Biodegradable dielectricmaterial using PLA and luffa ber modied by medical linear accelerator

Biodegradable electronic devices are presently in command in various sectors mainly in health care system.The present work comprehends the dielectric properties of biodegradable composites made from biodegradable polymer poly (lactic) acid (PLA) and natural fiber of luffa cylindrica (LC) fabricated using microcompounding and injection molding . LC fibers are an agricultural waste, rich in cellulose.LC fibers were exposed to 6 MeV electron beam of doses 0.5Gy, 1.0Gy, 2.0Gy, 4.0Gy and 10.0 Gy generated from medical linear accelerator(LINAC) in presence of air. Such low doses are normally used for treatment of cancer patients and not for modifying polymers where doses in the range of 20-200KGy are used. The effect of such low irradiation dose on fiber and study if any significant changes taking place is the innovative aspect of the present work. The effects of irradiation dose as constant and ac conductivity investigated at different and 80 o C while keeping frequency constant . The increase in dielectric constant from 57 in virgin PLA at 26 0 C,500Hz to a maximum of 84 in composite sample due to reinforcement of low dose irradiated LC fibers recording 49% increase is an important result of the investigation.


Introduction
Biodegradable material has recently plunged major impact in biomedical terrain such as drug delivery, therapeutics and tissue engineering. There is always a need of biodegradable dielectric material through which electrical signal can be transmitted in myocardial tissue and neurons.Biodegradable bioimplant devices for human body are currently in high yearning.The need of biodegradable electronic devises arise to eliminate the problem of disposal of synthetic electronic devices waste which cause serious environmental problem like soil pollution and contamination [1,2]. The current research uses the fruit of LC, a common tropical fruit which is basically a waste, as reinforcement in completely biodegradable PLA matrix producing green composites.PLA has captivated substantial research interests due to its biodegradable and biocompatible nature. The LC fibers are rich in cellulose(60%), cost effective, readily available with low CO2 emission along with high electrical resistance, good thermal and acoustic insulating properties. Most importantly they are biodegradable and can be recycled [3][4][5][6].However the LC fibers are hydrophilic in nature due to presence of cellulose, hemi-cellulose and lignin, with a tendency to absorb moisture from the surrounding making them less reactive having poor compatibility with the polymer matrices during fabrication of composite materials.Thus fiber modification is necessary prior to its use as reinforcement [4][5][6]. Ionizing radiation for modification of natural fiber hold great consideration because of no use of chemicals, low processing time leading to energy saving, clean waste free procedure leaving no bad impact on environment. Electron beams are used in radiation processing mostly, which bring significant structural changes in the fiber with very low irradiation dose [4,7,8]. Celluloses , which are the major component of plant fiber ,are long chain polymers consisting of identical gluco pyranose units which are linked to its neighbor by glucosidic linkage. When natural fibers rich in celluloses are exposed to electron beam and gamma rays, free radicals are formed on the surface of the fiber leading to enhanced bonding between fiber and matrix . Crosslinking and chain scisson are two major chemical changes observed when the cellulose rich natural fiber is irradiated with electron beam and gamma rays [4,9]. Further the increase in surface area due to defibrillation of cellulose fibers enhance the compatibility between fiber and matrix. The interaction of electrons with the solid can be soft interactions(excitation) or hard interactions or knockout interactions.When these radiations interact with solid molecules ,they transfer energy to the solid molecules leading to formation of reactive molecules in the excited states. This can be represented as where M is the solid molecule and M + is the ionized solid molecule as well as excited solid molecule. Finally the irradiated material is ionized during electron beam irradiation and large number of electrons are available on the surface, thereby generating many free radicals. Presence of free radicals and monomers enhance the chemical bonding between irradiated LC fiber and matrix when the fiber is used as reinforcement during fabrication of composites [8][9][10].
Normally polymers are modified with irradiation doses in the range from 20-200KGy and incase of some biopolymers which are highly sensitive, changes are observed from 1KGy irradiation dose. Cellulose, the major component in plant fiber is usually rather resistant to irradiation.The most significant and innovative matter concerning this investigation is doses applied for modification of LC fiber at the level from 0.5Gy to 10.0Gy.The objectives behind this work is to detect whether any significant changes are occurring at this low level of doses.
Such low doses are used for treatment of cancer patients and not normally for modifying polymers.Thus this is an innovative way of modifying surface of LC fiber using radiation obtained from medical linac. Electrons are accelerated to high energy of 6MeV The different components of a linac are given in Figure 1.

Figure 1:Schematic diagram of a medical linac
As shown in Figure 1,the linear accelerators are connected to a power supply ,supplying alternating currents to the modulator.The modulator converts the alternating current to direct current.The modulator is a pulse modulator that converts the dc voltage in terms of pulses of few microseconds duration. Linacs accelerate electrons to few mill volts of energy in pulsed manner and not in continuous manner. The output of the pulse modulator are fed to the electron gun as well as to the magnetron. The magnetron generates microwaves required to accelerate electrons to 6MeV coming from electron gun.These accelerated electron beams are further directed to a target having high Z to generate X-rays/gamma rays by bremsstrahlung interactions using microwave technology.
All agricultural materials such as natural fibers, foods conduct electric currents to some extent.
Knowledge of dielectric properties of these materials will determine the distribution of electromagnetic field in the materials in presence of external alternating field. How rapidly a material can be heated by radio waves or microwaves, can be ascertained from the evaluation of dielectric properties. All crystalline materials consist of two sublattices, ordered lattice exhibiting normal oscillations obeying Lyddane-Sachs-Teller (LST) model and disordered lattice having random oscillations obeying Debye's relaxations. Thus the crystal when exposed to external electric field is polarized in different ways .and the total polarization is expressed as p is the polarization due to ordered motion having contribution from dipole polarization , atomic polarization and ionic polarization while d p is the polarization due to disordered motion caused by jumping of particles between two positions also known as orientation polarization. r ε is the dielectric constant of the material and is sum of two terms contributing from ordered oscillation( ' ε ) and disordered oscillation( d ε ) i.e U ∆ is the energy barrier between the two states and T is the temperature.As temperature increases, the relaxation time decreases leading to increased relaxation frequency.
The dielectric constant of a material due to disordered state is a complex number having both real part and imaginary part. Separating the real part and imaginary part we get ε ∞ is the dielectric constant of the material at very high frequency of the applied electric field where the orientation of polar molecules are not possible due to much less time interval and s ε is the static dielectric constant at zero frequency of the applied electric field. In addition to real and imaginary part of dielectric constant, the knowledge of ac conductivity of a material is essential , whose expression can be obtained using Maxwell electromagnetic wave equation.
The displacement current in the dielectric is expressed as is known as ac conductivity of the material.
Dielectric behavior of a material relate the intrinsic interactions of the electromagnetic waves with the matter. The biodegradable nature of both PLA and natural fibers of LC fascinates us to carry out the work and to study the complex dielectric properties of the prepared blended materials with variation in irradiation dose, temperature and frequency.

Material
Polylactic acid (PLA) of grade 4042D (molecular weight Mw ~ 6, 00, 000) was acquired from Nature Works, USA. The LC fibers were obtained from local forest area.

Electron beam irradiation
Electron beam of energy 6 MeV generated from the medical LINAC (Millenium True Beam Linear Accelerator, Varian) installed in Health Care Global Panda Cancer Hospital, Cuttack, India. The LC fiber was mounted below cotton gauge of 3 inch thickness and was irradiated with a rate of 600MU/min to attain doses of 0.5Gy, 1.0Gy, 2.0Gy, 4.0Gy and 10.0Gy.

Composite processing and fabrication
The PLA pellets and the electron beam irradiated LC fibers were left for drying in vacuum at 80° C for 24 h before using. The PLA and LC fiber were mixed mechanically at 100 rpm with a micro compounding molding equipment at 170°C for 10 minutes. After extrusion through a preheated cylinder, the molten composite samples were transferred to the mini injection molder in order to obtain the desired specimen samples for studying various properties .

Variety of samples for characterization
Composite samples were prepared with PLA matrix and different doses of electron beam irradiated LC fibers. in E1, E2, E3, E4, E5 samples, the PLA and 5% wt fibers are mixed with electron beam irradiation dose of 0.5Gy, 1.0Gy, 2.0Gy, 4.0Gy and 10.0Gy respectively.

X-Ray diffraction
WXRD/SHIMADZU/JAPAN, goniometer facilitated with scintillation counter records the the X-ray diffractograms at 26˚C with bragg's angle ranging from 10 0 to 80 0 using Ni filtered Cu Kα radiation of wavelength of 0.1542 nm.

Dielectric properties measurements
Rectangular specimens of 10mm × 10mm × 2mm were prepared and coated with conductive silver paint for study of electrical properties. The test samples were fixed between two electrodes and kept inside the sample holder. Measurements were carried out at 26 0 C,40 0 C, 60 0 C and 80 0 C temperature keeping constant frequency from 500 Hz to 5MHz to examine various dielectric properties such as dielectric constant and ac conductivity.

Results and discussion
X-ray diffraction pattern of virgin PLA             and the minimum ac conductivity is found to be 0.127nSi for virgin PLA at 500Hz and 80 0 C.Again the observance of such wide range of ac conductivity explores many oppertunities in use of these composite materials.

Conclusions
The