted LDPE at different irradiation doses (DG%) in the region 200–900 nm is shown in Figure 3. α = 2.303A/t was calculated using the model Davis and Mott (32) where A is the UV-Vis absorbance and t is the film thickness. No absorption bands have been observed for LDPE in the studied wavelength range. A shoulder band at ∼260 nm, with different absorption intensities, has been observed for the irradiated LDPE-g-MMA/Py films. This band is attributed to π–π* of unsaturated C═O and C═C bonds confirmed by FTIR spectroscopy for MMA and Py, respectively, and indicates the interaction of MMA/Py with LDPE matrix (33). The observed results show different absorption edges, at different wavelengths (edg) in Table 2, changes with DG% due to the formation of intermolecular and intramolecular hydrogen bonding in the polymer matrix. Suppose the energy of UV photons is insufficient for electron transfer (its energy is lower than the value of gap energy). In that case, an increase in α value occurs, followed by an exponential decrease in the density of states, creating an absorption edge called the Urbach edge and ending with a tail (Urbach tail) (33). These shifts in the absorption edge at different λ _edg support the variation in the energy gap (E _g) values and microstructural changes with the change in the irradiation dose (34). The Urbach tail width characterizes Urbach energy (E _U), which gives information about the level of defects (short-range crystalline order) in the forbidden gap (33).

Figure 3

The absorption coefficient vs wavelength of LDPE and grafted LDPE.

Table 2

UV-Vis parameters; absorption edge, absorption maximum, Urbach energy and optical energy band gap of ungrafted and grafted LDPE

DG (%)	Absorption edge (λ _edg) (nm)	λ _max (nm)	E _U (eV)	Energy gap E _g (eV)
LDPE (0 kGy)	280	252	0.362	5.1
35.0 (5 kGy)	288	258	0.369	4.2
90.0 (10 kGy)	288	259	0.491	4.1
118.5 (15 kGy)	294	259	0.521	4.0
132.0 (20 kGy)	300	261	0.540	4.0
140.0 (30 kGy)	303	261	0.572	4.0

The Urbach tail, E _U, was calculated for all samples using the relation (33):

(2) α ( ν ) = β exp h ν E U

where β denotes a constant, h is Plank’s constant, and (v) represents the frequency of the incident UV-Vis rays. A relation between the photon energy (E = hv) and ln(α) was plotted for all films and E _U was calculated from the slope of the exponential edge as shown in Figure 4. The obtained results are given in Table 2. It was observed that the E _U values of grafted LDPE are higher than that of blank LDPE and increase with DG%. The increase in E _U values indicates an increase in localized defect states in the band gap (disorder) in the films upon grafting.

Figure 4

The logarithm of the absorption coefficient, ln(α), against photon energy for LDPE and grafted LDPE.

Generally, the value E _g of the irradiated grafted polymers depends on the density and length of grafted polymer chains as well as the change in microstructure of the main chain due to irradiation. In contrast, E _U depends on the polymer matrix’s disordering (defect level).

E _g values were determined from the absorbance measurements of UV-Vis spectrophotometry using the relation (32):

(3) α h ν = α 0 ( h ν − E g ) n

where α _o is a constant related to the band tail range and n is an empirical index that characterizes the optical transition. n is equal to 1/2 and 3/2 for directly allowed and direct forbidden and equal to 2 and 3 for indirectly allowed and indirect forbidden in the quantum mechanical sense, respectively. Plots of (αhv)^1/n vs photon energy (hv) using the data of absorption coefficient of all samples are presented in Figure 5. By extrapolating the linear portion (best fit when n = 2) of the obtained data to the point ((αhv)^1/2 = 0), the optical band gap of the sample was determined. The E _g values were correlated with the absorption edge position (λ _edg) of the grafted irradiated polymers’ UV-Vis absorbance, as shown in Table 2. As shown in Table 2, λ _edg is shifted toward the longer wavelength region while E _g values are decreased with increasing irradiation dose up to a dose of 15 kGy and then leveled off. This means that grafting performed to create the energy states induced by the effect of irradiation dose (DG%) and the irradiation dose of 15 kGy is recommended to obtain an LDPE-g-MMA/Py with suitable optically conductive films.

Figure 5

The plots of (αhv)^1/2 vs (hv) for all films.

3.5 TGA

The TGA from room temperature up to 600°C of LDPE and grafted films is shown in Figure 6. The TGA curve revealed one obvious stage for pure LDPE and the onset temperature was 376°C, after which a complete breakdown of LDPE backbone was observed. While the grafted films showed three-step degradation and their onset temperature depending on DG%. The multistage decomposition assigned to the (i) elimination of adsorbed moisture (dehydration), (ii) decomposition of graft side chains, and (iii) LDPE main backbone decomposition (breakdown of polymer). All samples tabulate the onset temperature and char residue at 600°C in Table 3. The first stage of weight loss is due to water and moistures in the sample while after 6,000°C is the char residue. It is observed that the onset temperature gradually decreases with DG%. This indicates that the thermal stability of the new film is higher than that of grafted film, which gradually decreases with the increase in DG%. This result indicates that the chains’ length of the grafted MMA/Py gathers with the increase in the grafting degree.

Figure 6

Initial TGA thermograms for LDPE and LDPE-g-MMA/Py.

Table 3

The TGA and the energy of activation (E _a) for the thermal decomposition of LDPE and grafted LDPE

DG (%)	Onset temp. (°C)	Char residue at 600°C (%)	Activation energies E _a (10⁻¹⁹ eV·mol⁻¹)
0	376	3.7	133.75
35.0	316	3.9	192.5
90.0	265	4.65	205.9
118.5	269	5.43	213.1
132.0	267	5.59	188.1
140.0	265	6.25	162.5

Horowitz and Metzger method (35) was used to investigate the energy of activation (E _a) for the thermal main chain decomposition for all samples. A relation of ln{ln((W ₀ – W _f)/(W _t – W _f))} vs θ(= T – T _s) was plotted (Figure 7) and E _a was calculated from the slope of the main decomposition region as follows:

(4) ln ln W o − W f W t − W f = E a θ R T s 2

where W ₀, W _f, and W _t are the initial, final weights, and the weight of the sample at time t, respectively, R denotes the gas constant (R = 8.314 J·K⁻¹·mol⁻¹), T represents the sample temperature, and T _s is a reference calculated from:

(5) W t − W f W o − W f = 1 e

Figure 7

Plot of ln{ln((W ₀ − W _f)/(W _t − W _f))} against θ for LDPE and LDPE-g-MMA/Py.

E _a was calculated for LDPE and the grafted samples and is presented in Table 3. It was observed that E _a of LDPE is higher than that is for grafted films. The values of E _a of the grafted films increase with DG% up to DG = 118.5% (at 15 kGy of irradiation dose) and then decrease gradually with increasing DG%. These results support the calculation of the optical band gap in which 15 kGy of radiation dose is appropriate to get LDPE-g-MMA/Py.

4 Conclusions

Gamma rays induced grafting of the MMA/Py (80/20) onto LDPE films with different irradiation doses (5, 10, 15, 20, and 30 kGy). The effect of irradiation dose on the grafting yield is increased. FTIR analysis of LDPE and grafted films at different irradiation doses was performed. Grafted films showed bands consistent with CH2 scissoring, wagging, and rocking modes of PMMA. There is a significant increase in white clusters with increasing radiation dose. In most grafted polymers, the base chain is linked with a function group to enhance a substantial property, such as electrical and optical conductivity. Investigation of optical absorption of polymeric materials is a useful technique to investigate the optical transition in the matrix. A shoulder band at 260 nm, with different absorption intensities, has been observed for the irradiated LDPE-g-MMA/Py films. This band is attributed to the formation of unsaturated C═O and C═C bonds. The morphological and optical properties and thermal stability of LDPE films that grafted with MMA/Py (80/20 v/v) mixture using gamma irradiation with different gamma doses (0–30 kGy) were investigated. The structural changes due to grafting were detected using the FTIR spectroscopy. The increase in γ dose leads to an increase in the number of the formed free radicals, which leads to an increase in the grafting degree. It was observed that the E _U values of grafted LDPE are higher than that of blank LDPE, increasing the gamma dose indicating disordering in the films upon grafting. The results obtained from E _g calculations recommended using an irradiation dose of 15 kGy to obtain an LDPE-g-MMA/Py film with suitable optical properties. The thermal stability of the blank LDPE film was higher than that of grafted LDPE film, which gradually decreases on irradiation and indicates that the chain length of the grafted MMA/Py gathers with the increase in the grafting degree.

Funding information: Author states no funding involved.
Author contributions: Samera Ali Al-Gahtany: writing – original draft, writing – review and editing, methodology, formal analysis.
Conflict of interest: Author states no conflict of interest.

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Received: 2022-08-11

Revised: 2022-09-18

Accepted: 2022-09-30

Published Online: 2022-11-23

This work is licensed under the Creative Commons Attribution 4.0 International License.

Articles in the same Issue

https://doi.org/10.1515/epoly-2022-0081

Keywords for this article

radiation; grafting; optical properties; kinetics of thermal decomposition; activation energy

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