Critical Insights into the Effect of Shear, Shear ... - ACS Publications

Subscriber access provided by UB + Fachbibliothek Chemie | (FU-Bibliothekssystem)

Article

Critical insights into the effect of shear, shear history and the concentration of a diluent on the polymorphism in PVDF Amanuel Gebrekrstos, Maya Sharma, Giridhar Madras, and Suryasarathi Bose Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b01896 • Publication Date (Web): 28 Feb 2017 Downloaded from http://pubs.acs.org on March 4, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Crystal Growth & Design is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 27

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

Crystal Growth & Design

Critical insights into the effect of shear, shear history and the concentration of a diluent on the polymorphism in PVDF Amanuel Gebrekrstos1, Maya Sharma2, Giridhar Madras1, Suryasarathi Bose3* 1

Department of Chemical Engineering,

2

Centre for Nanoscience and Engineering,

3

Department of Materials Engineering,

Indian Institute of Science, Bangalore-560012. India

*

Corresponding author. Email: [email protected]

1 ACS Paragon Plus Environment


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

Page 2 of 27

Abstract The effect of shear, shear history and addition of PMMA in inducing β-polymorph in PVDF was investigated systematically for melt-mixed PVDF/PMMA blends. Various techniques like polarized optical microscopy (POM) coupled to a hot stage and a shear cell; Fourier transform infrared spectroscopy (FTIR), differential thermal analysis (DSC), Melt-rheology and Dielectric relaxation spectroscopy (DRS) were used to gain a mechanistic insight for the observed polymorphism in PVDF. Different PVDF rich blends were prepared by melt mixing and were subjected to different shear history on a rheometer and the induction time was monitored with respect to PMMA content and the applied shear flow in the blends. The rheological measurements revealed that the induction time was significantly lower for blends with higher PVDF content (≥80 wt%) which was ascribed to the diluent effect of PMMA that restricts the chain mobility of PVDF and longer time is required to start the crystallization process. The crystalline morphology observed from POM demonstrates that the growth rate of spherulites was greatly reduced with increasing PMMA content in the blends. FTIR results were used to determine the amount of β phase in the blends before and after the shear history. The blends that were sheared at high temperature (220 oC) showed more β phase than the blends that were sheared around the crystallization temperature. This study clearly demonstrates the fact that shear, shear history and the content of PMMA significantly influence the conformational change that results in phase transformation in PVDF. Further, this study will help guide the researchers working on various aspects of polymer processing where the effect of blending and shear on polymorphism is very important. Keywords: shear history; β-phase; crystallinity; growth rate, PVDF/PMMA blends


Page 3 of 27

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


Introduction Due to piezoelectric and pyroelectric properties of polyvinylidene fluoride (PVDF), it is widely used for device applications such as sensors and actuators and has been attracting much attention in the recent development in polymer industries. These various applications are governed by the different crystalline structure of PVDF. PVDF is known to exist in four most common polymorphic phases, namely, α (trans-gauche-trans-gauche, TGTG), β (trans, TTT) planar zigzag), δ (TGTG) and γ (T3GT3G). When PVDF chains crystallize in parallel dipole orientation, it possesses a net dipole moment resulting in polar β and γ phases. However, in antiparallel dipoles, the net dipole moment is zero resulting in non-polar α phase1-4. The α polymorph usually forms upon crystallization from melt whereas the β polymorph is not easy to obtain5. As the pyro and piezoelectricity of PVDF depends on β polymorph, several approaches have been developed to obtain β phase in PVDF. For instance, on mechanical rolling of PVDF films, transformation of α phase to β phase was observed. This is because rolling induces orientation of the crystals and facilitates β phase formation in PVDF6, 7. Another study showed that β phase forms in PVDF by blending with miscible poly(1, 4-butylene succinate) (PBS) upon melt quenching at low temperature. The maximum amount of β phase obtained was at a critical composition of PBS (between 40 and 50 wt %). This was due to the morphology change at the critical PBS content8. The β phase can also be induced by adding PMMA to PVDF. The result showed that lower amounts of PMMA (< 30 wt%) favors the crystallization of the β phase. This is because addition of PMMA increases the quenching effect due to reduction of crystallization rate and the increase of glass transition temperature 9-12. It was also reported that β-PVDF films can be obtained from the melt by adding CoFe2O4 nanoparticles. This is due to the strong interaction between CH2 group of PVDF with negative charge of the nanoparticles which results in the polymer chains to align on the surface of the nanoparticles in an extended TTTT conformation i.e. β phase13. Recently, incorporating graphene oxide into PVDF has been proven to induce β phase formation. This is due to the residual oxygen containing GO crystal that can promote transformation of PVDF matrix from α to β phase14. It is well reported that the β PVDF can be obtained by shearing at different temperatures and isothermally crystallized at 155 oC. The higher β-phase in the sheared sample at 3 ACS Paragon Plus Environment


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

Page 4 of 27

high temperature could be attributed to the fact that at higher temperature, the viscosity of the melt is 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-conformation15. The physical properties of semi-crystalline polymers depend on the processing conditions16. During such processing, the polymer is subjected to shear, which affects predominantly the crystallization behavior and the resulting crystal morphology17. The enhancement of crystallization kinetics is due to nucleation density, induction time and crystal growth rate18,

19

. Increasing the shear rate and/or the shear strain enhances the crystallization

kinetics of poly(butylene terephthalate)20. Most of the studies on shear-induced β phase formation are focused on polyolefins, especially isotactic polypropylene (iPP). The formation of β iPP under melt shearing is caused by the melt shear that yields α row nuclei and the surface of these α row nuclei induce α- β transition so that along the fiber, a layer of enriched β may form21. Blends of poly(vinylidenefluoride) (PVDF) and poly(methyl methacrylate) (PMMA) are completely miscible due to specific chain interactions. This specific interaction can lead to the formation of β phase in the blend upon quenching from melt

22-25

. Another study on PVDF/PMMA blends

showed that, addition of PMMA to PVDF promotes formation of the polar β-phase over the nonpolar α-phase due to dipole-dipole interactions which also suppressed the rate of crystallization of PVDF 26. All of the existing studies focus on β-phase formation in PVDF/PMMA based on solution casting, poling, rolling and electro-spinning etc. However, limited information is available on the effect of shear, shear history and the concentration of a diluent on the polymorphism in PVDF. In the present work, different blends of PVDF/PMMA (90/10, 80/20, 70/30 and 60/40) were prepared by melt-mixing, which is an industrially viable route to synthesize blends. From melt-rheology and polarized optical microscope (POM) equipped with a hot stage and a shear cell, the crystallization kinetics was investigated and compared under different shear history (both temperature and the quench depth from the crystallization temperature). The polymorphism in PVDF was examined using Fourier transform infrared spectroscopy (FTIR) and differential 4 ACS Paragon Plus Environment

Page 5 of 27

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


scanning calorimetry (DSC) and the segmental relaxations was studied using broadband dielectric relaxation spectroscopy (DRS). Experimental Materials and sample preparation Both atactic PMMA (Atuglass V825T, with Mw of 95,000 g/mol and polydispersity of 2.1) and PVDF (Kynar-761, with Mw of 44,000 g/mol) was provided by Arkema. Blends of 90/10, 80/20, 70/30 and 60/40 PVDF/PMMA were prepared by melt mixing using a Minilab II HAAKE extruder CTW5 (7 cm3) at 220 °C with a rotational speed of 60 rpm for 20 min. Prior to mixing, PVDF/PMMA powder was dried in a vacuum oven at 80 °C for 12 h. Melt mixed samples were subsequently compression molded into thin films pressed for 5 min at 10 bar. These films were 120 µm thick for shear experiments and 1 mm thick for rheological measurements. Characterization The crystalline morphology was observed by polarising optical microscope, POM (OLYMPUS BX53) equipped with a (Linkam CSS450 shear stage) heating stage both at quiescent and shear conditions. The procedure for shear experiments was as follows. The shear stage provided precise control of shear experiments including temperature (heating and cooling rates) and sample thickness. Each compressed sample was placed on the shear stage between two quartz slides (upper and lower slides). For PVDF/PMMA (90/10) samples, they were heated from room temperature to 220 oC, at a rate of 30 K/min and kept for 3 min to erase previous history. A steady shear of 10 s-1 was imposed for 10 s and cooled to 156 oC at a rate of 10 K/min. At 156 oC, the samples were isothermally crystallized for 45 min. For other shear temperatures (160 ℃, 170 ℃, 180 ℃, 190 ℃ and 200 ℃), the samples were heated from room temperature to 220 oC at a rate of 30 K/min and held for 3 min to erase previous history. The samples were cooled to 200 oC at a rate of 10 K/min and steady shear of 10 s-1 for 10 s was imposed and cooled to 156 oC at a rate of 10 K/min. At 156 oC, the samples were isothermally crystallized for 45 min. The same procedure was used for other compositions of PVDF/PMMA (80/20, 70/30 and 60/40). The isothermal crystallization temperatures were selected as 152 oC , 146 oC and 142 oC respectively. 5 ACS Paragon Plus Environment


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

Page 6 of 27

For comparison, for all compositions PVDF/PMMA (90/10, 80/20, 70/30 and 60/40), samples without shear were directly cooled at the same rate to the selected isothermal crystallization to evaluate the effect of shear history on the crystalline morphology and conformational changes in PVDF. For all the samples, images of growing PVDF/PMMA spherulites were taken at different times during isothermal crystallization to record the complete evolution of crystalline morphology. Finally, after it was isothermally crystallized for 45 min, all the samples were immediately quenched in ice water. The quenched samples were further characterized using FTIR and differential scanning calorimetry (DSC). 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. Stress controlled rheological experiments (dynamic temperature ramps) were performed using Discovery Hybrid Rheometer (DHR-3, TA Instruments) with parallel plate geometry (25 mm in diameter and 1 mm gap distance) to evaluate the crystallization temperature of PVDF/PMMA blends. Various concentrations of PMMA were taken and were heated to 220 ℃; held for 3 min to erase the previous history, followed by cooling at 5 K/min. We tried to replicate the POM experiments with rheometer. The 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 (160 ℃, 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 (at a cooling rate of 5 K/min) to the selected isothermal crystallization temperature after cessation of shear. For all compositions PVDF/PMMA (90/10, 80/20, 70/30 and 60/40), isothermal dynamic time sweeps were carried out at 156 oC, 152 oC, 146 oC and 142 oC respectively to determine the evaluation of morphology and behavior of storage modulus (Gʹ) with time. Dielectric relaxation spectroscopy (DRS) was done using an Alpha-N Analyzer, Novocontrol (Germany) in the frequency range of 0.01 ≤ ω ≤107 Hz. The samples were heated


Page 7 of 27

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


from room temperature and the dielectric spectra were captured at different temperatures in the frequency range of 0.1-107 Hz. Results and discussion Crystallization temperatures of PVDF/PMMA blends by melt-rheology The viscoelastic properties of semi-crystalline polymers are highly sensitive to the evolution of microstructures during crystallization. Rheological experiments were conducted by heating followed by cooling, as explained in the experimental section. During cooling, the storage modulus as a function of temperature (G’ vs T) was recorded for each sample. A sharp increase in storage modulus at a particular temperature upon cooling from melt can be used to assign the crystallization temperature of PVDF/PMMA blends. For comparison, rheological measurements of neat PVDF was also included in this study. For neat PVDF, an abrupt increase in storage modulus was observed at 155 ℃ (see Figure 1) and ascribed to the onset of crystallization in PVDF. For the blends ranging PVDF/PMMA 10% to 40% PMMA, the Tc are found to be 150 ℃, 145 ℃, 140 ℃ and 135 ℃ respectively. According to this result, the crystallization temperature of PVDF decreases with increasing concentration of PMMA. This arises from the fact that PMMA restricts the chain mobility of PVDF that retards the thermal energy for crystallization. In order to study the crystallization kinetics and morphology of the blends, isothermal crystallization temperatures above Tc were selected as 156 oC, 152 oC, 146 oC and 142 oC, respectively. Below these selected temperatures, crystallization kinetics was very fast and it was not possible to capture the initial morphologies. On the other hand, isothermal crystallization will take long time above these temperatures.



0.25

Neat PVDF PVDF/PMMA 80/20 blends 0.20

0.15

G' (MPa)

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

Page 8 of 27

o

Tc= 155 C 0.10

o

Tc= 145 C

0.05

0.00

120

140

160

180

200

220

o

T ( C) Figure 1: Storage modulus, G’, evolution during dynamic temperature ramps for neat PVDF and PVDF/PMMA (80/10) blends with fixed frequency of 1.0 rad/s and cooling rate 5 K/min. Crystallization induction time of PVDF/PMMA blends The time evolution of the storage modulus were measured to understand the effect of shear on the isothermal crystallization of PVDF/PMMA blends, under a fixed shear rate of 10 s-1, as shown in Figure 2. Figure 2 represents the evolution of storage modulus with time for PVDF/PMMA blends with different content of PMMA under quiescent and shear conditions. Under quiescent conditions, samples were directly (without shear) cooled down to crystallization temperature at a cooling rate of 5 K/min. For shear conditions, samples were cooled at the same cooling rate after the cessation of shear. To assess crystallization kinetics, induction time has been calculated. Figure 2 shows that all the PVDF/PMMA blends investigated have sigmoidal shapes during isothermal crystallization. The G’ increases rapidly and then approaches a plateau at the end of the primary crystallization stage. The onset time for the rapid increase of G′ is often 8 ACS Paragon Plus Environment

Page 9 of 27

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


defined as the induction time, t0, for nucleation (illustrated by the crossover point of two dashdot lines in Figure 2a, b, and c), reflecting the energy barrier for crystallization kinetics. For 70/30 and 60/40 PVDF/PMMA blends, when the shear temperature was 180 oC, the induction time was not very different from the quiescent condition, as it is clearly seen in Table 1. On the other hand, for 90/10 and 80/20 PVDF/PMMA blends, when the shear temperature increases from 180 oC to 220 oC, the induction time significantly shortened as compared to quiescent condition. This shows that the overall crystallization rates of 90/10 and 80/20 blends are higher than that of 70/30 and 60/40 blends. After shearing at high temperature, the induction time of PVDF/PMMA significantly reduced at lower content of PMMA when compared to samples that were not sheared. However, for 60/40 blends, there is no change in induction time during quiescent and shear conditions (data not shown here). It can be noticed that the effect of shear is more pronounced in blends with PVDF content ≥80 wt% and below which there is no much difference in the induction time for quiescent and shear conditions. This shows that shear can accelerate the rate of crystallization of PVDF/PMMA blends up to a critical concentration of PMMA (20%) below which significant change in rate of crystallization occurs and above which no change in induction time or rate of crystallization have been observed. In general, shear and content of PMMA influences the crystallization induction time of PVDF/PMMA blends. Table 1: Induction time of PVDF/PMMA blends with different content of PMMA sheared at 10 s-1 for 10 s.

Blend composition PVDF/PMMA

Without

Sheared

Sheared

shear

at 180 oC

at 220 oC

Induction time of 90/10 isothermally crystallized at 156 oC

1030 s

683 s

250 s


950 s

390 s

270 s


760 s

700 s

670 s



30

o

(a)

Without shear captured at 156 C o o Sheared at 180 C captured at 156 C o Sheared at 220 C o captured at 156 C

G' (MPa)

20

10

0

t0 induction time 0

500

1000

1500

Time (s)

o

Without shear at 152 C o o Sheared at 180 C captured at 152 C o Sheared at 220 C o captured at 152 C

30

(b)

20

G' (MPa)

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

Page 10 of 27

10

0

t0 0

500

1000

induction time 1500

Time (s)


Page 11 of 27

o

Without shear at captured at 146 C o o Sheared at 180 C captured at 146 C o o Sheared at 220 C captured at 146 C

4

(c)

3

G' (MPa)

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


2

1

t0 induction time

0

0

500

1000

Time (s)

Figure 2: Evolution of storage modulus (G′) with crystallization time of PVDF/PMMA blends sheared at 10 s−1 for 10 s for (a) 90/10, (b) 80/20 and (c) 70/30 Radial growth rate: effect of PMMA and shear flow To study whether the spherulitic growth rate is affected either by addition of PMMA or shear flow, the changes in spherulite radius as a function of crystallization time for PVDF/PMMA blends were investigated under quiescent and shear conditions. Figure 3 displays the quantitative analysis of growth rate of spherulites with time after shearing at different temperature. For 90/10, 80/20, 70/30 and 60/40 PVDF/PMMA blends, the growth rate was determined from the slope of linearly fitted curve. Figure 3 shows that the radial growth of spherulite linearly increases with time. It appears that the growth rate for 90/10 was found to be 14, 18 and 28 nm/min for samples without shear and sheared at 180 oC and 220 oC, respectively (see Figure 3a). In addition, Figure 3b and 3c shows that for 80/20 blends the radial growth rate is 15 and 18 nm/min and for 70/30 blends it is 15 and 17 nm/min for samples without shear and sheared at 220 oC, respectively. But for 60/40 blends, constant spherulite growth has been observed. Interestingly, in all cases the increase in spherulitic growth rate becomes significant when sheared at high temperature. This could be due to the fact that when we sheared the samples at high temperature and allowed to crystallize at the selected temperature, the polymer 11 ACS Paragon Plus Environment


chains were relaxed and had sufficient time and thermal energy to grow. On the other hand, the growth rate of the spherulite decreases with increasing PMMA content, indicating that shear temperature and content of PMMA plays an important role in determining the spherulite morphology and radial growth rate. Increasing the content of PMMA decreases the melting point (driving force for the crystallization process) and acts as a diluent to restrict chain mobility of PVDF crystals, similar to that observed in PVDF/PBSA blends27. Thus both induction time and growth rate increases by shearing the blends at higher temperature and the result is more pronounced for 90/10 and 80/20 blends.

o

Without shear cap at 156 C o o Sheared at 180 C captured at 156 C o o Sheared at 220 C captured at 156 C

100

(a)

80

3

Radial growth (X 10 nm)

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

Page 12 of 27

60

40

20

0 25

30

35

40

45

50

Time (min)


Page 13 of 27

90

o

Without shear captured at 152 C o o Sheared at 220 C captured at 152 C

(b)

70

3

Radial growth (X10 nm)

80

60

50

40

30

20 25

30

35

40

45

50

Time (min)

80

o

Without shear at 146 C o o Sheared at 220 C captured at 146 C

(c)

70

3

Radial growth (X10 nm)

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


60

50

40

30

25

30

35

40

45

50

Time (min)

Figure 3: Variation of radial growth with crystallization time for PVDF/PMMA blends sheared at 10 s-1 for 10 s (a) 90/10, (b) 80/20, and (c) 70/30



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

Page 14 of 27

Crystalline Morphology: Polarizing optical microscopy POM equipped with a Linkam CSS450 shear stage was utilized, to assess the crystallization kinetics and crystalline morphology for PVDF/PMMA blends. Here, the POM micrographs of PVDF/PMMA blends with different content of PMMA (10, 20, 30 and 40%) were chosen. Prior to shear experiments, the onset of crystallization in 90/10, 80/20, 70/30 and 60/40 PVDF/PMMA blends were investigated by POM. It was found that, when the blend samples were cooled from melt at 10 K/min, the onset of crystallization was 140 oC, 135 oC, 130 oC and 125 oC for 90/10, 80/20, 70/30 and 60/40 blends, respectively. From our earlier study, the onset of crystallization of neat PVDF was found to be 145 oC. This shows that the onset of crystallization temperature of PVDF drastically decreases upon addition of PMMA. This arises from the fact that the miscible PMMA component restricts chain mobility of PVDF crystals.

Figure 4: The POM images of PVDF/PMM 90/10 blends (a1-a4) without shear, (b1-b4) sheared at 180 ℃, (c1-c4) sheared at 220 ℃. Images captured at fixed shear rate of 10 s-1 for 10 s and isothermally crystallized at 156 ℃. Images were taken at 15 min (a1, b1 and c1), 25 14 ACS Paragon Plus Environment

Page 15 of 27

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


min (a2, b2 and c2), 35 min (a3, b3 and c3), and 45 min (a4, b4 and c4). The red arrow indicates the flow direction. To realize the influence of shear on the crystalline morphology, the experiments were carried out using suitably selecting the isothermal crystallization temperature for each of the blends (90/10, 80/20, 70/30 and 60/40). For 90/10 blends the isothermal crystallization temperature was chosen to be 156 oC. Below 156 °C, crystallization kinetics was very fast and it was not possible to capture the initial morphologies. On the other hand, isothermal crystallization above 156 °C, will take longer time. Similarly, for the other blends like 80/20, 70/30 and 60/40, the isothermal crystallization temperature chosen was 152 oC, 146 oC and 142 oC respectively. Once the isothermal crystallization temperature was fixed, we varied the shearing temperature Tshear before quenching to Tc. The blends were sheared at higher temperature and allowed them to crystallize isothermally at a fixed temperature depending on the blend composition as mentioned above. The selected POM images 90/10 blends are shown in Figure 4. Under quiescent condition, sporadic spherulites are observed, indicating the nucleation density of PVDF/PMMA blends without shear is very small (see Figure 4a1-a4). After being sheared at 180 oC, an increased nucleation density is observed (see Figure 4b1-b4). Interestingly, pronounced effect was observed when the samples are sheared at 220 oC; that is nucleation density increases greatly (see Figure 4c1-c4). Thus one can observe that the number of spherulites increases with shear, which is consistent with the result obtained for isotactic polypropylene (iPP) under continuous shear flow28. But no change in crystalline morphology was observed. The crystalline morphology of PVDF/PMMA (90/10), isothermally crystallized at 156 oC from melt remains spherulitic (see Figure 4) regardless of the shear temperature. One can also see that with further increase in the content of PMMA (80/20), the crystallization process enhanced with shear temperature as compared to quiescent conditions as reflected from the increased number of activated nuclei after shear (see Figure S1). Similarly, a slight increase in nucleation density was observed for 70/30 blends as it is clearly seen in Figure S2. However, for 60/40, very small spherulite and no change in nucleation density under quiescent and shear conditions was observed (see Figure S3). Thus the enhancements in crystalline density were observed at low content of PMMA combined with shear flow in agreement with rheological results. 15 ACS Paragon Plus Environment


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

Page 16 of 27

Crystalline phases of PVDF: FTIR & DSC To investigate the content of α and β phases, quantitative analysis of PVDF/PMMA blends was performed by FTIR. The FTIR spectra of PVDF clearly shows the bands at 763 cm-1 (CF2 bending and skeletal bending), 795 cm-1 (CH2 rocking), 975 cm-1 (CH2 twisting) which corresponds to the α- phase and 510 cm-1 (CF2 bending), 840 cm-1 (CH2 rocking) which represents β phase29. The α phase of PVDF has a unique IR absorption band at 763 cm−1. The presence of exclusively β phase can be observed through the presence of bands at 510 and 840 cm−1. Thus, absorption bands at 763 cm−1 and 840 cm-1 are used to evaluate the changes in the fraction of β-phase in all the samples. Before any treatment, FTIR spectra of PVDF/PMMA blends have been obtained to determine the content of α and β phases. The results revealed that only α phase existed for all the compositions. This shows that the β phase can be obtained under some critical conditions such as shear or quenching from melt during crystallization. To investigate the effect of shear temperature and addition of PMMA on polymorphism of PVDF/PMMA blends, samples were sheared at different temperature and allowed to crystallize at a selected temperature. Interestingly, all samples with and without shear exhibited both α and β phases. During quiescent condition (when we crystallized from melt), the amount of β phase of the blends was higher than that of neat PVDF. This revealed that mere addition of PMMA results in more β phase content. The possible explanation for this could be due to intermolecular interactions between PVDF and PMMA. Figure 5a shows the FTIR of PVDF/PMMA blend with 10% PMMA sheared at various temperatures and isothermally crystallized at 156 oC for 45 min. It is clearly seen from Figure 5 that, without shear, the amount of β phase was very less. After being sheared at high temperature (220 oC), the β phase significantly increased. In Figure 5a, for the samples sheared at higher temperature and allowed to crystallize at 156 ℃, α phase decreases and β-phase increases as observed from the peak at 840 cm-1. Similar trend was obtained in the other blends (80/20 and 70/30, see Figure 5b, and 5c) where shearing at high temperature favored the β phase formation. However, the addition of PMMA of more than 40% (see figure 5d) showed no change in the amount of β phase during quiescent and shear conditions. The presence of higher β-phase in the 16 ACS Paragon Plus Environment

Page 17 of 27

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


sheared sample at high temperature could be due to the fact that at higher temperature, samples had longer relaxation time (from Tshear to Tcry) than other temperatures (200, 190, 180 and 160 o

C). This longer relaxation time allowed more oriented crystals. 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), respectively. Assuming that IR absorption follows the Lambert-Beer law, the Aα and Aβ absorbencies, at 763 and 840 cm-1, respectively, can be estimated as, Aα =

(1)

=Kα C Xα L

Aβ = =Kβ C Xβ L

(2)

where L is sample thickness and C is an average total monomer concentration. The subscripts α and β refer to the two crystalline phases. The incident and transmitted intensity radiations are given by I° and I. K is the absorption coefficient while X represents the degree of crystallinity of each phase. The Aα and Aβ values were determined by I° and I at 763 and 840 cm-1, respectively. Kα and Kβ are the absorption coefficient of the respective bands30 (Kα =6.1 × 104 and Kβ =7.7 × 104 cm2/mol), Xα and Xβ are the % crystallinity of the respective phases. The relative β fraction, F (β) was calculated as () =

= (

/ )

= (.)

(3)



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

Page 18 of 27

Figure 5: FTIR spectra of PVDF/PMMA (a) 90/10 (b) 80/20 (c) 70/30 and (d) 60/40 isothermally crystallized at 156, 152, 146 and 142 ℃ respectively with fixed shear rate 10 s-1 for 10 s. Comparisons have been made in order to understand the influence of shear and addition of PMMA. As quantified by FTIR, Figure 6 showed that the content of β-phase fraction increases on increasing the shear temperature at a given composition. From Figure 6, the maximum value of β phase was observed when the blends were sheared at 220 ℃. Interestingly, the β phase increases with PMMA addition as well; although, higher PMMA concentrations (20 and 30%) are reflected in higher β-phase at higher temperature, but was lesser as compared to 90/10 blends. This may be explained as the content of PMMA increases, the glass transition temperature increases significantly. As the temperature increases, the growth of the α phase is favored because thermodynamically it is stable at higher temperature.


Page 19 of 27

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


From FTIR, we can conclude that shearing at higher temperature results in longer relaxation time and favors β phase formation. An explanation for increased fraction of β-phase at higher temperature is as follows: as we sheared the sample at 220 ℃ and crystallized at selected temperature, the duration between Tshear and the isothermal crystallization temperature was higher. The polymer chains were relaxed and had sufficient time and/or thermal energy to form the meta-stable β phase in PVDF/PMMA blends aided by shear forces. 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 packing of CH2-CF2 dipoles (TTT). The maximum β phase was obtained at PMMA concentration of 10%.

Figure 6: Variations of β-phase content with shearing temperature at a fixed shear rate of 10 s-1 and shear time 10 s. It is well documented that different crystallization behavior can be achieved by varying the shear rate and shear time31-33. To detect the variation in degree of crystallinity, samples with and without shear were considered for DSC scan. The degree of crystallinity (∆Xc) was calculated according to the following equation, ∆

∆"# ! = ∆"

(4) 19

ACS Paragon Plus Environment


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

Page 20 of 27

In eq. (4), ∆Hm and ∆H100 are the melting enthalpies for the sample and 100% crystalline sample (∆H100 = 104.50 J/g for PVDF)34, respectively. Table 2 shows that, without shear, the degree of crystallinity of PVDF/PMMA blends continuously decreases with increasing PMMA content that is consistent with POM and rheological measurements. This can be explained based on the fact that as we increase content of PMMA, the chain mobility gets reduced or restricted by the amorphous PMMA. Interestingly, when sheared the degree of crystallinity increases. This show that upon shear, the orientation of molecules was strengthened leading to high % crystallinity. Table 2 Percent of crystallinity measured by Differential Scanning Calorimetry for samples sheared at different temperatures. Samples

% crystallinity (Xc)

PVDF/PMMA Without

Sheared

at Sheared

at Sheared at Sheared

shear

160 oC

180 oC

200 oC

220 oC

90/10

43

46

46

46

49

80/20

41

41

43

44

47

70/30

38

38

38

39

41

at

Change in segmental dynamics with shearing: Broadband dielectric relaxation spectroscopy Dielectric relaxation spectroscopy was used to probe the dielectric relaxations in 90/10 PVDF/PMMA blends before and after shearing. From rheology, POM and FTIR, we established that among all the blends studied here, 90/10 PVDF/PMMA blends showed maximum β phase. Furthermore, shearing 90/10 PVDF/PMMA blends at 220 oC results in maximum β phase. Therefore, we choose two samples: quiescent 90/10 PVDF/PMMA blends and sheared 90/10 PVDF/PMMA blends for studying dynamics response. Our earlier papers22, 35 have discussed the various relaxations exhibited by PVDF/PMMA blends. Therefore, we will only discuss the alteration in dielectric response from shearing 90/10 PVDF/PMMA blends. Figure 7(a-b) shows dielectric loss (ε″) as a function of frequency at different temperatures for neat 90/10 PVDF/PMMA as well as for sheared 90/10 samples. From our earlier work, we know that36, 90/10 blends show two or three relaxations depending on the temperatures. In 90/10 blends, the second relaxation is a combination of αβ from PMMA and αm from the amorphous miscibility in 20 ACS Paragon Plus Environment

Page 21 of 27

the blends. Similarly, we were able to monitor three distinguishable relaxations at room temperature which further merge at high temperature and form two distinguishable relaxations. When the control PVDF sample was sheared, as reported elsewhere, 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 phase, which are defective 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 this work, where αc relaxations shifted to higher frequency (showed by the black dotted lines in the figure a and b) and eventually merged with αβ relaxations of PMMA, and form a broad and diffuse segmental relaxation. This indicates the imperfections in the crystalline phases of PVDF (β polymorph) which relaxes faster and merged with αβ of PMMA.

PVDF/PMMA 90/10 30 α Relaxation 50 70 90 CCCC

1

ε′ ′

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


0.1 αβ Relaxation

0.01 -1 10

0

10

1

10

2

10

3

10

4

10

5

10

6

10

Frequency (Hz)



PVDF/PMMA 90/10 o sheared @220 C 30 50 70 90 110

ε′ ′

1

α Relaxation CCCC

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

Page 22 of 27

0.1

αβ Relaxation

0.01 -1 10

0

10

1

10

2

10

3

10

4

10

5

10

6

10

Frequency (Hz)

Figure 7: Dielectric loss spectra as a function of frequency with different temperatures (a) 90/10 PVDF/PMMA blend without shear, (b) 90/10 PVDF/PMMA blend sheared at 220 oC and captured at 156 oC. Conclusions The effect of shear, shear history and addition of PMMA on the polymorphism and overall crystallization behavior of PVDF was investigated systematically in PVDF/PMMA blends. Under quiescent condition, the rate of crystallization was lowered in presence of PMMA. In contrast, under shear flow the rate of crystallization (characterized by induction time and growth rate) drastically increased. This phenomenon was more pronounced when the blends were sheared at higher temperature. However, the final crystalline morphology remained the same irrespective of the quench depth from the crystallization temperature. From FTIR, quantitative analyses have been made to determine the amount of β phase. Under quiescent condition (when we crystallized from melt), the amount of β phase in the blends was higher than that of the control PVDF possibly due to intermolecular interactions between PVDF and PMMA or due to higher quenching effect by PMMA that favors the formation of β phase. Once sheared at high temperature (220 oC), the β phase increased significantly and this phenomenon was observed to be more prominent at lower content of PMMA in the blends. As the content of PMMA increases (beyond 20 wt%), the amount of β phase decreased although they are higher 22 ACS Paragon Plus Environment

Page 23 of 27

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


than the as pressed samples. It is envisaged that as the content of PMMA increases, the glass transition temperature of the blends increases significantly and as a result the blends predominantly show only α phase which is thermodynamically stable at high temperature. The observed increase in β phase upon shearing the blend samples at high temperature is also supported by DRS studies where the relaxations shifted to higher frequency when compared to quiescent samples as small and defective crystals orient faster than those of α- phase. Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: The crystalline morphology of 80/20, 70/30 and 60/40 PVDF/PMMA blends from POM. Acknowledgements The authors would like to thank the Department of Science and Technology (DST/1362) India for the financial support. Giridhar Madras thanks DST for the J.C. Bose fellowship.



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

Page 24 of 27

References (1) Shukla, N.; Shukla, A., Low temperature ferroelectric behaviour of PVDF based composites. Indian J. Eng. Mater. Sci. 2008, 15, 126. (2) Salimi, A.; Yousefi, A., Analysis method: FTIR studies of β-phase crystal formation in stretched PVDF films. Polym. Test. 2003, 22, 699-704. (3) 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, 514-525. (4) Gu, M.; Zhang, J.; Wang, X.; Ma, W., Crystallization behavior of PVDF in PVDF‐DMP system via thermally induced phase separation. J. Appl. Polym. Sci. 2006, 102, 3714-3719. (5) Yu, L.; Cebe, P., Crystal polymorphism in electrospun composite nanofibers of poly (vinylidene fluoride) with nanoclay. Polymer 2009, 50, 2133-2141. (6) Sharma, M.; Madras, G.; Bose, S., Process induced electroactive β-polymorph in PVDF: effect on dielectric and ferroelectric properties. Phys. Chem. Chem. Phys. 2014, 16, 1479214799. (7) Sajkiewicz, P.; Wasiak, A.; Gocłowski, Z., Phase transitions during stretching of poly (vinylidene fluoride). Eur. Polym. J. 1999, 35, 423-429. (8) Wang, B.; Yin, M.; Lv, R.; Na, B.; Zhu, Y.; Liu, H., Critical Composition of the β Form of Poly(vinylidene fluoride) in Miscible Crystalline/Crystalline Blends. J. Phys. Chem. B 2015, 119, 14303-14308. (9) Gregorio Jr, R.; de Souza Nociti, N. C. P., Effect of PMMA addition on the solution crystallization of the alpha and beta phases of poly (vinylidene fluoride)(PVDF). J. Phys. D: Appl. Phys. 1995, 28, 432. (10) Kim, K. J.; Cho, Y. J.; Kim, Y. H., Factors determining the formation of the β crystalline phase of poly (vinylidene fluoride) in poly (vinylidene fluoride)-poly (methyl methacrylate) blends. Vib. Spectrosc. 1995, 9, 147-159. (11) Sun, J.; Yao, L.; Zhao, Q.-L.; Huang, J.; Song, R.; Ma, Z.; He, L.-H.; Huang, W.; Hao, Y.-M., Modification on crystallization of poly (vinylidene fluoride)(PVDF) by solvent extraction of poly (methyl methacrylate)(PMMA) in PVDF/PMMA blends. Front. Mater. Sci. 2011, 5, 388400. (12) Song, D.; Yang, D.; Feng, Z., Formation of β-phase microcrystals from the melt of PVF2-PMMA blends induced by quenching. J. Mater. Sci. 1990, 25, 57-64. (13) Martins, P.; Caparros, C.; Gonçalves, R.; Martins, P. M.; Benelmekki, M.; Botelho, G.; Lanceros-Mendez, S., Role of Nanoparticle Surface Charge on the Nucleation of the Electroactive β-Poly(vinylidene fluoride) Nanocomposites for Sensor and Actuator Applications. J. Phys. Chem. C 2012, 116, 15790-15794. (14) Xu, X.-l.; Yang, C.-j.; Yang, J.-h.; Huang, T.; Zhang, N.; Wang, Y.; Zhou, Z.-w., Excellent dielectric properties of poly (vinylidene fluoride) composites based on partially reduced graphene oxide. Composites, Part B 2017, 109, 91-100. (15) Gebrekrstos, A.; Sharma, M.; Madras, G.; Bose, S., New Physical Insights into Shear History Dependent Polymorphism in Poly (vinylidene fluoride). Cryst. Growth Des. 2016, 16, 2937-2944. (16) Mago, G.; Fisher, F. T.; Kalyon, D. M., Deformation-induced crystallization and associated morphology development of carbon nanotube-PVDF nanocomposites. J. Nanosci. Nanotechnol. 2009, 9, 3330-3340.


Page 25 of 27

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


(17) Roozemond, P. C.; van Erp, T. B.; Peters, G. W., Flow-induced crystallization of isotactic polypropylene: Modeling formation of multiple crystal phases and morphologies. Polymer 2016. (18) Wang, J.; Yang, J.; Deng, L.; Fang, H.; Zhang, Y.; Wang, Z., More dominant shear flow effect assisted by added carbon nanotubes on crystallization kinetics of isotactic polypropylene in nanocomposites. ACS Appl. Mater. Interface 2015, 7, 1364-1375. (19) Koscher, E.; Fulchiron, R., Influence of shear on polypropylene crystallization: morphology development and kinetics. Polymer 2002, 43, 6931-6942. (20) Li, L.; de Jeu, W. H., Shear-Induced Crystallization of Poly(butylene terephthalate): A Real-Time Small-Angle X-ray Scattering Study. Macromolecules 2004, 37, 5646-5652. (21) Varga, J.; Karger-Kocsis, J., Rules of supermolecular structure formation in sheared isotactic polypropylene melts. J. Polym. Sci., Part B: Polym. Phys. Edition 1996, 34, 657-670. (22) Sharma, M.; Sharma, K.; Bose, S., Segmental Relaxations and Crystallization-Induced Phase Separation in PVDF/PMMA Blends in the Presence of Surface-Functionalized Multiwall Carbon Nanotubes. J. Phys. Chem. C 2013, 117, 8589-8602. (23) Sasaki, H.; Bala, P. K.; Yoshida, H.; Ito, E., Miscibility of PVDF/PMMA blends examined by crystallization dynamics. Polymer 1995, 36, 4805-4810. (24) Elashmawi, I.; Hakeem, N., Effect of PMMA addition on characterization and morphology of PVDF. Polym. Eng. Sci. 2008, 48, 895. (25) Mijovic, J.; Sy, J.-W.; Kwei, T., Reorientational dynamics of dipoles in poly (vinylidene fluoride)/poly (methyl methacrylate)(PVDF/PMMA) blends by dielectric spectroscopy. Macromolecules 1997, 30, 3042-3050. (26) Li, M.; Stingelin, N.; Michels, J. J.; Spijkman, M.-J.; Asadi, K.; Feldman, K.; Blom, P. W. M.; de Leeuw, D. M., Ferroelectric Phase Diagram of PVDF:PMMA. Macromolecules 2012, 45, 7477-7485. (27) Qiu, Z.; Yan, C.; Lu, J.; Yang, W., Miscible Crystalline/Crystalline Polymer Blends of Poly(vinylidene fluoride) and Poly(butylene succinate-co-butylene adipate): Spherulitic Morphologies and Crystallization Kinetics. Macromolecules 2007, 40, 5047-5053. (28) Coccorullo, I.; Pantani, R.; Titomanlio, G., Spherulitic Nucleation and Growth Rates in an iPP under Continuous Shear Flow. Macromolecules 2008, 41, 9214-9223. (29) Andrew, J. S.; Clarke, D. R., Effect of Electrospinning on the Ferroelectric Phase Content of Polyvinylidene Difluoride Fibers. Langmuir 2008, 24, 670-672. (30) Gregorio, J. R.; Cestari, M., Effect of crystallization temperature on the crystalline phase content and morphology of poly(vinylidene fluoride). J. Polym. Sci., Part B: Polym. Phys. 1994, 32, 859-870. (31) Song, S.; Wu, P.; Feng, J.; Ye, M.; Yang, Y., Influence of pre-shearing on the crystallization of an impact-resistant polypropylene copolymer. Polymer 2009, 50, 286-295. (32) Elmoumni, A.; Winter, H. H., Large strain requirements for shear-induced crystallization of isotactic polypropylene. Rheol. Acta 2006, 45, 793-801. (33) Mago, G.; Fisher, F. T.; Kalyon, D. M., Effects of Multiwalled Carbon Nanotubes on the Shear-Induced Crystallization Behavior of Poly(butylene terephthalate). Macromolecules 2008, 41, 8103-8113. (34) Lanceros-Méndez, S.; Mano, J. F.; Costa, A. M.; Schmidt, V. H., Ftir And Dsc Studies Of Mechanically Deformed Β-Pvdf Films. J. Macromol. Sci., Part B: Phys. 2001, 40, 517-527.



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

Page 26 of 27

(35) Sharma, M.; Madras, G.; Bose, S., Cooperativity and Structural Relaxations in PVDF/PMMA Blends in the Presence of MWNTs: An Assessment through SAXS and Dielectric Spectroscopy. Macromolecules 2014, 47, 1392-1402. (36) Sharma, M.; Madras, G.; Bose, S., Anomalous structural relaxations in PVDF rich blends with PMMA in the presence of surface functionalized CNTs. Phys. Chem. Chem. Phys. 2014, 16, 23421-23430.


Page 27 of 27

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


For Table of Contents Use Only Critical insights into the effect of shear, shear history and the concentration of a diluent on the polymorphism in PVDF Amanuel Gebrekrstos, Maya Sharma, Giridhar Madras, Suryasarathi Bose Compression molded samples of PVDF/PMMA blends show only α phase whereas; upon shearing at high temperature β phase with enhanced rate of crystallization was observed. This phenomenon was observed to be dependent on the shear history as well.


Critical Insights into the Effect of Shear, Shear ... - ACS Publications

Recommend Documents