New Physical Insights into Shear History Dependent Polymorphism in

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New physical insights into shear history dependent polymorphism in PVDF Amanuel Gebrekrstos, Maya Sharma, Giridhar Madras, and Suryasarathi Bose Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00282 • Publication Date (Web): 04 Apr 2016 Downloaded from http://pubs.acs.org on April 12, 2016

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Crystal Growth & Design

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New physical insights into shear history dependent polymorphism in PVDF Amanuel Gebrekrstos1, Maya Sharma2, Giridhar Madras1, Suryasarathi Bose3* 1

Department of Chemical Engineering, 2Centre for Nanoscience and Engineering,

3

Department of Materials Engineering, Indian Institute of Science, Bangalore-560012, India

*Corresponding author: [email protected] (SB)

ACS Paragon Plus Environment


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Abstract Poly(vinylidene fluoride) (PVDF) is a semi-crystalline polymer that exists in four crystalline phases (α, β, γ, δ). Among these, the β-phase has received tremendous techno-commercial importance due to higher dipole moment as compared to the other phases and thus many strategies have been explored in the recent past to obtain the β-polymorph of PVDF. In this study, the effect of shear history on crystalline morphology and behavior of PVDF has been investigated systematically by polarized optical microscopy (POM) coupled to a hot stage, Fourier transform infrared spectroscopy (FTIR), differential thermal analysis (DSC), Rheometer and dielectric relaxation spectroscopy (DRS). Thin films of PVDF (120-150 µm) were sheared at different temperatures ranging from 155 to 220 ℃ and were allowed to isothermally crystallize at 155 ℃. When the samples were isothermally crystallized at 155 ℃, a remarkable increase in βphase content was observed. More interestingly, this phenomenon was observed to be shear history dependent. For instance, the samples which were sheared at high temperature (220 ℃) reflected in higher β fraction as compared to samples which were sheared at lower temperature (155 ℃). It is envisaged that the distance between Tshear (temperature at which the samples were sheared) and Tcry (crystallization temperature) significantly influences the content of β phase in PVDF. This study clearly demonstrates the fact that both shear history and the depth from Tc influences the conformational changes in PVDF. Keywords: Shear temperature; β-phase; Crystallinity; Morphology; Polarizing optical microscope.


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1. Introduction Polymers can be processed by variety of processing techniques like injection molding, extrusion, fiber spinning etc. During such processing, the polymer is subjected to shear, which affects predominantly its crystalline structure, and ultimately the final properties1-3. It is widely accepted that polymer melts, under the influence of a shear field (rate and total strain), exhibit an increased rate of crystallization and a different morphology, when compared with the quiescent melts4. The understanding of the principal parameters such as shear strain, shear rate and temperature that have a direct impact on the characteristic features of crystalline phase is very important. For example, previous studies5 on poly(phenylene sulfide) (PPS) reveal that there is a critical shear rate of 30 s-1, above which shish–kebab-like fibrillar crystals were formed. Furthermore, it has been shown that shear enhances crystallization kinetics and changes the overall crystalline morphology from spherulites to shish–kebab-like fibrillar crystals. The enhanced rate of crystallization upon shear was confirmed by a decrease in induction time when the first nuclei start to appear. Another study on shear induced crystallization has investigated on linear and long-chain branched polylactides. The results revealed that the crystal growth decreased and nucleation density increased upon increasing the shear rate and shear strain6. In general, when a semi-crystalline polymer is sheared, molecular chains are oriented and extended. The pre-ordered chain serves as primary nuclei facilitating crystal nucleation and eventually leading to increasing rate of crystallization by reducing the entropy. This raises the free energy with respect to the crystal. The increased free energy difference at a given rate of undercooling would lower the nucleation barrier, resulting in more nuclei and faster crystallization7. Poly (vinylidene fluoride) (PVDF) is one of the most versatile semi-crystalline polymer due to ease of processing, excellent chemical resistance and mechanical properties, pyro and



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piezoelectric properties. All these properties rely on different complex chain conformations in PVDF. Based on the crystal structures, PVDF has four most common polymorphic phases, namely α (trans-gauche-trans-gauche, TGTG), β (trans, TTT) planar zigzag), δ (TGTG) and γ (T3GT3G) and each of them can be obtained by different processing techniques. For instance, α, which is the thermodynamically more stable phase and non-polar, can be obtained directly by cooling from melt. The van der Waals’ forces acting between the atoms along the carbon backbone and between the molecules of the polymer, make the trans-gauche-trans-gauche structure of the α phase more stable because of the greater amount of space between the atoms8. Among all these phases, β phase which is highly polar (in which all C-F bonds are normal to the backbone chain), has higher dipole moment compared to other phases and is more desirable from technological point of view. In the β-phase, two chains are in an all-trans planar zigzag conformation, resulting in a significant dipole moment lateral to the chain axis. This high dipole moment renders good pyro and piezoelectric properties in PVDF9. It is due to this reason that many researchers10-13 have developed different processes to induce β phase formation. For example, this phase can be obtained by mechanical stretching of α phase, by poling or applying high electric fields, crystallizing PVDF from polar solvents. The β phase can also be formed by shear induced crystallization of 60/40 (wt/wt) PVDF/PMMA blends upon addition of 1% amine functionalized (NH2-CNTs) that was absent in the neat 60/40 (wt/wt) blends14. Another study described that with the addition of ionic fluorinated surfactants during electrospray deposition, a complete conversion of α phase into β phase was observed. This is due to the interaction of hydrogen or fluorine atom of PVDF with the charge group of surfactants15. The β phase can also be found by adding organically modified montmorillonite (OMMT) to PVDF and applying shear simultaneously. The β phase was found to increase upon adding 1 wt% and 2 wt% of OMMT16.


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Others17 also showed that the presence of carbon nanotubes in PVDF increased the amount of β phase in PVDF under shear. The enhanced β phase upon adding carbon nanotubes is attributed to heterogeneous nucleation effect of the nanotubes. It was also reported that high level of β-PVDF was obtained by incorporation of PMMA-g-CNTs. This is due to the strong interaction between the >C=O group of PMMA and the >CF2 group of PVDF during microinjection18. Overall, different processes such as rolling, mechanical stretching, adding nanoparticles etc. have been extensively employed to obtain β phase. However, shear induced crystallization or the effect of shear history of PVDF samples from melt did not receive much attention. Hence, the objective of this work was to study systematically the effect of shear history on the formation of β - polymorph of PVDF. To realize this, the samples were sheared at different temperatures at a fixed shear rate and for fixed shear duration and subsequently allowed to crystallize at two different temperatures. The morphology and orientation of PVDF sample was investigated using a polarized optical microscope (POM) equipped with a hot stage. The different phases were examined using Fourier transform infrared spectroscopy (FTIR), Differential scanning Calorimetry (DSC), rheometer and dielectric relaxation spectroscopy (DRS). 2. Experimental 2.1 Materials and Sample preparation PVDF (Kynar-761, with Mw of 440,000 g/mol) was procured from Arkema Inc. Prior to mixing, PVDF powder was dried in vacuum oven at 80 °C for 12 h. Dried samples were extruded using a Minilab II HAAKE extruder CTW5 (7 cm3) at 220 °C with rotational speed of 60 rpm for 20 min. To prevent oxidative degradation, all mixing was done under N2. Finally, compression



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molding was used to make thin films with thickness of 120 µm pressed at 220 °C for 5 min at 10 bar. 2.2. Characterization Polarising optical microscope, POM (OLYMPUS BX53) equipped with a (Linkam CSS450 shear stage) heating stage was employed to trace the crystalline morphologies both at quiescent and shear conditions. The shear stage provided precise control of shear experiments including temperature (heating and cooling rates) and sample thickness. Figure 1 illustrates the thermal and shear protocol employed in this study. Each compressed sample was placed on the shear stage between two quartz slides (upper and lower slides). The samples were heated from room temperature to 220 oC at a rate of 30 K/min and holding for 3 min to erase previous history. After the samples reached 220 oC, where we expect that the thermal history is erased, steady shear of 10 s-1 for 10 s was imposed and subsequently cooled to 155 oC at a rate of 10 K/min. At 155 oC, the samples were isothermally crystallized for 30 min. For other shear temperatures (155 ℃, 170 ℃, 180 ℃, 190 ℃ and 200 ℃), for example Tshear at 200 oC, the samples were heated from room temperature to 220 oC at a rate of 30 K/min and held for 3 min to erase the previous history. The samples were cooled to 200 oC at a rate of 10 K/min and a steady shear of 10 s-1 for 10 s was imposed and cooled to 155 oC at a rate of 10 K/min. At 155 oC, the samples were isothermally crystallized for 30 min. For comparison, samples without shear were directly cooled at the same rate to the set temperature of 155 °C to evaluate the effect of shear history on the crystalline morphology and conformational changes in PVDF. For all the samples, images of growing PVDF spherulites were taken at different times during isothermal crystallization to record the complete evolution


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of crystalline morphology. Finally, after it was isothermally crystallized for 30 min, all the samples were immediately quenched in ice water. The quenched samples were further characterized using FTIR, Differential Scanning Calorimetry (DSC) and dielectric relaxation spectroscopy (DRS). Before the FTIR measurements, water was wiped off using tissue and samples were air dried. FTIR spectra on the films were recorded on a Perkin-Elmer frontier by accumulating 16 scans over a range of 600-4000 cm−1 to obtain information about the different crystalline forms. Differential Scanning Calorimetry (DSC; TA Q2000) was used to determine the thermal properties of samples. For the experiment, 5– 10 mg samples were cut and sealed in copper pans and heated to 220 ℃ at a heating rate of 10 K/min. Dielectric relaxation spectroscopy (DRS) was done using an Alpha-N Analyzer, Novocontrol (Germany) in the frequency range of 0.01 ≤ ω ≤107 Hz.

Figure 1: Experimental procedure for samples crystalline morphology captured at 155 oC x = 1, 5, 10, 20, 30, and 40 .



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Viscoelastic properties and induction time calculations of the samples were studied using stress controlled Discovery Hybrid Rheometer (DHR-3, TA Instruments) with parallel plate geometry (25 mm in diameter and 1 mm gap distance). We tried to replicate the POM experiments with rheometer. Sample was heated to 220 °C and held at this temperature for 3 min to erase previous history. Once the selected shear temperature was reached (155 ℃, 170 ℃, 180 ℃, 190 ℃, 200 ℃ and 220 ℃), a steady shear (flow peak hold, 10 s-1) was applied at a fixed shear time of 10 s. Sheared samples were cooled down (at a cooling rate of 10 K/min) to the selected isothermal crystallization temperature after cessation of shear. Isothermal dynamic time sweeps were carried out at 155 °C to determine the evaluation of morphology and behavior of storage modulus (Gʹ) with time. 3. Results and discussion Shear induced β in PVDF It is well reported that the rate of crystallization and crystalline morphology of polymer sample are affected by shear time and shear rate. For optimizing the shear duration, different shearing times were considered (10, 60, 120 and 300 s). Interestingly, all the samples showed the same trend i.e. upon increasing the shear duration, the rate of crystallization increased. However, the final morphology of all samples remained the same irrespective of the shear duration. From these experiments, we found that longer shear duration and higher shear rate enhanced the rate of crystallization. We observed that, although the crystallization rate increases with time, the final morphology remains the same under all shear durations (not shown here). Shearing films for longer duration also causes physical damage (as film thickness is very less). Hence, we limit our studies to only shear duration of 10 s.


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To optimize shear rate, samples were sheared at different shear rates (5, 10, 20, 30 and 40 s-1) and fixed shear duration 10 s. From FTIR (see table 1), we found that at lower shear rates, the amount of oriented β crystal increases slightly and reached maximum for a shear rate of 10 s-1. It is interesting to note that when the shear rate exceeds 10 s-1, the content of β phase decreased. This is due to the fact that as the shear increases, growth of α nuclei from samples is so rapid that the growth of β crystals are restrained. A shear rate of 10 s-1 was chosen for further investigation for isothermal crystallization at 155 oC. Table 1: Content of β-phase with different shearing rate at constant crystallization temperature of 155 ℃ and fixed shear time 10 s. Samples sheared at Y s-1

β-phase content, F(β) (%)

crystallized at 155 ℃, fixed shear duration 10 s Without shear 155 ℃

38

Y= 5

43

Y= 10

64

Y= 20

52

Y= 30

50

Y= 40

46

After optimizing the shear rate and shear duration (10 s-1 for 10 s), samples were sheared at different temperatures. Figure 2a illustrates the FTIR spectra of PVDF samples sheared at different temperatures. The FTIR spectra of PVDF for α and β phases appears at their respective



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vibration bands of 763 cm-1 (CF2 bending and skeletal bending), 795 cm-1 (CH2 rocking), 975 cm-1 (CH2 twisting) predominantly the α- phase and 510 cm-1 (CF2 bending), 840 cm-1 (CH2 rocking) represent β phase. The α phase of PVDF had a unique IR absorption band at 763 cm−1 IR that was baseline separated from all other peaks. The presence of exclusively β phase can be observed through the presence of the 510 and 840 cm−1 absorption bands. Thus, absorption bands at 763 cm−1 and 840 cm-1 are used to evaluate the changes in the fraction of β-phase in any PVDF film sample19.

Figure 2: (a) FTIR spectra of neat PVDF isothermally crystallized at constant temperature of 155 ℃ (b) Variations of β-phase content with shearing temperature crystallize at 155 ℃ with applied shear rate of 10 s-1 for 10 s. All samples with and without shear exhibited both α and β phases. As it is clearly seen from figure 2a, for the samples sheared at higher temperature and allowed to crystallize at 155 ℃, α phase is observed to be diminished and prominent peak appears at 840 cm-1 corresponds to the βphase. A plausible reason for the presence of higher β-phase in the sheared sample at high temperature could be attributed to the fact that at higher temperature, the viscosity of the melt is


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low leading to enhanced chain mobility. Due to this, it would be easier for PVDF chains to align in the flow direction and take all trans-conformation. In order to determine the fraction of β-phase present in each sample, IR absorption bands at 763 and 840 cm-1 was chosen (characteristic of the α and β phases) respectively20. Assuming that IR absorption follows the Lambert-Beer law, the Aα and Aβ absorbance, at 763 and 840 cm-1, respectively, can be estimated as,

Aα = =Kα C Xα L

(1)

Aβ = =Kβ C Xβ L

(2)

for a sample of thickness L and an average total monomer concentration C. The α and β subscripts refer to the two crystalline phases, I° and I are the incident and transmitted intensity radiations, respectively, K is the absorption coefficient at the respective wavenumber and X the degree of crystallinity of each phase. The Aα and Aβ values were determined by I° and I at 763 and 840 cm-1, respectively21. Kα and Kβ are the absorption coefficient of the respective bands22 (Kα =6.1 × 104 and Kβ =7.7 × 104 cm2/mol), Xα and Xβ are the % crystallinity of the respective phases. Using the above equations, the relative β fraction, F (β) in the sample can be calculated as follows: =

=

/

= .

(3)

As it is quantified by FTIR, Figure 2b showed that the content of β-phase fraction increases markedly upon increasing the shear temperature. Interestingly, from figure 2b, the maximum value of β-phase content that is 84.4% occurs at 220 ℃. The high β phase at higher shear



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temperature was attributed to the shear induced orientation of polymer chains. Such an enhancement of crystallization may be achieved by adding effective nucleating agents. However, according to our study, the same enhancement could also be realized by appropriate pre-shearing which is strongly contingent on the shear history. An explanation for increased fraction of βphase at high temperature is as we sheared at 220 ℃ and crystallized at 155 ℃, the depth between Tshear and the isothermal crystallization temperature (i.e. 155 oC) was higher, the polymer chains were relaxed and had sufficient time/thermal energy to form the meta-stable β phase in PVDF due to the shear force. This helps in the formation of trans (TTT) conformation PVDF or TGTG conformation transformed into a zigzag TT i.e. β phase. This essentially suggests that temperature could lead to preferable orientated packing of CH2-CF2 dipoles (TTT) conformation on the surface. To detect the variation of the degree of crystallinity and melting enthalpy, samples without shear at 155 ℃, sheared at 180 ℃ and 200 ℃ were considered for DSC scan. The DSC thermograms are presented in figure 3. The melting enthalpy and degree of crystallinity are illustrated in table 2. The result revealed that neat PVDF without shear exhibits multiple isothermal peaks. The double melting endotherms appeared at 161 ℃ and 167 ℃, that correspond to the melting temperature of α and β phases, respectively. The double endotherm is due to the melting of two different crystalline phases coexisting or from the melting of two kinds of PVDF crystals with different extents of perfection23. As it is clearly seen in figure 3, the melting point of β-phase is higher than that of α -phase. This is due to its all trans planar zigzag conformation, higher cell density that provides more packing and higher melting temperature crystals. Interestingly, all sheared samples exhibit single melting endotherms. Consequently, we can conclude that the β -phase was predominant in all the samples. In all the samples, the melting


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point of β was the same. The degree of crystallinity (∆Xc) of each sample was calculated according to the following equation,

∆ =

∆

(4)

∆

In eq. (4), ∆Hm is the melting enthalpy of the sample and ∆H100 is the melting enthalpy for a 100% crystalline sample (∆H100 = 104.50 J/g for PVDF)24. Table 2: Melting Temperature (Tm) and enthalpy measured by Differential Scanning Calorimetry for samples sheared at different temperatures Samples sheared at Y ℃, crystallized at 155 ℃ and quenched Without shear at 155 ℃ Y= 180 Y= 200

Tm, °C

Hm, J/g

Crystallinity, %

167

37

36

167 167

44 38

41 38

It is observed that the % crystallinity is similar in all samples. But at 180 ℃, crystallinity and the melting enthalpy are high. To ascertain the result, samples were repeated three times and the same results were obtained.



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o

Heat Flow (mW)

PVDF at 200 C

o

PVDF at 180 C

PVDF without shear endo

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

100

120

140

160

180

200

Temperature (°C)

Figure 3: Effect of shear temperature on the melting behaviors of PVDF samples at a shear rate of 10 s-1 for 10 s. Crystalline morphology by POM In this section, we discuss the morphology of PVDF after cessation of shear and influence of shear temperature on the samples isothermally crystallized at 155 ℃. It is generally envisaged that shear affects the crystallization kinetics because of high orientation of polymer chains upon shear flows by decreasing the entropy of the melt. To study the effect of shear history on the crystalline morphology of PVDF samples, POM equipped with a (Linkam CSS450 shear stage) hot stage was used. In this study, different samples were sheared at various temperatures and crystallized at 155 ℃. Prior to the shear experiments, crystallization temperature of PVDF using POM was investigated. It was found that when samples were cooled from the melt at a rate of 10 K/min, the onset of crystallization was 145 oC. To ensure this temperature, we repeated the


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experiments thrice. Hence, for further studies, 145 ℃ was taken as the crystallization temperature of PVDF as determined from POM. It is well reported that the degree of orientation and extension of the chain segments depends on the shear rate and the duration of shear25. To ascertain the effect of shear on morphology and crystallization kinetics, samples were sheared at different shear rates (1, 5, 10, 20, 30 and 40 s-1) and different duration. To realize the influence of isothermal crystallization temperature on the morphology and crystallization kinetics, 155 °C was chosen to study the effect of shear history. Below 155 °C, crystallization kinetics was very fast and it was not possible to capture the initial morphologies. On the other hand, isothermal crystallization above 155 °C, will take longer time. As we have mentioned above, the degree of orientation in polymer melt is affected by the different shear parameters like shear rate and shear strain. We have optimized the shear rate and shear duration for the samples crystallized isothermally at 155 °C. Figure 4 illustrates the effect of different shear rates on the morphology of PVDF samples captured at isothermal crystallization (at 155 ℃). From Figure 4, it is clearly observed that the morphology of all samples represents Maltese cross pattern after cessation of shear irrespective of the shear rate. One can observe that for samples sheared at 10 s-1 (see figure 4a1-a2), spherulites were lesser in number but with time, the spherulites start impinging each other. On the contrary, when the samples were sheared at 20 and 30 s-1, a relatively higher nucleation density (smaller sizes of spherulites) was observed (figure 4b1-b2 and c1-c2). From this, we can conclude that the higher is the shear rate, the higher is nucleation density.



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Figure 4: The POM of images PVDF taken at 15 min and 30 min (a1-a2) sheared at 10 s-1, (b1-b2) Sheared at 20 s-1 and (c1-c2) sheared at 30 s-1. All images captured at 155 ℃ and shear time 10 s. (Scale bar is 100 µm in all samples.) The red arrow indicates the flow direction. After optimizing the shear rate and shear duration (10 s-1 for 10 s), we attempted to investigate the effect of shear temperature on the evolving crystalline morphologies. We sheared samples at different temperatures (Tshear) of 155 ℃, 170 ℃, 180 ℃, 190 ℃, 200 ℃ and 220 ℃ and allowed them to crystallize isothermally at 155 ℃. We sheared samples up to 220 oC and above this temperature, we found difficulty in applying shear as the films suffered physical damage. A series of micrographs was taken during the isothermal crystallization process. Figure 5 illustrates the evolution of spherulitic morphology with time for different shear temperatures


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(Tshear). At the first 7 min, without shear no generation of nuclei could be observed and the same was applicable for all samples sheared at different Tshear. After 15 min (see figure. 5a2), under quiescent condition samples, small number of nuclei was generated. In contrast (see figure 5b2, c2 and d2), many nuclei were generated by imposing shear. Moreover, the effect is more pronounced with increase in Tshear (crystallization rate appeared to be enhanced at higher temperature). After 25 min, no new spherulites were formed but the existing ones grew in size. In sheared samples, after 25 min, spherulites had already fully grown in the direction of shear and reached a saturation point (see Figure 5d3). After 25 min, the size of spherulites of sheared samples was smaller than that obtained without shear. This is because, in sheared samples, more nuclei are evolved and leads to higher nuclei density. This is elucidated from Figure 5a3, b3, c3 and d3 that shows that samples sheared at high temperature are much smaller in size than that sheared at low temperature. In general, samples sheared at higher temperature showed enhanced nuclei density and showed smaller spherulites as compared to that observed without shear. This phenomenon is consistent with the available literature3, 26.



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Figure 5: The POM of images PVDF (a1-a3) without shear, (b1-b3) sheared at 155 ℃, (c1-c3) sheared at 180 ℃, (d1-d3) sheared at 220 ℃. Images captured at fixed shear rate of 10 s-1 and isothermally crystallized at 155 ℃. (Scale bar is 100 µm.) The red arrow indicates the flow direction. Crystallization kinetics Induction time and growth rate from melt rheology Viscoelastic properties and induction time calculations of the samples were studied using stress controlled Discovery Hybrid Rheometer (DHR-3, TA Instruments). Figure 6 shows the changes of storage modulus (G′) with time at a crystallization temperature of 155 ℃ under quiescent and shear conditions. From figure 6, it is clearly seen that the changes of G′ with crystallization time show a sigmoidal shape, exhibiting a progression of storage modulus from about constant values before the starting of crystallization to a rapid increase and then approaching the plateau values at the ending of the primary crystallization stage. The onset time for the rapid increase of G′ is


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19

often defined as the induction time, t0, for nucleation (illustrated by the crossover point of two dash-dot lines in Figure 6), reflecting the energy barrier for crystallization kinetics. For the quiescent condition at the temperature of 155 °C, the induction time needed for the onset of crystallization is about 854 s. When a shear is applied at 200 ℃, the storage modulus curve shifts to the shorter time region, and the induction time is significantly reduced to 785 s. The decrease in the crystallization induction time indicates shear flow can play an important role in the crystallization kinetics of PVDF. As shear can cause molecular chains to orient, less time is needed to form stable nuclei for oriented chains. Thus the crystallization induction time decreases.

o

Without shear captured at 155 C o o Sheared at 200 C ,captured at 155 C

7

1.8x10

G' (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

6

9.0x10

0.0

t0

0

induction time

1000

2000

3000

Time (s)

Figure 6: Change of storage modulus (G′) with crystallization time of neat PVDF sheared at 10 s-1 for 10 s. Figure 7 shows time dependent radial growth of PVDF crystallized at 155 oC under quiescent and sheared conditions (180 oC and 200 oC). As it is clearly seen from figure 7, samples under quiescent and sheared conditions, a linear increase in radial growth with time was observed. The growth rates, calculated from the slope of the linearly fitted curve, are 23 nm/s, 25 nm/s and 31



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20

nm/s for samples without shear, sheared at 180 oC and 200 oC, respectively. It seems that the growth rate increases with shear temperature.

o

50

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Figure 7: Radial growth with time for samples sheared at 10 s-1 for 10 s and crystallized at 155 oC. Segmental dynamics from dielectric relaxation spectroscopy Broadband dielectric relaxation spectroscopy was carried out to probe the dielectric relaxations in PVDF. From POM and FTIR, we established that shearing PVDF at 200 oC results in maximum β phase. We are interested in investigating the dielectric response from α and β phases


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of PVDF. Hence, we chose two samples: one quiescent PVDF (predominantly rich in α phase) and the other sheared PVDF (predominantly rich in β phase) for dielectric relaxation spectroscopy studies. Figure 8(a-b) shows the dielectric permittivity (ε″) as a function of frequency at different temperatures for neat PVDF as well as for sheared PVDF samples. It is known that PVDF exhibits three different types of relaxations: relaxations associated with glass transition temperature (αa); relaxations associated with crystallization of PVDF (αc), and secondary β relaxations, often observed at low temperatures (only below glass transition temperature). The samples were heated from room temperature and the dielectric spectra was captured at different temperatures in the frequency range of 0.1-107 Hz. Dielectric permittivity for neat PVDF as a function of frequency has been illustrated in Figure 8a. For all samples, the dielectric relaxation peak intensity increases with temperature that corresponds to the increase in chain mobility with temperature. PVDF showed a single relaxation centered around 1 Hz corresponding to αc relaxations, originating from crystalline segments of PVDF. From our earlier work27, it has been established that αc becomes sharper and shifts to higher frequency with increasing temperature. Interestingly, sheared samples also showed single relaxation in the DRS spectra for all temperatures as illustrated in Figure 8b. Interestingly, the relaxations shifted to higher frequency. This is due to imperfection in the crystalline lamellae. Interestingly, the relaxations shifted to higher frequency. The plausible reason of this high frequency shift is due to formation of imperfect crystals in the presence of shear21. Sheared samples exhibited higher fraction of β fraction (~ 82%), which are defective crystals and can orient faster. These small and defective crystals orient faster than those of α- phase and shift the relaxations to higher frequency. Similar results were obtained in our earlier studies, where small amount of PMMA



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shifted the relaxations of PVDF to higher frequencies. The relaxation in neat PVDF is diffuse and broad that indicates the existence of both α and β- phase in PVDF (and corroborates well with FTIR results). On the contrary, sheared samples showed a single intense peak that indicates the existence of predominantly a single phase.

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Figure 8: Dielectric loss spectra as a function of frequency with different temperatures (a) PVDF neat, (b) PVDF sheared at 200 oC at a rate of 10 s-1 for 10 s and captured at 155 oC. Dielectric relaxation spectra were fitted using the Vogel-Fulcher (VF) equation28: τ = τo exp

!

(5)

where T∞ is Vogel temperature, το represents relaxation time at infinite temperature; B is material constant which corresponds to apparent activation energy. Figure 9a-b illustrates that both samples (PVDF without shear and PVDF sheared at 200 oC) followed VF equation, the solid red line represents the VF fit. Although both samples follow the VF Fit, the apparent activation energy (B) varied in both cases. PVDF sheared at 200 oC quenched at 155 oC showed lower activation energy (9 kJ/mol) as compared to quiescent PVDF samples (20 kJ/mol). In PVDF, the β relaxation is associated with the Brownian motion of the amorphous structures. Under shear (at 200 ℃), the chains of the amorphous PVDF are arranged into a less disordered conformation and the Brownian motion of the amorphous PVDF could be reduced, as reflected in the lower apparent activation energy. When sheared at 200 ℃, PVDF chain would not entangle much and this pre-ordering of chains possibly can provide more sites for nucleation and further reflects in higher β fraction as compared to quiescent samples. Near the crystallization of PVDF (at 155 ℃), the apparent activation energy is higher than the thermal activation energy often obtained from Arrhenius fits around Tg. Such observations seem to be quite general for the kinetics in the solid (or viscous) state encompassing the complexity of the movements of structural units. These findings reveal new physical insights in understanding the altered chain conformation in PVDF under different shear conditions and more specifically on the shear history.



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Figure 9: VFT fit of segmental relaxation for different PVDF samples (a) PVDF samples without shear captured @ 155 oC (b) PVDF sheared @ 200 oC captured @ 155 oC.


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Conclusions We have systematically investigated the effect of shear history (both shear rate and depth from the crystallization temperature) on the polymorphism of PVDF. This study clearly demonstrates new physical insights as to how shearing at different temperature influences the rate and the evolution of crystalline morphology, orientation and spherulitic growth rate in PVDF using POM, FTIR, DSC and DRS. The results suggest that, under quiescent conditions, the rate of crystallization was lower. Once shear was applied, enhanced rate of crystallization was observed in PVDF. However, the final morphology remained the same irrespective of the depth from the crystallization temperature where the samples were sheared. Among the samples sheared at different temperatures, a significant increase in β-phase was observed. Quantitatively, without shear, the fraction of β-phase was found to be around ca. 38%. Once shear was imposed at higher temperature, the fraction of β-phase increased markedly to ca. 84%. This increment is attributed to the fact that at higher temperature (200 ℃), PVDF chain would not entangle much and this pre-ordering of chains possibly reflects in higher β fraction as compared to quiescent samples. This is also reflected from the lower apparent activation energy in the sheared samples. However, near the crystallization temperature of PVDF (i.e. at 155 ℃), the apparent activation energy is higher and impedes the formation of β polymorph in PVDF. Taken together, these findings should help guide theories and simulations on understanding the altered chain conformation in semicrystalline polymer such as PVDF under different shear conditions and more specifically on the shear history. More specifically, the distance between Tshear (temperature at which the samples were sheared) and Tcry (crystallization temperature) significantly influences the content of β phase in PVDF. Acknowledgements



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The authors thank the Department of Science and Technology (DST/1362) India for the financial support. The authors would also like to acknowledge Prof. Rajeev Ranjan (Materials Engineering, IISc) for extending the dielectric spectroscopy facility. Giridhar Madras thanks DST for the J.C. Bose fellowship. References (1) Zhang, C.; Wang, B.; Yang, J.; Ding, D.; Yan, X.; Zheng, G.; Dai, K.; Liu, C.; Guo, Z., Synergies among the self-assembled β-nucleating agent and the sheared isotactic polypropylene matrix. Polymer 2015, 60, 40-49. (2) Pantani, R.; Coccorullo, I.; Volpe, V.; Titomanlio, G., Shear-Induced Nucleation and Growth in Isotactic Polypropylene. Macromolecules 2010, 43, (21), 9030-9038. (3) Huo, H.; Jiang, S.; An, L.; Feng, J., Influence of shear on crystallization behavior of the β phase in isotactic polypropylene with β-nucleating agent. Macromolecules 2004, 37, (7), 24782483. (4) Somani, R. H.; Hsiao, B. S.; Nogales, A.; Srinivas, S.; Tsou, A. H.; Sics, I.; Balta-Calleja, F. J.; Ezquerra, T. A., Structure development during shear flow-induced crystallization of i-PP: in-situ small-angle X-ray scattering study. Macromolecules 2000, 33, (25), 9385-9394. (5) Zhang, R.-C.; Xu, Y.; Lu, A.; Cheng, K.; Huang, Y.; Li, Z.-M., Shear-induced crystallization of poly(phenylene sulfide). Polymer 2008, 49, (10), 2604-2613. (6) Najafi, N.; Heuzey, M.-C.; Carreau, P.; Therriault, D., Quiescent and shear-induced crystallization of linear and branched polylactides. Rheol. Acta 2015, 1-15. (7) Anthony, J. R.; Nicholas, J. T.; Fairclough, J. P. A., A Scattering Study of Nucleation Phenomena in Homopolymer Melts. In Scattering from Polymers, American Chemical Society: 1999; Vol. 739, pp 201-217. (8) Hapuarachchi, T. D.; Peijs, T.; Bilotti, E., Thermal degradation and flammability behavior of polypropylene/clay/carbon nanotube composite systems. Polym. Adv. Technol. 2013, 24, (3), 331-338. (9) Sencadas, V.; Gregorio Jr, R.; Lanceros-Méndez, S., α to β phase transformation and microestructural changes of PVDF films induced by uniaxial stretch. J. Macromol. Sci. 2009, 48, (3), 514-525. (10) Salimi, A.; Yousefi, A. A., Analysis Method: FTIR studies of β-phase crystal formation in stretched PVDF films. Polym. Test. 2003, 22, (6), 699-704. (11) Mohammadi, B.; Yousefi, A. A.; Bellah, S. M., Effect of tensile strain rate and elongation on crystalline structure and piezoelectric properties of PVDF thin films. Polym. Test. 2007, 26, (1), 42-50. (12) Yu, S.; Zheng, W.; Yu, W.; Zhang, Y.; Jiang, Q.; Zhao, Z., Formation Mechanism of βPhase in PVDF/CNT Composite Prepared by the Sonication Method. Macromolecules 2009, 42, (22), 8870-8874. (13) Gomes, J.; Nunes, J. S.; Sencadas, V.; Lanceros-Mendez, S., Influence of the β-phase content and degree of crystallinity on the piezo- and ferroelectric properties of poly(vinylidene fluoride). Smart Mater. Struct. 2010, 19, (6), 065010.


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Table of Content

New physical insights into shear history dependent polymorphism in PVDF Amanuel Gebrekrstos, Maya Sharma, Giridhar Madras, Suryasarathi Bose

β-phase was induced in PVDF when sheared near the bulk crystallization temperature (Tc) and induced significantly when sheared far from the Tc. PVDF chain would not entangle much and this pre-ordering of chains possibly reflects in higher β fraction as compared to quiescent samples. This is also reflected from the lower apparent activation energy in the sheared samples in contrast to higher activation energy near the Tc.


New Physical Insights into Shear History Dependent Polymorphism in

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