Polymer Association Structures - American Chemical Society

Chapter 16

Downloaded by UNIV OF CINCINNATI on May 31, 2016 | http://pubs.acs.org Publication Date: December 30, 1989 | doi: 10.1021/bk-1989-0384.ch016

Phase-Separation Dynamics of Aqueous Hydroxypropyl Cellulose Solutions 1

2

Thein Kyu, Ping Zhuang , and Partha Mukherjee

Center for Polymer Engineering, University of Akron, Akron, OH 44325

Time-resolved light scattering studies have been undertaken to elucidate the mechanism of thermally induced phase separation in aqueous hydroxypropyl cellulose (HPC) solutions of different molecular weight. The phase diagram was established on the basis of cloud point determination. The phase separation behavior resembles an LCST (lower critical solution temperature), but there appears a discontinuity in the cloud point curve at intermediate compositions. The phase diagram consists of three d i s t i n c t regions: (1) the isotropic regime at low concentrations, (2) the anisotropic color regime at intermediate concentrations, and (3) the gel regime at high concentrations. The presence of the superstructure in the l a t t e r two regimes has complicated the phase separation dynamics; therefore, at t h i s time we will report only the time-resolved light scattering studies on the dynamics of phase separation and phase dissolution in a 10 wt% HPC solution. The time-evolution of scattering curves were analyzed in the context of the linearized theory proposed by Cahn-Hilliard. The linearized theory describes many of the qualitative features of phase separation phenomena, but shows some deficiency in the quantitative comparison. The late stage of spinodal decomposition i s dominated by non-linear behavior and the results are interpreted in comparison with recent scaling theories. The phenomenon of phase separation in l i q u i d c r y s t a l l i n e polymer solutions has been the subject of recent interest. This phenomenon was noticed in a number of lyotropic l i q u i d c r y s t a l 1Current address: Department of Chemistry, University of Kentucky, Lexington, KY 40506 Current address: Institute of Polymer Science, University of Akron, Akron, OH 44325

2

0097-6156/89/0384-0266$06.00/0 « 1989 American Chemical Society

El-Nokaly; Polymer Association Structures ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


16. KYUETAL.

Phase-Separation Dynamics of Cellulose Solutions

systems, notably in poly-7-benzyl L-glutamate (PBLG)/dimethyl formamide (DMF) (1), poly-p-phenylene benzobisthiazole (PBT)/methane sulfonic acid (MSA) (2) or s u l f u r i c acid (3), and hydroxypropyl cellulose (HPC)/water systems (4-6). However, the phenomenon of phase separation has been complicated by additional factors such as phase transitions, gelation, and the presence of superstructure. M i l l e r and co-workers (1) found that poly-7-benzyl L-glutamate in dimethyl formamide reveals phase behavior (temperature-composition) covering isotropic and anisotropic l i q u i d c r y s t a l l i n e phase. When lyotropic solutions are brought into the biphase region by temperature quenching, gelation takes place. This phenomenon i s associated with phase separation between polymer- r i c h and -poor regions and i s similar in character with an upper c r i t i c a l solution temperature (UCST). Another rigid-rod polymer system that exhibits gelation associated with phase separation i s the poly-p-phenylene benzobisthiazole in methane sulfonic acid (2). Similar observation was also made in the case of PBT-sulfuric acid solution (3), in which the biphase structure changes to a single phase during heating. It returns to a two-phase regime upon cooling, thus i s reversible in character. Aqueous hydroxypropyl cellulose i s another type of rod-like material reported to undergo phase separation with heating (4). The phase behavior i s similar to that of a lower c r i t i c a l solution temperature (LCST), hence i t i s different from the above systems. The HPC/water system i s an interesting model system because of the r i c n variety of phase structure of the material. HPC i s a semicrystalline polymer in the s o l i d state (7), but exhibits thermotropic l i q u i d c r y s t a l l i n e character at elevated temperatures below the melting point (8). It shows isotropic phase in d i l u t e solutions, but forms an ordered l i q u i d c r y s t a l l i n e phase with cholesteric structure in concentrated solutions (4). The phenomenon of thermally induced phase separation in aqueous HPC solutions has been recognized f o r some time (4), the mechanism by which i t occurs was not known u n t i l recently ( 6 ) . According to our previous study (6), the process of phase separation at the isotropic regime (10 wt/« aqueous HPC solution) is the spinodal decomposition (SD). In t h i s review a r t i c l e , we continue our e f f o r t s to elucidate the dynamics of phase separation as well as the reverse process of phase dissolution using time-resolved light scattering. The technique i s f a s t , non-destructive and provides a good s t a t i s t i c a l average of phase domains, thus permitting one to follow the rapid spinodal decomposition process. Moreover, the time evolution of scattering function can be analyzed in terms of linear (9. 10) and non-linear scaling theories (11-17) of c r i t i c a l phenomena and related matter in s t a t i s t i c a l physics. Materials and Methods Two different molecular weight HPC specimens, M ~ 60,000 (HPC-E, Hercules Inc.) and M - 100,000 (HPC-L, Aldrich Chemical Co.) were used in t h i s study. Various aqueous HPC solutions, t y p i c a l l y from 17· to 807o concentrations, were prepared by dissolving them in w

w


267

POLYMER ASSOCIATION STRUCTURES


268

d i s t i l l e d water. Concentrated solutions were obtained by evaporating dilute solutions and the percent concentration was calculated on the basis of weight change. The solutions were sealed in a demountable c e l l (Wilmad Glass Co., Model WG-20/C) of 1 t o 0.1 mm path length. A time-resolved light scattering set-up, schematically shown in Figure l a , was u t i l i z e d (18). It consists of a laser light source (He-Ne laser with a wavelength of 6328 A°), a set of polarizer and analyzer, a sample hot stage and a screen. Scattered patterns from polymers may be photographed using a Polaroid instant camera (Polaroid Land Film Holder 545). The scattered intensity can be quantitatively monitored by means of a two-dimensional Vidicon camera (1254 B, EGfeG Co.) coupled with a detector controller (Model 1216, EG&G Co.). The analogue signal is d i g i t i z e d and analyzed on an OMA III (Optical Multichannel Analyzer) system. The scan rate i s t y p i c a l l y 30 ms f o r one-dimensional scan and about 0.5 to 1.5 s i o r two-dimensional mode depending on the number of pixels chosen f o r grouping. Various modes of data acquisition are available f o r selection to commensurate with experimental configurations. The raw data are further transferred to an o f f - l i n e computer (IBM-XT) f o r post data treatments such as background correction, data smoothing, rescaling, etc. Generally, a set of sample hot stages i s used f o r temperature jump studies; one i s controlled at an experimental temperature and the other i s preheated below phase separation points. The schematic drawing of the hot stage i s depicted in Figure l b . Cloud point phase diagrams were established at various heating rates and the equilibrium cloud points were estimated by extrapolating the data to the zero heating rate. Various T-jump experiments were carried out from room temperature to 44 - 48°C at one degree intervals and the reverse T-quench experiments were undertaken from 45 to 43 ~ 40°C. Phase E q u i l i b r i a and Phase Morphology The dilute aqueous HPC solutions were transparent at ambient temperature, but turned milky upon heating to 45°C, indicating the occurrence of thermally induced phase separation. In the light scattering study, a scattering halo developed in the V (vertical polarizer with v e r t i c a l analyzer) configuration, however, no pattern was seen in the H (horizontal polarizer with v e r t i c a l analyzer) polarization. This kind of behavior can be expected i f the scattering arises predominantly from concentration or density fluctuations rather than orientation fluctuations. Figure 2 shows two and three dimensional perspective plots of a scattering halo for the 10 wt7« aqueous HPC-L solution without using any polarizers. The interconnected biphase structure has been seen under optical microscope (6), but the picture i s not shown here. The revelation of scattering halo suggests that the process of phase separation occurs v i a spinodal decomposition. The alternative v

v


16. K Y U E T A L

Phase-Separation Dynamics of Cellulose Solutions 269

DETECTOR


CONTROLLER VIOICON CAMERA LENS

π

MONITOR

A ΙΌ CONV ERTER

copperMock

(a)

(b)

Figure 1. (a) Schematic diagram of time-resolved light scattering and (b) sample hot-stage.


POLYMER ASSOCIATION STRUCTURES

270

mechanism of nucleation and growth, i f i t occurs, would have led to the scattering maximum at zero angle which i s not seen here. For the determination of cloud point, the intensity at the scattering wavelength q=3.1 μπτ , at which the scattering maximum f i r s t developed, was plotted against temperature i n Figure 3. The temperature at which the change of intensity occurs has been regarded as a cloud point. This temperature naturally depends on the rate of heating as can be seen in Figure 4. The equilibrium cloud point was estimated by extrapolating the points to zero heating rate. The cloud point phase diagram obtained f o r various HPC/water systems i s i l l u s t r a t e d in Figure 5. The phase behavior is reminiscent of an LCST (lower c r i t i c a l solution temperature), but there i s no clear maximum or c r i t i c a l point. Instead, a discontinuity i s observed at moderate concentrations of 40 to 60 wt7. HPC; i . e . , the region where the iridescent color i s seen. The phase diagram i s extremely complex, consisting of three d i s t i n c t regions: (1) the isotropic region at low concentrations, (2) the anisotropic color region at intermediate concentrations, and (3) the gelation region at high concentrations. The complex structure of HPC solutions may be best explained by the evolution of the Hy scattering patterns during the course of solvent evaporation as shown in Figure 6. At low concentrations, there appears no scattering indicative of the single phase structure. The concentration rapidly increases around 40 wt7« where the overlapping structure of a four-lobe clover at odd multiples of 45° and a small + type pattern in 0 and 90° directions appears. The four-lobe pattern may arise from the collection of rod-like l i q u i d c r y s t a l l i n e entities probably having f a i r l y uniform dimension, whereas the inner + type pattern may be a consequence of inter-rod interference. The tour-lobe pattern further moves to low scattering angles with continued drying and eventually merges with the + type pattern to form an incomplete spherulite or sheaf structure. At this stage, the concentration levels off around 85 wt7« and the sheaf structure persists u n t i l the f i l m i s completely dried. The interpretation of the structural evolution of HPC solutions i s by no means straightforward because of the strong scattering power of l i q u i d c r y s t a l l i n e e n t i t i e s . The region 40 wt7«, at which the dual scattering pattern appeared, corresponds to the color region with the cholesteric structure. The wide-angle scattering may be attributed to the aggregates of rod-like scattering e n t i t i e s , whereas the inner scattering (+ type) pattern may be due to the inter-rod interference. Hashimoto and coworkers scattering patterns during solvent evaporation in terms of the changes in the size of the rods, the anisotropy of the rods, the number of rods in the assembly (growth of the assembly leading to an increase of overall c r y s t a l l i n i t y ) , and inter-rod interference. With continued solvent evaporation, the four-lobe (x type) and the + type patterns merge to form a sheaf-like structure. In


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KYU ET AL.



20°

10%

HPC

Figure 2. Two- and three-dimensional perspective plots of a scattering halo f o r the 107· HPC-L solution at 45°C.

43

44

45

46

TEMPERATURE

Figure 3. The change of scattered intensity as a function of temperature obtained at l°C/min.

HEATING RATEe approximated by linear slopes with the help of q = q /2. The apparent d i f f u s i v i t i e s as estimated from the intercepts were plotted against temperature in Figure 12. A negative d i f f u s i v i t y s i g n i f i e s the u p - h i l l and the positive the down-hill d i f f u s i o n . The temperature at which the d i f f u s i v i t y becomes zero i s regarded as the spinodal point ( T ) . Near the T , the d i f f u s i v i t y D may be scaled as: c

2

2

2

m

2

c

s

D = D . 0

v

s

(5)

e

with e = (T-T )/T s

s


(6)

275

276

POLYMER ASSOCIATION STRUCTURES 600

400


200

Figure 9. Time-evolution of scattering curves during phase dissolution of the 107. HPC-L solution following a T-quench from 45 to 43°C. qs 2.1

o°°°

..·-:::*···

Λ

φφ©φ€©°

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β

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φ φ

θ

φφφΦ

θ θ θ θ θ θ θ θ

100

Φ

θ

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Φ

θ θ

200 TIME (sec)

TIME (sec)

Figure 10. Log I versus t f o r the 10% aqueous HPC-L solution during (a) phase separation following a T-jump from 23 to 44°C and (b) phase dissolution following a T-quench from 45 to 43°C.



KYU ET AL.

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cr Downloaded by UNIV OF CINCINNATI on May 31, 2016 | http://pubs.acs.org Publication Date: December 30, 1989 | doi: 10.1021/bk-1989-0384.ch016

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Figure 11. R(q)/q versus q plot f o r (a) the phase separation and (b) the pnase dissolution at various temperatures. 0.06

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PHASE SEPARATION

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Mw = 100.000

φ

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Polymer Association Structures - American Chemical Society

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