The physical aging behaviour of two epoxy-thiol shape memory polymers has been studied: diglycidyl ether of bisphenol-A cross-linked with pentaerythritol tetrakis, denoted 4-thiol, with a calorimetric glass transition temperature, Tg, of 51 °C; and the same system modified with tri (2,3-epoxypropyl)isocyanurate in a 30 wt% proportion, denoted 4-thiol-30%iso, with Tg = 63 °C. The aging of both polymers has been measured by torsional creep for aging temperatures between room temperature and the Tg of each polymer. The creep behaviour of each polymer is characterised by a discrete distribution of relaxation times, which shifts with aging time according to a double logarithmic aging rate, µ, and with aging temperature according to an activation energy. The distribution of creep relaxation times for 4-thiol is rather symmetrical, and slightly narrower than that for 4-thiol-30%iso, which is also skewed to longer times. The dependence of µ on temperature displays a peak as the aging temperature reduces below Tg, and is sharper for 4-thiol, in accordance with the narrower distribution, as predicted by a theoretical model based upon structural relaxation kinetics. The other parameters defining the aging behaviour, namely the reduced activation energy (¿h*/R = 92 kK for 4-thiol and 82 kK for 4-thiol-30%iso) and the non-linearity parameter of structural relaxation (x = 0.35 for 4-thiol and 0.25 for 4-thiol-30%iso), have been determined experimentally and are compared with the predictions of the theoretical model. These parameters can be used to predict the effects of physical aging on the shape memory response.
The structural modifications of a commercially available poly(lactid acid) grade were induced through reactive extrusion using a multi-epoxide reactive agent in a pilot plant. The statistical nature of the chemical reactions led to the generation of several types of non-uniform molecular architectures. Even though conventional spectroscopic (NMR) or chromatographic (SEC-static light scattering) techniques are placed at the forefront of the molecular characterization, both methods usually failed in characterizing non-uniform structures. In this study, a promising approach was applied to automatically classify modified PLA samples. It is based on the analysis of FTIR spectral data by means of multivariable methods, including feature extraction and classification algorithms. The fast and accurate results presented in this paper show the potential of the proposed approach.
The fracture of a ductile polymer film, a heterophase ethylene-propylene block copolymer, has been studied, combining a range of characterisation methods in an attempt to provide a better understanding of the intricate details that play an important role in the repeatability and reproducibility of the essential work of fracture test. The experimental factors that have a strong influence on the resulting parameters are clearly explained, with particular attention to the effect of the quality of the notches, the non-collinearity of the two edge notches in double edge notched tension specimens, and the lack of alignment of the specimen with the load axis once it is mounted on the load train. Furthermore, the influence of these experimental factors on the registered stress-displacement curves is also studied, and a criterion and the method for separating non-valid specimens are established.
The effect of three different organically modified layered silicate clays (Nanomer I.30E, Cloisite 30B and Nanofil SE 3000) on the exfoliation process and on the thermal properties and nanostructure of cured trifunctional epoxy resin based nanocomposites was studied. Optical microscopy showed that the best and poorest qualities of clay distribution in the epoxy matrix were obtained with Nanofil SE 3000 and Nanomer I.30E, respectively. However, the isothermal differential scanning calorimetry scans show that, of the three systems, it is only the Nanomer clay that promotes intra-gallery reaction due to homopolymerisation, appearing as an initial rapid peak prior to the cross-linking reaction. This rapid intra-gallery reaction is not present in the curing curve for the Cloisite and Nanofil systems. This fact implies that the fully cured nanostructure of the Cloisite and Nanofil system is poorly exfoliated, which is confirmed by small angle X-ray scattering which shows a scattering peak for these systems at around 2.53°, corresponding to about 3.5 nm d-spacing.
A unique set of double-edge notched tension specimens of a Polyethylene Terephthalate Glycol-modi¿ed ¿lm was tested in mode I, plane stress. The load was registered on a universal testing machine. The displacements, ligament lengths, and video frames were recorded by a Digital Image Correlation system. With these registered data, the essential work of fracture, J-integral, and crack tip opening displacement (CTOD) fracture concepts have been applied. The onset of crack initiation was through a complete yielded ligament. The analysis showed that the intrinsic speci¿c work of fracture, we, is the speci¿c energy just up to crack initiation, which is an initiation value. we has both a coincident value and the same con-ceptual meaning as Jo, the J-integral at the onset of crack initiation. The relationship between Jo and CTOD is also determined. The in¿uence on the notch quality when the specimens were sharpened by two different procedures, femtosecond laser ablation and razor blade sliding, was analysed in detail.
In this work the phenomena involved with the microwave devulcanization of ground tyre rubber (GTR) were investigated. During studies three types of GTR characterized by different content of organic compounds (elastomers, plasticizers, etc..), carbon black and ash have been analyzed. The chemical structure of GTR before and after microwave devulcanization process was studied by Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). Furthermore, efficiency of microwave devulcanization conducted at different time was evaluated based on the crosslinking density and sol content values. FTIR spectroscopy results shown that devulcanization of GTR causes a decrease in carbon black with generation of CO2 due to its thermo oxidation, a decrease in structural groups of elastomeric components (mainly methylene and methine) and a breaking of C-S groups and S-S bridges. The presented results indicate the strong correlation between content of SiO2 in GTR and its degree of devulcanization. It was observed that GTR with a high content of SiO2 are easier devulcanized than samples with low content of SiO2, which suggest the presence of silica fillers improve microwave devulcanization efficiency
A reactive extrusion-calendering process was used in order to manufacture sheets with a nominal thickness of 1 mm of poly(lactic acid) and its nanocomposite with 2.5% of an organo-modified montmorillonite. During processing, the properties of the melt were stabilized and enhanced by the addition of 0.5% of a styrene-acrylic multi-functional-epoxide oligomeric reactive agent. The general analytical equation has been used in order to evaluate the kinetic parameters of the thermal degradation of poly(lactic acid) obtained by reactive extrusion and its nanocomposite. Various empirical and theoretical solid-state mechanisms have been tested to elucidate the best kinetic model. In order to reach this goal, master plots have been constructed by means of standardized conversion functions. Given that it is not always easy to visualize the best accordance between experimental and theoretical values of standardized conversion functions, a new index has been developed to quantitatively discern the best mechanism. By doing that, it has been possible to determine the right activation energy of the thermal degradation. It has been demonstrated that the best theoretical mechanism was the random scission of macromolecular chains within the polymer matrix. This was also in accordance with an empirical kinetic model based on an autocatalytic kinetic model. The presence of montmorillonite nanoparticles has been beneficial and has enhanced the thermal stability of poly(lactic acid).
Several commercial polyolefin-based flexible foams produced by extrusion foaming were characterized in terms of their cellular morphology and fracture behaviour using the concept of the Essential Work of Fracture (EWF), focusing on the influence of foam's chemical nature, expansion ratio and cellular structure on the values of the fracture parameters. Correction procedures were proposed in order to take into account the complexity of foams in the obtained fracture parameters, particularly a correction procedure based on their expansion ratio, and a second one based on the fraction of polymer
present in the foams determined from cellular structure characterization. Although doubts remain about the applicability of the EWF methodology to LDPE foams, the correction procedure based on the expansion ratio seemed to provide more accurate results than that
based on polymer fraction, with EWF effectively distinguishing between polyolefin foams having different chemical nature. Comparatively, foams based on a P-E copolymer presented the highest values of the essential work of fracture in the MD direction, while significant differences were only observed in the TD direction for foams having a highly oriented cellular structure. All PP-based foams showed similar non-essential work of fracture values in both MD and TD directions.
The analysis of the fracture behaviour of polymeric films involves the determination of the specific essential work of fracture, which requires preparation, testing and evaluation of a large number of notched specimens. All these tasks demand significant time and costs; therefore, the use of alternative testing procedures that offer savings of both materials and time are becoming increasingly attractive options. Recently, the Small Punch Test (SPT) has experienced significant development as an alternative to conventional testing procedures in the field of steels and some metallic alloys. This work considers the applicability of SPT to the fracture characterisation of polylactide acid reinforced with organic-modified montmorillonite clay, using several weight percentages of nanoclay. The applicability of the experimental methodology is evaluated through comparison of the results obtained by SPT and conventional testing (tensile and essential work of fracture).
The fracture toughness of poly (ethylene terephthalate) modified glycol (PETG) has been evaluated using notch sharpening techniques which could be grouped into contact and non-contact procedures. Razor blade tapping, razor blade sliding, razor pressing and broaching are part of the first group, while the femtolaser technique belongs to the second one. Not all the contact procedures generated valid sharp cracks for fracture parameter assessment; indeed none of the samples sharpened via razor sliding generated acceptable sharp cracks. The results revealed that the non-contact femtolaser technique produced the sharpest cracks in this polymer, with crack tip radii of only 0.5 µm, leading to the lowest fracture toughness values. On the contrary, the traditional notch sharpening technique based on razor tapping, recommended in ISO, ESIS and ASTM protocols and standards, generated larger crack tip radii than those introduced via femtolaser and, consequently, resulted in higher fracture toughness values. Both broaching and pressing methods created specimens with smaller crack tip radii compared to those obtained by razor tapping, and hence resulting in intermediate fracture toughness values
The fracture toughness of poly (ethylene terephthalate) modified glycol (PETG) has been evaluated using notch sharpening techniques which could be grouped into contact and non-contact procedures. Razor blade tapping, razor blade sliding, razor pressing and broaching are part of the first group, while the femtolaser technique belongs to the second one. Not all the contact procedures generated valid sharp cracks for fracture parameter assessment; indeed none of the samples sharpened via razor sliding generated acceptable sharp cracks. The results revealed that the non-contact femtolaser technique produced the sharpest cracks in this polymer, with crack tip radii of only 0.5 μm, leading to the lowest fracture toughness values. On the contrary, the traditional notch sharpening technique based on razor tapping, recommended in ISO, ESIS and ASTM protocols and standards, generated larger crack tip radii than those introduced via femtolaser and, consequently, resulted in higher fracture toughness values. Both broaching and pressing methods created specimens with smaller crack tip radii compared to those obtained by razor tapping, and hence resulting in intermediate fracture toughness values.
Carrasco, F.; Perez-Maqueda, L.A.; Sanchez-Jimenez, P.E.; Perejón, A.; Santana, O.; Maspoch, M. Polymer testing Vol. 32, num. 5, p. 937-945 DOI: 10.1016/j.polymertesting.2013.04.013 Data de publicació: 2013-08 Article en revista
An enhanced general analytical equation has been developed in order to evaluate the kinetic parameters of the thermal degradation of poly(lactic acid) (PLA) at various linear heating rates and at constant rate conditions. This improvement consisted of replacing the n-order conversion function by a modified form of the Sestak-Berggren equation f(a) = c(1-a)nam, which led to better adjustment of experimental data, and also adequately represented the conventional mechanisms for solid-state processes. The kinetic parameters so obtained have been compared to those determined by conventional differential and isoconversional methods. Given that the thermal degradation of PLA has been argued to be caused by random chain scission reactions of ester groups, the conversion function (a) = 2(a1/2-a), corresponding to a random scission mechanism, has been tested.
An enhanced general analytical equation has been developed in order to evaluate the kinetic parameters of the thermal degradation of poly(lactic acid) (PLA) at various linear heating rates and at constant rate conditions. This improvement consisted of replacing the n-order conversion function by a modified form of the Sestak-Berggren equation f(α) = c(1−α)nαm, which led to better adjustment of experimental data, and also adequately represented the conventional mechanisms for solid-state processes. The kinetic parameters so obtained have been compared to those determined by conventional differential and isoconversional methods. Given that the thermal degradation of PLA has been argued to be caused by random chain scission reactions of ester groups, the conversion function (α) = 2(α1/2−α), corresponding to a random scission mechanism, has been tested.
Santana, O.; Rodríguez, C.; Belzunce Varela, Javier; Gamez, J.; Carrasco, F.; Maspoch, M. Polymer testing Vol. 29, num. 8, p. 984-990 DOI: 10.1016/j.polymertesting.2010.09.004 Data de publicació: 2010-12 Article en revista
The applicability of the essential work of fracture (EWF) in SENB geometry as an alternative to the J-integral methodology has been assessed. An amorphous poly(lactic acid) (PLA) with ductile behaviour promoted by a de-ageing heat treatment has been used. The results demonstrate that the EWF technique can be used as a simple alternative methodology to determine the J0.2 value in plane strain conditions, using the ESIS-2000 protocol and ASTME813-87 standard. As a validation of the range of ligament length to be used in the EWF analysis, the SENB solution of the Slip Line Field theory could be employed.
The influence of the notching procedure on the fracture toughness measured via Elastic-
Plastic Fracture Mechanics has been analyzed on four different ethylene-propylene block
copolymers with two distinct dimensions, paying special attention to the morphology of
the area surrounding the crack tip front. Two sharpening techniques were evaluated: the
traditional steel razor blade and the femtolaser ablation process. The fracture toughness of the razor blade sharpened samples was always higher than that of the femtolaser sharpened specimens. Also, the fracture toughness of the razor blade samples was dependent on the thickness of the samples, whereas the fracture toughness of the femtolaser sharpened specimens was not influenced by the dimensions of the test specimens.
The microscopic analysis of non-tested samples showed that the crack tip radii were
similar for both type of sharpened samples but the damage and its extension ahead of the
crack tip was dependent on the notching technique, the copolymer type and the dimensions of the analyzed specimen. The femtolaser sharpened samples presented a very tinyheat affected zone ahead of the crack tip, the size of which was independent of the copolymer type and the dimensions of the test specimen. On the other hand, the steel razor blade sharpened samples showed an area surrounding the crack tip formed by
plastic deformation, the length of which increased for the smaller size of sample and for higher ethylene content in the copolymer.
The effect of the notch sharpening procedure on the fracture toughness computed under
linear-elastic and elastic-plastic conditions (at low and high loading rates, respectively) of a bulk ethylene–propylene block copolymer has been investigated. The specimens utilized for fracture characterization were sharpened using a steel razor blade and the femtosecond
laser ablation technique. The crack growth initiation parameter, JIC, determined with the help of the elastic-plastic fracture approach, and the fracture toughness, KIC, obtained with specimens verifying linear-elastic fracture mechanics assumptions, of the steel razor blade sharpened samples were higher than that of the femtolaser sharpened specimens. The plastic deformation occurring ahead of the crack tip during the sharpening method seems to be the reason for the dissimilarity of the fracture values. Moreover, the strain hardening suffered by the material close to the notch tip reduces the stable crack growth resistance
evaluated under elastic-plastic conditions.
Heatsetting is a thermal treatment applied to most textile fabrics to stabilise structure and dimensions. During heatsetting, a process of crystallization takes place. In thermoplastic fibres, the determination of the effective temperature of heatsetting and the crystallinity increase due to this treatment can be calculated by the study of the pre-melting endothermic peak (PEP) that appears in the corresponding DSC thermograms. In the case of substrates subjected to very intense treatments, the PEP and the main melting peak are overlapped and, then, those parameters are difficult to calculate. This paper develops a method to determine the effective temperature of heatsetting and crystallinity of the PEP of heatset substrates.
Arencon, D.; Velasco J.I.; De Redondo, V.; De Sousa Pais, M.; Maspoch, M. Polymer testing Vol. 26, num. 6, p. 761-769 DOI: 10.1016/j.polymertesting.2007.03.009 Data de publicació: 2007-09 Article en revista
Aretxabaleta, L.; Aurrekoetxea, J.; Urrutibeascoa, I.; Sanchez-Soto, M. Polymer testing Vol. 24, num. 2, p. 145-151 DOI: 10.1016/j.polymertesting.2004.09.014 Data de publicació: 2005-04 Article en revista