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Synthesizing Radiation-Hard Polymer and Copolymers Using Laccol Monomers Extracted From Lacquer Tree Toxicodendron succedanea via Cationic Polymerization.

INTRODUCTION

For centuries, "oriental" lacquer has been used as coatings [1-3] and paints [1, 4] in Asian countries due to its high solvent resistance [1], excellent toughness [1], and high durability [1, 3, 5, 6]. Lacquer trees belong to the genus Toxicodendron (formerly Rhus) and family Anacardiaceae with more than 73 genera and 600 species all over the world. Out of these varieties, only three kinds of lacquer trees are able to produce lacquer sap. The first variety is Toxicodendron vernicifluum, which grows in the regions of China, Japan, and Korea with the main component of "urushiol." Second, Toxicodendron succedanea, which grows in Vietnam and Chinese Taiwan, has "laccol" as its main component. The third variety is Gluta usitata, which grows in Myanmar, Laos, Cambodia, and Thailand having the "thitsiol" as the main liquid component [7]. In recent years, although many synthetic polymers and coatings have been developed, lacquer sap is still attracting the attention because of its renewable ability, eco-friendly nature, and promising physicochemical properties [3].

The Toxicodendron succedanea tree sap from Vietnam was utilized in this study to synthesize polymer and copolymers. This lacquer sap has slow drying speed [7, 8] compared with urushiol, under 60%-80% humidity environment via enzyme-catalyzed reaction [6]. The use of filtered lacquer sap for the experiments and processing with laccase enzyme that was involved in polymerization process is widely held in reported literature [1-6, 8]. In contrast to this practice, the "laccol" (dark brown color liquid main component) extracted with acetone (C[H.sub.3]COC[H.sub.3]) from the tree sap was utilized for all the experiments conducted herein. It has C17 unsaturated hydrocarbon chain with 0-3 olefins at the 3-position of the catechol ring [3, 7, 9] and 38.7% of the monomer mixture contains three olefins in the side chain [7]. The laccase enzyme that is insoluble in C[H.sub.3]COC[H.sub.3] was removed during the extraction process as "acetone powder" [10]. The main constituent in laccol mixture (C[H.sub.3]COC[H.sub.3] soluble components) that has three olefins in the side chain was significantly involved in the cationic polymerization process incorporated herein.

Cationic polymerization is highly feasible with species containing double bonds and reactions are considerably faster than the anionic and radical polymerizations [11]. According to the predicted mechanisms in Schemes 1 and 2, water ([H.sub.2]O) interacts with aluminum chloride-ethyl acetate (Al[Cl.sub.3].EtOAc) coinitiator to generate the proton. Then, the electrophilic addition of proton to double bond takes place to give the active center of polymerization. This active center then initiates polymerization and propagation ensues. Initiation of the cationic polymerization can be achieved by using protonic acids (Bronsted) or Lewis acids [11]. A widely used Lewis acid, which is effective in cationic polymerization of styrene, isobutylene, and other monomers, is aluminum trichloride (Al[Cl.sub.3]). This Al[Cl.sub.3] initiator is involved in rapid propagation reactions in the systems including chain transfer and termination due to its strong Lewis acid nature. However, Al[Cl.sub.3] has poor solubility in organic solvents [11, 12]. To enhance the selectivity of polymerization reaction, ethyl acetate (EtOAc) was introduced to Al[Cl.sub.3] for developing an Al[Cl.sub.3].EtOAc complex [13, 14]. This coinitiator complex was utilized herein to synthesize laccol polymer (LP) and laccol-styrene copolymers. Al[Cl.sub.3].EtOAc was activated during the reaction by the residual water in laccol mixture and carbocation was formed specifically in the conjugated double bond region of the side chain of catechol ring. Two hydroxyl groups in the catechol ring which are acidic and has able to form phenoxide anions to react with carbocation species in the side chain. Additionally, formation of macrocyclic compounds is also possible due to phenoxide anions and unsaturated groups (eis and trans double bonds) in side chain as discussed by Rozentsvet et al. [15]. These phenomena were investigated using infrared (1R) and nuclear magnetic resonance (NMR) data for LP and laccol-styrene copolymers. The possible mechanisms for these processes are illustrated in Schemes 1 and 2. The synthesized polymer and copolymers were exposed to gamma radiation [16] and investigated the characteristic properties resulted from radiation treatment.

The overall hypothesis behind this work is that the lacquer studied herein will exhibit superior radiation resistance. This is substantiated by the fact that Toxicodendron succedanea contains hydroxyl phenyl groups that act as radical scavengers, enhancing radiation resistance via their ability to convert excitation energy into nonchemistry inducing energy in lacquer that has been cured [17, 18]. Further, Rogner and Langhals have shown via IR and mass spectroscopy that doses of 300 kGy (30 Mrad) electron beam (EB) do not result in measurable damage in qi-lacquer (urushi) [19]. Additionally, Harmon's group has shown that nanotubes enhance radiation hardness of organic polymer matrices exposed to gamma rays [20-22].

Irradiated polymers could undergo various reactions forming cations, anions, gases, and other species. The two main types of reactions considered herein were crosslinking and chain scission. The crosslinking can be physical, chemical, or biological [23]. The chemical crosslinking occurs between adjacent polymer molecules by forming new bonds and physical crosslinks occur due to secondary interactions like hydrogen bonding, ionic interactions, hydrophobic interactions, and due to entangle chains [24]. The biological crosslinking is also a newly immerging area, however not yet developed in industrial level as chemical crosslinking [23]. These processes could increase hardness of the polymer until it forms insoluble three-dimensional network. Chain scission decreases the hardness and increases the solubility [25]. This makes them ideal candidates for use in radiation environments such as nuclear reactors, machines for radiation therapy, industrial sterilization, and space [26]. According to the literature, radiation hardness of lacquer sap has not been investigated and references regarding lacquer sap (urushiol and laccol) are limited to polymerization/ curing studies primarily via UV and EB radiation [4, 6, 19, 27]. Therefore, in this article, the effort was made to synthesize radiation-hard LP and laccol-styrene copolymers using Toxicodendron succedanea lacquer sap (from Vietnam) via cationic polymerization. The prepared materials were investigated in two stages as cured control samples and irradiated samples by LR, NMR, gel permeation chromatography (GPC), Rheometer, thermal gravimetric analysis (TGA), DSC, wide-angle X-ray (WAXS), and Shore A hardness methods.

EXPERIMENTAL

Materials

Raw lacquer sap was imported from Viet Lacquer Interior Co. Ltd., Vietnam. Reagent grade C[H.sub.3]COC[H.sub.3] and dichloromethane (C[H.sub.2][Cl.sub.2]) were used as solvents. Anhydrous calcium chloride (Ca[Cl.sub.2]) granular, anhydrous sodium chloride (NaCl), anhydrous magnesium sulfate (MgS[O.sub.4]), analytical grade sulfuric acid, hydrochloric acid, sodium hydroxide (NaOH), and 99.99% HPLC grade tetrahydrofuran (THF) were obtained from Fischer Scientific. Reagent Plus grade 99% aluminum chloride (Al[Cl.sub.3]), 99.9% HPLC grade EtOAc, and 99% styrene monomer were purchased from Sigma Aldrich and inhibitor was removed using 15% NaOH.

Preparation Procedures

Extraction of Laccol from Lacquer Sap [5]. A 100 g of filtered lacquer sap was mixed with 300.0 mL of C[H.sub.3]COC[H.sub.3] and stirred for 2 h. The mixture was vacuum-filtered and the filtrate was collected. After much of the solvent C[H.sub.3]COC[H.sub.3] was evaporated, two layer separations were observed due to the water content. The water layer (bottom layer of the separatory funnel) was removed and the organic layer was further dried using anhydrous Ca[Cl.sub.2] granular. The vacuum evaporation was used to remove the excess C[H.sub.3]COC[H.sub.3]. Up to 60%-65% yield of laccol can be obtained from this extraction technique.

Synthesis of an Aluminum Chloride and Ethyl Acetate Complex [13, 14], EtOAc (2.3 mL, 2.25 X [10.sup.-2] mol) was added dropwise to slurry of Al[Cl.sub.3] (3 g, 2.25 X [10.sup.-2] mol) in 20.2 mL of C[H.sub.2][Cl.sub.2] for 5-10 min. The reaction was allowed to stir for 30-60 min up to complete dissolving of Al[Cl.sub.3] to give a solution of complex (Al[Cl.sub.3].EtOAc) in C[H.sub.2][Cl.sub.2] (~1 M).

Preparation of Laccol Polymer. Polymerization reactions were carried out in Erlenmeyer flasks inside an ice bath. As an example of a typical procedure, polymerization was initiated by adding a solution of Al[Cl.sub.3].EtOAc in C[H.sub.2][Cl.sub.2] (3.0 mL, ~1 M) to a mixture of laccol (10.0 g, -6.07 X [10.sup.-2] mol) and C[H.sub.2][Cl.sub.2] (7.0 mL) while stirring. After 15 min of stirring, the sample had become more viscous and then it was transferred to a polymer releasing paper that was folded to hold the sample. This was dried under vacuum oven for 4 days at 60[degrees]C and partially cured samples were then dried under hot air oven for 4 days at ~100[degrees]C. Proposed mechanism for the synthesis is illustrated in Scheme 1.

Preparation of Polystyrene. Styrene monomer was purified by solvent extraction process with 15% (W/V) NaOH and dried the organic layer using anhydrous MgS[O.sub.4] [28]. The polymerization was initiated by adding Al[Cl.sub.3].EtOAc in C[H.sub.2][Cl.sub.2] (2.50 mL, ~1 M) to the mixture of styrene (11.0 mL) and C[H.sub.2][Cl.sub.2] (6.5 mL) while stirring in an ice bath under inert nitrogen atmosphere. The mixture was stirred until it got more viscous and then it was kept under nitrogen atmosphere for overnight. This was dried in vacuum oven for 4 days at 60[degrees]C.

Copolymers of Laccol and Styrene. The copolymers were prepared by mixing different monomer ratios of laccol and styrene, as illustrated in Table 1. S-10, S-15, S-30, S-50, S-70, and S-90 sample names corresponded to 10%, 15%, 30%, 50%, 70%, and 90% (w/v) laccol in styrene, respectively. These samples were dried under vacuum oven for 4 days at 60[degrees]C and partially cured samples were then dried under hot air oven for 4 days at ~100[degrees]C.

Proposed mechanism for laccol-styrene copolymers formation is illustrated in Scheme 2.

Characterization Methods

Several methods were incorporated herein to identify the formation of polymer material and its chemical, thermal, and physical characteristics. Further, the effects resulted due to radiation exposure were also investigated to identify the suitable polymer materials for applications.

Fourier Transform Infrared Analysis. For the Fourier transform infrared (FTIR) study, a Perkin Elmer UATR Two spectrometer and solid samples were used having the scan range of 400-4,000 [cm.sup.-1] set at a resolution 4 [cm.sup.-1] and 16 average scans.

Nuclear Magnetic Resonance Spectroscopy. NMR analysis was conducted on LP and its copolymers dissolved in chloroform-d, using a Varian INOVA 400 spectrometer. Instrumentation parameters were employed as follows: Temperature maintained at 298 K, spin set and maintained at 20 Hz, 16 transients used in block sizes of 8, dl relaxation time 2.000 s.

Gel Permeation Chromatography. The molecular weight and molecular weight distribution of the LP and laccol-styrene copolymers were determined by GPC (PL-GPC 50 Integrated GPC System, Polymer Laboratories). The partially cured samples were dissolved in THF to give the concentration of 4 mg/mL, and after 6 h, these samples were passed through 0.45 [micro]m filters separately. A 100 [micro]L from each sample was injected at 30[degrees]C using THF as mobile phase with a flow rate of 0.5 mL/min. A calibration curve was generated using monodispersed polystyrene (PS) standards (Polymer Laboratories PS-2) to obtain the relative GPC data for the samples.

Dynamic Mechanical Analysis [29]. Testing articles (rectangular bars with width 10.5 mm, length 50 mm, and thickness 2 mm) were molded in a heated Carver hydraulic press with slow cooling to room temperature under pressure. The isothermal strain-sweep test was performed at -100[degrees]C to determine the linear viscoelastic region (LVR) using AR 2000 (TA Instruments) rheometer. Within the measured LVR, 0.3% strain was selected to characterize the samples with a temperature ramp in oscillation mode to identify the glass transition temperature ([T.sub.g]). Ramp conditions were -90[degrees]C to 200[degrees]C at 5[degrees]C [min.sup.-1] for LP and copolymers, and for synthesized PS, it was 0[degrees]C to 200[degrees]C at 10 [degrees]C [min.sup.-1] with liquid nitrogen for cooling. Further, frequency-sweep experiment was conducted using rectangular bars for LP and its copolymers, having the frequency range from 0.1-10.0 Hz, 0.300% strain, and starting temperature of -- 15[degrees]C with 5[degrees]C increments each time until the sample collapse, and for synthesized PS, strain was 0.200% and starting temperature was 0[degrees]C. Using the frequency-sweep experimental data, activation energy was calculated for each sample associated with glass transition temperature region.

Time-sweep experiment was also performed for all the samples using parallel plate setup of the rheometer with compression molded discs (diameter 1 inch and thickness 0.1 inch) having 2% strain and 1.0-Hz frequency for 30 min at 150[degrees]C to observe the viscosity difference with time at a specific temperature. Resulting data were analyzed with the software available from TA Instruments.

Thermal Gravimetric Analysis. TGA was performed on a TGA model TA Q50. A weight loss versus temperature scans for different polymers and copolymers were recorded at a ramp rate of 10[degrees]C [min.sup.-1] in air at the range between room temperature (-22[degrees] C) and 700[degrees] C.

Shore Hardness. ASTM D2240 Shore A was used to test the hardness of polymerized samples. Testing articles (disks with diameter 1 inch and thickness 0.1 inch) were punched from sheets that were compression-molded using a Carver laboratory press (Model C) equipped with heating elements. Hardness of the samples was measured before and after the y-irradiation, before and after rheometer experiments, and before and after temperature treatment in dry oven at 200[degrees]C.

Radiation Studies and Microscopic Analysis. Co-60 Gamma Irradiator at University of Florida, Gainesville, was used for the radiation studies. A 1.25-MeV gamma rays source was utilized, delivering about 200-250 krad/h (-30 Gy/min) to 6-8 adjacently placed samples per each exposure. Samples were exposed to 9.8 Mrad in air; this required 336 h of exposure time.

Leica Microsystems Wetzlar GmbH instrument equipped with the Leica DFC 290 camera (made in Germany) was used to investigate the surfaces of prepared samples. Magnitude was set to PL FLUOTAR 5X/0.12P and images were captured using the Leica FireCam software.

Wide-Angle X-Ray Scattering [30]. To investigate the degree of crystallization of synthesized LP, laccol-styrene copolymers, and PS, powder X-ray diffraction (WAXS) was used. X-ray diffraction patterns were collected using the Broker D8 Focus x-ray diffractometer in the range of 2[degrees] to 70[degrees] with a step of 0.020[degrees] at 25[degrees]C.

RESULTS AND DISCUSSION

Cationic polymerization with AlClj.EtOAc coinitiator successfully incorporated to develop LP and laccol-styrene copolymers and this was confirmed from the results obtained from IR, NMR, GPC, and other characterization methods. These polymers and copolymers were exposed to gamma radiation and reinvestigated the properties to identify the behavior of materials after the radiation treatment. It was clearly indicated that neat laccol polymer (NLP), S-90, and S-70 copolymers showed promising results toward radiation hardening. Materials were further cured and crosslinked (physically and/or chemically) due to gamma radiation without deteriorating and the obtained results were analyzed in detail hereafter.

FTIR Analysis

Identification of polymerizing process of lacquer-based materials was frequently investigated using FTIR data [1, 4-8, 23, 31-33], Figure 1(a) has the IR spectra of laccol extract (LE) and NLP. It shows peaks at 3,450 [cm.sup.-1] due to hydroxyl groups (O--H), at 3,010 [cm.sup.-1] due to C--H stretching vibrations of unsaturated groups (cis and trans double bonds) in side chain, at 2,940 [cm.sup.-1] due to methylene (C--H), at 1,621, 1,596, and 1,470 [cm.sup.-1] due to vibrations of phenyl ring (aromatic skeletal), at 988 [cm.sup.-1] (trans) due to the bending vibrations of conjugated alkene, at 966 [cm.sup.-1] (trans) and 732 [cm.sup.-1] (cis) due to alkenes in side chain [6, 7, 27], Figure 1b is IR spectrum of purified styrene monomer, which represents the peaks at 3,010-3,083 [cm.sup.-1] for C--H in aromatic group, at 1,630 [cm.sup.-1] for C=C, 907, and 990-1,000 [cm.sup.-1] for bending vibrations of R--CH=C[H.sub.2].

The bending vibration for conjugated alkene at 988 [cm.sup.-1] (trans), which is presented in the laccol side chain, was involved in the reaction via cationic polymerization process. The peak at 988 [cm.sup.-1] completely disappeared after the polymerization process and the intensity of the peak at 966 [cm.sup.-1] was reduced. This is possible due to the formation of LP and copolymers. This was clearly identified with FTIR spectrum that was obtained for LP as illustrated in Fig. 1a as NLP. The peaks at 1,278 and 1,184 [cm.sup.-1] can be attributed as [gamma]O-H for laccol and its copolymers. Comparing the curves LE and NLP in Fig. 1a, respective peaks for [gamma]O-H significantly decreased in NLP because of the formation of dimers and polymers [6]. This scenario can be observed clearly from IR spectra c - e for the laccol and styrene copolymers as well in Fig. 2. The peak at 1,352 [cm.sup.-1], which is related to [beta]O-H [6], also decreased in spectra "b - e" compared with the curve 'a' in Fig. 2. The peak at 3,010 [cm.sup.-1], which can be attributed as the stretching vibration of the unsaturated group in the side chain, was significantly reduced possibly due to the crosslinking process occurred in the side chain. The peaks at 1,621 and 1,596 [cm.sup.-1], which are related to vibration of -C=C- bonds in phenyl ring, were significantly changed in Fig. 1a NLP graph due to the formation of dimers and polymers. However, all the synthesized materials are melt-processable.

Samples were reinvestigated after the irradiation process using FTIR and the peak at 966 [cm.sup.-1] decreased, as illustrated in Fig. 2. The peak at 1,690 [cm.sup.-1] shifted to 1,710 [cm.sup.-1] and broadened after the irradiation because of more laccol quinones formation [6]. The peaks at 1,621 and 1,596 [cm.sup.-1] were further changed after gamma irradiation due to the formation of dimers and polymers [6]. The peaks in the region of 1,350-1,100 [cm.sup.-1] also decreased in intensity after irradiation. These observations suggest that the LP and its copolymers were further cured and have been crosslinked (physically and chemically) due to the gamma radiation without deteriorating the material. Most promising results were observed for NLP, S-90, S-70, and S-50 samples.

NMR Analysis

Cationic polymerization of LP and its copolymers were further confirmed by [sup.1]H NMR analysis. Completely crosslinked polymer and copolymers were insoluble in any organic solvents or water, and the partially dissolved samples of NLP and laccokstyrene copolymers were investigated in this experiment. LE was completely dissolved in CD[Cl.sub.3] and the obtained spectrum was compared with the NLP spectrum, as illustrated in Fig. 3.

The peaks relevant to the protons on the 13' to 16' carbon atoms were detected around 5.5-6.0 ppm completely disappeared after the LP formation. Also peaks relevant to cis double bond at carbon number 10' and 11' reduced and new peaks appeared in the region of 4.0-4.3 ppm (-C-O-C- bond formation) [2, 8], These new -C-O-C- bonds could form through the side chain and also through the phenyl ring [6] due to cationic polymerization (Scheme 1). For the laccol and styrene copolymers, peaks relevant to cis double bond (10', 11') could be observed even after the polymerization process and peaks relevant to trans conjugated double bonds (13'-16') were completely utilized during the reaction. The -C-O-C- bond formation was also observed as a minor product in the polymerized material (Scheme 2). Possibilities for this observation are macrocyclic compound formation [15] and crosslinking.

Reinvestigation of the samples was carried out after the irradiation process and it was confirmed that the materials were further cured due to gamma radiation without deteriorating (Fig. 4). Peak at 4.0 ppm increased and it was evident for -C-O-C- bond formation [8] and possibility of making macrocyclic compounds. Peaks relevant to cis double bond for copolymers were still appearing after the gamma irradiation, but in less intensity. One favorable possibility for this observation is inaccessible cis double bond due to random orientation of molecules in the physically and chemically crosslinked three-dimensional network. Other possibilities are insufficient curing time or insufficient amount of styrene/laccol monomer to allow the complete reaction.

GPC Analysis

Completely cured samples could not be dissolved in any solvent due to its high crosslinking property, and therefore, partially cured samples dissolved in THF were used for the experiment. Molecular weight buildup was identified in each sample at the very beginning stage of polymerization process. According to the results illustrated in Table 2, all the samples were shown a molecular-weight buildup and it was significant for NLP and S-90 even in the very beginning stage. This was again proved the successful incorporation of cationic initiator for the polymerization.

Dynamic Mechanical Analysis

The temperature region where the long-chain segments slips is identified as the glass transition temperature ([T.sub.g]). The [T.sub.g] was identified for samples using the tan 5 curve from the temperature ramp experiment for the rectangular shaped solid samples in rheometer. NLP showed the [T.sub.g] value of 15.40[degrees]C, and for the copolymers, Ts values were decreased as S-30 > S-50 > S-90 [approximately equal to] S-70. According to the trend observed, it was clear that the high styrene monomer consisting samples showed the higher [T.sub.g] value comparatively due to more ordered packing of the material. Also with the increase in temperature, a significant rubbery plateau was observed for S-30, S-50, S-70, S-90, and NLP samples, which are indicative of physical and/ or small amounts of chemical crosslinking (Fig. 5c).

The resulting graph (from frequency-sweep data) created using the time-temperature superposition software with shift factor versus temperature, as shown in Fig. 5a,b, is an evidence of WLF behavior and used to calculate the activation energy for each sample using Eq. 1. [C.sub.1], [C.sub.2] are material constants, R is the universal gas constant (8.314 J [mol.sup.-1] [K.sup.-1]), T is temperature in Kelvin (K), and [E.sub.a] is the activation energy [34, 35]. Required energy to induce a large segment slippage associated with glass transition was illustrated in Table 3. Highest activation energy was observed for the S-90 sample and least was given by the S-70. NLP showed a 244.3 kJ [mol.sup.-1] value in the glass transition temperature region.

[DELTA][E.sub.a] = {2.303)([C.sub.1]/[C.sub.2]) R[T.sup.2] (1)

The master curves were created using the above software and the storage modulus (G') always predominant the loss modulus (G"). Figure 5b is an example for the master curve created for NLP. Because of the high crosslinking property, completely cured samples were unable to test using GPC to identify the molecular weights of final products, as mentioned in the GPC Analysis section.

According to the data obtained from time-sweep experiment, the viscosity increased with time at 150[degrees]C for all the samples and significant increment resulted from NLP, S-90, and S-70 samples. This observation suggests a possibility of further curing of materials at a higher temperature as reported in literature [3].

[T.sub.g] of an amorphous polymer can be strongly influenced by molecular weight, tacticity, sample weight, laboratory processing conditions, and used ramp rate for the experiment [36-39]. The [T.sub.g] of synthesized PS was identified using the temperature ramp experiment in rheometer. The obtained experimental value was 102.8[degrees]C {Mw 20,438) and this was inside the reported range of [T.sub.g] values from 96[degrees]C to 106[degrees]C for PS [37, 40-43].

TGA Analysis

The change in sample mass is determined as a function of temperature or time in this technique. Commonly used method is dynamic thermogravimetry where the sample is heated at a linear rate in predetermined temperature changing environment [44]. Figure 6a illustrates the thermal stability increment after addition of styrene. Figure 6b illustrates the results obtained for NLP and S-90 samples after irradiation as a comparison to control samples. The degradation of NLP could be described using two temperature stages [2, 6]. The first stage of the NLP-control sample scan (50[degrees]C to 150[degrees]C) with a weight loss of 3.63% was due to the removal of residual water and other small molecules [2, 6]. The second stage (150[degrees]C to 500[degrees]C) with a weight loss of 83.4% can be attributed to the degradation of laccol oligomer and polymer. The irradiated NLP sample noted in the weight loss at the beginning of the scan and after 150[degrees]C was less than the NLP-control sample, 2.87% and 76.6%, respectively. Compared with NLP, observed TG curves were similar for its copolymers. S-90 copolymer was chosen as an example to demonstrate the data with NLP (LP). For S-90 control sample, 2.02% weight loss was observed in the first stage, and in the second, it was 80.0%. After the irradiation process, the weight loss after 200[degrees]C was reduced with the value of 77.4%. Onset temperatures of both irradiated NLP and S-90 samples were increased with the values of 384[degrees]C and 370[degrees]C, respectively, compared with control samples. Additionally, a considerable amount of residue was observed even after the 700[degrees]C for irradiated samples (NLP irradiated: 17.7%, S-90 irradiated: 17.4%) comparative to controls (NLP control: 11.8%, S-90 control: 16.5%). These observations indicated that the irradiated samples possessed higher thermal stability possibly due to further curing and crosslinking.

Shore Hardness Analysis

Shore A durometer was used for investigations due to the soft nature of synthesized materials. Higher number of the scale from 0-100 indicates the greater resistance to indentation, and thus harder materials [45]. According to the obtained results as shown in Table 4, addition of styrene increased the hardness of the materials and irradiation further increased the physical and chemical crosslinks, hence resulting in harder materials. Higher laccol monomer containing samples such as NLP, S-90, S-70 demonstrated significant increments of the hardness after irradiation compared with initial readings.

The effect of increased temperature during the experimental steps and processing to hardness property was also analyzed parallel to this study and those data illustrated an increment of hardness for the NLP, S-90, S-70, and S-50 samples.

Microscopic Analysis

The cured and processed discs were analyzed using an optical microscope before and after the gamma irradiation. Results were illustrated in Fig. 7. It was clearly shown that more wrinkles appeared after the irradiation significantly for NLP, S-90, S-70, and S-50 samples. This can be observed from the micrographs that illustrated more reflecting points than control samples. The mechanism for the wrinkle formation and factors controlling the surface properties are not clearly identified [4]. Curing process of the material could possibly lead to wrinkle formation due to the evaporation of water and excess solvent. Exposure to gamma radiation could possibly elicit more growing points for polymerization and remarkably increase the crosslinking density, resulting in a hard cured material [4].

Powder X-Ray Analysis

A polymer is a macromolecule composed of many repeating subunits. The arrangement of these subunits in a bulk polymer is controlled by molecular weight, chemical composition, spatial orientation, and processing conditions. Based on these factors, polymers could show amorphous or partially crystalline phase states. In crystalline state, the arrangement of molecules is more ordered, and in the amorphous state, it is more random. Amorphous polymers do not include long-range order, thus consisting of characteristically identifiable short-range order. As a result of this, powder X-ray diffraction provides a diffuse peak signal [46]. According to the obtained results as shown in Fig. 8, controlled samples (NLP and S-90) have more crystallinity in the matrix before exposing them to radiation. These diffused peaks are possible due to the local packing arrangement of the molecules [47]. The distinct peak observed at 20[degrees] repetitively for all the compositions could possibly due to the main backbone chain of the polymer matrix (Scheme 1). After exposing the samples to gamma radiation, the peak observed at 20[degrees] is preserved and other peaks were diffused. The exact reason for the peak diffusion is not clearly identified and possibilities could be different spatial orientation of the molecules, crosslinking, processing conditions, or chain scission.

CONCLUSIONS

LP and laccol-styrene copolymers were synthesized successfully incorporating Al[Cl.sub.3].EtOAc cationic coinitiator. The unsaturated hydrocarbon chain in 3-position of laccol was effectively cured with cationic coinitiator to produce physically and small amounts of chemically crosslinked polymeric material. The changes occurred to IR peaks at 988, 966, and 732 [cm.sup.-1] (Figs. 1 and 2) and the NMR peaks at 5.5-6.0 ppm range for NLP (Fig. 3), and copolymers after polymerization confirmed this statement. Flexibility of these amorphous polymers was restricted below their [T.sub.g] values [46], Observed [T.sub.g] values for polymer and copolymers were ranging from 12.90[degrees]C to 29.60[degrees]C (Table 3). For synthesized PS, it was 102.8[degrees]C. When the styrene monomer content increased in the copolymers, they became hard and processability of the materials was increased, specifically for S-90, S-70, S-50, and S-30 copolymers. In addition, incorporating styrene to laccol monomer enhances the thermal stability of copolymers. Exposure to gamma radiation enhances more growing points for polymerization and significantly increases the crosslinking density that leads to increment of the hardness. The IR and NMR data suggested the stable condition of materials after irradiation and evidences for more physical and small amounts of chemical crosslinking. This was further justified by the data obtained from temperature ramp experiment (tan 5 curve), which illustrated a significant rubbery plateau (Fig. 5c). TGA and shore hardness data (Table 4) were evidenced the thermal stability and hardness improvements of the materials. TG curves obtained for irradiated NLP and S-90, clearly indicated higher onset temperatures and high residue weight even after 700[degrees]C. NLP, S-90, and S-70 samples show significant hardness increments after the irradiation. Microscopic observations also illustrated higher wrinkle formations after the irradiation process, suggesting the fact of solvent evaporation and crosslinking. The wide-angle X -ray data exemplified significant differences due to gamma irradiation. As per the objective of the study, synthesizing radiation-hard polymer and copolymers was achieved and the prepared materials were further cured through physical and chemical crosslinking without deteriorating after exposed to gamma radiation. Considering the factors like processing ability, thermal stability, and radiation-hard ability, the NLP and S-90, S-70, S-50 copolymers are the promising materials that illustrated the most profound outcome during this study. The developed materials have a great potential for serving as protective layer or coating and radiation shield against gamma radiation in the advanced high-energy radiation environments like nuclear reactors, machines for radiation therapy, industrial sterilization, and space [26].

ACKNOWLEDGMENTS

Authors gratefully acknowledge Professor M. Luis Muga, University of Florida, Chemistry Department, Gainesville, for performing gamma irradiation experiments and Professor Lukasz Wojtas, University of South Florida, Chemistiy Department, for analyzing powder X-ray data.

Imalka H. A. M. Arachchilage, Milly K. Patel, Julie P. Harmon (iD) Department of Chemistry, University of South Florida, Tampa, Florida, 33620

Correspondence to: J. P. Harmon; e-mail: harmon@usf.edu

DOI 10.1002/pen.25159

Published online in Wiley Online Library (wileyonlinelibrary.com).

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Caption: Sch 1. Proposed mechanism for laccol polymerization. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: Sch 2. Proposed mechanism for laccol and styrene copolymers formation.

Caption: FIG. 1. IR spectra of (a) LE and NLP and (b) styrene monomer. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 2. IR spectra of various laccol compositions: (a) LE-control, (a') LE-after irradiation, (b) NLP-control, (b') NLP-after irradiation, (c) S-90_control, (c') S-90_after irradiation, (d) S-70_control, (d') S- 70_after irradiation, (e) S-50_control, (e') S- 50_after irradiation, (f) NPS-control, (f') NPS-after irradiation. NPS*, neat PS. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 3. NMR spectra for (a) LE and (b) NLP (*acetone).

Caption: FIG. 4. (a) Laccol:Styrene (S-50) copolymer-control and (b) LaccohStyrene (S-50) copolymer-after irradiation.

Caption: FIG. 5. (a) NLP-WLF behavior graph, (b) master curve developed for NLP, and (c) tan o curves for materials from temperature ramp experiment (rheometer). [Color figure can be viewed at wileyonlinelibrary.com!

Caption: FIG. 6. TG curves for NLP and S-90 samples.

Caption: FIG. 7. Optical microscopy observations of polymers and copolymers (magnitude: 5X/0.12P).

Caption: FIG. 8. Powder X-ray diffraction data for NLP and S-90.
TABLE 1. Different monomer contents
that used to prepare the copolymers.

Sample     Laccol      Styrene    Initiator
name     extract (g)    (ml)     complex (ml)

S-10        4.00        39.6        10.20
S-15        6.00        37.4        11.15
S-30        12.0        30.8        10.60
S-50        20.0        22.0        11.00
S-70        28.0        13.2        11.50
S-90        36.0        4.40        11.80

Sample    C[H.sub.2]       Total
name      [Cl.sub.2]    volume (ml)
         solvent (ml)

S-10         5.20          55.0
S-15         1.45          50.0
S-30         8.60          50.0
S-50         6.00          40.0
S-70         5.30          30.0
S-90         13.8          30.0

TABLE 2. The [M.sub.w] of NLP and its copolymers
at initial polymerization stage.

Sample name   [M.sub.w]   [M.sub.n]   PD (a)
                (Da)        (Da)

S-10           18,716       9,549      1.96
S-30            4,059       2,331      1.74
S-50            2,218       1,760      1.26
S-70            2,942       2,079      1.42
S-90            6,195       2,755      2.25
NLP             4,268       2,327      1.83
NPS            20,438       3,524      5.80
LE               461         434       1.06

(a) Polydispersity, PD = [M.sub.w]/[M.sub.n].

TABLE 3. [T.sub.g] values and related activation
energies obtained from rheometer.

Sample name     [T.sub.g]     Activation energy
              (tan [delta])   ([DELTA][E.sub.a])
               [degrees]C          (kJ/mol)

S-30              29.60             250.8
S-50              15.60             236.0
S-70              12.90             199.2
S-90              13.00             253.5
NLP               15.40             244.3
NPS               102.8             446.9

TABLE 4. Shore A hardness data for LP and copolymers;
before and after irradiation.

Sample    Sample   Hardness of       Hardness of
number    name     control samples   irradiated samples

1          NLP          19.0                51.0
2          S-90         11.5                45.6
3          S-70         38.9                70.4
4          S-50         85.3                91.6
5          S-30         89.8                93.5
6          S-15         90.3                92.4
7          NPS          80.0                83.1
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Author:Arachchilage, Imalka H.A.M.; Patel, Milly K.; Harmon, Julie P.
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:1USA
Date:Aug 1, 2019
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