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Continuous wave laser induced third-order nonlinear optical properties of conducting polymers.

INTRODUCTION

The studies on third-order nonlinear optical properties of organic materials are of great significance due to their high damage threshold, possibility of tailoring material properties, ultrafast response time, etc. [1]. A wide variety of organic and inorganic materials were studied to determine their nonlinear optical properties [2-11]. Among organic materials, polymers have received prodigious interest due to their potential application in photonic devices. The nonlinear optical properties of polymers are of importance mainly because of their potential applications in light-emitting diodes, sensors, batteries, electrochemical super capacitors, photonics, etc. [12]. Polymers are good substitute for inorganic materials as they are light weight, inexpensive, fracture tolerant, pliable, easy processing, and manufacturing [13]. Also, they display remarkable optical and electrical properties and a broad change in colour due to their conjugated double bonds, which originates from their both conducting and non-conducting forms.

Electrically conducting polymers are also known as synthetic metals. Among the conducting polymers, polyaniline, poly(otoluidine), poly(o-anisidine), and polypyrrole materials belong to the class of ionic electroactive polymers. The conductive polymers conductivity is directly associated with the presence of a conjugated backbone which enables electron delocalization. A conjugated backbone signifies a primary axis of carbon atoms interconnected by alternating single and double bonds. These are the flexible polymers with characteristics that are similar to semiconductors. Research has shown that with suitable doping, conducting polymers undergoes transition from insulating to conducting state [14], Usually polymers are insulators in their neutral state and by the introduction of suitable electron acceptors/donors by a process known as doping, the polymers transforms to conductors. In other words, polymers become conductive which effectively reduces the band gap and their conductivity gets enhanced with the suitable addition or removal of electrons from the carbon backbone [15]. Thus, the synthesis of polymers with suitable doping results in with the properties of a semiconductor and metal combined in one material with the ease and low cost of preparation and fabrication.

[PI]-conjugated polymer depicts large third-order optical nonlinearities which are ascribed to the [pi]-electron delocalization along the chain [16]. Polyaniline and polypyrrole are considered as one of the promising conducting polymer due to its easy synthesis, comparatively stable in environment, good conductivity, variety of applications such as light emitting diodes, sensors, electro chromic devices, rechargeable batteries, corrosion resistant paints, etc. [17], Poly(o-toluidine) and poly(o-anisidine) are the derivatives of polyaniline which contains the -C[H.sub.3] (methyl) and -OC[H.sub.3] (methoxy) group in the ortho position of the aromatic ring of the aniline monomer. Among the ring-substituted polyaniline derivatives, poly(o-toluidine) and poly(o-anisidine) have been probably the most widely studied compounds due to their interesting electro-optical properties [18]. In our previous study, we have determined the nonlinear optical properties of conducting polymers polyaniline and poly(o-toluidine) which is reported elsewhere [19]. Beside many advantages of polyaniline, insolubility in solvents is the major disadvantage; to overcome this and to enhance the processability, various techniques have been introduced. Recently we adopted the copolymerization technique and the result involved the work on the nonlinear optical properties of copolymers which is reported elsewhere [20]. In the present study, we have employed another technique in which the addition of a functional group as a substituent to polyaniline results in a material with good solubility and processability. The substitutions provide a distortion in the polymer backbone and helps in increasing electron localization [21].

In this study we present the synthesis, the results of third-order optical nonlinearity and optical limiting properties of poly(o-anisidine) (POA) and polypyrrole (PPy) for the first time. CW He-Ne laser at 633 nm was used as source and [[chi].sup.(3)] was evaluated using z-scan technique. The induced selfdiffraction rings were studied experimentally. To the best of our knowledge, the evaluation of [[chi].sup.(3)] and optical power limiting of POA and PPy under CW laser has not been reported till date.

[FIGURE 1 OMITTED]

EXPERIMENTAL

Materials Synthesis and Methods

The monomers O- anisidine and Pyrrole were purchased from Sigma Aldrich and were distilled before use. The monomer solutions were prepared by dissolving 2 ml of o- anisidine (molecular weight 123.15 g/mol, chemical formula is C[H.sbp.3]O[C.sub.6][H.sub.4]N[H.sub.2]) and pyrrole (molecular weight 67.09 g/mol, chemical formula is [C.sub.4][H.sub.5]N) in 0.1 N Sulfuric acid ([H.sub.2]S[0.sub.4]) separately. The synthesis procedure of conducting polymers POA and PPy are similar to that of polyaniline and poly(o-toluidine) as reported in our previous report [19]. For determining the absorptive and refractive nonlinearities, 0.25 mg of POA and PPy samples were weighed and dissolved in 5 ml research grade (V, N-Dimethyl Formamide (DMF) separately. For optical limiting studies 0.25, 5, 10, 20, and 40 mg of samples were dissolved in 5 ml research grade DMF separately and labelled as a, b, c, d, and e respectively. The molecular structure of polymers are shown in Fig. 1. The optical characterization of the polymers POA and PPy was studied by recording the electronic spectra in the wavelength range 250-1050 nm using UV-1601 PC Shimadzu spectrophotometer are depicted in Figs. 2 and 3. The FTIR measurements were studied via making pellets by mixing the POA and PPy samples with pure potassium bromide (KBr) in the ratio of 1:100 separately. The FTIR study spectrum was taken in the mid IR region of 400-4000 [cm.sup.-1] using FTIR 8400S Shimadzu spectrophotometer with DLATGS detector with 16 scan speed are depicted in Fig. 4.

[FIGURE 2 OMITTED]

z-Scan Technique

Sheik Bahae et al. [22, 23] developed z-scan technique was employed to determine the third-order nonlinear susceptibility [[chi].sup.(3)] of polymers in DMF. Using z-scan technique one can get both nonlinear absorption (NLA) and nonlinear refraction (NLR) measurements of the samples simultaneously. This is a simple and very sensitive technique. Optical power limiting measurements were carried out to investigate the power limiting and clamping behaviours of the polymers. The z-scan and optical limiting experimental procedure is similar to that given in our previous reports [19], The experiments were performed by using Thor labs HRP350-EC-1 CW He-Ne laser at 633 nm wavelength as an excitation source. The laser beam was focused to a spot size of 36.78 [micro]m and the Rayleigh length ([Z.sub.R]) of 6.71 mm using a 5 cm focal length lens with input power 21.7 mW and the resultant output power through the samples was recorded using a photo-detector fed to Thor labs PM320E dual channel optical power and energy meter. The samples were placed in a cuvette of 1 mm thickness and thus the thin sample approximation is valid [22, 23].

RESULTS AND DISCUSSION

UV-VIS and FTIR Spectroscopic Measurements

UV-VIS spectroscopy is a powerful tool for the elucidation of the interactions between the solvent, the dopant and the polymer chains [24], Figs. 2 and 3 shows the optical absorption spectra of POA & PPy. For the lowest concentration (a), there are two peaks in the visible region and a slight hump in the near infrared region (inset figure). With increase in concentration, the absorption curves get changed. That is, the spectra become wide, broader and almost it covers the UV, visible and near infrared regions and hence the polymers are a promising candidate for photo-voltaic applications. The wide absorption peaks indicate high degree of conjugation and high intra chain order in the polymers. The band in the UV region corresponds to the [pi]-[[pi].sup.*] transition and the band in the visible region corresponds to the inter-ring charge transfer associated with excitation from benzenoid to quinoid moieties. From the figure one can see that the spectra show bathochromic (or red) shift. The red shift in the absorption maxima indicates, increase in the conjugation length [25], Thus we can say that the conjugation length of POA is greater than that of PPy sample.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

The FTIR spectra for the POA & PPy polymers are shown in Fig. 4. Several characteristic peaks associated with POA and PPy samples are observed. The absorption peak at 3139.90, 3186.18, 3440.77, 3510.20 [cm.sup.-1] for POA and 3186.18, 3440.77 [cm.sup.-1] for PPy is due to the N-H stretching vibration peaks and indicates the presence of -NH group in the o-anisidine and pyr-role units [26]. The peak at 2923.88 [cm.sup.-1] for POA is due to C-H stretching vibration of methoxy group. The absorption peak at 1635.52 [cm.sup.-1] for PPy is attributed to the -C-N stretching vibration of pyrrole. The peaks at 1504.37, 1581.52 [cm.sup.-1] for POA and peaks at 1558.23, 1404.08 [cm.sup.-1] for PPy is ascribed to -C=C stretching mode for quinoid and benzoid rings [27, 28]. The peaks at 1411.80 [cm.sup.-1] for POA and 1211.21, 1118.64 [cm.sup.-1] is owed to C-N stretching vibrations in quinoid units. The presence of o-methoxy group in POA is attributed to the peak 1203.50 [cm.sup.-1] [29], Peaks at 1041.49 and 930.12 [cm.sup.-1] is ascribed to -C-H- in-plane and out-of-plane deformation in the pyrrole units [30]. The absorption peaks at 1126.35 and 1010.63 [cm.sup.-1] is attributed to 1,4- substitution on benzene ring. The peaks at 840.91 [cm.sup.-1] and peaks between 800 to 700 [cm.sup.-1] reveal the occurrence of 1,2- substitution on benzene ring [27], The peak observed for o-methoxy group in the FTIR spectrum indicates the presence of o-anisidine unit in the POA chain. The spectra obtained for the synthesised polymers are in good agreement with the literatures. These peaks depict the presence of various groups in the structure and thus confirm the structure of synthesised polymers.

Nonlinear Measurements

In z-scan technique, open aperture and closed aperture experiments are conducted to determine the NLA and NLR behaviours of the sample. By performing the NLA and NLR experiments, one can determine the nonlinear absorption coefficient [B.sub.eff] and nonlinear index of refraction [n.sub.2].

The nonlinear absorption z-scan traces of POA and PPy are shown in Fig. 5. The samples were taken in a cuvette and made to sail from one end of farfield to the other end. When the sample is at farfield the laser intensity is low and hence the normalized transmittance is linear. At the focus, the intensity is high and as the sample sail through the focus, the transmittance decreases forming a valley. This indicates the reverse saturable absorption (RSA) type of process. This type of process is represented by positive nonlinear absorption coefficient [B.sub.eff]. There are various mechanisms resulting RSA process [31], Along with two-photon absorption (TPA), another process called excited state absorption (ESA) can also occur. If the observed nonlinear absorption behaviour is only due to TPA alone, then the variation in the [B.sub.eff] values with input intensity [I.sub.0] would be a straight line. But from Fig. 6, it is visible that, the [B.sub.eff] values are not constant but it is dependent on [I.sub.0]. This confirms that the observed nonlinear absorption is not due to TPA alone, but rather a combination of TPA and ESA. Hence the observed nonlinear absorption is ascribed to TPA and ESA assisted RSA behaviour.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

The pure nonlinear refraction traces of POA and PPy samples are shown in Fig. 7. The traces reveal a peak followed by valley behaviour that is self-defocusing property for the samples and it is represented by negative nonlinear refractive index [n.sub.2]. Thus from this z-scan trace we can easily confirm the sign of nonlinear index of refraction. In order to estimate the nonlinear index of refraction [n.sub.2], the peak-valley separations for the samples were measured and it is found to be ~2 ZR. A peak-valley separation of more than 1.7 times [Z.sub.R] is the clear indication of thermal nonlinearity and indicates the observed nonlinear effect is the third-order process.

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

The nonlinear absorption coefficient [[beta].sub.eff] and nonlinear index of refraction [n.sub.2] were calculated using Figs. 5 and 7. Using the [[beta].sub.eff] and [b.sub.2] values, the real [[chi].sub.R.sup.(3)] and imaginary [[chi].sub.I.sup.(3)] parts of third-order nonlinear susceptibility [[chi].sup.(3)] were calculated. The determined values of [[Beta].sub.eff], [n.sub.2], [b.sub.2] are tabulated in Table 1. We conducted z-scan experiment on the DMF and found a negligible contribution to the observed nonlinearity. Therefore any contribution from the solvent to the observed nonlinearity is negligible at the laser input intensity used.

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

In the Figs. 5 and 7, the absorptive and refractive nonlinearities of POA are better than PPy. The methoxy and the amino groups act as electron donors. On the other hand, phenyl acts as electron acceptors. The presence of donor and/or acceptor groups enhances the electron density [32]. As a result, an increase in the magnitude of dipole moment and efficient charge transfer takes place from these dopants to the benzene rings. In POA, the presence of large number of donor groups like NH and OC[H.sub.3], in turn enhances the charge transfer. Due to efficient charge transfer in the structure, the conjugation length increases. With the increase in conjugation length, the electron delocalization increases and this in turn increases the nonlinear optical property. These nonlinearities arise mainly from the [pi]-conjugated electron moieties [33], The observed increase in nonlinearity may be due to the electron donating ability of the groups in the structure. The [pi]-electron delocalization and efficient charge transfer, along with the increase in conjugation length make the compounds to exhibit a large molecular hyperpolarizability and thus contributes to large [[chi].sup.(3)] value. Also, from UV-VIS spectra, it confirms that POA has greater conjugation length than PPy and hence POA reveals large nonlinearity.

[FIGURE 12 OMITTED]

[FIGURE 13 OMITTED]

Optical Limiting and Induced Diffraction Rings

Optical limiters are of great importance, as they limit and clamp the high intensity laser light, in order to protect the optical sensors and human eyes from damage. The optical power limiting and clamping of polymers were investigated under CW laser illumination. In this experiment, the sample is located at the focal plane of the lens and the transmitted power through the sample is measured for different input powers. The optical limiting curves as a function of incident power varying from 0.17 mW to 21.7 mW of the POA and PPy samples are depicted in Figs. 8 and 9, respectively. Optical limiting and clamping were observed for different concentrations of POA and PPy samples and are tabulated in Table 2. From Table 2, it is clear that power limiting threshold and clamping values are inversely proportional to the concentration of the sample and with the increase in concentration; reduction in linear transmission below a threshold and better optical clamping is achieved. The maximum clamping and limiting obtained for the highest concentration (d) of POA is 0.04 and 1 mW and for concentration (e) of PPy is around 0.12 and 2 mW. We could not perform the limiting and clamping experiment for the concentration (e) of POA, as the sample was deep in colour (not transparent) and there was almost complete absorption of light (output power was feeble). The experimental setup was of energy absorbing type of optical limiter and thus, the major nonlinear mechanism employed is RSA.

The refractive index of a nonlinear sample changes when a high intensity light like laser passes through it. They act on the propagation of light and induce diffraction. Thus, the light diffraction is self-induced. The sample effectively absorbs laser radiation and gives rise to the temperature gradient across the laser beam. Since the thermal nonlinearity leads to decrease in the refractive index of the medium, the nonlinear negative phase shift appears in the zone of beam of action. This phase shift acts on the laser radiation as a defocussing lens or thermal lens and leads to the self-diffraction of radiation and results in the formation of interference fringes in the far field. The induced ring patterns for the various concentrations of samples POA and PPy from (a) to (e) are recorded using a digital camera and are shown in Figs. 10 and 11. The rings number increases with increase in concentration, which indicates that when laser passes through the sample, the sample acts like a negative lens [19], This is because with increase in concentration, more number of molecules per unit volume will participate in the interaction and this is due to the induced nonlinear phase shift due to the intensity dependence of the refractive index and thermal lensing. We could not record the diffraction rings pattern for the highest concentration (e) of POA, as the sample was completely blocking the light and the output was very weak.

The change in laser spot size with respect to position of the sample was also studied experimentally. The samples were positioned at various places, (a) far from focus, (b) pre-focus transmittance maximum, (c) post-focus transmittance minimum, and (d) away from focus were recorded. The recorded photographs are shown in Figs. 12 and 13. Figure (a) and (d) are recorded at farfield -z and +z positions, where the nonlinear effects are absent due to low intensity. The scale lines in the figure (a-d) are of 5 mm width each. With the help of scale bar, we can infer that the laser spot size is smaller when the sample was near to focus. We observed self-focusing and self-defocusing ring patterns with naked eye, which confirms the nonlinear behaviour of the material before conducting the experiment. The contribution of cuvette used for the studies did not induce any change in the far field intensity and hence, its contribution is not considered in the analysis. The samples were examined using optical microscopy before and after the laser irradiation to check any damage in the samples and we found no damage of the samples at the input intensity used.

CONCLUSIONS

We report the synthesis, the third-order optical nonlinearity, optical limiting and clamping, self-diffraction rings and variation in laser spot size of conducting polymers POA and PPy under CW He-Ne laser irradiation. UV-VIS measurements represent wide and broad bands with good conjugation length. FTIR measurements were carried out for the elucidation of structure. The samples were characterized with negative nonlinear refraction (self-defocusing) property. We observed, increase in induced self-diffraction ring patterns with increase in concentration due to refractive index change and thermal lensing. Also, variation in the laser beam spot size of sample with respect to its position was observed. Strong optical power limiting and clamping phenomenon was observed. From these studies we can infer that the conducting polymers investigated here emerge as possible candidate for optical device applications.

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S. Pramodini, P. Poornesh

Department of Physics, Nonlinear Optics Research Laboratory, Manipal Institute of Technology, Manipal University, Manipal, Karnataka 576 104, India

Correspondence to: P. Poornesh; e-mail: poomeshp@gmail.com

Contract grant sponsor: DAE-BRNS (Sanction no. 2010/20/34/7/BRNS/ 2263).

DOI 10.1002/pen.24128
TABLE 1. Third-order nonlinear optical parameters of polymers in DMF.

          [[beta].sub.eff]     [n.sub.2]
              (cm/W)         ([cm.sup.2]/W)  [n.sub.2](esu)
Sample     X [10.sup.-2]     X [10.sup.-7]   X [10.sup.-5]

POA (a)         1.65             -2.85            9.57
PPy (a)         1.21             -1.85            6.20

          [[chi square].sub.   [[chi square].sub.   [[chi square].
           R.sup.(3) (esu)      I.sup.(3) (esu)     sup.(3) (esu)
Sample      X [10.sup.-7]        X [10.sup.-7]      X [10.sup.-7]

POA (a)         -10.02                2.98              10.46
PPy (a)         -6.47                 2.14               6.82

TABLE 2. Optical power limiting and clamping of (a) 0.25, (b) 5,
(c) 10, (d) 20, and (e) 40 mg of poly(o-anisidine) and polypyrrole
in 5 ml DMF.

                Optical limiting    Optical
Sample          threshold (mW)      clamping (mW)
concentration
(wt%)           POA and PPy         POA and PPy

(a)             ~10       ~12.5     ~7.4      ~11
(b)              ~3        ~6       ~0..38     ~4
(c)              ~1.9      ~4       ~0.1       ~1.4
(d)              ~1.0      ~3       ~0.04      ~0.5
(e)              --        ~2        --        ~0.12
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