Advanced Vibrational Microspectroscopic Study of Conformational

Article pubs.acs.org/Macromolecules

Advanced Vibrational Microspectroscopic Study of Conformational Changes within a Craze in Poly(ethylene terephthalate) Gonzalo Santoro,* Isabel M. Ochando, and Gary Ellis Instituto de Ciencia y Tecnología de Polímeros, CSIC, c/Juan de la Cierva 3, E-28006 Madrid, Spain S Supporting Information *

ABSTRACT: Crazes constitute one of the most common failure mechanisms in polymers. They act as fracture precursors, severely degrading the mechanical properties of the material. Thus, a deep understanding of the chain rearrangement occurring inside crazes is of utmost fundamental and practical importance. We have employed synchrotron infrared microspectroscopy (SIRMS) and Raman microspectroscopy to investigate the conformational changes inside micron-sized crazes in poly(ethylene terephthalate), PET. A promotion of the full-extended chain conformational structure at the expense of mainly the trans amorphous mesomorphic phase along with an increase in crystallinity of around 4% has been found. These results differ from what we observed across the deformation neck during PET cold drawing, where no promotion of the all-trans crystalline conformation occurred for slow drawing speeds at temperatures well below the glass transition. Our results show the tremendous capabilities of advanced vibrational microspectroscopy techniques to investigate microscale phenomena in polymer science.

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INTRODUCTION

crazes, for both the crystalline and amorphous phases, is still missing. Contrary to common diffraction or microscopy techniques, vibrational spectroscopy (infrared and Raman techniques) provides very valuable information on both the amorphous and crystalline phases, being intrinsically highly sensitive to the conformational structures present in the sample. Moreover, when using polarized light, vibrational spectroscopy is capable of providing concurrent information on the orientation, conformation, and crystallinity of polymers when precisely applied and the spectra correctly analyzed. During the past 10− 15 years the emergence and maturing of advanced vibrational microspectroscopies, especially the development of imaging techniques such as synchrotron infrared microspectroscopy (SIRMS)20−23 or coherent anti-Stokes Raman scattering (CARS),24−26 has increasingly contributed to biology and medical sciences. However, the application of these powerful techniques in polymer science is still not fully developed.27−29 In this sense, vibrational microspectroscopies may provide very valuable information with adequate spatial resolution on the crazing phenomena in semicrystalline polymeric materials. Conformationally, the poly(ethylene terephthalate) (PET) repeat unit presents four different conformers: trans−gauche conformations respecting to O−CH2 and CH2−O bonds (t and g), trans−gauche conformations with respect to CH2−CH2 bond (T and G), and cis−trans conformations of ester groups regarding the phenyl ring (TB and GB).30 The crystalline

Mechanical failure in semicrystalline polymeric materials is a highly complex phenomenon that, as well as depending on intrinsic parameters such as polymer chain structure, molecular weight, and processing history, can involve a variety of microstructural features.1 Among these, one of the most common failure precursors in polymeric materials are crazes. They propagate perpendicularly to the direction in which the external load is applied and are usually described as consisting of elongated voids and fibrils. The fibrils are formed of highly oriented polymeric chains, whereas the presence of voids is due to cavitation processes that arise from competition between the hydrostatic component of the applied stress and the internal cohesion.2−5 The tensile overload on the fibrils finally provokes chain disentanglement and/or chain scission processes. Crazes are initiated at local heterogeneities, e.g., internal flaws, where a stress concentration takes place.3,6 The generally accepted model for craze propagation after the craze has been formed is the meniscus instability model in which the propagation occurs through the formation of a fluid meniscus at the craze tip.7 At a local critical stress the craze fails through the so-called craze−crack transition mechanism,8,9 initiating the macroscopic mechanical failure of the material. Crazing phenomena in polymers have been mainly investigated by microscopy or scattering techniques.10−18 The role of conformational transitions to enhance chain mobility and cooperative motion has been suggested as having a strong impact in the nucleation process for craze initiation and subsequent craze evolution.4,19 However, a detailed conformational description of the polymeric chain rearrangement inside © XXXX American Chemical Society

Received: October 28, 2014 Revised: December 3, 2014

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DOI: 10.1021/ma502193t Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules structure is composed only of the all-trans conformation, tTtTB,31 leading to an extended chain structure nearly coplanar to the phenyl ring, whereas in the amorphous regions all the possible conformational combinations coexist. On the other hand, a two-phase model considering only one amorphous and one crystalline phase is not sufficient to explain the properties of PET. Based on Fourier transform infrared spectroscopy (FT-IR) and wide-angle X-ray scattering (WAXS), a structural model comprising three different phases has been proposed for PET, enabling a better description of the material properties.32,33 These three phases have been identified as a crystalline phase, a mesomorphic amorphous phase consisting of ethylenic trans conformers (usually referred to as a rigid amorphous phase), and a pure amorphous phase constituted only by ethylenic gauche conformers. From a spectroscopic point of view, Cole et al. proposed a very useful infrared spectral basis for PET,34,35 i.e., a set of base spectra defining a spectral space in which any spectrum can be expressed as a linear combination of the base spectra, in the same way a vector basis defines a vector space. The proposed spectral basis employs the aforementioned structural model and can be used to provide a concise description of the conformational distribution of the system. The spectral basis has three different elements denoted by G, TC, and TX. The base spectrum G is assigned to chain units with gauche conformers in the ethylenic segment (xGx), the TC spectrum represents the fully extended chain conformation (tTtTB), and the TX spectrum corresponds to an intermediate state where the ethylenic segment adopts the trans conformation, as in TC, but the rest of the chain remains disordered (xTx), as in G. Although both FT-IR and Raman microspectrocopies are highly sensitive to the conformational structure of PET, the maximum achievable spatial resolution of each technique differs due to the different illumination wavelengths employed. While in the case of Raman microspectroscopy submicron spatial resolution can be routinely achieved, in FT-IR microspectroscopy the spatial resolution is usually defined by the use of remote microscope apertures that define the illuminated sample area. For conventional IR thermal sources the maximum spatial resolution is in practice limited to around 15−20 μm, when point-by-point mapping is performed, since the signal-to-noise ratio degrades considerably as the aperture size is reduced because of the limited source brightness, i.e., photon flux per unit area and solid angle. However, it is possible to take advantage of the extraordinary brightness of synchrotron radiation (up to 3 orders of magnitude in comparison to conventional thermal sources) to achieve diffraction-limited spatial resolution,27,36−38 especially effective when using a confocal configuration.39 In this respect, it is important to mention the development of focal plane array (FPA) detectors, now routinely commercially available, since by using them remote apertures are not required for IR microspectroscopy. FPA detectors allows one shot fast imaging of the illuminated sample area with pixel-defined diffraction limited resolution.40 Current technology using commercial IR sources in combination with FPA detectors can achieve high spatial contrast and “hi-fidelity” imaging through oversampling, albeit with lower sensitivity. Nevertheless, the high brightness of synchrotron IR radiation sources guarantees high signal-to-noise ratio and fully diffraction limited spatial resolution when compared to conventional IR sources.37 Indeed, so-called “high definition” IR imaging, which can provide spatial contrast below the diffraction limit, has been

recently achieved only through the combination of FPA detectors with multiple synchrotron beams.22 In the case of IR synchrotron radiation, the so-called bending magnet radiation (BMR), produced by curving the trajectory of relativistic charged particles in a constant magnetic field, presents quasi-linear polarization properties that can be exploited to perform orientation analysis without the use of any further optical elements, thus taking advantage of the full source intensity, crucial when employing small apertures.41−44 The polarized nature of the synchrotron radiation is an important fact to consider when conformational information is to be extracted in oriented samples since polarized spectra retain effects due to orientation. Liang et al.45 have shown that for oriented samples true conformational information is revealed by the structural factor absorbance while unpolarized spectra also possess some effects due to molecular orientation. Thus, it is necessary to acquire spectra at two orthogonal polarization configurations to mathematically build up the socalled structural factor spectrum to suppress the orientation related effects.46 In this work we present synchrotron infrared microspectroscopy (SIRMS) and Raman microspectroscopy data on the conformational rearrangement of PET chains inside micron-sized crazes naturally occurring when the polymer is subjected to uniaxial deformation. Apart from the results on the crazing mechanism in PET, our results also demonstrate the high value of the information extracted from vibrational microspectroscopy, from both SIRMS and Raman. Our microspectroscopy approach to a classical problem in polymer science such as crazing can also be applied to new and expanding questions in polymer science, e.g., the interaction of proteins in polymeric scaffolds for tissue engineering or the orientation and conformational rearrangement of polymeric chains in nanoconfined media, to cite a few.

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EXPERIMENTAL SECTION

Sample Preparation. A thin film of a commercial PET polymer (LSB S.A., Barcelona, Spain; Mv = 57 000 g/mol; polydispersity index of ca. 2 (technical specifications from the producer); melting temperature, Tm = 260 °C; glass transition temperature, Tg = 80 °C) with thickness around 20 μm was prepared by compression molding using a heated press COLLIN P200 P (Dr. Collin GmbH, Ebersberg, Germany) at 280 °C and 240 bar. After 5 min at these conditions, the PET thin film was rapidly cooled between aluminum water-cooled plates (≈15 °C) for 5 min. A strip of 0.5 × 40 mm2 was then cut and subjected to uniaxial deformation in an INSTRON 3366 (Instron, Norwood, MA) dynamometer at 23 °C and 50% relative humidity. The sample was stretched up to a draw ratio of 2.5 employing a strain rate of 0.0017 s−1. The sample was maintained at this deformation for 10 min to allow for the relaxation of the material. Synchrotron Infrared Microspectroscopy (SIRMS). SIRMS was performed at the SMIS beamline47 of the French synchrotron SOLEIL (L’Orme des Merisiers Gif-sur-Yvette, Paris, France) using a Thermo Nicolet CONTINUUM XL (Thermo Scientific, Waltham, MA) microscope with a 32× Schwartzchild objective (numerical aperture, NA = 0.6) coupled to a NEXUS 5700 FTIR (Thermo Scientific, Waltham, MA) spectrometer and a liquid nitrogen cooled mercury cadmium telluride (MCT) detector. Two different regions were scanned in transmission mode. For the first, an aperture size of 10 × 10 μm2 was used to scan an area of 25 × 25 μm2 with a step size of 5 × 5 μm2. The spectral resolution was set at 4 cm−1, and 64 scans were added at each spatial point. The second region was scanned using an aperture of 4 × 4 μm2 to map an area of 20 × 8 μm2 with a step size of 2 × 2 μm2. The spectral resolution was 4 cm−1, and in this case 256 scans were summed up at each spatial point. In both cases the oversampling technique was used to increase the contrast fidelity. B

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Macromolecules Because of the intrinsic polarization of the BMR used as IR source, two spectral maps were acquired for each scanned regionone with the polarization axis parallel and another one with the polarization axis perpendicular to the deformation axisby precise sample rotation and positioning using visual markers. Thus, from the polarized spectra the structural factor spectra were derived. The structural factor spectra were used for the conformational and crystallinity analysis. Where necessary, spectral peak band deconvolution was performed using simple Gaussian curves. The deconvolution was constrained by peak positions that were held constant to within ±0.5 cm−1. The bandwidth of the 1470 cm−1 band was constrained to be lower than 10 cm−1, those at 1439, 1454, and 1475 cm−1 lower than 20 cm−1, and that at 1461 cm−1 lower than 30 cm−1. The constraints in bandwidths were based on the values reported in refs 34 and 35. The intensities were allowed to fit. Raman Microspectroscopy. Raman microspectroscopy was performed in the Raman Microspectroscopy Laboratory of the Characterization Service in the Institute of Polymer Science and Technology, CSIC, using a Renishaw InVia Reflex Raman System (Renishaw plc., Wotton-under-Edge, UK). A laser wavelength of 785 nm was used with a 100× objective with a numerical aperture of NA = 0.85. The laser spot at the sample was 1 μm in diameter, leading to a power density at the sample position of approximately 2.0 mW/μm2. A step size of 1 × 1 μm2 was employed to scan an area of 20 × 20 μm2, with an acquisition time of 1 s and 20 accumulations coadded at each spatial point. The spectral resolution was better than 1 cm−1, and a pinhole of 10 μm was employed. To ensure accurate sample focusing throughout the spectral mapping, the FocusTrack tool from the Wire 3.0 software package (Renishaw plc., Wotton-under-Edge, UK) was used. In this case, to suppress orientation related effects due to the polarization of the incident laser, a quarter-wave plate was used to depolarize the laser beam.

As was previously mentioned, due to the intrinsic polarization of the BMR, the spectra acquired using SIRMS retained orientation effects that blur the precise conformational information. Therefore, from the polarized spectra we have calculated the structural factor spectra at each spatial point so as to suppress orientation-related effects. In the case of uniaxially drawn specimens, the cylindrical symmetry of the deformation set the structural factor absorbance, A0, as

A0 =

2A⊥ + A 3

(1)

where A⊥ and A∥ denote the absorbance when the stretching direction is perpendicular and parallel to the polarization axis of the illuminating radiation, respectively.46 Figure 2a shows the polarized spectra as well as the structural factor spectrum at position (15, 5) μm2. Large differences in the absorbance of some bands, e.g., the 875 and 1340 cm−1 bands, are clearly visible in the case of the polarized spectra, showing strong IR dichroism. The chain orientation inside micron-sized crazes in PET using SIRMS and exploiting the intrinsic polarization properties of the BMR, i.e., without the use of external polarizing elements, has already been qualitatively described by us27 using the dichroic ratio, R = A∥/A⊥, of selected IR bands. Quantitative results are presented here (Figure 2b) for the 1340 cm−1 band by calculating the orientation parameter, S. The orientation parameter is related to the dichroic ratio through the relationship35,45 R−1 2 S = ⟨P2(cos θ )⟩ = 2 (2) R + 2 3 cos α − 1

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RESULTS AND DISCUSSION Figure 1 shows an overview of the PET thin film investigated in this work after being subjected to uniaxial drawing. Several

where ⟨P2(cos θ)⟩ is the mean value of the second-order Legendre polynomial of cos(θ), θ represents the angle between the main chain axis and the deformation axis, R is the dichroic ratio of the considered IR band, and α is the angle between the main chain axis and the dipole moment involved in the IR absorption process. An angle α of 18° has been assumed for the 1340 cm−1 band.35,45 As expected, a higher orientation was observed inside the craze (increase of S from 0.40 to 0.49). However, the raise in orientation was not homogeneous within the craze which may be attributed to a nonhomogeneous distribution of the load around the craze. An increase of the orientation parameter from 0, implying a randomly oriented film, to 0.40 has been obtained across the deformation neck (see Supporting Information, Figure S4), in good agreement with the values reported by Liang et al.45 A semiquantitative conformational analysis was performed using the structural factor spectra by ratioing the integrated absorbance of the 1340 and 1371 cm−1 bands (Figure 3a). Both are assigned to CH2 wagging modes (ethylenic segment),48,49 but the former is jointly related to TX and TC structures whereas the latter arises from the G conformational structure.34,35 Three representative spectra at selected positions are shown in Figure 3a. The spectra have been normalized to the 1410 cm−1 band, assigned to the C−C stretching mode of the phenyl ring,48,49 which is known to be insensitive to the conformational structure, thus being commonly used as a reference band.35,45 The spatial distribution of the 1340 to 1371 cm−1 band ratio as a 2D false color map is depicted in Figure 3b superimposed on a microscopy image. An increase of the 1340 to 1371 cm−1 band ratio from a value around 2.9 to a value around 3.3 was found inside the micronsized craze relative to its surroundings. This implies that the

Figure 1. Optical microscopy image of the uniaxially drawn PET thin film. The stretching direction corresponds to the horizontal direction. The crazes investigated by SIRMS (labeled as 1) and Raman microspectroscopy (labeled as 2) are indicated in the figure.

naturally developed micron-sized crazes perpendicular to the stretching direction can be clearly observed in the image. Although some defects were observed in the elastically deformed region, the crazes investigated were formed during the stretching process. C

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Figure 2. (a) Polarized, A∥ and A⊥, and structural factor, A0, spectra at the position (15, 5) μm2. (b) Orientation parameter, S, spatial distribution as a 2D false color map. The map is superimposed on a microscopy image.

conformation (T) at the expense of the gauche (G) conformation. However, the ratio of these two bands cannot distinguish between the TX and TC conformational structures. Therefore, a more detailed fully quantitative conformational analysis has been performed in the spectral region corresponding to the CH2 bending modes (from 1420 to 1500 cm−1).48,49 The broad band that appears in this spectral region is composed of five different bands, all assigned to CH2 bending modes but responding to different conformational structures. The bands at 1439 and 1454 cm−1 are due to the G conformational structure, those at 1461 and 1475 cm−1 are due to the TX conformational structure, and the band at 1470 cm−1 corresponds to the TC conformational structure.34,35 Therefore, through the spectral deconvolution of this broad feature (Figure 4a) it is possible to obtain a detailed picture of the conformational distribution by ratioing the integrated absorbance corresponding to each of the conformational structures to the total area of the CH2 bending modes. The results obtained are presented in Figure 4b−d as axonometric projections. The G conformational structure (Figure 4b) dominates over the complete mapped area, as expected for a rapidly cooled

Figure 3. (a) Structural factor spectra at several x positions (indicated in the figure in μm) along the line y = 5 μm. The spectra have been normalized to the 1410 cm−1 band. (b) Spatial distribution of the 1340 to 1371 cm−1 band ratio as a 2D false color map. The map is superimposed on a microscopy image.

chain rearrangement inside the micron-sized craze occurred through the conformational promotion of the CH2−CH2 trans

Figure 4. (a) Deconvolution of the CH2 bending mode spectral region at position (5, 15) μm2. Spatial distribution of the (b) G, (c) TC, and (d) TX conformational structures. D

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Macromolecules PET sample. The TC structure represents an average value of around 10%, in accordance with the values obtained by solidstate nuclear magnetic resonance (NMR) for PET samples thermally quenched from the melt.50 Within the micron-sized craze an increase of the contribution of the TC conformational structure was observed (Figure 4c), and the relative asymmetry observed may be explained by considering that the external deformation load is not necessarily evenly transferred to a local heterogeneity in the material such as a craze, which is supported by the orientation distribution (Figure 2b). On the other hand, the TX structure (Figure 3d) decreases considerably inside the craze at the same spatial positions that correspond to the increase in TC. The G conformational structure also decreases at the same positions as TC increases, in agreement with the results obtained by ratioing the 1340 and 1371 cm−1 bands, but the variation in %G is relatively smaller than that of %TC, indicating that the conformational rearrangement to full extended chain conformation (tTtTB) occurs mainly through the transformation of the TX (xTx) structure and to a less extent of the G structure (xGx). This behavior, at the low strain rate used here, differs from what was found across the deformation neck (see Supporting Information, Figure S4). We did not observe an increase in the all-trans TC conformational structure across the necking region (TC around 10%), although it has been reported that, even below Tg, a raise in the degree of crystallinity may occur in uniaxially stretched PET samples.34,51−53 The pergentage of G conformers dramatically decreased from around 71% to around 57%, and at the same time the TX conformational fraction increased by 14% (from around 19% to around 33%), revealing that the conformational redistribution of the system occurred through a G to TX conformational transformation. Moreover, the degree of crystallinity did not change showing a constant value of around 8%. To confirm whether the conformational chain rearrangement inside the microsized craze produced variations in the local crystallinity, we focused on the changes in the CO stretching overtone since the full width at half-maximum (fwhm) of this band can be related to crystallinity.34,35 The fact that this band appears at 3434 cm−1 (wavelength ≈3 μm) allows the higher diffraction-limited spatial resolution of SIRMS to be taken advantage of, using smaller microscope remote apertures. Thus, a spectral map with an aperture size of 4 × 4 μm2 was obtained on a scanned area that corresponded to the region x ∈ [3 μm, 23 μm] and y ∈ [14 μm, 22 μm] in Figures 2 and 3. After calculating the structural factor spectra, the fwhm of the CO stretching overtone was analyzed at each spatial position (Figure 5). To quantify the crystallinity degree, we performed a correlation between the fwhm of the CO stretching overtone and the degree of crystallinity (see Supporting Information, Figures S1 and S2). For that purpose we prepared several PET thin films under the same experimental conditions described in the Experimental Section but with different crystallinity by adding a small amount of carbon nanotubes due to its known nucleation ability even at high cooling rates.54 An increase in the degree of crystallinity of around 4% (from around 8% to 12%) was observed within the craze revealing that the promotion of the all-trans conformational structure implied a slight raise in the local crystallinity of PET. This again differs from what we observed across the necking region (Figure S4d).

Figure 5. (a) The 3434 cm−1 band (CO stretching overtone) at selected x positions (indicated in the figure in μm) along the line y = 0 μm. To highlight the changes in fwhm, the spectra have been normalized to the absorbance of the 3434 cm−1 band. (b) Spatial distribution of the degree of crystallinity as a 2D false color map. The map is superimposed on a microscopy image.

To confirm that the orientation changes, conformational redistribution and crystallinity development within crazes are not unique to the sample presented here, several other crazes were studied by SIRMS line mapping. The overall results are in good agreement with those presented here (see Supporting Information, Figure S5). Raman microspectroscopy was also used to study the conformational changes inside a micron-sized craze. In this case, the ratio of the 1097 cm−1 (assigned to a combination of the C(O)−O stretching mode and the C−C stretching mode of the ethylenic moiety)48,49 and the 1117 cm−1 band (assigned to a combination of the C−C stretching mode of the phenyl ring and the C(O)−O stretching mode)48,49 intensities was employed. The 1097 cm−1 band is related to tTt conformations and is insensitive to the cis−trans conformers of the ester groups.30 On the other hand, the band at 1117 cm−1 is commonly used as a reference since it has been shown to be independent of the thermal history and degree of crystallinity of the sample.55,56 Therefore, the ratio of both bands gives an idea of the tTt conformational content in the sample. The results are presented in Figure 6b as a 2D false color map. On the other hand, the fwhm of the 1726 cm−1 band (CO stretching mode)48,49 is related to the degree of crystallinity.57,58 This is due to the fact that the carbonyl groups are coplanar with the phenyl ring in the crystalline phase that produces a narrowing of the band, the same reason why the IR CO stretching overtone narrows due to crystalline development. Therefore, the local crystallinity changes were investigated by the variation in the fwhm spatial distribution of the CO stretching mode. As for the SIRMS data, a correlation between the fwhm and the crystallinity degree was performed (see Supporting Information, Figures S1 and S2) instead of using the relationships found in the literature52,57 since more accurate results were found when using our own internal correlation. The results are presented in Figure 6d as a 2D false color map. The ratio of the 1097 and 1117 cm−1 bands experienced an increase inside the micron-sized craze, and at the same time, the 1726 cm−1 was observed to narrow. Thus, in agreement with the results obtained by SIRMS, within the micron-sized craze the amount of tTt conformers increased with respect to the surroundings and the conformational rearrangement led to an increase of the local crystallinity in the sample. The local crystallinity extracted from the Raman microspectroscopic E

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presented here, where crystalline development has been observed within crazes with much lower changes in orientation and in increase in extended chain conformation, suggests that different conformational redistribution pathways may take place within crazes, which could be attributed to stress concentration at the craze and at its surroundings and/or to the constraints imposed by the size of the fibrils within the craze.

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CONCLUSIONS Synchrotron infrared microspectroscopy (SIRMS) and Raman microspectroscopy have been applied to investigate orientation, conformation, and crystallinity changes inside micron-sized crazes in PET. An increase of the all-trans conformational structure (extended chain conformation) to the detriment of mainly the trans amorphous mesophase and to a less extent to the purely amorphous phase has been observed. The conformational chain rearrangement inside the crazes produced an increase in crystallinity of around 4%. These results are consistent with a local strain-hardening phenomenon, although some dissimilarities have been observed regarding the conformational rearrangement in drawn PET samples below Tg. Our results suggest that different conformational transformation pathways could occur during crazing. Further, it has been shown that vibrational microspectroscopy techniques are very powerful tools to investigate local heterogeneities in polymeric materials. They can provide explicit information on the orientation and conformation of the polymeric chains, from both the amorphous and crystalline phases, and the degree of crystallinity with spatial resolutions in the micrometer range. The ongoing developments toward higher spatial resolutions, especially in the use of synchrotron radiation as IR source, may provide new insights into polymer materials science.

Figure 6. (a) Raman spectra at selected x positions (indicated in the figure in μm) along the line y = 10 μm. The curves have been shifted for clarity. (b) Spatial distribution of the 1097 to 1117 cm−1 intensity band ratio as a 2D false color map. The map is superimposed on a microscopy image. (c) The 1726 cm−1 band (CO stretching mode) at selected x positions (indicated in the figure in μm) along the line y = 10 μm. The curves have been shifted for clarity. (d) Spatial distribution of the degree of crystallinity as a 2D false color map. The map is superimposed on a microscopy image.

measurements agrees well with the one calculated from SIRMS (within the experimental error; note that the fitting error for the degree of crystallinity and fwhm correlation is larger in the case of Raman spectroscopy than in the case of IR spectroscopy; see Supporting Information, Figure S2), and a degree of crystallinity enhancement of around 4% (from around 9.5% to 13.5%) was also found for the micron-sized craze investigated using Raman microspectroscopy. Overall, the aforementioned results are consistent with a local strain-hardening process within the micron-sized crazes. From a conformational point of view, crazes in PET behave as local heterogeneities where the external load is concentrated, and therefore in these regions the polymeric chains undergo a higher amorphous trans to full extended chain conformational transformation than the surroundings, as well as a higher gauche to trans conformational rearrangement. Further, these conformational changes together with a rise in the number of oriented chains can be accompanied by crystal development when the local stress reaches values high enough to promote highly packed ordered structures. Although some similarities can be observed with previous studies on the conformational redistribution in stretched PET samples drawn below Tg,34,35,51−53 the changes in orientation reported are significantly higher than the ones observed within crazes in the present work. In the case of drawn PET films, first the T conformers of the amorphous phase orient rapidly adopting an extended chain conformation. After this process, the stress applied causes the conversion of the G conformers into new T oriented conformers. Subsequently, the increase in oriented chains in the all-trans conformation can lead to the development of crystalline structures. However, the results

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ASSOCIATED CONTENT

S Supporting Information *

Correlation between the fwhm of IR and Raman bands and degree of crystallinity; fwhm spatial distribution of the bands associated with crystallinity; orientation and conformational changes in PET across the deformation neck; SIRMS results from a different craze. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (G.S.). Present Address

G.S.: Photon Science, DESY, Notkestr. 85, D-22607 Hamburg, Germany. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge SOLEIL for the provision of synchrotron radiation facilities, and we thank Paul Dumas, Ibraheem Yousef, Frederic Jamme, Christophe Sandt, and Stephane Lefrançois for their inestimable assistance in using at the SMIS beamline. Travel, accommodation, and subsistence expenses during the synchrotron experiments were funded by the ELISA program of the European Union (FP7/2007-2013). The work was funded through the projects MAT2006-13149-C02-01 and MAT201021070-C02-01 of the Spanish Ministry of Economy and F

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Macromolecules

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Competitiveness (MINECO). Finally, G.S. acknowledges a research grant through the FPI program of the MINECO.

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ABBREVIATIONS BMR, bending magnet radiation; FT-IR, Fourier transform infrared spectroscopy; NA, numerical aperture; NMR, nuclear magnetic resonance; PET, poly(ethylene terephthalate); SIRMS, synchrotron infrared microspectroscopy; WAXS, wide-angle X-ray scattering.

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DOI: 10.1021/ma502193t Macromolecules XXXX, XXX, XXX−XXX