Journal of Crystal Growth 373 (2013) 45–49

Contents lists available at SciVerse ScienceDirect

Journal of Crystal Growth

0022-02

http://d

n Corr

E-m

journal homepage: www.elsevier.com/locate/jcrysgro

Directional crystallization of dioxane in the presence of PVDFproducing porous membranes

Byoung Soo Kim, Jonghwi Lee n

Department of Chemical Engineering and Material Science, Chung-Ang University, Seoul 156-756, South Korea

a r t i c l e i n f o

Available online 12 September 2012

Keywords:

A1. Crystal morphology

A1.Directional solidification

A1. Stresses

B1. Organic compounds

B1. Polymers

48/$ - see front matter & 2012 Elsevier B.V. A

x.doi.org/10.1016/j.jcrysgro.2012.09.005

esponding author. Tel.: þ82 2 8165269.

ail address: jong@cau.ac.kr (J. Lee).

a b s t r a c t

Melt crystallization methods offer high purity and energy-efficient purification for many chemicals. The

solubility of solutes in a crystal phase is extremely low, but they can be engulfed by crystal phases.

Therefore, the control of crystal morphology and size is critical in melt crystallization. In this work, we

investigated the crystallization behavior of dioxane in the presence of a polymer solute under a

controlled temperature gradient. By controlling the movement of a sample toward a liquid nitrogen

reservoir, the nucleation and growth of crystals were regulated to produce uniformly distributed

cylindrical crystals. Upon cooling, two crystallization steps were observed. While the first crystal-

lization step mainly involved solidification of the bulk solution, the second crystallization at a lower

temperature was accompanied by a further reduction in transparency and micro-cracking. This two-

step crystallization behavior was used to make defect-free polymer structures with a unique cylindrical

morphology of through-thickness pores, regardless of the dioxane removal method. Such under-

standing of directional crystallization will facilitate the future development of melt crystallization

routes for dioxane and novel preparation methods for porous polymeric materials.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

Melt crystallization differs from other crystallization methodsin that the operating temperature is close to the melting tem-perature of the main component [1]. The melt crystallizationprocess for low molecular weight molecules has been investi-gated for many decades, primarily for purification purposes.However, typical solvent molecules have seldom been the focusof melt crystallization research. For example, water has rarelybeen purified by melt crystallization, although the melt crystal-lization of water is an important process for life within the brinyhabitat of sea ice or geochemical and physical processes operatingin the Earth’s crust and mantle [2]. Similarly, 1,4-dioxane hasmainly been purified by distillation after its synthesis via thedehydration of ethylene glycol with sulfuric acid or heating ofethylene oxide (or bis(b-chloroethyl)ether) with NaOH, whichcreates typical impurities such as acetaldehyde, ethylene acetal,acetic acid, water, and peroxides [3,4].

In general, melt crystallization requires relatively little energy,and the heat of transition in crystallization is typically two to fivetimes lower than that in distillation [1]. Recently, increas-ing environmental and energy concerns have forced modernindustries to adopt more crystallization unit operations than

ll rights reserved.

distillation procedures. Another characteristic of melt crystal-lization is its high selectivity due to the intrinsic low solubilityof solutes in the crystal phase. However, solutes can be engulfedby crystal phases and thus, may remain as impurities. Therefore,crystal morphology and size are the major factors determiningthe purification efficiency of melt crystallization.

In this study, we developed a novel method to control thetemperature gradient in two perpendicular directions so thatuniformly distributed dioxane crystals with a cylindrical mor-phology could be prepared. Polyvinylidene fluoride (PVDF) wasused as a solute since it facilitated the observation of crystalmorphology by solidifying polymer phases after the removal ofdioxane [5,6]. Furthermore, the cylindrical crystal morphologycan produce through-thickness pores in the resulting polymermembranes. These pores can in turn be developed for novelpolymer membrane technologies.

2. Experimental section

2.1. Materials

PVDF (average Mw 534,000 g/mol by GPC) used in this studywas purchased from Sigma-Aldrich (MO, USA), while dehydrated1,4-dioxane (Wako Chemicals, Tokyo, Japan) was used as asolvent. HPLC grade methanol (J.T. Baker, NJ, USA) was used asan etchant to remove dioxane crystals.

Fig. 1. Directional crystallization equipment (above) and a schematic illustration (below) showing a temperature gradient imposed in two perpendicular directions for the

columnar growth of dioxane crystals. This crystallization step was followed by freeze drying (or solvent etching). (For interpretation of the references to color in this figure,

the reader is referred to the web version of this article.)

B.S. Kim, J. Lee / Journal of Crystal Growth 373 (2013) 45–4946

2.2. Directional crystallization of dioxane

A polymer solution (7 wt%) was first prepared by dissolvingPVDF in 1,4-dioxane at 60 1C and 300 rpm for 3 days. Thesolution was then poured onto a silicon wafer (thickness¼50–55 mm, p-type, WaferTec, Seoul, Korea) and spread to athickness of 50–80 mm using a doctor blade (HaeChang, Seoul,Korea). The custom-made apparatus shown in Fig. 1 (DaihanScientific, Seoul, Korea) was employed to move the silicon waferwith a polymer solution film toward a liquid nitrogen reservoirat a controlled speed of 200 mm/s. As the wafer nears thereservoir (3.7 cm between the wafer to the surface of liquidnitrogen), a liquid-to-solid phase transition makes the polymerfilm turn cloudy. The average cooling temperature of the waferwas measured to be approximately �5 1C/min. A digital CCDcamera (DSC-WX5, Sony, Tokyo, Japan) was employed to observethe transparency change of the polymer solution due to the meltcrystallization of dioxane. The solid phase of dioxane wasremoved by two different methods. In one approach, freezedrying (FDU-2200, EYELA, Tokyo, Japan, trap refrigerationtemperature¼�85.6 1C, 4.6 Pa) was applied for 24 h. In theother method, the membrane was etched in methanol (1 L) at�2071 1C for 24 h under stirring, washed with fresh methanolseveral times, then vacuum dried for 24 h.

2.3. Characterization

Differential scanning calorimetry (DSC 6100, Seiko Instru-ments, Osaka, Japan) was used to observe the two-step crystal-lization of 1,4-dioxane. A polymer solution (2 mg) was sealed inan aluminum (Al) pan and cooled from 50 to �50 1C at �5 1C/min.For crystal morphology analysis, the resulting polymer membraneswere coated with platinum via ion sputtering (E-1030, Hitachi, Tokyo,Japan) and examined with a field electron scanning electron micro-scope (FESEM, S-4700, Hitachi, Tokyo, Japan) at a 5 kV accelerationvoltage. Infrared spectroscopy (FT-IR, Nicolet 6700, Thermo Scientific,USA) analysis was conducted on the membranes so as to observe anychange in the polymorphs.

3. Results and discussion

Dioxane crystallized when a thin film of solution was depos-ited on the surface of a silicon wafer. A temperature gradient wascreated by moving the sample over a liquid nitrogen reservoir at aconstant speed (Fig. 1). If we vertically lower a sample directlytowards the top of the reservoir, only a single temperaturegradient is created along the z-axis (Fig. 1) perpendicular to thesurface of the film, thereby inducing crystal nucleation from the

B.S. Kim, J. Lee / Journal of Crystal Growth 373 (2013) 45–49 47

bottom. In real trials, crystal nucleation indeed started from thebottom and grew along the z-axis, but crystallization was notuniform in the x- and y-directions. The inevitable establishmentof a small temperature difference over the x–y plane of the siliconwafer provides uncontrollable spreading of crystallization overthe plane.

Sliding a sample over the liquid nitrogen reservoir at acontrolled speed (see Fig. 1) enabled cylindrical crystal growthin the z-direction with uniform propagation of nucleation alongthe �y-direction. The nucleation of dioxane crystallization startsfrom the right side of the film in Fig. 1 and propagates at a speedrelated to the movement of the sample at 200 mm/s. The tem-perature gradient has two components imposed along the z- and�y-directions (red arrows in Fig. 1), and the crystal growth willreflect the direction of the temperature gradient.

The crystallization temperature of dioxane is reported as11.8 1C, while its glass transition temperature is �38.2 1C [7,8].However, in mixtures, dioxane crystallizes at temperatures lowerthan 11.8 1C due to the effects of melting point depression and thecooling rate [9]. Two crystallization temperatures have beenreported for mixtures of dioxane, possibly one for dioxane phaseI and the other for dioxane phase II or polymer-rich phase [7,8].

As shown in Fig. 2, two crystallization temperatures wereclearly observed in our experiment. As a solution film wasbrought towards the liquid nitrogen reservoir, a cloudy solidphase started to form in the area closest to the reservoir, andthe solidification spread to the rest of film. This corresponds tothe first crystallization of dioxane forming a dioxane-rich phase.

Fig. 2. The observed crystallization phenomena: (a) 2 s after the sample slides

The temperature measured using a thermocouple attached to thesilicon wafer when the solid phase formed was between 0 and5 1C. No liquid–liquid phase separation (cloudy point) before thecrystallization of dioxane was observed. This is common in dilutepolymer solutions [9,10].

Although the first change in cloudiness produced a solid phase,a second change in the cloudiness of the film was clearly noticedupon further cooling (Fig. 2(d) and (e)). Such a change corre-sponded to a measured temperature between �10 and �15 1C.This decrease in the transparency of the film during the secondcrystallization indicates the formation of a phase with a differentrefractive index, i.e., crystallization of dioxane phase II or vitrifi-cation of a PVDF-rich phase. Interestingly, the first crystallizationdid not create significant microcracks, but the second crystal-lization triggered massive microcracking throughout the films.Therefore, it seems that a liquid or at least a liquid-like phaseremained from the first crystallization and completely solidifiedin the second crystallization. While the liquid-like phase couldadsorb internal stress caused by the first crystallization, it lost itsstress-adsorbing ability after the second crystallization. Instead,the second crystallization created a significant freezing stress thattriggered microcracking.

The two-step crystallization behavior can be confirmed by DSC(Fig. 3). At a �5 1C/min cooling rate, the first and second crystal-lizations occurred at 2 and �22 1C, respectively. Different crystalstructures could develop at each crystallization temperature. Inthis case, the heat of crystallization could be different depend-ing on the crystal polymorphs. Dioxane has two monoclinic

over the liquid nitrogen reservoir, (b) 6 s, (c) 23 s, (d) 52 s, and (e) 190 s.

B.S. Kim, J. Lee / Journal of Crystal Growth 373 (2013) 45–4948

crystalline phases, phase I and phase II, and the former exists inthe temperature range from 278 (5) to 285 (12) K, and the latterfrom below 133 to 278 (5) K [7,8]. The other possibility is that thecrystallization of restricted dioxane molecules occurs at thesecond crystallization after the first crystallization. This couldpossibly occur in cryo-concentrated regions (PVDF-rich phase). Ifthe heat of crystallization is similar, majority of dioxane appearsto crystallize at the first peak, since the first crystallization peak ismuch larger than the second one. Therefore, it is a naturalconsequence that the vitrification of the solution is seeminglycomplete after the first crystallization.

The morphology of the dioxane crystals was examined usingporous structures of PVDF polymer after the removal of the

-50 -40 -30 -20 -10 0 10

-5 oC/min

Temperature (oC)

Hea

t Flo

w (e

ndo

up)

2.44 oC

-21.93 oC

Fig. 3. DSC thermogram of dioxane upon cooling at a rate of �5 1C/min.

50 µm

50 µm

Fig. 4. SEM micrographs of PVDF porous structures after removing

dioxane crystals. To examine any possible changes in morphologyduring the removal steps, two different removal methods wereemployed: freeze drying and solvent etching. Freeze drying is oneof most reliable methods to keep internal structures intact duringdrying. However, a significant desiccant stress can still develop,which in turn could create internal damage. As shown in Fig. 4,the structures prepared from the two removal methods did notexhibit significantly different characteristics. In most cases, aporosity of 80–95% and a pore size of 15–30 mm were obtained.The small differences that can be observed in the morphologies ofthe pores and wall surfaces were similar to the variations noticedat different positions on a sample.

The overall temperature gradient formed radially from thereservoir, not perfectly perpendicular to the surface of film.However, in Fig. 4, the growth direction of dioxane crystals seemsto be almost perpendicular to the surface of the film due to thesample movement and the nature of the thin film. The controlledmovement of the sample in the y-direction (Fig. 1) suppressedcrystal growth in the x–y direction and promoted the orderednucleation of cylindrical crystals only in the x�y plane. Theresulting pores are rather uniformly distributed on the x–y planeand have through-thickness structures with low tortuosity.FTIR analysis revealed the existence of b or g phase PVDF in themembranes, while most of the bulk PVDF showed only the aphase (data not shown). This result indicates that the controlledgrowth condition of dioxane crystals promotes the developmentof a specific crystal polymorph, i.e. b or g phase, in the PVDFphase, which was also confirmed by the XRD analysis (data notshown).

The existence of through-thickness pore structures can makethis controlled crystallization method for dioxane useful forpreparation of novel polymer membranes, which are hard toprepare by the conventional membrane prep approaches of phaseseparation [1,6,11]. Various applications such as water purifica-tion, lithium batteries, fuel cells, and electro-chemical devices

50 µm

50 µm

dioxane crystals by (a) solvent etching and (b) freeze drying.

B.S. Kim, J. Lee / Journal of Crystal Growth 373 (2013) 45–49 49

could benefit from this novel approach using directional crystal-lization. The efficient and uniform separation of polymer anddioxane by controlled crystal nucleation and growth can also beused for the future development of melt crystallization processes.

4. Conclusion

The melt crystallization of dioxane under the control of tempera-ture gradients was investigated for the development of an efficientseparation and novel membrane preparation method. By controllingthe movement of the sample toward a liquid nitrogen reservoir,uniformly distributed cylindrical crystals were obtained. Dioxanemolecules underwent two crystallization steps, the first one near2 1C and the second near �20 1C.The observation of sample transpar-ency and the DSC results both showed that the majority of dioxanecrystallized at the first crystallization temperature with solidificationof the samples, while the remaining dioxane crystallized at thesecond crystallization temperature with the appearance of significantmicrocracking. The resulting through-thickness pores showed thatthe crystals had a cylindrical morphology throughout the samplethickness. This understanding could provide a useful strategy forefficient melt crystallization and the preparation of defect-freemembranes with through-thickness pores.

Acknowledgment

This study was supported by grants from the National ResearchFoundation of Korea (NRF), funded by the Korea government (MEST)(#2012–014107), and the Korea Healthcare technology R&D Project,Ministry of Healthcare & Welfare (#A103017).

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