An NMR Investigation of a Polymer Melt under Shear - ACS Publications

Chapter 26

Downloaded by MICHIGAN STATE UNIV on August 27, 2013 | http://pubs.acs.org Publication Date (Web): October 14, 2011 | doi: 10.1021/bk-2011-1077.ch026

An NMR Investigation of a Polymer Melt under Shear Ute Böhme and Ulrich Scheler* Leibniz-Institut für Polymerforschung Dresden e.V., Hohe Str. 6, 01069 Dresden, Germany *E-mail: [email protected]

Mechanical shear is part of processing steps of polymers in the melt. Shear influences polymer dynamics and chain order in the melt. The effect of mechanical shear on the NMR relaxation behaviour of polymer melts has been studied. A dedicated probehead capable for high-temperature rheo NMR has been developed and applied. Both the transverse and the longitudinal relaxation time exhibit a strong temperature dependence. The prolongation of the longitudinal relaxation time T1 with increasing temperature is attributed to slowed down spin diffusion as a function of the enhanced motion. The enhanced mobility of polymer segments between the entanglements is directly reflected in the prolongation of the transverse relaxation time T2. Shearing the polymer sample results in an increase of the transverse relaxation time, indicating enhanced molecular mobility which implies, that as an effect of the shear a fraction of entanglements in the polymer have been lost, resulting in longer chain segments, these in turn exhibit an increased molecular mobility as manifested in a prolongated T2.

Introduction The flow behaviour of polymer melts is critical for various steps of the processing and defines the materials properties in a wide range of applications. Therefore rheological measurements are an important source of information on the behaviour of polymer materials (1–3). For a further understanding molecular insight into the origin of materials properties is desirable. The combination of © 2011 American Chemical Society In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


rheology with X-Ray scattering (rheo SAXS) has proven to be a valuable tool for the investigation of molecular order resulting from shear of the material (4–6). The combination of rheology with nuclear magnetic resonance (NMR) as rheo NMR permits to link macroscopic properties as measured in rheology with molecular parameters and thus yields insight in the rheological behaviour of materials (7) with all the information content of NMR on structure, order and mobility. Applications to liquid crystalline materials show shear-induced orientation and the competition between orientation in the shear field to magnetically-induced orientation (8, 9). The formation of multilamellar vesicles has been followed as well (10). Applications to polymer melts sofar have been limited to the room temperature range. Callaghan and coworkers showed, that the NMR relaxation of all protons in the polymer is affected by the shear. Results are complicated by the formation of shear bands (11). Other studies showed, that shearing a polymer melt may result in loosening of entanglements (12). Especially the transverse relaxation time T2 is sensitive to slow motions (13) as they are present in the polymer chain segments between chemical or physical crosslinks (14, 15). Another efficient measure of the polymer dynamics is the evaluation of the residual dipolar coupling between protons via the generation of double quantum coherences (16–18). A major advantage of the double quantum experiments is the fact, that a buildup of the double quantum signal is observed as opposed to the different kinds of decays like in the relaxation time experiments. The data may directly be converted into a dynamic order parameter. Relaxation experiments are feasible, even whan the residual dipolar couplings are too weak, i.e. in liquid-like systems, they can often be performed much faster.

Experimental Section For the rheo NMR experiments an in-house built high-temperature rheo NMR probe head with an integrated Couette cell has been used. The probe head fits in the micro 2.5 gradient system of a Bruker Avance 300 WB NMR spectrometer operating at a Larmor frequency of 300 MHz for protons. The micro 2.5 gradient system generates magnetic field gradients of up to 1 T/m in three axis. The Couette cell is composed of a 10 mm NMR tube as the outer cylinder with an inner diameter of 8.6 mm and exchangeable rotating inner cylinders with a diameter of 6 mm, generating a gap of 1.3 mm. The rotor is made of PEEK with a structured surface to minimize slip. Shear is applied by driving the rotor with an external servo motor located on the top of the magnet and controlled by the spectrometer. The sample is heated by hot air. The gradient system, the shims and the superconducting magnet are protected by a cooling gas flow. Between the sample and the gradient coils a temperature gradient of 10 K/mm has to be maintained. A dedicated temperature control system is in place to protect the probehead, the gradient system and the superconducting magnet monitoring the entire experiment. Initially temperatures have been calibrated in a an off-line experimente, because temperature sensors would detoriate the sensitivity of the NMR experiment. Gas flow and gas temperatures have been monitored to adjust the sample temperature. 432 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


The sample has been molten in the outer cylinder outside of the magnet. Subsequent the components of the rheo cell were assembled in a dedicated holder centering the rotor in the melt and placed into the probehead. Imaging and velocity profiles, derived from a combination of NMR imaging with PFG NMR, have been used to view the melt in the gap and to monitor the steady shear. Due to the visco-elasticity, increasing rotational velocity causes the polymer melt to climb up the rotor (Weissenberg effect) and thus disrupting the shear in the gap. The used rotor frequencies were 0.5, 1, 2 and 5 rps, yielding shear rates at the wall of the inner cylinder between 12 and 122 s-1.

Figure 1. Intensities of the double quantum signal of poly(propylene) as a function of the sample temperature.

The influence of different temperatures and shear rates of the melt has been investigated by relaxation and DQ measurements without spatial resolution. CPMG, spin echo and inversion recovery sequences have been used for T2 and T1 experiments, for the double quantum experiments a pulse sequence based on (16, 17) has been applied. The 90° pulse length was 28 µs and a repetition delay of 3 s was employed. The number of points has been restricted to limit the time duration for which the sample is exposed to high temperatures in order to avoid sample degradation. The sample used in this study was isotactic poly(propylene) (i-PP), blended with 3% PP-g-maleic anhydride (Mw = 250 kg/mol, Mw/Mn = 4.1), which is often used as an additive in composite materials. The melt temperature of 163.5 °C and the crystallinity of 50 % (based on the melting enthalpy of 207 J/g of 100% crystalline i-PP (19)), were analyzed by DSC. 433 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

Results and Discussion


Homonuclear double quantum spectra have been recorded as a measure of the residual dipoar coupling during the melting process of the polymers. As is depicted in Fig. 1 the intensity of the double quantum signal diminishes, when the polymer melts. The mobility in the polymer becomes sufficiently high, that the dipolar coupling is averaged to a degree, that no double quantum intensity is observable. Unexpectedly no double qunatum signals have been observed under the shear rates applied in the present study, indicating, that only limited order of the polymer chains has been induced by the shear.

Figure 2. Longitudinal relaxation time (T1) experiments by inversion recovery of poly(propylene) as a function of the sample temperature.

Fig. 2 shows the temperature dependence of the longitudinal relaxation time, the lines show the fitted curves with the values shown in Table 1. At temperatures below the nomimal melting point (160 °C) biexponential decay is observed. Above 170 °C T1 can be described by a single exponential. The increase of T1 with increasing temperature is attributed to slowing down spin diffusion due to the enhanced molecular mobility. The small difference of the two components below the melting point and the fact that above the melting point a single component is observed, show that spin diffusion is important for T1. The major relaxation mechanism is the methyl rotation and thus the efficiency of the spin diffusion from CH and CH2 protons is important for the longitudinal relaxation. The parameters used to fit the experimental data of all T1 experiments a summarized in Table 1. The temperature dependence of the transverse relaxation time T2 is depicted in Fig. 3 showing a strong increase of T2 upon melting. T2 increases further with increasing temperature showing the enhanced mobility of polymer chain segments between entanglements as expected. T2 shows a multiexponential behaviour as for most polymers, and can sufficiently be described by a biexponential fit. The results from the fit are shown in Table 2. A short component in the order of 20 to 80 ms 434 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


and the longer component raises from 200 ms to 300 ms. The parameters used to fit the experimental data of all T2 experiments a summarized in Table 2.

Figure 3. Decay curves from CPMG experiments as a function of the sample temperature.

Figure 4. T1 as a function of the shear rate of poly(propylene) at 180 °C.

If the inner cylinder is rotated with a torque applied from the servo motor, the poymer melt in the gap between the two cylinders is sheared. The melt sticks to both cylinders, thus the polymer is at rest on the outer wall and rotates with the inner cylinder. 435 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


Figure 5. Decay curves from CPMG experiments at 180 °C as a function of the shear rate. Shearing the polymer at a constant sample temperature of 180 °C does not result in a significant change of T1 as depicted in Fig. 4. Comparing to the strong temperature dependence of the longitudinal relaxation seen in Fig. 2 one can conclude, that the sample temperature has not been increased by shearing the highly viscous polymer melt. From the viscosity a temperature increase of up to 30 K would have been anticipated. Because the sample is heated by an air flow of 17 l/min, the additional heat is taken away from the sample by convection.

Table 1. Results from the fits for the longitudinal relaxation time T1 T

rotation

T1 [s]

T1 [s]

[°C]

[rev/s]

component 1

component 2

160

0

0.73

0.06

170

0

0.73

0.10

180

0

0.74

190

0

0.83

180

0.5

0.70

180

1

0.63

180

2

0.64

180

5

0.64

436 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


Table 2. Results from the fits for the transverse relaxation time T2 T

rotation

T2 [ms]

T2 [ms]

[°C]

[rev/s]

component 1

component 2

160

0

19

224

170

0

25

227

180

0

34

242

190

0

53

375

180

0.5

47

258

180

1

50

281

180

2

65

285

180

5

83

269

The effect of shearing on T2 is shown in Fig. 5, showing an increase in T2 as a function of the shear. The signal decay curves under shear are well-described by a double exponential decay. Both components increase from 40 ms to 60 ms and 150 ms to about 300 ms respectively. The fraction of the component with a larger T2 increases with the shear rate. The increase in T2 implies an increase in the mobility of chain segments as a result of the shear. A decrease in T2 would have been indicative for possible chain ordering, which has not been observed. The increase of T2 and thus of chain mobility is explained by loosing some of the entanglements of the polymer chains as a result of the shear-induced motion of the polymer chains.

Conclusions Rheo NMR of polymer melts at high temperatures has been demonstrated. Both the longitudinal and the transverse relaxation times exhibit a strong temperature dependence in the temperature range of the melting point. The observed temperature dependence of the longitudinal relaxation time has been utilized to exclude a shear-induced rise in the sample temperature in the present study. The transverse relaxation time T2, which is sensitive to slow molecular motions typical for polymer chain segments between crosslinks or entanglements, is becoming longer. The polymer network looses entanglements as a result of the shear. The absence of a significant double quantum signal together with the increase in T2 indicate, that chain ordering, which would be anticipated, is not the dominating effect. The effect of orientation of the polymer chains is negligible. The most reasonable explaination for the prolongation of T2 is the partial loss of entanglements resulting in longer chain segments between entanglements.


Acknowledgments This work has been supported by the Deutsche Forschungsgemeinschaft (DFG) under grant SCHE 524/9 in the Materials World Network. The sample has been provided by Dr. Harkin-Jones (Queen’s University Belfast).

References


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An NMR Investigation of a Polymer Melt under Shear - ACS Publications

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