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1. Centro de Investigación en Ingeniería y Ciencias Aplicadas (CIICAp), Universidad Autónoma del Estado de Morelos, Cuernavaca, Morelos, México
2. Laboratorio de Investigación y Desarrollo de Materiales Avanzados (LIDMA), Facultad de Química, Universidad Autónoma del Estado de México, Edo. de México, México
3. Síntesis y Caracterización de Materiales, Instituto Nacional de Investigaciones Nucleares, Edo. de México, México
Received: February 28, 2012 / Accepted: March 19, 2012 / Published: April 25, 2012.
Abstract: Nylon is an important industrial material, and gamma irradiation is used for material modification. In this work, different gamma radiation dose were applied to crystalline fibres of nylon 6-12 at room temperature under argon atmosphere. Post-radiation effect was also studied, to know the effects of time. Some physical properties were measured obtaining the degree of modification with respect to the applied dose. X-ray values showed increment on the crystallinity and crystal size with dose increase up to 300 kGy. Brill Transition can be appreciated as a result of the in-situ X-ray with temperature study. Thermal analyses and AFM roughness measurements corroborate the modifying mechanisms in terms of the applied dose. For 15 and 50 kGy re-polymerization and crosslinking are the main mechanisms. Increment in the melting point at 15 kGy occurs, returning at 50 kGy to the original value. For 100 to 300 kGy the chain scission is the dominant mechanism because the melting point is lower than the original one. Both mechanisms take place in the amorphous zone.
Aliphatic polyamides, or nylons, are important industrial materials, valued for their high strength and processability. The polymer chains contain amide groups, which separating by alkane sequences in regular arrangements. Each nylon type depends on the alkane lengths. Nylon 6-12 has six methylene units between the di-amine and ten between the dicarboxylic acid segments. Special attention on the directionally specific interchain hydrogen bonds need to be attended; as we know such hydrogen bonds are formed between neighbouring amide groups and they are responsible of the improvement on the physical properties and substantially enhanced melting points, in comparison with those of the poly-olefins and polyesters [1].
The effect of different dose of γ-radiation on nylons has been followed by X-ray diffraction and calorimetry[2-4]. The main new finding is the occurrence of a hexagonal phase transition on heating; this transition temperature is an increasing function of crystal thickness, and for all samples decreases with increasing radiation dose. As a sideline, the present findings lead to corroborate the existence of Brill Transition temperature in nylon 6-12, as already has been demonstrated using Raman spectroscopy [5].
Some researchers establish that the main process in polymers, due to high radiation energy, is that of crosslinking [6, 7]. Others propose the chain scission as the main effect [8, 9] and even some others show that
and 0.44 nm. These spacing are due to the intersheet distance between the sheets and the projected interchain distance within the sheet, respectively. The nylons can also exist in hexagonal structure and is known as γ-phase. The characteristic of γ-phase is that it exhibits only one strong reflection corresponding to d spacing at about 0.42 nm.
In the present work the effect of different gamma irradiation dose, applied to crystalline fibres of nylon 6-12 at room temperature under argon atmosphere are studied, as well as the confirmation of the Brill transition temperature, obtained from X-ray with temperature in-situ and related to gamma radiation dose.
With higher irradiation dose, more chain scission occurs [26-28].
This process allows the production of more crystalline areas in the amorphous zones [17, 27, 28] because the oligomers formed act as nucleating agents, forming secondary hydrogen bonds between them. Some authors claim [29-31] that crystallinity increment can be attributed to the increase in temperature provoked by molecular collisions between gamma radiation and matter. In this case, it can be noticed that[100] peak at 20°, has more intensity than the [010] peak at 23°, because it is in the growing direction.
Fig. 3 shows the diffractograms obtained from X-ray diffraction applied in transverse position, as opposed to Fig. 2. First of all, the intensity peaks are lower than in the lateral position, which is due to the observed anisotropy of nylon 6-12 of its triclinic crystallographic system. For the same reason, peaks in both crystallographic directions have almost the same intensities.
Crystal size was obtained using the Debye-Scherer equation which makes a relationship between the width of the peak and its crystallinity considering the mean height width. Fig. 4 shows the crystal mean size in the lateral position for both: FI and 3YI samples, in the[100] crystallographic direction.
For the FI fibres, crystal mean size tends to diminish probably because chain scission is generating more crystalline areas but with less size. Almost the same behaviour is observed in the 3YI fibres, where crystal mean size tends to diminish from 15 to 50 kGy. Above 100 kGy there are not observed changes. The
differences between FI and 3YI fibres are the crystallites sizes. With time, crystal size increase meaning that whatever the reaction mechanism is taking place. After three years generation of new crystalline areas still goes on.
Behaviour of mean crystal size in the [010] crystallographic direction can be seen in Fig. 5, for both: FI and 3YI fibres. In this direction there are no significant changes, which are in agreement with the X-ray results (Fig. 3). Irradiated fibres at 15 kGy in the 3YI analysis show the only difference. Nevertheless, both types of fibres have sizes between 30 and 33 ?.
On the other hand, the melting point has a general trend to decrease with no significant differences in temperature, just a slight increase as a function of time for the 3YI condition (Fig. 6). But, the peculiar low dose response can be observed again. At 15 kGy and 50 kGy the melting point for gamma irradiated fibres is above or at least at the same temperature compared to the NI fibres. This phenomenon is attributed to the crosslink or partial damage [4, 30], and the decrease in the fusion temperature is ascribed to the chain scission or permanent damage that yield oligomers [4, 31].
The heat of fusion is the parameter that shows significant differences, because contrary to the expectation, it has an increment with radiation, but with time diminishes, as can be seen in Fig. 7. Heat of fusion
process. In both cases, the number and depth of scratches are increasing when the radiation dose increases too. The scratches are consequence of the chain scissions, oligomers formation, chain reorganization and creation of new hydrogen bonds and are more evident with time. This leads to the formation of new crystalline domains. Moreover, the scanning electron microscopy is a useful tool for to evidence the effect of gamma irradiation across the time.
Surface roughness, obtained with AFM technique, is another convenient parameter that provides important
It can be established that when gamma irradiation increasing, changes in physical and chemical properties of nylon 6-12 are established, namely: primary and secondary bonds, crystallinity degree, crystal size, melting point, heat of fusion as well as surface roughness and pattern. It was confirmed that there are two different mechanism of reaction, depending on the irradiation dose. The irradiated fibres at low dose show crosslinking and partial damage, but for those irradiated at higher dose chain scission and permanent damage take place. Moreover, reversible Brill transition was observed at 110 °C, changing from triclinic to pseudo-hexagonal crystallographic systems.
The authors are deeply thanked to Dr. Marco Antonio Espinoza, from Instituto Mexicano del Petróleo, for the TGA/DTA equipment. Financial support of the National Council of Science and Technology of Mexico (CONACyT), Mexico City, by Grant No. 52572 and scholarship for the students involved is acknowledged.
[1] N.A. Jones, S.J. Cooper, E.D.T. Atkins, M.J. Hill, L. Franco, Temperature-induced changes in chain-folded lamellar crystals of aliphatic polyamides, Investigation of Nylons 2-6, 2-8, 2-10, and 2-12, J. Poly. Sci. Part A: Polym. Chem. 35 (4) (1997) 675-688.
[2] C. Menchaca, L. Rejón, A. álvarez-Castillo, M. Apátiga, V.M. Casta?o, Structural analysis of crystalline nylon 6-12 exposed to gamma radiation, Int. J. Polym. Mater. 48(2000) 135-143.
[3] C. Menchaca, A. Alvarez-Castillo, H. López-Valdivia, H. Carrasco, V.H. Lara, P. Bosch, et al., Radiation-induced morphological changes in polyamide fibers, Int. J. Polym. Mater. 51 (9) (2002) 769-781.
[4] C. Menchaca, A. álvarez-Castillo, G. Martinez-Barrera, H. López-Valdivia, H. Carrasco, V.M. Casta?o, Mechanisms for the modification of nylon 6-12 by gamma irradiation. IJMPT 19 (6) (2003) 521-529.
[5] C. Menchaca, B. Manoun, G. Martinez-Barrera, V.M. Casta?o, In-situ high-temperature raman study of crystalline nylon 6-12 fibers gamma-irradiated in argon atmosphere, J. Phys. Chem. Solids 67 (9-10) (2006) 2111-2118.
[6] A.I. Balabanovich, S.V. Levchik, G.F. Levchik, W. Schnabel, C.A. Wilkie, Thermal decomposition and combustion of γ-irradiated polyamide 6 containing phosphorus oxynitride or phospham, Poly. Degrad. Stab. 64 (2) (1999) 191-195.
[7] A. Charlesby, Atomic Radiation and Polymers, Pergamon Press, Oxford, 1960.
[8] B. Bittner, K. M?der, C. Kroll, H.H. Borchert, T. Kissel, Tetracycline-HCl-loaded poly(DL-lactide-co-glycolide) microspheres prepared by a spray drying technique: Influence of γ-irradiation on radical formation and polymer degradation, J. Controlled Release. 59 (1) (1999) 23-32.
[9] D.M. Timus, C. Cincu, D.A. Bradley, G. Craciun, E. Mateescu, Modification of some properties of polyamide-6 by electron beam induced grafting, Appl. Radiat. Isot. 53 (4-5) (2000) 937-944.
[10] V.G. Barkhudaryan, Effect of γ-radiation on the molecular characteristics of low-density polyethylene, Polymer 41 (2)(2000a) 575-578.
[11] V.G. Barkhudaryan, Alterations of molecular characteristics of polyethylene under the influence ofγ-radiation, Polymer 41 (7) (2000b) 2511-2514.
[12] C.W. Delley, A.E. Woodward, J.A. Saver, Effect of irradiation on dynamic mechanical properties of 6-6 nylon, J. Appl. Phys. 28 (1957) 1124-1130.
[13] M.C. Gupta, V.G. Deshmukh, Radiation effects on poly(lactic acid), Polymer 24 (7) (1983) 827-830.
[14] B. Li, L. Zhang, γ-Radiation damage to nylon 1010 containing neodymium oxide, Polym. Degrad. Stab. 55 (1)(1997) 17-20.
[15] A. Valenza, S. Piccarolo, G. Spadaro, Influence of morphology and chemical structure on the inverse response of polypropylene to gamma radiation under vacuum, Polymer 40 (4) (1999) 835-841.
[16] X.C. Zhang, M.F. Butler, R.E. Cameron, The ductile-brittle transition of irradiated isotactic polypropylene studied using simultaneous small angle X-ray scattering and tensile deformation, Polymer 41 (10)(2000) 3797-3807.
[17] C. Menchaca-Campos, G. Martínez-Barrera, M.C. Resendiz, V.H. Lara, W. Brostow, Long term irradiation
2. Laboratorio de Investigación y Desarrollo de Materiales Avanzados (LIDMA), Facultad de Química, Universidad Autónoma del Estado de México, Edo. de México, México
3. Síntesis y Caracterización de Materiales, Instituto Nacional de Investigaciones Nucleares, Edo. de México, México
Received: February 28, 2012 / Accepted: March 19, 2012 / Published: April 25, 2012.
Abstract: Nylon is an important industrial material, and gamma irradiation is used for material modification. In this work, different gamma radiation dose were applied to crystalline fibres of nylon 6-12 at room temperature under argon atmosphere. Post-radiation effect was also studied, to know the effects of time. Some physical properties were measured obtaining the degree of modification with respect to the applied dose. X-ray values showed increment on the crystallinity and crystal size with dose increase up to 300 kGy. Brill Transition can be appreciated as a result of the in-situ X-ray with temperature study. Thermal analyses and AFM roughness measurements corroborate the modifying mechanisms in terms of the applied dose. For 15 and 50 kGy re-polymerization and crosslinking are the main mechanisms. Increment in the melting point at 15 kGy occurs, returning at 50 kGy to the original value. For 100 to 300 kGy the chain scission is the dominant mechanism because the melting point is lower than the original one. Both mechanisms take place in the amorphous zone.
Aliphatic polyamides, or nylons, are important industrial materials, valued for their high strength and processability. The polymer chains contain amide groups, which separating by alkane sequences in regular arrangements. Each nylon type depends on the alkane lengths. Nylon 6-12 has six methylene units between the di-amine and ten between the dicarboxylic acid segments. Special attention on the directionally specific interchain hydrogen bonds need to be attended; as we know such hydrogen bonds are formed between neighbouring amide groups and they are responsible of the improvement on the physical properties and substantially enhanced melting points, in comparison with those of the poly-olefins and polyesters [1].
The effect of different dose of γ-radiation on nylons has been followed by X-ray diffraction and calorimetry[2-4]. The main new finding is the occurrence of a hexagonal phase transition on heating; this transition temperature is an increasing function of crystal thickness, and for all samples decreases with increasing radiation dose. As a sideline, the present findings lead to corroborate the existence of Brill Transition temperature in nylon 6-12, as already has been demonstrated using Raman spectroscopy [5].
Some researchers establish that the main process in polymers, due to high radiation energy, is that of crosslinking [6, 7]. Others propose the chain scission as the main effect [8, 9] and even some others show that
and 0.44 nm. These spacing are due to the intersheet distance between the sheets and the projected interchain distance within the sheet, respectively. The nylons can also exist in hexagonal structure and is known as γ-phase. The characteristic of γ-phase is that it exhibits only one strong reflection corresponding to d spacing at about 0.42 nm.
In the present work the effect of different gamma irradiation dose, applied to crystalline fibres of nylon 6-12 at room temperature under argon atmosphere are studied, as well as the confirmation of the Brill transition temperature, obtained from X-ray with temperature in-situ and related to gamma radiation dose.
With higher irradiation dose, more chain scission occurs [26-28].
This process allows the production of more crystalline areas in the amorphous zones [17, 27, 28] because the oligomers formed act as nucleating agents, forming secondary hydrogen bonds between them. Some authors claim [29-31] that crystallinity increment can be attributed to the increase in temperature provoked by molecular collisions between gamma radiation and matter. In this case, it can be noticed that[100] peak at 20°, has more intensity than the [010] peak at 23°, because it is in the growing direction.
Fig. 3 shows the diffractograms obtained from X-ray diffraction applied in transverse position, as opposed to Fig. 2. First of all, the intensity peaks are lower than in the lateral position, which is due to the observed anisotropy of nylon 6-12 of its triclinic crystallographic system. For the same reason, peaks in both crystallographic directions have almost the same intensities.
Crystal size was obtained using the Debye-Scherer equation which makes a relationship between the width of the peak and its crystallinity considering the mean height width. Fig. 4 shows the crystal mean size in the lateral position for both: FI and 3YI samples, in the[100] crystallographic direction.
For the FI fibres, crystal mean size tends to diminish probably because chain scission is generating more crystalline areas but with less size. Almost the same behaviour is observed in the 3YI fibres, where crystal mean size tends to diminish from 15 to 50 kGy. Above 100 kGy there are not observed changes. The
differences between FI and 3YI fibres are the crystallites sizes. With time, crystal size increase meaning that whatever the reaction mechanism is taking place. After three years generation of new crystalline areas still goes on.
Behaviour of mean crystal size in the [010] crystallographic direction can be seen in Fig. 5, for both: FI and 3YI fibres. In this direction there are no significant changes, which are in agreement with the X-ray results (Fig. 3). Irradiated fibres at 15 kGy in the 3YI analysis show the only difference. Nevertheless, both types of fibres have sizes between 30 and 33 ?.
On the other hand, the melting point has a general trend to decrease with no significant differences in temperature, just a slight increase as a function of time for the 3YI condition (Fig. 6). But, the peculiar low dose response can be observed again. At 15 kGy and 50 kGy the melting point for gamma irradiated fibres is above or at least at the same temperature compared to the NI fibres. This phenomenon is attributed to the crosslink or partial damage [4, 30], and the decrease in the fusion temperature is ascribed to the chain scission or permanent damage that yield oligomers [4, 31].
The heat of fusion is the parameter that shows significant differences, because contrary to the expectation, it has an increment with radiation, but with time diminishes, as can be seen in Fig. 7. Heat of fusion
process. In both cases, the number and depth of scratches are increasing when the radiation dose increases too. The scratches are consequence of the chain scissions, oligomers formation, chain reorganization and creation of new hydrogen bonds and are more evident with time. This leads to the formation of new crystalline domains. Moreover, the scanning electron microscopy is a useful tool for to evidence the effect of gamma irradiation across the time.
Surface roughness, obtained with AFM technique, is another convenient parameter that provides important
It can be established that when gamma irradiation increasing, changes in physical and chemical properties of nylon 6-12 are established, namely: primary and secondary bonds, crystallinity degree, crystal size, melting point, heat of fusion as well as surface roughness and pattern. It was confirmed that there are two different mechanism of reaction, depending on the irradiation dose. The irradiated fibres at low dose show crosslinking and partial damage, but for those irradiated at higher dose chain scission and permanent damage take place. Moreover, reversible Brill transition was observed at 110 °C, changing from triclinic to pseudo-hexagonal crystallographic systems.
The authors are deeply thanked to Dr. Marco Antonio Espinoza, from Instituto Mexicano del Petróleo, for the TGA/DTA equipment. Financial support of the National Council of Science and Technology of Mexico (CONACyT), Mexico City, by Grant No. 52572 and scholarship for the students involved is acknowledged.
[1] N.A. Jones, S.J. Cooper, E.D.T. Atkins, M.J. Hill, L. Franco, Temperature-induced changes in chain-folded lamellar crystals of aliphatic polyamides, Investigation of Nylons 2-6, 2-8, 2-10, and 2-12, J. Poly. Sci. Part A: Polym. Chem. 35 (4) (1997) 675-688.
[2] C. Menchaca, L. Rejón, A. álvarez-Castillo, M. Apátiga, V.M. Casta?o, Structural analysis of crystalline nylon 6-12 exposed to gamma radiation, Int. J. Polym. Mater. 48(2000) 135-143.
[3] C. Menchaca, A. Alvarez-Castillo, H. López-Valdivia, H. Carrasco, V.H. Lara, P. Bosch, et al., Radiation-induced morphological changes in polyamide fibers, Int. J. Polym. Mater. 51 (9) (2002) 769-781.
[4] C. Menchaca, A. álvarez-Castillo, G. Martinez-Barrera, H. López-Valdivia, H. Carrasco, V.M. Casta?o, Mechanisms for the modification of nylon 6-12 by gamma irradiation. IJMPT 19 (6) (2003) 521-529.
[5] C. Menchaca, B. Manoun, G. Martinez-Barrera, V.M. Casta?o, In-situ high-temperature raman study of crystalline nylon 6-12 fibers gamma-irradiated in argon atmosphere, J. Phys. Chem. Solids 67 (9-10) (2006) 2111-2118.
[6] A.I. Balabanovich, S.V. Levchik, G.F. Levchik, W. Schnabel, C.A. Wilkie, Thermal decomposition and combustion of γ-irradiated polyamide 6 containing phosphorus oxynitride or phospham, Poly. Degrad. Stab. 64 (2) (1999) 191-195.
[7] A. Charlesby, Atomic Radiation and Polymers, Pergamon Press, Oxford, 1960.
[8] B. Bittner, K. M?der, C. Kroll, H.H. Borchert, T. Kissel, Tetracycline-HCl-loaded poly(DL-lactide-co-glycolide) microspheres prepared by a spray drying technique: Influence of γ-irradiation on radical formation and polymer degradation, J. Controlled Release. 59 (1) (1999) 23-32.
[9] D.M. Timus, C. Cincu, D.A. Bradley, G. Craciun, E. Mateescu, Modification of some properties of polyamide-6 by electron beam induced grafting, Appl. Radiat. Isot. 53 (4-5) (2000) 937-944.
[10] V.G. Barkhudaryan, Effect of γ-radiation on the molecular characteristics of low-density polyethylene, Polymer 41 (2)(2000a) 575-578.
[11] V.G. Barkhudaryan, Alterations of molecular characteristics of polyethylene under the influence ofγ-radiation, Polymer 41 (7) (2000b) 2511-2514.
[12] C.W. Delley, A.E. Woodward, J.A. Saver, Effect of irradiation on dynamic mechanical properties of 6-6 nylon, J. Appl. Phys. 28 (1957) 1124-1130.
[13] M.C. Gupta, V.G. Deshmukh, Radiation effects on poly(lactic acid), Polymer 24 (7) (1983) 827-830.
[14] B. Li, L. Zhang, γ-Radiation damage to nylon 1010 containing neodymium oxide, Polym. Degrad. Stab. 55 (1)(1997) 17-20.
[15] A. Valenza, S. Piccarolo, G. Spadaro, Influence of morphology and chemical structure on the inverse response of polypropylene to gamma radiation under vacuum, Polymer 40 (4) (1999) 835-841.
[16] X.C. Zhang, M.F. Butler, R.E. Cameron, The ductile-brittle transition of irradiated isotactic polypropylene studied using simultaneous small angle X-ray scattering and tensile deformation, Polymer 41 (10)(2000) 3797-3807.
[17] C. Menchaca-Campos, G. Martínez-Barrera, M.C. Resendiz, V.H. Lara, W. Brostow, Long term irradiation