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Stability of Amorphous PEEK in Organic Solvents.

Byline: Aqeel Ahmad, Tanveer Iqbal, Saima Yasin, Rabia Hanif, Sheema Riaz and Paul F Luckham

Summary: Poly ether ether ketone (PEEK) is one of the most promising thermoplastics for high performance applications such as biomedical surgical components, seal valves and various components of pump. Chemical and thermal behavior of amorphous PEEK in different chemical environments has been investigated in the present study. Commonly used organic solvents were selected based on Hildebrand solubility parameter for stability analysis of the polymer. Polymeric sheets placed in the solvents for 2 months were analyzed through Fourier Transform Infra -Red (FTIR) Spectroscopy, Thermogravimetric Analysis (TGA) and Differential Scanning Calorimeter (DSC). Chemical structure of the polymer was seen to be altered by toluene, benzene and tetrahydrofuran.

The polymer remains resistant to thermal attack after exposure to most of the solvents. Chloroform, THF, toluene and acetone affects stability of the polymer based on the experimental results. Thus, prolonged interaction of PEEK with these solvents requires some additional treatment to make PEEK sustainable in these harsh environments.

Keywords: PEEK, Plasticization, Crystallinity, Organic Solvents.

Introduction

PEEK is one of the most commonly used specialty thermoplastic. Owing to its high strength to weight ratio, resistance to chemical and thermal attacks, it is widely used in process, electronics and biomedical industries. Major applications of the polymer include its use in manufacturing of aircraft and turbine blades, pump and compressor parts, automobile components, as insulation in wire coating, in orthopedic, trauma and spine implants and as a material of construction of prosthesis [1-6]. Chemical structure of PEEK contains three aromatic rings in its monomer with two functional groups; ether (R-O-R) and ketone (R-CO-R) as shown in Fig. 1.

The polymer is commonly obtained by the reaction of 4'4-difluorobenzophenone with disodium salt of hydroquinone in the presence of aprotic solvents like diphenylsulfone at 300 AdegC [7] as shown in Fig. 2.

Amorphous PEEK lacks ordered structure of molecules, whereas the molecules of semicrystalline polymer possess fraction of ordered (crystalline) and fraction of disordered (amorphous) arrangements. PEEK is known for comparatively higher melting temperature (343AdegC) and glass transition temperature (Tg =143AdegC) [8]. Studies have shown that most commonly used industrial grades of PEEK have a crystalline content in the range of 30-35% [9-10]. Maximum crystalline limit that can be induced in PEEK polymer matrix is ~ 48% [11]. Specific gravity of the polymer is 1.265 for amorphous form while for semicrystalline polymer it is reported to be 1.320 for maximum achievable crystallization. Amorphous regions of polymer are more prone to chemical and thermal attack [11].

Common organic solvents {acetone (19.7), chloroform (18.7), benzene (18.7), toluene (18.3), and tetrahydrofuran (18.5)} were selected for analysis on the basis of Hildebrand solubility criteria and also on the basis of likelihood of exposure during commercial applications [12]. Hildebrand solubility parameter of a liquid is a numerical digit that depicts the solvency behavior of that specific liquid [13].

Solvents on interaction with polymers can result in inducing crystallization, swelling or dissolution of the polymer in the liquids [11]. For liquids, Hildebrand parameter can be calculated through their heat of vaporization, however, polymers usually degrade before it is possible to measure heat of vaporization. Therefore, swelling behavior or change in physical characteristics of polymers in liquids is the most common method of assigning Hildebrand values to polymers as shown in Fig. 3. Solubility parameter of PEEK has been reported to be in the range of 18-20 MPa [14]. Effects on the physical, chemical, morphological and thermal behavioral of PEEK by the considered solvents were not investigated much in the literature and therefore this study analyzed these parameters for evaluating the sustainability of the amorphous PEEK in applications involving interaction of the polymer with harsh solvent environments.

Experimental

Powdered PEEK purchased from Victrex PLC, UK was heated to a temperature of 385 o C to get molten PEEK in a melt press (Model: Laboratory Press, Serial number: PRE-2004055) in a 4*4 inches mold for 10 minutes [15]. The molten polymer was quenched in water at room temperature to obtain amorphous polymer. Afterwards, the samples were air dried and stored at room temperature for further experimentation. Acetone (99.5%), THF (99.9%), chloroform (99.5%), benzene (99.5%) and toluene (99.5%) lab scale solvents selected based on Hildebrand solubility parameter were supplied by Sigma-Aldrich. The polymer sheets were cut into small pieces and placed in the chosen solvents at room temperature for a period of 2 months to establish equilibrium solvent uptake and morphology can be noticed prominently [14].

All the samples were air dried and stored at room temperature before characterization. Three experiments were performed for each sample.

Fourier Transform Infrared Spectroscopy (FTIR) studies were carried out to analyze for any physical adsorption of chemical functionalities on the polymer because of solvent interaction at room temperature, scanning over a range of 450-4000 cm-1 with data interval of 0.24 cm-1 using a Perkin Elmer FTIR L160000A diamond lens ATR unit. Thermal behavior of the polymer was studied with 100 ml/min flow rate of nitrogen gas on a SDT Q-600 DSC/TGA analyzer. Ramp temperature was set to 900AdegC at a scanning rate of 10AdegC/min [17] with isothermal condition for 5 minutes, first at 105AdegC and then at 900AdegC [18]. 10 mg of each sample was used in aluminum pans for the DSC/TGA experimentation.

Results and Discussion

FT-IR spectra of PEEK were used to identify whether any of the used solvents caused a surface reduction or additional functionalization of polymer matrix. Graphical representation of TGA was used to monitor any destabilization caused in thermal behavior of the polymer in solvents. DSC results provide information about crystallization induced by these solvents in the amorphous polymer.

Fourier Transform Infrared (FT-IR) Spectroscopy

FT-IR was used to identify functional groups associated with PEEK. If the polymer decomposes, it would not be expected to observe functional groups associated with the virgin polymer. Fig. 4 shows the FT-IR spectrum of the untreated polymeric sample. A pair of peaks around 1500 cm-1 and 1600 cm -1 possibly depict presence of the aromatic ring in the structure i.e. benzene rings in the polymer. Absorbance at wave numbers 900 cm -1 to 670 cm -1 corresponds to C-H out of the plane bend vibrations. Also, several sharp peaks in the region 1225 cm -1 - 950 cm -1 may be possibly due to C-H in-plane bend vibrations. Small less intense peaks around 1250 cm -1 and 1730 cm -1 are due to the presence of ether and ketone functional groups, respectively [19]. In the range of 2800 cm -1 to 3000 cm -1 , two less intense peaks may be due to aliphatic C-H stretch vibrations [20].

FTIR study of benzene, acetone and toluene immersed polymer samples are shown in Figs. 5a - d, respectively. It is evident that the polymeric functional groups associated with the virgin polymer are still present, indicating that the decomposition was minimal. A clear change exists in Fig. 5d where THF immersed polymer showed emergence of a peak at wave numbers 3700 cm-1 -3600 cm-1. The region corresponds to the presence of O-H stretch vibrations which may be due to water absorbed in the sample when it was experimented on FTIR [19]. Moreover, a decrease in the absorbance of peaks at wave numbers 3000 cm-1 - 2800 cm-1 is obvious from the graph in the fingerprint region. This shows possible degradation and a surface modification effect of the polymer by the solvent. Being a cyclic ether, THF is weakly basic in nature hence it has a potential to interact with polymers either in the form of swelling, adsorption or absorption [21].

However, a decrease of absorbance intensity of C-H vibrational band at 3000 cm-1 - 2800 cm-1 in case of benzene, toluene and THF is clear from Fig. 5a-d. This may also correspond to chemical degradation of the polymer via ether or ketone bond breakage by these solvents [22]. A small amount of solvent might have entrapped but that amount was very minute and not even identified by gravimetric method. Therefore, its effect on the FTIR spectra of modified PEEK was neglected.

Thermogravimetric Analysis

Thermogravimetric Analysis (TGA) was used to determine the thermal decomposition of the amorphous PEEK and the solvent immersed polymeric samples in an oxidative thermal environment as shown in Fig. 6. The polymer decomposition into range of organic compounds in all cases have been observed to initiate at approximately 530AdegC. The decomposition rate was higher at incipient decomposition where almost 40% of the mass was lost in the temperature range 525AdegC to 597AdegC. From 600AdegC to 900AdegC slope of the decomposition curve decreased and an almost linear trend was observed with a final mass content of 6 % at 900AdegC. This mass may be referred to as ash content of the polymer but possibility of further decrease of mass with increasing temperature cannot be ignored. THF, benzene and toluene immersed polymer depicted similar degradation behavior as that of the virgin amorphous PEEK.

Initial weight (before dipping in solvents) and final weight (after dipping in solvents) of sample were measured to determine absorption of solvents. No appreciable uptake of the solvents has been observed based on the gravimetric method.

Chloroform and acetone immersed polymeric samples showed a three-step decomposition behavior with stable intermediates. A lower initial rate of degradation initially at 50AdegC - 60AdegC with a weight loss of 9% for chloroform after which it became stable up to 500AdegC and then followed the same trend as that of the untreated polymeric sample. At the onset of first decomposition step, in case of chloroform, is occurring near its boiling point so this initial decomposition step might also be linked to the evaporation of small quantity of entrapped solvents, which was not identified by the gravimetric method. However, the chloroform immersed polymer degraded completely at 900AdegC in contrast to other samples. This illustrates that chloroform somewhat reduced the thermal oxidizing stability of PEEK. In case of acetone, a lower decomposition was observed in the first step where only 2% mass was lost from 150AdegC to 230AdegC, and at 900AdegC, 9% mass of the sample persisted.

Patel et. al [22] proposed that thermal degradation of PEEK can be initiated by ketone or ether chain scission with major volatile products as phenols, 4-phenoxyphenol, 1-4-diphenoxybenzene, carbon monoxide and carbon dioxide in the first decomposition step [23]. Temperature plays a vital role in breaking bonds adjacent to carbonyl and ether linkages and in this way free oxygen r adical could be generated. These carbonyl or ether free radicals can then attack C-H bond of aromatic ring to form phenols and aldehydes. However, since most of the volatile products contain phenols or hydroxyl groups, therefore, it is also proposed that among ketone and ether, the latter linkage would be more susceptible to breakage through end chain scission [22]. Most rapid decomposition occurred just below 600AdegC after which second decomposition step starts. This step is supposed to be due to oxidation of carbonaceous char formed.

Above 650AdegC, low molecular weight products including carbon monoxide, carbon dioxide, phenols, methyl phenols, dibenzofuran/ derivatives of benzofuran, biphenyls, benzophenone, hydroquinone and benzoquinone may be formed [24,25]. Formation of carboxylic acid through oxidation of carbonyl radical in air has also been reported in the literature [22]. Carboxylic acid formed during decomposition may in turn de-carboxylate to carbon dioxide at higher temperatures. Based on the experimental observations the polymer can be used in applications involving an upper service temperature of 200AdegC [26] in each studied fluid environment except for chloroform and acetone environment where the operating temperature may need to be further lowered.

Derivative thermogravimetric analysis (DTG) directly relates the rate of mass loss with temperature. Region around 225 AdegC is showing moisture in toluene and pure PEEK sample. When weight of sample begins to lose, an abrupt increase in the derivative was observed as can be seen in Fig.7. Similar degradation trends of TGA curves (Fig. 6) were observed for the untreated, benzene and toluene immersed polymeric samples, whereas different behaviors of DTA curves were observed for the samples as can be seen in Fig. 7. Higher rate of mass losses at 585AdegC i.e. 4% higher for benzene and 3% higher in case of toluene in comparison to the untreated polymeric samples. After this rapid weight loss, at 600AdegC, the rate of mass loss becomes almost constant at a value of 1.7%.

Differential Scanning Calorimetry (DSC)

The DSC curves were utilized to calculate the percentage crystallinity and hence morphology of the polymer in the solvents. All samples showed a major decomposition peak at 570AdegC and an endothermic melting peak at 334AdegC as can be seen in Fig. 8. Zhao et. al. [27] already reported that amorphous poly aryl ether ketone (PAEK) polymers can undergo solvent induced crystallization. The percentage of crystalline content in the polymer can be calculated for each sample, using heat of fusion value from endothermic peak area and taking 130 Jg-1as a reference for pure crystalline PEEK by the following equation [28]:

% Crystallinity= (Hm - Hc)/ Href * 100

Where,

Hm = Heat of fusion computed by integrating area under the melt peak

Hc = Heat of crystallization computed by integrating area of the crystallization peak

Href = reference heat of fusion if PEEK was 100% crystalline

Percentage crystallinity of amorphous PEEK was found out to be 12% whereas an increase in percentage crystallinity was observed for each solvent immersed sample. The magnitude of induced crystallization was different for each solvent as tabulated in Table-1 based on 10AdegC/min heating rate. Hence these solvents tend to disturb the morphological stability of amorphous PEEK [29-31].

Table-1: DSC results of treated and untreated amorphous PEEK.

###% weight

###Initial decomposition###Maximum derivative weight percent###Percentage

###Sample###remaining at

###temperature (AdegC)###(%/min)###Crystallinity (%)

###900 AdegC

###PEEK###530###6###10###12

Benzene-PEEK###530###10###14###16.2

Toluene-PEEK###530###10###13###14.5

THF-PEEK###530###10###10###14.2

Acetone-PEEK###180###9###10###14.9

Chloroform-

###109###0###10###14.4

###PEEK

Conclusion

PEEK is one of the stable aromatic thermoplastics belonging to the class of PAEK. Investigation of the amorphous PEEK behavior in various aggressive environments is carried out to evaluate the polymer stability in high performance applications. These applications often involve interaction of polymer based industrial components with liquid solvents. Examples include flow of chemicals through pipelines, valves, bearings, seals, membranes etc. As a result of prolonged interaction of the polymer with chemicals, there are chances of alterations in the characteristics along with the possible consequences of abrasion of the polymer and leaching of components.

This work demonstrated the disturbance caused in chemical, thermal and morphological stability of amorphous PEEK by different solvents. Changes induced in the properties of solvent interacted PEEK during interaction with different fluid environments for 2 months were examined through FTIR and TGA analysis. The experimental results from the FTIR illustrated that no chemical change on surface was caused by the solvents except for THF which introduced a non-bonded O-H group on the polymer surface due to water absorbed in the sample during experimentation. TGA analysis revealed the effect of different solvents on thermal decomposition behavior of the polymer. Chloroform and acetone were observed to exhibit a reduction of upper service temperature of the polymer. The solvents were observed to induce crystallization in the polymer.

Therefore, based on the experimental investigation the polymer has been affected by chloroform, THF, toluene and acetone. Thus, prolonged interaction of PEEK with these solvents, either at room temperature or at elevated temperatures requires additional treatment like surface modification in order to make the polymer sustainable in harsh environments.

Acknowledgement

The authors grateful to Dr. Muhammad Asif, Associate Professor, Basic Sciences Department, UET, Lahore for his help with the experimentation and useful discussion.

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