Study on the Glass Transition Process of Polymer System Using Differential Scanning Calorimetry and Fourier Transform Infrared Spectroscopy

ANALYTICAL SCIENCES SEPTEMBER 2017, VOL. 33 1071 2017 © The Japan Society for Analytical Chemistry

Yong-jin PENG,*† Yu-ling LIU,* Qiang WU,** and Ping-chuan SUN**

*Teaching and Research Section of Physics, College of Comprehensive Studies, Jinzhou Medical University, Jinzhou 121001, P. R. China **College of Chemistry, Nankai University, Tianjin 300071, P. R. China

The change in the infrared spectrum of polymer samples with temperature and their differential scanning calorimetry (DSC) experimental results are analyzed. According to the van’t Hoff equation at constant pressure, the changes in the absorbance ratio corresponding to high and low vibrational states are calculated, and the apparent enthalpy differences of the vibration energy states transformation of the characteristic group can be obtained. From the experimental results, we can find that characteristic vibration modes of a chemical group in a polymer are under the influence of the glass transition process of the polymer with a different extent. The characteristic vibration modes of the same chemical group behave differently due to the influence of the polymer system at which the chemical moiety is situated.

Keywords Polymer, glass transition process, polystyrene, polyvinyl methyl ether, FTIR micro-spectroscopy

(Received March 15, 2017; Accepted June 2, 2017; Published September 10, 2017)

model has its reasonable parts. But, because most of these Introduction theories can only explain part of the glass transition behavior, a consensus still can not be obtained concerning the theory of Glass transition is a common nature of all materials. Any glass transitions.4,8,9,13 Developed in the 1970s, Fourier transform material in the liquid state can be passing through the glass infrared spectroscopy (FTIR) is a kind of interferometric transition process and be frozen into the glass state (amorphous) infrared spectroscopy. As a kind of effective method for the as long as it is under the appropriate cooling rate.1–4 Although identification and characterization of molecular structure, the essence of the glass transition still can not be really FTIR is a measurement with a high signal-to-noise ratio, high understood so far, the glass transition process has been used for resolution, high scanning speed and wide measurable wavelength production and in life service by people with a long history. range. Since its development, FTIR has been widely used Generally a glass transition denotes a transition from the in chemical research, energy exploration, astronomical equilibrium liquid state to a non-equilibrium solid state in a observations, weather analysis, environment monitoring, multibody system.5,6 This kind of transition does not involve a geological research, drug development, agricultural management, change of a thermodynamic, but a dynamic sense, at least within and other fields of analytical determination.29–33 The molecular the time scope of the experimental observation. An amorphous orientation arrangement and aggregation inside the material will solid is in a static state in a dynamic sense, and can not relax change with the corresponding change of the external conditions within the time scale of an observation.7–11 The typical glass (load, temperature, pressure, etc.). The microscopic structure forming material is an ultra-cooled molecular liquid at under the change and its energy contribution in the macro process can be crystal transition temperature. The density of the molecular obtained through calculations and analysis of the infrared liquid increases with the decrease of temperature. For the same spectra using the dynamic FTIR technique. There is much reason it is also found that the glass transition exists in a series research on polymer systems using the FTIR technique; some of of particle systems with different sizes, from a micellar them are combined with other techniques, such as NMR, X-ray suspension to a cell biology system. The glass transition is a etc., which show that FTIR is a powerful tool for analyzing the general phenomenon of a multibody system and is very microscopic structure and energy change in, but not limited to, important in condensed matter physics. the polymer system.34–44 Yabe and coauthors studied the There are lots of theoretical models concerning glass temperature effect on the molecular structure of spider’s capture transitions like as free volume theory, dynamic frustrated theory, thread by using microscopic FT-IR spectroscopy. By measuring mode-coupling theory, energy landscape mode and twinkle the FT-IR spectral change from 40 to 220°C, they found that the fraction theory etc.8,9,12–28 All of these theoretical models can molecular conformation of silk fiber protein of a capture thread help us to understand the nature of the glass transition, and each is denatured at over 60°C, whereas the viscid droplets on a capture thread retain their structure.44 This work demonstrates † To whom correspondence should be addressed. the power of the microscopic FT-IR technique at the micro E-mail: [email protected] level. The glass transition process can commonly occur in a 1072 ANALYTICAL SCIENCES SEPTEMBER 2017, VOL. 33 polymer system along with the change of parameters, such as of 10°C/min, and was kept at this temperature to eliminate the the temperature or pressure, which would be accompanied by a thermal history. Then, the polymer sample was cooled to a micro energy change and a conformation transformation.9,45–49 temperature which was about 50°C below its glass transition This can help us to understand the glass transition process temperature at a cooling rate of 1°C/min, and was kept at this deeply through studying the micro change process of each group temperature for 10 min before being heated up to 50°C above in the polymer system. But within the traditional analysis the glass transition temperature again at a rate of 1°C/min. methods of thermodynamic and dynamic commonly used in the Polymer samples were infrared sampled under the wave number study of glass transition process, most popular method can only range from 750 to 4000 cm–1. The sampled data were analyzed obtain macroscopic change information in the glass transition using Bio-Rad Win IR Pro 2.7 Ver. software. According to the process of a polymer system. The functional molecular group, infrared spectrum of the polymer system in the process of whether participating in glass transition process or not, can temperature changing, the characteristic vibration absorption seldom be specified at the micro level. The molecular orientation frequency peak of each functional group in molecule had a change of the arrangement and aggregation inside the material certain degree of shift. This shift reflects the fact that mutual can not be recognized very well either. By analyzing the change transformation between two different energy states of the of the infrared spectra of the polymer system with the vibration pattern of a certain functional group in the molecule: temperature, we can study the participation information of corresponding to the upper state A at a higher temperature and different molecular moieties in a homopolymer (polystyrene, the low-energy state B at a lower temperature. PS110K) and miscible polymer blend (polystyrene/polyvinyl The van’t Hoff equation can be used to well describe the methyl ether, PS110K/PVME), and we can also study the relationship between the concentration of the upper state A (CA) 33 influence of the interaction between the molecular moieties on and the low-energy state B (CB): the glass transition behavior of the polymer system, which can deepen our understanding on the polymer glass transition CA ⎡ ΔG ⎤ = exp⎢− ⎥ , process at the micro level. CB ⎣ RT ⎦

where T is the absolute temperature, R is the gas constant and Experimental ΔG is the difference value of the Gibbs free energy between the two different energy states of the vibration pattern of a certain Polystyrene was purchased from Polymer Source (Canada) with functional group in the molecule. ΔG can be described using a molecular weight of about 110 kg/mol, and used directly after the following formula: being purchased. Polyvinyl methyl ether with the molecular weight of 67 kg/mol, purchased from TCI Company (Japan) in ΔG = ΔH – TΔS, a 50% methanol solution, was used after removing methanol. PS110K/PVME of 3:1 mass ratio was dissolved in a toluene In the above formula, ΔH and ΔS are the enthalpy and entropy solution at a concentration of 1% (g/ml), and was under magnetic differences of the two different energy states of the vibration stirring at room temperature for 24 h until completely dissolved. pattern, respectively. The relation between the absorption

After being rotary evaporated to be less solvent the solution strength AA(upper state)/AB(lower energy state) and the was put in a vacuum oven at a temperature of 40°C under a concentration of upper energy state (CA)/ lower energy state vacuum for one week and at 60°C again for more than 48 h until (CB) are as follows: its mass no longer changed. A transparent blend film was made, which showed that the blend of PS110K and PVME had good AA = aACAb, AB = aBCBb. compatibility, which could also be seen from the DSC results. The DSC thermograms were measured using a Mettler-Toledo Here a is the molar light absorption coefficient of the polymer DSC1 STARe calorimeter. The temperature and heat ﬂow were sample and b is the length of the light absorption path of the calibrated before the measurement. All samples were heated up polymer sample. Then the following equation can be obtained: to 50°C above its glass transition temperature at a rate of 10°C/min in a nitrogen atmosphere and was kept at that temperature to AaBA ⎡ ΔΔH S ⎤ = exp⎢− + ⎥. eliminate the thermal history. Then, the polymer sample was AaAB ⎣ RTe R ⎦ cooled to a temperature of about 50°C below its glass transition temperature at a cooling rate of 1°C/min, and was kept at this Take the logarithm on both sides: temperature for 10 min before being heated up to 50°C above its glass transition temperature again with a heating rate of 1°C/min. ⎡ AB ⎤ ΔΔΗ S aA ln ⎢ ⎥ = − + − ln . A FT S6000 Fourier transform infrared spectrometer, UMA500 ⎣ AA ⎦ RT R aB type infrared microscope with an aperture unit of 250 μm in diameter and narrowband MCT detector, were used in our The amount of polymer sample is very small relative to the experiment. The scanning rate was 10 kHz, electronic filtering heating stage. Therefore, it is reasonable to think the enthalpy was 5 kHz, 16 scans (finished in about 22 s) were coadded in and entropy change between the different energy states of the each scan set for every temperature point, spectral resolution vibrational mode of the sample molecules, can be neglected which was decided by the moving distance of the movable within a small temperature interval. Also, the molar light mirror was 4 cm–1 and the measurement accuracy was decided absorption coefficients of two different energy states have a by an infrared laser interferometric fringe is of 0.01 cm–1. similar relationship with temperature, so that the enthalpy A Linkam THMSE 600 heating stage was used to control the change between two different energy states of the vibrational temperature to within ±1°C. The polymer sample (10 μg) was mode of polymer sample can be written as placed on a heating stage and clamped with a KBr microscope ∂[ln(AAAB/ )] slide and a glass coverslip. Firstly the polymer sample was ΔΗ = R . heated up to 50°C above its glass transition temperature at a rate ∂()1/T ANALYTICAL SCIENCES SEPTEMBER 2017, VOL. 33 1073

Fig. 1 Comparison of DSC thermograms for the PS110K/PVME Fig. 3 FTIR spectra of PS110K/PVME (800 – 1000 cm–1); the blend and the corresponding pure original species in the second temperature denoted in the figure is in degrees Celsius. heating process.

Table 1 L/H of the characteristic vibration modes in different polymer system analyzed in this work (Car stands for C in benzene ring)

L/cm–1 H/cm–1 Vibration mode Polymer system

842.8 839.0 Main chain C–C PS110K 2850.9 2850.1 Main chain C–C PS110K 907.6 904.9 Car–H out of benzene ring PS110K 943.6 941.3 Car–H out of benzene ring PS110K 1804.8 1798.3 Car–H out-of-plane bending PS110K 1944.8 1939.1 Car–H out-of-plane bending PS50K 3001.9 3000.1 Car–H stretch PS110K 3083.2 3079.0 Car–H stretch PS50K 907.9 905.6 Car–H out of benzene ring PS110K/PVME 946.4 944.0 Car–H out of benzene ring PS110K/PVME 2851.0 2849.7 Main chain C–C PS110K/PVME

Fig. 2 FTIR spectra of PS110K (800 – 1000 cm–1); the temperature denoted in the figure is in degrees Celsius. It can be seen from Figs. 4 and 5 that at a temperature of 105°C the values of ln(A839.0/A842.8) and ln(A2850.1/A2850.9) both have an obvious turning point. The corresponding temperature (105°C) of the inflection point is very close to the glass transition Results and Discussion temperature of PS110K, which indicates that the main chain C–C vibration of the homopolymer polystyrene is under the DSC thermograms measured for PS110K/PVME, pure PS110K, influence of the glass transition process. When the temperature and pure PVME samples in the second heating process are is lower than the glass transition temperature of polystyrene, the reproduced in Fig. 1. It can be seen that the PS110K and PVME main chain C–C vibration is weakened, subject to the constraint in toluene solvent form a miscible polymer blend with glass of the surrounding glassy environment, the electric dipole transition temperature between the glass transition temperatures moment of C–C vibration is decreased and the frequency peak of monomer. Due to the effect of self-concentration and the value of the vibration band changes. Reflecting on the infrared concentration fluctuation with the internal density inhomogeneity spectrum, there is a turning point near to the glass transition the PS110K/PVME blend has a broad glass transition temperature of PS110K in its ln(AL/AH) corresponding to the temperature. The FTIR of PS110K (800 – 1000 cm–1) and main chain C–C vibration band. This clearly shows that this PS110K/PVME (800 – 1000 cm–1) are shown in Figs. 2 and 3, vibration is affected by the system’s glass transition process. respectively. It can be seen in Figs. 2 and 3 that the positions Two typical plots of ln(AL/AH) value versus the temperature and shapes of the studied spectral bands change as a function of corresponding to C–H out of the benzene ring surface vibration the temperature. For simplicity, the other FTIR bands of the patterns of PS110K are represented in Figs. 6 and 7, respectively. polymer system are shown in Supporting Information (SI). The same as main chain C–C vibration mentioned above, C–H

AL/AH (A stands for the light absorption strength and L/H stands out of the benzene ring surface vibration also has an obvious for light absorption frequency peak value of lower/higher energy turning point at a temperature of about 105°C, which is near to state of a polymer sample) of the characteristic vibration modes the glass transition temperature of PS110K. This indicates that in different polymer systems analyzed in this work are listed in the C–H out of the benzene ring surface vibration mode of Table 1. homopolymer polystyrene is also affected by its glass transition 1074 ANALYTICAL SCIENCES SEPTEMBER 2017, VOL. 33

Fig. 6 Typical plot of ln(A904.9/A907.6) versus temperature represented Car–H out of benzene ring surface vibration patterns of homopolymer

Fig. 4 Typical plot of ln(A839.0/A842.8) versus temperature represented polystyrene. main chain C–C vibration of the homopolymer polystyrene.

Fig. 7 Typical plot of ln(A941.3/A943.6) versus temperature represented Fig. 5 Typical plot of ln(A2850.1/A2850.9) versus temperature Car–H out of benzene ring surface vibration patterns of homopolymer represented the main chain C–C vibration of the homopolymer polystyrene. polystyrene.

the literature50 and can help to deepen our understanding of the process. All of the significant vibration modes corresponding to polymer glass transition at the micro level. Figures 8 and 9 the different groups in homopolymer polystyrene PS110K were show two typical plots of the ln(AL/AH) value of C–H out of the analyzed by us (some shown in SI). It can be concluded that benzene ring surface vibration pattern of PS110K in a miscible basically all of the significant vibration modes in the PS110K polymer blend PS110K/PVME versus the temperature, that we analyzed were affected by its glass transition process, respectively. It can be seen from the above two figures that in but within different degrees. This may be due to the strong π–π the miscible blend PS110K/PVME, due to the interaction interaction between the benzene ring in the polystyrene, which between the polystyrene and polyvinyl methyl ether, the makes the molecular vibrations to be easily restricted by the ln(AL/AH) value of C–H out of benzene ring surface vibration has interaction. an obvious turning point near 50°C (about the PS110K/PVME When the temperature is higher than the glass transition blend glass transition temperature), instead of 105°C, at which temperature of polystyrene, the percolated network structure the turning point occurs within the ln(AL/AH) value of inside the polystyrene begins to collapse, and the constraint on homopolymer polystyrene. While in the blend, there is no each group becomes weak. Thus the characteristic vibration apparent slope change at 105°C within the curves of the corresponding to each group will be under the influence of the ln(AL/AH) value of these two vibration modes versus temperature. glass transition process. We also conducted the same test and These results suggest that the effect of the interaction on the analysis on other homopolymer polystyrenes with different polystyrene C–H out of the benzene ring vibration by the molecular weights by means of variable-temperature FTIR polyvinyl methyl ether is very significant. This is consistent micro spectroscopy (shown in SI). The same results were with the conclusion that the hydrogen bonding interaction obtained as the PS110K, which illustrates the universality of the between the hydrogen atoms of polystyrene’s benzene ring and above conclusion. This conclusion is consistent with results in oxygen atoms in the polyvinyl methyl ether is the main cause of ANALYTICAL SCIENCES SEPTEMBER 2017, VOL. 33 1075

Fig. 10 Typical plot of ln(A2849.7/A2851.0) versus temperature represented main chain C–C vibration of the polystyrene in miscible Fig. 8 Typical plot of ln(A905.6/A907.9) versus temperature represented Car–H out of benzene ring surface vibration pattern of polystyrene in blend PS110K/PVME. miscible blend PS110K/PVME.

blend, the percolated system network structure is collapsed, which leads to an abrupt slope change in the curve of almost all

characteristic vibrational modes’ ln(AL/AH) value versus temperature. PS110K rigid clusters remain at this temperature and begin to melt gradually with the temperature increasing. At the polystyrene’s glass transition temperature, the constraint on the characteristic vibration corresponding to the main chain in PS110K is further decreased to be weak, and another obvious turning point occurs near to this temperature.

Conclusions

Through the analysis of the change of polymer samples’ infrared spectra with temperature and their DSC experimental results, we can find that the characteristic vibration modes are under the influence of their glass transition process to different extent. The characteristic vibration modes of the same chemical group

Fig. 9 Typical plot of ln(A943.96/A946.4) versus temperature represented behave differently due to the influence of the polymer system at Car–H out of benzene ring surface vibration pattern of polystyrene in which the chemical moiety is situated. This conclusion can help miscible blend PS110K/PVME. us to understand the polymer glass transition process at the micro level. the miscibility in the PS110K/PVME blend. Above the blend Acknowledgements PS110K/PVME glass transition temperature, the constraint on the characteristic vibration of C–H in polystyrene benzene ring This work was financially supported by the National Science is very weak, and so in the blend there is no apparent slope Fund for Distinguished Young Scholars (No. 20825416), and the change at 105°C (polystyrene glass transition temperature) National Natural Science Foundation of China (No. 21374051), within its curve of ln(AL/AH) value versus temperature. 973 program (No. 2012CB821503), PCSIRT (No. IRT1257) and While the characteristic vibration modes corresponding to the CERS-1-61. PS110K main chain in the blend PS110K/PVME have different features. It can be seen from Fig. 10, except for an obvious slope change near 50°C (PS110K/PVME blend glass transition Supporting Information temperature) within the curve of ln(AL/AH) value versus temperature, there is another turning point near 105°C (PS110K This material is available free of charge on the Web at http:// glass transition temperature). This shows that the characteristic www.jsac.or.jp/analsci/. vibration modes corresponding to the main chain of the polystyrene in PS110K/PVME blend are under the influence of the blend PS110K/PVME glass transition and polystyrene, References itself, glass transition (it is not shown in the DSC curve of the PS110K/PVME blend) simultaneously. When the temperature 1. M. Beiner, J. Korus, H. Lockwenz, K. Schröter, and E. is above the glass transition temperature of PS110K/PVME Donth, Macromolecules, 1996, 29, 5183. 1076 ANALYTICAL SCIENCES SEPTEMBER 2017, VOL. 33

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