Dynamic infrared spectroscopy, a tool to detect hydrogen bonds in polymers?

Dynamic infrared spectroscopy, a tool to detect hydrogen bonds in polymers?

PII: Eur. Polym. J. Vol. 34, No. 11, pp. 1571±1577, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0014-3057/99 $ - s...

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PII:

Eur. Polym. J. Vol. 34, No. 11, pp. 1571±1577, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0014-3057/99 $ - see front matter S0014-3057(98)00011-1

DYNAMIC INFRARED SPECTROSCOPY, A TOOL TO DETECT HYDROGEN BONDS IN POLYMERS? MICHAEL KISCHEL,* D. KISTERS G. STROHE and W. S. VEEMAN Department of Physical Chemistry, Gerhard-Mercator-UniversitaÈt Duisburg, Lotharstr. 1, 47057 Duisburg, Germany (Received 10 July 1997; accepted in ®nal form 4 September 1997) AbstractÐDynamic infrared linear polarised spectra of thermoplastic polyester±polyurethane and nylon-6 ®lms are recorded under a varying sinusoidal strain. Dichroic spectra are calculated from the dynamic 08 and 908 polarised spectra. The large bipolar bands in the dichroic in-phase spectra, caused by large frequency shifts of the original monopolar absorption bands, are ascribed to hydrogen bonds. This shows that especially the hydrogen bonds are a€ected by stretching of the material. # 1998 Elsevier Science Ltd. All rights reserved

INTRODUCTION

Dynamic Infrared Spectroscopy [1±4] is a wellknown tool to enhance the information of vibrational spectra. Dynamic linear dichroic spectra [5, 6] provide information about stress-induced deformations of the sample on a molecular level. For example, molecular reorientations and frequency shifts of distinct absorption bands induced by the stress can be observed. Especially weak chemical forces like H-bonds should be very sensitive to the applied external perturbations. The properties of hydrogen-bonding in polyurethane [7±12] and nylon [12±18] have been studied intensively in the literature. Generally, hydrogenbonding involves the interaction between a proton acceptor (R1±Y) and a proton donor group (H±X± R2). It may be described schematically by: R1 ±Y    H±X±R2 The hydrogen-bonding forces entail an increase of the R1±Y and H±X bond length. Consequently, the n(R1Y) and n(HX) stretching frequencies will be lowered. Compared to other stretching frequencies, these frequencies may be very susceptible to deformations of the material. Therefore, we describe here experiments which can detect small changes in IR frequencies upon periodic stretching of the sample, in order to establish whether hydrogen bonds are induced more sensitive to mechanical perturbation than other bands.

THEORY

Dynamic spectra In a dynamic experiment changes in the environment of functional groups are caused by a small external time-dependent perturbation. These vari*To whom all correspondence should be addressed.

ations can be observed as changes in the infrared absorption. The resulting spectra are called dynamic spectra. If the external perturbation is small and reversible, the time dependent infrared absorption A(, t) measured at the wavelength  can be written as sum of two independent components ~ , t† A…, t† ˆ A…† ‡ A…

…1†

The ®rst term A() refers to the quasistatic absorption corresponding to the ``normal'' absorption spectrum without any external perturbation. The second term, AÄ(, t) describes the perturbation induced modulation of absorption. Dynamic IR linear dichroism (DIRLD) In general IR absorption is caused by the interaction between the IR electric ®eld vector and the molecular dipole transition moment related to certain molecular vibrations. When the electric ®eld vector and the dipole transition moment are parallel to each other, absorption reaches its maximum. In case of a perpendicular orientation the absorption is zero. The dichroic di€erence DA(), which serves as a convenient measure of the optical anisotropy of the sample, is given by DA…† ˆ Ak …† ÿ A? …†

…2†

The directional absorbances A6() and A_() are measured with linear polarised IR beams. The terms parallel and perpendicular refer to the orientation of the polarised IR beam to a reference axis, in this case the stretching direction. DA() yields information about the average orientation of dipole transition moments of the corresponding functional groups. If the dichroic di€erence is positive (i.e. parallel dichroism), the average of the dipole transition moments are oriented along the reference axis. It becomes negative (i.e. perpendicular dichroism), when the overall orientation is perpendicular. For an optical isotropic sample the dichroic di€erence vanishes.

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Dynamic IR linear dichroic (DIRLD) experiments can be considered as a combination of dynamic and dichroic measurements. The response of the sample to the time dependent perturbation is monitored as changes of directional absorptions. For small perturbations the dynamic dichroic di€erence, DA(, t) can also be considered as a sum of a quasistatic and a dynamic component.  † ÿ DA… ~ , t† DA…, t† ˆ DA…

…3†

 † ˆ A  k …† ÿ A  ? …† DA…

…4†

~ , t† ˆ A~ k …, t† ÿ A~ ? …, t† DA…

…5†

with

The dynamic dichroic di€erence DAÄ(, t) is caused by the response of the system to the applied external perturbation. Variations in the amplitude, the line width or in the resonance frequency of the absorption lines may occur [19, 20]. These variations yield characteristic line shapes in the dynamic spectra [21]. Sinusoidal perturbation An applied sinusoidal stress with frequency os can be described as ~ ˆ s^ sin o s t s…t†

…6†

where s^ is the maximum stress. The dynamic dichroic di€erence of a system is in¯uenced by the stretching as follows ~ , t† ˆ DA… ^ †sin‰o s t ‡ b…†Š DA…

…7†

DAÃ(n) refers to the amplitude of the dynamic dichroic signal, whereas b() represents the phase lag between the observed response of the sample and the perturbation. The dynamic dichroic di€erence can be expressed in terms of a sum of two orthogonal components ~ , t† ˆ DA 0 …†sin o s t ‡ DA0…†cos o s t DA…

…8†

^ †cos b…† DA 0 …† ˆ DA…

…9†

^ †sin b…† DA0…† ˆ DA…

…10†

with

DA'() and DA0() form the in-phase and the quadrature spectrum, respectively, associated with the response simultaneously and 908 out of phase to the external perturbation. Modulation/demodulation experiments Spectra gained by modulation/demodulation experiments can be approximated by the di€erence of two spectra taken at the extreme positions of the modulation cycle [1]. This is true if the phase delay b of equation (7) is independent of the absorption frequency. In this case the 08 and 908 polarised inphase spectra can be approximated by  st …† ÿ A  re …† Ak 0 …†  A k k

…11†

 st …† ÿ A  re …† A? 0 …†  A ? ?

…12†

The exponents ``st'' and ``re'' express the state of maximum (st = stretched) and minimum (re = relaxed) elongation of the polymer ®lm in a static measurement. The di€erence of equations (11) and (12) DA 0 …† ˆ Ak 0 …† ÿ A? 0 …†

…13†

leads to the DIRLD in-phase spectrum. Hence, the dynamic in-phase spectrum can be described as a dichroic di€erence of two di€erence spectra, obtained by static measurements. EXPERIMENTAL

Sample preparation The thermoplastic polyester polyurethane (TPU) ®lms are made of Elastollan1 C64D. Approximately 0.04 g TPU was dissolved in 10 ml DMF under re¯ux. The polymer solution was cast on a Te¯on mould with an area of 52.6 cm2. After drying for 5 h at 708 in a dry-box the ®lm was brought into boiling water to remove residues of DMF and then dried again [22]. The samples were prestretched to two times their original length under heating with a hot-air gun. The nylon samples were made of a 2% solution of nylon-6 granulate (DSM) in formic acid (97%). 5 ml of this solution was cast on a glass plate with an area of 10 cm2. DSC measurements have shown that the dried ®lms consist of a mixture of a- and g-modi®cation.

Experimental setup The experimental setup has been described before [21]. All spectra were recorded with a Bio-Rad FTS-60A FT-IR spectrometer, equipped with a direct current mercury±cadmium±telluride detector (DC-MCT). The interferometer was operated in step-scan mode at a step-scan frequency of 0.25 Hz with 400 Hz phase-modulation. The amplitude of modulation was 0.92 mm, corresponding to 1.5 He±Ne laser wavelengths. One symmetric scan with 1973 data points and a resolution of 8 cmÿ1 was recorded in approximately 132 min. A stretching frequency of 10 Hz was chosen for the BioRad polymer stretcher device with a force-controller (model 9203 with charge ampli®er model 5011 of Kistler). The sinusoidal varying stress of the sample was displayed on an oscilloscope. The amplitude of the stress remained constant during the measurements, which ensures that no irreversible stretching of the polymer ®lm occurred. The amplitude of stretching was 100 mm, corresponding to a deformation of approximately 0.4% for a 2.5 cm long polymer sample. At the aperture of the sample compartment a gold wire grid polarisator was mounted to polarise the IR-radiation parallel (08) or perpendicular (908) with respect to the stretching direction. An optical ®lter (UDR 4 ®lter), attenuating wavelengths above 3900 cmÿ1, was used to avoid folding e€ects. First, a Bio-Rad lock-in ampli®er (LIA1), referenced to the phase modulation frequency of 400 Hz demodulates the doubly modulated detector signal. To maximise the output signal, the phase angle was adjusted. The roll-o€ of the low-pass ®lter was set to 25 Hz. The output of LIA1 is digitised to obtain the single-beam spectra. The output of LIA1 passes an electronic ®lter that cuts o€ the dc-o€set and attenuates large spikes, due to the mirror movement in the interferometer. The singly modulated signal is further demodulated by a two-channel lock-

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Fig. 1. Dichroic and dichroic in-phase spectrum of 100% prestretched TPU, collected at a stretching frequency of 10 Hz. in ampli®er (LIA2, model SR850, Stanford Research), which is referenced to the stretching frequency. The inphase and quadrature signal is digitised, to obtain the dynamic IR-spectra. The phase angle of the phase sensitive detector LIA2 is adjusted so that the quadrature component becomes minimum. The interferograms collected from the output of LIA1 were Fourier transformed by aid of the Mertz phase correction. The phase spectra calculated from these interferograms were stored and used for the phase correction of the dynamic spectra. To normalise the dynamic spectra, they were divided by the corresponding single beam spectra [23]. Afterwards the dichroic di€erence was calculated in order to eliminate signals due to the variation of the sample thickness. All data traces were collected simultaneously with a sampling frequency of 11.1 kHz by an external data station.

RESULTS AND DISCUSSION

Monopolar bands TPU. The DIRLD in-phase and dichroic spectra of 100% prestretched TPU are presented in Fig. 1. The band assignment, based on work of Ishihara et al. [11], Srichatrapimuk et al. [24] and Nakayama et al. [25], is presented in Table 1. The dichroic spectrum contains bands with positive and negative intensities. The absorptions of the N±H, C±H, CH2 and C.O stretching vibrations are positive, corresponding to a net orientation of the dipole transition moments perpendicular to the stretching direction. The other dipole transition moments, almost parallel to the chain axis, align parallel to the stretching direction. This implies that an average orientation of the polymer backbone in the prestretched sample is in the direction of the stretching axis.

The dichroic in-phase spectrum contains changes in absorption caused by the mechanical perturbation. It shows several monopolar and two strong bipolar bands. The origin of the bipolar bands will be discussed below. In our convention, positive monopolar bands in DIRLD-spectra refer to reorientation movements of dipole transition moments to the direction perpendicular to the stretching axis (movement of the axis B, perpendicular to the chain axis, towards x in Fig. 2). A movement towards the 08 axis with respect to the stretching direction yields a negative band (movement of the chain axis A towards y in Fig. 2). In the in-phase spectrum in Fig. 1 the vibrations of n (CH2) at 2960 cmÿ1, g (C±H) at 817 cmÿ1 and g (COO) at 772 cmÿ1 have positive signals. This means that their dipole transition moments move in the direction perpendicular to the external perturbation. The other bands have negative signs. Consequently, their dipole transition moments tend to align with the external perturbation. The dipole transition moments of bands with a positive sign in the dichroic spectrum are approximately perpendicular to the polymer backbone. The axis of the polymer chain lies in the plane built by phenyl rings and carboxyl groups. Therefore, out of plane vibrations of these groups are perpendicular to the polymer backbone. Almost parallel to the polymer chain are the dipole transition moments of signals with negative sign. These are the CC stretching in aromatic rings, the stretching of COCgroups, the CH2 wagging, the in plane bending of CH in aromatic rings and the d(NH) + n(CN) vibration. This shows clearly the movement of the polymer chains upon stress towards the stretching axis due to the external perturbation.

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M. Kischel et al. Table 1. Absorption band assignment of the polyester±polyurethane Frequency (cmÿ1) 3331 3043 2960 2874 1732 1703 1597 1532 1415 1359 1310 1254 1230 1183 1145 1112 1081 1019 817 772

Relative intensitya

Main assignmentb

S W S M, she VS VS S VS M W M W

n (N±H) bonded N±H n (C±H) in benzene ring na (CH2) ns (CH2) n (C.O) free C.O n (C.O) bonded C.O n (C.C) in benzene ring d (N±H) + n (C±N) n (C±C) in benzene ring o (CH2) d (NH) + n (C±N) o (CH2), n (C±O±C) in hard- and soft segment d (N±H) + n (C±N) n (C±O±C) ester group in soft segment n (CH2)n n (CH2)n n (C±O±C) in hard segment b (C±H) in benzene ring g (C±H) in benzene ring g (COO)

W W, she W, she W, she M W W W

a

Relative intensity in dichroic spectrum based on sample at room temperature. W = weak; M = medium; S = strong; VS = very strong; she = shoulder. b n = stretching; ns=symmetric stretching; na=asymmetric stretching; d = bending; o = wagging; b = in plane bending; g = out of plane bending.

Nylon. The DIRLD in-phase and dichroic spectra of nylon-6 are presented in Fig. 3. The band assignment, based on work of Arimoto [26] and Matsubara et al. [27], is presented in Table 2. Bands in the region from 1500 to 1650 cmÿ1 are not taken into consideration, because they show absorptions larger than 1.4 AU [1, 3]. The dichroic spectrum contains only very weak bands, corresponding to an almost optical isotropic sample. The dynamic in-phase spectrum contains monopolar bands with positive and negative signs and a bipolar band, discussed later. The CH2 stretching

Fig. 2. Schematic representation of orientation movements in a polymer chain upon stress. Stretching direction is parallel to Y.

has a positive peak, consequently their dipole transition moments move to an axis perpendicular to the stretching direction. The very weak bands of the deformation vibrations in the in-phase spectrum of Fig. 3 are too weak to discuss in detail. It can be assumed that similar motions as in TPU take place in nylon-6. Bipolar bands In the in-phase spectra of TPU (at 3330 and 1703 cmÿ1) and nylon-6 (at 3310 cmÿ1) intensive bipolar bands can be observed. The corresponding quadrature spectra contain only very weak peaks. This shows the independence of b (see equations (9) and (10)) on the absorption frequency. In this case, the dynamic in-phase spectra can be approximated by the di€erence of two static measurements taken at the extreme positions of a stretching cycle (equation (13)). Ingemey et al. have shown that bipolar bands occur in dynamic spectra if the frequency of an absorption maximum shifts upon stress [21, 28]. Negative bands, with negative absorption on the high frequency side and positive bands with positive absorption on the high frequency side are distinguished in dichroic dynamic spectra. The negative bipolar bands of TPU and nylon-6 are caused by frequency shifts of originally negative absorption bands to higher wavenumbers upon stress. In Figs 4 and 5 can be seen, that the bipolar character is detected only in the 08 polarised st spectrum, i.e., the maximum of A6 () is at a higher re wavenumber than the maximum of A6 () (equation (11)), while the perpendicularly polarised spectrum shows only a monopolar band due to the modulation of the ®lm thickness. This means that the maximum of A_() is not shifted by the stretching (equation (12)). The bipolar bands are assigned to the n (N±H) stretching in TPU and nylon-6 and the n (C.O) stretching in TPU, respectively. These functional groups take part in hydrogen bonds, where the N±

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Fig. 3. Dichroic and dichroic in-phase spectrum of nylon-6, collected at a stretching frequency of 10 Hz.

H group interacts with the C.O group [7±18]. It can be assumed, that during the stretching cycle the hydrogen bonds within the sample are weakened, and consequently the N±H and C.O bonds are tightened. Their absorption bands are therefore at higher wavenumbers in the stretched than in the relaxed state. Furthermore, this e€ect occurs only in 08 polarised dynamic spectra of TPU and nylon6 (see Figs 4 and 5). This means, that only the Hbonds of N±H in nylon-6 and N±H and C.O groups in TPU that are aligned almost parallel to the external perturbation, are in¯uenced by the stretching. The dichroic in-phase spectrum of TPU in Fig. 1 shows two bipolar bands with the same sign from the two interacting groups. Asymmetries of these bands are due to underlying monopolar bands of unbounded N±H and C.O groups. In the dichroic in-phase spectrum of nylon-6 the bipolar band of the C.O group does not appear. The spectral region

where this band is expected has total absorption in the corresponding absorption spectrum and the spectral information is therefore not reliable. Consequently, the small changes of the line shape in the absorption spectrum caused by the modulation cannot be analysed in the in-phase spectrum. In an earlier work Singhal and Fina [29] studied melt-crystallised nylon-11 ®lms on Te¯on1 with 2D-IR spectroscopy. They recorded 08 polarised dynamic spectra with a spectral resolution of 16 cmÿ1. 12 scans were coadded and no normalisation with the single-beam spectrum was performed. The dichroic di€erence was not calculated and signals due to the variation of the sample thickness were not eliminated. In the N±H stretching region they assigned the bipolar band at 3300 cmÿ1 to two distinct peaks at 3300 and 3332 cmÿ1. From the synchronous and asynchronous 2D-IR correlation plots, they assigned the 3300 cmÿ1 absorption to dipole transition moments in primarily ordered

Table 2. Absorption band assignment of nylon-6 ÿ1

Frequency (cm ) 3331/3283 3075 2949 2880 1481 1460 1421 1260 1209 728 a

Relative intensitya

Main assignmentb

VS M

n(NH) Fermi resonance n(NH) with overtone Amid II na(CH) ns(CH) d (CH2) d (CH2) d (CH2) Amid III gt (CH2) or o(CH2) gr (CH2)

S W W W W W W W

Relative intensity in dynamic spectrum based on sample at room temperature. W = weak; M = medium; S = strong; VS = very strong; she = shoulder. n = stretching; ns=symmetric stretching; na=asymmetric stretching; d = bending; o = wagging; gt=twisting; gr=rocking.

b

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Fig. 4. 08 and 908 in-phase spectrum of 100% prestretched TPU, collected at a stretching frequency of 10 Hz.

(crystalline) regions. For the 3332 cmÿ1 peak they suggested a response of dipole transition moments in ordered and disordered (amorphous) phases. We assign these peaks to a negative bipolar band with a minimum at 3334 cmÿ1 and a maximum at 3285 cmÿ1. Such a spectral feature can be explained by a slight frequency shift of one absorption band. As discussed above, in our view the origin of this bipolar band is due to the perturbation of hydrogen bonds caused by the cyclic stretching.

CONCLUSIONS

In dichroic dynamic infrared spectra of prestretched thermoplastic polyester±polyurethane and unstretched nylon-6 ®lms strong bipolar bands are observed. These bipolar bands are ascribed to hydrogen bonds. The large intensities of the bipolar bands show that some of the hydrogen bonds are strongly a€ected by the periodic stretching of the ®lms during the dynamic experiment. From the fact

Fig. 5. 08 and 908 in-phase spectrum of nylon-6, collected at a stretching frequency of 10 Hz.

Dynamic infrared spectroscopy

that the bipolar bands are only observed in the 08 polarised dynamic in-phase spectra, we conclude that only the H-bonds that are aligned almost parallel to the external perturbation are a€ected by the periodic stress. The dynamic dichroic infrared spectroscopy is particularly suited to detect hydrogen bonds, as this work proofs, since the weak hydrogen bonds are a€ected relatively strong by the mechanical perturbation. REFERENCES

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